NTSB Safety Recommendation P-21-012

NTSB safety recommendation P-21-012.

TO ATMOS ENERGY CORPORATION: Without delay, assess your integrity management program, paying particular attention to the areas identified in this investigation, and revise the program to appropriately consider: (1) threats that degrade a system over time, and (2) the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure.

Priority: CLASS II

Overall Status: Closed - Acceptable Action

Issued Date: 2021-02-08

Adopted Date: 2021-01-12

Overall Date Closed: 2023-09-06

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: ATMOS Energy Corporation

Addressee Status: Closed - Acceptable Action

Addressee Date Closed: 2023-09-06

Addressee Organization Type: P-Private Industry

Communication Date: 2021-02-08

Communication Type: Transmittal Letter

Communication Contents: The National Transportation Safety Board (NTSB) is an independent federal agency charged by Congress with investigating every civil aviation accident in the United States and significant accidents in other modes of transportation—railroad, highway, marine, and pipeline. We determine the probable cause of the accidents and issue safety recommendations aimed at preventing future accidents. In addition, we carry out special studies concerning transportation safety and coordinate the resources of the federal government and other organizations to provide assistance to victims and their family members affected by major transportation disasters. 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 recommendation may be found in the attached report, which can also be accessed at http://www.ntsb.gov. For more information about NTSB and our recommendation process, please see the attached one-page summary. 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 recommendation by number (for example, P 21 8 through -12). 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. All communications regarding safety recommendations will be stored by the NTSB and viewable by the public. Please do not send privileged or confidential communications in response to this recommendation. Responses marked as confidential or privileged (or similar designations) will be considered nonresponsive. If you have concerns about this protocol, please contact us at ExecutiveSecretariat@ntsb.gov. The National Transportation Safety Board (NTSB) is an independent federal agency charged by Congress with investigating every civil aviation accident in the United States and significant accidents in other modes of transportation—railroad, highway, marine, and pipeline. We determine the probable cause of the accidents and issue safety recommendations aimed at preventing future accidents. In addition, we carry out special studies concerning transportation safety and coordinate the resources of the federal government and other organizations to assist victims and their family members affected by major transportation disasters. We are providing the following information to urge Atmos Energy Corporation (Atmos) to act on the safety recommendation in this letter because we believe your organization can help reduce the risk of future accidents. For more information about the NTSB and our recommendation process, please see the attached one-page summary. 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 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 Atmos. Additional information regarding these recommendations can be found in the noted sections of the report. • Provide initial and recurrent training to Dallas Fire-Rescue Department arson investigators and firefighters on the local natural gas distribution system and associated hazards. (P 21 8) (See section 2.3.1.) • Develop and implement more rigorous inside leak investigation requirements in response to fires and explosions when gas involvement cannot be excluded, including clear guidance on pressure testing and inside gas measurements and the potential need to return to the property after firefighters have departed. (P 21-9) (See section 2.3.2.1.) • Develop a clear procedure to coordinate with local emergency responders when investigating all fires and explosions that may be gas related to conclusively determine whether your system can be excluded as a potential contributor, and collecting the necessary evidence to support the conclusion of your investigations. (P 21-10) (See section 2.3.2.3.) • Revise your policies and procedures for responding to leaks, fires, explosions, and emergency calls to address the challenges caused by wet weather conditions. The revised policies and procedures should include: (1) leak investigation methods that are reliable in wet weather; (2) leak investigation procedures that assess all viable gas migration paths; (3) criteria for when to shut down or isolate gas distribution systems and pressure test main and service lines; and (4) an alternate safe response such as evacuation when reliable leak investigations are not possible due to wet weather or other circumstances. (P-21-11) (See section 2.4.) • Without delay, assess your integrity management program, paying particular attention to the areas identified in this investigation, and revise the program to appropriately consider: (1) threats that degrade a system over time, and (2) the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure. (P 21 12) (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 recommendatio

