Torsional failure of a wire rope mooring line during installation in deep water

\ PERGAMON

Engineering Failure Analysis 5 "0887# 56Ð71

Torsional failure of a wire rope mooring line during installation in deep water C[R[ Chaplin Department of Engineering\ University of Reading\ Reading RG5 5AY\ U[K[ Received 7 October 0887^ accepted 01 October 0887

Abstract During the deployment of mooring components for an FPSO in deep water excessive torsional distortion was induced which led to failure of all six spiral strand mooring ropes[ A mechanism has been investigated which accounts for the failures[ The following issues are fundamental to the incident being considered] , the di}erent torque:tension characteristics of chain and wire rope , the sensitivity of di}erent components to twist , the interactions between chain and a work wire[ Þ 0888 Elsevier Science Ltd[ All rights reserved[ Keywords] Fatigue assessment^ Fatigue design^ Load history^ O}shore failures^ Rope failures

0[ Introduction Beneath the South Atlantic waters o} the coast of Brazil there are several large reservoirs of oil[ Much of this oil is located in water depths of between 499 and 1999 m[ This makes the Brazilian o}shore oil_elds the deepest in the world\ in terms of water depth\ and it is inevitable therefore that ~oating production systems installed by Petrobras are continually breaking new ground[ This is especially true of the mooring and anchoring systems employed\ and the methods used to install them[ This paper analyses a particular problem encountered during the installation of compound chain and spiral strand mooring lines on the FPSO "~oating production\ storage and o/oading system# P23 in the Barracuda _eld of the Campos Basin ð0Ł[

1[ Background Wire ropes are used in combination with chain\ anchors and now _bre ropes\ not only as component parts of mooring systems\ but also as the principal tension element for raising or lowering mooring components] the work wire[ Whether installed as components in a mooring line\ S0249Ð5296:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S 0 2 4 9 Ð 5 2 9 6 " 8 7 # 9 9 9 3 2 Ð 9

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or used for raising and lowering\ wire rope commonly acts in series with other components[ These di}erent components operate in static and dynamic equilibrium with each other\ transmitting mechanical loading along the line between surface and seabed[ The di}erent classes and sizes of components inevitably have di}erent mechanical characteristics\ and while such properties as strength\ sti}ness\ mass\ and even di}erences in fatigue are well understood\ the responses to twist and applied torsion\ though widely appreciated as a potential source of problems\ are generally not well understood[ And it is in the torsional response of the di}erent components that some of the most signi_cant di}erences in behaviour can be observed[ The interplay between these di}erences is important in determining the overall behaviour of a system\ but not only as regards the behaviour of the system which has been conceived by the mooring designer[ The historical sequence of operations\ starting from the installation of the _rst component\ can e}ectively alter the con! _guration\ and thus the response\ of the _nal system[ Because of the relative sizes and weights of the di}erent components used\ water depth becomes especially important when considering torsional behaviour during deployment operations[ These problems have parallels in the _eld of mine hoisting where the torsional response of wire rope has long been recognised as an important consideration ð1Ð3Ł[ The considerations given to the mine hoisting problem\ especially in the context of developments for ultra deep shafts "2999Ð3999 m# in South African gold mines ð4Ð6Ł\ has been a valuable source of information for understanding the mooring system and work wire handling problems[

