OTC JIP Key Findings

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Copyright 2005, Offshore Technology Conference

This paper was prepared for presentation at the 2005 Offshore Technology Conference held inHouston, TX, U.S.A., 25 May 2005.

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Abstract

Over the last two years Noble Denton has been undertaking aJoint Industry Project (JIP) to investigate how to improve theintegrity of the moorings used by Floating Production Systems

(FPSs). The JIP has surveyed the world wide performance ofall types of FPS mooring systems including FPSOs, semi

submersible production units and Spars. Wide rangingsupport from 23 sponsoring organizations including operators,floating production contractors, regulatory authorities,equipment suppliers and inspection companies has enabledaccess to a significant pool of data.

This paper utilizes the JIP data to discuss the following:

Causes of system degradation

Consequences of mooring failure

Key areas to check on a mooring system

Fatigue implications of friction induced bending

Options for in-water inspection The importance of connector design

Methods to detect line failure

Contingency planning

A few pioneering floating production units have now been onstation for many years. Review of inspection data from theseunits shows that selective repair may be needed to maintainthe design specification right up to the end of the operationallife. It has been found that wear can be faster on leeside, asopposed to windward lines and that certain weighted chain

designs are susceptible to damage.

The likelihood of line failure and the implications need to bebetter appreciated. Following failure, it may well take severalmonths to implement a full repair, due to a lack ofspares/procedures and possible non-availability of suitable

vessels. However, it has been found that carefully plannedand coordinated inspection operations can detect potentialissues early on before more serious deterioration takes place.In general, mooring monitoring/instrumentation and access forin-water inspection seem not to be as advanced as might beexpected for a system which is safety critical. Hence goodpractice recommendations are included which can be applied

to both existing and planned future units.

Introduction

Unlike trading ships, Floating Production Systems (FPSs),stay at fixed positions year after year without regular dry

docking for inspection and repair. Since they cannot move offstation, they must withstand whatever weather is thrown atthem. Hence at times, depending on their location, theirmooring systems need to withstand high storm loadings.Typically during design, mooring systems for harshenvironments do not have much reserve capacity above what

is required to withstand survival conditions. Thereforedeterioration of the lines over time can increase the likelihoodof single or multiple line failures. Multiple line failure couldconceivably result in a FPS breaking away from the mooringsand freely drifting in the middle of an oil field.

The Mooring Integrity JIP has been concerned with assessinghow mooring systems have performed in the field to identifythe level of degradation which has taken place. Hence theproject has looked at FPSOs, Semi submersible productionunits and Spars through out the world. The key objectiveshave been:

To feedback operational and inspection experience tothe industry and to mooring designers

To publicize how hard moorings work, theirimportance and potential vulnerability

To disseminate best practice guidance

OTC 17499

Floating Production Mooring Integrity JIP Key FindingsMartin G. Brown, Noble Denton Europe Limited

Tony D. Hall, Welaptega Marine Limited

Douglas G. Marr, Balmoral Marine Limited

Max English, U.K. Health and Safety ExecutiveRichard O. Snell, B.P. Exploration


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From the survey it has become apparent that certain problemshave occurred and thus the JIP wishes to publicise these sothat they can be taken account of during inspection of existing

units and during the design of future units. Taking dueaccount of past experience is particularly important when a

design premise or specification is being developed for a newproject.

International Survey

Significant effort was made to try and ensure that the

international survey was as simple and straight forward aspossible for respondents. To this end a custom designedspreadsheet based questionnaire with drop down boxes wasdeveloped. This spreadsheet was partially completed by

Noble Denton, using information in the public domain, beforebeing emailed out for checking and final completion.

As well as the questionnaire face to face interviews werecarried out with key personnel from different areas of the

industry. Conference papers, in-house data and journals werealso consulted. Response to the questionnaire was reasonable,but could have been better particularly for non North Sea

regions. This perhaps gives some indication of the prioritylevel that at present seems to be associated with mooringsystems. Initially it was believed that offshore based staffwould be able to complete the questionnaires. However, itbecame apparent that in some assets there is little in-depthknowledge about the make up and history of their mooring

systems. Overall though, in summary, good data wasobtained, but not on as many units as had been originallyplanned.

Degradation Mechanisms

Intrinsically mooring lines present a fairly simple system forkeeping a floating object on station. However, experiencefrom the field has shown that mooring is in fact a particularlydifficult dynamic problem. Figure 1 illustrates a number ofthe degradation mechanisms which a mooring system will beexposed to every day of its operational life. Inevitably theperformance of the system will decrease over time. Despite

this, at the end of the field life, which in certain circumstancescould be in excess of 20 years, the mooring system normallystill needs to be capable of withstanding 100 year return periodstorm conditions. This represents a stern test for any 20 yearold mechanical system. It is also logical that the longer amooring system is out there, the higher is the probability that it

will encounter extreme weather conditions.

Many of the mooring issues mentioned in this paper refer tochain. This is because chain is normally selected at the twomost challenging locations, namely the vessel interface andthe sea-bed touch down. Since the loading regime is severe

degradation may sometimes occur. However, experience overthe years has shown that using wire in these areas does notgive a true long term solution. The same would almostcertainly apply to the use of fibre ropes.

Figure 1 Mooring degradation and the key areas to inspect

Historical Incidents

Given these degradation mechanisms a search was made ofhistorical records to see what lessons could be learnt from pastincidents. This search identified the following incidents which

could have implications for present day systems, althoughparticularly for the SALM the failure mechanism was unique

to the system concerned:

Argyll Transworld 58 production semi, completebreak away in 1981

Fulmar FSU, complete break away in 1988 from theSALM (Single Anchor Leg Mooring).