Addressee Organization Type: P-Private Industry

Communication Date: 2026-03-26

Communication Type: Recommendation Mention

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: 1.4 Expansive Soil At the time of the two accidents, four of Atmos’s six regional divisions, including the Mississippi Division, had distribution areas that included expansive soil, or what the US Geological Survey referred to as soil with “high-swelling potential.” Figure 8 shows a US map of Atmos’s headquarters and regional divisions transposed over a map of expansive-soil locations. Yazoo clay, an expansive clay, is prevalent in Jackson, where the accidents occurred. A 1988 Mississippi Department of Natural Resources paper noted that Yazoo clay had “long been” associated with the city of Jackson and that the expansive nature of the clay caused soil movement, which was the primary reason for the foundation and structure issues in the city (Stover, Williams, and Peel 1988). The paper noted that expansive clays can absorb large amounts of water (or swell) during periods of prolonged rainfall. A 1993 Mississippi State University (MSU) paper reaffirmed the abundance of expansive clay soil in Mississippi and the threat that it posed to structures, asserting that the soil caused “major problems for foundations, roads, sidewalks, pipelines, excavations, and industrial and agricultural operations” (MSU MAFES 1993). Footnote: A Bristol Boulevard resident told the NTSB that there was soil movement near his home before the accident. In addition, a Bristol Boulevard affected home resident told the NTSB that the soil in her backyard had moved, creating a large hole; she did not indicate whether the soil movement had occurred before or after the accident. One such problem that expansive soil causes for natural gas distribution pipeline systems, though not described in either paper, occurs when rain oversaturates the soil and natural gas migrates from an active leak, and the leak becomes hazardous. The NTSB’s Dallas investigation described how this can happen as follows: Under dry conditions, natural gas from an active natural gas leak may find a path to vent from the ground and disperse in the atmosphere without becoming hazardous (without reaching explosive levels). However, when the natural gas venting pathway to the surface becomes obstructed (which can occur when heavy rains oversaturate the soil), the gas cannot vent through the soil, so it migrates laterally in search of an area to vent, where it may enter a home or occupied structure and accumulate to explosive levels (NTSB 2021). Footnote: See section 1.10.1.3 for more information on the NTSB’s Dallas investigation. At the corporate level and in the field, Atmos’s employees were aware of the potential threat that expansive soil posed to pipelines. In 2022, in response to NTSB Safety Recommendation P-21-12, which recommended that Atmos assess and revise its distribution integrity management program, Atmos reported that it had updated its distribution integrity management risk model to consider the swell potential of clay soil. Footnote: Safety Recommendation P-21-12 is classified Closed—Acceptable Action. (See section 1.10.1.3 for more information on this safety recommendation.) (See section 1.9.3.2 for more information on Atmos’s distribution integrity management risk model.) In addition, in interviews with the NTSB, two Atmos employees in Jackson with field experience noted the effect that expansive soil could have on pipeline compression couplings during certain weather conditions. An Atmos operations manager who oversaw Atmos leak management in the city said that, in his experience, the cases of complete compression coupling failure occurred when the weather changed from drought conditions to rain conditions because the soil in the area was “Yazoo clay,” which was “expansive.” Footnote: The operations manager had 28 years of Atmos experience. Another Atmos employee, an operations supervisor of construction, made a similar statement, saying the following: Right here in Jackson, I mean -- I've worked in the Delta too -- it's almost the same kind of soil. You know, it's when, you know, seasons change, drought, a lot of rain, the ground moves, and that's when -- if there's a slip or something like that [a compression coupling failure] -- that's when it usually occurs.

