Probabilistic fatigue model for design and life extension of mooring chains, including mean load and corrosion effects

A probabilistic model for mooring chain fatigue damage is developed based on the S–N approach. The effects of mean load and corrosion condition on the fatigue capacity of the chains are included by adopting a parameterized S–N curve intercept parameter, and the model allows for the uncertainties and time dependencies of these to be addressed. Uncertainties in fatigue loads are also accounted for, including the annual variability which may be of importance in certain cases. Furthermore, the resulting model distinguishes between damage due to prior known loads and future unknown loads, to allow for reduced uncertainties in case that the load history is available from measurements or calculations. Measures are taken to ensure that the correlation between mean and cyclic loads is handled implicitly. A case study based on extensive hindcast-based simulations for a realistic mooring system is performed, and the respective effects of uncertainties in fatigue capacity, corrosion development and fatigue loads are presented and discussed.