Study On Dynamic Response Characteristics Of Offshore Wind Power Monopile Foundation In Heterogeneous Soil Under Horizontal Cyclic Loading

In recent years,China's offshore wind power industry has developed rapidly.Large diameter single pile foundation is a potential offshore wind turbine foundation,which accounts for more than 75%of the global offshore wind farms.During its service,it will inevitably bear the horizontal cyclic loads generated by the action of waves,ocean currents and wind on the structure.The loading conditions and soil geological conditions are complex.At present,the research on the failure mechanism and mechanical deformation characteristics of monopile foundation of offshore wind power in heterogeneous soil under horizontal cyclic loading is not perfect,and the theory is not yet mature.Therefore,the dynamic response characteristics of offshore wind power monopile foundation in heterogeneous soil under horizontal cyclic loading are studied.The research results can provide a reasonable and reliable theoretical basis for the design and construction of monopile foundation of offshore wind turbines,which has important theoretical and practical significance.Taking an offshore wind farm in the East China Sea as an example,the numerical calculation model of offshore wind power monopile foundation in heterogeneous soil was established by using finite element analysis software ABAQUS.In the model,the wave,ocean currents and wind loads on the pile foundation were equivalent to the bidirectional symmetrical cyclic load,and the soil mass model were selected as three layer soil,and the initial geostress balance was carried out.Therefore,the dynamic response characteristics of the offshore wind power monopile foundation in the heterogeneous soil under the action of horizontal cyclic loading were studied.Firstly,the change rules along the buried depths of horizontal displacement,shear force,bending moment along the pile shaft and soil resistance of pile inner and outer wall in different cycling processes were compared and studied.Meanwhile,the time history curves of different cycles at mud surface,soil interface and pile bottom were compared and analyzed.Secondly,the distribution characteristics of soil resistance of pile inner and outer wall along the circular direction of pile in different burial depths of the same cycle and different cycles at the same burial depth were contrastive studied.Then the numerical calculation results were compared with the existing research results to verify the rationality of the numerical calculation model and the reliability of the numerical calculation results.Finally,the effects of cyclic load ratio,load frequency,loading height,loading mode,vertical load,pile wall thickness and pile diameter on the dynamic response characteristics of offshore wind power monopile foundation in heterogeneous soil were analyzed in detail.The main results are as follows:(1)The rotation of the pile around a point on the pile axis shows the property of rigid pile.With the increase of cycle times,the time lag of maximum horizontal displacement,maximum shear force and maximum bending moment along the pile shaft is produced.The absolute values of the horizontal displacement,shear force and bending moment along the pile shaft at the mud surface gradually increase,and the increasing amplitude becomes smaller and smaller,and gradually tends to be stable.(2)The soil resistance of the pile outer wall increases significantly at the interface between silt soil and mealy sand soil due to the great difference in soil layer properties.The resistance of soil outside the pile wall not only depends on the relative displacement of pile and soil,but also depends on the buried depth and the nature of soil layer at the pile side.Soil resistance along the buried depth change of pile outer wall at different time points is mainly concentrated in a certain range from mud surface and pile tip,and with the increase of cycle times,the variation range of soil resistance with time increases.The variation curves of soil resistance along the buried depth of pile outer wall with time presents a cut-off point,which is located at about 2/3 of the buried depth.The variation law of soil resistance within the upper and lower boundaries of the cut-off point with time is just the opposite.(3)At the interface of silt soil and mealy sand soil,the soil resistance of pile inner wall increases first and then decreases alternately along the circular direction of pile.The soil resistance of the inner and outer walls of the pile body changes periodically along the circular direction of pile with time.When the depth is relatively shallow,the passive soil pressure on the pile outer wall changes greatly along the circular direction of pile with the number of cycles.As the depth increases,the active soil pressure changes greatly along the circular direction of pile with the number of cycles.With the increase of the number of cycles,the position of the main load on the inner wall of the pile turns from both sides to the middle.(4)Under different cyclic load ratios,the zero point of pile displacement moves down gradually along the buried depth.The change of cyclic load ratio has a significant effect on the passive soil pressure of the pile outer wall.With the increase of load frequency,the load borne by the inner wall of the pile body in the forward loading area increases,while the load borne by the inner wall of the pile body in the reverse loading area decreases.At different loading heights,the pile displacement intersected above the zero point,and the shear force anti-bending point moved up gradually along the buried depth.Under different loading methods,the passive soil pressure of pile outer wall varies greatly,while the active soil pressure varies little.(5)The vertical load has more influence on the horizontal displacement along the pile shaft under negative loading than that under positive loading.The maximum value of soil resistance of the pile shaft inner and outer wall increases significantly with the increase of vertical load.In order to reduce the horizontal displacement along the pile shaft,variable section steel pipe piles with "upper thickness,lower thinness" and "upper coarseness,lower fineness" can be designed in the range above the zero of displacement.At different pile wall thickness,the pile shaft shear force curves intersect at the buried depth of about 6D.Pile diameter and pile wall thickness have great influence on distribution characteristics of soil resistance of pile inner and outer wall along the buried depth and along the circular direction of pile.The change of pile diameter has a significant effect on the deformation of soil in passive zone of pile outer wall and the stress of surrounding piles.