Addressee Organization Type: P-Private Industry

Communication Date: 2021-05-07

Communication Type: Official Correspondence

Communication Contents: Kevin Akers, President and CEO: Atmos Energy submits this letter and attachments in response to the National Transportation Safety Board (NTSB)’s letter dated February 8, 2021 concerning the NTSB’s January 12, 2021 Report on the incident that occurred in Dallas, Texas on February 23, 2018. As the attached responses reflect, we have either already, or soon will be, implementing the NTSB Recommendations P-21-8 through P-21-12. Our focus on safety has continued throughout this investigation, including voluntary initiatives to reduce the risk of third-party damage to our pipelines. In addition to the enhancements reflected in the Safety Recommendations, because third party damage remains one of the greatest threats to natural gas distribution systems, we have been and will continue to be a champion for damage prevention. We are committed to continuous improvement on our journey to achieve our vision to be the safest provider of natural gas services. Atmos Energy appreciates the opportunity to respond to the NTSB and we look forward to continuing to work closely with the NTSB to further enhance the safe delivery of natural gas services. Atmos Energy appreciates the work of the NTSB in connection with its response to the incident on February 23, 2018, its investigation of the probable cause, and its efforts to enhance industry safety. As we did throughout the investigation, Atmos Energy will continue to work collaboratively with the NTSB in response to the Recommendations. Atmos Energy’s commitment to safety is a core value. Our holistic approach to managing safety involves observing, evaluating, and adapting to changing and challenging conditions. We are committed to continuous improvement as we work to achieve our vision of being the safest provider of natural gas services. It is in that spirit that we voluntarily implemented the safety initiatives outlined in the Supplement to Atmos Energy Corporation’s Proposed Findings, Probable Cause, and Recommendations to the National Transportation Safety Board dated September 4, 2020, including efforts to assess and strengthen our distribution integrity management program. The purpose of Atmos Energy’s Distribution Integrity Management Plan (“IMP”) is to achieve continual improvements to enhance safety. In doing so, Atmos Energy’s IMP identifies current and potential threats, assesses and mitigates risk, and measures the overall effectiveness of the program. Atmos Energy’s comprehensive IMP includes a risk model which quantifies threats relative to discrete portions of the distribution system based on factors and weightings, a plan for addressing such threats, and complementary programs to mitigate threats that would increase risk to the integrity of its distribution systems. As confirmed by the investigation, Atmos Energy’s IMP for the Mid-Tex division in effect at the time of the incident met or exceeded regulatory requirements, was consistent with industry standards, and was not directly linked to the event on February 23, 2018. Further, factors that inform threats that degrade a system over time were present and accounted for in the risk model. The factors that support an understanding of degradation include pipe material, coating type, total leaks, leak type/cause, and corrosion and coating conditions. Although Atmos Energy’s understanding of the unprecedented system performance experienced in Northwest Dallas in 2018 differs from that of the NTSB, following this event Atmos Energy pulled together a team of experienced integrity management members and began assessing its Mid-Tex IMP. To date, Atmos Energy has implemented enhancements to the risk model to address both the geophysical issues experienced in NW Dallas as well as threats that can degrade a system over time. The completed work includes: • The engagement of a geotechnical engineering firm to better understand the potential cause for the sudden and unprecedented leaks experienced in Northwest Dallas and to assist in developing a response. As a result of that effort, a new geological risk factor, or Earth Factor (EF) was developed and incorporated in the 2019 Mid-Tex risk model. The EF divides the operating system into individual, discrete grids to account for and quantify certain static risks for potential differential movement, including expansive soils and soil hydrology. The geological and geophysical considerations within the EF serve to elevate risk in the model when conditions warrant. Atmos Energy has since completed the development of an EF for its remaining states of operation. • The implementation of additional factors in Atmos Energy’s Mid-Tex division that inform threats that can degrade a system over time and elevate risk in the model when conditions warrant. These new factors take into consideration age, susceptibility to corrosion, and the presence of mechanical joints. While none of these factors alone (or any factor in isolation) are dispositive of the condition of the system or even degradation of the pipe, they were incorporated into the model after 2018 to further inform and enhance the Mid-Tex division’s risk modeling. Historically, because of regulation and other factors, Atmos Energy has had multiple Distribution Integrity Management Plans. Atmos Energy has also undertaken work to consolidate its various IMP risk models into a single consolidated risk model for its natural gas distribution systems in its eight states of operation. The new model will consider approximately 130 factors across the eight defined categories of threats. Each factor will be weighted for its contribution to the relative likelihood and/or consequences within each threat. Atmos Energy engaged a third-party expert who reviewed these input factors and weighting for relevance and applicability against industry standards and best practices. As part of this effort, Atmos Energy is also exploring approaches to enhance the previously existing methods through which the risk model considers threats that can degrade a system over time, as well as the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure. The basic principle underlying integrity management is that operators know their system, identify and understand the threats to their system, and apply their safety resources commensurate with the relative risk of each threat. As such, we will continue to assess our integrity management programs and incorporate new factors and approaches that contribute to our understanding of threats to the system as they reveal themselves to better apply our safety resources. To date a number of enhancements have already been implemented which are responsive to Recommendation P-21-12, and additional work is underway to further consider and incorporate the items outlined in this Recommendation. We will continue to monitor the work of the Gas Piping Technology Committee (GPTC) in response to Recommendation P-21-14 and consider any additional guidance that is developed related to distribution integrity management.

