Durability of Polyester Ropes Used as Deepwater Mooring Lines

Four failure modes have concerned others as potentially limiting the service life of polyester ropes used as deepwater mooring lines: axial compression fatigue, creep rupture, hysteresis heating, and internal abrasion. Based on extensive investigations and testing, the first three modes are of no concern with polyester rope. Internal abrasion is the only failure mode of possible concern. It is not a problem in large parallel-strand or jacketed wire-rope construction polyester ropes. Under the relatively high mean loads and relatively low amplitude load patterns normally experienced in typical deepwater mooring lines, the strength reduction caused by internal abrasion is negligible. Internal abrasion might be a problem with poorly designed ropes or rope terminations. Thus prototype rope testing is still necessary. Extensive cyclic load testing shows that the cycles-to-failure (ctf) of polyester rope at moderate load ranges is about fifty times the steel rope ctf curve given in API RP-2SM, "Deepwater Synthetic Mooring Line Guidelines". But the slope of the ctf curve for polyester rope in the present edition of that guideline may be nonconservative at very low load ranges. The axial compression fatigue test now specified in API RP2SM is of no value for polyester ropes. The polyester maximum allowable low-tension axial compression fatigue cycles criterial should be eliminated. The creep rupture criteria specified for polyester in RP2SM is meaningless