The selective water withdrawal structure installed and operating at Round Butte Dam posed a variety of challenges in terms of design, construction and installation. Today, the structure allows juvenile fish numbering in the hundreds of thousands to pass the dam each year.
By Walter N. Bennett, Vince Rybel, Mike Jenkins, Kerry Donohue, Richard E. Riker and Doug Sticka
Design and installation of a one-of-a-kind modification to an existing hydropower intake system was completed by a diverse team of engineers and constructors. This modification, called the Round Butte Dam selective water withdrawal (SWW) project, is already helping restore salmon and steelhead runs to their ancestral spawning grounds. The facility consists of five large steel structures constructed in an inland reservoir.
The SWW system consists of a selective withdrawal bottom (SWB) structure that is hydraulically connected to the power intake; a 40-foot-diameter, 135-foot-long vertical flow conduit; a floating top structure that contains the surface withdrawal and fish collection components; a 230-foot-long access bridge; and a floating fish transfer facility.
Located in Lake Billy Chinook, the reservoir impounded by Round Butte Dam, the SWW project was needed to reestablish anadromous fish runs above the dam. The dam was originally constructed in the early 1960s with a fish passage system for upstream and downstream migration, but the downstream migration was not effective. Instead, in 1966 that fish passage system was abandoned and a hatchery was built at the base of the dam.
Round Butte Dam is part of the 465-MW Pelton Round Butte Hydroelectric Project on the Lower Deschutes River in Oregon, which is co-owned by Portland General Electric and the Confederated Tribes of the Warm Springs Reservation of Oregon. CH2M HILL provided design and construction oversight for the project.
This work presented many unique challenges, not the least of which involved connecting the structure to the powerhouse intake, 270 feet under water.
|A pontoon barge with a central moon pool was used to assemble the selective withdrawal bottom structure, 50 feet offshore at the right abutment of Round Butte Dam.|
Designing the structure
The three-unit powerhouse at Round Butte Dam has a peak flow of 14,000 cfs through a deep-water intake. The intake is a concrete structure with three trashracks that create a face area 54 feet wide by 70 feet high. The SWB needed to enclose this entire intake in such a way that no gaps larger than ¼ inch were created. The exact dimensions of the intake were not known.
ASI Group performed a preconstruction three-dimensional (3D) sonar survey during the summer of 2006 to map the intake structure, trashracks and bottom contours. Because of the logistics of the tower and intake structure, a truss and winch system was designed and installed on the tower deck so that the sonar equipment used to survey the intake could be hung from the tower. The sonar system was deployed and operated to map the four pier noses and bottom contour for the excavation work would be needed to install the SWW. Images featuring thousands of cloud points were plotted to complete the 3D mosaic.
The structure also had to be designed to be assembled in the dry at a site where no land was available. The only shoreline access available was a small pad on the dam’s right abutment, just enough space to operate one crane and unload materials. From this location, all five large structures were assembled, using multiple techniques. Once the 1.4-million-pound structure was erected, it had to be placed at the bottom of the lake and sealed against the intake while keeping powerhouse outages to a minimum.
PGE and CH2M HILL conducted several workshops in 2005 with industry experts to identify viable alternatives. After the workshops, the team decided the construction methods used to build and place the SWB structure would be critical to the design. As a result, the general contractor, Barnard Construction Co. of Bozeman, Mt., and several specialty subcontractors — including Dix Corp. of Spokane, Wash., Associated Underwater Services (AUS) of Spokane, Wash., and Thompson Metal Fabricators (TMF) of Vancouver, Wash. — were hired at the 25 percent design phase to outline the means and methods to be used in construction and to collaborate on the final design. A series of investigations was then undertaken to define the design variables and understand the design and construction constraints.
The geometry of the structure was complex as a result of the large flow areas needed to pass water through the control gates and from the 40-foot-diameter vertical flow conduit that conveyed water from the top to the bottom of the structure. This complex geometry also meant designing unconventional framing and bracing to support the high gravity and seismic loads. The team designed the structure to be hung from the top of four columns during construction and placement, supported off four jack locations temporarily when first set on the bottom, and then anchored with 11 steel pipes in the final operating condition.
Building the structure
Because of the configuration of the dam, there is virtually no work space available on the dam crest. To overcome site limitations, designers decided to use a pontoon barge with a central moon pool to assemble the SWB structure in the water, 50 feet offshore at the right abutment. Materials were delivered to the barge using a land-based ringer crane and a barge-mounted crane.
Because dive work at depths of 200 to 270 feet is time-consuming, expensive and a significant safety concern, the structure was designed so that remotely operated vehicles could perform nearly all the underwater work. Performing this excavation, drilling and grouting without making a single dive was a major challenge. All equipment was controlled from the barge and observed using ROVs while divers were kept on standby for emergency situations.
