海洋环境荷载下裙式吸力基础长期承载机理和动力特性研究

The skirted suction caisson is an innovative offshore wind turbine foundation. The skirted suction caissons are required to withstand several environmental loads (wind, wave and dynamic loads) during operation, resulting in the complex long-term cyclic bearing behavior and dynamic responses. A series of model tests, numerical simulations and theoretical analysis well be carried out to investigate the responses of the accumulation of rotation, cyclic stiffness, natural frequency and the accumulation and dissipation of pore pressure in sand to the loading cycles of skirted suction caissons in saturated marine sand under combination environmental loads. Parametric studies will be conducted to explore the influences of foundation sizes, wind and wave misalignment effects, relatively densities of sand and the extreme environment loads on the long-term bearing capacities and the dynamic responses of skirted suction caissons. Based on the results, the optimal dimension for the skirted suction caissons can be obtained. Expressions to calculate the accumulation of rotation, the cyclic stiffness, the natural frequency of the skirted suction caisson will be also presented. The liquefaction and the strength reduction mechanisms of sand around the main structure and skirted structure is also proposed by carrying the long-term cyclic sand-foundation interface shear test. In addition, the method to evaluate the stability of the skirted suction caissons will be proposed considering the effects of the cyclic bearing behavior, dynamic response, the liquefaction and the strength reduction of sand. Results obtained herein will guide the research and the construction of offshore wind turbine foundations in China.

裙式吸力基础是一种新型海上风电塔架的基础形式受到多种环境荷载（风荷载、波浪荷载、叶轮转动动力荷载）共同作用，其长期循环承载特性和动力特性十分复杂。针对我国典型砂质海床地基，以模型试验为主，结合理论分析和数值模拟等研究手段，分析多种海洋环境荷载共同作用下裙式吸力基础累积变形、刚度、固有频率变化和基础内外砂土孔隙水压力累积消散规律；研究基础尺寸、长期荷载组合、波浪荷载和风荷载不共向性、砂土相对密实度以及极端环境荷载发生时间等因素的影响。得到基础长期循环承载力、循环累积转角和刚度计算方法，揭示裙式吸力基础长期循环承载机理；得到裙式吸力基础固有频率变化预测公式，阐述其动力特性；开展室内土体与基础侧壁长期界面剪切特性试验，获得基础周围砂土液化触发条件和土体循环弱化特性；提出裙式吸力基础长期循环承载稳定性评价方法。研究成果将对我国海上风电塔架基础的理论研究和工程应用起到指导作用。

针对新型海上风电裙式吸力基础，开展模型试验和数值模拟，研究其在砂土中的复杂长期循环承载机理以及基础土体之间相互作用。研究结果表明，裙式吸力基础水平循环承载力较传统吸力基础显著提升，且显著控制了基础水平累积位移，水平累积变形减小70%。基础在单向和非对称循环荷载作用下，累积转角同循环荷载幅值和次数呈正相关，但随着循环次数的增加，其增长的速率呈衰减性。非对称循环加载方式中传统及裙式吸力基础产生最大的累积转角，相较于单向循环加载分别增加了44%、23%。当荷载偏转角度小于等于90°时，变方向水平循环加载的总累积位移大于水平单向循环荷载；当荷载偏转角度大于等于120°时，变方向水平循环加载的总累积位移小于水平单向循环荷载。当荷载偏转角度为30°至45°时，基础的总累积位移较水平单向循环荷载增大5%~10%，基础累积位移达到最大值，说明此时为基础最不利荷载组合。变方向加载将导致基础的累积位移方向与加载方向不一致的现象。随着荷载偏转角度的增大，累积位移方向和荷载方向间的夹角先增大后减小。提出了基础水平循环累积变形和刚度变化规律，揭示了变向水平加载条件下基础周围土体变形规律，阐述了基础在变向荷载作用下基础的运动特性。开展数值模拟得到裙式吸力基础在地震等动力荷载作用下基础的位移及土体内部孔压累积及消散规律。此外，开展GDS界面剪切试验，研究了循环服役阶段，基础-土体界面相互作用规律。基础服役阶段，在沉贯速率影响下，服役阶段界面循环正向剪应力随着沉贯速率增大而降低。随着循环次数的增加，法向压力越大界面强度弱化现象越明显。在位移幅值影响下，界面正向剪应力随着位移幅值的增大逐渐增大，且随着法向压力增大而增大。项目相关研究成果已经成功应用于我国广东、福建、广西等海上风电及海洋边际油田吸力基础的设计领域，同时作为我国漂浮式风电锚固基础的技术储备。相关模型试验装置已获批发明专利，且实现成果转化和推广应用。