Numerical Analysis Of Scour Mechanical Behavior Of Pile Foundation Of Offshore Wind Turbine

Under the corresponding policies issued by the state,the rapid development of offshore wind power generation,the installed capacity of offshore wind turbines continues to increase,and the small wind turbines gradually develop to large wind turbines,and the capacity of a single unit in the country gradually transition from 3MW to 13 MW,and its diameter and buried depth length continue to increase.The single pile foundation is subjected to the combined action of long-term ocean current and wave load,which will cause local erosion around the pile foundation,resulting in the change of the bearing capacity of the pile foundation,and even the damage or instability of the pile foundation.Therefore,it is of great practical significance to study the simulation of pile foundation erosion protection.This paper mainly conducts a study on scour protection of single pile foundation through FLOW-3D software numerical simulation and production experiment data.The main research contents and results are as follows:Study on local scour of single pile foundation under sea current load.The coupling model of pile foundation,seabed and fluid is established.The governing equations of fluid movement are N-S equation and RNG k-ε turbulence model.The governing equations of sediment movement on seabed are bedload.The local scour of single pile is studied,and the changes of velocity near pile foundation,shear stress on seabed surface,bending moment of pile foundation and scour formation process around pile foundation are discussed.The study says: In the scouring process around the pile foundation,horseshoe vortexes are formed in front of the pile foundation,tail vortexes are formed in back of the pile foundation and fluid is accelerated to flow through both sides of the pile foundation.However,the accelerated flow of fluid through both sides of the pile foundation in the early stage increases the effect of shear stress on the soil surface,which makes the erosion first occur on both sides of the pile foundation in the early stage.However,as the scouring process progresses,the shear stress on the soil surface decreases.The erosion rate around pile foundation decreases.With the increase of water depth,pile pressure,shear force and combined moment increase.The dynamic response and soil surface scour of pile foundation under wave and current loads are studied under different working conditions.It is found that under the combined action of wave and sea current,the scour depth of soil surface is less than that under the sole action of sea current,but the accumulation height is obviously increased,which reflects that the scour depth around pile foundation is mainly caused by sea current,and the waves have a greater impact on the sediment accumulation around pile foundation.Through studying the influence of wave height on pile scouring simulation,it is found that the shear stress of soil surface around pile foundation is significantly reduced by the combined action of wave and current compared with the single action of sea current,and this phenomenon is more obvious with the increase of wave height.However,the shear stress changes with time in the form of wave motion.In addition,the pile bending moment and fluid combination moment increase obviously with the increase of wave height.The scour response of pile foundation was studied by changing the water depth and wavelength.The scour depth around pile foundation increased when the water depth doubled,the sediment accumulation height behind pile foundation decreased,the surface shear stress of soil surface increased,and the bending moment of pile foundation increased.In addition,the maximum erosion depth around pile foundation is smaller when the wavelength is reduced by one quarter than that when the water depth is reduced by half,and the sediment accumulation height is smaller,which reflects that the change of wavelength has a greater impact on the erosion around pile foundation than the change of water depth.Under the same water depth condition,the bending moment of pile foundation increases slightly with the decrease of wavelength,but the shear force,pressure and combined moment of pile foundation decrease obviously.The response of the change of flow direction to pile scouring is studied.The flow direction changes the scouring direction around pile foundation,but has little influence on pile scouring depth.Compared with the condition where the flow direction does not change,the wave form of the shear stress on the sand surface around the pile foundation disappears,and the change of shear stress is more inclined to the condition without wave motion.In addition,the pressure and bending moment of pile foundation in the direction of wave crest X increase,and the pressure,shear force and combined moment of pile foundation as a whole decrease obviously,and the combined moment presents a wave form with time.According to the measurement of water depth distribution around pile foundation and the riprap effect in the process of riprap protection in practice production,the paper analyzes the water depth distribution around pile foundation.In actual production,the riprap protection around the unit in a wind site cannot be completed at one time,and 2-3 riprap is generally adopted,which is determined by the characteristics of the construction schedule.The first riprap is a sacrifice to prevent the increase of scouring depth around the pile foundation and to save the cost of later riprap.The second time will be based on the first final protective riprap.Even the third riprap is to reach the design acceptance requirements for riprap.The scour around the pile foundation measured before the first riprap reflects that the scour around the pile foundation is more serious within 8m range,and the scour outside 8m range gradually decreases.The scour depth around the pile is also affected by the main flow direction.In the main flow direction,the scour depth on both sides of the pile foundation is larger.With the constant change of the flow direction,the scour around the pile foundation tends to be a semi-ellipsoid shape.The uneven riprap around pile foundation will cause local sediment accumulation and change its flow state.