Estimation of accumulated lateral displacement and settlement are critical in design of wind turbine foundation. However, there have been few studies exploring the response of bucket foundation to long-term cyclic loading. We perform a series of three-dimensional finite element analyses of bucket foundations installed in sands and clays. An empirical formulation which captures the stiffness degradation observed in cyclic triaxial and simple shear tests is implemented into the finite element analysis in the form of a user subroutine. It is shown that the cyclically accumulated rotation of the bucket foundation increases with the number of cycles and cyclic amplitude. In particular, the magnitude of the moment and the position of the horizontal load are shown to significantly influence the cyclic response. Extensive numerical simulations are required to develop design charts for predicting the accumulated deformation of bucket foundation under long-term cyclic loading.
Offshore wind energy has been widely developed as an alternative source of energy in the last few decades. Various types of bottom-fixed offshore wind turbines have been designed and constructed. The monopile foundation is the most common type, although bucket foundations have also been used (Zhu et al., 2013). The bucket foundation, which is typically installed using pressure difference between inside and outside of structure, has the advantage of simple and economic installation. A large body of literature on bearing capacity of bucket foundation has been published. Model tests, field tests, and numerical simulations have been performed to determine the bearing capacities of bucket foundations under vertical, horizontal, and moment loadings (Bransby and Randolph, 1998; Yun and Bransby, 2007; Hung and Kim, 2012; Hung and Kim, 2014).
If the bucket foundation is used as a wind turbine foundation, fatigue design is a very important issue. The effect of wind and wave cyclic loading to changes of soil properties has to be considered. However, the long-term cyclic response and the estimation of accurate accumulated rotation are the most important issues for design.