2[ The selection of rope construction There is a signi_cant range of di}erent wire rope constructions supplied by rope manufacturers for o}shore mooring use\ each with di}erent combinations of attributes[ When selecting wire rope for di}erent applications a number of di}erent considerations will apply and the _nal selection will inevitably represent a compromise[ The essential characteristic of a rope is that it has high axial strength and sti}ness\ in relation to its weight\ combined with low ~exural sti}ness[ This combination is achieved in a wire rope by using a large number of steel wires\ each of which is continuous throughout the rope length\ which when loaded axially in parallel provide the tensile strength and sti}ness\ but when deformed in bending have low combined bending sti}ness provided their bending deformation is decoupled[ To facilitate handling it is necessary to ensure that the rope has some integrity as a structure\ rather than being merely a set of parallel wires^ this is achieved by twisting the wires together[ Whilst this gives the rope coherence\ it also creates lateral forces within the rope which increase with the axial tension[ Consequently\ each wire is gripped between its immediate neighbours so that when there is a local fracture of an individual wire\ the clamping forces generate frictional shears[ As a result within a short distance of the break\ the broken wire is carrying its full share of the rope load[ This clamping can also increase bending sti}ness raising wire stresses associated with operation over pulleys and drums[ To minimise this e}ect and so maintain bending ~exibility\ every wire in the rope must have some freedom to slide along its path within the rope construction\ e}ectively to cancel out the di}erences in bending strain implied by simple beam bending con! siderations "this is the essential di}erence between a rope and a solid bar#[ The above bending requirements would be satis_ed by a simple twisted bundle of wires^ however\

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Fig[ 0[ Typical rope constructions[

such a structure would\ if freely suspended\ simply untwist[ It would also be very vulnerable to damage of the outer wires\ which if broken at one location\ would just fall away and serve no further contribution to the load bearing function of the rope[ The resolution of these problems provides the basic motivation for the design of di}erent rope constructions[ Wire rope can be constructed to be more or less {torque balanced| "or non!spin\ or spin resistant# to minimise any axial torque generated by tensile load\ and any tendency to rotate when suspended[ However such constructions tend to have other disadvantages\ so before proceeding to a detailed consideration of torsional behaviour of wire ropes\ it is relevant to consider the relative merits of the three generic classes of rope construction[ 2[0[ Spiral strand Geometrically\ perhaps the simplest of rope constructions\ spiral strand consists of concentric helical layers of wire "Fig[ 0"a##[ The outer layers of a spiral strand\ which constitute the bulk of the cross section\ generally have wires of the same diameter\ opposite helical senses\ and the same\ or similar\ helix angle "but consequently di}erent helical pitch\ or lay length# with a core often geometrically similar to that of conventional stranded ropes "of mixed wire sizes but common pitch and sense#[ The characteristics of spiral strand\ as relevant to o}shore mooring applications\ can be summarised as follows] , spiral strands o}er high strength and sti}ness for a given diameter and wire grade^ , spiral strands can provide a high degree of torque balance^ , wires tend to be of larger diameter than in stranded ropes of comparable diameter*this bene_ts corrosion resistance but can limit the tensile strength of the wire employed "especially if shaped wires are used as in mining or ropeway applications#^ , the outer rope surface is essentially cylindrical which facilitates sheathing in polymer to provide long term corrosion protection^ , exposed outer wires are vulnerable to damage and this construction is unsuitable for operation on and o} multi!layer winch drums at more than low tension "unless having {full lock| outer wires\ and especially if sheathed#^

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, the construction is considered generally unsuitable for operation at pulley and winch diameter ratios less than about 24 ] 0^ , the construction is inherently expensive to manufacture because of the number of separate operations needed to build up the layers\ and the high capital cost of the large scale equipment needed^ , this construction is not tolerant to imposed twist\ which is quite di}erent to being torque balanced\ as discussed below[ 2[1[ Six strand This is the wire rope construction that is most widely used for general engineering purposes[ Six "or eight strand# ropes "Fig[ 0"b## are manufactured in essentially two stages] wires are twisted together to form the strands "which have an equal lay length to allow di}erent wire diameters to nest together#\ then six "or eight# strands are twisted together over a core to make the rope[ Two main categories of core can be used] a _bre core\ as in general engineering rope "easier to splice and cheaper to make#\ and wire rope core which provides higher strength\ and higher axial\ as well as transverse\ sti}ness\ the latter being especially important in resisting crushing when wound at high load onto multi!layer winches[ Another major feature of the construction of six strand ropes is the helical sense of the strands in the rope relative to the wires in the strand] where these are di}erent the outer wires appear to align with the rope axis and the construction is termed ordinary "or regular# lay^ where the helical sense is opposite\ along the strand crowns the wires are at a greater angle to the rope axis and the construction is termed Lang|s lay[ For o}shore use in connection with moorings\ when a stranded rope is used\ ordinary lay constructions are used almost exclusively[ The relevant characteristics of such ropes are as follows] , the rope has a high degree of damage tolerance and can be used at high loads on multi!layer winches^ , for any given ultimate breaking load this construction tends to be the cheapest of all^ , because of the problems of sheathing\ and thinner wires\ these ropes are not generally selected for very long term exposure to seawater^ , six strand ropes are not torque balanced "though that is possible*three strand ropes are manu! factured for oceanographic use at very high tensions in very deep water where a high degree of torque balance is essential#^ , ordinary lay ropes have better torque balance than Lang|s lay ropes^ , this construction is tolerant to torsion\ though perhaps not to the same degree as Lang|s lay ropes[ 2[2[ Multi!strand Multi!strand ropes "Fig[ 0"c## have two or more layers of strands\ the sense and lay of which are selected to achieve a maximum degree of torque balance[ The strands of these ropes are sometimes compacted "either by drawing or rolling# to improve the outer pro_le of the rope\ and the strand to strand contact stresses[ These ropes are used o}shore for applications requiring ~exibility and torque balance\ such as single fall crane ropes or diving bell hoist ropes[ The characteristics of multi!strand ropes are as follows]

