Ice Gouging over a Buried Pipeline: Superposition Error of Simple Beam-and-Spring Models

Abstract

Two types of models, coupled and uncoupled, are currently used to determine the extent to which it is necessary to bury subsea pipelines deeper than the maximum expected depth of ice gouges. In the uncoupled model, the soil is modeled by nonlinear Winkler springs attached to the pipe at one end, with the subgouge displacement imposed at the other end of the springs. In coupled models, the soil is modeled as a three-dimensional (3D) continuum, simultaneously capturing the processes of gouging (with associated very large deformations) and the pipeline resisting the soil displacements. This paper pinpoints the main reason for differences in predictions between the coupled and uncoupled model. It is not the coupling errors (attributable to directional coupling between Winkler springs in the axial, lateral, and vertical directions, and slice-to-slice coupling), but, rather, the superposition error, which arises in the uncoupled model by adding the soil displacements attributable to the load the pipe exerts on the soil to the subgouge deformations, despite the strongly nonlinear behavior of the soil.