CHARLESTON, W.Va. -- It's still not clear how NiSource Gas Transmission and Storage officials were alerted to a massive explosion at a 20-inch-diameter transmission line near Sissonville, investigators from the National Transportation Safety Board said Thursday.
Investigators said previously that no alarm went off Tuesday afternoon at the gas company's control room that would notify its personnel the pipeline had ruptured near Columbia Gas Transmission's Lanham Compressor Station at Rocky Fork.
The blast and resulting fires destroyed about five homes and engulfed a large section of Interstate 77. No one was killed but several people were treated for minor injuries.
NTSB member Robert Sumwalt told reporters Thursday evening at the Holiday Inn in South Charleston that investigators are interviewing company personnel and are reviewing control room records to determine why the alarms didn't go off.
Investigators discovered that pressure measured within the pipe dropped significantly at about the time the first 911 call came in about the explosion.
"All we are saying at this point is, because we haven't gotten a detailed look at the records, we just know there was a pressure decrease [but] we don't know the magnitude of that decrease," he said. "We want to find out how much the decrease was and if it reached the level of the alarms, why the alarms did not go off."
NTSB investigators will focus on whether NiSource officials acted adequately in shutting down gas to the ruptured line.
Based on preliminary data, Sumwalt said the explosion occurred at 12:41 p.m. and gas to the pipeline was shut off at about 1:45 p.m., 64 minutes later.
A 10-member team of NTSB investigators visited the crater site Thursday for a preliminary examination of the ruptured pipe, Sumwalt said.
It appears the pipe was about 70 percent thinner than it should have been to sustain pressure of 921 pounds per square inch measured at the time, he said.
An area along the bottom of the pipe, running about 6 feet long, was measured at less than one-tenth of an inch thick, he said. This thinning indicates that some segments along the pipe wall were about 30 percent thinner than required.
"There are many things that can cause pipe wall thickness to deteriorate and that is exactly what we will be looking at . . . ," Sumwalt said. "What caused this pipe wall to become deteriorated? What caused it and what finally led to the actual rupture and explosion?"