STANDARD
DNV-ST-N001
Edition December 2023
Marine operations and marine warranty
The PDF electronic version of this document available at the DNV website dnv.com is the official version. If there
are any inconsistencies between the PDF version and any other available version, the PDF version shall prevail.
DNV AS
FOREWORD
DNV standards contain requirements, principles and acceptance criteria for objects, personnel,
organisations and/or operations.
© DNV AS December 2023
Any comments may be sent by e-mail to rules@dnv.com
This service document has been prepared based on available knowledge, technology and/or information at the time of issuance of this
document. The use of this document by other parties than DNV is at the user's sole risk. DNV does not accept any liability or responsibility
for loss or damages resulting from any use of this document.
This document supersedes the September 2018 edition of DNVGL-ST-N001.
The numbering and/or title of items containing changes is highlighted in red.
Changes December 2023
Topic
Reference
Description
Structure and style
All
Restructured, including renumbered, some sections to increase
readability.
Introduction
Sec.1
Restructured to increase readability.
[1.3.1]
Updated to reflect the scope of this standard.
[1.4.9]
Clarified guidance regarding the use of safe working load (SWL)
and working load limit (WLL) within the standard.
[1.6.2]
Added the following definitions:
— air cushion
— ancillary equipment
— certification, verification, re-validation and re-certification.
Updated the following definitions:
— alpha factor
— indeterminate lift
— matched pairs (to clarify it applies to steel slings)
— minimum bending radius.
Planning and execution /
environmental conditions and
criteria / ballast and other
systems, technical updates
and clarifications
[1.6.3]
γsf renamed as 'design factor', previously labelled as 'combined
factors'.
[2.3.2.5]
Clarified that only the documents as agreed with MWS are
required.
[2.3.6.1]
Clarified that marine operations manual and referenced
document shall be available.
[2.3.7.2]
Modified item x) to include places of refuge.
[2.6.5.1]
New guidance note to clarify.
[2.6.5.3],
[2.6.5.4]
New clauses defining required for operation durations longer
than allowed for weather restricted operations.
Moved and edited guidance note that used to be in [2.6.6.1].
[2.6.6.1]
Moved guidance note describing situations when restricted
weather criteria may be used for longer duration oerations to
[2.6.5.3].
[2.6.7]
Clarified requirements for weather restricted operations.
[2.6.7.1]
New clause referencing [11.8] for voyages subject to heading
control.
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Changes - current
CHANGES – CURRENT
Reference
Description
[2.6.7.4],
Figure 2-4
Updated figure and guidance note added to clarify requirements
for considering issuing of weather forecasts in operation
duration.
[2.6.7.6]
Revised to include consideration of conditions that cannot be
adequately forecast.
[2.6.7.8],
[2.6.7.9]
Updated and restructured requirements for weather restricted
operations that may be stopped.
[2.6.9.1],
[2.6.9.2]
Restructured previous [2.6.9.1] into two clauses. Clarified
section applies to wave height and wind speed uncertainty.
[2.6.9.7]
New requirement for vessels requiring earth fixed heading
during operation.
[2.6.11],
[2.6.12]
Introduced wind alpha factor for <1 hour (subject to on-site
monitoring) with:
— Table 2-5 alpha factors updated
— [2.6.11.2] and Table 2-8 updated
— Table 2-12 alpha factors updated
— [2.6.12.2] added and Table 2-15 updated.
[2.7.1.5]
New requirement for issue time for weather forecasts.
[2.7.3.2]
New requirement for all relevant parameters to be forecast.
[2.9.2.2]
New guidance note regarding wind speed measurements.
[2.9.4.1]
Added relaxation to the requirements regarding monitoring
device location for the alpha factor related monitoring when two
(2) buoys are used.
[2.11]
Clarified that class should be by a recognized classification
society.
[3.3.4]
Added guidance note regarding stochastic waves for weather
restricted operations and range of wave period selection.
[3.3.5]
Updated STF to 2.3 for longer operation reference periods.
Removed formula to estimate range of wave periods associated
with Hmax.
[3.4.3]
Revised to clarify requirements related to return periods and
calculation of metocean data.
[3.4.3.1]
Updated to exclude the return periods in this section being used
for platforms left in-situ during decommissioning and Sec.18
referenced.
[3.4.3.2]
Clarified the use of the return period values from Table 3-1.
[3.4.3.4]
Added guidance note to explain alignment of LRFD and ASD/
WSD.
[3.4.3.5]
Updated requirements for data for following month.
Added guidance note regarding delays.
[3.4.3.6]
Clarified required probability level.
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Changes - current
Topic
Reference
Description
[3.4.3.7],
Table 3-1
Added guidance note explaining why wind and wave return
periods differ.
Updated table notes.
[3.4.3.10]
Added minimum return periods for LRFD cases.
[3.4.4.1]
Added requirement for return periods for moorings during
installation.
[3.4.6]
Revised wind velocity profile formulae and clarified reference
dimension in guidance note 1.
[3.4.11]
Restructured section to allow better distinction between
general, deterministic and stochastic requirements.
New requirement in [3.4.11.2].
Added new guidance note to [3.4.11.7.2].
Revised range of wave periods requirements in [3.4.11.7].
[3.4.17.5]
New guidance note giving minimum speed for use of weather
routeing.
[3.4.20.1]
Updated probability from voyage simulations to align with
[3.4.17] and [3.4.19].
[3.6]
Added reference to DNV-ST-0111 for definition of Beaufort
Force 5.
[4.3.1.1]
Updated guidance note to include ballast water.
[4.3.6.1]
Added guidance note regarding ballast pumping capacity.
[4.4.3]
Updated bumper and guide strength requirements for better
alignment between ASD/WSD and LRFD.
[B.1.4],
Table B-2
New row for class society recommendations (conditions).
[B.2.2],
Table B-3
Documentation required for lifting, towing and mooring gear,
and technical clarification. Added reminder in the comments
column that the classification society rules apply for moorings.
[B.3.1.3]
Clarified that towing relates to iceberg.
[B.4.4]
Revised requirement for number of ensemble forecasts.
[B.4.7]
Added guidance about verification of ensemble forecasts.
[B.4.9],
[B.4.10]
New requirements for ensemble forecasts.
[C.3.1]
Clarified guidance note for operations/transports passing
through several areas.
[C.3.3.1]
Corrected equation for v to include the '^2' for the (T1/Tz)
term.
[C.3.4.3]
Revised text to reference [3.4.11.7.6].
Guidance notes deleted.
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Changes - current
Topic
Strength assessment,
technical updates and added
clarifications
Reference
Description
[5.3.5]
New requirements regarding bolted connections for
seafastening.
[5.4.4.3]
Deleted item g (less conservative material factor if more tests)
from guidance note.
[5.5.6.1]
Added road transportation to list of environmental loads.
[5.5.6.3]
Updated to reflect change to [5.5.6.1].
[5.6.2]
Updated subsection generally to reflect changes to ISO
19905-1.
Clarified weighing requirements for nominally identical items.
[5.6.3.2]
Updated to ensure worst case accounting for heel is considered.
[5.6.5.5]
Updated guidance note regarding consideration of green water
effects.
[5.6.9.3]
Updated referenced DNV code section.
[5.6.9.6]
Updated references for bolts.
[5.6.11.2]
Clarified distance between supports in item b).
[5.6.15.4]
Updated to clarify applicability.
[5.6.16]
Updated default motion criteria guidance.
[5.6.16.2]
Updated to clarify applicability.
[5.9.2.1]
Updated to clarify application of LS1 and LS2.
[5.9.3.3.3]
Added slings to the list of items for which the characteristic
resistance is applicable.
[5.9.4.1.5]
Clarified guidance note 2 on when new structures are
susceptible to fatigue, including additional pairs of nominal
peak stress and SCF.
[5.9.7.3]
Updated to clarify application of LS1 and LS2.
[5.9.7.4]
New clause to clarify application of LS1, LS2 and LS2A.
[5.9.7.5],
Table 5-7
Removed references to LS1A.
[5.9.7.6],
[5.9.8.4.5]
Updated weld permissible usage factors and removed weld
location, 'Good "shop" conditions'.
[5.9.7.7]
Clarified that requirement applies to vessels using pre 2016-07
DNV class rules.
[5.9.8.5.2]
Clarified what may be considered as 'other fibre materials'.
Replaced guidance note with new guidance note regarding
OCIMF advice that both polyethylene and polypropylene are
generally not recommended as a mooring line.
[5.9.8.5.3]
Added D-links to list.
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Changes - current
Topic
Reference
Description
[5.9.9]
Updated title to clarify that the material factors for friction are
applicable for ASD/WSD and LRFD.
Removed guidance note 2 from [5.9.9.2].
[5.10.1.3.3]
Included DNV-ST-0378 as an option (if applicable) for material
types.
[E.1.1.6]
Updated guidance note in particular clarifying application of
other codes.
[E.2.2.2]
New exception for operational equipment and added related
guidance note.
[E.2.5.2],
[E.2.5.3]
Updated requirement to 'shall'.
[E.2.10.1]
Updated requirement for re-use of bolts.
[7.1.1.1]
Clarified Item f).
[7.1.1.2]
Clarified Item 6).
[7.1.2.2],
Table 7-1
Added new reference to design code for steel risers, umbilicals
and subsea power cables.
[7.3.2.1.13]
Updated guidance note related to S-Lay.
[7.4.6.1]
Added guidance note to reference DNV-RP-0232 regarding
certification and verification of pipeline and cable laying
equipment and systems.
[7.4.7.5]
Added requirement that MBL of Chinese fingers shall be at least
that required for the connected wire.
[7.4.14.2]
Updated requirement for calibration and testing of essential
equipment for consistency with DNV-ST-F101.
[7.9.10.2.3]
Updated safety factor for sliding from 1.0 to 1.1 in the guidance
note.
[7.13.8.2]
Added pipe tracking procedure to the list of items.
[8.1.1]
Added reference to relevant DNV codes related to offshore
wind.
[8.3.1]
Added guidance note related to reference information on
required clearances
[8.4.2]
Updated to reflect changes in [5.6.2] related to weight and
centre of gravity
[8.6.1.3]
Updated to refer to bolted connections in [5.3.5].
Road transport
[9.3.2]
Updated to cover self-propelled modular transport.
Load-out operations, technical
updates and clarifications
[10.2.2.4]
New item related to fender friction added.
[10.3.7]
Updated requirement in the last list item updated to 'shall'.
[10.4.1.1]
Updated to clarify which load combinations apply.
Cables, pipelines, risers and
umbilicals, General technical
upgrades and clarifications
Offshore wind farm installation
operations
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Changes - current
Topic
Sea voyages, technical
upgrades, clarification and
additional guidance
Reference
Description
[10.5.2]
Clarified requirements to hold back/braking and added
associated new clauses.
[10.5.3.3]
Updated guidance note for when an objects needs to be turned
prior to loadout.
[10.5.3.5]
Clarified both static and dynamic loads apply.
[10.5.3.7]
Clarified external effects includes turning.
[10.5.3.15]
Clarified requirements (including guidance note) for trailer
stability.
[10.5.3.17]
Added new guidance note to clarify application of 2°
uncertainty. Correction in formula in guidance note 3.
[10.5.3.18]
Clarified application of tilt angles.
[10.5.3.22]
New clause providing an alternative to the tipping angle
approach including referencing ESTA publication.
[10.5.8]
Updated and expanded to reflect changes to [17.10.6].
[10.8.2.4]
Clarified guidance note related to link beams.
[11.3.4.1]
Included requirement to consider the effects of free surfaces if
more onerous.
[11.4]
Updated to clarify how which structural checking approaches
should be used with the different default motion options.
[11.5]
Updated for clarity regarding the use of the IMO Code of Safe
Practice for Cargo Stowage and Securing.
[11.6]
Updated to give a better distinction between pre and post
2016-07 editions of ship rules as approach altered.
[11.7.2]
Updated requirements to clarify and included new requirement
to consider roll natural period.
[11.7.2.2],
Table 11-4
Updated case 2 of the default motion criteria (ASD/WSD
approach) for different block coefficients. Clarification added
regarding full cycle period.
[11.7.3.2],
[11.7.3.10],
Table 11-5,
Clarified that the accelerations for the North Sea Barge are
applicable for barges that are larger in one or more dimensions.
Table 11-6,
Table 11-7
[11.8.2]
New guidance note on redundancy requirements.
[11.9.1.1]
Updated guidance note to cover structures that will experience
different loads during transport to their operational phase.
[11.9.1.3]
New clause to clarify requirement to account for pretension in
lashings.
[11.9.1.4]
Updated for clarity.
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Changes - current
Topic
Reference
Description
[11.9.2.9]
New clause to allow friction and no seafastening for weather
restricted voyage.
[11.9.2.11]
Added coefficient of friction for concrete.
[11.9.5.2.4]
New requirement related to hatch covers.
[11.9.5.2.15]
Clarified application of uplift factor in guidance note resulting in
different requirements for ASD/WSD and LRFD.
[11.9.5.2.18]
Updated requirement (including guidance note) for when
lashings may be acceptable.
[11.9.5.3.4],
Table 11-11
Updated to:
— remove LS1A case,
— reduce permissible usages,
— list other fibre materials (including nylon (polyamide),
polyethylene and polypropylene), and
— include D-lnks and complete web lashing assemblies.
[11.9.5.3.7]
Text updated for clarity.
[11.9.5.3.9]
Updated requirement regarding hook usage for smaller cargoes.
[11.9.9.1]
Replaced list of items to consider with reference to [11.9.1].
[11.9.9.2]
Updated friction coefficient for 'Uncoated steel - uncoated steel'
pipes and references.
[11.9.9.5]
Updated requirement related to the stowage of pipes on the
vessel.
[11.9.9.9]
Updated requirement and guidance note for larger diameter
pipe.
[11.10]
Updated and simplified content related to vessel stability.
[11.10.1.8]
Added reference in guidance note 2 regarding floating stability
of vessel engaged in lifting operations.
[11.12.2.3],
[11.12.2.4]
Updated current speed requirement.
[11.12.2.14]
Moved short towline requirements to [11.14.1.10] and added
cross reference.
[11.12.5]
Moved catenary and seabed clearance requirements to
[11.14.1.10].
[11.12.6]
Included new reference to DNV-ST-0111 for Beaufort Force
definition.
[11.13]
Restructured requirements related to towing arrangement and
equipment.
[11.13.3.1]
Clarified which towing components MBL applies to.
[11.13.3.5],
[11.13.3.6]
Updated and restructured for clarity.
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Changes - current
Topic
Reference
Description
[11.13.4],
[11.13.5]
Updated title and restructured text for increased readability.
[11.13.6.4]
Clarified that aluminium ferrules are not allowed on penannts or
tow lines.
[11.13.6.5]
New clause specifying required strength.
[11.13.8.1]
New guidance note related to soft shackles added.
[11.13.9]
Clarified minimum MBL required for pennants.
[11.13.10.4]
New requirement for spare stretcher.
[11.13.13]
Revised to clarify emergency towing gear requirements.
[11.13.14.4]
New reference added.
[11.13.14.6]
Wire rope eyes, and technical clarification.
Rope terminations clarified. Previous subsections [11.13.14.6]
and [11.13.14.7] removed.
[11.13.14.8],
Table 11-19
Reference added in table to [11.13.14.1].
[11.14.1.5]
Included new reference to DNV-ST-0111 for Beaufort Force
definition.
[11.14.1.10]
Updated catenary and seabed clearance requirements now
located here.
[11.14.2.4]
Included new reference to DNV-ST-0111 for Beaufort Force
definition.
[11.14.4.2]
New requirements for weather routed voyage including
application of alpha factors.
[11.14.4.4.3]
Added new guidance note to clarify that for generic departure
limits alpha factors are not applicable.
[11.14.14]
Updated to include place of refuge.
[11.14.22.2]
Added guidance note regarding channel width and restricted
manoeuvrability.
[11.18.7.2]
Updated bollard pull requirement for multiple tugs to one tow.
[11.24.1.5]
Updated terminology related to ports and places of shelter.
[11.30],
Table 11-25
Added new item 7.6 related to MOC.
[11.31.6.3]
Included new reference to DNV-ST-0111 for Beaufort Force
definition.
[11.32.2.5]
Added:
— new item related to catenary management
— new guidance note on ports and places of refuge
— new item related to MOC.
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Changes - current
Topic
Reference
Description
[K.7]
Updated example of cribbing/seafastening force calculations.
[K.8.4.3]
Added reference to Figure K-6.
Tow out of dry-dock or building [12.4.2]
basin
Clarified to make clear that environmental loads in addition to
manoeuvring loads shall be considered.
Jacket Installation
[13.1.1]
Added guidance note regarding decommissioning requirements
of jackets.
[13.10.1.1]
Updated to clarify that section applies to installation and not
decommissioning. Updated guidance note to reflect this.
[13.10.1.5]
Added text regarding the use of seasonal data.
[13.10.5.3]
New requirement for pile run cases shall be considered.
[15.1.3]
Updated guidance note 2.
[15.2.2.5]
Updated requirement related to safe condition to 'shall'.
[15.5.3.2]
Clarified the stability requirement to make better distinction
between different vessel types.
[15.9.6.5.1]
Revised guidance note 3 to increase readability.
[16.2]
Changed the term 'safety factor' to 'design factor' where
applicable.
[16.2.1.2],
Figure 16-1
Updated item g) in 'Obtain' box to include both in-air and
subsea lift to cover the range of deployment.
[16.2.5.6]
Updated lifting DAFs from elevated jack-ups in Table 16-2 for
better consistency with Table 16-1.
[16.2.6.1]
Clarified that sling elongation needs to consider the effects of
any change in stiffness with use for slings/grommets.
[16.2.6.4]
Updated requirement to ensure the effect of the pin is used in
the measurement of the sling/grommet.
[16.2.6.12]
Clarified that requirements apply to cable laid wire rope slings
and grommets.
[16.2.6.13]
New requirement for skew loads when using fibre slings.
[16.3.2.8]
Clarified that both static and dynamic loads are to be
considered.
[16.3.3.1]
New guidance note for lift points at different elevation.
[16.3.3.2]
Expanded guidance note 2 to address spreader bars.
[16.3.3.3],
[16.3.4]
Clarified that requirements apply to all rigging and not just
slings.
[16.3.4.4]
Added reference to guidance note 2 in [16.3.3.2].
[16.4.2],
[16.4.3]
Changed the term 'nominal safety factor'/'safety factor' (γsf) to
'design factor' where applicable.
[16.4.3.3]
Added 'with manufacturer's agreement'.
Lift-off, mating and float-over
operations
Lifting operations, technical
clarifications, upgrades and
additional guidance
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Changes - current
Topic
Reference
Description
[16.4.8.4]
Revised requirements to MBL reduction of fibre slings due to
bending.
[16.4.9]
Updated to reference [5.9.8.5.2] for material factors. Revised
guidance on fibre slings.
[16.5.2.2]
Included an explanation of the acceptance criteria in this
standard for shackles.
[16.5.2.6]
Expanded guidance note to explain acceptance criteria in this
standard.
[16.5.2.7],
[16.5.2.8]
New requirement for bow-bow connected shackles.
[16.6.2]
Clarified requirements related to lifting tools.
[16.6.3]
Added 'plate shackles' to 'other lifting equipment design'.
[16.6.4]
Revised and expanded to clarify requirements for ancillary
lifting equipment.
[16.7.2.4]
Added fifth bullet point.
[16.7.3]
Clarified requirements related to hook load.
[16.7.4]
Active heave compensation subsection re-written for clarity.
[16.7.5.4]
New clause relating to vessel stability.
[16.8.2.4]
Clarified use of a reduced load factor.
[16.8.3.1]
Clarified requirements for tested structure and ancillary lifting
equipment.
[16.8.5]
Deleted [16.8.5.2]. Added guidance note to [16.8.5.1] to
explain that lifting equipment can be verified against the
requirements of this standard.
[16.9.3]
Updated to clarify lateral load during lift. In particular:
— [16.9.3.1]: updated guidance note for spread bars.
— [16.9.3.1]: new guidance note to clarify application of 3%.
— [16.9.3.2]: clarified tilt applies to both the lifted object and
the vessel it is being lifted from.
— [16.9.3.5]: clarified to include effects of waves and current
as well as buoyancy for subsea lifts.
— [16.9.3.7] and [16.9.3.8]: new requirements.
[16.9.4.1]
Added guidance note regarding consideration of the natural
flattening of fibre slings.
[16.9.5.4]
Updated requirement regarding use of asymmetric cheek
plates.
[16.11.2.11],
[16.11.3.11]
Updated to clarify requirement for wet stowing of lifting
equipment.
[16.11.3.2]
Added guidance note regarding material characteristics to be
documented.
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Changes - current
Topic
Reference
Description
[16.11.3.4]
Clarified testing certification process for fibre slings.
[16.11.3.8]
Revised testing requirements for fibre slings.
[16.11.3.11],
[16.11.3.12],
New requirements for fibre slings.
[16.11.3.13]
[16.11.5.2]
Clarified that material properties of lifting tools are to be in
accordance with recognised design code.
[16.11.5.4]
Clarified that pressure testing for lifting tools may alternatively
be in accordance with recognised design code.
[16.11.6.2]
Updated requirement to include reference to [5.10].
[16.11.7.3]
Clarified material selection for ancillary lifting equipment.
Clarified SWL and WLL in guidance note.
[16.12.1.2]
Added requirement to identify recognised design code used as
basis of certification.
[16.12.2]
Updated requirements for certification, inspection and
revalidation of slings and grommets.
Previous [16.12.3.6]
Deleted.
[16.12.5]
Updated section to 'documentation' instead of 'certification'
to clarify that this is only applicable to items considered as
structure.
[16.12.5.4]
Added 'Records of prior use and inspections shall be made
available to the MWS'.
[16.12.6.3]
Added reference to inspection requirements for lifting tools.
[16.13.2.1]
Added reference to [16.13.1].
[16.13.3]
Added clearance requirements for lifting from floating vessel.
[16.13.4.1]
Clarified that the clearances apply for weather restricted lifting
operations and take precedence over requirements in [17.2.6].
[16.13.6]
Updated to refer to [16.10.8] for clearances during fabrication
yard lifts.
[16.14.3]
New requirements for motion limiting systems.
[16.14.5.2],
Table 16-6
Updated table note.
[16.14.5.4]
New guidance note for insignificant relative motion.
[16.16.1.5]
Added safety plan to the list of items.
[16.16.10.1]
Requirement updated to 'shall' for items c), k), and m).
[16.16.10.3]
Clarified use of extra shackles and shackle-shackle connections
in lifting rigging.
[16.17.1.2]
Included jackets in list of applicable items.
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Changes - current
Topic
Reference
Description
[16.17.2]
Revised requirements related to minimum tension and snap
loads for subsea and in water lifting.
[16.17.3]
Updated to clarify lifting/lowering analyses methods.
[P.1]
Corrected formulae.
[P.2.7.2],
[P.2.7.3]
Changed requirements related to padeye bearing pressure
(Dpin/DH change over now 0.94, equation for > 0.94 updated)
[P.2.7.4]
Clarified application of β = 0.7 (including new guidance note).
[P.3.4.2]
Updated skew load factor for fibre slings to reflect changes in
[16.2.6.13].
Mooring, technical updates and [17.2.1.2.4]
clarification
Revised with deletion of text relating to API RP 2SK being
included in ISO 19901-7.
[17.2.1.2.6]
Added guidance note regarding regional requirements.
[17.2.3.4.2]
Added guidance note referencing DNV-OS-E301.
[17.2.4.2.2],
Table 17-1
Added note regarding requirements for dynamic analyses in
Norway.
[17.2.5.4.4]
Clarified that Table 17-3 values are limited to wire, chain and
fibre rope and is not applicable for quayside and inshore.
[17.2.5.5.1]
Clarified to ensure design covers all relevant load directions for
anchor capacity.
[17.2.5.5.2]
Added guidance note regarding regional requirements.
[17.2.5.5.3]
Added reference to DNV-RP-E301 for anchor capacity.
[17.2.6.8]
New requirements for releasable mooring systems.
[17.2.7.6.3]
Updated (including new guidance note) for added clarity.
[17.2.7.9]
New requirements for releasable mooring systems.
Previous [17.8.9.7]
Deleted.
[17.2.9.1.1]
Updated to clarify that BS 6349-6 is withdrawn. Added
guidance note to reference [Q.1].
[17.2.9.2.7]
Updated to reference [5.9.9] for material factors for friction.
[17.2.9.6]
Consolidated specific requirements for moorings during loadouts in [10.5.8].
[17.2.9.8.7]
Updated requirements (including guidance note) for wire clips/
grips in moorings.
[17.2.9.9.1]
Added guidance note regarding DNV service and deliverable for
shore bollard testing.
[17.3.3.1]
Updated CMID list item for eCMID and to allow alternatives.
[17.3.5.1]
Added new guidance note for DP vessel clearances.
[Q.1.9.4]
Updated to reference [17.2.9].
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Changes - current
Topic
Decommissioning and removal
of offshore installations
Reference
Description
Previous [Q.1.9.11]
Deleted.
[Q.1.12.1]
Clarified that text applies to wire ropes.
[Q.2.9],
Table Q-2
Previous note 1) deleted.
[18.1]
Clarified scope of section.
[18.2.1.2]
Added new item, e), to the list.
[18.2.5]
Updated to include return periods for drag dominated structures
in new Table 18-1. Reduced load factor permitted when
unmanned.
Editorial corrections
In addition to the above stated changes, editorial corrections may have been made.
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Changes - current
Topic
Changes – current.................................................................................................. 3
Section 1 Introduction.......................................................................................... 24
1.1 Introduction.................................................................................... 24
1.2 Objective.........................................................................................24
1.3 Scope.............................................................................................. 24
1.4 Application...................................................................................... 24
1.5 References...................................................................................... 26
1.6 Definitions and abbreviations......................................................... 27
Section 2 Planning and execution......................................................................... 55
2.1 Introduction.................................................................................... 55
2.2 General project requirements......................................................... 55
2.3 Technical documentation................................................................ 57
2.4 Risk management........................................................................... 63
2.5 Planning of marine operations........................................................66
2.6 Operation and design criteria......................................................... 67
2.7 Weather forecast............................................................................ 80
2.8 Organization of marine operations................................................. 83
2.9 Monitoring.......................................................................................85
2.10 Inspections and testing................................................................ 87
2.11 Vessels.......................................................................................... 89
Section 3 Environmental conditions and criteria................................................... 92
3.1 Introduction.................................................................................... 92
3.2 Design environmental condition..................................................... 92
3.3 Design environmental conditions for weather restricted
operations.............................................................................................93
3.4 Design environmental conditions for weather unrestricted
operations.............................................................................................94
3.5 Weather/metocean forecast requirements................................... 108
3.6 Benign weather areas................................................................... 108
Section 4 Ballast and other systems................................................................... 111
4.1 Introduction.................................................................................. 111
4.2 System and equipment design...................................................... 111
4.3 Ballasting systems........................................................................ 113
4.4 Guiding and positioning systems.................................................. 119
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Contents
CONTENTS
Section 5 Loading and structural strength.......................................................... 126
5.1 Introduction.................................................................................. 126
5.2 Design principles.......................................................................... 126
5.3 Specific design considerations...................................................... 127
5.4 Testing.......................................................................................... 130
5.5 Load categorisation...................................................................... 132
5.6 Loads and load effects (responses).............................................. 134
5.7 Failure modes............................................................................... 149
5.8 Analytical models..........................................................................150
5.9 Strength assessment.................................................................... 150
5.10 Materials and fabrication............................................................ 162
Section 6 Gravity based structure....................................................................... 167
6.1 Introduction.................................................................................. 167
6.2 Floating gravity based structure stability and freeboard............... 167
6.3 Structural strength....................................................................... 170
6.4 Instrumentation............................................................................ 171
6.5 Gravity based structure installation..............................................172
Section 7 Cables, pipelines, risers and umbilicals............................................... 176
7.1 Introduction.................................................................................. 176
7.2 Design philosophy.........................................................................178
7.3 Installation engineering................................................................181
7.4 Vessel and installation equipment................................................ 191
7.5 Load-out and offshore transfer..................................................... 200
7.6 Transport...................................................................................... 206
7.7 Route............................................................................................ 207
7.8 Surveys......................................................................................... 208
7.9 Lay operations.............................................................................. 210
7.10 Other installation activities.........................................................230
7.11 Protection and post-lay intervention...........................................235
7.12 Product testing and pre-commissioning......................................240
7.13 Documentation requirements......................................................240
Section 8 Offshore wind farm installation operations......................................... 248
8.1 Introduction.................................................................................. 248
8.2 Planning........................................................................................ 248
8.3 Offshore wind farm installation vessels........................................ 250
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Contents
4.5 Remontely operated vehicle systems............................................121
8.5 Load-outs of offshore wind farm components...............................254
8.6 Transport of offshore wind farm components............................... 254
8.7 Installation of offshore wind farm components............................ 256
8.8 Lifting operations and lifting tools................................................258
8.9 Information required for marine warranty survey approval.......... 258
Section 9 Road transport.................................................................................... 259
9.1 Introduction.................................................................................. 259
9.2 Requirements................................................................................259
9.3 Information required.................................................................... 260
Section 10 Load-out............................................................................................ 262
10.1 Introduction................................................................................ 262
10.2 General....................................................................................... 262
10.3 Loads.......................................................................................... 264
10.4 Design calculations..................................................................... 267
10.5 Systems and equipment..............................................................269
10.6 Vessels........................................................................................ 279
10.7 Operational aspects.................................................................... 282
10.8 Special cases...............................................................................286
10.9 Information required.................................................................. 288
Section 11 Sea voyages...................................................................................... 293
11.1 Introduction................................................................................ 293
11.2 Towage or transport design/approval flow chart........................293
11.3 Motion response......................................................................... 294
11.4 Default motion criteria – general................................................296
11.5 Default motion criteria – IMO..................................................... 296
11.6 Default motion criteria – ships................................................... 297
11.7 Default motion criteria – specific cases...................................... 300
11.8 Directionality and heading control.............................................. 304
11.9 Design and strength................................................................... 307
11.10 Floating stability....................................................................... 323
11.11 Transport vessel or barge selection.......................................... 332
11.12 Tug selection............................................................................ 333
11.13 Towing arrangements and equipment.......................................341
11.14 Voyage planning....................................................................... 352
11.15 Bilge and ballast pumping systems...........................................364
11.16 Anchors (and alternatives) and mooring arrangements............ 367
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Contents
8.4 Planning and execution................................................................ 252
11.18 Specific for multiple towages....................................................371
11.19 Specific for towing in ice.......................................................... 375
11.20 Specific for towage in the Caspian Sea..................................... 385
11.21 Specific for voyages in inland waterways................................. 389
11.22 Specific for FSUs (FPSOs, FSOs, FLNG facilities, FRSUs etc.)..... 389
11.23 Specific for jacket voyages....................................................... 393
11.24 Specific for ship towage........................................................... 394
11.25 Specific for voyage to scrapping............................................... 396
11.26 Specific for towing of pipes and submerged objects................. 397
11.27 Specific for deep draught towages............................................404
11.28 Specific for jack-up voyages..................................................... 405
11.29 Approaching a jack-up location.................................................411
11.30 Rig move procedures (for all mobile offshore units)................. 416
11.31 Specific for semi-submersible voyages..................................... 419
11.32 Information required................................................................ 422
Section 12 Tow out of dry-dock or building basin...............................................426
12.1 Introduction................................................................................ 426
12.2 Dry dock/construction basin.......................................................426
12.3 Design and strength................................................................... 427
12.4 Mooring and handling lines for tow-out...................................... 427
12.5 Intact and damage stability........................................................ 428
12.6 Under-keel clearance for leaving basin....................................... 428
12.7 Side clearances........................................................................... 429
12.8 Under-keel clearance outside basin............................................ 429
12.9 Towage and marine considerations.............................................429
12.10 Information required................................................................ 429
Section 13 Jacket installation operations............................................................431
13.1 Introduction................................................................................ 431
13.2 Environmental conditions............................................................431
13.3 Strength...................................................................................... 432
13.4 Jacket buoyancy, stability and seabed clearance........................ 433
13.5 Jacket lift.................................................................................... 437
13.6 Jacket launch.............................................................................. 438
13.7 Floating controlled upend and set-down ballasting..................... 444
13.8 Jacket position and set-down..................................................... 445
13.9 Buoyancy tank............................................................................ 447
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Contents
11.17 Manned voyages....................................................................... 369
13.11 Information required................................................................ 452
Section 14 Construction afloat............................................................................ 455
14.1 Introduction................................................................................ 455
14.2 Loads and structures.................................................................. 455
14.3 Stability and damage stability.................................................... 456
14.4 Mooring and fendering................................................................457
14.5 Construction spread.................................................................... 457
14.6 Operational requirements........................................................... 458
14.7 Information required.................................................................. 458
Section 15 Lift-off, mating and float-over operations......................................... 460
15.1 Introduction................................................................................ 460
15.2 General....................................................................................... 461
15.3 Loads.......................................................................................... 462
15.4 Systems and equipment..............................................................463
15.5 Vessels........................................................................................ 464
15.6 Operational aspects.................................................................... 465
15.7 Specific for lift-off operations..................................................... 467
15.8 Specific for mating operations.................................................... 470
15.9 Specific for float-over operations................................................477
15.10 Specific for docking operations.................................................488
15.11 Information required................................................................ 493
Section 16 Lifting operations.............................................................................. 498
16.1 Introduction................................................................................ 498
16.2 Load factors................................................................................ 498
16.3 Derivation of hook, lift point and rigging loads........................... 507
16.4 Sling and grommet design.......................................................... 510
16.5 Shackle design............................................................................ 515
16.6 Other lifting equipment design................................................... 517
16.7 Cranes and installation vessels...................................................519
16.8 Structural analysis...................................................................... 523
16.9 Lift point design..........................................................................525
16.10 Fabrication yard lifts.................................................................530
16.11 Fabrication of rigging and lifting equipment............................. 533
16.12 Certification/documentation and inspection of rigging and
lifting equipment................................................................................ 539
16.13 Clearances.................................................................................543
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Contents
13.10 On-bottom stability and piling.................................................. 448
16.15 Heave compensation................................................................. 549
16.16 Operations and practical considerations................................... 551
16.17 Subsea lifting and installation.................................................. 560
16.18 Information required................................................................ 576
Section 17 Mooring and dynamic positioning systems........................................ 579
17.1 Introduction................................................................................ 579
17.2 Mooring....................................................................................... 579
17.3 Dynamic positioning systems......................................................606
Section 18 Decommissioning and removal of offshore installations.................... 612
18.1 Introduction................................................................................ 612
18.2 General principles....................................................................... 612
Section 19 Bibliography...................................................................................... 615
19.1 Bibliography................................................................................ 615
Appendix A Introduction..................................................................................... 621
Appendix B Planning and execution.................................................................... 622
B.1 Documentation and certification for marine vessels..................... 622
B.2 Documentation required for lifting, towing and mooring gear.......625
B.3 Iceberg management operations.................................................. 628
B.4 Ensemble forecasting - informative.............................................. 632
Appendix C Environmental conditions and criteria.............................................. 634
C.1 General......................................................................................... 634
C.2 Wind conditions............................................................................ 634
C.3 Wave conditions........................................................................... 635
Appendix D Ballasting and other systems........................................................... 642
Appendix E Structural strength........................................................................... 643
E.1 Fillet eld checking......................................................................... 643
E.2 Bolted connections........................................................................647
Appendix F Gravity based structure (GBS)..........................................................655
Appendix G Cables, pipelines, risers and umbilicals............................................656
Appendix H Offshore wind farm installations - informative.................................657
H.1 Introduction................................................................................. 657
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Contents
16.14 Bumpers and guides................................................................. 547
H.3 Cable challenges/cables............................................................... 660
H.4 Specific challenges/considerations for array cables..................... 663
H.5 Exclusions from marine warranty scope....................................... 663
Appendix I Land transport.................................................................................. 664
Appendix J Load-out............................................................................................665
Appendix K Sea voyages..................................................................................... 666
K.1 Example of main tow bridle with recovery system........................666
K.2 Example of emergency towing gear..............................................667
K.3 Example of Smit-type clench plate............................................... 668
K.4 Emergency anchor mounting on a billboard..................................669
K.5 Alternatives to the provision and use of an emergency anchor..... 669
K.6 Alternative arrangements for towing connections for ship
towages.............................................................................................. 671
K.7 Example of cribbing/seafastening force calculations informative......................................................................................... 672
K.8 Good practice recommendations for the tie-down of lifting slings
- informative.......................................................................................675
K.9 Good practice recommendations for towing - informative.............678
K.10 Ice classification - informative................................................... 681
K.11 Options for mobile offshore unit voyages in ice - informative..... 683
Appendix L Tow out of dry-dock or construction basin....................................... 684
Appendix M Jacket Installation........................................................................... 685
Appendix N Construction afloat...........................................................................686
Appendix O Float-over, mating and float-off operations..................................... 687
Appendix P Lifting operations - informative........................................................688
P.1 2-hook lift - load factors and derivation of lift point loads............ 688
P.2 Padeye calculations...................................................................... 692
P.3 Calculation of skew load factor.....................................................696
Appendix Q Mooring and dynamic positioning systems.......................................699
Q.1 Good practice recommendations for quayside mooring Informative......................................................................................... 699
Q.2 Dynamic positioning systems - informative.................................. 712
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Contents
H.2 General......................................................................................... 657
Changes – historic.............................................................................................. 716
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Contents
Appendix R Decommissioning and removal of offshore installations...................715
SECTION 1 INTRODUCTION
1.1 Introduction
DNV Noble Denton marine services is a global provider of marine warranty services and has set the industry
standard for marine operations and marine assurance activities for the last 50 years. DNV's collective
industry best practice and guidance documentation is referenced and used all over the world.
1.2 Objective
This standard is intended to ensure marine operations are designed and performed in accordance with
recognized safety levels and to describe “current industry good practice”. Where applicable, this standard can
be used in the approval of the marine operation(s) for marine warranty survey purposes.
1.3 Scope
1.3.1
This standard addresses the marine operations that can occur during the development or decommissioning
of an offshore asset or when objects are moved by water from one place to another. It addresses marine
warranty survey requirements relevant to load-out, construction afloat, voyages and installation/removal and
the load cases that should be addressed in the design and planning of these marine operations.
1.3.2
The integrity of the final structure in the installed condition is the responsibility of the assured and would
normally be verified and accepted by the certifying authority. The marine warranty survey company takes no
responsibility for the installed condition unless the marine warranty survey scope specifically addresses this
case e.g. for jack-up location approval.
1.3.3
With the exception of location approval of mobile offshore units (MOUs) which are covered in DNV-STN002, /39/, this standard covers most offshore assets and operations that are likely to require MWS
approval.
1.4 Application
1.4.1
Where DNV Noble Denton marine services is the marine warranty survey provider, this standard should
be read in conjunction with DNV-SE-0080, /38/, which provides a description of the process used by DNV
Noble Denton marine services when providing marine warranty survey (MWS) services to evaluate whether a
marine operation can be accepted for the purposes of insurance-related MWS. It addresses both ‘project’ and
MODU/MOU related MWS.
1.4.2
The use of this standard presupposes and does not replace the application of industry knowledge, experience
and know-how throughout the marine operation activities. It should solely be used by competent and
experienced organizations, and does not release the organizations involved from exercising sound
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professional judgment. DNV has however no obligations or responsibility for any services related to this
standard delivered by others. DNV has a qualification scheme mandatory to approval engineers and
surveyors providing services related to this standard. This ensures that all approvals and certificates
delivered are carried out by well qualified personnel who understand the intention behind the standard, the
limitations and the correct interpretations. The use of this document is at the user's sole risk. DNV does not
accept any liability or responsibility for loss or damages resulting from any use of this document.
1.4.3
In some cases risk assessments can be used to justify project-specific deviations from the standard criteria
provided that the results are acceptable. When such risk assessments show that the risk levels are increased
relative to those inherent in the standard criteria, the operation may be approved subject to disclosure by the
customer to, and agreement by, the insurance underwriters.
1.4.4
Execution of operations not adequately covered by this standard shall be specially considered in each case.
1.4.5
Fulfilment of all requirements in this standard does not guarantee compliance with international and national
(statutory) regulations, rules, etc. covering the same subjects/operations.
1.4.6
This standard should if required be used together with other recognized codes or standards applicable for
marine operations.
1.4.7
In case of conflict between other codes or standards and this document, the latter shall be governing if this
provides a higher level of safety or serviceability.
1.4.8
By recognized codes or standards are meant national or international codes or standards applied by the
majority of professionals and institutions in the marine and offshore industry.
1.4.9 Safe working load and working load limit
1.4.9.1 Both working load limit (WLL) and safe working load (SWL) are in common use, along with lashing
capacity and rated capacity/force amongst others. There are no standard definitions for WLL and SWL with
these terms sometimes being used interchangeably. In general, they are defined along the lines of 'maximum
working load of lifting equipment that should not be exceeded' (IMO Resolution A.918(22) - IMO Standard
Marine Communication Phases, /149/). Some definitions refer to mass or weight rather than load and
normally no specific mention is made as to whether the load referred to is static or dynamic.
The value of WLL can vary depending on the configuration in which the equipment is being used.
Confusion can arise due to the very different safety factors being assumed by different equipment
manufacturers and for different uses (e.g. mooring, lifting or towing). Therefore, whenever possible this
standard uses minimum breaking load (MBL) or ultimate load capacity (ULC) to avoid these discrepancies.
Where necessary the term WLL has been adopted in this standard although referenced documents may
refer to SWL. Whether the WLL is to be compared against a static or dynamic load depends on the operation
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being undertaken and is therefore specified in the relevant section. See [16.5] for use of shackles during
engineered lifts (as defined in [16.1.4]).
1.4.9.2 If the WLL or SWL of a shackle or other equipment is documented but the MBL or ULC is not, the
owner or operator should obtain a document from the manufacturer stating the minimum safety factor defined as (MBL or ULC) / (WLL or SWL as appropriate).
1.5 References
1.5.1 Normative references
Table 1-1 lists normative DNV references used in this document which, through reference in this text,
constitute provisions of this standard.
Table 1-1 DNV references
Document code
Title
DNV-OS-C101
Design of offshore steel structures, general – LRFD method
DNV-ST-N002
Site specific assessment of mobile offshore units for marine warranty
Table 1-2 lists other normative references used in this document.
Table 1-2 Other references
Document code
Title
ANSI/AISC 360-22
Specification for Structural Steel Buildings
EN 1993 series
Eurocode 3: Design of steel structures
IMO IMDG Code
International Maritime Dangerous Goods Code
IMO IS Code
Intact Stability Code
IMO Load Lines
International Convention on Load Lines
IMO COLREG
International Regulations for Preventing Collisions at Sea
IMO ISM Code
International Safety Management Code
IMO ISPS Code
International Ship and Port Facility Security Code
IMO Res. A.1024(26)
Guidelines for ships operating in polar waters
IMO Polar Code
International Code for Ships Operating In Polar Waters
ISO 19901-5
Petroleum and Natural Gas Industries - Specific requirements for offshore structures – Part
5: Weight management.
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1.5.2 Informative references
1.5.2.1 All references are listed in Sec.19.
1.6 Definitions and abbreviations
1.6.1 Definition of verbal forms
The verbal forms defined in Table 1-3 are used in this document.
Table 1-3 Definition of verbal forms
Term
Definition
shall
verbal form used to indicate requirements strictly to be followed in order to conform to the
document
should
verbal form used to indicate that among several possibilities one is recommended as
particularly suitable, without mentioning or excluding others
may
verbal form used to indicate a course of action permissible within the limits of the document
Where guidance notes have been included they are used for giving additional information, clarifications or
advice to increase the understanding of preceding text. Therefore guidance notes shall not be considered as
giving binding or defining requirements. Any values in guidance notes are not a requirement and shall be
considered as an initial recommendation.
1.6.2 Definition of terms
The terms defined in Table 1-4 are used in this document. Terms defined elsewhere in Table 1-4 are within
single quotation marks.
Table 1-4 Definition of terms
st
1
Term
Definition
intercept (angle)
first angle of static inclination at which the wind overturning moment is equal to the righting
moment, see Figure 11-3 and Figure 11-4
24-hour move
nd
2
intercept (angle)
'jack-up' move taking less than 24 hours between entering the water and reaching a safe
air gap with at least two very high confidence good weather forecasts for the 48 hours after
entering the water, having due regard to area and season
second angle of static inclination at which the wind overturning moment is equal to the
righting moment, see Figure 11-3 and Figure 11-4
9-part sling
sling made from a 'single laid sling' braided nine times with the sling rope and eyes forming
each eye of the 9-part sling
A&R winch
abandonment and retrieval winch on a lay 'vessel' whose primary purpose is to lower the
'pipeline' to the seabed and to retrieve it back to the lay 'vessel' with sufficient working
tension to control the pipe catenary within safe code limits at all stages
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Term
Definition
accidental limit state
limit state related to an accidental event
This can apply to either the intact structure resisting accidental loads (including operational
failure) or the load carrying capacity of the structure in a damaged condition.
added mass
'added mass' or virtual mass is the inertia added to a system because an accelerating or
decelerating body shall move some volume of surrounding water as it moves through it, since
the object and fluid cannot occupy the same physical space simultaneously
This is normally calculated as mass of the water displaced by the structure multiplied by the
'added mass coefficient'.
added mass coefficient
non-dimensional coefficient dependant on the overall shape of the structure
air cushion
air pumped into under base compartments of a structure
They are typically used between any skirts below gravity base structures during float out.
Since these compartments are open at the bottom, the air pressure and volume varies with
the draught at any time.
alpha factor
factor applied to the 'operational limiting criteria' to account for weather forecasting
inaccuracies, see [2.6.9]
ancillary lifting equipment as defined in [16.6.4]
Items considered as ancillary lifting equipment in this standard may be termed lifting
accessories, loose gear or similar in recognised design codes. For the purposes of this
standard requirements for shackles are specially considered, see [16.5]. For spreader bars/
frame and lifting beams see [16.6.3].
angle of loll
static angle of inclination after flooding, without wind heeling, see Figure 11-4
approval
act, by the designated the 'MWS company' representative, of issuing a 'certificate of approval'
array cable(s)
generic term collectively used for inter turbine cables and collector cables
See also 'infield cables'.
asset
structure or object subject to an insurance warranty or at risk from an operation
assured
person who has been insured by some insurance company, or underwriter, against losses or
perils mentioned in the policy of insurance
barge
non-propelled 'vessel' commonly used to carry 'cargo' or equipment
barrel
cylindrical/conical structure that the 'product' is wrapped around
base weight
calculated weight of a structure, excluding all allowances and contingencies
Sometimes known as net weight.
(cable) basket
mobile non-rotating horizontal circular storage facility with a vertical 'barrel' and side wall(s)
'Baskets' can be used to for storage, transportation and onshore or offshore installation.
Product is loaded by 'coiling'.
bend restrictor
device with several interlocking elements that lock when a design radius is achieved
bend strain reliever
tapered plastic sleeve fitted to a flexible pipe, umbilical or cable at the transition between a
stiff section (typically an end fitting or connector) and the normal body of the pipe, umbilical
or cable
Also known as bend stiffener.
bending factor γb
partial safety factor that accounts for the reduction in strength caused by bending round a
shackle, 'trunnion', padear, diverter or crane hook
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Term
Definition
benign (weather) area
area with benign weather as described in [3.6]
bifurcated tow
method of towing 2 (or more) 'tows', using one tow wire, where the second (or subsequent)
'tow(s)' is connected to a point on the tow wire ahead of the preceding 'tow', and with each
subsequent towing pennant passing beneath the preceding tow, see [11.18.1.4]
bird-caging
phenomenon whereby armour wires locally rearrange with an increase and/or decrease
in pitch circle diameter as a result of accumulated axial and radial stresses in the armour
layer(s)
bollard pull
'certified' continuous static 'bollard pull' of a 'tug'
The mean bollard pull developed in a test by a 'tug' at 100% of the 'maximum continuous
rating' (MCR) of main engines over a period of 10 minutes. This is used for the selection of
'tugs' and sizing of towing equipment.
Maximum bollard pull (at 110% of MCR) should not be used for tug selection.
buckle 'wet'/'dry'
local collapse of pipe cross section in the span of pipe between the lay 'vessel' and the seabed
'Dry' means that the pipe wall is not breached and 'wet' means that the pipe wall is breached
and seawater floods into the pipe.
bundle
configuration of two or more 'pipelines' joined together and either strapped or contained
within a carrier or 'sleeve pipe'
burial assessment survey
survey to assess the expected burial depths on a cable route using purpose built sledges
equipment with bottom penetrating sonar equipment or by towing a miniature plough
burial protection index
process to optimise 'cable burial' depth requirements based on a 'risk assessment' of threats
to the cable and the soil strengths in the location of each risk
cable burial
submarine power cable is trenched into the seabed and covered with soil providing complete
burial of a cable
cable grips
used to pull or support cables and pipes
They work on the principle of the harder the pull, the tighter the grip.
cable tank
circular storage area where cable is coiled
cable-laid grommet
single length of 'unit rope' laid up 6 times over a core, as shown in IMCA M 179, /81/, to form
an endless loop
Sometimes known as an endless sling.
cable-laid sling
sling made up of 6 'unit ropes' laid up over a core 'unit rope', as shown in IMCA M 179, /81/,
with a hand spliced eye at each end
cargo
where the item to be transported is carried on a vessel, it is referred to throughout this
standard as the cargo
If the item is towed on its own buoyancy, it is referred to as the tow.
cargo overhang
distance from the side of the 'vessel' to the extreme outer edge of the 'cargo'
cargo ship safety
certificates
(safety construction)
(safety radio)
certificates issued by a certifying authority to attest that the 'vessel':
(safety equipment)
— complies with the cargo ship construction and 'survey' regulations
— has radiotelephone equipment compliant with requirements
— carries safety equipment that complies with the rules applicable to that 'vessel' type
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Term
(basket or reel) carousel
Definition
rotating horizontal circular platform with a vertical 'barrel' used in conjunction with a
'turntable'
Either:
a)
a 'basket carousel' with side wall(s) where the 'product' is loaded by 'winding' or
b)
a 'reel carousel' with an upper flange where the 'product' is loaded by 'reeling/spooling'.
The upper flange can be plated (fully or partially) or of spoke design. The upper flange
can be adjustable or supplemented by an intermediate flange.
'Carousels' can be mobile or integrated with the 'turntable'. They can be used for storage,
transportation and onshore or offshore installation.
carrier or sleeve pipe
outer casing of a 'bundle' or 'pipe-in-pipe'
cats-paw
extreme type of loop thrown into cables where a combination of low tension and residual
torsion forms a twisted loop
Commonly seen at repair 'Final Splice' locations where the 'Final Splice' is lowered too
quickly.
certificate of approval
formal document issued by a 'MWS company surveyor' stating that, in their judgement and
opinion, all reasonable checks, preparations and precautions have been taken to keep risks
within acceptable limits, and an 'operation' may proceed
certification
third-party issue of a statement, based on a decision following review, that fulfilment of
specified requirements has been demonstrated related to products, processes or systems
certified
having (or proved by) a certificate from an acceptable source
For vessels, see [2.11].
For other equipment (e.g. towing gear, lifting equipment) this is normally done by certificates
issued or endorsed by bodies approved by a recognized classification society or other body
accepted by the MWS company. Certification of towing, mooring and lifting equipment need
to be certified. This includes design verification and manufacturing surveillance including FAT.
characteristic load
reference value of a load to be used in the determination of load effects
The 'characteristic load' is normally based upon a defined fractile in the upper end of the
distribution function for the load.
Chinese fingers
also known as pulling socks are used to pull or support cables and pipes
They work on the principle of the harder the pull, the tighter the grip.
classification
system to ensure ships are built and maintained in accordance with the rules of a particular
classification society
Although not an absolute legal requirement, the advantages (especially as regards insurance)
mean that almost all 'vessels' are maintained in class.
coiling
operation where the 'product' (typically cable) is placed in layers of concentric rings into a
'tank'/'basket' that does not rotate, such that it is twisted 360 degrees per turn
Note: not all cables can be coiled.
cold stacking
where the unit is expected to be moored or jacked-up for a significant period of time and will
have minimum or, in some cases, no services or personnel available
column stabilised unit
MOU which floats on its columns during 'operation' or transit (e.g. 'semi-submersible')
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Term
competent person
Definition
'competent person' carrying out a thorough examination/assessment /analysis/certification
shall have such appropriate practical and theoretical knowledge and experience of the
equipment and/or activity
Although the 'competent person' may often be employed by another organisation, this is not
necessary, provided they are sufficiently independent and impartial to ensure that in-house
examinations are made without fear or favour. However, this should not be the same person
who undertakes routine maintenance of the equipment as they would then be assessing their
own maintenance work.
Note: Where local or national regulations define a 'competent person' with more onerous
requirements, then the definition in these local or national regulations shall apply.
consequence factor γc
factor applied in the design of critical components to ensure that these components have an
increased factor of safety in relation to the consequence of their failure
contingency operation
operation performed in response to an emergency situation, see [2.5.4]
Also the term for bringing an object into a 'safe condition' for operations that can be halted,
see [2.6.5.2].
controlled depth tow
special towing 'operation' where the pipe string or 'bundle' is made almost buoyant and towed
at a controlled depth within the water column, suspended between a lead and trail 'tug'
crane vessel
'vessel', ship or 'barge' on which 'lifting appliance' is mounted
For the purposes of this document it is considered to include: crane barge, crane ship, derrick
barge, floating shear-leg, heavy lift vessel, 'semi-submersible' crane vessel (SSCV) and 'jackup' crane vessel.
cribbing
arrangement of timber baulks, secured to the deck of a 'barge' or 'vessel', formally designed
to support the cargo, generally picking up the strong points in 'vessel' and/or 'cargo'
cross linked polyethylene
type of AC cable conductor insulation commonly used on submarine power cables
cross sectional area
normally the CSA of a single conductor in a submarine power cable x 3
2
For example a submarine power cable with 3 mm x600 mm in its designation would be a
2
cable with three conductors each of 600 mm .
customer
company to which the 'MWS company' is contracted to perform marine warranty or
consultancy activities
dead man anchor
anchor or multiple anchors (which may be clump weights, sometimes buried), typically used
to initiate 'pipelay'
deck mating
act of installing integrated topsides over a substructure, generally by 'float-over' and
ballasting
Deck mating may take place inshore or offshore, onto a floating or a previously installed
substructure.
deck support unit
installed on the 'vessel' 'grillage' to support the structure before and during the 'float-over'
It can be designed to either provide a rigid vertical support and allow horizontal movement or
utilise elastomers to absorb vertical and horizontal installation motions and forces.
deep water
determined on a case by-case-basis but for installation of subsea equipment it is generally
taken as greater than 500 m
demolition towage
'towage' of a 'dead' 'vessel' for scrapping
design environmental
condition
design wave height, wave period, wind speed, current and other relevant environmental
conditions specified for the design of a particular 'voyage' or 'operation'
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Term
Definition
design load
load or load condition which forms basis for design and design verification
determinate lift
lift where the slinging arrangement is such that the sling loads are statically determinate, and
are not significantly affected by minor differences in sling length or elasticity e.g. two and
three point lifts
double tow
'operation' of towing two 'tows' with two separate tow wires by a single 'tug', see [11.18.1.2]
dry towage
'operation' of transporting a 'cargo' on a 'barge'
(cable) drum
cylinder (the 'barrel'), normally with horizontal axis, with two flanges onto which a 'product' is
wrapped by 'reeling'/'spooling
.Used for storage, transport and onshore or offshore installation. A 'drum' is normally
unpowered and normally made of wood or steel.
Also see 'reel'.
dunnage
typically inexpensive material used to protect 'cargo' during 'transport'
'Dunnage' also refers to material used to support loads and prop tools and materials, see
'cribbing'.
dynamic amplification
factor
factor by which the weight is multiplied, to account for accelerations and impacts during the
operation
dynamic angle
smallest angle at which the area ratio in [11.10.3.1] is satisfied
dynamic hook load
'static hook load' multiplied by the DAF
engineered lift
lift which is planned, designed and executed in a detailed manner, with thorough supporting
documentation, see [16.1.4]
export cable(s)
submarine power cables connecting the offshore wind farm transformer station to a landfall
connection
factored weight
calculated weight of a structure, including all allowances and contingencies
Sometimes known as gross weight.
fatigue limit state
'limit state' related to the capacity of the structure to resist accumulated effect of repeated
loading
field joint coating
refers to single or multiple layers of coating applied to girth welds and associated cutback of
the 'line pipe' coating
Coating can be applied in factory or field.
final splice
location where a second joint is inserted into a cable system during a repair and includes the
excess slack in the cable where the two ends of the 'final splice' come to the surface
flag state
state under which a commercial 'vessel' is registered or licensed
It has the responsibility to enforce regulations over 'vessels' registered under its flag,
including inspections, certification and issuance of safety or pollution prevention documents.
floating off-load
reverse of 'floating on-load'
floating on-load
'operation' of transferring a 'cargo', which itself is floating, onto a 'vessel' or 'barge', which is
submerged for the purpose
floating production
system
including FPV, FPU, FPSO, FGSO, spar (buoy) or TLP
float-over
'operation' of installation/removal of a structure onto or from a fixed 'host structure' by
manoeuvring and ballasting the transport 'vessel' to effect load transfer
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Term
Definition
flying-lead
type of 'umbilical jumper' that is normally lowered to the seabed using a supporting frame
and then installed using ROV
forecasted operational
criteria
metocean limits used when assessing weather forecasts to determine the acceptability of
proceeding with (each phase of) an 'operation' beyond the next 'point of no return'
For a 'weather restricted operation/voyage' these equal the 'operational limiting criteria'
multiplied by an 'alpha factor'.
For 'weather unrestricted operation/voyage' see [2.6.6.2].
freeboard
distance from the waterline to the 'watertight' deck level
In commercial 'vessels', it is measured relative to the ship's 'load line'.
'Effective freeboard' is the minimum vertical distance from the still water surface to any
opening (e.g. an open manhole) or downflooding point, after accounting for 'vessel' trim and
heel.
global positioning system
satellite based system providing geographic coordinate location
grillage
structure, secured to the deck of a 'barge' or 'vessel', formally designed to support the 'cargo'
and distribute the loads between the 'cargo' and 'barge' or 'vessel'
heave
'vessel' motion in a vertical direction
heavy transport vessel
'vessel' which is designed to ballast down to submerge its main deck, to allow self-floating
'cargo(es)' to be on-loaded and off-loaded
host structure
'host structure' (e.g. 'jacket', GBS, TLP) onto which the structure or structure deck will be
floated and supported, or from which it will be removed
hoist line
all components of the lift system from the crane tip to the lifted object i.e. crane falls, hook
and rigging below the hook
hydro-acoustic
positioning reference
through water acoustic link between a 'vessel' and a seabed beacon
Used to locate and track vehicles in the water column and can be used as a DP reference.
indeterminate lift
any lift where the sling loads are not statically determinate, typically lifts using four or more
'lift points' connected directly to the hook
inshore mooring
mooring 'operation' in relatively sheltered coastal waters, but not at a quayside
inspection and test plan
plan in which all test, witness and hold points for all aspects of a cable installation are listed
insurance warranty
clause in the insurance policy for a particular venture, requiring the 'assured' to seek approval
of a 'marine operation' by a specified independent survey house
International Association
of Classification Societies
listing of IACS members is given on the IACS web site
http://www.iacs.org.uk/explained/members.aspx
International Cable
Protection Committee
trade body representing and lobbying on behalf of subsea cable owners
For historical reasons membership is predominately comprised of telecom companies.
International Convention
for the Safety Of Life At
Sea SOLAS
international treaty concerning the safety of merchant and other ships and MOUs
International Maritime
Organization
The United Nations specialized agency with responsibility for the safety and security of
shipping and the prevention of marine pollution by ships
International Safety
Management
The 'ISM Code' provides an international standard for the safe management and operation of
ships and for pollution prevention
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Term
Definition
intersection point
point at which two straight sections or tangents to a 'pipeline' curve, or two slings, meet
when extended
ISM code
'International Safety Management Code', /92/, - the International Management Code for
the Safe Operation of Ships and for Pollution Prevention - Mandatory code referenced by
SOLAS, /139/, Chapter IX (Management for the safe operation of ships).
I-tube
vertical tube fitted to offshore structures to install 'product' between the seabed and the
structure topsides
jacket
sub-structure, positioned on the seabed, generally of tubular steel construction and secured
by piles, designed to support topsides facilities
jack-up
self-elevating MODU, MOU or similar, equipped with legs and jacking systems capable of
lifting the hull clear of the water
J-Lay
laying method where the pipe joints are raised to a nearly vertical angle in a tower mounted
on a 'pipelay' 'vessel' in a tower, assembled and lowered, curved through approximately 90°
(J shape) to lie horizontally on the sea-bed
J-tube
J shaped tube fitted to offshore structures to install 'product' between the seabed and the
structure topsides
jumper
short length of pipe used to connect subsea assets
kilometre point
position of on 'pipeline' route at a given distance from an agreed reference point, typically at
or near one end
lay-back
horizontal offset from the last pipe support on the lay 'vessel' to the 'touch down' point on the
seabed
lay chute
'lay chute' is generally a fixed plate shaped to protect the 'product' MBR
A 'lay chute' may also incorporate rollers.
lay sheave
'lay sheave' is a sheave with a radius greater than or equal to the 'product' MBR that may for
example be at the top of a lay tower to guide the product from the reel into the lay tower or
at the stern to guide the 'product' into the water
leg mating unit
designed and installed between the structure and the 'host structure' in order to absorb
vertical and horizontal installation motions and forces
The units are normally either installed on the 'host structure' legs to receive the structure, or
on the structure leg stubs, in order to interface with the 'host structure' legs. LMU’s can be
also installed on the removal 'vessel'.
lift point
connection between the 'rigging' and the structure to be lifted
May include 'padear', 'padeye' or 'trunnion'.
lifting appliance
machine or appliance used for the purpose of lifting objects, or in special modes, personnel
e.g. crane
lifting beam/lifting frame
structure designed to be connected to a 'lifting appliance' at a single point; the object being
lifted is connected to the bottom of the beam at two or more 'lift points'
The beam/frame shall resist the bending moments. It is not designed to carry compression
loads.
lifting equipment
general expression including 'lifting appliances', 'lifting gear' and 'loose gear' and other lifting
attachments; used separately or in combination
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Term
Definition
lifting factor
equivalent to a 'load factor' but applied only in the design of slings and 'grommets' used for
lifting operations
lifting gear
load carrying equipment designed, tested and used with a specific 'lifting appliance' but not
necessarily permanently attached to the 'lifting appliance'
The examples under 'loose gear' may be considered as lifting gear when used in this way.
'Lifting gear' that is not for a specific 'lifting appliance' shall be considered as 'loose gear'.
lightship weight
weight of the hull plus permanently installed items
limit state
state beyond which the product or component no longer satisfies the given acceptance
criteria
limit state 1
ASD/WSD design condition where the loading is gravity dominated; also used when the
exclusions of [5.9.7.4] apply
limit state 2
ASD/WSD design condition where the loading is dominated by environmental/storm loads,
e.g. at the 10 year or 50 year return period level or, for 'weather restricted operations',
(where the 'forecasted limiting criteria' are less than the 'design environmental condition' due
to the application of an 'alpha factor', see [2.6.9])
line pipe
coated or uncoated steel pipe sections, intended to be assembled into a 'pipeline'
linear cable engine
industry term commonly used to refer collectively to cable lay 'tensioners'
link beam/link span
connecting beam between the quay and the 'barge' or 'vessel'
It may provide a structural connection, or be intended solely to provide a smooth path for
'skidshoes' or 'trailers'/SPMTs.
load controlled condition
condition in which magnitude and direction of bending is governed by only the loads applied
to the 'product', e.g. in the 'touchdown' region
load factor
factor applied to a 'characteristic load' in a 'limit state' analysis
In 'rigging' design it is termed 'lifting factor' to differentiate between the values that shall
be applied in a limit state analysis to those that are applied in the design of slings and
'grommets' used for lifting operations.
load line
maximum depth to which a ship may be loaded in the prevailing circumstances in respect to
zones, areas and seasonal periods
A load line certificate is subject to regular 'surveys', and remains valid for 5 years unless
significant structural changes are made.
load transfer operation
operation to transfer the load (i.e. an object) from/to 'vessel(s)' without using cranes, i.e. by
using (de-)ballasting
Typical l'oad transfer operations' are 'load-out', lift-off, mating and 'float-over'.
load-in
transfer of an assembly, module, pipes or component from a 'barge' or 'vessel', e.g. by
horizontal movement or by lifting
load-out
transfer of an assembly, module, pipes or component onto a 'barge' or 'vessel', e.g. by
horizontal movement or by lifting
load-out support frame
structural frame that supports the structure during fabrication and 'load-out' and may support
the structure on a 'barge'/'vessel' above 'grillage'
load-out, floating
'load-out' onto a floating 'vessel'
load-out, grounded
'load-out' onto a grounded 'vessel'
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Term
Definition
load-out, lifted
'load-out' performed by crane
load-out, skidded
'load-out' where the structure is skidded, using a combination of 'skidways', 'skidshoes' or
runners, propelled by jacks or winches
load-out, trailer
'load-out' where the structure is wheeled onto the 'vessel' using 'trailers' or SPMTs.
location move
move of a MODU or similar, which, although not falling within the definition of a field '24hour move', may be expected to be completed with the unit essentially in 24-hour field move
configuration, without overstressing or otherwise endangering the unit, having due regard to
the length of the move, and to the area (including availability of 'shelter points') and season
loose gear
load carrying equipment used to attach the lifted object to the 'lifting appliance' that are not
part of the lifted object or used with a specific 'lifting appliance'
Includes devices and steel structures such as, but not limited to:
a)
grabs
b)
spreaders
c)
traverses
d)
lifting magnets
e)
attachment rings
f)
shackles
g)
swivels
h)
balls
i)
pins
j)
sheaves
k)
blocks (including hook-blocks)
l)
hooks
m)
load cells
n)
chains
o)
claws
p)
clamps
q)
pliers
r)
load fastening ropes (slings/strops)
s)
lifting straps, etc.
magnetic particle
inspection
'non-destructive testing' (NDT) process for detecting surface and slightly subsurface
discontinuities in ferroelectric materials such as iron
marine operation
see 'operation'
marine warranty survey
company
MWS company
company that is specified on an 'insurance warranty' and has been contracted to approve
specified 'operations' as a condition of the insurance
marine warranty survey
company surveyor (MWS
company surveyor)
surveyor employed to review the proposed 'procedures' and equipment and, when satisfied
that they and the weather forecasts are suitable, to issue a 'certificate of approval' for each
relevant 'operation'
The surveyor may also attend during such 'operations' to monitor that the 'procedures' are
followed or to agree any necessary changes.
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Term
Definition
matched pair of slings
the following definition apply for steel slings: 'matched pair of slings' is fabricated or designed
so that the difference in length does not exceed 0.5d for cable laid slings or 'grommets' and
1.0d for 'single laid slings' or 'grommets', where d is the nominal diameter of the sling or
'grommet'
See Section 3.2 of IMCA M 179, /81/ for cable laid details.
material factor γm
factor used on a material’s yield stress in a 'limit state' analysis and is also a factor used in
the design of slings and 'grommets' used for lifting operations
Note: For slings and grommets, the 'material factor' is a function of the age, certification and
material type.
maximum continuous
rating
manufacturer’s recommended 'maximum continuous rating' of the main engines
mechanical termination
sling eye termination formed by use of a ferrule that is mechanically swaged onto the rope
See ISO 2408 and ISO 7531, /104/ and /105/.
minimum bend radius
the minimum radius to which 'product' or sling/grommet can be bent without compromising
its integrity as specified by the manufacturer
This is the minimum radius to which a flexible, 'umbilical' or cable can be bent without
compromising its integrity.
minimum breaking load
minimum value of breaking load for a particular sling, 'grommet', wire or chain, shackle etc.
mobile mooring
'mooring system', generally retrievable, intended for deployment at a specific location for a
short-term duration, such as those for 'mobile offshore units'
mobile offshore unit
for the purposes of this document, the term may include mobile offshore drilling units
(MODUs), and non-drilling mobile units such as accommodation, construction, lifting or
production units including those used in the offshore renewables sector
monopile
tubular structure used as foundation for offshore wind turbine generator
moored vessel
within the scope of this document refers to any structure which is being moored
mooring system
consists of all the components in the 'mooring system' including shackles windlasses and
other jewellery and, in addition, 'rig'/'vessel' and shore attachments such as bollards
most probable maximum
extreme
value of the maximum of a variable with the highest probability of occurring over a period of
3 hours
Note: The most probable maximum is the value for which the probability density function of
the maxima of the variable has its peak. It is also called the mode or modus of the statistical
distribution. It typically occurs with the same frequency as the maximum wave height
associated with the design sea state.
multiple towage
operation of towing more than one 'tow' by a single 'tug', or more than 1 'tug' towing one
'tow', see [11.18]
nacelle
part of the wind turbine on top of the 'tower', where the hub, gearbox, generator and control
systems are located
non-destructive testing
ultrasonic scanning, 'magnetic particle inspection', eddy current inspection or radiographic
imaging or similar
Can also include visual inspection.
not to exceed weight
sometimes used in projects to define the maximum weight of a structure for an operation,
see [5.6.2.2]
object
item subject to an insurance warranty or at risk from an operation
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Term
Definition
off-hire survey
survey carried out at the time a 'vessel', 'barge', 'tug' or other equipment is taken off-hire, to
establish the condition, damages, equipment status and quantities of consumables, intended
to be compared with the 'on-hire survey' as a basis for establishing costs and liabilities
off-load
reverse of 'load-out'
offshore converter station offshore converter station transforms the collected energy from the 'offshore transformer
stations' (several wind parks) to direct current in order to send it to a land based converter
station
offshore pull
pulling of a 'pipeline' away from the shore using a lay 'vessel'
offshore transformer
station
'offshore transformer station' is transforming the collected energy from the wind turbines to a
higher voltage
on-hire survey
survey carried out at the time a 'vessel', 'barge', 'tug' or other equipment is taken onhire, to establish the condition, any pre-existing damages, equipment status and quantities
of consumables. It is intended to be compared with the 'off-hire survey' as a basis for
establishing costs and liabilities
It is not intended to confirm the suitability of the equipment to perform a particular
'operation'.
operation reference
period
'planned operation period', plus the contingency period, see [2.6.2] to [2.6.4]
operation, marine
operation
generic term covering, but not limited to, the following activities which are subject to the
hazards of the marine environment:
a)
'load-out'/'load-in'
b)
'voyage'
c)
lift/lowering (offshore/inshore)
d)
tow-out/tow-in
e)
'float-over'/float-off
f)
'jacket' launch/'jacket' upend
g)
'pipeline' installation
h)
construction afloat
operational limiting
criteria
maximum environmental criteria which the operation is design to be performed in, see
[2.6.9]
For a 'weather restricted operation'/'voyage' only these are the maximum environmental
conditions in which it may be possible to perform the operation, however to account for
uncertainty in the weather forecast they shall be reduced by the applicable alpha factor, see
[2.6.8].
padear
shaped 'lift point' around which a sling or grommet can be passed
The shaping of the bearing surface is normally elliptical to allow for flattening of the sling
or grommet under load and incorporates a flange/keeper within the design. Padears are
normally cast and can be either a single padear or a double padear. A retaining device is also
included to secure the sling/grommet. For a typical arrangement of a padear, see Figure 16-3
in [16.9.4.2]. See also trunnion.
padeye
'lift point' consisting essentially of a plate, reinforced by cheek plates if necessary, with a hole
through which a shackle may be connected
permanent mooring
'mooring system' normally used to moor floating structures deployed for long-term
'operations', such as those for a 'floating production system'
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Term
Definition
pigging
practice of passing a device known as a 'pig' through a 'pipeline' for maintenance (e.g. for
cleaning, gauging or inspection) without stopping the flow in the 'pipeline'
pipe carrier
'vessel' specifically designed or fitted out to transport 'line pipe'
pipe-in-pipe
single rigid pipe held within a 'carrier pipe' by spacers and/or solid filler
pipelay
operation of assembling and laying the 'pipeline' on the seabed, from start-up point to laydown point
pipeline
any marine pipeline system for the carriage of oil, gas, water or other process fluids. It may
be of rigid material or flexible layered construction
For the purposes of this document the term pipeline includes flowlines as defined in API RP
1111, /3/.
planned operation period
planned duration of the operation from the forecast before either the operation start or 'point
of no return', as appropriate, to a condition when the 'operations'/structures can safely
withstand a seasonal design storm (also termed 'safe to safe' duration) this excludes the
contingency period
platform
completed steel or concrete structure complete with topsides
point of no return
last point in time, or a geographical point along a route, at which an 'operation' could be
aborted and returned to a 'safe condition'
port (or point) of shelter
see 'shelter point'
port or place of refuge
location where a 'towage' or a 'vessel' seeks refuge, as decided by the master, due to events
which prevent the 'towage' or 'vessel' proceeding towards the planned destination
A safe haven where a 'towage' or 'voyage' may seek shelter for 'survey' and/or repairs, when
damage is known or suspected.
pre-loading
testing of soil foundations or anchors by loading to check that they can take subsequent loads
For 'jack-up' foundations it is often done be adding water ballast to pre-load tanks or (with
units with more than 3 legs) by pre-driving by removing load from other legs in turn.
procedure
documented method statement for carrying out an 'operation'
product
generic term used within this standard to reference 'pipelines' (rigid and flexible), risers,
'jumpers', 'umbilicals' and submarine cables
product storage
equipment
generic term for 'drum', 'reel', 'carousel' ('basket' and 'reel') and 'basket'
pull back method
'J-tube' pull-in operation where the pull-in winch is mounted on the installation 'vessel' and
the end of the pull-in wire connected to the cable runs from the 'vessel' to the 'J-tube' bottom
end up and over a sheave and back to the installation 'vessel' pull-in winch
quadrant
structure, usually with rollers, to limit the MBR as the cable travels over or though it
and changes direction, typically during loading or laying during second end J tube pull in
operations
quadratic transfer
function
refers to the matrix that defines second order mean wave loads on a 'vessel' in bi-chromatic
waves
When combined with a wave spectrum, the mean wave drift loads and low frequency loads
can be calculated.
quayside mooring
mooring that locates a 'vessel' alongside a quay (usually at a sheltered location)
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Term
Definition
recertification
process by which a piece of equipment is certified after it has been modified and/or repaired
The new/renewed parts normally require design verification and manufacturing surveillance
followed by FAT, both with surveyor attendance. See also revalidation.
recognized classification
society
member of IACS with recognized and relevant competence and experience in specialised
'vessels' or structures, and with established rules and procedures for classification/
certification of such 'vessels'/structures under consideration
reduction factor, γr
reduction factor used in the design of slings or 'grommets' representing the largest values of
redundancy check
check of the failure load case associated with the applicable extreme (survival) environment,
e.g. the one line broken case
reel
similar to a horizontal axis 'drum' but powered (by reel drive system, RDS) and usually larger
Used for storage, transport and onshore or offshore installation normally made of steel. See
'reel carousel' for when the 'barrel' axis is vertical.
reel lay (for rigid pipe)
laying method where the pipeline is pre-assembled into long strings or stalks and wound onto
a large reel with the pipe experiencing plastic deformation when wound on and off the reel
and straightened when reeled off
Typical lay angles of 20 to 90 degrees are achieved.
reeling
operation of wrapping the product in layers around a rotating 'barrel' with back tension
The 'product' can be guided back-and-forth along the 'barrel' to complete each layer.
γb and γs
The operation normally uses either a 'drum' or 'reel'.
registry
indicates who may be entitled to the privileges of the national flag, gives evidence of title
of ownership of the ship as property and is required by the need of countries to be able to
enforce their laws and exercise jurisdiction over their ships
The certificate of registry remains valid indefinitely unless name, flag or ownership changes.
remotely (controlled)
operated vehicle
device deployed subsea on a 'tether' or umbilical, typically equipped with a subsurface
acoustic navigation system and thrusters, to control its location and attitude, and a lighting
and video system
Additional devices such as manipulators, acoustic scanning for 'touch-down' monitoring, etc.,
may also be provided.
responsible person
in accordance with ILO C 152, /136/, 'a person appointed by the employer, the master of the
ship or the owner of the gear, as the case may be, to be responsible for the performance of
a specific duty or duties and who has sufficient knowledge and experience and the requisite
authority for the proper performance of the duty or duties'
response amplitude
operator
defines the vessel’s (first order) response in regular waves and allows calculation of 'vessel'
wave frequency (first order) motion in a given sea state using spectral analysis techniques
revalidation
process by which a piece of equipment is confirmed as acceptable for continued service
considering its current condition e.g. thorough examination of lifting equipment. See also
recertification.
rig
general reference term often used to describe a 'jack-up' or 'semi-submersible' (mobile
offshore drilling unit or MODU) see MOU) e.g. ‘rig move procedures’
rigging
slings, shackles and other devices including spreaders used to connect the structure to be
lifted to the crane
rigging weight
total weight of 'rigging', including slings, shackles and spreaders, including contingency
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Term
Definition
righting arm
righting moment divided by the displacement
risk assessment
method of hazard identification where all factors relating to a particular 'operation' are
considered
rope
assembly of 'strands' wrapped around a core
When a 'rope' is used for 'cable-laid sling' or 'cable-laid grommet' it is referred to as a 'unit
rope' (as per IMCA M 179, /81/).
rotor
configuration consisting of the complete set of blades, connected to the hub
route planning list
tabularised list of the coordinates defining the route along which a submarine cable shall be
installed and the planned installation slack
A post installation RPL will record the as-built cable route coordinates, installed slack and
burial depths.
routine lift
'everyday' lift, without detailed design, planning or documentation, such as general cargo
lifting operations or lifting portable units on/off a supply vessel, see [16.1.4]
safe condition
condition where the object is considered to be exposed to a normal level of risk of damage or
loss
See guidance note to [2.5.1.2].
safe working load
see [1.4.9]
safety management
system
structured and documented system enabling company personnel to implement the company
safety environmental protection policy
sand jacks
compartment filled with sand that is incorporated into the LMU to allow the final controlled
lowering of the structure onto the 'host structure'
scour pit
result of scour around a pile, leg etc.
See 'dynamics of scour pits and scour protection', /119/.
sea room
distance that a disabled 'vessel' or 'tow' in bad weather can drift before grounding, see
[11.14.1.5]
seafastenings
means of restraining movement of the loaded structure on or within the 'barge' or 'vessel'
self-propelled modular
transporter
'trailer' system having its own integral propulsion, steering, jacking, control and power
systems
semi-submersible
floating structure normally consisting of a deck structure with a number of widely spaced,
large cross-section, supporting columns connected to submerged pontoons
serviceability limit state
design condition where the structure is required to fulfil its primary operational function
setback
space on the derrick floor where stands of drill pipe or tubing are 'setback' and racked in the
derrick
It can also mean the amount of drill pipe etc. in this area.
shelter point (or port
of shelter, or point of
shelter)
area or safe haven where a 'towage' or 'vessel' may seek shelter, in the event of actual or
forecast weather outside the design limits for the 'voyage' concerned
A planned holding point for a 'staged voyage'.
shore pull
pulling of a cable or 'pipeline' to the shore from a lay 'barge'/'vessel'
simultaneous operations
'operations' usually involving various parties and 'vessels' requiring co-ordination and
definitions of responsibilities
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Term
Definition
single laid sling
normally made up of 6 'strands' laid up over a core, as shown in ISO 2408 and ISO 7531,
(/104/ and /105/), with terminations each end
single tow
operation of towing a single 'tow' with a single 'tug'
site move
operation to move a structure or partially assembled structure in the yard from one location
to another
The site move may precede a 'load-out' if carried out as a separate 'operation' or may form
part of a 'load-out'. The site move may be subject to approval if so desired.
skew load factor
factor to account for additional loading caused by 'rigging' fabrication tolerances, fabrication
tolerances of the lifted structure and other uncertainties with respect to asymmetry and
associated force distribution in the 'rigging' arrangement
skidshoe
bearing pad attached to the structure which engages in the 'skidway' and carries a share of
the vertical load
skidway
lower continuous rails, either on the quay or on the 'vessel', on which the structure is loaded
out, via the 'skidshoes'
slack management
generalized term used by the submarine cable installation industry to refer to the control of
cable pay-out out against a pre-defined installation plan
slamming loads
transient loads on the structure due to wave impact when lifting through the splash zone
S–Lay
laying method where the pipe is assembled horizontally, fed out of the stern or bow of the
'barge' or 'vessel', typically over a stinger
Can also be without stinger at certain depths or at the end of the 'shore pull' before the water
depth increases to a depth where stinger becomes necessary, and then makes a double curve
(shallow S shape) to lie horizontally on the sea-bed.
sling design load
maximum calculated dynamic axial load in a lifting sling, including all relevant 'load factors'
sling eye
loop at each end of a sling, either formed by a 'splice' or 'mechanical termination'
specified minimum yield
stress
minimum yield stress specified in standard or specification used for purchasing the material
splice
that length of sling where the 'rope' (or 'unit rope' for 'cable-laid sling') is connected back into
itself by tucking the tails of the strands (or 'unit ropes') back through the main body of the
'rope' (or 'unit ropes'), after forming the 'sling eye'
spool (piece)
short length of rigid pipe used to connect pipe (e.g. 'pipeline', riser) and/or subsea asset
spooling
similar to 'reeling'
See 'winding' and 'coiling' for where the 'product' is wrapped without back tension.
spreader bar (also known
as spreader beam)/
spreader frame
spreader bar or frame is a structure designed to resist the compression forces induced by
angled slings, by altering the line of action of the force on a lift point into a vertical plane
The bar (beam)/frame shall also resist bending moments due to geometry and tolerances.
spud
large metal post which penetrates the seabed under its own weight and is used to prevent
lateral movement of a 'barge'
A dredge 'barge' will typically have two spuds in guides near its stern.
squeeze load
radial force applied to the 'product' by a 'tensioner', clamp or similar
staged voyage
weather restricted 'voyage' in which there is a commitment to seek shelter (or 'jack-up' at a
stand-by location) on receipt of a weather forecast in excess of the operational criteria, see
[11.14.4.1]
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Term
Definition
static hook load
weight plus the 'rigging weight' (see [16.3.2])
This load is suspended by a crane hook during lifting operations.
strand
assembly of wires wound together to create a 'strand'
Wire rope consists of multiple 'strands' wound together. For example: 6x36 wire rope
construction indicates that the wire rope consists of 6 strands, each having 36 wires.
structure
object to be transported, lifted or installed, or a sub-assembly, component or module
submerged weight
weight of the 'structure' minus the weight of displaced water
suitability survey
'survey' intended to assess the suitability of a 'tug', 'barge', 'vessel' or other equipment to
perform its intended purpose
Different and distinct from an 'on-hire survey'.
surge
'barge' or 'vessel' motion in the longitudinal direction OR
A change in water level caused by meteorological conditions.
survey
attendance and inspection by a 'MWS company surveyor'
Other surveys which may be required for a 'marine operation', including suitability,
dimensional, structural, navigational and class surveys.
surveyor
'MWS company' representative carrying out a ‘survey’ or an employee of a contractor
or classification society performing, for instance, a suitability, dimensional, structural,
navigational or class 'survey'
sway
'vessel' motion in the transverse direction
system pressure test
pressure test at a pressure normally at a 1.25 to 1.5 times the 'pipeline' design pressure
(for rigid 'pipelines'), which is made after installation operations are substantially or wholly
completed, to provide proof of pressure and strength integrity of the 'pipeline' and 'spools'
tandem tow
operation of towing two or more 'tows' in series with one tow wire from a single 'tug', the
second and subsequent 'tows' being connected to the stern of the 'tow' ahead
tangent point
point where the bend of a 'pipeline' begins or ends
(cable) tank
static non-rotating circular storage area (on vessel, on land), which is loaded by 'coiling'
tensioner
equipment to keep and control tension in the 'product' during installation operation
termination factor γs
partial safety factor that accounts for the reduction in strength caused by a 'splice' or
'mechanical termination'
tether
mooring line used for pulling and mooring the installation /removal 'vessel' into the required
position
It may also be the 'umbilical' to an ROV or part of a TLP’s 'mooring system'.
tidal range
where practicable, the tidal range referred to in this document is the predicted tidal range
corrected by location-specific tide readings obtained for a period of not less than one lunar
cycle before the 'operation'
tonnage
measurement of a 'vessel' in terms of the displacement of the volume of water in which it
floats, or alternatively, a measurement of the volume of the 'cargo' carrying spaces on the
'vessel'
Tonnage measurements are principally used for freight and other revenue based calculations.
Tonnage certificates remain valid indefinitely unless significant structural changes are made.
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Term
Definition
tonnes
metric tonnes of 1,000 kg (approximately 2,204.6 lbs) are used throughout this document
The necessary conversions shall be made for equipment rated in long tons (2,240 lbs,
approximately 1,016 kg) or short tons (2,000 lbs, approximately 907 kg).
touchdown
seabed location at which a submarine 'pipeline', 'umbilical' or cable touches down on the
seabed during installation, or a mooring line during 'operation'
tow
item being towed
This can be a 'barge' or 'vessel' (laden or un-laden) or an item floating on its own buoyancy.
towage
operation of towing a non-propelled 'barge' or 'vesse'l (whether laden or not,) or other
floating object (wet 'tow') by 'tug(s)'
towed bundle
'pipeline' system comprising one or more 'pipelines', tubes or cables contained within a
'carrier pipe', and fitted with towing and trailing heads
The 'bundle' is usually assembled on land and launched. The 'bundle' may be towed offbottom, on surface, or at an intermediate controlled depth.
tower
tubular element from the top of the flange on the foundation to the bottom of the flange
below the 'nacelle', generally built up of several sections
towing arrangements
hardware from the towing winch to the towing connections plus the bridle recovery and
emergency towing equipment
(They do not normally include the towing 'procedures'.)
towline connection
strength
'ultimate load capacity' of towline connections, including connections to 'vessel', bridle and
bridle apex
towline pull requiredP
towline pull computed to hold the 'tow', or make a certain speed against a defined weather
condition
trailer
system of steerable wheels, connected to a central spine beam by hydraulic suspension which
can be raised or lowered
Trailer modules can be connected together and controlled as a single unit. Trailers generally
have no integral propulsion system, and are propelled by tractors or winches. See also SPMT.
transition piece
tubular structure on top of a 'monopile' to provide support for the 'tower'
transport
'operation' of transporting a 'cargo' on a powered 'vessel'
trunnion
'lift point' consisting of a horizontal tubular stub with a flange/keeper plate at the end, around
which a 'sling' or 'grommet' is passed
A retaining device is also included to secure the sling/grommet. The stub is normally welded
to the supporting member.
An upending trunnion can be used to rotate a structure from horizontal to vertical, or vice
versa, and the trunnion forms a bearing round which the 'sling', 'grommet' or another
structure will rotate. See also padear.
tug
'vessel' performing a 'towage' (including tug supply and anchor handling towing 'vessels')
'Approval' by the 'MWS company' of the tug will normally include consideration of the general
design, classification, condition, towing equipment, bunkers and other consumable supplies,
emergency communication and salvage equipment, and manning.
tug efficiency (Te or Teff)
effective bollard pull produced in the weather considered divided by the 'certified' continuous
static 'bollard pull'
tug management
positioning system
system installed on the AHV and the anchoring 'vessel' to allow the accurate placing of the
'tug' and anchors
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Term
Definition
turntable
active drive system used in conjunction with a mobile or integrated 'carousel' (both basket
and reel)
ultimate limit state
'limit state' related to the maximum load carrying capacity
Also see 'limit state 1' and 'limit state 2'. (ULS).
ultimate load capacity
ultimate load capacity of a wire 'rope', chain or shackle or similar is the 'certified' 'minimum
breaking load'
The 'load factors' allow for good quality 'splices' in wire rope.
Ultimate load capacity of a 'padeye', clench plate, triangular plate or similar structure, is
defined as the load, which will cause general failure of the structure or its connection into the
'barge' or other structure.
ultrasonic testing
detection of flaws or measurement of thickness by the use of ultrasonic pulse-waves through
steel or some other materials
umbilical
typically a combination of cables and flexible pipes used to provide energy and/or chemicals
and remote control for equipment (e.g. subsea), or to provide communications and life
support for a diver
under-keel clearance
clearance below the keel or base of a vessel or structure, after allowances for motions, and
the seabed (or the 'host structure' during mating operations)
unit rope
'rope' from which a 'cable-laid sling' or 'cable-laid grommet' may be constructed, made from
either 6 or 8 'strands' around a steel core, as indicated in ISO 2408 and ISO 7531, (/104/
and /105/) and IMCA M 179, /81/.
variable load
weight added to the 'lightship weight' to obtain the total weight for a particular 'towage' or
operation, including 'cargo', liquids and temporary equipment
verification
confirmation, through the provision of objective evidence, that specified requirements have
been fulfilled
Verification is 'project specific' and therefore different to 'certification'.
vessel
marine craft designed for the purpose of 'transporting' by sea or construction activities
offshore
This can include ships and 'barges'.
voyage
for the purposes of this standard, voyage covers both 'towages' and 'transport' from one
place to another
watertight
watertight opening is an opening fitted with a closure designated by class as watertight, and
maintained as such, or is fully blanked off so that no leakage can occur when fully submerged
wear factor, γw
factor used in the design of slings and 'grommets' used for lifting operations to account for
physical condition of the sling or 'grommet'
weather restricted
operation
operation for which (any of) the applied characteristic environmental conditions are less than
the characteristic 'environmental conditions' calculated based on the statistical extremes for
the area and season
See also [2.6.7].
weather restricted
voyage
'voyage' for which the strength or stability will not meet the weather unrestricted
environmental criteria
It can either be or staged (see [11.14.4.1]) or weather-routed (see [11.14.4.2]) depending
on the 'sea room' and 'shelter point' availability.
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Term
weather routed voyage
Definition
'weather restricted voyage' in which a weather forecasting organisation advises the relevant
captain on the best route to avoid weather exceeding the 'operational limiting criteria', see
[11.14.4.2]
Weather routeing may also be used for non-weather restricted 'voyages' to reduce fuel costs
or 'voyage' time.
weather unrestricted
operation
operation for which (all of) the applied characteristic environmental conditions are calculated
based on the statistical extremes for the area and season
See also [2.6.6].
weather unrestricted
towage
any 'towage' which does not fall within the definition of a weather restricted 'towage', or any
'towage' of a MODU or similar which does not fall within the definition of a '24-hour move' or
'location move'
weather unrestricted
voyage
any 'voyage' which does not fall within the definition of a 'weather restricted voyage'
weather window
period that the forecasted environmental conditions are less than or equal to OPWF (the
'forecast operation criteria')
weathertight
weathertight opening is an opening closed so that it is able to resist any significant leakage
from one direction only, when temporarily immersed in green water or fully submerged
web lashing assembly
lashing device which comprises end connections that cannot disconnect if the tension is lost,
webbing and a tensioning device, usually certified as a system/assembly.
These are sometimes known as web lashings, ratchet straps or cargo straps.
weighing contingency
factor
factor applied to the weighed weight of an object to account for uncertainties in the weighing
equipment
weight contingency factor
factor applied to the weight of an object, when an object is not to be weighed, to account for
uncertainties related to the design and fabrication of the object
wet towage
operation of transporting a floating object by towing it with a 'tug'
wind heeling arm
wind heeling moment divided by the displacement
winding
operation where the 'product' is placed into a rotating 'carousel' in layers of concentric rings
without back tension
See 'reeling' and 'spooling' for where the product is wrapped with back tension.
working load limit
see [1.4.9]. The maximum load which a piece of equipment is authorized to sustain in
general service when the 'rigging' and connection arrangements are in accordance with the
design
1.6.3 Symbols
= factor for ratio of mean to specified bolt pretension
Du
= modulus of Elasticity
E
FSD = sling or grommet design load
= factor for fillers in bolted connections
hf
Hmax = height of maximum wave
= hole clearance factor
ks
= number of slip planes for bolted connections
Ns
OPLIM = operational limiting criteria
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OPWF = forecasted operational criteria
= minimum fastener pretension for bolted connections
Tb
= contingency period
TC
Te or = tug efficiency'
Teff
= peak period
Tp
TPOP = planned operational period (without contingencies, TC)
TR
= operation reference period (including contingencies, TC)
Tsafe = time to safely cease the operation
TWF = time between weather forecasts
= zero-up crossing period for waves
Tz
Wcc = allowable static hook load from the crane curves
Wld = lower bound design weight
Wud = upper bound design weight
= design resistance
Rd
= nominal slip resistance
Rn
= permissible usage factor
η
= 'bending factor '
γb
= 'consequence factor'
γc
= 'load factor'
γf
= 'lifting factor'
γh
= 'material factor'
γm
= 'reduction factor'
γr
= 'termination factor'
γs
= design factor (i.e. combined: lifting, consequence, reduction, wear, and material and twist)
γsf
= 'wear factor'
γw
γweight= 'weight contingency factor' (unweighed objects only)
= hole factor for slip resistant bolts
Ф
= design dynamic factor for cranes (see DNV-ST-0378, /129/)
ψ
1.6.4 Abbreviations
Abbreviations within single quotation marks in Table 1-5 are defined in Table 1-4.
Table 1-5 Abbreviations
Abbreviation
Description
A&R
abandonment and retrieval
ABS
American Bureau of Shipping
AC
alternating current
ADL
absolute minimum deployable length (of towline)
AHC
active heave compensation
AHV
anchor handling vessel
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Abbreviation
Description
AISC
American Institute of Steel Construction
ALARP
as low as reasonably practicable
ALS
'accidental limit state'
AMS
anchor management system
ANSI
American National Standards Institute
API
American Petroleum Institute
ARPA
automatic radar plotting aid
ASD
allowable stress design (effectively the same as WSD)
ASOG
activity specific operations guidelines
ASPPR
Arctic Shipping Pollution Prevention Regulations
ATA
automatic thruster assist
AUT
automatic ultrasonic testing
AWS
American Welding Society
AWTI
above water tie-in
BAS
'burial assessment survey '
BF
Beaufort Force, see DNV-ST-0111 Table 2-1, /22/
BBL
bridle breaking load
BHP
brake horse power
BP
'bollard pull'
BPI
'burial protection index'
BS
British Standard
BSR
'bend strain reliever'
BV
Bureau Veritas
BWM
ballast water management
CAM
critical activity mode of operation
CAMO
now being replaced by CAM
CASPRR
Canadian Arctic Shipping Pollution Prevention Regulations
CCTV
closed circuit television
CDT
'controlled depth tow'
CFD
computational fluid dynamics
CFM
controlled failure mode
CMID
Common Marine Inspection Document
CoB
centre of buoyancy
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Abbreviation
Description
CoG
centre of gravity
COMOPS
combined operations
COSHH
control of substances hazardous to health
CRBL
calculated rope breaking load
CSA
'cross sectional area'
CSBL
calculated sling breaking load
DAF
'dynamic amplification factor'
DFF
design fatigue factor
DGPS
differential global positioning system
DHL
dynamic hook load
DMA
'dead man anchor'
DOC
depth of cover
DOL
depth of lowering
DP
dynamic positioning or dynamically positioned
DR
design review
DSC
digital selective calling
DSU
'deck support unit'
DTL
deployable towline length
ECA
engineering criticality assessment
EER
escape, evacuation and rescue
EIPS
extra improved plow steel
EP HAZOP
early procedure hazard and operability study
EPIRB
emergency position indicating radio beacon
ESD
emergency shut down
FAT
factory acceptance tests
FBE
fusion bonded epoxy
FEA
finite element analysis
FEED
front end engineering design
FGSO
floating gas storage and offloading vessel
FJC
'field joint coating '
FLNG
floating liquefied natural gas
FLS
'fatigue limit state '
FMEA
failure modes and effects
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Abbreviation
Description
FMECA
failure modes, effects and criticality analysis
FoS
factor of safety
FPS
'floating production system'
FPSO
floating production, storage and offloading vessel
FPU or FPV
floating production unit or floating production vessel
FRSU
floating storage re-gasification unit
FSO
floating storage and offloading vessel
FSU
floating storage unit (including FPSO, FSO, FLNG facility, FRSU etc.)
GA
general arrangement
GBS
gravity base structure (foundation)
GM
initial metacentric height
GMDSS
global maritime distress and safety system
G-OMO
guidelines for offshore marine operations
GPS
'global positioning system'
GZ
'righting arm'
HAT
highest astronomical tide
HAZMAT
hazardous materials
HAZID
hazard identification
HAZOP
hazards and operability study
HDD
horizontal directional drilling
HDPE
high-density polyethylene
HIRA
hazard identification and risk assessment
HISC
hydrogen induced stress cracking
HMPE
hHigh-modulus polyethylene
HPR
'hydro-acoustic positioning reference'
HPU
hydraulic power unit
HSE
health, safety and environment or health and safety executive (in context of UK HSE)
HTV
'heavy transport vessel'
(Not to be confused with HLV (heavy lift vessel) which has heavy lifting appliance(s).)
HVDC
high voltage direct current
IACS
'International Association of Classification Societies'
IAPP
International Air Pollution Prevention
ICPC
'International Cable Protection Committee'
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Abbreviation
Description
IEC
International Electrotechnical Commission
ILO
International Labor Organization
ILT
internal lifting tool
IMCA
International Marine Contractors Association
IMDG Code
International Maritime Dangerous Goods Code
IMO
'International Maritime Organization'
IOPP Certificate
International Oil Pollution Prevention Certificate (see also MARPOL)
ISM
'International Safety Management'
ISO
International Standards Organisation
ISPS
International Ship and Port Facility Security
ITP
inspection test plan
JIP
joint industry project
JONSWAP
Joint North Sea Wave Project
JSA
job safety analysis
LARS
launch and recovery system
LAT
lowest astronomical tide
LBL
ong baseline array
LCE
'linear cable engine'
LCG
longitudinal centre of fravity
LMU
'leg mating unit'
LOA
length over all
LR
Lloyd’s Register
LRFD
load and resistance factor design
LS1
'limit state 1'
LS2
'limit state 2'
LSF
'load-out support frame'
MARPOL
International Convention for the Prevention of Pollution from Ships 1973/78, as amended
MBL
'minimum breaking load'
MBR
'minimum bend radius'
MCR
'maximum continuous rating'
MDR
master document register
MOC (procedure)
management of change (procedure)
MODU
mobile offshore drilling unit
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Abbreviation
Description
MOU
'mobile offshore unit'
MPI
'magnetic particle inspection'
MPME
'most probable maximum extreme'
MRU
motion reference unit
MSL
mean sea level
MTS
Marine Technology Society
MWS
'marine warranty survey'
n/a
not applicable
ND
Noble Denton
NDT
'non-destructive testing'
NMA
Norwegian Maritime Authority
NTE (weight)
'not to exceed (weight)'
OCIMF
Oil Companies International Marine Forum
OIM
offshore installation manager
OSS
out of straightness survey
OVID
offshore vessel inspection database
OWF
offshore wind farm
PHC
passive heave compensation
PIC
person in charge
PLEM
pipeline end manifold
PLET
pipeline end termination
PMS
positioning monitoring system
PNR
'point of no return'
PRT
pipeline recovery tooling/tool
QC
quality control
QTF
quadratic transfer function
RAO
'response amplitude operator'
RCS
'recognized classification society'
RCSC
Research Council on Structural Connections
RIB
rigid inflatable boat
ROV
'remotely (controlled) operated vehicle'
RPL
route planning list
RTBL
required towline breaking load
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Abbreviation
Description
SAR
synthetic aperture radar
SART
search and rescue radar transponder
SCF
stress concentration factor
SCR
steel catenary riser
SE
shore end
SF
safety factor
SHL
'static hook load'
SIMOPS
'simultaneous operations'
SJA
safe job analysis
SKL
'skew load factor'
SLS
'serviceability limit state'
SMC
safety management certificate
SMO
safest mode of operation
SMS
'safety management system'
SMYS
'specified minimum yield stress'
SOLAS
International Convention for the Safety Of Life At Sea, /139/
SOPEP
shipboard oil pollution emergency plan
SPMT
'self-propelled modular transporter'
SPS
special purpose ship
SQRA
semi-quantitative risk analysis
SSCV
semi-submersible crane vessel
STF
storm factor
SWL
'safe working load'
TA
thruster assist
TAM
task appropriate mode
TBL
towline breaking load
TCG
transverse centre of gravity
TDP
touchdown point
TLP
tension leg platform
TMPS
'tug management positioning system'
TMS
tether management system
TPR
'towline pull required'
UHF
ultra high frequency
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Abbreviation
Description
UK
United Kingdom
UKC
'under-keel clearance'
UL
Underwriter Laboratories Inc.
ULC
'ultimate load capacity'
ULS
'ultimate limit state'
USBL
ultra short baseline array
UT
'ultrasonic testing'
UTM
universal transverse mercator
UV
ultra violet
UXO
unexploded ordnance
VCG
vertical centre of gravity
VHF
very high frequency
VIV
vortex induced vibration
VLA
vertical load anchors
VMO
VERITAS Marine Operations
WD
water depth
WF
weather forecast
WHA
'wind heeling arm'
WLL
'working load limit'
WMO
World Meteorological Organisation
WPS
welding procedure specification
Wrt
with respect to
WSD
working stress design (effectively the same as ASD)
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SECTION 2 PLANNING AND EXECUTION
2.1 Introduction
This section includes the general requirements for planning, organization, execution and documentation of
marine operations.
2.2 General project requirements
2.2.1 Project organisation
2.2.1.1 An appropriate project organisation chart shall be set up, illustrating how the marine operations
integrate with the rest of the project.
2.2.1.2 All project interfaces between (key) contractors shall be clearly defined.
2.2.1.3 For organisation during the marine operation see [2.8].
2.2.2 Health, safety and environment
Personnel safety shall be duly considered throughout the marine operation(s). This subject shall be managed
by the customer or their nominated contractor in accordance with local jurisdiction, as well as appropriate
guidelines and specifications regarding health, safety and the environment (HSE).
Guidance note:
By following the recommendations in this standard it is assumed that the safety of personnel and an acceptable working
environment are ensured in general during the operations. However, specific personnel safety issues are not covered.
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2.2.3 Jurisdiction
2.2.3.1 Marine operations are subject to national and international regulations and standards on personnel
safety and protection of the environment. It should also be noted that a marine operation can involve more
than one nation’s area of jurisdiction, and that for barges and vessels the jurisdiction of the flag state will
apply. Documented relevant regulatory approval is a prerequisite to MWS approval.
2.2.3.2 If a part of the marine operations is to be carried out near other facilities or their surroundings any
safety zone(s) defined by the owner shall be duly considered.
