Farrow, G., Potts, A.E., Kilner, A., Kurts, P., Dimopoulos, S., and Jal, E., Proceedings of the ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering OMAE2019, Glasgow, Scotland, June 9-14, 2019.

Abstract OMAE2019-96426

Phase one 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. An accurate estimate of the chain in its as-new condition is critical to the ability to evaluate the residual fatigue life of a degraded chain, thereby providing a benchmark for any loss in fatigue life associated with severe corrosion or wear. The available test data was collated for comparison and to establish underlying trends in mooring chain fatigue response. A non-linear multi-axial Finite Element Analysis (FEA) fatigue assessment method was developed to correlate against available fatigue test data. The study; established the basis for development of an FEA based method of assessing mooring chain fatigue; developed an FEA numerical modelling method for assessment of the fatigue life of as-new studlink and studless chains; and validated the numerical model method with physical fatigue test data of as-new links. ? This paper will present the basis for development of the FEA method of fatigue assessment, describe the method developed to achieve correlation with the available test data, and present the correlation achieved between the predicted and measured test data. A number of key findings will be presented, including a method of assessing fatigue based on FEA analysis, developed with respect to the DNV B1 material curve which correlates well with the available physical fatigue test data. It will also discuss employing a non-linear chain link FEA and multi-axial fatigue calculation method to determine an equivalent Stress Multiplication Factor (SMF) achieving good correlation of predicted utilisations and associated cycles-to-failure with fatigue test data, and in respect of critical locations with evidenced failure locations. The paper will also present the method of equivalent SMF calculation accounting for significant effects such as proof load induced residual stress, mean stress levels, and the increase in material fatigue endurance associated with increased steel UTS. The analytical method developed in this study, which achieved a high degree of correlation with fatigue test data, should enable the accurate prediction of fatigue stresses around a link and in particular for irregular geometry associated with corrosion degraded chain link. ? This paper will discuss current Code formulation and how it may be expanded to reduce uncertainty and allow for the reduction in a fatigue safety factor and the economic benefits of employing a consistent, optimised proof load specification and a Code specified formulation for chain fatigue that accounts for both material strength increase and mean mooring chain tension.


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