Characterization of Polyester Mooring Lines

Fibre ropes are extensively used in marine applications. One critical area of interest is their application as mooring lines for floating offshore platforms, for which primarily polyester is now employed in various regions (offshore Brazil - now for 10 years, West Africa, Gulf of Mexico). Evaluating the response of the system requires a description of the load-elongation properties of the rope. A practical model involving two sets of stiffness data is currently used, and procedures for their measurement are available. This paper presents an overview of this model, then focuses on recent work on the quasi-static stiffness of polyester ropes. This is addressing the variations of the mean tension in the lines, at a very slow rate, under changing weather conditions. Extensive tests were performed, principally on polyester sub-rope samples. Some tests were also performed on a full size 800-ton MBS rope. Besides standard tests, specific tests were performed over an extended range of loading, to cover the situations that may be found in a wide range of systems and design conditions. The factors (measurement accuracy, test conditions, etc...) affecting the values are discussed along with the presentation of tests and results. Results are interpreted to provide practical data for mooring analysis, in the form of a quasi-static load-elongation characteristic. These results also give a better insight into the visco-elasto-plastic response of polyester fibre ropes. For the dynamic stiffness of polyester ropes, an overview of recent and earlier test data is presented. The dependence of dynamic stiffness on testing parameters is discussed, highlighting mean load as the principal parameter under real stochastic loading, and confirming the current practice for modelling dynamic stiffness in design.