ANTAN AUDIT REPORT - ukooa.co.uk · 1.2.1 A number of mooring line, turret and thruster failures were investigated during the course of the study. The following trends were identified:

Noble Denton Europe Ltd. Marine and Engineering

Consultant and Surveyors

42

42 Victoria Street Aberdeen AB10 1XA Telephone 01224 642255 Fax 01224 647046 Telex 885802 www.nobledenton.com

NOBLE DENTON

REPORT

UNITED KINGDOM OFFSHORE OPERATORS ASSOCIATION

FPSO COMMITTEE

FPSO MOORING SYSTEM

INTEGRITY STUDY

No: A3792/01/NDE/APC

Prepared by

NOBLE DENTON EUROPE LTD.

External Distribution: UKOOA Working Group, Industrial Questionnaire Respondents Internal Distribution: WS 40/64/4792, Bomel

Date Rev. Description Prepared by Authorised by 8th January 2002 00 Draft Final RBB/APC RBB 22nd March 2002 01 Final for Issue RBB/APC RBB 27th March 2002 02 For General Issue RBB/APC RBB

NOBLE REV 02 FPSO MOORING SYSTEM DENTON INTEGRITY STUDY

CONTENTS 1 SUMMARY................................................................................................................................................... 3

2 INTRODUCTION ........................................................................................................................................ 7

3 INDUSTRY QUESTIONNAIRE ................................................................................................................ 8

4 EXTERNAL LITERATURE SEARCH................................................................................................... 13

5 NOBLE DENTON IN HOUSE DATA ..................................................................................................... 29

6 WORKSHOP DETAILS............................................................................................................................ 33

7 DRAFT GUIDANCE.................................................................................................................................. 34

8 RECOMMENDATIONS ........................................................................................................................... 35

APPENDIX

A Presentation Overheads B Data Gathering Questionnaire C Incident Reports D Survey Data E Workshop Notes F Draft Best Practice Document

I:\Documents\Reports\Reports 2002\A3792-02.doc 18/07/02 13:42

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1 SUMMARY 1.1 This report

1.1.1 This report describes the results of a FPSO mooring integrity study performed by Noble Denton Europe Ltd. (NDE) on behalf of the UK Offshore Operators Association FPSO Steering Committee. The study includes input from 30 different organisations and covers 9 UK sector FPSO's (giving a total of 40 operating years).

1.2 Station Keeping Failure History

1.2.1 A number of mooring line, turret and thruster failures were investigated during the course of the study. The following trends were identified:

• Most failures have been associated with terminations, fairleads, connectors, pin retention details (i.e. "one off" design details not standard chain/wire elements).

• There have also been similar “one off” defects with turret and thruster systems

• Where there have been line failures there has typically been evidence of degradation in all similar components (i.e. the potential exists for multiple failure) rather than the single failures historically associated with MODU's).

• The causes have included poor detailing, fatigue sensitivity, inadequate corrosion protection, in-appropriate materials and lack of adequate manufacturing integrity.

• Most of the failures first appeared within the first three years of operation.

1.3 Risk Exposure(s).

1.3.1 Data published by DnV suggests an average historical rate of FPSO mooring failure about once every 5-6 operating years and this is consistent with the frequency of failures identified in the study. However, the UK FPSO fleet is relatively new and has not yet experienced anything like its design fatigue exposure so failure rates could increase in the future. On the other hand the failures investigated in this study were generally due to weaknesses in detailed design of the sort that are unlikely to affect all units to the same extent. Future rates of failure therefore remain uncertain.

1.3.2 None of the failures have resulted in any serious Health Safety and Environmental (HSE) exposures and this will remain the case provided that the primary importance of FPSO position monitoring and riser isolation is recognised. This in turn requires the application of strict performance standards (and/or heavy weather policies) that will ensure that all hydrocarbons are controlled and safely isolated prior to exceeding the riser's safe operating envelope.

1.3.3 However, each of the failures has resulted in commercial exposure due to the cost of repairs downtime etc. and these exposures could be serious in the event of total mooring system failure. A total mooring system failure is still considered "unlikely" but the study has identified weaknesses in the current "best practice" that could lead to a serious multiple line failure in certain circumstances.


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1.4 Fundamental Concern

1.4.1 The fundamental concern is that the mooring integrity strategies adopted by most UK FPSO operators cannot always anticipate or prevent the widespread degradation of mooring components that could lead to multiple line failure(s) in storm conditions. This weakness has be illustrated by many of the failures that have occurred and is likely to be relevant for the long term fatigue failures that may occur in the future.

1.4.2 None of the permanently moored FPSO's/FSU's investigated in this study have experienced multiple line failures but several failure histories showed evidence of characteristic damage on more than one line that had been developing for some time. In no case was the initial line failure immediately recognised and the absence of spares, replacement procedures, intervention vessels etc. meant that the period between the initial failure and the repair of the unit was typically quite long.

1.4.3 In such cases the integrity of the mooring/riser system depends on the system being repaired before experiencing a major storm. This "joint probability" exposure has already been identified as a key driver in theoretical work on mooring system reliability but now there is strong practical evidence to confirm this observation.

1.4.4 There is also one example of a multiple line failure on a UK sector FPSO. The case is not entirely representative because the unit was operating a mobile production role and the design was not typical of the majority of UK FPSO's. Nevertheless the installation suffered a total of 4 mooring line failures in one storm and was only able to maintain station by using multiple thusters and the main propulsion. There was a temporary loss of production, but other wise no major consequences and repairs to the mooring system commenced within hours of the initial incident.

1.5 Key Issues

1.5.1 The main reasons why multiple mooring line failures remain a potential concern for UK sector FPSO's can be summarised as follows:

• Inspection and Maintenance. The majority of FPSO's have not been designed to facilitate the replacement of individual mooring line(s). Consequently most operators rely on In Water Survey (IWS) of the mooring system by ROV (in preference to inspection in air either on the beach or in the field). This method cannot access all critical parts of the system and will not identify fatigue damage or wear prior to the appearance of a gross defect or a line rupture. Survey frequency is generally annual or in some cases less (i.e. once very 2 years). There are therefore long periods in which multiple defects could remain undetected.

