Frequency change and accumulated inclination of offshore wind turbine jacket structure with piles in sand under cyclic loadings

Jacket structures are generally recommended as the foundations for offshore wind turbines (OWTs) with water depth more than 30m. In this paper, the load sharing characteristic for typical jacket foundation OWT is firstly analyzed. It is found that the vertical soil resistance along the pile shaft (namely the t-z springs) has the most significant contribution for the anti-overturning moment of OWT. Considering the effect of cyclic loadings, a novel numerical calculation method is proposed based on a cyclic t-z spring model to study the frequency change and accumulated inclination for OWT structure in storm situations. The novel calculation method for the evaluations of accumulated inclination and load sharing characteristic are verified by one centrifuge test. Furthermore, a series of numerical studies using the novel calculation method are performed. The results show that the accumulated inclination should be the controlling factor rather than the natural frequency in the design of jacket foundation OWT considering cyclic loading effects. For the load sharing characteristic under cyclic loadings, the contribution of anti-overturning moment by the t-z springs decreases obviously. This is mainly due to the continuous degradation of t-z springs, companied by gradual mobilization of the Q-z spring at the pile tip.