4851.32.493NR_2012-04

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Classification of MooringSystems for Permanent

Offshore Units

April 2012

Rule NoteNR 493 DT R02 E

Marine Division92571 Neuilly sur Seine Cedex France

Tel: + 33 (0)1 55 24 70 00 Fax: + 33 (0)1 55 24 70 25Marine website: http://www.veristar.comEmail: [email protected]

2012 Bureau Veritas - All rights reserved

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ARTICLE 1

1.1.- BUREAU VERITAS is a Society the purpose of whose Marine Division (the "Society") is the classi-fication (" Classification ") of any ship or vessel or structure of any type or part of it or system therein col-lectively hereinafter referred to as a "Unit" whether linked to shore, river bed or sea bed or not, whetheroperated or located at sea or in inland waters or partly on land, including submarines, hovercrafts, drillingrigs, offshore installations of any type and of any purpose, their related and ancillary equipment, subseaor not, such as well head and pipelines, mooring legs and mooring points or otherwise as decided by theSociety.

The Society:

prepares and publishes Rules for classification, Guidance Notes and other documents (Rules);

issues Certificates, Attestations and Reports following its interventions (Certificates);

publishes Registers.

1.2.- The Society also participates in the application of National and International Regulations or Stand-ards, in particular by delegation from d ifferent Governments. Those activities are hereafter collectively re-ferred to as " Certification ".

1.3.- The Society can also provide services related to Classification and Certification such as ship andcompany safety management certification; ship and port security certification, training activities; all activi-ties and duties incidental thereto such as documentation on any supporting means, software, instrumen-tation, measurements, tests and trials on board.

1.4.- The interventions mentioned in 1.1., 1.2. and 1.3. are referred to as " Services ". The party and/or itsrepresentative requesting the services is hereinafter referred to as the " Client ". The Services are pre-pared and carried out on the assumption that the Clients are aware of the International Maritime

and/or Offshore Industry (the "Industry") practices.

1.5.- The Society is neither and may not be considered as an Underwriter, Broker in ship's sale or char-tering, Expert in Unit's valuation, Consulting Engineer, Controller, Naval Architect, Manufacturer, Ship-builder, Repair yard, Charterer or Shipowner who are not relieved of any of their expressed or impliedobligations by the interventions of the Society.

ARTICLE 2

2.1.- Classification is the appraisement g iven by the Society for its Client, at a certain date, following sur-veys by its Surveyors along the lines specified in Articles 3 and 4 hereafter on the level of compliance ofa Unit to its Rules or part of them. This appraisement is represented by a class entered on the Certificatesand periodically transcribed in the Society's Register.

2.2.- Certification is carried out by the Society along the same lines as set out in Art icles 3 and 4 hereafterand with reference to the applicable National and International Regulations or Standards.

2.3.-It is incumbent upon the Client to maintain the condition of the Unit after surveys, to presentthe Unit for surveys and to inform the Society without delay of circumstances which may affect thegiven appraisement or cause to modify its scope.

2.4.- The Client is to give to the Society all access and information necessary for the safe and efficientperformance of the requested Services. The Client is the sole responsible for the conditions of presenta-

tion of the Unit for tests, trials and surveys and the conditions under which tests and trials are carr ied out.

ARTICLE 3

3.1.- The Rules, procedures and instructions of the Society take into account at the date of theirpreparation the state of currently available and proven technical knowledge of the Industry. Theyare not a standard or a code of construction neither a guide for maintenance, a safety handbookor a guide of professional practices, all of which are assumed to be known in detail and carefullyfollowed at all times by the Client.

Committees consisting of personalities from the Industry contribute to the development of those docu-ments.

3.2. - The Society only is qualified to apply its Rules and to interpret them. Any reference to themhas no effect unless it involves the Society's intervention.

3.3.- The Services of the Society are carried out by professional Surveyors according to the applicableRules and to the Code of Ethics of the Society. Surveyors have authority to decide locally on matters re-lated to classification and certification of the Units, unless the Rules provide otherwise.

3.4.- The operations of the Society in providing its Services are exclusively conducted by way ofrandom inspections and do not in any circumstances involve monitoring or exhaustive verifica-tion.

ARTICLE 4

4.1.- The Society, acting by reference to its Rules:

reviews the construction arrangements of the Units as shown on the documents presented by the Cli-ent;

conducts surveys at the place of their construction;

classes Units and enters their class in its Register;

surveys periodically the Units in service to note that the requirements for the maintenance of class aremet.

The Client is to inform the Society without delay of circumstances which may cause the date or theextent of the surveys to be changed.

ARTICLE 5

5.1. - The Society acts as a provider of services. This cannot be construed as an obligation bearingon the Society to obtain a result or as a warranty.

5.2. - The certificates issued by the Society pursuant to 5.1. here above are a statement on the levelof compliance of the Unit to its Rules or to the documents of reference for the Services providedfor.

In particular, the Society does not engage in any work relating to the design, building, productionor repair checks, neither in the operation of the Units or in their trade, neither in any advisory serv-ices, and cannot be held liable on those accounts. Its certificates cannot be construed as an im-plied or express warranty of safety, fitness for the purpose, seaworthiness of the Unit or of its valuefor sale, insurance or chartering.

5.3. - The Society does not declare the acceptance or commissioning of a Unit, nor of its construc-tion in conformity with its design, that being the exclusive responsibility of its owner or builder,respectively.

5.4.- The Services of the Society cannot create any obligation bearing on the Society or constitute anywarranty of proper operation, beyond any representation set forth in the Rules, of any Unit, equipment ormachinery, computer software of any sort or other comparable concepts that has been subject to any sur-vey by the Society.

ARTICLE 6

6.1.- The Society accepts no responsibility for the use of information related to its Services which was notprovided for the purpose by the Society or with its assistance.

6.2.- If the Services of the Society cause to the Client a damage which is proved to be the directand reasonably foreseeable consequence of an error or omission of the Society, its liability to-wards the Client is limited to ten times the amount of fee paid for the Service having caused thedamage, provided however that this limit shall be subject to a minimum of eight thousand (8,000)Euro, and to a maximum which is the greater of eight hundred thousand (800,000) Euro and oneand a half times the above mentioned fee.

The Society bears no liability for indirect or consequential loss such as e.g. loss of revenue, lossof profit, loss of production, loss relative to other contracts and indemnities for termination of oth-er agreements.

6.3.- All claims are to be presented to the Society in writing within three months of the date when the Serv-ices were supplied or (if later) the date when the events which are relied on of were first known to the Client,and any claim which is not so presented shall be deemed waived and absolutely barred. Time is to be in-terrupted thereafter with the same periodicity.

ARTICLE 7

7.1.- Requests for Services are to be in writing.7.2.- Either the Client or the Society can terminate as of right the requested Services after givingthe other party thirty days' written notice, for convenience, and without prejudice to the provisionsin Article 8 hereunder.

7.3.- The class granted to the concerned U nits and the previously issued certificates remain valid until thedate of effect of the notice issued according to 7.2. here above subject to compliance with 2.3. here aboveand Article 8 hereunder.

7.4.- The contract for classification and/or certification of a Unit cannot be transferred neither assigned.

ARTICLE 8

8.1.- The Services of the Society, whether completed or not, involve, for the part carried out, the paymentof fee upon receipt of the invoice and the reimbursement of the expenses incurred.

8.2. Overdue amounts are increased as of right by interest in accordance with the applicable leg-islation.

8.3. - The class of a Unit may be suspen ded in the event of non -payment of fee after a first unfru itfulnotification to pay.

ARTICLE 9

9.1.- The documents and data provided to or prepared by the Society for its Services, and the informationavailable to the Society, are treated as confidential. However:

clients have access to the data they have provided to the Society and, during the period of classifica-tion of the Unit for them, to the classification fileconsisting of survey reports and certificates whichhave been prepared at any time by the Society for the classification of the Unit;

copy of the documents made available for the classification of the Unit and of available survey reportscan be handed over to another Classification Society, where appropriate, in case of the Unit's transferof class;

the data relative to the evolution of the Register, to the class suspension and to the survey status of theUnits, as well as general technical information related to hull and equipment damages, are passed onto IACS (International Association of Classification Societies) according to the association workingrules;

the certificates, documents and information relative to the Units classed with the Society may bereviewed during certificating bodies audits and are disclosed upon order of the concerned governmen-tal or inter-governmental authorities or of a Court having jurisdiction.

The documents and data are subject to a file management plan.

ARTICLE 10

10.1.- Any delay or shortcoming in the performance of its Services by the Society arising from an eventnot reasonably foreseeable by or beyond the control of the Society shall be deemed not to be a breach ofcontract.

ARTICLE 11

11.1.- In case of diverging opinions during surveys between the Client and the Society's surveyor, the So-ciety may designate another of its surveyors at the request of the Client.

11.2.- Disagreements of a technical nature between the Client and the Society can be submitted by theSociety to the advice of its Marine Advisory Committee.

ARTICLE 12

12.1. - Disputes over the Services carried out by delegation of Governments are assessed within theframework of the applicable agreements with the States, international Conventions and national rules.

12.2.- Disputes arising out of t he payment of the Society's invoices by the Client are submitted to the Courtof Nanterre, France.

