Main characteristics and safety assessment of girth welds in high grade steel pipelines

Abstract

Girth weld is an indispensable part of long-distance pipelines. Girth weld failure is caused by the interaction of load, defects, local fracture toughness, strength matching, weld geometry and residual stress, and the welding for high steel grade pipelines is particularly difficult which increases the failure possibility of girth welds. In this case, the load, defect and strength matching characteristics of girth weld were analyzed, the strain concentration mechanism of low matching girth welds or softening girth welds in heat affected zone was revealed, and four failure modes including brittle fracture, net section collapse before yield, net section collapse after yield and base metal necking of girth weld were explained. Based on the comparison of the applicability of girth weld safety assessment in GB/T 19624—2019、BS 7910-2015、API 579-1/ASME FFS-1-2016、API 1104-2013 and other domestic and foreign standards, the strain-based girth weld safety assessment method was proposed, and the influence of crack size, strength matching coefficient and other factors on girth weld crack driving force was discussed. The research demonstrates that the load or displacement of the girth weld should be the premise of the safety assessment, and improving the strain capacity of the girth weld can enhance the adaptability of high grade steel pipelines to adverse geological conditions. It is suggested to carry out the research on the initiation and growth mechanism of the girth weld crack defects, develop the strain-based fracture theory, optimize the strength matching of the girth weld, and quantify the relation between the strain capacity of the girth weld and the geometric and mechanical performance parameters of the weld.

Keywords

• high grade steel
• girth welds
• strength matching
• failure mode
• strain concentration