A series of semi sub multiple line failures in thestorms of Oct. 1991 and January 1992, see ref 5.

Petrojarl 1, 1994, 2 lines failed at the same time whenhit by a 20 to 25m wave 10 off port bow.

The TW58 and the Fulmar 210,658dwt storage tanker both

broke away after 6 years and 7 years on station. Thesedurations tie in surprisingly well with the failure statistics

reported later on. The TW58 was designed to and haddisconnected its risers before breakaway, but it was still freedrifting for 1.5 days in the North Sea before it was possible toattach a tow line to it. The Fulmar FSU did not have

propulsion and was drifting for 5 hours before tow lines couldbe attached.

Reference 5 is informative since it gives an idea of how muchdamage can be inflicted by unusually severe, but not freak,

storms. The Petrojarl incident is significant since it shows thatif there is a common degradation mechanism multiple linefailure may occur virtually at the same time. In this case

redesign of the chain guides and up-rating the chain resolvedthe particular problem.

Consequences of Mooring Failure

Environmental Impact

The design premise of the majority of FPSs is that they shouldbe able to withstand a single mooring line failure without theresulting increased vessel offset causing damage to the risers.Multiple line failure is only likely to occur if a failure has gone

un-detected (see later) or if there is general degradation which

Bending & Tension

Highest Tensions

Corrosion

Impact & Abrasion

Wear & fatigue


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is affecting all lines in a particular quadrant to approximatelythe same extent, see Figure 2.

Figure 2 Possible Line Failure and Repair Scenarios

In the unlikely event of multiple mooring line failure causing

rupture of one or more risers, the extent of hydrocarbonrelease will be strongly dependant upon whether or not therisers are still pressurized. Typically it is assumed thatmooring line failure will be progressive and thus there will besufficient time to shut down production and depressurize therisers, before the resulting increased vessel offset causes

damage. However, the multiple mooring line failure whichoccurred on Petrojarl 1, when hit by a shock-inducing steepwave, shows that loss of position keeping on a non DP assistedvessel could occur remarkably quickly. This could possibly bein wave heights below survival criteria. Hence, it is

recommended that on-board emergency procedures shouldidentify what action should be taken in case of single ormultiple riser rupture while the risers are still pressurized.

If the risers are depressurized when rupture occurs, the extentof possible hydrocarbon release ranges from 100m3 to2,500m3. This depends on field specific architecture such as

the number of risers and the step out distance of the flowlines.

Business Interruption Impact

The business interruption cost of a single mooring line failureis not insignificant when the cost of anchor handling tugs,ROV or dive support vessels, new components and deferred

production is taken into account. For example the followingcosts have been estimated for two typical cases.

2M for a 50,000 bpd N. Sea FPSO

10M for a 250,000 bpd W. African FPSO

Multiple line failure which does not cause breakaway, butresults in shut down for an extended period, would cost muchmore than the figures outlined above.

Causes of System Degradation - Case StudiesCorrosion and Wear North Sea Production Semi

A fascinating insight into the possible future performance ofmodern FPSs is provided by a purpose designed new buildNorth Sea production unit which has been in continuous

operation for coming up to 20 years. During this time the FPShas experienced three mooring failures, plus significantdefects have been found on two other lines during inspection.

Interestingly all three line failures have been on lines whichare defined as leeside lines based on prevailing weather

conditions (see Figure 3). Leeside lines are in general underless tension and this seems to result in greater relative rotation/

motion between chain links and thus more wear. On firstthought it might be expected that greater wear would be

expected on the more heavily loaded windward lines.However, a bar tight line will in fact see less relative rotation

between links than a slacker line subject to the samemovement of the surface platform.

Figure 3 Illustration of Windward and Leeward Lines

On this unit the failures have typically been on chain which atthe no load equilibrium position is somewhat above the touchdown point. Hence, in-water inspection during calm weathershould make sure that this area is carefully inspected.Accelerated degradation in this area is highlighted by a morerecent ROV inspection which has revealed that a studded

chain has shed studs see Figure 5. This is interesting, sinceit proves that studded chains can lose studs in situ rather than

just during the relatively harsh handling that chain receivesduring a recovery operation by an anchor handling tug.

Figure 4 shows a recovered link which was close to the link

which failed in service. The failed link could not be found onthe sea-bed. On the photograph it is interesting to note that the

area of maximum wear is not at the point of contact betweentwo links under tension, otherwise known as the inter griparea. Instead it is part way down the inner face of one side ofthe link. Damage was also noted on the crowns of other links.

This suggests that some form of dynamic impact/grindingaction is occurring which is wearing down the links.

Significant inter link motion is thought to have been a factorcontributing to the shackle pin failure illustrated on Figure 9.

Losing material in this area is significant, since a finite

elements analysis of a link will confirm that this is a highlystressed area. This is one of the reasons why it is

recommended that tests should be undertaken to determine theactual break strength of worn mooring components.


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Figure 4 Example of Wear and Corrosion on a Chain Link fromthe Sea-bed Touch Down Zone

Based on the original nominal diameter of this chain, which it

is appreciated can vary; the combined wear and corrosion rateover its years of use has been estimated to be 0.6mm/year.This wear has occurred around the chain touch down area at

the sea-bed, otherwise known as the thrash zone. 0.6mm/yearof wear/corrosion is 50% higher than the maximum valuesfound in APIs RP2SK and DNVs OSE301 (refs. 3 and 4). Itis interesting to note that a corrosion rate of 0.3 to

0.88mm/year for uncoated steel has been quoted on a long-term inshore project where sulphate reducing bacteria (SRB)

induced corrosion might be experienced.

