Mooring Integrity Guidance - O&GUK - RevF

Oil & Gas UK MOORING INTEGRITY GUIDANCE

Rev A B C D E F

Date 12/Mar/07 15/Apr/07 13/Feb/08 17/Mar/08 31/Mar/08 6/Apr/08

Status Text draft for review After WG for review Incl WG 1st review After WG 3rd meeting Incl WG comments Minor typo corrections

Author IDW IDW IDW IDW IDW IDW

Checked For circ to Working Group For circ to Working Group For circ to Working Group For final comments from WG For FPSO Group presentation For FPSO & Corr Group review

Oil & Gas UK Mooring Integrity Guidance 080406 RevF

Table of Contents (i) Background, Purpose and Scope ...........................................................................................................4 i.i

Background ............................................................................................................................................ 4

i.ii Purpose .................................................................................................................................................. 5 i.iii

Scope ................................................................................................................................................. 6

(ii) Executive Summary ................................................................................................................................7 1.

Existing Guidance....................................................................................................................................9

2.

Influence of design and deployment stages ......................................................................................... 12

3.

4.

5.

2.1

Design stage influences ................................................................................................................... 12

2.2

Deployment stage impact ............................................................................................................... 15

Performance Standards and Operational Procedures ....................................................................... 17 3.1

Background ...................................................................................................................................... 17

3.2

Mooring Performance Standard contents ...................................................................................... 18

3.3

Operational Procedures .................................................................................................................. 21

3.4

Heading control FPSOs .................................................................................................................... 22

Failure Procedures and Contingency Planning .................................................................................. 23 4.1

Failure Procedures ........................................................................................................................... 23

4.2

Contingency Planning ...................................................................................................................... 24

4.3

Planned disconnection .................................................................................................................... 25

Assessing Risks to Mooring Components ............................................................................................ 26 5.1

Level of Risk Review ........................................................................................................................ 28

5.2

Choice of component level .............................................................................................................. 29

5.3

Hazard Identification ....................................................................................................................... 30

5.4

Risk estimation – consequence and likelihood ............................................................................... 31

5.5

Risk reduction options ..................................................................................................................... 34

6.

Mooring Monitoring.............................................................................................................................. 35

7.

Mooring Inspection Methods ............................................................................................................... 37

8.

Mooring Integrity Management System ............................................................................................. 40

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Appendix A

Abbrevations ..................................................................................................................... 41

Appendix B

Working Group Terms of Reference .............................................................................. 43

Appendix 1 Participating Organisations ................................................................................................. 45 Appendix 2

Synopsis of mooring system failure modes ......................................................................... 47

Appendix 3

Example Performance Standards ....................................................................................... 54

Appendix 4

Example procedure in the event of a mooring line failure ............................................... 72

Appendix 5

Example mooring risk assessment process......................................................................... 73

Appendix 6

Example mooring monitoring systems ............................................................................. 103

Appendix 7

Example mooring inspection systems ............................................................................... 111



(i)

Background, Purpose and Scope

i.i

Background There are a number of authoritative documents dealing with the design of a mooring system for an installation, including API RP 2SK, DNV E301, ISO 19901-7 and a varied collection of other Class rules. These documents are relevant to the initial design analysis of a mooring, but provide little guidance or structure to the detailed equipment design or the much longer phase of the operation of the mooring. API RP 2I does provide some inspection standard and retirement criteria but is aimed solely at out of water inspection capability. The FPSO industry, both UK and internationally, has experienced a number of mooring line failures, and a larger number of damages to mooring lines. Some of these failure mechanisms had not been anticipated during design nor even envisaged, and illustrate that existing practices in design, installation and subsequent monitoring could benefit from being more robust. Recent work by the industry, specifically sessions held under the auspices of UKOOA in 2005 and 2006, and also the JIP on Mooring Integrity which was completed in December 2005 (and now available on the Oil & Gas UK and HSE websites) has highlighted many of these incidents. At the meeting held on 7th June 2006 UKOOA, in conjunction with the HSE, recognised that the safety of FPSOs could be enhanced by providing Guidance to Operators for the design of new moorings, and the integrity management of existing moorings. This was ratified at the UKOOA meeting held 2nd October 2006, from which a Working Group was created. This present Guidance has been written by the Working Group, assisted by some expert consultants, reviewed by all Oil & Gas UK members, and is now issued for the potential benefit of all those with an interest in FPSOs.

The subject of FPSO Mooring Integrity is not a recent one; there have been a number of previous study reports which have concluded with some key recommendations that have contributed to this present Guidance. A full list of references, and other reading material, is included in Appendix 1.



i.ii

Purpose This document is termed Guidance as it is intended to provide guidance to all those with an interest in the integrity of a mooring system. The overall aim is to promote the understanding of the factors involved in maintaining a mooring system so that its functionality is not impaired and it can continue to safely moor the vessel; by raising the profile of mooring integrity issues throughout the industry, Oil & Gas UK wishes to raise the level of attention paid to mooring systems closer to the level of attention given to other areas of a n Installation such as topsides production plant and its safety regime. This document is based on information as understood at the time of its writing – the subject of Mooring Integrity at a detailed level is an evolving subject on a worldwide basis and mooring system users should not rely on this document as being complete. The principal aims of this present document are therefore to:

widely promote the practice of effectively managing the integrity of the mooring system for every floating Installation; to this end not only have Oil & Gas UK members been involved in the preparation of this document, but a much wider worldwide correspondent group;

provide a source of reference for those seeking to continue to improve the safe operation of all elements of their floating facility. It has been found that Mooring Integrity has not received much focus to date at an industry wide level and for those not already involved it is not an easy subject to identify appropriate input, because there is not much freely available that encompasses the whole subject;

to suggest a means of a structured mooring integrity review that can assist in the component choice at the mooring design stage, as well as for an existing mooring in assessing mooring integrity risks;

identify the key elements that Oil & Gas UK suggest should be considered by each Operator in determining the best practice for their own individual unit. Each Operator is free to act as they see fit for the safety and operation of their unit, but this Guidance is intended to provide guidance on factors that should be considered by Operators;

present the information in a user friendly approach, so that necessarily lengthy descriptions or examples, are given in Appendices;

provide a list of references so that users can research the subject further as they may feel is appropriate to their installation. Users should recognise that details of commercial companies or the products and services they offer will change over time and may not be correct into the future.



i.iii

Scope Expand The scope of this Guidance is a Mooring System for any floating unit; the document was original conceived for principally permanently moored ship-shaped FPSOs but much of the Guidance is applicable to any mooring system. The term “Mooring System” is interpreted to mean the collection of mooring components including everything that is not steel structure, ie including as applicable to individual systems: chain; rope; wire; shackles; weights; buoys; pennants; means of anchoring; but also including the structural interfaces with those components, and hence including: chain stoppers and trumpets; hawse pipes; chain jacks; fairleads; winches; tension & position monitoring; and all other similar or related equipment that is necessary for the efficient working of the whole mooring system. Some aspects of Mooring Integrity are specific to Heading control type turret moored FPSOs (for example using drag chains or thrusters); these specific aspects are considered in Section 3.4 and reference can usefully be made to the UKOOA Tandem Loading Guidelines, various sections but specifically Appendices B6 and B7. In addition to the principle of active or passive FPSO heading control, other features of the whole FPSO concept are also relevant to the Mooring System, including: friction of turret bearing systems; corrosion allowance; coatings; impressed current cathodic protection system; sacrifical anodes; although the turret design itself is not considered part of this Guidance. Fluid transfer or containment is also not included other than as an interface to the mooring system. Similarly detail of how to conduct design analyses are not included, although the appropriateness of analyses at different stages of the life of a mooring system are referred to.



(ii) Executive Summary The mooring system of a FPSO and other offshore installation units is a Safety Critical Element, within the definition of the Safety Case Regulations in the UK, but also in general terms since it is normally the sole means of holding the unit on location so that well fluids can be brought safely on board. Because a mooring system is mostly, and in some cases, completely, under the water and therefore largely out of sight, it’s condition, and any maintenance or repair that may be required, is often considered as more difficult – and costly - to undertake. This is however one reason why sufficient attention should be given not only to the design of the mooring system and the verification of that design, but most importantly to the ongoing confirmation of the suitable operating condition of that mooring system. Historically the number of incidents that have occurred with FPSO moorings would suggest that there is room for improvement in the management of FPSO moorings in general. This document recommends that an Operator should, in respect of a FPSO mooring: • have a recognition that manufacturing and deployment activities can contribute to a reduced life of a mooring system, even though this may not be visually apparent at the time (Chapter 2); • have a clear, internally promulgated strategy as to the operational intent of the mooring system, not just as regards strength and fatigue life, but inspectability, operational and survival limits; and most importantly with a clear, unambiguous set of appropriate steps in the event of these being breached. For an existing mooring system this may be something more than can be extracted from traditional design reports (Chapters 3 and 4); • conduct a detailed risk review, component by component; this is a larger, more in depth, study than a traditional FMEA on the whole mooring system. Different formats for such a review may suit different facilities, but this document gives an example of a detailed and systematic approach (Chapter 5); • assess the need for frequent monitoring of the mooring system, whether this should be real, or close to real, time monitoring, and whether it should be just to determine the mooring is fully intact; or additionally to confirm actual behaviour with the design as part of design verification (Chapter 6); • have a clear inspection scope of work, driven by the results of the detailed risk review, and that there is an appropriate level of competency of the personnel carrying out each survey (refer to the Mooring Integrity JIP Phase II which is developing this subject more). A suggested checklist of features to look for during an inspection is given but this may not be complete for all facilities, or because experience in this field is growing. Modern technology is leading to ways of measurement of mooring system features which should be included in this scope of work; currently available methods to date are outlined for guidance (Chapter 7); • a suitable system to track the findings from surveys, comparison with previous findings, and to identify anomalies (Chapter 8). Unlike some other aspects of Offshore Installations mooring systems have not to date generated a specific recognised area of expertise, but have generally be grouped together either with marine or with subsea, often not in a defined manner. Knowledgeable, experienced mooring personnel are therefore not always present within the organisation of every Operator, and each Operator should assess if he has access to appropriate expertise to fully understand all aspects of the mooring. C:\Documents and Settings\WME - Ian\My Documents\123 Current\199 JIPs\2006 HSE-UKOOA Mooring Guidance\Master Copies\080430 1m file idw Guidance RevF.docx Page 7 of 113


For most existing UK Installation mooring systems the above approach has not been fully followed in whole or part, and a typical work process is described in the following chart:

Develop day to day operational philosophy

Identify the design philosophy Identify deployment practices

Chapter 2

Performance Standards

Carry out a component by component risk review Chapter Further engineering Chapter 6 Operational monitoring

Chapter

3

Develop a damaged operational philosophy and procedure Chapter

4

5 Verification of Performance Standard

Inspection workbook Chapter 7

All the above issues may be assembled into a single Mooring Integrity Management System, or different elements may be incorporated within a range of existing management system documents, but an understood and complete approach to Mooring Integrity as evidenced by the above key features is considered to be good practice.

As more and more work is performed by Operators into their moorings, sharing of experience is to be encouraged. As many real life examples as practicable have been included within this Guidance to illustrate many of the areas of potential concern that can arise with a mooring. Additional guidance can be obtained from a number of sources and activities which are listed within this document.



1.

Existing Guidance This Chapter provides a summary of all the known Regulations and guidance specifically in respect to FPSO Mooring System Integrity. Where applicable/allowable, extracts of these are given in Appendix 1, intended as sufficient for a user to obtain a full copy and research further for himself if he desires. However the conclusion from this list is that there is little guidance, except in one instance of the Mooring Integrity JIP, that deals with the subject at anything but a high level. These are generally insufficient for an Operator to readily capture the full picture of his mooring integrity, with insufficient detail to create a cohesive mooring integrity plan. The contents of this present Guidance are believed to be in accordance with all of these high level requirements, but to offer a methodology to take them to a sufficient level of detail for an Operator to increase the confidence in the integrity of his mooring system, without the need to consult all the individual documents listed in Appendix 1. Note that this Guidance does not repeat detailed information given elsewhere, but seeks to summarise the key aspects of it, and then make specific reference to where detail is available. Sources of written regulations and guidance on mooring integrity actually come from a limited number of sources as given below. It is recognised that considerably more expertise lies with a range of commercial companies and some of their specific staff.

HSE The regulatory regime in the UK for the safety of offshore installations is driven principally by a suite of Regulations around the principle of a Safety Case. While this present Guidance is written within the context of the UK Safety Case regime, such a regime does not affect the potential relevance of this Guidance to any FPSO. These Regulations can be purchased through the HSE website. As part of their work to promote safe operation, the UK Health and Safety Executive (HSE) has also conducted a number of research projects and over the years has issued a number of reports; many good sources can be found within the HSE Offshore Technology reports or Research Reports. From time to time the HSE also issue Safety Notices. Most of these documents are freely available for pdf download through the HSE website. HSE also publish general guidance on Integrity Management and Risk Review methodology although none of these documents specifically deal with the issues of mooring systems.

Class Societies The major Classification Societies all have regulations for the detailed analysis design of moorings but less as regards physical matching of components and only a high level about inspections. Against these an Operator can undertake design and ongoing surveys, and be granted a notation confirming that the mooring complies with these standards. All Operators should note however that while having a valid Class Notation for a mooring system is one step towards having confidence in that system, it is recognised by many of these Class Societies themselves that in the light of incidents over the last few years that their Rules may not be sufficient to ensure the integrity of a FPSO mooring system. C:\Documents and Settings\WME - Ian\My Documents\123 Current\199 JIPs\2006 HSE-UKOOA Mooring Guidance\Master Copies\080430 1m file idw Guidance RevF.docx Page 9 of 113


In general terms Class Rules in respect to FPSO mooring systems have been derived from the established Rules for ships, where a prescriptive regime exists. Similarly the experience from MODUs is not transferable to FPSOs. As the moorings for a FPSO operate continuously rather than intermittently, and in a completely different manner than standard ship anchors and chains, and as is being identified by experience, each FPSO mooring is individual, the validity of the present sets of Class Rules for ensuring on-going mooring system integrity is limited. Some Class societies recognise this and are taking steps to enhance their existing Rules. However, if a FPSO Operator diligently studies the integrity needs of his mooring system, and applies them accordingly, whether using this present Guidance or not, it is likely that compliance with these needs would easily surpass the present requirement of Classification Rules.

API API RP 2SK, now in its third edition, is the culmination of considerable work principally on the analysis design methodology for FPSO moorings, and the most recent edition makes some limited mention of on-going inspections. Of course the setting of strength factors of safety have an impact on the future integrity of a mooring system, but as these have not changed during the life of most worldwide FPSOs, the present mooring failure rate is with these factors already being used. Fatigue factors of safety have been increased in the most recent revision however these solely consider axial tension since data is not available for fatigue life in bending and torsion. API RP 2I is aimed more clearly at in-service inspection of moorings, in this case mobile offshore drilling units. It provides some quantified retirement criteria for stud-link chain and wires, but it is aimed specifically at out-of-water inspections, although the criteria could equally be applied to measurements taken underwater. It does however represent a prescriptive approach not always suited to FPSOs.

ISO Until 2006 ISO had not published (ref ISO 19901-7:2005) any document relating to the design of moorings, although various national documents exist for the design of the chain, and some other components. As with API, the new ISO document general deals with analysis design however it does state two relevant issues to the subject of FPSO Mooring Integrity as dealt with in this present document: Firstly that a “Structural Integrity Management System” is recommended for a permanent mooring system (Section 12) with a structure broadly as outlined in this present Guidance, and that a means of either tension measurement or failure detection should be in place (Section 11.3.1). In particular the main components of a mooring Integrity Management System stated by ISO at a high level, are identical to the ones treated in more detail in this present Guidance, allowing for alternative terminology.

Oil & Gas UK Under the auspices of Oil & Gas UK a number of studies have already been undertaken and published as guidance – refer to the Bibliography in this document.



Commercial companies A number of companies have undertaken research into specific aspects of moorings. It is suspected that only a small proportion of this work has ever reached the public domain, typically through publication as Papers at conferences – and then typically these are only readily available to those who have attended that specific conference. Some known Papers, judged as those that add to the knowledge of how to deal with FPSO mooring failure modes and integrity issues, are referenced in Appendix 1, but direct quotes are not given due to potential copyright issues. The list given is recognised as just a sample of the Papers that deal with moorings and that a wealth of further background material does exist. Oil & Gas UK would always welcome additions to this list. Historically some companies appear to work on the basis that publication of their own work may damage their reputation, and thus only severe incidents are those typically circulated and then from other sources. Fortunately it would seem that there is a growing industry feeling that sharing of mooring issues at least is something that not only can benefit the whole industry, but the individual company with the issue - since it is unlikely that any one company has a sufficiently wide enough range of mooring systems in their control that they will have experienced all known issues. The principle of sharing should be encouraged so that the experience base can be widened, and where solutions, design or operation, have been identified for one FPSO, the probability for these to be applied to prevent an issue on another FPSO can be much higher simply because they are known about.

Joint Industry Projects This is one of the key areas where exchange of experiences on mooring systems has recently been promulgated. There is one key JIP that has taken place in recent years that provides much of the detail behind many of the failure mechanisms presented in this present Guidance. This JIP, entitled “FPS Mooring Integrity” and coordinated by Noble Denton Europe Ltd, Aberdeen, was issued in December 2005 and has 309 pages of extremely useful information behind many aspects of mooring integrity. This document is freely available for pdf download from the HSE website as RR444. At the time of issue of this Guidance Phase 2 of this JIP is underway. Other JIPs in the area of mooring integrity have taken place in the past, and others are currently underway or proposed. Coordination and cooperation between JIPs can avoid either duplication of effort and contradictions in approach, which therefore would assist the industry further.


Oil & Gas UK FPSO Mooring Integrity Guidance 080406 RevF

2.

Influence of design and deployment stages


Chapter 2

Develop day to day operational philosophy Performance Standards

Develop a damaged operational philosophy and procedure

Carry out a component by component risk review

Further engineering Operational monitoring

Verification of Performance Standard

Inspection workbook

2.1

Design stage influences A typical design process for a FPSO has traditionally included the following elements. Chronologically many of these will be happening concurrently: 1. a mooring system concept selection process is undertaken; 2. tender documents are issued to those companies the client believes can provide such a system; 3. a contractor is awarded the work, typically partly on functionality, partly on price; 4. the 100 year metocean condition is defined, and sometimes an Operating Philosophy; 5. a design analysis standard is chosen, perhaps most commonly DNV’s E301 (Posmoor) or API RP 2SK; 6. the contractor, or his sub-contractor, performs the bad weather analyses, and the fatigue calculations, required by the chosen standard; a report is written and typically provided to a Class Society for agreement; 7. components are chosen, principally for their strength against the above analyses; 8. a procurement process involving the few major suppliers available is handled by the contractor; 9. Class are typically appointed as an “expert” third party witness to manufacturing tests and subsequently issue certificates; 10. contractor delivers the mooring system; client signs off that it is in accordance with the contractual scope;


Identify the design philosophy

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Inspection activities

11. client appoints an Operator who receives the design document package as his input; from which he writes his Operational Safety Case.