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, multi!strand constructions can combine su.cient bending ~exibility and crushing resistance to operate on multi!layer winches\ and possess good torque balance^ , these ropes have smaller outer wires than other ropes of comparable strength\ so tend to have inferior corrosion resistance^ , because of the high contact stresses at the crossed contacts between the strands in di}erent layers\ fatigue damage in these ropes tends to develop internally rendering them prone to high levels of strength loss without externally visible signs^ , these ropes are signi_cantly more expensive than conventional six strand ropes of similar strength[ Multi!strand ropes have been used for mooring a ~oating o}shore production platform\ but with limited success[ 3[ The torsional response of wire rope Under conditions of rotational restraint\ conventional six strand ropes develop a torque which is approximately proportional to the tensile load\ however this torsional response is modi_ed by twisting or untwisting the rope[ This characteristic is illustrated in Fig[ 1 in which axial rope torque "M# is plotted as a function of axial tension "F# for di}erent levels of twist "df: dz#\ where a positive twist implies a reduction in lay length[ As would be expected\ a reduction in lay length increases the torque generated by applied tension[ The o}sets on the torque axis show the torque at zero tension] this is a function of the torsional sti}ness[ A typical characteristic for a torque balanced rope is also shown for comparison] note that torque balance is never perfect due to the various geometrical changes associated with tensile deformation "both radial and axial#[ Simplifying these torque characteristics for six strand rope to a set of parallel lines makes it possible to describe the relationships by an equation of the form]

Fig[ 1[ A schematic representation of axial torque as a function of rope tension showing the characteristics typical of six strand rope "as manufactured\ twisted up\ and untwisted#\ and a nominally torque balanced rope for comparison "the dashed line#[

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M  C = F¦T

df dz

where C "the torque factor# and T "the torsional sti}ness# are constants "in practice these are not constants and the relationships are much more complex being highly non!linear as well as having di}erent slopes and signi_cant hysteresis\ consequently being a}ected by the amplitude of any deformation#[ In this simpli_ed form\ both {constants| are a function of rope diameter and construction\ C begin directly proportional to diameter and T increasing with the fourth power of diameter for any given construction[ One example of the consequences of this type of behaviour is seen in the rotational deformation of a rope in a very deep mine shaft where the maximum tension in the rope\ at the shaft headframe\ derives from two components of similar magnitude] attached mass and rope mass[ The result is a linear fall in tension down the rope[ With a rope that is not torque balanced\ this would suggest a similar gradient in torque\ but that cannot be sustained so there is a counter rotation of the upper part with respect to the bottom to provide a constant torque throughout the rope[ In o}shore applications there is a need to understand such behaviour in order to anticipate when torsional problems can arise[ A simple knowledge of torque factor "constant C in the expression above#\ which may be supplied for di}erent rope constructions by the rope manufacturer\ only gives the torque generated as a function of tension when the rope is in its manufactured state of twist and constrained from rotation\ and does not indicate torsional sti}ness[ A more informative way of looking at the torsional characteristics of rope is to review the torsional sti}ness as a function of tensile load\ state of rotation\ and torsion amplitude[ This kind of response has been modelled by Rebel ð6Ł who has measured torsional response in this way\ as illustrated in Fig[ 2\ for _bre cored ropes with six triangular strands\ as used for mine hoisting in South Africa[ An alternative to Rebel|s highly complex model is another linear expression to characterise torque:tension behaviour of six stranded rope\ which has been developed by Feyrer and Schi}ner ð3Ł[ This is somewhat more sophisticated than the two term expression above\ re~ecting the changes in torsional sti}ness associated with changes in tension[ In the expression] M  c0Fd¦c1Fd 1

