BOZEMAN – NorthWestern Energy crews misread a map of the company’s pipelines in downtown Bozeman, contributing to a delay in shutting off natural gas that fueled a fire after an explosion last year, the Montana Public Service Commission found.
It took more than 16 hours to stem the flow of natural gas, and firefighters were unable to extinguish a blaze sparked by the March 5 explosion until the gas feeding the fire could be cut off.
The explosion and fire killed a woman and leveled half a city block.
NorthWestern Energy spokeswoman Claudia Rapkoch said the stressful situation apparently fueled confusion in reading the maps, leading crews to install a fitting to stop the leak on the wrong pipe. A second fitting was installed nearby and was effective.
“The maps were accurate, but it was really more a function of people working in very stressful conditions,” Rapkoch said.
Rapkoch said the mistake delayed gas shut-off for only three or four hours and that other factors were involved.
“When you shut a gas system off, it’s still pressurized,” Rapkoch said. “You still have to let the system purge, and (with) all that gas that’s packed into that line, it’s not instantaneous.”
Instead of shutting down the entire downtown zone, affecting thousands of customers, the utility company decided to let gas burn while they installed shut-off fittings.
Authorities based that decision on the expected cold weather and the time it would have taken to bleed gas out of the entire zone and shut off individual meters one by one.
Rapkoch said 8,000 to 10,000 customers would have been without heat or hot water for weeks if the company had turned off the zone.
Rapkoch said welding the fittings in a live natural gas line also required careful work that took time.
The crew was “working in an emergency situation with a high-volume, high-pressure pipeline with welding equipment, and they were working as quickly as they could given their situation,” Rapkoch said. “But at no time would they take any shortcuts” to compromise their safety or that of anyone else,” she said.
PSC investigators found no corrosion on pipes removed from the area near the blast site, and said a coupling on a 2-inch service line “was believed to fail as a result of environmentally assisted cracking — possibly frost heave,” the report said.