Farrow, G., Potts, A.E., Kilner, A., Jal, E., and D’Arcy Evans, N., Proceedings of the ASME 2019 38th International Conference on Ocean, Offshore and Arctic Engineering OMAE2019, Glasgow, Scotland, June 9-14, 2019.

Abstract OMAE2019-95875

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. Available test data was collated for comparison and to establish underlying trends in mooring chain fatigue response, and a non-linear multi-axial Finite Element Analysis (FEA) fatigue assessment method was developed to correlate against available fatigue test data The study sought through collation and review of available fatigue test data to; identify relationships between chain fatigue performance and the key input parameters of chain type, grade and environmental conditions; compare and validate the fatigue test data against the current Code formulations for chai fatigue endurance; and determine chain nominal stress S-N fatigue endurance curves against which to valid a numerical model developed as part of the Chain FEARS JIP for the assessment of as-new chain link fatigue endurance. This paper will present the collated data and findings, including the correlation demonstrated between the two standard deviation lower bound of the mean curves and current Code formulation design curves, as well as the mean curves of the collated fatigue test data (considered representative of the overall fatigue performance of chain links and as such formed a good basis for the validation of FEA model for the assessment of chain fatigue endurance). It will also look at the collated fatigue data separated into environmental condition data groups for in-air and free corrosion in seawater testing indicated as anticipated that in-air fatigue endurance is greater than in seawater testing. The collated fatigue data is separated into groups associated with offshore mooring chain type (i.e. stud and studless), grade (ie ORQ, R3, R4 and R5) and environmental conditions (ie free corrosion in seawater and in-air) for subsequent correlation of fatigue assessment. In addition, this paper will discuss how the multi-axial fatigue formulation enabled a strong correlation of predicted failure location with actual test data as a function of mean load and load range, even where residual compressive stress regime suggested that crack propagation should not occur.

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