# Atmos Energy Corporation Natural Gas-Fueled Explosion - **operation:** document - **citation:** PLD18FR002 - **title:** Atmos Energy Corporation Natural Gas-Fueled Explosion - **source type:** incident - **agency:** National Transportation Safety Board - **status:** current - **official:** true - **published on:** 2021-01-12 - **effective on:** 2018-02-23 - **summary:** Accident. in Dallas, TX, USA. on 2018-02-23. Atmos Energy. Leak/explosion/fire - **machine formats:** - **json:** https://regulus.evalyn.ai/document/ntsb-case-pld18fr002.json - **markdown:** https://regulus.evalyn.ai/document/ntsb-case-pld18fr002.md - **app url:** https://regulus.evalyn.ai/document/ntsb-case-pld18fr002 - **source url:** https://www.ntsb.gov/investigations/Pages/PLD18FR002.aspx **body:** NTSB investigation PLD18FR002. Event Type: Accident Event Date: 2018-02-23 Event City: Dallas Event State Or Region: TX Event Country: USA Pipeline Operator: Atmos Energy Pipeline Type: Distribution Accident Type: Leak/explosion/fire Completion Status: Completed Report Number: 64964 Report Date: 2021-01-12 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. Tier1Name: Emergency response Tier2Name: Evacuation Tier1Name: Emergency response Tier2Name: Emergency shutoff Tier1Name: System operating Tier2Name: Fire/explosion (post-release) Tier1Name: System maintenance Tier2Name: Maintenance event Tier1Name: System operating Tier2Name: Product leak/release Finding Tier1Name: Organizational Finding Tier2Name: Management Finding Tier3Name: Policy/procedure Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Management - Policy/procedure - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Safety programs Finding Modifier Name: Pipeline operator Finding Report Text: Organizational - Support/oversight/monitoring - Safety programs - Pipeline operator Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Training Finding Modifier Name: State/Local agency Finding Report Text: Organizational - Support/oversight/monitoring - Training - State/Local agency Finding Tier1Name: Organizational Finding Tier2Name: Management Finding Tier3Name: Policy/procedure Finding Modifier Name: State/Local agency Finding Report Text: Organizational - Management - Policy/procedure - State/Local agency Finding Tier1Name: Organizational Finding Tier2Name: Support/oversight/monitoring Finding Tier3Name: Safety programs Finding Modifier Name: Federal agency Finding Report Text: Organizational - Support/oversight/monitoring - Safety programs - Federal agency Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline structure Finding Tier3Name: Pipe Finding Modifier Name: Damaged/degraded Finding Report Text: Pipeline - Pipeline structure - Pipe - Damaged/degraded Finding Tier1Name: Environment/Infrastructure Finding Tier2Name: Conditions/weather/phenomena Finding Tier3Name: Precipitation Finding Modifier Name: Effect on equipment Finding Report Text: Environment/Infrastructure - Conditions/weather/phenomena - Precipitation - Effect on equipment Finding Tier1Name: Environment/Infrastructure Finding Tier2Name: Physical environment Finding Tier3Name: Soil Finding Modifier Name: Effect on operation Finding Report Text: Environment/Infrastructure - Physical environment - Soil - Effect on operation Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline operation/capability Finding Tier3Name: Pipeline integrity/capacity Finding Modifier Name: Not attained/maintained Finding Report Text: Pipeline - Pipeline operation/capability - Pipeline integrity/capacity - Not attained/maintained Finding Tier1Name: Pipeline Finding Tier2Name: Pipeline handling/service Finding Tier3Name: Maintenance/inspection Finding Modifier Name: Inadequate inspection Finding Report Text: Pipeline - Pipeline handling/service - Maintenance/inspection - Inadequate inspection Official NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations. What Happened On February 23, 2018, about 6:38 a.m. local time, a natural gas-fueled explosion occurred at 3534 Espanola Drive, Dallas, Texas, injuring all five occupants, one fatally. The one-story two-bedroom residence sustained major structural damage. Following the explosion, National Transportation Safety Board investigators located a through-wall crack in the 71-year-old natural gas main that served the residence and positive gas measurements leading from this crack to 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. What We Found 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. What We Recommended As a result of this investigation, we made the following new safety recommendations. To the Pipeline and Hazardous Materials Safety Administration: 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) 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 and limit the effectiveness of leak management programs. (P-21-2) Assist the Railroad Commission of Texas in conducting the audit recommended in Safety Recommendation P-21-4. (P-21-3) To the Railroad Commission of Texas: With assistance from the Pipeline and Hazardous Materials Safety Administration, conduct a comprehensive audit of Atmos Energy Corporation’s incident-reporting practices; policies and procedures for responding to leaks, fires, explosions, and emergency calls; and integrity management programs. (P-21-4) To the Dallas Fire-Rescue Department: Revise the continuing