NTSB Safety Recommendation P-21-002

NTSB safety recommendation P-21-002.

TO THE PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION: Evaluate industry’s implementation of the gas distribution pipeline integrity management [DIMP] requirements and develop updated guidance for improving their effectiveness. The evaluation should specifically consider factors that may increase the likelihood of failure such as age, increase the overall risk (including factors that simultaneously increase the likelihood and consequence of failure), and limit the effectiveness of leak management programs.

Priority: CLASS II

Overall Status: Open - Acceptable Response

Issued Date: 2021-02-08

Adopted Date: 2021-01-12

Synopsis: On February 23, 2018, at 6:38 a.m. local time, a natural gas–fueled explosion occurred at 3534 Espanola Drive, Dallas, Texas. The residence sustained major structural damage, but when first responders arrived on scene at 6:44 a.m., they observed no smoke or fire. Four family members were injured, and one was killed in the explosion. Following the explosion, National Transportation Safety Board (NTSB) investigators located a through-wall crack in the 71 year old natural gas main that served the residence. In the 2 days before this explosion, two gas-related incidents occurred on the same block at houses that were served by the same natural gas main, each resulting in significant structural damage and burn injuries to one occupant. The first occurred on February 21, 2018, at 5:49 a.m., and resulted in one injury involving second-degree burns and significant structural damage to 3527 Durango Drive. The second incident occurred on February 22, 2018, at 10:21 a.m., and resulted in one injury involving second-degree burns and significant structural damage to 3515 Durango Drive. As a result of this investigation, the NTSB issued new safety recommendations to the Pipeline and Hazardous Materials Safety Administration, the Railroad Commission of Texas, the Dallas Fire-Rescue Department, Atmos Energy Corporation, and the Gas Piping Technology Committee. The NTSB is also reiterating safety recommendations to the International Code Council, the National Fire Protection Association, and the Gas Technology Institute

Probable Cause: The National Transportation Safety Board determines that the probable cause of the explosion at 3534 Espanola Drive was the ignition of an accumulation of natural gas that leaked from the gas main that was damaged during a sewer replacement project 23 years earlier and was undetected by Atmos Energy Corporation’s investigation of two related natural gas incidents on the 2 days prior to the explosion. Contributing to the explosion was Atmos Energy Corporation’s insufficient wet weather leak investigation procedures. Contributing to the severity of the explosion was Atmos Energy Corporation’s inaction to isolate the affected main and evacuate the houses. Contributing to the degradation of the pipeline system was Atmos Energy Corporation’s inadequate integrity management program.

