During the $1.5 million rehab project at Boundary Dam, workers removed 5 tons of silt and debris buildup from the maintenance gate. The gate was returned to service in September 2011.
By Michael Haynes and Scott Thomsen
Two-hundred feet below the surface of the Pend Oreille River in northeast Washington sits an important piece of equipment for Seattle City Light’s Boundary Dam.
A 312-ton sluice maintenance gate at the base of the dam is used to seal the dam, to remove any of the seven gates that control the amount of water in the reservoir for maintenance or to work inside one of the sluices that carry that water.
That important work was brought to light by a year-long maintenance project for the gate, which City Light completed in September 2011. It was the first time the gate had been removed since the early 1980s.
With a capacity of 1,046.8 MW, Boundary Dam accounts for almost 60 percent of the electricity Seattle City Light generates, which makes regular maintenance of all its parts a critical function for the utility.
“Maintenance of this kind is not easy,” superintendent Jorge Carrasco said. “Our employees must coordinate dam operations with other river stakeholders and regional utilities for power production and recreational uses even before they can begin to manipulate the massive gate with hoists, boats and heavy equipment to make repairs.”
|Seattle City Light removes this sluice maintenance gate from the base of Boundary Dam for a major rehabilitation. Workers removed five tons of gook from the 312-ton gate. (Photo courtesy of Seattle City Light)|
Work started in the fall of 2010, but planning for the project started in January of 2009.
Organizations concerned with lowering and raising the reservoir elevation include the Washington State Department of Fish and Wildlife, which is the permitting authority; the U.S. Army Corps of Engineers, which operates Albeni Falls Dam and issued a 404 permit for excavation of the gate removal ramp in the Boundary recreation area; Pend Oreille Public Utility District, which operates Box Canyon Dam; Pend Oreille County Community Development Department, which issued a shoreline development permit; Washington State Department of Ecology, which is another permitting authority; and BC Hydro, which operates 7 Mile Dam downstream of Boundary in Canada.
Extensive internal coordination also was needed among Seattle City Light dispatchers, power marketers and the operators at the plant.
Permits for the work were obtained in September 2009. Coordination of reservoir elevation changes occurred daily during the work.
Because the section of the Pend Oreille River immediately below Boundary Dam flows north into Canada, BC Hydro needed to know what flows to expect coming from Boundary. Dams located upstream of Boundary cooperated by lowering and increasing flows when Seattle City Light needed to raise and lower its water elevations.
Seattle City Light’s Power Marketing group and Boundary staff handled the day-to-day river flow coordination, and the utility’s Environmental Affairs staff led by Senior Environmental Analyst Margee Duncan applied for and received the permits for the work.
“Once all the planning was in place, most everything went like clockwork,” Senior Mechanical Engineer Kevin Marshall said. “The combination of bidding periods, purchasing uncertainty, contract negotiations and site logistics with different projects within the main gate project, such as sand blasting, painting, machining, testing and a variety of piping, created most of the sleepless nights.
“When we started planning for the sluice maintenance gate work, Boundary personnel had just completed dredging of the forebay, which involved obtaining the same kind of permits that were needed for sluice maintenance gate removal and replacement,” said Marshall. “So, our permitting people were already familiar with the people and agencies involved, which helped make the permitting part go smoothly.”
Seattle City Light has submitted its application for a new operating license at Boundary Dam. The Federal Energy Regulatory Commission is expected to make a decision on that relicensing application in early 2012.
|Workers hoist a 312-ton sluice maintenance gate from the reservoir at Boundary Dam in September 2010. (Photo courtesy of Seattle City Light)|
Removing the gate
When it came time to remove the gate, Knight Construction helped to clear the gate removal bay of silt to move the gate into position for raising. They employed high pressure air nozzles to blast the many years of muck out of the way.
City Light crews, dam operators and contractors used a series of changes in the reservoir’s water level to float the gate to the surface, tow it into position with a boat, and then lower it onto a framework of heavy beams supported and driven by eight hydraulically driven “trucks” or dollies to haul it onto land at Boundary’s Forebay Recreation Area.
“This was delicate work with giant pieces of equipment,” said Lonnie Johnson, a supervisor at Boundary Dam. “Safety was paramount. A mistake could send the massive gate crashing into the dam.”
Each of the “trucks” had eight wheels, and each could be independently steered, and driven in forward and reverse. Hydraulic power was supplied by a large diesel engine-driven hydraulic pump, temporarily mounted on the side of the gate. A ninth truck was on site in case it was needed.
The trucks, beams and other equipment for the short move across land were supplied and operated by McClure and Sons of Mill Creek, Wa., which has extensive experience moving large pieces of equipment. The work was performed under a $250,000 contract for removal and reinstallation that was awarded in June 2010 after a bid process that started in March 2010.