The above process has some potential pitfalls, and in choosing the optimum sequence for the design of a FPSO mooring system, it is prudent for a company to consider at least the following factors (where the paragraph numbers refer to the outline steps above): 1/ When choosing the concept for the mooring system, the level of potential robustness and redundancy should be considered. Typically a review should be carried out to determine that the risks of the chosen concept have been minimised taking into account the specific field, location and life-cycle requirements. In the UK for the Safety Case regime – and as may be applied for any worldwide location - the risks should be determined as being ALARP; the principles of ALARP are well documented and reference should be made to the HSE Oil and Gas website where a suite of Alarp guidance can be found. 2/ Whether the use of a design contractor that has no direct involvement in, or responsibility for, the future operation of the system, will achieve the optimum performance of that mooring system throughout its life; ie will the system design be made in parallel with the practicality of the inspection regime; 3/ That the potential life of the mooring system, not just its life costs, are evaluated against the original bid costs. Some elements of a design can either directly provide for a longer life (such as the use of higher factors of safety on strength and/or fatigue); but other design concepts may permit a much greater flexibility to allow for unforeseen events, or for upgrades or time extensions to the originally envisaged functionality. For example a design that requires divers to be used in order to change out chains could be considered from a total risk point of view against a different system design where the same operation could be carried out with access from the deck of a FPSO; 4/ Where practicable the design should result in “fail-safe” conditions in the event of a system failure. A less than optimised example could be for a heading control FPSO where in the event of an electrical black-out the vessel swings to the weather so that she is beam on. 4/ Future operations using the mooring system should be considered. Turrets are often designs that have limited space to fit all the required equipment, and considerations of repair and maintenance for some of the equipment for later operations can be easily overlooked at considerable later difficulty, cost, or even interruption to production operations. Example A turret will typically have one or more windlasses in order to tension the chains during initial deployment. This windlass can sometimes be literally “builtin” to the structure and the surrounding equipment. Later in the life of the FPSO a need for her to be moved is going to occur, and hence future use of the windlass. Designing sufficient access for maintenance access is one aspect that should be included, also access to repair elements of the windlass; but consideration should be given to completely replacing it. 5/ Whether the sole use of the chosen design standard is adequate to provide sufficient operational input, such as for the OIM or Marine Control Room Operators to be confident that the system is performing as designed; for example is the expected data concerning offset in different weather conditions presented (if at all) in a format that is practicable for use offshore;




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Chapter 2







6/ some mooring designs only include the metocean conditions that may occur, sometimes only those that may be required by Class recommendations. However obtaining the best possible metocean data, specifically time-series data as is available from hindcast analysis, can provide a better list of all headings that the vessel can reasonably be expected to take up compared to the potential wind, wave and current directions. Even if no time-series data is available general knowledge of the weather patterns in the area can lead to a prudent choice of relative weather combinations; and hydrodynamic software is now well advanced to undertake a heading analysis. Example At least one FPSO mooring (and structural) design considered headings of up to 45º from the 100 year wind direction, but did not include a load case where the current could make the FPSO sit across moderate swell waves in calm conditions; heavy rolling conditions result which although at no time approach the peak mooring line tensions, mooring integrity considerations may require an understanding of the motions of the mooring legs and could give rise to mooring failure mechanisms other than just tension. Clearly a poor choice of load cases is relevant to other design aspects of a FPSO, including: • hull structural integrity – an issue that has arisen on at least one North Sea FPSO, where beam seas were demonstrated as generating the highest load ranges in some hull brackets; • operational uptime – such as fluid movements within process vessels, or inability of Diesel Generators or Gas Turbines to continue to operate. 6/ whether the design analysis review carried out by Class as is typically done, is of sufficient scope to verify the completeness of the design, components and analysis, and not just the accuracy of the calculations actually carried out; 7/ whether the applicability of the standard components typically used for each leg are the optimum components not just for axial strength, but for all the other potential failure modes (such as those included within Appendix 2 of this document); 8/ when the choice of mooring equipment (such as chain nominal diameter) has to be made early in the project due to lead times, this may compromise the future operating limits for the vessel. While the schedule of projects would rarely allow for each design step to be taken in sequence one after the other, a commercial structure that does not lead to the choice of the lowest priced components – and hence often those that “only just” comply with the chosen Standard – could permit a mooring system with a much higher integrity confidence limit. 9/ whether the standard manufacturing checks carried out by Class are sufficient to provide confidence in the longevity of the mooring system. One example is to consider the extent of proof loading carried out by the chain manufacturer. A study report suggests that the fatigue life of chain can be reduced by a factor of up to 5 if proof loading is not carried out. Example It has been demonstrated on at least one North Sea FPSO by subsequent physical testing that some (though perhaps not all) of the chain did not receive the full proof loading at original manufacturing;

In all cases it should be noted that verification of the suitability of the design choices is part of the verification processes that should be carried out during operation of the unit. Refer to Chapter 3 and others of this Guidance as to how verification can be practicably carried out.




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2.2

Deployment stage impact FPSO mooring systems are typically installed by large installation contractors and a considerable amount of procedural planning is undertaken. What is not always so evident is control of some of the more hands-on practices that may take place. The following are examples of poor practice that in some instances has either been the cause of, or contributed to, an early (and unexpected) loss of mooring integrity Example Over-zealous application of force when putting in safety pins in shackles, causing hairline fractures in the safety pin, resulting in their failure, and in turn permitting the main shackle nut to come off, and the shackle pin to become detached. This particular example from the UK was identified by inspection prior to actual mooring line failure, but did result in a production interruption to re-instate the system; Example Tack welding of the main shackle nut to the body of a shackle with the mistaken aim that this would stop the nut coming undone. In reality what happened was that local stresses were imposed into the shackle, and this stress concentration factor led to an initiation point for a fatigue crack, causing premature failure of that shackle; Example Wrong sizing of shackles (ie due to a late design change), albeit by just a few millimetres, meant that a shackle would not fit over its intended padeye. The chosen solution as used on two FPSOs as known was to jack open the jaws of the D shackles so that the bow plastically deformed and hence the shackle fitted. While these examples have not yet led to a known mooring leg failure, the potential from an increased stress concentration factor in the bow of the shackle is evident; Example During an interruption to the operation to pull chain through stopper dogs (ie flapper plates) a chain link was damaged and some material was forcibly removed leaving behind a very rough surface. If this link was to remain in the tensioned part of the mooring leg not only is the remaining material thickness a potential source of weakness and hence a higher stress, the rough surface could act as a fracture initiation point. The above are real examples, and lack of attention to all details can result in considerable risk to the future integrity of a mooring system – and hence potentially put offshore personnel lives at risk. In the majority of the above examples, these have occurred due to lack of expert knowledge or awareness of the personnel physically doing the work on deck. This can be interpreted as lack of appropriate and experienced supervision. Operators and Contractors should carefully consider the competence of their staff in these circumstances in the light of the sort of examples given above – where the personnel meant well, but failed to keep their element of the work in perspective of the bigger picture. Operators and Contractors should consider keeping detailed records of the deployment activities; not just a detailed log of events and a list of which equipment tag numbers/IDs are fitted in which position, but for example to taking photos or videos of every operation. This activity has sometimes been considered as unnecessary and not cost-effective; but it does become a real alternative if having the correct experience offshore is not practicable.




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Other considerations can also be envisaged, which while to date are not known to have caused a mooring incident, could easily contribute: • on some occasions delays to a project schedule mean the chain or other component is manufactured well in advance of when it is actually deployed. To minimise free corrosion occurring in air consideration should be given to the supplier applying a coating at manufacture; • use of inappropriate gypsy sizes; this can easily be the case where non-standard link sizes are used, a reasonably common practice to take advantage of a different geometry and a different weight per metre. It is normally clear that working chain links over the wrong size gypsy pockets while under tension has a large risk of the chain running away, but even low tensions can cause bending stresses to be imposed into a single link positioned just wrongly on a gypsy; • use of sacrificial wire slings to lower chain, or to overboard particular components to avoid damage. After deployment a ROV would typically cut the wire sling and remove it, but on occasion complete removal may be difficult. Inexperience in placing the wire sling in the first place can easily lead to this problem, causing effectively a foreign body to be inserted between two mooring components that otherwise had a deliberate surface contact – hence additional wear, additional stresses. There is a correct method for insertion of a wire sling into chain to avoid it becoming trapped; • tightening of bolts where insulated sleeves and washers have been used to ensure that components receive the correct level of cathodic protection. These sleeves often cannot accept full bolt compression and nominal bolt tension values should be specified as appropriate. In general it is suggested that during the planning for a mooring system installation the following activities are given due attention: a) sufficient training of the deck personnel should be undertaken and guidance provided on specific handling aspects of the equipment they have. It is too easy to assume that some practices, such as the welding of shackle nuts as given above, is now widely acknowledged as poor practice; but it only takes a couple of deck hands to think it is a good idea for a whole mooring system to be compromised a few years down the line; b) any mechanism that could create an unusual Stress Concentration Factor in a link, are avoided, especially when these are on an already highly stressed part of a link. The difference in the stress patterns of studless and studded links should be understood in this consideration:

High stresses can arise from loose studs that can then move out of position; considerations of handling of chain (where done) and studs should be considered. c) a good as-built survey is carried out that is aimed at looking not just for the major and obvious problems, but specifically at some of the smaller issues such as those exampled above. C:\Documents and Settings\WME - Ian\My Documents\123 Current\199 JIPs\2006 HSE-UKOOA Mooring Guidance\Master Copies\080430 1m file idw Guidance RevF.docx Page 16 of 113



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Performance Standards and Operational Procedures





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3.1

Background As required by the UK Safety Case legislation, all Operators are required to have carried out a Major Accident risk review, to have determined Safety Critical Elements, and to have a demonstrable verification scheme for each of these. Non-UK Operators may elect to follow the same type of principles. The definition1 of a Safety Critical Element is: “such parts of an installation and such of its plant (including computer programmes), or any part thereof – (a) the failure of which could cause or contribute substantially to; or (b) a purpose of which is to prevent, or limit the effect of, a major accident.” And a major accident2 means: “(a) a fire, explosion or the release of a dangerous substance involving death or serious personal injury to persons on the installation or engaged in an activity on or in connection with it; (b) any event involving major damage to the structure of the installation or plant affixed thereto or any loss in the stability of the installation; (c) the collision of a helicopter with the installation; (d) the failure of life support systems for diving operations in connection with the installation, the detachment of a diving bell used for such operations or the trapping of a diver in a diving bell or other subsea chamber used for such operations; or (e) any other event arising from a work activity involving death or serious personal injury to five or more persons on the installation or engaged in an activity in connection with it.” 1 2

SCR Regulation 2 (1) - Definitions SCR Regulation 2 (1) - Definitions




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Major damage to the mooring system clearly falls within the definition of paragraph (b) above and therefore constitutes a Major Accident Hazard; a definition of “major damage” is not given in the Regulations, however it is clearly feasible for any mooring system to be not only damaged, but even completely lost. As the mooring system is part of an Installation (both because it is generally within 500m of it and also affixed to its main structure3) and could cause a Major Accident, it is clearly a Safety Critical Element. As a Safety Critical Element the Safety Case Regulations require it must be subject to a verification scheme for ensuring that it is “suitable”, and that it remains “in good repair and condition”4. To verify the above it is normal practice to create a Performance Standard for the Safety Critical Element which clearly states what the operational functionality should be; the same or another document would then typically describe how this level of functionality is demonstrated or verified (the “Written Scheme”). The remaining chapters of this Guidance provide specific input for various elements of verification of what an Installation is likely to have written in their Mooring system Performance Standard; this present chapter provides guidance on the functionality that a Performance Standard would be expected to cover. Draft mooring system Performance Standards are given in Appendix 3.

3.2

Mooring Performance Standard contents In general Performance Standards should be “SMART”5: •

Specific – precise about what is to be achieved

•

Measurable – there must be a way to show that the Standard is being achieved;

•

Agreed – with all personnel who have to deliver them;

•

Realistic – failure to achieve the Standard should not be because they are not attainable;

•

Timed – should be set against a suitable timescale.

The following subjects should be considered for inclusion in a robust Performance Standard. Draft Performance Standards and some commentary are given below in Appendix 3. A method of providing assurance for each of the functionalities and their criteria is necessary. Verification of these assurance activities, as required in the UK regime to be undertaken by an Independent Competent Person (ICP), should also be provided. Additionally mooring systems typically have Design Verification activities that have to be carried over into the operational phase of a project, as actual measurements are the only means of determining with confidence that the original design is sufficiently correct. Such activities are considered vital to building confidence in the integrity of an operating mooring system and are included in the following notes and Appendix 3.

3

MAR Regulation 3 (3) (c) SCR Regulation 2(5) 5 Refer to HSG65 “Successful Health and Safety Management”, particularly Chapter 4. 4




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Vessel offset. A typical functional parameter is the maximum allowable vessel offset – perhaps expressed as the turret centre position. This parameter is likely to feature also in the philosophy in case of mooring failure. These can usefully be expressed as a traffic light arrangement linked to actions that the Installation should take as the offset increases. Assurance activities for offset are typically provided by DGPS equipment and a display given in the Control Room. Specific for example turret centre from the “at rest” location; determination of the “centre” may only be possible after a period of time of vessel motions, as during deployment a single snapshot may not be the correct centre; Measurable such as by a DGPS with a specified accuracy; note that use of a DGPS at a large radius from the rotation point (eg turret centre) also requires reliance on the measurement of vessel heading. The level of accuracy should be considered based on factors such as the gradient of the load/offset curve; Agreed ensure that the criteria used meet all requirements not just for moorings, but at least for risers and umbilicals; Realistic consider some sort of “historian” and alarm system rather than rely on manual observations; Timed measurement interval is important since vessel motions may limit the duration any given offset is exceeded, but the damage may still be done. Design Verification: the actual offsets for at least one period in a known metocean condition should be established and compared to the original design (typically best done as a comparison of mean and standard deviation); and then extrapolated to establish that the worst case conditions can also be predicted as being within the design, or other, acceptable values. Time to detect mooring failure; ie that the mooring system remains intact. As discussed in Chapter 6 below it is recommended that each Operator should assess the need for operational monitoring of the mooring lines. The output of this assessment can usefully be included in a Mooring System Performance Standard. Minimum required strength of the mooring system. This is typically defined as the peak tension that has been identified in the design analysis, ie in the design worst case condition and the factor of safety against the minimum break load. Different tensions occur at different parts of a mooring system; commonly these are at the top of the mooring leg where the mass of the leg is acting; however some moorings can experience the highest tension in other parts of a mooring leg, perhaps due to buoyancy or due to dynamic motions. Thus a minimum acceptable breakload for a mooring leg (perhaps different for different azimuths due to directional metocean characteristics) may be necessary; to equate this to chain measurement is unlikely to be appropriate as there are too many factors affecting the strength of a mooring component for explanation within a Performance Standard; instead an Operator should have a process for assessment of the effect on remaining strength of any wear, corrosion, or other anomaly identified. If the Performance Standard refers only to design strengths, then this does not provide for a measurable item; similarly if only the highest design tension is given, for example at the top of a mooring leg, with a factor of safety or a stated Standard, then this is not sufficiently specific as it is unnecessarily penalising other components that should never experience such loads. Assurance activities for component strengths are typically visual inspections, although other inspection techniques including chain measurement are now readily available and should be considered – refer to Chapter 7 and Appendix 7. Design Verification activities for component strengths typically take place during manufacture using established rules of material testing, proof loading and sample break loading, and are not typically taken forward into the operational phase. However testing of spare mooring equipment, or of actual components if the opportunity is presented, should be considered as adding to the confidence in the integrity of a mooring system.




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Fatigue life. The minimum required fatigue life of the mooring system and clarity on the factor of safety used to derive this should be given within the Performance Standard; in Appendix 3 this is part of the Design Verification. While fatigue damage or fatigue life are not items that in themselves can be readily measured, the factors inputting to the fatigue design of a component (tension range and frequency of occurrence) can often be measured or assessed in order to provide a measure of Design Verification. Reliance on a design stage fatigue assessment without demonstration during operation of the validity of the design assumptions may not be a satisfactory assurance as these components cannot generally be suitably inspected to get early warning of early fatigue failure. Example: It should be noted that fatigue calculations are based on tension ranges, not on the absolute tension value. As has been demonstrated on one North Sea FPSO recently the location of the highest tension range does not coincide with the location of the highest tension. Dynamic motions of some parts of a chain catenary may give rise to higher wear rates in specific sections of chain, and if these coincide with the location of highest tension ranges this may give rise to a substantially reduced fatigue life. To identify the above it is necessary to have carried out time domain analysis for the whole of the chain catenary and perform fatigue calculations at several positions – a piece of analysis that is not often checked. Note also that high load ranges may be associated with areas of increased wear and measurements of chain and other components throughout a mooring system should be considered to obtain the correct load range as a percentage of the actual break load at that position of the mooring.

Turret friction. Freely weathervaning turrets are generally assumed to be close to frictionless, with the vessel rotating smoothly around the geo-stationary turret. Detailed design engineering typically identifies a maximum angle that the turret should rotate before its bearings release; simple monitoring of the turret vs vessel heading can monitor this, and for some units this may be very applicable in demonstrating that design conditions have not been breached. A similar need applies for heading controlled FPSOs but in these systems the turret friction is usually much greater and the vessel is usually rotated around the turret using push/pull grippers. If power to the thruster system is lost then there is a danger that as the vessel heading yaws due to the wind and current moments, the turret can twist up the moorings and in extreme cases the moorings can come into contact with the risers. Hence such systems should have a Performance Standard that at a given twist of the turret and mooring the applied torque will overcome the bearing friction and allow the turret to rotate before clashing occurs.

Metocean conditions The design metocean condition is perhaps useful information to be included in the description of the mooring within the Performance Standard, and certainly within an operational document, but it is not something that can typically be changed in any way, other than with a disconnectable mooring and riser system. However many of the activities described above, particularly Design Verification activities, require a knowledge of the actual metocean conditions. The Verification activity connected with this may be monitoring of the output from a wave measuring tool if on board or from another but nearby installation, or a wave rider buoy, or hindcast met data, but it could be as simple as monitoring of the regularly received weather forecasts. Note that other Design Verification activities may not be possible without actual (live or retrospective) metocean data input.




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Other factors may be applicable for the Performance Standard depending on the individual system design. These might include:

minimum amount of buoyancy for different positions on the mooring leg;

maximum depth of any seabed trenching at the touchdown;

the time required to replace a failed mooring line – and hence drive what arrangements are in place for alternative or replacement equipment (refer to Chapter 4).

Example Performance Standards for a Freely Weathervaning FPSO and a Heading Controlled FPSO are given in Appendix 3. Each Operator will have their own Performance Standard layout, but the factors referred to above and in the examples should be included or appropriately referenced except where established as not applicable for a particular installation.

3.3

Operational Procedures For many freely weather-vaning FPSOs an operating philosophy for the mooring system may not initially appear as relevant when in comparison to systems with the following or similar features:

thruster assist;

drag chain;

winch mounted mooring legs.

NB: For aspects specific to such FPSOs refer to Section 3.4 below. However, even on passively moored FPSOs there are a few aspects that can make some changes in vessel behaviour. These include: vessel draft; vessel trim. These should not be ignored lightly and consideration should be given for analysis to consider their effect. For some ships at anchor, for example, it has been reportedly shown by a Japanese study that a head trim can substantially reduce anchor chain loads. The intent of the setting of all operational parameters, and the derivation of the operating practices from the design parameters of the mooring system, should have been established during the design stage of any mooring system. The way systems are operated can have a major impact on the behaviour and life of the mooring system, and its ability to withstand these human induced forces and motions in addition to the environmental loads. Thus it is important that the design philosophies and design limitations are available for the operators of the FPSO with its mooring system. For example typically mooring design analyses will have determined the peak tensions that the mooring system should experience, and the peak offsets that the vessel will go to. However these extreme conditions occur, by definition, very rarely, and Operators should consider the benefit that can be obtained by also undertaking mooring analyses in a variety of typical operational metocean and draft conditions, so that the personnel on board or ashore can readily compare the vessel behaviour on any day with what should be expected according to the original design. Not only does this aid Design Verification of offsets (as referred to above in Section 3.2, it also provides confidence





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to the personnel offshore and may become a relevant influence for decision making in extreme conditions. All relevant operating parameters should be included in an accessible operational document. This document should include a suitable description of the mooring system, as well as how it should be operated, so that it can be readily understood by offshore personnel, noting that many personnel are unlikely to have seen the components involved and may have little understanding of the detailed implications of operating beyond the design parameters.

3.4

Heading control FPSOs In addition to the above the Performance Standard for a FPSO with a means of heading control should also consider: •

the reliability and redundancy of the thruster and power generation/distribution systems. UKOOA’s Tandem Loading Guidelines provides information on redundancy issues for heading controlled FPSO’s, including FMEA’s, proving trials etc. Reliability issues associated with power generation should also include scenarios where large angles of roll can be experienced such as in non-colinear seas.

•

limits of vessel heading vs weather direction, subject to those weather conditions, taking into account the strength of the turret system, including the capability of the chain fairleads to accept side loading, and their absolute limit for rotation;

•

additional offsets that may arise as a result of additional forces on the vessel partially across the weather direction, and the resulting plan view of the mooring lines in relation to other seabed architecture;

•

the reliability of the heading control mechanisms, with due consideration to the consequences if the turret locking or rotating mechanism should fail.

Operationally, consideration should also be given to the effects of heading control on the operations of other vessels, both supply vessels and offtake tankers where applicable.



4.

Failure Procedures and Contingency Planning Identify the design philosophy Identify deployment practices


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The typical design standards adopted for a mooring system require that the system should be capable of keeping the vessel moored even with one line broken6. Typically a reduced factor of safety is applicable in such conditions, and sometimes a transient factor of safety is specified to acknowledge that the highest tension that is experienced during a single leg mooring failure is during the time that the unit is settling down to its new mooring arrangement. Some vessels have been deliberately designed to have a greater level of mooring redundancy than such standards and on occasion reference is made to equivalent means, such as the use of the main propulsion to counteract some or all of the metocean forces.

