Rainy weather and firefighting activity in the days before a fatal 2018 natural gas explosion in Dallas created wet ground conditions that hampered efforts to identify an unusual number of gas leaks — including on a cracked gas main near the site of a fatal blast, Atmos Energy Corp. executives and workers told federal investigators.
The insight into the company's efforts leading up to the Feb. 23, 2018, incident are laid out in dozens of documents made public by the National Transportation Safety Board, or NTSB, on Aug. 4. The documents reveal new details about a damaged gas main found near the site of the incident, as well as Atmos's attempts to identify the cause of an explosion and fires that occurred on a neighboring block during the preceding two days.
The company's inability to find the source of the incidents in the roughly 48 hours between the first explosion and the blast that killed 12-year-old Linda "Michellita" Rogers was one of the more unusual aspects of the pipeline accident. At this stage in the investigation, the technical reports in the NTSB docket do not draw conclusions about the probable cause but offer a fuller picture of the difficulty Atmos experienced.
Atmos workers attempted to conduct bar hole tests in the unpaved alley between accident sites where service lines tied into a natural gas distribution main. Source: Atmos Energy; National Transportation Safety Board |
Atmos staff struggled to pinpoint leaks
Starting with an inspection following a Feb. 21, 2018, explosion and fire at 3527 Durango Drive, Atmos staff had trouble conducting so-called bar hole tests, in which a worker inserts into the ground a probe connected to a handheld combustible gas indicator. An Atmos service technician said the ground was too wet to get an accurate reading, according to the NTSB's July 22 operations and integrity management report. He neither got a positive reading while conducting an above-ground survey with his combustible gas indicator, nor observed bubbles in pooled water that would indicate a leak.
After a gas fire three doors down at 3515 Durango Drive on Feb. 22, 2018, a Dallas firefighter asked an Atmos service technician to call in additional staff to "investigate what was going on," the report said. More than a dozen Atmos workers were eventually on site, with personnel working throughout the night to address numerous leaks identified in the area, many linked to service lines.
But wet ground conditions and rain once again hampered efforts to conduct bar hole tests, including in an unpaved alley behind the homes under which a 2-inch gas distribution main ran. Workers described creating probe holes that immediately filled with water and scooping out water with buckets as they dug.
According to Atmos survey specialists, the rainy conditions left them largely dependent on equipment such as remote methane leak detectors, which identify the presence of methane but do not pinpoint its subsurface source. One specialist said readings from the less precise above-ground surveying equipment did not raise concerns about the integrity of the main beneath the alley. A second survey specialist said he had never surveyed in similar conditions or seen so many leaks during a single incident response.
Atmos staff was still on site when the Feb. 23, 2018, explosion occurred on the opposite side of the alley at 3534 Espanola Drive in the early morning. The company isolated the main and later decided to replace all service lines and mains in the area. Atmos Energy continued to identify leaks in the following days.
"We'd say get a crew out there and take care of that leak and you go back over that area again and there was new leaks. So it was something that was unexplained. I've never seen it in my 25 years of working for the gas company," Atmos Energy Vice President of Technical Services Jeffrey Knights told the NTSB in an April 2018 interview.
New details about failed gas main
The NTSB excavated a section of the gas main where it found a crack along the circumference of the pipe beneath a lateral sewer line. Source: Atmos Energy; National Transportation Safety Board |
In the weeks following the blast, NTSB investigators located a crack in the 2-inch diameter gas main installed beneath the alley running behind homes on the block. The damaged section was directly behind the fatal accident site at 3534 Espanola Drive.
The NTSB's July 20 accident summary revealed that the crack resulted from a dent in part of the main positioned half an inch beneath a 6-inch diameter lateral sewer line installed in 1995. NTSB investigators also discovered five "major gouges" along a 22-inch stretch of the main near the dent. The line failed when NTSB staff subjected it to a pressure test.
The steel main was installed in 1946, but Atmos did not have records of its manufactured date or its manufacturer, according to the NTSB's operations and integrity management report. Company data showed no indication of leaks from the main in the 10 years prior to the incident, though Atmos addressed several leaks on service lines tied into the main during that period. Atmos repaired the last recorded leak from the main in 1997.
A geotechnical review by Bryant Consultants Inc. suggested a rainy period "likely exacerbated" movement at the intersection of two geological formations underlying northwest Dallas, and the forces "caused unanticipated external loadings" on Atmos's distribution system. A separate review by the U.S. Army Corps of Engineers observed the subsurface soils in the neighborhood are mostly high plasticity clay, which swells and shrinks depending on moisture level, putting stress on underground structures.