Study On The Influence Of Scour On The Mechanical Performance Of Offshore Wind Turbine Monopile Foundation

With the development of economy,energy shortage has become a big problem in the world.At present,the foundation form of offshore wind power mostly adopts monopile foundation,which seriously affects the dynamic characteristics and mechanical performance of the structure and even causes the capsizing of the structure due to the scouring effect of seawater.Therefore,it's necessary to evaluate the effect of scour on offshore wind power monopile foundation.In this paper,the influence of scour depth,sand density and pile embedding depth on displacement,horizontal limit bearing capacity,pile stress and the first natural frequency of monopile foundation is studied through small scale model test under 1g condition.Then,ABAQUS software is used to establish a three-dimensional finite element model consistent with the experimental model,and the reliability of the numerical model is verified through comparative analysis with the model test results.Taking a 5MW fan supported by a monopile foundation as an example,the influence of scour on the deformation and dynamic characteristics of a monopile foundation is studied.By using the same amount of steel and taking the character of monopile foundation in unscoured state as the target value,the design parameters of pile body are optimized.Finally,the influence of scour protection device on horizontal displacement,corner Angle and stress of monopile foundation is evaluated.The following conclusions are obtained:(1)The test shows that when the maximum equilibrium scour depth is reached,the scour can reduce the first natural frequency of single pile foundation by 12.5%;The angle of single pile foundation at mud surface can be increased at most 93%.Reduce the ultimate bearing capacity of single pile foundation by 33%.The scour protection device reduce the corner of single pile foundation at mud surface by up to 60%;The ultimate bearing capacity of single pile foundation is increased by 10.8%.The stress of the pile at three measuring points decreased by an average of 12.78%.(2)In the engineering case of numerical simulation,scour can reduce the first natural frequency of monopile foundation by 8.6%;Increase the Angle of monopile foundation at mud surface by 10.3%;The pile stress at the maximum scour depth increased by 8.4%.The scour protection device increases the first natural frequency of monopile foundation by 0.75%.The angle of monopile foundation at mud surface is reduced by 0.2%.The stress of the pile under the mud surface decreased by 3.7%.(3)By controlling the same amount of steel used in a monopile foundation,the effect of three measures of increasing pile length,wall thickness and pile diameter on offsetting the impact of scour is obtained.The results show that the measures of increasing pile diameter can reach the target value when there is no scour from the aspects of pile displacement,corner,stress and natural frequency.