Influence of groundwater level on the first natural frequency of monopile

The variation of groundwater level caused by tides is a unique feature of intertidal wind turbines (IWTs). The groundwater level affects the natural frequency of the IWTs' monopile by changing the seabed characteristics. Therefore, it is necessary to reveal the influence of groundwater level on the natural frequency of monopile to ensure the stable operation and maintenance of IWTs. Based on model experiments, the laws of the change in groundwater level and the number of groundwater level cycles on the first natural frequency of the monopile in fine sand foundation is revealed, and is explained according to the stiffness and damping characteristics of the fine sand foundation. Moreover, the stiffness and damping characteristics of the fine sand foundation have different effects on the first natural frequency under different variation states of groundwater level: The stiffness of sand foundation is the main factor affecting the first natural frequency; with the groundwater level rising, the influence of the damping of sand foundation on the first natural frequency is getting stronger. In the designing and maintenance of IWTs, the seabed groundwater level should be focused on.