12.3.- Other disputes over the present General Conditions or over the Services of the Society areexclusively submitted to arbitration, by three arbitrators, in London according to the ArbitrationAct 1996 or any statutory modification or re-enactment thereof. The contract between the Societyand the Client shall be governed by English law.

ARTICLE 13

13.1.-These General Conditions constitute the sole contractual obligations binding together theSociety and the Client, to the exclusion of all other representation, statements, terms, conditionswhether express or implied. They may be varied in writing by mutual agreement.

13.2. - The invalidity of one or more stipulations of the present General Conditions does not affect the va-lidity of the remaining provisions.

13.3.- The definitions herein take precedence over any definitions serving the same purpose which mayappear in other documents issued by the Society.

BV Mod. Ad. ME 545 k - 17 December 2008

M A R I N E D I V I S I O N

G E N E R A L C O N D I T I O N S

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April 2012

RULENOTENR 493

NR 493Classification of Mooring Systems

for Permanent Offshore Units

SECTION 1 GENERAL

SECTION 2 CLASSIFICATIONREQUIREMENTS

SECTION 3 DESIGNOFMOORINGSYSTEM

SECTION 4 COMPONENTSOFMOORINGLINES

APPENDIX1 CHARACTERISATIONOFTHELINERESPONSE

APPENDIX2 COMBINATIONOFMETOCEANPARAMETERS

APPENDIX3 STRUCTURALSTRENGTHCRITERIA

APPENDIX4 GEOTECHNICALCAPACITYOFANCHORINGDEVICES(PARTIALFACTORFORMAT)

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2 Bureau Veritas April 2012

Section 1 General

1 Subject 7

1.1 General1.2 Rules and related documents1.3 Other References

Section 2 Classification Requirements

1 General 9

1.1 Context of classification1.2 Applicability of POSA notation1.3 Limits of POSA notation and interfaces with Class

2 Scope of activities for POSA notation 9

2.1 General2.2 Certificates2.3 Design2.4 Components of mooring system2.5 Survey of installation and deployment2.6 Documents to be submitted

3 In service surveys 11

3.1 General3.2 Annual Survey of anchoring lines

3.3 Intermediate Survey of anchoring lines3.4 Class Renewal Surveys of anchoring lines3.5 Survey summary3.6 Renewal criteria for chains, steel wire ropes and fibre ropes of permanent

installations

Section 3 Design of Mooring System

1 General 15

1.1 Subject1.2 Review of design

1.3 General methodology

2 Methods of evaluation 15

2.1 Objective2.2 Quasi-static analysis2.3 Quasi-dynamic analysis2.4 Dynamic line response2.5 Other methods of analysis2.6 Model tests

3 Environment, actions and motions 16

3.1 Environment

3.2 Actions3.3 Unit response

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4 Mooring System 20

4.1 Mooring pattern and initial tensions4.2 Mooring response4.3 Mooring stiffness

5 Line response 215.1 Quasi-static line response5.2 Dynamic line analysis5.3 Characterization of the line response5.4 Dynamic line response

6 Design tensions 22

6.1 Intact condition6.2 One-line damaged condition6.3 One-line failure (transient) condition6.4 Two-lines damaged condition6.5 Thruster failure

6.6 Design tension in line components6.7 Minimum tension

7 Fatigue analysis 24

7.1 Tension range7.2 In/Out of Plane Bending (OPB/IPB)

8 Strength of line 24

8.1 General8.2 Breaking strength of line components8.3 Tension-Tension fatigue endurance8.4 In/Out of plane bending endurance

9 Selection of design conditions 269.1 General9.2 System configuration9.3 Metocean conditions9.4 Extreme metocean conditions9.5 Operating conditions9.6 Transient conditions9.7 Fatigue analysis

10 Criteria 28

10.1 Mooring Line10.2 Anchors

10.3 Clearance10.4 Fatigue10.5 Test loading of anchor and lines

Section 4 Components of Mooring Lines

1 General 30

1.1 Subject1.2 Scope1.3 General requirements

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2 Chains and standard fittings 31

2.1 General2.2 Designation2.3 Design2.4 Manufacturing and testing

2.5 Installation and service conditions

3 Steel wire ropes 32

3.1 General3.2 Designation3.3 Design of steel wire rope3.4 Design of Terminations3.5 Manufacturing and testing3.6 Installation and service conditions

4 Fibre ropes 33

4.1 General

4.2 Design4.3 Manufacturing and testing4.4 Installation and service conditions

5 Non standard fittings 33

5.1 General5.2 Design5.3 Manufacturing and testing

6 Anchoring devices 35

6.1 General6.2 Design6.3 Manufacturing

6.4 Installation

7 Items at the on-vessel end 36

7.1 General7.2 Design7.3 Manufacturing and testing

8 Ancillary elements 38

8.1 General8.2 Service conditions

Appendix 1 Characterisation of the Line Response

1 General 39

1.1 Test run1.2 Characterisation

Appendix 2 Combination of Metocean Parameters

1 General 40

1.1 Subject

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2 Metocean data 40

2.1 Metocean conditions and design data2.2 Intensity and direction2.3 Sea-states2.4 Wind

2.5 Current

3 Metocean design conditions 42

3.1 Principles3.2 Directions3.3 Intensities3.4 Operating conditions

4 Extra-tropical conditions 43

4.1 Applicability4.2 Typical design conditions4.3 Selection of return periods

4.4 Reduction factors4.5 Conditions with swell

5 Equatorial conditions 44

5.1 Applicability5.2 Typical design conditions5.3 Waves and wind5.4 Selection of return periods5.5 Directions

6 Tropical storm conditions 45

6.1 Applicability6.2 Typical design conditions

6.3 Data for the intensity of the elements

Appendix 3 Structural Strength Criteria

1 General 47

1.1 Subject1.2 Design loads1.3 Elastic design1.4 Elastic plastic design1.5 Design based on elasto-plastic analysis

Appendix 4 Geotechnical Capacity of Anchoring Devices (Partial Factor

Format)

1 General 49

1.1 Scope of application1.2 Format

2 Actions 49

2.1 General2.2 Design tensions2.3 Partial load factors

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3 Capacity 50

3.1 Ultimate capacity3.2 Components of ultimate capacity

4 Other factors 51

4.1 Factor A4.2 Uplift factor4.3 Safety factors

5 Criteria 51

5.1 General

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NR 493, Sec 1


SECTION 1 GENERAL

1 Subject

1.1 General

1.1.1 The subject of this Note is the mooring system (sta-tion keeping system) of floating offshore Units that are per-manent installations as defined in NR445 Rules for theClassification of Offshore Units, Part A, Chapter 1.

This Note gives technical requirements, criteria and guid-ance on the design, construction and installation of moor-ing systems, as a complement to the below mentioned

Rules, for the granting of the additional notation POSA tothe Unit.

Additional notation POSA-HR (higher redundancy) is alsocovered by this Note.

Operational procedures for installation, anchoring linedeployment, desinstallation, maintenance, etc. are not cov-ered herein.

The integrity of risers, if any, connected to the moored Unitis not addressed.

The adequacy of the type of anchoring point selected, e.g.drag anchor, driven pile, suction caisson, etc., with the soilcapacity is not discussed.

1.2 Rules and related documents

1.2.1 Rules

a) NR216 Rules on Materials and Welding for the Classifi-cation of Marine Units

Hereafter referred to as the Rules on Materials andWelding

b) NR445 Rules for the Classification of Offshore Units

Hereafter referred to as the Offshore Rules

c) NR467 Rules for the Classification of Steel Ships

Hereafter referred to as the Ships Rules.

1.2.2 Guidance and Rule Notes

a) NI 425 Recommendations on the Quality of SoftwareOn Board.

b) NI 432 Certification of Fibre Ropes for Deepwater Off-shore Services.

c) NR320 Approval and Inspections at Works of Materialsand Equipment for the Classification of Ships and Off-shore Units.

d) NR426 Construction Survey of Steel Structures of Off-shore Units and Installations.

e) NR494 Rules for the Classification of Offshore Loadingand Offloading Buoys.

1.2.3 IACS Unified Requirements

a) IACS UR W18 (2004): Anchor chain cables and acces-sories including chafing chain for emergency towingarrangements.

b) IACS UR W22 (2009): Offshore Mooring Chains.

c) IACS UR Z17 (2008): Procedural Requirements for Ser-vice Suppliers.

1.2.4 IACS Recommendations

a) IACS Rec. 34 (2001): Standard Wave Data.

b) IACS Rec. 38 (1995): Guidelines for the Survey of Off-shore Mooring Chain cable in Use.