2.2.4 Quality assurance and administrative procedures
A quality management system in accordance with the current version of ISO 9001, /106/, or equivalent
should be adopted by the designer(s) and installation contractor(s) and be in place.
2.2.5 Technical procedures
2.2.5.1 Technical procedures shall be in place to control engineering related to the marine activities.
2.2.5.2 The technical procedures shall consider the planning and design process. For this process it is
recommended that the following sequence is adopted:
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a)
b)
c)
d)
e)
f)
Identify relevant and applicable regulations, rules, company specifications, codes and standards, both
statutory and self-elected.
Identify physical limitations. This may involve pre-surveys of structures, local conditions and soil
parameters.
Plan the overall operation i.e. evaluate operational concepts, available equipment, limitations, economic
consequences, etc.
Describe/define unambiguously with adequate detailing the design basis and main assumptions, see
[2.2.7].
Carry out engineering and design analyses.
Develop operation procedures.
2.2.5.3 The indicated sequence is illustrated in Figure 2-1 where the engineering and design verification and
the operational procedures should be considered as an iterative process.
Figure 2-1 Planning and design sequence
2.2.5.4 A review of all operations, from start to finish of the marine operation, shall be performed in order to
identify the analysis scope.
Guidance note:
In the initial phase of design, simplified checks of critical operations are typically performed, while in the final design, detailed
analyses are typically required.
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2.2.5.5 Analyses shall be performed to ensure that relevant limit state criteria are satisfied for
environmental conditions up to and including the operational limiting criteria, OPLIM. The analyses shall form
the basis for establishing applicable operational parameters (e.g. lay angle, lay-back) to ensure that relevant
limit state criteria are not exceeded for product, vessel, equipment and installation aids.
Guidance note:
Where applicable, the hydrodynamic modelling of product and components could be done in accordance with DNV-RP-C205 /46/.
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2.2.5.6 The procedures shall include sufficient information to ensure agreement and uniformity on all
relevant matters such as:
a)
b)
c)
d)
e)
f)
g)
h)
International and national standards and legislation
Certifying authority/regulatory body standards
Marine warranty survey company standards or guidelines
Project criteria
Design basis
Metocean criteria
Calculation procedures
Change management.
Guidance note:
It will also normally be applicable to include requirements to assure compliance, where relevant, with any peer-reviewed best
industry practice, e.g. IMCA, MTS, G-OMO, NORSOK.
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2.2.6 New technology
2.2.6.1 Design and planning of marine operations shall as far as feasible be based on well proven principles,
techniques, systems and equipment.
2.2.6.2 If new technology or existing technology in a new environment is used, this technology shall be
documented through an acceptable qualification process, e.g. in DNV-RP-A203, /45/.
2.2.7 Design basis and design brief
2.2.7.1 A design basis and/or a design brief shall be developed and provided for early acceptance in order to
obtain a common basis and understanding for all parties involved during design, engineering and verification.
2.2.7.2 The design basis should describe the basic input parameters, main assumptions, characteristic
environmental conditions, characteristic loads/load effects, load combinations and load cases, including those
for the proposed marine operations.
2.2.7.3 The Design Brief(s) should describe the planned verification activities, analysis methods, software
tools, input specifications, acceptance criteria, etc.
2.3 Technical documentation
2.3.1 General
2.3.1.1 Fulfilment of all the requirements in this standard applicable for the considered marine operation(s)
shall be properly documented. Guidance on required documentation is given throughout this standard.
However, it shall always be thoroughly evaluated if additional documentation is required.
2.3.1.2 A document plan describing document hierarchy, issuance schedule and scope for each document
should be provided for major marine operations/projects.
Guidance note:
Normally this will be in the form of MDR(s) that are distributed for review/mark-up by involved parties including the MWS
company.
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2.3.1.3 A system/procedure ensuring that all required documentation is produced in due time and
distributed according to plan, should be implemented.
2.3.1.4 It shall be ensured that all the documentation pertaining to a specific marine operation has been
accepted by authorities, company, other contractors and MWS, as relevant, before any operation starts.
2.3.2 Documentation required
2.3.2.1 The design basis shall be clearly documented, see [2.2.7].
2.3.2.2 Environmental conditions for the actual area shall be documented by reliable statistical data, see
Sec.3.
2.3.2.3 The acceptability of the following shall be documented: the object, all equipment, temporary or
permanent structures, vessels, etc. involved in the operation. Recognized certificates (e.g. classification
documents) are normally acceptable as documentation if the basis for certification is clearly stated and
complies with the philosophy and intentions of this standard.
Guidance note 1:
By basis for certification it is meant acceptance standard, basic assumptions, design loads, including dynamics, limitations, etc. For
items without certificates see [2.3.2.4].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Note that all elements of the marine operation should be properly documented. This also includes onshore facilities such as quays,
bollards and foundations.
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2.3.2.4 Design calculations/analysis shall be documented by design reports and drawings.
2.3.2.5 The condition of all involved equipment, structures and vessels shall be documented as acceptable
by means of certificates, (load) tests, surveys and/or NDT reports as relevant and in agreement with MWS.
Guidance note:
For vessels, such documentation may be recent inspections to acceptable industry standards, e.g. OVID or CMID, provided all
relevant non-conformances are closed out. See also [2.11.2].
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2.3.2.6 Operational aspects shall be documented in form of operation manuals and records.
2.3.2.7 Relevant qualifications of key personnel shall be documented.
2.3.2.8 Required third party verification, e.g. to fulfil the warranty clause, shall be properly documented. See
also [2.4.4].
2.3.3 Documentation quality and schedule
2.3.3.1 An integrated document numbering system for the entire project is recommended, including
documents produced by customer, contractors, sub-contractors and vendors.
2.3.3.2 Documents relating to marine operations should be grouped into levels according to their status, for
example:
a)
b)
Criteria and design basis documents.
Procedures and operations manuals.
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c)
Supporting documents, including engineering calculations, systems operating manuals and equipment
specifications and certificates.
2.3.3.3 The documentation shall demonstrate that philosophies, principles and requirements of this standard
are complied with. This documentation shall be provided to the MWS company.
Guidance note:
The operation and document type dictates the level of review by the MWS company. The following terms have been used as an
indication of the level of detail:
—
Documented – An in-depth document that is subjected to a detailed review by the MWS company e.g. analysis reports,
—
Submitted – A document that is provided to the MWS company in advance where the checking is limited e.g. a certificate to
procedures and operation manuals
confirm that piece of equipment has the required capacity. In some cases this could be immediately prior to the operation but
this may lead to delays if the documents are incorrect and/or insufficient.
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2.3.3.4 Documentation for marine operations shall be self-contained, or clearly refer to other relevant
documents.
2.3.3.5 The quality and details of the documentation shall be such that it allows for independent reviews of
plans, procedures and calculations, for all parts of the operation.
2.3.3.6 All significant updates shall be clearly identified in revised documents.
2.3.3.7 The document schedule shall allow for the required (agreed) time for independent reviews.
Guidance note:
The time available for review should be at least 10 working days, and more for complex documents.
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2.3.4 Input documentation
Applicable input documentation, such as:
— documents covering the aspects described in [2.2.5]
— relevant parts of contractual documents
— concept descriptions
— basic/FEED engineering results
— environmental studies including weather window analysis for weather restricted operation.
should be identified before any detailed design work is performed.
2.3.5 Output documentation
Documentation shall be prepared to prove that all relevant design and operational requirements are fulfilled.
Typical output documentation is:
a)
b)
c)
d)
Planning documents including design briefs and basis, schedules, concept evaluations, general
arrangement drawings and specifications.
Design documentation including motion analysis, load analysis, global strength analysis, local design
strength calculations, stability and ballast calculations and structural drawings.
Operational manuals/procedures, see [2.3.7] and [2.9.5].
Operational records, see [2.3.8].
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2.3.6 Availability of technical documentation
2.3.6.1 All relevant documentation shall be available and accessible on site or on board during execution of
the operation. This shall include the marine operations manual and any documents referenced therein.
2.3.6.2 The top level procedure document should define the On-Scene Commander in the event of an
emergency situation and the interfaces between the various parties involved.
2.3.6.3 Vessel and equipment certificates and NDT reports shall be submitted. See [B.1] and [B.2] for the
information that is typically required.
Guidance note:
In order to avoid possible delays due to unacceptable or incomplete documentation, it is recommended that such documentation is
submitted for review as soon as possible.
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2.3.6.4 Procedure documents, intended to be used as an active tool during marine operations should include
a section which clearly shows their references to higher and lower level documents, and should list all interrelated documents.
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Guidance note:
A document organogram is often helpful as shown in Figure 2-2.
Figure 2-2 Example of document organogram
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2.3.7 Marine operation manuals
2.3.7.1 An operational procedure shall be developed for the planned operation, and shall reflect
characteristic environmental conditions, physical limitations, design assumptions and tolerances.
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Guidance note:
For complex operations it is recommended that a high level presentation of the marine operation is made available as an animation
or picture series. See also [2.8.3].
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2.3.7.2 The operational procedures shall be described in a marine operation manual covering all aspects of
the operation and should include the following, as applicable:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
reference documents
general arrangement
permissible load conditions
outline execution plan
organogram and lines of command, see [2.2.1]
job-descriptions for key personnel
safety plan, see [2.3.7.5]
authorities and permits including notification and approval requirements
contractual approvals and hand over, see also [2.3.7.4]
environmental criteria, including design and operational criteria
weather (forecast) and current/wave reporting
operational bar chart, showing the anticipated duration of each activity, inter-related activities, key
decision points, hold points
m) specific step-by-step instructions (procedures/task plans) for each phase of the operation including
sequence, timing, resources and check lists
n) reference to related drawings and calculations, e.g. environmental loads, moorings, ballast, stability,
bollard pull
o) permissible draughts, trim, and heel and corresponding ballasting plan
p) how to handle any changes in the procedure during the operation, see also [2.2.5.6] h).
q) contingency and emergency procedures, see [2.5.4]
r) emergency preparedness bridging document
s) monitoring during the operation, see [2.9.5]
t) clearances and tolerances
u) systems and equipment including layout
v) systems and equipment operational instructions
w) vessels involved
x) tow routes and ports and/or places of refuge
y) navigation
z) safety equipment
aa) recording and reporting routines
ab) sample forms
ac) equipment operation history
ad) check lists for preparation and performance of the operation.
2.3.7.3 Operational limiting criteria for marine operations or parts thereof shall be clearly stated in the
manual.
2.3.7.4 The manual shall describe the decision point for issuing the CoA from the MWS company. It may also
be found relevant to include (other) 'gates' at which agreement from representatives of the principal parties
involved should be obtained before continuing to next stage of operation.
2.3.7.5 A safety plan shall be included in the operation manual. This plan consists of the safety rules that
apply to minimise the following risks encountered during each operation:
a)
Risks inherent from the metocean conditions
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b)
c)
d)
e)
Risks incurred by construction, transport, installation and commissioning activities
Risks to the environment
Risks due to simultaneous operations (SIMOPS), see IMCA M 203, /83/
Risks due to working on live assets, etc.
2.3.7.6 Essential documentation in the form of certificates, release notes and classification documents for all
equipment and vessels involved in the marine operation shall be enclosed and/or listed in the manual, see
also [2.3.6.3].
2.3.8 Operation records and reporting
2.3.8.1 The execution of marine operations shall be logged. Recording form templates shall be included in
the marine operations manual.
2.3.8.2 The following should as a minimum be recorded during the operation:
a)
b)
c)
d)
e)
log of (main) tasks carried out
any modifications in the agreed procedure
unexpected events and any deviations from or alterations of procedure imposed by such
environmental conditions
critical monitoring results.
2.3.8.3 Any significant modifications in the agreed procedure shall be reported promptly to the MWS
company.
Guidance note:
It is recommended that all changes to previously agreed/approved procedures are signed off by the principal representatives of the
parties involved. See also [2.3.7.2] p), and that this is described in the MOC procedure.
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2.3.8.4 For larger projects, communications to the customer (and the MWS company) on site should be
confirmed in writing, e.g. by daily reports.
2.3.8.5 Regular, at least daily, reports shall be issued to the MWS company from operations (e.g. towage)
where the MWS company is not attending.
2.3.8.6 Any incidents, accidents or near-misses relevant to the safety of the structure or future marine
operations shall be reported to the MWS company.
2.4 Risk management
2.4.1 General
2.4.1.1 Risk management shall be applied to the project to reduce the overall risk. The preferred approach is
to address the following:
a)
b)
c)
d)
Identification of potential hazards.
Preventative measures to avoid hazards wherever possible.
Controls to reduce the potential consequences of unavoidable hazards.
Mitigation of the consequences, should hazards occur.
2.4.1.2 The overall responsibility for risk management shall be clearly defined when planning marine
operations.
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Guidance note:
It is recommended that risk management is performed according to DNV-RP-N101, /54/, in order to ensure a systematic
evaluation and handling of risk. It is also a premise for a successful risk management that a project team with sufficient
competence to understand the marine operation and the potential risk/hazard is mobilized, see [2.8].
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2.4.1.3 Risk evaluations shall be carried out at an early stage for all marine operations in order to define
the extent of risk management required, and to identify and mitigate risk as early in the design process as
possible.
Guidance note 1:
The type and amount of risk evaluations should be based on the complexity of each marine operation. DNV-RP-N101 [D.5], /54/,
gives advice on how to carry out initial risk evaluations. The effect of (planned) redundancy, back-up, safety barriers, and
emergency procedures should be taken into account in the (initial) risk estimates. Contingency situations with a documented
(joint) probability of occurrence less than 10
-4
per operation may be disregarded.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Ideally, each of the various studies outlined should be managed by a competent independent person familiar with the overall
concept, but outside the team carrying out the relevant system or structure design or operational management.
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2.4.1.4 Risk assessments shall be documented and the mitigated risks accepted by the MWS company.
2.4.1.5 Detailed hazard studies should include the personnel and organisations involved in the design of
structures and systems, as well as those involved in the marine operation and the MWS company. The
studies shall be performed for:
a)
b)
Each major marine operation.
Each major system essential to the performance and safety of marine operations. For example, the
power generation and the ballast and compressed air systems.
Guidance note:
Hazard identification activities (see [2.4.2]) may be used to systematically evaluate risk applicable to any operation, to
compare levels of risk between alternative proposals or between known and novel methods, and to enable rational choices to
be made between alternatives.
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2.4.2 Hazard identification activities
2.4.2.1 Risk identification techniques and methods shall be used as applicable for the intended operation.
Examples of applicable techniques and methods are:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Preliminary risk assessment in order to assess concepts and methods
Hazard identification analysis (HAZID)
Early procedure hazard and operability study (EP HAZOP)
Hazard identification and risk assessment (HIRA)
Design review (DR)
System HAZOP
Failure mode effect (and criticality) analysis (FMEA/FMECA)
Procedure HAZOP
Semi-quantitative risk analysis (SQRA)
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j)
Safe job analysis (SJA) / job safety analysis (JSA).
Guidance note:
DNV-RP-N101 App.B, /54/, defines and describes most of the risk identifying activities listed above in detail. The HAZOP is
not only focused on possible hazards, but also on issues related to the operability of an activity or operation, the plant or
system, including possible improvements.
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2.4.2.2 All identified possible hazards shall be reported and properly managed.
2.4.3 Risk reducing activities
2.4.3.1 Relevant corrective actions from the risk identifying activities shall be implemented in the planning
and execution of the operations.
2.4.3.2 The following risk reducing activities for marine operations shall be used as applicable for the
intended operation:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
Operational feasibility assessments
Document verification
Familiarisation
Personnel safety plans
Emergency preparedness
Marine readiness verification
Inspection and testing
Survey of vessels
Toolbox talk
Safe job analysis / job safety analysis
Survey of operations.
Guidance note:
DNV-RP-N101 App.C, /54/, describes the above listed risk reducing activities in detail. Note that Safe Job Analysis is in DNVRP-N101, /54/, mentioned only in DNV-RP-N101 App.B - Hazard Identification Activities.
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2.4.4 Third party verification and marine warranty survey
2.4.4.1 As a part of the risk management the requirements for third party verification of calculations,
procedures, vessels, equipment, etc. and survey of the operations shall be defined.
2.4.4.2 If applicable a marine warranty survey company shall be contracted to ensure that the marine
warranty clause is fulfilled.
2.4.4.3 It shall be ensured that the MWS (marine warranty survey) Company’s (minimum) scope of work
has been adequately defined to fulfil the intention of the marine warranty clause. Specific requirements of
warranty clause to be given to MWS as early as possible.
2.4.4.4 Thorough knowledge of this standard shall be documented in order to carry out marine warranty
survey with the intention of confirming compliance with this standard.
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2.5 Planning of marine operations
2.5.1 Philosophy
2.5.1.1 Marine operations shall be planned according to safe and sound practice, and according to defined
codes and standards.
2.5.1.2 A marine operation shall be designed to bring an object from one defined safe condition to another.
Guidance note:
'Safe condition' is defined as a condition where the object is considered to be exposed to a normal level of risk of damage or loss
(i.e. the risk is similar to that expected for the in-place condition). Normally this will imply a (support) condition for which it is
documented that the object fulfils the design requirements applying the relevant weather unrestricted, see [2.6.6], environmental
loads.
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2.5.1.3 Risk management, see [2.4], should normally be included in the planning.
2.5.2 Type of operation
2.5.2.1 To define the (sub-) operations as either weather unrestricted or weather restricted can have a great
impact on the safety and cost of the operation. Hence, the type of operation should, if possible, be defined
early in the planning process. See also [2.6.5].
2.5.2.2 The planning and design of marine operations should normally be based on the assumption that
it can be necessary to halt the operation and bring the object to a safe condition e.g. by reversing the
operation.
2.5.2.3 For operations passing a point where the operation cannot be reversed, a point of no return (PNR)
shall be defined. The first safe condition after passing a PNR shall be defined and considered in the planning.
2.5.3 Operations in ice areas
2.5.3.1 The risk of significant ice shall be considered in the operation planning. I.e. operations in ice areas
should be subject to suitable ice management operations, details of which appear in [B.3].
2.5.3.2 Towages in ice are considered in [11.19] and voyages in [K.11].
2.5.3.3 The evacuation from rigs/offshore structures in ice shall be properly planned.
Guidance note:
ISO 19906, /103/ Clause 18 and Annex A.18 provide appropriate normative requirements and informative guidance for escape,
evacuation and rescue (EER) operations from Arctic offshore structures.
Additional guidance on the design of an appropriate EER system may be found in Barents 2020 (2012), /21/, Chapter 4. This
includes performance standards for emergency response vessels and guidance for Arctic evacuation methods.
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2.5.4 Contingency and emergency planning and procedures
2.5.4.1 All possible emergency situations shall be identified, and contingency procedures or actions shall be
prepared for these situations.
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Guidance note:
Foreseeable emergencies and contingencies can include:
a)
Severe weather
b)
Planned precautionary action in the event of forecast severe weather
c)
Structural parameters approaching pre-set limits
d)
Stability parameters approaching pre-set limits
e)
Failure of mechanical, electrical or control systems
f)
DP or power failure 'black ship'
g)
Fire
h)
Collision, grounding
i)
Leakage, flooding
j)
Pollution
k)
Structural failure
l)
Equipment failure
m)
Mooring failure
n)
Icebergs, excessive ice (see also [2.5.3.3])
o)
Human error
p)
Man overboard
q)
Personnel accidents or medical emergencies
r)
Terrorism and sabotage.
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2.5.4.2 Possible emergency situations to be considered may be defined or excluded based on conclusions
from risk identifying activities, see [2.4.2].
2.5.4.3 Contingency and emergency planning shall consider redundancy, back-up equipment, supporting
personnel, emergency procedures and other relevant preventive measures and actions.
2.5.4.4 The contingency procedures should form part of the operational procedures.
2.6 Operation and design criteria
2.6.1 Introduction
2.6.1.1 Marine operations shall be executed ensuring that the assumptions made in the planning and design
process are fulfilled.
2.6.1.2 Marine operations shall be classified as weather restricted or as weather unrestricted (see [2.6.5]).
Guidance note:
The main difference between these operations is how the environmental loads are selected. See Table 5-1.
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2.6.2 Operation reference period - TR
2.6.2.1 The duration of marine operations shall be defined by an operation reference period, TR:
TR = TPOP+TC
where:
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TR
TPOP
TC
= operation reference period
= planned operation period
= estimated maximum contingency time.
2.6.2.2 The start and completion points for the intended operation or parts of the operation shall be clearly
defined. See also [2.6.7.4] and [2.6.7.5].
2.6.3 Planned operation period – TPOP
2.6.3.1 The planned operation period, TPOP, shall if possible be based on a detailed schedule for the
operation.
Guidance note:
In cases (e.g. in the early planning phase) were a detailed schedule is not available TPOP can be based on experience with similar
operations.
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2.6.3.2 The time estimated for each task in the schedule should be based on a reasonably conservative
assessment of experience with same or similar tasks.
Guidance note:
Normally a probability of (maximum) 10-20% of exceeding TPOP during the actual operations should be aimed at.
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2.6.3.3 Time delaying incidents that are experienced frequently should be included in TPOP.
2.6.4 Estimated contingency time – TC
2.6.4.1 Contingency time, TC, shall be added to cover:
a)
b)
c)
General uncertainty in the planned operation time, TPOP.
Unproductive time during the operation, e.g. to solve unforeseen procedural problems.
Possible contingency situation(s), see [2.5.4], that will require additional time to complete the operation.
Guidance note:
It is normally not necessary to add the estimated additional time from several (two) rare independent contingency situations.
However, it can be relevant to consider that more than one of the frequently experienced incidents mentioned in [2.6.3.3]
(e.g. equipment malfunction) may occur.
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2.6.4.2 If TPOP uncertainties and the required time for contingency situations is not assessed in detail the
operation reference period should normally be taken to be at least twice the planned operation period,
i.e.TR ≥ 2 × TPOP.
Guidance note:
A contingency time TC of 50% of TPOP can normally be accepted for:
—
Operations with an extensive experience basis from similar operations, e.g. positioning (anchoring) of MOUs.
—
Towing operations with redundant tug(s) and properly assessed towing speed, see Sec.11 for more information.
—
Repetitive operations where TPOP has been accurately defined based on experience with the actual operation and vessel.
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2.6.4.3 A contingency time TC less than 6 hours is normally not acceptable unless thoroughly documented.
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Guidance note:
TC < 6 hours is unlikely to be acceptable except for short simple marine operations involving only robust equipment.
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2.6.5 Weather unrestricted and restricted operations
2.6.5.1 An operation shall be defined as weather unrestricted, see [2.6.6], or weather restricted, see
[2.6.7]. See [2.5.2] and Figure 2-3 for further guidance.
Guidance note:
By weather it is meant all relevant environmental parameters that are predicted in the received weather forecast. Wind and waves
are described in detail in [2.6], see also [2.6.5.4].
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2.6.5.2 Operations with a duration that is too long to be planned as weather restricted, see [2.6.7.2], may
still be defined as weather restricted if a continuous surveillance of actual and forecasted weather conditions
is implemented, and the operation can be halted and the object brought into a safe condition within the
maximum allowable period for a weather restricted operation. See flowchart in Figure 2-3.
Guidance note 1:
If found beneficial also operations with TPOP < 72h and TR < 96h may also be planned with possible stops, see also [2.6.7.8].
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The indicated maximum allowable period for a weather restricted operation, as per [2.6.7.2], is a theoretical value. For most
continuous operations a considerably shorter period should be documented in order to make the operation feasible without risking
too much delay.
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Figure 2-3 Flow chart to determine whether an operation is weather restricted or weather
unrestricted
2.6.5.3 A weather restricted criteria for operations with longer duration than the maximum allowed, see
[2.6.7.2], could in some situations be acceptable based on active use of the (long term) weather forecast.
However, an accidental load case (ALS) considering extreme value statistics should normally be included.
Guidance note:
Typical situations are:
—
Operations in areas and seasons where it has been shown and documented that the long term weather forecasts can predict
—
Open (Ocean) voyages where the vessel speed is sufficient to avoid extreme weather conditions and where weather routeing is
any extreme weather conditions within the defined TR for the operation.
used, see [3.4.17.5], [3.4.20.3] and [11.14.4].
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2.6.5.4 Wind and wave limitations are what normally will define a marine operation as weather restricted.
If other forecasted parameters, see [2.7.1.6], are crucial for a safe operation these shall be included in the
weather restricted or unrestricted evaluation.
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2.6.6 Weather unrestricted operations
2.6.6.1 Marine operations that cannot be defined as weather restricted (see [2.6.5] and [2.6.7]) shall be
defined as weather unrestricted operations. Environmental criteria for these operations should be based on
extreme value statistics, see Sec.3. If found beneficial, operations of shorter duration may also be defined as
weather unrestricted.
2.6.6.2 For operations where the design environmental condition is based on extreme value statistics, the
operation could theoretically be performed with acceptable risk without considering the weather forecast.
However, it is normally not recommended that an operation is started if extreme weather conditions are
expected, and a start criterion may apply.
Guidance note:
Note that certain operations require a start criterion although designed for weather unrestricted conditions. Further information is
given for the respective operations in this standard.
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2.6.7 Weather restricted operations
2.6.7.1 For voyages that are weather restricted due to the need for heading control, see [11.8].
2.6.7.2 Marine operations with a reference period (TR) less than 96 hours and a planned operation time
(TPOP) less than 72 hours may normally be defined as weather restricted. However, in areas and/or seasons
where the duration of the reliable weather forecast is less than 96 hours, the maximum allowable TR is the
duration of the reliable forecast.
Guidance note:
The above indicated limits for TR and TPOP define the maximum allowable period for a weather restricted operation.
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2.6.7.3 A weather restricted operation shall be planned to be executed within a reliable weather window, see
Figure 2-4.
2.6.7.4 The planned operation period start point for a weather restricted operation shall normally be defined
as being at the issuance of the last weather forecast. See Figure 2-4.
Guidance note:
The issuance of WF is the point in time indicated on the WF. For operations with appropriate wave/wind monitoring and TPOP ≤ 4
hours, TPOP could be considered from start of operation.
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Figure 2-4 Operation periods
2.6.7.5 The operation shall only be considered completed when the object is in a safe condition, see
[2.5.1.2].
2.6.7.6 Restricted operations may be planned with the design environmental conditions which can/will be
(weather) forecasted, selected independent of statistical data, i.e. set by owner, operator or contractor.
Environmental conditions which cannot be adequately forecasted, e.g. current, should be based on extreme
value statistics, see Sec.3.
Guidance note:
If the weather restricted design environmental condition is too low, severe waiting on weather delays can occur. The design
environmental condition should be selected based on an overall evaluation of operability i.e. there should be an acceptable
probability of obtaining the required weather window. See also [3.3].
For weather restricted operations, it is recommended that the current speed is selected based on statistical data, see [3.4.15], to
ensure an acceptable probability of obtaining the required weather window. For towline pull calculation, see [11.12.2.5].
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2.6.7.7 The start of a weather restricted operation is conditional on an acceptable weather forecast, see
[2.7.3].
2.6.7.8 Operations that could be carried out within the maximum allowed period may be planned with
(possible) stops in a stand-by position in case of period(s) with weather conditions above the OPLIM (or
forecasted above the OPWF) for the normal operation. Increased risk due to the following shall be considered:
a)
b)
c)
additional stop/start(s).
'nature' of stand-by (quasi-safe) condition, see [2.6.7.9].
longer total operation period.
2.6.7.9 If the planning indicated in [2.6.7.8] is implemented the alpha (α) factors shall be adjusted
accordingly, e.g.:
— Total planned operation period (including stops) to be used for defining alpha factors. See also [2.6.10.2].
— Apply alpha factor to define the OPWF for the stand-by position.
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— If the stand-by position is a safe condition, alpha factor philosophy for continuous operations may be
applied. See [2.6.9.3].
2.6.8 Operational limiting criteria
2.6.8.1 Operational limiting environmental criteria (OPLIM) shall be established and clearly described in the
marine operation manual.
2.6.8.2 The OPLIM shall not be taken greater than the minimum of:
— The design environmental conditions. See [3.3].
— Maximum wind and waves for safe working and object handling (e.g. on vessel deck) or transfer
conditions for personnel.
— Weather restrictions for equipment (e.g. ROV and cranes).
Guidance note:
Weather restrictions for equipment should be based on specified limitations if available. They may also be assessed and/or
refined based on items as criticality, back-up equipment and contingency procedures.
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— Limiting weather conditions of diving system (if any).
— Limiting conditions for position keeping systems.
— Any limitations identified, e.g. in HAZID/HAZOP, based on operational experience with involved vessel(s),
equipment, tools.
— Limiting weather conditions for carrying out identified contingency plans.
2.6.9 Forecasted and monitored operational limits, alpha factor (α)
2.6.9.1 Uncertainty in both the monitoring and the weather forecasting shall be considered for all weather
restricted environmental parameters, see [2.6.5.4].
2.6.9.2 Wave height and wind speed uncertainty should be considered by defining a forecasted (and, if
applicable, monitored at the operation start) operational criteria - OPWF, defined as OPWF = α × OPLIM.
Guidance note:
To ensemble weather forecasts which identify the expected ‘spread’ of weather conditions and assess the probability of particular
weather events could be an alternative for applying the tabulated alpha factors. Such weather forecasts will anyhow give useful
additional information to evaluate uncertain weather situations. Further description of ensemble forecasting is in [B.4].
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2.6.9.3 The planned operation period (TPOP, see [2.6.3]) from issuance of the weather forecast to the
operation is completed shall be used as the minimum time for selection of the alpha factor. See Figure 2-4.
2.6.9.4 For operations that can be halted, see [2.6.5.2], the alpha factor can normally be selected based on
a TPOP defined as the time between weather forecasts plus the required time to safely cease the operation
and bring the handled object into a safe condition. If a proper procedure based on continuously reliable (see
[2.9.3]) monitoring readings, is established the time between weather forecasts can normally be disregarded
in the estimation of TPOP. However, the maximum expected reaction time from monitoring readings above
OPWF to initiation of ceasing of the operation, shall be included in TPOP. A reaction time below 2 hours should
normally not be considered.
2.6.9.5 The following should be used as guidelines for selecting the appropriate alpha factor for waves:
a)
The expected uncertainty in the weather forecast should be calculated based on statistical data for the
actual site and the operation schedule, i.e. TPOP. The alpha factor should be calibrated to ensure that
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b)
the probability of exceeding the operational environmental limiting criteria (OPLIM) by more than 50% in
-4
LRFD (see [2.6.11]) is less than 10 .
Reliable wave and/or vessel response monitoring system(s) and applied weather forecast level, see
[2.7.2], could be taken into account.
2.6.9.6 Special considerations should be made regarding uncertainty in the wave periods i.e. if the operation
is particularly sensitive to some wave periods (e.g. swell), the uncertainty in the forecasted wave periods
shall also be considered.
Guidance note:
The alpha factors given herein relate only to uncertainty in the wave height. Traditionally, calculations for marine operations have
been carried out for the complete range of possible wave periods for a wave height (see [3.4.11]). Hence, there was no need to
include uncertainty in the wave period. This approach can be too conservative and therefore different limiting wave heights each
with corresponding period ranges have been specified in more recent projects. In such cases, and where the period is critical, it is
important that there is ample margin between the forecasted and allowable period.
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2.6.9.7 For vessels requiring earth fixed heading during the operation, forecast uncertainty in wave
heading(s) shall be duly considered.
2.6.10 Selection of alpha factors
2.6.10.1 The (tabulated) alpha factor(s) shall be selected based on:
— The applicable table, see [2.6.10.4] and Table 2-1.
— Operational limiting criteria, OPLIM, see [2.6.8].
— The planned operational period, TPOP, see [2.6.9.3].
2.6.10.2 The alpha factor can be assumed to vary in time for one operation, e.g. for an operation with TPOP=
36 hours, Hs= 4.0 m, the alpha factor is 0.79 for the first 12 hours, 0.76 for the next 12 hours and 0.73 for
the last 12 hours of the operation.
2.6.10.3 Design wave heights less than one (1) meter are normally not applicable for offshore operations. If
a smaller design wave height nevertheless has been applied the alpha factor may be duly considered in each
case.
2.6.10.4 In the North Sea and the Norwegian Sea the alpha factor table to be used should normally be
selected using the tables referenced in Table 2-1 considering the applied weather forecast (WF) level, see
[2.7.2], applicable environmental monitoring, see [2.9.3], and design method (LRFD or ASD/WSD).
2.6.10.5 The uncertainty in forecasted and actual weather conditions shall be considered also in other
offshore areas than mentioned in [2.6.10.4]. If reliable data is not available to establish alpha factors, see
[2.6.9.5], the approach in [2.6.10.4] should also be used for other areas.
Guidance note:
The tabulated alpha factors are based on the work performed in a Joint Industry Project during the years 2005-2007 with the aim
to establish a revised set of α-factors for European waters. For details of the JIP see DNV Report 2006_1756 Rev. 03, DNV Marine
Operation Rules, Revised Alpha Factor JIP Project.
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Table 2-1 Selection of alpha factor table(s)
WF level
A1
A2 and B
C
Environmental monitoring?
Yes
No
Yes
No
Yes
No
Wave alpha factor – LRFD
Table 2-7
Table 2-6
Table 2-5
Table 2-4
Table 2-3
Table 2-2
Wave alpha factor – ASD/WSD
Table 2-14
Table 2-13
Table 2-12
Table 2-11
Table 2-10
Table 2-9
Wind alpha factor – LRFD
Table 2-8
Wind alpha factor – ASD/WSD
Table 2-15
2.6.11 Tabulated alpha factor – LRFD method
2.6.11.1 The alpha factor for waves applying LRFD, see [5.9.8], shall be selected according to Table 2-1 and
are given in Table 2-2 through Table 2-7. Values for wind are in Table 2-8.
Table 2-2 LRFD alpha factor for waves, level C – no environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
TPOP ≤ 12
0.65
TPOP ≤ 24
0.63
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.76
0.71
4 < Hs < 6
0.79
0.73
Linear
interpolation
Hs = 4
0.80
0.76
Linear
interpolation
0.73
Hs≥6
0.78
Linear
interpolation
TPOP ≤ 36
0.62
0.76
TPOP ≤ 48
0.60
0.68
0.71
0.74
TPOP ≤ 72
0.55
0.63
0.68
0.72
Table 2-3 LRFD alpha factor for waves, level C – with environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.90
0.95
1.00
1.00
TPOP ≤ 12
0.72
0.84
0.87
0.88
TPOP ≤ 24
0.66
TPOP ≤ 36
0.62
TPOP ≤ 48
0.60
0.68
0.71
0.74
TPOP ≤ 72
0.55
0.63
0.68
0.72
Linear
interpolation
0.77
0.71
Linear
interpolation
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0.80
0.73
Linear
interpolation
0.82
0.76
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Table 2-4 LRFD alpha factor for waves, level A2 or B – no environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
TPOP ≤ 12
0.68
TPOP ≤ 24
0.66
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.80
0.75
4 < Hs < 6
0.83
0.77
Linear
interpolation
Hs = 4
0.84
0.80
Linear
interpolation
0.77
Hs≥6
0.82
Linear
interpolation
TPOP ≤ 36
0.65
0.80
TPOP ≤ 48
0.63
0.71
0.75
0.78
TPOP ≤ 72
0.58
0.66
0.71
0.76
Table 2-5 LRFD alpha factor for waves, level A2 or B – with environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.90
0.95
1.00
1.00
TPOP ≤ 12
0.75
0.88
0.91
0.92
TPOP ≤ 24
0.69
TPOP ≤ 36
0.65
TPOP ≤ 48
0.63
0.71
0.75
0.78
TPOP ≤ 72
0.58
0.66
0.71
0.76
Linear
interpolation
0.81
0.75
Linear
interpolation
0.84
0.77
Linear
interpolation
0.86
0.80
Table 2-6 LRFD alpha factor for waves, level A1 – no environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
TPOP ≤ 12
0.72
TPOP ≤ 24
0.69
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.84
0.78
4 < Hs < 6
0.87
0.80
Linear
interpolation
Hs = 4
0.88
0.84
Linear
interpolation
0.86
Linear
interpolation
TPOP ≤ 36
0.68
TPOP ≤ 48
0.66
0.75
0.78
0.81
TPOP ≤ 72
0.61
0.69
0.75
0.79
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0.80
Hs≥6
0.84
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Table 2-7 LRFD alpha factor for waves, level A1 – with environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.90
0.95
1.00
1.00
TPOP ≤ 12
0.78
0.91
0.95
0.96
TPOP ≤ 24
0.72
TPOP ≤ 36
0.68
TPOP ≤ 48
0.66
0.75
0.78
0.81
TPOP ≤ 72
0.61
0.69
0.75
0.79
Linear
interpolation
0.84
0.78
Linear
interpolation
0.87
0.80
Linear
interpolation
0.90
0.84
2.6.11.2 The appropriate alpha factor for wind should be selected (estimated) considering the following:
— Statistical data and local experience for the actual site.
— Planned operation period from issuance of weather forecast, TPOP.
— The availability of reliable on-site monitoring.
— Applied wind speed compared with the maximum possible wind speed, i.e. 10 year return wind speed.
— Criticality of exceeding the design wind speed, e.g. by considering the contribution from wind on the total
design load.
2.6.11.3 If no reliable data is available the alpha factors indicated in Table 2-8 shall be considered as the
maximum allowable.
Table 2-8 LRFD recommended alpha factor for wind
Planned operation period
1)
Operational limiting (OPLIM) wind speed – Vd
Vd < 0.5 x V10 year return
1)
Vd > 0.5 x V10 year return
0.95
1)
TPOP ≤ 1
0.90
TPOP ≤ 24
0.80
0.85
TPOP ≤ 48
0.75
0.80
TPOP ≤ 72
0.70
0.75
These values shall be used only when the weather is stable and there is reliable on-site monitoring.
2.6.11.4 The possibility for unpredictable strong wind, e.g. squalls and polar lows, should be duly considered
in the selected alpha factor for wind (and if relevant also for waves). Alternatively, if possible, operational
contingency actions that sufficiently reduce the criticality of such wind, could be planned.
2.6.12 Tabulated alpha factor - ASD/WSD method
2.6.12.1 The alpha factors for waves and wind applicable to the ASD/WSD, see [5.9.7] design approach shall
be selected based on Table 2-1 and are shown in Table 2-9 through Table 2-15. These factors are calibrated
for the ASD/WSD format, with the objective of ensuring that a given structure will be treated equally under
ASD/WSD and LRFD. The alpha factors for ASD/WSD are therefore lower than the values given in [2.6.11]
because the inherent safety margin in ASD/WSD checks is less than that in LRFD checks, so higher design
values are needed to achieve this equivalence.
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Where the operational limiting criteria are not the result of strength considerations the alpha factors given in
[2.6.11] may be used.
Table 2-9 ASD/WSD alpha factor for waves, level C – no environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
TPOP ≤ 12
0.58
TPOP ≤ 24
0.56
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.68
0.63
4 < Hs < 6
0.70
0.65
Linear
interpolation
Hs = 4
0.71
0.68
Linear
interpolation
0.65
Hs≥6
0.69
Linear
interpolation
TPOP ≤ 36
0.55
0.68
TPOP ≤ 48
0.53
0.61
0.63
0.66
TPOP ≤ 72
0.49
0.56
0.61
0.64
Table 2-10 ASD/WSD alpha factor for waves, level C – with environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.80
0.85
0.89
0.89
TPOP ≤ 12
0.64
0.75
0.77
0.78
TPOP ≤ 24
0.59
TPOP ≤ 36
0.55
TPOP ≤ 48
0.53
0.61
0.63
0.66
TPOP ≤ 72
0.49
0.56
0.61
0.64
Linear
interpolation
0.69
0.63
Linear
interpolation
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0.71
0.65
Linear
interpolation
0.73
0.68
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Table 2-11 ASD/WSD alpha factors (waves) - level A2 or B – no environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
TPOP ≤ 12
0.61
TPOP ≤ 24
0.59
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.71
0.67
4 < Hs < 6
0.74
0.69
Linear
interpolation
Hs = 4
0.75
0.71
Linear
interpolation
0.69
Hs≥6
0.73
Linear
interpolation
TPOP ≤ 36
0.58
0.71
TPOP ≤ 48
0.56
0.63
0.67
0.69
TPOP ≤ 72
0.52
0.59
0.63
0.68
Table 2-12 ASD/WSD alpha factor for waves, level A2 or B – with environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.80
0.85
0.89
0.89
TPOP ≤ 12
0.67
0.79
0.82
0.83
TPOP ≤ 24
0.62
TPOP ≤ 36
0.58
TPOP ≤ 48
0.56
0.63
0.67
0.69
TPOP ≤ 72
0.52
0.59
0.63
0.68
Linear
interpolation
0.72
0.67
Linear
interpolation
0.75
0.69
Linear
interpolation
0.77
0.71
Table 2-13 ASD/WSD alpha factors (waves) - level A1 – no environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
TPOP ≤ 12
0.64
TPOP ≤ 24
0.61
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.75
0.69
4 < Hs < 6
0.77
0.71
Linear
interpolation
Hs = 4
0.78
0.75
Linear
interpolation
0.77
Linear
interpolation
TPOP ≤ 36
0.61
TPOP ≤ 48
0.59
0.67
0.69
0.72
TPOP ≤ 72
0.54
0.61
0.67
0.70
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Hs≥6
0.75
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Table 2-14 ASD/WSD alpha factors (waves) - level A1 – with environmental monitoring
Planned
operation
period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.80
0.85
0.89
0.89
TPOP ≤ 12
0.69
0.81
0.85
0.85
TPOP ≤ 24
0.64
TPOP ≤ 36
0.61
TPOP ≤ 48
0.59
0.67
0.69
0.72
TPOP ≤ 72
0.54
0.61
0.67
0.70
0.75
Linear
interpolation
Linear
interpolation
0.69
0.77
Linear
interpolation
0.71
0.80
0.75
2.6.12.2 The appropriate alpha factor for wind should be selected (estimated) considering the following:
— Statistical data and local experience for the actual site.
— Planned operation period from issuance of weather forecast, TPOP.
— The availability of reliable on-site monitoring.
— Applied wind speed compared with the maximum possible wind speed, i.e. 10 year return wind speed.
— Criticality of exceeding the design wind speed, e.g. by considering the contribution from wind on the total
design load.
2.6.12.3 If no reliable data is available the alpha factors indicated in Table 2-15 shall be considered as the
maximum allowable in ASD/WSD. See also [2.6.11.2] and [2.6.11.4].
Table 2-15 ASD/WSD alpha factors (wind - all forecast requirements)
Planned operation period
Operational limiting wind speed (Vd)
Vd < 0.5 x V10 year return
1)
1)
1)
Vd > 0.5 x V10 year return
0.85
1)
TPOP ≤ 1
0.80
TPOP ≤ 24
0.71
0.76
TPOP ≤ 48
0.67
0.71
TPOP ≤ 72
0.62
0.67
These values shall be used only when the weather is stable and there is reliable on-site monitoring.
2.7 Weather forecast
2.7.1 General
2.7.1.1 Arrangements shall be made for receiving weather forecasts at regular intervals before, and during,
the marine operations. Such weather forecasts shall be from recognized sources and be project specific.
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Guidance note:
Public domain weather forecast(s) may be found acceptable as level C forecasting, but the inherent increased uncertainty should
be considered. Applicable alpha factors are found by multiplying the factors in Table 2-2 (Table 2-9) and Table 2-15 (Table 2-16)
with 0.75.
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2.7.1.2 Independent weather forecasts shall be taken from different weather providers. The providers shall
be different organizational bodies. Each body shall document which different atmospheric and oceanographic
models have been evaluated and taken into account in the generation of the forecasts.
2.7.1.3 The weather forecasts (WF) shall be area/route specific. For non-stationary marine operations (e.g.
sea voyages or subsea laying operations) it shall be ensured that weather forecasts comprise the position (at
the time of the WF) of the transport vessel/barge and all alternative routes that could be chosen in the period
covered by the weather forecast.
2.7.1.4 Weather forecast procedures should consider the nature and duration of the planned operation, see
[2.7.2.1].
2.7.1.5 The weather forecasts shall be in writing. The point in time made/issued and the confidence level(s)
should be stated.
2.7.1.6 In addition to a general description of the weather situation and its predicted development, the
weather forecast shall, as relevant, include:
— wind speed and direction
— waves and swell, significant and maximum height, mean or peak period and direction
— rain, snow, lightning, ice etc.
— tide variations and/or storm surge
— visibility
— temperature
— barometric pressure
— possibility for unpredictable strong wind, see [2.6.11.4].
for each 12 hours for a minimum of the TR plus 24 hours. In addition an outlook for at least the next 24
hours should normally be included.
2.7.1.7 The forecast shall clearly define forecasted parameters, e.g. average time and height for wind,
characteristic wave periods (Tz or Tp). The content and format of the weather forecast should be agreed with
the meteorologist in due time before the operation starts.
2.7.2 Weather forecast levels
2.7.2.1 The required weather forecast level shall be selected based on the operational sensitivity to weather
conditions and the operation reference period (TR). The following weather forecast levels are defined in this
standard:
— Level A that applies to major marine operations sensitive to environmental conditions.
— Level B that applies to environmental sensitive operations of significant importance with regard to value
and consequences.
— Level C that applies to conventional marine operations less sensitive to weather conditions, and carried
out on a regular basis.
2.7.2.2 For operations that require a level A weather forecast it shall be thoroughly considered to have the
dedicated meteorologist present on site. See Table 2-16 for further advice regarding selection of the forecast
level and for requirements to the weather forecast procedure.
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Table 2-16 Weather forecast levels
Weather forecast level
A1
A2
Operation sensitivity
Examples
High
C
Moderate
Low
— mating operations
— tow-out operations
— offshore float over
— multi barge towing
— weather routed sea
transports
— major (e.g. GBS) tow out
operations
— offshore lifting
— subsea installation
— offshore installation operations
— semi-submersible rig
moves
— jack-up rig moves.
Meteorologist on site
Yes
No
Dedicated
meteorologist
Yes
Yes
2
Maximum WF interval
12 hours
— load-out operations
— short tows in
sheltered waters/
harbour tows
— standard sea
transports without
any specified wave
restrictions.
No
2)
No
5)
1
12 hours
12 hours
No
4)
Minimum independent
2)
WF sources
— onshore/inshore
lifting
— standard laying
operations.
— sensitive laying operations
1)
B
2
6)
1)
See [2.7.1.1] guidance note.
2)
Meteorologist shall be consulted if the weather situation is unstable and/or close to the defined limit.
3)
See [2.7.1.2] for definition of independent WF sources.
4)
It is assumed that the dedicated meteorologist (and other involved key personnel) will consider weather
information/forecasts from several (all available) sources.
5)
The most severe weather forecast shall be used.
6)
Based on sensitivity with regards to weather conditions smaller intervals may be required. However, see
[2.7.3.6].
2.7.3 Acceptance criteria
2.7.3.1 The acceptance criteria for the weather forecast(s) shall clearly define the applicable limitations, see
[2.6.9] and the minimum required weather window, see [2.6.2] and Figure 2-4. The acceptance criteria shall
be included in the marine operation manual.
2.7.3.2 Acceptance criteria shall be defined for all relevant weather forecasted parameters, see [2.6.5.4].
2.7.3.3 If the weather forecasts received from the two sources do not agree the most severe weather
forecast should be considered governing, unless otherwise justified. If the discrepancy between the forecasts
is significant the weather situation should be carefully evaluated to determine whether it is too uncertain to
safely start an operation.
2.7.3.4 Based on the available weather forecasts the weather situation shall be assessed according to a
worst case scenario development. This is particularly important for unstable weather situations and for
forecasts which are stated (considered) to be of low confidence.
2.7.3.5 Uncertainties in forecasted weather window duration shall be duly considered i.e. the forecasted
weather window duration should be conservatively assessed.
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2.7.3.6 Weather forecasts are based on extensive computer analyses. In cases where forecast updates are
made at intervals of less than 12 hours it shall be documented that the updates are based on sufficient data
to be as accurate as ordinary forecasts.
2.8 Organization of marine operations
2.8.1 General
2.8.1.1 The organisation and responsibility of key personnel involved in marine operations shall be
established and described before execution of marine operations. The responsibilities and duties of each
function shall be clearly defined to minimise uncertainties and overlapping responsibilities.
2.8.1.2 Organisation charts, including names and functional titles of key personnel, shall be included in the
marine operations manual. Authority during the operation shall be clearly defined.
2.8.1.3 Operations shall be carried out in accordance with the conditions for design, the approved
documentation, and sound practice, such that unnecessary risks are avoided. This is the responsibility of the
operation superintendent or manager.
2.8.1.4 Responsibilities in possible emergency situations shall be described.
2.8.1.5 Access to the area for the operation should be restricted. Only authorised personnel should be
allowed into the operation area.
Guidance note:
Where necessary, a suitable security and tracking system should be in use to record personnel on the structure or vessels, to track
their whereabouts.
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2.8.2 Qualification and training
2.8.2.1 Operation supervisors shall possess thorough knowledge and have experience from similar
operations. Other key personnel shall have knowledge and experience within their area of responsibility.
2.8.2.2 CVs for supervisors and key personnel involved in major marine operations shall be submitted.
2.8.2.3 Vessel manning and personnel qualifications shall as a minimum fulfil statutory requirements.
Additional manning shall be considered for complex operations or to satisfy specific project requirements.
2.8.2.4 Adequate training appropriate to each individual’s function and situation should be given, including
job training, site safety training and briefings, marine safety and survival training.
2.8.2.5 A qualification matrix is recommended for correct tracking and control of personal qualifications.
2.8.2.6 Computer simulation and training, and/or model tests can give valuable information for the
personnel carrying out the operation. Where relevant, a full-mission simulation should be undertaken.
2.8.3 Familiarisation and briefing
2.8.3.1 Operation supervisors shall familiarise themselves with all aspects of the planned operations and
possess a thorough knowledge with respect to limitations and assumptions for the design.
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2.8.3.2 Key personnel shall familiarise themselves with the operations. A thorough briefing by the
supervisors regarding responsibilities, communication, work procedures, safety and other items of
importance shall be performed.
2.8.3.3 Other personnel participating in the operations shall be briefed about the operation with emphasis on
their assigned tasks/responsibilities and safety.
Guidance note:
The use of visual aids for presenting complex marine operations is highly recommended, either through picture series and/or
animations.
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2.8.3.4 For complex marine operations a separate and detailed familiarisation program shall be prepared and
thoroughly implemented involving all personnel.
Guidance note:
Familiarisation should for offshore operations normally be initiated prior to vessel mobilisation. The familiarisation should cover all
involved personnel, including marine crew, project personnel and third party, and should address all aspects of the operation.
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2.8.4 Communication and reporting
2.8.4.1 Communication lines and primary and secondary means of communication shall be defined,
preferably in a communication chart, including as appropriate:
— Customer’s representative and third party/MWS representative (if relevant)
— Overall project management
— Operation management
— Involved vessels
— Mooring systems and marine spread
— Ballast system operation
— Monitoring
— Weather forecasting
— Support services
— Field engineers providing expertise as required
— Safety
— Statutory, regulatory and approving bodies
— Emergency response.
2.8.4.2 Communication systems, including radio channels, telephone numbers, e-mail addresses and out-ofhours numbers shall be identified and checked for accuracy.
2.8.4.3 The primary operational communication system should be used only for information needed for
managing and controlling the operation. Important information should be given dedicated lines/channels.
2.8.4.4 The planned flow of information during the operation shall be described.
2.8.4.5 A common language understood by all personnel involved should be used for VHF/UHF
communication. Radio channels should be allocated early to avoid possible interference.
Guidance note:
If a common language could lead to misunderstandings, it can be acceptable to use two or more languages. Such communication
needs to be duly planned and rehearsed.
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2.8.4.6 Communication of important information that may be misunderstood, e.g. monitoring results, should
be confirmed in writing.
2.8.4.7 All communication and reporting should be made available for continuous monitoring by the MWS
during the operation. (See also [2.3.8]).
2.8.5 Shifts
2.8.5.1 For operations with a planned duration exceeding 12 hours, a shift plan shall be established.
2.8.5.2 Where personnel changes occur during the course of an operation because of shift changes, these
shall be identified. Every effort should be made to avoid changes of key personnel during critical stages of
the operation.
2.8.5.3 Where transfer of responsibility is involved, times of and procedures for hand-over from one
organisation to another (e.g. from fabrication to marine operations, from on-shore to offshore) shall be
identified.
2.8.5.4 When continuous operations using more than 1 shift are not standard practice then special provision
to prevent fatigue shall be made for operations that could continue beyond normal working hours. This
includes provision of suitably experienced and briefed alternate personnel with good hand-overs at each shift
change.
2.9 Monitoring
2.9.1 General
2.9.1.1 Actual parameters should be monitored and compared against those used in design to as great an
extent as practicable during and also if applicable before marine operations.
2.9.1.2 The monitoring methods should duly reflect the required accuracy (i.e. acceptable monitoring
tolerances).
2.9.1.3 Target values and maximum deviations from target values, i.e. tolerances, for monitoring should be
clearly defined.
Guidance note:
Maximum allowable measured deviations should normally be within 75% of ‘deviations considered in the design’ less the
‘monitoring tolerance’.
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2.9.1.4 General and back-up requirements to monitoring instrumentation systems are given in [4.2].
2.9.2 Environmental conditions
2.9.2.1 Environmental conditions can be monitored by both direct monitoring of environmental conditions
and by monitoring responses caused by environmental effects, see [2.9.3].
2.9.2.2 For marine operations particularly sensitive to environmental conditions such as wind, waves,
swell, current, tide etc., systematic monitoring of these conditions before and during the operation shall be
arranged.
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Guidance note 1:
In some areas, tide behaviour can vary considerably locally. In such cases a local tide variation curve should be established based
on extensive tide monitoring including at least one period with the same lunar phase as for the planned operation. Tidal variations
should be plotted against established astronomical tide curves. Any discrepancies should be evaluated, considering barometric
pressure and other weather effects. See also [3.4.15.3].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Wind speed measurements at higher elevations should be used to ensure that the operational limits are not exceeded for
operations sensitive to the variation of wind speed with elevation. Such measurements can be made using anemometers and/or
other devices, e.g. LIDAR.
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2.9.2.3 Expected values, for the remaining time of the operation, of significant environmental conditions
should be continuously predicted during execution of a marine operation. Such predictions should, as
relevant, be based on the monitored variations, tabulated values and weather forecasts.
2.9.3 Loads and/or responses
2.9.3.1 Full scale monitoring can be used for the determination of responses (e.g. accelerations on a vessel)
or loading effects (e.g. strain-gauge measurements). All full scale load and/or response monitoring shall be
carried out according to agreed procedures, see e.g. [2.9.5].
Guidance note:
Full scale monitoring is normally carried out to meet one or both of the following objectives:
—
To obtain valuable design information for future projects.
—
To control that design criteria (ULS or FLS) are not exceeded during an operation.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.9.3.2 During full scale monitoring it can be difficult to accurately measure the load which causes the
measured response. The information obtained may therefore be of a statistical nature, and the use of
statistical methods can be necessary in order to draw conclusions.
Guidance note:
Full scale monitoring has limitations, e.g. as indicated above, that need to be duly considered if such monitoring is used as an
(assisting) operational means of control.
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2.9.4 Alpha factor related monitoring
2.9.4.1 It shall be documented that monitoring systems and procedures used as a means to increase the
alpha factor for waves have adequate accuracy and reliability. Normally this implies fulfilment of all the
following:
— Continuous monitoring.
— The monitoring device should be adequately located (e.g. no shielding effects) to give correct readings
and not in any case more than 3 (three) nautical miles from the location of the operation. If the highest
recording from two (2) devices on opposite sides of the location are used the sum of the distance may be
up to 12 nautical miles.
— Documented monitoring accuracy better than ±5% of the measured maximum values.
— Statistical treatment of the results which continuously indicate the expected maximum value within a
defined time period (normally 3 hours).
— It should be possible to relate the response monitoring results to the wave conditions. See also [2.9.3].
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— A secondary system and/or procedure that will detect any significant erroneous results produced by the
primary system.
2.9.4.2 A procedure shall be made that describes how the interface between monitoring results and weather
forecasts shall be handled.
Guidance note:
The procedure should, as a minimum, cover the following:
—
Discrepancies between weather forecast for the present time and monitoring results.
—
How to calibrate the weather forecast for the coming hours based on the monitoring results.
—
Feed-back to meteorologist(s).
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2.9.5 Monitoring procedure
2.9.5.1 A monitoring procedure describing at least monitoring methods and intervals, responsibilities,
reporting and recording shall be prepared.
2.9.5.2 Any unforeseen monitoring results shall be reported without delay.
2.9.6 Back-up and contingency
2.9.6.1 The requirements of [4.2.1.10] apply.
2.9.6.2 If the monitoring back-up system does not have the same accuracy as the original system this
should be considered in the contingency planning.
2.10 Inspections and testing
2.10.1 General
2.10.1.1 Testing and inspection of equipment, structures, systems and vessels shall be carried out according
to relevant and recognized codes/standards and/or relevant specifications, functional requirements and
assumptions for the design.
2.10.1.2 Inspection during the operation shall include a systematic review and evaluation of monitoring
results, see [2.9].
2.10.1.3 The MWS company shall identify any inspections and tests to be witnessed by its own
representatives.
2.10.2 Test program
2.10.2.1 The required inspections and tests both in the preparation phase and during the operation shall be
described in a test and inspection program.
2.10.2.2 The test and inspection results shall be documented.
Guidance note:
The inspections and testing can be documented by reports and completed checklists.
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2.10.2.3 For larger operations it is recommended that a test/commissioning program is developed specifying
the planned inspections and tests. The test program should indicate expected characteristics, and state
acceptance criteria based on the design assumptions.
Guidance note:
Acceptance criteria for tests may also be functional requirements.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.10.3 Systems
2.10.3.1 All systems and their back-up shall be tested before the start of an operation. Such tests shall
demonstrate that they function as intended. If critical, the capacity of the system shall be adequately
checked.
2.10.3.2 Change over from a primary to a secondary system shall be tested.
2.10.3.3 Instrumentation systems shall be calibrated and tested before the operation. The calibration
procedure may be subject to review.
2.10.3.4 Essential systems shall be function and capacity tested in their final configuration and connected to
the same power supply/HPU as intended to be used during operation. If several consumers are connected to
the same power supply/HPU, the test should be performed realistically with all consumers running in order to
test capacity.
2.10.3.5 Emergency systems/functions and fail-safe configurations shall, as far as practically possible, be
tested in a realistic scenario with adequate loading.
2.10.4 Communication
2.10.4.1 Primary and secondary means of communication shall be tested before operation.
2.10.4.2 For operations with complex communication and reporting procedures, or where proper information
flow is vital, a run-through of communication routines shall be carried out.
Guidance note:
This rehearsal should be performed with the nominated personnel and under conditions similar to those expected during the actual
operation. See also [2.8.4.5].
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2.10.5 Inclining tests
2.10.5.1 The requirement to perform inclining and/or displacement tests shall be agreed with the MWS
company.
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Guidance note:
Vessels with a valid Trim and Stability booklet, including all modifications since the last inclining test, do NOT normally require an
inclining test when conservative estimates of cargo weight and centre of gravity show adequate reserves of intact and damage
stability.
Where ideally an inclining test would be performed but may not give sufficiently accurate results the calculations may be based
on outputs from the weight control programme checked against a displacement test. This would only apply if there is a sufficient
reserve of stability to cover possible inaccuracies.
Where a number of very similar units are constructed at the same place, the requirement for inclining tests on the later units may
be reduced after a study of weight variations (from displacement tests) and centre of gravity variations (from inclining tests) of the
previous units, and agreement with the MWS company.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.10.5.2 Where inclining and/or displacement tests are required:
a)
b)
c)
d)
e)
f)
g)
They should be performed before any marine operation where the displacement, centre of gravity or
stability may be critical.
They should be performed according to the guidance in IMO Intact Stability Code 2008, /89/, Part B
Annex 1.
if applicable, an allowance shall be made for the presence and compressibility of any air cushion
if the vessel is not axisymmetric then inclining tests may be required about two axes, as agreed with the
MWS company. (This normally applies to bodies with an irregular shaped plan view, not vessels with a
list).
Upon completion of the inclining test, a report containing measurements/readings and corresponding
calculations of displacement (and light displacement if relevant), metacentric height (GM), and the
position of the centre of gravity of the structure, should be prepared.
The output from the inclining test should be used to check and calibrate the output from the weight
control programme. A rigorous weight control system should be enforced from the inclining test until the
relevant marine operation is completed.
A sensitivity analysis of the parameters affecting the test results should be performed.
2.11 Vessels
2.11.1 General
2.11.1.1 This subsection [2.11], includes general requirements for vessels involved in marine operations.
Where applicable, further requirements are given for each type of operation vessel in Sec.6 through Sec.18.
2.11.1.2 Vessels shall satisfy the relevant hydrostatic stability requirements given in [11.10].
2.11.1.3 A general description of the vessel systems to be used shall be documented. Ballast and towing
equipment/systems shall be described in detail if used.
2.11.1.4 Vessels shall be suitable for their planned tasks during the operation.
Guidance note:
If there is any doubt about the vessel suitability for a specific operation it is recommended to carry out an independent suitability
survey of the vessel.
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2.11.1.5 See [17.3] for further requirements to Dynamic Positioned vessels.
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2.11.2 Condition and inspections
2.11.2.1 All vessels shall be in acceptable condition and with valid certificates, see [B.1].
2.11.2.2 All vessels involved in the operations should be inspected before the operation to confirm
compliance with the design assumptions, validity of certificates, suitability (see [2.11.1.4]) and acceptable
condition.
2.11.2.3 The global and local condition of the vessels with respect to corrosion shall be confirmed and
considered in strength verifications.
2.11.3 Structural strength
2.11.3.1 Adequate global and local structural strength shall be documented for all vessels.
Guidance note:
The strength may be documented by either ensuring that the vessel is operated within the requirements of a recognized
classification society, see [2.11.4], or by calculating the strength according to the relevant requirements in Sec.5.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.11.3.2 If the allowable deck load is based on load charts, the limitations and conditions for these with
respect to number of loads and simultaneousness of loads shall be clearly stated. The applied design factors
shall be specified.
2.11.4 Class requirements
2.11.4.1 Where a vessel is classed by a recognized classification society it shall be operated in accordance
with requirements from the Society. The limitations for class as given in 'Appendix to Class Certificate' or
similar shall be submitted.
2.11.4.2 For mobile offshore units the following annexes (or similar) to the maritime certificates shall be
submitted:
— Annex I - Operational limitations
— Annex II - Resolutions according to which the unit has been surveyed, and possible deviations from these.
2.11.4.3 Valid recommendations (conditions) given by the recognized classification society shall be
submitted.
Guidance note:
Modifications to vessel structure or equipment can require approval from the classification society.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.11.4.4 If it is planned to use a vessel or its equipment (e.g. crane) outside the limitations stated by class,
a statement of acceptance from class shall be submitted.
2.11.5 Certificates
All required certificates shall be valid, or relevant exemptions shall be submitted.
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Guidance note:
The documents (certificates) to be carried on board different types of vessels can be found in IMO Joint FAL/MEPC/MSC/LEG
Circular List of certificates and documents required on board ships, /163/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.11.6 Collision avoidance
2.11.6.1 All vessels and towed objects (unless submerged) shall meet the requirements of the International
Regulations for Preventing Collisions at Sea, (COLREGS), /91/ and any local regulations. These requirements
include the carrying of lights and shapes and, for manned vessels and towed objects, sound-signalling
devices.
2.11.6.2 Navigation lights shall be independently powered (e.g. from an independent electric power sources
or from gas containers). Fuel or power sources shall be adequate for the maximum duration of the towage,
plus a reserve. Spare mantles or bulbs should be carried, even if the tow is un-manned.
Guidance note:
Solar powered navigation lights should be compliant with UL 1104, /164/, and/or EN 14744, /165/. Additional power provided by
solar panels may be considered if an adequate track record is documented.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.11.6.3 Where possible, a duplicate system of lights should be provided.
2.11.6.4 Towed objects which may offer a small response to radar, such as barges or concrete caissons with
low freeboard, should be fitted with a radar reflector. The reflector should be mounted as high as practical.
Octahedral reflectors should be mounted in the 'catch-rain' orientation.
2.11.7 Contingency situations
2.11.7.1 All vessels shall be selected with due consideration to possible contingency situations.
Guidance note:
This could e.g. result in the selection of redundant (twin screw) tugs for towing operations in narrow waters. See also the
operation-specific requirements in Sec.10 to Sec.18 of this standard for further guidance.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
2.11.7.2 Where several tugs (vessels) are involved, a stand-by tug to assist or remove vessels in case of
black-out, engine failure, etc. should be considered.
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SECTION 3 ENVIRONMENTAL CONDITIONS AND CRITERIA
3.1 Introduction
3.1.1 General
3.1.1.1 This Section addresses the design environmental conditions applicable for marine operations. The
focus is on the criteria applicable to weather unrestricted marine operations however, design environmental
criteria for weather restricted marine operations are addressed in [3.3].
3.1.1.2 Metocean criteria are generally used for analysis to a recognized standard (including relevant safety
factors). In this standard, the environmental criteria to be used for the ASD/WSD approach are different to
those to be used for the LRFD approach.
3.1.1.3 Each marine operation shall be designed to withstand the loads caused by the most adverse
environmental conditions expected. In the case of a voyage this shall account for the areas and seasons
through which it will pass. Any agreed mitigating measures may be taken into account.
3.1.1.4 For each phase of a voyage or marine operation, the design criteria should be defined, consisting
of the design wave or sea state, design wind and, if relevant, design current. It should be noted that the
maximum wave and maximum wind may not occur in the same geographical area, in which case it may be
necessary to check the extremes in each area, to establish governing load cases.
3.1.2 Scope
3.1.2.1 The design environmental conditions should be established dependent on the duration of each
discreet phase of a marine operation, which may be a weather restricted or a weather unrestricted operation
as defined in [2.6.5].
3.1.2.2 Section [3.4] defines the default return periods that can be used to determine applicable
environmental criteria. App.C gives more detailed approaches for the determination of design winds and
waves as a function of the exposure duration and location-specific metocean parameters.
3.2 Design environmental condition
3.2.1
The design environmental condition consists of the wave height, wind speed, current and other relevant
environmental conditions specified for the design of a particular marine operation.
3.2.2
A weather unrestricted operation is not limited by practical aspects, and therefore the operational criteria
are the design environmental condition. In this case the design environmental condition is based on extreme
statistical data and is addressed in [3.4].
3.2.3
The environmental design data should be representative of the geographical area or site and operation in
question.
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3.2.4
Where it is impractical and/or uneconomical to design marine operations based on extreme statistical data,
the design environmental condition can generally be set independent of extreme statistical data for weather
restricted operations - see [2.6.7] and [3.3].
3.3 Design environmental conditions for weather restricted
operations
3.3.1
For weather restricted operations the design wind may be selected independent of statistical data.
Guidance note:
Characteristic wind velocities less than 10 m/s are generally not recommended. See also [3.3.4] for general considerations.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.3.2
The ratio between forecasted wind and design wind should be determined in accordance with Table 2-8 or
Table 2-15 as applicable.
3.3.3
Wave conditions for weather restricted operations, i.e. operations with wave heights (and/or periods)
selected independent of statistical data, should be as described by [C.3.4].
3.3.4
The significant wave height(s) and associated period(s) should be selected considering:
— Feasibility and safety of the planned operation.
— Typical weather conditions at the site.
— Operation period.
— Uncertainties in weather forecasts.
Guidance note 1:
Other factors such as the length of delay that can be accepted due to waiting on weather, and possible contractual obligations
should be considered as found relevant.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
When using stochastic waves for weather restricted operations the methods discussed in [3.4.11.6] may be used as the basis
for selecting the range of periods and appropriate spectrums provided thorough evaluations are undertaken, e.g. comparing with
scatter diagrams for the applicable area. Whilst the significant wave height can be selected independently of return period the
metocean data for return period(s) with similar wave heights can be considered when selecting the range of wave periods for
weather restricted operations. For deep water, the steepness criterion in DNV-RP-C205 [3.5.4], /46/, can be used to define the
shortest wave period.
For design/regular waves, the range of periods should be sufficiently wide and include resonance periods that could be excited.
A reduced range of periods may be considered. This is, however, likely to reduce operability, see [2.6.9.5].
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3.3.5
Maximum wave height for weather restricted operations should be calculated according to the following
equation:
Hmax = STF × Hs
where:
Hmax = maximum wave height in metres
STF = 2.0 for operation reference periods up to 72 hours
STF = 2.3 for longer operation reference periods
= significant wave height in metres.
Hs
Guidance note:
For short reference periods STF < 2.0 may be acceptable. See DNV-RP-N102 Table 2-2, /55/, for guidelines.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
An appropriate range of wave periods associated with Hmax should be considered.
3.3.6
Where relevant, applicable information from [3.4] may be used e.g. [3.4.12].
3.4 Design environmental conditions for weather unrestricted
operations
3.4.1 General
Whilst an operation may be defined as weather unrestricted, specific portions can be dependent on suitable
weather forecasts, e.g. the departure of a tow from safe haven as described in [11.14.1.4]. Such restrictions
shall be agreed before the start of an operation and are normally included on the certificate of approval.
3.4.2 Environmental statistics
3.4.2.1 Environmental phenomena are usually described by physical variables of statistical nature. Statistical
data should as far as possible be used to establish characteristic environmental conditions. The statistical
description should reveal the extreme conditions as well as the long and short-term variations.
3.4.2.2 Statistical data used as basis for establishing characteristic environmental criteria shall cover a
sufficiently long time period.
Guidance note:
For meteorological and oceanographic data a minimum of three to four years of data collection is recommended. When using
seasonal data longer periods are required. See DNV-RP-C205 /46/ for more info.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.2.3 The validity of older (typically more than 20 years) statistical data should be carefully considered
with respect to both monitoring methods/accuracy and possible long term climate changes.
3.4.2.4 If statistical environmental data are assumed to follow a two-parameter Weibull distribution, the
regression analysis should be performed with emphasis on a correct representation of the extreme values.
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Guidance note:
Regression analysis of two-parameter Weibull distributions are recommended based on the 30% highest data points, i.e. P(x > X)
= 0.3.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.3 Return periods for determining environmental criteria (apart from
moorings, except during their installation)
3.4.3.1 The return periods that shall be used for determining environmental criteria for weather unrestricted
marine operations (apart from moorings (except during their installation), the elevated operation of jack-ups
and platforms that remain in-situ during decommissioning prior to the marine operation to remove them),
should be related to its operation reference period, as defined in [2.6.2]. For design criteria for moorings see
[3.4.4], for the elevated operation of jack-ups see DNV-ST-N002, /39/, and for platforms that remain in-situ
during decommissioning prior to the marine operation to remove them see Sec.18.
3.4.3.2 As general guidance, the criteria in Table 3-1 may be applied provided that the independent
extremes are considered concurrently. The return period values from Table 3-1 are those associated with
the all-year metocean extreme hazard curve. These shall only be used with seasonal or monthly data if the
values are based on the calculation in [3.4.3.6].
3.4.3.3 The intention of the return periods and load, safety and material factors used in the LRFD approach
-4
is to ensure a probability for structural failure less than 1/10000 per operation (10 probability). Note that
this probability level defines a structural capacity reference. When the probability of operational errors is
included, the total probability of failure is increased.
Guidance note:
When including operational errors, the level of probability of total loss per operation cannot be accurately defined. However,
the recommendations and guidance given in this standard are introduced in order to obtain a probability of total loss As Low As
Reasonable Practicable (ALARP principle).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.3.4 The return periods for the ASD/WSD approach have been calibrated with the objective of ensuring
that a given structure will be treated equally under ASD/WSD and LRFD. The inherent safety margin in
ASD/WSD checks is less than that in LRFD checks, so that higher design values are needed to achieve this
equivalence.
Guidance note:
—
For loads that are linearly proportional to wave height: WSD load = 1.12 *LRFD load.
—
For loads that are quadratically proportional to wind or wave height: WSD load = 1.12 * LRFD load.
2
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.3.5 Seasonal and/or directional variations may be used subject to the requirements of [3.4.3.6] and
[3.4.3.7]. Data for the month(s) of the operation and the following month shall be used. If the operation is to
be carried out in the first 10 days of the month, the data used shall include the preceding month, but there is
no need to include the data for the following month.
Guidance note:
If an operation is delayed and will take place in, or extend into, later months:
—
with lower metocean criteria then there is no need to increase the originally planned operation reference period used in the
—
with higher metocean criteria then, the higher criteria shall be used with the applicable planned operation reference period.
design and thereby the associated metocean return period
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3.4.3.6 When seasonal variations are taken into account, this shall not imply a shorter return period,
as would occur if the monthly return period values are derived from only the data in that month without
adjustment of the target probability level. There are differing approaches to obtaining the monthly or
seasonal data at required return period (e.g. the 'one year return'). One approach is to perform an extreme
value analysis by month/season, and consider a conditional probability corresponding to that month/season.
For example, to determine the N-year return period extremes for say March, perform extreme value analysis
on the subset of data for March, consisting of 3 hr sea-states, 240 per month in the data, and fit a Weibull
curve to the cumulative distribution function. Select the required probability level for the N-yr extreme
calculated as: 1/(365.25*8*N*C) where C = 1. However the result may be limited to the all-year values.
Guidance note:
Traditionally C = 1/12, however, to comply with the requirement 'this shall not imply a shorter return period', it is necessary to
take C = 1.
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3.4.3.7 Similarly, when directional variations are taken into account, this shall not imply a shorter return
period.
Table 3-1 Metocean minimum design return periods, Td – unrestricted marine operations
Operation
reference
period
ASD / WSD
Wind
1)
3)
LRFD
Wave
2)
and current
Wind
1)
3)
Wave
2)
and current
Up to 3
4)
days
Td≥5 year
Td≥3 month
Td≥10 year
Td≥1 month
3 to 7 days
Td≥10 year
Td≥1 year
Td≥10 year
Td≥3 month
7 days to 1
month
Td≥25 year, (or obtain
from 10 yr and 50 yr
environmental criteria
values using: 10 yr +
0.7*(50 yr-10 yr) )
Td≥10 year
Td≥10 year
Td≥1 year
1 month to
1 year
Td≥75 year (or obtain
from 50 yr and 100 yr
environmental criteria
values using: 50 yr
+0.7*(100 yr-50 yr) )
Td≥50 year
Td≥100 year
Td≥10 year
More than
1 year
100 year return
Td≥100 year
Td≥100 year
Td≥100 year
NOTE:
— When seasonal or monthly data are used these shall comply with the requirements of [3.4.3.6], as discussed in
[3.4.3.2]
— Return periods for platforms that remain in-situ during decommissioning prior to the marine operation to remove
them are given in [18.2.5].
1)
More accurate design wind speeds may be determined as a function of the operation reference period and sitespecific metocean parameters using the method shown in [C.1].
2)
More accurate design waves may be determined as a function of the operation reference period and site-specific
metocean parameters using the method shown in [C.3].
3)
See [3.4.3.4].
4)
Operations up to three (3) days may also be defined as weather restricted operations, see [2.6.7].
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Guidance note:
The return periods for wind and wave differ because wave forces are assumed linear with wave height whilst wind forces vary
with the square of the wind speed, the probability of exceeding the characteristic wind speed therefore needs to be less than the
probability of exceeding the characteristic wave height to achieve the target probability limit for failure.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.3.8 If conditions are determined using the joint probability of different parameters, then the return
period should be increased by a factor of 4 i.e. 10 years to say 50 years and 50 years to 200 years, unless
the loadings are dependent on a single parameter in which case the value of that parameter shall be taken
from a joint probability combination in which it is maximised.
3.4.3.9 For voyages that are governed for ULS and ALS by a single sea area, the operation reference period
may be taken as 7 days to 1 month. For FLS the whole voyage shall be considered, see [11.9.12].
3.4.3.10 For voyages, the design extremes may be reduced below the required seasonal storm return period
(10 year for ASD/WSD or 1 year for LRFD), to give the same probability of encounter as a 30 day exposure
to a 10 year seasonal storm. In this case the 'adjusted' design extremes are defined in terms of the 10% risk
level, see [3.4.17.3]. The design extremes for weather unrestricted voyages shall not be reduced below the
one (1) year seasonal return for ASD/WSD or the three (3) month seasonal return for LRFD.
3.4.4 Return periods for determining environmental criteria for moorings
3.4.4.1 Table 3-2 identifies minimum return periods applicable to a various of mooring types for weather
unrestricted operations. The return periods specified in this document are based on ISO 19901-7 /100/,
however the selection of return period will depend on the choice of the design code (See [17.2.1] for
acceptable mooring codes) and the associated factor of safety. For weather restricted operations, see [3.3];
during installation [3.4.3] may be used. More onerous, local requirements can override the requirements
stated in Table 3-2, for example ISO 19901-7, Annex B.
Table 3-2 Return periods for determining environmental criteria for moorings
Mooring type
Quayside/inshore
Return period
Table 3-1 (LRFD)
Offshore - mobile near another asset
10 year
Offshore - mobile in open location
5 year
1)
1)
Where the unit is capable of leaving the quay on receipt of poor weather
forecast a lower return period can be acceptable.
3.4.4.2 For mobile moorings deployed for a duration extending beyond the inspection cycle of the
components of the mooring system, the system and its components should be assessed against the
requirements for designing a permanent mooring system.
3.4.4.3 Joint probability data should only be used when permitted by the referenced standard.
3.4.4.4 Mobile moorings should generally be designed with reference to a 10 year return period when in the
vicinity of any other infrastructure. Where a mobile mooring is in an open location, with reduced consequence
from mooring failure, a five year return period may be acceptable. Where applicable seasonal/monthly and/or
directional metocean data as in [3.4.5] can be used with the specified return period.
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3.4.4.5 When evaluating the consequence of failure, consideration should be given to whether risers will
be connected, proximity to other installations and the type of operation being undertaken. For pipe laying
operations, the expected duration of the operation, plus a suitable contingency value, should be addressed.
3.4.5 Use of seasonal/directional metocean data for moorings
3.4.5.1 Metocean data specific to the month(s) or season(s) during which the mooring will be utilised may
be used where appropriate.
3.4.5.2 Directional metocean data may also be used with suitable spreading functions to reflect directional
divergence in the design environment.
3.4.6 Wind
3.4.6.1 The averaged wind velocity over a defined time is referred to as the mean wind velocity. As the
mean wind velocity also varies with height above the sea surface or height above ground both the averaging
time and the reference height shall always be specified.
Guidance note:
Forecasted wind velocity is normally given as the 10 minute mean wind (tmean = 10 min) at a reference height of 10 m (z = 10 m).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.6.2 The wind velocity profile in open sea may normally be related to a reference height (H) and mean
time period (T0) according to the Frøya wind profile model /68/. For extreme mean wind speeds, the Frøya
model implies that the following expression can be used for conversion of the one-hour mean wind speed U0
at reference height H above sea level to the mean wind speed U with averaging period T at height z above
sea level, provided T is less than T0 (1 hour):
(3.1)
where:
z
H
T
T0
U(T, z)
= height above sea surface
U0
= 1-hour mean wind speed at height H above sea level in m/s
C
=
IU(z)
=
= reference height above sea level - usually 10 m
= averaging period for design (< 1 hour) in same units as T0
= 1 hour (reference averaging period)
= mean wind velocity with averaging period T at height z above sea level in m/s
for H=10m
.
Guidance note:
The Frøya wind profile model is valid for moderate and strong nearly neutrally stable atmospheric wind conditions. For moderate
conditions (U0 < 15 m/s - 20 m/s) and non-neutral stability conditions the wind profile can deviate significantly from the above. It
is not valid for the description of squall winds.
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3.4.6.3 For land-based, nearshore and (partly) sheltered inshore operations it is recommended that
location-specific wind velocity profiles are determined applicable to the wind velocities of interest, noting that
the profile can vary with wind direction.
3.4.6.4 For static wind force calculations, the characteristic averaging time shall be selected accounting
for the nature of the operation, the size of the structure involved and the response characteristics of the
structure to wind.
Guidance note 1:
The following averaging times are given as examples:
Averaging
Item/object description
Units
time
Local loads on individual components e.g. structural member, item of equipment
3
[s]
Structures with a maximum dimension < 50 m
5
[s]
Structures with a maximum dimension > 50 m
15
[s]
For any structure if wave loading dominates
1
[minute]
Quay mooring, small vessels/objects
15
[s]
Quay mooring, large (wind area > 2000 m ) vessels/objects
1
[minute]
Floating stability calculations, normally
1
[minute]
Catenary mooring of vessels/objects
10
[minutes]
Catenary mooring of GBS
60
[minutes]
2
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
OCIMF Mooring Equipment Guidelines, /113/, gives further guidance with respect to mean wind periods to be used for quay
mooring of vessels. For static wind calculations on lifted objects the recommendations for fixed structures above normally apply.
See also DNV-ST-0378 App.A, /129/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
3.4.6.5 For dynamic wind analysis the mean wind period recommended for the applied wind spectrum should
be used. See [3.4.6.7].
3.4.6.6 Gust wind: For elements or systems sensitive to wind oscillations (e.g. where dynamics or fatigue
governs the design) the short and long term wind variations should be considered.
3.4.6.7 The wind variations may be described by a wind spectrum. See e.g. DNV-RP-C205, /46/; NORSOK
N-003, /111/ or ISO 19901-1, /98/.
3.4.6.8 Squalls: If squalls are possible during a marine operation maximum realistic (in the actual area)
characteristic wind speeds during squalls shall be considered in the planning and execution of the operation.
Guidance note:
Squalls are strong winds (22 knots or more) characterised by a sudden onset, duration of minimum 1 minute, and then a rather
sudden decrease in speed. Squalls are caused by advancing cold air and are associated with active weather such as thunderstorms.
Their formation is related to atmospheric instability and is subject to seasonality.
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3.4.7 Wind for moorings
3.4.7.1 In addition to the requirements in [3.4.6], for permanent moorings the more onerous of the
following should be considered:
— Steady one minute mean velocity; or
— 1-hour mean plus a suitable gust spectrum. Generally the ISO 19901-1 gust spectrum, /98/, would be
applicable unless an alternative can be clearly justified.
3.4.7.2 For mobile moorings either a steady state wind speed or a suitable gust spectrum may be used
depending upon the stiffness of the mooring system.
3.4.7.3 For inshore or quayside moorings care shall be taken to ensure that all natural periods of response
of the system have been considered. Some of the mooring system response periods may be shorter than one
minute but on the other hand the use of shorter gust periods may not represent a sustained design wind that
will act at the same time across the whole of the structure. The representative design wind sampling period,
therefore, has to be carefully established on a case-by-case basis for inshore and quayside moorings, but the
averaging time shall not be longer than 1 minute if applying static wind load.
3.4.7.4 For locations prone to squall events, system design should include assessment for squall events.
Guidance on squall assessment is provided in DNV-OS-E301, /27/.
3.4.8 Waves - design methods
3.4.8.1 Wave conditions are defined by characteristic wave height, Hc, or the significant wave height, Hs, and
corresponding periods.
3.4.8.2 Wave conditions for design may be described either by a deterministic design wave method, or by a
stochastic method.
3.4.8.3 In the deterministic method the design sea states are represented by regular periodic waves
characterised by wave length (or period), wave height and possible shape parameters.
3.4.8.4 In the stochastic method the design sea states are represented by wave energy spectra
characterised by main parameters Hs and Tz or Tp.
3.4.9 Waves - weather unrestricted operations, general
3.4.9.1 Characteristic wave conditions for weather unrestricted operations shall be based on long term
statistical data.
3.4.9.2 Long term variations of waves may be described by a set of sea states characterised by the wave
spectrum parameters.
3.4.10 Wind seas and swell
3.4.10.1 All possible combinations of wind seas and swell should be considered.
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Guidance note:
The wave conditions in a sea state can be divided into two classes, i.e. wind seas and swell. Wind seas are generated by local
wind, while swell have no relationship to the local wind. Swells are waves that have travelled out of the areas where they were
generated. Note that several swell components may be present at a given location.
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3.4.11 Characteristic waves for weather unrestricted operations
3.4.11.1 Characteristic values shall be based on the defined operation reference period. Periods less than
3 days shall not be used. These can be based on the return periods given in Table 3-1 or Table 3-2 as
applicable. Alternatively, the characteristic significant wave height, Hs, c may be taken according to [C.3.2.1]
and the corresponding maximum wave height, Hmax, c, may be taken according to [C.3.2.2].
Guidance note 1:
The significant wave height
where m0 is the sea surface variance. In sea states with only a narrow band of wave
frequencies, Hs is approximately equal to
(the mean height of the largest third of the zero up-crossing waves).
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Guidance note 2:
The Hmax, c corresponds to an approximate 10% probability of exceeding this individual wave height during the anticipated
operation reference period. If an alternative method is applied it should be documented that this corresponds to an equal or less
probability.
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3.4.11.2 The range of wave periods to be analysed shall consider all probable wave periods including swell
(if applicable), see [3.4.14].
3.4.11.3 For operations involving phases sensitive to extreme sea states, such as temporary on-bottom
stability or green water assessment, the maximum wave height and associated period should be used.
3.4.11.4 For precise operations sensitive to small fluctuations of the sea level even under calm sea state
conditions, the occurrence of long period, small amplitude swell on the site should be checked and its effects
on the operations evaluated.
3.4.11.5 Attention should also be paid to areas prone to strong currents acting against the waves which
would amplify the steepness of the sea state (i.e. reduce the wave encounter period that drives dynamic
response).
3.4.11.6 Deterministic method (regular wave)
3.4.11.6.1 When a regular wave analysis is applicable, the design maximum wave shall be the most
probable highest individual wave in the design sea state, assuming an exposure of three (3) hours. The
determination of the height, crest elevation and kinematics of the maximum wave should be determined from
an appropriate higher-order wave theory and account for shallow water effects. For most practical purposes
the kinematics of regular deterministic waves can be described by the following theories:
h/λ ≤ 0.1
0.1 < h/λ ≤ 0.3
h/λ > 0.3
Solitary wave theory for particularly shallow water.
Stokes 5
th
order wave theory or stream function wave theory.
Linear wave theory (or Stokes 5
th
order).
where:
h
= water depth
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λ
= wave length.
Guidance note:
For more information on the kinematics of regular waves, see DNV-RP-C205, /46/.
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3.4.11.6.2 A range of wave height-period combinations shall be investigated, including those that can cause
resonance, see [C.3.3].
3.4.11.7 Stochastic method (irregular waves)
3.4.11.7.1 Sea states shall include all relevant spectra up to and including the design storm sea state for the
construction site or voyage route. Long-crested seas shall be considered unless there is a justifiable basis for
using short-crested seas or these are more critical, see [3.4.12]. Consideration should be given to the choice
of spectrum.
3.4.11.7.2 Wave spectra defined by the Jonswap or the Pierson-Moskowitz spectra are most frequently used.
Wave conditions with combined wind sea and swell may be described by a double peak wave spectrum. See
DNV-RP-C205, /46/, for further guidance.
Guidance note:
See DNV-RP-C205 [3.5.5.3] /46/, for when the JONSWAP spectrum is expected to be a reasonable model. See DNV-RP-C205
[3.5.5.5], /46/, for use of the JONSWAP spectrum for pure swell seas.
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3.4.11.7.3 When using stochastic/irregular seas for weather unrestricted operations the range of wave
periods to be considered may generally be selected based on one of the following methods:
1)
2)
3)
The Hs-Tp contour for the design return period (see [3.4.11.7.4])
The scatter diagram for actual location (see [3.4.11.7.5])
The equations in [3.4.11.7.6]
Guidance note:
Methods 2 and 3 incorrectly assume that all periods are equally probable. As a result these methods should generally produce
higher design responses than would be the case when using the more robust Hs-Tp method described in [3.4.11.4.4]
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3.4.11.7.4 In the IFORM method a contour is constructed within the Hs-Tp plane that identifies equally
probable combinations of Hs and Tp for the design return period. This contour should also cover swell. The
contour should be checked for accuracy e.g. against the theoretical constraints on wave breaking. Hs-Tp
combinations from around the contour should be tested in motion response calculations to identify the worst
case response (there is no need to consider a range of Tp with each Hs).
3.4.11.7.5 If a suitable scatter diagram of the location(s) is available then this may be used to define the
range of wave periods for the design seastate. The range of periods should be extended to consider lower
wave heights if these periods could be critical e.g. cause resonance.
3.4.11.7.6 In the absence of other data, the following equations may be used to define the range of periods:
for Hs≤ 5.7 m
for Hs> 5.7 m
where:
Hs
= significant wave height in metres
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Tz
g
= zero up-crossing period in seconds
= acceleration due to gravity in m/s.
If justified, provided the route or site-specific data and natural periods allow a reduced range of wave periods
may be used.
Guidance note 1:
For spectral peak periods, the equivalent above range can normally be taken as:
for Hs≤ 5.7 m
for Hs> 5.7 m
where:
Hs =
significant wave height in metres
Tp =
zero up-crossing period in seconds
g =
acceleration due to gravity in m/s.
These equations assume that Tp = 1.24Tz for steep waves (gamma = 5) and Tp = 1.4Tz for long waves (gamma = 1.0). The
relation between zero-crossing period Tz and the spectral peak period Tp can be found in DNV-RP-N103 [2.2.6], /56/ or DNV-RPC205 [3.5.5], /46/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The range of periods defined by the above sets of equations (i.e. those for both Tz and Tp) can lead to
values outside
those for which the JONSWAP spectrum is expected to be a reasonable model, see DNV-RP-C205 [3.5.5.3], /46/, and therefore
should be used with caution when outside this range.
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3.4.11.7.7 The relationship between the peak period Tp and the zero-up crossing period Tz is dependent on
the spectrum. For a mean JONSWAP spectrum (γ=3.3) Tp/Tz = 1.286; for a Pierson-Moskowitz spectrum
(γ=1) Tp/Tz = 1.41.
3.4.12 Short crested seas
3.4.12.1 A directional short crested wave spectrum, see the equation below, may be applied based on nondirectional spectra.
where:
= wave spectrum, see [3.4.11.7]
θ
= angle between direction of elementary wave trains and the main direction of the short crested
wave system
= directional short crested wave power density spectrum
= directional function.
3.4.12.2 Energy conservation requires that the directional function fulfils;
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In absence of more reliable data the following directional function may be applied for wind sea,
where:
Γ( )
= gamma function. Due consideration should be taken to reflect an accurate correlation between the
actual sea-state and the constant n. Typical values for wind seas are n = 2 to n = 10. Swell should
normally be taken as long crested, n > 10.
Guidance note:
For cases where long crested seas are conservative, it is recommended that long crested seas are used for the original design
work. If short crested seas are introduced in connection with estimating extremes, the exponent, n, should not be taken lower
than 10 without more detailed documentation. Swell seas should be taken as long crested. For fatigue assessment, where low and
moderate sea states are governing the fatigue accumulation, n could be taken as the most unfavourable value between 2 and 6.
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3.4.12.3 Short crested seas should not be considered for significant wave heights exceeding 10 m, unless
they cause more onerous response(s).
3.4.13 Waves for moorings
3.4.13.1 In addition to the requirements in [3.4.8], for mobile moorings it is generally acceptable to
consider a single extreme significant wave height and a range of associated peak periods corresponding to
the relevant return period for a location.
3.4.13.2 For permanent moorings a number of Hs-Tp combinations along the 100 year return period contour
line shall be considered in the analysis. If a contour plot is not available, a sensitivity study by varying peak
period for the 100 year return period sea state is required. This is to ensure that extreme line tensions due to
low frequency motion at lower periods are captured in the analysis, especially for ship shaped floaters.
3.4.13.3 Long crested waves shall be assumed for analysis unless otherwise documented.
3.4.14 Swell
3.4.14.1 Swell type waves should be considered for operations sensitive to long period motion or loads.
3.4.14.2 Swell type waves may be assumed regular in period and height, and may normally also be
assumed independent of wind generated waves.
3.4.14.3 Critical swell periods should be identified and considered in the design verification.
3.4.14.4 Characteristic height(s) and period(s) for swell type waves for weather restricted operations may
be selected independently of statistical data.
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3.4.14.5 Characteristic height(s) and period(s) for swell type waves for weather unrestricted operations
should be based on statistical data and the applicable return periods.
3.4.15 Current
3.4.15.1 The design current shall be the rate at mean spring tides, taking account of variations with depth
and increases caused by the design environmental condition, storm surge, fluvial (river) and wind-driven
components.
3.4.15.2 Currents can be divided into two different categories:
— Tidal currents.
— Residual currents that remain when the tidal component is removed, including river outflows, surge, wind
drift, loop and eddy currents.
3.4.15.3 Tidal currents can be predicted reliably, subject to long term measurement (at least one complete
lunar cycle at the same season of the year as the actual planned operation). Residual currents can only be
reliably predicted or forecast using sophisticated mathematical models.
3.4.16 Other parameters
Other factors including the following may be critical to the design, operations or voyages and should be
addressed:
— water level including tide and surge
— sea icing, icing on superstructure
— exceptionally low temperature
— large temperature differences
— water density and salinity
— bad visibility.
3.4.17 Calculation of adjusted design extremes, weather unrestricted
voyages
3.4.17.1 The risk of encounter of extreme conditions on a particular voyage is dependent on the length of
time that it spends in those route sectors where extreme conditions are possible. If the length of time is
reduced, then the probability of encountering extreme conditions is similarly reduced.
3.4.17.2 It is generally accepted that for a prolonged weather unrestricted voyage the wind and wave design
criteria should be those with a probability of exceedance per voyage of 0.1 or less. For a voyage of 30 days
(or more), through meteorologically and oceanographically consistent areas, this corresponds to the 10 year
monthly extreme.
3.4.17.3 Many voyages last less than 30 days, or are potentially exposed to the most severe conditions for
less than 30 days. Consequently, for shorter exposures, the 10 year monthly extreme may be adjusted for
reduced exposure. This value is equivalent to the 10 voyage extreme and is also referred to as the 10% risk
level extreme. This shall not be confused with the 10% exceedance value for the voyage, as discussed in
[3.4.19.6].
3.4.17.4 When the 10% risk level extremes are less than the 1 year return monthly extremes, the 1 year
monthly extremes are the minimum that shall be used for design.
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3.4.17.5 If the 10 year extremes are due to a tropical cyclone it may not be appropriate to design to
adjusted extremes. This is likely to be the case for barge or MODU towages that are not able to respond
effectively to weather routeing.
Guidance note:
For transports (as defined [1.6]) to avoid TRS activity in otherwise benign areas (see [3.6]), a calm-water vessel speed of 12
knots accounting for the effects of the vessel’s cargo(es) is normally regarded as the minimum safe-side limit that makes routeing
possible. The actual vessel speed required should be determined via a risk assessment accounting for the route; proposed weather
routeing, forecasting and monitoring; the vessel/cargo combination and its seakeeping ability; and anything else pertinent.
For weather routeing in other areas see [11.14.4.2].
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3.4.18 Calculation of exposure
3.4.18.1 For the purpose of the calculation of 'adjusted' extremes the exposure time to potentially extreme
or near extreme conditions is calculated taking consideration of the following points:
— The initial 48 hours of the voyage is assumed to be covered by a reliable departure weather forecast and
is excluded.
— The speed of the voyage is reduced by taking the monthly mean wave heights along the route into
consideration as described in [3.4.18.3].
— The speed of the voyage is adjusted to take into consideration the mean currents as described in
[3.4.18.4].
— A contingency time of 25 per cent of the time is added. This allowance is to account for severe adverse
weather, for tug breakdowns or other operational difficulties.
— A minimum exposure time of 3 days is considered.
3.4.18.2 The voyage duration in each route sector shall be calculated using the speed in the monthly mean
sea state for each route sector and shall allow for adverse currents and adverse prevailing winds as described
in [3.4.18.3].
3.4.18.3 The effect of the mean sea state on the voyage speed in each route sector shall be calculated as
a function of the wave height in which the voyage is assumed to come to a dead stop, b (metres). This can
typically be taken 5 m for barge towages, and 8 m for ships. The speed in the each route sector can be taken
as the calm weather speed is multiplied by the factor, F, for that route sector defined by:
where Hm is the monthly mean wave height in that route sector.
3.4.18.4 The effect of the mean current on the voyage speed in each route sector shall be calculated by
adding the current vector (resolved with respect to the voyage heading).
3.4.18.5 For the calculation of exposure to the extreme conditions only prevailing winds or currents which
act to delay the voyage shall be taken into account.
3.4.19 Calculation of adjusted extremes
3.4.19.1 The probability of non-exceedance of a value of wind speed or significant wave height in a
particular route sector is expressed as a cumulative frequency distribution (e.g. a Weibull distribution).
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3.4.19.2 The probability that during some 3 hour period for waves (or 1 hour for wind) the voyage will
experience a significant wave height (or wind speed) less than some value x is given by Fx(X).
3.4.19.3 If it takes M hours to pass through the route sector and making the assumption that consecutive
wave height and wind speed events are independent then the probability of not exceeding the value x is
given by
where N = M/T where T = 1 hour is applied for winds and T = 3 hours for waves, which are a more persistent
form of energy.
3.4.19.4 If it is reasonable to expect that extremes of wind speed or wave height could occur in more than
one route sector then the probability of not exceeding the value x is given by the product
3.4.19.5 The probability of encountering an extreme value of wind speed or significant wave height, during
a particular voyage, that is reached or exceeded once on average for every 10 voyages, is 0.1. The value of x
is varied until
to give the 10 voyage extreme for the voyage or towage.
3.4.19.6 This value is also referred to as the 'adjusted' extreme for the voyage, or as having a risk level of
10%. The method can be adjusted to give other risk levels (e.g. 1% or 5%). This should not be confused
with the percentage exceedance (see guidance note to [3.4.19.7]).
3.4.19.7 The extremes used for design shall not be less than the 1 year return monthly extremes.
Guidance note:
The percentage exceedance is obtained as follows:
—
Given a series of values of wind speed or significant wave height, as may be observed during a complete voyage, some value y
will be exceeded at some times but not others and the percentage exceedance of value y is equal to:
—
If each observed value of wind speed or significant wave height is assumed to last for some duration (typically 1 hour for winds
and 3 hours for waves) then for example, during a voyage lasting 10 days there will be 240 wind events and 80 wave events.
On the voyage, if a wind speed greater than 30 knots is observed during 24 separate, hourly occasions then the percentage
exceedance of 30 knots is 10%.
—
The 10% risk level (as defined in [3.4.17.3]) for a voyage along a specific route, departing on a specific date is expected to
occur only once, on average, in every 10 voyages. However a value with a 10% exceedance level for the same route and
departure date is likely to occur on average for 10% of the time on every voyage.
—
Thus a 10% exceedance value is far more likely to occur than a 10% risk level value, or an adjusted, 10 year extreme value.
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3.4.20 Criteria from voyage simulations
3.4.20.1 If continuous time series of winds and waves are available along the entire voyage route (e.g.
from hindcast data or satellite observations), an alternative way to develop criteria with a specified risk
of exceedance in a single voyage is to perform tow simulations. A large number of simulations can be
performed, with uniformly spaced (in time) departure times during the specified month of departure over the
number of years in the database. For each simulated voyage, the maximum wind speed and the maximum
wave height experienced somewhere along the tow route are retained. Then the probability distribution of
these voyage-maxima can be used to determine the design value with a specified risk of exceedance. For
example, the value exceeded once in every 10 voyages, on average, can be determined by reading off the
th
value of wave height from the distribution of voyage-maximum wave heights at the 90 percentile level.
3.4.20.2 If fatigue during tow is an issue, the complete distributions of winds and waves experienced during
the simulated voyages (not just the voyage-maximum values) can be retained. These can be used to give
scatter diagrams of wave height against period and/or direction, and wind speed against direction.
3.4.20.3 The voyage simulation method can be made to be very realistic and account for variation of speed
due to inclement weather or ocean currents, weather avoidance en-route through forecasting/routeing
services, or the use of safe havens, etc. If the voyage simulator cannot accommodate all these features, a
reasonably conservative estimate of criteria can be derived by using a conservative (slow) estimate of the
average speed. Care should be taken when choosing the average speed estimate - a slow speed may not
be conservative if it results in the vessel apparently arriving in a route sector late enough to miss severe
weather, which might have been encountered if arrival had been earlier.
3.4.21 Metocean database bias
Regardless of whether the method described in [3.4.19] or the method described in [3.4.20] is used, it is
important to know the accuracy of the metocean database being used. Specifically, if there is a known bias in
the wind or wave statistics for any segment of a tow, it is essential to adjust the criteria accordingly.
3.4.22 Metocean data for bollard pull requirements
The design extremes are not normally used for calculation of bollard pull requirements except when there is
limited sea room. Bollard pull requirements are covered in [11.12.2].
3.5 Weather/metocean forecast requirements
The requirements for weather forecasting are given in [2.7] and the requirements for environmental
monitoring in [2.9].
3.6 Benign weather areas
Areas considered benign are shown in Table 3-3 and Figure 3-1 for different months. In general they have
the following characteristics:
— virtually free of monsoons, tropical revolving storms or tropical cyclones
— exceeding Beaufort Force 5 for < 20% of any month (in a 'typical' year), see DNV-ST-0111 Table 2-1 /22/
— these areas may experience sudden vicious squalls and very rare tropical storms or cyclones.
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Table 3-3 Northern and Southern boundaries of benign weather areas by month
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Figure 3-1 Map showing benign weather areas
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SECTION 4 BALLAST AND OTHER SYSTEMS
4.1 Introduction
This section includes general requirements to system and equipment design. It covers all (temporary)
systems, see [4.2.1.7], used during marine operations, with emphasis on ballast systems.
4.2 System and equipment design
4.2.1 General
4.2.1.1 Systems and equipment shall be designed, fabricated, installed, and tested in accordance with
relevant codes and standards.
4.2.1.2 Systems and equipment shall, as far as possible, be designed to be fail-safe and arranged such that
a single failure in one system or unit cannot spread to another unit. The most probable failures, e.g. loss of
power or electrical failures, shall result in the least critical of any possible new conditions.
4.2.1.3 Alarm system(s) should be incorporated for essential functions and be audible/visible at operators’
station.
4.2.1.4 Work stations shall be arranged to provide the user with good visibility and easy access to controls
required for the operations.
4.2.1.5 Systems and equipment shall be selected based on a thorough consideration of functional and
operational requirements for the complete operation. Emphasis shall be placed on reliability and the expected
behaviour in possible contingency situations.
4.2.1.6 Depending on the complexity and duration of the operation, and the structure itself, risk evaluations
may be required to determine the systems and equipment required for a safe operation, see [2.4.2]. Such
studies shall include normal operations as well as emergency situations.