• Status Monitoring. The majority of UK sector FPSO's have no line tension monitoring system and there have been some reliability problems with those that do. Three units are equipped with sonar probes to visually identify line failures from the centre of the turret but deployment is intermittent and the probes may not identify failures that


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occur well outboard of the turret. Excursion monitoring systems are rarely sufficiently accurate to immediately identify single line failures. Consequently a mooring line failure might not be detected for some time (especially if it initially occurred in relatively calm conditions).

• Risk Awareness and Preparedness Most operators class station keeping failures as "highly unlikely" events although statistics suggest a medium probability of failure at least once on average during a typical FPSO's design life. Many FPSO mooring components are non-standard "long lead" items, yet the majority of operators do not carry any spares.

• Design Codes and Standards Over the years there has been considerable work on load calculations and on the material properties of standard components (i.e. chains, wires etc). However, there is little or no effective guidance on the design and durability of connections, terminations, fair leads and stopper details. Guidance on inspection repair and maintenance is largely based on MODU practice and principally depends on inspection in air. Finally there are weakness in the treatment of FPSO heading variation and active thruster control systems all of which can potentially impact mooring system integrity.

1.6 Summary Conclusions

1.6.1 Whilst the study has identified a number of failures and outlined some key concerns the evidence suggests that existing station keeping hazard management strategies are still effective (at least so far as any potential health, safety and environmental exposures are concerned). However, there are some clear opportunities to further reduce HSE exposures by careful review of the performance standard(s) for position monitoring and riser isolation.

1.6.2 The remaining commercial exposure(s) (e.g. due to repair costs, down time etc) are potentially significant but could be reduced by addressing key weaknesses in current "best practice" for mooring integrity management. Potential benefits for existing FPSO's will obviously vary from case to case but the recommendations given in the report should be of some value both to existing operators and to those engaged in new FPSO development projects.

1.7 Recommendations

1.7.1 A large number of detailed recommendations have been given in the report. The key points have been summarised below. Based upon the investigations completed in this study it is suggested that UK FPSO operators should:

• Conduct a risk based review of their unit(s) mooring system to determine whether the design features any of the key causes of failure detailed in this study.

• Carefully re-examine performance standards (and/or heavy weather policies) for FPSO position monitoring and riser isolation.

• Consider program(s) of rotation and replacement of individual mooring lines or


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system components for inspection on the beach (a risk based evaluation should be used to determine which parts of the system should be included).

• Review their mooring system sparing philosophy and consider setting up a spares club for those installations that feature common components (again a risk based approach should be used to determine the initial sparing requirements).

• Consider developing the detailed recommendations given in this document into a more formal "best practice" document for mooring system integrity management.

• Consider establishing a formal system for the reporting/sharing and collation of station keeping system failure and near miss data.

1.8 Further Work

1.8.1 Finally it should be noted that some of the technical issues require further research work to provide a platform for the continual improvement of future design(s). There are therefore opportunities for joint industry technology projects in some areas. Potentially promising subjects areas include:

- Design/durability of mooring system connectors, fairleads & stoppers.

- Development of effective and reliable line tension monitoring systems.

- Review and development of ROV IWS technology.

- Investigation of FPSO heading behaviour

- Investigation of code requirements for active control of thrusters.


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2 INTRODUCTION 2.1 Background

2.1.1 A significant proportion of UKCS hydrocarbon reserves are being produced via turret moored FPSO’s that are intended to remain on location for periods between 10 and 25 years. There is considerable variation in the station keeping solutions adopted for each unit and in the approach to mooring inspection, sparing and replacement.

2.1.2 Harsh environment operating experience for FPSO’s was limited at the time when most of the current designs were completed and a large number of new mooring line material specifications have been introduced during the last 10 years.

2.1.3 There is now a need to learn the lessons associated with the last 10 years of UK FPSO operations and develop new guidance on FPSO station keeping integrity management. This document describes the results of a mooring system integrity study performed by Noble Denton Europe Ltd. (NDE) on behalf of the UKOOA FPSO Steering Committee (UKOOA).

2.2 Objectives

2.2.1 The key objectives of the work were as follows: • To establish the mooring specification/operating philosophy of UK Sector FPSO’s • To gather lessons learned from UKCS operating history to date. • To review current practice for its effectiveness. • To develop recommendations for the improvement of mooring system integrity.

2.3 Scope and Methodology

2.3.1 The agreed scope was as detailed in NDE’s proposal (ref AQN-71/00/19 Rev 2 dated 03.04.01). The study focuses on the station keeping systems of UK Sector FPSOs including seabed anchors, mooring lines, turret interfaces and any associated active systems (i.e. winches, thrusters etc). Mooring integrity during design, construction, and operation was considered and a particular emphasis was placed on mooring line condition monitoring, inspection, corrosion protection, sparing and replacement.

2.3.2 The work was completed as a desk study. Data was gathered by means of a questionnaire, a workshop meeting and an internal/external literature search. The conclusions and recommendations were validated by internal review within the study group. An external review was performed by the UKOOA steering committee and by Dr Whim Morris of Bomel Limited.

2.4 Instructions and Contract

2.4.1 Initial instructions were given on 19.05.01 by Jennifer Stewart (UKOOA Committee Secretary). The work was performed under contract reference FPSO.JIP/01/06.


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3 INDUSTRY QUESTIONNAIRE 3.1 Overview

3.1.1 This section describes a questionnaire that was developed in order to gather relevant lessons from the FPSO industry. A copy of the blank form is given in Appendix B. This document was circulated electronically to a total of 70 companies and organisations in the following sectors: • Facilities Operators • FSPO Licence Partners • Equipment Suppliers/Designers • EPIC Contractors • Installation Contractors • Inspection Contractors • Verification & Regulatory Bodies • Consultants

3.1.2 A total of 31 responses were received containing feedback on ten UK Sector FPSOs. The responses cover one West of Shetland unit and nine North Sea units. They are based upon a total of 40 operating years.

3.1.3 All input was of a confidential non-attributable nature unless a specific release was given by the respondent(s).

3.2 Interviews

3.2.1 Detailed interviews were undertaken with the following bodies in order to clarify the feedback and obtain additional details.

