{"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.\n\nEvent Type: Accident\n\nEvent Date: 2018-02-23\n\nEvent City: Dallas\n\nEvent State Or Region: TX\n\nEvent Country: USA\n\nPipeline Operator: Atmos Energy\n\nPipeline Type: Distribution\n\nAccident Type: Leak/explosion/fire\n\nCompletion Status: Completed\n\nReport Number: 64964\n\nReport Date: 2021-01-12\n\nProbable 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.\n\nTier1Name: Emergency response\n\nTier2Name: Evacuation\n\nTier1Name: Emergency response\n\nTier2Name: Emergency shutoff\n\nTier1Name: System operating\n\nTier2Name: Fire/explosion (post-release)\n\nTier1Name: System maintenance\n\nTier2Name: Maintenance event\n\nTier1Name: System operating\n\nTier2Name: Product leak/release\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Management\n\nFinding Tier3Name: Policy/procedure\n\nFinding Modifier Name: Pipeline operator\n\nFinding Report Text: Organizational - Management - Policy/procedure - Pipeline operator\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Support/oversight/monitoring\n\nFinding Tier3Name: Safety programs\n\nFinding Modifier Name: Pipeline operator\n\nFinding Report Text: Organizational - Support/oversight/monitoring - Safety programs - Pipeline operator\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Support/oversight/monitoring\n\nFinding Tier3Name: Training\n\nFinding Modifier Name: State/Local agency\n\nFinding Report Text: Organizational - Support/oversight/monitoring - Training - State/Local agency\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Management\n\nFinding Tier3Name: Policy/procedure\n\nFinding Modifier Name: State/Local agency\n\nFinding Report Text: Organizational - Management - Policy/procedure - State/Local agency\n\nFinding Tier1Name: Organizational\n\nFinding Tier2Name: Support/oversight/monitoring\n\nFinding Tier3Name: Safety programs\n\nFinding Modifier Name: Federal agency\n\nFinding Report Text: Organizational - Support/oversight/monitoring - Safety programs - Federal agency\n\nFinding Tier1Name: Pipeline\n\nFinding Tier2Name: Pipeline structure\n\nFinding Tier3Name: Pipe\n\nFinding Modifier Name: Damaged/degraded\n\nFinding Report Text: Pipeline - Pipeline structure - Pipe - Damaged/degraded\n\nFinding Tier1Name: Environment/Infrastructure\n\nFinding Tier2Name: Conditions/weather/phenomena\n\nFinding Tier3Name: Precipitation\n\nFinding Modifier Name: Effect on equipment\n\nFinding Report Text: Environment/Infrastructure - Conditions/weather/phenomena - Precipitation - Effect on equipment\n\nFinding Tier1Name: Environment/Infrastructure\n\nFinding Tier2Name: Physical environment\n\nFinding Tier3Name: Soil\n\nFinding Modifier Name: Effect on operation\n\nFinding Report Text: Environment/Infrastructure - Physical environment - Soil - Effect on operation\n\nFinding Tier1Name: Pipeline\n\nFinding Tier2Name: Pipeline operation/capability\n\nFinding Tier3Name: Pipeline integrity/capacity\n\nFinding Modifier Name: Not attained/maintained\n\nFinding Report Text: Pipeline - Pipeline operation/capability - Pipeline integrity/capacity - Not attained/maintained\n\nFinding Tier1Name: Pipeline\n\nFinding Tier2Name: Pipeline handling/service\n\nFinding Tier3Name: Maintenance/inspection\n\nFinding Modifier Name: Inadequate inspection\n\nFinding Report Text: Pipeline - Pipeline handling/service - Maintenance/inspection - Inadequate inspection\n\nOfficial NTSB investigation data. NTSB findings determine probable cause and make safety recommendations; they do not adjudicate civil liability or regulatory violations.\n\nWhat Happened\nOn 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.\n\nIn 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.\n\nWhat We Found\nThe 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.\n\nWhat We Recommended\nAs a result of this investigation, we made the following new safety recommendations.\nTo the Pipeline and Hazardous Materials Safety Administration:\n\nExpand 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)\nEvaluate 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)\nAssist the Railroad Commission of Texas in conducting the audit recommended in Safety Recommendation P-21-4. (P-21-3)\n\nTo 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)\n\nTo the Dallas Fire-Rescue Department:\nRevise the continuing education requirements for your arson investigators to include training on building fuel gas systems. (P-21-5)\nRevise your procedures to require gas monitoring after the occurrence of a gas\u0002related structure fire or explosion. (P-21-6)\nDevelop 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)\n\nTo Atmos Energy Corporation:\nProvide 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)\nDevelop 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)\nDevelop 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)\nRevise 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)\nWithout 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)\n\nTo the Gas Piping Technology Committee:\nDevelop 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)\nDevelop 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)\n\nWe reiterated the following safety recommendations.