4.1

Failure Procedures Whatever the procedures for each FPSO, this should be readily available to all relevant staff, and should represent a thought through process covering all eventualities. Such a procedure and associated philosophy, would likely include the following issues:

how to identify that a failure has occurred (see also Section 6 of this Guidance);

what action to take when such a failure is suspected; this would likely include a means to check that the failure has occurred (unless obvious), who to notify, whether to shut down production, etc

6

Some standards, for example Norway, in some circumstances require more than one line broken to be considered.



what action to take when such a failure has been confirmed; this would likely include at least a temporary production shut down for full evaluation, and subject to the weather forecast or other circumstances, may include down-manning.

Design analyses for many FPSOs will typically have assessed the metocean conditions for which the reduced mooring strength is now capable, in accordance with the design Standard, of withstanding. However Operators should bear in mind that there is a reason for the failure of a mooring and historically most failure mechanisms (refer to Section 5 and Appendix 2 to this Guidance) involve an element of commonality between all mooring legs. Operators should therefore consider the applicability of the following factors in setting the actions to take in the event of a mooring leg failure:

how clear is the identification of the failure mechanism; has there been any previous failures; are there any known issues that may be the cause;

is it common to the remaining mooring legs;

what is the probability (typically a qualitative assessment) of that commonality causing a second failure in the time it is expected to take to restore the full, or at least an increased, mooring capability;

what are the consequences to personnel, the environment (for example an offset beyond the integrity of any of the risers or umbilicals) and other assets, of a second mooring leg failure;

how significant is the total risk of such an event.

In assessing these Operators may wish to bear in mind that all the known UK FPSO mooring leg failures between 2000 and 2008 have been due to reasons that could equally occur in other mooring legs. None of these have led to a second mooring leg failure, although elsewhere in the world this has occurred.

4.2

Contingency Planning For mooring systems with no more than one line redundancy Operators should note that if the risk of a second line failure is considered significant, that a shut down of production could continue until an improved mooring capability can be restored; hence the financial consequences could be substantial as well as the safety implications for the personnel remaining on board. Some of the methods to mitigate the commercial implications, and hence to limit the commercial pressures affecting any decision on actions to be taken in the event of a mooring leg failure, are as follows:

have a clear and accessible procedure to be followed in the case of such an event; ie as described in sub-section 4.1 above;

have a suitable call-off contract with one or more companies capable of confirming and investigating such an event. These may include, for example, a ROV operator and a marine consultancy;

maintain a sufficient level of spare equipment to carry out repairs promptly. At least on a



temporary basis it may not be necessary to replace like for like;

consider a pooled arrangement of spare mooring equipment with other Operators, or with a major marine equipment supplier;

consider the level of applicable maintenance for winches and other relevant equipment needed for reinstatement, as to refurbish or replace these in a failed mooring situation may govern the schedule for resuming production.

There are other steps that are applicable, such as the ability to call out helicopters and/or anchor handling vessels, but these arrangements typically exist for other operational reasons and are not specific to mooring contingencies.

4.3

Planned disconnection During the life of a FPSO a number of factors may lead to Operators considering removing her from station and taking her to port, whether repair berth or drydock. To date no UK examples of this being done solely for reasons of the mooring system condition are known, but on several occasions FPSOs are taken off station for other planned upgrades or repairs. Advantage should be taken of these events to ensure that the condition of the mooring system is fully checked, and where applicable or practicable, to consider an appropriate upgrade or modification.




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5.

Assessing Risks to Mooring Components Develop day to day operational philosophy




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Many elements of an Offshore Installation are the subject of detailed risk reviews; this applies no more so than to topsides. Over the years considerable development has taken place of Hazop methodologies for topsides and also data to enable quantified risk analysis of instruments, pumps, pipework, valves and other equipment. Traditionally, mooring systems have not received anything like such close review and while some individual companies have performed detailed studies, this has not become a standard industry practice, even though a mooring system is just as much a Safety Critical Element as a pressure retaining topsides system (as established in Chapter 3 above). There appears little justification for such lack of focus on moorings, even if the reason of “out of sight, out of mind” probably represents an element of the background. For many Operators of course not only are safety issues a concern, but other severe consequences cannot be ignored, including what the newspaper headlines tomorrow might be. Ideally a risk review process should be done at the design stage; but is just as applicable for existing mooring systems to determine the mooring integrity strategy. A Mooring Integrity Risk Review feeds many decisions including design, operation, inspection, maintenance, failure philosophy etc and this process should be considered the hub of a Mooring Integrity Management System, as it sets the whole context for the mooring system. In view of this reduced industry attention to moorings over the years compared to other Safety Critical Elements, there is little at the present time that can be drawn on to conduct a detailed risk review. The HSE Offshore Information Sheet 3/2006, “Guidance on Risk Assessment for Offshore Installations”, is generic and does not refer to the lack of data on such systems as moorings, resulting in the impossibility to conduct a full Quantified Risk Assessment for a mooring system.




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Although this section of this Guidance provides some general guidance taken from various UK HSE publications, including the Offshore Information Sheet 3/2006, Marine Risk Assessment (HSE Offshore Technology Report 2001/063) and “Reducing Risks, Protecting People”, but tailored specifically to a mooring system risk review, the general approach is considered applicable to mooring systems of any geographical location. Appendix 5 provides a specific example of a Mooring System Semi-Quantitative Review that can be, and has previously been, carried out. Both of these contents are intended to provide Guidance to individual Operators so that they can tailor their mooring risk review to the individual mooring system being considered. The main stages in the risk assessment process are dealt with below as separate sub-sections: 5.1

Level of risk review

5.2

Choice of component level

5.3

Hazard Identification

5.4

Risk estimation – consequence and likelihood

5.5

Consideration of risk reduction options

Two of the most common risk reduction methods are monitoring and inspection. Further guidance on these aspects can be found in Sections 6 and 7 of this Guidance respectively.

Other risk assessment methods than those outlined below may be considered appropriate by individual Operators. In electing for such methods great care should be exercised to ensure that the method is fully applicable to a mooring system. Following the suggested risk review procedure in this Document is expected, based on experience, to take circa two days; one day of a competent person with mooring expertise and experience, and a group session to ensure all disciplines are included.




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5.1

Level of Risk Review The HSE Information Sheet 3/2006 states that the level of risk assessment should be proportionate to the magnitude of the perceived risk; since a mooring system is currently an indeterminate risk (as very few mooring risk assessments have been carried out) it is therefore good practice to carry out a high quality risk assessment. In general a semi-quantitative method is considered to be more in-depth than a purely qualitative approach, and thus semi-quantitative would be the general expectation for a mooring system. The current lack of an industry mooring database and the limited knowledge of mooring system issues throughout the industry are considered to form an effective barrier to undertaking a full Quantitative Risk Assessment. For an Operator to make a decision on the level of Risk Review to carry out the following factors can be considered: Issue

Factors tending towards more detailed risk assessment approach

Factors tending towards less detailed approaches

- Design - Initial operation - Extended life

- End of field life - Short duration systems

- Low Factors of Safety - Severe weather conditions - Low redundancy

- Proven integrity - Benign conditions - High redundancy

Level of detailed analysis

- Novel concepts - Previously failed designs - No available analysis

- Proven designs - Proven analysis methods - Onshore inspections

Complexity

- Many components - Branched legs (eg buoys) - Mixed materials

- Legs of few components - One material grade

Persons on board (POB)

- High POB - Permanently manned - Few spare components

- Low POB - Occasionally manned - Available tugs & helos

Stage in lifecycle Mooring conditions

Notes: 1. In the above table a word of caution is expressed about the word “proven” in relation to mooring systems. In some senses even standard chain links could be considered a “proven” design; however recent experience suggests that there are several modes of chain link failure that have not until recently been understood as relevant. 2. Analysis methods for mooring systems have been developed to a high degree of complexity and for some aspects have been well validated against model and real life examples. However for some aspects, to date there are still no accurate methods of analysis. For example the detailed behaviour of a pennant buoy is not fully modelled by traditional mooring analysis software – this would require Computational Fluid Dynamic (CFD) modelling; yet there are no CFD programs that can also model the aspects of structures required in a mooring analysis. Due to the complexity of most UK permanent Installation mooring systems and the complexity of the failure modes at component level, it is considered that a Mooring Integrity Risk Review should be a semi-quantitative process; effectively the best quality approach that is practicable with the current industry knowledge.




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This approach for mooring systems fits well with the general HSE guidance that the use of quantitative risk assessment can have a number of pitfalls: • that the sample may be too small or too narrow; • that the time period of the data may be too short – which is certainly the case for many of the mooring failures referred to in this document; • that the time period may be too long – for example Grade 4 chain when it was first introduced had a number of issues, all of which were solved in the mid-nineties, but some mooring failures may refer back to causes that are no longer prevalent; • that the data may not include sufficient description of the cause – which is without doubt the case as regards many of the mooring failures in recent years. Thus the use of numerical estimates of risk as regards mooring systems, currently, could easily result in misleading conclusions and hence led to decisions that do not meet adequate levels of integrity.

5.2

Choice of component level Historically a level of Risk Review is generally carried out for a mooring system for any floating Installation. Quite often this is a group Hazid type technique looking at the whole mooring system. Rarely have mooring systems been considered at the component level; this is surprising in view of the detailed Hazops that are carried out on piping systems, where each section of that system is considered one by one. A similar approach to a Hazop should be considered for carrying out a risk review for a mooring system, noting that some hazards apply to very specific components within a mooring system. For example out of plane bending of a chain link does not apply to the main length of a section of chain, but it does apply to any link that is being restrained from free movement; thus the last held link from a chain stopper or a fairlead should be considered as an individual component, while the main length of chain to the next point of note may be considered as another separate component. Being able to identify the level at which to split a mooring system for risk assessment can be made if the risk assessor has a sufficient level of technical knowledge of the particular mooring system. However if the Hazard Identification process (see sub-section 5.3) is properly carried out it would also become obvious during this process that hazards are being widely applied rather than to the whole “component”. In this context therefore a “component” means a section of the mooring system that has failure modes acting on its whole, rather than on separate parts. When choosing the list of “components” on which to carry out the risk review for a mooring system, the following factors should be considered: Is there metocean directionality so that not all mooring legs can be assumed to behave in the same way ? For example do the strengths of wind, waves and current mean different vessel behaviour on different headings ? This is not as simple as assuming the worst case conditions, since some mooring failures have occurred on the leeward, or less heavily laden, mooring legs. Is there directionality due to an asymmetrical mooring layout, or seabed features ? Are there any parts of a potential “component” that may be subject to a significantly different set of hazards ? Will the failure of all parts of the “component” lead to the same consequence ? It is therefore important that a sufficient level of information is available about the mooring system; not just drawings of the components, but sufficient metocean information, vessel behaviour etc. An example list of components is shown as a worked example in Appendix 5.




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5.3


In carrying out hazard identification for a mooring system, the following features should be considered as to their relevance for the specific system being considered:

it is likely that for a mooring Hazid there will be an element of a review by a mooring expert, and an element involving a group of relevant but different discipline personnel. This is because at the present time there are relatively few industry personnel with a wide understanding of moorings, but at the same time group interactions can stimulate consideration of hazards that could be overlooked by an individual;

it is usually a qualitative exercise based primarily on expert judgement;

use should be made of previous incident experience. In this specific regard reference should be made to Appendix 2;

a structured approach should be used so that comprehensive coverage of all hazards is performed and hence as far as practicable avoid skipping less obvious problem areas;

care should be taken that combinations of hazards are not excluded; ie that the hazards are not just taken singly. Thus wear/erosion not only weakens a chain link, but it also reduces the fatigue life of that component.

A proposed list of specific hazards to mooring components is given below. Further background to each of these in considerable detail can be found in Appendix 2. Strength

Factor of Safety Snatch load Means of securing Wear/erosion With connected item With other item Seabed Fatigue Not proof loaded Axial Bending Torsion Corrosion Galvanic Bilogoical/SRB Chemicals Hydrogen embrittlement Contact With offtake tanker With supply vessel With fishing vsl/gear Dropped object From FPSO From offtake From supply vessel During inspection Motion Non-design of attachment Seabed scouring Manufacture defect Deployment




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5.4

Risk estimation – consequence and likelihood

Consequence It is too pessimistic to assume that the only consequence of a failure of a mooring component is the failure of that whole mooring leg, and hence the potential failure of the whole system. A number of sets of consequences can be imagined. The appropriate set for any particular mooring system may be considered from among the following – generally intended as in decreasing order of severity:

Safety

Repair Cost

Reputation

Common cause leading to inevitable sequential failure

> ~10 million

Major international impact

Common cause leading to latent sequential failure

> ~10 million

Major national impact

Immediate failure of one leg

> ~1 million

Considerable impact

Latent failure of one leg

> ~1 million

Limited impact

Degradation of Factor of Safety

~ 100k

Slight impact

Minor

0

Zero impact

The above follows the principles of the Consequence bandings in ISO 17776, but specifically adapted to mooring systems. Other column headings may also be People and Environment; however these have not been included in the above as for floating Installations the assessment of injuries or fatalities is extremely difficult since the vessel still floats in the event of a mooring system failure; and consequential effects to a riser system is difficult to relate directly to a mooring system failure. In any mooring system risk assessment, it should never be overlooked that the ultimate consequence is the installation breaking free and being a danger to other installations (and that drift routes are very difficult to predict with accuracy) is enhanced asset, personnel and pollution risks, and hence raised risk levels also to personnel on board. An Operator is free to utilise any Consequence banding he feels appropriate for the particular mooring system being considered. A worked example is given in Appendix 5.




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Probability A standard risk assessment tends to use Likelihood bandings such as: rarely occurred in the E & P industry; happened several times per year in industry; has occurred in operating company; happened several times per year in operating company; happened several times per year in location. Such bandings are difficult to sustain for a mooring system because: most Operators generally have a very limited number of operated units; the industry has a limited number of operating units; mooring integrity information has not historically been readily shared between different Operators; such assessments are relatively new, and a database of hazards and frequencies has not been maintained. A different approach to the ranking of the probability of an event occurring is thus necessary for the semi-quantitative assessment of a mooring component. The following factors may be used as questions, having a “yes”, “no” or “possible” as the only answers: ¾ Has the problem been known, in the industry, UK or elsewhere, to occur before ? ¾ Does realistic engineering fail to rule out possible lack of integrity ? ¾ Can a realistic mechanism or sequence be envisaged ? ¾ Is inspection at appropriate interval technical infeasible, unacceptably risky or unreasonable cost ? It is generally logical as to which answer increases the likelihood of a failure; a Yes answer to any of the above questions scores 2; and a “No” answer scores zero; an in-between answer would score 1 but generally if the answer cannot be given with certainty it should score 2. The sum of the scored answers would then represent a Probability scoring, which can be split into as many bands as is considered appropriate.

A worked example is given in Appendix 5.




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Risk rating As is commonplace with such matrices, the Consequence and Probability scores should be combined in a table, with the combination resulting in an overall risk rating. A potential 4 x 3 matrix (as used in the worked example in Appendix 5) is as follows:

Consequence

1 2 3 4

L Acceptable Acceptable Acceptable Make Alarp

Probability M Acceptable Make Alarp Make Alarp Make Alarp

H Acceptable Make Alarp Make Alarp Intolerable

A more complex matrix may be considered more appropriate by some Operators. A summary of such a structure was issued by the HSE in its “Tolerability of Risk” framework back in 1992: “Above a certain level, a risk is regarded as intolerable and cannot be justified in any ordinary circumstances. Below such levels, an activity is allowed to take place provided that the associated risks have been made as low as reasonably practicable. In pursuing any further safety improvements to demonstrate ALARP account can be taken of cost. It is in principle possible to apply formal cost-benefit techniques to assist in making judgements of this kind.” Considerable further guidance is available on risk ranking from the HSE and other sources. In applying such guidance care should be taken that it is fully applicable to a mooring system.




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Chapter 5 Further engineering Operational monitoring



5.5

Risk reduction options One of the key purposes of the risk review is to identify any mooring components that have an intolerable risk; it is then obvious that some actions have to be taken to reduce the risk to the integrity of the mooring system. The components that have a risk rating below the intolerable level however should still be reviewed to reduce the risk further if practicable. A hierarchical approach to risk reduction should be taken, in the following preferred order: 1. elimination and minimisation by design. For existing Installations such opportunity may be limited, but should not automatically be discarded since some changes can be achieved; it should be a priority for new build mooring systems. Sometimes additional engineering analysis can demonstrate if a design eliminates a specific hazard or not; analysis that may not have been undertaken at the original design stage, either due to omission or such engineering capability had developed in the industry at that time. 2. prevention; ie reduction of likelihood. This does not necessarily mean a change of a component, it could also mean such changes as a corrected tension balance between mooring legs that would ensure that too high a tension in a single leg would not occur. A change in operating practices can also prevent risks. The options are likely to be system specific but may include such as changing, in rotation, the link that may be sitting on a fairlead; or limiting the vessel heading on a drag chain or thruster assisted arrangement. These actions can be used for a direct effect on potential peak tensions, but may be more relevant to limit the number of times a chain link may see a bending moment, or in general benefit the fatigue life of a component. 3. detection; such as permanent monitoring or inspection intervals. The available methods for mooring system inspection are increasing all the time as more industry focus is given to mooring systems. Some of these are specific to individual mooring designs, and methods to investigate less frequent mooring integrity issues have less commercial benefit and so may be developed later than more common issues; however there is a large mooring industry community and it is likely that someone else may have come across the same or a similar problem. Inspection frequency, the periods between sequential inspections, can be reduced, so that if a problem does occur the chances of identifying it before integrity is breached is enhanced. Such a strategy of course should be backed up with a plan as to what the remedial actions will be. 4. control; limiting the scale, intensity or duration of the risk. Included in this would be such measures as limiting the consequences by shutting down production flow at seabed in the event of stated events occurring; 5. mitigation of consequences – protection from the effects; and ultimately emergency response. Some of the above may be used in combination; for example the risk of a component failing due to fatigue between inspections could be limited by carrying out crack propagation analysis; the results of this can set an appropriate inspection interval.



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Further engineering Chapter 6 Operational monitoring



6.

Mooring Monitoring Identify the design philosophy Identify deployment practices






Inspection workbook

Operators should review how they can detect that their installation may have lost one or more mooring legs. It is not considered prudent that a reduction in mooring capability goes undetected for more than an acceptable period. Such an assessment should include consideration of the following factors:

the reliability of the chosen method(s); this may include a system that can be checked, or made double or triple redundant. For safety, but also for commercial reasons, this may be important to be confident that an alarm or an indication is not spurious;

the risk for different elements of the mooring system, and the ability of the chosen method to detect a failure in that area; ie not all failure detection methods have the same capability for detection;

the ease of interpretation by the FPSO operating personnel on board;

the time interval between detection sweeps. Factors to consider in arriving at this interval are likely to include: o is 24 hour coverage required; for example visual systems are less effective at night; o the level of mooring redundancy of the whole system; both in terms of factors of safety (which may not apply to all components) and number of mooring legs; o the level of mooring integrity risk identified; o the consequential risk to personnel (on this or other installations).



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As a guide to what may be considered appropriate for detection intervals, the following examples may be helpful: Example A: A FPSO mooring has intact strength factors of safety that just meet the chosen design standard, and can only accept one broken mooring line before a cascade failure may occur. A mooring monitoring system that can provide an indication in real time or at no more than one minute may be applicable, on the basis that: • there are a number of common failure modes that are not well understood in the present day; • a cascade failure can occur very quickly (less than a minute) and often quicker than the subsea system will fail close; • the consequences are potentially so severe that all available time to take appropriate actions should be made available, for the purposes of either saving life or to limit commercial exposure. Example B: A FPSO mooring has a higher factor of safety than the chosen design code, and can still be considered safely moored after the loss of two mooring legs. A mooring monitoring system that provides an indication at daily intervals may be applicable. For the purposes of bounding the risk assessment thought processes for monitoring systems, a 24 hour interval should be taken as the initial limiting benchmark. Some monitoring methods permit a greater functionality than just detection of one or more failed lines. Direct measurement of tension, or measurement of angle that leads to a calculated tension, on one or more legs, may be considered an advantage in providing input data for a more in-depth analysis. Such an analysis may be used to confirm actual behaviour that is consistent with the design, or could be used as direct input to a fatigue analysis; such design verification can be considered as good practice and is included in the draft Performance Standards in Appendix 3.

A list of potential mooring monitoring methods, with a view of some key capabilities, is included in Appendix 6 as a starting point for each facility to judge what may be most applicable.




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Inspection activities Chapter 7

7.

