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Xcel Energy’s Cherokee Station is a coal-fired, steam-electric generating station with three operating units located just north of Denver Colorado near 58th Avenue and Interstate 25. As one of Xcel Energy’s largest power plants in Colorado, it is capable of generating 611 megawatts (MW). Cherokee Station began operating in 1957 when Unit 1 went into service. It was followed by Unit 2 in 1959, Unit 3 in 1962, and Unit 4 in 1968. The plant is also capable of burning natural gas as fuel.
Xcel Energy is beginning to implement its Colorado Public Utilities Commission-approved Clean Air – Clean Jobs Plan. The plan will early retire Cherokee Units 1, 2, and 3 (365 MW), which operate on coal, and install a cleaner, more efficient natural gas plant (569 MW) at the existing plant site. Xcel will also switch Cherokee Unit 4 from coal to natural gas (352 MW).
As part of the project, a conduit installation was required to connect the proposed Generator # 7 to the Cherokee Substation Bay # 13. The power cable system will operate at nominal 115,000 volts. Overhead power lines were considered, but logistics rendered the option impractical. Instead, Xcel decided that underground power lines would be a better solution.
The proximity of the substation to the adjacent power plant was the largest hurdle to overcome for successful installation of the six 115 kV lines. The substation sits relatively close to the power plant, but is elevated roughly 100 feet above the ground floor of the power plant, with an extremely steep side slope (more than 60% grade) between the two locations. Operating heavy equipment on the slope would have been almost impossible and even an underground installation would prove challenging.
Xcel turned to horizontal directional drilling (HDD) contractor Rick Allison of Pinnacle Development to seek a trenchless solution for the conduit alignment between the substation and the power plant. After reviewing the site topography and soils, Allison determined that an HDD Installation would be feasible. The original plan called for HDPE conduits to be installed, with a strict requirement that a mandrel with an outside diameter ½-inch smaller than the conduit pipe inside diameter be successfully pulled through each conduit following installation. Conduit pipe failing to meet the post-installation mandrel test would be rejected.
Allison knew that pulling an HDD bundle of eight 6-inch conduits (six lines, plus two spares) and five 2-inch conduits (for grounding cables and communication lines) would require a very large borehole with high hydraulic forces downhole during pull-back. While bore distance was relatively short at 450 LF, managing the elevation change and handling the 13-pipe bundle would be challenging. Allison’s extensive experience with HDD projects led him to question the critical buckling capacity of HDPE conduit pipe for this installation. If the downhole pressure during installation caused the conduit pipe to oval even slightly, the mandrel would not pass and the installation would have to be repeated with new pipe. Ultimately Allison recommended to Xcel that Pinnacle use Fusible PVC® pipe for the conduits, in lieu of HDPE.
When the mandrel passed through the eighth and final 6-inch Fusible PVC® conduit, Allison knew he had made the right choice: “UGSI was more of a partner than a pipe supplier on this project. Their fusion technician and construction manager helped my crew avoid unwanted surprises during fusion, internal de-beading, and installation. The bundle of Fusible PVC® conduits held up well through pull-in and we were delighted that the mandrel easily passed through each of the eight conduits after installation.”
|Pipeline Details and Project Summary|
|Project:||Cherokee Power Station Conduit HDD|
|Length and Pipe Size:||3,720 LF 6” DR18 (8 x 465 LF|
|Pressure Test:||3 psi air test for 2 hours|
|Engineer:||Power Engineers (Mike Durkin)|
|Contractor:||Pinnacle Development (Rick Allison)|
|UGSI Contact:||Steve Austin (303) 563-9467 firstname.lastname@example.org|