1.2.5 Other Industry documents

a) API RP2 SK: Recommended practice for design andanalysis of Station Keeping systems for floating struc-tures, third edition, 2005.

b) API Spec 9A: Specification for Wire Rope, Twenty-fifth edition, February 2004.

c) API RP 2I: Recommended Practice for In-serviceInspection Of Mooring Hardware for Floating Struc-

tures, third edition, 2007.

d) API RP 2A LRFD: Recommended Practice for Planning,Designing and Constructing Fixed Offshore Platforms -Load and Resistance Factor Design, 1993.

e) API RP 2A WSD: Recommended Practice for Planning,Designing and Constructing Fixed Offshore Platforms -Working Stress Design, 2005.

f) ISO 1704:2008: Shipbuilding - Stud-link anchor chains

g) ISO 2232:1990: Round drawn wire for general purposenon-alloy steel wire ropes and for large diameter steelwire ropes - Specifications.

h) ISO 10425:2003: Steel wire ropes for the petroleumand natural gas industries - Minimum requirements andterms of acceptance.

i) ISO 18692:2007: Fibre ropes for offshore stationkeep-ing - Polyester.

j) ISO TS 14909: CFibre ropes for offshore stationkeeping- High modulus polyethylene (HMPE).

k) ISO 199017:2005: Stationkeeping systems for floatingoffshore structures and mobile offshore units.

l) OCIMF 2008: Mooring Equipment Guidelines (MEG3),Oil Companies International Marine Forum.

m) NORSOK U-104: Remotely operated vehicle (ROV) ser-vices, 2003.

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NR 493, Sec 1


1.3 Other References

1.3.1

a) L. Leblanc, J.L. Isnard, H. Wilczynski: A complete andconsistent methodology for the assessment of mooringsystems, OTC 7709, May 1995.

b) M. Franois & al: Statistics of extreme and fatigue loadsin deep water moorings, OMAE01-2162, June 2001.

c) X. B. Chen: Approximation on the quadratic transferfunction of low-frequency loads, 7th InternationalBOSS conference, 1994.

d) M. Le Boulluec & al.: Recent advances on the slow-drift damping of offshore structures, 7th InternationalBOSS conference, 1994.

e) M. Francois, P. Davies, F. Grosjean, F. Legerstee: Mod-eling Fiber Rope Load-Elongation Properties: Polyesterand Other Fibers, OTC 2010.

f) P.J. Clark, S. Malenica and B. Molin (1993): An heuris-tic approach to wave drift damping, Applied OceanResearch, 15, 0141-1187/93, p. 53-55.

g) B. Molin (1994): Second-order hydrodynamics appliedto moored structures A state-of-the-art survey ShipTechnology Res. Vol. 41.

h) B. Molin (2002): Hydrodynamique des structures off-shore, Editions Technip.

i) C. Morandini, F. Legerstee, C. Raposo:Criteria for Anal-ysis of Offloading Operation, OTC14311, 2002.

j) M. Franois & al.: Multi-variate I-FORM contours forthe design of offshore structures (Practical Methodologyand application to a West Africa FPSO), ISOPE-2007-

JSC-434.

k) F. Legerstee, M. Franois, C. Morandini, S. Le-Guennec:Squall: Nightmare for designers of deepwater west afri-can mooring systems, OMAE2006-92328.

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NR 493, Sec 2


SECTION 2 CLASSIFICATIONREQUIREMENTS

1 General

1.1 Context of classification

1.1.1 Within the context of the classification of a floatingoffshore Unit as defined in Offshore Rules, the POSAnota-tion is addressing the station keeping capability of the Unit,within the limits of applicability defined in [1.2] and [1.3].

1.1.2 For floating Units such as FPUs, F(P)SOs, offloading

buoys (this list is not limitative), that are considered as per-manent installations, the station keeping capability isdeemed a Safety Critical Element and the compliance toPOSAnotation is a Classification Requirement.

The notation POSA-HR(higher redundancy) could also begranted. This notation covers strength analysis of mooringsystem with two lines damaged (see Sec 3, [6.4.1]) in addi-tion to usual POSA notation criteria. This notation is notmandatory but could be requested by Company applyingfor classification to cover more stringent criteria.

1.1.3 General provisions

The general provisions of Offshore Rules, where the princi-ples conditions and other aspects of the Classification pro-cess are defined, are fully applicable, as relevant.

1.1.4 Maintenance of Class

Conditions for the maintenance of Class, as defined in Off-shore Rules, Part A, Chapter 2, also apply to POSAnotation,with specific requirements as given in Offshore Rules,Part A, Ch 2, Sec 9, [3] (see also [3] hereafter).

1.2 Applicability of POSA notation

1.2.1 The POSAnotation covers, in general terms, the sta-tion keeping system of a free-floating body by means of aprincipally passive system.

This notation covers all the possible types of anchoring pat-terns (such as spread mooring, internal or external turret,etc.), line make up, and materials (such as chain, wires,fibre ropes, in catenary or taut configuration, etc.).

The POSAnotation does not cover however a Tension LegPlatform, that is not deemed a free-floating body, nor its(tendons) mooring system.

ThePOSA

notation also covers assisted mooring (see [1.3.3]).However, Dynamic Positioning is covered by the DYNAPOSnotation (refer to Ships Rules, Part E, Ch 10, Sec 6).

1.3 Limits of POSA notation and interfaces

with Class

1.3.1 The POSAnotation covers all the outboard elementsof a mooring system, namely:

anchors, whatever type (drag anchors, piles, suctionpiles, etc.)

all the components of load bearing lines, including linesegments and connecting devices

all the ancillary components, such as buoys, sinkers, andtheir attachment to the main lines, excluding those that are

used solely at time of deployment of the mooring system.

1.3.2 POSAnotation also covers:

fairleads and stoppers on the Unit (in whole, i.e. includ-ing Unit-side female support parts)

any associated monitoring and control systems.

1.3.3 The POSA notation does not cover windlass,winches, sheaves, used for deployment of the system or foroccasional handling of lines during Unit operation, norassociated monitoring and control systems (separate certifi-cation may be performed on request).

However, the foundations of these items into hull or on theturret are considered as part of the Unit hull and covered bythe Main Class of the Unit.

Note 1: During the classification process, organization of classifi-cation activities will take due account of the respective scopes ofContractors, in case the limits of Contractors' scope differ fromthose above.

Equipment and systems related to thruster assistance, if any,are to be separately covered under the classification mark forMachinery, as a condition for the granting of POSAnotation.

For other configurations that are not explicitly addressedhereabove, the limits of POSAnotation are to be specifiedon a case by case basis.

2 Scope of activities for POSA notation

2.1 General

2.1.1 As for other disciplines or systems, Classificationactivities span over all phases of a project:

design of the mooring system and of principal compo-nents (engineering): proposed design and related docu-mentation are reviewed by the Society

detailed design, manufacturing, and testing of all com-ponents (procurement): detailed design is reviewed, andSurveys are made of the manufacturing and testing

installation on Unit and deployment of the system at site(deemed equivalent to the construction of a structure):installation activities are Surveyed

in service inspection, for the maintenance of Class.

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NR 493, Sec 2


2.2 Certificates

2.2.1 Upon satisfactory completion of the installation ofthe system and of all activities before, and of related surveysby the Society, the POSAnotation is granted and entered inthe Initial Hull Classification Certificates of the Unit.

2.3 Design

2.3.1 General

In accordance with the provisions of Offshore Rules, Part A,Chapter 1 and Offshore Rules, Part B, Chapter 2, the partyapplying for classification is to provide the Society with theclassification data and assumptions.

The design of the mooring system is to be performed on thebasis of Unit design data, operational data and environmen-tal data, as specified in Offshore Rules, Part B, Ch 2, Sec 1.

2.3.2 Site Data

As specified for permanent installations in Offshore Rules,Part A, Chapter 1 and Offshore Rules, Part B, Chapter 2, theparty applying for classification is to specify the site atwhich the Unit will operate.

2.3.3 Operating conditions and loads

The data on Unit operation are to include the followinginformation:

a) environmental conditions:

extreme environmental conditions (survival condition)

fatigue environmental conditions (operational con-dition).

b) Unit characteristics (the range of loading conditions andassociated responses of these Units are to be also speci-fied).

c) mooring lines descriptions from anchor to stopper (themooring lay out is to be also precisely described).

d) following loads, in all relevant conditions quoted in b):

environmental loads

anchoring/mooring loads

hawser line loads

risers loads

loads induced by other equipments.

Note 1: Operational condition is to be understood as usual condi-tion (day to day condition). This notion does not take into accountany process or production consideration.

2.3.4 Methodology

The mooring system is to be designed in accordance withthe provisions and criteria specified in the present docu-ment, where guidance is also given on the methodology forthe analysis.

Statements of design review are issued following the rele-vant procedures.

2.4 Components of mooring system

2.4.1 Line components

Design specifications and design documentation are to besubmitted to the Society for review.

The manufacturing of materials and sub-components, andthe construction of components are to be performed undersurvey by the Society, according to an approved program.

Certificates will be delivered to each set of items, upon sat-isfactory completion of all related reviews and Surveys.

2.4.2 Load control system

For deep water moorings, taut systems, fibre rope moorings,and other cases where the verification of line pre-tensionscannot be achieved by conventional methods, a permanentload monitoring device is to be fitted on each line, for thecontrol of line pre-tensions at the time of periodical surveys.

System may include the transmission of an alarm in case ofline failure, or capability for continuous recording oversome time, e.g. for re-tensioning operations. Associatedcomputer software is to be in accordance with applicableprovisions of NI 425 Recommendations on the Quality ofSoftware On Board (see Sec 1, [1.2.2]).

