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

Abstract

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. INTRODUCTIONThis paper presents the results of tests conducted on a polyester rope in the DNV industry-sponsored JIP "Improving Fiber Mooring Design Practices". 1, 2 Further information on that JIP, including the test method for determining the six change-in-length properties (6 CILP) and the actual properties for a particular polyester rope, are given in the companion paper. 3 The six properties were discussed in detail in several earlier papers. 4, 5, 6This paper describes how to use the 6 CILP method to calculate important mooring system performance parameters. The lengths of mooring lines are calculated both during and immediately after installation. The maximum and minimum mooring line tensions experienced during maximum storm-induced platform offset are calculated. These can also be used to calculate fatigue of mooring line components. The mooring line length at the end of service life is also calculated.

Modeling nonlinear time-dependent behaviors of synthetic fiber ropes under cyclic loading

“…Falkenberg et al (2011) proposed to introduce the empirical model which consists of the spring and dashpot into the frequency analysis of the synthetic fiber rope mooring systems. Flory and Ahjem (2013a, 2013band 2013c proposed six change-in-length properties (6 CILP) empirical method together with the test method to identify the parameters to describe how the length of a fiber rope changes under various tension conditions and histories. The 6 CILP method can account for permanent length change due to construction stretch and long term polymer stretch as well as the dynamic strain and temporary creep and recovery effects.…”

Testing polyester fiber rope for six change-in-length properties (6 CILP)

“…3, 4, 5 Use of the properties in mooring system design is described in a companion paper. 6 The objective of the JIP was to develop a universal test method to determine the change-in-length properties of fiber rope. The results should be usable for any mooring system design scenario.…”