Fatigue durability of nylon rope for permanent mooring design

Over the last nine years three major research studies have led to the development of a new design of nylon rope suitable for permanent moorings with design lives in the region of at least 20 to 30 years. Prior to this research nylon rope designs had a fatigue performance which was only adequate for temporary moorings typically lasting six months to a few years. This paper reviews two pivotal very long duration tests (10 and 20 million cycles) which were inspected for damage following completion of the load cycling, before measuring the residual breaking strength. In addition to these very long duration tests which were undertaken with load ranges more representative of wave loading, to account for all the ranges experienced through life it is necessary to be able to calculate any strength loss. Hence, a series of “partial life” fatigue tests was conducted to determine the strength loss for a series of load ranges. In these tests rope samples were cycled to a representative number of cycles which the rope would experience in service before stopping the test to assess the accumulated damage by means of a series of rope yarn tests. The residual breaking strength data is presented for critical layers through the cross-section which clearly identifies how yarns in different positions in the rope are exposed to varying degrees of inter-strand abrasion. A preliminary strength loss relationship was developed which could then be used to calculate strength loss for the fatigue spectrum based on an example mooring analysis of a WEC device. This work was successfully undertaken under the umbrella of UK Government funded Research and Development test programmes driven by consultancy firm Tension Technology International (TTI), as part of a team with rope-maker, certification body and wave energy converter (WEC) developer. The results of these projects have created a new technology enabling improved mooring systems with the additional benefit of significant cost reduction for the renewables industry. This technology can also be applied to any offshore mooring system where first order motions create high peak line loads.