Model tests on sinking technique of composite bucket foundations for offshore wind turbines in silty clay

The composite bucket foundation (CBF) is a new type of for offshore wind turbines, which can be adapted to the loading characteristics and development needs of offshore wind farms due its special structural form. The composite bucket foundation in the large-scale tests has an outer diameter of 3.5 m and a clear wall height of 0.9 m. There are seven rooms divided inside the CBF by steel bulkheads, which are arranged in a honeycomb structure. The six peripheral rooms with the skirt have the same proportions while the middle orthohexagonal one is a little larger. With the seven-room structure, the CBF has reasonable motion characteristics and towing reliability during the wet-tow construction process. Moreover, the pressure inside the compartments can control the levelness of the CBF during suction installation. Several large-scale model tests on suction installation and adjusting levelness of CBF were carried out in saturated silty clay off the coast of Jiangsu in China. During the sinking process under negative pressure, the tubes valves of all the compartments were controlled to level the foundation in a timely operation. When the foundation started to tilt, the feasibility of the bucket tilt adjusting technique was explored by applying suction/positive pressure and intermittent pumping among the seven rooms. When the required levelness of the foundation was achieved by suction-assisted lowering the high compartments and/or positive pressures raising the low compartments, all of the valves were opened so that the entire foundation could sink. Test results show that the reciprocating adjustment process can be repeated until the CBF is completely penetrated into a designed depth.