Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Civil and Environmental Engineering

Supervisor

Wenxing Zhou

Abstract

Pipelines are the safest and most efficient way to transport large volumes of oil and gas from extraction fields, to refineries, industry and home consumption. Extensively used to transport fluids over long distances, pipelines may pass through terrain features exposed to geohazards. The performance of buried pipelines in areas subjected to ground displacements constitute a criterion for design, assessment and management of gas pipelines, to ensure public, environment and property safety in a cost-effective manner. Modern surveying and sampling techniques allow for better geotechnical characterization of ground movements and the variability of the soil properties with confidence. The statistical data enables reliable models to correlate the inspection measurements with the overall safety of the buried pipelines.

Random field theory is widely used to model spatial variability of soil properties that affect the probability of failure of pipelines. A limit state for onshore gas pipelines laid down over hill-type features is Upheaval Buckling (UHB). In this study, the spatial variability of the soil properties is considered in a simplified manner. The soil correlation structure is idealized as a multivariable cross-correlated Gaussian random field. A parametric example illustrates the impact of the spatiality variability of the soil on the failure probability due to UHB and the applicability of simple empirical equations to account for the spatial variability of soil. The analysis results suggest that accounting for the soil spatial variability of the soil may lead to less overconservative estimation of failure probability due to the risk of UHB.

A practical approach to analyze probability of failure of pipelines susceptible to landslides is presented. Soil displacements can impose significant loads to pipelines, which might eventually result in the failure of pipelines along unstable slopes. A simple procedure to estimate tensile rupture and compressive local buckling is presented. The soil is characterized as random field. The probability of failure is obtained by numerical simulation, a particular critical slip surface is considered at each realization.

The research objective is to understand the effects of the soil spatial variability on the overall safety of pipelines laydown over hill-type imperfections. And pipelines susceptible to landslides.

Summary for Lay Audience

Pipelines are considered the safest and most efficient way to transport large volumes of oil and gas from extraction fields to refineries, industry and home consumption. Thus, pipelines are critical infrastructure and have been used extensively. The high efficiency of pipelines can be explained by the low energy cost of pumping pressurized fluids with reduced viscosity at high temperatures over long distances. Buried gas pipelines usually transport fluid at elevated temperatures to optimize the productivity of the wells. Due to these conditions, an overall compressive force is induced along the pipes. This compression, may cause the pipeline to buckle upward or even break out of the ground if the soil cover or restraining measures are not sufficient.

Additionally, out-of-straightness imperfections or geometric features along the line topography can further reduce the buckling resistance of the pipeline. Trenched or buried pipelines are design to protect the pipe from external actions and ensure structural stability. The current state of the art allows for estimations of the critical axial buckling force considering different types of soil, pipe materials and the pipeline geometric conditions.

Different from engineering materials, natural soil deposits have mechanical properties that vary orders of magnitude more. This uncertainty has to be taken into account to assess the optimal operation of pipelines in a safe and efficient manner. The main objective of this research is to investigate the effects of the soil inherent variability on the stability of buried pipelines.

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