2.5 Survey of installation and deployment

2.5.1 Installation on Unit

The installation of Unit-side items (fairleads, stoppers,chainhawses, etc.) and on-board equipment and relatedsystems is to be performed under survey by the Society, in

accordance with applicable provisions of Offshore Rules(these activities are to be usually carried out within theframe of Unit Classification Surveys).

Survey will cover quality of construction work (particularlythrough weld Non Destructive Tests - NDT). Load tests arenormally not required but, if performed, will be attended.

2.5.2 Deployment at site (installation)

Survey of installation is performed on the basis of generalprovisions of Offshore Rules, and particularly those of Off-shore Rules, Part B, Ch 3, Sec 6.

The installation procedures prepared by the relevant Con-

tractor are to be submitted to the Society for examination.Installation tolerances are to be specified in the installationprocedures, and duly taken into account in design calcula-tions.

The installation operations will be surveyed, including, butnot limited to:

installation of anchors

deployment of mooring lines

traceability of components

test loading of anchor and lines (see Sec 3, [10.5])

connection to Unit and tensioning

post-installation inspection of the system (by diversand/or ROV Survey).

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Reviews and Surveys address only the issues under thescope of Classification, particularly:

conformity of all components to Classification require-ments (as attested by Inspection Certificates)

integrity of installed parts

conformity to design of the system as installed, particu-larly the setting of line pretensions.

Surveys will include attendance of operations at site by thesurveyor of the Society, following an agreed program.

The surveyor of the Society will review records and otherdocumentation of the installation operations, prior to thedelivery of a Certificate.

Note 1: Survey by the Society is by no way intended to substitute toInsulators duty to fully document installation of the mooring system.

2.6 Documents to be submitted

2.6.1 The Submitted documentation is to include the fol-lowing information, in addition to what is specified in Off-shore Rules, Part A, Chapter 1:

a) Design criteria and data, as defined in [2]

Metocean data, soil data, and background informa-tion (see Offshore Rules, Part B, Ch 2, Sec 2)

Unit characteristics and range of loading conditions

reports of design analysis.

b) General drawings:

layout description including positions of the anchorpoints, water depth, surrounding equipment (riser,well head, ), layout of other Units in close proxim-ity (see Sec 3, [9.6.1].

mooring lines description (from anchor to stopper)

location of fairleads and stoppers

turret structure (if any).

c) Structural drawings, specifications and supporting docu-ments:

mooring systems foundations (fairleads, stoppers,

winches, bollards, etc.) as applicable.d) Mooring fittings drawings and specifications:

connecting systems

ancillary elements

anchoring systems

e) Model Tests (when performed)

specification

final report.

f) Monitoring and control system

monitoring system description

control system description.

3 In service surveys

3.1 General

3.1.1 The use of ROVs for carrying out the in-water survey

is acceptable. The company providing this service will haveto be approved as a service supplier for carrying out in-water survey, as per the requirements of IACS UR Z17 (seeSec 1, [1.2.3]).

ROV must be equipped in order to produce dimensionallymeasurement and NDT. ROV capacity should comply withSec 1, [1.2.5] item m) or equivalent.

The surveys of the mooring system of Units granted withadditional service feature POSAis normally carried out theUnit being on location, no disruption of Unit's operationbeing required.

For all dimensional check or NDT inspection to be per-

formed by sampling, the number of link surveyed is to beagreed by the Society.

For intermediate and renewal surveys, a specific inspectionprogramme is to be submitted to and agreed by the Society,according to the nature and arrangement of the mooringsystem and other relevant parameters. This programme is tobe submitted in written format and agreement signed wellin advance of the inspection campaign.

3.2 Annual Survey of anchoring lines

3.2.1 The Surveyor reviews at each annual survey therecords of operation of the station keeping equipment and

of the examination carried out by the Unit's crew at times oftensioning changes or modifications, if any.

3.2.2 The examination of the mooring components (chainor wire) adjacent to winches or windlasses, stoppers andfairleads is to be performed.

3.2.3 In the case of significant damages revealed by theabove examinations, or if the Surveyor determines thatproblems have been experienced since last annual survey, amore extensive survey may be required by the Surveyor.

3.3 Intermediate Survey of anchoring lines

3.3.1 General

Intermediate survey is usually to be performed two and a half

years (21/2) between two renewal surveys. Due to seasonalternance, tolerance of +/-9 months could be accepted.

The overall integrity of the system should be examined, e.g.by general visual inspection of selected lines, over their fulllengths, and of all lines in critical areas.

Examination of the integrity of critical components withrespect to corrosion, wear, overload, fatigue and other pos-sible modes of degradation, by visual inspection and otherappropriate methods have to be done.

The condition of corrosion protection systems, as applica-ble, should be verified.

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Pretension setting of each line (or angle measurement)should be confirmed.

Additional inspections/tests in accordance with the specificinspection program could also be performed.

3.3.2 Global anchoring line

The Surveyor reviews the records of operation of the stationkeeping equipment and of the examination carried out bythe Units crew at times of handling, if any.

3.3.3 Connection with the stopper

As far as practicable, visual inspection has to be performedfor links the closest to the stopper, in all lines. This require-ment could be mitigated in case of difficulty to access thispart of the chain (e.g. bell hawse stopper).

3.3.4 Above water chain segment

For Units having fairleads or stoppers above water level, thefollowing should be verified:

Visual examination of the whole length of the mooringline above water for all lines

Dimensional checks of at least one link in the last 5meters of chains above water in all lines (two measure-ments per link is deemed sufficient).

Additional dimensional checks of sample of links of thisportion in all lines

Measurement of chain angle (or tension) at top.

3.3.5 Upper line segment

For all types of anchoring systems, the following inspectionsshould be performed:

Visual examination of the 10 first meters of chain under-water for all lines

For chains, dimensional checks of a sample of links ofthe 10 first meters of this portion, in a representativenumber of lines (at least one per bundle in case of bun-dle configuration).

Specific provisions applicable to Fibre Rope mooring linesare given in NI432 (See Sec 1, [1.2.2]).

3.3.6 Bottom line segment

For all types of anchoring systems, the following inspectionsshould be performed:

Examination by divers or ROV of a representative chainlength close to the contact with seabed for all lines.

3.3.7 Jewellery

A general examination of all jewelleries (socket, shackle,...)is to be performed for a representative number of lines.Refer to Sec 4 for additional information.

3.4 Class Renewal Surveys of anchoring

lines

3.4.1 General

At renewal survey (every five years), the overall integrity of thesystem should be examined, e.g. by general visual inspection

of all lines, over their full lengths (by ROV or divers).

Examination of the integrity of critical components withrespect to corrosion, wear, overload, fatigue and other pos-sible modes of degradation, by visual inspection and otherappropriate methods has to be done.

The condition of corrosion protection systems, as applica-ble, should be verified.

Confirmation of the pretension setting of each line (or anglemeasurement) should be confirmed.

Additional inspections/tests in accordance with the specificinspection program is also be performed.

3.4.2 Global anchoring lineSpecial survey at the fifth year (first term), tenth, fifteenth,twentieth year and subsequently (other terms) if satisfactoryextra margin of corrosion is considered.

The Surveyor reviews at each annual survey the records ofoperation of the station keeping equipment and of theexamination carried out by the Unit's crew at times of ten-sioning changes or modifications, if any.

3.4.3 Connection with the stopper

As far as practicable, inspection, dimensional check andNDT inspection has to be performed for links connected tothe stopper, in all lines.

3.4.4 Above water chain segmentFor Units having fairleads or stoppers above water level, thefollowing items should be verified:

Visual inspection of the stopper adjacent portion

Dimensional checks of all links of this portion in all lines

Measurement of chain angle (or tension) at top.

3.4.5 Upper line segment (first 10 meters ofunderwater chain)

In all lines of the mooring system, following inspectionsshould be performed:

Visual inspection of the stopper adjacent portion (incase of underwear stopper)

Dimensional checks of this portion

Measurement of chain angle (or tension) at top (in caseof underwater stopper).

These items should be covered with a Unit at minimumdraft.

3.4.6 Wire rope

In case of sheating, visual inspection should be performedfor all lines all along the wire rope segments.

In case wire is not sheated, diameter measurement shouldbe performed for all lines all along the wire rope segments.

3.4.7 Fibre rope

Specific provisions applicable to Fibre Rope mooring linesare given in NI 432 (Cf. Sec 1, [1.2.2]).

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3.4.8 Anchors and buried chains

For anchors and buried chains, the following inspectionsshould be performed:

Visual inspection of the soil around the anchors, in par-ticular to check the absence of scouring in the vicinityof anchors

Buried part of the chain and the connection withanchors being generally not inspectable, examination ofoperations logs has to be done to confirm that each legwas recently subject to a significant loading giving indi-cations that the segment has not failed.

Note 1: In addition to that, IACS Rec. 38 (See Sec 1, [1.2.4]) shouldbe applied.

3.4.9 Jewellery

All jewelleries (socket, shackle,...) should be visually

inspected for all lines. Confer to Sec 4 for additional infor-mation.

3.5 Survey summary

3.5.1 Intermediate and renewal surveys

Fig 2 summarises the intermediate and renewal surveys tobe performed along the anchoring lines.

