Evaluation of high vibration mode and hybrid foundation reinforcement effects for monopile-supported offshore wind turbines under earthquake loading

With the growing demand for clean energy, the number of offshore wind farms in seismic regions has been increasing globally, particularly in regions such as Japan, China, India, and South Korea. Offshore wind turbines (OWTs) are high-rise structures and are sensitive to lateral loads during earthquakes. In this study, numerical simulations of OWTs are carried out using the OpenSees framework. The PDMY03 constitutive model for saturated soils under cyclic loading is calibrated against centrifuge test results. Then, the validated numerical model is applied to the seismic analysis of a 10 MW OWT, considering both monopile and hybrid foundations. The latter consists of a monopile and a bearing plate in the form of a circular collar. The contribution of high-frequency vibration to the structural response under seismic shaking is investigated. Moreover, the performance of the hybrid foundation under combined wind and earthquake loadings is evaluated. Simulation results indicate that mudline rotation and superstructure-induced inertial forces are dominated by the second mode of vibration. The reinforcing effect of the hybrid foundation is evident in the reduction of lateral displacement and rotation. However, the circular collar increases the bending moment in the monopile near the mudline during earthquake shaking due to the interaction between the collar and the monopile. Although the circular collar reduces the required monopile embedded depth while maintaining comparable lateral stiffness under normal operating conditions (wind loading only), this may compromise global stability under extreme loading conditions (wind and earthquake loadings) due to excess pore water pressure generation.