Effects of Uniform and Mega Pitting Corrosion on Residual Strength of Degraded Offshore Mooring Chain

Abstract

The first phase of the Chain FEARS (Finite Element Analysis of Residual Strength) Joint Industry Project (JIP) aimed to develop guidance for the determination of a rational discard criteria for mooring chains subject to severe pitting corrosion which, based on current code requirements, would otherwise require immediate removal and replacement. Critical to the development of rational discard criteria is the ability to accurately estimate the residual strength of a corrosion and wear degraded chain, and to have as a benchmark for loss in strength, an accurate estimate of the chain in its as-new condition. A Finite Element Analyses (FEA) residual capacity assessment method was first developed and correlated against available break strength test data of corrosion degraded links [1], and the Predicted Break Load (PBL) of as-new links was established as a benchmark for loss of strength [2]. The validated FEA method for the determination of chain residual capacity was employed as a basis to establish an understanding of the effects of corroded chain and to determine the underlying relationship between loss of chain capacity and both uniform and mega pitting corrosion levels. The investigation sought through the conduct a series of FEA assessments employing a parametric model of idealized degraded chain links to derive the theoretical relationship beteen the residual capacity for varying levels of uniform corrosion, and the residual capacity for varying size, locations and shapes of mega pit corrosion degradation. This paper presents the findings of the investigation into the effects of uniform and mega pitting corrosion on degraded chain residual capacity and the correlation of the relationship with physical break test data, the findings of which forms the basis for development of a rational guideline for chain discard criteria whereby degraded mooring chain can be assessed in respect of the need for replacement.