考虑浅埋破坏的拖曳锚在黏土中安装运动特性分析

Kinematic Analysis on Drag Anchor Installation in Clay Considering Shallow Failure Effects

拖曳锚是海洋工程中一种常见的系泊基础,因造价低廉和高承载特性而得到广泛应用.其在海床中的安装轨迹和运动特性受到锚与土体之间复杂相互作用的影响,使得精确定位仍存在挑战.目前已有的塑性屈服面方法被广泛用于计算拖曳锚的运动特性,即假设整个拖曳过程为深埋板在不同深度的破坏过程.实际上,拖曳锚的安装是从浅埋到深埋的连续贯入,因此该方法不能考虑浅埋破坏对拖曳锚运动特性的影响,从而可能导致预测的轨迹不准确.通过有限元分析研究了锚板方位角及埋深比对单向承载和复合荷载下屈服面的影响,确定了锚板浅埋破坏时的屈服面,补充了塑性屈服面法对浅埋破坏效应的考虑;进一步地,考察了锚板方位角、承载系数、浅埋区域大小对拖曳锚轨迹预测和运动特性的影响,并与传统仅假设深埋破坏情况对比分析.结果表明:浅埋破坏时锚板方位角与浅埋区域大小决定了锚板的运动特性和轨迹;合理考虑浅埋破坏后,与纯假设深埋破坏情况比,锚板在达到稳定状态之前的预测埋深和锚链力要小,但极限嵌入深度一致.

The drag anchor is widely used in offshore engineering as a foundation for the mooring system due to high capacity and low cost. The prediction of kinematic behavior and trajectory is still challenging due to the complex interaction of the anchor fluke with soil. The existing plastic yield envelope method is generally used for the kinematic analysis,where the deep anchor failure is assumed to occur during the whole dragging process. In reality,the penetration is a process of anchor fluke being dragged into seabed soil continuously from shallow to deep depths. Obviously,the existing yield envelope method could not capture the effect of shallow anchor failure,which may lead to inaccurate prediction of the anchor trajectory.Finite element analyses were conducted firstly to obtain the influences of the anchor embedment depth and angle of the fluke on the drag anchor behaviors under unidirectional and combined loadings.The yield envelope for the shallow anchor failure was determined accordingly,with which the shallow failure effect can be taken into account for the yield envelop method. The effects of the fluke angle,the bearing capacity factor and the shallow zone size were investigated. The predicted trajectories with and without the shallow failure effect were compared. It is shown that the fluke angle and the shallow failure zone size influence significantly the kinematic behaviors and the trajectory of the drag anchor. The consideration of shallow failure results in a shallower predicted anchor embedment depth and a smaller anchor line force before the anchor penetration depth stabilizes,but hardly changes the ultimate embedment depth.