Mooring Inspection Methods Identify the design philosophy Identify deployment practices






Inspection workbook Chapter 7

Appendix 7 provides a brief synopsis of mooring inspection methods that are available at the date of this revision of these Guidelines; this list may not be complete. Please note that the inclusion of a company contributing to this document is provided for convenience of Installation Operators, and does not in any way suggest any level of endorsement of any company. In assessing which inspection techniques are applicable for a specific mooring system inspection, the following factors should be included:

the failure or damage risks to each component as identified in the Mooring Integrity Risk Review (see Section 5), and how the inspection method can reduce the perceived risks;

any risks to the mooring that may arise as a result of the mooring method. Such risks may be from physical damage, but less direct effects should also be considered, such as whether, for example, cleaning of all marine growth and scale may promote corrosion on cleaned links, or in some circumstances may reduce the risks of bacterial corrosion;

the accuracy and repeatability of the method; this may only apply to chain measurement techniques, but results from subsequent inspections would be misleading if confidence in both sets was not good. A later set of measurements giving higher readings than an earlier set would devalue both;

for qualitative inspection methods, sufficient means of distinction should exist, or be created, to be sure of a comparison between not just sequential surveys, but also between adjacent legs.

Several inspection methods are relatively new and without a long track record; this does not necessarily make them less valuable or reliable but Operators should evaluate all methods with care.




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Even some tried and tested methods are open to a wide variety of standards; even general visual inspection techniques can vary greatly from one inspection to another, and some of the following aspects should be considered;

camera quality – in terms of lens size, zoom capability and specifically the number of pixels;

recording quality – many large ROVs use slip rings for the umbilical and this can degrade the quality of the picture being recorded. Fibre optics can provide a higher quality of transmission but traditionally suffer from more downtime;

ROV manoeuverability – not all ROVs have the same thruster configuration and have different capabilities of movement in three axes. Not being able to thrust sideways without changing heading may be important to ensure that if an anomaly is found, a range of view angles can be achieved, or to hold position against a current direction;

method of recording – use of VHS tapes is now old fashioned and recording to DVD, DV tape or direct to hard drive can achieve a higher quality;

overlay capability – to be able to readily see on screen the chain or component being inspected, as well as depth, heading and other ROV data, can ease the review process.

One type of mooring inspection that can be of immense benefit because it permits accurate computer modelling, is to ensure that as-built data is available. Obtaining such information is not straightforward, often for reasons including the following. There are methodologies that can overcome these aspects:

true lengths of components; manufacturers measured lengths are as accurate as their equipment allows but are always summations of a series of shorter lengths, and generally at tensions less than even at rest conditions once installed. When even a change of 1m in length can make a significant difference in mooring leg tension, accuracy considerably better than is typically shown on manufacturers certificates is necessary;

FPSO position; as positions at all relevant points on a mooring leg cannot be taken simultaneously, and any floating body will not be entirely stationary, careful normalisation during the period of an individual leg inspection is necessary;

touchdown position; this is a largely ill-defined spot, since judgement of when a chain or wire is touching the seabed is a subjective issue, and made more difficult with any level of seabed trenching;

position system accuracy; correct techniques of determining the mean position of a point are established in the surveying industry, but practicable issues of what is meant by the position of a component should be addressed – the centre, one end etc.

Marking of a long component can be advantageous; applicable locations may be determined by analysis, or at least a single marker as a reference point. If this is done during deployment this aids future inspections, but choice of a robust method (rather than just white paint) is worthwhile. If marking is to be done retroactively, then care should be taken not to give rise to any issues, such as use of dis-similar metals. One available method for marking of chain is the use of polymeric clips which can be installed by ROV; these may be considered not just for a known position, but to mark specific links that have been measured.



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The benefit of inspection in air should not be overlooked. This permits a much more detailed inspection to be possible and generally in a much quicker time and at lower cost. Such opportunities may arise from a variety of possibilities, including: •

on an adjustable mooring system, the ability to winch some mooring legs out and others in, so that some elements come into a zone where access is much easier than in the water column;

•

where there is a ready means of disconnection on a mooring leg, or there is sufficient slack in the mooring legs, that one or more may be partially lifted to the surface. Care should be taken to assess that the risks of this procedure are appropriately less than the perceived risks that are being checked;

•

when a floating Installation leaves site for other reasons, the mooring legs are inevitably disconnected at the FPSO or other permanent facility end, providing opportunity for detailed in-air inspections for a majority of their length.

The opportunity for inspection in air may also be judged beneficial as a result of a Mooring Integrity Risk Review. While there are risks to achieving this it may be judged in some circumstances that the possibility of a failure mechanism occurring in an otherwise un-inspectable location may be higher; in this event consideration should be given to having a replacement available at the time of the inspection.


Mooring Integrity Management System Develop day to day operational philosophy


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8.

Mooring Integrity Management System Mooring Integrity Management System Identify the design philosophy Identify deployment practices







Each Operator will have its own unique structure for its Safety Management System. Within this structure it is expected that the following elements in connection with the mooring system exist. These need not be in a single document; how the total Mooring Integrity Management System is arranged throughout the whole Safety Management is a matter for the individual Operator, but in every case appropriate links between each of the key elements should be referenced. The key parts of a Mooring Integrity Management System are in summary : System description – so that there is no doubt as to what is in place Performance Standard – refer to Section 3.2 Operational Procedures – refer to Section 3.3 – that are consistent with the Performance Standard Written Scheme that formally identifies the methods to be used to confirm compliance with the parameters in the Performance Standard – such as on-going analyses, inspections etc Failure Philosophy – refer to Section 4.1 Contingency Plan – refer to Section 4.2 All the above should be under-pinned by: Mooring Integrity Risk Review – refer to Section 5 – component by component, that acts as a means of having determined the extent of an inspection program, which in turn is likely to be one of the verification methods to demonstrate compliance with the Performance Standard. Mooring Monitoring review – refer to Section 6 - which could be incorporated within the Risk Review. Mooring Inspection Workbook, Anomaly Tracking Register and a working procedure for closeout of mooring system anomalies – reference to the on-going Mooring Integrity JIP Phase 2. C:\Documents and Settings\WME - Ian\My Documents\123 Current\199 JIPs\2006 HSE-UKOOA Mooring Guidance\Master Copies\080430 1m file idw Guidance RevF.docx Page 40 of 113


Appendix A

Abbrevations

ALARP

As Low as Reasonably Practicable – a key concept of the UK Safety Case regime

API

American Petroleum Institute

AUV

Autonomous Underwater Vehicle

DCR

Offshore Installations and Wells (Design and Construction, etc) Regulations 1996 as amended

FPSO

Floating Production, Storage and Offtake, ie a vessel, which could be of any size or shape

HAZID


HSE

Health and Safety Executive (UK body)

IMO

International Maritime Organisation

ISO

International Standards Organisation

JIP

Joint Industry Project – typically where members of the JIP pay a set fee for one company to undertake specific work and produce a report on behalf of all the members

MAR

Offshore Installations and Pipeline Works Administration) Regulations 1995 as amended

MODU

Mobile Offshore Drilling Unit

Oil & Gas UK

Oil & Gas UK is the leading representative organisation for the UK offshore oil and gas industry. Its members are companies licensed by the Government to explore for and produce oil and gas in UK waters and those who form any part of the industry's supply chain.

PFEER

Offshore Installations (Prevention of Fire and Explosion, and Emergency Response) Regulations 1995 as amended

POB

Personnel onboard (ie the number of)

QRA

Quantitative Risk Assessment

ROV

Remotely Operated Vehicle – ie an underwater craft controlled via an umbilical

SCR

Safety Case Regulations – or in full “Offshore Installations (Safety Case) Regulations 2005 as amended

(Management

and



UK

United Kingdom

UKOOA

United Kingdom Offshore Operators Association, the previous name for Oil & Gas UK.



Appendix B

Working Group Terms of Reference

Background The FPSO industry, both UK and internationally, has experienced a number of mooring line failures, and a larger number of damages to mooring lines. Some of these failure mechanisms had not been anticipated and illustrate that existing practices in design, installation and subsequent monitoring could benefit from being more robust. Recent work by the industry, specifically sessions held under the auspices of UKOOA in 2005 and 2006, and also the JIP on Mooring Integrity which was completed in December 2005 (and now available on the Oil & Gas UK website) has highlighted many of these incidents. At the meeting held on 7th June 2006 UKOOA, in conjunction with the HSE, has recognised that the safety of FPSOs could be enhanced by providing Guidance to Operators for the design of new moorings, and the integrity management of existing moorings. This was ratified at the UKOOA meeting held 2nd October 2006. In broad terms this document is intended to provide Guidance in the same way as the following documents do for their subjects: 2002: Guidance Note on Monitoring Methods and Integrity Assurance for Unbonded Flexible Pipe 2002: Tandem Loading Guidelines 2003: Ship/Installation Collision Avoidance This document provides the Terms of Reference for the Working Group to develop this Guidance.

Terms of Reference (i)

The FPSO Mooring Integrity Guidance is to provide Guidance as regards: 1. a review of existing guidance and regulations, including major Classification Societies’ requirements; 2. the typical physical aspects of mooring systems and their components; 3. how consideration at the design and deployment stages can benefit the long term integrity of moorings; 4. mooring system Performance Standards; operational philosophies; 5. philosophies and readiness in the event of failure; contingency planning; 6. a structured means of assessing the risks to mooring components; 7. currently available means of mooring monitoring; 8. good practice for, and currently available, mooring inspection techniques, visual, measurement and other; 9. mooring system integrity management system; Specifically what is not intended to be included in any detail is how to conduct design analyses, although the appropriateness of analyses would be referred to.

(ii)

The scope of this Guidance is the Mooring System for any floating unit, principally ship-shaped FPSOs but much of the Guidance is applicable to any mooring system. The Mooring System is made up of mooring components including everything that is not steel structure, ie all chain, rope, wire, shackles, weights, buoys, pennants and means of anchoring, but also including the structural



interfaces with those components, and hence including chain stoppers and trumpets, hawse pipes, chain jacks, fairleads and similar. The effect of the turret design (passive/active) as it affects the failure modes of the mooring system is applicable; structural aspects onto these mooring components is also considered, including the benefits of a corrosion protection system (eg corrosion allowance, coatings, impressed current or sacrificial anodes); however the turret design itself is not part of this Guidance. Fluid transfer or containment is not included other than as an interface to the mooring system. (iii)

The Working Group consists of personnel from Oil & Gas UK members, or personnel co-opted with the agreement of the Oil & Gas UK members of that Working Group.

(iv)

Input to the Working Group should encompass a balance of engineering, inspection and operational experience; this can be obtained from a combination of all of the following: in-house experience of the Working Group members; experiences of any other FPSO operator; mooring equipment manufacturers; mooring inspection companies.

(v)

Where input is obtained from any commercial enterprise, all identified competitors in that field should also be invited to provide input. The input required is to describe available technology and is not to be treated in any way as a means of marketing. The only reference to a company name will be in the acknowledgements within the document.

(vi)

If the Working Group considers that the use of a paid consultant is beneficial, then this shall be referred to Oil & Gas UK management for approval prior to making any commitments.

(vii) It is anticipated that the majority of the work can be completed by correspondence. Physical meetings of the Working Group are desirable and will be arranged at the discretion of the Working Group Chairman at intervals he feels appropriate. (viii) The target date for completion of this Guidance is September 2007. (ix)

The completed Guidance will be made freely available to Oil & Gas UK members and the FPSO community at large.

(x)

The Working Group Chairman:

Ian Williams, Wood Group Engineering (North Sea) Ltd Tel: +44 7939 647574 Em: [email protected]

Oil & Gas UK contact:

Samantha Reid

Working Group members are:

Max English, HSE John Awater, Bluewater Paul Drummond, Lloyds Register EMEA Mike Efthymiou, Shell Donald Smith, Maersk Contractors Peter Nugent, Ugland Stena Storage



Appendix 1 Participating Organisations

To be included in the final version.



Deliberately blank for now.



Appendix 2

Synopsis of mooring system failure modes


FAILURE MODE MAIN GROUP SUB-GROUP EXAMPLES Strength

Factor of safety Design calculation method (eg quasi-static where dynamics may be better) Quasi-static where dynamic method may be better Frequency domain vs time domain No design method really available (eg pennants for buoys) Design calculation errors Error in input values Wrong conventions used Design load cases poorly chosen I ffi i t mett data Insufficient d t available il bl Poor assumptions on wind, wave & current combinations Draft or trim choices not covering full spread Imbalance between lines Not balanced at first installation Seabed touchdown moved sideways / doglegs Ground length laid with some slack now pulled out Snatch load At one or more parts of the dynamic catenary, chain goes slack Design calcs assumed top of chain is worst position Unexpected movement of attachment point Means of securing Shackle safety pin (bar or split) fractured due to excessive bending Excessive bending on installation Shear from main shackle nut rotating Shackle safety pin galvanic corrosion with main pin Plate securing bolts Galvanic corrosion Electrically isolated from CP due to bolt thread material Damage during deployment Weight securing bolt failure Insufficient strength, perhaps as no analysis method available Galvanic action Lack of CP

Known to have occured ?

Y Y Y Y Y Y Y Y Y

Y Y

Y Y Y Y Y Y Y Y Y

FAILURE MODE MAIN GROUP SUB-GROUP EXAMPLES Wear / erosion

With connected item Chain - inter-link wear Due to axial tension ranges Due to chain bending Due to chain torsion Foreign object between links Crown against crown (slack chain) Poor fitting of pin Size Sh Shape Shackle or other pinned connector Pin rotating in shackle body Pin rotating in chain/connecting link Poorly matched hardness criteria Insufficient pin clearances to allow for thermal expansions to allow for corrosion to allow for corrosion products to be cleared away Poor choice of shackle type With other item Seabed Sand particles between chain links Fine particles inside fibre ropes Fairlead Pocketed gypsy Bending shoe Trumpet Riser Adjacent mooring Parallel weight chains With seabed Other seabed asset Debris Rock


Y Y Y Y Y Y

Y Y

Y Y Y Y Y

Y

FAILURE MODE MAIN GROUP SUB-GROUP EXAMPLES Fatigue

Axial Poor analysis of load ranges Poor choice of methodology Insufficient load cases Directions Wave height / period bandings Inappropriate fatigue curve chosen (particularly for connectors) SCF poorly identified Design calcs assumed top of chain is worst position D b i iin iinterlink Debris t li k area causing i hi high h SCF Wrong size or shape of pin causing high SCF Insufficient recognition of variability, ie too low a factor of safety chosen Particles inside wire or fibre ropes Bending Vertical link inboard end held, held next (horizontal) link moving horizontally Vertical link inboard end held, two links out (vertical link) moving vertically Horizontal link inboard end held, next (vertical) link moving vertically Horizontal link inboard end held, two links out (horiz link) moving horizontally Around a bending shoe / ramp Turret stiction bending a link at end of a trumpet Poorly design fairlead / gypsy Seabed trenching at touchdown Torsion Static torsion causing higher SCF Rotation of chain around its own axis (eg weathervaning) Bird-caging of wires Poorly matched torsional stiffness characteristics Rotation of buoys in current or waves Poor choice of swivel type

Known to have occured ? Y

Y Y Y Y

Y

Y Y

Y Y Y Y Y

FAILURE MODE MAIN GROUP SUB-GROUP EXAMPLES Corrosion

General Insufficient corrosion allowance made mild steel shackle safety split pins Area of oxygenated water such as from an outflow Unintended insulation from sacrificial anodes Bolt thread coating Corrosion Coating Galvanic I Innappropriate i t materials t i l in i electrical l t i l contact t t Chain/shackle Shackle/safety pin Shackle/rope thimble


Y

Y

Use of steel clips Biological / SRB From seabed From effluent outflow

Y

Chemicals In seabed In effluent flow Seabed debris Hydrogen embrittlement Passes too close to impressed current anode

Y

FAILURE MODE MAIN GROUP SUB-GROUP EXAMPLES Contact

With FPSO / offtake tanker Anchoring in wrong place With supply vessel With fishing gear Trawl wires Trawl nets Temporary operations Drilling or intervention rig moorings Seabed S d particles Sand ti l between b t chain h i links li k causing i wear


Y Y

Dropped object From FPSO From supply vessel From offtake tanker During inspection ROV Diving bell

Motion

Of attachment point Sudden rotation of the turret Seabed touchdown Different weather directions Seabed trenching Of external item Adjacent mooring line Riser Seabed scouring Exposure of anchors Build up of seabed material

Y

Y

FAILURE MODE MAIN GROUP SUB-GROUP EXAMPLES


Manufacturing

Not fully proof loaded, so fatigue characteristics reduced Poor or insufficient bar material quality Heat treatment inconsistent Minimum material testing

Y Y Y

Deployment

Physical damage due to poor handling Side loading on chain links or shackles causing high SCFs, reduced fatigue life Cold bending of materials causing reduced fatigue life Local heating (eg from spot welding) causing reduced fatigue life B di below Bending b l minimum i i bbend d radius di ffor wire i or fib fibre rope Use of inappropriate shackle sizes through chain links at high tensions

Y Y Y Y Y


Appendix 3

Example Performance Standards

The following pages are provided as further detailed guidance, in a typical Performance Standard format, of what items should be considered for inclusion. Two versions are given, one for a Passive Mooring System (Freely Weathervaning), and one for a Heading Control Mooring System. The latter has the same entries as the former but with some added lines.

Appendix 3.1

Example Performance Standard (Passive Mooring System)

Performance Standard Tag Reference Performance Standard/SCE Description

Role

Description

Ref 1

ABC123

'FPSO/FSO/FPF MOORING SYSTEM/STATION KEEPING' Comments • Outline of main high-level safety crirical functions of the mooring system, including: • To maintain the vessel’s location within the excursion limits defined for the flexible pipeline risers p to select and maintain the vessel’s heading g relative to the turret with respect p to the weather • To allow the operator (some passive systems) • To transmit environmental loads from the FPSO hull structure to the mooring system foundations and connect the subsea risers, power and signal cables to the topside facilities. Outline description of mooring system, including safety critical components and primary load path from mooring anchors/foundations and lines to FPSO hull i.e.: • Number of mooring legs (grouped / equal spread) & outline of leg components (chain / wire rope incl sizes); • Mooring line jewellery (joints eg shackles, deltaplates, kenters, spelter sockets), mooring termination , fairlead/gypsy wheels; • Turret terminations / spider; • Turret layout & bearing details (Upper / Lower turret roller / journal bearings / jacking systems & races, including turret turning/breakout management facilities (active/passive); • Heading Control Facilities (eg stern thruster) if applicable.

Consideration can be given to including the design metocean condition, typically characterised by a significant wave height. A fuller statement of the worse case conditions including Hs, Hmax, Tp, wind speed and current speed, as determined by the design analysis to represent the worst case for design, would provide a ready reference in case of potential exceedance but may equally be included in other documentation, referred to in the Performance Standard. For some vessels the worst case condition for design is not the traditionally referred to “100 year storm” or other maxima, and may be for example a steep wave condition.

TURRET / MOORING SYSTEM FUNCTIONALITY

Function 1.01 To moor the turret within safe excursion envelope (e.g. riser operating limits / hull interactions etc)

Acceptance Criteria Define excursion limits, (alarm set points and limiting excursion criteria), as defined by system overstress / riser safe operating limits, as applicable.

Assurance There shall be effective continuuous monitoring of turret excursion from neutral point (eg DGPS, Hydro acoustic). Define unit specific required (critical) levels of accuracy.

Verification Demonstration of functioning mooring excursion monitoring system, within requisite levels of accuracy, cross referenced to other system data as available.

Comments For typical UKCS depths a benchmark of +/-2m is a recognised benchmark) Specify details of design basis, in terms of fundamental assumptions on critical mooring line / hull interactions and project phase assurance (eg designer assurance, ICP / Class Society verification / approval) Define field environmental / design conditions (eg 50/100 year storm predictted for specified location). Specify whether tension needs to be maintained in turret mooring lines to maintain position / line integrity. Describe critical tension limits to mitigate / prevent line / jewellery snatch loading. Mooring line tensions for the 1 / 10 year storm condition(s) may be specified, but details of the line tensions for the field design g (50/100 ( yyear)) condition should be included as a minimum requirement here. There should be a comparison made between the limiting mooring line performance / code compliance as certified and verified (model correlation), and the design storm condition derived projections for mooring line tension.

Appendix 3.1


Function 1.02 To provide continuous assurance to vessel operators that the mooring lines are intact.

Acceptance Criteria Assurance Define means by which each Ability to confirm that mooring mooring line's ongiong functional line(s) are still intact in acordance performance is demonstrated to with Mooring Guidance section 6. the vessel crew (e.g. mooring line inclinometers, mooring line tension monitoring alarms,) - ref Mooring Guidance section 6 and Appendix 6.

1.03 To provide adequate mooring from seabed to vessel and to transmit those loads into the vessel hull.

Specify the lowest allowable minimum breaking load (MBL) of the mooring legs / turret system as limited by the weakest leg / element (specify which) and its eg a o into o the e vessel esse hull u integration structure and anchor point/foundations . Refer to mooring line component procurement assurance documentation (eg certification, describing performance compliance).

