A Two-yield Surface Progressive Hardening Elastoplastic Model And Accumulative Deformation Law For Offshore Wind Turbine Pile Foundation

In complex marine environment,offshore wind turbine pile foundation is always subjected to strong cyclic loading from wind,wave and current loads.These lateral cyclic loads can easily lead to accumulative deformation of offshore wind turbine pile foundation,and affects its fatigue life and even leads to overall instability.In addition,deep soft clay is widely distributed in the coastal ocean of eastern China.The soil surrounding piles under lateral cyclic loading is prone to cause accumulation of excess pore water pressure and strength degradation,thus affecting the soil-pile interaction.In this study,accumulative deformation law of offshore wind turbine pile foundation in soft clay under is analyzed in detail by combining theory,experiment and numerical methods.Based on this,a two-yield surface progressive hardening elastoplastic model for clay and sand,namely UACC2,is proposed,and applied to the numerical analysis for single pile under lateral cyclic loading.Moreover,on the basis of the test and numerical results,a prediction model is established to predict the accumulative deformation of pile with soil-pile relative stiffness,cyclic amplitude,cyclic number and loading height.Then,a control method for the accumulative deformation of offshore wind turbine pile foundation is proposed.The main contents and research results of this thesis are as follows:(1)Based on the Modified Cam-clay model,a progressive hardening mechanism of inter yield surface is employed to realize the smooth transformation of the plastic modulus from the loading yield surface to the outer yield surface.A unified state parameter and a new yield surface function are established for the proposed progressive hardening elastoplastic model.In order to realize unification of clay and sand,the normal compression curve(NCL)and the limit compression curve(LCC)in e-ln p'plane is used as the reference stress level respectively to unify relative distance between the current stress point and the reference stress point for clay and sand.A shape parameter of yield surface related to relative density of sand is introduced to consider various initial relative density and simulation undrained liquefaction characteristics of sand.If this shape parameter is not considered,the yield surface function degenerates to that for clay.For cyclic behavior of soil,the nonlinear smallstrain stiffness and a special accumulative control parameter for the hardening law of loading yield surface are introduced to realize the closed hysteretic behavior,cyclic shakedown and degradation of natural clay under cyclic loading.For natural clay,the elastic anisotropy and structure are added for influence of deposition,stress history and structure degradation during triaxial shearing.(2)The main feature of the proposed UACC2 model is that its constitutive equations can be simply formulated based on the consistency condition for the inner yield surface,instead of the mapping rule and interpolation theorem of classical elastoplastic models.Then,it can be efficiently implemented using a stress integration scheme similar to that of the Modified Cam-clay model,and a cutting plane implicit integration algorithm is employed to write code for numerical software.(3)Considering different conditions for pile and clay,the accumulative deformation law of single pile subjected to lateral cyclic loading in clay is analyzed in detail using FLAC3 D with the proposed UACC2 model.It is found that the maximum soil resistance around piles appears at the depth of 2D-2.5D below the ground surface.And the cyclic degradation of the soil resistance around piles is mainly concentrated within the depth of 5D.Below this depth,the soil resistance around will not weaken.Then,a power function is introduced to calculate the accumulative deformation of pile head,and a prediction model,related to ultimate control load,curvature(initial stiffness),undrained shear strength,pile diameter,pile length and design load(cyclic amplitude)is established.Subsequently,the accumulative deformation of offshore wind turbine pile foundation can be effectively predicted by the load-displacement curve obtained by simple static load test or static numerical simulation.(4)A set of centrifuge model tests was conducted to discuss the lateral response of a tetrapod jacket foundation embedded in Kaolin.The test results mainly reveal that the loading history has a great influence on the lateral response of the jacket foundation.Under the low stress level,the bending moment and deformation of the jacket foundation after the soil disturbance is significantly greater than that under independent monotonic loading.However,in continuous cyclic loading,the lateral accumulative deformation rate of jacket top during post cyclic process is more stable than that during primary cyclic process.It is mainly due to the complex coupling of pore water pressure dissipation and clay reconsolidation under multiple cycles,resulting in the progressive hardening of clay surrounding piles.In addition,the lateral displacement and bending moment of piles increase linearly with cycles in doublelogarithmic coordinate,while the vertical displacement increases linearly with cycles in Cartesian coordinate.The loading sequence is the main Factor affecting the accumulation rate of the lateral displacement,while the accumulation rate of vertical displacement and the shear force of pile top depend on the position of piles.Moreover,the cyclic p-y curve shows typical hysteresis and degradation characteristics.The cyclic degradation of soil reaction surrounding piles is mainly concentrated in 5D depth,and the cyclic degradation of shallow clay is more striking.(5)The static p-y method suggested by API underestimates the initial stiffness and lateral ultimate bearing capacity of offshore wind turbine pile foundation.By comparison,the hyperbolic tangent p-y method is more suitable for the calculation of soil-pile interaction for offshore wind turbine pile foundation.For pile group effect of jacket foundation in soft clay,p-multiplier of the leading-row pile and the trailing-row pile can be taken as 0.93 and 0.76,respectively.The values in soft clay is obviously lower than that in sand.Moreover,an attenuation model of soil reaction surrounding piles under cyclic loading is established by reducing static p-y curve,and the range of cyclic degradation factor is suggested.In addition,it is found that the accumulative deformation law of jacket foundation is similar to that of single pile.Compared with the test results of jacket foundation,it further verifies the effectiveness of the prediction method by using static load-displacement curve for jacket foundation.