Upheaval buckling is a potentially serious problem which can be encountered during the operation of buried oil and gas pipelines operating at high temperature and pressure. Temperature increase causes axial expansion, and if it is restrained by the axial resistance offered by soil or end fixity, the axial force might be relieved by upheaval buckling. The only resistance towards the upheaval buckling of the pipeline is the vertical uplift soil resistance. In the current industry practice (e.g. ALA 2005, PRCI 2009), this vertical soil resistance on pipelines is generally represented by vertical nonlinear springs defining both vertical uplift and vertical bearing parts. The force-displacement relationships for the vertical uplift spring developed in these guidelines are based on small scale laboratory tests and theoretical models. For this reason, the applicability of the equations used in these guidelines is limited to relatively shallow soil cover. Furthermore, the uplift soil resistance comes from the complex pipe-soil interaction behaviour based on the failure mechanisms of the surrounding soil. To analyze/design a buried pipeline against upheaval buckling, an in-depth understanding of this uplift pipe-soil interaction mechanism is, therefore, significantly important.
In this study, the detailed failure mechanisms of soil around the pipe during a pipe uplift in dense sand are investigated through advanced finite element (FE) analysis using Abaqus/Explicit FE software. Results show that both maximum and post-peak uplift resistances are dependent on the failure mechanism of the soil. The vertical inclination of the failure plane plays an important role on the uplift soil resistance. A simplified method is proposed to estimate the maximum uplift resistance for a wide range of pipe sizes and soil covers to define the vertical soil springs for dense sand.
Abstract
This study conducted finite-element (FE) modeling of uplift resistance from dense backfill sand. The prepeak hardening, postpeak softening, density, and confining-pressure-dependent soil behavior were implemented in FE analysis to simulate the progressive development of shear bands. The location of the shear bands was identified from soil failure mechanisms for a range of burial depths. For shallow buried pipelines, the inclination of the slip planes to the vertical was found to be approximately equal to the maximum dilation angle when the peak uplift resistance was mobilized and then decreased with an upward movement, resulting in a postpeak reduction of uplift resistance. For deeper conditions, in addition to two inclined slip planes, logarithmic spiral-type shear bands formed above the pipe. Based on mobilized shear strength parameters and inclination of slip planes, a method to calculate the peak and postpeak uplift resistances, using an equivalent angle of internal friction, is presented.
