Safety Effects of Less Prescriptive Requirements for Low-Stress Natural Gas Transmission Pipelines Are Uncertain

GAO-12-389R: Published: Feb 16, 2012. Publicly Released: Feb 16, 2012.

Additional Materials:


Susan A. Fleming
(202) 512-2834


Office of Public Affairs
(202) 512-4800

What GAO Found

Applying PHMSA’s new distribution integrity management requirements to low-stress transmission pipelines would result in less prescriptive safety requirements for these pipelines. Overall, requirements for distribution pipelines are less prescriptive than requirements for transmission pipelines in part because the former operate at lower pressure and pose lower risks in general than the latter. For example, the integrity management regulations for transmission pipelines allow three types of in-depth physical inspections. In contrast, distribution pipeline operators can customize their integrity management programs to the complexity of their systems, including using a broader range of methods for physical inspection. While PHMSA officials stated that “less prescriptive” does not necessarily mean less safe, they also stated that distribution integrity management requirements for distribution pipelines can be more difficult to enforce than integrity management requirements for transmission pipelines. Currently, PHMSA can grant special permits to modify requirements for individual pipelines, if merited, but applying the new distribution integrity management requirements to low-stress transmission pipelines would affect all such pipelines.

If PHMSA’s requirements for low-stress transmission pipelines changed, operators whose systems consist almost entirely of distribution pipelines and include only a short low-stress transmission pipeline segment could benefit because they would be subject to one set of integrity management requirements instead of two. This might allow them to apply more resources to other safety priorities. However, the effect of such a change on pipeline safety is unclear. While the consequences of a low-stress transmission pipeline failure are generally not severe because these pipelines are more likely to leak gradually rather than rupture, the point at which a gas pipeline fails by rupture is uncertain and depends on a number of factors in addition to pressure. For example, the size or type of defect and the materials used to construct the pipeline also influence whether a pipeline leaks or ruptures. In addition, the mileage and location of pipelines that would be affected by such a regulatory change are currently unknown, although PHMSA recently changed its reporting requirements to collect such information. The concern is that because distribution pipelines are located in highly populated areas, the low-stress transmission pipelines that are connected to them could also be located in highly populated areas. Overall, officials we contacted from state pipeline safety agencies and PHMSA supported the current integrity management requirements for low-stress transmission pipelines. Specifically, about 58 percent of the officials from state pipeline safety agencies we surveyed (30 of 52), responded that the current requirements would best apply to low-stress transmission pipelines—affirming the current regulatory environment. By comparison, 10 of 52 officials were in favor of changing the regulatory environment by applying distribution integrity management requirements to low-stress transmission pipelines, and 12 of 52 officials stated no opinion on the issue. In light of the uncertain safety effects of changing safety requirements for low-stress transmission pipelines and the opinion of state pipeline safety officials we surveyed, the current regulatory approach of applying more prescriptive transmission pipeline requirements—with an option for operators to apply for a special permit—appears reasonable.

Why GAO Did This Study

More than 2.6 million miles of pipelines form a nationwide network to transport the majority of natural gas consumed in the United States. This extensive gas pipeline network includes several different types of pipelines, including

  • transmission pipelines, which transport product over long distances at high pressure from sources to communities;

  • distribution pipelines, which operate at lower pressures to deliver natural gas to homes and businesses; and

  • low-stress transmission pipelines, which typically connect transmission pipelines to distribution pipelines and may operate at pressures that are similar to the latter.

The Pipeline and Hazardous Materials Safety Administration (PHMSA), within the Department of Transportation (DOT), is responsible for establishing safety requirements for these pipelines and has traditionally included low-stress transmission pipelines in its regulations for all transmission pipelines. In 2004, PHMSA implemented a risk-based regulatory approach called “integrity management” for natural gas transmission pipelines, including low-stress transmission pipelines.

In 2010, PHMSA extended integrity management to distribution pipelines, but modified the requirements to account for differences in transmission and distribution pipelines, such as differences in pipeline size and operating pressure. Some stakeholders have suggested that the requirements for distribution pipelines should apply to low-stress transmission pipelines because of the similarity in operating pressures of these two types of pipelines. Congress asked that we consider the safety implications to the public of this proposal. Accordingly, this report focuses on how applying distribution integrity management requirements to low-stress gas transmission pipelines might affect the safety of these pipelines.

For more information, contact Susan Fleming at (202) 512-2834 or

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