Ntsbnumber: PLD18FR002

Report Number: PAR-21-01

Addressee Name: PHMSA

Addressee Status: Open - Acceptable Response

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2025-04-08

Communication Type: Recommendation Reiteration

Communication Contents: Safety Recommendation P-21-002 was reiterated in the report UGI Corporation Natural Gas-Fueled Explosion and Fire, West Reading, Pennsylvania, March 24, 2023, PIR-25-1, publication date April 8, 2025: The vulnerability to slow crack growth, also called brittle-like cracking, of early vintage Aldyl A and other early vintage polyethylene piping materials under certain environmental (such as high ground temperatures), installation, and service conditions has been extensively documented.98 In 1998, the NTSB issued a special investigation report, Brittle-Like Cracking in Plastic Pipe for Gas Service, which concluded that the procedure used in the United States to rate the strength of plastic pipe may have overrated the strength and resistance to brittle-like cracking of much of the plastic pipe manufactured and used for gas service from the 1960s through the early 1980s. The report found that much of this early vintage plastic piping may therefore be susceptible to premature brittle-like cracking failures when subjected to stress intensification (NTSB 1998). In response, PHMSA and its predecessor, the Research and Special Programs Administration, issued four advisory bulletins addressing brittle-like cracking in plastic pipe materials. The 2002 bulletin Notification of the Susceptibility to Premature Brittle-Like Cracking of Older Plastic Pipe warned that “brittle-like cracking (also known as slow crack growth) can substantially reduce the service life of polyethylene piping systems” (Research and Special Programs Administration 2002). The bulletin specifically cited certain Aldyl A piping material manufactured by DuPont Company before 1973—the same material as the retired Building 2 service tee—as potentially susceptible to brittle-like cracking.99 A 2007 update to the advisory bulletin added Delrin-insert tapping tees to the list of polyethylene pipe materials susceptible to brittle-like cracking. District heating systems that use underground steam pipes like the one used by Palmer can be found throughout the United States, particularly in large cities like New York; San Francisco, California; Philadelphia, Pennsylvania; and Denver, Colorado. Of the 68 district heating systems still operating, just over half were built before 1950—with one-quarter built before 1900 (Pierce 2022). The extent of district heating systems nationwide means that other natural gas pipeline operators may have assets near steam pipes. Research has established that elevated temperatures can affect the pressure rating of polyethylene plastic piping, with one study citing the adverse effects of district heating systems on polyethylene gas pipelines (Akhmerova and others 2021). Early vintage Aldyl A piping is limited to operating conditions below 100°F, and operating outside these conditions increases the risk of slow crack growth. For plastic piping in general, the risk of damage grows as temperatures increase above typical ground temperatures. Modern plastics (including later vintages of Aldyl A) are more resistant to damage at higher temperatures than earlier vintages. Operators base the maximum operating pressure for plastic piping on the properties of the pipe and an assumed maximum environmental temperature; as seen in this accident, the release of steam can raise that temperature, creating an environment in which the piping was not originally designed to operate. To address the risks associated with plastic piping, pipeline operators must be aware of where these assets are located in their system and which ones may be susceptible to slow crack growth or other degradation from outside factors, such as heat. Before the accident, UGI had evaluated the threat and consequences of early vintage Aldyl A to be the same as other polyethylene fittings in its risk models, counter to PHSMA guidance. UGI was not able to conduct a complete inventory of its plastic assets, including manufacturer, with available records. The Palmer steam pipe had not been recorded or identified in UGI’s records, precluding UGI from identifying the elevated temperature environment as a threat. The NTSB concludes that because UGI did not have sufficient threat information available for analysis in its DIMP, it could not effectively evaluate and address the risk to pipeline integrity of its plastic piping in elevated temperature environments. UGI strengthened its data collection and record correction procedures and is working to remediate Aldyl A service tees with Delrin inserts as they are discovered in the field, using new operational procedures and electrofusion repair fittings developed specifically for the tees. UGI is also conducting a complete analysis of all its assets that may be exposed to elevated temperature environments to evaluate and address this threat to pipeline integrity, but this effort needs to be completed. Therefore, the NTSB recommends that UGI inventory all its plastic natural gas assets that may be located in elevated temperature environments and address the risk associated with these assets. The NTSB is concerned that the extent of the use of plastic natural gas assets throughout the country, including Aldyl A, and their susceptibility to degradation in elevated temperature environments raise the risk of an accident like this one. This accident demonstrates the need to quantify the extent of plastic piping assets in natural gas pipeline systems that are at risk of exposure to elevated temperatures. Historical asset records on pipe installed more than 40 years ago may not be accurate, possibly complicating operators’ efforts to assess the extent of plastic piping throughout their systems, as UGI experienced. A 2014 study of natural gas pipeline operators in California demonstrated uncertainty similar to UGI’s regarding the extent of the operators’ Aldyl A assets (Haine 2014). The NTSB concludes that given the widespread adoption of plastic piping, including Aldyl A assets, and the unreliability of historical asset records, operators may not be aware of the locations of their plastic natural gas assets that are vulnerable to degradation in elevated temperature environments, thus appropriate mitigations may not be in place. Specific guidance from PHMSA on identifying and evaluating the risks associated with plastic piping in elevated temperature environments would reduce the chances of a similar accident occurring in the future. Once pipeline operators have identified the extent of the threat in their systems, they can evaluate risks and implement mitigations where necessary. Therefore, the NTSB recommends that PHMSA issue an advisory bulletin to all regulated natural gas distribution pipeline operators referencing DIMP regulations and encouraging operators to: • Complete a one-time inventory of all plastic assets that are located in environments that experience or are at risk of elevated temperatures; • Continue, during maintenance and new construction projects, to identify plastic assets that are in elevated temperature environments; and • Evaluate and mitigate risks to deter the degradation of these assets. Although the failure of Aldyl A tees with Delrin inserts is well documented, this is the first accident NTSB has investigated in which thermal degradation of an Aldyl A service tee with Delrin insert resulted in a fracture that released a substantial amount of natural gas and led to an explosion. Less-severe Delrin insert and cap failures have been documented: 2 years after the 2007 PHMSA advisory bulletin, a 2009 Gas Technology Institute report detailed several insert and cap failures in Aldyl A service tees with Delrin inserts (Mamoun, Maupin, and Miller 2009). Further, data reported to the Plastic Pipe Database Committee show that about 20% of failures of Aldyl A fittings manufactured by DuPont and Uponor were likely caused by the tee with the Delrin insert (American Gas Association 2024). The NTSB acknowledg

