Analysis On Horizontal Cumulative Displacement Of Monopile Foundations In Saturated Soft Soils

With the rapid economic development and the continuous improvement of living standards, human demand for energy has widened. Wind power currently offers a very competitive source of renewable energy, and therefore the market for onshore and offshore wind farms is projected to expand rapidly within the next decade. It is characteristic for offshore wind turbines that the substructure will be subjected to strong cyclic loading, originating from the wind and wave loads. In recent projects, usually a maximum permanent rotation of a monopile at mudline of 0.5° was required. However, the estimation of accumulated displacements is one of the most difficult tasks for engineering design.Although many methods have been proposed for estimating lateral deformation of a pile in sand under cyclic lateral load, no generally accepted design procedure yet exists. The standard design procedure for a laterally loaded offshore pile is the p-y curve method, which is described in the guidelines of the American Petroleum Institute (API) and explicit method. However, these methods are based on small-scale laboratory pile test results or field tests with a limited number of cycles, and suitability for a monopile with a large diameter under cyclic lateral loading is not clear. A FEM method is provided in this paper to analyze the behavior of monopole embedded in saturated soft clay. In this paper, the main work is as follows:(1) The dynamic characteristics of saturated soft clay and the influence of pore water pressure on the accumulation of soil deformation characteristics is studied. The stiffness degradation model for soft clay considering the influence pore water pressure is proposed. 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.(2)The stress of the soil mass is partitioned with the threshold cyclic stress ratio as the threshold value. The soil partition which the stress is lower than the threshold cyclic stress ratio is analyzed by stiffness degradation model considering pore water pressure, and vice versa.(3) Three dimensional finite element model of monopile foundation was presented and degradation soil stiffness model was embedded in the 3-D FEM model. The development of lateral displacement of monopile foundation under long-term lateral cyclic loading was studied. The effect of diameter, embedded depth, number of cycles, the frequency and size of cyclic load on lateral displacement of monopile were mainly considered.