Creep and Relaxation of Polyester Mooring Lines

Abstract

Abstract This paper presents results from an experimental study of the time related behavior of polyester fibres and ropes. Results from creep and relaxation tests performed at different constituent levels are given, from single fibre tests through yarn, rope yarn and sub-rope up to tests on full size 500 ton break load parallel strand ropes. Results will be discussed in terms of applications, and the usefulness of subscale testing will be evaluated. A system to allow strains to be measured continuously in service is proposed. Introduction In order to design synthetic moorings with confidence it is essential to take account of the viscoelastic behaviour of the polymer fibres which make up these cables. This may be revealed as an increasing elongation with time under load (creep), which may require the inclusion of additional fairlead chain in the line, or as a decrease in tension in the line (relaxation) which will impose periodic re-tensionning. Viscoelastic effects will also influence line stiffness. A further more dramatic consequence of time-dependent behaviour, at high tensions, may be the appearance of creep rupture, though this should be avoidable by appropriate safety factors in design. Creep effects have also been shown to dominate the high load (above 60-70% break load) cyclic fatigue behaviour [1]. The present project is a joint co-operative study between IFREMER, Bureau Veritas, IFP, Bouygues Offshore, Doris Engineering and Technip. Two aspects have been studied. First, stiffness of large ropes (450 to 710 tons MBL) has been determined under a wide range of loading conditions. This work is described elsewhere [2]. In the second part of the project the viscoelastic response of polyester fibres and ropes has been characterized. The tests performed range from the tensile loading of a single polyester fibre through to tests on 500 ton ropes (containing over nine million fibres). One of the aims of this research is to evaluate the influence of the viscoelastic nature of constituent polymer fibres (polyester and other materials) on their operational response for mooring lines. The integration of this data in modelling and design analyses is also being studied. Viscoelastic behaviour of polymer fibres and ropes The viscoelastic behaviour of polymer fibres is not a new subject. Meredith proposed a model for creep rupture of fibres in the 1950's [3], while Ward and co-workers have been characterizing creep of different fibres since the 1960's [4]. For polyester creep behaviour has been correlated with fibre structure, and the importance of crystallinity has been shown [5]. The creep deformation of polyester is described as linear with log of elapsed time from the start of the load plateau. Recent creep data from the 'Tethers 2000' project [6] and results from Del Vecchio [7] give creep rates in % per decade. Typical values for polyester yarns loaded at 30% of Minimum Break Load (the lower bound strength value guaranteed by the ropemaker, MBL) were 0.12% for 103 to 104 seconds, decreasing to 0.07 %/decade after one year (between 106 to 107 seconds).