Wear and Fatigue of Nylon and Polyester Mooring Lines

Abstract

Wear and fatigue in synthetic fiber lines depend on a multitude of mechanisms, including cumulative creep to rupture, hysteretic heating due to cyclic loading of filaments or to relative displacement of structural rope elements, internal wear caused by relative movement between yarns or strands, and external abrasion against deck hardware or rough surfaces. Studies undertaken to evaluate fiber, yarn, and rope responses to each of these mechanisms, singly or in combination, vary significantly with the test conditions including cyclic load level (both axial and transverse), cyclic strain level, and ambient conditions, i.e., temperature and moisture content (whether dry or wet in fresh water or salt). Cyclic load limits depend on wave height, wind velocity, and mooring line dimensions and properties. The relative durability of nylon versus polyester lines is considered as the lines are subjected to tensile cycling under storm conditions. While tensile fatigue as a creep phenomenon is operative over the free length of the mooring line, the more critical location of rope deterioration is at its interactive contact with deck hardware. An ideal mooring system is composed of two segments. The one between the elements of deck hardware (cleats and chocks) should possess maximum abrasion resistance and high tensile stiffness. The other segment should possess high tensile compliance so as to minimize cyclic load levels induced by waves and wind. A review of yarn/yarn wear tests and rope abrasion tests is provided to form the foundation for recommendations.