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NOBLE DENTON MARINE SERVICES
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 DNVGL-SE-0080 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
insurance-related 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 know-how throughout the marine operation activities. It should solely be used by
competent and experienced organizations, and does not release the organizations involved from exercising
sound professional judgment.
Full version of Standard - DNVGL-ST-N001 & DNVGL-ST-N002
DNVGL-ST-N001
Full version of Standard - DNVGL-ST-N001 & DNVGL-ST-N002
DNVGL-ST-N001 Marine operations and marine warranty (Edition: 2016-06)
SECTION 0
CHANGES – CURRENT
SECTION 1
Introduction
1.1
General
1.2
Objective
1.3
Scope
1.4
References
1.5
Definitions
1.6
Acronyms, abbreviations and symbols
SECTION 2
Planning and execution
2.1
Introduction
2.2
General project requirements
2.3
Technical documentation
2.4
Risk management
2.5
Planning of marine operations
2.6
Operation and design criteria
2.7
Weather forecast
2.8
Organization of marine operations
2.9
2.10
2.11
SECTION 3
Monitoring
Inspections and testing
Vessels
Environmental conditions and criteria
3.1
Introduction
3.2
Design environmental condition
3.3
Design environmental criteria for weather restricted operations
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3.4
Design criteria for weather unrestricted operations
3.5
Weather/metocean forecast requirements
3.6
Benign weather areas
SECTION 4
Ballast and other systems
4.1
Introduction
4.2
System and equipment design
4.3
Ballasting systems
4.4
Guiding and positioning systems
4.5
ROV systems
SECTION 5
5.1
Loading and structural strength
Introduction
5.2
Design principles
5.3
Specific design considerations
5.4
Testing
5.5
Load categorisation
5.6
Loads and load effects (responses)
5.7
Failure modes
5.8
Analytical models
5.9
Strength assessment
5.10
SECTION 6
Materials and fabrication
Gravity based structure (GBS)
6.1
Introduction
6.2
Floating GBS stability and freeboard
6.3
Structural strength
6.4
Instrumentation
6.5
SECTION 7
GBS installation
Cables, pipelines, risers and umbilicals
7.1
Introduction
7.2
Codes and standards
SECTION 8
8.1
Offshore wind farm (OWF) installation operations
Introduction
8.2
Planning
8.3
OWF installation vessels
8.4
Planning and execution
8.5
Load-outs of OWF components
8.6
Transport of OWF components
8.7
Installation of OWF components
8.8
Lifting operations and lifting tools
8.9
Information required for MWS approval
SECTION 9
9.1
Road transport
Introduction
9.2
Requirements
9.3
Information required
SECTION 10
Load-out
10.1
Introduction
10.2
General
10.3
Loads
10.4
Design calculations
10.5
Systems and equipment
10.6
Vessels
10.7
Operational aspects
10.8
Special cases
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Information required
SECTION 11
Sea voyages
11.1
Introduction
11.2
Towage or transport design/approval flow chart
11.3
Motion response
11.4
Default motion criteria – General
11.5
Default motion criteria – IMO
11.6
Default motion criteria – Ships
11.7
Default motion criteria – Specific cases
11.8
Directionality and heading control
11.9
Design and strength
11.10
Floating stability
11.11
Transport vessel or barge selection
11.12
Tug selection
11.13
Towing equipment
11.14
Voyage planning
11.15
Bilge & ballast pumping systems
11.16
Anchors (and alternatives) and mooring arrangements
11.17
Manned voyages
11.18
Specific for multiple towages
11.19
Specific for towing in ice
11.20
Specific for towage in the Caspian Sea
11.21
Specific for FSUs (FPSOs, FSOs, FLNG facilities, FRSUs etc.)
11.22
Specific for jacket voyages
11.23
Specific for ship towage
11.24
Specific for voyage to scrapping
11.25
Specific for towing of pipes and submerged objects
11.26
Specific for deep draught towages
11.27
Specific for jack-up voyages
11.28
Approaching a jack-up location
11.29
Rig move procedures (for all MOUs)
11.30
Specific for semi-submersible voyages
11.31
Information required
SECTION 12
Tow out of dry-dock or building basin
12.1
Introduction
12.2
Dry dock/construction basin
12.3
Design and strength
12.4
Mooring and handling lines for tow-out
12.5
Intact & damage stability
12.6
Under-keel clearance for leaving basin
12.7
Side clearances
12.8
Under-keel clearance outside basin
12.9
Towage and marine considerations
12.10
SECTION 13
Information required
Jacket installation operations
13.1
Introduction
13.2
Environmental conditions
13.3
Strength
13.4
Jacket buoyancy, stability and seabed clearance
13.5
Jacket lift
13.6
Jacket launch
13.7
Floating controlled upend and set-down ballasting
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13.9
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Jacket position and set-down
Buoyancy tank
13.10
On-bottom stability and piling
13.11
Information required
SECTION 14
Construction afloat
14.1
Introduction
14.2
Loads and structures
14.3
Stability and damage stability
14.4
Mooring and fendering
14.5
Construction spread
14.6
Operational requirements
14.7
SECTION 15
Information required
Lift-off, mating and float-over operations
15.1
Introduction
15.2
General
15.3
Loads
15.4
Systems and equipment
15.5
Vessels
15.6
Operational aspects
15.7
Specific for lift-off operations
15.8
Specific for mating operations
15.9
Specific for float-over operations
15.10
Specific for docking operations
15.11
Information required
SECTION 16
Lifting operations
16.1
Introduction
16.2
Load factors
16.3
Derivation of hook, lift point and rigging loads
16.4
Sling and grommet design
16.5
Shackle design
16.6
Other lifting equipment design
16.7
Crane and installation vessel
16.8
Structural analysis
16.9
16.10
Lift point design
Fabrication yard lifts
16.11
Fabrication of rigging and lifting equipment
16.12
Certification and inspection of rigging and lifting equipment
16.13
Clearances
16.14
Bumpers and guides
16.15
Heave compensation
16.16
Operations and practical considerations
16.17
Subsea lifting and installation
16.18
Information required
SECTION 17
17.1
Mooring and dynamic positioning systems
Introduction
17.2
Codes and standards
17.3
Design environmental conditions
17.4
Environmental loads and motions
17.5
Mooring analysis
17.6
Design and strength
17.7
Clearances
17.8
Mooring equipment
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Procedural considerations
17.10
Special considerations for inshore & quayside moorings
17.11
Weather restricted mooring considerations
17.12
Information required
17.13
Dynamic positioning systems
SECTION 18
Decommissioning and removal of offshore installations
18.1
Introduction
18.2
General principles
SECTION 19
References
APPENDIX A
Introduction
APPENDIX B
Planning and execution
B.1
Documentation and certification for marine vessels
B.2
Documentation required for lifting, towing and mooring gear - Informative
B.3
Iceberg management operations
B.4
Ensemble forecasting - informative
APPENDIX C
C.1
Environmental conditions and criteria
General
C.2
Wind conditions
C.3
Wave conditions
APPENDIX D
Ballasting and other systems
APPENDIX E
Structural strength
E.1
Fillet weld checking
E.2
Bolted connections
APPENDIX F
Gravity based structure (GBS)
APPENDIX G
Cables, pipelines, risers and umbilicals
APPENDIX H
Offshore wind farm installations - Informative
H.1
Introduction
H.2
General
H.3
Cable challenges/cables
H.4
Specific challenges/considerations for array cables
H.5
Exclusions from marine warranty scope
APPENDIX I
Land transport
APPENDIX J
Load-out
APPENDIX K
Towage and sea transport
K.1
Example of main tow bridle with recovery system
K.2
Example of emergency towing gear
K.3
Example of Smit-type clench plate
K.4
Emergency anchor mounting on a billboard
K.5
Alternatives to the provision & use of an emergency anchor
K.6
Alternative arrangements for towing connections for ship towages
K.7
Example of cribbing / seafastening force calculations - Informative
K.8
Good practice recommendations for the tie-down of lifting slings - Informative
K.9
Good practice recommendations for towing - Informative
K.10
Ice Classification - Informative
K.11
Options for MOU voyages in ice - Informative
APPENDIX L
APPENDIX M
Tow out of dry-dock or construction basin
Jacket Installation
APPENDIX N
Construction afloat
APPENDIX O
Float-over, mating and float-off operations
APPENDIX P
Lifting operations - Informative
P.1
2-Hook lift - load factors and derivation of lift point loads
P.2
Padeye calculations
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P.3
Calculation of SKL
APPENDIX Q
Q.1
Mooring and dynamic positioning systems
Good practice recommendations for quayside mooring - Informative
Q.2
Dynamic positioning systems - Informative
APPENDIX R
Decommissioning and removal of offshore installations
DNVGL-ST-N001 Marine operations and
marine warranty (Edition: 2016-06)
SECTION 0 CHANGES – CURRENT
This document (DNVGL-ST-N001 - Edition 2016-06) replaces the legacy DNV-OS-H-series and all legacy GL
Noble Denton Guidelines except 0009/ND, 0016/ND, which are addressed in the DNVGL-ST-N002 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:
• DNV-OS-H101
• 0001/ND.
Sec.3 Environmental conditions and criteria
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and legacy DNV-OS-Hseries standards.
Sec.4 Ballast and other systems
This section replaces the following parts of the VMO Standard and the ND Guidelines:
•
•
•
•
•
DNV, Marine Operations, General, DNV-OS-H101
DNV, Load Transfer Operations, DNV-OS-H201
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
GL Noble Denton, Guidelines for Load-outs, 0013/ND
GL Noble Denton, Guidelines for Float-over 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 DNV-OS-Hseries 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, DNV-OS-H201.
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 Load-out
This section replaces the applicable sections of the following legacy documents:
• DNV-OS-H201, Load transfer operations
• GL Noble Denton, Guidelines for Load-outs, 0013/ND
Sec.11 Sea voyages
This section replaces the applicable sections of the following legacy documents:
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• DNV-OS-H202, Sea transport operations
• DNV-OS-H203, Transit and Positioning of Offshore Units
• GL Noble Denton, Guidelines For Marine Transportations, 0030/ND.
Sec.12 Tow out of dry-dock 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, DNV-OS-H201.
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 2-4), DNV-OS-H204
• GL Noble Denton, Guidelines for Steel Jacket Transportation & Installation, 0028/ND.
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 DNV-OS-H201 Load Transfer Operations.
Sec.15 Lift-off, mating and float-over operations
This section replaces the applicable sections of the following legacy documents:
• GL Noble Denton, Guidelines For Float-Over Installations / Removals, 0031/ND
• DNV Offshore Standard DNV-OS-H201 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 DNV-OS-H205 Lifting Operations (VMO Standard – Part 2-5)
• DNV Offshore Standard DNV-OS-H206 Load-out, transport and installation of subsea objects (VMO
Standard – Part 2-6).
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
DNV-OS-H101 Marine Operations, General
DNV-OS-H102 Marine Operations, Design and Fabrication
DNV-OS-H203 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, DNV-OS-H203.
Sec.18 Decommissioning and removal of offshore installations
This section replaces Section 14 of 0001/ND “General Guidelines for Marine Projects”.
SECTION 1 Introduction
1.1
General
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 DNVGL-SE-0080 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 know-how throughout the marine operation activities. It should solely be used by competent and
experienced organizations, and does not release the organizations involved from exercising sound 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 project-specific deviations from the standard criteria
provided that the results are acceptable. When such risk assessments show that the risk levels are increased
relative to those inherent in the standard criteria, the operation may be approved subject to disclosure by the
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
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.
1.1.12
SWL and WLL:
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a. Safe Working Load (SWL) has generally been superseded by Working Load Limit (WLL) though both are in
common use during the change-over 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.
b. 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).
c. 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.
1.2
Objective
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.
1.3
Scope
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 load-out, 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 jack-up location approval.
1.3.3
With the exception of location approval of MOUs (Mobile Offshore Units) which are covered in DNVGL-STN002, /39/, this standard covers most offshore assets and operations that are likely to require MWS approval.
1.4
References
1.4.1
Normative (i.e. mandatory) references
1.4.1.1
The standards and guidelines in Table 1-1 include provisions, through which reference in this text constitute
provisions of this standard.
Table 1-1 Normative (i.e. mandatory) standards
Id
Name
Date
Revision
14
AISC: 360/10
Specification for Structural Steel Buildings, (included in
AISC Steel Construction Manual 14th Edition)
2010
DNVGL-OS-C101
Design of offshore steel structures, general – LRFD
method
2015
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DNVGL-ST-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 jack-ups”
applies]
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
IMO ISPS Code
International Ship and Port Facility Security Code
(amendment to SOLAS convention)
2002
(effective
2004)
IMO Resolution
A.1024(26)
Guidelines for ships operating in polar waters
Jan 2010
ISO 19901-5
Petroleum and Natural Gas Industries “Specific
requirements for offshore structures – Part 5: Weight
control during engineering and construction”.
1.4.2
2016
2006
2008 and later
amendments
2002
2016
Informative references
1.4.2.1
All references appear in Sec.19.
1.5
Definitions
1.5.1
Verbal forms
Table 1-2 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.
1.5.2
Terms
1.5.2.1
Underlined definitions are defined elsewhere in Table 1-3.
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Table 1-3 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 11-3 and Figure 11-4)
24-hour Move
A jack-up move taking less than 24 hours between entering the water and reaching
a safe air gap with at least two very high confidence good weather forecasts for the
48 hours after entering the water, having due regard to area and season.
2nd intercept (angle)
The second angle of static inclination at which the wind overturning moment is
equal to the righting moment (see Figure 11-3 and Figure 11-4)
9-Part sling
A sling made from a single laid sling braided nine times with the sling rope and
eyes forming each eye of the 9-part sling.
A&R Winch
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
Non-dimensional 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 11-4)
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 non-propelled vessel commonly used to carry cargo or equipment.
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]
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Bird-caging
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.
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.
Cable-laid 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
Cable-laid 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 pipe-in-pipe.
Cats-paw
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.
Client
The company to which the MWS company is contracted to perform marine warranty
or consultancy activities.
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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 in-house examinations are made without fear or favour. However, this
should not be the same person who undertakes routine maintenance of the
equipment as they would then be 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 shear-leg, heavy lift vessel, semi-submersible 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.
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 float-over
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
float-over. It can be designed to either provide a rigid vertical support and allow
horizontal movement or utilise elastomers to absorb vertical and horizontal
installation motions and forces.
Deep water
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.
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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.
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 off-load
The reverse of floating on-load
Floating on-load
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
Float-Over
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 on-loaded and off-loaded.
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.
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Hydro-acoustic
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/
I-tube
A vertical tube fitted to offshore structures to install product between the seabed
and the structure topsides.
Jacket
A sub-structure, positioned on the seabed, generally of tubular steel construction
and secured by piles, designed to support topsides facilities.
Jack-up
A self-elevating MODU, MOU or similar, equipped with legs and jacking systems
capable of lifting the hull clear of the water.
J-Lay
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 sea-bed.
J-tube
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
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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
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 load-out,
lift-off, mating and float-over.
Load-in
The transfer of an assembly, module, pipes or component from a barge or vessel,
e.g. by horizontal movement or by lifting.
Load-out
The transfer of an assembly, module, pipes or component onto a barge or vessel,
e.g. by horizontal movement or by lifting.
Load-out Support
Frame (LSF)
A structural frame that supports the structure during fabrication and load-out and
may support the structure on a barge/vessel above grillage.
Load-out, floating
A Load-out onto a floating vessel.
Load-out, grounded
A Load-out onto a grounded vessel.
Load-out, lifted
A Load-out performed by crane.
Load-out, skidded
A Load-out where the structure is skidded, using a combination of skidways,
skidshoes or runners, propelled by jacks or winches.
Load-out, trailer
A Load-out 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 24-hour move, may be expected to be completed with the unit essentially in
24-hour field move configuration, without overstressing or otherwise endangering
the unit, having due regard to the length of the move, and to the area (including
availability of shelter points) and season.
Magnetic Particle
Inspection (MPI)
A Non-Destructive Testing (NDT) process for detecting surface and slightly
subsurface discontinuities in ferroelectric materials such as iron
Marine operation
See Operation
Marine Warranty
Survey company
MWS Company
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.
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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 short-term 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 non-drilling mobile units such as accommodation,
construction, lifting or production units including those used in the offshore
renewables sector.
Monopile
Tubular structure used as foundation for offshore wind turbine generator.
Moored Vessel
Within the scope of this document refers to any structure which is being moored.
Mooring System
Consists of all the components in the mooring system including shackles
windlasses and other jewellery and, in addition, rig/vessel and shore attachments
such as bollards.
Most Probable
Maximum Extreme
(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.
Non-Destructive
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]
Off-hire 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 on-hire survey as a basis for
establishing costs and liabilities.
Off-load
The reverse of load-out
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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.
On-hire 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 off-hire survey as
a basis for establishing costs and liabilities. It is not intended to confirm the
suitability of the equipment to perform a particular operation.
Operation reference
period
The Planned Operation Period, plus the contingency period. See [2.6.2] to [2.6.4]
Operation, marine
operation
Generic term covering, but not limited to, the following activities which are subject
to the hazards of the marine environment:
a. Load-out/load-in
b. Voyage
c. Lift/Lowering (offshore/inshore)
d. Tow-out/tow-in
e. Float-over/float-off
f. Jacket launch/jacket upend
g. Pipeline installation
h. Construction afloat
Operational Limiting
Criteria
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 long-term
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
Pipe-in-Pipe
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 start-up
point to lay-down 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
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
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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.
Pre-Loading
The testing of soil foundations or anchors by loading to check that they can take
subsequent loads. For jack-up foundations it is often done be adding water ballast
to pre-load tanks or (with units with more than 3 legs) by pre-driving by removing
load from other legs in turn.
Procedure
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 J-tube pull-in operation where the pull-in winch is mounted on the installation
vessel and the end of the pull-in wire connected to the cable runs from the vessel to
the J-tube bottom end up and over a sheave and back to the installation vessel
pull-in winch.
Quadrant
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.
Reel Lay (for rigid
pipe)
A laying method where the pipeline is pre-assembled into long strings or stalks and
wound onto a large reel with the pipe experiencing plastic deformation when
wound on and off the reel and straightened when reeled off. Typical lay angles of
20 to 90 degrees are achieved.
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 jack-up 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.
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Rope
An assembly of strands wrapped around a core. When a rope is used for cable-laid
sling or cable-laid grommet it is referred to as a unit rope (as per IMCA M 179 /81/).
Rotor
Configuration consisting of the complete set of blades, connected to the hub.
Route Planning List
(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 as-built cable route coordinates, installed slack and burial depths.
Routine lift
“Everyday” lift, without detailed design, planning or documentation, such as
general cargo lifting operations or lifting portable units on/off a supply vessel. See
[16.1.1.4].
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
Self-Propelled Modular
Transporter (SPMT)
A trailer system having its own integral propulsion, steering, jacking, control and
power systems.
Semi-submersible
A floating structure normally consisting of a deck structure with a number of widely
spaced, large cross-section, supporting columns connected to submerged
pontoons.
Serviceability Limit
State (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.
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 load-out if carried out as a
separate operation or may form part of a load-out. The site move may be subject to
approval if so desired.
Skew Load Factor
(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
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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 pay-out out against a pre-defined installation plan.
Slamming loads
Transient loads on the structure due to wave impact when lifting through the splash
zone.
S–Lay
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 sea-bed.
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 cable-laid sling) is connected
back into itself by tucking the tails of the strands (or unit ropes) back through the
main body of the rope (or unit ropes), after forming the sling eye
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.
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 stand-by 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 sub-assembly, 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 on-hire
survey.
Surge
Barge or vessel motion in the longitudinal direction OR
A change in water level caused by meteorological conditions
Survey
Attendance and inspection by a MWS company surveyor.
Other surveys which may be required for a marine operation, including suitability,
dimensional, structural, navigational and Class surveys.
Surveyor
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.
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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.
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 location-specific 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 non-propelled 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.
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.
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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
Under-Keel 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 cable-laid sling or cable-laid grommet may be constructed,
made from either 6 or 8 strands around a steel core, as indicated in ISO 2408 and
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 non-weather 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.
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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.
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].
1.6
Acronyms, abbreviations and symbols
1.6.1
Underlined acronyms and abbreviations in Table 1-4 are defined in Table 1-3.
Table 1-4 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
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 Tie-In
BAS
Burial Assessment Survey
BBL
Bridle Breaking Load
BHP
Brake Horse Power
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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
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
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FPSO
Floating Production, Storage and Offloading Vessel
FPU or FPV
Floating Production Unit or Floating Production Vessel
FRSU
Floating Storage Re-gasification Unit
FSD
Sling or grommet design load
FSE
Free Surface Effect
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
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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
Hydro-acoustic Positioning Reference
HSEQ
Health, Safety, Environment and Quality
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)
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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
Load-out Support Frame
MAOP
Maximum Allowable Operating Pressure
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
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OSS
Out of Straightness Survey
OTDR
Optical Time Domain Reflectometry
OWF
Offshore Wind Farm
PHC
Passive Heave Compensation
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/,
SOPEP
Shipboard Oil Pollution Emergency Plan
SPMT
Self-Propelled Modular Transporter
SSCV
Semi-submersible 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
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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
Zero-up crossing period for waves
UKC
Under-Keel 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
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
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SECTION 2 Planning and execution
2.1
Introduction
2.1.1
Scope
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2.1.1.1
This Section includes the general requirements for planning, organization, execution and documentation of
marine operations.
2.1.2
Revision history
2.1.2.1
This section replaces the following parts of the VMO Standard and the ND Guidelines:
• DNV-OS-H101
• 0001/ND.
2.2
General project requirements
2.2.1
Project organisation
2.2.1.1
An appropriate Project organisation chart shall be set up, illustrating how the marine operations integrate with
the rest of the project.
2.2.1.2
All project interfaces between (key) contractors shall be clearly defined.
2.2.1.3
For organisation during the marine operation see [2.8].
2.2.2
Health, safety and environment
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
2.2.3
Jurisdiction
2.2.3.1
Marine operations are subject to national and international regulations and standards on personnel safety and
protection of the environment. It should also be noted that a marine operation can involve more than one
nation’s area of jurisdiction, and that for barges and vessels the jurisdiction of the flag state will apply.
Documented relevant regulatory approval is a prerequisite to MWS approval.
2.2.3.2
If a part of the marine operations is to be carried out near other facilities or their surroundings any safety zone(s)
defined by the owner shall be duly considered.
2.2.4
Quality assurance and administrative procedures
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.
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2.2.5
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Technical procedures
2.2.5.1
Technical procedures shall be in place to control engineering related to the marine activities.
2.2.5.2
The technical procedures shall consider the planning and design process. For this process it is recommended
that the following sequence is adopted:
a. Identify relevant and applicable regulations, rules, company specifications, codes and standards, both
statutory and self-elected.
b. Identify physical limitations. This may involve pre-surveys of structures, local conditions and soil
parameters.
c. Plan the overall operation i.e. evaluate operational concepts, available equipment, limitations, economic
consequences, etc.
d. Describe/define unambiguously with adequate detailing the design basis and main assumptions, see
[2.2.7].
e. Carry out engineering and design analyses.
f. Develop operation procedures.
2.2.5.3
The procedures shall include sufficient information to ensure agreement and uniformity on all relevant matters
such as:
a.
b.
c.
d.
e.
f.
g.
h.
International and national standards and legislation
Certifying authority/regulatory body standards
Marine warranty survey company standards 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
peer-reviewed best industry practice, e.g. IMCA, MTS, GOMO, NORSOK, etc.
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2.2.6
New technology
2.2.6.1
Design and planning of marine operations shall as far as feasible be based on well proven principles, techniques,
systems and equipment.
2.2.6.2
If new technology or existing technology in a new environment is used, this technology should be documented
through an acceptable qualification process, e.g. in DNV-RP-A203, /45/.
2.2.7
Design basis and design brief
2.2.7.1
A design basis and/or a design brief shall be developed and provided for early acceptance in order to obtain a
common basis and understanding for all parties involved during design, engineering and verification.
2.2.7.2
The Design Basis should describe the basic input parameters, main assumptions, characteristic environmental
conditions, characteristic loads/load effects, load combinations and load cases, including those for the proposed
marine operations.
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2.2.7.3
The Design Brief(s) should describe the planned verification activities, analysis methods, software tools, input
specifications, acceptance criteria, etc.
2.3
Technical documentation
2.3.1
General
2.3.1.1
Fulfilment of all the requirements in this Standard applicable for the considered marine operation(s) shall be
properly documented. Guidance on required documentation is given throughout this Standard. However, it shall
always be thoroughly evaluated if additional documentation is required.
2.3.1.2
A document plan describing document hierarchy, issuance schedule and scope for each document should be
provided for major marine operations/projects.
Guidance note:
Normally this will be in the form of MDR(s) that are distributed for review/mark-up by involved parties including
the MWS Company.
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2.3.1.3
A system/procedure ensuring that all required documentation is produced in due time and distributed
according to plan, should be implemented.
2.3.1.4
It shall be ensured that all the documentation pertaining to a specific marine operation has been accepted by
Authorities, Company, other Contractors and MWS, as relevant, before any operation starts.
2.3.2
Documentation required
2.3.2.1
The design basis shall be clearly documented, see [2.2.7].
2.3.2.2
Environmental conditions for the actual area shall be documented by reliable statistical data, see Sec.3.
2.3.2.3
The acceptability of the following shall be documented: the object, all equipment, temporary or permanent
structures, vessels, etc. involved in the operation. Recognized certificates (e.g. classification documents) are
normally acceptable as documentation if the basis for certification is clearly stated and complies with the
philosophy and intentions of this Standard.
Guidance note 1:
By basis for certification it is meant acceptance standard, basic assumptions, design loads, including dynamics,
limitations, etc. For items without certificates see [2.3.2.4].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Note that all elements of the marine operation should be properly documented. This also includes onshore
facilities such as quays, bollards and foundations.
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2.3.2.4
Design calculations/analysis shall be documented by design reports and drawings.
2.3.2.5
The condition of all involved equipment, structures and vessels shall be documented as acceptable by means of
certificates 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 non-conformances are closed out. See also [2.11.2].
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2.3.2.6
Operational aspects shall be documented in form of operation manuals and records.
2.3.2.7
Relevant qualifications of key personnel shall be documented.
2.3.2.8
Required 3rd Party verification, e.g. to fulfil the warranty clause, shall be properly documented. See also [2.4.4].
2.3.3
Documentation quality and schedule
2.3.3.1
An integrated document numbering system for the entire project is suggested, including documents produced
by client, contractors, sub-contractors and vendors.
2.3.3.2
Documents relating to marine operations should be grouped into levels according to their status, for example:
a. Criteria and design basis documents
b. Procedures and operations manuals
c. Supporting documents, including engineering calculations, systems operating manuals and equipment
specifications and certificates.
2.3.3.3
The documentation shall demonstrate that philosophies, principles and requirements of this Standard are
complied with. This documentation shall be provided to the MWS Company.
Guidance note:
The operation and document type dictates the level of review by the MWS company. The following terms have
been used as an indication of the level of detail:
• Documented – An in-depth document that is subjected to a detailed review by the MWS company e.g.
analysis reports, 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.
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2.3.3.4
Documentation for marine operations shall be self-contained, or clearly refer to other relevant documents.
2.3.3.5
The quality and details of the documentation shall be such that it allows for independent reviews of plans,
procedures and calculations, for all parts of the operation.
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2.3.3.6
All significant updates shall be clearly identified in revised documents.
2.3.3.7
The document schedule shall allow for the required (agreed) time for independent reviews.
Guidance note:
The time available for review should be at least 10 working days, and more for complex documents.
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2.3.4
Input documentation
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.
2.3.5
Output documentation
2.3.5.1
Documentation shall be prepared to prove that all relevant design and operational requirements are fulfilled.
Typical output documentation is:
a. Planning documents including design briefs and basis, schedules, concept evaluations, general
arrangement drawings and specifications.
b. Design documentation including motion analysis, load analysis, global strength analysis, local design
strength calculations, stability and ballast calculations and structural drawings.
c. Operational manuals/procedures, see [2.3.7] and [2.9.5].
d. Operational records, see [2.3.8].
2.3.6
Availability of technical documentation
2.3.6.1
All relevant documentation shall be available and accessible on site or on board during execution of the
operation. 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 On-Scene Commander in the event of an emergency
situation and the interfaces between the various parties involved.
2.3.6.3
Vessel and equipment certificates and NDT reports shall be submitted. See [B.1] and [B.2] for the information
that is typically required.
Guidance note:
In order to avoid possible delays due to unacceptable or incomplete documentation, it is recommended that
such documentation is submitted for review as soon as possible.
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2.3.6.4
Procedure documents, intended to be used as an active tool during marine operations should include a section
which clearly shows their references to higher and lower level documents, and should list all inter-related
documents.
Guidance note:
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A document organogram is often helpful as shown in Figure 2-1.
Figure 2-1 Example of document organogram
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2.3.7
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Marine operation manuals
2.3.7.1
An operational procedure shall be developed for the planned operation, and shall reflect characteristic
environmental conditions, physical limitations, design assumptions and tolerances.
Guidance note:
For complex operations it is recommended that a high level presentation of the marine operation is made
available as an animation or picture series. See also 2.8.3.
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2.3.7.2
The operational procedures shall be described in a marine operation manual covering all aspects of the
operation and should include the following, as applicable:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
r.
s.
t.
u.
v.
w.
x.
y.
z.
aa.
ab.
ac.
ad.
reference documents
general arrangement
permissible load conditions
outline execution plan
organogram and lines of command
job-descriptions for key personnel
safety plan, see [2.3.7.5]
authorities and permits including notification and approval requirements
contractual approvals and hand over, see also [2.3.7.4]
environmental criteria, including design and operational criteria
weather (forecast) and current/wave reporting
operational bar chart, showing the anticipated duration of each activity, inter-related activities, key
decision points, hold points
specific step-by-step 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:
a. Risks inherent from the metocean conditions
b. Risks incurred by construction, transport, installation and commissioning activities
c. Risks to the environment
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d. Risks due to simultaneous operations (SIMOPS) – see IMCA M 203, /83/
e. Risks due to working on live assets, etc.
2.3.7.6
Essential documentation in the form of certificates, release notes and classification documents for all equipment
and vessels involved in the marine operation shall be enclosed and/or listed in the Manual. See also 2.3.6.3.
2.3.8
Operation records and reporting
2.3.8.1
The execution of marine operations shall be logged. Recording form templates shall be included in the marine
operations manual.
2.3.8.2
The following should as a minimum be recorded during the operation:
a.
b.
c.
d.
e.
log of (main) tasks carried out
any modifications in the agreed procedure
unexpected events and any deviations from or alterations of procedure imposed by such
environmental conditions 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 near-misses relevant to the safety of the structure or future marine operations shall be
reported to MWS company.
2.4
Risk management
2.4.1
General
2.4.1.1
Risk management shall be applied to the project to reduce the overall risk. The preferred approach is to address
the following:
a.
b.
c.
d.
Identification of potential hazards
Preventative measures to avoid hazards wherever possible
Controls to reduce the potential consequences of unavoidable hazards
Mitigation of the consequences, should hazards occur.
2.4.1.2
The overall responsibility for risk management shall be clearly defined when planning marine operations.
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Guidance note:
It is recommended that risk management is performed according to DNV-RP-H101, /54/, in order to ensure a
systematic evaluation and handling of risk. It is also a premise for a successful risk management that a project
team with sufficient competence to understand the marine operation and the potential risk/hazard is mobilized,
see [2.8].
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2.4.1.3
Risk evaluations shall be carried out at an early stage for all marine operations in order to define the extent of risk
management required, and to identify and mitigate risk as early in the design process as possible.
Guidance note 1:
The type and amount of risk evaluations should be based on the complexity of each marine operation. DNV-RPH101, /54/, Appendix D.5 gives advice on how to carry out initial risk evaluations. The effect of (planned)
redundancy, back-up, safety barriers, and emergency procedures should be taken into account in the (initial) risk
estimates. Contingency situations with a documented (joint) probability of occurrence less than 10-4 per
operation may be disregarded.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Ideally, each of the various studies outlined should be managed by a competent independent person familiar
with the overall concept, but outside the team carrying out the relevant system or structure design or operational
management.
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2.4.1.4
Risk assessments shall be documented and the mitigated risks accepted by the MWS company.
2.4.1.5
Detailed hazard studies should include the personnel and organisations involved in the design of structures and
systems, as well as those involved in the marine operation and the MWS company. The studies shall be
performed for:
a. Each major marine operation.
b. Each major system essential to the performance and safety of marine operations. For example, the power
generation and the ballast and compressed air systems.
Guidance note:
Hazard identification activities (see [2.4.2]) may be used to systematically evaluate risk applicable to any
operation, to compare levels of risk between alternative proposals or between known and novel methods,
and to enable rational choices to be made between alternatives.
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2.4.2
Hazard identification activities
2.4.2.1
Risk identification techniques and methods shall be used as applicable for the intended operation. Examples of
applicable techniques and methods are:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
Preliminary risk assessment in order to assess concepts and methods
Hazard Identification Analysis (HAZID)
Early Procedure Hazard and Operability study (EP HAZOP)
Hazard Identification and Risk Assessment (HIRA)
Design Review (DR)
System HAZOP
Failure Mode Effect (and Criticality) Analysis (FMEA/FMECA)
Procedure HAZOP
Semi-Quantitative Risk Analysis (SQRA)
Safe Job Analysis (SJA) / Job Safety Analysis (JSA).
Guidance note:
DNV-RP-H101, /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.
2.4.3
Risk reducing activities
2.4.3.1
Relevant corrective actions from the risk identifying activities shall be implemented in the planning and execution
of the operations.
2.4.3.2
The following risk reducing activities for marine operations shall be used as applicable for the intended
operation:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
Operational feasibility assessments
Document verification
Familiarisation
Personnel safety plans
Emergency preparedness
Marine readiness verification
Inspection and testing
Survey of vessels
Toolbox talk
Safe Job Analysis / Job Safety Analysis
Survey of operations.
Guidance note:
DNV-RP-H101, /54/, Appendix C describes the above listed risk reducing activities in detail. Note that Safe
Job Analysis is in DNV-RP-H101, /54/, mentioned only in Appendix B - Hazard Identification Activities.
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2.4.4
3rd 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.
2.5
Planning of marine operations
2.5.1
Philosophy
2.5.1.1
Marine operations shall be planned according to safe and sound practice, and according to defined codes and
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standards.
2.5.1.2
A marine operation shall be designed to bring an object from one defined safe condition to another.
Guidance note:
“Safe Condition” is defined as a condition where the object is considered to be exposed to a normal level of risk
of damage or loss (i.e. the risk is similar to that expected for the in-place condition). Normally this will imply a
(support) condition for which it is documented that the object fulfils the design requirements applying the
relevant weather unrestricted, see [2.6.6], environmental loads.
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2.5.1.3
Risk management, see [2.4], should normally be included in the planning.
2.5.2
Type of operation
2.5.2.1
To define the (sub-) operations as either weather unrestricted or weather restricted can have a great impact on
the safety and cost of the operation. Hence, the type of operation should, if possible, be defined early in the
planning process. See also [2.6.5].
2.5.2.2
The planning and design of marine operations should normally be based on the assumption that it can be
necessary to halt the operation and bring the object to a safe condition e.g. by reversing the operation.
2.5.2.3
For operations passing a point where the operation cannot be reversed, a point of no return (PNR) shall be
defined. The first safe condition after passing a PNR shall be defined and considered in the planning.
2.5.3
Operations in ice areas
2.5.3.1
The risk of significant ice shall be considered in the operation planning. I.e. operations in ice areas should be
subject to suitable ice management operations, details of which appear in [B.3].
2.5.3.2
Towages in ice are considered in [11.19] and voyages in [K.11].
2.5.3.3
The evacuation from rigs/offshore structures in ice shall be properly planned.
Guidance note:
ISO 19906, /103/ Clause 18 and Annex A.18 provide appropriate normative requirements and informative
guidance for escape, evacuation and rescue (EER) operations from Arctic offshore structures.
Additional guidance on the design of an appropriate EER system may be found in 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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2.5.4
Contingency and emergency planning and procedures
2.5.4.1
All possible emergency situations shall be identified, and contingency procedures or actions shall be prepared
for these situations.
Guidance note:
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Foreseeable emergencies and contingencies can include:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
r.
Severe weather
Planned precautionary action in the event of forecast severe weather
Structural parameters approaching pre-set limits
Stability parameters approaching pre-set limits
Failure of mechanical, electrical or control systems
DP or power failure "black ship"
Fire
Collision, grounding
Leakage, flooding
Pollution
Structural failure
Equipment failure
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, back-up equipment, supporting personnel,
emergency procedures and other relevant preventive measures and actions.
2.5.4.4
The contingency procedures should form part of the operational procedures.
2.6
Operation and design criteria
2.6.1
Introduction
2.6.1.1
Marine operations shall be executed ensuring that the assumptions made in the planning and design process are
fulfilled.
2.6.1.2
Marine operations shall be classified as weather restricted or as weather unrestricted (see [2.6.5]).
Guidance note:
The main difference between these operations is how the environmental loads are selected. See Table 5-1.
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2.6.2
Operation reference period - TR
2.6.2.1
The duration of marine operations shall be defined by an operation reference period, TR:
TR = TPOP+TC
where
TR
TPOP
TC
=
Operation reference period
=
Planned operation period
=
Estimated maximum contingency time.
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2.6.2.2
The start and completion points for the intended operation or parts of the operation shall be clearly defined. See
also [2.6.7.3] and [2.6.7.4].
2.6.3
Planned operation period – TPOP
2.6.3.1
The planned operation period, TPOP, shall if possible be based on a detailed schedule for the operation.
Guidance note:
In cases (e.g. in the early planning phase) were a detailed schedule is not available TPOP can be based on
experience with similar operations.
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2.6.3.2
The time estimated for each task in the schedule should be based on a reasonably conservative assessment of
experience with same or similar tasks.
Guidance note:
Normally a probability of (maximum) 10-20% of exceeding TPOP during the actual operations should be aimed at.
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2.6.3.3
Time delaying incidents that are experienced frequently should be included in TPOP.
2.6.4
Estimated contingency time – TC
2.6.4.1
Contingency time, TC, shall be added to cover:
a. General uncertainty in the planned operation time, TPOP
b. Unproductive time during the operation, e.g. to solve unforeseen procedural problems
c. 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.
• 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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2.6.5
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Weather unrestricted and restricted operations
2.6.5.1
An operation shall be defined as weather unrestricted, see [2.6.6], or weather restricted, see [2.6.7]. See [2.5.2]
and Figure 2-2 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 2-2.
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
Figure 2-2 Flow chart to determine whether an operation is weather restricted or weather unrestricted
2.6.6
Weather unrestricted operations
2.6.6.1
Marine operations that cannot be defined as weather restricted (see [2.6.5] and [2.6.7]) shall be defined as
weather unrestricted operations. Environmental criteria for these operations should be based on extreme value
statistics, see Sec.3. If found beneficial, operations of shorter duration may also be defined as weather
unrestricted.
Guidance note:
A reduction in the weather criteria based on extreme value statistics could in some situations be acceptable
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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.
Guidance note:
Note that certain operations require a start criterion although designed for weather unrestricted conditions.
Further information is given for the respective operations in this Standard.
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2.6.7
Weather restricted operations
2.6.7.1
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 2-3.
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 2-3.
Figure 2-3 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].
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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 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:
a. Increased risk for halting (and re-starting) due to additional operations.
b. Increased risk due to the nature of the “temporary” safe position of the object.
c. 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.
2.6.8
Operational limiting criteria
2.6.8.1
Operational limiting environmental criteria (OPLIM) shall be established and clearly described in the marine
operation manual.
2.6.8.2
The OPLIM shall not be taken greater than the minimum of:
• The 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, back-up equipment and contingency procedures.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--• 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.
2.6.9
Forecasted and monitored operational limits, alpha factor (α)
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:
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To ensemble weather forecasts which identify the expected ‘spread’ of weather conditions and assess the
probability of particular weather events could be an alternative for applying the tabulated alpha factors. Such
weather forecasts will anyhow give useful additional information to evaluate uncertain weather situations. Further
description of ensemble forecasting is in [B.4].
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2.6.9.2
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 2-3.
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:
a. The expected uncertainty in the weather forecast should be calculated based on statistical data for the
actual site and the operation schedule, i.e. TPOP. The Alpha Factor should be calibrated to ensure that the
probability of exceeding the operational environmental limiting criteria (OPLIM) by more than 50% in LRFD
(see [2.6.11]) is less than 10-4.
b. 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.
2.6.10
Selection of alpha factors
2.6.10.1
The (tabulated) Alpha Factor(s) shall be selected based on:
• The applicable table, see [2.6.10.4] and Table 2-1
• Operational limiting criteria, OPLIM, see [2.6.8]
• The planned operational period, TPOP, see [2.6.9.2]
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,
Hs= 4.0 m, the Alpha Factor is 0.79 for the first 12 hours, 0.76 for the next 12 hours and 0.73 for the last 12 hours
of the operation.
2.6.10.3
Design wave heights less than one (1) meter are normally not applicable for offshore operations. If a smaller
design wave height nevertheless has been applied the Alpha Factor 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 2-1
considering the applied weather forecast (WF) level, see [2.7.2], applicable environmental monitoring, see
[2.9.3], and design method (LRFD or ASD/WSD).
2.6.10.5
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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 α-factors for European waters. For details of the JIP see DNV
Report 2006_1756 Rev. 03, “DNV Marine Operation Rules, Revised Alpha Factor JIP Project”.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Table 2-1 Selection of Alpha Factor table(s)
WF level
A1
A2 & B
C
Environmental monitoring?
Yes
No
Yes
No
Yes
No
Wave Alpha Factor – LRFD
Table 2-7
Table 2-6
Table 2-5
Table 2-4
Table 2-3
Table 2-2
Table
2-14
Table
2-13
Table
2-12
Table
2-11
Table
2-10
Table 2-9
Wave Alpha Factor –
ASD/WSD
Wind Alpha Factor – LRFD
Table 2-8
Wind Alpha Factor –
ASD/WSD
Table 2-15
2.6.11
Tabulated alpha factor – LRFD method
2.6.11.1
The Alpha Factor for waves applying LRFD, see [5.9.8], shall be selected according to Table 2-1 and are given in
Table 2-2 through Table 2-7. Values for wind are in Table 2-8.
Table 2-2 LRFD Alpha Factor for waves, Level C – No Environmental Monitoring
Planned
Operation
Period [h]
Hs = 1
TPOP ≤ 12
0.65
TPOP ≤ 24
0.63
Operational limiting (OPLIM) significant wave height [m]
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.76
0.71
4 < Hs < 6
0.79
0.73
Linear
Interpolation
Hs = 4
0.80
0.76
Linear
Interpolation
0.73
Hs≥6
0.78
Linear
Interpolation
TPOP ≤ 36
0.62
0.76
TPOP ≤ 48
0.60
0.68
0.71
0.74
TPOP ≤ 72
0.55
0.63
0.68
0.72
Table 2-3 LRFD Alpha Factor for waves, Level C – With Environmental Monitoring
Planned
Operation
Period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.90
0.95
1.00
1.00
TPOP ≤ 12
0.72
0.84
0.87
0.88
TPOP ≤ 24
0.66
TPOP ≤ 36
0.62
TPOP ≤ 48
0.60
0.68
0.71
0.74
TPOP ≤ 72
0.55
0.63
0.68
0.72
Linear
Interpolation
0.77
0.71
Linear
Interpolation
0.80
0.73
Linear
Interpolation
0.82
0.76
Table 2-4 LRFD Alpha Factor for waves, Level A2 or B – No Environmental Monitoring
Planned
Operation
Period [h]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 12
0.68
Linear
0.80
Linear
0.83
Linear
0.84
Operational limiting (OPLIM) significant wave height [m]
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TPOP ≤ 24
0.66
0.77
TPOP ≤ 36
0.65
TPOP ≤ 48
0.63
0.71
0.75
0.78
TPOP ≤ 72
0.58
0.66
0.71
0.76
Interpolation
0.75
0.80
0.77
Interpolation
0.82
Interpolation
0.80
Table 2-5 LRFD Alpha Factor for waves, Level A2 or B – With Environmental Monitoring
Planned
Operation
Period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.90
0.95
1.00
1.00
TPOP ≤ 12
0.72
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 2-6 LRFD Alpha Factor for waves, Level A1 – No Environmental Monitoring
Planned
Operation
Period [h]
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
Operational limiting (OPLIM) significant wave height [m]
1 < Hs < 2
Linear
Interpolation
Hs = 2
0.78
2 < Hs < 4
Linear
Interpolation
Hs = 4
0.80
4 < Hs < 6
Linear
Interpolation
Hs≥6
0.84
Table 2-7 LRFD Alpha Factor for waves, Level A1 – With Environmental Monitoring
Planned
Operation
Period [h]
Operational limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.90
0.95
1.00
1.00
TPOP ≤ 12
0.78
0.91
0.95
0.96
TPOP ≤ 24
0.72
TPOP ≤ 36
0.68
TPOP ≤ 48
0.66
0.75
0.78
0.81
TPOP ≤ 72
0.61
0.69
0.75
0.79
Linear
Interpolation
0.84
0.78
Linear
Interpolation
0.87
0.80
Linear
Interpolation
0.90
0.84
2.6.11.2
The appropriate Alpha Factor for wind should be selected (estimated) considering the following:
•
•
•
•
Statistical data and local experience for the actual site.
Planned operation period from issuance of weather forecast, TPOP.
Applied wind speed compared with the maximum possible wind speed, i.e. 10 year return wind speed.
Criticality of exceeding the design wind speed, e.g. by considering the contribution from wind on the total
design load.
2.6.11.3
If no reliable data is available the Alpha Factors indicated in Table 2-8 shall be considered as the maximum
allowable.
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Table 2-8 LRFD Recommended Alpha Factor for wind
Operational limiting (OPLIM) wind speed – Vd
Planned Operation Period
Vd < 0.5 x V10 year return
Vd > 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.
2.6.12
Tabulated alpha factor - ASD/WSD method
2.6.12.1
The Alpha factors for waves and wind applicable to the ASD/WSD, see [5.9.7] design approach shall be selected
based on Table 2-1 and are shown in Table 2-2 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 2-9 ASD/WSD Alpha Factor for waves, Level C – No Environmental Monitoring
Planned
Operation
Period [h]
Hs = 1
TPOP ≤ 12
0.58
TPOP ≤ 24
0.56
Operational Limiting (OPLIM) significant wave height [m]
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.68
0.63
4 < Hs < 6
0.70
0.65
Linear
Interpolation
Hs = 4
0.71
0.68
Linear
Interpolation
0.65
Hs≥6
0.69
Linear
Interpolation
TPOP ≤ 36
0.55
0.68
TPOP ≤ 48
0.53
0.61
0.63
0.66
TPOP ≤ 72
0.49
0.56
0.61
0.64
Table 2-10 ASD/WSD Alpha Factor for waves, Level C – With Environmental Monitoring
Planned
Operation
Period [h]
Operational Limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.80
0.85
0.89
0.89
TPOP ≤ 12
0.64
0.75
0.77
0.78
TPOP ≤ 24
0.59
TPOP ≤ 36
0.55
TPOP ≤ 48
0.53
0.61
0.63
0.66
TPOP ≤ 72
0.49
0.56
0.61
0.64
Linear
Interpolation
0.69
0.63
Linear
Interpolation
0.71
0.65
Linear
Interpolation
0.73
0.68
Table 2-11 ASD/WSD Alpha factors (waves) - Level A2 or B – No Environmental Monitoring
Planned
Operation
Period [h]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 12
0.61
0.71
Linear
Interpolation
0.75
0.59
Linear
Interpolation
0.74
TPOP ≤ 24
Linear
Interpolation
TPOP ≤ 36
0.58
Operational Limiting (OPLIM) significant wave height [m]
0.69
0.67
0.71
0.69
0.73
0.71
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TPOP ≤ 48
0.56
0.63
0.67
0.69
TPOP ≤ 72
0.52
0.59
0.63
0.68
Table 2-12 ASD/WSD Alpha Factor for waves, Level A2 or B – With Environmental Monitoring
Planned
Operation
Period [h]
Operational Limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.80
0.85
0.89
0.89
TPOP ≤ 12
0.64
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
0.71
Linear
Interpolation
0.69
Linear
Interpolation
0.73
0.71
Table 2-13 ASD/WSD Alpha factors (waves) - Level A1 – No Environmental Monitoring
Planned
Operation
Period [h]
Hs = 1
TPOP ≤ 12
0.64
TPOP ≤ 24
0.61
Operational Limiting (OPLIM) significant wave height [m]
1 < Hs < 2
Hs = 2
2 < Hs < 4
0.75
0.69
4 < Hs < 6
0.77
0.71
Linear
Interpolation
Hs = 4
0.78
0.75
Linear
Interpolation
0.71
Hs≥6
0.77
Linear
Interpolation
TPOP ≤ 36
0.61
0.75
TPOP ≤ 48
0.59
0.67
0.69
0.72
TPOP ≤ 72
0.54
0.61
0.67
0.70
Table 2-14 ASD/WSD Alpha factors (waves) - Level A1 – With Environmental Monitoring
Planned
Operation
Period [h]
Operational Limiting (OPLIM) significant wave height [m]
Hs = 1
1 < Hs < 2
Hs = 2
2 < Hs < 4
Hs = 4
4 < Hs < 6
Hs≥6
TPOP ≤ 4
0.80
0.85
0.89
0.89
TPOP ≤ 12
0.69
0.81
0.85
0.85
TPOP ≤ 24
0.64
TPOP ≤ 36
0.61
TPOP ≤ 48
0.59
0.67
0.69
0.72
TPOP ≤ 72
0.54
0.61
0.67
0.70
Linear
Interpolation
0.75
0.69
Linear
Interpolation
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 2-15 shall be considered as the maximum
allowable in ASD/WSD. See also [2.6.11.2] and [2.6.11.4].
Table 2-15 ASD/WSD Alpha factors (wind - all forecast requirements)
Operational Limiting Wind Speed (Vd)
Planned Operation Period
Vd < 0.5 x V10 year return
Vd > 0.5 x V10 year return
TPOP ≤ 24
0.71
0.76
TPOP ≤ 48
0.67
0.71
TPOP ≤ 72
0.62
0.67
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2.7
Weather forecast
2.7.1
General
Page 51 of 543
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.
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2.7.1.2
Independent weather forecasts shall be taken from different weather providers. The providers shall be different
organizational bodies. Each body shall document which different atmospheric and oceanographic models have
been evaluated and taken into account in the generation of the forecasts.
2.7.1.3
The weather forecasts (WF) shall be area/route specific. For non-stationary marine operations (e.g. sea voyages
or subsea laying operations) it shall be ensured that weather forecasts comprise the position (at the time of the
WF) of the transport vessel/barge and all alternative routes that could be chosen in the period covered by the
weather forecast.
2.7.1.4
Weather forecast procedures should consider the nature and duration of the planned operation, see [2.7.2.1].
2.7.1.5
The weather forecasts shall be in writing and the confidence level(s) should be stated.
2.7.1.6
In addition to a general description of the weather situation and its predicted development, the weather forecast
shall, as relevant, include:
•
•
•
•
•
•
•
•
wind speed and direction
waves and swell, significant and maximum height, mean or peak period and direction
rain, snow, lightning, ice etc.
tide variations and/or storm surge
visibility
temperature
barometric pressure
possibility for unpredictable strong wind, see [2.6.11.4].
for each 12 hours for a minimum of the TR plus 24 hours. In addition an outlook for at least the next 24 hours
should normally be included.
2.7.1.7
The forecast shall clearly define forecasted parameters, e.g. average time and height for wind, characteristic
wave periods (Tz or Tp). The content and format of the weather forecast should be agreed with the meteorologist
in due time before the operation starts.
2.7.2
Weather forecast levels
2.7.2.1
The required weather forecast level shall be selected based on the operational sensitivity to weather conditions
and the operation reference period (TR). The following weather forecast levels are defined in this standard:
• Level A that applies to major marine operations sensitive to environmental conditions.
• Level B that applies to environmental sensitive operations of significant importance with regard to value
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and consequences
• Level C that applies to conventional marine operations less sensitive to weather conditions, and carried
out on a regular basis.
2.7.2.2
For operations that require a Level A weather forecast it shall be thoroughly considered to have the dedicated
meteorologist present on site. See Table 2-16 for further advice regarding selection of the forecast level and for
requirements to the weather forecast procedure.
Table 2-16 Weather forecast levels
Weather Forecast
Level
A1
Operation
Sensitivity
A2
High
•
•
•
•
mating operations
offshore float over
multi barge towing
major (e.g. GBS) tow out
operations
• offshore installation
operations
• jack-up rig moves.
• sensitive laying
operations
Examples
Meteorologist on
site
Yes
No
Dedicated
Meteorologist
Yes
Yes
C1)
Moderate
Low
• tow-out
operations
• weather routed
sea transports
• offshore lifting
• subsea
installation
• semisubmersible rig
moves
• standard laying
operations.
• onshore/inshore
lifting
• load-out 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
Minimum
independent WF
sources2)
Maximum WF
interval
B
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].
2.7.3
Acceptance criteria
2.7.3.1
The acceptance criteria for the weather forecast(s) shall clearly define the applicable limitations, see [2.6.9] and
the minimum required weather window, see [2.6.2] and Figure 2-3. 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.
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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.
2.8
Organization of marine operations
2.8.1
General
2.8.1.1
The organisation and responsibility of key personnel involved in marine operations shall be established and
described before execution of marine operations. The responsibilities and duties of each function shall be clearly
defined to minimise uncertainties and overlapping responsibilities.
2.8.1.2
Organisation charts, including names and functional titles of key personnel, shall be included in the marine
operations manual. Authority during the operation shall be clearly defined.
2.8.1.3
Operations shall be carried out in accordance with the conditions for design, the approved documentation, and
sound practice, such that unnecessary risks are avoided. This is the responsibility of the operation
superintendent or manager.
2.8.1.4
Responsibilities in possible emergency situations shall be described.
2.8.1.5
Access to the area for the operation should be restricted. Only authorised personnel should be allowed into the
operation area.
Guidance note:
Where necessary, a suitable security and tracking system should be in use to record personnel on the structure
or vessels, to track their whereabouts.
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2.8.2
Qualification and training
2.8.2.1
Operation supervisors shall possess thorough knowledge and have experience from similar operations. Other
key personnel shall have knowledge and experience within their area of responsibility.
2.8.2.2
CVs for supervisors and key personnel involved in major marine operations shall be submitted.
2.8.2.3
Vessel manning and personnel qualifications shall as a minimum fulfil statutory requirements. Additional
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manning shall be considered for complex operations or to satisfy specific project requirements.
2.8.2.4
Adequate training appropriate to each individual’s function and situation should be given, including job training,
site safety training and briefings, marine safety and survival training.
2.8.2.5
A qualification matrix is recommended for correct tracking and control of personal qualifications.
2.8.2.6
Computer simulation and training, and/or model tests can give valuable information for the personnel carrying
out the operation. Where relevant, a full-mission simulation should be undertaken.
2.8.3
Familiarisation and briefing
2.8.3.1
Operation supervisors shall familiarise themselves with all aspects of the planned operations and possess a
thorough knowledge with respect to limitations and assumptions for the design.
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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2.8.4
Communication and reporting
2.8.4.1
Communication lines and primary and secondary means of communication shall be defined, preferably in a
communication chart, including as appropriate:
•
•
•
•
•
•
•
•
•
•
•
•
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
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• Emergency response.
2.8.4.2
Communication systems, including radio channels, telephone numbers, e-mail addresses and out-of-hours
numbers shall be identified and checked for accuracy.
2.8.4.3
The primary operational communication system should be used only for information needed for managing and
controlling the operation. Important information should be given dedicated lines/channels.
2.8.4.4
The planned flow of information during the operation shall be described.
2.8.4.5
A common language understood by all personnel involved should be used for VHF/UHF communication. Radio
channels should be allocated early to avoid possible interference.
Guidance note:
If a common language could lead to misunderstandings, it can be acceptable to use two or more languages.
Such communication needs to be duly planned and rehearsed.
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2.8.4.6
Communication of important information that may be misunderstood, e.g. monitoring results, should be
confirmed in writing.
2.8.4.7
All communication and reporting should be made available for continuous monitoring by the MWS during the
operation. (See also [2.3.8]).
2.8.5
Shifts
2.8.5.1
For operations with a planned duration exceeding 12 hours, a shift plan shall be established.
2.8.5.2
Where personnel changes occur during the course of an operation because of shift changes, these shall be
identified. Every effort should be made to avoid changes of key personnel during critical stages of the operation.
2.8.5.3
Where transfer of responsibility is involved, times of and procedures for hand-over from one organisation to
another (e.g. from fabrication to marine operations, from on-shore to offshore) shall be identified.
2.8.5.4
When continuous operations using more than 1 shift are not standard practice then special provision to prevent
fatigue shall be made for operations that could continue beyond normal working hours. This includes provision
of suitably experienced and briefed alternate personnel with good hand-overs at each shift change.
2.9
Monitoring
2.9.1
General
2.9.1.1
Actual parameters should be monitored and compared against those used in design to as great an extent as
practicable during and also if applicable before marine operations.
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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 back-up requirements to monitoring instrumentation systems are given in [4.2].
2.9.2
Environmental conditions
2.9.2.1
Environmental conditions can be monitored by both direct monitoring of environmental conditions and by
monitoring responses caused by environmental effects, see [2.9.3].
2.9.2.2
For marine operations particularly sensitive to environmental conditions such as 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.
