The Performance of the Plate Anchor to Adapt the Trenching Seabed

The seabed trenching is prevalent in many floating moorings. The seabed around the dip-down point of the mooring line is tunnelled due to the chain-soil-water interaction. The trenching presence may significantly reduce the holding capacity of the anchoring foundations, which threatens the safety of the floating system. The plate anchor is relatively mobile in comparison with a fixed suction caisson. With appropriate padeye offset, the plate anchor can dive deeper into stronger soils with higher strength under extreme conditions. Therefore, it is reasonable to assume that plate anchors provide an alternative to adapt the seabed trenching. Once the seabed trenching is severely developed, the mooring line connected to the plate anchor is further tensioned by a tug, forcing the anchor to dive deeper to gain additional holding capacity. Two issues should be addressed to illustrate the performance of plate anchors in trenched seabed. The first is the anchor stability in trenched seabed, i.e. how to determine the critical trench depth for the anchor to maintain stability within the seabed? The second is the further keying property of the plate anchor in trenched seabed, i.e. does the anchor still exhibit diving behaviour in the trenched seabed? The present study performed model tests to address the above issues. First, the progressive development of the seabed trenching was simulated, which aimed to investigate the stability of the plate anchor in the trenched seabed. Second, the plate anchor was further keyed in the trenched seabed, which aimed to investigate the further keying behaviour of the anchor. The test results indicate that the plate anchor does not fail with the trench depth approaching 0.90 times the padeye embedment depth, and still exhibits diving behaviour in the trenched seabed. Overall, plate anchors are relatively mobile and are useful to restrain the effect of seabed trenching on the holding capacity of the anchoring system.