By Dave Carlson, Southeast Alaska Power Agency, and Greg Huffman, Dryden & LaRue
Alaska is blessed with immense energy resources including fossil fuels and renewables. Its unique geography and sheer size coupled with a relatively small population, however, create challenges in transporting or transmitting these resources to the population base.
The Swan-Tyee intertie, a 57-mile 138 kV transmission line through a rugged and road-less portion of the nation’s largest national forest, the Tongass, exemplifies the challenges facing any infrastructure project in Alaska. The area’s geography is dominated by islands, mountainous terrain, and inland waters of the Pacific and receives up to 200 inches of annual rainfall.
Hydroelectric power generation dominates the area, created by the Energy Program for Alaska initiative and initially funded using oil revenues. As a result, the Tyee Lake and Swan Lake projects in Southeast Alaska were built in the early 1980s. Both Tyee and Swan are approximately 20 MW projects with Swan Lake serving Ketchikan via a 30-mile 115kV transmission line and Tyee providing power to Wrangell and Petersburg through a 73-mile transmission line including four separate submarine cable crossings.
Swan Lake has less capacity and storage than Tyee; it’s almost fully utilized to meet Ketchikan’s power requirements. Tyee Lake, on the other hand, is larger and has more energy capacity than Swan Lake while serving smaller communities. Thus, a surplus of power exists at Tyee Lake. The Swan-Tyee intertie interconnects these two hydro projects to transmit the Tyee surplus to Ketchikan.
With this transmission line complete, an interconnected system in southern Southeast Alaska exists for the first time.
Ownership and Funding
The Energy Program for Alaska fizzled out when oil prices plummeted in the early 1980s. Ketchikan initially pursued construction of the Swan-Tyee intertie but disliked that the city’s connection would tie together two state-owned projects. In 1995, four local cities began discussions to buy the projects from the state, and the purchase was completed in 2002 with the projects placed under the jurisdiction of the Four Dam Pool Power Agency (FDPPA), a joint action agency for the cities. The Swan-Tyee intertie project was then transferred from Ketchikan to FDPPA. After some issues with distance and scope for the FDPPA, some projects were sold and transferred, and the remaining regional projects were placed under a renamed FDPPA, now the Southeast Alaska Power Agency (SEAPA).
With the intertie given a green light, funding remained the final hurdle. When oil prices rose in 2007, the legislative session approved a $46.2 million dollar appropriation in the 2008 budget for the project. This state support, along with some bonds sold by SEAPA in 2009, covered the final project costs.
The Project Starts
Ketchikan contracted with an out-of-state engineering firm and began routing and engineering efforts for the Swan-Tyee Intertie in 1995. By 2002, they had successfully completed an environmental impact statement (EIS) with an approved route through the Tongass National Forest. Other than a few existing service roads, the entire route was inaccessible by vehicles; no new roads were allowed to be constructed. Construction for the entire 57-mile intertie route, therefore, would have to be performed using floating work camps and helicopters. In the summer of 2003, most of the route was cleared.
In 2003, the city solicited line construction bids using a design incorporating braced H-frame structures. Bidders expressed concerns regarding the extreme difficulty and expense to construct such a design. Braced H-frame structures are heavy and rigid, and the hillside slopes would require unique leg length combinations for almost every structure, along with dual foundations per frame. Due to the concern, the city sought design modifications.
Dryden & LaRue (D&L) was asked to participate in modifying the design. D&L proposed a Y-tower concept be used in lieu of all but a few of the H-frames. The Y-tower is lighter, more helicopter-compatible, requires only one foundation and is not sensitive to side-slope terrain.
After another round of construction bids, Wilson Construction was chosen.
Line Design Process
Wilson embraced the Y-tower concept and offered suggestions to make the structures more helicopter-compatible, including the “stove pipe” concept for setting structures. This involves having a slip joint for the structure just a few feet off the foundation which allows the light stove pipe (or base section) to be set with a relatively inexpensive helicopter. The remainder of the structure can be assembled at staging yards, flown to the site and set on the stove pipe with a much larger and expensive helicopter. With this and other changes to the process, construction costs were reduced by nearly $10 million before ground was broken.
Since the right of way was mostly cleared, it was decided to confirm the original profile survey using LIDAR technology. Minor reroutes, structure relocations and structure eliminations were then incorporated into the design. Total structure quantity was reduced by 37 from the original design. Of the 263 final structures, 220 are Y-towers, 33 are three-pole dead ends, five are H-frames, four are A-frames and one, the new structure inside Swan Lake switchyard, is a four-leg frame structure.
System studies for the intertie project resulted in a design voltage of 138 kV with initial operation at 115 kV based on a peak load transfer of 30 MW. This is a relatively small load for these voltage levels, and conductor size for the new line is consequently on the small end of the typical range for high voltage transmission systems.