2.9.3
Loads and/or responses
2.9.3.1
Full scale monitoring can be used for the determination of responses (e.g. accelerations on a vessel) or loading
effects (e.g. strain-gauge measurements). All full scale load and/or response monitoring 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:
• 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:
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Full scale monitoring has limitations, e.g. as indicated above, that need to be duly considered if such monitoring
is used as an (assisting) operational means of control.
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2.9.4
Alpha factor related monitoring
2.9.4.1
It shall be documented that monitoring systems and procedures used as a means to increase the Alpha Factor
for waves have adequate accuracy and reliability. Normally this implies fulfilment of all the following:
• Continuous monitoring.
• The monitoring device should be adequately located (e.g. no shielding effects) to give correct readings
and not in any case more than 3 (three) nautical miles from the location of the operation.
• 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.
• Feed-back to meteorologist(s)
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2.9.5
Monitoring procedure
2.9.5.1
A monitoring procedure describing at least monitoring methods and intervals, responsibilities, reporting and
recording shall be prepared.
2.9.5.2
Any unforeseen monitoring results shall be reported without delay.
2.9.6
Back-up and contingency
2.9.6.1
The requirements of [4.2.1.10] apply.
2.9.6.2
If the monitoring back-up system does not have the same accuracy as the original system this should be
considered in the contingency planning.
2.10
Inspections and testing
2.10.1
General
2.10.1.1
Testing and inspection of equipment, structures, systems and vessels shall be carried out according to relevant
and recognized codes/standards and/or relevant specifications, functional requirements and assumptions for the
design.
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2.10.1.2
Inspection during the operation shall include a systematic review and evaluation of monitoring results, see [2.9].
2.10.1.3
The MWS company shall identify any inspections and tests to be witnessed by its own representatives.
2.10.2
Test program
2.10.2.1
The required inspections and tests both in the preparation phase and during the operation shall be described in
a test and inspection program.
2.10.2.2
The test and inspection results shall be documented.
Guidance note:
The inspections and testing can be documented by reports and completed checklists.
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2.10.2.3
For larger operations it is recommended that a test/commissioning program is developed specifying the
planned inspections and tests. The test program should indicate expected characteristics, and state acceptance
criteria based on the design assumptions.
Guidance note:
Acceptance criteria for tests may also be functional requirements.
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2.10.3
Systems
2.10.3.1
All systems and their back-up shall be tested before the start of an operation. Such tests shall demonstrate that
they function as intended. If critical, the capacity of the system shall be adequately checked.
2.10.3.2
Change over from a primary to a secondary system shall be tested.
2.10.3.3
Instrumentation systems shall be calibrated and tested before the operation. The calibration procedure may be
subject to review.
2.10.3.4
Essential systems shall be function and capacity tested in their final configuration and connected to the same
power supply/HPU as intended to be used during operation. If several consumers are connected to the same
power supply/HPU, the test should be performed realistically with all consumers running in order to test capacity.
2.10.3.5
Emergency systems/functions and fail safe configurations should, as far as practically possible, be tested in a
realistic scenario with adequate loading.
2.10.4
Communication
2.10.4.1
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Primary and secondary means of communication shall be tested before operation.
2.10.4.2
For operations with complex communication and reporting procedures, or where proper information flow is vital,
a run-through of communication routines shall be carried out.
Guidance note:
This rehearsal should be performed with the nominated personnel and under conditions similar to those
expected during the actual operation. See also [2.8.4.5].
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2.10.5
Inclining tests
2.10.5.1
The requirement to perform inclining and/or displacement tests shall be agreed with the MWS Company.
Guidance note:
Vessels with a valid Trim and Stability booklet, including all modifications since the last inclining test, do NOT
normally require an inclining test when conservative estimates of cargo weight and centre of gravity show
adequate reserves of intact and damage stability.
Where ideally an inclining test would be performed but may not give sufficiently accurate results the calculations
may be based on outputs from the weight control programme checked against a displacement test. This would
only apply if there is a sufficient reserve of stability to cover possible inaccuracies.
Where a number of very similar units are constructed at the same place, the requirement for inclining tests on the
later units may be reduced after a study of weight variations (from displacement tests) and Centre of Gravity
variations (from inclining tests) of the previous units, and agreement with the MWS company.
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2.10.5.2
Where inclining and/or displacement tests are required:
a. They should be performed before any marine operation where the displacement, centre of gravity or
stability may be critical.
b. They should be performed according to guidelines in IMO Intact Stability Code 2008, /89/, Part B Annex 1.
c. if applicable an allowance shall be made for the presence and compressibility of any air cushion
d. 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).
e. 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.
f. 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.
g. A sensitivity analysis of the parameters affecting the test results should be performed.
2.11
Vessels
2.11.1
General
2.11.1.1
This 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.
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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
See [17.13] for further requirements to Dynamic Positioned vessels.
2.11.2
Condition and inspections
2.11.2.1
All vessels shall be in acceptable condition and with valid certificates, see [B.1].
2.11.2.2
All vessels involved in the operations should be inspected before the operation to confirm compliance with the
design assumptions, validity of certificates, suitability (see [2.11.1.4]) and acceptable condition.
2.11.2.3
The global and local condition of the vessels with respect to corrosion shall be confirmed and considered in
strength verifications.
2.11.3
Structural strength
2.11.3.1
Adequate global and local structural strength shall be documented for all vessels.
Guidance note:
The strength may be documented by either ensuring that the vessel is operated within the 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.
2.11.4
Class 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.
2.11.5
Certificates
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.426-MSC/Circ.1151.
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2.11.6
Navigation 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 un-manned.
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 “catch-rain” orientation.
2.11.7
Contingency situations
2.11.7.1
All vessels shall be selected with due consideration to possible contingency situations.
Guidance note:
This could e.g. result in the selection of redundant (twin screw) tugs for towing operations in narrow waters. See
also the operation-specific requirements in Sec.10 to Sec.18 of this Standard for further guidance.
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2.11.7.2
Where several tugs (vessels) are involved, a stand-by tug to assist or remove vessels in case of black-out, engine
failure, etc. should be considered.
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SECTION 3 Environmental conditions and criteria
3.1
Introduction
3.1.1
General
3.1.1.1
This Section 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.
3.1.2
Scope
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
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 location-specific metocean parameters.
3.1.3
Revision history
3.1.3.1
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and legacy DNV-OS-Hseries standards.
3.2
Design environmental condition
3.2.1
The design environmental condition consists of the wave height, wind speed, current and other relevant
environmental conditions specified for the design of a particular marine operation.
3.2.2
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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].
3.3
Design 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.3.2
The ratio between forecasted wind and design wind should be determined in accordance with Table 2-8 or
Table 2-15 as applicable.
3.3.3
Wave conditions for weather restricted operations, i.e. operations with wave heights (and/or periods) selected
independent of statistical data, should be as described by [C.3.4].
3.3.4
The significant wave height(s) and associated period(s) should be selected considering:
•
•
•
•
Feasibility and safety of the planned operation.
Typical weather conditions at the site.
Operation period.
Uncertainties in weather forecasts.
Guidance note:
Other factors such as the length of delay that can be accepted due to waiting on weather, and possible
contractual obligations should be considered as found relevant.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.3.5
Maximum wave height for weather restricted operations should be calculated according to the following
equation:
Hmax = STF × Hs
where
STF
=
2.0 for all reference periods.
Guidance note:
For short reference periods STF < 2.0 may be acceptable. See DNV-RP-H102, /55/, Table 2.2 for guidelines.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--An appropriate range of wave periods associated with Hmax should be considered. In the absence of other data,
the range of Tass can be taken as:
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3.3.6
Where relevant, applicable information from [3.4] may be used e.g. [3.4.12].
3.4
Design criteria for weather unrestricted operations
3.4.1
General
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.
3.4.2
Environmental statistics
3.4.2.1
Environmental phenomena are usually described by physical variables of statistical nature. Statistical data should
as far as possible be used to establish characteristic environmental conditions. The statistical description should
reveal the extreme conditions as well as the long and short-term variations.
3.4.2.2
Statistical data used as basis for establishing characteristic environmental criteria shall cover a sufficiently long
time period.
Guidance note:
For meteorological and oceanographic data a minimum of three to four years of data collection is
recommended. When using seasonal data longer periods are required. See DNV-RP-C205 /46/ for more info.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.4.2.3
The validity of older (typically more than 20 years) statistical data should be carefully considered with respect to
both monitoring methods/accuracy and possible long term climate changes.
3.4.2.4
If statistical environmental data are assumed to follow a two-parameter Weibull distribution, the regression
analysis should be performed with emphasis on a correct representation of the extreme values.
Guidance note:
Regression analysis of two-parameter Weibull distributions are recommended based on the 30% highest data
points, i.e. P(x > X) = 0.3.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.4.3
Return 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 jack-ups see DNVGL-ST-N002, /39/.
3.4.3.2
As general guidance, the criteria in Table 3-1 may be applied provided that the independent extremes are
considered concurrently.
3.4.3.3
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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:
When including operational errors, the level of probability of total loss per operation cannot be accurately
defined. However, the recommendations and guidance given in this Standard are introduced in order to obtain a
probability of total loss As Low As Reasonable Practicable (ALARP principle).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.4.3.4
The return periods for the ASD/WSD approach have been calibrated with the objective of ensuring that a given
structure will be treated equally under ASD/WSD and LRFD. The inherent safety margin in ASD/WSD checks is
less than that in LRFD checks, so that higher design values are needed to achieve this equivalence.
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 N-year
return period extremes for say March, perform extreme value analysis on the subset of data for March, consisting
of 3 hr sea-states, 240 per month in the data, and fit a Weibull curve to the cumulative distribution function.
Select the required probability level for the N-yr extreme calculated as: 1/(365.25*8*N*C) where C = 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 all-year 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 3-1 Metocean minimum design return periods, Td – unrestricted operations
Operation
reference
period
ASD / WSD 3)
LRFD 3)
Wind 1)
Wave 2) and
Current
Wind 1)
Wave 2) and
Current
Up to 3 days 4)
Td≥5 year
Td≥3 month
Td≥10 year
Td≥1 month
3 to 7 days
Td≥10 year
Td≥1 year
Td≥10 year
Td≥3 month
7 days to 1
month
Td≥25 year, (or obtain
from 10 yr and 50 yr
environmental criteria
values using: 10yr +
0.7*(50 yr-10 yr) )
Td≥10 year
Td≥10 year
Td≥1 year
1 month to 1
year
Td≥75 year (or obtain
from 50 yr and 100 yr
environmental criteria
values using: 50yr
+0.7*(100 yr-50 yr) )
Td≥50 year
Td≥100 year
Td≥10 year
More than 1
year
100 year return
Td≥100 year
Td≥100 year
Td≥100 year
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Notes:
1. More accurate design wind speeds may be determined as a function of the operation reference period
and site-specific metocean parameters using the method shown in [C.1].
2. More accurate design waves may be determined as a function of the operation reference period and
site-specific metocean parameters using the method shown in [C.3].
3. See [3.4.3.6].
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.
3.4.4
Return periods for determining environmental criteria for moorings
3.4.4.1
Table 3-2 identifies minimum return periods applicable to a various of mooring types for weather unrestricted
operations. The return periods specified in this document are based on ISO 19901-7 /100/, however the
selection of return period will depend on the choice of the design code (See 17.2 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 3-2, for example ISO 19901-7, Annex B.
Table 3-2 Return periods for determining environmental criteria for moorings 1)
MOORING TYPE
Quayside/Inshore
RETURN PERIOD
100 year 1)
Offshore - Mobile near another asset
10 year
Offshore - Mobile in Open Location
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
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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.
3.4.5
Use of seasonal/directional metocean data for moorings
3.4.5.1
Metocean data specific to the month(s) or season(s) during which the mooring will be utilised may be used
where appropriate.
3.4.5.2
Directional metocean data may also be used with suitable spreading functions to reflect directional divergence
in the design environment.
3.4.6
Wind
3.4.6.1
The averaged wind velocity over a defined time is referred to as the mean wind.
Guidance note:
Forecasted wind velocity is normally given as the 10 minute mean wind (tmean = 10 min) at a reference height of
10 m (z = 10 m).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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]
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.4.6.3
For dynamic wind analysis the mean wind period recommended for the applied wind spectrum should be used.
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See [3.4.6.7].
3.4.6.4
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 (tr, mean)
according to the equation below, see also Table 3-3 and ISO 19901-1 “Metocean design and operational
considerations”, /98/.
Where:
z
=
zr
=
tmean
=
tr, mean
=
U(z, tmean) =
U(zr, tr,
=
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 3-3 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.4.6.6
Gust wind: For elements or systems sensitive to wind oscillations (e.g. where dynamics or fatigue governs the
design) the short and long term wind variations should be considered.
3.4.6.7
The wind variations may be described by a wind spectrum. See e.g. DNV-RP-C205, /46/; NORSOK N-003, /111/
or ISO 19901-1, /98/.
3.4.6.8
Squalls: If squalls are possible during a marine operation maximum realistic (in the actual area) characteristic
wind speeds during squalls shall be considered in the planning and execution of the operation.
Guidance note:
Squalls are strong winds (22 knots or more) characterised by a sudden onset, duration of minimum 1 minute, and
then a rather sudden decrease in speed. Squalls are caused by advancing cold air and are associated with active
weather such as thunderstorms. Their formation is related to atmospheric instability and is subject to seasonality.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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3.4.7
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Wind for moorings
3.4.7.1
In addition to the requirements in [3.4.6], for permanent moorings the more onerous of the following should be
considered:
• Steady one minute mean velocity; or
• One hour mean plus a suitable gust spectrum. Generally the ISO 19901-1 gust spectrum, /98/, would be
applicable unless an alternative can be clearly justified.
3.4.7.2
For mobile moorings either a steady state wind speed or a suitable gust spectrum may be used depending upon
the stiffness of the mooring system.
3.4.7.3
For inshore or quayside moorings care shall be taken to ensure that all natural periods of response of the system
have been considered. Some of the mooring system response periods may be shorter than one minute but on
the other hand the use of shorter gust periods may not represent a sustained design wind that will act at the
same time across the whole of the structure. The representative design wind sampling period, therefore, has to
be carefully established on a case by case basis for inshore and quayside moorings, but the averaging time shall
not be longer than 1 minute if applying static wind load.
3.4.7.4
For locations prone to squall events, system design should include assessment for squall events. Guidance on
squall assessment is provided in DNVGL-OS-E301, /27/.
3.4.8
Waves - design methods
3.4.8.1
Wave conditions are defined by characteristic wave height, Hc, or the significant wave height, Hs, and
corresponding periods.
3.4.8.2
Wave conditions for design may be described either by a deterministic design wave method, or by a stochastic
method.
3.4.8.3
In the deterministic method the design sea states are represented by regular periodic waves characterised by
wave length (or period), wave height and possible shape parameters.
3.4.8.4
In the stochastic method the design sea states are represented by wave energy spectra characterised by main
parameters Hs and Tz or Tp.
3.4.9
Waves - weather unrestricted operations, general
3.4.9.1
Characteristic wave conditions for weather unrestricted operations shall be based on long term statistical data.
3.4.9.2
Long term variations of waves may be described by a set of sea states characterised by the wave spectrum
parameters.
3.4.10
Wind seas and swell
3.4.10.1
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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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
3.4.11
Characteristic waves for weather unrestricted operations
3.4.11.1
Characteristic values shall be based on the defined operation reference period. Periods less than 3 days shall not
be used. These can be based on the return periods given in Table 3-1 or Table 3-2 as applicable. Alternatively,
the Characteristic significant wave height, Hs, c may be taken according to [C.3.2.1] and the corresponding
maximum wave height, Hmax, c, may be taken according to [C.3.2.2].
Guidance note 1:
The significant wave height
where m0 is the sea surface variance. In sea states with only a narrow
band of wave frequencies, Hs is approximately equal to
(the mean height of the largest third of the zero
up-crossing waves).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The Hmax, c corresponds to an approximate 10% probability of exceeding this individual wave height during the
anticipated operation reference period. If an alternative method is applied it should be documented that this
corresponds to an equal or less probability.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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 should be determined from an appropriate higher-order wave
theory and account for shallow water effects. For most practical purposes the kinematics of regular deterministic
waves can be described by the following theories:
h/λ ≤ 0.1
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 height-period combinations shall be investigated, including those that can cause resonance, see
[C.3.3].
For more information on the kinematics of regular waves, see DNV-RP-C205, /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. Long-crested 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 Pierson-Moskowitz spectra are most frequently used. Wave
conditions with combined wind sea and swell may be described by a double peak wave spectrum. See DNV-RPC205, /46/, for further guidance.
3.4.11.5
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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 Hs ≤ 5.7 m
for Hs > 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 zero-crossing period Tz and the spectral peak period Tp can be found in Table 3-4. See also DNV-RPH103, /56/, Sec.2.2.6 or DNV-RP-C205, /46/, Sec.3.5.5.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 site-specific 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 Hs-Tp 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 Hs-Tp plane that identifies equally
probable combinations of Hs and Tp for the design return period. This contour should also cover swell. The
contour should be checked for accuracy e.g. against the theoretical constraints on wave breaking. Hs-Tp
combinations from around the contour should be tested in motion response calculations to identify the worst
case response (there is no need to consider a range of Tp with each Hs).
3.4.11.9
The relationship between the peak period Tp and the zero-up crossing period Tz is dependent on the spectrum.
For a mean JONSWAP spectrum (γ=3.3) Tp/Tz = 1.286; for a Pierson-Moskowitz spectrum (γ=1) Tp/Tz = 1.41.
3.4.11.10
Table 3-4 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 3-4 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
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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
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).
3.4.12
Short crested seas
3.4.12.1
A directional short crested wave spectrum, see the equation below, may be applied based on non-directional
spectra.
where
θ
=
Wave spectrum, see [3.4.11.4].
=
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 sea-state and the constant n. Typical values for wind seas are n = 2 to n = 10. Swell
should normally be taken as long crested, n > 10.
Guidance note:
For cases where long crested seas are conservative, it is recommended that long crested seas are used for the
original design work. If short crested seas are introduced in connection with estimating extremes, the exponent,
n, should not be taking lower than 10 without more detailed documentation. Swell seas should be taken as long
crested. For fatigue assessment, where low and moderate sea states are governing the fatigue accumulation, n
could be taken as the most unfavourable value between 2 and 6.
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3.4.12.3
Short crested seas should not be considered for significant wave heights exceeding 10 m, unless they cause
more onerous response(s).
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Waves for moorings
3.4.13.1
In addition to the requirements in [3.4.8], for mobile moorings it is generally acceptable to consider a single
extreme significant wave height and a range of associated peak periods corresponding to the relevant return
period for a location.
3.4.13.2
For permanent moorings a number of Hs-Tp combinations along the 100 year return period contour line shall be
considered in the analysis. If a contour plot is not available, a sensitivity study by varying peak period for the 100
year return period sea state is required. This is to ensure that extreme line tensions due to low frequency motion
at lower periods are captured in the analysis, especially for ship shaped floaters.
3.4.13.3
Long crested waves shall be assumed for analysis unless otherwise documented.
3.4.14
Swell
3.4.14.1
Swell type waves should be considered for operations sensitive to long period motion or loads.
3.4.14.2
Swell type waves may be assumed regular in period and height, and may normally also be assumed
independent of wind generated waves.
3.4.14.3
Critical swell periods should be identified and considered in the design verification.
3.4.14.4
Characteristic height(s) and period(s) for swell type waves for weather restricted operations may be selected
independently of statistical data.
3.4.14.5
Characteristic height(s) and period(s) for swell type waves for weather unrestricted operations should be based
on statistical data and the applicable return periods.
3.4.15
Current
3.4.15.1
The design current shall be the rate at mean spring tides, taking account of variations with depth and increases
caused by the design environmental condition, storm surge, fluvial (river) and wind-driven components.
3.4.15.2
Currents can be divided into two different categories:
• Tidal currents
• Residual currents that remain when the tidal component is removed, including river outflows, surge, wind
drift, loop and eddy currents.
3.4.15.3
Tidal currents can be predicted reliably, subject to long term measurement (at least one complete lunar cycle at
the same season of the year as the actual planned operation). Residual currents can only be reliably predicted or
forecast using sophisticated mathematical models.
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3.4.16
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Other 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.
3.4.17
Calculation of “adjusted” design extremes, weather unrestricted voyages
3.4.17.1
The risk of encounter of extreme conditions on a particular voyage is dependent on the length of time that it
spends in those route sectors where extreme conditions are possible. If the length of time is reduced, then the
probability of encountering extreme conditions is similarly reduced.
3.4.17.2
It is generally accepted that for a prolonged weather unrestricted voyage the wind and wave design criteria
should be those with a probability of exceedance per voyage of 0.1 or less. For a voyage of 30 days (or more),
through meteorologically and oceanographically consistent areas, this corresponds to the 10 year monthly
extreme.
3.4.17.3
Many voyages last less than 30 days, or are potentially exposed to the most severe conditions for less than 30
days. Consequently, for shorter exposures, the 10 year monthly extreme may be adjusted for reduced exposure.
This value is equivalent to the 10 voyage extreme and is also referred to as the 10% risk level extreme. This shall
not be confused with the 10% exceedance value for the voyage, as discussed in [3.4.19.6].
3.4.17.4
When the 10% risk level extremes are less than the 1 year return monthly extremes, the 1 year monthly extremes
are the minimum that shall be used for design.
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.
3.4.18
Calculation of exposure
3.4.18.1
For the purpose of the calculation of “adjusted” extremes the exposure time to potentially extreme or near
extreme conditions is calculated taking consideration of the following points:
• The initial 48 hours of the voyage is assumed to be covered by a reliable departure weather forecast and is
excluded
• The speed of the voyage is reduced by taking the monthly mean wave heights along the route into
consideration as described in [3.4.18.3].
• The speed of the voyage is adjusted to take into consideration the mean currents as described in
[3.4.18.4].
• A contingency time of 25 per cent of the time is added. This allowance is to account for severe adverse
weather, for tug breakdowns or other operational difficulties
• A minimum exposure time of 3 days is considered.
3.4.18.2
The voyage duration in each route sector shall be calculated using the speed in the monthly mean sea state for
each route sector and shall allow for adverse currents and adverse prevailing winds as described in [3.4.18.3].
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3.4.18.3
The effect of the mean sea state on the voyage speed in each route sector shall be calculated as a function of the
wave height in which the voyage is assumed to come to a dead stop, b (metres). This can typically be taken 5 m
for barge towages, and 8 m for ships. The speed in the each route sector can be taken as the calm weather
speed is multiplied by the factor, F, for that route sector defined by:
where Hm is the monthly mean wave height in that route sector.
3.4.18.4
The effect of the mean current on the voyage speed in each route sector shall be calculated by adding the
current vector (resolved with respect to the voyage heading).
3.4.18.5
For the calculation of exposure to the extreme conditions only prevailing winds or currents which act to delay the
voyage shall be taken into account.
3.4.19
Calculation of “adjusted” extremes
3.4.19.1
The probability of non-exceedance of a value of wind speed or significant wave height in a particular route sector
is expressed as a cumulative frequency distribution (e.g. a Weibull distribution).
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]).
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3.4.19.7
The extremes used for design shall not be less than the 1 year return monthly extremes.
Guidance note:
The percentage exceedance is obtained as follows:
• Given a series of values of wind speed or significant wave height, as may be observed during a complete
voyage, some value y will be exceeded at some times but not others and the percentage exceedance of
value y is equal to:
• If each observed value of wind speed or significant wave height is assumed to last for some duration
(typically 1 hour for winds and 3 hours for waves) then for example, during a voyage lasting 10 days there
will be 240 wind events and 80 wave events. On the voyage, if a wind speed greater than 30 knots is
observed during 24 separate, hourly occasions then the percentage exceedance of 30 knots is 10%.
• The 10% risk level (as defined in [3.4.17.3]) for a voyage along a specific route, departing on a specific date
is expected to occur only once, on average, in every 10 voyages. However a value with a 10% exceedance
level for the same route and departure date is likely to occur on average for 10% of the time on every
voyage.
• Thus a 10% exceedance value is far more likely to occur than a 10% risk level value, or an adjusted, 10 year
extreme value.
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3.4.20
Criteria from voyage simulations
3.4.20.1
If continuous time series of winds and waves are available along the entire voyage route (e.g. from hindcast data
or satellite observations), an alternative way to develop criteria with a specified risk of exceedance in a single
voyage is to perform tow simulations. A large number of simulations can be performed, with uniformly spaced (in
time) departure times during the specified month of departure over the number of years in the database. For
each simulated voyage, the maximum wind speed and the maximum wave height experienced somewhere
along the tow route are retained. Then the probability distribution of these voyage-maxima can be used to
determine the design value with a specified risk of exceedance. For example, the value exceeded once in every
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 voyage-maximum values) can be retained. These can be used to give scatter diagrams of
wave height against period and/or direction, and wind speed against direction.
3.4.20.3
The voyage simulation method can be made to be very realistic and account for variation of speed due to
inclement weather or ocean currents, weather avoidance en-route through forecasting/routeing services, or the
use of safe havens, etc. If the voyage simulator cannot accommodate all these features, a reasonably
conservative estimate of criteria can be derived by using a conservative (slow) estimate of the average speed.
Care should be taken when choosing the average speed estimate - a slow speed may not be conservative if it
results in the vessel apparently arriving in a route sector late enough to miss severe weather, which might have
been encountered if arrival had been earlier.
3.4.21
Metocean database bias
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.
3.4.22
Metocean data for bollard pull requirements
3.4.22.1
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]
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3.5
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Weather/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].
3.6
Benign weather areas
3.6.1
Areas considered benign are shown in Table 3-5 and Figure 3-1 for different months. In general they have the
following characteristics:
• virtually free of monsoons, Tropical Revolving Storms or Tropical Cyclones
• exceeding Beaufort Force 5 for <20% of any month (in a “typical” year)
• However these areas may experience sudden vicious squalls and very rare tropical storms or cyclones.
Table 3-5 Northern and Southern boundaries of benign weather areas by month
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Figure 3-1 Map showing benign weather areas
SECTION 4 Ballast and other systems
4.1
Introduction
4.1.1
Scope
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.
4.1.2
Revision history
4.1.2.1
This section replaces the following parts of the VMO Standard and the ND Guidelines:
•
•
•
•
•
4.2
DNV, Marine Operations, General, DNV-OS-H101
DNV, Load Transfer Operations, DNV-OS-H201
GL Noble Denton, General Guidelines for Marine Projects, 0001/ND
GL Noble Denton, Guidelines for Load-outs, 0013/ND
GL Noble Denton, Guidelines for Float-over Installations / Removals, 0031/ND
System and equipment design
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General
4.2.1.1
Systems and equipment shall be designed, fabricated, installed, and tested in accordance with relevant codes
and standards.
4.2.1.2
Systems and equipment shall, as far as possible, be designed to be fail safe and arranged such that a single
failure in one system or unit cannot spread to another unit. The most probable failures, e.g. loss of power or
electrical failures, shall result in the least critical of any possible new conditions.
4.2.1.3
Alarm system(s) should be incorporated for essential functions and be audible/visible at operators’ station.
4.2.1.4
Work stations shall be arranged to provide the user with good visibility and easy access to controls required for
the operations.
4.2.1.5
Systems and equipment shall be selected based on a thorough consideration of functional and operational
requirements for the complete operation. Emphasis shall be placed on reliability and the expected behaviour in
possible contingency situations.
4.2.1.6
Depending on the complexity and duration of the operation, and the structure itself, risk evaluations may be
required to determine the systems and equipment required for a safe operation, see [2.4.2]. Such studies shall
include normal operations as well as emergency situations.
4.2.1.7
The following systems shall be considered where applicable:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
power supply
fuel supply
electrical distribution systems
machinery control systems
alarm systems
valve control systems
bilge and ballast systems
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
◦ under-keel clearance and
◦ penetration/settlements.
Guidance note:
Object guiding and positioning systems, including structural and functional requirements are covered in
[4.4]. If applicable, the requirements in 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].
4.2.2
Back-up
4.2.2.1
All essential systems, parts of systems or equipment shall have back-up or back-up alternatives. Necessary time
for a change over to the back-up system or equipment shall be assessed.
Guidance note:
It is recommended that the marine operation manual includes an inventory of main spare parts available on site.
It is also recommended to assess the necessity of having repair or service personnel available on site during
operations.
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4.2.2.2
All back-up systems should be designed and fabricated to the same standard as the primary systems.
4.2.2.3
Back-up systems should be adequately separated from the main system such that failure of any component does
not adversely affect the safe conduct of the operation.
4.2.2.4
For systems consisting of multiple independent units, back-up may be provided by having a sufficient number of
available spare units available on site.
4.2.2.5
If umbilicals are necessary to provide power and/or hydraulic services during any marine operation, adequate
back-up capability shall be provided, and fail-safe systems shall be incorporated into critical controls.
4.2.2.6
Automatic control systems shall be provided with a possibility for manual overriding.
4.3
Ballasting systems
4.3.1
General
4.3.1.1
This sub-section is mainly applicable for ballasting and de-ballasting of vessel(s) involved in load transfer
operations.
Guidance note:
See [11.15] regarding pumping capacity requirements during voyages. 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:
•
•
•
•
•
•
lay-out and reliability of the system
tank capacities including contingency situations
ballasting capacity including contingency situations
strength limitations
easily controllable ballasting
tide
4.3.1.4
General requirements to (de)ballasting systems are given in [4.2.1].
4.3.1.5
Adequate testing of the ballast system considering the actual operation shall be carried out, see [2.10].
4.3.2
Ballast system power supply
4.3.2.1
Adequate power supply considering the actual operation shall be provided for the ballast system.
4.3.2.2
The need for emergency power supply due to the following situations shall be considered:
a. Breakdown of any one power unit
b. Breakdown of the common energy supply
c. Unexpected increase in the consumption of energy above the expected value.
Guidance note:
The back-up capacity for accidental conditions represented by a) and b) may be spare units in stand-by
position. The back-up capacity for conditions represented by c) may be spare capacity in the main unit or a
back-up unit installed to assist the main unit.
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4.3.2.3
Sufficient main and back-up power supply capacity should be documented by calculations.
Guidance note:
Necessary power supply for ballasting should be based in the required ballasting capacity given in Table 4-2 for
the relevant load-out class. For evaluations of back-up requirements, an independent power supply source
should be regarded as a “pump system”, see note 3) in Table 4-2.
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4.3.3
Operation classes
4.3.3.1
An operation class should be defined for load transfer operations see Table 10-1 for load-outs and Table 15-1 for
lift-off, mating and float-over operations.
4.3.4
Ballast system lay-out and reliability
4.3.4.1
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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 de-ballasting by air pressurisation (or ballasting by vacum – low pressure) of ballast tanks shall be avoided
during load transfer phases.
Guidance note:
Ballasting by flooding during load transfer phases where accurate control of ballast amount is crucial may be
allowed if the system has sufficient redundancy (e.g. double valves to compensate a failure to close a valve)
and/or back-up ballast plans are available where mechanical failures can be compensated by an alternative
ballast procedure.
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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 de-pressurise a tank should be
taken into account, as should any limitations on differential pressure across a bulkhead. It should be
remembered that compressed air systems cannot always fill a tank beyond the external waterline. Air pressurised
vessel tanks shall be fitted with safety (pressure relief) valves.
4.3.4.5
Hoses, umbilicals and power cables shall be placed with due consideration to other ongoing activities during the
load transfer.
4.3.4.6
Required access throughout the load transfer for (possibly) needed equipment, e.g. fork lifts for replacing
pumps, should be demonstrated.
4.3.4.7
All internal compartments shall be cleaned of debris before ballasting starts.
4.3.4.8
When inlets are near the seabed, care shall be taken to avoid sucking in mud or sand that can block the pumping
systems or filters.
4.3.4.9
Where inlets or outlets are near the seabed, care shall be taken to avoid scour that could have adverse effects on
foundations of any structure or grounded vessel, or reduce under-keel clearances.
4.3.4.10
Except when in use for inlet or discharge, all openings to sea shall be protected by a double barrier.
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
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All essential pipework in temporary systems should be of permanent-type construction and shall be
hydrostatically tested to a minimum of 1.3 times the design pressure. Temporary flexible hoses shall only be
used when a risk assessment, in accordance with [2.4], demonstrates the acceptability of the system.
Guidance note:
For offshore operations temporary flexible hoses are not generally permitted unless their use cannot be avoided,
for instance for supply of back-up compressed air from a compressor barge alongside.
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4.3.4.13
Permanent-type ballast 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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4.3.5
Ballast tank capacity
4.3.5.1
The ballast tanks shall meet the capacity requirements in Table 4-1 for the required floating position(s)
throughout the operation for both planned and contingency situations.
4.3.5.2
A reasonable amount of residual water in the tanks should be taken into account.
Guidance note:
The amount to be considered will depend on details and location of the pumping intake(s), heel/trim of the
vessel and structural elements at the tank bottom. For tanks in use during the load transfer without any special
arrangements allowing easy tank stripping, the minimum water head should be taken equal to the height of the
tank bottom stiffeners plus 0.05 metres.
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4.3.5.3
The required tank capacities should include relevant spare capacity to compensate for the following:
a. Tide levels below or above the predicted values.
b. Total vessel weight, including vessel lightship, consumables and temporaries (e.g. project equipment,
grillages, etc.), being higher or lower than expected
c. Possible object weight and CoG variations
d. Operational delays.
Table 4-1 Tank capacity requirements
Operation
Class
The tank capacity shall be adequate for the following scenarios (see Table 10-1 for load-out
classes and Table 15-1 for float-ons and float-offs).
All
• Normal (planned) operations
• Spare tank capacity to cover items [4.3.5.2] and [4.3.5.3] shall be ensured in all
situations.
• Any necessary pumping capacity contingency involving modifications in ballasting
procedures. See Table 4-2.
1
• See All
• Reversing of the operation. Tide compensation if stop of load transfer, considering
maximum possible (defined) duration of the load transfer.
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.
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3
• See All
• Ballasting through a complete tide cycle at any stage of the load transfer. Maximum tide
variations for at least the coming 3-5 days shall be considered.
4
• See All
• Reversing of the operation, if applicable.
5
• See All
4.3.6
Ballast pumping capacity
4.3.6.1
The ballast pumping capacity shall meet the capacity requirements in Table 4-1 for the required floating position
(s) throughout the operation for both planned and contingency situations. Pump capacity shall be based on the
published pump performance curves, taking account of the maximum head for the operation and pipeline
losses.
4.3.6.2
Adequate capacity shall be documented considering the requirements to nominal, spare and back-up capacity
given in this sub-section.
4.3.6.3
The nominal ballasting capacity shall be determined by the worst combination of expected tide rise/fall and
planned load transfer velocity.
4.3.6.4
For operation classes 2 and 3, it shall be documented that the ballast systems have capacity to compensate for
the tide rise/fall through one complete tide cycle with the object in any position.
Guidance note:
If the tidal range increases in the days following the planned operation start, this should be considered when
evaluating the consequences of a delayed start or delays during the operation.
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4.3.6.5
Pumps which require to be moved around the barge in order to be considered as part of the back-up capacity,
shall be easily transportable, and may only be so considered if free access is provided at all stages of load-out
between the stations at which they may be required. Adequate resources shall be available to perform this
operation.
4.3.6.6
Spare pumps should normally be installed and tested in the position they are planned to be used as back-up.
However, for pumps that may be replaced during the operation, spare pumps in stand-by position that require
minimum replacement time may be used. Required number of spare pumps should be conservatively assessed.
The replacement time shall be documented. See [4.3.4.6].
4.3.6.7
Requirements for minimum total ballasting capacity, including back-up, are given in Table 4-2, including the
notes.
Table 4-2 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.
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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 back-up requirement X% capacity in all tanks could be covered by a modified ballast procedure
giving X% capacity in all tanks involved in this modified procedure.
4.3.7
Vessel strength considerations
4.3.7.1
All ballast conditions shall be checked against longitudinal strength requirements. Any hull beam strength
limitations shall be considered in the ballast procedure.
4.3.7.2
The effect of hull beam deflections on the object support load distribution shall be considered, see [5.6.11].
4.3.7.3
Differential pressures across bulkheads shall be demonstrated to be within allowable values.
4.3.7.4
Any restrictions, e.g. any requirement to mimic the vessel voyage condition, on ballast condition(s) during
welding of seafastening shall be considered.
4.3.7.5
Possible significant strength reduction due to cut outs (e.g. for ballast hoses, pumps or other equipment) in
structural elements shall be considered.
4.3.8
Ballasting control
4.3.8.1
A straightforward ballasting control system and procedure shall be used.
Guidance note:
It is recommended that it is possible to operate the ballast pumps from one control centre during operation. For
multi barge operations, a control centre on each barge may be applicable. However, the control centre at one of
the barges should be defined as the master ballast control centre. The arrangement should be such that
simultaneous de-ballasting can be effected for all the relevant tanks at each stage.
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4.3.8.2
It shall be thoroughly documented how the ballasting will be done (controlled) for all possible combinations of
tide level and load transferred.
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4.3.8.3
Each tank should preferably be used to compensate one effect (see Guidance Note) only. To use a system/tank
for compensation of more than two effects shall be avoided.
Guidance note:
In order to maintain maximum control with the ballasting it could be advisable to use separate systems/tanks for
compensation of the effects of tide variation, weight transferred, and CoG position in both directions (trim and
heel).
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4.3.8.4
A ballasting control monitoring system including back-up shall be established. A communication system shall be
established when pumps are operated manually away from the control centre.
4.3.9
Ballast calculations
4.3.9.1
Ballast calculations shall be carried out in order to establish required nominal capacity (i.e. the 100% capacity,
see note 1 in Table 4-2) pumping capacities.
4.3.9.2
For ballast calculations the expected CoG and weight without any contingencies should normally be used as the
base case. However, the effect of possible weight and CoG variations shall be considered, see [5.6.2].
4.3.9.3
The ballast calculations shall include sufficient steps to accurately define the required ballasting throughout the
(load transfer) operation.
4.3.9.4
All considered contingency situations shall also be covered with an adequate number of ballast calculation steps.
4.3.9.5
The results of the ballast calculations, i.e. required pumping in all steps, shall be clearly outlined in ballast
procedure(s).
4.3.10
Contingency and back-up
4.3.10.1
Means for adequate handling of all ballast system contingencies identified in the risk management process shall
be provided.
4.3.10.2
The contingencies indicated in Table 4-3 shall be considered. Minimum requirements to back-up have also been
indicated.
Table 4-3 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 4-2 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 4-1 and Table 4-2 for specific
requirements.
3
Reversal of operation, if required.
Ballast procedures/calculations with
corresponding pump lay-out and tank capacities
for this case shall be documented.
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4
Reduced pump capacity.
Spare pump capacity. See Table 4-2 for specific
requirements in %.
5
Breakdown of ballast pump(s).
Spare pump(s) or spare capacity in the main pump
(s). See Table 4-2 for specific requirements.
6
Breakdown of power supply, including cables.
Back-up required, see [4.3.2.2], or adequate
pump capacity, see Table 4-2, considering any
power supply unit failed shall be documented.
7
Failure of any control panel/switchboard.
8
Failure of any ballast valve or hose/pipe.
Sufficient back-up to fulfil the requirements in
Table 4-2 for one pump system failure. Alternative
pump/valve control methods (locations and
procedures) could also be accepted as back-up.
See Notes.
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.
4.4
Guiding and positioning systems
4.4.1
General
4.4.1.1
This sub section applies for design and verification of (object) guiding and positioning systems to be used for
marine operations.
Guidance note 1:
Guiding systems are often designed with a primary and secondary system. The primary system is normally
designed to absorb possible impact energy, and provide guiding onto the secondary system. The secondary
system is normally designed to ensure accurate and controlled positioning of the object.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Additional operational specific guidance and requirements to guiding and positioning systems for lifting may be
found in [16.14]. Requirements to positioning systems for vessels are given in Sec.17.
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4.4.1.2
Guides and bumpers shall have sufficient strength and ductility to resist impact and guiding loads during
positioning without causing operational problems (e.g. excessive positioning tolerances), and without
overloading members of the supporting structure. Plastic deformation of guides and bumpers due to impact
loads may be allowed. The possibility and consequences of multiple impacts shall be considered.
4.4.1.3
After the design impact(s), guides and bumpers shall be able to resist loads due to the environmental conditions
during operation, and operational loads from tugger lines, mooring lines etc.
4.4.1.4
After the design impact(s), guides and bumpers shall also provide a positive clearance towards neighbouring
and supporting structure, and maintain their functionality.
4.4.1.5
DNV-RP-H102, /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
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The stiffness of bumper and guide members should be as low as possible, in order that they may deflect
appreciably without yielding.
4.4.1.7
Design of bumpers and guides should cater for easy sliding motion of the guide in contact with a bumper.
Sloping members should be at an acute angle to the vertical. Ledges and sharp corners should be avoided in
areas of possible contact, and weld beads should be ground flush.
4.4.1.8
As-built bumper and guide dimensions shall be documented.
4.4.2
Characteristic loads
4.4.2.1
Characteristic impact loads for bumpers should be based on impact and deformation energy considerations.
Alternatively for lifts in air only, the characteristic guide loads may be calculated according to the simplified
method in [16.14.4].
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
The characteristic loads shall be used as the basis for determining the maximum entry speed of the lifted object
into the guiding system.
4.4.3
Design verification
4.4.3.1
Structural strength of guiding and positioning systems should be verified according to Sec.5.
4.4.3.2
The connection into the object and the members framing the bumper or guide location should be at least as
strong as the bumper or guide.
4.4.3.3
The bumpers and guides shall be designed as either
• To the ASD/WSD approach LS2 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,
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• 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.
4.4.4
ALS 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.
4.4.5
Position monitoring systems
4.4.5.1
The positioning equipment system accuracy and redundancy shall be specified. System accuracy shall be
suitable for congested areas or where dimensional tolerances become tighter, e.g. for tie-ins, capture of docking
piles.
4.4.5.2
System redundancy shall be in accordance with [4.2.1.10] appropriate to safety criticality and operational
criticality requirements.
4.4.5.3
Sub-surface positioning of ROV’s or other targets shall interface with the surface positioning system and should
display on the same equipment. Subsea acoustic transceivers/beacons shall be separately identifiable and on
coordinated channels. Survey systems using line-of-sight shall recognise and cater for crossing surface vessels
possibly occluding the system. Survey systems should be commissioned and calibrated before start of
installation operations.
4.4.5.4
Normally, two independent on board positioning monitoring systems (PMSs) shall be utilized for operational
monitoring and control purposes. Both systems shall be in operation at any time, each serving as the back-up for
the other. Each should be fed by an independent power source.
4.4.5.5
Where underwater accuracy is important, at least one PMS shall be an underwater, hydro-acoustic reference
system.
4.5
ROV systems
4.5.1
Planning
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.
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4.5.1.2
When planning for a subsea operation, the following ROV limitations and recommendations should be noted:
a. Minimum practical operational depth in the expected wave conditions, also considering possible wake
from vessel thrusters.
b. ROV working range, i.e. maximum horizontal offset vs. available tether length, considering the worst
expected current conditions.
c. Planning and design of the ROV operation shall as far as possible minimise the operational influence of the
ROV operator's skill and experience.
d. Poor visibility due to e.g. disturbed soil conditions, stirred up by contact or thruster or tool use close to
seabed.
e. Access to working site.
4.5.1.3
Planned ROV downtime and statistical uptime of ROV shall be taken into consideration when establishing TPOP,
see [2.6.3]. If statistical data for ROV uptime is not available a conservative estimate shall be made.
4.5.1.4
For subsea operations where the operation reference period (TR, see [2.6.2])is based on using ROVs (i.e. ROV
activities are on critical path), ROV contingencies shall be documented and available. This can include a back-up
ROV spread on an independent system, i.e. there shall be no possible single failure identified that may cause an
unacceptable long downtime for both ROV spreads.
4.5.1.5
The need for backup of essential ROV tools shall be assessed, and if applicable, the time needed to switch ROV
tools/skids between ROVs shall be considered in the planning.
4.5.1.6
ROV tooling shall be provided with sufficient spares and back-up tooling to allow the work to proceed with
minimum delay.
4.5.1.7
For operations requiring assistance of both ROV(s) and diver(s), any restrictions on simultaneous working shall be
considered and be clarified in advance.
4.5.2
Stationkeeping and positioning
4.5.2.1
The stationkeeping capability and manoeuvrability of the ROV during operation shall be considered. If the ROV
is carrying equipment or is equipped with tooling packages/skids, this needs to be accounted for.
Guidance note:
Any ROV manipulator or tooling operation that requires the pilot to actively control the position of the ROV, e.g.
if the target is moving, during performance of the task should be avoided. See also 4.5.2.3.
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4.5.2.2
The required ROV thrust capacity shall be documented by verified capability plots (if available) and/or detailed
calculations considering:
•
•
•
•
maximum current speeds at applicable depth(s), see 3.4.3.
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
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dcab
lcab
AROV
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
Grab bars to aid ROV positioning for manipulative or observation tasks should be provided where critical path
ROV operations are planned.
4.5.3
Testing
4.5.3.1
For complex and critical stages of the installation that are dependent on ROV operations, Client/Contractor shall
demonstrate ROV capability of executing the planned intervention. This can be demonstrated by used of 3D
models, mock-up tests, previous experience, etc.
Guidance note:
This may involve the manufacture of mock-ups. If mock-ups are used, great care shall be taken to ensure that the
mock-ups replicate the actual item.
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4.5.3.2
System Integration Testing should be carried out onshore to prove that the integration of all components and
tooling can be achieved.
4.5.3.3
Dry tests and FAT should be carried out for critical and complex systems, the failure of which would result in
significant and unacceptable schedule delay.
4.5.3.4
Before acceptance of ROV operations, maintenance records and dive logs for each ROV should be submitted.
Sufficient spares should be available.
4.5.4
Launch and recovery system
4.5.4.1
Once installed, the launch and recovery system (LARS) shall be load tested according to the applied
design/certification standard.
4.5.4.2
ROV launching and recovery restrictions shall be defined based on the capacity of the launch and recovery
system, including capacity of the umbilical. In addition any restrictions related to operational aspects need to be
considered.
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 2.5-3.0 m (Hs) if the ability to operate in a safe manner under more severe conditions has
not been documented. Higher waves may be applicable if the launch and recovery always may take place
on leeward, for Moon-pool ROV operations and if heavy weather side rail systems are used.
• High wind speeds, and operational aspects (e.g. risk of entanglement) may also be critical.
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4.5.4.3
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The over-boarding system shall be safely operated within its intended design limit and due consideration of ROV
recovery needs to accounted for in the definition of the weather criteria.
4.5.4.4
Launch and recovery shall as much as practically possible take place at safe distance from sensitive subsea
infrastructure. See [5.6.6.6].
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.
4.5.5
Monitoring
4.5.5.1
Video monitoring of all subsea operations should in general be provided, e.g. ROV, diver-operated, etc. Any
critical part of the operation should be performed with such monitoring.
4.5.5.2
All diving and complex Work-ROV operations should be monitored by independent ROV with monitoring as its
only task in the period it is carrying out such critical monitoring.
4.5.5.3
The ROV used for monitoring subsea operations should, as far as practically possible, be operated from the
installation vessel.
4.5.5.4
If the ROV operation has to be performed by a vessel other than the installation vessel, the stability and reliability
of the video-link system between the vessels shall be proven under the given conditions.
Guidance note:
Some operations can require a large horizontal distance between the installation vessel and the observation
ROV, thus necessitating a separate ROV vessel. The video-link should be tested before start of operation.
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4.5.5.5
Means for locating and tracking of the ROV from the surface are required for navigational purposes and
emergency recovery.
4.5.6
Human factors
4.5.6.1
The feasibility of subsea operations often relies on the correct completion of tasks by ROV - it should therefore
be ensured that ROV operators have the necessary experience and skills.
4.5.6.2
If complex operations reliant on the skill of the ROV operator alone cannot be avoided, ROV operator experience
shall be evaluated. Training sessions specially adapted for the proposed operation can be appropriate.
4.5.7
Deepwater 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.
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• 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 5 Loading and structural strength
5.1
Introduction
5.1.1
General
5.1.1.1
This section addresses loading categorisation, load effects, load cases and load combinations.
5.1.1.2
The requirements for structural strength are given, mainly related to steel structures. For structures of other
materials, adequate safety levels shall be achieved by use of recognized standards.
5.1.2
Scope
5.1.2.1
This section presents the requirements for strength checking of steel structures using both Allowable Stress
Design (ASD) / Working Stress Design (WSD) and Load and Resistance Factor Design (LRFD). Alternatively,
probabilistic methods can be used.
5.1.2.2
The ASD/WSD and LRFD checks have differing inherent levels of safety. To compensate, this Standard has
differing requirements for the design loading. It is therefore important that the applied environmental loading is
determined using the return period applicable to the checking method selected.
5.1.3
Revision history
5.1.3.1
This section replaces the applicable sections of the legacy GL Noble Denton Guidelines and legacy DNV-OS-Hseries standards.
5.2
Design principles
5.2.1
Introduction
5.2.1.1
The object subject to marine warranty survey, together with the associated equipment shall be shown to possess
adequate strength to resist the loads imposed during the marine operation.
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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 DNVGL-OS-C401, /26/, or
another recognized standard.
5.2.1.6
Structural components and details should be designed so that the structure behaves, as far as possible, in a
ductile manner.
Guidance note:
A structure or a structural element, can exhibit brittle behaviour even if it is made of ductile materials e.g. when
there are sudden changes in section properties, when exposed to low temperatures.
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5.3
Specific design considerations
5.3.1
Connections
5.3.1.1
Connections should be designed with smooth transitions and proper alignment of elements. Stress
concentrations should be avoided as far as possible.
5.3.1.2
The transmission of tensile stresses through the thickness of rolled steel elements (plates, beams etc.) should be
avoided unless materials with proven (tested) z-quality are applied. Alternatively, the material can be subject to
non-destructive testing (NDT) using UT to demonstrate that it is free of laminations, see [5.10.2.3 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.
5.3.2
Penetrations
5.3.2.1
The object shall be reinforced as necessary in the area adjacent to any penetrations (e.g. for risers or J-tubes)
below the water line against hydrostatic pressures and against accidental impact from dropped objects and
vessel impact if likely at any draught.
5.3.2.2
Penetrations shall be positively sealed to prevent the ingress of water whilst the structure is afloat.
5.3.3
Doubler plates
5.3.3.1
Doubler plates are generally recommended for use:
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• When attaching seafastenings or sacrificial anodes to permanent steel work subject to fatigue or if the
permanent structure could be damaged when the attachments are burnt off after use.
• To avoid welding onto other welds.
5.3.3.2
Doubler plates are generally NOT recommended for use when tension can cause overstress in the doubler plate
or the structure to which it is attached.
5.3.4
Tension connections
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 through-thickness properties of the deck plate.
Guidance note:
The welds between vessel deck plates and under deck stiffeners/bulkheads (including cut out infills) are normally
small and can limit the capacity.
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5.3.5
Bolted connections for seafastening
5.3.5.1
Appendix [E.2] gives the requirements for bolted connections for seafastenings which involving cyclic loading
due to the dangers of progressive collapse.
5.3.6
Light-weight metallic and composite structures
5.3.6.1
The designers or manufacturers shall specify any handling/connection requirements which shall appear in the
relevant procedures and towing/transport manuals.
Guidance note:
Tugger line systems are especially important when handling light-weight alloy, composite and other items in
order to avoid any impact with seafastening, grillage or offshore structures which could cause plastic
deformations.
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5.3.6.2
The structural strength of objects of innovative design and/or material shall be documented.
Guidance note:
Particular attention should be given to local strength in way of supports, seafastening etc.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.3.7
Compressed air
5.3.7.1
Compressed air may be used to resist hydrostatic head on internal or external walls during ballasting, for
reducing draught, or for reducing overall bending moments by air cushions in skirt cells under well controlled
conditions. However its absence should not, in general, result in structural collapse i.e. it should be used only to
increase structural safety factors.
5.3.7.2
Where the requirements of [5.3.7.1] cannot be met, then a risk assessment shall be carried out to determine
possible causes and probabilities of loss of compressed air. Mitigating measures to reduce the risks to an
acceptable level shall be agreed with the MWS company.
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5.3.7.3
Some practical considerations on the use of compressed air are given in [12.6.2].
5.3.8
Inspection
5.3.8.1
Sufficient access for inspection, maintenance, and repair shall be provided during planning of the operation.
5.3.8.2
Instrumentation (monitoring) can be used as a supplement to other inspection, see [2.9].
5.3.9
Existing structures
5.3.9.1
Strength calculations for marine operations often include the verification of existing steel structures of e.g.
barges, other vessels and objects for dismantling. The calculations shall account for any reductions in the design
capacity. Examples of possible causes include:
• corrosion
• damage
• modifications not shown on drawings.
5.3.9.2
Existing structures should normally be inspected in order to assess possible reductions in the design capacity,
see [5.3.9.4], [5.9.8.4], [5.10.2.2], and [5.10.2.3 5)]. See DNV-RP-H102, /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 as-built drawings less the vessel’s class corrosion allowance, or reduced by 0.2
mm per year from each side. For new vessels with a proper corrosion protection system, e.g. painting or coating,
no thickness reduction need to be considered for the first five years of the vessel’s life.
Guidance note:
Typical corrosion allowance requirements can be found in 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:
a. Class acceptance for these welds can be required, especially for new/reinforced welds.
b. All loads (force components) normal to the deck plate should generally be considered transferred to the
under deck welds. However, when the force is only compressive, i.e. there is no tension force in any load
combination, this force component may be assumed to be transferred through direct contact between the
deck plate and the web frames/bulkheads, and the weld may be checked for shear stress only, see item f).
If the force varies between compression and tension, the weld should be able to transfer also the
compression force in order to ensure intact welds, unless the capacity of the seafastening system is
documented in ALS assuming that the connection under consideration is broken.
c. All loads (force components) parallel to the deck plate can be disregarded, see however item f).
d. The dispersion angle through the deck plate should be taken as maximum 45° unless a greater dispersion
can be justified.
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e. 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.
f. 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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5.3.10
Protection against accidental damage
5.3.10.1
The structure shall be protected against accidental damage by application of the following two principles:
• reduction of damage probability
• 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.4
Testing
5.4.1
General
5.4.1.1
Testing can be used in order to establish or verify design parameters. Material and weld testing should be
carried out according to a relevant recognized standard, e.g. DNVGL-OS-C401, /26/, see also [5.10] which
summarises key requirements.
5.4.1.2
Adequate and reliable test data should be used to verify/correlate values that are considered unreliable based
on theoretically calculations only. This is particularly relevant for geometrically complex structures and for new
design or operational concepts.
5.4.1.3
For marine operations, such (project) specific testing is normally most relevant to determine or verify:
• response, e.g. motions by model testing,
• loads, e.g. by direct measuring of loads in model tests and
• resistance, e.g. by load testing or testing of friction.
5.4.2
Model testing
5.4.2.1
Model testing is most frequently used for the determination of response and loading effects but can also be used
for determination of structural resistance.
5.4.2.2
Model tests should be carried out according to a verified test program/procedure using:
•
•
•
•
models representing the object(s), vessel(s) and real conditions as accurately as required,
qualified test personnel,
adequate testing facilities, and
calibrated monitoring equipment with sufficient accuracy.
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5.4.2.3
Normally the testing should be combined with theoretical calculations.
5.4.2.4
The laws of similarity shall be considered in order to ensure that the quantities measured in the model test can
be correctly transformed.
5.4.2.5
Effects that can influence the measured quantities and that are not represented in the model test shall be
identified and the consequences of these effects should be evaluated.
Guidance note:
For example, the correct relative stiffness (of vessels/structures) will normally not be obtainable in model tests
and effects of this on the results should be evaluated.
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5.4.3
Full scale load testing
5.4.3.1
Full scale load testing should be carried out according to agreed procedures.
5.4.3.2
Requirements for standardised load testing, e.g. of lifting appliances, are not described in this standard. Such
testing should be carried out as described in the relevant standard, e.g. DNV 2.22, /16/, and DNV 2.7-3, /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.
• Local soil capacity (deflection), e.g. of load-out tracks.
• Existing (steel) structures with no/limited inspection access.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.4.3.5
For such tests the load should normally be at least 0.9 times the maximum design load (i.e. including load factor)
for the item. All relevant load directions should be tested.
5.4.3.6
A thorough inspection shall be carried out of items that have been subject to testing. Defects that could reduce
the strength (capacity) shall not be allowed.
5.4.4
Testing of friction
5.4.4.1
Testing may be carried out in order to establish applicable friction coefficients. The testing conditions should
represent the expected friction surface and load intensity as close as possible.
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5.4.4.2
In marine operations the dynamic friction coefficient will normally be the most relevant and testing of this should
hence be included unless it is not needed for the particular application.
5.4.4.3
Where testing is carried out, a detailed test procedure shall be documented.
Guidance note:
The test procedure should consider the following:
a. Possible variations in applicable conditions (e.g. wet and dry surfaces). See [5.4.4.1] and [5.4.4.2].
b. Dynamic friction, if applicable, should be tested and measured by a recognised method.
c. 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.
d. At least 5 test pieces should be made, and each tested at least twice for each actual condition.
e. The design friction coefficient is calculated using the characteristic friction coefficient and an appropriate
material factor. See [5.9.8.6], [5.9.5.3] and [5.9.6.2].
f. Where fewer tests are performed e.g. because of the scale, more conservative material factors should be
used.
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5.5
Load categorisation
5.5.1
Introduction
5.5.1.1
This section defines load categories and describes loads of general interest for marine operations.
5.5.1.2
The appropriate characteristic value should be defined (calculated) for all relevant loads.
5.5.1.3
More detailed descriptions of the loads to be considered are given for each type of marine operation/object
type in Sec.6 to Sec.18.
5.5.1.4
See [5.6] for load combinations, [5.7] for the failure modes to be considered, [5.8] for guidance on analytical
models and [5.9] for strength assessment.
