ABSTRACT

In ice gouging analyses, the seabed is usually represented by a uniform material domain ignoring the complexities and implications that may arise from layered seabeds that are quite common in many of the Arctic geographical locations. In this study, the response of different layered seabed comprising soft over stiff clay, stiff over soft clay, and the loose and dense sand over soft and stiff clay to the ice gouging were investigated by performing large deformation finite element (LDFE) analysis using a Coupled Eulerian Lagrangian (CEL) algorithm. The study showed the significance of considering seabed layers in ice gouging analysis.

INTRODUCTION

Floating icebergs or ice ridges during their long trip from Arctic oceans reach the shallow waters and start scouring the sea bottom by the ice keel tip. This is called "ice gouging", an Arctic seabed geohazard that may jeopardize the structural integrity of subsea pipelines. Pipelines are buried below the deepest potential gouge depth for physical protection. However, due to the shear strength of the soil, the subgouge soil displacement may significantly extended deep through the soil. Determining the optimum burial depth for protection against iceberg attack is a challenging design aspect of Arctic offshore pipelines. The pipe response to ice gouging is currently determined by a decoupled approach in engineering practice. For this purpose, first, a free-field (with no pipeline) ice gouging analysis is conducted using continuum large deformation finite element analysis (LDFE). Then the obtained subgouge soil deformations are transferred to the end of a set of springs connected to a simple beam-spring model to capture the pipeline structural response (Woodworth-Lynas et al., 1996; Phillips and Barrett, 2012). Although the accuracy of this approach suffer from the superposition of idealization and directional load decoupling as two sources of errors (Konuk and Gracie, 2004; Nobahar et al., 2007b; Lele et al., 2011; Peek and Nobahar, 2012; Phillips and Barrett, 2012; Eltaher, 2014; Pike and Kenny, 2016), but this is still cost-effective solution that compromise some level of accuracy and is followed by the pipeline industry. The accurate simulation of free-field ice gouging can have a significant impact on the ultimate results in the decoupled method. In free-field ice gouging analysis, the seabed stratum is usually simplified by uniform soil. This can be a gross simplification in the areas with complex layered seabed strata. Modeling the ice gouging in layered seabed such as soft over stiff clay, stiff over soft clay, and sand over clay has not been sufficiently explored in the literature while such a non-uniform soil strata have been broadly observed in offshore Arctic areas with lots of gouging signatures (e.g., Chukchi Sea (Winters and Lee, 1984; C-CORE, 2008); Alaskan Beaufort Shelf (C-CORE, 2008); Russian Sakhalin Island (C-CORE, 1995e) etc.) (see Figure. 1).

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