Foundations and Anchors
Foundations and anchors for the structures use drilled micro-piles. Micro-piles were selected because they can be installed with lightweight equipment transported by small-lift helicopters and because of the micro-piles’ versatility in varying soil and rock conditions. Wilson subcontracted the foundation and anchor work to Crux Subsurface.
This was the first transmission project of this magnitude to be constructed solely using micro-pile foundations. D&L and Crux designed piles and patterns compatible with Crux’s equipment which accommodated the different structure types and loadings in the varying geotechnical conditions from deep organics (muskeg), soft silts and clays, to hard bedrock.
Each tower foundation installation utilized an adjustable leg platform (for the diverse slope angles) with a drop-in guide template that allowed drilling to match the pile caps that were installed later.
During this process, flat deck barges and barge camps were utilized as mobile living quarters, maintenance and machine shops, management and dispatch offices, helicopter landing zones, and platforms for grout batch plants and materials storage yards.
The project sat idle for three years after 2004, unfortunately, until funds were secured in 2007. Crux returned to complete foundation and anchor installation in the spring of 2008.
A total of 357 foundations with 1,919 total micro-piles were installed on the project. Each foundation consisted of three micro-piles to nine micro-piles bolted to a pile cap. There are 494 guy anchors on the project. Total drilling length on the project was over 56,000 feet.
In 2002, a contract to procure steel structures based on the original H-frame design was bid and awarded to V&S Schuler. No procurement activity was initiated until early 2004 when D&L began working with V&S Schuler to change the structure designs from H to Y. Using the revised designs and the micro-pile pattern developed by D&L and Crux, V&S Schuler designed and fabricated pile caps for connecting the structures to the micro-piles. By the fall of 2004, over 430 tons of pile caps had been delivered to Ketchikan.
Due to the project postponement in 2004, no further material procurement occurred until 2008. Steel structure procurement was re-bid using the redesigned structures, and Valmont-Penn Summit (VPS) was awarded the contract to supply tubular steel structures for the project.
By the end of May 2008, all 222 stove pipe sections were delivered to Ketchikan. VPS first focused on fabricating the Y-tower structures. The other structures, all consisting of multiple legs, could not be designed and fabricated until foundation as-built information was received at the end of the 2008 construction season.
Steel structures were trucked from Pennsylvania to Seattle, and then barged to Ketchikan. Wilson received and assembled some of the structures in Ketchikan, stacked the structures on barges and staged them in waterways close to the intertie route. Other structures were assembled and staged at land sites accessible by barge. Insulators and stringing travelers were also installed on the structures at the staging areas.
An Erickson air crane arrived on site in late May 2009, and structure setting began shortly thereafter. The Y-towers were picked from the staging areas, flown to the structure sites and set onto the stove pipes without the need for any ground support. A small helicopter was used to spot foundations for the air crane and help guide the structure onto the stove pipe. Ground crews came in later and jacked the structure tightly onto the stove pipe. The air crane set as many as 60 structures in a day, setting all 263 structures, staging conductor reels and stringing equipment in less than nine days.
Two conductor types were used on the project. A special 397.5 one thousand circular mils (kcmil), 30/7 stranding, aluminum alloy, alumoweld steel-reinforced conductor (AACSR/AW) was used for the low altitude sections. This conductor has a diameter of 0.81 inches and a rated breaking strength of 27,500 pounds. It was used for 42 of the line’s 57 miles. Aluminum alloy strands had the strength required to support the line’s heavy ice and wind loading design on relatively small diameter conductors.
For the 15 miles of high altitude section and long spans, an even stronger conductor was needed. The route crossed four large water bodies requiring spans over one mile; the longest span was 6,890 feet. A 37 strand, number eight alumoweld steel conductor was selected for the long span and high altitude sections. It has a diameter of 0.90 inches and a rated breaking strength of 84,200 pounds. Stringing tensions in the long spans approached 20,000 pounds and the maximum design sag was approximately 650 feet.
Conductors were manufactured in India by APAR Industries and furnished on reels in prescribed lengths provided by Wilson to fit their wire setup locations. Dead ends for the all-alumweld conductors and splices for both conductor types were specially manufactured incorporating implosive technology. Special conductor pullers were designed and manufactured in Italy, and further modified in Wilson’s Oregon shop to be more easily flown from site to site.
After all the structures were set and wire reels and equipment staged by the air crane, it took only three months for Wilson to shake down the structures, install guy wires, and perform wire work. In mid-August 2009, more than 14 years after engineering began, the Swan-Tyee intertie was completed and ready to energize.
Carlson is CEO of the Southeast Alaska Power Agency, an agency of three remote Alaska utility members (Ketchikan, Petersburg and Wrangell municipalities); the agency was formed to own and operate hydroelectric projects in the region.
Huffman, P.E., is an engineer with Dryden & LaRue, consultants who have been providing total electrical project services to Alaska’s utilities since 1977.
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