Body Type BP Schiehallion Operator Texaco, Captain FPSO Operator NSP, North Sea Producer Operator Maersk Contractors, Maersk Curlew Operator Balmoral Equipment Supplier Stolt Offshore Installation Contractor Fugro UDI Inspection Contractor

Table: 4.1 Interviews

3.3 Summary Responses

3.3.1 The responses to the questionnaire have been summarised in Appendix D.


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3.4 Key Matters Arising

3.4.1 Key matters arising from the responses to the questionnaire were as follows: • General

The first FPSO or FSO covered in the survey was installed in 1993 and the most recent during 2000. The range of mooring design life varied between 10 and 25 years. The majority of the mooring systems were designed to Lloyds rules but some units also comply with API RP2SK and DnV POSMOOR. The units are mostly ship shaped hulls although there are two transom stern barges. There is a mixture of new builds and intercept or trading conversions. The largest unit has a displacement of 170,000 te whereas the smallest is only 32,300 te. Ship lengths vary between 120m and 258m.

• Design Basis Most units have been designed to 100yr return omni-directional environmental extremes (although two installations are based on 50 years). The range of extreme design values is as follows: 1 hour Mean wind speed: 33 - 40 m/s Significant Waveheight: 12.0 - 17.1 m Zero Crossing Period: 11.1 - 14.1 secs Current Speed: 0.53 - 1.8 m/s

• Mooring and Station Keeping Systems

Most units are internal freely rotating turrets although there was one internal drag chain unit. The majority of the turrets are located within 15% of the ship length aft of the forward perpendicular although there are three unit turrets located at 30%, 35% & 44% aft respectively. Turret diameters vary at between 4m and 22m.

The moorings were all multiple element spread catenary systems with between 8 and 14 lines. All but one are composite chain/wire systems, the odd one being a purely chain system. The majority have drag embedment anchors although there are 3 equipped with suction piles and one with driven piles. One unit had one winch per mooring leg the others had a minimum of one winch and a maximum of three.

• Water Depth 9 out of the 10 FPSOs for which questionnaires were filled in operate in water depths of less than 150m. The deepest water FPSO was operating in 400m.

• Failures There have been mooring system failures amongst the units that have been examined in this study, fairly in line with the number that published statistics would suggest for the number of operating years being considered.

In particular it was noted that the number of failures was higher amongst components that were designed ‘one offs’ connection or termination details and


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the following common features were identified:

- The failure to take proper account of the friction, impact, wear etc. that can occur between line elements that are subject to large relative motions.

- The failure to properly predict and accommodate the complex fatigue and corrosion mechanisms that may exist in "one off" design details and line connections.

- The failure to account for the concentrated stress and degradation that may occur in pocketed chain cable fair leads.

There were also a number of failures of FPSO thruster units, mooring tension monitoring devices and a number of turret problems. Concerning the turret it was noted that few, if any, problems have been experienced with freely rotating turrets.

Further details of the failures and incidents covered by this study are given in Appendix C.

• Design for inspection In general very little provision has been made in the design for the in service inspection of the mooring system, and in particularly the interface between the mooring system and turret. This is probably largely in response to the considerable practical difficulties of in water survey(s).

• Design for replacement It is clear from the design and operation of the vast majority of these units that the potential need to replace mooring components was discounted when the design and installation of the turret and mooring interface was undertaken. Further to this, the majority of operators do not think that there is a significant risk of loosing a mooring line, although the statistics show that all of the UKCS FPSOs can expect to experience at least one mooring system failure during their design lives.

• Design Standardisation It would be beneficial to operators, installation contractors, manufactures and mooring designers if there was standardisation in mooring systems. For example the vast majority of UKCS FPSOs use 120mm stud-less chain with special end links connected to spiral wound wire rope through D-shackles and connector plates. Consequently, a standardisation of mooring equipment would gives several benefits, namely: type approval and bench testing of standard components with regard to fatigue, inter-changeability of mooring components between FPSOs thus allowing for a common stockpile of components, and easier installation methods.

• Failure Identification It has become abundantly clear that the units do not have an adequate means of identifying a single line failure. This is unfortunate as semi-submersible


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experience would dictate, and is borne out by one of the failures in this study, that a single line failure is most likely to happen in benign conditions. Consequently, if the line failure could be identified then appropriate action could be taken whilst the conditions were still benign.

Instead the units principally rely on the position monitoring equipment to show that the offsets are greater than some set alarm limit, which would only go off as the weather starts to rise thus putting the integrity of the remaining lines to the test.

It is acknowledged that some units do have line tension monitoring but this appears to be unreliable and often un-calibrated. In this study one line failure was identified from line tension monitoring was only found because the crew suspected something was wrong and asked for the data to be analysed. Three weeks later after much signal processing a 10 tonne reduction in tension on the remaining lines was found and so the line failure was identified. It is perhaps worth noting that that reason so much signal processing was required was because the failure occurred in benign conditions.

It is also acknowledged that some of the units carry sonar probes that are released through the turret and can therefore check that the mooring system is still intact. However, where these units are fitted they can only be deployed intermittently and it relatively good weather conditions. Finally there is also some doubt if they would identify a failure that occurs well outboard of the turret in benign weather conditions.

• Inspection Philosophy The majority of FPSO's have not been designed to facilitate the replacement of individual mooring line(s). Consequently most operators rely on In Water Survey (IWS) of the mooring system by ROV (in preference to inspection in air either on the beach or in the field). This method cannot access all critical parts of the system and will not identify fatigue damage or wear prior to the appearance of a gross defect or a line rupture.

ROV survey techniques are constantly evolving and certain sub-sea diagnostics are already available. Cathodic potential measurements for example are relatively common place and this technique can be helpful in monitoring the status of the corrosion protection system. Tools are also being developed to allow for the sub-sea measurement of mooring lines (i.e. the API 3 or 5 link measurement) and some limited sub-sea capability exists to identify loose studs in mooring chains. However, none of these techniques can identify the main failure mode for long term moorings (i.e. fatigue).

Other methods of inspection need to be explored in order to ensure the continuing integrity of the mooring systems, but in the short term it must be recognised that destructive testing in air is the only truly reliable method of inspection.

• Line Replacement Preparation


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Only a very small number of the operators have a procedure in place for the replacement of a line should a line failure occur. An even smaller number actually own a spare line and know its location. Certain operators have suggested that a lack of preparation is not a problem because the design of their mooring system is such that they can stay on location, and in some cases even continue producing, in the one line failed condition.

This does not take account of the fact that many of the line failures to date were caused by problems that affected more than one line, and so the potential for multiple line failure was high. This is borne out by the statistics for known anchor system problems for FPSOs where 17% of the failures to date resulted in multiple line failure. In addition to which the minimum lead-time on a new anchor chain or cable can be up to 18 months.

