Nonlinear finite element analysis of upheaval buckling of buried offshore pipelines in medium dense sand with fines

Abstract

Offshore pipelines that transport oil and gas in different areas of the world are often buried in trenches to provide stability and protection against upheaval buckling. Hydrocarbons in the pipeline are transported at high temperatures and pressures to facilitate flow and prevent potential solidification. Such conditions of transport, however, generate increases in axial compressive forces in the pipeline, which may lead to upward buckling (in the direction of the least soil resistance). Upheaval buckling can result in pipeline failure causing severe environmental and economic losses. The potential for upheaval buckling is mitigated by the resistance of the soil overlying the pipeline. This resistance is a function of several parameters. In this study, a series of 3D displacement-controlled finite element analyses were conducted using AbaqusTM to investigate the effects of pipeline diameter, pipeline embedment depth, and fines content on the soil resistance against uplift buckling. In the analyses, the offshore pipeline was assumed to be buried in medium dense sand with fines and was pulled upward along its entire length to simulate plane-strain conditions. The response of the pipeline was: (1) studied through the variation of the normalized uplift soil resistance with respect to normalized pipeline displacement, and (2) compared to the predicted response from available analytical resistance models. The results show that the uplift resistance, along with the normalized mobilization distance, depends on the pipeline diameter, embedment soil depth and fines content. Moreover, analytical design methods which assume mobilized soil blocks with inclined slip surfaces better capture the behavior observed in the numerical models, when compared to methods in which vertical slip surfaces are considered.