Experimental and numerical study on motion characteristics of a bucket foundation during immersion process

To ensure that the hexagonal bucket foundation (HBF) for offshore wind turbines (OWTs) can be immersed and landed safely at different installation water depths, an experimental study with a 1:60 scale model was conducted to investigate the motion characteristics of the HBF during the immersion process. Self-stability tests, free decay tests, and motion response tests in regular waves were performed. Then, a numerical immersion model of the HBF considering different gas ratios is proposed and verified by comparing with experimental results, indicating that the accuracy of the numerical method improves as the gas ratio decreases. The synthesized experimental and numerical results show that the stability of the HBF decreases significantly at the immersion depth up to 17.7 m. Moreover, for a broader application of the HBF in transitional water (the water depths of 20-50 m), a novel immersion technology utilizing a combined system of hoisting and mooring (CSHM) is proposed. A numerical HBF-CSHM coupling immersion model is established to investigate the feasibility of this proposed method, focusing on effects of different wave conditions and the layout of the CSHM on the motion responses of the HBF at different immersion depths. The results show that the CSHM is feasible and effective in reducing the response amplitude of the HBF for both vertical and rocking motions at the sudden change in the area of waterplanes, compared with the conventional hoisting method. The rotational and translational responses patterns of the HBF with the wave period are different. In addition, the HBF tends to suffer from higher motion responses in State II and state III during the whole immersion process, which can be effectively reduced by setting the initial angle of the mooring wires to the vertical to 20 degrees.