education requirements for your arson investigators to include training on building fuel gas systems. (P-21-5) Revise your procedures to require gas monitoring after the occurrence of a gasrelated structure fire or explosion. (P-21-6) Develop and implement a formal process to alert appropriate local, state, and federal agencies of potential systemic safety issues that should be investigated further. (P-21-7) To Atmos Energy Corporation: 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) 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) 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) 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) 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) To the Gas Piping Technology Committee: Develop additional guidance that identifies steps gas distribution operators can take to safely respond to leaks, fires, explosions, and emergency calls, considering the limitations due to wet weather conditions, that includes: (1) criteria for when to shut down or isolate gas distribution systems, pressure test main and service lines, and begin evacuations; (2) leak investigation methods that are reliable in wet weather, (3) require an alternate safe response, such as an evacuation when reliable leak investigations are not possible due to wet weather, and (4) leak investigations that assess all viable gas migration paths, including granular backfill and crawlspaces. (P-21-13) Develop guidance that identifies steps that gas distribution operators can take to ensure that their gas distribution integrity management program, at a minimum, appropriately considers: (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-21-14) We reiterated the following safety recommendations. To the International Code Council: In coordination with the Gas Technology Institute and the National Fire Protection Association, incorporate provisions in the International Fuel Gas Code that requires methane detection systems for all types of residential occupancies with gas service. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-006) To the National Fire Protection Association: In coordination with the Gas Technology Institute and the International Code Council, revise the National Fuel Gas Code, National Fire Protection Association 54 to require methane detection systems for all types of residential occupancies with gas service. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-007) To the Gas Technology Institute: In coordination with the National Fire Protection Association and the International Code Council, work to develop standards for methane detection systems for all types of residential occupancies in both the International Fuel Gas Code and the National Fuel Gas Code, National Fire Protection Association 54. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-008) PAR-21-01 <<>> Atmos Energy Corporation Natural Gas-Fueled Explosion Dallas, Texas February 23, 2018 Accident Report NTSB/PAR-21/01 PB2021-100901 National Transportation Safety Board <<>> NTSB/PAR-21/01 PB2021-100901 Notation 64964 Adopted January 12, 2021 Pipeline Accident Report Atmos Energy Corporation Natural Gas-Fueled Explosion Dallas, Texas February 23, 2018 National Transportation Safety Board 490 L’Enfant Plaza, S.W. Washington, D.C. 20594 <<>> National Transportation Safety Board. 2021. Atmos Energy Corporation Natural Gas-Fueled Explosion, Dallas, Texas, February 23, 2018. Publication Type NTSB/PAR-21/01. Washington, DC: NTSB Abstract: 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. The National Transportation Safety Board (NTSB) is an independent federal agency dedicated to promoting aviation, railroad, highway, marine, and pipeline safety. Established in 1967, the agency is mandated by Congress through the Independent Safety Board Act of 1974, to investigate transportation accidents, determine the probable causes of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety effectiveness of government agencies involved in transportation. The NTSB makes public its actions and decisions through accident reports, safety studies, special investigation reports, safety recommendations, and statistical reviews. The NTSB does not assign fault or blame for an accident or incident; rather, as specified by NTSB regulation, “accident/incident investigations are fact-finding proceedings with no formal issues and no adverse parties … and are not conducted for the purpose of determining the rights or liabilities of any person” (Title 49 Code of Federal Regulations section 831.4). Assignment of fault or legal liability is not relevant to the NTSB’s statutory mission to improve transportation safety by investigating accidents and incidents and issuing safety recommendations. In addition, statutory language prohibits the admission into evidence or use of any part of an NTSB report related to an accident in a civil action for damages resulting from a matter mentioned in the report (Title 49 United States Code section 1154(b)). For more detailed background information on this report, visit the NTSB investigations website and search for NTSB accident ID PLD18FR002. Recent publications are available in their entirety on the NTSB website. Other information about available publications also may be obtained from the website or by contacting— National Transportation Safety Board Records