Axial Compression Fatigue in Long-Lay-Length Fiber Mooring Ropes

Abstract

Abstract Rope failure due to axial compression fatigue was found in two recent rope investigations. One of these was an HMPE mooring line on a gas carrier and the other was a nylon SPM hawser. In both incidents, the unusual rope design, with very long rope and strand lay lengths, was the root cause of the failure. This paper first describes axial compression fatigue and how it can occur in fiber ropes. It briefly discusses a past failure of an aramid rope caused by axial compression fatigue. It describes the two more recent failure incidents. It describes how long-lay-length rope designs caused or contributed to the axial compression fatigue which resulted in rope failure. This paper describes the axial compression fatigue test method. It reviews the axial compression fatigue performance for aramid and HMPE yarns from the Fiber Tethers 2000 JIP and presents new data for HMPE yarns and also for nylon yarns, which have never before been tested for this property. Recommendations are made for fiber rope designs. Unless they are properly designed for the application, ropes with long rope lay and long strand lay lengths should not be used in the following situations: *Cyclic tension with many cycles down to low trough loads.*Cyclic bend over sheaves.*Cyclic rotation of the rope.