Xcel Energy, supplying electric and natural gas energy to eight states in the Midwest is close to completing one of the largest electrical power projects to be built in the state of Minnesota.
Once completed, the 7-year, $1 billion project will improve air quality and efficiency at three key electrical generating plants in the St. Paul/Minneapolis metro area.
Part of the Minnesota Metro Emissions Reduction Project (MERP), Xcel Energy will replace old, out-dated coal fired electrical generating equipment at the St. Paul High Bridge and Minneapolis Riverside plants with gas-fired combined-cycle equipment.
The third plant, located in Oak Park Heights on the St. Croix River 20 mi. (32 km) east of St. Paul, added pollution control equipment and rehabilitated existing electrical generating equipment in 2007.
Construction at the Riverside plant started this year and will replace two of its existing coal fired units with the natural gas combined-cycle arrangement.
The combined-cycle equipment installed at the High Bridge and Riverside plants burns natural gas as the fuel source.
The most extensive construction occurred alongside Xcel Energy’s existing High Bridge plant, located on the shoreline of the Mississippi River just south of St. Paul’s downtown business and commercial district and straddling an extensive city park system.
Eating up $500 million of the $1 billion dollar budget, the new plant was recently completed and is now under going testing. Once testing is completed, the new plant will go online and the old plant, serving St. Paul customers since 1923, will be demolished.
No longer will there be piles of dirty coal sitting on the site. The new plant also will be smaller and eliminate a huge eye sore to area residents; a 565-ft. (172 m) stack.
Inside the new plant, the pair of combustion turbines running in tandem with the heat recovery generators will crank out 575 megawatts of electricity, an increase of 280 megawatts from the existing plant or enough power to supply 300,000 typical homes.
LG Constructors of Atlanta, Ga., is the engineering, procurement and construction contractor on site. According to Don Hardison, senior construction manager for the project, LG Constructors specializes in this type of construction and has built similar power plants throughout the nation.
It also builds other types of power plants along with manufacturing and processing facilities.
More than 40 sub-contractors will team with LG Constructors to complete the building, eventually consuming more than 1,000,000 man-hours. Workers will pound piles, pour concrete, piece together steel, hang siding, place the roof and install internal systems.
Standing 11 stories tall and covering three acres (1.2 ha), iron workers swung into place 4,500 lbs. (2,041 kg) of beams and trusses along with 150,000 sq. ft. (13,935 sq m) of siding to complete the structure.
Inside the shell, trade workers installed millions of feet of wiring and thousands of feet of piping and moved in high tech electrical generating equipment, weighing more than 788 tons (800 t).
LG Constructors brought in countless pieces of heavy equipment to get the job done. More than a dozen cranes — 70- to 270-ton (63 to 244 t) Manitowoc, Link-Belt and American cranes — picked up steel and lifted the mammoth generating equipment into place.
Pile drivers on American cranes smashed hundreds of piles deep into the ground.
Tracked and rubber tired Komatsu, John Deere, Cat, Case, JCB, Kobelco, Volvo and Daewoo excavators dug the pit and cut the ditches for the water and gas supply lines.
Also on hand were Dresser, Galion, John Deere and Cat graders, Blaw Knox and Barber Greene pavers and hydraulic cranes from “Carry Decks” to 90-ton (81 t) machines, featuring the Broderson, Lorain, Grove, Pettibone, Liebherr and Tadano names.
Despite facing design challenges early on caused by an unpredictable steel market and some significant revisions in parts of the new building, LG Constructors was right on schedule, remarked Hardison.
“Material supplies in today’s wildly fluctuating market turned out to be quite a challenge,” Hardison explained. “We worked very hard developing steel quantities to get a purchase order together for fabricated steel. However, by the time we were able to do that, mill scheduling started reflecting six-month delivery times on some of the shakes that we needed. So we had to do a little creative engineering to get to a different design and order heavier members that were available sooner to accomplish the same task.”
Another problem cropped up when foundation design changes increased the number of piles required for the building and equipment foundations. Hardison credited a jump start in the piling operations due to the mild winter two years ago, or in his words, “dodging the winter bullets” to keep the schedule on track.
With a project of this scope, size and technical components, lining up skilled labor can sometimes be a problem and is one of the first challenges for a project like this, Hardison noted. Yet, coming into this project, he was confident that the right mix of skilled labor would be available.