• Unsheathed Wire Rope A number of the installations were found to have unsheathed spiral wound galvanised wire rope in their systems “electrically isolated” from the rest of the mooring system by PTFE type bushes. It was found however, that there did not appear to have been any wear calculations undertaken to shown how long the bushes might last or that a careful design study had been undertaken to ensure total electrical isolation for the unsheathed wire. This is a concern as these units were also using impressed current systems for the protection of the hull from corrosion and an unsheathed cable can therefore act as an anode and rapidly deteriorate.

In one case the electrical isolation had been entirely omitted in an unsheathed wire system owing to a design omission. The result was that the electrical potential of the ground chain to which the wire was attached quickly exhausted the permanent anodes on the wire and was making a start on the galvanising of the wire when the problem was identified by annual ROV survey.

• Rudder Failure It appears that rudder damage/failure is common amongst FPSOs in the UKCS that have been converted from trading tankers. This is probably because the rudder system was never designed to take the magnitude of lateral loading that it experiences when the FPSO is weather vaning.


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4 EXTERNAL LITERATURE SEARCH 4.1 Overview

4.1.1 This section describes the results of a search of relevant public domain information.

4.1.2 The search was targeted in the following main areas: • Relevant codes standards and guidelines. • Published papers articles etc • Joint Industry Studies and initiatives.

4.2 Relevant Codes Standards and Guidelines

4.2.1 Relevant codes, standards and guidelines are listed in Table 5.1 over leaf. In each case a short commentary is given on the relevant sections of the code and their application to mooring line integrity.

4.3 Key Matters Arising:

• Proliferation There are a large number of codes, standards and guidelines available, all of which have subtly different requirements. Efforts to establish consistency under ISO have not yet fed through the system to the design community.

• Applicability Many codes are still largely applicable to MODUs rather than FPSOs/FSOs. Where efforts have been made to modify or draft new codes the MODU based philosophy sometimes remains. In simple terms the codes are not yet fully mature.

• Analysis In general there is plenty of guidance on analysis and factors of safety against tensile failure and fatigue for standard line elements, but guidance in certain key practical areas of detailed deign or inspection repair and maintenance (IRM) is either weak or non-existent.

• Vessel Heading The requirements for and guidance on determining vessel heading is generally weak. This can have an effect on the prediction of line tensions and in particular motion at the fairlead or chain table.

• Active Control The rules for the analysis of systems with active control of winches, thrusters, DP systems etc are generally poorly defined.

• Inspection Inspection and discard requirements are fairly well defined for MODUs, although procedures for chain cables are much better defined than for wire ropes, connectors and jewellery. However, there are no clear warnings that non-destructive examination methods will not identify all the critical aspects prior to


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failure. More significantly there is very little guidance on the inspection, discard and sparing requirements for FPSOs. Several classification societies allow in water surveys (IWS) but at present the survey regime must be developed on an ad-hoc basis for each installation.

• Terminations, Connectors Fairleads There is little or no practical guidance on terminations, connections, fairleads etc, even though experience suggests that these items are the most likely to fail, which follows the general trend of looking at the moorings being simply made up of long lengths or chain and/or cable without any jewellery. There has admittedly been some work on the comparative fatigue testing of "D-Shackles" versus kenters, baldts etc. but the majority of FPSO mooring systems contain terminations and connector types that have never been tested at all. Furthermore small variations in type and detail can have a very large effect upon long-term fatigue performance of those element types that have been tested. A type approval process giving some kind of long term fatigue justification for connectors and terminations would be desirable.

• Status and Position Monitoring Similarly there is no effective guidance on mooring line status and position monitoring. This might explain why many units do not have effective systems to verify line status.

4.4 Summary

4.4.1 Overall it can be concluded that design to existing codes, standards and verification systems will not always guarantee the required levels of integrity because of:

• Human error leading to non compliance • Ambiguity in the codes and standards • Gaps in critical areas which are not adequately covered

4.4.2 There is therefore a place for improved guidance, however at present station keeping system integrity rests principally upon the competence of the designer, the verification agency and the ICP. It is hoped that this document might in some small way assist the process of experience transfer.


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Ref Title Body Date Commentary – Draft & Preliminary API RP2SK Recommended Practice for Design

and Analysis of Stationkeeping Systems for Floating Structures

American Petroleum Institute

1996 It is a good source of data for those undertaking analysis, although by no means all FPSOs in UKCS are analysed in accordance with API criteria as Lloyds and DNV criteria are also extensively used.

API RP 9B Recommended Practice on Application, care and use of wire rope for oil-field service


1980 Although not generally applicable to mooring lines this RP contains useful guidance on the care of wire ropes, but there is no guidance on how in water inspection might be conducted and what criteria should apply.

API RP 21 Recommended Practice for In-service inspection of mooring hardware for floating drilling units


1987 Useful information on common faults to look out for and inspection procedures. No guidance on IWS (see above)

API RP 2 Recommended Practice for Design, Analyse, and Maintenance of Mooring for Floating Production Systems


1989 This RP gives good guidance in addition to that given by API RP 2 SK & RP 2P, into how to design, build and maintain an FPS. Guidance on the maintenance of the mooring system is ver limited.

API RP 2P Recommended Practice for the analysis of Spread Mooring Systems for floating Drilling Units


1987 This RP gives good background to the questions it is asking of the analyst in order to help them produce a conservative and consistent analysis. However, mainly focusses on MODUs and not directly applicable to FPSOs.

Rules & Regulations for the Classification of a Floating Offshore Installation at a Fixed Location

Lloyds Register of Shipping

1999 Possibly the most applicable of all the rules to FPSOs due to Lloyds large market share of UKCS FPSOs.

Rules for the Classification of Mobile Offshore Units

Det Norske Veritas

1989 Most widely used classification rules within UKCS. The mooring code (POSMOOR) has basically developed for MODUs and has been modified for FPSOs/FSOs. A new code


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has now been issued that supersedes this document.

Cert. Notes No.2.6

Certification of Offshore Mooring Chain

Det Norske Veritas

1985 Class requirements for chain manufacture and inspection. This code was a major step forward in quality control of chain cable manufacture.