\n\nTo 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\nthat 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)\n\nTo 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\nwith gas service. At a minimum, the provisions should cover the installation, maintenance, placement of the detectors, and testing requirements. (P-19-007)\n\nTo 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)\n\nPAR-21-01\n<<<PAGE 1>>>\n\nAtmos Energy Corporation Natural Gas-Fueled Explosion\nDallas, Texas\nFebruary 23, 2018\nAccident Report\nNTSB/PAR-21/01\nPB2021-100901\nNational\nTransportation\nSafety Board\n\n<<<PAGE 2>>>\n\nNTSB/PAR-21/01\nPB2021-100901\nNotation 64964\nAdopted January 12, 2021\nPipeline Accident Report\nAtmos Energy Corporation Natural Gas-Fueled Explosion\nDallas, Texas\nFebruary 23, 2018\nNational\nTransportation\nSafety Board\n490 L’Enfant Plaza, S.W.\nWashington, D.C. 20594\n\n<<<PAGE 3>>>\n\nNational Transportation Safety Board. 2021. Atmos Energy Corporation Natural Gas-Fueled\nExplosion, Dallas, Texas, February 23, 2018. Publication Type NTSB/PAR-21/01. Washington, DC:\nNTSB\nAbstract: On February 23, 2018, at 6:38 a.m. local time, a natural gas–fueled explosion occurred at 3534\nEspanola Drive, Dallas, Texas. The residence sustained major structural damage, but when first\nresponders arrived on scene at 6:44 a.m., they observed no smoke or fire. Four family members were\ninjured, and one was killed in the explosion. Following the explosion, National Transportation Safety\nBoard (NTSB) investigators located a through-wall crack in the 71-year-old natural gas main that served\nthe residence. In the 2 days before this explosion, two gas-related incidents occurred on the same block at\nhouses that were served by the same natural gas main, each resulting in significant structural damage and\nburn injuries to one occupant. The first occurred on February 21, 2018, at 5:49 a.m., and resulted in one\ninjury involving second-degree burns and significant structural damage to 3527 Durango Drive. The\nsecond incident occurred on February 22, 2018, at 10:21 a.m., and resulted in one injury involving\nsecond-degree burns and significant structural damage to 3515 Durango Drive. As a result of this\ninvestigation, the NTSB issued new safety recommendations to the Pipeline and Hazardous Materials\nSafety Administration, the Railroad Commission of Texas, the Dallas Fire-Rescue Department, Atmos\nEnergy Corporation, and the Gas Piping Technology Committee. The NTSB is also reiterating safety\nrecommendations to the International Code Council, the National Fire Protection Association, and the\nGas Technology Institute.\nThe National Transportation Safety Board (NTSB) is an independent federal agency dedicated to promoting\naviation, railroad, highway, marine, and pipeline safety. Established in 1967, the agency is mandated by Congress\nthrough the Independent Safety Board Act of 1974, to investigate transportation accidents, determine the probable\ncauses of the accidents, issue safety recommendations, study transportation safety issues, and evaluate the safety\neffectiveness of government agencies involved in transportation. The NTSB makes public its actions and decisions\nthrough accident reports, safety studies, special investigation reports, safety recommendations, and statistical\nreviews.\nThe NTSB does not assign fault or blame for an accident or incident; rather, as specified by NTSB regulation,\n“accident/incident investigations are fact-finding proceedings with no formal issues and no adverse parties … and\nare not conducted for the purpose of determining the rights or liabilities of any person” (Title 49 Code of Federal\nRegulations section 831.4). Assignment of fault or legal liability is not relevant to the NTSB’s statutory mission to\nimprove transportation safety by investigating accidents and incidents and issuing safety recommendations. In\naddition, statutory language prohibits the admission into evidence or use of any part of an NTSB report related to an\naccident in a civil action for damages resulting from a matter mentioned in the report (Title 49 United States Code\nsection 1154(b)).