海上风机筒型基础循环弱化与支撑结构长期振动特性研究

Developing offshore wind energy has great meaning to improve the energy structure of China and realize sustainable development. During the operation period of offshore wind turbines, the natural frequency of support structure may be obviously changed due to the variation of its foundation soil properties, which can be a serious threat to long-term safe operation of the offshore wind turbine. Therefore, the following studies on the long-term performance of the novel caisson foundation support structure are carried out from the respects of theoretical analysis, numerical calculation and laboratory experiments: analyze the characteristics of offshore wind, wave and current loads, and obtain their coupling forms; on the basis of support structure vibration model tests and flume experiments coupling caisson vibration and wave effect, the dynamic impedance of caisson foundation, excess pore pressures and soil stresses around the caisson are thoroughly investigated; a series of soil cyclic loading tests considering specific stress path are carried out to stduy the cyclic degraded mechanism of foundation soil, then the nonlinear dynamic impedance of caisson foundation can be calculated, and finally the substructure model for analyzing the vibration response of support structure is built; this substructure model is verified and modified according to the results of flume experiments coupling vibration and wave effect, then the key factors causing the evolution of support structure natural frequency are studied, and thus the safety evaluating system for the long-term performance of offshore wind turbine can be improved. The research results of this project can provide the necessary scientific basis for the design of support structure of offshore wind turbines in China.

大力发展海上风能对于改善我国当前能源结构，实现可持续发展意义重大。在海上风机运营期内，其支撑结构的固有频率可能因地基土体特性改变而产生显著变化，这将严重威胁海上风机长期安全运行。鉴于此，本项目采用理论分析、数值计算和室内试验等手段，针对新型吸力式筒型基础支撑结构的长期振动特性开展如下研究：分析获得近海风、浪、流荷载基本特征及耦合作用形式；进行支撑结构循环振动试验、耦合筒基振动和波浪荷载效应的支撑结构水槽试验，研究筒基动力阻抗变化规律及筒基周围各区域土体的孔压、应力路径特征；开展考虑应力路径的土体单元循环加载试验，揭示筒周土体的循环弱化机理，计算分析筒基的非线性动力阻抗，建立支撑结构振动响应子结构分析模型；基于支撑结构水槽试验结果验证并优化所建子结构模型，分析导致支撑结构振动特性长期演化的关键因素，完善海上风机安全运营评价体系。本项目的研究成果可为我国海上风机支撑结构设计提供必要的科学依据。

海上风机的支撑结构是一种自身动力特性受环境荷载、基础刚度等因素影响异常敏感的高耸结构物。本项目针对海上风机运营期内，其支撑结构固有频率因地基弱化不断演化的规律及其机制开展研究，旨在为我国海上风机设计提供必要的科学依据。研究团队基于无量纲化的相似性理论，构建了海上风机动力特性模型试验平台，试验研究了在长期循环荷载作用下的海上风机支撑结构自振频率和系统阻尼的变化规律。通过理论分析和试验观测，揭示了支撑结构自振频率变化的内在机制：在循环加载初期，土体颗粒密实效应将增大支撑结构的自振频率；之后，由于基础周围土体下陷导致埋置桩长减小，这显著减小了支撑结构的自振频率。这是影响海上风机支撑结构振动频率演化的两个主要因素。项目组依次研究了单桩基础、重力式浅基础、不同长径比的吸力筒基础、对称与非对称三脚架结构及四脚导管架等“多足”基础所支撑的模型结构动力特性及在循环荷载下动力特性变化规律。基于研究成果，给出了实际工程中海上风机基础选型以及支撑结构设计上的有益建议。此外，针对吸力式筒型基础在粉土、粘土海床中的安装控制技术和安装后的set-up效应展开研究。依托本项目研究，已发表学术论文15篇（其中SCI期刊论文8篇），授权国家发明专利4项，软件著作权3项。