A natural gas pipeline that exploded during safety testing near Woodside Sunday afternoon (Nov. 6), causing a mudslide that shut down northbound Interstate Highway 280, had a pre-existing dent at the point where it ruptured, according to PG&E.
Pacific Gas & Electric was conducting hydrostatic pressure tests on a 3- mile section of the 24-inch transmission line between Menlo Park and Woodside at the time of the rupture, said utility spokesman Brian Swanson.
The tests involve emptying the pipe of gas and pumping water through at high pressure to check for any leaks or weak points.
The pipeline, Line 132, which runs from Milpitas to San Francisco, including through Palo Alto, is the same line that ruptured in San Bruno in October 2010, killing eight people and injuring many more. It normally operates at a pressure somewhere lower than 375 pounds per square inch but has been operating at a reduced pressure of less than 300 pounds per square inch, Swanson said.
"In order to help meet higher winter demands, we want to bring the pressure level back up to normal levels, around 375, but before we do that we need to ensure that the pipeline is safe," Swanson said.
Pressure in the pipeline had been raised to around 550 pounds per square inch at the time of Sunday's rupture, well above normal operating levels, he noted.
The rupture, which occurred around 3:20 p.m., was first reported as a large geyser of water that was getting cars wet on Interstate Highway 280 near Farm Hill Boulevard. A short time later an explosion was reported in the same area, along with a large volume of flowing water, according to Woodside Fire Battalion Chief Kevin Butler.
The rupture had caused a mudslide, sending mud, rocks and debris across the northbound lanes of the freeway. Several lanes on the freeway were closed by the slide, some for more than four hours.
There were no injuries, but one vehicle was damaged by pieces of dirt and rock, Butler said.
Fire crews located a 5 foot by 5 foot crater on the hillside east of the freeway, in an easement running behind a group of homes. The closest home was about 100 yards away, Butler said.
PG&E has tested around 120 miles of pipeline this year, Swanson said. Last week the utility found a 1 millimeter leak in Line 132 in Palo Alto, and last month a line ruptured in Bakersfield during testing. However, Sunday's rupture was far more disruptive.
"We'll definitely look into what happened today," Swanson said, noting that there were crews on the scene. "Obviously we don't want a hydrostatic test to disrupt traffic on 280 like it did today, and we will look at ways to minimize disruption."
PG&E will remain on the scene for several more days to make repairs and complete testing on the pipeline.