Abstract

A computer program is now available to model the strength and stretch properties of new fiber ropes. The program can be used to predict changes in these properties due to tension cyclic loading, even after many years of service. It accounts for internal abrasion, which is believed to be the principal potential cause of strength reduction in large polyester ropes. It accounts for creep, which affects stretch properties and might cause strength reduction after many years. The program can also account for the effect of external damage to a rope.

The program performance has been demonstrated by comparison with data from cyclic load testing of polyester ropes used in a Gulf of Mexico permanent platform mooring system. It has also been verified by comparison with data from polyester ropes removed from another deepwater mooring system.

This program can now be used together with actual mooring load data to predict the long term performance and remaining strength of polyester rope mooring lines on deepwater platforms. It would be a more practical and reliable means of assuring the integrity of such mooring lines than periodic testing of tails removed from the upper part of the mooring line.

Introduction
Summary

Synthetic fiber ropes and other textile tension members can now be analyzed using the Fibre Rope Modeller (FRM) computer program. FRM calculates the tension-stretch and torquetwist behavior of a rope using basic yarn properties and rope construction details.

The FRM program can model laid (twisted, helical, wire rope construction), braided (plaited), and parallel rope constructions. In addition to the stiffness (stress-strain) and break strength (tenacity) characteristics of the fiber or yarn, it accounts for abrasion, fatigue, creep and hysteresis. In modeling rope construction, it accounts for the shape, packing and compaction of strands. It includes the effects of friction, which inhibits or permits relative motion between strands, and abrasion, which results from this relative motion. It accounts for changes in lay length brought about by compaction, movement, and stretch.

This paper describes the FRM program. It presents three examples in which the change-in-length characteristics of large fiber ropes predicted by FRM compared favorably with those measured in rope tests.

Background

The FRM computer program has been developed through a number of research programs over the last twenty years.

Dr. Leech developed the original concept and computer code while conducting rope research and testing in the mid 1980s. That work was sponsored by a UK Engineering and Physical Science Research Council grant. While studying the geometrical aspects of rope construction, he made the first application of virtual work principals to the modeling of twisted rope structures.

Tension Technology International (TTI) was awarded a US Navy contract in 1988 to further develop the program for use in design and analysis of large ropes. Internal friction modes were incorporated into the program to model abrasion damage. This enabled prediction of rope strength loss and failure caused by cyclic loading.

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