\nFor more detailed background information on this report, visit the NTSB investigations website and search for\nNTSB accident ID PLD18FR002. Recent publications are available in their entirety on the NTSB website. Other\ninformation about available publications also may be obtained from the website or by contacting—\nNational Transportation Safety Board\nRecords Management Division, CIO-40\n490 L’Enfant Plaza, SW\nWashington, DC 20594\n(800) 877-6799 or (202) 314-6551\nCopies of NTSB publications may be downloaded at no cost from the National Technical Information Service, at the\nNational Technical Reports Library search page, using product number PB2021-100901. For additional assistance,\ncontact—\nNational Technical Information Service\n5301 Shawnee Rd. Alexandria, VA 22312\n(800) 553-6847 or (703) 605-6000\nNTIS website\n\n<<<PAGE 4>>>\n\nContents\nFigures ........................................................................................................................................... iii\nAbbreviations and Acronyms ..................................................................................................... iv\nExecutive Summary .................................................................................................................... vii\nProbable Cause.............................................................................................................................. vii\nSafety Issues................................................................................................................................. viii\nFindings.......................................................................................................................................... ix\nRecommendations .......................................................................................................................... xi\nNew Recommendations .......................................................................................................... xi\nPreviously Issued Recommendations Reiterated in this Report ........................................... xiii\n1. Factual Information .................................................................................................................1\n1.1 Synopsis ....................................................................................................................................1\n1.2 Background ..............................................................................................................................2\n1.3 Narrative ...................................................................................................................................2\n1.4 Events Leading Up to the Explosion ........................................................................................3\n1.4.1 3527 Durango Drive Incident ........................................................................................5\n1.4.2 3515 Durango Drive Incident ........................................................................................7\n1.4.3 Atmos Energy Response Following Second Incident ....................................................7\n1.5 Emergency Response to 3534 Espanola Drive .......................................................................11\n1.6 System Isolation and Pipe Segment Replacement .................................................................13\n1.7 Examinations After the Explosion .........................................................................................17\n1.7.1 On-site Integrity Tests..................................................................................................17\n1.7.2 On-site Gas Measurements ..........................................................................................19\n1.7.3 Gas Odorization Testing ..............................................................................................20\n1.7.4 Gas Regulator Testing..................................................................................................20\n1.7.5 NTSB Testing ..............................................................................................................21\n1.7.6 Soil Testing ..................................................................................................................25\n1.7.7 Incidents at 3527 and 3515 Durango Drives................................................................27\n1.8 Prior Leak History ..................................................................................................................28\n1.9 Atmos Policies and Procedures ..............................................................................................29\n1.9.1 Gas Leak Surveys ........................................................................................................30\n1.9.2 Emergency Response Procedures ................................................................................31\n1.9.3 Gas Distribution Pipeline Integrity Management Program .........................................32\n1.10 Regulatory and Municipal Requirements ...............................................................................33\n1.10.1 PHMSA Regulatory Requirements ..............................................................................33\n1.10.2 RRC Regulatory Requirements....................................................................................35\n1.10.3 DFR Procedures and Training .....................................................................................35\n1.11 Actions After the Explosion ...................................................................................................36\n1.11.1 Regulatory Actions After the Explosion ......................................................................36\n1.11.2 Atmos Actions After the Explosion .............................................................................37\n1.11.3 DFR Actions After the Explosion ................................................................................39\n64964\n\n<<<PAGE 5>>>\n\nNTSB Pipeline Accident Report\n2. Analysis ...................................................................................................................................41\n2.1 Introduction ............................................................................................................................41\n2.2 Condition of the Natural Gas Main ........................................................................................42\n2.3 Investigation of the Two Incidents that Preceded the Explosion ...........................................46\n2.3.1 DFR’s Investigation of the First Two Incidents ..........................................................46\n2.3.2 Atmos’s Investigation of the First Two Incidents ........................................................48\n2.3.3 NTSB’s Evaluation of Causal Factors for the First Two Incidents .............................50\n2.4 Leak Investigations and Repairs Prior to the Explosion ........................................................53\n2.5 Methane Detection .................................................................................................................57\n2.6 Incident Reporting ..................................................................................................................60\n2.6.1 Atmos Incident Reporting ............................................................................................60\n2.6.2 DFR Incident Reporting ...............................................................................................62\n2.7 Integrity Management ............................................................................................................62\n3. Conclusions .............................................................................................................................69\n3.1 Findings ..................................................................................................................................69\n3.2 Probable Cause .......................................................................................................................71\n4. Recommendations ..................................................................................................................72\n4.1 New Recommendations ..........................................................................................................72\n4.2 Previously Issued Recommendations Reiterated in this Report .............................................74\nBoard Member Statement ...........................................................................................................76\nAppendixes....................................................................................................................................78\nAppendix A. The Investigation ......................................................................................................78\nAppendix B. Consolidated Recommendation Information ............................................................78\nReferences .....................................................................................................................................83\nii\n\n<<<PAGE 6>>>\n\nNTSB Pipeline Accident Report\nFigures\nFigure 1. Location of explosion, preceding incidents, and shared utilities. ................................... 2\nFigure 2. Timeline of the events leading up to the explosion. ....................................................... 5\nFigure 3. Gas measurements taken in the block surrounding the site prior to the explosion. ....... 9\nFigure 4. Leaks identified prior to the explosion. ........................................................................ 11\nFigure 5. Dallas Fire-Rescue map of the four exclusionary zones after the February 23\nexplosion. ...................................................................................................................................... 