Wind over deep water

Research into floating wind turbine systems promises to expand composites to towers and platforms.
#windblades #gerenewable


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Last year the U.S. became a world leader in wind-generated electric power. Running a distant fifth just a few years ago, the U.S. vaulted over the leading European nations, Great Britain, Germany and Denmark, to rank second only to China in 2010. Remarkably, it did so on the strength of its onshore (land-based) wind farm development alone. European wind advocates have long had offshore farms, particularly in the shallow continental shelf of the North Sea region, but the U.S. has yet to commission a single offshore turbine. This apparent lack of emphasis on offshore wind, however, is just that: apparent.

In February of this year, the U.S. became officially bullish about offshore wind. U.S. Secretary of the Interior Ken Salazar and U.S. Secretary of Energy Steven Chu on Feb. 7 announced significant support for offshore wind energy in the U.S. and backed it with dollars — $50.5 million — for projects that support offshore wind energy deployment and several high-priority Wind Energy Areas in the mid-Atlantic U.S. The move finally paves the way legally —  despite some very high-profile opposition (see “U.S. clears way ....,” under "editor's Picks," at top right) — for the first U.S. wind farm offshore from Martha’s Vineyard in Massachusetts.

To this point, however, offshore installations have been confined to relatively shallow waters in which a turbine can be mounted on a pedestal with a foundation sunk into the sea floor. But some of the best wind is found farther from shore. According to the National Renewable Energy Laboratory (NREL, Golden, Colo.), 61 percent of U.S. offshore wind resources are over deep seawater and are capable of producing about 1,533 GW of electric power (see Fig. 1).

With that in mind, researchers at the University of Maine (UMaine, Orono, Maine), and its associated DeepCwind Consortium of 35 U.S. companies, are already hard at work on technologies to support giant offshore turbines in waters too deep to make pedestal-mounted turbines practical or affordable. And that is good news for the composites industry because the need to trim topside weight and protect these far-from-shore floating structures from corrosion will almost certainly dictate the expanded use of composites in the wind energy market.

Floating a new idea

UMaine and its partners have completed testing of three 1:50-scale 5-MW floating wind turbine prototypes for deepwater offshore installations. The tests were conducted in the Offshore Basin at the Maritime Research Institute Netherlands (MARIN, Wageningen, The Netherlands). A major milestone in Phase 1 of UMaine’s five-phase plan for design and validation of floating wind farms, these prototypes prefigure those the Consortium hopes will be capable of harvesting the wind in deep water off the coast of Maine.

The floating-platform prototypes are based on design proposals that had been solicited by the Consortium from 14 U.S. companies. The proposals received — primarily for spar-buoy, semisubmersible and tension-leg platform designs — were evaluated by a DeepCwind blue ribbon panel consisting of UMaine, NREL, the U.S. Department of Energy (DoE, Wash. D.C.), Technip USA (Houston, Texas), MARIN USA (Houston, Texas), Cianbro (Pittsfield, Maine), BIW (Bath, Maine) and the Maine Maritime Academy (Castine, Maine). Seven designs were selected as viable for construction, and three of these were selected for the tests (Fig. 2).

UMaine and Consortium members designed and built the 1:50-scale prototypes and the test equipment; they also designed the test protocols and supervised the test procedures.

“This was the first extensive scale-model test program in the world of this type for floating turbines,” reports Dr. Habib J. Dagher, director of UMaine’s Advanced Structures and Composites Center, emphasizing the importance of these tests for wind energy production. “Until now,” he emphasizes, “no available models have been in the public domain to predict behavior of floating wind platforms in deep water.”

The next step is deployment of intermediate 1:3-scale floating platforms in water near Monhegan Island in the Gulf of Maine. “We have all our permits for the Monhegan test site and we also have done thorough studies on wind and wave data in this area,” Dagher says. He adds that his team, leaving little to chance, performed additional studies of the “ocean floor, seismic activity, fish, birds and aircraft.” The extra work has yielded useful findings. Consultations with top bird experts from the National Audubon Society, for example, indicate there is less risk of bird strike farther out to sea.

“We’re excited about this intermediate-scale platform,” says Walter Musial, supervisor of Offshore Wind and Ocean Power Systems for NREL, noting that it has “potential for real proof of concept for offshore wind platforms.” The first intermediate prototype, scheduled for installation in July 2012, will be a 225-kW turbine on a tension-leg platform (TLP). The turbine will have an 88.6-ft/27m diameter rotor and will sit atop a 100-ft/30m tower. Assembled on shore, the platform will be towed about 10 miles/16 km off the coast of Maine to a water depth of 200 ft to 400 ft (61m to 122m).

In Phase 2, from 2011 to 2015, full-scale 3-MW to 5-MW floating wind system prototypes will be designed, built and deployed in the Gulf of Maine test area, and these are expected to incorporate more composites applications.

Phase 3 plans call for construction of the first 25-MW floating “stepping-stone” wind farm in the world, 10 to 20 miles offshore, from 2013 to 2016. “We have received bids and anticipate selecting a bidder by the end of 2012,” Dagher says, adding that although bidding is now closed for platform construction, the Consortium still needs a developer for this project.

Phase 4 calls for expanding the stepping-stone farm to a 500-MW to 1,000-MW commercial wind farm, comprising as many as 200 5-MW turbines. And Phase 5 looks ahead to building a network of four to eight similar commercial wind farms in the 2020 to 2030 time frame. These farms could range from 10 to 50 miles offshore, each rated at 500 MW to 1,000 MW, foreseeing a total installed capacity of 5,000 MW by 2030 (see Fig. 3).

