Mechanism and model tests on protection of submarine pipelines using rock armor berms

Submarine pipelines are likely to be exposed to the risk of damage from drag anchors of ships that are out of control during severe storms. Rock armor berms are commonly adopted to protect the pipelines by deflecting drag anchors. To understand the protection mechanism, both theoretical studies and model tests are carried out. A limit equilibrium method is put forward to deal with the interaction between the drag Hall anchor and the developed soil wedges. The polygons of forces on the soil wedges induced by both the anchor crown and the anchor fluke are established respectively. The total drag force on the anchor is the sum of the drag force on the anchor crown and that on the anchor fluke. The minimum total drag force on the anchor can be solved by using the hill climbing algorithm. Model tests are carried out using three model anchors with different linear scales. The drag forces measured in model tests agree well with those calculated by the theoretical approach. With the obtained forces, the moment on the anchor can be calculated. It is found that when the anchor is dragged into the rock armor berm from the foundation soil, unbalanced moment on the anchor will take place,which will cause the drag anchor to rotate and the fluke tip to rise up. The deflecting of the anchor trajectory can make the fluke tip keep a safe distance from the buried pipeline, which explains the protection mechanism. Using the proposed approach, the position of the fluke tip and the drag force on the anchor under equilibrium conditions can be predicted, which may be helpful to the berm designers.