13\nFigure 6. Leaks identified as Grade 1 or 2 and prior to system replacement. .............................. 15\nFigure 7. Crack in natural gas main at 3539 Durango Drive sewer lateral crossing. .................. 18\nFigure 8. Gas measurements taken in the block surrounding the site after the explosion. .......... 19\nFigure 9. Gas measurements taken on and near the property after the explosion........................ 20\nFigure 10. Natural gas main recovered near 3539 Durango Drive sewer lateral crossing. ......... 21\nFigure 11. Side view of the damaged natural gas main showing the dent with a crack. ............. 21\nFigure 12. Top surface of the natural gas main showing the five major gouges. ........................ 22\nFigure 13. Crack and dent on top surface of natural gas main after removing surface deposits. 22\nFigure 14. West face of pipe fracture after separating mating faces of circumferential crack. ... 23\nFigure 15. West face of the pipe fracture, after cleaning procedure. ........................................... 23\nFigure 16. Natural gas main, with service line recovered from behind 3524 Espanola Drive. ... 25\nFigure 17. Leaks identified as Grade 1 or Grade 2 which were beyond the scope of this\ninvestigation. ................................................................................................................................. 63\niii\n\n<<<PAGE 7>>>\n\nNTSB Pipeline Accident Report\nAbbreviations and Acronyms\nAGA American Gas Association\nANSI American National Standards Institute\nAPI American Petroleum Institute\nASME American Society of Mechanical Engineers\nAtmos Atmos Energy Corporation\nBCI Bryant Consultants, Inc.\nCFM cubic feet per minute\nCFR Code of Federal Regulations\nCGI combustible gas indicator\nDFR Dallas Fire-Rescue Department\nDIRT Damage Information Reporting Tool\nDOT U.S. Department of Transportation\nEDS energy dispersive spectroscopy\nGPTC Gas Piping Technology Committee\nhazmat hazardous materials\nHAZWOPER Hazardous Waste Operations and Emergency Response\nHMCRP Hazardous Materials Cooperative Research Program\nHMRT Hazardous Materials Response Team\nHUD US Department of Housing and Urban Development\nHVAC heating, ventilation, and air conditioning\nIC incident commander\nICC International Code Council\nIFGC International Fuel Gas Code\niv\n\n<<<PAGE 8>>>\n\nNTSB Pipeline Accident Report\nIG inspector general\nIM integrity management\nIM Plan Distribution Risk and Integrity Management Plan\nLEL lower explosive limit\nMAOP maximum allowable operating pressure\nMNOPS Minnesota Department of Public Safety, Office of Pipeline Safety\nNFPA National Fire Protection Association\nNFPA 54 National Fuel Gas Code\nNRC National Response Center\nNTSB National Transportation Safety Board\nO&M operations and management\nOQ operator qualifications\nOSHA Occupational Safety and Health Administration\nPHMSA Pipeline and Hazardous Materials Safety Administration\nppm parts per million\npsig pounds per square inch gauge\nPSMS Pipeline Safety Management System\nRMLD remote methane leak detector\nRRC Railroad Commission of Texas\nRSPA Research and Special Programs Administration\nSEM scanning electron microscopy\nSME subject matter expert\nSOP standard operating procedure\nTAC Texas Administrative Code\nTCFP Texas Commission of Fire Protection\nv\n\n<<<PAGE 9>>>\n\nNTSB Pipeline Accident Report\nTRB Transportation Research Board\nUEL upper explosive limit\nUSACE US Army Corps of Engineers\nU.S.C. United States Code\nUSCB US Census Bureau\nvi\n\n<<<PAGE 10>>>\n\nNTSB Pipeline Accident Report\nExecutive Summary\nOn February 23, 2018, about 6:38 a.m. local time, a natural gas-fueled explosion\noccurred at 3534 Espanola Drive, Dallas, Texas, injuring all five occupants, one fatally. The\none-story two-bedroom residence sustained major structural damage. Following the explosion,\nNational Transportation Safety Board investigators located a through-wall crack in the\n71-year-old natural gas main that served the residence and positive gas measurements leading\nfrom this crack to the residence.1\nIn the 2 days before this explosion, two gas-related incidents occurred on the same block\nat houses that were served by the same natural gas main, each resulting in significant structural\ndamage and burn injuries to one occupant.\n2 The first occurred on February 21, 2018, at\n5:49 a.m., and resulted in one injury involving second-degree burns and significant structural\ndamage to 3527 Durango Drive. The second incident occurred on February 22, 2018, at\n10:21 a.m., and resulted in one injury involving second-degree burns and significant structural\ndamage to 3515 Durango Drive.