Review and discussion of strength mismatch of girth welds in high strength pipelines

Abstract

Pipelines may be subject to displacement-controlled external loads exceeding yield strength of materials due to many causes, e.g., soil settlements and ground movement. It is important to ensure that strength of girth welds even- or over-matches that of pipe steel to avoid strain concentration in the weld leading to a premature failure of a pipeline. Unfortunately, a number of high strength pipeline failure incidents primarily associated with weld strength under-match had been reported. Currently most standards do not explicitly specify strength match requirements for girth welds in pipelines and it is a challenge to achieve weld strength over-match in high strength pipelines. This paper provides a comprehensive review on the characteristics of strength mismatch, the difficulties faced in achieving weld strength over-match in high strength pipelines and the impact of weld strength under-match on structural integrity of pipelines. It also introduces the research, failure incidents, operation practice and the relevant standards/codes associated with weld strength mismatch in high strength pipelines in China. The paper starts by briefly reviewing the historic origin for weld strength requirements, different definitions for weld strength mismatch are then described, the acceptance requirements for pipe steel and girth welds in several major standards and codes are introduced and their inadequacies are discussed. This paper also discusses macro- and micro-scale weld strength mismatch, the influence of weld strength under-match on failure of girth weld and both design and engineering construction requirements for weld strength over-match. Possible mitigation measures to avoid strength under-match are finally proposed.