4.2.1.7 The following systems shall be considered where applicable:
1) power supply
2) fuel supply
3) electrical distribution systems
4) machinery control systems
5) alarm systems
6) valve control systems
7) bilge and ballast systems
8) compressed air systems
9) firefighting systems
10) cooling systems
11) ROV systems
12) lifting systems
13) positioning systems, see guidance note
14) communications systems
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15) instrumentation systems for monitoring of:
— loads and/or deformations
— environmental conditions, such as tide
— ballast and stability conditions
— heel, trim, and draught
— position (navigation)
— tide
— under-keel clearance and
— penetration/settlements.
Guidance note:
Object guiding and positioning systems, including structural and functional requirements are covered in [4.4]. If applicable,
the requirements in [4.2] should be considered regarding mechanical parts and operation of such systems. Vessel position
systems are described in Sec.17.
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4.2.1.8 Computerised control or data acquisition systems should be equipped with uninterruptible power
supply system (UPS).
4.2.1.9 All systems shall be inspected and tested according to [2.10].
4.2.1.10 Where a permanent system is complimented by a temporary system, the integration of the two
systems shall be inspected and tested according to [2.10].
4.2.2 Back-up
4.2.2.1 All essential systems, parts of systems or equipment shall have back-up or back-up alternatives.
Necessary time for a change over to the back-up system or equipment shall be assessed.
Guidance note:
It is recommended that the marine operation manual includes an inventory of main spare parts available on site. It is also
recommended to assess the necessity of having repair or service personnel available on site during operations.
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4.2.2.2 All back-up systems should be designed and fabricated to the same standard as the primary
systems.
4.2.2.3 Back-up systems should be adequately separated from the main system such that failure of any
component does not adversely affect the safe conduct of the operation.
4.2.2.4 For systems consisting of multiple independent units, back-up may be provided by having a sufficient
number of available spare units available on site.
4.2.2.5 If umbilicals are necessary to provide power and/or hydraulic services during any marine operation,
adequate back-up capability shall be provided, and fail-safe systems shall be incorporated into critical
controls.
4.2.2.6 Automatic control systems shall be provided with a possibility for manual overriding.
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4.3 Ballasting systems
4.3.1 General
4.3.1.1 This subsection is mainly applicable for ballasting and de-ballasting of vessel(s) involved in load
transfer operations.
Guidance note:
See [11.15] regarding pumping capacity requirements during voyages and [11.14.18] for planning related to ballast water. For
jacket installations additional requirements apply, see [13.7.2]. For ballasting of (crane) vessels during lifting, see Sec.16.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.1.2 Regardless of any requirement to change draught during construction, towage or installation
operations, floating structures should normally be fitted with a means of pumping out water from all
compartments.
4.3.1.3 The (de)ballasting system design shall properly consider the operation class (see [4.3.3]) as well as
functional requirements related to:
— lay-out and reliability of the system
— tank capacities including contingency situations
— ballasting capacity including contingency situations
— strength limitations
— easily controllable ballasting
— tide.
4.3.1.4 General requirements to (de)ballasting systems are given in [4.2.1].
4.3.1.5 Adequate testing of the ballast system considering the actual operation shall be carried out, see
[2.10].
4.3.2 Ballast system power supply
4.3.2.1 Adequate power supply considering the actual operation shall be provided for the ballast system.
4.3.2.2 The need for emergency power supply due to the following situations shall be considered:
a)
b)
c)
Breakdown of any one power unit.
Breakdown of the common energy supply.
Unexpected increase in the consumption of energy above the expected value.
Guidance note:
The back-up capacity for accidental conditions represented by a) and b) may be spare units in stand-by position. The back-up
capacity for conditions represented by c) may be spare capacity in the main unit or a back-up unit installed to assist the main
unit.
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4.3.2.3 Sufficient main and back-up power supply capacity should be documented by calculations.
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Guidance note:
Necessary power supply for ballasting should be based in the required ballasting capacity given in Table 4-2 for the relevant loadout class. For evaluations of back-up requirements, an independent power supply source should be regarded as a 'pump system',
see Note 3) in Table 4-2.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.3 Operation classes
An operation class should be defined for load transfer operations see Table 10-1 for load-outs and Table 15-1
for lift-off, mating and float-over operations.
4.3.4 Ballast system lay-out and reliability
4.3.4.1 The ballast pumps may be the vessel’s internal pumps, pumps purposely installed for the operation/
project, or a combination of these. Internal vessel pumps that are not frequently in use, as the primary
pumping means, should be carefully considered and demonstrated fit for purpose.
Guidance note:
Internal vessel pumps can have unreliable service records. Also, permanent piping systems are inherently inflexible.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.4.2 Where accurate control of the ballast amount is crucial, ballasting by flooding (i.e. opening of bottom
valves) and/or de-ballasting by air pressurisation (or ballasting by vacuum – low pressure) of ballast tanks
shall be avoided during load transfer phases.
Guidance note:
Ballasting by flooding during load transfer phases where accurate control of ballast amount is crucial may be allowed if the system
has sufficient redundancy (e.g. double valves to compensate a failure to close a valve) and/or back-up ballast plans are available
where mechanical failures can be compensated by an alternative ballast procedure.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.4.3 Umbilicals used for air pressurisation of submerged vessel compartments should be connected to
valves at the vessel tanks.
4.3.4.4 Where a compressed air system is used, the time lag needed to pressurise or de-pressurise a tank
should be taken into account, as should any limitations on differential pressure across a bulkhead. It should
be remembered that compressed air systems cannot always fill a tank beyond the external waterline. Air
pressurised vessel tanks shall be fitted with safety (pressure relief) valves.
4.3.4.5 Hoses, umbilicals and power cables shall be placed with due consideration to other ongoing activities
during the load transfer.
4.3.4.6 Required access throughout the load transfer for (possibly) needed equipment, e.g. fork lifts for
replacing pumps, should be demonstrated.
4.3.4.7 All internal compartments shall be cleaned of debris before ballasting starts.
4.3.4.8 When inlets are near the seabed, care shall be taken to avoid sucking in mud or sand that can block
the pumping systems or filters.
4.3.4.9 Where inlets or outlets are near the seabed, care shall be taken to avoid scour that could have
adverse effects on foundations of any structure or grounded vessel, or reduce under-keel clearances.
4.3.4.10 Except when in use for inlet or discharge, all openings to sea shall be protected by a double barrier.
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4.3.4.11 Any external valves and pipework shall be protected against collision and fouling by towlines,
mooring lines or handling wires.
4.3.4.12 All essential pipework in temporary systems should be of permanent-type construction and shall be
hydrostatically tested to a minimum of 1.3 times the design pressure. Temporary flexible hoses shall only be
used when a risk assessment, in accordance with [2.4], demonstrates the acceptability of the system.
Guidance note:
For offshore operations temporary flexible hoses are not generally permitted unless their use cannot be avoided, for instance for
supply of back-up compressed air from a compressor barge alongside.
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4.3.4.13 Permanent-type ballast systems used in marine operations should fulfil the class requirements for
construction and testing.
Guidance note:
For permanent ballast systems not subject to class approval the requirement in [4.3.4.12] apply.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
4.3.5 Ballast tank capacity
4.3.5.1 The ballast tanks shall meet the capacity requirements in Table 4-1 for the required floating
position(s) throughout the operation for both planned and contingency situations.
4.3.5.2 A reasonable amount of residual water in the tanks should be taken into account.
Guidance note:
The amount to be considered will depend on details and location of the pumping intake(s), heel/trim of the vessel and structural
elements at the tank bottom. For tanks in use during the load transfer without any special arrangements allowing easy tank
stripping, the minimum water head should be taken equal to the height of the tank bottom stiffeners plus 0.05 metres.
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4.3.5.3 The required tank capacities should include relevant spare capacity to compensate for the following:
a)
b)
c)
d)
Tide levels below or above the predicted values.
Total vessel weight, including vessel lightship, consumables and temporaries (e.g. project equipment,
grillages), being higher or lower than expected.
Possible object weight and CoG variations.
Operational delays.
Table 4-1 Tank capacity requirements
Operation
class
All
The tank capacity shall be adequate for the following scenarios
(see Table 10-1 for load-out classes and Table 15-1 for float-ons and float-offs)
— Normal (planned) operations.
— Spare tank capacity to cover items [4.3.5.2] and [4.3.5.3] shall be ensured in all situations.
— Any necessary pumping capacity contingency involving modifications in ballasting procedures.
See Table 4-2.
1
— See All.
— Reversing of the operation. Tide compensation if stop of load transfer, considering maximum
possible (defined) duration of the load transfer.
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2
— See All.
— Ballasting through a complete tide cycle at any stage of the load transfer. Maximum tide
variations within the operation period (TR) shall be considered. Reversing of the operation, if
applicable.
3
— See All.
— Ballasting through a complete tide cycle at any stage of the load transfer. Maximum tide
variations for at least the coming 3-5 days shall be considered.
4
— See All.
— Reversing of the operation, if applicable.
5
— See All.
4.3.6 Ballast pumping capacity
4.3.6.1 The ballast pumping capacity shall meet the capacity requirements in Table 4-1 for the required
floating position(s) throughout the operation for both planned and contingency situations. Pump capacity
shall be based on the published pump performance curves, taking account of the maximum head for the
operation and pipeline losses.
Guidance note:
Compressed air systems may be included in calculating pumping capacity subject to adequate demonstration of the system.
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4.3.6.2 Adequate capacity shall be documented considering the requirements to nominal, spare and back-up
capacity given in this sub-section.
4.3.6.3 The nominal ballasting capacity shall be determined by the worst combination of expected tide rise/
fall and planned load transfer velocity.
4.3.6.4 For operation classes 2 and 3, it shall be documented that the ballast systems have capacity to
compensate for the tide rise/fall through one complete tide cycle with the object in any position.
Guidance note:
If the tidal range increases in the days following the planned operation start, this should be considered when evaluating the
consequences of a delayed start or delays during the operation.
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4.3.6.5 Pumps which require to be moved around the barge in order to be considered as part of the back-up
capacity, shall be easily transportable, and may only be so considered if free access is provided at all stages
of load-out between the stations at which they may be required. Adequate resources shall be available to
perform this operation.
4.3.6.6 Spare pumps should normally be installed and tested in the position they are planned to be used
as back-up. However, for pumps that may be replaced during the operation, spare pumps in stand-by
position that require minimum replacement time may be used. Required number of spare pumps should be
conservatively assessed. The replacement time shall be documented. See [4.3.4.6].
4.3.6.7 Requirements for minimum total ballasting capacity, including back-up, are given in Table 4-2,
including the notes.
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Table 4-2 Ballast pumping capacity requirements
Normal operation
Load transfer as planned
Tide compensation
Load transfer unexpectedly stopped
1
Minimum 200% capacity with intact system and
minimum 120% capacity in all tanks with any one
pump system failed.
Minimum 150% capacity with intact system and
minimum 100% capacity in all tanks with any one
pump system failed.
2
Minimum 130% capacity with intact system and
minimum 100% capacity in all tanks with any one
pump system failed.
As for class 1.
3
Minimum 130% capacity with intact system and a
contingency plan covering pump system failure.
As for class 1.
4
As for class 2.
No requirements.
5
As for class 3.
No requirements.
Operation
class
1)
100% pump capacity during normal operation is the capacity required to carry out the operation at the planned
speed. The required pump capacity for a reduced speed could be acceptable as '100%', if ballast calculations
are documented for this case, and the impact of the increased activity duration is properly taken into account.
2)
100% pump capacity during tide compensation is the capacity required to compensate for the maximum
expected tidal rate of change.
3)
A pump system includes the pump(s) which will cease to operate due to a single failure in any component.
4)
The back-up requirement X% capacity in all tanks could be covered by a modified ballast procedure giving X%
capacity in all tanks involved in this modified procedure.
4.3.7 Vessel strength considerations
4.3.7.1 All ballast conditions shall be checked against longitudinal strength requirements. Any hull beam
strength limitations shall be considered in the ballast procedure.
4.3.7.2 The effect of hull beam deflections on the object support load distribution shall be considered, see
[5.6.11].
4.3.7.3 Differential pressures across bulkheads shall be demonstrated to be within allowable values.
4.3.7.4 Any restrictions, e.g. any requirement to mimic the vessel voyage condition, on ballast condition(s)
during welding of seafastening shall be considered.
4.3.7.5 Possible significant strength reduction due to cut outs (e.g. for ballast hoses, pumps or other
equipment) in structural elements shall be considered.
4.3.8 Ballasting control
4.3.8.1 A straightforward ballasting control system and procedure shall be used.
Guidance note:
It is recommended that it is possible to operate the ballast pumps from one control centre during operation. For multi barge
operations, a control centre on each barge may be applicable. However, the control centre at one of the barges should be defined
as the master ballast control centre. The arrangement should be such that simultaneous de-ballasting can be effected for all the
relevant tanks at each stage.
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4.3.8.2 It shall be thoroughly documented how the ballasting will be done (controlled) for all possible
combinations of tide level and load transferred.
4.3.8.3 Each tank should preferably be used to compensate one effect (see guidance note) only. To use a
system/tank for compensation of more than two effects shall be avoided.
Guidance note:
In order to maintain maximum control with the ballasting it could be advisable to use separate systems/tanks for compensation of
the effects of tide variation, weight transferred, and CoG position in both directions (trim and heel).
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4.3.8.4 A ballasting control monitoring system including back-up shall be established. A communication
system shall be established when pumps are operated manually away from the control centre.
4.3.9 Ballast calculations
4.3.9.1 Ballast calculations shall be carried out in order to establish required nominal capacity (i.e. the 100%
capacity, see Note 1 in Table 4-2) pumping capacities.
4.3.9.2 For ballast calculations the expected CoG and weight without any contingencies should normally be
used as the base case. However, the effect of possible weight and CoG variations shall be considered, see
[5.6.2].
4.3.9.3 The ballast calculations shall include sufficient steps to accurately define the required ballasting
throughout the (load transfer) operation.
4.3.9.4 All considered contingency situations shall also be covered with an adequate number of ballast
calculation steps.
4.3.9.5 The results of the ballast calculations, i.e. required pumping in all steps, shall be clearly outlined in
ballast procedure(s).
4.3.10 Contingency and back-up
4.3.10.1 Means for adequate handling of all ballast system contingencies identified in the risk management
process shall be provided.
4.3.10.2 The contingencies indicated in Table 4-3 shall be considered. Minimum requirements to back-up
have also been indicated.
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Table 4-3 Contingency requirements
Contingency situation
No.
Minimum requirement
1
Tidal velocities above (or below) the predicted values.
Spare pump(s) or spare capacity in the main
pump(s). See Table 4-2 for specific requirements.
2
Unplanned stops in load transfer (e.g. object
movement stopped due to repair work)
Adequate tank and pump capacities to handle the
situation. See Table 4-1 and Table 4-2 for specific
requirements.
3
Reversal of operation, if required.
Ballast procedures/calculations with corresponding
pump lay-out and tank capacities for this case shall be
documented.
4
Reduced pump capacity.
Spare pump capacity. See Table 4-2 for specific
requirements in %.
5
Breakdown of ballast pump(s).
Spare pump(s) or spare capacity in the main
pump(s). See Table 4-2 for specific requirements.
6
Breakdown of power supply, including cables.
Back-up required, see [4.3.2.2], or adequate pump
capacity, see Table 4-2, considering any power supply
unit failed shall be documented.
7
Failure of any control panel/switchboard.
8
Failure of any ballast valve or hose/pipe.
Sufficient back-up to fulfil the requirements in Table
4-2 for one pump system failure. Alternative pump/
valve control methods (locations and procedures)
could also be accepted as back-up. See notes below.
1)
All remotely controlled valves shall be capable of operation by a secondary, preferably manual system. Any
automatic or radio controlled system shall have a manual override system.
2)
The secondary valve operation system may be by ROV, provided that ROV access and a suitable ROV are
available at all stages of the operation. The time for the ROV to get to and operate the valve shall ensure that
the valve can be operated before the flow through it is critical.
4.4 Guiding and positioning systems
4.4.1 General
4.4.1.1 This sub section applies for design and verification of (object) guiding and positioning systems to be
used for marine operations.
Guidance note 1:
Guiding systems are often designed with a primary and secondary system. The primary system is normally designed to absorb
possible impact energy, and provide guiding onto the secondary system. The secondary system is normally designed to ensure
accurate and controlled positioning of the object.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Additional operational specific guidance and requirements to guiding and positioning systems for lifting may be found in [16.14].
Requirements to positioning systems for vessels are given in Sec.17.
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4.4.1.2 Guides and bumpers shall have sufficient strength and ductility to resist impact and guiding loads
during positioning without causing operational problems (e.g. excessive positioning tolerances), and without
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overloading members of the supporting structure. Plastic deformation of guides and bumpers due to impact
loads may be allowed. The possibility and consequences of multiple impacts shall be considered.
4.4.1.3 After the design impact(s), guides and bumpers shall be able to resist loads due to the
environmental conditions during operation, and operational loads from tugger lines, mooring lines etc.
4.4.1.4 After the design impact(s), guides and bumpers shall also provide a positive clearance towards
neighbouring and supporting structure, and maintain their functionality.
4.4.1.5 DNV-RP-N102 [3.3.5], /55/, contains more recommendations and guidelines especially related to
guiding systems used during removal of offshore platforms.
4.4.1.6 The stiffness of bumper and guide members should be as low as possible, in order that they may
deflect appreciably without yielding.
4.4.1.7 Design of bumpers and guides should cater for easy sliding motion of the guide in contact with a
bumper. Sloping members should be at an acute angle to the vertical. Ledges and sharp corners should be
avoided in areas of possible contact, and weld beads should be ground flush.
4.4.1.8 As-built bumper and guide dimensions shall be documented.
4.4.2 Characteristic loads
4.4.2.1 Characteristic impact loads for bumpers should be based on impact and deformation energy
considerations. Alternatively for lifts in air only, the characteristic guide loads may be calculated according to
the simplified method in [16.14.5].
4.4.2.2 Realistic impact velocities, impact positions and deformation patterns shall be assumed.
4.4.2.3 Characteristic loads for the guiding and positioning phase shall be based on environmental conditions
during operation, in addition to operational loads from tugger lines, mooring lines etc.
4.4.2.4 Combination of horizontal and vertical loads during guiding shall be considered in the design load
cases. Realistic friction coefficients shall be used.
4.4.2.5 Characteristic loads for positioning lines (tugger lines, mooring lines, etc.) and attachments
(padeyes, brackets etc.) shall be the expected maximum line tension. Possible dynamic effects shall be
considered.
4.4.2.6 The characteristic loads shall be used as the basis for determining the maximum entry speed of the
lifted object into the guiding system.
4.4.3 Design verification
4.4.3.1 Structural strength of guiding and positioning systems should be verified according to Sec.5.
4.4.3.2 The connection into the object and the members framing the bumper or guide location should be at
least as strong as the bumper or guide.
4.4.3.3 The bumpers and guides shall be designed as either:
— to the ASD/WSD approach LS2
— to the LRFD approach ULS. The ULS load factor of 1.3 may be reduced to 1.1 when characteristic loads
based on Table 16-6 are used.
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4.4.3.4 To avoid overloading the supporting structure it shall be designed for 1.3 times the strength of
the attached bumpers and/or the design loads for the attached bumpers and/or guides from [4.4.3.3] and
checked either:
— to the ASD/WSD approach LS2
— to the LRFD approach ULS.
4.4.3.5 Positioning padeyes should be designed to behave in a ductile manner in case of overloading.
4.4.3.6 Submerged brackets or padeyes shall be arranged such that failure will not breach any tank or
compartment.
4.4.4 Accidental limit state conditions
4.4.4.1 If greater impact loads (velocities) than used in the ULS verification are considered possible, the
guide system should be verified for accidental limit state (ALS).
4.4.4.2 If the ALS (impact) load considered can cause failure (extensive damage) in the guiding system, it
should be documented that installation of the object still will be feasible. Alternatively it should be possible to
reverse the operation and return the object to a safe condition.
4.4.5 Position monitoring systems
4.4.5.1 The positioning equipment system accuracy and redundancy shall be specified. System accuracy
shall be suitable for congested areas or where dimensional tolerances become tighter, e.g. for tie-ins, capture
of docking piles.
4.4.5.2 System redundancy shall be in accordance with [4.2.1.10] appropriate to safety criticality and
operational criticality requirements.
4.4.5.3 Sub-surface positioning of ROV’s or other targets shall interface with the surface positioning system
and should display on the same equipment. Subsea acoustic transceivers/beacons shall be separately
identifiable and on coordinated channels. Survey systems using line-of-sight shall recognise and cater
for crossing surface vessels possibly occluding the system. Survey systems should be commissioned and
calibrated before start of installation operations.
4.4.5.4 Normally, two independent on board positioning monitoring systems (PMSs) shall be utilized for
operational monitoring and control purposes. Both systems shall be in operation at any time, each serving as
the back-up for the other. Each should be fed by an independent power source.
4.4.5.5 Where underwater accuracy is important, at least one PMS shall be an underwater, hydro-acoustic
reference system.
4.5 Remontely operated vehicle systems
4.5.1 Planning
4.5.1.1 Remotey operated vehicle (ROV) systems and tooling shall be selected based on the environmental
conditions that are to be expected at the worksite during the planned and contingency intervention/
observation tasks.
4.5.1.2 When planning for a subsea operation, the following ROV limitations and recommendations should be
noted:
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a)
b)
c)
d)
e)
Minimum practical operational depth in the expected wave conditions, also considering possible wake
from vessel thrusters.
ROV working range, i.e. maximum horizontal offset vs. available tether length, considering the worst
expected current conditions.
Planning and design of the ROV operation shall as far as possible minimise the operational influence of
the ROV operator's skill and experience.
Poor visibility due to e.g. disturbed soil conditions, stirred up by contact or thruster or tool use close to
seabed.
Access to working site.
4.5.1.3 Planned ROV downtime and statistical uptime of ROV shall be taken into consideration when
establishing TPOP, see [2.6.3]. If statistical data for ROV uptime is not available a conservative estimate shall
be made.
4.5.1.4 For subsea operations where the operation reference period (TR, see [2.6.2]) is based on using
ROVs (i.e. ROV activities are on critical path), ROV contingencies shall be documented and available. This
can include a back-up ROV spread on an independent system, i.e. there shall be no possible single failure
identified that may cause an unacceptable long downtime for both ROV spreads.
4.5.1.5 The need for backup of essential ROV tools shall be assessed, and if applicable, the time needed to
switch ROV tools/skids between ROVs shall be considered in the planning.
4.5.1.6 ROV tooling shall be provided with sufficient spares and back-up tooling to allow the work to proceed
with minimum delay.
4.5.1.7 For operations requiring assistance of both ROV(s) and diver(s), any restrictions on simultaneous
working shall be considered and be clarified in advance.
4.5.2 Stationkeeping and positioning
4.5.2.1 The stationkeeping capability and manoeuvrability of the ROV during operation shall be considered.
If the ROV is carrying equipment or is equipped with tooling packages/skids, this needs to be accounted for.
Guidance note:
Any ROV manipulator or tooling operation that requires the pilot to actively control the position of the ROV, e.g. if the target is
moving, during performance of the task should be avoided. See also [4.5.2.3].
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4.5.2.2 The required ROV thrust capacity shall be documented by verified capability plots (if available) and/
or detailed calculations considering:
— maximum current speeds at applicable depth(s), see [3.4.3].
— appropriate drag areas and -factors for ROV, cable and any tools
— all relevant relative ROV and current directions
— need for spare capacity, to be at least 30% for crucial ROV operations.
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Guidance note:
If detailed calculations are not made the horizontal current force on the ROV and the submerged cable may be taken as:
[kN]
where:
dcab=
diameter of submerged cable [m]
lcab=
projected length of submerged cable [m]
=
AROV
projected cross sectional area of ROV including any tools [m ]
vcur=
maximum current velocity [m/s].
2
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4.5.2.3 Grab bars to aid ROV positioning for manipulative or observation tasks should be provided where
critical path ROV operations are planned.
4.5.3 Testing
4.5.3.1 For complex and critical stages of the installation that are dependent on ROV operations, customer/
contractor shall demonstrate ROV capability of executing the planned intervention. This can be demonstrated
by used of 3D models, mock-up tests, previous experience, etc.
Guidance note:
This may involve the manufacture of mock-ups. If mock-ups are used, great care shall be taken to ensure that the mock-ups
replicate the actual item.
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4.5.3.2 System integration testing should be carried out onshore to prove that the integration of all
components and tooling can be achieved.
4.5.3.3 Dry tests and FAT should be carried out for critical and complex systems, the failure of which would
result in significant and unacceptable schedule delay.
4.5.3.4 Before acceptance of ROV operations, maintenance records and dive logs for each ROV should be
submitted. Sufficient spares should be available.
4.5.4 Launch and recovery system
4.5.4.1 Once installed, the launch and recovery system (LARS) shall be load tested according to the applied
design/certification standard.
4.5.4.2 ROV launching and recovery restrictions shall be defined based on the capacity of the launch and
recovery system, including capacity of the umbilical. In addition any restrictions related to operational
aspects need to be considered.
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Guidance note:
The following should be considered as rough guidance when establishing the ROV restrictions:
—
The launch and recovery system should incorporate a (guide/cursor) system that ensures adequate clearance with vessel side
—
Overboard launching and retrieval of large ROV's is not generally recommended to take place in sea states exceeding 2.5-3.0
during lowering through the splash zone in the limiting wave conditions.
m (Hs) if the ability to operate in a safe manner under more severe conditions has not been documented. Higher waves may be
applicable if the launch and recovery always may take place on leeward, for Moon-pool ROV operations and if heavy weather
side rail systems are used.
—
High wind speeds, and operational aspects (e.g. risk of entanglement) may also be critical.
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4.5.4.3 The over-boarding system shall be safely operated within its intended design limit and due
consideration of ROV recovery needs to accounted for in the definition of the weather criteria.
4.5.4.4 Launch and recovery shall as much as practically possible take place at safe distance from sensitive
subsea infrastructure. See [5.6.6.3].
4.5.4.5 A tether management system (TMS) should be used in deep water sites to ease the deployment of
the ROV to the worksite. The tether shall be of sufficient length to allow the ROV to get from the TMS to the
worksite.
4.5.5 Monitoring
4.5.5.1 Video monitoring of all subsea operations should in general be provided, e.g. ROV, diver-operated,
etc. Any critical part of the operation should be performed with such monitoring.
4.5.5.2 All diving and complex work-ROV operations should be monitored by independent ROV with
monitoring as its only task in the period it is carrying out such critical monitoring.
4.5.5.3 The ROV used for monitoring subsea operations should, as far as practically possible, be operated
from the installation vessel.
4.5.5.4 If the ROV operation has to be performed by a vessel other than the installation vessel, the stability
and reliability of the video-link system between the vessels shall be proven under the given conditions.
Guidance note:
Some operations can require a large horizontal distance between the installation vessel and the observation ROV, thus
necessitating a separate ROV vessel. The video-link should be tested before start of operation.
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4.5.5.5 Means for locating and tracking of the ROV from the surface are required for navigational purposes
and emergency recovery.
4.5.6 Human factors
4.5.6.1 The feasibility of subsea operations often relies on the correct completion of tasks by ROV - it should
therefore be ensured that ROV operators have the necessary experience and skills.
4.5.6.2 If complex operations reliant on the skill of the ROV operator alone cannot be avoided, ROV operator
experience shall be evaluated. Training sessions specially adapted for the proposed operation can be
appropriate.
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4.5.7 Deepwater remotely operated vehicle operations
4.5.7.1 ROV equipment capacities shall be chosen to suit the relevant depth and consider the following:
— Both the ROV and any ROV tooling should be 'depth rated', and their stated depth limitation should not be
exceeded.
— General wear on the complete ROV spread during deep water operations is more extensive than during
moderate depth operations, it is important therefore that all required maintenance is done before
operation.
— During deep water operations special attention shall be given to lubrication systems which can be affected
by the external water pressure.
4.5.7.2 Current forces acting on the umbilical and ROV shall be defined, see guidance note in [4.5.2.2].
4.5.7.3 Potential effects due to resonance in wires, cables, umbilicals, etc. shall be investigated and
accounted for in the design.
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SECTION 5 LOADING AND STRUCTURAL STRENGTH
5.1 Introduction
5.1.1 General
5.1.1.1 This section addresses loading categorisation, load effects, load cases and load combinations.
5.1.1.2 The requirements for structural strength are given, mainly related to steel structures. For structures
of other materials, adequate safety levels shall be achieved by use of recognized standards.
5.1.2 Scope
5.1.2.1 This section presents the requirements for strength checking of steel structures using both
allowable stress design (ASD) / working stress design (WSD) and load and resistance factor design (LRFD).
Alternatively, probabilistic methods can be used.
5.1.2.2 The ASD/WSD and LRFD checks have differing inherent levels of safety. To compensate, this
standard has differing requirements for the design loading. It is therefore important that the applied
environmental loading is determined using the return period applicable to the checking method selected.
5.2 Design principles
5.2.1 Introduction
5.2.1.1 The object subject to marine warranty survey, together with the associated equipment shall be
shown to possess adequate strength to resist the loads imposed during the marine operation.
5.2.1.2 The overall design shall be performed with due consideration to the execution of marine operations.
5.2.1.3 Structures shall be robustly designed such that an incident does not lead to consequences
disproportional to the original cause.
5.2.1.4 Simple load and stress patterns shall be aimed for in the design.
5.2.1.5 Structural elements should be fabricated according to the requirements given in DNV-OS-C401, /26/,
or another recognized standard.
5.2.1.6 Structural components and details should be designed so that the structure behaves, as far as
possible, in a ductile manner.
Guidance note:
A structure or a structural element, can exhibit brittle behaviour even if it is made of ductile materials e.g. when there are sudden
changes in section properties, when exposed to low temperatures.
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5.3 Specific design considerations
5.3.1 Connections
5.3.1.1 Connections should be designed with smooth transitions and proper alignment of elements. Stress
concentrations should be avoided as far as possible.
5.3.1.2 The transmission of tensile stresses through the thickness of rolled steel elements (plates, beams
etc.) should be avoided unless materials with proven (tested) z-quality are applied. Alternatively, the material
can be subject to non-destructive testing (NDT) using UT to demonstrate that it is free of laminations, see
[5.10.2.2.5].
5.3.1.3 Structural details above the still water level shall be so arranged that water will not be trapped in the
structure if this can cause damage such as e.g. rupture due to freezing of the water, when the operation is in
an area and season when this can occur.
5.3.2 Penetrations
5.3.2.1 The object shall be reinforced as necessary in the area adjacent to any penetrations (e.g. for risers
or J-tubes) below the water line against hydrostatic pressures and against accidental impact from dropped
objects and vessel impact if likely at any draught.
5.3.2.2 Penetrations shall be positively sealed to prevent the ingress of water whilst the structure is afloat.
5.3.3 Doubler plates
5.3.3.1 Doubler plates are generally recommended for use:
— When attaching seafastenings or sacrificial anodes to permanent steel work subject to fatigue or if the
permanent structure could be damaged when the attachments are burnt off after use.
— To avoid welding onto other welds.
5.3.3.2 Doubler plates are generally NOT recommended for use when tension can cause overstress in the
doubler plate or the structure to which it is attached.
5.3.4 Tension connections
Where tension connections to a vessel deck are required, attention shall be given to the connection between
the deck plate and underdeck members. In cases of any doubt about the condition, an initial visual inspection
should be undertaken, to establish that fully welded connections exist, and that the general condition is fit
for purpose. Further inspection may be required, depending on the stress levels imposed and the condition
found. See also [5.10.2.2.5] regarding through-thickness properties of the deck plate.
Guidance note:
The welds between vessel deck plates and under deck stiffeners/bulkheads (including cut out infills) are normally small and can
limit the capacity.
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5.3.5 Bolted connections for seafastening
5.3.5.1 The design of bolted connections shall be 'gap free' to avoid any bolts becoming loose. All gaps due
to imperfections shall be filled in with shim plates but not more than two (2) shim plates should be used at
any gap. When shim plates are not retained by bolts they shall be secured by welding.
5.3.5.2 Bolts used for seafastening shall generally be used one time only due to the potential for fatigue
during voyages or from re-installation. In special cases, re-use can be accepted as described in [E.2] but
only if the pretension targeted during installation does not exceed 60% of the bolt yield strength and the
loads during the transport have not exceeded the maximum design values. For re-use of bolts, a detailed
inspection plan with regular NDT including rejection criteria and exchange intervals should be documented.
As a minimum, bolts should be visually inspected after each transport and with magnetic particle inspection
(MPI) after every third transport unless fatigue calculations accepted by the MWS company show that less
frequent inspections are acceptable.
5.3.5.3 The requirements of [E.2] will apply for bolted connections used for seafastening. The strength of
bolted connections may be assessed to [E.2] or, subject to [E.2.3.4], to DNVGL-OS-C101 July 2018 Ch.2
Sec.11 [2] , /24a/ or Eurocode 3 /61/. See also [5.9.7.8] or [5.9.8.5.4].
5.3.6 Light-weight metallic and composite structures
5.3.6.1 The designers or manufacturers shall specify any handling/connection requirements which shall
appear in the relevant procedures and towing/transport manuals.
Guidance note:
Tugger line systems are especially important when handling light-weight alloy, composite and other items in order to avoid any
impact with seafastening, grillage or offshore structures which could cause plastic deformations.
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5.3.6.2 The structural strength of objects of innovative design and/or material shall be documented.
Guidance note:
Particular attention should be given to local strength in way of supports, seafastening etc.
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5.3.7 Compressed air
5.3.7.1 Compressed air may be used to resist hydrostatic head on internal or external walls during
ballasting, for reducing draught, or for reducing overall bending moments by air cushions in skirt cells under
well controlled conditions. However its absence should not, in general, result in structural collapse i.e. it
should be used only to increase structural safety factors.
5.3.7.2 Where the requirements of [5.3.7.1] cannot be met, then a risk assessment shall be carried out to
determine possible causes and probabilities of loss of compressed air. Mitigating measures to reduce the risks
to an acceptable level shall be agreed with the MWS company.
5.3.7.3 Some practical considerations on the use of compressed air are given in [12.6.2].
5.3.8 Inspection
5.3.8.1 Sufficient access for inspection, maintenance, and repair shall be provided during planning of the
operation.
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5.3.8.2 Instrumentation (monitoring) can be used as a supplement to other inspection, see [2.9].
5.3.9 Existing structures
5.3.9.1 Strength calculations for marine operations often include the verification of existing steel structures
of e.g. barges, other vessels and objects for dismantling. The calculations shall account for any reductions in
the design capacity. Examples of possible causes include:
— corrosion
— damage
— modifications not shown on drawings.
5.3.9.2 Existing structures should normally be inspected in order to assess possible reductions in the design
capacity, see [5.3.9.4], [5.9.8.4], [5.10.2.2], and [5.10.2.2.5]. See DNV-RP-N102, /55/, for further guidance
on existing structures and their inspection.
5.3.9.3 Project related strength verifications of vessels should normally be carried out conservatively with
either the as-built thickness reduced to account for possible corrosion or based on detailed inspections
including thickness measurements. Where the thickness is reduced to account for corrosion the thickness
used in calculations should be the thickness indicated on the as-built drawings less the vessel’s class
corrosion allowance, or reduced by 0.2 mm per year from each side. For new vessels with a proper corrosion
protection system, e.g. painting or coating, no thickness reduction need to be considered for the first five
years of the vessel’s life.
Guidance note:
Typical corrosion allowance requirements can be found in DNV-RU-SHIP Pt.3 Ch.3 Sec.3, /35/.
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5.3.9.4 Weld capacity should be calculated according to [5.9.7.6] for ASD/WSD or [5.9.8.4] for LRFD, as
applicable.
Guidance note:
When checking vessel welds the following should be noted:
a)
Class acceptance for these welds can be required, especially for new/reinforced welds.
b)
All loads (force components) normal to the deck plate should generally be considered transferred to the under deck welds.
However, when the force is only compressive, i.e. there is no tension force in any load combination, this force component
may be assumed to be transferred through direct contact between the deck plate and the web frames/bulkheads, and the
weld may be checked for shear stress only, see item f). If the force varies between compression and tension, the weld should
be able to transfer also the compression force in order to ensure intact welds, unless the capacity of the seafastening system
is documented in ALS assuming that the connection under consideration is broken.
c)
All loads (force components) parallel to the deck plate can be disregarded, see however item f).
d)
The dispersion angle through the deck plate should be taken as maximum 45° unless a greater dispersion can be justified.
e)
Size reduction due to possible corrosion should be considered. If not otherwise documented the size should be as shown on
f)
Note that shear stress in stiffener/girder welds due to local bending/shear in these should be included in the equivalent stress
the drawing less the class corrosion allowance.
(the effects due to global vessel behaviour can be ignored).
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5.3.10 Protection against accidental damage
5.3.10.1 The structure shall be protected against accidental damage by application of the following two
principles:
— reduction of damage probability
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— reduction of damage consequences.
5.3.10.2 If damage to piping, equipment, structures, etc. could lead to severe consequences (e.g. accidental
flooding, explosion, fire or pollution) such items shall be protected to minimise the risk of accidental damage.
Guidance note:
Protection may be established by methods such as providing a sheltered location, by local strengthening of the structure, or by
appropriate fender systems.
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5.4 Testing
5.4.1 General
5.4.1.1 Testing can be used in order to establish or verify design parameters. Material and weld testing
should be carried out according to a relevant recognized standard, e.g. DNV-OS-C401, /26/, see also [5.10]
which summarises key requirements.
5.4.1.2 Adequate and reliable test data should be used to verify/correlate values that are considered
unreliable based on theoretically calculations only. This is particularly relevant for geometrically complex
structures and for new design or operational concepts.
5.4.1.3 For marine operations, such (project) specific testing is normally most relevant to determine or
verify:
— response, e.g. motions by model testing,
— loads, e.g. by direct measuring of loads in model tests and
— resistance, e.g. by load testing or testing of friction.
5.4.2 Model testing
5.4.2.1 Model testing is most frequently used for the determination of response and loading effects but can
also be used for determination of structural resistance.
5.4.2.2 Model tests should be carried out according to a verified test program/procedure using:
— models representing the object(s), vessel(s) and real conditions as accurately as required,
— qualified test personnel,
— adequate testing facilities, and
— calibrated monitoring equipment with sufficient accuracy.
5.4.2.3 Normally the testing should be combined with theoretical calculations.
5.4.2.4 The laws of similarity shall be considered in order to ensure that the quantities measured in the
model test can be correctly transformed.
5.4.2.5 Effects that can influence the measured quantities and that are not represented in the model test
shall be identified and the consequences of these effects should be evaluated.
Guidance note:
For example, the correct relative stiffness (of vessels/structures) will normally not be obtainable in model tests and effects of this
on the results should be evaluated.
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5.4.3 Full scale load testing
5.4.3.1 Full scale load testing should be carried out according to agreed procedures.
5.4.3.2 Requirements for standardised load testing, e.g. of lifting appliances, are not described in this
standard. Such testing should be carried out as described in the relevant standard, e.g. DNV-ST-0378, /129/,
and DNV-ST-E273, /17/.
5.4.3.3 Full scale load testing may be carried out by loading test pieces to destruction. The characteristic
th
th
strength should normally be defined based on the 5 or the 95 percentile of the test results, whichever is
the most conservative.
5.4.3.4 If sufficient design documentation is not available to verify the strength (capacity) of an item, it can
be acceptable to document the strength of the item by means of a load test.
Guidance note:
Typical items for which this type of testing could be applicable include:
—
Anchors for which no holding power calculations have been carried out.
—
Shore bollards without relevant certificates or where the underground design and workmanship is not documentation.
—
Holding power of clamps or other types of connections.
—
Local soil capacity (deflection), e.g. of load-out tracks.
—
Existing (steel) structures with no/limited inspection access.
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5.4.3.5 For such tests the load should normally be at least 0.9 times the maximum design load (i.e.
including load factor) for the item. All relevant load directions should be tested.
5.4.3.6 A thorough inspection shall be carried out of items that have been subject to testing. Defects that
could reduce the strength (capacity) shall not be allowed.
5.4.4 Testing of friction
5.4.4.1 Testing may be carried out in order to establish applicable friction coefficients. The testing conditions
should represent the expected friction surface and load intensity as close as possible.
5.4.4.2 In marine operations the dynamic friction coefficient will normally be the most relevant and testing
of this should hence be included unless it is not needed for the particular application.
5.4.4.3 Where testing is carried out, a detailed test procedure shall be documented.
Guidance note:
The test procedure should consider the following:
a)
Possible variations in applicable conditions (e.g. wet and dry surfaces). See [5.4.4.1] and [5.4.4.2].
b)
Dynamic friction, if applicable, should be tested and measured by a recognized method.
c)
The characteristic friction coefficient should be defined based on the 5
th
or the 95
th
percentile confidence level of the test
results, whichever is the most conservative.
d)
At least 5 test pieces should be made, and each tested at least twice for each actual condition.
e)
The design friction coefficient is calculated using the characteristic friction coefficient and an appropriate material factor. See
f)
Where fewer tests are performed e.g. because of the scale, more conservative material factors should be used.
[5.9.9], [5.9.5.3] and [5.9.6.2].
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5.4.4.4 In cases where full-scale tests are not viable, the characteristic friction shall be documented by
an engineering assessment based on an adequate number of applicable small-scale tests using a validated
calculation methodology.
5.5 Load categorisation
5.5.1 Introduction
5.5.1.1 This subsection [5.5], defines load categories and describes loads of general interest for marine
operations.
5.5.1.2 The appropriate characteristic value should be defined (calculated) for all relevant loads.
5.5.1.3 More detailed descriptions of the loads to be considered are given for each type of marine operation/
object type in Sec.6 to Sec.18.
5.5.1.4 See [5.6] for load combinations, [5.7] for the failure modes to be considered, [5.8] for guidance on
analytical models and [5.9] for strength assessment.
5.5.2 Load categories
5.5.2.1 Loads and load effects shall be categorised as follows:
— Permanent loads - G
— Variable functional loads - Q
— Deformation loads - D
— Environmental loads - E
— Accidental loads - A.
5.5.2.2 The characteristic values of loads shall be selected as indicated in Table 5-1 for all applicable loads.
Table 5-1 Characteristic load selection
Limit states
Load category
– temporary design conditions
ALS
1)
ULS
Permanent (G)
FLS
Intact
structure
Damaged
structure
SLS
Expected maximum and minimum values (weight/buoyancy)
3)
Specified
3)
value
Specified
load history
Environmental (E) –
weather restricted
Specified value
Specified
load history
Environmental (E) –
weather unrestricted
4)
operations
Based on
statistical
5)
data
Expected
load history
n/a
n/a
Variable (Q)
2)
Accidental (A)
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Specified
3)
value(s)
Specified value(s)
n/a
Specified value(s)
Specified
value
Based on
statistical
5) and 6)
data
n/a
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Deformation (D)
Expected
extreme value
Expected
load history
1)
See [5.5.3] to [5.5.7] for definitions of load categories.
2)
See [5.9.1.3] for definitions of limit states.
Specified value(s)
3)
The specified value (load history) shall, if relevant be justified by calculations. See also [5.6.6].
4)
See [2.6.6].
5)
See Sec.3.
6)
Joint probability of accident and environmental condition could be considered.
5.5.3 Permanent loads (G)
5.5.3.1 Permanent loads are loads which will not be moved or removed during the phase of the marine
operation being considered. Such loads can be due to:
— weight of stationary structures
— weight of permanent ballast and equipment that cannot be removed
— external/internal hydrostatic pressure of permanent nature
— pretension.
5.5.3.2 Characteristic permanent loads shall be based on reliable data. For weight see [5.6.2].
5.5.4 Variable functional loads (Q)
5.5.4.1 Variable functional loads are loads that can be moved, removed or added. Such loads can be due to:
— operation of winches
— pull/push forces
— weight of moving structures
— loads from adjacent vessels
— ballasting
— operational impact loads
— stored materials, equipment or liquids.
5.5.4.2 Characteristic variable functional loads shall be specified with maximum and minimum values, which
shall be considered as necessary to determine the worst case(s).
5.5.5 Deformation loads (D)
5.5.5.1 Deformation loads are associated with inflicted deformations. Such loads can be caused by:
— installation or set down tolerances
— barge hull beam global deformations caused by moving ballast water (or temperature)
— structural restraints between structures
— differential settlements
— temperature deformations.
5.5.5.2 Characteristic deformation loads shall be maximum or minimum specified values, which shall be
considered as necessary to determine the worst case(s). The specified values shall, if applicable, be based on
results from analysis considering extreme conditions.
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5.5.6 Environmental loads (E)
5.5.6.1 All loads caused by environmental phenomena shall be categorised as environmental loads. Such
loads can be due to phenomena including:
— wind
— waves
— current
— storm surge
— tide
— ice
— movement/propulsion/(road) transportation.
Guidance note:
Accelerations due to movement/propulsion/(road) transportation are considered as E loads so that they are treated in the same
way as loads resulting from accelerations due to motions in a seaway, i.e. using the same load factors and allowable strength.
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5.5.6.2 Where applicable, see [5.6.11], seafastening (and grillage/cribbing) reactions due to barge hull
beam global deformations caused by waves should be considered as environmental loads. See also [5.6.17].
5.5.6.3 Gravity load components caused by the roll and pitch angles of a floating object due to wind and
waves or other causes of dynamic inclination e.g. movement/propulsion/(road) transportation, shall be
categorised as environmental loads.
5.5.6.4 The environmental design loads shall be calculated based on a process involving, as applicable:
— definition of characteristic conditions - see [2.2.7]
— calculation of characteristic loads – see [5.5] and [5.6]
— load analysis - see [5.6.2] to [5.6.11]
— motion analysis - see [5.6.12]
— selection of load cases - see [5.6.13]
— load factors - see [5.9].
5.5.7 Accidental loads (A)
5.5.7.1 Accidental loads are loads associated with exceptional or unexpected events or conditions. Such
loads can be due to:
— collisions from vessels
— dropped objects
— loss of hydrostatic stability
— flooding
— loss of internal pressure.
5.5.7.2 Characteristic accidental loads shall be based on realistic accidental scenarios. See also [5.6.6].
5.6 Loads and load effects (responses)
5.6.1 General
This subsection [5.6],describes the loads and load effects that should be considered.
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5.6.2 Weight and centre of gravity
5.6.2.1 Introduction
5.6.2.1.1 For calculation purposes, conservative values of weight and centre of gravity (CoG) should be used.
5.6.2.1.2 Weight control shall be performed by means of a well-defined and documented system. The weight
control system should comply with ISO 19901-5 – Weight management, /99/.
5.6.2.1.3 Weight reports should reflect the project phase and be issued in accordance with ISO
19901-5, /99/. Contents and format of weight reports that are not in accordance shall be agreed with the
MWS company at an early stage of the project.
5.6.2.2 Weight considerations
5.6.2.2.1 An upper bound design weight (Wud) shall be defined for all operations. Where the minimum weight
could be critical in an operation e.g. voyage motions, a lower bound design weight (Wld) shall be defined.
Guidance note 1:
The upper/lower bound design weights are normally defined to cover the expected range of weights in the weight report and
accounting for the factors in [5.6.2.2.2] or [5.6.2.2.3] for unweighed and weighed objects respectively, as shown in Figure 5-1.
The expected range of weights should include margin(s) to account for uncertainties during the design and fabrication process.
Appendix J of ISO 19901-5, /99/, presents examples of net weight allowances which can be considered when defining the
uncertainties.
Figure 5-1 Typical behavior of expected range of weights
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Guidance note 2:
Where a not to exceed (NTE) weight has been defined and used as the upper bound design weight the actual maximum
permissible value is less than the NTE weight due to the weight margin factors. If the NTE weight is multiplied by the appropriate
weight margin factor the resulting weight can be considered as the upper bound design weight.
In addition to any in-place considerations, the following can control the NTE weight:
—
draught and stability for tow-out, towages, mating operations and installation
—
allowable stresses in the structure for marine operations
—
limitations due to crane, load-out trailers, other equipment or ground-bearing capacity.
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5.6.2.2.2 Where an object (excluding piles) is not to be weighed, the following shall be true for the as-built
weight report:
WReport including uncertainties ≤ Wud/γWeight
WReport excluding uncertainties ≥ Wld γWeight(where applicable)
where:
WReport including
uncertainties
WReport excluding
uncertainties
Wud
Wld
γWeight
= from weight report including uncertainities
= from weight report excluding uncertainties
= upper bound design weight
= lower bound design weight
= unweighed object weight margin factor based on factors in Table 5-2 and reflecting
the proportion of weighed and estimated parts of the object.
Table 5-2 Unweighed object weight margin factors
γWeight
γWeighing
Weighed weights of parts of the object
Estimated weights - high confidence in weight report
As agreed with the MWS company
but generally not less than 1.05
Estimated weights - low confidence in weight report
1.10
Guidance note:
For objects of a nominally identical series where a reduced number of weighings has been agreed, see [5.6.2.4], the object may be
considered as weighed and the requirements in [5.6.2.2.3] applied.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.2.2.3 Where an object (excluding piles) is to be weighed, the following shall be true for the final weighed
condition (corrected for any post weighing modifications):
WWeighed ≤ Wud/γWeighing
WWeighed ≥ WldγWeighing (where applicable - see [5.6.2.2.1])
where:
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WWeighed
Wud
Wld
= weighed weight
γWeighing
=
= upper bound design weight
= lower bound design weight
factor to account for weighing equipment inaccuracy i.e. (
) where the %
inaccuracy is the most onerous weighing equipment inaccuracy of the load cells used for the
weighing.
Guidance note:
When an object has already been weighed, the upper bound design weight may be taken as the weighed weight multiplied by the
factor to account for weighing equipment inaccuracy, γWeighing. A similar approach may be used for the lower bound design weight
except the weighed weight is divided by the factor, γWeighing.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.2.2.4 The weight contingency factors for piles shall be agreed with the MWS company and shall consider
the following as a minimum:
— plate thickness tolerance
— fabrication tolerances.
5.6.2.3 Centre of gravity factors
5.6.2.3.1 Inaccuracy in CoG position shall be considered in the design loads. A CoG envelope should be
applied to allow for CoG inaccuracies.
5.6.2.3.2 The size of the CoG envelope should reflect the operational and structural sensitivity to CoG
variations and the most conservative centre of gravity position within the envelope should be taken.
Guidance note 1:
For early design stages, too small an envelope should be avoided and envelope sizes should generally be no less than 0.05L x
0.05B x 0.05H, where L, B and H are the length, breadth and height of the structure. As the design progresses the envelope size
could be reduced however this could increase the risk that the CoG becomes outside of the envelope used in the calculations. For
fabricated objects an envelope size of 0.025L x 0.025B x 0.025H or ±1 m may be appropriate depending on the actual object and
operation being considered. Contractors may have their own requirements that would also need to be considered.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For operations with a linear relation between shift in CoG and resulting load effects, or operations less sensitive to CoG shifts, the
inaccuracy in estimated CoG may alternatively be accounted for by an inaccuracy factor applied to the weight. Where there is high
confidence in the accuracy of the weight report a lower factor may be agreed with the MWS company but should not be less than
1.05. Where there is a lack of confidence in the accuracy this factor should normally be taken as 1.10.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.2.3.3 The CoG contingency factors for piles shall be determined considering the pile length and the plate
manufacturer’s plate thickness tolerance specification.
5.6.2.3.4 Normal weighing operations can be used only to identify the CoG position in a horizontal plane. The
envelope should be applied for the vertical CoG.
5.6.2.3.5 Inaccuracies in the vertical CoG position should be specially considered for operations that are
sensitive to the vertical CoG position. If applicable, the vertical CoG should be verified by means of an
inclining test (see [2.10.5]). Once the COG has been verified by means of an inclining test a reduced vertical
envelope, considering the accuracy of the equipment and test, may be applied.
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5.6.2.4 Weighing and weight control
5.6.2.4.1 The actual weight and CoG position shall be determined by weighing unless agreed otherwise with
the MWS company.
Guidance note:
Gravity based structures and launched jackets are generally excluded from being weighed.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.2.4.2 A weighing procedure for the structure shall be produced and include the specification, including
accuracy, for all equipment. The accuracy of the weighing equipment shall be certified by a competent body.
The weighing should preferably be carried out a minimum of three (3) times with the load cells interchanged
between each of the weighing operations.
5.6.2.4.3 Before any structure is weighed, a predicted weight and CoG report shall be issued, so that the
weighed weight and CoG can immediately be compared with the predicted results. The cause(s) of significant
deviations between the weighed and predicted results (both weight and CoG) shall be investigated and
reported.
5.6.2.4.4 Where weight is added to/removed from the structure after weighing, a weight control system shall
be adopted to ensure that the weight and CoG details based on the weighing are updated with any changes.
The weight changes due to items that are added and removed shall include their weighing contingency
factors.
5.6.2.4.5 The final calculated or weighed weight and CoG values shall be documented. Where the calculated
or weighed weight, including weighing and contingency factors, or the CoG is outside the design values
considered, the effects of the deviations shall be quantified and the operational procedures and documents
modified as required.
5.6.2.4.6 For nominally identical items, built with very good quality control, a reduced requirement for
weighing may be agreed with the MWS company provided rigorous quality control is in place and predictions
of final weights in initial weighings are demonstrated to be accurate. In this case, the weight control
programme should show the effects of all potential variations on the final weights. Subsequent random
weighings as agreed with the MWS company shall be used to confirm consistency of manufacture.
Guidance note:
For objects of a nominally identical series where a reduced number of weighings has been agreed, the object may be considered as
weighed and the requirements in [5.6.2.2.3] applied (instead of [5.6.2.2.2]).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.2.4.7 See [18.2.1.2] for weight control for decommissioning/removal.
5.6.2.5 Buoyancy
5.6.2.5.1 Buoyancy (hydrostatic external load) normally counteracts another load and shall be categorised
accordingly.
5.6.2.5.2 Where the buoyancy or distribution of buoyancy is critical to the marine operation, the dimensional
and buoyancy control and monitoring shall be maintained to an appropriate degree of accuracy.
5.6.2.5.3 The buoyancy of the object and the position of the centre of buoyancy should be determined on the
basis of an accurate geometric model.
5.6.2.5.4 Characteristic buoyancy loads should be based on maximum and/or minimum expected values.
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5.6.2.5.5 Buoyant cargoes, particularly where the buoyancy contributes to stability requirements, shall be
adequately secured against lift-off unless it can be shown that lift-off will not occur. See [11.10.1.12].
5.6.3 Wind loads
5.6.3.1 Wind loads shall be calculated based on the characteristic wind speed, see Sec.3, and recognized
calculation methods.
5.6.3.2 Wind induced loads shall be based on projected area. The total wind load shall consider both lateral
and parallel load components. The effects of wind-induced inclination on the wind area shall be considered
e.g. when computing the wind overturning moment for floating stability calculations.
5.6.3.3 The possibility of lift effects and their magnitude shall be considered.
5.6.3.4 The gravity components due to wind induced heeling shall be considered.
Guidance note:
DNV-RP-C205, /46/, gives further information with respect to shape coefficients as well as to effects of wind direction relative to
member, solidification and shielding.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.4 Current loads
5.6.4.1 Current loads shall be calculated based on the characteristic current velocity, see Sec.3, and
recognized methods.
5.6.4.2 The increase in current velocities/loads due to shallow waters or narrow channels shall be
considered.
Guidance note:
DNV-RP-C205, /46/, gives further information with respect to shape coefficients as well as to effects of flow direction relative to
member, solidification and shielding.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.5 Wave-current loads
5.6.5.1 Combined loads
Combined wave-current induced drag loads shall be calculated considering the vector sum of the current and
wave particle velocities.
5.6.5.2 First order wave loads
5.6.5.2.1 Wave loads should be estimated according to a deterministic or stochastic design method. A wave
period range according to [3.4.11.6.2] or [3.4.11.7] should be investigated.
Guidance note:
If any responses are found governing for
the response should be checked in these areas with
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.5.2.2 Wave loads shall be determined using methods applicable for the location and operation, taking into
account the type of structure, its size and shape and its response characteristics.
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5.6.5.2.3 The effects of wave elevation shall be evaluated, and if necessary included in the design.
5.6.5.2.4 Wave slamming, see [5.6.5.4], hydrodynamic and hydrostatic loads on members protruding
over the vessel side shall be considered. The effect of such loads on the motion characteristics and on the
seafastenings and grillage/cribbing shall be taken into account.
5.6.5.3 Second order wave loads
5.6.5.3.1 Second order wave drift forces can be important in the design of some marine operations. The
effects of second order drift forces shall be considered in these cases, which include large volume structures,
mooring and positioning systems, towing resistance estimates, etc. Second order wave loads consist of mean
wave drift forces and slow varying wave drift forces.
5.6.5.3.2 Long period responses excited by slow drift forces shall be investigated.
5.6.5.4 Slamming loads and breaking waves
5.6.5.4.1 Cargo overhangs and elements in the splash zone or overhanging the periphery of the floating body
shall be investigated with regards to possible slamming loads and/or immersion, see also [5.6.2.5].
5.6.5.4.2 The effect of shock pressures on surfaces in the splash zone, caused by breaking waves, shall be
investigated for conditions up to the design sea state for all headings.
5.6.5.4.3
The effect of vibrations due to wave slamming loads on the vessel hull and/or on overhanging cargo shall be
assessed. Typical consequences include:
— Reduction in the effectiveness of friction restraint.
— That seafastening is needed to prevent swinging/vibration of slender members/equipment/pipes.
— Unintended unscrewing of nuts/bolts.
5.6.5.4.4 Loads due to slamming and breaking waves should normally be calculated according to DNV-RPC205, /46/.
Guidance note:
Further information regarding slamming loads and breaking waves can be found in DNV-RU-SHIP Pt.3 Ch.10, /35/, and NORSOK
N-003, /111/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.5.5 Green water
5.6.5.5.1 The possible effects of green water (extensive amounts of water on deck due to waves), shall be
considered. The effects on both the structure and stability (weight and free surface) shall be investigated.
Guidance note 1:
See e.g. NORSOK N-003, /111/, for further information regarding green water effects. In lieu of voyage-specific calculations the
design forces for sea pressure from green water can be based on requirements for deck houses, see DNV-RU-SHIP Pt.3 Ch.4 Sec.5
[3], /35/, in which case the reduction factors in Table 11-1 can be applied.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The effects of green water are not normally expected for voyages that are staged-tows with shelter available (see [11.14.6] for
requirements related to the availability of shelter) and where the vessel is at a draught less than the load line draught.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.5.5.2 Deck cargoes vulnerable to damage from green water on deck should be protected by breakwaters
or increasing freeboard.
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5.6.5.6 Swell
The effects of loads and motions due to swell shall be considered. See [3.4.14] and [5.6.18]. Swell can be
governing for operations designed for small irregular waves (e.g. weather restricted tows). In such cases
swell operational limits and forecasting shall be established.
5.6.6 Accidental loads
5.6.6.1 General
5.6.6.1.1 Accidental loads should be defined based on relevant accidental scenarios. In many cases the
probability of accidental scenarios can be reduced to a level such that there is no need to consider them
further.
5.6.6.1.2 The accidental load design principles indicated in DNV-OS-A101, /40/, should be considered as
applicable for the planned marine operation. DNV-RP-C204, /31/, gives further guidance related to design
philosophy and calculation of relevant accidental loads due to e.g. collisions and dropped objects.
5.6.6.1.3 Load effects due to all possible accidental scenarios/conditions shall be considered. Accidental cases
and contingency situations may be defined or excluded based on results from HAZOP’s or risk evaluations/
assessments.
5.6.6.1.4 DNV-OS-A101, /40/, is, in general, based on annual probabilities, whilst this standard is based
on probability per operation. This can be considered when the (magnitude of) applicable accidental loads
are defined. However, unless a justification for lower loads is documented the loads indicated in DNV-OSA101, /40/, should be considered.
5.6.6.2 Vessel collision
5.6.6.2.1 Characteristic collision loads shall be estimated from energy considerations. Estimates of the
collision energy should be based on reasonable assumptions of possible collision scenarios, velocities,
directions, ship or object type, size, mass, and added mass. Estimates of deformation energy should be
based on the most likely impact points and probable deformation patterns.
5.6.6.2.2 The behaviour of the vessels or structures during the impact, and thus the distribution of impact
energy between kinetic rotation and translation and deformation energy, should be considered by dynamic
equilibrium or energy considerations.
5.6.6.2.3 Local effects (deformation, damage, etc.) and global load effects (acceleration, global stress, etc.)
shall be considered.
Guidance note:
In some cases collisions will have been covered under the design and classification of the vessel.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.6.3 Dropped objects
5.6.6.3.1 Loads caused by dropped objects can be relevant for some ALS load cases. The characteristic
load due to a dropped object should be based on the weight of objects that could fall and their potential fall
height.
5.6.6.3.2 For objects falling through water maximum possible impact velocity should be considered. The
maximum velocity is normally the terminal (free fall in water) velocity. See DNV-RP-N103 [4.7.3.5], /56/,
and DNV-RP-F107 [5.3], /52/, for guidance.
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5.6.6.3.3 Loads on subsea items due to dropped objects may be ignored if operations that could cause
dropped objects are carried out at a safe distance. The safe distance should be calculated considering the
maximum possible dispersion angle for each type of object falling through the water. The effect of current
should be considered. Risk analysis may be used in order to eliminate physical possible high dispersion
angles by showing that the risk of hitting specified critical locations is acceptably low for such high angles.
See DNV-RP-F107, /52/, for further risk assessment guidance. If detailed assessments are not made, the
safe distance can normally be taken as the larger of 50 meters or that determined from a dispersion angle of
20° to the vertical.
5.6.6.4 Other causes
5.6.6.4.1 Other relevant accidental loadings shall be considered. These can include, but are not limited to,
cases such as: 'one line broken'; 'one compartment damaged'; malfunction of critical systems e.g. heave
compensation, leaking valves; erroneous operation e.g. the use of the wrong valve; unexpected values of
parameters e.g. deformations, friction, vessel GM, tidal variation, weights and CoG’s.
5.6.6.4.2 The static loads resulting from any one compartment damage, as described in [11.10.4] to
[11.10.6], shall be considered and, if significant, designed for as a LS2 or ULS case.
5.6.7 Dynamics
5.6.7.1 The potential for dynamic response shall be investigated, and the effects shall be included in the
design analysis when they are of significance. Dynamic response is typically caused by wave forces, wind
loads (gusts), vortex shedding in air or water, slamming loads, etc.
5.6.7.2 Dynamics shall be investigated by recognized methods using realistic assumptions for the natural
period, damping, material properties etc.
5.6.7.3 The response to dynamic effects e.g. structural stress and deflections can be relevant for all limit
states.
5.6.7.4 Means of determining whether vortex shedding could be critical for any particular member are
contained in DNV-RP-C205 Sec.9, /46/ and Section 7.2 of Dynamics of Fixed Marine Structures, /122/.
5.6.8 Non-linearities
5.6.8.1 Non-linear effects shall be considered in cases where these significantly influence the estimated
responses. Non-linear effects are typically caused by:
— non-linear materials
— non-linear geometry (large-displacement effects)
— non-linear damping
— non-linear combination of load components or response components
— wave elevation e.g. due to wave-in-deck, non-linear effects of drag-loading (especially with current).
5.6.8.2 Non-linear load effects due to combinations of environmental loads should be taken into account e.g.
wave-current drag forces are a function of the square of the sum of the wave and current particle velocities.
5.6.9 Friction
5.6.9.1 Possible unfavourable effects of friction shall be considered. Well documented favourable effects of
friction may be included in the design.
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5.6.9.2 A friction coefficient range, i.e. both a maximum and a minimum friction coefficient, should be
considered in the design calculations or it should be proven that a conservative minimum (or maximum)
coefficient suffices.
5.6.9.3 The characteristic friction coefficient range shall be defined according to recognized industry
standards or tests, see [5.4]. Indicative operation-specific values are given Table 10-2, [11.9.2.11],
Table 11-10, Table 11-12, Table 11-23, Table 13-5 and in DNV-RP-N102 Table 2-4, /55/. For soil-material
interfaces, guidance is provided in DNV-RP-F109 Sec.7, /53/, and DNV-RP-F114, /51/.
5.6.9.4 The lower bound design friction coefficient (μld) shall be the lower bound characteristic value (μlc)
divided by a material factor.
5.6.9.5 The upper bound design friction coefficient (μud) shall be the upper bound characteristic value (μuc)
divided by a material factor less than 1.0.
5.6.9.6 The appropriate material (safety) factor for friction shall be selected dependent upon the limit state
considered and the risk involved in exceeding (or going below) the design friction. See [5.9.5], [5.9.6] and
[5.9.9]. These factors are applicable to both LRFD and ASD/WSD. For bolts see [E.2.4] and [5.9.7.8] or
[5.9.8.5] for ASD/WSD and LRFD respectively.
5.6.9.7 The minimum design friction force shall be taken as the minimum design load (i.e. including relevant
load factors) perpendicular to the friction surface multiplied by μld.
5.6.9.8 The maximum design friction force shall be taken as the maximum design load (i.e. including
relevant load factors) perpendicular to the friction surface multiplied by μud.
5.6.9.9 If the friction coefficient range is based on uncertain data the consequences of the maximum
possible variation in friction coefficients shall be evaluated. See [5.6.14].
5.6.9.10 Vibrations, varying or uncertain surface conditions etc. affecting the friction shall be considered.
5.6.9.11 Restraint effects caused by combination of friction and global deflections shall be considered.
5.6.10 Tolerances
5.6.10.1 Loads caused by operational or fabrication tolerances exceeding the tolerances stated in the design
standards/codes shall be considered. Typical examples include:
— set-down tolerances (load-out, positioning)
— shimming tolerances
— uncertain deformation (in load distributing material)
— fabrication tolerances, see [5.10.1.4].
5.6.10.2 Loads caused by effects described in [5.5.5].
5.6.11 Relative deflections
5.6.11.1 The effects of relative deflections between structures shall be considered and included in the design
whenever applicable. These can be of particular significance when they induce loads in connections and
supports such as grillages and seafastenings. The causes of relative deflections include:
— vessel deflection (longitudinal bending) in waves,
— ballasting, de-ballasting or re-distribution of ballast,
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— temperature differences,
— relative deflections that need to be considered during the operation.
5.6.11.2 For sea voyages the potential effects of longitudinal wave bending effects should always be
considered when:
a)
b)
c)
d)
e)
The towed hull is not a classed, seagoing vessel or barge, or
rd
The longitudinal distance between the aftermost and foremost cargo supports is greater than about 1/3
of the transport barge or vessel length, or
The cargo is supported longitudinally on more than 2 groups of supports, or
The relative stiffness of the hull and cargo could cause unacceptable stresses to be induced in either, or
The seafastening design allows little or no flexibility between cargo and vessel.
5.6.11.3 Some cargoes, such as large steel jackets, can be inherently much stiffer than the barge, and will
reduce vessel deflections, at the expense of increased cargo stresses.
5.6.11.4 See also [11.9.3.2] for friction, [11.9.5] for seafastening design and [11.28.4.3] for jack-ups.
5.6.11.5 The restraint loads should be defined in the same category as the load that causes the relative
deflections, i.e. restraint loads caused by environmental conditions should be defined as E-loads, see [5.5.6].
5.6.12 Motion analysis
5.6.12.1 General
5.6.12.1.1 Motions of floating objects shall be determined for the relevant environmental conditions and
loads. These may be from simplified conservative estimates, however it is normally recommended that the
analysis (and tests) described in this subsection are carried out.
Guidance note:
Detailed analyses and model tests are not normally needed for the transportation of smaller cargoes on standard vessels.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.12.1.2 Inertia loads due to motion should be calculated for all six degrees of freedom.
Guidance note:
This also includes an evaluation of mass (rotational) inertia effects from roll and pitch. These effects should as a minimum be
quantified, and the effect evaluated. This is particularly relevant for barge voyages with large roll motions.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.12.1.3 Testing of models, see [5.4.2], or full scale structures, see [5.4.3], may be carried out where the
accuracy of theoretical approaches is uncertain, or where the design is particularly sensitive for motions.
Guidance note:
Estimation of motions from model testing or by theoretical calculation has associated advantages and disadvantages. The two
approaches are generally to be considered as complimentary rather than as alternatives.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.12.1.4 It is recommended that theoretical calculations are correlated against relevant model test data
(if available) in cases where strongly non-linear behaviour is expected. Such cases can occur when, for
example:
— overhanging cargo is occasionally submerged, or
— there are large changes in the waterplane area with draught.
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5.6.12.1.5 The analytical models should be checked with respect to sensitivity to input parameters, see
[5.6.14].
5.6.12.1.6 Recognized and well proven six-degree of freedom linear or linearized computer programs,
utilising the strip theory or 3D sink source techniques are generally recommended. Special consideration shall
be given to non-linear damping effects. The effect of forward speed shall be evaluated, where this is more
onerous.
5.6.12.1.7 Computer programs shall be validated against a suitable range of model test or full scale results
in irregular seas. When using new software or for new or unconventional applications or new problems, this
validation shall be documented. Similarly justification of drag coefficients, added mass and damping shall be
documented.
Guidance note:
Guidance on drag and added mass coefficients for a range of standard shapes can be found in DNV-RP-C205, /46/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.12.1.8 First-order motion response analysis programs generally report heave in a global fixed axis
system. In these cases heave shall be assumed to be parallel to the global vertical axis and therefore the
component of heave parallel to the deck at the computed roll or pitch angle (theta) is additive to the forces
caused by the static gravity component and by the roll or pitch acceleration.
5.6.12.1.9 In general, motion response calculations should be based upon a 3D panel model of the vessel. If
a 2D strip theory model is used, the computer program needs to include the proper treatment of head/stern
sea wave excitation loads. Simplified calculations should only be applied for non-critical routine operations or
screening purposes.
5.6.12.2 Wave headings
5.6.12.2.1 The full range of wave headings shall be analysed. Spacing between the analysed wave headings
should not exceed 45°.
Guidance note:
For the cases where reduced design wave heights are acceptable from some headings, see [11.8], this applies to all headings.
However, symmetry can be considered when relevant provided appropriate means of accounting for cargo CoG offset are included.
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5.6.12.2.2 Short crested sea shall be considered for wave analysis where all headings are not carried out
with equal wave heights i.e. typically motion analysis in order to find limiting installation wave heights for
different vessel headings.
Guidance note:
If short crested waves are considered the spacing between analysed wave headings should normally not exceed 22.5°. See also
[3.4.12].
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5.6.12.2.3 Short crested sea may be considered for wave analysis where all headings are included with equal
wave height i.e. typically motion analysis for sea voyages without any heading restrictions.
5.6.12.3 Wave periods
A wave period range with corresponding wave heights, see [3.4], shall be considered when evaluating
characteristic motions and accelerations.
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5.6.12.4 Response amplitude operators
5.6.12.4.1 Response amplitude operators (RAO’s) for the basic six degrees of freedom can be utilised to
calculate displacements, velocities, accelerations, and reaction forces for points in a body fixed co-ordinate
system, or to establish RAO’s for these points. These RAO’s may be used for calculation of significant and
maximum responses.
5.6.12.4.2 When combining different responses, the phase angle between the different components may be
considered.
5.6.12.4.3 The gravity component shall be considered when determining the RAO’s for inertia loads (e.g.
transverse accelerations).
5.6.13 Load cases and load combinations
5.6.13.1 Loads and load effects shall be combined to form load cases that are applicable to and physically
feasible for the actual object(s) and type of operation under consideration.
5.6.13.2 All possible load cases which can influence the feasibility of the marine operation shall be
considered in the design.
5.6.13.3 Characteristic loads may be combined taking into account their probability of simultaneous
occurrence.
5.6.13.4 Characteristic static (mean) load components and characteristic dynamic (varying) load
components which are statistically independent may be combined according to the formulae below.
where:
Fi,mean
Fi,amp
= characteristic static load components
= amplitude of dynamic load components.
Guidance note:
Dynamic load components in the above formulae are normally restricted to loads with periods less than 10 minutes. The maximum
values of dynamic loads with periods greater than 10 minutes are normally added as static loads (i.e. Fi,mean equal to the
maximum load, and Fi,amp =0).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.13.5 Correlated dynamic load components shall be added as vectors, unless statistical data of
simultaneous occurrence are available. Load components due to first order motions should be considered to
be correlated. The combination of these components is described in [5.6.15].
5.6.14 Sensitivity analysis
5.6.14.1 The load cases shall include a parametric sensitivity analyses whenever a single load or parameter
significantly affects the design or selection of the method or equipment to determine whether small changes
significantly affect the design.
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5.6.14.2 Where the operational safety is critically dependent on a sensitive input, conservative characteristic
values shall be used.
5.6.15 Loads due to motions and wind
5.6.15.1 Load cases for each heading shall be derived by the addition of fluctuating loads resulting from
wind and wave action to static loads resulting from gravity and still water initial conditions.
5.6.15.2 In lieu of a refined analysis the worst possible combination of the individual responses for the same
heading, including components from the self-weight and wind, shall be combined, i.e.:
where:
Sd
S( )
Fx, Fy, Fz
= design load or load effect
Fwx, Fwy
= wind forces (vectors), in x and y directions including relevant load factors. The horizontal load
components due to wind induced heel or trim shall be included
= response/load effect function
= inertia forces (vectors), in x, y and z directions including relevant load factors and gravity
components
Where ballasting during transport is possible to counteract wind heel, and this is documented
in the procedures, wind induced heel on the vessel can be computed based on difference
between 1-minute mean and the 1-hour average wind-force.
W
= load due to self-weight (vectors).
5.6.15.3 Alternatively, the fluctuating components shall be the worst possible combination of the loads
resulting from calculations or model tests carried out in accordance with [11.3.7.1] through [11.3.7.3], with
due account to be taken of the effects of phase. All influential loadings shall be considered, however the
following static and environmental loadings are the most likely to be of importance:
S1
F1
= loadings caused by gravity including the effects of the most onerous ballast condition on the voyage
F2
F3
F4
F5
F6
F7
F8
= loadings caused by surge and sway acceleration
= loadings caused by the wind heel and trim angle. Where ballasting during transport is possible to
counteract wind heel, and this is documented in the procedures, wind induced heel on the vessel
can be computed based on difference between 1-minute mean and the 1-hour average wind-force
= loadings caused by pitch and roll acceleration
= loadings caused by the gravity component of pitch and roll motion
= loadings caused by direct wind
= loadings caused by heave acceleration, including heave.sin(theta) terms
= loadings caused by wave induced bending
= loadings caused by slam and the effects of immersion.
5.6.15.4 Except when default motion criteria are used, see [11.4] to [11.7], the method in this paragraph
shall be used to determine the design loadings.
The effects of phase differences between the various motions can be considered, if resulting from model test
measurements, or if the method of motion response calculation has been suitably validated.
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In cases where it is not convenient or possible to determine the relative phasing of extreme wind loadings
and heave accelerations with roll/sway or pitch/surge maxima, a reduction of 10% may be applied to
fluctuating load cases F1 through F8 which combine maximum wind and wave effects. However, if the wind
induced load or the wave induced load exceeds the reduced load, then the greatest shall be considered.
The total loads may be calculated by combination of loads as follows:
where:
Fmot
F#(1 hour)
F#(1 min)
F#
= maximum load due to wind and wave motions
= loads based on 1-hour mean wind speed
= loads based on 1 minute mean wind speed
= F1 through F8 as applicable.
Where ballasting during transport is possible to counteract wind heel, and this is documented in the
procedures, the wind induced heel due to the 1-hour average wind speed can be ignored:
5.6.15.5 Where transfer functions for motions are available these may be combined to a transfer function for
the actual response or load effect. The phasing between the different components may be considered.
Guidance note:
This method requires careful evaluation of the responses to be analysed. All responses which will be governing for the design
should be considered.
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5.6.16 Default motion criteria
5.6.16.1 For motions in accordance with [11.5], the loads shall be calculated using the associated load
calculation in IMO Code of Safe Practice for Cargo Stowage and Securing, /87/.
5.6.16.2 For accelerations and wind loads computed in accordance with [11.6] and [11.7], the loads applied
to the cargo shall be:
where: S1, F1, F3, F4, F5 and F6 are as defined in [5.6.15.3].
When default motion criteria for ships are used, see [11.6], the loads S1, F3, F4 and F6are computed from the
acceleration combinations given in [11.6.5] to which F1 and F5 should be added.
Guidance note:
For motions in accordance with [11.7.2], F5 is generally omitted however see [11.7.2.2].
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5.6.16.3 The effects of buoyancy and wave slam loading shall also be considered if appropriate.
5.6.16.4 For the default motion criteria in [11.4] to [11.7], roll and pitch cases shall be considered
separately. For the default motion criteria in [11.7.2] combined roll and pitch are generally not required.
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Guidance note:
Quartering seas should also be included if deemed critical for any structural element (see also IMO Res. A.714(17), Annex
13 regarding allowable angles of securing devices.) Quartering seas can be included by combining 60% of the horizontal
transverse and 60% of the longitudinal acceleration with both the minimum and maximum vertical acceleration i.e.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.6.17 Loads due to restraint deflections, vessel motions and wind
Restraint loads due to vessel deflections in waves, see [5.6.11], loads due to vessel motions and wind may
be combined as shown below.
where:
Ftot = total design load
Fdef = maximum loads due to deflections
Fmot = maximum load due to wave motions and wind.
5.6.18 Loads due to irregular waves and swell
Loads and load effects from irregular waves and swell shall be combined. These loads and load effects may
normally be combined assuming that they are statistically independent. See [5.6.13.4].
5.7 Failure modes
5.7.1
All relevant failure modes shall be investigated. A failure mode is relevant if it is considered possible and the
anticipated consequence(s) of the failure cannot be disregarded.
5.7.2
The relevant failure modes can be grouped as either as global (total system) or local (individual members) as
indicated in the following subsections [5.7.3] and [5.7.4] respectively.
5.7.3
Global modes of failure include:
— structural collapse
— overturning
— sliding
— lift-off
— loss of hydrostatic or hydrodynamic stability
— sinking
— settlement
— free drift.
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5.7.4
Local modes of failure include:
— plastic deformation (yield)
— buckling
— fracture
— large deflections
— excessive vibration.
5.8 Analytical models
5.8.1
The analytical models used for evaluation of loads, responses, structural behaviour and resistance shall be
relevant considering: the design philosophy, the type of operation and the possible failure modes. The models
should satisfactorily simulate the behaviour of the object’s structures, its supports and the environment.
5.8.2
Design analyses shall be carried out considering all relevant loads and failure modes, see [5.7].
5.8.3
The design analysis shall be thoroughly documented that the results shown to satisfy the relevant
requirements and criteria.
5.9 Strength assessment
5.9.1 General
5.9.1.1 Structural strength can be assessed using either ASD/WSD methodology or LRFD methodology.
These are discussed below.
5.9.1.2 Whichever methodology is applied, the loading conditions/limit states shown in Table 5-1 shall be
considered when verifying structural strength.
5.9.1.3 A limit state is commonly defined as a state in which the structure ceases to fulfil the function, or to
satisfy the conditions, for which it was designed. See also DNV-OS-C101 Ch.2 Sec.1 [3], /24/ (Ed: July 2019
Amended 2021).
5.9.1.4 Limit states shall be defined for all possible failure modes, see [5.7].
5.9.1.5 The FLS and SLS load cases requirements are the same for ASD/WSD and for LRFD. It is however
important that the load cases assessed for the ALS and LS / ULS are developed using the applicable
environmental inputs for ASD/WSD or LRFD.
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Table 5-3 Description of loading conditions/limit states
Loading condition / limit state
ASD / WSD name
LRFD name
Maximum capacity, usually for maximum environmental and
functional loads (permanent, variable, deformation).
LS1
LS2
ULS-a
ULS-b
Loading history – important for structures exposed to significant
cyclic/repetitive loading.
FLS
FLS
Intact structure subjected to loads from an accidental event.
ALS-I
ALS-I
Damaged structure subjected to post-damage loading.
ALS-D
ALS-D
Serviceability checks (alignment, clearances, deflection, vibration,
etc.).
SLS
SLS
5.9.2 Design approach
5.9.2.1 The format of the ASD/WSD method implies that strength/capacity verification of structures or
systems involves the following steps:
— Identify all relevant limit states, see [5.9.1].
— Identify all relevant loading conditions, see [5.6.13].
— For each loading condition define the relevant characteristic loads, see [5.5.2], and design conditions, see
Table 5-1.
— For each loading condition and failure mode, see [5.6] and [5.7], find the design loads
— For each loading condition determine the design load effect, see [5.6]
— Ensure adequate safety by proving that the design load effect does not exceed the allowable, as described
in [5.9.4], [5.9.5], [5.9.6] and [5.9.7].
LS2 is applicable only when the loading is dominated by environmental/storm loads, e.g. for weather
unrestricted operations when the extreme loads are due to the applicable design return period environmental
criteria, see Table 3-1 or for weather restricted operations provided an alpha factor according to [2.6.9] shall
be applied. Any LS2 load case may be treated as a gravity-load dominated limit state (LS1).
LS2A is applicable only when the loading is determined from classification society rules, see [11.6]. Any LS2A
load case may be treated as a gravity-load dominated limit state (LS1).
5.9.2.2 The format of the LRFD method implies that strength/capacity verification of structures or systems
involves the following steps:
— Identify all relevant limit states, see [5.9.1].
— For each limit state define the relevant characteristic loads, see [5.5.2], and design conditions, see Table
5-1.
— For each limit state find the design loads by applying the relevant load/design factors, see [5.9.4.2],
[5.9.5.2], [5.9.6.2] and [5.9.8.3].
— For each limit state determine the design load effect, see [5.6] and [5.9.3.2.2].
— For each limit state determine the characteristic resistance, see [5.9.3.3].
— For each limit state determine the design resistance, see [5.9.3.2.4].
— Ensure adequate safety by proving that the design load effect does not exceed the design resistance, See
[5.9.3.2.1].
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5.9.3 LRFD checks
5.9.3.1 General
Where the LRFD (load and resistance factor design) method is used for design verification the load and
material factors specified in this subsection [5.9.3], shall be used according to the principles of the method.
Guidance note:
The safety factor format applied for lifting slings in Sec.16 could be regarded as an ASD/WSD (permissible stress) method, but the
safety level is correlated according to the applicable LRFD factors.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.3.2 Acceptance criteria
5.9.3.2.1 The level of safety is considered to be satisfactory if the design load effect, Sd, does not exceed the
design resistance, Rd, i.e.:
Sd ≤ Rd for all limit states
The equation Sd = Rd defines the respective limit state.
5.9.3.2.2 A design load effect is an effect (e.g. stress, mooring line load, sling load, deformation, overturning
moment, cumulative damage) due to the most unfavourable combination of design load(s) i.e.:
where:
Sd
Fd
S
= design load effect
= design load(s)
= load effect function.
5.9.3.2.3 A design load (Fd) is obtained by multiplying the characteristic load (Fc) by the appropriate load
factor, see [5.9.8.3], [5.9.4.2], [5.9.5.2] and [5.9.6.2].
5.9.3.2.4 A design resistance (Rd) is obtained by dividing the characteristic resistance (Rc), see [5.9.3.3], by
a material or design factor, see [5.9.8.3], [5.9.4.1.7], [5.9.5.2] and [5.9.6.2].
5.9.3.3 Characteristic resistance
5.9.3.3.1 Rc shall be calculated based on the characteristic values of the relevant parameters or determined
th
th
by testing. Characteristic values should be based on the 5 or the 95 percentile of the test results,
whichever is the most conservative. See also [5.4].
Guidance note 1:
The resistance for a particular load effect is, in general, a function of parameters such as structural geometry, material properties,
environment and load effects (interaction effects).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Rc for materials not mentioned e.g. concrete, concrete reinforcement, wood, synthetic materials, soil, may normally be based on
recommendations/requirements in the applied design code or standard. For soil see DNV-OS-C101 Ch.2 Sec.10 [1.3], /24/ (Ed:
July 2019 Amended 2021).
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5.9.3.3.2 The characteristic static resistance of steel, fc, used in the calculation of design resistance shall be
taken as the smaller of:
— the guaranteed minimum yield stress, fy, or
— 0.85 times minimum tensile strength of the material.
5.9.3.3.3 Rc for (wire and fibre) ropes, slings and chains should be taken as the certified MBL.
5.9.4 Fatigue limit states
5.9.4.1 General
5.9.4.1.1 For all structures exposed to significant cyclic loads during a marine operation the possibilities and
effects of fatigue should be considered.
5.9.4.1.2 The fatigue limit states (FLS) design conditions should be based on the defined operation period
and the anticipated or expected load history during the marine operation. See Table 5-3.
5.9.4.1.3 Possible dynamic load effects due to e.g. slamming and vortex shedding should be investigated.
See [5.6.7].
5.9.4.1.4 Restraint loads, see [5.6.17], may be important and shall hence be thoroughly evaluated and
included in the FLS calculations.
5.9.4.1.5 The FLS shall be evaluated according to procedures given in a recognized code or standard. See
e.g. DNV-OS-C101 Ch.2 Sec.5, /24/ (Ed: July 2019 Amended 2021), for general requirements for checking of
fatigue limit states.
Guidance note 1:
See DNV-RP-C203, /29/, and DNV-CG-0129, /20/, for practical details with respect to fatigue design. Information about fatigue for
bolted is in [E.2.7].
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Guidance note 2:
For new structures, including objects, grillages and seafastenings, that are susceptible to fatigue:
—
2
If the nominal peak stress range is < 350 N/mm and SCF ≤ 2.5 [i.e. hot-spot stress range including geometric and weld SCF
2
< 875 N/mm ] (e.g. nominal peak stress can be checked against W3 SN curve with free corrosion) then there is normally no
need to run fatigue analysis for voyage durations less than 50 days.
—
2
If the nominal peak stress range is < 350 N/mm and SCF ≤ 1.6.1 [i.e. hot-spot stress range including geometric and weld
2
SCF < 560 N/mm ] (e.g. nominal peak stress can be checked against F3 SN curve with free corrosion) then there is normally
no need to run fatigue analysis for voyage durations less than 200 days.
—
2
If the nominal peak stress range is < 525 N/mm and SCF ≤ 2.5 [i.e. hot-spot stress range including geometric and weld SCF
2
< 1300 N/mm ] (e.g. nominal peak stress can be checked against W3 SN curve with free corrosion) then there is normally no
need to run fatigue analysis for voyage durations less than 15 days.
—
2
If the nominal peak stress range is < 550 N/mm and SCF ≤ 1.6.1 [i.e. hot-spot stress range including geometric and weld
2
SCF < 885 N/mm ] (e.g. nominal peak stress can be checked against F3 SN curve with free corrosion) then there is normally
no need to run fatigue analysis for voyage durations less than 50 days.
The 'nominal peak-stress range' is the stress range accounting for geometric effects that are not included in the referenced S-N
curve. In deriving the above, the method set out in DNV-RP-C203 was used, assuming that:
—
The voyage is weather unrestricted, see [2.6.6] and there is no heading control, see [11.8].
—
The stress range is the MPME for the most onerous heading from a 3-hour storm for the design environmental conditions applicable
—
The stress range includes ULS partial factors for LRFD or a load factor of 1.0 for ASD/WSD (as applicable).
to the voyage duration in accordance with [3.4].
—
Tz = 6.3 second average zero-up-crossing period.
—
DFF = 1.0
—
Weibull distribution parameter h = 0.8; this is selected to make allowance for variations in vessel heading relative to the most
critical seastate direction and marine weather routeing practice. The indicated Weibull parameter should normally be justified/
evaluated for the actual transport.
Otherwise, or if any of the assumptions above are inapplicable, a fatigue analysis is advisable.
Shorter durations can be applicable when the structure is subject to fatigue before or after the considered marine operation, see
[5.9.4.2].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
New-build MOU's are normally verified for fatigue for the initial delivery voyage in the classification process and a separate analysis
is not normally required for this voyage. For subsequent voyages, it is desirable to undertake a fatigue analysis, however in many
cases there is insufficient time and/or data regarding prior use. In such cases it is good practice to undertake a thorough NDT
inspection of fatigue-critical areas before the voyage and to repair any cracks, see [11.28.5.3].
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5.9.4.1.6 For mooring systems, the FLS is mainly of concern for steel components where fatigue endurance
limits the design. For fibre-rope segments, the time-dependent strength can limit the design; consequently
stress rupture or creep failure should be incorporated in the checks for ULS and ALS as appropriate. See also
DNV-OS-E301, /27/.
5.9.4.1.7 Where structural items e.g. grillages and seafastenings, are to be re-used they should be
demonstrated to have sufficient fatigue life for the sequence of planned operations, including all previous
operations. An appropriate inspection regime shall be proposed including NDT at appropriate intervals e.g.
close visual examination after every use and NDT after every 10 uses; if there are highly utilised areas, more
frequent NDT may be appropriate. For bolts, see [E.2].
5.9.4.2 Design factors - fatigue limit states
5.9.4.2.1 All load factors shall be:
γf = 1.0s
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5.9.4.2.2 Design fatigue factors (DFF) shall be applied to increase the probability of avoiding fatigue failures.
5.9.4.2.3 The calculated cumulative damage ratios for the defined design conditions times the applicable DFF
according to Table 5-4 shall be less than or equal to 1.0.
5.9.4.2.4 Lower values for the Miner’s sums than 1.0 can be relevant if the structure has been or will be
subjected to fatigue loading before or after the considered marine operation. In such cases the maximum
allowable Miner’s sum for the actual marine operations shall be determined by considering the total load
history the structure will be exposed to.
Table 5-4 Design fatigue factors (DFF)
Inspection during operation
(and repair) planned
Elements in inspection category I
Elements in inspection
categories II and III
Yes
2.0
1.0
No
3.0
2.0
1)
The elements shall be categorised according to the definitions in Table 5-12.
2)
Higher DFF than indicated may be applicable based on other (project) governing codes.
3)
The indicated DFF are applicable only for the fatigue utilization during the considered marine operation. Hence,
if the fatigue utilization is combined with the utilization from other phases, see [d)], a different DFF may be
applicable.
5.9.5 Accidental limit states
5.9.5.1 General
5.9.5.1.1 Accidental limit states (ALS) for marine operations include verification of:
— ALS-I: the intact structure or system for the defined accidental load effect(s) combined with other
relevant load effects, see Table 5-1 (i.e. loads of type E may be ignored).
— ALS-D: the damaged structure or system, see [5.9.5.1.2], for relevant design load effects, see Table 5-1.
Guidance note:
See also Table 5-3 for definition of ALS-I and ALS-D.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.5.1.2 The damage to the structure or system in ALS-D is normally defined by either:
— the damage caused by the defined accidental load effect(s) or,
— a defined damaged or an accidental condition/scenario, see [5.6.6].
5.9.5.2 Design approach and load and resistance factors
5.9.5.2.1 Accidental loads are defined in [5.6.6].
5.9.5.2.2 Design against accidental loads shall primarily consider global failure modes, see [5.7.3]. E.g.
increasing of local strength which may reduce the safety against overall failure of the structure should be
avoided.
5.9.5.2.3 Load factors should in ALS normally, see d), be taken according to Table 5-5 or Table 5-6.
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5.9.5.2.4 Load factors greater than 1.0 shall be considered if an LRFD method ALS load or condition is not
considered to have a sufficient low, i.e. ≤ 10-4 per operation, probability. If working to the approach, the
factors should be similarly increased.
5.9.5.2.5 The characteristic environmental load (E) in the ALS-D load condition should/may be defined
considering the probability of the analysed accident/damage and the anticipated maximum period (i.e.TR, see
[2.6.2]) the damaged situation will remain.
Table 5-5 ASD/WSD load factors for ALS member strength checks using ANSI/AISC 360-22
Type
ANSI/AISC 360-22 ASD/WSD option strength checking
ALS-I
0.6
ALS-D
0.6
1)
The load factor of 0.6 for the ASD/WSD case arises because the basic allowable stress in ANSI/AISC
360-22 is 0.6*yield. In order to effectively work to yield, the load is multiplied by 0.6 and used with the
standard allowable of 0.6*yield.
Table 5-6 LRFD load factors for ALS
Load
condition
Load categories
G
Q
D
E
A
ALS-I
1.0
1.0
1.0
n/a
1.0
ALS-D
1.0
1.0
1.0
1.0
n/a
1)
Load categories G, Q, D, E and A are described in [5.5.2].
5.9.5.3 Material factor - accidental limit states
The material factor may in ALS generally be taken equal to:
γm, ALS = γm/1.15
where γm = the applicable material factor in ULS, see [5.9.8.3].
Guidance note:
E.g. the ALS material factor for steel wire ropes may be taken as γm, ALS = 1.5/1.15 = 1.3.
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5.9.6 Serviceability limit states
5.9.6.1 General
5.9.6.1.1 For some marine operations it is relevant to check serviceability limit states (SLS) related to the
feasibility of the operation. Such serviceability limit states may be associated with required clearances, push/
pull capacities and vessel (barge) level (compared e.g. with quay height).
5.9.6.1.2 See DNV-OS-C101 Ch.2 Sec.7, /24/ (Ed: July 2019 Amended 2021), for typical SLS requirements
for offshore steel structures.
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5.9.6.2 Safety factors
5.9.6.2.1 For SLS related to feasibility the load factors are normally equal to 1.0. Relevant safety factors/
margins should be defined considering the actual operation. See Sec.6 to Sec.18 for guidance.
5.9.6.2.2 SLS for structural elements shall normally be checked applying load and material factors equal to
1.0.
Guidance note:
In SLS the object (or equipment/vessel) owner is free to define higher load- and material factors if this is found applicable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.7 ASD/WSD strength checks to LS1 or LS2/LS2A loading
5.9.7.1 The ASD/WSD design approach is described in [5.9.2.1].
5.9.7.2 The primary structure and any critical temporary works like lifting attachments, spreader bars and
seafastenings shall be of high quality structural steelwork with full material certification and NDT inspection
certificates showing appropriate levels of inspection.
5.9.7.3 Still-water and other load cases that are gravity dominated shall be checked to LS1. Other load cases
may be checked to LS1.
Guidance note:
In many situations still-water and static load cases are covered by the in-place design.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.7.4 LS2 is applicable only when the loading is dominated by environmental/storm loads, e.g. for weather
unrestricted operations when the extreme loads are due to the applicable design return period environmental
criteria, see Table 3-1 or for weather restricted operations provided an alpha factor according to [2.6.12]
is to be applied. Any LS2 load case may be treated as a gravity-load dominated limit state (LS1). LS2A is
applicable only when the loading is determined from classification society rules, see [11.6]. Any LS2A load
case may be treated as a gravity-load dominated limit state (LS1). LS2 and LS2A shall not be used for:
a)
b)
Steelwork subject to deterioration and/or limited initial NDT unless the condition of the entire load path
has been verified, for example the underdeck members of a barge or vessel.
Steelwork subject to NDT before elapse of the recommended cooling and waiting time as defined by the
welding procedure specification (WPS) and NDT procedures. In cases where this cannot be avoided by
means of a suitable WPS, it may be necessary to increase the strength or impose a reduction on the
design/permissible sea state.
which shall be checked to LS1.
Guidance note:
For steelwork supporting bumpers and guides see [4.4.3].
For spreader bars, lift points and primary steelwork of lifted items see [16.8.2.4].
For structures during a load-out see [10.4.1.1].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.7.5 Traditionally AISC has also been considered a reference code, e.g. by API RP 2A, /166/. If the ANSI/
AISC 360-22 American National Standard Specification for Structural Steel Buildings of August 2022 is used,
the allowables shall be not less than the member stresses determined using a load factor on all loads (dead,
live, environmental, etc.) of no less than the applicable of those detailed in Table 5-7.
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Table 5-7 Load factors for ASD/WSD method strength checks using ANSI/AISC 360-22
Type
ANSI/AISC 360-22 ASD/WSD option strength checking
Limit state 1 (LS1)
1.00
1)
1), 2), 3)
Limit state 2 (LS2)
0.75
Limit state 2A (LS2A)
0.84
1), 4)
1)
Where the Sec.11 loads are due to accelerations determined according to DNV or DNV class rules, see [11.6],
LS1 or LS2A shall be used.
2)
The load factor of 0.75 for ASD/WSD in the LS2 case arises because the basic allowable stress in ANSI/AISC
360-22 is 0.6*yield and the traditional 1/3 increase to 0.8*yield (i.e. to 0.6*yield*4/3) for environmental load
cases is not included. As an alternative, the load is multiplied by 3/4 and used with the standard allowable of
0.6*yield in order to achieve the safety levels that have been used and accepted over many years.
3)
Any load LS2 case may be treated as a gravity-load dominated limit state LS1.
4)
Any load LS2A case may be treated as a gravity-load dominated limit state LS1.
5)
Where the loads are determined using IMO, then the strength shall be checked using IMO requirements.
Guidance note:
nd
The API RP 2A-WSD 22
edition references the 9
th
Edition of AISC Steel Construction Manual, which includes the traditional '1/3
increase' for infrequent environmentally dominated load cases (API says 'Where stresses are due in part to the lateral and vertical
forces imposed by design environmental conditions ...'). The AISC Steel Construction Manual 15
th
Edition does not reference the
1/3 increase, instead it allows the referencing code to specify load factors.
The LS2 load factors herein effectively allow the 1/3 increase.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.7.6 Stresses in welds determined from an analysis using the applicable load factor from Table 5-7 shall
have utilisations no greater than the applicable value from Table 5-8 and be assessed according to either:
a)
b)
the method given in DNVGL-OS-C102 Ch.2 Sec.9 [2.5], /25/, or equivalent, or
the method illustrated by the example given for the assessment of fillet welds for brackets given in [E.1].
Guidance note:
For welds made on board the vessel the permitted utilisation for welds made at a fabrication site is acceptable provided that the
welding conditions are good, see [5.10.2.1.3], and there is good weld fit-up (e.g. control of correct/no gaps to deck plate).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Table 5-8 Weld permissible usage factors for ASD/WSD method
Weld permissible usage factor, η
Location of
welding operation
Weld type
Yield stress
Ultimate stress
Full penetration
Any
Use design resistance of the weaker of the joined parts
Partial penetration and fillet
Fabrication site
0.60
0.53
Onboard the vessel
0.52
0.46
1)
Where the loads are determined using IMO, then the strength shall be checked using IMO requirements.
2)
Where the Sec.11 loads are due to accelerations determined according to DNV class rules, see [11.6.2] or [11.6.3]
(as applicable), LS1 or LS2A shall be used as per [11.6.6].
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5.9.7.7 Below deck welds in vessels classed to DNV rules for classification of ships (up to 2016-07) may
be checked against 90fw in shear on the weld throat and 160fw for normal stress perpendicular to the weld
throat, where fw is the material factor for the applicable strength group as given in /15/.
5.9.7.8 The allowable strength of slip critical bolted connections shall be assessed according to the method
given in [E.2] and have utilisations no greater than the permissible usage factors in Table 5-9. The loads shall
be determined from an analysis using the applicable load factor from Table 5-7.
Table 5-9 Slip critical bolted connections permissible usage factors for ASD/WSD method
Permissible usage factors, η
Bolt hole type
DNV or DNV class rules, see [11.6]
2)
Other approaches given in this standard
1)
(excluding IMO)
LS1
LS2A
LS1
LS2
Standard hole clearances
0.55
0.41
0.55
0.41
Oversize or slotted holes
0.49
0.37
0.49
0.37
1)
Where the loads are determined using IMO, then the strength shall be checked using IMO requirements.
2)
Where the Sec.11 loads are due to accelerations determined according to DNV or DNV class rules, see [11.6.2]
and [11.6.6], LS2A may be used.
3)
The permissible usage factors for LS2 and LS2A are less than those for LS1 because it is required that bolts are
effectively checked to LS1.
5.9.7.9 The design of non-tubular connections shall be in accordance with an appropriate standard such as
AISC/AISC 360-22, /2/, using a consistent safety format and factors.
5.9.8 Load and resistance factor design strength checks to ultimate limit
states loading
5.9.8.1 General
DNV-OS-C101 Ch.2 Sec.4, /24/ (Ed: July 2019 Amended 2021), gives provisions for checking of ultimate
limit states (ULS) for typical structural elements used in offshore steel structures.
5.9.8.2 Load factors - ultimate limit states
5.9.8.2.1 For the ultimate limit states (ULS) the two (2) load conditions 'ULS-a' and 'ULS-b' as given in the
Table 5-10 shall be considered.
Table 5-10 Load factors for ULS
Load
condition
Load categories
G
Q
D
E
A
ULS-a
1.3
1.3
1.0
0.7
n/a
ULS-b
1.0
1.0
1.0
1.3
n/a
1)
Load categories G, Q, D, E and A are described in [5.5].
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5.9.8.2.2 For loads and load effects that are well controlled a reduced load factor γf = 1.2 may be used for
the G and Q loads instead of 1.3 in load condition ULS-a.
Guidance note:
Examples where γf = 1.2 may be applicable are:
—
External hydrostatic pressure caused by an accurately defined water level.
—
Loads due to an accurately distributed (i.e. static determinate) well defined self-weight.
—
Functional loads accurately defined (limited) by the maximum (possible) capacity of equipment.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.8.2.3 Where a permanent load G (e.g. self-weight or hydrostatic pressure) causes favourable load
effects, a load factor γf = 1.0 shall be used for this load in load condition ULS-a. See also [5.6.2.2] and
[5.6.2.3].
5.9.8.2.4 In cases where the load is the result of counteracting and independent large hydrostatic pressures
the appropriate load factor shall be applied to the pressure difference. However, the pressure difference
should not be taken less than 0.1 times the hydrostatic pressure.
5.9.8.2.5 In dynamic problems the application of load factors should be given special consideration. In lieu
of a probabilistic analysis, the load effects may be found by application of load factors after having found the
responses, e.g. after having solved the equations of motion for vessel motion response analysis.
5.9.8.3 Ultimate limit state material factors
5.9.8.3.1 Applicable material factors in ULS are given in [5.9.8.4] to [5.9.9]. Material factors for materials
not mentioned in [5.9.8.4] to [5.9.9] e.g. concrete, concrete reinforcement, wood, synthetic materials, soil,
etc. shall be in accordance with a recognized code or standard. See also [5.9.3.3].
5.9.8.3.2 If a material factor γm = 1.0 is found more unfavourable than the indicated values, γm = 1.0 shall
be used.
5.9.8.4 Material factors for structural steel
5.9.8.4.1 In ULS the material factors for steel structures should be taken as minimum: γm = 1.15.
5.9.8.4.2 For members in compression a higher material factor may be applicable. The material factor should
normally be chosen according to the applied design code, but never smaller than 1.15.
