Failure Assessment for Girth Weld Defects of Pipeline

This paper studies some key issues of fitness-for-service assessment for pipelines constructed in 1970s in Northeastern China, which were found to have large amount of weld defects resulting in leakage and rupture accidents. The mechanical behaviors of tensile strength, Charpy V-notch impact energy and fracture toughness are tested for the spiral weld metal and pipeline steel, showing that the state standards of steel for pressure vessel are still met after serving for more than 30 years. The safety limit line of the failure assessment diagram (FAD) is derived according to BS7910: level 2B assessment, based on the obtained stress-strain curve of weld metal. The chemical composition of pipeline steel analyzed by the energy spectrum method indicates the pipeline was made of 16Mn steel. The metallographic examination reveals that the metallurgical structure of weld metal was composed of ferrite and pearlite with five different zones. The morphology of tensile fractured surface shown by ductile dimples indicates the tensile fracture is of ductile type, which implies the weld metal and pipeline steel after long time service have not yet become brittle. The stress magnification factor for the bulge effect of through-wall girth defect is extracted from the stress intensity factor evaluated by the finite element simulation method, indicating the applicable ranges of Kastner solution and Schulze et al solution. The stress magnification factor caused by the bending stress of the misalignment imperfection of girth weld joint is calculated by FEM to review the applicability of relevant formulas given in BS7910 for engineering critical assessment. Finally, it is concluded by FE simulation that though the stress magnification effect of shallow cracks in weld toe zone is significant, it may be insignificant in its fracture failure assessments.