Influence Of Soft Topsoil On Static And Dynamic Characteristics Of The Bucket Foundation For Offshore Wind Turbines

With the rapid development of offshore wind power,to further reduce cost and increase efficiency,and develop high-quality wind energy resources,the wind power industry presents a trend of large-scale development toward deeper waters.Compared with the shallow water,offshore wind turbine foundations in deep and ultradeep waters are faced with greater load and more complex geological conditions.Simultaneously,there is an overlying silt layer in the coastal areas of China,the thickness of which can reach 8～10 m.The bearing capacity of the bucket foundation is mainly carried by the top cover.Therefore,considering the characteristics of wide distribution and large thickness of soft topsoil in China’s coastal areas,this paper studies the bearing characteristics,dynamic response and differential settlement of bucket foundations in the soft topsoil,so as to provide some reference for the subsequent design and research work of the bucket foundation.The specific research contents are as follows:(1)Through finite element simulation,ultimate bearing capacity,failure mode and distribution of soil pressure in soft topsoil of varying thickness(0～10 m)and strength(5～40 k Pa)under vertical,horizonal and bending moment are investigated.The results show that when the thickness of the top layer is less than H/2(H is the bucket height),the ultimate bearing capacity of the bucket foundation linearly decreases with the increasing thickness of the soft topsoil.When the thickness of the top layer is greater than H/2,the reduction rate increases.Affected by the soft layer,the plastic collapse area of the soil decreases and the plastic failure in soft topsoil becomes more obvious.Under the vertical load,the top-bearing capacity decreases first and then increases with the increasing topsoil thickness,exactly opposite to that of the bucket side.Under horizontal load and bending moment,the decrease of passive soil pressure on the bucket side is the main factor that causes the reduction in bearing capacity.(2)The two-layered clay-bucket foundation-tower model is established to study the influence of structural parameters,sour depth,thickness and strength of the soft topsoil on the natural frequency of the whole machine,and to explore the vibration response of the whole structure under different geological conditions under random wind,wave and current loads.The results show that the natural frequency is not sensitive to the soft topsoil and scour.However,when the soft topsoil thickness grows,the downwind displacement of the structure under wind,wave and current loads and the foundation inclination increase markedly.(3)The bucket foundation in soft topsoil under the combined loading in practical engineering is simulated,and the influence of soft topsoil on differential settlement is quantitatively analyzed.Furthermore,the improvement effect of vertical load and soilreinforcement measures on differential settlement is discussed.The results show that when the thickness of soft topsoil is less than H/2,the differential settlement increases linearly with the increase of soft soil thickness.When the thickness is greater than H/2,the rate of differential settlement increases obviously.Appropriate vertical load is beneficial to differential settlement,but the vertical load should not exceed 0.05 times of vertical ultimate bearing capacity of foundation.The differential settlement of bucket foundation can be reduced by reinforcing the topsoil and the end soil under the bucket wall in soft foundation.