Numerical Analysis Of Bearing Performance Of Offshore Wind Power Steel Cylinder Foundation Under Different Load Conditions

President Xi Jinping announced at the seventy-fifth session of the United Nations General Assembly that China is striving to peak carbon dioxide by 2030 and achieve carbon neutrality by2060^[1].Under the background of"dual carbon"strategic target,offshore wind power construction has become an economic and effective way to boost the realization of"dual carbon"target.According to the statistics of the United States energy department,the cost of wind power infrastructure accounts for 1/3 of the total cost,so it is urgent to support the development of wind power with a foundation structure.At present,the offshore wind turbine cylinder foundation structure is most common in shallow sea areas with rock strata.The structure is bottom opening and top cover.The bucket foundation is characterized by the efficient use of shallow soil bearing capacity,and the negative pressure sinking does not require too cumbersome construction procedures,which is the main development of wind power in shallow sea areas in China.The offshore wind power steel cylinder foundation is subjected to vertical load(V),transverse load(H)and tilting moment(M)imposed on it by wind power structure,wind and wave in practical engineering.In this paper,a three-dimensional finite element model of offshore wind turbine steel cylinder foundation and surrounding soil is established based on the large general finite element software ABAQUS.Monotonic load and composite load were applied to the finite element model of steel cylinder foundation by simulating the actual working conditions,and the model structure(aspect ratio,buried depth,loading height)and cylinder soil parameters were quantitatively changed.Through simulation calculation,the load-displacement curves and failure envelope curves under different working conditions are obtained,and the failure mechanism and ultimate bearing capacity of steel cylinder foundation under different load spaces are studied.The research conclusion can reveal the inherent bearing performance law of offshore wind power steel cylinder foundation,improve the basic theoretical data of similar cylinder,and provide theoretical basis for the practical engineering of similar offshore areas in China.(1)The failure mechanism of steel cylinder foundation under vertical load is that the bottom forms a coherent arc failure zone;The failure mechanism of steel cylinder foundation under lateral load and bending moment load is similar as follows,the cylinder in wedge-shaped active failure zone of cylinder foundation is separated from the surrounding soil to form cracks;the cylinder in wedge-shaped passive failure zone of cylinder foundation squeezes the surrounding soil to form uplift;and the corresponding tensile zone and compression zone are formed in the bottom area of cylinder.Through the analysis of the load-displacement curve,it can be seen that the bottom of the steel cylinder foundation under vertical load is punching failure,so the curve is a slow-varying type,and there is no obvious inflection point.The ultimate load can only be determined by the maximum displacement.The load-displacement curves under lateral and bending moment loads are steep,and there is an obvious inflection point,that is,the load corresponding to the inflection point is the lateral ultimate bearing capacity.Therefore,the ultimate bearing capacities V_ult,H_ult and M_ult in three directions under monotonic loading can be determined.(2)The sensitivity analysis of quantitative change of soil parameters and model structure is carried out to analyze the influence of soil parameters on the ultimate bearing capacity in all directions.The influence of cylinder soil model structure on the ultimate bearing capacity of steel cylinder foundation is studied to optimize the design of steel cylinder foundation in practical engineering.The failure mechanism of steel cylinder foundation under composite load space is explored,and the influence relationship between different load components is analyzed according to the load displacement curve.The failure envelope of the steel cylinder foundation in V-H,V-M,H-M load space and the normalized envelope surface of the bearing capacity of the cylinder foundation in V-H-M load space are obtained by the load-displacement joint search method.The failure mechanism of tube foundation under H-M and V-M load space is similar,and the shape of the failure envelope surface is roughly the same,but the V-M failure envelope surface has a yield point.Increasing the displacement in the bending direction has obvious weakening effect on the ultimate bearing capacity in both directions.The failure envelope surface under V-H-M load space can judge the stability state of steel cylinder foundation under different load combinations,which is approximately a quarter of the spherical surface.