5.5.2
Load categories
5.5.2.1
Loads and load effects shall be categorised as follows:
•
•
•
•
•
Permanent Loads - G
Variable Functional Loads - Q
Deformation Loads - D
Environmental Loads - E
Accidental Loads - A.
5.5.2.2
The characteristic values of loads shall be selected as indicated in Table 5-1 for all applicable loads.
Table 5-1 Characteristic load selection
Limit states 2) – Temporary design conditions
ALS
Load category 1)
ULS
FLS
Intact
structure
Damaged
structure
SLS
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Variable (Q)
Specified2)
value
Specified2)
load history
Environmental (E) –
Weather restricted
Specified
value
Specified
load history
Environmental (E) –
Weather unrestricted
Operations 4)
Based on
statistical
data 5)
Expected
load history
NA
NA
Expected
extreme
value
Expected
load history
Accidental (A)
Deformation (D)
Page 100 of 543
Specified2) value(s)
Specified value(s)
NA
Based on statistical data5) & 6)
Specified
value
NA
NA
Specified value(s)
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.
5.5.3
Permanent loads (G)
5.5.3.1
Permanent loads are loads which will not be moved or removed during the phase of the marine operation being
considered. Such loads can be due to:
•
•
•
•
weight of stationary structures
weight of permanent ballast and equipment that cannot be removed
external/internal hydrostatic pressure of permanent nature
pretension.
5.5.3.2
Characteristic permanent loads shall be based on reliable data. For weight see [5.6.2].
5.5.4
Variable functional loads (Q)
5.5.4.1
Variable functional loads are loads that can be moved, removed or added. Such loads can be due to:
•
•
•
•
•
•
•
operation of winches
pull/push forces
weight of moving structures
loads from adjacent vessels
ballasting
operational impact loads
stored materials, equipment or liquids.
5.5.4.2
Characteristic variable functional loads shall be specified with maximum and minimum values, which shall be
considered as necessary to determine the worst case(s).
5.5.5
Deformation loads (D)
5.5.5.1
Deformation loads are associated with inflicted deformations. Such loads can be caused by:
• installation or set down tolerances
• barge hull beam global deformations caused by moving ballast water (or temperature)
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• structural restraints between structures
• differential settlements
• temperature deformations.
5.5.5.2
Characteristic deformation loads shall be maximum or minimum specified values, which shall be considered as
necessary to determine the worst case(s). The specified values shall, if applicable, be based on results from
analysis considering extreme conditions.
5.5.6
Environmental loads (E)
5.5.6.1
All loads caused by environmental phenomena shall be categorised as environmental loads. Such loads can be
due to phenomena including:
•
•
•
•
•
•
wind
waves
current
storm surge
tide
ice.
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].
5.5.7
Accidental loads (A)
5.5.7.1
Accidental loads are loads associated with exceptional or unexpected events or conditions. Such loads can be
due to:
•
•
•
•
•
collisions from vessels
dropped objects
loss of hydrostatic stability
flooding
loss of internal pressure.
5.5.7.2
Characteristic accidental loads shall be based on realistic accidental scenarios. See also [5.6.6].
5.6
Loads and load effects (responses)
5.6.1
General
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5.6.1.1
This section describes the loads and load effects that should be considered.
5.6.2
Weight and centre of gravity (CoG)
5.6.2.1
Introduction
1. For calculation purposes, conservative values of weight and CoG should be used.
2. Weight control shall be performed by means of a well-defined and documented system, complying with
ISO 19901-5 – Weight control during engineering and construction, /99/.
3. ISO 19901-5 states (inter alia) that:
◦ “Class A (weight control) will apply if the project is weight or CoG-sensitive 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.
5.6.2.2
Weight 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 in-place considerations, the following can control the NTE weight:
◦ Draught and stability for tow-out, towages, mating operations and installation;
◦ Allowable stresses in the structure for marine operations;
◦ Limitations due to crane, load-out trailers, other equipment or ground-bearing capacity.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--2. Where an object (excluding piles) is not to be weighed, the following shall be true for the as-built weight
report:
WReport, Factored ≤ Wud/γWeight
WReport, Base ≥ Wld γWeight (where applicable)
Where:
WReport,
Factored
=
Factored weight in weight report
=
Base weight in weight report
=
Upper bound design weight
=
Lower bound design weight
=
Unweighed object weight margin factor as per Table 5-2
WReport,
Base
Wud
Wld
γWeight
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 =
Wud
=
Wld
=
Net weight in weight report
γWeighing
Factor to account for weighing equipment inaccuracy i.e. (
=
Upper bound design weight
Lower bound design weight
)
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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 5-2 Unweighed object weight margin factors
Weight Class
(as defined by ISO 19901-5, /99/)
γWeight
A
1.05
B and C
1.10
5.6.2.3
Centre of gravity factors
a. 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.
b. 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For operations with a linear relation between shift in CoG and resulting load effects, or operations less
sensitive to CoG shifts, the inaccuracy in estimated CoG may alternatively be accounted for by an
inaccuracy factor applied to the weight. This factor should normally not be taken less than 1.05.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--c. 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.
d. The CoG contingency factors for piles shall be determined considering the pile length and the plate
manufacturer’s plate thickness tolerance specification.
e. 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]).
5.6.2.4
Weight control
a. 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. 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.
c. 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.
d. 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.
e. 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.
f. 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.
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g. See [18.2.1.2] for weight control for decommissioning/removal.
5.6.2.5
Buoyancy
a. Buoyancy (hydrostatic external load) normally counteracts another load and shall be categorised
accordingly.
b. 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.
c. The buoyancy of the object and the position of the centre of buoyancy should be determined on the basis
of an accurate geometric model.
d. Characteristic buoyancy loads should be based on maximum and/or minimum expected values.
e. Buoyant cargoes, particularly where the buoyancy contributes to stability requirements, shall be
adequately secured against lift-off unless it can be shown that lift-off will not occur.
5.6.3
Wind 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:
DNV-RP-C205, /46/, gives further information with respect to shape coefficients as well as to effects of wind
direction relative to member, solidification and shielding.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.6.4
Current loads
5.6.4.1
Current loads shall be calculated based on the characteristic current velocity, see Sec.3, and recognised
methods.
5.6.4.2
The increase in current velocities/loads due to shallow waters or narrow channels shall be considered.
Guidance note:
DNV-RP-C205, /46/, gives further information with respect to shape coefficients as well as to effects of flow
direction relative to member, solidification and shielding.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.6.5
Wave-current loads
5.6.5.1
Combined wave-current induced drag loads shall be calculated considering the vector sum of the current and
wave particle velocities.
5.6.5.2
First order wave loads
a. 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.
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Guidance note:
If any responses are found governing for
the response should be checked in these
areas with
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. 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.
c. The effects of wave elevation shall be evaluated, and if necessary included in the design.
d. Wave slamming, see [5.6.5.4], hydrodynamic and hydrostatic loads on members protruding over the
vessel side shall be considered. The effect of such loads on the motion characteristics and on the
seafastenings and grillage/cribbing shall be taken into account.
5.6.5.3
Second order wave loads
a. 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.
b. Long period responses excited by slow drift forces shall be investigated.
5.6.5.4
Slamming loads and breaking waves
a. 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.
b. 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.
c. Loads due to slamming and breaking waves should normally be calculated according to DNV-RPC205, /46/.
Guidance note:
Further information regarding slamming loads and breaking waves can be found in DNV GL Rules for
classification: Ships /37/ Pt.3 Ch.10 and NORSOK N-003, /111/.
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5.6.5.5
Green water
a. 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 N-003, /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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. Deck cargoes vulnerable to damage from green water on deck should be protected by breakwaters or
increasing freeboard.
5.6.5.6
Swell
a. The effects of loads and motions due to swell shall be considered. See [3.4.14] and [5.6.18]. Swell can be
governing for operations designed for small irregular waves (e.g. weather restricted tows). In such cases
swell operational limits and forecasting shall be established.
5.6.6
Accidental loads
5.6.6.1
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 DNV-OS-A101, /40/, should be considered as applicable for
the planned marine operation. DNVGL-RP-C204, /31/, gives further guidance related to design philosophy and
calculation of relevant accidental loads due to e.g. collisions and dropped objects.
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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
DNV-OS-A101, /40/, is, in general, based on annual probabilities, whilst this Standard is based on probability per
operation. This can be considered when the (magnitude of) applicable accidental loads are defined. However,
unless a justification for lower loads is documented the loads indicated in DNV-OS-A101, /40/, should be
considered.
5.6.6.5
Vessel collision
a. 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.
b. 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.
c. 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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5.6.6.6
Dropped objects
a. 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.
b. For objects falling through water maximum possible impact velocity should be considered. The maximum
velocity is normally the terminal (free fall in water) velocity. See DNV-RP-H103, /56/, [4.7.3.5] and DNV-RPF107, /52/, [5.3] for guidance.
c. Loads on subsea items due to dropped objects may be ignored if operations that could cause dropped
objects are carried out at a safe distance. The safe distance should be calculated considering the
maximum possible dispersion angle for each type of object falling through the water. The effect of current
should be considered. Risk analysis may be used in order to eliminate physical possible high dispersion
angles by showing that the risk of hitting specified critical locations is acceptably low for such high angles.
See DNV-RP-F107, /52/, for further risk assessment guidance. If detailed assessments are not made, the
safe distance can normally be taken as the larger of 50 meters or that determined from a dispersion angle
of 20° to the vertical.
5.6.6.7
Other causes
a. 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.
b. 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.
5.6.7
Dynamics
5.6.7.1
The potential for dynamic response shall be investigated, and the effects shall be included in the design analysis
when they are of significance. Dynamic response is typically caused by wave forces, wind loads (gusts), vortex
shedding in air or water, slamming loads, etc.
5.6.7.2
Dynamics shall be investigated by recognised methods using realistic assumptions for the natural period,
damping, material properties etc.
5.6.7.3
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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 “DNV-RP-C205 Environmental Conditions and Environmental Loads”, /46/ and Section 7.2 of
“Dynamics of Fixed Marine Structures” - Barltrop and Adams, /122/.
5.6.8
Non-linearities
5.6.8.1
Non-linear effects shall be considered in cases where these significantly influence the estimated responses. Nonlinear effects are typically caused by:
•
•
•
•
•
non-linear materials
non-linear geometry (large-displacement effects)
non-linear damping
non-linear combination of load components or response components
wave elevation e.g. due to wave-in-deck, non-linear effects of drag-loading (especially with current), etc..
5.6.8.2
Non-linear 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.
5.6.9
Friction
5.6.9.1
Possible unfavourable effects of friction shall be considered. Well documented favourable effects of friction may
be included in the design.
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 operation-specific values are given Table 10-2, [11.9.2], Table 11-8, Table 11-20, Table 13-5
and in DNV-RP-H102, /55/, Table 2-4. For soil-material interfaces, guidance is provided in DNV-RP-F109, /53/,
Section 3.4.6 and DNV-RP-F105, /51/, Section 7. Pipe-Soil 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.
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5.6.9.8
The maximum design friction force shall be taken as the maximum design load (i.e. including relevant load
factors) perpendicular to the friction surface multiplied by μud.
5.6.9.9
If the friction coefficient range is based on uncertain data the consequences of the maximum possible variation in
friction coefficients shall be evaluated. See [5.6.14].
5.6.9.10
Vibrations, varying or uncertain surface conditions etc. affecting the friction shall be considered.
5.6.9.11
Restraint effects caused by combination of friction and global deflections shall be considered.
5.6.10
Tolerances
5.6.10.1
Loads caused by operational or fabrication tolerances exceeding the tolerances stated in the design
standards/codes shall be considered. Typical examples include:
•
•
•
•
set-down tolerances (load-out, positioning)
shimming tolerances
uncertain deformation (in load distributing material)
fabrication tolerances, see [5.10.1.4].
5.6.10.2
Loads caused by effects described in [5.5.5].
5.6.11
Relative deflections
5.6.11.1
The effects of relative deflections between structures shall be considered and included in the design whenever
applicable. These can be of particular significance when they induce loads in connections and supports such as
grillages and seafastenings. The causes of relative deflections include:
•
•
•
•
vessel deflection (longitudinal bending) in waves,
ballasting, de-ballasting or re-distribution of ballast,
temperature differences,
relative deflections that need to be considered during the operation.
5.6.11.2
For sea voyages the potential effects of longitudinal wave bending effects should always be considered when:
a.
b.
c.
d.
e.
The towed hull is not a classed, seagoing vessel or barge, or
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
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.
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restraint loads caused by environmental conditions should be defined as E-loads, see [5.5.6].
5.6.12
Motion analysis
5.6.12.1
General
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 sub-section are carried out.
Guidance note:
Detailed analyses and model tests are not normally needed for the transportation of smaller cargoes on
standard vessels.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--3. Testing of models, see [5.4.2], or full scale structures, see [5.4.3], may be carried out where the accuracy of
theoretical approaches is uncertain, or where the design is particularly sensitive for motions.
Guidance note:
Estimation of motions from model testing or by theoretical calculation has associated advantages and
disadvantages. The two approaches are generally to be considered as complimentary rather than as
alternatives.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--4. It is recommended that theoretical calculations are correlated against relevant model test data (if available)
in cases where strongly non-linear behaviour is expected. Such cases can occur when, for example:
◦ overhanging cargo is occasionally submerged, or
◦ there are large changes in the waterplane area with draught.
5. The analytical models should be checked with respect to sensitivity to input parameters, see [5.6.14].
6. Recognised and well proven six-degree of freedom linear or linearized computer programs, utilising the
strip theory or 3D sink source techniques are generally recommended. Special consideration shall be
given to non-linear damping effects. The effect of forward speed shall be evaluated, where this is more
onerous.
7. Computer programs shall be validated against a suitable range of model test or full scale results in
irregular seas. When using new software or for new or unconventional applications or new problems, this
validation shall be documented. Similarly justification of drag coefficients, added mass and damping shall
be documented.
Guidance note:
Guidance on drag and added mass coefficients for a range of standard shapes can be found in DNV-RPC205 /46/.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--8. First-order 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 non-critical routine operations
or screening purposes.
5.6.12.2
Wave headings
a. 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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b. 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].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--c. Short crested sea may be considered for wave analysis where all headings are included with equal wave
height i.e. typically motion analysis for sea voyages without any heading restrictions.
5.6.12.3
Wave periods
a. A wave period range with corresponding wave heights, see [3.4], shall be considered when evaluating
characteristic motions and accelerations.
5.6.12.4
Response amplitude operators (RAO’s)
a. RAO’s for the basic six degrees of freedom can be utilised to calculate displacements, velocities,
accelerations, and reaction forces for points in a body fixed co-ordinate system, or to establish RAO’s for
these points. These RAO’s may be used for calculation of significant and maximum responses.
b. When combining different responses, the phase angle between the different components may be
considered.
c. The gravity component shall be considered when determining the RAO’s for inertia loads (e.g. transverse
accelerations).
5.6.13
Load cases and load combinations
5.6.13.1
Loads and load effects shall be combined to form load cases that are applicable to and physically feasible for the
actual object(s) and type of operation under consideration.
5.6.13.2
All possible load cases which can influence the feasibility of the marine operation shall be considered in the
design.
5.6.13.3
Characteristic loads may be combined taking into account their probability of simultaneous occurrence.
5.6.13.4
Characteristic static (mean) load components and characteristic dynamic (varying) load components which are
statistically independent may be combined according to the formulae below.
where
Fi,mean
Fi,amp
=
Characteristic static load components
=
Amplitude of dynamic load components
Guidance note:
Dynamic load components in the above formulae are normally restricted to loads with periods less than
10 minutes. The maximum values of dynamic loads with periods greater than 10 minutes are normally added as
static loads (i.e. Fi,mean equal to the maximum load, and Fi,amp =0).
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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].
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5.6.14
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Sensitivity analysis
5.6.14.1
The load cases shall include a parametric sensitivity analyses whenever a single load or parameter significantly
affects the design or selection of the method or equipment to determine whether small changes significantly
affect the design.
5.6.14.2
Where the operational safety is critically dependent on a sensitive input, conservative characteristic values shall
be used.
5.6.15
Loads due to motions and wind
5.6.15.1
Load cases for each heading shall be derived by the addition of fluctuating loads resulting from wind and wave
action to static loads resulting from gravity and still water initial conditions.
5.6.15.2
In lieu of a refined analysis the worst possible combination of the individual responses for the same heading,
including components from the self-weight and wind, shall be combined, i.e.:
where
Sd
=
S( )
=
Fx, Fy, Fz =
Design load or load effect.
Response/load effect function.
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 self-weight (vectors).
5.6.15.3
Alternatively, the fluctuating components shall be the worst possible combination of the loads resulting from
calculations or model tests carried out in accordance with [11.3.7.1] through [11.3.7.3], with due account to be
taken of the effects of phase. All influential loadings shall be considered: however the following static and
environmental loadings are the most likely to be of importance:
S1
=
Loadings caused by gravity including the effects of the most onerous ballast condition on the
=
Loadings caused by the wind heel and trim angle.
=
Loadings caused by surge and sway acceleration
=
Loadings caused by pitch and roll acceleration
=
Loadings caused by the gravity component of pitch and roll motion
=
Loadings caused by direct wind
=
Loadings caused by heave acceleration, including heave.sin(theta) terms
=
Loadings caused by wave induced bending
=
Loadings caused by slam and the effects of immersion.
voyage.
F1
F2
F3
F4
F5
F6
F7
F8
5.6.15.4
One of the following four methods in this paragraph shall be used to determine the design loadings:
a. 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.
b. 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.
c. The total loads may be calculated by combination of loads as follows:
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where:
Fmot
F#(1 hour)
F#(1 min)
F#
=
Maximum load due to wind and wave motions
=
Loads based on 1 hour mean wind speed
=
Loads based on 1 minute mean wind speed
=
F1 through F8 as applicable
d. 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.
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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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5.6.16
Default motion criteria
5.6.16.1
For loads computed in accordance with [11.4], the loads applied to the cargo shall be:
S1+F1+F3+F4+F6
where: S1, 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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5.6.17
Loads due to restraint deflections, vessel motions and wind
5.6.17.1
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
5.6.18
=
Total design load
=
Maximum loads due to deflections
=
Maximum load due to wave motions and wind.
Loads 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].
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5.7
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Failure modes
5.7.1
All relevant failure modes shall be investigated. A failure mode is relevant if it is considered possible and the
anticipated consequence(s) of the failure cannot be disregarded.
5.7.2
The relevant failure modes can be grouped as either as global (total system) or local (individual members) as
indicated in the following sections.
5.7.3
Global modes of failure include:
•
•
•
•
•
•
•
•
structural collapse
overturning
sliding
lift-off
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.
5.8
Analytical models
5.8.1
The analytical models used for evaluation of loads, responses, structural behaviour and resistance shall be
relevant considering: the design philosophy, the type of operation and the possible failure modes. The models
should satisfactorily simulate the behaviour of the object’s structures, its supports and the environment.
5.8.2
Design analyses shall be carried out considering all relevant loads and failure modes, see [5.7].
5.8.3
The design analysis shall be thoroughly documented that the results shown to satisfy the relevant requirements
and criteria.
5.9
Strength assessment
5.9.1
General
5.9.1.1
Structural strength can be assessed using either ASD/WSD methodology or LRFD methodology. These are
discussed below.
5.9.1.2
Whichever methodology is applied, the loading conditions/limit states shown in Table 5-1 shall be considered
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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 DNVGL-OS-C101, /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 5-3 Description of loading conditions/limit states
Loading condition / limit state
ASD / WSD name
LRFD name
Maximum capacity, usually for maximum environmental and
functional loads (permanent, variable, deformation)
LS1
LS2
ULS-a
ULS-b
Loading history – important for structures exposed to
significant cyclic/repetitive loading
FLS
FLS
Intact structure subjected to loads from an accidental event
ALS-I
ALS-I
Damaged structure subjected to post-damage loading
ALS-D
ALS-D
Serviceability checks (alignment, clearances, deflection,
vibration, etc.)
SLS
SLS
5.9.2
Design approach
5.9.2.1
The format of the ASD/WSD method implies that strength/capacity verification of structures or systems involves
the following steps:
• Identify all relevant limit states, see [5.9.1].
• Identify all relevant loading conditions, see [5.6.13].
• For each loading condition define the relevant characteristic loads, see [5.5.2], and design conditions, see
Table 5-1.
• For each loading condition and failure mode, see [5.6] and [5.7], find the design loads
• For each loading condition determine the design load effect, see [5.6]
• Ensure adequate safety by proving that the design load effect does not exceed the allowable, as
described in [5.9.4], [5.9.5], [5.9.6] and [5.9.7],
LS2 is applicable only when the loading is dominated by environmental/storm loads, e.g. for weather
unrestricted operations the extreme loads due to the applicable design return period environmental criteria, see
Table 3-1; 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
5-1.
• For each limit state find the design loads by applying the relevant load/design factors, see [5.9.4.2],
[5.9.5.2], [5.9.6.2] and [5.9.8.3].
• For each limit state determine the design load effect, see [5.6] and [5.9.3.2 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)].
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5.9.3
LRFD checks
5.9.3.1
General
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a. 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.9.3.2
Acceptance criteria
a. The level of safety is considered to be satisfactory if the design load effect, Sd, does not exceed the design
resistance, Rd, i.e.:
Sd ≤ Rd for all limit states
The equation Sd = Rd defines the respective limit state.
b. A design load effect is an effect (e.g. stress, mooring line load, sling load, deformation, overturning
moment, cumulative damage) due to the most unfavourable combination of design load(s) i.e.:
where
Sd
Fd
S
=
design load effect
=
design load(s)
=
load effect function.
c. 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].
d. 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].
5.9.3.3
Characteristic resistance
a. 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).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 DNVGL-OS-C101 /24/ Section 10 1.3.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. Rc for (wire & fibre) ropes and chains should be taken as the certified MBL.
5.9.4
Fatigue limit states – FLS
5.9.4.1
General
a. For all structures exposed to significant cyclic loads during a marine operation the possibilities and effects
of fatigue should be considered.
b. 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 5-3.
c. Possible dynamic load effects due to e.g. slamming and vortex shedding should be investigated. See
[5.6.7].
d. Restraint loads, see [5.6.17.1], could be important and shall hence be thoroughly evaluated and included
in the FLS calculations.
e. The FLS shall be evaluated according to procedures given in a recognised code or standard. See e.g.
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DNVGL-OS-C101, /24/, Ch.2 Sec.5 for general requirements for checking of fatigue limit states.
Guidance note 1:
Reference can be made to DNVGL-RP-C203, /29/, and DNV CN 30.7, /20/, for practical details with respect
to fatigue design.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 peak-stress range is increased
to 550 N/mm2 then a fatigue analysis is advisable for voyages over about 10 days.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
New-build MOU's are normally verified for fatigue for the initial delivery voyage in the classification
process and a separate analysis is not normally required for this voyage. For subsequent voyages, it is
desirable to undertake a fatigue analysis, however in many cases there is insufficient time and/or data
regarding prior use. In such cases it is good practice to undertake a thorough NDT inspection of fatiguecritical areas before the voyage and to repair any cracks, see [11.27.4.4].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--f. For mooring systems, the FLS is mainly of concern for steel components where fatigue endurance limits
the design. For fibre-rope segments, the time-dependent strength can limit the design; consequently
stress rupture or creep failure should be incorporated in the checks for ULS and ALS as appropriate. See
also DNVGL-OS-E301, /27/.
g. Where structural items e.g. grillages and seafastenings, are to be re-used they should be demonstrated to
have sufficient fatigue life for the sequence of planned operations, including all previous operations. An
appropriate inspection regime shall be proposed including NDT at appropriate intervals e.g. close visual
examination after every use and NDT after every 10 uses; if there are highly utilised areas, more frequent
NDT could be appropriate. For bolts, see [E.2].
5.9.4.2
Design factors - FLS
a. All load factors shall be:
γf=1.0
b. Design fatigue factors (DFF) shall be applied to increase the probability of avoiding fatigue failures
c. The calculated cumulative damage ratios for the defined design conditions times the applicable DFF
according to Table 5-4 shall be less or equal to 1.0.
d. Lower values for the Miner’s sums than 1.0 can be relevant if the structure has been or will be subjected to
fatigue loading before or after the considered marine operation. In such cases the maximum allowable
Miner’s sum for the actual marine operations shall be determined by considering the total load history the
structure will be exposed to.
Table 5-4 Design fatigue factors (DFF)
Inspection during operation (and
repair) planned
Elements in inspection category I
Elements in inspection categories
II & 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.
5.9.5
Accidental limit states – ALS
5.9.5.1
General
1. Accidental limit states for marine operations include verification of:
◦ ALS-I: The intact structure or system for the defined accidental load effect(s) combined with other
relevant load effects, see Table 5-5 (i.e. loads of type E may be ignored).
◦ ALS-D: The damaged structure or system, see [5.9.5.1 2)], for relevant design load effects, see Table
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5-5.
Guidance note:
See also Table 5-3 for definition of ALS-I and ALS-D.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--2. The damage to the structure or system in ALS-D is normally defined by either:
◦ the damage caused by the defined accidental load effect(s) or,
◦ a defined damaged or an accidental condition/scenario, see [5.6.6].
5.9.5.2
Design approach and load and resistance factors
a. Accidental loads are defined in [5.6.6].
b. 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.
c. Load factors should in ALS normally, see [d)], be taken according to Table 5-5 or Table 5-6.
d. Load factors greater than 1.0 shall be considered if an LRFD method ALS load or condition is not
considered to have a sufficient low, i.e. ≤10-4 per operation, probability. If working to the ASD/WSD
approach, the factors should be similarly increased.
e. The characteristic environmental load (E) in the ALS-D load condition should/may be defined considering
the probability of the analysed accident/damage and the anticipated maximum period (i.e.TR, see [2.6.2])
the damaged situation will remain.
Table 5-5 ASD/WSD Load factors for ALS
Type
AISC 14th WSD option strength checking allowables
ALS-I
0.6
ALS-D
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 5-6 LRFD Load factors for ALS
Load Categories
Load
Condition
G
Q
D
E
A
ALS-I
1.0
1.0
1.0
NA
1.0
ALS-D
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]
5.9.5.3
Material 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.9.6
Serviceability limit states – SLS
5.9.6.1
General
a. 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).
b. See DNVGL-OS-C101, /24/, Ch.2 Sec.7 for typical SLS requirements for offshore steel structures.
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5.9.6.2
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Safety factors
a. 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.
b. SLS for structural elements shall normally be checked applying load and material factors equal to 1.0.
Guidance note:
In SLS the object (or equipment/vessel) owner is free to define higher load- and material factors if this is
found applicable.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.9.7
ASD/WSD strength checks for structural steel subject to LS1 or LS2 loading
5.9.7.1
Design 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:
a. 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.
b. 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.
c. Steelwork supporting sacrificial bumpers and guides.
d. Spreader bars, lift points and primary steelwork of lifted items.
e. Structures during a load-out.
4. Traditionally AISC has also been considered a reference code, e.g. by API RP2A. If the ANSI/AISC 360-10
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
5-7.
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--5. Stresses in welds shall be assessed according to either:
a. The method given in DNVGL-OS-C102 Ch.2 Sec.9.2.5, /25/, or equivalent, or
b. The method illustrated by the example given for the assessment of fillet welds for brackets given in
[E.1].
c. 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 5-3):
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
5-3):
0.78 for welds made at fabrication site
0.67 for welds made on board the vessel.
d. 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 fit-up (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.
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---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 5-7, are as follows:
a. 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.
b. 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 non-tubular connections shall be in accordance with an appropriate standard such as AISC /2/,
using a consistent safety format and factors.
Table 5-7 Load factors for use the ASD/WSD method and AISC 14th edition
AISC 14th WSD option strength checking allowables
Type
Limit State 1 (LS1)
1.00 3)
Limit State 2 (LS2)
0.75 3)
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 gravity-load 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.
5.9.8
LRFD strength checks for structural steel subject to ULS loading
5.9.8.1
General
DNVGL-OS-C101, /24/, Ch.2 Sec.4 gives provisions for checking of ultimate limit states for typical structural
elements used in offshore steel structures.
5.9.8.2
Load factors - ULS
For the ultimate limit states (ULS) the two load conditions “ULS-a” and “ULS-b” as given in the Table 5-8 shall be
considered.
Table 5-8 Load factors for ULS
Load Categories
Load
Condition
G
Q
D
E
A
ULS-a
1.3
1.3
1.0
0.7
NA
ULS-b
1.0
1.0
1.0
1.3
NA
Notes:
1. Load categories G, Q, D, E and A are described in [5.5].
a. For loads and load effects that are well controlled a reduced load factor γf = 1.2 may be used for the G and
Q loads instead of 1.3 in load condition ULS-a.
Guidance note:
Examples where γf = 1.2 may be applicable are:
◦ External hydrostatic pressure caused by an accurately defined water level.
◦ Loads due to an accurately distributed (i.e. static determinate) well defined self-weight.
◦ Functional loads accurately defined (limited) by the maximum (possible) capacity of equipment.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. Where a permanent load G (e.g. self-weight or hydrostatic pressure) causes favourable load effects, a load
factor γf = 1.0 shall be used for this load in load condition a. See also [5.6.2.2] and [5.6.2.3].
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c. 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.
d. In dynamic problems the application of load factors should be given special consideration. In lieu of a
probabilistic analysis, the load effects may be found by application of load factors after having found the
responses, e.g. after having solved the equations of motion for vessel motion response analysis.
5.9.8.3
ULS material factors
a. 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].
b. If a material factor γm = 1.0 is found more unfavourable than the indicated values, γm = 1.0 shall be used.
5.9.8.4
Material factors for structural steel:
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
DNVGL-OS-C101, /24/, Ch.2 Sec.4 for steel structures and DNVGL-OS-C101, /24/, Ch.2 Sec.8 for welded
connections shall be applied.
Guidance note:
See also Table 6-1 in NORSOK N-004, /112/, for applicable material factors.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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-OS-C101 Ch.2 Sec.8, /24/, EN 1993-1-8 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 fit-up (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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.9.8.5
Material 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:
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).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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
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acceptable; however, γm should not be less than 1.65
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--4. When using DNVGL-OS-C101 /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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.9.8.6
Material factors for friction
a. 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.
b. 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.10
Materials and fabrication
5.10.1
Design considerations
5.10.1.1
Applicable codes
a. In general material selection, fabrication method, and non-destructive testing should be carried out
according to a recognised offshore code, e.g. DNVGL-OS-C101, /24/, or DNVGL-OS-C401, /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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. Independent of the applied code, it shall be documented that the requirements in this section [5.10] are
fulfilled.
5.10.1.2
Structural 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 DNVGL-OSC101, /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
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--3. For materials in temporary structures used for marine operations, the following apply:
◦ The design temperature, see DNVGL-OS-C101, /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 6-1 for guidelines regarding selection of structural category. See also DNVGL-OSC101, /24/, Ch.2 Sec.3.3.
◦ For materials that could be welded under adverse conditions the yield strength (SMYS) should not
exceed 355 MPa.
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5.10.1.3
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Material quality
a. Selection of steel types shall be determined based on the structural application and the required category
Table 5-9.
b. All steel materials shall be suitable for the intended service conditions and shall have adequate properties
of strength, ductility, toughness, weldability and corrosion resistance.
c. Material types and qualities should comply with requirements in DNV-OS-B101, /23/.
d. Non-structural steels shall have mechanical properties and weldability suitable for the intended
application.
Table 5-9 Structural categories
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
Recommended
structural
category
NORSOK N-004
Equivalent /112/
4)
DNVGL-OS-C101
• Padeyes and
other lifting
points
• Seafastening
elements
without
redundancy
• Spreader bars
Structures for
connection of:
• Mooring and
towing lines
• Grillages
• Redundant2)
seafastening
elements
• Bumpers and
guides
• Fender
structures
• Redundant2)
(parts of)
grillages
Insp.
Cat.,
DNV
GL
Special
DC1 – SQL1
I
Primary (Special)
DC2 – SQL2
(SQL1)3)
I or
II5)
DC3 – SQL2
(SQL1)3)
II
DC4 – SQL3
(SQL1)3)
II
DC5 – SQL4
III
3)
Primary (Special)
3)
Primary (Special)
3)
Secondary
Notes:
1. Complex joints are joints where the geometry of connected elements and weld type leads to high
restraint and to tri-axial stress pattern.
2. Residual strength (redundant) means that the structure meets requirements corresponding to the
damaged condition in the check for ALS, with failure in the actual joint or component as the defined
damage.
3. Selection where the joint strength is based on transference of tensile stresses in the through thickness
direction of the plate.
4. The design classes and material selection according to NORSOK M-120, /110/ should be considered as
guidance only.
5. Extent of NDT to be according to DNV 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 M-101, /109/ inspection category B is normally acceptable.
5.10.1.4
Tolerances
a. As-built deviations shall not exceed fabrication tolerances assumed in the applied structural codes and
standards, or in the design analysis, unless specially considered on a case-by-case basis.
b. Acceptance of any as-built deviations exceeding specified tolerances shall be confirmed in writing by, as
applicable, the owner, designer, installation contractor, etc.
c. DNVGL-OS-C401, /26/, Ch.2 Sec.2.5 indicates fabrication tolerances that are normally acceptable.
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d. 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 DNVGL-OS-C401, /26/
5.10.2
Fabrication
5.10.2.1
Workmanship
a. Workmanship during fabrication shall be of good standard and according to accepted practice. See also
DNVGL-OS-C401, /26/, Ch.2, Sec.1 and Sec.2.1 through 2.5.
b. Guidelines regarding assembly and welding can be found in DNVGL-OS-C401, /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 fit-up 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:
a. Welding procedure specifications should be qualified by welding procedure tests carried out under
conditions representative of the actual working environment; see DNVGL-OS-C401, /26/, Ch.2 Sec.1.2.5.
b. Thorough inspections of fit-up and welding should be planned for.
c. Weather conditions and forecast to indicate acceptable conditions for welding considering the welding
method and available shelter at the welding locations.
d. Use of increased weld size in order to compensate for inaccurate fit-up (i.e. over-sized gaps) to be
considered.
e. Robust and well proven welding methods and procedures to be applied.
f. Use of material with improved weldability; see DNVGL-OS-C101, /24/, Ch.2 Sec.3.4.2, to be considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
5.10.2.3
Weld inspection
1. All NDT (non-destructive 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.
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
(Z-quality). Where this is not feasible, the material under through-thickness tension should be checked for
laminations after the recommended cooling and waiting time as defined by the Welding Procedure
Specification (WPS) and NDT procedures. The reason for waiting is that laminations can also be subject to
hydrogen embrittlement, the same as welds, see SSC-290, /118/, for more details of lamellar tearing. If
access is not possible after welding, pre-welding checks could be acceptable.
Guidance note 1:
For non-critical seafastenings and their supports, through-thickness testing should be carried out when the
tensile stress normal to any plate exceeds 100 MPa.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The tensile stress should be calculated in a section between the deck plate and the weld (i.e. not in the
critical weld section). If the under deck weld is smaller, this weld should be used as a reference, see also
Guidance note to [11.9.5.27]. 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 non-destructive testing (NDT) of welds can be found in DNVGL-OS-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/D1.1M-2015 “Structural welding code – steel” /8/.
7. Minimum extent of inspection should be as shown in DNVGL-OS-C401, /26/, Ch.2 Sec.3 Table 1 with
“Inspection Category” as defined in Table 5-9. See also Table 5-10 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 M-101, /109/, Sec.9.1 and DNVGL-OS-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 5-10 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Table 5-10 Minimum extent of NDT and waiting time
Minimum extent of NDT
Minimum waiting time before NDT
Inspection
Category
Visual
Other1)
SMYS2) ≤355
MPa3)
SMYS2) > 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-OS-C401, /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 6 Gravity based structure (GBS)
6.1
Introduction
6.1.1
General 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 6-1 are covered. Depending on the type of structure and method of construction,
some or all of the following sections will give the relevant requirements.
Table 6-1 Requirements for different GBS phases
General requirements
Stability and freeboard (all phases)
Structural strength
Temporary ballasting and compressed air systems
See Sec.2 to Sec.4
See [6.2]
See [6.3] and Sec.5
See [4.3]
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Construction basin and tow-out
See Sec.12
Construction and/or solid ballasting afloat
See Sec.14
Deck-mating (inshore or offshore)
See Sec.15
Towage(s)
See Sec.11
Instrumentation
See [6.4]
Installation at location
See [6.5]
Ensuring on-bottom stability
6.1.2
See [13.10]
Revision 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, DNV-OS-H201
6.2
Floating GBS stability and freeboard
6.2.1
General
6.2.1.1
Sufficient positive stability and reserve buoyancy shall be ensured during all stages of the marine operations.
Both intact and damage stability shall be evaluated, on the basis of an accurate geometric model. This shall
include inclining tests of the 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:
a.
b.
c.
d.
e.
Free surface
Air cushion
Icing
Influence of moorings, including a check on the consequences of failure.
Temporary Loads and Structures (including any cantilevered structures)
6.2.1.5
The number of openings in buoyant elements adjacent to the sea shall be kept to a minimum. Where
penetrations are necessary for access, piping, ventilation, electrical connections, etc. arrangements shall be
made to maintain watertight integrity. During construction phases, particular attention should be paid to
openings near the waterline, which will vary as construction proceeds.
6.2.1.6
Damage stability requirements shall be evaluated considering the operation procedure, environmental loads
and responses, the duration of the operation and the consequences of possible damage. Compartments that
may be subject to flooding or partial flooding include:
a. Compartments adjacent to the sea
b. Compartments inside the structure, crossed by seawater filled pipes
c. Skirt compartments containing compressed air.
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6.2.1.7
Special attention should be paid to flooding which may be caused by:
a.
b.
c.
d.
Impact loads from vessels
Damage to structure or pipework from dropped objects
Mechanical system failure
Human error.
6.2.1.8
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:
a.
b.
c.
d.
e.
Limiting the exposure period
Providing additional local structural strength
Providing additional protection, such as fendering
Minimising vessel movements near the structure
Dedicated procedures and experienced personnel.
6.2.1.11
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 back-up or protection systems, or special procedures should be
assessed.
6.2.2
Intact stability
6.2.2.1
The initial GM shall not be less than 0.5 m (after allowing for all possible inaccuracies in measuring it) unless
agreed with MWS Company.
6.2.2.2
The maximum inclination of the floating GBS or platform should not exceed 5° in the design environmental
condition as defined in [3.1] apart from possible exceptions during installation as described in the guidance note
to [6.5.4.4]. Calculation of maximum inclination should take into account:
a. Maximum amplitude of pitch or roll motion in the design sea state, plus
b. Inclination due to design wind, plus
c. Inclination due to mooring line tensions or required towline pull.
Guidance note:
The maximum inclination of 5° is due to the large height of GBS structures and the corresponding motion
experienced at this height.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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°.
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6.2.2.4
The area under the righting moment curve shall be not less than 140% of the area under the overturning
moment curve as shown in Figure 11-2. Both curves shall be bounded by the least of:
a. The second intercept of the righting and overturning moment curves
b. The angle of downflooding
c. 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.
d. The angle at which allowable stresses are reached in any part of the structure, construction equipment,
topsides or topsides attachments, if applicable.
6.2.2.5
The wind used for overturning moment calculations should be the design wind for the operation, as defined in
[3.3]. Short duration operations during construction or towage may be considered as weather restricted
operations, provided the structure can achieve or be returned to a safe condition, within the operation reference
period
6.2.3
Effective freeboard
6.2.3.1
For inshore towages and construction afloat, the effective freeboard, as defined in Table 1-3, shall not be less
than the greater of:
a. 1 m above the design wave crest height, with allowance for run-up, all around the structure, under the
design storm loading from the most critical direction,
b. 6 m in the intact condition, if the unit does not have one-compartment damage stability.
6.2.3.2
For offshore towages, after damage, an effective freeboard of not less than 5 m shall remain above the design
wave crest height, with allowance for run-up, all around the structure, from the most critical direction. Calculation
of the freeboard shall account for motions experienced as a result of the design environmental conditions and
mooring line tensions or required towline pull.
6.2.4
Damage stability for tow-out and inshore tows
6.2.4.1
For tow-out from dry-dock, one-compartment damage stability is not required as it is a controlled operation and
the under-keel clearance is limited.
6.2.4.2
For other inshore tows the structure should have one-compartment damage stability, as defined in [6.2.1.6]
through [6.2.1.9].
6.2.4.3
If one-compartment damage stability requirements cannot be fulfilled, the requirements for construction afloat in
[6.2.5.2] shall apply.
6.2.5
Damage stability during construction afloat
6.2.5.1
During the period of construction afloat, the platform shall possess one-compartment damage stability, for as
much of the construction period as is practical.
6.2.5.2
When the platform does not possess one-compartment damage stability, then in addition to [6.2.1.10]:
a. A means should be available to compensate for inclination due to flooding of any compartment, and
b. There shall be sufficient structural strength in the outer walls to withstand impact loads from the
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construction spread and vessels, which may be in close proximity to the platform, and
c. Fendering may be used to reduce impact loads in critical areas, and
d. 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
e. Any objects or equipment on barges alongside, which if dropped, could endanger the platform shall be
similarly identified and additional precautions taken, and
f. 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
g. At all times there shall be adequately trained personnel on board the platform, and
h. As per [6.2.1.11], a risk assessment of flooding shall be carried out in accordance with [2.4].
6.2.6
Damage stability for offshore tows and installation
6.2.6.1
When towing on the caisson or columns the platform should possess one-compartment damage stability.
6.2.6.2
It is acknowledged that for an offshore tow, the requirement in [6.2.6.1] might be impractical, in which case:
a. 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
b. Rigorous procedures shall be developed to minimise the risk of flooding, and
c. A risk assessment of flooding shall be carried out in accordance with [2.4].
6.2.6.3
It is acknowledged that during installation, it might be impractical to provide reinforcement against collision over
the full range of waterlines. Planning and risk assessment shall include a procedure to return the structure to the
reinforced waterline should the installation operation be aborted.
6.3
Structural strength
6.3.1
Concrete gravity structures - load cases
6.3.1.1
The requirements of Sec.5 apply.
6.3.1.2
Load cases shall be derived by the addition of fluctuating loads resulting from wind, wind heel, wave action and
the effect of towline pull or mooring loads to the static forces resulting from gravity and hydrostatic loads for the
following temporary phases before it is safely installed:
1.
2.
3.
4.
5.
tow-out from construction basin or dry-dock (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.
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6.3.1.5
The unit shall be able to safely withstand a static heel angle of 10°, or any greater angle required during
construction, towage or installation. If it has damage stability, the unit shall also be able to withstand the static
and dynamic loads caused by the flooding of any one compartment in the lesser of the 10-year return period
environmental conditions or a 25 m/s wind and associated waves. These should be assessed as LS1 or ULS
conditions, unless it is demonstrated that alternative criteria apply.
6.3.1.6
Hydrostatic loads on the substructure at the deepest draught during deck-mating can be the governing load
case. It shall be demonstrated that a thorough independent check of the calculations covering this load case has
been carried out, and that the design and reinforcement details assumed in the calculations concur with the 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.
6.3.2
Structural concrete
6.3.2.1
The strength of concrete and its reinforcement including any pre- or post-tensioning 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 DNV-OS-C502
– 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.
6.4
Instrumentation
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:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
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.
6.5
GBS installation
6.5.1
General
6.5.1.1
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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:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
The size and design of the platform
Water depth
The positioning tolerances required in all 6 degrees of freedom
The positioning/stationkeeping system proposed
Whether cranes, winches or external buoyancy is required for lowering and/or positioning
Whether the operation involves docking over a template, docking piles or other structures
Stability at all stages of immersion
Whether a vertical or inclined installation is required
Tolerances on differential ballast levels
The skirt design, and penetration method
Whether under-base grouting is required
Whether solid ballast or scour protection is required.
6.5.2
Survey
6.5.2.1
The position of the site location shall be given in both geographical and grid coordinates.
6.5.2.2
The water depth and bathymetric tolerances shall be determined.
6.5.2.3
When determining the extent of the survey area, the following shall be accounted for:
a.
b.
c.
d.
e.
f.
g.
Tolerances on site survey position
Inaccuracy of position monitoring systems during installation
Operational tolerances
The approach corridor
Whether a holding location is required close to the site
Whether an inclined installation, with previous off-site touch-down is required
The proximity of any other platforms or subsea assets at or near the location.
6.5.2.4
The bottom topography shall be established by swathe bathymetry, high resolution echo sounder techniques,
side scan sonar, and checked by magnetometer and ROV video for obstructions and possible unexploded
ordnance. The extent of any required levelling or other seabed preparation should be decided at the design
stage.
Guidance note:
Swathe bathymetry is now available in portable units and is installed on most survey vessels so should be used as
standard on all survey projects. Due to constraints imposed by calibration and processing requirements (single
point obstructions may be removed in processing), conventional high-resolution bathymetry and side scan sonar
should be run in conjunction.
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6.5.2.5
The seabed and sub-seabed conditions shall be established by coring, magnetometer, in-situ testing, lab testing
and sub-bottom profiling.
6.5.2.6
Sufficient current surveys shall be completed to determine the current profile with depth.
6.5.2.7
The area should be checked to ensure that there are no travelling sand-waves or other seabed erosion/accretion
that could affect the structure during installation.
6.5.2.8
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A site survey of the installation area covering the full area of any anchor pattern, carried out not more than 4
weeks before the start of installation, shall be provided to verify the location of all subsea infrastructure, debris
and obstructions.
6.5.3
Seabed preparation
6.5.3.1
The required tolerances for level and compaction shall be documented at an early stage.
6.5.3.2
Where surveys shows the seabed is out of tolerance it shall be prepared to correct for uneven levels or
consistency. Description of the preparation works, including details of how tolerances shall be achieved, shall be
documented.
Guidance note:
Typical seabed preparation methods include:
a. Controlled dumping and compacting of gravel before final levelling
b. Placing sand-bags
c. Excavating of unsuitable soils before replacing as in a) or b).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
6.5.4
Installation method principles
6.5.4.1
In general it is desirable for all installation phases to be reversible though this may not always be possible,
especially if there are temporary unstable phases.
6.5.4.2
The approval criteria shall be agreed with the MWS Company. The agreed criteria shall depend on the
installation methods and consider the following:
a.
b.
c.
d.
The required external assistance (e.g. temporary buoyancy, winches, cranes, 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
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
6.5.4.5
In the event of an inclined installation the following shall be considered:
a.
b.
c.
d.
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
e. If the skirt touch down is on the final site, accurate position control may be difficult in the inclined condition
f. If skirt touch down is remote from the final site, the deballast capability required by [4.3.5] will be used.
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Positioning and position monitoring systems
6.5.5.1
The positioning system shall be designed to meet the required installation tolerances. This will normally be by
means of tugs, often the tow fleet is rearranged into a star configuration.
6.5.5.2
Where more precise positioning is required, the tugs may be connected at the bow to pre-laid anchors though
other mooring systems are possible. Mooring systems shall comply with Sec.17.
6.5.5.3
Where the position and orientation tolerances are not critical, the tugs may be in free floating configuration.
6.5.5.4
Where docking piles are to be used the requirements in [13.8.4] apply
6.5.5.5
A position monitoring system in accordance with [4.4.5] shall be provided. The system shall allowing monitoring
of capturing docking piles if being used.
6.5.6
Ensuring on-bottom stability/skirt penetration
6.5.6.1
The requirements in [13.10.1] apply including specifying the depth(s) of any required penetration(s).
6.5.6.2
Calculations shall be documented to demonstrate that the base or skirts will penetrate to the required depths.
The calculations shall specify if negative pressure is required in addition to gravity/buoyancy loads. Additionally
the calculations should consider the following:
a.
b.
c.
d.
expected (and maximum and minimum) soil friction
expected (and maximum) suction versus penetration depth
soil sealing differential pressure versus penetration depth
capacity of suction pumps
6.5.6.3
A venting system sufficient to ensure foundation integrity shall be provided to allow water in the skirt
compartments to escape and where required to allow negative pressure to be applied.
Guidance note:
Design of the pipework should take into account the requirements for removal on decommissioning.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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:
a. differential pressure (suction)
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b. penetration
c. flow rate
6.5.7
Anti-scour precautions
6.5.7.1
All locations, especially with high current speeds, should be investigated to see if scour could cause problems
during the installation and subsequent temporary stages.
6.5.7.2
Details of anti-scour precautions where required shall be documented. Possible solutions to scour include:
• Controlled rock dumping or placing sand-bags immediately after the unit is installed. Care shall be taken
to avoid any damage to the unit especially near penetrations, pipelines, cables or other sub-sea 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 7 Cables, pipelines, risers and umbilicals
7.1
Introduction
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
• DNV-OS-H206 ,DNV Offshore Standard, Load-out, transport and installation of subsea objects (VMO
Standard Part 2-6).
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 DNV-OS-H206 is applied then DNV-OS-H101, and DNV-OS-H102 and DNV-OS-H205 also apply
along with any other referenced documents.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
7.2
Codes 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: DNV-OS-F101, /42/,
Dynamic risers: DNV-OS-F201, /43/,
Flexible pipe systems: ISO 13628-2, /95/, or ISO 13628-11, /97/,
Umbilicals: ISO 13628-5, /96/,
Subsea power cables: see Guidance note 4.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
Guidance note 2:
Generally the default for rigid pipeline system design and approval is DNV-OS-F101 Submarine Pipeline
Systems. DNV-OS-F101 Sec.10 gives requirements for installation/offshore construction of submarine pipeline
systems. Parts of DNV-OS-F101 Sec.10 are also generally applicable for flexible pipes and risers.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
Detailed guidance regarding installation of cables may be found in DNV-RP-J301 Sec. 6.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
SECTION 8 Offshore wind farm (OWF) installation
operations
8.1
Introduction
8.1.1
General
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.
8.1.2
Scope
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 jack-up platforms.
8.1.3
Revision history
8.1.3.1
This section replaces the applicable sections of the following legacy document:
• 0035/ND Guidelines for Offshore Wind Farm Infrastructure Installation.
8.2
Planning
8.2.1
General
8.2.1.1
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See Sec.2 for general planning requirements and Sec.3 for environmental conditions and criteria.
8.2.2
Tolerances and criteria
8.2.2.1
Tolerances and criteria should be agreed with the MWS company at an early stage of the project.
Guidance note 1:
The selection of many installation tolerances and criteria will be a trade-off between reducing the cost of
manufacture and reducing the costs of delays waiting for good weather in consequence. Manufacturers often
prefer tighter installation tolerances which require better weather criteria for installation. It is generally beneficial
to select the transport/installation contractors before such tolerances and criteria are fixed as they may
significantly affect the installation methods, risks and costs.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
8.2.2.2
Such tolerances may include:
a. Position and orientation of monopiles, pile templates, jackets and other structures.
b. Pile or structure verticality.
c. Clearances between piles inside pile sleeves, including allowances for weld beads and grout keys.
8.2.2.3
Such criteria may include:
a. Wind speeds (at specified heights and gust durations) for critical lifts.
b. 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.
c. Degree of acceptable damage to grout keys during piling.
d. Any restrictions on helicopter or vessel movements within the field in bad visibility or other adverse
conditions.
e. Any restrictions on transfer of people and equipment onto fixed or floating installations by various means.
f. 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).
g. Piling operations – sound effects on sea life.
8.2.3
Vulnerable items or areas
8.2.3.1
Due to the many parties and vessels working in close proximity, it is necessary that each party understands what
items are particularly vulnerable to actions by others. These items need to be identified at an early stage so that
they can be considered in the relevant risk assessments. The list of vulnerable items needs to be updated and
promulgated as required during the life of the wind farm.
8.2.3.2
Typical vulnerable items or areas may include:
a. J-tube entry holes being covered with soil or debris.
b. Changes in seabed level (from scour, dredging, jack-up footprints, drill cuttings, etc.) varying the natural
frequency of foundations.
c. Scour can also affect jack-up foundations, cables, anchors etc. Scour model tests may be required in areas
with high current speeds and soft or sandy seabeds.
d. Damage to grout seals and back-up seals.
e. External fittings (including anodes, J-tubes, etc.) being damaged by dropped objects, vessel collision or
mooring lines.
f. Operations of divers (vulnerable to propellers and propeller wash, noise and blast, bubble curtains, cables
and dropped or lowered objects).
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Planned 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.
8.3
OWF installation vessels
8.3.1
Jack-ups – general
8.3.1.1
Jack-up legs can be a major threat to cables. The as-laid cable routes should be updated as required and
properly distributed through the project in order to prevent cable damages. A suitable safe distance shall be
maintained between the as-laid cable route and the intended positions of the jack-up legs. This is of particular
importance in OWF developments where cable laying/installation is progressing near turbine installation
activities in a similar time frame.
8.3.2
Jack-ups in weather unrestricted operations
8.3.2.1
Jack-ups that are designed and classed for elevated operations in conditions in excess of those at the installation
site (either all year or for particular months) shall comply with the requirements of DNVGL-ST-N002, /39/
8.3.2.2
The jack-up can operate at a lower air gap than required for survival in a design storm as long as it is able to 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
8.3.3
Jack-ups in weather restricted operations
8.3.3.1
Jack-ups that cannot comply with [8.3.2] for a specific location and season shall comply with the requirements for
weather restricted operations in [2.6.5].
Guidance note:
Useful practical guidance on weather restricted jack-up operations is given in Section 5.3 of RenewableUK
Guidelines for Jack-ups, /115/, but note that [2.6.7] allows a greater operational window. This is summarised as:
a. Agree procedure documents which include limiting criteria, allowing for uncertainty due to monitoring
and the forecasting of the environmental conditions (see [2.6.9]), for relevant decision points and identify
suitable alternative jack-up locations between the site and safe ports.
b. The jack-up is only to leave a safe location to go to the installation site on receipt of a favourable weather
forecast with high confidence to cover the time (including a contingency for delays) from departure to
return to a safe location.
c. The jack-up 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 stand-by
location, including a contingency for delays.
d. If the jack-up cannot reach a safe port or location before meeting bad weather (above the laden jacking
limits of the jack-up, typically about 1 m to 1.5 m significant wave height), then it should jack-up to survival
air gap at a suitable shallow water location and evacuate the crew if necessary.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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
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[2.4].
8.3.3.3
Jack-ups can also operate on DP or when moored afloat to save time jacking up and down and pre-loading.
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
Jack-ups can operate in semi-jacked-up condition (vessel stabilised in water by a low leg pre-loading and a
reduced draught) under good weather conditions. This condition can make it feasible to operate the jack-up at
critical locations where the risk of punch through is high. It will require approval by the vessel’s classification
society as it is not typically a normal operating condition.
8.3.4
Crane vessels (seagoing)
8.3.4.1
Any crane vessel or sheerlegs shall be classed for operating in the relevant area. The design and operating
criteria shall be defined according to Sec.2.
8.3.4.2
Carrying a suspended load on a crane hook in transit offshore is not generally considered good practice, unless
it is for very short distances in calm weather. In bad weather the load can be very difficult to control, stability is
reduced and the crane can be overloaded. Approval of such operations will require agreement from the vessel’s
Classification Society and a risk assessment in accordance with [2.4].
8.3.5
Inshore crane vessels and barges
8.3.5.1
Inshore crane vessels and barges shall only be used if allowed by their class notation and:
a. 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]
b. 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.
c. The vessel to leave the installation site unless there is a confident good weather forecast to cover the
remaining time on site and to reach a safe port or location, including a contingency for delays.
8.3.6
Grounded 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:
a. The vessel’s classification society allows such operations.
b. The seabed is such that the vessel will not be damaged and it will not hold the vessel down when
attempting to refloat.
c. 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.
d. A confident good weather forecast is obtained before grounding to cover the period (including a suitable
allowance for delays) until float-off without exceeding the operational criteria.
8.3.7
Other OWF installation vessels
8.3.7.1
The following vessels usually do not require the approval of the MWS company unless their operations represent
a risk for other structures or operations.
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a. Crew transfer or accommodation vessels with proprietary crew access arrangements.
b. Escort and stand-by vessels can be needed in some areas to warn off other vessels, especially during
sensitive operations or transports.
c. Bubble curtain deployment and energising vessels which can be needed if regulations on piling noise
pollution apply (see [13.10.2]).
8.3.7.2
In some cases, it may be unclear whether the approval of the MWS company is required or not for smaller vessels
approaching existing structures. Planned operations should be discussed between the OWF owner, the
Underwriter and the MWS company in order to identify the major risks for the existing structure and decide case
by case the scope of the MWS company.
8.4
Planning and execution
8.4.1
Procedures and manuals
8.4.1.1
Technical documentation shall be completed for all operations. See [2.3] for details. In general, this should
include:
a. The anticipated timing and duration of each operation, including contingencies.
b. The limiting wave states, wind speeds and currents, and where applicable any visibility/day-light,
temperature and precipitation limits, as well as the site-specific equipment or methodology prescribed for
measuring each limit-state.
c. The transport route including shelter points.
d. The arrangements for control, manoeuvring and mooring of barges and/or other craft alongside
installation vessels.
e. Effects on and from any other simultaneous operations (SIMOPs – see IMCA M 203, /83/).
f. Contingency and emergency plans.
g. Requirements from the relevant MWS company standards for each individual phase.
8.4.2
Weight control
8.4.2.1
The requirements in [5.6.2] apply.
8.4.2.2
The manufacturer shall supply a weight statement with tolerance and CoG envelope for all weight-sensitive
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
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.
8.4.3
Weather restricted operations and weather forecasts
8.4.3.1
For requirements see [2.6.7] for requirements for weather restricted operations and [2.7] for weather forecasts.
8.4.3.2
For areas with high tidal currents there can be additional restrictions on operations due to the need to wait for
slack (or slacker) tides for current-sensitive operations such as:
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•
•
•
•
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Moving jack-ups on or off location
Stabbing piles or installing jackets, substructures or equipment on the seabed
Bringing cargo vessels alongside installation vessels.
