Strength matching factor of pipeline girth weld designed by reliability method

Abstract

Crack-like defects frequently form in pipes exposed to aggressive environments or during welding fabrication. Appropriate strength matching factors should be determined for welding process evaluation based on the reliability aspect. In this paper, the current research status of codes and standards related to the strength requirement of weld metals in the pipeline industry were reviewed. The results showed that almost all standards considered the lower limit of pipe tensile strength as a requirement for weld strength evaluation for specimens of cross-weld test breaking at the weld position. This was contradictory to the original intention of even-match or over-match of pipeline girth weld. The Monte Carlo Method was then employed to investigate the evolution of girth weld strength matching factor and pipeline failure probability under different high-low misalignments, fracture toughnesses, and standard deviations of pipe strength distribution. The data revealed significantly reduced probability of pipe failure as the weld metal strength matching factor increased. At the same strength matching factor, the failure probability of pipe decreased as fracture toughness of girth weld. The failure probability of pipe also declined as the misalignment and standard deviation of pipe strength distribution diminished. Based on the requirement of certain target reliability and strain demand, a semi-empirical prediction model of critical strength matching factor was proposed for engineering applications. The analysis of parameter sensitivity showed the relative importance of several parameters on the critical strength matching factor of girth weld, including high-low misalignment, standard deviation of pipe material strength distribution, and fracture toughness.