Abstract:As a clean energy, offshore wind power has attracted increasing attention and investments. Offshore wind generators with pile foundations are mainly subjected to horizontal loading coursed by wind, waves, ice, ocean currents, and earthquakes. Such complex loading that acts upon the structure-foundation system leads to complex stress changes within the foundation. Moreover, extremely strict requirements for the deformation behaviors of offshore wind generator foundation structures and adjacent soils are controlled in their design and usage. However, knowledge of the actual stress changes and the deformation laws are very limited. This paper systematically investigates the stress-time histories of typical points in the foundations around a typical offshore wind generator and the amplitude distribution of the stress-changing induced by wind and waves through three-dimensional (3D) finite element method (FEM) numerical analysis. The effects of the angle between the directions of horizontal wind loading acting on the structure and the waves and frequency of the two loading types are studied. The results indicate that the amplitude and the principal stress direction of the wind generator foundations are both changing. The angle and the frequencies of the two loadings significantly affect the stress state, particularly the principal stress rotation. 3D cyclic rotation of the principal stress axes within a limited rotating angle is determined to occur always in the foundation soil adjacent to the foundation structure of an offshore wind generator. In particular, it is shown to become a two-dimensional (2D) cyclic rotation of principal stress axes when two directions of the wind load and the wave propagation remain the same. However, the stress paths become more complex when the frequencies of the two cyclic loads differ.