Defined strategy / schedule of inspection, operational maintenance and testing routines, derived from the risk review in section 5 of the Mooring Guidance and a d assu assurance a ce / verification e ca o o of same.

Verification Verification of intact mooring lines at requisite frequency - ref Mooring Guidance section 6.

Comments Primary means of demonstrating functional performance includes online tension monitoring, inclinomoters, proximity switches, hydro-acoustic systems and various others as described in Section 6 and Appendix 6.

• Describe means by which mooring functional performance is described (e.g. mooring tension repeaters / alarms); • Specify alarm set points (high / low tension alarms, inclinometer set points, turret position monitoring system excursion alarm set points);; • Define the context of each set point in terms of the failure with which its initiation is associated (eg (i) low tension set point / inclinometer set point associated with weight of vertical chain of length equivalent to water depth at field location, (ii) high tension set point/ inclinometer set point equivalent to most probable maximum (MPM) load associated with design condition return period, in the intact [??line] condition, / Rule [ref] compliant factored Minimum Breaking Load (MBL) (iii) turret excursion set point associated with riser safe operational limits / mooring line limiting condition); • Refer to safety critical system component maintenance regime (incl planned maintenance/testing routine tags) • Describe means by which crew's attention is drawn to system alarms / functional noncompliance (visual / audible alarms). Results of mooring integrity assurance activities confirming ongoing conformance of mooring system features with specified acceptance criteria.

Clarify safe operating limits on each, mooring system component, including, where necessary, consideration of the following

-Sensitivities of mooring components combinations and their interactions; - Certain mooring turret / bearing systems are particularly sensitive in terms of being maintained centrally withiin the moonpool bearing race for safe opetration / load transfer. The measures by which this issue is managed and assured should be descriobed. - Depending on the particular vessel / mooring geometries, turret bearing systems exhibit 'turret breakout', whereby brearing friction causes the turret to be twisted with the yawing hull until the mooring line tensions break the turret free, allowing it to return to its own neutral heading. In some systems, this phenomenon is actively controlled, whilst in others the response in entirely passive. The safety critical aspects of this phenomenon (eg turret break out yaw offset versus mooring chain / riser safe operating/contact limits) should be described and the means by which ongoing safe operations are assured referenced. Additionally, means by which the turret heading (as opposed to vessel heading) is advised to the crew should be described and its critical maintenance referenced. - Some turret bearing use systems of jacks acting in horizontal and vertical directions.and t th turret t t / bearing b i race, requiring i i control t l systems t t preventt this. thi Thie Thi can overstress the to safety critical aspects of this systems's performance and corresponding assurance should be described here. - Provision of hydraulic power to the turret turning/bearing system during normal / emergency operations may be safety critical and any safety critical aspects should be outlined here.

Appendix 3.1


Function 2

Acceptance Criteria

Assurance

Verification

Comments

TURRET / MOORING SYSTEM AVAILABILITY

Function 2.01 Verification of System Design Assumptions (To maintain functionality of mooring system for the installation design life of

2.02 Verification of Component Design Assumptions (To g maintain mooring integrity for the installation design life of ?? years (specify)).

Acceptance Criteria The in-service mooring system performance data shall fall within the limiting envelope defined by the mooring system design assumptions: · Storm / exceedence of design condition;

Assurance Correlation of in service data (tensions, catenary profile, mooring offsets, hindcast data) with design assumptions (model tests, operational load offset Project / vendor supplied certification for mooring system components. Post storm-event response measures.

Verification Verification that in-service data from mooring integrity assurance activities matches results that would be associated with a mooring system which continues to perform as designed, with no leading indicators of vectors No indications / records of environmental events which exceed design / manufacturing certification specified operating for mooring system. Verification of satisfactory execution of post storm-event response procedures.

·

Project / vendor supplied certification for mooring system components.

No indications / records of environmental events which exceed design / manufacturing certification specified temperature limits

Temperature;

As a passive system, the availability of the mooring and turret system is quantified in terms of maintenance of the turret / FPSO system within a defined radius up to and including the design storm condition in either the intact or damaged condition,

Define limiting integrity vectors and describe system limiting sensitivity(ies) to vectors including: · Corrosion;

·

Fatigue;

Verification of design, construction, installation and operational assurance processes:

Comments Accuracy of correletion with design limits (e.g. statstical means & std deviations) within 20% represents a suitable benchmark. The limiting design condition (wind, wave, current etc) should be specified here. Post storm-event response measures can be defined within mooring system through-life integrity management strategy for maintenance / assurance / verification.

Operators may elect to put the detail of this section in a separate document as part of their Mooring Integrity Management System (Ref Section 8 of this Guidance).

- Design and construction completed in accordance with recognised standards, defined acceptance criteria. - Confirmation of performance within tolerable limits of design. '- Maintenance of mooring system components in accordance with criteria defined in mooring system integrity management strategy

Define mooring line component functional endurance characteristics and refer to relevant certification:

Verification that in-service data from mooring integrity assurance activities matches results that would be associated with a mooring system which continues to perform as designed, with no leading indicators of vectors associated with defined failure modes / degradation mechanisms.

The mooring system integrity management strategy should define the means by which the continued effectiveness of this system is assured. It should describe the processes of identifying, assessing and where necessary repairing any deterioration due to corrosion, fatigue, wear, scour, damage, temperature, manufacturing/installation defects, creep or over-loading (ref Mooring Guidance section 5 & Appeondix 2)

Corrosion protection measures.

Confirmation of performance of corrosion protection measures.

Mooring component design and operational fatigue endurance assessment(s).

Mooring component fatigue endurances verifiied as exceeding ongoing operational and projected field deployment fatigue utilisations.

Define corrosion protection measures / assurance (eg material corrosion allowances, cathodic protection systems, coating systems); Consideration of all potential corrosion vectors should be included (e.g. recent examples highlighted of anerobic sulphide reducing corrosion in UKCS based mooring spreads at hitherto unexpectedly low temperatures) Outline the mooring system design fatigue endurance and its assessment / assurance. List assumptions implicit in the fatigue assessment process.

Operational utilisation conforms with assumptions implicit within the fatigue

nb certain joining components such as kenters have considerably reduced fatigue lives when compared to common link.

Appendix 3.1 Function

Example Performance Standard (Passive Mooring System) Acceptance Criteria · Wear (incl intergrip wear & wear with other items);

Assurance Chain wear allowances / certification and through life inspection of wear.

·

Through life inspection of onbottom mooring system components. Through life inspection (scheduled and post-event) of mooring system sys e components. co po e s Mooring system project certification & through-life strategy for maintenance / assurance / verification;

Scour;

· Contact damage (incl other vessel operations / dropped objects); objec s); · Manufacturing / installation defect limits

Verification Confirmation that mooring system component dimensions continue to meet or exceed minimum allowable dimensions associated with safety critical performance. On-bottom mooring system components found as per project defined limits for scour. Mooring system componets found within acceptable limits for damage. Review of project / change management certification, confirming that manufacturing / installation procesess have not invalidated mooring system design basis/assumptions.

Comments

Any additional limits (weather, time) associated with damage noted in way of mooring system components should be expediently assessed and consequences for continued safe operation of the mooring system defined. Consider potentially critical manufacturing defect limits for material tensile properties, fracture toughness properties, chemical properties, manufactiring / fabrication defect acceptance criteria); Consider potentially critical installation defect limits for mooring line pre-tension / balancing of mooring spread, on-bottom seabed conditions (eg boulders, debris), mooring foundation installation acceptance criteria, mooring line/turret/hull installation acceptance criteria.

·

Creep;

Through life inspection of mooring line components.

·

Axial stiffness;


·

Turret bearing friction limits

Through life measurement / determination of bearing friction.

· Mooring termination geometrical limits;

· Mooring line snatch load / offload;

· Mooring chain/wire interactions wiith fairleads/wildcats/stoppers/moorin g line jewellery (tension-bending effects, fretting, galvanic corrosion etc); · Mooring spread imbalance / pre tension levels to optimise effects of mooring spread fatigue vs system fatigue endurance and

Alarms / records pertaining to relative displacements of hull / turret / mooring terminations / mooring lines / risers. Design features to mitigate snatch load/oflload and in-service monitoring of these feautures.

Through-life maintenance / inspection.

Through-life mooring system tension monitoring / balance optimisation measures, including maintenance and monitoring

Review of through life mooring line inspection records, confirming no evidence of leading vectors w.r.t. creep. Review of through life mooring line inspection records, verifying acceptable operational utilisation of mooring system components w w.r.t. rt defined limits for in-plane/out-of-plane bending. Review of friction measurement activities / calculations, confirming confirmance to design assumptions (acceptable limits for turret bearing friction). Confirmation that records show no relative movements of the hull / turret / mooring terminations / mooring lines / risers which exceed defined operational limits. Review of mooring line tension monitoring records / alarm event records to confirm that snatch load / offload has not occurred. Review of mooring line inspection records, verifying that any instances of mooring line damage (e.g. cable strand failures, birdcging) remain within limits defined by code / mooring component manfucturer. Review of through-life maintenance / inspection records & confirmation that any instances of tension-bending effects, fretting, galvanic corrosion etc are captured and assessed against limiting criteria. Review of through-life mooring system tension monitoring / balance optimisation records, confirming that mooring line pre-tensions align with mooring spread requirements for optimum

Particularly relevant for moorings involving polyester or other fibre ropes.

Mooring system inetgrity management strategy should include consideration of the sensitivity of mooring system components' fatigue endurance, in terms of potential reductions arising from in in-plane plane / out out-of-plane of plane bending bending.

Appendix 3.1 Function 3

Example Performance Standard (Passive Mooring System) Acceptance Criteria

Assurance

Verification

Assurance In practice, operational reliability is assured by the development and implementation of a mooring system inetgrity management strategy, which has considered d accounted t d ffor the th safety f t and critical vectors described within this performance standard and which defines a range of practically achievable schedule of inspection, maintenance and testing activities to assure the ongoing integrity of the (passive) system components.

Verification Review / survey of inspection and maintenance activities arising from implementation of mooring system inetgrity management strategy, confirming that these assurance activites are beig carried out at the requisite f frequency, with ith corrective ti measures ttaken k as and where necessary.

TURRET / MOORING SYSTEM RELIABILITY

Function 3.01 Passive elements to be 100% available for duration of field deployment.

Acceptance Criteria As a passive system, the reliability of the mooring and turret system is quantified in terms of maintenance of the turret / FPSO system within a defined radius di up tto and d iincluding l di th the design storm condition for the duration of the deployment to each specified field location.

3.02 Transmit mooring loads Turret structural integrity and Mooring turret bearing bearing systems need to be 100& maintenance regime. to the FPSO hull available structure.

Turret /vessel friction control / yaw breakout: the active / passive breakout of the turret should occur within defined safe yaw offset limits (define) with 100% availability on demand. 4

Comments

Comments Reference component risk assessment guidance and mooring system component failure modes in Mooring Guidance section 5 and Appendix 2). An element of reliability assurance is normally by design and construction in accordance with recognised standards and maintenance of systems in accordance with recognised t criteria, it i under d which hi h th the continued ti d effectiveness ff ti off thi this system t relies li on acceptance identifying, assessing and where necessary repairing any structural deterioration due to corrosion, fatigue or over-loading.

Review / survey of turret bearing maintenance / assurance activities (grease sampling and assessment, bearing friction measurements).

Reference component risk assessment guidance and mooring system component failure modes in Mooring Guidance section 5 and Appendix 2).

Through life inspection of turret internal and moonpol structures and associated upper/lower bearing systems

Review / survey of integrity assurance activities in way of turret internal and moonpol structures and associated upper/lower bearing support structures, with results confirming ongoing structural integrity in accordance with defined acceptance criteria.


Turret breakout yaw-offset testing/measurement/calculation (passive and, if applicable active)

Review / witness of turret breakout yaw-offset testing/measurement activities, confirming that passive /active (as applicable) breakout occurs within defined safe limits.

The mooring system integrity management strategy should consider the consequences of high turret breakout offset (chain / fairlead / stopper contact, in-plane / out-of-plane tension-bending, shear) reference Mooring Guidance section 5 and Appendix 2. The means by which the defined safe yaw offset breakout is assured should be defined.

TURRET / MOORING SYSTEM SURVIVABILITY

Function 4.01 To maintain the vessel on location as defined under the functionailty requirements outlined above for the 'intact case' condition, with an adequate margin of safety

Acceptance Criteria As defined under F101, F102, F103 and A201 above

Assurance Design 'intact case' determination and assessment of results with regard to design code requirements

Verification Confirmation of design approval.

Comments Class Rule requirements provide a suitable benchmark for assessing the adequacy of the mooring system in the 'intact case' condition.

Appendix 3.1 Function 4.02 To continue to maintain the vessel on location as defined under the functionailty requirements outlined above, with an adequate margin of safety following the loss of (??) mooring line(s) (specify).

Example Performance Standard (Passive Mooring System) Acceptance Criteria Assurance Moor the turret within safe Outline of design basis (National / excursion envelope (e.g. riser international codes, classification operating limits / hull interactions rules) etc) for the defined damaged condition:

Verification Confirmation that the continuuous monitoring of the turret excursion from neutral point (eg DGPS, Hydro acoustic) should continue to operate in the defined 'damaged case' condition.

Comments Ref Mooring Guidance chapter 4 Appendix 4

*** Repeat sections 1.01, 1.02, 1.03 and 2.01 for defined damaged condition , confirming the validity of each acceptance criteria in turn and revising where appropriate. Define excursion limitsfor the damaged (?? line(s) missing) condition, (alarm set points and limiting excursion criteria, as defined by system overstress / riser safe operating limits, as applicable). These can be defined for the 1/10 year storm condition, but should include details of the limiting excursion for the field design (50/100 year) condition as a minimum.

Confirmation of design approval.

• Specify 'damage case' alarm set points (high / low tension alarms, inclinometer set points, turret position monitoring system excursion alarm set points);; • Define, for 'damaged case', the context of each set point in terms of the failure with which its initiation is associated (eg (i) low tension set point / inclinometer set point associated with weight of vertical chain of length equivalent to water depth at field location, (ii) high tension set point/ inclinometer set point equivalent to most probable maximum (MPM) load associated with design condition return period, in the damagedt [?specify?line] condition, / Rule [ref] compliant factored Minimum Breaking Load (MBL) (iii) turret excursion set point associated with riser safe operational limits / mooring line limiting condition); • Define field environmental / design 'damage case' conditions (eg 50/100 year storm predicted for specified location)..

Design 'damaged case' determination and assessment of results with regard to design code requirements

Class Rule requirements provide a suitable benchmark for assessing the adequacy of the mooring system in the 'damaged case' condition. Emergency response measures can include production shutdown, riser depressurisation/bullheading, downmanning, locking the turret within the moonpool, additional power brought online for thruster assist, adding ballast within the turret. Cross reference may be made to the elements described under F1.03

Description of 'damage case' emergency response / emergency operating procedures / alarms & setpoints and their assurance / verification

4.03 To continue to operate following defined fire and explosion events.

The passive mooring line / turret termination components of the mooring system 'shall be' positioned away from the likely sources of fire and explosion. Turret bearings are generally sufficiently robust and occluded from fire / explosion sources to survive these events for sufficient duration to facilitate mustering and evacuation of the crew.

Review / verification of emergency response drills. Visual inspection of mooring Position of turret bearings confirmed as being line/terminations and their removed from topsiidesfire/explosion hazards. proximity to likely fire and explosion hazards.

Visual inspection of turret bearing / turning facilties and their proximity to likely fire and explosion hazards.

Position of turret bearings confirmed as being as far from topsiidesfire/explosion hazards as practicable.

Ref Mooring Guidance chapter 4 Appendix 4 The passive mooring line / turret termination components of the mooring system are generally (but not universally) positioned away from the likely sources of fire and explosion. However, some mooring systems terminate within a turret space which is vulnerable to explosion / fire hazards from the adjacent turret hydrocarbon / fuild transfer facilities Certain turret turning and yaw breakout control equipment may be vulnerble to fire / explosion events from adjacent fluid transfer facilities. The sensitivity of the system as a whole to this should be considered. In some cases, active turret yaw control is critical to maintaining hull and mooring line motion / loading responses within acceptable limits.

In some cases, active turret yaw control is required for turret yaw breakout within safe operating limits and in these cases these systeme have been seen to be vulnerable to


4.04 To continue to operate at defined angles of vessel heel and trim corresponding to defined limiting hull damaged flotation and stability criteria 4.05 To continue to operate following defined power loss events (up to 'black ship' condition)

5

Example Performance Standard (Passive Mooring System) Acceptance Criteria Where fitted, stern thrusters are positioned away from topsides / external hazards. However, by virtue of their location, typically low in the stern of the vessel, they can be vulnrable to engine room fire and flooding events.

Assurance Visual inspection of stern thruster(s) and their proximity to likely fire, explosion and flooding hazards.

Verification Position of stern thruster(s) confirmed as being as far from topsiidesfire/explosion hazards as practicable.

Limiting floatation and stability criteria for hulls are normally defined under the hull performance standard and are typically based on IMO statutory regulations for all operatring conditions. Passive weathervaning mooring systems continue to operate with 100% functionailty in the 'black ship' condition.

Review of defined performance limits / design basis of turret bearing/turning / locking / centring systems in terms of defined hull limits for heel and trim.

Verify limiting operating angles of hell and trimwithin operations manual.

Consideration may be required for non-standard values of heel and trim for vessels with more novel geometries. (Possible reference to MODU code criteria).

Verification of continuing functionailty post 'black ship' shutdown.

Verify that vessel continues to yaw around turret post 'black ship' shutdown.

Some "passive" mooring systems may have an element that requires power, such as a mechanism for the release of friction in turret bearings.

Comments

TURRET / MOORING SYSTEM

Interacting Systems 5.01 Inteacting systems typically include: · Underwater flexible risers including the midwater arch and anchor blocks · Flexible hoses in the turret transfer system · Main and emergency hydraulic power ·

E P Emergency Power

Dependence/Interaction

Reference

Verification

Comments

Inspection of hoses and buoyancy system to ensure containment of process fluids. Inspection and replacement of hoses to ensure containment of process fluids. Energises the turret bearings and the turret yaw breakout system..

P id power tto th th Provides the tturrett the turret yaw breakout system

Some passive mooring designs include facilities which accommodate small yaw offsets between the hull and turret to minimise bearing wear, actively 'breaking out' the turret and returning it to its neutral heading at a defined small yaw offset (eg 5 degrees).

Appendix 3.2

Example Performance Standard (Heading Control System)

ABC123

Performance Standard Tag Reference Performance Standard/SCE Description

Role

Description

Ref 1

'FPSO/FSO/FPF MOORING SYSTEM/STATION KEEPING' Comments • Outline of main high-level safety crirical functions of the mooring system, including: • To maintain the vessel’s location within the excursion limits defined for the flexible pipeline risers • To allow the operator to select and maintain the vessel’s heading relative to the turret with respect to the weather (some passive systems) • To transmit environmental loads from the FPSO hull structure to the mooring system foundations and connect the subsea risers, power and signal cables to the topside facilities. Outline description of mooring system, including safety critical components and primary load path from mooring anchors/foundations and lines to FPSO hull i.e.: • Active / passive heading control; • Number of mooring legs (grouped / equal spread) & outline of leg components (chain / wire rope incl sizes); • Mooring line jewellery (joints eg shackles, deltaplates, kenters, spelter sockets), mooring termination , fairlead/gypsy wheels; • Turret terminations / spider; • Turret layout & bearing details (Upper / Lower turret roller / journal bearings / jacking systems & races, including turret turning/breakout management facilities (active/passive); • Heading Control Facilities.

Consideration can be given to including the design metocean condition, typically characterised by a significant wave height. A fuller statement of the worse case conditions including Hs, Hmax, Tp, wind speed and current speed, as determined by the design analysis to represent the worst case for design, would provide a ready reference in case of potential exceedance but may equally be included in other documentation, referred to in the Performance Standard. For some vessels the worst case condition for design is not the traditionally referred to “100 year storm” or other maxima, and may be for example a steep wave condition.

TURRET / MOORING SYSTEM FUNCTIONALITY

Function 1.01 To moor the turret within safe excursion envelope (e.g. riser operating limits / hull interactions etc)

Acceptance Criteria Define excursion limits, (alarm set points and limiting excursion criteria), as defined by system overstress / riser safe operating limits, as applicable.

Assurance There shall be effective continuuous monitoring of turret excursion from neutral point (eg DGPS, Hydro acoustic). Define unit specific required (critical) levels of accuracy.

Verification Demonstration of functioning mooring excursion monitoring system, within requisite levels of accuracy, cross referenced to other system data as available.

