Air-Floating Characteristics of Large-Diameter Multi-Bucket Foundation for Offshore Wind Turbines

Abstract

In the present study, as a novel and alternative form of foundation for offshore wind turbines, the air-floating characteristics of a large-diameter multi-bucket foundation (LDMBF) in still water and regular waves are investigated. Following the theory of single degree of freedom (DOF)-damped vibration, the equations of oscillating motion for LDMBF are established. The spring or restoring coefficients in heaving, rolling and pitching motion are modified by a dimensionless parameter ϑ related to air compressibility in every bucket with the ideal air state equation. Combined with the 1/25 scale physical model tests and the numerically simulated prototype models by MOSES, the natural periods, added mass coefficients and damping characteristics of the LDMBF in free oscillations and the response amplitude operator (RAO) have been investigated. The results shown that the added mass coefficients between 1.2 and 1.6 is equal to or larger than the recommended values for ship dynamics. The coefficient 1.2 can be taken as the lower limit 1.2 for a large draft and 1.6 can be taken as the upper limit 1.6 for a small draft. The resonant period and maximum amplitudes for heaving and pitching motions decrease with increasing draft. The amplitudes of heaving and pitching movements decrease to a limited extent with decreasing water depth.