Study On Dynamic Characteristics Of Large Diameter Single Pile Foundation Of Offshore Wind Turbine

Offshore wind power pile foundations are mainly large-diameter single piles.Offshore wind power should withstand long-term horizontal cyclic loads such as wind and waves during service.The properties of soil around the foundation under cyclic loading will change,and the changes in soil properties will be changed.The influence of the change of the foundation stiffness on the dynamic characteristics of the upper support structure causes the cumulative lateral deformation of the fan,causing the upper structure to tilt,which seriously affects its normal use function.Therefore,it is of great theoretical significance and guiding significance to carry out research on the overall dynamic characteristics of the offshore wind turbine pile foundation-foundation-superstructure.In this paper,the large-scale indoor model test is used to study the stress and deformation characteristics of the large-diameter(open and closed)single pile foundation of the offshore wind turbine under horizontal static force and cyclic load.The wind turbine structure is subjected to cyclic loading dynamic characteristics.The dynamic triaxial test was carried out to study the variation law of cumulative displacement of sand.The main work and research results are as follows:(1)The large-diameter single pile sinking pile and horizontal static loading test in dry sand foundation were carried out to study the influence law of soil plug and surrounding soil during the pile-opening process of open and closed piles and the internal force variation characteristics of pile body under static loading.The results show that the soil plug in the open pile is slowed down with the pile sinking,and the height of the soil plug accounts for 76.6% of the pile height.The influence of the open pile on the surrounding soil during the pile sinking process is smaller than that of the closed pile.The influence range of the two on the surrounding soil of the pile is about7-8 times the pile diameter.Under the action of horizontal static force,the bending moment of the pile body,the resistance of the surrounding soil of the pile and the displacement of the pile body increase with the increase of the loading force,and the maximum position of the bending moment has a downward trend;the sand recommended by the API specification is found.The initial stiffness of the single pile py curve is obviously too large.It is recommended to use the hyperbolic p-y curve toanalyze the horizontal deformation of the pile foundation.(2)The horizontal cyclic loading model test of large-diameter single-pile foundation of offshore wind turbines in dry sand foundation was carried out.The effects of different loading frequency,cyclic load ratio and different pile end modes on the large-diameter single-pile foundation and structural horizontal dynamic characteristics of offshore wind turbines were considered.In order to deeply analyze the cyclic loading characteristics of large-diameter single pile foundations and structures in dry sand foundation.The results show that during the cyclic loading process,the cumulative displacement extreme value of the large-diameter single pile foundation,the maximum bending moment approximation of the pile body and the logarithmic linear relationship of the cycle number,the bearing characteristics of the large-diameter single pile are reduced to some extent by the cyclic loading;The natural vibration frequency of the fan support structure shows an upward trend under cyclic loading.The damping ratio can produce 50% decrease under cyclic loading.The variation law is roughly opposite to the natural vibration frequency.Under the lateral cyclic load,the offshore fan is self-oscillated.The phenomenon of frequency decline should be fully considered in the design of offshore wind turbines.(3)Carry out a series of dynamic triaxial tests on the drainage conditions of saturated sand,and study the effects of confining pressure,dynamic stress ratio and deviatoric stress on the cyclic cumulative deformation characteristics of the model test sand.The results show that the concomitant pressure,dynamic stress ratio and deviatoric stress are larger,and the cumulative deformation of sand is larger.The larger the confining pressure and static deviator stress,the larger the dynamic elastic modulus,and the dynamic elastic modulus with dynamic strain.Increase and decrease,stiffness softening occurs.