Study On Dynamic Bearing Characteristics Of Offshore Wind Power Monopile Foundation Under Wind-Wave-Gravity Loads

In order to thoroughly implement the strategy of strengthening the ocean power and reduce the environmental pollution caused by traditional energy utilization,the offshore wind energy industry is experiencing vigorous development.As the most commonly used offshore wind power foundation form,monopile foundation is subjected to the combined action of wind load,wave load and gravity load of the upper wind turbine tower structure during its service,which involves the coupling of static load,dynamic load and overturning moment load.Under complex loading conditions,the dynamic load-bearing characteristics of pile body such as horizontal displacement,settlement,bending moment and pile top acceleration are significantly different from those under single load.However,at present,there is little analysis on the dynamic load-bearing of offshore wind power monopile foundation under the combined action of wind,wave and gravity load,and there is still a lot of theoretical research work to be done.Therefore,the research on the dynamic load-bearing characteristics of monopile foundation in marine environment is carried out in order to provide scientific and reasonable reference for the design and construction of monopile foundation for offshore wind turbines.Taking a proposed wind farm in Xiangshui sea area of East China Sea as the actual environmental background of the project,ABAQUS numerical analysis software is used to establish a solid numerical finite element numerical calculation model of wind power monopile foundation in the marine environment.The wave load,wind load and vertical load analysis theories are reasonably applied to calculate the environmental load in the sea area of the wind farm,and the environmental load is equivalent to horizontal cyclic load,overturning moment load and gravity load for application.On this basis,some calculation results of the numerical model are compared with the existing research results to verify the scientific rationality of pile-soil finite element model.Then,according to the changes of dynamic bearing characteristics such as horizontal displacement and cumulative displacement of pile body,pile foundation settlement,bending moment of pile body and acceleration of pile top with wind load and gravity load under the action of different wave loads,research and analysis are carried out,and the change law of dynamic bearing characteristics of offshore wind power monopile foundation under the combined action of wind-wave-gravity load is summarized by comparing the results of time-history curve and special time-point curve.Finally,the control variable method is adopted to study the dynamic bearing characteristics of offshore wind power monopile foundation under different pile diameters,different embedded depths and different wall thicknesses.The influences of various factors on the horizontal displacement and cumulative displacement of monopile foundation,pile foundation settlement,pile bending moment and pile top acceleration are analyzed,and the influence change rules are summarized.The main results are as follows:(1)Under different working conditions,the cumulative amount of positive and negative horizontal displacement of pile body increases with the increase of lateral wave load cycles.The amplitude of transverse wave load is the main controlling factor of horizontal cumulative displacement.Gravity load has a restraining effect on pile displacement accumulation,but when wind wave load increases,the restraining effect of gravity load decreases.The wind-induced overturning moment load will increase the cumulative velocity of horizontal displacement of the pile body on the active side,but has little effect on the cumulative displacement.(2)The positive and negative horizontal displacement and bending moment of the pile body increase in proportion with the increase of wave height of transverse wave load.The wind-induced overturning moment load will increase the horizontal displacement and bending moment of the pile body on the active side,while decrease the horizontal displacement and bending moment on the other side.Gravity load can reduce the absolute value of horizontal displacement and bending moment when wind wave load is small.With the wind and wave load gradually increasing,due to the influence of p-Δ effect,gravity load will cause additional displacement and additional bending moment of monopile foundation.(3)When the amplitude of wave load increases,the acceleration at the pile top of monopile foundation will change in shape,and the positive and negative acceleration will gradually develop from an irregular double peak curve to a sine function curve.The acceleration of pile top is less affected by wind load and gravity load.(4)The horizontal load caused by wave and wind will increase the settlement of offshore wind power monopile foundation.With the increase of wave load wave height and wind load wind speed,the settlement of pile foundation will further develop and the settlement of pile foundation is more sensitive to wind-induced overturning moment load.(5)When the pile diameter of the pile foundation subjected to wind-wave-gravity load gradually increases,the cumulative displacement of the pile body in positive and negative directions decreases,but the decrease amplitude is smaller and smaller,and the influence of wind-induced overturning moment load and gravity load is smaller and smaller.The positive and negative horizontal displacements of the pile body decrease greatly,the deformation characteristics approach to the rigid pile,and the negative influence of p-Δ effect decreases gradually.The pile bending moment increases numerically with the increase of pile diameter,the positive effect of gravity load gradually strengthens,and the sensitivity of pile bending moment to wind-induced overturning bending moment load gradually decreases.The acceleration at the top of the pile continues to decrease in numerical value,and the shape characteristic gradually approaches to sinusoidal function curve from irregularity.The influence of gravity load and overturning moment is small.The settlement of monopile foundation decreases continuously,and the negative influence of wind-induced overturning moment on settlement decreases gradually.(6)When the wall thickness of monopile foundation subjected to wind-wave-gravity load increases gradually,the positive and negative horizontal cumulative displacement of pile body basically remains stable first and then decreases gradually,and the influence of wind load and gravity load also decreases accordingly.The positive and negative horizontal displacement of the pile body decreases,the rigidity of the pile body increases,the P-Δ effect weakens with the decrease of flexural deformation,and the wind-induced overturning moment load has less and less influence under various working conditions.The bending moment of the pile body increases numerically with the increase of the wall thickness,the restraining effect of gravity load on the bending moment of the pile body gradually develops,and the bending moment variation of the pile body caused by wind load continuously decreases.The acceleration at the top of the pile continues to decrease in value and is less affected by gravity load and wind load.The settlement of pile foundation is basically stable,and the relationship curve between settlement and wall thickness under various working conditions is approximately parallel.(7)When the pile foundation subjected to wind-wave-gravity load increases gradually,the cumulative horizontal displacement of pile body in positive and negative directions basically remains unchanged under the influence of wind-induced overturning moment load and gravity load.The positive and negative horizontal displacement values of the pile body decrease,the reverse bending point of the horizontal displacement of the pile body moves up first and then tends to be stable.The restraining effect of gravity load on the development of the horizontal displacement of the pile body gradually exceeds the negative effect caused by the p-Δ effect,and the influence degree of wind-induced overturning moment load on the horizontal displacement is basically stable.The bending moment of pile body increases numerically with the increase of embedded depth,the influence of p-Δ effect decreases with the decrease of flexural deformation,and the in fluence of wind-induced overturning bending moment load on the bending moment of pile body does not change with the increase of embedded depth.The acceleration of pile top continues to decrease in numerical value,and the influence of gravity load and overturning moment is small.The settlement of monopile foundation firstly decreases and then tends to be stable,and the negative influence of wind-induced overturning moment on settlement has the same rule.