Comments For typical UKCS depths a benchmark of +/-2m is a recognised benchmark) Specify details of design basis, in terms of fundamental assumptions on critical mooring line / hull interactions and project phase assurance (eg designer assurance, ICP / Class Society verification / approval) Define field environmental / design conditions (eg 50/100 year storm predictted for specified location). Specify whether tension needs to be maintained in turret mooring lines to maintain position / line integrity. Describe critical tension limits to mitigate / prevent line / jewellery snatch loading. Mooring line tensions for the 1 / 10 year storm condition(s) may be specified, but details of the line tensions for the field design (50/100 year) condition should be included as a minimum requirement here. There should be a comparison made between the limiting mooring line performance / code compliance as certified and verified (model correlation), and the design storm condition derived projections for mooring line tension.


Example Performance Standard (Heading Control System) Acceptance Criteria

Assurance

1.02 To provide continuous assurance to vessel operators that the mooring lines are intact.

Ability to confirm that mooring Define means by which each mooring line's ongiong functional line(s) are still intact in acordance with Mooring Guidance section 6. performance is demonstrated to the vessel crew (e.g. mooring line inclinometers, mooring line tension monitoring alarms,) - ref Mooring Guidance section 6.

1.03 To provide adequate mooring from seabed to vessel and to transmit those loads into the vessel hull.

Specify the lowest allowable minimum breaking load (MBL) of the mooring legs / turret system as limited by the weakest leg / element (specify which) and its integration into the vessel hull structure and anchor point/foundations . Refer to mooring line component procurement assurance documentation (eg certification, describing performance compliance).

Defined strategy / schedule of inspection, operational maintenance and testing routines, derived from the risk review in section 5 of the Mooring Guidance and assurance / verification of same.

Verification Verification of intact mooring lines at requisite frequency - ref Mooring Guidance section 6.

Comments Primary means of demonstrating functional performance includes online tension monitoring, inclinomoters, proximity switches, hydro-acoustic systems and various others as described in Section 6 and Appendix 6.

• Describe means by which mooring functional performance is described (e.g. mooring tension repeaters / alarms); • Specify alarm set points (high / low tension alarms, inclinometer set points, turret position monitoring system excursion alarm set points);; • Define the context of each set point in terms of the failure with which its initiation is associated (eg (i) low tension set point / inclinometer set point associated with weight of vertical chain of length equivalent to water depth at field location, (ii) high tension set point/ inclinometer set point equivalent to most probable maximum (MPM) load associated with design condition return period, in the intact [??line] condition, / Rule [ref] compliant factored Minimum Breaking Load (MBL) (iii) turret excursion set point associated with riser safe operational limits / mooring line limiting condition); • Refer to safety critical system component maintenance regime (incl planned maintenance/testing routine tags) • Describe means by which crew's attention is drawn to system alarms / functional noncompliance (visual / audible alarms).

Results of mooring integrity assurance activities confirming ongoing conformance of mooring system features with specified acceptance criteria.

Clarify safe operating limits on each, mooring system component, including, where necessary, consideration of the following -Sensitivities of mooring components combinations and their interactions; - Certain mooring turret / bearing systems are particularly sensitive in terms of being maintained centrally withiin the moonpool bearing race for safe opetration / load transfer. The measures by which this issue is managed and assured should be descriobed. - Depending on the particular vessel / mooring geometries, turret bearing systems exhibit 'turret breakout', whereby brearing friction causes the turret to be twisted with the yawing hull until the mooring line tensions break the turret free, allowing it to return to its own neutral heading. In some systems, this phenomenon is actively controlled, whilst in others the response in entirely passive. The safety critical aspects of this phenomenon (eg turret break out yaw offset versus mooring chain / riser safe operating/contact limits) should be described and the means by which ongoing safe operations are assured referenced. Additionally, means by which the turret heading (as opposed to vessel heading) is advised to the crew should be described and its critical maintenance referenced.

- Some turret bearing use systems of jacks acting in horizontal and vertical directions.and can overstress the turret / bearing race, requiring control systems to prevent this. Thie safety critical aspects of this systems's performance and corresponding assurance should be described here. - Provision of hydraulic power to the turret turning/bearing system during normal / emergency operations may be safety critical and any safety critical aspects should be outlined here.

Appendix 3.2 Function 1.04 To maintain the vessel at a selected heading.

Example Performance Standard (Heading Control System) Acceptance Criteria Describe the safety critical functions of the unit's Heading Control System (HCS) as it supports the vessel, eg 'The vessel shall maintain a selected heading automatically. The vessel shall be able to change heading as selected by the crew.'

Assurance HCS Risk Assessment / FMEA documentation. Heading Control System safety critical maintenance and testing regime.

Verification

Comments A heading control system is usually fitted to installations with mooring systems whose safe Confirmation that safety critical maintenance operation is only possible when the mooring line tensions are maintained below defined and testing of the Heading Control System is critical values. Examples include systems where the turret is positioned toward amidships carried out according to the strategy/schedule derived from the HCS Risk Assessment / FMEA, rather than a bow/stern mounted turret/mooring spread and cases where a large installation is operating in a harsh environment coupled with shallow water depth. In most with no outstanding actions affecting the safety cases, the fitting of an HCS dictates a system redundancy equivalent to DP (AA) / NMD2. critical functionality. This in turn requires the development and verification of various Risk Assessment / FMEA documents, all of which should be described and referenced here. Finally the safety critical maintenance and testing regime need to be referenced to a level which satisfactorily clarifies the basic performance requirements of this system to both operator and verifier.

Also annual / biennial testing regime as specified under the Tandem Offloading Guidelines. Confirmation that inspection and safety critical maintenance / testing is carried out according to the strategy/schedule derived from the HCS Risk Assessment / FMEA, and where applicable, Classification requirements with no outstanding actions affecting the safety critical functionality.

Consideration should be given here to clearly describing the relationship between heading control performance and thruster performance. In most cases, HCS performance can only be maintained in a 1-5year storm condition when certain thrusters (i.e. those with larger turning moments around the turret) are on line.

Verification Verification that in-service data from mooring integrity assurance activities matches results that would be associated with a mooring system which continues to perform as designed, with no leading indicators of vectors associated with defined failure modes / degradation mechanisms.

Comments Accuracy of correletion with design limits (e.g. statstical means & std deviations) within 20% represents a suitable benchmark.

· Storm / exceedence of design Project / vendor supplied condition; certification for mooring system components. Post storm-event response measures.

No indications / records of environmental events which exceed design / manufacturing certification specified operating for mooring system. Verification of satisfactory execution of post storm-event response procedures.

Post storm-event response measures can be defined within mooring system through-life integrity management strategy for maintenance / assurance / verification.

·

No indications / records of environmental events which exceed design / manufacturing certification specified temperature limits

Strategy for thruster inspection, Describe the disposition and safety critical performance (power operational maintenance routines and / thrust and directional limitations) assurance. of each thuster.

2

TURRET / MOORING SYSTEM AVAILABILITY

Function 2.01 Verification of System Design Assumptions (To maintain functionality of mooring system for the installation design life of ?? years (specify)).

Acceptance Criteria The in-service mooring system performance data shall fall within the limiting envelope defined by the mooring system design assumptions:

Temperature;

Assurance Correlation of in service data (tensions, catenary profile, mooring offsets, hindcast data) with design assumptions (model tests, operational load offset curves) and confirmation that operational performance is within design defined limits.

Project / vendor supplied certification for mooring system components.

The limiting design condition (wind, wave, current etc) should be specified here.

Appendix 3.2


Function

Acceptance Criteria

Assurance 2.02 Verification of Verification of design, As a passive system, the construction, installation and Component Design availability of the mooring and Assumptions (To turret system is quantified in terms operational assurance processes: maintain mooring of maintenance of the turret / integrity for the FPSO system within a defined - Design and construction installation design life of radius up to and including the completed in accordance with ?? years (specify)). design storm condition in either the intact or damaged condition, recognised standards, defined for the duration of the deployment acceptance criteria. to the specified field location

Define limiting integrity vectors and describe system limiting sensitivity(ies) to vectors including: · Corrosion;

·

Fatigue;

Verification

Comments Operators may elect to put the detail of this section in a separate document as part of their Mooring Integrity Management System (Ref Section 8 of this Guidance).

Define mooring line component functional endurance characteristics and refer to relevant certification:

- Confirmation of performance within tolerable limits of design. '- Maintenance of mooring system components in accordance with criteria defined in mooring system integrity management strategy

Verification that in-service data from mooring integrity assurance activities matches results that would be associated with a mooring system which continues to perform as designed, with no leading indicators of vectors associated with defined failure modes / degradation mechanisms.

The mooring system integrity management strategy should define the means by which the continued effectiveness of this system is assured. It should describe the processes of identifying, assessing and where necessary repairing any deterioration due to corrosion, fatigue, wear, scour, damage, temperature, manufacturing/installation defects, creep or over-loading (ref Mooring Guidance section 5 & Appeondix 2)

Corrosion protection measures.

Confirmation of performance of corrosion protection measures.

Mooring component design and operational fatigue endurance assessment(s).

Mooring component fatigue endurances verifiied as exceeding ongoing operational and projected field deployment fatigue utilisations.

Define corrosion protection measures / assurance (eg material corrosion allowances, cathodic protection systems, coating systems); Consideration of all potential corrosion vectors should be included (e.g. recent examples highlighted of anerobic sulphide reducing corrosion in UKCS based mooring spreads at hitherto unexpectedly low temperatures) Outline the mooring system design fatigue endurance and its assessment / assurance. List assumptions implicit in the fatigue assessment process.

Operational utilisation conforms with assumptions implicit within the fatigue endurance assessment.

nb certain joining components such as kenters have considerably reduced fatigue lives when compared to common link.

· Wear (incl intergrip wear & wear with other items);

Chain wear allowances / certification and through life inspection of wear.

Confirmation that mooring system component dimensions continue to meet or exceed minimum allowable dimensions associated with safety critical performance.

·

Through life inspection of onbottom mooring system components.

On-bottom mooring system components found as per project defined limits for scour.

· Contact damage (incl other vessel operations / dropped objects);

Through life inspection (scheduled and post-event) of mooring system components.

Mooring system componets found within acceptable limits for damage.

Any additional limits (weather, time) associated with damage noted in way of mooring system components should be expediently assessed and consequences for continued safe operation of the mooring system defined.

· Manufacturing / installation defect limits

Mooring system project certification & through-life strategy for maintenance / assurance / verification;

Review of project / change management certification, confirming that manufacturing / installation procesess have not invalidated mooring system design basis/assumptions.

Consider potentially critical manufacturing defect limits for material tensile properties, fracture toughness properties, chemical properties, manufactiring / fabrication defect acceptance criteria);

Scour;

Appendix 3.2


Function

Acceptance Criteria

Assurance

·

Creep;


·

Axial stiffness;


·

Turret bearing friction limits

Through life measurement / determination of bearing friction.

· Mooring termination geometrical limits;

· Heading Control System failure;

· Mooring line snatch load / offload;

· Mooring chain/wire interactions wiith fairleads/wildcats/stoppers/moorin g line jewellery (tension-bending effects, fretting, galvanic corrosion etc); · Mooring spread imbalance / pre tension levels to optimise effects of mooring spread fatigue vs system fatigue endurance and assurance of same;

Verification

Review of through life mooring line inspection records, confirming no evidence of leading vectors w.r.t. creep. Review of through life mooring line inspection records, verifying acceptable operational utilisation of mooring system components w.r.t. defined limits for in-plane/out-of-plane bending.

Review of friction measurement activities / calculations, confirming confirmance to design assumptions (acceptable limits for turret bearing friction). Alarms / records pertaining to Confirmation that records show no relative relative displacements of hull / movements of the hull / turret / mooring turret / mooring terminations / terminations / mooring lines / risers which mooring lines / risers. exceed defined operational limits. HCS Design Basis & verification of Approval of HCS Design Basis, Risk same;; Assessment and FMEA documentation. Project phase commissioning FATs/HATs/SATs. Design features to mitigate snatch Review of mooring line tension monitoring load/oflload and in-service records / alarm event records to confirm that monitoring of these feautures. snatch load / offload has not occurred.

Through-life maintenance / inspection.

Through-life mooring system tension monitoring / balance optimisation measures, including maintenance and monitoring activities.

Review of mooring line inspection records, verifying that any instances of mooring line damage (e.g. cable strand failures, birdcging) remain within limits defined by code / mooring component manfucturer. Review of through-life maintenance / inspection records & confirmation that any instances of tension-bending effects, fretting, galvanic corrosion etc are captured and assessed against limiting criteria. Review of through-life mooring system tension monitoring / balance optimisation records, confirming that mooring line pre-tensions align with mooring spread requirements for optimum fatigue endurance.

Comments Consider potentially critical installation defect limits for mooring line pre-tension / balancing of mooring spread, on-bottom seabed conditions (eg boulders, debris), mooring foundation installation acceptance criteria, mooring line/turret/hull installation acceptance criteria.

Particularly relevant for moorings involving polyester or other fibre ropes.

Mooring system inetgrity management strategy should include consideration of the sensitivity of mooring system components' fatigue endurance, in terms of potential reductions arising from in-plane / out-of-plane bending.

Appendix 3.2 Function 3


Assurance

Verification

Comments

Assurance In practice, operational reliability is assured by the development and implementation of a mooring system inetgrity management strategy, which has considered and accounted for the safety critical vectors described within this performance standard and which defines a range of practically achievable schedule of inspection, maintenance and testing activities to assure the ongoing integrity of the (passive) system components. Reference should be made here to the means by which the vaildity and implementation of this strategy is verified.

Verification Review / survey of inspection and maintenance activities arising from implementation of mooring system inetgrity management strategy, confirming that these assurance activites are beig carried out at the requisite frequency, with corrective measures taken as and where necessary.

Comments Reference component risk assessment guidance and mooring system component failure modes in Mooring Guidance section 5 and Appendix 2).

Review / survey of turret bearing maintenance / assurance activities (grease sampling and assessment, bearing friction measurements).


Review / survey of integrity assurance activities in way of turret internal and moonpol structures and associated upper/lower bearing support structures, with results confirming ongoing structural integrity in accordance with defined acceptance criteria.


TURRET / MOORING SYSTEM RELIABILITY

Function 3.01 Passive elements to be 100% available for duration of field deployment.

Acceptance Criteria As a passive system, the reliability of the mooring and turret system is quantified in terms of maintenance of the turret / FPSO system within a defined radius up to and including the design storm condition for the duration of the deployment to eeh specified field location. This is normally assured by design and construction in accordance with recognised standards and maintenance of systems in accordance with recognised acceptance criteria, under which the continued effectiveness of this system relies on identifying, assessing and where necessary repairing any structural deterioration due to corrosion, fatigue or over-loading.

3.02 Transmit mooring loads Turret structural integrity and Mooring turret bearing to the FPSO hull bearing systems need to be 100& maintenance regime. structure. available

Through life inspection of turret internal and moonpol structures and associated upper/lower bearing systems

Turret /vessel friction control / yaw Turret breakout yaw-offset breakout: the active / passive testing/measurement/calculation breakout of the turret should occur (passive and, if applicable active) within defined safe yaw offset limits (define) with 100% availability on demand.

Review / witness of turret breakout yaw-offset testing/measurement activities, confirming that passive /active (as applicable) breakout occurs within defined safe limits.

The mooring system integrity management strategy should consider the consequences of high turret breakout offset (chain / fairlead / stopper contact, in-plane / out-of-plane tensionbending, shear) reference Mooring Guidance section 5 and Appendix 2.

Appendix 3.2 Function 3.03 Active system elements to be 100% available for duration of field deployment.

4

Example Performance Standard (Heading Control System) Acceptance Criteria The HCS and Turret (Bearing / Turning / Locking / Centring) Systems are active components required to ensure the safe operation of the installation. In terms of reliability, the HCS should be designed and installed in accordance with the recommendations of the project phase FMEA and Risk Assessment documentation. The HCS mechanical, electrical and control system components and position referencing systems data inputs should be duplicated such that no loss of the HCS should arise from a fault or damage in any single active component up to loss of a single compartment from fire or flooding (equivalent to DP(AA) / NMD2. Test and maintenance procedures should be prioritised accordingly within the maintenance management system

Assurance Heading Control System safety critical maintenance and testing regime.

Verification

Comments

Confirmation that safety critical maintenance and testing of the Heading Control System is carried out according to the strategy/schedule derived from the HCS Risk Assessment / FMEA, with no outstanding actions affecting the safety critical functionality. Review of safety critical maintenance and testing and confirmation of no leading indicators / trends affecting system'd future availabilty

TURRET / MOORING SYSTEM SURVIVABILITY

Function 4.01 To maintain the vessel on location as defined under the functionailty requirements outlined above for the 'intact case' condition with an 4.02 To continue to maintain the vessel on location as defined under the functionailty requirements outlined above, with an adequate margin of safety

Acceptance Criteria As defined under F101, F102, F103 and A201 above

Assurance Design 'intact case' determination and assessment of results with regard to design code requirements

Verification Confirmation of design approval.

Comments Class Rule requirements provide a suitable benchmark for assessing the adequacy of the mooring system in the 'intact case' condition.

Moor the turret within safe excursion envelope (e.g. riser operating limits / hull interactions etc) for the defined damaged condition:

Outline of design basis (National / international codes, classification rules)

Confirmation that the continuuous monitoring of the turret excursion from neutral point (eg DGPS, Hydro acoustic) should continue to operate in the defined 'damaged case' condition.

Ref Mooring Guidance chapter 4 Appendix 4

*** Repeat sections 1.01, 1.02, 1.03 and 2.01 for defined damaged condition , confirming the validity of each acceptance criteria in turn and revising where appropriate. Define excursion limitsfor the damaged (?? line(s) missing) condition, (alarm set points and limiting excursion criteria, as defined by system overstress / riser safe operating limits, as applicable). These can be defined for the 1/10 year storm condition, but should include details of the limiting excursion for

Design 'damaged case' determination and assessment of results with regard to design code requirements

Confirmation of design approval.

• Specify 'damage case' alarm set points (high / low tension alarms, inclinometer set points, turret position monitoring system excursion alarm set points);; • Define, for 'damaged case', the context of each set point in terms of the failure with which its initiation is associated (eg (i) low tension set point / inclinometer set point associated with weight of vertical chain of length equivalent to water depth at field location, (ii) high tension set point/ inclinometer set point equivalent to most probable maximum (MPM) load associated with design condition return period, in the damagedt [?specify?line] condition, / Rule [ref] compliant factored Minimum Breaking Load (MBL) (iii) turret excursion set point associated with riser safe operational limits / mooring line limiting condition); • Define field environmental / design 'damage case' conditions (eg 50/100 year storm predicted for specified location)..



Assurance

Verification

Description of 'damage case' emergency response / emergency operating procedures / alarms & setpoints and their assurance / verification

Ref Mooring Guidance chapter 4 Appendix 4

Review / verification of emergency response drills.

4.03 To continue to operate following defined fire and explosion events.

The passive mooring line / turret termination components of the mooring system 'shall be ' positioned away from the likely sources of fire and explosion. Turret bearings are generally sufficiently robust and occluded from fire / explosion sources to survive these events for sufficient duration to facilitate mustering and evacuation of the crew.

Where fitted, stern thrusters are positioned away from topsides / external hazards. However, by virtue of their location, typically low in the stern of the vessel, they can be vulnrable to engine room fire and flooding events. For active heading control based moornig systems, survivability requirements are normally achieved by diverse positioning of the thruster systems along the length of the hull. Additionally, control systems are positioned away from the most likely and severe fire / explosion hazards and are connected to the thrusters / position sensors via diverse

Comments Class Rule requirements provide a suitable benchmark for assessing the adequacy of the mooring system in the 'damaged case' condition. Emergency response measures can include production shutdown, riser depressurisation/bullheading, downmanning, locking the turret within the moonpool, additional power brought online for thruster assist, adding ballast within the turret. Cross reference may be made to the elements described under F1.03

Visual inspection of mooring line/terminations and their proximity to likely fire and explosion hazards.

Position of turret bearings confirmed as being removed from topsiidesfire/explosion hazards.

Visual inspection of turret bearing / turning facilties and their proximity to likely fire and explosion hazards.

Position of turret bearings confirmed as being as far from topsiidesfire/explosion hazards as practicable.

Visual inspection of stern thruster(s) and their proximity to likely fire, explosion and flooding hazards.

Position of stern thruster(s) confirmed as being as far from topsiidesfire/explosion hazards as practicable.

Visual inspection of thruster disposition / machinery rooms and HCS control hardware / cabling.