Diving operations.
8.4.3.3
When high currents are combined with shallow water then additional current forces will be caused by “blockage”
effects. These shallower conditions also lead to increased seabed turbulence due to wave action, and additional
contingency measures can be necessary to make allowances for accelerated scouring around jack-legs and
spudcans. However suitable moorings, stabbing guides and other aids can help to reduce the sensitivity to
currents and decrease downtime waiting for slack tide.
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 sub-sea
currents with acoustic Doppler or similar systems should be considered when the operational limits of ROVs, and
drag on piles during stabbing can lead to operational delays.
8.4.4
Site and route survey requirements
8.4.4.1
As well as ensuring that all positional, bathymetric, soil and current surveys are performed using the same datum
and coordinate systems, various requirements to ensure sufficient accuracy like the frequency of survey
equipment calibration (for salinity, temperature etc.) shall be agreed. There shall be an agreed procedure for
ensuring that all survey results are disseminated to all relevant parties as required.
8.4.4.2
The “as built” locations of structures, cables and subsea equipment shall be recorded accurately on charts using
a common survey datum used by all parties. These charts shall be kept updated, including all jack-up footprints
as soon as they are made and issued to all vessels operating in the field. “No anchoring” zones shall be well
marked.
8.4.4.3
In advance of the final detailed design being carried out for the foundations, the seabed material, geophysical,
and geotechnical surveys of the sub-bottom profile should have been carried out, as well as magnetometer
surveys for ferrous objects, including UXO. The Cone Penetrometer Test results and other appropriate survey
details for each foundation location should be documented, to jack-up vessel operators. This will allow them to
carry out site-specific assessments in accordance with ISO 19905-1, /102/, and to assess the possibility of
scouring around jack-legs and spudcans.
8.4.4.4
Unexploded ordnance (UXO) disposal, although important, is not generally subject to a Marine Warranty and is
normally excluded. However it is recommended that it will be managed in accordance with the requirements of
‘Risk Management Framework’ provided in CIRIA C754, Assessment and management of unexploded ordnance
(UXO) risk in the marine environment, /13/ or similar.
8.4.4.5
Additional requirements for the cable route surveys are given in Sec.7.
8.4.5
Scour protection
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8.4.5.1
If scour is a possible problem, procedures or contingency procedures shall be prepared and anti-scour materials
stockpiled and deployment equipment prepared for mobilisation. See [8.4.3.3] and [8.4.4.3] for information that
will help in prediction of scour.
Guidance note 1:
“Dynamics of scour pits and scour protection”, /119/ gives the results of research into scour on early UK offshore
wind farms.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Cables are generally be trenched or otherwise protected in scour-prone areas. However additional precautions
can be required close to J-tubes or I-tubes at monopiles or platforms, especially immediately after laying.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
Scour around jack-up legs can make them more vulnerable to punch-through and around cables can make them
more vulnerable to damage.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
8.4.5.2
Care shall be taken when laying scour protection to ensure that bad weather and/or high currents during the
installation phase do not cause damage to the lower layers.
8.4.6
Wet storage of jackets or 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.
8.5
Load-outs of OWF components
8.5.1
Structure load-out
8.5.1.1
Load-outs shall be in accordance with Sec.6. However the following special cases apply, as applicable.
a. Special consideration should be given to purpose-built lifting appliances for blades. The lifting tool
Certificate shall specify the maximum load and any limits regarding the overall dimensions of the lifted
item and any environmental limitations (e.g. maximum wind speed).
b. 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.
c. Bolts used for removable lifting lugs shall generally be used one time only. In special cases, re-use can be
accepted as described in [E.2] but only if initial pretensioning does not exceed 60% of the bolt yield
strength and the loads during lifting have not exceeded the maximum design values. For re-use of bolts, a
detailed inspection plan with regular NDT including rejection criteria and exchange intervals should be
documented. As a minimum, bolts should be visually inspected after each lift and with MPI (Magnetic
Particle Inspection) after every 3 lifts unless fatigue calculations accepted by the MWS company show that
less frequent inspections are acceptable.
d. Re-useable lifting lugs shall be tested in accordance with [16.9.7].
8.6
Transport of OWF components
8.6.1
General
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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
The requirements of [E.2] will apply for bolted connections used for seafastening. The strength of bolted
connections may be assessed to DNVGL-OS-C101 /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].
8.6.2
Transport 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Normally the voyage and installation planning considers some or all of the following:
a. The rotor being designed to safely withstand the accelerations (from [11.3]).
b. Reducing to negligible the probability of wave slam on the blades by securing them well above the still
water level.
c. Selecting vessels that can be ballasted to reduce the motions in likely wind and wave combinations.
d. Doing motion response calculations to optimise the loading and ballasting arrangements so as to
minimise the probability of wave slam on the blades in likely wind and wave combinations.
e. 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).
f. 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
8.6.3
Transport of tall vertical cargoes
8.6.3.1
Seafastening of the Transition Piece flanges on barges or ships is often critical for many projects. 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
Bolts used for Seafastening shall generally be used one time only. In special cases, re-use 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 re-use of bolts, a detailed inspection
plan with regular NDT including rejection criteria and exchange intervals should be documented. As a minimum,
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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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
8.6.4
Other OWF wet towages
8.6.4.1
Larger Concrete Gravity Structures will generally be built in a dry-dock or building basin with construction often
completed afloat. The MWS requirements are given in Sec.12
8.6.4.2
Smaller gravity structures may be built on barges and floated off or lifted off by crane or sheerlegs. They can also
be lowered to the seabed by purpose-built installation units. Where these are not covered by existing MWS
company standards, suitable criteria can be developed by the MWS company at an early design stage.
8.6.4.3
It will often be impracticable to provide one-compartment damage stability for floating piles, transition pieces
and suction anchors by introducing temporary bulkheads. In this case, a risk assessment, in accordance with
[2.4], shall be carried out to determine the major causes of flooding and to reduce the risk to acceptable levels,
as described in [11.10.7.3].
8.7
Installation of OWF components
8.7.1
Monopiles and transition pieces installation
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. On-bottom 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]).
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◦ Contingency plans and equipment (e.g. fishing tools) for a broken drill string.
9. Approval of grouting operations (see [H.5.3]).
8.7.2
Piling templates
8.7.2.1
Piling templates are often used to help locate piles before driving and to ensure that piles are driven vertically or
at the right inclination. They are normally placed on the seabed but may be attached to the side of a jack-up, with
the facility to be lowered or raised and may use the jack-up legs as a positioning guide.
8.7.2.2
If transported attached to a jack-up then the template and its attachment shall be able to withstand the design
accelerations according [11.3] as well as the hydrodynamic forces acting on the structure. Its effect on trim and
stability shall also be checked.
8.7.2.3
Special transit procedures can need to be developed to reduce the risk of collisions or grounding if the attached
template increases the combined draught or beam, especially if not visible above water.
8.7.2.4
The template shall be capable of being levelled if there is a sloping or uneven seabed. 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.
8.7.3
Suction 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
Equipment and procedures shall be documented to ensure that:
a. the foundations can be safely lowered through the splash zone (buoyancy should be considered) to the
seabed and located within tolerances
b. there is no “piping” (soil erosion due to seepage) through the soil between outside and inside, or between
individual compartments, if any, during installation
c. that any out of verticality can be corrected to within the required tolerances (possibly using crane
assistance)
d. 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.
e. the operation should be made reversible so as to be able to extract the suction bucket foundation and
relocate if there is a risk of refusal (no further penetration at maximum pump capacity). The risk of refusal
should be determined from a penetration analysis using the latest soil data.
8.7.4
J-tubes and I-tubes
8.7.4.1
Installing cables through J-tubes and I-tubes is covered in Sec.7.
8.7.5
Turbine installation
8.7.5.1
Requirements in this standard shall apply unless novel installation techniques are proposed.
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8.7.6
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Towers
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 de-rigging
Partial bolting
Lifting points certification for multiple use (load-out, installation, maintenance, decommissioning)
Verification that there will be no ovalisation of structure tubular members due to local seafastening forces
in higher sea states
• Transport frames
• Requirements and criteria for upending from the horizontal to vertical mode.
8.7.7
Nacelles
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 de-rigging
Partial bolting.
8.7.8
Blades
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:
• Infra-red release systems which shall be shown to be reliable in releasing and, more importantly, not liable
to early release from any cause
• Limiting criteria.
• Boom tip motions, See [16.17.3.1 4)]
• Partial bolting.
8.7.9
Complete 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.
8.8
Lifting 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.
8.9
Information required for MWS approval
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8.9.1
See subsections at the end of each relevant section, e.g. lifting, voyages, etc.
SECTION 9 Road transport
9.1
Introduction
9.1.1
General
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.
9.1.2
Scope
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.
9.1.3
Revision history
9.1.3.1
This section is new.
9.2
Requirements
9.2.1
Statutory 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.
9.2.2
Loads and accelerations
9.2.2.1
Table 9-1 will cover most countries with published requirements for tie-down requirements. These shall apply in
the absence of more stringent criteria, depending on jurisdiction or other requirements.
9.2.2.2
Possible additional limitations on wind (or road speed) shall be checked for structures where strength or stability
can be an issue.
Table 9-1 Typical road tie-down acceleration requirements
Direction
Requirements
Transverse acceleration
0.5 g
Forward acceleration
0.8 g
Backward acceleration
0.5 g
Vertical acceleration
1.2 g (1.0 g in some areas)
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9.2.3
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Securing
9.2.3.1
All securing equipment should be in accordance with the principles and requirements of seafastening design
strength in [11.9.5].
9.2.3.2
Friction can be permitted as part of the securing system, subject to justification and where permitted by local
legislation.
9.2.4
Stability
9.2.4.1
Stability in accordance with [10.5.3.15] shall be demonstrated.
9.3
Information required
9.3.1
Object information
9.3.1.1
For the object:
a.
b.
c.
d.
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
e. Padeyes, where used as lashing points, or other lashing points on the cargo to be verified against
transport design forces.
9.3.2
Trailer or SPMTs
9.3.2.1
Requirements are given in [10.5.3].
9.3.2.2
For the trailer or SPMTs:
a.
b.
c.
d.
e.
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.
f. Demonstration that stroke length is adequate to prevent grounding (cargo/trailer/SPMTs) or tyres losing
contact with roadway.
9.3.3
Securing
9.3.3.1
For the securing arrangement:
a. Details including WLL/SWL and MBL of all items in the securing system including tensioners.
b. General arrangement drawing of the securing plan including cargo CoG location while positioned on
trailer/SPMT and clearly defined required lashing angles.
c. Design acceleration definition and justification.
d. Securing Calculations documented and found adequate.
e. Demonstration of lashing/stopper adequacy against uplift and horizontal forces, including friction
assumptions and description of friction material and description of blocking if used.
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9.3.4
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Route
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 de-powering as required.
◦ Any relevant limitations on bridge loadings
◦ Any relevant limitations on timing.
◦ Significant inclines and declines.
3. Permit obtained if required.
4. Max speed defined if not stipulated in a permit. Max speed to allow for trailer /SPMT levelling as needed.
5. Requirements for strength of ramps where used, allowable ground pressure should take in to
consideration any limits on buried culverts, utilities etc.
6. Allowable ground pressures for the route defined. Special attention regarding ground pressure capacity
should be made to areas where the route is changing ground type (e.g. asphalt to cement). Tyre and
ground pressures should comply with the allowables for the entire route.
7. If the transit passes an airfield and the cargo is of sufficient height, evidence that co-ordination with the
airfield, including any required aviation warning lights, has been included in the transport procedure.
9.3.5
Risk assessment
9.3.5.1
A risk assessment in accordance with [2.4] of the transport.
SECTION 10 Load-out
10.1
Introduction
10.1.1
Scope
10.1.1.1
This section presents the requirements for load-out operations involving transfer of heavy objects from land and
onto a vessel (often a barge) either by skidding or by use of trailers. General requirements and guidance is given
in Sections [10.1] to [10.8]. Section [10.8] gives additional requirements and guidance for the following special
cases:
•
•
•
•
•
grounded load-outs
load-in, i.e. a reversed load-out
vessel to vessel load transfers
transverse load-outs
site moves.
10.1.2
Other types of load-out
10.1.2.1
For load-out operations carried out by crane lifting, see Sec.16
10.1.2.2
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For other load transfer operations, see Sec.15.
10.1.3
Revision history
10.1.3.1
This section replaces the applicable sections of the following legacy documents:
• DNV-OS-H201, Load transfer operations
• GL Noble Denton, Guidelines for Load-outs, 0013/ND
10.2
General
10.2.1
Load-out class
10.2.1.1
Requirements to load-out equipment are defined according to load-out class. The load-out operation shall,
based on tide conditions and weather restrictions, be classified according to Table 10-1.
Table 10-1 Load-out (operation) class
Tide range 1)
Tide restricted? 2)
Weather restricted? 3)
Load-out Class
Significant
Yes
No/Yes
1
Significant
No
Yes
2
Significant
No
No
3
Zero
No
Yes
4
Zero
No
No
5
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 load-out shall be defined as
tide restricted.
3. If weather restrictions apply, then the load-out shall be categorized as weather restricted, see [2.6]. If
there are no weather restrictions to the object movement/ballasting phase the load-out class may be
selected accordingly.
10.2.1.2
The possibility for water level differences due to environmental effects shall be duly considered. If such effects
could be significant during the load-out, then the tide range in Table 10-1 should normally be regarded as
significant even if the astronomical tide variation is defined as zero.
10.2.1.3
For grounded load-outs, see [10.8.1], the tide range in Table 10-1 shall be defined as significant if ballasting is
required in order to maintain ground reactions within acceptable limits.
10.2.2
Planning
10.2.2.1
General requirements for planning of marine operations are given in Sec.2.
10.2.2.2
Start and end points for a load-out shall be safe conditions and clearly defined, see [2.5].
Guidance note:
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A load-out from one safe to another safe condition could include many sub-operations, such as “lift-off from
construction supports”, “site move”, “move onto barge”, “temporary seafastening phase”, “turning of barge” and
“final mooring of barge”. Hence, it should be thoroughly evaluated if it may be possible and beneficial to split the
load-out into two (or more) operations with safe condition(s) in-between, see [2.5].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.2.2.3
Tide variation is normally a critical parameter for load-outs. Extreme tide levels and rates of change should be
considered. All environmental effects that can influence tide levels, in addition to the astronomical tide variation,
shall also be evaluated and duly considered.
10.2.2.4
The following should be given due attention when planning load-out operations:
1.
2.
3.
4.
5.
6.
Yard lay-out, including position of object
Transport vessel dimensions and strength
Object position and support height on transport vessel
Load-out route survey regarding clearances and obstructions
Water depths
Local environmental effects, e.g.:
◦ the possibility of waves/swell
◦ currents during and following the operation, including blockage effects if applicable
◦ the possibility for squalls and/or thunderstorms; design wind speeds should account for such effects
when relevant
7. Quay strength and condition
8. Load-out site soil strength and condition
9. Skidway levelness tolerances
10.2.2.5
A load-out operation could involve several construction-, transport- and load transfer (main)
contractors/responsible parties. Interface planning should be given due attention.
10.2.3
Risk management
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.3
Loads
10.3.1
General
10.3.1.1
Loads and load effects are generally defined in [5.5] and [5.6]. It shall be thoroughly evaluated if any other loads
and load effects not described in Sec.5 need to be considered.
10.3.1.2
The design principles and methods described in Sec.5 shall be adhered to.
10.3.1.3
All relevant limit states as defined in Sec.5 shall be included in the design calculations/analysis.
10.3.2
Weight and CoG
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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].
10.3.3
Weight of load-out equipment
10.3.3.1
The weight of the load-out equipment (Weq) should be accurately assessed.
Guidance note:
Weq is the total weight of equipment and support structures which moves with the transported object. Such
equipment may be support beams, grillages, skidding shoes, trailers, push/pull jacks, hydraulic power packs, etc.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.3.3.2
Any uncertainties in weight and CoG of load-out equipment shall be considered by applying conservative
estimates in the load-out calculations, see however [4.3.9.2].
10.3.4
Environmental loads
10.3.4.1
All load effects caused by tide variations shall be considered.
10.3.4.2
Load effects caused by wind and current shall be considered.
10.3.4.3
Load-out operations should normally not be carried out in significant waves and swell conditions.
Guidance note:
Applicable loads due to waves and swell for transport vessel mooring before and after the load-out operation to
be considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.3.5
Skidding loads
10.3.5.1
The loads required to break loose and continue moving the object can be expressed as:
F1 = μ1(W+Weq) + P1
F2 = μ2(W+Weq) + P2
Where:
F1
F2
μ1
=
Required break-out load
=
Load required to continue moving the object
=
Upper bound design friction coefficient or rolling resistance for break-out, see [10.3.5.4]
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μ2
W
Weq
P1
P2
=
Upper bound design friction coefficient/rolling resistance for the move, see [10.3.5.4]
=
Object weight, see [10.3.2]
=
Equipment weight, see [10.3.3]
=
Any other load occurring during break-out, see [10.3.5.2]
=
Any other load occurring during skidding/trailing, see [10.3.5.2]
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10.3.5.2
The following load effects should be considered:
• Inertial loads
• Environmental loads
• Loads caused by the slope of the skidding or rolling surface
10.3.5.3
If two or more propulsion systems are used then the effect of maximum possible differential push/pull loads shall
be considered.
10.3.5.4
The upper bound design friction coefficients/rolling resistance values used should not be taken less than
specified in Table 10-2 unless adequate in-service documentation indicates that other values may be used, see
also [5.6.9].
Guidance note:
The indicated friction coefficients for moving include re-starting after short stops during the load-out operation.
Break-out friction is the maximum friction expected after an extended (construction) period with the object
supported at the friction surfaces, see also [10.3.5.5].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Table 10-2 Upper bound design friction coefficients/rolling resistance
Breakout
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
Sliding surfaces
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 break-out. See also [10.3.5.5].
3. The values are valid only for contact
stresses lower or equal to the allowable
contact stresses for the considered
medium. Allowable contact stresses
should be obtained from the
manufacturer or from an applicable code
or standard.
4. Wood should normally be surface
treated by wax or by other adequate
means in order to avoid that the
lubrication is absorbed by the wood.
10.3.5.5
Where 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
break-out requirements than anticipated. The effects of skidway deformation shall also be considered.
10.3.6
Skew load
10.3.6.1
Skew load is the extra loading at object support points due to inaccuracies in the level of the skidways, rolling
surfaces, supports, etc. Such loads shall be considered.
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Guidance note:
Skew loads could normally be disregarded for load-out operations where the object has a 3 point support
system. This could be obtained by including a reliable load equalising system.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For SPMT load-outs using 4 support groups, the effect of skew loading across diagonals should be assessed to
account for the possibility that the groupings may not be coplanar due to incorrect pressure in the SPMT groups,
the stiffness of the 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%.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
For skidded load-outs it is recommended to verify the object and supports for the following minimum
deflections:
• Subsidence of any single object “corner” support with respect to the other “corner” supports by 25 mm.
• Subsidence of any single object support with respect to the other supports by 15 mm.
Dimensional survey measurement before (and if applicable during) the operation should substantiate that the
actual relative deflection will be within 75% of the deflections assumed in structural verifications.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.3.7
Other 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:
•
•
•
•
•
Hydrostatic loads on transport vessel(s)
Impact loads
Local support loads on grounded vessel hulls
Mooring loads
Guiding loads.
10.4
Design calculations
10.4.1
General
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].
10.4.2
Load cases
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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 load-out operation consists of a sequence of different load cases. In principle, the entire load-out sequence
should be considered step-by-step and the most critical load case for each specific element should be identified,
e.g. 25%, 50% and 75% of travel, steps of 5 axles, half jacket node spacing, etc. as appropriate. However, the
force distribution during a load-out may normally be represented by static load cases distributing the object
weight and any environmental and equipment loads to relevant elements in the analyses.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.4.2.2
For skidded load-outs, analyses of the skidded object should consider the elasticity, alignment and as-built
dimensions of the shore and vessel skidways. See also [10.3.6.2] GN-3.
10.4.2.3
For trailer load-outs, the reactions imposed by the trailer configuration on the transported object shall be taken
into account.
Guidance note:
Support reactions for the transported object will be governed by the trailer arrangement. It should be
remembered that trailer axle loads within each hydraulic group will be uniform and that the trailers spine stiffness
may influence the support reactions. See also [10.3.6.2] GN-2.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.4.2.4
The design load cases for link beams, link beam attachments and the quay should consider mooring forces,
skidding forces and vessel movements when relevant, including any situations where the object or vessel can be
jammed.
10.4.3
Quays
10.4.3.1
Allowable horizontal and vertical load capacities of load-out quays should be documented according to a
recognized code or standard.
10.4.3.2
Calculations showing that the actual loads during load-out are not more than the allowable loads should be
documented.
Guidance note:
If information about the quay is limited and it is therefore difficult to document its capacity by calculations, then
an alternative approach where quay capacity is documented by historical records of previous load-outs over the
quay may be considered. Detailed information about the previous load-out(s) will be needed for an adequate
comparison.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.4.4
Soil
10.4.4.1
Strength and settlement calculations/evaluations for the ground in the load-out area should be documented.
Guidance note:
The risk of differential ground settlements which may influence the loads during load-out should be considered
and minimised by means such as:
• pre-loading of ground in load-out tracks
• load spreading e.g. by concrete slabs or steel plates.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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10.4.4.2
Soil bearing capacity should normally be tested before construction or load-out of the object. Alternatively,
relevant site investigation should be documented.
10.4.4.3
Geotechnical calculations and testing should be carried out according to a recognized standard, e.g. EN 1997
Eurocode 7, /67/.
10.4.4.4
For trailer transport, the soil strength requirements apply for the whole planned path/track plus at least 2 meters
at each side.
10.4.4.5
If there is any doubt as to the soil capacity, then a loaded SPMT test drive should be done before the load-out.
10.4.4.6
For load-outs involving grounded vessel, the seabed should be evaluated with respect to topography, bearing
capacity, settlement, etc.
10.5
Systems and equipment
10.5.1
General
10.5.1.1
Systems and equipment to be used during load-out should comply with the requirements given in Sec.4.
10.5.2
Propulsion systems
10.5.2.1
Propulsion systems shall be able to break loose and push/pull the object to the final position on the transport
vessel.
Guidance note:
Propulsion systems can for skidded load-outs be for instance wire and winch, hydraulic jacks or strand jacks.
Trailer load-outs can be by self-propelled trailers (SPMT) or trailers. Trailers can be propelled by a wire and winch
system or by tractors/trucks.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.2.2
The propulsion system capacity for break-out shall be not less than the required break-out load (F1), see
[10.3.5.1]. For objects that cannot be considered to be in a safe condition if the break-out system fails the
capacity and redundancy requirements in Table 10-3 apply also to the break-out system.
Guidance note:
Adequate break-out capacity may be obtained by combining e.g. jacks with the continuous propulsion system.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.2.3
The propulsion system used to move the object shall satisfy requirements specified in Table 10-3.
10.5.2.4
Propulsion systems should act in a synchronised manner in the transfer direction. A minimum required load-out
velocity shall be identified considering:
• Maximum allowable load-out duration
• Dynamic friction coefficient
• Length of the load-out track
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• 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
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 load-out 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
Back-up propulsion system capacity should be able to compensate for the following conditions:
a. Breakdown of one arbitrary self-contained propulsion unit
b. Unexpected increase in the skidding loads above the expected nominal value
Guidance note:
Back-up capacity for accidental conditions of type a) may be achieved by:
◦ Spare capacity in the main propulsion units
◦ Separate back-up propulsion units with sufficient capacity
◦ Spare parts for the main propulsion units and an acceptable and proven repair/replacement time
The back-up capacity for conditions of type b) may be:
◦ Spare capacity in the main propulsion units
◦ Back-up propulsion units
Detailed requirements to be complied with are in Table 10-3.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.2.7
Any required modifications during the operation, e.g. removal of pull bars of the push/pull system lay-out should
be proven feasible. Normally, lay-out modifications should be avoided with the object supported both at the
quay and transport vessel.
10.5.2.8
A pull-back system and a procedure for pulling the object back on shore shall be available for Load-out Class 1.
10.5.2.9
A pull-back system and procedure shall be available for Load-out classes 2 and 4 unless otherwise justified by
risk assessment, see [10.2.3] and [2.4].
Guidance note:
One acceptable option may be to substantiate that a retrieval system could be made operative to retrieve the
object within the Operation Reference Period (TR).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Table 10-3 Propulsion system requirements
Load-out
Class
Intact System
Capacity
System Redundancy Requirement after
breakdown of any one component
1
160%
130% capacity or repair possible within 30
minutes
Required
2
140%
120% capacity or repair possible within 2 hours
See [10.5.2.9]
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
Pull-back System
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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 pull-back system is achieved by de-rigging and re-rigging the pull on system, the time required
to achieve this shall be estimated, clearly defined and duly considered.
10.5.3
Trailers
10.5.3.1
Trailers (multi wheel bogies) should be used in accordance with the manufacturer's specifications.
10.5.3.2
The hydraulic suspension layout (linking) should be thoroughly considered. Normally a layout giving a three
point support condition for the object, e.g. a statically determinate system, is recommended. However, it should
be noted that a 3-point support system is generally less stable than a 4-point support system.
10.5.3.3
The trailer configuration should have adequate manoeuvring capabilities for the intended load-out (including
site move) route.
Guidance note:
Where 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/load-out support frame (LSF)/cribbage.
• It should be demonstrated by the load-out 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 associated loads.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.4
The trailer axle load calculations shall consider:
•
•
•
•
•
•
•
•
Weight of object
Weight of object supports on the trailers
Weight of the trailers themselves
Extreme positions of CoG
Hydraulic suspension lay-out
Maximum overturning effect caused by relevant “external” horizontal loads, see [10.5.3.7]
Possible operating errors, see e.g. [10.5.3.8]
Contingency situations, see [10.5.3.12].
Guidance note:
It some cases it may be found beneficial to plan for possible rearrangement of the trailer after lift-off should the
load distribution between the trailer groups not be as expected.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.5
The following shall be documented for the trailer axle loads calculated according to [10.5.3.4]:
a. Maximum axle loading shall be shown to be within the trailer manufacturer's recommended limits.
b. Trailer moment and shear force within the manufacturer’s specified limits or the global (spine) strength to
be documented by calculations.
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10.5.3.6
The support lay-out on each trailer shall ensure stability in both directions of the trailer.
Guidance note:
A trailer with a fully linked hydraulic suspension needs to be regarded more as a distributed load than as a
support. The supports on such trailers should be checked for the vertical loading from the trailers combined with
maximum “external” and “internal” horizontal loads acting on the trailers, see [10.5.3.7] and [10.5.3.8].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.7
The trailers should be properly supported to withstand horizontal loads. Such loads are caused by:
a. External effects, i.e. reaction loads from wind, inertia (e.g. acceleration during start and stop) and ground
slope (including vessel heel/trim).
b. Internal effects such as differential traction and steering inaccuracies.
10.5.3.8
Trailer inclinations due to improper co-ordination in operation of the hydraulic suspension system shall be
considered.
10.5.3.9
The traction system, either the trailers are self-propelled or pushed/pulled by trucks/winches, should fulfil the
requirements in [10.5.2]. Ground surface conditions should be duly considered.
10.5.3.10
It should be documented that the trailer hydraulic suspension will work well within the stroke limits. Support
heights, ground slopes/conditions and defined vessel levels/motions (see [10.6.5]) should be considered.
Guidance note:
Normally the planned operational stroke should be limited to 70% of the total theoretically available stroke.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.11
Contingency/repair procedures should be documented for at least:
•
•
•
•
•
•
Hydraulic system failure
Hose rupture/leakage
Tyre puncture
Steering problems
Traction failure, see [10.5.2]
Failure of power pack.
10.5.3.12
The trailer load calculations shall consider that any one axle does not take load due to e.g. tyre puncture.
Guidance note:
If repair is possible 10% overload could normally be accepted. For Class 1 load-out the loading should be within
the stated maximum trailer loading.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.13
Link span bridge capacity shall be demonstrated by calculation, see [10.4.2.4].
10.5.3.14
Special caution/consideration should be given to steel plates used as link span bridge between the quay and the
vessel. The following should be considered when ensuring their suitability:
• Vessel ballasting should be carried out to minimise the difference in level between the vessel deck and the
quay.
• The distance between the vessel and the quay should be minimised to avoid excessive deformation of the
steel plates caused by the reactions from the trailers or SPMTs.
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• Effectively maintaining of the vessel position on the quay e.g. using mooring winches
• Securing the plates to the vessel or quay to prevent their slippage during load-out.
10.5.3.15
Adequate global stability of the 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°.
Guidance note:
The COG to be used in these calculations is the combined CoG for trailers and object.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.3.16
For load case A the following shall be considered:
a. The most extreme possible horizontal/vertical location of the centre of gravity.
b. When transiting on land: Any known inclination of the route increased by 2° to account for uncertainties in
the route profile.
c. 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:
a. The most extreme possible horizontal/vertical location of the centre of gravity.
b. The characteristic horizontal load due wind and inertia, see 10.5.3.7 a).
c. When transiting on land: Any known inclination of the route increased by 2° to account for uncertainties in
the route profile.
d. 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
e. Possible change of heel or trim due to hang-up between the vessel and the quay, or dynamic response
after release of hang-up.
f. 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 hang-up between vessel and quay:
No
Free surface effects:
None
Calculation for example case:
Design slope:
3°+2° = 5°
Load case A
Virtual correction of COG:
(10x300 x sin5)/300 = 0.87 m
Horizontal distance from virtual COG to
tipping line:
2.5-0.87= 1.63 m
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Minimum tipping angle:
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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
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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 hold-down equal to the calculated hold-down force plus 25% of the static reaction. When there is no
uplift, the remaining contact reaction can be taken into account. The strength of the restraints shall be assessed
to LS1 (ASD/WSD method) or ULS (LRFD method).
10.5.3.19
Special attention shall be given to load-out operations where the CoG of the 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
Load-outs 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 de-ballasting.
10.5.4
Skidding equipment
10.5.4.1
Skid shoes, steel wheel bogies and steel rollers are in this subsection defined as skidding equipment. Any part of
such equipment used for the horizontal movement of the object is defined as part of the propulsion system, see
[10.5.2].
10.5.4.2
Adequate strength and stability of skidding equipment should be documented. All possible combinations of
vertical load, horizontal load and support reaction distribution should be verified. Sufficient articulation or
flexibility of skid shoes shall be provided to compensate for level and slope changes when crossing from shore
to vessel.
Guidance note:
Skidding equipment may be connected in order to reduce internal horizontal loads transferred through the
object. The effect of possible rotation of skidding equipment should be considered.
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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:
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• Sliding interfaces should be suitably lubricated unless this is not required by the supplier of any
specialised equipment used for the load-out
• 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 load-out tidal window the design of the
link beams shall be such that when the loaded unit is in its final position they are not trapped, i.e. they are free for
removal.
10.5.4.5
For hydraulic suspension systems, see [10.5.3.2] and [10.5.3.10].
10.5.4.6
The nominal computed load on winching systems shall not exceed the certified working load limit (WLL), after
taking into account the requirements of [10.5.2] and [10.3.5] and after allowance for splices, bending, sheave
losses, wear and corrosion. If no certified WLL is available, the nominal computed load shall not exceed one third
of the breaking load of any part of the system.
10.5.4.7
The winching system should be capable of moving the structure from fully on the shore to fully on the vessel
without re-rigging
Guidance note:
If re-rigging cannot be avoided, then this should be included in the operational procedures, and adequate
resources should be available.
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10.5.5
Ballasting systems
10.5.5.1
The requirements for the ballasting systems are given in [4.3].
Guidance note 1:
The load-out classes defined in Table 10-1 corresponds to the operation classes referred to in [4.3.2].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Normally, vessel pumps should not be considered for the primary ballast system but may be taken into account
in the back-up provision
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.6
Power 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
load-out.
Guidance note:
Need for additional power supply to e.g. lighting and welding should be considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.7
Testing
10.5.7.1
See general requirements in [2.10] with respect to testing/commissioning, test procedures and test reporting.
10.5.7.2
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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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.7.3
For load-out operations of Class 1 a complete test run of the ballast system following the procedure for the loadout should be carried out.
10.5.7.4
The propulsion units including the spare units should be tested in both push and pull mode before the load-out
operation in order to verify the estimated friction forces and functioning/capacities of the equipment.
10.5.7.5
If the considered back-up necessitate replacement of equipment (e.g. pumps and propulsion units) then this
should be included in the test program.
10.5.8
Mooring and fendering
10.5.8.1
General design requirements to mooring systems are given in Sec.17. Additional requirements applicable for
load-outs 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 load-out from quay to vessel should be designed for the limiting (design)
weather conditions, see [2.6], in combination with the maximum loads from the pushing or pulling of the
structure.
10.5.8.4
Mooring before and after load-out should normally be considered a weather unrestricted operation. Weather
unrestricted moorings should be designed to the return periods given in Table 3-2 and in accordance with
Sec.17.
10.5.8.5
Facilities for re-tensioning of mooring lines should be present and in stand-by during the load-out.
Guidance note:
Such facilities may be winches, jacks for tensioning, etc.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.5.8.6
The mooring system stiffness shall limit the movements of the load-out vessel(s) to those that are acceptable
during the load-out in particular when the object is supported both on the quay and vessel.
10.5.8.7
Adequate strength, stiffness and layout of fenders should be documented.
Guidance note:
Fender design solutions should at least consider:
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•
•
•
•
Page 162 of 543
Requirement for a stiff mooring system during load-out, 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 load-outs care should be taken to ensure that minimum friction exists between the vessel and quay
face. Where the quay has a rendered face, steel plates should be installed in way of the vessel fendering system.
The interface between the vessel and vessel fendering should be liberally lubricated with grease or other
substitute which complies with local environmental rules.
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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 load-outs,
see [10.8.1] shall be properly documented.
Guidance note:
The calculations of friction effect should at least consider:
• The documented lower bound design friction, see [5.6.9]
• Minimum vertical load on the pad considering all relevant ballast, tide level and deck loading
combinations
• Any limitations due to interaction between mooring system and the friction effect
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10.6
Vessels
10.6.1
General
10.6.1.1
General requirements for vessel(s) are given in [2.11]. These requirements are applicable to any vessel involved
in the load-out.
10.6.1.2
See Section [10.9.3.2] for requirements to vessel documentation.
10.6.1.3
For tugs involved in the load-out the applicable sections from [11.12] apply as relevant for the actual tug work
tasks.
10.6.1.4
Approved tugs shall be available or in attendance as required, for vessel movements, removal of the vessel from
the load-out berth in the event of deteriorating weather, or tug back-up to the moorings, see also [10.7.2.4].
10.6.1.5
For the load-out vessel the requirements in Sec.11 apply as relevant.
10.6.2
Class
10.6.2.1
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
If the barge will be grounded during load-out then it should be ensured that the classification society is informed
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and that any requirements to inspection of the vessel after grounding are adhered to.
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10.6.2.2
Vessels that are intended to be totally immersed during load-out should be classed for such use by a recognised
classification society.
10.6.2.3
Where a load-out operation temporarily invalidate the class or load line certificate then a statement of
acceptance from the classification society should be submitted, see [2.11.4.4].
10.6.2.4
Any items temporarily removed for load-out shall be reinstated after the load-out is completed and the vessel
shall be brought back into class before sailaway.
Guidance note:
This may apply if, for instance, holes have been cut in the deck for ballasting, if towing connections have been
removed or, in some instances, after grounding on a pad.
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10.6.3
Structural strength
10.6.3.1
The load-out vessel global strength shall be documented for all possible ballast conditions, see also [2.11.3].
10.6.3.2
The strength should be documented for all parts of the vessel exposed to local loads. Such parts are typically:
a.
b.
c.
d.
e.
f.
g.
Link beam/plate support area
Skidway, including support area
Deck plate for wheel loading
Jacking system connection points
Hull locally for horizontal loads from the quay
Bottom structure, if grounded load-out
Bollards/mooring brackets
10.6.4
Stability afloat
10.6.4.1
Sufficient stability afloat shall be ensured during load-out.
Guidance note 1:
Generally load-out should be performed with a minimum GM of 1 m at all stages. The accuracy requirements to
ballasting will tend to increase with decreasing GM.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
• Normally there is no requirement to document damage stability during load-out. 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 “hang-up” by resisting
the heel or trim until the pull-on reaction is released, or the friction force is overcome, whereupon a
sudden change of heel or trim may result.)
• Due attention should be given to situations where a change of wind velocity may cause a significant
change of heel or trim during the operation.
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10.6.4.2
For load-out operations the minimum “effective freeboard”, should for vessels be
fmin=0.5 + 0.5Hmax
where
fmin
Hmax
=
Minimum effective freeboard in metres, see the Guidance Note.
=
Maximum anticipated wave height in metres at the site during load-out.
Guidance note:
• The “effective freeboard” is defined as the minimum vertical distance from the still water surface to any
opening, e.g. an open manhole or deck area where personnel access could be required. A maximum
possible tide level and any possible vessel heel/trim should be considered. Coamings/bundings at
openings could be installed to increase the “effective freeboard”.
• In order to use a vessel with less freeboard than defined by the load line certificate, approval from class is
required. The freeboard should be sufficient to maintain the vessel’s water-plane 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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10.6.5
Load-out vessel draught and motions
10.6.5.1
Nominal values and allowable tolerances for the load-out vessel(s) level, trim and heel shall be clearly defined for
all stages of the load-out.
10.6.5.2
It should be documented that the values defined according to [10.6.5.1] are adequate to prohibit unexpected
effects or load effects.
10.6.5.3
Significant wave/swell induced motions of the load-out vessel are normally not acceptable during the operation,
see [10.3.4.3].
10.6.6
Maintenance
10.6.6.1
A vessel (barge) handling procedure should normally be documented. The procedure should as a minimum
describe:
•
•
•
•
Berthing and if applicable relocation
Vessel surveys e.g. on-hire and off-hire surveys, condition surveys
Installation and inspection of moorings
General watch keeping
10.6.6.2
A barge engineer familiar with operation and maintenance of the barge equipment should be present if any
barge equipment is used (or considered as back-up) during critical phases of the load-out.
10.6.6.3
Where relevant, precautions to avoid freezing in tanks and ballast systems shall be taken.
Guidance note:
Such arrangements may be heating devices (in pump rooms), additive anti-freeze solution, or any other devices
or actions serving the above purpose.
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10.7
Operational aspects
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10.7.1
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General
10.7.1.1
The general requirements for planning and execution of the operation in Sec.2 apply.
Guidance note:
The remaining paragraphs in [10.7] include some additional requirements and/or emphasise on requirements
considered especially important for load-out operations.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.7.1.2
Manhole covers which are opened for ballast water transfer or other reasons shall be closed watertight as soon
as practical after use. Any holes cut for ballasting purposes shall be closed as soon as practical, see also
[10.6.2.3].
10.7.2
Preparations
10.7.2.1
All structures and equipment necessary for the operation shall be correctly rigged and ready to be used.
10.7.2.2
Means (e.g. steel plates) and personnel (e.g. welders) for general repair work shall be available during the
operations.
10.7.2.3
For operations or phases of operations that may be carried out in darkness sufficient lighting shall be arranged
and be available during the entire operation.
10.7.2.4
Additional tugs that may be employed for critical tasks (e.g. as planned contingency measures) during the loadout operation should be nominated and comply with the requirements of Section [11.12] and be available for
inspection as required before the operation.
10.7.3
Clearances
10.7.3.1
Adequate minimum clearances, including clearances under water, for all phases of the operation shall be
defined and properly documented by calculations and surveys before and during the operation.
Guidance note:
Welding/erection of “last minute” items should not be allowed without a proper re-check of the clearances.
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10.7.3.2
Sufficient under-keel clearance should be documented for vessel(s) during and after the load-out operation.
Normally the clearance should not be less than 1.0 meters.
Guidance note:
If the vessel under-keel clearance is considered as critical, then the seabed should be inspected by divers or by
other adequate survey method. Where there is a risk of debris, inspections should be done immediately before
the vessel berthing. If confidence in the lowest predicted water levels and in the survey of the load-out area is
high, then the minimum clearance requirement could be reduced to 0.5 m.
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10.7.3.3
The required land area and sea room shall be checked for obstacles. All obstacles that could cause damages
and/or which may delay the operation shall be removed.
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10.7.3.4
If relevant, adequate tug air draught shall be ensured.
Guidance note:
The nominal air draught should be minimum 0.5 metres. All positions, including needed access routes that may
be required for the tug(s) should be considered. Possible emergency situations should be included in the
considerations.
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10.7.4
Environmental effects
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.
10.7.5
Marine 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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10.7.6
Organisation and personnel
10.7.6.1
General requirements for organisation, personnel qualifications and communication are given in [2.8].
Guidance note:
Load transfer operations will often involve personnel that do not regularly participate in this type of operation.
Personnel training and briefing are hence of great importance, see [2.8.3].
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10.7.6.2
Load transfer operations may involve complicated equipment. Hence, equipment operators should have the
required experience, (see [10.6.6.2] for barge engineers).
10.7.6.3
Proper working conditions for personnel shall be ensured throughout the load transfer operation.
Guidance note:
Load transfer operations may last for many hours or sometimes for several days and they may be carried out in
areas with limited permanent facilities. Hence, the following may be important to consider:
• Easy access to food, drinking water and toilets in order to allow for proper continuous work execution
• Adequately sheltered/heated/cooled working location(s) for required paper/PC work during the operation
• Safe access to all areas were work, including inspections, may be required.
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10.7.7
Load-out site
10.7.7.1
Due attention shall be paid to the possibility of the vessel “hanging” on the fenders or the quay structures, see
[10.5.8.7] and [10.7.8.2].
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10.7.7.2
A level survey of the site area should be performed for load-outs with trailers to ensure that the level tolerances
of the trailers will not be exceeded.
10.7.7.3
Planned trailer tracks should provide an adequate surface condition and the tracks should be marked on the
ground and vessel.
Guidance note:
Before any load-out it should be ensured that:
• pot holes are filled and compacted
• debris and obstructions to the load-out path are identified and removed
• the load-out path and at least 2 m either side of it is freshly graded
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10.7.7.4
The movement of the structure should not be stopped in areas with the potential for settlement due to e.g.
consolidation or adverse weather.
10.7.8
Supports and skidways
10.7.8.1
Levels of supports (and, if applicable, skidways and temporary supports) and horizontal dimensions on the 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/load-out 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 load-out vessel.
10.7.8.4
Suitable shims should be available on the load-out vessel for filling of gaps if required during set down.
10.7.8.5
The skidway surface condition shall be checked to be as assumed in the friction coefficient estimate.
10.7.9
Grillage and seafastening
10.7.9.1
The main requirements for the grillage and seafastening structures of the transported object are in [11.9].
10.7.9.2
The set down procedure for the object should be documented and it should ensure that the grillage and
seafastening design assumptions are fulfilled.
10.7.9.3
The seafastening should start immediately after final position of the object on the load-out (transport) vessel is
confirmed. However, see [4.3.7.2].
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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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
10.7.9.5
Final seafastening connections should be made with the vessel ballast condition as close as practical to the
voyage condition. See [11.9.5] for towing section requirements.
10.7.10 Recording and monitoring
10.7.10.1
During the operation a detailed log should be prepared and kept, see [2.3.8].
10.7.10.2
Monitoring shall be carried out according to [2.9].
10.7.10.3
The following load-out parameters should, as applicable, be monitored and recorded before and during the
operation:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
tide
push/pull force
straightness and levelness of skidding tracks
inclination of link beam
level and vertical deflections of the object
horizontal position of the object
vessel draught and/or level
vessel heel and trim
water level in vessel tanks
hydraulic pressure and stroke on any support/equalising jack, e.g. trailer hydraulic suspension.
10.7.10.4
The line and level of the skidways and skidshoes should be documented by dimensional control surveys and
reports. The line and level should be within the tolerances defined for the load-out operation and
skidway/skidshoe design.
10.7.10.5
Normally a remote reading sounding system should be used for tank water level control. A back-up system but
not necessarily remotely controlled (e.g. hand ullaging) should be provided. If access to any tank is obstructed,
e.g. by seafastening supports, alternative access should be arranged.
10.7.10.6
For tidal load-outs, an easily readable tide gauge should be provided adjacent to the load-out quay in such a
location that it will not be obscured during any stage of the load-out operation. Where the tide level is critical,
the correct datum should be established.
10.7.10.7
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It shall be possible to continuously monitor hydraulic pressures.
10.8
Special cases
10.8.1
Grounded load-outs
10.8.1.1
If the barge (or load-out vessel) is supported at the seabed during the load transfer phase then the operation is
defined as a “grounded load-out”.
10.8.1.2
Seabed support pad(s) should be prepared considering:
a.
b.
c.
d.
e.
Any protruding elements (e.g. anodes and bilge keels) on the vessel bottom
Soil bearing capacities, see also [10.4.4]
Stability and global deflections of the vessel
Vessel bottom local strength
Required sliding resistance (friction)
10.8.1.3
Acceptable safety margins should be documented for all relevant load effects, see [10.4] and [10.6.3].
Guidance note:
Maximum vessel bottom loading at the extreme low tide throughout the period should be considered.
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10.8.1.4
Where the margin against sliding is low mooring lines shall be maintained between the vessel and quayside
when the vessel is grounded.
10.8.1.5
The plan area of the grounding pad with respect to the vessel keel shall be of sufficient extent to ensure stability
of the edges of the grounding pad. Both geotechnical site investigation data and geotechnical calculations
demonstrating the capacity of the grounding pad shall be documented.
Guidance note:
The grounding pad elevation should be defined based on the actual depth of the vessel and not the moulded
vessel depth.
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10.8.1.6
Condition and level survey(s) of the support pad(s) shall be performed in due time before load-out.
10.8.1.7
A diver or side-scan survey should be carried out shortly before the vessel is positioned. This to ensure that there
is no debris in the area that can damage the vessels bottom plating.
Guidance note:
If a bar sweep survey is done, then it is recommended that this is supported by a diver’s inspection.
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10.8.1.8
The vessel should be positioned and ballasted onto the pad several tidal periods before the load-out to allow for
consolidation and settlement. Pad loading to reflect the load-out loading condition(s) and vessel levels to be
monitored during this period.
Guidance note:
Pre-loading in excess of the maximum loading during load-out may be used to reduce the required period for
pad consolidation and settlement.
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10.8.1.9
A detailed procedure covering both positioning on the pads and the float-off operation following the load-out
shall be made.
10.8.1.10
Final skidway levels shall be measured and confirmed to be within tolerances compatible with assumptions used
for structural analysis as in [10.3.6].
10.8.1.11
Between load-out and sailaway, the vessel keel should be inspected, either by diver survey or by internal tank
inspection. This is to ensure that no damage has occurred during the load-out.
10.8.2
Transverse vessel load-outs
10.8.2.1
Generally transverse load-outs are sensitive to variations in object weight and CoG as well as to inaccuracies
(between theoretical and actual) moved distance, ballasting and tide levels. This shall be duly considered both in
the ballast calculations and in the monitoring/control procedures. See also [10.3.2].
Guidance note:
Ballasting calculations for transverse load-outs should be based on the weighed weight and CoG and include
load combinations addressing weight and CoG contingencies. See also [10.3.2].
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10.8.2.2
A small GM may be more critical than for an end-on load-out as the heel may change significantly due to minor
inaccuracies. Hence, it is recommended that the GM is as high as possible and that the moment to change the
vessel heel by 0.1 m is computed (and shown in the operation manual) for all stages of the load-out.
10.8.2.3
As the vessel (accidental) heel can be significant, a braking system for the (skidded) object shall be provided. See
[10.5.2.5].
10.8.2.4
A risk assessment, see [10.2.2.5], should consider the effects of potential errors in ballasting, and of friction
between the vessel and the quay.
Guidance note:
Friction between the side of the vessel and the quay may be more critical than for an end-on load-out.
Snagging or hang-up could potentially lead to ballasting getting out of synchronisation with the 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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10.8.3
Load-in
10.8.3.1
Requirements to load-out operations are generally applicable for load-in operations as well.
10.8.3.2
Special attention should be given to selecting the optimal tide phase for starting the load-in operation.
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Guidance note:
Normally load-ins are scheduled to be started on a falling tide.
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10.8.4
Vessel to vessel load transfer
10.8.4.1
A vessel to vessel load transfer operation is defined as the activities necessary to transfer an object between
vessel(s) doing mainly a horizontal movement of the object.
10.8.4.2
Requirements to load-out operations are generally applicable for vessel to vessel load transfer operations as
well.
10.8.4.3
Vessel to vessel load transfer operations could be complex involving more than two vessel(s), and different
support conditions on one or more of the vessel(s). Due attention should be paid to this fact during planning,
design and execution of the operation.
Guidance note:
For these operations measurements of the vessel(s) draught, trim and heel may not be sufficient to control the
load distribution.
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10.8.4.4
Tide effects can be neglected for operations involving only floating vessel(s) if sufficient bottom clearance is
ensured. Hence, the operation could be defined as load-out class 3 or 4.
10.8.5
Site moves
10.8.5.1
The entire route for the site move shall be clearly defined.
10.8.5.2
Any variation in ground slope along the route shall be duly considered.
10.8.5.3
It shall be ensured that condition and capacity of the ground is satisfactory along the entire route, see [10.4].
10.8.5.4
The route should be marked up and barriered off.
10.8.5.5
It shall be ensured that clearances are sufficient to all parts of the transported object along the entire route.
10.8.5.6
If the site move involves crossing of a road with traffic or a move on a road with traffic, then this shall be duly
planned for.
Guidance note:
Relevant authorities should be informed and any required approvals should be in place.
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10.9
Information required
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10.9.1
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General
10.9.1.1
General requirements to documentation are given in [2.3].
10.9.2
Design documentation
10.9.2.1
The following design documentation is normally required:
• Analyses/calculations/certificates/statements adequately documenting the necessary strength and
capacity of all involved equipment and structures, see also [10.9.2.3]
• Documentation of civil elements (soil, quay, bollards, etc.) by e.g. engineering calculations, approved
drawings or certificates, see also [10.9.2.3] and [10.9.5]
• Vessel (barge) stability and (local) strength verifications, see also [10.9.3.2]
• Ballast calculations covering the planned operation as well as contingency situations, see also [10.9.4] and
[4.3.8.4].
• Weight report(s).
10.9.2.2
Where parameters are monitored, the expected monitoring results should be documented together with the
acceptable tolerances and the contingency measure to be applied should the acceptable tolerances be
exceeded.
10.9.2.3
Structural analysis report for the object to be loaded out should normally include at least:
•
•
•
•
•
•
•
Structural drawings, also of any additional load-out steelwork
Description of analyses programs used
Description of the structural model
Description of boundary conditions
Description of load cases
Unity checks for members and joints
Local analyses for support points, padeyes and winch connection points.
10.9.3
Equipment, fabrication and vessel(s)
10.9.3.1
Acceptable fabrication and acceptable condition of equipment/vessel(s) involved in the load-out operation shall
be documented by:
• Certificates
• Test-, survey- and NDT reports
• Classification documents.
10.9.3.2
For the load-out vessel:
•
•
•
•
•
•
•
•
•
•
•
•
General arrangement drawing
Hull structural drawings, including drawings of any internal reinforcements
Limitations for evenly distributed load and point loads on barge deck
Limitations on skidway loadings, if applicable
Equipment data and drawings
Hydrostatic data (either in curves or tables)
Tank plan, including ullage (or sounding) tables
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
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For tug(s) supporting the load-out a general specification should be submitted and include information about
the tugs bollard pull and towing equipment.
10.9.3.4
For trailered load-outs:
•
•
•
•
•
•
•
•
•
Trailer specification and configuration
Details of any additional supporting steelwork, including link span bridges and attachments
Allowable and actual axle loadings
Allowable and actual spine bending moments and shear forces
Schematic of hydraulic interconnections
Statement of hydraulic stability of trailer or SPMT system, with supporting calculations
For SPMTs, details of propulsion axles and justification of propulsion capacity
Details of set up coordinates for the trailer or SPMT grouping
Specifications of tractors if used.
10.9.3.5
For skidded load-outs:
•
•
•
•
•
Jack/winch specification
Layout of pull-on system
Layout of pull-back and braking systems
Details of power sources and back-up equipment
Calculations showing friction coefficient, allowances for bending and sheaves, loads on attachment points
and safety factors
• Reactions induced between vessel and quay.
10.9.3.6
For the pumping system:
• Specification and layout of all pumps, including back-up pumps
• Pipe schematic and details of manifolds and valves where applicable
• Pump performance curves.
10.9.3.7
For the load-out vessels mooring:
a.
b.
c.
d.
A statement showing capacity of all mooring bollards, winches and other attachments used.
Mooring arrangement drawings for the load-out operation and for the post-load-out condition.
Mooring design calculations, see [10.5.8].
Certificates for all mooring arrangement component, e.g. wires, ropes, shackles, fittings and chains (issued
or endorsed by a body approved by a recognized classification society or other certification body
accepted by the MWS Company).
e. Specification for winches, details and design of winch foundation/securing arrangements.
f. Fender arrangement, including lubrication arrangements if applicable.
10.9.3.8
If the object is weighed, then weighing results and load cell calibration certificates shall be submitted.
10.9.4
Operation manual
10.9.4.1
An operation manual shall be prepared, see [2.3.7] for general requirements to operation manuals.
10.9.4.2
The items listed below will normally be essential for successful execution of a load-out and shall be emphasized
in the manual:
a. A detailed operational communication chart (and/or description) showing clearly the information flow
throughout the operation.
b. Monitoring procedures describing equipment set-up, recording, expected readings (including acceptable
deviations) and reporting routines during the operation.
c. Detailed ballast procedures, see also [4.3.9.5] and [10.9.4.4].
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d. Operation bar chart showing time and duration of all critical activities.
Guidance note:
The operation bar-chart should include the following as applicable:
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
◦
Mobilisation of equipment
Testing of pumps and winches
Testing of pull-on and pull-back systems
Barge movements
Initial ballasting
Structure movements
Load-out operation
Trailer removal
Seafastening
Re-mooring
Decision points.
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10.9.4.3
The manual should highlight metocean conditions/directions to which the operation is sensitive.
10.9.4.4
The manual should include ballasting information as follows:
1. Planned date, time and duration of the load-out, with alternative dates, tidal limitations and windows
2. Ballast calculations for each stage showing:
◦ Time
◦ Tidal level
◦ Structure position
◦ Weight on quay, link beam and vessel
◦ Ballast distribution
◦ Vessel draught, trim and heel
◦ Pumps in use and pump rates required
◦ Moment required to change heel and trim
3. Stages to be considered should minimum include:
◦ Start condition with 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
Jack-winch failure
Trailer/skidshoe power pack failure
Trailer/skidshoe hydraulics failure
Trailer tyre failure
Tractor failure
Failure of any computerised control or monitoring system
Mooring system failure
Structural failure
Deteriorating weather
Quay failure.
10.9.5
Site
10.9.5.1
For the load-out location:
• Site plan, showing load-out quay, position of object, route to quay edge, position of mooring bollards and
winches used, reinforced areas etc.
• Section through quay wall
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• Drawing showing heights above datum of quay approaches, object support points, vessel, link beams, pad
(if applicable) and water levels (the differential between civil and bathymetric datums should be clearly
shown)
• Statement of maximum allowable loadings on quay, quay approaches, wall, grounding pads and
foundations
• Specification of capacity for all mooring bollards and other attachment points used
• Bathymetric survey report of area adjacent to the quay and passage to deep water
• Bathymetric survey of pad (for grounded load-outs)
• Structural drawings of skidways and link beams, with statement of structural capacity, construction
(material and NDT reports) and supporting calculations
• Method of fendering between vessel and quay, showing any sliding or rolling surfaces and their
lubrication.
SECTION 11 Sea voyages
11.1
Introduction
11.1.1
General
11.1.1.1
This section covers MWS requirements for sea voyages which include:
a.
b.
c.
d.
dry towages of objects on barges
transport of objects on self-propelled vessels
wet towages of objects floating on their own buoyancy, including floating or submerged pipes or similar.
Location moves of jack-ups (though approval of the locations will be covered in DNVGL-ST-N002, /39/).
11.1.1.2
It does not normally cover “standard” or “routine” cargoes such as bulk liquids, bulk solids, refrigerated cargoes,
containers or vehicles (on ferries) or supply vessels unless they are subject to marine warranty.
11.1.2
Scope
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.
11.1.3
Revision history
11.1.3.1
This section replaces the applicable sections of the following legacy documents:
• DNV-OS-H202, Sea transport operations
• DNV-OS-H203, Transit and Positioning of Offshore Units
• GL Noble Denton, Guidelines For Marine Transportations, 0030/ND.
11.2
Towage or transport design/approval flow chart
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11.2.1
The flow chart in Figure 11-1 shows the steps in the approval process and references the sections in this
standard.
Figure 11-1 Voyage design/approval flowchart
11.3
Motion response
11.3.1
General
11.3.1.1
Design motions shall be derived by means of motion response analyses, from model tank testing, or by using the
default equivalent motion values shown in [11.4].
11.3.1.2
See [3.2] for design sea states. The range of periods associated with the extreme sea state shall be in accordance
with [3.4.11].
11.3.2
Vessel heading and speed
11.3.2.1
The analyses shall be carried out for zero vessel speed for a range of headings.
Guidance note:
Normally head, bow quartering, beam, stern quartering and stern seas should be considered.
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11.3.2.2
Additionally, the analysis should be carried out for non-beam sea cases for the maximum service speed of the
vessel or the maximum speed that can be maintained in the design sea state. Where this cannot be handled
directly by the software, a zero speed analysis can be carried out with the range of probable peak wave periods,
Tp, adjusted for the speed of the vessel as follows:
where
Tp, lower
Tp, upper
VSHIP
θ
11.3.3
=
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).