\nProbable Cause\nThe National Transportation Safety Board determines that the probable cause of the\nexplosion at 3534 Espanola Drive was the ignition of an accumulation of natural gas that leaked\nfrom the gas main that was damaged during a sewer replacement project 23 years earlier and was\nundetected by Atmos Energy Corporation’s investigation of two related natural gas incidents on\nthe 2 days prior to the explosion. Contributing to the explosion was Atmos Energy Corporation’s\ninsufficient wet weather leak investigation procedures. Contributing to the severity of the\nexplosion was Atmos Energy Corporation’s inaction to isolate the affected main and evacuate\nthe houses. Contributing to the degradation of the pipeline system was Atmos Energy\nCorporation’s inadequate integrity management program.\n1 (a) For more information, see the factual information and analysis sections of the report. Additional\ninformation about the accident investigation can be found in the public docket for the accident (NTSB case number\nPLD18FR002) by accessing the Accident Dockets link for the Docket Management System at www.ntsb.gov. For\nmore information on our safety recommendations, see the Safety Recommendation Database at www.ntsb.gov.\n(b) Through-wall crack refers to a crack that extends between the inner- and outer-diameter of a pipe; the\ncommodity cannot leak from a crack in the pipe until it extends through the wall.\n2 Throughout this report, the term “explosion” is used to refer to the explosion at 3534 Espanola Drive, whereas\nthe term “incident” is used to refer to the earlier events at 3527 Durango Drive and 3515 Durango Drive. The term\n“incident” is used in accordance with Title 49 Code of Federal Regulations 831.40(a)(2) and does not indicate that\nthese events meet the Pipeline and Hazardous Materials Safety Administration’s definition of incident in Title 49\nCode of Federal Regulations 191.3.\nvii\n\n<<<PAGE 11>>>\n\nNTSB Pipeline Accident Report\nSafety Issues\nThis report focuses on the following safety issues:\n• Incident investigation. Neither the Dallas Fire-Rescue Department nor Atmos\nEnergy Corporation identified the causes of the two incidents that occurred in the\ndays immediately preceding the explosion. Dallas Fire-Rescue Department arson\ninvestigators and Atmos Energy Corporation technicians did not effectively\ninvestigate, communicate, or collaborate to determine the cause of either incident.\nFurther, Atmos Energy Corporation did not gather enough evidence to determine if\ngas migrated from their piping and fueled the first two incidents.\n• Leak investigations and repairs. Atmos Energy Corporation dedicated significant\nresources to its response following the second incident, finding 13 leaks determined\nto present an existing or probable future hazard. However, none of its employees\nquestioned the integrity of the system. As a result, Atmos Energy Corporation did not\ntake appropriate action to secure the safety of the area and its residents. This was\nattributed, in part, to inadequate procedures for performing leak investigations in wet\nweather conditions.\n• Methane detection. Although Atmos Energy Corporation added odorant to its gas\ndistribution system in a manner consistent with Pipeline and Hazardous Materials\nSafety Administration regulations, none of the residents at any of the affected homes\nsmelled gas. Although odorant can act as an early warning of a gas release to prevent\nan explosion and fire, it is known to become depleted if it travels through soil.\n• Incident reporting. Incident reporting requirements mandated by the Pipeline and\nHazardous Materials Safety Administration rely on the judgement of the operator to\ndetermine whether an incident resulted from a leak in their system and do not specify\nthe level of investigation necessary to make the determination. While operators have\nan option to report events that may have been caused by their system, Atmos Energy\nCorporation relied on an incomplete investigation to support its position not to report\nthe first two incidents.\n• Integrity management. Although Atmos Energy Corporation’s integrity\nmanagement program was generally consistent with regulatory requirements and\nindustry practice, the program did not adequately evaluate and address the risk of its\n71-year-old system. This failure to adequately address risk was illustrated by the\n26 leaks determined to present an existing or probable future hazard in the area\naround the explosion, as well as the additional 740 leaks found in northwest Dallas in\nthe weeks that followed.\nviii\n\n<<<PAGE 12>>>\n\nNTSB Pipeline Accident Report\nFindings\n• None of the following were factors in the explosion: (1) ongoing maintenance\nactivities; (2) overpressurization of the gas distribution system; (3) materials used for\nthe construction of the gas main and external coating; and (4) natural gas\ncomposition.