3.6 Renewal criteria for chains, steel wire

ropes and fibre ropes of permanent

installations

3.6.1 For all the following paragraphs, measurementshould be performed after cleaning of the marine growth.

Figure 1 : Survey summary

I : Intermediate survey

R : Renewal survey

3.6.2 As a rule, for studless or studlink chains, a linkshould be considered as defective if one of the followingcriteria is not satisfied:

the average of the two measured diameters (90 degreesapart) is to be more than 95% of the as-built diameter

diameter in any direction is to be more than 90% of theas-built diameter

The final criteria to replace chains is linked to initial com-putation hypotheses (mainly hypothesis on wear and corro-sion during design of mooring system).

3.6.3 In the case of mooring system designed taking intoaccount a corrosion margin, the following criteria should befulfilled: the lowest measured diameter Dm should behigher than the as-built diameter D reduced by the totaldesign corrosion margin C (annual corrosion rate times theinitial design life)

Dm > 95%(D - C)

3.6.4 At least one measurement at the interlink (See Fig 1)and one in an other location visually judged as the worstshould be performed.

For criteria at interlink, consideration on fabrication toler-ance should be given.

Figure 2 : Interlink measurement

3.6.5 In case criterion in [3.6.3] is not fulfilled and in orderto avoid to replace a defective segment, the following anal-yses should be provided for review:

Anchoring lines extreme tensions analysis

Anchoring lines fatigue analysis

Strength analysis of link.

Analyses should account for the observed diameter and a

provision of expected corrosion/wear for the five (5) follow-ing years, in line with rates previously observed.

Strength analysis aims at providing information on the resid-ual resistance of the corroded/weared link and is to be per-formed following Sec 3, [8.2].

In case the corrosion and wear of a link is higher than thevalue defined during the mooring system design (end ofdesign life), following considerations apply:

as a rule, the theoretical minimum breaking strength ofan equivalent "as-built" link of the reduced diameter(Dm-corrosion) cannot be taken into account

special consideration may be given in case of appropri-

ate documentation of the remaining resistance of thechain (based on actual corroded link dimensions).

I & R

I & R

R

I & R

10 m

I & R

R

I & R

10 m

10 m

10 m

Interlink

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3.6.6 For studlink chain, if a stud is missing, the link is con-sidered as defective.

3.6.7 Replacement of a link considered as defective is to be

based on a plan subject to Society approval.

3.6.8 Damage on the core of the wire rope or on thesheathing should be analysed on a case by case basis (SeeAPI RP 2I Sec 1, [1.2.5]for criteria).

Note 1: For the purpose of [3.6], it could be referred to API RP 2I

(See Sec 1, [1.2.5]).

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SECTION 3 DESIGNOFMOORINGSYSTEM

Symbols

TP : Peak period of the wave spectrum, in s

Tz : Zero-up crossing period of the wave spectrum,in s

T0 : Largest natural period of the system for motionsin the horizontal plane, in s

nf : Number of elementary Airy wave components.

1 General

1.1 Subject

1.1.1 The purpose of the present Section is to provide Classrequirements related to the design of a mooring system,with a view to the assignment of the notation POSA to afloating offshore unit.

The present Section includes:

guidance methodology for mooring analysis

design criteria.

Alternative methodologies will be given consideration, on acase by case basis, provided they are demonstrated to pro-vide a Safety Level equivalent to that resulting from theapplication of the present document.

Note 1: Unless otherwise specified, documents quoted in Sec 1,[1.2.5] are for general reference and may complement, but not

replace, the requirements of the present document.

1.2 Review of design

1.2.1 For the granting of the notation POSA, the proposeddesign and supporting documentation are reviewed, includ-ing documents in Sec 2, [2.6.1].

Verification is generally performed by independent analysis,following the methodology of this document.

Note 1: Independent analysis is by no way intended to substitute todesigner's duty to fully document his des ign.

1.3 General methodology

1.3.1 Mooring lines POSA assessment should cover follow-ing items:

Global lines

Mooring components

Fairleads / Stoppers

Anchors.

Global line should be assessed for strength (intact, damagedand transient) and fatigue (TT and OPB/IPB) purposes.

Additional considerations for the integrity of the lines arealso to be considered (contact of connectors with soil, up-lift at anchor, clashing, synthetic rope minimum tension ).All these items are covered in this note.

2 Methods of evaluation

2.1 Objective

2.1.1 The objective of analysis is to obtain information onUnit motions, the resulting excursions and line tensions,under some specified metocean conditions, that are repre-sentative of:

either the extreme conditions at intended site, or somelimit operating conditions, for the evaluation of design(extreme) values

or more frequently occurring conditions, for the assess-ment of fatigue.

2.1.2 Available methods

The available methods of analysis vary by the approachtaken to evaluate:

overall system response and resulting excursions andUnit motions

line response and resulting tensions.

Model Tests are another possible source of information, as

discussed below.

Guidance and criteria in the present document are madewith reference to the Quasi-dynamic and the Quasi-dynamic/Dynamic line response methods, as definedbelow.

2.2 Quasi-static analysis

2.2.1 In a quasi-static analysis, the line tensions are evalu-ated from the static line response to loads/displacementsthat are applied on Unit as static actions.

This method is often used at an initial planning stage, but isnot deemed acceptable for system design nor for Classassessment.

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2.3 Quasi-dynamic analysis

2.3.1 Methodology

A methodology for analysis of mooring systems, the Quasi-dynamic analysis, has been developed by the Society, and

presented in a) of Sec 1, [1.3] and [3.3].

This method is generally considered as the most adequatefor moorings in shallow and moderate water depths, in theconditions specified in [2.3.2]and [5.3].

Background information is presented in reference b) ofSec 1,[1.3].

2.3.2 Limitations of the calculation methodology

The mooring system is assumed not to be subject to reso-nance at the wave frequency. In addition, out-of-horizontal-plane low frequency motions are supposed to be negligible.Hence, this methodology may not be appropriate to Spars

or certain types of semi-submersibles operating at very largedrafts.

It is supposed that horizontal low and wave frequency phe-nomena do not interfere. Compliance with this assumptionis reasonably satisfied if the natural period of the mooringsystem in surge, sway and yaw is greater than five times thezero-up crossing period of the wave.

The variation of the suspended line weight with the motionof the moored Unit is supposed not to significantly modifythe average Unit draft, trim or list angles.

2.4 Dynamic line response

2.4.1 In a Dynamic line response analysis, the dynamic Unitresponse is computed by the same method as in Quasi-dynamic analysis, but the line tensions are evaluated from aDynamic analysis of the line response to the fairlead motion.

This method is applicable to deep water moorings, or veryharsh metocean conditions, where the criteria of accept-ability of Quasi-dynamic analysis are not met, and, in allcases, to fatigue analysis.

2.5 Other methods of analysis

2.5.1 Fully coupled analysis might be used when couplingsare deemed important, or for calibration purpose.

2.6 Model tests

2.6.1 As a rule, tunnel (or basin) tests are to be performedin order to obtain load coefficients for wind and current onthe Unit.

As a rule, model tests in the basin are to be carried out forthe validation of the overall behavior of the system and thecalibration of analyses.

Consideration may be given, on a case by case basis, tomodel tests performed on a very similar system, in equiva-lent metocean conditions and water depth.

Model tests however will not be generally deemed suffi-cient to fully document a design, due to practical limita-tions in the modelling of the system (e.g. with respect towater depth), and in the number of system configurationsand combinations of metocean parameters that can beaddressed within a testing program.

Note 1: These tests are considered as a source of information fordesign and do not form part of physical testing activities duringmanufacturing and construction.

3 Environment, actions and motions

3.1 Environment

3.1.1 Waves

Waves are defined by the parameters of a wave energyspectrum. In some areas, it will be relevant to split the

incoming energy in two (or more) parts (e.g. swell and windsea), modelled by two (or more) spectra with different direc-tions of approach.

For modelling of waves by elementary Airy wave compo-nents, using the technique of random frequency and ran-dom phase, the number nf of elementary Airy wave

components (in each spectrum) is not to be taken less than:

nf 100

not less than:

provided the range of circular frequency ( = M m,with Mand m- the Maximum and minimum circular fre-

quency for wind spectrum, in Hz) does not exceed 15/Tp(otherwise, nfis to be increased accordingly).

3.1.2 Wind

A description by a constant speed V10-min (without windspectrum) may be used for initial evaluations or when thenatural period T0is not very large.

In case of lower T0, V1-minshall be considered.

Otherwise, e.g. in deep waters, a description by an appro-

priate wind spectrum combined with a wind speed V1-hourshould be used.

In the discretisation of spectrum, the minimum frequency,in Hz, is not to be taken less than the frequency correspond-ing to a 1 h period, i.e.:

fm= 2,8 10-4

The upper frequency fMmay be taken in the range of 0,03 to0,05 Hz. However, a higher frequency content is to betaken into account if the smallest natural period of the sys-tem (for horizontal motions) is lower than 1 mn.

Number of frequencies is to be selected so that the fre-

quency interval f satisfy:

f < 0,1 / T0and f < fm

nf 30 T0 Tp

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3.1.3 Squalls

Strong and sudden winds (squalls) occurring in inter tropi-cal convergence zone (ITCZ) are to be modelled by repre-sentative time series of wind speed and direction.