5.9.8.4.3 If EN 1993 (Eurocode 3) /61/ is used for calculation of structural resistance, the material factors
listed in DNV-OS-C101 Ch.2 Sec.4, /24/ (Ed: July 2019 Amended 2021), for steel structures and DNV-OSC101 Ch.2 Sec.8, /24/ (Ed: July 2019 Amended 2021), for welded connections shall be applied.
Guidance note 1:
See also Table 5 in NORSOK N-004, /112/, for applicable material factors.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The plastic design provisions in Eurocode 3, /61/, should be used only when the loading is applied in a constant direction and, if
there will be repeated loading then only for lower strength materials (i.e. yield no greater than 355 MPa).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.8.4.4 In ULS the material factor for static strength of tubular joints should be chosen according to the
applied design code, but never smaller than 1.15.
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5.9.8.4.5 An increased material factor shall be considered if the production is carried out in an environment
where reduced control of dimensions, materials and fabrication could be expected, e.g. welding on board
vessels. The minimum material factors, γmW, given in Table 5-11 apply when the weld capacity is calculated
according to DNV-OS-C101 Ch.2 Sec.8, /24/ (Ed: July 2019 Amended 2021), EN 1993-1-8 or [E.1].
Table 5-11 Weld material factors for LRFD
Weld material factor, γmW
Location of
welding operation
Weld type
Yield stress
Ultimate stress
Full penetration
Any
Use design resistance of the weaker of the joined parts
Partial penetration and fillet
Fabrication site
1.15
1.3
Onboard the vessel
1.3
1.5
Guidance note:
For welds made on board the vessel γmW for welds made at a fabrication site is acceptable provided that the welding conditions
are good, see [5.10.2.2], and there is good weld fit-up (e.g. control of correct/no gaps to deck plate).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.8.4.6 When [5.9.7.4] a) or b) apply the material factor shall be multiplied by 1.3.
5.9.8.5 Material factors for ropes, chain and bolts
5.9.8.5.1 In ULS the material factor for certified steel wire ropes and chains should normally be taken as:
γm = 1.5
Guidance note:
γm = 1.15/0.85/0.9 = 1.5
where:
1.15 is the general steel material factor,
0.85 is a factor to account for the characteristic strength of ropes and chains being based on the tensile strength (MBL), see
[5.9.3.3] guidance note 2, and
0.9 is a general factor because wire ropes and chains are considered more vulnerable to 'undetectable' wear and material
irregularities than regular steel structures. For as-new ropes to be used in lifting and with a '3.2 certificate' it may be acceptable to
use 1.0, see [16.4.9.2]. (Note also that an additional wear factor could be applicable).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.8.5.2 For fibre ropes and webbing straps the material factor depends on the material and relevant failure
mode. The following minimum factors apply (but see [16.4.9.4] for fibre lifting slings):
— Polyester: 1.65
— HMPE and aramid: 2.0
— Other fibre materials: 2.5, including nylon (polyamide), polyethylene and polypropylene.
For fibre ropes subject to a robust certification process, other material factors may be considered acceptable;
however the material factor should not be taken to be lower than that required for polyester ropes.
Guidance note:
OCIMF Mooring Equipment Guidelines, /113/, advises that both polyethylene and polypropylene are generally not recommended as
a mooring line.
HMPE (high-modulus polyethylene) may also be referred to as ultra-high-molecular-weight polyethylene (UHMWP).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
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5.9.8.5.3 When used in lashings, the minimum material factor for shackles, turnbuckles, load binders, D-links
and web lashing assemblies should normally be taken as:
γm = 1.95
Additionally, for 'web lashing assemblies', the webbing should be checked using the applicable material factor
from [5.9.8.5.2].
5.9.8.5.4 When using DNVGL-OS-C101 July 2018 Ch.2 Sec.11 [2], /24a/, Eurocode 3 /61/ or [E.2], the
material factor for slip resistant bolt connections shall be taken as minimum:
— γm = 1.25 for standard clearances in the direction of the force.
— γm = 1.4 for oversize holes or long slotted holes in the direction of the force.
Guidance note:
[E.2] provides for further information regarding slip resistant bolt connections and an alternative methodology.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.9.9 Material factors for friction (ASD/WSD and LRFD)
5.9.9.1 A material factor of minimum γm = 1.4 should normally be used to calculate the lower bound design
friction coefficient for load bearing friction effects.
5.9.9.2 A material factor of maximum γm = 0.8 should normally be used to calculate the upper bound design
friction coefficient. See [5.4.4].
Guidance note:
In each case, the design friction coefficient should obtained in accordance with [5.6.9].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.10 Materials and fabrication
5.10.1 Design considerations
5.10.1.1 Applicable codes
5.10.1.1.1 In general material selection, fabrication method, and non-destructive testing should be carried
out according to a recognized offshore code, e.g. DNV-OS-C101, /24/, or DNV-OS-C401, /26/.
Guidance note:
Recognized codes or standards are meant to be national or international codes or standards applied by the majority of professional
people and institutions in the marine and offshore industry.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.10.1.1.2 Independent of the applied code, it shall be documented that the requirements in subsection
[5.10] are fulfilled.
5.10.1.2 Structural categories
5.10.1.2.1 Structural elements and connections shall be grouped in categories determined according to:
— type of stress
— presence of cyclic loading
— presence of stress concentrations
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— presence of restraint
— loading rate
— consequences of failure
— redundancy.
5.10.1.2.2 Guidelines for selection of applicable materials for offshore steel structures can be found in DNVOS-C101 Ch.2 Sec.3, /24/ (Ed: July 2019 Amended 2021).
Guidance note:
For steel with yield stress below 500 MPa, the test temperature need not be taken lower than -40°C.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.10.1.2.3 For materials in temporary structures used for marine operations, the following apply:
— The design temperature, see DNV-OS-C101 Ch.2 Sec.3 [2], /24/ (Ed: July 2019 Amended 2021), should
be defined based on the season and location(s) of the marine operation. Note that a design temperature
above 0ºC may be applicable.
— See Table 5-12 for guidelines regarding selection of structural category. See also DNV-OS-C101 Ch.2
Sec.3 [3], /24/ (Ed: July 2019 Amended 2021).
— For materials that could be welded under adverse conditions the yield strength (SMYS) should not exceed
355 MPa.
5.10.1.3 Material quality
5.10.1.3.1 Selection of steel types shall be determined based on the structural application and the required
category, see Table 5-12.
5.10.1.3.2 All steel materials shall be suitable for the intended service conditions and shall have adequate
properties of strength, ductility, toughness, weldability and corrosion resistance.
5.10.1.3.3 Material types and qualities should comply with requirements in DNV-OS-B101, /23/ or, where
applicable, DNV-ST-0378, /129/.
5.10.1.3.4 Non-structural steels shall have mechanical properties and weldability suitable for the intended
application.
Table 5-12 Structural categories
Selection criteria for
structural category
Examples for typical structures
involved in marine operations
Recommended
structural
category
Failure
consequence
Structural part
Substantial,
the structure
possesses limited
2)
residual
strength
Complex joints
— Padeyes and other lifting
points
Special
Simple joints
and members
— Seafastening elements
without redundancy
Special
or
1)
— Spreader bars
DNV-OS-C101
3)
primary
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NORSOK N-004
equivalent /112/
Insp.
cat.,
DNV
DC1 – SQL I
I
4)
SQL I
or
3)
I or
5)
II
DC2 – SQL II
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Not substantial,
the structure
possesses
2)
residual
strength
Un-substantial,
as local failure
will be without
substantial
consequences
Complex joints
1)
Structures for connection of:
— Mooring and towing lines
— Grillages
Simple joints
and members
Any structural
part
— Redundant
elements
2)
seafastening
— Bumpers and guides
Special
or
3)
primary
Special
or
3)
SQL I
or
3)
II
DC3 – SQL II
SQL I
or
3)
primary
DC4 – SQL III
Secondary
DC5 – SQL IV
II
III
— Fender structures
— Redundant
grillages
2)
(parts of)
1)
Complex joints are joints where the geometry of connected elements and weld type leads to high restraint and
to tri-axial stress pattern.
2)
Residual strength (redundant) means that the structure meets requirements corresponding to the damaged
condition in the check for ALS, with failure in the actual joint or component as the defined damage.
3)
Selection where the joint strength is based on transference of tensile stresses in the through thickness direction
of the plate.
4)
The design classes and material selection according to NORSOK M-120, /110/, should be considered as guidance
only.
5)
Extent of NDT to be according to DNV category I in Table 5-13, but category II may be used as 'input' in Table
5-13 regarding waiting time for these welds. Regarding extent of inspection according to NORSOK M-101, /109/
inspection category B is normally acceptable.
5.10.1.4 Tolerances
5.10.1.4.1 As-built deviations shall not exceed fabrication tolerances assumed in the applied structural codes
and standards, or in the design analysis, unless specially considered on a case-by-case basis.
5.10.1.4.2 Acceptance of any as-built deviations exceeding specified tolerances shall be confirmed in writing
by, as applicable, the owner, designer, installation contractor, etc.
5.10.1.4.3 DNV-OS-C401 Ch.2 Sec.6 [11], /26/, indicates fabrication tolerances that are normally
acceptable.
5.10.1.4.4 Some marine operations procedures can be difficult (or impossible) to execute when standard
tolerances are applied. In these cases consideration may be given to defining and documenting the
consequences of using tolerances that are less onerous than those indicated in DNV-OS-C401, /26/.
5.10.2 Fabrication
5.10.2.1 Workmanship
5.10.2.1.1 Workmanship during fabrication shall be of good standard and according to accepted practice. See
also DNV-OS-C401 Ch.2 Sec.2 [3], /26/.
5.10.2.1.2 Guidelines regarding assembly and welding can be found in DNV-OS-C401 Ch.2 Sec.6 [8], /26/,
and DNV-OS-C401 Ch.2 Sec.6 [9], /26/.
5.10.2.1.3 Environmental conditions during marine construction work can be unfavourable and the time
available is often limited. Also accurate fit-up can be difficult to obtain e.g. due to a dented barge deck. Such
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issues regarding marine work shall be duly considered in the planning of the work. See also [5.9.7.6] or
[5.9.8.4].
Guidance note:
Due to the special conditions during marine construction work, the following precautions are recommended:
a)
Welding procedure specifications should be qualified by welding procedure tests carried out under conditions representative
b)
Thorough inspections of fit-up and welding should be planned for.
c)
Weather conditions and forecast to indicate acceptable conditions for welding considering the welding method and available
d)
Use of increased weld size in order to compensate for inaccurate fit-up (i.e. over-sized gaps) to be considered.
of the actual working environment; see DNV-OS-C401 Ch.2 Sec.5 [2.2], /26/.
shelter at the welding locations.
e)
Robust and well proven welding methods and procedures to be applied.
f)
Use of material with improved weldability; see DNV-OS-C101 Ch.2 Sec.3 [4.2], /24/ (Ed: July 2019 Amended 2021), to be
considered.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
5.10.2.2 Weld inspection
5.10.2.2.1 All non-destructive testing (NDT) of structures and structural components shall be carried out by
qualified personnel and covered by written specifications and procedures.
5.10.2.2.2 Personnel evaluating results from NDT shall possess thorough knowledge and experience with
NDT.
5.10.2.2.3 The NDT method selected shall be suitable for detection of the type of defects considered
detrimental to the safety and integrity of the structures.
5.10.2.2.4 The extent of NDT shall be based upon the importance of the connection in question. Aspects
which shall be considered in specifying the extent of NDT are:
— stress level and stress direction
— cyclic loading
— material toughness
— redundancy of the member
— overall integrity of the structure
— accessibility for examination.
5.10.2.2.5 Where through thickness properties of the steel are used, the material should be certified
accordingly (Z-quality). Where this is not feasible, the material under through-thickness tension should be
checked for laminations after the recommended cooling and waiting time as defined by the welding procedure
specification (WPS) and NDT procedures. The reason for waiting is that laminations can also be subject to
hydrogen embrittlement, the same as welds, see SSC-290, /118/, for more details of lamellar tearing. If
access is not possible after welding, pre-welding checks may be acceptable.
Guidance note 1:
For non-critical seafastenings and their supports, through-thickness testing should be carried out when the tensile stress normal to
any plate exceeds 100 MPa.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The tensile stress should be calculated in a section between the deck plate and the weld (i.e. not in the critical weld section). If the
under deck weld is smaller, this weld should be used as a reference, see also guidance note to [11.9.5.4]. Stresses greater than
100 MPa, caused by e.g. a local moment on seafastening brackets can generally be accepted in limited areas without lamination
testing.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
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5.10.2.2.6 Requirements to non-destructive testing (NDT) of welds can be found in DNV-OS-C401 Ch.2
Sec.7, /26/. Equivalent standards may be used e.g. EEMUA 158 Construction specification for fixed offshore
structures in the North Sea /59/ and AWS D1.1/D1.1M Structural welding code – steel, /8/.
5.10.2.2.7 Minimum extent of inspection should be as shown in DNV-OS-C401 Ch.2 Sec.7 Table 2, /26/, with
'inspection category' as defined in Table 5-12. See also Table 5-13 for a summary and especially note 4) to
the table.
5.10.2.2.8 Normally final inspection and NDT of welds shall not be carried out before 48 hours after
completion. A shorter minimum waiting time may be acceptable depending on material thickness and yield
strength. See NORSOK M-101, /109/, Sec.9.1 and DNV-OS-C401 Ch.2 Sec.7 [1.2], /26/, for further details.
5.10.2.2.9 For marine operations with weld inspection on the critical path, the minimum waiting time should
be selected according to Table 5-13 however, the decreased waiting may only be used if the precautions
listed in [5.10.2.1.3] are fulfilled.
Guidance note:
Weld inspection may be completed after a voyage has commenced provided that procedures are in place to remediate or mitigate
any defects that are found.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Table 5-13 Minimum extent of NDT and waiting time
Inspection
category
Minimum extent of NDT
Minimum waiting time before NDT
1)
SMYS
2)
≤355 MPa
3)
SMYS
2)
Visual
Other
> 355 MPa
I
100%
100%
24 hours
4)
48 hours
4)
II
100%
20%
5)
Cold weld
6)
24 hours
4)
III
100%
5%
5)
Cold weld
6)
24 hours
4)
3)
1)
Test method to be selected according to the type of connection, see DNV-OS-C401 Ch.2 Sec.7 Table 2, /26/.
2)
SMYS to be defined according to the specification for the actual material used and not according to the
minimum required design value.
3)
For thickness less than 40 mm the limiting SMYS is 420 MPa.
4)
The use of PWHT (post weld heat treatment) can reduce the required waiting time.
5)
An increased % extent shall be evaluated if defects are found and/or the weld conditions and precautions, see
[5.10.2.2], are not fully satisfactory.
6)
The NDT may start when the weld is cold, but it is recommended to wait as long as practicable.
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SECTION 6 GRAVITY BASED STRUCTURE
6.1 Introduction
6.1.1
This section is mainly applicable to 'Condeep'-type gravity based structures (with one or more columns above
a submerged base). However the principles will apply to most types of steel and concrete gravity based
platforms.
6.1.2
The areas shown in Table 6-1 are covered. Depending on the type of structure and method of construction,
some or all of the following sections will give the relevant requirements.
Table 6-1 Requirements for different GBS phases
General requirements
See Sec.2 to Sec.4
Stability and freeboard (all phases)
See [6.2]
Structural strength
See [6.3] and Sec.5
Temporary ballasting and compressed air systems
See [4.3]
Construction basin and tow-out
See Sec.12
Construction and/or solid ballasting afloat
See Sec.14
Deck-mating (inshore or offshore)
See Sec.15
Towage(s)
See Sec.11
Instrumentation
See [6.4]
Installation at location
See [6.5]
Ensuring on-bottom stability
See [13.10]
6.2 Floating gravity based structure stability and freeboard
6.2.1 General
6.2.1.1 Sufficient positive stability and reserve buoyancy shall be ensured during all stages of the marine
operations. Both intact and damage stability shall be evaluated, on the basis of an accurate geometric model.
This shall include inclining tests of the gravity base structure (GBS) in accordance with [2.10.5] at stages
agreed with the MWS company.
6.2.1.2 In calculations of stability and reserve buoyancy/freeboard, due allowance shall be included for
uncertainty in mass, buoyancy, volume, location of centre of gravity, density of liquid and solid ballast, and
density of seawater.
6.2.1.3 The output of the weight control programme as described in [5.6.2] shall be taken into account.
6.2.1.4 Stability calculations should include corrections and allowances for:
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a)
b)
c)
d)
e)
Free surface
Air cushion
Icing
Influence of moorings, including a check on the consequences of failure.
Temporary loads and structures (including any cantilevered structures)
6.2.1.5 The number of openings in buoyant elements adjacent to the sea shall be kept to a minimum. Where
penetrations are necessary for access, piping, ventilation, electrical connections, etc. arrangements shall
be made to maintain watertight integrity. During construction phases, particular attention should be paid to
openings near the waterline, which will vary as construction proceeds.
6.2.1.6 Damage stability requirements shall be evaluated considering the operation procedure,
environmental loads and responses, the duration of the operation and the consequences of possible damage.
Compartments that may be subject to flooding or partial flooding include:
a)
b)
c)
Compartments adjacent to the sea
Compartments inside the structure, crossed by seawater filled pipes
Skirt compartments containing compressed air.
6.2.1.7 Special attention should be paid to flooding which may be caused by:
a)
b)
c)
d)
Impact loads from vessels
Damage to structure or pipework from dropped objects
Mechanical system failure
Human error.
6.2.1.8 Flooding as a result of vessel impact is assumed to occur in a zone bounded by two horizontal planes
normally positioned 5 m above and 8 m below the waterline. These levels should be reviewed if deep draught
vessels are likely to be operating nearby.
6.2.1.9 For operations where the structure cannot meet damage stability criteria, measures shall be taken to
minimise the risk, by:
a)
b)
c)
d)
e)
Limiting the exposure period
Providing additional local structural strength
Providing additional protection, such as fendering
Minimising vessel movements near the structure
Dedicated procedures and experienced personnel.
6.2.1.10 For operations where at any stage stability or reserve buoyancy is critical or where damage stability
cannot be obtained, a risk assessment in accordance with [2.4] shall be carried out. The duration of the
critical condition should be minimised. Requirements for back-up or protection systems, or special procedures
should be assessed.
6.2.2 Intact stability
6.2.2.1 The initial GM shall not be less than 0.5 m (after allowing for all possible inaccuracies in measuring
it) unless agreed with the MWS company.
6.2.2.2 The maximum inclination of the floating GBS or platform should not exceed 5° in the design
environmental condition as defined in [3.1] apart from possible exceptions during installation as described in
the guidance note to [6.5.4.4]. Calculation of maximum inclination should take into account:
a)
b)
Maximum amplitude of pitch or roll motion in the design sea state, plus
Inclination due to design wind, plus
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c)
Inclination due to mooring line tensions or required towline pull.
Guidance note:
The maximum inclination of 5° is due to the large height of GBS structures and the corresponding motion experienced at this
height.
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6.2.2.3 During towing, the static inclination in still water when subjected to 50% of required towline pull
should not normally exceed 2°. Differential ballasting may be used to reduce the static inclination resulting
from towline pull only by not more than 1°.
6.2.2.4 The area under the righting moment curve shall be not less than 140% of the area under the
overturning moment curve as shown in Figure 11-2. Both curves shall be bounded by the least of:
a)
b)
c)
d)
The second intercept of the righting and overturning moment curves
The angle of downflooding
The angle which would cause any part of the GBS to touch bottom in the minimum water depth at the
construction site or along the towage route. This requirement may be deleted for installation at the
offshore site.
The angle at which allowable stresses are reached in any part of the structure, construction equipment,
topsides or topsides attachments, if applicable.
6.2.2.5 The wind used for overturning moment calculations should be the design wind for the operation,
as defined in [3.3]. Short duration operations during construction or towage may be considered as weather
restricted operations, provided the structure can achieve or be returned to a safe condition, within the
operation reference period
6.2.3 Effective freeboard
6.2.3.1 For inshore towages and construction afloat, the effective freeboard, as defined in Table 1-4, shall
not be less than the greater of:
a)
b)
1 m above the design wave crest height, with allowance for run-up, all around the structure, under the
design storm loading from the most critical direction,
6 m in the intact condition, if the unit does not have one-compartment damage stability.
6.2.3.2 For offshore towages, after damage, an effective freeboard of not less than 5 m shall remain above
the design wave crest height, with allowance for run-up, all around the structure, from the most critical
direction. Calculation of the freeboard shall account for motions experienced as a result of the design
environmental conditions and mooring line tensions or required towline pull.
6.2.4 Damage stability for tow-out and inshore tows
6.2.4.1 For tow-out from dry-dock, one-compartment damage stability is not required as it is a controlled
operation and the under-keel clearance is limited.
6.2.4.2 For other inshore tows the structure should have one-compartment damage stability, as defined in
[6.2.1.6] through [6.2.1.8].
6.2.4.3 If one-compartment damage stability requirements cannot be fulfilled, the requirements for
construction afloat in [6.2.5.2] shall apply.
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6.2.5 Damage stability during construction afloat
6.2.5.1 During the period of construction afloat, the platform shall possess one-compartment damage
stability, for as much of the construction period as is practical.
6.2.5.2 When the platform does not possess one-compartment damage stability, then in addition to
[6.2.1.9]:
a)
b)
c)
d)
e)
f)
g)
h)
A means should be available to compensate for inclination due to flooding of any compartment, and
There shall be sufficient structural strength in the outer walls to withstand impact loads from the
construction spread and vessels, which may be in close proximity to the platform, and
Fendering may be used to reduce impact loads in critical areas, and
Lifting of heavy objects shall be carefully controlled. Protection shall be provided against dropped
objects. Any lifts which, if dropped, could endanger the platform shall be identified and additional
precautions taken, and
Any objects or equipment on barges alongside, which if dropped, could endanger the platform shall be
similarly identified and additional precautions taken, and
Rigorous procedures shall be developed to minimise the risk of flooding. These shall include
consideration of collision, leakage through the ballast or other systems, reliability and redundancy of
pumping arrangements and power supplies, and
At all times there shall be adequately trained personnel on board the platform, and
As per [6.2.1.10], a risk assessment of flooding shall be carried out in accordance with [2.4].
6.2.6 Damage stability for offshore tows and installation
6.2.6.1 When towing on the caisson or columns the platform should possess one-compartment damage
stability.
6.2.6.2 It is acknowledged that for an offshore tow, the requirement in [6.2.6.1] might be impractical, in
which case:
a)
b)
c)
The structure shall be locally reinforced within the zone defined in [6.2.1.8], to withstand impact from
the largest towing or attending vessel, and/or
Rigorous procedures shall be developed to minimise the risk of flooding, and
A risk assessment of flooding shall be carried out in accordance with [2.4].
6.2.6.3 It is acknowledged that during installation, it might be impractical to provide reinforcement against
collision over the full range of waterlines. Planning and risk assessment shall include a procedure to return
the structure to the reinforced waterline should the installation operation be aborted.
6.3 Structural strength
6.3.1 Concrete gravity structures - load cases
6.3.1.1 The requirements of Sec.5 apply.
6.3.1.2 Load cases shall be derived by the addition of fluctuating loads resulting from wind, wind heel,
wave action and the effect of towline pull or mooring loads to the static forces resulting from gravity and
hydrostatic loads for the following temporary phases before it is safely installed:
1)
2)
tow-out from construction basin or dry-dock (with and without any air cushion)
the most critical construction afloat stages
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3)
4)
5)
any towages, with or without a deck
deep submergence for deck mating
installation on the seabed, including:
— any impact with the seabed including any rocks or debris during installation
— penetration and grouting phases
— any impact with scour protection during its placement.
— Any other critical phase as agreed with the MWS company.
6.3.1.3 Accidental loadings shall also be considered for all of the phases in [6.3.1.2].
6.3.1.4 The specific load cases considered shall be documented. For all load cases it shall be documented
that the design (global and local) is acceptable.
6.3.1.5 The unit shall be able to safely withstand a static heel angle of 10°, or any greater angle required
during construction, towage or installation. If it has damage stability, the unit shall also be able to withstand
the static and dynamic loads caused by the flooding of any one compartment in the lesser of the 10-year
return period environmental conditions or a 25 m/s wind and associated waves. These should be assessed as
LS1 or ULS conditions, unless it is demonstrated that alternative criteria apply.
6.3.1.6 Hydrostatic loads on the substructure at the deepest draught during deck-mating can be the
governing load case. It shall be demonstrated that a thorough independent check of the calculations
covering this load case has been carried out, and that the design and reinforcement details assumed in the
calculations concur with the as-built condition.
6.3.1.7 Any limitations on the maximum allowable duration of deep immersion due to concrete creep, in
relation to the structural stability of the unit, should be established and the procedures planned accordingly.
6.3.2 Structural concrete
6.3.2.1 The strength of concrete and its reinforcement including any pre- or post-tensioning shall comply
with a recognized and appropriate concrete design code, such as those listed in ISO 19903, /101/. Any timedependent properties of the materials shall be taken into account. Adequate global and local strength shall be
documented.
6.3.2.2 The strength of the structure in the installed condition should be covered by the relevant certifying
authority or classification society who will normally refer to a suitable offshore structural code or rules such
as DNV-ST-C502, /41/.
6.3.2.3 Testing of concrete for permanent works should be covered by the certifying authority and testing for
temporary works should follow the same requirements.
6.4 Instrumentation
Instrumentation shall be in accordance with [4.2] and adequate instrumentation shall be installed to monitor
the following, as applicable, during the operation to ensure loads, etc., remain within analysis and/or
operational limits and assumptions:
a)
b)
c)
d)
e)
f)
the water level in all compartments, quantity and percentage
status of all valves
pump status and flow rates
main and emergency power supply status
platform draught, heel and trim
compartment air pressure
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g)
h)
i)
j)
compressor status
air cushion pressure
water seal level in skirt compartments
status of access doors and manholes.
6.5 Gravity based structure installation
6.5.1 General
This subsection describes the general requirements for the installation of a concrete gravity platform at its
final offshore location. The installation procedures will vary, depending on parameters including:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
the size and design of the platform
water depth
the positioning tolerances required in all 6 degrees of freedom
the positioning/stationkeeping system proposed
whether cranes, winches or external buoyancy is required for lowering and/or positioning
whether the operation involves docking over a template, docking piles or other structures
stability at all stages of immersion
whether a vertical or inclined installation is required
tolerances on differential ballast levels
the skirt design, and penetration method
whether under-base grouting is required
whether solid ballast or scour protection is required.
6.5.2 Survey
6.5.2.1 The position of the site location shall be given in both geographical and grid coordinates.
6.5.2.2 The water depth and bathymetric tolerances shall be determined.
6.5.2.3 When determining the extent of the survey area, the following shall be accounted for:
a)
b)
c)
d)
e)
f)
g)
tolerances on site survey position
inaccuracy of position monitoring systems during installation
operational tolerances
the approach corridor
whether a holding location is required close to the site
whether an inclined installation, with previous off-site touch-down is required
the proximity of any other platforms or subsea assets at or near the location.
6.5.2.4 The bottom topography shall be established by swathe bathymetry, high resolution echo sounder
techniques, side scan sonar, and checked by magnetometer and ROV video for obstructions and possible
unexploded ordnance. The extent of any required levelling or other seabed preparation should be decided at
the design stage.
Guidance note:
Swathe bathymetry is now available in portable units and is installed on most survey vessels so should be used as standard on all
survey projects. Due to constraints imposed by calibration and processing requirements (single point obstructions may be removed
in processing), conventional high-resolution bathymetry and side scan sonar should be run in conjunction.
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6.5.2.5 The seabed and sub-seabed conditions shall be established by coring, magnetometer, in-situ testing,
lab testing and sub-bottom profiling.
6.5.2.6 Sufficient current surveys shall be completed to determine the current profile with depth.
6.5.2.7 The area should be checked to ensure that there are no travelling sand-waves or other seabed
erosion/accretion that could affect the structure during installation.
6.5.2.8 A site survey of the installation area covering the full area of any anchor pattern, carried out not
more than 4 weeks before the start of installation, shall be provided to verify the location of all subsea
infrastructure, debris and obstructions.
6.5.3 Seabed preparation
6.5.3.1 The required tolerances for level and compaction shall be documented at an early stage.
6.5.3.2 Where surveys shows the seabed is out of tolerance it shall be prepared to correct for uneven levels
or consistency. Description of the preparation works, including details of how tolerances shall be achieved,
shall be documented.
Guidance note:
Typical seabed preparation methods include:
a)
Controlled dumping and compacting of gravel before final levelling
b)
Placing sand-bags
c)
Excavating of unsuitable soils before replacing as in a) or b).
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6.5.4 Installation method principles
6.5.4.1 In general it is desirable for all installation phases to be reversible though this may not always be
possible, especially if there are temporary unstable phases.
6.5.4.2 The approval criteria shall be agreed with the MWS company. The agreed criteria shall depend on the
installation methods and consider the following:
a)
b)
c)
d)
The required external assistance (e.g. temporary buoyancy, winches, cranes)
Range of positive stability at all stages of installation. Also see [6.5.4.4].
Length of weather windows required and sensitivity to bad weather or strong currents
Possible requirement of scour protection immediately after emplacement (see [6.5.7]).
6.5.4.3 For structures towed on their side, an agreed Up–End procedure shall be documented.
6.5.4.4 Ideally platforms should be shown to be stable at all phases of the installation.
Guidance note:
Shallow draught platforms frequently undergo a phase of instability during submergence of the base, and an inclined installation
procedure may then be used in which case the requirements of [6.5.4.5] will apply. Sometimes it may be necessary to touch down
on one edge to achieve stability or to use temporary buoyancy or crane /winch assistance.
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6.5.4.5 In the event of an inclined installation the following shall be considered:
a)
b)
All machinery, systems and personnel, if aboard, shall be able to work efficiently in the inclined condition
Monitoring of ballast levels, and allowable differential levels
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c)
d)
e)
f)
Structural capacity of the skirt at touch down, and possible impact loads imposed
Skirt touch down, if on the final site, may disturb the seabed, and prejudice the final skirt penetration or
base slab bearing
If the skirt touch down is on the final site, accurate position control may be difficult in the inclined
condition
If skirt touch down is remote from the final site, the deballast capability required by [4.3.5] will be used.
6.5.5 Positioning and position monitoring systems
6.5.5.1 The positioning system shall be designed to meet the required installation tolerances. This will
normally be by means of tugs, often the tow fleet is rearranged into a star configuration.
6.5.5.2 Where more precise positioning is required, the tugs may be connected at the bow to pre-laid
anchors though other mooring systems are possible. Mooring systems shall comply with Sec.17.
6.5.5.3 Where the position and orientation tolerances are not critical, the tugs may be in free floating
configuration.
6.5.5.4 Where docking piles are to be used the requirements in [13.8.4] apply
6.5.5.5 A position monitoring system in accordance with [4.4.5] shall be provided. The system shall allowing
monitoring of capturing docking piles if being used.
6.5.6 Ensuring on-bottom stability/skirt penetration
6.5.6.1 The requirements in [13.10.1] apply including specifying the depth(s) of any required penetration(s).
6.5.6.2 Calculations shall be documented to demonstrate that the base or skirts will penetrate to the
required depths. The calculations shall specify if negative pressure is required in addition to gravity/buoyancy
loads. Additionally the calculations should consider the following:
a)
b)
c)
d)
expected (and maximum and minimum) soil friction
expected (and maximum) suction versus penetration depth
soil sealing differential pressure versus penetration depth
capacity of suction pumps.
6.5.6.3 A venting system sufficient to ensure foundation integrity shall be provided to allow water in the skirt
compartments to escape and where required to allow negative pressure to be applied.
Guidance note:
Design of the pipework should take into account the requirements for removal on decommissioning.
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6.5.6.4 Skirts shall be shown to meet the requirements of [4.4.5.1] for all expected loads during the
installation process.
6.5.6.5 If differential pressure or suction is applied, then it shall be demonstrated that an adequate seal can
be obtained at the skirt tip, with minimal risk of 'piping' between outside and inside each skirt compartment.
6.5.6.6 Requirements to minimum pumping pressure and flow rate should be established
6.5.6.7 All relevant parameters shall be controlled, monitored and recorded during the installation. This shall
include:
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a)
b)
c)
differential pressure (suction)
penetration
flow rate.
6.5.7 Anti-scour precautions
6.5.7.1 All locations, especially with high current speeds, should be investigated to see if scour could cause
problems during the installation and subsequent temporary stages.
6.5.7.2 Details of anti-scour precautions where required shall be documented. Possible solutions to scour
include:
— Controlled rock dumping or placing sand-bags immediately after the unit is installed. Care shall be taken
to avoid any damage to the unit especially near penetrations, pipelines, cables or other subsea assets.
Scour may start immediately after installation, especially in bad weather.
— Artificial seaweed or other seabed stabilisation methods. This solution needs to be demonstrated to be
successful under these conditions.
— Increased skirt lengths, though this should have been determined at an early design stage.
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SECTION 7 CABLES, PIPELINES, RISERS AND UMBILICALS
7.1 Introduction
7.1.1 Scope
7.1.1.1 This section addresses the requirements for marine operations involving pipelines, risers, jumpers,
umbilicals and submarine cables (refered to a product, see [1.6.2]), including:
a)
b)
c)
d)
e)
f)
Load-outs of rigid and flexible product via lifting of drums/reels/baskets/carousels or reeling/spooling/
coiling/winding (including transpooling operations).
Transporting rigid and flexible products.
S-Lay or J-Lay of rigid single or piggy-back pipelines or pipe-in-pipe using laying vessels assembling the
line from joints of pipe and held on station by moorings or dynamic positioning.
Reel lay of rigid single pipes or piggy-back pipelines or pipe-in-pipe where a straightener is used.
Laying cables, umbilicals, flexible pipelines and risers using vessels which carry the completed product
on reel(s) or carousel(s), or in turntable(s) or basket(s)/tank(s). These can be laid using chutes or
purpose built lay towers.
Shore and offshore pulls including pull-ins.
Guidance note:
See Table 1-4 for definitions used in this standard in particular reeling, spooling, winding, coiling, carousel, turntable, basket, tank,
drum and reel.
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7.1.1.2 Requirements for the following specific activities are covered as well:
1)
Marine lay operations including:
— vessel mooring or stationkeeping on DP
— pipe handling
— lay initiation
— lay-down and recovery
— pipeline and cable crossings
— above water tie-ins (AWTI) and
— landfalls.
2)
Burial of cables, pipelines, risers and umbilicals by trenching and backfill, jetting or rock placement or
dumping.
3)
Seabed survey and route.
4)
The installation of flexible or rigid risers into external clamps or J-tubes, and steel catenary risers and
flexible risers between subsea infrastructure and floating units, including riser arches or other buoyancy
units.
5)
Installation of deep water steel catenary risers (SCR), including pull-in and hang-off.
6)
Installation of PLET/PLEMs (pipeline end terminations/pipeline end modules) and inline structures as an
integral part of pipeline catenary. PLETs and PLEMs installed separately are considered a single subsea
structure.
7)
Shore pull and offshore pull, which are often used for connection to onshore section or facilities.
8)
Installation of spools and jumpers.
9)
Subsea tie-ins.
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Guidance note:
Transportion and installation of subsea assets separately to any product are covered elsewhere in this standard, notably Sec.11
and Sec.16.
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7.1.1.3 This standard does not address the following:
a)
b)
c)
d)
e)
f)
Fitness for purpose.
Manufacture of cables, pipelines, risers and umbilicals and their permanent appurtenances (although
protection of product integrity during installation is covered by this standard).
Welding, non-destructive testing (NDT), field joint coating or anodes for cathodic protection.
Diving procedures.
Tunnelled and drilled shore crossings.
Cables, pipelines, risers and umbilicals in the Arctic and other similar cold regions that are subject to sea
ice, iceberg and/or icing conditions (additional requirements could apply in these cases).
7.1.1.4 Specific requirements for transport/installation of pipelines and pipeline bundles by wet/submerged
towing are covered in [11.26]. Requirements for towage and transport in Sec.11 also apply.
7.1.1.5 For any lay operation supported by lifting operations, the relevant requirements of Sec.16 apply.
Specific requirements for subsea lifting are covered in [16.17].
7.1.2 Codes and standards
7.1.2.1 A number of recognized standards and design codes covering products are already in existence.
These recognized design codes shall be used to establish the limit state criteria for the product. This
document presents requirements related only to ensuring the limit state criteria for the product are not
exceeded during any marine operations (e.g. load-out, transportation and installation). Any pre-service
conditions requirements also specified in the recognized design code shall be satisfied.
7.1.2.2 Table 7-1 gives codes that may be considered as recognized design codes.
Table 7-1 Typically acceptable product recognized design codes
Product
Design code
Rigid pipelines (submarine)
DNV-ST-F101, /42/
Steel risers
DNV-ST-F201, /43/
API STD 2RD, /4/
Rigid pipelines (not specifically offshore)
API RP 1111, /3/
EN 14161, /10/
Flexible pipes
ISO 13628-2, /95/
API Spec 17J, /134/
ISO 13628-11, /97/
API RP 17B, /133/
Umbilicals
ISO 13628-5, /96/
API Spec 17E, /146/
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Subsea power cables
Relevant IEC standards
Cigré Technical Brochure Nos. 194 /147/, 490 /11/, 496 /137/,
623 /12/ and 862 /148/
DNV-ST-0359, /132/
Guidance note 1:
The default standard for rigid pipeline system design and approval is DNV-ST-F101, /42/, Submarine pipeline systems. DNV-STF101 Sec.10, /42/, gives requirements for installation/offshore construction of submarine pipeline systems. Parts of DNV-ST-F101
Sec.10, /42/, are also generally applicable for flexible pipes and risers. Hence, requirements in DNV-ST-F101, /42/, are referred to
and/or repeated in this section where applicable. It is intended for this standard to be consistent with DNV-ST-F101 Sec.10, /42/,
and therefore the requirements presented there are normally acceptable in their entirety.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Flexible pipes may be used for both static and dynamic applications. A flexible dynamic pipe will normally be categorized as a
dynamic riser.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
Guidance regarding installation of subsea power cables in shallow water can be found in DNV-RP-0360, /58/. Subsea power cables
for wind power plants are also covered in DNV-ST-0359, /132/.
Additional guidance for wind farm installations can be found in Sec.8 of this standard. It is intended for this standard to be
consistent with DNV-ST-0359, /132/ and therefore the requirements presented there are normally acceptable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 4:
A design code that specifies design loads and criteria for typical failure modes applicable to installation and operation of subsea
power cables is currently not available. The IEC standards and Cigré recommendations typically applied in design of subsea power
cables focus primarily on requirements to electrical performance after exposure to relevant electrical, thermal and mechanical
loads. Limit state criteria applicable to installation and operation, such as minimum allowable bending radius etc., are typically
established by the cable supplier based on previous experience, and the cable design qualified and/or verified by testing.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.1.2.3 For some products, it may be necessary to refer to a recognized design code for a similar product
type. Care shall be taken to use coherent input data, analysis methods and safety factors; in general this
means that these should be taken together from a single source. Combining the least conservative options
from different sources is not acceptable.
7.2 Design philosophy
7.2.1 Main principles
7.2.1.1 This subsection [7.2], applies to all marine operations for product and is supplementary to the
general requirements for the planning and execution of marine operations given in Sec.2.
7.2.1.2 DNV’s basic concept for design of marine operations is to ensure that the level of safety specified in
relevant design codes is not jeopardised. This is achieved through careful planning of operations to ensure
that the loads experienced are in line with the exceedence probability specified in the design codes. The
overall objective is to ensure safety of personnel, equipment and product.
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7.2.2 Planning of operations
7.2.2.1 General
7.2.2.1.1 In order to achieve the required level of safety, an operation shall be planned in such a manner
that the product can be brought into a safe condition if the limiting weather conditions for the operation or
sub-operation are exceeded. A safe condition is defined as a condition in which weather conditions in excess
of the limiting weather conditions for an operation will not jeopardise the required level of safety for the
product. A safe condition may be established by:
— completing the operation
— reversing the operation
— abandoning the operation according to a safe and pre-planned contingency procedure, which should allow
for a recovery of the object (e.g. cutting and lay-down of pipeline), or
— establishing a stand-by configuration that ensures that product integrity is maintained until normal
operations can be resumed.
7.2.2.1.2 The planning shall include risk management including relevent requirements of [2.4] and the
recognized design code.
Guidance note:
A HAZID is often the preferred tool to identify critical operations and required contingency operations. Normally the items in DNVST-F101 [10.1.3.4], /42/, may be considered as part of the risk management process. See also DNV-RP-N101, /54/.
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7.2.2.1.3 Contingency operations shall be identified through HAZID/HAZOP studies (see also [2.4.2]) and/or
defined by the customer. Contingency and emergency planning requirements in [2.5.4] should be considered.
7.2.2.1.4 Any limitations related to the product’s properties (structural strength, size etc.), product
components and/or installation equipment shall be considered in the planning of operations, both in relation
to limiting weather conditions and contingency procedures, e.g. waiting on weather. Any such limitations shall
be identified in the procedures. If relevant, the limitations in [2.6.8.2] should be considered.
Guidance note:
For example, the horizontal and vertical angle coverage of a chute may restrict vessel heading during laying of flexible products.
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7.2.2.1.5 Relevant operation parameters and limiting weather conditions to ensure safe operations
for personnel, product and installation equipment shall be specified in the installation procedures. The
installation criteria shall account for uncertainties in the weather forecast.
7.2.2.1.6 Environmental criteria for the installation operations shall be in accordance with Sec.3 and shall be
appropriate to the location, the season and the expected duration of the installation operations. See [2.6]
for requirements and associated operational limiting criteria for weather restricted and weather unrestricted
operations.
7.2.2.2 Operation duration
7.2.2.2.1 Lay operations with a planned duration exceeding the limitation for weather restricted operations,
see [2.6.5], may still be defined as such subject to the following:
— Continuous surveillance of actual and forecast weather conditions is implemented.
— The handled object can be brought into a safe condition within the maximum allowable period for a
weather restricted operation. A safe condition (see [2.5.1.2]) may be established by; reversing the
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operation, abandoning the operation or establishing a stand-by configuration that ensures that product
integrity is maintained until normal operations can be resumed.
Guidance note:
See also [2.6.7.8] and [2.6.7.9], for lay operations with a planned duration less than the limitation for weather restricted
operations.
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7.2.2.2.2 As per [2.6.5.2], if the duration is too long to be considered as a weather restricted operation
then procedures shall be developed to establish a safe condition in the event that the weather forecast
should indicate that the forecasted operational criteria may be exceeded prior to completion. Time needed to
establish the safe condition shall be considered, as well as the safety of personnel working on deck under the
specified limiting weather conditions.
Guidance note:
In this case weather in excess of the limiting conditions specified for an operation is not considered an emergency, as procedures
for establishing a safe condition shall be in place.
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7.2.2.3 Stand-by
7.2.2.3.1 Where a weather stand-by condition is planned, the temporary configuration shall be such that
relevant limit state criteria are satisfied for weather conditions exceeding the limiting installation conditions.
Guidance note:
Lowering and suspending a rigid pipeline from the vessel is an example of a typical stand-by configuration, as is use of temporary
buoyancy attached to umbilicals and subsea power cables to establish a lazy wave configuration.
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7.2.2.3.2 Maximum allowable stand-by duration due to weather, equipment failure, repair or similar,
shall be specified, if applicable, considering the product’s fatigue budget (total and proportion assumed
for installation). Unless free vessel heading can be demonstrated, a conservative wave heading shall be
assumed.
Guidance note:
DNV-ST-F101 [5.4.8.14], /42/, provides guidance on allowable fatigue utilization during the construction phase for various safety
categories of rigid pipeline safety criticality.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.2.2.3.3 In the event that a stand-by configuration is to be established for weather conditions in excess of
the limiting conditions for an operation, it shall be demonstrated by analysis that relevant limit state criteria
are satisfied for the maximum allowable stand-by conditions specified for the operation.
7.2.2.3.4 Relevant installation parameters shall be established as input to the operational procedures.
7.2.2.3.5 It shall be ensured that the parameters of the stand-by configuration determined for a given depth
interval are applicable for the full range of water depths within the interval (typically maximum and minimum
water depth of the interval).
7.2.2.4 Recovery
7.2.2.4.1 An operation shall, as far as possible, be planned to enable reversal and recovery of the product. If
reversal/recovery of the product is not possible, this shall be catered for through contingency planning.
7.2.2.4.2 In order to ensure sufficient capacity of both product and installation equipment, loads related to
recovery shall account for effects of the following, as applicable:
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— friction over a chute or similar
— break-out force, e.g. in relation to a halt in a shore pull or pull-in operation.
7.2.2.4.3 Fatigue damage due to recovery and re-installation shall be considered with respect to the fatigue
budget.
7.2.3 Operational procedures
7.2.3.1 The operation manual shall provide a full suite of documentation for the project and comprise
manuals, specifications, method statements, procedures, drawings, calculations, records, certificates, etc.
necessary to fully define the safe execution of the operations. See [2.3.7].
7.2.3.2 Limiting weather conditions and maximum allowable duration of the planned operation, including
any planned stand-by or contingency configuration, shall be specified along with applicable operational
parameters to ensure that relevant limit state criteria are not exceeded.
Guidance note:
See [3.4.3.5] for requirement about use of monthly environmental data.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.2.3.3 Operational procedures shall be supported by analyses to ensure that relevant limit state criteria
are satisfied for weather conditions up to and including the limiting conditions for the operation and/or
sub-operation. The uncertainty of predicted weather increases with the duration of the forecast, and this
uncertainty shall be considered in the planning of operations.
Guidance note:
The uncertainity of predicted weather is normally accounted for by applying an alpha factor. Further guidance on the use of alpha
factors in operations is in [2.6] and in particular [2.6.7], [2.6.8] and [2.6.9]. See also [7.3.1].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.2.3.4 Procedure changes required by events in the field shall be controlled by a suitable management
of change procedure. Any major change to procedure or document subject to the specifications of the
management of change procedure which can critically affect the intergrity of the product or operability and
relevant for the MWS company shall be informed to the MWS company before implementation. If the MWS
company is not present then the MWS company office should be contacted.
Guidance note:
It is common that the management of change procedure is part of the marine operation manual, as per [2.3.7.2].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.3 Installation engineering
7.3.1 Design process
Relevant limit state criteria defined in the recognized design code shall be checked applying the loads and/or
load effects determined in the analyses and using the format applied in the recognized design code. Where
the alpha factors in [2.6] shall be used and unless steel work has been designed in accordance with the ASD/
WSD requirements then the LRFD alpha factor may be used (see also [2.6.12.1]).
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Guidance note:
The different design codes apply different design formats:
—
The standards for steel risers and pipelines, respectively DNV-ST-F201, /43/, and DNV-ST-F101, /42/, apply a partial safety
—
The ISO standards for design of unbonded flexible pipes (ISO 13628-2) and umbilicals (ISO 13628-5) use a single
factor format called load and resistance factor design (LRFD) format.
(permissible) utilization factor approach.
The most obvious difference in the two formats is that LRFD applies separate safety factors for different load components and
the resistance (capacity), reflecting the uncertainty in each of them, while the a single (permissible) utilization factor applies one
safety factor and is akin to the ASD/WSD methods.
The main types of load components relevant in a capacity check of product are:
—
pressure (internal and/or external)
—
tension (some apply the effective tension directly)
—
moment or curvature.
Some design codes distinguish between functional loads and environmental loads. For rigid pipes designed to DNV-ST-F101, /42/
see guidance note to [7.3.4.2].
A functional load is typically well defined and associated with less uncertainty than environmental loads. The LRFD format accounts
for this lower uncertainty using a lower safety factor for functional loads than for environmental loads.
Environmental loads are defined as loads resulting directly or indirectly from wind, waves and current. The effect of an
environmental load is the difference between the total dynamic response and the static response.
In some cases other special load components may be relevant. Guidance is provided in the relevant design code.
For rigid pipelines, the capacity checks combine the effect of the main load components. See applicable design codes, e.g. DNV-STF101, /42/, for more details.
For flexible pipes, the capacity check is performed for single load effects, e.g. characteristic tension to be less than specified tensile
strength and characteristic curvature to be less than maximum allowed curvature.
For umbilicals and subsea power cables the load effects tension, curvature and internal/external pressure (as applicable) are
considered simultaneously in the capacity checks.
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7.3.2 Laying/installation analysis
7.3.2.1 General
7.3.2.1.1 The laying/installation analyses shall demonstrate the suitability of the proposed installation spread
and proposed installation method.
7.3.2.1.2 The analyses shall establish operational parameters for a range of configurations and/or weather
conditions, e.g. minimum allowable lay-back for sea states up to and including the limiting sea state for an
operation. The sophistication of the analyses, and hence software, shall be appropriate to the criticality of the
operations, recognising technical complexity or novelty, reserves of strength and stability in the product and
in the equipment involved and prior industry experience.
Guidance note:
Lay analyses may vary from static linear elastic to dynamic, stochastic non-linear elasto-plastic modelling.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.3.2.1.3 The vessel RAOs used in the laying/installation analyses shall be conservative and representative
for the relevant conditions of the vessel during the installation. This may be done through sensitivity analysis.
The need for applying more than one set of RAOs in the analysis should be considered as vessel response
may vary depending on loading condition, water depth etc. Appropriate RAO’s shall be used for shallow water
cases.
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Guidance note:
The vessel loading condition may have changed significantly from the beginning of the operation to the final phases due to effects
such as reduction in product on board, reduction in consumables and fuel etc. Such aspects may require analysis based on more
than one set of RAOs.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.3.2.1.4 The effect of any variation in parameters (section, weight, coating, etc.) along the length of the
product shall be considered. Any significant change of mechanical properties or stiffness shall be accounted
for in analyses.
Guidance note:
Normally, field joint coating or variation between insulation and centraliser in a pipe-in-pipe may be excluded from the analysis.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.3.2.1.5 As stiffness properties of flexible products in tension and axial compression can differ, non-linear
stiffness properties shall be considered in the analyses if there is a non-negligible impact on the operational
criteria derived from this analysis. This should also be considered for bending stiffness.
7.3.2.1.6 For rigid pipe, any discontinuity in the pipe properties, except those permitted by the recognized
design code (e.g. dimensional tolerances) shall be considered. Any such discontinuity shall not result in the
exceedance of any limit state during reeling/unreeling operations and should be justified by appropriate tests
and/or FEA.
Guidance note 1:
For example a discontinuity may occur due to changes in tangential stiffness, different wall thickness, varying material properties
or insulation coating thickness. Where a discontinuity occurs there is an increased potential for buckles to occur.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For non-sacrificial transition spools detailed non-linear elasto-plastic analyses may be required of any transitions between pipe
joints of differing dimensions (greater than mill tolerance on the specified wall thickness either side of the joint) or through a
tapering transition piece or at differing specified minimum yield strengths (differing by more than 5% of the lesser) and at any
pipe joints containing internal or external design geometry changes. For complex cases e.g. outer diameter changes, one wrap on
the reel and through the straightener may need to be included in the analyses.
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7.3.2.1.7 The effect of any in-line bulkheads, connectors, joints, bend restrictors or other structures on the
product shall be considered. Special care shall be taken for lifting of ends or inline structures with a large
relative mass compared to the product. The consequences of relative movements of such objects relative to
the product (e.g. pendulum motions, rotation of lifted object) should be closely considered. Focus should also
be given to situations where tension and/or handling loads are provided by more than one source to ensure
that this does not become a risk to the product integrity (e.g. minimum bending radius).
7.3.2.1.8 Where installation aids are required, they should be considered and included in the analysis when
they can impact the product response and/or the lay configuration.
7.3.2.1.9 Sensitivity analyses shall be performed to assess effects of tolerances and variations in product
tension, lay angle and/or lay-back, and thereby demonstrate robustness of the proposed procedure.
7.3.2.1.10 Product can be damaged due to over-bending at the chute exit or against the side flanges of the
chute. Therefore the angle of coverage of a chute, both in the vertical and the horizontal planes should be
considered. The calculated lead angles shall include the effect of cross currents and the product checked for
bending over edges.
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7.3.2.1.11 Installation analyses shall cover the expected range of water depths, considering relevant or
conservative environmental conditions, lay vessel draughts and trims. Laying in shallow water should be
considered especially, with particular focus on effects of vessel motion (both vertical and horizontal).
7.3.2.1.12 Lay tables shall be established with relevant installation parameters for the governing weather
conditions and wave headings for the full range of water depths along the lay route. A sufficient number of
water depths shall be included to reflect the variations in the product configuration and response. It shall
be ensured that the installation parameters determined for a given depth interval are applicable for the full
range of water depths within the interval (typically maximum and minimum water depth of the interval).
7.3.2.1.13 Dynamic movement in installation equipment, such as stingers and lay chutes, shall be
considered. For S-Lay methods, if the vessel includes a hinged buoyant stinger, specific analysis by a time
domain analysis shall be completed to verify that the stinger will provide the support required and to
determine the weather threshold for abandonment of laying operations. The limiting sea state shall be set
below the conditions which causes the product to lift off and bump onto the support rollers in the stinger.
Guidance note:
Movement in this equipment, such as loss of pipe contact with the supports, can determine the limits rather than overloading,
noting that such dynamics can cause pipe overbending, fatigue or cracking or damage to the pipe coatings or anodes.
For S-Lay methods, the lift-off point of the pipeline should be at the next to last roller, so that in case of small loss of tension, the
contact of the pipe will be with the next to last roller and the last roller could also provide support to the pipe, thereby avoiding
excessive bending at the last roller. With a camera at the last roller, if the separation between the pipe and the last roller is seen to
be too small, the tension can be increased to maintain an acceptable lay curvature of the line.
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7.3.2.1.14 For spooled/reeled lay, the optimum balance between lay tension applied at the tensioner and on
the drum shall be assessed, included in the lay analyses and installation procedures and controlled during
laying.
7.3.2.2 Flexible product
For flexible products passing over rollers whilst under tension, then the loads from the rollers on the product
shall be verified by the manufacturer to ensure that product internals are not damaged unless negligible
compared to manufacturer’s allowable value.
7.3.2.3 Rigid pipe
7.3.2.3.1 For rigid pipelines, checks shall be documented for potential local buckling during pipelay and for
subsequent propagation buckling initiation and running in accordance with a recognized design code. The
external pressure should be checked against the propagation pressure. Where propagation is expected,
appropriate buckle arrestors shall be installed.
Guidance note:
See DNV-ST-F101, /42/, for guidance.
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7.3.2.3.2 For rigid pipelines, where the recognized design code (e.g. DNV-ST-F101, /42/) allows the
controlled yielding of the pipe in the section supported by the stinger, consideration should be given to
residual flattening, torsion or twisting that can be formed in the pipe during installation. Plastic strain
history shall be considered for design of parts of the line which will or can experience fatigue loading during
operation. Buckling consequences and mitigations shall be clearly documented including control and recovery
measures for collapse and buckle propagation.
Guidance note:
The elastic-plastic bending behaviour between the reel and straightener is highly non-linear and sometimes highly susceptible to
changes in stiffness of the pipe.
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7.3.2.3.3 For rigid pipeline that is to be reeled/spooled, where the pipe will be required to deform plastically,
the bend radius shall be sufficient to avoid local pipe buckling or coating failure. An operation specific reeling/
spooling analysis should be perfomed, and be:
— backed-up by demonstation bend tests or small scale reeling simulation testing and/or
— demonstration of successful track record of similar product and arrangement.
Guidance note:
For low criticality pipelines the requirement for analysis may be waived subject to adequate documentation demonstrating
successful track record of similar product and arrangement.
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7.3.2.3.4 Residual ovalisation shall be accounted for in the pipelay strength and buckling analyses, and in the
analyses for operating conditions.
7.3.2.3.5 Ovalisation due to bend strain should be calculated in accordance with DNV-ST-F101, /42/.
Guidance note:
The installation contractor may offer other bases supported by documented testing or by prior successful application for
determination of ovalisation due to reeling/spooling, and for the extent or recovery from ovalisation due to straightening.
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7.3.2.3.6 The total strain shall be assessed as per DNV-ST-F101, /42/.
7.3.2.3.7 Installation strain history shall be recognized in analyses of any fatigue loadings during operation.
Total strain induced at reeling/spooling and laying shall be accounted for in the fatigue design as reeling/
spooling can lead to reduction in the fatigue life due to increased size of existing flaws in a weld.
7.3.2.3.8 A fatigue check is required to account for periods when laying is halted (e.g. for extended welding,
weather downtime, equipment failure) and the product is subject to vessel motions. Consideration of
cumulative fatigue damage from operations shall be considered.
7.3.2.3.9 For rigid pipelines, minimum allowable lay rate (slow lay) shall be determined considering the
available fatigue budget, if applicable.
7.3.2.4 Abandonment and recovery
7.3.2.4.1 Abandonment and recovery (A&R) analysis shall be performed at suitable water depth intervals
representative of the lay route.
7.3.2.4.2 The configuration at the beginning of the abandonment analysis should be the same as the
configuration just prior to abandonment, considering the most conservative lay-back and weather conditions,
as a minimum. The number of steps analysed should be sufficient to ensure a smooth lay-down or recovery
and avoid exceeding relevant limit state criteria.
Guidance note 1:
For recovery of rigid pipe using the S-Lay method, the tension may be slightly increased (by approximately 10%) when the head
approaches the stinger to increase the clearance of the head with the stinger in order to avoid the risk of clash between the A&R
head and the stinger.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Good practice is to change the tension in 5 to 10 tonne steps.
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7.3.2.5 End termination assemblies/in-line components
7.3.2.5.1 Assesments (supported by analyses as required) shall be performed to establish or validate a
procedure for safe handling of the component/assembly attached to the product from the vessel storage
location to the installed condition including through the splash zone. It shall be ensured that the most critical
load condition has been considered for the most critical configuration and demonstrated that relevant limit
state criteria are satisfied for the limiting weather conditions specified for the operation.
Guidance note:
As per [7.1.1.5], the lifting requirements in Sec.16 and in particular those for subsea lifting in [16.17] apply.
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7.3.2.5.2 If critical, the effects of wave slamming on end termination assemblies and in-line components
shall be considered, as well as snap loads in the lifting wire.
Guidance note:
Guidance with respect to lifting and lowering of structures may be found in DNV-RP-N103, /56/.
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7.3.2.5.3 Contact with stinger rollers, radius controllers, chutes or similar shall be considered, with special
attention to the transition in bending stiffness at the interface between the product and end fitting/
termination, joint or component. Correct modelling of stiffness properties (also bend restrictors and bend
stiffeners) shall be ensured.
7.3.2.6 Clump weights
7.3.2.6.1 If one or more clump weights are attached to the product to pull down a buoyant riser section,
product response, particularly at the vessel interface and in the touchdown area, shall be analysed to
establish the effect of:
— attaching each clump weight to riser
— landing each clump weight on the seabed.
7.3.2.6.2 The part of the operation involving attachment and landing of a clump weight is of relatively short
duration, and may therefore be considered as transient.
7.3.2.6.3 The effect of sudden impact between clump weight and seabed due to vessel heave motion can
cause axial compression in the product at the vessel interface. Attachment point and length of clump weight
wire should therefore be determined considering the product response as the clump weight is landed.
7.3.2.6.4 Size of clump weight shall be determined considering interference with the product.
7.3.3 Limit state criteria
7.3.3.1 As per [5.9.1.3], a limit state is commonly defined as a state in which the structure ceases to fulfil
the function, or to satisfy the conditions, for which it was designed.
Guidance note:
As per [5.9.1.2], the limit states are typically grouped into the following four categories:
—
Ultimate limit states (ULS) corresponding to the ultimate resistance for carrying loads.
—
Fatigue limit states (FLS) related to the possibility of failure due to the effect of cyclic loading.
—
Accidental limit states (ALS) corresponding to damage to components due to an accidental event or operational failure.
—
Serviceability limit states (SLS) corresponding to the criteria applicable to normal use or durability.
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7.3.3.2 As per [5.7], all relevant failure modes shall be investigated. A failure mode is relevant if it is
considered possible and the anticipated consequence(s) of the failure cannot be disregarded.
Guidance note:
Possible causes of failure include (specific applicability depends on product type):
—
excessive bending
—
excessive tensile or axial compressive loads
—
excessive torsion/twist
—
excessive crushing/bearing loads (e.g. tensioner squeeze load)
—
excessive point loads
—
local and global buckling
—
fatigue
—
excessive ovalisation
—
excessive pressure
—
excessive displacement (in relation to on-bottom stability)
—
excessive free span, and
—
excessive residual curvature (e.g. for pull-in operations, fitting of components).
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7.3.3.3 Installation loads and limit state criteria shall comply with the requirements of a recognized design
code, see [7.1.2]. A coherent set of criteria shall be adopted throughout. The selected criteria shall be
specified in an installation design basis. Where the recognized design code includes a safety class this should
be considered.
7.3.3.4 Flexible product
7.3.3.4.1 For flexible products, the limit state criteria relevant for installation design are presented in Table
7-2. Product specific limit state criteria shall be provided by the supplier in accordance with the recognized
design code, see [7.1.2].
Guidance note:
The limit state criteria are required input to installation design, and are typically outlined in data sheets and/or a handling
specification provided by the manufacturer of the product.
For umbilicals, a capacity curve specifying allowable curvature/bending radius for different tensions is common.
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Table 7-2 Limit state criteria applicable to flexible products
Limit state
ULS
Limit state criterion
Maximum allowable tension
Minimum allowable bending radius
Minimum allowable storage radius
Maximum allowable axial compression
Maximum allowable tensioner squeeze load
Maximum allowable point load
Maximum allowable twist
FLS
Maximum allowable stand-by duration
ALS
Minimum breaking load
7.3.3.4.2 For allowable tension and allowable bending radius, the following shall be specified as a minimum:
a)
b)
Minimum allowable bending radius at maximum installation tension for both load controlled (bending in
free space) and displacement controlled (bending over support) conditions. Unless otherwise specified, a
conservative estimate of maximum installation tension may be assumed.
Minimum allowable bending radius at a tension representative of the touchdown region and conditions
during storage and handling.
7.3.3.4.3 If axial compression is allowed, the product supplier shall specify axial and bending stiffness of the
product in compression (as well as tension).
7.3.3.4.4 The need for providing the maximum allowable point load limit state criterion should be agreed
with the customer. If required, details of the contact surface shall be specified.
7.3.3.4.5 The bending radius shall not be less than the displacement controlled or load controlled, as
applicable, minimum bend radius (MBR) at any time. Details of the specification of the MBR, including any
margins, shall be documented. MBR to be adhered to shall be those provided by the product manufacturer.
7.3.3.4.6 Unless it is justified that fatigue is negligible, the product supplier shall specify maximum allowable
duration of a stand-by condition. Alternatively maximum fatigue damage over the cross-section, for specified
load combinations (i.e. tension and curvature) shall be provided by the installation contractor to the supplier
for input to their fatigue calculations in an agreed format.
7.3.3.5 Rigid pipe
7.3.3.5.1 For rigid pipelines, the limit states shall be as per the recognized code e.g. DNV-ST-F101, /42/.
The following typical design conditions shall be checked, as applicable, against the relevent limit state in the
recognized code depending on installation method:
— on-reeling (reeling installation only)
— over bend (as applicable)
— stinger tip (as applicable)
— sag bend (all installation methods).
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Guidance note:
Reel-lay presents additional design and material requirements for the pipe compared to S-Lay and J-Lay. The main considerations
are:
a)
the pipe and coatings’ suitability for the strain cycling for reeling/spooling onto and off the vessel’s reel
b)
limiting the D/t ratio to prevent buckling when reeling/spooling and
c)
resistance to collapse and propagation buckling due to residual flattening of the pipe section following reeling/spooling, unreeling and straightening.
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7.3.3.5.2 Anti-corrosion coatings and concrete weight coatings shall be specified and proven to be resistant
to the range of shear and crushing loads applied at the pipelay tensioners and support rollers. Pipe tensioners
impart very high loads into the pipeline which shall be analysed and/or pre-qualified before implementing
them in the field.
Guidance note 1:
Individual tensioner loads are likely to be higher for J-Lay than for S-Lay, so it is important that inter-coating shear bond and crush
strengths are specified, and demonstrated by project specific or prior representative qualification testing.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For pipe-in-pipe, the flooded cases to be analysed and tensions to be used for squeeze settings normally are:
—
when laid empty considering either flooding of annulus or bore, whichever is greater
—
when laid with flowline flooded with both flowline and annulus considered flooded.
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7.3.3.5.3 Where the J-Lay system requires collars to suspend the pipe, the collar system shall be suitable for
the maximum design load, including any waterfilled condition, and in accordance with a recognized design
code.
Guidance note:
Collars are normally located close to every butt weld in order to suspend the pipe.
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7.3.3.5.4 Other failure modes that shall be checked, as applicable, are:
— squeeze and slippage in tensioner/clamp
— concrete crushing
— rotation
— anode integrity
— residual ovality
— residual curvature
— bottom tension and curve stability.
7.3.4 Loads and load effects
7.3.4.1 The specification and design of product is only of direct concern to the MWS approval process in that
the design shall be capable of being safely installed, in accordance with verified design documentation and
the approved procedures and good industry practice.
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Guidance note:
The specification and design of the product are generally determined by required performance in the service condition in the design
environment. However, installation method often influences certain parameters, such as wall thickness, material properties and
protection coating type and thickness. For pipe-in-pipe and carrier pipe bundles, the installation method largely determines the
carrier pipeline system design.
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7.3.4.2 The general requirements for loading and structural strength in Sec.5 should be considered.
Requirements for load categories are given in [5.5] and for loads and load effects in [5.6]. All loads relevant
for the objects covered in subsection [7.3.4] shall be taken into account.
Guidance note:
It should be noted that DNV-ST-F101, /42/, and DNV-ST-F201, /43/, categorize loads somewhat differently than this standard, as
indicated below:
—
both G and Q loads are called functional loads in DNV-ST-F101 [4.2], /42/
—
E loads have the same definition, i.e. environmental loads, see DNV-ST-F101 [4.3], /42/
—
DNV-ST-F101 [4.5], /42/, defines interference loads. These can be considered as accidental loads with probability greater than
1/10 000 per operation.
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7.3.4.3 All applicable load combinations specified in the recognized design code shall be considered.
Guidance note:
For example for rigid pipe the ULS condition b) in DNV-ST-F101 Table 4-4, /42/, or the ULS condition in DNV-ST-F201 Table
5-2, /43/, would normally be considered whenever relevant in addition to the load combinations defined in [5.9.8]. Where
considered relevant this is normally fulfilled by applying the load combinations in [5.9.8], with a load factor of 1.1 on G and Q
loads in ULS combination b).
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7.3.4.4 The most unfavourable load scenarios for all relevant installation phases and conditions shall be
considered.
7.3.4.5 All loads and forced displacements which can influence the product shall be taken into account.
For each cross-section or part of the system to be considered and for each possible mode of failure to be
analysed, all relevant combinations of loads which can act simultaneously shall be considered.
7.3.4.6 Heavy items, structures and end terminations stored with the product can influence the CoG of the
reel/basket/carousel significantly and shall be taken into account when undertaking the design for lifting and
transport.
7.3.4.7 When considering the environmental load, the most unfavourable relevant combination, position and
direction of simultaneously acting environmental loads shall be used when documenting the integrity of the
system, see also [5.6.13].
Guidance note:
This includes the effect of temperature gradients.
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7.3.4.8 Any possible accidental loads shall be considered, see also [5.5.7].
Guidance note:
Accidental loads are described in DNV-ST-F101 [4.6], /42/, and interference loads in DNV-ST-F101 [4.5], /42/.
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7.3.4.9 Requirements for vessel motion analysis are given in subsection [5.6.12].
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7.4 Vessel and installation equipment
7.4.1 General
The general requirements for system design in [4.2] apply.
7.4.2 Vessels
7.4.2.1 These requirements apply to all vessels performing product installation operations, and supporting
operations, such as trenching and burial.
7.4.2.2 The vessel requirements given in [2.11] apply. The installation spread for vessels performing
installation of products as described in [7.1] shall comply with the requirements of the recognized design
code.
Guidance note:
For example DNV-ST-F101 [10.4], /42/, gives requirements for vessels, position reference systems/ navigation reference systems,
anchor systems, anchor patterns, anchor handling, dynamic positioning, cranes and lifting equipment, lay vessel arrangement,
laying equipment, instrumentation, mobilization activities, qualification of vessel and equipment, calibration and testing.
For further guidance regarding installation spread for cables see DNV-RP-0360 [6.2], /58/.
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7.4.2.3 Vessel(s) shall have adequate documented stationkeeping and manoeuvering characteristics for all
planned and contingency situations.
7.4.2.4 Vessel information, including but not limited to, the information listed in [7.13.2] shall be submitted.
Additionally, equipment safety, operating and maintenance manuals and critical equipment maintenance
records with any current backlog listing should be submitted.
7.4.2.5 Where a vessel has additions or modifications for a project specific mobilisation, these may require a
separate suitability survey (in addition to that required by [2.11.2.2]).
Guidance note:
This is separate to any class survey and is to ensure the suitability of the equipment for the planned operation. Items covered by
any class survey e.g. vessel strength would normally be excluded from the suitability survey.
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7.4.3 Mooring equipment
7.4.3.1 All mooring equipment, including, but not limited to, mooring winches, control system, mooring
wires, anchors, pennant wires, buoys, and ancillary fittings, shall be in serviceable condition with valid
certification in place as appropriate. See [17.2.7].
7.4.3.2 A mooring control system and equipment FMEA shall be produced. The MWS company shall be
notified of any critical findings or results.
7.4.3.3 The lay vessel should have a centralised mooring control and monitoring system, monitoring and
displaying winch operational status with tested alarms of line out and line tension reporting into a central
control room or bridge. The mooring control station shall be equipped with an open communications system
to all relevant control points and shall have monitors with feeds from the survey suite and the AMS (anchor
management system).
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7.4.3.4 Where a lay vessel does not have a centralised mooring control and monitoring system, a controlled
procedure for reporting parameters from the local control and monitoring systems shall be implemented.
Guidance note:
This is normally only acceptable where mooring equipment has been installed on the vessel for a specific project only.
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7.4.3.5 Mooring winches shall have local line out recorders and tension monitors with read outs at the winch
control station.
7.4.3.6 Mooring winches shall be monitored by CCTV with screens installed at the mooring control station.
7.4.3.7 Where mooring equipment has been installed on vessel only for a specific project, the requirements
of [7.4.3.5] and [7.4.3.6] may be exempted when there are acceptable localized watch arrangements. In
such cases, readouts shall be regularly communicated to the mooring control station, where a log book will
be kept.
7.4.3.8 Mooring and anchor handling operations shall be monitored and controlled with an anchor monitoring
system (AMS) continuously logging anchor positions. The mooring control station and the anchor handling
vessel navigation bridge shall be equipped with screens showing anchor positions.
7.4.3.9 The AMS shall be capable of showing when an anchor has slipped from its deployed position.
7.4.3.10 The lay vessel survey display and the AMS shall be integrated to provide real time position displays
of the vessel, anchor locations, installed cables, and other subsea assets/hazards, including third party
assets.
7.4.4 Dynamic positioning system
The following requirements apply for operations using DP:
a)
b)
c)
The general requirements for DP operations in [17.3].
The vessel should have minimum DP equipment class 2. See Table 17-9.
DP capability analyses shall consider the effect of lay tension, and any ploughing or trenching forces.
7.4.5 Monitoring equipment
A sufficient amount of instrumentation and measuring devices shall be installed to ensure that relevant lay
parameters are monitored and maintained within specified limits at all times, and stored for documentation
purposes.
Guidance note 1:
Monitoring requirements for specific installation equipment are specified in [7.4.7] to [7.4.13].
Requirements for monitoring during laying are in [7.9.8].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
It is preferable that the output from monitoring equipment goes to a central control room or the bridge. For monitoring of mooring
equipment see [7.4.3].
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7.4.6 Installation equipment – general
7.4.6.1 The requirements in subsection [7.4.6], apply to all installation equipment for product related marine
operations. Requirements for specific installation equipment aboard vessels relevant to the particular method
of installation are given in [7.4.7] to [7.4.13].
Guidance note:
DNV-RP-0232 provides guidance for certification and verification of the design, materials, fabrication, safety, testing of the pipeline
and cable laying equipment and systems
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7.4.6.2 Calibration and testing requirements are in [7.4.13.1].
7.4.6.3 Equipment used for operations involving product shall be designed to prevent any relevant limit
state(s) being exceeded.
Guidance note:
Measurement of the MBR is not typically practicable and is ensured by the following of lay tables and layback ranges from analysis.
Sometimes specific handling tools/aids (such as a quadrant, shoe-in tool, bend restrictors, chutes or rollers) are used to limit the
bending radius.
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7.4.6.4 Installation equipment shall be arranged such that the insertion of tees or inline valves does not
interfere with the operation of the equipment.
7.4.6.5 All equipment shall be set to the parameters determined by the installation analyses and/or as
specified in the operation manual.
Guidance note:
For example for rigid pipelines, equipment includes pipe tensioners, clamps, welding stations, NDT stations, field joint coating
stations, pipe handling systems (see [7.4.10.3]) and any multiple jointing stations.
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7.4.6.6 Temporary installation equipment shall also comply with Sec.11, for both transport and operational
loads.
7.4.7 Lifting equipment
7.4.7.1 Lifting equipment shall be in accordance with Sec.16.
Guidance note:
For cranes see [16.7.2] and [16.7.3].
For slings, shackles and other rigging certification requirements see [16.12].
See [16.4] for sling design, and [16.5] for shackle (and other connecting equipment) design.
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7.4.7.2 Lifting equipment used to handle in-line and end structures/end terminations including pull-in frames
and winches shall be designed in accordance with Sec.16. The design of lifting equipment and lift points
on such structures should be given special consideration to ensure that all possible load directions and
distributions between different parts of the rigging are considered.
7.4.7.3 Lifting magnets shall only be used when certified and approved for the work and proven not
to permanently magnetize the pipe, unless a controlled de-gaussing is planned to remove residual
magnetisation before offshore welding.
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7.4.7.4 Lifting and pulling points of A&R heads, lay-down heads and similar, shall be designed in accordance
with Sec.16, including the consequence factors (see [16.8.3]). These shall be designed for the maximum
dynamic tension expected and consider the worst possible load directions. See also [7.9.15.14] for pull
heads.
7.4.7.5 Chinese fingers, cable grips etc. shall be suitable for the product including the load capacity and used
in accordance with manufacturer’s requirements. The MBL of a Chinese finger should not be less than the
MBL required for a steel sling/wire connected to it that is designed to [7.4.7.4].
7.4.7.6 For well planned one time pull-in operations and if permitted by the recognized design code the
consequence factor may be discarded provided that the following conditions are satisfied:
a)
b)
c)
a risk assessment of the operation has been carried out to evaluate potential consequences of winch and
winch wire failure, and
it can be documented that a failure of the end termination/fitting, pull-in head or installation aid will not
cause personnel injury, environmental pollution or damage to vessel, installation equipment or other
installations, and
necessary contingency procedures are in place, supported by analyses as appropriate.
Guidance note:
Consequence factor during pull-in operation is addressed in [7.9.13.1].
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7.4.7.7 The A&R system including the wire shall be in serviceable condition with valid certification with line
out and tension read-outs calibrated. The A&R system should be able to abandon the product safely including
any waterfilled condition. In case the A&R system is incapable of recovering the product, alternative methods
should be available.
Guidance note:
For pipe-in-pipe systems, it is normally acceptable to assume only that the inner pipe or the annulus is flooded, not both.
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7.4.8 Tensioners
7.4.8.1 The tensioner shall be fail-safe and shall be rated to hold the heaviest flooded product contingency
case. The tensioners should also be able to lower the heaviest flooded product to the seabed. If not,
mitigation measures shall be implemented to ensure safety in case of a flooding scenario and include
contingency plans for retrieving the water filled pipe.
Guidance note:
In deep waters it may not be possible/practical to design the installation equipment for the loads associated with a flooded pipe.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.4.8.2 The tensioner’s capacity to support product tension shall account for the following as applicable:
— the product’s squeeze load capacity
— number of tensioner tracks
— squeeze load applied by tensioner tracks
— contact length between tensioner tracks and product
— geometry and material properties of track pads
— friction between tensioner tracks and product externals
— friction between the product outer layer(s) and tensile components
— creep of polymer materials in the product cross-section
— effects of temperature, e.g. on creep rate, and
— wet conditions including product internal annular flooding (if applicable).
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Guidance note 1:
Geometry and material properties of the tensioner pads (i.e. pad length and separation, angle of pad groove) are required input to
the product supplier for establishing maximum allowable tensioner squeeze load and minimum tensioner holding capacity.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Stresses in the outer layer(s) of a flexible product, resulting from tensioner squeeze loads and product tension, generally causes
the layer material (typically a polymer) to relax and creep. At low stresses, the creep effect is negligible, but increases with
increasing squeeze and/or tensile load. In order to prevent excessive elongation and sudden rupture of the outer layer, potentially
resulting in uncontrollable slippage of the product internals (bundle) through the tensioner (and outer layer), it is important to
have control of the creep rate at maximum allowable squeeze load and installation tension (combined). See [7.4.8.6] and [7.4.8.7]
for further details.
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7.4.8.3 It shall be ensured that a tensioner’s holding capacity is sufficient to prevent product slippage
through the tensioner during installation for all planned and contingency operations including any water filled
condition (if applicable).
7.4.8.4 In order to ensure that the tensioner squeeze load capacity is sufficient to support maximum product
tension, the tolerance of the squeeze load shall be considered.
7.4.8.5 The minimum squeeze load (alternatively track contact length) required to prevent product slippage
through the tensioner(s) shall be established by using the minimum documented and submitted friction
coefficient. The material factor, γm, applied to the characteristric friction coefficient should generally be taken
as 1.5.
7.4.8.6 Where applicable the control of creep effects shall be ensured by specifying the maximum allowable
product tension for a specified tensioner squeeze load based on an acceptable creep rate (see guidance note
to [7.4.8.7]). The material factor, γm, applied to establish maximum allowable product tension for a given
squeeze load should be taken as 1.5.
Guidance note:
This requirement is normally applicable to flexible product and may be applicable to coated rigid pipelines. It is not applicable to
uncoated rigid pipelines.
Creep is the tendency of a material to slowly deform permanently as a result of internal stress. The external layer(s) of a product
will typically be subjected to compressive stress from the tensioner pads and shear stress resulting from the transfer of product
tension from the strength elements (e.g. armour wires) via the external layer(s) to the tensioner pads. For example, this may
occur in polymer materials (e.g. external layer).
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7.4.8.7 Lower material factors than those in [7.4.8.5] and [7.4.8.6] may be accepted where there are welldocumented tests (see [5.4.4]) that account for all the items listed in [7.4.8.2]. Where the tests comply with
the minimum requirements of [5.4.4], the material factor, γm, may be taken as 1.4. A further reduction may
be accepted where there has been more extensive testing validated by engineering and/or it can be shown
by risk assessment (see [2.4]) that the consequences of slippage/creep in the tensioner (including risk to
personnel, equipment, infrastructure, product and schedule) are low or can be mitigated; in such cases the
minimum acceptable material factor, γm, is 1.25.
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Guidance note:
When testing the friction coefficient and creep rate (where applicable) between the externals and the tensile components on a
complete product:
a)
The externals should be removed over a short length on either side of the test pads to ensure that the resistance measured is
b)
To capture material creep, displacement transducers should be placed on the pad(s) and the bundle/tensile components to
c)
For a given squeeze load, product tension should be increased in steps up to at least the maximum expected installation
due to friction and/or material creep alone.
continuously log and capture minor changes in their separation.
tension. The minimum number of load steps should be 10, and minimum holding period at each load step should be at least 5
minutes.
d)
For a given squeeze load, the maximum allowable characteristic creep rate (where relevant) should to defined to ensure that
the operation is controllable,i.e.: – that a small change in tension or squeeze load (e.g. due to uncertainties or tolerances)
will not lead to a disproportionate increase in the creep rate, and – that the accumulated creep during a halt in operations,
i.e. maximum stand-by period, will not lead to failure, e.g. rupture of the external layer. The maximum allowable tension is
that associated with the characteristic creep rate divided by the material factor.
e)
Typically, polymer coatings soften with temperature; in such cases the test temperatiure should exceed the maximum
temperature expected offshore, and it should be ensured that the product being tested is at the required temperature over
the cross-section.
For rigid pipe, where fully representative project specific testing using the actual product with the actual tensioner pads for a
sufficient number of squeeze loads each using different areas of the product has been performed this may be considered as
extensive testing provided that the results at different squeeze loads are fully consistent.
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7.4.8.8 Where there are contingency plans to continue the operation after the failure of one pad or one line
of pads, then the capacity of these shall be omitted from the design calculations. It shall be documented that
continuing to lay after these items have failed will cause no harm to the product and that the required grip
can be achieved from the resulting asymetric pad configuration.
7.4.8.9 If tensioners are to be used, demonstration tests may be required to confirm the product will be
unharmed during installation unless tensioning system review tests are documented as per DNV-ST-F101
[10.4.11], /42/.
Guidance note:
The scope and nature of such demonstration tests is to be agreed between the customer and the installer, based upon current
practice and previous track record of lay operations using this tensioner system and similar product properties.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.4.8.10 The tensioner system shall be qualified for the actual product dimension to be installed.
7.4.8.11 Variations in the outer diameter of the product due to production tolerances and dimensional
transitions should be within the working range of the tensioner system.
Guidance note:
Following risk assessments, it is in some circumstances acceptable to lay products with diameter deviations out of tensioner
specification.
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7.4.8.12 The maximum force the product handling equipment can exert shall be set and calibrated at a load
point that ensures the maximum allowable squeeze load cannot be exceeded. The MWS company should:
a)
b)
be provided with a written statement from the product supplier confirming the maximum allowable
tensioner squeeze load at maximum installation tension in accordance with acceptance criteria specified
in the applicable design code
be provided with detailed equipment specifications including items listed in [7.4.8.2]
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c)
d)
e)
be provided with valid calibration certificates, in accordance with [7.4.13.1] for the handling machinery
compression force setting, measurement and display functions
be provided the intended compression settings through the installation documentation, and
witness (if present), prior to the actuation of the system, the inputting of the settings into the lay
system.
7.4.9 Straighteners
For rigid pipe, any pipe straightener shall be set to ensure that the out of straightness is within the
tolerance(s) in the analyses for the installation and in-place condition. The settings should be proven by trial
conducted at the yard/offshore.
Guidance note:
If proven by track record and/or engineering for a similar pipe design, the requirement for straightening trials can be waived. If
proven by engineering alone, calibration trials of the analyses should be performed and attended by a competent third party.
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7.4.10 Stingers, ramps and lay towers
7.4.10.1 Rollers and tracks shall be faced with a material that will not damage the product. Rollers shall be
in good rolling condition, both in air and subsea, with maintenance records submitted.
7.4.10.2 The height and spacing of the rollers shall be adjusted to ensure a smooth transition from the
vessel to the water column. The rollers should be spaced and their heights set to maintain loads in the
product within the limits given in the installation analyses.
7.4.10.3 The pipelay vessel’s stern ramp orientation, and stinger geometry, buoyancy, mass and stiffness
shall be set to suit the planned lay with roller heights/openings set to suit also.
7.4.10.4 A stinger should be equipped with a monitoring systems that feeds back to the control room to
confirm that the pipe is not experiencing high point or dynamic contact loads at the end of the stinger.
Guidance note:
Suitable monitoring systems include load cells and video cameras, preferably fitted on the last roller to monitor the clearance with
the last roller. If unavailable, a full dynamic analysis is required in addition to onsite monitoring of roller reactions and stinger angle
and TDP monitoring.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.4.10.5 All lay towers, lay ramps and stingers shall be equipped with tower, ramp and stinger angle
indicators.
7.4.10.6 Where stingers incorporate ballast tanks, suitable indication of tank fill status shall be provided.
7.4.11 Lay chutes, lay sheaves and radius controllers
7.4.11.1 The radius of lay chutes, lay sheaves and radius controllers shall be sufficient to prevent any limit
state criteria being exceeded. Any rollers shall be in accordance with [7.4.10.1].
Guidance note:
The requirements in [7.4.11], are generally only applicable for flexible product. The equipment’s radius is to be equal to or larger
than the specified displacement controlled MBR of the product.
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7.4.11.2 Maximum contact load between product and chute/sheave/radius controller (sometimes referred to
as side-wall pressure) shall be verified to be within acceptable limits specified by the product supplier.
7.4.11.3 In cases where the product lead angle is not in line with the lay chute or lay sheave plan centre
line, the lay chutes/sheaves shall have side walls to protect the MBR. The side walls shall accommodate the
maximum lead angle expected during laying operations.
Guidance note:
The laying may be planned such that the lay vessel might need to weather vane or crab along the route to ensure that the
influence of wind, waves and current does not cause a limit state to be exceeded.
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7.4.12 Reels, drums, carousels, baskets, tanks and turntables
7.4.12.1 The structural integrity of equipment such as reels, drums, carousels, baskets, tanks and turntables
shall be documented as adequate for all phase(s) of the marine operation(s).
7.4.12.2 Portable reels/drums for flexible product which will be operated on marine vessels should be of the
driven spindle type. However, under-roller type reels may be used if extra care is taken during installation to
ensure slip and escape from the turning mechanism does not occur.
7.4.12.3 When transport is on board the installation vessel and the installation equipment, including product
storage equipment, also acts as permanent/temporary seafastening, e.g. reel drive systems, the mechanical/
hydraulic capacity of the installation equipment shall be documented as sufficient for the relevant transport
loads. Special attention should be given to systems depending on hydraulic pressure, gears and roller/
bearing systems exposed to high loads in a static condition.
7.4.12.4 The seafastening arrangements for product storage equipment not supported and seafastened via
the installation equipment (e.g a reel-drive system) shall be documented for all phases.
Guidance note:
A seafastening release procedure should document that the reel is adequately secured until the reel drive system is mounted.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.4.12.5 Floors of carousels, turnables, baskets and tanks shall be levelled to prevent damage to the product
when fully loaded.
Guidance note:
Floors may be lined with ply wood sheets to protect the product against irregularities of the floor.
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7.4.12.6 Partitioning of reels/drums, carousels, turntables, baskets and tanks shall be designed with
sufficient strength to contain the product in the designated partition for all phases of the marine operation(s).
Guidance note:
For reel carousels, this applies to any adjustable flange.
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7.4.13 Hang-off/installation clamps
7.4.13.1 Where required, temporary product hang-off shall be well planned, using dedicated equipment only.
The hang-off system should normally consist of well supported hang off collar/clamp. The equipment shall be
suitable for the maximum design loads including any waterfilled condition (if applicable).
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Guidance note:
Collars for J-lay of rigid pipe are in [7.3.3.5.3].
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7.4.13.2 The clamp’s capacity to support product tension shall account for the following, as applicable:
— the product’s squeeze load capacity
— number and arrangement of pads
— squeeze load applied by pads
— contact length between pads and product
— geometry and material properties of pads
— friction between pads and product externals
— friction between the product outer layer(s) and tensile components
— creep of polymer materials in the product cross-section
— effect of temperature, e.g. on expansion/contraction, creep rate, and
— wet conditions including product internal annular flooding (if applicable).
Guidance note 1:
Geometry and material properties of the tensioner pads (i.e. pad length and separation, angle of pad groove) are required input to
the product supplier for establishing maximum allowable tensioner squeeze load and minimum tensioner holding capacity.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Stresses in the outer layer(s) of a flexible product, resulting from tensioner squeeze loads and product tension, generally causes
the layer material (typically a polymer) to relax and creep. At low stresses, the creep effect is negligible, but increases with
increasing squeeze and/or tensile load. In order to prevent excessive elongation and sudden rupture of the outer layer, potentially
resulting in uncontrollable slippage of the product internals (bundle) through the tensioner (and outer layer), it is important to
have control of the creep rate at maximum allowable squeeze load and installation tension (combined). See [7.4.8.6] and [7.4.8.7]
for further details.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.4.13.3 Minimum required squeeze load to prevent slippage between clamp and product shall be
established by using the minimum documented and submitted external friction coefficient. The material
factor, γm, applied to the characteristic friction coefficient should generally be taken as 1.5.
7.4.13.4 Where applicable the control of creep effects shall be ensured by specifying the maximum allowable
product tension for a specified squeeze load based on an acceptable creep rate (see guidance note to
[7.4.8.7]). The material factor, γm, applied to the friction coefficient can generally be taken as 1.5.
Guidance note:
This requirement is normally applicable to flexible product and may be applicable to coated rigid pipelines. It is not applicable to
uncoated rigid pipelines.
For definition of creep see guidance note to [7.4.8.6].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.4.13.5 Lower material factors than those in [7.4.13.3] and [7.4.13.4] may be accepted where there
are well-documented tests (see [5.4.4]) that account for all the items listed in [7.4.13.2]. Where the tests
comply with the minimum requirements of [5.4.4], the material factor, γm, may be taken as 1.4. A further
reduction may be accepted where there has been more extensive testing validated by engineering and/or
it can be shown by risk assessment (see [2.4]) that the consequences of slippage/creep in the tensioner
(including risk to personnel, equipment, infrastructure, product and schedule) are low or can be mitigated; in
such cases the minimum acceptable material factor, γm, is 1.25. See guidance note to [7.4.8.7].
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7.4.13.6 Where there are contingency plans to continue the operation after the failure of one pad or one line
of pads, then the capacity of these shall be omitted from the design calculations. It shall be documented that
continuing the operation after these items have failed will cause no harm to the product and that the required
grip can be achieved from the resulting asymetric pad configuration.
7.4.14 Calibration and testing
7.4.14.1 The general requirements for equipment testing and calibration in [2.10] apply. Testing and
calibration of laying equipment shall be done according to DNV-ST-F101 [10.4], /42/, or equivalent.
7.4.14.2 All loading equipment controls, sensors and monitoring devices shall have valid calibration
certificates. The calibration and testing of essential equipment should as a minimum be performed for each
project and on an annual basis.
7.4.14.3 Unless covered by valid certificates, the following shall be done before lay operations:
1)
2)
3)
4)
5)
Combined positioning system/tensioning system tests shall be done by simulating product pull,
tensioning system failures and redundancy tests during pull.
Any tensioner load cells should be calibrated. (as per DNV-ST-F101 [10.4.7], /42/).
The A&R winch and any product support load cells should be calibrated. (As per DNV-ST-F101
[10.4.7], /42/.)
Equipment should be tested and calibrated to the maximum equipment capacity or alternatively up to
maximum expected dynamic or accidental loads + 50%, whichever is less.
During testing and calibration the complete load range should be covered. For rigid pipes:
— For linear trends at least five load steps should be applied up to maximum expected dynamic loads,
more steps are required when testing to maximum capacity.
— Non-linear trends require higher numbers of load steps. Cyclic loading should be considered.
6)
For tensioners and A&R winches, prior to lay operations, in-date calibration certificates and proof load
test certificates shall be available. Fail-safe operations during event of a power black-out should be
demonstrated.
Guidance note 1:
Loss of main power and loss of signal should be tested by removing fuses or cables.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Calibration shall occur not more than one year in advance of the expected completion of operations.
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7.5 Load-out and offshore transfer
7.5.1 General
7.5.1.1 The limit states of the product (see [7.3.3]) shall not be exceeded during any phase of the loadout. This shall be confirmed by analysis where necessary. See [7.3] for details on installation engineering
including analysis requirements.
7.5.1.2 If required by the recognized design code or customer or similar a tracking system should be
implemented. Individual pipe joints shall be identified and manifested, as they are stowed into or on the
marine transport vessel, to ensure traceability.
7.5.1.3 Any damage during load-out shall be made good or otherwise addressed.
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7.5.1.4 Adequate stationkeeping of all vessels shall be documented. See Sec.17 and in particular [17.2.9]
for quayside moorings.
7.5.2 Handling and lifting
7.5.2.1 Product handling and lifting shall be planned so that product limit state criteria are not exceeded.
The requirements in the rest of subsection [7.5.2] shall be applied as appropiate.
Guidance note:
For bundled product see [7.9.10.3.4].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.2.2 The outer layer of flexible product and pipeline coating shall be protected against damage that will
adversely affect the inservice condition of the product.
Guidance note:
Protection may be by the use of smooth facings (manufacturer approved facings) on all parts coming into contact with the product,
including rollers, tensioners/cable engines, reel drums and flanges. Bolts and fixtures with potential to contact the product may be
recessed or protected.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.2.3 Use of hold back rigging (choked slings, Chinese fingers etc.) directly on the outer layer of flexible
product shall be confirmed as acceptable by the product manufacturer. Similarly, the use of installation
clamps should be confirmed acceptable by the product manufacturer and only be used according to their
instructions.
Guidance note:
Normally, the use of hold back-rigging directly on the product is acceptable. Incorrect use of Chinese fingers and chokes slings
could result in damage/slippage of the outer layer and/or otherwise compromise the product integrity.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.2.4 Where flexible product is being handled, the product shall be moved over rollers or bodily lifted,
without dragging, unless all the product/support interfaces are smooth (e.g. deflectors, chutes, prepared
deck surfaces) and no excessive contact force is applied. Tension and minimum bend radii shall be controlled
by means of lift and transfer arrangements and procedures that are proven, e.g. through analyses and/or
monitoring, not to exceed the design of the product. Similar care is required during deck handling of end or
midline structures.
7.5.2.5 Lifting operations of components attached to a product shall be planned to ensure the limit state
criteria of the product (see [7.3.3]) are not exceeded. In particular the following should be considered:
a)
b)
c)
The relative movements between the lifted component and product.
The need for additional control measures during the lift, (e.g. tugger winches [16.16.12], lifting guides
[16.14]).
The gradual change in CoG for the lift where the component is gradually carrying more/less of the
product weight, including means to compensate and control of the catenary/bend radius (e.g. through
additional support rigging, change in tension).
Guidance note:
These considerations are particularly important for components with large relative weight compared to the product itself (e.g. end
terminations, inline components)
See [7.4.7] for requirements to lifting equipment.
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7.5.2.6 Where long strings or stalks of rigid pipeline are being moved, they should do so on soft faced rollers
or soft faced pipe racks demonstrated to be arranged so as not to over stress the pipe and to ensure no
damage is feasible to the pipe coatings, with forward and backward control of the pipe movement/tension.
7.5.2.7 Pipe joints shall be lifted using equipment and/or tools systems that do not damage the coatings or
prepared pipe ends, such as:
— soft faced hooks
— soft slings
— suitable spreader bars
— certified electro-magnets.
7.5.3 Product load-out by reeling, spooling, winding and coiling
7.5.3.1 This subsection [7.5.3] applies to load-out of product by:
— reeling or spooling onto horizontal or vertical reels/drums/reel carousels with back tension
— winding into a rotating basket carousel/turntable without back tension
— coiling into a non-rotating basket without back tension.
Guidance note:
See also [7.4.12] for requirements to product storage equipment (e.g. reels, drums, carousels, baskets and tanks).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.2 The complete load-out pathway including equipment shall be inspected for acceptability and set up
for the actual product.
Guidance note 1:
Depending on the site layout the pathway can start at the manufacturing facility or storage location or spool base and ends at
product storage equipment
Examples of equipment to consider are reeling/spooling/winding/coiling system, tensioners, on board equipment and spooling
tower roller boxes (for rigid pipelines only).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The following are normally included in the inspection for acceptability:
—
non-conforming bends (e.g. less than the product MBR)
—
loose/stiff rollers
—
sharp protrusions
—
coarse sliding surfaces
—
sections where the product can jump off the loading pathway and
—
sections that are out of sight.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.3 The loading pathway should be monitored continuously as a minimum at critical sections and bends.
Guidance note:
CCTV and/or competent personnel may be considered as suitable monitioring methods.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.4 For product storage below deck, it shall be documented that there is sufficient clearance between
top layer of basket/carousel/tank and deck opening to allow for handling of the product within its design
limitations (e.g. bending radius and twist). Special focus shall be given to handling of stiff end or midline
structures on the product.
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7.5.3.5 The product catenary between shore and vessel shall be monitored to prevent excessive product
bending or tension in case of unsynchronised line speed onshore and on vessel, or a sudden stop in the line.
Responsibility for managing the load-out catenary between shore and the vessel shall be clearly defined.
Guidance note:
Minimum and maximum height of bottom point on any catenary may be specified and monitored.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.6 A watch shall be maintained to ensure that the wake from passing vessels cannot endanger the
operation. The catenary shall have sufficient length to preserve the product if the vessel moves excessively
due to wake from passing traffic.
7.5.3.7 Product reeled/spooled onto reels/drums/reel carousels shall be wound under tension to prevent
loose wraps. The required back-tension to prevent this shall be documented. Documentation may be means
of track record made available to customer/MWS if required.
Guidance note:
For rigid pipe to be reeled/spooled see [7.9.10.4.1] for requirement to length required for lead and tail sections.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.8 Flexible product loaded by winding or coiling into a basket carousel, basket or tank shall be tightly
packed together to minimise gaps. Gaps should be packed out to prevent movement at sea and to prevent
overlying product being forced into gaps between product in underlying layers. Any packing material shall not
damage the outer layer in any way that will adversely affect the inservice condition of the product.
7.5.3.9 Where product is to be coiled the manufacturer shall specify that the product is suitable for coiling
and the loading procedures shall specify the coiling direction considering the armour layup.
Guidance note:
Coilable product is designed to prevent buckling and/or locking of concentric layers within the cross-section as it is twisted (360°
per turn/coil). A torque balanced cross-section is typically not coilable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.10 Suitable arrangements for storage, seafastening and protection of the end terminations shall be
made and documented. See [7.6.1.3].
7.5.3.11 Swivels use shall be evaluated based on sling nature and product nature considering torque
balance. Generally swivels should be used on the tension side when pulling products using ropes or slings
that are not torque balanced, and when pulling flexible products that are not torque balanced.
Guidance note:
In all cases inclusion of swivels in pull-in/hold-back systems is to be carefully evaluated with respect to the wire construction.
Rotation resistant ropes are not affected when free-rotating swivels are introduced into the system, while non rotation resistant
and semi rotation resistant ropes will be adversely affected by free-rotating swivel. When such wire ropes are loaded and
unloaded, the swivel will allow the rope to rotate back and forth causing tension-torsion fatigue of the rope. As a result these wire
ropes will operate at a reduced design factor and will in addition fail from the inside out. For non rotation resistant ropes there is
generally more than 30% reduction of rope MBL and endurance by a factor of about ten.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.12 The requirements for transpooling operations of flexible product between vessels are the same as
for reeling/spooling quayside operations, with the following additions:
— The product catenary between the two vessels shall be monitored to ensure that relative motion between
the two vessels does not jeopardise specified minimum product bending radius or cause snap loads in the
product.
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— Mooring lines should be checked on a regular basis and the checking interval should be in accordance with
the prevailing environmental conditions and passing marine traffic. Mooring line checks and adjustment
shall be recorded in the reeling/spooling log (in addition to the vessel bridge log).
— The operation should be in sheltered waters with the relative motion between the vessels minimised.
Where the operation is to take place in open waters limiting environmental conditions shall be specified to
prevent any limit state being excluded.
Guidance note:
If a spacer barge is used between the vessels to ease the catenary, the same general principles apply. Spacer barge shall be
adequate against the expected berthing and/or mooring forces.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.3.13 Before start of load-out, any cable ends shall be sealed against water ingress or terminated by
competent technicians using junction box, splice box, subsea connector or similar. The sealing or termination
shall be in accordance with the cable manufacturer’s procedures and/or termination box/connector supplier
specification.
7.5.3.14 Upon completion of post load-out testing, cable seals, tube fittings, end caps and pull-in
arrangements , as appropriate, shall be properly fitted to the product ends by competent technicians in
accordance with the manufacturer’s procedures.
7.5.3.15 If any damage occurs during load-out of a flexible product, testing shall be performed to determine
the extent of product damage. The scope of the testing can be agreed at the time of an incident and
will depend on the type of incident. The product shall be accepted by the owner after such damage, and
subsequent testing and repairs approved before continuing the operation.
7.5.3.16 When pre-installed pull-in heads are fitted at the manufacturer’s premises appropriate
documentation defining the load capacity shall be submitted.
7.5.3.17 Any damage to the flexible product during load-out shall be rectified. Any signs of bird-caging
armour wires, kinks and buckling in the cable and any tendency for the product to try to form loops in the
catenary between the load-out gantry and the lay vessel load-out chute are causes for concern and should be
investigated.
7.5.4 Lifted load-outs and offshore transfer
7.5.4.1 Loaded drums should be inspected for signs of physical damage on arrival at the delivery location
before load-out. If a drum is damaged then the cable in the damaged area should be inspected and, if
deemed necessary, the cable should be tested.
7.5.4.2 Lifted load-outs and offshore transfer lifts from the transport vessel to the installation or lay vessel
shall be in accordance with the applicable parts of Sec.16. This includes lifting of pipe joints and product
stowed on reels/drums/reel carousels and in baskets or basket carousels.
Guidance note:
See also [7.5.2] for requirements to handling and lifting.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.4.3 Where pipe joints are lifted as a stack the height shall be restricted to prevent damage to pipe/
coatings and by vessel constraints. Access to the top of the pipe stack shall be arranged to minimise risk of
injuries to riggers.
Guidance note:
See [7.5.1.2] for tracking system requirements.
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7.5.4.4 For offshore transfer of pipe joints from pipe carrier to installation vessel, the limiting weather
conditions shall be specified considering relative motion between vessels.
7.5.4.5 Offshore transfer lifts shall not be executed above subsea assets without due consideration for the
effect dropped objects. For lifting operations taking place over a vessel side, a safe over boarding distance
from any subsea assets shall be established in accordance with [5.6.6.3].
7.5.5 Testing
7.5.5.1 Subsection [7.5.5] covers the requirements for pre load-out and post load-out testing of umbilicals
(including cables) or similar. It is not applicable to rigid or flexible pipelines.
Guidance note:
For other product types, the testing is in accordance with the recognized desing code and any contract between customer and
supplier.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.5.2 Umbilicals shall be tested before and after load-out normally in accordance with the recognized
design code with the results submitted. Generally the following would be considered as the minimum testing
requirements.
Guidance note:
See ISO 13628-5, /96/, for guidance.
Other parties (e.g. supplier and installation contractor) may also require testing to document product integrity before/after loadout.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.5.3 Before start of load-out, the umbilical shall be function tested at the load-out site.
Guidance note 1:
Normally the function tests are in accordance with ISO 13628-5, /96/.
If the load-out site is at the manufacturer’s facility then the tests do not need to be repeated.
Any tests older than three months may need to be repeated.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
In fibre optic cables it may be acceptable to test an agreed percentage of the fibre optics unless there is an incident during loadout that might have damaged the cable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.5.4 After load-out, the umbilical should be function tested.