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2026-03-26

Communication Type: Recommendation Reiteration

Communication Contents: From the Report, “Atmos Energy Corporation Natural Gas-Fueled Home Explosions and Fires Jackson, Mississippi, January 24, 2024, and January 27, 2024” published on March 26, 2026. Inadequate Distribution Integrity Management Program System Data As discussed in 1.7.1.2, over 15 years ago, the Pipeline and Hazardous Materials Safety Administration introduced distribution integrity management requirements to enhance safety by identifying and reducing pipeline integrity risks. The first required element for natural gas distribution pipeline integrity management programs is system data (or system knowledge), and Atmos lacked system data for its Mississippi Division. After the Jackson accidents, Atmos told the NTSB that it did not have service-line installation records for over 63% of the service lines in its Mississippi Division. Therefore, at the time of the accidents, Atmos did not have system data for over 193,024 of the 306,387 service lines in its Mississippi Division, including the accident service lines. Atmos’s lack of system data for its Mississippi Division affected its leak management program and its risk model. The lacking system data affected Atmos’s leak management program because, despite Atmos’s knowledge of the threat of compression coupling failure in expansive-soil environments, like Jackson, Mississippi, (see section 2.2 for more information on this), its lack of system data for its Mississippi Division service lines prevented it from identifying the locations of most of the compression couplings in that system, including the accident couplings. The lacking system data affected Atmos’s risk model because system data was a primary input in the model. Although Atmos’s risk model assigned higher risk-factor weights to service lines that were missing system data (see section 1.9.3.2 for more on this), the model was incapable of determining which of the data lacking service lines posed the greatest threat to the system because of all the missing data. (See section 2.5.2 for more information on how Atmos’s lack of system data impacted its risk model.) As of this report, Atmos is still missing about 63% of the service-line data for its Mississippi Division. As discussed in section 1.9.3.1, Atmos also has significant shortfalls in service-line data at several other divisions, including its Kentucky/Mid States Division, which is missing 73% of the service-line data in Kentucky. Atmos has acquired legacy pipeline systems from companies that may not have kept records of their pipeline assets (federal regulations did not require operators to keep installation records until the mid 1970s). However, as the current owner of property that provides hazardous materials to the public, Atmos has a responsibility to learn about its system, so it can appropriately assess and address the risk to that system and to public safety. Therefore, the NTSB concludes that Atmos’s failure to gather relevant information about its service line records prevented it from effectively assessing the risk to its assets. The two accidents in Jackson and Atmos’s reported rates of missing service line data in its other divisions indicate that Atmos needs to take a more focused approach to gain data about its system. Atmos currently gathers system data through routine operations and maintenance activities, including when an employee identifies an opportunity to correct or complete a pipeline-asset record. In addition to administrative strategies, some of which Atmos currently employs, technology-based strategies, some of which the ASCE guide describes, offer other methods of gaining system data that do not involve excavating buried pipelines. A natural gas distribution pipeline operator in Louisiana, Missouri, developed a plan that allowed it to gain additional service-line data, and Atmos could do the same. Therefore, the NTSB recommends that Atmos develop and implement a program to proactively identify and collect missing service-line information for all its operating divisions. The program should (1) identify one or more methods for gaining additional system data and (2) establish and make public the milestones and timeline for acquiring the unknown system data. Risk Model As discussed in section 1.7.1.2, federal regulations required natural gas distribution pipeline operators to evaluate and rank risk in their pipeline systems, and Atmos did not effectively evaluate and rank system risk in its system. In 2023, the year before the two accidents, Atmos repaired 182 hazardous (grade 1) leaks in Jackson. Although the repaired hazardous leaks were confirmed risks to Atmos’s system, its risk model, in large part, did not rank the locations of the hazardous leaks as high risk. Figure 23 shows the hazardous leaks that Atmos repaired in 2023 within a 5-mile radius of Jackson overlaid with the Atmos risk model’s high-risk grids that were current on that date; not many of the hazardous leaks are in high-risk grids. On January 24, 2024, the day of the Bristol Boulevard accident, there were 289 open, nonhazardous leaks (unrepaired leaks that Atmos knew about) in a 5-mile radius of Jackson, including the accident leaks. On January 24, 2024, the Bristol Boulevard accident occurred. Three days later, on January 27, 2024, the Shalimar Drive accident occurred. Although these two catastrophic accidents were confirmed risks to Atmos’s system, resulting from (open) leaks that Atmos was aware of, its risk model did not rank the accident locations as high risk. Figure 24 shows the open, nonhazardous leaks within a 5-mile radius of Jackson as of January 24, 2024, overlaid with the Atmos risk model’s high-risk grids that were current on that date; not many of the open, nonhazardous leaks, including the accident leaks, are in high risk grids. The figure also shows the grade 3 leaks that met the criteria for higher grade leaks (grade 1 and grade 2 leaks) when Atmos reevaluated them. Footnote: As discussed in section 1.9.3.2, the Atmos risk model’s high-risk grids that were current on January 24, 2024, were the same high-risk grids that were current at the end of 2023. In 2023, the year that Atmos’s risk model did not identify many of the areas with hazardous leaks as high risk, Atmos told the NTSB that in response to Safety Recommendation P-21-12, it had updated its risk model to consider threats posed by expansive soil, like the soil in Jackson, a factor that the NTSB determined played a role in Atmos’s 2018 accident in Dallas. The NTSB closed Safety Recommendation P 21-12 based on the information that Atmos provided. However, in light of the two accidents in Jackson and the number of leaks that became a higher grade before Atmos repaired them (about 15% of the leaks Atmos knew about as of the day of the Bristol Boulevard accident), it is likely that the updates that Atmos reported to have made to its risk model, whether to address expansive soil or any other threat, did not result in an effective model. Footnote: The two accidents in Jackson indicate that Atmos’s risk model did not adequately consider expansive-soil risks, contrary to Atmos’s correspondence with the NTSB regarding Safety Recommendation P-21-12. This investigation did not evaluate the reason for the discrepancy between Atmos’s statements about the updates to its risk model and the model’s performance. A distribution integrity management risk model that effectively evaluated and ranked risk would have produced results that helped Atmos identify potentially hazardous leaks and take corrective actions before the leaks became hazardous, which Atmos’s risk model did not do. In addition to Atmos’s risk model not adequately evaluating the risks to its system, this investigation found that shortfalls in Atmos’s leak management program contributed to the two Jackson accidents (see section 2.3 for more information on this), and federal regulations require distributi

