Study On Coupling Load Bearing Characteristics Of Large Diameter Steel Pipe Pile Foundation For Offshore Wind Turbine Under Combined Load

Large diameter steel pipe pile foundation has become one of the most widely used foundation forms of offshore wind power because of its clear principle,simple production,convenient construction and economical cost.However,at present,there are relatively few studies on the bearing characteristics of pile foundation under the combined action of horizontal loads such as wind,wave and flow and vertical self-load loads such as fans and towers.Moreover,the existing methods decompose the combined loads into independent calculation of vertical and horizontal bearing capacity,ignoring the mutual influence between horizontal loads and vertical loads,resulting in unpredictable effects on the real combined bearing performance.Therefore,it is necessary to study the coupling bearing characteristics of large diameter steel pipe pile foundation of offshore fan under combined load.In this thesis,with the support of Postgraduate Research & Practice Innovation Program of Jiangsu Province(SJCX21＿1794)and National Natural Science Foundation of China(52201324、51808112),laboratory model test,numerical simulation analysis and theoretical analysis were carried out.The main work and conclusions of this paper are as follows:(1)Based on the self-developed multi-dimensional servo loading test equipment,the experimental research scheme of model pile bearing characteristics under combined loads is developed,which lays a foundation for the establishment of envelope curve of single pile bearing capacity.The successful implementation of the experiment in this paper proves the reliability and efficiency of the independently developed equipment,and solves the problem of applying the combined load of the traditional model pile.(2)Based on the model pile test,the load-displacement curves of pile top and the distribution curves of pile bending moment,shear force and axial force under different loading angles and buried depths were obtained,and the envelope curves of ultimate combined bearing capacity of single pile were further obtained.The test results show that the bearing performance of the model pile combination is significantly affected by the loading Angle,which mainly depends on the game between the vertical component and the horizontal component of the combined load.The 60° loading angle is the turning point of the game,that is,when the loading angle is less than 60°,the horizontal component of the load takes the dominant position in controlling the bearing performance of the combination,and when the loading Angle is greater than 60°,the vertical component is controlled.(3)Based on the finite element simulation study of ABAQUS,the influence factors of different loading angles on the bearing characteristics of single pile composite were revealed,and finally the envelope curve of composite bearing capacity based on the finite element was formed.The finite element results show that the pile length has a greater effect on the load failure of pile foundation,the pile length is less prone to the whole rotation,and the plastic zone of the upper soil is reduced,but the internal force of the long pile is larger.According to the envelope curve of combined bearing capacity,vertical load will not affect the horizontal bearing performance of pile foundation when the loading angle is less than 60°.When the loading angle is between 60° and 75°,the vertical and horizontal components play a leading role together.When the loading angle is greater than 75°,the combined load performance is mainly controlled by the vertical load component.(4)Based on the principle of matrix transfer method,the differential equation of pile foundation force considering Timoshenko beam and vertical load is established.The matrix transfer solution of pile horizontal bearing response is derived by the idea of dimensionality reduction,and the correctness of the proposed method is verified by comparison.Finally,parameter analysis is carried out and the simplified envelope formula of combined bearing capacity is established.The theoretical analysis results show that when the pile diameter is greater than 3m,the proportion of pile shear deformation in the total pile deformation increases rapidly,and the pile shear deformation cannot be ignored in the design of large diameter pile foundation.When the horizontal load is constant and the vertical load is twice the horizontal load,the internal force and deformation of large diameter pile foundation will increase significantly with the increase of vertical load.When the combined load Angle is less than 65°,the bearing characteristics of the combined load pile are controlled by the horizontal load component,and when the combined load Angle is greater than 65°,the vertical load component is controlled.The simplified formula of envelope of combined bearing capacity is simple and easy to use,and the calculated results are between those of finite element and traditional elliptic equation,which has good application value.