Effects 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 water-plane area that can cause heave-induced roll the effects shall be
quantified by analysis and/or model tests.
11.3.4
Effects of free surfaces
11.3.4.1
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)
11.3.5
Effects 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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11.3.6
Motion 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)].
11.3.7
Results of model tests
11.3.7.1
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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.
11.4
Default motion criteria – General
11.4.1
If neither a motions study nor model tests are performed, then for standard configurations and subject to
satisfactory marine procedures, default motion criteria 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].
11.5
Default 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:
Smaller cargoes are typically under 100 tonnes weight.
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11.6
Default motion criteria – Ships
11.6.1
For ships the default motion/acceleration criteria from classification society rules may be acceptable e.g. DNV 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.
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• 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 10-8 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, ay-env 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
World-wide
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
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 ULS-a and ULS-b 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 DNV-OS-H202, /44/.
11.7
Default motion criteria – Specific cases
11.7.1
Alternative design methods
11.7.1.1
Where the ASD/WSD approach is used for structural checks the values in [11.7.2] apply. For LRFD the criteria in
[11.7.3] apply.
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ASD/WSD default motion criteria
Table 11-1 Default motion criteria (ASD/WSD approach)
Nature of Voyage
Weather
unrestricted
(these values to be
used unless cases 7
to 15 apply)
Case
LOA
(m)
B 1)
(m)
L/B
1)
1
> 140 & >
30
n/a
2
> 76 & > 23
n/a
3
≤ 76 or ≤
23
≥2.5
<
2.5
Heave
Roll
Pitch
10
20°
10°
0.2 g
Any
10
20°
12.5°
0.2 g
15°
0.2 g
3)
30°
10
25°
< 0.9
≤ 76 or ≤
23
Single amplitude
3)
≥0.9
6
Weather restricted
operations in nonbenign areas for a
duration <24 hours
(see [11.7.2.1 6)].
For L/B < 1.4 use
unrestricted case.
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.
< 0.9
Full
cycle
period
(secs)
< 0.9
4
5
Block
Coeff
3)
30°
30°
25°
25°
10
≥0.9
0.2 g
7
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
10
Any
<
2.5
≥1.4
Any
10
5°
5°
0.1 g
Inland and
sheltered water
voyages (see
[11.7.2.1 8)]). For
L/B < 1.4 use
unrestricted case.
11
Any
≥1.4
Any
Static
Independent leg
jack-ups, weather
unrestricted tow on
own hull. For L/B >
1.4 use unrestricted
Cases 1 to 6
12
n/a
>
23
<
1.4
n/a
10
20°
20°
0.0
Independent leg
jack-ups, 24-hour or
location move. For
L/B > 1.4 use Case 7
or 8 as applicable
13
n/a
>
23
<
1.4
n/a
10
10°
10°
0.0
Mat-type jack-ups,
weather
unrestricted tow on
own hull. For L/B >
2.5 the pitch angle
can be reduced to
8°
14
n/a
>
23
<
1.4
n/a
13
16°
16°
0.0
Mat-type jack-ups,
24-hour or location
move.
15
n/a
>
23
n/a
n/a
13
8°
8°
0.0
Equivalent to 0.1 g in
both directions
0.0
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Notes:
1. B = maximum moulded waterline breadth, L = waterline length. n/a = not applicable
2. Block coefficient = 0.9 is the cut-off between barge-shaped hulls (>0.9) and ship-shaped hulls.
3. See [11.6] for alternative criteria for ship-shaped hulls.
11.7.2.1
The default motion criteria shown in Table 11-1 shall only be applied in accordance with the following:
1. Vessels to have typical geometry for their type. For example, vessels with high freeboard are excluded
because they will not experience deck-edge immersion, and consequent damping.
2. The cargo-vessel 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 one-compartment damage condition.
9. The additional heel or trim caused by the design wind (with a default value of 52 m/sec or 100 knots)
should be considered. For most voyages, it is permissible to omit the effects of direct wind load when
computing the forces on the cargo (see [5.6.15] and [5.6.16]). If the total effect of the wind on the cargo
due to direct loading and wind heel are more than 10% of the loads from the default motion criteria, then
they shall be added.
11.7.3
LRFD default motion criteria
11.7.3.1
The characteristic accelerations given in Table 11-2 to Table 11-4 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 self-weight.
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 11-2 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 11-3 and Table 11-4 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)
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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 11-3 and Table
11-4 are divided by the same factor
11.7.3.7
All 3 cases (roll/quartering/pitch) in Table 11-2 to Table 11-4 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 11-2 to Table 11-4:
x
y
d
=
z
ay
ax
az
distance from vessel/barge mid ship
=
distance from vessel/barge centreline
=
distance used for calculating az in quartering sea,
=
height above waterline.
=
transverse acceleration parallel with barge deck
=
longitudinal acceleration parallel with barge deck
=
acceleration normal to the barge deck.
Table 11-2 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
ay at waterline
ay increase for each metre (z) above waterline
az at centre (C) barge
Table 11-3 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
az incr. each metre (y, d or x respectively) from C
Wind pressure
0.017 g/m
0.011 g/m
0.006 g/m
2
2
0.5 kN/m2
0.5 kN/m
0.5 kN/m
Roll Case
Quartering
Table 11-4 Criteria for Hs ≤ 4 m for larger barges (LRFD approach)
Acceleration/wind force
Pitch Case
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ay at waterline
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0.26 g
0.20 g
0
0.017 g/m
0.013 g/m
0
ax at waterline (wl)
0
0.08 g
0.12 g
ax increase for each metre (z) above waterline
0
0.003 g/m
0.004 g/m
0.15 g
0.12 g
0.08 g
az incr. each metre (y, d or x respectively) from C
0.017 g/m
0.009 g/m
0.004 g/m
Wind pressure
0.3 kN/m2
0.3 kN/m2
0.3 kN/m2
ay increase for each metre (z) above waterline
az at centre (C) barge
11.8
Directionality and heading control
11.8.1
The incident weather shall be considered to be effectively omni-directional, as stated in [11.3.2]. No relaxation in
the design sea states from the bow-quartering, beam and stern-quartering directions shall be considered for:
a. Any voyage where the default motion criteria are used, in accordance with [11.4], or similar
b. Single tug towages, or voyages by vessels with non-redundant propulsion systems (see [11.8.3]).
c. 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
d. Any towage in a Tropical Revolving Storm area and season
e. Any un-manned towage
f. Any transport where the vessel does not have sufficient redundant systems to maintain any desired
heading in all conditions up to and including the design storm, taking account of the windage of the
cargo.
Guidance note:
For c) examples are crossing of the Indian Ocean or Arabian Sea in the South-West monsoon
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11.8.2
When the relaxation exclusions in [11.8.1] do not apply, relaxation in the non-head sea cases can be considered
for:
1. Manned, multiple tug towages, where after breakdown of any one tug or breakage of any one towline or
towing connection, the remaining tug(s) still comply with the bollard pull requirements of [11.12.2].
2. Voyages by self-propelled vessels with redundant propulsion systems. A vessel with a redundant
propulsion system is defined as having, as a minimum:
◦ 2 or more independent main engines
◦ 2 or more independent fuel supplies
◦ 2 or more independent power transmission systems
◦ 2 or more independent switchboards
◦ 2 or more independent steering systems, or an alternative means of operation of a single steering
system (but excluding emergency steering systems that cannot be operated from the bridge)
◦ the ability to maintain any desired heading in all conditions up to and including the design storm,
taking account of the windage of the cargo and assuming the failure of any one component.
11.8.3
Any vessel not complying with all the requirements in [11.8.1] and [11.8.2] shall be considered non-redundant.
11.8.4
For voyages by self-propelled vessels a survey should be performed to confirm the propulsion system
redundancy is acceptable.
Guidance note:
If there is any doubt as to whether or not a vessel can be considered to have a redundant propulsion system the
survey should be performed at an early stage of the project.
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11.8.5
In general, where a relaxation is allowed in accordance with [11.8.2], Table 11-5 is a guide to the acceptable sea
state values. This should be confirmed by the MWS company as being acceptable on a case-by-case basis.
Table 11-5 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
Such relaxation shall only apply to considerations of accelerations, loads and stresses. It shall not be applied to
considerations of stability.
11.8.8
For any voyage where a relaxation is allowed in accordance with [11.8.2] and [11.8.5], the voyage manual shall
contain, in a format of use to the Master:
a. The limitations on critical parameters see Guidance Note
b. Procedures for monitoring and recording of critical parameters, possibly by accelerometers on barges
with radio links to the lead tug(s)
c. Procedures for heading control
d. Results of the risk assessment, and any recommendations arising
e. 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.
11.9
Design and strength
11.9.1
Computation of loads
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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 dry-transported. Its centre of gravity can be higher above the roll centre in the dry-transport condition than
in any of its floating service conditions. Even though the voyage motions can appear to be less than the service
motions, the loads on cargo components and ship-loose items can be greater.
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11.9.1.2
The loads shall include components due to the distribution of mass and rotational inertia of the cargo.
Guidance note:
This is of particular importance in the calculation of shear forces and bending moments in the legs of jack-up
units and similar tall structures.
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11.9.1.3
If the computed loads are less than the “Minimum allowable seafastening force” shown in Table 11-6, 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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11.9.2
Friction – general
11.9.2.1
Friction forces on the cargo supports/cribbing may be allowed to contribute to a reduction in the seafastening
design loads provided that the entire load path, including the potential sliding surfaces, are documented as
being capable of withstanding the loading generated.
11.9.2.2
Uncertainty in the load distribution between (seafastening) members and friction forces shall be taken properly
into account.
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 wave-induced bending and
torsion of transport vessel shall be considered.
11.9.2.4
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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.
• 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 11-6. If the effects of vibrations and hull beam deflections can be proved to be insignificant,
consideration can be given to reducing this requirement.
Table 11-6 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 steel-steel, 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 11-7, as a function of the cargo weight and overhang.
Table 11-7 Max allowable upper bound design friction coefficients
Maximum cargo
Cargo weight, W, tonnes
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overhang
W<1,000
1,000 <W<
5,000
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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 vice-versa.
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 self-weight reaction the total normal reaction shall be reduced by 10% to allow
for adverse phasing.
• When heave reduces the self-weight reaction, the normal reaction shall be taken as weight less heave as
any effects of phasing will cause an increase in the normal reaction.
11.9.3
ASD/WSD friction
11.9.3.1
When using the ASD/WSD method, friction shall NOT be used if the loadings are computed in accordance with
the default criteria in [11.4] and [5.6.16], except as allowed by [11.9.9].
11.9.3.2
When using the ASD/WSD method, friction effects can be incorporated to reduce seafastening requirements for
cargos supported on timber cribbing, subject to the following:
1. 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 11-7 can be assumed to act in any direction relative to the cribbing
provided that:
◦ the cribbing is reasonably well balanced in terms of the proportion in the fore-aft and transverse
directions, AND
◦ each of these groups is reasonably well balanced about the cargo CoG in plan.
3. Provided that the conditions in [2)] are met, for cribbing heights between 600 mm and 900 mm, with a
width not less than 300 mm, then the percentage computed friction force at right angles to the
longitudinal axis of a cribbing beam shall not exceed (900 - H)/3%, where H = the height of cribbing above
deck, in mm. In the direction of the longitudinal axis of a cribbing beam, the full friction force can be used.
4. For wood cribbing over 900 mm high, or with a width less than 300 mm, no friction force shall be assumed
to act in a direction at right angles to the longitudinal axis of a cribbing beam.
5. If greater cribbing friction is required than available according to [3)] and [4)], stanchions can be fitted to
provide transverse cribbing restraint. Where such stanchions are fitted, they should be designed to carry
loads due to a friction coefficient of 0.5 (to ensure they are able to carry loads due to upper-bound friction
assumptions).
6. The underlying assumption in the approach given above is that the seafastenings have sufficient flexibility
to deflect in the order of at least 2 mm in the horizontal direction of loading without failing. This will be
reasonable in most cases, but when this is not the case the more detailed approach given in [7)] shall be
used.
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7. As an alternative to [2)] through [5)], a more detailed approach can be used. In such cases, the friction
coefficient permitted in Table 11-7 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)].
11.9.4
LRFD friction
11.9.4.1
When using the LRFD method, friction can be used if the loadings are computed in accordance with the default
criteria in [11.4] and [5.6.16]
11.9.4.2
When using the LRFD method the following approach shall be used if friction effects are to be incorporated to
reduce seafastening requirements.
11.9.4.3
Design loading on the seafastening can be reduced by considering relevant friction effects on the cribbing, see
[11.9.7].
11.9.4.4
Where friction on the cribbing is considered, see [5.6.9], the characteristic friction coefficient shall be
documented and a material factor applied to find the design friction coefficient.
Guidance note:
In the areas and directions where full friction effect could be mobilized a design friction coefficient of 0.3 can
normally be applied between wood and steel on cargo. Any special effects (e.g. wood treating, type of surface
treatment on cargo, and risk of oil/lubricant present) that can reduce the friction significantly should be
evaluated.
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11.9.4.5
Due to low wood shear stiffness and strength, friction forces transverse to the cribbing (soft wood) boards should
only be accounted for if properly documented.
Guidance note:
If a thorough evaluation including cribbing shear stiffness and seafastening design (stiffness) has not been
carried out the following apply:
a. For cribbing with H (height)≥1.5B (breadth) zero contribution should be considered from friction in the
transverse cribbing direction.
b. For cribbing with H < 1.5B contribution from friction in the transverse direction could be considered with
(1.5B – H)/1.5B x 100%.
c. Normally 100% contribution from friction could be considered in the longitudinal cribbing direction.
However, see [11.9.2.2].
d. The mean design friction coefficient considered should in any case not exceed 0.2, but see [11.9.2.10].
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11.9.5
Seafastening 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 load-out method.
• Cargoes floated over a submersible barge or vessel, are frequently supported by timber cribbing or
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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 load-carrying 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 vessel-stiffened
panels for support loads from cargo should include the stresses caused by hull bending moments and shear
forces.
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11.9.5.5
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]
load-out trailer layout
needed space for (operation of) load-out systems, e.g. pumps, hoses, pull/push units
set down tolerances and shimming requirements
cutting/handling offshore
securing of object before lift, see [11.9.5.7]
possible need for set down of the object again and re-instate 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 load-out procedure.
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11.9.5.7
Seafastening design for offshore or inshore installation operations should allow for easy release and provide
adequate support and horizontal restraints until the object can be lifted clear of the vessel, or launched as
applicable as described in 11.9.6].
11.9.5.8
Elements providing horizontal and/or vertical support after cutting/removal of seafastening shall be verified for
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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 DNV-RP-H102, /55/.
11.9.5.12
If re-instating of the transport seafastening may be required offshore this should be taken into account in the
design and in the cutting/release procedure.
11.9.5.13
Seafastenings shall be designed to accept deflections of the barge or vessel in a seaway, principally due to
longitudinal bending. In general, longitudinal bending should be considered for the cases described in [5.6.11].
Guidance note:
Where longitudinal bending is a consideration, suitable seafastening designs include:
a.
b.
c.
d.
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 vessel-seafastenings-cargo, 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 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
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When required by [5.6.11.2], and in the absence of more detailed information, it should be assumed that the
vessel will incur bending and shear deflection as if unrestrained by the cargo; the seafastenings and the object
should be checked assuming quasi-static vessel hogging and sagging due to a wave of length, Lw, equal to the
vessel length, and height:
where Lw is in metres.
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 pre-tensioning and length/stiffness variations in statically
indeterminate seafastening arrangements shall be taken into account. The design loads for lashings should be
multiplied by a skew load factor not less than 1.5 if skew load effects are not accurately calculated.
Guidance note:
A skew load factor of 1.5 is considered adequate if lashings carrying the same (quasi-static) load component
between them have approximately the same stiffness and similar means of pre-tensioning. If not, a conservative
assessment should be conducted to estimate the applicable skew load factor(s).
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11.9.5.21
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 quasi-static 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:
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a. D-links and shackles shall be used instead of hooks (which can unhook)
b. 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.
c. 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.
d. The fittings shall be of the correct shape and size to ensure that the straps are not damaged
e. 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.
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 load-out with the barge or vessel in the
voyage ballast condition, or a condition giving a similar longitudinal bending situation. If not practical, then the
additional stresses which can be caused by the change in ballast condition shall be considered.
11.9.5.31
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
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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
Guide posts should not be used for seafastenings unless specifically designed for that purpose.
11.9.6
Seafastenings to be removed offshore
11.9.6.1
For cargoes that will be installed offshore, the seafastenings should be capable of being released in stages, such
that the cargo remains secure for all anticipated angles and motions. The release of seafastenings, and the
removal of any one object, should not disturb the seafastenings of any other object.
Guidance note:
For lifts, see [16.16.9.5] for the design of the restraints/seafastenings that remain after all cutting has been
completed. For other operations, 10° is normally sufficient.
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11.9.6.2
Where the installation is in an area more benign than that for which the seafastenings were designed then,
subject to the agreement of the MWS company, some seafastenings can be removed after entering that area and
before the Installation Certificate of Approval is issued. In this case, unless weather-routed:
a. The remaining seafastenings shall be designed for the design criteria for the installation area and the route
to a sheltered area if required, and
b. The seafastenings to be removed early shall be clearly marked as such.
11.9.6.3
Removal of seafastenings shall not normally start until the Installation Certificate of Approval has been issued.
This requirement can be relaxed in special circumstances subject to a risk assessment in accordance with [2.4].
The seafastenings to be removed early shall be clearly marked as such and identified in the seafastening removal
procedures.
11.9.7
Cribbing
11.9.7.1
Where the cargo is supported on wooden cribbing or dunnage, rather than steel-to-steel supports, then
sufficient plan area and height of material should be provided to distribute the loads to ensure that the
underside of the cargo and to the deck of the transport vessel are not overstressed. The loads shall include the
static loadings and the design environmental loadings as shown in [11.3] and Sec.5.
Guidance note:
Cribbing designed to pick up structural members in the underside of larger transported objects e.g. MOUs, the
vessel deck, or both, and fixed to the deck of the vessel, should not normally be less than 200 mm high.
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11.9.7.2
A minimum clearance of 0.075 m, after accounting for vessel deflections, should be provided between the
lowest protrusion of the cargo and the deck of the barge or vessel.
Guidance note:
Where the dimensions and locations of protrusions from the cargo are well documented the minimum clearance
can be reduced.
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11.9.7.3
Unless it can be demonstrated that the cargo, vessel and cribbing (without crushing), can withstand 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.
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11.9.7.4
The selected timber should withstand the computed cribbing pressures without crushing. Localised crushing to
accommodate cargo and cribbing imperfections is permissible.
Guidance note:
A satisfactory arrangement can consist of hardwood for the main cribbing structure, topped by a soft packing
layer, typically 50 mm thick.
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11.9.7.5
In the case of a random or herring-bone dunnage layout supporting a flat-bottomed cargo, without 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 on-load and off-load. Minimum angles of 5º should be considered.
11.9.8
Cargo strength requirements
11.9.8.1
The cargo shall meet the requirements in Sec.5 for the loads imposed during the voyage. Additionally the cargo
shall be shown to have adequate strength to withstand the local cribbing/grillage and seafastening loads, see
[11.9.5].
11.9.8.2
Any additional loadings caused by any overhang of the cargo over the side of the transport vessel, buoyancy
forces and wave slam loadings shall be included.
11.9.9
Securing of pipe and other tubular goods
11.9.9.1
This 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
jack-ups is covered in [11.27.11]. The design of securing shall consider the following:
•
•
•
•
the type of vessel,
the nature of the cargo,
the duration of the towage or voyage and
the 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 11-8.
Table 11-8 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
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Uncoated steel - uncoated steel
0.15
Epoxy coated pipe – timber
0.1
Epoxy coated pipe - epoxy coated pipe
0.05
11.9.9.3
Where sand can be present between the friction surfaces, the friction coefficient should be considerably
reduced.
11.9.9.4
Friction coefficients (both wet and dry) for other materials should be justified or the beneficial effects of friction
should be ignored.
11.9.9.5
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
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.
a. For weather restricted operations, and 24-hour or location moves of jack-ups, 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.
b. For weather unrestricted operations, including voyages of jack-ups, steel strongbacks should be fitted over
the top layer, and each stow (group of pipes) set up hard by driving wooden wedges between the
strongbacks and the top layer of pipe. End stops or bulkheads shall be provided.
11.9.9.8
Line pipe on pipe carrier vessels can be stacked between the existing stanchions/crash barriers, on the wooden
sheathed deck. Timber dunnage or wedges should be used to chock off any clearance between the pipes and
the stanchions.
a. For weather restricted operations, provided it can be demonstrated that adequate friction exists to prevent
longitudinal movement, no end stops need be provided.
b. 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.
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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:
a. the vessel’s stability shall meet the requirements of [11.10] with including the effects of entrapped water,
and
b. the deck and pipe layers shall not be overstressed.
Guidance note:
Where the requirements are not met a possible solution is to seal the pipe ends of at least the lowest level
of the stack.
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11.9.9.13
Where the trim and stability booklet includes suitable example loading conditions these should be considered.
11.9.10 Inspection of welding and seafastenings
11.9.10.1
Principal seafastening welds shall be visually checked and the weld sizes confirmed against the agreed design.
11.9.10.2
Non-destructive testing (NDT) shall be carried out on the structural members of the seafastenings. Specific
requirements for weld inspection are given in [5.10.2.3].
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 re-tested.
11.9.11 Use of second hand steel seafastenings
11.9.11.1
When second hand steel seafastenings are used, any wastage caused during previous removal(s) or use should
not affect its fitness for purpose. There should be sufficient documentation to ensure the traceability of the steel
and in particular documentation relating to the grade of steel.
11.9.11.2
There should be NDT inspection reports to demonstrate no cracking or lamellar tearing in critical areas.
Guidance note:
Areas to consider included regions of previous fabrication, old welds, burnt off attachments etc.,
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11.9.11.3
Should sufficient documentation of the type of steel (e.g. EN10025) be unavailable, coupon testing is acceptable
to determine the steel type. The guaranteed minimum properties of this type of steel shall be used.
Guidance note:
Tested values should not be used as they may not be representative of the rest of the steel.
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11.9.12 Fatigue
11.9.12.1
See [5.9.4] for requirements for fatigue analysis.
11.9.12.2
The FLS design waves (and wind) should be carefully selected based on a “worst case scenario” regarding
weather conditions during the voyage.
Guidance note:
For calculating the maximum expected fatigue damage for a voyage it is recommended that weather conditions
are selected that do not have more than 10% probability of being exceeded with regards to cumulative fatigue
damage.
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11.9.12.3
A reasonably conservative exposure time should be selected for calculating the maximum expected transport
fatigue damage.
Guidance note:
The following exposure times should normally be considered:
• For transports from one sheltered location to another: 1.5 x TPOP ; when TPOP exceeds 30 days,
TPOP + 15 days can be 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 pre-dominant sea state directions relative to the vessel
the exposure time and analysed directions can be selected according to the below table. Where applicable,
symmetry can be considered to reduce number of load cases/directions.
Sea direction
Head
H Port Q
Port Beam
S Port Q
Stern
S Stbd Q.
Stbd Beam
H, Stbd 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
10
15
15
10
10
10
15
Analysed
direction
Exposure in
%
Where H and S denote head seas and stern seas respectively and Q denotes quartering (45°) seas.
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11.9.12.6
For fatigue critical transports, it is recommended to maintain control of the (anticipated) fatigue damage during
the transport. This is especially important if the assumed stress range distribution could be unconservative.
Guidance note:
Fatigue damage could be controlled by regular inspections and/or by verifying that the actual fatigue stress
range is less critical than the stress range applied in the calculations. The stress range could be controlled by
setting up systems that compare the actual to the applied:
•
•
•
•
exposure time
wave scatter diagram considering relative vessel/sea directions
vessel motions, e.g. calculated vs MRU readings
member loads/stresses.
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11.9.12.7
Whenever relevant, mitigation actions to avoid excessive transport fatigue shall be defined.
Guidance note:
Potential mitigation actions include: heading control and/or weather routing. Regular inspections combined with
repair possibilities could/should also be considered.
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11.9.13 Vortex shedding
11.9.13.1
All voyages should be checked for wind-induced Vortex Induced Vibration (VIV), see [5.6.7.4]. Where the
potential is identified, mitigating measures shall be taken.
Guidance note:
Typical items that can be susceptible include:
• slender members in jackets that will be submerged in the in-place condition and which are therefore not
checked for in-place VIV, or
• single-tube jack-up legs (which can be fitted with spoilers to prevent VIV).
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11.9.14 Condition 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:
a. An extended, in depth, survey of the vessel structure involving one or more specialist surveyor(s). Facilities
for a close-up survey of inaccessible parts of the hull structure may be required.
b. 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.
c. A MWS company review of classification society approved scantling drawings.
d. 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.
e. A dry dock survey of the vessel can be necessary should there be any doubt as to the condition of the tow.
11.10
Floating stability
11.10.1 General
11.10.1.1
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Free-trimming 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
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:
a. the effects of free surface liquids in unit and cargo,
b. residual free surface due to incomplete venting, such as can occur if ballasting when trimmed
c. 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
Multi-vessel 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
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accepted for Class by a Recognized Classification Society.
11.10.1.11
Requirements for the different asset types in transit are given in Table 11-9.
Table 11-9 Stability requirements for different asset types in transit
Jack-up
Semi-sub
Intact range
Cargo on ships
and barges
See [11.10.3]
Damage (general)
See [11.10.4]
See
[11.10.5]
GBS
40° 1)
See [11.10.2]
Wind overturning
(intact)
Damage (specific)
Jacket
wet tow
See [6.2]
See [11.10.6]
See [11.10.7]
Compartmentation and
watertight integrity
See [11.10.8]
Draught and trim
See [11.10.9]
[11.10.7] 1)
Notes:
1. Subject to agreement with the MWS company once full details are known
11.10.2 Intact 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 self-floating 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
11-10. 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 11-10 Intact stability range
Vessel or towed object, type and size
Intact range
Large and medium vessels, LOA > 76 m and B 1) > 23 m
36º
Large cargo barges, LOA > 76 m and B 1) > 23 m
36º
Small cargo barges, LOA < 76 m or B 1) < 23 m
40º
Small vessels, LOA < 76 m or B 1) < 23 m
44º
MOU’s including jack-ups and semi-submersibles
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
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Notes:
1. B = maximum moulded waterline beam.
11.10.2.5
Requirements for objects which do not fall into the categories shown in Table 11-10, which are non-symmetrical,
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 jack-ups 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].
11.10.3 Wind overturning (intact condition - all units)
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 11-2.
11.10.3.2
The wind velocity used for intact wind overturning calculations for the survival condition shall be the 1-minute
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.
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Figure 11-2 Wind overturning criteria (intact case)
11.10.4 Damage 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 jack-ups
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:
a. 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.
b. Damage shall be assumed to extend from the baseline upwards without limit.
c. 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.
d. 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 case-bycase 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.
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11.10.5 Damage stability for jack-ups
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 11-3.
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.
Figure 11-3 Damage stability for jack-ups
11.10.6 Damage 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
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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 11-4) at the same angle.
Figure 11-4 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.
a. The angle of loll shall be less than 25º for any axis.
b. The residual range of stability from the angle of loll to the downflooding angle shall be not less than 7º.
Figure 11-5 Damage stability for column stabilised unit (Case B)
11.10.7 Damage stability (apart from jack-ups 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 11-6.
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Figure 11-6 Damage stability (apart from jack-ups and column-stabilised)
11.10.7.2
One-compartment 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 one-off towage of short duration, carried out under controlled conditions, and suitable precautions
are taken, which can include:
• Areas vulnerable to collision should be reinforced or fendered to withstand collision from the largest
towing or attending vessel, at a speed of 2 m/s.
• Projecting hatches, pipework and valves are protected against collision or damage from towing and
handling lines.
• Emergency towlines are provided, with trailing pick-up lines, to minimise the need for vessels to approach
the structure closely during the tow.
• Emergency pumping equipment is provided.
• Potential leaks via ballast or other systems are minimised.
• Ballast intakes and discharges, and any other penetrations through the skin of the vessel or object, shall be
protected by a double barrier system, or blanked off.
• Vulnerable areas are conspicuously marked and Masters of all towing and attending vessels are aware of
the vulnerable areas.
• A guard vessel is available to warn off other approaching vessels.
• A risk assessment in accordance with [2.4] shall be carried out.
11.10.7.4
The relaxations allowed by [11.10.7.2] and [11.10.7.3] do not apply in ice-affected 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:
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.10.8 Compartmentation and watertight integrity
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11.10.8.1
All external openings below the static intact and any one-compartment-damaged 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 11-2 is
satisfied), or
• 4 m above all required static one-compartment-damaged 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
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
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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.
11.10.9 Draught 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 on-load and off-load 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 11-11.
Table 11-11 Minimum recommended draught and trim for barges
Length of Towed Vessel
Minimum Draught Forward
Minimum Trim by Stern
30 m
1.0 m
0.3 m
60 m
1.7 m
0.6 m
90 m
2.4 m
0.8 m
120 m
3.1 m
1.0 m
150 m
3.7 m
1.2 m
200 m plus
4.0 m
1.5 m
11.10.9.5
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 ship-shaped units (where LOA is the overall length of the unit in metres) the forward draught should
be greater than:
for LOA≥200 m 2.0 m+ 0.015 x LOA
for LOA <200 m as for barges in Table 11-11
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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 in-operation 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, re-painted in the area above the
waterline.
11.10.9.10
Where the tow is un-manned, and in order that the tug can monitor any increased draught during the towage, a
broad distinctive line of contrasting colour should be painted around the bow approximately 0.5 m above the
waterline.
11.11
Transport vessel or barge selection
11.11.1 Selection 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:
a. 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.
b. 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.
c. The barge or vessel as loaded shall have sufficient freeboard to give reasonable protection to the cargo.
d. If a floating load-out is planned, there shall be sufficient water depth to access and leave the load-out
berth and the load-out can be carried out in accordance with Sec.10.
e. If a submerged load-out is planned, the barge or vessel can be submerged, within its Class limitation, so as
to give adequate clearance over the deck, and adequate stability at all stages, within the water depth
limitations of the load-out location.
f. There shall be adequate pumping capacity to comply with [11.15], or be suitable for the use of additional
pumping equipment.
g. Submersible barges. Barges that can be totally immersed in the intact condition should be classed as
submersible barges. Submersible barges are normally classed as such by a RCS (Recognized Classification
Society).
h. The deck strength shall be adequate, including stiffener, frame and bulkhead spacing and capacity, for
load-out and transport loads.
i. For a vessel, securing of seafastenings shall not need welding in way of fuel tanks.
j. For a barge, it shall be properly equipped with main and emergency towing connections, recovery gear,
pumping equipment, mooring equipment, anchors, lighting and access ladders.
k. The motion responses as calculated shall not cause overstress of the cargo.
l. All required equipment and machinery shall be in sound condition and operating correctly.
m. The barge or vessel shall possess the relevant, in date, documentation as set out in Table B-2.
n. Unclassed barges shall be subject to appropriate project-specific structural, equipment and machinery
checks. They shall have a valid load line, or load line exemption, certificate.
11.11.2 Suitability and on-hire surveys
11.11.2.1
In their interest, the charterer is advised to have a suitability survey and an on-hire survey of the barge or vessel
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carried out before acceptance of the charter.
11.12
Tug selection
11.12.1 General
11.12.1.1
The tug(s) selected should comply with the minimum bollard pull requirements shown in [11.12.2], and should
also comply with the appropriate Category in Table 11-12. The appropriate category should be agreed with the
MWS company.
Table 11-12 Tug categories
Category
Used for
U - Unrestricted
Single tug towages in benign or non-benign weather areas.
They shall have very good seakeeping qualities including good propeller immersion in
bad weather. These qualities are unlikely to be satisfied with a Length Over All (LOA)
less than 40 m and a displacement of less than 1,000 tonnes.
C - Coastal
Towages in benign weather areas or staged tows
R1 - Restricted
Assisting in multi-tug towages
R2 - Restricted
Benign weather area towages
R3 - Restricted
Assisting in multi-tug towages in benign weather areas
ST – Salvage Tug
11.12.1.2
Vessels in all categories shall be of such a design to allow them to operate safely and effectively in their
designated areas and shall be purpose-built for towing operations or be of a multi-purpose design having
towing capability.
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. Ice-breaking tugs can be less than this
and the MWS company should be consulted.
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
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carry out the following duties:
• Act as a guardship, to protect the tow, and advise approaching vessels that they can be running into
danger
• In the event of mechanical failure or towline breakage, assist in removing the failed tug from the towing
spread. In this case it is desirable for all the main tugs to have towing connections forward and appropriate
rigging deployed. See [11.18.7.5] for procedure for tug breakdowns in multi-tug tows.
• Take over the duties of the failed tug
• Provide any other required assistance in an emergency.
11.12.2 Bollard pull requirements
11.12.2.1
Table 11-13 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 11-13 Meteorological criteria for calculating TPR (towline pull required)
Section
Condition
Hs(m)
Wind (m/sec)
Current (m/sec)
11.12.2.2
Limited sea room
Design
Design (1 hour
mean)
0.5 or predicted
current if greater
11.12.2.3
Continuous adverse
current or weather
11.12.2.4
Standard
5
20
0.5
11.12.2.5
<24 hour staged
tow or
<24 hour jack-up
move
3
15
0.5 or predicted
current if greater
11.12.2.6
Benign weather
areas
11.12.2.7
Sheltered from
waves
As agreed with the MWS company to ensure a reasonable speed in
moderate weather.
As agreed with the MWS company but not less than:
2
15
0.5
As agreed with the MWS company.
11.12.2.2
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 jack-up moves and every stage of a staged tow),
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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
=
(BP ×
Teff)/100 =
Σ
=
the tug efficiency in the sea conditions considered, %
the contribution to TPR of each tug
the aggregate of all tugs assumed to contribute.
11.12.2.9
Only those tugs connected so they are capable of pulling effectively in the forward direction shall be assumed to
contribute. Stern tugs shall be discounted from the calculation in [11.12.2.8].
11.12.2.10
Tug efficiency, Teff, depends on the size and configuration of the tug, the sea state considered and the towing
speed achieved. In the absence of alternative information, Teff can be estimated for good ocean-going tugs
according to the following equation:
where
LOA
BP
=
tug length overall in metres (using 45 m for LOA > 45 m)
=
Static continuous bollard pull in tonnes (with BP > 20 tonnes, and using 100 when BP >100
=
significant wave height (with 1 m < Hs < 5 m).
tonnes)
Hs
Note that all tugs will generally have very low efficiencies with Hs > 5 m since they should be protecting their
towing gear. Tugs with less sea-kindly characteristics will have significantly lower values of Teff in all sea states.
11.12.2.11
These efficiencies are shown graphically in Figure 11-7 for tugs of LOA > 45 m in different significant wave
heights up to 5 m.
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Figure 11-7 Tug efficiencies in various wave heights (tug LOA ≥ 45 m)
11.12.2.12
The resulting effective bollard pull in different wave heights for tugs with LOA ≥ 45 m and LOA = 20 m is shown
in Figure 11-8.
Figure 11-8 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].
11.12.3 Main 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].
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11.12.4 Tailgates/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.
11.12.5 Towline 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
On square-sterned tugs, it is preferred that mechanically or hydraulically operated stops be fitted near the aft
end of the bulwarks, to prevent the towline slipping around the tug's quarter in heavy weather.
11.12.5.4
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.
11.12.6 Workboat
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.
11.12.7 Communication 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.
11.12.8 Navigational equipment
11.12.8.1
Tugs shall be provided with:
• all necessary navigational instruments and up-to-date charts (for which an IMO-approved electronic chart
display and information system (ECDIS) is acceptable), and
• publications that can be required on the particular towage, including information for possible diversion
ports and their approaches.
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11.12.9 Searchlight
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.
11.12.10 Portable pump
11.12.10.1
On any tow outside coastal limits, the tug shall carry at least one portable pump, equipped with means of suction
and delivery and having a self-contained power unit with sufficient fuel for 12 hours usage at the pump’s
maximum rating. The pump shall be suitable for the requirements outlined in [11.15.2] to [11.15.4] but cannot be
considered to be a substitute for the pump(s) required in [11.15.2] as it may be difficult to deploy in bad weather.
The methods and feasibility of deployment should be considered.
11.12.11 Additional equipment
11.12.11.1
Anti-chafe gear should be fitted as necessary. Particular attention should be paid to contact between the towline
and towing pods, tow bars and stern rail and any other sharp edges (e.g. in the gap between hull and rollers)
that could damage the towline.
11.12.11.2
All tugs should be equipped with burning and welding gear for use in emergency.
11.12.12 Bunkers and other consumables
11.12.12.1
The tug should carry fuel and other consumables including potable water, lubricating oil and stores, for the
anticipated duration of the towage, taking into account the area and season, plus a useable reserve of at least 5
days’ supply (excluding any unpumpable). For tows likely to take more than 20 days the reserve should be
increased to 7 days.
11.12.12.2
If refuelling en-route is proposed, then suitable arrangements shall be made before the towage starts, and
included in the towing procedures (see [11.14.7]).
11.12.13 Tug manning and accommodation
11.12.13.1
Vessels in all categories shall be manned to meet the minimum requirements laid down by Statutory Regulations
or those required by State or Port Authorities.
11.12.13.2
Manning levels for vessels in all categories shall be subject to the requirements of a specific towage.
11.12.13.3
Where vessels are required to undertake long duration towages, difficult towages or where the tow is 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 life-saving 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.
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11.12.13.5
Category ST. To satisfy category ST, certified accommodation and life-saving appliances shall be provided for a
minimum of twelve (12) persons.
11.12.13.6
Vessels in category ST shall, when engaged in towing operations, carry a minimum of five (5) certificated officers.
These should be the Master, two (2) Deck Officers and two (2) Engineer Officers.
11.12.13.7
Categories U, C and R1. To satisfy categories U, C and R1, certified accommodation and life-saving 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.
11.13
Towing equipment
11.13.1 Flowchart
11.13.1.1
Figure 11-9 is a flowchart for determining the required strength of the towing gear for a specific tug.
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Figure 11-9 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:
• Part-built or damaged ships, or any structure when the bow sections could be vulnerable to wave damage.
• Part-built ships, converted ships or FPSOs without a rudder or skeg, or with a turret or spider fitted
forward, where better directional stability can be obtained if towed by the stern.
• 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.
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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.
11.13.2 Number of towlines
11.13.2.1
Table 11-14 gives the minimum number of towlines for each category of tug.
Table 11-14 Tug wire requirements
Category
Main Wire
Spare
ST – Salvage Tug
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
11.13.3 Strength of towline and towline connections (outside ice areas)
11.13.3.1
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 11-15 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 11-15 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 11-15 due to problems in safely handling the large towlines required. In these cases the effective towing
bollard pull for selecting the towline MBL can be reduced to not less than 280 tonnes provided that:
•
•
•
•
the vessel is fitted with towline tension monitoring,
the tug Master is in agreement,
the reduction is documented in the towing procedures and Certificate of Approval,
the tug master shall take extra care in bad weather to protect the towline.
and if practicable:
• the winch should be adjusted to pay out at 80% of the towline MBL, and
• the engines should be mechanically or electronically limited to produce a maximum static bollard pull of
not more than 50% of the towline MBL (i.e. the effective bollard pull).
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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
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
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related to the required BP rather than the actual BP but should allow for the effective length of the towline used.
Such relaxation shall be with the express agreement of the Master of the tug, and shall be noted in the towing
procedures and Certificate of Approval. It shall not apply for towages in ice areas (see [11.19.23]).
11.13.4 Relationship 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”:
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.
11.13.5 Towline 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].
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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 quick-release type, then the fairlead design shall allow all the released parts
to pass easily through the fairlead.
11.13.6 Bridle legs
11.13.6.1
Each bridle leg should be of stud link chain or composite chain and wire rope. If composite, the chain should 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.
11.13.7 Bridle 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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11.13.8 Shackles
11.13.8.1
The documented MBL of shackles forming part of the towline (including any shackle between the towline and the
bridle apex) shall be at least 130% of the 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.
11.13.9 Intermediate pennant or surge chains
11.13.9.1
An intermediate wire rope pennant can be fitted between the main towline and the bridle or chain pennant. All
wire rope pennants shall have hard eyes or sockets, and be of the same lay (i.e. left or right hand) as the main
towline.
Guidance note:
Its main use is for ease of connection and reconnection.
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11.13.9.2
A synthetic spring, if used, should not normally replace the intermediate wire rope pennant.
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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].
11.13.10 Synthetic 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
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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11.13.11 Bridle recovery system
11.13.11.1
A system shall be fitted to recover the bridle or chain pennant, to allow reconnection in the event of towline
breakage. The recovery system should consists of a winch and a recovery line connected to the bridle apex, via a
suitable lead, preferably an A-frame.
Guidance note:
The preferred type of bridle recovery system is shown in [K.1].
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11.13.11.2
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The recovery winch shall be capable of handling at least 100% of the weight of the bridle, plus attachments
including the apex and the intermediate pennant. It shall be suitably secured to the structure of the tow. Except
for very small barges, the winch should have its own power source. Sufficient fuel should be carried, including a
reserve.
Guidance note:
A well-sized recovery winch can also be useful for initial connection of the towline.
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11.13.11.3
If the winch is manually operated, it should be fitted with ratchet gear and brake, and should be geared so that
the tow bridle apex can be recovered by 2 persons.
11.13.11.4
Should no power source be available, and manual operation is deemed impractical, then arrangements shall be
made, utilising additional pennant wires as necessary, to allow the tug to reconnect.
11.13.11.5
The MBL of the recovery wire, shackles, leads etc. shall be at least 6 times the weight of the bridle, apex and
intermediate pennant. The winch barrel should be adequate for the length and size of the wire required.
11.13.12 Towing winches
11.13.12.1
Tugs in all categories shall be provided with at least one towing winch, (two towing winch drums for category
ST).
11.13.12.2
The towing winch and its connection to the vessel shall be strong enough to withstand a force equal to the actual
MBL of the tow wire acting at its maximum height above deck, without over-stressing either the winch or the
deck connections
11.13.12.3
If the power for the towing winch is supplied via a main engine shaft generator during normal operating
conditions, then another generator shall be available to provide power for the towing winch in case of main
engine or generator failure.
11.13.12.4
If a multi-drum winch is used, then each winch drum shall be capable of independent operation.
11.13.12.5
The towing winch drum(s) shall have sufficient capacity to stow the required minimum length of the tow wire(s).
11.13.12.6
A spooling device shall be provided such that the tow wire(s) is effectively spooled on to the winch drum(s).
11.13.12.7
The towing winch brake shall be capable of preventing the towing wire from paying out when the vessel is
towing at its maximum bollard pull and shall not release automatically in case of a power failure.
11.13.12.8
The winch shall be fitted with a mechanism for emergency release of the tow wire.
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.
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11.13.12.10
If there is only one towing winch then the crew shall be able to demonstrate that a spare tow wire can be safely
run onto the towing winch within 6 hours of a towline break in bad weather.
11.13.13 Emergency towing gear
11.13.13.1
Emergency towing gear shall be provided in case of towline failure, bridle failure or inability to recover the
bridle. 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:
a. The towing connection should be on or near the centreline of the tow, over a bulkhead or other suitable
strong point
b. Closed fairlead should be provided
c. The emergency pennant should be at least 80 m, with hard eyes or sockets. See Guidance Note.
d. An extension wire to prevent the float line chafing on the stern of the tow should be provided.
e. A float line, to extend 75 m to 90 m abaft the stern of the tow should be provided
f. Conspicuous pick-up buoy, with reflective tape, on the end of the float line should be provided.
Guidance note:
The pennant is preferably in one length. The pennant length can be reduced for small barges and in
benign areas
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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
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of the towage demand it:
a. Heaving lines
b. Line throwing equipment
c. Spare shackles.
11.13.14 Certification and inspection
11.13.14.1
Valid certificates (less than 5 years old) shall be submitted for all towing gear hardware (e.g. chains, wires and
shackles) from the towing winch to the towing connections. Certificates shall be issued or endorsed by bodies
approved by a Recognized Classification Society or other body accepted by the MWS company. For 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.13.14.2
Apart from towing bridles or pennants connected to underwater connections (such as on semi-submersible
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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
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 re-socketed at sea for (temporary) contingency reasons socketing shall only be
done by a certified specialist, approved by a Recognized Classification Society. Renewed means the wire
cropped back to steel that shows no sign of deterioration and the use of either a new socket or one which has
undergone rigorous NDT.
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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 11-16 summarises the required expiry times for the above certificates and inspections shown above.
Table 11-16 Certificate and inspection document requirements
Certificate valid
for
Time since documented inspection by a
competent person (unless new)
Bollard pull
<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
Item
11.13.15 Access to tows
11.13.15.1
Whether a tow is manned or not, suitable access shall be provided. This can include at least one permanent steel
ladder on each side, from main deck to below the waterline.
11.13.15.2
Where practical, ladders should be recessed, back painted for ease of identification, be clear of overhanging
cargo, and faired off to permit access by the tug’s workboat.
11.13.15.3
Alternatives can be accepted if it can be demonstrated that they will provide a safe and reliable means of access
during the towage.
Guidance note:
For example, a pilot ladder on each side or over the stern, secured to prevent it being washed up on deck, can
be accepted for short tows or where it can be deployed from a manned tow.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.13.15.6
A boarding party shall be appropriately equipped such as survival suits, lifejackets and communication
equipment.
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11.13.15.7
Un-manned tows should have lifesaving appliances on board, appropriate to the hazards a boarding party could
experience.
11.13.15.8
Notwithstanding the potential for piracy in some areas, means of boarding shall still be available.
11.13.16 Damage control and emergency equipment
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:
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
q.
r.
Burning gear
Welding equipment
Steel plate - various thicknesses
Steel angle section - various sizes and lengths
Plywood sheets – 25 mm thick
Lengths of 3” x 3” (75 mm x 75 mm) timber
Caulking material
Sand and cement (suitably packaged)
Nails - various sizes
Wooden plugs – various sizes
Wooden wedges – various sizes
A selection of tools, including a hydraulic jack, hammers, saws, crowbars, Tirfors.
Portable coamings 60 cm minimum height, with a flange and boltholes to suit the manhole design. The
top should be constructed to avoid damage to hoses and cables
A sounding tube extension, of 60 cm minimum height, threaded so that it can be screwed into all
sounding plug holes
Sounding tapes
Fire-fighting equipment as appropriate
Personal protection equipment - gloves, goggles, hard hats, survival suits etc.
Emergency lighting.
11.14
Voyage planning
11.14.1 General
11.14.1.1
The following requirements apply to the way in which the towage or voyage shall be conducted. The Certificate
of Approval is based on agreed towage or voyage arrangements, which shall not be deviated 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])
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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 up-current of any platforms or other isolated
obstacles, especially for single tug tows.
11.14.1.7
The required sea room and the basis for its calculation should be included in the 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:
In many cases in rough weather areas and seasons the required sea room can be many hundreds of miles. It may
be impractical to plan a route with adequate sea room and so a staged towage can be required (in which there is
a commitment to seek shelter or jack-up at a stand-by location on receipt of a bad weather forecast). However a
staged towage may be impracticable due to the problems of finding suitable places of shelter or safely
approaching them on a lee shore.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
11.14.2 Sea room
11.14.2.1
Unless a tow’s sea room, for the case in [11.14.2.2] to [11.14.2.5] is greater than both of the following, the tow
should be considered to be “higher risk” and the underwriters informed as in [1.1.6]:
• 75% of the required sea room within 1 day after the end of the reliable good-weather period of the
departure forecast, and
• 100% of the required sea room within 3 days after the end of the reliable good-weather period of the
departure forecast (a reduction can be agreed for short periods during the duration of the tow).
Guidance note:
Adequate sea room is typically defined as the distance that a disabled transport or tow in bad weather can safely
drift, without grounding.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.14.2.2
Case 1 - In bad weather (outside good weather forecast periods): The required sea room is the distance drifted
whilst the significant wave height is greater than 5 m in a storm for that section of the towage including the effect
of any associated currents. The design storm for determining required sea room should be at least the 1 year
return after the end of a good weather forecast.
Guidance note 1:
A method of calculating the sea room is described in [K.9.2].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
A 1 year storm has been selected as many storms will not blow towards the nearest shoals.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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
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or stern to wind.
11.14.2.4
It shall be assumed that the tugs develop negligible effective pull in waves over 5 m significant height, unless it
can be shown that the actual tugs can safely do so without overloading their deployed towing gear in the
relevant water depths.
Guidance note:
Most tugs cannot develop significant bollard pull without overloading their towing gear in weather much more
than BF 8 (typically 20 m/s wind, 5 m sig wave height) since tugs should normally be in “survival mode” if caught
in these conditions.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
These results can also be used to validate towline dynamic simulations to extrapolate the results for other
conditions.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.14.2.6
Case 2 - When approaching a potential lee shore (with a good weather forecast): The required sea room is the
distance drifted in the worst acceptable forecast conditions during the time taken to replace and reconnect a
broken towline and/or disabled tug.
Guidance note:
See Figure K-12 for an example of sea room requirements against time taken to reconnect for a range of
weather conditions.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.14.2.7
The acceptable forecast conditions should be included within the towage procedures/manual for a particular
case. These shall be determined by applying the appropriate Alpha factor to the theoretical limit to allow for
uncertainties in the forecast.
11.14.2.8
Approaching a potential lee shore should only be attempted without a good weather forecast if there is no
practicable alternative in an emergency situation.
Guidance note:
An additional (connected) tug can be used to guard against a towline breakage or disabling of a single tug when
approaching or leaving a lee shore.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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.
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11.14.3 Routeing and piracy
11.14.3.1
Routeing procedures shall be agreed with the Master before the start of the voyage, taking into account:
•
•
•
•
•
the transport vessel or tug’s capacity,
fuel consumption,
the weather and current conditions,
normal good navigation and seamanship and
where possible avoiding the potential for piracy.
11.14.3.2
Anti-piracy procedures shall be included in the towing/transport manual/procedures unless there is a low risk of
piracy.
11.14.3.3
Where anti-piracy measures are warrantable the requirements should be advised to the MWS.
Guidance note 1:
Piracy is prevalent in many areas and these vary with time. Guidance on mitigation of piracy can be found in:
a. “BMP4 - Best Management Practices for Protection against Somalia Based Piracy” (or later version). This
can be downloaded from websites of sponsoring organisations including
http://www.intertanko.com/Topics/Security/Security-/BMP4-forProtection-against-Somalia-Based-Piracy/
(http://www.intertanko.com/Topics/Security/Security-/BMP4-forProtection-against-Somalia-Based-Piracy/)
While this guidance was created to address the Somalian situation, this document’s good practice should
be considered for any high risk area.
b. Website http://www.lmalloyds.com/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.
c. 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”)
d. IMB Piracy Reporting Centre website http://www.icc-ccs.org/piracy-reporting-centre (http://www.iccccs.org/piracy-reporting-centre)
e. Flag-state, vessel insurance and P&I club requirements. The MWS company should be advised by the
insured at an early stage if there are any relevant insurance warranty requirements.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Key mitigations of piracy include:
a. Awareness of the sea areas and ports affected by piracy and armed robbery and, at the very least,
ensuring proper all round vessel lookouts are in place and maintained, using every means possible, while
in these areas. Previous incidents of piracy clearly demonstrate that slow vessels (typically less than about
18 knots) or tows, especially with low freeboards (typically less than about 8 m) or easy access over the
side or stern, are particularly at risk.
b. Maintaining sufficient distance from land throughout the voyage can help to reduce this risk and also
ensures there is sufficient sea room in case of emergency. However, given that attacks now regularly occur
many miles from the coastline (up to 1,500 nautical miles), it is essential vessels considering transiting
these areas prepare well in advance for the possibility of an attack.
c. Careful consideration by Masters of their route and the risks and implementation at an early stage all
necessary measures to reduce the likelihood of their vessel becoming a target. A full route analysis should
be conducted taking into account previously reported incidents of piracy, as part of the passage planning
process.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
11.14.4 Weather routeing and forecasting
11.14.4.1
Staged voyages shall have a commitment to seek shelter (or jack-up at a stand-by location) on receipt of a
weather forecast in excess of the operational limiting criteria incorporating an Alpha factor. A staged voyage
shall have sufficient suitable ports of shelter (or stand-by locations) along the route.
11.14.4.2
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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:
These are particularly important for weather restricted voyages (either staged or weather routed) for which the
strength or stability will not meet the weather unrestricted environmental criteria.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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 2-16.
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 11-17 for the first 24 hours of the towage.
Table 11-17 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
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forecast before reducing their sea room requirements.
11.14.5 Departure
11.14.5.1
Before departure, a departure condition report for the tow or vessel shall be submitted by the owners or their
agents, to the Master and the MWS company surveyor. This report should contain as a minimum:
•
•
•
•
•
•
•
•
•
The documentation referred to in Table B-2 as appropriate
Lightship weight
Tabulation and distribution of ballast, consumables, and cargo, including any hazardous materials
Calculated displacement and draughts
Actual draughts and displacement
A statement that the longitudinal bending and shear force are within the allowable seagoing limits
Calculated VCG
Calculated GM and confirmation that it is within allowable limits
GZ Curve and confirmation that it is within allowable limits.
11.14.5.2
In the departure condition, the tow shall have acceptable stability with proper allowance made for any slack
tanks.
11.14.5.3
If no stability documentation is available then it can be necessary to perform an inclining test to check that the
GM is satisfactory. Calculations can be needed to establish righting and overturning lever curves.
11.14.5.4
It shall be verified that the tow floats in a proper upright attitude and at a draught and trim appropriate to the
calculated weight and centre of gravity.
11.14.5.5
The Certificate of Approval shall be issued on agreed readiness for departure and receipt of a suitable weather
forecast.
11.14.6 Ports of shelter, shelter areas, holding areas
11.14.6.1
Ports of shelter, or shelter areas on or adjacent to the route, with available safe berths, mooring or holding areas,
shall be agreed before departure and all necessary permissions obtained.
11.14.6.2
Where such shelter points are required as part of a weather restricted operation, as described in [2.6.7], they
shall be capable of entry in worsening weather.
11.14.7 Bunkering
11.14.7.1
Bunkering ports, if required, shall be agreed before departure.
11.14.7.2
If it is not practical to take the tow into port, then alternative arrangements shall be agreed and included in the
approved towage procedures. Unless agreed otherwise the requirements of [11.12.2], shall apply at all times:
Guidance note:
Possible alternative arrangements include:
• Where the towage is by more than one tug, each tug in turn can be released to proceed to a nearby port
for bunkers, subject to a favourable weather forecast. The remaining tug(s) should meet the requirements
of [11.12.2], or some other agreed criterion.
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• 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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11.14.8 Assisting 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.
11.14.9 Pilotage
11.14.9.1
The Master shall engage local pilotage assistance during the towage or voyage, as appropriate.
11.14.10 Log
11.14.10.1
A detailed log of events shall be maintained during the towage or voyage.
11.14.11 Inspections during the towage or voyage
11.14.11.1
Unless the tow is manned, it should be boarded on a regular basis by the crew of the tug, particularly after a
period of bad weather, in order to verify that all the towing arrangements, condition of the cargo, seafastenings
and watertight integrity of the tow are satisfactory. Suitable access shall be provided - see [11.13.15].
11.14.11.2
For manned tows, and self-propelled vessels, the above inspections should be carried out on a daily basis as
relevant - see also [11.17.5].
11.14.11.3
Any adjustable seafastenings or lashings shall be re-tensioned as necessary.
11.14.12 Reducing excessive movement and shipping water
11.14.12.1
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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11.14.13 Notification 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.
11.14.14 Diversions
11.14.14.1
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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.
11.14.15 Responsibility
11.14.15.1
The towmaster is responsible for the overall conduct of a tow, and towing arrangements during the towage.
Similarly the master of the transport vessel is responsible for the overall conduct of the voyage. Nothing in this
document shall set aside or limit the authority of the Master who remains solely responsible for 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.
11.14.16 Tug 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.
11.14.17 Hazardous materials
11.14.17.1
The carriage of hazardous materials should be avoided, unless it can be shown that the materials are effectively
controlled. For un-manned voyages, hazardous materials should be stowed accounting for the limited remedial
actions available in the case of inadvertent release.
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.
11.14.18 Ballast water
11.14.18.1
Voyage planning shall account for any need to change ballast water, including all local laws, before or at the
arrival port.
Guidance note:
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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 de-ballasting 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.
11.14.19 Restricted depths, heights and manoeuvrability
11.14.19.1
The clearance requirements for each towage should be assessed, taking into account
•
•
•
•
environmental conditions,
length of areas of restricted manoeuvrability,
any course changes within the areas of restricted manoeuvrability,
cross section of areas of restricted manoeuvrability in relation to underwater area/shape of the base
structure, and
• capability of the tugs.
11.14.19.2
The clearances in [11.14.20] to [11.14.21] are the generally acceptable minimum values. Any reduction of these
shall be agreed with MWS company at an early stage and it shall be proven that the reduced values give an
acceptable level of risk.
11.14.19.3
Calculation of clearances shall account for the effects of
•
•
•
•
•
•
•
•
•
•
•
•
•
Roll, pitch and initial heel and trim,
heave,
tow-line pull,
inclination due to wind,
tolerance on bathymetry (which can be over 10 m on old surveys)
changes in draught of the transport vessel or towed object,
differences in water density,
tidal height variations,
squat effects,
deflections of the structure
errors in measurement
surge (negative for under-keel clearances and positive for air draught), and
any protrusions below the bottom of the asset.
11.14.19.4
For areas where the under-keel or side clearance is critical, a survey that is not older than 3 months should be
documented.
