Performance-based evaluation of the impact of scour erosion on the offshore wind turbine monopile foundation in silty-sandy seabed

The bearing performance of offshore wind turbine monopile foundations under scour erosion is typically assessed using recommended scour depths (e.g. S = 1.3D or 1.5D) provided in design specifications, without accounting for site-specific geological conditions. To address this limitation, this study proposes a coupled computational fluid dynamics (CFD) and finite element method (FEM) approach to evaluate actual scour patterns and their structural consequences more accurately. First, the local scour process around a monopile under current-induced erosion is simulated via CFD and the equilibrium scour depth obtained is S = 1.04D, which is notably shallower than the code-specified values. Subsequently, static and dynamic bearing behaviors of the monopile under this realistic scour condition are investigated through FEM incorporating full structure–soil interaction. Results demonstrate that employing specification-recommended scour depths leads to considerably conservative estimates of static performance. Specifically, the horizontal displacement at the mudline is overestimated by 18.4% and 39.5% for S = 1.3D and S = 1.5D, respectively, compared to the CFD-derived S = 1.04D condition. A similar trend is observed for the rotation angle. In contrast, the natural frequency remains relatively insensitive to scour depth, with deviations of only 2.0% and 3.7% relative to the CFD-based case. These findings highlight the importance of incorporating site-specific scour predictions into the design process to avoid overly conservative foundation designs while maintaining safety and performance standards.