FEA-Based Study of Pipeline Protection From Anchors

Abstract

This paper discusses the use of Finite Element Analysis in rock berm design for pipeline protection against anchor impact. Submarine pipelines located near harbour areas and in major shipping lanes are likely to be exposed to potential damage due to anchors and dropped object impact. To protect these pipelines, they are buried and protected with rock armour berms. The design of these rock berms has been traditionally based on model testing because there has been no analytical solution available to resolve the complex interaction between the anchor, anchor chain, seabed soil and rock armour. The authors are currently working on a Finite Element Analysis (FEA) based procedure that can be used to assess the potential anchor damage to a pipeline protected by rock backfill. The 3-D non-linear numerical analysis code ABAQUS/Explicit is used to simulate the entire process from the anchor being dropped into the seabed to the anchor being dragged out of the seabed and through the rock protection cover. The interactions between the anchor, chain, soil, rock and pipeline are all taken into consideration. From the anchor dragging trajectory within the seabed soil and protection rock cover, the clearance between the anchor fluke tip and pipeline can be obtained. This along with the pushing forces on the pipeline can be used to assess the potential damage to the pipeline. This paper presents the FEA model for an anchor dragging across a buried pipeline resting on soft clay soil and covered by rock berm. These results have been successfully validated against results obtained from physical model tests. The Finite Element Analysis procedure allows the investigation of a wide range of configurations to optimize the rock berm design, such as the effect of off-design of armour rock shape and trench profile, rock size and grading variation etc. With a better understanding of the pipeline, anchor, soil and rock berm interaction, the design of the pipeline armour rock protection against anchor damage can be carried out in a more definitive manner. This paper also presents a recent application of the FEA model to study the effectiveness of the rock berm protection due to change in seabed profile.