Abstract: Bucket foundation is an important foundation type of offshore structures, especially for offshore wind turbines. Study on its penetration law is the key to successful application of this kind of foundation type. Previous methods for penetration suction are CPT-Based method and analytical calculation method, which are based on cone penetration test (CPT) and force balance in the penetration process, respectively. However, results from previous methods deviate from practical engineering data. In this paper, three bucket foundations with different wall thicknesses were penetrated by suction pressure. Earth pressure and pore pressure inside and outside the bucket wall, as well as the tip resistance were measured in the experiments. The influence of key parameters on the two calculation methods was analyzed and reasonable suggestions were given. According to measured pore pressure compared with numerical simulation analysis, a critical pressure calculation formula is established. Studies have shown that coefficient of friction between side wall and the soil is about 0.2. For CPT-Based method，outside-wall friction parameter kf is in the range of 0.002 6? 0.007 0；inside-wall friction parameter kf is 0.001; tip resistance coefficient kp is in the range of 0.2?0.6. For analytical calculation method, lateral earth pressure coefficient K should be between the stationary earth pressure coefficient and passive earth pressure coefficient, and tip resistance coefficient Nq is 40. The critical suction pressure formula based on protection mechanism for seepage failure can effectively increase the critical suction pressure，which is close to the actual situation.

Key words: offshore wind turbines, bucket foundation, penetration resistance, critical suction pressure