Addressee Organization Type: P-Private Industry

Communication Date: 2022-01-11

Communication Type: Official Correspondence

Communication Contents: -From John McDill, Senior Vice President Utility Operations: Atmos Energy submits this letter and attachments to supplement its May 7, 2021 response to the National Transportation Safety Board (NTSB) concerning the NTSB’s January 12, 2021 Report on the incident that occurred in Dallas, Texas on February 23, 2018. As the attached supplemental responses reflect, we have completed our work relative to the NTSB Recommendations P-21-8 through P-21-12. We are committed to continuous improvement on our journey to achieve our vision to be the safest provider of natural gas services. Atmos Energy appreciates the opportunity to have participated in this investigation alongside the NTSB and other party participants to further enhance the safe delivery of natural gas services. Atmos Energy appreciates the opportunity to update the NTSB on the outcome of our integrity management program assessment. We remain committed to managing safety and continually improving to achieve our vision of being the safest provider of natural gas service. As outlined in our May 7, 2021 response to the NTSB, the purpose of Atmos Energy’s Distribution Integrity Management Plan (“IMP”) is to achieve continual improvements to enhance safety. In doing so, Atmos Energy’s IMP identifies current and potential threats, assesses and mitigates risk, and measures the overall effectiveness of the program. Atmos Energy’s comprehensive IMP includes a risk model which quantifies threats relative to discrete portions of the distribution system based on factors and weightings, a plan for addressing such threats, and complementary programs to mitigate threats that would increase risk to the integrity of its distribution systems. As confirmed by the investigation, Atmos Energy’s IMP for the Mid-Tex division in effect at the time of the incident met or exceeded regulatory requirements, was consistent with industry standards, and was not directly linked to the event on February 23, 2018. Although our understanding of the unprecedented system performance experienced in Northwest Dallas in 2018 differs from that of the NTSB, following this event Atmos Energy formed a team of experienced integrity management members and began assessing its Mid-Tex IMP. Atmos Energy then implemented enhancements to its risk model to address both the geophysical issues experienced in Northwest Dallas as well as threats that can degrade a system over time, all as described in our May 7, 2021 response. Since our May 7, 2021 response, Atmos has completed its work to consolidate its various IMP risk models into a single risk model for its natural gas distribution systems in its eight states of operation. The new consolidated model considers approximately 130 factors across the eight defined categories of threats. Each factor is weighted for its contribution to the relative likelihood and/or consequences within each threat. Atmos Energy also engaged a third-party expert who reviewed these input factors and weighting for relevance and applicability against industry standards and best practices. We have developed the appropriate enhancements to fully address the NTSB’s safety recommendation to consider threats that can degrade a system over time and to address threats that can simultaneously increase the likelihood and consequence of failure. Specifically, our new model: • Includes the existing “Earth Factor Risk Model” that quantifies the risk for potential differential movement that could impact our distribution system based on a combination of factors such as geology, hydrology, expansive soils, topography, climactic rating and seismic activity. In application, areas that are more susceptible to differential movement will realize higher relative threat-level values. • Accounts for age as a consideration to help inform the potential for the degradation of assets over time. In application, older assets realize higher relative threat-level values. • Incorporates potential consequences related to rain through the consideration of the potential for conditions related to soil, moisture and wetness as indicated by the NASA/NOAA National Land Data Assimilation System to inhibit measurement and venting as well as increase migration. In application, areas where these conditions make the operating environment more susceptible to effects realize higher relative threat-level values. • Considers the opportunity for threats to interact in such a way that their concurrent presence or simultaneous action increases threat levels and overall risk. In application, threat and risk levels are elevated according to the likelihood that threats may be coincident and that an interaction may occur. • Incorporates the potential for historical excavation activity to have resulted in unreported non-leak damage to assets (latent damage), such as a gouge, dent, or coating damage by resourcing historical locate ticket data. In application, higher volumes of opportunities for non-leak damage as indicated by ticket volumes in a local area realize higher relative threat level values.