Management Division, CIO-40 490 L’Enfant Plaza, SW Washington, DC 20594 (800) 877-6799 or (202) 314-6551 Copies of NTSB publications may be downloaded at no cost from the National Technical Information Service, at the National Technical Reports Library search page, using product number PB2021-100901. For additional assistance, contact— National Technical Information Service 5301 Shawnee Rd. Alexandria, VA 22312 (800) 553-6847 or (703) 605-6000 NTIS website <<>> Contents Figures ........................................................................................................................................... iii Abbreviations and Acronyms ..................................................................................................... iv Executive Summary .................................................................................................................... vii Probable Cause.............................................................................................................................. vii Safety Issues................................................................................................................................. viii Findings.......................................................................................................................................... ix Recommendations .......................................................................................................................... xi New Recommendations .......................................................................................................... xi Previously Issued Recommendations Reiterated in this Report ........................................... xiii 1. Factual Information .................................................................................................................1 1.1 Synopsis ....................................................................................................................................1 1.2 Background ..............................................................................................................................2 1.3 Narrative ...................................................................................................................................2 1.4 Events Leading Up to the Explosion ........................................................................................3 1.4.1 3527 Durango Drive Incident ........................................................................................5 1.4.2 3515 Durango Drive Incident ........................................................................................7 1.4.3 Atmos Energy Response Following Second Incident ....................................................7 1.5 Emergency Response to 3534 Espanola Drive .......................................................................11 1.6 System Isolation and Pipe Segment Replacement .................................................................13 1.7 Examinations After the Explosion .........................................................................................17 1.7.1 On-site Integrity Tests..................................................................................................17 1.7.2 On-site Gas Measurements ..........................................................................................19 1.7.3 Gas Odorization Testing ..............................................................................................20 1.7.4 Gas Regulator Testing..................................................................................................20 1.7.5 NTSB Testing ..............................................................................................................21 1.7.6 Soil Testing ..................................................................................................................25 1.7.7 Incidents at 3527 and 3515 Durango Drives................................................................27 1.8 Prior Leak History ..................................................................................................................28 1.9 Atmos Policies and Procedures ..............................................................................................29 1.9.1 Gas Leak Surveys ........................................................................................................30 1.9.2 Emergency Response Procedures ................................................................................31 1.9.3 Gas Distribution Pipeline Integrity Management Program .........................................32 1.10 Regulatory and Municipal Requirements ...............................................................................33 1.10.1 PHMSA Regulatory Requirements ..............................................................................33 1.10.2 RRC Regulatory Requirements....................................................................................35 1.10.3 DFR Procedures and Training .....................................................................................35 1.11 Actions After the Explosion ...................................................................................................36 1.11.1 Regulatory Actions After the Explosion ......................................................................36 