风能被认为是一种十分有利用价值的、可持续的清洁能源。近三十年来，全球海上风电发展迅速，已建成不少固定式基础的海上风电场，但开发成本仍然十分昂贵。当前，大容量风机的研发与应用研究以及漂浮式基础的深水风电场是研究的热点，如何权衡结构的安全稳定性与风电场开发成本的关系是工程界和学术界共同关注的核心问题，准确估计风机支撑结构的水动力载荷是解决这一核心问题的首要条件之一。因此，开展海上风机支撑结构水动力载荷特性研究对发展海上风电场，推动海上风能开发利用具有重要的意义。本文主要针对海上风机固定式支撑结构，利用数值模拟方法，研究复杂海洋环境下典型支撑结构所受波浪力的描述理论与方法，为大容量风机的应用提供理论依据。

    首先，利用OpenFOAM开源CFD开发平台，基于粘性流理论的Navier-Stokes方程，考虑自由表面效应，采用VOF方法捕捉自由表面，并利用k-ω SST模型处理结构近壁面的流动，建立了全非线性数值波浪水池，可准确有效地模拟强非线性波、破碎波等及其与复杂结构的相互作用过程，从而可获得结构的水动力载荷。

    运用建立的数值波浪水池，研究了最具代表性的直立圆柱式单桩结构的波浪力。通过对海洋工程结构波浪力现有计算方法的分析，不难发现现有大、小尺度结构物的界限并不十分统一。而大容量风机的大尺寸支撑结构的尺度可能恰好介于传统的小尺度与大尺度之间，传统理论对其非线性波浪载荷计算的适用性值得探讨。因此，我们提出了中等尺度工程结构的概念，利用数值波浪水池分别求解带自由面的Navier-Stokes方程和Euler方程，实现了波浪粘性力的准确分离，通过数值实验详细分析线性波条件下波浪粘性力、惯性力、绕射效应的显著作用区间，得到了中等尺度工程结构的明确尺度界限，并给出了相应的波浪力计算方法和载荷系数取值。进一步详细研究了线性波和非线性波条件下的光滑直立圆柱结构波浪力随结构尺度、波陡、水深等参数变化的规律和内在机理，表明波浪非线性效应对高阶惯性力和绕射效应均有显著影响，并且对高阶惯性力的影响大于绕射效应。

    针对我国海上风电场广泛采用的高桩承台支撑结构，研究了这种复杂结构的波浪力特征。首先采用线性波浪理论和绕射理论，对承台和下部桩柱波浪力进行了探讨。然后进一步考虑波浪非线性效应、复杂自由液面、以及波浪冲击的作用，利用数值波浪水池模型，对高桩承台结构和强非线性波相互作用过程进行模拟，得到了高桩承台各部分的波浪载荷特征，分析了承台和桩柱的相互影响，发现了桩柱上的附加冲击载荷，并揭示了其力学机理。此外，我们对波流共存和波浪破碎情况下的结构水动力载荷进行了初步研究。

    最后采用数值实验研究了桩柱所受附加冲击载荷的变化规律，详细讨论了承台底部高程对波浪附加冲击载荷的影响，以及承台开孔卸压对减小波浪冲击载荷的效果，据此提出了降低高桩承台结构波浪力的结构改进措施。

    本文对直立圆柱中等尺度结构非线性波浪载荷的研究，解决了海上风机基础结构现有计算方法不匹配的问题，完善了水动力载荷系数在中等尺度的范围取值的缺漏，有利于工程设计的应用；对高桩承台结构非线性波浪载荷的研究，发现桩柱上的附加冲击载荷和提出改进措施，对实际工程设计中降低支撑结构波浪载荷、提高结构安全稳定性有重要意义。

    Wind energy is a very friendly, sustainable and clean energy source. In the last three decades, offshore wind power exploitation has been greatly developed worldwide with a number of offshore wind farms built, but the construction costs are still very expensive. It is of great significance to explore wave loads of offshore wind turbine foundations for developing offshore wind farm and promoting the efficiency of offshore wind power exploitation.

    In this thesis, via the Navier-Stokes equations and the volume of fluid (VOF) technique, a fully nonlinear numerical wave tank is established to investigate the wave forces and moments. Using this numerical wave tank, we can accurately and effectively simulate the interaction process between nonlinear waves, breaking waves and complex offshore structures.

    Based on the analysis of the methods for wave force estimation in engineering, we have found their weakness in estimating nonlinear wave loads on large diameter structures used for supporting offshore wind turbines of megawatts capacity. For this reason, we have estalished a 3D numerical wave tank, simulating complex wave motions and their interaction with complex structures. We have proposed a practical approach to obtain viscous force of wave motion acting on offshore structures by solving the Navier-Stokes and Euler equation, respectively. Then, we have carefully analized the effects of viscosity, inertia and diffraction for structures of different scales. Based on this analysis, we have proposed a new concept, namely the intermediate scale structures, and a new classification criteria for offshore structures. We have further recommended wave load calculation methods for small, intermediate and large scale structures.

    A new type of bottom-fixed structure, the so-called high-rise pile cap foundation, has been widely used to support offshore wind turbines in China. Engineers are unaware of the wave load mechanisms for this new structure. Therefore, we have investigated its wave loads by using linear wave theory, diffraction theory and the established numerical wave tank as well. The numerical wave tank model is used to simulate the high-rise pile cap structure and strong nonlinear waves interaction. The wave load characteristics of each part of the high-rise pile cap are obtained. Particularly, additional impact loads on the piles is found. The hydrodynamic load of wave-current and broken waves condition is preliminarily studied.

    Finally, by means of numerical experiments, several cases of different cap bottom elevation are tested to analyze its influence on the additional impact wave force. By modification of structural parameters and types of the high-rise pile cap structure, we have proposed new improvement schemes to reduce the additional impact wave loads of the structure.