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Deep water offshore wind turbines are coming to the coast of California. Figuring out how to secure the platforms to the ocean floor will be an engineering challenge.

Clean Power

Floating Wind Farms Off California Will Require New Tethering Technology

Deep water offshore wind turbines are coming to the coast of California. Figuring out how to secure the platforms to the ocean floor will be an engineering challenge.

Offshore wind farms have a big advantage over land-based installations. The wind out over the ocean tends to be stronger and more consistent than over land. It also tends to pick up near the end of the day just when electricity from solar farms is beginning to decrease.  Last Friday, the Interior Department took the first steps toward leasing areas off the coast of California for deep water wind development. It will take a while for the process to be completed. The first wind turbines probably won’t begin producing electricity until 2024.

The West Coast Is Different From The East Coast

Many east coast states are pursuing offshore wind, but there is an important difference. The Continental Shelf along the eastern seaboard is only a few hundred feet below the surface and extends many miles out to sea. That means the foundations for wind turbines can be sunk directly into the ocean floor.

In shallow waters, such as those off the coast of Block Island, Rhode Island, a single foundation tube is sunk directly into the sea bed. In deeper water, a foundation with three legs is often used. But at depths of 300 feet or more, a floating platform tethered to the bottom is necessary.

Things are quite different off the coast of California, where the land plunges to great depths just a short distance offshore. Only floating wind turbines will do in such conditions, which will require lots of new engineering solutions to keep the turbines stable even in severe weather. “They would be in much deeper water than anything that has been built in the world so far,” Karen Douglas, a member of the California Energy Commission, tells the New York Times.

The problem is exacerbated by the desire of the natives not to have to have the turbines spoil their pristine view of the Pacific Ocean. They can live with thousands of oil derricks scattered throughout southern California, millions of utility poles, billions of miles of wires, and the post-modern beauty of nuclear power plants, but Heaven forbid they should see a wind turbine blade spinning offshore! To avoid offending their tender sensibilities, the turbines will be located 15 to 20 miles offshore, mostly hidden from view behind the curvature of the Earth.

Proper Tethering Is Key To Offshore Wind

How to configure the tethering system for a deep water floating wind turbine is a challenge for engineers. HR Wallingford, a global civil engineering firm headquartered in the UK, is developing new tools to assess the forces floating wind turbine platforms will be subjected to in the real world, particularly as Category 5 typhoons become as common as thunderstorms in a warming world.

Working with Coastal and Hydraulics Laboratory in the US, it has created a toolkit designers can use to configure the optimal platform tethering system for any location. The toolkit utilizes computational fluid dynamics based on Proteus open source software and multi-body dynamics using the Chrono open source solver. Here is a video that shows the toolkit in action.

Dr Aggelos Dimakopoulos, senior engineer at HR Wallingford, says, “We have put a special focus on the fully dynamic simulation of mooring cables, as they can significantly affect station‑keeping and the overall response of the device, which in turn affects its energy extraction efficiency.”

Michael Case, head of the renewable energy section, adds, “Using a CFD model at an early stage can reduce costs in design optimisation, by performing full-scale simulations under realistic sea states, before performing laboratory tests which may be subject to practical limitations. This is important as it provides a further opportunity to drive down the levelized cost of energy.”

Offshore wind farms will benefit from the fact that California already has a number of generating stations located along the Pacific Coast, so the cables bringing electricity ashore can connect to the grid through existing substations. Many of those existing power plants are scheduled to be shut down as California seeks to reach its goal of 100% renewable energy by 2045. One of the facilities scheduled for decomissioning is the nuclear power station at Diablo Canyon, the last functioning nuke in the state.

Three Coastal Areas Identified

The Bureau of Ocean Energy Management has identified three potential areas for leases — a parcel off Humboldt County in Northern California and two sites in the Morro Bay area on the central coast near Hearst Castle and Diablo Canyon.

Redwood Coast, a government-run utility serving 60,000 customers in the mostly rural area of Humboldt County, expects to spend about $500 million for the wind farm. “That level of generation would be a significant chunk of our energy load,” says Matthew Marshall, the utility’s executive director. “Offshore wind is really the big untapped resource.”

“California has very good offshore wind,” says Walt Musial, a principal engineer and manager of offshore wind efforts at the National Renewable Energy Laboratory. “If we look at the cost breakdown structures of a floating project or fixed-bottom project, they’re using a lot of the same components. There’s no big element that makes floating more expensive. In fact, there are some elements that might make floating cheaper,” he says.

Who Will Build Deep Water Offshore Wind Farms?

Three companies with offshore wind experience are expected to bid on the leases when they become available in about 18  months — Equinor, which used to be Statoil, the Norwegian state owned energy company, Trident Winds, and Magellan Wind, which is working with Copenhagen Infrastructure Partners. Trident winds envisions a 100-turbine installation west of Morro Bay.

Dan Reicher, a former Energy Department official who has been an adviser to Magellan, said he believed that California was starting one of its greatest initiatives in developing clean power. “In California, we’re not used to falling behind other states when it comes to renewable energy. That is the case when it comes to offshore wind. I think all of that will change with these floating systems,” he says.

Let The Review Process Begin!

The path forward will not be without its challenges. In addition to federal reviews, the wind projects must be approved by the California Coastal Commission for impact on federal and state waters, the California State Lands Commission, and the California Department of Fish and Wildlife because of concern about protected species.

“I would have some questions whether those cables would mean that whales would not use the area the same way as they have,” says Francine Kershaw, a marine mammal scientist at the Natural Resources Defense Council. In general, the NRDC supports wind power, including offshore development, but Kershaw says “collisions with sea birds is probably the major concern.” There are also concerns about whales and other aquatic animals getting entangled in the cables bringing the electricity from offshore wind turbines to the mainland.

The review process will be long and torturous. But offshore wind will inevitably be a a crucial factor in meeting California’s renewable energy standard.

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Steve writes about the interface between technology and sustainability from his home in Florida or anywhere else The Force may lead him. He is proud to be "woke" and doesn't really give a damn why the glass broke. He believes passionately in what Socrates said 3000 years ago: "The secret to change is to focus all of your energy not on fighting the old but on building the new."


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