土体流动对自升式平台桩靴极限承载力的影响

The Influence of Mud Flow on the Ultimate Bearing Capacity of Jack-up Platform Spud Can

现有自升式平台强度设计中地基承载力采用静态设计法,将地基作为一种不变的边界条件,将桩土作用简化为泥面下简支约束,从而忽略了地基土的非线性及低强度特性。桩土贯入过程属于动态高度非线性、大变形问题,土体流动特性对桩端承载影响很大。建立桩土有限元模型,采用任意拉格朗日-欧拉法(ALE)模拟桩土贯入,利用罚函数法求解桩土面-面动力接触效应。分析结果表明:桩靴上部空腔破坏,土体回流冲击桩靴上端面,并在桩靴上下端面形成速度逆差,造成桩靴下部形成瞬态吸力,使桩端承载骤增。因此采用静态设计法得到地基承载能力低于实际贯桩桩端承载,由此提出了这样的问题:按照现行规范进行地基承载能力设计,会导致地基承载能力不足。

In the existing jack-up platform strength design the static design method has been adopted for foundation beating capacity, which takes the foundation as a fixed boundary condition while the spud can boundary condition is simplified to be a simple support under mud, thus neglecting the nonlinearity and low strength characteristic of the foundation soil. The pile-soil penetration process is dynamic, highly nonlinear and with large deformation, so the influence of mud flow characteristics on the spud can bearing capacity is very large. This paper establishes the foundation finite element model, uses the arbitrary Lagrange Euler method （ALE） to simulate the pile-soil penetration, and adopts the penalty function method to solve the face-to-face dynamic contact effect. The analysis result demonstrates that the spud can upper vacuum is damaged, back-flow of mud impacts the upper spud can surface and creates a velocity gradient deficit between the upper and lower ends of the spud can, leading to a transient suction at the lower part of the spud can, increasing the spud can beating capacity abruptly. Therefore, in accordance with the current design regulation the foundation beating capacity obtained by static design method would be lower than the actual value, demonstrating a deficient foundation beating.