Document Type

Thesis - Open Access

Award Date

2020

Degree Name

Master of Science (MS)

Department / School

Mechanical Engineering

First Advisor

Zhong Hu

Keywords

ANSYS, Deformation, Displacement, Failure, Pipeline, Strain-based

Abstract

Landslide displacement is one of the major threats to the structural integrity of buried oil and natural gas pipelines that are often located far from major markets with terrains prone to permanent ground deformations. These pipelines can experience large longitudinal strains and circumferential deformation resulting from the differential ground movements thereby potentially impacting pipeline safety by adversely affecting structural capacity and leak tight integrity. In order to proffer theoretical basis for the design, safety evaluation and maintenance of pipelines, the failure analysis and mechanical behavior of buried API X65 steel pipeline perpendicularly crossing landslide area was investigated with the Finite Element Method (FEM), considering soil – pipeline interaction using the strain-based approach in this thesis. The soil – pipe interaction system was rigorously modeled through finite elements using the ANSYS Parametric Design Language (APDL) Mechanical Finite Element (FE) software, which accounts for large strains, displacement, non-linear material behavior and special conditions of contact and friction on the soil – pipe interface. Various diameter – thickness ratios (D/t-96 and D/t-72) pipeline models was used. This thesis focuses on the influence of various soil and pipeline parameters on the structural response of the pipeline, with particular emphasis on identifying pipeline failure (excessive longitudinal strains). The influence of soil strength and stiffness, and internal pressure on the structural response was also examined. Furthermore, a comparison of the conventional stress-based design approach versus strain-based approach was made. The results show that there are two high strain areas on the buried pipeline sections where the bending deformations are bigger. The maximum strains on the pipeline were mostly tensile at the maximum soil displacement of 0.5 m in the deformation process. The compressive strains resulted in local buckling of the pipeline. Buried pipeline in the landslide bed with hard soil (non-cohesive) is more prone to failure. The biggest deformations appear on the pipeline sections that are on either side of the interface between the sliding soil and the stable surrounding soil at around 20 m and 16 m, respectively. The maximum displacement of the pipeline is smaller than the landslide displacement due to soil-pipe interaction. Bending deformations and tensile strain of the pipeline increase with landslide displacement increase. An increase in the soil’s elastic modulus, cohesion (changing the soil from cohesive to non-cohesive) and diameter thickness (D/t) ratio of the pipeline resulted in increased bending deformation and tensile strain of the pipeline. Comparing stress-based to strain-based analysis of the pipeline showed that the stress-based approach is more conservative and attained the yield limit over two times earlier in the deformation process when compared to the strain-based approach which maximizes the plastic and ductile properties of steel pipes under landslide displacement. The strain limit of ε_(x,max) ≤ 2% is in the strain-based approach in accordance with the strain-based design codes of DNV-OS-F101 (2000), CZA-Z662-07 and ASCE (2005). The results are presented in diagrams, tables, and plot curves form.

Library of Congress Subject Headings

Underground pipelines.
Failure analysis (Engineering)
Finite element method.
Landslide hazard analysis.

Format

application/pdf

Number of Pages

238

Publisher

South Dakota State University

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Rights Statement

In Copyright