1.11.2 Atmos Actions After the Explosion .............................................................................37 1.11.3 DFR Actions After the Explosion ................................................................................39 64964 <<>> NTSB Pipeline Accident Report 2. Analysis ...................................................................................................................................41 2.1 Introduction ............................................................................................................................41 2.2 Condition of the Natural Gas Main ........................................................................................42 2.3 Investigation of the Two Incidents that Preceded the Explosion ...........................................46 2.3.1 DFR’s Investigation of the First Two Incidents ..........................................................46 2.3.2 Atmos’s Investigation of the First Two Incidents ........................................................48 2.3.3 NTSB’s Evaluation of Causal Factors for the First Two Incidents .............................50 2.4 Leak Investigations and Repairs Prior to the Explosion ........................................................53 2.5 Methane Detection .................................................................................................................57 2.6 Incident Reporting ..................................................................................................................60 2.6.1 Atmos Incident Reporting ............................................................................................60 2.6.2 DFR Incident Reporting ...............................................................................................62 2.7 Integrity Management ............................................................................................................62 3. Conclusions .............................................................................................................................69 3.1 Findings ..................................................................................................................................69 3.2 Probable Cause .......................................................................................................................71 4. Recommendations ..................................................................................................................72 4.1 New Recommendations ..........................................................................................................72 4.2 Previously Issued Recommendations Reiterated in this Report .............................................74 Board Member Statement ...........................................................................................................76 Appendixes....................................................................................................................................78 Appendix A. The Investigation ......................................................................................................78 Appendix B. Consolidated Recommendation Information ............................................................78 References .....................................................................................................................................83 ii <<>> NTSB Pipeline Accident Report Figures Figure 1. Location of explosion, preceding incidents, and shared utilities. ................................... 2 Figure 2. Timeline of the events leading up to the explosion. ....................................................... 5 Figure 3. Gas measurements taken in the block surrounding the site prior to the explosion. ....... 9 Figure 4. Leaks identified prior to the explosion. ........................................................................ 11 Figure 5. Dallas Fire-Rescue map of the four exclusionary zones after the February 23 explosion. ...................................................................................................................................... 13 Figure 6. Leaks identified as Grade 1 or 2 and prior to system replacement. .............................. 15 Figure 7. Crack in natural gas main at 3539 Durango Drive sewer lateral crossing. .................. 18 Figure 8. Gas measurements taken in the block surrounding the site after the explosion. .......... 19 Figure 9. Gas measurements taken on and near the property after the explosion........................ 20 Figure 10. Natural gas main recovered near 3539 Durango Drive sewer lateral crossing. ......... 21 Figure 11. Side view of the damaged natural gas main showing the dent with a crack. ............. 21 Figure 12. Top surface of the natural gas main showing the five major gouges. ........................ 22 Figure 13. Crack and dent on top surface of natural gas main after removing surface deposits. 22 Figure 14. West face of pipe fracture after separating mating faces of circumferential crack. ... 23 Figure 15. West face of the pipe fracture, after cleaning procedure. ........................................... 23 Figure 16. Natural gas main, with service line recovered from behind 3524 Espanola Drive. ... 