近海软弱土层中大直径单桩风电基础环境荷载灾变机理研究

The offshore soft seabed and water depth in China make the steel-pipe monopile be the first choice in the design of offshore wind turbine. The application of offshore wind turbine is in the early stage in China, and the research achievements in west European area are not suitable here due to the distinct stratum conditions. For the lack of systematic research as well as intensive study, there may exist unnecessary waste and potential catastrophe might occur. This research project is directed towards the essential scientific problems pertaining to the steel-pipe monopile foundations for offshore wind turbines, covering the bearing properties of a monopile affected by penetration, the static and dynamic response of the steel-pile pile foundation under offshore environmental load. Through centrifuge model tests, numerical simulations as well as theoretical methods, research efforts will be focused on the accumulative water pressure and soil deformation of soft marine soils under cyclic load and the corresponding static and dynamic influence on the pile-soil interaction. A theoretical system could be established, including CEL large-deformation numerical method for the installation process of steel-pipe piles, consideration of pile installation effect on their bearing behavior. Analysis methods for evaluating the performance of steel-pipe pile offshore wind turbine foundations under long-term cyclic and extreme storm load conditions will also be put forward. The outcome of this proposed research can furnish scientific support for more rational and economic design for steel-pipe monopile and the safety of offshore wind turbines under long-term operational state.

我国近海的软弱地层条件以及水深条件，使得大直径钢管桩单桩基础成为海上风电基础的首选。海上风电在我国还在起步阶段，因地层条件差异，无法照搬西欧的相关研究成果。由于缺乏系统性的深入研究，海上风电基础的设计施工产生了不必要的浪费和潜在灾变。本项目针对近海软弱土中大直径单桩海上风电基础的海洋环境荷载效应开展研究，主要关注大直径钢管桩沉桩效应对水平承载特性的影响，低幅循环加载的拟静态响应和极端风暴荷载的瞬时动态响应。通过离心模型试验、数值模拟以及理论分析，研究近海软弱土的孔压累积和塑性应变累积对桩土系统的静力动力响应，建立沉桩过程CEL大变形数值技术以及承载特性沉桩效应下单桩基础分析方法，提出长期环境循环荷载和强风暴荷载下钢管桩风电基础灾变分析方法，为更合理经济的大直径钢管桩风电基础设计、保障海上风机长期运行安全提供科学支持。

针对海上大直径钢管桩风电基础的环境荷载灾变机理研究方面存在的不足，通过自主研发砂土制样和循环加载设备，开展一系列低频循环加载模型试验，对加载幅值二次修正，建立了考虑循环加载幅值、冲刷效应和桩土刚度影响的大直径单桩长期累积变形预测经验公式。考虑砂土的循环加载特性，采用R-O模型和修正Masing准则构建土体的加卸载应力应变关系，提出桩周土体加卸载刚度模型。基于显式累积应变确立循环演化参数，利用有限元和弹性地基梁模型，建立大直径单桩的长期循环加载特性理论计算方法。将土体累积应变视为自发应变，在土体静载本构关系中减掉该量，满足平衡及变形协调条件，建立了大直径单桩长期循环加载的等效有限元方法。分别利用激振系统和自主研发的齿轮循环加载装置开展大直径单桩风电的共振频率模型试验，探讨了埋入比和循环加载次数对风电共振频率的影响。进一步建立了考虑塔筒变截面和塔筒桩基刚度区别的风电共振频率差分法、桩基阻抗作为塔筒边界的共振频率求解子结构法和考虑不同桩顶阻抗条件的共振频率能量法，探讨了大直径单桩的埋入比、桩周土体剪切模量和不同的桩基边界条件对风电共振频率的影响。自主研发可实现用户自定义输入不规则波形的精确动载控制加载量测系统，开展了砂土中大直径单桩的极端风载动力响应模型试验，分别探讨了结构因素和风载因素对其动力特性的影响。通过输入不同的风载功率谱的谐波叠加法、叶素动量理论计算叶片荷载，考虑塔筒非定常模式计算塔筒荷载，建立极端风载条件下作用于风机结构的随机气动荷载。进一步基于差分格式和桩基的Winkler地基梁模型，建立了海上风电大直径单桩动力响应的理论计算模型，对比了风载功率谱、塔筒荷载模式、固定和桩基模型、叶片和集中质量模型对风机结构动力响应最大弯矩、最大位移、功率谱密度函数的影响。本研究工作可为大直径单桩海上风电的发展提供相关参考，并可进一步拓展至海上风电的其它基础型式。