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NOBLE DENTON MARINE SERVICES
DNV GL Noble Denton marine services warranty standards wizard includes the
following standards per today; DNVGL­ST­N001 Marine operations and marine
warranty.
Disclaimer
The extracted sections below are based on your selections in the wizard. DNV GL do not take
on any responsibility for your selection related to your project scope and DNV GL expressly
disclaims any liability if the outcome of the selection does not encompasses the need or does
not fit for purpose.
Where DNV GL Noble Denton marine services is the Marine Warranty Survey provider, it
should be read in conjunction with DNVGL­SE­0080 Noble Denton marine services – marine
warranty survey, which provides a description of the process used by DNV GL Noble Denton
marine services when providing marine warranty survey (MWS) services to evaluate whether
a marine operation can be accepted for the purposes of insurance­related MWS. It addresses
both ‘project’ and MODU/MOU related MWS.
The use of our standard presupposes and does not replace the application of industry
knowledge, experience and know­how 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 professional judgment.
SECTION 0CHANGES – CURRENT
This document (DNVGL­ST­N001 ­ Edition 2016­06) replaces the legacy DNV­OS­H­series
and all legacy GL Noble Denton Guidelines except 0009/ND, 0016/ND, which are addressed
in the DNVGL­ST­N002 standard and 0021/ND which will be addressed in a service
specification.
The following is a summary provided for guidance on where the contents of the legacy
documents can be found in this standard.
Sec.1 Introduction
Sec.2 Planning and execution
This section replaces the following parts of the VMO Standard and the ND Guidelines:
DNV­OS­H101
0001/ND.
Sec.3 Environmental conditions and criteria
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and
legacy DNV­OS­H­series standards.
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Sec.4 Ballast and other systems
This section replaces the following parts of the VMO Standard and the ND Guidelines:
DNV, Marine Operations, General, DNV­OS­H101
DNV, Load Transfer Operations, DNV­OS­H201
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
GL Noble Denton, Guidelines for Load­outs, 0013/ND
GL Noble Denton, Guidelines for Float­over Installations / Removals, 0031/ND.
Sec.5 Loading and structural strength
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and
legacy DNV­OS­H­series standards.
Sec.6 Gravity based structure (GBS)
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, Guidelines for concrete gravity structure construction & installation,
0015/ND
DNV Offshore Standard, Load transfer operations, DNV­OS­H201.
Sec.7 Cables, pipelines, risers and umbilicals
Sec.8 Offshore wind farm (OWF) installation operations
This section replaces the applicable sections of the following legacy document:
0035/ND Guidelines for Offshore Wind Farm Infrastructure Installation.
Sec.9 Road transport
This section is new.
Sec.10 Load­out
This section replaces the applicable sections of the following legacy documents:
DNV­OS­H201, Load transfer operations
GL Noble Denton, Guidelines for Load­outs, 0013/ND
Sec.11 Sea voyages
This section replaces the applicable sections of the following legacy documents:
DNV­OS­H202, Sea transport operations
DNV­OS­H203, Transit and Positioning of Offshore Units
GL Noble Denton, Guidelines For Marine Transportations, 0030/ND.
Sec.12 Tow out of dry­dock or building basin
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
DNV Offshore Standard, Load Transfer Operations, DNV­OS­H201.
Sec.13 Jacket installation operations
This section replaces the applicable sections of the following legacy documents:
DNV Offshore Standard, Offshore Installation Operations (VMO Standard Part 2­4),
DNV­OS­H204
GL Noble Denton, Guidelines for Steel Jacket Transportation & Installation, 0028/ND.
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Sec.14 Construction afloat
This section replaces the applicable sections of the following legacy documents:
0015/ND Guidelines for concrete gravity structure construction & installation
DNV Offshore Standard DNV­OS­H201 Load Transfer Operations.
Sec.15 Lift­off, mating and float­over operations
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, Guidelines For Float­Over Installations / Removals, 0031/ND
DNV Offshore Standard DNV­OS­H201 Load Transfer Operations.
Sec.16 Lifting operations
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, Guidelines For Marine Lifting & Lowering Operations, 0027/ND
DNV Offshore Standard DNV­OS­H205 Lifting Operations (VMO Standard – Part 2­5)
DNV Offshore Standard DNV­OS­H206 Load­out, transport and installation of subsea
objects (VMO Standard – Part 2­6).
Sec.17 Mooring and dynamic positioning systems
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, Guidelines for Moorings , 0032/ND
DNV­OS­H101 Marine Operations, General
DNV­OS­H102 Marine Operations, Design and Fabrication
DNV­OS­H203 Transit and Positioning of Offshore Units.
Section [17.13] replaces the applicable Dynamic Positioning related sections of the following
legacy documents:
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
DNV Offshore Standard, Transit and Positioning of Offshore Units, DNV­OS­H203.
Sec.18 Decommissioning and removal of offshore installations
This section replaces Section 14 of 0001/ND “General Guidelines for Marine Projects”.
SECTION 1Introduction
1.1General
1.1.1
DNV GL 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. Our collective industry best practice and guidance documentation is referenced
and used all over the world. This document includes the harmonized legacy DNV standards
and legacy GL Noble Denton guidelines, with the exception of those for MODU/MOU site
specific assessment.
1.1.2
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Where DNV GL Noble Denton marine services is the Marine Warranty Survey provider, it
should be read in conjunction with DNVGL­SE­0080 Noble Denton marine services – marine
warranty survey, /38/, which provides a description of the process used by DNV GL Noble
Denton marine services when providing marine warranty survey (MWS) services to evaluate
whether a marine operation can be accepted for the purposes of insurance­related MWS. It
addresses both ‘project’ and MODU/MOU related MWS.
1.1.3
This document may be used in its complete form using the relevant sections based on the
asset type and/or operation. It is recommended that the reader uses the Noble Denton
marine services wizard available through My DNV GL (https://my.dnvgl.com/
(https://my.dnvgl.com/)) for easier access and to obtain the relevant sections based on asset
type and/or operation.
1.1.4
The use of this standard presupposes and does not replace the application of industry
knowledge, experience and know­how 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 professional judgment. DNV GL has however no
obligations or responsibility for any services related to this standard delivered by others.
DNV GL 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 GL does not accept any liability or responsibility for loss or damages
resulting from any use of this document.
1.1.5
Further provisions and background information are contained in the appendices.
1.1.6
In some cases risk assessments can be used to justify project­specific 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 client to, and agreement by, the
insurance underwriters.
1.1.7
Execution of operations not adequately covered by this Standard shall be specially
considered in each case.
1.1.8
Fulfilment of all requirements in this Standard does not guarantee compliance with
international and national (statutory) regulations, rules, etc. covering the same
subjects/operations.
1.1.9
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This Standard should if required be used together with other recognized codes or standards
applicable for marine operations.
1.1.10
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.
1.1.11
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.
1.1.12SWL and WLL:
Safe Working Load (SWL) has generally been superseded by Working Load Limit (WLL)
though both are in common use during the change­over period. However 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). Whenever
possible this standard uses minimum breaking load (MBL) or ultimate load capacity
(ULC) to avoid these problems.
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).
There is often some confusion about the differences between WLL and SWL. SWL is a
derated value of WLL, following an assessment by a competent person of the maximum
static load the item can sustain under the conditions in which the item is being used.
SWL may be the same or less than WLL but can never be more.
1.2Objective
1.2.1
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.
1.3Scope
1.3.1
This standard addresses the marine operations that can occur during the development of an
offshore asset or when objects are moved by water from one place to another. It addresses
the Marine Warranty Survey requirements relevant to load­out, construction afloat, voyages
and installation and the load cases that should be addressed in the design.
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 jack­up location
approval.
1.3.3
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1.3.3
With the exception of location approval of MOUs (Mobile Offshore Units) which are covered in
DNVGL­ST­N002, /39/, this standard covers most offshore assets and operations that are
likely to require MWS approval.
1.4References
1.4.1Normative (i.e. mandatory) references
1.4.1.1
The standards and guidelines in Table 1­1 include provisions, through which reference in this
text constitute provisions of this standard.
Table 1­1 Normative (i.e. mandatory) standards
Id
Date
Revision
Specification for Structural Steel Buildings,
(included in AISC Steel Construction Manual
14th Edition)
2010
14
DNVGL­OS­
C101
Design of offshore steel structures, general –
LRFD method
2015
DNVGL­ST­
N002
Site specific assessment of mobile offshore
units [due to be issued in 2016, until then GL
Noble Denton 0009/ND “Guidelines for site
specific assessments of jack­ups” applies]
2016
EN 1993
Eurocode 3, Design of steel structures
IMO IMDG
International Maritime Dangerous Goods Code
IMO Intact
Stability Code
Intact Stability Code
IMO
International
Convention on
Load Lines
IMO International Convention on Load Lines,
Consolidated Edition 2002
IMO COLREGS
IMO International Regulations for Preventing
Collisions at Sea, 1972 (amended July 2015)
(COLREGS)
1972
(amended
July 2015)
IMO ISM Code
IMO International Safety Management Code ­
ISM Code ­ and Revised Guidelines on
Implementation of the ISM Code by
Administrations
2002
AISC: 360/10
Name
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2006
2008 and
later
amendments
2002
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IMO ISPS Code
International Ship and Port Facility Security
Code (amendment to SOLAS convention)
IMO Resolution
A.1024(26)
Guidelines for ships operating in polar waters
ISO 19901­5
Petroleum and Natural Gas Industries “Specific
requirements for offshore structures – Part 5:
Weight control during engineering and
construction”.
2002
(effective
2004)
Jan 2010
2016
1.4.2Informative references
1.4.2.1
All references appear in Sec.19.
1.5Definitions
1.5.1Verbal forms
Table 1­2 Definitions 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, or that a certain
course of action is preferred but not necessarily required
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.
1.5.2Terms
1.5.2.1
Underlined definitions are defined elsewhere in Table 1­3.
Table 1­3 Definition of terms
Term
Definition
1st intercept
(angle)
The first angle of static inclination at which the wind overturning
moment is equal to the righting moment (see Figure 11­3 and
Figure 11­4)
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24­hour Move
A jack­up 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.
2nd intercept
(angle)
The second angle of static inclination at which the wind overturning
moment is equal to the righting moment (see Figure 11­3 and
Figure 11­4)
9­Part sling
A sling made from a single laid sling braided nine times with the
sling rope and eyes forming each eye of the 9­part sling.
A&R Winch
The 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.
Accidental Limit
State
The 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
Non­dimensional coefficient dependant on the overall shape of the
structure
Alpha Factor
The maximum ratio of operational criteria/design environmental
condition to allow for weather forecasting inaccuracies. See [2.6.9]
Angle of Loll
The static angle of inclination after flooding, without wind heeling
(see Figure 11­4)
Approval
The 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
An structure or object subject to an insurance warranty or at risk
from an operation
Assured
The Assured is the person who has been insured by some insurance
company, or underwriter, against losses or perils mentioned in the
policy of insurance.
Barge
A non­propelled vessel commonly used to carry cargo or equipment.
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Base weight
The calculated weight of a structure, excluding all allowances and
contingencies. Sometimes known as net weight
Bend Restrictor
A device with several interlocking elements that lock when a design
radius is achieved.
Bend Strain
Reliever (BSR)
A 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
A partial safety factor that accounts for the reduction in strength
caused by bending round a shackle, trunnion, diverter or crane hook.
Benign (weather)
area
An area with benign weather as described in [3.6]
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. See [11.18.1.4]
Bird­caging
A 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 (BP)
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”
A 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
A configuration of two or more pipelines joined together and either
strapped or contained within a carrier or sleeve pipe.
Burial Assessment
Survey (BAS)
A 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 (BPI)
A 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
A submarine power cable is trenched into the seabed and covered
with soil providing complete burial of a cable.
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Cable Grips
Cable Grips are used to pull or support cables and pipes. They work
on the principle of the harder the pull, the tighter the grip.
Cable Tank
A circular storage area where cable is coiled.
Cable­laid
grommet
A 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
Cable­laid sling
A 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)
(Safety
Equipment)
Certificates issued by a certifying authority to attest that the vessel
complies with the cargo ship construction and survey
regulations,
has radiotelephone equipment compliant with requirements
and
carries safety equipment that complies with the rules
applicable to that vessel type.
Carrier or Sleeve
pipe
The outer casing of a bundle or pipe­in­pipe.
Cats­paw
An 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 (CoA)
A formal document issued by a MWS company surveyor stating that,
in his/her judgement and opinion, all reasonable checks,
preparations and precautions have been taken to keep risks within
acceptable limits, and an operation may proceed.
Certified
Having (or proved by) a certificate from an acceptable source
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
A system of ensuring 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.
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Client
The company to which the MWS company is contracted to perform
marine warranty or consultancy activities.
Cold Stacking
Cold stacking is 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
A MOU which floats on its columns during operation or transit (e.g.
semi­submersible).
Competent person
A 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 in­house 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 responsible for
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 γb
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.
Controlled Depth
Tow (CDT)
A 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
The vessel, ship or barge on which lifting equipment is mounted. For
the purposes of this document it is considered to include: crane
barge, crane ship, derrick barge, floating shear­leg, heavy lift vessel,
semi­submersible crane vessel (SSCV) and jack­up crane vessel.
Cribbing
An 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
(XLPE)
A type of AC cable conductor insulation commonly used on
submarine power cables.
Cross Sectional
Area (CSA)
Normally the CSA of a single conductor in a submarine power cable x
3. For example a submarine power cable with 3x600 mm2 in its
designation would be a cable with three conductors each of
600 mm2.
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Dead Man Anchor
(DMA)
Anchor or multiple anchors (which may be clump weights,
sometimes buried), typically used to initiate pipelay.
Deck mating
The act of installing integrated topsides over a substructure,
generally by float­over and ballasting. Deck mating may take place
inshore or offshore, onto a floating or a previously installed
substructure.
Deck Support Unit
(DSU)
Unit installed on the vessel grillage to support the structure before
and during the float­over. 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
This is 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
The design wave height, wave period, wind speed, current and other
relevant environmental conditions specified for the design of a
particular voyage or operation.
Determinate lift
A 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
The operation of towing two tows with two separate tow wires by a
single tug. See [11.18.1.2]
Dry Towage
The operation of transporting a cargo on a barge.
Dunnage
Typically dunnage is 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 (DAF)
The factor by which the weight is multiplied, to account for
accelerations and impacts during the operation
Dynamic Angle
The 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
A lift which is planned, designed and executed in a detailed manner,
with thorough supporting documentation. See [16.1.1.4].
Export Cable(s)
Submarine power cables connecting the offshore wind farm
transformer station to a landfall connection.
Factored weight
The calculated weight of a structure, including all allowances and
contingencies. Sometimes known as gross weight.
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Fatigue Limit State
The limit state related to the capacity of the structure to resist
accumulated effect of repeated loading.
Field Joint Coating
(FJC)
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
The 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
The state under which a commercial vessel is registered or licenced.
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 off­load
The reverse of floating on­load
Floating on­load
The operation of transferring a cargo, which itself is floating, onto a
vessel or barge, which is submerged for the purpose.
Floating Production
System (FPS)
Including FPV, FPU, FPSO, FGSO, spar (buoy) or TLP
Float­Over
The 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
Forecasted
Operational
Criteria
The 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.
Freeboard
Freeboard is defined as the 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 (GPS)
A satellite based system providing geographic coordinate location.
Grillage
A 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 (HTV)
A vessel which is designed to ballast down to submerge its main
deck, to allow self­floating cargo(es) to be on­loaded and off­loaded.
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Host Structure
The 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.
Hydro­acoustic
Positioning
Reference (HPR)
A 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
Inshore Mooring
A mooring operation in relatively sheltered coastal waters, but not at
a quayside.
Inspection and
Test Plan (ITP)
A plan in which all test, witness and hold points for all aspects of a
cable installation are listed.
Insurance
Warranty
A 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 (IACS)
A listing of IACS members is given on the IACS web site
http://www.iacs.org.uk/explained/members.aspx
(http://www.iacs.org.uk/explained/members.aspx)
International Cable
Protection
Committee (ICPC)
A 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, /92/
An international treaty concerning the safety of merchant and other
ships and MOUs.
International
Maritime
Organization (IMO)
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 (ISM)
The ISM Code provides an International standard for the safe
management and operation of ships and for pollution prevention.
Intersection Point
The point at which two straight sections or tangents to a pipeline
curve, or two slings, meet when extended.
ISM Code
International Safety Management Code ­ the International
Management Code for the Safe Operation of Ships and for Pollution
Prevention ­ SOLAS Chapter IX, /92/
I­tube
A vertical tube fitted to offshore structures to install product
between the seabed and the structure topsides.
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Jacket
A sub­structure, positioned on the seabed, generally of tubular steel
construction and secured by piles, designed to support topsides
facilities.
Jack­up
A self­elevating MODU, MOU or similar, equipped with legs and
jacking systems capable of lifting the hull clear of the water.
J­Lay
A 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 sea­bed.
J­tube
A J shaped tube fitted to offshore structures to install product
between the seabed and the structure topsides.
Kilometre Point
The position of on pipeline route at a given distance from an agreed
reference point, typically at or near one end.
Lay Back
The horizontal offset from the last pipe support on the lay vessel to
the touch down point on the seabed.
Leg Mating Unit
(LMU)
Unit that is 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
The connection between the rigging and the structure to be lifted.
May include padear, padeye or trunnion
Lifting Beam
A lifting beam is a structure designed to be connected to a lifting
appliance at a single point, and structure being lifted is connected to
the bottom of the beam at two or more lift points. The beam shall
resist the bending moments. It is not designed to carry compression
loads.
Lightship weight
The weight of the hull plus permanently installed items.
Limit state
A state beyond which the product or component no longer satisfies
the given acceptance criteria
Limit State 1
(LS1)
An ASD/WSD design condition where the loading is gravity
dominated; also used when the exclusions of [5.9.7.1 3)] apply.
Limit State 2
(LS2)
An 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
operational limiting criteria are less than the design environmental
criteria 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
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Linear Cable
Engine (LCE)
An industry term commonly used to refer collectively to cable lay
tensioners.
Link beam/link
span
The 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 Factor (LF)
A factor used on a design load in a limit state analysis and is also
used in the design of slings and grommets used for lifting
operations.
Load line
The 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
The operation to transfer the load (i.e. an object) from/to vessel(s)
without using cranes, i.e. by using (de­)ballasting. Typical load
transfer operations are load­out, lift­off, mating and float­over.
Load­in
The transfer of an assembly, module, pipes or component from a
barge or vessel, e.g. by horizontal movement or by lifting.
Load­out
The transfer of an assembly, module, pipes or component onto a
barge or vessel, e.g. by horizontal movement or by lifting.
Load­out Support
Frame (LSF)
A structural frame that supports the structure during fabrication and
load­out and may support the structure on a barge/vessel above
grillage.
Load­out, floating
A Load­out onto a floating vessel.
Load­out,
grounded
A Load­out onto a grounded vessel.
Load­out, lifted
A Load­out performed by crane.
Load­out, skidded
A Load­out where the structure is skidded, using a combination of
skidways, skidshoes or runners, propelled by jacks or winches.
Load­out, trailer
A Load­out where the structure is wheeled onto the vessel using
trailers or SPMTs.
Location move
A 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 24­hour 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.
Magnetic Particle
Inspection (MPI)
A Non­Destructive Testing (NDT) process for detecting surface and
slightly subsurface discontinuities in ferroelectric materials such as
iron
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Marine operation
See Operation
Marine Warranty
Survey company
MWS Company
The Marine Warranty Survey (MWS) company is one 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)
An MWS company surveyor is 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. He /she may also attend during such
operations to monitor that the procedures are followed or to agree
any necessary changes.
Matched pair of
slings
A 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 2.2 of
IMCA M 179 /81/ for cable laid details
Material Factor γb
A 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
(MCR)
Manufacturer’s recommended Maximum Continuous Rating of the
main engines.
Mechanical
Termination
A sling eye termination formed by use of a ferrule that is
mechanically swaged onto the rope. See ISO 2408 and 7531, /104/
and /105/.
Minimum Bend
Radius (MBR)
Specified by the manufacturer of a flexible pipe, umbilical or cable.
This is the minimum radius to which a flexible, umbilical or cable can
be bent without compromising its integrity.
Minimum Breaking
Load (MBL)
The 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 short­term duration, such as those for mobile
offshore units.
Mobile Offshore
Unit (MOU)
For the purposes of this document, the term may include Mobile
Offshore Drilling Units (MODUs), and non­drilling 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.
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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
(MPME)
The 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 associated with the design sea state.
Multiple towage
The operation of towing more than one tow by a single tug, or more
than 1 tug towing one tow. See [11.18]
Nacelle
The part of the wind turbine on top of the tower, where the hub,
gearbox, generator and control systems are located.
Non­Destructive
Testing (NDT)
Ultrasonic scanning, magnetic particle inspection, eddy current
inspection or radiographic imaging or similar. Can also include visual
inspection.
Not To Exceed
(NTE) weight
Sometimes used in projects to define the maximum weight of a
structure for an operation. See [5.6.2.2]
Off­hire survey
A survey carried out at the time a vessel, barge, tug or other
equipment is taken off­hire, to establish the condition, damages,
equipment status and quantities of consumables, intended to be
compared with the on­hire survey as a basis for establishing costs
and liabilities.
Off­load
The reverse of load­out
Offshore Converter
Station
The 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
The pulling of a pipeline away from the shore using a lay vessel
Offshore
Transformer
Station
The offshore transformer station is transforming the collected energy
from the wind turbines to a higher voltage.
On­hire survey
A 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 off­hire 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
The Planned Operation Period, plus the contingency period. See
[2.6.2] to [2.6.4]
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Operation, marine
operation
Generic term covering, but not limited to, the following activities
which are subject to the hazards of the marine environment:
Load­out/load­in
Voyage
Lift/Lowering (offshore/inshore)
Tow­out/tow­in
Float­over/float­off
Jacket launch/jacket upend
Pipeline installation
Construction afloat
Operational
Limiting Criteria
The 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 design
environmental condition multiplied by an Alpha factor.
Padear
A lift point consisting of a central member, which may be of tubular
or flat plate form, with horizontal trunnions round which a sling or
grommet may be passed
Padeye
A 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 long­term operations, such as those for a floating production
system.
Pigging
The 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
A vessel specifically designed or fitted out to transport Line pipe
Pipe­in­Pipe
A single rigid pipe held within a carrier pipe by spacers and/or solid
filler.
Pipelay
The operation of assembling and laying the pipeline on the seabed,
from start­up 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
The 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
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Platform
The completed steel or concrete structure complete with topsides
Point of No Return
(PNR)
The 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 of refuge
A 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.
Pre­Loading
The testing of soil foundations or anchors by loading to check that
they can take subsequent loads. For jack­up foundations it is often
done be adding water ballast to pre­load tanks or (with units with
more than 3 legs) by pre­driving by removing load from other legs in
turn.
Procedure
A documented method statement for carrying out an operation
Product
A generic term used within this standard to reference pipelines (rigid
and flexible), risers, jumpers, umbilicals and submarine cables.
Pull Back Method
A J­tube pull­in operation where the pull­in winch is mounted on the
installation vessel and the end of the pull­in wire connected to the
cable runs from the vessel to the J­tube bottom end up and over a
sheave and back to the installation vessel pull­in winch.
Quadrant
A 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 (QTF)
Refers to the matrix that defines second order mean wave loads on a
vessel in bi­chromatic waves. When combined with a wave spectrum,
the mean wave drift loads and low frequency loads can be calculated.
Quayside Mooring
A mooring that locates a vessel alongside a quay (usually at a
sheltered location).
Recognized
Classification
Society (RCS)
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
The Reduction Factor used in the design of slings or grommets
representing the largest values of γb and γs.
Redundancy Check
Check of the failure load case associated with the applicable extreme
(survival) environment, e.g. the one line broken case.
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Reel Lay (for rigid
pipe)
A laying method where the pipeline is pre­assembled 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.
Registry
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
(ROV)
A 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.
Response
Amplitude
Operator (RAO)
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.
Rig
General reference term often used to describe a jack­up or semi­
submersible (Mobile Offshore Drilling Unit or MODU)see MOU) e.g.
‘Rig move procedures’
Rigging
The slings, shackles and other devices including spreaders used to
connect the structure to be lifted to the crane
Rigging weight
The total weight of rigging, including slings, shackles and spreaders,
including contingency.
Righting Arm (GZ)
Righting Moment divided by the displacement
Risk assessment
A method of hazard identification where all factors relating to a
particular operation are considered.
Rope
An assembly of strands wrapped around a core. When a rope is used
for cable­laid sling or cable­laid 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
(RPL)
A tabularised list of the coordinates defining the route along which a
submarine cable is to be installed and the planned installation slack.
A post installation RPL will record the as­built 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 [16.1.1.4].
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Safe condition
A 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
(SWL)
SWL is a derated value of WLL, following an assessment by a
competent person of the maximum static load the item can sustain
under the conditions in which the item is being used. See [1.1.12]
Safety
Management
System (SMS)
A structured and documented system enabling Company personnel
to implement the Company safety environmental protection policy.
Sand Jacks
A 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
The result of scour around a pile, leg etc. See “Dynamics of scour
pits and scour protection”, /119/
Sea room
The distance that a disabled vessel or tow in bad weather can drift
before grounding. See [11.14.1.5]
Seafastenings
The means of restraining movement of the loaded structure on or
within the barge or vessel
Self­Propelled
Modular
Transporter
(SPMT)
A trailer system having its own integral propulsion, steering, jacking,
control and power systems.
Semi­submersible
A floating structure normally consisting of a deck structure with a
number of widely spaced, large cross­section, supporting columns
connected to submerged pontoons.
Serviceability Limit
State (SLS)
A design condition where the structure is required to fulfil its
primary operational function.
Setback
The 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)
An 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
The pulling of a cable or pipeline to the shore from a lay barge/vessel
Simultaneous
Operations
(SIMOPS)
Operations usually involving various parties and vessels requiring co­
ordination and definitions of responsibilities.
Single Laid Sling
A sling normally made up of 6 strands laid up over a core, as shown
in ISO 2408 and 7531, (/104/ and /105/), with terminations each
end.
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Single tow
The operation of towing a single tow with a single tug.
Site Move
An operation to move a structure or partially assembled structure in
the yard from one location to another. The site move may precede a
load­out if carried out as a separate operation or may form part of a
load­out. The site move may be subject to approval if so desired.
Skew Load Factor
(SKL)
A 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
A bearing pad attached to the structure which engages in the
skidway and carries a share of the vertical load
Skidway
The lower continuous rails, either on the quay or on the vessel, on
which the Structure is loaded out, via the Skidshoes.
Slack Management
A generalized term used by the submarine cable installation industry
to refer to the control of cable pay­out out against a pre­defined
installation plan.
Slamming loads
Transient loads on the structure due to wave impact when lifting
through the splash zone.
S–Lay
A 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 sea­bed.
Sling design Load
The maximum calculated dynamic axial load in a lifting sling,
including all relevant load factors.
Sling eye
A loop at each end of a sling, either formed by a splice or mechanical
termination
Specified Minimum
Yield Stress
(SMYS)
The 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 cable­laid 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
Spreader beam or
bar (frame)
A 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 structure
shall also resist bending moments due to geometry and tolerances.
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Spud
A 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.
Staged voyage
A weather restricted voyage in which there is a commitment to seek
shelter (or jack­up at a stand­by location) on receipt of a weather
forecast in excess of the operational criteria. See [11.14.4.1].
Static Hook Load
(SHL)
The weight plus the rigging weight (see [16.3.2]). This load is
suspended by a crane hook during lifting operations.
Strand
An 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
The object to be transported, lifted or installed, or a sub­assembly,
component or module.
Submerged
Weight
Weight of the Structure minus the weight of displaced water.
Suitability survey
A survey intended to assess the suitability of a tug, barge, vessel or
other equipment to perform its intended purpose. Different and
distinct from an on­hire 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
The 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
A 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
The 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
The point where the bend of a pipeline begins or ends.
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Tensioner
Equipment to keep and control tension in the product during
installation operation.
Termination factor
γs
A partial safety factor that accounts for the reduction in strength
caused by a splice or mechanical termination.
Tether
A tether is a 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 location­specific tide readings
obtained for a period of not less than one lunar cycle before the
operation.
Tonnage
A 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.
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).
Touch Down (TD)
Seabed location at which a submarine pipeline or cable touches down
on the seabed during installation, or a mooring line during operation.
Tow
The item being towed. This can be a barge or vessel (laden or un­
laden) or an item floating on its own buoyancy.
Towage
The operation of towing a non­propelled barge or vessel (whether
laden or not,) or other floating object (wet tow) by tug(s).
Towed bundle
A 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 off­ bottom, on surface, or at an intermediate
controlled depth.
Tower (OWF)
The 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
The 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.
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Towline Pull
Required (TPR)
The towline pull computed to hold the tow, or make a certain speed
against a defined weather condition.
Trailer
A 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
A tubular structure on top of a monopile to provide support for the
tower.
Transport
The operation of transporting a cargo on a powered vessel.
Trunnion
A lift point consisting of a horizontal tubular cantilever, round which
a sling or grommet may be passed. An upending trunnion is 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.
Tug
The 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
(TMPS)
A system installed on the AHV and the anchoring vessel to allow the
accurate placing of the tug and anchors.
Ultimate Limit
State (ULS)
The limit state related to the maximum load carrying capacity. Also
see Limit State 1 and Limit State 2. (ULS)
Ultimate Load
Capacity (ULC)
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, delta 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
(UT)
Detection of flaws or measurement of thickness by the use of
ultrasonic pulse­waves 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
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Under­Keel
Clearance (UKC)
The 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
The rope from which a cable­laid sling or cable­laid grommet may be
constructed, made from either 6 or 8 strands around a steel core, as
indicated in ISO 2408 and 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.
Vessel
A 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
A 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
A 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
An 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
A voyage for which the strength or stability will not meet the
weather unrestricted environmental criteria (typically 10 year
return). It can either be or staged (see [11.14.4.1]) or weather­
routed (see [11.14.4.4]) depending on the sea room and shelter
point availability.
Weather routed
voyage
A 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
[11.14.4.4]).
Weather routeing may also be used for non­weather restricted
voyages to reduce fuel costs or voyage time.
Weather
unrestricted
operation
An operation for which (all of) the applied characteristic
environmental conditions are calculated based on the statistical
extremes for the area and season. See also 2.6.62.6.5.
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 24­hour move or location move.
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Weather
unrestricted
voyage
Any voyage which does not fall within the definition of a weather
restricted voyage
Weather Window
A period that the forecasted environmental conditions are less than
or equal to OPWF (the Forecast Operation Criteria).
Weathertight
A 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.
Weighing
Contingency Factor
A factor applied to the weighed weight of an object to account for
uncertainties in the weighing equipment.
Weight
Contingency Factor
A 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
The operation of transporting a floating object by towing it with a
tug.
Wind Heeling Arm
(WHA)
Wind Heeling Moment divided by the displacement
Working Load
Limit (WLL)
The maximum static 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. See [1.1.12].
1.6Acronyms, abbreviations and symbols
1.6.1
Underlined acronyms and abbreviations in Table 1­4 are defined in Table 1­3.
Table 1­4 Acronyms and abbreviations
Short Form
In full
ABS
American Bureau of Shipping
ADL
Absolute minimum Deployable Length (of towline)
AHC
Active Heave Compensation
AHV
Anchor Handling Vessel
AISC
American Institute of Steel Construction
ALARP
As Low As Reasonably Practicable
ALS
Accidental Limit State
AMS
Anchor Management System
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API
American Petroleum Institute
ASD
Allowable Stress Design (effectively the same as WSD)
ASOG
Activity Specific Operations Guidelines (for DP – See [17.13.4.1 11))
ASPPR
Arctic Shipping Pollution Prevention Regulations
ATA
Automatic Thruster Assist
AUT
Automatic Ultrasonic Testing
AWTI
Above Water Tie­In
BAS
Burial Assessment Survey
BBL
Bridle Breaking Load
BHP
Brake Horse Power
BP
Bollard Pull
BPI
Burial Protection Index
BSR
Bend Strain Reliever
CAMO
Critical Activity Mode of Operation (for DP – See [17.13.4.1 11))
CASPRR
Canadian Arctic Shipping Pollution Prevention Regulations
CBP
Continuous Bollard Pull
CDT
Controlled Depth Tow
CGBL
Calculated Grommet Breaking Load
CoB
Centre of Buoyancy
CoG
Centre of Gravity
COMOP
Combined Operations
COSHH
Control of Substances Hazardous to Health
CR
Continuity Resistance
CRBL
Calculated Rope Breaking Load
CSA
Cross Sectional Area
CSBL
Calculated Sling Breaking Load
CSV
Construction Support Vessel
DAF
Dynamic Amplification Factor
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DMA
Dead Man Anchor
DP
Dynamic Positioning or Dynamically Positioned
DSU
Deck Support Unit
DSV
Diving Support Vessel
DTL
Deployable Towline Length (see [11.13.4.3])
Du
Factor for ratio of mean to specified bolt pretension
ECA
Engineering Criticality Assessment
EPC
Engineering, Procurement and Construction
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
FMECA
Failure Modes, Effects and Criticality Analysis
FOI
Floating Offshore Installation
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 Re­gasification Unit
FSD
Sling or grommet design load
FSE
Free Surface Effect
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FSO
Floating Storage and Offloading Vessel
FSU
Floating Storage Unit (including FPSO, FSO, FLNG facility, FRSU etc.)
Gamma b, γb
Bending Factor
Gamma c, γc
Consequence Factor
Gamma f, γf
Load Factor
Gamma m,
γm
Material Factor
Gamma r, γr
Reduction Factor
Gamma s, γs
Termination Factor
Gamma sf, γsf
Combined factors (Load, Consequence, Reduction, Wear, and Material and
Twist)
Gamma w, γw
Wear Factor
Gamma
weight,
γweight
Weight Contingency Factor (unweighed objects only)
GBS
Gravity Base Structure (foundation)
GM
Initial metacentric height
GMDSS
Global Maritime Distress and Safety System
GN
Guidance Note
GPS
Global Positioning System
GZ
Righting Arm
HAT
Highest Astronomical Tide
HAZID
Hazard Identification
HAZOP
HAZards and OPerability study
HDD
Horizontal Directional Drilling
hf
Factor for fillers in bolted connections
HIRA
Hazard Identification and Risk Assessment
HPR
Hydro­acoustic Positioning Reference
HSEQ
Health, Safety, Environment and Quality
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HTV
Heavy Transport Vessel. (not to be confused with HLV (Heavy Lift Vessel)
which has heavy lifting gear)
HVAC
High Voltage Alternating Current
HVDC
High Voltage Direct Current
IACS
International Association of Classification Societies
ICPC
International Cable Protection Committee
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)
IR
Insulation Resistance
ISM
International Safety Management
ISO
International Standards Organisation
ITP
Inspection Test Plan
JSA
Job Safety Analysis
ks
Hole clearance factor
LARS
Launch And Recovery System
LAT
Lowest Astronomical Tide
LBL
Long Baseline Array
LCE
Linear Cable Engine
LMU
Leg Mating Unit
LOA
Length Over All
LRFD
Load and Resistance Factor Design
LS1
Limit State 1
LS2
Limit State 2
LSF
Load­out Support Frame
MAOP
Maximum Allowable Operating Pressure
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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
MLWS
Mean Low Water Spring Tides
MoC
(procedure)
Management of Change (procedure)
MODU
Mobile Offshore Drilling Unit
MOU
Mobile Offshore Unit
MPI
Magnetic Particle Inspection
MPME
Most Probable Maximum Extreme
MRU
Motion Reference Unit
MSL
Mean Sea Level
MWS
Marine Warranty Survey
n/a
Not Applicable
NDT
Non Destructive Testing
NMD
Norwegian Maritime Directorate
Ns
Number of slip planes for bolted connections
NTE (weight)
Not To Exceed (weight)
OCIMF
Oil Companies International Marine Forum
OD
Outside Diameter
OPLIM
Operational limiting criteria
OPWF
Forecasted operational criteria
OSS
Out of Straightness Survey
OTDR
Optical Time Domain Reflectometry
OWF
Offshore Wind Farm
PHC
Passive Heave Compensation
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PIC
Person In Charge
PLEM
Pipeline End Manifold
PLET
Pipeline End Termination
PNR
Point of No Return
PRT
Pipeline Recovery Tooling/Tool
PSA
Petroleum Safety Authority Norway
QC
Quality Control
QCFAT
Quality Control Factory Acceptance Test
QRA
Quantified Risk Analysis
QTF
Quadratic Transfer Function
RAO
Response Amplitude Operator
RCS
Recognized Classification Society
ROV
Remotely (Controlled) Operated Vehicle
RPL
Route Planning List
RTBL
Required Towline Breaking Load
SART
Search and Rescue Radar Transponder
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
SMS
Safety Management System
SMYS
Specified Minimum Yield Stress
SOLAS
International Convention for the Safety Of Life At Sea, /92/,
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SOPEP
Shipboard Oil Pollution Emergency Plan
SPMT
Self­Propelled Modular Transporter
SSCV
Semi­submersible crane vessel
SWL
Safe Working Load
TA
Thruster Assist
TAM
Task Appropriate Mode
Tb
Minimum fastener pretension for bolted connections
TBL
Towline Breaking Load
TC
Contingency period
TD
Touch Down
TDR
Time Domain Reflectometry
Te or Teff
Tug efficiency
TLP
Tension Leg Platform
TMPS
Tug Management Positioning System
TMS
Tether Management System
Tp
Peak period
TPOP
Planned operational Period (without contingencies, TC)
TPR
Towline Pull Required
TR
Operation Reference Period (including contingencies, TC)
Tsafe
Time to safely cease the operation
TWF
Time between weather forecasts
Tz
Zero­up crossing period for waves
UKC
Under­Keel Clearance
UKCS
United Kingdom Continental Shelf
ULC
Ultimate Load Capacity
ULS
Ultimate Limit State
UNCLOS
United Nations Law of the Sea
USBL
Ultra Short Baseline Array
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UT
Ultrasonic Testing
UTM
Universal Transverse Mercator
UXO
Unexploded Ordnance
VIV
Vortex Induced Vibration
VLA
Vertical Load Anchors
WF
Weather Forecast
WHA
Wind Heeling Arm
Wld
Lower bound design weight
WLL
Working Load Limit
WMO
World Meteorological Organisation
WROV
Work class Remotely Operated Vehicle
Wrt
with respect to
WSD
Working Stress Design (effectively the same as ASD)
WTG
Wind Turbine Generator
Wud
Upper bound design weight
SECTION 2Planning and execution
2.1Introduction
2.1.1Scope
2.1.1.1
This Section includes the general requirements for planning, organization, execution and
documentation of marine operations.
2.1.2Revision history
2.1.2.1
This section replaces the following parts of the VMO Standard and the ND Guidelines:
DNV­OS­H101
0001/ND.
2.2General project requirements
2.2.1Project organisation
2.2.1.1
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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].
2.2.2Health, safety and environment
2.2.2.1
Personnel safety shall be duly considered throughout the marine operation(s). This subject
shall be managed by the client or his 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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
2.2.3Jurisdiction
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.
2.2.4Quality assurance and administrative procedures
2.2.4.1
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.
2.2.5Technical procedures
2.2.5.1
Technical procedures shall be in place to control engineering related to the marine activities.
2.2.5.2
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The technical procedures shall consider the planning and design process. For this process it
is recommended that the following sequence is adopted:
Identify relevant and applicable regulations, rules, company specifications, codes and
standards, both statutory and self­elected.
Identify physical limitations. This may involve pre­surveys 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 procedures shall include sufficient information to ensure agreement and uniformity on
all relevant matters such as:
International and national standards and legislation
Certifying authority/regulatory body standards
Marine warranty survey company standards and 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 peer­reviewed best industry practice, e.g. IMCA, MTS, GOMO,
NORSOK, etc.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
2.2.6New 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
should be documented through an acceptable qualification process, e.g. in DNV­RP­A203,
/45/.
2.2.7Design 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
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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.
2.3Technical documentation
2.3.1General
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/mark­up by
involved parties including the MWS Company.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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.
2.3.2Documentation 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.
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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].
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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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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 and test, survey and NDT reports.
Guidance note:
For vessels, such documentation may be recent inspections to acceptable industry standards,
e.g. OVID or CMID, provided all relevant non­conformances 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 3rd Party verification, e.g. to fulfil the warranty clause, shall be properly
documented. See also [2.4.4].
2.3.3Documentation quality and schedule
2.3.3.1
An integrated document numbering system for the entire project is suggested, including
documents produced by client, contractors, sub­contractors and vendors.
2.3.3.2
Documents relating to marine operations should be grouped into levels according to their
status, for example:
Criteria and design basis documents
Procedures and operations manuals
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:
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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 in­depth document that is subjected to a detailed review by the
MWS company e.g. analysis reports, procedures and operation manuals
Submitted – A document that is provided to the MWS company in advance where the
checking is limited e.g. a certificate to 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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
2.3.3.4
Documentation for marine operations shall be self­contained, 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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2.3.4Input documentation
2.3.4.1
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.
2.3.5Output documentation
2.3.5.1
Documentation shall be prepared to prove that all relevant design and operational
requirements are fulfilled. Typical output documentation is:
Planning documents including design briefs and basis, schedules, concept evaluations,
general arrangement drawings and specifications.
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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].
2.3.6Availability 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. In addition to the marine operations manual this should include
the documents referenced therein.
2.3.6.2
The top level procedure document should define the On­Scene 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.
Guidance note:
A document organogram is often helpful as shown in Figure 2‑1.
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Figure 2­1 Example of document organogram
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2.3.7Marine 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.
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:
reference documents
general arrangement
permissible load conditions
outline execution plan
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organogram and lines of command
job­descriptions 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, inter­related
activities, key decision points, hold points
specific step­by­step instructions (procedures/task plans) for each phase of the
operation including sequence, timing, resources and check lists
reference to related drawings and calculations, e.g. environmental loads, moorings,
ballast, stability, bollard pull
permissible draughts, trim, and heel and corresponding ballasting plan
how to handle any changes in the procedure during the operation, see also 2.2.5.3 h).
contingency and emergency plans
emergency preparedness bridging document
monitoring during the operation, see [2.9.5]
clearances and tolerances
systems and equipment including layout
systems and equipment operational instructions
vessels involved
tow routes and ports of refuge
navigation
safety equipment
recording and reporting routines
sample forms
equipment operation history
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:
Risks
Risks
Risks
Risks
Risks
inherent from the metocean conditions
incurred by construction, transport, installation and commissioning activities
to the environment
due to simultaneous operations (SIMOPS) – see IMCA M 203, /83/
due to working on live assets, etc.
2.3.7.6
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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.
2.3.8Operation 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:
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 and
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 client (and 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 MWS company from operations (e.g.
towage) where the MWS company is not attending.
2.3.8.6
Any incidents, accidents or near­misses relevant to the safety of the structure or future
marine operations shall be reported to MWS company.
2.4Risk management
2.4.1General
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:
Identification of potential hazards
Preventative measures to avoid hazards wherever possible
Controls to reduce the potential consequences of unavoidable hazards
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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.
Guidance note:
It is recommended that risk management is performed according to DNV­RP­H101, /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. DNV­RP­H101, /54/, Appendix D.5 gives advice on how to carry out initial risk
evaluations. The effect of (planned) redundancy, back­up, 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.
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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:
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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2.4.2Hazard identification activities
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2.4.2Hazard 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:
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
Semi­Quantitative Risk Analysis (SQRA)
Safe Job Analysis (SJA) / Job Safety Analysis (JSA).
Guidance note:
DNV­RP­H101, /54/, Appendix B 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.
2.4.3Risk 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:
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:
DNV­RP­H101, /54/, Appendix C describes the above listed risk reducing activities in
detail. Note that Safe Job Analysis is in DNV­RP­H101, /54/, mentioned only in
Appendix B ­ Hazard Identification Activities.
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rd
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2.4.43
party verification and MWS
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2.4.43rd party verification and MWS
2.4.4.1
As a part of the risk management the requirements for 3rd 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.
2.5Planning of marine operations
2.5.1Philosophy
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 in­place
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.
2.5.2Type 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
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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.
2.5.3Operations 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 DNVGL
Barents 2020 (2012), /21/, Chapter 4. This includes performance standards for emergency
response vessels and guidance for Arctic evacuation methods.
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2.5.4Contingency 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.
Guidance note:
Foreseeable emergencies and contingencies can include:
Severe weather
Planned precautionary action in the event of forecast severe weather
Structural parameters approaching pre­set limits
Stability parameters approaching pre­set limits
Failure of mechanical, electrical or control systems
DP or power failure "black ship"
Fire
Collision, grounding
Leakage, flooding
Pollution
Structural failure
Equipment failure
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Mooring failure
Icebergs, excessive ice (see also [2.5.3.3])
Human error
Man overboard
Personnel accidents or medical emergencies
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, back­up 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.
2.6Operation and design criteria
2.6.1Introduction
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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2.6.2Operation 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
TR
TPOP
TC
=
=
=
Operation reference period
Planned operation period
Estimated maximum contingency time.
2.6.2.2
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The start and completion points for the intended operation or parts of the operation shall be
clearly defined. See also [2.6.7.3] and [2.6.7.4].
2.6.3Planned 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) 10­20% 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.
2.6.4Estimated contingency time – TC
2.6.4.1
Contingency time, TC, shall be added to cover:
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 [2.5.3], 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.
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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.
Guidance note:
TC < 6 hours is unlikely to be acceptable except for short simple marine operations involving
only robust equipment.
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2.6.5Weather 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 2­2 for further guidance.
2.6.5.2
Operations with a duration that is too long to be planned as weather restricted, see
[2.6.7.1], 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 2­2.
Guidance note:
The indicated maximum allowable period for a weather restricted operation, as per [2.6.7.1],
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 2­2 Flow chart to determine whether an operation is weather restricted or weather
unrestricted
2.6.6Weather 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.
Guidance note:
A reduction in the weather criteria based on extreme value statistics could in some situations
be acceptable based on active use of the (long term) weather forecast. Such typical
situations are:
Operations in areas and seasons where it has been shown and documented that the
long term weather forecasts can predict any extreme weather conditions within the
defined TR for the operation.
Open (Ocean) voyages where the vessel speed is sufficient to avoid extreme weather
conditions.
Such a reduction in the design criteria may be accepted by the MWS company, but normally
an accidental load case (ALS) considering extreme value statistics should be included.
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2.6.6.2
For operations where the design environmental condition is based on extreme value
statistics, the forecasted operational limiting criteria may theoretically be taken equal to the
design environmental condition. However, it is normally not recommended that an operation
is started if extreme weather conditions are expected, and a start criterion may apply.
