Probabilistic structural integrity assessment of a floating offshore wind turbine under variable amplitude loading

The objective of the present study is to assess the structural integrity of a semisubmersible-type floating offshore wind turbine probabilistically based on fracture mechanics. Although floating offshore wind turbines have been historically much more expensive than their fixed bottom peers, the levelised cost of energy of floating offshore wind turbines has decreased dramatically due to increasing wind turbine size, higher capacity factors, innovative designs, and optimal operational strategies. However, there is still much to be done to bring the cost down, especially from the operational and maintenance standpoint. In this regard, a damage-tolerant approach is a strong alternative to a safe-life design approach that leads to overdesigning. The present study assesses the structural integrity of a semisubmersible-type floating platform with an initial crack assumption using a probabilistic framework, resulting in the quantification of the uncertainty involved in the fatigue crack growth under variable amplitude loading. The present study outcomes can then be used to optimise the structural design, inspection, and maintenance plan. The cycle-by-cycle crack growth is simulated using the modified Paris' law, which allows accounting for the overload-induced retardation on the crack propagation. Furthermore, the factors influencing crack growth are subjected to a great deal of uncertainty. The present study focuses on the variability caused by the load sequence and interaction effect. To this end, Monte Carlo Simulation is performed to quantify the uncertainty associated with the crack growth by first considering the load sequence effect and then the overload-induced retardation effect.