11.14.19.5
Where the survey report in [11.14.19.4] is not available, the tow route shall be surveyed with a width of 5 times
the beam, with a minimum of 500 m. Side-scan sonar and bathymetric data should be documented. The
equipment used shall be of a recognized industry standard. The spacing between depth contour lines should be
appropriate for the purpose. Current surveys should be made in restricted parts of the tow route.
11.14.19.6
The survey requirements can be relaxed if it can be shown that the on board bathymetry measurement systems
and position management systems have sufficiently high precision.
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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).
11.14.20 Under-keel clearances
11.14.20.1
The under-keel 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 under-keel clearance can be
reduced in very benign conditions after agreement with the MWS company.
11.14.20.2
Under-keel clearances for departure from dry-docks or building basins are covered in [12.6].
11.14.20.3
If sections of the passage are tidally dependent, safe holding areas should be identified in the vicinity with
adequate sea room and water depth to maintain the minimum under-keel clearance at low tide. Any delay time
waiting for the tide shall be included in the overall planning.
11.14.20.4
Immediately before critical sections of the passage the tidal level shall be confirmed by measurement.
11.14.20.5
Where an air cushion is used to reduce draught then the following shall be considered:
a. Any conceivable loss of air not increasing the draught by more than the reserve on under-keel clearance,
and
b. The recommendations contained in [12.6.2] on air cushions.
Guidance note:
Use of air cushions is generally only acceptable to reduce draught to assist in crossing localised areas of
restricted water depth.
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11.14.21 Air draught
11.14.21.1
When passing under obstructions, the overhead clearance shall be calculated accounting for the items listed in
[11.14.19.3] excluding squat and shall be greater than one metre plus dimensional tolerances.
11.14.21.2
Where clearance is limited then a dimensional survey of the barge/vessel and structure shall take place just
before sailaway in order to ensure that the required clearance exists.
11.14.21.3
For power cables the minimum allowable clearance shall be specified by the transmission company (but not be
less than 1 m) and be measured to the lowest possible catenary position.
Guidance note 1:
Power cables need a 'spark gap', as well as a physical clearance.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
The catenary of the power cable will change depending on the electrical load being carried in the cable.
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11.14.21.4
The actual clearance shall be confirmed with all appropriate authorities including those responsible for the
obstruction.
11.14.21.5
Immediately before the passage the tidal level shall be confirmed by measurement unless the calculated
overhead clearance from [11.14.21.1] is greater than two metres plus dimensional tolerances at HAT (Highest
Astronomical Tide).
11.14.22 Channel width and restricted manoeuvrability
11.14.22.1
The minimum channel width along any inshore legs of the tow route with the under-keel clearance and air
draught required in [11.14.20] and [11.14.21] should be three times the maximum width of the towed object plus
allowances for yaw and sway. Additional channel width can be required
• in exposed areas
• if there are significant cross currents
• for tugs on either side to assist in manoeuvring if required.
11.14.22.2
Narrower channels may be agreed with the MWS company on a cases by case basis for ideal conditions (e.g.
sheltered straight short channels and no tight time restrictions).
11.14.22.3
Side clearances for departure from dry-docks or building basins are covered in [12.7].
11.15
Bilge & ballast pumping systems
11.15.1 Pumping arrangements – General
11.15.1.1
For classed vessels, the drainage system and (bilge) pumps should as a minimum comply with the Rules of the
Classification Society.
11.15.1.2
Tugs towing outside coastal limits shall also comply with [11.12.10].
11.15.1.3
The general requirements in [4.2] shall be applied as applicable.
11.15.2 Pumping arrangements - Emergency
11.15.2.1
Emergency pumping arrangements shall be installed and operable on for any vessel to deal with any leakage
after collision, grounding, structural failure or other accident. The requirements in [11.15.3] apply to:
a. Towed vessel or barge
b. Unclassed vessels or those operating outside their conditions of class.
11.15.2.2
For other wet tows, the need for and specifications of the emergency pumping systems shall be determined by a
risk assesment in accordance with [2.4] considering the requirements in [11.15.3].
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Guidance note:
The risk assessment would also normally consider the details of the towage and the extent and availability of any
installed system.
Examples of other tows are self-floating objects, MOU’s, FSU’s and disabled ships,
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11.15.2.3
Some relaxation can be possible, as agreed with the MWS company, on a case-by-case basis, for a towage
considered as a weather restricted operation.
11.15.2.4
Whether or not a tow is manned, the emergency pumping system shall be available at short notice and deliver
pumping times and capacities shown in [11.15.3.5] to [11.15.3.7].
Guidance note:
For an un-manned tow, short notice is considered to start after boarding (which could be by helicopter).
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11.15.2.5
Where a tow is not manned, then the tug master and chief engineer shall be aware of the available pumping
system. Members of the tug crew shall be familiar with the systems, and be able to board the tow and run the
pumps at short notice. Procedures for pumping shall be known and available, including any restrictions arising
from considerations of stability or hull stresses, and any vents, which shall be opened before pumping starts.
11.15.3 Pumping system requirements - Emergency
11.15.3.1
Vessels should have one of the following systems to meet the capacity requirements of [11.15.3.5] to [11.15.3.7],
able to pump into and from all critical spaces (as defined in [11.15.3.2]) in order of preference:
• Two independent pump rooms or one protected pump room, as described in [11.15.3.3] and [11.15.3.4]
• An unprotected pump room with an independent emergency system that can pump out the pump room
• A system of portable pumps.
11.15.3.2
A critical space is defined as any tank or compartment which:
1. when flooded or emptied, at any stage of the voyage, can lead to:
◦ non-compliance with intact or damage stability criteria, or
◦ non-compliance with structural load limits, or
◦ heeling or trimming that can prevent the tow from continuing its passage safely and free from
obstructions in shallow water, or
◦ maximum allowable transit draught being exceeded.
2. can be required for ballasting/de-ballasting so that the barge or vessel can safely continue its passage
after any single compartment is damaged.
11.15.3.3
Independent pump rooms should have separate power supply, pumps, control and access. Each pump room
should be able to work into all spaces.
11.15.3.4
To be considered protected, a pump room, and any compartment required for access, should be separated from
the bottom plating by a watertight double bottom not less than 0.60 m deep and from the outer shell by other
compartments or cofferdams not less than 1.5 m wide.
11.15.3.5
The total capacity of the fixed and portable pumps should be such that any one wing space (or other critical
space as defined in [11.15.3.2]) can be emptied or filled in 4 hours for an un-manned tow, or 12 hours for a
manned tow. Any time required for connection or warm-up should be included in the pumping times shown.
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11.15.3.6
Except where there is a protected pump room, at least two pumps shall be provided.
11.15.3.7
Whatever type of pumps are fitted or supplied, sufficient fuel shall be carried for at least 72 hours continuous
operation.
11.15.3.8
If portable pumps are used then either they should be portable enough to be moved around the vessel (and
cargo) by two men, or enough pumping equipment should be carried so that any critical compartment can be
reached.
11.15.3.9
Each portable pump should be able to pump out from the deepest space (with portable coaming installed).
Portable submersible pumps shall be able to fit through tank manholes.
Guidance note:
This requires submersible pumps for vessels over about 6 m depth, due to suction head.
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11.15.3.10
Any compressed air system should have a compressor on board and available, connected into the permanent
lines.
Guidance note:
The use of a vessel compressed air system may not be practicable for all these or emergency cases, especially if
manhole covers have been removed, or the vessel is holed above the waterline.
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11.15.4 Pumping arrangements – Non-emergency
11.15.4.1
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):
a.
b.
c.
d.
e.
f.
g.
h.
i.
Ballasting before, during and after load-outs
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
De-ballasting after accidental grounding
Access to flooded compartment (e.g. pump or anchor winch room).
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11.15.5 Watertight manholes
11.15.5.1
If manholes to critical compartments are covered by cargo then either alternative manholes should be fitted, or
cutting gear should be installed and positions marked for making access. Welding gear and materials shall be
carried to restore watertight integrity.
11.15.5.2
Where the vessel is classed, the owner should inform the classification society in good time of any holes to be cut
or any structural alterations to be made.
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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)].
11.15.6 Sounding systems
11.15.6.1
Sounding of and pumping into or from critical spaces (as defined in [11.15.3.2]) in severe weather should be
feasible. The following shall be provided on all critical spaces:
•
•
•
•
Pumping system
Watertight manholes
Portable coamings
Sounding plugs, extensions and tapes or rods. An additional remote sounding system can be needed for
compressed air ballasting systems
• Vents to all compartments.
11.15.6.2
For vessels or barges with compressed air ballast systems, gauges shall be provided in lieu of sounding pipes.
11.15.6.3
A sounding plug shall be installed in each compartment (in manhole covers if necessary) to avoid removing
manhole covers. Sounding tapes and chalk shall be carried on board the tow.
11.15.6.4
For spaces that will be sounded regularly, a tube and striker plate should be available.
11.15.7 Vents
11.15.7.1
All compartments connected to a pumping system shall have vents fitted. The vents should be of an approved,
automatic, self-closing type. If not automatic, then the vents should be sealed for towage with wooden bungs or
steel blanks, but with a 6 mm diameter breather hole fitted.
Guidance note:
This will give audible warning or reduce pressure differentials in event of mishap, and compensate for
temperature changes. The breather hole can be drilled into the gooseneck of the vent or through the wooden
bung used to close the vent.
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11.16
Anchors (and alternatives) and mooring arrangements
11.16.1 Emergency anchors
11.16.1.1
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Emergency anchors have traditionally been required to reduce the risk of a tow running aground if a tug is
disabled or a towline broken. However in many cases the disadvantages (described in [K.5]) associated with
using such anchors can outweigh the advantages.
11.16.1.2
If a tow passes through an area of restricted sea room, a comparative risk assessment should be performed to
determine the preferred arrangements. [K.5] sets out topics to be taken into account in this risk assessment. One
possible outcome can be the provision of suitably sized extra tugs for some sectors of the tow.
11.16.1.3
The same requirements apply for towed ships, including demolition towages. See [11.23.3]. Where such
towages may need to wait for a few days on arrival at the end of a voyage before documentation is completed
then, if this is in a high-current area, anchoring or mooring arrangements can be required.
11.16.1.4
For jack-up platforms, see also [11.27.16].
11.16.2 Size and type of anchor
11.16.2.1
For classed vessels and barges, the anchor(s) fitted in accordance with Class requirements should be acceptable
unless there is deck cargo.
Guidance note:
For open deck vessels and barges, the anchor is designed to hold the vessel or barge only and does not account
for deck cargo windage.
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11.16.2.2
In other cases the minimum weight of the emergency anchor should be 1/10 of the towline pull required (TPR)
for the tow, as defined in [11.12.2]. A high holding power anchor with anti-roll stabilisation should be used.
11.16.3 Anchor cable length
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.
11.16.4 Anchor cable strength
11.16.4.1
For cable on a winch, or capstan, which can be paid out under control, the MBL of the cable should be 15 times
the weight of the anchor, or 1.5 times the holding power of the anchor if greater.
11.16.4.2
For cable flaked out on deck, the MBL of the cable should be 20 times the weight of the anchor, or twice the
holding power if greater.
Guidance note:
The increase from the requirements in [11.16.4.1] is to allow for the extra shock load.
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11.16.4.3
The last few flakes of cable on deck should have lashings that will break and slow down the cable before it is fully
paid out.
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11.16.5 Attachment of cable
11.16.5.1
The inboard end of the cable should be led through a capped fairlead near the vessel centre line and be
securely fixed to the vessel. Precautions should be taken to minimise chafe of the cable.
11.16.5.2
The MBL or ULC of the connections of the cable to padeye or winch, and padeye or winch to the vessel structure
should be greater than that of the cable.
11.16.5.3
For towed ships, and tows with similar arrangements, the anchor cable(s) shall be properly secured, with the
windlass brake(s) applied. Any additional chain stopper arrangements that are fitted shall be utilised, or
alternatively, removable preventer wires should be deployed.
11.16.5.4
Spurling pipes into chain lockers should be made watertight with cement plugs, or another satisfactory method.
11.16.6 Anchor mounting and release
11.16.6.1
If there is no suitable permanent anchor housing the anchor should be mounted on a billboard, as shown in
[K.4], at about 60° to the horizontal.
11.16.6.2
The anchor should be held on the billboard in stops to prevent lateral and upwards movement. It should be
secured by wire rope and/or chain strops that can be easily released manually without endangering the
operator.
11.16.6.3
The billboard should normally be mounted on the stern. It should be positioned such that on release the anchor
will drop clear of the vessel and the cable will pay out without fouling.
11.16.6.4
For any system, it shall be possible to release the anchor safely, without the use of power to release pawls or dog
securing devices. If the anchor is held only on a brake, an additional manual quick release fastening should be
fitted.
11.16.6.5
The anchor arrangement should be capable of release by one person. Adequate access shall be made available.
11.16.7 Mooring arrangements
11.16.7.1
All vessels and floating objects should be provided with at least four mooring positions (bollards/staghorns etc.)
on each side of the vessel unless it is impracticable to moor them, e.g. because of draught limitations.
11.16.7.2
If fairleads to the bollards are not installed then the bollards should be provided with capping bars, horns, or
head plate to retain the mooring lines at high angles of pull. Suitable chafe protection should be fitted as
required e.g. to the deck edge for low angles of pull.
11.16.7.3
At least four mooring ropes in good condition of adequate strength and length should be provided.
Guidance note:
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Typically the mooring ropes are about 50 mm to 75 mm diameter polypropylene or nylon, and each 60 m to
90 m long.
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11.16.7.4
Mooring ropes should be stowed in a protected but accessible position.
11.16.7.5
Objects with very large freeboard such as FSUs should be fitted with mooring and towing connection points
along the side, at a convenient height above the towage waterline. The connection points should not damage, or
be damaged by, attending vessels.
Guidance note:
These can provide a more convenient connection for mooring lines and harbour tugs than bollards at deck level.
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11.17
Manned voyages
11.17.1 General
11.17.1.1
Manning of tows should generally be limited to those where early intervention by a riding crew can be shown to
reduce the risks to the tow, for example tows of MOU’s, passenger ships and Ro-Ro vessels.
11.17.1.2
Where a riding crew is carried on a tow for commissioning and/or maintenance, sufficient marine personnel shall
be included to operate the equipment listed in [11.17.4] and to carry out the duties in [11.17.5].
Guidance note:
A riding crew can be carried on an FPSO or FSU for similar reasons.
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11.17.1.3
Riding crew carried on any dry transport shall be within the carrying vessel’s Flag State limits for life saving
appliances; any exceedance of the Flag State limit shall be approved by the Flag State in advance.
Guidance note:
There is sometimes a requirement for a riding crew on a dry transport to maintain or commission systems or to
carry out general maintenance.
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11.17.1.4
The transport contractor shall provide the MWS company documented flag state approval for the proposed
number of riding crew. The underwriters should also be informed if a large riding crew is proposed.
Guidance note:
The transport contractor should therefore obtain this Flag State approval in good time.
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11.17.1.5
The health and safety of the riding crew shall be ensured at all times.
11.17.1.6
A risk assessment shall be carried out, in accordance with [2.4], to demonstrate the acceptability of the proposed
arrangements.
11.17.2 International regulations
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11.17.2.1
Accommodation, consumables, lifesaving appliances, pumping arrangements and communication facilities with
the tug shall comply with International Regulations.
11.17.3 Riding crew carried on the cargo
11.17.3.1
Where a riding crew is carried on the cargo, for instance a maintenance crew on a dry-transported jack-up rig,
additional precautions shall be considered including:
• Access to/from the cargo/rig forward and aft, and to the evacuation or escape area(s)
• The cargo/rig’s life rafts and lifeboats should be relocated and the falls lengthened, if necessary, so that on
launching they will land in the water.
• A firewater supply should be made available to the cargo/rig.
• The cargo/rig’s and vessel’s alarm systems should be linked, so that an alarm on the cargo/rig is repeated
on the vessel, and vice versa.
11.17.4 Safety and emergency equipment
11.17.4.1
Notwithstanding the requirements of SOLAS, /92/, and any or all international regulations for Life Saving
Appliances and Fire-Fighting Equipment, the minimum complement of safety and emergency equipment carried
aboard the tow shall be as follows:
• Certified life rafts located on each side of the tow, clear of any possible wave action, provided with means
of launching and fitted with hydrostatic releases. The life raft or life rafts on each side of the tow shall be
capable of taking the full crew complement. Adequate means of access to the water shall be provided
• 4 lifebuoys, two located on each side of the tow and including two fitted with self-igniting lights and two
with a buoyant line
• Approved life jackets to be provided for each crew member plus 25% reserve
• If appropriate, a survival suit to be provided for each crew member
• First aid kit
• Fire-fighting equipment, which can consist of an independently powered fire pump with adequate hoses,
and portable fire extinguishers as appropriate.
• 6 parachute distress rockets and 6 hand held flares
• A daylight signalling lamp and battery
• 2 portable VHF radios, fitted with all marine VHF channels, with appropriate battery charging equipment
• Hand held GPS (Global Positioning System) receiver
• 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.
11.17.5 Manned 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.
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11.18
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Specific for multiple towages
11.18.1 Definitions
11.18.1.1
This section expands on the definitions in Table 1-3 for multiple towages:
11.18.1.2
Double tow – 2 tows each connected to the same tug with separate towlines. One towline is of sufficient length
that the catenary to the second vessel is below that of the first.
11.18.1.3
Tandem tow – 2 (or more) tows in series behind 1 tug, i.e. the second and following tows connected to the stern
of the previous one.
11.18.1.4
Bifurcated tow - the method of towing 2 (or more) tows, using one tow wire, where the second (or subsequent)
tow(s) is connected to a point on the tow wire ahead of the preceding tow, and with each subsequent towing
pennant passing beneath the preceding tow.
11.18.1.5
Two tugs (in series) towing one tow – where there is only 1 towline connected to the tow and the leading tug is
connected to the bow of the second tug.
11.18.1.6
More than 1 tug (in parallel) towing one tow – each tug connected by its own towline, pennant or bridle to the
tow.
Figure 11-10 Multiple towage types (not to scale)
11.18.2 General
11.18.2.1
Compared with single towages, multiple towages have additional associated problems including those of:
• Manoeuvring in close quarter situations, especially at the start and end of a tow.
• Reconnecting the towlines after a breakage
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• Maintaining sufficient water depth for the longer and deeper catenaries required.
11.18.2.2
With the exception of the cases described in [11.18.1.6], multiple towages can only be approvable in certain
configurations, areas and seasons, and subject to a risk assessment.
11.18.2.3
When approval is sought, then full details of the operation, including detailed drawings, procedures and
equipment specifications shall be documented. An initial assessment of the method will then be made, and if the
basic philosophy is sound, recommendations can be made for the approval process to continue.
11.18.2.4
Approval can be declined if any doubt exists as to the viability of the operation proposed.
11.18.2.5
For those multiple towages that are approvable, each tow shall be prepared as described in this standard.
11.18.2.6
Additional factors can be applied to the towing arrangements, so that the probability of breakage is further
reduced.
11.18.2.7
The bollard pull requirement of the tug shall be according to the number and configuration of the tows
connected. The Towline Pull Required (TPR) should be the sum of those required for each tow. The towing
arrangements on each tow shall have sufficient capacity for the Bollard Pull (BP) of the tug(s).
11.18.2.8
The tug shall be equipped as in [11.13], although additional or stronger equipment and longer towlines can be
necessary. Where longer towlines are required, these can be formed by the utilisation of pennant wires of no less
Ultimate Load Capacity than the main tow wires.
11.18.2.9
Where the towing configuration requires the use of 2 towlines from 1 tug, a third tow wire shall be carried on
board the tug, stowed in a protected position, whence it can be safely transferred at sea to either towing winch.
11.18.2.10
Consideration should be given to including (surge) chain or a stretcher to improve the spring, or to provide the
required catenary in any towing arrangement.
11.18.2.11
If a synthetic stretcher is included in any towing arrangement, it shall comply with [11.13.10]. A spare stretcher
shall be carried aboard the tug for each stretcher utilised in the towing arrangement.
11.18.2.12
For multiple tows being towed behind a single tug, special arrangements shall be made on the deck of the tug
to separate the towlines.
Guidance note:
This requirement is because the tows can yaw in different directions.
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11.18.2.13
Special procedures shall be agreed and included in the towing manual for reconnection.
Guidance note:
It is particularly difficult to reconnect to a tow that has broken loose when another tow or tows are connected to
the same tug.
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11.18.2.14
Due to the difficulties that will be encountered if a towline breakage should occur, the number of crew on the tug
should be increased over that required for a single tow.
11.18.3 Double tows
11.18.3.1
These should only be considered when:
a. the area is benign
b. the towage duration is short and covered by good weather forecasts
c. Where there is sufficient water depth along the tow route to allow for the catenary required for the second
tow.
11.18.3.2
The tug should be connected to each tow with a separate towline on a separate winch drum. It shall also carry a
spare towline, stowed on a winch, or capable of being spooled onto a winch at sea.
11.18.4 Tandem tows
11.18.4.1
These should only be considered when in very benign areas or in ice conditions where the towed barges will
follow each other.
11.18.4.2
In ice conditions the towlines between tug and lead tow and between tows will normally be short enough for the
line to be clear of the water. Procedures shall be in place to avoid tows over-running each other, or the tug.
11.18.5 Bifurcated tow
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.
11.18.6 Two tugs (in series) towing one tow
11.18.6.1
The first tug should be smaller and connected to the bow of a larger, less manoeuvrable second tug.
Guidance note:
This arrangement is used to improve steering/manoeuvring.
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11.18.6.2
This configuration should only be considered when:
a. 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
b. The second tug is significantly heavier than the leading tug (to avoid girding the second tug).
11.18.7 Multiple tugs to one tow
11.18.7.1
Each tug should have a separate towline to the vessel (via bridles or pennants as required).
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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 three-tug
spread blacks out and can be over-ridden by the tow with catastrophic consequences. Suitable emergency
procedures and tow equipment shall be available to mitigate such a possibility.
11.19
Specific for towing in ice
11.19.1 General
11.19.1.1
This 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 ice-covered
waters.
11.19.1.2
It is recognized that towing in ice-covered water is a unique marine operation and that all vessels and towages in
ice are different - making this standard general in nature. Each approval will depend on the result of an in-depth
review of the towing manual as well as an equipment inspection/attendance by a MWS company surveyor to
identify any particular problems that can exist for the specific vessel(s) and towage in question.
11.19.1.3
Structural safety and towing performance will require careful consideration of the size and shape of the tow,
especially with respect to the beam of the tow in comparison to the beam of the tug and the shape of the bow of
the tow. The beam difference will affect the level of ice protection provided by the tug to the tow, as well as the
ice interaction and towing resistance caused when the beam of the tow is greater than that of the tug and/or of
any independent icebreaker support. In addition, special towing techniques used in ice and manoeuvrability
restrictions caused by the ice require that experienced personnel plan and execute the tow.
11.19.1.4
Except as allowed by [11.9.14], any vessel that is operated and/or towed in ice shall be in Class with a
Recognized Classification Society and have a current Load Line Certificate.
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11.19.1.5
After complying with the requirements of [11.19.1.1] to [11.19.1.4], the MWS company can deem that the
vessel/object is unfit for tow and decline to issue a Certificate of Approval. For example, the towage of any tow
which is damaged below the waterline, is suspected of being damaged below the waterline or has suffered other
damage or deterioration which could affect the structural strength and/or watertight integrity will not be
approved for towage in ice. Alternatively, the vessel/object can only be considered fit for tow after specified
repairs and suitable ice strengthening has been carried out.
11.19.2 Vessel ice classification
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
11.19.3 Towage without independent icebreaker escort
11.19.3.1
Where no independent icebreaker escort is identified in the towing manual for the intended voyage, the tug and
tow shall be of appropriate ice classification and power to maintain continuous headway in the anticipated ice
conditions. When a tow is anticipated to take more than three (3) days (the maximum for a reasonably accurate
weather/ice forecast) or longer in ice conditions that includes a concentration of five (5) tenths or more of limiting
ice types, the towing manual shall indicate the location of the nearest icebreaker support and the anticipated
time before independent icebreaker assistance (Coast Guard or Commercial) can be provided.
11.19.3.2
With the exception of a vessel pushed ahead (push-towed), the ice classification requirement for the towed
object can be considered for reduction if it is determined that the tug has a higher than necessary level of ice
classification and can protect the tow from potentially damaging ice interaction.
11.19.4 Conventional ice towing operations
11.19.4.1
The tug shall have sufficient power and hull strength (ice classification) to be capable of safely maintaining
continuous towing headway through the worst anticipated ice conditions including, if necessary, the breaking of
large diameter floes and deformed ice with no requirement for ramming.
11.19.4.2
The towing manual shall show that the towage should not be subjected to ice pressure.
11.19.5 Close-couple ice towing operations
11.19.5.1
Close-couple towing is an operation that allows a specially designed icebreaker to combine towing and
icebreaking assistance. The stern of the icebreaker has a heavily fendered ‘notch’ into which the bow of a ship is
pulled by the icebreaker’s towline. The towline remains attached and the icebreaker steams ahead, usually with
additional power provided by the towed vessel in the notch. In this way an icebreaker can tow a low-powered
and low ice classed ship quickly (up to 3 times faster than conventional towing in ice) and safely (better
protection of the towed vessel and less risk of collision due to over-running) through high concentrations of
difficult ice. For close-couple towages:
• The beam of the icebreaker shall be more than that of the towed ship in order to avoid shoulder damage
to the towed vessel and excessive towline stress and:
• The icebreaker is fitted with a constant tension winch or equipment that will reduce the effects of shockloading:
• The bow of the towed ship shall be compatible with the notch design of the icebreaker. Preferably the
entrance of the towed ship is not so sharp as to apply excessive force on the stem when going straight
ahead. Freedom of movement of the towed ship’s bow can cause manoeuvring difficulties as well as
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applying heavy side forces on the towed ship’s bow when turning. The bow should not be so bluff that all
the force is concentrated in localized areas. In addition the towed ship cannot have a bulbous bow
because the underwater protrusion could damage the icebreakers propellers and:
• The displacement and freeboard of the towed vessel should not be so disproportionate with that of the
icebreaker that the manoeuvring characteristics of the icebreaker are seriously compromised:
• The anticipated ice conditions should not require ramming or passage through areas where high levels of
ice pressure can be experienced without independent icebreaker assistance.
11.19.6 Push-tow operations
11.19.6.1
Push-Tow operations should be carried out using either rigid connection (composite unit) or flexible connections
(a push-knee erected at the stern of the pushed vessel). Where the design and ice strength of the tug and tow is
acceptable, especially when experiencing ice pressure, consideration should be given to pushing rather than
towing in ice.
Guidance note:
Pushing enables headway to be maintained and to remove the stress from the towline. Push towing can also be
more efficient.
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11.19.6.2
Where the push-towing technique is to be used, the pushed vessel shall have acceptable ice strengthening,
particularly in the bow and shoulder areas.
11.19.6.3
The ice classification of a tug that is engaged in a ‘push-tow’ operation with no independent icebreaker support
can be reduced if:
• the vessel being pushed has appropriate ice classification and strength for unescorted transit in the
anticipated ice conditions and:
• the beam of the pushed vessel is greater than that of the tug. The beam of the pushed vessel should be at
least one third greater than that of the tug to allow suitable manoeuvring for a flexible connection and:
• the connection between tug and tow is of suitable strength for emergency stops and:
• the towing manual shows that the ‘push-tow’ will not enter, or be exposed to, an area where ice pressure
can be encountered of sufficient severity to stop the continuous forward progress of the push-tow without
independent icebreaker assistance.
11.19.7 Towage operations with independent icebreaker escort
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.
11.19.8 Manning
11.19.8.1
In addition to [11.12.13] concerning manning, special consideration should be given to the number, qualification
and experience of personnel required on the navigating bridge to ensure safe navigation including steering and
engine control, lookout, operation of searchlights and, emergency operation of the towing winch abort system.
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11.19.8.2
The master in charge of a tow (tow-master) should typically have at least 3 years’ experience of towing in ice
conditions similar to those anticipated for the proposed towage. Other navigating officers on tugs involved in a
towage in ice should also have previous experience of towages in ice.
11.19.9 Multiple towages
11.19.9.1
Multiple towages in ice are subject to the requirements set out in this 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 in-depth risk
assessment would be required and the risks shown to be acceptable.
11.19.9.2
In addition to the provisions in [11.19.9.1] for towages using more than one tug or multiple tows:
1. To avoid collision or over-running each tug shall have a quick release and re-set system as described in
[11.19.11.1] and [11.19.11.2].
2. The most experienced tug Master shall be designated as the tow-master and give directions to the other
vessels. All other tug Masters and senior navigating officers involved in the multi-tug towage should have
an appropriate level of experience of towing in ice and be familiar with the associated difficulties and
hazards.
3. A multi-tug towing manual that does not include independent icebreaker escort assistance shall
demonstrate clearly why it is not considered necessary. As an acceptable example, the tow could be
configured such that one or more tugs with the capability to perform ice management (escort duties) can
be released, and the remaining tug(s) have sufficient BP to continue making towing progress. In some
circumstances a towing manual can include the contingency of releasing one or more tugs that are towing
in the conventional manner to push-tow provided that:
◦ the towed vessel is appropriately ice strengthened:
◦ the towed vessel is appropriately designed and strengthened in the pushing location(s):
◦ the tugs are designed and adequately fendered for pushing:
◦ such action would only be considered in a high ice concentration where there is no influence by sea
or swell.
11.19.9.3
When two tugs are towing in series as described in [11.18.6] in an ice infested area, the towing connections on
the foredeck of the second tug shall be strong enough for any shock loading that may result from the lead tug to
breaking through ice floes of varying thickness.
11.19.9.4
For a tandem tow (as described in [11.18.4.2]) where the presence of ice increases the potential for rapid
changes to the towing speed, good fenders shall be in place between each unit in the tow due to the close
connection.
Guidance note:
This is sometimes referred to as ice-coupled.
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11.19.10 Towing equipment
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 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
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• large heading deviations due to manoeuvring through and around ice and
• unintentional tug interaction with heavy ice floes which can result in shock-loading to towing components
due to whiplash and the tow taking charge.
It is for these reasons that additional provisions concerning towing equipment strength, type and configuration
are necessary.
11.19.11 Additional equipment requirements for towing in ice
11.19.11.1
In addition to [11.12.5] a tug involved in towing in ice infested waters shall be fitted with an operational towline
quick release/reset system (tow-wire abort system) when:
• towing in ice that could rapidly reduce towing speed or
• a tug is involved in a multiple tow or
• a tug is involved with a multi-tug tow.
11.19.11.2
The towline quick release system should be capable of immediate winch brake release for pay out of tow-wire as
well as winch brake re-set from the navigation bridge and the winch control station (if different).
11.19.11.3
With reference to [11.12.9], a tug involved in a towage in ice should be fitted with at least two searchlights that
can be directed from the navigation bridge.
11.19.11.4
As required by [11.12.11.2] and [11.13.16], every tug that is towing in ice shall be equipped with burning and
welding gear for ice damage control and repair.
11.19.12 Strength of towline
11.19.12.1
With reference to [11.13.3.1] for a tug that is planning a conventional single towline towage in ice the towline
MBL should be as follows:
Table 11-18 Minimum towline MBL in ice
Bollard Pull (BP)
MBL (tonnes)
BP ≤ 40 tonnes
40 < BP ≤ 150 tonnes
BP > 150 tonnes
2 × BP+60
11.19.12.2
An exception can be made for short tows in very thin ‘new’ ice or in very low concentrations (<3/10ths) of
medium or thick ‘rotten’ ice. In these circumstances the towline MBL should be computed as shown for a ‘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
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Further, ALL tugs involved in a towage in ice shall carry a spare tow-wire of the same length and strength as the
main tow-wire that is immediately available on a reel to replace the main tow-wire. In addition, there shall be
enough competent personnel, equipment and spares on board to crop and re-socket the main tow-wire at least
once.
11.19.13 Special cases of reduced tow-wire strength
11.19.13.1
The minimum size of tow-wire that is typically used by icebreaking tugs of 160t BP for 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 tow-wire, is made fast to each bow of the vessel
being towed. The tow-wire goes from the towing winch to the floating block on the bridle and back via a fairlead
to a towing damper on the tug. For larger powered tugs, the tow-wire can be doubled up again by passing the
wire through a standing block on the tug’s deck and around a second sheave on the floating block before it is
made fast to the towing damper. This makes the bridle wire the ‘weak link’ in the system and because of this an
icebreaking tug shall carry sufficient spare bridle wires, typically at least 6.
11.19.13.2
To meet the minimum towline strength criteria a tug that has an appropriate bollard pull can, in exceptional
circumstances, be considered for approval of a conventional towage in calm waters containing ice using two
towlines provided that:
• Each of the two independent towlines is a minimum of 90% of the required strength and
• Each tow-wire is on a separate towing winch that can be adjusted, quick released and reset independently
from the other and
• Each tow-wire meets the requirements of a single tow-wire in terms of minimum length, construction etc.
and
• Each tow-line has a monitoring system to enable load sharing.
11.19.14 Towing winches
11.19.14.1
Towing winches shall be provided due to the typical manoeuvring restrictions and hazards that are inherent to
towing in ice.
Guidance note:
Towing hooks do not allow for the rapid adjustment of towline length.
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11.19.14.2
Each towing winch should have sufficient pull to allow the towline to be shortened under tension. The navigating
bridge and winch operator should be provided with continuous readouts of towline length deployed and towline
tension.
11.19.14.3
Winch controls and winch operating machinery should be suitably protected from environmental conditions,
particularly low temperatures that can result in winch malfunction.
11.19.14.4
Towing winches shall have a quick release and re-set system as described in [11.19.11.1] and [11.19.11.2].
11.19.15 Chain bridles
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 extra-long chain pigtail can be
considered appropriate. Wire pennants and bridles are sometimes used for small barge and vessel tows,
especially when the close-couple or ice-couple towing technique is anticipated.
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11.19.16 Synthetic rope
11.19.16.1
Synthetic rope shall not be used in a towing system for an in-ice towage, therefore [11.13.9.2] and all parts of
[11.13.10] do not apply in ice transits or in very low temperatures where icing can occur.
Guidance note:
Synthetic rope is prone to rapid cutting both internally by ice crystals and externally by ice edges.
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11.19.17 Bridle recovery system
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
tri-plate.
• The fuel mentioned in [11.13.11.2] for a motorized recovery winch shall be appropriate for the anticipated
temperatures.
11.19.18 Emergency towing gear
11.19.18.1
For all towages in ice the emergency towing gear should be fitted and arranged to tow from the bow unless it
can be shown that the object being towed is designed for multi-directional towing. With reference to [11.13.13],
special arrangements can be required for the emergency towing gear, especially on an un-manned tow
proceeding in ice.
11.19.18.2
The emergency tow gear arrangement shall not be susceptible to being cut and lost or snagged by ice and
pulled clear of retaining soft lashings or metal clips, especially in high concentrations of ice.
Guidance note:
For example, an intermediate wire can be attached to the end of the emergency tow-wire and lightly secured to
a pole extended astern at least 5 m. The eye of the intermediate wire is suspended above the surface of the ice
approximately 1 m above the aft working deck of the tug where it can be captured for connection to a tuggerwinch wire. The float line and pick-up buoy are shackled to the emergency tow-wire in the same way as
described in [11.13.13.3] but remain coiled on the deck of the tow for deployment once the tow arrives in open
water.
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11.19.19 Access 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 de-ice recessed access arrangements and ladders to tows. Pilot
ladders used as a short term alternative should be closely inspected for ice damage before being used.
Guidance note:
Typically, a pilot ladder secured at the stern of the tow is subject to the least amount of ice interaction.
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11.19.20 Towing equipment certification and special precautions
11.19.20.1
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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 non-destructively tested (NDT)
before the towage.
11.19.20.3
Before departure a visual inspection of the tow-wire shall be performed, and based on the results the MWS
company surveyor can also require that the tow-wire is cropped and re-socketed before the towage.
11.19.21 Safety equipment for the workboat
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.
11.19.22 Bollard 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 (un-benign) towage, as shown in [11.12.2].
11.19.23 Oversized 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 tow-master shall be fully aware of any strength
reduction to the connections, carry adequate replacement spares and the towing procedures and any Certificate
of Approval should identify the maximum power setting that can be applied.
11.19.24 Cargo loadings
11.19.24.1
Special attention should be given to cargo overhangs on a case-by-case basis.
11.19.24.2
For towage in ice, the cargo shall not overhang unless it can be shown that the cargo is adequately protected so
that no ice interaction can occur.
11.19.24.3
To determine the potential for ice interaction, calculations shall show that the cargo has at least three meters
clearing height above the maximum height of ice deformity that can be experienced during the tow. In all ice
concentrations this minimum clearing height shall be maintained in all conditions of roll, pitch and heave (see
[11.3] and [5.2]). Due to the potential for ice impact and resulting damage cargo overhang cannot be allowed to
immerse under any circumstance, so that [5.6.5.4 a)], [11.3.4.2], [11.10.2.8] and [11.10.2.9] are not applicable.
11.19.25 Seafastening design and strength - motions
11.19.25.1
The cargo mass shall include the effect of ice accretion calculated in accordance with the IMO Intact Stability
Code 2008, /89/, Chapter 5.
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11.19.25.2
In low ice concentrations, the motions of a vessel transiting should be assumed to be as severe as those
experienced in clear open water storm conditions.
Guidance note:
Swell waves can persist for many miles even into an ice edge of very high ice concentration.
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11.19.25.3
In high ice concentrations the strength of cargo and sea-fastenings for voyages in ice conditions shall be of
acceptable design and not less than that required for weather unrestricted voyages in non-benign areas - see
[11.3] and [5.2].
Guidance note:
Despite no waves being evident, impact or over-running of thick ice floes can cause sudden deceleration,
heading deflections, listing and rolling of the tow.
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11.19.26 Inspection of welding and seafastenings
11.19.26.1
With reference to 5.10.2], welding procedures and techniques shall account for very cold temperatures,
particularly for sea-fastening installation.
11.19.27 Pipes 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.
11.19.28 Stability in ice
11.19.28.1
Stability calculations for vessels, including tugs and tows, operating in very cold temperatures and in ice
conditions shall be documented and reviewed against the IMO Intact Stability Code 2008, /89/, 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 ice-infested 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
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].
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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 self-propelled) shall not be excessively trimmed. On
manned tows the trim should be appropriate to provide watch personnel with as much forward visibility as
possible for observation of approaching ice conditions and the movements of other vessels involved in the
towage to reduce the potential for ice impact and/or collision damage.
11.19.29 Ballasting in ice
11.19.29.1
Unless otherwise agreed the forepeak should be ballasted to above the waterline of tug(s) and towed vessel(s).
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.
11.19.30 Voyage planning in ice
11.19.30.1
In addition to the requirements listed in [11.14], a written voyage plan or towing manual should be documented
in advance of a proposed towage in an ice-infested region.
11.19.30.2
The plan should include:
•
•
•
•
•
•
•
•
•
•
•
•
•
A general description of the proposed voyage (manned/un-manned towage etc.)
Tug and tow particulars including ice classifications and certification
Research documentation indicating the anticipated ice/weather conditions
Routeing including shelter and holding locations
Navigation and communications equipment appropriate for the region
Summary of tow-master and senior officer experience
Arrangements for receiving weather and ice information and/or routeing
Voyage speed and fuel calculations including any bunkering requirements and procedures to comply with
National regulations
Contingency fuel, hydraulic and lubricating oils of suitable viscosity for the low ambient temperatures
Main and emergency towing arrangements and certification
Stability calculations and location of all cargoes, consumables, ballast and pollutants for the tug and tow
Sea-fastening (cargo securing) arrangements
Arrangements for assist tugs for docking etc. and for ice management as required
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• 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 tow-master 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
The towing manual shall indicate compliance with the International, National and Local regulations and
guidelines concerning the carriage of oil cargoes, the allowable quantity and distribution of fuel oil or any other
pollutant or dangerous cargo. In addition, where a towing manual indicates the requirement to re-fuel the tug
from the tow or from another vessel this will normally require special approval from a National authority and also
require that the tug carries appropriate pollution containment and clean-up equipment. The re-fuelling approval
from the appropriate jurisdiction, as well as the re-fuelling procedure and equipment, shall be documented in
the towing manual.
11.19.31 Weather/ice restricted operations
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.
11.19.32 Damage 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 de-icing equipment
Space heaters
Extension ladders
Chain falls
Collision mat materials.
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11.20
Page 258 of 543
Specific for towage in the Caspian Sea
11.20.1 Background
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 11-11), 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--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 11-11 Northern and Intermediate Caspian Sea areas
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11.20.2 Towage requirements for all Caspian Sea areas
11.20.2.1
Single propeller tugs should not be used, unless there are suitable additional (redundant) tugs in attendance to
replace them.
Guidance note:
The whole Caspian Sea suffers from a large number of unmarked fishing nets which provide a serious hazard to
tugs which can be immobilised by these nets fouling their propellers.
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11.20.2.2
Pusher tugs should not be used for pushing in open waters.
Guidance note:
Many of the tugs found in this region are pusher tugs
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11.20.3 Towage requirements within northern Caspian Sea
11.20.3.1
GENERAL: The following changes from the requirements in [11.4], [11.12] and [11.13] can be accepted for tows
that take place totally within the Northern area (North of 45ºN latitude).
11.20.3.2
BOLLARD PULL REQUIREMENTS: Because of the limited wave heights (due to the shallow water) the
meteorological criteria for calculating the Towline Pull Required (TPR) referred to in [11.12.2] when there is no
ice, can be taken as:
• Hs = 2.5 m
• Wind = 20 m/s
• Current = 0.5 m/s
provided that the tow will have adequate sea room after the initial departure. If there will not be adequate sea
room, then [11.12.2.2] 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, purpose-built quick-release towing connections are preferred.
Where bollards have to be used as the towline connection:
• The capacity of the bollards and their foundations shall comply with the requirements of [11.13.5].
• Suitable fairleads and anti-chafe arrangements shall be used.
• A keeper plate, capping bar or other means of keeping the towing bridle connected to the bollards shall
be provided and this shall be suitable for any vertical loads likely to be encountered.
• The design shall also allow for quick release of the keeper plate, capping bar or another proven method to
rapidly clear a fouled bridle.
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11.20.3.6
WORKBOAT: A twin screw tug fitted with a bow thruster and two anchors in accordance with Class requirements
can be exempt from the requirement for a workboat in [11.12.6] provided the voyage can be completed within a
favourable weather forecast. The tug shall also be able to come alongside the tow at sea so that crew can board
with any necessary equipment for pumping, repairs, dropping the vessel’s anchor or reconnecting a towline.
11.20.3.7
BUNKERS: The requirement for 5 days reserve in [11.12.12] can be reduced to 3 days (pumpable reserve)
provided that:
• the towage can be completed within a good weather forecast period, and
• there are suitable bunkering ports within 3 days sailing at all times, and
• there are suitable tugs available to take over the tow if required during a diversion for refuelling.
11.20.3.8
DEFAULT MOTION RESPONSE: The following default values will apply, where applicable, for voyages entirely
within the Northern Caspian Sea Area.
Table 11-19 Default motion criteria for Northern Caspian
Nature of Voyage
Weather
unrestricted
Case
LOA (m)
B
1)
(m)
Block
Coeff
Full
cycle
period
(secs)
Single amplitude
Heave
Roll
Pitch
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
Notes:
1. B = maximum moulded waterline beam.
11.20.4 Towage 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.
11.20.5 Requirements for towages between Caspian Sea areas
11.20.5.1
Many shallow draught tugs that are designed for working in the shallow Northern area will be unable to carry
towing gear suitable for towing in the Southern area. When it is not practicable for towages to change tugs when
travelling between these areas whilst within the intermediate area defined in [11.20.1.1], 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.
11.21
Specific for FSUs (FPSOs, FSOs, FLNG facilities, FRSUs etc.)
11.21.1 General and background
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11.21.1.1
This section addresses the specific marine-related issues associated with the towage of these units, not already
addressed in this standard. Although it is recognized that there are many more marine activities in an FSU
development, towage to field or operating location is a critical and often long operation, which shall be
addressed by the project team early in the schedule.
Guidance note:
• Some FSU developments are ‘fast-track’, resulting in construction and commissioning activities being
completed during the tow.
• New-build 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
Project-specific fit-for-purpose criteria shall be agreed in each case.
11.21.2 Route 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
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.
11.21.3 Structural 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 dry-docking for their entire working life, usually in the order of
20 years. A commercial vessel is usually assumed, for design purposes, to spend about 20% of its life in port, and
is periodically dry-docked. These differences place much greater emphasis on ensuring the quality of the hull.
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11.21.3.2
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
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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.
11.21.4 Tug 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
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].
11.21.5 Ballast, 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 ship-shape hull with the forefoot well immersed. The ballast distribution shall be checked to ensure that the
shear and longitudinal bending moment are within acceptable limits.
Guidance note 1:
Having the forefoot will immersed will reduce slamming in heavy weather.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Directional stability under tow can be compromised resulting in the FSU veering off the course line. This is due to
various factors related to the design and construction of the FSU, including but not limited to:
• The presence of a mooring or riser turret, below the keel of the vessel, generally at the forward end or
mid-length.
• The removal of the vessel’s rudder, where the FSU is a conversion
• The hull design of purpose-built FSUs
• High windage structure at the fore end.
The lack of directional stability can be hazardous due to:
• Lack of sea room in congested and/or confined waters, e.g. Dover Strait
• Accelerated deterioration of the towing gear caused by excessive movement, especially wear of chains.
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11.21.5.2
Consideration should also be given to attaching a tug at the stern of the FSU (see also [11.21.5.3] below).
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11.21.5.3
The design of the towing gear should minimise the directional instability.
11.21.5.4
Consideration can be given to towing by the stern. If this is proposed then any motions analysis or model testing
shall recognise this configuration. The strength of the hull in way of the stern shall be checked to ensure that:
• The stern can withstand the anticipated slamming loads
• Suitably sized towing connections and fairleads are or can be attached.
11.21.6 Towing 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 wide-spaced, preferably more than one-half of the beam
• The chafe chains should be generously oversized (typically +50%) to allow for accelerated wear during the
voyage.
11.21.6.4
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.
11.21.7 Self-propelled or thruster-assisted 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
A risk assessment shall be undertaken, in accordance with [2.4], to determine the need for assisting tugs.
11.21.8 Manning and certification
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11.21.8.1
The documentation set out in Table B-2 should be submitted.
Guidance note:
Most FSUs are not classed as ships during their service life.
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11.21.8.2
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 un-manned, the unit shall be fitted with appropriate means of boarding, in
accordance with [11.13.15]
11.21.9 Emergency anchor
11.21.9.1
The general emergency anchor requirements of [11.16] shall apply.
Guidance note:
FSU mooring systems (whether turret-type or spread), being only for in-place conditions, are not configured to
act as emergency moorings during transit. On a conversion the permanent anchors will often be removed. For
many designs the deck space where an emergency anchor might be sited is taken up with the permanent
mooring equipment.
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11.21.10 Moorings
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.
11.22
Specific for jacket voyages
11.22.1 Introduction
11.22.1.1
This section gives requirements specific for jacket voyages and not already addressed in this standard.
11.22.2 Fatigue, 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].
11.22.3 Equipment seafastenings
11.22.3.1
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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 de-rigging 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.
11.22.4 Transport on deck of crane vessel
11.22.4.1
Jackets and piles are sometimes transferred to the deck of the crane vessel for final transport to the installation
location, or at the location itself to facilitate installation. The weight of the grillages and seafastening shall be
accounted for.
11.22.4.2
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
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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).
11.22.5 Wet 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.
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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
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.
11.23
Specific for ship towage
11.23.1 General 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 B-2. 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 dry-docking 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. Ro-Ro ships can also pose particular
problems, on account of the potentially large free surface in the event of flooding. Passenger ships and Ro-Ro
ships will generally only be approved for towage if the tow is manned, to permit early intervention in the event of
any problems.
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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 stern-first 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.
11.23.2 Towlines and towing connections
11.23.2.1
Each ship or vessel towage is unique and it is therefore not possible to specify the connection equipment to be
used or how it is to be attached for every case. Alternative systems are suggested in [K.6]. Any equipment used
for the towage shall be fit for purpose and shall be agreed between the owner of the tow, the tug master and the
MWS company. In particular it shall be shown that towing connections and their foundations, above and below
deck, comply with [11.13.3.4]. 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 backed-up 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.
11.23.3 Anchors
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.
11.23.4 Securing 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
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The propeller shaft shall be immobilised, or disconnected, to prevent damage to machinery during the towage.
11.23.4.5
Every effort shall be made to limit the carriage of any loose deck equipment to an absolute minimum. Where
equipment is carried on an exposed deck then it shall be protected and secured against movement using
welded brackets, chain or wire. Equipment in other areas shall also be secured.
11.23.4.6
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.
11.23.5 Carriage 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 seaman-like 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.
11.24
Specific for voyage to scrapping
11.24.1 Anchoring
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:
This will normally be a class requirement for classed vessels.
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11.24.2 Towage
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 load-line certificate or
exemption.
11.24.3 Own 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.
11.25
Specific for towing of pipes and submerged objects
11.25.1 Introduction
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 system-dependent approach to the design to make the pipe suitable for
the intended installation method.
• Bottom (or on-bottom) 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 off-bottom tow.
• Off-bottom and CDT tow methods require great care to control the control the submerged weights of the
assembly within a narrow band.
• Surface tow, near-surface 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.
11.25.2 General 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 DNV-RP-H103, /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.
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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
Break-out 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 break-out 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
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. DNV-OS-F101, /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.
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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 tow-out.
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 sub-surface tow of pipelines etc. see [11.25.9]
Submerged tow of objects attached to the installation vessel, see [11.25.10]
11.25.3 Launch
11.25.3.1
For off-bottom 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
For off-bottom 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 post-launch 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 build-up 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 “off-bottom tow mode”, friction between the ballast
chain and seabed cannot be used to counteract lateral deflection of the pipe.
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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 load-out.
11.25.3.12
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 11-20 Typical upper bound design friction coefficients for launch and installation
Conditions
Break-out
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 11-21 Load factor for uncertainties during launch
Computed load (L)
L <150 tonnes
150 < L < 300 tonnes
L > 300 tonnes
Load factor
1.5
1.5 - 0.002*(L - 150)
1.2
11.25.3.14
The minimum factored static load at either end for launch should normally be taken as 100 tonnes.
11.25.4 Route and weather restrictions
11.25.4.1
A tow route corridor pre-survey 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 lay-down
areas.
11.25.4.2
If pre-design survey was made in more than 90 days before the tow, a follow-up 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.
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11.25.4.3
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 off-bottom 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 lay-down 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
The minimum required Under-Keel 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 off-bottom 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.
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11.25.5 Tug 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 vessel-specific data, the reduction in available towline pull at the required towing speed should
be taken into account as follows:
Table 11-22 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.
11.25.6 Towing 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 tri-plates 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
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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)
In-line 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
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.
11.25.7 General 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 stand-by configuration (e.g. wet-parking)
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 back-up 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.
11.25.8 Controlled depth tow (CDT)
11.25.8.1
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Further to the general pipeline design requirements for tow methods provided in [11.25.2] to [11.25.4] for on
and off-bottom 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
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.
11.25.9 Surface and near-surface 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 calm-weather 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
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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
• A suitable work class ROV shall be present in the field during installation operations with a package of
spares.
11.25.9.5
Hang-off rigging between the buoy and the object shall be designed in accordance with [16.2]. The possibility of
fatigue should be considered.
11.25.10 Submerged 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
DNV-RP-H103, /56/.
11.25.10.2
Hang-off rigging shall be designed in accordance with [16.2].
11.25.10.3
Possible fatigue shall be considered for:
• hang-off 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 hang-off rigging and any anti-rotation line(s).
11.25.10.5
Acceptable clearances to the sides of vessels or moonpools shall be documented by calculations if applicable.
11.26
Specific for deep draught towages
11.26.1 General
11.26.1.1
This section covers the special requirements for towages of deep draught structures, e.g. GBS’s, towages not
covered above.
11.26.2 Deep 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).
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11.27
Page 278 of 543
Specific for jack-up voyages
11.27.1 General
11.27.1.1
This section covers the special requirements of jack-up, not covered above for both wet towages and dry
towages/transport. The terms 24-hour 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 Self-Elevating Offshore Installations”, /121/ and “The Safe Approach,
Set-Up And Departure of Jack-Up Rigs to Fixed Installations”, /120/, describe good practice for jack-up moves
within the North Sea. Many of these practices should be followed in all areas.
11.27.1.3
The requirements for jack-up 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].
11.27.2 Loadings and strength
11.27.2.1
Loads in legs, guides, jack-houses and jack-house 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 jack-ups 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.
11.27.3 Seafastenings 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.
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11.27.3.3
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 conventionally-placed 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.
11.27.4 Hull strength – wet towage
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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 quasi-static 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].
11.27.5 Stress levels
11.27.5.1
Stress levels in legs, guides, jack-houses, 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
Before an weather unrestricted voyage of a jack-up, 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 fatigue-critical 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
jack-house connections to the deck and connections of the spudcans or mudmats to the leg chords. Note that
new-build MOU's should normally have been verified for fatigue for the initial delivery voyage.
11.27.5.4
Local areas of jack-up 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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11.27.6 Stability 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 jack-ups 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 pre-load 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 non-return 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
Free surface in the mud pits is not generally acceptable, except for very short 24-hour 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.
11.27.7 Tugs, towlines and towing connections
11.27.7.1
Tugs shall be selected in general accordance with [11.12] as applicable for:
a. weather unrestricted towages
b. 24-hour 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.
11.27.8 Securing of legs
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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 24-hour 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
For jacking systems fitted with elastomeric pads, clearances should be shimmed or pre-load applied in
accordance with the manufacturer’s specifications.
11.27.8.6
For tilt-leg jacking systems, tie bars shall be fitted to by-pass the tilt mechanism.
11.27.8.7
Where lowering of legs or jacking on a stand-by 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.
11.27.9 Drilling 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 24-hour 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 24-hour 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.
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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.
11.27.10 Helideck
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.
11.27.11 Securing 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 24-hour 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.
11.27.12 Spudcans
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11.27.12.1
For 24-hour and location moves, the spudcans should normally be full. See also [11.27.6.6] for stability
calculations.
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.
11.27.13 Pumping 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, self-contained, self-priming salvage pumps
shall be on board, with not less than 30 m each of suction and delivery hose.
11.27.14 Manning
11.27.14.1
Jack-ups towed on their own buoyancy should usually be manned, and the general manning requirements of
[11.17] shall apply. In general the jack-up should be down-manned to the minimum complement essential for the
safe conduct of the jack-up move, including jacking operations.
11.27.14.2
Jack-ups 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.
11.27.15 Protection 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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11.27.16 Anchors
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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 quick-release system should be
passed through the anchor shackle and secured on deck. The turnbuckle and quick-release system shall be on
deck and accessible.
11.27.17 Safety 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 Fire-Fighting 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.
11.27.18 Use of a stand-by 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, pre-loading 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 stand-by locations which may not have been surveyed recently.
11.27.18.3
Such stand-by 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 pre-approved 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 stand-by 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
As a minimum, the following shall be documented for each proposed stand-by location:
a. Specific procedures, if required, for approach.
b. Sufficient information to satisfy the location approval requirements in DNVGL-ST-N002 - Site specific
assessment of mobile offshore units, /39/.
11.28
Approaching a jack-up location
11.28.1 Background
11.28.1.1
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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 “soft-pinning” and “leg dragging”, dynamic
positioning or mooring.
Guidance note:
The common stationkeeping methods are briefly described below:
a. 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.
b. “Soft-pinning” – 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.
c. “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.
d. Dynamic positioning – the unit’s own thrusters are used to keep station under control of a dynamic
positioning system.
e. 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 jack-up locations:
a. Stationkeeping procedures , calculations and drawings (with risk assessments as required)
b. Contingency plans (with risk assessments as required)
c. Documentation confirming platforms status; live and producing, hydrocarbons above seabed, fully shut
down situations
d. Emergency response plan.
11.28.1.4
In addition to the information in [11.28.1.3], as a minimum, the following shall be documented for jack-ups
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 pre-move meetings on the unit.
11.28.2 Requirements for use of tugs only
11.28.2.1
The tugs shall be able to hold the jack-up 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):
a. 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
b. 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.
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11.28.3 Requirements for use of “soft-pinning”
11.28.3.1
Soft-pinning shall only be used under the following conditions:
a. The site within which the soft-pin 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.
b. A suitable weather window exists to allow for the running of anchors, pre-loading and jacking up.
c. 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 soft-pin location near a platform in the dark the following additional requirements will apply.
a. At least one lead tug plus 2 stern tugs with adequate bollard pull attached to the (aft) quarters
b. Platform to be adequately lit by its own power, with back-up, and not likely to dazzle or blind an
approaching rig crew
c. Platform shut in with lines depressurized
d. 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, current and visibility criteria to be documented in the rig
move procedures.
e. No pipelines or cables close to the platform or soft-pin location that would require the legs to be kept at
higher clearance and thus extend the time to reach the seabed
f. 100 m minimum separation between the rig and the platform or any other subsea assets apart from
pipelines or cables already covered in e)
g. 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.
h. 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.
i. Contingency /back-up in place in case of sudden power or other equipment failure on any tug (especially
the lead tug)
j. 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.
k. Navigation package to be working properly with suitable back-up systems
l. 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
m. 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
n. Priority to be given to the safety of the rig, platform or crew rather than commercial considerations.
11.28.4 Requirements 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).
11.28.5 Requirements for use of DP
11.28.5.1
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The dynamic positioning requirements in [17.13] shall apply.