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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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2.6.7Weather restricted operations
2.6.7.1
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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2.6.7.2
A weather restricted operation shall be planned to be executed within a reliable weather
window, see Figure 2­3.
2.6.7.3
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 2­3.
Figure 2­3 Operation Periods
2.6.7.4
The operation shall only be considered completed when the object is in a safe condition, see
[2.5.1.2].
2.6.7.5
Restricted operations may be planned with design environmental conditions selected
independent of statistical data, i.e. set by owner, operator or contractor.
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
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the required weather window. See also [3.3].
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2.6.7.6
The start of a weather restricted operation is conditional on an acceptable weather forecast,
see [2.7.3].
2.6.7.7
Operations that could be carried out within the maximum allowed period may be planned
with (possible) stops in (case of) periods with weather conditions above the OPLIM. The
following shall be taken into account:
Increased risk for halting (and re­starting) due to additional operations.
Increased risk due to the nature of the “temporary” safe position of the object.
Increased weather risk due to an increased total operation period.
2.6.7.8
If the planning indicated in [2.6.7.7] is implemented the Alpha (α) factors shall be adjusted
accordingly, e.g.:
Depending on the risk evaluations in [2.6.7.7 b)] and [2.6.7.7 c)] it may be applicable
to reduce the Alpha factor for the final stage of the operation due to an increased total
operation period.
If no significant increased risk is identified due to [2.6.7.7 a)] and [2.6.7.7 b)] alpha
factor(s) according to [2.6.9.3] applies.
2.6.8Operational 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 environmental design criteria. 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, back­up
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, etc.
Limiting weather conditions for carrying out identified contingency plans.
2.6.9Forecasted and monitored operational limits, alpha factor
(α)
2.6.9.1
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2.6.9.1
Uncertainty in both the monitoring and the forecasting of the environmental conditions shall
be considered. This should be done 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].
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
2.6.9.2
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 2­3.
2.6.9.3
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.
2.6.9.4
The following should be used as guidelines for selecting the appropriate Alpha Factor for
waves:
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 the probability of exceeding the operational
environmental limiting criteria (OPLIM) by more than 50% in LRFD (see [2.6.11]) is less
than 10­4.
Reliable wave and/or vessel response monitoring system(s) and applied weather
forecast level, see [2.7.2], could be taken into account.
2.6.9.5
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.
2.6.10Selection 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 2­1
Operational limiting criteria, OPLIM, see [2.6.8]
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The planned operational period, TPOP, see [2.6.9.2]
2.6.10.2
The Alpha Factor could be assumed to vary in time for one operation, e.g. for an operation
with TPOP= 36 hours, H s= 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
should 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 shall be selected
using Table 2­1 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 2.6.9.4, 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 2005­2007 with the aim to establish a revised set of α­factors for European
waters. For details of the JIP see DNV Report 2006_1756 Rev. 03, “DNV Marine Operation
Rules, Revised Alpha Factor JIP Project”.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Table 2­1 Selection of Alpha Factor table(s)
WF level
Environmental
monitoring?
A1
A2 & B
C
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 2­8
Wind Alpha Factor –
ASD/WSD
Table 2­15
2.6.11Tabulated alpha factor – LRFD method
2.6.11.1
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The Alpha Factor for waves applying LRFD, see [5.9.8], shall be selected according to Table
2­1 and are given in Table 2­2 through Table 2­7. Values for wind are in Table 2­8.
Table 2­2 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
0.76
0.79
0.80
TPOP ≤ 24
0.63
0.73
0.76
0.78
TPOP ≤ 36
0.62
TPOP ≤ 48
0.60
0.68
0.71
0.74
TPOP ≤ 72
0.55
0.63
0.68
0.72
1 < Hs < 2
Linear
Interpolation
Hs = 2
0.71
2 < Hs < 4
Linear
Interpolation
Hs = 4
0.73
4 < Hs < 6
Linear
Interpolation
H s≥6
0.76
Table 2­3 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
H s≥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
0.80
0.73
Linear
Interpolation
0.82
0.76
Table 2­4 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
0.80
0.83
0.84
TPOP ≤ 24
0.66
0.77
0.80
0.82
TPOP ≤ 36
0.65
TPOP ≤ 48
0.63
0.71
0.75
0.78
TPOP ≤ 72
0.58
0.66
0.71
0.76
1 < Hs < 2
Linear
Interpolation
Hs = 2
0.75
2 < Hs < 4
Linear
Interpolation
Hs = 4
0.77
4 < Hs < 6
Linear
Interpolation
H s≥6
0.80
Table 2­5 LRFD Alpha Factor for waves, Level A2 or B – With Environmental Monitoring
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Planned
Operation
Period [h]
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Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
H s≥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
0.65
TPOP ≤ 48
0.63
0.71
0.75
0.78
TPOP ≤ 72
0.58
0.66
0.71
0.76
Linear
Interpolation
0.77
0.75
Linear
Interpolation
0.80
0.77
Linear
Interpolation
0.82
0.80
Table 2­6 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
0.84
0.87
0.88
TPOP ≤ 24
0.69
0.80
0.84
0.86
TPOP ≤ 36
0.68
TPOP ≤ 48
0.66
0.75
0.78
0.81
TPOP ≤ 72
0.61
0.69
0.75
0.79
1 < Hs < 2
Linear
Interpolation
Hs = 2
0.78
2 < Hs < 4
Linear
Interpolation
Hs = 4
0.80
4 < Hs < 6
Linear
Interpolation
H s≥6
0.84
Table 2­7 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
H s≥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:
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Statistical data and local experience for the actual site.
Planned operation period from issuance of weather forecast, TPOP.
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 2­8 shall be considered as
the maximum allowable.
Table 2­8 LRFD Recommended Alpha Factor for wind
Operational limiting (OPLIM) wind speed – V d
Planned Operation Period
V d < 0.5 x V10 year return
V d > 0.5 x V10 year return
TPOP ≤ 24
0.80
0.85
TPOP ≤ 48
0.75
0.80
TPOP ≤ 72
0.70
0.75
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.
2.6.12Tabulated 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 2­1 and are shown in Table 2­2 through Table 2­
8. 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.
Table 2­9 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
0.68
0.70
0.71
TPOP ≤ 24
0.56
0.65
0.68
0.69
TPOP ≤ 36
0.55
TPOP ≤ 48
0.53
1 < Hs < 2
Linear
Interpolation
Hs = 2
0.63
0.61
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2 < Hs < 4
Linear
Interpolation
Hs = 4
0.65
0.63
4 < Hs < 6
Linear
Interpolation
H s≥6
0.68
0.66
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0.49
0.56
0.61
0.64
Table 2­10 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
H s≥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
0.71
0.65
Linear
Interpolation
0.73
0.68
Table 2­11 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
0.71
0.74
0.75
TPOP ≤ 24
0.59
0.69
0.71
0.73
TPOP ≤ 36
0.58
TPOP ≤ 48
0.56
0.63
0.67
0.69
TPOP ≤ 72
0.52
0.59
0.63
0.68
1 < Hs < 2
Linear
Interpolation
Hs = 2
0.67
2 < Hs < 4
Linear
Interpolation
Hs = 4
0.69
4 < Hs < 6
Linear
Interpolation
H s≥6
0.71
Table 2­12 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
H s≥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
0.58
TPOP ≤ 48
0.56
0.63
0.67
0.69
TPOP ≤ 72
0.52
0.59
0.63
0.68
Linear
Interpolation
0.69
0.67
Linear
Interpolation
0.71
0.69
Table 2­13 ASD/WSD Alpha factors (waves) ­ Level A1 – No Environmental Monitoring
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Linear
Interpolation
0.73
0.71
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Table 2­13 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
0.75
0.77
0.78
TPOP ≤ 24
0.61
0.71
0.75
0.77
TPOP ≤ 36
0.61
TPOP ≤ 48
0.59
0.67
0.69
0.72
TPOP ≤ 72
0.54
0.61
0.67
0.70
1 < Hs < 2
Linear
Interpolation
Hs = 2
2 < Hs < 4
Linear
Interpolation
0.69
Hs = 4
0.71
4 < Hs < 6
Linear
Interpolation
H s≥6
0.75
Table 2­14 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
H s≥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
Linear
Interpolation
0.75
Linear
Interpolation
0.69
0.77
0.71
Linear
Interpolation
0.80
0.75
2.6.12.2
If no reliable data is available the Alpha Factors indicated in Table 2­15 shall be considered
as the maximum allowable in ASD/WSD. See also [2.6.11.2] and [2.6.11.4].
Table 2­15 ASD/WSD Alpha factors (wind ­ all forecast requirements)
Planned Operation Period
Operational Limiting Wind Speed (V d )
V d < 0.5 x V10 year return
V d > 0.5 x V10 year return
TPOP ≤ 24
0.71
0.76
TPOP ≤ 48
0.67
0.71
TPOP ≤ 72
0.62
0.67
2.7Weather forecast
2.7.1General
2.7.1.1
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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.
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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 non­stationary 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 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.
2.7.2Weather forecast levels
2.7.2.1
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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 2­16 for further advice
regarding selection of the forecast level and for requirements to the weather forecast
procedure.
Table 2­16 Weather forecast levels
Weather
Forecast Level
A1
A2
Operation
Sensitivity
Examples
High
mating operations
offshore float over
multi barge towing
major (e.g. GBS)
tow out operations
offshore installation
operations
jack­up rig moves.
sensitive laying
operations
Meteorologist on
site
Yes
No
Dedicated
Meteorologist
Yes
Yes
Minimum
independent WF
sources2)
Maximum WF
interval
B
C1)
Moderate
Low
tow­out
operations
weather
routed sea
transports
offshore
lifting
subsea
installation
semi­
submersible
rig moves
standard
laying
operations.
onshore/inshore
lifting
load­out
operations
short tows in
sheltered
waters/harbour
tows
standard sea
transports
without any
specified wave
restrictions.
No
No2)
No
24)
25)
1
12 hours6)
12 hours
12 hours
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Notes:
1. See 2.7.1.1 GN.
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 [2.7.3.5].
2.7.3Acceptance 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
2­3. The acceptance criteria shall be included in the marine operation manual.
2.7.3.2
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.
2.7.3.3
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.
2.7.3.4
Uncertainties in forecasted weather window duration shall be duly considered i.e. the
forecasted weather window duration should be conservatively assessed.
2.7.3.5
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.
2.8Organization of marine operations
2.8.1General
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
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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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
2.8.2Qualification 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 full­mission simulation should be
undertaken.
2.8.3Familiarisation and briefing
2.8.3.1
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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.
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 thouroughly 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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2.8.4Communication 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:
Client’s representative and 3rd 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, e­mail addresses and
out­of­hours numbers shall be identified and checked for accuracy.
2.8.4.3
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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]).
2.8.5Shifts
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 hand­over from one
organisation to another (e.g. from fabrication to marine operations, from on­shore 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 hand­overs at each shift change.
2.9Monitoring
2.9.1General
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
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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 back­up requirements to monitoring instrumentation systems are given in
[4.2].
2.9.2Environmental 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 waves, swell,
current, tide etc., systematic monitoring of these conditions before and during the operation
shall be arranged.
Guidance note:
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.
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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.
2.9.3Loads 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. strain­gauge measurements). All full scale load and/or
response monitoring should 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:
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To obtain valuable design information for future projects.
To control that design criteria (ULS or FLS) are not exceeded during an operation.
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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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2.9.4Alpha 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.
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].
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 is to 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.
Feed­back to meteorologist(s)
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2.9.5Monitoring 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
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Any unforeseen monitoring results shall be reported without delay.
2.9.6Back­up and contingency
2.9.6.1
The requirements of [4.2.1.10] apply.
2.9.6.2
If the monitoring back­up system does not have the same accuracy as the original system
this should be considered in the contingency planning.
2.10Inspections and testing
2.10.1General
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.
2.10.2Test 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.
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2.10.3Systems
2.10.3.1
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2.10.3.1
All systems and their back­up 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 should, as far as practically
possible, be tested in a realistic scenario with adequate loading.
2.10.4Communication
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 run­through 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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2.10.5Inclining tests
2.10.5.1
The requirement to perform inclining and/or displacement tests shall be agreed with the
MWS Company.
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.
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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.
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2.10.5.2
Where inclining and/or displacement tests are required:
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 guidelines 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.
2.11Vessels
2.11.1General
2.11.1.1
This section 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 should 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
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2.11.1.5
See [17.13] for further requirements to Dynamic Positioned vessels.
2.11.2Condition 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.
2.11.3Structural 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
Class requirements, see [2.11.4], or by calculating the strength according to the relevant
requirements in Sec.5.
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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.
2.11.4Class requirements
2.11.4.1
Where a vessel is classed by a 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 Classification Society shall be submitted.
Guidance note:
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Modifications to vessel structure or equipment can require approval from the Classification
Society.
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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.
2.11.5Certificates
2.11.5.1
All required certificates shall be valid, or relevant exemptions shall be submitted.
Guidance note:
The documents (certificates) to be carried on board different types of vessels can be found in
IMO FAL.2/Circ.87­MEPC/Circ.426­MSC/Circ.1151.
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2.11.6Navigation lights and shapes
2.11.6.1
All vessels and towed objects (unless submerged) shall carry the lights and shapes, towed
objects required by the International Regulations for Preventing Collisions at Sea, 1972
amended 1996 (COLREGS, /91/) and any local regulations.
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 un­manned.
Guidance note:
Solar powered navigation lights should be compliant with UL 1104 (USCG) and/or EN14744
(EU Marine Equipment Directive). Additional power provided by solar panels may be
considered if an adequate track record is documented.
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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 “catch­rain”
orientation.
2.11.7Contingency situations
2.11.7.1
All vessels shall be selected with due consideration to possible contingency situations.
Guidance note:
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This could e.g. result in the selection of redundant (twin screw) tugs for towing operations in
narrow waters. See also the operation­specific requirements in Sec.10 to Sec.18 of this
Standard for further guidance.
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2.11.7.2
Where several tugs (vessels) are involved, a stand­by tug to assist or remove vessels in case
of black­out, engine failure, etc. should be considered.
SECTION 3Environmental conditions and
criteria
3.1Introduction
3.1.1General
3.1.1.1
This Section refers to the environmental design criteria 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 recognised 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.
3.1.2Scope
3.1.2.1
The environmental design criteria 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
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This section 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 location­specific
metocean parameters.
3.1.3Revision history
3.1.3.1
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and
legacy DNV­OS­H­series standards.
3.2Design 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.
3.2.4
Where it is impractical and/or uneconomical to design marine operations based on extreme
statistical data, the design environmental condition can be set independent of extreme
statistical data for weather restricted operations ­ see [2.6.7] and [3.3].
3.3Design environmental criteria for weather restricted
operations
3.3.1
For weather restricted operations the design wind could 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.
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3.3.2
The ratio between forecasted wind and design wind should be determined in accordance with
Table 2­8 or Table 2­15 as applicable.
3.3.3
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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:
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.
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3.3.5
Maximum wave height for weather restricted operations should be calculated according to
the following equation:
H max = STF × H s
where
STF
=
2.0 for all reference periods.
Guidance note:
For short reference periods STF < 2.0 may be acceptable. See DNV­RP­H102, /55/, Table 2.2
for guidelines.
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An appropriate range of wave periods associated with H max should be considered. In the
absence of other data, the range of Tass can be taken as:
3.3.6
Where relevant, applicable information from [3.4] may be used e.g. [3.4.12].
3.4Design criteria for weather unrestricted operations
3.4.1General
3.4.1.1
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.
3.4.2Environmental statistics
3.4.2.1
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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 short­term 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
RP­C205 /46/ for more info.
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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 two­parameter Weibull distribution,
the regression analysis should be performed with emphasis on a correct representation of the
extreme values.
Guidance note:
Regression analysis of two­parameter Weibull distributions are recommended based on the
30% highest data points, i.e. P(x > X) = 0.3.
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3.4.3Return periods for determining environmental criteria
(apart from moorings)
3.4.3.1
The return periods that shall be used for determining environmental criteria for weather
unrestricted marine operations (apart from moorings and the elevated operation of jack­
ups), should be related to its operation reference period, as defined in [2.6.2]. For design
criteria for moorings see [3.4.4], and for the elevated operation of jack­ups see DNVGL­ST­
N002, /39/.
3.4.3.2
As general guidance, the criteria in Table 3­1 may be applied provided that the independent
extremes are considered concurrently.
3.4.3.3
The intention of the return periods and load, safety and material factors used in the LRFD
approach is to ensure a probability for structural failure less than 1/10000 per operation (10­
4 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:
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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).
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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.
3.4.3.5
Seasonal and/or directional variations may be used. 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.
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 N­year return period extremes for say March, perform extreme value analysis
on the subset of data for March, consisting of 3 hr sea­states, 240 per month in the data,
and fit a Weibull curve to the cumulative distribution function. Select the required
probability level for the N­yr extreme calculated as: 1/(365.25*8*N*C) where C =
conditional probability for month = 1/12. Another approach is to obtain relative weightings
of the severity of each month in a year, and scale the monthly or seasonal values such that
the worst month in the year has the same extremes as the all­year value at the required
return period.
3.4.3.7
Similarly, when directional variations are taken into account, this shall not imply a shorter
return period.
Table 3­1 Metocean minimum design return periods, Td – unrestricted operations
Operation
reference
period
Up to 3
days 4)
3 to 7
days
ASD / WSD
Wind
1)
3)
Wave 2) and
Current
LRFD
Wind
1)
3)
Wave 2) and
Current
Td ≥5 year
Td ≥3 month
Td ≥10 year
Td ≥1 month
Td ≥10 year
Td ≥1 year
Td ≥10 year
Td ≥3 month
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7 days to
1 month
Td ≥25 year, (or
obtain from 10 yr
and 50 yr
environmental
criteria values
using: 10yr +
0.7*(50 yr­10 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: 50yr +0.7*
(100 yr­50 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
Notes:
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 site­specific metocean parameters using the method shown in
[C.3].
3. See [3.4.3.6].
4. Operations up to 3 days may also be defined as weather restricted operations. See
Section [2.6.7].
5. 1 year return period for a 3 month seasonal period will normally be acceptable.
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 10 year seasonal return, 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 1 year seasonal return.
3.4.4Return periods for determining environmental criteria for
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3.4.4Return periods for determining environmental criteria for
moorings
3.4.4.1
Table 3­2 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 19901­7 /100/, however the selection of return period will depend on the choice of the
design code (See 17.2 for acceptable mooring codes) and the associated factor of safety. For
weather restricted operations, see [3.3]. More onerous, local requirements can override the
requirements stated in Table 3­2, for example ISO 19901­7, Annex B.
Table 3­2 Return periods for determining environmental criteria for moorings
Mooring Type
Quayside/Inshore
Offshore ­ Mobile near another asset
Offshore ­ Mobile in Open Location
1)
Return Period
100 year
1)
10 year
5 year
Notes:
1. Where the exposure is limited to less than 30 days, or unit capable of leaving the
quay on receipt of poor weather forecast, 10 year return period extremes can be
used in the assessment.
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 sytem.
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.
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.
3.4.5Use 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
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3.4.5.2
Directional metocean data may also be used with suitable spreading functions to reflect
directional divergence in the design environment.
3.4.6Wind
3.4.6.1
The averaged wind velocity over a defined time is referred to as the mean wind.
Guidance note:
Forecasted wind velocity is normally given as the 10 minute mean wind (t mean = 10 min) at a
reference height of 10 m (z = 10 m).
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3.4.6.2
The design wind speed shall generally be the 1 minute mean velocity at a reference height of
10 m above sea level. A longer or shorter averaging time should be used for design
depending upon 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;
­ Fixed structures L < 50 m
3
[s]
­ Fixed structures L > 50 m
15
[s]
­ For any structure if wave load dominating
1
[minute]
­ Quay mooring, small vessels/objects
15
[s]
­ Quay mooring, large (Wind area > 2000 m2)
vessels/objects
1
[minute]
­ Stability calculations, normally
1
[minute]
­ Catenary mooring of vessels/objects
10
[minutes]
­ Catenary mooring of GBS
60
[minutes]
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
OCIMF (2007) 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 DNV2.22, /16/, Appendix A.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3.4.6.3
For dynamic wind analysis the mean wind period recommended for the applied wind
spectrum should be used. See [3.4.6.7].
3.4.6.4
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The mean wind velocity varies with the averaging time and height above the sea surface or
height above ground (yard lift). For these reasons, the averaging time for wind speeds and
the reference height shall always be specified.
3.4.6.5
The wind velocity profile in open sea can be related to a reference height (zr) and mean time
period (t r, mean) according to the equation below, see also Table 3­3 and ISO 19901­1
“Metocean design and operational considerations”, /98/.
Where:
z
zr
t mean
t r, mean
U(z,
t mean)
U(zr, t r,
mean )
=
=
=
=
Height above sea surface.
Reference height 10 [m].
Averaging time for design.
Reference averaging time 10 [minutes].
=
Average wind velocity.
=
Reference wind speed.
Table 3­3 Wind profile, U(z, tmean)/ U(zr , tr, mean)
Averaging time
z (m)
3s
15 s
1 min.
10 min.
1 hour
1
0.93
0.86
0.79
0.69
0.60
5
1.15
1.08
1.01
0.91
0.82
10
1.25
1.17
1.11
1.00
0.92
20
1.34
1.27
1.20
1.10
1.01
50
1.47
1.39
1.33
1.22
1.14
100
1.56
1.49
1.42
1.32
1.23
Guidance note:
The wind profile given in Table 3‑3 is for open sea and should not be considered applicable to
(partly) sheltered inshore locations. Wind profiles for such locations should be selected based
on local data.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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. DNV­RP­C205, /46/;
NORSOK N­003, /111/ or ISO 19901­1, /98/.
3.4.6.8
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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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3.4.7Wind 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
One hour mean plus a suitable gust spectrum. Generally the ISO 19901­1 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 DNVGL­OS­E301, /27/.
3.4.8Waves ­ design methods
3.4.8.1
Wave conditions are defined by characteristic wave height, H c, or the significant wave height,
H s, 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
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3.4.8.4
In the stochastic method the design sea states are represented by wave energy spectra
characterised by main parameters H s and Tz or Tp .
3.4.9Waves ­ 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.
3.4.10Wind seas and swell
3.4.10.1
All possible combinations of wind seas and swell should be considered.
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3.4.11Characteristic 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 3­1
or Table 3­2 as applicable. Alternatively, the Characteristic significant wave height, H s, c may
be taken according to [C.3.2.1] and the corresponding maximum wave height, H max, 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, H s is approximately equal to
(the mean
height of the largest third of the zero up­crossing waves).
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
The H max, 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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3.4.11.2
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 3 hours.
The determination of the height, crest elevation and kinematics of the maximum wave
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should be determined from an appropriate higher­order 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
Solitary wave theory for particularly shallow water
0.1 < h/λ ≤ 0.3
Stokes 5th order wave theory or Stream Function wave theory.
h/λ > 0.3
Linear wave theory (or Stokes 5th order)
where
h
λ
=
=
water depth.
wave length.
A range of wave height­period combinations shall be investigated, including those that can
cause resonance, see [C.3.3].
For more information on the kinematics of regular waves, see DNV­RP­C205, /46/.
3.4.11.3
Sea states shall include all relevant spectra up to and including the design storm sea state
for the construction site or voyage route. Long­crested seas shall be considered unless there
is a justifiable basis for using short­crested seas or these are more critical, see [3.4.12].
Consideration should be given to the choice of spectrum.
3.4.11.4
Wave spectra defined by the Jonswap or the Pierson­Moskowitz spectra are most frequently
used. Wave conditions with combined wind sea and swell may be described by a double peak
wave spectrum. See DNV­RP­C205, /46/, for further guidance.
3.4.11.5
In the simplest method the peak period (Tp ) for all sea states considered, should be varied.
In areas where swell is insignificant, the range of Tp can be taken as:
in areas where swell is significant, the range of Tp can be taken as:
for H s ≤ 5.7 m
for H s > 5.7 m
where:
Hs
Tp
=
=
significant wave height in metres
wave peak period in seconds
Guidance note:
The equations for areas where swell could be significant are based on the equations for Tz
given in [C.3.4.3], assuming that Tp = 1.24Tz for steep waves (gamma = 5) and Tp = 1.4Tz
for long waves (gamma = 1.0). The relation between zero­crossing period Tz and the spectral
peak period Tp can be found in Table 3‑4. See also DNV­RP­H103, /56/, Sec.2.2.6 or DNV­
RP­C205, /46/, Sec.3.5.5.
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Alternatively, see [C.3.4.3].
3.4.11.6
The effects of swell, see [3.4.14], should also be considered if not already covered in this
peak period range. A reduced range of Tp may be used if the route or site­specific data and
natural periods allow.
3.4.11.7
However, [3.4.11.5] incorrectly assumes that all periods are equally probable. As a result
this method should generally produce higher design responses than would be the case when
using the more robust H s­Tp method described in [3.4.11.8], which may be used when
desired.
3.4.11.8
In the alternative method, a contour (IFORM) is constructed within the H s­Tp plane that
identifies equally probable combinations of H s 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. H s­Tp 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 H s).
3.4.11.9
The relationship between the peak period Tp and the zero­up 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.
3.4.11.10
Table 3­4 indicates how the characteristics of the JONSWAP wave energy spectrum vary over
the range of recommended sea states. The constant, K, varies from 13 to 30 as shown in the
equation in [3.4.11.4]. T1 is the mean period (also known as Tm).
Table 3­4 Value of JONSWAP γ, ratio of Tp :Tz and Tp : T1 for each integer value of K
Constant
K
γ
Tp / Tz
Tp / T1
Constant
K
γ
Tp / Tz
Tp / T1
13
5.0
1.24
1.17
22
1.4
1.37
1.27
14
4.3
1.26
1.18
23
1.3
1.39
1.28
15
3.7
1.27
1.19
24
1.1
1.40
1.29
16
3.2
1.29
1.20
25
1.0
1.40
1.29
17
2.7
1.31
1.21
26
1.0
1.40
1.29
18
2.4
1.32
1.23
27
1.0
1.40
1.29
19
2.1
1.34
1.24
28
1.0
1.40
1.29
20
1.8
1.35
1.25
29
1.0
1.40
1.29
21
1.6
1.36
1.26
30
1.0
1.40
1.29
3.4.11.11
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3.4.11.11
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.
3.4.11.12
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.
3.4.11.13
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).
3.4.12Short 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 [3.4.11.4].
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;
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 sea­state 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 taking lower than 10 without more
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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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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).
3.4.13Waves 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 H s­Tp 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.
3.4.14Swell
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.
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.
3.4.15Current
3.4.15.1
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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 wind­driven 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.
3.4.16Other parameters
3.4.16.1
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.
3.4.17Calculation 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
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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.
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.
3.4.18Calculation 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 H m 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.
3.4.19Calculation of “adjusted” extremes
3.4.19.1
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The probability of non­exceedance 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).
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
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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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3.4.20Criteria 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
voyage­maxima can be used to determine the design value with a specified risk of
exceedance. For example, the value exceeded once in every 20 voyages, on average, can be
determined by reading off the value of wave height from the distribution of voyage­
maximum wave heights at the 95th 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 voyage­maximum 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 en­route 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.
3.4.21Metocean database bias
3.4.21.1
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.
3.4.22Metocean data for bollard pull requirements
3.4.22.1
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The design extremes are not normally used for calculation of bollard pull requirements
(except when there is limited sea room), which is covered in [11.12.2]
3.5Weather/metocean forecast requirements
3.5.1
The requirements for weather forecasting are given in [2.7] and the requirements for
environmental monitoring in [2.9].
3.6Benign weather areas
3.6.1
Areas considered benign are shown in Table 3­5 and Figure 3­1 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)
However these areas may experience sudden vicious squalls and very rare tropical
storms or cyclones.
Table 3­5 Northern and Southern boundaries of benign weather areas by month
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Figure 3­1 Map showing benign weather areas
SECTION 4Ballast and other systems
4.1Introduction
4.1.1Scope
4.1.1.1
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.
4.1.2Revision history
4.1.2.1
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4.1.2.1
This section replaces the following parts of the VMO Standard and the ND Guidelines:
DNV, Marine Operations, General, DNV­OS­H101
DNV, Load Transfer Operations, DNV­OS­H201
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
GL Noble Denton, Guidelines for Load­outs, 0013/ND
GL Noble Denton, Guidelines for Float­over Installations / Removals, 0031/ND
4.2System and equipment design
4.2.1General
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.
2.
3.
4.
5.
6.
7.
power supply
fuel supply
electrical distribution systems
machinery control systems
alarm systems
valve control systems
bilge and ballast systems
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8.
9.
10.
11.
12.
13.
14.
15.
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compressed air systems
firefighting systems
Cooling systems
ROV systems
lifting systems
positioning systems, see Guidance Note
communications systems and
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
under­keel 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 this section
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].
4.2.2Back­up
4.2.2.1
All essential systems, parts of systems or equipment shall have back­up or back­up
alternatives. Necessary time for a change over to the back­up 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 back­up systems should be designed and fabricated to the same standard as the primary
systems.
4.2.2.3
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Back­up 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, back­up 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 back­up capability shall be provided, and fail­safe systems shall be
incorporated into critical controls.
4.2.2.6
Automatic control systems shall be provided with a possibility for manual overriding.
4.3Ballasting systems
4.3.1General
4.3.1.1
This sub­section is mainly applicable for ballasting and de­ballasting of vessel(s) involved in
load transfer operations.
Guidance note:
See [11.15] regarding pumping capacity requirements during voyages. For jacket
installations additional requirements apply, see [13.7.2]. For ballasting of (crane) vessels
during lifting see Sec.16.
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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 [4.3.2]) as
well as functional requirements related to:
lay­out 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].
4.3.2Ballast system power supply
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4.3.2Ballast 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:
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 back­up capacity for accidental conditions represented by a) and b) may be spare
units in stand­by position. The back­up capacity for conditions represented by c) may
be spare capacity in the main unit or a back­up unit installed to assist the main unit.
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4.3.2.3
Sufficient main and back­up power supply capacity should be documented by calculations.
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 back­up requirements,
an independent power supply source should be regarded as a “pump system”, see note 3) in
Table 4‑2.
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4.3.3Operation classes
4.3.3.1
An operation class should be defined for load transfer operations see Table 10­1 for load­outs
and Table 15­1 for lift­off, mating and float­over operations.
4.3.4Ballast system lay­out 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.
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4.3.4.2
Where accurate control of the ballast amount is crucial, ballasting by flooding (i.e. opening of
bottom valves) and/or de­ballasting by air pressurisation (or ballasting by vacum – low
pressure) of ballast tanks shall be avoided during load transfer phases.
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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 back­up ballast plans are available where
mechanical failures can be compensated by an alternative ballast procedure.
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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 de­pressurise 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.
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 permanent­type 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.
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Guidance note:
For offshore operations temporary flexible hoses are not generally permitted unless their use
cannot be avoided, for instance for supply of back­up compressed air from a compressor
barge alongside.
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4.3.4.13
Permanent­type ballast sytems 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.
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4.3.5Ballast tank capacity
4.3.5.1
The ballast tanks shall meet the capacity requirements in Table 4­1 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:
Tide levels below or above the predicted values.
Total vessel weight, including vessel lightship, consumables and temporaries (e.g.
project equipment, grillages, etc.), being higher or lower than expected
Possible object weight and CoG variations
Operational delays.
Table 4­1 Tank capacity requirements
Operation
Class
All
The tank capacity shall be adequate for the following scenarios (see Table
10‑1 for load­out classes and Table 15‑1 for float­ons and float­offs).
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 4­2.
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1
See All
Reversing of the operation. Tide compensation if stop of load transfer,
considering maximum possible (defined) duration of the load transfer.
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 3­5 days shall
be considered.
4
See All
Reversing of the operation, if applicable.
5
See All
4.3.6Ballast pumping capacity
4.3.6.1
The ballast pumping capacity shall meet the capacity requirements in Table 4­1 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.
4.3.6.2
Adequate capacity shall be documented considering the requirements to nominal, spare and
back­up capacity given in this sub­section.
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
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Pumps which require to be moved around the barge in order to be considered as part of the
back­up capacity, shall be easily transportable, and may only be so considered if free access
is provided at all stages of load­out 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 back­up. However, for pumps that may be replaced during the operation, spare
pumps in stand­by 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 back­up, are given in Table 4­
2, including the notes.
Table 4­2 Ballast pumping capacity requirements
Operation
Class
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
Notes:
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 back­up 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.
4.3.7Vessel strength considerations
4.3.7.1
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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.
4.3.8Ballasting 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 de­ballasting 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 back­up shall be established. A
communication system shall be established when pumps are operated manually away from
the control centre.
4.3.9Ballast calculations
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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 4­2) 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).
4.3.10Contingency and back­up
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 4­3 shall be considered. Minimum requirements to
back­up have also been indicated.
Table 4­3 Contingency requirements
No
Contingency situation
Minimum requirement
1
Tidal velocities above (or below) the
predicted values.
Spare pump(s) or spare capacity in the
main pump(s). See Table 4­2 for specific
requirements.
2
Unplanned stops in load transfer (e.g.
object movement stopped due to repair
work, etc.)
Adequate tank and pump capacities to
handle the situation. See Table 4­1 and
Table 4­2 for specific requirements.
3
Reversal of operation, if required.
Ballast procedures/calculations with
corresponding pump lay­out and tank
capacities for this case shall be
documented.
4
Reduced pump capacity.
Spare pump capacity. See Table 4­2 for
specific requirements in %.
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5
Breakdown of ballast pump(s).
Spare pump(s) or spare capacity in the
main pump(s). See Table 4­2 for specific
requirements.
6
Breakdown of power supply, including
cables.
Back­up required, see [4.3.2.2], or
adequate pump capacity, see Table 4­2,
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 back­up to fulfil the
requirements in Table 4­2 for one pump
system failure. Alternative pump/valve
control methods (locations and
procedures) could also be accepted as
back­up. See Notes.
Notes:
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.
4.4Guiding and positioning systems
4.4.1General
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.
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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 overloading members of the supporting structure. Plastic
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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
DNV­RP­H102, /55/, Sec. 3.3.5 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
As­built bumper and guide dimensions shall be documented.
4.4.2Characteristic 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 [16.14.4].
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
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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.
4.4.3Design 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 or,
To the LRFD approach ULS.
4.4.3.4
To avoid overloading the supporting structure it shall be designed either
To the ASD/WSD approach LS1 or,
To the LRFD approach ULS with an additional load factor of 1.3.
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.
4.4.4ALS 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 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.
4.4.5Position 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 tie­ins, capture of docking piles.
4.4.5.2
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System redundancy shall be in accordance with [4.2.1.10] appropriate to safety criticality
and operational criticality requirements.
4.4.5.3
Sub­surface 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 line­of­
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 back­up 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.
4.5ROV systems
4.5.1Planning
4.5.1.1
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:
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
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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 back­up 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 back­up 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.
4.5.2Stationkeeping 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.
approprate 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.
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
lcab
A ROV
vcur
=
=
=
=
diameter of submerged cable [m]
projected length of submerged cable [m]
projected cross sectional area of ROV including any tools [m2]
maximum current velocity [m/s]
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4.5.2.3
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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.
4.5.3Testing
4.5.3.1
For complex and critical stages of the installation that are dependent on ROV operations,
Client/Contractor shall demonstrate ROV capability of executing the planned intervention.
This can be demonstrated by used of 3D models, mock­up tests, previous experience, etc.
Guidance note:
This may involve the manufacture of mock­ups. If mock­ups are used, great care shall be
taken to ensure that the mock­ups 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.
4.5.4Launch 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.
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 during lowering through the splash zone in
the limiting wave conditions.
Overboard launching and retrieval of large ROV's is not generally recommended to take
place in sea states exceeding 2.5­3.0 m (H s) 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 Moon­pool
ROV operations and if heavy weather side rail systems are used.
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High wind speeds, and operational aspects (e.g. risk of entanglement) may also be
critical.
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4.5.4.3
The over­boarding 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 [5.6.6.6].
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.
4.5.5Monitoring
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 Work­ROV 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 video­link 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 video­link 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.
4.5.6Human 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.
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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.
4.5.7Deepwater ROV 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.
SECTION 5Loading and structural strength
5.1Introduction
5.1.1General
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.
5.1.2Scope
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
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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.
5.1.3Revision history
5.1.3.1
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and
legacy DNV­OS­H­series standards.
5.2Design principles
5.2.1Introduction
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 DNVGL­OS­
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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5.3Specific design considerations
5.3.1Connections
5.3.1.1
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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) z­quality are applied.
Alternatively, the material can be subject to non­destructive testing (NDT) using UT to
demonstrate that it is free of laminations, see [5.10.2.3 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.
5.3.2Penetrations
5.3.2.1
The object shall be reinforced as necessary in the area adjacent to any penetrations (e.g. for
risers or J­tubes) 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.
5.3.3Doubler 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.
5.3.4Tension connections
5.3.4.1
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
[5.10.2.3 5)] regarding through­thickness 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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5.3.5Bolted connections for seafastening
5.3.5.1
Appendix [E.2] gives the requirements for bolted connections for seafastenings which
involving cyclic loading due to the dangers of progressive collapse.
5.3.6Light­weight 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 light­weight 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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5.3.7Compressed 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].
5.3.8Inspection
5.3.8.1
Sufficient access for inspection, maintenance, and repair shall be provided during planning of
the operation.
5.3.8.2
Instrumentation (monitoring) can be used as a supplement to other inspection, see [2.9].
5.3.9Existing structures
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5.3.9Existing 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 [5.10.2.3 5)]. See DNV­RP­H102,
/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 as­built 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 as­built 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 the DNV GL Rules for classification:
Ships, /35/, Jan 2015, Pt.3 Ch.3 Sec.3. Normally a total thickness allowance of 3 mm is
applicable for the top 1.5 m of ballast tanks.
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5.3.9.4
Weld capacity should be calculated according to [5.9.7.1] for ASD/WSD or [5.9.8.4] for
LRFD, as applicable.
Guidance note:
When checking vessel welds the following should be noted:
Class acceptance for these welds can be required, especially for new/reinforced welds.
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.
All loads (force components) parallel to the deck plate can be disregarded, see however
item f).
The dispersion angle through the deck plate should be taken as maximum 45° unless a
greater dispersion can be justified.
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Size reduction due to possible corrosion should be considered. If not otherwise
documented the size should be as shown on the drawing less the Class corrosion
allowance.
Note that shear stress in stiffener/girder welds due to local bending/shear in these
should be included in the equivalent stress (the effects due to global vessel behaviour
can be ignored).
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5.3.10Protection 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
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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5.4Testing
5.4.1General
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. DNVGL­OS­
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.
5.4.2Model testing
5.4.2.1
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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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5.4.3Full 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 2.22, /16/, and DNV 2.7­3, /17/.
5.4.3.3
Full scale load testing may be carried out by loading test pieces to destruction. The
characteristic strength should normally be defined based on the 5th or the 95th 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.
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Local soil capacity (deflection), e.g. of load­out 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.
5.4.4Testing 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:
Possible variations in applicable conditions (e.g. wet and dry surfaces). See [5.4.4.1]
and [5.4.4.2].
Dynamic friction, if applicable, should be tested and measured by a recognised
method.
The characteristic friction coefficient should be defined based on the 5th or the 95th
percentile confidence level of the test results, whichever is the most conservative.
At least 5 test pieces should be made, and each tested at least twice for each actual
condition.
The design friction coefficient is calculated using the characteristic friction coefficient
and an appropriate material factor. See [5.9.8.6], [5.9.5.3] and [5.9.6.2].
Where fewer tests are performed e.g. because of the scale, more conservative material
factors should be used.
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5.5Load categorisation
5.5.1Introduction
5.5.1.1
This section defines load categories and describes loads of general interest for marine
operations.
5.5.1.2
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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.
5.5.2Load 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 5­1 for all applicable
loads.
Table 5­1 Characteristic load selection
Limit states
Load category
2)
– Temporary design conditions
ALS
1)
ULS
Permanent (G)
Variable (Q)
Environmental (E) –
Weather restricted
FLS
Intact
structure
Damaged
structure
SLS
Expected maximum and minimum values (weight/buoyancy)
Specified2)
value
Specified2)
load
history
Specified
value
Specified
load
history
Specified2) value(s)
Specified value(s)
NA
Environmental (E) –
Weather unrestricted
Operations 4)
Accidental (A)
Deformation (D)
Based on
statistical
data 5)
Expected
load
history
NA
NA
Expected
extreme
value
Expected
load
history
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Based on statistical data5)
& 6)
Specified
value
NA
NA
Specified value(s)
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Notes:
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.
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.
5.5.3Permanent 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].
5.5.4Variable 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).
5.5.5Deformation 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
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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.
5.5.6Environmental 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.
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, 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].
5.5.7Accidental 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].
5.6Loads and load effects (responses)
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5.6Loads and load effects (responses)
5.6.1General
5.6.1.1
This section describes the loads and load effects that should be considered.
5.6.2Weight and centre of gravity (CoG)
5.6.2.1Introduction
1. For calculation purposes, conservative values of weight and CoG should be used.
2. Weight control shall be performed by means of a well­defined and documented system,
complying with ISO 19901­5 – Weight control during engineering and construction,
/99/.
3. ISO 19901­5 states (inter alia) that:
“Class A (weight control) will apply if the project is weight or CoG­sensitive for
lifting and marine operations or during operation (with the addition of
temporaries), or has many contractors with which to interface. Projects may also
require this high definition if risk gives cause for concern”.
“Class B (weight control) shall apply to projects where the focus on weight and
CoG is less critical for lifting and marine operations than for projects where Class
A is applicable”.
“Class C (weight control) shall apply to projects where the requirements for
weight and CoG data are not critical”.
4. Class A weight control shall apply unless it can be shown and agreed with the MWS
company that a particular structure and all its marine operations are not weight or CoG
sensitive.
5. Weight reports should be issued in accordance with Section 6 of /99/. Contents and
format of weight reports that are not in accordance shall be agreed with MWS company
at an early stage of the project.
5.6.2.2Weight considerations
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 with additional margins to account for
uncertainties during the design process and the factors in [2)] or [5.6.2.2 3)] for
unweighed and weighed objects respectively.
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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.
In addition to any in­place considerations, the following can control the NTE weight:
Draught and stability for tow­out, towages, mating operations and installation;
Allowable stresses in the structure for marine operations;
Limitations due to crane, load­out trailers, other equipment or ground­bearing
capacity.
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2. Where an object (excluding piles) is not to be weighed, the following shall be true for
the as­built weight report:
WReport, Factored ≤ Wud /γWeight
WReport, Base ≥ Wld γWeight (where applicable)
Where:
WReport,
Factored weight in weight report
Factored =
WReport,
= Base weight in weight report
Base
Wud
= Upper bound design weight
Wld
= Lower bound design weight
γWeight = Unweighed object weight margin factor as per Table 5­2
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 ≥ Wld γWeighing (where applicable)
Where:
WWeighed= Net weight in weight report
Wud
= Upper bound design weight
Wld
= Lower bound design weight
γWeighing= Factor to account for weighing equipment inaccuracy i.e. (
)
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.
Table 5­2 Unweighed object weight margin factors
Weight Class
(as defined by ISO 19901­5, /99/)
γWeight
A
1.05
B and C
1.10
5.6.2.3Centre of gravity factors
For weight Class A and B structures, see [5.6.2.1 3)], a CoG envelope shall be applied
to allow for CoG inaccuracies. For Class C structures a CoG envelope is recommended.
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.
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Guidance note 2:
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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. This factor
should normally not be taken less than 1.05.
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For Class C, if a CoG envelope is not used then a CoG inaccuracy factor of 1.10 shall be
applied to the weight. Where it can be documented that a lower CoG inaccuracy factor
is applicable, this should be agreed with the MWS company.
The CoG contingency factors for piles shall be determined considering the pile length
and the plate manufacturer’s plate thickness tolerance specification.
Normal weighing operations can be used only to identify the CoG position in a
horizontal plane. Consequently, 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 can be verified by means of an inclining test (see [2.10.5]).
5.6.2.4Weight control
The actual weight and CoG position shall be determined by weighing unless agreed
otherwise with MWS company.
Guidance note:
Gravity based structures and launched jackets are generally excluded from being
weighed.
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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 3 times with the load cells interchanged between each of the weighing
operations.
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.
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.
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.
When the installation of a large number of nominally identical items is to be approved,
the weight control programme should be documented to show the effects of all
potential variations on the final weights and the results documented by a competent
person.
See [18.2.1.2] for weight control for decommissioning/removal.
5.6.2.5Buoyancy
Buoyancy (hydrostatic external load) normally counteracts another load and shall be
categorised accordingly.
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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.
The buoyancy of the object and the position of the centre of buoyancy should be
determined on the basis of an accurate geometric model.
Characteristic buoyancy loads should be based on maximum and/or minimum expected
values.
Buoyant cargoes, particularly where the buoyancy contributes to stability
requirements, shall be adequately secured against lift­off unless it can be shown that
lift­off will not occur.
5.6.3Wind loads
5.6.3.1
Wind loads shall be calculated based on the characteristic wind speed, see Sec.3, and
recognised 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.
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:
DNV­RP­C205, /46/, gives further information with respect to shape coefficients as well as to
effects of wind direction relative to member, solidification and shielding.
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5.6.4Current loads
5.6.4.1
Current loads shall be calculated based on the characteristic current velocity, see Sec.3, and
recognised methods.
5.6.4.2
The increase in current velocities/loads due to shallow waters or narrow channels shall be
considered.
Guidance note:
DNV­RP­C205, /46/, gives further information with respect to shape coefficients as well as to
effects of flow direction relative to member, solidification and shielding.
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5.6.5Wave­current loads
5.6.5.1
Combined wave­current induced drag loads shall be calculated considering the vector sum of
the current and wave particle velocities.
5.6.5.2First order wave loads
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5.6.5.2First order wave loads
Wave loads should be estimated according to a deterministic or stochastic design
method. A wave period range according to [3.4.11.5] and [3.4.11.2] should be
investigated.
Guidance note:
If any responses are found governing for
the response should be
checked in these areas with
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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.
The effects of wave elevation shall be evaluated, and if necessary included in the
design.
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.
5.6.5.3Second order wave loads
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.
Long period responses excited by slow drift forces shall be investigated.
5.6.5.4Slamming loads and breaking waves
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.
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.
Loads due to slamming and breaking waves should normally be calculated according to
DNV­RP­C205, /46/.
Guidance note:
Further information regarding slamming loads and breaking waves can be found in
DNV GL Rules for classification: Ships /37/ Pt.3 Ch.10 and NORSOK N­003, /111/.
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5.6.5.5Green water
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:
See e.g. NORSOK N­003, /111/, for further information regarding green water effects.
