1):
Gathering pipelines. Class 1 locations  are in rural areas and Class 4 locations are in densely populated areas  (see table 1.). All combustible gases transported by distribution pipelines  are required to be odorized because these pipelines are primarily in  populated areas. The risk- based integrity management programs for natural gas transmission  pipelines require operators to systematically identify and mitigate risks to  pipeline segments located in high-consequence areas. Officials and Stakeholders Said That Odorizing Gas in Pipelines Improves Public Safety, but Can Impede Some Industrial Processes

		Pipeline Gas Odorization Facilitates Early Detection, Particularly in Populated Areas

Almost all officials and stakeholders we interviewed and the state pipeline  safety officials we surveyed told us that the advantage of using sulfur- based odorants to odorize combustible gas transported by pipeline is  public safety. Accordingly, that expert  stated that odorization is the most effective safety method for alerting the  public of a possible gas leak. Additionally, a third expert noted that (1)  electronic detectors can be difficult to place in certain areas and (2)  odorants allow the public to quickly detect gas leaks without acquiring or  maintaining external equipment. Officials from both federal safety regulatory  agencies we interviewed (PHMSA and NTSB); approximately half of state  pipeline safety officials surveyed; and about half of the stakeholders  interviewed reported that sulfur-based odorants used in transmission  pipelines can cause an adverse chemical reaction during processing for  some industries. Officials and Stakeholders Had Mixed Views on Need to Modify Odorization Requirements

		Many Officials and Stakeholders Agreed That Federal Distribution Pipeline Odorization Regulations Do Not Need to be Modified

General consensus exists among those we spoke with (including federal  regulatory and safety officials, experts identified by the National  Academies, and industry stakeholders) that federal requirements to  odorize all gases in distribution pipelines are sufficient as written and do  not need to be modified. PHMSA and NTSB officials we interviewed and  many commenting stakeholders articulated this view. While the majority of stakeholders and state survey respondents did not  see a need to odorize gas in gathering pipelines, a third of the state  safety officials and three other stakeholders said all gathering pipelines  should be odorized for additional safety regardless of any technical  challenge. Officials from NTSB as well  as about half of the stakeholders we contacted said the current  regulations for odorizing gas in certain transmission pipelines in  populated areas were sufficient. These  inspection methods are designed to detect issues that could cause a gas  leak before the leak occurs. While odorants could be added in addition to integrity management  requirements, PHMSA officials said that integrity management more  effectively helps assure an acceptable level of safety for transmission  pipelines than an odorant could because the risk assessments focus on  the potential causes of leaks and ruptures for these types of pipelines  and, therefore, are more preventative than odorizing. However, as part of the ongoing two rulemakings: the Pipeline  Safety: Safety of Gas Transmission Pipelines, MAOP Reconfirmation,  Expansion of Assessment Requirements and Other Related Amendments  and the Pipeline Safety - Safety of Gas Transmission Pipelines, Repair  Criteria, Integrity Management Improvements, Cathodic Protection,  Management of Change, and Other Related Amendments Rulemaking,  PHMSA also plans to strengthen and expand requirements for the gas  integrity management program for transmission pipelines. The 2016 PIPES Act includes a  mandate for GAO to review PHMSA’s gas integrity management program  as soon as PHMSA publishes the final rule. Those with some familiarity  offered the following advantages and disadvantages.
Why GAO Did This Study

The nation's gas pipeline network moves about 74 billion cubic feet of combustible gas to homes and businesses daily. To alert the public of a gas leak before an explosion occurs, PHMSA has different requirements for odorizing gas. All gas transported by distribution pipelines throughout communities must be odorized. Gas transported across many miles by transmission pipelines is required to be odorized only in certain populated areas. There are no requirements to odorize gas in gathering pipelines. Congress included a provision in statute for GAO to review odor requirements for all pipelines.
This report presents the views of federal and state pipeline safety officials and industry and safety stakeholders on: (1) the advantages and disadvantages of odorizing combustible gases in pipelines; and (2) whether and how federal requirements for odorizing pipelines should be modified. GAO reviewed relevant regulations and reports; surveyed officials in 48 states and the District of Columbia; and interviewed PHMSA and NTSB officials. GAO also interviewed 34 stakeholders, including 14 experts identified by the National Academies, and 20 other industry and safety stakeholders.

What GAO Found

Pipeline and Hazardous Materials Safety Administration (PHMSA) and National Transportation Safety Board (NTSB) officials, state officials, and stakeholders GAO contacted cited safety as the main advantage to odorizing combustible gases in pipelines, primarily for distribution pipelines in densely populated areas (see figure). Specifically, adding a chemical with a distinctive odor to gas allows the public to generally detect leaks before an explosion can occur. The most frequently cited disadvantage was that commonly used sulfur-based odorants must be removed—primarily from gas in transmission pipelines—before the gas can be used in certain processes, such as producing fertilizer.
While federal odorization requirements follow a risk-based approach by focusing on pipelines in populated areas, the officials and stakeholders GAO contacted disagreed on the need to modify these requirements for some pipelines. Specifically, because distribution pipelines run through populated areas, everyone GAO contacted generally agreed that these pipelines should be odorized for safety, as currently required. For gathering pipelines, the majority of officials and stakeholders did not see a need to modify regulations because these pipelines would be technically challenging to odorize and are primarily located in rural areas. However, about two-thirds of state officials and about half of stakeholders said that additional transmission pipelines should be odorized for public safety.
Conversely, officials from PHMSA and NTSB and about half of the stakeholders contacted noted that, because transmission pipelines operate at high pressure and generally rupture rather than leak, it is unlikely that odorant could mitigate risk. Instead, other required safety practices—such as internal pipeline inspections—can provide more preventative, risk-based safety management, according to PHMSA officials. In this regard, PHMSA officials said that they plan to strengthen risk-based safety requirements for transmission and gathering pipelines as part of on-going rulemakings. PHMSA anticipates issuing these rules in 2019.