Generic fully coupled framework for reliability assessment of offshore wind turbines under typical limit states

Offshore wind turbines (OWTs) generally experience harsh marine environments with uncertainties in marine loads, soil properties, and material properties; therefore, the structural integrity assessment of OWTs is a challenging task. Most of the previously proposed methods for reliability assessment are time-consuming and neglect the coupling effects of aerodynamics, hydrodynamics, and OWT structural dynamics under complex environmental loads. In this study, a fully coupled framework for the reliability assessment of OWTs is proposed, in which a dynamic simulation tool in the time domain is combined with surrogate models. The established Kriging, multivariate regression, and support vector regression surrogate models are compared to determine the most feasible model. Furthermore, the influence of different solving methods, such as first-order reliability method (FORM), importance sampling (IS), and subset simulation (SS), on the estimation of structural reliability indexes are discussed. The superiority of the estimated reliability indexes in the safety assessment of OWTs is proved using the SS method based on the Kriging surrogate model. Subsequently, the potential failure modes of an OWT under different operational states are revealed, indicating that excessive tower vibrations must be eliminated for stable power production from OWTs. Furthermore, the SS method is found to be feasible as it ensures the estimation accuracy of the reliability indexes of the parked OWT based on Kriging and the other surrogate models. Finally, the significance of soil-structure interactions in the reliability index estimation is investigated, and the equivalent coupled spring boundary is proposed from the perspectives of structural safety and simulation accuracy.