Figure 5 In water inspection showing a Studded Chain whichhas lost its Stud in situ

If the combined wear and corrosion rate is higher than thatspecified in mooring design codes this may well havesignificant implications for the true long term integrity of FPSmoorings. It is appreciated that the wear rate reported heremay well not be appropriate for all regions and platformtypes/system pre-tensions. However, the fact that this level of

wear/corrosion has been experienced does highlight theimportance of obtaining more data on wear/corrosion for other

long-term moored units. The options for in-water inspectionare discussed later on. There are, however, some limitationsand hence it would be highly desirable if comprehensive

inspection, including dimension checking, could be

undertaken of mooring components whenever a FPS comes offstation or has repairs done to its moorings.

Mooring line Configuration at the Vessel Interface

The design of the vessel interface needs to minimize thepotential for wear, corrosion or other forms of degradation.

However, experience is demonstrating that this is not alwaysbeing achieved. This is discussed below. The key points are

relevant to mooring systems in general, not just to oneparticular type.

Although there are a number of different turret mooringsystem designs, including both internal and external turrets, itis possible to categorize them as follows:

a) Non adjustable permanently locked off chains at theturret base

b) Adjustable chains which come up through the turretand are stored in a chain locker.

On Type a) systems the line tensions are not intended to be

changed at any time throughout the field life. Type b) systemsuse a wildcat at the base of the turret similar to that found on adrilling rig running chains. Type b) FPSOs typically adjust

their lines lengths and tensions either annually or evenmonthly. On some designs of spread-moored FPSOs the linelengths are also not intended to be adjusted and the requiredequipment for adjustment may not normally be present.

If the line lengths are never adjusted during the field life this

means that the same links in the thrash zone and at the turretinterface will need to withstand the majority of thedegradation. In addition, inspecting lines in situ is moredifficult, since the chain is relatively inaccessible inside the

trumpet/chain stopper. It is also much more difficult withsuch designs to pick up the chain off the sea-bed to make it

more accessible for in water inspection.

Being able to adjust line lengths can introduce its own perils.During a regular line tension adjustment operation on oneNorth Sea FPSO there was a failure of the lifting and lockingmechanism. This was partly due to a late change in chain sizeand the fact that the tolerances of forged chain links had not

been properly taken account of. The failure resulted in thecomplete line being whipped out of the turret and falling downthrough the water column to the sea bed. Fortunately no onewas hurt and there was no damage to subsea architecture.Modifications to the lifting and locking mechanisms shouldprevent another incident of this type occurring. It is worth

noting that line run-outs are far from unknown on semi-submersible drilling rigs. This incident highlights theimportance of reviewing all similar mechanical systems tocheck that, during the course of a long period of operation,chain/stopper wear or link dimensional variation may notjeopardize the integrity of the mechanism.

Wear at Trumpet Welds Internal and External Turrets

On two type a) turret configurations wear has beenexperienced where the chains have been rubbing against theweld beads where the bell mouth joins with the parallel

Localised Wear


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trumpet section (see Figure 6). This was first experienced onan early S.E. Asian external turret moored FPSO and morerecently on an internal turret moored N. Sea FPSO. For the

internal turret a slight shadow was seen on one of the chainsduring the annual workclass ROV chain inspection

programme. To check out this anomaly a test tank mock up ofthe chain and trumpet assembly was built so that the capability

of using a football sized micro-ROV to get in close to the bellmouth could be evaluated. This concept proved to be

successful as can be seen from the photograph taken by amicro-ROV in the field, see Figure 7.

Figure 6 - Test Tank Mock-Up of Micro-ROV inspection of ChainEmerging from Turret Trumpet

In the case of the external turret, in air access was such that itwas possible to shroud the chains where they were rubbingagainst the weld beads with a replaceable material (ultra highmolecular weight polyethylene sheeting). However, for thesubmerged trumpets on the North Sea unit a more long-termrepair was needed which involved changing out the worn

chain at the trumpet with larger diameter chain with aspecially applied hardened coating (cobalt chromium) toreduce the severity of any future wear. A special connector(see Figure 15) was developed to allow the new chain to beconnected up to standard common link chain. This approachavoided disturbing the wire section of the mooring line on the

sea-bed, which is relatively susceptible to damage (birdcage).The original system designer was included in the review

process for the repair operation. This represents good practicewhich, where possible, it is recommended should be followedfor any future FPS mooring repair operations.

On type a) systems the trumpets are typically pivoted about asingle axis so as to minimize chain rotation and wear. Since

the rotation is only about one axis and the trumpets arearranged around an approximate circle, the pivoting actioncannot eliminate chain rotation for all the lines at the sametime. Thus, to minimize wear over a long field life, there may

be arguments for selecting a design which can pivot about twoaxes, although this would be mechanically more complicated.

Figure 7 - Micro-ROV Photograph of Chain Wear Notches whereChain Emerges at the Trumpet Bell Mouth

Trumpets or guides are included on type a) FPSO designs to

help guide the chain into the chain stopper. The trumpetsthemselves may include angle iron guides to ensure that thechain is in the right orientation when it enters the chain

stopper. Once the chains are tensioned the trumpets have noreal purpose unless they are required in the future for a new

chain pull in operation. Interestingly, the pivoting chainstopper design which was adopted for the Brent Spar buoy didnot include trumpets to help guide in the chain. The BrentSpar mooring was a successful design with a 19 yearoperational life and minimum wear on the chains at thestoppers when they were examined when the Spar was cut up

in Norway. There was one failure but this was at a kenterconnecting link. Such a failure is not surprising, since

standard kenters are known to have low fatigue lives. Thereare, fortunately, now new designs of kenters with improvedfatigue lives, but these still do not at present haveclassification society approval for long-term mooring.