Integrating composite solutions

Prototypes for the 1:50 scale tests were built using fiberglass composite rotor blades and a steel platform, and the first 1:3-scale intermediate prototype will be of similar construction. However, additional intermediate platforms, which will be built and deployed in the 2013 to 2014 time frame, are expected to incorporate more composite materials, especially in the tower and platform foundation. A major objective of UMaine’s plan for future full-scale offshore turbines is to integrate more durable, lighter-weight hybrid composite materials — wherever feasible — into sea anchors, mooring lines, the platform foundation and floor and the turbine tower and nacelle for optimal advantages in strength, weight and maintenance-reducing corrosion resistance.

The goal is to reduce the cost of offshore wind power to between $0.08 and $0.10 per kWh. “Composites offer the biggest cost reduction opportunity,” says UMaine research engineer Anthony Viselli. “Wherever we can reduce weight above the waterline, we can reduce loads at the bottom and potentially reduce cost.”

The DeepCwind Consortium includes several composite materials suppliers and manufacturing companies and is open to additional members. Dagher believes that “this will be a major opportunity for composites,” but he cautions, “We are not looking at something happening tomorrow morning, but expect solid progress and opportunities in the next three years.”

“Probably the biggest trade-off, or challenge, of deepwater wind generation is the motion of the platform due to the waves, and its effect on the turbine and generator equipment at deepwater sites,” Musial says. Wave motion will subject deepwater platforms, towers and turbines to much higher loads than those borne by land-based turbines. “This has to be mitigated by controls,” he says. Control methods that could mitigate the motion of platforms even in extreme wave conditions are under development. “That’s a big complexity that has to be part of the concept,” Musial points out. “We can’t just stick the generator out on a raft and hope it floats!”

An ultimate objective of the five-phase plan is to validate coupled aeroelastic/hydrodynamic (numerical) models. Aeroelastic simulation tools are used to design and analyze wind airflow, aerodynamics and other physical/mechanical loads and effects that impact onshore wind turbines. Hydrodynamic models, on the other hand, analyze the loads and effects of regular and irregular seas on the dynamic coupling between the motions of the support platform and the motions of the wind turbines and mooring systems for offshore turbines on floating platforms.

To this end, NREL developed a computational engineering analysis tool, called FAST. “FAST simulates the behavior of the wind turbine coupled to a floating base in the water — called a coupled system,” Musial explains. “It simulates the dynamic response of the coupled system in the presence of aerodynamic and hydrodynamic forces that are acting on the turbine and platform.”

The recent MARIN test was an important first step toward achieving this objective. UMaine researchers used FAST to simulate the anticipated performance of the models before the prototypes were tested at MARIN, and they will do the same in the intermediate 1:3-scale program. “If the simulated performance of FAST matches the actual performance data of the MARIN test, and the performance of the upcoming Monhegan Island prototypes, this will serve to validate the simulation software — that is, it will give us confidence that FAST provides an accurate simulation of actual performance,” Musial says. “UMaine is giving us the first opportunity in the U.S. to provide that kind of validation data.”

This validation process will no doubt encourage UMaine and the Consortium to proceed and feel more secure in their ability to control floating wind turbines in deep water.

Floating turbines in Europe

Alstom (Paris, France), Siemens (Munich, Germany) and other European companies are also researching means of harvesting the wind over deepwater sites. In 2009, Siemens and Statoil AG (Stavanger, Norway) installed a floating wind turbine, called Hywind, off the coast of Norway at a water depth of about 220m/722 ft. But the turbulent storms endemic to the North Sea region will make deepsea farms there a particularly daunting challenge. Thus far, with this exception, wind farms off European shores are in shallow waters.

In November 2011, Alstom will take delivery (by sea) from LM Wind Power (Kolding, Denmark) of the largest blade to date, 73.5m/241-ft long with a rotor diameter of 150m/492 ft. Alstom is building two 6-MW prototypes using this blade: one for installation onshore in St. Nazaire, France; the second for installation on a fixed platform offshore of Belgium at a water depth of about 35m/115 ft. Pep Prats, VP of advanced technologies for Alstom Wind in Barcelona, Spain, says the company is working on a floating platform for a water depth of about 100m/300 ft. Alstom wind turbines have fiberglass composite blades and nacelles, but Prats says Alstom is not currently pursuing composites for towers and platforms.

Predicting the wind

Wind energy is already a good market for fiberglass composites. Chris Red, Composites Forecasts and Consulting LLC (Gilbert, Ariz.) reports that nearly 9,000 composite blades were needed for U.S. wind farms in 2010, or about 210 million lb (more than 95,250 metric tonnes) of finished composites, worth $3.1 billion. But this figure was down from 14,400 in 2009, due to the global recession and anticipated policy changes, especially the expected expiration of U.S. tax incentives in 2012. Wind-industry advocates agree that the greatest need is a set of firm long-term priorities, including clean-air and energy standards and an energy transmission policy (a plan to connect wind farms to national grids) that puts wind on a competitive footing with other incentive-supported resources, including fossil fuels.

This as-yet unmet need notwithstanding, good growth is likely in the wind energy market. Red and other analysts disagree about the predicted extent, but they agree on the direction. During the years from 2011 to 2020, Red expects blade unit production to reach about 175,000 per annum, representing some 4.0 billion lb (more than 1.8 million metric tonnes) of composite structure worth $59 billion.

If lawmakers pave the way with favorable long-term policy and wind energy production moves into deep water, then turbine manufacturers are certain to seek composites to increase rotor diameter but limit mass; form taller towers yet reduce weight above the waterline; and float seaworthy and seawater-resistant platforms.

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