Mechanical Integrity Evaluation of Unequal Wall Thickness Transition Joints in Transmission Pipeline

Abstract

Transition welds joining pipe segments of unequal wall thickness are typically designed using back-bevel butt welds in accordance with industry recommended practices. An alternative approach, for joining transition pieces, would be the use of a counterbore-taper design, which has been successively utilized by TransCanada PipeLines. In comparison with the back-bevel joint design, the counterbore-taper design provides a simple geometry that facilitates the welding process for joints of unequal wall thickness, improves the NDT quality and reliability, and increases the process efficiency for welding and NDT tasks. The counterbore-taper design reduces the effect of stress concentrations at the weldment and enhances fatigue life. A parameter study, using continuum based finite element methods, was conducted to comparatively examine the mechanical performance of a pipe joint, using back-bevel and counterbore-taper designs, with unequal wall thickness and different material grade. The parameters examined include pipe diameter, D/t ratio, axial force and moment. The numerical study assessed the mechanical stress response, including stress path, initial yield and onset of plastic collapse, for back-bevel and counterbore-taper joint designs. Based on these preliminary investigations, the performance of each transition joint design was evaluated and guidance on the selection of the joints design method was provided.