No.2.13 Standard for certification of offshore mooring fibre ropes

Det Norske Veritas

1999 Good guidance & information on the design & use of fibre ropes for offshore moorings.

BS 365:1968

Galvanized Steel Wire Ropes for Ships

British Standards Institute

1968 Although this standard is now quite old its themes can be seen in the more modern standards and criteria.

ISO 2408 Steel wire ropes for general purpose - Characteristics

International Standards Organisation

1973 Essentially very similar to the British Standard above, this standard lays down the minimum to try and ensure suitable quality of wire rope is used.

NI 432 DTO ROO E-1997

Certification of synthetic fibre ropes for mooring systems

Bureau Veritas

SI No. 289 The Offshore Installations(Construction & Survey) Regulations

Department of Energy

1974 Now superseded –

HSE Rules Offshore Installations: Guidance on Design, Construction, and Certification, 4th Edition

Department of Energy

Now superseded -

FPSO Rules Guide for building & classing floating production & storage systems

American Bureau of Shipping

1996

SV Rules Rules for building and classing steel vessels

American Bureau of Shipping

1996


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CFR 46 Code of Federal Regulations (CFR) Shipping, Title 46, Subchapter I-A-Mobile offshore Drilling Units, Parts 107-109

United States Coast Guard

Regulations that USCG apply to FPSOs, particularly with regard to their mooring systems. Obviously developed with MODUs principally in mind.

CFR 46 Code of Federal Regulations (CFR) Shipping – Navigation & Navigable Waters, Title 33, Parts 135, 136, 151-156

United States Coast Guard

Regulations that USCG apply to FPSOs.

HSE Rules The Offshore Installations (Safety Case) Regulations

Health & Safety Executive

1992 Safety case rules that govern all UKCS FPSOs.

Table 4.1 Relevant Codes Standards and Guidelines


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4.5 Published Papers Articles etc

4.5.1 A number of published papers identified during the literature search are detailed in Table 5.2 over leaf. In each case a short commentary is given on each reference and its application to FPSO mooring line integrity.

4.5.2 The review of published papers has shown up a number of general items:

• Failure Statistics DnV have recently published a welcome update to their statistics on mooring failure with the result that a large body of data is now available for the period 1980-1998. According to information given in this database there have been 7 reported anchor system failures on FPSOs/FSUs operating in the UKCS. This gives rise to an anchoring system failure frequency of 0.186 per unit per operating year or one failure every 5.4 operating years for FPSOs, based on consolidated data for the period 1980-1998. It is also worth noting that one of these failures was a multiple line failure, and the majority of the other failures had the potential to become a multiple failure because of the widespread damage to mooring components that had occurred. However, some care is required in drawing too many conclusions from this work in view of the relatively small data set of FPSO's.

• Chain There is a great deal of published information on chains and in particular their fatigue lives and failure mechanisms. In addition there is useful information on inspection methods for chains and how this is best achieved. Chain connectors are rarely considered though, with only Helvoirt (1982) and later NDE performing comparative fatigue tests on a limited number of standard connectors.

• Synthetic Fibre Ropes As the interest synthetic fibre ropes grows so has the number of papers on the subject, particularly with regard to the unique fatigue life properties of these ropes and their dimensional change over time. Today there are established design codes for fibre rope moorings and more detailed published work on terminations and installation techniques than exists for conventional moorings.

• FPSO responses A number of operators have shared their experiences in various papers, in general though these do not give any information on the mooring system.

• Operational experience A number of operators have shared their experiences in various papers, in general though these do not give any information on the mooring system.

4.5.3 In summary, there have been a number of studies undertaken into ensuring the integrity of the chains and the cables in the mooring systems, particularly with regard


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to fatigue. However, there are gaps in the available research work regarding the design and durability of connectors, terminations and fair leads.


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Title and Reference Date Commentary New Mooring Chain Design, Canada L., Vicinay J., Sanz A., Lopez E, OTC 8148

This paper looks at how altering the link shape the strength and fatigue life of mooring chain can be improved.

Toward Improving the reliability of anchor chain and accessories, Irving L., Stern F., Michael, F., OTC 3206

1978 This study looks at anchor chain failures and sees how integrity can be improved.

Static & Fatigue tests on Chain Links & Chain Connecting Links, Helvoirt L.C., OTC 4179

1982 Test data on chain failures, which shows that connecting links are more prone to fatigue failure than common links

Anchor Chain Fractures, Taraldsen A., OTC 5059

1985 Gives historical mooring line fracture data for semi-submersibles. This paper laid the foundation for the regime of manufacturing quality control introduced under DNV Cert. Note 2.6.

Summary of the 4-year research project: Anchor Chain Cables Offshore, Lereim J., OTC 5060

1985 This project looks at cables in service and how they withstand to the rigours of an actual environment.

Mooring Line Failures on Semi-submersibles, Garen O., Aune A., DSO-258-248-89

1989 Data relating line failure to line tensions for k3 & k4 chain., 1989 so a little out of date

A Probabilistic Dynamic & Fatigue Analysis Procedure for Mooring Systems, ein Y.S.D, Lohr C.J., Bruen F.J., Huang K., OTC 6036

1989 The study looks at analysing the mooring system from a fatigue point of view and has data that can be applied to FPSOs in the future.

Review of reported mooring incidents for Department of Energy, Miller B.L., GM-513-0690-919

1990 Incident data for mooring system failures, largely applicable to MODUs.


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Experience from stud welding of offshore mooring chains, Sohlstrom M.

1990 Information on welding studs into offshore mooring chain and the reasons for stud weld failures.

Influence of Chain Reliability on Mooring Design and Approval for the North Sea, Bush R.B., Luo Y. & Jack R.L., Ramnas Chain Cable Seminar, London

1991 Probabilities of mooring line failures against time for MODUs

Gives mooring line failure data up to 1990 for chain cable systems

Lessons learned from inspection of offshore mooring chains, Chainco

Henk H.J., International Offshore Mooring Seminar

1991

Design Practice for Mooring of Floating Production Systems, Kwan C.T.

1991 Insight into the thinking behind “API Recommended Practice for Design, Analyse, and Maintenance of Mooring for Floating Production Systems”. It contains useful background data that could be useful in initial design of the mooring system.

The influence of Proof Loading on the Fatigue Life of Anchor Chain, Shoup F.J., Tiption S.M., Sorem J.R., OTC 6905

1992 A practical look at how high loads on anchor chains can shorten their fatigue life.

Experience with mooring integrity assessment for semi-submersible, Inglis R.B.