Design forces for sea pressure from green water can be based on requirements for deck
houses, see DNV GL Rules for classification: Ships, /36/, Pt.3 Ch.4 Sec. 5.3.
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Deck cargoes vulnerable to damage from green water on deck should be protected by
breakwaters or increasing freeboard.
5.6.5.6Swell
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.
5.6.6Accidental loads
5.6.6.1
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.
5.6.6.2
The accidental load design principles indicated in DNV­OS­A101, /40/, should be considered
as applicable for the planned marine operation. DNVGL­RP­C204, /31/, gives further
guidance related to design philosophy and calculation of relevant accidental loads due to e.g.
collisions and dropped objects.
5.6.6.3
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.
5.6.6.4
DNV­OS­A101, /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 DNV­OS­A101, /40/, should be considered.
5.6.6.5Vessel collision
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.
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.
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.
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5.6.6.6Dropped objects
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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.
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 DNV­RP­H103, /56/, [4.7.3.5] and DNV­RP­F107, /52/, [5.3] for guidance.
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
DNV­RP­F107, /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.
5.6.6.7Other causes
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 & CoG’s, etc.
The static loads resulting from any one compartment damage, as described in
[11.10.4] to [11.10.7], shall be considered and, if significant, designed for as a LS2 or
ULS case.
5.6.7Dynamics
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 recognised 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 Section 9 of “DNV­RP­C205 Environmental Conditions and Environmental
Loads”, /46/ and Section 7.2 of “Dynamics of Fixed Marine Structures” ­ Barltrop and Adams,
/122/.
5.6.8Non­linearities
5.6.8.1
Non­linear effects shall be considered in cases where these significantly influence the
estimated responses. Non­linear effects are typically caused by:
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non­linear materials
non­linear geometry (large­displacement effects)
non­linear damping
non­linear combination of load components or response components
wave elevation e.g. due to wave­in­deck, non­linear effects of drag­loading (especially
with current), etc..
5.6.8.2
Non­linear load effects due to combinations of environmental loads should be taken into
account e.g. wave­current drag forces are a function of the square of the sum of the wave
and current particle velocities.
5.6.9Friction
5.6.9.1
Possible unfavourable effects of friction shall be considered. Well documented favourable
effects of friction may be included in the design.
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 recognised industry
standards or tests, see [5.4]. Indicative operation­specific values are given Table 10­2,
[11.9.2], Table 11­8, Table 11­20, Table 13­5 and in DNV­RP­H102, /55/, Table 2­4. For
soil­material interfaces, guidance is provided in DNV­RP­F109, /53/, Section 3.4.6 and DNV­
RP­F105, /51/, Section 7. Pipe­Soil Interaction.
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) multiplied by a material factor.
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 [5.9.7] or [5.9.8.6], [5.9.5] and [5.9.6]. These are also applicable to both ASD/WSD.
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
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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.
5.6.10Tolerances
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:
set­down tolerances (load­out, 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].
5.6.11Relative 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, de­ballasting or re­distribution of ballast,
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:
The towed hull is not a classed, seagoing vessel or barge, or
The cargo is longer than about 1/3rd 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
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See also [11.9.3.2] for friction, [11.9.5] for seafastening design and [11.27.4.3] 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 E­loads, see [5.5.6].
5.6.12Motion analysis
5.6.12.1General
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.
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2. Inertia loads due to motion should be calculated for all six degrees of freedom.
Guidance note:
This includes also 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.
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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.
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4. It is recommended that theoretical calculations are correlated against relevant model
test data (if available) in cases where strongly non­linear 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.
5. The analytical models should be checked with respect to sensitivity to input
parameters, see [5.6.14].
6. Recognised and well proven six­degree 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 non­linear damping effects. The
effect of forward speed shall be evaluated, where this is more onerous.
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:
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Guidance on drag and added mass coefficients for a range of standard shapes can be
found in DNV­RP­C205 /46/.
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8. First­order motion response analysis program 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.
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 non­critical routine operations or screening purposes.
5.6.12.2Wave headings
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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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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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.
5.6.12.3Wave periods
A wave period range with corresponding wave heights, see [3.4], shall be considered
when evaluating characteristic motions and accelerations.
5.6.12.4Response amplitude operators (RAO’s)
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 co­ordinate
system, or to establish RAO’s for these points. These RAO’s may be used for calculation
of significant and maximum responses.
When combining different responses, the phase angle between the different
components may be considered.
The gravity component shall be considered when determining the RAO’s for inertia
loads (e.g. transverse accelerations).
5.6.13Load cases and load combinations
5.6.13.1
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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).
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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 [5.6.15.2]
and [5.6.15.4].
5.6.14Sensitivity 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.
5.6.14.2
Where the operational safety is critically dependent on a sensitive input, conservative
characteristic values shall be used.
5.6.15Loads due to motions and wind
5.6.15.1
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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 self­weight and wind, shall be combined, i.e.:
where
Sd
= Design load or load effect.
S( )
= Response/load effect function.
Fx, Fy,
Fz
= Inertia forces (vectors), in x, y and z directions including relevant load
factors and gravity components.
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.
W
= Load due to self­weight (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
= Loadings caused
condition on the voyage.
F1
= Loadings caused
F2
= Loadings caused
F3
= Loadings caused
F4
= Loadings caused
F5
= Loadings caused
F6
= Loadings caused
F7
= Loadings caused
F8
= Loadings caused
by gravity including the effects of the most onerous ballast
by
by
by
by
by
by
by
by
the wind heel and trim angle.
surge and sway acceleration
pitch and roll acceleration
the gravity component of pitch and roll motion
direct wind
heave acceleration, including heave.sin(theta) terms
wave induced bending
slam and the effects of immersion.
5.6.15.4
One of the following four methods in this paragraph shall be used to determine the design
loadings:
Except as noted in [11.7.2.1], the effects of phase differences between the various
motions can be considered, if resulting from model test measurements, or if the
method of calculation has been suitably validated.
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 percent may be applied to fluctuating load cases F1 through F8 which
combine maximum wind and wave effects. However, if wind induced or wave induced
loads individually exceed the reduced load, then the greatest single effect shall be
considered.
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The total loads may be calculated by combination of loads as follows:
where:
Fmot
F#(1
=
Maximum load due to wind and wave motions
hour)
=
Loads based on 1 hour mean wind speed
F#(1
= Loads based on 1 minute mean wind speed
F#
= F1 through F8 as applicable
For deck cargo units carried on ships assessed using DNV GL Rules for the Classification
of Ships, /36/, Part 3, Chapter 4, Section 3, see [11.6].
Guidance note:
If the deck cargo is carried on a vessel classed an earlier edition of the DNV Rules for
the Classification of Ships, the earlier version can be used.
min)
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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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5.6.16Default motion criteria
5.6.16.1
For loads computed in accordance with [11.4], the loads applied to the cargo shall be:
S 1+F1+F3+F4+F6
where: S 1, F1, F3, F4 and F6 are as defined in [5.6.15.3].
The effects of buoyancy and wave slam loading shall also be considered if appropriate.
As stated in [11.7.2.1] roll and pitch cases are to be considered separately. Combined roll
and pitch are not required.
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 80% of the horizontal transverse and 60% of
the longitudinal acceleration with both the minimum and maximum vertical acceleration.
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5.6.17Loads due to restraint deflections, vessel motions and
wind
5.6.17.1
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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
Fdef
Fmot
=
=
=
Total design load
Maximum loads due to deflections
Maximum load due to wave motions and wind.
5.6.18Loads due to irregular waves and swell
5.6.18.1
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].
5.7Failure 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 sections.
5.7.3
Global modes of failure include:
structural collapse
overturning
sliding
lift­off
loss of hydrostatic or hydrodynamic stability
sinking
settlement
free drift.
5.7.4
Local modes of failure include:
plastic deformation (yield)
buckling
fracture
large deflections
excessive vibration.
5.8Analytical models
5.8.1
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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.
5.9Strength assessment
5.9.1General
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 5­1
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 DNVGL­OS­C101,
/24/, Ch.2 Sec.3.
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 for assessed for the ALS and LS / ULS are developed
using the applicable environmental inputs for ASD/WSD or LRFD.
Table 5­3 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
ULS­a
ULS­b
Loading history – important for structures exposed
to significant cyclic/repetitive loading
FLS
FLS
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Intact structure subjected to loads from an
accidental event
ALS­I
ALS­I
Damaged structure subjected to post­damage
loading
ALS­D
ALS­D
SLS
SLS
Serviceability checks (alignment, clearances,
deflection, vibration, etc.)
5.9.2Design 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 5­1.
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 the extreme loads due to the applicable design return period
environmental criteria, see Table 3­1; for weather restricted operations, where an Alpha
Factor according to [2.6.9] is to be applied. Any LS2 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 5­1.
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 b)].
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 d)].
Ensure adequate safety by proving that the design load effect does not exceed the
design resistance, See [5.9.3.2 a)].
5.9.3LRFD checks
5.9.3.1General
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Where the LRFD (load and resistance factor design) method is used for design
verification the load and material factors specified in this section 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.
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5.9.3.2Acceptance criteria
The level of safety is considered to be satisfactory if the design load effect, S d , does not
exceed the design resistance, Rd , i.e.:
S d ≤ Rd for all limit states
The equation S d = Rd defines the respective limit state.
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
= design load effect
Fd
= design load(s)
S
= load effect function.
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].
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 g)], [5.9.5.2] and
[5.9.6.2].
5.9.3.3Characteristic resistance
Rc shall be calculated based on the characteristic values of the relevant parameters or
determined by testing. Characteristic values should be based on the 5th or the 95th
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).
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
The characteristic static resistance of steel, fc, is to be taken as the smaller of:
the guaranteed minimum yield stress, fy, or
0.85 times minimum tensile strength of the material.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 3:
Rc for materials not mentioned e.g. concrete, concrete reinforcement, wood, synthetic
materials, soil, etc. could normally be based on recommendations/requirements in the
applied design code or standard. For soil see DNVGL­OS­C101 /24/ Section 10 1.3.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Rc for (wire & fibre) ropes and chains should be taken as the certified MBL.
5.9.4Fatigue limit states – FLS
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5.9.4Fatigue limit states – FLS
5.9.4.1General
For all structures exposed to significant cyclic loads during a marine operation the
possibilities and effects of fatigue should be considered.
The FLS design conditions should be based on the defined operation period and the
anticipated or expected load history during the marine operation. See Table 5­3.
Possible dynamic load effects due to e.g. slamming and vortex shedding should be
investigated. See [5.6.7].
Restraint loads, see [5.6.17.1], could be important and shall hence be thoroughly
evaluated and included in the FLS calculations.
The FLS shall be evaluated according to procedures given in a recognised code or
standard. See e.g. DNVGL­OS­C101, /24/, Ch.2 Sec.5 for general requirements for
checking of fatigue limit states.
Guidance note 1:
Reference can be made to DNVGL­RP­C203, /29/, and DNV CN 30.7, /20/, for practical
details with respect to fatigue design.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
For new structures that are susceptible to fatigue, it is advisable to check for adequate
fatigue life by analysis for voyages over about 50 days, including possible waiting time
at sea, where the nominal peak­stress range is less than 350 N/mm2 and the SCF does
not exceed 2.5. If the peak­stress range is increased to 550 N/mm2 then a fatigue
analysis is advisable for voyages over about 10 days.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 3:
New­build 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 fatigue­critical areas before
the voyage and to repair any cracks, see [11.27.4.4].
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
For mooring systems, the FLS is mainly of concern for steel components where fatigue
endurance limits the design. For fibre­rope segments, the time­dependent 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 DNVGL­OS­E301,
/27/.
Where structural items e.g. grillages and seafastenings, are to be re­used 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 could be
appropriate. For bolts, see [E.2].
5.9.4.2Design factors ­ FLS
All load factors shall be:
γf=1.0
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Design fatigue factors (DFF) shall be applied to increase the probability of avoiding
fatigue failures
The calculated cumulative damage ratios for the defined design conditions times the
applicable DFF according to Table 5­4 shall be less or equal to 1.0.
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 5­4 Design fatigue factors (DFF)
Inspection during operation
(and repair) planned
Elements in inspection
category I
Elements in inspection
categories II & III
Yes
2.0
1.0
No
3.0
2.0
Notes:
1. The elements shall be categorised according to the definitions in Table 5‑9.
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.
5.9.5Accidental limit states – ALS
5.9.5.1General
1. Accidental limit states for marine operations include verification of:
ALS­I: The intact structure or system for the defined accidental load effect(s)
combined with other relevant load effects, see Table 5­5 (i.e. loads of type E may
be ignored).
ALS­D: The damaged structure or system, see [5.9.5.1 2)], for relevant design
load effects, see Table 5­5.
Guidance note:
See also Table 5‑3 for definition of ALS­I and ALS­D.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
2. The damage to the structure or system in ALS­D 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].
5.9.5.2Design approach and load and resistance factors
Accidental loads are defined in [5.6.6].
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.
Load factors should in ALS normally, see [d)], be taken according to Table 5­5 or Table
5­6.
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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. ≤10­4 per operation,
probability. If working to the ASD/WSD approach, the factors should be similarly
increased.
The characteristic environmental load (E) in the ALS­D 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 5­5 ASD/WSD Load factors for ALS
Type
AISC 14th WSD option strength checking allowables
ALS­I
0.6
ALS­D
0.6
Notes:
1. The load factor of 0.6 for the ASD/WSD case arises because the basic allowable
stress in AISC WSD 14th edition 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 5­6 LRFD Load factors for ALS
Load
Condition
Load Categories
G
Q
D
E
A
ALS­I
1.0
1.0
1.0
NA
1.0
ALS­D
1.0
1.0
1.0
1.0
NA
Notes:
1. Load categories G, Q, D, E and A are described in [5.5.2]
5.9.5.3Material factor ­ ALS
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
5.9.6Serviceability limit states – SLS
5.9.6.1General
For some marine operations it is relevant to check SLS related to the feasibility of the
operation. Such serviceability limit states could be associated with required clearances,
push/pull capacities and vessel (barge) level (compared e.g. with quay height).
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See DNVGL­OS­C101, /24/, Ch.2 Sec.7 for typical SLS requirements for offshore steel
structures.
5.9.6.2Safety factors
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.
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
5.9.7ASD/WSD strength checks for structural steel subject to
LS1 or LS2 loading
5.9.7.1Design approach
1. The ASD/WSD design approach is described in 5.9.2.1.
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.
3. The infrequent load cases, generally limited to survival and damaged cases, including
design cases for weather restricted operations where an Alpha factor according to
[2.6.12] is to be applied, may be treated as an LS2 case (environmental load
dominated). This does not apply to:
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.
Steelwork supporting sacrificial bumpers and guides.
Spreader bars, lift points and primary steelwork of lifted items.
Structures during a load­out.
4. Traditionally AISC has also been considered a reference code, e.g. by API RP2A. If the
ANSI/AISC 360­10 American National Standard “Specification for Structural Steel
Buildings” of June 2010 (in the AISC 14th edition) is used, the allowables shall be
compared against 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 5­7.
Guidance note:
The API RP2A 22nd edition references the 9th Edition of AISC, which includes the
traditional “1/3 increase” for infrequent environmentally dominated load cases. The
14th 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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
5. Stresses in welds shall be assessed according to either:
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The method given in DNVGL­OS­C102 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].
The permissible usage factors for a) and b) are as follows:
Where the loads are due to accelerations determined according to Class
Rules, see [11.6]:
0.60 for welds made at fabrication site
0.52 for welds made on board the vessel.
Where the loads are determined using other approaches given in this
standard:
LS1 (cases where the loading is gravity dominated – see Table 5­3):
0.58 for welds made at fabrication site
0.51 for welds made on board the vessel.
LS2 (cases where the loading is dominated by environmental/storm
loads – see Table 5­3):
0.78 for welds made at fabrication site
0.67 for welds made on board the vessel.
Below deck welds in vessels classed to DNV ship rules may be checked against
90f1 in shear on the weld throat and 160f1 for normal stress perpendicular to the
weld throat, where f1 is the material factor for the applicable strength group as
given in /15/.
Guidance note:
If good welding conditions, see [5.10.2.2], and weld fit­up (e.g. control of
correct/no gaps to deck plate) on board the vessel are ensured by procedures and
well planned inspection it could be acceptable to increase the permitted
utilisations to those applicable for welds made at a fabrication site.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
6. The allowable strength of slip critical bolted connections shall be assessed according to
the method given in [E.2]. The permissible usage factors for slip critical bolted
connections, assuming all loads are assessed using the LS1 condition as shown in Table
5­7, are as follows:
Where the loads are due to accelerations determined according to Class Rules,
see [11.6]::
η = 0.48 for joints made with standard hole clearances.
η = 0.42 for joints made with oversize or slotted holes.
Where the loads are determined using other approaches given in this standard:
η = 0.62 for joints made with standard hole clearances.
η = 0.55 for joints made with for oversize or slotted holes.
The design of non­tubular connections shall be in accordance with an appropriate standard
such as AISC /2/, using a consistent safety format and factors.
Table 5­7 Load factors for use the ASD/WSD method and AISC 14th edition
Type
AISC 14th WSD option strength checking allowables
Limit State 1
(LS1)
1.00
3)
Limit State 2
(LS2)
0.75
3)
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Notes:
1. The load factor of 0.75 for ASD/WSD in the LS2 case arises because the basic
allowable stress in AISC WSD 14th edition 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.
2. Any load case may be treated as a gravity­load dominated limit state (LS1).
3. Where the loads are due to accelerations determined according to DNV and DNV GL
Class Rules, see [11.6], LS2 shall be used with a load factor of 1.2.
5.9.8LRFD strength checks for structural steel subject to ULS
loading
5.9.8.1General
DNVGL­OS­C101, /24/, Ch.2 Sec.4 gives provisions for checking of ultimate limit states for
typical structural elements used in offshore steel structures.
5.9.8.2Load factors ­ ULS
For the ultimate limit states (ULS) the two load conditions “ULS­a” and “ULS­b” as given in
the Table 5­8 shall be considered.
Table 5­8 Load factors for ULS
Load
Condition
Load Categories
G
Q
D
E
A
ULS­a
1.3
1.3
1.0
0.7
NA
ULS­b
1.0
1.0
1.0
1.3
NA
Notes:
1. Load categories G, Q, D, E and A are described in [5.5].
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 ULS­a.
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Where a permanent load G (e.g. self­weight or hydrostatic pressure) causes favourable
load effects, a load factor γf = 1.0 shall be used for this load in load condition a. See
also [5.6.2.2] and [5.6.2.3].
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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.
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.
5.9.8.3ULS material factors
Applicable material factors in ULS are given in [5.9.8.4] to [5.9.8.6]. Material factors
for materials not mentioned in [5.9.8.4] to [5.9.8.6] e.g. concrete, concrete
reinforcement, wood, synthetic materials, soil, etc. shall be in accordance with a
recognised code or standard. See also [5.9.3.3].
If a material factor γm = 1.0 is found more unfavourable than the indicated values, γm
= 1.0 shall be used.
5.9.8.4Material factors for structural steel:
1. In ULS the material factors for steel structures should be taken as minimum: γm=1.15.
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.
3. If EN 1993 (Eurocode 3) /61/ is used for calculation of structural resistance, the
material factors listed in DNVGL­OS­C101, /24/, Ch.2 Sec.4 for steel structures and
DNVGL­OS­C101, /24/, Ch.2 Sec.8 for welded connections shall be applied.
Guidance note:
See also Table 6­1 in NORSOK N­004, /112/, for applicable material factors.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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.
5. An increased (i.e. larger than 1.15) 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 following minimum
material factors, γmW, apply when the weld capacity is calculated according to DNVGL­
OS­C101 Ch.2 Sec.8, /24/, EN 1993­1­8 or [E.1]:
For welds made at fabrication site: γmW = 1.3
For welds made on board the vessel: γmW = 1.5
Guidance note:
If good welding conditions, see [5.10.2.2], and weld fit­up (e.g. control of correct/no
gaps to deck plate) on board the vessel are ensured by procedures and well planned
inspection γmW = 1.3 could be found adequate.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
5.9.8.5Material factors for ropes, chain and bolts
1. The design load in any chain, wire or webbing strap used for seafastening should not
exceed the certified (lifting) Working Load Limit (WLL) of the seafastening.
2. In ULS the material factor for certified steel wire ropes and chains should normally be
taken as:
γm = 1.5
Guidance note:
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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 that the characteristic strength, see [5.9.3.3] Guidance
Note 2, of ropes and chains is based on the tensile strength (MBL), and
0.9 is a general factor because wire ropes are considered more vulnerable to
“undetectable” wear and material irregularities than regular steel structures. For new
ropes with a 3.2 certificate it may be acceptable to use 1.0, see [15.10]. (Note also
that an additional wear factor could be applicable).
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3. For fibre ropes the material factor depends on the material and relevant failure mode.
The following minimum factors apply:
Polyester: 1.65
HMPE and Aramid: 2.0
Other fibre materials: 2.5.
Guidance note:
For fibre slings subject to a robust certification process, other material factors may be
considered acceptable; however, γm should not be less than 1.65
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
4. When using DNVGL­OS­C101 /24/, Ch 2 Sec 4.8, 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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
5.9.8.6Material factors for friction
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.
A material factor of maximum γm = 0.8 should normally be used to calculate the upper
bound design friction coefficient. See [5.4].
Guidance note:
In each case, the design friction coefficient should obtained by dividing the
characteristic friction coefficient by the material factor.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
5.10Materials and fabrication
5.10.1Design considerations
5.10.1.1Applicable codes
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In general material selection, fabrication method, and non­destructive testing should
be carried out according to a recognised offshore code, e.g. DNVGL­OS­C101, /24/, or
DNVGL­OS­C401, /26/.
Guidance note:
Recognised 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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Independent of the applied code, it shall be documented that the requirements in this
section [5.10] are fulfilled.
5.10.1.2Structural categories
1. Structural elements and connections shall be grouped in categories determined
according to:
type of stress
presence of cyclic loading
presence of stress concentrations
presence of restraint
loading rate
consequences of failure
redundancy.
2. Guidelines for selection of applicable materials for offshore steel structures can be
found in DNVGL­OS­C101, /24/, Ch.2 Sec.3.
Guidance note:
For steel with yield stress below 500 MPa, the test temperature need not be taken
lower than ­40° C
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
3. For materials in temporary structures used for marine operations, the following apply:
The design temperature, see DNVGL­OS­C101, /24/, Ch.2 Sec.3.2, 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 6­1 for guidelines regarding selection of structural category. See also
DNVGL­OS­C101, /24/, Ch.2 Sec.3.3.
For materials that could be welded under adverse conditions the yield strength
(SMYS) should not exceed 355 MPa.
5.10.1.3Material quality
Selection of steel types shall be determined based on the structural application and the
required category Table 5­9.
All steel materials shall be suitable for the intended service conditions and shall have
adequate properties of strength, ductility, toughness, weldability and corrosion
resistance.
Material types and qualities should comply with requirements in DNV­OS­B101, /23/.
Non­structural steels shall have mechanical properties and weldability suitable for the
intended application.
Table 5­9 Structural categories
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Selection criteria for
structural category
Failure
consequence
Substantial,
the structure
possesses
limited
residual2)
strength
Not
substantial,
the structure
possesses
residual2)
strength
Un­
substantial,
as local
failure will be
without
substantial
consequences
Structural
part
Complex1)
joints
Simple
joints and
members
Complex1)
joints
Simple
joints and
members
Any
structural
part
Examples for typical
structures involved in
marine operations
Padeyes and
other lifting
points
Seafastening
elements without
redundancy
Spreader bars
Structures for
connection of:
Mooring and
towing lines
Grillages
Redundant 2)
seafastening
elements
Bumpers and
guides
Fender structures
Redundant 2)
(parts of)
grillages
Recommended
structural
category
NORSOK N­
004
Equivalent
/112/ 4)
Insp.
Cat.,
DNV
GL
Special
DC1 –
SQL1
I
Primary
(Special)3)
DC2 –
SQL2
(SQL1)3)
I or
II5)
Primary
(Special)3)
DC3 –
SQL2
(SQL1)3)
II
Primary
(Special)3)
DC4 –
SQL3
(SQL1)3)
II
Secondary
DC5 –
SQL4
III
DNVGL­OS­
C101
Notes:
1. Complex joints are joints where the geometry of connected elements and weld type
leads to high restraint and to tri­axial 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 M­120, /110/ should
be considered as guidance only.
5. Extent of NDT to be according to DNV GL category I in Table 5‑10, but category II
may be used as “input” in Table 5‑10 regarding waiting time for these welds.
Regarding extent of inspection according to NORSOK M­101, /109/ inspection
category B is normally acceptable.
5.10.1.4Tolerances
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As­built deviations shall not exceed fabrication tolerances assumed in the applied
structural codes and standards, or in the design analysis, unless specially considered on
a case­by­case basis.
Acceptance of any as­built deviations exceeding specified tolerances shall be confirmed
in writing by, as applicable, the owner, designer, installation contractor, etc.
DNVGL­OS­C401, /26/, Ch.2 Sec.2.5 indicates fabrication tolerances that are normally
acceptable.
Some marine operations procedures can be difficult (or impossible) to execute when
standard tolerances are applied. In these cases consideration can be given to defining
and documenting the consequences of using tolerances that are less onerous than
those indicated in DNVGL­OS­C401, /26/
5.10.2Fabrication
5.10.2.1Workmanship
Workmanship during fabrication shall be of good standard and according to accepted
practice. See also DNVGL­OS­C401, /26/, Ch.2, Sec.1 and Sec.2.1 through 2.5.
Guidelines regarding assembly and welding can be found in DNVGL­OS­C401, /26/,
Ch.2 Sec.2.6.
5.10.2.2
Marine work Environmental conditions during marine construction work can be unfavourable
and the time available is often limited. Also accurate fit­up can be difficult to obtain e.g. due
to a dented barge deck. Such issues regarding marine work shall be duly considered in the
planning of the work. See also [5.9.8.4].
Guidance note:
Due to the special conditions during marine construction work, the following precautions are
recommended:
Welding procedure specifications should be qualified by welding procedure tests carried
out under conditions representative of the actual working environment; see DNVGL­
OS­C401, /26/, Ch.2 Sec.1.2.5.
Thorough inspections of fit­up and welding should be planned for.
Weather conditions and forecast to indicate acceptable conditions for welding
considering the welding method and available shelter at the welding locations.
Use of increased weld size in order to compensate for inaccurate fit­up (i.e. over­sized
gaps) to be considered.
Robust and well proven welding methods and procedures to be applied.
Use of material with improved weldability; see DNVGL­OS­C101, /24/, Ch.2 Sec.3.4.2,
to be considered.
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5.10.2.3Weld inspection
1. All NDT (non­destructive testing) of structures and structural components shall be
carried out by qualified personnel and covered by written specifications and
procedures.
2. Personnel evaluating results from NDT shall possess thorough knowledge and
experience with NDT.
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.
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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. Where through thickness properties of the steel are used, the material should be
certified accordingly (Z­quality). Where this is not feasible, the material under
through­thickness 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 SSC­290, /118/, for more details of
lamellar tearing. If access is not possible after welding, pre­welding checks could be
acceptable.
Guidance note 1:
For non­critical seafastenings and their supports, through­thickness testing should be
carried out when the tensile stress normal to any plate exceeds 100 MPa.
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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 [11.9.5.27]. 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.
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6. Requirements to non­destructive testing (NDT) of welds can be found in DNVGL­OS­
C401, /26/, Ch.2 Sec.3. Equivalent standards may be used e.g. EEMUA 158
“Construction specification for fixed offshore structures in the North Sea” /59/ and AWS
D1.1/D1.1M­2015 “Structural welding code – steel” /8/.
7. Minimum extent of inspection should be as shown in DNVGL­OS­C401, /26/, Ch.2
Sec.3 Table 1 with “Inspection Category” as defined in Table 5­9. See also Table 5­10
for a summary and especially note 4) to the table.
8. Normally final inspection and NDT of welds shall not be carried out before 48 hours
after completion. However, for materials with yield strength of 355 MPa or less this
could be reduced to 24 hours. See NORSOK M­101, /109/, Sec.9.1 and DNVGL­OS­
C401, /26/, Ch.2 Sec.3. 2 for further details.
9. For marine operations with weld inspection on the critical path, the minimum waiting
time should be selected according to Table 5­10 however, the decreased waiting may
only be used if the precautions listed in [5.10.2.2] are fulfilled.
Guidance note:
Weld inspection can be completed after a voyage has commenced provided that
procedures are in place to remediate or mitigate any defects that are found.
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Table 5­10 Minimum extent of NDT and waiting time
Inspection
Category
Minimum extent of NDT
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Minimum waiting time before NDT
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Visual
Other1)
SMYS 2) ≤355
MPa3)
SMYS 2) > 355
MPa3)
I
100%
100%
24 hours4)
48 hours4)
II
100%
20%5)
Cold weld6)
24 hours4)
III
100%
5%5)
Cold weld6)
24 hours4)
Notes:
1. Test method to be selected according to the type of connection, see DNVGL­
OS­C401, /26/, Ch.2 Sec.3, Table C1.
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 can start when the weld is cold, but it is recommended to wait as long
as practicable.
SECTION 6Gravity based structure (GBS)
6.1Introduction
6.1.1General and scope
6.1.1.1
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.
6.1.1.2
The areas shown in Table 6­1 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 6­1 Requirements for different GBS phases
General requirements
Stability and freeboard (all phases)
Structural strength
Temporary ballasting and compressed air
systems
Construction basin and tow­out
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See Sec.2 to Sec.4
See [6.2]
See [6.3] and Sec.5
See [4.3]
See Sec.12
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Construction and/or solid ballasting afloat
See Sec.14
Deck­mating (inshore or offshore)
See Sec.15
Towage(s)
See Sec.11
Instrumentation
See [6.4]
Installation at location
See [6.5]
Ensuring on­bottom stability
See [13.10]
6.1.2Revision history
6.1.2.1
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, Guidelines for concrete gravity structure construction & installation,
0015/ND
DNV Offshore Standard, Load transfer operations, DNV­OS­H201
6.2Floating GBS stability and freeboard
6.2.1General
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 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:
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
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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:
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:
Impact loads from vessels
Damage to structure or pipework from dropped objects
Mechanical system failure
Human error.
6.2.1.8
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.
6.2.1.9
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.
6.2.1.10
For operations where the structure cannot meet damage stability criteria, measures shall be
taken to minimise the risk, by:
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.
6.2.1.11
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 back­up or
protection systems, or special procedures should be assessed.
6.2.2Intact 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 MWS Company.
6.2.2.2
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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:
Maximum amplitude of pitch or roll motion in the design sea state, plus
Inclination due to design wind, plus
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 11­2. Both curves shall be bounded by
the least of:
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
6.2.3Effective freeboard
6.2.3.1
For inshore towages and construction afloat, the effective freeboard, as defined in Table 1­3,
shall not be less than the greater of:
1 m above the design wave crest height, with allowance for run­up, 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 one­compartment damage
stability.
6.2.3.2
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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 run­up, 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.
6.2.4Damage stability for tow­out and inshore tows
6.2.4.1
For tow­out from dry­dock, one­compartment damage stability is not required as it is a
controlled operation and the under­keel clearance is limited.
6.2.4.2
For other inshore tows the structure should have one­compartment damage stability, as
defined in [6.2.1.6] through [6.2.1.9].
6.2.4.3
If one­compartment damage stability requirements cannot be fulfilled, the requirements for
construction afloat in [6.2.5.2] shall apply.
6.2.5Damage stability during construction afloat
6.2.5.1
During the period of construction afloat, the platform shall possess one­compartment
damage stability, for as much of the construction period as is practical.
6.2.5.2
When the platform does not possess one­compartment damage stability, then in addition to
[6.2.1.10]:
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 [6.2.1.11], a risk assessment of flooding shall be carried out in accordance with
[2.4].
6.2.6Damage stability for offshore tows and installation
6.2.6.1
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When towing on the caisson or columns the platform should possess one­compartment
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:
The structure shall be locally reinforced within the zone defined in [6.2.1.9], 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.
6.3Structural strength
6.3.1Concrete 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.
3.
4.
5.
tow­out from construction basin or dry­dock (with and without any air cushion)
the most critical construction afloat stages
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
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compartment in the lesser of the 10­year 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 deck­mating 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 as­built 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.
6.3.2Structural concrete
6.3.2.1
The strength of concrete and its reinforcement including any pre­ or post­tensioning shall
comply with a recognised 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 DNV­OS­C502 – Offshore Concrete Structures, /41/, or the
GL Rules, /68/.
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.
6.4Instrumentation
6.4.1
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:
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
Compressor status
Air cushion pressure
Water seal level in skirt compartments
Status of access doors and manholes.
6.5GBS installation
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6.5GBS installation
6.5.1General
6.5.1.1
This section 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:
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 under­base grouting is required
Whether solid ballast or scour protection is required.
6.5.2Survey
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:
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 off­site touch­down 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
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processing), conventional high­resolution bathymetry and side scan sonar should be run in
conjunction.
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6.5.2.5
The seabed and sub­seabed conditions shall be established by coring, magnetometer, in­situ
testing, lab testing and sub­bottom 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 sand­waves 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.
6.5.3Seabed 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:
Controlled dumping and compacting of gravel before final levelling
Placing sand­bags
Excavating of unsuitable soils before replacing as in a) or b).
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6.5.4Installation 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:
The required external assistance (e.g. temporary buoyancy, winches, cranes, etc.)
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
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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:
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
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.
6.5.5Positioning 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.
6.5.6Ensuring on­bottom stability/skirt penetration
6.5.6.1
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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:
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:
differential pressure (suction)
penetration
flow rate
6.5.7Anti­scour 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 anti­scour precautions where required shall be documented. Possible solutions to
scour include:
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Controlled rock dumping or placing sand­bags 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.
SECTION 7Cables, pipelines, risers and
umbilicals
7.1Introduction
7.1.1
This section is currently under development and therefore for work related to cables,
pipelines, risers or umbilicals the following legacy documents apply:
0029/ND, GL Noble Denton, Guidelines for Submarine Pipeline Installation
0035/ND, Section 10 (for cables), of GL Noble Denton, Guidelines for Offshore Wind
Farm Infrastructure Installation, and
DNV­OS­H206 ,DNV Offshore Standard, Load­out, transport and installation of subsea
objects (VMO Standard Part 2­6).
7.1.2
The legacy documents shall be used in their entirety including any referenced documents
and NOT the equivalent sections of this Standard.
Guidance note:
For example if DNV­OS­H206 is applied then DNV­OS­H101, and DNV­OS­H102 and DNV­
OS­H205 also apply along with any other referenced documents.
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7.1.3
For the installation by lifting of other subsea equipment the requirements of this document
should apply unless agreed otherwise.
Guidance note:
Generally, where subsea equipment is installed by lifting as part of a project using the
documents referenced in [7.1.1] then the legacy documents would apply.
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7.2Codes and standards
7.2.1
A number of recognised standards and design codes covering pipelines, risers and umbilicals
are already in existence and should be considered.
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Guidance note 1:
The following are examples of relevant industry standard codes:
Pipelines in general: API RP 1111, /3/and BS EN 14161, /10/,
Risers in general: API RP 2RD, /4/
Submarine pipelines: DNV­OS­F101, /42/,
Dynamic risers: DNV­OS­F201, /43/,
Flexible pipe systems: ISO 13628­2, /95/, or ISO 13628­11, /97/,
Umbilicals: ISO 13628­5, /96/,
Subsea power cables: see Guidance note 4.
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Guidance note 2:
Generally the default for rigid pipeline system design and approval is DNV­OS­F101
Submarine Pipeline Systems. DNV­OS­F101 Sec.10 gives requirements for
installation/offshore construction of submarine pipeline systems. Parts of DNV­OS­F101
Sec.10 are also generally applicable for flexible pipes and risers.
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Guidance note 3:
Detailed guidance regarding installation of cables may be found in DNV­RP­J301 Sec. 6.
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SECTION 8Offshore wind farm (OWF)
installation operations
8.1Introduction
8.1.1General
8.1.1.1
This section gives the MWS requirements for installing offshore wind farm 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.
8.1.2Scope
8.1.2.1
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 jack­up platforms.
8.1.3Revision history
8.1.3.1
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This section replaces the applicable sections of the following legacy document:
0035/ND Guidelines for Offshore Wind Farm Infrastructure Installation.
8.2Planning
8.2.1General
8.2.1.1
See Sec.2 for general planning requirements and Sec.3 for environmental conditions and
criteria.
8.2.2Tolerances 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 trade­off 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:
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:
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.
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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.
8.2.3Vulnerable 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:
J­tube entry holes being covered with soil or debris.
Changes in seabed level (from scour, dredging, jack­up footprints, drill cuttings, etc.)
varying the natural frequency of foundations.
Scour can also affect jack­up 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 back­up seals.
External fittings (including anodes, J­tubes, 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).
8.2.4Planned moorings
8.2.4.1
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.
8.3OWF installation vessels
8.3.1Jack­ups – general
8.3.1.1
Jack­up legs can be a major threat to cables. The as­laid 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 as­laid cable route and the intended
positions of the jack­up legs. This is of particular importance in OWF developments where
cable laying/installation is progressing near turbine installation activities in a similar time
frame.
8.3.2Jack­ups in weather unrestricted operations
8.3.2.1
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8.3.2.1
Jack­ups 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 DNVGL­ST­N002, /39/
8.3.2.2
The jack­up can operate at a lower air gap than required for survival in a design storm as
long as it is able to jack­up 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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8.3.3Jack­ups in weather restricted operations
8.3.3.1
Jack­ups 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 jack­up operations is given in Section 5.3 of
RenewableUK Guidelines for Jack­ups, /115/, but note that [2.6.7] allows a greater
operational window. This is summarised as:
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 jack­up locations between the
site and safe ports.
The jack­up is only to leave a safe location to go to the installation site on receipt of a
favourable weather forecast with high confidence to cover the time (including a
contingency for delays) from departure to return to a safe location.
The jack­up is to leave the installation site unless there is a confident good weather
forecast to cover the remaining time on site and to return to a safe port or to elevate
to a safe air gap at a suitable stand­by location, including a contingency for delays.
If the jack­up cannot reach a safe port or location before meeting bad weather (above
the laden jacking limits of the jack­up, typically about 1 m to 1.5 m significant wave
height), then it should jack­up 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].
8.3.3.3
Jack­ups 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 classification society.
8.3.3.4
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Jack­ups can operate in semi­jacked­up 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 jack­up 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.
8.3.4Crane 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].
8.3.5Inshore crane vessels and barges
8.3.5.1
Inshore crane vessels and barges shall only be used if allowed by their class notation and:
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.
8.3.6Grounded OWF installation vessels and barges
8.3.6.1
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:
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 float­off without exceeding the
operational criteria.
8.3.7Other OWF installation vessels
8.3.7.1
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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.
Crew transfer or accommodation vessels with proprietary crew access arrangements.
Escort and stand­by 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.
8.4Planning and execution
8.4.1Procedures and manuals
8.4.1.1
Technical documentation shall be completed for all operations. See [2.3] for details. In
general, this should include:
The anticipated timing and duration of each operation, including contingencies.
The limiting wave states, wind speeds and currents, and where applicable any
visibility/day­light, temperature and precipitation limits, as well as the site­specific
equipment or methodology prescribed for measuring each limit­state.
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.
8.4.2Weight 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.
8.4.2.3
When a large number of virtually identical items are built with very good quality control,
reduced weight contingency factors can be agreed with the MWS company based on the
standard deviation from weighing of initial items, with random subsequent weighing used to
confirm consistency of manufacture.
8.4.2.4
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8.4.2.4
Where rigorous quality control is in place, and predictions of final weights in initial weighings
are demonstrated to be accurate, a reduced requirement for weighing can be agreed with
the MWS company.
8.4.3Weather 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 current­sensitive operations such as:
Moving jack­ups 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 jack­legs 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.
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.
8.4.4Site 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
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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 jack­up 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 sub­bottom 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 jack­up vessel operators. This will allow them to carry out site­
specific assessments in accordance with ISO 19905­1, /102/, and to assess the possibility of
scouring around jack­legs 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.
8.4.5Scour 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.
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Guidance note 2:
Cables are generally be trenched or otherwise protected in scour­prone areas. However
additional precautions can be required close to J­tubes or I­tubes at monopiles or platforms,
especially immediately after laying.
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Guidance note 3:
Scour around jack­up legs can make them more vulnerable to punch­through 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.
8.4.6Wet storage of jackets or OWF foundations
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8.4.6Wet storage of jackets or OWF 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.
8.5Load­outs of OWF components
8.5.1Structure load­out
8.5.1.1
Load­outs shall be in accordance with Sec.6. However the following special cases apply, as
applicable.
Special consideration should be given to purpose­built 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, re­use can be accepted as described in [E.2] but only if initial pretensioning does
not exceed 60% of the bolt yield strength and the loads during lifting have not
exceeded the maximum design values. For re­use 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.
Re­useable lifting lugs shall be tested in accordance with [16.9.7].
8.6Transport of OWF components
8.6.1General
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
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The requirements of [E.2] will apply for bolted connections used for seafastening. The
strength of bolted connections may be assessed to DNVGL­OS­C101 /24/, Ch 2 Sec 4.8,
Eurocode 3 /61/ or [E.2]
8.6.1.4
Minimum clearance between cargo items to be lifted is given in [16.13.2] and [16.13.3].
8.6.2Transport of complete rotor
8.6.2.1
Rotors with diameters of well over 100 m may be transported horizontally (rotor axis
vertical) on vessels or barges of only about 30 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.
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Guidance note 2:
Normally the voyage and installation planning considers some or all of the following:
The rotor being designed to safely withstand the accelerations (from [11.3]).
Reducing to negligible the probability of wave slam on the blades by securing them
well above the still water level.
Selecting vessels that can be ballasted to reduce the motions in likely wind and wave
combinations.
Doing motion response calculations to optimise the loading and ballasting
arrangements so as to minimise the probability of wave slam on the blades in likely
wind and wave combinations.
Weather routing the transport to avoid any weather that could cause wave slam on the
blades. (This cannot always be practicable for some seasons and longer routes between
suitable shelter points).
Developing procedures to avoid blade collision damage when coming alongside loading
quays, entering ports of shelter (as part of the weather routing) 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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8.6.3Transport of tall vertical cargoes
8.6.3.1
Seafastening of the Transition Piece flanges on barges or ships is often critical for many
projects. The design of the bolted connection 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 2
shim plates should be used at any gap.
8.6.3.2
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Bolts used for Seafastening shall generally be used one time only. In special cases, re­use
can be accepted as described in [E.2] but only if initial pretensioning does not exceed 60%
of the bolt yield strength and the loads during the transport have not exceeded the
maximum design values. For re­use 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 MPI (Magnetic Particle
Inspection) after every 3 transports unless fatigue calculations accepted by the MWS
company show that less frequent inspections are acceptable.
8.6.3.3
Pretension bolts in seafastenings shall be used only once due to fatigue during voyages.
8.6.3.4
Seafastenings shall be designed to allow safe removal offshore without endangering the
cargo or personnel. See also [11.9.6].
8.6.3.5
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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8.6.3.6
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.
8.6.4Other OWF wet towages
8.6.4.1
Larger Concrete Gravity Structures will generally be built in a dry­dock 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 purpose­built installation units. Where
these are not covered by 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 one­compartment 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 [11.10.7.3].
8.7Installation of OWF components
8.7.1Monopiles and transition pieces installation
8.7.1.1
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8.7.1.1
The following items shall be addressed and agreed with the MWS company:
1. Position and orientation tolerances (see [8.2.2]).
2. Release of seafastenings which will normally require a specific procedure, especially for
tall objects transported vertically.
3. Sea bed soil condition and scour protection requirements (see [6.5.7] and [8.4.5])
4. Levelling arrangements for the transition pieces.
5. Grippers, handling and upending equipment.
6. On­bottom stability of the unpiled Monopile in the pile gripper.
7. Stability of the Transition Piece on the Monopile before grouting (see [13.10] for the
criteria).
8. 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]).
8.7.2Piling 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 jack­up, with the facility to be lowered or raised and may
use the jack­up legs as a positioning guide.
8.7.2.2
If transported attached to a jack­up 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. Mud
mats 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.
8.7.3Suction bucket foundations
8.7.3.1
The requirement for any seabed preparation before installation shall be determined at any
early stage.
8.7.3.2
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Equipment and procedures shall be documented to ensure that:
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.
8.7.4J­tubes and I­tubes
8.7.4.1
Installing cables through J­tubes and I­tubes is covered in Sec.7.
8.7.5Turbine installation
8.7.5.1
Requirements in this standard shall apply unless novel installation techniques are proposed.
8.7.6Towers
8.7.6.1
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 de­rigging
Partial bolting
Lifting points certification for multiple use (load­out, 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.
8.7.7Nacelles
8.7.7.1
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 de­rigging
Partial bolting.
8.7.8Blades
8.7.8.1
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8.7.8.1
Installation lifting requirements are covered in Sec.16. In addition the following items shall
be addressed, if applicable, and agreed with the MWS company:
Infra­red release systems which shall be shown to be reliable in releasing and, more
importantly, not liable to early release from any cause
Limiting criteria.
Boom tip motions, See [16.17.3.1 4)]
Partial bolting.
8.7.9Complete rotor assembly installation
8.7.9.1
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.
8.8Lifting operations and lifting tools
8.8.1
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 to
the MWS at the beginning of the project. Refer to [16.12] for more information.
8.9Information required for MWS approval
8.9.1
See subsections at the end of each relevant section, e.g. lifting, voyages, etc.
SECTION 9Road transport
9.1Introduction
9.1.1General
9.1.1.1
This section gives the requirements for objects subject to road transport on public roads,
which are generally subject to national or local requirements/legislation.
9.1.2Scope
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9.1.2Scope
9.1.2.1
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.
9.1.3Revision history
9.1.3.1
This section is new.
9.2Requirements
9.2.1Statutory requirements
9.2.1.1
Most countries have specific legislation containing criteria for transport of large items by
road. These shall be obtained and complied with.
9.2.2Loads and accelerations
9.2.2.1
Table 9­1 will cover most countries with published requirements for tie­down 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 9­1 Typical road tie­down 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)
9.2.3Securing
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.
9.2.4Stability
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9.2.4Stability
9.2.4.1
Stability in accordance with [10.5.3.15] shall be demonstrated.
9.3Information required
9.3.1Object information
9.3.1.1
For the object:
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.
9.3.2Trailer or SPMTs
9.3.2.1
Requirements are given in [10.5.3].
9.3.2.2
For the trailer or SPMTs:
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.
9.3.3Securing
9.3.3.1
For the securing arrangement:
Details including WLL/SWL 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.
