An investigation by the state of Washington’s Utilities and Transportation Commission (UTC) concluded Wednesday that a downed city utility electrical line punched a hole in a natural gas distribution pipeline, resulting in an explosion in a north Seattle home in September 2011.

An investigative report released by UTC pipeline safety staff said natural gas leaking from the pipeline likely migrated into a crawlspace, eventually igniting and causing the explosion and fire that destroyed a house and resulted in injuries to two homeowners Sept. 26, 2011. The explosion also caused property damage to surrounding homes in Seattle’s Northgate section.

The staff report recommends that gas utility Puget Sound Energy (PSE) be assessed unspecified penalties for violating gas safety rules, but the three-member UTC must still review the report and ultimately take any actions regarding the utility.

Bellevue, WA-based PSE speculated in the immediate aftermath of the incident that a wind-toppled tree caused a gas leak at a home blocks away that eventually resulted in an explosion (see Daily GPI, Sept. 29, 2011).

Given the continuing series of high-profile natural gas pipeline incidents following the transmission pipeline rupture in San Bruno, CA, a year earlier, the Seattle incident involving a distribution main and various service lines has received more than the usual news media attention.

“Following a comprehensive investigation, pipeline engineers and safety experts with the UTC found the most likely cause of the explosion was gas leaking from PSE’s service pipeline located outside of the Pinehurst home,” a regulatory commission spokesperson said.

A downed power line that fell a day before the explosion had energized a metal fence post, an abandoned buried metal water pipe and finally PSE’s utility gas distribution pipeline. “The leak that led to the explosion was attributed to a problem known as ‘electrical arcing,’ in which a powerful jolt of stray electricity energized the metal natural gas piping,” the UTC spokesperson said.

Ultimately the electricity in the gas distribution piping caused leaks in four separate locations, creating “finger-sized holes” in the pipe at each home site. The day before the explosion, PSE gas utility workers found three of the leaks, but they did not find the fourth one until after the house explosion. “Neither the couple in the home that they explored nor any of their neighbors reported a possible gas leak before the explosion,” the staff report said.

While the utility promptly responded to odor reports and conducted a leak survey in the area, the UTC staff report concluded that PSE “failed to leak survey all the gas pipelines located in the area, including the service line to the home that exploded,” along with the gas main located in and under an adjoining cul-de-sac.

UTC safety staff monitor gas pipeline safety regulations on PSE’s 12,000 miles of distribution pipelines in Washington state, and the UTC commissioners have the power to assess fines against pipeline operators. PSE serves 785,000 gas utility customers, primarily in the Puget Sound area of Washington state.

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