Moving the massive sluice maintenance gate from the water to its rehab location and back again was a slow process.
When loaded on the dollies, the gate moved at about 2 feet per minute. It took about 2-1/2 hours to cover the 250 feet from the repair area to the removal ramp where the gate was floated.
With the gate out of the water, the project team got its first real look at the wear and tear from more than two decades of underwater duty.
“The thing that was amazing to me was that the gate still worked when we took it out,” Marshall said. “There was a lot of damage to it.”
One of the gate’s six guide wheels was broken. On the other five, the tires came right off when they were tested. Two of the massive springs that work with the wheels to keep the gate away from the dam when it is being moved were completely broken off.
|Fully rehabilitated and painted with a fresh coat of “safety green” paint, workers prepare to transport this 312-ton sluice maintenance gate to Boundary Dam in September 2011. (Photo courtesy of Seattle City Light)|
Preparing for the rehab
Once on land, Seattle City Light built a temporary building around the maintenance gate. Completing that aspect of the project became more challenging than anticipated.
Construction of the concrete pad and repair building qualified as a Public Works project, requiring a bid process for the work. The project team underestimated how long it would take for a building contract to be generated as a result of that Public Works bidding process.
“Our original idea was to issue a single contract for installation of the concrete pad, and building construction and movement. This turned out to not be practical, as it was difficult to anticipate and make accommodations for all the potential problems that could arise,” Marshall said. “Instead, we broke the work up into four separate phases.”
Adjusting those initial plans kept the project on schedule.
Centennial Construction oversaw the construction of the concrete pad the gate rested on during refurbishment. Centennial employed Dan Dawson Construction and Concrete to do the excavation and WM Winkler Co. for the finishing.
Throughout the year, workers blasted old paint, corrosion, silt and other debris from the metal gate. Workers used 100,000 pounds of blasting abrasive to clean the gate and removed about 5 tons of gook that had built up over the 28 years since its last service.
“Every part that moved was removed,” Marshall said. “We refurbished it and put it back in.”
In addition to the guide wheels, two broken gate springs also had to be replaced. Each of the new springs was made from 14 feet of 2-3/4 inch stainless steel.
“We were not aware that any of the 18 springs that are installed in the gate needed to be replaced until after the gate was removed from the water in September 2010,” Marshall said.
“We did not know who made the original springs. The raw material needed for their manufacture is a special alloy in an uncommon size that is difficult to obtain, and there are only a couple of companies in business that have the equipment required to bend 2-3/4 inch diameter high-strength stainless steel into the shape required.”
Warden Fluid Dynamics rebuilt the hydraulics of the gate’s gear drive system.
|Workers tow this 312-ton sluice maintenance gate back to Boundary Dam after a full rehabilitation. (Photo courtesy of Seattle City Light)|
Gate returned to operation
Renovations complete, the gate was painted “safety green” so divers could spot it easily during inspections. It was then ready to return to duty deep below the surface.
In September 2011, crews reversed the removal process. Over 10 days, they moved the gate back to the water on the hydraulic trucks, then lowered and raised the water level to take the refurbished gate off the beams and float it into position on concrete guide slots on the dam.
Once the gate’s wheels were captured in the guide slots, a ballast chamber on one end of the floating gate was filled with water to position the gate vertically. With a 140-ton hoist attached, a chamber on the other end of the gate was then filled with water, allowing crews to lower the gate.
When it reached its operating position, timing pins on the gate meshed the gate’s toothed rack with operating gears on the dam and it was ready to return to work. In all, the job involved about 40 people, including engineers, dam operators, machinists and movers, to complete.
Garco Construction erected and pulled down the containment structure, and Associated Underwater Services monitored all underwater operations.
The rehabilitation work cost about $1.5 million, not counting costs associated with the loss of electricity generation from the dam.
Seattle City Light lost about $850,000 in revenue from the loss of generation capacity associated with the project, about $610,000 during the 2010 removal and about $240,000 during the 2011 reinstallation.
During both drawdowns, the loss of generation forced Seattle City Light to move energy from peak periods to off-peak periods when the utility would normally shape the maximum amount of energy possible into the peak period. In 2011, the on/off price spread was about 54 percent of the 2010 spread during the outage. The duration of the period affecting generation shaping was also shorter in 2011, further reducing the cost of the 2011 reinstallation outage.
During the drawdowns, the water level in the reservoir would change by as much as 35 feet from its normal elevation of 1,990 feet above sea level.
At the low point, the drawdown uncovered the namesake falls near the town of Metaline Falls and other portions of the reservoir not seen since 1982.
Mike Haynes is power production director and Scott Thomsen is a senior strategic advisor for Seattle City Light
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