The SWB was erected first, in two stages. During the first stage, the outer framing was built on the pontoon barge, supported by a temporary floor. Once the framing work was completed, the SWB was lifted by the rod jacks and suspended by the truss system to allow removal of the temporary floor. As work on the SWB was completed, it was partially lowered into the water to provide better access to the top of its 70-foot frame.
Other structures then built were the top structure, access bridge, fish transfer facility and vertical flow conduit. Total construction time was 26 months.
|This artist’s rendering shows the final configuration of the selective withdrawal structure at Round Butte Dam.|
Installing the structure
Organizing the work approach, given the project’s limited onshore workspace, was one of the most challenging aspects of the Round Butte SWW project. The completed structure was to be massive — more than 300 feet high and weighing roughly 5 million pounds. Yet the only shoreline access available was a small pad on the dam’s right abutment.
From this location, all five large steel structures were assembled using multiple techniques to build them over water. Each of the five required a different method of floatation to address their unique configurations. Barnard Construction, working with PGE and the subcontractors, developed various solutions for temporary and permanent floatation needs.
A system had to be devised to lower the SWB to the appropriate depth and attach it to the concrete intake, and the same system would need to be used to provide a working platform for assembling the 1.4 million pounds of steel. A custom pontoon system with steel framing was used to build a barge with a moon pool large enough for the entire structure to fit through. The pontoon system was outfitted with a tower and truss system that supported a heavy-lift rod jack system capable of lifting and lowering the SWB.
One major challenge of this project involved setting the SWB so close to the intake, 270 feet below the surface. Construction photos taken just before the reservoir was filled in the 1960s provided useful information on the configuration of the intake tower and the surrounding area.
Preparation work involved excavating (underwater) nearly 200 cubic yards of solid rock without blasting. Conventional methods of preparing a foundation on which to land the structure would have been prohibitively expensive. Instead, the team designed the SWB to be lowered from the pontoon barge and settle on top of the existing intake using strong backs equipped with hydraulic jacks. Two landing pads were poured on the lake bottom to act as footings for the legs of the structure furthest from the intake.
The SWB was pushed via its pontoon barge out into the lake above the intake structure. Four units of 400-ton rod-lowering equipment were used to lower the SWB. Each unit consisted of four 1.5-inch-diameter rods. The system lowered the SWB in 18-inch increments. After lowering each increment, rod tensions and measurements to the structure were taken to check that the SWB was maintaining level and balance. Lowering the SWB the initial 250 feet required multiple days of constant lowering.
A combination of hydraulic and hand winches was used to position the barge on the surface and to control movement of the SWB, handing 250 feet below as it approached the existing intake. ROVs equipped with cameras were used to align and observe the position of the SWB. As the SWB approached the intake, the lower portion of the cheek plates engaged the upper half of the existing intake bull noses. ROVs monitored this approach to be sure the cheek plates were in their proper position.
After the cheek plates were engaged, the SWB was lowered to within 6 inches of its final elevation and position. The SWB was then lowered until the strong backs contacted the top of the intake structure. This lowering was conducted so that a slight load was placed on the strong back’s vertical jacks (less than 1,500 psi). Then the horizontal jacks were used to align the center of the SWB with the center of the intake. The strong back’s north/south positioning jacks were used to pull the SWB tight to the intake and slightly compress the seal along the roof. After the SWB roof engaged the intake, the rear support vertical position jacks were deployed to engage the grout landing pads. Final adjustments to the position of the SWB were made using these two vertical jacks along with the two vertical position jacks that first engaged the top of the intake.
With the seals, the challenge was to provide a seal that was fish-tight without tying the two structures together. The concrete intake and the steel SWB have very different stiffness and mass and will therefore respond differently in an earthquake. To satisfy this requirement, three different systems for the top, side and bottom seals were used to allow the structures to respond independently without one loading the other. The side seals, cheek plates were high-strength steel plates that were clamped to the sides of the two outboard bull noses to provide a seal with a flexible connection. The top seal is a steel panel faces with a rubber fender that is designed to slide when loaded, preventing large loads from transferring through this seal.
Mechanically activated clamping devices and grout-filled fabric bags were used to achieve a seal with the face of the power intake while maintaining seismic isolation between the intake and the SWB structure. All grout bags were installed above water to achieve proper fit and seal. The ROV performed multiple inspections on the bags as the structure was lowered to monitor their condition.