Guidance note:
Normally the function tests are in accordance with ISO 13628-5, /96/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.5.5 The post load-out test procedure should be taken into account in the deck layout, as well as the
reeling/spooling, load-out and transpooling procedures.
Guidance note:
Depending on the post load-out test scope, it can be necessary for both ends of the loaded umbilical to be accessible for testing.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.5.5.6 Post load-out test results should be compared with the FAT results and abnormalities investigated.
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7.6 Transport
7.6.1 General
7.6.1.1 These requirements are valid for the transport of product on both installation and transport vessels.
7.6.1.2 General requirements for the transport are contained in Sec.11.
7.6.1.3 All products, including in-line assemblies, end structures/terminations, buoyancy modules, clamps
and other accessories shall be seafastened according to requirements in Sec.11.
7.6.1.4 Limit state criteria shall not be exceeded during any phase of the voyage. The effects of vessel
motions should be considered when evaluating crush loads.
7.6.1.5 Temperature limitations during storage and/or transport should be considered, not least related to
the preservation fluids that have been used. The need for over pressure protection of products that are filled
with preservation fluids should be considered (e.g. due to effect of temperature differences).
7.6.1.6 See [7.4.12] for reel/drum/carousel/basket/turntable requirements.
7.6.1.7 The product shall have been loaded-out in accordance with the applicable sections of [7.5].
7.6.1.8 Drummed product ends shall be protected from external damage by use of wooden planks, or
similar, and securely fitted between the drum flanges.
7.6.1.9 Crush loads imparted by overlying product shall be within limits set by the manufacturer.
Calculations confirming crush loads are within set limits shall be submitted and shall include the effects of
vessel motions which should not be less than the design environmental condition in [3.2].
7.6.1.10 Unless part of the vessel’s permanent equipment, design calculations shall be submitted to
demonstrate that the reel/carousel/basket/tank structure can withstand the loads imparted by product due
to vessel motions. The conditions used for the calculations shall be not less than the design environmental
condition in [3.2].
7.6.2 Pipe joint specific
7.6.2.1 Stowing and stacking of pipe joints for transport shall be in accordance with [11.9.9] or
API RP 5LW, /7/, or equivalent.
7.6.2.2 Pipe joints should be protected against corrosion.
Guidance note:
Plastic end closers may be fitted before load-out for long marine voyage (particularly via tropical conditions), as experience has
shown that significant corrosion can occur in the pipe bore in these circumstances.
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7.6.2.3 Additionally to seafastening requirements in [11.9.9], where the pipe joint is bare or coated with a
hard slick product such as FBE (fusion bonded epoxy) or polyethylene either:
— adequate friction against axial pipe movement shall be documented or
— restraint against axial movement shall be provided.
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7.7 Route
7.7.1
The intended route of the product shall be submitted showing all relevant details of the area, including but
not limited to:
1)
2)
3)
4)
5)
shoreline
bathymetry
existing and projected platforms
subsea assets, and
any obstructions such as:
— wrecks
— rock or coral outcrops
— glacier scars
— seabed pock-marks
— sand wave or mudslide areas
— fault lines
— dumping grounds including munitions
— unexploded ordnance (UXO)
— fishing
— anchoring
— naval exercise
— other restricted areas
— areas where free-spanning may occur
6)
third party concession owners.
Guidance note:
This may be in the form of alignment sheets or similar.
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7.7.2
Drawings shall be to a suitable scale, and all locations shall be to a common coordinate system such as UTM
and geodetic datum (e.g. European Datum 1950 or World Geodetic System 1984).
7.7.3
The route shall be clearly defined by coordinate points, straight and curved sections, kilometre points,
tangent points and intersection points as necessary. All existing structures, crossed lines and the target areas
or target box for the start-up and lay-down locations/targets shall be clearly identified.
Guidance note 1:
The start-up and lay-down targets are normally rectangular boxes whose centre is the nominal coordinate for the as-laid location
of a particular pipe spool, subsea termination unit, or similar. Lateral and axial tolerances on this coordinate set in the approved
design drawings define the target box size.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For cable lay operations this may be a route positioning list (RPL). See also DNV-RP-0360, /58/.
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7.7.4
Where applicable, plough routes should be based on
and should avoid wrecks, substantial obstacles, UXO, environmentally sensitive areas (such as Sabellaria
worm habitats), and difficult soil conditions. Small objects may be removed, but should be avoided if
practicable.
7.7.5
Where applicable, existing jack-up footprints shall be identified and new footprints should be accurately
plotted when jack-ups are deployed.
Guidance note:
This is of particular interest for cable laying in offshore wind farm developments where turbine installation and cable laying
operations may occur in parallel.
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7.7.6
The seabed shall be prepared in accordance with the recognized design code, as applicable, to ensure that no
limit state of the product is exceeded.
Guidance note:
See DNV-ST-F101 [10.3.2], /42/.
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7.8 Surveys
7.8.1 General
7.8.1.1 The surveys in subsection [7.8], shall be submitted. These are required for approval of the marine
operation, however any additional requirements in the recognized design code shall be considered.
Guidance note:
More information regarding survey requirements and their purpose can be found in the following documents/sections:
—
For pipelines: DNV-ST-F101 Sec.3, /42/, and DNV-ST-F101 Sec.10, /42/
—
For subsea cables: DNV-RP-0360 [3.4], /58/
—
Submerged pipeline towing: DNV-ST-F101 [10.6.5], /42/
—
Bundles: [11.32.4] of this standard.
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7.8.1.2 The survey vessel, survey equipment, the extent of survey, and survey tolerances shall be suitable
for the surveys to be performed.
7.8.2 Route survey
7.8.2.1 For product tows, route surveys shall be carried out along the total length of the planned route to
provide sufficient data for design and installation related activities.
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7.8.2.2 Areas where permanent constructions or temporary installation activities might be restricted shall be
identified. Areas can be restricted for a variety of reasons such as environmentally sensitive habitats, political
reasons, fishing activity, military ranges, or unexploded ordnance (UXO).
7.8.3 Geotechnical survey
7.8.3.1 A geotechnical survey shall be carried out in sufficient detail to establish the characteristics of the
seabed along the chosen pipeline route.
Guidance note:
If the product is to be laid on the seabed without burial, this survey may be limited to confirm general seabed soil characteristics
and stability and to identify rock or coral and any presence of sulphate reducing bacteria or other aggressive soil conditions.
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7.8.3.2 If the product is to be trenched or trenched and buried, detailed information on the soil
characteristics shall be documented to justify the trenching method, trenching tool proposed and its
performance to meet trenching specification in terms of trench depth and depth of cover.
Guidance note:
This is known as a burial assessment survey (BAS) in the cable lay industry.
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7.8.4 Crossing surveys
Third party cables and pipelines crossing the proposed route shall be identified. Their location and condition
shall be established in detail in order that agreement can be reached on the most effective means to protect
the existing line and the new line during installation and future operating conditions.
Guidance note:
If the existing line is buried it may have to be located by means of acoustic or magnetic tools deployed to the seabed or from prior
surveys.
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7.8.5 Pre-installation survey
7.8.5.1 A pre-installation survey of the work-site or route may be required in addition to the route survey
required for design purposes (see [7.8.2]) if:
— time elapsed since the route survey is significant
— a change in seabed conditions is considered likely
— the installation site/route is located in areas with heavy marine activity
— new installations or facilities are present in the area
— seabed preparation work is performed within the route corridor after previous survey.
Guidance note:
The pre-installation survey can be achieved by performing a grapnel run for cable laying. This should not be used for oil and gas
products.
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7.8.5.2 The pre-installation survey, if required, shall determine/confirm:
— potential new hazards
— location of wrecks, submarine installations and other obstructions such as mines, debris, rocks and
boulders that might interfere with, or impose restrictions on, the installation operations
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— the size as well as the location of the seabed objects and infrastructure
— that the present seabed conditions confirm those of the survey required in [7.8.2] and [7.8.3]
— previously unidentified hazards related to the nature of the installation operations.
7.8.5.3 The extent of, and the requirements for, the pre-installation installation site survey/route survey
shall be specified.
7.8.6 Post-lay surveys
7.8.6.1 Post lay survey(s) shall be completed to confirm the as-laid route and the visible condition of the
product. This may be done either by continuous touchdown point monitoring during pipe laying or by a
separate survey. In case continuous touchdown monitoring is used, product positioning shall be confirmed
after completion of product installation.
Guidance note:
Continuous touchdown monitoring does not always identify possible horizontal curve pull-out, and it can therefore be necessary to
confirm the positioning after completion of installation activities.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.8.6.2 Any free spans shall be identified with their length and height assessed to determine if any support
or protection is needed. Subsequent surveys may be required after trenching to confirm product disposition
and condition in the trench and the final elevation profile of the product.
7.8.6.3 Further detail on the product’s elevation relative to LAT and out of straightness survey (OSS) can be
needed for lines susceptible to upheaval buckling in order to determine surcharge loading or backfill required
to inhibit upheaval. Following such surcharge, another survey can be required to confirm surcharge depth
achieved.
Guidance note:
The last of the above post-lay surveys, or the accumulation of them, will constitute the as-built survey. The results of the
survey(s) are normally required to be captured on video and on as-built alignment sheets for issue to the customer together with
comprehensive as-built documentation of the full construction and installation phase.
Details of the installed protection, if any, are included.
For power cables, an as-built survey might not be required if burial levels were previously recorded. In this case, vessel location
can be sufficiently accurate for as-built records. Required accuracy of location is project specific.
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7.9 Lay operations
7.9.1 General
For laying operation, the requirements of [7.2], [7.3] and [7.4] apply.
7.9.2 Planning of laying operations
7.9.2.1 The requirements for planning in [7.2] apply. The planning shall consider the requirements in the
rest of [7.9].
7.9.2.2 Seabed preparation requirements in accordance with the recognized design code shall be considered
and included in the planning and/or documented as completed prior to commencement of the laying
operation.
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Guidance note:
See also DNV-ST-F101 [10.3.2], /42/.
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7.9.2.3 The direction of lay shall be determined with consideration for:
a)
b)
c)
d)
e)
f)
g)
operational considerations
installation and precommissioning methodology
seabed structures/subsea facilities
location and water depth of planned offshore jointing operations
steep slopes (uphill lay/downhill lay)
offshore pull-in/subsea tie-in/shore pull operations including end termination design, and
prevailing environmental conditions.
7.9.2.4 Steep slopes
7.9.2.4.1 The risk of product slippage on steep slopes shall be considered, as this can lead to straightening of
curves and the formation of free spans uphill of the slope, as well as axial compression or buckling at the foot
of the slope. The risk of free spans at the foot of the slope due to excessive tension during uphill lay should
also be considered.
7.9.2.4.2 On steep slopes, the need for seabed anchors should be evaluated, considering effects of tension
variations both during downhill lay and in service.
7.9.2.4.3 The difficulty of establishing the point of contact with the seabed (touchdown) during laying on
steep slopes shall be considered with respect to lay method and installation parameters. The installation
procedure shall be established in such a manner as to ensure control of the catenary at all times, e.g. by use
of a step-by-step lay table.
7.9.2.5 End termination assemblies/in-line components
Procedures for safe overboarding and lay tables for lowering and landing shall be established to ensure a safe
operation for the limiting weather conditions specified for the operation.
7.9.3 Contingencies
7.9.3.1 The operation manual shall contain a set of contingency procedures, which shall be agreed between
MWS, customer and contractor.
7.9.3.2 Detailed contingency procedures for each critical operational step should be documented.
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Guidance note:
For laying operations in general, typical steps requiring contingency procedures can include
a)
failure of dynamic positioning system
b)
failure of anchors or anchor lines,
c)
coating repair
d)
anode repair
e)
failure of tensioning system
f)
repair of lay equipment - emergency hang off
g)
product becoming stuck during pull-in/tie-in
h)
ROV breakdown
i)
breakdown/failure of position reference systems/navigation reference system
j)
weather conditions in excess of operational limiting criteria
k)
third party marine activity and
l)
critical or emergency situations identified in FMEA or HAZOP studies.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.3.3 For power cables and umbilicals, contingency procedures should address abandonment and recovery,
including contingency cutting and subsequent repair splicing (as applicable).
Guidance note:
Where contingency cutting and subsequent repair splicing is not possible alternative procedures can be required.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.3.4 For rigid pipelines, contingency procedures shall address the following (as applicable):
a)
b)
c)
d)
e)
dry buckle lay-down and retrieval
wet buckle lay-down and retrieval
repair
accidental flooding
any other abandonment and recovery situations identified in FMEA or HAZOP studies.
Guidance note:
Due diligence should be exercised to ensure buckling does not occur, however a small residual risk remains that the pipe will
buckle during installation. Procedures and equipment are required to be in place to safely and efficiently recover from such an
event. The presence of a buckle should be detected and signalled to the vessel as per [7.9.10.5].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.3.5 Contingency procedures shall address the situation where lay operations are suspended, but the
product is held in-situ and not abandoned. The procedures shall detail how fatigue damage will be minimized
at the over boarding and touchdown points.
Guidance note:
Fatigue damage can be minimized by paying in/out product to change the contact point/bend at the over boarding point, such that
fresh product is moved into the dynamic section and the hardworked product moves to the static section. Where multiple over
boarding points are possible, the most favourable shall be used, considering the direction of prevailing current. To minimize fatigue
damage at the touchdown point, visual monitoring shall be maintained such that corrective action can be implemented if anything
unfavourable occurs.
If the lay vessel stands by with cable deployed over the lay chute or the lay sheave then the cable shall be picked up or paid
out a few metres at set periods of time to prevent cable damage where the cable is stationary on the lay chute or lay sheave –
freshening the nip.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.3.6 The schedule should include contingency time for possible repairs (e.g. of damaged outer layer/
coating).
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7.9.3.7 Provision of pipeline recovery tooling (PRT), dewatering equipment and pipeline cutting tools shall
be defined at the start of the project. The availability of this equipment shall be reflected in the contingency
procedures.
Guidance note:
For major pipeline projects in deep water, it can be beneficial to have a full stand-by spread of pigging and air compression
equipment to de-water the pipeline, to enable rapid recovery and repair of a wet buckle.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.4 Procedures
7.9.4.1 Procedures for laying, supported by analyses, shall be documented.
Guidance note:
Typically the following (as applicable) are included in the procedures:
a)
initiation of lay
b)
normal continuous lay, including around curves and changes in heading
c)
lay-down/termination of laying operation
d)
flooding and system pressure test
e)
abandonment and recovery (empty and flooded cases for pipelines)
f)
reeling/spooling
g)
trenching, ploughing and back-filling/burial
h)
tie-in operations
i)
pull-in to structure
j)
contingency procedures
k)
temporary conditions
l)
riser and spool installation
m)
laying of in-line structures
n)
shore pull and offshore pull
o)
andfalls
p)
close proximity operations
q)
SIMOPS
r)
DP/mooring operations
s)
diver/ROV operations
t)
control and monitoring
u)
operational limiting criteria for each operation phase.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.4.2 Laying procedures shall cover the whole range of water depths and environmental cases up to the
marginal environmental condition at which abandonment and lay-down will start using the A&R winch. Laydown and recovery procedures are also required, addressing the controlled step-wise operation to abandon
and return to normal lay conditions. The weather threshold required before recovery operations starting shall
also be documented in the marine operation manual.
7.9.5 Stationkeeping
7.9.5.1 In accordance with [7.2.2.2], the vessel shall be documented to have sufficient station keeping
capabilities to maintain its required position within the specified installation criteria (such as lay-back,
heading etc.) for all planned and contingency operations.
7.9.5.2 For laying operations, the vessel should either be:
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a)
b)
conventionally moored with one or more pull ahead winches (see also [7.9.6]) or
be dynamically positioned (see also [7.9.7]).
Guidance note:
Mooring equipment requirements are in [7.4.3]. Dynamic positioning system requirements are in [7.4.4].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.5.3 The stationkeeping system shall have sufficient capacity to manoeuvre the lay vessel, considering:
a)
b)
c)
limiting environmental condition
lay tension, and
ploughing loads (if applicable).
7.9.6 Mooring operations for product laying
7.9.6.1 A mooring procedure shall be documented with every critical/congested anchor location pre-defined
along the pipe route. For open field locations, typical anchor plans shall be submitted. The number of anchor
handling vessels shall be appropriate to the proposed mooring operations.
Guidance note:
As lay operations involve a near-continuous process of moving moorings and the lay vessel is normally attended by two or more
anchor handling vessels (AHV) to facilitate adjustment of the mooring pattern.
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7.9.6.2 A mooring analysis shall be documented to justify the mooring spread used, and identify the
limiting environmental conditions for both planned and contingency operations including any stand-by
condition. Critical configurations and load cases shall be determined for the given route and addressed,
allowing for planned anchor retrieval and re-lay operations. Consideration shall be given to contingency
cases with reference to allowable vessel offsets and anchor holding capacity. The mooring analysis shall be in
accordance with Sec.17.
7.9.6.3 The mooring analysis shall include the redundancy (single point failure) requirements in [17.2.5.2].
The reduced stationkeeping capability after/during a mooring failure, and any effects on product integrity,
shall be considered and documented. Attending anchor handling vessels may be considered for the
redundancy case.
7.9.6.4 AHV specifications and anchor handling procedures shall be documented. AHV(s) shall be subject to
MWS suitability survey before the start of the operations. Anchor handling vessels shall be equipped with:
a)
b)
c)
A surface positioning reference system of sufficient accuracy. High accuracy is required for anchor drops
in areas with strict requirements to control of anchor position, typical within safety zone of existing
installations, proximity of pipelines or areas of archeological or environmental importance.
Computing and interfacing facilities for interfacing with lay vessel, trenching vessel or other anchored
vessels.
Latest revision of charts for the whole area of operation.
7.9.6.5 Live anchors, i.e. anchor handling vessels used for stationkeeping, may be used to manoeuvre the
lay vessel. Live anchor operations shall be carefully planned and the live anchor handling procedure shall be
documented. The procedure shall include anchor position drawings showing the planned positions of anchors
for all phases of live anchor operations. When a DP anchor handling vessel is used as a live anchor, it shall be
DP class 2 at a minimum. See [17.3.2].
7.9.6.6 Contingency plans shall mitigate the potential impact on the product in the case of a mooring line
failure. Mooring line failure can govern the limiting environmental conditions if vessel excursions exceed the
permissible values for the product being laid. See [17.3.2].
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7.9.6.7 Clearances around mooring lines and anchors should comply with the requirements of Table 17-7. If
any of the clearances specified within Table 17-7 are impractical because of the mooring configuration, water
depth or seabed layout, a risk assessment shall be carried out and special precautions taken as necessary,
noting that agreement will be required for any affected third party equipment owner’s for such risks.
Guidance note:
This applies to both anchor running and laid conditions.
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7.9.6.8 Moorings should avoid contact with subsea assets. If contact/crossing is unavoidable then:
a)
b)
c)
it shall be demonstrated that the mooring will not damage the asset
suitable means of protection shall be installed, and
permission shall be obtained from the asset owner.
7.9.6.9 If an anchor is run out over a subsea asset, the anchor shall be securely stowed on the deck of
the anchor handling vessel. In circumstances where either gravity anchors or closed stern tugs are used,
and anchors cannot be stowed on deck, the anchors shall be double secured through the additional use of a
safety strap or similar.
7.9.6.10 Anchors shall be deployed with the vessel as far from other assets as safely possible, given the
constraints of the mooring system. Clearances between anchors/mooring line and the subsea asset shall be
in accordance with [17.2.6].
7.9.6.11 The anchor position, drop zone and wire catenary should be established taking into account the
water depth, wire tension and wire length. Confirmation of anchor positioning after installation shall be
confirmed at regular intervals depending on required accuracy and criticality, and counteractive measures
taken when found required. Monitoring systems shall be considered.
7.9.6.12 Mooring and pull-ahead anchors shall be selected to provide the required holding capability at the
limiting operating conditions in the soil conditions expected along the entire route. See [17.2.5].
7.9.6.13 Each anchor shall be loaded after deployment to a tension as per mooring analysis/procedure, and
position and tension shall be monitored to ensure no dragging taking place. See [17.2.8.2].
7.9.7 Dynamic positioning operations
7.9.7.1 The DP footprint shall be considered when defining the lay control parameters to ensure sufficient
margin is taken to ensure that this footprint is within the limits of the installation analyses/procedures.
Contingency plans shall address action required in event of a DP malfunction and shall include the actions
required by the product installation equipment operators. The reduced DP capability after a DP malfunction,
and any effects on product integrity, shall be considered and documented. See [7.9.3].
Guidance note:
In the event that the lay vessel requires a change of heading under high wind or current conditions, it not always feasible for the
DP centre of rotation to remain at the product over boarding point (or equivalent remote location). Such an operation should be
carefully planned to avoid vessel movement causing damage to the product, and documented in the contingency plans.
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7.9.7.2 Great care should be taken when choosing and prioritizing the various DP position reference
systems, taking into account the specific situations and critical parameters that need controlling.
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Guidance note:
Relative positioning system should be used when monitoring distances to other floaters and/or moving objects while absolute
positioning systems would typically be important if object is to be landed in an absolute position, such as target box or when
working close to a fixed platform.
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7.9.8 Installation monitoring
7.9.8.1 Lay monitoring
7.9.8.1.1 The lay monitoring methods should be documented considering the actual lay operation and
[7.9.8.2] to [7.9.8.4] as applicable. Also see [7.4.5].
7.9.8.1.2 The lay configuration and loads shall be controlled in order to ensure that these are within
established limits during installation. The configuration and loads may be controlled by various means and
shall be clearly described including allowable ranges for the specific installation. Redundancy in monitoring
method or contingency plans shall be available in the event of failure of monitoring equipment.
Guidance note:
The lay configuration may be controlled by tension, stinger tip clearance and/or lay-back distance/touchdown monitoring.
Depending on the installation vessel and type of product, the preferred method can alter.
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7.9.8.1.3 ROV monitoring should be performed for inline structures and PLET/PLEMs during lay-down and
initiation operations.
Guidance note:
In the case of multiple structures during lay-down, monitoring the lower structure close to TDP and lay-down area is normally
adequate.
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7.9.8.2 Touchdown monitoring
Where the following requirements to touchdown monitoring cannot be achieved due to operational
constraints (e.g. water depth) then alternatives to ensure product integrity shall be considered:
a)
b)
Touchdown monitoring should be accomplished by ROV or other means to ensure the configuration in the
touchdown area remains within the identified limits.
Touchdown monitoring shall be performed for critical operations and critical sections representing a risk
to the product or existing infrastructure. Critical operations and critical sections shall be identified in the
operational HAZID.
Guidance note:
Critical operations and critical sections can typically be:
—
initiation
—
laying around curves/altering course
—
narrow lay corridor
—
crossings
—
laying towards counteracts,
—
boulder areas and
—
lay-down.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
c)
Laying in congested areas, in the vicinity of existing installations and at crossings, shall be carried out
using positioning systems with required accuracy. Measures shall be taken to avoid damage to existing
infrastructure. ROV should be used to continuously monitor such operations.
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d)
e)
In order to enable continuous lay operations, adequate contingency measures for touchdown monitoring
shall be established in case of primary monitoring system failure.
Any ROV used shall meet the requirements of [4.5] and it shall have sufficient working range to observe
the critical areas such as touchdown point, turning points etc.
7.9.8.3 Vessel monitoring
7.9.8.3.1 The position monitoring systems requirements in [4.4.5] apply.
7.9.8.3.2 Monitoring equipment for the following should also be available:
— vessel movements such as roll, pitch, sway, heave
— water depth
— vessel floating position (e.g. draught and trim)
— direct or indirect indication of sagbend curvature and strain
— laying equipment, [7.4]
— for vessels to be moored see [7.4.3].
7.9.8.4 Load monitoring
7.9.8.4.1 Tension levels shall be monitored and should be recorded throughout installation to ensure
allowable values are not exceeded unless suitable risk assessment and mitigations are in place.
7.9.8.4.2 For stinger or ramp, the following (if applicable) should be monitored :
— product and A&R wire position with respect to the last roller or guide
— roller reaction loads on the roller introducing the stinger curvature
— roller reaction loads (vertical and horizontal), as a minimum for stinger tip
— stinger and ramp configuration.
Guidance note:
Pipeline and wire position with respect to the last roller or guide may be monitored using underwater camera(s), sonar, ROV or
diver.
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7.9.9 Lay initiation
7.9.9.1 The procedures for initiating the lay, accounting for any end termination, shall be documented. The
lay initiation procedure shall demonstrate that laying will start in the correct location. For laying operations
where initiation is via transfer and pull-in to a host structure the procedure shall consider the requirements in
[7.9.13].
Guidance note:
For subsea power cables in shallow water see DNV-RP-0360 [6.5.3], /58/.
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7.9.9.2 Any rigging and connection point e.g. initiation head used during the initiation shall meet the
requirements of [7.4.7]
7.9.9.3 For un-bonded flexible pipe risers, annulus pressure relieving vents at end fittings shall be confirmed
fitted and in good condition, before over-boarding the riser.
7.9.9.4 The rest of subsection [7.9.9] is applicable when the laying is initiated using an initiation point i.e.
without connection to a previously installed asset.
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Guidance note:
Piles or dead man anchors may be used as an initiation point.
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7.9.9.5 The initiation point design shall be in accordance with a recognized code.
Guidance note:
DNV-RP-E301, /48/, DNV-RP-E302, /49/, DNV-RP-E303, /50/, and DNV-RP-C212, /19/, may be considered.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.9.6 Where used, dead man anchors (DMA) should be documented as being stable against tilting under
load.
7.9.9.7 The initiation point shall be of adequate design and have sufficient structural capacity to resist the
initiation load in the direction of lay, given a lay tolerance of +/-5° from the proposed lay direction.
Guidance note:
In general the characteristic start-up tension is based on results from full dynamic analysis of the lay process. For early design the
characteristic environmental loads may be taken as half the functional loads with appropriate load and material factors.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.9.8 Purpose made initiation points, should be load tested to the maximum characteristic bottom
tension during start-up before start of lay operation. The direction of pull during test shall reflect the actual
conditions during initiation. Holding time should be at least 15 minutes.
Guidance note:
Load tests are generally required, unless it is proven by design, soil conditions, geotechnical analysis and quality checks, that there
is a sufficient margin available against the maximum characteristic bottom tension. For initiation piles, normally embedment of the
pile, design and as built fab dossier are sufficient to prove the fitness of the initiation point.
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7.9.9.9 Calculations shall demonstrate that the length of the initiation rigging shall ensure that product
landing will occur in the designated target box. The initiation wire should be tangential to the seabed, unless
the initiation point is used and designed to take loads in the vertical plane.
7.9.9.10 If a diver-less latch/sheave type of initiation system is used, ROV monitoring of the sheave is
imperative throughout the whole initiation phase. Further, crossings between running wires and other wires,
pipelines, mooring equipment etc. shall be continuously monitored.
7.9.9.11 The initiation head structure should have means of preventing or mitigating the consequences of
overturning. Structures with CoG above pipeline centre shall be specially considered.
7.9.9.12 Pulling heads fitted with valves, flanges, stabs etc. (used afterwards for pigging etc.) should be
protected to eliminate the risk of entrapment in the soil/seabed or designed to cope up with any possible
rotation and entrapment.
7.9.10 Laying
7.9.10.1 Catenary
7.9.10.1.1 The method(s) of maintaining an acceptable catenary shall be documented and based on lay
analyses.
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Guidance note:
Generally it includes the allowable lay-back range and top tensions required to maintain a safe seabed tension at all water depths,
and for all phases of the operation.
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7.9.10.1.2 For laying, the lay-back shall be maintained within determined limits to ensure no limit state is
exceeded.
7.9.10.1.3 The lay vessel shall not drop back onto the catenary unless the operation is planned and laying
equipment e.g. linear cable engine/tensioners can recover product from the seabed at least at the same
speed as the vessel drops back.
Guidance note:
This type of movement, when unplanned and uncontrolled, reduces tension and increases the risk of loops being thrown in the
cable and buckling in pipelines.
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7.9.10.1.4 If an unplanned drop back over the product occurs, the product shall be inspected at the seabed
and tested, if deemed necessary, to ensure product integrity before the catenary is re-established and laying
continues.
7.9.10.2 Curves
7.9.10.2.1 Touchdown position when laying in curves shall take account of the tendency for tension in the
product during laying to pull the touchdown point closer to the installation vessel track line.
7.9.10.2.2 Due to potential lateral slip of rigid pipelines over the seabed at the tangent points, checks shall
be performed to assure that the curve radius is acceptable.
Guidance note:
S-Lay may limit the minimum pipeline route radii, as does the proximity of any turns in the route relative to the ends of the
pipeline.
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7.9.10.2.3 In order to ensure that a curve remains stable on the seabed the curve radius, R, should be
greater than the minimum stable radius, i.e.
where:
= safety factor (reflecting the consequences of sliding)
SF
TTDP = tension at touchdown point
Wsub = submerged weight per unit length
= lower bound friction coefficient.
μf
Passive soil resistance may be calculated based on DNV-RP-F109 [7.5], /53/.
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Guidance note:
SF for sliding is normally taken as 1.1 (particularly if tension at touch down point is taken as static bottom tension). When pipelay
is in close proximity to existing subsea structures, it is recommended to use 1.5.
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7.9.10.3 Tensioners
7.9.10.3.1 Tensioners used for laying shall be in accordance with [7.4.8].
7.9.10.3.2 Tensioner track faces bearing on the pipe shall be maintained with adequate bearing area.
Tensioner control systems shall be set to the limits documented in the operation manual.
7.9.10.3.3 During normal lay and as much as is practically possible, sufficient tensioners should be closed
and utilised so that failure of one tensioner will not cause the product to slip (or at least slippage will be
limited).
7.9.10.3.4 Where cables are installed in bundles (e.g. HVDC circuits) the difficulty in handling such cables
compared to single cables shall be considered.
7.9.10.4 Rigid pipe
7.9.10.4.1 Where the pipe is to be reeled/spooled, sufficient line pipe shall be available for lead and tail
sections for reeling/spooling onto and off the vessel and for when straightening trials are required.
7.9.10.4.2 Welding, non-destructive testing (NDT), field joint coating and installation of cathodic protection
procedures/activities shall be documented including details of inspections and QC procedures for approval
of laying operations. The field joint coating (FJC) procedure shall be documented and shall be approved as
agreed between contractor, customer and the MWS company.
Guidance note 1:
These activities are not subject to MWS approval however the effect of them on the installation as a whole is.
In general the FJC procedure clearly:
a)
Reflects the manufacturer's specification, particularly for the temperature of application which is of paramount importance.
b)
Shows how linepipe coating is prepared to provide adequate adherence of FJC to linepipe coating and ensure continuity of
c)
Details the method for FJC inspection.
protection.
DNV-ST-F101 [10.5] describes the welding and non-destructive testing requirements for pipelines in accordance with that
standard.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Anode attachments should in general be made on circular doubler pads, matching the line pipe material and preferably welded at
the neutral axis of the pipe on the reel; however welding is not always permissible for materials susceptible to HISC.
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7.9.10.5 Buckle detection
7.9.10.5.1 For laying non reeled/spooled rigid pipe in shallow water, pipe buckle detection, may be by means
of a buckle sensor pulled along the bore of the pipe on a cable, located just beyond the touchdown and
continuously monitored in the control room.
7.9.10.5.2 If a buckle detector is not used a suitable alternative shall be provided. Alternatives include but
are not limited to:
a)
increased factor of safety against buckling using design guidance given in DNV-ST-F101 [10.6.7], /42/
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b)
c)
d)
continuous ROV monitoring at touchdown
lay parameters control
acoustic buckle detection.
Guidance note:
Some installation contractors are reluctant to use traditional in-line buckle detection methods, especially in deep water installations
due to the hazards introduced by breakage of the line connecting the buckle arrestor to the pipelay vessel.
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7.9.10.6 Bundles
7.9.10.6.1 Where the line is installed strapped to another load carrying line, such as a pipeline or stronger
flexible, loads from the strapping/clamping shall be acceptable.
7.9.10.6.2 It shall be confirmed that no limit state is exceeded for all lines within the strapped bundle.
Guidance note:
In most cases one of the lines in the bundle will carry the installation loads as it is convenient to use only one tension system. The
strapping/clamping loads and load transfer from the carried line to the carrying pipe may determine the strength requirements of
the pipe.
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7.9.11 Abandonment and recovery
7.9.11.1 This subsection [7.9.11] applies to all abandonment and recovery (A&R) operations. A&R
operations shall be documented in normal and/or contingency procedures; see [7.3.3.2].
Guidance note:
Abandonment and recovery operations could be:
—
in response to emergency situations
—
to enable the vessel to collect more product
—
as a temporary safe condition in response to a weather forecast in excess of the forecasted operational criteria.
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7.9.11.2 A detailed abandonment and recovery procedure is required in the operation manual, covering the
full range of water depth and product weight cases that may be required over the route. This procedure shall
define in stages the:
a)
b)
c)
d)
e)
required A&R winch tension
A&R line-out
lay-back
lay-down head depth, and
roller support loads, where applicable
to get the pipe safely down to the seabed. The analysis shall define the steps for abandonment and recovery
with the aforementioned parameters and shall identify if limit states are exceeded. Where a limit state is
exceeded it shall be shown that it is possible to safely abandon and recover the product.
7.9.11.3 The procedure should present the same steps with the same tensions, line-out, lay-back distance
and head depth.
7.9.11.4 An acceptance threshold for tension should be defined for each piece of pipelay equipment/
operational phase such that in conjunction with the installation methods and risk assessment, the decision for
emergency A&R is clear at the start of pipelay.
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7.9.11.5 The reference point for the A&R line-out (length of wire being paid out) shall be clearly specified in
the analysis/procedure. This reference shall be available or directly readable by the Superintendent or winch
operators so that the tension can be reduced/increased in accordance with the steps defined in the analysis.
7.9.11.6 A&R lay-down areas should be defined as it may not be possible to abandon the pipeline within the
lay corridor. Areas of the route where abandonment is not possible shall be documented.
7.9.11.7 Before abandonment of a rigid pipeline, all internal equipment shall be removed from the pipeline
and all welds filled sufficiently to achieve the strength determined for abandonment and recovery. If a buckle
detector is being used it may be left in the line with its cable made secure to the abandonment head. The
abandonment and recovery head shall be fully welded to the line and tested in accordance with relevant
welding and NDT procedure.
7.9.11.8 Should it be necessary to release the A&R wire to the sea, a buoy and pennant shall be fitted to the
wire for subsequent recovery. Alternatively, an ROV hooking loop may be used on the wire end.
7.9.11.9 The method of establishing the product’s condition and ensuring there has been no damage
(including buckling) anywhere between the location of the touchdown point before abandonment and the
lay-down head shall be documented reflecting the nature of the abandonment and the implemented lay
monitoring.
Guidance note:
Generally and in particular when abandonment commenced during environmental conditions approaching the limiting criteria or in
response to an emergency situation the product is to be surveyed prior to recovery.
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7.9.12 Temporary condition including on-bottom stability
7.9.12.1 The product shall be demonstrated as adequate for other temporary conditions experienced
including confirmation that the product has sufficient on-bottom stability for any relevant temporary condition
(e.g. untrenched, air filled) and that the on bottom roughness and free span limits are not exceeded. The
applicable project specific design basis should identify the relevant recognized codes to be satisfied in this
regard.
Guidance note:
Further information on the on-bottom stability of submarine pipelines can be found in DNV-RP-F109, /53/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.12.2 The as-laid product on the seabed shall be stable on the seabed in the design environment. The
risk of product slippage in steep slopes should be considered. For a temporary phase with duration less
than 12 months but in excess of three days, a 10-year return period for the actual seasonal environmental
condition should apply. An approximation to this condition is to use the most severe of the following two
combinations:
a)
b)
The seasonal 10-year return condition for waves combined with the seasonal 1-year return condition for
seasonal current.
The seasonal 1-year return condition for waves combined with the seasonal 10-year return condition for
current.
Guidance note:
This is consistent with DNV-RP-F109, /53/. Where the recognized design code has other requirements these may be acceptable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.12.3 The season covered by the environmental data shall be sufficient to account for uncertainties in the
beginning and ending of the temporary condition, e.g. delays. For a temporary phase less than three days an
extreme load condition may be specified based on reliable weather forecasts.
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7.9.13 Pull-in to structures
7.9.13.1 General
7.9.13.1.1 General requirements for product pull-in into structures are given, with additional requirements
for various pull-in operations as follows:
— pull-in through J-tubes, see [7.9.13.2]
— installation of steel risers (including pull-in through I-tubes), see [7.9.13.3].
Guidance note:
Pull-in is usually through J-tubes or I-tubes. There are two types of J-tube/I-tube pull-in methods:
—
pull-in winches mounted on the offshore structure
—
pull-in winches mounted on the lay vessel with a sheave arrangement mounted on the offshore structure (pull back method).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.1.2 Other requirements within this document to ensure product integrity shall be applied as found
relevant, in particular those related to not exceeding any limit state (see [7.3.3]).
7.9.13.1.3 The operation manual shall document careful planning of close proximity operations and shall
include contingency plans to be implemented in the event that the lay vessel moves out of control toward or
away from a structure.
Guidance note:
Lay vessels typically approach close enough to offshore structures to transfer pull-in aids using the lay vessel crane.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.1.4 Vessel stand-off or offset at all stages of the installation shall be defined and drawn to scale in
plan, and elevation if necessary, to demonstrate that the installation is clash free from any adjacent risers
or moorings or other facilities and to guide the vessel crew. Such drawings shall be included in the operation
manual. This shall also include different vessel headings to account for various wave directions, if applicable.
7.9.13.1.5 Clearances to nearby subsea assets should be monitored during pull-in of the product.
7.9.13.1.6 Catenary geometry at these critical locations and vessel offset to achieve them shall be
documented in supporting analyses of the installation.
7.9.13.1.7 Where applicable, contingency procedures for safe rapid disconnection of the installation vessel
from the product shall be established and approved for the duration of physical connection of the installation
vessel to the receiving installation. Procedures for recovery from circumstances such as dead ship, DP run-off
or failed winches shall be established and accepted before mobilisation.
7.9.13.1.8 The pull-in load shall be found through analysis, including the effects of friction, vessel motions
and back tension, and any other relevant loads.
7.9.13.1.9 The strength of the J-tube/I-tube, pull-in aids, pull-in winch, foundation and sheaves shall be
confirmed capable of carrying the maximum loads placed upon it during product pull-in.
7.9.13.1.10 Pull-in winch and rigging capacity on the receiving installation shall have a practical margin (see
guidance note) between the predicted peak pull load and the winch’s capacity at the loaded diameter under
consideration, subject to any uncertainties governing assumed friction loads.
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Guidance note:
A 10% increase (i.e. a safety factor of 1.1) may typically be acceptable for this practical margin.
The nominal capacity can be stated for an empty drum whereas the peak pull load is likely to be with the drum full of wire when
the winch’s capacity is lower.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.1.11 The pull-in winch shall have a render setting so that excess tension in the pull-in rigging caused
by lay vessel movements does not over tension the pull-in rigging.
7.9.13.1.12 Before mobilisation the pull-in winch should be load and function tested. A commissioning report
should be submitted if the testing is not witnessed by the MWS company.
7.9.13.1.13 Pull-in rigging and components, including the pull head, loaded by the pull-in shall be checked in
accordance with the criteria in [7.4.7].
Guidance note:
The design of the pull in arrangement should minimise the possibility of snagging. Sharp corners and/or protruding elements shall
be avoided. Contingency plans should include actions to be taken in case of stuck product during pull-in, failure of pull-in winch,
actions to be taken in case of topside evacuation etc.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.1.14 Pull-in heads or grips/Chinese fingers shall be of adequate strength for the transfer operation.
The MBL of a Chinese finger should not be less than the required steel wire MBL of the sling/wire connected
to it.
Guidance note:
Depending on the geometry of the pull-in route (i.e. if it is not in a single vertical plane), the pull-in head may be required to have
equal strength across all axes.
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7.9.13.1.15 Messenger lines, connecting shackles and other rigging should be fully certified and graded for
predicted loads. Pre-installation of messenger wires before pull-in should consider as-left conditions and the
risk of entanglement/damage.
Guidance note:
Messenger lines are used to initiate the pull-in operation. It is recommended that messenger lines are marked top and bottom to
ensure the correct line is connected to the pull-in winch wire.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.1.16 Product shall not be pulled in by the pre-installed messenger lines. The messenger line shall be
used to install the dedicated pull-in lines.
7.9.13.1.17 Prior to pull-in of products into J-tubes, the position of the J-tube, clamps, supports and product
plus ancillaries shall be confirmed and evaluated with respect to the design. Bend restrictors shall be fitted
at the correct location on the product and follow the manufacturer’s installation instructions to ensure the
interface with the J-tube/I-tube engages properly.
Guidance note:
Careful measurements and calculations are required to establish the position at which bend restrictors and product protection are
to be attached, and these should be verified by an approved and proven QA/QC check process on-board the installation vessel..
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.1.18 Factors including the following shall be considered when determining the pull-in operational
limiting sea states and other limiting conditions:
— offshore structure access/egress for pull-in personnel
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— vessel motions and impact on crane operations for transferring pull-in aids and other equipment to
offshore structures
— pull-in tension caused by lay vessel motions
— ablility to safely land the pull-in head on the seabed.
7.9.13.1.19 The entry of the product into the structure shall be continuously monitored, and the pull-in
tension shall be within specified limits.
7.9.13.2 Pull-in through J-tubes
7.9.13.2.1 J-tube bell mouths can be raised above the seabed. The product MBR shall be protected in the
free span between the J-tube bell mouth and the touchdown point.
7.9.13.2.2 Protection against dropped objects at the approach to offshore structures should be provided on a
project specific basis.
7.9.13.2.3 Pull-in loads shall account for the frictional soil loads from the seabed (as discussed in [7.9.15.5]),
frictional loads (as discussed in d) and e)), and the back-tension from the tensioners.
7.9.13.2.4 Friction between the J-tube and the product shall be calculated according to the principles in
[5.6.9].
Guidance note:
Indicative coefficients are in Table 7-3. The internal coating and geometry are not accounted for in the below but should be
considered if relevant
Table 7-3 Typical upper bound design friction coefficients for J-tubes
Friction interface
Static coefficient
Sliding coefficient
Steel wire
0.8
0.5
Bare pipe or pull head
0.9
0.6
FBE (fusion bonded epoxy) coated pipe
0.5
0.4
PP (polypropylene) coated pipe
0.5
0.4
Nylon, PE (polyethylene) or polyurethane
0.5
0.4
Cable or umbilical
0.5
0.4
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7.9.13.2.5 If lubricant is planned to be used during pull-in to reduce friction, the compatibility of the
lubricant with the surfaces in question both with respect to chemical reactions and friction reduction shall
be documented. Testing may be carried out in order to establish applicable friction coefficients following the
principles in [5.4].
7.9.13.2.6 It shall be ensured that the J-tube is free of any obstacles that can cause the pulling head to get
stuck during the pull-in operation.
Guidance note:
This may be achieved by pigging the J-tube in at least in one direction, for instance during the retrieval of the messenger line.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.2.7 If the clearance between the J-tube internal diameter and the pull-in head is small, a gauge
run with a dummy pulling head should be performed. The gauge plate should be sized slightly below the
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minimum J-tube bore, e.g. 95% of the I.D., according to its fabrication tolerances. Also, if the J-tube has
been in place for several years, its internal condition should be assured by means of remote visual spot check
inspection.
7.9.13.2.8 For flexible product pull-ins, classical theory for the tension amplification around a frictional
bollard and elastic beam bending theory are usually sufficient for the purpose.
7.9.13.2.9 For rigid riser pipe, the dimensions of the J-tube are such that the pipe will be plastically strained
as it passes through each bend in the J-tube, acquiring residual plastic and elastic bending after each bend,
and large contact/bearing loads will set up in the middle and each side of these bends during the pull-in.
Analysis shall be performed to calculate the pull-in loads.
Guidance note:
For calculation of the pull-in loads for a rigid riser pipe reference may be made to A Design Basis for the J-tube Method of Riser
Installation, /123/, or to Design and Installation of the Oseberg Flowline Bundle, /124/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.9.13.2.10 Cumulative rigid pipe riser ovalisation due to bending shall be checked and documented in the
operation manual to ensure that local buckling or collapse is not a risk, particularly at the deepest bends in
the J-tube. Similarly, the riser pipe material shall be verified as suitable for the strains accumulated, including
those during J-tube pull-in. For a J-tube pull-in after a reel lay, the pipe will be deformed plastically twice
more. It is important to document the total accumulated strain and compare it with the requirement for ECA
or additional tests. See [7.3.2.3.6].
7.9.13.3 Dynamic flexible riser/umbilical installation (including pull-in through I-tubes)
7.9.13.3.1 The path of the riser, cable or umbilical through the installation shall be guided through cones
and smooth bores of known dimensions and tolerances, as far as practicable, to avoid hang-ups. Also,
consideration of the approach angle off-vertical or wire offset from centre of pull-tubes/I-tubes, etc. shall be
included in the planning and procedure to avoid hang-up.
7.9.13.3.2 In the case of installation into a floating installation such as an FPSO which is free to rotate
around a turret mooring, specific procedures and controls of the weather-vaning shall be documented if the
installation vessel is to enter the swing circle (swept envelope created by the weather-vaning installation and
its maximum radial excursion on its mooring system).
7.9.13.3.3 Lift points should be properly engineered and designed according to the specific rules. Where it is
necessary to attach to the line between hard points, this should be by means of purpose made clamps or by
suitably sized and rated Chinese fingers.
7.9.13.3.4 Chinese fingers should be used only for axial and minor lateral loadings after acceptance for this
has been provided by the product manufacturer including design limits for such use, see [7.5.2.3]. Similarly,
soft webbing strops choked around the body of the line shall only be used for minor loads and only after
acceptance by the product manufacturer and according to their instructions included with the installation
procedure.
7.9.14 Crossings
7.9.14.1 For all crossings identified on the crossing survey, [7.8.4], a detailed procedure shall be
documented that ensures a safe crossing for both the crossing and the crossed submarine plant (pipeline or
cable). The procedure shall include adequate monitoring to allow the crossing to be installed correctly.
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Guidance note:
The key principle for crossings is to demonstrate that the existing line will be protected from harm and can continue to operate
without interruption. This could include prevention of physical contact, prevention of galvanic action and electrical or cathodic
protection interference.
Cable or pipeline crossings are normally constructed with mattresses, rock dumping, polyurethane products or small concrete
bridges.
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7.9.14.2 The crossing shall be designed in accordance with the recognized design code(s) for both the
crossed and crossing product.
7.9.14.3 Construction and protection methods, crossing angles and operational zone dimensions shall be
agreed between the respective submarine plant owners.
Guidance note:
Typically owners of the crossed submarine plant require that plough operations cease and restart at specified distance from the
crossing point location. Jetting is normally allowed within the agreed no-plough zone.
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7.9.14.4 Preparations for crossing of pipelines and cables shall be carried out according to a specification
detailing the measures adopted to avoid damage to both installations. The operations should be monitored to
confirm proper placement and configuration of the supports. Support and profile over the existing installation
shall be in accordance with the accepted design.
7.9.14.5 As-built records and visual and written records of the construction sequence shall be submitted to
provide evidence of the conditions with respect to the crossed line.
7.9.14.6 An accurate position of the crossing shall be obtained.
Guidance note:
Visual monitoring and/or transponders are typically acceptable. Where the crossing is close to the final end/lay-down area, the
procedure should state the number of transponders (where required) to obtain an accurate positioning at the crossing area and to
ensure that the pipeline will also land in the target box.
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7.9.15 Shore crossings/landfalls
7.9.15.1 Shore crossings are made by one of the following methods, on the surface or seabed (bottom):
a)
b)
Pull from shore (offshore pull).
Pull to shore (shore pull).
Guidance note:
a)
Surface pull is when the pipeline is made to float on the surface assisted by floating devices such as individual buoyancy
b)
Bottom pull is when the pipeline is pulled along the seabed and is often performed in a trench to mitigate the impact of
tanks. This method is commonly used for short distance crossings in a calm sea environment and for relative fast fabrication.
lateral load due to current and wave. This method may also be performed from shore to shore, crossing a strait, bay or river.
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7.9.15.2 Pulls require trenching or dredging operations before or after the product is in position and backfill
to stabilise and protect the product against the environment and against other traffic in the area.
Guidance note:
Pull in can also be done through pre-installed horizontal conduits (steel or HDPE conduits pre-installed in a trench or drilled hole).
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7.9.15.3 For shore crossings using horizontal directional drilling (HDD) ducts, the relevant requirements of
[7.9.13] shall apply.
Guidance note:
HDD ducts are often required for landfall sites with sea defence systems.
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7.9.15.4 The pull loads shall be calculated in order to establish:
— required capacity of winch, tensioner or similar
— loads on product and installation aids
— the need for guiding arrangements and roller supports and
— corresponding design loads.
Guidance note:
The pull loads are essentially static forces but the product span between the seabed and the lay vessel will be dynamic. Depending
upon the equipment used and the local ocean environment, it can be necessary to consider the dynamics. The maximum static pull
load includes the following components:
a)
leading rigging friction, including in any sheaves
b)
leading pull head friction
c)
product friction on launchway, shore and seabed
d)
any hold back tension.
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7.9.15.5 A significant part of the rigging and product will rest on the seabed. Friction shall be calculated
according to the principles in [5.6.9]. Also, effects of soil penetration of pull-in head and product shall be
considered, if applicable, as well as required break-out force due to start-up after a temporary stop in the
pulling operation.
Guidance note:
Indicative coefficients are in Table 7-4.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Table 7-4 Typical upper bound design friction coefficients for shore crossings
Conditions
Breakout
Running
Pull wires on seabed and shore
1.0
0.85
Pipe on seabed and shore
1.2
0.85
Cable on seabed and shore
1.2
0.85
Wheel bogies on rail track
0.01
0.008
Pull wires on sleepers
0.5
0.5
7.9.15.6 The maximum on-bottom nominal pipe weight should be used, accounting for any increase due to
seawater absorption.
Guidance note:
For concrete coating, a 3% increase in weight may normally be considered.
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7.9.15.7 Analyses shall be performed to establish buoyancy requirements and design loads for product
attachment points. In cases where buoyancy aids are used, analyses shall be performed to demonstrate
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the integrity of the product with an appropriate loss of buoyancy considering the buoyancy fastening plan.
Seabed clearance and stability of buoyancy tanks during floating of pipelines shall be considered.
7.9.15.8 The effect of drag on product and buoyancy elements due to current and wind shall be considered.
7.9.15.9 Effects of contact loads at the entrance to a HDD duct or micro tunnel shall be considered, including
stress concentrations at interface between product and pull-in head, and increase in required winch tension
due to friction.
7.9.15.10 Sensitivity with respect to vessel stand-off distance, friction coefficients and drag should be
considered in order to ensure a robust solution.
7.9.15.11 For shore pull, a design factor shall be applied to the computed total static force to account for
uncertainties.
Guidance note:
An indicative value for this design factor is 1.3.
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7.9.15.12 The MBL of the pull wire should be twice the calculated factored static load.
7.9.15.13 Shackles, end fittings such as spelter sockets and other certified rigging items should have an
MBL at least 30% greater than the pull wire.
7.9.15.14 Structural steel items such as padeyes and the load path through the pull head to the pipe should
have an ultimate load capacity not less than:
— pull wire required MBL + 40 tonnes for MBL > 160 tonnes or
— pull wire required MBL x 1.25 for MBL ≤ 160 tonnes.
7.9.15.15 In particular circumstances, where the available pull wire is oversized with regard to the design
shore pull requirement, the design of the rest of the components can be related to the required MBL rather
than the actual MBL of the pull wire. Such relaxation shall be noted in the pull in procedures and appropriate
measures put in place to avoid exceeding the design pull.
7.9.15.16 In the event of the pull stopping due to a build-up of soil ahead of the pull head or on top of the
product, the peak allowable pull load may be increased to 60% of the MBL of the weakest part of the launch
rigging. This upper limit should be clearly stated in the pull procedure as a contingency case. It may only be
used subject to accurate monitoring of the actual force required. Any such operation shall be subject to a risk
assessment, see [2.4].
7.9.15.17 Documented calculations shall justify the acceptability of the structure or equipment (i.e. walls
made of sheet piles, mooring lines etc.) used for holding back tension
7.9.15.18 A representative vessel stand-off distance shall be established considering:
— seabed topography
— tide
— vessel draft for a representative loading condition
— hull/seabed clearance.
7.9.15.19 If a lay vessel can approach close enough to the landfall position or in case the shore pull area is
sheltered area like bay/gulf, product can be floated ashore using a pulling wire from the landfall with floats
attached to the product. This is called a direct landfall. Detailed marine planning is required to ensure the lay
vessel can set up close enough to the landfall position for the duration of the landfall operation. Care shall be
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taken in cross currents which can cause excessive tension in the product leading to loss of control of the pullin operation.
7.9.15.20 In shallow water, where the lay vessel is allowed to bottom out during low water phases of the
tidal cycle then the following shall be considered:
a)
b)
c)
d)
e)
f)
g)
h)
Careful planning is required to ensure the seabed is flat with no obstructions that might in anyway cause
damage to the lay vessel hull.
Adequate pre-set mooring shall be provided.
The work shall be carefully planned and timed to ensure the vessel does not become trapped by falling
tides.
A pull-ahead anchor should be deployed offshore of the lay vessel
A stand-by anchor handling vessel should be on site to assist the lay vessel if it needs to pull off the setup position in an emergency.
Consideration should be given to seabed suction forces when re-floating the lay vessel.
Hull strength shall be adequate to withstand the loads arising.
Required vessel systems shall be operable.
Guidance note:
It is common to float the lay vessel in on a rising tide until the lay vessel is close enough to the landfall for a pull-in operation
Often anchors are placed on land due to the close proximity to land of the lay vessel set up position.
It is normal for an operation where it is planned for the lay vessel to bottom out for it to approach the landfall site with minimum
product on-board. This requires the product to be laid from the offshore structure towards the land.
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7.9.15.21 Alternatively, a small low draught vessel can be used to pre-install a separate shore end cable
when the main lay vessel cannot approach close enough for a direct landfall operation. There are two
methods for implementing a pre-installed shore end installation:
a)
b)
A shore end cable is laid by a small vessel between the landfall and a position where the main lay vessel
can set up for jointing operations or,
The cable is turned-over from the main lay vessel onto a small vessel. The small vessel lays the cable
toward the landfall location. This method presents complications at the landfall end as the cable will need
to be turned-over a second time to bring the landfall cable end to the top of the cable tank.
7.9.15.22 Landfall sites with extensive shallow water will require a bespoke solution adapted to the
challenges of each landfall. A range of methods have been adopted including very shallow draught vessels
and tracked vehicles with cable drums mounted on the vehicle. The combination of HDD ducts and extensive
shallow water adds a high level of construction risk.
7.10 Other installation activities
7.10.1 Tie-in operations
7.10.1.1 General
The requirements for tie-in operations in DNV-ST-F101 [10.9], /42/, apply.
7.10.1.2 Above water tie in operations
For pipeline above water tie-in (AWTI) operations, procedures and engineering calculations shall be
documented to demonstrate suitability of the following:
a)
b)
The capacity of the lifting davits and/or cranes to be within allowable limits.
The capacity of any lifting slings and lifting appliances involved in the operation to be within allowable
limits.
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c)
d)
e)
f)
g)
The capacity of any temporary supports for the pipeline sections during tie-in operations to be within
allowable limits.
The capacity of the pipeline to be within requirement of pipeline capacity check according to relevant
design code including all dynamic lift-lowering effects.
An hourly time schedule for the AWTI operation.
Limiting criteria for recovery and abandonment of the pipeline sections.
Vessel stationkeeping analyses.
7.10.2 Steel catenary riser installation
7.10.2.1 This subsection [7.10.2] specifies the additional requirements for the installation of steel catenary
risers (SCR) over those elsewhere in Sec.7. The applicable design codes for SCR’s are API RP 2RD, /4/, and
DNV-ST-F201, /43/.
Guidance note:
Steel catenary risers require sufficient water depth for the riser to flex to follow the surface installation without overstressing and
are therefore only feasible in deep water (generally greater than 500 m). They may be installed as part of the pipeline using the
same type of vessels and methods as the rigid pipe laid to the location.
The weight of thick walled SCRs is an issue for installation. There are only a limited number of installation vessels able to install
thick walled SCRs in deep water and the number of suitable installation vessels diminishes rapidly with increasing depth and pipe
wall thickness.
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7.10.2.2 In this application, the pipelay process is critically important to the available life of the system
and its performance. Residual strains, strain history, residual torque, residual ovalisation, local stress
concentrations and fatigue life consumption from the installation process are key and shall be compared to
the allowances provided by the system designers.
7.10.2.3 SCR transfer operations shall be planned for all contingencies identified during hazard identification.
Where the operation is planned based on an empty SCR, contingency procedures based on the SCR
accidentally becoming flooded shall be prepared to enable the SCR to be brought into a safe condition.
The installation aids shall be sized to enable the SCR to be bought into a safe condition for all planned and
contingency operations.
7.10.2.4 The capacity of the SCR porch shall be adequate for all planned and/or contingency operations
including the flooded SCR condition.
7.10.2.5 Where a flexible joint or taper stress joint or similar is provided at the top of the riser as the
connection to the host facility:
a)
b)
the capacity of the joint shall be acceptable for all phases of the marine operation and
the joint shall be included in the analyses, procedures and operation manual.
7.10.2.6 The effects of locked in torque shall be evaluated.
Guidance note:
This joint is vulnerable to any locked in torque left in the pipeline and riser so the load should be determined from the installation
analysis and confirmed to be within the design specification of the joint.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.2.7 If automatic ultrasonic testing (AUT) is employed, the procedure, equipment and personnel shall be
qualified by an authorized body.
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Guidance note:
The use of SCRs in deep water results in thick wall risers with complex welding procedure requirements for the offshore field joints.
Automatic ultrasonic testing (AUT) is typically employed as part of the offshore NDT program and is considered one of the most
effective methods of inspection offshore to detect critical flaw sizes as small as 0.5 mm to 1.0 mm in height, which is a possible
requirement for fatigue sensitive SCR welds. See DNV-ST-F101 App.D, /42/, and DNV-ST-F101 App.E, /42/, for guidance.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.2.8 The planning shall consider the complexity of the handover operation and the close proximity of the
pipelay vessel to the host facility. If the SCR is laid down before arrival of host facility for later recovery and
hand-over, the lay route should be finalized considering the requirements of vessel movements and clearance
issues during hand-over after arrival of host facility.
Guidance note:
Generally SCRs are installed once the host facility is in place.
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7.10.2.9 Additionally to any point of no return, the procedures shall identify any point of the operation where
the SCR cannot be brought into a safe condition (including abandonment) without reversing the operation to
that point. Where reversing the operation could be required this shall be included in the procedures.
Guidance note:
During assembly of the SCR, typically there will be a point beyond which the SCR cannot be abandoned (the point of no
return or PNR) as it would interfere with the host platform’s field architecture (moorings, in-field pipeline, wells, templates,
etc.). Abandonment beyond this point will first require the SCR to be cut back (or reeled back for reel-lay vessels) before it is
abandoned.
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7.10.2.10 The installation time from any point of no return to final handover and hang off of the SCR onto
the host facility shall be kept to a minimum.
Guidance note:
The key factor for the final transfer phase is to ensure an adequate weather window is in place to successfully complete the SCR
assembly and its subsequent transfer to the host facility. At a deep water facility, the SCR installation can take up to five days. This
is longer than currently available weather forecasts.
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7.10.3 Rigid spool/jumper installation
7.10.3.1 This subsection [7.10.3] is applicable to transportation and installation of rigid spools/jumpers (see
[1.6.2]). See DNV-ST-F101 [10.6], /42/, for guidance.
7.10.3.2 Transport and seafastening shall be in accordance with Sec.11.
Guidance note:
Short spools/jumpers can be accommodated and seafastened on deck by lashings or weldments and clamps as appropriate. Larger
spools/jumpers can be cantilevered or carried on over-side seafastenings welded to the installation vessel or to a transport vessel.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.3.3 Lifting of all but the shortest spools/jumpers shall be in accordance with Sec.16 and in particular
[16.17].
7.10.3.4 Lift point design shall consider any restrictions on welding directly to the pipe.
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7.10.4 Flexible jumper and flying lead installation
7.10.4.1 The loadout, transportation and installation of flexible jumpers and flying leads shall be designed
and executed in such a manner as to prevent the exceedance of any limit state, see [7.3.3]. The same
principles for flexible product lay apply for these items, as described in [7.9].
7.10.4.2 Fully certificated rigging in accordance with [16.12] shall be used. Lift points and rigging shall
be checked for condition before use. The lift should be analysed and proven suitable for the design
environmental conditions and vessel. See Sec.16.
7.10.4.3 The structural capacity of baskets, pallet structures or similar used to transfer the product to the
seabed shall be documented as adequate.
Guidance note:
Valid load test certificates may be used to document adequate strength as an alternative to a full design and fabrication package.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.4.4 Lifting shall be in accordance with Sec.16 and in particular [16.17].
7.10.4.5 It shall be clear in the procedures and drawings that the line is being installed sealed closed or
free-flooding and arrangements made to suit.
7.10.5 Cable jointing and repair
7.10.5.1 Submarine power cable repair operations involve a number of complex manoeuvres that shall be
carefully planned with documented procedures. The proposed repair method(s) shall be determined and
included in the marine operation manual. The required personnel, tools, equipment and consumables shall be
available.
Guidance note 1:
See also DNV-RP-0360 [7.6], /58/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Wind farm cable repairs can be either by complete replacement of a cable or by cutting out the damaged/faulty section of cable
and inserting a new section. Replacement is most likely to be used for array cables whereas inserting a new segment will most
likely apply to export cables.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.5.2 The repair vessel shall be capable of supporting all phases of a cable repair operation. The general
vessel requirements of [7.3] apply.
7.10.5.3 Repair vessels shall provide a stable working platform, have fine control when manoeuvring and be
capable of holding station over the repair location for the time required to complete a cable joint.
7.10.5.4 An enclosed jointing space with sufficient room for performing a cable joint shall be provided.
7.10.5.5 Repair joints shall be qualified and type tested in accordance with a recognized design code.
7.10.5.6 Jointing personnel shall be qualified and jointers’ qualifications shall be submitted.
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Guidance note:
It is recommended that repair joint kits are opened in a clean environment onshore, that all parts are checked against the repair
joint contents list, and that a dummy assembly of the joint is done to ensure that the procedures are correct. The shelf life of
compounds and/or resins should be checked and shown to be valid.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.5.7 Dynamic analyses of the suspended cable(s) shall be performed in accordance with [7.3.2] for the
entire repair operation, from the time when the damaged cable is recovered until the repaired cable has been
redeployed to the seabed. This includes the time when the catenaries are suspended from the vessel during
the final splice.
7.10.5.8 If the cable is cut subsea and is not of a water-blocked design, then the potential for and the
consequences of water ingress shall be considered in the procedures.
Guidance note 1:
If cable tension prevents the cable from being lifted to the surface the cable will need to be cut on the seabed.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Submarine power cables are either wet, dry or semi-dry designs and each has different levels of tolerance to water ingress. The
cable manufacturer should be consulted if the repair strategy involves cutting the cable on the seabed.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.5.9 Any cable end returned to the seabed shall be secured to prevent movement due to currents.
Guidance note:
Typically this is achieved by attaching a chain and/or a clump weight to the cable end.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.5.10 The method for retrieving a cable end from the seabed shall be documented. If a surface buoy is
used to aid location and recovery of the ground tackle, the buoy should have sufficient buoyancy to remain
at the surface in all anticipated environmental conditions. If the cable end is to be lifted by use of the rope,
wire or chain to connect the buoy to the cable, the rope/wire/chain shall be designed according to the
requirements for lifting rigging in Sec.16.
7.10.5.11 The cable(s) shall be stoppered to secure points on the repair vessel, having sufficient capacity to
support maximum cable tension during the cable jointing process. The stopper type shall be approved by the
cable supplier, and the stopper attachment point shall be in line with the cable to prevent the stopper from
exerting non-conforming forces on the cable.
7.10.5.12 If an in-line joint is to be deployed over a lay chute or sheave, the mechanical strength of the
joint shall be confirmed adequate for the chute’s/sheave's radius and cable tension during deployment.
Extreme tensions can be generated in the cable where it enters the joint housing as the joint enters and
leaves the chute/sheave. Consideration should be given to using shaped supports or a spreader beam and
crane to deploy the joint over the repair chute/sheave.
7.10.5.13 In the case of an omega joint, the recovered cable shall be cut back sufficienty to ensure good
cable before cutting the spare cable to length and performing the second joint.
7.10.5.14 During deployment of an omega joint, the joint and cables shall be supported to prevent damage
during lowering, e.g. using a spreader beam. The joint lowering speed shall be coordinated with vessel
movements to ensure the final splice is laid down on the seabed without exceeding any limit state criteria
and without throwing loops or leaving sections of the cable standing clear of the seabed. The subsea lifting
requirements of [16.17] apply.
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Guidance note 1:
Deployment and landing of an omega joint is a hazardous operation during which a combination of cable slack and residual
torsion in the cable can cause loops, cat-paws and standing sections of cable. It is recommended that the lay-down points are
continuously monitored by ROVs in order to maintain control of lay-back and further mitigate the risk for the forming of loops,
overbending, etc.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
An omega joint typically adds an additional cable length of 2-3 times the water depth into a cable system.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.10.5.15 Deployment rigging shall be removed from the cable and joint after installation to avoid corrosion
issues and ensure the rigging does not become entangled with cable jetting equipment or similar.
7.10.5.16 The cable catenaries formed during an omega jointing operation shall have sufficient tension to
ensure the cable MBR is not jeopardised at the seabed touchdown point, but not higher than the maximum
allowable tension specified in the operational procedures.
7.10.5.17 If the cable is to be trenched and/or rock dumped, the whole of the repaired section shall be
protected. The protection installation shall be in accordance with [7.11].
Guidance note:
This can be achieved by trenching into the seabed or rock-dumping. The omega joint layout should be surveyed using an
inspection ROV and any loops, standing loops and/or cats-paws should be recorded and if necessary an alternative protection to
jetting should be developed.
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7.10.5.18 The repaired cable shall be tested and consideration should be given to retaining the cable repair
vessel on site until the tests are complete and the cable repair is shown to be acceptable.
7.11 Protection and post-lay intervention
7.11.1 General
7.11.1.1 Any requirements to protection and/or post-lay intervention shall be documented with
corresponding operational procedures as applicable.
Guidance note:
Generally only the installation related aspects of the protection system (including the design of installation aids) are of interest to
the MWS company.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
7.11.1.2 Limiting weather conditions for the installation operation of protection, as well as any survey
operations, shall be documented and clearly stated in the operational procedures.
7.11.1.3 Deployment of protection and deployment/recovery of any associated equipment (e.g. trenching
equipment) shall be managed as any over-side lift in the proximity of unprotected subsea assets, see
[16.17.2.14]. The lifting requirements of [16.17] apply.
7.11.1.4 The product shall be adequatly protected in accordance with the recognized design code. Varying
ground conditions can require a combination of protection methods to achieve acceptable levels of protection.
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Guidance note:
Various means of protection are employed to protect a product from excessive loads during installation and/or in service. The most
common means of protection are mechanical protection systems, trenching, gravel installation/rock dumping, and installation of
mattresses and grout bags.
Mechanical protection systems are generally devices attached to, or structures applied over/under, the product for mechanical
protection during installation and/or in service, typically designed to protect the product against abrasion or impact, e.g. from
dropped objects, or to limit product bending. A mechanical protection system may be attached to and/or installed together with
the product, or installed prior to and/or after the product is installed.
A bend restrictor assembly attached to a product end fitting/termination or component, is an example of a mechanical protection
system that is installed together with the product. Other examples are various types of abrasion/impact protection, e.g. sleeving
and split pipe assemblies. Such systems will typically affect the submerged weight, bend stiffness and drag properties of the
product, during installation and potentially also in service. Various types of structures and covers are examples of mechanical
protection systems that are typically installed prior to or after installation of the product.
Trenching is generally performed to lower a product into the seabed, typically to protect the product from dropped anchors or
trawling equipment, and/or to ensure on-bottom stability of the product, i.e. to prevent lateral movement as a result of wave and
current action. The most common methods of trenching are ploughing and jetting, and to some extent also cutting (in partuclarly
hard soils) and excavation (typically in the landfall area). The preferred method, and whether trenching is performed prior to,
during or after a product lay operation, is typically a function of soil conditions, water depth, environmental conditions and cost.
Required depth of burial may be governed by a number of site specific parameters such as regulatory requirements, marine
activities, and geotechnical, loads from ice, seabed and tidal conditions on site.
Gravel installation/rock dumping is typically a means of protecting a product in areas not suitable for trenching, e.g. in hard soils
or close to seabed structures etc. It is also a means of levelling out an uneven seabed to avoid or reduce free spans and ensuring
on-bottom stability. Gravel installation/rock dumping generally requires the use of purpose built vessels.
Mattresses placed over a product can serve as protection from dropped objects, a means of stabilising the product on the seabed,
preventing abrasion, reducing erosion, or ensuring a certain separation between products at crossings. In general, installation of
mattresses does not require a purpose built vessel, and is thus often a more cost effective solution than gravel installation/rock
dumping, particularly in cases where the scope is limited.
Grout bags are typically used to provide support and/or protection of a product at shorter free spans and exposed areas, and can,
in certain applications, be an alternative to mattresses or gravel installation/rock dumping (primarily where the scope is limited).
Installation of grout bags generally does not require a purpose built vessel.
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7.11.1.5 For requirements to trenching of rigid pipelines see also DNV-ST-F101 [10.8], /42/.
7.11.2 Mechanical protection systems
7.11.2.1 It shall be verified that the mechanical protection system(s) meet(s) the requirements of relevant
approved design reports and/or specifications.
7.11.2.2 Mechanical protection systems that are attached to and/or installed together with the product will
normally affect product response, and thus loads, on both the product and the protection system itself, as
well as any joints, terminations, or similar. These effects shall therefore be considered in the installation
analyses to demonstrate that all relevant system loads are acceptable for the proposed installation
procedure.
7.11.2.3 Installation of protective covers or structures shall be managed as any over-side lift in the
proximity of unprotected subsea assets. The lifting requirements of [16.17] apply.
7.11.3 Trenching
7.11.3.1 The suitability of the trenching equipment with respect to the applicable product(s) and soil
conditions shall be documented (also see [7.8.3.2]).
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Guidance note:
The term trenching equipment refers to ploughs, jetting machines, and cutters, used to generate a trench in which to lower a
product, either prior to, during, or after the product laying operation.
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7.11.3.2 The trenching equipment monitoring system shall be calibrated and include:
— devices to measure depth of pipe
— a monitoring system and control system preventing horizontal loads on the pipeline or devices to measure
and record all vertical and horizontal forces imposed on the pipeline by trenching equipment, and devices
to measure the proximity of the trenching equipment to the pipeline, horizontally and vertically relative to
the pipeline
— underwater monitoring systems enabling the trenching equipment operator to view the pipeline and
seabed profile forward and aft of the trenching equipment
— measuring and recording devices for trenching equipment tow force
— devices monitoring pitch, roll, depth, height and speed of the trenching equipment.
7.11.3.3 Jet sleds shall have a control and monitoring system for the position of the jetting arms and the
overhead frame, horizontally and vertically relative to the pipeline. The location of the sled shall not be
controlled by the force between sled and pipeline. Devices indicating tension in the tow line and showing the
depth of the trench shall be installed.
7.11.3.4 It shall be documented that the trencher, including any guides, rollers, depressors, ploughshares,
hydraulic jets, or cutters, will not damage the product or any components attached to the product (e.g.
anodes) under normal and contingency operations. Special care shall be taken during trenching operations
of piggy back and bundle pipelines, so that strapping arrangements are not disturbed/damaged during
trenching.
7.11.3.5 It shall be documented that no limit states of the product are exceeded during all phases of the
trenching operation. In particular, the transition into and out of the trench shall not induce excessive product
bending or free spans. The following phases of the trenching as a minimum should also be considered:
— during docking and un-docking of the trencher
— during start-up and stopping of the trenching operation
— in transition zones
— during any other normal and contingency operation.
7.11.3.6 Required depth of lowering (DOL) and depth of cover (DOC), as applicable, shall be clearly stated in
the operational procedures.
Guidance note:
To avoid confusion related to definition of burial depth, use of the following terms is recommended:
—
Depth of lowering (DOL) is the vertical distance from mean seabed level to bottom of product.
—
Depth of cover (DOC) is the vertical distance from mean back-fill level to top of product.
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7.11.3.7 It shall be verified that the DOL and/or DOC specified in the trenching procedures are in accordance
with relevant approved design reports and/or specifications.
7.11.3.8 Limiting weather conditions shall consider the deployment and recovery of trenching equipment.
Guidance note:
Lifting and handling of a trencher may pose a risk to the safety of personnel, the product, and the trenching equipment itself.
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7.11.3.9 Where it is planned to leave trenching equipment on the seabed during weather stand-by,
the limiting weather conditions for stand-by shall be clearly stated in the operational and/or emergency
abandonment procedures.
7.11.3.10 Loads applied to a product by a trencher, e.g. guides, depressors, or other, shall be monitored by
calibrated load cells with alarms and trips on the motive power to the trencher, if applicable. These alarms
shall be relayed by fail-safe radio telemetry to the support vessel control center. These alarms shall be
calibrated and tested in accordance with [7.4.13.1].
7.11.3.11 For towed ploughs, the required bollard pull shall be considered for the expected conditions and
the actual seabed. Consideration shall be given to the impact this can have on the ability of the DP vessel to
support plough operations for both planned and contingency operations in the operational limiting criteria.
Guidance note:
Towed ploughs can require high bollard pulls, therefore trials are recommended to ensure that the selected plough is suitable and
to verify that the support vessel can provide sufficient bollard pull or power supply for the plough to penetrate the seabed to the
required depth of lowering.
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7.11.3.12 The towline between a plough and the support vessel shall be designed in accordance with
[11.13].
7.11.3.13 Where moving cutters are involved, product position sensors and automatic trip sensors shall
be utilised to trigger alarms and cut the power to the cutters and forward thrust (if possible) in the event
that the cutters come too close to the product. These sensors, the alarm and trip should be calibrated in
accordance with [7.4.13.1].
7.11.3.14 The procedures shall consider that unexpected or sudden changes in the soil may:
— affect the performance of the trenching equipment
— cause damage to the trencher and/or the product.
Guidance note:
Buried debris and glacial rocks in clay are examples of the hazards which require constant diligence from the operators of the
trenching equipment who will need to adjust or stop operations as these are encountered.
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7.11.3.15 If an uncontrolled drop back occurs during ploughing of a flexible product, the product shall be
inspected and tested, if deemed necessary, to ensure product integrity before re-starting the ploughing
operation.
7.11.3.16 The method of monitoring trenching equipment seabed position and plotting of as-laid coordinates
shall be considered.
Guidance note:
For example, trenchers can carry HPR acoustic beacons for this.
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7.11.3.17 Achieved DOL and DOC, as applicable, shall be documented for as-built purposes and to enable
an assessment of the need for mitigating measures, such as an additional pass or an alternative protection
method.
7.11.3.18 A post trench survey, using an active or passive tracking system (as applicable), should be
performed to document achieved DOL and DOC, as applicable. Alternative methods of documenting achieved
DOL and/or DOC shall be described in the operational procedures.
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7.11.3.19 Seabed profiles showing product position relative to trench/seabed should be provided at regular
intervals along the line to document that relevant protection requirements are satisfied. Profile intervals
shall be defined in project procedures, depending on site and operational constraints. Calibration of relevant
instruments shall be documented. Where applicable, a video survey record should also be made to enable
detection of free spans or other anomalies.
7.11.4 Gravel installation/rock dumping
7.11.4.1 Materials used for gravel installation/rock dumping shall meet the specified requirements to specific
gravity, composition and grading in order to prevent product damage due to impact or over-loading in the
buried condition.
7.11.4.2 Required DOC, alternatively minimum size/profile of gravel/rock berm and relative position of the
product, shall be clearly stated in the operational procedures.
Guidance note:
For crossings both products are normally considered.
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7.11.4.3 It shall be verified that the acceptance criteria, e.g. DOC, size/profile, specified in the operational
procedures are in accordance with the recognized design code.
7.11.4.4 The limiting weather conditions for gravel installation/rock dumping operations, shall consider the
installation and recovery of of the fall pipe and shall be clearly stated in the operational procedures.
7.11.4.5 Gravel installation shall be performed in a continuous and controlled manner. Existing infrastructure
should not be disturbed or interfered with.
7.11.4.6 The position of the bottom of the fall pipe relative to the product, seabed and any nearby seabed
structures shall be controlled and monitored to ensure that the fall pipe system does not damage the product
or any other subsea asset.
7.11.4.7 Achieved DOC shall be documented for as-built purposes and to enable an assessment of the need
for dumping additional gravel/rock.
7.11.4.8 A post gravel installation/rock dump survey, using an active or passive tracking system (as
applicable), should be performed to document the achieved DOC. Alternative methods of documenting
achieved DOL and/or DOC shall be described in the operational procedures.
Guidance note:
For rigid pipelines, see also DNV-ST-F101 [10.8.5.7], /42/.
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7.11.4.9 Seabed profiles showing product position within the gravel/rock berm should be provided
at adequate intervals (normally a maximum 5 m) along the line to document that relevant protection
requirements are satisfied. Calibration of relevant instruments shall be documented. Where applicable, a
video survey record should also be made to enable detection of exposed product or other anomalies.
7.11.5 Installation of mattresses and grout bags
7.11.5.1 It shall be verified that mattresses and/or grout bags meet the requirements of the recognized
design code.
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7.11.5.2 Placing of grout bags and concrete mattresses shall be performed in a controlled manner with
suitable monitoring, such that the bags or mattresses are placed as required. Restrictions on vessel
movements during the operation shall be considered.
7.11.5.3 For grout bags, the following shall be clearly stated in the operational procedures:
— required amount of grout bags and
— any acceptance criteria related to positioning, e.g. relative to the product.
7.11.5.4 For mattresses, the following shall be clearly stated in the operational procedures:
— acceptable tolerances for positioning of mattresses relative to the product and/or other components or
structures
— the position of a mattress and lifting frame relative to the product, seabed and any nearby seabed
structures.
7.11.5.5 Any fibre ropes or slings used for lifting shall be checked to confirm fitness for purpose, noting that
the fibres are degraded by ultra violet radiation from sunshine.
7.12 Product testing and pre-commissioning
7.12.1
Product testing and precommissioning is not typically part of the MWS scope. However it does influence
other activities within the MWS scope, and possibly the warranty if a damaged product is installed
undetected. Therefore MWS involvement is limited to ensuring that testing is completed and results and/or
remedial actions are acceptable to relevant parties (such as owner, installation/transport contractor and/or
manufacturer).
Guidance note:
Product testing in this refers to the testing used to confirm the product has not been damaged during the marine operations and
that performance is satisfactory.
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7.12.2
The product testing and precommissioning programme shall be in accordance with the recognized design
code and include any remedial actions and hold points. The programme shall be reflected in the schedule and
operation manual.
7.12.3
The testing programme shall be fully documented and agreed by relevant parties for the extent of testing at
each stage of the operation, such as pre/post load-out, pre/post transport, during lay operations, and post
installation.
7.13 Documentation requirements
7.13.1 General
Information noted in this subsection is what may be required to achieve MWS approval for the given
operation with exact list to be agreed on a project specific basis. Other parties may have different
requirements.
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7.13.2 Lay vessel
For the lay vessel:
a)
b)
c)
d)
General arrangement of vessel, principal dimensions and operational draught.
Deck layout of all permanent and moveable equipment items.
Outline specification of vessel, accommodation and machinery.
Cranes – including load-radius covers, winches including abandonment and recovery (A&R) and records
of maintenance.
e) Stinger, lay tower or ramp geometry and rating.
f) Reel lay system or reel drive system.
g) Carousels capacity and drive systems.
h) Structures (PLET/ILT) handling systems.
i) Tensioner capacity and settings.
j) Full details of the mooring system, winches, anchors, control and monitoring system, a mooring
procedures manual, mooring lines certificates and records of maintenance.
k) Dynamic positioning system information.
l) For DP vessels, the last annual DP trials including recommendations and close-outs, the latest DP FMEA
detailing single point failures.
m) ROV class and capacities.
n) ROV handling system.
o) Rollers, load cell arrangement.
p) Video and other control and monitoring equipment.
q) Weather or motion limitations for laying.
r) Motion response characteristics, preferably as tabulated response amplitude operators (RAO’s) against
frequency and vessel heading.
s) MRUs and motion measurement system.
t) Current meters.
u) Where available, the latest common marine inspection document.
v) Certification as appropriate, including:
w) Certificate of class for hull and machinery.
x) Registry, load line and tonnage certificates.
y) Safety construction, safety equipment and safety radio certificate.
z) Certificates for cranes, davits, winches, rigging and other items relevant to the operation such as DMA/
initiation cable not covered by the normal classification requirements.
7.13.3 Mooring/dynamic positioning procedures
7.13.3.1 For DP vessels, documents in accordance with [17.3].
7.13.3.2 Mooring information required is in [17.2] and summarized below:
a)
b)
c)
d)
e)
f)
g)
General procedures for mooring and anchor handling.
General anchor patterns and catenary curves during straight lay and on radius.
Specific procedures for working anchors close to shore.
Specific anchor patterns during start-up and lay-down, crossing points and where working close to
existing structures and pipelines, risers, subsea assets and other prohibited zones.
Command structure, communications and working frequencies, anchor movement logging procedures
and proformas.
Contingency procedures for bad weather, mooring failure, power failure of positioning system, etc.
Details of anchor handling vessels.
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h)
i)
j)
Vessel position survey procedures.
Survey equipment calibration procedures.
Survey equipment specifications.
7.13.4 Route
For the route following information shall be provided :
a)
b)
Start-up and lay-down target areas.
Route between the target areas, clearly showing the theoretical route, straight and curved sections, radii
and tangent points, existing platform(s), pipelines, cables, and shoreline (if applicable).
c) Lay corridor.
d) Alignment with predominant current and wave headings.
e) Shipping lanes and other restricted areas due to military or political reasons.
f) Presence of any submarine exercise areas, fishing, mine fields, dredging and wrecks.
g) If applicable, levels and type of fishing activity.
h) Mooring line/riser crossings.
i) Minimum distance from existing subsea assets.
j) Minimum distance to existing fixed/floating platforms and platform approach drawings.
k) Any other potential hazards or obstructions, such as mines or munitions dumping areas.
l) Seabed features such as possible huge and multiple spans, coral growths, rock out crops, soft liquefiable
soils, sand dunes, pockmarks, iceberg scour marks and other seafloor obstructions such as boulder
fields.
m) RPL.
7.13.5 Survey
7.13.5.1 A hydrographic and sidescan survey of the route for the design corridor showing:
a)
b)
c)
Water depth along the route and general seabed topography.
Any obstacles within the proposed corridor.
Any areas where free-spanning is anticipated, or where scour or sand waves are anticipated.
7.13.5.2 A geotechnical survey of the route showing:
a)
b)
c)
General seabed composition and stability.
Identification of rock or coral outcrops, sand waves or mudslide areas.
Sufficient subsurface information to justify trenching and/or burial procedures.
7.13.5.3 As-built surveys of cables/pipelines to be crossed.
7.13.5.4 Procedures and specifications for the following:
a)
b)
c)
Pre-lay survey.
Post-lay surveys.
As-built survey.
7.13.6 Product design and construction as related to marine operations
7.13.6.1 For rigid pipe:
a)
b)
Internal and external diameter and wall thickness.
Weight in air and in water (empty and flooded).
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c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
m)
n)
o)
Allowable stress, strain and fatigue.
Design environmental conditions.
Design, operating and test pressures.
Structural design during installation and commissioning load cases, stresses and buckling.
Pipeline material specification.
Pipeline corrosion and weight coating.
field joint coatings specifications.
Anode type and method of fixing.
Piggyback lines.
On bottom pipeline stability analysis for temporary conditions.
Free spans analysis.
Free spans and rectification procedures.
Trenching/burial requirements.
7.13.6.2 For flexible pipe:
a)
Drawings and dimensions of the flexible product and all ancillary equipment and components to be
installed onto it.
b) Internal and external diameter and wall thickness.
c) Weight in air and in water (empty and flooded).
d) Allowable stress, strain, fatigue, axial compression, twist and MBR.
e) Bend stiffness.
f) Design environmental conditions.
g) Design, operating and test pressures.
h) Structural design during installation and commissioning load cases, stresses and buckling.
i) Flexible pipe data sheet.
j) Anode type and method of fixing.
k) Piggyback lines.
l) On bottom pipeline stability analysis for temporary conditions.
m) Free spans analysis.
n) Free spans and rectification procedures.
o) Trenching/burial requirements.
7.13.6.3 For cables:
a)
b)
c)
d)
e)
f)
g)
h)
Operating performance specification.
Weight in air and in water.
Allowable stress, strain, fatigue, axial compression, twist and MBR.
Bend stiffness.
Design environmental conditions.
Structural design during installation and commissioning load cases, stresses and buckling repair
specifications.
Trenching/burial requirements.
Jointers qualifications.
7.13.7 Loadout and spooling
7.13.7.1 For lifted loadouts:
a)
b)
c)
Loadout procedure.
Rigging design report.
Stationkeeping/mooring arrangement.
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7.13.7.2 For spooling/reeling:
a)
b)
c)
d)
e)
Spooling procedure.
Spooling analysis.
Rigging design report.
Stationkeeping/mooring arrangement.
Stationkeeping/mooring analysis.
7.13.8 Lay operations
7.13.8.1 For all lay operations (as applicable):
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
m)
n)
o)
p)
q)
r)
s)
t)
Laying procedure.
Laying schedule covering expected durations and contingency time from safe to safe condition.
Installation analysis.
Operational limit.
Provisions for weather forecasting.
Line installation stress and fatigue calculations.
Special procedures for laying and attaching piggyback line.
Procedures for attachment of anodes, etc.
Installation aids and their structural design.
Slack and catenary management.
Start-up procedures and analyses.
Calculations for start-up sheave or pile and rigging.
Touchdown monitoring procedures.
Lay-down procedures and analyses.
Emergency lay-down and recovery procedures and analyses.
Contingency protection methods in the event of temporary abandonment.
Contingency planning.
SIMOPS.
Risk management plan.
Initiation rigging and lifting/pulling equipment:
— arrangement
— calculations
— certification showing WLL and/or MBL
— calculations justifying length of initiation wire [7.9.9.9].
7.13.8.2 For all pipelay operations (as applicable):
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
Buckle detection procedures.
Dry and wet buckle contingencies.
Stringing, welding, NDT and field joint coating procedures.
Procedures and analyses for installation of hot tap tee, in-line tee, wyes, pipeline end tees and other
structures in the line.
Free span limitations.
Pre-commissioning procedure including pigging and hydrotest acceptance criteria (if applicable).
Design, operating and test pressures.
Details of pulling heads, pig receivers, end caps, etc.
Dewatering and drying procedures.
Pipe tracking procedure
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7.13.8.3 The following items shall be checked and documented for S-Lay and J-Lay of rigid pipe:
a)
b)
c)
d)
e)
f)
g)
Soil friction for route curvatures mid-line and in proximity to pipeline ends (see [7.9.10.2.2]).
Local buckling strength at seabed touchdown recognising bending and hydrostatic loads and ovalisation
from manufacturing tolerance and plastic bending strain from passage through the stinger (see
[7.3.2.3.1]).
Propagation buckling initiation pressure and sustained running pressure (see [7.3.2.3.1]).
Requirement and extent of buckle arrestor provision based upon initiation and running pressures and
hydrostatic pressures along the route (see [7.3.2.3.1]).
Static pipelay analyses for the load cases combining tension ranges, water depth ranges, trim, draught,
current, pipe weight and effects of concrete stiffness (where applicable) to determine critical cases for
over-bend stress/strain on the stinger, and sagbend stress near the touchdown (see [7.3.2.3.2]).
Dynamic pipelay analyses for critical stress cases that determine weather limits for stress/strain in the
pipe and other limit states such as pipe dynamics in the stinger due to wave motions and plastic strain
history limits (see [7.3.4]).
Analysis of bearing loads in the concrete weight coating (where applicable) from tensioner pads and
tracks and for inter-coating shear between the concrete coating and the underlying anti-corrosion
coating (see [7.4.8]).
7.13.8.4 The following items should be checked for reel and carousel lay of rigid pipe, in addition to those for
S-Lay and J-Lay in [7.13.8.3]:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
Linepipe steel qualification testing to be performed to confirm suitable toughness for reeling strains on
the intended installation vessel.
Coating qualification testing to demonstrate suitability for the strain cycles on the intended installation
vessel.
Minimum reelable wall thickness for linepipe or strength against local buckling at the maximum reeling
strain.
Total strain and requirements for ECA or additional testing.
Cumulative ovalisation and partial recovery of circular section through reeling/spooling, un-reeling and
straightening.
Local buckling strength at seabed touchdown recognising bending and hydrostatic loads and ovalisation
from manufacturing tolerance and residual ovalisation from reeling/spooling.
Propagation buckling initiation pressure and sustained running pressure (recognising that it is not
practical to include buckle arrestors in a reeled pipe).
Anode doubler plates shall be circular, matching the pipe material.
Quasi-static pipelay analyses for the load cases combining tension ranges, water depth ranges, trim,
draught, ramp angles, current and pipe weight to determine critical cases for peak and minimum axial
tension in the product and maximum curvatures.
Dynamic pipelay analyses for critical stress cases that determine weather limits for axial tension in the
pipe and other limit states such as pipe dynamics in the stinger or over boarding chute/roller due to
wave motions.
Analysis of crushing and shear loads in the product from tensioner pads and tracks and for inter-layer
shear within the product.
Analysis of crushing and shear loads in the product at any straps or clamps attaching to other lines or
installation aids.
7.13.8.5 For all cable lay operations (as applicable):
a)
b)
Testing procedures and schedule.
Cable handling requirements.
7.13.9 Pull-in through J-tubes or I-tubes
For J-tubes or I-tubes:
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a)
b)
Procedures.
Analysis:
— pull-in tension
— stress/MBR check or product.
c)
J-tubes/I-tubes details:
— dimensions
— strength
— bend radius
— friction.
d)
Messenger line details.
e)
Pull-in winch details including capacity of the pull-in winch, winch wire and other parts (sheaves, blocks,
foundations etc.).
f)
Pull-in winch load cell calibration.
g)
Pull-in winch operational procedure.
h)
Winch commissioning report.
i)
Drawings and details or rigging and pull-in assembly.
7.13.10 Pipeline and cable crossings
For pipeline and cable crossings:
a)
b)
c)
d)
e)
f)
Crossing procedure including inspection and monitoring (may be included in lay procedure).
Overall chart of crossing location, identifying coordinates of the crossing point (UTM), crossing angle and
sizes of crossed and crossing cables and pipelines.
Specification of minimum cover between and above crossed and crossing cables and pipelines.
Geometry of supporting structures, methods to be used for protection, seabed morphology and soil
characteristics at the crossing location.
Mooring procedures whilst anchors and mooring lines are in the vicinity of the crossed cable or pipeline.
Check of stress and span at crossing location.
7.13.11 Shore crossing
For shore crossing:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Pulling procedure and analysis.
Governing weather criteria.
Tide tables.
Details of launching supports on land, vessels (such as pull barges), any special equipment, and manning
arrangements.
Special procedures for messenger and pull wire laying and attaching installation aids such as buoyancy
tank etc.
Installation aids (pull head, riggings etc.) and their structural design.
Contingency protection methods in the event of temporary abandonment.
Holding back tension analysis for structure or mooring lines.
Dredging and pre-trenching procedure.
7.13.12 Riser or subsea tie-in
For riser or subsea tie-in installation:
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a)
Tie-in analysis report including calculations of, maximum allowable landing speed and for riser clamps
and rigging.
b) Stress analysis where pipe is to be lifted for connection.
c) Riser or subsea tie-in installation procedures.
d) Installation tolerances.
e) Information of target boxes for installation.
f) Details regarding tie-in equipment and tie-in capacities.
g) Allowable tie-in loads.
h) Capacity of interfacing hubs/structures.
i) Contingency procedures in case of tie-in load limits are reached.
j) Check tie-in equipment, including inspection, monitoring and testing.
k) Welding and NDT procedures for approval in the situation where welding will be performed under a
suspended load in order to see how this is addressed. This should normally be risk-assessed.
l) Bolting procedures.
m) Temporary hang-off procedures.
n) Riser, subsea spool and/or structure configuration, weight (in-air and in seawater) and CoG.
o) Need for installation aids (e.g. buoyancy, lifting slings, seabed anchors, low friction plates/ gratings.)
p) Need for seabed preparations, e.g. rock dumping or similar.
7.13.13 Protection and post-lay intervention
For protection and post-lay intervention (if applicable):
a)
b)
c)
d)
e)
f)
g)
Specification for burial.
Specification of plough, trenching machine, jet sled and support vessel as applicable.
Available bollard pull and tow force.
Deployment/recovery procedures and analyses of plough/sled/trencher.
Trenching and backfilling procedures and analyses.
Monitoring procedures.
Analysis to confirm product design allowables are not exceeded during trenching operations i.e. pick-up,
transition-in, normal trenching and transition-out.
h) Confirming that transition in and out of the trench will not induce over-long free spans at the ends of
trench.
i) Protection against contact with trencher specifically for piggyback lines or gross lateral deflection.
j) Protection against cutter coming in contact with or too close to product.
k) Contingency procedures.
l) Upheaval pipe buckling assessment.
m) Seabed rock installation vessel specification and operating procedures.
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SECTION 8 OFFSHORE WIND FARM INSTALLATION OPERATIONS
8.1 Introduction
8.1.1 General
8.1.1.1 This section gives the MWS requirements for installing offshore wind farm (OWF) infrastructure
(apart from cables which are covered in Sec.7). Operators should also consider national and local regulations,
which can be more stringent. Background information is in App.H.
8.1.1.2 In addition to the requirements of this standard, recognized design codes shall be used to establish
the limit state criteria for offshore wind structures. Any pre-service condition requirements also specified in
the recognized design codes shall be satisfied.
Guidance note:
The following documents may be considered:
—
DNV-ST-0054 Transport and installation of wind power plants (onshore and offshore), /151/
—
DNV-ST-0119 Floating Wind Turbine Structures, /152/
—
DNV-ST-0126 Support structures for wind turbines, /153/
—
DNV-ST-0145 Offshore substations, /154/
—
DNV-ST-0361 Machinery for wind turbines, /155/
—
DNV-ST-0376 Rotor blades for wind turbines, /156/.
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8.1.2 Scope
This standard provides requirements and guidance for installation of offshore wind farms, in particular:
— Foundations including monopiles, steel jackets, gravity bases, suction bases, floating bases including
spars, TLPs and semisubmersibles.
— Towers, turbines and blades to be installed on foundations.
— Offshore substations, offshore converter platforms, offshore transformer station, control and other
platforms, including those on jack-up platforms.
8.2 Planning
8.2.1 General
See Sec.2 for general planning requirements and Sec.3 for environmental conditions and criteria.
8.2.2 Tolerances and criteria
8.2.2.1 Tolerances and criteria should be agreed with the MWS company at an early stage of the project.
Guidance note 1:
The selection of many installation tolerances and criteria will be a trade-off between reducing the cost of manufacture and reducing
the costs of delays waiting for good weather in consequence. Manufacturers often prefer tighter installation tolerances which
require better weather criteria for installation. It is generally beneficial to select the transport/installation contractors before such
tolerances and criteria are fixed as they may significantly affect the installation methods, risks and costs.
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Guidance note 2:
The MWS company normally has input to the selection to ensure that the tolerances and criteria are not so severe that there is a
possibility that the equipment may never be able to be installed without taking unacceptable risks.
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8.2.2.2 Such tolerances may include:
a)
b)
c)
Position and orientation of monopiles, pile templates, jackets and other structures.
Pile or structure verticality.
Clearances between piles inside pile sleeves, including allowances for weld beads and grout keys.
8.2.2.3 Such criteria may include:
a)
b)
c)
d)
e)
f)
g)
Wind speeds (at specified heights and gust durations) for critical lifts.
Any restrictions on current speeds or wave heights (and how they will be measured) for specific
operations. These could include stabbing piles or jackets into templates.
Degree of acceptable damage to grout keys during piling.
Any restrictions on helicopter or vessel movements within the field in bad visibility or other adverse
conditions.
Any restrictions on transfer of people and equipment onto fixed or floating installations by various
means.
Requirements for disposal of any dredged materials, drilling cuttings or soil plugs removed from piles (to
comply with national or international laws or conventions, and to avoid problems with other contractors).
Piling operations – sound effects on sea life.
8.2.3 Vulnerable items or areas
8.2.3.1 Due to the many parties and vessels working in close proximity, it is necessary that each party
understands what items are particularly vulnerable to actions by others. These items need to be identified at
an early stage so that they can be considered in the relevant risk assessments. The list of vulnerable items
needs to be updated and promulgated as required during the life of the wind farm.
8.2.3.2 Typical vulnerable items or areas may include:
a)
b)
c)
d)
e)
f)
J-tube entry holes being covered with soil or debris.
Changes in seabed level (from scour, dredging, jack-up footprints, drill cuttings, etc.) varying the natural
frequency of foundations.
Scour can also affect jack-up foundations, cables, anchors etc. Scour model tests may be required in
areas with high current speeds and soft or sandy seabeds.
Damage to grout seals and back-up seals.
External fittings (including anodes, J-tubes, etc.) being damaged by dropped objects, vessel collision or
mooring lines.
Operations of divers (vulnerable to propellers and propeller wash, noise and blast, bubble curtains,
cables and dropped or lowered objects).
8.2.4 Planned moorings
Geotechnical and bathymetric surveys should determine at an early design stage if the seabed will provide
good anchor holding and may determine the type of anchors that will be needed. If anchor holding is poor
(leading to a high probability of dragging anchors damaging cables) then prelaid or piled anchors may be
desirable. Allowable anchor locations should be agreed at the same time as the cable routes.
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8.3 Offshore wind farm installation vessels
8.3.1 Jack-ups – general
Jack-up legs can be a major threat to cables. The as-laid cable routes should be updated as required and
properly distributed through the project in order to prevent cable damages. A suitable safe distance shall
be maintained between the as-laid cable route and the intended positions of the jack-up legs. This is of
particular importance in OWF developments where cable laying/installation is progressing near turbine
installation activities in a similar time frame.
Guidance note:
ISO 19905-1, /102/, and Renewable UK Guidelines for the Selection and Operation of Jack-ups in the Marine Renewable Energy
Industry, /115/, have information on required clearances.
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8.3.2 Jack-ups in weather unrestricted operations
8.3.2.1 Jack-ups that are designed and classed for elevated operations in conditions in excess of those at
the installation site (either all year or for particular months) shall comply with the requirements of DNV-STN002, /39/.
8.3.2.2 The jack-up can operate at a lower air gap than required for survival in a design storm as long as it
is able to jack-up to a safe air gap for a design storm before bad weather.
Guidance note:
If a breakdown prevents jacking up, then the crew may need to be evacuated.
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8.3.3 Jack-ups in weather restricted operations
8.3.3.1 Jack-ups that cannot comply with [8.3.2] for a specific location and season shall comply with the
requirements for weather restricted operations in [2.6.5].
Guidance note:
Useful practical guidance on weather restricted jack-up operations is given in section 5.3 of Renewable UK Guidelines for Jackups, /115/, but note that [2.6.7] allows a greater operational window. This is summarised as:
a)
Agree procedure documents which include limiting criteria, allowing for uncertainty due to monitoring and the forecasting
of the environmental conditions (see [2.6.9]), for relevant decision points and identify suitable alternative jack-up locations
between the site and safe ports.
b)
The jack-up is only to leave a safe location to go to the installation site on receipt of a favourable weather forecast with high
c)
The jack-up is to leave the installation site unless there is a confident good weather forecast to cover the remaining time on
confidence to cover the time (including a contingency for delays) from departure to return to a safe location.
site and to return to a safe port or to elevate to a safe air gap at a suitable stand-by location, including a contingency for
delays.
d)
If the jack-up cannot reach a safe port or location before meeting bad weather (above the laden jacking limits of the jack-up,
typically about 1 m to 1.5 m significant wave height), then it should jack-up to survival air gap at a suitable shallow water
location and evacuate the crew if necessary.
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8.3.3.2 The procedures and criteria described in [8.3.3.1] shall be the subject of a risk assessment in
accordance with [2.4].
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8.3.3.3 Jack-ups can also operate on DP or when moored afloat to save time jacking up and down and preloading. These operations require favourable weather and shall follow the weather restricted operations
requirements in [2.6.7]. The use of the crane in floating mode shall be specified in the vessel’s operation
manual with the associated allowable environmental limits and approved by the recognized classification
society.
8.3.3.4 Jack-ups can operate in semi-jacked-up condition (vessel stabilised in water by a low leg pre-loading
and a reduced draught) under good weather conditions. This condition can make it feasible to operate the
jack-up at critical locations where the risk of punch through is high. It will require approval by the vessel’s
classification society as it is not typically a normal operating condition.
8.3.4 Crane vessels (seagoing)
8.3.4.1 Any crane vessel or sheerlegs shall be classed for operating in the relevant area. The design and
operating criteria shall be defined according to Sec.2.
8.3.4.2 Carrying a suspended load on a crane hook in transit offshore is not generally considered good
practice, unless it is for very short distances in calm weather. In bad weather the load can be very difficult
to control, stability is reduced and the crane can be overloaded. Approval of such operations will require
agreement from the vessel’s classification society and a risk assessment in accordance with [2.4].
8.3.5 Inshore crane vessels and barges
Inshore crane vessels and barges shall only be used if allowed by their class notation and:
a)
b)
c)
The MWS company has agreed procedure documents which include limiting environmental criteria for
relevant decision points and identifies safe ports or locations. These criteria shall take into consideration
the alpha factors described in [2.6.9]
The vessel is only to leave a safe port or location to go to the installation site on receipt of a confident
good weather forecast to cover the period from departure to safe return, including a contingency for
delays.
The vessel to leave the installation site unless there is a confident good weather forecast to cover the
remaining time on site and to reach a safe port or location, including a contingency for delays.
8.3.6 Grounded offshore wind farm installation vessels and barges
Some vessels working in shallow water may need to be grounded at low water or over one or more tidal
cycles. This can only be approved provided that:
a)
b)
c)
d)
The vessel’s classification society allows such operations.
The seabed is such that the vessel will not be damaged and it will not hold the vessel down when
attempting to refloat.
