Cyclic fatigue and wear in deepwater polyester mooring systems

Fatigue life of deepwater mooring lines is a very important issue in the design of mooring systems for offshore structures. Polyester ropes experience cyclic loading over the life of the mooring, and the remaining strength of those ropes is reduced over time as individual fibers comprising the rope are broken. Fatigue and wear testing of polyester fibers and polyester subropes has shown that two distinct mechanisms are at work: (1) true fatigue failures in single polyester fibers and (2) fiber damage due to cyclic wear caused by subrope strands rubbing against one another near the toe of eye splices. This paper compares and contrasts these key issues of fiber fatigue and subrope strand-on-strand cyclic wear. A new method was developed to test fretting wear of subrope strands subjected to cyclic loading. This method, called the 20-Hurricane method, has been demonstrated and found to constitute a strand-on-strand abrasion wear method conceptually similar to the yarn-on-yarn abrasion test methods used for yarns, rather than strands. The test can measure both strength reduction and maximum elongation performance of polyester subrope designs due to damage from a series of 20 hurricanes over a 20-year period. Test results for various subrope designs are described, and the designs are evaluated in terms of reduction in subrope strength due to the effects of cyclic wear. The authors propose that the 20-Hurricane test be adopted as a ¿benchmark¿ by industry for qualification testing of different designs and brands of polyester rope, and types of splices and marine finishes. Other potential uses of the 20-Hurricane test method for synthetic fiber ropes are also proposed.