Small-diameter, low-pressure pipelines (operated at pressures below 40 percent of specified minimum yield strength [SMYS]) are unlikely to experience a rupture but may be vulnerable to leaks. In general, gas leaks result in less significant consequences to public safety than ruptures; however, under certain circumstances leaks may represent a significant threat. In particular, leaks resulting in belowground gas migration from low-pressure pipelines to a nearby confined space (e.g. the basement of a nearby structure to which the pipeline is not physically connected) may result in significant consequences as accumulation of gas to flammable or explosive levels may be possible. This paper explores the methods by which an Operator may address the threat of gas migration from leaks on low pressure pipelines in its risk assessment and leak management programs.
Research was conducted into the specific conditions that contribute to belowground gas migration and included: a literature review of studies into gas migration phenomena, and a review of historical incidents within the United States on similar pipeline systems which resulted in significant property damage, injuries, or fatalities. The research included consideration of the effect of pipeline operating conditions, pipeline attributes (including diameter and depth of cover), leak duration, proximity of nearby structures, attributes of nearby structures (including presence of basements, slabs, and openings in the foundations), leak severity (e.g. pinhole, full separation), ground conditions (including soil permeability and presence of belowground features), and gas odorization. Key conditions impacting the potential for belowground gas migration are identified, and risk assessment methods and leak management practices are recommended based on the likelihood of migration.
Based on the research conducted and the environmental conditions identified, a quantitative risk assessment approach, taking into account the likelihood of a leak on a low-pressure pipeline, the likelihood of belowground gas migration, and the potential consequences to public safety, is proposed.