自升式平台桶形桩靴踩脚印稳定性研究

A Jack-up unit obtains its bearing capacity by penetrating its legs and spudcan foundations to a certain depth into seabed. After operation on one site, a Jack-up unit can extract its legs and be towed to the next site. The penetration and extraction of its legs and spudcan foundations leave significant seabed depressions or footprints, and the soil strength in the footprint is reduced by remoulding. In recent years, Jack-up units often have to return to the same site and be installed near existing footprints. The existing footprint leads to an eccentric load on the spudcan during reinstallation, and may cause structural failures within the jack-up legs. The jack-up reinstallation problem is perceived by the industry as a significant problem with impact on time, costs and safety of structures and personnel. As an alternative to spudcans, skirted foundations have potential to mitigate the instability of legs during reinstallation near existing footprints., since it can provide more lateral resistance. To date, there are limited publications in the literature focusing on the spudcan-footprint interation. The effect of an exiting footprint on the stability of a reinstalled jack-up foundation is still not very clear. The study on the behavior of skirted foundations near existing footprints are more rare..In this study, three dimentional large deformation numerial analyses and centrifuge model tests are to be carried out to simulate the penetration and extraction processes of skirted foundations. The forming mechanism of a footprint are to be investigated. Equations are to be sumarised to evaluat the shape of a footprint and the soil strength profile in the footprint.The reinstrallation precesses of skirted foundations near existing footprints are also to be simulated. The comprehensive effects of the footprint’s geometry, the footprint’s soil properties and the structural properties of the jack-up unit on the reinstallation stability are to be investigated. Equations are to be summarised to evaluate the response of a reinstalled skirted foundation near an existing footprint. The effect of water jetting on the extration behavior of skirted foundations are to be studied by centrifuge model tests. Suggestions on the skirt length and the positions of water jetting are to be proposed. This study has important significances in both geotechnical research and practical offshore engineeing.

自升式平台插桩过程中常遇到桩靴基础踩脚印失稳问题。相对于工程中常用的纺锤形桩靴，桶形桩靴由于桶壁插入地基，能够提供更大的侧向抗滑力，有利于提高踩脚印插桩的稳定性。目前国际上关于桩靴踩脚印问题的相关研究还不多，桩靴踩脚印的失稳机理尚不明确，桶形桩靴踩脚印的相关研究更加少见。因此，本项目拟采用三维大变形数值分析和离心机模型试验相结合的手段，模拟插拔桩形成桩坑和二次插桩踩脚印过程，揭示桩坑的形成和演化机制，建立桩坑几何尺寸和土体强度特性的预测模型；揭示桶形桩靴的踩脚印失稳机理，提出最大水平推力和弯矩预测模型；建立复杂地基-桩靴-平台系统耦合作用数值分析方法，揭示其相互作用机制并提出平台水平位移预测模型；进行冲桩过程离心机试验研究，对桶壁高度和冲桩喷水口位置提出合理建议。本项目研究在土与结构相互作用领域具有重要科学研究意义，在自升式海洋平台的设计和施工方面具有重要工程应用价值。

自升式平台插桩过程中常遇到桩靴基础踩脚印失稳问题。相对于工程中常用的纺锤形桩靴，桶形桩靴由于桶壁插入地基，能够提供更大的侧向抗滑力，有利于提高踩脚印插桩的稳定性。目前国际上关于桩靴踩脚印问题的相关研究还不多，桩靴踩脚印的失稳机理尚不明确，桶形桩靴踩脚印的相关研究更加少见。本项目根据申请书中拟定的研究内容和研究方案展开展研究，经过四年的努力，1）发展了能考虑土体应变软化和土体变形速率的土与结构大变形相互作用分析方法；2）基于此开展了系列大变形数值模拟和离心机实验，揭示了插拔桩形成的桩坑的演化机制，建立了桩坑几何特性和土体强度特性的预测模型；3）揭示了纺锤桩靴和桶形桩靴踩脚印失稳机理，建立了纺锤形和桶形桩靴踩脚印所受最大水平推力和最大弯矩预测模型，结果表明，与平底桩靴和纺锤桩靴相比，桶形桩靴二次插桩过程中可以提供最大的竖向力，但水平力和弯矩最小，证明了桶形桩靴在缓解桩靴踩脚印问题中的优势；4）建立了复杂地基-桩靴-平台系统的耦合相互作用大变形数值分析方法，揭示其相互作用机制，建立桩靴踩脚印过程中桩靴位移预测模型。结果表明，在相同桩腿抗弯刚度下，与平底桩靴和纺锤桩靴相比，桶形桩靴的水平力和水平侧移最小，说明桶形桩靴在缓解踩脚印带来的危害方面有一定的优势；5）完成了二次插桩及喷桩实验，提出桶形桩靴桶壁高度和冲桩喷水口位置的合理建议。6) 根据研究成果，提出了一种新型导向桩靴，可以用来缓解踩脚印带来的危害。7）针对极地冻土插桩过程，结合工程实际，对自升式平台桩靴基础在极地海洋冻土层中极限承载力特性开展了研究，通过建立桩靴与冻土的热-力顺序耦合有限元模型来模拟桩靴在海洋冻土中热传导过程，探究竖向极限承载力变化规律，分析了冻土融化后强度折减系数对极限承载力的影响。研究成果具有重要的学术与工程应用价值，为自升式海洋平台的精细化设计和施工提供了参考。