Thompson Falls 50 MWaddition goes commercial
In a joint venture with Kiewit Western Co., Stone & Webster Engineering recently completed a 50 MW addition to the Thompson Falls Hydroelectric station. The Thompson Falls facility began operation in 1915, with expansions in 1916 and 1917, building to a 40 MW generating station on the Clark`s Fork of the Columbia River adjacent to Thompson Falls.
Initially, much of its output powered the electric locomotives of the Milwaukee railroad. The original powerhouse design and construction program, while addressing the needs of the times, accounted for the challenges of remote location, mountainous terrain and cost-effective solutions. These factors were also considered in the design of the addition, done under contract with Montana Power.
Construction of the powerhouse presented numerous challenges. The powerhouse was built on a 7-acre island, bounded to the north by the existing intake channel and to the south by an overflow channel dam, requiring the construction of a 135-foot-long access bridge. The bridge allowed crews to access the work site and begin the excavation of argilite rock in the winter of 1993-1994. “The material we had to remove for the powerhouse was the equivalent of a four-story-high football field,” said Paul Giroux, Kiewit resident engineer. “The crews met the challenges of winter weather, the hazards of high walls and the close tolerances required for the structure on a multiple-shift basis that kept us on schedule.”
The town has grown up over the years, with development clustered near the waterfront area, so the construction team was literally working in homeowners` backyards. Both the timetable and the construction site were very tight. Multiple phases of the project were being executed simultaneously and the ramifications of a single glitch on one aspect of the project had far-reaching consequences that affected the entire team.
The river flow at Clark Fork during the 32-month project varied from a low of 6,000 cubic feet per second (cf/s) to a high of around 75,000 cf/s. The existing six-unit plant uses approximately 11,000 cf/s and the new 50 MW unit uses an additional 12,000 cf/s.
During the final check-out and start-up phase of the project, Thompson Falls was hammered with unseasonable, prolonged heavy rains which created extremely high river flows. The workers were challenged with winds exceeding 100 mph and all four transmission lines tied to the project went out of service while the new unit was first being tested and connected to the transmission grid. “We didn`t set out to design the new unit to start without an outside supply of power, but with the spirit of teamwork and real-field condition innovation that developed over the course of the project, our team took this in stride too,” said John Yale, lead start-up engineer.