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Feds offer offshore leases for floating wind turbines

Northern California has some of the strongest offshore winds in the U.S., and they offer immense potential for production of clean energy. But its continental shelf drops off so quickly that anchoring traditional wind turbines to the seafloor ranges from expensive to impossible.

Once water gets more than about 200 feet deep – roughly the height of an 18-story building – so-called “monopile” structures are pretty much out of the question. A potential solution is now being tested in several locations around the world — floating versions.

In California, where drought has put pressure on the hydropower supply, the state is moving forward on a plan to develop the nation’s first floating offshore wind farms. On Dec. 7, the federal government auctioned off five lease areas about 20 miles off the California coast to companies proposing to develop floating wind farms.

The bids were lower than those off the Atlantic coast, where turbines can be anchored to the seafloor, but still significant. In all, they topped $757 million.

So, how do floating wind farms work?

A floating wind turbine works just like other wind turbines. Wind turns the blades, driving a generator that creates electricity.

But instead of having its tower embedded directly into the ground or the seafloor, a floating turbine sits on a platform with mooring lines, such as chains or ropes connecting to anchors in the seabed below. These mooring lines hold the turbine in place against the wind and keep it connected to the cable that sends its electricity back to shore.

Most of the stability is provided by the floating platform itself. The trick is to design the platform so the turbine doesn’t tip too far in strong winds or storms.

There are three main types of platforms:

A spar buoy platform features a long hollow cylinder extending downward from the turbine tower. It floats vertically in deep water, weighted with ballast in the bottom to lower its center of gravity. It’s then anchored in place, but with slack lines that allow it to move with the water to avoid damage.

Spar buoys have been used by the oil and gas industry for years for offshore operations.

Semisubmersible platforms have large floating hulls that spread out from the tower, also anchored to prevent drifting. Designers have been experimenting with multiple turbines on some of these hulls.

Tension leg platforms have smaller platforms with taut lines running straight to the floor below. These are lighter, but more vulnerable to earthquakes or tsunamis because they rely more on the mooring lines and anchors for stability.

Each platform must support the weight of the turbine and remain stable while the turbine operates. It can do this in part because the hollow platform, often made of large steel or concrete structures, provides buoyancy to support the turbine.

Since some can be fully assembled in port and towed out for installation, they figure to be significantly cheaper than fixed-bottom structures requiring specialty vessels for installation on site.

The University of Maine has been experimenting with a small floating wind turbine, about one-eighth scale, on a semisubmersible platform. It has been working in partnership with RWE, one of the winning bidders.

Floating platforms can support wind turbines generating 10 megawatts or more of power. That’s similar to onshore turbines and mounted offshore turbines.

Why do we need floating turbines?

Some of the strongest wind resources are away from shore in locations with hundreds of feet of water below, such as off the U.S. West Coast, the Great Lakes, the Mediterranean Sea and the coast of Japan. And some of the strongest offshore wind power potential in the U.S. is in areas where the water is too deep for fixed turbines, particularly off the West Coast.

Lease areas auctioned off in early December cover about 583 square miles in two regions — one off central California’s Morro Bay and the other near the Oregon state line.

Farms in those five areas could provide about 4.6 gigawatts of clean electricity, enough to power 1.5 million homes, according to government estimates. And the winning companies suggested they could produce even more.

But getting turbines on the water will take time. The winners of the lease auction will undergo a Justice Department anti-trust review and then a planning, permitting and environmental review process that typically takes several years.

Globally, several full-scale demonstration projects are already operating in Europe and Asia. The Hywind Scotland project became the first commercial-scale offshore floating wind farm in 2017, with five 6-megawatt turbines supported by spar buoys designed by the Norwegian energy company Equinor.

Equinor Wind US submitted one of the winning bids off Central California. Another winning bidder was RWE Offshore Wind Holdings, which operates wind farms in Europe and supports three floating wind turbine demonstration projects.

The other companies involved in the project are Copenhagen Infrastructure Partners, Invenergy and Ocean Winds. They hold Atlantic Coast leases or operate existing offshore wind farms.

While floating offshore wind farms are becoming a commercial technology, there are still technical challenges that need to be solved. The platform motion may cause higher forces on the blades and tower, and more complicated and fluctuating aerodynamics. Also, as water depths get deeper, the cost of the mooring lines, anchors and electrical cabling goes up.

But we can expect to see more offshore turbines supported by floating structures in the near future.

From The Conversation, an online repository of lay versions of academic research findings found at https://theconversation.com/us. Used with permission.

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