Modeling Cyclic Embedment of Deep-Water Pipelines Using Large Displacement Limit Analysis

Abstract

The as-laid embedment is a key design element of deep-water pipelines, as it significantly affects the longitudinal and transverse pipe–soil interaction and thermal insulation. Due to the small-amplitude oscillations driven by the sea state as well as lay vessel motions, the as-laid embedment is significantly greater than the predictions derived from static penetration analyses. This paper presents a numerical study into the cyclic embedment behavior of deep-water pipelines in soft clay using a recently developed sequential limit analysis (SLA) technique. It directly models the evolution of model geometry and material properties through a consecutive series of small displacement analyses and incorporates the effect of strain softening and strain rate on soil strength. A rigid plane-strain pipe section is pushed-in-place and then subjected to small-amplitude cyclic lateral displacements, with an accumulated lateral pipe displacement of up to 60 diameters. Careful comparisons between the numerical results with published centrifuge data are provided to demonstrate the robustness of SLA, in terms of pipe invert trajectory and soil resistance, followed by an extensive parametric study investigating the effect of strain rate, strain softening, oscillation amplitude, and loading history. More detailed aspects of the cyclic loading behavior are discussed with reference to soil failure mechanisms and bearing capacity yield envelopes. A strong correlation between accumulated lateral pipe displacement and pipe embedment is identified, regardless of the varying oscillation amplitudes, and soil buoyancy is found to play the most significant role in determining the final pipe embedment. The present study aims to assist the development of a macro-element plasticity pipe–soil model that can be incorporated into simplified three-dimensional (3D) pipeline–seabed models, as well as to promote the application of SLA to model complex large-displacement pipe–soil interaction problems.