The influence of layered soil on ice-seabed interaction: Soft over stiff clay

Ice gouging is one of the main threats for the structural integrity of the offshore pipelines in the territories that floating icebergs can reach. The safe and cost-effective burial depth of pipelines for protection against the iceberg attack depends on accurate assessment of the subgouge soil deformation and the ice keel-seabed contact forces. Layered seabed soil strata that are found in many geographical locations adds to the complexity of assessing the ice keel-seabed interaction process by signifying the effect of strain rate dependence and strainsoftening in the soil layers interaction areas and ice-seabed contact zone. The existing design codes simplify the seabed condition by assuming a uniform soil strata. This in turn affects the accuracy and cost-effectiveness of the design. In this paper, large deformation finite element (LDFE) analysis was conducted using Coupled Eulerian Lagrangian (CEL) approach to investigate the free-field ice gouging in layered (soft over stiff clay) cohesive seabed. A modified Tresca soil model was coded in a user subroutine to account for the effects of strain rate and strain-softening on the mobilised undrained shear strength. A comprehensive parametric study was conducted to examine the effect of gouging features and soil shear strength profiles on the subgouge soil deformation, developed plastic shear strains and the resultant ice keel reaction forces. The analysis results showed that the seabed response to ice gouging is heavily dependent on the soil strata, where the layers interaction effects can override the usual response expected from a uniform seabed. Practical solutions were proposed to improve the pipeline design in layered seabed soils.