It is significant to note that the chain stopper on type a)designs is typically inboard of the pivot point. This means thatthe trumpet assembly does not automatically follow themotion of the chain. In fact it is contact between the chain andthe outer face of the bell mouth which causes the trumpet to

rotate. It is this contact, plus an associated sliding/sawingaction, which seems to have led to the chain notches shown onFigure 7.

Intrinsically there does not seem to be any reason why thechain stopper should be inboard of the pivot point. If it isoutboard of the pivot point movement of the chain should

cause movement of the trumpet without the need for chaincontact with the bell mouth. This type of arrangement hasbeen adopted on some more recent spread-moored FPSOs.

For chain stoppers which are inboard of the pivot points itwould appear that long trumpets are not helpful after the

completion of the installation process. Thus it isrecommended that careful checks should be made on any unitswhich fit this category.


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In general achieving compatible chain surface hardness isimportant for long term integrity, since it affects wear.Unfortunately, at present chain hardness and wear do not seem

to be evaluated in any detail. These factors should be takenaccount of during detailed design, but more work is needed on

this area before it becomes part of the standard design process.

Friction Induced BendingWhen a chain is under tension there will be friction and local

yielding between the links which will inhibit inter linkrotation. It is found that the higher the tension in the line, the

greater the frictional forces. This friction can result in out ofplane bending on individual links, see Figure 8.

Figure 8 - Illustration of Friction Induced Bending

Thus out of plane bending tends to become more of an issue aswater depths and line pre-tensions increase. Over timecyclical out of plane loading can cause fatigue damage. Thishas been illustrated by a number of fatigue failures which have

occurred on a taut moored CALM buoy off West Africa.

Historically, mooring line fatigue has not been evaluated,partly due to the complexity, since MODUs work in differentgeographical locations areas on relatively short assignments.Today, for long term moored units, a fatigue assessment is

typically carried out (refs. 3, 4 and 6). Such an analysis isnormally in terms of tension loading cycles; it does notconsider the combined effects of bending and tension. Forlong term moored units it is clear that friction induced bendingfatigue should be evaluated. This is particularly important fordeep water taut moored systems, but will still have some

relevance for units in more moderate water depths. Physicaltesting has been undertaken to evaluate suitable friction

coefficients for chain subject to out of plane bending

9

.

In field experience has shown that the orientation of the linkswhere they emerge from the bell mouth can significantly

affect fatigue life. Improved fatigue life can be obtained if thedynamic link just outboard of the bell mouth is in a verticalplane. In other words the oval face of the link is at 90 to thesea surface.

Excursion Limiting Weighted Chain and Mid Line Buoys

From a mooring design perspective increasing the chainweight for a section of mooring line in the thrash zone can bea beneficial solution to reduce vessel offsets. This tends to be

particularly applicable for moderate water depths in harsh

environments, which represents a particularly taxing mooringproblem. There are a number of ways in which this can beachieved. However, from the international survey it is clear

that great care is needed to select a robust system if such anapproach is adopted.

One way of increasing the chain weight, is to hang off short

chain lengths from the main mooring chain. This was thesolution adopted on one harsh environment FPSO. However,

Figure 9 illustrates the damage that has been caused to one ofthe pins. It is believed that this damage may well have been

caused by a dynamic pinching/grinding action of adjacentlinks.

Figure 9 Photograph of a Partial Failure of a Hang-Off ShacklePin

Another possible approach to increasing the line weight over acertain section is to attach clump weights to the chains.

illustrates half of a clump weight from a FPSO mooring linewhich utilized such a system. In this instance it can be seenthat the bolts which kept the two half shells together havefailed and the clump weight has thus split open. Again the

dynamic loading of the line is thought to have led to the failureof the restraining bolts.

Figure 10 - Chain Clump which has become detached only onehalf of the Clump Weight Visible


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Other systems for increasing chain weight locally include aparallel chain system with triplates or using a larger chain size.Both of these systems appear to have worked successfully,

although there is a need for careful design of connectors. Thisis because enhanced wear may be experienced due to an

increased rotation resulting from a change in the weight permetre at the connectors.

An alternative way of reducing FPS excursions due to mean

wind, current and wave drift forces is to add buoys on to themooring lines. However, problems have been experienced on

one FPSO with the buoys becoming disconnected from thelines over time. Interestingly this seems to have been onleeward lines, which indicates that that the increased motion ofthe less tensioned lines may be contributing to the problem.

Connector Failure Unintended Line Disconnection

Careful detailed design of long term mooring connectors isvital to ensure that they are fit for purpose. Figure 11

illustrates an unintended line disconnection on a FSU. Thissocket was at the transition from wire rope to chain. Hence,there was a weight per metre discontinuity which resulted in

extra rotation at the connector. In this instance the socket pinwas restrained from rotating by relatively small bolts. The pinwanted to rotate and it eventually sheared the bolts on the endcap which allowed the whole pin to work loose. It isinteresting to note the size of the locking-pins which make upthe double locking system on the purpose designed connector

shown on Figure 15. The substantial size of these pins wasbased on hand calculations utilizing the expected line loadsand an estimated friction factor. In the case of the unintendeddisconnection, at times, depending on vessel offset, the

connectors would have been in the thrash zone. They wouldhave experienced repeated lift up/set down contact with the

sea bed.