\n• The natural gas main was damaged by mechanical excavation equipment, likely when\nthe sanitary sewer lateral was replaced in 1995.\n• A circumferential crack in the main propagated through the pipe wall prior to the first\nincident, allowing natural gas to leak into the surrounding environment for an\nextended period.\n• Soil absorbed and depleted the natural gas odorant, eliminating the opportunity for\noccupants to detect it.\n• Natural gas leaking from Atmos Energy Corporation’s cracked gas main in the alley\nbehind 3534 Espanola Drive migrated through the soil and into the house where it\nwas ignited by an unknown source.\n• Dallas Fire-Rescue Department’s initial misclassification of the first incident delayed\nthe sharing of information that could have helped Atmos Energy Corporation identify\nthe origin of the leak.\n• Had the Dallas Fire-Rescue Department’s arson investigators been adequately trained\non natural gas systems, their investigation findings may have provided more timely\nand accurate assistance to Atmos Energy Corporation in locating the source of the gas\nleak.\n• Timely pressure testing of the customer piping by Atmos Energy Corporation could\nhave eliminated potential sources of the gas leaks and helped focus their efforts on\noutside leak detection to locate the damaged and leaking gas system piping more\nquickly.\n• Atmos Energy Corporation did not adequately investigate the first two gas-related\nincidents that occurred at 3527 and 3515 Durango Drive.\n• Damage to the structure involved in the first incident on 3527 Durango Drive was\nconsistent with a fuel gas/air mixture explosion, which was most likely caused by\nnatural gas that migrated from underneath the structure.\n• Fuel gas was involved in both incident homes; there was insufficient evidence to\nexclude natural gas from Atmos Energy Corporation’s system from either incident,\nevidence of leaks present prior to the first two incidents occurring, and the probability\nof two or three structure fires/explosions occurring independently on the same block\nduring the same week is very low. Therefore, the two prior incidents that occurred on\nix\n\n<<<PAGE 13>>>\n\nNTSB Pipeline Accident Report\nthe same block on subsequent days and the explosion at 3534 Espanola Drive were all\nlikely related.\n• Limitations of the equipment and procedures due to the wet weather conditions on the\nability of Atmos Energy Corporation to reliably detect the presence of leaked gas\nduring its response to the first two incidents, and the number and severity of leaks\nidentified following the first two incidents and prior to the explosion, should have\nprompted Atmos Energy Corporation to shut down or isolate the pipeline.\n• Had Atmos Energy Corporation pressure tested the main in the alley behind the first\ntwo incident homes on February 21 or 22, it could have found that the main did not\nhold pressure, spurring additional protective actions that could have prevented the\nfatal injury at 3534 Espanola Drive.\n• Atmos Energy Corporation’s wet weather leak investigation procedures were\ninsufficient given the known limitations of its equipment.\n• The assistance of the Dallas Fire-Rescue Department’s Hazardous Materials\nResponse Team, particularly after the second incident, could have enhanced Atmos\nEnergy Corporation’s leak investigation.\n• Had methane detectors been installed at the residences located on Durango and\nEspanola Drives, an alarm would have alerted residents to a gas release, reducing the\npotential for and consequences of the resulting natural gas fires and explosions.\n• The lack of official reporting of the first two incidents by Atmos Energy Corporation\ndelayed the response from regulatory authorities, the Railroad Commission of Texas\nand the Pipeline and Hazardous Materials Safety Administration.\n• The Pipeline and Hazardous Materials Safety Administration does not provide clear\nrequirements regarding the level of investigation necessary to determine whether an\nevent is subject to its reporting requirements, potentially resulting in the\nunderreporting of natural gas incidents.\n• If Dallas Fire-Rescue Department reported the first two incidents in a timely manner,\nit could have prompted further investigation or regulatory oversight prior to the\nexplosion.\n• The high number of leaks observed in northwest Dallas after the explosion were due\nto the degradation of Atmos Energy Corporation’s gas distribution system, not\nsudden, unanticipated geologic loadings.\n• Atmos Energy Corporation did not adequately consider or mitigate against threats\nthat were degrading its pipeline system, the likelihood of failure associated with these\nthreats, or the potential consequences of such a failure as required by gas distribution\nintegrity management requir","truncated":true,"body_characters":272095}