海上风机超大直径薄壁桶基在侧向动荷载作用下的初始动阻抗研究

The implementation of the ocean energy development strategy has accelerated the steps of offshore wind power development in our country, how to build wind turbines better is one of the most important issues we are facing. Calculating natural frequencies of wind turbines accurately is the key point to avoiding the damage from loads with various frequencies, such as winds, waves, currents and earthquakes, etc. The initial dynamic impedances of wind turbine foundation affect the natural frequencies of turbines directly; however, we don’t know initial dynamic impedances of subsea foundations enough, especially for large diameter and thin walled bucket foundations. As a type of new foundations for offshore wind turbines, there are so few results that engineers are still calculating approximately using p - y curves obtained from bearing capacity tests of small diameter and slender piles. Based on analytical theories, with model tests, this project will do researches on the initial dynamic impedances of offshore bucket foundations: we will propose a coupled seawater - bucket foundation - seafloor half space vibration theory, and a bucket foundation is decomposed to a rigid disc and an elastic shell foundation. The model will be solved through 3-D wave theories and (singular) integral equation theories, and then using model tests to testify the theoretical results. At last, combining theoretical and model test results to analyse the mechanism of the bucket impedances. We will also provide some useful suggestions on the values of the dynamic impedances of field offshore bucket foundations. This project will help keeping the turbine running safely and saving the costs a lot.

海洋能源开发战略的实施加快了我国海上风能开发的进程，如何更好更经济地建造海上风机是当前面临的重要问题。精确计算风机自振频率是避免风机在风、浪、流及地震等频率迥异的荷载下发生共振破坏的关键，然而对风机基础初始动阻抗这一直接影响风机自振频率的重要因素了解仍不充分。特别是作为海上风机新型基础的大直径薄壁桶基，应用前景广泛，但目前研究成果匮乏，工程师仍按小直径细长桩基静力加载试验得到的p-y曲线近似计算，误差较大且偏于保守。本项目拟立足于解析理论，并辅以试验对桶基的初始动阻抗进行研究：提出全耦合的海水-桶基-海床半空间振动理论，将桶基看成刚性圆盘和弹性壳体的叠加，通过三维波动理论及（奇异）积分方程理论进行求解；并通过模型试验验证解析理论中的假设和结论；最终结合理论及试验分析桶基初始动阻抗变化机理及其影响因素，给出实用取值建议。本项目将为保障海上风机安全运行和降低海上风电成本提供科学依据。

随着海上风机功率的增加，风机结构的尺寸变得越来越高、越来越柔，极易跟环境荷载产生共振，从而产生疲劳损伤。海上风机的振动特性在很大程度上受基础动阻抗控制，而超大直径薄壁桶基作为一种极有发展前景的新型基础形式，目前研究较为缺乏，故本项目针对该新型基础形式开展了理论研究并进行了试验验证。理论研究方面，本项目立足于解析理论，根据流体力学、固体力学及土力学相关理论及恰当的边界条件，建立了近海风机超大直径桶基在侧向动荷载作用下与海水、海床的动力相互作用理论，通过积分方程方法求解得出超大直径桶基在竖向、水平动荷载及动弯矩作用下的初始动阻抗及相应的海水、超大直径桶基与海床的动力响应，并进行了参数分析。揭示了影响超大直径桶基侧向初始动阻抗的关键因素主要为桶基壳体与土体的动力相互作用，桶基顶部顶板发挥的作用在壳体非常小的时候较为显著，并随着桶基壳体长度的增加而减小，故将桶基看成刚性顶板与壳体的叠加时，顶板部分的影响应乘一个在（0-1）之间变化的刚度发挥系数。为验证理论模型的正确性，项目中搭建了海洋基础-土体动力相互作用试验平台，对大直径桶基的初始动阻抗进行研究，验证了不同条件下大直径桶基理论模型的正确性。此外，本项目还研究了冲刷对海上风机基础动力特性的影响、波浪对基础动力特性的影响。依托本项目研究，共发表论文14篇（其中SCI 刊源8篇、6篇已检索，均为第一作者），授权国家发明专利2项，培养已毕业硕士研究生2名。