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Demonstration of lashing/stopper adequacy against uplift and horizontal forces,
including friction assumptions and description of friction material and description of
blocking if used.
9.3.4Route
9.3.4.1
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.
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 de­powering 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.
9.3.5Risk assessment
9.3.5.1
A risk assessment in accordance with [2.4] of the transport.
SECTION 10Load­out
10.1Introduction
10.1.1Scope
10.1.1.1
This section presents the requirements for load­out 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 load­outs
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load­in, i.e. a reversed load­out
vessel to vessel load transfers
transverse load­outs
site moves.
10.1.2Other types of load­out
10.1.2.1
For load­out operations carried out by crane lifting, see Sec.16
10.1.2.2
For other load transfer operations, see Sec.15.
10.1.3Revision history
10.1.3.1
This section replaces the applicable sections of the following legacy documents:
DNV­OS­H201, Load transfer operations
GL Noble Denton, Guidelines for Load­outs, 0013/ND
10.2General
10.2.1Load­out class
10.2.1.1
Requirements to load­out equipment are defined according to load­out class. The load­out
operation shall, based on tide conditions and weather restrictions, be classified according to
Table 10­1.
Table 10­1 Load­out (operation) class
Tide range
1)
Tide restricted?
2)
Weather restricted?
Load­out Class
3)
Significant
Yes
No/Yes
1
Significant
No
Yes
2
Significant
No
No
3
Zero
No
Yes
4
Zero
No
No
5
Notes:
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 load­out
shall be defined as tide restricted.
3. If weather restrictions apply, then the load­out shall be categorized as weather
restricted, see [2.6]. If there are no weather restrictions to the object
movement/ballasting phase the load­out class may be selected accordingly.
10.2.1.2
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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 load­out, then the tide range in
Table 10­1 should normally be regarded as significant even if the astronomical tide variation
is defined as zero.
10.2.1.3
For grounded load­outs, see [10.8.1], the tide range in Table 10­1 shall be defined as
significant if ballasting is required in order to maintain ground reactions within acceptable
limits.
10.2.2Planning
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 load­out shall be safe conditions and clearly defined, see [2.5].
Guidance note:
A load­out from one safe to another safe condition could include many sub­operations, such
as “lift­off 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 load­out into two (or
more) operations with safe condition(s) in­between, see [2.5].
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10.2.2.3
Tide variation is normally a critical parameter for load­outs. 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 load­out operations:
1.
2.
3.
4.
5.
6.
Yard lay­out, including position of object
Transport vessel dimensions and strength
Object position and support height on transport vessel
Load­out 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. Load­out site soil strength and condition
9. Skidway levelness tolerances
10.2.2.5
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A load­out operation could involve several construction­, transport­ and load transfer (main)
contractors/responsible parties. Interface planning should be given due attention.
10.2.3Risk management
10.2.3.1
Operational risk should be evaluated and handled in a systematic way, see [2.4].
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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10.3Loads
10.3.1General
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.
10.3.2Weight and CoG
10.3.2.1
Weight (W) and 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].
10.3.3Weight of load­out equipment
10.3.3.1
The weight of the load­out equipment (Weq ) should be accurately assessed.
Guidance note:
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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 load­out equipment shall be considered by applying
conservative estimates in the load­out calculations, see however [4.3.9.2].
10.3.4Environmental 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
Load­out operations should normally not be carried out in significant waves and swell
conditions.
Guidance note:
Applicable loads due to waves and swell for transport vessel mooring before and after the
load­out operation to be considered.
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10.3.5Skidding 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
=
[10.3.5.4]
μ2
=
[10.3.5.4]
W
=
Weq
=
P1
=
P2
=
Required break­out load
Load required to continue moving the object
Upper bound design friction coefficient or rolling resistance for break­out, see
Upper bound design friction coefficient/rolling resistance for the move, see
Object weight, see [10.3.2]
Equipment weight, see [10.3.3]
Any other load occurring during break­out, 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
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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 10­2 unless adequate in­service documentation indicates
that other values may be used, see also [5.6.9].
Guidance note:
The indicated friction coefficients for moving include re­starting after short stops during the
load­out operation. Break­out 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 10­2 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
Steel wheels/Steel
0.02
0.02
Rubber tyres/Steel
0.02
0.02
Rubber tyres/Asphalt
0.03
0.03
Rubber tyres/Compacted
gravel
0.05
0.05
Rolling surfaces
Notes:
1. It is assumed that sliding
surfaces are properly lubricated.
2. Long term effects such as
adhesion, settlements, etc. are
included in values for break­out.
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 a structure 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 break­out requirements
than anticipated. The effects of skidway deformation shall also be considered.
10.3.6Skew load
10.3.6.1
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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 load­out operations where the object has a 3
point support system. This could be obtained by including a reliable load equalising system.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
For SPMT load­outs 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 structure and/or uneven
conditions beneath the SPMTs.
Appropriate limitation in pressures should be defined and the structure 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%.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 3:
For skidded load­outs 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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10.3.7Other loads
10.3.7.1
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 may include:
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Hydrostatic loads on transport vessel(s)
Impact loads
Local support loads on grounded vessel hulls
Mooring loads
Guiding loads.
10.4Design calculations
10.4.1General
10.4.1.1
Structures and structural elements shall be checked against the requirements in [5.2]. for
the load cases in [10.3].
10.4.1.2
Mooring system design is covered in [10.5.8].
10.4.2Load 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.
Guidance note:
A load­out operation consists of a sequence of different load cases. In principle, the entire
load­out sequence should be considered step­by­step 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 load­outs, analyses of the skidded object should consider the elasticity,
alignment and as­built dimensions of the shore and vessel skidways. See also [10.3.6.2] GN­
3.
10.4.2.3
For trailer load­outs, 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]
GN­2.
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10.4.2.4
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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.
10.4.3Quays
10.4.3.1
Allowable horizontal and vertical load capacities of load­out quays should be documented
according to a recognized code or standard.
10.4.3.2
Calculations showing that the actual loads during load­out 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 load­outs over the quay may be considered. Detailed
information about the previous load­out(s) will be needed for an adequate comparison.
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10.4.4Soil
10.4.4.1
Strength and settlement calculations/evaluations for the ground in the load­out area should
be documented.
Guidance note:
The risk of differential ground settlements which may influence the loads during load­out
should be considered and minimised by means such as:
pre­loading of ground in load­out 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 load­out of the object.
Alternatively, relevant site investigation should be documented.
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 load­out.
10.4.4.6
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For load­outs involving grounded vessel, the seabed should be evaluated with respect to
topography, bearing capacity, settlement, etc.
10.5Systems and equipment
10.5.1General
10.5.1.1
Systems and equipment to be used during load­out should comply with the requirements
given in Sec.4.
10.5.2Propulsion 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 load­outs be for instance wire and winch, hydraulic jacks
or strand jacks. Trailer load­outs can be by self­propelled 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 break­out shall be not less than the required break­out
load (F1), see [10.3.5.1]. For objects that cannot be considered to be in a safe condition if
the break­out system fails the capacity and redundancy requirements in Table 10­3 apply
also to the break­out system.
Guidance note:
Adequate break­out 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
10­3.
10.5.2.4
Propulsion systems should act in a synchronised manner in the transfer direction. A
minimum required load­out velocity shall be identified considering:
Maximum allowable load­out duration
Dynamic friction coefficient
Length of the load­out 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
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The propulsion system shall be able to provide adequate braking capacity at any time.
Required braking capacity shall be evaluated assessing conservatively the possible
(combined) effects of:
Track slope, including maximum possible (accidental) inclinations of the load­out vessel
Low friction, e.g. by using (steel) wheels/rollers or surfaces with low friction
Elasticity in pull system, i.e. high elasticity (e.g. long winch wires) combined with
temporary jamming could result in a “catapult effect”.
10.5.2.6
Back­up propulsion system capacity should be able to compensate for the following
conditions:
Breakdown of one arbitrary self­contained propulsion unit
Unexpected increase in the skidding loads above the expected nominal value
Guidance note:
Back­up capacity for accidental conditions of type a) may be achieved by:
Spare capacity in the main propulsion units
Separate back­up propulsion units with sufficient capacity
Spare parts for the main propulsion units and an acceptable and proven
repair/replacement time
The back­up capacity for conditions of type b) may be:
Spare capacity in the main propulsion units
Back­up propulsion units
Detailed requirements to be complied with are in Table 10‑3.
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10.5.2.7
Any required modifications during the operation, e.g. removal of pull bars of the push/pull
system lay­out should be proven feasible. Normally, lay­out modifications should be avoided
with the object supported both at the quay and transport vessel.
10.5.2.8
A pull­back system and a procedure for pulling the object back on shore shall be available for
Load­out Class 1.
10.5.2.9
A pull­back system and procedure shall be available for Load­out 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).
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Table 10­3 Propulsion system requirements
Load­out
Class
Intact
System
Capacity
System Redundancy Requirement after
breakdown of any one component
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Pull­back
System
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1
160%
130% capacity or repair possible within 30
minutes
Required
2
140%
120% capacity or repair possible within 2
hours
See [10.5.2.9]
3
120%
Repair possible
Not required
4
120%
100% capacity or repair possible within 6
hours
See [10.5.2.9]
5
100%
Repair possible
Not required
Notes
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 load­out without repairing the component, or it shall
be possible to repair the component within the timeframe indicated.
3. Where a pull­back system is achieved by de­rigging and re­rigging the pull on
system, the time required to achieve this shall be estimated, clearly defined and
duly considered.
10.5.3Trailers
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 3­point support system is generally less
stable than a 4­point support system.
10.5.3.3
The trailer configuration should have adequate manoeuvring capabilities for the intended
load­out (including site move) route.
Guidance note:
Where a structure 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 structure/load­out support frame (LSF)/cribbage.
It should be demonstrated by the load­out contractor that the steering coordinates
used in the trailers or SPMTs set up are correct, with the details of the set up
coordinates contained in the procedures.
The cornering speed should be kept to a minimum to avoid the potential for loads due
to lateral accelerations affecting the stability of the structure or SPMTs. Alternatively, a
limiting turn speed should be specified and the stability assessed accounting for the
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associated loads.
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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 lay­out
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 lift­off should the load distribution between the trailer groups not be as expected.
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10.5.3.5
The following shall be documented for the trailer axle loads calculated according to
[10.5.3.4]:
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 lay­out 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:
External effects, i.e. reaction loads from wind, inertia (e.g. acceleration during start
and stop) 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 co­ordination in operation of the hydraulic suspension
system shall be considered.
10.5.3.9
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The traction system, either the trailers are self­propelled 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 load­out the
loading should be within the stated maximum trailer loading.
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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 load­out.
10.5.3.15
Adequate global stability of the hydraulic system shall be ensured. 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°.
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Guidance note:
The COG to be used in these calculations is the combined CoG for trailers and object.
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10.5.3.16
For load case A the following shall be considered:
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.
10.5.3.17
For load case B the following shall be considered:
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 hang­up between the vessel and the quay, or
dynamic response after release of hang­up.
Any free surface liquids within the structure.
Guidance note 1:
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.
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Guidance note 2:
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 hang­up between vessel
and quay:
No
Free surface effects:
None
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Calculation for example case:
Design slope:
3°+2° = 5°
Load case A
Virtual correction of COG:
(10x300 x sin5)/300 = 0.87 m
Horizontal distance from virtual
COG to tipping line:
2.5­0.87= 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 sin5°))/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
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10.5.3.18
For virtual COG location as in load case A or B in [10.5.3.15], it shall be demonstrated that
the structure itself is stable on the trailer bed. 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 restraint system is suitable. The restraint shall be designed to provide
hold­down equal to the calculated hold­down 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 load­out operations where the CoG of the structure 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 structure 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 is to be checked accounting for the slope and the horizontal load from the structure
on to the trailers or SPMTs.
10.5.3.21
Load­outs with high slender structures 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 de­ballasting.
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10.5.4Skidding 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.
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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 the load­out
Side (lateral) guides are normally provided along the full length of skidways.
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10.5.4.4
Where a vessel, because of tidal limitations, has to be turned within the load­out 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 structure from fully on the shore to
fully on the vessel without re­rigging
Guidance note:
If re­rigging cannot be avoided, then this should be included in the operational procedures,
and adequate resources should be available.
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10.5.5Ballasting systems
10.5.5.1
The requirements for the ballasting systems are given in [4.3].
Guidance note 1:
The load­out classes defined in Table 10‑1 corresponds to the operation classes referred to in
[4.3.2].
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Guidance note 2:
Normally, vessel pumps should not be considered for the primary ballast system but may be
taken into account in the back­up provision
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10.5.6Power supply
10.5.6.1
The power requirements in [4.3.2] shall apply for both the ballast pumps and the propulsion
units during the load­out.
Guidance note:
Need for additional power supply to e.g. lighting and welding should be considered.
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10.5.7Testing
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
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.
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10.5.7.3
For load­out operations of Class 1 a complete test run of the ballast system following the
procedure for the load­out should be carried out.
10.5.7.4
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The propulsion units including the spare units should be tested in both push and pull mode
before the load­out operation in order to verify the estimated friction forces and
functioning/capacities of the equipment.
10.5.7.5
If the considered back­up necessitate replacement of equipment (e.g. pumps and propulsion
units) then this should be included in the test program.
10.5.8Mooring and fendering
10.5.8.1
General design requirements to mooring systems are given in Sec.17. Additional
requirements applicable for load­outs are given below.
10.5.8.2
For additional load cases to be considered, see [10.4.2.4].
10.5.8.3
Moorings for the duration of the actual load­out 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 structure.
10.5.8.4
Mooring before and after load­out should normally be considered a weather unrestricted
operation. Weather unrestricted moorings should be designed to the return periods given in
Table 3­2 and in accordance with Sec.17.
10.5.8.5
Facilities for re­tensioning of mooring lines should be present and in stand­by during the
load­out.
Guidance note:
Such facilities may be winches, jacks for tensioning, etc.
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10.5.8.6
The mooring system stiffness shall limit the movements of the load­out vessel(s) to those
that are acceptable during the load­out in particular when the object is supported both on
the quay and vessel.
10.5.8.7
Adequate strength, stiffness and layout of fenders should be documented.
Guidance note:
Fender design solutions should at least consider:
Requirement for a stiff mooring system during load­out, see [10.5.8.6]
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 load­outs 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
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fendering should be liberally lubricated with grease or other substitute which complies with
local environmental rules.
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10.5.8.8
Friction between the vessel and support pad considered as a part of the mooring system in
grounded load­outs, 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
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10.6Vessels
10.6.1General
10.6.1.1
General requirements for vessel(s) are given in [2.11]. These requirements are applicable to
any vessel involved in the load­out.
10.6.1.2
See Section [10.9.3.2] for requirements to vessel documentation.
10.6.1.3
For tugs involved in the load­out 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 load­out berth in the event of deteriorating weather, or tug
back­up to the moorings, see also [10.7.2.4].
10.6.1.5
For the load­out vessel the requirements in Sec.11 apply as relevant.
10.6.2Class
10.6.2.1
Generally it is recommended that a vessel classed by a recognized classification society is
used, see also [2.11].
Guidance note 1:
If the vessel is not classed by a recognised 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 load­out 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 load­out should be classed for such
use by a recognised classification society.
10.6.2.3
Where a load­out 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 load­out shall be reinstated after the load­out 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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10.6.3Structural strength
10.6.3.1
The load­out 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:
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 load­out
Bollards/mooring brackets
10.6.4Stability afloat
10.6.4.1
Sufficient stability afloat shall be ensured during load­out.
Guidance note 1:
Generally load­out 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 load­out.
However, it is recommended to consider if/how incorrect operation of the ballast
system may influence stability.
Due attention should be given to situations with small metacentric height where an
offset centre of gravity may induce a heel or trim as the structure 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 “hang­up” by resisting the heel or trim until the pull­on 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 load­out operations the minimum “effective freeboard”, should for vessels be
fmin=0.5 + 0.5H max
where
fmin
H max
=
=
Minimum effective freeboard in metres, see the Guidance Note.
Maximum anticipated wave height in metres at the site during load­out.
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 freeboard should be sufficient to maintain the
vessel’s water­plane area.
Procedures to monitor freeboard at all 4 quarters of the vessel should be in place;
where this is not implemented fmin should be increased by 0.3 m.
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10.6.5Load­out vessel draught and motions
10.6.5.1
Nominal values and allowable tolerances for the load­out vessel(s) level, trim and heel shall
be clearly defined for all stages of the load­out.
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 load­out vessel are normally not acceptable
during the operation, see [10.3.4.3].
10.6.6Maintenance
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10.6.6Maintenance
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. on­hire and off­hire surveys, condition surveys
Installation and inspection of moorings
General watch keeping
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 back­up) during critical phases of
the load­out.
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 anti­freeze solution,
or any other devices or actions serving the above purpose.
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10.7Operational aspects
10.7.1General
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 load­out 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].
10.7.2Preparations
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
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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 load­out operation should be nominated and comply with the
requirements of Section [11.12] and be available for inspection as required before the
operation.
10.7.3Clearances
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 re­check of
the clearances.
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10.7.3.2
Sufficient under­keel clearance should be documented for vessel(s) during and after the
load­out operation. Normally the clearance should not be less than 1.0 meters.
Guidance note:
If the vessel under­keel 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 load­out 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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10.7.4Environmental effects
10.7.4.1
Effects caused by (unexpected) swell and tide could be of significant importance for load
transfer operations and shall be duly considered.
10.7.5Marine traffic
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10.7.5Marine traffic
10.7.5.1
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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10.7.6Organisation 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.
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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10.7.7Load­out 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 [10.5.8.7] and [10.7.8.2].
10.7.7.2
A level survey of the site area should be performed for load­outs with trailers to ensure that
the level tolerances of the trailers will not be exceeded.
10.7.7.3
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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 load­out it should be ensured that:
pot holes are filled and compacted
debris and obstructions to the load­out path are identified and removed
the load­out 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 structure should not be stopped in areas with the potential for
settlement due to e.g. consolidation or adverse weather.
10.7.8Supports and skidways
10.7.8.1
Levels of supports (and, if applicable, skidways and temporary supports) and horizontal
dimensions on the load­out 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/load­out frames are moved from link beams and onto the
vessel, see also [10.7.7.1] and [10.5.8.7].
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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 load­out 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.
10.7.9Grillage 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
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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 load­out
(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 one­compartment 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 density) will not cause displacements of the
object that may jeopardize the integrity of the object vertical supports.
Classification society acceptance required for moving of the vessel if out of class.
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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.
10.7.10Recording 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 load­out parameters should, as applicable, be monitored and recorded before
and during the operation:
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
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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 load­out operation and skidway/skidshoe design.
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 load­outs, an easily readable tide gauge should be provided adjacent to the load­out
quay in such a location that it will not be obscured during any stage of the load­out
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.
10.8Special cases
10.8.1Grounded load­outs
10.8.1.1
If the barge (or load­out vessel) is supported at the seabed during the load transfer phase
then the operation is defined as a “grounded load­out”.
10.8.1.2
Seabed support pad(s) should be prepared considering:
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
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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
load­out.
10.8.1.7
A diver or side­scan 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
load­out to allow for consolidation and settlement. Pad loading to reflect the load­out loading
condition(s) and vessel levels to be monitored during this period.
Guidance note:
Pre­loading in excess of the maximum loading during load­out 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 float­off operation
following the load­out 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 load­out 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.
10.8.2Transverse vessel load­outs
10.8.2.1
Generally transverse load­outs 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
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procedures. See also [10.3.2].
Guidance note:
Ballasting calculations for transverse load­outs 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 end­on load­out 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 load­out.
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 end­on
load­out.
Snagging or hang­up could potentially lead to ballasting getting out of synchronisation with
the move of the structure. Release of snagging load could potentially lead to instability and
failures.
Where a winch or strand jack system is used to pull the structure 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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10.8.3Load­in
10.8.3.1
Requirements to load­out operations are generally applicable for load­in operations as well.
10.8.3.2
Special attention should be given to selecting the optimal tide phase for starting the load­in
operation.
Guidance note:
Normally load­ins are scheduled to be started on a falling tide.
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10.8.4Vessel to vessel load transfer
10.8.4.1
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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 load­out 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 load­out class 3 or 4.
10.8.5Site 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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10.9Information required
10.9.1General
10.9.1.1
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10.9.1.1
General requirements to documentation are given in [2.3].
10.9.2Design 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 load­out 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.
10.9.3Equipment, 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 load­out 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
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Guidelines 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 load­out a general specification should be submitted and include
information about the tugs bollard pull and towing equipment.
10.9.3.4
For trailered load­outs:
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 load­outs:
Jack/winch specification
Layout of pull­on system
Layout of pull­back and braking systems
Details of power sources and back­up 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 back­up pumps
Pipe schematic and details of manifolds and valves where applicable
Pump performance curves.
10.9.3.7
For the load­out vessels mooring:
A statement showing capacity of all mooring bollards, winches and other attachments
used.
Mooring arrangement drawings for the load­out operation and for the post­load­out
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).
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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.
10.9.4Operation 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 load­out and
shall be emphasized in the manual:
A detailed operational communication chart (and/or description) showing clearly the
information flow throughout the operation.
Monitoring procedures describing equipment set­up, 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].
Operation bar chart showing time and duration of all critical activities.
Guidance note:
The operation bar­chart should include the following as applicable:
Mobilisation of equipment
Testing of pumps and winches
Testing of pull­on and pull­back systems
Barge movements
Initial ballasting
Structure movements
Load­out operation
Trailer removal
Seafastening
Re­mooring
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. Planned date, time and duration of the load­out, with alternative dates, tidal
limitations and windows
2. Ballast calculations for each stage showing:
Time
Tidal level
Structure position
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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 structure 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
Jack­winch 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
Deteriorating weather
Quay failure.
10.9.5Site
10.9.5.1
For the load­out location:
Site plan, showing load­out 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 load­outs)
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.
SECTION 11Sea voyages
11.1Introduction
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11.1Introduction
11.1.1General
11.1.1.1
This section covers MWS requirements for sea voyages which include:
dry towages of objects on barges
transport of objects on self­propelled vessels
wet towages of objects floating on their own buoyancy, including floating or submerged
pipes or similar.
Location moves of jack­ups (though approval of the locations will be covered in DNVGL­
ST­N002, /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.
11.1.2Scope
11.1.2.1
This section covers the requirements for:
Motion response
Design and strength
Floating stability
Transport & tug selection
Towing equipment
Voyage planning
Pumping and anchoring equipment
Manning
Multiple tows
Additional requirements for specific asset types
Information required for MWS approval.
11.1.3Revision history
11.1.3.1
This section replaces the applicable sections of the following legacy documents:
DNV­OS­H202, Sea transport operations
DNV­OS­H203, Transit and Positioning of Offshore Units
GL Noble Denton, Guidelines For Marine Transportations, 0030/ND.
11.2Towage or transport design/approval flow chart
11.2.1
The flow chart in Figure 11­1 shows the steps in the approval process and references the
sections in this standard.
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Figure 11­1 Voyage design/approval flowchart
11.3Motion response
11.3.1General
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].
11.3.2Vessel heading and speed
11.3.2.1
The analyses shall be carried out for zero vessel speed for a range of headings.
Guidance note:
Normally head, bow quartering, beam, stern quartering and stern seas should be considered.
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11.3.2.2
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Additionally, the analysis should be carried out for non­beam 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 =
V SHIP =
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).
11.3.3Effects of low GM and waterplane area
11.3.3.1
Any effects of low GM giving wind heeling should be considered
11.3.3.2
Where there are large changes in water­plane area that can cause heave­induced roll the
effects shall be quantified by analysis and/or model tests.
11.3.4Effects of free surfaces
11.3.4.1
For motions analyses, free surface corrections to reduce metacentric height (GM) and hence
to increase natural roll period should not be considered. The effect of any reduction in GM
shall, however, 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)
11.3.5Effects of cargo immersion
11.3.5.1
The effect of cargo immersion on the motion response should be considered.
Guidance note:
Cargo immersion increases the GM and damping. The increase in GM reduces the natural roll
period.
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11.3.6Motion response computer programs
11.3.6.1
Motion response programs and their application are discussed in [5.6.12.1 6)] to [5.6.12.1
9)].
11.3.7Results of model tests
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11.3.7Results 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 non­Gaussian 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.
11.4Default 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 may be acceptable.
11.4.2
When criteria in [11.5] or [11.6] are used the criteria adopted shall be applicable to the
actual case in question. The associated loading and strength calculations shall also be used
and not those in [5.6] and [11.9.1].
11.5Default motion criteria – IMO
11.5.1
For smaller cargoes, IMO Code of Safe Practice for Cargo Stowage and Securing, /87/, may
be acceptable
Guidance note:
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Smaller cargoes are typically under 100 tonnes weight.
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11.6Default motion criteria – Ships
11.6.1
For ships the default motion/acceleration criteria from classification society rules may be
acceptable e.g. DNV GL Rules for the Classification of Ships, /36/, Part 3, Chapter 4, Section
3.
11.6.2
Where the motions from DNV GL Rules for the Classification of Ships, /36/, Part 3, Chapter
4, Section 3 are used then the following should be considered:
The assumptions in Part 3 Chapter 1 Section 2 [3.2] of /36/ shall apply.
The heavy object (reduced) 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.
Part 3, Chapter 4, Section 3 [3.3] (Envelope Accelerations) of /36/ shall apply and not
[3.2] (Accelerations for dynamic load cases).
11.6.3
The motions and accelerations obtained from DNV GL Rules for the Classification of Ships are
based on loads at the 10­8 probability level, and are therefore conservative for marine
operations. Reduced accelerations may be applied to represent the maximum expected wave
loads for the actual operation.
Guidance note:
For ships with length greater than or equal to 100 m it is normally acceptable to multiply
the accelerations from the DNV GL Rules (ax‑env, ay­env and az‑env) by the values shown in the
table below.
For ships with length of 50 m or less a value of 1.0 should 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.
Duration in days (TPOP)
TPOP ≤ 7
7 < TPOP ≤ 30
30 < TPOP ≤
180
180 < TPOP
World­wide
0.67
0.67
0.80
1.00
Harsh conditions
0.67
0.80
0.90
1.00
For transports that can seek shelter in the case of forecasted extreme weather conditions and
will do so according to the operation procedure, TPOP ≤ 7 days may be applied.
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11.6.4
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11.6.4
Characteristic loads due to these accelerations shall be combined and analysed according
[5.6.15.2] (for both ASD/WSD and LRFD) and [5.9.8.2] (for LRFD), accounting for the
following. See also Guidance note.
LRFD Method: The accelerations in the longitudinal and transverse directions should
be factored in accordance with the ULS load cases ULS­a and ULS­b and combined with
both the maximum and minimum vertical acceleration i.e. gravity +/­ vertical
acceleration.
WSD Method: The accelerations in the longitudinal and transverse acceleration should
be combined with both the minimum and maximum vertical acceleration where the
maximum vertical acceleration is given by gravity + heave and the minimum by
0.85*gravity ­ heave.
Guidance note:
The maximum horizontal acceleration should be combined with both the minimum and
maximum vertical acceleration. Beam and head seas may be treated as two separate load
cases. Quartering sea 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 sea could be included by combining 80% of the horizontal transverse and 60% of
the longitudinal acceleration with both the minimum and maximum vertical acceleration.
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11.6.5
If the deck cargo is to be carried on a vessel classed to DNV Rules for the Classification of
Ships, /15/, then Pt3 Ch1 of those rules may be used subject to the requirements of Sec
6.2.2 of DNV­OS­H202, /44/.
11.7Default motion criteria – Specific cases
11.7.1Alternative 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.
11.7.2ASD/WSD default motion criteria
Table 11­1 Default motion criteria (ASD/WSD approach)
Nature of Voyage
Case
1
LOA
(m)
1)
B
(m)
> 140 & >
30
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L/B
1)
n/a
Block
Coeff
< 0.9
3)
Full
cycle
period
(secs)
Single
amplitude
Roll
Pitch
10
20°
10°
Heave
0.2 g
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2
3
> 76 & >
23
n/a
≥2.5
7
Weather restricted
operations in benign
areas, as defined in
[3.6], (see [11.7.2.1
7)]. For L/B < 1.4 use
unrestricted case.
3)
<
2.5
3)
12.5°
0.2 g
15°
0.2 g
30°
25°
< 0.9
≤ 76 or ≤
23
20°
10
≥0.9
6
Weather restricted
operations in non­
benign areas for a
duration <24 hours (see
[11.7.2.1 6)]. For L/B <
1.4 use unrestricted
case.
10
< 0.9
≤ 76 or ≤
23
4
5
Any
30°
30°
10
≥0.9
0.2 g
25°
25°
Any
≥2.5
Any
10
10°
5°
0.1 g
8
Any
<
2.5
≥1.4
Any
10
10°
10°
0.1 g
9
Any
≥2.5
Any
10
5°
2.5°
0.1 g
Any
<
2.5
≥1.4
Any
10
5°
5°
0.1 g
10
Inland and sheltered
water voyages (see
[11.7.2.1 8)]). For L/B
< 1.4 use unrestricted
case.
11
Independent leg jack­
ups, weather
unrestricted tow on own
hull. For L/B > 1.4 use
unrestricted Cases 1 to
6
12
n/a
Independent leg jack­
ups, 24­hour or location
move. For L/B > 1.4 use
Case 7 or 8 as
applicable
13
Mat­type jack­ups,
weather unrestricted
tow on own hull. For L/B
> 2.5 the pitch angle
can be reduced to 8°
14
Any
Equivalent
to 0.1 g in
both
directions
0.0
≥1.4
Any
Static
>
23
<
1.4
n/a
10
20°
20°
0.0
n/a
>
23
<
1.4
n/a
10
10°
10°
0.0
n/a
>
23
<
1.4
n/a
13
16°
16°
0.0
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Mat­type jack­ups, 24­
hour or location move.
15
n/a
>
23
n/a
n/a
13
8°
8°
0.0
Notes:
1. B = maximum moulded waterline breadth, L = waterline length. n/a = not applicable
2. Block coefficient = 0.9 is the cut­off between barge­shaped hulls (>0.9) and ship­
shaped hulls.
3. See [11.6] for alternative criteria for ship­shaped hulls.
11.7.2.1
The default motion criteria shown in Table 11­1 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 deck­edge immersion, and
consequent damping.
2. The cargo­vessel interface shall have friction coefficients no less than those of typical of
unlubricated steel­steel interfaces.
3. Roll and pitch axes shall be assumed to pass through the centre of floatation.
4. Gravity and heave shall be assumed to be parallel to the global vertical axis, see
[5.6.12.1 8)].
5. Phasing shall be assumed to combine, as separate load cases, the most severe
combinations of
roll +/­ heave
pitch +/­ heave.
6. For Cases 7 and 8, the departure shall be limited to a maximum of Beaufort Force 5,
with an improving forecast for the following 48 hours. The voyage duration including
contingencies, should not be greater than 24 hours.
7. For Cases 9 and 10, 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.
8. For Case 11, 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 one­compartment 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.
11.7.3LRFD default motion criteria
11.7.3.1
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The characteristic accelerations given in Table 11­2 to Table 11­4 can normally be applied to
a standard “North Sea Barge” (300’ × 90’ × 20’) and bigger barges for the wave heights
shown (either design values or OPLIM for weather restricted tows). The accelerations include
the component for self­weight.
11.7.3.2
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.
11.7.3.3
If accelerations corresponding to an OPLIM are used, an appropriate OPWF shall be defined for
the planned tow duration and procedure.
11.7.3.4
Table 11­2 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.
11.7.3.5
For barges smaller than a “North Sea Barge”, the limiting wave heights in Table 11­3 and
Table 11­4 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)
11.7.3.6
Alternatively, the limiting wave heights (6 m and 4 m) can be used, if the accelerations from
Table 11­3 and Table 11­4 are divided by the same factor
11.7.3.7
All 3 cases (roll/quartering/pitch) in Table 11­2 to Table 11­4 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.
11.7.3.8
Gravity shall be assumed to be normal to the vessel’s deck.
11.7.3.9
The following key applies to Table 11­2 to Table 11­4:
x
y
=
=
distance from vessel/barge mid ship
distance from vessel/barge centreline
d
=
distance used for calculating az in quartering sea,
z
ay
ax
=
=
=
height above waterline.
transverse acceleration parallel with barge deck
longitudinal acceleration parallel with barge deck
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az
=
acceleration normal to the barge deck.
Table 11­2 Weather unrestricted criteria worldwide (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/m2
1.0 kN/m2
1.0 kN/m2
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
az incr. each metre (y, d or x respectively)
from C
0.017 g/m
0.011 g/m
0.006 g/m
Wind pressure
0.5 kN/m2
0.5 kN/m2
0.5 kN/m2
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
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
Table 11­3 Criteria for Hs ≤ 6 m for larger barges (LRFD approach)
Acceleration/wind force
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
Table 11­4 Criteria for Hs ≤ 4 m for larger barges (LRFD approach)
Acceleration/wind force
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
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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/m2
0.3 kN/m2
0.3 kN/m2
11.8Directionality and heading control
11.8.1
The incident weather shall be considered to be effectively omni­directional, as stated in
[11.3.2]. No relaxation in the design sea states from the bow­quartering, beam and stern­
quartering directions shall be considered for:
Any voyage where the default motion criteria are used, in accordance with [11.4], or
similar
Single tug towages, or voyages by vessels with non­redundant 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 un­manned 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 South­West
monsoon
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11.8.2
When the relaxation exclusions in [11.8.1] do not apply, relaxation in the non­head 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].
2. Voyages by self­propelled 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.
11.8.3
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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 self­propelled 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 11­5 is a guide
to the acceptable sea state values. This should be confirmed by the MWS company as being
acceptable on a case­by­case basis.
Table 11­5 Reduced sea state v heading
Incident angle
(Head Seas = 0°)
Applicable Hs, as % of design sea state
(adjusted as appropriate)
0° to ± 30°
100%
± (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
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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:
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 he can accept that the effects of these restrictions are
practicable and acceptable.
11.9Design and strength
11.9.1Computation 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], 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 dry­transport 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 ship­loose items can be greater.
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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 jack­up units and similar tall structures.
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11.9.1.3
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11.9.1.3
If the computed loads are less than the “Minimum allowable seafastening force” shown in
Table 11­6, then the values in the Table shall apply.
Guidance note:
A simplified example of cribbing/seafastening calculations is shown in [K.7].
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11.9.2Friction – 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.
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 restrain forces as a result of ballasting to transport condition, see [11.9.5.30]
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 wave­induced bending and torsion of transport vessel
shall be considered.
11.9.2.4
The effect of vibrations due to wave entry (slamming) loads on the vessel hull and/or on
overhanging cargo shall be assessed. Typical effects could be:
Reduction of “efficient” friction.
Seafastening is needed to prevent swinging/vibration of slender
members/equipment/pipes.
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Unintended unscrewing of nuts/bolts.
11.9.2.5
For cargoes with weight less than 1000t 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 need to 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
The minimum seafastening capacity, without considering friction, shall be sufficient to resist
the accelerations shown in Table 11­6. If the effects of vibrations and hull beam deflections
can be proved to be insignificant, consideration can be given to reducing this requirement.
Table 11­6 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 very short duration moves in sheltered water, such as turning a vessel back alongside
the quay after a load­out, 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.
11.9.2.9
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 steel­steel, unless the friction surfaces can be guaranteed to remain dry.
Guidance note:
The following maximum upper bound 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
It is assumed that the friction surfaces are free from oil or other lubricating fluids.
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11.9.2.10
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 11­7, as a function of the cargo weight and overhang.
Table 11­7 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
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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11.9.2.11
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 vice­versa. When the provisions of
[5.6.15.4 b) and c)] are used, the normal reaction should be determined conservatively, as
follows:
When heave adds to the self­weight reaction the total normal reaction shall be reduced
by 10% to allow for adverse phasing.
When heave reduces the self­weight reaction, the normal reaction shall be taken as
weight less heave as any effects of phasing will cause an increase in the normal
reaction.
11.9.3ASD/WSD friction
11.9.3.1
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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. Loadings are computed in accordance with [5.6.12] to [5.6.15].
2. 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 11­7 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 fore­aft
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 upper­bound 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 11­7 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 of friction and seafastening reactions BEFORE the seafastenings are
overstressed. Where stanchions are used, they shall comply with [5)].
11.9.4LRFD 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
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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:
For cribbing with H (height)≥1.5B (breadth) zero contribution should be considered
from friction in the transverse cribbing direction.
For cribbing with H < 1.5B contribution from friction in the transverse direction could
be considered with (1.5B – H)/1.5B x 100%.
Normally 100% contribution from friction could be considered in the longitudinal
cribbing direction. However, see [11.9.2.2].
The mean design friction coefficient considered should in any case not exceed 0.2, but
see [11.9.2.10].
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11.9.5Seafastening design
11.9.5.1
Introduction. This section 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 section also applies to fastenings for
land transport though with different accelerations.
Guidance note:
Grillage and seafastening design is influenced by the load­out method.
Cargoes floated over a submersible barge or vessel, are frequently supported by timber
cribbing or dunnage to distribute the loads and allow for minor undulations in the deck
plating.
Cargoes lifted onto the transport barge or vessel are either supported on timber
cribbing/dunnage or grillage depending on type and size of cargo.
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.
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11.9.5.2
Design Principles. The grillage elements, including shimming plates, shall be used to
distribute a concentrated deck load to a sufficient number of load­carrying elements. The
grillage or cribbing height shall allow for any projections below the cargo support line.
11.9.5.3
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 section. 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.
11.9.5.4
The seafastening and grillage design shall duly reflect the structural strength limitations of
both the objects and transport vessel.
Guidance note:
The effect of global loads on local strength should be considered; e.g. a buckling check of
vessel­stiffened panels for support loads from cargo should include the stresses caused by
hull bending moments and shear forces.
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11.9.5.5
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 could be related to:
available heights
strict tolerances, etc. imposing requirements for the erection/welding sequence, see
also [11.9.5.10]
load­out trailer layout
needed space for (operation of) load­out systems, e.g. pumps, hoses, pull/push units
set down tolerances and shimming requirements
cutting/handling offshore
securing of object before lift, see [11.9.5.7]
possible need for set down of the object again and re­instate seafastening offshore.
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11.9.5.6
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 load­out procedure.
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11.9.5.7
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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].
11.9.5.8
Elements providing horizontal and/or vertical support after cutting/removal of seafastening
shall be verified for characteristic environmental conditions applicable for the installation
operation.
11.9.5.9
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.
11.9.5.10
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 [11.9.5.30]
11.9.5.11
For special precautions to seafastening after back loading offshore see DNV­RP­H102, /55/.
11.9.5.12
If re­instating of the transport seafastening may be required offshore this should be taken
into account in the design and in the cutting/release procedure.
11.9.5.13
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:
Chocks which allow some movement between the vessel and cargo
Pitch stops at one point only along the cargo, with other points free to slide or deflect
longitudinally
Vertical supports at only 2 positions longitudinally
An integrated structure of vessel­seafastenings­cargo, capable of resisting the loads
induced by bending and shear.
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11.9.5.14
The (assumed) force distribution in seafastening and grillage shall correspond to the
considered reaction forces for vessel and transported object strength verifications.
11.9.5.15
Possible uplift due to overturning of the object and/or relative deflections shall be prevented
by seafastening where required. See also [5.3.4].
Guidance note:
Uplift seafastening is always required if the object overturning moment is greater than the
object restoring moment in the “worst” applicable ULS load combination. Also, if “first uplift”
represents LS2 or ULS, an additional safety factor corresponding to a “material factor” should
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be applied. This could be done by applying a load factor of 0.85 on G loads in the uplift load
case(s). 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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11.9.5.16
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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11.9.5.17
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 quasi­static vessel
hogging and sagging due to a wave of length, Lw, equal to the vessel length, and height:
where Lw is in metres.
11.9.5.18
Seafastenings should be 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 [11.9.5.19] to [11.9.5.26].
Guidance note:
Smaller cargoes are typically less than 100 tonnes for chain seafastening and 50 tonnes for
webbing lashings.
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11.9.5.19
Seafastening – Lashings: Chain binders, ratchets or turnbuckles shall be tensioned before
departure to spread the load between the seafastenings and secured so that they cannot
become slack. Lashings 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 should not be used for un­manned voyages since they
are difficult to inspect regularly.
11.9.5.20
Possible skew loads in lashings due to uneven pre­tensioning 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:
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A skew load factor of 1.5 is considered adequate if lashings carrying the same (quasi­static)
load component between them have approximately the same stiffness and similar means of
pre­tensioning. If not, a conservative assessment should be conducted to estimate the
applicable skew load factor(s).
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11.9.5.21
The design load in ropes, chains and lashings shall be assessed accounting for the
requirements of [5.9.8.5].
11.9.5.22
The good practice for lashings and similar devices in the IMO Code of Safe Practice for Cargo
Securing and Stowing, /87/ should be followed if relevant.
11.9.5.23
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 quasi­static load
distribution combined with an appropriate skew load factor, see [11.9.5.20].
Guidance note:
Applicable design friction coefficients are listed in [11.9.2].
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11.9.5.24
Lashing equipment (chains, wires, shackles, turnbuckles etc.) shall have certificates, giving
the ultimate capacity, WLL or SWL, 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.
11.9.5.25
Synthetic webbing should only be used for smaller cargoes on manned voyages. Where
synthetic webbing ratchet straps are used, then:
D­links and shackles shall be used instead of hooks (which can unhook)
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.
11.9.5.26
If chains are used, (and not properly documented otherwise) then:
1. 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.
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2. The effective MBL of doubled chains that are bent more than 90° around connection
points should be reduced as indicated below:
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).
11.9.5.27
Seafastening ­ welded: 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].
11.9.5.28
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].
11.9.5.29
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.
11.9.5.30
So far as is practical, seafastening connections should be made after load­out 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.
11.9.5.31
Welding of seafastenings should not be carried out in wet conditions. Weather protection
should be used to minimise the effects of wet conditions.
11.9.5.32
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.
11.9.5.33
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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11.9.5.34
To prevent items moving inside structures or modules, 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.
11.9.5.35
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11.9.5.35
Guide posts should not be used for seafastenings unless specifically designed for that
purpose.
11.9.6Seafastenings 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:
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.
11.9.7Cribbing
11.9.7.1
Where the cargo is supported on wooden cribbing or dunnage, rather than steel­to­steel
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.
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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 a greater pressure, the nominal bearing pressure on the cribbing should not
exceed 2 N/mm2 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.
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 herring­bone dunnage layout supporting a flat­bottomed cargo,
without taking into account the strong points, then the maximum cribbing pressures should
not exceed 1 N/mm2, 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 on­load and off­load. Minimum angles of 5º should be
considered.
11.9.8Cargo 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.
11.9.9Securing of pipe and other tubular goods
11.9.9.1
This section 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 jack­ups is covered in [11.27.11]. The design of
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securing shall consider the following:
the
the
the
the
type of vessel,
nature of the cargo,
duration of the towage or voyage and
weather conditions expected.
11.9.9.2
For these types of cargoes, friction can be assumed to resist longitudinal seafastening loads
(i.e. from pitch), and [11.9.1.1] and [11.9.1.3] do not apply. The design friction coefficients
shall be in accordance with [5.6.9] and should not exceed the coefficients in Table 11­8.
Table 11­8 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
0.15
Epoxy coated pipe – timber
Epoxy coated pipe ­ epoxy coated pipe
0.1
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
Generally, pipes should be stowed in the fore and aft direction.
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.
Guidance note:
Reference can be made to API RP 5LW “Recommended practice for transportation of line pipe
on barges and marine vessels”, /7/.
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11.9.9.7
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11.9.9.7
Smaller diameter pipes such as drill pipe can 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.
For weather restricted operations, and 24­hour or location moves of jack­ups, the stack
can 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 jack­ups, 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.
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) are often individually chocked, and end stops provided.
Unless proven that the piles cannot roll out of the chocks further restraints shall be provided.
Guidance note:
It may be possible to provide end stops at one end only.
Further restraints to retain the pipes could be individual wire or chain lashings, stanchions or
strongbacks.
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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:
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.
11.9.10Inspection 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
Non­destructive testing (NDT) shall be carried out on the structural members of the
seafastenings. Specific requirements for weld inspection are given in [5.10.2.3].
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 re­tested.
11.9.11Use 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
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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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11.9.12Fatigue
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 considered.
For transports to offshore (wave exposed) location ample time should be added to
account for the maximum expected waiting 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.
Guidance note:
For transports with sailing routes for which there are no pre­dominant 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.
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Sea direction
Head
H
Port
Q
Representing
range
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 %
Port
Beam
S Port
Q
Stern
S Stbd
Q.
Stbd
Beam
H,
Stbd Q
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
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 routing. Regular
inspections combined with repair possibilities could/should also be considered.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.9.13Vortex shedding
11.9.13.1
All voyages should be checked for wind­induced 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 in­place condition and which
are therefore not checked for in­place VIV, or
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single­tube jack­up legs (which can be fitted with spoilers to prevent VIV).
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11.9.14Condition of unclassed tows
11.9.14.1
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 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:
An extended, in depth, survey of the vessel structure involving one or more specialist
surveyor(s). Facilities for a close­up 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.
11.10Floating stability
11.10.1General
11.10.1.1
Free­trimming stability programs can give misleading results when the trim is significant
relative to the heel due to the hull geometry. Stability calculations using fixed trim can be
used provided that a sufficient number of axes of rotation are considered to identify the
most severe heeling axes. The most severe heeling axes are the ones for which the
maximum righting arm or range of stability is lowest, ignoring possible downflooding.
11.10.1.2
The lightship data used in the stability calculations shall accurately reflect the current status
of the unit.
Guidance note:
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
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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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11.10.1.3
The stability calculations shall also take into consideration any addition or removal of
mooring chain from the system that will impact the final loads during passage/departure and
arrival conditions.
11.10.1.4
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.
11.10.1.5
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.
11.10.1.6
The effects of free surface shall be considered in all stability calculations. These shall include:
the effects of free surface liquids in unit and cargo,
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.
11.10.1.7
Vessels shall comply with the mandatory parts of the IMO Intact Stability Code 2008, /89/,
and the IMO International Convention on Load Lines, Consolidated Edition 2002 /90/.
11.10.1.8
Multi­vessel combinations can be considered as one vessel providing that the strength of the
combination meets the requirements of Sec.5.
11.10.1.9
Any cases where stability or damage IMO Intact Stability Code 2008, /89/, requirements
cannot be met should be agreed with the MWS company at an early stage.
11.10.1.10
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.
11.10.1.11
Requirements for the different asset types in transit are given in Table 11­9.