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2021-02-08

Communication Type: Transmittal Letter

Communication Contents: The attached letter from the NTSB Chairman provides information about the NTSB’s January 12, 2021, report Atmos Energy Corporation Natural Gas-Fueled Explosion, Dallas, Texas, February 23, 2018, PAR-21/01. The details of this accident investigation and the resulting safety recommendations may be found in the attached report, which can also be accessed at http://www.ntsb.gov. The NTSB is vitally interested in these recommendations because they are designed to prevent accidents and save lives. We would appreciate a response within 90 days of the date of this letter, detailing the actions you have taken or intend to take to implement these recommendations. When replying, please refer to the safety recommendations by number (for example, P 21-1 through -3). We encourage you to submit your response to ExecutiveSecretariat@ntsb.gov. If your reply exceeds 20 megabytes, including attachments, please e mail us at the same address for instructions on how to send larger documents. Please do not submit both an electronic copy and a hard copy of the same response. This letter provides information about the National Transportation Safety Board’s (NTSB) January 12, 2021, report Atmos Energy Corporation Natural Gas-Fueled Explosion, Dallas, Texas, February 23, 2018, PAR-21/01. The details of this accident investigation and the resulting safety recommendations may be found in the attached report, which can also be accessed at http://www.ntsb.gov. As a result of this investigation, we identified the following safety issues: • Incident investigation. Neither Dallas Fire-Rescue Department (DFR) nor Atmos Energy Corporation (Atmos) identified the causes of the two incidents that occurred in the days immediately preceding the explosion. DFR arson investigators and Atmos technicians did not effectively investigate, communicate, or collaborate to determine the cause of either incident. Further, Atmos did not gather enough evidence to determine if gas migrated from their piping and fueled the first two incidents. • Leak investigations and repairs. Atmos dedicated significant resources to its response following the second incident, finding 13 leaks determined to present an existing or probable future hazard. However, none of its employees questioned the integrity of the system. As a result, Atmos did not take appropriate action to secure the safety of the area and its residents. This was attributed, in part, to inadequate procedures for performing leak investigations in wet weather conditions. • Methane detection. Although Atmos added odorant to its gas distribution system in a manner consistent with Pipeline and Hazardous Materials Safety Administration (PHMSA) regulations, none of the residents at any of the affected homes smelled gas. Although odorant can act as an early warning of a gas release to prevent an explosion and fire, it is known to become depleted if it travels through soil. • Incident reporting. Incident reporting requirements mandated by the PHMSA rely on the judgement of the operator to determine whether an incident resulted from a leak in their system and do not specify the level of investigation necessary to make the determination. While operators have an option to report events that may have been caused by their system, Atmos relied on an incomplete investigation to support its position not to report the first two incidents. Accordingly, the NTSB makes the following safety recommendations to PHMSA. Additional information regarding these recommendations can be found in the noted sections of the report. • Expand incident reporting requirements in Title 49 Code of Federal Regulations Part 191 so that events that may meet the definition of “incident” are immediately reported to the National Response Center even when the source of the natural gas has not been determined. (P 21-1) (See section 2.6.1.) • Evaluate industry’s implementation of the gas distribution pipeline integrity management requirements and develop updated guidance for improving their effectiveness. The evaluation should specifically consider factors that may increase the likelihood of failure such as age, increase the overall risk (including factors that simultaneously increase the likelihood and consequence of failure), and limit the effectiveness of leak management programs. (P 21-2) (See section 2.7.) • Assist the Railroad Commission of Texas in conducting the audit recommended in Safety Recommendation P-21-4. (P-21-3) (See section 2.7.) The NTSB is vitally interested in these recommendations because they are designed to prevent accidents and save lives. We would appreciate a response within 90 days of the date of this letter, detailing the actions you have taken or intend to take to implement these recommendations. When replying, please refer to the safety recommendations by number (Safety Recommendation(s) P-21-1 through -3). We encourage you to submit your response to ExecutiveSecretariat@ntsb.gov. If your reply, including attachments, exceeds 20 megabytes, please e mail us at the same address for instructions. Please do not submit both an electronic copy and a hard copy of the same response.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2023-07-06

