Appraisal of Lightweight Moorings for Deep Water
Authors: Chaplin C.R. , Del Vecchio C.J.M. 
Published: October 2008

This paper outlines the potential advantages of using lightweight fiber ropes for mooring floating production platforms. The influence of tether properties on mooring system response is discussed. It is shown that a comparison of alternative ropes in terms of their mechanical properties is inadequate; relative merit can only be assessed through comparison of full systems that have been designed to minimum cost, and to common performance specifications. For catenary systems, it is found that a lightweight option using polyester fiber can outperform both traditional and more exotic solutions. The significant mechanical performance characteristics Governing mooring systems design have been measured for a model tether.

Introduction

Work tm this topic was initiated at Reading University in 1989 with support through a joint industry project on "lightweight materials for offshore station keeping" [1]. The primary objective of that study had been to rank the Relative merits of mooring components manufactured from lightweight materials using available fiber and rope technology, in relation to conventional offshore platform systems. The investigations carried out in the course of that study demonstrated that simplistic comparison of rope properties was potentially misleading. 'Ib facilitate any meaningful comparison it was found essential to perform an analysis of the complete mooring system, optimised in terms of cost and designed to comparable criteria.

The function of a mooring system is to restrict the lateral motions of the floating structure within specified operational constraints. Thus the overall design parameters are typically in terms of maximum permissible offsets, varying according to the operating conditions and the riser specification, with limitations on maximum tether loads and other more specific constraints on say anchor uplift.

The design loads on the overall system are generated by wind, waves and currents, all of which are complex functions of time and stochastic in nature.

Essentially any mooring system operates by developing horizontal components at the points of attachment tothe floating structure. The loads in the mooring system will change in response to lateral displacements, thereby generating the restoring forces. The primary performancecriterion of a mooring system is therefore its lateral stiffness - its station keeping performance. The details ofthe system performance are a function of the interactive frequency response of the vessel and mooring system. The loads induced in the mooring components as a result of the platform motions, in turn determinethe safety factor between component strength and the Maximum loads under extreme conditions; and, the spectrum of repeating loads in relation to fatigue performance.

Mooring concepts

For operating in deepwater there are two alternative mooring concepts which can be broadly categorised as catenary or TLl. The stiffness and frequency response ofthe two classes are fundamentally different, particularly in the first order wave frequency range. Because of these differences and the way in which the mechanical properties of the mooring components influence loads and therefore design, the two types of system must be considered separately.