Table 11­9 Stability requirements for different asset types in transit
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Jack­up
Intact range
Semi­sub
Cargo on ships
and barges
See [11.10.2]
40°
Wind overturning
(intact)
See [11.10.3]
Damage (general)
See [11.10.4]
Damage (specific)
See
[11.10.5]
See
[11.10.6]
Jacket
wet tow
See [11.10.7]
Compartmentation
and watertight
integrity
See [11.10.8]
Draught and trim
See [11.10.9]
GBS
1)
[11.10.7]
1)
See
[6.2]
Notes:
1. Subject to agreement with the MWS company once full details are known
11.10.2Intact stability (apart from GBS’s and floating jackets)
11.10.2.1
This section does not cover stability of GBSs (for which see [6.2]) or self­floating structures
(if not MOU, barge or ship shaped) for which the criteria should be agreed with the MWS
company once full details are known.
11.10.2.2
Where there is a significant difference between the departure, arrival or any intermediate
condition, then the most severe should be considered, including the effects of any ballast
water changes during the voyage.
11.10.2.3
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.
11.10.2.4
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 the values shown in Table 11­10. When assessing the range of stability,
downflooding does not need to be taken into account provided that the watertight and
weathertight requirements of [11.10.8] and [11.27.6] are met.
Table 11­10 Intact stability range
Vessel or towed object, type and size
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Large and medium vessels, LOA > 76 m and B
Large cargo barges, LOA > 76 m and B
Small cargo barges, LOA < 76 m or B
Small vessels, LOA < 76 m or B
1)
1)
1)
1)
> 23 m
36º
> 23 m
36º
< 23 m
40º
< 23 m
MOU’s including jack­ups and semi­submersibles
44º
To satisfy [11.10.3]
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
Notes:
1. B = maximum moulded waterline beam.
11.10.2.5
Requirements for objects which do not fall into the categories shown in Table 11­10, which
are non­symmetrical, or which have an initial heel or trim which is not close to 0º, shall be
agreed with MWS company.
11.10.2.6
Alternatively for barges, 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:
15+(15/GM)+θ
where GM is in metres and θ = 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 caused by the design wind, in
degrees.
11.10.2.7
Additional requirements for jack­ups are given in [11.27.6].
11.10.2.8
Cargo overhangs shall generally not immerse as a result of inclination from a 15 m/s wind in
still water conditions (but see [11.19.28.3] for ice areas)
11.10.2.9
Subject to [11.10.2.8], [11.10.4.2], [5.6.2.5] and [11.19.24.2] (for ice areas), buoyant
cargo overhangs can be assumed to contribute to the range of stability requirement of
[11.10.2.4], but see [5.6.2.5 e)].
11.10.2.10
In areas and seasons prone to icing of superstructures, the effects of icing on stability shall
be considered as described in [11.19.28].
11.10.3Wind overturning (intact condition ­ all units)
11.10.3.1
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11.10.3.1
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 (30% for column stabilised
units). 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 be less than both the second
intercept and the downflooding angle as shown in Figure 11­2.
11.10.3.2
The wind velocity used for intact wind overturning calculations for the survival condition shall
be the 1­minute design wind speed, as described in [3.2]. In the absence of other data,
52 m/s (100 knots) shall be used. A 36 m/s (70 knot) wind can be used for operating
conditions as long as the unit can always change to a survival condition within an adequate
time scale.
Figure 11­2 Wind overturning criteria (intact case)
11.10.4Damage stability background (all except for column­
stabilised)
11.10.4.1
This section gives the common requirements for damage stability before the specific
requirements for jack­ups in [11.10.5] and others in [11.10.7]
11.10.4.2
The wind velocity used for overturning moment calculations in the damage condition shall be
26 m/s (50 knots) or the wind used for the intact calculation if less. It shall be applied in the
most critical direction.
11.10.4.3
All units (except for those covered in [11.10.7.1]) shall have positive stability about any
horizontal axis with damage caused by an assumed minimum penetration of 1.5 m from any
external plating, between effective watertight bulkheads, with the following:
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.
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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.
Where damage of a lesser extent than in [a)] to [c)] results in a more severe condition
such lesser extent shall be assumed.
11.10.4.4
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.
11.10.4.5
The extent and adequacy of the precautions necessary for a particular towage shall be
assessed on a case­by­case basis.
11.10.4.6
Transports on multiple vessels. 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.
11.10.5Damage stability for jack­ups
11.10.5.1
All units shall have positive stability about any horizontal axis with any one compartment
flooded or breached.
11.10.5.2
The residual range of damage stability (ignoring downflooding and wind inclination) about
any axis from the angle of loll to the maximum angle of positive stability shall be not less
than (7º + 1.5 x angle of loll) with a minimum of 10º as shown in Figure 11­3.
11.10.5.3
The downflooding angle shall be greater than the first intercept (the angle of loll plus wind
inclination, with the wind speed in [11.10.4.2).
11.10.5.4
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.
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Figure 11­3 Damage stability for jack­ups
11.10.6Damage stability for column stabilised units
11.10.6.1
Damage shall be considered for 2 separate cases, A and B for any transit or operating
draught and for the most critical horizontal axis and wind direction.
11.10.6.2
Case A (including wind heel using the wind speed in [11.10.4.2]) covers damage on exposed
portions of columns, underwater hulls and braces on the periphery of the unit. (Exposed
means outboard of a line through the centres of the periphery columns). The damage shall
be assumed to have a horizontal penetration of 1.5 m and a vertical extent of 3 m occurring
at any level between 5 m above and 3 m below the transit or operating draught being
considered. The following shall be assumed damaged:
1. Any horizontal flat between these levels.
2. 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.
3. Any vertical bulkheads within the following distances of another which is considered
intact:
3 m, or
column perimeter/8 measured around the outer skin at the waterline (when
within a column) if greater than 3 m.
11.10.6.3
The inclination at the first intercept (the angle of loll plus wind heel) for any axis shall be
less than 17º and less than the downflooding angle.
11.10.6.4
The residual range of stability from the first intercept to the second (ignoring downflooding,
but see [11.10.8.2]) shall be not less than 7º.
11.10.6.5
The righting arm at some inclination before downflooding or the second intercept (if less)
shall be at least twice the Wind Heel Arm (shown as WHA in Figure 11­4) at the same angle.
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Figure 11­4 Damage stability for column stabilised unit (Case A)
11.10.6.6
Case B covers flooding of any compartment adjacent to the sea, or with pumps, or with
machinery with salt water cooling. No wind heel need be included.
The angle of loll shall be less than 25º for any axis.
The residual range of stability from the angle of loll to the downflooding angle shall be
not less than 7º.
Figure 11­5 Damage stability for column stabilised unit (Case B)
11.10.7Damage stability (apart from jack­ups and column
stabilised)
11.10.7.1
Except as described in [11.10.7.2] and [11.10.7.3], the unit should have sufficient reserve
stability in a damaged condition to withstand the wind heeling moment using the wind
speed in [11.10.4.2] superimposed from any direction and the damage as described in
[11.10.4.3]. 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 11­6.
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Figure 11­6 Damage stability (apart from jack­ups and column­stabilised)
11.10.7.2
One­compartment damage stability is not always achievable without impractical design
changes, for the wet towages of the following and similar structures:
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.
11.10.7.3
For those structures listed in [11.10.7.2], or similar, damage stability requirements can be
relaxed, provided the towage is a one­off 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 pick­up 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.
11.10.7.4
The relaxations allowed by [11.10.7.2] and [11.10.7.3] do not apply in ice­affected areas,
where the vessel or structure should comply with [11.19.28].
11.10.7.5
The damage stability recommendations of this section do not apply to transport of cargos on
flagged trading vessels, sailing at the assigned ‘B’ freeboard or greater.
Guidance note:
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The ‘B’ freeboard is the minimum freeboard assigned to a Type B vessel, which is generally
defined as any vessel not carrying a bulk liquid cargo. Reduced freeboards can be assigned
to a Type B vessel over 100 m in length, depending on the arrangements for protection of
crew, freeing arrangements, strength, sealing and security of hatch covers, and damage
stability characteristics. See the IMO International Convention on Load Lines, Consolidated
Edition 2002, /90/, for further details.
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11.10.8Compartmentation and watertight integrity
11.10.8.1
All external openings below the static intact and any one­compartment­damaged waterlines
from [11.10.3] to [11.10.7] with wind applied in the most onerous directions, but no waves,
shall be fitted with watertight closing appliances in operable condition.
11.10.8.2
Weathertight closing appliances in operable condition shall be fitted to all external openings
that are not required to be watertight by [11.10.8.1] and are below either:
the static intact waterline at the dynamic angle (the smallest angle at which the area
ratio in Figure 11­2 is satisfied), or
4 m above all required static one­compartment­damaged waterlines.
All horizontal axes should be considered with the wind applied in the most onerous direction
for each case.
11.10.8.3
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.
11.10.8.4
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.
11.10.8.5
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.
11.10.8.6
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.
11.10.8.7
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.
11.10.8.8
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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.
11.10.8.9
Windows which could be exposed to wave action shall be plated over, or similarly protected.
11.10.8.10
All tank top and deck manhole covers and their gaskets shall be in place, checked in good
condition, and securely bolted down.
11.10.8.11
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.
11.10.8.12
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.
11.10.8.13
All holds, void spaces and engine room bilges shall be checked before departure and should
be pumped dry.
11.10.8.14
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.
11.10.9Draught and trim
11.10.9.1
For vessels and barges with a load line certificate, the draught shall not normally exceed the
appropriate load line draught, without flag state exemption, except for temporary on­load
and off­load operations under controlled conditions.
11.10.9.2
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.
11.10.9.3
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.
11.10.9.4
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 11­11.
Table 11­11 Minimum recommended draught and trim for barges
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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
11.10.9.5
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.
11.10.9.6
For towed ship­shaped 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 11­11
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.
11.10.9.7
Draught should be carefully selected for FSU’s etc. that will have deeper in­operation
draughts than for towage. This can give higher accelerations in the installed modules etc.
when under tow.
11.10.9.8
It can be preferable to tow structures such as floating docks at minimum draught with zero
trim, in order to minimise longitudinal bending moments.
11.10.9.9
Draught marks forward and aft shall be easily readable and, if necessary, re­painted in the
area above the waterline.
11.10.9.10
Where the tow is un­manned, 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.
11.11Transport vessel or barge selection
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11.11Transport vessel or barge selection
11.11.1Selection criteria
11.11.1.1
The transport barge or vessel selection, including identification of any necessary repairs or
upgrades, should be undertaken considering the following:
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.
If a floating load­out is planned, there shall be sufficient water depth to access and
leave the load­out berth and the load­out can be carried out in accordance with Sec.10.
If a submerged load­out 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 load­out location.
There shall be adequate pumping capacity to comply with [11.15], or be suitable for
the use of additional pumping equipment.
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).
The deck strength shall be adequate, including stiffener, frame and bulkhead spacing
and capacity, for load­out and transport loads.
For a vessel, securing of seafastenings shall not need welding in way of fuel tanks.
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.
The motion responses as calculated shall not cause overstress of the cargo.
All required equipment and machinery shall be in sound condition and operating
correctly.
The barge or vessel shall possess the relevant, in date, documentation as set out in
Table B­2.
Unclassed barges shall be subject to appropriate project­specific structural,
equipment and machinery checks. They shall have a valid load line, or load line
exemption, certificate.
11.11.2Suitability and on­hire surveys
11.11.2.1
In their interest, the charterer is advised to have a suitability survey and an on­hire survey
of the barge or vessel carried out before acceptance of the charter.
11.12Tug selection
11.12.1General
11.12.1.1
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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 11­12. The
appropriate category should be agreed with the MWS company.
Table 11­12 Tug categories
Category
ST – Salvage
Tug
U ­ Unrestricted
Used for
Single tug towages in benign or non­benign 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 multi­tug towages
R2 ­ Restricted
Benign weather area towages
R3 ­ Restricted
Assisting in multi­tug 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 purpose­built for towing operations or be of
a multi­purpose design having towing capability.
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. 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.
Ice­breaking tugs can be less than this and the MWS company should be consulted.
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2. 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
An additional tug can be recommended for high value tows or towages through areas with
limited sea room, 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 multi­tug tows.
Take over the duties of the failed tug
Provide any other required assistance in an emergency.
11.12.2Bollard pull requirements
11.12.2.1
Table 11­13 summarises the different conditions to be considered. The most severe
conditions that apply to a particular towage should be used. The conditions are described in
more detail in the indicated sections.
Table 11­13 Meteorological criteria for calculating TPR (towline pull required)
Section
Condition
Hs(m)
Wind (m/sec)
Current
(m/sec)
Design (1 hour
mean)
0.5 or
predicted
current if
greater
11.12.2.2
Limited sea room
11.12.2.3
Continuous adverse
current or weather
11.12.2.4
Standard
11.12.2.5
<24 hour staged tow
or
<24 hour jack­up
move
11.12.2.6
Benign weather areas
Design
As agreed with the MWS company to ensure a
reasonable speed in moderate weather.
5
3
0.5
15
0.5 or
predicted
current if
greater
As agreed with the MWS company but not less
than:
2
11.12.2.7
20
Sheltered from waves
15
0.5
As agreed with the MWS company.
11.12.2.2
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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 minimum 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 maximum predicted surface current if greater.
11.12.2.3
If the tow route passes through an area of continuous adverse current or weather, or if a
particular towing speed is required in calm or moderate weather, a greater TPR can be
appropriate and agreed with the MWS company. In any event, an assessment should be
made that a reasonable speed can be achieved in moderate weather.
11.12.2.4
For 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:
5.0 m significant sea state, and
20 m/s wind, and
0.5 m/s current, or the maximum predicted surface current if greater.
11.12.2.5
For tows which are planned to take less than 24 hours (including jack­up moves and every
stage of a staged tow), the following reduced criteria, acting simultaneously, can 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.
11.12.2.6
For benign weather areas, the criteria for calculation of TPR shall be agreed with the MWS
company. Generally these should not be less than:
2.0 m significant sea state, and
15 m wind, and
0.5 m current.
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
=
the tug efficiency in the sea conditions considered, %
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(BP ×
Teff)/100=
Σ
=
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 ocean­going 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)
Hs
= significant wave height (with 1 m < H s < 5 m).
Note that all tugs will generally have very low efficiencies with H s > 5 m since they should be
protecting their towing gear. Tugs with less sea­kindly characteristics will have significantly
lower values of Teff in all sea states.
11.12.2.11
These efficiencies are shown graphically in Figure 11­7 for tugs of LOA > 45 m in different
significant wave heights up to 5 m.
Figure 11­7 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 11­8.
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Figure 11­8 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].
11.12.3Main and spare towing wires and towing connections
11.12.3.1
The main and spare towing wires, pennants and connections shall be in accordance with
[11.13.3].
11.12.4Tailgates/stern rails
11.12.4.1
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.
11.12.5Towline control and seabed clearance
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
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On square­sterned 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.
11.12.5.4
Tug masters should be cognizant of the towline catenary at all times, but particularly in
shallow 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.
11.12.5.5
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.
11.12.6Workboat
11.12.6.1
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. 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.
11.12.7Communication equipment
11.12.7.1
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.
11.12.8Navigational equipment
11.12.8.1
Tugs shall be provided with:
all necessary navigational instruments and up­to­date 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.
11.12.9Searchlight
11.12.9.1
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.
11.12.10Portable pump
11.12.10.1
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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 self­contained 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] 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.
11.12.11Additional equipment
11.12.11.1
Anti­chafe 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.
11.12.12Bunkers 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 en­route is proposed, then suitable arrangements shall be made before the
towage starts, and included in the towing procedures (see [11.14.7]).
11.12.13Tug 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 un­manned, 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
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Category ST. To satisfy category ST, certified accommodation and life­saving 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.
11.13Towing equipment
11.13.1Flowchart
11.13.1.1
Figure 11­9 is a flowchart for determining the required strength of the towing gear for a
specific tug.
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Figure 11­9 Flowchart for determining towing gear required strength and lengths
11.13.1.2
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].
11.13.1.3
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:
Part­built or damaged ships, or any structure when the bow sections could be
vulnerable to wave damage.
Part­built ships, converted ships or FPSOs without a rudder or skeg, or with a turret or
spider fitted forward, where better directional stability can be obtained if towed by the
stern.
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Any structure with overhanging or vulnerable equipment near the bow, which could be
vulnerable to wave damage, or could interfere with the main and emergency towing
connections.
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11.13.1.4
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.
11.13.1.5
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.
11.13.1.6
For any systems in [11.13.1.4] and [11.13.1.5], a recovery system should be provided for
the connection point for each tug.
11.13.1.7
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.
11.13.2Number of towlines
11.13.2.1
Table 11­14 gives the minimum number of towlines for each category of tug.
Table 11­14 Tug wire requirements
Category
Main Wire
Spare
Two (on separate winch
drums)
One
U – Unrestricted
One
One
C – Coastal
One
One
R1 – Restricted
One
Not applicable
R2 – Restricted
One
One
R3 – Restricted
One
Not applicable
ST – Salvage Tug
11.13.3Strength of towline and towline connections (outside ice
areas)
11.13.3.1
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The Minimum Breaking Loads (MBL) of the main and spare towlines, and the ultimate load
capacity of the towline connections to the tow including each bridle leg, shall be related to
the continuous static bollard pull (BP) of the actual tug to be used. Table 11­15 gives the
minimum required breaking load of the towlines and wire intermediate pennants (BP, MBL
and ULC are in tonnes) but see [11.13.4.4] for shorter towlines.
Table 11­15 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
11.13.3.2
For tugs with very large bollard pulls (typically over 280 tonnes) it can be difficult to satisfy
the requirements of Table 11­15 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
the
the
the
vessel is fitted with towline tension monitoring,
tug Master is in agreement,
reduction is documented in the towing procedures and Certificate of Approval,
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).
11.13.3.3
For specific towages in benign weather areas and in deep water that allows long towlines to
be deployed, the effective towing bollard pull in [11.13.3.2] can be further reduced to not
less than 250 tonnes after agreement with the MWS company.
11.13.3.4
The Ultimate Load Capacity (ULC), in tonnes, of towline connections to the tow, including
each bridle leg, connectors (apart from shackles and bridle apex which are covered in
[11.13.8]), chain pennants, and fairleads, where fitted, shall be not less than:
ULC = 1.25 x required towline MBL for the actual tug (for MBL ≤ 160 tonnes) or
ULC = required towline MBL for the actual tug + 40 (for MBL > 160 tonnes).
11.13.3.5
See [11.13.4.4] for shorter towlines and [11.13.6.2] for bridle apex angle≥90º.
11.13.3.6
See [11.13.14.4] when bridles and pennants cannot be inspected annually.
11.13.3.7
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Any towline connections below or near the towing waterline shall be designed to fail without
allowing flooding.
11.13.3.8
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.
11.13.3.9
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 [11.13.3.4].
11.13.3.10
Where no fairleads are fitted, the towing connections shall be similarly designed.
11.13.3.11
If a fairlead or towing connection is to be used either with or without a bridle, it should be
designed for both cases.
11.13.3.12
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.
11.13.3.13
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.
11.13.3.14
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]).
11.13.4Relationship between towline length and strength
11.13.4.1
Except in benign areas and 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 [11.13.4.3]) be less than
650 m, apart from coastal towages within a good weather forecast when this can be reduced
to 500 m.
11.13.4.2
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 [11.13.4.3]) be less than
500 m.
11.13.4.3
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.
11.13.4.4
The towline MBL as shown in [11.13.3.1] shall be increased if required to allow L to comply
with [11.13.4.1] or [11.13.4.2]. In such cases the ULC for the bridle, fairleads and towing
connections [11.13.3.4] shall be correspondingly increased.
11.13.5Towline connection points
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].
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.13.5.2
Towline connections and fairleads shall be designed to the requirements of [11.13.3.4].
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
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Where towing connections are of a quick­release type, then the fairlead design shall allow all
the released parts to pass easily through the fairlead.
11.13.6Bridle 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 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 shall be a special enlarged link, not a normal link with the stud
removed.
11.13.6.4
All wire ropes shall have hard eyes or sockets but not aluminium or alloy ferrules.
11.13.7Bridle apex
11.13.7.1
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
should 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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11.13.8Shackles
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 required MBL of the towline
to be used.
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 [1.1.12] if the MBL of any
equipment is not known.
11.13.9Intermediate pennant or surge chains
11.13.9.1
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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.
Guidance note:
Its main use is for ease of connection and reconnection.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.13.9.2
A synthetic spring, 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 that required for the main towline.
11.13.9.5
Any “fuse” or “weak link” pennant shall have a strength not less than that required 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 [11.13.6.3]). 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].
11.13.10Synthetic springs
11.13.10.1
Where a synthetic spring is used, its MBL shall be at least 1.5 times that required 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 springs
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 spring 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
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11.13.10.3
All synthetic springs shall have hard eyes. A synthetic spring 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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11.13.11Bridle 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 A­frame.
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 well­sized 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.
11.13.12Towing 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
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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 over­stressing 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 multi­drum 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.
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.
11.13.13Emergency 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. It 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]. Precautions should be taken to
minimise chafe of all wire ropes.
11.13.13.2
For a bridle arrangement the same strength requirements as the main bridle shall apply.
11.13.13.3
If a system as shown in [K.2] is to be used the following shall apply:
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The towing connection should be on or near the centreline of the tow, over a bulkhead
or other suitable strong point
Closed fairlead 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 pick­up 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 can be reduced for small
barges and in benign areas
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11.13.13.4
The strength of items [a)] and [b)] above should be as for the main towline connections, as
shown in [11.13.3.4]. The MBL of the handling system, items [d)] and [e)] above 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.
11.13.13.5
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.
11.13.13.6
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.
11.13.13.7
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.
11.13.13.8
Whatever the arrangement agreed, precautions shall be taken so no chafe can occur to the
floating line when deployed.
11.13.13.9
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.
11.13.13.10
The following reconnection equipment should also be considered, and placed on board if the
duration and area of the towage demand it:
Heaving lines
Line throwing equipment
Spare shackles.
11.13.14Certification and inspection
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11.13.14Certification 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 Delta plates, less than 5 years old,
calculations agreed with the MWS company in advance can be acceptable instead of
certification.
Guidance note:
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
inspection by a competent person not more than 12 months before use and shall be
thoroughly visually inspected before each use. Any significant wear or damage shall be
repaired and thoroughly inspected again, or replaced, before use.
11.13.14.3
Additionally all Delta plates, master links and shackles shall be inspected less than 2 years
before each use with MPI and UT to confirm there are no defects.
11.13.14.4
For any towing gear that cannot be inspected 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 inspections. 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.
Guidance note:
For example with a submerged bridle pennant with a 5 year planned life, rated for a 100 t
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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11.13.14.5
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).
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.
11.13.14.6
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 rigorous inspection of the whole wire by an independent competent person
appropriately certified to do such inspections.
11.13.14.7
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11.13.14.7
The closed socket (normally spelter type) if used to form the towline termination shall be
renewed at intervals not exceeding two years (excluding time before fitting when new on the
tug), irrespective of the condition of the socket and its wire. Except when re­socketed 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.
11.13.14.8
Aluminium or alloy ferrules shall not be used on any pennant or towline.
11.13.14.9
The MWS company surveyor can reject any items that appear to be unfit for purpose, or are
lacking valid certification.
11.13.14.10
Table 11­16 summarises the required expiry times for the above certificates and inspections
shown above.
Table 11­16 Certificate and inspection document requirements
Item
Certificate valid
for
Time since documented inspection
by a competent person (unless
new)
<10 years
Not applicable. See [11.12.1.6]
Delta plates, master links and
shackles
<5 years
< 12 months and MPI & UT < 2
years
Pennants, bridles and towlines
<5 years
< 12 months
Submerged bridles
<5 years
See [11.13.14.4]
Lashing equipment
<4 years
< 12 months
Spelter sockets
<2 years
< 12 months
Bollard pull
11.13.15Access 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.
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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
Un­manned 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.
11.13.16Damage control and emergency equipment
11.13.16.1
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:
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.
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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
Fire­fighting equipment as appropriate
Personal protection equipment ­ gloves, goggles, hard hats, survival suits etc.
Emergency lighting.
11.14Voyage planning
11.14.1General
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 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 (as defined in page 9 of IMO Resolution
A.1024(26), /94/) shall comply with the mandatory IMO Polar Code adopted in May 2015
and once it has come into force (due on 1st January 2017).
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 towages 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) at LAT, should be plotted in
advance. For sea room calculations in [11.14.2], the contours should be the underkeel
clearance in a 1 year return storm.
11.14.1.6
The route should be planned to avoid passing too close upwind or up­current 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 towing
procedures/manual for the guidance of the tug captain(s)/towmaster.
11.14.1.8
The actual tow route can safely deviate from the planned route if the weather forecasts are
favourable as long as the tow can obtain the required sea room before bad weather is likely
to arrive.
Guidance note:
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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 jack­up at
a stand­by 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.
11.14.2Sea 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 [1.1.6]:
75% of the required sea room within 1 day after the end of the reliable good­weather
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.
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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].
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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
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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) 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.
Guidance note 1:
Towline yield stress is typically about 40% of the wire break load.
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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.
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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:
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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.
11.14.3Routeing 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
Anti­piracy procedures shall be included in the towing/transport manual/procedures unless
there is a low risk of piracy.
11.14.3.3
Where anti­piracy measures are warrantable the requirements should be advised to the
MWS.
Guidance note 1:
Piracy is prevalent in many areas and these vary with time. Guidance on mitigation of piracy
can be found in:
“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
against­Somalia­Based­Piracy/
(http://www.intertanko.com/Topics/Security/Security­/BMP4­forProtection­against­
Somalia­Based­Piracy/)While this guidance was created to address the Somalian
situation, this document’s good practice should be considered for any high risk area.
Website http://www.lmalloyds.com/Web/market_places/marine/JWC/Joint_War.aspx
(http://www.lmalloyds.com/Web/market_places/marine/JWC/Joint_War.aspx) (Lloyd’s
Market Association/Joint War Committee website). This also gives current piracy risk
areas.
IMO website http://www.imo.org/OurWork/Security/SecDocs/Pages/Maritime­
Security.aspx (http://www.imo.org/OurWork/Security/SecDocs/Pages/Maritime­
Security.aspx) (then select “Piracy”)
IMB Piracy Reporting Centre website http://www.icc­ccs.org/piracy­reporting­centre
(http://www.icc­ccs.org/piracy­reporting­centre)
Flag­state, vessel insurance and P&I club requirements. The MWS company should be
advised by the insured at an early stage if there are any relevant insurance warranty
requirements.
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­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
Key mitigations of piracy include:
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.
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.
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.
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11.14.4Weather routeing and forecasting
11.14.4.1
Staged voyages shall have a commitment to seek shelter (or jack­up at a stand­by
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 stand­by locations) along the route.
11.14.4.2
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.
11.14.4.3
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.
11.14.4.4
Weather routed voyages shall only be approved if the sea room requirements in [11.14.2]
can be achieved and the vessel’s speed enables it to avoid weather in excess of the
operational limiting criteria.
11.14.4.5
Forecasts – General: 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:
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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.
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11.14.4.6
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.
11.14.4.7
Forecasts – Departure: 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.
11.14.4.8
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 2­16.
11.14.4.9
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 11­17 for
the first 24 hours of the towage.
Table 11­17 Typical maximum initial towage departure weather forecasts
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)
11.14.4.10
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.
11.14.4.11
Towages approaching a potential lee shore or areas with restricted sea room shall obtain a
favourable weather forecast before reducing their sea room requirements.
11.14.5Departure
11.14.5.1
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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 B­2 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.
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.
11.14.6Ports 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.
11.14.7Bunkering
11.14.7.1
Bunkering ports, if required, shall be agreed before departure.
11.14.7.2
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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 tug can proceed to a nearby port for
bunkers.
Bunkering at sea from a visiting vessel, subject to suitable procedures and calm
weather conditions.
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11.14.8Assisting tugs
11.14.8.1
Assisting tugs shall be engaged at the start of the towage, at any intermediate bunkering
port and at the arrival destination, as appropriate.
11.14.9Pilotage
11.14.9.1
The Master shall engage local pilotage assistance during the towage or voyage, as
appropriate.
11.14.10Log
11.14.10.1
A detailed log of events shall be maintained during the towage or voyage.
11.14.11Inspections 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].
11.14.11.2
For manned tows, and self­propelled 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 re­tensioned as necessary.
11.14.12Reducing excessive movement and shipping water
11.14.12.1
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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.
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11.14.13Notification of unusual or abnormal events
11.14.13.1
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.
11.14.14Diversions
11.14.14.1
Should any emergency situation arise during the towage or voyage which necessitates
diversion to a port 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.
11.14.15Responsibility
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 his 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.
11.14.16Tug change
11.14.16.1
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.
11.14.17Hazardous materials
11.14.17.1
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The carriage of hazardous materials should be avoided, unless it can be shown that the
materials are effectively controlled. For un­manned voyages, hazardous materials should be
stowed accounting for the limited remedial actions available in the case of inadvertent
release.
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.
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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.
11.14.18Ballast 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 of 2004 (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.
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11.14.18.2
The necessary ballast plan and records should be submitted to any attending MWS company
surveyor.
11.14.19Restricted 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
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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 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,
tow­line 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,
tidal height variations,
squat effects,
deflections of the structure
errors in measurement
surge (negative for under­keel clearances and positive for air draught), and
any protrusions below the bottom of the asset.
11.14.19.4
For areas where the under­keel 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. Side­scan 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, due to restricted water
depth, weather restrictions shall be defined (to reduce peak loads due to relative motions of
tug and tow).
11.14.20Under­keel clearances
11.14.20.1
The under­keel clearance shall be not less than the greater of one metre or ten percent of
the maximum draught (with a maximum of 3 m) accounting for the items listed in
[11.14.19.3]. The under­keel clearance can be reduced in very benign conditions after
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agreement with the MWS company.
11.14.20.2
Under­keel clearances for departure from dry­docks 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 under­keel
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:
Any conceivable loss of air not increasing the draught by more than the reserve on
under­keel 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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11.14.21Air 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.
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Guidance note 2:
The catenary of the power cable will change depending on the electrical load being carried in
the cable.
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11.14.21.4
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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).
11.14.22Channel width and restricted manoeuvrability
11.14.22.1
The minimum channel width along any inshore legs of the tow route with the under­keel
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).
11.14.22.3
Side clearances for departure from dry­docks or building basins are covered in [12.7].
11.15Bilge & ballast pumping systems
11.15.1Pumping 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.
11.15.2Pumping 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:
Towed vessel or barge
Unclassed vessels or those operating outside their conditions of class.
11.15.2.2
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11.15.2.2
For other wet tows, the need for and specifications of the emergency pumping systems shall
be determined by a risk assesment 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 self­floating 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 case­by­case 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 un­manned 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.
11.15.3Pumping 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
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:
non­compliance with intact or damage stability criteria, or
non­compliance 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
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maximum allowable transit draught being exceeded.
2. can be required for ballasting/de­ballasting 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
un­manned 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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11.15.4Pumping arrangements – Non­emergency
11.15.4.1
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In addition to the emergency pumping arrangements described in [11.15.2], suitable
pumping arragnements shall be provided for all planned (including contingency) ballasting
operations.
Guidance note:
Generally the following ballasting operations should be considered (as applicable):
Ballasting before, during and after load­outs
Ballasting to the agreed departure condition and subsequent ballasting to towing
condition
Restoration of draught and trim before/during/after discharge (e.g. lift off from barge
offshore)
Adjusting draught or trim due to shallow waters or air draught restrictions
(De­)Ballasting to change draught at end of towage (e.g. reducing draught to enter
port)
Trimming to allow inspection and repair below normal waterline.
Correction of unintended flooding
De­ballasting after accidental grounding
Access to flooded compartment (e.g. pump or anchor winch room).
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11.15.5Watertight 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 [11.13.16.1 m)].
11.15.6Sounding 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:
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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.
11.15.7Vents
11.15.7.1
All compartments connected to a pumping system shall have vents fitted. The vents should
be of an approved, automatic, self­closing 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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11.16Anchors (and alternatives) and mooring
arrangements
11.16.1Emergency 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
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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 high­current area, anchoring or mooring
arrangements can be required.
11.16.1.4
For jack­up platforms, see also [11.27.16].
11.16.2Size 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
anti­roll stabilisation should be used.
11.16.3Anchor cable length
11.16.3.1
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.
11.16.4Anchor 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.
11.16.5Attachment of cable
11.16.5.1
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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.
11.16.6Anchor 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.
11.16.7Mooring 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
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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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11.17Manned voyages
11.17.1General
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 Ro­Ro 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.
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.
11.17.2International regulations
11.17.2.1
Accommodation, consumables, lifesaving appliances, pumping arrangements and
communication facilities with the tug shall comply with International Regulations.
11.17.3Riding crew carried on the cargo
11.17.3.1
Where a riding crew is carried on the cargo, for instance a maintenance crew on a dry­
transported jack­up 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.
11.17.4Safety and emergency equipment
11.17.4.1
Notwithstanding the requirements of SOLAS, /92/, and any or all international regulations
for Life Saving Appliances and Fire­Fighting 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
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Fire­fighting 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
GMDSS radio (Global Maritime Distress and Safety System)
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.
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.
11.17.5Manned routine
11.17.5.1
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.
11.18Specific for multiple towages
11.18.1Definitions
11.18.1.1
This section expands on the definitions in Table 1­3 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
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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.
Figure 11­10 Multiple towage types (not to scale)
11.18.2General
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
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11.18.2.5
For those multiple towages that are approvable, each tow shall be prepared as described in
this standard.
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
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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.
11.18.3Double tows
11.18.3.1
These should only be considered when:
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.
11.18.4Tandem 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.
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
over­running each other, or the tug.
11.18.5Bifurcated tow
11.18.5.1
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
MWS company.
11.18.6Two 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:
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).
11.18.7Multiple tugs to one tow
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11.18.7Multiple 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 should normally be within 10% of that of each other.
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 three­tug spread blacks out and can be over­ridden by the tow with catastrophic
consequences. Suitable emergency procedures and tow equipment shall be available to
mitigate such a possibility.
11.19Specific for towing in ice
11.19.1General
11.19.1.1
This section 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 ice­covered waters.
11.19.1.2
It is recognized that towing in ice­covered 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 in­depth review of the towing manual as well as an
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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.
11.19.2Vessel ice classification
11.19.2.1
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
11.19.3Towage 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.
11.19.3.2
With the exception of a vessel pushed ahead (push­towed), 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.
11.19.4Conventional ice towing operations
11.19.4.1
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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.
11.19.5Close­couple ice towing operations
11.19.5.1
Close­couple 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 low­powered 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 over­running) through high
concentrations of difficult ice. For close­couple 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.
11.19.6Push­tow operations
11.19.6.1
Push­Tow operations should be carried out using either rigid connection (composite unit) or
flexible connections (a push­knee 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
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11.19.6.2
Where the push­towing 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 ‘push­tow’ operation with no independent
icebreaker support can be reduced if:
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 ‘push­tow’ 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 push­tow without independent icebreaker assistance.
11.19.7Towage operations with independent icebreaker escort
11.19.7.1
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.
11.19.8Manning
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.
11.19.9Multiple towages
11.19.9.1
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Multiple towages in ice are subject to the requirements set out in this section 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 in­depth 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. To avoid collision or over­running each tug shall have a quick release and re­set system
as described in [11.19.11.1] and [11.19.11.2].
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 multi­tug towage should have an appropriate level of experience of
towing in ice and be familiar with the associated difficulties and hazards.
3. A multi­tug 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) 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 push­tow 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 ice­coupled.
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11.19.10Towing equipment
11.19.10.1
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
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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 shock­loading 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.
11.19.11Additional 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 (tow­wire 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 multi­tug tow.
11.19.11.2
The towline quick release system should be capable of immediate winch brake release for pay
out of tow­wire as well as winch brake re­set 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.
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.
11.19.12Strength 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 11­18 Minimum towline MBL in ice
Bollard Pull (BP)
MBL (tonnes)
BP ≤ 40 tonnes
40 < BP ≤ 150 tonnes
BP > 150 tonnes
11.19.12.2
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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 ‘non­benign’ 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 [11.13.3.4].
11.19.12.5
Further, ALL tugs involved in a towage in ice shall carry a spare tow­wire of the same length
and strength as the main tow­wire that is immediately available on a reel to replace the
main tow­wire. In addition, there shall be enough competent personnel, equipment and
spares on board to crop and re­socket the main tow­wire at least once.
11.19.13Special cases of reduced tow­wire strength
11.19.13.1
The minimum size of tow­wire that is typically used by icebreaking tugs of 160t BP for close­
couple 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 tow­wire, is made fast to each bow of the vessel being towed. The tow­wire 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 tow­wire 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
Each tow­wire is on a separate towing winch that can be adjusted, quick released and
reset independently from the other and
Each tow­wire meets the requirements of a single tow­wire in terms of minimum
length, construction etc. and
Each tow­line has a monitoring system to enable load sharing.
11.19.14Towing 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.
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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 re­set system as described in [11.19.11.1]
and [11.19.11.2].
11.19.15Chain bridles
11.19.15.1
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 extra­long chain pigtail can be considered appropriate. Wire pennants and
bridles are sometimes used for small barge and vessel tows, especially when the close­couple
or ice­couple towing technique is anticipated.
11.19.16Synthetic rope
11.19.16.1
Synthetic rope shall not be used in a towing system for an in­ice 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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11.19.17Bridle recovery system
11.19.17.1
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 tri­plate.
The fuel mentioned in [11.13.11.2] for a motorized recovery winch shall be appropriate
for the anticipated temperatures.
11.19.18Emergency towing gear
11.19.18.1
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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 multi­directional
towing. With reference to [11.13.13], special arrangements can be required for the
emergency towing gear, especially on an un­manned 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 tow­wire
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 tugger­winch wire. The float line and
pick­up buoy are shackled to the emergency tow­wire in the same way as described in
[11.13.13.3] but remain coiled on the deck of the tow for deployment once the tow arrives in
open water.
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11.19.19Access to tows
11.19.19.1
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 de­ice 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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11.19.20Towing 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 tow­wire shall be performed, and based on the
results the MWS company surveyor can also require that the tow­wire is cropped and re­
socketed before the towage.
11.19.21Safety equipment for the workboat
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11.19.21Safety equipment for the workboat
11.19.21.1
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.
11.19.22Bollard pull requirements
11.19.22.1
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 (un­benign)
towage, as shown in [11.12.2].
11.19.23Oversized tug
11.19.23.1
For all towages in ice, [11.13.3.14] 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.
11.19.24Cargo loadings
11.19.24.1
Special attention should be given to cargo overhangs on a case­by­case 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 a)], [11.3.4.2], [11.10.2.8] and [11.10.2.9] are
not applicable.
11.19.25Seafastening 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/, Chapter 5.
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.
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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 sea­fastenings for voyages in ice
conditions shall be of acceptable design and not less than that required for weather
unrestricted voyages in non­benign areas ­ see [11.3] and [5.2].
Guidance note:
Despite no waves being evident, impact or over­running of thick ice floes can cause sudden
deceleration, heading deflections, listing and rolling of the tow.
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11.19.26Inspection of welding and seafastenings
11.19.26.1
With reference to 5.10.2], welding procedures and techniques shall account for very cold
temperatures, particularly for sea­fastening installation.
11.19.27Pipes and tubulars
11.19.27.1
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.
11.19.28Stability 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/, Chapter 5.
11.19.28.2
The intact range of stability of a towed vessel (see [11.10.2]) shall never be less than 36°,
including inland and sheltered towages.
11.19.28.3
For transit in ice­infested waters, the statement in [11.10.2.8] 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
[11.10.2.9] referring to buoyant cargo overhangs does not apply to transits in ice.
11.19.28.5
In addition to the requirements of [11.10.4], towed objects shall have positive stability with
any two compartments flooded or breached.
11.19.28.6
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11.19.28.6
The damaged stability relaxations for towed objects in [11.10.7.2] and [11.10.7.3] do not
apply in any area where ice interaction can occur as stated in [11.10.7.4].
11.19.28.7
The integrity of all underwater compartments of a tug and compartments subject to down­
flooding 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 [11.10.9] 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 self­propelled) 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.
11.19.29Ballasting 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).
Guidance note:
T is 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.
11.19.30Voyage planning in ice
11.19.30.1
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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 ice­infested region.
11.19.30.2
The plan should include:
A general description of the proposed voyage (manned/un­manned 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
Sea­fastening (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 un­
manned 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 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
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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 re­fuel 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 re­fuelling approval
from the appropriate jurisdiction, as well as the re­fuelling procedure and equipment, shall
be documented in the towing manual.
11.19.31Weather/ice restricted operations
11.19.31.1
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.
11.19.32Damage control and emergency equipment in ice
11.19.32.1
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
[11.13.16.1], the following additional equipment should be available for a towage in ice:
Portable generator
Portable compressor
Portable salvage pump(s)
Bracing shores
Portable de­icing equipment
Space heaters
Extension ladders
Chain falls
Collision mat materials.
11.20Specific for towage in the Caspian Sea
11.20.1Background
11.20.1.1
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 11­11), 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:
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:
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 11­11 Northern and Intermediate Caspian Sea areas
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11.20.2Towage 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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11.20.3Towage requirements within northern Caspian Sea
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11.20.3Towage 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:
H s = 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 [11.12.2.2] 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 [11.13.4.3]) be less than
200 m.
Guidance note:
Because of the very shallow water depths and limited wave heights the minimum towline
lengths required in [11.13.4.1] 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 [11.13.4.4]. The towing connection capacities in [11.13.3.4] 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, purpose­built quick­release 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 anti­chafe arrangements shall be used.
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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.
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 11­19 Default motion criteria for Northern Caspian
Nature of
Voyage
Case
LOA
(m)
B 1)
(m)
Block
Coeff
Full
cycle
period
(secs)
Single
amplitude
Roll
Pitch
Heave
1
> 37 m and > 15
m
any
10
13.5º
7.5º
0.1g
2
< 37 m and > 15
m
any
10
13.5º
13.5º
0.1g
Weather
unrestricted
Notes:
1. B = maximum moulded waterline beam.
11.20.4Towage requirements for remaining Caspian Sea areas
11.20.4.1
All tows in this area should follow the requirements in [11.3], [11.11] and [11.12] for
weather unrestricted tows outside benign weather areas, as applicable.
11.20.5Requirements for towages between Caspian Sea areas
11.20.5.1
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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 [11.20.1.1], 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.
11.21Specific for FSUs (FPSOs, FSOs, FLNG facilities,
FRSUs etc.)
11.21.1General and background
11.21.1.1
This section addresses the specific marine­related 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 ‘fast­track’, resulting in construction and commissioning
activities being completed during the tow.
New­build or converted FSUs usually undertake a limited number of towages only,
following construction or conversion. There can be a further towage at the end of their
working life.
Frequently the design weather conditions for towage are more severe than the service
conditions. There is a natural reluctance to build in additional strength or equipment
which will have no practical value during the service life.
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11.21.1.2
Project­specific fit­for­purpose criteria shall be agreed in each case.
11.21.2Route and weather conditions
11.21.2.1
Metocean design criteria should be carefully established early in the project, in accordance
with [3.2].
11.21.2.2
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Mitigation of the design extremes for shorter routes may be achieved by the use of staged
towage, in accordance with [11.14.4.1].
11.21.2.3
The need for appropriate additional tugs for passage through restricted or busy waters shall
be considered and agreed with MWS company.
11.21.3Structural issues
11.21.3.1
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 dry­docking 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 dry­docked. These differences place
much greater emphasis on ensuring the quality of the hull.
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11.21.3.2
For long towages, fatigue damage can need to be considered (see [5.9.4]).
11.21.3.3
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.
11.21.3.4
Equipment foundations shall be designed for the temporary phase operations. Fatigue
damage to the connections between the topsides and hull should be considered.
11.21.3.5
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.
11.21.3.6
Green water damage or slamming damage on temporary equipment should be considered in
the location of equipment.
11.21.4Tug selection
11.21.4.1
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.
11.21.4.2
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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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11.21.4.3
The use of additional tug(s) can be required in restricted waters.
11.21.4.4
If it is not possible or practical to provide an emergency anchor, then additional or larger
tugs can be required. See also [11.21.9].
11.21.5Ballast, trim and directional stability
11.21.5.1
To limit the loss of directional stability the hull shall be carefully ballasted, trimmed by the
stern and in the case of a ship­shape 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.
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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 mid­length.
The removal of the vessel’s rudder, where the FSU is a conversion
The hull design of purpose­built 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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11.21.5.2
Consideration should also be given to attaching a tug at the stern of the FSU (see also
[11.21.5.3] below).
11.21.5.3
The design of the towing gear should minimise the directional instability.
11.21.5.4
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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.
11.21.6Towing equipment
11.21.6.1
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.
11.21.6.2
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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11.21.6.3
As noted in [11.21.5], 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 wide­spaced, 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.
11.21.6.4
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.
11.21.7Self­propelled or thruster­assisted vessels
11.21.7.1
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.
11.21.7.2
The specification of the thruster units, power supplies and manning shall be suitable for the
voyage requirements and documented at an early stage.
11.21.7.3
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A risk assessment shall be undertaken, in accordance with [2.4], to determine the need for
assisting tugs.
11.21.8Manning and certification
11.21.8.1
The documentation set out in Table B­2 should be submitted.
Guidance note:
Most FSUs are not classed as ships during their service life.
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11.21.8.2
If the towage is to be manned, then the requirements of [11.17] shall be considered.
11.21.8.3
A dedicated marine riding crew should be provided, regardless of the presence of
construction or commissioning personnel, as shown in [11.17.1.4].
11.21.8.4
In all cases, whether manned or un­manned, the unit shall be fitted with appropriate means
of boarding, in accordance with [11.13.15]
11.21.9Emergency anchor
11.21.9.1
The general emergency anchor requirements of [11.16] shall apply.
Guidance note:
FSU mooring systems (whether turret­type or spread), being only for in­place 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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11.21.10Moorings
11.21.10.1
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.
11.22Specific for jacket voyages
11.22.1Introduction
11.22.1.1
This section gives requirements specific for jacket voyages and not already addressed in this
standard.
11.22.2Fatigue, wave slam and vortex shedding
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11.22.2Fatigue, wave slam and vortex shedding
11.22.2.1
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].
11.22.3Equipment seafastenings
11.22.3.1
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.
11.22.3.2
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.
11.22.3.3
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 de­rigging case, when high
impact loads can be expected, shall also be considered.
11.22.3.4
Rigging shall be adequately secured to rigging platform structural members or jacket
members accounting for the elevation of the rigging (see [11.22.3.1]). 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]).
11.22.3.5
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.
11.22.3.6
Items which could be exposed to wave action during either voyage or launch shall be suitably
secured and protected against the expected loadings.
11.22.3.7
Flexible control lines and cables for the ballast and/or grout systems should be protected
from wave action.
