Northwest Pipeline Corp. is seeking federal approval for a $446 million expansion of its natural gas pipeline that runs from northwest New Mexico to the Canadian border north of Seattle.

The planned expansion would include spending $91.5 million on the Utah portions of the 3,574-mile pipeline. The expansion would add 627 miles of new pipeline plus improvements to another 104 miles. system would increase total capacity by 534 million cubic feet per day.Salt Lake-based Northwest is a subsidiary of the Williams Companies of Tulsa, Okla.

The existing system starts in the San Juan Basin of northwest New Mexico, snakes along the Utah-Colorado border to a point north of Vernal, cuts diagonally through the southwest corner of Wyoming and across southern Idaho to the Columbia River basin on the Oregon-Washington border. The pipeline follows the Columbia River to the Portland area where it forks, with one leg running due north through the Tacoma-Seattle area to the Canadian border and the other fork following the Oregon coast to a point near the California border. Another branch takes off from the starting point along the Columbia River and runs northeast into the Idaho panhandle.

Much of the project involves looping parts of the existing system and adding compressor stations to increase the amount of gas that can be pushed through the pipeline.