Bearing Characteristics Of Suction Caisson Foundation Under Oblique Cyclic Load

With the construction of offshore wind farms gradually developing into deep water,various floating wind turbines came into being,such as tension-leg type,semi-submersible type and so on.The key to the safe and stable operation of floating wind turbine lies in the anchor foundation on the seabed,which directly determines the bearing capacity of the anchor system and the dynamic response of the superstructure.When the suction caisson foundation is applied to different types of floating wind turbines as an anchor foundation,it will be subjected to oblique loads with different inclination angles from anchor cables,including static loads and cyclic loads caused by wind and waves.The cyclic softening effect will reduce the bearing capacity of the foundation and affect the safety of the superstructure.Therefore,it is of great significance to study the bearing characteristics of suction caisson foundation under oblique static loads and cyclic loads.The main research contents and achievements are as follows:(1)Through static load model test and finite element simulation,the failure modes of suction caisson foundation in soft clay under oblique static loads with different loading positions and different inclination angles were revealed.It is found that there are mainly four types: the overall overturning failure mode,the combined failure mode of rotation and translation,the translation failure mode at the optimal mooring point and the overall failure mode of uplift.The test load-displacement curves of foundation under oblique static loads were also analyzed.The results show that for the suction caisson foundation with length-diameter ratio of 3,when the included angle between the load direction and the horizontal direction is less than 30 °,the buried depth of the loading position has a greater impact on the ultimate bearing capacity of the foundation,and when the included angle is greater than 45 °,the buried depth of the loading position has a smaller impact on the ultimate bearing capacity of the foundation.(2)Through the cyclic load model test in soft clay,the effects of average load ratio,cyclic load ratio and number of cycles on the cyclic bearing characteristics of suction caisson foundation were studied.It is found that under oblique loads at different angles,with the increase of average load ratio,cyclic load ratio and number of cycles,the ultimate bearing capacity of the foundation after cycling will decrease.And when the number of cycles is constant,the cumulative displacement of foundation will increase with the increase of average load ratio and cyclic load ratio.(3)Based on ABAQUS software,a three-dimensional numerical model of suction caisson foundation under oblique load was established,and its rationality was verified by comparing with the model test results.Then,by changing parameters,the factors affecting the ultimate bearing capacity of the foundation were explored.It is found that under oblique loads at different angles,the ultimate bearing capacity of the foundation will increase with the increase of the friction coefficient between foundation and soil,the length-diameter ratio of the foundation and the undrained shear strength of the soil,while the influence of the loading position on the ultimate bearing capacity of the foundation is related to the load inclination angle.In addition,for the suction caisson foundation with length-diameter ratio between 3 and6,its optimal mooring point was determined through numerical simulation.The results show that it is about 0.65 times the buried depth of the foundation.(4)Load control mode,swipe displacement loading mode and fixed displacement ratio loading mode were used to explore the envelopes of normalized static ultimate bearing capacity of suction caisson foundations with different length-diameter ratio when loaded at the optimal mooring points.It is found that the envelopes are approximately a quarter ellipse.Furthermore,according to the results of cyclic load model test,the weakening coefficient of cyclic bearing capacity δ was obtained,and the calculation method of oblique cyclic ultimate bearing capacity of suction caisson foundation considering the effects of average load ratio,cyclic load ratio and number of cycles was proposed.