Experimental Study On Installation Characteristics And Lateral Cyclic Loading Response Of Bucket Foundation For Offshore Wind Turbine

With the implementation of the national"emission peak and carbon neutrality"clean energy development strategy,offshore wind power has entered a period of rapid development.As a preferred type of offshore wind turbine foundation,it has the advantages of easy construction,reusable,low construction costs,and can be transported by self-floating towing.Therefore,it has been applied in some offshore wind farms in recent years.At present,the study on the upper and lower critical suction pressure of the bucket foundation sustainable installation in saturated sand is lagging behind engineering practice.The study on the cyclic bearing characteristics of single bucket and multi-bucket foundations under complex environmental load conditions and long-term cyclic loading is less.Thus,the research on the stiffness and accumulated rotation of the bucket foundation under unidirectional and multidirectional cyclic loads,and other influencing factors of the bearing capacity is not yet complete.Based on the above problems,experimental tests and finite element methods combined with theoretical analysis methods are used to study the penetrating characteristics and cyclic bearing characteristics of the bucket foundation in saturated sand.The main research contents and results include:（1）According to the calculation method of the critical suction pressure of the bucket foundation,a series of sustainable model tests of the bucket foundation in saturated sand were carried out.The research results show that the suction loss occurs in the soil inside the bucket foundation,and the suction loss is linear with installation depth.The relationship between the internal hydraulic gradient of the bucket foundation and the installation depth was established.The relationship between the k_fac ratio of the soil permeability coefficient inside and outside the foundation and installation depth was proposed.The calculation method of the upper/lower critical suction pressure of the bucket foundation was proposed,and finally the applicability of calculation method was verified by the four-bucket foundation installation test.（2）For the novel four-bucket foundation with board connection structure,model tests and finite element methods were used to study the horizontal bearing mode of the novel four-bucket foundations and single buckets in saturated sand.Then the influence factors of the bearing capacity were analyzed.The research results show that the horizontal bearing capacity of the four-bucket foundation can be increased by more than10%in undrained conditions according to the additional board connection structure.The 45-degree loading direction and the aspect ratio have significant effects on the bearing capacity of the four-bucket foundation in the undrained conditions.The failure mode of the four-bucket foundation was obtained by the experiments and finite element analysis.Finally,a calculation method for the horizontal bearing capacity of the four-bucket foundation was proposed and its validity was verified.（3）In this study,the intelligent multi-directional cyclic loading system was designed and carried out.Based on this loading system,the single bucket foundation was tested by cyclic loading under unidirectional and multi-directional cyclic loading.The influences of the loading direction,vertical load and drainage conditions were analyzed.Moreover,the changes of the accumulated rotation,cyclic stiffness,rotation point position and monotonic bearing capacity of the foundation after the cyclic load were obtained.The research results show that the accumulated rotation angle can be reduced significantly by the multi-directional cyclic load,and the reduction range can reach more than 50%.After unidirectional or multidirectional cyclic loading,the load-bearing capacity of single bucket foundation can be increased by 30%or more than20%,respectively.The relationships between the vertical load,the number of cyclic loadings and the normalized accumulated rotation angle were established,and the calculation method for the vertical load and the bearing capacity of the foundation after the cyclic loading was proposed.（4）Series of model tests were carried out to study the cyclic bearing characteristics of the four-bucket foundation under unidirectional/multidirectional cyclic load,and the aspect ratio,cyclic load amplitude ratioζ_b,drainage conditions,and cyclic load direction during the cyclic loading process were analyzed.The changes of the accumulated rotation angle,the cyclic stiffness and the horizontal bearing capacity of the four-bucket foundation after cyclic load were studied.The test results show that at the beginning of the cyclic loading the accumulated rotation angle of the four-bucket foundation was completed more than 80%of the total accumulated rotation,and the calculation method for the normalized accumulated rotation angle and number of cycles was established.The horizontal bearing capacity of the four-bucket foundation was increased by about 15%-20%after cyclic loading,and the accumulated rotation angle of the two-way and four-way cyclic load can be reduced by up to 30%and more than40%,respectively.The cyclic load amplitude ratioζ_b has a significant influence on the accumulated rotation angle of the foundation and the bearing capacity after cyclic loading.The relationship between the cyclic load amplitude ratioζ_b and the final cyclic accumulated rotation angle and the bearing capacity after cyclic load were established.