Use of six change-in-length properties (6 CILP) in designing fiber rope mooring systems

This paper discusses recent advances in understanding how the length of a fiber rope changes under various tension conditions and histories. The change-in-length characteristics of polyester rope can be completely represented by six properties: original stiffness, static stiffness, dynamic stiffness, construction strain, polymer strain, and working strain. This is called the 6 CILP method. This paper describes how to use the 6 CILP method to calculate fiber rope deepwater platform mooring system performance parameters, such as moored platform offset, highest mooring line load, and line length at end of service. The test method for these properties is discussed in the companion paper “Testing Polyester Fiber Rope for Six Change-in-Length Properties (6 CILP).” Using the 6 CILP method, the stretch of the mooring line and thus its overall length can be calculated at a given time and at a given tension. The spring rate of the mooring line can be calculated under various conditions, for example, before and during cycling. The peak and trough tensions and the maximum platform offsets can be calculated under various conditions, for example, during an extreme storm event. The properties can thus be used to analyze the performance of the fiber rope design in any mooring system and in any environment. The paper will be of interest to all designers of fiber rope systems. It will be of particular interest to deepwater mooring system designers.