df df ¦c2Gd 3 dz dz

where c0\ c1 and c2 are constants\ which have been determined for di}erent rope constructions^ d is rope diameter^ G is wire shear modulus^ F and M are tension and torque respectively\ as before[

4[ Sensitivity of wire rope to torsional distortion Whether a rope is torque balanced or not the manner of use can result in the imposition of torsional distortion[ Of course this is more likely for ropes that are not torque balanced\ than those which are\ but torque balance is not a safeguard[ In practice the feature common to most torque balanced constructions of having concentric layers\ whether of wires or strands\ renders such ropes very vulnerable to twist[ There are two classes of distortion damage that can result from twist being imposed on a rope]

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Fig[ 2[ Results of series of torsional tests at constant tensile loads on 35 mm triangular strand Lang|s lay rope\ from Rebel ð6Ł[

a loop "or kink\ but also termed a hockle# formed in the rope as a whole^ and constructional deformation involving hockling of individual strands or wires\ or strands forming a {birdcage| caused by local buckling[ In either case\ the instability can be prevented by maintaining adequate tension[ The most serious deformations occur at very low tension[ Uncontrolled reloading of a rope once it has developed a torsional instability is likely to lead to severe permanent deformation which can reduce strength dramatically\ and damage fatigue resistance catastrophically[ The overall\ {whole rope|\ hockle can be removed by skilled handling if spotted before reloading\ but the smaller scale\ strand hockling cannot generally be removed without serious permanent degradation[ The tendency to overall or constructional deformation is a function of the bending sti}ness of the rope components\ and the loading caused by imposed twist[ In a six stranded rope\ untwisting whilst maintaining tensile load will eventually lead to local de!stranding of the rope "Fig[ 3# which\ when tension is relaxed\ will allow the torque carried by the individual strands to cause local strand hockles "Fig[ 4#[ If torque is imposed at low tension\ whether untwisting or twisting up\ then the whole rope will hockle[ However\ six strand ropes can sustain very severe torsional deformation\ provided a minimum tension is maintained[ Six strand Lang|s lay mine hoist ropes have been reported ð6Ł operating with lay length changes from −29) to ¦69) without any major compromise to service life[ Torque balanced spiral strand and multi!strand ropes both tend to have outer elements with diameters\ and consequently bending sti}ness\ which are a much smaller proportion of rope diameter than the six strand equivalent[ These outer elements also have a helical sense that opposes the layer underneath[ Consequently\ as already indicated\ these ropes are more susceptible to

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Fig[ 3[ Destranding of a six strand mooring rope[

Fig[ 4[ Hockles formed in the individual strands of an eight strand work wire[

{birdcage| damage through imposed twisting[ But twisting\ even without instabilities\ is damaging to these ropes because of the e}ect of unbalancing the load distribution between wires or strands in di}erent layers[ This can reduce fatigue endurance signi_cantly[ By contrast\ the wires in six strand ropes retain a better load balance under twisting\ so their fatigue performance is maintained[

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Fig[ 5[ Schematic representation of the proposed torsional behaviour of chain as a function of tensile load\ compared with a six strand wire rope[

