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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. Page 2 of 121,NOBLE REV 02 FPSO MOORING SYSTEM,DENTON INTEGRITY STUDY. 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. I Documents Reports Reports 2002 A3792 02 doc 18 07 02 13 42. Page 3 of 121,NOBLE REV 02 FPSO MOORING SYSTEM,DENTON INTEGRITY STUDY. 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. I Documents Reports Reports 2002 A3792 02 doc 18 07 02 13 42. Page 4 of 121,NOBLE REV 02 FPSO MOORING SYSTEM,DENTON INTEGRITY STUDY. 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. I Documents Reports Reports 2002 A3792 02 doc 18 07 02 13 42. Page 5 of 121,NOBLE REV 02 FPSO MOORING SYSTEM,DENTON INTEGRITY STUDY. 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. I Documents Reports Reports 2002 A3792 02 doc 18 07 02 13 42. Page 6 of 121,NOBLE REV 02 FPSO MOORING SYSTEM,DENTON INTEGRITY STUDY. 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. I Documents Reports Reports 2002 A3792 02 doc 18 07 02 13 42. Page 7 of 121,NOBLE REV 02 FPSO MOORING SYSTEM,DENTON INTEGRITY STUDY. 3 INDUSTRY QUESTIONNAIRE,3 1 Overview,3 1 1 This secti.
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

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