Long-term scouring effects on the lateral loading behaviour of monopiles using field monitoring data and finite element modelling

Scour of the seabed induced by currents and waves poses a critical threat to the serviceability and lifespan of offshore wind turbines. Despite its significance, the mechanisms and temporal evolution of offshore scour remain insufficiently understood, primarily due to the limited availability of high-resolution, long-term field monitoring data. Moreover, the influence of scour on foundation stiffness and load-bearing capacity has not been comprehensively quantified. This study presents a series of consecutive scour inspections conducted in one-year period following the installation of foundations at a representative offshore wind farm. Based on the acquired field data, an empirical model was developed to predict key geometric parameters of scour holes over time, including maximum scour depth (MSD), maximum scour extension perpendicular to the current (MSE-PDC), and maximum scour extension in the direction of the current (MSE-IDC). These predictions were integrated into three-dimensional finite element models, incorporating the advanced SANISAND-MS constitutive model to characterize sand behavior. A detailed analysis was performed to assess the impact of scour on monopile response, focusing on lateral load-displacement behavior, static soil reaction curves, and monopile rotation. The results demonstrate that scour development within the first 12 months significantly reduces foundation stiffness and capacity. Additionally, lateral soil resistance at constant depths declines with prolonged scour exposure, primarily due to the loss of shallow soil volume and the associated reduction in mean effective stress. These findings provide valuable insights into scour development, foundation performance, and the design of effective scour protection systems for offshore wind infrastructure.