5[ Torsional characteristics of chain A common practice is the use of mooring systems which combine wire rope and chain in series[ The torsional properties of chain are somewhat of a mystery other than a general acceptance that relative rotation of the order of 2> per link is possible with only nominal torque resulting[ Because of the torque reaction which can be generated in rope\ unless there is perfect torque isolation between components\ the response of chain is signi_cant[ However\ because of the dearth of speci_c information it is necessary to speculate\ and approximate the behaviour of chain[ Under a given constant tensile loading chain can be expected to have a limited range of rotation "the accumulated 2> per link# at more or less constant torque[ This essentially frictional torque will increase with tensile load[ Once the limit of this rotation has been reached the chain will rapidly become very sti}\ but eventually a limit will be reached at which the chain will start to knot up\ and if the tensile load is constant this process might be expected to continue at a more or less constant torque[ At zero tension there is e}ectively no resistance to rotation and chain can absorb rotation almost inde_nitely[ These characteristics are illustrated diagrammatically in Fig[ 5[ 6[ Interactions in installed mooring systems With six strand rope and chain in series\ and assuming both ends are prevented from rotating as tension increases\ the rope will generate a torque which will be transmitted through the whole line[ Initially\ the chain will take up its 2> per link\ allowing some corresponding unlaying of the rope\ but as the chain sti}ens\ further transfer of rotation will meet increasing resistance and the torque will increase more rapidly[ In a quasi!static situation this should not cause any problems\ but the dynamic response may well be di}erent[ Most experienced engineers have a natural feel

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for potential dynamic resonances\ but with very large structures and large masses these intuitions can be misleading[ The torsional sti}ness of wire rope mooring lines\ as compared with equivalent solid bars\ is so low that the period of natural torsion oscillation can be quite high\ and there is reason to believe from observed mooring line failures\ that such motions can sometimes be excited by the environmentally induced motions of ~oating structures[ If the torsional amplitudes are large enough\ and vessel motions such as to allow very low tensions to occur\ then the kind of torsional instability described above can develop[ Indeed\ the destranding in the mooring rope illustrated in Fig[ 3 is thought to have been induced by just such behaviour[ This kind of dynamic torsional problem is initiated by the torque developed in the rope which then causes rotation because of the di}erent characteristics of the adjoining component[ The use of a torque balanced rope will e}ectively remove the rotational response and so overcome the problem\ but operating procedures that always maintain a minimum tension would also avoid the rope being damaged[ It is also worth noting that the components in series which excite this kind of counter rotation need not be rope and chain[ Any torsionally mismatched pair will be susceptible\ be they six strand and torque balanced ropes\ unbalanced ropes of di}erent construction\ or even two ropes of the same construction but di}erent diameter[

7[ Problems encountered during installation Mooring installation operations provide numerous opportunities for torsion related problems[ Apart from the tendency for a stranded rope to rotate under tension\ one common mechanism for inducing twist is dragging the rope along the seabed ð7Ł[ This can happen when installing a wire rope mooring line deployed from a mobile unit[ The problem only becomes apparent when the tension in the rope is relaxed\ and distortion is induced between fairlead and a deck mounted winch[ Reeling back onto the winch then causes miscoiling which can subsequently lead to severe crushing damage[ The mechanisms which are the principal interest in this paper are those associated with raising and lowering components\ and speci_cally\ operations involving six strand {work wires|[ Numerous operations which involve wire rope coupled to a chain that is lying on the seabed can induce twist in the chain\ which can\ at a later time\ be transferred into yet another component[ If that component is sensitive to imposed twist then the consequences can be very serious[ An example of such a deployment\ based on the problems encountered during deployment of the P23 mooring system ð0Ł\ will be considered in detail and a hypothetical mechanism for the torsional damage to the P23 spiral strand mooring lines analysed[ The operation to be analysed involves the installation of a mooring lined in 0999 m water depth[ The original intent was that the completed line should consist of a conventional drag embedment anchor attached to somewhat in excess of 0999 m of chain\ attached in turn to a spiral strand and thus to a ~oating production\ storage and o/oading vessel "FPSO#[ The anchor and chain were to be pre!installed with the free end of the chain lying on the seabed attached to a six strand wire rope supported by a surface buoy[ The particular stage of the operation being considered is the

