Bearing Characteristics Of Composite Bucket Foundation For Offshore Wind Turbine

As a new type of offshore wind power foundation, wide-shallow composite bucket foundation is realized to be built on land and rapidly installed offshore. The problems faced with developing offshore wind resources can be well solved due to the foundation’s strong anti-overturning ability and the adaptability to all kinds of ground soil. The ground bearing capacities of classical soil mechanics can hardly match the bearing mechanism of the new type of foundation. The compartment structure leads to a better collaborative bearing mechanism between the foundation and the soil, which is quite different from the previous relevant research of foundation. In this paper, the bearing capacity of composite bucket is preliminary determined through tests, then the one-dimensional, two-dimensional and three-dimensional bearing capacities and the buckling failure mechanisms are calculated and analyzed by finite element simulation. Finally, specific to the 3MW composite bucket foundation, the practical and theoretical ultimate bearing capacities are analyzed and the bearing mechanism and load transfer mechanism are obtained. The specific research contents are as follows:The horizontal bearing capacity tests of composite bucket foundation are carried out in silt clay and sandy soil, the hierarchical loading mechanism is used to conform to tests expectations, and the variations of earth pressure and displacement are obtained respectively. Then the tests are analyzed and compared by finite element simulation. According to analyses, the rotation center of composite bucket foundation gradually moves from the central axis to the bottom of compartment boards along the loading direction with the increase of load and finally locates in the distance of 0.8L(cylinder height) from the cylinder cap. Under ultimate horizontal load, the earth pressure distribution of composite bucket foundation’s main regions is obtained through tests, finite element method and theoretical formula, respectively. The bearing capacity of composite bucket foundation is calculated according to the earth pressure distribution and the calculation formula is also revised.The vertical load and bending load are calculated by finite element simulation respectively. Under vertical load effect, the bucket foundation and the soil inside are regarded as a whole, the corresponding buckling failure mode is obtained. The ultimate vertical bearing capacity is calculated by empirical formula and theoretical formula and the theoretical formula is also revised by finite element results. Under bending load, the rotation center of composite bucket foundation in the region close to the bucket bottom gradually moves from the left of the central axis(reverse to the loading direction) to nearby the compartment boards along the loading direction. The vertical and bending ultimate bearing capacities can be improved by appropriate vertical load, and the biggest increase in silt clay is about two times, which is greater in sandy soil.According to the load values of 3MW offshore wind turbines, the diameter of the wide-shallow composite bucket foundation is generally more than 27 m and the height is preferable between 7 m to 12 m. The influence of soil parameters and the contact characteristics between the bucket and soil on bucket foundation’s bearing capacity is also analyzed. Considering two cases of allowable and ultimate ground deformations, the horizontal, bending and vertical ultimate capacities of 3MW composite bucket foundation are calculated respectively as well as the corresponding two-dimensional envelopes and three-dimensional enveloping surfaces. Then the comparative analysis is conducted, the allowable upper limit and the theoretical limit of bearing capacities are obtained respectively. It can be concluded that the composite bucket foundation is relatively safe in practical engineering. According to finite element analysis, the load bearing mode is determined as “top-bearing type”, which means the top cap carrying the load primarily and the cylinder wall taking subsidiary effects. The huge bending load at the top of the transition section can be transformed to relatively small tension and compression stresses at the bottom through the arc transition section and the prestressed reinforcement, which demonstrates the load transfer mechanism of the prestressed concrete arc transition section.