There is a method (e.g. moorings or 'spuds') for holding the vessel on location when grounding and
floating off in the design conditions agreed with the MWS company at the design stage without damaging
any cables or other structures or equipment.
A confident good weather forecast is obtained before grounding to cover the period (including a suitable
allowance for delays) until float-off without exceeding the operational criteria.
8.3.7 Other offshore wind farm installation vessels
8.3.7.1 The following vessels usually do not require the approval of the MWS company unless their
operations represent a risk for other structures or operations.
a)
Crew transfer or accommodation vessels with proprietary crew access arrangements.
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b)
c)
Escort and stand-by vessels can be needed in some areas to warn off other vessels, especially during
sensitive operations or transports.
Bubble curtain deployment and energising vessels which can be needed if regulations on piling noise
pollution apply (see [13.10.2]).
8.3.7.2 In some cases, it may be unclear whether the approval of the MWS company is required or not for
smaller vessels approaching existing structures. Planned operations should be discussed between the OWF
owner, the underwriter and the MWS company in order to identify the major risks for the existing structure
and decide case-by-case the scope of the MWS company.
8.4 Planning and execution
8.4.1 Procedures and manuals
Technical documentation shall be completed for all operations. See [2.3] for details. In general, this should
include:
a)
b)
c)
d)
e)
f)
g)
The anticipated timing and duration of each operation, including contingencies.
The limiting wave states, wind speeds and currents, and where applicable any visibility/day-light,
temperature and precipitation limits, as well as the site-specific equipment or methodology prescribed for
measuring each limit-state.
The transport route including shelter points.
The arrangements for control, manoeuvring and mooring of barges and/or other craft alongside
installation vessels.
Effects on and from any other simultaneous operations (SIMOPs – see IMCA M 203, /83/).
Contingency and emergency plans.
Requirements from the relevant MWS company standards for each individual phase.
8.4.2 Weight control
8.4.2.1 The requirements in [5.6.2] apply.
8.4.2.2 The manufacturer shall supply a weight statement with tolerance and CoG envelope for all weightsensitive items.
8.4.3 Weather restricted operations and weather forecasts
8.4.3.1 For requirements see [2.6.7] for requirements for weather restricted operations and [2.7] for
weather forecasts.
8.4.3.2 For areas with high tidal currents there can be additional restrictions on operations due to the need
to wait for slack (or slacker) tides for current-sensitive operations such as:
— Moving jack-ups on or off location
— Stabbing piles or installing jackets, substructures or equipment on the seabed
— Bringing cargo vessels alongside installation vessels.
— Diving operations.
8.4.3.3 When high currents are combined with shallow water then additional current forces will be caused by
'blockage' effects. These shallower conditions also lead to increased seabed turbulence due to wave action,
and additional contingency measures can be necessary to make allowances for accelerated scouring around
jack-legs and spudcans. However suitable moorings, stabbing guides and other aids can help to reduce the
sensitivity to currents and decrease downtime waiting for slack tide.
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8.4.3.4 Weather forecasts shall follow the requirements in [2.7]. Forecasts for wind speed shall specify the
height (to be agreed in advance) and wind speeds measured on site should be corrected to that height for
direct comparison. The swell height, direction, and period should also be included, as well as the probability
of precipitation, fog and lightning within the next 24 hours. The time of sunrise and sunset, and the phase of
the moon can be advantageous though these will normally be found in nautical almanacs.
8.4.3.5 For subsea lifts in areas where it is known that high currents exist in the water column, in-field
monitoring of currents (speed and direction) should be considered to enhance the regular forecasts. The
monitoring of subsea currents with acoustic Doppler or similar systems should be considered when the
operational limits of ROVs, and drag on piles during stabbing can lead to operational delays.
8.4.4 Site and route survey requirements
8.4.4.1 As well as ensuring that all positional, bathymetric, soil and current surveys are performed using the
same datum and coordinate systems, various requirements to ensure sufficient accuracy like the frequency
of survey equipment calibration (for salinity, temperature etc.) shall be agreed. There shall be an agreed
procedure for ensuring that all survey results are disseminated to all relevant parties as required.
8.4.4.2 The 'as built' locations of structures, cables and subsea equipment shall be recorded accurately on
charts using a common survey datum used by all parties. These charts shall be kept updated, including all
jack-up footprints as soon as they are made and issued to all vessels operating in the field. 'No anchoring'
zones shall be well marked.
8.4.4.3 In advance of the final detailed design being carried out for the foundations, the seabed material,
geophysical, and geotechnical surveys of the sub-bottom profile should have been carried out, as well as
magnetometer surveys for ferrous objects, including UXO. The Cone Penetrometer Test results and other
appropriate survey details for each foundation location should be documented, to jack-up vessel operators.
This will allow them to carry out site-specific assessments in accordance with ISO 19905-1, /102/, and to
assess the possibility of scouring around jack-legs and spudcans.
8.4.4.4 Unexploded ordnance (UXO) disposal, although important, is not generally subject to a Marine
Warranty and is normally excluded. However it is recommended that it will be managed in accordance with
the requirements of ‘Risk Management Framework’ provided in CIRIA C754, Assessment and management of
unexploded ordnance (UXO) risk in the marine environment, /13/ or similar.
8.4.4.5 Additional requirements for the cable route surveys are given in Sec.7.
8.4.5 Scour protection
8.4.5.1 If scour is a possible problem, procedures or contingency procedures shall be prepared and antiscour materials stockpiled and deployment equipment prepared for mobilisation. See [8.4.3.3] and [8.4.4.3]
for information that will help in prediction of scour.
Guidance note 1:
Dynamics of scour pits and scour protection, /119/, gives the results of research into scour on early UK offshore wind farms.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Cables are generally be trenched or otherwise protected in scour-prone areas. However additional precautions can be required
close to J-tubes or I-tubes at monopiles or platforms, especially immediately after laying.
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Guidance note 3:
Scour around jack-up legs can make them more vulnerable to punch-through and around cables can make them more vulnerable
to damage.
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8.4.5.2 Care shall be taken when laying scour protection to ensure that bad weather and/or high currents
during the installation phase do not cause damage to the lower layers.
8.4.6 Wet storage of jackets or offshore wind farm foundations
8.4.6.1 Any unpiled jackets or foundations should be able to comply with the requirements in [13.10] for the
return period applicable to the operation reference period given in [3.4]. This can require any chosen location
to be sheltered from high waves and currents.
8.4.6.2 A constant exclusion zone for marine traffic shall be enforced.
8.5 Load-outs of offshore wind farm components
Load-outs shall be in accordance with Sec.10. However the following special cases apply, as applicable:
a)
b)
c)
d)
Special consideration should be given to purpose-built lifting appliances for blades. The lifting tool
certificate shall specify the maximum load and any limits regarding the overall dimensions of the lifted
item and any environmental limitations (e.g. maximum wind speed).
In the event of structural modifications to an item of lifting equipment, it shall be re-approved by a
recognized classification society before further use.
Bolts used for removable lifting lugs shall generally be used one time only. In special cases, re-use can
be accepted as described in [E.2] but only if the pretension targeted during installation does not exceed
60% of the bolt yield strength and the loads during lifting have not exceeded the maximum design
values. For re-use of bolts, a detailed inspection plan with regular NDT including rejection criteria and
exchange intervals should be documented. As a minimum, bolts should be visually inspected after each
lift and with MPI (Magnetic Particle Inspection) after every 3 lifts unless fatigue calculations accepted by
the MWS company show that less frequent inspections are acceptable.
Re-useable lifting lugs shall be tested in accordance with [16.9.7].
8.6 Transport of offshore wind farm components
8.6.1 General
8.6.1.1 Sea voyages are covered in Sec.11 and road transport in Sec.9. The rest of [8.6] describes items
specific to OWF components.
8.6.1.2 Seafastening of blades and other fragile components require special care to avoid damage from
welding or locating guides. Where friction is required to resist some or all of the seafastening forces, the
coefficients of friction shall be shown to be adequate in both the wet and dry states. See [11.9.2].
8.6.1.3 Where bolted connections are used see [5.3.5].
8.6.1.4 Minimum clearance between cargo items to be lifted is given in [16.13.2] and [16.13.3].
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8.6.2 Transport of complete rotor
Rotors with diameters of well over 100 m may be transported horizontally (rotor axis vertical) on vessels or
barges of only about 30 m to 40 m beam. The voyage and installation planning shall account for the large
overhangs in particular avoiding wave slam on the blades.
Guidance note 1:
The blades will generally be very vulnerable to wave slam, especially when the vessel rolls and/or pitches into a wave.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Normally the voyage and installation planning considers some or all of the following:
a)
The rotor being designed to safely withstand the accelerations (from [11.3]).
b)
Reducing to negligible the probability of wave slam on the blades by securing them well above the still water level.
c)
Selecting vessels that can be ballasted to reduce the motions in likely wind and wave combinations.
d)
Doing motion response calculations to optimise the loading and ballasting arrangements so as to minimise the probability of
e)
Weather routeing the transport to avoid any weather that could cause wave slam on the blades. (This cannot always be
f)
Developing procedures to avoid blade collision damage when coming alongside loading quays, entering ports of shelter (as
wave slam on the blades in likely wind and wave combinations.
practicable for some seasons and longer routes between suitable shelter points).
part of the weather routeing) and coming alongside the offshore lifting vessel. These procedures include advance liaison with
any suitable shelter points (to agree the conditions under which the transport can enter, e.g. problems when meeting other
vessels in the approach channel, clearances at harbour entrance and mooring at a quay). Escort vessels may also be required
to reduce the probability of collision with other shipping, especially at night.
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8.6.3 Transport of tall vertical cargoes
8.6.3.1 Seafastening of the transition piece flanges on barges or ships is often critical for many projects.
Where bolted connections are used see [5.3.5].
8.6.3.2 Seafastenings shall be designed to allow safe removal offshore without endangering the cargo or
personnel. See also [11.9.6].
8.6.3.3 Clearance (air draught) under any bridges or power cables shall be considered. The safe distance
from live power lines shall be considered with input from the power line operator.
Guidance note:
The power line catenary will change if power is shut off.
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8.6.3.4 High towers, when transported vertically, can be vulnerable to vortex induced vibrations. Analysis
shall be carried out to evaluate the risk for the structure and the seafastening frame, see [5.6.7.4]. If
required, protection devices shall be installed to reduce the risk of vibrations.
8.6.4 Other offshore wind farm wet towages
8.6.4.1 Larger concrete gravity structures will generally be built in a dry-dock or building basin with
construction often completed afloat. The MWS requirements are given in Sec.12
8.6.4.2 Smaller gravity structures may be built on barges and floated off or lifted off by crane or sheerlegs.
They can also be lowered to the seabed by purpose-built installation units. Where these are not covered by
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existing MWS company standards, suitable criteria can be developed by the MWS company at an early design
stage.
8.6.4.3 It will often be impracticable to provide one-compartment damage stability for floating piles,
transition pieces and suction anchors by introducing temporary bulkheads. In this case, a risk assessment, in
accordance with [2.4], shall be carried out to determine the major causes of flooding and to reduce the risk
to acceptable levels, as described in [11.10.6.5].
8.7 Installation of offshore wind farm components
8.7.1 Monopiles and transition pieces installation
The following items shall be addressed and agreed with the MWS company:
1)
2)
3)
4)
5)
6)
7)
8)
Position and orientation tolerances (see [8.2.2]).
Release of seafastenings which will normally require a specific procedure, especially for tall objects
transported vertically.
Sea bed soil condition and scour protection requirements (see [6.5.7] and [8.4.5])
Levelling arrangements for the transition pieces.
Grippers, handling and upending equipment.
On-bottom stability of the unpiled Monopile in the pile gripper.
Stability of the Transition Piece on the Monopile before grouting (see [13.10] for the criteria).
If drilling is required for installing piles then:
— Disposal of cuttings (see [8.2.2]).
— Contingency plans and equipment (e.g. fishing tools) for a broken drill string.
9)
Approval of grouting operations (see [H.5.3]).
8.7.2 Piling templates
8.7.2.1 Piling templates are often used to help locate piles before driving and to ensure that piles are driven
vertically or at the right inclination. They are normally placed on the seabed but may be attached to the side
of a jack-up, with the facility to be lowered or raised and may use the jack-up legs as a positioning guide.
8.7.2.2 If transported attached to a jack-up then the template and its attachment shall be able to withstand
the design accelerations according [11.3] as well as the hydrodynamic forces acting on the structure. Its
effect on trim and stability shall also be checked.
8.7.2.3 Special transit procedures can need to be developed to reduce the risk of collisions or grounding if
the attached template increases the combined draught or beam, especially if not visible above water.
8.7.2.4 The template shall be capable of being levelled if there is a sloping or uneven seabed. Mudmats can
also be needed for a soft seabed.
8.7.2.5 When templates are liable to settle in clay or silt, provision shall be made for jetting or other means
to overcome adhesion during subsequent extraction.
8.7.3 Suction bucket foundations
8.7.3.1 The requirement for any seabed preparation before installation shall be determined at any early
stage.
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8.7.3.2 Equipment and procedures shall be documented to ensure that:
a)
b)
c)
d)
e)
the foundations can be safely lowered through the splash zone (buoyancy should be considered) to the
seabed and located within tolerances
there is no 'piping' (soil erosion due to seepage) through the soil between outside and inside, or between
individual compartments, if any, during installation
that any out of verticality can be corrected to within the required tolerances (possibly using crane
assistance)
there is sufficient redundancy to allow installation to continue after flooding of any compartment or
breakdown of any item of equipment. If there is insufficient redundancy a risk assessment in accordance
with [2.4] should be completed.
the operation should be made reversible so as to be able to extract the suction bucket foundation and
relocate if there is a risk of refusal (no further penetration at maximum pump capacity). The risk of
refusal should be determined from a penetration analysis using the latest soil data.
8.7.4 J-tubes and I-tubes
Installing cables through J-tubes and I-tubes is covered in Sec.7.
8.7.5 Turbine installation
Requirements in this standard shall apply unless novel installation techniques are proposed.
8.7.6 Towers
Installation lifting requirements are covered in Sec.16. In addition the following items shall be addressed, if
applicable, and agreed with the MWS company:
— Access for de-rigging
— Partial bolting
— Lifting points certification for multiple use (load-out, installation, maintenance, decommissioning)
— Verification that there will be no ovalisation of structure tubular members due to local seafastening forces
in higher sea states
— Transport frames
— Requirements and criteria for upending from the horizontal to vertical mode.
8.7.7 Nacelles
Installation lifting requirements are covered in Sec.16. In addition the following items shall be addressed, if
applicable, and agreed with the MWS company:
— Lift points
— Tugger lines arrangement
— Access for de-rigging
— Partial bolting.
8.7.8 Blades
Installation lifting requirements are covered in Sec.16. In addition the following items shall be addressed, if
applicable, and agreed with the MWS company:
— Infra-red release systems which shall be shown to be reliable in releasing and, more importantly, not
liable to early release from any cause
— Limiting criteria.
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— Boom tip motions, See [16.17.3.1.4]
— Partial bolting.
8.7.9 Complete rotor assembly installation
The following aspects need special consideration:
— Upending and lifting devices
— Tugger lines arrangement
— Partial bolting
— Horizontal and vertical movement during positioning
— High windage area effect on dynamic loads.
8.8 Lifting operations and lifting tools
Lifting operations are covered in Sec.16 and lifting tools in [16.6.2]. However, due to the high number of
repetitive lifting operations carried out in the offshore wind industry, special attention should be paid to the
regular inspections of lifting gear. Replacement of slings and grommets as well as the provision of sufficient
spares along the project will prevent project delays and offshore downtime. An inspection plan including
the detailed scope of inspections and rejection criteria should be documented by the lifting operator and
submitted to the MWS company at the beginning of the project. Refer to [16.12] for more information.
8.9 Information required for marine warranty survey approval
See subsections at the end of each relevant section, e.g. lifting, voyages.
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SECTION 9 ROAD TRANSPORT
9.1 Introduction
9.1.1 General
This section gives the requirements for objects subject to road transport on public roads, which are generally
subject to national or local requirements/legislation.
9.1.2 Scope
This section gives the basic default design criteria for transport on roads, together with the information
typically required for MWS approval. However additional local or technical requirements can apply.
9.2 Requirements
9.2.1 Statutory requirements
Most countries have specific legislation containing criteria for transport of large items by road. These shall be
obtained and complied with.
9.2.2 Loads and accelerations
9.2.2.1 Table 9-1 will cover most countries with published requirements for tie-down requirements. These
shall apply in the absence of more stringent criteria, depending on jurisdiction or other requirements.
9.2.2.2 Possible additional limitations on wind (or road speed) shall be checked for structures where strength
or stability can be an issue.
Table 9-1 Typical road tie-down acceleration requirements
Direction
Requirements
Transverse acceleration
0.5 g
Forward acceleration
0.8 g
Backward acceleration
0.5 g
Vertical acceleration
1.2 g (1.0 g in some areas)
9.2.3 Securing
9.2.3.1 All securing equipment should be in accordance with the principles and requirements of seafastening
design strength in [11.9.5].
9.2.3.2 Friction can be permitted as part of the securing system, subject to justification and where permitted
by local legislation.
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9.2.4 Stability
Stability in accordance with [10.5.3.15] shall be demonstrated.
9.3 Information required
9.3.1 Object information
For the object:
a)
b)
c)
d)
e)
Weight, CoG and envelopes considered
Description and dimensions
Definition of allowed lashing points on cargo, or specification of those locations which are forbidden
Support point requirements and cargo general strength when transported. For multiple supports,
allowance shall be made for possible loss of support due to trailer deflections
Padeyes, where used as lashing points, or other lashing points on the cargo to be verified against
transport design forces.
9.3.2 Trailer or self-propelled modular transports
9.3.2.1 Requirements are given in [10.5.3].
9.3.2.2 For the trailer or self-propelled modular transports (SPMTs):
a)
b)
c)
d)
e)
f)
Trailer or SPMTs specifications including lashing anchor point capacity and spine load capacity.
Bending moment and shear calculations of applied load on trailer or SPMTs spine.
Tyre ground pressure calculations and axle utilizations.
Hydraulic grouping details and trailer or SPMTs (loaded) stability calculations.
Demonstration that there is enough power/traction/braking capacity in SPMT or trucks to conduct the
transport along the planned route accounting for any inclines or turns.
Demonstration that stroke length is adequate to prevent grounding (cargo/trailer/SPMTs) or tyres losing
contact with roadway.
9.3.3 Securing
For the securing arrangement:
a)
b)
c)
d)
e)
Details including WLL (or similar) and MBL of all items in the securing system including tensioners.
General arrangement drawing of the securing plan including cargo CoG location while positioned on
trailer/SPMT and clearly defined required lashing angles.
Design acceleration definition and justification.
Securing Calculations documented and found adequate.
Demonstration of lashing/stopper adequacy against uplift and horizontal forces, including friction
assumptions and description of friction material and description of blocking if used.
9.3.4 Route
For the route:
1)
Transport procedure in place which includes contingency plans for prime mover failure, schedule for
arrival at way points and destination, as well as if police escort is required.
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2)
Route mapped - an overview of the entire route with the following:
— Start location and destination
— Critical turns planned to show no collisions with roadside obstructions.
— Adequate overhead clearance when passing under bridges or overpasses.
— Overhead power and utility lines along with relevant traffic signals and street signs, including a plan
for de-powering as required.
— Any relevant limitations on bridge loadings
— Any relevant limitations on timing.
— Significant inclines and declines.
3)
Permit obtained if required.
4)
Max speed defined if not stipulated in a permit. Max speed to allow for trailer /SPMT levelling as needed.
5)
Requirements for strength of ramps where used, allowable ground pressure should take in to
consideration any limits on buried culverts, utilities etc.
6)
Allowable ground pressures for the route defined. Special attention regarding ground pressure capacity
should be made to areas where the route is changing ground type (e.g. asphalt to cement). Tyre and
ground pressures should comply with the allowables for the entire route.
7)
If the transit passes an airfield and the cargo is of sufficient height, evidence that co-ordination with the
airfield, including any required aviation warning lights, has been included in the transport procedure.
9.3.5 Risk assessment
A risk assessment in accordance with [2.4] of the transport.
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SECTION 10 LOAD-OUT
10.1 Introduction
10.1.1 Scope
This section presents the requirements for load-out operations involving transfer of heavy objects from land
and onto a vessel (often a barge) either by skidding or by use of trailers. General requirements and guidance
is given in sections [10.1] to [10.8]. Section [10.8] gives additional requirements and guidance for the
following special cases:
— grounded load-outs
— load-in, i.e. a reversed load-out
— vessel to vessel load transfers
— transverse load-outs
— site moves.
10.1.2 Other types of load-out
10.1.2.1 For load-out operations carried out by crane lifting, see Sec.16
10.1.2.2 For other load transfer operations, see Sec.15.
10.2 General
10.2.1 Load-out class
10.2.1.1 Requirements to load-out equipment are defined according to load-out class. The load-out
operation shall, based on tide conditions and weather restrictions, be classified according to Table 10-1.
Table 10-1 Load-out (operation) class
Tide range
1)
Tide restricted
2)
Weather restricted
3)
Load-out class
Significant
Yes
No/Yes
1
Significant
No
Yes
2
Significant
No
No
3
Zero
No
Yes
4
Zero
No
No
5
1)
If ballasting is required in order to compensate for tide variation, then the tide range shall be defined as
significant, see also [10.2.1.2] and [10.2.1.3].
2)
If the ballast system cannot compensate for a complete tide cycle, then the load-out shall be defined as tide
restricted.
3)
If weather restrictions apply, then the load-out shall be categorized as weather restricted, see [2.6]. If there
are no weather restrictions to the object movement/ballasting phase the load-out class may be selected
accordingly.
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10.2.1.2 The possibility for water level differences due to environmental effects shall be duly considered. If
such effects could be significant during the load-out, then the tide range in Table 10-1 should normally be
regarded as significant even if the astronomical tide variation is defined as zero.
10.2.1.3 For grounded load-outs, see [10.8.1], the tide range in Table 10-1 shall be defined as significant if
ballasting is required in order to maintain ground reactions within acceptable limits.
10.2.2 Planning
10.2.2.1 General requirements for planning of marine operations are given in Sec.2.
10.2.2.2 Start and end points for a load-out shall be safe conditions and clearly defined, see [2.5].
Guidance note:
A load-out from one safe to another safe condition could include many sub-operations, such as 'lift-off from construction supports',
'site move', 'move onto barge', 'temporary seafastening phase', 'turning of barge' and 'final mooring of barge'. Hence, it should be
thoroughly evaluated if it may be possible and beneficial to split the load-out into two (or more) operations with safe condition(s)
in-between, see [2.5].
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10.2.2.3 Tide variation is normally a critical parameter for load-outs. Extreme tide levels and rates of change
should be considered. All environmental effects that can influence tide levels, in addition to the astronomical
tide variation, shall also be evaluated and duly considered.
10.2.2.4 The following should be given due attention when planning load-out operations:
1)
2)
3)
4)
5)
6)
Yard lay-out, including position of object.
Transport vessel dimensions and strength.
Object position and support height on transport vessel.
Load-out route survey regarding clearances and obstructions.
Water depths.
Local environmental effects, e.g.:
— the possibility of waves/swell
— currents during and following the operation, including blockage effects if applicable
— the possibility for squalls and/or thunderstorms; design wind speeds should account for such effects
when relevant.
7)
Quay strength and condition.
8)
Load-out site soil strength and condition.
9)
Skidway levelness tolerances.
10) The effect of fender friction (vessel 'hang' on the fenders or quay structure) which could affect other
items (e.g. mooring lines, reaction forces, link beam design, braking system).
10.2.2.5 A load-out operation could involve several construction-, transport- and load transfer (main)
contractors/responsible parties. Interface planning should be given due attention.
10.2.3 Risk management
Operational risk should be evaluated and handled in a systematic way, see [2.4].
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Guidance note:
The risk assessment should at least demonstrate that all necessary tasks can be safely performed under all environmental
conditions planned and designed for.
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10.3 Loads
10.3.1 General
10.3.1.1 Loads and load effects are generally defined in [5.5] and [5.6]. It shall be thoroughly evaluated if
any other loads and load effects not described in Sec.5 need to be considered.
10.3.1.2 The design principles and methods described in Sec.5 shall be adhered to.
10.3.1.3 All relevant limit states as defined in Sec.5 shall be included in the design calculations/analysis.
10.3.2 Weight and centre of gravity
10.3.2.1 Weight (W) and centre of gravity (CoG) of the object shall be determined as described in [5.6.2].
10.3.2.2 The appropriate weights and CoGs to be used may be evaluated separately for strength and ballast
purposes, see [4.3.9.2].
10.3.2.3 Any possible CoG position shall be considered for support layouts or systems sensitive to CoG
shifts, see [5.6.2].
10.3.2.4 If there are significant uncertainties regarding weight and CoG position, sensitivity analysis should
be carried out, see [5.6.14].
10.3.3 Weight of load-out equipment
10.3.3.1 The weight of the load-out equipment (Weq) should be accurately assessed.
Guidance note:
Weq is the total weight of equipment and support structures which moves with the transported object. Such equipment may be
support beams, grillages, skidding shoes, trailers, push/pull jacks, hydraulic power packs, etc.
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10.3.3.2 Any uncertainties in weight and CoG of load-out equipment shall be considered by applying
conservative estimates in the load-out calculations, see however [4.3.9.2].
10.3.4 Environmental loads
10.3.4.1 All load effects caused by tide variations shall be considered.
10.3.4.2 Load effects caused by wind and current shall be considered.
10.3.4.3 Load-out operations should normally not be carried out in significant waves and swell conditions.
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Guidance note:
Applicable loads due to waves and swell for transport vessel mooring before and after the load-out operation to be considered.
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10.3.5 Skidding loads
10.3.5.1 The loads required to break loose and continue moving the object can be expressed as:
F1 = μ1(W+Weq) + P1
F2 = μ2(W+Weq) + P2
where:
F1
F2
μ1
μ2
W
Weq
P1
P2
= required break-out load
= load required to continue moving the object
= upper bound design friction coefficient or rolling resistance for break-out, see [10.3.5.4]
= upper bound design friction coefficient/rolling resistance for the move, see [10.3.5.4]
= object weight, see [10.3.2]
= equipment weight, see [10.3.3]
= any other load occurring during break-out, see [10.3.5.2]
= any other load occurring during skidding/trailing, see [10.3.5.2].
10.3.5.2 The following load effects should be considered:
— Inertial loads
— Environmental loads
— Loads caused by the slope of the skidding or rolling surface.
10.3.5.3 If two or more propulsion systems are used then the effect of maximum possible differential push/
pull loads shall be considered.
10.3.5.4 The upper bound design friction coefficients/rolling resistance values used should not be taken less
than specified in Table 10-2 unless adequate in-service documentation indicates that other values may be
used, see also [5.6.9].
Guidance note:
The indicated friction coefficients for moving include re-starting after short stops during the load-out operation. Break-out friction
is the maximum friction expected after an extended (construction) period with the object supported at the friction surfaces, see
also [10.3.5.5].
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Table 10-2 Upper bound design friction coefficients/rolling resistance
Sliding surfaces
Break-out
Moving
Steel/steel
0.30
0.20
Steel/teflon
0.25
0.10
Stainless steel/teflon
0.20
0.07
Teflon/unwaxed wood
0.40
0.10
Teflon/waxed wood
0.25
0.08
Steel/waxed wood
0.28
0.15
Rolling surfaces
Steel wheelswSteel
0.02
0.02
Rubber tyres/wteel
0.02
0.02
Rubber tyres/asphalt
0.03
0.03
Rubber tyres/compacted gravel
0.05
0.05
1)
It is assumed that sliding surfaces are
properly lubricated.
2)
Long term effects such as adhesion,
settlements, etc. are included in values for
break-out. See also [10.3.5.5].
3)
The values are valid only for contact stresses
lower or equal to the allowable contact
stresses for the considered medium. Allowable
contact stresses should be obtained from the
manufacturer or from an applicable code or
standard.
4)
Wood should normally be surface treated by
wax or by other adequate means in order to
avoid that the lubrication is absorbed by the
wood.
10.3.5.5 Where an object is supported for an extended period on a skidway system, the effect of the
degradation of the lubricant between the support and the skidway system should be investigated. This is
particularly important where unwaxed wood is used as part of the interface as the lubricant can disperse into
the wood giving higher break-out requirements than anticipated. The effects of skidway deformation shall
also be considered.
10.3.6 Skew load
10.3.6.1 Skew load is the extra loading at object support points due to inaccuracies in the level of the
skidways, rolling surfaces, supports, etc. Such loads shall be considered.
Guidance note:
Skew loads could normally be disregarded for load-out operations where the object has a 3 point support system. This could be
obtained by including a reliable load equalising system.
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10.3.6.2 For cases without 3 point support systems skew load effects shall be determined by considering the
stiffness of the object, the supporting structure, the tolerances of skidways, rolling surfaces and supports,
deflections/movement of transport vessel and link beams, transport vessel inaccuracies and the operational
procedure.
Guidance note 1:
In lieu of a more refined analysis, the skew load may be determined considering the object supported on 3 points only. It may be
required to assume various possible 3 point support situations.
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Guidance note 2:
For SPMT load-outs using 4 support groups, the effect of skew loading across diagonals should be assessed to account for the
possibility that the groupings may not be coplanar due to incorrect pressure in the SPMT groups, the stiffness of the object and/or
uneven conditions beneath the SPMTs.
Appropriate limitation in pressures should be defined and the object should be checked to ensure that these limitations do not
cause overstress. During the operation it should be controlled that the measured pressure variations are within 75% of the set
limitations. E.g. if the limiting load (i.e. pressure) variation across a diagonal is 20% of the combined nominal value for that
diagonal, the measured variation should not exceed 15%.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
For skidded load-outs it is recommended to verify the object and supports for the following minimum deflections:
—
Subsidence of any single object 'corner' support with respect to the other 'corner' supports by 25 mm.
—
Subsidence of any single object support with respect to the other supports by 15 mm.
Dimensional survey measurement before (and if applicable during) the operation should substantiate that the actual relative
deflection will be within 75% of the deflections assumed in structural verifications.
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10.3.7 Other loads
Any other significant loads, not covered above should be considered in the design of the object and in the
planning of the operation. Such loads alternatively use can include:
— Hydrostatic loads on transport vessel(s).
— Impact loads
— Local support loads on grounded vessel hulls
— Mooring loads
— Guiding loads.
— Lateral loads caused by any known inclination of the ground or vessel, increased by 2 degrees to account
for uncertainties in the route profile for transport over land or ballasting operations when on the barge,
shall be used in the design of any structure or equipment used for the operation. Further details on
stability for a SPMT loadout, are given in sections [10.5.3.16] and [10.5.3.17].
10.4 Design calculations
10.4.1 General
10.4.1.1 Structures and structural elements shall be checked against the requirements in Sec.5 for the load
cases in [10.3]. For ASD/WSD, LS1 shall be used. For the LRFD approach the structure shall be verified for
the standard ULS-a and ULS-b conditions.
10.4.1.2 Mooring system design is covered in [10.5.8].
10.4.2 Load cases
10.4.2.1 Relevant load cases shall be selected in order to identify design conditions for the object, skidding
equipment or trailers, support structures and transport vessel.
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Guidance note:
A load-out operation consists of a sequence of different load cases. In principle, the entire load-out sequence should be considered
step-by-step and the most critical load case for each specific element should be identified, e.g. 25%, 50% and 75% of travel,
steps of 5 axles, half jacket node spacing, etc. as appropriate. However, the force distribution during a load-out may normally be
represented by static load cases distributing the object weight and any environmental and equipment loads to relevant elements in
the analyses.
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10.4.2.2 For skidded load-outs, analyses of the skidded object should consider the elasticity, alignment and
as-built dimensions of the shore and vessel skidways. See also [10.3.6.2], guidance note 3.
10.4.2.3 For trailer load-outs, the reactions imposed by the trailer configuration on the transported object
shall be taken into account.
Guidance note:
Support reactions for the transported object will be governed by the trailer arrangement. It should be remembered that trailer axle
loads within each hydraulic group will be uniform and that the trailers spine stiffness may influence the support reactions. See also
[10.3.6.2], guidance note 2.
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10.4.2.4 The design load cases for link beams, link beam attachments and the quay should consider mooring
forces, skidding forces and vessel movements when relevant, including any situations where the object or
vessel can be jammed.
10.4.3 Quays
10.4.3.1 Allowable horizontal and vertical load capacities of load-out quays should be documented according
to a recognized code or standard.
10.4.3.2 Calculations showing that the actual loads during load-out are not more than the allowable loads
should be documented.
Guidance note:
If information about the quay is limited and it is therefore difficult to document its capacity by calculations, then an alternative
approach where quay capacity is documented by historical records of previous load-outs over the quay may be considered.
Detailed information about the previous load-out(s) will be needed for an adequate comparison.
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10.4.4 Soil
10.4.4.1 Strength and settlement calculations/evaluations for the ground in the load-out area should be
documented.
Guidance note:
The risk of differential ground settlements which may influence the loads during load-out should be considered and minimised by
means such as:
—
pre-loading of ground in load-out tracks
—
load spreading e.g. by concrete slabs or steel plates.
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10.4.4.2 Soil bearing capacity should normally be tested before construction or load-out of the object.
Alternatively, relevant site investigation should be documented.
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10.4.4.3 Geotechnical calculations and testing should be carried out according to a recognized standard, e.g.
EN 1997 Eurocode 7, /67/.
10.4.4.4 For trailer transport, the soil strength requirements apply for the whole planned path/track plus at
least 2 meters at each side.
10.4.4.5 If there is any doubt as to the soil capacity, then a loaded SPMT test drive should be done before
the load-out.
10.4.4.6 For load-outs involving grounded vessel, the seabed should be evaluated with respect to
topography, bearing capacity, settlement, etc.
10.5 Systems and equipment
10.5.1 General
Systems and equipment to be used during load-out should comply with the requirements given in Sec.4.
10.5.2 Propulsion systems
10.5.2.1 Propulsion systems shall be able to break loose and push/pull the object to the final position on the
transport vessel.
Guidance note:
Propulsion systems can for skidded load-outs be for instance wire and winch, hydraulic jacks or strand jacks. Trailer load-outs can
be by self-propelled trailers (SPMT) or trailers. Trailers can be propelled by a wire and winch system or by tractors/trucks.
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10.5.2.2 The propulsion system capacity for break-out shall be not less than the required break-out load
(F1), see [10.3.5.1]. For objects that cannot be considered to be in a safe condition if the break-out system
fails the capacity and redundancy requirements in Table 10-3 apply also to the break-out system.
Guidance note:
Adequate break-out capacity may be obtained by combining e.g. jacks with the continuous propulsion system.
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10.5.2.3 The propulsion system used to move the object shall satisfy requirements specified in Table 10-3.
10.5.2.4 Propulsion systems should act in a synchronised manner in the transfer direction. A minimum
required load-out velocity shall be identified considering:
— Maximum allowable load-out duration.
— Dynamic friction coefficient.
— Length of the load-out track.
— Conservatively estimated duration of repair work (if such work is accepted as back up), or documented
installation time for back up equipment.
10.5.2.5 For any loadout, the need for providing an additional braking system and (if applicable) its
sizing shall be conservatively assessed based on the possibility of loss of control of the object taking in to
consideration the following:
— Track slope, including maximum possible (accidental) inclinations of the load-out vessel.
— Low friction, e.g. by using (steel) wheels/rollers or surfaces with low friction.
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— Elasticity in pull system, i.e. high elasticity (e.g. long winch wires) combined with temporary jamming
could result in a 'catapult effect'.
— Sudden release of friction between the vessel and the quay face causing a sudden change in the vessel
heel or trim.
— Tolerance in the methods used to determine the heel/trim of the vessel.
— Inaccuracies in the amount of ballast in the vessel tanks.
— Speed of module movement going too fast or slow with regard to ballast pumping.
— Tolerances on link beam movement (see [10.7.8.2]).
— Large changes in the heel of the vessel during a side loadout due to a low metacentric height causing a
large vessel heel for a small transverse movement of the module (see [10.6.4.1]).
10.5.2.6 The horizontal load and the associated braking capacity requirements shall be calculated for the
following conditions:
— When transiting on land: any known inclination of the route increased by 2˚ to account for uncertainties in
the route profile.
— When transiting on a vessel or bridge link: the total of the expected values and the dynamic effects
on heel/trim caused by the relevant conditions given in [10.5.2.5] increased by 2˚ to account for
uncertainties in the ballasting, tolerance on the methods to record heel/trim values and environmental
conditions.
If this horizontal load is greater than the resistance from a friction value of 0.5* the minimum friction load
based on a lower bound coefficient of friction, then an independent braking system shall be provided to
resist the additional load in excess of this friction load. The braking system should be such that it allows the
movement of the object but can be quickly deployed in the event that it is required.
Guidance note:
1)
Very low friction surfaces, and loadouts using steel wheels/rollers should be avoided where possible due to possible loss of
2)
When using winches, strand jacks or jacks, if the land route profile is accurately surveyed for level, then the surveyed level can
control of the object.
be used to determine the horizontal load and it is not necessary to include the 2˚ due to uncertainties in the route profile.
3)
To mitigate against uncontrolled forward movement when using winches, strand jacks or jacks, a nominal inclination of the
4)
Systems which cannot tolerate any change in heel or trim should be avoided.
vessel could be considered so that the object is moving up the slope.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.2.7 To ensure that the effects of friction between the vessel and the quayside are minimised, the
tension in the pulling system should be slackened off prior to the object commencing load transfer on to the
vessel. Whilst the object is moving from the quayside on to the vessel, the pulling tension shall be slackened
off every time the object moves:
th
— 1/30 of the vessel width for a side loadout
th
— 1/15 of the vessel length for a longitudinal loadout.
10.5.2.8 The final movement of the object on to the vessel shall be specifically considered.
Guidance note:
Normally, prior to the final movement of the object on to the vessel, the skidding should be stopped, the pulling system slackened
off and the final load transfer completed by ballasting of the barge to lift off the object from the quayside. The amount of the
object/skid shoe remaining on the quay is to be determined ensuring that the increased loading on the vessel side or stern, and
the object/skid shoe does not exceed their allowable capacities.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.2.9 Should the tension in a pulling system increase by more than 25% above the value used during
the previous stages of the skidding operation, the operation shall be stopped and the reason for the increase
determined. Where this occurs on a load-out class 1 operation (see Table 10-1), preparations should
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commence for recovery of the object to the quayside should the reason for the increase not be known or any
associated corrective action not be possible.
10.5.2.10 Back-up propulsion system capacity should be able to compensate for the following conditions:
a)
b)
Breakdown of one arbitrary self-contained propulsion unit
Unexpected increase in the skidding loads above the expected nominal value
Guidance note:
Back-up capacity for accidental conditions of type a) may be achieved by:
—
Spare capacity in the main propulsion units.
—
Separate back-up propulsion units with sufficient capacity.
—
Spare parts for the main propulsion units and an acceptable and proven repair/replacement time.
The back-up capacity for conditions of type b) may be:
—
Spare capacity in the main propulsion units.
—
Back-up propulsion units.
Detailed requirements to be complied with are in Table 10-3.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.2.11 Any required modifications during the operation, e.g. removal of pull bars of the push/pull
system lay-out should be proven feasible. Normally, lay-out modifications should be avoided with the object
supported both at the quay and transport vessel.
10.5.2.12 A pull-back system and a procedure for pulling the object back on shore shall be available for
load-out class 1.
10.5.2.13 A pull-back system and procedure shall be available for load-out classes 2 and 4 unless otherwise
justified by risk assessment, see [10.2.3] and [2.4].
Guidance note:
One acceptable option may be to substantiate that a retrieval system could be made operative to retrieve the object within the
operation reference period (TR).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
Table 10-3 Propulsion system requirements
Loadout class
Intact system
capacity
System redundancy requirement after
breakdown of any one component
1
160%
130% capacity or repair possible within 30 minutes
Required
2
140%
120% capacity or repair possible within 2 hours
See [10.5.2.13]
3
120%
Repair possible
Not required
4
120%
100% capacity or repair possible within 6 hours
See [10.5.2.13]
5
100%
Repair possible
Not required
Pull-back system
1)
Nominal (100%) system capacity is the load (F2) required to continue moving the object in the intact case, see
[10.3.5.1].
2)
Breakdown of any one mechanical component, hydraulic system, control system or prime mover/power source
shall be considered. After such a breakdown it shall either be possible to proceed with the load-out without
repairing the component, or it shall be possible to repair the component within the timeframe indicated.
3)
Where a pull-back system is achieved by de-rigging and re-rigging the pull on system, the time required to
achieve this shall be estimated, clearly defined and duly considered.
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10.5.3 Trailers
10.5.3.1 Trailers (multi wheel bogies) should be used in accordance with the manufacturer's specifications.
10.5.3.2 The hydraulic suspension layout (linking) should be thoroughly considered. Normally a layout
giving a three point support condition for the object, e.g. a statically determinate system, is recommended.
However, it should be noted that a 3-point support system is generally less stable than a 4-point support
system.
10.5.3.3 The trailer configuration should have adequate manoeuvring capabilities for the intended load-out
(including site move) route.
Guidance note:
Where an object cannot be loaded out directly onto a barge or vessel without turning:
—
Turning radii should be maximised where possible.
—
For small turning radii, lateral supports/restraints should be installed between the trailer and the object/load-out support frame
—
It should be demonstrated by the load-out contractor that the steering coordinates used in the trailers or SPMTs set up are
—
The cornering speed should be kept to a minimum to avoid the potential for loads due to lateral accelerations affecting the
(LSF)/cribbage.
correct, with the details of the set up coordinates contained in the procedures.
stability of the object or SPMTs. Alternatively, a limiting turn speed should be specified and the stability assessed accounting
for the associated loads.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.4 The trailer axle load calculations shall consider:
— Weight of object
— Weight of object supports on the trailers
— Weight of the trailers themselves
— Extreme positions of CoG
— Hydraulic suspension lay-out
— Maximum overturning effect caused by relevant 'external' horizontal loads, see [10.5.3.7]
— Possible operating errors, see e.g. [10.5.3.8]
— Contingency situations, see [10.5.3.12].
Guidance note:
It some cases it may be found beneficial to plan for possible rearrangement of the trailer after lift-off should the load distribution
between the trailer groups not be as expected.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.5 The following shall be documented for the trailer axle loads (static and dynamic) calculated
according to [10.5.3.4]:
a)
b)
Maximum axle loading shall be shown to be within the trailer manufacturer's recommended limits.
Trailer moment and shear force within the manufacturer’s specified limits or the global (spine) strength
to be documented by calculations.
10.5.3.6 The support lay-out on each trailer shall ensure stability in both directions of the trailer.
Guidance note:
A trailer with a fully linked hydraulic suspension needs to be regarded more as a distributed load than as a support. The supports
on such trailers should be checked for the vertical loading from the trailers combined with maximum 'external' and 'internal'
horizontal loads acting on the trailers, see [10.5.3.7] and [10.5.3.8].
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10.5.3.7 The trailers should be properly supported to withstand horizontal loads. Such loads are caused by:
a)
b)
External effects, i.e. reaction loads from wind, inertia (e.g. acceleration during start/stop and during
turning operations) and ground slope (including vessel heel/trim).
Internal effects such as differential traction and steering inaccuracies.
10.5.3.8 Trailer inclinations due to improper co-ordination in operation of the hydraulic suspension system
shall be considered.
10.5.3.9 The traction system, either the trailers are self-propelled or pushed/pulled by trucks/winches,
should fulfil the requirements in [10.5.2]. Ground surface conditions should be duly considered.
10.5.3.10 It should be documented that the trailer hydraulic suspension will work well within the stroke
limits. Support heights, ground slopes/conditions and defined vessel levels/motions (see [10.6.5]) should be
considered.
Guidance note:
Normally the planned operational stroke should be limited to 70% of the total theoretically available stroke.
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10.5.3.11 Contingency/repair procedures should be documented for at least:
— Hydraulic system failure
— Hose rupture/leakage
— Tyre puncture
— Steering problems
— Traction failure, see [10.5.2]
— Failure of power pack.
10.5.3.12 The trailer load calculations shall consider that any one axle does not take load due to e.g. tyre
puncture.
Guidance note:
If repair is possible 10% overload could normally be accepted. For class 1 load-out the loading should be within the stated
maximum trailer loading.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.13 Link span bridge capacity shall be demonstrated by calculation, see [10.4.2.4].
10.5.3.14 Special caution/consideration should be given to steel plates used as link span bridge between the
quay and the vessel. The following should be considered when ensuring their suitability:
— Vessel ballasting should be carried out to minimise the difference in level between the vessel deck and the
quay.
— The distance between the vessel and the quay should be minimised to avoid excessive deformation of the
steel plates caused by the reactions from the trailers or SPMTs.
— Effectively maintaining of the vessel position on the quay e.g. using mooring winches
— Securing the plates to the vessel or quay to prevent their slippage during load-out.
10.5.3.15 Adequate global stability of the system, comprising the trailer(s) and the object, shall be ensured.
Checking the global stability of the system shall be by one of the following methods:
— Load cases A and B as specified in [10.5.3.16] and [10.5.3.17] shall be considered. For each of these load
cases a minimum tipping angle shall be calculated. Unless otherwise justified, the minimum tipping angles
for load case A shall be ≥ 7° and for load case B shall be ≥ 5°.
— The method shown in [10.5.3.22].
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Guidance note 1:
The CoG to be used in [10.5.3.16] and [10.5.3.17] is the combined CoG for trailers and object (the CoG to be used in [10.5.3.18]
is the CoG of the object).
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For transports with untraditional trailer layouts and hydraulic suspension divisions/groups, the stability should be checked with
both (i.e. [10.5.3.22] and load cases A and B) methods.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.16 For load case A the following shall be considered:
a)
b)
c)
The most extreme possible horizontal/vertical location of the centre of gravity.
When transiting on land: any known inclination of the route increased by 2° to account for uncertainties
in the route profile.
When transiting on a vessel or bridge link: any predicted inclination of the vessel and link under the
design wind and ballast conditions, increased by 2° to account for uncertainties in the ballasting and
wind speed.
See guidance notes to [10.5.3.17].
10.5.3.17 For load case B the following shall be considered:
a)
b)
c)
d)
e)
The most extreme possible horizontal/vertical location of the centre of gravity.
The characteristic horizontal load due wind and inertia, see [10.5.3.7] a).
When transiting on land: any known inclination of the route increased by 2° to account for uncertainties
in the route profile.
When transiting on a vessel or bridge link: the defined maximum acceptable level inaccuracies/
motions of the vessel and bridge link increased by 2° to account for uncertainties in the ballasting and
environmental conditions
Possible change of heel or trim due to hang-up between the vessel and the quay, or dynamic response
after release of hang-up.
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f)
Any free surface liquids within the object.
Guidance note 1:
The 2° uncertainty allowance in [10.5.3.16] and [10.5.3.17] needs to be added only once to the known inclinations to check
the effect for the nearest 'critical' tipping line.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Where the hydraulic support system allows for the trailer bed to be levelled horizontally to account (partly) for a known
inclination, the effect of the known inclination can be reduced to account for this, provided this capability is demonstrated
and contained in the procedures. This may be considered also for case A.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
Example case:
Total weight of object and trailer assembly:
300t
Extreme CoG, vertical location:
10 m above ground level
Extreme CoG, horizontal location:
2.5 m from tipping line
Design wind load on the assembly:
11t at 12 m above ground level
Known maximum route inclination:
3°
Risk for hang-up between vessel and quay:
No
Free surface effects:
None
Calculation for example case:
Design slope:
3°+2° = 5°
Load case A
Virtual correction of CoG:
(10x300 x tan5°)/300 = 0.87 m
Horizontal distance from virtual CoG to tipping line:
2.5-0.87= 1.63 m
Minimum tipping angle:
arctan(1.63/10) = 9.3° > 7°, i.e. OK
Load case B
Virtual correction of CoG:
(10x300 x tan5°))/300 = 0.87 m for slope
12x11/300 = 0.44 m for wind load
0.87 + 0.44 = 1.31 m in total
Horizontal distance from virtual CoG to tipping line:
2.5 - 1.31 = 1.19
Minimum tipping angle:
arctan(1.19/10) = 6.8° > 5°, i.e. OK
The above example only demonstrates how the effect of lateral load due to wind is considered. For lateral loads due to inertia
(see [10.5.3.7]), these lateral loads are to be treated similarly to how the effect of wind is included in the stability and are
additive to any wind load.
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10.5.3.18 For virtual CoG location of the object determined as in load case B in [10.5.3.17] i.e. accounting
for the known tilt plus 2° together with the wind and inertia loading, it shall be demonstrated that the
object is stable on the trailer bed and does not have uplift at its support locations. Where any object support
reaction on the trailer gives uplift or a value of less than 25% of the static support reaction, a means of
securing the object to resist the uplift shall be provided and calculations documented to show that the uplift
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restraint system is suitable. The restraint shall be designed to provide hold-down equal to the calculated
hold-down force plus 25% of the static reaction. When there is no uplift, the remaining contact reaction can
be taken into account. The strength of the restraints shall be assessed to LS1 (ASD/WSD method) or ULS
(LRFD method).
10.5.3.19 Special attention shall be given to load-out operations where the CoG of the object is very close to
the centre of a group or grouping of trailers or SPMTs and the CoG has a low elevation.
10.5.3.20 For movements of the object where slopes are expected and these cannot be compensated by
stroking of the SPMTs, the stability of the group or grouping of trailers or SPMTs shall be checked accounting
for the slope and the horizontal load from the object on to the trailers or SPMTs.
10.5.3.21 Load-outs with high slender objects on narrow support bases, or offset from the vessel centreline,
shall be subject to special attention in terms of the effects of uncertainties in ballasting and de-ballasting.
10.5.3.22 An acceptable alternative to the tipping angle approach given in [10.5.3.15] to [10.5.3.18]
(inclusive) is to base the global stability verification on the minimum calculated load on any trailer suspension
grouping (also termed hydraulic zone). For this method, the minimum calculated load should not be less than
60% of the calculated nominal load on the same system. For this check, the loads given in [10.5.3.17] shall
be considered.
Guidance note 1:
The nominal load on a trailer suspension grouping is the loading considering neither horizontal loads nor any CoG tolerance, from
the transported unit on the system.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For further guidance on global stability, see ESTA Guideline Modular Hydraulically Suspended Trailers – Transport stability, /144/.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.4 Skidding equipment
10.5.4.1 Skid shoes, steel wheel bogies and steel rollers are in this subsection defined as skidding
equipment. Any part of such equipment used for the horizontal movement of the object is defined as part of
the propulsion system, see [10.5.2].
10.5.4.2 Adequate strength and stability of skidding equipment should be documented. All possible
combinations of vertical load, horizontal load and support reaction distribution should be verified. Sufficient
articulation or flexibility of skid shoes shall be provided to compensate for level and slope changes when
crossing from shore to vessel.
Guidance note:
Skidding equipment may be connected in order to reduce internal horizontal loads transferred through the object. The effect of
possible rotation of skidding equipment should be considered.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.4.3 Skidway levelness tolerances, surface condition and side guides shall be adequate for the applied
skidding equipment.
Guidance note:
—
Sliding interfaces should be suitably lubricated unless this is not required by the supplier of any specialised equipment used for
—
Side (lateral) guides are normally provided along the full length of skidways.
the load-out.
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10.5.4.4 Where a vessel, because of tidal limitations, has to be turned within the load-out tidal window the
design of the link beams shall be such that when the loaded unit is in its final position they are not trapped,
i.e. they are free for removal.
10.5.4.5 For hydraulic suspension systems, see [10.5.3.2] and [10.5.3.10].
10.5.4.6 The nominal computed load on winching systems shall not exceed the certified working load limit
(WLL), after taking into account the requirements of [10.5.2] and [10.3.5] and after allowance for splices,
bending, sheave losses, wear and corrosion. If no certified WLL is available, the nominal computed load shall
not exceed one third of the breaking load of any part of the system.
10.5.4.7 The winching system should be capable of moving the object from fully on the shore to fully on the
vessel without re-rigging
Guidance note:
If re-rigging cannot be avoided, then this should be included in the operational procedures, and adequate resources should be
available.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.5 Ballasting systems
The requirements for the ballasting systems are given in [4.3].
Guidance note 1:
The load-out classes defined in Table 10-1 corresponds to the operation classes referred to in [4.3.2].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Normally, vessel pumps should not be considered for the primary ballast system but may be taken into account in the back-up
provision.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.6 Power supply
The power requirements in [4.3.2] shall apply for both the ballast pumps and the propulsion units during the
load-out.
Guidance note:
Need for additional power supply to e.g. lighting and welding should be considered.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.7 Testing
10.5.7.1 See general requirements in [2.10] with respect to testing/commissioning, test procedures and test
reporting.
10.5.7.2 Commissioning of the ballast pumps should at least include:
— Capacity control
— Final functional testing not more than two hours before start of the operation.
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Guidance note:
Pump capacity control should be carried out with equal or greater head and similar hose lengths as planned used during the
operation. If tank ullages are used as capacity measuring means, the pumped volumes should be sufficient to obtain minimum
300 mm difference in ullages before and after pumping.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.7.3 For load-out operations of class 1 a complete test run of the ballast system following the procedure
for the load-out should be carried out.
10.5.7.4 The propulsion units including the spare units should be tested in both push and pull mode before
the load-out operation in order to verify the estimated friction forces and functioning/capacities of the
equipment.
10.5.7.5 If the considered back-up necessitate replacement of equipment (e.g. pumps and propulsion units)
then this should be included in the test program.
10.5.8 Mooring and fendering
10.5.8.1 General design requirements to mooring systems are given in Sec.17 and in particular [17.2.9] for
quayside moorings. Additional requirements applicable for load-outs are given below.
10.5.8.2 For additional load cases to be considered, see [10.4.2].
10.5.8.3 Moorings for the duration of the actual load-out from quay to vessel should be designed for the
limiting (design) weather conditions, see [2.6], in combination with the maximum loads from the pushing or
pulling of the object.
10.5.8.4 Mooring before and after load-out should normally be considered a weather unrestricted operation.
Weather unrestricted moorings should be designed to the return periods given in Table 3-2 and in accordance
with Sec.17.
10.5.8.5 Mooring calculations for load-out conditions shall take into account the wind area of the loaded out
structure.
Guidance note:
Simplified calculations by resolving forces and moments based only on the orientation of the mooring lines (without considering
the relative stiffness of the lines) are not acceptable. Such calculations may be accepted only if it can be shown that at least
one mooring line is capable of providing the required restraint for a specific environment direction in that direction considered
without exceeding the allowable loads and failure of this line will not lead to unacceptable vessel displacements or rotation for the
operation.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.8.6 Facilities for re-tensioning of mooring lines should be present and in stand-by during the load-out.
Guidance note:
Such facilities may be winches, jacks for tensioning, etc.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.8.7 The mooring system stiffness shall limit the movements of the load-out vessel(s) to those that are
acceptable during the load-out in particular when the object is supported both on the quay and vessel.
10.5.8.8 Adequate strength, stiffness and layout of fenders should be documented.
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Guidance note:
Fender design solutions should at least consider:
—
Requirement for a stiff mooring system during load-out, see [10.5.8.7].
—
Effect of extreme tide variations.
—
Possible impact loads.
—
The possibility that the vessel could 'hang' on the fenders, see also [10.7.7.1].
For floating load-outs care should be taken to ensure that minimum friction exists between the vessel and quay face. Where the
quay has a rendered face, steel plates should be installed in way of the vessel fendering system. The interface between the vessel
and vessel fendering should be liberally lubricated with grease or other substitute which complies with local environmental rules.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.5.8.9 Friction between the vessel and support pad considered as a part of the mooring system in
grounded load-outs, see [10.8.1] shall be properly documented.
Guidance note:
The calculations of friction effect should at least consider:
—
The documented lower bound design friction, see [5.6.9].
—
Minimum vertical load on the pad considering all relevant ballast, tide level and deck loading combinations.
—
Any limitations due to interaction between mooring system and the friction effect.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
10.6 Vessels
10.6.1 General
10.6.1.1 General requirements for vessel(s) are given in [2.11]. These requirements are applicable to any
vessel involved in the load-out.
10.6.1.2 See section [10.9.3.2] for requirements to vessel documentation.
10.6.1.3 For tugs involved in the load-out the applicable sections from [11.12] apply as relevant for the
actual tug work tasks.
10.6.1.4 Approved tugs shall be available or in attendance as required, for vessel movements, removal of
the vessel from the load-out berth in the event of deteriorating weather, or tug back-up to the moorings, see
also [10.7.2.4].
10.6.1.5 For the load-out vessel the requirements in Sec.11 apply as relevant.
10.6.2 Class
10.6.2.1 Vessels should be classed by a recognized classification society, see also [2.11].
Guidance note 1:
If the vessel is not classed by a recognized classification society, then there should be particular emphasis on documentation of
structural strength for the vessel, see also [2.11] and [10.6.3]. In such cases a detailed survey of the barge by the MWS company
may be required.
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Guidance note 2:
If the barge will be grounded during load-out then it should be ensured that the classification society is informed and that any
requirements to inspection of the vessel after grounding are adhered to.
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10.6.2.2 Vessels that are intended to be totally immersed during load-out should be classed for such use by
a recognized classification society.
10.6.2.3 Where a load-out operation temporarily invalidate the class or load line certificate then a statement
of acceptance from the classification society should be submitted, see [2.11.4.4].
10.6.2.4 Any items temporarily removed for load-out shall be reinstated after the load-out is completed and
the vessel shall be brought back into class before sailaway.
Guidance note:
This may apply if, for instance, holes have been cut in the deck for ballasting, if towing connections have been removed or, in some
instances, after grounding on a pad.
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10.6.3 Structural strength
10.6.3.1 The load-out vessel global strength shall be documented for all possible ballast conditions, see also
[2.11.3].
10.6.3.2 The strength should be documented for all parts of the vessel exposed to local loads. Such parts
are typically:
a)
b)
c)
d)
e)
f)
g)
Link beam/plate support area
Skidway, including support area
Deck plate for wheel loading
Jacking system connection points
Hull locally for horizontal loads from the quay
Bottom structure, if grounded load-out
Bollards/mooring brackets.
10.6.4 Stability afloat
10.6.4.1 Sufficient stability afloat shall be ensured during load-out.
Guidance note 1:
Generally load-out should be performed with a minimum GM of 1 m at all stages. The accuracy requirements to ballasting will tend
to increase with decreasing GM.
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Guidance note 2:
—
Normally there is no requirement to document damage stability during load-out. However, it is recommended to consider if/
—
Due attention should be given to situations with small metacentric height where an offset centre of gravity may induce a heel
how incorrect operation of the ballast system may influence stability.
or trim as the object transfer is completed, i.e. when any transverse moment ceases to be restrained by the shore skidways or
trailers.
—
Friction forces between the vessel and the quay, contributed to by the reaction from the pull on system and the moorings,
should be given due attention. (Large friction forces may cause 'hang-up' by resisting the heel or trim until the pull-on reaction
is released, or the friction force is overcome, whereupon a sudden change of heel or trim may result.)
—
Due attention should be given to situations where a change of wind velocity may cause a significant change of heel or trim
during the operation.
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10.6.4.2 For load-out operations the minimum 'effective freeboard', should for vessels be
fmin=0.5 + 0.5Hmax
where:
fmin
Hmax
= minimum effective freeboard in metres, see guidance note
= maximum anticipated wave height in metres at the site during load-out.
Guidance note:
—
The 'effective freeboard' is defined as the minimum vertical distance from the still water surface to any opening, e.g. an open
manhole or deck area where personnel access could be required. A maximum possible tide level and any possible vessel heel/
trim should be considered. Coamings/bundings at openings could be installed to increase the 'effective freeboard'.
—
In order to use a vessel with less freeboard than defined by the load line certificate, approval from class is required. The
—
Procedures to monitor freeboard at all 4 quarters of the vessel should be in place; where this is not implemented fmin should be
freeboard should be sufficient to maintain the vessel’s water-plane area.
increased by 0.3 m.
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10.6.5 Load-out vessel draught and motions
10.6.5.1 Nominal values and allowable tolerances for the load-out vessel(s) level, trim and heel shall be
clearly defined for all stages of the load-out.
10.6.5.2 It should be documented that the values defined according to [10.6.5.1] are adequate to prohibit
unexpected effects or load effects.
10.6.5.3 Significant wave/swell induced motions of the load-out vessel are normally not acceptable during
the operation, see [10.3.4.3].
10.6.6 Maintenance
10.6.6.1 A vessel (barge) handling procedure should normally be documented. The procedure should as a
minimum describe:
— Berthing and if applicable relocation
— Vessel surveys e.g. on-hire and off-hire surveys, condition surveys
— Installation and inspection of moorings
— General watch keeping.
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10.6.6.2 A barge engineer familiar with operation and maintenance of the barge equipment should be
present if any barge equipment is used (or considered as back-up) during critical phases of the load-out.
10.6.6.3 Where relevant, precautions to avoid freezing in tanks and ballast systems shall be taken.
Guidance note:
Such arrangements may be heating devices (in pump rooms), additive anti-freeze solution, or any other devices or actions serving
the above purpose.
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10.7 Operational aspects
10.7.1 General
10.7.1.1 The general requirements for planning and execution of the operation in Sec.2 apply.
Guidance note:
The remaining paragraphs in [10.7] include some additional requirements and/or emphasise on requirements considered especially
important for load-out operations.
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10.7.1.2 Manhole covers which are opened for ballast water transfer or other reasons shall be closed
watertight as soon as practical after use. Any holes cut for ballasting purposes shall be closed as soon as
practical, see also [10.6.2.3].
10.7.2 Preparations
10.7.2.1 All structures and equipment necessary for the operation shall be correctly rigged and ready to be
used.
10.7.2.2 Means (e.g. steel plates) and personnel (e.g. welders) for general repair work shall be available
during the operations.
10.7.2.3 For operations or phases of operations that may be carried out in darkness sufficient lighting shall
be arranged and be available during the entire operation.
10.7.2.4 Additional tugs that may be employed for critical tasks (e.g. as planned contingency measures)
during the load-out operation should be nominated and comply with the requirements of section [11.12] and
be available for inspection as required before the operation.
10.7.3 Clearances
10.7.3.1 Adequate minimum clearances, including clearances under water, for all phases of the operation
shall be defined and properly documented by calculations and surveys before and during the operation.
Guidance note:
Welding/erection of 'last minute' items should not be allowed without a proper re-check of the clearances.
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10.7.3.2 Sufficient under-keel clearance should be documented for vessel(s) during and after the load-out
operation. Normally the clearance should not be less than 1.0 meters.
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Guidance note:
If the vessel under-keel clearance is considered as critical, then the seabed should be inspected by divers or by other adequate
survey method. Where there is a risk of debris, inspections should be done immediately before the vessel berthing. If confidence in
the lowest predicted water levels and in the survey of the load-out area is high, then the minimum clearance requirement could be
reduced to 0.5 m.
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10.7.3.3 The required land area and sea room shall be checked for obstacles. All obstacles that could cause
damages and/or which may delay the operation shall be removed.
10.7.3.4 If relevant, adequate tug air draught shall be ensured.
Guidance note:
The nominal air draught should be minimum 0.5 metres. All positions, including needed access routes that may be required for the
tug(s) should be considered. Possible emergency situations should be included in the considerations.
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10.7.4 Environmental effects
Effects caused by (unexpected) swell and tide could be of significant importance for load transfer operations
and shall be duly considered.
10.7.5 Marine traffic
In areas with other marine traffic necessary precautions should be taken to avoid:
— possible collisions (e.g. with the object, involved vessel(s) or mooring lines)
— significant wash from passing vessel(s).
Guidance note:
Port authority approval for the operation may be required. It may also be necessary to ask local harbour authorities to put
restrictions on the marine traffic.
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10.7.6 Organisation and personnel
10.7.6.1 General requirements for organisation, personnel qualifications and communication are given in
[2.8].
Guidance note:
Load transfer operations will often involve personnel that do not regularly participate in this type of operation. Personnel training
and briefing are hence of great importance, see [2.8.3].
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10.7.6.2 Load transfer operations may involve complicated equipment. Hence, equipment operators should
have the required experience, (see [10.6.6.2] for barge engineers).
10.7.6.3 Proper working conditions for personnel shall be ensured throughout the load transfer operation.
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Guidance note:
Load transfer operations may last for many hours or sometimes for several days and they may be carried out in areas with limited
permanent facilities. Hence, the following may be important to consider:
—
Easy access to food, drinking water and toilets in order to allow for proper continuous work execution.
—
Adequately sheltered/heated/cooled working location(s) for required paper/PC work during the operation.
—
Safe access to all areas were work, including inspections, may be required.
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10.7.7 Load-out site
10.7.7.1 Due attention shall be paid to the possibility of the vessel 'hanging' on the fenders or the quay
structures, see [10.5.8.8] and [10.7.8.2].
10.7.7.2 A level survey of the site area should be performed for load-outs with trailers to ensure that the
level tolerances of the trailers will not be exceeded.
10.7.7.3 Planned trailer tracks should provide an adequate surface condition and the tracks should be
marked on the ground and vessel.
Guidance note:
Before any load-out it should be ensured that:
—
pot holes are filled and compacted
—
debris and obstructions to the load-out path are identified and removed
—
the load-out path and at least 2 m either side of it is freshly graded.
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10.7.7.4 The movement of the object should not be stopped in areas with the potential for settlement due to
e.g. consolidation or adverse weather.
10.7.8 Supports and skidways
10.7.8.1 Levels of supports (and, if applicable, skidways and temporary supports) and horizontal dimensions
on the load-out vessel should be thoroughly checked to be within acceptable tolerances.
10.7.8.2 Tolerances on link beam movement shall be shown to be suitable for anticipated movements of the
vessel during the operation.
Guidance note:
Design of link beam hinges should ensure that it is not possible for the link beams to get stuck when the last skid shoes/load-out
frames are moved from link beams and onto the vessel, see also [10.7.7.1] and [10.5.8.8].
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10.7.8.3 Nominal set down position and set down tolerances should be marked on the supports on the loadout vessel.
10.7.8.4 Suitable shims should be available on the load-out vessel for filling of gaps if required during set
down.
10.7.8.5 The skidway surface condition shall be checked to be as assumed in the friction coefficient
estimate.
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10.7.9 Grillage and seafastening
10.7.9.1 The main requirements for the grillage and seafastening structures of the transported object are in
[11.9].
10.7.9.2 The set down procedure for the object should be documented and it should ensure that the grillage
and seafastening design assumptions are fulfilled.
10.7.9.3 The seafastening should start immediately after final position of the object on the load-out
(transport) vessel is confirmed. However, see [4.3.7.2].
10.7.9.4 Before moving the vessel to another location at the same site for further seafastening, the object
should be secured to the vessel/barge to withstand possible impact loads and/or any heel and trim (due
to wind or one-compartment damage). This condition shall be checked with load and material factors for
relevant failure mode(s) in LS1 (ASD/WSD method) or ULS (LRFD method).
Guidance note:
—
Normally a horizontal characteristic acceleration of minimum 0.1g in any direction will be sufficient.
—
Friction may be considered in the calculations of necessary seafastening capacity, as described in [5.6.9]. The possibility of
contaminants such as grease, water or sand (which may reduce friction between sliding surfaces) should then be assessed and
duly considered.
—
It should be justified that impacts (e.g. between vessel and quay, ground or nearby vessels (in areas of high marine traffic
—
Classification society acceptance required for moving of the vessel if out of class.
density) will not cause displacements of the object that may jeopardize the integrity of the object vertical supports.
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10.7.9.5 Final seafastening connections should be made with the vessel ballast condition as close as
practical to the voyage condition. See [11.9.5] for towing section requirements.
10.7.10 Recording and monitoring
10.7.10.1 During the operation a detailed log should be prepared and kept, see [2.3.8].
10.7.10.2 Monitoring shall be carried out according to [2.9].
10.7.10.3 The following load-out parameters should, as applicable, be monitored and recorded before and
during the operation:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
tide
push/pull force
straightness and levelness of skidding tracks
inclination of link beam
level and vertical deflections of the object
horizontal position of the object
vessel draught and/or level
vessel heel and trim
water level in vessel tanks
hydraulic pressure and stroke on any support/equalising jack, e.g. trailer hydraulic suspension.
10.7.10.4 The line and level of the skidways and skidshoes should be documented by dimensional control
surveys and reports. The line and level should be within the tolerances defined for the load-out operation and
skidway/skidshoe design.
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10.7.10.5 Normally a remote reading sounding system should be used for tank water level control. A backup system but not necessarily remotely controlled (e.g. hand ullaging) should be provided. If access to any
tank is obstructed, e.g. by seafastening supports, alternative access should be arranged.
10.7.10.6 For tidal load-outs, an easily readable tide gauge should be provided adjacent to the load-out
quay in such a location that it will not be obscured during any stage of the load-out operation. Where the tide
level is critical, the correct datum should be established.
10.7.10.7 It shall be possible to continuously monitor hydraulic pressures.
10.8 Special cases
10.8.1 Grounded load-outs
10.8.1.1 If the barge (or load-out vessel) is supported at the seabed during the load transfer phase then the
operation is defined as a 'grounded load-out'.
10.8.1.2 Seabed support pad(s) should be prepared considering:
a)
b)
c)
d)
e)
Any protruding elements (e.g. anodes and bilge keels) on the vessel bottom
Soil bearing capacities, see also [10.4.4]
Stability and global deflections of the vessel
Vessel bottom local strength
Required sliding resistance (friction).
10.8.1.3 Acceptable safety margins should be documented for all relevant load effects, see [10.4] and
[10.6.3].
Guidance note:
Maximum vessel bottom loading at the extreme low tide throughout the period should be considered.
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10.8.1.4 Where the margin against sliding is low mooring lines shall be maintained between the vessel and
quayside when the vessel is grounded.
10.8.1.5 The plan area of the grounding pad with respect to the vessel keel shall be of sufficient extent to
ensure stability of the edges of the grounding pad. Both geotechnical site investigation data and geotechnical
calculations demonstrating the capacity of the grounding pad shall be documented.
Guidance note:
The grounding pad elevation should be defined based on the actual depth of the vessel and not the moulded vessel depth.
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10.8.1.6 Condition and level survey(s) of the support pad(s) shall be performed in due time before load-out.
10.8.1.7 A diver or side-scan survey should be carried out shortly before the vessel is positioned. This to
ensure that there is no debris in the area that can damage the vessels bottom plating.
Guidance note:
If a bar sweep survey is done, then it is recommended that this is supported by a diver’s inspection.
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10.8.1.8 The vessel should be positioned and ballasted onto the pad several tidal periods before the load-out
to allow for consolidation and settlement. Pad loading to reflect the load-out loading condition(s) and vessel
levels to be monitored during this period.
Guidance note:
Pre-loading in excess of the maximum loading during load-out may be used to reduce the required period for pad consolidation and
settlement.
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10.8.1.9 A detailed procedure covering both positioning on the pads and the float-off operation following the
load-out shall be made.
10.8.1.10 Final skidway levels shall be measured and confirmed to be within tolerances compatible with
assumptions used for structural analysis as in [10.3.6].
10.8.1.11 Between load-out and sailaway, the vessel keel should be inspected, either by diver survey or by
internal tank inspection. This is to ensure that no damage has occurred during the load-out.
10.8.2 Transverse vessel load-outs
10.8.2.1 Generally transverse load-outs are sensitive to variations in object weight and CoG as well as to
inaccuracies (between theoretical and actual) moved distance, ballasting and tide levels. This shall be duly
considered both in the ballast calculations and in the monitoring/control procedures. See also [10.3.2].
Guidance note:
Ballasting calculations for transverse load-outs should be based on the weighed weight and CoG and include load combinations
addressing weight and CoG contingencies. See also [10.3.2].
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10.8.2.2 A small GM may be more critical than for an end-on load-out as the heel may change significantly
due to minor inaccuracies. Hence, it is recommended that the GM is as high as possible and that the moment
to change the vessel heel by 0.1 m is computed (and shown in the operation manual) for all stages of the
load-out.
10.8.2.3 As the vessel (accidental) heel can be significant, a braking system for the (skidded) object shall be
provided. See [10.5.2.5].
10.8.2.4 A risk assessment, see [10.2.2.5], should consider the effects of potential errors in ballasting, and
of friction between the vessel and the quay.
Guidance note:
Friction between the side of the vessel and the quay may be more critical than for an end-on load-out.
Snagging or hang-up could potentially lead to ballasting getting out of synchronisation with the movement of the object. Release of
snagging load could potentially lead to instability and failures.
Where a winch or strand jack system is used to pull the object onto the vessel, the effects of the pulling force on the friction on the
fenders should be duly considered.
For sliding surfaces between the vessel and the quay, particular attention should be paid to lubrication and use of low friction or
rolling fenders.
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10.8.3 Load-in
10.8.3.1 Requirements to load-out operations are generally applicable for load-in operations as well.
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10.8.3.2 Special attention should be given to selecting the optimal tide phase for starting the load-in
operation.
Guidance note:
Normally load-ins are scheduled to be started on a falling tide.
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10.8.4 Vessel to vessel load transfer
10.8.4.1 A vessel to vessel load transfer operation is defined as the activities necessary to transfer an object
between vessel(s) doing mainly a horizontal movement of the object.
10.8.4.2 Requirements to load-out operations are generally applicable for vessel to vessel load transfer
operations as well.
10.8.4.3 Vessel to vessel load transfer operations could be complex involving more than two vessel(s), and
different support conditions on one or more of the vessel(s). Due attention should be paid to this fact during
planning, design and execution of the operation.
Guidance note:
For these operations measurements of the vessel(s) draught, trim and heel may not be sufficient to control the load distribution.
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10.8.4.4 Tide effects can be neglected for operations involving only floating vessel(s) if sufficient bottom
clearance is ensured. Hence, the operation could be defined as load-out class 3 or 4.
10.8.5 Site moves
10.8.5.1 The entire route for the site move shall be clearly defined.
10.8.5.2 Any variation in ground slope along the route shall be duly considered.
10.8.5.3 It shall be ensured that condition and capacity of the ground is satisfactory along the entire route,
see [10.4].
10.8.5.4 The route should be marked up and barriered off.
10.8.5.5 It shall be ensured that clearances are sufficient to all parts of the transported object along the
entire route.
10.8.5.6 If the site move involves crossing of a road with traffic or a move on a road with traffic, then this
shall be duly planned for.
Guidance note:
Relevant authorities should be informed and any required approvals should be in place.
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10.9 Information required
10.9.1 General
General requirements to documentation are given in [2.3].
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10.9.2 Design documentation
10.9.2.1 The following design documentation is normally required:
— Analyses/calculations/certificates/statements adequately documenting the necessary strength and
capacity of all involved equipment and structures, see also [10.9.2.3]
— Documentation of civil elements (soil, quay, bollards, etc.) by e.g. engineering calculations, approved
drawings or certificates, see also [10.9.2.3] and [10.9.5]
— Vessel (barge) stability and (local) strength verifications, see also [10.9.3.2]
— Ballast calculations covering the planned operation as well as contingency situations, see also [10.9.4]
and [4.3.8.4].
— Weight report(s).
10.9.2.2 Where parameters are monitored, the expected monitoring results should be documented together
with the acceptable tolerances and the contingency measure to be applied should the acceptable tolerances
be exceeded.
10.9.2.3 Structural analysis report for the object to be loaded out should normally include at least:
— Structural drawings, also of any additional load-out steelwork
— Description of analyses programs used
— Description of the structural model
— Description of boundary conditions
— Description of load cases
— Unity checks for members and joints
— Local analyses for support points, padeyes and winch connection points.
10.9.3 Equipment, fabrication and vessel(s)
10.9.3.1 Acceptable fabrication and acceptable condition of equipment/vessel(s) involved in the load-out
operation shall be documented by:
— Certificates
— Test-, survey- and NDT reports
— Classification documents.
10.9.3.2 For the load-out vessel:
— General arrangement drawing
— Hull structural drawings, including drawings of any internal reinforcements
— Limitations for evenly distributed load and point loads on barge deck
— Limitations on skidway loadings, if applicable
— Equipment data and drawings
— Hydrostatic data (either in curves or tables)
— Tank plan, including ullage (or sounding) tables
— Instructions for air pressurised barge tanks, if used
— Guidelines, if applicable, for grounded barge condition
— Specification and capacity of all mooring bollards
— Details of any additional steelwork such as grillages or winch attachments
— Details of vessel pumping system, see also [10.9.3.6].
10.9.3.3 For tug(s) supporting the load-out a general specification should be submitted and include
information about the tugs bollard pull and towing equipment.
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10.9.3.4 For trailered load-outs:
— Trailer specification and configuration
— Details of any additional supporting steelwork, including link span bridges and attachments
— Allowable and actual axle loadings
— Allowable and actual spine bending moments and shear forces
— Schematic of hydraulic interconnections
— Statement of hydraulic stability of trailer or SPMT system, with supporting calculations
— For SPMTs, details of propulsion axles and justification of propulsion capacity
— Details of set up coordinates for the trailer or SPMT grouping
— Specifications of tractors if used.
10.9.3.5 For skidded load-outs:
— Jack/winch specification
— Layout of pull-on system
— Layout of pull-back and braking systems
— Details of power sources and back-up equipment
— Calculations showing friction coefficient, allowances for bending and sheaves, loads on attachment points
and safety factors
— Reactions induced between vessel and quay.
10.9.3.6 For the pumping system:
— Specification and layout of all pumps, including back-up pumps
— Pipe schematic and details of manifolds and valves where applicable
— Pump performance curves.
10.9.3.7 For the load-out vessels mooring:
a)
b)
c)
d)
e)
f)
A statement showing capacity of all mooring bollards, winches and other attachments used.
Mooring arrangement drawings for the load-out operation and for the post-load-out condition.
Mooring design calculations, see [10.5.8].
Certificates for all mooring arrangement component, e.g. wires, ropes, shackles, fittings and chains
(issued or endorsed by a body approved by a recognized classification society or other certification body
accepted by the MWS company).
Specification for winches, details and design of winch foundation/securing arrangements.
Fender arrangement, including lubrication arrangements if applicable.
10.9.3.8 If the object is weighed, then weighing results and load cell calibration certificates shall be
submitted.
10.9.4 Operation manual
10.9.4.1 An operation manual shall be prepared, see [2.3.7] for general requirements to operation manuals.
10.9.4.2 The items listed below will normally be essential for successful execution of a load-out and shall be
emphasized in the manual:
a)
b)
c)
A detailed operational communication chart (and/or description) showing clearly the information flow
throughout the operation.
Monitoring procedures describing equipment set-up, recording, expected readings (including acceptable
deviations) and reporting routines during the operation.
Detailed ballast procedures, see also [4.3.9.5] and [10.9.4.4].
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d)
Operation bar chart showing time and duration of all critical activities.
Guidance note:
The operation bar-chart should include the following as applicable:
—
Mobilisation of equipment
—
Testing of pumps and winches
—
Testing of pull-on and pull-back systems
—
Barge movements
—
Initial ballasting
—
object movements
—
Load-out operation
—
Trailer removal
—
Seafastening
—
Re-mooring
—
Decision points.
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10.9.4.3 The manual should highlight metocean conditions/directions to which the operation is sensitive.
10.9.4.4 The manual should include ballasting information as follows:
1)
2)
Planned date, time and duration of the load-out, with alternative dates, tidal limitations and windows
Ballast calculations for each stage showing:
— Time
— Tidal level
— object position
— Weight on quay, link beam and vessel
— Ballast distribution
— Vessel draught, trim and heel
— Pumps in use and pump rates required
— Moment required to change heel and trim.
3)
Stages to be considered should minimum include:
— Start condition with object entirely on shore
— A suitable number of intermediate steps
— 100% of weight on vessel
— Any subsequent movements on vessel up to the final position.
4)
Stages requiring movement or reconnection of pumps should be clearly defined.
10.9.4.5 The manual should include contingency plans for all eventualities identified during risk assessment
process, including as appropriate:
— Pump failure
— Mains power supply failure
— Jack-winch failure
— Trailer/skidshoe power pack failure
— Trailer/skidshoe hydraulics failure
— Trailer tyre failure
— Tractor failure
— Failure of any computerised control or monitoring system
— Mooring system failure
— Structural failure
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— Deteriorating weather
— Quay failure.
10.9.5 Site
For the load-out location:
— Site plan, showing load-out quay, position of object, route to quay edge, position of mooring bollards and
winches used, reinforced areas etc.
— Section through quay wall.
— Drawing showing heights above datum of quay approaches, object support points, vessel, link beams, pad
(if applicable) and water levels (the differential between civil and bathymetric datums should be clearly
shown).
— Statement of maximum allowable loadings on quay, quay approaches, wall, grounding pads and
foundations.
— Specification of capacity for all mooring bollards and other attachment points used.
— Bathymetric survey report of area adjacent to the quay and passage to deep water.
— Bathymetric survey of pad (for grounded load-outs).
— Structural drawings of skidways and link beams, with statement of structural capacity, construction
(material and NDT reports) and supporting calculations.
— Method of fendering between vessel and quay, showing any sliding or rolling surfaces and their lubrication.
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SECTION 11 SEA VOYAGES
11.1 Introduction
11.1.1 General
11.1.1.1 This section covers MWS requirements for sea voyages which include:
a)
b)
c)
d)
dry towages of objects on barges
transport of objects on self-propelled vessels
wet towages of objects floating on their own buoyancy, including floating or submerged pipes or similar.
Location moves of jack-ups (approval of the locations is covered in DNV-ST-N002, /39/).
11.1.1.2 It does not normally cover 'standard' or 'routine' cargoes such as bulk liquids, bulk solids,
refrigerated cargoes, containers or vehicles (on ferries) or supply vessels unless they are subject to marine
warranty.
11.1.2 Scope
This section covers the requirements for:
— motion response
— design and strength
— floating stability
— transport and tug selection
— towing equipment
— voyage planning
— pumping and anchoring equipment
— manning
— multiple tows
— additional requirements for specific asset types
— information required for MWS approval.
11.2 Towage or transport design/approval flow chart
The flow chart in Figure 11-1 shows the steps in the approval process and references the sections in this
standard.
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Figure 11-1 Voyage design/approval flowchart
11.3 Motion response
11.3.1 General
11.3.1.1 Design motions shall be derived by means of motion response analyses, from model tank testing,
or by using the default equivalent motion values shown in [11.4].
11.3.1.2 See [3.2] for design sea states. The range of periods associated with the extreme sea state shall be
in accordance with [3.4.11].
11.3.2 Vessel heading and speed
11.3.2.1 The analyses shall be carried out for zero vessel speed for a range of headings.
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Guidance note:
Normally head, bow quartering, beam, stern quartering and stern seas should be considered.
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11.3.2.2 Additionally, the analysis should be carried out for non-beam sea cases for the maximum service
speed of the vessel or the maximum speed that can be maintained in the design sea state. Where this cannot
be handled directly by the software, a zero speed analysis can be carried out with the range of probable peak
wave periods, Tp, adjusted for the speed of the vessel as follows:
where:
Tp, lower
Tp, upper
VSHIP
θ
= lower Tp for zero forward speed
= upper Tp for zero forward speed
= ship speed in m/s
= ship’s heading in degrees (0° = head seas, 180° = stern seas).
11.3.3 Effects of low initial metacentric height and waterplane area
11.3.3.1 Any effects of low initial metacentric height (GM) giving wind heeling should be considered
11.3.3.2 Where there are large changes in water-plane area that can cause heave-induced roll the effects
shall be quantified by analysis and/or model tests.
11.3.4 Effects of free surfaces
11.3.4.1 Free surface corrections to reduce metacentric height (GM) and hence increase natural roll
period, should not be considered in motion analysis, unless the inclusion results in more onerous motion
responses. The effect of any reduction in GM shall however always be considered in intact and damage
stability calculations.
11.3.4.2 RAO’s for vessels with roll reduction tanks (for example) are permissible if this is the actual loading
condition and the roll damping effects are documented (say by model tests).
11.3.5 Effects of cargo immersion
The effect of cargo immersion on the motion response shall be considered.
Guidance note:
Cargo immersion increases the GM and damping. The increase in GM reduces the natural roll period.
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11.3.6 Motion response computer programs
Motion response programs and their application are discussed in [5.6.12].
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11.3.7 Results of model tests
11.3.7.1 Model tests can be used to derive design motions, provided the tests pass the usual review of
overall integrity, see [5.4.2]. Generally, for voyage analyses, the model test results should present the
standard deviation of the relevant responses. The standard deviation of the responses should then be
multiplied by
, where N is the number of zero-upcrossings, to obtain the most probable maximum
extreme (MPME) in 3 hours, which is required for design. This applies to Gaussian responses, however where
the response is significantly non-Gaussian then alternative methods should be used.
Guidance note:
The individual measured maxima from model tests should generally not be used in design as these vary between different
realisations of the same sea conditions, and are therefore unreliable for use as design values. However, the maxima from a series
of tests can be analysed statistically to determine a design value. The number of tests in the series should be sufficient to achieve
stable results.
Most wave frequency motion responses can be considered as Gaussian responses.
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11.3.7.2 Maximum values of global loads, motions or accelerations from model test results can be used
provided ten similar realisations, or greater, are carried out to ensure that variations between individual tests
are accounted for. The mean and standard deviations of the maxima should be calculated. The design value
should be the mean plus two standard deviations.
11.3.7.3 Scale effects should also be accounted for by increasing the design values by a further 10% or a
mutually agreed value.
11.4 Default motion criteria – general
11.4.1
If neither a motions study nor model tests are performed, then for standard configurations and subject to
satisfactory marine procedures, default motion criteria given in [11.5] to [11.7] may be acceptable provided
they are applicable to the actual case in question.
11.4.2
When criteria in [11.5] are used the associated loading calculations shall also be used and not those in
[5.6.15] or [5.6.16].
For the criteria in [11.6] and [11.7] the loads should be calculated in accordance with [5.6.16] for all relevant
load combinations.
11.5 Default motion criteria – IMO
For smaller cargoes, IMO Code of Safe Practice for Cargo Stowage and Securing, /87/, may be acceptable.
Guidance note:
Smaller cargoes are typically under 100 tonnes weight. However, the IMO Code of Safe Practice for Cargo Stowage and
Securing, /87/, may be used to calculate accelerations for cargoes not exceeding 400 tonnes weight, but lashings alone should not
be used for cargoes of 100 tonnes weight or more, see [11.9.5.2.18].
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11.6 Default motion criteria – ships
11.6.1
For ships the default motion/acceleration criteria from classification society rules may be acceptable e.g. DNV
rules for classification of ships (current) DNV-RU-SHIP Pt.3 Ch.4 Sec.3, /35/, and DNV rules for classification
of ships (up to 2016-07), /15/, Pt.3 Ch.1 Sec.4, B600, B700 and B800.
11.6.2
Where accelerations from DNV-RU-SHIP Pt.3 Ch.4 Sec.3, /35/, are used then the following should be
considered:
— The assumptions in DNV-RU-SHIP Pt.3 Ch.1 Sec.2 [3.2], /35/, shall apply.
— The heavy object accelerations consider the 'normal' behaviour of the vessel captain, i.e. extreme weather
conditions are avoided if possible and any extreme vessel motions are reduced by adequate vessel
manoeuvring.
— The vessel GM influences the transverse accelerations significantly and it should be ensured that it is
within the analysed value throughout the transport. As this can be difficult to ensure, it is recommended
that a conservative value is applied and that GM < B/13 is normally not considered.
— Likewise, if not known, the Cb factor shall be chosen conservatively.
— DNV-RU-SHIP Pt.3 Ch.4 Sec.3 [3.3] (envelope accelerations) of /35/ shall apply and not DNV-RU-SHIP
Pt.3 Ch.4 Sec.3 [3.2] (accelerations for dynamic load cases).
11.6.3
If the cargo is to be carried on a vessel classed to DNV rules for classification of ships (up to 2016-07), /15/,
then Pt.3 Ch.1 Sec.4 may be used. The following should be considered:
— The factor for active roll damping should not be used.
— The heavy object accelerations consider the 'normal' behaviour of the vessel captain, i.e. extreme weather
conditions are avoided if possible and any extreme vessel motions are reduced by adequate vessel
manoeuvring.
— The vessel GM influences the transverse accelerations significantly and it should be ensured that it is
within the applied value throughout the transport. As it may be difficult to ensure this, it is recommended
that a conservative value is applied and that a GM < B/13 is normally not considered. Likewise, if not
known, the Cb factor shall be chosen conservatively.
— The referenced part of the DNV ship rules that states the simplified accelerations are valid only for ships
with a length L > 100 m. For ships with length greater than or equal to 100 m it is normally acceptable
to multiply the accelerations from the DNV rules (al, at, av, apz and arz) by the values shown in [11.6.4]
below. For ships with length of 50 m or less a value of 1.0 shall be assumed for all TPOP. For ships with
length between 50 m and 100 m, linear interpolation of the 50 m and 100 m values may be used.
11.6.4
The accelerations obtained from both DNV rules (al, at, av, apz and arz) and DNV rules for classification
-8
of ships up to 2016-07 (ax‐env, ay‐env and az‐env) are based on loads at the 10 probability level, and are
therefore conservative for marine operations. Reduced accelerations may be applied to represent the
maximum expected accelerations for the actual operation. The reduction factors given in Table 11-1 may be
used.
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Guidance note:
The reduction factor related to service area restrictions in the DNV ship rules are considered weather restrictions (see DNV-RUSHIP Pt.1 Ch.2 Sec.5 and DNV-RU-SHIP Pt.3 Ch.4 Sec.3) as the safe distance to port requirement is there to make it possible
for the ship to avoid extreme weather conditions. Therefore such voyages shall be considered as weather restricted and the
necessary procedures presented for approval to apply service area reduction factors. The values in Table 11-1 should not be used
in conjunction with reductions associated with service area restrictions
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Table 11-1 Reduction factors as a function of voyage duration
Duration in days (TPOP)
World-wide
1)
Harsh conditions
2)
TPOP ≤ 7
7 < TPOP ≤ 30
30 < TPOP ≤ 180
180 < TPOP
0.67
0.67
0.80
1.00
0.67
0.80
0.90
1.00
1)
Where a voyage passes through area(s) of harsh conditions for a part of the voyage, then the time spent in the
harsh conditions area(s) shall be addressed separately from the rest of the voyage which may be treated as
world-wide.
2)
Voyage entirely in an area with harsh conditions; such areas include the North Atlantic, Norwegian Continental
Shelf and Northern North Sea outside the summer season.
For transports that can seek shelter in the case of forecasted extreme weather conditions and will do so
according to the operation procedure and within the reliable forecast period, TPOP ≤ 7 days may be applied.
11.6.5
For both ASD/WSD and LRFD, loads due to these accelerations (which may be reduced according to Table
11-1), shall be combined as shown in Table 11-2 or Table 11-3, as applicable:
Table 11-2 Load cases for accelerations based on DNV rules for classification of ships (up to
2016-07)
ax
ay
Head sea 1 (mainly pitch)
al
0
Head sea 2 (mainly pitch)
al
0
Beam sea 1 (mainly roll)
0
at
Beam sea 2 (mainly roll)
0
at
0.6al
0.6at
Load case
Oblique/quartering 1
az
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Oblique/quartering 2
0.6al
0.6at
Table 11-3 Load cases for accelerations based on DNV rules for classification of ships (current)
ax
ay
az
Head sea 1 (mainly pitch)
ax‐env
0
g + az‐env‐pitch
Head sea 2 (mainly pitch)
ax‐env
0
g - az‐env‐pitch
Beam sea 1 (mainly roll)
0
ay‐env
g + az‐env‐roll
Beam sea 2 (mainly roll)
0
ay‐env
g - az‐env‐roll
Oblique/quartering 1
0.6ax‐env
0.6ay‐env
g + az‐env
Oblique/quartering 2
0.6ax‐env
0.6ay‐env
g - az‐env
Load case
Where:
az‐env is the envelope vertical acceleration in m/s2, at any position:
where:
apitch‐z = apitch(1.08x – 0.45L)
aroll‐z = aroll y.
All as defined in the DNV rules for classification of ships (current).
az‐env‐pitch is the envelope vertical acceleration in head seas, in m/s2, at any position:
az-env-roll is the envelope vertical acceleration in beam seas, in m/s2, at any position:
11.6.6
The strength assessment (of all items, including steel structure, welds, bolted connections and lashings) shall
be as follows:
— WSD method: The loads due to the combinations given in [11.6.5] shall be assessed as an LS1 or LS2A
condition (see [5.9.7] and Table 5-7 to Table 5-9 therein).
— LRFD method: The loads due to the combinations given in [11.6.5] shall be taken as characteristic loads
i.e. assessed using the normal load factor(s) applied to the environmental accelerations and the gravity
loading and the normal resistance factors.
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11.7 Default motion criteria – specific cases
11.7.1 Alternative design methods
11.7.1.1 Where the ASD/WSD approach is used for structural checks the values in [11.7.2] apply. For LRFD
the criteria in [11.7.3] apply.
11.7.1.2 Where green water loads are expected these shall be added to the loads due to the motions in
[11.7.2] and [11.7.3]. See also [5.6.5.5].
11.7.2 ASD/WSD default motion criteria
11.7.2.1 LS2 checks may be used for the default motion criteria in Table 11-4 subject to the requirements of
[11.7.2.2].
Guidance note:
LRFD checks in accordance with [5.9] may also be acceptable if the loads resulting from the motions in Table 11-4 are considered
as characteristic loads and appropriate ULS load factors are applied. The requirements of [11.7.2.2] still apply.
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11.7.2.2 The default motion criteria shown in Table 11-4 shall only be applied in accordance with the
following:
1)
Vessels to have typical geometry for their type. For example, vessels with high freeboard are excluded
because they will not experience deck-edge immersion, and consequent damping.
Guidance note:
High freeboard vessels are those where, in flat water, the deck edge will not immerse at the applicable single amplitude roll
angle from Table 11-4 less 13 degrees.
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2)
3)
4)
5)
The cargo-vessel interface shall have friction coefficients no less than those of typical of unlubricated
steel-steel interfaces, however see [11.9.3.1].
Roll and pitch axes shall be assumed to pass through the centre of floatation.
Gravity and heave shall be assumed to be parallel to the global vertical axis, see [5.6.12.1.8].
Phasing shall be assumed to combine, as separate load cases, the most severe combinations of
— roll +/- heave
— pitch +/- heave.
6)
For cases 8 and 9, the departure shall be limited to a maximum of Beaufort Force 5 (for both wind and
waves, see DNV-ST-0111 Table 2-1), with an improving forecast for the following 48 hours. The voyage
duration including contingencies, should not be greater than 24 hours. The actual criteria should be
agreed with the MWS company, taking into account the nature of the vessel or barge and cargo, the
voyage route, the weather conditions which can be encountered, the shelter available and the weather
forecasting services to be utilised.
7)
For cases 10 and 11, the criteria stated is given as general guidance for short duration voyages as there
are too many variables associated with weather routeing. The actual criteria should be agreed with the
MWS company, taking into account the nature of the vessel or barge and cargo, the voyage route, the
weather conditions which can be encountered, the shelter available and the weather forecasting services
to be utilised.
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8)
For case 12, the design loading in each direction shall be taken as the most onerous due to:
— a 0.1g static load parallel to the deck, or
— the static inclination caused by the design wind, or
— the most severe inclination in the one-compartment damage condition.
9)
The additional heel or trim caused by the design wind (with a default value of 52 m/sec or 100 knots)
should be considered. For most voyages, it is permissible to omit the effects of direct wind load when
computing the forces on the cargo (see [5.6.15] and [5.6.16]). If the total effect of the wind on the
cargo due to direct loading and wind heel are more than 10% of the loads from the default motion
criteria, then they shall be added.
10) Where default motion criteria are used the effects of ballasting to counter-act wind heel shall be ignored.
Table 11-4 Default motion criteria (ASD/WSD approach)
Nature of voyage
Weather unrestricted
(these values to be used unless cases
8 to 16 apply).
Case
1
2
3
4
5
6
7
1)
LOA
[m]
B
[m]
L/B
> 140 & > 30
n/a
1)
> 76 & > 23
n/a
≤ 76 or ≤ 23
≥2.5
≤ 76 or ≤ 23
< 2.5
Block
coeff
< 0.9
2)
< 0.9
2)
≥0.9
< 0.9
Roll
Pitch
10
20°
10°
0.2 g
12.5°
0.2 g
15°
0.2 g
10
2)
≥0.9
< 0.9
Full
cycle
period
4)
[secs]
10
2)
≥0.9
10
Single
amplitude
25°
20°
30°
25°
30°
30°
25°
25°
Heave
0.2 g
Weather restricted operations in nonbenign areas for a duration <24 hours
(see [11.7.2.2] 6). For L/B < 1.4 use
unrestricted case.
8
Any
≥2.5
Any
10
10°
5°
0.1 g
9
Any
< 2.5
≥1.4
Any
10
10°
10°
0.1 g
Weather restricted operations in
benign areas, as defined in [3.6],
(see [11.7.2.2] 7). For L/B < 1.4 use
unrestricted case.
10
Any
≥2.5
Any
10
5°
2.5°
0.1 g
11
Any
< 2.5
≥1.4
Any
10
5°
5°
0.1 g
Equivalent
to 0.1 g
in both
directions
0.0
Inland and sheltered water voyages
(see [11.7.2.2] 8). For L/B < 1.4 use
unrestricted case.
12
Any
≥1.4
Any
Static
Independent leg jack-ups, weather
unrestricted tow on own hull. For L/B
≥ 1.4 use unrestricted cases 1 to 6.
13
n/a
> 23
< 1.4
n/a
10
20°
20°
0.0
Independent leg jack-ups, 24-hour or
location move. For L/B ≥ 1.4 use case
7 or 8 as applicable.
14
n/a
> 23
< 1.4
n/a
10
10°
10°
0.0
Mat-type jack-ups, weather
unrestricted tow on own hull. For L/B
≥ 2.5 the pitch angle can be reduced
3)
to 8° .
15
n/a
> 23
< 1.4
n/a
13
16°
16°
0.0
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Nature of voyage
Case
Mat-type jack-ups, 24-hour or location
move.
16
1)
LOA
[m]
B
[m]
L/B
n/a
> 23
n/a
1)
Block
coeff
Full
cycle
period
4)
[secs]
Single
amplitude
Roll
Pitch
n/a
13
8°
8°
Heave
0.0
1)
B = maximum moulded waterline breadth, L = waterline length. n/a = not applicable.
2)
Block coefficient = 0.9 is the cut-off between barge-shaped hulls (>0.9) and ship-shaped hulls; see [11.6] for
alternative criteria for ship-shaped hulls.
3)
Linear interpolation may be used between 16° at L/B = 1.4 and 8° at L/B = 2.5.
4)
Where the natural roll period is less than 10 seconds, then the natural roll period shall be be used for roll
accelerations.
If not obtained by other means (e.g. model tests or motion response calculations), then the roll natural period
(TNRoll ) may be calculated from the following approximate expression:
, where k is the
system roll gyradius (in metres) including the effect of roll added mass, and GMt is the transverse metacentric
height (in metres).
11.7.3 LRFD default motion criteria
11.7.3.1 Loads due to the accelerations given in this section shall be checked using the load and resistance
factors given in [5.9.8].
11.7.3.2 The characteristic accelerations given in Table 11-5 to Table 11-7 can normally be applied to a
standard 'North Sea Barge' (300’ × 90’ × 20’) and barges that are larger in one or more dimensions for the
wave heights shown (either design values or OPLIM for weather restricted tows). The transverse/longitudinal
accelerations include the gravitational component due to roll/pitch angle.
Guidance note:
ASD/WSD checks in accordance with [5.9] may also be acceptable if loads due to the accelerations given in Table 11-5 to Table
11-7 are considered as design loads and the LS1 load and permissible usage factors in [5.9] are applied.
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11.7.3.3 If the effect of rotational inertia is negligible, the accelerations can be calculated at the CoG of the
cargo. If not, they should be calculated at carefully selected 'mass locations' on the cargo, in order to include
the effect of rotation.
11.7.3.4 If accelerations corresponding to an OPLIM are used, an appropriate OPWF shall be defined for the
planned tow duration and procedure.
11.7.3.5 Table 11-5 can also be used for smaller barges with B > 20 m and L > 50 m for most normal
cargoes and configurations. However for unusual towages it would be prudent to check with analysis or
model testing.
11.7.3.6 For barges smaller than a 'North Sea Barge', the limiting wave heights in Table 11-6 and Table 11-7
shall be reduced by multiplying them by the factor which is the lesser of:
L/LNSB or B/BNSB - using the same units (feet or metres)
where:
L is the length of the barge and LNSB is 300’ (91.4 m)
B is the breadth of the barge and BNSB is 90’ (27.4 m).
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11.7.3.7 Alternatively, the limiting wave heights (6 m and 4 m) can be used, if the accelerations from Table
11-6 and Table 11-7 are divided by the same factor
11.7.3.8 All 3 cases (roll/quartering/pitch) in Table 11-5 to Table 11-7 should be considered. In each
case, all possible combinations of directions of the indicated ax, ay and az accelerations shall be taken into
account. Wind force should be added. However it can be acceptable to omit the quartering case based on
engineering judgement if agreed with the MWS company. At least the seafastening forces and maximum
vertical support reaction should be evaluated.
11.7.3.9 Gravity shall be assumed to be normal to the vessel’s deck.
11.7.3.10 The following key applies to Table 11-5 to Table 11-7:
x
y
= distance from barge mid ship
d
= distance used for calculating az in quartering sea,
z
ay
ax
az
= height above waterline.
= distance from barge centreline
= transverse acceleration parallel with barge deck
= longitudinal acceleration parallel with barge deck
= acceleration normal to the barge deck.