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Fig[ 6[ Schematic representation of anchor handling vessel "AHV# lifting the anchor chain with six strand wire rope {work wire|[ The increasing tension as more chain is lifted causes rotation at the point of connection between rope and chain\ the turns accumulating in the chain at zero tension on the seabed[

recovery of the end of the chain for the purpose of attaching the spiral strand[ This is achieved by hauling the rope pendant onto the anchor handling vessel "AHV# winch[ The chain is sized for the mooring application\ but the rope is a work wire\ and for this use has to support only the weight of the chain[ At the start of this process all the chain is on the seabed and the rope is just supporting its own weight[ As the rope is wound onto the winch the tension\ which is always greatest at the surface\ gradually increases as the heavy chain is raised[ This increase in tension causes the rope to untwist since there is no torsional restraint from the slack chain on the seabed "see Fig[ 6#[ As the chain is progressively lifted\ the untwisted rope is wound onto the drum and the opposing twist transferred along the chain\ accumulating in the _nal grounded section adjacent to the anchor[ Once the end of the chain is on the deck of the AHV\ the spiral strand is attached from a second AHV and the combination lowered back to the seabed as the spiral strand is paid out[ The arrangement with the three components and two AHVs prevents further rotation[ Finally the work wire is disconnected and the spiral strand tensioned[ The increase in tension in the chain transforms its torsional sti}ness\ causing the accumulated twist to be transferred to the lower sti}ness spiral strand[ When the system is again relaxed to await hook up to the FPSO\ the spiral strand can no longer sustain the imposed twist and forms kinks or hockles[ When these hockles are pulled straight for _nal connection to the FPSO the rope is e}ectively destroyed\ deforming as shown in Fig[ 7[ To try to quantify the turns transformed from the work wire to the chain\ the torsion model de_ned by Feyrer and Schi}ner ð3Ł has been used with values the authors obtained for a rope of appropriate construction but\ at 19 mm\ of rather smaller diameter[ Other quantities for the base case are as given below\ and relate to the incident referred to by Komura ð0Ł in so far as details are known[

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Fig[ 7[ Characteristic damage induced in spiral strand by pulling out a hockle or kink[

Rope] construction diameter mass in seawater c0 c1 c2

5×25 with IWRC 53 mm 09 kg:m 9[974 from ð3Ł 9[076 from ð3Ł 9[999420 from ð3Ł

"Note] in the expression for torque:twist units of N and mm have been used throughout and twist "df: dz# is de_ned in radians:mm[# Chain] diameter mass in seawater water depth

78 mm 049 kg:m 0999 m[

The calculation has been made on an incremental basis for each 49 m of chain raised\ calculating the degree of twist in the corresponding 49 m of rope wound onto the AHV winch[ This twist is based on the mean calculated for the start and _nish of each increment\ and is determined by considering the twist at each tension associated with torque value of zero\ the boundary condition determined from an assumption of zero tension where the chain touches the seabed[ The calculation indicates that a total of 83 turns will have accumulated in the chain on the seabed by the time the end of the chain reaches the surface[ On the assumption that the chain can accommodate 2> of rotation per link\ then\ the number of turns subsequently transferred to the spiral strand will be reduced by 08\ to 64[ In practice the chain may develop some resistance to the progressive transfer of so many turns\ possibly lifting knotted clumps above the seabed\ which would further increase

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Fig[ 8[ Number of turns transferred from the chain when tensioned as a function of water depth[

the suspended weight but still not change the minimal torsional sti}ness where the chain tension is zero[ Figure 8 shows how reduction in water depth reduces the number of turns transferred from the chain when tensioned\ with the other parameters as speci_ed initially[ Figure 09 shows how using ropes of di}erent diameter can in~uence the turns transferred\ with results shown for two di}erent water depths\ 0999 and 699 m[ In both _gures\ allowance has been made for an assumed capacity for the chain to accommodate 2> of rotation per link before adopting a high torsional sti}ness[ This capacity of chain to absorb induced turns can overcome the problem completely in shallower water "as seen by the e}ective cut!o} in Fig[ 8#[ It is evident from Fig[ 09 that signi_cant reductions in turns can be achieved from the use of rope with greater diameter and hence higher torsional

