Tension/Torsion Interactions in Multicomponent Mooring Lines
Authors: Chaplin C.R. , Rebel G. , Ridge I.M.L. 
Published: October 2008

Six strand ropes generate torque under tension. This can lead to transfer of twist to other mooring line components, either permanently, or dynamically. The effects of this imposed twist can seriously affect strength and fatigue endurance. In order to predict the torsional interactions between components it is necessary to quantify the tension/torsion behavior of all the different categories of mooring line component, not just the six strand rope. This paper discusses the problems of torsional interaction and presents results of measurements of tension/torsion behavior of six strand rope, stud link chain and a PET fiber rope.

Introduction

Mooring systems for floating facilities are becoming more complex, especially for floating production systems placed in deeper water and with the adoption of PET components. One feature of these trends is a tendency to combine different types of component in a single "composite" line, whether this be a temporary expedient to facilitate deployment, or a final assembly. It is useful to identify four different categories of component which are used:chain;six (or possibly eight) strand wire rope;spiral strand; and,PET fiber rope (of various constructions).

These different classes of component share properties which make them suitable for use in mooring systems: they all have high axial strength and stiffness combined with low bending stiffness. However, for any given axial strength the different categories have very different weights (especially when submerged), they have different costs, different degrees of "ruggedness" and exhibit some different axial stiffness properties. These differences are what lead to the selection of components for different elements in a composite mooring line. But there is another category of mechanical response that comes as a consequence of the geometry and material properties of these components: torsional behavior.

The torsional stiffness, tension/torsion response, and sensitivity to twist differ appreciably between these different components. This is especially important due to the particular behavior of six strand rope which is used both as an installed component as well as for work wires and pendants during deployment: six stranded rope is not "torque balanced" which means that under tensile loading, if restrained, it will develop an axial torque, but if unrestrained it will untwist to maintain zero torque. This torque reaction is caused by the helical nature of the rope's construction. Because of this somewhat unusual behavior it is important to understand the torsional restraint presented to six strand rope by the other components to which it might be attached in a mooring line. This is necessary to predict the transfer of twist from one component to another, and its consequences.

These problems have been a pre-occupation of the rope research group at the University of Reading for the past few years, and some aspects of the group's studies have been published previously1,2,3. This paper discusses some of the torsional mechanisms which seem to be increasingly prevalent offshore; presents details of the measured response of different components at laboratory scale; and, sets out methodologies for making quantitative predictions of these effects.