1992 Discussion on the factors involved in mooring integrity albeit for semi-submersibles but still applicable to FPSOs.

Regulatory, Statutory & Classification Requirements of Floating Production Storage Systems, Sember W.J., Ghosh S.

1993 Overview of the role of the different agencies in the integrity management of an FPSO, although written with reference to USOCS, the majority of the roles still apply to UKCS but with different agencies involved.

Mooring Integrity:- a state of the art report, NDE on behalf of Shell

1993 Review of the integrity of mooring systems for mobile offshore drilling units.

Posisjonering Av Plattformer og Skip, 1994 Post 1985 chain cable failure probabilities & mooring component failure data up to 1993


Kjettingbrudd, A1994, Fargernes

ndreassen R., NIF Conference

Performance & Testing of Components of the Ivanhoe/Rob Roy Floating Production System Mooring, MacGregor J.R., Smith S.N., Paton J.E., Kenny J.P., OTC 7192

1994 In service data for various mooring components for a FPS.

Anchor Chain – New Data, New Design Lines & Practical Details, Bolt H.M., Billington C.J., Humphries M.J., OTC 7707

1995 An overview of the JIP managed by Bomel into a ‘Design Guidelines for Anchor Chains’. This work encompassed a review of the methods employed in the manufacture of chain, a review of design codes and regulations, and fatigue testing of samples in air for various parameters.

Fatigue of Mooring Chain in Air & Water – Results & Analysis, Stiff J.J., Smith D.W., Casey N.F., OTC 8147

1996 This investigation took chain samples and subjected them to fatigue tests to try and establish fatigue data for modern anchor chains.

The development of a response-based design methodology for FPSOs in exposed locations, Standing R.F., Thomas D.O., Ahilan R.V., Corr R.B., Snell R.O.

1997 Study on the weather used in the design of FPSO mooring systems and whether it is overly conservative.

A Review of Inspection of Chain Mooring Systems, Shaw D., Dingwall J.

1999 Overview of regulations and the duties placed on people. In addition to which it has a discussion on typical mooring chain problems and how these are meant to be dealt with according to API and what the currently accepted practice is.


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Review of model testing requirements for FPSO’s, BMT on behalf of HSE

2000 Guidance on items to be considered during and after model tests to find the coefficients needed for numerical analysis.

FPSO Integrity: JIP on FPSO Fatigue Loads, Bultema S., Boom Henk van den, Krekel M., OTC 12142

2000 Full scale FPSO hull fatigue measurements and discussion on results.

FPSO Integrity: Structural Monitoring on Glas Dowr, Boom Henk van den, Krekel M., Aalberts P., OTC 12143

2000 Full scale fatigue loading measurements on the hull of the Glas Dowr

Background and status of the FPSO fatigue capcity JIP, Lotsberg I., OTC 12144

2000 Details the approach used and the philosophy of the project.

The Structural Integrity of FPSO’s/FSU’s – A regulators view, Millar J.L., White R.J., OTC 12142

2000 This paper identifies the approach that the HSE is taking to try and ensure the integirty of FPSO hulls. From this it can be seen that it only a matter of time before they take similr interest in the mooring systems.

Risk-based Inspection of an FPSO, Landet E., Lotsberg I., Sigurdsson F., OTC 12146

2000 Details the approach that DNV are considering for the structural inspection of FPSO based on a risk approach.

Extreme responses of Turret Moored Tankers, Barr J.M., Heyl C.N., Rodenbusch F., OTC 12147

2000 This paper looks at the problem of trying to establish the most likely extreme motions for a turret moored tanker based on the worst non-colinear combination of wind, waves and current.


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FPSO integrity, Comparitive Study of Fatigue Analysis Results, Francois M., Fricke W., Mitchell K., Healy B., OTC 12148

2000 A comparative study of the different fatigue analysis methods used for the hull form, undertaken by the 5 major classification societies as part of the FPSO Integrity JIP.

Mooring System: From Initial Design to offshore Installtion, Potella R.B., OTC 12174

2000 Description of the mooring system design from initial design concept to the offshore installation for the Petrobras 40 unit. Although the unit involved is a semi-submersible this paper gives a good general oversight into the whole design, build and install method.

Fatigue Curves for Polyester Moorings, Banfield S., Versavel T., Snell R.O., Ahilan R.V., OTC 12175

2000 An examination of all the available fatigue data for Polyester Ropes, which includes polyester specific T-N curves.

Creep and relaxtion of Polyester Mooring Lines, Davies P., Huard F., Grosjean F., Francois M., OTC 12176

2000 This paper presents results from an experimental study into the time related behaviour of polyester ropes.

Short- and Long-Term Property Behaviour of Polyester Rope, Casey N.F., Belshaw R., Paton A.F., OTC 12177

2000 This paper gives the results of an investigation into the mechanical behaviour and fatigue performance of polyester rope.

Development of API RP 2 SM for Synthetic Fibre Rope Moorings, Lee M., Devlin P., Kwan C.T., OTC 12178

2000 This paper details the work undertaken by ABS in consolidating the existing design guides, SIP results and industry operating experience into a single guide.

Operation of North Sea FPSOs: 1995-2000 and Beyond, Mapes F., Casias C, OTC 12054

2000 Details the experiences of Bluewater operating FPSOs in the North Sea.

Operating Experiences with the Captain FPSO, Bradley R., Sanders C.J., OTC 12055

2000 Details the operational problems that this installation has suffered from in addition to production data and highlights the problems of operating an FPSO

Pierce Field Development by FPSO solution, Hayes M.R.L., Tangvald T.B., OTC 12056

2000 This paper details the development of the Pierce field but includes a description & schematics of the unique turret system used on this vessel.


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Operating experience with Navion FPSO at the Lufeng Field, Tangvald T.B., Wiik E., Boge H., OTC 12057

2000 Although this vessel is operating in the South China Seas it has a turret system similar to the Berge Hugin FPSO. The paper includes details of the operational and theoretical aspects of disconnecting the turret system.

Operational Experience with the FPSO Ocean Producer, Thompson R., OTC 12058

2000 Operating experience for an FPSO West of Africa.

Accident Statistics for Mobile Offshore Units on the UK Continental Shelf, Det Norke Veritas on behalf of HSE, OTR 2000/091

2000 Failure statistics from various sources for MODUs, FPSOs, FSUs etc. This is a useful document for assessing the relative safety levels of different type of mobile unit.