11.28.6 Requirements for use of moorings
11.28.6.1
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 pre-loading each anchor to an agreed value - typically 100+ tonnes for pre-laid 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.
11.28.7 Required 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 DNVGL-ST-N002 - 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 jack-up 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].
11.28.8 Approaching 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 pre-loaded 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
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:
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a. Do all involved units including tugs and their captains have a history of reliable operations with good
communications?
b. Will the jack-up be pinned at a stand-off position before making a final close approach to the platform?
c. 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?
d. 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?
e. If there is any risk of punch-through/ Has there been an impact assessment of contact between Jack-up
and Platform as a result of a punch-through, sliding or interaction with existing footprints, and is it
acceptable to the MWS company?
f. 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?
g. 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)
h. 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.
i. 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.
j. Are an Emergency Response Plan and Communication protocols in place for SIMOPs?
k. Are all 3rd Party (including platform and pipeline owners) written consents in place?
l. Have any other risks due to local conditions been identified and addressed?
m. 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.
a. Platform not to be “live” and with all hydrocarbons bled off and pipelines depressurized
b. 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.
c. No congestion in the area
d. 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.
e. High confidence in the survey /navigation package and operators
f. Adequate mitigating measures in place.
11.28.9 Approaching a live platform or pipeline
11.28.9.1
Before positioning a jack-up 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 worst-case 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.
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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]:
a. Would the location be approvable by the MWS company if the platform were not live?
b. Have the quantities, locations and pressures of all hydrocarbons been identified?
c. Are all risers and hydrocarbon inventory well protected from a collision with the jack-up 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)?
d. Has a geotechnical engineer in the MWS company evaluated the location for risk of punch-through 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.
e. Is there an Emergency Shut Down system in place which will not be obstructed by the Jack-up 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.)
f. 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”.)
g. Has down-manning the jack-up and platform been considered for the final approach?
11.29
Rig move procedures (for all MOUs)
11.29.1
Rig move procedures shall be accepted in advance by the MWS company and include as a minimum:
Table 11-23 Rig move procedure contents
Ref
1
Item (* indicates that it is for jack-ups 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 jack-ups)
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
Co-ordinates of the departure & arrival locations with air gap and heading details
2.2
Unambiguous reference point for the co-ordinates 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 pre-loading 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.
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2.10
Clearances from subsea assets (vertical at LAT and horizontal) identified for key stages of the
approach.
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 stand-by locations needed en-route together with the necessary data (soils, water depth,
etc.)
3.2
Confirmation that permissions are in place for the stand-by 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)
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 in-line 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.
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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
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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 client-specific requirements
7.4
Any peculiarities or inconsistencies e.g. wrt to mooring equipment /make-up/or capabilities identified
from the most recent rig move report for the unit
7.5
Evidence that procedures been reviewed by appropriate marine personnel from the rig management
(ashore and offshore) and that any resulting comments have been addressed
8
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 soft-pin 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.
11.30
Specific for semi-submersible voyages
11.30.1 General
11.30.1.1
This section gives requirements specific for semi-submersible MOUs not already covered in this standard.
11.30.1.2
“Guidelines for Offshore Marine Operations (G-OMO)” /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 semi-submersible 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.
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11.30.2 Loadings and strength
11.30.2.1
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
• I-beams.
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11.30.3 Stability 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.
11.30.4 Tugs 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 (G-OMO)” /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 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
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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.
11.30.5 Anchors
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.
11.30.6 Deep draught
11.30.6.1
There may be occasions when a semi-submersible 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:
• 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 22-27 knots and wave height of 3 m should be considered as an upper limit
weather condition for deep draught rig moving.
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11.30.7 Transport or tow plan
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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.
11.30.8 Requirements for use of DP
11.30.8.1
The dynamic positioning requirements in [17.13] shall apply.
11.30.9 Approaches 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.
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11.30.9.2
For towed units, the tight tow should be suspended at a safe distance from the arrival location. For self-propelled
units the sea passage should be suspended at this time.
11.30.9.3
The approach to the location and run-in on the first anchor should be undertaken at a controlled speed and
normally achieved by one AHV on the bridle and another on the run-in 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 semi-submersible units (towed or self-propelled) 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 non-benign 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.
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11.31
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Information required
11.31.1 General
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.
11.31.2 Transport 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.
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.
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• Reporting details: who to, how often and content.
• 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.
11.31.3 Towed 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 fire-fighting equipment (see [11.17.4]).
11.31.4 Towed 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 lay-down, parking and stand-by
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
• Launch analysis and details of software used
• Dynamic towage analysis and details of software used
• Installation analysis and details of software used.
11.31.5 Transport 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:
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• [11.11] for transport vessels or barges
• [11.12] for tugs and
• [11.13] for towing equipment as applicable.
11.31.6 Proposed 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].
11.31.7 Strength
11.31.7.1
Analysis or specifications for strength of transported or towed object, grillage, seafastening and overall strength.
See Sec.5.
11.31.8 Stability
11.31.8.1
Stability Manual or calculations for stability and damage stability. See [11.10].
11.31.9 Other
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 Jack-Ups (see
[11.26] to [11.29]).
SECTION 12 Tow out of dry-dock or building basin
12.1
Introduction
12.1.1
General 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.
12.1.2
Revision 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, DNV-OS-H201.
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Dry 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
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 run-up 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 run-off from surrounding land)
and spray overtopping and allow for loss of pumps or power sources.
12.3
Design 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].
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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.
12.4
Mooring and handling lines for tow-out
12.4.1
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].
12.5
Intact & 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.
12.6
Under-keel clearance for leaving basin
12.6.1
General
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 under-keel 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 tow-out should be completed before high
water.
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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.
12.6.2
Air cushion
12.6.2.1
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 non-return valves
• Back-ups 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.
12.7
Side 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.
12.8
Under-keel clearance outside basin
12.8.1
Once outside the building basin the minimum under-keel clearance shall be as required by [11.14.20] until final
emplacement.
12.9
Towage and marine considerations
12.9.1
Tugs, towage and marine considerations for tow out of dock shall be in accordance with Sec.11.
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12.9.2
An exclusion zone for marine traffic should be agreed with the Port Authority and any other relevant authorities.
12.10
Information required
12.10.1 Object
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 back-up
systems, and compartment status-monitoring systems.
• Structural strength and stability analyses covering all phases of construction
• Ballasting calculations
12.10.2 Towage
12.10.2.1
As per [11.30.9] (as applicable).
12.10.3 Basin
12.10.3.1
For the basin:
• Drawings of basin and route
• Flooding procedure
• Design Documents including details of criteria used.
SECTION 13 Jacket installation operations
13.1
Introduction
13.1.1
General
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
With the exception of stability requirements, the tow of self-floating jackets is covered in Sec.11. The intact and
damaged stability requirements shall be agreed on a case-by-case with MWS company.
13.1.1.4
A self-upending jacket is one that after launch rotates to a near-vertical attitude without an intermediate
horizontal phase. This can be achieved by distribution of buoyancy and/ or free-flooding compartments.
13.1.1.5
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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 near-vertical attitude.
13.1.2
Revision 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 2-4), DNV-OS-H204
• GL Noble Denton, Guidelines for Steel Jacket Transportation & Installation, 0028/ND.
13.2
Environmental conditions
13.2.1
All phases – excluding on-bottom 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 well-controlled 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. 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 co-linearly.
Guidance note:
Where appropriate directionality could be considered.
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13.2.2
Launch
13.2.2.1
The sea state for launch shall be limited to the lesser of:
a. The maximum sea state allowed by jacket stresses, dive depth, rocker arm reactions and barge
submergence, or
b. 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
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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.
13.3
Strength
13.3.1
All 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).
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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.
13.3.2
Launch 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.
13.3.3
On-bottom
13.3.3.1
Jacket strength for the on-bottom condition should be treated as an LS2 or ULS limit state for the same design
environmental conditions as the on-bottom stability checks in [13.10.1]. All foreseen ballasting arrangements
shall be considered including any resulting from single compartment damage.
13.4
Jacket buoyancy, stability and seabed clearance
13.4.1
Intact 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
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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.
13.4.2
Minimum 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 13-1.
Table 13-1 Minimum GM
GM
Phase
Intact / ULS
During towage to field (self-floating 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
During upend, transverse
0.5 m
During upend, longitudinal
After upending, before final positioning,
both directions
> 0.0 m
0.2 m
0.2 m
1)
0.5 m
> 0.0 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 one-compartment 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 up-end 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.
13.4.3
Reserve buoyancy after launch and during upend
13.4.3.1
The combined reserve buoyancy is defined as:
based on generally the upper bound design jacket weight, as defined in [5.6.2].
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13.4.3.2
The reserve buoyancy shall be achieved whilst maintaining the minimum seabed clearances shown in Table 13-4.
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 13-2 and should be greater than those in Table 13-2 for intact conditions,
based on nominal total intact buoyancy.
Table 13-2 Reserve buoyancy for operations without crane assistance
Minimum reserve buoyancy
Case
Intact / ULS
Damaged / ALS
Launched jacket after launch 1) or
Lifted jacket if required to be re-rigged before upend
10%
5%
During upend by ballasting
8%
4%
Absolute minimum, subject to agreed risk assessment results
5%
2.5%
Notes:
1. Applicable where the jacket is to be upended using ballast both with crane and without crane
assistance. Also applicable to self-upending 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 13-3 and should be greater than those in Table 13-3 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 13-3 Reserve buoyancy with crane assistance
Minimum reserve buoyancy
Case (see Notes 1 and 2)
Intact / ULS
Damaged / ALS
8%
4%
12%
6%
Lifted jacket, with static and dynamic analysis carried out and
contingency procedures in place
Lifted jacket, with only static analysis carried out
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.
13.4.4
Seabed 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 single-compartment 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.
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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 side-scan 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.
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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 13-4.
Table 13-4 Minimum seabed clearance
Clearance after allowing for all tolerances in [13.4.4.1]
Case
During launch including upending of selfupending jackets and free floating position
During upend by controlled ballasting, with or
without crane assist
13.4.5
Intact / ULS
Damaged /
ALS
Greater of 10% of water depth or 5 m.
>2m
>2m
>2m
Jacket 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 set-down 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 un-controlled filling.
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13.4.5.4
In the event that jacket appurtenances (risers, conductors, J-tubes, 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.
13.4.6
Temporary items
13.4.6.1
All temporary items (e.g. rigging platforms, spreader bars, rigging) shall be adequately secured for all the
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following conditions:
•
•
•
•
all possible combinations of trim and heel angles
slam loads
against voyage loadings
loads during launch, upending and setting.
13.4.7
Freeboard
13.4.7.1
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.
13.4.8
Rubber 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.
13.5
Jacket lift
13.5.1
Lifting 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.
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13.5.2
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In-water dynamic behaviour
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 in-water lifting loads are derived, taking into account the weight, centre of gravity,
buoyancy, damaged cases, entrained mass, boom-tip velocities and accelerations, inertia and drag forces,
or:
• Demonstrate that the in-water case is not critical.
13.5.2.3
During the in-water 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.
13.5.3
Underwater disconnection
13.5.3.1
Where a remotely operated system for disconnection of rigging is used, a back-up system which is fully
independent of the primary system shall be available. For further details, refer to [16.16.11].
13.5.4
Re-use of lifting equipment
13.5.4.1
Where lifting rigging and lift points have been used before the installation operation, for load-out or transfer to
the crane vessel without incident, all rigging and lift points should be visually re-inspected by a competent
person before re-use. 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].
13.5.5
Free 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 re-rigging) then the principles outlined in [13.2], [13.3], [13.4], [13.6.9] and [13.7.2] apply.
13.6
Jacket launch
13.6.1
Analysis 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
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
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• Slamming velocities on critical items.
13.6.1.2
The launch analysis should be carried out using a 3-D 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:
a. To validate computer analyses,
b. To quantify parameters which are difficult to derive analytically,
c. 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].
13.6.2
Launch analysis model requirements
13.6.2.1
The jacket hydrodynamic, buoyancy and mass model should account for all items including main members,
buoyancy tanks, secondary members, and 'non-structural items' such as caissons, pile guides etc.
13.6.2.2
It should be ensured that the jacket buoyancy model is not over-buoyant 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:
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• 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.).
13.6.3
Structural/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 wave-induced 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
When selecting the load cases for analysis it is important that the pre-launch ballast condition is considered, in
conjunction with the allowable launch sea state, as this may lead to the most onerous case for jacket-barge loads
at the jacket nodes nearest the bow of the barge. The barge strength should also be verified for the post-launch
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.
13.6.4
Stability 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.
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• 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
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.
13.6.5
Launch barge trim angle
13.6.5.1
The pre-launch 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 self-launch should normally not exceed 4° whilst satisfying all
other parameters (barge draught, barge shear forces and bending moments).
13.6.6
Friction 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 13-5 and include the
necessary material factors.
Table 13-5 Design Friction Coefficients
Type
Static
Moving
Surface
Min
Typical
Max
Min
Typical
Max
Waxed wood-grease-steel
0.1
0.2
0.28
0.05
0.1
0.15
Waxed wood-grease-Teflon
0.08
0.14
0.25
0.03
0.05
0.08
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.
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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 (break-out) 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.
• 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
break-out requirements than anticipated.
13.6.6.5
The dynamic friction coefficient shall, if possible, be verified through monitoring of required pull/push force
during load-out. If the friction coefficient calculated based on the load-out monitoring is outside the range used
for launch analysis then the documentation, based on the measured values shall be updated.
13.6.7
Winching, 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 self-launching 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.
13.6.8
Self-upending 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.
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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 de-ballast 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 set-down.
13.6.9
Practical 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 night-time working.
• VHF radios for communication between work parties, and with the installation vessel, tugs and supporting
vessels.
• Equipment necessary to re-pressurise 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 pre-connected to handling wires before initiating launch.
13.6.9.4
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
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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 un-piled 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.
13.6.10 Launch 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
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.
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13.7
Floating controlled upend and set-down ballasting
13.7.1
Methods
Page 315 of 543
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 self-upending jackets are in [13.6.8].
13.7.1.3
The floating controlled upend and set-down 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].
13.7.2
Ballasting 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 fail-safe - 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 one-off operation reversibility, although desirable, is
not a requirement. De-ballasting 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 back-up.
13.7.2.6
Umbilical systems should have a manual or ROV back-up.
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.
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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
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.
13.7.3
Crane-assisted upend and set-down ballasting
13.7.3.1
Where a crane is used to assist the controlled upend and/or set-down ballasting the requirements for rigging, lift
points and practical as per [15.10] and [13.5] as appropriate.
13.8
Jacket position and set-down
13.8.1
Surveys
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 sand-wave 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.
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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 sub-surface 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
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.
13.8.2
Positioning 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).
13.8.3
Positioning 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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13.8.4
Positioning over pre-installed/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 and loads.
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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.
13.9
Buoyancy tank
13.9.1
Removal 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 pre-installed piles are not overloaded
13.9.1.3
Ballasting of buoyancy tanks shall not adversely affect jacket verticality.
13.9.2
Removal 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 back-up method or system should be available.
13.9.2.5
Removal by lifting should be in accordance with [15.10], taking account of crane vessel dynamics, in-water 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].
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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 back-up 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 de-watered 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].
13.10
On-bottom stability and piling
13.10.1 Unpiled and partially piled on-bottom 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 on-bottom stability shall be sufficient to allow safe personnel
boarding and working on the structure in the design environmental conditions.
13.10.1.4
The operation reference period should account for all piling and grouting activities, including an allowance for
waiting on weather. The return period for on-bottom stability should be selected from Table 3-1 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:
a. minimise the time taken to achieve the storm capability
b. 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
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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 change-over 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 un-piled 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 set-down ballast conditions,
Water levels and water density,
Penetration
Removal of buoyancy tanks
Pile stick-up
Hammer weight including follower
Pile hang-off
Pile installation schedule/sequence
Sequential piling sequence for correcting jacket level and
Soil strength parameters.
13.10.1.9
Using the LRFD approach, the on-bottom 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 13-6.
Table 13-6 Intermediate on-bottom stability safety factors
Safety Factor
Mode
ASD/WSD
LRFD 4)
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 mud-mat skirts and/or suction. 1), 2)
1.0
1.0
Sliding (soil failure) Where applicable, account may be taken of the
capacity of un-grouted piles to resist sliding 2)
1.5
1.0
Mud-mat/foundation V-H combined bearing capacity check 3)
1.5
1.0
Structure buoyancy (lower bound design weight excluding
contingency divided by buoyancy)
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 V-H 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.
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Figure 13-1 Mudmat capacity checks
13.10.1.11
Particular attention should be paid to the on-bottom 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 on-bottom 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:
• Benign areas Hs = 2.5 m
• Non benign areas Hs = 5.0 m.
Wave/current forces shall be calculated from the maximum wave (Hmax) 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 1-minute
averaging period should be used for computation of wind forces.
13.10.2 Piling 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.
13.10.3 Pile 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 add-ons should be shown to be fit for purpose
and properly commissioned. See [16.9.7], [16.11.5] and [16.12.4].
13.10.4 Pile structural analysis
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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 un-driven pile in the pile sleeve and the
maximum possible inclination of the jacket when performing pile stick-up analyses
13.10.4.4
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.
13.10.5 Self-penetration 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 self-penetration of piles (sometimes referred to as dropfalls/runaway).
13.10.5.2
Self-penetration analyses should account for weight of installation equipment (e.g. lifting tools, hammers,
followers etc.)
13.10.6 Pile 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
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For grouted connections the following provisions should be incorporated into the jacket:
a.
b.
c.
d.
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,
e. 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.
13.11
Information required
13.11.1 Manuals 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:
• 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 set-up, 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 set-down activities including buoyancy tank removal procedures.
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13.11.2 Jacket 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 back-up
systems, and compartment status-monitoring systems.
• Structural strength and stability analyses for the jacket during all phases covering load-out, voyage and
installation.
• Ballasting calculations for the jacket installation, including contingencies, where ballasting is required
13.11.3 Transport 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.
13.11.4 Installation 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.
13.11.5 Launching (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.
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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
13.11.6 Lifting (if applicable)
13.11.6.1
Lifting and upending analysis reports and other information requested in [16.18].
13.11.7 Unpiled/partly piled on-bottom 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 14 Construction afloat
14.1
Introduction
14.1.1
General 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 tow-out 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 semi-permanent 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
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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 inter-related,
including:
•
•
•
•
•
•
•
Structural loads and resistance
Draught, heel/trim, displacement, ballast condition and stability
Mooring loads and resistance
Marine spread requirements
Weather conditions
Other on-going activities and access restrictions.
Environmental restrictions imposed by local authorities.
14.1.2
Revision 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 DNV-OS-H201 Load Transfer Operations
14.2
Loads and structures
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.
14.3
Stability 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:
a. Maximum and minimum draught
b. Differential ballast levels in adjacent buoyancy compartments, or any compartment and the sea
c. Weight distribution
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d. Structural limitations on heel or trim, which may therefore lead to limitations on draught, stability or
environmental conditions. The age of any time-dependent construction material, such as concrete, shall be
considered.
e. Free surface limitations
f. Phases during which one-compartment damage stability does not exist, and the requirements given in
[6.2.5.2] apply.
14.3.4
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.
14.4
Mooring 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 direct-embedment 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
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.
14.5
Construction spread
14.5.1
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The construction spread may include barges and other floating equipment moored alongside or near the
platform, to serve the following functions:
a.
b.
c.
d.
e.
f.
g.
h.
i.
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.
14.6
Operational 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.
14.7
Information required
14.7.1
Object
14.7.1.1
For the object:
a.
b.
c.
d.
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 back-up
systems, and compartment status-monitoring systems.
e. Structural strength and stability analyses covering all phases of construction
14.7.2
Mooring arrangement
14.7.2.1
The information listed in [17.12] (as applicable).
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Other
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 15 Lift-off, mating and float-over
operations
15.1
Introduction
15.1.1
Scope
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:
• Lift-off operations: Load transfer of an object, e.g. a module, from land or seabed supports to supports
placed on one or more vessel(s). Lift-off 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.
• Float-over 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 off-load.
Guidance note:
Note that float-on to a Heavy Transport Vessel (HTV) falls under this definition and that these requirements apply
for on- and offloading of HTVs by float-on/float-off method, see also [15.1.1.4].
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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 float-over operations the load-transfer is not necessarily by ballasting alone. It could be that other equipment
is used, e.g. hydraulic jacks.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
In docking operations the object is a floating one and could be for instance a vessel.
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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 on-loading 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 float-out operations, see Sec.12.
15.1.1.7
For construction afloat, see Sec.14.
15.1.1.8
For load-out operations (e.g. by skidding or trailers), see Section 10.
15.1.2
Revision history
15.1.2.1
This section replaces the applicable sections of the following legacy documents:
• GL Noble Denton, Guidelines For Float-Over Installations / Removals, 0031/ND
• DNV Offshore Standard DNV-OS-H201 Load Transfer Operations.
15.2
General
15.2.1
Operation class
15.2.1.1
An Operation class should be defined according to Table 15-1.
Table 15-1 Operation class
Tide range 1)
Tide restrictions? 2)
Weather restrictions? 3)
Operation Class
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 load-out shall be defined as
tide restricted.
3. If weather restrictions apply, then the load-out shall be categorized as weather restricted, see [2.6]. If
there are no weather restrictions to the object movement/ballasting phase the load-out class may be
selected accordingly.
15.2.2
Planning and design basis
15.2.2.1
General requirements to planning and execution of load-transfer operations are given in Sec.2.
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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
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 sub-operations. 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.
15.2.3
Risk 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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15.3
Loads
15.3.1
General
15.3.1.1
Loads and load effects are generally defined in Sec.5. It shall be thoroughly evaluated if any other loads and load
effects not described in [5.6] need to be considered.
15.3.1.2
The design principles and methods described in Sec.5 shall be adhered to.
15.3.1.3
All relevant limit states as defined in Sec.5 shall be included in the design calculations/analysis.
15.3.2
Weight and CoG
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15.3.2.1
Weight and CoG, as well as buoyancy and CoB, shall be determined as described in [5.6.2].
15.3.2.2
Weight control shall be implemented as described in [5.6.2].
15.3.2.3
For removal of a structure the requirements of Sec.18 shall apply.
15.3.3
Environmental loads
15.3.3.1
All relevant wave lengths and periods, including swell type wave lengths shall be considered.
15.3.3.2
First order wave loads need to be considered for stiff securing/mooring systems, such as:
• mooring arrangements including short lines without catenary, and
• objects partly supported by vessel(s) and partly by land/seabed supports.
15.3.4
Skew loads
15.3.4.1
Skew loads are here defined as the variation in support reactions due to fabrication- and operation inaccuracies.
All possible skew loads should be evaluated and included in the relevant strength calculations if the effect
cannot be proven insignificant.
Guidance note 1:
Operational precautions such as shimming, monitoring, etc., may be used before and during the operation in
order to reduce/eliminate potential skew loads.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Items which may cause skew load effects are:
•
•
•
•
•
•
•
fabrication tolerances for the object and for the vessel supports
fabrication tolerances for the vessel(s)
vertical offset of the object for each support condition
vessel heel and trim variations
movement of vessel centre of buoyancy, gravity and flotation relative to draught and ballast configuration
inaccurate positioning of vessel(s) relative to the object supports
deformation of the object and the vessel(s) including the possible introduction of horizontal loads.
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15.3.5
Other loads
15.3.5.1
All loads which may occur due to effects such as hydrostatic pressure, impacts, mooring, guiding, pulling by tugs
and winches, etc. should be considered in the design of the object and in the planning of the operation.
15.3.5.2
The value of other loads should be determined considering operational and equipment limitations. For
determination of accidental loads possible failure modes should be sought.
15.4
Systems and equipment
15.4.1
General
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15.4.1.1
The systems and equipment used for load transfer operations should be designed, fabricated, installed and
tested according to [4.2].
Guidance note:
Specific requirements for systems are given in the respective subsections.
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15.4.2
Ballasting systems
15.4.2.1
General requirements to the ballasting systems are given in [4.3].
Guidance note:
The operation classes defined in Table 15-1 correspond to the operation classes referred to in [4.3.2].
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15.4.3
Positioning systems
15.4.3.1
A positioning system ensuring accurate (i.e. within the specified tolerances) and safe guidance and positioning of
the object/vessel(s) shall be provided. The requirements for guiding/positioning systems are given in [4.4] and in
Sec.17. The requirements for DP positioning systems are given in [17.13].
15.5
Vessels
15.5.1
General
15.5.1.1
General requirements to vessel(s) are given in [2.11] and requirements in [10.6] also apply as applicable.
15.5.2
Structural strength
15.5.2.1
The load transfer vessel(s) structural strength shall be documented for all possible ballast conditions, see also
[2.11.3].
15.5.2.2
The vessel deflections should be maintained within an acceptable range during the load transfer by selecting
adequate ballast configurations for the vessel(s).
15.5.2.3
Tolerances for the vessel deflections should be established considering the maximum allowable skew loads at
the vessel supports.
15.5.3
Stability afloat
15.5.3.1
Sufficient stability afloat should be ensured for single vessels during positioning. See [11.10] for general
requirements for stability.
15.5.3.2
The following requirements apply for barges:
• GM≥1.0 m
• fmin=0.3 m+0.5Hmax where fmin is the minimum effective freeboard
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15.5.3.3
For load transfer operations carried out with open barge manholes the minimum “effective freeboard” (fmin)
during load transfer, including any defined “stop point”, should be
fmin=0.5 m+0.5Hmax
15.5.3.4
Stability checks should be carried out for the full range of probable GM values, object weight and centre of
gravity predicted during the operation. The checks shall include the effects of vessel deballasting and jacking of
object, where applicable.
Guidance note:
Particular attention should be paid to :
• operations with a small metacentric height, where an offset centre of gravity (object) may induce a heel or
trim during the ballasting/weight transfer i.e. when any transverse/longitudinal moment ceases to be
restrained by the host structure.
• cases where a change of wind velocity or wave direction may cause a significant change of heel and trim
during the installation/removal.
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15.5.3.5
Special attention should be paid to accurate interpretation and application of hydrostatic data for vessel(s),
substructure and/or (floating) object involved in the operation. For complicated operations, inclining tests (see
[2.10.5]) can be used to verify the hydrostatic stability parameters.
15.6
Operational aspects
15.6.1
General
15.6.1.1
The general requirements to planning and execution of the operation in Sec.2 apply.
Guidance note:
The following paragraphs include some additional requirements and/or emphasise on requirements considered
especially important for load transfer operations.
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15.6.2
Preparations
15.6.2.1
All structures and equipment necessary for the operation shall be correctly rigged and ready to be used.
15.6.2.2
It should be ensured that means (e.g. steel plates) and personnel (e.g. welders) for general repair work will be
available during the operations.
15.6.2.3
For operations or phases of operations that may be carried out in darkness, sufficient lighting shall be arranged
and be available during the entire operation.
15.6.2.4
All tugs that may be employed for critical tasks (i.e. including planned contingency measures) during the load
transfer operation should be nominated in due time, comply with the requirements of [11.12] and be available
for inspection as required before the operation.
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15.6.3
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Clearances
15.6.3.1
Adequate minimum clearances, including clearances under water, for all phases of the load transfer operation
shall be defined and properly documented by calculations and surveys before and during the operation.
Guidance note:
More detailed requirements to clearances and type of surveys are indicated for each type of load transfer
operation in [15.7] to [15.9]. Welding/erection of “last minute” items should not be allowed without a proper recheck of the clearances.
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15.6.3.2
The involved land- and sea areas shall be checked for obstacles. All obstacles that could cause damages and/or
which may unduly delay the operation shall be removed.
15.6.3.3
If relevant, adequate tug air draught shall be ensured.
Guidance note:
The nominal air draught should be minimum 0.5 m. 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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15.6.4
Recording and monitoring
15.6.4.1
During the operation a detailed log should be prepared and kept, see [2.3.8].
15.6.4.2
Monitoring shall be carried out according to [2.9].
15.6.5
Environmental effects
15.6.5.1
Effects caused by (unexpected) swell and tide could be of significant importance for load transfer operations and
shall be duly considered.
15.6.6
Marine traffic
15.6.6.1
In areas with other marine traffic necessary precautions to avoid possible collisions (e.g. with the object, involved
vessel(s) or mooring lines) should be taken.
15.6.6.2
Possible significant waves from passing vessel(s) should be prevented.
Guidance note:
If required, local harbour authorities should be requested to put restrictions on the marine traffic.
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15.6.7
Organisation and personnel
15.6.7.1
General requirements to organisation, personnel qualifications and communication are given in [2.8].
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15.6.7.2
A readiness meeting shall be held shortly before the start of the operation, attended by all involved parties.
Guidance note:
Load transfer operations will in many cases involve personnel which are not participating in this type of operation
on a frequent basis. Personnel exercising and briefing are hence of great importance, see [2.8.3].
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15.6.7.3
Load transfer operations may involve rather complicated equipment. Hence, it should be ensured that
equipment operators have the required experience.
15.6.7.4
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:
• In order to allow for proper continuous work execution easy access to food, drinking water and toilets
should be arranged.
• 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 should be ensured.
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15.7
Specific for lift-off operations
15.7.1
General
15.7.1.1
This subsection gives specific requirements for lift-off operations as defined in [15.1.1].
15.7.1.2
Lift-off includes all activities from vessel positioning until the object is lifted to an acceptable height for tow out
(or safe mooring) above the construction supports.
Guidance note:
The weight of the object is normally transferred from the supports to the vessel(s) by de-ballasting of the vessel(s)
at rising tide.
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15.7.2
Planning and design basis
15.7.2.1
See [15.2] for general requirements. Operation Class shall be defined, see Table 15-1.
15.7.2.2
Other items of importance for the lift-off planning are normally:
•
•
•
•
•
•
•
•
layout of object on construction supports before lift-off
layout of object on board vessel(s) after lift-off
requirements to support heights and lay-out of vessel supports and vessel(s)
vessel(s) dimensions and strength
water depths
quay and ground strength/condition
accidental conditions
structural limitations for object, vessel supports, and vessel(s).
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15.7.2.3
Requirements for documentation are given in [2.3] and [15.11].
15.7.3
Load cases and load effects
15.7.3.1
General requirements for loads and load analysis are given in [15.3].
15.7.3.2
The lift-off operation, from initial contact through completed lift-off, represents theoretically an infinite number of
load cases for both the object and the vessel(s). Hence, the entire lift-off operation should be considered stepby-step and the most critical load case for each specific member of the object should be identified.
15.7.3.3
Local load effects due to ballast content in the vessel(s) tanks and due to global deformations of the object and
the vessel(s) should be considered.
15.7.3.4
Accidental load conditions should be identified, see [5.5.7]. Identified accidental loads that cannot be neglected
due to low probability (see [2.4.1]), should be included in the design calculations.
15.7.3.5
The load cases required to adequately address and combine all identified load effects should be analysed as
static load cases by distributing the self-weight, vessel support forces, and other loads to the actual members of
the object.
15.7.3.6
Local loads on the object and on the vessel(s) during positioning and mooring at the construction site after lift-off
should be included as found relevant in the calculations/analysis.
15.7.3.7
Forces in anchoring, mooring and fendering equipment/structures due to functional and environmental loads
should be considered.
15.7.3.8
The force distribution in the object and in the vessel(s), and their global deflections, should preferably be
determined by a 3-dimensional analysis.
15.7.4
Structures
15.7.4.1
Structures and structural elements shall be verified according to principles and requirements in [5.2].
15.7.4.2
Special attention should be paid to the assessment of local support loads from the vessel supports and other
external loads.
15.7.4.3
Vertical deflection tolerances should be specified from the structural analysis of the object such that
unacceptable vertical deflections may be avoided. The selected deflection tolerances shall consider the practical
limitations of the shimming procedure.
15.7.5
Object supports
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15.7.5.1
The object’s construction supports should have sufficient strength to withstand the object self-weight and
relevant skew loads, relevant impact loads from vessel(s), mooring forces, forces due to environmental loads,
etc., occurring during the lift-off operation.
15.7.5.2
The object’s vessel supports should have sufficient strength to withstand all vertical and horizontal forces during
lift-off.
Guidance note:
The horizontal forces may be reduced by decreasing the horizontal restraint by means of e.g. low friction
surfaces.
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15.7.5.3
The vertical load distribution to all supports should be controllable, i.e. it should be ensured that the support
reactions throughout the load transfer are within the allowable reaction loads.
Guidance note:
The reactions could be controlled by one or a combination of the following means:
• support load monitoring
• hydraulic load distribution system
• shimming of the vessel supports in accordance with an appropriate procedure. (Possible as-built
deviations and calculations inaccuracies, etc. should be accounted for.)
• a flexible support system to be used between the top of the vessel supports and the object. (The flexible
support system may be obtained by using crushing tubes, lead plates, wood, wedge systems or similar.)
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15.7.6
Mooring and positioning systems
15.7.6.1
General design requirements for mooring and positioning systems are given in Sec.17 and [4.4]. Other
additional requirements applicable for lift-off are given below.
15.7.6.2
The load cases described in [15.7.3.6] and [15.7.3.7] should be considered.
15.7.6.3
Horizontal load bearing capacity between the object and the construction supports considered as part of the
mooring shall be thoroughly documented.
15.7.6.4
Facilities to re-tension mooring lines should be available and in stand by position during the lift-off. Such facilities
may be winches, jacks for tensioning, etc.
15.7.6.5
Fendering structures should be arranged on the vessel sides or the construction pillars to prevent damages to
the vessel(s) during the lift-off operation.
15.7.6.6
The vessel(s) should be equipped with guides to ensure accurate positioning underneath the object before
starting the lift-off operation.
15.7.6.7
The positioning and mooring system should provide for correct alignment and securing of the vessel(s) during all
phases of the operation.
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15.7.7
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Monitoring and monitoring systems
15.7.7.1
The following lift-off parameters should as applicable be monitored and recorded, see [15.6.4], before and
during the operation:
•
•
•
•
•
•
•
•
•
•
tide
swell
support reactions
object deflections
vessel deflections and draught
water level in vessel tanks
air pressure in air pressurised vessel compartments
clearance between the vessel supports and the object
seabed clearances
clearance between construction supports and the related object.
Guidance note 1:
Normally a remote reading sounding system should be used for tank water level control. A back-up 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.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Support reaction measurements and comparison of the results with the actual ballast water and tide situation
should be performed continuously during the lift-off. The actual deviation in total load and moments should be
noted for each measurement and compared with agreed tolerances.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.7.8
Operational requirements
15.7.8.1
The operational requirements in [15.6] are generally applicable.
15.7.8.2
The lift-off site including the seabed should be surveyed before installation of the vessel(s). The survey should
verify that the vessel(s) vertical and lateral clearances are acceptable for the planned operation, see [15.7.8.6] to
[15.7.8.10].
15.7.8.3
If it is planned for tug(s) to enter into a dock during or after the lift-off operation, then it shall be documented that
clearances are adequate for this, see also [15.6.3.3].
15.7.8.4
Obstacles that may damage the vessel(s) or impede the operation should be removed.
15.7.8.5
If grounded vessel(s) will be used then this should be considered in the site preparations, see also [10.8.1].
15.7.8.6
Sufficient vertical clearance, considering any possible heel, trim and/or motion, shall be maintained between the
underside of the object and the top of the vessel supports during positioning of vessel(s) and before the weight
transfer operation.
Guidance note:
This clearance should relative to a reference tide level, be greater than 25% of the tidal range and 0.25 m. The
reference tide level should be defined taking adequately into account the operation procedure/schedule
including contingencies.
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15.7.8.7
During possible mooring at the construction supports after weight transfer from these to the vessel(s), sufficient
clearance shall be ensured between the underside of the object and the top of the construction supports.
Guidance note:
The minimum vertical clearance at low tide should greater than 25% of the tidal range whilst moored and 0.25 m.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.7.8.8
Sufficient horizontal clearance between vessel(s) and construction supports should be ensured throughout the
operation.
15.7.8.9
Sufficient under-keel clearance should be documented for vessels during positioning. Normally the clearance
should not be less than 0.5 m.
15.7.8.10
During the weight transfer operation and after the lift-off operation a minimum under-keel clearance of 0.5 m
shall be maintained.
15.8
Specific for mating operations
15.8.1
General
15.8.1.1
This subsection gives specific requirements for mating operations as defined in [15.1.1].
15.8.1.2
Mating includes ballasting of the floating substructure, positioning, load transfer (e.g. of the topside weight) from
vessel(s) to the floating substructure and de-ballasting of the substructure to final draught.
15.8.2
Planning and design basis
15.8.2.1
See [15.2] for general requirements. Operation Class shall be defined, see Table 15-1. Mating operations are
normally Operation Class 4.
15.8.2.2
The following parameters should be considered in relation to operational feasibility and structural limitations of
the object on vessel(s) and the substructure:
•
•
•
•
•
environmental conditions
time limitations determined by the weather forecasting period
topographical limitations
structural limitations for object, vessel(s), vessel supports, substructure, etc.
freeboard and hydrostatic stability.
15.8.2.3
Requirements for documentation are given in [2.3] and [15.11].
15.8.3
Load cases and load effects
15.8.3.1
General requirements for loads and load analysis are given in [15.3].
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15.8.3.2
The requirements for skew loads in [15.3.4] shall be considered.
Guidance note:
The items listed in [15.3.4] should be considered as relevant. In addition, fabrication tolerances (including
supports) and possible heel and trim variations of the sub-structure should be considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.3.3
The load transfer procedure shall consider any requirements to limiting “built-in” skew load effects.
Guidance note:
Analyses should be performed as required to find the skew loading effects that could remain as permanent
(“built-in”) loads after completion of the mating.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.3.4
The basic load cases for the object on vessel(s) and the substructure should be determined by evaluating the
following activities:
•
•
•
•
Ballasting of the substructure to mating draught.
Positioning of the object on vessel(s) above the substructure.
De-ballasting of the substructure to contact with the object.
Object weight transfer from the vessel(s) to the substructure by combined de-ballasting of the substructure
and ballasting of the vessel(s).
• Removal of the vessel(s) and de-ballasting of the substructure to the defined inshore safe condition/towing
draught.
15.8.3.5
Each phase of the mating operation should be considered step-by-step and the most critical load case for each
specific member of the structures should be identified.
15.8.3.6
The vessel loading condition for each stage of mating shall be determined as from the first contact to 100%
transfer. These stages shall be analysed for the vessel at intermediate draughts, to allow for ballasting.
15.8.3.7
The basic load cases for the substructure are determined by loads from;
• external/internal hydrostatic pressure,
• internal transfer of ballast water and
• object self-weight.
15.8.3.8
The basic load cases for the object on vessel(s) are determined by loads from;
• transfer of object self-weight from the vessel(s) to the substructure, and
• transfer of ballast water in the vessel(s).
15.8.3.9
The load cases given in [15.8.3.7] and [15.8.3.8] may be analysed as static load cases.
15.8.3.10
Positioning and mooring loads acting on the substructure or the object on vessel(s) should be considered.
Adequate protection against positioning loads should be ensured.
15.8.3.11
Motion amplitudes due to waves should be determined according to [5.6.12].
15.8.3.12
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All realistic accidental load conditions should be identified, see [5.5.7]. Identified accidental loads that cannot be
neglected due to low probability (see [2.4.1]), should be included in the design calculations.
15.8.4
Structures
15.8.4.1
Structures and structural elements shall be verified according to principles and requirements in [5.2].
15.8.4.2
Adequate horizontal support between object and substructure shall be ensured from the start of the load
transfer.
Guidance note:
The positioning system, see [15.8.6], could be considered in the load transfer phase. The effects of friction may
be taken into account.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.4.3
The horizontal restraint (support) capability shall be designed considering all relevant loads including effect of
maximum heel/trim due to defined damage cases. Where friction is taken into consideration, a safety factor
against sliding of at least 3 shall be documented.
Guidance note:
Damage cases that cannot be disregarded due to low probability should be considered. It could also be relevant
just to define a maximum heel/trim as an accidental design case. Normally it is acceptable to consider damage
cases only in the phase after de-ballasting to the planned (minimum safe condition) draught. Wind heel and
possible effects of current and waves should be considered. Horizontal restraints should be verified for ULS
and/or ALS according to the defined loads and load cases. See Table 5-1 in [5.5.2].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.4.4
Vessel supports should have sufficient strength to withstand all vertical and horizontal forces introduced by
deflections of the object and the vessel(s) during object weight transfer.
15.8.4.5
The substructure should be protected against possible accidental loads such as mooring line failure (not relevant
if the mooring lines are slack during mating), flooding of buoyant compartment(s), dropped objects, collision
loads, etc., during the mating operation.
15.8.4.6
Dimensional control of the height and locations of the structure and substructure mating points shall allow for
possible variations.
Guidance note:
This may be due to due to temperature differences, hog or sag during mating (compared to when measured)
and any horizontal movement of columns during deep submergence.
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15.8.4.7
Any limitations on the maximum allowable duration of deep immersion e.g. due to concrete creep, in relation to
the structural stability of the substructure, should be established and the procedures planned accordingly.
15.8.5
Systems, equipment and vessels
15.8.5.1
Requirements for systems and equipment are given in [15.3.5.2], and for vessels in [15.5].
15.8.5.2
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The de-ballast systems shall have sufficient capacity to complete the mating operation within the planned
operation period (TPOP).
Guidance note:
See Table 4-2 in [4.3.6] for guidance.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.5.3
The substructure without the deck should be capable of being deballasted to a freeboard at which the host
structure has damage stability within 24 hours. An initial deballasting capability of not less than 2 m per hour is
recommended.
15.8.5.4
Failure of one valve used for ballasting/de-ballasting shall not cause uncontrolled filling/draining of tanks on selffloating structures not complying with the one compartment damage stability requirement, see [11.10.4].
15.8.5.5
Adequate back-up shall be available for all ballast pumps, compressors, and generators.
Guidance note:
See Table 4-3 in [4.3.10] for guidance.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.5.6
The ballasting systems should be capable of levelling the structure by eccentric ballasting/ de-ballasting to
compensate for any shift in the centre of gravity during the mating operation
15.8.5.7
Pipe systems and valves should be designed to prevent accidental cross flooding and uncontrolled ingress of
water.
15.8.5.8
Sealings around cables, pipes etc. penetrating a water tight bulkhead should be designed for the maximum
possible differential pressure duly considering all phases of the operation.
15.8.5.9
Ballast compartments, which are intended to remain dry, should have adequate drainage capability to eliminate
free surface effect from possible ingress of water. Water detection sensors/equipment should be evaluated.
Guidance note:
If the filling rate could (i.e. in case of accidental type ingress of water) be higher than the drainage capability then
this should be considered in a damage stability check.
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15.8.5.10
Air venting systems from cells and ballast compartments should have adequate monitoring and control to
prevent excess structural loading during ballasting and de-ballasting of compartments.
15.8.5.11
Umbilicals for remote power and control should be adequately protected and be backed up by additional
systems to cover breakdowns or rupture.
15.8.5.12
Power and control systems should have adequate redundancy to cover failures and to ensure object transfer
within the defined period.
15.8.5.13
Immersion trials should be performed at selected draughts before the mating operation. These trials should be
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used to test the performance of the pumps, power/control systems and water tightness of the structure.
Guidance note:
Some items that should be considered are:
• Selected draughts should normally at least include the deepest draught during mating.
• Where to check/inspect for leakages/water ingress (pump rooms, along piping, at valves, where pipes etc.
penetrate tank walls, in bottom of access shafts) to be carefully evaluated.
• Check of tank levels, draughts, heel, trim etc. over a time interval; e.g. remain at max submergence
draught for a minimum time.
• How to do, with sufficiently high accuracy, draught readings at columns when there are waves at mating
site?
• How to ensure that computer tank monitoring system works properly and show correct water level in all
tanks? E.g. check against calculations and/or check sensor readings by other means of tank level readings.
• Proper monitoring of all relevant parameters should be done, see [15.8.7.1] for guidance.
• Primary positioning system.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.5.14
Any temporary ballasting equipment used for the substructure shall be designed, constructed and operated in
accordance with [4.3].
15.8.6
Mooring, guiding and positioning systems
15.8.6.1
General design requirements for mooring and positioning systems are given in Sec.17 and [4.4]. Other
additional requirements applicable for mating are given below.
15.8.6.2
The substructure and the object on vessel(s) should be secured by primary positioning systems, which normally
are:
• a permanent mooring system for the substructure, see Sec.17
• the towing fleet for the object on vessel(s), see Sec.11.
15.8.6.3
The primary positioning system should be capable of securing the structures in the event that the mating
operation is interrupted.
15.8.6.4
The primary positioning system should be sufficiently accurate to ensure safe navigation and positioning of the
object on vessel(s) close to the substructure.
15.8.6.5
The secondary positioning system should ensure accurate and well controlled positioning of the object on vessel
(s) above the substructure.
Guidance note:
It should be documented that the positioning could take place without contact with unprotected areas of the
substructure, and without local impact loads exceeding the energy absorption capability of positioning
bumpers/fenders. The environmental effects should be considered. Especially varying wind and current may be
of significant importance. (See also [15.8.8.14] and [15.8.8.15]).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.6.6
The secondary positioning system (winches, wires, jacks, fenders, etc.) should have sufficient capacity to resist
inertia (impact) forces, wind forces, current forces, friction forces, etc. (See [15.8.6.1]).
15.8.6.7
See Table 15-2 for requirements to redundancy and back-up. Mating is normally defined as operation class 4.
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15.8.7
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Monitoring and monitoring systems
15.8.7.1
General requirements to recording and monitoring are given in [15.6.4]
15.8.7.2
The following mating parameters should be monitored manually or by monitoring systems during mating
operations:
• Relative position, orientation, and clearances of substructure and object before and during positioning.
• Clearances between vessel-object supports (e.g. between substructure and underside of module and
between barges and substructure) .
• Environmental conditions (monitoring should begin well in advance of the operation).
• Seabed clearances.
The vessel’s
•
•
•
•
water level in tanks
air pressure in compartments, if applicable
open/closed status for valves
trim, heel and draught.
The substructure's
•
•
•
•
•
•
water level in cells/tanks
air pressure in cells/tanks
open/closed status for valves
leakages
heel, trim and draught
submergence rate and motions.
Guidance note 1:
Normally a remote reading sounding system should be used for tank water level control. A back-up system, but
not necessarily remotely controlled (e.g. measuring ullages by hand) should be provided.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
Where possible, the support reaction measurements and comparison of the results with the actual vessel(s) and
substructure ballast situation should be performed continuously during the mating. The actual deviation in total
load and moments should be noted for each measurement and compared with agreed tolerances.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.8
Operational requirements
15.8.8.1
The operational requirements in [15.6] are generally applicable.
15.8.8.2
The minimum freeboard and reserve buoyancy for the substructure during the mating operation shall be
adequate and shall be agreed with the MWS company at an early stage of the project.
Guidance note:
For semi-submersibile type host structures generally the minimum freeboard is 4 m but not less than that
required to maintain 5% reserve buoyancy of the substructure.
For large (concrete) gravity base structures with open shafts generally the minimum freeboard is 6 m.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.8.3
During mating, the relative movements of the structures due to environmental loads should be carefully
considered.
15.8.8.4
All back-up systems should be ready for immediate activation during the critical stages of the mating operation.
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15.8.8.5
For mating operations involving substructure draughts greater than normally acceptable the schedules for
mating should be carefully planned in order to minimise the time at the maximum draught.
Guidance note:
In event of delays the substructure should be returned to an acceptable stand-by draught. For gravity base
structure the minimum freeboard should not be less than 20 m or the reserve buoyancy should be minimum 10%
at this draught. The substructure should have the capability of remaining at the stand-by draught for an indefinite
period.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.8.6
The following criteria should be considered in the selection of the mating site:
•
•
•
•
Environmental conditions.
Magnitude and direction of wind, waves, and current, protection against swell, etc.
Geographical limitations.
Feasibility of towing the object on vessel(s) to the mating site, sea room for mooring, minimum water
depth, etc.
15.8.8.7
The seabed at the mating site should be surveyed before submergence of the substructure to mating draught, if
the seabed clearance is considered critical.
15.8.8.8
The location where mating will take place should be investigated for the possibility of variations in the density of
the water. If rapid changes in density is possible, density measurements should be performed before and during
the mating.
15.8.8.9
The requirements for preparations in [15.6.2] apply.
15.8.8.10
All connections between the vessel(s) and the object, which may hamper the lift-off, should be properly removed
before start of weight transfer.
15.8.8.11
A seabed survey at the site shall be documented, covering the total excursion area. The depth contour lines shall
be drawn in sufficient detail to give an adequate indication of seabed profile, considering the seabed slopes and
actual clearances encountered.
15.8.8.12
Sufficient under-keel clearance to the seabed for the substructure should be ensured at the maximum mating
draught considering minimum tide and any possible heel, trim and/or motions.
Guidance note:
The bottom clearance should normally be at least 2 m.
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15.8.8.13
The extension of the area giving adequate bottom clearance shall be defined. Positioning accuracy, maximum
excursions caused by the environmental loads plus an adequate margin should be considered.
Guidance note:
Normally “adequate margin” should be defined as minimum half the diameter of the substructure at its lower
end.
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15.8.8.14
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Sufficient clearances between unprotected parts of the substructure and both the object and vessel(s) should be
ensured considering any possible heel, trim and/or motions.
Guidance note:
The following minimum values are recommended:
• 0.5 m sideways clearance during positioning
• 0.25 m vertical clearance between the underside of the object and the top of the substructure during
positioning
• 0.5 m under-keel clearance between the vessel and substructure. (If the substructure has underwater
elements limiting the water depth).
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.8.15
Adequate clearances shall be ensured between object or vessel(s) and the substructure throughout positioning,
load transfer and removal of vessel(s).
Guidance note 1:
Contact (i.e. zero clearance) between the vessel(s) and protected (i.e. by fenders/bumpers) parts of the
substructure is allowed if properly planned for. See [15.8.6.5]. The effect of friction between vessel(s) and fenders
should be considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
When towing a barge in between the columns of a substructure, clearances can be tight. If there are strong
currents at the mating site then this can be a challenge. The possibility for the barge to get jammed between the
substructure columns should be given due attention in such cases
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.8.8.16
Transport vessel(s) may get a relative (to the mated object) trim/heel during the final phase of the load transfer.
Clearances at the support points shall be adequate to handle such relative trim/heel.
Guidance note:
Normally the transport vessel(s) should be ballasted to minimize the relative trim/heel.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.9
Specific for float-over operations
15.9.1
General
15.9.1.1
This subsection gives specific requirements for float-over operations as defined in [15.1.1].
15.9.1.2
Float-over includes positioning and ballasting of the transport vessel and load transfer of the object (e.g.
platform deck) from the transport vessel to the fixed host structure.
15.9.2
Planning and design basis
15.9.2.1
See [15.2] for general requirements. Operation Class shall be defined, see Table 15-1.
15.9.2.2
Strict environmental limitations normally apply for a float-over. Such conditions could be difficult to obtain
offshore and this should be duly considered in the planning.
15.9.2.3
The Planned Operational Period (TPOP) should be as short as practically possible, and if relevant the point of no
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return should be clearly defined. (See [15.2])
15.9.2.4
In addition to limitations addressed in [15.9.2.2] and [15.9.2.3] the following should be considered in relation to
operational feasibility:
• Structural limitations for object, vessel, vessel supports, host structure, etc.
• Clearances between barge, object and host structure
• Impact loads; possible need for shock dampers (e.g. leg mating units), fendering etc.
15.9.2.5
Requirements for documentation are given in [2.3] and [15.11].
15.9.3
Load cases and load effects
15.9.3.1
General requirements for loads and load analysis are given in [15.3].
15.9.3.2
The requirements for skew loads in [15.3.4] shall be considered as relevant.
15.9.3.3
The basic load cases should be determined by evaluating the following activities:
•
•
•
•
•
•
(De)-ballasting of vessel before positioning
Positioning of vessel and object (e.g. platform deck) above the host structure
Ballasting of vessel to first contact between object and host structure
Load transfer of object weight from vessel to host structure by further ballasting of vessel
Last contact between object and supports on the vessel
Further ballasting and removal of vessel from host structure
Guidance note:
The description above assumes load transfer is by ballasting only. If load transfer is aided by other means, such
as for instance jacking, then the sequence of activities will be different and consequently so will the load cases to
be considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.9.3.4
The load transfer should be considered step-by-step and the most critical load case for each specific member of
the structures should be identified.
15.9.3.5
Positioning and mooring loads acting on the host structure or the object to be installed should be considered.
Adequate protection against positioning loads should be ensured.
15.9.3.6
All realistic accidental load conditions should be identified, see [5.5.7]. Identified accidental loads that cannot be
neglected due to low probability (see [2.4.1]) should be included in the design calculations.
Guidance note:
If the float-over operation is planned to be executed without the use of fenders then the global and local capacity
of the host structure should normally be documented for an accidental impact scenario.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.9.3.7
Wave loads and motions due to waves should be considered for mooring, guide and support reaction load
calculations.
15.9.3.8
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Motion amplitudes due to waves should be determined according to [5.6.12].
15.9.3.9
The stiffness of the mooring system should be taken into account in the motion response analysis.
15.9.3.10
An adequate analysis model and method shall be used to establish both horizontal and vertical dynamic (impact)
reaction loads during the positioning and load transfer phases considering the following:
1. It is recommended that the motions of the transport vessel and associated docking, mooring line and
fender loads are analysed in the time domain for pre-docking, docking, load transfer and undocking
positions.
2. Non-linear effects of the stiffness of the host structure/object/vessel, mooring configuration, shock
absorbers, fendering system, etc. should be considered.
3. A Monte Carlo simulation or multiple seed simulation is recommended performed to define maximum
values. The simulation period for each stationary stage should reflect the actual operational period
multiplied by a factor of two to capture a contingency period. The time step to be used should be selected
so as to achieve results that differ by no more than a few percent when the time step is halved and be
sufficiently small to ensure that the maximum peak motion is identified.
◦ When a Monte Carlo simulation is used the design value should have a probability of exceedance of
not more than 63% and the number of simulations should be such that the design values change by
no more than 10% when the number of simulations is doubled.
◦ When a multiple seed simulation is performed, the number of seeds should be no fewer than 10 and
the average of the maxima should be used as the design value.
Guidance note:
Design values determined are applicable only when the operational metocean limits are reduced below
the design values with the applicable Alpha Factor(s) from [2.6].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.9.3.11
Where float-over operations are conducted in the shelter of a breakwater (e.g. for tanker loading facilities at
coastal locations), the adverse effects of the breakwater on the waves and current should be considered when
determining the environmental loading on the installation vessel.
15.9.3.12
An assessment of the speed at which the object and vessel can separate during installation should be made. As
the vessel starts to separate from the object there will be a tendency for re-contact due to the vessel motions.
Mitigations shall be considered to avoid damage to object. (For removal operations the vertical speed before
load transfer should be assessed.)
15.9.4
Structures
15.9.4.1
General
a. Structures and structural elements shall be verified according to principles and requirements in Sec.5.
b. Horizontal restraint should be provided between the vessel and the host structure to absorb any possible
impact loads during float-over and to prevent lateral movement of the object (e.g. platform deck) after
initial engagement with the host structure.
c. The effects of un-even load distribution during the float-over operation shall be considered.
15.9.4.2
Seafastenings
1. Seafastenings on the installation vessel shall be designed to:
◦ Resist seafastening forces during the voyage to the float-over location, see [11.9.5]
◦ Minimise offshore cutting or welding, possibly by the use of mechanical devices
◦ Provide restraint after cutting equivalent to 5% of the structure weight acting horizontally
◦ Permit installation without fouling.
2. A design case shall be established for any seafastenings that remain after initial seafastening cutting with
the installation vessel in a stand-off position before the vessel being manoeuvred into the docking slot.
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3. Where a jacking system is used to achieve clearances during the initial docking and subsequent
operations, the jacking system shall be suitable to provide lateral restraint equivalent to 10% of the
structure weight acting horizontally.
4. Where mechanical seafastening systems are used their capacity to resist design loads shall be
demonstrated.
5. All the equipment on the installation vessel, including ship loose items, shall be properly fastened to the
deck for the tow and float-over phases.
6. All seafastening cut lines should be clearly marked. If cutting in 2 stages, the two sets of cut lines should
preferably be marked in different colours.
15.9.4.3
Removal operations
For removal (decommissioning) operations reduced load factors may be acceptable both for the object to be
removed (e.g. the platform deck) and for the host structure. Local damage to host structure and/or object may
also be acceptable for such operations. For further information please see Sec.18 and DNV-RP-H102 “Marine
Operations During Removal of Offshore Installations”, /55/.
15.9.5
Systems, equipment and vessels
15.9.5.1
General
a. General requirements for systems and equipment are given in [15.4] and for vessels in [15.5].
b. Shock absorbers may be used between object and host structure and/or between object and installation
vessel. This in order to dampen vertical and horizontal motions and help distribute load evenly.
c. A jacking system or a rapid ballast system may be used in combination with a mechanism which allows for
rapid transfer of the object to the host structure and establishment of clearance between the object and
the installation vessel. Such systems should be optimized to reduce both the risks of weather downtime
and the potential for impact damage between object, vessel and host structure.
d. If any passive or active heave-compensation systems are used to compensate for relative motions, then
specification, capacity and design of these systems shall be stated in the operational procedures.
15.9.5.2
Installation vessel
1. For requirements to ballasting and gauging systems on the vessel, see [15.9.5.3].
2. The equipment installed on the installation vessel (e.g. winches, fairleads, towing and mooring lines, etc.)
shall comply with the requirements of the MWS company and have valid certificates.