Addressee Organization Type: P-Private Industry

Communication Date: 2023-04-25

Communication Type: Official Correspondence

Communication Contents: -From John McDill, Senior Vice President Utility Operations, Atmos Energy: Atmos Energy submits this letter and attachments to supplement its prior responses to the National Transportation Safety Board (NTSB) concerning the NTSB’s January 12, 2021, Report on the incident that occurred in Dallas, Texas on February 23, 2018. The attached supplemental responses reflect the work we have undertaken to implement Recommendations P-21-11 and P-21- 12. We are committed to continuous improvement on our journey to achieve our vision to be the safest provider of natural gas services. Atmos Energy appreciates the opportunity to have participated in this investigation alongside the NTSB and other party participants to further enhance the safe delivery of natural gas services. This addresses the NTSB’s May 11, 2022 request for additional information about the scope of our revised distribution integrity management program (DIMP). Our new DIMP model specifically incorporates swell/shrink and moisture infiltration characteristics of the soil in a localized area (based on the USDA’s Natural Resource Conservation Service Soil Survey) to quantify the potential for stress and leakage on our pipelines. The model is configured so that soil groups with the largest amounts of clay receive the highest risk ranking relative to other soil types. In this regard, Atmos’ DIMP model explicitly considers the swell potential of clay soil where such soil is present. With respect to the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure, our DIMP also considers the potential for soil moisture to inhibit measurement and venting, as well as increase the potential for migration. Using soil moisture data from the National Land Data Assimilation System (NLDAS), our DIM model attributes a higher relative risk ranking to assets in areas with greater soil moisture. In this regard, Atmos’ DIMP model explicitly considers the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure by considering soil moisture’s ability to inhibit gas measurement and venting and to facilitate migration. Since 2018, our Mid-Tex division’s leak management program has continued to expand the use of Advanced Mobile Leak Detection (AMLD) technology for compliance and special leak surveys, particularly in areas with steel assets (mains or services). AMLD technology is more sensitive than traditional leak survey equipment which enhances Atmos’ ability find and repair leaks, simultaneously mitigating system risks as well as providing information related to leak cause and pipe condition that is used within Atmos’ DIMP. This information will continue to enhance our risk model’s predictive capabilities and results over time. Our Mid-Tex division also leverages AMLD technology and processes to improve our ability to identify and respond to emerging leakage trends. Our compliance group has developed an AMLD leak dashboard that provides a visual alert when we encounter leaks in a localized area that are above our established threshold. The dashboard attributes a score to each leak discovered (based on its grade classification) and provides a visual alert on our ESRI GIS application if the score for all leaks within that area exceeds the threshold. When we receive an alert, members of our Operations and Technical Services groups meet to review the data and determine an appropriate response including if accelerated actions are needed. This dashboard-based alert process and the associated response promotes integrity management and complements our DIM risk modeling process. Finally, our Mid-Tex division has also developed and implemented a dynamic soil stability alert system which considers changes in soil moisture relative to the underlying soil type and provides a notice of areas where rain or other weather conditions could be causing soils to shrink or swell and increase the potential for pipe stress and susceptibility leakage. The soil stability alert system uses soil moisture data that is updated daily in order to capture the impact of current weather events. Upon receiving a notification, we perform a review of our assets in the area identified including the high relative risk areas identified by DIMP. Based on this review, we deploy a leak survey team to assess the area. Post survey, we expedite all below grade leaks found and evaluate leak cause for any accelerated action necessary. In sum, our DIMP enhancements appropriately consider threats that may degrade our system, such as the swell potential of the clay soil, we have implemented changes to our leak management program that will improve our DIMP model, and we have identified and appropriately considered the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure, such as rain. Accordingly, we believe we have fully addressed the NTSB’s safety recommendation P-21-12.

