Ageing-dependent, multi-hazard fragility of monopile-supported offshore wind turbines

This paper presents time-dependent multi-hazard fragility functions for a monopile-supported 5 MW offshore wind turbine (OWT) under combined stochastic wind, wave and seismic loads. Ageing is considered by explicitly modelling two major deterioration phenomena: support structure zonal corrosion and monopile foundation scouring. A Latin Hypercube sampled, cloud-based, dual-intensity-measure (IM) fragility assessment framework is employed to produce multi-hazard fragility surfaces at nine evenly separated instants (0–40 years). Various model uncertainties were accounted for via a pre-defined multivariate probabilistic distribution. Failure probabilities at different ages were derived using Gaussian Process Regression (GPR) for selected Engineering Demand Parameters (EDPs), where the full range of operational inflow wind speeds (3–25 m/s) was considered. Over time, the modal characteristics of the soil-foundation-structure system deviate from its original state. The probability of an OWT exceeding the ultimate limit state (ULS) criterion when subjected to a design-level combination of wind, wave and earthquake loads is not just nonnegligible but can increase considerably owing to ageing: by 66 % after 10 years of operation and 100 % after a typical 25-year design life. The results indicate the importance of multi-hazard coupling and provide a robust framework for assessing time-evolving fragility under joint earthquake-wind-wave loading.