黏土地基上海洋平台桩靴峰值阻力上限分析

在自升式平台的预压载过程中,桩靴在层状地基上较易发生“穿刺”现象,很大程度上影响着平台的安全运行。准确地分析桩靴峰值阻力,避免平台桩靴发生“穿刺”是非常重要的。采用极限分析上限定理,合理构建运动许可速度场,从理论上推导了层状地基上桩靴峰值阻力的上限解答。为了进一步验证峰值阻力理论解答的准确性,采用ABAQUS有限元软件构建了“桩靴—弹塑性海床”的三维数值模型,对桩靴贯入海床的过程进行了数值模拟,分析桩靴周围土体的塑性变形演变规律,研究土体的破坏机理。研究结果表明：推导的桩靴峰值阻力上限解答,能够较好地计算层状地基极限承载能力,通过与离心机试验和数值结果对比,计算误差在18%以内;给出的运动许可速度场能够较好地反映桩靴周围土体破坏模式;桩靴阻力达到峰值时,下层软土中的土塞高度约为桩靴直径的0.2倍。

Upper limit analysis of spudcan peak resistance of offshore platform on clay foundation

In the preloading process of jack-up platform, the "punch-through" phenomenon of spudcan is more likely to occur on the layered foundation, which greatly affects the safe operation of the platform. Therefore, accurate analysis of the peak resistance of spudcan becomes the key to avoid the "punch-through" of platform spudcan. The upper bound theorem of limit analysis is used to construct kinematic admissible velocity field reasonably, and the upper bound solution of peak resistance of spudcan on layered foundation is deduced theoretically. In order to further verify the accuracy of the theoretical solution of the peak resistance, the three-dimensional numerical model of "spudcan-elastoplastic seabed" is constructed by ABAQUS finite element software. The process of spudcan penetrating into the seabed is numerically simulated, the plastic deformation evolution law of the soil around the spudcan is analyzed, and the failure mechanism of the soil is studied. The research result shows that the upper bound solution of peak resistance of spudcan can better calculate the ultimate bearing capacity of layered foundation, and the calculation error is within 18% compared with centrifuge test and numerical results. The given kinematic admissible velocity field can better reflect the failure mode of soil around the spudcan. When the spudcan resistance reaches the peak value, the soil plug height in the lower soft soil is about 0.2 times the diameter of the spudcan.