CLASSIFICATION OF FIBRE ROPE MOORINGS FOR … OF FIBRE ROPE MOORINGS FOR ... M. A. Pereira Bureau Veritas do Brasil ... integrity of the mooring system is a corner stone in the Classification

CLASSIFICATION OF FIBRE ROPE MOORINGS FOR

FLOATING PRODUCTION UNITS

M. Francois Bureau Veritas ParisM. A. Pereira Bureau Veritas do BrasilC.Raposo Bureau Veritas do Brasil

Abstract— With the exploitation of Offshore oil resources moving to deeperand deeper waters, new systems for the mooring of Floating ProductionUnits, have been developed, based on innovative technology for componentssuch as mooring lines and anchor points, including fibre ropes, suction pilesand Vertically Loaded Anchors, taut leg mooring system, etc...Several of such systems are to be installed by PETROBRAS OffshoreBRAZIL, in 1997.

Mooring integrity being a corner stone in the Classification of FPUs, in thispaper are presented the new approach, procedures and requirements forClassification of the mooring system which have been developed, in closecooperation with ongoing projects, in order to take the challenge ofinnovation, whilst maintaining the level of safety which is the objective ofClassification.The framework of classification being briefly reminded, the issues on thedesign and operation, and on the analysis of mooring system, which arespecific to mooring systems using synthetic fibre ropes are reviewed anddiscussed.A description is made, of the fibre ropes, as now proposed by Manufacturersfor use in mooring lines, and of the most salient properties of this product.The procedures which have been developed by Bureau Veritas, based onapplication in recent projects, for the Certification of Fibre Ropes for mooringsystem are presented.

Transactions on the Built Environment vol 29, © 1997 WIT Press, www.witpress.com, ISSN 1743-3509

156 Offshore Engineering

INTRODUCTION

The move in the exploitation of Offshore Oil and Gas resources into deeperand deeper waters have triggered the development of new systems for themooring of Floating Production Unit, such as the concept of taut legmooring. These developments are based on innovative technology forcomponents of mooring lines and anchor points, such as fibre ropes, suctionpiles and VLAs, etc...After a number of studies, experimental work and field trials in the past tenyears, Operators are now moving to full scale implementation, and several ofsuch systems are to be installed by PETROBRAS Offshore BRAZIL, in 1997.i

In order to take the challenge of innovation, whilst maintaining the level ofsafety which is the objective of Classification, and considering that theintegrity of the mooring system is a corner stone in the Classification ofFPUs, new approach, procedures and requirements for Classification of themooring system have been developed within Bureau Veritas, in closecooperation with ongoing projects.In this paper are presented the specific criteria for the design and analysis ofmooring system for FPUs, using synthetic fibre ropes, and the procedureswhich have been developed for the Certification of Fibre Ropes, withexamples of application to recent projects.

CLASSIFICATION OF FLOATING PRODUCTION UNITS

The objective of classification activities is for the Classification Society toexpress an independant opinion on the fitness for purpose of a unit(traditionally a trading ship, nowadays also a drilling rig, a production unit,etc...)This advice is based on classification activities, that include the appraisal ofthe design, the certification of equipment and material at Vendors' premisesworldwide, the survey of construction and installation, and the surveys of theunit in serviceAs described in the Rules * , classification covers all issues relevant to theoverall safety of the unit, such as buoyant stability, hull structural strength,systems and equipment essential to the safety of the unit, and for a FPU, thesafety of the process installations.For a FPU, and especially for a permanent system, intended for long durationactivity at a given site, e.g. 20 years, such as the PETROBRAS XIX semisubmersible floating production unit, the integrity of the mooring system isalso a corner stone in the Classification process, taking into account the


Offshore Engineering 15 7

hazard that a failure of the mooring system would put on riser integrity,nearby installations, etc...

Classification of mooring systemThe scheme of classification of the mooring system, for the delivery of aPOSA notation to the Unit, includes :design appraisal of the mooring system, and of mooring system components(e.g. anchors),certification / inspection at works of ropes, and other components,survey of installation activities,

periodical survey of system in operation, including monitoring activities, asdiscussed later.

