OEM Report: Energy

The mono bucket challenge

By Nick Wright

Above: Mono Bucket foundations will anchor six wind turbines in Lake Erie, due to be installed in 2018. Pictured above: Mono Buckets awaiting install in the North Sea for the Dogger Bank wind farm in 2013.

Lake Erie’s Project Icebreaker relies on a novel method of setting steel wind turbine foundations

December 2015 - Doubtless you’ve driven across swaths of America—maybe Iowa, Texas or North Dakota—and tried to count miles and miles of wind turbines dotting the landscape. All told, about 49,000 of them spread across 39 states crank out 68,000 MW, according to the American Wind Energy Association. That makes the United States the largest wind energy producer in the world, ahead of China and Spain.

When it comes to offshore wind turbines, the electricity output isn’t as bright, because U.S. coastlines are devoid of wind turbines. Within the next five years, however, that will change—first, with a project off the coast of Rhode Island that will have blades spinning in late 2016. Others are in early planning stages. 

One proposal will install six turbines on the floor of Lake Erie. Dubbed Icebreaker, the turbines will sit about seven miles north off the shore of Cleveland. As of late summer 2015, Lake Erie Energy Development Corp. (LEEDCo) took core samples from 75 ft. below the lake bottom to assess anchoring options. These will be the first wind turbines in the Great Lakes. LEEDCo was formed in 2009, and in 2012 received a $4 million grant, which was extended with a $2.8 million cooperative agreement from the U.S. Department of Energy in 2014 for the project.

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This past August, LEEDCo crews began taking soil samples from the bottom of Lake Erie.

To install the six turbines, each of which will generate 3 MW, LEEDCo has turned to the Mono Bucket. It’s a foundation design that uses a giant steel bucket, which is pulled by suction into the lake bottom. It is cheaper than driving steel piles into the bedrock and less disruptive than underwater excavating.

LEEDCo tapped the expertise of Norwegian firm Fred. Olsen Windcarrier and the Fred. Olsen affiliated company Universal Foundation, based in Denmark, to advise and guide the installation. 

In some ways, the Mono Bucket resembles a large-scale plunger. Lowered from a barge, the Mono Bucket is set, open-side down, on the lake bottom. As water is sucked out of the steel bucket component using pumps, the ensuing combo of gravity and vacuum sinks the steel structure down into the ground. During lowering, a jetting system helps control vertical alignment so it accurately lands. When the Mono Bucket hits the target depth, the pumps are disconnected and reeled back to the barge.

Reversing the process—pumping water back into the cavity—allows the foundations to be removed later.

In many ways, Lake Erie, the shallowest of the Great Lakes, offers ideal conditions for the Mono Bucket: The installation site is only 60 ft. deep (the average depth of the lake), the soft clay and sand deposits aren’t especially resistant to a steel foundation, and the lake has minimal tides, doesn’t swell or generate waves the way oceans do. One big question left to answer is the whether the effects of ice loads on the tower during winter will be problematic. But, again, the lack of turbulence on Lake Erie makes it viable. 

Building the bucket

The Mono Bucket consists of two main components: the shaft and the bucket. Each is fabricated separately and then welded together on shore but close to the installation site.

According to Universal Foundation, the Mono Bucket has a multishell foundation with vertical stiffeners and a robust lid. The shaft is a cylindrical (or conical) tubular structure made from rolled and welded plate, which forms the full length of the tubular tower. Next, it’s fitted with outside platforms, ladder and boat landing pieces. Both the plate and ancillary bits are carbon steel.

The bucket piece involves a bit more. It’s a fabricated combination of steel bars, plates and bent/rolled plate. After it’s all welded together to form the bucket, crews mount pipes that are used during installation and decommissioning phases.

Kristian Ascanius Jacobsen, head of business development at Universal Foundation, says the shaft surface is treated for rust prevention before being welded to the bucket. “Both the shaft and bucket are fitted with a cathodic protection system to further improve corrosion resistance.”

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A crane lifts the shaft and places it on top of the bucket for welding. Next, the foundation is fitted with ladders, platforms and electrical components. The secondary parts are needed for the turbine’s operation and to support technicians working on it later. Once complete, the foundation is lifted onto a barge or similar vessel to be towed to the installation site. 

Dave Karpinski, vice president of operations for Cleveland-based LEEDCo, says the Mono Bucket proved to be the most cost-effective solution even considering fabrication and installation. The Mono Bucket relies on a simple suction pump, ruling out the need for pile hammers, upending tools, noise mitigation and grouting systems associated with the monopile method.