11.22.4Transport on deck of crane vessel
11.22.4.1
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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.
11.22.4.2
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.
11.22.4.3
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.
11.22.4.4
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.
11.22.4.5
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.
11.22.4.6
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).
11.22.5Wet towed jackets
11.22.5.1
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.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.22.5.2
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.
11.22.5.3
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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.
11.23Specific for ship towage
11.23.1General considerations
11.23.1.1
This Section 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.
11.23.1.2
Minimum certification and documentation requirements are shown in Table B­2. If the towed
vessel is not in Class with a recognised 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 dry­docking
are required.
11.23.1.3
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.
11.23.1.4
Passenger ships and warships, because of the complex nature of their systems, pose
particular problems with respect to their compartmentation, and require special
consideration. Ro­Ro ships can also pose particular problems, on account of the potentially
large free surface in the event of flooding. Passenger ships and Ro­Ro ships will generally
only be approved for towage if the tow is manned, to permit early intervention in the event
of any problems.
11.23.1.5
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 ports of shelter shall be noted and taken into account in the towage
procedures. To minimise the risk of pollution, the requirements of the IMO “Guidelines for
Safe Ocean Towing”, /92/, paragraph 13.19, shall be taken into account as far as is practical.
11.23.1.6
If a stern­first towage is required (see [11.13.1.3]) then special care shall be taken
regarding towing connections, draught, trim and the control and protection of the tow
during the towage.
11.23.2Towlines and towing connections
11.23.2.1
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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 [11.13.3.4]. If necessary, reinforcements shall be fitted to achieve the required
capacity, otherwise alternative arrangements shall be made.
11.23.2.2
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 load­sharing 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.
11.23.3Anchors
11.23.3.1
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.
11.23.3.2
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.
11.23.3.3
Spurling pipes into chain lockers shall be made watertight with cement plugs or another
satisfactory method.
11.23.4Securing of equipment and moveable items
11.23.4.1
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].
11.23.4.2
See [11.27.11.6] for securing and use of cranes and lifting derricks.
11.23.4.3
The rudder shall be positioned in the amidships position, or as agreed with the Tug Master,
and immobilised.
11.23.4.4
The propeller shaft shall be immobilised, or disconnected, to prevent damage to machinery
during the towage.
11.23.4.5
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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.
11.23.4.6
For large equipment, engineering calculations shall be carried out in order to verify that the
securing of items is satisfactory.
11.23.4.7
Additional protection or securing can be required for equipment exposed to wave slam.
11.23.5Carriage of cargo
11.23.5.1
The carriage of manifested cargo on the tow shall not normally be approved unless the tow is
manned and is fully classed by a Classification Society, including the possession of a current
International Load Line Certificate.
11.23.5.2
International Load Line Regulations shall be strictly followed. Approval shall not be given to
any towage where the prescribed Load Line draught is exceeded.
11.23.5.3
The cargo plan shall be documented.
11.23.5.4
The cargo shall be loaded in a seaman­like manner making proper allowances for load
distribution both during loading and for the duration and route of the towage. Longitudinal
strength requirements shall be complied with.
11.23.5.5
Bulk cargoes shall be properly trimmed to prevent shifting in a seaway. Shifting boards or
other preventative methods shall be utilised where appropriate.
11.23.5.6
All other cargoes shall be secured in accordance with [11.3] and Sec.5.
11.23.5.7
Particular attention shall be paid to the securing of scrap steel, which if carried shall be
properly seafastened. If carried in a hold, it shall not be treated as a bulk cargo.
11.24Specific for voyage to scrapping
11.24.1Anchoring
11.24.1.1
In addition to the emergency requirements in [11.23.3], the anchoring equipment shall be
shown to be in good working order if there is a possibility of having to anchor at the final or
intermediate locations.
Guidance note:
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This will normally be a class requirement for classed vessels.
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11.24.2Towage
11.24.2.1
The additional risks of vessels at the end of their useful lives being towed for scrapping shall
be considered in the risk assessment especially if they are no longer in class or do not have a
current load­line certificate or exemption.
11.24.3Own power or manned
11.24.3.1
Vessels sailing under their own power, or manned towages, shall require a load line or
Exemption certificate issued on behalf of the Flag State. The manning requirements in
[11.17] shall apply.
11.25Specific for towing of pipes and submerged objects
11.25.1Introduction
11.25.1.1
This section gives specific requirements for towages of pipes and submerged objects not
already addressed in this standard. It covers the launch and towage of pipes and other long
slender elements including bundles, TLP tethers, riser towers and hybrid risers. For
simplicity in this section these are called “pipes” unless referring to specific items.
11.25.1.2
Pipe tow methods are wide ranging in approach . The simplest and longest standing method
is bottom tow. Other methods involve increasing complexity of construction and execution
with the most sophisticated being Controlled Depth Tow (CDT) where a bundle of lines,
sometimes including hydraulic and electrical control and service lines, are housed in a carrier
pipe. The bundle is generally fitted with a towhead and trailhead with ballasting and
production system functions.
11.25.1.3
Other tow methods generally require a system­dependent approach to the design to make
the pipe suitable for the intended installation method.
Bottom (or on­bottom) tows when lengths of ballast chain drag on the seabed. At
higher tow speeds the uplift from the towline and hydrodynamic lift forces makes this
become an off­bottom tow.
Off­bottom and CDT tow methods require great care to control the control the
submerged weights of the assembly within a narrow band.
Surface tow, near­surface tow and CDT generate greater fatigue loadings in the
installation phase which control axial design strength of the carrier pipe.
11.25.1.4
For the simplest methods the main considerations affecting pipe design are tow depth, which
can be greater than that at the destination site so can control collapse and buckle strength
design, and tow force which can control axial design strength of the pipe.
11.25.2General design for launch and towage phases
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11.25.2General design for launch and towage phases
11.25.2.1
All parameters for submerged tows that could be critical shall be considered during the
modelling and analysis of a submerged tow. See DNV­RP­H103, /56/, Sec. 7.3 for examples
of critical parameters.
11.25.2.2
The strength of the towed object, including the tow head, other towing connections and
wires should be designed based on dynamic analysis of the launch, towing and holdback
forces using the largest bollard pull to be used and all possible towing and trailing vessel
headings.
11.25.2.3
Calculated effects should include maximum rigging forces, maximum stresses in the pipe,
the accumulation of fatigue damage from launch through to installation, and the definition of
limiting weather criteria for the tow operation.
11.25.2.4
For LRFD, calculated characteristic values of loads should have a maximum 10% probability
of being exceeded. In order to verify that the corresponding design load is adequate it is also
necessary to check the tail of the distribution.
11.25.2.5
The pipe itself shall be proven to be acceptably loaded during all phases of the operation.
Calculations shall be documented to justify strength during launch, including axial and
bending stresses, sag bending as the towing head moves forward, and any reverse bending
at the water line.
11.25.2.6
Break­out forces shall be conservatively estimated. The effects of launch track
slope/settlement, mechanical resistance, launch bogie/roller condition and other relevant
parameters that influence the break­out force shall be considered.
11.25.2.7
The strength of the pipe should be documented as adequate for all potential situations,
including that where it is hanging freely supported only at each end.
11.25.2.8
The specification, method and limitations of the analysis program should be documented.
Behaviour during tow should as far as possible be estimated during design. Inline structures
should as far as possible be designed in a way that will minimise the generation of
hydrodynamic drag and lift forces that could cause an instable/fluctuating pipe configuration
during tow.
11.25.2.9
Sensitivity studies shall be carried out for essential parameters such as weight, ballast,
buoyancy, salinity, cross current, towing speed, back tension, internal pressure loss etc. for
relevant phases.
11.25.2.10
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Pipe deflection and anchorage forces (required to stabilise the pipe in any predefined holding
locations) shall be analysed for the characteristic current conditions and loads.
11.25.2.11
Pipe behaviour following towline failure should be assessed and used as a basis for evaluating
and generating and appropriate contingency procedures. Qualification testing should be
based on a product and configuration representative of the actual pipe and towing conditions
anticipated.
11.25.2.12
Fatigue life utilisation during the tow needs to be consistent with the assumptions of the
pipeline designer’s assumptions regarding fatigue life allocation to the various phases of the
life of the pipeline. DNV­OS­F101, /42/, Section 5, Clause D800 provides guidance on
allowable fatigue utilization during the construction phase for various safety categories of
pipeline safety criticality.
11.25.2.13
Steady state and dynamic towing forces should generally be computed using a suitable
dynamic analysis. The analysis should include the effects of waves, currents and forces
induced in the pipe by the trail tug.
11.25.2.14
The floating and directional stability of the towed object and tow heads/structures shall be
calculated for all stages of the launch, tow installation and flooding. Side current forces,
hydrodynamic effects during tow and free surface effects during flooding operations should
be considered.
11.25.2.15
Coatings and anodes for bottom towed pipe need care in selection and testing as they need
to resist abrasion from the seabed during tow­out.
11.25.2.16
The following requirements apply to:
Launch of pipelines etc. see [11.25.3]
Route and weather restrictions, see [11.25.4]
Tug selection and operation, see [11.25.5]
Towing rigging, see [11.25.6]
General towing procedures and requirements, see [11.25.7]
Controlled depth tow (CDT), see [11.25.8]
Surface or sub­surface tow of pipelines etc. see [11.25.9]
Submerged tow of objects attached to the installation vessel, see [11.25.10]
11.25.3Launch
11.25.3.1
For off­bottom tow, the entry of the towing head, pipe string and trailing head into the
water should be monitored by divers and, if necessary, by an inshore survey boat. Internal
pressures should be checked after launch during the ballasting and trimming operations.
11.25.3.2
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For off­bottom tow, all connections of towing equipment and drag chains should be checked
by ROV or divers after launch. The launch bridle and pennant should not be used for the tow
if the launch procedure can have caused mechanical damage or overstressing of the gear, or
the post­launch checks reveal actual damage.
11.25.3.3
The launch shall start on a weather forecast with assurance that it will reach a safe condition
within the foreseeable forecast, taking into account local conditions, currents and pipe string
deflection. Following completion of launch, a decision will be taken on whether to start the
tow to field or to “park” the pipe.
11.25.3.4
A maximum tug efficiency of 80% of the continuous bollard pull should be assumed for the
tug(s) used for launch, assuming calm conditions ­ see [11.12.2.10].
11.25.3.5
In the event of the launch stopping due to build­up of sand ahead of the towhead, the peak
load can be increased to 60% of the MBL of the weakest part of the launch rigging. This
upper limit should be clearly stated in the launch procedure as a contingency case. It can
only be used subject to accurate monitoring of the actual force applied and full briefing of all
personnel involved.
11.25.3.6
Local environmental conditions at the launch site, such as wave directions/patterns, tide and
current forces should be considered.
11.25.3.7
The launch area, including an adequate corridor to allow for the necessary deflection of the
pipe, shall be surveyed before the operation.
11.25.3.8
Tow deflection due to side current shall be analysed for different stages of the launch. The
tug offset positions required to counteract the predicted pipe deflection shall be established.
11.25.3.9
When a towed object is towed in its axial direction in an “off­bottom tow mode”, friction
between the ballast chain and seabed cannot be used to counteract lateral deflection of the
pipe.
11.25.3.10
Pipe support and bending restrictions shall be defined, based on structural pipe analysis,
consideration of local soil conditions, the launch track characteristics and tow weight and
stiffness. Variable conditions such as scour and erosion in wave effected zone and
consolidation of the soil shall be considered. Acceptable departure angles from the launch
way, in both horizontal and vertical direction, shall be defined.
11.25.3.11
Adequate means of monitoring environmental conditions and limiting launch parameters
shall be established and tested before start of load­out.
11.25.3.12
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Coefficients of friction as listed below should be taken into account when computing launch
and installation loads (noting that those below are not true for all locations and projects,
project specific values if available should be used instead):
Table 11­20 Typical upper bound design friction coefficients for launch and installation
Conditions
Break­out
Running
Launch/tow wires on seabed
1.0
1.0
Towhead on seabed
1.0
1.0
Towhead skids on launchway rails
0.15
0.12
Wheel bogies on rail track
0.01
0.008
Holdback wire on track sleepers
0.5
0.5
Ballast chains on seabed
1.0
1.0
11.25.3.13
For launch, a load factor as shown below should be applied to the computed total static force
to account for uncertainties:
Table 11­21 Load factor for uncertainties during launch
Computed load (L)
Load factor
L <150 tonnes
150 < L < 300 tonnes
1.5
1.5 ­ 0.002*(L ­ 150)
L > 300 tonnes
1.2
11.25.3.14
The minimum factored static load at either end for launch should normally be taken as 100
tonnes.
11.25.4Route and weather restrictions
11.25.4.1
A tow route corridor pre­survey is required and should be made before detailed design
completion. This survey should provide seabed bathymetry and side scan seabed images (or
a swathe survey) of the whole transit corridor, and soil type and soil stiffness properties
measured at regular intervals along the corridor. The surveyed area shall allow for likely
deviations of the tow and the necessary deflection of the pipe and temporary lay­down areas.
11.25.4.2
If pre­design survey was made in more than 90 days before the tow, a follow­up survey is
needed no more than 90 days before the tow operation with a swathe or side scan to confirm
the route is clear of debris and other construction activity.
11.25.4.3
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Tow wires are generally relatively short, and cannot comply with the requirements of
[11.13.3]. The motion of the tug in waves will cause dynamic loads to be applied to the
towheads. A dynamic analysis should be carried out, which will normally result in limitations
being placed on the sea states in which the pipe can be towed.
11.25.4.4
For off­bottom tow it is usual to provide buoyancy tanks attached to both tow and trail
heads. These and any along the pipe are to be subdivided so that loss of any single
compartment does not lead to an irrecoverable situation. Buoyancy tanks should be
pressurised to the maximum water depth likely to be encountered during towage.
Consideration shall be given to possible loss of buoyancy tanks in analysis and procedure
(25% loss of buoyancy should be considered).
11.25.4.5
It is essential that:
Advance notice of the operation should be given through Notices to Mariners, or local
equivalent, and military and fishing interests should be advised well ahead of the
proposed date of the operation.
The tow shall be accompanied by a guard vessel at all times.
The tugs shall display shapes and lights in accordance with IMO International
Regulations for Preventing Collisions at Sea (COLREGS), /91/
11.25.4.6
Weather for the tow shall be limited to that in which the towing/trail tugs can maintain the
required tension to keep the tow string within the tow corridor. In practice, the weather can
be limited by the ability of the trail tug to maintain station relative to the position of the
pipe.
11.25.4.7
The limiting sea state, current, wind speed, etc., for the operation should be clearly defined
and suitable contingencies included to account for forecast and analytical uncertainty. The
towage should move from place of safety to place of safety (usually predefined parking
areas) within a foreseeable weather window.
11.25.4.8
The required weather window shall be documented in detail for comparison with forecasts.
11.25.4.9
In order to control the attitude and position during tow, instrumentation shall be provided,
including as a minimum:
Transponder/Depth and Hydro acoustic Positioning Reference sensors on tow and trail
heads, and at least three (typically at least 1 per km) distributed along the pipeline to
monitor the position and configuration of the line. The system should have sufficient
redundancy to ensure that loss of any one transponder does not prejudice the
capabilities of the system to determine the position of the pipe.
Pressure gauges on all pressurised compartments and lines, with transducers fitted at
the lay­down points within the tow and trail head structures.
All positioning and monitoring equipment should be centrally monitored on the
command vessel. Positioning equipment should be capable of giving good visual and
plotted indication of the pipe position, shape and depth at all times.
11.25.4.10
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The minimum required Under­Keel Clearance (UKC, between the lowest part of the pipeline
and LAT), apart from bottom tows, will depend on the controllability of the tow depth. This
needs to be determined and agreed with the MWS company at an early stage of the project
but will typically be at least 10 m.
11.25.4.11
Safe conditions before and after the tow shall be clearly defined. It can be necessary for
planned or contingency reasons to park the tow string at a designated parking position.
11.25.4.12
For the off­bottom tow string, consideration should be given to the fatigue damage that can
be experienced in any parking position allowing for foreseeable spells of waiting on weather.
11.25.5Tug selection and operation
11.25.5.1
The required bollard pull of the lead tow tug(s) should be derived using the estimated
efficiency factors shown in [11.12.2.10] taking into account the defined limiting weather
criteria.
11.25.5.2
In the absence of vessel­specific data, the reduction in available towline pull at the required
towing speed should be taken into account as follows:
Table 11­22 Reduction in effective bollard pull with speed
Certified Continuous Bollard Pull (BP),
tonnes
Reduction in Bollard Pull with Speed
(tonnes/knot)
BP < 120 tonnes
7 tonnes/knot
120 < BP < 280 tonnes
7 + 0.03125*(BP ­ 120) tonnes/knot
BP > 280 tonnes
12 tonnes/knot
11.25.5.3
The tugs are to be highly constrained and shall keep their position accurately with respect to
each other and the towing and trailing heads so that the tow string is maintained in the tow
corridor and at the required tow depth. Particular care should be taken during alterations of
course.
11.25.5.4
In cases where the bollard pull of the tug can exceed 50% of the MBL of any of the wires
through which the tug is connected to the pipe, the following requirements shall be
incorporated into the operational procedures:
The tug shall have a recently calibrated and operational means of displaying the actual
towline force/actual bollard pull. If this is based on winch torque, then compensation
shall be included for the layer on the winch from which the rope is being pulled.
The peak load applied by the tug shall not be allowed to exceed 50% of the MBL of the
weakest link through which it is connected to the pipe.
The master of the tug shall be fully briefed on the permissible peak load which can be
applied.
11.25.6Towing rigging
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11.25.6Towing rigging
11.25.6.1
The MBL of the tow rigging shall be calculated using the peak dynamic tow force multiplied
by a factor of 2.0. Shackles and other certified rigging/connecting items such as tri­plates
and padeyes shall have a MBL at least 30% greater than that required for launch or tow wire
as applicable.
11.25.6.2
Structural steel items and connections such as padeyes and the load path from through the
towhead should have an Ultimate Load Capacity not less than the lesser of:
Tow wire required MBL + 40 tonnes, (for MBL > 160 tonnes) or
Tow wire required MBL x 1.25 (for MBL < 160 tonnes)
11.25.6.3
Trail end tug behaviour is not currently amenable to dynamic analysis; therefore the
following assumptions are often used:
Steady hold back force = 30 tonnes
Dynamic hold back force = 90 tonnes
MBL of trail tow rigging = 180 tonnes
MBL of shackles and other certified rigging items = 30% greater than tow wire
Structural steel items shall be in accordance with the same principles as the towhead
11.25.6.4
Cumulative fatigue damage in the pipe also needs to be assessed, recognising stress cycling
from traversing the seabed bottom over the tow route. Note that fatigue of partial
penetration butt welds used in one application caused failure of a tow.
11.25.6.5
Launch and installation loads are essentially static forces. The maximum static launch load is
based on the largest structure, generally the leading towhead, just leaving the launchway,
and includes the following components:
Leading towhead friction (towhead just left launchway)
Pipe friction (bogies on rails)
In­line friction, if any (bogies on rails)
Trailing towhead friction (bogies on rails)
Trailing rigging friction (wire on sleepers)
Hold back tension.
11.25.6.6
The maximum static launch load in the rigging consists of the towhead load as detailed
above, plus the leading rigging friction. A significant part of the leading rigging will rest on
the seabed.
11.25.6.7
The maximum static installation (final positioning) load at the towhead and trail head should
allow for movement in both forward and reverse directions, allowing for the following:
Ballast chain friction
Trailing rigging friction (if any)
Hold back tension.
11.25.6.8
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11.25.6.8
For installation (final positioning), a load factor of 1.0 can be applied, subject to assumptions
for chain and wire friction against the seabed being conservative.
11.25.7General towing procedures and requirements
11.25.7.1
Adequate means of monitoring environmental conditions, tow parameters and pipe
configuration shall be established and tested before the start of the tow. Current speed (and
direction) should be monitored at regular intervals during tow and holding periods, unless
extreme current values are used in the analysis of pipe behaviour. Contingency procedures
should be documented and mitigating actions employed in case the current speed exceeds
the design values. Before starting the tow the pipe shall be ballasted to an acceptable
configuration for the tow.
11.25.7.2
The towage should move from safe to safe condition, see [2.5.1.2], within a foreseeable
weather window unless the towage can safely continue in a design storm or by establishing a
stand­by configuration (e.g. wet­parking) that ensures that product integrity is maintained
until normal operations can be resumed. Pipe parameters, configuration and feedback shall
be systematically checked after start of the tow. Deviations from expected values shall be
recorded and any possible effects on the towing procedure and pipe evaluated.
11.25.7.3
If external ballast is used (normally chain) the pipe shall be sufficiently robust to accept
some loss of ballast during tow, without undue effect on the pipe configuration.
11.25.7.4
Adequate back­up systems shall be available. Adequate abandonment equipment shall be
carried on board the lead tug(s) and trailing tug, to enable controlled laydown and
abandonment if necessary.
11.25.8Controlled depth tow (CDT)
11.25.8.1
Further to the general pipeline design requirements for tow methods provided in [11.25.2]
to [11.25.4] for on and off­bottom tows, this section addresses the additional specifics for
tows where the pipe buoyancy is engineered to enable the pipe assembly to be towed
suspended between trail and lead tugs at a controlled depth.
11.25.8.2
The position survey package shall be carefully developed to ensure it will meet the needs of
the operations. In particular it shall demonstrate the clearance to any fixed hazards at the
location and shall always include transponders fitted to the string that allow its profile to be
identified in three dimensions.
11.25.8.3
Pipe stresses shall be determined and calculations documented to show adequate strength
allowing for the curvature of the pipe during towage. Effective pipe stiffness and local
stresses at bulkheads are to be determined.
11.25.8.4
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Considerations should be given to pipeline crossings in terms of checks and vessel status.
11.25.8.5
Trail tug motion characteristics should be considered as a criterion for obtaining operational
limits.
11.25.8.6
Suitability of ROV support vessel and associated equipment to be thoroughly checked via
vessel assurance survey or similar.
11.25.8.7
The Towmaster and Marine Representatives should have clear understanding of the
behaviour and response of the pipe to the various corrective actions that can be undertaken
during the tow.
11.25.8.8
The pipe profile should be continuously monitored and any remedial actions taken in a timely
manner.
11.25.8.9
Manning on board all the vessels in the tow fleet should be adequate to allow the tow to be
constantly monitored and so that no person is overloaded with several tasks.
11.25.8.10
Consideration to be given as to whether the Guard Vessel can be utilised to monitor the pipe
attitude in the event of a failure on board the ROV support vessel.
11.25.9Surface and near­surface tow
11.25.9.1
This section addresses the additional requirements for tows where the pipe is floated and
held between trail and lead tugs.
11.25.9.2
Removal of buoyancy elements is a calm­weather operation that shall be carefully addressed,
to avoid loss of control of pipe or elements.
11.25.9.3
Pipe stresses and fatigue need particular attention as the string will experience a large
number of stress cycles if left at the surface for some time in all but sheltered waters.
11.25.9.4
The following information will normally be required for approval:
Fatigue life calculations for the tow, with assumptions made.
A clearly documented heavy weather procedure
The full and comprehensive leak testing of all buoyancy elements and pipeline closures
A minimum overall reserve buoyancy of 25%
The ability to withstand the loss of 25% of the buoyancy elements including any four
adjacent elements
All connections between the buoyancy elements and the pipeline are to be completed
with robust connections that are not sensitive to fatigue
All tensile connections are to be subject to 100% NDT
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A suitable work class ROV shall be present in the field during installation operations
with a package of spares.
11.25.9.5
Hang­off rigging between the buoy and the object shall be designed in accordance with
[16.2]. The possibility of fatigue should be considered.
11.25.10Submerged tow when attached to installation vessel
11.25.10.1
Design considerations for submerged tow of object attached to installation vessel are given
in Section 7.3.3 of DNV­RP­H103, /56/.
11.25.10.2
Hang­off rigging shall be designed in accordance with [16.2].
11.25.10.3
Possible fatigue shall be considered for:
hang­off point(s) on vessel
lift points and elements supporting lift points on towed object
components on towed object, e.g. internal piping, due to VIV.
11.25.10.4
Measures shall be taken to prevent abrasion of hang­off rigging and any anti­rotation line(s).
11.25.10.5
Acceptable clearances to the sides of vessels or moonpools shall be documented by
calculations if applicable.
11.26Specific for deep draught towages
11.26.1General
11.26.1.1
This section covers the special requirements for towages of deep draught structures, e.g.
GBS’s, towages not covered above.
11.26.2Deep draught towages
11.26.2.1
The procedures and equipment shall be designed to avoid excessive load on the stern of the
tugs if the towing connections are submerged too deep, either during any towage stage or
the subsequent installation.
Guidance note:
Possible solutions include the use of vertical bridles or floats attached to the towline (s).
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.27Specific for jack­up voyages
11.27.1General
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11.27.1General
11.27.1.1
This section covers the special requirements of jack­up, not covered above for both wet
towages and dry towages/transport. The terms 24­hour move, location move, weather
unrestricted towage and weather unrestricted voyage have the meanings shown in Table 1­
3.
11.27.1.2
“UKOOA Guidelines for Safe Movement of Self­Elevating Offshore Installations”, /121/ and
“The Safe Approach, Set­Up And Departure of Jack­Up Rigs to Fixed Installations”, /120/,
describe good practice for jack­up moves within the North Sea. Many of these practices
should be followed in all areas.
11.27.1.3
The requirements for jack­up move procedures are given in [11.29] and for approaches to a
location in [11.28]. Requirements for documentation of longer voyages are given in
[11.31.2].
11.27.2Loadings and strength
11.27.2.1
Loads in legs, guides, jack­houses and jack­house connections into the hull, as appropriate,
shall be derived in accordance with one of the methods set out in [5.2].
11.27.2.2
For jack­ups transported on a barge or vessel, the loads in cribbing and seafastenings shall
be similarly derived in accordance with [5.2].
11.27.2.3
Hull and superstructure construction, details, materials and workmanship shall be shown to
be in accordance with sound marine practice, and shall be in sound condition.
11.27.3Seafastenings for dry towages and transport
11.27.3.1
Seafastenings for dry towages and transport shall be designed for sustain the loadings
determined in accordance with [11.27.2].
11.27.3.2
Seafastenings shall only be provided at strong points on the cargo and the arrangement
balanced about the centre of gravity.
Guidance note:
Examples of strong points are:
against the sideshell or leg wells in way of bulkheads of frames
against thicker areas of bottom plating.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.27.3.3
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Seafastenings placed against the spudcans are not normally accepted by the MWS company.
This is due to the difficulties of fully eliminating guide clearances, and thus the possibility of
hull movement on the cribbing before the spudcan seafastenings begin to act. In such cases
any conventionally­placed seafastenings are likely to be overloaded before the spudcan
seafastenings begin to act. If such solutions are proposed, they cannot be accepted unless
such issues are adequately addressed to the satisfaction of the MWS company.
11.27.4Hull strength – wet towage
11.27.4.1
For units towed on their own buoyancy, either the hull shall be built to the requirements of a
recognised Classification Society, and be in Class or verified to comply with Class building
and inspection requirements. Otherwise the requirements of [11.27.4.2] through
[11.27.4.4] shall apply.
11.27.4.2
If not in Class, the hull shall be demonstrated to be capable of withstanding the following
loadings:
Static loading, afloat in still water, with all equipment, variable load and legs in towage
position, plus either:
Longitudinal or transverse bending, as derived from [11.27.4.3], or
Loads imposed on the hull and guide support structures by the legs, when subjected to
the agreed motion criteria.
11.27.4.3
Longitudinal and transverse bending can be derived by quasi­static methods, assuming a
wave length, Lw, equal to the unit’s length or beam, and height:
where Lw is in metres.
11.27.4.4
External plating shall be demonstrated to have adequate strength to withstand the
hydrostatic loads due to the immersion of the section of shell plating considered, to a depth
equivalent to that which would be caused by inclining the hull, in towage condition, to the
static angle equal to the amplitude of motion as considered in [11.4].
11.27.5Stress levels
11.27.5.1
Stress levels in legs, guides, jack­houses, hull and all temporary securing arrangements shall
comply with Sec.5. The hull in way of seafastenings to a barge or transport vessel shall also
be checked to comply with [11.9.7]. See also the caution for dry transport in [11.9.1.1]
Guidance Note.
11.27.5.2
A critical motion curve should be drawn up, or be in the Operations Manual, reflecting the
motion limits for the legs or any other component. This can be used as a guide during the
towage or voyage, indicating whether course or speed should be changed, or the legs
lowered, as appropriate.
11.27.5.3
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Before an weather unrestricted voyage of a jack­up, an inspection programme, including
NDT, for critical structural areas shall be undertaken if the unit has previously been subject
to one or more transoceanic voyages and there has been no subsequent recent detailed
inspection of the fatigue­critical areas. Typically, the inspection should include, as
appropriate, the areas of legs from just below the lower guides to 2 bays above the upper
guides, with the legs in any proposed voyage condition. It should also include the guide
connections, the jack­house connections to the deck and connections of the spudcans or
mudmats to the leg chords. Note that new­build MOU's should normally have been verified
for fatigue for the initial delivery voyage.
11.27.5.4
Local areas of jack­up platforms can be particularly prone to fatigue damage as described in
[11.27.5.3].
Guidance note:
Normally fatigue damage is excluded from any MWS company approval, unless specific
instructions are received from the client to include it in the scope of work.
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11.27.6Stability and watertight integrity – wet towages
11.27.6.1
The following practical considerations apply in addition to the requirements in [11.10].
11.27.6.2
For weather unrestricted towages, the compartmentation and watertight integrity
requirements of [11.10.8.1] shall be particularly addressed, in particular for engine room
intake vents and exhausts. Other special considerations for jack­ups include:
All compartments and their vents, intakes, exhausts and any other appurtenances or
openings should be effectively watertight up to the waterlines described in
[11.10.8.1], and weathertight up to 3 m above main deck level, if higher.
All compartments and their vents, intakes, exhausts and any other appurtenances or
openings should be structurally capable of withstanding hydrostatic pressure due to
inclination to the minimum required downflooding angle, and direct loadings from
green water.
All air intakes and exhausts for equipment that shall be kept running and/or which
shall be available for emergency use should extend above the waterline associated with
the minimum required downflooding angle, or 4 m above main deck level, whichever is
the higher.
Any jetting lines and pumping nipples in lines shall be checked closed and watertight
before departure.
All pre­load dump valves shall be closed and secured.
Mud return lines from shale shaker pumps etc., leading below main deck, shall be
blanked off.
Dump valves in mud pits shall be checked closed secured.
Overboard discharges shall be blanked off, or fitted with non­return valves.
11.27.6.3
For all towages, liquid variable loads shall be minimised and shall be in pressed up tanks
where possible.
11.27.6.4
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Free surface in the mud pits is not generally acceptable, except for very short 24­hour moves
in controlled conditions.
11.27.6.5
Free surface effects of all remaining liquid variables, except those in pressed up tanks, shall
be taken into account in the stability calculations.
11.27.6.6
Stability calculations shall accurately reflect the position and buoyancy of the spudcans.
Spudcan water shall be taken into account in weight and centre of gravity calculations,
where appropriate.
11.27.7Tugs, towlines and towing connections
11.27.7.1
Tugs shall be selected in general accordance with [11.12] as applicable for:
weather unrestricted towages
24­hour or location moves.
11.27.7.2
The particular requirements for manoeuvring on and off location should be taken into
account when selecting the towing fleet, unless additional tugs are used for manoeuvring.
Additional tugs should be connected when in congested waters or when approaching a lee
shore when there may not be sufficient time to reconnect a tug after a broken towline or
breakdown in the forecast weather conditions.
11.27.7.3
Towlines and towing connection MBLs shall, as a minimum, be in accordance with [11.13].
The cautions in [11.13.3.12] (for vertical loads) and [11.13.3.13] (for larger tugs) shall be
noted.
11.27.8Securing of legs
11.27.8.1
For weather unrestricted voyages, legs shall be properly secured against excessive horizontal
movement by means of shimming in the upper and lower guides, or by means of an
approved locking device. Shim material specification should take into account the pressures
expected, particularly for units with guides having a small contact area.
11.27.8.2
For 24­hour and location moves, leg position and securing arrangements shall be agreed,
and shall comply with designers’ recommendations.
11.27.8.3
For electric jacking systems, all motors should be checked for torque and equalised in
accordance with manufacturers’ instructions.
11.27.8.4
Hydraulic and pneumatic jacking systems shall be secured in accordance with manufacturers’
recommendations.
11.27.8.5
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For jacking systems fitted with elastomeric pads, clearances should be shimmed or pre­load
applied in accordance with the manufacturer’s specifications.
11.27.8.6
For tilt­leg jacking systems, tie bars shall be fitted to by­pass the tilt mechanism.
11.27.8.7
Where lowering of legs or jacking on a stand­by location is envisaged during the towage,
then any leg securing arrangements shall be quickly removable.
11.27.8.8
Where a critical motion curve, or equivalent limitation, is provided for the legs, it can be
necessary to lower the legs in order to comply. Instructions and limitations for this operation
shall be clearly defined in the Operations Manual, taking into account any lesser motion
limitation during the lowering operation. The lowering operation shall be carried out well
before the onset of forecast bad weather.
11.27.9Drilling derrick, substructure and cantilever
11.27.9.1
The drilling derrick, substructure and cantilever shall be shown to be capable of withstanding
the motions as derived from [5.2] and [11.3]. For 24­hour and location moves the crown
block can be left in place. For weather unrestricted voyages the derrick shall be considered in
the condition proposed for the voyage, with the crown block lowered if required. Other
machinery and equipment are to be similarly considered.
11.27.9.2
For weather unrestricted voyages and location moves, no setback shall be carried.
11.27.9.3
For 24­hour moves, towage with setback in the derrick can be considered, provided it can be
demonstrated that all of the following apply:
The derrick, with the setback proposed and after suitable allowance for wear, corrosion
or fatigue, can withstand the motion criteria derived from [11.3].
All pipes, collars and other equipment racked in the derrick are secured to meet the
same criteria.
The seabed conditions at the arrival location are confirmed as presenting virtually zero
risk of a punch­through.
The stability of the unit can meet the requirements of [11.27.6].
The carriage of setback in the derrick is clearly documented. The limitations thereof,
the securing method, and any special precautions shall be clearly stated.
11.27.9.4
For weather unrestricted voyages the travelling block and/or top drive should be lowered and
secured. The drill line should be tightened, and secured against movement.
11.27.9.5
The cantilever and substructure shall be skidded to their approved positions for tow, and
secured in accordance with manufacturers’ recommendations.
11.27.10Helideck
11.27.10.1
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11.27.10.1
For weather unrestricted towages, it shall be shown that at an inclination in still water of 20°
about any horizontal axis, no part of the helideck plating or framing is immersed.
11.27.10.2
Alternatively, model tests can be used to demonstrate that the helideck remains at least
1.5 m clear of wave action, in any sea state up to the design sea state as defined in [3.1].
11.27.10.3
If neither [11.27.10.1] nor [11.27.10.2] can be satisfied, then all or part of the helideck
shall be removed for the towage.
11.27.11Securing of equipment and solid variable load
11.27.11.1
Weight of equipment variable load carried on board shall not exceed the maximum variable
load allowed for jacking.
11.27.11.2
All items of equipment above and below decks shall be secured to resist the motions
indicated in [11.3].
11.27.11.3
For 24­hour and location moves, drill pipe, collars and other tubulars shall be properly
stowed on the pipe deck and in the bays provided with stanchions erected. Chain lashings
over each stack shall be used. See also [11.9.9].
11.27.11.4
For weather unrestricted voyages, drill pipe, collars and other tubulars shall be stowed in the
pipe racks to a height above the rack beams of no more than 1.8 m. Drill pipes should
normally be stowed on top of collars. Timber battens should be placed between each layer of
pipe. See also [11.9.9].
11.27.11.5
For weather unrestricted voyages, the well logging unit shall be secured in position and stops
fitted to prevent rotation.
11.27.11.6
All crane and lifting derrick booms shall be laid in secure boom rests. For weather
unrestricted voyages, the booms should be shimmed or wedged against transverse and
vertical movements, but left free to move axially. Fitted brake systems for prevention of
crane rotation shall be implemented. Electric power shall be isolated at the main
switchboard. Cranes shall not be used at sea except in an emergency.
11.27.11.7
Deepwell and leg well pumps shall be fully raised and secured.
11.27.12Spudcans
11.27.12.1
For 24­hour and location moves, the spudcans should normally be full. See also [11.27.6.6]
for stability calculations.
11.27.12.2
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11.27.12.2
For weather unrestricted towages, the spudcans may be full or empty­ see [11.27.6.6]. If
empty, and if the towage procedures call for lowering of legs (see [11.27.8.7]), then the
lowering procedures shall include procedures for filling the spudcans.
11.27.12.3
For dry transports, the spudcans should be empty and vented. Safety notices should be
posted at each spudcan, and at the control panel.
11.27.13Pumping arrangements
11.27.13.1
For units towed on their own buoyancy, the general pumping requirements of [11.15] shall
apply. The requirements of [11.27.13.2] and [11.27.13.3] shall also apply.
11.27.13.2
All spaces should be capable of being pumped by the unit’s own pumping systems.
Sufficient generator capacity should be available to operate bilge and ballast systems
simultaneously.
11.27.13.3
Additionally for weather unrestricted towages, 2 no x 3 inch portable, self­contained, self­
priming salvage pumps shall be on board, with not less than 30 m each of suction and
delivery hose.
11.27.14Manning
11.27.14.1
Jack­ups towed on their own buoyancy should usually be manned, and the general manning
requirements of [11.17] shall apply. In general the jack­up should be down­manned to the
minimum complement essential for the safe conduct of the jack­up move, including jacking
operations.
11.27.14.2
Jack­ups transported on a barge or vessel need not be manned. However, it can be
advantageous for person(s) familiar with the unit’s structure, machinery and systems to be
on board the tug or the transport vessel, and to inspect the unit periodically.
11.27.15Protection of machinery
11.27.15.1
Where practical, and where the unit is manned, main and auxiliary machinery should be run
periodically during the voyage.
11.27.15.2
For weather unrestricted voyage, electrical equipment which cannot be run, including
motors, switchgear and junction boxes, should have dehumidifying chemicals placed inside,
and then be wrapped against wetting damage. Heaters, where fitted, should be run
periodically.
Guidance note:
Additionally, instructions from the manufacturer should be followed.
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11.27.16Anchors
11.27.16.1
The general emergency anchor/risk assessment requirements of [11.16] shall apply.
11.27.16.2
For weather unrestricted towages where anchors are fitted, the forward anchors should
normally be removed, and secured on deck. The aft anchors should be left in place and
stopped on the racks to prevent lateral movement. A retaining wire tightened by a
turnbuckle and incorporating a quick­release system should be passed through the anchor
shackle and secured on deck. The turnbuckle and quick­release system shall be on deck and
accessible.
11.27.17Safety equipment
11.27.17.1
For towages on a unit’s own buoyancy, safety equipment in accordance with SOLAS, /92/,
and any or all regulations for Life Saving Appliances and Fire­Fighting Equipment shall be
carried. Consideration should be given to any additional safety and emergency equipment
listed in [11.17.4.1].
11.27.17.2
For weather unrestricted towages, it can be necessary to relocate life rafts stowed forward or
overboard to a secure area protected from wave action. Securing arrangements for life rafts
stowed aft should be checked.
11.27.18Use of a stand­by location
11.27.18.1
Where the towage or location move includes the possibility of jacking up at any intermediate
location, suitable procedures shall be written to cover location feasibility, pre­loading
requirements, air gap requirements, local permissions/clearances and Customs formalities,
etc.
11.27.18.2
Consideration should be made of fitting scanning sonar on forward leg(s) to be used to check
for major debris when approaching any stand­by locations which may not have been
surveyed recently.
11.27.18.3
Such stand­by locations should typically be every 12 hours towing time along the route, with
one near the final location if there is a platform there, and be pre­approved by the MWS
company. Additional emergency jacking locations can also be identified for extra safety in
case of problems, but these will typically not be approved by the MWS company in advance.
11.27.18.4
In many cases the stand­by locations are “owned” by other concession holders or authorities
who shall be consulted in advance to ensure that they can be used.
11.27.18.5
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As a minimum, the following shall be documented for each proposed stand­by location:
Specific procedures, if required, for approach.
Sufficient information to satisfy the location approval requirements in DNVGL­ST­N002
­ Site specific assessment of mobile offshore units, /39/.
11.28Approaching a jack­up location
11.28.1Background
11.28.1.1
The rig move procedures shall include justification of the stationkeeping method to be used
for the approach and show that control of the unit can be maintained in case of any single
failure of the chosen positioning system (tug, towing gear, DP system, jacking system,
mooring system or communications, etc.). It shall be demonstrated that such a failure will
not result in damage to the unit or any nearby assets for all conditions up to the operational
limiting environmental criteria, (OPLIM­ see [2.6.8]). The justification will normally be a risk
assessment, in accordance with [2.4]. Justification for approaching a platform shall include
the extra items in [11.28.8] and for approaching a live asset (including live pipelines) in
[11.28.9].
11.28.1.2
Stationkeeping is normally achieved by use of tugs only and/or “soft­pinning” and “leg
dragging”, dynamic positioning or mooring.
Guidance note:
The common stationkeeping methods are briefly described below:
Use of tugs only – main tugs, and possibly smaller more manoeuvrable tugs, hold the
unit on location while lowering the legs. This is often used at open sea sites with no
other infrastructure or obstacles and with steady currents.
“Soft­pinning” – the legs are partially lowered into the seabed close to the final
location. It is usually used as a stage (80 ­100 metres from a platform) in order to
‘run’ anchors or stabilise the rig.
“Leg dragging” – from the soft pin position the tugs are used to overcome soil
resistance (which acts as a brake) while manoeuvring onto the final location.
Dynamic positioning – the unit’s own thrusters are used to keep station under control
of a dynamic positioning system.
Mooring – the unit’s anchors and winches are used to assist in stationkeeping and to
prevent sudden or unexpected movements. If there are many pipelines in the area or
there is poor anchor holding then prelaid anchors or anchor piles can be used.
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11.28.1.3
As a minimum, the following shall be documented, as part of the rig move procedures, for
approval for approaching all jack­up locations:
Stationkeeping procedures , calculations and drawings (with risk assessments as
required)
Contingency plans (with risk assessments as required)
Documentation confirming platforms status; live and producing, hydrocarbons above
seabed, fully shut down situations
Emergency response plan.
11.28.1.4
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11.28.1.4
In addition to the information in [11.28.1.3], as a minimum, the following shall be
documented for jack­ups approaching platform locations with live wells, risers or
hydrocarbon processing equipment:
Emergency shutdown procedures (for live and producing platforms).
11.28.1.5
The approved rig move procedures shall be discussed and agreed at the pre­move meetings
on the unit.
11.28.2Requirements for use of tugs only
11.28.2.1
The tugs shall be able to hold the jack­up stationary at the correct heading and correct
location in the agreed operational limiting environmental criteria, (OPLIM), for the approach
before the legs/spudcans engage with the seabed.
11.28.2.2
If approaching another asset (e.g. a platform or well head):
the tugs shall have sufficient power and manoeuvrability to control the unit in the
agreed operational limiting environmental criteria, (OPLIM), after any (worst case)
single failure of tug, gear or control systems
the measured current speed and direction shall be checked against predictions before
the start of the approach. The approach shall not have the current flowing towards the
other asset.
11.28.3Requirements for use of “soft­pinning”
11.28.3.1
Soft­pinning shall only be used under the following conditions:
The site within which the soft­pin location is placed has been assessed by the MWS
company and found not to exhibit lateral variability or vertical layering that could give
rise to rapid penetration during soft­pinning.
A suitable weather window exists to allow for the running of anchors, pre­loading and
jacking up.
If the anchors are being run, the jacking panel to be manned at all times to maintain
the hull in the water.
11.28.3.2
If approaching a soft­pin location near a platform in the dark the following additional
requirements will apply.
At least one lead tug plus 2 stern tugs with adequate bollard pull attached to the (aft)
quarters
Platform to be adequately lit by its own power, with back­up, and not likely to dazzle or
blind an approaching rig crew
Platform shut in with lines depressurized
Very calm weather and sea conditions. Good visibility. No rain or squalls in the vicinity
or forecast for the approach period with contingencies. Predictable low and steady
current, not flowing towards the platform during the approach. Maximum wind, wave,
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current and visibility criteria to be documented in the rig move procedures.
No pipelines or cables close to the platform or soft­pin location that would require the
legs to be kept at higher clearance and thus extend the time to reach the seabed
100 m minimum separation between the rig and the platform or any other subsea
assets apart from pipelines or cables already covered in e)
Tugmasters to be confident of their own and their crews’ ability to carry out such an
operation and in agreement to carry out a night approach.
Towmaster to be confident and in agreement with a night approach and be able to
make judgements on visible (relative) bearings rather than relying on the navigation
displays.
Contingency /back­up in place in case of sudden power or other equipment failure on
any tug (especially the lead tug)
A nominated vessel to constantly check the distance from the rig to the platform by
radar and advise the towmaster immediately if there is any significant discrepancy
between it and the planned or navigation package readings.
Navigation package to be working properly with suitable back­up systems
Company representative, Rig OIM, Platform OIM, Rig Manager, MWS company
surveyor and any other key personnel to be in full agreement for a night approach after
consideration of the above items
Any one of the key personnel to have the right to abort the night approach if the safety
of the rig, platform or crew appear to be compromised
Priority to be given to the safety of the rig, platform or crew rather than commercial
considerations.
11.28.4Requirements for use of leg “dragging”
11.28.4.1
Dragging of legs shall only be done under the following conditions:
1. The leg strength has been confirmed as suitable for leg “dragging” and any
restrictions, maximum allowable speeds or maximum forces documented.
2. The soils have been assessed by the MWS company and found to be sufficiently soft
and consistent (e.g. with no debris or boulders) to allow the legs to be dragged gently
through the upper layer(s) without damage to the legs.
3. The seabed is sufficiently flat that none of the pinned legs can lift out of the seabed
with any likely roll and pitch during the approach.
4. There are no pipelines, other subsea assets or obstructions that could be damaged by,
or cause damage to, the legs.
5. The tugs’ power shall be controlled to maintain both:
the towing speed to not more than 0.1 m/s (0.2 knots) or the maximum
allowable speed from 1)], if less,
the total net towing force to not more than any maximum in 1).
11.28.5Requirements for use of DP
11.28.5.1
The dynamic positioning requirements in [17.13] shall apply.
11.28.6Requirements for use of moorings
11.28.6.1
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The capacity of the mooring system to hold and control the unit in the agreed operational
limiting environmental criteria, (OPLIM), for the approach shall be demonstrated in the
Operations Manual or other documentation agreed with the MWS company before the move.