Vryhof Anchor Manual 2000 Overall guide to anchors: the way they work, their design intent, their selection for specific environments etc.

Operational Safety of FPSOs: Initial Summary Report, Vinnem J.E. on behalf of HSE

2001 This report outlines the work to identify hazard scenarios/events and potential associated human errors and develop models and tools in order to integrate human reliability science into predictive models.

Appraisal of API RP 2FPS (Recommended practice for planning designing and constructing floating production systems), MSL Engineering Limited on behalf of HSE

2001 The purpose of the study was to assess the suitability of the API code for application to the design of FPS’s in the UKCS.

Table 5.2 Published Papers Articles etc


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4.6 Joint Industry Studies and Initiatives (Non ND Group).

4.6.1 This section summarises relevant joint industry studies and initiatives that were identified during the course of the literature search study. It details only those JIP's which were not organised and managed by one of the Noble Denton Group of Companies. Each relevant JIP is summarised in Table 5.3 Details of Noble Denton JIPs are given in the following section.

4.6.2 In summary, the Joint Industry Projects to date when concerned with the mooring system have concentrated on the integrity and fatigue sensitivity of the chains and cables in the system and have effectively ignored the rest of the jewellery.


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Title and Reference Date Commentary Risk and Reliability of a FPSO in Deepwater Gulf of Mexico,Bechtel/Safetec

2001 US administration(s) investigation of the risks associated with the introduction of FPSO's into the Gulf of Mexico.

Design Guidelines for Anchor Chains, Bomel

1989 A review of the methods employed in the manufacture of chain, a review of design codes & regulations to determine the extent of guidance on chain fatigue, tension / bending fatigue experiments in air, and an appraisal of the inspection, maintenance and repair procedures.

Full Scale Anchor Tests, Aker-Omega 1991 Detailed results of full scale anchor tests.

FPSO Integrity, Marin 1997 on

The aim of the JIP is to produce a validated FPSO lifetime prediction model and a comparison of existing calculation procedures with real-world data. Only indirectly applicable to station keeping systems.

DP, Marin 1998 on

This study aims to combine full scale monitoring with model test correlation, and use that as a basis for validating improved DP control methods. Some relevance to heading control FPSOs.

Full Scale Fibre Rope Testing, NEL 1999 Data on elastic modulus that could be used for calibration of numerical models.

Operational Experience Database, Lloyds Register

1999 on

Basic data for all FPSOs & FSUs worldwide.

Major design, fabrication and project execution issues for FPSOs in the Gulf of Mexico, Aker Engineering Inc.

1999 on

The study aims to consolidate lessons learnt and experience from previous FPSO projects on major technical and project execution issues so that operators do not "reinvent the wheel". Only indirectly applicable to station keeping systems.

FPSO Fatigue Capacity, DnV 2000 on

Reduced and full-scale S-N data on typical fatigue-sensitive details in FPSOs and tankers, based on experimental assessment of test pieces. Only indirectly applicable to station keeping systems.

Wave Impact on FPSOs and Floating Platforms, Marintek

2000 on

The aim is to come up with a consistent design tool to take account of wave impact on the vessel. Only indirectly applicable to station keeping systems.

Table 5.3 Joint Industry Studies and Initiatives (non ND Group)


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5 NOBLE DENTON IN HOUSE DATA 5.1 Introduction

5.1.1 This section describes the key items of in house data held by the Noble Denton Group (NDG) of companies. The main data areas are as follows: • Joint Industry Projects Managed or Completed by NDG Companies • Client studies or activities performed by the NDG of Companies. • Data supplied by Clients or collaborators in connection with the above.

5.1.2 Of the above, some data remains client confidential and therefore has not been included in this study however there remains a substantial portion which has either been released by the client or where the term of confidentiality has expired allowing the data to come into the pubic domain.

5.2 NDE Managed Joint Industry Projects

5.2.1 The Noble Denton Group has managed a number of Joint Industry Projects, a large number of which have involved mooring components or systems. In particular the ‘Discard Criteria For Offshore Moorings’ study is widely used an inspection and replacement tool.

5.2.2 In summary the various JIPs show that mooring systems cannot last forever and even the most well designed system may not see out its design life due to detail design issues and a lack of full understanding of the fatigue loads experienced by various components in the mooring system. They do, however, like some of the non-NDG JIPs, go some way to ensuring the integrity of the overall mooring system by ensuring the integrity of the chains and cables within the system.


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Ref Title Body Date Commentary Semi-Submersible Based Floating

Production System Mooring And Installation Analysis

Noble Denton Europe Limited

1985 Initial development of analysis techniques

Deep Water Catenary Mooring Systems


1986 Initial development of analysis techniques

5-21-1314 Prediction Of Wire Rope Endurance Noble Denton Europe Limited

1986 UBL & fatigue testing of large diameter ropes to produce guidelines and data for long term mooring design.

Tanker-Based Floating Production Systems


1986 Review of technologies for monohull and semi-sub floating production.

Prediction Of Chain Fatigue Endurance


1989 UBL & fatigue testing of anchor chains to establish S-N curves.

Hi- Tech Fibre Ropes For Offshore Moorings


1990 UBL & fatigue testing of new fibres, construction and sockets to establish selection and design guidelines.

Discard Criteria For Offshore Moorings


1990 Gives guidance on mooring line expected serviceable life.

Dynamics of Catenary Moorings Noble Denton Europe Limited

1991 Analysis techniques for deepwater moorings.

Evaluation of Mooring Line Damping Noble Denton Europe Limited

1992 Analytical study of the influence of damping, with the production of new models.


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The Evaluation Of Wire Mooring

Line Strength And Endurance Noble Denton Europe Limited

1993 Initial testing of wire ropes.

H3687/NDAI/DWS

Calibration of ABS, Api, Dnv, Hse (Den) And NMD Mooring Design Codes For Floating Drilling and Production Platforms

Noble Denton and Associates Houston

1993 This comparative study shows which of the codes is most applicable to which region of the world, and how in the end the results from one code will be very similar to the results from another code.