Table 11-5 Weather unrestricted barge motion acceleration criteria worldwide for 'North Sea' and
larger (see [11.7.3.2]) barges (LRFD approach)
Acceleration/wind force
Roll case
Quartering
Pitch case
0.50 g
0.40 g
0
0.017 g/m
0.013 g/m
0
ax at waterline (wl)
0
0.15 g
0.25 g
ax incr. each metre (z) above waterline
0
0.005 g/m
0.007 g/m
0.20 g
0.15 g
0.10 g
az incr. each metre (y, d or x respectively) from C
0.017 g/m
0.012 g/m
0.007 g/m
Wind pressure
1.0 kN/m
2
1.0 kN/m
2
1.0 kN/m
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
2
Table 11-6 Motion acceleration criteria for Hs ≤ 6 m for 'North Sea' and larger (see [11.7.3.2])
barges (LRFD approach)
Acceleration/wind force
Roll case
Quartering
Pitch case
0.37 g
0.28 g
0
0.017 g/m
0.013 g/m
0
ax at waterline (wl)
0
0.12 g
0.17 g
ax incr. each metre (z) above waterline
0
0.004 g/m
0.006 g/m
0.20 g
0.15 g
0.10 g
0.017 g/m
0.011 g/m
0.006 g/m
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
az incr. each metre (y, d or x respectively) from C
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Wind pressure
0.5 kN/m
2
0.5 kN/m
2
0.5 kN/m
2
Table 11-7 Motion acceleration criteria for Hs ≤ 4 m for 'North Sea' and larger (see [11.7.3.2])
barges (LRFD approach)
Acceleration/wind force
Roll case
Quartering
Pitch case
0.26 g
0.20 g
0
0.017 g/m
0.013 g/m
0
ax at waterline (wl)
0
0.08 g
0.12 g
ax increase for each metre (z) above waterline
0
0.003 g/m
0.004 g/m
0.15 g
0.12 g
0.08 g
az incr. each metre (y, d or x respectively) from C
0.017 g/m
0.009 g/m
0.004 g/m
Wind pressure
0.3 kN/m
2
0.3 kN/m
2
0.3 kN/m
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
2
11.8 Directionality and heading control
11.8.1
The incident weather shall be considered to be effectively omni-directional, as stated in [11.3.2]. No
relaxation in the design sea states from the bow-quartering, beam and stern-quartering directions shall be
considered for:
a)
b)
c)
d)
e)
f)
Any voyage where the default motion criteria are used, in accordance with [11.4], or similar
Single tug towages, or voyages by vessels with non-redundant propulsion systems (see [11.8.3]).
Any voyage where the design conditions on any route sector are effectively beam on or quartering, of
constant direction, and of long duration, see guidance note
Any towage in a tropical revolving storm area and season
Any un-manned towage
Any transport where the vessel does not have sufficient redundant systems to maintain any desired
heading in all conditions up to and including the design storm, taking account of the windage of the
cargo.
Guidance note:
For c) examples are crossing of the Indian Ocean or Arabian Sea in the South-West monsoon.
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11.8.2
When the relaxation exclusions in [11.8.1] do not apply, relaxation in the non-head sea cases can be
considered for:
1)
Manned, multiple tug towages, where after breakdown of any one tug or breakage of any one towline or
towing connection, the remaining tug(s) still comply with the bollard pull requirements of [11.12.2].
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2)
Voyages by self-propelled vessels with redundant propulsion systems. A vessel with a redundant
propulsion system is defined as having, as a minimum:
— 2 or more independent main engines
— 2 or more independent fuel supplies
— 2 or more independent power transmission systems
— 2 or more independent switchboards
— 2 or more independent steering systems, or an alternative means of operation of a single steering
system (but excluding emergency steering systems that cannot be operated from the bridge)
— the ability to maintain any desired heading in all conditions up to and including the design storm,
taking account of the windage of the cargo and assuming the failure of any one component.
Guidance note:
For power transmission and switchboards to be independent either system is capable of operating on its own to provide the
required power and maintain heading control. There is no specific requirement to operate in open bus but they should be able
to operate independently. Typically vessels with at least DP 2 notation will be easily able to demonstrate this. Those without can
attempt to present a robust case to demonstrate this capability. There is no specific requirement to operate in open bus however
the expectation is that the switchboards are fault tolerant when operating closed bus and can operate independently in the event
of a failure to one switchboard.
For fuel supplies to be independent there should be at least two independent fuel supply systems from tanks, pumps, pipework
to engine. This includes static and dynamic components. This independence can be demonstrated using a single fuel system
or multiple fuel systems. Common tanks, pipework and pumps will not fulfil this. This is easily demonstrated if the vessel is in
compliance to at least IMO DP 2 class, /93/. In all other cases a robust demonstration using an FMECA or similar may be expected.
The redundancy requirements above are not exhaustive. Documentation submitted for vessels seeking exemption will be subject
to review by MWS. Any anomalies will be questioned and should be clarified to an adequate level to ensure that all risks are ALARP
before relaxation in design sea states is permitted.
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11.8.3
Any vessel not complying with all the requirements in [11.8.1] and [11.8.2] shall be considered nonredundant.
11.8.4
For voyages by self-propelled vessels a survey should be performed to confirm the propulsion system
redundancy is acceptable.
Guidance note:
If there is any doubt as to whether or not a vessel can be considered to have a redundant propulsion system the survey should be
performed at an early stage of the project.
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11.8.5
In general, where a relaxation is allowed in accordance with [11.8.2], Table 11-8 is a guide to the acceptable
sea state values. This should be confirmed by the MWS company as being acceptable on a case-by-case
basis.
Table 11-8 Reduced sea state v heading
Incident angle
(head seas = 0°)
Applicable Hs, as % of design sea state
(adjusted as appropriate)
0° to ± 30°
100%
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± (30° to 60°)
Linear interpolation between 100% and 80%
± 60°
80%
± (60° to 90°)
Linear interpolation between 80% and 60%
± 90°
60%
± (90° to 120°)
Linear interpolation between 60% and 80%
± 120°
80%
± (120° to 150°)
Linear interpolation between 80% and 100%
± (150° to 180°)
100%
11.8.6
For any voyage where a relaxation is allowed in accordance with [11.8.2] and [11.8.5], a risk assessment in
accordance with [2.4] shall be carried out.
Guidance note:
For any voyage where a relaxation is allowed in accordance with [11.8.2] and [11.8.5] having an independent cargo owner’s
representative is on board to witness events could be beneficial. The representative should be qualified to discuss with the master
weather conditions forecast and encountered, routeing advice received and avoidance techniques adopted.
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11.8.7
Such relaxation shall only apply to considerations of accelerations, loads and stresses. It shall not be applied
to considerations of stability.
11.8.8
For any voyage where a relaxation is allowed in accordance with [11.8.2] and [11.8.5], the voyage manual
shall contain, in a format of use to the master:
a)
b)
c)
d)
e)
The limitations on critical parameters see guidance note
Procedures for monitoring and recording of critical parameters, possibly by accelerometers on barges
with radio links to the lead tug(s)
Procedures for heading control
Results of the risk assessment, and any recommendations arising
Contingency actions in the event of any breakdown.
Guidance note:
Critical parameters should be observable or measurable by the master.
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11.8.9
The master shall confirm that the effects of these restrictions are practicable and acceptable.
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11.9 Design and strength
11.9.1 Computation of loads
11.9.1.1 The loads acting on grillages, cribbing, dunnage, seafastening and components of the cargo shall
be derived from the loads acting on the cargo, according to Sec.3, Sec.5, and [11.3] to [11.7], as applicable.
Guidance note:
Care should be taken in cases where the cargo has be designed for service loads in the floating condition, but is being drytransported. Its centre of gravity can be higher above the roll centre in the dry-transport condition than in any of its floating
service conditions. Even though the voyage motions can appear to be less than the service motions, the loads on cargo
components and ship-loose items can be greater.
It should be noted that fishfarms, vessels and other structures that in their operational phase are floating will under transport
experience very different loads than during the operational phase that they are primarily designed for.
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11.9.1.2 The loads shall include components due to the distribution of mass and rotational inertia of the
cargo.
Guidance note:
This is of particular importance in the calculation of shear forces and bending moments in the legs of jack-up units and similar tall
structures.
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11.9.1.3 Account shall be taken of the effects of pretension in all lashings used in the system supporting and
restraining the object, see also [11.9.5.3].
Guidance note:
Lashings can typically be used to:
—
directly restrain the object to the vessel
—
restrain the object to supporting frames or grillages
—
restrain supporting frames or grillages to the vessel.
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11.9.1.4 The higher of the computed loads or the required capacity in accordance [11.9.2.7] or [11.9.2.8]
(as applicable) shall apply.
Guidance note:
A simplified example of cribbing/seafastening calculations is shown in [K.7].
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11.9.2 Friction – general
11.9.2.1 Friction forces on the cargo supports/cribbing may be allowed to contribute to a reduction in the
seafastening design loads provided that the entire load path, including the potential sliding surfaces, are
documented as being capable of withstanding the loading generated.
11.9.2.2 Uncertainty in the load distribution between (seafastening) members and friction forces shall be
taken properly into account.
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Guidance note:
Force/load distribution between friction supports and seafastening can be calculated (assessed) by comparing deflection needed to
mobilize friction with seafastening stiffness. If this is not done the following precautions should be implemented:
—
Seafastening members should be designed to tolerate possible “overloading”, see [11.9.2.3].
—
In FLS, friction should not be used to reduce the seafastening loads in any sea state up to the sea state giving seafastening
load without friction equal to the ULS characteristic load with friction.
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11.9.2.3 The magnitude of restraint loads, especially if caused (entirely or partly) by friction effects, could
be difficult to calculate accurately. Hence, the following precautions should be taken:
— Avoid if possible designs/layouts that cause restraint forces.
— Minimize restraint forces as a result of ballasting to transport condition, see [11.9.5.4.4].
— The end connection of seafastening elements with significant restraint forces should be made stronger
than the element itself.
— A thorough evaluation of 'worst case distribution' of restraint forces between seafastening elements should
be carried out. Reasonably conservative assumptions regarding force distributions should be considered
in FLS calculations. Deformation loads on the cargo due to the wave-induced bending and torsion of
transport vessel shall be considered.
11.9.2.4 The reduction in the effectiveness of friction restraint due to vibrations caused by wave slamming
on the vessel hull and/or on overhanging cargo shall be assessed.
Guidance note:
Other slamming effects are given in [5.6.5.4.3].
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11.9.2.5 For cargoes with weight less than 1000 t and/or unusual high volume (potential buoyancy) weight
relationship the possible effect of buoyancy (and green water, see [5.6.5.5]) should always be evaluated on a
case-by-case basis.
11.9.2.6 FLS calculations shall be based on the actual (linear elastic) stress (range) distribution. Hence, the
effect of friction and restraint forces (vessel deflection) on the stresses shall be adequately calculated. See
[5.6.11].
11.9.2.7 For voyages in open water, the minimum seafastening capacity, without considering friction,
shall be sufficient to resist the accelerations shown in Table 11-9. If the effects of vibrations and hull beam
deflections can be proved to be insignificant, consideration can be given to reducing this requirement.
Table 11-9 Minimum seafastening capacity as a function of Cargo Weight, W
W < 1000t
1000t ≤ W
< 5000t
5000t ≤ W
< 20000t
20000t ≤
W < 40000t
W ≥ 40000t
Transverse
0.15g
Linear
0.10g
Linear
0.05g
Longitudinal
0.10g
Linear
0.05g
Linear
0.03g
Direction/weight
11.9.2.8 For voyages in sheltered inland water, the minimum transverse seafastening capacity shall be taken
as 0.1g plus the effect of any heel, but need not exceed the requirements of Table 11-9.
11.9.2.9 For weather restricted voyages (see [2.6.5], [2.6.7] and [3.3]), if the available friction capacity
is at least twice the calculated required total seafastening design loads, then the MWS company may, on a
case-by-case basis, agree to dispense with the minimum seafastening capacity requirements of [11.9.2.7].
In such cases, the entire load path, including the potential sliding surfaces, shall be documented as being
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capable of withstanding twice the larger of the loadings from the motions analyses and the minimum
seafastening capacity requirements of Table 11-9. See also [5.6.9].
11.9.2.10 For very short duration moves in sheltered water, such as turning a vessel back alongside the
quay after a load-out, then friction can be allowed to contribute. The entire load path, including the potential
sliding surfaces, shall be demonstrated to be capable of withstanding the loading generated, including
collision with nearby vessels in areas of high marine traffic density. Where friction is applied for this case, any
seafastenings shall have sufficient flexibility to allow the friction to develop.
11.9.2.11 Where friction is considered, see [5.6.9], the characteristic friction coefficient shall be documented
and a material factor applied to find the design friction coefficient. The effects of lubricating fluids or similar
shall be considered when establishing the design friction coefficient. Friction shall not be used to reduce
the design loads when the potential friction interfaces are steel-steel, unless the friction surfaces can be
guaranteed to remain dry.
Guidance note:
The following maximum design friction coefficients for calculation of favourable friction forces can/should normally be considered:
—
Steel to steel, wet: 0.0
—
Steel to steel (wet and dry) if vibrations (see [11.9.2.4]) can occur: 0.0
—
Steel to steel dry: 0.1
—
Steel to wet timber: 0.2
—
Steel to dry timber or rubber (wet or dry): 0.3
—
Timber to timber: 0.4
—
Concrete to steel: 0.1
—
Concrete to wood: 0.3
—
Concrete to concrete: 0.4.
It is assumed that the friction surfaces are free from oil or other lubricating fluids.
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11.9.2.12 Where the cargo is supported on cribbing alone, the friction contribution may be determined using
a simplified approach provided that a mean design friction coefficient of 0.2 is found applicable according to
either [11.9.3.2] for ASD/WSD or sections [11.9.4.4] and [11.9.4.5] for LRFD. In such cases the assumed
friction coefficient shall not exceed the value given in Table 11-10, as a function of the cargo weight and
overhang.
Table 11-10 Max allowable upper bound design friction coefficients
Cargo weight, W, tonnes
Maximum
cargo overhang
W < 1,000
1,000 <
W < 5,000
5,000 < W
< 10,000
10,000 <
W < 20,000
20,000 ≤ W
Maximum allowable coefficient of friction
None
0.10
0.20
0.20
0.20
0.20
< 15 m
0
0.10
0.20
0.20
0.20
15 – 25 m
0
0
0.10
0.20
0.20
25 – 35 m
0
0
0
0.10
0.20
35 - 45 m
0
0
0
0
0.10
> 45 m
0
0
0
0
0
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Guidance note:
The friction coefficients can be interpolated as a function of Maximum Cargo Overhang using the actual maximum overhang value.
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11.9.2.13 Friction forces shall be computed using the normal reaction between the vessel and cargo
compatible with the direction of the heave.sin(theta) term used in computing the forces parallel to the deck
in [5.6.15.3]. Thus, when heave.sin(theta) increases the force parallel to the deck, it also increases the
normal reaction and vice-versa. When the provisions of [5.6.15.4] are used, the normal reaction should be
determined conservatively, as follows:
— When heave adds to the self-weight reaction the total normal reaction shall be reduced by 10% to allow
for adverse phasing.
— When heave reduces the self-weight reaction, the normal reaction shall be taken as weight less heave as
any effects of phasing will cause an increase in the normal reaction.
11.9.3 ASD/WSD friction
11.9.3.1 When using the ASD/WSD method, friction shall NOT be used if the loadings are computed in
accordance with the default criteria in [11.4] and [5.6.16], except as allowed by [11.9.9].
11.9.3.2 When using the ASD/WSD method, friction effects can be incorporated to reduce seafastening
requirements for cargos supported on timber cribbing, subject to the following:
1)
2)
Loadings are computed in accordance with [5.6.12] to [5.6.15].
For wood cribbing less than 600 mm high, with a width not less than 300 mm, the friction force due to
the friction coefficient permitted in Table 11-10 can be assumed to act in any direction relative to the
cribbing provided that:
— the cribbing is reasonably well balanced in terms of the proportion in the fore-aft and transverse
directions, AND
— each of these groups is reasonably well balanced about the cargo CoG in plan.
3)
Provided that the conditions in 2) are met, for cribbing heights between 600 mm and 900 mm, with
a width not less than 300 mm, then the percentage computed friction force at right angles to the
longitudinal axis of a cribbing beam shall not exceed (900 - H)/3%, where H = the height of cribbing
above deck, in mm. In the direction of the longitudinal axis of a cribbing beam, the full friction force can
be used.
4)
For wood cribbing over 900 mm high, or with a width less than 300 mm, no friction force shall be
assumed to act in a direction at right angles to the longitudinal axis of a cribbing beam.
5)
If greater cribbing friction is required than available according to 3) and 4), stanchions can be fitted
to provide transverse cribbing restraint. Where such stanchions are fitted, they should be designed to
carry loads due to a friction coefficient of 0.5 (to ensure they are able to carry loads due to upper-bound
friction assumptions).
6)
The underlying assumption in the approach given above is that the seafastenings have sufficient
flexibility to deflect in the order of at least 2 mm in the horizontal direction of loading without failing.
This will be reasonable in most cases, but when this is not the case the more detailed approach given in
7) shall be used.
7)
As an alternative to 2) through 5), a more detailed approach can be used. In such cases, the
friction coefficient permitted in Table 11-10 can be doubled, provided that the distribution of loading
between the seafastenings and cribbing friction accounts for the relative flexibility of the cribbing and
seafastenings. The angle between the loading direction and the grain of the cribbing shall be taken
into account, e.g. when the loading is perpendicular to the grain the cribbing is more flexible. The
arrangements shall be such as to ensure that the required lateral load can be carried by the combination
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of friction and seafastening reactions BEFORE the seafastenings are overstressed. Where stanchions are
used, they shall comply with 5).
11.9.4 LRFD friction
11.9.4.1 When using the LRFD method, friction can be used if the loadings are computed in accordance with
the default criteria in [11.4] and [5.6.16]
11.9.4.2 When using the LRFD method the following approach shall be used if friction effects are to be
incorporated to reduce seafastening requirements.
11.9.4.3 Design loading on the seafastening can be reduced by considering relevant friction effects on the
cribbing, see [11.9.7].
11.9.4.4 Where friction on the cribbing is considered, see [5.6.9], the characteristic friction coefficient shall
be documented and a material factor applied to find the design friction coefficient.
Guidance note:
In the areas and directions where full friction effect could be mobilized a design friction coefficient of 0.3 can normally be applied
between wood and steel on cargo. Any special effects (e.g. wood treating, type of surface treatment on cargo, and risk of oil/
lubricant present) that can reduce the friction significantly should be evaluated.
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11.9.4.5 Due to low wood shear stiffness and strength, friction forces transverse to the cribbing (soft wood)
boards should only be accounted for if properly documented.
Guidance note:
If a thorough evaluation including cribbing shear stiffness and seafastening design (stiffness) has not been carried out the following
apply:
a)
For cribbing with H (height) ≥ 1.5B (breadth) zero contribution should be considered from friction in the transverse cribbing
b)
For cribbing with H < 1.5B contribution from friction in the transverse direction could be considered with (1.5B – H)/1.5B x
c)
Normally 100% contribution from friction could be considered in the longitudinal cribbing direction. However, see [11.9.2.2].
d)
The mean design friction coefficient considered should in any case not exceed 0.2, but see [11.9.2.12].
direction.
100%.
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11.9.5 Seafastening design
11.9.5.1 Introduction
This subsection [11.9.5] covers requirements for seafastenings which in this context, includes any grillage,
dunnage, cribbing or other supporting structure, roll, pitch and uplift stops, and the connections to the barge
or vessel. This subsection also applies to fastenings for land transport though with different accelerations.
Guidance note:
Grillage and seafastening design is influenced by the load-out method.
—
Cargoes floated over a submersible barge or vessel, are frequently supported by timber cribbing or dunnage to distribute the
—
Cargoes lifted onto the transport barge or vessel are either supported on timber cribbing/dunnage or grillage depending on
—
Cargoes loaded by skidding normally remain on the skidways, and are seafastened to the skidways and/or vessel.
—
Cargoes loaded out by trailers normally need a grillage structure higher than the minimum trailer height.
loads and allow for minor undulations in the deck plating.
type and size of cargo.
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11.9.5.2 Design principles
11.9.5.2.1 The grillage elements, including shimming plates, shall be used to distribute a concentrated deck
load to a sufficient number of load-carrying elements. The grillage or cribbing height shall allow for any
projections below the cargo support line.
11.9.5.2.2 Lateral and vertical bearing connections shall be tight against movement and fitted with shim
plates where necessary. Shim plates shall be secured in place.
11.9.5.2.3 Seafastenings shall be designed to withstand the global loadings from the transported objects
rotation (overturning) and sliding in any direction as computed in Sec.5 and the additional requirements
of this subsection [11.9]. Their strength shall be assessed using the applicable checks in [5.9]. Normally
seafastening calculations should be provided for any item heavier than 5 tonnes.
11.9.5.2.4 The seafastening and grillage design shall duly reflect the structural strength limitations of both
the objects and transport vessel. The strength of hatch covers used to provide support shall be assessed for
the point loadings they will sustain.
Guidance note:
The effect of global loads on local strength should be considered; e.g. a buckling check of vessel-stiffened panels for support loads
from cargo should include the stresses caused by hull bending moments and shear forces.
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11.9.5.2.5 Grillage and seafastening shall be designed (and installed) taking into account all the physical
limitations implied by the load transfer procedures/methods both to and from the transport vessel(s).
Guidance note:
Typical physical limitations may be related to:
—
available heights
—
strict tolerances, etc. imposing requirements for the erection/welding sequence, see also [11.9.5.2.10]
—
load-out trailer layout
—
needed space for (operation of) load-out systems, e.g. pumps, hoses, pull/push units
—
set down tolerances and shimming requirements
—
cutting/handling offshore
—
securing of object before lift, see [11.9.5.2.7]
—
possible need for set down of the object again and re-instate seafastening offshore.
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11.9.5.2.6 The design calculations shall include any positioning tolerances for the transported object on the
grillage including, if applicable, effect of vessel hull beam deflections.
Guidance note:
Positioning tolerances should be included in the load-out procedure.
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11.9.5.2.7 Seafastening design for offshore or inshore installation operations should allow for easy release
and provide adequate support and horizontal restraints until the object can be lifted clear of the vessel, or
launched as applicable as described in [11.9.6].
11.9.5.2.8 Elements providing horizontal and/or vertical support after cutting/removal of seafastening shall
be verified for characteristic environmental conditions applicable for the installation operation.
11.9.5.2.9 Wave entry (slamming) and exit loads shall be considered for overhang cargo in the seafastening
and cargo design (see [5.6.5.4]). See also [11.9.2.5] for uplift due to buoyancy.
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11.9.5.2.10 Vessel global deflection both due to waves and redistribution of ballast may impose significant
loads on grillage elements and seafastening. Both additional horizontal and vertical loads shall be considered,
see [11.9.5.4.4].
11.9.5.2.11 For special precautions to seafastening after back loading offshore see DNV-RP-N102, /55/.
11.9.5.2.12 If re-instating of the transport seafastening may be required offshore this should be taken into
account in the design and in the cutting/release procedure.
11.9.5.2.13 Seafastenings shall be designed to accept deflections of the barge or vessel in a seaway,
principally due to longitudinal bending. In general, longitudinal bending should be considered for the cases
described in [5.6.11].
Guidance note:
Where longitudinal bending is a consideration, suitable seafastening designs include:
a)
Chocks which allow some movement between the vessel and cargo
b)
Pitch stops at one point only along the cargo, with other points free to slide or deflect longitudinally
c)
Vertical supports at only 2 positions longitudinally
d)
An integrated structure of vessel-seafastenings-cargo, capable of resisting the loads induced by bending and shear.
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11.9.5.2.14 The (assumed) force distribution in seafastening and grillage shall correspond to the considered
reaction forces for vessel and transported object strength verifications.
11.9.5.2.15 Possible uplift due to overturning of the object and/or relative deflections shall be prevented by
seafastening where required. See also [5.3.4] and [11.9.5.4.1].
Guidance note:
Uplift seafastening is always required when, for the most onerous combination of motion induced loads, the object restoring
moment is less than the object overturning moment after it has been increased by 15% for LRFD or by 25% for ASD/WSD. For
LRFD the ULS factors in Table 5-10 should be included in the calculation. For ASD/WSD the characteristic loads should be used.
When uplift restraints are required they should be provided to resist the greater of the uplift from the above check and an
overturning moment equal to 15% of the restoring moment due to gravity in the still water condition. The resulting uplift load may
be considered as a design load for LRFD ULS checks or assessed using LS2 for ASD/WSD.
The need for prohibiting calculated 'local' uplift should be evaluated in each case. If not prohibited the effect of 'gaps' and
redistribution of reaction loads should be taken into account.
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11.9.5.2.16 Additionally, for towed objects which can have permanently installed modules with piping or
other connections between them, there should be adequate flexibility in the connections to avoid overstress.
In long modules carried as cargo, internal pipework should be similarly considered.
Guidance note:
It should be noted that the voyage wave bending condition can be more severe than the operating condition.
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11.9.5.2.17 When required by [5.6.11.2], and in the absence of more detailed information, it should
be assumed that the vessel will incur bending and shear deflection as if unrestrained by the cargo; the
seafastenings and the object should be checked assuming quasi-static vessel hogging and sagging due to a
wave of length, Lw, equal to the vessel length, and height:
where Lw is in metres.
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11.9.5.2.18 Seafastenings should generally be welded steel. For smaller cargoes, chain, wire or webbing
lashings with suitable tensioning devices can be acceptable and shall meet the requirements in [11.9.5.3].
Guidance note:
Smaller cargoes are typically less than 100 tonnes for wire and chain seafastening alone and 50 tonnes for webbing lashings alone.
For smaller cargoes up to 400 tonnes, a combination of lashing and welded stoppers can be acceptable as suggested by IMO Code
of Safe Practice for Cargo Stowage and Securing, /87/.
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11.9.5.3 Seafastening – lashings and bolts
11.9.5.3.1 Where chain binders, ratchets, turnbuckles or bolts are used:
— They shall be tensioned before departure to spread the load between the seafastenings and secured so
that they cannot become slack.
— They should be inspected regularly and after bad weather to ensure that tension is maintained.
— All mechanisms shall be adjustable without release unless there is sufficient redundancy.
— Wire lashings and webbing lashings should not be used for un-manned voyages since they can loosen and
are difficult to inspect regularly, see also [11.9.5.3.9].
— Where green water risk is identified, the risk of impact damage to lashings from loose deck equipment/
dunnage or similar shall be assessed and mitigated (eg. redundancy in design).
— Bolts shall be pre-tensioned and/or fitted with other means e.g. lock nuts to prevent loosening under
cyclic motions or higher frequency vibrations arising from rotating equipment, thrusters, ballast pumps,
engine room equipment, see [5.3.5], [5.6.5.4] and [E.2.2].
— Bolts and lashings shall not be re-used without fatigue assessment and/or rigorous inspection - see
[5.3.5], [E.2.2], [E.2.7], [E.2.9] and [E.2.10.1].
.
11.9.5.3.2 Possible skew loads in lashings due to uneven pre-tensioning and length/stiffness variations in
statically indeterminate seafastening arrangements shall be taken into account. The design loads for lashings
should be multiplied by a skew load factor not less than 1.5 if skew load effects are not accurately calculated.
Guidance note:
A skew load factor of 1.5 is considered adequate if lashings carrying the same (quasi-static) load component between them have
approximately the same stiffness and similar means of pre-tensioning. If not, a conservative assessment should be conducted to
estimate the applicable skew load factor(s).
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11.9.5.3.3 Calculations of characteristic loads in lashing seafastening shall take into account cargo CoG,
support lay out, friction and location/direction/stiffness of each lashing. In indeterminate seafastening
arrangements the loads can be calculated based on a quasi-static load distribution combined with an
appropriate skew load factor, see [11.9.5.3.2].
Guidance note:
Applicable design friction coefficients are listed in [11.9.2].
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11.9.5.3.4 Unless motions have been calculated in accordance with [11.5] (i.e. IMO Code of Safe Practice for
Cargo Stowage and Securing, /87/), then if the equipment certificate states the MBL the following applies to
both new and used items, provided that used items are inspected and found to be in good condition, and that
the manufacturer does not specify reduced capacity for re-use:
— For ASD/WSD the maximum design load computed using the applicable load factors from Table 5-7 shall
be less than the applicable value from Table 11-11.
— For LRFD the maximum design load shall be less than the design resistance where the design resistance is
the certified MBL divided by the relevant material factor, see [5.9.8.5].
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Table 11-11 Allowable design load as a function of MBL for ASD/WSD approach
Maximum allowable design load
to DNV class rules, see [11.6]
Maximum allowable
1)
ASD/WSD design load
LS2A
LS1 / LS2
Certified steel wire ropes
and chains
0.43MBL
0.43MBL
Polyester ropes
0.39MBL
0.39MBL
0.32MBL
0.32MBL
Ropes and webbing
straps of other fibre
materials, including nylon
(polyamide), polyethylene
2)
and polypropylene
0.26MBL
0.26MBL
Shackles, turnbuckles, load
binders, D-links and web
lashing assemblies.
For 'web lashing
assemblies', the webbing
should be checked using the
applicable allowable from
above.
0.33MBL
0.33MBL
Item type
HMPE and aramid ropes
1)
2)
1)
The design load shall be computed using the applicable load factors from Table 5-7.
2)
For fibre ropes subject to a robust certification process, other material factors may be considered acceptable;
however the maximum allowable design load should not be taken to be greater than that permitted for polyester
ropes.
11.9.5.3.5 For equipment in accordance with a recognized code where the certificate states a capacity other
than the MBL then:
a)
b)
It may be acceptable to use the above requirements where the MBL is taken as the breaking load (or
similar) specified in the code.
Where a) does not apply the characteristic load shall be less than the stated capacity (typically given as
WLL or similar).
11.9.5.3.6 When the motions have been calculated in accordance with [11.5] (i.e. IMO Code of Safe Practice
for Cargo Stowage and Securing, /87/), then strength of lashing equipment shall be assessed in accordance
with IMO requirements.
11.9.5.3.7 The good practice for lashings and similar devices in the IMO Code of Safe Practice for Cargo
Stowage and Securing, /87/ should be followed if relevant. Any hooks should be fitted with robust safety
catches or other means of positively retaining the hook.
11.9.5.3.8 Lashing equipment (chains, wires, shackles, turnbuckles etc.) shall have certificates, giving the
ultimate capacity, WLL (or similar), issued or endorsed by a body approved by a recognized classification
society or other certification body accepted by the MWS company. Certificates should be revalidated at
intervals of not more than 4 years and identify the equipment to which they apply. Each item is to be visually
inspected prior to each use by a responsible person.
11.9.5.3.9 Synthetic webbing should only be used for smaller cargoes on manned voyages. Where web
lashing assemblies (web lashings, ratchet straps or cargo straps) are used, then:
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a)
b)
c)
d)
e)
f)
D-links and shackles shall be used instead of hooks (which can unhook) or the hooks shall be fitted with
robust safety catches or other means of positively retaining the hook.
The straps shall be in good condition, with no rips or abrasion damage. They shall not have been, or
be likely to be, subject to chemical degradation or excessive sunlight (ultraviolet radiation). Note that
different types of synthetic materials (e.g. nylon or polyester) have very different resistance to acids,
alkalis, UV radiation, ripping and abrasion. Material design has also improved over the last few years.
There shall be no sharp edges to damage the straps. If sharp edges are protected by rubber or similar
materials then the materials shall be properly secured.
The fittings shall be of the correct shape and size to ensure that the straps are not damaged
Straps shall not be knotted or twisted through more than 90° unless allowed in the certification.
The strength of synthetic webbing doubled around a D-ring is reduced. This reduction shall be taken into
account; it can be documented by testing.
11.9.5.3.10 If chains are used, and it is not properly documented otherwise, then:
1)
2)
Chains should not be bent around edges with diameter less than 4 times the chain diameter. 2 times the
chain diameter may be acceptable for up to 90° edges.
The effective MBL of doubled chains that are bent more than 90° around connection points should be
reduced as indicated below:
a)
b)
c)
Point with diameter equal or less than 2 times (1.5 times if bend 90° or less) the chain pitch (inside
length of links): 50%.
Point with diameter equal or greater than 4 times (3 times if bend 90° or less) the chain pitch: 10%
(skew load between the two legs included).
Point with diameter greater than 2 (1.5) times and less than 4 (3) times the chain pitch: Linearly
between a) and b).
11.9.5.4 Seafastening - welded
11.9.5.4.1 Connections to the deck of a barge or vessel should be carefully considered, particularly tension
connections. Calculations should be documented to justify all connections. It should not be assumed, without
inspection, that underdeck connections between deck plating and stiffeners or bulkheads are adequate
especially in the region of tension connections, see [5.3.4].
11.9.5.4.2 It is not generally acceptable to land tubular seafastenings, liable to tension, on deck via a
doubler plate. A gusset connection should be used, landing on an underdeck member of suitable strength in
accordance with [5.3.4].
11.9.5.4.3 Seafastenings shall not be welded onto fuel oil tanks or oil cargo tanks, unless the tanks are
empty, and gas free certification has been obtained.
11.9.5.4.4 So far as is practical, seafastening connections should be made after load-out with the barge
or vessel in the voyage ballast condition, or a condition giving a similar longitudinal bending situation. If
not practical, then the additional stresses which can be caused by the change in ballast condition shall be
considered.
11.9.5.4.5 Adequate weather protection and heating and drying should be used to ensure the quality of
seafastening welds made in wet conditions, see also [5.9.7.6], [5.9.8.4.5] and [5.10.2.2].
11.9.5.4.6 Where a lift is made onto a vessel offshore, the seafastenings should be designed accordingly,
normally by means of guides or a cradle, which will hold the cargo whilst it is being seafastened.
11.9.5.4.7 Items of the cargo which are vulnerable to wave action, wetting or weather damage shall be
suitably protected.
Guidance note:
This can require provision of breakwaters or waterproofing of sensitive areas.
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11.9.5.4.8 Internal seafastenings shall be provided to prevent items moving inside structures or modules.
See also the caution in guidance note to [11.9.1.1] for dry transport.
11.9.5.4.9 Guide posts should not be used for seafastenings unless specifically designed for that purpose.
11.9.6 Seafastenings to be removed offshore
11.9.6.1 For cargoes that will be installed offshore, the seafastenings should be capable of being released
in stages, such that the cargo remains secure for all anticipated angles and motions. The release of
seafastenings, and the removal of any one object, should not disturb the seafastenings of any other object.
Guidance note:
For lifts, see [16.16.9.5] for the design of the restraints/seafastenings that remain after all cutting has been completed. For other
operations, 10° is normally sufficient.
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11.9.6.2 Where the installation is in an area more benign than that for which the seafastenings were
designed then, subject to the agreement of the MWS company, some seafastenings can be removed after
entering that area and before the installation certificate of approval is issued. In this case, unless weatherrouted:
a)
b)
The remaining seafastenings shall be designed for the design criteria for the installation area and the
route to a sheltered area if required, and
The seafastenings to be removed early shall be clearly marked as such.
11.9.6.3 Removal of seafastenings shall not normally start until the installation certificate of approval
has been issued. This requirement can be relaxed in special circumstances subject to a risk assessment in
accordance with [2.4]. The seafastenings to be removed early shall be clearly marked as such and identified
in the seafastening removal procedures.
11.9.7 Cribbing
11.9.7.1 Where the cargo is supported on wooden cribbing or dunnage, rather than steel-to-steel supports,
then sufficient plan area and height of material should be provided to distribute the loads to ensure that the
underside of the cargo and to the deck of the transport vessel are not overstressed. The loads shall include
the static loadings and the design environmental loadings as shown in [11.3] and Sec.5.
Guidance note:
Cribbing designed to pick up structural members in the underside of larger transported objects e.g. MOUs, the vessel deck, or
both, and fixed to the deck of the vessel, should not normally be less than 200 mm high.
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11.9.7.2 A minimum clearance of 0.075 m, after accounting for vessel deflections, should be provided
between the lowest protrusion of the cargo and the deck of the barge or vessel.
Guidance note:
Where the dimensions and locations of protrusions from the cargo are well documented the minimum clearance can be reduced.
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11.9.7.3 Unless it can be demonstrated that the cargo, vessel and cribbing (without crushing), can withstand
2
a greater pressure, the nominal bearing pressure on the cribbing should not exceed 2 N/mm for softwood.
The nominal bearing pressure on the cribbing should be calculated taking into account the deadweight of the
cargo plus the loads caused by the design environmental loadings.
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11.9.7.4 The selected timber should withstand the computed cribbing pressures without crushing. Localised
crushing to accommodate cargo and cribbing imperfections is permissible.
Guidance note:
A satisfactory arrangement can consist of hardwood for the main cribbing structure, topped by a soft packing layer, typically
50 mm thick.
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11.9.7.5 In the case of a random or herring-bone dunnage layout supporting a flat-bottomed cargo, without
2
taking into account the strong points, then the maximum cribbing pressures should not exceed 1 N/mm ,
subject to consideration of the overall allowable loads on the deck of the vessel and the underside of the
cargo.
11.9.7.6 For cargoes floated on and/or off a grounded or partially grounded transport barge or vessel, the
cribbing should be designed to withstand:
— line loads during initial phases of contact or final stages of separation and
— trim or heel angles during on-load and off-load. Minimum angles of 5º should be considered.
11.9.8 Cargo strength requirements
11.9.8.1 The cargo shall meet the requirements in Sec.5 for the loads imposed during the voyage.
Additionally the cargo shall be shown to have adequate strength to withstand the local cribbing/grillage and
seafastening loads, see [11.9.5].
11.9.8.2 Any additional loadings caused by any overhang of the cargo over the side of the transport vessel,
buoyancy forces and wave slam loadings shall be included.
11.9.9 Securing of pipe and other tubular goods
11.9.9.1 This subsection [11.9.9] refers to the transport of tubulars, including line pipe, casing, drill pipe,
collars, piles, conductors, marine risers and similar, hereafter called 'pipes', on vessels and barges. Transport
of drill pipe, collars etc. on jack-ups is covered in [11.28.11]. The design of pipe supports and securing shall
consider the applicable loads according to [11.9.1].
11.9.9.2 The design friction coefficients shall be in accordance with [5.6.9] and should not exceed the
coefficients in Table 11-12. The friction coefficients in the guidance note to [11.9.2.11] may be disregarded
for pipe transports.
Table 11-12 Typical upper bound design friction coefficients for pipe stowage
Materials in contact
Friction coefficient
Concrete coated pipe - concrete coated pipe
0.5
Concrete coated pipe – timber
0.4
Timber – timber
0.4
Uncoated steel – timber
0.3
Polypropylene coated pipe - timber or rope dunnage
0.3
Polypropylene coated pipe - Polypropylene coated pipe
0.15
Uncoated steel - uncoated steel
0.1
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Epoxy coated pipe – timber
0.1
Epoxy coated pipe - epoxy coated pipe
0.05
11.9.9.3 Where sand can be present between the friction surfaces, the friction coefficient should be
considerably reduced.
11.9.9.4 Friction coefficients (both wet and dry) for other materials should be justified or the beneficial
effects of friction should be ignored.
11.9.9.5 For pipes stowed longitudinally on the vessel, the longitudinal minimum seafastening requirement
in [11.9.2.7] does not apply.
Guidance note:
For pipes stowed transversely, seafastening in the pipe longitudinal direction is required to provide the greater of the seafastening
capacity required by Table 11-9 and that determined from a motions analysis (from which the effects of friction have been
deducted).
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11.9.9.6 Where pipes are stacked in several layers, the maximum permissible stacking height shall be
established, in order to avoid overstress of the lower layers. Safe access shall be provided to minimise risk of
injuries to riggers.
Guidance note:
See API RP 5LW Recommended practice for transportation of line pipe on barges and marine vessels, /7/.
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11.9.9.7 Smaller diameter pipes such as drill pipe may be stacked without individual chocking arrangements
and restrained transversely by means of vertical stanchions. Timber dunnage or wedges shall be used to
chock off any clearance between the pipes and the stanchions. The stanchions, taken collectively, shall be
capable of resisting the total transverse force computed.
a)
b)
For weather restricted operations, and 24-hour or location moves of jack-ups, the stack may be secured
by means of transverse chain or wire lashings over the top, adequately tensioned. Provided it can be
demonstrated that sufficient friction exists to prevent longitudinal movement, no end stops need be
provided.
For weather unrestricted operations, including voyages of jack-ups, steel strongbacks should be fitted
over the top layer, and each stow (group of pipes) set up hard by driving wooden wedges between the
strongbacks and the top layer of pipe. End stops or bulkheads shall be provided.
11.9.9.8 Line pipe on pipe carrier vessels can be stacked between the existing stanchions/crash barriers, on
the wooden sheathed deck. Timber dunnage or wedges should be used to chock off any clearance between
the pipes and the stanchions.
a)
b)
For weather restricted operations, provided it can be demonstrated that adequate friction exists to
prevent longitudinal movement, no end stops need be provided.
For weather unrestricted operations, steel strongbacks should be fitted over the top layer, and each stow
set up hard by driving wooden wedges between the strongbacks and the top layer of pipe. End stops or
bulkheads shall be provided.
Guidance note:
This is likely to apply to concrete coated pipe, but uncoated or epoxy coated pipe should be treated with caution.
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11.9.9.9 Larger diameter pipes (e.g. piles) should be individually seafastened and/or supported against
rolling and transverse sliding. Seafastening in the pile longitudinal direction should normally be provided.
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Guidance note:
Longitudinal seafastening of pipes stowed longitudinally may be omitted if adequate restraint due to friction is documented. Such
documentation should include:
—
Load calculations, see [11.9.1].
—
Applied friction coefficients, see [11.9.9.2].
—
Calculation/evaluation of the effect of transport vessel hull beam bending on horizontal support forces.
The following may be considered to mitigate vessel hull beam bending effects:
—
Additional vertical load (and hence friction) due to sufficiently pretensioned top lashings. Applied pretension should be ensured
—
If the pipes can move longitudinally (due to extreme loading/vessel bending) then the choking effect due to top lashings
throughout the voyage, see [11.14.11.3].
becoming inclined may be considered provided that the pipe to lashing friction and lashing strength are documented and
achieve the required securing effect.
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11.9.9.10 In all cases for the transport of coated line pipe, the transport and securing arrangements
shall be designed so that the coating will be protected from damage. The manufacturer’s and/or shipper’s
recommendations should be followed.
11.9.9.11 Where end stops are provided for pipes with prepared ends, the end preparation shall be
protected.
Guidance note:
Protection could be either by protectors on the pipe, or by wood sheathing on the end stops.
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11.9.9.12 When open ended pipes are carried as deck cargo and the pipes could become partially filled with
water, care should be taken to ensure that:
a)
b)
the vessel’s stability shall meet the requirements of [11.10] with including the effects of entrapped
water, and
the deck and pipe layers shall not be overstressed.
Guidance note:
Where the requirements are not met a possible solution is to seal the pipe ends of at least the lowest level of the stack.
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11.9.9.13 Where the trim and stability booklet includes suitable example loading conditions these should be
considered.
11.9.10 Inspection of welding and seafastenings
11.9.10.1 Principal seafastening welds shall be visually checked and the weld sizes confirmed against the
agreed design.
11.9.10.2 Non-destructive testing (NDT) shall be carried out on the structural members of the
seafastenings. Specific requirements for weld inspection are given in [5.10.2.2].
11.9.10.3 Any faulty welds discovered shall be removed or repaired in accordance with a qualified weld
repair procedure and qualified welders and re-tested.
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11.9.11 Use of second hand steel seafastenings
11.9.11.1 When second hand steel seafastenings are used, any wastage caused during previous removal(s)
or use should not affect its fitness for purpose. There should be sufficient documentation to ensure the
traceability of the steel and in particular documentation relating to the grade of steel.
11.9.11.2 There should be NDT inspection reports to demonstrate no cracking or lamellar tearing in critical
areas.
Guidance note:
Areas to consider included regions of previous fabrication, old welds, burnt off attachments etc.,
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11.9.11.3 Should sufficient documentation of the type of steel (e.g. EN10025) be unavailable, coupon
testing is acceptable to determine the steel type. The guaranteed minimum properties of this type of steel
shall be used.
Guidance note:
Tested values should not be used as they may not be representative of the rest of the steel.
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11.9.12 Fatigue
11.9.12.1 See [5.9.4] for requirements for fatigue analysis.
11.9.12.2 The FLS design waves (and wind) should be carefully selected based on a 'worst case scenario'
regarding weather conditions during the voyage.
Guidance note:
For calculating the maximum expected fatigue damage for a voyage it is recommended that weather conditions are selected that
do not have more than 10% probability of being exceeded with regards to cumulative fatigue damage.
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11.9.12.3 A reasonably conservative exposure time should be selected for calculating the maximum
expected transport fatigue damage.
Guidance note:
The following exposure times should normally be considered:
—
For transports from one sheltered location to another: 1.5 x TPOP ; when TPOP exceeds 30 days, TPOP + 15 days can be
—
For transports to offshore (wave exposed) location ample time should be added to account for the maximum expected waiting
considered.
time, including possible return(s) to an inshore holding location.
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11.9.12.4 Fatigue damage should be calculated for representative sea state directions relative to the vessel.
The spacing between analysed wave headings should not exceed 45°. Symmetry may be considered.
11.9.12.5 The most probable (percentage) exposure time for each sea state direction relative to the vessel
should be selected for calculating the maximum expected transport fatigue damage.
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Guidance note:
For transports with sailing routes for which there are no pre-dominant sea state directions relative to the vessel the exposure time
and analysed directions can be selected according to the below table. Where applicable, symmetry can be considered to reduce
number of load cases/directions.
Sea direction
Representing
range
Head
H port Q
Port beam
S port Q
Stern
S stbd Q.
Stbd beam
H, stbd Q
337.5-22.5
22.5-67.5
67.5-112.5
112.5-157.5
157.5-202.5
202.5-247.5
247.5-292.5
292.5-337.5
0
45
90
135
180
225
270
315
10
15
15
10
10
10
15
15
Analysed
direction
Exposure in %
Where H and S denote head seas and stern seas respectively and Q denotes quartering (45°) seas.
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11.9.12.6 For fatigue critical transports, it is recommended to maintain control of the (anticipated) fatigue
damage during the transport. This is especially important if the assumed stress range distribution could be
unconservative.
Guidance note:
Fatigue damage could be controlled by regular inspections and/or by verifying that the actual fatigue stress range is less critical
than the stress range applied in the calculations. The stress range could be controlled by setting up systems that compare the
actual to the applied:
—
exposure time
—
wave scatter diagram considering relative vessel/sea directions
—
vessel motions, e.g. calculated vs MRU readings
—
member loads/stresses.
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11.9.12.7 Whenever relevant, mitigation actions to avoid excessive transport fatigue shall be defined.
Guidance note:
Potential mitigation actions include: heading control and/or weather routeing. Regular inspections combined with repair possibilities
could/should also be considered.
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11.9.13 Vortex shedding
All voyages should be checked for wind-induced vortex induced vibration (VIV), see [5.6.7.4]. Where the
potential is identified, mitigating measures shall be taken.
Guidance note:
Typical items that can be susceptible include:
—
slender members in jackets that will be submerged in the in-place condition and which are therefore not checked for in-place
—
single-tube jack-up legs (which can be fitted with spoilers to prevent VIV).
VIV, or
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11.9.14 Condition of unclassed tows
Special cases can be considered for the towage of vessels with a load line exemption certificate or for objects
with no classification such as caissons and vessels with expired classification such as a demolition towage. In
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such special cases the object shall be in seaworthy condition, and therefore an inspection shall be carried out
in order to verify if the structural strength and watertight integrity of the tow is approvable for the intended
voyage. As such, the MWS company can require one or more of the following:
a)
b)
c)
d)
e)
An extended, in depth, survey of the vessel structure involving one or more specialist surveyor(s).
Facilities for a close-up survey of inaccessible parts of the hull structure may be required.
Thickness determination (gauging) of specified areas of the vessel structure. This survey may be in
limited areas or extend over large parts of the hull structure. Such surveys shall be carried out by a
reputable independent company. An existing survey report may be acceptable provided that it is not
more than 1 year old, and there is no evidence of damage or significant deterioration since that date.
A MWS company review of classification society approved scantling drawings.
Calculations to show that the structural strength of particular local areas of the vessel is adequate. The
extent of the calculation required to be determined by the results of the surveys and drawings review.
A dry dock survey of the vessel can be necessary should there be any doubt as to the condition of the
tow.
11.10 Floating stability
11.10.1 General
11.10.1.1 The requirements in this subsection [11.10] are applicable to the following:
— Classed vessels
— Jack-ups (self-elevating units)
— Column stabilised units
— Barges and pontoons
— Self-floating structures.
11.10.1.2 Requirements shall be agreed with the MWS company for objects which:
— do not fall into the categories shown in [11.10.1.1] or,
— are non-symmetrical or,
— have an initial heel or trim which is not close to 0º.
11.10.1.3 In areas and seasons prone to icing, the effects of icing on stability shall be considered and the
requirements in [11.19.28] shall additionally apply.
11.10.1.4 Both intact and damage stability shall be documented with the required wind applied from the
most onerous directions.
11.10.1.5 Free-trimming stability programs can give misleading results unless 'free-twist' or equivalent is
used. Two examples when this may occur are: (i) where hull geometry causes significant trim relative to the
heel angle, such as, in the case of a triangular shaped jack-up; (ii) where there is an asymmetric waterplane
area, such as, during off-load of a floating cargo aligned obliquely to the longitudinal axis of a transport
vessel. In such cases, fixed trim can be used for stability calculations provided a sufficient number of axes
of rotation (azimuth) are considered, thereby allowing the minimum GZ to be identified for each azimuth/
heel angle combination. Consideration should be given to the selection of the range and increment in azimuth
angle by refining the increment until no significant changes in results are apparent.
11.10.1.6 Whether using fixed or free trim, metacentric height (GM) should be calculated from the initial
slope of the righting arm (GZ) curve rather than from the second moment of waterplane area.
11.10.1.7 The effects of free surface shall be considered in all stability calculations. These shall include:
a)
The effects of free surface liquids in unit and cargo.
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b)
c)
Residual free surface due to incomplete venting, such as can occur if ballasting when trimmed.
Any air cushion effect from air trapped or introduced below any part of the hull which produces additional
buoyancy. The air cushion effect is in addition to the free surface effect from all standard closed tanks. It
reduces stability due to the compressibility of the air.
11.10.1.8 The lightship data used in the stability calculations shall accurately reflect the current status of
the unit.
Guidance note 1:
It is common practice to maintain a lightship alteration log to record minor iterations to light ships with modification/mutations
over a period of time from previous light ship survey. The weight and position of additions or removals in excess of 100 kg (220 lb)
should be recorded in the log. The details would typically include:
—
Date the modification was made
—
A description of the item
—
Weight (positive value for weight addition, negative value for removal)
—
Vertical centre of gravity (VCG)
—
Longitudinal centre of gravity (LCG)
—
Transverse centre of gravity (TCG)
—
Reference to modification, project or approval number as applicable.
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Guidance note 2:
Recommendations regarding the floating stability of ships engaged in lifting operations can be found in /89/ MSC 97/22/Add.1,
Annex 7, Section 2.9.
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11.10.1.9 All variable loads, including any variation that will impact the final loads during passage/departure
and arrival conditions, shall be included in the stability calculations.
Guidance note:
For example, addition or removal of mooring chain will impact the loads during passage/departure and arrival conditions. This can
be particularly significant for semi-submersibles, TLPs and GBSs.
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11.10.1.10 Where there is a significant difference between the departure, arrival and any intermediate
loading condition, then the most severe should be considered, including the effects of any ballast water
changes during the voyage.
11.10.1.11 Cargo overhangs shall generally not immerse as a result of inclination from a 15 m/s wind in still
water conditions, for ice areas, see [11.19.28.3].
11.10.1.12 If buoyancy of the cargo has been included to meet intact stability requirements, then loss of
cargo buoyancy or flooding of cargo compartments, shall be considered as a damage case, as appropriate.
In such cases and subject to [11.10.1.11], [5.6.2.5] and [11.19.24.2] (for ice areas), buoyant cargo
overhangs may be assumed to contribute to the range of stability requirement provided the cargo is securely
fastened, see [5.6.2.5.5].
11.10.1.13 The initial apparent metacentric height, GM0, shall be greater than 0.15 m and should be greater
than 1.0 m. The calculation of GM0 shall include adequate margins for computational and other inaccuracies.
Guidance note:
Verification of the calculated GM by means of an inclining experiment is likely to be required by the MWS when there is any doubt
or when it is close to the lower limit. The actual cargo should be considered. See also [2.10.5].
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11.10.1.14 The requirements to damage stability shall be evaluated considering the procedures,
environmental loads and responses, duration, consequences of possible damage, etc.
11.10.1.15 Attention shall be paid to ingress of water caused by e.g.:
— impact loads from vessels, dropped objects, etc.,
— mechanical system failure,
— accumulation of spray water,
— operational errors and
— deteriorating weather conditions.
11.10.1.16 The consequences of the escape of air from any air cushion shall be evaluated where applicable.
11.10.1.17 Loose solid ballast shall be prevented from moving and, where submerged, shall be protected
against dispersal by wave or current or towing action.
11.10.1.18 The extent and adequacy of the precautions against damage necessary for a particular towage
shall be assessed on a case-by-case basis.
11.10.1.19 Transports on multiple vessels
11.10.1.19.1 Multi-vessel combinations should meet both the intact and damage stability requirements for
barges and pontoons ([11.10.5]) and have a connection system that permits them to be considered as one
vessel. The connection system shall meet the requirements of Sec.5.
11.10.1.19.2 When cargo is transported on multiple vessels it shall be demonstrated that the flooding of any
one compartment of any vessel cannot cause the damaged vessel to change its heeling or trim angle relative
to the overall heeling or trim of the combined vessel assembly. In other words, the damaged vessel should
not pivot around any of the reaction points between it and the cargo or between it and another vessel, thus
losing contact at another reaction point.
11.10.2 Classed vessels
11.10.2.1 The MWS company will generally accept the stability of ships and MOUs when they are operated
within the limits accepted for class by a recognized classification society.
11.10.2.2 Classed vessels shall either:
— comply with the mandatory parts of the IMO Intact Stability Code 2008, /89/, and the IMO International
Convention on Load Lines, Consolidated Edition 2005 /90/ and their approved documentation or,
— for any voyages where these requirements cannot be fulfilled without impractical modifications, the
stability criteria shall be agreed with the MWS company at an early stage and the vessel shall have
an exemption from the classification society and flag state to use these criteria for the voyage being
considered.
Guidance note:
As an alternative to the IMO requirements, the MWS company would normally accept the criteria in [11.10.5].
Class and flag state should be consulted in use of alternative stability criteria prior to the voyage.
The underwriters should be informed of the use of alternative stability criteria and the class and flag state position on the use of
this.
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11.10.2.3 The approved stability calculations shall be presented upon request prior to the operation.
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11.10.2.4 For vessels whose stability is in accordance with the IMO Code for the Construction and
Equipment of Mobile Offshore Drilling Units, /140/, the most recent version (including all amendments) shall
apply.
11.10.3 Jack-ups (self-elevating units)
11.10.3.1 All jack-ups shall meet the requirements in the latest version of the IMO Code for the
Construction and Equipment of Mobile Offshore Drilling Units, /140/, or DNV-OS-C301, /141/.
Guidance note:
For jack-ups, the requirements in [11.28.6] also apply.
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11.10.3.2 Where a mat is fitted, the damage shall generally be assumed for either hull or mat. Simultaneous
damage shall be assumed if any part of the mat is within 1.5 m of the waterline or upper hull and the mat
extends less than 1.5 m horizontally outside the upper hull.
11.10.4 Column stabilised units
All column stabilised units shall meet the requirements in the latest version of the IMO Code for the
Construction and Equipment of Mobile Offshore Drilling Units, /140/, or DNV-OS-C301, /141/.
Guidance note:
For semi-submersibles, the requirements in [11.31.3] also apply.
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11.10.5 Barges and pontoons
11.10.5.1 In addition to any class and flag state requirements the following should normally be met for
barges and pontoons.
11.10.5.2 The wind velocity used for intact stability calculations shall be the 1-minute design wind speed, as
described in [3.2]. In the absence of other data, for the intact condition a wind speed of 52 m/s (100 knots)
may be used. The wind velocity used for stability calculations in the damage condition shall be 26 m/s (50
knot) or the wind speed used for the intact calculation if less. It shall be applied in the most critical direction.
Guidance note:
See also [C.2.1.2] for the Norwegian continental shelf.
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11.10.5.3 The intact range of stability, about any horizontal axis, defined as the range between 0°
inclination and the smallest angle at which the righting arm (GZ) becomes negative shall not be less than
either:
— the values shown in Table 11-13 or
— the range calculated in accordance with [11.10.5.4].
Guidance note:
When using the default motion criteria in [11.7] the values in Table 11-13 apply.
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Table 11-13 Intact stability range
Vessel or towed object, type and size
Large cargo barges, LOA > 76 m and B
Small cargo barges, LOA < 76 m or B
1)
1)
> 23 m
Intact range
36º
< 23 m
40º
Vessels and barges in inland and sheltered water (in ice areas)
36º
Vessels and barges in inland and sheltered water (out of ice areas)
24º
1)
B = maximum moulded waterline beam.
11.10.5.4 If maximum amplitudes of motion for a specific towage or voyage can be derived from model
tests or motion response calculations, the intact range of stability shall be not less than:
where GM is in metres and θ is the maximum amplitude of roll or pitch caused by the design sea state as
defined in [3.2], plus the static wind heel or trim angle caused by the design wind, in degrees.
11.10.5.5 When assessing the range of stability, downflooding does not need to be taken into account
provided that the watertight and weathertight requirements of [11.10.7] are fulfilled.
11.10.5.6 For the intact condition, the area under the righting moment curve shall not be less than 40%
in excess of the area under the wind overturning arm curve using the wind speed in [11.10.5.2]. The areas
shall be bounded by 0º inclination, and the dynamic angle (defined as the angle at which this condition is
met). The dynamic angle shall, as shown in Figure 11-2, be less than the least of:
— the second intercept of the two curves
— the downflooding angle
— the angle at which overloading of a structural member, including seafastening and grillage, occurs.
Guidance note:
See [11.10.7] for requirements of fitting watertight and weathertight closures to openings.
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Figure 11-2 Wind overturning criteria (intact case)
11.10.5.7 Damage stability evaluations shall be based on damage scenarios according to identified
contingency situations, see [11.10.1]. Collision, leakage and operational failure situations shall be evaluated
Guidance note:
As a minimum the barge should have an acceptable stability and reserve buoyancy and remain floating in an acceptable manner
with any one submerged or partly submerged compartment flooded.
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11.10.5.8 In addition to any damage scenarios identified in [11.10.5.7] barges shall meet the requirements
in [11.10.5.9] to [11.10.5.11] with damage caused by an assumed minimum penetration of 1.5 m from any
external plating, between effective watertight bulkheads, with the following:
a)
b)
c)
d)
e)
All piping and ventilation systems within the 1.5 m penetration in damaged compartments shall be
assumed damaged. Positive means of closure shall be provided to preclude the progressive flooding of
other spaces which are intended to be intact.
Damage shall be assumed to extend from the baseline upwards without limit.
The distance between effective watertight bulkheads or their nearest stepped positions which are
positioned within the assumed extent of horizontal penetration should not be less than 3 m; where there
is a lesser distance, one or more of the adjacent bulkheads shall be disregarded.
The consequences of water ballast escaping from any compartments above the waterline, or the escape
of air from any air cushion shall be evaluated where applicable.
Where damage of a lesser extent than in [a)] to [d)] results in a more severe condition such lesser
extent shall be assumed.
11.10.5.9 In all damage scenarios, the acceptable floating condition is determined by the following:
— the design resistance of any part of the barge, cargo seafastening or grillage should not be exceeded.
— the barge should have sufficient freeboard considering environmental effects to any open compartment,
where flooding may occur.
11.10.5.10 In accordance with [11.10.5.9], the barge should have sufficient reserve stability in a damaged
condition to withstand the wind heeling moment using the wind speed in [11.10.5.2] applied from any
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direction and the damage as described in [11.10.5.8]. In this condition the final waterline, after flooding and
wind heel, should be below the lower edge of any downflooding opening as shown in Figure 11-3.
Figure 11-3 Damage stability
11.10.5.11 In all damage scenarios, the area under the righting moment curve should not be less than 40%
in excess of the area under the wind overturning arm curve, using the lower of a 26 m/s (50 knots) wind
speed or 1-minute design wind speed, as described in [3.2], see [11.10.5.2]. The areas shall be bounded
by angle of inclination after damage, and the dynamic angle (defined as the angle at which this condition is
met). The dynamic angle shall be less than the least of:
— the second intercept of the two curves
— the downflooding angle, see Figure 11-4.
Figure 11-4 Damage stability area ratio requirement
11.10.6 Self-floating structures
11.10.6.1 This subsection applies to objects such as gravity base structures, jackets, offshore towers, etc.
which are supported by their own buoyancy.
11.10.6.2 Self-floating structures in ice-affected areas should comply with [11.19.28].
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11.10.6.3 For gravity base structures the requirements in [6.2] shall apply.
11.10.6.4 For other self-floating structures the requirements shall be agreed with the MWS company and the
requirements in [11.10.5] would normally apply.
11.10.6.5 For self-floating structures where one-compartment damage stability is not achievable without
impractical design changes, it may be permitted to omit this, provided the towage is a one-off towage
of short duration, carried out under controlled conditions, and suitable precautions are taken, which can
include:
— Areas vulnerable to collision should be reinforced or fendered to withstand collision from the largest
towing or attending vessel, at a speed of 2 m/s.
— Projecting hatches, pipework and valves are protected against collision or damage from towing and
handling lines.
— Emergency towlines are provided, with trailing pick-up lines, to minimise the need for vessels to approach
the structure closely during the tow.
— Emergency pumping equipment is provided.
— Potential leaks via ballast or other systems are minimised.
— Ballast intakes and discharges, and any other penetrations through the skin of the vessel or object, shall
be protected by a double barrier system, or blanked off.
— Vulnerable areas are conspicuously marked and masters of all towing and attending vessels are aware of
the vulnerable areas.
— A guard vessel is available to warn off other approaching vessels.
— A risk assessment in accordance with [2.4] shall be carried out.
Guidance note:
The follow are examples of structures where it may be permitted to omit damage stability:
—
Concrete gravity based structures, particularly when towing on the columns
—
Submerged tube tunnel sections
—
Bridge pier caissons
—
Outfall or water intake caissons
—
Monopiles, transition pieces (TPs) and suction bases for wind farm foundations.
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11.10.7 Compartmentation and watertight integrity
11.10.7.1 During towing, all watertight doors and openings on and underdeck on both the tug(s) and tow
shall be closed at all times. Where vessels are fitted with remote indication of watertight door position, this
shall be confirmed as operational.
11.10.7.2 The towed asset and tug(s) should have a systematic programme for the assurance that such
openings are closed prior to and throughout towing operations, and these arrangements referenced as
necessary in the tow plan.
11.10.7.3 Watertight closing appliances in operable condition shall be fitted on all external openings being
submerged at least up to an angle of heel equal to the greater of the first intercept in either the intact or any
damage condition with wind applied in the most onerous directions.
11.10.7.4 Weathertight closing appliances in operable condition shall be fitted to all external openings that
are not required to be watertight by [11.10.7.3] and are either:
— below the static intact waterline at the dynamic angle (the smallest angle at which the area ratio in Figure
11-2 is satisfied), or
— less than 4 m above all required static one-compartment-damaged waterlines.
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All horizontal axes should be considered with the wind applied in the most onerous direction for each case.
11.10.7.5 Where the watertight integrity of any tow is in question, particularly for demolition tows, part built
ships and MOU’s, it shall be checked by visual inspection, chalk test, ultrasonic test, hose test or air test as
considered appropriate by the attending MWS company surveyor.
11.10.7.6 Hatches, ventilators, gooseneck air pipes and sounding pipes shall be carefully checked for proper
closure and their watertight or weathertight integrity confirmed. Where such equipment could be damaged by
sea action or movement of loose equipment, then additional precautions shall be considered.
11.10.7.7 Outboard accommodation doors shall be carefully checked for proper closure and their watertight
or weathertight integrity confirmed. All dogs shall be in good operating condition and seals shall be
functioning correctly.
11.10.7.8 Watertight doors in holds, tween decks and engine room bulkheads, including shaft alleyway and
boiler room spaces, shall be checked for condition and securely closed.
11.10.7.9 Any watertight doors required to be opened for access during the voyage, shall be marked, on
both sides, 'To be kept closed except for access' or words to that effect. In some cases a length of bar or pipe
can be required to assist opening and closing.
11.10.7.10 Portholes shall be checked watertight. Porthole deadlights shall be closed where fitted. Any
opening without deadlights that can suffer damage in a seaway shall be plated over.
11.10.7.11 Windows which could be exposed to wave action shall be plated over, or similarly protected.
11.10.7.12 All tank top and deck manhole covers and their gaskets shall be in place, checked in good
condition, and securely bolted down.
11.10.7.13 All overboard valves shall be closed and locked with wire or chain. Where secondary or back-up
valves are fitted for double protection, they shall also be closed.
11.10.7.14 Closure devices fitted to sanitary discharge pipes, particularly near the waterline, shall be
closed. Any discharge pipe close to the waterline not fitted with a closure device, can need such a facility
incorporated, or be plated over.
11.10.7.15 All holds, void spaces and engine room bilges shall be checked before departure and should be
pumped dry.
11.10.7.16 All other spaces shall be sounded before departure. It is recommended that all spaces should be
either pressed up or empty. Slack tanks should be kept to a minimum.
11.10.8 Draught and trim
11.10.8.1 For vessels and barges with a load line certificate, the draught shall not normally exceed the
appropriate load line draught, without flag state exemption.
11.10.8.2 The draught should be small enough to give adequate freeboard and stability, and large enough
to reduce motions and slamming. Typically, for barge towages it will be between 35% and 60% of hull depth,
which is usually significantly less than the load line draught.
11.10.8.3 For barges and large towed objects, such as FSUs, the draught and trim should be selected to
minimise slamming under the forefoot, to give good directional control, and to allow for the forward trim
caused by towline pull.
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11.10.8.4 For guidance, and for discussion with the master of the tug, the tow should be ballasted to the
minimum draughts and trims for barges in Table 11-14.
Table 11-14 Minimum recommended draught and trim for barges
Length of towed vessel
Minimum draught forward
Minimum trim by stern
30 m
1.0 m
0.3 m
60 m
1.7 m
0.6 m
90 m
2.4 m
0.8 m
120 m
3.1 m
1.0 m
150 m
3.7 m
1.2 m
200 m plus
4.0 m
1.5 m
11.10.8.5 Where barges with faired sterns are fitted with directional stabilising skegs, it can be preferable to
have no trim. However allowance should be made for trim caused by the towline force and there should be
adequate freeboard at the bow (and possibly a breakwater) to minimise damage from 'green water' coming
over the bow.
11.10.8.6 For towed ship-shaped units (where LOA is the overall length of the unit in metres) the forward
draught should be greater than:
for LOA≥200 m 2.0 m+ 0.015 x LOA
for LOA <200 m as for barges in Table 11-14
but in both cases the mean draught shall not be less than the minimum class approved ballast draught.
Slamming pressure under the forefoot estimated for the metocean criteria for the tow route shall be less
than the bottom design pressure. For directional stability, a minimum aft trim of 0.75% of LOA is normally
recommended.
11.10.8.7 Draught should be carefully selected for FSU’s etc. that will have deeper in-operation draughts
than for towage. This can give higher accelerations in the installed modules etc. when under tow.
11.10.8.8 It can be preferable to tow structures such as floating docks at minimum draught with zero trim,
in order to minimise longitudinal bending moments.
11.10.8.9 Draught marks forward and aft shall be easily readable and, if necessary, re-painted in the area
above the waterline.
11.10.8.10 Where the tow is un-manned, and in order that the tug can monitor any increased draught
during the towage, a broad distinctive line of contrasting colour should be painted around the bow
approximately 0.5 m above the waterline.
11.11 Transport vessel or barge selection
In addition to the requirements in [2.11], the transport barge or vessel selection, including identification of
any necessary repairs or upgrades, should be undertaken considering the following:
a)
b)
c)
There shall be adequate deck space for all the cargo items planned, including room for seafastenings,
access between cargo items, access to towing and emergency equipment, access to tank manholes,
installation of cargo protection breakwaters if needed, and for lifting offshore if required.
The barge or vessel shall have adequate intact and damage stability with the cargo and ballast as
planned, including any requirement for ballast water exchange.
The barge or vessel as loaded shall have sufficient freeboard to give reasonable protection to the cargo.
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d)
If a floating load-out is planned, there shall be sufficient water depth to access and leave the load-out
berth and for the load-out to be carried out in accordance with Sec.10.
e) If a submerged load-out is planned, the barge or vessel can be submerged, within its class limitation, so
as to give adequate clearance over the deck, and adequate stability at all stages, within the water depth
limitations of the load-out location.
f) There shall be adequate pumping capacity to comply with [11.15], or be suitable for the use of additional
pumping equipment.
g) Submersible barges: Barges that can be totally immersed in the intact condition should be classed as
submersible barges. Submersible barges are normally classed as such by a RCS (recognized classification
society).
h) The deck strength shall be adequate, including stiffener, frame and bulkhead spacing and capacity, for
load-out and transport loads.
i) For a vessel, securing of seafastenings shall not need welding in way of fuel tanks.
j) For a barge, it shall be properly equipped with main and emergency towing connections, recovery gear,
pumping equipment, mooring equipment, anchors, lighting and access ladders.
k) The motion responses as calculated shall not cause overstress of the cargo.
l) All required equipment and machinery shall be in sound condition and operating correctly.
m) The barge or vessel shall possess the relevant, in date, documentation as set out in Table B-2.
n) Unclassed barges shall be subject to appropriate project-specific structural, equipment and machinery
checks. They shall have a valid load line, or load line exemption, certificate.
Guidance note:
To ensure compliance with [2.11], the charterer is advised to have a suitability survey and an on-hire survey of the barge or vessel
carried out before acceptance of the charter.
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11.12 Tug selection
11.12.1 General
11.12.1.1 The tug(s) selected should comply with the minimum bollard pull requirements shown in
[11.12.2], and should also comply with the appropriate category in Table 11-15. The appropriate category
should be agreed with the MWS company.
Table 11-15 Tug categories
Category
ST – salvage tug
Used for
U - unrestricted
Single tug towages in benign or non-benign weather areas.
They shall have very good seakeeping qualities including good propeller immersion in bad
weather. These qualities are unlikely to be satisfied with a length over all (LOA) less than
40 m and a displacement of less than 1,000 tonnes.
C - coastal
Towages in benign weather areas or staged tows.
R1 - restricted
Assisting in multi-tug towages.
R2 - restricted
Benign weather area towages.
R3 - restricted
Assisting in multi-tug towages in benign weather areas.
11.12.1.2 Vessels in all categories shall be of such a design to allow them to operate safely and effectively
in their designated areas and shall be purpose-built for towing operations or be of a multi-purpose design
having towing capability.
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11.12.1.3 The length and normal operating draught of the vessel shall be adequate to maintain propeller
effectiveness and reduce slamming in heavy weather conditions.
11.12.1.4 Vessels in category ST, U, C and R1 shall have a raised forecastle with a height of at least 2 m
above the freeboard deck. The forecastle shall be of such a design to ensure minimum water retention.
11.12.1.5 The tug(s) used for any towage to be approved by the MWS company should be inspected
by a MWS company surveyor before the start of the towage. The survey shall cover the suitability of the
vessel for the proposed operation, its seakeeping capability, general condition, documentation (including ice
classification if applicable), towing equipment, manning and fuel requirements.
11.12.1.6 Where the tug does not have a bollard pull test certificate giving the static continuous bollard pull,
issued or endorsed within the last 10 years by a body approved by a recognized classification society or other
certification body accepted by the MWS company, then it can be calculated as follows:
1)
2)
for tugs under 10 years old without a bollard pull certificate, the bollard pull can be estimated as 1
tonne/100 (Certified) BHP (Brake Horsepower) of the main engines. Ice-breaking tugs can be less than
this and the MWS company should be consulted.
for tugs over 10 years old, without a bollard pull certificate less than 10 years old, can be the greater of:
— the certified value reduced by 1% per year of age since the BP test, or
— the value from 1) above reduced by 1% per year of age greater than 10.
11.12.1.7 Additional tug(s) can be required for high value tows or towages through areas with limited sea
room or through high density traffic zones, to carry out the following duties:
— Act as a guardship, to protect the tow, and advise approaching vessels that they can be running into
danger
— In the event of mechanical failure or towline breakage, assist in removing the failed tug from the
towing spread. In this case it is desirable for all the main tugs to have towing connections forward and
appropriate rigging deployed. See [11.18.7.5] for procedure for tug breakdowns in multi-tug tows.
— Take over the duties of the failed tug
— Provide any other required assistance in an emergency.
11.12.2 Bollard pull requirements
11.12.2.1 The minimum towline pull required (TPR) shall be calculated based on the towing route (including
available sea room) and procedures. In all cases, an assessment should be made that a reasonable speed
can be achieved in moderate weather. The TPR calculations shall include the wind, wave drift and current (i.e.
relative speed between towed object and water) forces and should be calculated for all relevant combinations
of wind, wave and current directions relative to the towed object.
Guidance note:
Further guidance may be found in DNV-RP-N103 Sec.7, /56/.
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11.12.2.2 Table 11-16 summarises the different conditions to be considered. The conditions are described in
more detail in the indicated sections.
Table 11-16 Meteorological criteria for calculating TPR (towline pull required)
Section
Condition
Hs [m]
Wind [m/sec]
Current [m/sec]
[11.12.2.3]
Standard
5
20
0.5
[11.12.2.3]
Benign weather areas
As agreed with the MWS company but not less than:
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[11.12.2.4]
Limited sea room
[11.12.2.5]
Weather restricted towage
[11.12.2.6]
< 24 hour staged tow or
< 24 hour jack-up move
[11.12.2.7]
Sheltered from waves
2
15
0.5
Design
Design
(1 hour mean)
0.5 or predicted
current if greater
To document that the tow will maintain forward speed
under all possible weather conditions and to render
probable that the tow can be carried out within TPOP.
3
15
0.5 or predicted
current if greater
As agreed with the MWS company.
11.12.2.3 For weather unrestricted towages where adequate sea room can be achieved within the departure
weather forecast and maintained thereafter, the TPR shall be computed for zero forward speed against the
following acting simultaneously:
1)
For non-benign areas:
— 5.0 m significant sea state, and
— 20 m/s wind, and
— 0.5 m/s current, or the one year return period current if greater.
2)
For benign weather areas, the criteria for calculation of TPR shall be agreed with the MWS company and
generally these should not be less than:
— 2.0 m significant sea state, and
— 15 m/s wind, and
— 0.5 m/s current.
Guidance note:
If the tow route passes through an area of continuous adverse current or weather, a greater TPR may be required and agreed with
the MWS company.
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11.12.2.4 For towages which pass through an area of restricted navigation or manoeuvrability, outside the
validity of the departure weather forecast and which cannot be considered a weather restricted operation,
the Towline Pull Required (TPR) should be computed for zero forward speed against the following acting
simultaneously:
— the design wave height see [3.4.8] (but such towages should not be attempted if the design wave is more
than 5 m significant), and
— 1 hour design wind speed (see [3.4.6]), and
— 0.5 m/s current, or the one year return period current if greater.
11.12.2.5 For weather restricted towages, the TPR shall be the greater of the TPRs calculated for the
following sets of environmental conditions:
1)
to document that the tow will maintain forward speed under all possible weather conditions:
— the OPLIM significant wave height, OPLIM < 5 m (i.e. such towages should not be attempted if the
design wave is more than 5 m significant)
— the ten-minute sustained wind velocity of 1.2 times the OPLIM wind speed and
— a one year return period current + 0.5 m/s.
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2)
to render probable that the tow can be carried out within TPOP:
— the OPWF significant wave height
— the ten-minute sustained wind velocity of the OPWF wind speed and
— a one year return period current + the required forward speed to obtain TPOP.
Guidance note 1:
Where weather restricted towages pass through an area of restricted navigation or manoeuvrability increases to the above
environmental conditions can be required.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The alpha factor to be applied is the same as used in the structural checks.
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11.12.2.6 For towages which are planned to take less than 24 hours (including jack-up moves and every
stage of a staged tow) and have adequate marine procedures, the following reduced criteria, acting
simultaneously, may be used for the calculation of TPR:
— 3.0 m significant sea state, and
— 15 m/s wind, and
— 0.5 m/s current, or the maximum predicted surface current if greater.
11.12.2.7 For towages partly sheltered from wave action, but exposed to strong winds, the criteria shall be
agreed with the MWS company.
11.12.2.8 The effective continuous static bollard pull (BP) of the tug(s) proposed shall be greater than or
equal to TPR as shown by:
where:
Teff
(BP × Teff)/100
Σ
= the tug efficiency in the sea conditions considered, %
= the contribution to TPR of each tug
= the aggregate of all tugs assumed to contribute.
11.12.2.9 Only those tugs connected so they are capable of pulling effectively in the forward direction shall
be assumed to contribute. Stern tugs shall be discounted from the calculation in [11.12.2.8].
11.12.2.10 Tug efficiency, Teff, depends on the size and configuration of the tug, the sea state considered
and the towing speed achieved. In the absence of alternative information, Teff can be estimated for good
ocean-going tugs according to the following equation:
where:
LOA = tug length overall in metres (using 45 m for LOA > 45 m)
BP = static continuous bollard pull in tonnes (with BP > 20 tonnes, and using 100 when BP >100 tonnes)
= significant wave height (with 1 m < Hs < 5 m).
Hs
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Guidance note:
Towing vessels will generally have very low efficiencies with Hs > 5 m since they should be protecting their towing gear. Towing
vessels with less sea-kindly characteristics will have significantly lower values of Teff in all sea states.
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11.12.2.11 These efficiencies given in [11.12.2.10] are shown graphically in Figure 11-5 for tugs of
LOA > 45 m in different significant wave heights up to 5 m.
Figure 11-5 Tug efficiencies in various wave heights (tug LOA ≥ 45 m)
11.12.2.12 The resulting effective bollard pull in different wave heights for tugs with LOA ≥ 45 m and
LOA = 20 m is shown in Figure 11-6.
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Figure 11-6 Effective bollard pull v wave heights for tug LOA = 20 m and ≥ 45 m
11.12.2.13 The curves for 20 m LOA tugs do not imply that they are approvable for towages in the given
wave heights but are shown to demonstrate the effect on assumed efficiency. See also [11.12.1.1].
11.12.2.14 Short towlines: The effect of backwash from the towed object shall be taken into account when
short towlines are used. See also [11.14.1].
Guidance note:
DNV-RP-N103 [7.2.8], /56/, shows how to estimate the effects of interaction between propeller thrust and the towed object with
short towlines.
In an extreme case (a very wide and deep towed structure) the backwash may be greater than the towline force. This effect has
been observed in tank testing and full scale operations.
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11.12.2.15 Shallow water: Allowance shall be made for the additional towing resistance in shallow water.
Guidance note:
For small under-keel clearances the Cd values will be increased (doubled for draft = 0.9 x water depth). See Fig. 20 of OCIMF
Guidelines and Recommendations for Safe Mooring of Large Ships at Piers & Sea Islands 1978, /131/, for details.
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11.12.3 Main and spare towing wires and towing connections
The main and spare towing wires, pennants and connections shall be in accordance with [11.13.3].
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11.12.4 Tailgates/stern rails
Where a towing tailgate or stern rail is fitted, the radius of the upper rail shall be at least 10 times the
diameter of the tug’s main towline, and adequately faired to prevent snagging.
11.12.5 Towline control
11.12.5.1 Where a towing pod is fitted, its strength shall be shown to be adequate for the forces it is likely
to encounter. It should be well faired and the inside and ends shall have a minimum radius of 10 times the
towline diameter.
11.12.5.2 Where no pod is fitted, the after deck should be fitted with a gog rope, mechanically operated and
capable of being adjusted from a remote station. If a gog rope arrangement is fitted then a spare shall be
carried. Where neither a towing pod nor gog rope is fitted, then an alternative means of centring the tow line
should be provided.
11.12.5.3 On square-sterned tugs, it is preferred that mechanically or hydraulically operated stops be fitted
near the aft end of the bulwarks, to prevent the towline slipping around the tug's quarter in heavy weather.
11.12.6 Workboat
A powered workboat shall be provided for emergency communication with and transfer to the tow, and shall
have adequate means for launching safely in a sea state associated with Beaufort Force 4 to 5, see DNVST-0111 Table 2-1 /22/. An inflatable or RIB can be acceptable provided it has flooring suitable for carriage
of emergency equipment, including the portable pumps in [11.12.10] to the tow.
11.12.7 Communication equipment
In addition to normal authorities’ requirements, the tug shall carry portable marine VHF and/or UHF radios,
for communication with the tow when tug personnel are placed on board for inspections or during an
emergency. Spare batteries and a means of recharging them shall be provided.
11.12.8 Navigational equipment
Tugs shall be provided with:
— all necessary navigational instruments and up-to-date charts (for which an IMO-approved electronic chart
display and information system (ECDIS) is acceptable), and
— publications that can be required on the particular towage, including information for possible diversion
ports and their approaches.
11.12.9 Searchlight
The tug shall be fitted with a searchlight to aid night operations and for use in illuminating the tow during
periods of emergency or malfunction of the prescribed navigation lights. The searchlight(s) should provide
illumination both forward and aft, thereby allowing the tug to approach the tow either bow or stern on.
11.12.10 Portable pump
On any tow outside coastal limits, the tug shall carry at least one portable pump, equipped with means of
suction and delivery and having a self-contained power unit with sufficient fuel for 12 hours usage at the
pump’s maximum rating. The pump shall be suitable for the requirements outlined in [11.15.2] to [11.15.4]
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but cannot be considered to be a substitute for the pump(s) required in [11.15.2] as it may be difficult to
deploy in bad weather. The methods and feasibility of deployment should be considered.
11.12.11 Additional equipment
11.12.11.1 Anti-chafe gear should be fitted as necessary. Particular attention should be paid to contact
between the towline and towing pods, tow bars and stern rail and any other sharp edges (e.g. in the gap
between hull and rollers) that could damage the towline.
11.12.11.2 All tugs should be equipped with burning and welding gear for use in emergency. If this is not
possible then burning and welding gear should be accessible on the tow or other tugs.
11.12.12 Bunkers and other consumables
11.12.12.1 The tug should carry fuel and other consumables including potable water, lubricating oil and
stores, for the anticipated duration of the towage, taking into account the area and season, plus a useable
reserve of at least 5 days’ supply (excluding any unpumpable). For tows likely to take more than 20 days the
reserve should be increased to 7 days.
11.12.12.2 If refuelling en-route is proposed, then suitable arrangements shall be made before the towage
starts, and included in the towing procedures (see [11.14.7]).
11.12.13 Tug manning and accommodation
11.12.13.1 Vessels in all categories shall be manned to meet the minimum requirements laid down by
statutory regulations or those required by state or port authorities.
11.12.13.2 Manning levels for vessels in all categories shall be subject to the requirements of a specific
towage.
11.12.13.3 Where vessels are required to undertake long duration towages, difficult towages or where
the tow is un-manned, they shall have adequate certified accommodation to enable manning levels to be
increased. Any increase in manning levels shall be subject to the limitations of the regulations relating to lifesaving appliances.
11.12.13.4 In addition, consideration shall be given to the fact that in an emergency situation, two or more
of the tug crew can need to board and remain on the tow for an extended period. This should be taken into
account when approving the manning level of a tug.
11.12.13.5 Category ST: To satisfy category ST, certified accommodation and life-saving appliances shall be
provided for a minimum of twelve (12) persons.
11.12.13.6 Vessels in category ST shall, when engaged in towing operations, carry a minimum of five (5)
certificated officers. These should be the master, two (2) deck officers and two (2) engineer officers.
11.12.13.7 Categories U, C and R1: To satisfy categories U, C and R1, certified accommodation and lifesaving appliances shall be provided for a minimum of eight (8) persons.
11.12.13.8 Vessels in categories U, C and R1 shall, when engaged in towing operations, carry a minimum of
four (4) certificated officers. These should be the master, one (1) deck officer and two (2) engineer officers.
11.12.13.9 Vessels in categories R2 and R3 shall, when engaged in towing operations, carry a minimum of
three (3) certificated officers. These should be the master, one (1) deck officer and one (1) engineer officer.
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11.13 Towing arrangements and equipment
11.13.1 General
11.13.1.1 Towage should normally be from the forward end of the barge or tow via a suitable bridle as
shown in [K.1]. The components of the system are:
— Towline connections, including towline connection points, fairleads, bridle legs and bridle apex
— Intermediate pennant
— Bridle recovery system
— Emergency towing gear, see [11.13.13].
11.13.1.2 Where there is a case for towing an object or vessels by the stern, the decision should be based
on the results of a risk assessment in accordance with [2.4].
Guidance note:
The following could be favourable to tow by the stern:
—
Part-built or damaged ships, or any structure when the bow sections could be vulnerable to wave damage.
—
Part-built ships, converted ships or FPSOs without a rudder or skeg, or with a turret or spider fitted forward, where better
—
Any structure with overhanging or vulnerable equipment near the bow, which could be vulnerable to wave damage, or could
directional stability can be obtained if towed by the stern.
interfere with the main and emergency towing connections.
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11.13.1.3 If two tugs of different sizes are to be used for towing, then either:
— the larger tug should be connected to the bridle, and the smaller tug to a chain or chain/wire pennant set
to one side of the main bridle or
— two bridles can be made up, one for each tug.
11.13.1.4 For two balanced tugs, the bridle can be split and the tugs should tow off separate bridle legs, via
intermediate pennants. This approach should not be used for tows with rectangular bows.
11.13.1.5 For any systems in [11.13.1.3] and [11.13.1.4], a recovery system should be provided for the
connection point for each tug.
11.13.1.6 For tows where a bridle is not appropriate, such as multiple tug towages, then unless agreed
otherwise with MWS company each tug should tow off a chain pennant and an intermediate wire pennant.
11.13.1.7 Figure 11-7 is a flowchart for determining the required strength of the towing gear for a specific
tug.
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Figure 11-7 Flowchart for determining towing gear required strength and lengths
11.13.2 Number of towlines
Table 11-17 gives the minimum number of towlines for each category of tug.
Table 11-17 Tug wire requirements
Category
ST – salvage tug
Main wire
Spare
Two (on separate winch drums)
One
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U – unrestricted
One
One
C – coastal
One
One
R1 – restricted
One
Not applicable
R2 – restricted
One
One
R3 – restricted
One
Not applicable
11.13.3 Strength of towing equipment includingtowline connections
(outside ice areas)
11.13.3.1 The minimum breaking loads (MBL) or ultimate load capacity (ULC) of all items in the load path
between the tug and the towed object shall be related to the continuous static bollard pull (BP) of the actual
tug to be used. BP, MBL and ULC are in tonnes.
11.13.3.2 Table 11-18 gives the minimum required breaking load of the towlines and wire intermediate
pennants but see [11.13.4.7] for shorter towlines.
Table 11-18 Minimum required towline breaking loads (RTBL)
Continuous bollard pull (BP)
Benign areas
Other areas
BP < 40 tonnes
2.0 x BP
3.0 x BP
40 ≤ BP ≤ 100 tonnes
2.0 x BP
(220 - BP) x BP/60
BP > 100 tonnes
2.0 x BP
2.0 x BP
11.13.3.3 For tugs with very large bollard pulls (typically over 280 tonnes) it can be difficult to satisfy the
requirements of Table 11-18 due to problems in safely handling the large towlines required. In these cases
the effective towing bollard pull for selecting the towline MBL can be reduced to not less than 280 tonnes
provided that:
— the vessel is fitted with towline tension monitoring,
— the tug master is in agreement,
— the reduction is documented in the towing procedures and certificate of approval,
— the tug master shall take extra care in bad weather to protect the towline.
and if practicable:
— the winch should be adjusted to pay out at 80% of the towline MBL, and
— the engines should be mechanically or electronically limited to produce a maximum static bollard pull of
not more than 50% of the towline MBL (i.e. the effective bollard pull).
11.13.3.4 For specific towages in benign weather areas and in deep water that allows long towlines to be
deployed, the effective towing bollard pull in [11.13.3.3] can be further reduced to not less than 250 tonnes
after agreement with the MWS company.
11.13.3.5 The ULC in tonnes, of all items in the load path between the tug towline and the towed object,
including the tow connection(s) e.g. pad eye(s), Smit bracket(s) shall be not less than:
— ULC = 1.25 x RTBL for the actual tug (for RTBL ≤ 160 tonnes) or
— ULC = RTBL for the actual tug + 40 (for RTBL > 160 tonnes).
except for:
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— the bridle apex, see[11.13.7]
— shackles, see [11.13.8],
— intermediate pennants and surge chains see[11.13.9],
— synthetic stretchers, see [11.13.10].
Guidance note:
Items in the load path between the tug towline and the towed object can include tow connection(s), each bridle leg, connectors,
pennants and fairleads.
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For shorter towlines the increased RTBL given in [11.13.4.7] shall be used. When the bridle apex angle ≥ 90º
[11.13.6.2] shall apply.
Guidance note:
It should be noted that the above requirement represents the minimum values for towline connection strength. It can be prudent
to design the main towline connections to allow for the use of tugs larger than the minimum required.
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11.13.3.6 See [11.13.14.3] when bridles and pennants cannot be inspected annually.
11.13.3.7 In particular circumstances, where the available tug is oversized with regard to the Towline Pull
Required (TPR - see [11.12.2]), and the towline connections are already fitted to the tow, then the towline
connections, fairleads and bridle (but not the towline itself, pennants, stretchers or shackles between the
towline and bridle) can be related to the required BP rather than the actual BP but should allow for the
effective length of the towline used. Such relaxation shall be with the express agreement of the master of the
tug, and shall be noted in the towing procedures and certificate of approval. It shall not apply for towages in
ice areas (see [11.19.23]).
11.13.4 Towline certification, terminations and length
11.13.4.1 A certificate to demonstrate the MBL of each towline shall be submitted. MBL can be obtained by
testing, or by showing the aggregate breaking load of its component wires, with a spinning reduction factor.
This certificate shall be issued or endorsed by a body approved by a recognized classification society or other
certification body accepted by the MWS company. See also [11.13.14].
11.13.4.2 Tow wires shall be terminated with hard eye thimbles or closed sockets. Hard eye thimbles shall
be formed by an acceptable type of mechanical splice except where formed at sea in an emergency situation.
Aluminium or alloy ferrules shall not be used on any towline.
11.13.4.3 The closed socket (normally spelter type) if used to form the towline termination shall be renewed
at intervals not exceeding 2.5 (two and a half) years (excluding time before fitting when new on the tug),
irrespective of the condition of the socket and its wire. Except when re-socketed at sea for (temporary)
contingency reasons socketing shall only be done by a certified specialist, approved by a recognized
classification society. Renewed means the wire cropped back to steel that shows no sign of deterioration and
the use of either a new socket or one which has undergone rigorous NDT.
11.13.4.4 The deployable length shall not include the minimum remaining turns on the winch drum, and the
distance from the drum to the stern rail or roller. One full strength wire rope pennant which is permanently
included in the towing configuration can be considered when determining the deployable length.
11.13.4.5 Except in benign areas and for sheltered water towages, the minimum deployable length in
metres of each of the main and spare towlines (L) shall be determined from the 'European formula':
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except that in no case shall the deployable length (as defined in [11.13.4.4]) be less than 650 m, apart from
coastal towages within a good weather forecast when this can be reduced to 500 m.
11.13.4.6 For benign areas, the minimum deployable length in metres shall be not less than:
except that in no case shall the deployable length (as defined in [11.13.4.4]) be less than 500 m.
11.13.4.7 The towline MBL as shown in [11.13.3.1] shall be increased if required to allow L to comply with
[11.13.4.4] or [11.13.4.5]. In such cases the ULC for the bridle, fairleads and towing connections [11.13.3.5]
may need to be correspondingly increased.
11.13.5 Towline connection points and fairleads
11.13.5.1 Towline connections to the tow shall be of an approved type. They should be capable of quick
release under adverse conditions, including to allow a fouled bridle or towline to be cleared, but shall also be
secured against premature release.
Guidance note:
A typical bracket design is shown in [K.3].
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11.13.5.2 Towline connections and fairleads shall be designed to the requirements of [11.13.3.5].
11.13.5.3 Sufficient internal/underdeck strength shall be provided for all towline connections and fairleads.
11.13.5.4 Where fitted, fairleads should be of an approved type, located close to the deck edge. They should
be fitted with capping bars and sited in line with the towline connections, to prevent side load on the towing
connections.
11.13.5.5 Where the bridle might bear on the deck edge, the deck edge should be suitably faired and
reinforced to prevent chafe of the bridle.
11.13.5.6 Where towing connections are of a quick-release type, then the fairlead design shall allow all the
released parts to pass easily through the fairlead.
11.13.5.7 Any towline connections below or near the towing waterline shall be designed to fail without
allowing flooding.
11.13.5.8 Fairleads, where fitted, shall be designed to take
— transverse loadings from any likely tug pulling direction, and
— loadings along the line of the towline caused by a chain or shackle being caught in the fairlead using the
loads given in [11.13.3.5].
11.13.5.9 Where no fairleads are fitted, the towing connections shall be similarly designed.
11.13.5.10 If a fairlead or towing connection is to be used either with or without a bridle, it should be
designed for both cases.
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11.13.5.11 Where towing connections or fairleads can be subjected to a vertical load, the design shall take
account of the connection or fairlead elevation, the proportion of bridle and towline weight taken at the
connection or fairlead, and the towline pull, at the maximum pitch angle computed.
11.13.6 Bridle legs
11.13.6.1 Each bridle leg should be of stud link chain or composite chain and wire rope. If composite, the
chain should be of sufficient length to extend beyond the deck edge and prevent chafing of the wire rope.
11.13.6.2 The angle at the apex of the bridle should normally be between 45° and 60°. If it exceeds 90° (or
if either leg is more than 45° to the centreline of the tow) then the strength of the bridle legs, fittings and
towing connections shall be increased to allow for the increased resolved load in the bridle from the towline
force.
11.13.6.3 The end link of all chains should be a special enlarged link, not a normal link with the stud
removed. If there is no enlarged link at the end of the chain connected to the towing bracket, a new Kenter
or similar joining shackle may be used provided that care is taken to ensure that the retaining pin cannot
come out under tow and that the Kenter is in a more or less static position.
11.13.6.4 All wire ropes shall have hard eyes or sockets. Aluminium or alloy ferrules shall not be used.
11.13.6.5 The strength of each bridle leg shall comply with [11.13.3.5] and the strength of shackles and
connectors shall comply with [11.13.8.2].
11.13.7 Bridle apex
The bridle apex connection should be a towing ring or triangular plate or an enlarged bow shackle. Any
towing ring or shackle shall have documented evidence that they have been designed and certified for this
type of loading. The triangular plate shall not allow any shackle to rotate (see [K.9.1]). The minimum MBL
or ULC of the bridle apex connection shall be at least that required for shackles in the bridle as described in
[11.13.8].
Guidance note:
A triangular plate is also known as a Delta, Flounder or Monkey Plate
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11.13.8 Shackles
11.13.8.1 The documented MBL of shackles forming part of the towline (including any shackle between the
towline and the bridle apex) shall be at least 130% of the RTBL of the towline to be used.
Guidance note:
Soft shackles are not generally acceptable. The MWS company may subject to special consideration accept their use during
harbour/sheltered manoeuvres involving multiple tugs where manual handling is required to re-rig the towing arrangements due to
the nature of operations.
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11.13.8.2 The MBL or ULC of the bridle apex and shackles forming part of the bridle shall be not less than
130% of the required MBL of the connected parts. See [1.4.9] if the MBL of any equipment is not known.
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11.13.9 Intermediate pennant or surge chains
11.13.9.1 An intermediate wire rope pennant can be fitted between the main towline and the bridle or chain
pennant. All wire rope pennants shall have hard eyes or sockets, and be of the same lay (i.e. left or right
hand) as the main towline. Aluminium or alloy ferrules shall not be used on any pennant
Guidance note:
Its main use is for ease of connection and reconnection.
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11.13.9.2 A synthetic stretcher, if used, should not normally replace the intermediate wire rope pennant.
11.13.9.3 The length of the wire pennant should be such that it can be handled on the stern of most tugs
without the connecting shackle reaching the winch. Longer pennants can be needed in particular cases.
Guidance note:
For barge tows, pennants are normally 10 m to 15 m long.
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11.13.9.4 The MBL of the wire rope pennant shall not be less than the RTBL for the main towline.
11.13.9.5 Any 'fuse' or 'weak link' pennant shall have a strength not less than the RBTL for the towline.
Guidance note:
MWS companies do not normally recommend the use of a 'fuse' or 'weak link' pennant.
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11.13.9.6 A surge chain can be used, especially in shallow water when a long towline catenary cannot be
used, to provide shock absorption. If a surge chain is supplied then the MBL shall not be less than that of the
main towing wire. The surge chain shall be a continuous length of welded stud link chain with an enlarged
open link at each end (see [11.13.6.4]). A method of recovery of the chain shall be provided in case a tow
wire breaks. The length of the surge chain should allow recovery by the tug when the weight of bridle and
chain is at the limit of the recovery system in [11.13.11].
11.13.10 Synthetic stretchers
11.13.10.1 Where a synthetic stretcher is used, its MBL shall be at least 1.5 times the RTBL for the main
towline. It shall be in good condition and its use shall be in line with the requirements of the manufacturer,
especially with regards to storage and safety factors. Synthetic stretcher have a limited life due to
embrittlement and ageing, and shall be stored to protect them from wear, solvents and sunlight. See
[11.19.16] for towages when icing can occur.
11.13.10.2 If used, a synthetic stretcher should normally be connected between the main towing wire and
the intermediate pennant, rather than connected directly into the bridle apex.
11.13.10.3 All synthetic stretchers shall have hard eyes. A synthetic stretcher should be a continuous loop
with a hard eye at each end.
Guidance note:
This is generally preferable to a single line with an eye splice each end due to the reduced strength from splicing.
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11.13.10.4 If a synthetic stretcher is to be used then an equivalent spare should be carried.
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11.13.11 Bridle recovery system
11.13.11.1 A system shall be fitted to recover the bridle or chain pennant, to allow reconnection in the
event of towline breakage. The recovery system should consists of a winch and a recovery line connected to
the bridle apex, via a suitable lead, preferably an A-frame.
Guidance note:
The preferred type of bridle recovery system is shown in [K.1].
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11.13.11.2 The recovery winch shall be capable of handling at least 100% of the weight of the bridle, plus
attachments including the apex and the intermediate pennant. It shall be suitably secured to the structure of
the tow. Except for very small barges, the winch should have its own power source. Sufficient fuel should be
carried, including a reserve.
Guidance note:
A well-sized recovery winch can also be useful for initial connection of the towline.
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11.13.11.3 If the winch is manually operated, it should be fitted with ratchet gear and brake, and should be
geared so that the tow bridle apex can be recovered by 2 persons.
11.13.11.4 Should no power source be available, and manual operation is deemed impractical, then
arrangements shall be made, utilising additional pennant wires as necessary, to allow the tug to reconnect.
11.13.11.5 The MBL of the recovery wire, shackles, leads etc. shall be at least 6 times the weight of the
bridle, apex and intermediate pennant. The winch barrel should be adequate for the length and size of the
wire required.
11.13.12 Towing winches
11.13.12.1 Tugs in all categories shall be provided with at least one towing winch, (two towing winch drums
for category ST).
11.13.12.2 The towing winch and its connection to the vessel shall be strong enough to withstand a force
equal to the actual MBL of the tow wire acting at its maximum height above deck, without over-stressing
either the winch or the deck connections
11.13.12.3 If the power for the towing winch is supplied via a main engine shaft generator during normal
operating conditions, then another generator shall be available to provide power for the towing winch in case
of main engine or generator failure.
11.13.12.4 If a multi-drum winch is used, then each winch drum shall be capable of independent operation.
11.13.12.5 The towing winch drum(s) shall have sufficient capacity to stow the required minimum length of
the tow wire(s).
11.13.12.6 A spooling device shall be provided such that the tow wire(s) is effectively spooled on to the
winch drum(s).
11.13.12.7 The towing winch brake shall be capable of preventing the towing wire from paying out when the
vessel is towing at its maximum bollard pull and shall not release automatically in case of a power failure.
11.13.12.8 The winch shall be fitted with a mechanism for emergency release of the tow wire.
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11.13.12.9 There shall be an adequate means of communication between the winch control station(s) and
the engine control station(s) and the bridge.
11.13.12.10 If there is only one towing winch then the crew shall be able to demonstrate that a spare tow
wire can be safely run onto the towing winch within 6 hours of a towline break in bad weather.
11.13.13 Emergency towing gear
11.13.13.1 Emergency towing gear shall be provided in case of towline failure, bridle failure or inability
to recover the bridle. The method of retrieving the emergency towing gear should be deployed as soon as
practical and safe after commencement of the voyage unless it can be safely deployed from the tow in case
of an emergency during the voyage.
11.13.13.2 The strength requirements for the emergency towing arrangement shall be the same as for the
main towing arrangement.
11.13.13.3 The emergency towing gear should be fitted at the bow of the tow and consist of either a
separate bridle and pennant or a system as shown in [K.2]. For either configuration of the emergency towing
gear the following apply:
a)
b)
c)
d)
e)
f)
Precautions should be taken to minimise chafe of all wire ropes.
Closed fairlead(s) with the same strength as for the main towline connections, as shown in [11.13.3.5]
should be provided.
The emergency pennant should be at least 80 m, with hard eyes or sockets. See guidance note.
An extension wire to prevent the float line chafing on the stern of the tow should be provided.
A float line, to extend 75 m to 90 m abaft the stern of the tow should be provided.
Conspicuous pick-up buoy, with reflective tape, on the end of the float line should be provided.
Guidance note:
The pennant is preferably in one length. The pennant length may be reduced for small barges and in benign areas.
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11.13.13.4 For a bridle arrangement the same requirements as the main bridle shall apply to both the bridle
and towing connection points.
11.13.13.5 If the system shown in [K.2] is to be used the towing connection should be on or near the
centreline of the tow, over a bulkhead or other suitable strong point.
11.13.13.6 The MBL of the handling system, items [d)] and [e)] [11.13.13.3] should be not less than 25
tonnes (with shackles stronger by a factor of 1.3), and shall be sufficient to break the securing devices or
lashings.
11.13.13.7 If the emergency towline is attached forward, it shall be led over the main tow bridle. It should
be secured to the outer edge of the tow, outside all obstructions, with soft lashings, or metal clips opening
outwards, approximately every 3 m.
11.13.13.8 If the emergency towing gear is attached aft, the wire rope should be coiled or flaked near the
stern, so that it can be pulled clear. The outboard eye should be led over the deck edge to prevent chafe of
the float line.
11.13.13.9 For towage of very long vessels, alternative emergency arrangements can be approvable but
any arrangement shall be agreed with the master of the tug to ensure that reconnection is possible in an
emergency.
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11.13.13.10 Whatever the arrangement agreed, precautions shall be taken so no chafe can occur to the
floating line when deployed.
11.13.13.11 The connection of the float line to the pennant line or extension wire, and at the connection of
the float line to the buoy should have swivels.
11.13.13.12 The following reconnection equipment should also be considered, and placed on board if the
duration and area of the towage demand it:
a)
b)
c)
Heaving lines
Line throwing equipment
Spare shackles.