The issues specific to mooring systems using fibre ropes are hereunderpresented and discussed.

MOORING SYSTEM

Arrangement of Mooring SystemA mooring system will generally include, for each line:a top length, usually a chain length,a main fibre rope segment,a bottom chain, and an anchor point into sea bed.The top length will maintain fibre rope well below surface, and clear ofplatform fairleads.

The main fibre rope segment may be made in one or several segments, forpractical reasons of fabrication or handling.For bottom chain, a long enough segment is provided, in case of a catenarysystem, so as to get no, or very little uplift load at anchor.In case of a taut leg system, only a short, taut, bottom chain segment isneeded, so as to keep the fibre rope clear of sea bottom, thus avoidingdetrimental chaffing, and a positive tension in fibre rope.Of the principal advantages of a taut leg system, over conventional mooring,one is the saving in anchoring radius (limiting interference with theunderwater developments) and another is the benefit of smaller excursions onriser design.", *

OperationDuring operation, and primarily in the first months after installation, lines willslacken, due to creep, and retensioning of lines will be necessary, to resetpretension to the design value.The length of the top section is to be determined, taking into accountestimates of the initial permanent elongation of fibre rope, and of rope creep.



Besides, due to the present uncertainties on the long term performance offibre ropes, the main section is to incorporate short rope segments, that canbe recovered periodically, for inspection and retesting, in order to monitor thein service behavior of rope, unless other provisions are made to achieve sameobjective.

DESIGN OF MOORING SYSTEM

AnalysisThe mooring analysis is performed with a view to get tensions in componentsof lines, for line sizing, and vessel offsets that will be used, e.g., for the designof risers. The process of analysis is generally the same as for conventionalmooring. An important aspect is the careful estimate of loads on the unit: e.g.the load on risers may be, for a FPU, a significant part of the total load. Moredetails are given in *.The proper specification of line stiffness is a critical issue for analysis , and adifficult subject given the properties of fibre ropes, as described in the nextsection.

Using load extension curves, as currently available for hawsers, will notprovide a realistic representation of dynamic line stiffness.Two approaches have been proposed, based on elongation measurements:either use an upper bound and a lower bound estimates of stiffness, to getseparately conservative estimates of loads and offsets * ,or, as in analysis, decompose the loading into several components: static,slow drift, and first order motion, and use appropriate stiffness for eachterm \This is however an area where further testing or instrumenting in servicesystem would bring useful information.

Design criteriaLine The design criteria for the strength of line components depend on thecondition examined, but also on the methodology used for analysis (e.g.frequency domain analysis, or time domain simulation) and its degree ofrefinement: they have to be taken consistent with this methodology.However, in order to account for the uncertainties in stiffness, and in the longterm performance of fibre rope, an increase of Safety Factors, by around 20%, is recommended for the rope section.Besides, the length of the bottom chain segment is to be sufficient to keep apositive tension in rope, in the leeward lines, for all design conditions

Steel fittings the design of steel parts does not differ from conventionalmooring systems. Design is made to achieve the specified Minimum Breaking


Offshore Engineering 159

Load, usually with some conservatism, to avoid actual breaking during thebreak load test, or from assumptions in analysis.Besides, mention is to be made that, with the present trend of using morefrequently materials with higher and higher strength, and the long exposurescurrently considered, the risks of fatigue failure steadily increase, requiringspecial attention during design.

Anchors For taut leg system, the anchor point have to withstand asignificant amount of uplift. Whilst conventional piles could be used, newconcept, such as suction piles, and Vertically Loaded Anchors areimplemented in current projects.The design of drag anchors, in term of holding capacity, have beentraditionally based on testing. Same are new VLAs, with, together, aquantification of soil properties. Anchor piles are usually designed from a pilesoil analysis, following the method used for jackets. For suction piles, specificgeotechnical models were developed *.Unlike conventional anchors, load after failure will quickly drop, due to theuplift component, resulting in a total loss of capacity. To account for this, andthe increased uncertainties in analysis, about the combination of uplift failureand lateral failure, an increase of Safety Factors for the required ultimatecapacity, by say 40 %, is recommended.Concerning the structural strength, the nominal stresses in the body ofanchor, i.e. the parts that are exerting pressure on soil, are normally keptbelow material yield strength, under the required ultimate capacity, unless acombined elastoplastic FE analysis with both soil and structure is made. Forthose elements located in serie with mooring line, same criteria as describedearlier could apply.