Communication Type: NPRM Response

Communication Contents: The National Transportation Safety Board (NTSB) has reviewed the Pipeline and Hazardous Materials Safety Administration’s (PHMSA’s) notice of proposed rulemaking (NPRM) titled, “Pipeline Safety: Gas Pipeline Leak Detection and Repair,” published at 88 Federal Register 31890 on May 18, 2023. The NPRM proposes to amend portions of Title 49 Code of Federal Regulations (CFR) Parts 191, 192, and 193 to implement congressional mandates in the Protecting our Infrastructure of Pipelines and Enhancing Safety Act of 2020 to reduce methane emissions from new and existing gas transmission pipelines, gas distribution pipelines, regulated gas gathering pipelines, underground natural gas storage facilities, and liquefied natural gas facilities. In its NPRM, PHMSA documented a detailed evaluation of several of its regulations and proposed amendments to: • strengthen leakage survey and patrolling requirements; • establish performance standards for advanced leak detection programs; • clarify leakage survey, investigation, and repair personnel qualification requirements; and • codify congressional mandates in federal regulation. Some of PHMSA’s proposed amendments, if implemented, will help improve pipeline leak detection and mitigation, an item on the NTSB’s Most Wanted List for 2021–2023. Footnote: https://www.ntsb.gov/Advocacy/mwl/Pages/default.aspx The NTSB first identified the need for leak detection and mitigation methods about 50 years ago and is encouraged by proposals in the NPRM that may enhance the industry’s performance in detecting and safely responding to pipeline leaks. We offer comments in the following topic areas: leakage survey requirements and advanced leak detection programs, in-home methane detectors, leak-prone materials, and leak detection systems. Leak-Prone Materials In July 2017, the NTSB investigated a natural gas–fueled explosion that destroyed a home in Millersville, Pennsylvania, and significantly damaged six others. The accident killed one person and injured three others. In a safety recommendation report related to this investigation, the NTSB concluded that referencing the use of external sources of information in PHMSA’s Distribution Integrity Management: Frequently Asked Questions would help pipeline operators recognize and better understand how to reduce potential accidents. Footnote: (a) For more information, see Safety Recommendation Report, Installation of PermaLock Mechanical Tapping Tee Assemblies. NTSB/PSR-18/01. Washington, DC: NTSB. (b) The document Distribution Integrity Management: Frequently Asked Questions can be found at https://www.phmsa.dot.gov/pipeline/gas-distribution-integrity-management/gas-distribution-integrity-management-faqs. We continue to believe that external sources of information are useful to pipeline operators when identifying threats to pipeline integrity, including leak-prone materials. Similarly, joint government–industry efforts, such as the status reports produced by the Plastic Pipe Database Committee, can assist pipeline operators in identifying leak-prone materials. Footnote: The Plastic Pipe Database Committee coordinates the creation and maintenance of a database to proactively monitor the performance of plastic pipe and metal and plastic appurtenances contained within plastic piping systems. Organizations represented on the Plastic Pipe Database Committee include the American Gas Association, American Public Gas Association, Plastics Pipe Institute, National Association of Regulatory Utility Commissioners, National Association of Pipeline Safety Representatives, PHMSA, and the NTSB. Additionally, pipeline operators can use operator-provided information contained in PHMSA’s pipeline incident and annual reports to evaluate their systems’ susceptibility to leaks. Pipeline operators can proactively identify leak prone materials using these resources as well as their system knowledge and operational experience. The NTSB made the following safety recommendation to PHMSA as a result of the Dallas, Texas, investigation: Evaluate industry’s implementation of the gas distribution pipeline integrity management requirements and develop updated guidance for improving their effectiveness. The evaluation should specifically consider factors that may increase the likelihood of failure such as age, increase the overall risk (including factors that simultaneously increase the likelihood and consequence of failure), and limit the effectiveness of leak management programs. (P-21-2) Footnote: Safety Recommendation P-21-2 is currently classified Open—Acceptable Response based on PHMSA’s ongoing evaluation of the industry’s implementation of gas distribution integrity management program requirements. In the NPRM, PHMSA invites comments on the value of explicitly listing leak-prone materials, either in 49 CFR Part 192 or in periodically issued implementing guidance. PHMSA notes that pipeline operators could reference authoritative resources—state pipeline safety regulatory actions, PHMSA pipeline failure investigation reports and advisory bulletins, and NTSB findings, for example—to help identify pipeline materials known to leak. To the extent these references are publicly available, the NTSB agrees that they are valuable resources that can help pipeline operators identify leak-prone materials. Footnote: The availability of state pipeline safety regulatory actions varies from state to state, and many PHMSA pipeline failure investigation reports are not readily available to the public. Although PHMSA-issued advisory bulletins and implementing guidance are helpful resources, they cannot replace pipeline operators’ responsibility to identify threats and safely operate their systems. The NTSB believes that PHMSA’s ongoing work in response to Safety Recommendation P-21-2 will better position pipeline operators to identify leak-prone materials.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2023-10-27

