Experimental Study on Installation of Composite Bucket Foundations for Offshore Wind Turbines in Silty Sand

The composite bucket foundation (CBF) is a cost-competitive foundation for offshore wind turbines, which can be adapted to the loading characteristics and development needs of offshore wind farms due to its special structural form. There are seven sections divided inside the CBF by steel bulkheads, which are arranged in a honeycomb structure. The six peripheral sections with the skirt have the same proportions while the middle orthohexagonal one is a little larger. With the seven-section 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 of CBF have been performed in order to explore the feasibility of the tilt adjusting technique in saturated silty sand off the coast of Jiangsu in China. The composite bucket foundation in the tests has an outer diameter of 3.5 m and a clear wall height of 0.9 m. During the suction-assisted penetration process, the pressures in all the compartments were controlled to level the foundation in a timely operation. A convenient method is to improve the CBF inclination by controlling the inside differential pressure among the compartments. It can be commonly carried out by applying suction/positive pressure with intermittent pumping among the seven compartments. Another adjusting technique for a big tilt with deeper penetration is operated with decreasing the penetration depth achieved by suction-assisted lowering the relatively high compartments and positive pressures raising the relatively low compartments. Test results show that the reciprocating adjustment process can be repeated until the CBF is completely penetrated into a designed depth.