This paper assesses the ARELIS (Assured Residual Life Span) method for estimating residual creep life of polyester rope used in deepwater mooring lines. A statistical model has been developed to quantify the uncertainties in the method, such as the scatter in creep rupture test data and load sharing between sub-ropes. This model can be used to determine the required test load, duration and number of ARELIS tests, in order to guarantee a minimum creep life fo

INTRODUCTION

Polyester fiber ropes are increasingly being used as components of mooring lines for offshore structures in deep water. The use of such ropes in long-term mooring systems has raised the question of whether creep rupture should be considered a possible failure mechanism (Lo, Xü and Skogsberg, 1999). The theoretical creep life of polyester fiber mooring ropes operating at or below their design load is millions of years. However, exposure to extreme storm loading or damage to the rope could reduce this life to a much lower figure. To give confidence that the creep life of polyester moorings has not been compromised, Bosman (2001) has proposed a method of testing to assure that the residual rope life is adequate. This method avoids the major problem with creep testing, namely the length of time that creep tests take, and is described below. Polyester mooring lines can be constructed with removable insert sections fitted in series with the mooring line. After being in service for a length of time, some of the insert sections can be removed and taken ashore for investigation. In a rope construction consisting of parallel sub-ropes, different sub-ropes from the insert can be tested in different ways, including breaking load tests, internal examination and ARELIS (Assured Residual Life Span) testing. The aim of the latter tests is to demonstrate that the mooring lines have sufficient residual creep life remaining.

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