Study On The Long-term Cyclic Loading Response Of Suction Caissons For Offshore Wind Turbines In Sand Based On Physical Modelling Tests

The long-term cyclic loading response of suction caissons is a hot and difficult problem in the development of offshore wind power in sand,such as the prediction of accumulated rotation,the complexity of foundation-soil interaction and the effect of cyclic load on post-cyclic monotonic capacity.By means of physical modelling tests,this thesis has carried out suction caissons to lateral loading with one million cycles in sand.The accumulated rotation,the evolution of lateral displacement,the foundation stiffness,the soil damping and the post-cyclic monotonic capacity of the suction caissons are studied comprehensively from the theoretical analysis of the factors,such as the number of cycles,the order of the multi-amplitude cyclic load,the cyclic load parameters and the cyclic load direction.The prediction model of long-term accumulated rotation are proposed,the mechanism of pile-soil interaction under cyclic loads is revealed,and the calculation method of post-cyclic monotonic capacity is established.The main research work and achievements are as follows:(1)Based on the relationship between the shear modulus of sand and the stress level and Bolton equation,the scaling model construction method for sand is proposed,which provides a theoretical basis for the experiment from field scale to model scale.The uniform and repeatable saturated sand was obtained by sand rainer.The repeatability of the model tests were proved by the results of suction caissons' installation and monotonic loading tests.The simulation of the complex whole-life cyclic load on the suction caissons for offshore wind turbines is realized.The reliability of the model test construction method and the accuracy of simulating the complex whole-life cyclic load are verified.(2)The effect of the ordering of cyclic loading on the accumulated rotation of suction caissons is studied.The accumulated rotation of suction caissons in descending cyclic loading tests is two times of the result in ascending cyclic loading tests,which has revealed the mechanism about the lower-amplitude cyclic loading has caused the sand densification around the suction caissons.Based on the constant amplitude cyclic loading tests' results,the Miner's rule is applied to predict the accumulated rotation of suction caissons under multi-amplitude cyclic loading.It is found that the method can predict the accumulated rotation under descending loading.(3)The response of suction caissons under one-way and two-way cyclic loading is studied.The evolution of accumulated rotation,foundation stiffness and soil damping of suction caissons under complex cyclic load parameters are revealed.It is found that the accumulated rotation of suction caissons under two-way cyclic load enters the cyclic plastic stage earlier than that under one-way cyclic load,and the foundation stiffness under two-way cyclic load is greater than that under one-way cyclic load.The soil damping and foundation stiffness change in the opposite direction with the increase of the number of cycles.The prediction model of long-term cyclic accumulated rotation under complex cyclic load parameters is established,and the prediction curve of the accumulated rotation of suction caissons under complex cyclic load parameters is calculated.(4)The response of suction caissons under multi-directional cyclic load is studied.It is found that the accumulated rotation of suction caissons under multi-directional cyclic loading influenced by the loading direction and loading history,the results is between the results of one-way and two-way cyclic loading tests,which reveals the initial direction of cyclic loading history affecting on subsequent response by cyclic loading.It is found that when the direction of cyclic load changes greatly,the foundation stiffness and soil damping will change abruptly,and the change amplitude gradually decreases with the increase of the number of repeated loads.It is indicated that the foundation stiffness and soil damping change greatly with the change of load direction.(5)The effect of cyclic load on post-cyclic monotonic capacity of suction caissons is studied.It is found that the post-cyclic monotonic capacity of suction caissons increases linearly and exponentially with the increases of load amplitude and cycle numbers respectively,and decreseas with the increases of load symmetry.However,the post-cyclic monotonic capacity has no obvious quantitative relationship with the cyclic loading order and the cyclic loading direction.The mechanism of the increase of post-cyclic monotonic capacity of suction caissons was revealed.Based on the main factors of cyclic load amplitude,cyclic load symmetry and cyclic numbers,a method for calculating the normalized post-cyclic monotonic capacity of suction caissons under cyclic loading is established and verified.