It may take months to learn why a gas pipeline erupted in flame in south Minneapolis Thursday.
The blazing spectacle that riveted commuters and disrupted hundreds of lives Thursday became a grinding mystery by day's end as investigators began sorting out how an underground natural gas pipeline could suddenly rupture without warning.
It didn't involve construction or someone digging in the wrong place -- it just blew.
The plume of flames erupted over 60th Street at Nicollet Avenue about 8 a.m. when gas flowing at 175 pounds of pressure through a 20-inch underground steel pipe caught fire, blowing a crater in an asphalt road. Nearby vehicles were damaged by the intense heat. No buildings were destroyed and no one was hurt.
CenterPoint spokesperson Becca Virden said the gas company was not doing any work on or near the pipe, and neither was anyone else as far as the company could determine Thursday.
State investigators have already begun what will likely be a lengthy, painstaking investigation. They will likely use CSI-like methods such as laboratory analysis of the pipe section to discover what went wrong. Corrosion, a shift in the earth, a bad weld or improper operation are just a few of the many possibilities. It may take months to get a clear answer.
One thing is certain -- such blasts are almost always memorable and often cause havoc in a neighborhood. In the last 10 years there have been 527 nationally in large gas transmission lines like the one involved in Thursday's fire. During the same time period in Minnesota there have been 30, involving both large and smaller distribution pipelines, and the rate has remained stable.
A gas explosion in San Bruno, Calif., last year killed eight people and destroyed 38 homes -- an event that shocked the nation.
The Twin Cities has seen its share of fires and explosions in the last two years, without fatalities. They include a gas explosion last September that leveled a house in Richfield. In February 2010 a sewer contractor in St. Paul struck a gas line set inside a sewer pipe, setting of an explosion in a home. Later that same month a blast in Edina took down a house on Arden Avenue after a cable TV crew accidentally severed a CenterPoint Energy gas main.
On Thursday, roadside assistance worker Emmett DySart said he smelled gas about three hours before the fire erupted. He said in an interview later Thursday that he was changing a customer's tire at 61th Street and Blaisdell Avenue and reported a "real strong smell of gas" to a CenterPoint employee. DySart, who described himself as an independent contractor for Texas-based Auto Rescue, said, "Nobody took me seriously. When I saw the fire later on, I was flabbergasted."
CenterPoint had no record of complaints about a suspicious smell of gas in the immediate hours before the fire, Virden said. Customers with complaints should first call 911, she added.
"Our employees are well trained and they take that very seriously," Virden said.
Understanding the cause will be left to investigators with the Minnesota Office of Pipeline Safety. Because it's now an open investigation, safety officials Thursday declined to comment on possible causes.
"It could be just about anything," said Carl Weimer, executive director of the Pipeline Safety Trust, a national safety advocacy group. "It could have been hit in an excavation years ago, and finally failed. It could be corrosion. It could be a problem with the pipe or the way that CenterPoint was operating it. There isn't enough information to draw a conclusion at all."
The fire occurred near the recently completed major reconstruction of the Interstate 35W/Crosstown Hwy. 62 interchange, but Virden said there's no reason to believe the work involved was connected to the incident.
While many have voiced concern nationally about the aging pipeline system, Weimer said how a company operates and maintains pipelines is far more critical. That, too, will be part of the investigation, state officials said.
Bill Mahre, an explosion and pipeline investigator based in Maplewood, said pipeline blast investigations typically start at the debris field. Investigators look for burn and blast patterns that may reveal details about what caused the failure and ignition of the escaping gas.
Then a section of the damaged pipe is excavated and examined in a lab by metallurgical experts and chemists, he said.
"These guys are kind of CSI-types, " said Mahre, who generally works for insurance companies and attorneys and is not involved in investigating Thursday's incident. "They will look at welds or the component parts like a valve or a regulator."
The lab work often can tell what caused a pipe or component to fail by looking for imperfections in the metal, he said. But the results can take many months.
Underground steel pipes "sometimes just deteriorate from age," though many lines in cities such as Minneapolis are 80 years old and still functioning fine, he added.
In frost belt states, pipes also can shift, sometimes causing a failure, he added.
"Anything you put in the ground that is metallic is attacked by Mother Earth," Mahre said.
According to the federal regulatory agency that oversees pipelines, the most frequent causes of the 527 significant gas transmission pipeline accidents over the past 10 years were failures of welds, joints or other pipe components.
The deadly California explosion last fall has been linked to a defective pipe with a flawed weld that suddenly gave way. That case is still under investigation.
Staff writers Paul Walsh and Corey Mitchell contributed to this report. Josephine Marcotty • 612-673-7394 David Shaffer • 612-673-7090