Mobile Gantry Makes Dam Job's Heaviest Lifts

By Mike Larson, editor | September 28, 2010

To view picture of this project, click here.

Miron Construction Co., Inc., Neenah, WI, is using a hydraulic gantry system to place 58-ton hydroelectric-generating components as it re-powers and upgrades the Dells dam and power station on the Chippewa River in Eau Claire, WI, for Xcel Energy Services, Inc., Minneapolis, MN.

Miron, the project's general contractor, ranks among the top 155 contractors in the U.S. It provides construction management, design-build and general construction services to a wide variety of markets throughout the Midwest.

Renovations Add New Performance

Originally built in 1924, the Dells dam had previously undergone major maintenance work in 1979 and 2001.

The current re-powering project, valued at about $6 million, will replace the generating equipment in units 2, 3 and 4, and re-install a refurbished turbine generator in unit 5.

Work began in August 2007, and units 2 and 3 are expected to be back in operation by the end of summer. Completion of units 4 and 5 is scheduled for next June.

This re-powering will boost the plant's efficiency and increase its generating capacity from 9.3 megawatts to 12.5 megawatts. It will also offer increased safety for the facility during flood conditions.

Work for the project includes dewatering the entire tailrace for all four units, as well as extensive concrete demolition and repair, concrete overlays, erection of new steel structures and stop gates, replacement of the existing bridge crane and runway, removal of all existing mechanical equipment for units 2 through 5, installation of three new 2.5-megawatt horizontal turbine generators and associated equipment, and re-installation of one refurbished turbine generator.

Hydraulic Gantry System Lifts Heaviest Components

The work of removing the existing power-generation equipment, modifying the dam, replacing its bridge crane, and installing the new generating components requires lifting numerous components and structural members.

The larger components for each unit range in weight from 7.5 tons to 25 tons. They include two embedded draft tube liners, two halves of the upper elbow assembly, a runner and shaft for the turbine, a generator rotor, and a generator stator.

All of that lifting is being handled by a Miron-owned 275-ton-capacity Liebherr lattice-boom crawler crane.

But the largest component for each unit is the distributor, which controls the flow of water to the turbine. Measuring about 15.5 feet in diameter, 6 feet in width, and weighing 58 tons, the distributor is too heavy for the lattice-boom crawler crane to handle at the several-hundred-foot reach required place it.

Miron's manager for this project, Jason Rieth, P.E., explains, “We examined different solutions, including huge lattice-boom crawler cranes and large hydro cranes to handle those lifts, but they required more room than we have or were more costly to bring in and set up for these few lifts.”

“Our most effective option,” he continued, “was to rent a customized hydraulic traveling gantry system to carry the distributors across the top of the dam and then lower them onto their support foundations down inside.”

Miron rented the system from Rigging Gear Sales of Dixon, IL, which helped tailor it to Miron's requirements.

Customized Components Combine To Meet Needs

The hydraulic gantry system is built around four wheel-mounted 200-ton-capacity hydraulic jacks that form the four corner posts of the lifting frame.

The two front jacks are connected to each other by a 20-foot-long I-beam, and the two rear jacks are connected to each other by their own 20-foot I-beam. The resulting pairs of jacks form the front and rear of the gantry's lifting frame.

The frame is completed by two 40-foot-long box girders that connect the front pair of jacks to the rear pair and that also support the suspended load.

Miron also specified that the system be equipped with four 25-ton-capacity chain falls to provide the required vertical travel to lower each distributor 28 to 30 feet down inside the dam.

The entire gantry assembly travels atop a pair of heavy girder runway rails provided as part of the system. The gantry's hydraulic travel drive is powered by a propane engine and controlled from a single operator's console.

Specialized Hydro Components Handled With Care

Each of the massive distributors is a circular welded frame equipped with wicket gates that will control the amount of water flowing to the generator's turbine. If a distributor were damaged in transit or installation, it would take about a year and cost about $1 million to make a replacement.

Miron's millwrighting subcontractor on the project, United Kiser Services, LLC, Norway, MI, sent its own heavy-haul tractor-trailer truck to pick up each of the 15.5-foot-diameter, 58-ton distributors at the manufacturer in York, PA, then deliver it to the job site, a round trip of nearly 2,000 miles.

Each distributor arrived lying horizontally on the truck, then was lifted off the trailer and laid onto cribbing until needed.

When it was time for installation, a distributor was lifted off the cribbing and up-righted by the lattice-boom crawler crane, with a telescopic-boom truck crane doing the tailing.

Custom bolt-on lifting lugs designed by the distributor's manufacturer permitted secure rigging of the unit for up-righting.

Once vertical, the distributor was cribbed and re-rigged for connection to the hydraulic gantry system, which was standing nearby on two 115-foot-long parallel sections of runway rails.

After the distributor was suspended from the hydraulic gantry, the gantry traveled to a turning point, where the distributor was supported by cribbing while the runway rails were picked up and repositioned atop the dam.

The gantry and suspended distributor then traveled over the repositioned rails to the installation site, where the distributor was lowered into position in the dam's intake pit and bolted to a cast-in-place foundation ring.

From start to finish, assembling the lifting system and placing the distributor took about two days.

Says Rieth, “All four of the wheel systems are controlled independently, and our operator does a great job of keeping them all traveling in synch throughout each lift. All the lifts so far have gone smoothly from start to finish. It's a real credit to everyone involved.”