Plastic Collapse Assessment of Unequal Wall Joints in Pipeline Transitions

This paper presents the results of extensive numerical and analytical analyses considering the many differences in the flow properties of today’s steels with a view to determine if the code design basis developed for early steels remains appropriate in light of these changes. These analyses involved parametric study of steel grade, tensile strength, yield-to-tensile (Y/T) strength ratio, and joint geometry, for a range of transitions within as well as beyond current ASME code allowables. The numerical results indicate that the plastic-collapse failure conditions of unequal wall joints are controlled by the pipe or fitting remote to the weld, as would occur for high-quality slightly over-matched welds. Mismatch location, taper angle and anisotropy of unequal wall joints have limited influence on such failures. Based on trends in these results, a closed-form plastic collapse solution to predict internal pressure was developed as the pipe geometry, material hardening and tensile properties for both end opened and end capped pressurized pipes in reference to deformation instability, finite strain theory and deformation theory of plasticity. A plastic collapse criterion and the corresponding plastic collapse assessment diagram (PCAD) were then developed as a function of the wall thickness and tensile strength mismatch conditions to ensure plastic collapse failure in the thinner-wall, higher strength line pipe. General procedures to use the PCAD are outlined in this paper. Application of PCAD indicates that the high yield strength grades with high Y/T can be used within as well as beyond current code limitations on the transition wall-thickness mismatch for a wide range of strength mismatch.