海上风电裙式吸力桶形基础沉贯机理研究

The skirted suction caisson is a novel type of foundation for offshore engineering to fix platforms and floating facilities.It features simple construction, low cost and reuse;espcically,is suitable for the foundation of offshore wind turbines. Penetration of suction anchors, i.e. driven into the desired position in the seabed under suction, is a governing fact element for the design. The suction resluts from pumping out the entrapped water in the chamber of the skirted suction caisson.Suction caissons are effectively performed in a wide range of subsoil such as sand,silt and clay.In sand,seepage plays an important role in reducing soil resistance during penetration under the suction. However, for homogenous clay and clay over sand,the penetration mechanicism differs distinctly from that in homogeneous sand.Seepage should be taken into account when skirted suction caissons penetrate in sand and clay over sand excluding homogeneous clay. Because seepage can reduce effective stresses along inner side wall and at the tip of the skirted scuiton caisson,it can help significantly reduce soil resistance.In additon,the desired penetration depth is necessary for the bearing capacity and deformation to meet the needs. Therefore, it is essential for this project to aim at mechansims of installation of skirted suction caissons with a varity of geometrical sizes in seabed consisting of fine sand and soft clay over sand. To explore principles of some factors such as soil stresses,pore water pressures, soil plug,scution and soil deformation during penetration,a series of improved model tests under 1g state are performed.Especially,water levels in model tests are taken into account by compressing air via GDS pressure controller. A new approach to measuring soil plug is proposed.The influence of the upper soft clay on seepage in sand for the case of the layered soil is studied in detail.Reasonable approaches to analyzing decrease in soil resistance along the inside of the caisson wall and at its tip is put forward.Finally, the formulae of penetration resistance and allowable suction values are established on the basis of experimental results. In order to further study the mechanism on penetration,the finite element method is used to simulate the entire process of the penetration of skirted suction caisson.In this study,model tests are in combination with the analytical method and the numerical method to validate the proposed formulae of penetration resistance and suction. The range of aspect ratios of geometry of the skirted suction caisson is determined under the condition of easy penetration. Finally, tests on tripod skrited suction caissons with different mutual spacing are performed and compared to the single caisson. This project is an extension of the undertaking research.The results obtained herein will guide the construction of offshore wind turbines in China ocean waters,and promote the discipline of offshore soil mechanics.

裙式吸力桶形基础是一种新型的海洋工程基础形式，具有施工方便、造价低和可重复利用等优点,可作为系泊海上浮动式结构物、海洋油气平台的基础，尤其适合近海海上风力发电塔架的基础。裙式吸力桶形基础在海床中的沉贯能力是整个基础达到设计要求，满足地基基础设计承载力和变形控制在允许范围内的重要前提。针对我国典型海床地基如均质饱和细砂、饱和软土及细砂组成的层状土层，以改进的沉贯模型试验系统为主要研究手段，结合理论和数值模拟分析方法，研究裙式吸力桶形基础压贯和吸力沉贯过程中土压力、孔隙水压力、沉贯速率、土塞等各参量变化规律，揭示其沉贯机理；重点研究渗流及基础"尖端"对沉贯阻力的影响；建立沉贯阻力、吸力理论计算公式和吸力控制标准；深入研究基础在层状地基中的沉贯机理，揭示软土层的影响机制；确定易于沉贯的裙式吸力桶形基础几何比尺。研究成果将对我国海上风电工程的地基基础理论和工程应用起到指导作用。

针对所提出的新型海上风电吸力基础—裙式吸力基础，采用模型试验与数值模拟相结合的方法，研究其在砂土、黏性土以及分层土体中沉贯特性，分析沉贯过程中基础内部土塞高度变化规律，提出裙式吸力基础沉贯所需吸力及抗拔承载力计算公式。首先开展裙式吸力基础在砂土中沉贯模型试验，研究表明裙式吸力基础在砂土中有良好的沉贯性，主桶长径比为1.0的传统及裙式吸力基础最终沉贯深度约为0.89及0.87倍基础高度；采用仅抽取主桶内水体沉贯时，裙式吸力基础最终沉贯深度最大；沉贯过程中裙式吸力基础周围砂土颗粒沿主桶及裙结构外侧壁向下移动，最终涌入基础内部形成土塞。采用有限元方法，分析了裙式吸力基础沉贯过程中周围土体渗流场分布。基于滑移线理论和极限平衡理论，提出了吸力基础在砂土中沉贯所需最大吸力、最小吸力以及临界吸力的解析公式。然后进行模型试验和数值模拟，研究裙式吸力基础在饱和海洋黏性土中沉贯特性，结果表明相同主桶长径比下，裙式吸力基础沉贯阻力较传统吸力基础提高44.7%，沉贯所需吸力较传统吸力基础高约20%；沉贯结束时，裙式吸力基础主桶内部土塞高度大于传统吸力基础。基于静力平衡条件，提出了裙式吸力基础在黏性土中沉贯所需吸力的表达式。最后，开展吸力基础在分层土体中沉贯研究。研究发现分层土中裙式吸力基础沉贯所需吸力大于单一土层情况（黏性土或砂土）；层状土中主桶内部土体等势线分布密集且斜率较小外侧等势线分布稀疏且斜率较大沉贯中水头损失主要集中在主桶内部。当基础穿越砂土进入粘性土层时砂土中等势线分布变稀疏水头损失变小。对于上层为黏性土下层为砂土情况，研究发现当基础内部吸力过大时，黏性土层将产生裂隙；当“裙”结构高度小于上层土厚度时，裙式吸力基础沉贯特性与传统吸力基础一致。最后得到裙式吸力基础在层状土中沉贯沉贯所需吸力计算公式。本研究所得规律可为我国海上风力发电工程提供必要的科学依据，具有重要的工程应用价值。