FIBRE ROPES

BackgroundSynthetic fibre ropes are nowadays commonly used as mooring lines forships. A noteworthy application is Offshore mooring of large tankers tooffshore SPM's, for which rope manufacturers have developed efficient ropeconstructions, based on the "high performance" fibres now offered by fibremakers.In the past ten years, a number of studies, experimental work, and field trials,have been conducted, towards using fibre ropes for anchoring systems in deepwaters, taking benefit from the combined low density, high strength andmoderate stiffness of most synthetic fibre ropes V .



Make up of synthetic fibre ropesFibres A number of different synthetic fibre material are currently producedfor the making of synthetic fibre ropes. Several of these could candidate foranchoring lines, given their mechanical properties, but for the present, takingperformances and cost into account, primarily polyester (PET) fibres areconsidered.During manufacture, a finish ( after-finish) is applied on yarn, in order toimprove rope strength and durability, by reducing yarn to yarn friction andabrasion, thus avoiding a detrimental heating of rope core during cyclicloading. A non water soluble finish (marine finish) must be used, but evidenceis missing that such properties will be retained in the long term.

Rope construction Synthetic fibre ropes are proposed in a number ofconstructions, with low twist constructions, having better performance andhigher stiffness than those for conventional hawsers, to meet the demand.Those currently proposed include parallel sub-ropes constructions, with arope core made of twisted or braided sub-ropes, and a braided cover. Thecover is holding elements together, and protecting rope core from mechanicaldamages during handling and in operation. Multi strands wire ropeconstructions, and parallel yarns constructions are also proposed, withextruded or braided cover.The strength of rope is depending on yarn strength, and on rope construction.The breaking strength of given ropes is to be established by testing ofprototype ropes.

Terminations Spliced termination is the conventional method for ropes.Thanks to a careful design, and high skill in the execution, the strength oftermination can be expected to approach the strength of rope current section.A thimble is used to hold the eye, with a shackle to connect the rope toadjacent items.Other termination methods such as resin socket, or cone and plug socketting(for parallel yarn ropes) could be considered, but the efficiency of suchterminations for large rope size is not well known.

Mechanical propertiesSome important aspects of the mechanical properties of fibre ropes areoutlined hereunder. It is important to acknowledge that these are based on thepresent knowledge, and that, until now, although available informationgenerally provide positive indications on the suitability of PET ropes foranchoring lines, the return of experience of actual, full size, installations islacking, to confirm to which extent, and how, these properties are affected bythe long term exposure to environment and loads, within a mooring system.



Load-elongation characteristics The knowledge of rope load-elongationcharacteristics is of primary importance for application to mooring.Unfortunately, the load-elongation characteristics of PET synthetic fibreropes is a complex function of load, load rate, and load history, which can beonly schematized.During initial loading of a rope, a mostly permanent, non recoverableelongation is observed.Under a constant load, creep will be observed, but, in the range of loadconsidered, the creep rate quickly decrease with time.Under cyclic loading, the stiffness was shown V to increase with higher meanload, but decrease with larger load range, up to more than five times theapparent stiffness at initial loading.The apparent rope stiffness depend on yarn stiffness, but also strongly onrope construction. For given ropes, stiffness at appropriate loads have to beobtained by measurements.

Fatigue strength At moderate mean load and load ranges, PET rope isexpected, from available results, to behave not worst than steel wire ropes.However, data on fatigue strength are not numerous, and it is desirable thatsystematic tests are performed.At higher loads, another rope failure mode (creep rupture)is observed, butsuch mode of failure, relevant for hawsers, is not expected for mooring lines.Apparently, the breaking strength of PET ropes is little affected by creep(under moderate load) and by cyclic loading.

