Study On Bearing Mechanism Of Suction Caissons For Offshore Wind Turbines

Suction caisson for offshore wind turbines is a bucket foundation inverted into the seabed and has very broad application prospects due to its advantages of low cost,convenient installation and recycling.The suction caisson that this paper focuses on is a bucket foundation with small aspect ratio,which is installed in seabed clay and can resist large moment load,so it is different from the suction anchor.Meantime,the suction caisson can be subjected to cyclic loads,which leads the clay surrounding the bucket to soften and even remold.Combined with the practical load conditions and the characteristics of clay,this paper makes a deep discussion on the bearing mechanism of suction caisson using the finite element method.Firstly,a numerical analysis of the bearing mechanism of suction caisson under monotonic lateral load is made.In this paper,by redeveloping the finite element software ABAQUS,the relation of the ultimate friction between the bucket and the clay and the undrained shear strength is built by using the user subroutine FRIC_COEF,and the finite element model is established reasonably and is validated with the experiment data.The failure mechanism of the foundation,earth pressure and friction distribution inside and outside the bucket and failure envelope are analyzed by changing the aspect ratio,load eccentricity,soil strength gradient and friction coefficients of the inside and outside bucket surfaces.Numerical results indicate that there are obvious spoon failure areas forming inside and under the bucket in homogeneous clay,while there is no obvious spoon failure area forming under the bucket in inhomogeneous clay.The failure mechanism is hardly affected by the load eccentricity and soil heterogeneity for the caisson with smaller aspect ratio,and soil failure mainly occurs at the top of the interior.With the load eccentricity increasing,the earth pressure acting on the front side wall outside the bucket and back side wall inside the bucket decreases,the earth pressure acting on the back side wall outside the bucket and front side wall inside the bucket rises,and the suction on the underside of the bucket lid decreases.The effects of friction coefficients on the horizontal bearing capacity increases with the load eccentricity increasing,soil strength gradient growing or aspect ratio decreasing.Then,a numerical analysis of the bearing mechanism of suction caisson under cyclic lateral load and dead weight is carried out.By redeveloping the finite element software ABAQUS,the strain-softening and strain rate-dependent model is built.Using two loading methods of displacement control and lateral force control,the cyclic bearing capacity of suction caisson,failure mechanism of the foundation,rotation center and settlement of the bucket are analyzed by changing the aspect ratio,load eccentricity and vertical loads.Numerical results indicate that when the deadweight of suction caisson rises,its horizontal bearing capacity decreases while moment bearing capacity almost keep unchanged.During the first forward and backward loading,the peak horizontal force,which is affected by the dead weight,occurs before the rotation angle increases to the maximum.Therefore,the dead weight must be considered for evaluating the horizontal bearing capacity.The rotation center of the soil inside the bucket can deviate from the axis line of the bucket for the bucket with bigger aspect ratio,and the deviation enlarges as the deadweight rises.Besides,as the cycle number increases,the rotation center of the soil inside the bucket moves upwards.The soil in the reversed spoon failure area inside the bucket degrades more quickly than the soil in the wedge failure area on both sides of the bucket.The rotation center of the bucket has a butterfly trajectory,and has greater vertical displacement compared with its very small horizontal displacement.The trajectory of rotation center of the bucket moves upwards with the load eccentricity and aspect ratio increases.The trajectory of rotation center of the bucket also moves upwards with the increasing cyclic number under the larger displacement amplitude while it is hardly affected by the cyclic number under the smaller displacement amplitude.During the unloading process,the bucket settles as the rotation angle decreases,while the uplift or settlement of the bucket is affected by the aspect ratio and displacement amplitude during the loading process.The effect of strain rate of soil strength enhances the bearing capacity of the suction caisson.Also,the effect of strain rate affects the extent of the degradation of horizontal and moment bearing capacity caused by the strain soften effect,and the effect relates to the aspect ratio.The rotation angle amplitude of suction caisson increases with the cyclic number rises.When the foundation failure is going to occur,the rotation angle may increase sharply and the settlement of the suction caisson may quicken as the cyclic number rises.