Plus, shallow bedrock at 80 ft. below the mudline and soft top layers are problematic for traditional monopiles. Ice and “modest” water depth render a jacket foundation unsuitable, Karpinski says. “The gravity base foundation would require expensive excavation and backfill, which also poses environmental challenges, to provide a suitable base.”

Series of firsts

Two major fabricators are engaged with LEEDCo to apply Universal Foundation’s design specifications to Icebreaker: American Tank & Fabricating Co., Cleveland, and Ventower Industries, Monroe, Michigan. The bucket’s initial design features a diameter of between 46 ft. and 52.5 ft. (14 m and 16 m). The bucket length to be penetrated will be between 33 ft. and 40 ft. (10 m and 12 m). The shaft length and shaft diameter will be 99 ft. by 16 ft. (30 m by 5 m), respectively.

Michael Ripich, president and CEO at AT&F, says the Mono Bucket design isn’t all that different from the massive rolled fabrications it creates for defense, mining and heavy construction applications. AT&F has some of the largest plate rolling equipment in the world. Among its machines is an Italian-made 14 ft. by 3.5 in. (4000 mm by 90 mm) Sertom roll bending machine that can form 8.5-in.-thick plate. 

As one of the largest fabricators in the Great Lakes region, AT&F linked up with LEEDCo a few years ago when Icebreaker was in the initial planning phase. 

“As we look to get into the offshore wind, we have excess capacity with large equipment,” says Ripich. In short, the 75-year-old AT&F already has the equipment, capacity and proximity to fabricate the steel. Investing in the machinery for just six wind turbines wouldn’t be worth it for most competitors. “If someone came in and said, ‘You need to make a capital investment to play in this market,’ that’d be prohibitive at this stage when there’s a couple dozen towers being installed in various markets of the U.S. People would be hesitant to make [that investment] to chase even 20 or 30 towers,” he says.

Ventower, located near the west end of Lake Erie and about 90 miles from Cleveland as the crow flies, specializes in utility-scale wind tower fabrication. Ventower benefits from the port in Monroe, which is scheduled for improvements in coming years, says Scott Viciana, Ventower’s vice president.

As offshore wind projects grow in the U.S., Icebreaker could be a practical example of how capable fabricators can build the Mono Buckets as kits so they can be economically shipped to the installation sites around the country for final assembly, Ripich says. 

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A Mono Bucket and tower being lowered into the North Sea, off the coast of the U.K., in 2013.

“If the industry wants to build a supply base using ourselves and others to be able to do that, then they must lean on more localized fabricators. The first couple of units will be somewhat more expensive because the economies of scale aren’t here initially. But we feel the market will develop, and we will be willing to make investments to capture the larger volume opportunities that follow.”

At least, initially, “we can prime the pump” to jumpstart the market for localized Mono Bucket fabrication as the first stage of a long-term strategy, says Ripich.

Tracing the energy

The Mono Bucket design came from an unlikely source, considering its intended use. The idea behind the technology originates from use of suction technology in oil and gas, where more than 2,000 foundations for that industry have been delivered. But the Mono Bucket discussed here is specifically designed for offshore wind. The first wind tower application was in 2001 off the coast of Denmark, supporting a Vestas V90 3 MW turbine. The largest turbine in existence at the time of its installation, the unit continues to operate. Others have since been successfully 

installed and decommissioned off Denmark’s coast. In 2014, Universal Foundation completed a trial installation campaign in the North Sea off the United Kingdom’s coast. There, a scaled-down version of the Mono Bucket and a reference structure were used. “The structures were successfully installed and removed 29 times in 24 days,” says Jacobsen.

In oil and gas applications, suction technology has been used at depths of 2,000 ft. to 6,000 ft., he adds. In theory, there’s no limit to the Mono Bucket’s depth. The deeper the bottom, the higher static pressure (more water pressing down on the Mono Bucket), which would make it easier to install. “In offshore wind, it’s naturally a question of economic feasibility and, so far, we have seen that the concept can be commercially feasible in depths up to 55 m (180 ft.) using the largest offshore wind turbines of 8 MW.”

Nor does anchoring a Mono Bucket require divers. In most cases, the sea bed (or lake bed) doesn’t need any preparation, and there’s no need for scour protection. The install doesn’t put marine life, installation crews or marine operators at risk. Looking far beyond 2018, when LEEDCo expects the first turbines to go up, the foundation could be pulled out by pumping water back into the foundation skirt for reuse or recycling. 

It’ll be a few years before anyone in Cleveland will catch a glimpse of spinning wind turbines on the Lake Erie horizon. And any future turbine will likely be placed further out of sight, according to LEEDCo. Six turbines is a good start toward a renewable energy solution. If the offshore wind turbine concept spins off to the rest of the country, it could be a great new market for U.S. fabricators. FFJ


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