CERTIFICATION OF ROPES

Approach

The objective of a certification of ropes is to provide, besides Manufacturerguaranty, a Third Party assurance that delivered lines will be suitable for theirintended use, within the present limits of knowledge on the application asanchoring lines.A scheme of certification of the ropes is proposed, in which are incorporateda comprehensive identification of the product, of its manufacture, and relatedQA-QC activities, and systematic testing.™Due reference is made to sound existing QA-QC practices for synthetic fibreropes, given in the OCIMF guideline for hawsers. These are however notconsidered as being sufficient for the present purpose, taking into account thecriticality of application, and the level of verification currently practiced in theOffshore industry, in consideration of that criticality.Testing of prototype rope are included in the scheme of certification, with theobjectives to verify Breaking Strength of proposed rope, get evaluations of



actual load-elongation properties of proposed rope, or confirm predictions,(noting that, for a given mooring system, required MBL is actually dependingon rope stiffness), and assess or confirm fatigue strength of rope.

Process of CertificationThe certification of ropes intended for a given project, is based on a twophase process:1) Rope design and approval, based on manufacturing and testing of a fullsize prototype rope, with same material, construction, terminations, and size ,than specified rope.A Certificate of Approval (Independent Review Certificate ) is delivered toManufacturer, for the specified rope, at the satisfactory completion ofcorresponding certification activities.2) Rope manufacturing and inspection, in/for conformity to approved design,resulting in the delivery of a Certificate of Inspection (Certificate ofConformity) to supplied ropes.The Certificate of Inspection will make reference to the Certificate ofApproval.For another supply of the same rope, the inspection is carried out in the sameway, without repeat of the testing and approval phase, as long as no deviationfrom approved design is made.Besides, in order to address design and manufacturing changes, and othersizes of the same rope design, a Product Approval procedure is proposed, bywhich the Manufacturer can take benefit of his expertise, and of theknowledge accumulated during the approval process, to optimize testing.

Rope design and approvalThe specification issued by rope Purchaser for a given project generallyinclude a limited amount of information, related to intended purchase, such asnumber and length of ropes, required MBL, and conditions of use at intendedsite.Based on this, the Manufacturer prepare the detailed design andmanufacturing specifications for the ropes, and the Quality Controlprocedures and Inspection Plans.These documents are reviewed, and prototype manufacturing surveyed ",with a view to ascertain that prototype rope conform to proposed design, andthat all aspects of design and manufacture, that could affect quality of finishedproduct, have been addressed and documented. At the same time, anevaluation of Factory, and of its quality system, for subject production, ismade.The design and manufacturing specifications are kept as confidentialdocuments. A supply specification is to be also issued by rope Manufacturer,to give to purchaser a general description of proposed product, and the



information which is necessary for mooring design, such as weigh data andstiffness predictions (later updated with test results).The testing of prototype ropes are witnessed, and, upon satisfactorycompletion, a Certificate of approval is issued.

Rope manufacture and QA/QCQA/QC and inspection activities during manufacture are expected to be, as aminimum, to the level of those of OCIMF Procedures for Hawsers, dulymodified to account for the particular rope construction, and subjectapplication.Yarns are supplied with yarn certificates attesting the conformity of supply toyarn specification, and are retested by Manufacturer, at random.Samples of Rope are taken during manufacturing, and inspected forconformity to design.Termination fittings and accessories are to be manufactured and tested, asmooring line fittings, in accordance with Rules ™

TestingTests are performed on sample ropes in full size, including same terminationsas final supply, so that the test can qualify the full assembly.All tests are performed in wet condition, with an appropriate sequence ofloading/unloading and cycling of rope, for setting of rope.The testing program include, as a minimum:A Breaking Strength test, on three test ropes,Load elongation measurements, on at least one test rope,A fatigue test, on at least two test rope

Additional tests may have to be done, in specific circumstancesThe load, the loading rate, and the overall elongation of rope under test, arecontinuously monitored.Load elongation measurements include initial elongation, at first loading, andstiffness measurements, under cyclic loading, with specified mean load, loadrange, and frequency. Several sets of conditions are applied, that arerepresentative of the various loading regimes expected in line. Elongation,stiffness and hysteresis of rope in each condition are obtained from records ofelongation over a gauge length, at mid span of test rope, free fromtermination.

