Scour-conditioned seismic fragility analysis of monopile-supported offshore wind turbines

Monopile-supported offshore wind turbines (MS-OWTs) are increasingly deployed in seismic coastal regions, where they face compound risks from earthquake loading and seabed scour. While past studies have addressed these hazards separately, seismic fragility under evolving scour conditions remains insufficiently understood. This study introduces a probabilistic framework to evaluate the seismic fragility of MS-OWTs across a range of scour depths, accounting for evolutionary changes in foundation stiffness, dynamic behavior, and structural vulnerability. Nonlinear dynamic analyses are performed using real ground motions within a cloud-based approach, and scour is modeled as a loss of lateral soil support. Structural and radiation damping mechanisms are treated separately to better reflect MS-OWT working conditions. A central contribution of this study is a comprehensive assessment of seismic intensity measures (IMs), including detailed analyses of efficiency and sufficiency. The results reveal that broadband spectral IMs provide more reliable and informative seismic demand predictions than traditional metrics such as peak ground acceleration (PGA) or peak ground velocity (PGV), particularly under scour-modified conditions. Fragility curves show how exceedance probabilities at both serviceability and ultimate limit states change with scour depth. These findings underscore the importance of integrating scour-induced uncertainties and IM selection into seismic design frameworks of MS-OWTs. The proposed approach contributes to more accurate risk-informed assessment and improved resilience of offshore wind infrastructure in multi-hazard coastal environments.