• Earth-moving work may have damaged a 52-year-old natural-gas pipeline that exploded April 17, injuring 13 people at a law-enforcement gun range.
•Digging without knowing where pipelines are buried is the top cause of gas pipeline damage in California over the last 20 years.
• The 12-inch diameter, asphalt-coated steel pipeline carried natural gas at a pressure of 370 pounds per square inch.
There have been 58 serious natural gas-pipeline breaches in California over the last 20 years, but last week’s Fresno blast was the first to cause injuries in California since the 2010 San Bruno disaster.
Thirteen people were hurt in the April 17 explosion at the Fresno County Sheriff’s Foundation gun range. In the week since, investigators have combed the site looking for answers. While the California Public Utilities Commission has not released any details yet about the cause, experts are pointing at earth-moving work that was happening at the time as a likely explanation — as it has been in most of the recent breaches in the state.
An analysis of data from the federal Pipeline and Hazardous Material Safety Administration indicates that of 58 significant incidents on natural-gas transmission lines in the state since 1995, 31 were the result of damage from excavation. The federal agency, a branch of the U.S. Department of Transportation, defines “significant incidents” as those in which people are killed or injured or that cause more than $50,000 in damage.
“If you look at the data, one of the top causes of pipeline failures (nationwide) continues to be excavation damage, like what happened in Fresno,” said Damon Hill, a spokesman for the pipeline safety office. “It’s especially those incidents that have an impact on people, where folks are injured or killed.”
Not every puncture of a pipeline ends in an explosion, but if there is a spark or an ignition source, the results can be catastrophic. The worst recent case in California was 2010 in San Bruno, where part of a neighborhood was obliterated, killing eight and injuring 51. That blast was attributed to a faulty pipeline.
Last week’s blast of a Pacific Gas & Electric Co. pipeline at the foundation’s gun range, next to the San Joaquin River just east of Highway 99 and the Union Pacific Railroad tracks, happened as a front-end loader was moving dirt around at the top of a berm above and behind the shooting targets at the range. The equipment operator was seriously burned, as well as two sheriff’s deputies and members of a crew of inmates doing maintenance work.
Denny Boyles, a PG&E spokesman, said the Fresno pipeline was operating normally, with gas flowing at a pressure of 370 pounds per square inch, until just moments before the explosion, when a drop in pressure was noted. “PG&E detected a slight change in pressure at 2:29 p.m., and was first notified about the incident at 2:36 p.m.” when the fire was reported, Boyles said Friday. By 3 p.m., crews were working to shut off the gas and had the flow stopped by 3:20. “Because of the remaining gas in the line,” Boyles added, “the flames were exhausted at 3:56 p.m.”
That pressure change preceding the blast suggests that a leak began shortly before something ignited the gas, said Michael Jenkins, a professor of mechanical engineering at Fresno State’s Lyles College of Engineering who specializes in materials and fracture mechanics.
The resulting explosion sent flames up to 200 feet into the sky, and the flow of high-pressure gas raged with such ferocity that the intense heat scorched nearby trees, ignited a grass fire, and damaged the adjacent railroad tracks. The radiant heat could also be felt by motorists driving several hundred feet away on Highway 99 before the road was closed.
“The evidence points to a gas pressure drop before the explosion,” Jenkins said. “It’s clear that there was a breach, that the pipe was leaking, and then something caused it to ignite.”
But like other types of high-pressure vessels, he added, a pressurized gas pipeline is engineered to minimize the chances of a major rupture. “Pressure vessels are designed with a lot of safety in mind,” he said. “They’re typically designed to leak slowly (if damaged) before they break.” A slow leak, Jenkins added, usually gives crews time to shut off the gas and prevent a more serious break.
PG&E’s pipeline was installed in 1962. The 12-inch diameter pipeline is made of quarter-inch-thick steel that was coated in asphalt as protection against exterior corrosion. The pipeline, cataloged by PG&E as Pipeline 118B, is buried 40 inches beneath the soil surface on a bluff that runs parallel to the southern border of the gun range, high above the berm behind the shooting targets. But at the range’s southwestern corner, the buried pipeline makes a nearly 90-degree turn and heads north to run parallel to the Union Pacific tracks, following the contours of the terrain and running beneath the San Joaquin River.
Aerial photos show that the explosion happened where the underground pipeline crosses under the end of the dirt road atop the man-made berm behind the shooting targets. Red-and-white striped signs mark the pipeline’s route.
Whether the front-end loader penetrated the pipeline won’t be definitively known until the PUC completes its investigation into the blast. But Hill, with the federal pipeline agency, said it doesn’t necessarily take a major impact to damage a gas line.
“It could be something minor like a scrape that can have an effect over time,” Hill said Friday. “Anything that comes into contact with a pipeline can affect it over time. If the protective coating is damaged, even a rock laying against the pipe could corrode it over time.”
“We’ve seen cases where a backhoe or some other big machine can hit a pipeline and doesn’t cause it to break, and so they don’t call it in,” he added. “But over time that causes corrosion and the pipeline could fail.”
Ironically, Hill said, the Fresno explosion happened in the middle of National Safe Digging Month, a campaign by federal and state pipeline regulators to encourage construction crews or anyone else doing excavating or earth-moving work to be aware of the dangers of buried pipelines. Contractors, utilities and others can call 8-1-1 to reach Underground Service Alert, or USA, an organization that coordinates utilities to help customers locate any buried utility lines — including power lines, gas pipelines and others — before digging begins.
Fresno County Sheriff Margaret Mims has said the equipment operator was not digging the day the blast occurred at the gun range. The lawyer for the operator, Ismael Arreazola, released a statement Thursday saying Arreazola “followed the instructions of his employer during his work day, which involved redistributing soil with no excavation.”
The federal pipeline safety agency has people in Fresno assisting the state utilities commission in the investigation, Hill said. Federal safety rules require that pipeline operators inspect their systems for leaks on a regular basis and to have leak-detection systems in place “so they can find leaks and fix them before they become a more serious issue,” Hill said.
PG&E crews did a ground survey of the pipeline on April 1 and an aerial survey the day before the explosion, Boyles said, and found no leaks in the 53-year-old line.