ApplicationA scheme of approval and inspection as described above was successfullyimplemented, in 1996, with two rope Manufacturers:Cordoaria Sao Leopoldo, in Brazil, for the supply of 500 t breaking loadUltraseven ropes, for the taut leg mooring system, in 620m water depth, ofthe Petrobras XEI, in CAMPOS Basin (Brazil)



Marlow Ropes, in UK, for the supply of 700 t breaking load Superline ropes,for the taut leg mooring system, in 770m water depth, of the Petrobras XIXFPU, and for deep water mooring (around 1400m) of the FPSO2, both inCAMPOS Basin (Brazil)

Product ApprovalThe Product Approval is a procedure that will aim at extending the approvalof one rope, in order to cover a range of diameters, for the same rope corefibre, same cover, same construction and fabrication process, and samemethod of termination, or to cover some variations in the above parameters,except the fibre.The Product Approval file can be open with available information, i.e. as aminimum, sizing criteria, strength and stiffness predictions for the proposedrange of rope sizes, when the activities of prototype approval have beencompleted, for one rope size of the same product. The file will be, from then,updated with the results of additional tests that will be performed by theManufacturer, under survey, for the purpose of approval, or for a specificproject, or taking into account the better knowledge that is expected to resultfrom in service experience.An Initial Product Approval Certificate will be delivered, based on the reviewof the initial file, and this certificate will be renewed, based on the content ofthe Product Approval file, when relevant.After a Product Approval Certificate has been delivered, the process ofapproval of one particular rope covered by the certificate will be greatlysimplified and shortened, with testing limited to breaking test, taking testropes out of the first length produced.

CONCLUSION

New systems for the mooring of Floating Production Unit, have beendeveloped, using synthetic fibre ropes as mooring lines, and several of suchsystems are to be installed in 1997.In this paper were presented the new approach, procedures and requirementsfor Classification of the mooring systems which have been developed, forsuch new systems, in close cooperation with ongoing projects, in order totake the challenge of innovation, whilst maintaining the level of safety whichis the objective of Classification.The issues on the design and operation, and on the analysis of mooringsystem, which are specific to these systems were reviewed.The procedures which have been developed by Bureau Veritas, based onapplication in recent projects, for the Certification of Fibre Ropes for mooringsystem were presented



AcknowledgmentsThe authors wish to thanks Petrobras, and Bureau Veritas for permission topublish this paper. The views expressed are those of the authors, and notnecessarily reflect those of Bureau Veritas.

REFERENCES

[1] CJ.M. Del Vecchio: « Taut leg mooring systems based on fibre ropes -Petrobras' experience and future developments », Aberdeen, June 1996[2] J.Formigli, S M Porciuncula: « Campos basin: 20 years of subsea andmarine hardware evolution », OTC8489, 1997[3] Bureau Veritas; « Rules for classification of Offshore Units », 1997[4] C.J.M. Del Vecchio: « Taut leg mooring systems for deep waters », BrazilOffshore 95,[5] M.Francois, F.Legerstee, C.Raposo, D.Berdin: « Classification of newsystems for deepwater mooring of floating production units », Aberdeen, June1997[6] J.Trickey, R.Stonor, A. Theophanatos: «Cost and performancecomparison between catenary and taut leg for ultra deepwater FPP's »,Singapore, feb 1997[7] C.R.Chaplin and CJ.M. Del Vecchio: «Appraisal of lightweightmoorings for deep waters », OTC6965, 1992[8] P Sparrevik: « Suction anchor piles - state of the art», Aberdeen, June1996[9] R.L.M.Bosman: « On the origin of heat built up in Polyester ropes »,1996[10] Bureau Veritas: Guidance note « Certification of synthetic fibre ropesfor mooring systems », May 1997[11] Bureau Veritas: Rule note «Approval and inspection at works ofmaterials and equipment», 1992[12] Bureau Veritas: « Rules for the classification of steel ships and offshoreunits - Materials », 1992


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CLASSIFICATION OF FIBRE ROPE MOORINGS FOR … OF FIBRE ROPE MOORINGS FOR ... M. A. Pereira Bureau Veritas do Brasil ... integrity of the mooring system is a corner stone in the Classification