11.28.6.2
The soil conditions at the location shall be shown to be suitable to provide adequate anchor
holding power before the approach is started. Relevant sections of Sec.17 shall apply.
Guidance note:
This may be done by pre­loading each anchor to an agreed value ­ typically 100+ tonnes for
pre­laid anchors, or the winch stall tension for the unit’s anchors. If the stall tension is not
sufficient then the OPLIM may need to be reduced.
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11.28.6.3
The horizontal and vertical clearances of mooring lines and anchors to other assets shall be in
accordance with [17.7], using suitable buoys if required to provide vertical clearances.
11.28.7Required clearances on approach
11.28.7.1
ROVs or divers (subject to risk assessments) shall be used to check clearances before final
leg lowering if pipelines or other subsea assets are within 25 m of legs or spudcans.
Guidance note:
The need to use ROVs or divers needs to be communicated to the relevant personnel in
sufficient time to mobilise them.
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11.28.7.2
Legs or spudcans shall not be placed within an unsafe distance from any pipeline, cable or
other subsea asset. The safe distance depends primarily on the nature of the soils supporting
both leg/spudcan and the other asset and shall be agreed with the MWS company when the
location is approved (see DNVGL­ST­N002 ­ Site specific assessment of mobile offshore
units, /39/). Any distance from the outer edge of the spudcan to the nearest edge of an
asset of less than one spudcan’s diameter (or twice the predicted penetration when this is
less), shall be studied further and the outcome agreed with the MWS company.
11.28.7.3
No part of the jack­up or tugs shall be planned to approach within 10 metres, plus any jack­
up motion allowances, of a live platform (or the platform operator’s required distance, if
greater) unless specifically accepted in a risk assessment in accordance with [2.4].
11.28.8Approaching a platform
11.28.8.1
Moving into a platform is considered to start when entering the safety zone and be complete
when the unit has fully pre­loaded its foundations and is at its final air gap. Moving away is
considered to start with the start of jacking down and considered complete when the last
vessel or unit has left the safety zone.
11.28.8.2
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A risk assessment shall be completed before the move, in accordance with [2.4], preferably
with the attendance of the towmaster, OIM and MWS company representative and be
accepted by the MWS company. It shall ensure that all the items in the following queries
have been addressed satisfactorily for approaching any platform:
Do all involved units including tugs and their captains have a history of reliable
operations with good communications?
Will the jack­up be pinned at a stand­off position before making a final close approach
to the platform?
Will there be a mooring system with redundancy in case of any one line or tug failure
used as a means of controlling the final approach?
Will the weather conditions and currents for the approach be measured and shown to
be well within the design capacity of the stationkeeping system after the worst single
failure?
If there is any risk of punch­through/ Has there been an impact assessment of contact
between Jack­up and Platform as a result of a punch­through, sliding or interaction
with existing footprints, and is it acceptable to the MWS company?
Have detailed Rig Move Procedures, including Management of Change (MOC) process,
been accepted by the MWS company surveyor before any attending surveyor joins the
unit?
Has there been a shore side Risk Assessment, specific to the operation agreed by the
MWS company? (It is to be conducted a minimum of three days before the start of the
operation and the MWS company shall attend and confirm acceptability of the Risk
Assessment findings)
Have arrangements been made to ensure that other key personnel will be suitably
rested before the approach to and departure from the platform? This can require 24
hour coverage for key positions, such as tow masters.
Any one of the key personnel to have the right to abort the night approach if the safety
of the rig, platform or crew appear to be compromised.
Are an Emergency Response Plan and Communication protocols in place for SIMOPs?
Are all 3rd Party (including platform and pipeline owners) written consents in place?
Have any other risks due to local conditions been identified and addressed?
Have all Risk Assessment action items been closed out?
11.28.8.3
If approaching platform in darkness. This is not considered to be good practice in most
areas, especially non­benign weather ones. However, if proposed, the following items shall be
considered in addition to those in [11.28.3.2] and [11.28.8.2] for low risk moves only.
Platform not to be “live” and with all hydrocarbons bled off and pipelines depressurized
Mooring system to be used with adequate redundancy in case of failure of any anchor,
line, winch, tug or towline. Mooring system to be designed for the maximum
operational criteria and anchors to be preloaded (or otherwise checked) in advance to
at least the maximum operational loads.
No congestion in the area
No pipelines or other subsea assets that could be damaged on either side of the rig
approach or escape route in case of abort, or by dragging anchors.
High confidence in the survey /navigation package and operators
Adequate mitigating measures in place.
11.28.9Approaching a live platform or pipeline
11.28.9.1
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Before positioning a jack­up close to any pipeline, full consideration shall be given to
depressurizing pipelines within 50 metres of the spudcans to reduce the risk of hydrocarbon
release in the event of a collision. The minimum distances depend on soil conditions, method
of positioning and should be fully risk assessed. The risk assessment shall take account of
change management e.g. weather getting worse, jacking system failure, etc. ­ the time to
depressurise can be very long, so the pipeline(s) should be readied for the worst­case
scenario. In any event, pipelines within 5 metres of a spudcan at any time shall be
depressurized.
11.28.9.2
In general a unit should not move into or away from platforms with live wells, risers or
hydrocarbon processing equipment. A platform shall be considered live unless the MWS
company has seen documented evidence that the platform is not live.
11.28.9.3
Exemption from the requirements in [11.28.9.2] shall only be given after a risk assessment
in accordance with [2.4] showing that the risks are acceptable before the move starts. The
major risks following a collision which releases hydrocarbons from a riser, pipe or other vessel
containing hydrocarbons include:
fire and/or
explosion and/or
pollution.
11.28.9.4
This risk assessment shall ensure that all the items in the following queries have been
satisfactorily addressed, in addition to those in [11.28.8.2]:
Would the location be approvable by the MWS company if the platform were not live?
Have the quantities, locations and pressures of all hydrocarbons been identified?
Are all risers and hydrocarbon inventory well protected from a collision with the jack­up
or any attending vessels from any likely direction with an impact speed of at least 0.5
m/sec (or maximum operational current speed during approach or departure if
greater)?
Has a geotechnical engineer in the MWS company evaluated the location for risk of
punch­through or rapid penetration and rated the risk as no higher than VERY LOW, in
particular for the legs nearest the jacket if contact could arise during a rapid
penetration.
Is there an Emergency Shut Down system in place which will not be obstructed by the
Jack­up coming alongside? (Live and Producing can only be accepted where an effective
Emergency Shutdown System is in place; otherwise production shall cease before
approach or departure.)
Has the Platform Operator demonstrated that the cost/benefit of remaining live is
substantial? (This should be undertaken for the applicable case which will normally be
for either “live and producing” or “no production but with platform hydrocarbon
inventory above sea level”.)
Has down­manning the jack­up and platform been considered for the final approach?
11.29Rig move procedures (for all MOUs)
11.29.1
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Rig move procedures shall be accepted in advance by the MWS company and include as a
minimum:
Table 11­23 Rig move procedure contents
Ref
1
Item (* indicates that it is for jack­ups only, ++ for moored MOUs)
GENERAL
1.1
Introduction or operational summary describing the key features of the move
1.2
Roles & responsibilities of Towmaster, OIM, client representative and positioning
contractor
1.3
Contact names & contact details of Responsible Persons Ashore & relevant offshore
contacts
1.4
Management of Change procedures
1.5
Results from rig move HIRA (see [2.4] and [11.28] for jack­ups)
1.6
Contingency plans for emergencies
1.7
Common language (e.g. English) specified and assurance that relevant parties and
can all use it effectively.
1.8
Details of all necessary permissions to be obtained or already obtained
2
LOCATIONS
2.1
Co­ordinates of the departure & arrival locations with air gap and heading details
2.2
Unambiguous reference point for the co­ordinates specified, e.g. drill centre
2.3
Field plan for departure and arrival locations if applicable
2.4
* Expected leg penetration(s)
2.5
* Confirmation that pre­loading procedures comply with location CoA requirements
2.6
Required positioning tolerances at the arrival location indicated (heading, distance
from platform or target coordinates)
2.7
Details of navigation equipment needed for achieving the tolerances required
2.8
Confirmation that the hazards noted in the Location CoA have been addressed (e.g.
proximity to pipelines, foundation difficulties etc.) with minimum clearances & extra
equipment (like ROV’s) identified.
2.9
Any additional requirements due to proximity of operations to another party’s
assets (subsea or topside) identified.
2.10
Clearances from subsea assets (vertical at LAT and horizontal) identified for key
stages of the approach.
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2.11
Catenary curves for mooring lines, or other means of checking clearances over
subsea assets available (if applicable)
2.12
++ Mooring pattern drawings ­ as laid (for present location) and proposed (for
destination) clearly showing exclusion zones and clearances (as applicable) from
subsea assets or obstructions.
2.13
Any restrictions on allowable rig motions or daylight /visibility for approaching the
location
3
ROUTE & TOWAGE
3.1
Voyage plan and suitable tow route
3.2
Details of stand­by locations needed en­route together with the necessary data
(soils, water depth, etc.)
3.2
Confirmation that permissions are in place for the stand­by locations
3.3
Bollard pull requirements and the tug names &/or specifications identified & tugs
matched (if required)
3.4
Evidence that there be sufficient tugs to control the tow and manoeuvre at either
end
3.5
Clearances from subsea assets (vertical at LAT and horizontal) identified for key
stages of the tow (allowing for rig motions in sea/swell)
3.6
Catenary management to ensure towline clearances from subsea assets as in 3.5
during tow and mooring clearances if applicable at departure and arrival
3.7
Confirmation that updated charts and current tide tables will be available on the rig
and lead tug
4
RIG
4.1
Confirmation that the latest updated MOU Marine Ops Manual in the MWS company
office
4.2
General description of the rig being moved
4.3
Any special features of the rig which may affect the move identified
4.4
Confirmation that mud pits will be empty or that contents are accordance with
Marine Ops Manual and acceptable to the MWS company.
4.5
Details of what will be within derrick and confirmation that this is allowed in Marine
Ops Manual
4.6
A suitable person identified on the rig, available for consultation on procedures
(e.g. for equipment information, review procedures, etc.)
5
MOORINGS (if applicable)
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5.1
Anchor plan showing the scopes of the mooring lines and the anchor coordinates
5.2
Proposed mooring pattern and anchor fluke angles (matching the supplied mooring
analysis)
5.3
++ Detailed step by step anchor recovery and deployment procedures, supported
with sketches and drawings as applicable.
5.4
Confirmation that proposed mooring can be installed without damage to the
mooring equipment (Particularly important with fibre ropes)
5.5
Test tension arrangements shown to be in­line with the requirements of the
mooring code used in the mooring analysis. If not are alternative procedures
acceptable?
5.6
Times required (if any) for anchor “soaking” or bedding in anchors
5.7
Details of any midline buoys, fibre rope inserts or chain extensions identified on the
anchor plan
5.8
Equipment lists, towing arrangements and anchor spread jewellery
5.9
Mooring make up diagram for each mooring line
5.10
Information regarding any existing mooring make up available and verified with
rig/shore management
5.11
++ Identify and detail any skidding requirements at any stage of the unmooring or
mooring operations.
6
WEATHER
6.1
Weather routing requirements and details (if applicable)
6.2
Weather forecast requirements & sources identified
6.3
* Limiting criteria for jacking
6.4
Limiting environmental or motion criteria for towing
6.5
Adverse weather procedures for each phase
7
OTHER
7.1
Confirmation that Notifications to Authorities have been or will be issued
7.2
Activity list for the move which shows the estimated time for each operation
7.3
Any client­specific requirements
7.4
Any peculiarities or inconsistencies e.g. wrt to mooring equipment /make­up/or
capabilities identified from the most recent rig move report for the unit
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7.5
8
Evidence that procedures been reviewed by appropriate marine personnel from the
rig management (ashore and offshore) and that any resulting comments have been
addressed
APPROACHING A LOCATION
8.2
* Summary of results of risk assessments to comply with [11.28], including
[11.28.8] if approaching platform or adjacent soft­pin location and [11.28.9] if
approaching a live platform or pipeline. These shall show that the risks are
acceptable to the MWS company, and include any resulting conditions, including
any required depressurizations.
8.2
* Procedures to comply with the results from 8.1 above
8.3
If approaching a live platform or pipeline, details of ESD for all relevant systems
with contact details for confirmation that the systems are in correct conditions at
the time of approach or departure.
11.30Specific for semi­submersible voyages
11.30.1General
11.30.1.1
This section gives requirements specific for semi­submersible MOUs not already covered in
this standard.
11.30.1.2
“Guidelines for Offshore Marine Operations (G­OMO)” /69/, chapter 11 describes good
practice for Anchor handling and MOU moves. The document is intended for use worldwide
and regional supplements are being developed and implemented separately.
11.30.1.3
The requirement summary for semi­submersible rig move procedures is given in [11.29] and
requirements for approaching locations are given in [11.30.9].
11.30.1.4
Generally wet towages of semisubmersible units are undertaken at transit draught with a
large air gap. However should the towage be undertaken at a deeper draught, arrangements
should be in place to ensure that a wave of 10% higher than a 50 year return period
maximum wave (or any wave of lesser height or period) will not strike the underside of the
deck or any other vulnerable item.
11.30.1.5
A risk assessment shall be carried out, in accordance with [2.4], to demonstrate the
acceptability of the proposed arrangements.
11.30.2Loadings and strength
11.30.2.1
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The drilling derrick, substructure and associated structures shall be capable of withstanding,
with the maximum pipe setback allowed in the operating manual, the loads caused by 10
year return period 1 minute sustained wind and either:
Those motions which model tests or motion analysis indicate to be a maximum at
towage draught, in conditions up to the 10 year return period; or
Those motions which structural calculations show to be the limiting motion for the
main hull structure ; or
Those motions shown in the operating manual to be the limiting condition for
remaining at towage draught.
11.30.2.2
Seafastenings shall be designed to sustain loadings as determined in accordance with
[11.30.2.1]. They shall be provided at strong points on deck and with certified equipment.
Guidance note:
Examples of Strong Points are;
Pad eyes
I­beams.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.30.3Stability and watertight integrity – wet towages
11.30.3.1
The following practical considerations shall apply in addition to the requirements in [11.10]
11.30.3.2
The stability shall be calculated for the following conditions:
Departure (transit draught)
An Intermediate position if duration of tow more than 72 hrs.
Arrival (transit draught)
and
A deep draught / survival draught condition that the unit would ballast down to during
the transit in the event of encountering heavy weather.
11.30.4Tugs and towing equipment
11.30.4.1
Tugs shall be selected in accordance with 11.12] and their towing equipment with [11.13].
When the tugs are also used for anchor handling on the move, their specific anchor handling
capability and capacity shall be assessed as part of the suitability/on hire inspection as per
[11.11.2]. These specific requirements should be included in the Rig Move procedures/Work
Specification as described in “Guidelines for Offshore Marine Operations (G­OMO)” /69/.
11.30.4.2
If the towage is to be undertaken using the rig’s mooring chains, the strength of the system
including chains, fairleads, winches and foundations shall be shown to comply with
[11.13.3.4]. Arrangements shall be made to ensure that any part of the tow connections
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that cannot pass through the fairlead is at a safe distance from the fairlead. This is
particularly important for deep draught rig moves using rig’s mooring line to tow instead of
the bridle.
Guidance note:
The footage counters measuring the amount of mooring line deployed should only be
considered accurate if they coincide with other indicators e.g. AHV distance off, taut chain,
and marking of chain
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.30.4.3
When more than one towing tug is used the requirements in [11.18.7] shall apply.
11.30.4.4
Arrangements shall be in place to ensure catenary management of the tow line during tow
particularly during departure and arrival at locations, to ensure appropriate vertical
clearances are maintained from subsea infrastructure.
11.30.5Anchors
11.30.5.1
The general emergency anchor/risk assessment requirements of [11.16] shall apply.
11.30.5.2
Anchors shall be racked on the bolster (or other alternative methods as per the Unit’s Marine
Operations Manual) and secured prior to the tow.
11.30.5.3
During deep draught in field rig moves, due to the anchor bolster being below water level,
anchors should normally be removed together with the permanent chasing pennant (PCP)
system if fitted and the chain tail handed back. The effect of this with respect to emergency
anchors in [11.16] and stability in [11.10] and [11.30.3] shall be considered. Hanging of
anchors down clear of the rig’s hull should only be considered as an exceptional measure.
This shall be subject to a separate assessment for MWS approval. In any case adjacent
anchors shall not be hung below on the same corner.
11.30.6Deep draught
11.30.6.1
There may be occasions when a semi­submersible unit would be required to be moved at
deep draught to improve operational efficiency. Deep draught is any draught where the
fairleads and/or anchor racks are not clearly visible.
11.30.6.2
Though primarily this has time saving and commercial benefits, i.e. there is large savings in
avoiding the double handling of cargo and at deep draught the rig is a steady platform for
other crane work e.g. especially pendant wire handling and this improves safety.
11.30.6.3
A deep draught rig move is typically undertaken for an infield move of short duration subject
to a positive result of a HIRA. The following shall be considered when a deep draught move
is to be undertaken:
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This shall not be expressly forbidden by the Units Operational Manual. Any specific
requirements in this aspect noted in the Operations Manual shall be considered for
such operations.
The rig move should start with a 5 day weather forecast covering recovering
anchors/moorings, tow and deploying anchors at destination.
Stability of the Unit [11.30.3].
Bollard Pull requirements at this draught [11.12.2].
Operational condition of the rigs ballasting system.
Weather limitations due to the requirements of this operation.
Securing/disconnection of anchors [11.30.5].
Emergency arrangements in case of failure of tow– e.g. emergency towing
arrangements, escort vessels, etc.
Guidance note:
Typically Beaufort 6 i.e. wind 22­27 knots and wave height of 3 m should be considered as
an upper limit weather condition for deep draught rig moving.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.30.7Transport or tow plan
11.30.7.1
A transport or tow plan as in [11.31.2] shall be completed for all voyages and shall
comprehensively address as aspects of the planned operation. For infield moves this may
form part of the rig move procedures.
11.30.8Requirements for use of DP
11.30.8.1
The dynamic positioning requirements in [17.13] shall apply.
11.30.9Approaches to location
11.30.9.1
The rig move procedures shall include details of the approach including any precautions and
preparations required as applicable.
Guidance note:
Some of the items that would typically be included would be;
Notification and approval requirements at arrival locations.
Shortening of tow lines and catenary management.
Identify distance or position of end of tow and commencement for approach.
Approach methodology. Typical approach speed is 1.5 knots when arriving at offshore
location for mooring.
Surface and subsurface obstructions and/or hazards.
Port arrival procedures including pilotage if applicable.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
11.30.9.2
For towed units, the tight tow should be suspended at a safe distance from the arrival
location. For self­propelled units the sea passage should be suspended at this time.
11.30.9.3
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The approach to the location and run­in on the first anchor should be undertaken at a
controlled speed and normally achieved by one AHV on the bridle and another on the run­in
anchor. For arrival in port/ quayside use of harbour tugs should be considered.
11.30.9.4
In any case the AHVs connected shall have sufficient power and manoeuvrability to control
the unit in the agreed environmental criteria.
11.30.9.5
The capacity of the mooring system to hold and control the unit in the agreed operational
limiting environmental criteria, (OPLIM), for the approach shall be demonstrated in the
Operations Manual or other documentation agreed with the MWS company.
11.30.9.6
All approach procedures shall be included in the rig move risk assessment, [11.30.1.5]
11.30.9.7
Generally arrival of semi­submersible units (towed or self­propelled) are not restricted to day
light operations only. This will largely be dictated by the operator/duty holder/authority who
provides the necessary consent to approach the location. It is generally not considered good
practice to make an approach in hours of darkness in all especially non­benign weather ones,
when the final location is in close proximity to a live platform. However, if proposed, the
following items shall be considered;
Tight position tolerances with respect to proximity to other surface assets.
SIMOPS or COMOPS.
Availability and reliability of the unit’s propulsion or thruster assist systems.
General meteorological visibility at that time.
11.31Information required
11.31.1General
11.31.1.1
The initial information in [11.31.2] to [11.31.9] is normally required for an approval of a
voyage by a MWS company. However it can vary depending on the size and complexity of
the project. Many items may need to be developed by, or agreed with the MWS company
once sufficient information is available, especially when innovative equipment or procedures
are proposed.
11.31.2Transport or towing manual
11.31.2.1
A transport or towing manual is required for all voyages for the following reasons:
It shall provide the Master with the key information that he needs, including the cargo
and route.
It shall describe the structural and any other limitations of the cargo.
It shall summarise contingency plans in the event of an emergency including contact
details.
It shall give approving bodies the key information that they require for approval.
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It shall define the responsibilities of different parties if parts of the transport/tow and
installation are performed by different contractors. The scope split between the
contractors shall be clearly defined, to ensure that all parties are aware of their
responsibilities, handover points and reporting lines.
11.31.2.2
The purpose of the transport or towing manual described in [11.31.2] is to give to:
the vessel or tug Master and
Persons In Charge (PIC), or Responsible Persons ashore for emergency response
planning in the event of an incident or accident,
information about:
The cargo,
Routeing, including possible deviations to shelter points if required,
What to do in an emergency,
Contact details (client, owner, local authorities, MWS company and MWS company
surveyor etc.),
Organogram showing the scope split between different contractors (if applicable).
These shall be clearly defined, to ensure that all parties are aware of their
responsibilities, handover points and reporting lines.
11.31.2.3
The contents of the manual shall be in a form and language that can be clearly understood
by the Master and senior officers undertaking the operations. Revisions should be clearly
marked and attached drawings, with their revision numbers noted in the main text.
11.31.2.4
Where a manual has been produced to satisfy local authority requirements then this should
take precedence, providing it satisfies the main requirements detailed below.
11.31.2.5
The list below is what the MWS company would expect to see in the transport or towage
manual. The list also includes the essential details needed by the vessel’s Master. Detailed
calculations and other documents can be in separate manuals referenced in the transport or
towage manual.
Introduction. What is the cargo, where is it being transported or towed, who for and
why.
Description of the vessel and cargo.
Proposed route (with plot or chart) including waypoints and any refuelling
arrangements, anticipated departure date, speed and ETA (Estimated Time or date of
Arrival).
Departure procedures.
Metocean criteria for the route for anticipated departure date (unless using default
motions).
Any limiting criteria and motions (roll, pitch and period etc.) for the transport or tow,
weather forecasting arrangements and weather routeing details if applicable.
Contact details and responsibilities.
Communication details with communication chart.
Reporting details: who to, how often and content.
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Summary of ballast conditions and stability (usually including anticipated departure
and arrival loading conditions) with corresponding stability calculations and GZ curves,
including any ballasting required for loading or discharging where applicable.
Motions and strength – detailed supporting calculations for the motions and
accelerations, longitudinal strength and strength of the seafastening and
cribbing/grillage.
Arrival details including procedures, contacts, field plan etc.
Contingency arrangements and planning, with ports of refuge (and limitations on their
use).
Drawings to include, where applicable, cargo, GA and other key drawings of vessel and
cargo, stowage plan, towing arrangement, cribbing/grillage arrangement, load­
out/discharge plan, seafastening arrangement, guidepost details etc.
Reference documents.
Tug bollard pull calculation (if applicable).
Tug or transport vessel specification.
11.31.3Towed or transported object
11.31.3.1
Details of size, construction, age, condition, weight and Centre of Gravity.
11.31.3.2
For vessels: name and IMO Number (if available), certification and other documentation
available, including stability manual.
11.31.3.3
If manned, details of crew, lifesaving and fire­fighting equipment (see [11.17.4]).
11.31.4Towed Pipes, bundles, risers etc.
11.31.4.1
For towed pipes, bundles, risers (see [11.25]):
Detailed route survey from launch to installation target area including lay­down,
parking and stand­by areas along the tow corridor
Drawings and specifications of carrier pipe, spacers, diaphragms and bulkheads
Drawings and specification of towing and trailing heads
Calculation of pipe, towing head and trailing head weight and buoyancy
Details of additional buoyancy and buoyancy control devices such as drag chains
Drawings, specification and certification of all padeyes and towing gear (including
emergency tow gear)
Details of tow and trail tugs, guard and command vessels, launching vessels (such as
pull barges), any special equipment, and manning arrangements
Towing procedures, including contingency procedures
Details of all positioning and tow monitoring equipment
Drawings and specifications of all land based works including soil conditions,
foundations, rollers or track ways and trolleys, support structures, etc.
Launch procedures
Trimming procedures
Lifting procedures and craneage for towing and trailing heads
Installation procedures at destination site
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Launch analysis and details of software used
Dynamic towage analysis and details of software used
Installation analysis and details of software used.
11.31.5Transport vessel/barge and/or tugs
11.31.5.1
Names and IMO Numbers (if available).
11.31.5.2
Further details will need to be seen before or during suitability survey(s) by the MWS
Company as in [2.3.6.3]. In particular see:
[11.11] for transport vessels or barges
[11.12] for tugs and
[11.13] for towing equipment as applicable.
11.31.6Proposed route, season(s) and environmental criteria
11.31.6.1
The proposed route and season(s)
11.31.6.2
The environmental criteria need to be developed by, or agreed with, the MWS Company in
accordance with Sec.3 once the proposed route and season(s) are determined.
11.31.6.3
If towage in ice areas is proposed then details of ice classification, icebreaker support & other
items in [11.19].
11.31.7Strength
11.31.7.1
Analysis or specifications for strength of transported or towed object, grillage, seafastening
and overall strength. See Sec.5.
11.31.8Stability
11.31.8.1
Stability Manual or calculations for stability and damage stability. See [11.10].
11.31.9Other
11.31.9.1
Other information will be required for specialised transports or towages, especially for
multiple towages (see [11.18]), Towage in ice (see [11.19]). FPSOs and FSUs (see [11.21]),
Jackets (see [11.22]), Ship Towages (see [11.23]), Vessels for scrapping (see [11.24]),
Pipelines and other submerged tows (see [11.25]) and Jack­Ups (see [11.26] to [11.29]).
SECTION 12Tow out of dry­dock or
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SECTION 12Tow out of dry­dock or
building basin
12.1Introduction
12.1.1General and scope
12.1.1.1
This section gives the requirements for bringing afloat an object constructed in a
dry dock/building basin and its subsequent tow out.
12.1.2Revision history
12.1.2.1
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
DNV Offshore Standard, Load Transfer Operations, DNV­OS­H201.
12.2Dry dock/construction basin
12.2.1
Before the start of any marine operation, the dry dock/basin shall be cleaned, i.e. items that
my cause blockages shall be removed.
12.2.2
The dry dock/construction basin shall be acceptable to the MWS company as it relates to
marine operations including for mooring and winching loads..
12.2.3
Where the scope of the MWS company includes the construction phase in the basin then
paragraphs [12.2.4] to [12.2.8] should be considered.
12.2.4
The surrounding walls of any construction basin shall be designed and fabricated in
accordance with accepted civil and geotechnical engineering practice, standards, codes and
standards.
12.2.5
Where materials are used whose stability characteristics can be affected by a change in pore
water pressure, suitable monitoring devices shall be installed. The data shall be retrieved
and analysed by competent geotechnical engineers to ensure continuing stability of the walls
throughout the period of the platform construction including bringing afloat and tow out.
12.2.6
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Meteorological design criteria for the basin design shall be at least the 100 year independent
extremes.
12.2.7
Consideration shall be given to the design of the basin walls, including but not limited to the
following:
1. The integrity of the walls shall remain stable when subjected to:
The highest astronomical tide, plus
Storm surge, plus
Maximum wave crest height, corrected for shoaling and run­up if applicable.
2. The walls shall be of adequate height to prevent overtopping, except by spray, in the
conditions described in 1).
3. Walls shall be protected against the effects of collision, scour (including propeller scour)
and wave action.
4. The effects of ice loading shall be considered when applicable.
5. Mooring and winching loads.
6. Equipment/crane loads on the edges and around the edges of the basin.
12.2.8
Pumping capacity shall be adequate for maximum seepage, rainfall (including run­off from
surrounding land) and spray overtopping and allow for loss of pumps or power sources.
12.3Design and strength
12.3.1
Weight, CoG, buoyancy, CoB and associated envelopes of the object to be brought afloat
shall comply with the requirements of [2.11].
12.3.2
The object shall be designed to meet the applicable structural strength requirements of
[4.4.5.1] and [6.3].
12.3.3
All loads which can occur due to effects such as hydrostatic pressure, impacts, mooring,
guiding, pulling by tugs and winches, etc. including those resulting from the operation shall
be considered in the design of the object and in the planning of the operation. The value of
the loads should be determined considering the operational end equipment limitations.
Accidental loads shall also be considered based on possible failure modes.
12.3.4
The object’s ballast system shall meet the requirements specified in [4.3]. All piping should
be protected against the ingress of debris.
12.4Mooring and handling lines for tow­out
12.4.1
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If the structure is to remain afloat at moorings inside the dock or basin, then the moorings
shall be designed in accordance with Sec.17.
12.4.2
Handling lines and winching equipment shall be designed to withstand the design loads
arising, assuming that handling operations are weather restricted operations. All wires shall
be designed with a safety factor on certified MBLs of not less than 3.0 against the maximum
line load from manoeuvring and handling. Higher safety factors, to be agreed with MWS
company, shall apply to lines made of other materials. Connections to the structure and to
the shore shall be designed in accordance with Sec.17.
12.4.3
The positioning and towing systems shall be designed to manoeuvre the object at the
clearances required in [12.7].
12.5Intact & damage stability
12.5.1
For GBS type structures the requirements of [6.2] apply.
12.5.2
For all other objects the requirements of [11.10] apply.
12.6Under­keel clearance for leaving basin
12.6.1General
12.6.1.1
If the unit is likely to be at moorings in the dock or basin at maximum draught for any
significant time, the effects of siltation and negative surge shall be considered.
12.6.1.2
When moored in, or leaving the dock or basin, the unit shall have a minimum under­keel
clearance of at least 0.5 m considering minimum tide during planned operation period,
required operation reference period, possible roll and pitch and minimum surge.
12.6.1.3
The planned operation period (TPOP, excluding contingencies) of the tow­out should be
completed before high water.
12.6.1.4
A tide gauge shall be installed on site to check that actual tidal levels correspond to those
predicted.
12.6.1.5
At least 4 visible draught scales shall be painted on or fixed to the object.
12.6.2Air cushion
12.6.2.1
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Compressed air can be used to form an air cushion to increase buoyancy and reduce
draught, or to reduce bending moments. If an air cushion is used, the following shall be
considered:
All piping shall be secure, protected and of adequate capacity and strength (temporary
flexible hoses should be avoided, but can be accepted after risk assessment and
mitigations)
Supply lines shall have non­return valves
Back­ups shall be provided for all critical valves and piping
Adequate reserve compressors shall be available so that air leakage from the skirts is
less than 5% of the available compressor capacity. The air leakage shall be monitored.
A venting system shall be provided to guarantee that all air is removed after use, to
ensure no residual free surface remains
Sufficient water seal (bottom of air cushion above bottom of skirt) shall be available to
prevent air escaping. Typically this should be a minimum of 0.5 m.
The air cushion should be isolated in separate compartments, so that failure of any
part of the system does not cause a large heel or trim in addition to loss of buoyancy.
12.6.2.2
For temporary flexible hoses, [4.3.4.12] applies.
12.7Side clearances
12.7.1
Depending on the positional control of the unit during the exit of the basin, the channel
width at full depth should normally be not less than:
1.2 × B when inside the basin, and
2.0 × B when immediately outside of the basin,
where B = maximum object dimension normal to the direction of travel.
12.7.2
The required clearances can be reduced to 1.05 × B if the object is winched out along a
fendered guide.
12.7.3
The required clearances can need to be increased if tugs are used instead of winches for
control inside the basin.
12.8Under­keel clearance outside basin
12.8.1
Once outside the building basin the minimum under­keel clearance shall be as required by
[11.14.20] until final emplacement.
12.9Towage and marine considerations
12.9.1
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Tugs, towage and marine considerations for tow out of dock shall be in accordance with
Sec.11.
12.9.2
An exclusion zone for marine traffic should be agreed with the Port Authority and any other
relevant authorities.
12.10Information required
12.10.1Object
12.10.1.1
For the object:
Drawings including plans, elevations and details
Weight report
Compartmentation and ballasting data
Details of ballast and control systems, including manual and remote operation systems
and back­up systems, and compartment status­monitoring systems.
Structural strength and stability analyses covering all phases of construction
Ballasting calculations
12.10.2Towage
12.10.2.1
As per [11.30.9] (as applicable).
12.10.3Basin
12.10.3.1
For the basin:
Drawings of basin and route
Flooding procedure
Design Documents including details of criteria used.
SECTION 13Jacket installation operations
13.1Introduction
13.1.1General
13.1.1.1
This section covers the installation at the field location of both lifted and launched jackets.
13.1.1.2
Buoyancy tank removal and piling is also covered.
13.1.1.3
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With the exception of stability requirements, the tow of self­floating jackets is covered in
Sec.11. The intact and damaged stability requirements shall be agreed on a case­by­case
with MWS company.
13.1.1.4
A self­upending jacket is one that after launch rotates to a near­vertical attitude without an
intermediate horizontal phase. This can be achieved by distribution of buoyancy and/ or free­
flooding compartments.
13.1.1.5
For the purposes of this document launch is finished once the jacket has reached its free
floating positon (also known as Post Launch Equilibrium Position). Therefore for self­
upending jackets the launch requirements shall apply until the jacket is free floating in a
near­vertical attitude.
13.1.2Revision history
13.1.2.1
This section replaces the applicable sections of the following legacy documents:
DNV Offshore Standard, Offshore Installation Operations (VMO Standard Part 2­4),
DNV­OS­H204
GL Noble Denton, Guidelines for Steel Jacket Transportation & Installation, 0028/ND.
13.2Environmental conditions
13.2.1All phases – excluding on­bottom stability
13.2.1.1
For each phase of the installation, design environmental conditions shall be defined and
analyses documented demonstrating the feasibility of the installation. Where transfer of
personnel is required the operational limiting criteria and corresponding design
environmental conditions shall allow for this to be done safely.
13.2.1.2
The environmental conditions (monitored/observed and forecast) shall be such that proposed
operation can be completed in a well­controlled manner, in accordance with the design
assumptions and operational limitations associated with the objects involved. The visibility
shall also be sufficient to allow the operation to proceed.
13.2.1.3
The limiting current speed for launch and subsequent installation activities shall be
determined to ensure that the launched/floating jacket can be controlled after launch with
suitably sized tugs and/or control lines from the vessel(s).
13.2.1.4
The design current velocity shall be based on local statistical data and experiences. Unless
more detailed evaluations of current velocity are made the design current shall be the taken
as the 1 year return value for the location. For a weather restricted jacket installation
operation, it could be applicable to define the maximum operational limiting current velocity.
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In this case current predictions and monitoring during operation are necessary in order to
ensure that the maximum operational limiting current velocity is not exceeded during the
operation.
13.2.1.5
When determining the bollard pull or winch line requirements, current, wind and waves shall
be considered to act simultaneously and co­linearly.
Guidance note:
Where appropriate directionality could be considered.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
13.2.2Launch
13.2.2.1
The sea state for launch shall be limited to the lesser of:
The maximum sea state allowed by jacket stresses, dive depth, rocker arm reactions
and barge submergence, or
The sea state which permits safe operation of tugs and workboats and safe transfer of
personnel to or from the launch barge and rigging platforms.
13.2.2.2
The limiting wind speed for launch shall be compatible with the limiting sea state. This wind
speed shall be used in launch related stability checks. It shall be demonstrated that this
wind speed does not cause unacceptable heel angles or additional stresses during launch.
13.3Strength
13.3.1All Phases
13.3.1.1
The requirements of Sec.5 apply for all installation phases.
13.3.1.2
Strength checks for buoyancy tanks shall consider all applicable stages including voyage,
launch, installation and the various stages of removal. The strength check for launch shall
allow for a possible 10% deeper submergence of the tank than the maximum calculated with
a minimum of 5 m (unless a lower value can be justified). The maximum calculated
submergence shall consider the variations listed in [13.4.4.1].
Guidance note:
Where applicable pile driving should be considered as part of the installation (including
fatigue effects).
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
13.3.1.3
The connections between the buoyancy tank(s) and the launched object shall be designed to
withstand the hydrodynamic, inertial and buoyancy loads acting on them during launch (see
also[13.3.1.2]). A consequence factor of 1.3 should be applied to the primary steel
attachments for all load conditions.
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13.3.2Launch and upending
13.3.2.1
Launching shall be treated as an LS1 or ULS limit state operation subject to the
environmental conditions being suitable for launch and subsequent installation activities. For
the free floating position single compartment damage (ALS) shall be considered. Single
compartment damage (ALS) does not need to be considered for earlier stages of launch.
13.3.2.2
Upending and placement shall be treated as an LS1 or ULS limit state operation subject to
the environmental conditions being suitable. Single compartment damage (ALS) shall also
be considered.
13.3.3On­bottom
13.3.3.1
Jacket strength for the on­bottom condition should be treated as an LS2 or ULS limit state
for the same design environmental conditions as the on­bottom stability checks in [13.10.1].
All foreseen ballasting arrangements shall be considered including any resulting from single
compartment damage.
13.4Jacket buoyancy, stability and seabed clearance
13.4.1Intact and damage conditions (all phases)
13.4.1.1
The intact condition (ULS) for the jacket shall take into account the most severe combination
of tolerances on jacket weight and centre of gravity, buoyancy and centre of buoyancy, and
water density.
13.4.1.2
The damage condition (ALS) shall assume damage (both flooding or emptying) of any one
jacket member or buoyancy element, considering the most severe combinations of tolerances
as indicated in [13.4.1.1].
13.4.1.3
Parametric studies may be required to determine the most severe combinations of tolerances
and damaged element for each stage of the upending sequence.
13.4.2Minimum stability (all phases)
13.4.2.1
The minimum metacentric height for the jacket at each stage shall be not less than that
shown in Table 13­1.
Table 13­1 Minimum GM
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GM
Phase
Intact / ULS
During towage to field (self­floating only)
Damaged / ALS
As agreed with MWS but damage
range is generally more critical than
GM
Before launch
See [13.6.4]
During launch
See [13.6.4]
After launch, transverse and longitudinal
0.5 m
0.2 m
During upend, transverse
0.5 m
0.2 m
During upend, longitudinal
After upending, before final positioning, both
directions
> 0.0 m
1)
> 0.0 m
0.5 m
1)
0.2 m
Notes:
1. see [13.4.2.4]
13.4.2.2
The sensitivity of the jacket to the effects of tolerances discussed in [13.4.1.1] shall be
investigated to demonstrate that minor changes of weight or buoyancy, or one­compartment
damage, do not cause an unacceptable jacket attitude which would hinder subsequent
operations, e.g. by making lift points or flooding valves inaccessible.
13.4.2.3
For hook assisted up­end operations the effects of the hook load shall be considered in the
calculations and procedures. For hook load requirements and their possible effect on the lift
points, refer to [13.4.3.4].
13.4.2.4
The jacket should be stable at all stages of the upending however a limited period during
upend when the jacket is metastable or unstable longitudinally might be acceptable,
provided the behaviour, including any potential effect on clearances (both seabed and
vessels), has been investigated and all interested parties are aware of and accept it. Practical
problems which may be encountered with attending vessels, or rigging and handling lines
should be resolved.
13.4.3Reserve buoyancy after launch and during upend
13.4.3.1
The combined reserve buoyancy is defined as:
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based on generally the upper bound design jacket weight, as defined in [5.6.2].
13.4.3.2
The reserve buoyancy shall be achieved whilst maintaining the minimum seabed clearances
shown in Table 13­4.
13.4.3.3
For operations without crane assistance, the reserve buoyancy for damaged/ALS conditions
shall be shown to be not less than that shown in Table 13­2 and should be greater than
those in Table 13­2 for intact conditions, based on nominal total intact buoyancy.
Table 13­2 Reserve buoyancy for operations without crane assistance
Minimum reserve buoyancy
Case
Intact / ULS
Damaged /
ALS
10%
5%
During upend by ballasting
8%
4%
Absolute minimum, subject to agreed risk assessment
results
5%
2.5%
Launched jacket after launch 1) or
Lifted jacket if required to be re­rigged before upend
Notes:
1. Applicable where the jacket is to be upended using ballast both with crane and
without crane assistance. Also applicable to self­upending jackets in free floating
position.
13.4.3.4
For operations with crane assistance, the reserve buoyancy for damaged/ALS conditions shall
be shown to be not less than that shown in Table 13­3 and should be greater than those in
Table 13­3 for intact conditions, The reserve buoyancy shall be based on nominal total intact
buoyancy, when the weight is that of the jacket minus
90% of the crane capacity at that radius or
total of 80% of each crane capacity for a two crane lift at the relevant lift radii.
Table 13­3 Reserve buoyancy with crane assistance
Minimum reserve buoyancy
Case (see Notes 1 and 2)
Lifted jacket, with static and dynamic analysis carried out
and contingency procedures in place
Lifted jacket, with only static analysis carried out
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Intact / ULS
Damaged /
ALS
8%
4%
12%
6%
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Notes:
1. Contingency procedures shall be in place to allow for corrective action should the
static hook load exceed the expected static loads.
2. For all conditions, the effect of the maximum possible load on the lift points and
jacket at each position analysed shall be considered in the design of these items.
13.4.4Seabed clearance during launch and upending
13.4.4.1
For computing clearances, the most onerous combination of the following shall be taken into
account:
tidal level (LAT to be assumed for planning the launch)
tolerance on water depth measurement
jacket weight and weight contingency
centre of gravity positions
variations in buoyancy,
centre of buoyancy positions
variations in water density
the worst single­compartment damaged case scenario, see [13.3.2.1].
Guidance note:
Variations in buoyancy and centre of buoyancy are usually accounted for by specifying
sufficiently large variations to weight and centre of gravity.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
13.4.4.2
The seabed topography shall be demonstrated to be suitable for launch and/or upend and be
free of obstructions, by means of bathymetric and side­scan surveys, as shown in [13.8.1].
The limits of the surveyed area shall be clearly delineated.
Guidance note:
If the survey of the launch or upending area is to be done at a late stage then it is important
that care is taken to ensure that a suitable area exists to comply with the upending
clearances required in Table 13‑4, using the reserve buoyancy requirements from Table 13‑2
or Table 13‑3 as applicable.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
13.4.4.3
Clearance during launch and upend operations, between the lowest jacket member or
appurtenance and the seabed shall be shown by calculations and/or model tests to be not
less than that shown in Table 13­4.
Table 13­4 Minimum seabed clearance
Clearance after allowing for all tolerances in
[13.4.4.1]
Case
Intact / ULS
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Damaged /
ALS
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During launch including upending of
self­upending jackets and free floating
position
Greater of 10% of water depth or
5 m.
>2m
During upend by controlled ballasting,
with or without crane assist
>2m
>2m
13.4.5Jacket compartmentation
13.4.5.1
Jacket and buoyancy tank compartmentation shall be determined and arranged to suit a
range of jacket weights, centre of gravity and buoyancy, so that the jacket with a damaged
single compartment can be shown to have sufficient reserve buoyancy and stability. Failure
of any individual appurtenance or member shall be considered as a single damage
compartment.
13.4.5.2
For each compartment that can be damaged, an upending and/or set­down procedure shall
show that hook loads, reserve buoyancy, stability and bottom clearances can still be
maintained as required by [13.4.1] to [13.4.4]. The attitude of the jacket in these damaged
conditions should be such that access to rigging, ballast control centres and valves can still
be maintained.
13.4.5.3
Jacket and buoyancy tank compartmentation should be arranged so that compartments are
either full or empty during intact ballasting stages.
Guidance note:
This facilitates ballasting offshore and reduces the consequences of un­controlled filling.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
13.4.5.4
In the event that jacket appurtenances (risers, conductors, J­tubes, pile sleeves) are
required to provide buoyancy to the jacket in the free floating or launch condition, the
methods of sealing and monitoring pressures in these appurtenances shall be provided.
13.4.6Temporary items
13.4.6.1
All temporary items (e.g. rigging platforms, spreader bars, rigging) shall be adequately
secured for all the following conditions:
all possible combinations of trim and heel angles
slam loads
against voyage loadings
loads during launch, upending and setting.
13.4.7Freeboard
13.4.7.1
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Safe and adequate access to and from the rigging platforms for all possible trim and heel
angles and jacket freeboards, including single compartment damage, shall be provided for
the maximum planned installation sea state. The rigging platforms shall be in a near
horizontal position for all cases considered.
13.4.7.2
For the intact condition (ULS) the minimum freeboard of rigging/control platform should be
the design significant wave height for installation plus 1.0 m. For single compartment
damage (ALS) condition the freeboard shall be sufficient to continue the installation without
problems.
13.4.8Rubber diaphragms
13.4.8.1
Rubber diaphragms shall have sufficient strength to withstand both internal and external
water head or air pressure, including loads due to temperature changes after assembly.
13.4.8.2
A test and inspection programme including short term and long term tests shall be carried
out to ensure adequate strength and integrity of the diaphragms. The tests should be
performed as close to sail away as possible and include the following:
Each individual diaphragm should be tested to 1.25 times the maximum working
pressure held over a minimum duration of 10 minutes.
One diaphragm of each type should be tested at 1.1 times the maximum working
pressure held over a minimum duration of 48 hours.
13.4.8.3
After the rubber diaphragms have been mounted on the jacket structure or buoyancy tanks
(if applicable), special attention shall be given to protect the rubber from the surrounding
environment, especially when hot work is being carried out.
13.4.8.4
Rubber diaphragm systems included in pile sleeves to increase the object's buoyancy shall
be designed so that they will not cause an obstruction to pile installation and neither will
they affect the grouting process or the level of grout in the pile sleeve.
13.5Jacket lift
13.5.1Lifting general
13.5.1.1
Lifting operations, including the design of lifting arrangements, lift points, rigging and crane
capacity, should be in accordance with Sec.16 and the requirements in the rest of [13.5] for
the design environmental conditions for each phase of the installation.
13.5.1.2
Where the jacket is to be transferred to the deck of the vessels the grillage and seafastening
structures may be transferred with the jacket, this may represent the governing lift case.
13.5.2In­water dynamic behaviour
13.5.2.1
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13.5.2.1
For the phase when part of the jacket is in the water, the requirements of [16.17] should be
considered.
13.5.2.2
Documentation should either:
Show how the total in­water lifting loads are derived, taking into account the weight,
centre of gravity, buoyancy, damaged cases, entrained mass, boom­tip velocities and
accelerations, inertia and drag forces, or:
Demonstrate that the in­water case is not critical.
