Pacific Gas and Electric Company (PG&E) examinations exposed vintage girth welds during maintenance opportunities.
Engineering critical assessment
(ECA) was applied on those that failed workmanship criteria. The ECA
considers the tolerance in defect measurement, operating stress, and other stresses at specified locations, such as a road crossing, near an elbow, thickness transition weld, and pipe axial bending.
Pacific Gas and Electric Company (PG&E) opportunistically examined and inspected fully exposed girth welds in pipelines that were excavated for direct examination or maintenance activities. The objective of the opportunistic inspections was to verify the quality of the exposed girth welds as well as to determine the accuracy and quality of prior radiographic inspection of girth welds.
Girth weld failures are infrequent occurrences within the US natural gas pipeline network. Major girth weld failures are typically precipitated by unusually large external loads acting on an individual weld containing some sort of imperfection that is usually not minor. The ability of a weld to withstand applied loadings is related to:
- the presence and size of imperfection;
- the weld’s inherent strength, ductility, and fracture toughness; and
- the magnitude of the applied load.
The standard for girth weld workmanship quality is API 1104 , which provides protocols for qualifying welding procedures, qualifying welder performance, qualifying weld inspection procedures, and evaluation of welds made in production. From historical experience, small imperfections are understood not to degrade a weld’s ability to safely tolerate expected loads acting on the pipeline under usually encountered conditions. Accordingly, API 1104 allows some imperfections to remain in a weld if they are not overly large or numerous. The specific allowable size limits of imperfections have evolved over time and depend on the types of imperfections. The standard criteria for workmanship quality in API 1104 were developed to promote welding of sufficient quality for most situations while maintaining a high rate of productivity.
- development of alternative quality acceptance standards for new or existing welds not meeting the conventional criteria for quality;
- development of quality specifications for welds in pipelines expected to experience unusual loadings; or
- development of load or strain limits in recognition of specific weld properties and quality.