A high-pressure natural gas line that runs though Snohomish County will be shut down over the next several weeks after federal safety regulators decided the pipeline is at risk of cracking.

Bev Chipman, spokeswoman with Williams Northwest Pipeline Co., said the company began reducing pressure in the 268-mile line last week. The gradual shutdown should be complete in about three weeks.

Natural gas delivery to homes and businesses should not be interrupted, the Washington Utilities and Transportation Commission said.

The federal Office of Pipeline Safety issued a rare shutdown order on Friday, following two ruptures on the line during the past eight months.

"It’s the first time it’s ever happened in our state, so it’s very unusual," Marilyn Meehan, spokeswoman for the Utilities and Transportation Commission, said of the shutdown order. She added that the order came on the recommendation of the state’s inspectors.

Until problems with the 46-year-old pipeline are resolved, Williams will rely on a newer, 30-inch gas pipeline along the same route. Both lines serve numerous utilities, such as Puget Sound Energy, and industrial customers, Chipman said.

The affected 26-inch pipeline runs from Washougal, on the border with Oregon, to Sumas, located on the Canadian border. In Snohomish County, the pipeline is roughly parallel to I-5 but farther east, running through Snohomish, west of Lake Stevens and northward through more rural areas.

The federal order also requires Williams to carefully inspect the line and ultimately replace each section in the state or prove it’s resistant to cracking.

Some portions of the pipeline, especially in more urban areas, will have to be replaced within three years, according to the order. The rest can be done over the next 10 years.

On May 1, the pipeline ruptured near Lake Tapps in Pierce County, causing the evacuation of about three dozen homes, a business and a school. Tests later showed the cause of the rupture was stress-corrosion cracking, which occurs in steel pipe because of a combination of factors including the environment.

After that rupture, Williams reduced the pressure in the pipe by about 20 percent to ease stress on the metal. On Dec. 13, however, it ruptured again near Toledo in Lewis County. According to the Washington Utilities and Transportation Commission, Williams has experienced nine failures in the 26-inch pipeline over the past decade.

Chipman said the pipeline’s age is a factor, but older natural gas pipelines are operating in other parts of the country without problems. The amount of moisture and acidity in the soil here, as well as other factors, play a role, too.

"It’s more a question of where the pipe is than how old it is," Chipman said.

The 30-inch parallel pipeline Williams uses with the older one was built in the late 1970s and early ’80s, she added. Having both gives the company flexibility in delivering natural gas across Western Washington, Chipman said.

The shutdown of the pipeline should have no effect on Williams’ proposal to build a 9-mile-long natural gas pipeline from Lake Stevens to Everett.

Puget Sound Energy has said that new line would improve its delivery of natural gas to the Everett area. But residents along the proposed route north of Lake Stevens have raised objections to the project.

Reporter Eric Fetters: 425-339-3453 or fetters@heraldnet.com.