Figure 11 - Unintended Line Disconnection due to the Failure of aSocket Restraining Mechanism

Dog Leg or Wavy Mooring Lines on the Seabed

During mooring line installation it is important that all lines

should be laid straight from the anchor to the fairlead at the noload equilibrium position. This requirement should be

emphasized in the installation procedures and reflected in anytug specifications. If dog legs or wavy lines do end up

being present and they are pulled out by storm loading, thiscan lead to unbalanced mooring line tensions. In other words

a system which was balanced originally with the dog legsmay no longer be so. If one line takes more of the load

coming in from a particular quadrant it is more likely to fail.If this originally taut line fails, the FPS may exceed itsallowable riser offset limit if the remaining lines are too slack.At present non straight mooring lines have been noted on two

North Sea FPSOs. On these units the initial pre-tensioningoperation and the storm loadings which have been experienced

have been insufficient to overcome the friction of the lines inthe sea-bed mud. But to date, these FPSOs have not yetexperienced storm line loadings as severe as the maximum

loadings evaluated during the mooring design process. It willbe interesting to see if, over the respective field lives, the doglegs/wavy lines are pulled straight or not and this should be

monitored during annual ROV surveys. If straightening doesoccur the implications for mooring behaviour should be fullyevaluated.

Unbalanced Set-Up Pretensions

On a long-term moored semi-submersible FPS, offshorepersonnel doubted the tension readouts on their mooring linewinches, since damage was occurring to the wires on thewinch drums. In addition, when grappling for certain

components on the mooring line they were not found at theexpected depth.

Therefore, in calm weather, an underwater ROV survey wasundertaken of the triplate connectors to obtain their X, Y andZ co-ordinates. From these positions and knowing thesubmerged weight of the line, it was possible to perform acatenary calculation to estimate the actual line tension. Thesetensions can then be compared to the winch tension readouts.

This process showed that in the worst instance the calculatedand the measured tensions were out by 160% !

Tension meters fitted to the base of pull in winches/windlassescan give a poor estimate of the tension in mooring lines, evenif properly calibrated, since the amount of friction in the

sheaves/fairleads is variable and difficult to quantify. Inaddition there is a possibility of full or partial seizure of thesubmerged lower sheaves or wildcats. To check this out,during a period of good weather, a carefully controlled LinePay-Out/Pull-In test was undertaken. In this test each line waspaid out in 2m increments and the line tensions were recorded.

The lines were then pulled in again the same amount and thewinch tensions noted. If this test is undertaken relativelyquickly in calm weather conditions it would be expected thatthe same line tension would be obtained for the same linepayouts. In actual fact this did not prove to be the case for allmooring lines, see for example Figure 12.


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Line No11

185.0

186.0

187.0

188.0

189.0

190.0

191.0

192.0

193.0

194.0

195.0

0.0 20.0 40.0 60.0 80.0 100.0 120.0

Tension (te)

Wirepayout(m)

Figure 12 Example of a Pay-Out/Pull-In Test for a Seized SubSea Sheave

Historically semi-sub drilling units have been subject torelatively frequent mooring line failures. The work reported inthis section shows that it is possible for a carefully set up Rigto have a seriously unbalanced mooring pattern which theOperators might not be aware of. Further information can be

found in ref. 5. It is hoped that Pay-Out/Pull-In tests can beundertaken for other semis to determine how wide ranging or

otherwise this occurrence could be.

For long-term moored units it is recommended that a ROVshould double check the line tension balance by measuring X,

Y and Z co-ordinates of known points on the line or the touchdown points. This should be done in good conditions and thena back calculation can be done of the line tensions.

Recent Multiple Line Failure Incidents

Unfortunately serious mooring incidents continue to occur.For example, a December 2004 North Sea storm resulted in adrilling rig losing two of its eight anchor chains. The resultingexcessive excursions ruptured the drilling riser.

During hurricane Ivan five MODUs broke free from their

moorings and were set adrift. One of the units was a fifthgeneration rig. Fortunately, as far as can be determined, Ivan

did not cause damage to the mooring systems on any of thelong term moored FPSs in the Gulf of Mexico.

Indicative Failure Statistics

Based on the limited response obtained during the

international survey, it is quite possible that only a fraction ofthe total number of mooring incidents which have occurredoutside the North Sea have been reported. In the North Seathere are statutory requirements for mooring incidents to be

reported to the UK Health and Safety Executive (HSE).Although the North Sea is a hostile climate, units intended foruse here are in general designed to a high standard. Inaddition, a number of units in the North Sea have been aroundlong enough for age related problems to start making an

appearance. It thus seems prudent to consider officialstatistics for this region to be a reasonable indicator of the

likelihood of mooring line failure. Based on reference 2 forthe period 1980 to 2001 it is reported that a drilling semi-

submersible might expect to experience a mooring failure (i.e.anchor dragging, breaking of mooring lines, loss of anchor(s),

winch failures) of once every 4.7 operating years, once every 9years for a production semi submersible and once every 8.8years for a FPSO. Thus it can be seen that although the failureprobability for production units is approximately half that of a

semi-submersible drilling unit, the statistics indicate that itwould not be totally unexpected for the crew on a FPS to

expect a mooring line failure at sometime during a field lifewhich exceeds 9 years. Exactly how these statistics can berelated to milder environments is difficult to quantify at

present.

Good Practice Recommendations

In Air-Inspection

Mobile Offshore Drilling Units (MODUs) need to recovertheir mooring lines and anchors on a regular basis when theymove from one location to another. This provides periodicopportunities to undertake in-air mooring line inspection when

the vessel is in sheltered water. Alternatively a spare line maybe bought or rented which can be swapped out with one of theexisting lines while the original line is taken to the shore forinspection and possible refurbishment.

FPSs spend much longer on location than MODUs. Hence,

their mooring lines are normally only recovered when the FPSmoves off location. It is possible to recover mooring lines partway through a field life but this has two disadvantages,namely:

1. The lines may be damaged either during recovery or re-installation

2. The whole operation is expensive since the services ofanchor handling and possibly heading control tugs will berequired for a number of days.