Fig[ 09[ Number of turns transferred from the chain when tensioned as a function of rope diameter\ the upper line for 0999 m water depth and the lower for 699 m water depth[

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sti}ness[ Obviously comparable reductions in chain diameter to reduce the rope tension would have equivalent e}ects[ The operation analysed here is only one example of a process which can induce rotation\ though this is perhaps the category of operation most likely to induce the higher amounts of twist[ Inevitably when working in even greater water depth much higher twists will be involved[ It must also be pointed out that the torque:tension model used for these calculations is fairly simple and has not been validated for ropes of the size used o}shore[ The analysis reported here has also been performed using the even simpler two!term model of rope response ð8Ł predicting a 24) lower rotation in the pendant rope\ but otherwise a very similar pattern of behaviour[ The reported facts relating to the P23 mooring line losses are that the deployment procedure was as described above\ and that the spiral strand was found to be torsionally damaged beyond recovery when connected to the FPSO[ The mechanism described here has been deduced from an analysis of the operation[ There are no observations that can con_rm\ or otherwise\ the relative states of twisting in di}erent components at each stage\ but no other explanation has been advanced[

8[ Steps to avoid induced torsion Apart from expensive solutions such as using two lines to di}erent AHVs\ or other devices attached to the connection between components to prevent the transfer of turns\ what else might be done to avoid this kind of problem< There are essentially three categories of solution] one involving rotating connectors "swivels#\ the second involving the use of torque balanced ropes\ and the third involving the selection of twist tolerant components[ The merits of each are considered below] 8[0[ Rotating connectors , Conventional swivels are of little use here because they have high friction and therefore really only operate at very low tension[ This is ideal to release torque to facilitate handling of a rope adjacent to a connector when restrained on the deck of an AHV[ , The use of special {low friction| swivels in the case study above may have little bene_t when coupled between chain and pendant work wire[ This is because of the combination of signi_cant tensile load and very low torque "associated with the low torsional sti}ness of the unloaded chain#[ Furthermore there is no validated quantitative data available for the relationship between load transmitted and {break!out| torque for these devices\ which makes any analysis impossible[ , Permanent installation of a {low friction| swivel between the chain and spiral strand should have the bene_t of limiting the transmission of accumulated turns from chain to rope as the mooring system is tensioned[ However\ such a policy would still run the risk of residual turns in the chain forming knotted clumps with serious loss of strength and fatigue performance "this risk is of course present with any option that does not prevent twisting of the chain in the _rst place\ and in fact one of the mooring chains in the P23 operation described above was broken at just such a knot during retrieval#[ , The use of a {low friction| swivel as a permanent connection in a mooring line which combines

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components having di}erent torque:tension characteristics "e[g[ chain and six strand ropes# is likely to result in torsional oscillations as the tension ~uctuates[ This will introduce additional fretting between the wires of the rope to compound fatigue[ There is no reliable published fatigue data to indicate how seriously this might a}ect endurance[ 8[1[ Torque balanced ropes , The choice of ropes with constructions having better torque balance characteristics\ for pendant ropes and work wires\ would have undoubted success in reducing the introduction of turns into chain\ and subsequently spiral strand[ However\ these ropes are signi_cantly more expensive than the six strand constructions currently employed[ , Ropes of such constructions are currently used routinely for diving bell hoisting and as single fall {whiplines| for cranes[ , There is some precedent for using torque balanced ropes as work wires during installation\ for example in lowering the clump weights for the Lena guyed tower in the early 0879s[ , Torsionally balanced ropes tend to have smaller outer wires than their six strand equivalents[ This makes them less robust and more vulnerable in aggressive mooring deployment operations[ 8[2[ Use of twist tolerant ropes , If installation procedures are likely to induce turns that can ultimately be transferred to com! ponents with a low tolerance to twist\ especially torque balanced wire rope\ then one remedy is the avoidance of such twist sensitive constructions for a mooring line[ , Current developments in moorings for deep water include the use of polyester _bre ropes[ Most of the constructions selected to date for this application comprise a braided outer cover for a set of essentially parallel sub!ropes which form the load bearing members[ At present there is no information available as to the torque:tension characteristics of these ropes\ but given the low level of twist in the sub!ropes\ a reasonable level of tolerance to imposed rotation might be expected[ It is of interest to note that\ necessity being the mother of invention and as a result of good fortune\ Petrobras were able to replace the damaged spiral strand by available polyester _bre ropes[

