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

135

3

induced loadings, as done in this study. Such representative loading sequence would allow universal fatigue and fracture testing procedures to be replicated worldwide. In addition to having an accurate model to assess crack growth, the crack life prediction is very sensitive to the nature of load cycles in terms of both frequency and magnitude. Especially for FOWTs, for which the coupling effect between wind and wave-induced loading is critical for accumulated total fatigue damage (Fan et al., 2020), the time domain load and structural response analysis is highly recommended despite the computational effort. This is due to the fact that coupling effects arising from wind turbine dynamics, wind-induced loading, wave-induced loadings, boundary conditions, and their interactions can only be captured by the time-domain analysis using multi-physics numerical tools (Patryniak et al., 2022). In the present study, the structural integrity assessment is carried out based on Fracture mechanics assuming the existence of an initial crack. To generate VAL, a fully coupled dynamic analysis of FOWT is conducted using openFAST considering different load scenarios, which are later translated into the hotspot stress time history. 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. These uncertainties can arise due to loading spectrum characteristics and load interactions. To deal with the probabilistic nature of the crack propagation, Monte Carlo Simulation is carried out to quantify the uncertainty in FCG throughout the service life of the FOWT. The results of the present study can aid the development of risk-based decision-making tools for structural asset management accounting for monetary consequences associated with the failure and cost of remedial actions. 2. Methodology The methodology introduced in the present study adopts a multidisciplinary approach that brings together the dynamic behaviour analysis of the FOWT, the crack growth analysis under the observed dynamic loading, and finally, the probabilistic analysis (see Fig. 1). The procedure starts with collecting the data associated with the design load case (DLC) for fatigue limit state, wind turbine data, characteristics of the offshore wind farm site. The collected data provides the necessary input to perform a fully coupled aero-hydro-servo-elastic (AHSE) simulation using OpenFAST (2023), resulting in six degrees of motion of the FOWT at the tower base. Subsequently, the global load analysis results are used to derive the global structural response at the tower base for each sea state.

Fig. 1. Methodology followed for the probabilistic structural integrity assessment