13.5.2.3
During the in­water phase, except when connecting or disconnecting, slings should be kept
under at least a moderate tension to avoid snatching and to facilitate positive control of the
structure.
13.5.2.4
For lifts which are to be performed under heave compensation, [16.15] applies.
13.5.3Underwater disconnection
13.5.3.1
Where a remotely operated system for disconnection of rigging is used, a back­up system
which is fully independent of the primary system shall be available. For further details, refer
to [16.16.11].
13.5.4Re­use of lifting equipment
13.5.4.1
Where lifting rigging and lift points have been used before the installation operation, for
load­out or transfer to the crane vessel without incident, all rigging and lift points should be
visually re­inspected by a competent person before re­use. Where any incident or deviation
from the planned prior operation has occurred the lifting rigging and lift points shall be re­
inspected. See also [16.9.5], [16.11] and [16.12].
13.5.5Free floating lifted jackets
13.5.5.1
Where a jacket is lifted from the transport vessel and relies on buoyancy for a period of time
(upending and/or jacket re­rigging) then the principles outlined in [13.2], [13.3], [13.4],
[13.6.9] and [13.7.2] apply.
13.6Jacket launch
13.6.1Analysis methods
13.6.1.1
A launch analysis shall be performed and documented. The analysis should report the
following:
The jacket and barge trajectories, including attitude after launch
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The skidway and rocker arm loads
The barge stern submergence
The maximum jacket dive depth including seabed clearance and buoyancy tank
submergence.
Loads for transfer to local and global structural analysis
Jacket translational and rotational velocities
Relative motions between jacket and barge
Details of the clearances during separation
Stability during launch
Rigging platform freeboard and angles
Slamming velocities on critical items.
13.6.1.2
The launch analysis should be carried out using a 3­D time domain computer program so
that all degrees of freedom are included.
13.6.1.3
In addition to the items listed in [13.4.4.1] (excluding single compartment damage) the
launch analysis should investigate the sensitivity to the following:
Hydrodynamic coefficients (jacket and barge)
Static and dynamic friction coefficients (See [13.6.6])
Initial trim
Initial draught
Jacket starting position on barge (where applicable).
13.6.1.4
The sensitivity analysis should build a better picture of the physics of a particular launch
configuration and establish that the configuration is not unduly sensitive to variations in any
particular parameter. A secondary benefit is the possibility of selecting the optimum launch
configuration.
13.6.1.5
The launch analysis should be validated by model tests and/or a separate analysis by a
different organisation using a different program. Such independent validation is essential
when analysis indicates that the design is approaching the limits of acceptability or
credibility. All analyses shall be documented.
13.6.1.6
Performance of launch model tests should be considered:
To validate computer analyses,
To quantify parameters which are difficult to derive analytically,
To confirm that no important operational facet of the operation has been overlooked.
13.6.1.7
Requirements for launch model tests are in [13.6.10].
13.6.2Launch analysis model requirements
13.6.2.1
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The jacket hydrodynamic, buoyancy and mass model should account for all items including
main members, buoyancy tanks, secondary members, and 'non­structural items' such as
caissons, pile guides etc.
13.6.2.2
It should be ensured that the jacket buoyancy model is not over­buoyant due to over­
simplistic modelling of the overlap of members at the joints.
13.6.2.3
Careful consideration should be given to the drag coefficients applied, especially for dense
areas of the structure or those with small aspect ratios (such as some buoyancy tanks). The
drag coefficients should be selected accounting for Reynolds Number effects and should be
realistic for the surface finish of the members. For a given Reynolds Number the drag
coefficients will typically be larger than for deterministic wave loading analysis as the
coefficients used for that analysis are normally reduced from measured values of 1.0 ­ 1.1 to
0.6 ­ 0.7 to allow for the over prediction of kinematics in standard wave theories.
13.6.2.4
The barge model should account for all items contributing to the barge mass and buoyancy.
It is important that items such as rocker arms and skidways are included even though they
may not be strictly buoyant (due to small drain holes etc.). As many launch programs do not
allow accurate modelling of the barge it is appropriate that:
The buoyancy model is verified as being reasonably accurate over the range of
draughts and trims encountered during the launch by comparison with the hydrostatic
results from a more detailed (stability) program model.
The added mass and drag modelling is verified against alternative data (e.g. motion
response program added mass etc.).
13.6.3Structural/strength checks
13.6.3.1
In addition to the requirements in [13.3] the requirements in this section ([13.3.2]) apply.
The variations described in [13.6.1.3] shall be considered.
13.6.3.2
The jacket (members, joints and attached items), buoyancy tanks, skidways, rocker arms
and barge structure should be analysed to verify their structural adequacy at various stages
of the launch procedure, allowing for their relative stiffness and including loads due to
weight, buoyancy, hydrodynamics and inertia. An allowance shall be made for additional
loads due to wave­induced motions. Further details for the structural strength checks are
given in Sec.5.
Guidance note:
Normally this implies the assessment of all hard truss points on jacket above rocker arm pin.
In addition the jacket could be analysed for some carefully selected cases following the
barge/jacket separation e.g. where buoyancy tanks are parallel to water surface.
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13.6.3.3
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When selecting the load cases for analysis it is important that the pre­launch ballast
condition is considered, in conjunction with the allowable launch sea state, as this may lead
to the most onerous case for jacket­barge loads at the jacket nodes nearest the bow of the
barge. The barge strength should also be verified for the post­launch condition.
13.6.3.4
Member checks should include hydrostatic pressure applying the largest submergence
draught during launch for all sensitivity analyses.
13.6.3.5
Additionally jacket members should be verified against slam loading and slender members
should be checked to ensure that vortex induced vibrations will not cause damage. These
subjects are covered in [5.6.5.4] (Wave Slam) and [5.6.7.4] (Vortex Shedding)
13.6.3.6
The barge stern should be verified as structurally adequate for the maximum predicted stern
submergence.
13.6.3.7
Calculations should demonstrate that the following parameters are within the specified
allowable values for the barge:
Rocker arm reactions
Barge stern submergence
Loads on skidways and barge structure
Barge longitudinal bending moment and shear force.
13.6.4Stability before, during and after launch
13.6.4.1
Before the initiation of jacket sliding, the stability of the jacket/barge combination shall
comply with the following:
Minimum range of static stability shall be not less than 15 + (10/GM) degrees or 20°,
whichever is higher, with GM in metres.
The area under the righting moment curve to the second intercept of the righting
moment and wind overturning moment curves or the downflooding angle, whichever is
less, shall be not less than 40% in excess of the area under the wind overturning
moment curve to the same limiting angle. For the short towage to the installation
location, the wind velocity used shall be 25 m/s or the design wind speed, whichever is
lesser.
13.6.4.2
The requirements of [13.6.4.1] should be met by the barge alone.
13.6.4.3
After initiation of jacket sliding, until the jacket starts to rotate relative to the barge, the
stability of the jacket/barge combination shall comply with the following:
Metacentric height of the jacket/barge combination shall be positive.
Angle of heel caused by 1.5 times the limiting launch wind speed shall be shown to be
acceptable.
13.6.4.4
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After launch (i.e. once in the free floating position), it shall be demonstrated that the jacket
will adopt a stable attitude, as required in [13.4]. The accidental flooding of any one jacket
member or buoyancy tank shall not cause a situation where the jacket cannot be
satisfactorily upended.
13.6.5Launch barge trim angle
13.6.5.1
The pre­launch trim angle should normally be selected to match the expected skidway
dynamic friction such that the launch is initiated by the winching or jacking system.
13.6.5.2
In no case shall the stern immersion during launch exceed any class limit, unless the
classification society provides a dispensation in writing.
13.6.5.3
The maximum launch barge trim angle to initiate self­launch should normally not exceed 4°
whilst satisfying all other parameters (barge draught, barge shear forces and bending
moments).
13.6.6Friction coefficients
13.6.6.1
For design and planning of the launch operation, the upper and lower bound design friction
coefficients should be established. The actual values should be determined in accordance
with [13.6.6.2] and [13.6.6.3] as applicable. Typical upper and lower bound design friction
coefficients are shown in Table 13­5 and include the necessary material factors.
Table 13­5 Design Friction Coefficients
Type
Static
Moving
Surface
Min
Typical
Max
Min
Typical
Max
Waxed wood­grease­steel
0.1
0.2
0.28
0.05
0.1
0.15
0.08
0.14
0.25
0.03
0.05
0.08
Waxed wood­grease­Teflon
13.6.6.2
The characteristic friction coefficients should normally be documented by:
manufacturer specifications;
experiences from similar operations and/or;
results from applicable friction tests.
13.6.6.3
Where testing is 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 and the static (break­out) friction and dynamic (moving) friction coefficients should
be included in any testing. The test procedure should consider the following:
Possible variations in applicable conditions e.g. wet and dry surfaces.
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All static and dynamic friction tests are to be undertaken and measured using
recognised methods.
The characteristic friction coefficient should be defined based on the most conservative
of the 5th or the 95th percentile confidence level of the test results.
At least 5 test pieces should be made, and each tested at least twice for each
condition.
The design friction coefficient shall be taken as the characteristic friction coefficient divided
(or multiplied) by an appropriate material factor. The applicable material factors are in
[5.9.8.6].
13.6.6.4
When a jacket is to be 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
considered. This is particularly important where unwaxed wood is used as part of the
interface as the lubricant may disperse into the wood giving higher break­out requirements
than anticipated.
13.6.6.5
The dynamic friction coefficient shall, if possible, be verified through monitoring of required
pull/push force during load­out. If the friction coefficient calculated based on the load­out
monitoring is outside the range used for launch analysis then the documentation, based on
the measured values shall be updated.
13.6.7Winching, jacking and jacket handling systems
13.6.7.1
Winching or jacking systems shall be capable of initiating jacket launch, taking into account
the anticipated range of friction coefficients, as described in [13.6.6] and at the initial barge
trim angle, after failure of any one system component. Tugs should not be used to initiate
the launch.
13.6.7.2
The final release system shall be designed to hold the jacket at the planned trim angle, in
the maximum launch sea state with the minimum friction coefficient used, and be capable of
being quickly released once the launch decision is taken. The final release, including any
cutting, shall be thoroughly coordinated.
13.6.7.3
If wires and winches are used for initiating launch, the wires shall be shown to release
cleanly from the jacket, when self­launching begins.
13.6.7.4
All wires, shackles, attachment points, winches and other components, including handling
and towing wires to be used after launch shall be designed so that the MBL or ULC of any
component is not less than 3 times the maximum anticipated load. Alternatively, the
maximum anticipated load shall not exceed the Certified Working Load Limit (WLL) of any
component. The maximum anticipated load should be based on the intact/ULS condition.
13.6.7.5
Wires and connections used for handling after launch shall be capable of withstanding loads
from all relevant directions.
13.6.8Self­upending jackets
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13.6.8Self­upending jackets
13.6.8.1
This system has no intermediate stages for checks and control, and is inherently irreversible.
Therefore the calculations for launch (including the upending) and free floating position shall
cover all reasonable variations of jacket weight, centre of gravity, and damage conditions. In
particular the requirements of [13.4.4] shall be considered.
13.6.8.2
The integrity of the jacket after launch can only be assessed from jacket draught readings
once the stable near­upright condition is achieved. Calculations should be carried out, and a
suitable tabular and/or diagrammatic presentation be included in the marine operation
manual so that a rapid assessment can be made of jacket weight and status from the post­
upend draught readings.
13.6.8.3
Provision for contingency air de­ballast of critical compartments may be required to ensure
adequate emplacement behaviour under all circumstances, but should not be relied on for
the "base case" undamaged operation.
13.6.8.4
The ballast system requirements in [13.7.2] apply to facilitate ballasting for set­down.
13.6.9Practical aspects
13.6.9.1
As a minimum, the barge shall be provided with the following:
Adequate boarding ladders on both sides of the barge, clear of any jacket overhangs,
for boarding, evacuation and transferring launching crew in safety.
Safety equipment for all launching crew members.
Adequate tools and equipment for cutting, removing, handling and securing
seafastening members.
Lights for night­time working.
VHF radios for communication between work parties, and with the installation vessel,
tugs and supporting vessels.
Equipment necessary to re­pressurise a compartment whose buoyancy is required for
the jacket in the free floating condition.
Crew facilities.
13.6.9.2
Barge ballasting and seafastening cutting shall be in accordance with a plan having defined
stages. Operations may take place simultaneously, but shall be planned, and equipment
provided, so as to minimise intrusion into the installation weather window. Operations should
be synchronised, with due regard to the safety of personnel cutting the aft seafastenings.
13.6.9.3
The towing tug should generally remain connected, and should maintain the barge heading
into the wind and sea. Tugs for handling the jacket after launch shall be pre­connected to
handling wires before initiating launch.
13.6.9.4
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The masters of all tugs should be aware of the predicted jacket and barge behaviour after
launch.
13.6.9.5
Equipment required for subsequent operations, such as lifting equipment, piles and pile
hammers, should be available before starting to cut seafastenings.
13.6.9.6
Valves and pressure gauges should be tested and checked closed before sailaway.
13.6.9.7
Any major compartment (jacket leg compartment, buoyancy, tank, pile sleeve), whose
buoyancy is required for intact and damaged stability, shall be pressurised to a minimum of
0.35 bar (5 psi). Compartment pressures shall be monitored for a period of three days
before jacket sailaway, and immediately before sailaway and immediately upon arrival at the
installation site. Variations in pressures should be within ranges expected due to
temperature changes. The method of monitoring the pressures shall be stated. Permanently
buoyant members (e.g. jacket braces) are excluded from this requirement.
13.6.9.8
Should the installation of piles be required to augment jacket un­piled stability, these piles
shall be in the field (on a vessel or the crane vessel) before starting jacket installation
together with the pile handling systems, pile hammers and any other necessary equipment
such as bear cages and welding sets.
13.6.9.9
If fitted, the removal of temporary vortex shedding devices shall be described in the
operation manual.
13.6.10Launch model tests
13.6.10.1
Where launch model test are performed they should meet the requirements in the rest of
[13.6.10].
13.6.10.2
Model scale should be not less than 1/100, and preferably greater than 1/60.
13.6.10.3
Jacket and barge models should represent accurately the prototypes in weight, location of
centre of gravity, and radii of gyration. A range of jacket weights and centre of gravity
positions should be considered.
13.6.10.4
In general, models are more rigid than the prototypes and have lower structural damping.
This should be taken into account in interpretation of measurements of accelerations and
rocker arm loads.
13.6.10.5
Instrumentation/equipment should be provided to measure/record:
Draught, trim and heel of the barge
Trim and heel of jacket
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Distance run by jacket
Rocker arm reactions and rotations
Digital video recording.
13.6.10.6
Real time facilities are required to provide:
Controlled adjustment of dynamic friction coefficient
Computer calculation of maximum barge submergence, maximum submergence at top
and bottom of jacket launch rail face
Quantification of maximum rocker arm reactions.
13.6.10.7
Seakeeping tests should be performed in the design launch sea state in head, quartering
and beam seas, for the selected barge condition, to quantify the dynamic magnification of
rocker arm forces.
13.6.10.8
The model test report should include, as a minimum:
All relevant prototype and model parameters
Statistical summary of each test, to include initial barge trim, friction coefficient, sea
state and heading, maximum rocker reaction, minimum seabed clearance or maximum
dive depth, maximum barge stern immersion
Jacket and barge trajectories
Rocker reaction and rotation time histories.
13.7Floating controlled upend and set­down ballasting
13.7.1Methods
13.7.1.1
Controlled upending of a horizontally floating structure to the vertical can be carried out in a
number of ways, such as:
Crane assisted, i.e. with intervention during upending, by crane assisted control alone
or in combination with controlled gravity flooding;
Ballasted, i.e. with intervention during upending, by controlled gravity flooding, or by
pumped flooding, or a combination of both.
13.7.1.2
The requirements for self­upending jackets are in [13.6.8].
13.7.1.3
The floating controlled upend and set­down ballast procedures shall meet the applicable
requirements of [13.4] including the compartmentation requirements in [13.4.5]. Partial
filling stages of compartments to be ballasted should be considered in the procedure.
13.7.1.4
The requirements for these methods are described in [13.7.2] and [13.7.3].
13.7.2Ballasting system
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13.7.2Ballasting system
13.7.2.1
These specific requirements are in additional to the general requirements in [4.3].
13.7.2.2
The ballasting system shall be designed to be fail­safe ­ the installation operation should not
be endangered or unduly delayed by the failure of any one component to operate correctly
(including loss of hydraulic fluid or similar).
13.7.2.3
Ideally, the system should be controllable at all stages. For a one­off operation reversibility,
although desirable, is not a requirement. De­ballasting may be required to assist with jacket
levelling.
13.7.2.4
Where a compartment is to be partially filled a system to report compartment fill levels, in
real time, should be provided.
13.7.2.5
Systems operated by telemetry should be duplicated, or have a manual, umbilical or ROV
back­up.
13.7.2.6
Umbilical systems should have a manual or ROV back­up.
13.7.2.7
Common system failure modes should be investigated and provisions made against failure.
13.7.2.8
Remotely operated flood valves should be duplicated in parallel, or an alternative flooding
system provided.
13.7.2.9
Remote valves which require to be closed to halt the flooding should be duplicated in series,
unless the compartment being ballasted can fill completely without serious consequences.
13.7.2.10
All valves should be accessible by ROV.
13.7.2.11
The ballasting systems, upending analyses and procedures should include alternatives to
allow installation after accidental flooding of any one jacket member or buoyant element.
13.7.2.12
Power reserves should be sufficient to achieve not less than twice the anticipated number of
valve operations.
13.7.2.13
Except for the simplest systems, the ballast system should be subjected to a formal system
investigation by means of FMEA and HAZOP techniques.
13.7.2.14
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Valves and pressure gauges should be tested and checked closed before sailaway.
13.7.2.15
Alternatives may be considered, provided the overall level of risk has been formally shown to
be acceptable.
13.7.3Crane­assisted upend and set­down ballasting
13.7.3.1
Where a crane is used to assist the controlled upend and/or set­down ballasting the
requirements for rigging, lift points and practical as per [15.10] and [13.5] as appropriate.
13.8Jacket position and set­down
13.8.1Surveys
13.8.1.1
Bathymetric, sidescan and geotechnical surveys should be performed at the design stage to
establish the water depth, foundation requirements and identify any obstructions in an area
around the jacket location and launch/upending sites if different. Where there is the
possibility of sand­wave mobility, scour and accretion after the initial surveys a pre­
installation bathymetric should be performed.
13.8.1.2
Debris surveys by a combination of sidescan, ROV and/or diver surveys as appropriate, shall
be carried out not more than 4 weeks before installation. These should identify and locate
any subsea infrastructure including pipelines and cables, and to identify and remove any
obstructions in the relevant areas.
13.8.1.3
In selecting the area(s) for survey, positioning errors during site investigation and jacket
installation should be taken into account. The surveys should cover sufficient area to allow
for possible drift during launch, upending and installation.
13.8.1.4
The jacket landing area shall be shown to be within tolerances, and free from debris likely to
cause problems with the installation.
13.8.1.5
Consideration should be given to the effects of local depressions such as pockmarks or jack­
up footing imprints.
13.8.1.6
Installation planning should be based on the geotechnical characteristics of the site. The
data should be acquired with due regard to installation requirements. The investigation
should provide information on the surface and sub­surface conditions within the zone
influenced by the installation. In addition to establishing the soil profile and the strength
and deformation characteristics, information should be acquired with which to assess risks
during the installation phase from mudslides, shallow gas and sediment transport.
13.8.1.7
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Clearances between subsea assets (pipelines, templates) shall be a minimum of 5 m after all
positioning tolerances (jacket base motion and positioning equipment errors) have been
taken into account.
13.8.2Positioning and position monitoring systems
13.8.2.1
Jacket positioning and position monitoring systems shall comply with [4.4] as applicable and
the rest of [13.8.2].
13.8.2.2
Jacket positioning systems shall be fit for purpose to achieve the specified tolerances on
position, verticality and orientation. The repeatability of positioning should be determined,
with regard to previous surveys.
13.8.2.3
Two independent position monitoring systems shall be provided. One of them shall be
independent of visibility. The systems shall be capable of the required accuracy at the
installation location.
13.8.2.4
If the soils survey or bathymetry suggest that the contracted jacket verticality tolerances
may not be achieved then consideration should be given to levelling the seabed or there
should be contingency plans for levelling the jacket or topsides (e.g. use of levelling tools
and grippers).
13.8.3Positioning over template
13.8.3.1
When the jacket is to be docked over a template, wellhead docking piles or similar, a docking
analysis shall be carried out to determine:
The jacket behaviour during docking
The loads and stresses on docking piles and jacket members
The limiting sea state and current speed for installation, taking into account the crane
vessel behaviour.
13.8.3.2
Until engagement of docking piles (or similar) the clearances between the jacket and a pre­
installed template and other subsea infrastructure should be in accordance with [13.8.1.7].
Guidance note:
Docking piles, guides or similar are normally provided to ensure clearances and positional
tolerances between the jacket and template and/or wellhead.
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13.8.4Positioning over pre­installed/docking piles
13.8.4.1
If docking piles are used for positioning, then suitable analyses shall be documented to
confirm the dynamics of the jacket, the feasibility of engaging with the pile(s), the strength
and elasticity of the piles, the behaviour of the piles in the soil, the jacket/pile interaction
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and loads.
13.8.4.2
The design of guidance system on the jacket should account for a minimum of:
lateral loads for bumpers (pin and bucket), and
wind, current and wave loads on the structure and piles, and
1° vertical tilt of the jacket and ensure that the jacket does not lock up on the
guidance system if this tilt occurs.
13.8.4.3
A study on boom tip motion from fluctuating wind loads and floating vessel motion response
should be performed to confirm that positioning can be accomplished to the required
accuracy.
13.9Buoyancy tank
13.9.1Removal schedule
13.9.1.1
Where options exist, removal of buoyancy tanks should be scheduled into the installation
sequence so that jacket safety is optimised. The following should be considered when
scheduling the removal:
Early removal may delay pile installation.
Late removal may mean increased wave forces on the jacket.
13.9.1.2
Where buoyancy tanks are ballasted, either to increase resistance against overturning or for
removal, documentation shall show that the soils under the mudmats or pre­installed piles
are not overloaded
13.9.1.3
Ballasting of buoyancy tanks shall not adversely affect jacket verticality.
13.9.2Removal method
13.9.2.1
The method of removal shall be detailed in the procedures and should cover the design of
the buoyancy tanks including attachments and the equipment available.
13.9.2.2
Connections between auxiliary buoyancy tanks and the object should be designed to ensure
the controlled and safe release of the devices.
13.9.2.3
The connections release system shall have at least two independent methods of release.
13.9.2.4
Separation from the jacket should be in a controlled manner. In general, where
disconnection is by means of remotely pulling connecting pins or burning, the tank should be
in a neutrally buoyant state at the instant of disconnection. Where remotely operated pins
are used a back­up method or system should be available.
13.9.2.5
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13.9.2.5
Removal by lifting should be in accordance with [15.10], taking account of crane vessel
dynamics, in­water loads and all ballast conditions.
13.9.2.6
Where the tanks are floated up and towed clear, sufficient control shall exist to avoid impact
with the jacket. The tank shall have adequate intact stability at all stages in accordance with
[13.4].
13.9.2.7
The position and orientation of the buoyancy tank shall be monitored during removal.
13.9.2.8
Where adjustment of buoyancy by means of ballast or compressed air is needed, then a
back­up method or system should be available.
Guidance note:
Flooding could be via diaphragms if rip out can be performed at required time in schedule.
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13.9.2.9
It is normally recommended that buoyancy tank compartments are flooded by valves and
de­watered via rubber diaphragms.
13.9.2.10
Some practical considerations on the use of compressed air are given in [12.6.2].
13.9.2.11
Wires and attachments shall be designed to the requirements of [13.6.7.4].
13.10On­bottom stability and piling
13.10.1Unpiled and partially piled on­bottom stability
13.10.1.1
This section applies to jackets and gravity structures using driven or suction piles, or relying
on suction or grouting to fill avoid spaces between the structure and the seabed.
13.10.1.2
The loads and behaviour of the structure after positioning and during installation shall be
investigated to determine the resistance of the structure to sliding, tilting or over­
penetration during the installation process. Installation includes pile installation, penetration
and ballasting as appropriate.
13.10.1.3
Where access to the jacket is required, the on­bottom stability shall be sufficient to allow
safe personnel boarding and working on the structure in the design environmental
conditions.
13.10.1.4
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The operation reference period should account for all piling and grouting activities, including
an allowance for waiting on weather. The return period for on­bottom stability should be
selected from Table 3­1 based on this operation reference period.
13.10.1.5
The objective is that the structure should achieve the ability to resist the storm loading as
quickly as possible, preferably within the structure installation weather window.
13.10.1.6
Where it is not feasible to achieve the storm loading within the installation weather window,
then the installation sequence should be optimised to:
minimise the time taken to achieve the storm capability
maximise the short term capability of the jacket, should a forecast be received during
the installation period which indicates severe weather.
13.10.1.7
Installation planning should anticipate construction problems with appropriate contingency
measures. Spare piling equipment including hammer(s), lifting tool(s) and pile follower(s) (if
required to achieve on bottom stability) shall be available, and the change­over time shall be
considered in the planning. Remedial measures which could adversely affect the final pile
capacity should not normally be considered.
13.10.1.8
The capability of the structure to resist environmental loadings in the un­piled and partly
piled or partly penetrated condition should be determined considering the most onerous
combination, as applicable, of:
Jacket weight (upper and lower bound),
Centre of gravity positions,
Buoyancy (including damaged case scenarios)
Post set­down ballast conditions,
Water levels and water density,
Penetration
Removal of buoyancy tanks
Pile stick­up
Hammer weight including follower
Pile hang­off
Pile installation schedule/sequence
Sequential piling sequence for correcting jacket level and
Soil strength parameters.
13.10.1.9
Using the LRFD approach, the on­bottom stability should be verified for the ULS condition.
The consequent of single compartment damage shall also be considered.
13.10.1.10
For each phase of the installation, it should be demonstrated that adequate safety factors
can be obtained against the failure modes, as shown in Table 13­6.
Table 13­6 Intermediate on­bottom stability safety factors
Safety Factor
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Mode
ASD/WSD
LRFD
Overturning (uplift of weather side leg(s) or corner). There
is no uplift when there is no tension required at the mud­
mat that cannot be resisted by skin friction on mud­mat
skirts and/or suction. 1), 2)
1.0
1.0
Sliding (soil failure) Where applicable, account may be
taken of the capacity of un­grouted piles to resist sliding
1.5
1.0
1.5
1.0
2)
Mud­mat/foundation V­H combined bearing capacity check
3)
Structure buoyancy (lower bound design weight excluding
contingency divided by buoyancy)
4)
1.1
Notes:
1. The Overturning safety factors should be increased to 1.5 when a full time domain
dynamic analysis is undertaken.
2. Where suction is included, the suction capacity shall be fully documented.
3. The Safety Factor is defined as the length of the vector from the still water (V, H)
point to the V­H capacity envelope divided by the vector from the same origin to the
unfactored storm load (V, H) point as shown in Figure 13‑1.
4. The LRFD checks shall include applicable load and soil material factors.
Figure 13­1 Mudmat capacity checks
13.10.1.11
Particular attention should be paid to the on­bottom stability of tripod jackets which tend to
be more sensitive to Centre of Gravity shifts, uneven foundation conditions and lateral
loading, including that due to some installation procedures. Unless the jacket can be safely
piled (and secured e.g. grouted if required) within an operation reference period, the
appropriate seasonal return period environmental conditions (see [3.4]) and associated range
of wave periods should be used for on­bottom stability calculations, to maintain the safety
factors shown in [13.10.1.8].
13.10.1.12
In any event, the structure shall be capable of withstanding the following minimum wave
heights (and associated range of wave periods) within 48 hours of the Point of No Return
(typically the decision to start cutting seafastenings); the seasonal 1 year return waves may
be used when they are smaller:
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Benign areas H s = 2.5 m
Non benign areas H s = 5.0 m.
Wave/current forces shall be calculated from the maximum wave (H max) in a 3 hour exposure
period. Wind forces shall be included, using a wind speed compatible with the sea state
considered in each case. The 1­minute averaging period should be used for computation of
wind forces.
13.10.2Piling noise and marine species protection
13.10.2.1
Where noise restrictions and any other marine species protection are in force during piling
the effect of this on the operation including any compliance/mitigation methods shall be
documented.
13.10.2.2
If bubble curtains are used then precautions shall be documented and taken to avoid
problems due to air in the water for the following as a minimum:
Man Over Board near the curtain (reduced buoyancy)
Vessels operating near the curtain (reduced buoyancy and/or stability)
DP thrusters losing efficiency due to cavitation.
13.10.3Pile handling
13.10.3.1
Pile lifting should be carried out in accordance with [15.10].
13.10.3.2
Specialised tools for pile lifting and upending, and welding of add­ons should be shown to be
fit for purpose and properly commissioned. See [16.9.7], [16.11.5] and [16.12.4].
13.10.4Pile structural analysis
13.10.4.1
Piles should be analysed to demonstrate that stresses during installation are within offshore
design code limits.
13.10.4.2
The following should not induce pile wall stresses in excess of the allowable value for static
conditions:
Lifting, upending and stabbing the pile
placing and supporting a hammer(including any follower) on the pile top
the effect of any free standing pile length.
13.10.4.3
Consideration shall be given to the maximum inclination of the un­driven pile in the pile
sleeve and the maximum possible inclination of the jacket when performing pile stick­up
analyses
13.10.4.4
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It may be necessary to consider wave and/or current induced oscillation and vortex shedding
during this phase ­ see [5.6.7.4]. Vertical skirt piles driven through the splash zone may
need special consideration.
Guidance note:
Large diameter piles are less susceptible to VIV.
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13.10.4.5
Dynamic stress caused by pile driving should be assessed using wave propagation analysis.
The sum of the static and dynamic driving stresses should not exceed the specified
minimum yield strength.
13.10.4.6
Fatigue due to driving shall be considered in the pile design.
13.10.5Self­penetration of piles
13.10.5.1
Particular attention should be paid to soil conditions (e.g. carbonate soils, crusted layers,
etc.) which may result in sudden self­penetration of piles (sometimes referred to as
dropfalls/runaway).
13.10.5.2
Self­penetration analyses should account for weight of installation equipment (e.g. lifting
tools, hammers, followers etc.)
13.10.6Pile connection to jacket
13.10.6.1
Details of the proposed connection method between the jacket and piles shall be
documented, including manufacturer’s instructions.
Guidance note:
Jackets can be connected to the piles by one of the following methods:
Grouted connections using cementitious grout. For example, individual grout lines can
be installed on the jacket (hard piped or flexible) in order to deliver the grout to the
sleeve. As an alternative, grout can be delivered using a temporary hose that docks
onto a subsea mateable connector located above the top of the pile sleeve cone. See
[13.10.6.2].
Mechanically swaged connections, where the pile is hydraulically deformed into
machined grooves in the pile sleeve. Swaged connections can incorporate jacket
levelling systems.
Welded pile to jacket pile shims. Where piles are installed through the legs of the
jacket, crown shims are welded between the pile and the jacket leg in order to effect
topside load transfer.
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13.10.6.2
For grouted connections the following provisions should be incorporated into the jacket:
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A passive seal or active (inflatable packer) seal in order to support the column of grout
whilst it cures,
A primary grout distribution ring located typically within 1.0 m above the seal,
A secondary grout distribution ring typically located 1.5 m above the primary ring,
A tertiary method in order to top up the sleeve in the event of grout slumping, or if
there is a blockage in any of the grout lines,
A means of determining when the connection is completely and properly full of full­
density grout.
Guidance note:
For e), this could be done by monitoring the top of the sleeve.
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13.10.6.3
The effects of the piling operation and resulting accelerations shall be considered for both
the jacket design (including all appurtenances) and in the design of the grouting system.
13.10.6.4
The grouting system (including active seals where fitted) should be pressure tested.
13.10.6.5
The operational limiting criteria for grouting operation shall be defined accounting for the
following:
vessel stationkeeping capabilities
grout system design
ROV operability
movement of the jacket.
Guidance note:
To prevent excessive movement of the jacket, grippers can be provided to isolate the
grouted connection during curing (see /112/, K.5.3). Grippers can also be used as part of a
jacket levelling operation.
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13.10.6.6
Grouting should only start once pile driving has been completed.
13.10.6.7
Testing shall show the achieved grout strength. Items shall not be transferred onto the
jacket until the required grout strength has been achieved or there is gripper capacity to
support them without movement. Limitations on the transfer of items during the curing
process should be documented and included in the marine operation manual.
13.11Information required
13.11.1Manuals and procedures
13.11.1.1
The items listed below are generally considered critical for the successful execution of an
offshore installation operation and should be emphasized in the manuals and procedures:
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Requirements of design/operational limitations and requirements for weather forecasts
as well as wind, wave and current monitoring.
Detailed operation schedule, relating to any specific weather window(s) and the
identification of all “safe conditions” as necessary.
Checklists ensuring that all required preparations have been carried out.
Correct positioning of the jacket and vessels.
Monitoring procedures describing equipment set­up, testing, recording, expected
readings including acceptable deviations and reporting requirements during the
operation.
Detailed ballasting procedures, including contingencies, for upending and installation of
the jacket.
Contingency procedures.
All post set­down activities including buoyancy tank removal procedures.
13.11.2Jacket information
13.11.2.1
For the jacket:
Drawings showing plans, elevations and details
Weight report
Compartmentation and ballasting
Jacket arrangement on vessel
Grillage and seafastening drawings
Details of any buoyancy tanks, piles or other equipment carried on the jacket, including
attachment details
Details of jacket ballast and control systems, including manual and remote operation
systems and back­up systems, and compartment status­monitoring systems.
Structural strength and stability analyses for the jacket during all phases covering
load­out, voyage and installation.
Ballasting calculations for the jacket installation, including contingencies, where
ballasting is required
13.11.3Transport vessel
13.11.3.1
For the Transport Barge or Vessel:
General arrangement drawing
Compartmentation plan
Plating, framing and skidway details, in particular in way of jacket support points and
seafastenings
Deck load capacity plan
Jacket grillage and seafastening details
Allowable bending moment and shear force
Lightship details, including rocker arms and launchways
Certification package
Pumping and ballasting specification
Hydrostatics
Towing connections where applicable
Guidelines for air pressurised barge tanks, if used
Crew documentation.
13.11.4Installation site(s)
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13.11.4Installation site(s)
13.11.4.1
Seabed bathymetric survey and soils data for the installation site and bathymetric survey
data for the launch site, the upending site and the route taken from these sites to the
installation site. (See [13.8.1])
13.11.4.2
A debris and infrastructure 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, to verify the
location of all subsea infrastructure, debris and obstructions. Confirmation will be required
that the bathymetry has not changed significantly since the initial surveys.
13.11.5Launching (if applicable)
13.11.5.1
Launching analysis report.
13.11.5.2
Jacket launch stress analysis report, showing overall and local loads and stresses.
13.11.5.3
Jacket member hydrostatic check.
13.11.5.4
In addition to the items in [13.11.3], the following shall be documented for the launch barge
Allowable rocker arm loads
Allowable stern submergence
13.11.6Lifting (if applicable)
13.11.6.1
Lifting and upending analysis reports and other information requested in [16.18].
13.11.7Unpiled/partly piled on­bottom stability
13.11.7.1
Report demonstrating adequate unpiled and partly piled stability, in accordance with
[13.10].
13.11.7.2
Seasonal environmental data.
13.11.7.3
Outline jacket and pile installation procedures and equipment.
13.11.7.4
Jacket levelling procedures and equipment and their effects on the local and global capacity
of the jacket.
SECTION 14Construction afloat
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SECTION 14Construction afloat
14.1Introduction
14.1.1General and scope
14.1.1.1
This section covers the marine operational aspects of construction and outfitting afloat.
Construction and outfitting afloat includes activities on a platform starting after tow­out
from initial construction site, mooring at outfitting site, and activities on the platform at the
outfitting site until departure for the offshore location. Construction afloat activities are
normally supported by a semi­permanent construction spread consisting of multiple barges
moored to, or adjacent to, the object under construction.
14.1.1.2
This Section is mainly intended to cover construction afloat of “Condeep”­type gravity
structures (with one or more columns above a submerged base). However the principles
apply to most construction afloat.
14.1.1.3
Inshore deck mating is covered in Sec.15.
14.1.1.4
Documentation and procedures for construction afloat shall follow the general principles of
Sec.2. Documentation and procedures shall above all ensure that those planning, authorising
and carrying out the work are fully informed about any limitations and constraints which
may be placed on the work by factors outside their own discipline.
14.1.1.5
All responsible parties shall remember that the platform cannot be treated as a normal
onshore construction activity. Any activity can be constrained by factors which change on a
daily basis, and which can be inter­related, including:
Structural loads and resistance
Draught, heel/trim, displacement, ballast condition and stability
Mooring loads and resistance
Marine spread requirements
Weather conditions
Other on­going activities and access restrictions.
Environmental restrictions imposed by local authorities.
14.1.2Revision history
14.1.2.1
This section replaces the applicable sections of the following legacy documents:
0015/ND Guidelines for concrete gravity structure construction & installation
DNV Offshore Standard DNV­OS­H201 Load Transfer Operations
14.2Loads and structures
14.2.1
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14.2.1
The following shall be taken into account when assessing the structural loading and stresses
during construction afloat:
Static loads
Hydrostatic loads
Tidal changes
Mooring loads
Differential ballasting
Environmental loads, including seasonal loads, such as snow and ice
Loads due to construction spread
Vessel impact loads
Guiding loads
Contingency loads, including accidental flooding, mooring line breakage, dropped
objects, as appropriate
Shallow water effects
14.2.2
Adequate approved precautions (guides, bumpers, reduction of ballast rate, etc.) should be
taken to avoid damages due to impact loads.
14.3Stability and damage stability
14.3.1
For GBS type structures, the requirements of [6.2] apply.
14.3.2
For all other objects or vessels the requirements of [11.10] apply.
14.3.3
Any structural, stability or draught limitations which lead to constraints on construction
operations shall be clearly defined, and written into the relevant operational procedures.
These may include:
Maximum and minimum draught
Differential ballast levels in adjacent buoyancy compartments, or any compartment
and the sea
Weight distribution
Structural limitations on heel or trim, which may therefore lead to limitations on
draught, stability or environmental conditions. The age of any time­dependent
construction material, such as concrete, shall be considered.
Free surface limitations
Phases during which one­compartment damage stability does not exist, and the
requirements given in [6.2.5.2] apply.
14.3.4
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Due attention shall be paid to continuous changes in weight, buoyancy, CoG and CoB during
construction.
14.3.5
Inclining tests (see [2.10.5] for details) shall be performed as required at different stages
during construction of floating structures in order to assess the position of the centre of
gravity. This is particularly relevant when the calculated value of the metacentric height is
close to the minimum value and if such a minimum condition is obtained by the transfer of
heavy loads.
14.3.6
Inclining tests for the substructure should normally be performed before major tows and
mating.
14.4Mooring and fendering
14.4.1
The mooring and fendering system for each item of the spread, and the unit under
construction should be designed in accordance with the requirements of Sec.17.
14.4.2
Where such a design is impractical, then the design and forecasted operational criteria for
the moorings should be clearly defined. Procedures should be developed to close down the
function of the affected equipment and remove it to a place of safety, before the operational
limit is reached. Adequate tugs and safe moorings should be available to perform this
operation.
14.4.3
The penetration depth of direct­embedment anchors should be verified (e.g. by ROV survey)
after the installation.
14.4.4
The position of the moored structure should be checked with regard to permanent
displacements, particularly in the first period after installation and after extreme weather
conditions.
14.4.5
Possible arrangement for emergency release of anchor lines should be considered in each
case.
14.4.6
Fairleads fitted between the stopper and the anchor should be of the roller type with swivel
provisions.
14.4.7
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Compensators based on steel springs, hydraulic/pneumatic spring systems, fibre ropes over
sheaves, etc., may be used. Compensators shall be of safe design and constructed of
certified materials and components.
14.5Construction spread
14.5.1
The construction spread may include barges and other floating equipment moored alongside
or near the platform, to serve the following functions:
Storage for construction materials and equipment
Concrete mixing plant
Temporary power supply
Temporary ballast control
Offices
Workshops
Personnel reception area and security
Berthing and unloading area for ferries, transport barges and other vessels
Safety and emergency facilities.
14.5.2
The number of barges moored alongside the platform shall be kept to a minimum. Where
practical, any redundant equipment shall be removed from the spread.
14.5.3
All floating equipment moored adjacent to the object under construction shall possess one­
compartment damage stability. The need for contingency pumping equipment on site should
be evaluated.
14.5.4
For stability requirements of floating equipment moored adjacent to the object under
construction, see [11.10].
14.5.5
A hazard identification study and risk assessments in accordance with [2.4] shall be carried
out for the entire spread involved during the construction and outfitting afloat.
14.6Operational requirements
14.6.1
Operational requirements are generally described in Sec.2.
14.6.2
All equipment and material on barges in the construction spread shall be secured to
minimise the risk of loss overboard. Any equipment which, if lost overboard, could cause
damage to the structure, shall be identified and handled so as to minimise the hazard.
14.7Information required
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14.7Information required
14.7.1Object
14.7.1.1
For the object:
Drawings including plans, elevations and details
Weight report covering stages of construction and final condition
Compartmentation and ballasting data
Details of ballast and control systems, including manual and remote operation systems
and back­up systems, and compartment status­monitoring systems.
Structural strength and stability analyses covering all phases of construction
14.7.2Mooring arrangement
14.7.2.1
The information listed in [17.12] (as applicable).
14.7.3Other
14.7.3.1
Depending on the marine warranty scope of work for the construction afloat stage,
information pertaining to other activities and operations may be required.
Guidance note:
Such activities can include heavy lifting and installation of solid ballast.
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SECTION 15Lift­off, mating and float­over
operations
15.1Introduction
15.1.1Scope
15.1.1.1
This section presents the requirements for operations collectively known as load transfer
operations. “Load transfer operation” is used in the text as a reference to any of following
four types of marine operations:
Lift­off operations: Load transfer of an object, e.g. a module, from land or seabed
supports to supports placed on one or more vessel(s). Lift­off operations addressed in
this section are assumed carried out in sheltered waters.
Mating operations: Load transfer of an object, e.g. a topside, supported by on one or
more vessel(s), pontoons, etc. to a floating substructure. Mating operations addressed
in this Section are assumed carried out inshore.
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Float­over operations: The operation of installation/removal of a structure, e.g. a
topside, onto or from a fixed host structure (e.g. a jacket or a concrete structure) by
manoeuvring and ballasting the transport vessel to effect load transfer
Inshore Docking operations: The positioning and setting of a floating object on
fixed or vessel mounted supports, see also [15.1.1.2] and [15.1.1.3].
15.1.1.2
Docking operations addressed in this Section include both docking onto support pads placed
on the seabed and docking onto supports placed on a floating vessel, e.g. a submersible
barge, a HTV or a floating dock. These operations are sometimes referred to as floating on­
load or floating off­load.
Guidance note:
Note that float­on to a Heavy Transport Vessel (HTV) falls under this definition and that
these requirements apply for on­ and offloading of HTVs by float­on/float­off method, see
also [15.1.1.4].
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
15.1.1.3
This section applies to objects such as offshore modules and deck structures transferred from
one support condition to another by ballasting alone, i.e. without the use of crane(s),
skidding or trailering.
Guidance note 1:
In float­over operations the load­transfer is not necessarily by ballasting alone. It could be
that other equipment is used, e.g. hydraulic jacks.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
Guidance note 2:
In docking operations the object is a floating one and could be for instance a vessel.
­­­e­n­d­­­of­­­G­u­i­d­a­n­c­e­­­n­o­t­e­­­
15.1.1.4
For platform removals it may be the reverse of operations described in [15.1.1.1] that shall
be performed. It may also be that it is for instance offloading rather than on­loading of a
HLV that shall be performed. Requirements in this section may, as applicable, be applied
also for such operations.
15.1.1.5
General requirements applicable for all load transfer operations are given in [15.2] to [15.5].
Requirements specific to each of the load transfer operation types described in [15.1.1.1] are
given in [15.7] to [15.10].
15.1.1.6
For dock float­out operations, see Sec.12.
15.1.1.7
For construction afloat, see Sec.14.
15.1.1.8
For load­out operations (e.g. by skidding or trailers), see Section 10.
15.1.2Revision history
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15.1.2Revision history
15.1.2.1
This section replaces the applicable sections of the following legacy documents:
GL Noble Denton, Guidelines For Float­Over Installations / Removals, 0031/ND
DNV Offshore Standard DNV­OS­H201 Load Transfer Operations.
15.2General
15.2.1Operation class
15.2.1.1
An Operation class should be defined according to Table 15­1.
Table 15­1 Operation class
Tide range
1)
Tide restrictions?
2)
Weather
restrictions?
Operation Class
3)
Significant
Yes
No/Yes
1
Significant
No
Yes
2
Significant
No
No
3
Zero
No
Yes
4
Zero
No
No
5
Notes:
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 load­out
shall be defined as tide restricted.
3. If weather restrictions apply, then the load­out shall be categorized as weather
restricted, see [2.6]. If there are no weather restrictions to the object
movement/ballasting phase the load­out class may be selected accordingly.
15.2.2Planning and design basis
15.2.2.1
General requirements to planning and execution of load­transfer operations are given in
Sec.2.
15.2.2.2
For each phase of a load transfer operation, the design environmental condition shall be
defined. All possible environmental conditions, see Sec.3, shall be evaluated and considered
in the planning (and design).
15.2.2.3
For Class 1 to 3 operations, tide variation is a critical parameter and shall be specially
evaluated.
15.2.2.4
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15.2.2.4
Any local environmental effects, e.g. the possibility of swell/waves at the operation site,
should be identified and considered.
15.2.2.5
The start and end points for the operation shall be safe conditions (see [2.5.1.2]), and they
should be clearly defined. Criteria for stopping or aborting each stage of the operation, and a
critical point of no return (PNR) for the operation shall be identified.
15.2.2.6
The load transfer operation could consist of several sub­operations. This shall be thoroughly
considered in the overall planning of the operation. It should be considered to define (and
design for) additional safe conditions in order to shorten the required weather window(s).
15.2.2.7
The load transfer operation could involve various construction, transport and load transfer
(main) contractors/responsible. This should be duly considered in the interface planning.
15.2.3Risk management
15.2.3.1
Operational risk should be evaluated and handled in a systematic way, see [2.4].
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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15.3Loads
15.3.1General
15.3.1.1
Loads and load effects are generally defined in Sec.5. It shall be thoroughly evaluated if