09[ Conclusions and recommendations , Wire ropes used as either mooring components or as work wires during installation can have a tendency to twist under tension[ This twist can be transferred from one component to another "especially during installation operations# with potentially serious consequences as regards twist sensitive components such as torque balanced wire rope\ and even chain in extreme cases[ , These mechanisms whereby turns can be generated are exacerbated by water depth\ indeed the capacity of chain to absorb some twist can overcome the problem completely in shallower water[ , Quantitative models of the torque:tension characteristics of all components employed are neces! sary to facilitate prediction of their torsional interactions[ However\ the _rst step in any such prediction is to appreciate that such mechanisms occur at all[

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, Possible steps to mitigate against this problem include] "0# the use of torque balanced ropes as pendants and work wires^ "1# the use of low friction swivels "although there is a dearth of data on such devices and furthermore the e}ect of cyclic rotation in permanent moorings has not been investigated#^ and "2# the avoidance of twist sensitive rope constructions as permanent mooring components[ , More information is needed to facilitate accurate prediction of these interactions[ This is especially the case of chain and swivels[ There is an understanding of the problem in the rope fraternity "both wire and _bre# where it has long been recognised particularly in the context of deep mine shafts\ but experimental data for realistic rope sizes is not currently available[ Acknowledgement The author acknowledges the invaluable input to this paper from discussions with engineers employed by Petrobras in Brazil[ References ð0Ł Komura AT[ Experiences in some installations of mooring lines with polyester rope in Campos Basin Brazil[ Proceedings of the Third International Conference on Continuous Advances in Mooring and Anchoring[ IBC\ Aberdeen\ June 0887[ ð1Ł Layland CL\ Rao BE\ Ramsdale HA[ Experimental investigation of torsion in stranded mining wire ropes[ Trans! actions of the Institution of Mechanical Engineers 0840]212Ð25[ ð2Ł Kollros W[ The relationship between torque\ tensile force and twist in ropes[ Wire 0865^08Ð13[ ð3Ł Feyrer K\ Schi}ner G[ Torque and torsional sti}ness of wire rope parts I and II[ Wire 0875^25"7#]207Ð19 and 0876^26]12Ð6[ ð4Ł Wainwright EJ[ The manufacture and current development of wire rope for the South African mining industry[ Proceedings of the International Conference on Hoisting of Men\ Materials and Minerals[ Canadian Institute of Mining and Metallurgy Toronto\ Canada\ June 0877[ ð5Ł McKenzie ID[ Steel wire hoisting ropes for deep shafts[ Proceedings of the International Deep Mining Conference] Technical Challenges in Deep Level Mining vol[ 1[ Johannesburg SAIMM\ 0889\ p[ 728Ð33[ ð6Ł Rebel G[ Torsional behaviour of triangular strand ropes for drum winders[ Proceedings of the Application of Endurance Prediction for Wire Ropes\ OIPEEC\ Reading\ September 0886\ p[ 024Ð59[ ð7Ł Chaplin CR[ The inspection + discard of wire mooring lines[ London] Noble Denton\ 0882[ ð8Ł Chaplin CR[ Torsion problems caused by wire rope during mooring installation operations in deep water[ Proceedings of the Mooring and Anchoring[ IBC Aberdeen\ June 0887[