A new 30-MW powerhouse was added to the existing powerhouse at the Lower Baker project to better manage flow releases for movement of migratory salmon.
By Monty Nigus, Shane Richards, Brian Taylor and Thomas Haag
Monty Nigus is project manager – hydroelectric services with Black & Veatch. Shane Richards is project manager with Puget Sound Energy. Brian Taylor is senior construction manager with PCL Construction Services. Thomas Haag is chief engineer – hydroelectric services with Black & Veatch.
Design-build delivery of the 30-MW Lower Baker Unit 4 Powerhouse was spawned by a need to improve flow releases for better movement of migratory salmon, but successful implementation of the newly completed addition is not a fish story in the sense of exaggeration. The Lower Baker Unit 4 redevelopment is in fact successful from safety, schedule, quality and cost perspectives.
The Baker River facilities, on the Baker River near Concrete in northwest Washington, collectively comprised the largest hydropower asset owned by Puget Sound Energy (PSE) even at the original 170 MW of capacity prior to construction of Unit 4. The facility has two dams, each with its own powerhouse. The 285-foot-high Lower Baker Dam, which creates Lake Shannon, was completed in 1925 and had a power generating capacity of about 80 MW prior to the construction of Unit 4. The 312-foot-high Upper Baker Dam, which creates Baker Lake, was completed in 1959 and has a power generating capacity of about 90 MW. The dams’ reservoirs are fed by runoff from the flanks of Mount Baker and Mount Shuksan.
To help PSE optimize use of the Baker River for power generation while meeting a flow release requirement in the Federal Energy Regulatory Commission (FERC) license, engineering consulting firm Black & Veatch teamed with PCL Construction Services (PCL) as a subcontractor to design a new 30-MW powerhouse and other hydropower components on a modified design-build basis for the Lower Baker Unit 4 Powerhouse project. Jacobs Associates was owners’ engineer for the project. PSE commissioned the new facilities in July 2013.
In developing the project, PSE evaluated a variety of options for meeting the minimum instream flow and downramping requirements into the Baker River before deciding to install a 30-MW vertical Francis turbine in an entirely new powerhouse (Unit 4). One of the considered options included a low-level outlet valve constructed into the downstream dam face. Another considered option entailed installation of two 15-MW horizontal units in six different locations onsite, including expansion of the existing Unit 3 powerhouse.
The three organizations worked closely to:
— Complete the project ahead of the schedule;
— Maintain operation of the existing Unit 3 powerhouse throughout construction;
— Ensure project site safety despite the adjacent steep rock hillside with historic concerns about slope stability and potential rockfalls;
— Develop a new 12-foot-diameter underground pressure tunnel from the existing 26-foot-diameter concrete-lined pressure tunnel for Unit 3 to the new Unit 4 powerhouse. This required a new underground bifurcation connection, which eliminated the need to penetrate the existing Lower Baker Dam for a new intake structure. The bifurcation was installed prior to construction of the new Unit 4 tunnel and in less than 30 days during an annual maintenance outage for Unit 3; and
— Build the powerhouse with a small site footprint, which demanded tight space allocation for construction equipment and installation.
Managing fish populations, water resources, and energy needs
The Baker River is a major tributary of the Skagit River, one of the state’s most prolific river systems for fish. The Skagit-Baker basin contains a variety of migratory fish species; the Baker River’s most abundant stocks are sockeye and coho salmon.
PSE has worked for decades to support the watershed’s fish populations. Advances in technology, greater knowledge of fish biology, ongoing PSE investments in fisheries systems, and continued collaboration with resource agencies and Northwest Indian tribes have produced significant gains in the river’s fish stocks over the years. The Baker River’s annual adult sockeye returns have averaged about 3,500 since the 1920s but plunged to a low return of just 99 fish in 1985, imperiling the stock. Since the mid-1980s, fish restoration efforts have had a dramatic effect in the recovery of Baker River sockeye. Eight of the 10 highest annual returns on record have occurred since 1994, including an all-time high of 48,367 in 2012.
