Chain FEARS JIP: Finite Element Analysis of Residual Strength of Degraded Chains

A recently identified problem facing operators and designers in the integrity of mooring chain is the effect of highly localized Microbiologically Influenced Corrosion (MIC) in tropical waters manifesting in large pits, as identified in the SCORCH JIP. These pits, if assessed against API-RP-2I, would necessitate immediate removal and replacement of the chain links. The conventional discard criteria in mooring codes were formulated based on uniform corrosion, and not highly localized material loss. The application of rigorous Finite Element techniques to determine the residual strength of these degraded links has not been recognised in existing codes of practice as an acceptable technique for assessing whether or not the chains can meet operational requirements. The Chain FEARS (Finite Element Analysis of Residual Strength) JIP aims to address this by developing guidance for the determination of discard criteria for mooring chains. This will assist in improving the reliability of prediction of the remaining life of degraded chain and better inform mooring replacement programs. This paper outlines the scope, objectives and an overview of the results of the first phase of the Chain FEARS JIP; investigating Finite Element methods for determining the strength loss of degraded mooring chains. This investigation has resolved issues including the effects of degradation on strength reduction and the challenges of establishing a robust methodology for assessing the remaining strength of degraded chains. The scope of the second phase of the Chain FEARS JIP, to develop tools for the prediction of the residual fatigue life of degraded chain, is also outlined. The findings of the Chain FEARS JIP are applicable to a wide variety of FPUs reliant on chain moorings and will enable the application of a more rational guidance for remaining chain life prediction and mooring line replacement.