Behavior of monopile-supported offshore wind turbines under normal operational and extreme-parked conditions in loose sand

Enhancing the performance and ensuring the long-time reliability of Offshore Wind Turbines (OWTs) requires a comprehensive understanding of OWT system response and interactions between OWT system components under realistic loading conditions. This paper presents Real-Time Hybrid Simulation (RTHS) results for a monopile-supported OWT under normal and extreme loading scenarios, which include aero-hydro-mechanical-geotechnical-structural component interactions. RTHS divides the OWT into analytical and experimental substructures, with dynamic equilibrium and compatibility maintained at the interfaces between substructures. The analytical substructures model the superstructure of the OWT above the mudline, along with the applied wave and wind loads, and the effects of the power-take-off unit using well-established numerical models, while the experimental substructure is the soil-monopile system that is physically tested in the laboratory. The RTHS results reveal that the OWT exhibits a ratcheting response due to a progressive accumulation of the monopile displacement under cyclic and dynamic loads. In addition, the stiffness and damping characteristics of the soil-monopile system change significantly, where the lateral stiffness increased by more than 100% and the damping ratio decreased by 76% relative to initial values during the RTHS. The results show that the OWT system response is dominated by wave loading and influenced by the soil-monopile response. The 1st fore-aft tower-monopile mode frequency was reduced by about 10% relative to a reference case without soil-monopile effects. The study shows that the soil-monopile response must be considered to accurately simulate the response of OWTs to various loading conditions.