A Coupled Eulerian Lagrangian Finite Element Model of Ice-Soil-Pipe Interaction Basel Abdalla, Kenton Pike, Ayman Eltaher, Paul Jukes Advanced Engineering Group, J P Kenny Houston, TX, USA ABSTRACT In ice environments, pipelines must be buried to provide protection from the keels of gouging ice features. A pipeline must not only be buried to avoid contact, but also to mitigate the effects of strains induced by soil displacement below gouge level. Previous studies have shown that the pipe should be buried in an intermediate zone below the scour depth and above the zone where soil only deforms elastically. There remains, however, uncertainty in the extent of this intermediate zone and in the determination of an economical minimum burial depth with acceptable risk. In this paper, the current state of the art, the Coupled Eulerian Lagrangian (CEL) method was adopted to make further advancements in ice gouge numerical modeling and to solve some of the uncertainty regarding pipeline burial depth. A three-dimensional (3D) finite element (FE) model was developed using the CEL formulation in ABAQUS, providing direct and explicit estimation of pipe stresses and strains. Simulation results from the developed model were validated by comparing the free field subgouge soil displacement to measured centrifuge test data and to results of FE models developed by others. Preliminary study of pipeline response to the forces generated by subgouge soil displacement was then presented. Trends were established between pipeline burial depth and pipeline strain for varying pipeline D/t ratios. KEY WORDS: ice gouge; ice scour; arctic pipeline; Coupled Eulerian Lagrangian, finite element methods, local buckling, limit states. INTRODUCTION The future of the oil and gas industry will see frontier oil and gas developments in Arctic regions. According to a recent report by the U.S. Geological Survey (USGS) the total mean undiscovered conventional oil and gas resources of the Arctic are estimated at approximately 90 billion barrels of oil, 1,669 trillion cubic feet of natural gas, and 44 billion barrels of natural gas liquids. These resources account for approximately 22% of the undiscovered, ‘technically recoverable’ resources in the world, where technically recoverable means able to be produced using currently available technology. It is further estimated that approximately 84% of the undiscovered oil and gas occurs offshore (USGS, 2008). The inevitable exploitation of these resources is driving a rush of technology development to meet the challenges presented when working and operating in such a harsh environment. Examples of key challenging characteristics that impact the oil and gas operations in the arctic may be found in Abdalla et al. (2008). One phenomenon that poses a significant threat to submarine pipelines in the arctic is ice gouging or ice scour. A pipeline must not only be buried to avoid contact, but also to mitigate the effects of strains induced by soil displacement below gouge level. Previous studies have shown that the pipe should be buried in an intermediate zone below the scour depth and above the zone where soil only deforms elastically. Despite the work done to date, there remains uncertainty on the magnitude and extent of subgouge soil deformations, and thus, uncertainty in target burial depth requirements, which directly impacts development cost and safety. Over the last two decades, the ice scour problem has been tackled using different means based on the available tools at the time. Such studies include small- and medium-scale experimental testing, analytical and empirical formulations, simplified structural analyses, and advanced numerical techniques. Konuk et al. (2007) reported several challenges in numerical modeling of ice gouge and pipeline response, such as proper understanding and modeling of the ice indenter, determination of initial geometry of the ice feature that gouges the seabed, selection or development of a constitutive soil model, selection of discretization method, definition of the interface processes, and contact mechanics between the ice ridge and seabed. Nevertheless, numerical formulations have proven the ability to model fully coupled ice/soil/pipe interactions with reliable results (Palmer et al., 2005). Different numerical formulations have been used to model the gouging problem, such as pure Lagrangian, Updated Lagrangian (UL), pure Eulerian, mesh-free, Arbitrary Lagrangian Eulerian (ALE), and others. The principle advantage of using a purely Lagrangian formulation is the precise definition of the interface between the ice ridge and subsoil (nodes are fixed within the material). However, this technique provided limited success due to mesh distortion and convergence problems as reported by Woodworth-Lynas et al. (1996). The use of Lagrangian analysis with re-meshing produced errors due to projecting the nodal variables to the nodes of new mesh (Konuk et al., 2005). To improve numerical modeling the ALE method was used. Examples of FE models using the ALE approach were presented in Kenny et al. (2004), Konuk et al. (2005), Nobahar et al. (2007) and others.