25 Figure 17. Leaks identified as Grade 1 or Grade 2 which were beyond the scope of this investigation. ................................................................................................................................. 63 iii <<>> NTSB Pipeline Accident Report Abbreviations and Acronyms AGA American Gas Association ANSI American National Standards Institute API American Petroleum Institute ASME American Society of Mechanical Engineers Atmos Atmos Energy Corporation BCI Bryant Consultants, Inc. CFM cubic feet per minute CFR Code of Federal Regulations CGI combustible gas indicator DFR Dallas Fire-Rescue Department DIRT Damage Information Reporting Tool DOT U.S. Department of Transportation EDS energy dispersive spectroscopy GPTC Gas Piping Technology Committee hazmat hazardous materials HAZWOPER Hazardous Waste Operations and Emergency Response HMCRP Hazardous Materials Cooperative Research Program HMRT Hazardous Materials Response Team HUD US Department of Housing and Urban Development HVAC heating, ventilation, and air conditioning IC incident commander ICC International Code Council IFGC International Fuel Gas Code iv <<>> NTSB Pipeline Accident Report IG inspector general IM integrity management IM Plan Distribution Risk and Integrity Management Plan LEL lower explosive limit MAOP maximum allowable operating pressure MNOPS Minnesota Department of Public Safety, Office of Pipeline Safety NFPA National Fire Protection Association NFPA 54 National Fuel Gas Code NRC National Response Center NTSB National Transportation Safety Board O&M operations and management OQ operator qualifications OSHA Occupational Safety and Health Administration PHMSA Pipeline and Hazardous Materials Safety Administration ppm parts per million psig pounds per square inch gauge PSMS Pipeline Safety Management System RMLD remote methane leak detector RRC Railroad Commission of Texas RSPA Research and Special Programs Administration SEM scanning electron microscopy SME subject matter expert SOP standard operating procedure TAC Texas Administrative Code TCFP Texas Commission of Fire Protection v <<>> NTSB Pipeline Accident Report TRB Transportation Research Board UEL upper explosive limit USACE US Army Corps of Engineers U.S.C. United States Code USCB US Census Bureau vi <<>> NTSB Pipeline Accident Report Executive Summary On February 23, 2018, about 6:38 a.m. local time, a natural gas-fueled explosion occurred at 3534 Espanola Drive, Dallas, Texas, injuring all five occupants, one fatally. The one-story two-bedroom residence sustained major structural damage. Following the explosion, National Transportation Safety Board investigators located a through-wall crack in the 71-year-old natural gas main that served the residence and positive gas measurements leading from this crack to the residence.1 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. 2 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. 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. 1 (a) For more information, see the factual information and analysis sections of the report. Additional information about the accident investigation can be found in the public docket for the accident (NTSB case number PLD18FR002) by accessing the Accident Dockets link for the Docket Management System at www.ntsb.gov. For more information on our safety recommendations, see the Safety Recommendation Database at www.ntsb.gov. (b) Through-wall crack refers to a crack that extends between the inner- and outer-diameter of a pipe; the commodity cannot leak from a crack in the pipe until it extends through the wall. 2 Throughout this report, the term “explosion” is used to refer to the explosion at 3534 Espanola Drive, whereas the term “incident” is used to refer to the earlier events at 3527 Durango Drive and 3515 Durango Drive. The term “incident” is used in accordance with Title 49 Code of Federal Regulations 831.40(a)(2) and does not indicate that these events meet the Pipeline and Hazardous Materials Safety Administration’s definition of incident in Title 49 Code of Federal Regulations 191.3. vii <<>> NTSB Pipeline Accident Report Safety Issues This report focuses on the following safety issues: • Incident investigation. Neither the Dallas Fire-Rescue Department nor Atmos Energy Corporation identified the causes of the two incidents that occurred in the days immediately preceding the explosion. Dallas Fire-Rescue Department arson investigators and Atmos Energy Corporation technicians did not effectively investigate, communicate, or collaborate to determine the cause of either incident. Further, Atmos Energy Corporation did not gather enough evidence to determine if gas migrated from their piping and fueled the first two incidents. • Leak investigations and repairs. Atmos Energy Corporation 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 Energy Corporation 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 Energy Corporation added odorant to its gas distribution system in a manner consistent with Pipeline and Hazardous Materials Safety Administration 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 Pipeline and Hazardous Materials Safety Administration 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 Energy Corporation relied on an incomplete investigation to support its position not to report the first two incidents. • Integrity management. Although Atmos