Numerical modelling of ice-seabed interaction in layered seabed

hashemi, Seyedhossein (2022) Numerical modelling of ice-seabed interaction in layered seabed. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Traveling icebergs may threat the structural integrity of the offshore pipelines in any territories that they can reach. Arctic offshore pipeline are usually buried for physical protection against ice gouging. Burying the pipeline in a trench immediately deeper than the maximum-recorded ice gouge depth is not sufficient to ensure the safety of the pipeline. The subgouge soil displacement is not limited to the depth of the ice keel tip. The shear resistance of the seabed soil causes the subgouge soil deformation to extend much deeper than the ice keel tip. This, in turn, threats the subsea pipelines and mandates achieving a sufficient burial depth to ensure the structural integrity of the pipeline. Finding the best burial depth satisfying the safety and economical consideration is a challenging aspect. In practice, a decoupled approach is usually undertaken by engineers, where, first, a continuum large deformation finite element analysis of free field ice gouging process is conducted. Then, the results of subgouge soil deformations are transferred to a simplified beam-spring model to obtain the structural response of the pipeline. Therefore, the free-field ice gouging analysis is a key part of the design practice. However, 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 (e.g., Chukchi Sea (Winters and Lee, 1984; C-CORE, 2008), Alaskan Beaufort Shelf (C-CORE, 2008), Russian Sakhalin Island (C-CORE, 1995e), etc.). Therefore, there exists a knowledge gap in the literature digging in to the ice gouging event in a complex non-uniform layered soil strata. 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 CEL allows the material to flow through the Eulerian fixed mesh and supress the mesh distortion and numerical instability issues. Despite the conventional studies that typically consider a uniform soil strata with an elastic perfect plastic seabed soil material obeying von Mises or Tresca criterion, the current study incorporated the strain rate and strain softening effects of the cohesive soil as well to improve the accuracy of the predictions. This was conducted by coding the modified soil model into a user-defined subroutine (VUSDFLD) and linking to the main model. Strain rate dependency and strain softening during shearing and remoulding are two natural behaviours of cohesive soils affecting the value of soil undrained shear strength. It is generally agreed that increasing shear strain rate leads to increase in undrained shear strength. Also, within large shear strains, the strain softening causes a gradual loss of shear strength. Including these effects in the current study improved the accuracy of simulating of ice gouging process in layered seabed as a high velocity geotechnical problem involving large deformations. During the analysis, the user subroutine is incrementally called by ABAQUS to update the undrained shear strength of the soil based on the incremental values of the currently accumulated absolute plastic shear strain and the calculated maximum shear strain rate with zero value adopted for friction and dilation angles. The performance of the modified soil model was verified through comparisons with published experimental studies and conventional soil models. Comprehensive parametric studies were conducted to examine the ice keel-seabed interaction in a range of layered seabed with different configurations. The effect of different input parameters including the ice keel geometry, gouge depth, seabed soil strength, and layering condition on the keel reaction forces, subgouge soil deformation, the side berm and frontal mound formations, and the progressive plastic shear strain distribution were examined through a large number of case scenarios. It was observed that replacing a layered seabed with a uniform seabed for simplicity could be significantly misleading resulting in non-reliable subgouge soil deformation magnitudes and keel reaction forces. The study showed that an interactive mechanism between the different soil layers might significantly affect the soil failure mechanism near the ice keel and cause unexpected subgouge soil deformations in underlying layers. The new findings of the conducted study are quite significant in terms of practical considerations for pipeline design. Based on the observations, several practical recommendations were made to improve the safety of the Arctic pipelines to be buried in complex layered seabed soil strata.

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