A study of mechanical integrity for unequal wall thickness transition joints in pipeline

Huo, Xiaotong (2016) A study of mechanical integrity for unequal wall thickness transition joints in pipeline. Masters thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (2MB)

Abstract

Unequal wall thickness transition joints are used in energy pipelines to connect straight pipe segments to thicker walled pipe sections such as cold bends and fittings. Due to the operational loads, changes in the pipe horizontal and vertical alignment, and variations in the soil type, axial loads and bending moments are generated along the pipeline. Through the wall thickness transition, stress concentrations develop due to the differences in pipe geometry, stiffness, material grade and mechanical strength. Current engineering practice and standards provide guidance on back-bevel design for wall thickness transitions. An alternative configuration, the counterbore-taper design recommended by TranCanada PipeLines, is intended to reduce stress concentration effect across the transition, facilitate welding processes, and improve NDT quality, productivity and reliability. Through a parametric study, using finite element methods, the relative mechanical response of the back-bevel and counterbore-taper wall thickness transition is evaluated. The numerical modelling procedures are verified with analytical equations and numerical simulations available in the public domain literature. The influence of element type, mesh topology, wall thickness mismatch (t₂/t₁), material grade on the limit load, pressure containment response, associated with the onset of plastic collapse, are evaluated. In terms of strength performance associated with stress concentration effects, the significance of element type, mesh topology, pipe diameter (D), pipe diameter to wall thickness ratio (D/t), wall thickness mismatch (t₂/t₁), material grade on the limit load, counterbore length, taper angle and radial girth weld offset (i.e., Hi-Lo) are examined. The improved performance of the counterbore-taper weld transition; relative to the back-bevel design as recommended by current practice, was demonstrated in this study through equivalent limit load capacity for pressure containment and reduction in the stress concentration factor for combined loading. The minimum counterbore length was found to be consistent with industry recommended practices, and was related to the pipe diameter and wall thickness mismatch. Guidance on the selection of joining methods to advance current engineering practice is provided.

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