Analysis On Horizonal Displacement Of Monopile Foundations Under Long-term Cyclic Later Loading

A monopile foundation of an offshore wind turbine will be subjected to long-term horizontal cyclic loading, such as wind and wave loads over the lifetime of the structure. The extensive low-frequency cyclic loading could lead to permanent lateral deformation of the monopile foundation. The wind driven generator will probably operate unnormally when a permanent rotation of the monopile foundation exceed0.5°at mudline. So, it is important to study accumulated lateral displacement of a monopile foundation for an offshore wind turbine subjected to wind and wave loads.At present, the standard design procedure for a laterally loaded pile is the p-y curve method, which is described in the guidelines of the American Petroleum Institute(API). The proposed p-y curves for cyclic loading conditions are based on field tests with small diameter pile and limited cycles.More and more studies show that the p-y curve method substantially will undervalue the accumulated displacement of monopile foundation. There is not a simple and practical way to analyze the deformation of the large-diameter monopile under long-term cyclic lateral loading. The main goal of this paper is to evaluate accumulated lateral displacement of a monopile foundation for an offshore wind turbine with a suitable method. The main work is listed as follows:(1) The deformation behavior of cohesionless soil was studied under long-term cyclic lateral loading and many kinds of cumulative plastic deformation model had also been analyzed. A degradation stiffness model which is based on the analysis of the dynamic triaxial test was used to evaluate long-term dynamic behavior of cohesionless soil. And then, the existing degradation stiffness model was improved.(2) The results of dynamic triaxial test were studied to get regression parameters of degradation soil stiffness model. Degradation soil stiffness model was realized by the secondary development of the finite element software ABAQUS. And the numerical calculation results were compared with laboratory test results. Finally, the parameters of calculation model were also analyzed.(3) Three dimensional finite element model of monopile foundation was presented and degradation soil stiffness model was embedded in the3-D model. The development of lateral displacement of monopile foundation under long-term horizontal cyclic loading was studied. The influence of diameter, embedded depth, number of cycles, the frequency and size of cyclic load on horizontal displacement of monopile were mainly considered. The availability of degradation soil stiffness model was also proved by comparing to laboratory model test results.