As a rule, the calculation methodology should follow the

steps below:

a) A set of squalls is defined for the considered site. Squallsare rescaled as per App 2, [2.4.2].

b) Each squall is considered as a governing element and socombined with wave(s) and current in accordance withApp 2, [5] and metocean specification. Directionalscanning interval and scanning methodology shall fol-low App 2, [3].

c) Design response (of tension, offset,...) is the maximumresponses obtained over all the squall cases except oth-erwise agreed.

Note 1: This methodology is based on the present knowledge aboutsqualls.

Note 2: When no time serie is available, using a constant wind isnot sufficient to assess squalls.

Note 3: Scanning methodology with an optimized step b) could besend to Class for agreement on a case by case basis

Note 4: Additional information about squalls can be found in Sec1, [1.3], k).

3.1.4 Current

A description of near-surface current by a constant speed isnormally sufficient (cf. App 2).

Sudden changes (local surface currents, loop currents)occurring in some areas may induce significant transienteffects and are to be modelled by representative time seriesof current intensity and direction.

3.2 Actions

3.2.1 Wave drift load

Quadratic Transfer Functions (QTF) are the second orderlow-frequency wave loads for floating body.

Some approximations can be used in given cases. The mostpopular one is the Newmans approximation. Newman'svariants Molin or BV are largely used in practice. Both arevalid for deep water and also soft mooring systems, butinvalid for stiff mooring systems and in shallow water.

Another approach called BV approximation allows dealingwith soft mooring system in shallow or deep water.

The most general way to reconstruct the 2nd order loads intime domain is to use the full QTF matrix (See papers c) andd) quoted in Sec 1, [1.3]). The low-frequency wave loadingis computed by a double summation (QTFC -complete - for-mulation). This requires the full QTF matrix determinationand can induce large computing time.

Consideration should be given to the use of this full QTF,

in the case where the Newman approximation might not besufficient (as a guidance: when slow drift natural period isless than 150 s, or in water depth less than 40m).

3.2.2 Wind and current loads

a) Load coefficients for wind and current loads are to beobtained from tunnel (or basin) tests. Consideration willbe given to derivation of data from tests on a very simi-lar model.

In such tests a model is maintained in a turbulent flowof adequate intensity and profile, in a fixed position,with a given incidence. The loads thus measured areprojected on the Unit axes:

Fx=Cx().V2

Fy=Cy().V2

M=C().V2

Where:

Fx : Force along vessel longitudinal axis

Fy : Force along vessel transverse horizontal axis

M

: Yaw moment

Cx, Cy, C: Corresponding force coefficients

V, : Flow (reference) velocity and incidence.

Note 1: Concerning yaw moment, particular attention should bepaid to the reference point where the moments are calculated(origin O of the Unit axis system, Unit centre of gravity G, mid-ship section, etc.).

When applicable, the data in Prediction of Wind andCurrent Loads on VLCC's, Oil Companies InternationalMarine Forum Sec 1, [1.2.5]) and Prediction of WindLoads on Large Liquefied Gas Carriers Sec 1, [1.2.5])can be used for tanker shaped units.

Note 2: These data are relevant for tankers or LNG Carriers, andnot applicable to different hull shapes or arrangement of super-structure.

At initial design stage, load coefficients may be obtainedfrom analytical expressions (e.g. extended DucheminFormula) or other heuristic expression, provided allthree components of force are taken into account(reduction to the in-line component is not consideredadequate in this respect).

b) Wind Load

Wind loads are evaluated by the above formulae, takinginto account the wind speed at a reference elevation(typically 10 m above sea level - same as in tests), and

the incidence of wind with respect to vessel.

c) Current Load

1) As the vessel is moving in water, the instantaneousloads (combining current load and drag induced bymotions) are evaluated by the above formulae, tak-ing into account the equivalent incidence cand the

equivalent current velocity Ucas follows.

Given Vc and c- the current velocity and inci-

dence (incoming direction w.r.t. vessel), the equiva-lent current intensity Uc and relative incidence care given by:

where V is the vector of vessel intant velocity.

Uc Vc V

+=

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2) When the Unit is fixed during current model tests,

the force coefficient Cdoes not include any effect

due to the rotation of the Unit in the fluid. In such

case, the Molins yaw moment (only valid for barge

or shipshape units) is therefore to be added to that

derived from model tests. Additional informationcan be found in Sec 1, [1.3], h).

Note 3: With the above relative velocity formulation and the equa-

tions of motions expressed in vessel axis, the hydrody-

namic yaw moment includes a steady component: the

Munk moment. As this moment is already included in the

moment calculated from current forces coefficients, it

should be substracted in the load balance.

3.2.3 Riser loads

Risers and other fluid carrying lines (e.g. an export line) are

generally kept under tension (possibly resulting in perma-nent pull on the Unit), and are subject to the action of cur-

rent over the water column. The resulting forces may

represent a significant part of the total load on the Unit.

The reactions on Unit may be obtained from a static analy-

sis of the lines.

Attention should be given to the effect of varying direction

and intensity of current along the water column.

The mean static load, that is depending upon the instanta-

neous low frequency position of the Unit, may be modelled

by:

either a static load, corresponding to a mean offset con-

dition, and dummy lines to represent variations of load

around this position, or

tabulated loads, for a range of positions around the

expected mean position (see ref. 4 in paper b) quoted in

Sec 1, [1.3]).

3.2.4 Damping

a) The sources of damping on a moored Unit are multiple

and of various nature. Different theories exist to explain

and model these effects. However, all of them are based

on either fully empirical or semi-analytical formulations

the range of validity of which is necessarily limited.

Any damping model requires to be calibrated and its

field of applicability clearly identified. Limitations of the

present approach are specified in [2.3.2].

Damping due to hull drag is modelled together with

current loads since these loads are calculated on the

basis of the relative fluid velocity (see [3.2.2]).

Other sources of damping can be modelled by a linear

dampins, i.e. forces proportional to the absolute speed

of the Unit, according to the following formulae:

FBx=-Bxxu

FBy=-Byyv

where:

u : Absolute velocity in surge of the origin O of

the Unit axis system

v : Absolute velocity in sway of the origin O of

the Unit axis system

: Function versus time of the Unit heading

Bxx : Linear damping coefficient in surge

Byy : Linear damping coefficient in sway

B : Linear damping coefficient in yaw.

F : Force induced by damping.

M : Moment induced by damipng.

b) The result of the simulation are quite sensitive to the lin-

ear damping coefficients. Great care must therefore be

paid to their evaluation.

The values proposed in Tab 1 tentatively account for

most of sources of damping (see d) below) [3.2.2].

c) Data in Tab 1 could be used as a preliminary approach,

unless more accurate data is available from model test,

or fully coupled analysis calibrated by model tests, to

assist in the calibration of the slow-drift damping coeffi-

cients.

These data applies only for usual mooring systems.

For Unit moored by only surface lines, as a Unit on asingle point mooring or a shuttle tanker moored to a

FPSO, the values from formula in Tab 1 for tanker

should be multiplied by a factor 0,37.

d) A direct assessment of damping terms requires that main

contributing terms are separately evaluated:

viscous damping on the hull is accounted for by the

relative velocity formulation of the equations of

manoeuvrability

wave drift damping may be obtained from the drift

forces and its derivates. Reference can be made to

the formulation in papers f) and g) of Sec 1, [1.3]. Inthese papers the quadratic transfer function matrix is

modified taking into account the slow drift velocity,

the current speed and the instantaneous heading

damping due to lines (risers and mooring lines) may

be estimated from line Dynamic calculations, or

inferred from a fully coupled analysis

bottom friction effects on mooring lines.

MB o Bddt-------

=

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Table 1 : Low-band linear damping coefficients for different types of usual mooring systems

3.2.5 Other loads

Thrusters are sometimes used to assist a passive mooringsystem.

The driving system of the thruster loads may be very simple(e.g. constant load in a constant direction relative to theUnit heading) or more complex.

Note 1: See ISO Sec 1, [1.2.5], k).

In any case, the loads that the thrusters actually produce onthe moored Unit should be computed at each time step as afunction of the applicable parameters.

Recognised methods should be used for the load calcula-

tions, with due account for possible interferences betweenthe thrusters themselves or between the thrusters and thehull and for all other phenomena that may modify, alter ordegrade the thruster performances.

3.3 Unit response

3.3.1 General

In a Quasi-dynamic analysis, the dynamic Unit response iscomputed using a mixed time-domain/frequency domainanalysis, taking into account quasi-static line response.

The calculation procedure consists of the determination ofthe low frequency response of the moored Unit under the

effect of waves, wind and current, by time domain simula-tions, followed by the superimposition of the wave fre-quency motions. It is assumed that low and wave frequencycomponents do not significantly interfere with each otherbecause of very different time scales (see [2.3.2]). As a con-sequence, they are assessed separately in the framework ofthis approximation and added together at the end of eachtime step of the simulation.

At the end of each time step, the line tensions are evaluatedfrom the quasi-static line response to the fairlead motion.

