The design of an offshore wind turbine (OWT) founded on a monopile foundation is principally based on dimensioning criteria related to its fundamental frequencies. These frequencies must remain outside the excitation frequencies to avoid resonance. For the calculation of the OWT natural frequencies, several studies exist, but few of them simultaneously consider both the real geometrical configuration of the OWT superstructure (tower, blades, and transition piece) and the three-dimensional (3D) soil domain and its interaction with the monopile foundation. This paper aims at filling this gap. A rigorous 3D finite element method-based model of a 10 MW DTU OWT installed in sand is developed. The aim is to perform a structural modal analysis of the wind turbine in parked condition. The obtained natural frequencies are compared with those corresponding to other simplified models available in literature for the foundation and the superstructure in the scope of giving an insight about how poorly the existing simplified models can predict the OWT natural frequencies. Finally, a parametric analysis is performed to study the effect of the water depth, the monopile dimensions (diameter, thickness, and embedded depth), the transition piece height, and the sandy soil relative density on the system natural frequencies.

中文翻译：

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用于估算沙中海上风力涡轮机固有频率的全三维模型

建立在单桩基础上的海上风力涡轮机 (OWT) 的设计主要基于与其基本频率相关的尺寸标准。这些频率必须保持在激励频率之外以避免共振。对于 OWT 固有频率的计算，存在多项研究，但很少同时考虑 OWT 上部结构（塔架、叶片和过渡件）的真实几何配置和三维 (3D) 土壤域及其相互作用与单桩基础。本文旨在填补这一空白。开发了安装在沙子中的 10 MW DTU OWT 的严格的基于 3D 有限元方法的模型。目的是对停机状态下的风力涡轮机进行结构模态分析。将获得的固有频率与文献中可用于基础和上部结构的其他简化模型进行比较，以了解现有简化模型对 OWT 固有频率的预测能力有多差。最后，通过参数分析研究了水深、单桩尺寸（直径、厚度和嵌入深度）、过渡段高度和沙土相对密度对系统固有频率的影响。