Advanced FE Modelling Approach for Pipeline Hooking Interaction of Dragged Anchors

Abstract

Offshore pipelines, located in areas with high/medium ship traffic, are exposed to threats such as dragged anchors, dropped objects, sinking ships, etc.. In the last years, there have been a few accidents where subsea pipelines or cables have been hooked and damaged by dragged anchors. These incidents have been caused both by emergency and accidentally dragged anchors. Two of the most serious incidents to pipelines caused by an accidentally dragged anchor involved the Trans Mediterranean gas pipelines in 2008 and the Kvitebjørn gas pipeline in the North Sea in 2007. The interaction mechanism between the pipeline and an anchor dragging on the seafloor is affected by different parameters (i.e. anchor size, chain length, water depth, soils geo-technics, anchor speed, incident angle between the pipeline and the anchor dragging direction). In common practice, the occurrence of hooking events is assessed by implementing specified interpretative model involving basically typical anchor dimensions for the different ship classes and expected soil pipe interaction scenarios. Major efforts have been made to develop a 3D Finite Element model with the aim of improving the simulation capabilities of engineers to address and anticipate the mechanism of pipeline and dragged anchor interaction. The model presented in this paper is based on the Coupled Eulerian Lagrangian (CEL) technique. With respect to typical simplified FE models, implementing the seabed as an analytic surface, this advanced technique (i.e. CEL) allows one to model the seabed as a deformable Eulerian domain, in which the anchor (and the pipeline) can penetrate. The seabed settlement and the displacement of soil particles due to anchor motion can be fully calculated. The FE Model includes the steel pipeline geometry and the external concrete coating, the surrounding water, the deformable seabed and the 3D anchor geometry with its chain. Scope of this assessment is to improve the understanding of the pipeline and anchor interaction event. In particular the focus is on the mechanisms and dynamics of accidental events which lead to the occurrence of anchor engagement with the pipeline. Eventually, the final goal is to refine and take into due account the contribution of accidentally dragged anchor scenario in the overall procedure for pipeline design against threats posed by commercial ship traffic. In this paper: • The state of the art about the classical analysis methodology recommended by standards to evaluate the interaction between the pipeline and the accidental dragged anchors is briefly described; • A FEM based analysis methodology is drawn and proposed to address the potential for pipeline hooking by the accidentally dragged anchor event; • A typical application is presented.