With the water depth exceeding 30m, jacket foundation is the suitable supporting structure for offshore wind turbine (OWT) compared to monopile. It has been well known that the OWT has strict requirements for natural frequency to avoid resonance. In this paper, a series of monotonic and cyclic interface shear tests for 500 cycles under const normal stiffness (CNS) condition are firstly performed. It found that the shear displacement corresponding to the mobilization of peak interface strength has little difference for different depths. For the cyclic interface shear tests, the rate of interface strength reduction is greater for deeper depth, larger pile diameter and larger displacement amplitude. Then, based on the test results, a general interface strength degradation model and a simple interface stiffness degradation model are both established considering the effects of depth, pile diameter and shear amplitude under cyclic loadings. After that, a simple method is proposed to evaluate the natural frequency of jacket supported OWT under storm. The detailed calculation procedure and an example of pile length optimization for a typical jacket supported OWT are presented. The proposed simple method can offer an effective tool to evaluate the natural frequency of jacket supported OWT under storm.
Wenjie Zhou, Zhen Guo, Lizhong Wang, Shengjie Rui, Jiahao Li, Yujie Li and Chencong Liao
Keywords: jacket supported OWT, cyclic interface shear tests, cyclic degradation model, natural frequency, storm
Publisher: DFI