In 2008, FERC issued PSE an amended 50-year operating license for the Baker River Hydroelectric Project. Eight years of collaborative consultation between PSE and 23 other parties — including government agencies, Indian tribes, and environmental groups — culminated in license revisions calling for major PSE initiatives to further enhance fish populations in the Skagit-Baker watershed. Provisions called for PSE to install new upstream and downstream fish-passage facilities, construct a new fish hatchery, protect riparian habitat and develop a flow implementation plan (FIP) that included redevelopment of the Lower Baker site. The Lower Baker project included a new Unit 4 to replaced Units 1 and 2, which were destroyed in a 1965 landslide.
PCL served as the design-build contractor, and Black & Veatch served as the engineer of record. PSE procured the turbine from Litostroj Power and the generator from Koncar GIM, while Black & Veatch specified and PCL procured the balance of the equipment. Black & Veatch also performed transient analyses for the water conveyance system through sub-consultants and provided three-dimensional powerhouse modeling.
The new Unit 4 powerhouse contains one turbine-generator unit with a capacity of 30 MW, increasing the installed capacity of the Lower Baker development to about 110 MW. The new unit is able to generate power from flow releases that are below the operating range of the 80-MW turbine-generator unit (Unit 3) at Lower Baker. In addition to generating power, Unit 4, operating in concert with Unit 3, allows PSE to closely regulate and maintain river flows through Lower Baker Dam to better accommodate the needs of fish and optimize the water resource.
A major element of the relicensing and stakeholder agreement was the development of an FIP for the total facility operation (Units 3 and 4) with two key features.
- To provide continuous minimum flow, an automated project bypass system was installed. This consisted of a 60-inch fixed cone pressure-reducing valve and penstock installed and operated in parallel with the generating units.
- When facility generating flow is reduced, the river level fluctuation downstream of the plant (1 to 2 inches per hour) is tightly controlled on an hourly basis. This function is called the “downramping” rate.
Black & Veatch automated the FIP and programmed it into the facility control system. This effort, which entailed more than 2,000 man-hours of programming time, provides PSE with the tools to optimize the power benefits and maintain compliance with the FIP.
Layered on top of the FIP and power generation planning is flood control. The Upper and Lower Baker projects are part of the Skagit River flood control plan administered by the U.S. Army Corps of Engineers. PSE is required to provide flood storage and cooperate with the Corps.
Building a powerhouse
Black & Veatch designed a 12-foot-diameter, 1,000-foot-long hard rock pressure tunnel that runs from the existing surge tank and Unit 3 power tunnel to the new Unit 4 powerhouse. The key project features also include the design and construction of a penstock and bifurcation; synchronous bypass valve; draft tube gates; installation of owner-furnished equipment; and startup and commissioning services. The work also included governor and control upgrades for Unit 3 at Lower Baker.
The new powerhouse, built about 400 feet downstream of the existing powerhouse site, is a subterranean, reinforced concrete structure. Powerhouse construction included the excavation of 10,000 cubic yards of overburden and rock to a depth of 60 feet below grade and 40 feet below the river and placement of 8,000 cubic yards of structural concrete on multiple levels to complete the structure.
The facility and unit control systems provide either combined or separate operation of Units 3 and 4.The minimum instream flow, maximum instream flow and downramping rate are variable based on time of year. Because the Baker River is a tributary of the Skagit, the flows and rates are a function of the Skagit River flow. An “Aquatics Table,” which summarizes the regulatory downramping control information requirements, was programmed into the PLC-based facility control system.
Some of the design and construction challenges merit special mention. Because of the limited site area, the design and construction of the semi-underground powerhouse and power tunnel could not be performed using open-cut methods. Instead, the powerhouse was constructed within an excavation support system using about 300 1-meter-diameter secant piles. This system encompassed the powerhouse construction area and provided a dry site during all construction phases. The design-build team evaluated various options, including diaphragm walls, traditional pile and lagging systems, and ground freezing. Secants were the most cost-effective option.