Communication Type: NPRM Response

Communication Contents: The National Transportation Safety Board (NTSB) has reviewed the Pipeline and Hazardous Materials Safety Administration’s (PHMSA) notice of proposed rulemaking (NPRM), titled “Pipeline Safety: Safety of Gas Distribution Pipelines and Other Pipeline Safety Initiatives,” published at 88 Federal Register 61746 on September 7, 2023. The NPRM proposes amendments to Title 49 Code of Federal Regulations (CFR) Parts 191, 192, and 198 in response to congressional mandates in the Leonel Rondon Pipeline Safety Act and an NTSB safety recommendation aimed at preventing catastrophic incidents resulting from overpressurization of low pressure gas distribution systems. The NTSB investigated two natural gas accidents that resulted in recommendations that are applicable to this NPRM. We describe these investigations and offer comments on the following topic areas: Clarifications and Updates to Distribution Integrity Management Program Plans, Updates to Emergency Response Communications, Updates to Operations and Maintenance Procedural Manuals, and Requirements for New Regulator Stations. Related NTSB Investigations In February 2018, the NTSB investigated a natural gas fueled explosion in Dallas, Texas, which resulted in 1 fatality; 4 injuries; a destroyed home; and the evacuation of 300 homes, 250 apartment units, and 600 students. In the 2 days before the explosion, two gas related incidents occurred on the same block at houses that were served by the same natural gas main, each resulting in one injury and significant structural damage. We determined that the probable cause of the accident was the ignition of natural gas that leaked from a gas main that was damaged 23 years earlier and was undetected by the operator’s investigation of the two related incidents that occurred in the 2 days before the explosion. In September 2018, the NTSB investigated the overpressurization of a natural gas distribution system in Merrimack Valley, Massachusetts, which resulted in fires and explosions, 1 fatality, 22 individuals being taken to local hospitals, 5 destroyed homes, 131 damaged structures, and the evacuation of 50,000 residents. We determined that the probable cause of the accident was the operator’s failure to adequately plan, review, sequence, and oversee a pipeline construction project. Clarifications and Updates to Distribution Integrity Management Program Plans NPRM Commentary In the NPRM, PHMSA proposes requiring that operators identify and minimize the risks to their systems from specific threats in their distribution integrity management program plans, such as (1) the presence of certain materials, like cast iron and other piping with known issues; (2) overpressurization of low pressure systems; and (3) extreme weather and other geohazards. PHMSA also proposes that operators consider the effect of age on those specific threats faced by a distribution pipeline. In addition, the NPRM proposes that, for operators of low pressure gas distribution systems, when evaluating and ranking risks, operators must also consider (1) the configuration of their overpressure protection, (2) the availability of gas pressure monitoring, and (3) the likelihood of any single event that could result in an overpressurization. NTSB Comment The NTSB’s investigations into both the Dallas and Merrimack Valley accidents demonstrate the importance of operators appropriately evaluating and mitigating risks to their pipeline systems. In our Dallas investigation, we determined that the operator’s inadequate integrity management program contributed to the degradation of the pipeline system. We noted that the operator did not adequately consider or mitigate against threats that were degrading its pipeline system, the likelihood of failure associated with these threats, or the potential consequences of such a failure as required by gas distribution integrity management rules. In addition, we concluded that while the operator’s periodic leak survey methodology and frequency complied with the minimum state and federal requirements, it did not identify the pipeline system degradation that authorities found after the accident. As a result, we recommended