Tension-tension fatigue in mooring ropes for offshore structures
The factors affecting the endurance of mooring ropes for large offshore structures are discussed. These large and expensive ropes are subjected to significant dynamic loading components. Although additional loading factors at the extremities are likely to be more critical, the Tension-Tension fatigue endurance is seen as a relevant parameter since it imposes an upper bound on endurance and therefore provides the reference against which to judge the other effects. For these reasons considerable attention has been given by the offshore community to Tension-Tension fatigue testing. This paper discusses the end effect problems, the comparability of results for different ropes and the issue of cumulative damage under variable amplitude loading. It has been found that central failures can be obtained; experience shows that results on different ropes from different laboratories correlate best when loads are expressed in terms of measured ultimate strength; it has also been found that cumulative damage effects can be modelled fairly well with a simple Miner's summation which for pseudo-random loading can be combined with a rain flow analysis. Much of the material presented here formed a part of a presentation at the OIPEEC technical meeting, "Endurance Calculation for Wire-Rope Selection", at the Seilprüfstelle in Bochum on 28th August, 1987. However the work described then on fatigue assessment of pseudo-random loading sequences has continued since the Bochum meeting and the later findings have been included here.
Author: C.R. Chaplin