Tetrapod suction buckets offer several advantages for offshore wind turbines (OWTs) foundations, including cost-effective installation and enhanced overturning resistance. OWTs are subject to various environmental loads such as wind and wave loads or even storm impacts during their service life. This study investigates the dynamic response of a 15 MW offshore wind turbine with an improved advanced hypoplastic cohesionless soil model to capture the nonlinear foundation-soil dynamic interaction under combined wind-wave loads via finite element simulations, taking into account different loading conditions and sandy soil densities. Results indicate that the foundation exhibits wind-direction rotation under wind load only, while vertical deformation dominates under wave and combined wind-wave loads. In addition, potential foundation overturning may occur under storm conditions. Besides, an increase in sandy soil density can effectively reduce the foundation deformation. These findings provide valuable insights for offshore wind turbines’ design.