| In recent years, with the negative effects of fossil energy towords environment increasing, wind energy gets lots of attention and expectation as one of the most promising renewable energy. Due to abundant wind resource in coastal area, offshore wind energy has huge potential than inland wind energy. Monopile is one of the most widely used foundation types in offshore wind turbine. Therefore, lots of scholars have carried safety study on monopile in ocean environment during its service life. The safety of the foundation is controlled by lateral deformation under seismic load, wind load and wave load during total life cycle. Besides, the system should avoid resonance caused by blade passing, and excessive lateral deformation.This paper mainly focuses on dynamic characteristics of monopile under seismic load and long-term cyclic load. Based on the site condition of Shanghai Donghai-bridge wind farm, three seismic waves were selected and inputed in a software, NERA, to analysis the site response. Based on the method proposed by El Naggar, this paper presents an analysis method using a beam on nonlinear Winkler foundation (BNWF) model. This model was incorporated with a finite element program, ABAQUS, and calculated the lateral deflection of monopile under seismic load.Based on available test data, empirical formula for stiffness degradation relationship of costal clay in China was presented. Then, a new model of stiffness-damping degradation of clay was proposed based on the formula. By using the new model, charactersitics of natural frequency and lateral deflection of monopile under long-term cyclic load were studied. After that, the calculated results were comparied with different control criterion. The presented method requires less computational efforts than continuum modeling, which also considers the dynamic characteristics of foundation better than quasi-static method. Therefore, it will provide a useful guide for future offshore engineering application in China. |
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