复杂地层条件下自升式钻井平台桩靴基础穿刺破坏机理及评估方法研究

A hazard of sudden uncontrolled punch-through failure of the spudcan foundation usually exists for a Jack-up rig to install (or operate) in the sediments with a strong layer overlying a relatively soft layer. Such a spudcan punch-through failure would seriously threaten safety of the rig structure, even cause casualties. The punch-through failure of spudcan under the complex strata conditions is quite a complex problem involved lots of factors, such as the spudcan-sediments interaction, the interaction between different layers, the effects from the spudcan penetration velocity and also the strain softening of the strata due to the large deformation. This project plans to build (or choose) the constitutive model for each stratum, respectively, according to its mechanical characteristics. And a numerical calculating model would be presented based on ABAQUS/Explicit. By back-analyses of the centrifuge tests, the FE model presented is validated. The failure mechanism of the spudcan is comprehensively investigated by LDFE simulations of the deep penetration of spudcan under the complex strata conditions. And quantitative descriptions of the 'soil plug' effects and the depth of the hole above the spudcan are presented by using the regression analysis methods in Matlab. On this basis, a conceptual model for the punch-through failure mechanism of the spudcan is presented. Based on the elastic-plastic solution of the normal stress on the shear slip plane in the overlying strong layer, the calculation model is presented for the limit bearing capacity of the spudcan. By calculating the q-d curve through the calculation model presented for the limit bearing capacity of the spudcan, the potential punch-through risk would be reasonably estimated.

自升式钻井平台在夹有上硬下软地层的作业区域插桩就位（或作业）过程中均存在着桩腿突然、不可控地贯入下伏地层的风险，即穿刺风险，严重影响着平台的作业安全，甚至导致灾难性事故。复杂地层条件下的穿刺问题涉及桩靴、地层及各地层间的非线性作用，加之桩靴贯入速率影响及土体应变软化效应，是一个涉及多因素非线性耦合的复杂问题。本项目拟通过对所涉及地层依据其力学特性，分别构建（或选择）本构模型，建立基于ABAQUS/Explicit的数值计算模型。通过反分析多组离心机实验，对所建模型进行验证。在此基础上，通过模拟不同复杂地层条件下，桩靴基础的大位移贯入过程，揭示桩靴基础破坏机理，并对“土塞”效应及桩坑深度给出定量描述。在此基础上，构建桩靴基础穿刺破坏概念模型,通过求解上履硬土层中剪切滑移面上法向应力场弹塑性解，建立桩靴基础极限承载力计算模型。进而通过计算q-d曲线对潜在穿刺风险进行合理评估。

自升式平台在夹有上硬下软地层的作业区域插桩就位过程中存在着桩腿突然、不可控地贯入下伏地层的风险，即穿刺风险，严重影响着平台的作业安全，甚至导致灾难性事故。本项目根据海床粘土、砂土不同的力学特性，对粘土、砂土分别构建了考虑应变软化效应和渐进破坏的本构模型，建立了基于ABAQUS/Explicit的数值计算模型，并通过对比系列离心机试验结果和已有计算结果，对所建模型的有效性进行了验证。全面、详尽地揭示了桩靴基础在砂土-黏土地层、含砂-黏地层的多地层等多种复杂地层条件下大位移贯入过程中的土体回流破坏机理、穿刺失效机理，并定量表征了桩靴底部的“土塞”效应。构建了考虑桩靴底部“土塞”效应和砂-黏地层交界面几何特征的桩靴基础穿刺失效概念模型，通过弹塑性力学分析，建立了综合考虑土体回流机理及穿刺机理的桩靴基础极限承载力计算模型。此外，结合平台潜在穿刺风险问题的工程实际，探究并分析了可能的应对措施。