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

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.