that PHMSA: Evaluate industry’s implementation of the gas distribution pipeline integrity management requirements and develop updated guidance for improving their effectiveness. The evaluation should specifically consider factors that may increase the likelihood of failure such as age, increase the overall risk (including factors that simultaneously increase the likelihood and consequence of failure), and limit the effectiveness of leak management programs. (P 21 2) In our Merrimack Valley investigation, we determined that a comprehensive and formal risk assessment would have identified the human error that caused the redundant regulators to open and overpressurize the system. As a result, we issued Safety Recommendation P 19 15 to PHMSA to issue an alert to operators of low pressure natural gas distribution systems notifying them of the possibility of a failure of overpressure protection and recommending that they use a failure modes and effects analysis or equivalent method to identify potential failures and take action to mitigate those failures. After PHMSA issued an advisory bulletin, titled “Overpressure Protection on Low Pressure Natural Gas Distribution Systems,” we classified the recommendation as Closed—Acceptable Action. The NTSB supports PHMSA’s proposal to add more stringent risk management requirements to distribution integrity management program plans. We also understand that PHMSA is still evaluating gas distribution pipeline integrity management requirements to develop the recommended guidance for improving their effectiveness, two components necessary to fully satisfy Safety Recommendation P 21 2. Summary The NTSB supports many of the NPRM’s proposed revisions to its pipeline safety regulations. However, we believe that the proposal has some shortcomings and encourage PHMSA to address our comments in its final rulemaking. Thank you for the opportunity to comment on this notice.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2021-07-16

Communication Type: Official Correspondence

Communication Contents: We note that you concur and will analyze the information reported by distribution pipeline operators to evaluate the industry’s implementation of the gas distribution integrity management program (DIMP) requirements. This evaluation will specifically consider factors, such as age, that may increase the likelihood of failure, increase overall risk, and limit the effectiveness of leak management programs. Additionally, PHMSA will work with the states to participate in DIMP inspections to identify areas of potential improvement. Based on the results of your analysis, you will update inspection questions and guidance to help improve the effectiveness of operators’ DIMPs. Pending completion of these actions, Safety Recommendation P-21-2 is classified OPEN-- ACCEPTABLE RESPONSE.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2022-05-02

Communication Type: Official Correspondence

Communication Contents: In our July 16, 2021, response to your May 3, 2021, letter about these recommendations, we classified Safety Recommendation P-21-1 “Open—Acceptable Alternate Response” and Safety Recommendations P-21-2 and -3 OPEN-- ACCEPTABLE RESPONSE.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2025-11-24

Communication Type: Official Correspondence

Communication Contents: We support PHMSA’s proposal to add more stringent risk management requirements to distribution integrity management program plans and provided comments on PHMSA’s September 7, 2023, notice of proposed rulemaking (88 Federal Register 61746). In the September 29 meeting, we also discussed how PHMSA could meet the intent of this recommendation outside of the regulatory process by developing and sharing best practices and updated guidance with pipeline operators based on PHMSA’s evaluation of the industry’s implementation of gas distribution pipeline integrity management requirements. Pending PHMSA updating its guidance to improve the effectiveness of gas distribution pipeline integrity management programs, the Board has voted to retain the classification of OPEN—ACCEPTABLE RESPONSE for Safety Recommendation P-21-2.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2022-01-14