Given that even in-air inspection will not necessarily detect allpossible cracks and defects which may be present; there is an

understandable interest among operators to undertake in-waterinspection. However, there will still be times when anomaliesare identified which can only be resolved with true confidenceby undertaking in-air inspection. One definite advantage of inwater inspection is that it is easy to identify which parts of thechain have been in the thrash zone and at the fairlead. This is

more difficult to determine for long lengths of chain lying on aquayside.

In-Water Inspection

To date chain mooring components have been the subject ofthe greatest effort to develop in-water inspection methods.


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This is because they are typically used in the sections ofmoorings subject to the greatest deteriorative forces,particularly at the seabed touchdown (thrash zone) and at the

vessel interface. Both windward and leeward lines should beinspected, but a particular check for wear should be

undertaken on the leeward lines, see Figure 3. Care is neededwhen inspecting the touchdown zone, since potential hazards

such as rocks or debris on the sea-bed can cause mooring lineabrasion. These hazards may be partially obscured by the sea

bed/mooring line and thus good visibility with powerfullighting is required.

In-Water Chain Measurement

A number of in water mooring chain measurement systems

have been developed with varying success, ranging fromsimple diver-deployed manual calipers to a prototype stand-

alone robotic system and ROV deployed systems.

Diver inspections are not a favoured option. Mooring chains

are highly dynamic and therefore are potentially dangerouswhen divers are in close proximity. Also diver inspection hasproven to generate inconsistent results and has inherent depth

limitations, for example, when checking the thrash zone.

A stand-alone robotic system has been developed, but so farthis has proven too large and cumbersome for practicaloffshore operations. In addition, it does not appear able toinspect the vital seabed touchdown or get in close to the

fairleads.

ROV-deployed systems include both mechanical caliper andoptical caliper systems. Mechanical calipers have met with

limited success, primarily because during deployment ontochain they have the potential to be knocked out of true and

consequently may well have to be recalibrated betweensuccessive measurements.

The most established ROV-deployable chain measurementsystem is effectively an optical caliper7, comprised ofmultiple high resolution video cameras and lights ondeployment frame, which is equipped with scale bars in pre-

assigned orientations and at set distances from each other andthe cameras (Figure 13). The system measures the chainparameters by calibrating from the tool scale bars andresolving dimensions and optical distortions using offlineimage analysis software.

This type of system has no depth limitation, requires nophysical recalibration and can be configured to measure notonly chain components at the seabed, but also in difficult toaccess regions such as the vessel interface. It can also beconfigured to measure other types of mooring jewelry suchas connectors, shackles and kenter links.

The optical caliper chain measurement technology is usedextensively by offshore operators and is accepted by a numberof offshore certification authorities. In this respect in at leastone instance it has been used as the basis for an extension of

the prescribed recertification period for an in-service FPSfacility.

Figure 13 - Illustration of ROV deployed optical calipermeasurement system

7

Loose Stud Detection

In studded chain, loose studs have been implicated in crack

propagation and fatigue. Accordingly studded chaininspection and recertification protocols require the assessmentof the numbers of loose studs and degree of looseness.However, there is no consensual industry opinion with respectto loose stud reject criteria. Traditionally chains have had to

be recovered for detailed loose stud determinations and haverelied on a manual test, either moving the stud by hand orusing a hammer to hit the studs. The resulting resonance (aping or thud) is used to assess whether a stud is loose ornot.

Recently an ROV-deployable loose stud detection system hasbecome commercially available7. The system uses an

electronically activated hammer to impact the stud and uses ahydrophone and a micro-accelerometer as sensors. A softwareprogram is used to distinguish between loose and tightresponses. Cross checks can be carried out in that very loosestuds can be detected using a ROV manipulator or a ROV

deployed high pressure water jet.

Component Condition Assessment

As well as chain dimension checking it is also important toassess link integrity and condition. The overall, or general,condition of mooring components often gives insights into the

types of deteriorative processes that are at play. For examplesurface pitting may be indicative of pitting corrosion,scalloping or indentations of wear, fretting corrosion, oranvil flattening, and unusual geometry may indicate frictionbending, or plastic deformation (e.g. stretch).

Camera

block

Underwater

light

Deployment

guide


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Underwater visual condition assessment by ROV isparticularly difficult because of the inherent flatness of videoimages from standard 2D inspection cameras. With 2D

cameras it is very difficult to distinguish whether a visualartifact on a surface is merely a mark, or a region from which

material has been lost (e.g. a pit).

The shortcomings of 2D video can be addressed by using 3Dvisualization, a long-time goal in the underwater inspection

sector. Over the last two decades a number of 3Dvisualization systems have been implemented but, until

recently, with limited success due to problems with usercomfort and impractical and cumbersome viewing systems.

Advances in 3D camera design and the development of user-

friendly viewing systems have led to the introduction of a newgeneration of 3D video systems7. These cameras come in a

range of configurations, sizes and depth ranges and haveproven very effective for the assessment of the surfacecondition and general geometry of mooring components.

Improvements have also been made in video assetmanagement, so that it is now easier to access data withouttrawling through hours and hours of video footage7.

Marine Growth Removal

A key challenge of conducting in-water inspection is gettingaccess to the component(s) to be inspected. Materials whichhave been in sea water for extended periods accumulate

varying levels of marine growth which can be heavy,depending on geography, water depth and season

10, (see

Figure 14). This growth needs to be removed so that theunderlying mooring components can be inspected.

Figure 14 Illustration of Marine Growth on Long Term DeployedChain

Cleaning options include manual brushing by divers, rotarybrushing with wire or synthetic fibre brushes and ROVdeployed high-pressure water or grit-entrained high pressurewater. Each system has its own pros and cons.

