Strength Assessment of Degraded Mooring Chains

Abstract

Abstract This paper presents a framework for strength assessment of degraded mooring chains, based upon laboratory break tests and finite element simulations of degraded chain. Four different modes of degradation, all observed in the field, are considered including: 1) general corrosion, 2) localized corrosion with large single and dual pits, 3) interlink wear, and 4) chain abrasion due to contact with the seabed. For each case, laser scanning was used to obtain the actual geometry of degraded chain taken from the field and the links were then loaded in tension to failure. Material properties were derived from coupon tests of adjacent links to those loaded to failure and the analytic predictions were compared with the actual break loads for each case, validating and demonstrating the robustness of the method. This paper discusses the strength capacity of mooring chains and does not discuss load or demand placed on the mooring system or fatigue endurance of degraded chains. However, mooring system demand will be an integral part of the holistic evaluation of any degraded chains. Case studies detailing the development of strength assessment curves for general corrosion, interlink wear, abrasion, and localized corrosion are presented. A comprehensive investigation covering these four degradation modes is not available to the industry at present. The assessment methodology has a strong technical basis and is expected to provide significant improvements to current industry practice.