Communication Type: Official Correspondence

Communication Contents: -From Tristan H. Brown, Deputy Administrator: PHMSA provided NTSB with an alternate response for this recommendation on May 3, 2021, and looks forward to NTSB accepting our response.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2021-05-03

Communication Type: Official Correspondence

Communication Contents: -From Tristan Brown, Acting Administrator: We concur. PHMSA will analyze data and trends available from the information reported by distribution pipeline operators to evaluate the industry’s implementation of the gas distribution integrity management program (DIMP) requirements. This evaluation will specifically consider factors that may increase the likelihood of failure (such as age of equipment/infrastructure), increase the overall risk (including factors that simultaneously increase the likelihood and consequence of failure), and limit the effectiveness of leak management programs. Additionally, when safe travel permits, PHMSA will work with the states to participate in DIMP inspections to identify areas of potential implementation improvement. Based on the results of our data analysis and state inspection participation, PHMSA will update inspection questions and guidance to help improve the effectiveness of operators’ DIMPs.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2025-07-01

Communication Type: Official Correspondence

Communication Contents: -From Ben Kochman, Acting Administrator: Concur. PHMSA published a notice of proposed rulemaking (NPRM) titled Safety of Gas Distribution Pipelines and Other Pipeline Safety Initiatives, (88 FR 61746; Sept. 7, 2023). In the NPRM, PHMSA proposed to address Safety Recommendation P-21-2 by “[revising distribution integrity management program] proposing requirements so that operators of gas distribution systems will improve their identification of existing and potential threats to their pipelines’ integrity, improve the accuracy of their risk analyses, and take meaningful, timely actions to remediate or mitigate the highest risks to their infrastructure.” PHMSA is reviewing comments from the NPRM and preparing for the statutorily mandated2 Gas Pipeline Advisory Committee (GPAC) meeting to review the technical feasibility, reasonableness, cost-effectiveness, and practicability of the NPRM.

Addressee Acronym: PHMSA

Addressee Organization Type: G-Federal Government

Communication Date: 2026-06-04

Communication Type: Official Correspondence

Communication Contents: -From Paul J. Roberti, Administrator: PHMSA Response to Reiterated Safety Recommendation P-21-2: Concur. State regulators review major distribution operators annually and all others every five years to verify DIMP cycle completion. PHMSA, which directly regulates specific systems in Alaska and Hawaii, advocates for all integrity programs to adopt a continual improvement cycle via Safety Management Systems. PHMSA is currently evaluating its technical guidance, pipeline inspection and enforcement priorities, NTSB’s investigative findings, and recent updates to ANSI/GPTC Z380.13 to identify areas for improvement. Footnote: PHMSA, Pipeline Safety: Inspection and Enforcement Priorities, available at: https://www.phmsa.dot.gov/regulatory-compliance/phmsa guidance/phmsa-pipeline-safety-inspection-and-enforcement-priorities (July 17, 2025). This memorandum announces PHMSA’s pipeline inspection and enforcement priorities identified through careful analysis of incident and accident data, inspection and investigation findings, and input from staff and other stakeholders. Gas Piping Technology Committee, Guide for Gas Transmission, Distribution, and Gathering Piping Systems, ANSI/GPTC Z380.1-2022, Addenda 2 and 4. This will result in a new ABD focused on integrity management. PHMSA is updating questions addressing the risks identified in P-21-2 to its Gas State Program Evaluation. Footnote: State regulators are the primary interface with distribution operators. PHMSA uses its State Program Evaluations to conduct annual, objective reviews of State pipeline safety programs, determining their compliance and eligibility to receive grant funds. PHMSA’s evaluation includes, but is not limited to, an on-site review of the State’s inspection, compliance, and incident investigation activities. PHMSA anticipates updating these questions during program evaluations of our State partners during calendar year 2027. CONCLUSION PHMSA believes the planned actions described above address new NTSB Safety Recommendation P-26-2, and reiterated Safety Recommendations P-21-2 and P-25-3. We value our continued partnership with NTSB as we work to ensure the safe and reliable operation of the Nation’s pipeline transportation system and realize the President’s vision for unleashing American energy.