Reliability Design Of Offshore Wind Turbine Monopile Foundation Based On Random Field Theory

With the exploration of renewable energy,people pay more attention to offshore wind power.Offshore wind turbine foundation is installed on the seabed with complex geological structure and bears random loads such as wind,wave and current,resulting in high cost and high risk.Therefore,it is necessary to analyze its reliability.The spatial variability of soil parameters has been identified as the main source of geotechnical engineering uncertainty.Among the types of offshore wind turbine foundation,large diameter monopile foundation is more and more widly applied.However,at present,there are few studies on the influence of the spatial variability of soil parameters on the bearing capacity of large diameter monopile foundation and the geotechnical investigation scheme of offshore wind turbine foundation.Therefore,this paper studies the above problems based on random field theory.The main research contents and conclusions are summarized as follows:(1)Based on the random field theory,the influence of the spatial variability of undrained shear strength in three-dimensional soil on the uniaxial ultimate bearing capacity of large diameter monopile foundation is studied by using random finite element method and Kriging surrogate model.The influence of the spatial variability of soil parameters on the ultimate bearing capacity of large diameter monopile foundation under combined load is studied by using probabilistic failure envelope.The results show that considering the spatial variability of soil parameters will reduce the mean value of the uniaxial ultimate bearing capacity of monopile foundation and change the size and shape of the failure envelope.It is safe to use H-M probabilistic failure envelope to design the foundation.The smaller the scale of fluctuation of undrained shear strength is,the stronger the spatial average effect is.The spatial average effect has little relationship with the variation coefficient of undrained shear strength.In this paper,it is suggested to adopt the 15% quantile of undrained shear strength for design of large diameter monopile foundation.(2)Taking the sea area of Weizhou Island as an example,a complete reliability design of the monopile foundation of NREL 5MW offshore wind turbine is carried out.The results show that the extreme load of the monopile foundation is dominated by wave load,and the resultant acting point of marine environmental load effect is the near sea level.The 15%quantile of undrained shear strength is used to design the pile length of large diameter monopile foundation,which meets the requirements of reliability design.(3)The difference of random response of uniaxial ultimate bearing capacity of large diameter monopile foundation in conditional random field and unconditional random field is compared,and the influence of longitudinal spacing,depth and horizontal distance of survey samples on the random response of uniaxial ultimate bearing capacity of the foundation is studied by using conditional random field.The results show that the mean value of uniaxial ultimate bearing capacity of the foundation calculated by conditional random field is closer to the ultimate bearing capacity of real soil than that calculated by unconditional random field.The variation coefficient of uniaxial ultimate bearing capacity of the monopile foundation calculated by conditional random field is significantly lower than that of unconditional random field.The appropriate geotechnical investigation of offshore wind turbine monopile foundation should be located at the center of the pile,after the horizontal distance between the survey point and the pile center exceeds 0.75 times the horizontal fluctuation range,the survey point data will not contribute to improving the prediction accuracy of ultimate bearing capacity of foundation,the longitudinal investigation interval should be less than 1 times the vertical scale of fluctuation,and the investigation depth should be 1 times the pile diameter D below the pile end.