Confirmation of diverse positioning of the thruster systems along the length of the hull. Verification that control systems are positioned away from the most likely and severe fire / explosion hazards and are connected to the thrusters / position sensors via diverse paths

The passive mooring line / turret termination components of the mooring system are generally (but not universally) positioned away from the likely sources of fire and explosion. However, some mooring systems terminate within a turret space which is vulnerable to explosion / fire hazards from the adjacent turret hydrocarbon / fuild transfer facilities Certain turret turning and yaw breakout control equipment may be vulnerble to fire / explosion events from adjacent fluid transfer facilities. The sensitivity of the system as a whole to this should be considered. In some cases, active turret yaw control is critical to maintaining hull and mooring line motion / loading responses within acceptable limits.

In some cases, active turret yaw control is required for turret yaw breakout within safe operating limits and in these cases, these systeme have been seen to be vulnerable to adjacent fire / explosion / dropped object hazards, resulting in the mooring system being overstressed

For active heading control based moornig systems, thrusters are typically positioned fore and aft and therefore separated by distance. With this disposition, a single event damaging all thrusters is unlikely. Similarly, power supplies to and motors driving the thrusters are housed in machinery spaces, which are themselves bounded by fire rated boundaries.


4.04 To continue to operate at defined angles of vessel heel and trim corresponding to defined limiting hull damaged flotation and stability criteria 4.05 To continue to operate following defined power loss events (up to 'black ship' condition)

Example Performance Standard (Heading Control System) Acceptance Criteria Turret turning and locking systems, positioning systems within moonpool and bearing overload protection systems are vulnerable to adjacent fire and explosion hazards. However, turret bearings should be sufficiently robust to withstand initial fire/exploson event and facilitate passive yaw break out for the time required to muster and evacuate the vessel crew. Limiting floatation and stability criteria for hulls are normally defined under the hull performance standard and are typically based on IMO statutory regulations for all operatring conditions. Passive weathervaning mooring systems continue to operate with 100% functionailty in the 'black ship' condition. Active HCS and DP assisted mooring systems may be vulnerable to significant and in some cases safety critical losses of performance under 'black ship' scenarios. This is normally mitigated by requiring levels of redundancy and diverse sourcing associated with DP(AA) / NMD2 notations, as outlined under the availabilty / reliability requirements Turret turning and locking systems, positioning systems within moonpool and bearing overload protection systems will be powered by batteries post black ship shutdown. Hydraulic accumulators will provide turret turning & centralizing / locking facilities for a limited time. After this time period, the turret bearings should still be sufficiently robust to withstand initial fire/exploson event and facilitate passive yaw break out for the time required to muster and evacuate the vessel crew.

Assurance

Verification

Maintenance regime in way of the turret bearing systems. Friction measurement activities / calculations.

Review of friction measurement activities / calculations, confirming confirmance to design assumptions (acceptable limits for turret bearing friction). Confirmation that bearing maintenance regime is being followed in accordance with schedule / strategy derived from identification and assessment of bearing failure modes.

Review of defined performance limits / design basis of turret bearing/turning / locking / centring systems in terms of defined hull limits for heel and trim.

Verify limiting operating angles of hell and trimwithin operations manual.

Verification of continuing functionailty post 'black ship' shutdown.

Verify that vessel continues to yaw around turret post 'black ship' shutdown.

Assurance defined under he availabilty / reliability requirements above

Confirmation that safety critical maintenance and testing of the Heading Control System is carried out according to the strategy/schedule derived from the HCS Risk Assessment / FMEA, with no outstanding actions affecting the safety critical functionality.

Maintenance regime in way of the turret bearing systems. Friction measurement activities / calculations. Maintenance regime in way of the turret bearing turning / locking positioning hydraulics systems

Review of friction measurement activities / calculations, confirming confirmance to design assumptions (acceptable limits for turret bearing friction). Confirmation that bearing maintenance regime is being followed in accordance with schedule / strategy derived from identification and assessment of bearing failure modes.

Comments

Consideration may be required for non-standard values of heel and trim for vessels with more novel geometries. (Possible reference to MODU code criteria).

Appendix 3.2


Function 5

Acceptance Criteria

Assurance

Verification

Comments

Dependence/Interaction

Reference

Verification

Comments

TURRET / MOORING SYSTEM

Interacting Systems 5.01 Inteacting systems typically include: · Underwater flexible risers including the midwater arch and anchor · Flexible hoses in the turret transfer system

Inspection of hoses and buoyancy system to ensure containment of process fluids. Inspection and replacement of hoses to ensure containment of process fluids. · Main and emergency Energises the turret bearings and hydraulic power the turret turning system grippers and jacks. · Emergency Power Provides power to the turret turning emergency powerpack and aft thruster. · Shuttle Tanker & Provides heading and positioning Position Keeping referencing to HCS · UPS Provides power for HCS, gyros and computer system.

The safety critical interrelationships between the HCS, DGPS and DARPS systems should be described.


Appendix 4

Example procedure in the event of a mooring line failure Detection of Mooring System Failure

Immediately detected failure

No

Weather forecast

Yes

If appropriate

Consider

Adjust mooring line tensions

Connect towing vessel

Initiate GPA

Moving off position

No

Monitor Position

Increased loading of intact mooring lines Yes

Stable

Operate propulsion

Identify Failure Mode

Downmanning

Immediate production shutdown

Unnacceptable

Risk of sequential line failure ? Acceptable

Controlled Production Shutdown

No

Real-time monitoring of mooring lines Yes

Consider flowline flushing

Repair / Replace

Continue monitoring of remaining moorings


Appendix 5

Example mooring risk assessment process

REF : XXX-YYY-ZZ-0001 REVISION : ? PAGE : 1 OF 29

MOORING INTEGRITY REVIEW REPORT

CONTENTS

1.0 2.0 3.0 4.0 5.0 6.0

INTRODUCTION ...................................................................................................................... 2 SCOPE ..................................................................................................................................... 2 PROCESS ................................................................................................................................ 3 RESULTS ................................................................................................................................. 6 ACTIONS ................................................................................................................................. 9 DETAILED RISK REVIEW SHEETS ...................................................................................... 12

A

Jun 07

After multi-discipline meeting

IDW

REV

DATE

DESCRIPTION

BY

CHKD

APPVD



1.0 INTRODUCTION Part of the verification activities for the FPSO is to undertake a Mooring Integrity Risk Review (MIRR). This is a systematic process looking at individual components of a mooring system and reviewing their performance against a list of potential failure modes. The process carried out as reported in this Report recognises the gap in identifying mooring system risks as highlighted by such organisations as UKOOA (United Kingdom Offshore Operators Assocation), recently re-badged as UK Oil and Gas.

2.0 SCOPE This MIRR considers all mooring components from the attachment point on FPSO “Example” to the anchors. There are some resulting actions from this MIRR which are taken forward to other processes. FPSO Example is spread moored to 12 lines consisting of chain near the FPSO, held in stoppers, then a length of polyester rope, and then a chain tail down to the seabed and a drag anchor. The following sketch indicates the arrangement of the line.

Stopper Vertical chain

Fairlead Upper H shackle Upper chain Polyester rope

Buoy D shackles Lower H shackle

Lower chain

Anchor



3.0 PROCESS The process is a semi-quantitative one, with an initial study carried out by a person familiar with mooring component issues, and then a session with a number of varied discipline personnel from the principal parties in the project. The first step is to break the mooring system into relevant components much as is done in choosing “nodes” for a Hazop; in doing this the following has been taken into account: Is there metocean directionality so that not all mooring legs can be assumed to behave in the same way ? For example do the strengths of wind, waves and current, mean different vessel behaviour on different headings ? This is not as simple as assuming the worst case conditions, since some mooring failures have occurred on the leeward, or less heavily laden, mooring legs. Are there any parts of a potential “component” that may be subject to a significantly different set of hazards ? Will the failure of all parts of the “component” lead to the same consequence ? Once the component choice is made, a list of hazards has to be considered. The following is the list used in this study which has been taken from a combination of previous industry sources, principally the Mooring Integrity JIP, plus additional experiences as collected by UKOOA. The following is the list of hazards used for the Example mooring system:

Strength

Factor of Safety Snatch load Means of securing Wear/erosion With connected item With other item Seabed Fatigue Not proof loaded Axial Bending Torsion Corrosion Galvanic Bilogoical/SRB Chemicals Hydrogen embrittlement Contact With offtake tanker With supply vessel With fishing vsl/gear Dropped object From FPSO From offtake From supply vessel During inspection Motion Non-design of attachment Seabed scouring Manufacture defect Deployment



x

x

jack

ower D

x x g9 x

x

Fairle ad

x

g7 g9 x

Chain

g7

Buoy

or

Strength

Uppe r&L

g1

Anch

g2 g2 x g4

End li nk

g7

r cha in

in H

g7 g9 x

Lowe

Link

rH

eyes bles &

Polye ster

Thim

Uppe rH

on fa irlead Uppe r cha in Link in H

Lowe

Factor of Safety Snatch load Means of securing Wear/erosion With connected item With other item Seabed Fatigue Not proof loaded Axial Bending Corrosion Galvanic Bilogoical/SRB Chemicals Hydrogen embrittlement Contact With offtake tanker With supply vessel With fishing vsl/gear Dropped object From FPSO From offtake From supply vessel During inspection Motion Non-design of attachment Seabed scouring Manufacture defect Deployment What if

Link

Failure mode

Verti

Link

in jaw

s

cal ch ain

A matrix of which hazards are considered potentially applicable to each component is then derived. The matrix used for this Example MIRR is as shown below:

x

x

z x

x x g1

g1

g1

g1

x g4

g1 x g4

g2 g2 g4

g3

g4

x g4 g5

g6

g1 x x

x x g4 g5

g5

g6 x

x x x x x x

x x

x x

x

x

x

x

x x x x

x

g8 x

g8 x

x

x

x

Any entry in the matrix indicates that a failure mechanism is considered to exist. A “g*” entry is a potential hazard but with a common mechanism to other components and so considered together. For each matrix entry a Risk Review has then been undertaken in a traditional way of consequence and probability and recorded on a sheet; the following is one example: Failure mode 1: Explanation:

Contact with supply vessel If the supply vessel comes too close it could make heavy contact with the chain, without grounding on the bilge keel etc. Existing safeguards are the standard practice of stand-off and a long reach crane; and that the normal handling position, stbd side of the platform, is between mooring legs.

Consequence score:

2

Probability: Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unnacceptably risky or unreasonable cost ? Probability score: Medium Risk Rating:

Make Alarp

Additional safeguards to consider: 1 Discuss with Asset Supervisors and Marine CROs 2

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 2 2

2

1

0

0 5



Consequences have been taken from the following simple table: Consequence Safety Potential common cause leading to sequential failure Failure of one leg (immediate or latent) Degradation of Factor of Safety Minor

Score and/or

Cost > ~10 million > ~ 1 million ~100k 0

4 3 2 1

And as can be seen on the above example Worksheet, the probabilities have been taken from the following straightforward questions: Probability (Yes / No questions) Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unnacceptably risky or unreasonable cost ?

Yes 2 2 2

Partial 1 1 1

No 0 0 0

2

1

0

These probability answer scores are then summed and graded into High, Medium and Low: Probability scoring: High Medium Low

7 2 0

8 6 1

The combination of Consequence and Probability is then used to arrive at the Risk Rating as per the following table: Overall risk rating:

Consequence

1 2 3 4

L Acceptable Acceptable Acceptable Make Alarp

Probability M Acceptable Make Alarp Make Alarp Make Alarp

H Acceptable Make Alarp Make Alarp Intolerable

Where an entry falls into the “Alarp” region, further considerations are taken, and these are then listed as Actions. The Actions are taken forward to be closed out. Where an entry falls into the “Intolerable” region these are taken forward to further engineering in order to mitigate against the failure modes and to reduce the Risk Ratings.



4.0 RESULTS The completed matrix on the next page is the output from the above MIRR process carried out on FPSO Example. Many of these matrix entries fall into the Alarp area and a number of actions result. These actions are listed in the following section. These are not design changes but the method and frequency of inspection. These will be taken forward to the Mooring Integrity Management System, which will be completed in line with the overall Installation Safety Management System. A few of the entries fall into the Intolerable range, and these have been taken forward for further engineering, ie design changes made. These are explained below.

Factor of Safety Snatch load Means of securing Wear/erosion With connected item With other item Seabed Fatigue Not proof loaded Axial Bending Corrosion Galvanic Bilogoical/SRB Chemicals Hydrogen embrittlement Contact With offtake tanker With supply vessel With fishing vsl/gear Dropped object From FPSO From offtake From supply vessel During inspection Motion Non-design of attachment Seabed scouring Manufacture defect Deployment What if

Strength

Failure mode

g1

x

g1

g6

x g4

g1

x

Link in jaw s Verti cal ch ain Link on fa irlead

x

x x x x

g6 x

x x

g8 x x

g4

g2 g2

x x

x g4

g1

x x

g4

Polye ster

x

x

g3

g8 x

x

g2 g2 x g4

g7 g9 x

Link in H

x

x g4

g1

x

g7

x

x

g5

x

g5

Anch or

x

g5

x

x x g4

x x g9 x

Uppe r&

Lowe rD

x

x

g1 x x

z x

Buoy

eyes Thim bles &

x

rH Lowe

r ch a in Lowe

jack

x

Chain

hain Uppe rc

g1

x

Uppe rH

g7 g9 x

ink End l

in H Link

g7

Fairle ad

x

MOORING INTEGRITY REVIEW REPORT REF : XXX-YYY-ZZ-0001 REVISION : ? PAGE : 7 OF 29



The “Intolerable” outcomes are all on the Lower H shackle and its attached shackles to the buoy. The original design, shown here:

The polyester mooring rope towards the FPSO is around the spool on the right of this image, shown in translucent pink; the first link of the chain to the seabed is shown to the left, and the bottom padeye on the steel shell buoy is attached into the upper small shackle. As a result of these findings the design of the H shackle and its attachment to the lower D shackle, and the lower D shackle design, have been amended as shown in the 2D drawing below:

This clamp arrangement increases the lever arm to avoid side loads occurring on the lower D shackle; it removes stresses from the D shackle, and increases the strength of the lower D shackle by using one with a lower jaw gap. The H shackle arrangement itself is a proven item in a number of other mooring systems.



5.0 ACTIONS The following two pages show the complete list of actions (total of 27) from the MIRR. Where the same action applies to several pieces of equipment these are grouped together. These Action sheets are a living document and will be updated from time to time. A final revision of this Report will be issued when all actions are closed out.

19

18

17

16

15

14

13

12

11

10

9

8

7

5 6

Upper chain

General

H shackles

General

Determine relative hardnesses of chain and fairlead; Have an operational procedure to move the chain from time to time If there is any way to ensure the semi-held link is a vertical link. Ensure a different link is on the fairlead after a suitable interval. Link on fairlead

Link on fairlead

Link on fairlead

Link on fairlead

General Thimbles & eyespolyester rope Lower H shackle Consider if the H shackles should have full UT H shackles inspection. Consider if a third party witness would be H shackles prudent. Use of bolts with nyloc nuts instead of split H & D shackles pins Use of an additional safety bolt through the H & D shackles nut. Ensure inspection regime includes this area Chain link in chain jack jaws Discuss with Asset Supervisors and Marine Vertical chain CROs Upper chain

Insulate uninspectable connections (anchor; safety pins) Check locations of impressed current system anodes relative to chains. Carry out dynamic analysis

FEA to determine SCF and make a realistic H shackles appraisal of fatigue life Confirm manufacturing testing regime Polyester Different material grades should be reviewed. General

4

3

2

General

Ensure inspection regime includes all areas inspected. Discuss this with Manufacturer so that proof load is held for 3 or 4 seconds Proof load H shackles

1

Applicable equipment

Action

Ref

Bending fatigue

Bending fatigue

Wear with other item

Wear with other item

Dropped object (from FPSO, supply vessel, tanker, inspection crew)

Contact with supply vessel

Wear with connected item

Shackle pin means of securing

Shackle pin means of securing

Manufacturing defect in H shackles

Wear with connected item Manufacturing defect in H shackles

Snatch load of shackles etc Wear with connected item

Hydrogen embrittlement

Galvanic corrosion

H shackles axial fatigue and not proof loaded H shackles axial fatigue and not proof loaded Polyester rope not proof loaded Galvanic corrosion

Chain not properly proof loaded

Chain not properly proof loaded

Failure mode

Close out action

Closed ?

By/when?


26 27

25

24

23

22

21

Hardness tests to be specified for Manufacturer Put a V shape in the end of the H shackle, instead of parallel sided Use an end link instead of a common link

20

Applicable equipment

Upper H shackle Lower H shackle Manufacturer to adjust heat treatment to match Upper H shackle hardnesses Assumes that seabed survey shows no debris, Lower (ground) chain but some sandy soil Ensure correct gypsy is sourced Lower (ground) chain Identify the optimum method for deployment, Buoy, specifically padeye for integrity and cost benefit

Chain link in Upper H shackle Chain link in Upper H shackle Chain link in Upper H shackle Consider overboarding shackles by crane or in Chain link in Upper H their own launch frame so pins are horizontal. shackle

Action

Ref

Deployment Deployment

Wear due to seabed

Deployment Deployment Wear with connected item

Deployment

Bending of link

Bending of link

Wear with connected item

Failure mode

Close out action

Closed ?

By/when?



6.0 DETAILED RISK REVIEW SHEETS




Item:

General issues

Failure mode 1: Explanation:

Chain not properly proof loaded More than one study has shown that proof loading increases the fatigue life of chain. The Manufacturer's standard equipment spikes the tension but it has been demonstrated that not all links see the load. This specific link has abnormal loading due to being held at the shoulders, but the same failure mode could exist on a number of other components.

Consequence score:

3

Probability: Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unacceptably risky or unreasonable cost ? Probability score: Medium Risk Rating:

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 0

2

1

0

0 4

Make Alarp

Additional safeguards to consider: 1 Ensure inspection regime includes all areas inspected. 2 Discuss this with Manufacturer so that proof load is held for 3 or 4 seconds


H shackles axial fatigue and not proof loaded The level of oversizing of these shackles is not known; if sufficiently high they may not need proof loading. However this is felt unlikely as a pin through a common link cannot be twice the chain mbl. This entry is to confirm that they will be proof loaded to at least the same as the chain.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

2 6

Make Alarp

Additional safeguards to consider: 1 Proof load H shackles 2 FEA to determine SCF and make a realistic appraisal of fatigue life


Polyester rope not proof loaded A break load of a polyester rope is rarely carried out as the released energy can destroy the test bed. This entry is to confirm whether the strength testing is just yarn, sub-ropes or the whole rope, with each batch of raw yarn material.

Consequence score:

4


Make Alarp

Additional safeguards to consider: 1 Confirm manufacturing testing regime 2

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6




Galvanic corrosion Different materials are used at different points. Combinations are chain/chain R5/R4; bolt/safety pin R4/AISI316; chain/fairlead R5/A487C; shackle/anchor R5/GS25CrNo5; shackle/buoy R4/S355J2G3. It is not known of the potential of all these are sufficiently comparable in seawater. Anchor connection is not inspectable. Split pins have little material to corrode.

Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

2 6

Make Alarp

Additional safeguards to consider: 1 Different material grades should be reviewed. 2 Insulate uninspectable connections (anchor; safety pins)


Biological corrosion A few fields have experienced bacteriological corrosion of chains and fittings when lying in the seabed, including to chain failure. This part of the North Sea has no known record of this behavious but this point is raised to ensure it is considered.

Consequence score:

3

Probability: Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unacceptably risky or unreasonable cost ? Probability score: Low Risk Rating:

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 0 0

2

1

0

0 1

Acceptable

Additional safeguards to consider: 1 2


Hydrogen embrittlement If chain material is subject to very high voltages it is currently believed that hydrogen embrittlement can occur; this is presently a study by Manufacturer. The components would have to be close to an impressed current anode on the hull. This issue is raised to check the locations of the Impressed Current system anodes.

Consequence score:

3

Probability: Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unacceptably risky or unreasonable cost ? Probability score: High Risk Rating:

Make Alarp

Additional safeguards to consider: 1 Check locations of impressed current system anodes relative to chains. 2

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8




Snatch load of shackles etc Because the H shackles represent a significant change of weight per metre, dynamic motions may be concentrated at these points.

Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6

Make Alarp

Additional safeguards to consider: 1 Carry out dynamic analysis 2


Manufacturing defect in H shackles All chain and D shackles are given MPI and UT. This issue is raised as it is not known if full thickness UT going to be given to the H shackles, rather than just surface NDT.

Consequence score:

3

Probability: Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unnacceptably risky or unreasonable cost ? Probability score: High Risk Rating:

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8

Make Alarp

Additional safeguards to consider: 1 Consider if the H shackles should have full UT inspection. 2 Consider if a third party witness would be prudent.