Once marine growth is removed it is possible to conductvarious levels of inspection including general visual

inspection, dimensional measurement and assessment ofmechanical fitness. Unfortunately cleaning off marine growthand scaling by high pressure water jetting may accelerate

corrosion by exposing fresh steel to the corrosive effects ofsalt water. At present there are currently no in-water

inspection methods for mooring components that do notrequire the prior removal of marine growth. This represents a

technology gap which warrants further investigation.

The time required to remove marine growth depends largelyon the cleaning option chosen and in light of the cost of ROV

vessels, can be a substantial component of the cost of aninspection program. Consequently it is essential that theplanning stage of mooring inspection campaigns shouldconsider the most suitable cleaning options for the expected

conditions.

Line Status Monitoring and Failure Detection

Given the safety critical nature of mooring lines one might

imagine that they would be heavily instrumented withautomatic alarms which would go off in case of line failure.In practice many FPSs are not provided with such

instrumentation/alarms see indicative statistics below. Ontype a) turrets in which the chains are permanently locked offunder the hull it is particularly difficult to monitor these linesin a reliable manner. For example, how do you readilydistinguish between mooring line and instrumentation failure,without direct intervention ?

Another factor which makes it difficult to be 100% sure of thecondition of a set of mooring lines is that line breaks do occuralong the sea-bed or in the thrash zone. If this happens the

line will drag through the mud until the friction exerted by thesoil surrounding the chain matches the tension in the chain at

its sea bed touchdown point. Experience has shown that highline pulls are required to drag large diameter chain through thesea-bed.

The following indicative statistics, based on data from themajority of North Sea based FPSOs, give an indication thatinstrumentation is not as prevalent as might be expected for

such a heavily regulated region:

50% of units cannot adjust line lengths,

50% of units cannot monitor line tensions in realtime,

33% of units cannot measure offsets from the no-loadequilibrium position,

78% of units do not have line failure alarms,

67% of units do not have mooring line sparesavailable.

The present position of the U.K. Health and Safety Executiveis that Operators should have in place suitable performancestandards for the time taken to detect a mooring line failure.This is particularly important as common mode failuremechanisms such as fatigue or wear are likely to be prevalent

on more than one mooring line and early detection of a line


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[OTC 17499] 11

failure with appropriate mitigation strategies could preventsystem failure. Depending on the inherent redundancy of themooring spread, the time taken to detect a failure could range

from virtually instantaneous detection to detection in a matterof days. It is clearly not appropriate to rely on annual ROV

inspection to check if a mooring line has failed. Monitoringthe excursion of a FPS, particularly using differential GPS is

inexpensive and will provide mariners with a feel for themooring integrity. But without real time monitoring of the

environment it is unlikely to indicate a line failure in anythingbut storm conditions, unless in deep water. Satellite drift is

also a potential factor affecting the reliability of offsetmonitoring.

New methodologies to detect a mooring line failure typically

feature acoustic transponders deployed through the turret,attached to the hull of the FPSO, or installed on the seabed to

provide an indication of the catenarys profile. Such systemsshould be trialed in the near future in the North Sea. Anotheroption may be a response learning system which takes into

account the expected performance in measured weatherconditions. The response will be different if a line fails due toa resulting change in the mooring system stiffness. Such an

approach requires further development work. But if theconcept proves successful this could prove to be a relativelysimple and inexpensive retrofit.

Contingency Planning - Spares and Procedures

Based on the indicative failure statistics reported earlier it isquite conceivable that a FPS may lose a line during itsoperational life. There is likely to be a several month leadtime to procure components such as large diameter chain,

wire/fibre rope or purpose built connectors, see for exampleFigure 15. Hence, to minimize FPS safety and business

exposure in case of line failure, it is believed to be wellworthwhile to have spare lines, connectors and proceduresavailable for immediate use if required. For deep waterprojects the procedures should ideally be developed which arebased on a generic anchor handling vessel rather than a highspecification installation vessel. Installation/constructionvessels are unlikely to be readily available at short notice and

tend to be expensive.

If a line does fail and no spares are available it may bepossible to mix and match making use of availableequipment from the established marine supply and rentalcompanies. However, the impact of introducing non standard

elements into a mooring system is best considered before afailure occurs. Long term mooring (LTM) shackles shouldideally be used as the connectors, but virtually any type ofshackle including alloy shackles would do in the short term.Repairs of this nature should give time for the procurement ofthe correct equipment, which may take around six months

depending on industry demand. Because the mooring systemhas been damaged and then modified, it may be necessary toobtain concessions from the relevant ClassificationSociety/Independent Competent Person (ICP). A reducedoperating envelope may have to be accepted during the periodthat the temporary repairs are effective.

Figure 15 Purpose designed connector for common link tocommon link chain allowing some compliance in two planes

Maximum Sea State for Continued Production Following

Line Failure

Once a mooring line fails it is believed to be no longerappropriate to apply the lower damaged system line safetyfactors. This is because, in most instances, the reason for theline failure will not be immediately apparent. Thus with theincrease in loading in the remaining lines there is an increasedchance of a further line failure. Hence, it is recommended that

the higher intact system line safety factors should be applied.Meeting the intact line safety factors with a degraded systemwill typically result in a reduction of the maximum allowablesea state. Data on the reduction in the maximum operational

sea state in case of line failure should be readily available onall units. The international survey indicates at the present timethis data is not generally available either with the designers or

on the units offshore.

