The offshore prefabrication and semi-wet towing of a bucket foundation for offshore wind turbines

Abstract

In recent years, the bucket foundation as a new form of supporting structure of offshore wind turbines (OWTs) has attracted increasing attention with the rapid development of offshore wind power. A synthetical prefabrication and transportation (P&T) technique is proposed in this paper to enable a broader application of bucket foundations efficiently and economically in the offshore wind market. The novel P&T technology consists of two key auxiliary pieces of equipment, i.e., an offshore prefabrication platform (OPP) and a U–K barge. The process of the offshore prefabrication is elaborated, and the requirement that need to be met for the monopod-bucket foundation with subdivisions (MBFS) to achieve offshore prefabrication utilizing the OPP is analyzed. Then, the concept of the semi-wet towing with a U–K barge is proposed, which can provide sufficient buoyancy and stability to ensure the safety of the MBFS during the towing operation. To investigate the seakeeping of the combined semi-wet system in moored and towing scenarios, a series of experiments were carried out to prove the reliability of the U–K barge design. Sensitivity analysis regarding towing speed, wave height, and wave period for the disintegration risk is performed, and three methods are proposed to avoid the disintegration of the U–K barge and MBFS at extreme sea cases. The results show that the proposed offshore prefabrication is feasible, and it is suggested that as the MBFS floats up, the water depth of the construction site should not be less than 6 m. In addition, it is proved that the combined semi-wet towing system composed of the U–K barge and the MBFS has an exceptional seakeeping capability with regard to the pitching angles, accelerations, and towing resistances, and that the wave height and the towing speed have greater effects on the disintegration risk than the wave period.