Shackle pin means of securing It is a Class requirement for long term mooring shackles to have two means of being secured. This is interpreted by some to mean the main nut and one safety pin or bolt; by others that the main nut does not count. This issue is raised to ensure it is considered. This has a relationship with potential galvanic corrosion. There has been a recent incident.

Consequence score:

4


Make Alarp

Additional safeguards to consider: 1 Use of bolts with nyloc nuts instead of split pins 2 Use of an additional safety bolt through the nut.

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6



Item:

Chain link in chain jack jaws


Wear with connected item The shoulders of one link are sitting on two plates; these are flat but have straightedged angled supports. These flat pieces of metal will cause plastic deformation of the link at the contact points.

Consequence score:

3

Is inspection at appropriate interval technically infeasible, unnacceptably risky or Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unnacceptably risky or unreasonable cost ? Probability score: Medium Risk Rating:

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 0 0

2

1

0

0 2

Make Alarp

Additional safeguards to consider: 1 Ensure inspection regime includes this area 2

Item:

Vertical chain


Contact with supply vessel If the supply vessel comes too close it could make heavy contact with the chain, without grounding on the bilge keel etc. Existing safeguards are the standard practice of stand-off and a long reach crane; and that the normal handling position, stbd side of the platform, is between mooring legs.

Consequence score:

2


Make Alarp


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 2 2

2

1

0

0 5



Item:

Link on fairlead


Wear with other item The same chain link will not be on the fairlead at all times as the mooring is adjusted for polyester creep. In the early life of the polyester no chain link will therefore stay on the fairlead, but after a year or so one link may stay for a considerable time. Dynamic motions of the outboad line could cause wear.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6

Make Alarp

Additional safeguards to consider: 1 Determine relative hardnesses of chain and fairlead; 2 Have an operational procedure to move the chain from time to time


Bending fatigue On a pocketed gypsy as this, one link is held, the other moves with the catenary. Experience shows that this movement need not be much, and even interlink friction is sufficient, to fatigue a chain link in bending.

Consequence score:

4


Make Alarp

Additional safeguards to consider: 1 If there is any way to ensure the semi-held link is a vertical link. 2 Ensure a different link is on the fairlead after a suitable interval.

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6



Item:

Upper chain


Contact with offtake tanker The tanker range is inside the upper chain radius, and contact between the hull and the chain could damage the chain, or cause a defect resulting in a future failure. Hand calc engineering has been done to keep the chains low enough; the result was to increase the chain length and thus make the catenary slightly lower at the FPSO end.

Consequence score:

3

Probability: Has the problem been known to occur in the industry before ? Does realistic engineering fail to rule out possible lack of integrity ? Can a realistic mechanism or sequence be envisaged ? Is inspection at appropriate interval technically infeasible, unnacceptably risky or unreasonable cost ? Probability score: Low Risk Rating:

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 0 1

2

1

0

0 1

Acceptable



Dropped object (from FPSO, supply vessel, tanker, inspection crew) A dropped container could impair one or two chains. Existing safeguard is that procedure is such that lifts are not normally done above the moorings.

Consequence score:

3


Make Alarp


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

0 4



Item:

Chain link in Upper H shackle


Wear with connected item The relative hardness of the R5 pin, because it is machined, will likely be different to that of the R5 chain which has not been machined after heat treatment. One or the other may wear with regular movements.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 2 2

2

1

0

0 5

Make Alarp

Additional safeguards to consider: 1 Hardness tests to be specified for Manufacturer 2


Bending of link Because of the weight discontinuity at the H shackle, the shackle and hence its attached link, may take large motions and cause bending of the attached link. This could be either in the vertical or horizontal direction as the orientation of the H shackle is not controlled.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6

Make Alarp

Additional safeguards to consider: 1 Put a V shape in the end of the H shackle, instead of parallel sided 2 Use an end link instead of a common link


Dropped object (from offtake tanker or supply vessel) Relatively small objects may get dropped off the vessels during their normal operation.

Consequence score:

1


Acceptable


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 0 0

2

1

0

0 0




Deployment If the H shackle pins were deployed over a stern roller with the pins vertical, the link in that pin may have the weight of the already deployed equipment on it and receive a high bending stress.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

0

Make Alarp

Additional safeguards to consider: 1 Consider overboarding shackles by crane or in their own launch frame so pins are horizontal. 2

4



Item:

Upper H shackle



Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 2 2

2

1

0

0 5

Make Alarp

Additional safeguards to consider: 1 Manufacturer to adjust heat treatment to match hardnesses 2


Dropped object Relatively small objects may get dropped off the vessels during their normal operation.

Consequence score:

1


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1

Acceptable



Deployment If the H shackle pins were deployed over a stern roller with the pins vertical, the link in that pin may have the weight of the already deployed equipment on it and the shackle arm would receive high stress.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

0

Make Alarp


4



Item:

Thimbles & eyespolyester rope


Wear with connected item The thimble may wear due to movement on the H shackle pin. The rope may make contact with the back of H shackle recess of the dynamic motions of the shackle are so violent to achieve this.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 2 2

2

1

0

0 5

Make Alarp

Additional safeguards to consider: 1 Dynamic analysis of this part of the mooring 2


Dropped object Relatively small objects may get dropped off the vessels during their normal operation.

Consequence score:

1


Acceptable


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1



Item:

Polyester Rope


Contact with fishing gear A trawl wire could easily pass over these ropes as they are beyond the 500m exclusion zone. Some tests have already been done and a different outer sheath is being used which shows much better wear properties.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6

Make Alarp



Dropped object during inspection The upper part of the ropes will be inspected by small ROV from the FPSO; the outer part from a large ROV from a DSV. The latter vessel could drop an object during an overside lift, but this is not considered would be large.

Consequence score:

2


Acceptable


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1



Item:

Lower H shackle



Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 1 2 2

2

1

0

0 5

Make Alarp

Additional safeguards to consider: 1 Perform dynamic analysis of this part of the mooring 2


Fatigue in bending The centre part of the H shackle is 170mm x 425mm and is a lot of steel. It has one 72mm hole (but see also Lower D shackle issues). This part may or may not be upright in operation. Dynamic motions of the chain below, polyester rope above, and buoy, may cause frequent bending across this part.

Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

2 6

Make Alarp

Additional safeguards to consider: 1 Carry out FEA to determine SCF for realistic load cases 2


Contact with fishing gear A trawl wire could easily pass over the polyester ropes and slide down to the shackle. However due to its size the risk of damage is seen as slight. Subject to the orientation of the shackle the main pin securing means could be damaged.

Consequence score:

2


Acceptable


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1




Deployment If the H shackle pins were deployed over a stern roller with the pins vertical, the link in that pin may have the weight of the already deployed equipment on it and the shackle arm would receive high stress.

Consequence score:

3


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 2 2

2

1

0

0

Make Alarp


4



Item:

Lower (ground) chain


Wear due to seabed The nature of the seabed can affect the amount of interlink wear. Rocks (or other debris) can cause damage to chains.

Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

0 6

Make Alarp

Additional safeguards to consider: 1 Assumes that seabed survey shows no debris, but some sandy soil 2 Probably is Alarp


Contact with fishing gear A trawl wire could easily pass over the ground chain, ie outboard of the buoy; the chain could thus be hit by a trawl board. Even such an impact however is not envisaged would cause serious damage.

Consequence score:

2


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1

Acceptable



Deployment This chain is 125mm nominal bar diameter, but 7d long. A different gypsy for deployment is necessary compared to the standard 120mm chain if any weight at all is going to be on the chain during deployment, for the sake of the chain slipping and damages to the surface, but also in case it runs off the deployment vessel.

Consequence score:

2


Make Alarp

Additional safeguards to consider: 1 Ensure correct gypsy is sourced 2

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 0 2

2

1

0

2 6



Item:

Lower D shackle (H shackle to upper D)


Insufficient strength, possible snatch load This is a mbl of circa 275te, with a proof load expected of circa 110te. The dynamic motions of the buoy may lead to snatch loadings on the shackle. The H shackle, hence the D shackle, may not always be upright, and thus these loadings will be on the side of the D.

Consequence score:

4

Rope lies on seabed, not allowable long term, and unbalanced load share


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8

Intolerable

Additional safeguards to consider: 1 Increase strength of the D shackle 2 Provide more lever arm to keep D shackle vertical 3 Reduce inspection intervals until confidence that shackle remains upright Failure mode 2: Explanation:

Wear of connected item The D shackle pin is R4; the H shackle R5. Some wear can be expected of the shackle pin.

Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8

Intolerable

Additional safeguards to consider: 1 Increase strength of the D shackle 2 Provide more lever arm to keep D shackle vertical 3 Reduce inspection intervals until confidence that shackle remains upright Failure mode 3: Explanation:

Fatigue - axial and bending As the buoy motions cannot accurately be ascertained by modelling, the exact load regime on these shackles cannot be known, potentially including snatching (see above).

Consequence score:

4


Intolerable

Additional safeguards to consider: 1 Increase strength of the D shackle 2 Provide more lever arm to keep D shackle vertical 3 Reduce inspection intervals until confidence that shackle remains upright

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8




Contact with fishing gear A trawl wire could easily slide down the polyester rope, over the end fitting and would then get stopped by the lower D shackle. If the trawl board comes this could do some damage, though perhaps not devastating.

Consequence score:

2


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1

Acceptable


Item:

Buoy, specifically padeye


Strength factor of safety - snatch load The dynamic behaviour of the buoy cannot be truly analysed, so the total load and bending moments on the padeye cannot be known.

Consequence score:

4


Intolerable

Additional safeguards to consider: 1 Review strength of the padeye 2 Further length between buoy and H shackle 3

Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8




Fatigue - axial and bending As the buoy motions cannot accurately be ascertained by modelling, the exact load regime on the padeye cannot be known, potentially including snatching (see above).

Consequence score:

4


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 2 2

2

1

0

2 8

Intolerable

Additional safeguards to consider: 1 Review strength of the padeye 2 Further length between buoy and H shackle


Contact with fishing gear A trawl could come each side of the buoy, or a wire could run on to it. Generally the fishing gear is likely to catch on a piece of the main mooring.

Consequence score:

2


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 0 1 0

2

1

0

0 1

Acceptable



Deployment The buoy has to be overboarded yet is on a very short leash to the main mooring. This issue is raised to understand if will be connected on surface or underwater.

Consequence score:

2


Yes 2 2 2

Partial 1 1 1

No 0 0 0

Score 2 0 2

2

1

0

0

Make Alarp

Additional safeguards to consider: 1 Identify the optimum method for deployment, for integrity and cost benefit 2

4


Appendix 6

Example mooring monitoring systems

Some of the factors that a FPSO may wish to consider when seeking or designing a mooring monitoring system include the following: A/

The ability to determine chain failure near the turret (ie little or no chain present, the majority now being on the seabed).

B/

The ability to determine chain failure near the catenary touchdown (ie the chain near the turret hanging vertically).

C/

The ability to determine other places of chain failure by detecting reasonable changes in catenary shape near the turret.

D/

Display of data in the Control Room; typically a considerable distance away from the turret centre.

E/

Ease of interpretation with confidence by the Marine Operators.

F/

Equipment uptime. Factors which may, but not necessarily, aid this are no moving parts, engineered for the 100 year storm condition and/or a track record of similar usage.

G/

Equipment life. This may be achieved with a level of redundancy and/or the ability to changeout critical elements of the equipment at low cost and without long ordering times.

H/

The ability to confirm the presence of all moorings within the required time frame. If this is not achieved then reduction of thinking time results hence placing more reliance on the damaged condition factor of safety in the mooring design.

I/

Ability for the system to be function tested in situ. For example: independent systems, one for each chain, may not tell the user if one system had failed, or whether the chain had failed.

J/

Relative ease of the system to be repaired, or replaced, without large cost (eg intervention vessel or production shutdown).

K/

Lead time to design and fabricate the system; thus off-the-shelf equipment may have an advantage.

L/

Cost of the system installation (including risk of weather downtime during the installation).

M/

Whether other benefits can arise from the system, rather than just confirm all moorings are present. For example some systems may demonstrate catenary shape, or tension pattern over time, which can be further used to check for abnormal behaviour.

One of the primary considerations may be whether the system is mounted on the geo-stationary side of the turret, or on the FPSO side; and how communications can be reliably and cost effectively achieved between the two. Indicative sketches of some of the possible arrangements are given below.



Hull-mounted Sonar: • hard wired magnetically mounted sonar heads; • single screen permanent display with built-in alarms; • 5 second refresh rate; • one head able to provide majority coverage (for redundancy) by cancelling time bar on displayed echoes;

Seabed-mounted Sonar: • three sonar heads at known azimuths for chain; • single screen permanent display with built-in alarms; • 20 second refresh rate; • able to also give full catenary shape; • large battery pack on crane deployed seabed unit.



Hull-mounted Cameras: • two hard wired magnetically mounted cameras; • single screen display either split or for one camera; • ma nual controlled pan facility so can check all chains; • adjustable lights fitted to each camera; • requires operator to use and no automatic alarms, but high confidence in what can be seen.

Pressure sensors: • low power redundant sensors at each location; • two-way comms to maximise battery life; • battery life circa 2 years; • seabed unit is crane deployed/recovered so large battery is feasible; • single magnetically fixed hard-wired hull mounted transducer;



Tension monitoring by turret strain: • sensors located inside turret, so less severe environment; • requires calibration during chain change-out; • requires slip ring onto FPSO; • may not be able to differentiate between a chain failure in moderate weather, and intact chain in extreme weather.

Tension monitoring in line at top: • requires physical insertion of shackle or special plate, so only done during chain changeout; • requires calibration; • requires turret penetration and slip ring; • track record underwater is questionable.



Chain strain measurement at top: • requires physical insertion of smart strain wire across a number of shackles; • requires calibration; • requires turret penetration and slip ring; • no track record underwater; robustness is questionable.

Alternating current field measurement: • requires physical insertion of a unit yet to be designed; • requires extensive calibration testing, but onshore; • requires turret penetration and slip ring; • no track record underwater or on chain; robustness is questionable.



Tension monitoring near touchdown: • single transducer magnetically mounted on hull; • transmitter with large battery crane-deployed on seabed; • questionable security of wire.

Seismic detection: • multi listening devices; • transmitter with large battery crane-deployed on seabed; • acoustic signature of falling chain is uncertain.



Tension monitoring at stopper: • either load bolts on stopper plates; • or compression cells under stopper plates; • requires calibration during chain change-out; • requires turret penetration and slip ring onto FPSO; • requires replacement stopper plate fabrication.

Hydro-acoustic: • either engine room mounted (preferable) or magnetically hull mounted, transducer; two transducers required to avoid shadow areas from risers; • transponders held in buckets on mooring chain; duplicated for redundancy; • battery life circa 1 year; • xyz position provided.



Distance Hydro-acoustic: • either engine room mounted (preferable) or magnetically hull mounted, transducer; two transducers required to avoid shadow areas from risers; • distance between units measured, only changes transmitted; • tx/rx units held in buckets on mooring chain; triplicated for redundancy; • battery life circa 1 year; • xyz position provided.

Strain wire: • wire threaded through chain links by micro-ROV and “happy hooker” arrangement; • hard wired to seabed unit via buoyant section to allow for vessel motions; • pulsed sensing of wires, and adjustable pulse rate via two-way comms, to maximise battery life; • seabed unit is crane deployed/recovered so large battery is feasible; • single magnetically fixed hard-wired hull mounted transducer.



Appendix 7

Example mooring inspection systems

There are three basic systems that are used for mooring inspections: Divers Within the North Sea arena diving has a very successful history but with considerable safety risks to personnel. Divers, whether near surface or using saturated techniques, require all the correct precautionary equipment and typically this would come with a dedicated dive vessel; hence any marine risks from multiple vessels in the field working within a FPSO swing circle need to be considered. ROV Traditionally these are large work-class ROVs operating from welded down platforms, either working from the FPSO or more likely from a dedicated vessel. They have a lot of capability but there is little standard tooling that will fit all ROVs. In the last few years the quality of smaller ROVs has increased and the capability of some of the midrange units that do not require hot work has increased exponentially, some even capable of fitting, size and weight, within the hold of a standard North Sea passenger helicopter. AUV To date there is no AUV designed to specifically inspect in detail the condition of a mooring system. AUVs at the time of writing are generally used for survey and mapping purposes. It is understood that a JIP has however been initiated in Norway to examine this possibility.

Using the above three basic platforms on which to mount equipment, the following generic methods are today available to FPSO operators for consideration in planning their mooring inspection requirements. These can feed directly back into Actions that can be considered as a result of the Mooring Integrity Risk Review. GVI - General Visual Inspection This is the most common method that is utilised for mooring inspections; it is a terminology that describes itself, normally carried out by a continuous slow swim past the items being inspected. It relies on having: • sufficient water clarity; • adequate camera quality and also of the feed back to the viewing point; • knowledge of the inspector (although tapes or other electronic storage is normally made of inspection footage, it is more convenient for inspectors offshore to review the images in real time during the work, especially as this allows for re-positioning to get a view from different angles if this is considered necessary). Many inspectors have considerable experience of chain and other mooring equipment inspections, but to date these personnel tend to be jack of all trades that do not specialise in moorings, but also cover risers, pipelines, hulls, wellheads, manifolds etc. Matching the capability of mooring inspectors to the required level of specific mooring knowledge may be a substantial factor in being able to identify anomalies.



CVI – Close Visual Inspection This term also speaks for itself. However it should be understood that not all ROVs are capable of sufficiently close inspection for some equipment, or that there may be too great a risk of unexpected motions for a diver to approach sufficiently close. Example A recent chain integrity concern on some FPSOs is the abrasion of the chain link that sits in the mouth of a trumpet or chain hawse on the bottom or side of a turret. Some of the larger work-class ROVs are unable to get in close enough, nor able to bring appropriate lights to bear, into the trumpet mouth to identify such abrasion. The use of different cameras, different light intensities and types, should also be considered. Taking shots from digital video footage does not necessarily give high clarity, and the use of a dedicated stills camera may be worthwhile.

3-dimensional imaging This can be very useful, particularly for identifying sizes of abrasion areas, corrosion pits etc. A number of companies offer this capability, which can be added to most ROVs. Its full potential is generally only possible after excellent cleaning of the inspection item, and with post processing accurate measurement between just about any two points is possible.

Chain measurement A number of systems have been created to measure the dimensions of chain links, Specific measurements that are often taken are: • chain link overall length. Some systems may measure the overall length for example of 5 links of chain, an early Class standard; while informative this may not provide as much information as measurement of those five individual links; • interlink distances (ie where the two bars are round each other); • chain bar diameter (although care must be taken not to confuse with the flash butt weld dimension). Some of these systems require post-processing of the data, some are real time measurements; some require cleaning of the chain, some do not. Traditionally chain measurements are taken at the top of a catenary, in the middle, and near the bottom; and then measurements taken at time intervals, typically two to three years, are used to derive corrosion and/or wear rates. Consideration should be given to matching the output of dynamic mooring analysis with the choice of location for chain measurements; ie dynamic analysis can show where there is the maximum motion of adjacent links (which could cause wear), or where the axial tension range is greatest (which may or may not be the point on the chain where the fatigue life is least, and which is not always at the top of the catenary), or where snatch loading may occur in some circumstances. Measurements as near as possible to chain touchdown can of course provide indication of abrasion from seabed material; but consideration should be given to identifying the touchdown at the time of the inspection, since if inspecting a leeward line the accessible chain for inspection may never have been on the seabed, while if inspecting a windward line, or a line deliberately raised by a FPSO offset, chain that is in the touchdown envelope can be measured. Consideration should also be given to identifying the links that have been measured. At the present time there is insufficient data available to know if high pressure cleaning of chain links and hence the removal of marine growth and scale accelerates apparent corrosion or not. If use of a chain marker is considered then this should take into account wear, galvanic action etc so that the marker does not do more harm than good. There is at least one marker design available in the market.



Sound Traditionally tapping of chain link studs during onshore chain inspection is a quick way for an experienced inspector to know if a stud is loose or not. This method is also available through one company providing underwater chain inspection. Nominally sound can also be used to provide a first pass integrity check of a whole link, although this is subject to many other factors such as the contact area and tension of the adjacent links. The Working Group is not aware of any underwater techniques for this.

Ultrasonics [HOLD]

ACFM (Alternating Current Field Measurement) ACFM is an electromagnetic technique for detecting and sizing surface breaking defects in metals. This can be deployed by both divers and ROV, but some calibration of the readings on a whole link will make the results more certain. [HOLD]

Interlink resistance Research is currently being undertaken to determine if the resistance between two chain links, and hence along a length of chain, can be used to indicate the condition of the interlink contact areas; ie if there is a larger and good condition surface contacting two adjacent links then the resistance between the two would be less. This may be detectable by existing cathodic potential measuring tools and provide a first pass at a chain condition.


Mooring Integrity Guidance - O&GUK - RevF

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