3.3.2 Low Frequency response

The mean and low-frequency responses (three motions) of

the Unit in the horizontal plane are calculated by resolving,in a time domain simulation, the equations of the dynamicequilibrium of the Unit (equation of manoeuvrability,

expressed typically in a system of axis linked to the Unit),for predefined time series of wave elevation and otherparameters.

This is made taking into account the actions of waves,wind,... as detailed in [3.2] above, all evaluated taken intoaccount the instant of low-frequency position and the rela-tive heading of the vessel with respect to each action, and:

the restoring mooring force (see [4.2] below)

the mass matrix and added mass of the Unit (addedmass calculated for -> 0)

a linear damping as per [3.2.4] above.

3.3.3 Wave Frequency motions

a) Unit motions (RAOs)

As as a pre-requisite to mooring analysis, the sixmotions of the vessel in the frequency domain (RAOs)should be determined.

The RAO's can be obtained by model tests or by a rec-ognised first order diffraction-radiation analysis pro-gram, with due account for the actual site water depth.

It is assumed that the wave frequency motions of theUnit are not significantly disturbed by the variation ofthe mooring stiffness with the low frequency offset. An

average mooring stiffness can therefore be used for pre-determining the Response Amplitude Operators (RAO's)of the Unit.

In motion analysis, account may be taken of hydrody-namic damping on the floating body, (e.g. roll damp-ing), by appropriate formulation.

The effect of suspended load of the mooring lines and ofrisers (vertical and horizontal components), around themean Unit position, should be accounted for, not asmass, but as terms in the stiffness matrix, together withthe stiffness of these systems.

Additional wave frequency dynamic effects of lines (pri-

marily damping) might be significant in some cases, andmay be estimated from line Dynamic calculations, orinferred from a fully coupled analysis.

Mooring system Bxx Byy B

Barge or tanker in spread mooring

Barge or tanker on a SPM (1) 0,083 L2Byy

Semi-submersible Unit at operating draft

(1) The values of Bxx, Byyand Bare given assuming that the origin O of the ship axis system is in the midship section.

Note 1:

KOxx, KOyy, KO: Diagonal terms of the mooring stiffness matrix [KO], evaluated at the average position of the Unit during the stormMaxx, Mayy, Ma: Diagonal terms of the asymptotic added mass matrix of the UnitI : Moment of inertia in yaw, in kgm

2, calculated at the centre of gravity G of the Unitm : Mass of the Unit, in kgL : Length of the Unit, in mB : Breadth of the Unit, in m.Note 2:For spread mooring, Bxxand Byycorrespond to 3% of the critical damping and Bto 5% of the critical damping.

0 06, KOx x m Ma xx+( ) 0 06, KOy y m Ma yy+( ) 0 10 KO I Ma m Ma yy+( )xG2

+ +[ ],

0 01m gL---, 0 02m gB

---,

0 20, KOx x m Ma xx+( ) 0 20, KOy y m Ma yy+( ) 0 10 KO I Ma m Ma yy+( )xG2

+ +[ ],

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b) Wave frequency motions

The wave frequency motions are obtained by linearsummation of those due to each component of thewaves, taking into account the instant low-frequencyposition and the relative heading of the vessel withrespect to waves.

At each time step of the simulation, the six wave fre-quency motions are superposed to the low frequencymotions, to get the instant position of the vessel, thusthe position of each fairlead from which the lineresponse (see [5]) can be evaluated.

4 Mooring System

4.1 Mooring pattern and initial tensions

4.1.1 The mooring pattern is the theoretical description ofthe mooring system as installed on site. The mooring pattern

thus includes the general layout of the mooring system andthe elevation of each line in its initial vertical plane. Toachieve such a description, the following information isneeded:

Site bathymetry

Unit position and heading

Unit draft and vertical centre of gravity

Fairlead positions

Anchor positions

Mooring line composition

Paid-out line lengths.

4.1.2 The knowledge of all items listed in [4.1.1] automati-cally settles the initial tensions since a relation of the fol-lowing form exists for each line as soon as its vertical planeis known:

where:

L : Paid-out length of the line

D : Horizontal distance between the anchor andthe fairlead

T : Tension at fairlead.

In practice, an iterative process is needed to set the initial

tensions to their prescribed values, and to ensure that the

three parameters are compatible together.

4.2 Mooring response

4.2.1 Following the assumptions in [5.1.1] below, the load

induced by any mooring line on the moored Unit depends

only on the anchor-to-fairlead distance.

The azimuth of a mooring line is defined by the relative

position of fairlead and anchor.

In the time domain simulation of low frequency motion, the

mooring restoring force (horizontal force and yaw moment)

at each time step is obtained by summation of the horizon-

tal components of line tensions at each fairlead, as resulting

from fairlead position under the low frequency motion.

4.3 Mooring stiffness

4.3.1 The mooring stiffness is the 6-by-6 matrix which links

elementary external loads applied to the Unit with its result-

ing elementary displacements around a given position.

The mooring stiffness is to be considered for the calculation

of unit motions RAOs. In most cases, the stiffness induced

by the mooring system for out-of-horizontal plane motions

is negligible in comparison with the hydrostatic stiffness.

The sensitivity to input is illustrated in Fig 1(example of unitsurge motion).

4.3.2 The stiffness matrix [K] is obtained by the summation

of the contributions of all mooring lines under six elemen-

tary displacement of the vessel.

Note 1: The stiffness matrix can be also obtained by multi-linear

regression, versus the time series of vessel motions, of a time series

of the mooring force, computed after an analysis taking into

account both low- frequency and wave frequency motions.

Figure 1 : Effect of mooring stiffness on RAO

f L D T, ,( ) 0=

Wave circular frequency (rad/s)

0,2 0,4 0,6 0,8 1,0 1,2 1,4

Motion(m/m)

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

0,0

Increasing mooring stiffness

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5 Line response

5.1 Quasi-static line response

5.1.1 Assumptions

The lower end of the mooring line is anchored to a fixedpoint. The upper end is connected to a fairlead of themoored Unit which can be either emerging or immersed.The mooring line cannot penetrate the seabed.

The mooring line is made up of a series of homogeneoussegments attached end-to-end, the bending stiffness ofwhich is negligible. A homogeneous segment is character-ised by constant mechanical properties over its wholelength. A buoy or a sinker may be connected at the upperend of any segment.

At any time, the mooring line is assumed to be in the verti-cal plane passing by its anchoring point and its fairlead.This implies that wave, current, wind and dynamic loads onany of the mooring line components are neglected. It alsoimplies that friction effects, transverse to those parts of theline laying on the seabed, are not taken into account.

5.1.2 Line elements

The quasi-static line response is based on the equations ofthe elastic catenary.

For chains and wire ropes, the elastic response is linear. Thiswrite:

Where dl is the elongation of an elementary length l of theline at rest, when submitted to a tension T at both ends, and is given by:

in which is the nominal diameter of the chain or wire ropeand E the equivalent Young modulus.

For some other materials (e.g. some synthetic material see[5.1.6]) the relation dl/l=f(T) is not linear: it can be approxi-mated e.g. by a polynomial function of the tension.

The elasticity properties of different materials are given in[5.1.4] to [5.1.6].

5.1.3 Wire ropes

Specific data are to be obtained from the manufacturersince the stiffness properties depend upon the wire ropedesign.

5.1.4 Fibre rope mooring lines

A model for the load-elongation characteristics of polyesterdeep water mooring lines is given in NI432 (See Sec 1,[1.2.2] and Sec 1, [1.3]).

The equivalent linear elastic properties are there expressedas a non-dimensional stiffness:

Kr = (T/MBS) / (dl / l)

where MBS is the breaking strength of the line.

Thus is given by:

= 1 / (Kr * MBS)

Note 1: A polynomial fit of the break load test load-elongationcurve is not appropriate in this case.

5.1.5 HawsersHawsers and other fibre ropes are generally substantiallymore compliant than deep water mooring ropes. In theabsence of better data, the load-elongation curve corre-sponding to a worked rope of the same material and con-struction may be used, but may still over-predict mean offsetand under-predict maximum load. Data taken from a newrope (first extension) are not acceptable.

Manufacturers data (usually presented as a curve giving thetension in percent of the breaking load (BL) versus the rela-tive elongation of the material) may be converted in a rela-tion as in [5.1.2] above

5.1.6 Buoys and sinkers

A buoy (respectively a sinker) induces an upward (respec-tively downward) load to the point of the line to which it isattached.

Buoys and sinker should be carefully modelled so that thenet action that they exert to the mooring line remains cor-rect whatever the tension in the line and the resulting posi-tion of the element with respect to sea level or seabed.

5.2 Dynamic line analysis

5.2.1 The Dynamic line response is obtained from a finite

element model of the line.

a) Hydrodynamic drag coefficient CD and inertia coeffi-cient CA may be taken as shown in Tab 3.

CD, respectively CA, is given based on the referencediameter, respectively volume per unit length, of a rodwith the effective diameter Deffbased on:

nominal chain diameter d, for chains

rope outside diameter D, for wire rope and fibrerope.