11.13.14 Certification and inspection
11.13.14.1 Valid certificates (less than 5 years old) shall be submitted for all towing gear hardware (e.g.
chains, wires and shackles) from the towing winch to the towing connections. Certificates shall be issued
or endorsed by bodies approved by a recognized classification society or other body accepted by the MWS
company. For triangular plates, less than 5 years old, calculations agreed with the MWS company in advance
can be acceptable instead of certification.
Guidance note:
It is preferable for certificates to detail any proof load testing (individual or batch) that has been performed.
Where certification is not submitted or attainable for minor items the MWS company can recommend that oversized equipment be
fitted.
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11.13.14.2 Apart from towing bridles or pennants connected to underwater connections (such as on semisubmersible pontoons) all towing gear hardware shall be subjected to a documented thorough examination
by a competent person not more than 12 months before use and shall be visually inspected before each use.
Any significant wear or damage shall be repaired and thoroughly examined again before use or the item
replaced. The competent person may require applicable testing in accordance with a recognized code e.g.
MPI and/or UT and/or proof load testing.
11.13.14.3 For any towing gear that cannot be thoroughly examined annually, an inspection regime shall
be agreed in advance with the vessel operator. Higher safety factors shall be agreed to allow for corrosion,
fatigue and longer times between examinations. The maximum age for such equipment shall be 5 years from
new and typically the safety factors should be increased by an extra 20% per year after the first year. Special
attention should be paid to any gear that spends time in the splash zone as it is subject to accelerated
corrosion.
Guidance note:
For example a submerged bridle pennant with a 5 year planned life, rated for a 100 tonnes BP tug, the required MBL would need to
be increased from 240 tonnes by a factor of 1.8 to 432 tonnes.
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11.13.14.4 Towlines shall not be in use longer than 100,000 nautical miles, of which no more than 50,000
miles shall have been in adverse weather conditions (nominally > Beaufort Force 6, see DNV-ST-0111 Table
2-1, /22/). Within 5 years from new or from any previous similar test about 10 m to 12 m of towline shall
be cut out and break tested or proof loaded to 1.5 x BP without yielding. Max towline life shall be 5 years if
not adequately documented in a towline log. Tow wires shall be terminated with hard eye thimbles or closed
sockets.
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Guidance note:
Guidance on recording of towing operations can be found in IMO MSC.1-Circ.884 Guidelines for Safe Ocean Towing, /138/. See
also Guidelines for Offshore Marine Operations (G-OMO), /69/, section 12.2.3.3.
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11.13.14.5 Anchor handling 'work' wires should generally not be used for towing due to the high probability
of damage. The only exception is when the wire log shows only very light use and after a thorough
examination of the whole wire by an independent competent person appropriately certified to do such
inspections.
11.13.14.6 All wire ropes shall have hard eyes or sockets. Aluminium or alloy ferrules shall not be used.
11.13.14.7 The MWS company surveyor can reject any items that appear to be unfit for purpose, or are
lacking valid certification.
11.13.14.8 Table 11-19 summarises the required expiry times for the above certificates and inspections
shown above.
Table 11-19 Certificate and inspection document requirements
Time since issue of original
certificates and either revalidation
or re-issue of original certificates
in accordance with [11.13.14.1]
Time since thorough
examination by a competent
person (unless new)
Bollard pull
< 10 years
Not applicable. See [11.12.1.6]
Triangular plates, master links and
shackles
< 5 years
< 12 months
Pennants, bridles and towlines
< 5 years
< 12 months
Submerged bridles
< 5 years
See [11.13.14.3]
Lashing equipment
< 4 years
< 12 months
< 2.5 years excluding time
before fitting when new
< 12 months
Item
Spelter sockets
11.13.15 Access to tows
11.13.15.1 Whether a tow is manned or not, suitable access shall be provided. This can include at least one
permanent steel ladder on each side, from main deck to below the waterline.
11.13.15.2 Where practical, ladders should be recessed, back painted for ease of identification, be clear of
overhanging cargo, and faired off to permit access by the tug’s workboat.
11.13.15.3 Alternatives can be accepted if it can be demonstrated that they will provide a safe and reliable
means of access during the towage.
Guidance note:
For example, a pilot ladder on each side or over the stern, secured to prevent it being washed up on deck, can be accepted for
short tows or where it can be deployed from a manned tow.
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11.13.15.4 Objects with high freeboard (e.g. over about 10 m) should have stairways. Where stairways are
not practical ladders should have resting platforms every 10 m and be enclosed, except within 5 m of the
towage waterline.
11.13.15.5 Where practical, a clear space should be provided and appropriately marked, with access ladders
if necessary so that, in an emergency, men can be landed or recovered by helicopter.
Guidance note:
If it is required to land a crew on board before entering port, for instance to start pumps and reduce draught, then a properly
marked and certified helideck or landing area would be an advantage.
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11.13.15.6 A boarding party shall be appropriately equipped such as survival suits, lifejackets and
communication equipment.
11.13.15.7 Un-manned tows should have lifesaving appliances on board, appropriate to the hazards a
boarding party could experience.
11.13.15.8 Notwithstanding the potential for piracy in some areas, means of boarding shall still be available.
11.13.16 Damage control and emergency equipment
When the length and area of the towage demand it, the following equipment should be carried on the tow in
suitable packages or in a waterproof container secured to the deck:
a)
b)
c)
d)
e)
f)
g)
h)
i)
j)
k)
l)
m)
n)
o)
p)
q)
r)
Burning gear
Welding equipment
Steel plate - various thicknesses
Steel angle section - various sizes and lengths
Plywood sheets – 25 mm thick
Lengths of 3” x 3” (75 mm x 75 mm) timber
Caulking material
Sand and cement (suitably packaged)
Nails - various sizes
Wooden plugs – various sizes
Wooden wedges – various sizes
A selection of tools, including a hydraulic jack, hammers, saws, crowbars, Tirfors.
Portable coamings 60 cm minimum height, with a flange and boltholes to suit the manhole design. The
top should be constructed to avoid damage to hoses and cables
A sounding tube extension, of 60 cm minimum height, threaded so that it can be screwed into all
sounding plug holes
Sounding tapes
Fire-fighting equipment as appropriate
Personal protection equipment - gloves, goggles, hard hats, survival suits etc.
Emergency lighting.
11.14 Voyage planning
11.14.1 General
11.14.1.1 The following requirements apply to the way in which the towage or voyage shall be conducted.
The certificate of approval is based on agreed towage or voyage arrangements, which shall not be deviated
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from without good cause, and where practical with the prior agreement of the MWS company. Deviations
should follow the MOC and/or contingency plans within the towing/transport manual/procedures.
11.14.1.2 Towages and voyages in the Arctic and Antarctic waters, as defined in SOLAS, /139/, Chapter
XIV (Safety measures for ships operating in polar waters), shall comply with the mandatory IMO Polar
Code, /130/.
Guidance note:
The maximum extent of Arctic and Antarctic waters are shown in Figure 1 and 2 of IMO Res. A.1024(26), /94/ or IMO Polar
Code, /130/.
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11.14.1.3 Planning of the voyage or towage shall be carried out in accordance with the requirements of the
IMO International Safety Management Code, /92/.
11.14.1.4 All voyages shall start on a reliable good weather forecast (see [11.14.4])
11.14.1.5 The critical depth contours for grounding, allowing for roll, pitch and heave in the worst expected
weather conditions (typically less than BF8/9, see DNV-ST-0111 Table 2-1, /22/) at LAT, should be plotted in
advance. For sea room calculations in [11.14.2], the contours should be the under-keel clearance in a 1 year
return storm.
11.14.1.6 The route should be planned to avoid passing too close upwind or up-current of any platforms or
other isolated obstacles, especially for single tug tows.
11.14.1.7 The required sea room and the basis for its calculation should be included in the voyage
procedures/manual for the guidance of the captain(s)/towmaster.
11.14.1.8 The actual route can safely deviate from the planned route if the weather forecasts are favourable
as long as the required sea room can be obtained before bad weather is likely to arrive.
Guidance note:
In many cases in rough weather areas and seasons the required sea room can be many hundreds of miles. It may be impractical
to plan a route with adequate sea room and so a staged towage can be required (in which there is a commitment to seek shelter
or jack-up at a stand-by location on receipt of a bad weather forecast). However a staged towage may be impracticable due to the
problems of finding suitable places of shelter or safely approaching them on a lee shore.
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11.14.1.9 If stretchers are used then their fatigue life (typically about 3 days in bad weather for a new
stretcher) shall be shown to be adequate.
11.14.1.10 For towages:
— Tug masters should be cognizant of the towline catenary at all times.
— The deployed towline length should consider the wave lengths and heights currently being encountered.
— In shallow water, the minimum static clearance between the towline and the seabed should be 10% of the
water depth with a minimum of 5 m in exposed waters or 2 m in sheltered or calm water to avoid towline
abrasion or snagging on the sea floor. Ideally this should be by monitoring the water depth, towline
tension and the deployed towline length from the tug stern combined with a method of calculating the
towline maximum depth below sea level.
— If short towlines have to be used, care shall be taken to avoid high shock loading due to lack of adequate
towline catenary, especially in bad weather.
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11.14.2 Sea room
11.14.2.1 Unless a tow’s sea room, for the case in [11.14.2.2] to [11.14.2.5] is greater than both of the
following, the tow should be considered to be 'higher risk' and the underwriters informed as in [1.4.3]:
— 75% of the required sea room within 1 day after the end of the reliable good-weather period of the
departure forecast, and
— 100% of the required sea room within 3 days after the end of the reliable good-weather period of the
departure forecast (a reduction can be agreed for short periods during the duration of the tow).
Guidance note:
Adequate sea room is typically defined as the distance that a disabled transport or tow in bad weather can safely drift, without
grounding.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.2.2 Case 1 - in bad weather (outside good weather forecast periods): The required sea room is the
distance drifted whilst the significant wave height is greater than 5 m in a storm for that section of the
towage including the effect of any associated currents. The design storm for determining required sea room
should be at least the 1 year return after the end of a good weather forecast.
Guidance note 1:
A method of calculating the sea room is described in [K.9.2].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
A 1 year storm has been selected as many storms will not blow towards the nearest shoals.
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11.14.2.3 The drift distance when lying broadside to the wind and waves should be used if it is greater than
that with bow or stern to wind.
11.14.2.4 It shall be assumed that the tugs develop negligible effective pull in waves over 5 m significant
height, unless it can be shown that the actual tugs can safely do so without overloading their deployed
towing gear in the relevant water depths.
Guidance note:
Most tugs cannot develop significant bollard pull without overloading their towing gear in weather much more than BF 8 (typically
20 m/s wind, 5 m sig wave height, see DNV-ST-0111 Table 2-1 /22/) since tugs should normally be in 'survival mode' if caught in
these conditions.
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11.14.2.5 For effective bollard pull to be included in the required sea room calculation, the results of towline
tension monitoring should show that the towline yield stress is not exceeded in the relevant weather and
towing conditions. The following details should be documented:
— significant wave heights and periods,
— water depths,
— towline deployed lengths
— other relevant towline properties and
— that the stretcher fatigue life is adequate for the towage including the duration of a 1 year return storm
if a stretcher is needed to reduce the shock loads in the towing equipment because of inadequate water
depth to deploy enough towline to provide a suitable catenary.
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Guidance note 1:
Towline yield stress is typically about 40% of the wire break load.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
These results can also be used to validate towline dynamic simulations to extrapolate the results for other conditions.
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11.14.2.6 Case 2 - when approaching a potential lee shore (with a good weather forecast): The required sea
room is the distance drifted in the worst acceptable forecast conditions during the time taken to replace and
reconnect a broken towline and/or disabled tug.
Guidance note:
See Figure K-12 for an example of sea room requirements against time taken to reconnect for a range of weather conditions.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.2.7 The acceptable forecast conditions should be included within the towage procedures/manual for a
particular case. These shall be determined by applying the appropriate alpha factor to the theoretical limit to
allow for uncertainties in the forecast.
11.14.2.8 Approaching a potential lee shore should only be attempted without a good weather forecast if
there is no practicable alternative in an emergency situation.
Guidance note:
An additional (connected) tug can be used to guard against a towline breakage or disabling of a single tug when approaching or
leaving a lee shore.
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11.14.2.9 The same philosophy should be followed when transiting 'choke points' with limited sea room, or
with a high collision risk, with the tow waiting for a suitable good weather window before committing to the
approach.
11.14.3 Routeing and piracy
11.14.3.1 Routeing procedures shall be agreed with the master before the start of the voyage, taking into
account:
— the transport vessel or tug’s capacity,
— fuel consumption,
— the weather and current conditions,
— normal good navigation and seamanship and
— where possible avoiding the potential for piracy.
11.14.3.2 Anti-piracy procedures shall be included in the towing/transport manual/procedures unless there
is a low risk of piracy.
11.14.3.3 Where anti-piracy measures are warrantable the requirements should be advised to the MWS.
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Guidance note 1:
Piracy is prevalent in many areas and these vary with time. Guidance on mitigation of piracy can be found in:
a)
BMP4 - Best Management Practices for Protection against Somalia Based Piracy (or later version). This can be downloaded
from websites of sponsoring organisations including http://www.intertanko.com/Topics/Security/Security-/BMP4forProtection-against-Somalia-Based-Piracy/ While this guidance was created to address the Somalian situation, this
document’s good practice should be considered for any high risk area.
b)
Website http://www.lmalloyds.com/LMA/Underwriting/Marine/Joint_War_Committee/lma/underwriting/marine/JWC/
c)
IMO website http://www.imo.org/OurWork/Security/SecDocs/Pages/Maritime-Security.aspx (then select 'Piracy')
d)
IMB Piracy Reporting Centre website http://www.icc-ccs.org/piracy-reporting-centre
e)
Flag-state, vessel insurance and P&I club requirements. The MWS company should be advised by the insured at an early
Joint_War.aspx (Lloyd’s Market Association/Joint War Committee website). This also gives current piracy risk areas.
stage if there are any relevant insurance warranty requirements.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Key mitigations of piracy include:
a)
Awareness of the sea areas and ports affected by piracy and armed robbery and, at the very least, ensuring proper all round
vessel lookouts are in place and maintained, using every means possible, while in these areas. Previous incidents of piracy
clearly demonstrate that slow vessels (typically less than about 18 knots) or tows, especially with low freeboards (typically
less than about 8 m) or easy access over the side or stern, are particularly at risk.
b)
Maintaining sufficient distance from land throughout the voyage can help to reduce this risk and also ensures there is
sufficient sea room in case of emergency. However, given that attacks now regularly occur many miles from the coastline (up
to 1,500 nautical miles), it is essential vessels considering transiting these areas prepare well in advance for the possibility of
an attack.
c)
Careful consideration by masters of their route and the risks and implementation at an early stage all necessary measures
to reduce the likelihood of their vessel becoming a target. A full route analysis should be conducted taking into account
previously reported incidents of piracy, as part of the passage planning process.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.4 Weather routeing and forecasting
11.14.4.1 Staged voyages
11.14.4.1.1 Staged voyages shall have a commitment to seek shelter (or jack-up at a stand-by location)
on receipt of a weather forecast in excess of the operational limiting criteria incorporating an alpha factor. A
staged voyage shall have sufficient suitable ports of shelter (or stand-by locations) along the route.
11.14.4.1.2 The voyage shall proceed in stages between shelter points, not leaving or passing each shelter
point unless there is a suitable weather forecast for the next stage. Subject to certain safeguards, each stage
can be considered a weather restricted voyage.
11.14.4.1.3 In such cases the towage route shall be planned to incorporate a series of shelter points,
meaning sheltered locations where the tow can safely ride out severe weather. It can also be necessary to
identify suitable bunker ports. These requirements can conflict with the requirement for adequate sea room,
and such conflicts shall be resolved.
11.14.4.2 Weather routed voyages
Weather routed voyages shall only be approved if the sea room requirements in [11.14.2] can be achieved
and:
— The vessel shall have sufficient sailing speed/manoeuvrability, accounting for any restrictions in
deteriorating weather conditions, to avoid conditions in excess of the operational limiting criteria in the
area(s) subject to weather routeing.
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— The vessel redundancy/contingency shall be adequate, i.e. the minimum voyage speed should account for
possible delays (the maximum speed should also be considered).
— Any vessel heading restrictions are appropriately considered. If heading control is to be used, then
redundancy is required, see [11.8.2].
— The set/agreed operational limiting criteria shall be sufficient to allow routeing around the prevailing/
possible weather conditions/patterns in the sailing area in the actual season (safetrans analysis including
the captains mimic could be used for guidance).
— The allowable conditions are not so low that (a) the voyage is not feasible and (b) the delays that could
result from the application of weather routeing are not acceptable to the project. Typically, the weather
routeing limit should be no less than 60% of the unrestricted conditions.
— Any potentially necessary points of shelter have been confirmed as accessible for the vessel and cargo.
— The relevant weather forecasting level is applied, and weather forecasting uncertainty is duly considered,
applying alpha factors from [2.6.11] or [2.6.12] as applicable. Typically the forecasting level will be A2/
B. The alpha factors to be used by the routeing service/software shall be based on the applicable table
in [2.6.11] or [2.6.12], with the 72 hour factor for the areas that could be reached in 72 hours, the 48hour factor for the areas that could be reached in 48 hours, etc.. In practice this likely means using the 72
hour alpha factor for all distances. Thus, for a WSD design under forecast category A2 or B and 4m Hs the
factor is 0.63, and 0.68 for 6m Hs (from Table 2-11 or Table 2-12).
— The voyage manual/procedure shall clearly describe all elements required for safe execution of the
weather routed voyage i.e. it shall thoroughly document how it will be ensured that the set operational
limit will not be exceeded. The limiting operational seastate shall be included. This is the design seastate
limit, reduced by a forecast accuracy factor (alpha factor).
Guidance note:
For a transport (as defined [1.6]) in areas not subject to TRS activity this typically requires that the vessel can achieve at least
12 knots in up to BF6, see DNV-ST-0111 Table 2-1, /22/ including the effects of the vessel’s cargo(es). The actual vessel speed
required should be determined via a risk assessment accounting for the route; proposed weather routeing, forecasting and
monitoring; the vessel/cargo combination and its seakeeping ability; and anything else pertinent. Additionally, this should be
considered in the motions analysis see [5.6.12.1.6] and [11.3.2.2].
For weather routeing to avoid TRS in otherwise benign areas see [3.4.17.5].
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.4.3 Forecasts – general
11.14.4.3.1 Requirements for weather forecasts for voyages should be in accordance with [2.7] and shall be
agreed with the MWS company in advance.
Guidance note:
These are particularly important for weather restricted voyages (either staged or weather routed) for which the strength or stability
will not meet the weather unrestricted environmental criteria.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.4.3.2 Arrangements shall be made for receiving suitable weather forecasts throughout any voyage
from a reputable source. If appropriate, a weather routeing service, provided by a reputable company, should
be arranged before the start of the towage or voyage. The utilisation of a weather routeing service can be a
requirement of the approval and shall be used for weather routed voyages.
11.14.4.4 Forecasts – departure
11.14.4.4.1 For any towage, the weather conditions for departure from the departure port or any
intermediate port or shelter area shall take into account the capabilities of the tug, the marine characteristics
of the tow, the forecast wind direction, any hazards close to the departure port or shelter area and the
distance to the next port or shelter area. Assistance from local pilots should be considered.
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11.14.4.4.2 Weather forecasts for the departure area should be started at least 48 hours before the
anticipated departure date and be level A or B as in Table 2-16.
11.14.4.4.3 Towage departures should take place with forecasts of good visibility, allowing for the effects of
fog, rain and snow, especially if the tow master is unfamiliar with the area. Unless otherwise justified the
forecast wind speeds should not exceed the values in Table 11-20 for the first 24 hours of the towage.
Table 11-20 Typical maximum initial towage departure weather forecasts limits
Type of tow
Maximum wind
Unusual tows with large wind area
15 knots (BF 4)
'Standard' tows
20 knots (BF 5)
'Standard' tows with small wind areas and a towing master familiar with the type of tow and
the towing route
25 knots (BF 6)
Guidance note:
For details of the Beaufort scale see DNV-ST-0111 Table 2-1 /22/.
The above values are forecast numbers and hence not subject to alpha factors. Additionally, if there is for any reason a specific
design wind speed or wave height limitation for the tow/transport this will need to be adhered to and the alpha factors apply.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.4.4.4 Additionally the longer term forecast shall enable the tow to obtain adequate sea room (or reach
a safe sheltered area) before bad weather can arrive.
11.14.4.4.5 Towages approaching a potential lee shore or areas with restricted sea room shall obtain a
favourable weather forecast before reducing their sea room requirements.
11.14.5 Departure
11.14.5.1 Before departure, a departure condition report for the tow or vessel shall be submitted by the
owners or their agents, to the master and the MWS company surveyor. This report should contain as a
minimum:
— The documentation referred to in Table B-2 as appropriate
— Lightship weight
— Tabulation and distribution of ballast, consumables, and cargo, including any hazardous materials
— Calculated displacement and draughts
— Actual draughts and displacement
— A statement that the longitudinal bending and shear force are within the allowable seagoing limits
— Calculated VCG
— Calculated GM and confirmation that it is within allowable limits
— GZ Curve and confirmation that it is within allowable limits.
11.14.5.2 In the departure condition, the tow shall have acceptable stability with proper allowance made for
any slack tanks.
11.14.5.3 If no stability documentation is available then it can be necessary to perform an inclining test
to check that the GM is satisfactory. Calculations can be needed to establish righting and overturning lever
curves.
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11.14.5.4 It shall be verified that the tow floats in a proper upright attitude and at a draught and trim
appropriate to the calculated weight and centre of gravity.
11.14.5.5 The certificate of approval shall be issued on agreed readiness for departure and receipt of a
suitable weather forecast.
11.14.6 Ports of shelter, shelter areas, holding areas
11.14.6.1 Ports of shelter, or shelter areas on or adjacent to the route, with available safe berths, mooring
or holding areas, shall be agreed before departure and all necessary permissions obtained.
11.14.6.2 Where such shelter points are required as part of a weather restricted operation, as described in
[2.6.7], they shall be capable of entry in worsening weather.
11.14.7 Bunkering
11.14.7.1 Bunkering ports, if required, shall be agreed before departure.
11.14.7.2 If it is not practical to take the tow into port, then alternative arrangements shall be agreed and
included in the approved towage procedures. Unless agreed otherwise the requirements of [11.12.2], shall
apply at all times:
Guidance note:
Possible alternative arrangements include:
—
Where the towage is by more than one tug, each tug in turn can be released to proceed to a nearby port for bunkers, subject
to a favourable weather forecast. The remaining tug(s) should meet the requirements of [11.12.2], or some other agreed
criterion.
—
Relief of the towing tug by another suitable tug, which itself is considered suitable to undertake the towage, so that the towing
—
Bunkering at sea from a visiting vessel, subject to suitable procedures and calm weather conditions.
tug can proceed to a nearby port for bunkers.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.8 Assisting tugs
Assisting tugs shall be engaged at the start of the towage, at any intermediate bunkering port and at the
arrival destination, as appropriate.
11.14.9 Pilotage
The master shall engage local pilotage assistance during the towage or voyage, as appropriate.
11.14.10 Log
A detailed log of events shall be maintained during the towage or voyage.
11.14.11 Inspections during the towage or voyage
11.14.11.1 Unless the tow is manned, it should be boarded on a regular basis by the crew of the tug,
particularly after a period of bad weather, in order to verify that all the towing arrangements, condition of the
cargo, seafastenings and watertight integrity of the tow are satisfactory. Suitable access shall be provided see [11.13.15].
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11.14.11.2 For manned tows, and self-propelled vessels, the above inspections should be carried out on a
daily basis as relevant - see also [11.17.5].
11.14.11.3 Any adjustable seafastenings or lashings shall be re-tensioned as necessary.
11.14.12 Reducing excessive movement and shipping water
The master should take any necessary measures to reduce excessive movement or the shipping of water
which can damage the cargo, cribbing or seafastenings.
Guidance note:
This can entail changes of course and/or speed.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.13 Notification of unusual or abnormal events
After departure of an approved towage or voyage, notification shall be sent to the MWS company regarding
any unusual or abnormal events, or necessary deviation from the agreed towing procedures.
11.14.14 Diversions
Should any emergency situation arise during the towage or voyage which necessitates diversion to a port
or place of refuge, then the MWS company shall be advised. The MWS company will review and advise any
mooring requirements and on the validity of the existing certificate of approval for continuing the towage
or voyage depending on the circumstances of the case. A further attendance at the port of refuge may be
required in order to re-validate the certificate of approval.
11.14.15 Responsibility
11.14.15.1 The towmaster is responsible for the overall conduct of a tow, and towing arrangements during
the towage. Similarly the master of the transport vessel is responsible for the overall conduct of the voyage.
Nothing in this document shall set aside or limit the authority of the master who remains solely responsible
for their vessel during the voyage in accordance with maritime laws.
11.14.15.2 If any special situations arise during the voyage and it is not possible to comply with any specific
recommendations, agreed procedures or international regulations, then such measures as appropriate for the
safety of life and property shall be taken. The MWS company shall be informed as soon as practical of any
such circumstances.
11.14.16 Tug change
The tug(s) approved for any towage, as noted on the certificate of approval, shall be the only tug(s)
approved for that specific towage and should remain with the tow throughout the towage. Should it be
required to change the tug(s) for any reason, except in emergency or where special arrangements have been
agreed for bunkering, the replacement tug shall be approved by the MWS company and a new certificate of
approval issued.
11.14.17 Hazardous materials
11.14.17.1 The carriage of hazardous materials should be avoided, unless it can be shown that the
materials are effectively controlled. For un-manned voyages, hazardous materials should be stowed
accounting for the limited remedial actions available in the case of inadvertent release.
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Guidance note:
Hazardous substances can be considered as materials which, when released in sufficient quantities or improperly handled, have the
potential to cause damage to the asset, personnel or the environment through chemical means or combustion.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.17.2 All hazardous materials shall be transported and stored in accordance with the IMO IMDG
(International Maritime Dangerous Goods) Code, /88/. The properties of such material are contained in the
COSHH (Control of Substances Hazardous to Health) data sheets.
11.14.17.3 Where identifiable hazardous material is found on board before a voyage taking place, it should
be controlled either through isolation or removal ashore.
11.14.18 Ballast water
11.14.18.1 Voyage planning shall account for any need to change ballast water, including all local laws,
before or at the arrival port.
Guidance note:
Vessels can need to change ballast water before or at their arrival port for operational reasons (loading/discharging). There can be
local laws that will have an impact on these activities. In the U.S.A. there are numerous state laws that cover these operations.
The IMO Ballast Water Convention (Resolution A.868(20)), /86/, requires the monitoring and recording of ballasting and deballasting operations. Vessels flagged in signatory states are required to have on board and to implement a Ballast Water
Management Plan. This plan is specific to each vessel and the record of ballast operations can be examined by the port state
authorities.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.18.2 The necessary ballast plan and records should be submitted to any attending MWS company
surveyor.
11.14.19 Restricted depths, heights and manoeuvrability
11.14.19.1 The clearance requirements for each towage should be assessed, taking into account
— environmental conditions,
— length of areas of restricted manoeuvrability,
— any course changes within the areas of restricted manoeuvrability,
— cross section of areas of restricted manoeuvrability in relation to underwater area/shape of the base
structure, and
— capability of the tugs.
11.14.19.2 The clearances in [11.14.20] to [11.14.21] are the generally acceptable minimum values. Any
reduction of these shall be agreed with the MWS company at an early stage and it shall be proven that the
reduced values give an acceptable level of risk.
11.14.19.3 Calculation of clearances shall account for the effects of
— Roll, pitch and initial heel and trim,
— heave,
— tow-line pull,
— inclination due to wind,
— tolerance on bathymetry (which can be over 10 m on old surveys)
— changes in draught of the transport vessel or towed object,
— differences in water density,
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— tidal height variations,
— squat effects,
— deflections of the structure
— errors in measurement
— surge (negative for under-keel clearances and positive for air draught), and
— any protrusions below the bottom of the asset.
11.14.19.4 For areas where the under-keel or side clearance is critical, a survey that is not older than 3
months should be documented.
11.14.19.5 Where the survey report in [11.14.19.4] is not available, the tow route shall be surveyed with
a width of 5 times the beam, with a minimum of 500 m. Side-scan sonar and bathymetric data should be
documented. The equipment used shall be of a recognized industry standard. The spacing between depth
contour lines should be appropriate for the purpose. Current surveys should be made in restricted parts of
the tow route.
11.14.19.6 The survey requirements can be relaxed if it can be shown that the on board bathymetry
measurement systems and position management systems have sufficiently high precision.
11.14.19.7 Passages through areas of restricted manoeuvrability and passing under bridges and power
cables should not generally take place in darkness.
11.14.19.8 For areas where it is not feasible to deploy adequate towline length, e.g. due to restricted water
depth, weather restrictions shall be defined (to reduce peak loads due to relative motions of tug and tow).
See also [11.12.2.14].
11.14.20 Under-keel clearances
11.14.20.1 The under-keel clearance shall be not less than the greater of 1 m or 10% of the maximum
draught (with a maximum of 3 m) accounting for the items listed in [11.14.19.3]. The under-keel clearance
can be reduced in very benign conditions after agreement with the MWS company.
11.14.20.2 Under-keel clearances for departure from dry-docks or building basins are covered in [12.6].
11.14.20.3 If sections of the passage are tidally dependent, safe holding areas should be identified in the
vicinity with adequate sea room and water depth to maintain the minimum under-keel clearance at low tide.
Any delay time waiting for the tide shall be included in the overall planning.
11.14.20.4 Immediately before critical sections of the passage the tidal level shall be confirmed by
measurement.
11.14.20.5 Where an air cushion is used to reduce draught then the following shall be considered:
a)
b)
Any conceivable loss of air not increasing the draught by more than the reserve on under-keel clearance,
and
The recommendations contained in [12.6.2] on air cushions.
Guidance note:
Use of air cushions is generally only acceptable to reduce draught to assist in crossing localised areas of restricted water
depth.
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11.14.21 Air draught
11.14.21.1 When passing under obstructions, the overhead clearance shall be calculated accounting for the
items listed in [11.14.19.3] excluding squat and shall be greater than one metre plus dimensional tolerances.
11.14.21.2 Where clearance is limited then a dimensional survey of the barge/vessel and structure shall
take place just before sailaway in order to ensure that the required clearance exists.
11.14.21.3 For power cables the minimum allowable clearance shall be specified by the transmission
company (but not be less than 1 m) and be measured to the lowest possible catenary position.
Guidance note 1:
Power cables need a 'spark gap', as well as a physical clearance.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The catenary of the power cable will change depending on the electrical load being carried in the cable.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.14.21.4 The actual clearance shall be confirmed with all appropriate authorities including those
responsible for the obstruction.
11.14.21.5 Immediately before the passage the tidal level shall be confirmed by measurement unless the
calculated overhead clearance from [11.14.21.1] is greater than two metres plus dimensional tolerances at
HAT (highest astronomical tide).
11.14.22 Channel width and restricted manoeuvrability
11.14.22.1 The minimum channel width along any inshore legs of the tow route with the under-keel
clearance and air draught required in [11.14.20] and [11.14.21] should be three times the maximum width
of the towed object plus allowances for yaw and sway. Additional channel width can be required
— in exposed areas
— if there are significant cross currents
— for tugs on either side to assist in manoeuvring if required.
11.14.22.2 Narrower channels may be agreed with the MWS company on a cases by case basis for ideal
conditions (e.g. sheltered straight short channels and no tight time restrictions).
Guidance note:
Agreement to a reduced channel width requires an acceptable risk assessment including applicable mitigations such as:
—
speed limitation to reduce squat and bank suction/yaw
—
additional tugs to assist with control e.g. fishtailing see [11.12.1.7]
—
analyses and/or model testing, etc.
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11.14.22.3 Side clearances for departure from dry-docks or building basins are covered in [12.7].
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11.15 Bilge and ballast pumping systems
11.15.1 Pumping arrangements – general
11.15.1.1 For classed vessels, the drainage system and (bilge) pumps should as a minimum comply with the
rules of the classification society.
11.15.1.2 Tugs towing outside coastal limits shall also comply with [11.12.10].
11.15.1.3 The general requirements in [4.2] shall be applied as applicable.
11.15.2 Pumping arrangements - emergency
11.15.2.1 Emergency pumping arrangements shall be installed and operable on for any vessel to deal with
any leakage after collision, grounding, structural failure or other accident. The requirements in [11.15.3]
apply to:
a)
b)
Towed vessel or barge
Unclassed vessels or those operating outside their conditions of class.
11.15.2.2 For other wet tows, the need for and specifications of the emergency pumping systems shall be
determined by a risk assessment in accordance with [2.4] considering the requirements in [11.15.3].
Guidance note:
The risk assessment would also normally consider the details of the towage and the extent and availability of any installed system.
Examples of other tows are self-floating objects, MOU’s, FSU’s and disabled ships,
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11.15.2.3 Some relaxation can be possible, as agreed with the MWS company, on a case-by-case basis, for
a towage considered as a weather restricted operation.
11.15.2.4 Whether or not a tow is manned, the emergency pumping system shall be available at short
notice and deliver pumping times and capacities shown in [11.15.3.5] to [11.15.3.7].
Guidance note:
For an un-manned tow, short notice is considered to start after boarding (which could be by helicopter).
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11.15.2.5 Where a tow is not manned, then the tug master and chief engineer shall be aware of the
available pumping system. Members of the tug crew shall be familiar with the systems, and be able to board
the tow and run the pumps at short notice. Procedures for pumping shall be known and available, including
any restrictions arising from considerations of stability or hull stresses, and any vents, which shall be opened
before pumping starts.
11.15.3 Pumping system requirements - emergency
11.15.3.1 Vessels should have one of the following systems to meet the capacity requirements of
[11.15.3.5] to [11.15.3.7], able to pump into and from all critical spaces (as defined in [11.15.3.2]) in order
of preference:
— Two independent pump rooms or one protected pump room, as described in [11.15.3.3] and [11.15.3.4]
— An unprotected pump room with an independent emergency system that can pump out the pump room
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— A system of portable pumps.
11.15.3.2 A critical space is defined as any tank or compartment which:
1)
when flooded or emptied, at any stage of the voyage, can lead to:
— non-compliance with intact or damage stability criteria, or
— non-compliance with structural load limits, or
— heeling or trimming that can prevent the tow from continuing its passage safely and free from
obstructions in shallow water, or
— maximum allowable transit draught being exceeded.
2)
can be required for ballasting/de-ballasting so that the barge or vessel can safely continue its passage
after any single compartment is damaged.
11.15.3.3 Independent pump rooms should have separate power supply, pumps, control and access. Each
pump room should be able to work into all spaces.
11.15.3.4 To be considered protected, a pump room, and any compartment required for access, should be
separated from the bottom plating by a watertight double bottom not less than 0.60 m deep and from the
outer shell by other compartments or cofferdams not less than 1.5 m wide.
11.15.3.5 The total capacity of the fixed and portable pumps should be such that any one wing space (or
other critical space as defined in [11.15.3.2]) can be emptied or filled in 4 hours for an un-manned tow, or
12 hours for a manned tow. Any time required for connection or warm-up should be included in the pumping
times shown.
11.15.3.6 Except where there is a protected pump room, at least two pumps shall be provided.
11.15.3.7 Whatever type of pumps are fitted or supplied, sufficient fuel shall be carried for at least 72 hours
continuous operation.
11.15.3.8 If portable pumps are used then either they should be portable enough to be moved around
the vessel (and cargo) by two men, or enough pumping equipment should be carried so that any critical
compartment can be reached.
11.15.3.9 Each portable pump should be able to pump out from the deepest space (with portable coaming
installed). Portable submersible pumps shall be able to fit through tank manholes.
Guidance note:
This requires submersible pumps for vessels over about 6 m depth, due to suction head.
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11.15.3.10 Any compressed air system should have a compressor on board and available, connected into
the permanent lines.
Guidance note:
The use of a vessel compressed air system may not be practicable for all these or emergency cases, especially if manhole covers
have been removed, or the vessel is holed above the waterline.
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11.15.4 Pumping arrangements – non-emergency
In addition to the emergency pumping arrangements described in [11.15.2], suitable pumping arrangements
shall be provided for all planned (including contingency) ballasting operations.
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Guidance note:
Generally the following ballasting operations should be considered (as applicable):
a)
Ballasting before, during and after load-outs
b)
Ballasting to the agreed departure condition and subsequent ballasting to towing condition
c)
Restoration of draught and trim before/during/after discharge (e.g. lift off from barge offshore)
d)
Adjusting draught or trim due to shallow waters or air draught restrictions
e)
(De-)Ballasting to change draught at end of towage (e.g. reducing draught to enter port)
f)
Trimming to allow inspection and repair below normal waterline.
g)
Correction of unintended flooding
h)
De-ballasting after accidental grounding
i)
Access to flooded compartment (e.g. pump or anchor winch room).
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11.15.5 Watertight manholes
11.15.5.1 If manholes to critical compartments are covered by cargo then either alternative manholes
should be fitted, or cutting gear should be installed and positions marked for making access. Welding gear
and materials shall be carried to restore watertight integrity.
11.15.5.2 Where the vessel is classed, the owner should inform the classification society in good time of any
holes to be cut or any structural alterations to be made.
11.15.5.3 Access shall always be available to pump rooms and other work areas.
11.15.5.4 For each manhole position, ladders to the tank bottom shall be provided.
11.15.5.5 Suitable tools shall be provided for removal and refastening of manhole covers and sounding
plugs. All manhole covers shall be properly secured with bolts and gaskets, renewed as necessary.
11.15.5.6 Portable coamings to suit the manhole design shall be carried, if required for operation with water
on deck, as in [11.13.16] m).
11.15.6 Sounding systems
11.15.6.1 Sounding of and pumping into or from critical spaces (as defined in [11.15.3.2]) in severe
weather should be feasible. The following shall be provided on all critical spaces:
— Pumping system
— Watertight manholes
— Portable coamings
— Sounding plugs, extensions and tapes or rods. An additional remote sounding system can be needed for
compressed air ballasting systems
— Vents to all compartments.
11.15.6.2 For vessels or barges with compressed air ballast systems, gauges shall be provided in lieu of
sounding pipes.
11.15.6.3 A sounding plug shall be installed in each compartment (in manhole covers if necessary) to avoid
removing manhole covers. Sounding tapes and chalk shall be carried on board the tow.
11.15.6.4 For spaces that will be sounded regularly, a tube and striker plate should be available.
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11.15.7 Vents
11.15.7.1 All compartments connected to a pumping system shall have vents fitted. The vents should be of
an approved, automatic, self-closing type. If not automatic, then the vents should be sealed for towage with
wooden bungs or steel blanks, but with a 6 mm diameter breather hole fitted.
Guidance note:
This will give audible warning or reduce pressure differentials in event of mishap, and compensate for temperature changes. The
breather hole can be drilled into the gooseneck of the vent or through the wooden bung used to close the vent.
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11.16 Anchors (and alternatives) and mooring arrangements
11.16.1 Emergency anchors
11.16.1.1 Emergency anchors have traditionally been required to reduce the risk of a tow running aground
if a tug is disabled or a towline broken. However in many cases the disadvantages (described in [K.5])
associated with using such anchors can outweigh the advantages.
11.16.1.2 If a tow passes through an area of restricted sea room, a comparative risk assessment should be
performed to determine the preferred arrangements. [K.5] sets out topics to be taken into account in this
risk assessment. One possible outcome can be the provision of suitably sized extra tugs for some sectors of
the tow.
11.16.1.3 The same requirements apply for towed ships, including demolition towages. See [11.23.3].
Where such towages may need to wait for a few days on arrival at the end of a voyage before documentation
is completed then, if this is in a high-current area, anchoring or mooring arrangements can be required.
11.16.1.4 For jack-up platforms, see also [11.28.16].
11.16.2 Size and type of anchor
11.16.2.1 For classed vessels and barges, the anchor(s) fitted in accordance with class requirements should
be acceptable unless there is deck cargo.
Guidance note:
For open deck vessels and barges, the anchor is designed to hold the vessel or barge only and does not account for deck cargo
windage.
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11.16.2.2 In other cases the minimum weight of the emergency anchor should be 1/10 of the towline pull
required (TPR) for the tow, as defined in [11.12.2]. A high holding power anchor with anti-roll stabilisation
should be used.
11.16.3 Anchor cable length
The effective length of anchor cable should be greater than 180 m, and should be mounted on a winch. If the
cable runs through a spurling pipe, or other access, to storage below decks, then the pipe or access should
be capable of being made watertight.
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11.16.4 Anchor cable strength
11.16.4.1 For cable on a winch, or capstan, which can be paid out under control, the MBL of the cable
should be 15 times the weight of the anchor, or 1.5 times the holding power of the anchor if greater.
11.16.4.2 For cable flaked out on deck, the MBL of the cable should be 20 times the weight of the anchor, or
twice the holding power if greater.
Guidance note:
The increase from the requirements in [11.16.4.1] is to allow for the extra shock load.
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11.16.4.3 The last few flakes of cable on deck should have lashings that will break and slow down the cable
before it is fully paid out.
11.16.5 Attachment of cable
11.16.5.1 The inboard end of the cable should be led through a capped fairlead near the vessel centre line
and be securely fixed to the vessel. Precautions should be taken to minimise chafe of the cable.
11.16.5.2 The MBL or ULC of the connections of the cable to padeye or winch, and padeye or winch to the
vessel structure should be greater than that of the cable.
11.16.5.3 For towed ships, and tows with similar arrangements, the anchor cable(s) shall be properly
secured, with the windlass brake(s) applied. Any additional chain stopper arrangements that are fitted shall
be utilised, or alternatively, removable preventer wires should be deployed.
11.16.5.4 Spurling pipes into chain lockers should be made watertight with cement plugs, or another
satisfactory method.
11.16.6 Anchor mounting and release
11.16.6.1 If there is no suitable permanent anchor housing the anchor should be mounted on a billboard, as
shown in [K.4], at about 60° to the horizontal.
11.16.6.2 The anchor should be held on the billboard in stops to prevent lateral and upwards movement. It
should be secured by wire rope and/or chain strops that can be easily released manually without endangering
the operator.
11.16.6.3 The billboard should normally be mounted on the stern. It should be positioned such that on
release the anchor will drop clear of the vessel and the cable will pay out without fouling.
11.16.6.4 For any system, it shall be possible to release the anchor safely, without the use of power to
release pawls or dog securing devices. If the anchor is held only on a brake, an additional manual quick
release fastening should be fitted.
11.16.6.5 The anchor arrangement should be capable of release by one person. Adequate access shall be
made available.
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11.16.7 Mooring arrangements
11.16.7.1 All vessels and floating objects should be provided with at least four mooring positions (bollards/
staghorns etc.) on each side of the vessel unless it is impracticable to moor them, e.g. because of draught
limitations.
11.16.7.2 If fairleads to the bollards are not installed then the bollards should be provided with capping
bars, horns, or head plate to retain the mooring lines at high angles of pull. Suitable chafe protection should
be fitted as required e.g. to the deck edge for low angles of pull.
11.16.7.3 At least four mooring ropes in good condition of adequate strength and length should be provided.
Guidance note:
Typically the mooring ropes are about 50 mm to 75 mm diameter polypropylene or nylon, and each 60 m to 90 m long.
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11.16.7.4 Mooring ropes should be stowed in a protected but accessible position.
11.16.7.5 Objects with very large freeboard such as FSUs should be fitted with mooring and towing
connection points along the side, at a convenient height above the towage waterline. The connection points
should not damage, or be damaged by, attending vessels.
Guidance note:
These can provide a more convenient connection for mooring lines and harbour tugs than bollards at deck level.
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11.17 Manned voyages
11.17.1 General
11.17.1.1 Manning of tows should generally be limited to those where early intervention by a riding crew
can be shown to reduce the risks to the tow, for example tows of MOU’s, passenger ships and Ro-Ro vessels.
11.17.1.2 Where a riding crew is carried on a tow for commissioning and/or maintenance, sufficient marine
personnel shall be included to operate the equipment listed in [11.17.4] and to carry out the duties in
[11.17.5].
Guidance note:
A riding crew can be carried on an FPSO or FSU for similar reasons.
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11.17.1.3 Riding crew carried on any dry transport shall be within the carrying vessel’s flag state limits for
life saving appliances; any exceedance of the flag state limit shall be approved by the flag state in advance.
Guidance note:
There is sometimes a requirement for a riding crew on a dry transport to maintain or commission systems or to carry out general
maintenance.
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11.17.1.4 The transport contractor shall provide the MWS company documented flag state approval for the
proposed number of riding crew. The underwriters should also be informed if a large riding crew is proposed.
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Guidance note:
The transport contractor should therefore obtain this flag state approval in good time.
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11.17.1.5 The health and safety of the riding crew shall be ensured at all times.
11.17.1.6 A risk assessment shall be carried out, in accordance with [2.4], to demonstrate the acceptability
of the proposed arrangements.
11.17.2 International regulations
Accommodation, consumables, lifesaving appliances, pumping arrangements and communication facilities
with the tug shall comply with International Regulations.
11.17.3 Riding crew carried on the cargo
Where a riding crew is carried on the cargo, for instance a maintenance crew on a dry-transported jack-up
rig, additional precautions shall be considered including:
— Access to/from the cargo/rig forward and aft, and to the evacuation or escape area(s).
— The cargo/rig’s life rafts and lifeboats should be relocated and the falls lengthened, if necessary, so that
on launching they will land in the water.
— A firewater supply should be made available to the cargo/rig.
— The cargo/rig’s and vessel’s alarm systems should be linked, so that an alarm on the cargo/rig is repeated
on the vessel, and vice versa.
11.17.4 Safety and emergency equipment
11.17.4.1 Notwithstanding the requirements of SOLAS, /139/, and any or all international regulations
for life saving appliances and fire-fighting equipment, the minimum complement of safety and emergency
equipment carried aboard the tow shall be as follows:
— Certified life rafts located on each side of the tow, clear of any possible wave action, provided with means
of launching and fitted with hydrostatic releases. The life raft or life rafts on each side of the tow shall be
capable of taking the full crew complement. Adequate means of access to the water shall be provided
— 4 lifebuoys, two located on each side of the tow and including two fitted with self-igniting lights and two
with a buoyant line.
— Approved life jackets to be provided for each crew member plus 25% reserve.
— If appropriate, a survival suit to be provided for each crew member.
— First aid kit.
— Fire-fighting equipment, which can consist of an independently powered fire pump with adequate hoses,
and portable fire extinguishers as appropriate.
— 6 parachute distress rockets and 6 hand held flares.
— A daylight signalling lamp and battery.
— 2 portable VHF radios, fitted with all marine VHF channels, with appropriate battery charging equipment.
— Hand held GPS (global positioning system) receiver.
— DSC VHF radio
— Charts covering the route
— An EPIRB (emergency position indicating radio beacon) emergency transmitter.
— 2 SARTs (search and rescue radar transponder).
— Heaving line(s) and/or line throwing apparatus if appropriate.
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11.17.4.2 All members of the riding crew shall be adequately trained in the use of the safety equipment. At
least 1 crew member shall possess the appropriate radio operator’s licences.
11.17.5 Manned routine
The riding crew shall take the following actions during the towage:
— Maintain a daily log and include all significant events.
— Inspect towing arrangements and navigation lights.
— Inspect all seafastenings and any other accessible, critical structures.
— Tension any adjustable seafastenings or lashings as necessary.
— Check soundings of all bilges and spaces.
— Monitor any unexpected or unexplained ingress of water.
— Pump out any ingress of water.
— Maintain regular contact by radio with the tug, reporting any. abnormalities
— Ensuring that the required sound signals are made during restricted visibility.
11.18 Specific for multiple towages
11.18.1 Definitions
11.18.1.1 This subsection [11.18.1] expands on the definitions in Table 1-4 for multiple towages:
11.18.1.2 Double tow – 2 tows each connected to the same tug with separate towlines. One towline is of
sufficient length that the catenary to the second vessel is below that of the first.
11.18.1.3 Tandem tow – 2 (or more) tows in series behind 1 tug, i.e. the second and following tows
connected to the stern of the previous one.
11.18.1.4 Bifurcated tow - the method of towing 2 (or more) tows, using one tow wire, where the second
(or subsequent) tow(s) is connected to a point on the tow wire ahead of the preceding tow, and with each
subsequent towing pennant passing beneath the preceding tow.
11.18.1.5 Two tugs (in series) towing one tow – where there is only 1 towline connected to the tow and the
leading tug is connected to the bow of the second tug.
11.18.1.6 More than 1 tug (in parallel) towing one tow – each tug connected by its own towline, pennant or
bridle to the tow.
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Figure 11-8 Multiple towage types (not to scale)
11.18.2 General
11.18.2.1 Compared with single towages, multiple towages have additional associated problems including
those of:
— Manoeuvring in close quarter situations, especially at the start and end of a tow.
— Reconnecting the towlines after a breakage.
— Maintaining sufficient water depth for the longer and deeper catenaries required.
11.18.2.2 With the exception of the cases described in [11.18.1.6], multiple towages can only be approvable
in certain configurations, areas and seasons, and subject to a risk assessment.
11.18.2.3 When approval is sought, then full details of the operation, including detailed drawings,
procedures and equipment specifications shall be documented. An initial assessment of the method will then
be made, and if the basic philosophy is sound, recommendations can be made for the approval process to
continue.
11.18.2.4 Approval can be declined if any doubt exists as to the viability of the operation proposed.
11.18.2.5 For those multiple towages that are approvable, each tow shall be prepared as described in this
standard.
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11.18.2.6 Additional factors can be applied to the towing arrangements, so that the probability of breakage
is further reduced.
11.18.2.7 The bollard pull requirement of the tug shall be according to the number and configuration of
the tows connected. The towline pull required (TPR) should be the sum of those required for each tow. The
towing arrangements on each tow shall have sufficient capacity for the Bollard Pull (BP) of the tug(s).
11.18.2.8 The tug shall be equipped as in [11.13], although additional or stronger equipment and longer
towlines can be necessary. Where longer towlines are required, these can be formed by the utilisation of
pennant wires of no less Ultimate Load Capacity than the main tow wires.
11.18.2.9 Where the towing configuration requires the use of 2 towlines from 1 tug, a third tow wire shall be
carried on board the tug, stowed in a protected position, whence it can be safely transferred at sea to either
towing winch.
11.18.2.10 Consideration should be given to including (surge) chain or a stretcher to improve the spring, or
to provide the required catenary in any towing arrangement.
11.18.2.11 If a synthetic stretcher is included in any towing arrangement, it shall comply with [11.13.10]. A
spare stretcher shall be carried aboard the tug for each stretcher utilised in the towing arrangement.
11.18.2.12 For multiple tows being towed behind a single tug, special arrangements shall be made on the
deck of the tug to separate the towlines.
Guidance note:
This requirement is because the tows can yaw in different directions.
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11.18.2.13 Special procedures shall be agreed and included in the towing manual for reconnection.
Guidance note:
It is particularly difficult to reconnect to a tow that has broken loose when another tow or tows are connected to the same tug.
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11.18.2.14 Due to the difficulties that will be encountered if a towline breakage should occur, the number of
crew on the tug should be increased over that required for a single tow.
11.18.3 Double tows
11.18.3.1 These should only be considered when:
a)
b)
c)
the area is benign
the towage duration is short and covered by good weather forecasts
Where there is sufficient water depth along the tow route to allow for the catenary required for the
second tow.
11.18.3.2 The tug should be connected to each tow with a separate towline on a separate winch drum. It
shall also carry a spare towline, stowed on a winch, or capable of being spooled onto a winch at sea.
11.18.4 Tandem tows
11.18.4.1 These should only be considered when in very benign areas or in ice conditions where the towed
barges will follow each other.
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11.18.4.2 In ice conditions the towlines between tug and lead tow and between tows will normally be short
enough for the line to be clear of the water. Procedures shall be in place to avoid tows over-running each
other, or the tug.
11.18.5 Bifurcated tow
This method should only be considered when in extremely benign areas, and additional safety factors with
respect to the capacity of the towing arrangements shall be agreed with the MWS company.
11.18.6 Two tugs (in series) towing one tow
11.18.6.1 The first tug should be smaller and connected to the bow of a larger, less manoeuvrable second
tug.
Guidance note:
This arrangement is used to improve steering/manoeuvring.
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11.18.6.2 This configuration should only be considered when:
a)
b)
All the towing gear (towline/pennants/bridles/connections etc.) between the second tug and the tow is
strong enough for the total combined bollard pull.
The second tug is significantly heavier than the leading tug (to avoid girding the second tug).
11.18.7 Multiple tugs to one tow
11.18.7.1 Each tug should have a separate towline to the vessel (via bridles or pennants as required).
11.18.7.2 Consideration should be given to matching the size and power of the tugs. If 2 tugs are towing
they should normally be sister vessels and/or with similar propulsion and equipment. The difference in
Bollard Pull shall normally be within 10% of that of each other unless otherwise documented that the
difference in applied bollard bull can be kept below this.
11.18.7.3 There should not be more than 3 tugs, except for the towage of very large objects, such as FPSOs
and concrete gravity structures, and for manoeuvring at either end of a towage.
11.18.7.4 The use of eccentric bridles can be advantageous but care shall be taken to avoid chafe.
11.18.7.5 The following procedures shall be in place:
— One tug shall be nominated as the lead tug and the tow plan shall describe the lead tug’s roles and
requirement to lead manoeuvres.
— A communication protocol shall be established.
— The tow plan shall describe tow wire length and separation of vessels to avoid tow wire entanglement
and/or collision, in particular in cases of a tug loss of propulsion and/or steering.
— There shall be a minimum separation distance prescribed once underway and enough sea room is
available.
Guidance note:
The minimum separation distance should normally be 100 m.
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11.18.7.6 Emergency procedures shall address the loss of a tug's power, in particular if the middle tug in
a three-tug spread blacks out and can be over-ridden by the tow with catastrophic consequences. Suitable
emergency procedures and tow equipment shall be available to mitigate such a possibility.
11.19 Specific for towing in ice
11.19.1 General
11.19.1.1 This subsection [11.19] sets out the special technical and marine aspects and issues not covered
elsewhere in this standard for the approval of the towage of ships, barges, MOU’s and any other floating
structure towed in ice-covered waters.
11.19.1.2 It is recognized that towing in ice-covered water is a unique marine operation and that all vessels
and towages in ice are different - making this standard general in nature. Each approval will depend on the
result of an in-depth review of the towing manual as well as an equipment inspection/attendance by a MWS
company surveyor to identify any particular problems that can exist for the specific vessel(s) and towage in
question.
11.19.1.3 Structural safety and towing performance will require careful consideration of the size and shape
of the tow, especially with respect to the beam of the tow in comparison to the beam of the tug and the
shape of the bow of the tow. The beam difference will affect the level of ice protection provided by the tug to
the tow, as well as the ice interaction and towing resistance caused when the beam of the tow is greater than
that of the tug and/or of any independent icebreaker support. In addition, special towing techniques used in
ice and manoeuvrability restrictions caused by the ice require that experienced personnel plan and execute
the tow.
11.19.1.4 Except as allowed by [11.9.14], any vessel that is operated and/or towed in ice shall be in class
with a recognized classification society and have a current load line certificate.
11.19.1.5 After complying with the requirements of [11.19.1.1] to [11.19.1.4], the MWS company can deem
that the vessel/object is unfit for tow and decline to issue a certificate of approval. For example, the towage
of any tow which is damaged below the waterline, is suspected of being damaged below the waterline or has
suffered other damage or deterioration which could affect the structural strength and/or watertight integrity
will not be approved for towage in ice. Alternatively, the vessel/object can only be considered fit for tow after
specified repairs and suitable ice strengthening has been carried out.
11.19.2 Vessel ice classification
The tug(s) and towed vessel shall have an appropriate ice classification or equivalent for transit through the
anticipated ice conditions identified in the towing manual and verified by the MWS company. See [K.10] for
more information
11.19.3 Towage without independent icebreaker escort
11.19.3.1 Where no independent icebreaker escort is identified in the towing manual for the intended
voyage, the tug and tow shall be of appropriate ice classification and power to maintain continuous headway
in the anticipated ice conditions. When a tow is anticipated to take more than three (3) days (the maximum
for a reasonably accurate weather/ice forecast) or longer in ice conditions that includes a concentration of
five (5) tenths or more of limiting ice types, the towing manual shall indicate the location of the nearest
icebreaker support and the anticipated time before independent icebreaker assistance (Coast Guard or
Commercial) can be provided.
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11.19.3.2 With the exception of a vessel pushed ahead (push-towed), the ice classification requirement for
the towed object can be considered for reduction if it is determined that the tug has a higher than necessary
level of ice classification and can protect the tow from potentially damaging ice interaction.
11.19.4 Conventional ice towing operations
11.19.4.1 The tug shall have sufficient power and hull strength (ice classification) to be capable of safely
maintaining continuous towing headway through the worst anticipated ice conditions including, if necessary,
the breaking of large diameter floes and deformed ice with no requirement for ramming.
11.19.4.2 The towing manual shall show that the towage should not be subjected to ice pressure.
11.19.5 Close-couple ice towing operations
Close-couple towing is an operation that allows a specially designed icebreaker to combine towing and
icebreaking assistance. The stern of the icebreaker has a heavily fendered ‘notch’ into which the bow of a
ship is pulled by the icebreaker’s towline. The towline remains attached and the icebreaker steams ahead,
usually with additional power provided by the towed vessel in the notch. In this way an icebreaker can tow
a low-powered and low ice classed ship quickly (up to 3 times faster than conventional towing in ice) and
safely (better protection of the towed vessel and less risk of collision due to over-running) through high
concentrations of difficult ice. For close-couple towages:
— The beam of the icebreaker shall be more than that of the towed ship in order to avoid shoulder damage
to the towed vessel and excessive towline stress and:
— The icebreaker is fitted with a constant tension winch or equipment that will reduce the effects of shockloading:
— The bow of the towed ship shall be compatible with the notch design of the icebreaker. Preferably the
entrance of the towed ship is not so sharp as to apply excessive force on the stem when going straight
ahead. Freedom of movement of the towed ship’s bow can cause manoeuvring difficulties as well as
applying heavy side forces on the towed ship’s bow when turning. The bow should not be so bluff that
all the force is concentrated in localized areas. In addition the towed ship cannot have a bulbous bow
because the underwater protrusion could damage the icebreakers propellers and:
— The displacement and freeboard of the towed vessel should not be so disproportionate with that of the
icebreaker that the manoeuvring characteristics of the icebreaker are seriously compromised:
— The anticipated ice conditions should not require ramming or passage through areas where high levels of
ice pressure can be experienced without independent icebreaker assistance.
11.19.6 Push-tow operations
11.19.6.1 Push-tow operations should be carried out using either rigid connection (composite unit) or
flexible connections (a push-knee erected at the stern of the pushed vessel). Where the design and ice
strength of the tug and tow is acceptable, especially when experiencing ice pressure, consideration should be
given to pushing rather than towing in ice.
Guidance note:
Pushing enables headway to be maintained and to remove the stress from the towline. Push towing can also be more efficient.
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11.19.6.2 Where the push-towing technique is to be used, the pushed vessel shall have acceptable ice
strengthening, particularly in the bow and shoulder areas.
11.19.6.3 The ice classification of a tug that is engaged in a ‘push-tow’ operation with no independent
icebreaker support can be reduced if:
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— the vessel being pushed has appropriate ice classification and strength for unescorted transit in the
anticipated ice conditions and:
— the beam of the pushed vessel is greater than that of the tug. The beam of the pushed vessel should be at
least one third greater than that of the tug to allow suitable manoeuvring for a flexible connection and:
— the connection between tug and tow is of suitable strength for emergency stops and:
— the towing manual shows that the ‘push-tow’ will not enter, or be exposed to, an area where ice pressure
can be encountered of sufficient severity to stop the continuous forward progress of the push-tow without
independent icebreaker assistance.
11.19.7 Towage operations with independent icebreaker escort
The ice classification requirements indicated in [K.10] for the tug(s) and towed vessel(s) can be considered
for reduction if it is determined that appropriate icebreaker escort assistance is provided for the duration of
the tow in ice and that:
— The icebreaker(s) has sufficient capability to allow the towage to maintain continuous headway through all
of the anticipated ice conditions and,
— The icebreaker(s) has a beam equal to, or greater than, the tug and tow combination or:
— The icebreaker(s) is fitted with suitable and operational equipment such as azimuthing main propulsion
units or compressed air systems that are capable of opening the track wider than the beam of the
escorted towage in the anticipated ice conditions or:
— More than one icebreaker will be used to provide a broken track equal to, or wider than, the beam of the
tug and tow combination.
11.19.8 Manning
11.19.8.1 In addition to [11.12.13] concerning manning, special consideration should be given to the
number, qualification and experience of personnel required on the navigating bridge to ensure safe navigation
including steering and engine control, lookout, operation of searchlights and, emergency operation of the
towing winch abort system.
11.19.8.2 The master in charge of a tow (towmaster) should typically have at least 3 years’ experience of
towing in ice conditions similar to those anticipated for the proposed towage. Other navigating officers on
tugs involved in a towage in ice should also have previous experience of towages in ice.
11.19.9 Multiple towages
11.19.9.1 Multiple towages in ice are subject to the requirements set out in this subsection [11.19.9]
regarding ice classification, equipment and suitable propulsion power as well as the general provisions
(particularly those in [11.18]). However, only in exceptional circumstances of very light ice and/or very low
ice concentration (trace) will a double tow (see [11.18.3]) or a bifurcated tow (see [11.18.5]) be considered
for approval. An in-depth risk assessment would be required and the risks shown to be acceptable.
11.19.9.2 In addition to the provisions in [11.19.9.1] for towages using more than one tug or multiple tows:
1)
2)
3)
To avoid collision or over-running each tug shall have a quick release and re-set system as described in
[11.19.11.1] and [11.19.11.2].
The most experienced tug master shall be designated as the towmaster and give directions to the other
vessels. All other tug masters and senior navigating officers involved in the multi-tug towage should
have an appropriate level of experience of towing in ice and be familiar with the associated difficulties
and hazards.
A multi-tug towing manual that does not include independent icebreaker escort assistance shall
demonstrate clearly why it is not considered necessary. As an acceptable example, the tow could be
configured such that one or more tugs with the capability to perform ice management (escort duties)
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can be released, and the remaining tug(s) have sufficient BP to continue making towing progress. In
some circumstances a towing manual can include the contingency of releasing one or more tugs that are
towing in the conventional manner to push-tow provided that:
— the towed vessel is appropriately ice strengthened
— the towed vessel is appropriately designed and strengthened in the pushing location(s
— the tugs are designed and adequately fendered for pushing
— such action would only be considered in a high ice concentration where there is no influence by sea or
swell.
11.19.9.3 When two tugs are towing in series as described in [11.18.6] in an ice infested area, the towing
connections on the foredeck of the second tug shall be strong enough for any shock loading that may result
from the lead tug to breaking through ice floes of varying thickness.
11.19.9.4 For a tandem tow (as described in [11.18.4.2]) where the presence of ice increases the potential
for rapid changes to the towing speed, good fenders shall be in place between each unit in the tow due to the
close connection.
Guidance note:
This is sometimes referred to as ice-coupled.
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11.19.10 Towing equipment
The towing techniques that are used in ice typically require a short distance between the tug and tow to
increase manoeuvrability and so that the propeller wash from the tug can assist in clearing ice accumulation
around the bow of the towed vessel. Because of the short towing distance and reduction of towline catenary
it is necessary for the towing arrangement to be suitable for the additional stress that can be experienced.
The stress on the towing arrangement can vary considerably with:
— the thickness and concentration of ice as well as ice pressure,
— the difference in beam between the tug and tow resulting in ice interaction on the shoulders of the towed
vessel and ice accumulation in front of the tow as well as the use and effectiveness of independent
icebreaker escort and
— large heading deviations due to manoeuvring through and around ice and
— unintentional tug interaction with heavy ice floes which can result in shock-loading to towing components
due to whiplash and the tow taking charge.
It is for these reasons that additional provisions concerning towing equipment strength, type and
configuration are necessary.
11.19.11 Additional equipment requirements for towing in ice
11.19.11.1 In addition to [11.12.5] a tug involved in towing in ice infested waters shall be fitted with an
operational towline quick release/reset system (tow-wire abort system) when:
— towing in ice that could rapidly reduce towing speed or
— a tug is involved in a multiple tow or
— a tug is involved with a multi-tug tow.
11.19.11.2 The towline quick release system should be capable of immediate winch brake release for pay
out of tow-wire as well as winch brake re-set from the navigation bridge and the winch control station (if
different).
11.19.11.3 With reference to [11.12.9], a tug involved in a towage in ice should be fitted with at least two
searchlights that can be directed from the navigation bridge.
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11.19.11.4 As required by [11.12.11.2] and [11.13.16], every tug that is towing in ice shall be equipped
with burning and welding gear for ice damage control and repair.
11.19.12 Strength of towline
11.19.12.1 With reference to [11.13.3.1] for a tug that is planning a conventional single towline towage in
ice the towline MBL should be as follows:
Table 11-21 Minimum towline MBL in ice
Bollard pull (BP)
MBL (tonnes)
BP ≤ 40 tonnes
40 < BP ≤ 150 tonnes
BP > 150 tonnes
2 × BP+60
11.19.12.2 An exception can be made for short tows in very thin ‘new’ ice or in very low concentrations
(< 3/10ths) of medium or thick ‘rotten’ ice. In these circumstances the towline MBL should be computed as
shown for a ‘non-benign’ tow in [11.13.3.1].
11.19.12.3 Consideration should be given to use of low temperature lubricants in the manufacture of
towlines for use in polar regions to reduce the probability of breakage.
11.19.12.4 The strength of all other towing connections and associated equipment should be appropriately
calculated as in [11.13.3.5].
11.19.12.5 Further, all tugs involved in a towage in ice shall carry a spare tow-wire of the same length
and strength as the main tow-wire that is immediately available on a reel to replace the main tow-wire. In
addition, there shall be enough competent personnel, equipment and spares on board to crop and re-socket
the main tow-wire at least once.
11.19.13 Special cases of reduced tow-wire strength
11.19.13.1 The minimum size of tow-wire that is typically used by icebreaking tugs of 160t BP for closecouple towing is for example, 64 mm EIPS rove through a multiple sheave floating ‘Nicoliev Block’ system.
In this system a single bridle wire, usually of the same size and strength as tug's main tow-wire, is made
fast to each bow of the vessel being towed. The tow-wire goes from the towing winch to the floating block
on the bridle and back via a fairlead to a towing damper on the tug. For larger powered tugs, the tow-wire
can be doubled up again by passing the wire through a standing block on the tug’s deck and around a second
sheave on the floating block before it is made fast to the towing damper. This makes the bridle wire the
‘weak link’ in the system and because of this an icebreaking tug shall carry sufficient spare bridle wires,
typically at least 6.
11.19.13.2 To meet the minimum towline strength criteria a tug that has an appropriate bollard pull can, in
exceptional circumstances, be considered for approval of a conventional towage in calm waters containing ice
using two towlines provided that:
— Each of the two independent towlines is a minimum of 90% of the required strength and
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— Each tow-wire is on a separate towing winch that can be adjusted, quick released and reset independently
from the other and
— Each tow-wire meets the requirements of a single tow-wire in terms of minimum length, construction etc.
and
— Each tow-line has a monitoring system to enable load sharing.
11.19.14 Towing winches
11.19.14.1 Towing winches shall be provided due to the typical manoeuvring restrictions and hazards that
are inherent to towing in ice.
Guidance note:
Towing hooks do not allow for the rapid adjustment of towline length.
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11.19.14.2 Each towing winch should have sufficient pull to allow the towline to be shortened under tension.
The navigating bridge and winch operator should be provided with continuous readouts of towline length
deployed and towline tension.
11.19.14.3 Winch controls and winch operating machinery should be suitably protected from environmental
conditions, particularly low temperatures that can result in winch malfunction.
11.19.14.4 Towing winches shall have a quick release and re-set system as described in [11.19.11.1] and
[11.19.11.2].
11.19.15 Chain bridles
A chain bridle is typically used for a towage in ice with a chain pigtail connected to a ‘fuse wire’ or directly to
the towline. In some circumstances where high shock loads are anticipated, an extra-long chain pigtail can
be considered appropriate. Wire pennants and bridles are sometimes used for small barge and vessel tows,
especially when the close-couple or ice-couple towing technique is anticipated.
11.19.16 Synthetic rope
Synthetic rope shall not be used in a towing system for an in-ice towage, therefore [11.13.9.2] and all parts
of [11.13.10] do not apply in ice transits or in very low temperatures where icing can occur.
Guidance note:
Synthetic rope is prone to rapid cutting both internally by ice crystals and externally by ice edges.
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11.19.17 Bridle recovery system
In addition to the requirements of [11.13.11.1]:
— To reduce direct ice interaction and disconnection of the bridle recovery wire, the wire should be lightly
secured to one leg of the bridle and the end shackled onto the apex or a chain link close to the apex of
the tri-plate.
— The fuel mentioned in [11.13.11.2] for a motorized recovery winch shall be appropriate for the anticipated
temperatures.
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11.19.18 Emergency towing gear
11.19.18.1 For all towages in ice the emergency towing gear should be fitted and arranged to tow from
the bow unless it can be shown that the object being towed is designed for multi-directional towing. With
reference to [11.13.13], special arrangements can be required for the emergency towing gear, especially on
an un-manned tow proceeding in ice.
11.19.18.2 The emergency tow gear arrangement shall not be susceptible to being cut and lost or snagged
by ice and pulled clear of retaining soft lashings or metal clips, especially in high concentrations of ice.
Guidance note:
For example, an intermediate wire can be attached to the end of the emergency tow-wire and lightly secured to a pole extended
astern at least 5 m. The eye of the intermediate wire is suspended above the surface of the ice approximately 1 m above the
aft working deck of the tug where it can be captured for connection to a tugger-winch wire. The float line and pick-up buoy are
shackled to the emergency tow-wire in the same way as described in [11.13.13.5] but remain coiled on the deck of the tow for
deployment once the tow arrives in open water.
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11.19.19 Access to tows
With reference to [11.13.15], whether a tow is manned or not, suitable access shall be provided. For towages
in ice, a permanent steel ladder should be provided at the stern from the main deck to just above the
waterline. As discussed in [11.13.15.2], ladders, particularly side ladders should be recessed to avoid ice
damage. A tug workboat should carry suitable equipment to de-ice recessed access arrangements and
ladders to tows. Pilot ladders used as a short term alternative should be closely inspected for ice damage
before being used.
Guidance note:
Typically, a pilot ladder secured at the stern of the tow is subject to the least amount of ice interaction.
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11.19.20 Towing equipment certification and special precautions
11.19.20.1 As described in [11.13.14], all equipment used in the main and emergency towing arrangements
for a towage in ice shall have valid certificates. Special precautions are necessary for equipment that has
been, or will be, used in extremely low temperatures.
11.19.20.2 Regardless of anticipated temperatures during the proposed towage, a MWS company
surveyor can request to have sockets, chains, flounder plates and shackles used in the towing process nondestructively tested (NDT) before the towage.
11.19.20.3 Before departure a visual inspection of the tow-wire shall be performed, and based on the
results the MWS company surveyor can also require that the tow-wire is cropped and re-socketed before the
towage.
11.19.21 Safety equipment for the workboat
In addition to [11.12.6], sufficient Arctic survival suits shall be carried on board the tug for all personnel
that can be operating the workboat and personnel transferred to the tow by the work boat. These additional
survival suits should be fitted with hard soled boots, belts and detachable gloves.
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11.19.22 Bollard pull requirements
The tug shall have a bollard pull appropriate for the anticipated ice and weather conditions. The calculated BP
should never be less than that necessary for an open ocean (un-benign) towage, as shown in [11.12.2].
11.19.23 Oversized tug
For all towages in ice, [11.13.3.7] concerning towing connections does not apply. In the case of an oversized
tug (in terms of TPR) all connections should be at least equal to the required towline MBL of the tug in use,
which in turn should comply with [11.19.12] and [11.19.13]. The towmaster shall be fully aware of any
strength reduction to the connections, carry adequate replacement spares and the towing procedures and
any certificate of approval should identify the maximum power setting that can be applied.
11.19.24 Cargo loadings
11.19.24.1 Special attention should be given to cargo overhangs on a case-by-case basis.
11.19.24.2 For towage in ice, the cargo shall not overhang unless it can be shown that the cargo is
adequately protected so that no ice interaction can occur.
11.19.24.3 To determine the potential for ice interaction, calculations shall show that the cargo has at least
three meters clearing height above the maximum height of ice deformity that can be experienced during the
tow. In all ice concentrations this minimum clearing height shall be maintained in all conditions of roll, pitch
and heave (see [11.3] and [5.2]). Due to the potential for ice impact and resulting damage cargo overhang
cannot be allowed to immerse under any circumstance, so that [5.6.5.4.1], [11.3.4.2], [11.10.1.11] and
[11.10.1.12] are not applicable.
11.19.25 Seafastening design and strength - motions
11.19.25.1 The cargo mass shall include the effect of ice accretion calculated in accordance with the IMO
Intact Stability Code 2008, /89/, Part B Chapter 6.
11.19.25.2 In low ice concentrations, the motions of a vessel transiting should be assumed to be as severe
as those experienced in clear open water storm conditions.
Guidance note:
Swell waves can persist for many miles even into an ice edge of very high ice concentration.
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11.19.25.3 In high ice concentrations the strength of cargo and sea-fastenings for voyages in ice conditions
shall be of acceptable design and not less than that required for weather unrestricted voyages in non-benign
areas - see [11.3] and [5.2].
Guidance note:
Despite no waves being evident, impact or over-running of thick ice floes can cause sudden deceleration, heading deflections,
listing and rolling of the tow.
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11.19.26 Inspection of welding and seafastenings
With reference to [5.10.2], welding procedures and techniques shall account for very cold temperatures,
particularly for sea-fastening installation.
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11.19.27 Pipes and tubulars
With reference to [11.9.9.6], stress on pipes in a stack, and [11.9.9.12], open ended pipes, special
consideration should be given to pipes filling with ice due to freezing spray and/or wave action in low
temperatures and the potential to overstress lower levels of pipe, seafastenings and deck structures. The
effect on the vessel stability should also be considered.
11.19.28 Stability in ice
11.19.28.1 Stability calculations for vessels, including tugs and tows, operating in very cold temperatures
and in ice conditions shall be documented and reviewed against the IMO Intact Stability Code 2008, /89/,
Part B Chapter 5.
11.19.28.2 The intact range of stability of a towed vessel (see [11.10.5]) shall never be less than 36°,
including inland and sheltered towages.
11.19.28.3 For transit in ice-infested waters, the statement in [11.10.1.11] of this standard shall
be modified to read ‘Cargo overhangs shall be such that no immersion is possible in the anticipated
environmental conditions’. See [11.19.24.3]
11.19.28.4 [11.10.1.12] referring to buoyant cargo overhangs does not apply to transits in ice.
11.19.28.5 In addition to the requirements of [11.10.6], towed objects shall have positive stability with any
two compartments flooded or breached.
11.19.28.6 The damaged stability relaxations for towed objects in [11.10.6.5] do not apply in any area
where ice interaction can occur.
11.19.28.7 The integrity of all underwater compartments of a tug and compartments subject to downflooding shall be safeguarded from flooding by watertight doors and hatches that access such compartments.
This is a critical requirement for an approval to conduct a towage in ice. All compartment accesses shall be
checked for watertight integrity and kept closed at all times throughout the towage.
11.19.28.8 The draughts mentioned in [11.10.8] are the minimum for open water operations. In an ice
environment, additional consideration shall be given to the location of any specially strengthened ‘ice belt’
and to the exposure of areas vulnerable to ice damage such as propulsion and steering equipment that can
require specific and/or deeper overall ice transit draughts.
11.19.28.9 A vessel being towed or pushed (regardless of being self-propelled) shall not be excessively
trimmed. On manned tows the trim should be appropriate to provide watch personnel with as much forward
visibility as possible for observation of approaching ice conditions and the movements of other vessels
involved in the towage to reduce the potential for ice impact and/or collision damage.
11.19.29 Ballasting in ice
11.19.29.1 Unless otherwise agreed the forepeak should be ballasted to above the waterline of tug(s) and
towed vessel(s).
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Guidance note:
This done to assist with ice impact load dispersal. This also provides protection against developing excessive trim by the head
in the event that a forward compartment is breached by ice and flooded. In addition, the emptying of a ballasted forward
compartment can assist with exposing damage for emergency repair or to raise the damaged area clear to avoid continued ice
interaction and escalation of damage.
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11.19.29.2 Structural damage caused by pressurizing compartments when ballasting and deballasting due
to water freezing in tanks or inside tank vent pipes shall be avoided.
11.19.29.3 The freezing of tank vents from coating with freezing spray in very low temperatures shall be
avoided.
11.19.30 Voyage planning in ice
11.19.30.1 In addition to the requirements listed in [11.14], a written voyage plan or towing manual should
be documented in advance of a proposed towage in an ice-infested region.
11.19.30.2 The plan should include:
— A general description of the proposed voyage (manned/un-manned towage etc.)
— Tug and tow particulars including ice classifications and certification
— Research documentation indicating the anticipated ice/weather conditions
— Routeing including shelter and holding locations
— Navigation and communications equipment appropriate for the region
— Summary of towmaster and senior officer experience
— Arrangements for receiving weather and ice information and/or routeing
— Voyage speed and fuel calculations including any bunkering requirements and procedures to comply with
National regulations
— Contingency fuel, hydraulic and lubricating oils of suitable viscosity for the low ambient temperatures
— Main and emergency towing arrangements and certification
— Stability calculations and location of all cargoes, consumables, ballast and pollutants for the tug and tow
— Sea-fastening (cargo securing) arrangements
— Arrangements for assist tugs for docking etc. and for ice management as required
— Damage and pollution control equipment as applicable
— Contingency procedures for ice damage, tug breakdown, fire, broken towline, man overboard and the
nearest icebreaker assistance.
11.19.30.3 In addition to the list in the previous section, before departure the towmaster of an unmanned towage shall be supplied with the appropriate drawings that indicate the basic structure, watertight
compartments, ballast system, cargo securing arrangements on the tow, and manuals that provide the
tug crew with operating procedures for emergency equipment such as ballast pumps (see [11.15]), the
emergency generator, the emergency anchor system and the tow bridle retrieval system.
11.19.30.4 Refuelling the tug: The towing manual shall indicate the calculated fuel usage during the tow for
the required power in the anticipated ice conditions.
11.19.30.5 For the portion of the voyage that will be carried out in ice conditions, in addition to the times
listed in [2.6.2] to [2.6.4] - the operation reference period, and [3.4.18] - calculation of voyage speed, the
planned duration shall account for:
— towing speeds of not more than 2 knots in ice covered areas as a conservative estimate where the actual
towing distance is unlikely to be direct. A towing speed of 5 knots can be used where it can be shown that
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the tow will only encounter very thin new ice or alternatively very low concentrations (< 3/10ths) of thick
rotting ice and:
— waiting for appropriate ice conditions for departure, transit and arrival and:
— up to 25% additional fuel (and other consumables) can be required (see [11.12.12]).
11.19.30.6 The towing manual shall indicate compliance with the International, National and Local
regulations and guidelines concerning the carriage of oil cargoes, the allowable quantity and distribution
of fuel oil or any other pollutant or dangerous cargo. In addition, where a towing manual indicates the
requirement to re-fuel the tug from the tow or from another vessel this will normally require special approval
from a National authority and also require that the tug carries appropriate pollution containment and cleanup equipment. The re-fuelling approval from the appropriate jurisdiction, as well as the re-fuelling procedure
and equipment, shall be documented in the towing manual.
11.19.31 Weather/ice restricted operations
In addition to the requirements of [2.5.3] for a towage in an ice infested area, dependable ice forecasts shall
be available and the tug shall have appropriate equipment on board to receive ice information including ice
maps, bulletins, advisories and forecasts.
11.19.32 Damage control and emergency equipment in ice
Special consideration should be given to the remoteness of the area and the anticipated ice conditions where
a towage will take place to determine the availability of emergency response, assistance and equipment. In
addition to the damage control equipment listed in [11.13.16], the following additional equipment should be
available for a towage in ice:
— Portable generator
— Portable compressor
— Portable salvage pump(s)
— Bracing shores
— Portable de-icing equipment
— Space heaters
— Extension ladders
— Chain falls
— Collision mat materials.
11.20 Specific for towage in the Caspian Sea
11.20.1 Background
For the purposes of this standard the Caspian Sea has been divided into the shallow Northern area (North
of 45ºN latitude as shown in Figure 11-9), an Intermediate area between 45ºN and Kuryk (approximately
43ºN), and the Southern area. The Intermediate area has been introduced for vessels travelling between the
Northern and the Southern areas with relaxations subject to suitable weather routeing.
Guidance note 1:
The Northern area contains 25% of the total Caspian Sea area but only 5% of the water volume. The shallow water (typically
3 m to 5 m deep, and very rarely more than 10 m) is a feature of the area which leads to the ready formation of ice in the winter
months. Although winds can be very strong, the limited fetches and shallow water do not allow significant wave heights above
about 3.5 to 4 m.
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Guidance note 2:
Because the water level depends on river inflows balancing the evaporation, there are long term and seasonal rises and falls in the
mean sea level and seawater density. As at 2005, the mean sea level (MSL) was 27 m below Baltic Datum (equivalent to global
mean sea level) and 1.0 m above Caspian Datum.
Figure 11-9 Northern and Intermediate Caspian Sea areas
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11.20.2 Towage requirements for all Caspian Sea areas
11.20.2.1 Single propeller tugs should not be used, unless there are suitable additional (redundant) tugs in
attendance to replace them.
Guidance note:
The whole Caspian Sea suffers from a large number of unmarked fishing nets which provide a serious hazard to tugs which can be
immobilised by these nets fouling their propellers.
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11.20.2.2 Pusher tugs should not be used for pushing in open waters.
Guidance note:
Many of the tugs found in this region are pusher tugs
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11.20.3 Towage requirements within northern Caspian Sea
11.20.3.1 General
The following changes from the requirements in [11.4], [11.12] and [11.13] can be accepted for tows that
take place totally within the Northern area (North of 45ºN latitude).
11.20.3.2 Bollard pull requirements
Because of the limited wave heights (due to the shallow water) the meteorological criteria for calculating the
towline pull required (TPR) referred to in [11.12.2] when there is no ice, can be taken as:
— Hs = 2.5 m
— wind = 20 m/s
— current = 0.5 m/s.
provided that the tow will have adequate sea room after the initial departure. If there will not be adequate
sea room, then [11.12.2.4] will apply.
11.20.3.3 Towline lengths
Within this area the minimum length in metres deployable for each of the main and spare towlines shall be
determined from the 'European formula':
except that in no case shall the deployable length (as defined in [11.13.4.4]) be less than 200 m.
Guidance note:
Because of the very shallow water depths and limited wave heights the minimum towline lengths required in [11.13.4.4] can be
reduced.
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11.20.3.4 Towline strength
Where the length is less than required by [11.20.3.3] and unless other methods of reducing the shock loads
are used, the towline MBL shall be increased in line with [11.13.4.7]. The towing connection capacities in
[11.13.3.5] shall be related to the increased required towline MBL.
Guidance note:
Because of the shorter towline length there will be little catenary to absorb shock loads in bad weather.
As an example, a deployed towline length of 200 m will require a towline MBL of 6 (=1,200/200) times the continuous static
bollard pull.
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11.20.3.5 Towing connections
Suitably positioned, purpose-built quick-release towing connections are preferred. Where bollards have to be
used as the towline connection:
— The capacity of the bollards and their foundations shall comply with the requirements of [11.13.5].
— Suitable fairleads and anti-chafe arrangements shall be used.
— A keeper plate, capping bar or other means of keeping the towing bridle connected to the bollards shall be
provided and this shall be suitable for any vertical loads likely to be encountered.
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— The design shall also allow for quick release of the keeper plate, capping bar or another proven method to
rapidly clear a fouled bridle.
11.20.3.6 Workboat
A twin screw tug fitted with a bow thruster and two anchors in accordance with class requirements can be
exempt from the requirement for a workboat in [11.12.6] provided the voyage can be completed within a
favourable weather forecast. The tug shall also be able to come alongside the tow at sea so that crew can
board with any necessary equipment for pumping, repairs, dropping the vessel’s anchor or reconnecting a
towline.
11.20.3.7 Bunkers
The requirement for 5 days reserve in [11.12.12] can be reduced to 3 days (pumpable reserve) provided
that:
— the towage can be completed within a good weather forecast period, and
— there are suitable bunkering ports within 3 days sailing at all times, and
— there are suitable tugs available to take over the tow if required during a diversion for refuelling.
11.20.3.8 Default motion response
: The following default values will apply, where applicable, for voyages entirely within the Northern Caspian
Sea Area.
Table 11-22 Default motion criteria for Northern Caspian
Nature of voyage
Weather unrestricted
1)
[m]
Block
coeff
Full cycle
period
[secs]
1
> 37 m and > 15 m
any
2
< 37 m and > 15 m
any
Case
LOA [m]
B
1)
Single amplitude
Heave
Roll
Pitch
10
13.5º
7.5º
0.1g
10
13.5º
13.5º
0.1g
B = maximum moulded waterline beam.
11.20.4 Towage requirements for remaining Caspian Sea areas
All tows in this area should follow the requirements in [11.3], [11.11], [11.12], [11.13] and [11.20.5] for
weather unrestricted tows outside benign weather areas, as applicable.
11.20.5 Requirements for towages between Caspian Sea areas
11.20.5.1 Many shallow draught tugs that are designed for working in the shallow Northern area will be
unable to carry towing gear suitable for towing in the Southern area. When it is not practicable for towages
to change tugs when travelling between these areas whilst within the intermediate area defined in [11.20.1],
and subject to suitable weather routeing, the following relaxations can be accepted:
— Deployable towline length to be at least 400 m, and
— Towline and towing connection strength requirements of [11.20.3.4] and [11.20.3.5] will apply, and
— Minimum bollard pull requirements as in [11.20.3.2].
11.20.5.2 Weather routeing will include:
— Voyage planning to avoid travelling too close to a lee shore and to identify sufficient suitable safe places of
shelter for different weather directions, and
— Receipt of regular marine weather forecasts and a commitment to go to a suitable safe place of shelter on
receipt of a bad weather forecast.
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11.21 Specific for voyages in inland waterways
11.21.1
The stability requirements in [11.10] shall apply.
11.21.2
Other standard criteria including the following may be relaxed with agreement of the MWS company but shall
never be less than the regulatory requirements:
— towline length
— towline termination
— spare and emergency towline
— under-keel clearance.
11.21.3
Where the voyage is pushed rather than towed consideration shall be given to the interface between the tug
and the barge including strength and fendering arrangements.
11.22 Specific for FSUs (FPSOs, FSOs, FLNG facilities, FRSUs etc.)
11.22.1 General and background
11.22.1.1 This subsection [11.22] addresses the specific marine-related issues associated with the towage
of these units, not already addressed in this standard. Although it is recognized that there are many more
marine activities in an FSU development, towage to field or operating location is a critical and often long
operation, which shall be addressed by the project team early in the schedule.
Guidance note:
—
Some FSU developments are ‘fast-track’, resulting in construction and commissioning activities being completed during the
—
New-build or converted FSUs usually undertake a limited number of towages only, following construction or conversion. There
—
Frequently the design weather conditions for towage are more severe than the service conditions. There is a natural reluctance
tow.
can be a further towage at the end of their working life.
to build in additional strength or equipment which will have no practical value during the service life.
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11.22.1.2 Project-specific fit-for-purpose criteria shall be agreed in each case.
11.22.2 Route and weather conditions
11.22.2.1 Metocean design criteria should be carefully established early in the project, in accordance with
[3.2].
11.22.2.2 Mitigation of the design extremes for shorter routes may be achieved by the use of staged
towage, in accordance with [11.14.4.1].
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11.22.2.3 The need for appropriate additional tugs for passage through restricted or busy waters shall be
considered and agreed with the MWS company.
11.22.3 Structural issues
11.22.3.1 The integrity of the FSU’s hull shall be maintained and precautions taken to ensure no damage
occurs during the tow, particularly the reliability, integrity and quality of the hull including its coating(s) other
than by reasonable wear and tear.
Guidance note:
FSUs are intended to remain at sea without dry-docking for their entire working life, usually in the order of 20 years. A commercial
vessel is usually assumed, for design purposes, to spend about 20% of its life in port, and is periodically dry-docked. These
differences place much greater emphasis on ensuring the quality of the hull.
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11.22.3.2 For long towages, fatigue damage can need to be considered (see [5.9.4]).
11.22.3.3 The capability of the FSU to withstand design environmental conditions for the towage shall be
demonstrated. Checks should include hull girder strength, local plating strength, operating limit states for
process equipment including rotating machinery.
11.22.3.4 Equipment foundations shall be designed for the temporary phase operations. Fatigue damage to
the connections between the topsides and hull should be considered.
11.22.3.5 Any temporary equipment aboard shall be secured to withstand the design environmental
conditions. If construction, completion, or commissioning work is performed during tow, then all the
scaffolding, temporary power packs, work containers etc. shall be installed to withstand the design
environmental conditions. Any scaffolding or other temporary works which cannot comply with the design
environmental conditions shall be dismantled or removed.
11.22.3.6 Green water damage or slamming damage on temporary equipment should be considered in the
location of equipment.
11.22.4 Tug selection
11.22.4.1 Tugs shall be selected, as a minimum, in accordance with [11.12], but due to their size FSUs will
often need a large total bollard pull requiring 2 or more tugs. See [11.18.7] for towages by more than one
tug.
11.22.4.2 There should be redundancy in the towing fleet.
Guidance note:
Redundancy in the towing fleet gives greater freedom for bunkering, where one tug can divert to bunker whilst the other(s)
continue(s) with the towage, as described in [11.14.7].
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11.22.4.3 The use of additional tug(s) can be required in restricted waters.
11.22.4.4 If it is not possible or practical to provide an emergency anchor, then additional or larger tugs can
be required. See also [11.22.9].
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11.22.5 Ballast, trim and directional stability
11.22.5.1 To limit the loss of directional stability the hull shall be carefully ballasted, trimmed by the stern
and in the case of a ship-shape hull with the forefoot well immersed. The ballast distribution shall be checked
to ensure that the shear and longitudinal bending moment are within acceptable limits.
Guidance note 1:
Having the forefoot will immersed will reduce slamming in heavy weather.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Directional stability under tow can be compromised resulting in the FSU veering off the course line. This is due to various factors
related to the design and construction of the FSU, including but not limited to:
—
The presence of a mooring or riser turret, below the keel of the vessel, generally at the forward end or mid-length.
—
The removal of the vessel’s rudder, where the FSU is a conversion.
—
The hull design of purpose-built FSUs.
—
High windage structure at the fore end.
The lack of directional stability can be hazardous due to:
—
Lack of sea room in congested and/or confined waters, e.g. Dover Strait
—
Accelerated deterioration of the towing gear caused by excessive movement, especially wear of chains.
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11.22.5.2 Consideration should also be given to attaching a tug at the stern of the FSU (see also [11.22.5.4]
below).
11.22.5.3 The design of the towing gear should minimise the directional instability.
11.22.5.4 Consideration can be given to towing by the stern. If this is proposed then any motions analysis
or model testing shall recognise this configuration. The strength of the hull in way of the stern shall be
checked to ensure that:
— The stern can withstand the anticipated slamming loads.
— Suitably sized towing connections and fairleads are or can be attached.
11.22.6 Towing equipment
11.22.6.1 Requirements for assisting tugs to provide additional manoeuvring control, and to assist with
berthing or connection to the permanent mooring system shall be assessed for:
— Departure
— Any intermediate ports
— Any shelter areas
— Bunkering
— Arrival.
11.22.6.2 The towing equipment shall be configured to accommodate additional and assisting tugs and to
allow connection and disconnection when required.
Guidance note:
These activities can dictate the equipment on board the unit. For example, tugger winches, davits or cranes could be needed.
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11.22.6.3 As noted in [11.22.5], FSUs can exhibit a lack of directional stability during towage, therefore the
following should be incorporated into the tow gear design:
— The towing brackets on the vessel need to be wide-spaced, preferably more than one-half of the beam.
— The chafe chains should be generously oversized (typically +50%) to allow for accelerated wear during
the voyage.
11.22.6.4 At least one emergency towline shall be provided. A means to recover each bridle after any
breakage shall be provided. The manning levels of the vessel shall be considered in the type and location of
any recovery gear.
11.22.7 Self-propelled or thruster-assisted vessels
11.22.7.1 In some cases, the FSU can have its own propulsion, which can be either the original ship’s
system or thruster units to be used in service. If these are to be used for the voyage to site, the vessel shall
comply fully with all regulatory requirements.
11.22.7.2 The specification of the thruster units, power supplies and manning shall be suitable for the
voyage requirements and documented at an early stage.
11.22.7.3 A risk assessment shall be undertaken, in accordance with [2.4], to determine the need for
assisting tugs.
11.22.8 Manning and certification
11.22.8.1 The documentation set out in Table B-2 should be submitted.
Guidance note:
Most FSUs are not classed as ships during their service life.
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11.22.8.2 If the towage is to be manned, then the requirements of [11.17] shall be considered.
11.22.8.3 A dedicated marine riding crew should be provided, regardless of the presence of construction or
commissioning personnel, as shown in [11.17.1.4].
11.22.8.4 In all cases, whether manned or un-manned, the unit shall be fitted with appropriate means of
boarding, in accordance with [11.13.15]
11.22.9 Emergency anchor
The general emergency anchor requirements of [11.16] shall apply.
Guidance note:
FSU mooring systems (whether turret-type or spread), being only for in-place conditions, are not configured to act as emergency
moorings during transit. On a conversion the permanent anchors will often be removed. For many designs the deck space where an
emergency anchor might be sited is taken up with the permanent mooring equipment.
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11.22.10 Moorings
The need for moorings before, during or immediately after the towage shall be considered. Design and layout
of such quayside moorings should be incorporated into the overall arrangement of the vessel as described in
[11.16.7]. See Sec.17 for mooring design.
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11.23 Specific for jacket voyages
11.23.1 Introduction
This subsection [11.23] gives requirements specific for jacket voyages and not already addressed in this
standard.
11.23.2 Fatigue, wave slam and vortex shedding
These following items shall be specifically considered: fatigue see [5.9.4], wave slam, see [5.6.5.4] and
vortex shedding, see [11.9.13].
11.23.3 Equipment seafastenings
11.23.3.1 Equipment which does not form part of the permanent jacket structure shall be seafastened to
withstand the same motion criteria as the jacket. When determining the design accelerations, particular
attention shall be given to the location of the item on the jacket structure as during the voyage the
acceleration of an elevated item can be much higher than the acceleration at the jacket centre of gravity.
For sizeable items, its inertial moment (about its own neutral axes) shall need to be considered to correctly
determine the additional load on the support points due to rotational effects.
11.23.3.2 Piles or similar items carried in pile sleeves or guides shall be secured so that movement does not
cause fatigue of the attachments. Wooden wedges shall not be assumed to prevent movement.
11.23.3.3 Rigging platforms, and their attachments to the jacket, shall be designed to support their own
weight and the weight of all rigging attached to them. The de-rigging case, when high impact loads can be
expected, shall also be considered.
11.23.3.4 Rigging shall be adequately secured to rigging platform structural members or jacket members
accounting for the elevation of the rigging (see [11.23.3.1]). The rigging shall not impinge on control lines/
equipment. Any such control lines/equipment shall be secured separately. Lashing should be of manila
rope lashings with a minimum of three crossovers at no more than 2.5 m centres. Alternatives, including
engineered seafastenings can be accepted (see [K.8]).
11.23.3.5 Shackles shall be individually secured to the jacket members to avoid possible impact on the
jacket during the voyage which could cause damage to the jacket.
11.23.3.6 Items which could be exposed to wave action during either voyage or launch shall be suitably
secured and protected against the expected loadings.
11.23.3.7 Flexible control lines and cables for the ballast and/or grout systems should be protected from
wave action.
11.23.4 Transport on deck of crane vessel
11.23.4.1 Jackets and piles are sometimes transferred to the deck of the crane vessel for final transport
to the installation location, or at the location itself to facilitate installation. The weight of the grillages and
seafastening shall be accounted for.
11.23.4.2 Seafastening loads should be derived taking into account the motions of the crane vessel and
wind loads. It should be demonstrated that these are no more severe than the voyage design loads.
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11.23.4.3 The sea state for deriving the crane vessel motions shall be the return period storm applicable
for the operation reference period and for the route or location, whichever is the more severe (see [3.2]).
Reduced exposure criteria shall not be applied for transport or awaiting installation on the crane vessel.
11.23.4.4 If it is necessary to change the draught of the crane vessel to minimise motions and thereby limit
loads on the jacket or seafastenings, this shall be incorporated into the marine procedures.
11.23.4.5 For relatively small and inherently stable items temporarily transferred to the deck of an SSCV,
it can be practical to dispense with seafastening provided the sea state is below, and is forecast to remain
below, a defined limit. If so, this shall be incorporated into the marine procedures.
11.23.4.6 The global strength of the crane vessel should be checked for lifting from its own deck as the load
shift from deck to crane hook could cause exceedance of the maximum allowable bending moment and/or
shear capacity. (This is particularly relevant to crane vessels converted from other uses).
11.23.5 Wet towed jackets
11.23.5.1 Where a jacket is to be wet towed the towing procedures should be documented at an early
stage. Depending on the jacket draught, tow route, tow duration and likely exposure, the MWS company can
specify additional requirements on a case by-case-basis.
Guidance note:
A jacket can be 'wet towed' vertically or horizontally on its own buoyancy to the installation site. This can either be achieved with
most of the jacket members submerged, OR with the jacket lower face bracing being close to or at the waterline.
---e-n-d---o-f---g-u-i-d-a-n-c-e---n-o-t-e---
11.23.5.2 A full and concise HAZID, HAZOP and risk assessment, in accordance with [2.4], shall be carried
out to document the risk mitigation measures that shall be in place during tow and installation.
11.23.5.3 Notification of the towage shall be given to all the necessary authorities including the military
authorities as the tow route can be subject to submarine activity or low flying aircraft.
11.24 Specific for ship towage
11.24.1 General considerations
11.24.1.1 This subsection [11.24] sets out the technical and marine aspects, in addition to the general
requirements above, which would be need to be considered for the towage of ships, including demolition
towages.
11.24.1.2 Minimum certification and documentation requirements are shown in Table B-2. If the towed
vessel is not in class with a recognized classification society, or does not possesses a current load line or load
line exemption certificate then further surveys shall be required as in [11.9.14] to ensure that the vessel is
suitable to be towed or if further repairs or dry-docking are required.
11.24.1.3 The towage of any vessel which is damaged or suspected of being damaged below the waterline,
or has suffered other damage or deterioration which could affect the structural strength will not normally be
approvable except where it is clearly shown by survey and calculation that the strength of the vessel and its
watertight integrity is satisfactory for the intended towage.
11.24.1.4 Passenger ships and warships, because of the complex nature of their systems, pose particular
problems with respect to their compartmentation, and require special consideration. Ro-Ro ships can also
pose particular problems, on account of the potentially large free surface in the event of flooding. Passenger
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ships and Ro-Ro ships will generally only be approved for towage if the tow is manned, to permit early
intervention in the event of any problems.
11.24.1.5 Any heavy fuel oil within the tanks of the vessel shall be identified and shall be minimised where
possible. In the event of heavy fuel oil being carried, possible limitations on entry to ports of refuge and
places of shelter shall be noted and taken into account in the towage procedures. To minimise the risk of
pollution, the requirements of the IMO MSC.1-Circ.884 Guidelines for Safe Ocean Towing, /138/, paragraph
13.19, shall be taken into account as far as is practical.
11.24.1.6 If a stern-first towage is required (see [11.13.1.2]) then special care shall be taken regarding
towing connections, draught, trim and the control and protection of the tow during the towage.
11.24.2 Towlines and towing connections
11.24.2.1 Each ship or vessel towage is unique and it is therefore not possible to specify the connection
equipment to be used or how it is to be attached for every case. Alternative systems are suggested in [K.6].
Any equipment used for the towage shall be fit for purpose and shall be agreed between the owner of the
tow, the tug master and the MWS company. In particular it shall be shown that towing connections and their
foundations, above and below deck, comply with [11.13.3.5]. If necessary, reinforcements shall be fitted to
achieve the required capacity, otherwise alternative arrangements shall be made.
11.24.2.2 Where mooring bitts are utilised to secure chain to the tow, and in order to ensure that the towing
arrangement is securely anchored on the vessel and does not slip on the bitts, the chain should be backedup to further bitts abaft the main connection points using suitable wire pennants locked into position with
clips. If such an arrangement is used then the first bitts used shall have the required ultimate capacity,
unless positive load-sharing can be achieved. Bitts and fairleads shall be capped with welded bars or plates of
sufficient strength to prevent equipment jumping off or out of the arrangement.
11.24.3 Anchors
11.24.3.1 An emergency anchor shall be provided if required as a result of the risk assessment described in
[11.16.1.2] and appropriate access afforded for deployment by one person.
11.24.3.2 Port and starboard anchor cables shall be properly secured with the windlass brake applied. Any
additional chain stopper arrangements that are fitted shall be utilised or, alternatively, removable preventer
wires shall be deployed.
11.24.3.3 Spurling pipes into chain lockers shall be made watertight with cement plugs or another
satisfactory method.
11.24.4 Securing of equipment and moveable items
11.24.4.1 In general, all equipment shall be secured to meet the appropriate motion requirements of [11.3],
and seafastenings of loose items designed in accordance with [5.2] and [11.9.1].
11.24.4.2 See [11.28.11.6] for securing and use of cranes and lifting derricks.
11.24.4.3 The rudder shall be positioned in the amidships position, or as agreed with the tug master, and
immobilised.
11.24.4.4 The propeller shaft shall be immobilised, or disconnected, to prevent damage to machinery during
the towage.
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11.24.4.5 Every effort shall be made to limit the carriage of any loose deck equipment to an absolute
minimum. Where equipment is carried on an exposed deck then it shall be protected and secured against
movement using welded brackets, chain or wire. Equipment in other areas shall also be secured.
11.24.4.6 For large equipment, engineering calculations shall be carried out in order to verify that the
securing of items is satisfactory.
11.24.4.7 Additional protection or securing can be required for equipment exposed to wave slam.
11.24.5 Carriage of cargo
11.24.5.1 The carriage of manifested cargo on the tow shall not normally be approved unless the tow is
manned and is fully clas