A significant proportion of the transmission pipelines in the United States was built before 1970. Not all girth welds in these pipelines were required to be inspected using non-destructive examination (NDE) techniques for the pipelines constructed before 1970. Large weld flaws occasionally remain undetected in these pipelines. Over time, as applied stresses on those welds evolve, such as in the case of changing pipe support conditions, welds with large flaws can fail in service. In some cases, leaks or ruptures occurred decades after construction.
The operational history...
The operational history of vintage pipelines has demonstrated that most vintage girth welds are safe for the intended service. This safety record can be attributed to at least two factors: (1) low stresses experienced by most girth welds in buried pipelines and (2) overall good integrity of girth welds due to reasonable toughness and, perhaps more significantly, weld cap reinforcement.
When vintage girth welds are subjected to modern construction acceptance criteria, a large proportion of those welds may not pass. However, it is not practical to replace those welds. More importantly, those welds could be perfectly sound for their intended service.
Since vintage pipelines account for a sizable portion of transmission pipelines, it is in the interest of all stakeholders to identify and mitigate high-risk factors before failures could occur. Fitness-For-Service (FFS) assessment is a preferred approach for evaluating the integrity of vintage girth welds. While inspections (e.g., X-ray) and associated acceptance criteria play a critical role in ensuring girth weld integrity, weld imperfections are only one of the contributing factors to weld integrity. FFS assessment takes all key factors into consideration.
FFS assessment requires material properties and flaw characteristics as inputs. Before this work, very little data of this kind were available in the public domain or to the companies that comprised this project team, presenting a formidable challenge for conducting FFS. The data generated in this project, in conjunction with the assessment procedures developed under PRCI project SIA-1-7, fill critical gaps in predicting the behavior of vintage girth welds. Cost-effective maintenance decisions can be made when integrity assessment can be performed with high confidence. For instance, many welds that fail modern X-ray criteria could pass FFS assessment, demonstrating integrity while avoiding unnecessary remediation actions.