Numerical modeling of seabed ice scour

The integrity of subsea pipelines in the Arctic regions are threatened by ice-related subsea geohazard. A common practice for physical protection of the pipelines against the ice loads is to burying them inside the subsea trenches. However, determining the minimum burial depth of the pipeline to minimize the construction cost is a challenging design aspect of Arctic offshore pipelines. This requires an in-depth understanding of the ice-soil-pipe interaction, which in turn is significantly affected by ice-soil interaction and the foucuse of this thesis.
Mohr-Coulomb soil model is conventionally used for continuum modeling of dense sand by adopting the constant friction and dilation angles. However, this approach neglects the pre-peak hardening and the post-peak softening behavior of dense sand. In this study, two smart self-correcting soil models were incorporated into an advanced Coupled Eulerian-Lagrangian (CEL) analysis to automatically update the shear strength parameters by the magnitude of plastic strains.
The analysis was conducted using ABAQUS/ Explicit scheme incorporating the soil model that was coded into a user-defined subroutine. The soil strength parameters are self-corrected to model the nonlinear hardening, softening and pressure dependency behavior of dense sand by considering the ice keel bearing pressure and octahedral shear strain. The pre-peak hardening, and post-peak softening behavior of dense sand were captured through a series of free-field ice gouging analysis. Tow comprehensive parametric studies were conducted, one investigated the ice properties and anther one was foucued on the soil parameters.

The response of dense sand to the ice gouging event

Ice Scour is a phenomenon that occurs when ice comes into contact with seabed. Ice Scour has been reported to have caused damage to communication cables and water intake pipelines. It will be very important to acknowledge the mechanisms. We have performed a series of small-scale (1-g) model experiments. Furthermore, we performed a series of medium-scale model experiments on ice scour events at a sandy beach in order to try to determine the effect of scale. The model ridge-keel was placed in the sea and pulled at a constant velocity toward the shore by a crane, and the bulldozing force and the motion of the model keel were measured, Furthermore, a model pipeline to which a strain gauge had been attached was buried at a depth of 15-20 cm from the predicted scour surface, and stress and strain acting on the model pipeline during a gouging event were measured. We showed our mechanical model could be applied to scour events not only on a small scale but also on a medium scale. We also discuss similarity law between medium-scale test results and small-scale test results, stress and deformation acting on the model pipeline.

Medium-scale model tests on ice scour event

Large areas of the continental shelf of northern oceans are frequently scoured or gouged by moving bodies of ice such as icebergs and sea ice keels associated with pressure ridges. This phenomenon presents a formidable challenge when the route of a submarine pipeline is intersected by the scouring ice. It is generally acknowledged that if a pipeline, laid on the seabed, were hit by an iceberg or a pressure ridge keel, the forces imposed on the pipeline would be much greater than it could practically withstand. The pipeline must therefore be buried to avoid direct contact with ice, but it is very important to determine with some assurance the minimum depth required for safety for both economical and environmental reasons. The safe burial depth of a pipeline, however, cannot be determined directly from the relatively straight forward measurement of maximum scour depth. The major design consideration is the determination of the potential sub-scour deformation of the ice scoured soil. Forces transmitted through the soil and soil displacement around the pipeline could load the pipeline to failure if not taken into account in the design. If the designer can predict the forces transmitted through the soil, the pipeline can be designedmore » to withstand these external forces using conventional design practice. In this paper, the authors outline a design methodology that is based on phenomenological studies of ice scoured terrain, both modern and relict, laboratory tests, centrifuge modeling, and numerical analysis. The implications of these studies, which could assist in the safe and economical design of pipelines in ice scoured terrain, will also be discussed.« less

M.J. Paulin

Papers

Development of a design methodology for pipelines in ice scoured seabeds