The sleeve bags were filled with grout through the inside of each of the 11 sleeves. A 2-inch tremie pipe was lowered until it touched the bottom. Grout was pumped into each bag until the grout level was about 12 inches above the sleeve base as verified by a vent pipe being monitored using the ROV. Once set up, the level grout inside the sleeve provided a good surface to enable the drill bit to get started. The drill was advanced through the grout and then about 30 feet into the rock below the structure. Grout bags were filled in a predetermined sequence, focusing on the seal along the existing intake and the four sleeves designated as critical for powerhouse operation to resume.
During the connection to the intake, ROVs also were used to cut tethers for the SWB seal and release the jacking rods from the SWB. Divers were kept on standby for emergency situations but were not needed. After the SWB was released, two deep dives were required to secure the side seals.
Once the structure was in place, it was anchored using 11 steel pipe piles. The columns for the structure were 3-foot-diameter open pipe sections, through which 2-foot-diameter piles were inserted. A 30-inch drill bit went through these sleeves to drill the sockets for the piles.
Drilling more than 300 vertical feet of 30-inch-diameter rock socket and installing and grouting 11 24-inch pipe piles in water depths up to 300 feet without diving and while limiting powerhouse shutdowns required careful planning and experienced resources. One of the most important decisions was designing the SWB structure to be used as a template/stabilizer for the drill equipment. Geologic conditions required using a rotary reverse-circulation drill system. Steven M. Hain Equipment Co. supplied the drilling equipment for this operation.
Grouting work, including the grout bags and piles, involved placing nearly 200 cubic yards of high-strength underwater grout. This was accomplished using a land-based grout mixer and pump that delivered grout to the construction barge 300 feet offshore, where the crane was used to suspend a 330-foot-long tremie grout pipe to deliver grout to the bottom of the pile sockets.
The structure began operating in December 2009.
Operations during construction
It cost $112 million to develop this structure, including design and construction.
Minimizing the impact on power generation during construction was important because outages mean lost revenue. The structure was erected in a corner of the forebay that was remote from the intake tower, so this work did not affect generation. Foundation preparation was accomplished on night shifts, when the powerhouse was not operating.
During the day, while the powerhouse was in operation, the SWB was lowered to just above the intake and held 100 feet from the trashracks. That position was held until the start of a night shift, when the powerhouse was shut down and crews went to work to set and seal the structure. This task was completed in one night shift. The powerhouse was then operated at a restricted flow rate until the first four piles were installed and grouted, at which time all flow restrictions were removed.
Accolades and accomplishments
The Pelton Round Butte SWW project is the only known floating surface fish collection facility coupled with power generation in the world. As a result of this work, the Pelton Round Butte project has been certified by the Low Impact Hydropower Institute as a source of green power. And for the first time in 40 years, chinook, sockeye and steelhead salmon are able to complete their life cycles, with juvenile fish passing downstream to the Deschutes River basin and then returning as adults to spawn naturally upstream of Round Butte Dam. More than 400,000 fish were captured and transferred downstream in 2011.
In May 2011, PGE received an Outstanding Stewards of America’s Waters award from the National Hydropower Association in the category of recreational, environmental and historical enhancement for the SWW project.
In April 2011, the project received the American Council of Engineering Companies’ Grand Award. This award is given to engineering feats that exhibit uniqueness; technical, social and economic value; complexity; and success meeting project goals. PGE also received a domestic Edison Award from the Edison Electric Institute in June 2010 for completion of the SWW project.
The authors thank Portland General Electric and the Confederated Tribes of Warm Springs for granting permission to use project-related graphics. They also acknowledge the contribution of the design and construction team comprised of PGE staff, consulting engineers from numerous firms, and the contractors.
Round Butte Dam Selective Water Withdrawal, Design Basis Report, prepared by CH2M HILL Inc. for Portland General Electric and The Confederated Tribes of Warm Springs, 2007.
Donohue, Kerry, Michael Jenkins Jr., Richard Riker, Vince Rybel, and Walter Bennett, “Retrofitting Existing Hydropower Intake Nearly 300 Feet Deep,” Proceedings of HydroVision International 2011, PennWell Corp., Tulsa, Okla, 2011.
Wally Bennett is a senior project manager and principal structural engineer and Vince Rybel, P.E., is a geotechnical engineer with CH2M HILL. Mike Jenkins is a project manager with Barnard Construction Co. Inc. Kerry Donohue is vice president of Associated Underwater Services. Rick Riker, P.E., is a geotechnical engineer with CH2M HILL. Doug Sticka is project manager for Portland General Electric. For the Round Butte selective water withdrawal project, Bennett was lead structural engineer and project manager, Rybel was lead geotechnical engineer, Jenkins was project manager, Donohue was lead marine supervisor and ROV operator, and Riker was quality manager.More HRW Current Issue Articles
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