Addressee Organization Type: P-Private Industry

Communication Date: 2021-08-04

Communication Type: Official Correspondence

Communication Contents: We note that, since the Dallas incident, you implemented enhancements to your risk model that include additional risk factors that address geological risk, age, susceptibility to corrosion, and the presence of mechanical joints. We further note that you are consolidating your various distribution integrity management plan (DIMP) risk models into a single consolidated risk model for your natural gas distribution systems in your eight states of operation. Your new model will consider approximately 130 factors across the eight defined categories of threats, and each factor will be weighted for its contribution to the relative likelihood or consequences within each threat. We understand that you have engaged a third-party expert to review these input factors and weight them for relevance and applicability against industry standards and best practices. Lastly, you report that you are also exploring approaches to enhance the previously existing methods through which the risk model considers threats that can degrade a system over time, as well as the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure. Pending the completion of your work to assess and revise your DIMP to appropriately consider threats that degrade a system over time and factors that can combine to increase the likelihood and consequence of failure, Safety Recommendation P-21-12 is classified OPEN-- ACCEPTABLE RESPONSE.

Addressee Organization Type: P-Private Industry

Communication Date: 2022-05-11

Communication Type: Official Correspondence

Communication Contents: We note that you have consolidated your various distribution integrity management plan (DIMP) risk models into a single risk model for the natural gas distribution systems in your eight states of operation. Your new consolidated DIMP considers approximately 130 factors across the eight defined threat categories. We further note that you engaged a third-party expert to review these input factors and weighting for relevance and applicability against industry standards and best practices. Your revised DIMP reportedly: • Includes an “Earth Factor Risk Model” that quantifies the risk for potential differential movement that you believe could impact your distribution system; • Takes age into account when determining threat levels; • Incorporates an environment’s susceptibility to conditions that may inhibit measurement and venting as well as increase migration, such as moisture; • Considers the opportunity that threats that may interact and increase overall risk; and • Accounts for historical excavation activity that may have resulted in latent damage. Although the NTSB concluded that the high number of leaks observed in northwest Dallas were due to degradation of your gas distribution system, not sudden, unanticipated geologic loadings, your existing “Earth Factor Risk Model” is based on a theory that these leaks were “sudden” and “unanticipated.” As a result, it is not clear that your revised DIMP appropriately considers threats that may have degraded your system prior to the explosion, such as the swell potential of the clay soil found in this area. If you have implemented changes to your leak management program that increase the sensitivity and frequency of leak surveys, they may improve your DIMP model’s predictive capabilities going forward, but such details have not been provided. Similarly, this explosion demonstrated that rain presented a threat, given the preexisting leak, and simultaneously increased the consequences by inhibiting venting, reducing the effectiveness of natural gas measurements, and increasing the tendency of natural gas to migrate laterally where it could be stripped of odorant and create a hazardous condition. Other threats may also reduce mitigative capability, such as working near explosive natural gas/air mixtures, excavating near gas lines, etc. It is not clear that your revised DIMP has identified and appropriately considered the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure. These actions may satisfy our recommendation; however, we ask that you provide the specific changes you have made to your DIMP and explain how the revisions satisfy this recommendation so that we can independently assess the effect of these changes. Pending the receipt and evaluation of this information, Safety Recommendation P-21-12 remains classified OPEN-- ACCEPTABLE ACTION.

Addressee Organization Type: P-Private Industry

Communication Date: 2023-09-06

Communication Type: Official Correspondence

Communication Contents: We note that you revised the risk model in your consolidated integrity management program to appropriately consider threats that degrade a system over time and the increased risk that can result from factors that simultaneously increase the likelihood and consequence of failure. Accordingly, Safety Recommendation P-21-12 is classified CLOSED-- ACCEPTABLE ACTION.