Conclusions

Moorings on FPSs are category 1 safety critical systems.Multiple mooring line failure could put lives at risk both on

the drifting unit and on surrounding installations. There isalso a potential pollution risk. Research to date indicates thatthere is an imbalance between the critical nature of mooringsystems and the attention which they receive. On many FPSs

there is an important need to improve the knowledge base ofoffshore personnel on the intricacies of their mooring systems

and their potential vulnerability. This will help to ensure thatmooring systems receive the amount of attention they deserve,particularly during inspection operations.

The interface between the surface vessel and the mooring linerequires particular attention for all types of FPS. Carefully

planned innovative inspection making use of all possible toolshas been demonstrated to be able to detect problems relativelyearly on before they become a potential source of failure. Theuse of micro-ROVs to gain access to restricted areas notaccessible by conventional ROVs and divers has been part of

the key to this success. The inspection which has been


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12 [OTC 17499]

undertaken has shown the importance of achieving compatiblesurface hardness since it affects wear. Unfortunately, atpresent chain hardness and wear do not seem to be considered

in any detail.

In situ in-water inspection techniques are continuing toimprove, but further developments are needed to provide

dimensional data on links away from the inter-grip area and toimprove the marine growth cleaning off speed. At present no

in-water techniques exist to check for possible fatigue cracksand the development of such technology should be

encouraged. Inspection access needs to be improved anddesign briefs should assign a higher priority to designingsystems which are easier to inspect.

On one long term deployed North Sea unit chain wear andcorrosion in the thrash zone has been found to be significantly

higher than what is specified by most mooring design codes.This wear seems to be more pronounced on less heavilyloaded leeward lines compared to the more loaded windward

lines. Hence, it appears that more interlink rotation isoccurring on the leeward lines. More data is needed to findout if this is a general finding which could have long term

implications for other FPSs in the North Sea and elsewhere.

At present there is little data available which indicates how thebreak strength of long term deployed mooring componentswill be reduced by wear, corrosion including pitting and thepossible development of small fatigue cracks. Thus to assess

long term integrity with any confidence it is recommendedthat break tests on a statistically representative sample numberof worn components should be undertaken. Recovered linesfrom the thrash zone and from the fairleads/chain stopper area

would be ideal for testing. Such material is likely to beavailable whenever a FPS comes off station or has repairs

done to its moorings. As well as break tests, MPI,photographs and comprehensive dimension measurementsshould be undertaken. It is important that this data should befed back to the industry. Certain North Sea Operators haveshown a willingness to make this data available.

Offset monitoring has limitations in quickly detecting line

failure unless a FPS is in deep water. However, it is cheap andeasily installed. Hence it should be installed as standard on allunits. In addition, all units should know the maximum seastate in which they can continue to produce in case one linefails. On board emergency procedures should identify whataction should be taken in case of riser rupture while the risers

are still pressurized, although the likelihood of this happeningis low.

A possible contributory mechanism for the relatively highfailure line failure rate among drilling semi-submersibles hasbeen identified. This is believed to be due to rigs thinking

they have set up balanced pre-tensions when in fact this hasnot been achieved. Hence, it is recommended that Pay-In/Pay-Out tests should be undertaken to check whether the linetension readings can be relied upon,

Finally a general lack of suitable spare lines, connectors andrepair procedures has been noted. Given the substantialprocurement lead-time associated with these items it isrecommended that Operators should review their assets to seehow they could deal in the short term with one or more failed

lines. The reported statistics show that line failures have beenhigher than might normally be expected for custom designed

systems which are not regularly recovered and redeployed.Thus the business interruption potential due to mooringproblems should not be underestimated.

Acknowledgements

The crucial support to this project provided by the followingsupporting organizations is gratefully acknowledged: B.P.,Chevron Texaco, ENI, Norsk Hydro, PetroCanada, Statoil,Bluewater, SBM, Maersk Contractors/North Sea Production

Company, Wood Group/Amerada Hess, Bureau Veritas, ABS,Lloyds Register, U.K. Health and Safety Executive (HSE),Craig Group/IMS, Vicinay Cadenas, Ansell Jones/Oceanside,

MARIN, OIL/Zhengmao, Welaptega Marine, BalmoralMarine, BMT/SMS, National Oilwell-Hydralift/BLM,Hamanaka Chains and in particular to Williams Marine

Enterprises.

The project Steering Committee itself has been exceptionallystrong and it is hoped that it will be possible for the committeeto continue to meet during future FPSO Forum/JIP Weeks.This will provide a continuing reporting/recording mechanism

as more data becomes available. New participants to thiscommittee will be welcome.

References

1. FPS Mooring Integrity JIP Report, A4163, 2005, Noble DentonEurope Limited, Aberdeen.

2. Analysis of Accident Statistics for Floating Monohull and FixedInstallations HSE Research Report 047, 2003.

3. Recommended Practice for Design and Analysis of Station-keeping Systems for Floating Structures, API RP 2SK, 1997.

4. Position Mooring, DNV Offshore Standard OS E301, June 20015. Design and Integrity Management of Mobile Installation

Moorings, HSE Research report 219, 20046. Station-keeping systems for floating offshore structures and

mobile offshore units, ISO Draft International Standard,ISO/DIS 19901-7, Part 7, 2004

7. Cost Effective Mooring Integrity Inspection Methods, Hall,

A.D., OTC 2005, May 2-5, Houston, paper 174988. Review of Mooring Incidents in the Storms of October 1991 and

January 1992, HSE Offshore Technology Report OTO 92 013.9. Failure of Chains by Bending on Deepwater Mooring Systems,

Philippe, J., OTC 2005, paper17238.10.Marine Bio-deterioration : an interdisciplinary Study, Costlow,

J.D., and Tipper R.C. (Eds.), pp. 384, Naval Institute Press,

Annapolis, Maryland, 1988.

Documents

OTC JIP Key Findings