Note 1: The force coefficient CMN= 1 + CANis also used to specify

the normal coefficient.

b) Length of elements l iin finite element model should notexceed, for each segment of line:

where:

mNi : Total transversal (normal) mass per unit

length of the line segment, in water, in kg/m.

mNmay be obtained from Tab 3 based on

the (in air) mass per unit length m of the linesegment

Fmean : Mean line tension, in kN.

Note 2: A smaller length of elements is generally necessary in thetouchdown area.

dll

----- T=

24

E2

------------=

l i TPFmeanmNi

------------=

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Table 2 : Hydrodynamic coefficients

5.2.2 The mean position is set to get the mean tension Fmeanfrom the Unit response analysis, and the 3D fairlead motionis applied as an imposed displacement at the top of the line.

Current and time depending water particle kinematics arealso applied, but this latter term gives a marginal contribu-tion and can be neglected.

5.2.3 Time domain analysis involves iterations at each timestep, and the iteration parameters (particularly the maxi-mum number of iterations) must be set so as to get anunspoiled solution.

5.3 Characterization of the line response

5.3.1 When the Quasi-dynamic line response has beenobtained, a test-run of Dynamic response is to be per-

formed, in order to characterize the Dynamic response, andevaluate, or confirm, if the Quasi-dynamic response can beused for the evaluation of extreme loads.

Details of methodology and criteria for this evaluation aregiven in App 1.

Such evaluation may be omitted for chain moorings in mod-erate water depths (less than 150 m), if all the other condi-tions specified in [2] are met, and accordingly the safetyfactors for Quasi-dynamic analysis are used.

5.4 Dynamic line response

5.4.1 For a given simulation, the maximum tension overthe duration of the simulation (at least three hours) is to beobtained.

This can be achieved by performing Dynamic analyses overa limited number of windows, each with a duration not lessthan T0, the natural period of Unit low frequency motion.

Three to five windows are to be selected, based on the max-ima of dTqd/ dt, or of Tqdif relevant (see App 1).

Alternatively, when the correlation of Tdynwith dTqd/ dt isclearly established, an option is to analyse only one win-dow corresponding to the expected maximum.

5.4.2 The maximum tension for the simulation is thentaken as the maximum over the several windows, or themaximum in the selected one-window, as relevant.

6 Design tensions

6.1 Intact condition

6.1.1 For any sea-state to be considered, n simulations of at

least three hours each should be performed using differentsets of elementary waves representative of the whole spec-trum. If a wind gust spectrum is used, the same provisionapplies.

Note 1: For squalls analysis, duration of the simulation will dependon squall time serie record.

Note 2: Guidances for metocean combinations are given in App 2

The response signals should be built up with a time stepequal to or less than one tenth of the peak or zero-up cross-ing period of the wave spectrum, whichever is the mostappropriate.

The design tension of a line in intact condition, for a speci-fied set of Unit and metocean conditions, is defined from the

mean and the standard deviation of the n maxima Tk, eachobtained from n simulations, using different seeds, i.e. dif-ferent sets of elementary waves and wind components.

The maxima are either the maxima of the Quasi-dynamictension, or the maxima of the Dynamic tension, obtained asdefined in [5.4] above.

The design load TDfor the condition analysed is given by:

TD= TM+ a TS

where:

TMis the mean of Tk :

TSis the (n 1) standard deviation, given by:

a is a factor, given in Tab 4, as a function of the type ofanalysis actually performed, and the number of simula-tions

Table 3 : Factor a

6.2 One-line damaged condition

6.2.1 The design tension of a line in damaged condition isobtained by the same method as the intact tension, con-sidering a system with any one line removed, or a thrusterfailure as specified in [6.5].

Note 1: The failure of an ancillary line component (buoy or sinker,etc.) is also a damaged condition. However in most cases suchcondition is covered by the above analysis.

DeffCDN

(1) (3)

CAN

(1)

CAL(1)

mN

Chain 1,8 d 0,8 1 0,5 1,13 m

Wire rope D 0,7 1 0 1,20 mFibre rope(2) 1,1 0,15 2,00 m

(1) Suffix N is for Normal (transversal) direction

Suffix L is for Longitudinal (tangential) direction

(2) For fibre rope, CANand CALare inclusive of entrapped

water.

(3) CDNare specified as lower bound, to avoid unconser-

vative over-estimate of damping effects.

Method of analysisNumber of simulations n

5 10 20 30

Dynamic 0,60 0,30 0,10 0

Dynamic 1 window 1,20 0,80 0,55 0,45

Quasi-dynamic 1,80 0,90 0,50 0,40

Note 1: For intermediate numbers, a can be obtained byinterpolation with n

TM1

n--- Tk

=

TS2 1

n 1------------ Tk TM( )

2

=

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6.3 One-line failure (transient) condition

6.3.1 The design tension of a line in a failure (transient)condition is obtained as the average over a set of possiblefailure instants, of the maximum transient tension (see also[9.6]).

The procedure described in a) and b) should be repeated foreach line:

a) For the sea-state to be considered and for each mooringline, the simulation to be selected for the study of theone-line failure case should satisfy the following criteria:

the maximum fairlead tension is the closest to itsdesign tension in intact condition as defined in[6.1.1], and

this maximum occurs while the low frequency com-ponent of the tension is increasing.

If the second criterion is not met, the selection proce-dure should be resumed with the highest second maxi-mum in the same simulation or the closest secondsimulation.

The two instants around the maximum tension wherethe low frequency component of the tension is respec-tively minimum and maximum should be identified. SeeFig 2.

b) Using always the same sets of Airy waves and windcomponents as those used in the simulation identified ina), five simulations are repeated; during these simula-tions, the line should be broken at different timesequally distributed between the two instants identifiedin a) with the objective of catching maximum.

All five simulations should be run from the beginning in

order to ensure the same initial numerical transient.They can be terminated after two low frequency cyclesfollowing the line failure.

The maximum tension obtained at the fairlead of eachremaining line after the line failure should be identified

for all five simulations. For this particular one-line-fail-ure case, the tension in damage condition of eachremaining line is the average of its five maxima thusobtained.

c) For a given line, the design tension in damage conditionis the maximum of all possible one-line failure cases.

6.4 Two-lines damaged condition

6.4.1 Residual strength based on a system with two linesremoved has to be estimated. The design tension of a line intwo-lines damaged condition is obtained by the samemethod as the intact tension, considering a system withtwo adjacent lines removed

This criteria is only part of the POSA-HRnotation and is notrequested for POSAnotation.

6.5 Thruster failure

6.5.1 The thruster failure is assumed not to be concomitantwith a mooring line failure.

6.5.2 Two kinds of failure should be investigated:

a) the total loss of one thruster, the other thrusters havingtwo thirds of their maximum thrust capacity available,

b) loss of half the total thrust capacity.

Note 1: These two cases lead to the same remaining thrust capacityfor a unit equipped with four identical thrusters.

6.5.3 The design tensions in the mooring lines should bedetermined by a method similar to that described in [6.1]

for the one-line failure case. The instants of failure, how-ever, can be randomly selected during the simulation iden-tified in [6.3.1] with a sufficient number to ensure that thestatistics derived from the response samples are reasonablyrepresentative.

Figure 2 : Range of time to be selected for the one-line failure case

2250

2300

2350

2400

2450

2500

2550

2600

2650

2700

2750

10000

10100

10200

10300

10400

10500

10600

10700

Maximum tension inintact condition

Half low frequencyperiod of interest

Time (s)

Tension (kN)

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6.6 Design tension in line components

6.6.1 The tensions at different locations along the lines, asrequired for sizing of line components, are to be obtainedby the same method as defined in [6.1] to [6.3].

When Quasi-dynamic analysis is performed, design tensionand other parameters (e.g. uplift angle at anchor point) maybe obtained from the catenary with a fairlead position cor-responding to the situation leading to the design tension atfairlead.

Note 1: There may be several positions, with different combina-tions of offset and vertical position, that are governing for differentlocations along the line.

6.7 Minimum tension

6.7.1 When required (fibre rope mooring), the minimumtension TDm is to be obtained from line Dynamic analysis,by the same method as maximum tension, as:

TDm= TMa TS

7 Fatigue analysis

7.1 Tension range

7.1.1 General

For each environmental condition selected as specified in[9.7], the distributions of tension ranges are to be obtainedfrom line dynamic analyses (see App 1, with a minimumduration of 30 min each and a minimum of 10.T0. Both

windward and leeward lines shall be analysed.

An appropriate cycle counting method, accounting for bothlow and wave frequency cycles, such as Rainflow, is to bebe used.

7.1.2 Miner summation

Fatigue damage in any component of the mooring line isobtained by means of the Miners ratio calculated for oneyear (31 557 600 seconds):

where:

Dj : Fatigue damage accumulated over one year bythe component under the environmental condi-tion number j

pj : Probability of occurrence of the environmental

condition number j (the sum of the probabilitiesof all selected environmental conditions shouldbe equal to 1)

dj : Duration of the simulation of the environmental

condition number j (normally 10 800 secondsas specified in [9.7]

njk : Number of cycles within the tension range inter-

val number k encountered by the componentunder the environmental condition number j

Nk : Number of cycles to failure at tension range k asgiven by the appropriate T-N curve.

The total damage D accumulated over one year by the com-ponent is then give

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