Energy Corporation’s integrity management program was generally consistent with regulatory requirements and industry practice, the program did not adequately evaluate and address the risk of its 71-year-old system. This failure to adequately address risk was illustrated by the 26 leaks determined to present an existing or probable future hazard in the area around the explosion, as well as the additional 740 leaks found in northwest Dallas in the weeks that followed. viii <<>> NTSB Pipeline Accident Report Findings • None of the following were factors in the explosion: (1) ongoing maintenance activities; (2) overpressurization of the gas distribution system; (3) materials used for the construction of the gas main and external coating; and (4) natural gas composition. • The natural gas main was damaged by mechanical excavation equipment, likely when the sanitary sewer lateral was replaced in 1995. • A circumferential crack in the main propagated through the pipe wall prior to the first incident, allowing natural gas to leak into the surrounding environment for an extended period. • Soil absorbed and depleted the natural gas odorant, eliminating the opportunity for occupants to detect it. • Natural gas leaking from Atmos Energy Corporation’s cracked gas main in the alley behind 3534 Espanola Drive migrated through the soil and into the house where it was ignited by an unknown source. • Dallas Fire-Rescue Department’s initial misclassification of the first incident delayed the sharing of information that could have helped Atmos Energy Corporation identify the origin of the leak. • Had the Dallas Fire-Rescue Department’s arson investigators been adequately trained on natural gas systems, their investigation findings may have provided more timely and accurate assistance to Atmos Energy Corporation in locating the source of the gas leak. • Timely pressure testing of the customer piping by Atmos Energy Corporation could have eliminated potential sources of the gas leaks and helped focus their efforts on outside leak detection to locate the damaged and leaking gas system piping more quickly. • Atmos Energy Corporation did not adequately investigate the first two gas-related incidents that occurred at 3527 and 3515 Durango Drive. • Damage to the structure involved in the first incident on 3527 Durango Drive was consistent with a fuel gas/air mixture explosion, which was most likely caused by natural gas that migrated from underneath the structure. • Fuel gas was involved in both incident homes; there was insufficient evidence to exclude natural gas from Atmos Energy Corporation’s system from either incident, evidence of leaks present prior to the first two incidents occurring, and the probability of two or three structure fires/explosions occurring independently on the same block during the same week is very low. Therefore, the two prior incidents that occurred on ix <<>> NTSB Pipeline Accident Report the same block on subsequent days and the explosion at 3534 Espanola Drive were all likely related. • Limitations of the equipment and procedures due to the wet weather conditions on the ability of Atmos Energy Corporation to reliably detect the presence of leaked gas during its response to the first two incidents, and the number and severity of leaks identified following the first two incidents and prior to the explosion, should have prompted Atmos Energy Corporation to shut down or isolate the pipeline. • Had Atmos Energy Corporation pressure tested the main in the alley behind the first two incident homes on February 21 or 22, it could have found that the main did not hold pressure, spurring additional protective actions that could have prevented the fatal injury at 3534 Espanola Drive. • Atmos Energy Corporation’s wet weather leak investigation procedures were insufficient given the known limitations of its equipment. • The assistance of the Dallas Fire-Rescue Department’s Hazardous Materials Response Team, particularly after the second incident, could have enhanced Atmos Energy Corporation’s leak investigation. • Had methane detectors been installed at the residences located on Durango and Espanola Drives, an alarm would have alerted residents to a gas release, reducing the potential for and consequences of the resulting natural gas fires and explosions. • The lack of official reporting of the first two incidents by Atmos Energy Corporation delayed the response from regulatory authorities, the Railroad Commission of Texas and the Pipeline and Hazardous Materials Safety Administration. • The Pipeline and Hazardous Materials Safety Administration does not provide clear requirements regarding the level of investigation necessary to determine whether an event is subject to its reporting requirements, potentially resulting in the underreporting of natural gas incidents. • If Dallas Fire-Rescue Department reported the first two incidents in a timely manner, it could have prompted further investigation or regulatory oversight prior to the explosion. • The high number of leaks observed in northwest Dallas after the explosion were due to the degradation of Atmos Energy Corporation’s gas distribution system, not sudden, unanticipated geologic loadings. • Atmos Energy Corporation 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 requir - **truncated:** true - **body characters:** 272095