Abstract

This paper reports on the results of an investigation into the stiffness behaviour of polyester ropes in the size range 600 to 1,000 tonne. Both quasi-static and dynamic stiffness data have been presented. Effects of dynamic stiffness have been quantified for a wide range of loading conditions under both sinusoidal and stochastic loading. It has been demonstrated that for the test conditions used and the associated analysis techniques employed, strain amplitude does exist as a variable. However, for stochastic loading its effect is sufficiently small that it could be neglected when considering dynamic stiffness behaviour for mooring design under low strain amplitudes up to 0.3%. (Strain amplitudes around 0.25% are relevant for taut moorings of typical floaters in 1,500 m harsh environment water depth). At higher strain amplitudes, around 0.6% the effect should not be ignored. It should also be noted that material and construction effects affect dynamic modulus. Therefore, for some materials and constructions at higher strain amplitudes of 0.6%, it may be possible to ignore the strain amplitude dependence. For a broad comparison, all the materials and constructions reported herein fall within a relatively narrow axial stiffness band for a given set of test conditions. This generic data will be useful for mooring system designers enabling preliminary designs to be conducted with confidence. To ensure the correct stiffness values are determined, detailed engineering should be based on test data generated on full scale rope and at loading conditions representative of the actual mooring.

Introduction

Platform or vessel offsets and mooring system forces are highly dependent on the mechanical properties of the mooring lines used. During recent years increased attention has therefore focussed on establishing the mechanical property behaviour of fiber rope (particularly polyester), as they are becoming increasingly considered for mooring temporary and permanent floating production systems. One of the findings of the Fiber Tethers 2000 Study1 was that polyester exhibited fatigue and mechanical property characteristics that make it a good contender for use within the mooring line systems of offshore platforms, especially the taut mooring line system. Following on from the Norsk Hydro Managed JIP2 into the Heat Build-up and Mechanical Property Behaviour of Full Scale Polyester Rope, the Testing and Optimisation JIP3 was set up in 1997. This study was designed to look more closely at the property behaviour of full scale fiber ropes under installation, drift and storm conditions, including the effects of zero load in leeward lines during storm conditions upon rope properties. Also, as part of the study, suitable eye/bush assemblies were produced that are compatible with existing mooring hardware. This study was reported in 19994-5. The confidentiality period for this JIP has now expired and this paper presents some of the main aspects from that study. Ten ropes in the size range 600 to 1,500 tonne were supplied from five manufacturers, with each manufacturer supplying two samples. Seven of the ropes were polyester, two were HMPE and one was aramid.

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