# Influencing Factors of Motion Responses for Large-Diameter Tripod Bucket Foundation
## 1 Introduction  
With the continuous development of offshore wind power, diverse foundations, such as gravity foundation, pile foundation, tripod foundation, jacket foundation, and even floating foundation, have been evaluated as the foundation of offshore wind turbines (OWTs) [1; 2]. In recent years, bucket foundation has attracted much attention because of its easy construction, convenient transportation, reusability, and good soil applicability. Furthermore, the cost of piling operation can be saved and noise pollution during piling can be avoided. A single bucket with a large diameter or multi-bucket foundations combining several bucket foundations in a regular polygon shape are a potential alternative foundation to improve the bearing capacities of foundations for OWTs. Figure 1 shows the three main types of bucket foundation applied to the engineering practices of OWTs [3; 4; 5; 6; 7]. A mono bucket foundation (e.g., developed by the Universal Foundation) was applied to the Frederikshavn wind farm in Denmark in 2002; it was used as the foundation of meteorological mast at the Horns Rev2 in Denmark in March 2009 and the Dogger Bank in the UK in September 2012. A suction bucket jacket (SBJ) (developed by DONG Energy) was installed at theBorkum Riffgrund 1 offshore wind farm in Germany in October 2014. A mono bucket with multi compartments (jointly developed by Tianjin University and Daoda Company) was installed in Qidong city. Moreover, a composite bucket foundation (CBF) was used with one-step transportation and installation technology in Sanxia Xiangshui and Dafeng offshore wind farm in China. For the mono bucket foundation with several compartments or multi-bucket foundation, air floating technology can be used to transport the structure from the dock to the construction site. This is one of the key technologies for the cost-effective development of bucket foundation for OWTs, as shown in Figure 2. The key scientific issues related air floating are the interactions between marine environment, construction loads, and structure. Because of open bottom and air compressibility, the air floating characteristics and mechanics of bucket foundation are different from those of conventional rigid bottom float. An ordinary floating body is equivalent to a structure with the foundation supported on a water spring. On the other hand, bucket foundation is equivalent to a structure with a flexible foundation supported on a series of springs coupled with compressible air spring and water spring [8; 9; 10; 11; 12; 13; 14]. Therefore, to predict the air floating performance of bucket foundations accurately, it is incorrect to use the same parameters and calculation methods used in predicting the hydrodynamic performance and response of traditional rigid bottom platforms or ship structures. In recent years, studies on air-cushion-supported structures mainly focused on the hydrostatic and hydrodynamic characteristics of structures. In the hydrostatic aspect, based on the perfect air law, [PERSON] introduced an air-pocket factor to describe the relationship between the compressibility of trapped air and the resulting hydrostatic stiffness of the system [15]. By considering the air floating structure as a single freedom rigid body and spring system, [PERSON] et al. developed an air floating reducing coefficient,  
Figure 1: Offshore wind turbines (OWTs) with different types of bucket foundations: (**a**) Monobucket foundation, (**b**) suction bucket jacket (SBJ), (**c**) composite bucket foundation (CBF) with multi compartments. Figure 2: Transportation of bucket foundation: (**a**) Monobucket with seven rooms as air cushions, (**b**) self-floating towing, (**c**) multibucket foundation, (**d**) air floating towing, (**e**) one-step transportation and installation technology. accounting for the difference in the restoring force coefficient between the air floating structures and the conventional buoy.