This paper explains and defines the synthetic fiber rope stretch and stiffness change-in-length properties which are important for deepwater platform mooring system design. It introduces the concept of accumulated elastic stretch, which causes temporary stretching and stiffening of the rope during tension cycling. It proposes how these properties can be determined by rope testing and by computer modeling. It proposes how the change-in-length properties should be used in mooring system design and analysis. Background The offshore industry and the fiber rope manufacturers have now had some experience in using fiber ropes in deepwater platform moorings. Petrobras pioneered this in the mid 1990s. In the late 1990s several class societies 1 2 3 and then API 4 published guidelines for fiber rope deepwater moorings. Tension Technology International with others also published fiber rope design guidelines.5 Those guidelines were based on what was then perceived to be the best design methods and the best test methods for the rope properties needed to carry out those methods. Since those guides were published, several fiber rope mooring systems - Mad Dog 6 7 and Red Hawk - have been designed for the Gulf of Mexico. A number of large fiber ropes have been tested and analyzed for use in those and other projects. Also, findings are now available from several significant fiber rope research programs which were conducted while and after those guides were written. Introduction The unique stretch and stiffness characteristics of synthetic fiber ropes are important in the design and analysis of deepwater platform mooring systems. Mooring system designers are experienced in designing with wire rope and chain catenary mooring lines. The stiffness characteristics of wire rope and chain are essentially linear. These components do not experience permanent stretch until high tension. With these components, the principal mooring system restoring force is produced by the catenary effect. As mooring water depths increase, there are incentives for using fiber ropes, as discussed in other papers. But with fiber rope, the principal mooring system restoring force is produced by the tension vs. stretch (stiffness) characteristics of the rope instead of by the catenary effect. Mooring system designers are still trying to understand the unique stretch and stiffness characteristics of fiber ropes and to incorporate them into traditional mooring design procedures. The stretch and stiffness characteristics of typical synthetic fiber ropes are time dependent and non-linear. Fiber ropes experience both elastic and permanent stretch. Conflicting and confusing terminologies are sometimes used. Some important rope properties have only recently been adequately understood and explained. Here the term stretch refers to any change in rope length hich is produced by tension. Stiffness is the relationship between stretch and applied tension. Other terminology is introduced in the text and summarized in Nomenclature at the end of this paper.

SummaryThis paper brings together two experiences from TTI (Tension Technology International) and the UoE (University of Exeter) and aims to quantify the damage that could be experienced by fibre mooring lines during the transit and commissioning phase of Marine Renewable Energy (MRE) devices.1. While deploying a new prototype of anchoring system for a MRE device, UoE experienced the failure of a new polyester mooring line during transit to site (90 minute duration). A thorough visual inspection supported by Scanning Electron Macroscopy (SEM) analysis and tensile test have established the physical conditions of the fibre rope before its failure.2. Wear testing led by TTI has investigated and established a relation between the degree of abrasion sustained and the residual strength of Dyneema ® fibre ropes. The ropes were submitted to friction forces via cycling over a roller and a fairlead with different abrasive surfaces (from 7.6 to 254.3μm). Negligible wear occurred with the low roughness surfaces and very high wear at the higher roughness surfaces.The load experienced by the polyester line during the deployment of the anchoring system was simulated with the software OrcaFlex TM . Dynamic modelling using representative conditions indicates that the polyester mooring line was loaded between 3 and 20% of its Minimum Breaking Load (MBL) which is well below the design load of the rope. IntroductionTo date there are only a few MRE designs which have achieved a pre-commercial development status. Experience in deploying such devices and specific guidance are missing. The approach of designers is thus conservative and based on the offshore oil and gas industry and anecdotal evidence [4].The use of synthetic fibre ropes for mooring systems has seen a sudden rise in marine applications over the last two decades [5] albeit mainly for deep water mooring [6]. Experience and feedback from the use of polyester ropes for MRE device moorings is still limited in terms of publications produced. This paper focuses on the commissioning phases of MRE devices, with a specific focus on the deployment of a novel anchoring system. Handling, lifting and rigging of mooring lines could result in damage during transport or installation as discussed in [7] and [8]. The strength of a rope will inevitably degrade from mishandling. Several research programs have been carried out to investigate the abrasion and twisting damage by rope manufacturers for towing systems. For example incorrect handling during transport could cause significant damage to the rope due to abrasion and the application of sudden high loads is one of the most severe conditions that a rope can experience [9].Certification standards and specific test methods have been developed to test the abrasion resistance of yarns [10] - [11]. The abrasion mechanism was described in [12] as a sequence of fibre peeling induced by surface shear forces. However the abrasion resistance of a fibre rope is not determined by standards and knowledge is only based on empirical test data.The exte...

Four failure modes have concerned others as potentially limiting the service life of polyester ropes used as deepwater mooring lines: axial compression fatigue, creep rupture, hysteresis heating, and internal abrasion. Based on extensive investigations and testing, the first three modes are of no concern with polyester rope.Internal abrasion is the only failure mode of possible concern. It is not a problem in large parallel-strand or jacketed wire-rope construction polyester ropes. Under the relatively high mean loads and relatively low amplitude load patterns normally experienced in typical deepwater mooring lines, the strength reduction caused by internal abrasion is negligible. Internal abrasion might be a problem with poorly designed ropes or rope terminations. Thus prototype rope testing is still necessary.Extensive cyclic load testing shows that the cycles-tofailure (ctf) of polyester rope at moderate load ranges is about fifty times the steel rope ctf curve given in API RP-2SM, "Deepwater Synthetic Mooring Line Guidelines". But the slope of the ctf curve for polyester rope in the present edition of that guideline may be nonconservative at very low load ranges.The axial compression fatigue test now specified in API RP2SM is of no value for polyester ropes. The polyester maximum allowable low-tension axial compression fatigue cycles criterial should be eliminated. The creep rupture criteria specified for polyester in RP2SM is meaningless.

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