NDA 002 00 Fatigue Testing of Large Diameter Mooring Anchor Chains


1994 Good fatigue data for large diameter chains based on full scale testing.

12-21-1363 Seal -Subsea Electro-Magnetic Appraisal Of Mooring Lines


1995 Investigates the use of calibrated magnetic devices to measure the cross-sectional area of a wire rope and additional sensors to detect broken strands.

12-21-2035 Engineering Design Guide of Fibre Ropes


1995 Provides effective guidance on the design of fibre rope mooring systems.

Integrated Mooring And Riser Design

Noble Denton Europe Limited / MCS

1996 Review of the techniques of joint mooring and riser design for FPSO's and development of new techniques with associated reliability based calibration.

Studless Chain Corrosion Fatigue Testing Programme


1996 on

Gives fatigue data for the new generation of studless mooring chains. This data is useful for both design houses and operating companies who already have this type of chain in use.

Table 6.1 NDE Managed JIPs


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5.3 Key Client Studies and Client Data

5.3.1 NDE have had close involvement either with the design, inspection, corrective action or audit of the station keeping systems on the following UK sector Floating production and storage units • Emerald FSU • Banff FPSO • Schiehallion FPSO • Chevron Alba FSU • Foinaven FPSO • Pierce FPSO • Captain FPSO • Balmoral FPU • Harding FSU

5.3.2 Various non-attributable lessons learned from these projects have been incorporated into the discussion, conclusions and recommendations of this report.


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6 WORKSHOP DETAILS 6.1 Overview

6.1.1 A workshop meeting was held as part of the study to discuss the preliminary conclusions of the work and develop further lines of enquiry. This section describes the key issues identified during the course of the workshop session. Details and notes of the discussion are given in Appendix E.

6.2 Key Discussion Items

6.2.1 The key items identified for discussion were split into "General Issues" and "Specific Items" as follows:

General Issues • Current failure rates. • Consequence and criticality. • Key Failure modes • The design process. • Codes and Standards. • Manufacturing and construction. • Installation. • Monitoring. • Inspection. • Discard • FPSO heading and response • Human factors Specific Items • Terminations and Fairleads • Chain cables • Wire ropes • Fibre ropes • Connectors and jewellery • Anchors • Buoyancy


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7 DRAFT GUIDANCE 7.1 General

7.1.1 Part of the agreed scope of work for the study was to develop draft guidance on FPSO mooring system integrity a form that might subsequently be developed into a UKOOA "best practice" document. The results of this exercise are given in Appendix F.

7.1.2 However, the contents of Appendix F must be considered as a preliminary first pass document that has not yet received the wide industry consultation and detailed technical PEER review required to be issued in anything other than draft form.

7.1.3 It should not therefore be used by UKOOA members or others for guidance until it has been properly completed.


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8 RECOMMENDATIONS

8.1 The following section details the key recommendations of the study with associated explanatory notes:

8.1.1 Based upon the investigations completed in this study it is suggested that UK FPSO operators should:

1. Risk Based Review of Existing FPSO/FSU Mooring System(s)

UK FPSO/FSU operators should conduct a careful risk based review of their unit(s) mooring system to determine whether the design features any of the key causes of failure detailed in this study.

This report and the attached appendices contain a large number of useful lessons learned for UK FPSO/FSU operators, however, the conclusions and recommendations are of a general rather than a specific nature. Operators should therefore conduct a risk based review of their existing station keeping systems to determine whether any of the failures, observations and lessons learned are relevant for their particular unit(s).

2. Performance Standards and Heavy Weather Policies

UK FPSO/FSU operators should carefully re-examine key performance standards (and/or heavy weather policies) for FPSO position monitoring and riser isolation to ensure that they are sufficiently robust.

The management of hazards associated with excessive FPSO excursion ultimately rests upon those systems and procedures that are designed to monitor vessel excursion and ensure that all hydrocarbons are isolated and controlled prior to exceeding the safe operating limit of the riser(s). Once this key requirement has been secured, other potential consequences of station keeping failure should be largely commercial in nature. The performance standards, heavy weather policies and procedures therefore need to carefully examined to ensure that they will indeed be effective in the event of an unexpected multiple line failure that occurs in limiting operating conditions. A careful re-examination of this key area is considered prudent because of the weaknesses identified in the other prevention systems (i.e. in detailed design, third party verification, in service inspection and line status monitoring).

3. Inspection Repair and Maintenance

UK FPSO/FSU operators should consider program(s) of rotation and replacement of individual mooring lines or system components for inspection on the beach (a risk based evaluation should be used to


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determine which parts of the system should be included).

One of the key conclusions of the study is that the in service IRM philosophies adopted by the majority of UK operators cannot prevent exposure to long periods in which critical (multiple) defects could remain undetected. If there are components or details where the risk of undetected degradation is high, then it is obviously prudent to replace, rotate and inspect items on the beach. This approach also has the benefit that spares will automatically be provided. However, as most FPSO mooring designs do not facilitate the replacement of individual lines or components and the potential consequences of failure are largely commercial, it is obvious that a risk based cost benefit assessment is required to determine reasonable practicability.

4. Mooring Spares Club

UK FPSO/FSU operators should review their mooring system sparing philosophy and consider setting up a spares club for those installations that feature common components (again a risk based approach should be used to determine the initial sparing requirements).

Very few UK sector operators carry spare mooring components and many of the subject components are long lead items. The potential commercial exposure due to mooring line failure is directly related to the speed with which a given failure can be identified and repaired. Several of the FPSO mooring designs feature common components so perhaps a spares club could be an economically attractive solution for some operators.

5. Mooring Integrity "Best Practice" Document

UKOOA members should consider developing the detailed recommendations given in this document into a more formal "best practice" document for mooring system integrity management.

A draft best practice document has been given in Appendix F to this report, however, much wider consultation internal peer review is required prior to issue as a best practice document.

6. Station Keeping Failures and Near Miss Data

UK FPSO/FSU operators should consider establishing a formal system for the reporting/sharing and collation of station keeping system failure and near miss data.


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This report is intended for the sole use of the person or organisation to whom it is addressed and no liability, of any nature whatsoever,

shall be assumed to any other party in respect of its contents.

NOBLE DENTON EUROPE LTD.

Original Signed by : Original Signed & on file A. Comley, B.Eng. Project Engineer Counter signed by : Original Signed & on file R B Bush BSc, CEng. FRINA Project Director

Aberdeen 22.03.02


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