Centrifuge modelling of large soil deformation due to ice scour

Lach, Paul Richard (1996) Centrifuge modelling of large soil deformation due to ice scour. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (71MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

Ice scouring occurs when drifting ice masses impinge upon and move through seabed sediments. It is a prevalent phenomenon over vast areas of the continental shelf regions of the Arctic Ocean and the Canadian east coast. This ice - seabed interaction represents a critical design consideration for marine pipelines associated with the production of offshore hydrocarbon reserves. Pipelines must be designed to accommodate loading transmitted by scour - induced soil deformation below the depth of ice intrusion in a safe and cost - effective manner. A rational design methodology must therefore incorporate a deterministic model which provides reliable predictions of the magnitude and extent of the soil displacements generated during a scouring event. -- This dissertation describes experimental and numerical investigations undertaken to gain a better understanding of ice scouring effects on seabed soil. The experimental programme comprised a series of nine centrifuge model tests. In each test, an idealized scouring condition was simulated in an instrumented specimen of saturated clay to permit measurements of the stress and deformation Fields developed in the soil, the contact pressures and resultant forces acting on the model ice feature, and the qualitative effects of scour on buried model pipeline segments. Test variables included the prescribed soil stress history, the attack angle and width of the model keel, and the scour depth attained for steady - state conditions. Centrifuge modelling was established as a valuable tool by which to obtain insight into the mechanics of the ice - soil interaction, and yielded quantitative data applicable to well - defined events. Soil displacement measurements exhibited variation which was dependent upon the initial state of the soil, and was also influenced by differences in applied stress paths resulting from changes to boundary conditions in individual scouring events. -- The finite element method was evaluated as a means for prediction of the soil response under idealized scouring conditions. The adopted two - dimensional numerical representation incorporated a finite strain formulation and the soil was modelled as a two - phase nonlinear elastic - plastic material. Preliminary verification of the numerical approach was provided through comparison of the analysis results with data acquired in representative centrifuge tests. Appropriate characterization of the effects of scouring required simulation of large movements associated with steady - state conditions, which imposed constraints on implementation of the analysis and discretization of the soil domain. The numerical representation provided adequate approximation of the effects of scouring for compressible soil behaviour, where an event was characterized by continuum distortion or flow, and volume change due to loss of material in the scour path was balanced primarily through compressive deformation beneath the incision.

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