PSI - Issue 57

Baran Yeter et al. / Procedia Structural Integrity 57 (2024) 133–143 Baran Yeter & Feargal Brennan / Structural Integrity Procedia 00 (2023) 000 – 000

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Fig. 7. FCG simulations accounting for overload-induced retardation (blue lines) compared to the FCG without retardation (red line) 5. Conclusion The present study investigated the structural integrity of a 15MW semisubmersible FOWT using probabilistic methods. Fracture mechanics was adopted to assess crack growth under variable amplitude load. The variable amplitude loading approximating the stochastic service loading of FOWT was generated using the weighted sum of the short-term structural responses. The cycle-by-cycle crack growth was simulated using the Paris law modified by the Wheeler retardation model in order to study the influence of the load sequence effect and overload-induced plasticity on the crack growth, in turn, the remaining crack life. The results showed that FCG was highly varied, even considering only the load sequence effect. The variation became substantial, especially after 15 years of service life of the FOWT, which can be considered within the fast crack growth phase. The variability of the crack size at a given point in service life was estimated in terms of CoV, which can be included in a fatigue reliability calculation as part of the modelling uncertainties. In addition to the significant difference in terms of the predicted crack depth, the variability of FCG grew even further when the overload-induced plasticity was introduced in the crack growth simulation. This result can be explained by the fact that the FOWT is subjected to a great number of low amplitude stresses whose contributions to the crack growth can be diminished by the existence of a precedent overload. Even though the effect of retardation on the variability of FCG is expected to be less for a relatively short crack, it is advisable that the fatigue reliability analysis considers the crack growth under VAL as a critical part of the modelling uncertainties, which would be future work. Acknowledgements This work was performed within the scope of the “Ocean REFuel” project, which is funded by “ the “ Engineering and Physical Sciences Research Council ” with the reference number " EP/W005212/1”.