Because it is subterranean, all required operating and maintenance facilities needed to be within the substructure of the powerhouse. Combined with the tight site and higher cost of underground powerhouse structures, the design-build team was faced with a space-allocation challenge. Use of 3D modeling tools enabled the team to minimize the powerhouse footprint and meet PSE’s operating and maintenance requirements.
|Design-build delivery of the 30-MW Lower Baker Unit 4 Powerhouse (foreground) will help Puget Sound Energy more effectively manage fish populations as well as optimize energy generation. (Courtesy Francis Zera, zeraphoto.com)|
During a period from 2011 to 2012, PSE and a team of geotechnical consulting engineers were monitoring the steep hillside directly adjacent to the Unit 4 powerhouse construction area when ground movement, as measured by inclinometers, began to accelerate. Activities were immediately shut down and site access was limited. The cause for the movement was identified as both shallow and deep sliding planes that were lubricated by perched groundwater. Mitigation against sudden slope failure included installation of additional inclinometers that were tied to an onsite alarm and warning system and installation of a retaining wall and drainage system at the foot of the hillside to mitigate the shallow failure scenario.
|Installation of a tied-back secant pile wall system enabled deep excavation and dry construction of the 30-MW Lower Baker Unit 4 powerhouse. (Courtesy PCL Construction Services)|
These efforts successfully stabilized the movement, and construction work resumed one to two months after the shutdown. Toward the end of the project, when the tailrace area was excavated, groundwater seepage into the tailrace excavation adjacent to the hillside was occurring, and a drainage system was installed to further mitigate slope movement.
The new 12-foot-diameter Unit 4 power tunnel bifurcates from the existing 23-foot-diameter tunnel at the existing surge tank. The bifurcation interface had to be completed before the Unit 4 tunnel work started, and it was completed during a 2011 plant outage as one of the first construction activities. The work was performed using drill/blast methods with a steel liner and redundant dished head temporary closures. In 2012, the power tunneling from the Unit 4 powerhouse end met up with the bifurcation within +/-0.5 inch.
|A 12-foot diameter, 1,000-foot long hard-rock pressure tunnel connects the existing surge tank and Unit 3 power tunnel to the new Unit 4 powerhouse at the Lower Baker project. (Courtesy Sue Bednarz, Jacobs Associates)|
An automated project bypass system was installed to provide continuous minimum flow and assist in the downramping of Units 3 and 4 in accordance with the FIP. The system consisted of a 60-inch fixed-cone pressure-reducing valve and installation of a penstock that operates in parallel to the generating units.
|Development of a three-dimensional model of Lower Baker Unit 4 helped communicate design intent among project participants, avoid conflicts and minimize the powerhouse footprint. (Courtesy Black & Veatch)|
The project management team believes the most important success factor was the excellent communication among team members. Design workshops, weekly construction meetings and owner leadership during commissioning all helped the team maintain progress with schedule, cost, quality and most importantly safety.
The design phase coordination used pre-planned workshops, and the 3D powerhouse model allowed such stakeholders as PSE engineering, operations, maintenance and project management to apply timely value engineering and buy in to the design. This prevented significant rework, conflicts and cost escalation during construction. Engineering decisions were value-based and timely. Design was phased to match the construction schedule.
The procurement plan was well-executed. PSE controlled key performance requirements by directly purchasing the turbine-generator unit. PCL procured the remaining plant equipment, which was coordinated with the phased design and specification process. This provided for detailed equipment procurement scoping and just-on-time deliveries. Basing procurements on final design minimized procurement change orders.
Even with the shutdown for the hillside slope mitigation, the project team maintained schedule using targeted overtime and replanning of construction. The team met the commercial operation schedule of July 2013 — ahead of the adjusted completion schedule of October 2013.
Safety was “job 1” on this project. With a tight working area next to the river and underground tunneling ongoing concurrently with the powerhouse construction, everyone had to focus on safety first and look out for each other. This team effort was the reason the project recorded 210,000 man-hours, a lost-time frequency rate of 0.0, and a total recordable incident rate of 2.05.
The high level of quality throughout design and construction earned the Lower Baker Unit 4 Powerhouse an Excellence in Concrete Construction for Public Works – Civil Projects award from the Washington Aggregates & Construction Association.
The authors would like to acknowledge:
— FERC’s Portland Regional Office, for its cooperation and timely review and response;
— Board of Consultants, led by Kim DeRubertis, who provided important feedback and guidance;
— PSE’s Lower Baker operations team members for their assistance and understanding throughout construction; and
— All construction personnel, subcontractors and suppliers for their excellent safety and quality performance.