Investigations Into Fatigue of OPB Loaded Offshore Mooring Chains

The Chain FEARS (Finite Element Analysis of Residual Strength) JIP developed a correlated FEA method for determining fatigue life of Tension-Tension (T-T) loaded offshore mooring chain. The developed first principles method incorporated the non-linear effects of proof loading, accounted for mean chain tension, accounted for material grade, employed a multi-axial fatigue method to account for complex stress fields within the chain, and was based on a parent material S-N curve. It was anticipated that this high fidelity approach could be applied more universally to successfully address a broader spectrum of emergent factors experienced in-field which have caused chain fatigue life reduction and adversely impacted mooring system integrity. These emergent factors include an array of chain degradation modes including; uniform, pitting and mega pitting corrosion; preferential butt weld corrosion; abrasive, contact and interlink wear; and load duties other than Tension-Tension (TT) such as Out-Of-Plane Bending (OPB), In-Plane-Bending (IPB) and Bending-Tension (BT). The objective of the investigations conducted by AMOG Consulting following completion of the Chain FEARS JIP research was to explore the extent to which the developed fatigue method could be applied more universally to address these emergent factors as they pertained to residual fatigue life. Application of the method identified a number of trends in fatigue performance for links subject to hawse pipe and fairlead interaction, and demonstrated good correlation against available guidance on fairlead performance and against OPB fatigue test data. The investigation confirms that the first principles FEA fatigue method can be applied more universally to successfully address a broader spectrum of emergent factors experienced in-field relating to fatigue life reduction.