“It was our feeling coming into this project that there were adequate numbers of skilled crafts people in the St. Paul/Minneapolis area based on the strength of the labor unions and the kind of work that’s been done in this part of the world,” Hardison said. “We’ll just say that it’s part of the good old Midwestern work ethic you hear so much about.”
Besides all the initial challenges, construction scheduling and the continual phasing of sub-contractors in and out of the site are key to moving the project forward, Hardison said.
“You have to have a good piling contractor to make this all happen. Our objective was to get the two heat recovery steam generators set as soon as possible, because Xcel had the equipment scheduled to get here by barge very early on,” Hardison said.
After the piling was done, a site clearing contractor came in to excavate a 300-ft. (91 m) by 400-ft. (121 m) long by 5-ft. (1.5 m) deep hole.
“Xcel’s contractor cut a ramp in and a ramp out and enough room for a retaining ring so crews could stand up the forms and pour the concrete.”
Though there are no sub levels to the High Bridge plant, the project still required a lot of excavation because deep foundations are necessary to support the mammoth and heavy turbine and steam components needed to make the plant hum with electricity, according to Hardison.
“We wanted to have those foundations ready,” Hardison added, “so we could take the equipment from the river and get it on those foundations as quickly as possible.”
The heavy duty foundations carried the largest of the components, the two heat recovery steam generators, each weighing 2,500 tons (2,260 t).
Each of the foundations consumed 2,400 cu. yd. (1,835 cu m) of concrete. It required a continuous, nine hour pour in a planning process that Hardison likened to professional sports because of the tight timing and coordination to complete the pour without interruption.
Likewise, the combustion combine generators required 1,600 cu. yd. (1,223 cu m) of concrete.
LG Constructors established a game plan to prepare for the pours, Hardison said.
“We met with all the players to review all the motions of the game plan,” Hardison explained. “We took the schedule and broke it down to the hour. The time is set and everybody needs to be there. Whatever problems come along the way, we need to manage them when they happen.”
To put the timing and planning in perspective, Hardison noted that three separate batch plants supplied the concrete and 245 trucks cycled in and out of the plant site to deliver the concrete for each heat recovery steam generator pad.
The careful timing, planning, coordination and phasing the dozens of sub-contractors into and out of the site paid off, Hardison added.
The road on the fast track continued when steel arrived by the truckload from Wisconsin to begin building the structure.
“As soon as Knutson was off the foundations and they were cured and stripped we had Merrill Steel out of Wisconsin come in with the steel,” Hardison said. “Truck loads of steel came in and the guys started taking the steel off and shaking it out into the various erection groups.”
Once the ironworkers completed one section of the structure, set the roof trusses and hung sheeting to shade the workers from the summer sun, ATCO, based in Canada, came in to place the siding and install the sound attenuation walls, Hardison said.
“Part of the our scheme was to have this building closed in enough where we could work people through the winter,” Hardison said. “By and large we were successful and with the mild weather that year, we were able to work through the winter.”
The installation of the gas supply line became a project in itself. The 20-in. (51 cm) pipe came from a distribution hub more than 2 mi (3.2 km) away, with a portion of it jacked under the Mississippi River.
Also, more than 900 ft. (274 m) of a pair of water pipes was laid in the same ditch to supply cooling water to the plant and return the waste water to the river. Each 84-in. (213 cm) diameter pipe is 20 ft. (6.1 m) in length and weighs approximately 7,000 lbs. (3,200 kg).
Each weld completed on the inside and outside of the concrete encased pipe took 50 hours.
While seemingly contrary to the nation’s effort to conserve its natural gas resources, the new combined-cycle equipment will reduce sulfur dioxide and nitrogen dioxide emissions by nearly 100 percent and mercury emissions by 100 percent, according to Xcel Energy documents.
This design also increases production efficiency to generate electricity, Xcel officials explained, by burning natural gas to fire a pair of combustion turbines which are similar to jet engines.
“The natural gas is burned to spin one generator and create energy while the hot exhaust creates steam to turn another generator,” explained Ron Brevig, High Bridge plant director. “That’s why it’s called a ’combined-cycle’ because you’re generating electricity twice from the same fuel.”
“Integrating combustion turbine and steam turbine technology provides an extremely efficient electricity production process and is about 30 percent more efficient than a traditional plant, Xcel Energy officials said.
Testing on the Oak Park Heights plant will continue through the summer months. The plant is scheduled to go online at the end of this year. Asbestos is now being removed from the old plant with demolition to follow, scheduled for next year. CEG
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