Abstract: Bucket foundation is an innovative type for supporting offshore wind turbines (OWT), and more than 60 foundations of this type have been installed in the wind farms in China due to its low cost and efficient installation. With the developing trend of large scale and deep water, the size of foundation grows, resulting in a high buckling failure probability of its steel bucket skirts during prefabrication, transportation and sinking. This paper develops a thick-walled bucket foundation that adopts a concrete skirt and steel bulkheads to further reduce its cost and skirt buckling possibility. However, as the penetration resistance of this foundation increases, how to reduce it becomes the key to successful installation. In this work, we conduct a laboratory test of the foundation in silty clay, and examine the resistance reduction effect of two measures: ring stiffeners and thick-skirt water flushing. And a calculation method of penetration resistance is developed to take the reduction effect into account. Results show that the ring stiffeners with the same size as the steel bulkheads can reduce the friction drag by 50%, and the flushing measure can reduce the end resistance by 88%. To avoid the influence on the suction operation caused by synchronous water flushing, an alternate operation of the two processes is suggested.

Key words: foundation, offshore wind power, thick-walled bucket foundation, penetration resistance, drag reduction method