3. The installation vessel shall have electrical, hydraulic and/or pneumatic power plants with an independent
100% back up to supply all power for installation operations. It shall have sufficient lighting to illuminate
the complete vessel deck and other operating areas to allow the float-over operation to proceed safely on
a 24-hour basis. In particular, all critical systems shall be shown to have adequate:
◦ Reserve capacity
◦ Back-up power
◦ Testing and commissioning before use
◦ Failure mode identification and acceptability
◦ Fail-safe condition (where practicable)
◦ Over-rides and alternative controls for emergencies
◦ Marinisation of key components
4. Where DP vessels are planned to be used to execute a float-over operation, the requirements of
[15.9.5.10] will apply.
15.9.5.3
Ballasting systems
a. General requirements for ballasting systems are given in [15.4.2] and [4.3].
b. The capacity- and redundancy requirements to the ballasting system shall be based on the operation class.
See [15.2.1].
Guidance note 1:
Operation Class 1 should normally be avoided for float over operations.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
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These requirements are based on load transfer by pumping of ballast only. If a rapid ballast system is used
or if load transfer is done by jacking, then these requirements may have to be modified. This should then
be agreed with the MWS Company.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--c. For float-over operations offshore the installation vessel shall have a permanent ballast system which may
be supplemented by temporary systems.
d. Control of the pumping systems and ballast valves shall be from a centralised ballast control room.
e. The installation vessel shall have a remote tank gauging system capable of continuously monitoring the
level of liquids in all ballast tanks simultaneously. It should be possible to take all readouts at one single
location, i.e. normally in the ballast control room. The ballast tanks shall also be fitted with sounding tubes
or ullage access to allow manual measuring of the tank levels.
f. A detailed ballasting procedure shall be developed for each stage of the float-over. The ballast
calculations shall include the quantity of water in each ballast tank for each stage of the operation. The
ballast procedure shall consider float-over clearances, keel clearance, load transfer, tidal range, expected
timings and vessel freeboard.
g. Where “drop tanks” are used to change vessel trim and draught, the operational features and control of
these tanks as part of the ballasting system shall be documented, see also [4.3.4.2].
h. All pumps and systems shall be tested and shown to be operational before the transport to the installation
site commence. At the discretion of the MWS Company verification of pump capacity may be required.
i. Provision shall be made for the detection of any likely movements of fresh water (freshets) that could cause
significant draught changes.
15.9.5.4
Assist tugs and support vessels
a. Where the installation vessel is a barge, a main tug capable of controlling the barge shall be provided. This
tug should be an AHV or equivalent.
b. Additional tugs may be required for anchor handling and/or for assistance during positioning of
installation vessel above host structure. When required these shall be highly manoeuvrable tractor tugs
with a specification to meet the needs of the operations. The tugs shall be classed for offshore work (if
appropriate) and crewed for 24 hour operations.
c. Any AHV used for anchor handling shall be fitted with a Tug Management Positioning System (TMPS)
which is sufficiently accurate to allow anchors to be positioned within 5 m of target.
d. An accommodation/work vessel may be required for offshore personnel and to permit host structure
preparations prior to, during and after the structure float-over. The vessel specification shall be developed
to suit the specific requirements of the project.
e. A work boat for personnel transfer shall be operated by a competent trained coxswain and have sufficient
crew members to assist during personnel transfers.
15.9.5.5
Fenders, tethers and guides
a. Devices to assist or control the safe entry of the installation vessel into the host structure slot should
normally be provided. These devices may be on the installation vessel and/or host structure.
Guidance note:
When DP is the primary method of stationkeeping for the installation vessel it may be that such devices are
not necessary, but this requires demonstration of acceptable risk level, see [15.9.5.10], [15.2.3] and [2.4].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. Design should be such that it acts to reduce the motions of the installation vessel, provide protection to
the host structure and guide the entry of the installation vessel into the host structure slot.
c. Design loads should be derived from detailed analysis of possible vessel motions, see also [15.9.3].
Design friction coefficient(s) used shall account for any facings applied to fenders.
d. The engineering properties (strength, stiffness, damping, hysteresis, elastomeric creep) of all the
components and systems should normally be verified by tests which cover the full range of conditions (e.g.
forces, displacements) anticipated for the float-over.
e. In most cases a suitable fendering system will be required to reduce stresses in the host structure and the
installation vessel in the event of contact (by spreading the load and limiting the relative motions). Fenders
on the host structure should be of sufficient depth to ensure that they engage the side of the vessel at all
stages of the float-over operation.
f. In addition to reducing the installation vessels impact loads on the host structure, sway and surge fenders
may also be used to limit the installation vessels motions, for instance when it is within the confines of a
jackets legs.
Guidance note:
During the load transfer operation the vessel may be held in position by tethers connected to the host
structure reacting against surge fenders, see also [g)] and [15.9.5.8 c)].
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To reduce clearances between the host structure and the vessel during the load transfer operation sway
fenders may be fitted to the vessel sides and so improve the lateral positioning of the vessel.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--g. Tethers may be used to limit the installation vessels motions and hold the vessel in position prior to, during
and after the load transfer phase, see also [15.9.5.8].
Guidance note:
Tethers may be used alone (in both directions) or for instance in combination with surge fenders, see also
[f)].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--h. It should be considered if it will be beneficial to provide guides on the installation vessel and/or the host
structure in order to facilitate entry of the installation vessel into the host structure slot.
15.9.5.6
Positioning systems
1. General requirements to positioning systems are given in [15.4.3].
2. A positioning system ensuring accurate, i.e. within the specified tolerances, and safe guidance and
positioning of the object/vessel shall be provided.
3. Mooring and positioning of the vessel into the host structure may be with a vessel with a DP system, see
[15.9.5.10], or by one or more of the following:
◦ pre-laid mooring lines/anchors, see [15.9.5.7]
◦ tethers, see [15.9.5.8]
◦ the installation vessel’s propulsion systems, see [15.9.5.8]
◦ tugs, see [15.9.5.9]
4. Applicable design loads due to inertia (impact), live loads (e.g. maximum winch pull), wind, current, waves,
etc. both in ULS and ALS should be defined for all parts (winches, wires, jacks, fenders, etc.) of the
positioning system. The design loads shall be defined based on all phases of the positioning. Adequate
resistance (safety factors) of all parts of the positioning system shall be documented.
5. Redundancy and back-up requirements to the positioning system shall be based on the Operation Class,
see [15.2].
6. For positioning systems it should be considered to incorporate damping systems in order to control
motions and potential impact loads.
Table 15-2 Positioning system requirements
Operation Class
1
The positioning system shall fulfil the following main requirements:
•
•
•
•
The design loads (see [4)]) shall be multiplied with a consequence factor of 1.3.
Reversing of the operation shall be possible.
ALS fulfilled for any single failure.
The positioning could be completed without significant delay after a single failure in
the system.
2
• Reversing of the operation shall be possible.
• ALS fulfilled for any single failure.
• The positioning could be completed without more than 2 hours delay after a single
failure in the system.
3
• ALS fulfilled for any single failure.
4
• Reversing of the operation shall be possible.
• ALS fulfilled for any single failure.
• The positioning could be completed without more than 6 hours delay after a single
failure in the system.
5
• No critical damages and the object/vessel(s) remain in a stable condition after a
single failure in the system.
Notes:
1. Fulfilment of ALS means; 1) no unacceptable damages and 2) the operation could be completed or the
object/vessel(s) brought to a safe condition within the available operation period.
2. If the requirement to reversing of the operation is not possible to fulfil throughout the operation the
point of no return should be clearly defined.
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15.9.5.7
Positioning by use of pre-laid mooring lines/anchors
a. A stand-off mooring system shall be provided.
b. When required, the installation vessel mooring system shall be designed to resist the environmental loads,
allowing the vessel to maintain position before load transfer.
c. The mooring system shall be verified both for operational conditions and for extreme environment standby cases.
d. A mooring analysis shall be carried out for the installation vessel at the stand-off location and at the
incremental stages that comply with the installation procedural steps. The mooring analyses shall
document compliance with Table 15-2 and Sec.17 as applicable.
e. All installation vessel mooring lines and tethers shall be capable of being tensioned by the use of winches
or capstans.
f. Clearances around mooring lines and anchors should comply with Table 17-7. Exemptions may be
considered for the final float-over stages when close to the host structure, but such exemptions should be
avoided to the extent practically possible. Exemptions shall be subject to risk assessment in accordance
with [2.4].
g. All anchor lines shall be pre-installed and pre-loaded to maximum operating loads with a safety factor and
holding period to be agreed.
15.9.5.8
Position keeping by use of tethers
a. The tethers shall be designed to hold the vessel in the load transfer position and ensure that vessel
motions do not exceed positioning tolerances. The characteristics of the tethers shall be accurately
modelled in the analysis.
Guidance note:
When positioning a platform deck above a jacket and using LMU’s it should be documented that motions
do not exceed the capture radius of the LMU’s.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. Temporary mooring tethers shall be designed for the maximum analysed dynamic tensions and sized
based on a factor of safety of 1.67 against the certified MBL.
c. The vessel may also be held in position using the vessel’s propulsion systems with constant thrust against
surge fenders.
15.9.5.9
Positioning by use of tugs
A combination of mooring lines and tug(s) may be used for vessel positioning. Tugs connected to pre-laid
moorings may be used to provide extra control in variable currents, using their tow winches to adjust the vessel
position.
15.9.5.10
Dynamic positioning
a. If the vessel has dynamic positioning capability (minimum DP Class 2), consideration can be given to the
use of DP in place of vessel moorings, subject to review of stationkeeping analyses and DP operating
procedures. The requirements in [17.13] will apply.
b. When DP is used rigorous risk assessment is always required, see [15.2.3] and [2.4].
Guidance note:
The probability and consequence of impact between the vessel (or the object on the vessel) and the host
structure should be assessed. Contact velocities and forces should be determined from a comprehensive
range of realistic scenarios. The possible damages to the vessel, object and host structure should be
quantified and assessed against the probability of the incident occurring in order to ensure a sufficiently
low risk level.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--c. Minimum static vertical and horizontal clearances between the host structure and the installation vessel
should be established.
Guidance note:
As a guide, minimum static horizontal clearances of 5 m between the extremities of the host structure and
the installation vessel should be provided if using vessels with a Class 3 notation and with DP reference
systems that meet the Class 3 notation. See also [15.9.6.5].
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
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15.9.5.11
Leg Mating Units (LMU)
a. Leg mating units (LMUs) may be required installed between the object and the host structure. This in order
to adequately dampen the maximum expected vertical and horizontal motions and aid even distribution of
loads.
Guidance note:
LMUs are shock absorbing devices specially designed for installation of platform decks on jacket legs.
They generally take both lateral misalignment jerks and vertical loads and may also aid positioning of the
platform deck on the jacket and load distribution between jacket legs. Such units could for instance consist
of vertical and horizontal elastomers assembled in metal fabricated cans. LMUs could be provided
within/at top of jacket legs or inside/at bottom of the topsides legs.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. The Leg Mating Unit (LMU) design should consider the loads, stroke and motion response expected
during the load transfer operation. The LMU performance characteristics should be considered in analyses
for the load transfer of the object from vessel to host structure.
c. Once the object weight is fully transferred to the host structure, final lowering to achieve steel/steel
contact may be required, often after vessel removal.
d. Host structure leg access platforms shall be incorporated with safe access from the sea for operation and
inspection of LMU’s. These platforms can also be used for host structure to deck leg weld out.
15.9.5.12
Deck support units (DSU):
a. Deck support units (DSUs) may be provided at the interface between the object and the support structures
on the vessel. The DSUs can be supplied with or without shock absorbers. Where DSUs are supplied with
shock absorbers, this will dampen vertical motions and help to distribute the loads evenly. Where there
are no shock absorbers in the DSU, but there is a low friction sliding surface, this decouples vessel mass
from object so that vessel inertia does not add lateral loadings to the host structure.
Guidance note:
The DSUs should provide robust sliding support during the load transfer operation. Sliding surfaces
should be treated/polished with suitable products so that low friction/smooth transition of forces is
ensured.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. The Deck Support Unit (DSU) design should consider the loads, stroke and motion response expected
during load transfer operations. The DSU performance characteristics should be considered in analyses for
the load transfer of the object from vessel to host structure.
Guidance note:
If DSUs are configured without shock absorbers, but has a low friction mating surface to allow the vessel to
move freely during the float-over operation, then the mass of the topside can be considered independent
from the vessel.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.9.5.13
Jacking systems:
1. If a jacking system enabling fast load transfer is applied, then detailed HAZIDs of the system shall be
carried out.
2. The following requirements apply for the jacking system:
◦ System strength, capacity and control means shall be documented
◦ The system shall be designed to ensure the stability and restraint of the object as it is raised above
its transport position
◦ Redundancy shall be provided so that there is no single point of failure in the system
15.9.5.14
Markings:
a. For host structures with columns, like e.g. jackets, the identity of each leg shall be clearly marked with row
and line reference.
b. Draught mark elevations shall be painted on the host structure (legs). After host structure installation, a
survey shall note corrections to be made to the markings for accurate tide measurement. Level markings
shall be floodlit so that they are clearly visible during darkness.
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c. Tide boards can be used if the painted (leg) markings on the host structure are not adequate. Design
elevations shown on the host structure legs shall relate to the lower edge of the mark, and shall be clearly
visible at a distance of not less than 50 m and shall include increments at a maximum of 200 mm.
Corrections by which these marks may be related to MSL, HAT or LAT shall be known.
d. Design elevations shown on the host structure legs shall relate to the lower edge of the mark, and shall be
clearly visible at a distance of not less than 50 m and shall include increments at a maximum of 200 mm.
Corrections by which these marks may be related to MSL, HAT or LAT shall be known.
15.9.5.15
Equipment for monitoring of motions/clearances:
1. The following critical factors shall be monitored using an MRU (Motion Reference Unit) for the float-over
installation/removal:
◦ The six degrees of freedom motions of the vessel in a free-floating mode. This is to ensure that the
motions can be compared with those predicted by the motion analysis. These are usually the
motions and accelerations at the system centre of gravity and should be used to check that the loads
and clearances remain at acceptable levels.
◦ The vertical clearance between the leg mating units (LMUs) on the structure, and the docking cones
on the underside of the host structure during the initial entry of the vessel into the host structure, or
any other critical vertical clearances during installation or removal operations.
2. MRU,s shall be calibrated and tested before sailaway to the mating/removal site.
15.9.5.16
Environmental monitoring systems
1. The environmental monitoring system has two primary functions:
◦ To confirm that conditions are suitable for the docking and mating operations to proceed.
◦ To provide input for the vessel or vessel’s DP system (if applicable).
2. The secondary function of the environmental monitoring system is to predict weather and environmental
trends before and during the float-over.
3. The environmental conditions which require monitoring are:
◦ Wind speed and direction
◦ Wave and swell heights and periods
◦ Current speed and direction
◦ Tidal height against time
4. A tide gauge should be installed in the field, as close to the host structure as is practical, and should be
monitored for at least two tide cycles before installation/removal to allow actual levels and cycle times to
be compared with predictions. During installation/removal corrections derived from this comparison shall
be used in conjunction with visual readings of the level marks on the host structure legs. A tide gauge may
also be fitted to the host structure for reference purposes.
5. To enhance operability an infield directional wave rider buoy or suitably positioned wave radar system
should be provided along with associated hardware recording wave height, direction, period and spectral
energy.
15.9.6
Operational requirements
15.9.6.1
General: Operational requirements are generally described in [15.6].
15.9.6.2
Weather forecasting arrangements: For offshore float-over a level A weather forecast should be provided, see
[2.7.2].
15.9.6.3
Draught: The maximum draught of the installation vessel during float-over shall not exceed the maximum load
line draught, without a class exemption. However, this is normally not required for semi-submersible heavy lift
barges or vessels.
15.9.6.4
Freeboard: Adequate freeboard to avoid green water shall be ensured for all phases of the operation.
Guidance note 1:
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The minimum freeboard is defined as the minimum distance from the waterline to the watertight deck level after
accounting for static trim and heel. The minimum freeboard shall be sufficient to maintain the vessel’s waterplane area and to ensure sufficient stability range to meet the requirements of [15.5] at all stages of the
operation, the minimum freeboard should usually be at least 1.0 m. A lower minimum freeboard may be
acceptable if adequate precautions and procedures are in place to ensure that the stability of the vessel is
maintained. The minimum freeboard used during the operation shall be confirmed with the vessel owner.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
For operations involving semi-submersible barges or heavy lift ships with watertight main decks, wave crests may
be allowed to over-top the vessel deck provided that all hatches and downflooding points are suitably protected
and that raised walkways are added to all areas affected by water on deck where personnel movement is
required.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.9.6.5
Clearances:
a. Adequate clearances shall be defined considering maximum expected motions, applied positioning
system and provided fendering/guiding.
Guidance note 1:
During approach for installation the minimum vertical clearance between the object stabbing cone and
host structure receptacles/jacket legs/piles should be minimum 0.5 m after accounting for maximum
dynamic vertical motions.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 2:
To allow safe removal of the installation vessel the minimum clearance between the keel of the vessel and
any part of the submerged host structure should be minimum 1.0 m after accounting for vessel maximum
motions at maximum draught.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--Guidance note 3:
The minimum vertical clearance between the LSF and the underside of the object following completion of
load transfer should be minimum 0.5 m after accounting for vessel maximum motions to allow safe
removal of the installation vessel.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. A system for controlling the clearances and support loads during the operation should be established.
c. Motions shall be controlled by monitoring before and during the operation, see also [15.9.5.14] and
[15.9.5.15]. Action(s) to be taken if the motions exceed the maximum expected motions shall be defined.
Guidance note:
The maximum vertical/horizontal movement of the stabbing cone should not normally exceed +/-0.5 m
during entry and weight transfer unless suitable systems and/or engineering are provided to compensate
for movements in excess of +/-0.5 m.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--d. There will be a tendency to re-contact between transport vessel and object as they start to separate.
Mitigations to avoid damages shall be considered.
e. Exclusion zones should be defined in the early phase of the project in order to minimise and avoid clashes
during the installation/removal operation.
f. The as-built clearances between the stabbing cones and the host structure receptacles/jacket legs shall be
checked after load-out of object to installation vessel and the procedures modified, if necessary. (Similarly
for removal operations the relevant measurements shall be checked before operations start.)
g. The as-built levels of the host structure should be verified as they can differ from the nominal values due to
seabed tolerances, especially where dredging operations are carried out.
h. The minimum clearances shall be calculated based upon the design draught of the vessel. The actual
vessel draughts shall be verified before starting the operation to confirm that they are in accordance with
requirements and that acceptable clearances are achievable. Note that the datum used for vessel draughts
may need to be corrected to account for any parts of the vessel that extend beneath the datum. Possible
freshets shall also be considered, see [15.9.5.3 i)].
15.10
Specific for docking operations
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15.10.1 General
15.10.1.1
This subsection gives specific requirements for docking operations as defined in [15.1.1].
15.10.1.2
Docking here refers to the positioning and setting of a floating object on under bottom supports. Both docking
onto seabed supports and onto floating vessels are addressed. Operations are generally assumed performed
inshore.
15.10.1.3
This subsection includes specific requirements for on- and offloading of HTVs by float-on/float-off type of
operation.
15.10.2 Planning and design basis
15.10.2.1
See [15.2] for general requirements. Operation Class shall be defined, see Table 15-1.
15.10.2.2
Items of importance for planning of docking onto seabed supports are normally:
•
•
•
•
•
layout and capacity of seabed supports
positioning of the object (e.g. vessel) on the seabed supports
soil conditions
structural limitations for the object
accidental conditions.
15.10.2.3
Items of importance for docking on floating vessels are normally:
•
•
•
•
•
lay-out of object (e.g. cargo) and supports on board the vessel
positioning of the object on the vessel supports
vessel and object dimensions, clearances
structural limitations for object, supports and vessel
accidental conditions.
15.10.2.4
Requirements for documentation are given in [2.3] and [15.11].
15.10.3 Load cases and load effects
15.10.3.1
General requirements for loads and load analysis are given in [15.3]. Requirements in Section [11.9] apply as
applicable.
15.10.3.2
Docking, from initial contact through completed load transfer, represents theoretically an infinite number of load
cases. Hence, the entire operation should be considered step-by-step and the most critical load case for each
specific member of the structures involved should be identified.
15.10.3.3
Accidental load conditions should be identified, see [5.5.7]. Identified accidental loads that cannot be neglected
due to low probability (see [2.4.1]), should be included in the design calculations.
15.10.3.4
Local load effects due to ballast content in object tanks and due to global deformations of the object should be
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considered.
15.10.3.5
Positioning and mooring loads acting on the object should be considered. Adequate protection against
positioning loads should be ensured.
15.10.3.6
Motion amplitudes due to waves should be determined according to [5.6.12].
15.10.3.7
If other vessels such as barges are to be transported by HTV then relevant contingencies on weight shall be
included to account for effects such as residual ballast water, marine growth etc.
15.10.4 Structures
15.10.4.1
Seabed supports should be prepared considering:
•
•
•
•
•
Any protruding elements (e.g. anodes and bilge keels) on the bottom of the object (e.g. vessel)
Soil bearing capacities
Stability and global deflections of the object
Local strength of object bottom
Required sliding resistance (friction).
15.10.4.2
Supports on floating vessels, e.g. HTVs should be prepared considering
•
•
•
•
•
•
•
Any protruding elements (e.g. anodes and bilge keels) on the bottom of the object (e.g. vessel)
Stability and global deflections of the object (e.g. cargo)
Local strength of object bottom
Structural capacity of vessel deck
Required sliding resistance (friction), see also [15.10.4.4]
Stability of cribbing during the load transfer, see also [15.10.4.5]
Clearances during positioning of object.
15.10.4.3
Adequate stability of the object on the bottom supports shall be documented. This is particularly relevant for
docking of objects with a rounded type bottom onto a floating vessel.
15.10.4.4
Design of supports shall take into account possible horizontal loads during positioning of the object.
15.10.4.5
Requirements to cribbing in [11.9.7] applies for operations involving HTVs. For on- and offloading operations it
shall be ensured that:
• grillage design (height of supports/cribbing) take into account any protruding parts (e.g. anodes and
spud-cans) on the cargo
• the size of the cribbing is adequate to account for possible inaccuracies in the positioning of cargo,
placement of guides, etc.
• the placing and width of the cribbing are such that no local overloading of the cargo or vessel will occur
• the cribbing strength and deformation characteristics are adequate for the intended load bearing and
load spreading
• wood cribbing and other “floating materials” are properly secured to counteract buoyancy forces.
15.10.4.6
Wood cribbing (and other “floating materials”) shall be properly secured to counteract buoyancy forces.
15.10.4.7
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Drawing(s) of the support (e.g. cribbing) lay-out shall be made and both horizontal and vertical position
tolerances shall be defined.
15.10.4.8
The local strength of the object at vertical support points shall always be verified.
Guidance note:
For docking on subsea supports the maximum object bottom loading at the extreme low tide should be
considered.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.10.4.9
The local strength of the floating vessel (e.g. HTV) at vertical support points shall always be verified.
15.10.5 Systems, equipment and vessels
15.10.5.1
General: General requirements for systems and equipment are given in [15.4]. General requirements for vessels
are given in [15.5].
15.10.5.2
Vessels:
a. All the particulars regarding strength and stability afloat and all systems and equipment should comply
with the requirements of the vessel's classification society.
b. General ballast system requirements are given in [15.4.2] and general stability requirements in [15.5.3].
c. Adequate stability and reserve buoyancy shall be documented for all docking operations / all HTV on- and
off-loading operations.
d. Step-by-step ballast calculations, including stability verifications shall be documented.
15.10.5.3
Positioning and guidance system(s) – general:
a. A system ensuring accurate, i.e. within the specified tolerances, and safe positioning of the object shall be
provided.
b. Required redundancy of the positioning system shall be based on the operation class, see Table 15-2 for
requirements.
c. Positioning and guidance systems for on- and offloading of HTVs are addressed in more detail in
[15.10.5.4]. Requirements in [15.10.5.4] apply, as applicable, also for docking onto e.g. a semisubmersible barge or a floating dock.
15.10.5.4
Positioning and guides for on- and offloading of HTVs:
a. A primary positioning system (normally tugs) should be capable of ensuring safe navigation and the
positioning of the object close to the HTV, where the secondary positioning system could be connected.
b. The secondary positioning system should ensure the accurate and well-controlled positioning of the
object above the HTV.
Guidance note:
It should be documented that the positioning could take place without unintended contact with the HTV
including any items on the HTV deck, and without loads exceeding the capability of positioning guides.
Environmental effects should be considered. Varying wind and current may be of especially significant
importance.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--c. The sufficient capacity of the secondary positioning system (normally winches) should be documented.
Guidance note:
It is not recommended to include tugs in the secondary positioning system so the pull/push force from tug
should not be included in the capacity check.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--d. The guide posts shall be designed both to withstand maximum loads imposed by winch line loads etc. and
to absorb a relevant amount of energy. See [4.4] for guidance.
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e. A conservatively assessed/calculated design load shall be applied for non-redundant (see [g)] guide posts.
f. The guide posts shall be of sufficient height to receive bumpers or similar and shall be clearly visible
during the float-on/float-off operations.
Guidance note:
Normally, the guide posts should be visible about 2 m above the waterplane at the deepest draught
during the operation.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--g. Adequate redundancy and/or contingency procedures covering single failure(s) in the position system
should be considered.
15.10.6 Operational aspects
15.10.6.1
General:
a. General operational requirements are given in [15.6].
b. If no wave load analysis has been carried out then operational limiting criteria ensuring insignificant
motions should be applied.
Guidance note:
The following is normally applicable as limiting criteria:
◦ zero (insignificant) swell
◦ significant wave height, Hs ≤ 0.5 meters.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--c. Condition and level survey(s) of subsea supports shall be performed in due time before such docking
operations. A diver or side-scan survey should be carried out shortly before the object (e.g. vessel) is
positioned. This to ensure that there is no debris in the area that can damage the bottom of the object.
Guidance note:
If a bar sweep survey is done, then it is recommended that this is supported by a diver’s inspection.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--d. Operational aspects for on- and offloading of HTVs are addressed in more detail in [15.10.5.4].
Requirements in [15.10.5.4] apply, as applicable, also for docking onto e.g. a semi-submersible barge or a
floating dock.
15.10.6.2
On- and offloading of HTVs:
a. The float-on/off shall be carried out at a location where the limiting loading criteria are (easily) obtainable
and with adequate bottom clearance in a sufficient area for adequate manoeuvring of the HTV and the
object to be loaded on board the HTV.
Guidance note:
If the reserve buoyancy of the HTV is considered critical, a location with depth and bottom conditions that
could allow the HTV to be supported at the bottom as a contingency is recommended.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--b. A detailed operation procedure should be prepared for the on-/offloading.
c. Limiting environmental criteria shall be established for the on-/offloading operation.
Guidance note:
Normally the limiting criteria should be insignificant current, maximum waves/motions as indicated in
[15.10.6.1 b)] and a maximum wind speed of 15 knots.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--d. The documented minimum nominal clearance between the cargo and top of the cribbing should be 0.5
metres during float-on/float-off. If the effect on the clearance of motions, tolerances and deflections could
be significant, the minimum tolerance should be increased accordingly.
Guidance note:
All possible relative horizontal positions of the object and HTV during float-on/off should be considered.
Any protruding elements on the object and HTV deck should be accounted for.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e--e. A survey of the loading/unloading site should be performed to ensure a sufficient water depth during the
loading/unloading operation.
f. It shall be confirmed by survey that all supports (cribbing) and guide posts are correctly positioned (and
secured) within defined horizontal and vertical tolerances.
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15.11
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Information required
15.11.1 General
15.11.1.1
General requirements to documentation are given in [2.3].
15.11.2 Design documentation
15.11.2.1
The following design documentation is normally required:
• Analyses/calculations/certificates/statements adequately documenting the necessary strength and
capacity of all involved equipment and structures
• Strength verifications of vessels and structures involved, including (local) strength verifications of object
and vessel, e.g. at object supports
• Documentation of civil elements (soil, bollards, etc.) by e.g. engineering calculations, approved drawings
or certificates
• Stability verifications for vessel(s), substructure and/or (floating) object
• Ballast calculations covering the planned operation as well as contingency situations.
15.11.2.2
Allowable environmental criteria and vessel motions shall be established for each phase of the load transfer
operation, by analysis. The decision to proceed from one phase of the operation to the next shall be based on a
comparison between the allowable environmental criteria for the next phase, the data obtained from the
environmental monitoring systems, MRU and weather forecasts.
Guidance note:
Operation stages to be considered include (as relevant):
•
•
•
•
•
Positioning
First contact
Intermediate load transfer, initial range 10-60%
Last contact
Vessel exit.
---e-n-d---of---G-u-i-d-a-n-c-e---n-o-t-e---
15.11.2.3
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.
15.11.2.4
Structural analysis reports for objects, host structures, substructures etc. involved in load transfer operations
should, as relevant, include:
•
•
•
•
•
•
•
•
•
•
•
Structural drawings, also of any additional steelwork for the load transfer operation
Drawings of host structure, substructure and or vessel(s) involved
Drawings of the object including plans, elevations and details
Description of analyses programs used
Description of structural models
Description of boundary conditions
Description of load cases
Structural strength checks for members, joints and connections
Justification of any over-stressed members or joints
Detailed design of structure support points, padeyes, winch connection points etc.
Proposals for structure reinforcement, if required
15.11.3 Equipment, fabrication and vessel(s)
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15.11.3.1
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.
15.11.3.2
For vessels, objects and/or substructures that will be (de)ballasted during the operation, the following
documentation should be provided, as relevant:
•
•
•
•
•
•
•
•
general arrangement and compartmentation drawings
hull structural drawings, including any internal reinforcement
limitations for evenly distributed load and point loads on vessel deck
equipment data and drawings
hydrostatic data (either curves or tables)
tank plans, including ullage (or sounding) tables
guidelines for air pressurised vessel tanks, if used
Details of ballast and control systems, including manual and remote operation systems and back-up
systems and compartment status-monitoring systems.
15.11.3.3
Documentation for vessels should, as relevant, also include:
•
•
•
•
•
•
•
•
•
•
•
•
Details of class
Trim and Stability booklet
Vessel allowable still water bending moment and shear force values
Allowable deck loadings and skidway loadings if applicable
Specification and capacity of all mooring bollards
Details of any additional steelwork such as grillages or winch attachments
Structural strength checks for grillage/cribbing, seafastening, additional steelwork and load-transfer areas
Details of vessel pumping system
Vessel boarding ladders (4 minimum) for the range of draughts in question and wave height range
Office/control room container suitability and equipment
Vessel power sources (generators) and redundant equipment
Method of fendering between vessel and host structure/substructure showing any sliding or rolling
surfaces
• Specification and layout of all pumps, including back-up pumps and control systems
• Pipe schematic and details of manifolds and valves where applicable
• Pump performance curves.
15.11.3.4
For mooring systems the information listed in [17.12] as applicable, including, as relevant at least:
• Limiting design and operational weather conditions for the load transfer operation
• Mooring arrangements for the load transfer operation and for stand-by position (if applicable)
• Calculations showing environmental loads, line tensions and attachment point loads for limiting weather
condition for each stage of the load transfer operation
• Specification and certificates of all wires, ropes, shackles and chains
• Specification for winches, and details of foundation/securing arrangements.
15.11.3.5
Details of any supporting tugs including bollard pull, thrusters and towing equipment.
15.11.3.6
The documentation of jacking, winching & load transfer equipment should, as relevant, include:
•
•
•
•
•
•
Jack/winch specification
Layout of jacking/winching systems including power-packs
Layout of contingency systems
Calculations showing friction coefficient and loads on attachment points and safety factors
Details of LMUs and any heave-compensation equipment
Details of any other load transfer equipment.
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15.11.4 Operation manual
15.11.4.1
A comprehensive operation manual shall be prepared, see [2.3.7]. The manual shall identify all aspects of the
operations in detail, cover all likely contingencies and clearly specify how the load transfer operation will be
conducted.
15.11.4.2
The items listed below will normally be essential for a successful execution of a load transfer operation 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 set-up, recording, expected readings including acceptable
deviations and reporting routines during the operation, see also [4.3.8.4].
• Detailed ballast procedures
• Operation bar chart showing time and duration of all critical activities.
15.11.4.3
The following should normally (as a minimum) be included in the manual(s):
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Organisational structure for the load transfer operation
Roles and responsibilities of key personnel
Communication procedure
Key contacts and personnel information
Emergency response procedures
Environmental limitations for operations
Tidal and current predictions
Weather forecasting procedure
Support facilities and vessel information
DP design and operational requirements (as in [17.13])
Vessel stationkeeping procedures
Preparation check lists
Pre-departure activities
Preparations activities on site prior to the actual load transfer
First contact during load transfer (e.g. docking)
The actual transfer of load
Final stages of load transfer (e.g. release of installation vessel after load is transferred to a host structure)
Installation/removal related drawings
Ballast procedure
Change procedure
Installation sequence drawings
Anchor patterns and catenaries
Specifications for all installation equipment and systems
General arrangement drawings of LMU, LSF, seafastenings, fenders etc.
Detailed make up drawings and specifications of all mooring lines and tethers
Specifications for all installation equipment and systems.
15.11.4.4
The procedure shall include detailed step by step procedures and contingency procedures for each phase of the
load transfer operation including all operational and limiting environmental conditions (e.g. minimum and
maximum tidal heights at all stages of a float-over operation). Required weather windows for critical operations
shall be stated, referenced to detailed hourly installation/removal schedules.
15.11.4.5
Criteria for stopping or aborting each stage of the operation (see [15.11.2.2]) and a critical “point of no return”
for the operation shall be identified.
15.11.4.6
If the operations are performed by different contractors, then 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.
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15.11.4.7
The installation crew shall be fully trained on the details of the installation procedures and the operation of all
related equipment.
15.11.4.8
For float-over operations detailed installation vessel mooring and anchor running procedures shall be
documented (when such systems are used) taking due account of the AHV and assist tugs being provided.
15.11.5 Site
15.11.5.1
For lift-off locations:
• A site plan showing e.g. the dock, position of object above the dock, other equipment in the area (e.g.
cranes near/above the dock), position of mooring bollards, winches etc.
• Drawings showing depths (e.g. inside the dock) and water levels
• Specification of capacity for all mooring bollards, winches etc. used
• Survey reports for the load transfer area (e.g. dock) confirming sufficient depths and no obstructions.
15.11.5.2
For mating locations:
• A site plan showing substructure position, substructure mooring system and any subsea infrastructure
(documented by recent reliable surveys)
• Drawings showing water depths and water levels.
15.11.5.3
For docking locations (e.g. for on- and offloading of HTVs):
• A site plan showing the intended load transfer location and any subsea infrastructure (documented by
recent reliable surveys)
• Drawings showing water depths and water levels.
15.11.5.4
For float-over locations:
• A site plan showing host structure position, infield pipelines, flowlines and subsea infrastructure
(documented by recent reliable surveys)
• Drawings showing heights above datum of host structure legs, LMUs, structure support points, vessel and
water levels
• Recent bathymetric survey report of area adjacent to the host structure (related to the same datum as
drawings).
15.11.6 Weight information
15.11.6.1
Weight report for object and results of weighing operation
15.11.6.2
Checks on the effect of any weight changes after weighing or final weight calculations on the load transfer
operation
SECTION 16 Lifting operations
16.1
Introduction
16.1.1
General and scope
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16.1.1.1
This section gives requirements for the MWS approval of marine lifting operations, including subsea lifting.
16.1.1.2
The section covers lifting operations by floating crane vessels, including crane barges, crane ships, semisubmersible crane vessels and jack-up crane vessels. It also covers subsea installations using a crane, winch or
derrick in [16.17].
16.1.1.3
The requirements also apply for lifting operations by land-based cranes for the purpose of lifted load-outs. See
[16.10].
16.1.1.4
The requirements are intended for engineered lifts. Engineered lifts are those which are planned, designed and
executed in a detailed manner, with thorough supporting documentation. Routine lifts are “everyday” lifts,
without detailed design, planning or documentation, such as general cargo lifting operations, or lifting portable
units on/off a supply vessel. As such, routine lifts typically have higher safety factors due to the lack of detailed
engineering. Routine lifts are not addressed in this standard.
16.1.2
Revision history
16.1.2.1
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 DNV-OS-H205 Lifting Operations (VMO Standard – Part 2-5)
• DNV Offshore Standard DNV-OS-H206 Load-out, transport and installation of subsea objects (VMO
Standard – Part 2-6).
16.2
Load factors
16.2.1
Introduction
16.2.1.1
For any lift, the calculations carried out shall include allowances, safety factors, loads and load effects as
described in this standard.
16.2.1.2
The various factors and their application are illustrated in Figure 16-1. This flowchart is for guidance only, and is
not intended to cover every case. In case of any conflict between the flowchart and the text, the text shall govern.
Figures in parentheses relate to sections in this standard.
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Figure 16-1 Lift calculation flowchart
16.2.2
Weight contingency and centre of gravity factors
16.2.2.1
Weight Contingency and Centre of Gravity control requirements are given in [5.6.2.2] and [5.6.2.3] which in turn
reference ISO Standard 19901-5, /98/.
16.2.2.2
For lifting operations carried out where the Centre of Gravity of the lifted object is above the lift points, care
should be taken to ensure that the stability of the lifting arrangement is considered in the design. This is of
particular concern where spreader bars or spreader frames are used as part of the lift system. Stability should be
demonstrated for these conditions allowing for both vertical and horizontal offsets in the position of the Centre
of Gravity.
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16.2.3
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Module tilt for single crane lifts
16.2.3.1
Object tilt due to CoG position and/or imposed horizontal loads (see [16.2.6.16] for possible causes of horizontal
loads) will influence the sling load distribution for most rigging configurations. The effect of tilt should be
considered in the load calculations where relevant.
16.2.3.2
The rigging geometry shall normally be configured so that the maximum tilt of the structure does not exceed 2°
for level lifts, however see [16.2.3.4] for lifts at a known tilt. The sling angle should normally be as described in
[16.3.4]. Where calculated maximum tilt is less than 2°, it is normally not necessary to consider related effects in
the sling load calculations.
16.2.3.3
Variable sling elongation, sling length and lift point fabrication tolerances could increase object tilt. Where lifting
points are located below the vertical CoG of the object, forces in the most utilised slings will tend to increase due
to sling elongation; in this case a suitable factor should be determined.
16.2.3.4
In special circumstances (e.g. flare booms, flare towers and cantilevered modules) the design angle of tilt may be
required to be greater than 2° to permit the effective use of installation aids. These structures shall be reviewed
as special cases.
16.2.3.5
Where long slings are used and there are small distances between the lift points, the effect of the sling tolerance
on new build slings is to be checked to ensure that excessive tilts are not introduced into the lifted structure.
16.2.3.6
The effect of module tilt on multi hook lifts is covered in [16.2.4].
16.2.4
2-hook lifts
16.2.4.1
A tilt effect shall be calculated to account for the increased sling loading caused by rotation of the object about a
horizontal axis and the effect of out-of-plumb hoist lines. The tilt effect should be based on possible tilt caused by
maximum hook height tolerances and hoist line deviations from the vertical. More guidance for the derivation of
the effect of tilt is given in [P.1].
16.2.4.2
For a 2-hook lift with hooks on one or two cranes on the same vessel, the static hook load at each hook should be
the more onerous condition of:
• a tilt of 3°
• a hook elevation difference of ±1.0 m.
Guidance note:
Reduced factors can be accepted by the MWS company, subject to supporting analyses, limiting sea state
criteria and installation procedure steps/controls. See [P.1] for a sample calculation.
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16.2.4.3
For a 2-hook lift with the cranes on separate vessels, the static hook load at each hook for offshore lifts shall be
the more onerous condition of a tilt of 5° or a hook elevation difference determined by analysis. For inshore lifts,
the static hook load at each hook shall be the more onerous condition of a tilt of 5° or a hook elevation difference
of ±1.0 m.
16.2.4.4
For multi-hook lifts carried out by the same sheerleg crane vessel (non-rotating crane), where the hook elevations
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are closely synchronised, the factors in [16.2.4.2] can be reduced by 50%.
16.2.4.5
To account for increased sling loading due to rotation of the object about a vertical axis; a minimum yaw effect
factor of 1.05 should be applied. For lifts with small sling opening angles at the hooks and/or significant
wind/tugger line loads a greater yaw effect factor may be applicable. Note, the yaw effect for a 2-hook lift only
applies when there is more than one sling connected to the hook.
Guidance note:
For lifts with no planned boom movements, such as slewing or booming up/down, the yaw factor specified in
[16.2.4.5] can be reduced to 1.0. One example where this can apply is a 2-hook lift where both cranes are on a
single sheerleg type vessel.
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16.2.5
Dynamic amplification factors (DAF)
16.2.5.1
Dynamic loading shall be applied to account for global dynamic effects resulting from vessel motions, boom,
wire and rigging stiffness, boom tip location and motions, crane movements and wind loading. This is typically
expressed as a Dynamic Amplification Factor (DAF).
16.2.5.2
For subsea lifts, see [16.17.2]
16.2.5.3
For offshore lifts by 2 or more vessels the DAF shall be determined by operation-specific analysis or model
testing.
16.2.5.4
For offshore lifts by 2 or more hooks on the same crane boom, a dynamic analysis should be completed.
Guidance note:
However, in some cases, a DAF (or DAF increased by an additional factor) from Table 16-1 can be accepted by
the MWS company.
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16.2.5.5
For all other lifting operations, the DAF should be determined by operation-specific analysis or model testing. In
lieu of such determination the DAFs in Table 16-1 (onshore or floating crane vessel) or Table 16-2 (elevated jackups) can generally be used as minimum values for lifts by a single crane hook in air, provided the lift will not take
place in adverse weather conditions. The DAFs in Table 16-1 and Table 16-2 shall only be used when the effect
of all variables influencing dynamic loading are well understood.
Guidance note:
The DAFs in Table 16-1 and Table 16-2 cannot apply to every lift because of the many variables noted in
[16.2.5.1] which influence dynamic loading. In particular, vessel size, weather conditions (see Guidance note 1 of
[16.2.5.6]), and boom tip/lift-off location have significant impact on DAF values.
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16.2.5.6
The DAF indicated in Table 16-1 also apply to the following in air lift combinations of vessels, cranes and
locations:
•
•
•
•
For lifts by 2 cranes on the same vessel
For onshore lifts by 2 or more cranes
For lifts by 2 or more hooks on the same crane boom excluding offshore lifts (see [16.2.5.4])
For inshore lifts, in totally sheltered waters, by 2 or more vessels.
Table 16-1 Dynamic amplification factors (DAF) in air (excluding elevated Jack-ups)
Static Hook Load (SHL)
(tonnes)
DAF
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Onshore 2),
3)
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Inshore 4), 6)
Offshore 5), 6)
3 1)
<
SHL
≤
100
1.10
100
<
SHL
≤
300
1.05
1.12
1.25
300
<
SHL
≤
1000
1.05
1.10
1.20
1000
<
SHL
≤
2500
1.03
1.08
1.15
1.03
1.05
1.10
SHL > 2500
Notes:
1. For lifted items weighing less than 3 tonnes, it is recommended to assume that the item weighs 3 tonnes
and this is used throughout the calculations for the rigging design.
2. For onshore crawler cranes travelling with load, possible dynamic effects should be evaluated
thoroughly. Crane speeds and surface conditions should be considered. If not documented, the factors
for “inshore lifts” should be used
3. Onshore is also applicable to a lift to/from a vessel moored alongside a quay using a land-based crane.
If a ship’s crane is used, inshore factors apply.
4. Inshore is applicable to a lift with a crane vessel to/from a vessel in sheltered waters and is also
applicable to lifting from the deck of a crane vessel onto a fixed platform at an offshore location
5. Offshore is applicable to a lift by a crane vessel from another vessel to a fixed platform.
6. SHL refers to the Static Hook Load (see [16.3.2.2] and [16.3.2.3]).
Guidance note:
For offshore lifts using monohull vessels greater than 80 m in length, adverse weather is suggested as follows:
For SHL > 100 tonnes,
• Significant swell (i.e. swell with a period and height creating significant crane vessel motions),
• For lifts not involving ballasting of crane vessels during lift-off, or lifts using a rapid ballast system during
lift-off: waves with Hs > 2-2.5 m.
• For lifts involving conventional ballasting of crane vessels during lift-off: waves with Hs > 1-1.5 m
For SHL < 100 tonnes,
• Waves with Hs > 2.5-3.5 m (highest value for small SHL).
• Swell/waves that are creating significant motions of the crane vessel.
Table 16-2 Dynamic amplification factors (DAF) in air (elevated Jack-ups only)
DAF for elevated jack-ups
Inshore
Static Hook Load (SHL)
(tonnes)
Own Deck
to/from
FIXED
structure
Offshore
Own Deck
to/from
FIXED
structure
To/from
FLOATING
structure
To/from FLOATING structure
3 1)
<
SHL
≤
100
1.10
100
<
SHL
≤
300
1.05
1.10
1.15
300
<
SHL
≤
1000
1.05
1.10
1.12
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<
SHL
≤
1000
SHL > 2500
2500
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1.03
1.08
1.10
1.03
1.05
1.10
Notes:
1. For lifted items weighing less than 3 tonnes, it is recommended to assume that the item weighs 3 tonnes
and this is used throughout the calculations for the rigging design.
2. SHL refers to the Static Hook Load (see [16.3.2.2] and [16.3.2.3]).
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16.2.6
Skew load factor (SKL)
16.2.6.1
Skew loads are 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. The skew load factor (SKL) is a load distribution factor based on:
•
•
•
•
•
•
•
rigging length manufacturing tolerances,
sling/grommet measurement tolerances over measuring pins,
rigging arrangement and geometry,
fabrication tolerances for lift points,
sling/grommet elongation,
crane hook geometry,
Deflections of lifted object (see [16.8.6]).
and should be considered for any rigging arrangement and structure (see [16.8.2.4]) that is not 100%
determinate. A significantly higher SKL factor may be required for new slings used together with existing slings
as one sling may exhibit more elongation than the others.
16.2.6.2
For rigging configurations involving slings from more than 4 lift points connected to a single hook, skew load
effects shall be calculated on a case by case basis.
16.2.6.3
When determining the length of a sling or grommet, the effect of the pin used in the measurement of the
sling/grommet should be considered as the connection points for the sling/grommet may have a different
diameter to the pin causing the in-use length to be different to the measured length.
16.2.6.4
When determining the rigging lengths and angles, the effect of the hook geometry and hook prong diameter
should be considered as these will affect the working points for the rigging when determining lengths and the
hook prong diameter may affect the measured length of the sling/grommet (see [16.2.6.3]).
16.2.6.5
For statically determinate lifts (with or without a single spreader bar), the SKL may be taken to be 1.0, provided it
can be demonstrated that sling length tolerances do not significantly affect the load attitude or lift system
geometry. The permitted length tolerance on the slings/grommets for the use of the SKL of 1.0 is such that the
lengths shall be within ±0.5% of their nominal length. Where the tolerance is outside this, the effect of the sling
length should be considered on the load distribution to the lift points incorporating any tilt effects caused by the
sling length tolerances.
16.2.6.6
For a lift system using matched pairs of slings and incorporating 2 or more spreader bars, a SKL of 1.10 is
applicable provided the following conditions are achieved:
a. An approximately symmetric rigging geometry is utilised.
b. The sling lengths are within ± 0.5% of their nominal length.
c. The calculated axial load in the spreader bar is at least 15% of the sling load
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d. If the stated conditions are not met the SKL should normally be found by calculation. However, generally if
the length tolerance is stricter than stated, the minimum axial load requirement in the spreader bars could
be relaxed.
16.2.6.7
For lifts where more than two hooks are used and each hook is connected to a single spreader bar, a SKL of 1.1
should be used. A reduced value may be used, provided the hook elevations can be shown to be individually
controlled, and subject to evaluation of sling length tolerances, the rigging arrangement and crane operating
procedures.
16.2.6.8
For indeterminate 4-sling lifts using matched pairs of cable laid slings or grommets, a Skew Load Factor (SKL) of
1.25 shall be applied to each diagonally opposite pair of lift points in turn provided the following are applicable:
For Cable Laid Slings:
• The slings are fabricated with a length tolerance of ±1.5d and the difference between a matched pair of
slings shall not be more than 0.5d where d is the sling diameter in consistent units;
• The slings are of a standard construction
• The slings are installed so that the longer slings of each matched pair are not on the same diagonal.
• Sling utilisation when checking with the termination factor (see [16.4.3.1] and [16.4.7.1]) and a skew factor
of 1.25 should be more than 0.6.
• The sling length shall be greater than 100 x d.
For (Cable Laid) Grommets:
• The grommets are fabricated with a circumferential length tolerance of ±3.0d and the difference between
a matched pair of grommets shall not be more than 1.0d where d is the grommet diameter in consistent
units;
• The grommets are of a standard construction.
• The grommets are installed so that the longer grommets of each matched pair are not on the same
diagonal.
• Grommet utilisation when checking with a termination factor of 1.0 (see [16.4.3.1]), and a skew factor of
1.25 should be more than 0.6.
• The grommet total (circumferential) length shall be greater than 200 x d
Note: where sling or grommet utilisations are less than 0.6, whilst a higher skew factor will not overload the
slings/grommets, the load on the lift point can increase and the effect of this shall be included in the design for
the lift points.
16.2.6.9
In lieu of the skew factors used in [16.2.6.8], the actual skew factor may be determined using a more detailed
analysis allowing for actual rigging properties, extreme tolerances for new build rigging and hook rotation.
Where possible, the analysis should include the lifted structure so that the effect of the structure’s stiffness can be
considered or where this is not carried out, the structure can be considered infinitely stiff and thus offers no
reduction to the skew value determined.
16.2.6.10
For indeterminate 4-sling lifts using four cable laid slings of un-equal length, the skew load shall be calculated
using an elastic modulus, E, of 25,000 N/mm2 with the sling area used based on a value of 0.785 x d2, where d is
the sling diameter in mm, and the sling lengths based on the most onerous fabrication tolerances.
16.2.6.11
For indeterminate 4-grommet lift using four cable laid grommets of un-equal length, the skew load shall be
calculated using an elastic modulus, E, of 25,000 N/mm2 with the grommet area used based on a value of 1.57 x
d2, where d is the diameter in mm of one leg of the grommet, and the grommet lengths based on the most
onerous fabrication tolerances.
16.2.6.12
For indeterminate 4-sling lifts using matched pairs of wire single laid slings, a Skew Load Factor (SKL) of 1.25
shall be applied to each diagonally opposite pair of lift points in turn provided the following are applicable:
• The slings are fabricated with a length tolerance of ±2.0d and the difference between a matched pair of
slings shall not be more than 1.0d where d is the sling diameter.
• The slings are of a standard construction and meet the criteria of 230xW/d2 <1.0 where W is the weight in
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kilograms per metre of the sling and d is the sling diameter.
• The slings are installed so that the longer slings of each matched pair are not on the same diagonal.
• Sling utilisation when checking with the termination efficiency factor (see [16.4.3.1] and [16.4.7.1]) and a
skew factor of 1.25 should be more than 0.6.
• The sling length shall be greater than 200 x d.
Note, where utilisations are less than 0.6, whilst a higher skew factor will not overload the slings, the load on the
lift point may increase and the effect of this shall be included in the design for the lift points.
16.2.6.13
For indeterminate 4-sling lifts using four single laid slings of un-equal length, the skew load shall be calculated
using an elastic modulus, E, of 80,000 N/mm2 with the sling area used based on a value of 0.785 x d2, where d is
the sling diameter in mm, and sling lengths based on the most onerous fabrication tolerances.
16.2.6.14
For fibre slings, standard values of skew load factors cannot be established due to different stiffnesses for
different materials and fabrication tolerances. Therefore SKL for fibre slings shall be determined on a lift or
project specific basis when sling properties are known.
16.2.6.15
Two prong or asymmetric four prong hooks may reduce the skew load in four sling lifts as the hook may rotate.
This can be considered in skew load calculations.
16.2.6.16
Guidance on direct calculation of SKL is in [P.3].
16.2.7
Special loads
16.2.7.1
When appropriate, allowances for special loads should be made in the derivation of loads on the lifted structure,
lift points and rigging system. Examples of special loads include tugger line loads, guide loads, wind loads,
hydrostatic loads, hydrodynamic loads, suction loads, friction loads etc.
16.2.8
Sling load distribution
16.2.8.1
Where a doubled sling other than at a termination, or grommet passes over, round or through a shackle,
trunnion, padear or crane hook, the total double sling/grommet force should be distributed into each part in the
ratio 45:55% to account for frictional losses over the bend.
Guidance note:
Equal loading on each part can be considered valid for single hook lifts where the slings are allowed to adjust
during a “slow” tensioning phase and do not involve upending/tilting (i.e. no rotation of the slings over a fixed
trunnion or similar after the slings are loaded will occur). It is assumed that each part has the same axial stiffness.
For lifts with rapid tensioning of the slings a 45:55% distribution should be assumed. Where a double sling or
grommet passes over a rotating greased sheave on a trunnion a 49:51% may be considered also for lifts with
rapid tensioning.
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16.2.8.2
Where upending a structure requires the doubled sling or grommet to slide over a trunnion or crane hook the
total sling force shall be distributed into each part in the ratio 32.5:67.5%. For this condition, the ratio may be
reduced if the lifting contractor can demonstrate through documented evidence or testing that a lesser value is
suitable.
Guidance note:
Friction coefficient values less than 0.22 for well-greased steel slings should be documented. For slings with a
dry surface higher friction coefficient values should be considered. For a 180° contact area and a friction
coefficient of 0.22, the load distribution will theoretically be 32.5:67.5.
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16.2.8.3
Where slings are used in any more than a double configuration e.g. doubled-doubled or grommets are used
doubled, calculations to justify the arrangement shall be documented. The calculations shall allow for the
frictional losses contained in [16.2.8.1] or [16.2.8.2] (e.g. when determining the highest sling load for a 45:55%
distribution in a double-doubled sling would be 0.55 x 0.55 on the design load in each leg of the sling or
grommet).
16.2.8.4
If a lift rigging includes two parallel slings (i.e. two slings connected between the same lift points), the load
distribution shall be calculated considering the maximum sling length difference between the two slings and the
maximum sling modulus of elasticity (E).
16.2.8.5
When using fibre slings (i.e. Round slings or webbing slings) in a doubled configuration or grommets, the
doubled sling factor referenced in [16.2.8.1] shall be used for guidance, but the specific recommendations of the
sling supplier should govern, based on the planned mode of use and the specifics of the sling type.
16.3
Derivation of hook, lift point and rigging loads
16.3.1
Introduction
16.3.1.1
The following sections determine the loads to be used for confirming the suitability of the cranes and for the
design of rigging components using the parameters laid down in [16.2].
16.3.2
Hook loads
16.3.2.1
The total loading on the crane hook(s) shall be based on the Upper Bound Design Weight of the lifted object as
defined in [5.6.2.2].
16.3.2.2
For single crane lifts, the hook loads are as follows:
SHL = Wud + Wrigging + Effect of special Loads – see [16.2.7.1]
DHL = SHL × DAF
Where
SHL
Wud
Wrigging
DHL
DAF
=
Static Hook Load
=
Upper bound design weight
=
Rigging Weight
=
Dynamic Hook Load
=
Dynamic Amplification Factor
16.3.2.3
For twin hook lifts whether cranes are on the same vessel, or multiple vessels, or the structure is suspended from
two hooks on the same crane on the same vessel, the load to each hook shall be based on the Upper Bound
Design Weight proportioned by the geometric distance of the centre of gravity from each of the hooks allowing
for the effect of the module tilt/hook elevation tolerances given in [16.2.4]. Where a CoG envelope is used (see
[5.6.2.3]), the hook loads should be calculated for a CoG position at the extremes of the CoG envelope. Where
no CoG envelope is used, the hook loads are to be increased by the factor given in [5.6.2.3 c)].
16.3.2.4
The final static hook load is then determined by the additional rigging weight connected to the hook and the
effect of special loads in accordance with [16.2.7.1].
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16.3.2.5
The dynamic hook load is then determined in a similar way to the formula for the dynamic hook load in [16.3.2.2].
16.3.2.6
Rigging weight includes all items between the lift points and the crane hook, including slings, shackles, lifting
tools and spreader bars or frames as appropriate.
16.3.2.7
For lifting operations involving pivoting and/or upending manoeuvres (e.g. roll-up operation, jacket upending
operation etc.), an adequate number of steps shall be analysed to ensure that the critical load cases for the
derivation of hook loads are identified. Where it is noted that there is the possibility for higher loads to occur
between the angles selected, then intermediate steps between the selected angles should be considered.
Guidance note:
For some stages of the upending consideration of only a few degrees between each step may be necessary; a
maximum of 15° between each step should normally be adopted
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16.3.2.8
The calculated hook loads are to be checked against the crane capacities - see [16.7.3].
16.3.3
Lift point loads
16.3.3.1
The vertical lift point load is the load at a lift point, taking into account the Upper Bound Design Weight as given
in [16.3.2] proportioned by the geometric distance of the centre of gravity, accounting for
• Where a CoG envelope is used (see [5.6.2.3 a)]), the lift point loads should be calculated for a CoG
position at the extremes of the CoG envelope. For twin hook lifts, the effect of tilt/hook elevation
tolerances given in [16.2.4] should be accounted for.
• Where no CoG envelope is used, the lift point loads are to be increased by the factor given in [5.6.2.3 c)].
For twin hook lifts, the effect of tilt/hook elevation tolerances given in [16.2.4] should be accounted for.
16.3.3.2
The lift point design load is calculated from the vertical lift point load and lift geometry, and increased by the
following factors:
•
•
•
•
•
•
Weight and CoG Contingency Factors/Envelopes (see [16.2.2])
Module tilt for single crane lifts (see [16.2.3])
Yaw Factor for twin hook lifts (see [16.2.4.5])
Dynamic Amplification Factor (see [16.2.5])
Skew Load Factor (see [16.2.6])
Effec