Line Contractor

The Way the Offshore Wind Blows: Launching a new energy source requires infrastructure

Published On
Jun 14, 2021

President Joe Biden’s ambitious plans to cut U.S. greenhouse gas emissions in half over 2005 levels in just the next nine years will have energy planners scrambling to ensure adequate electricity supplies, especially as that effort seeks to add to demand with the rapid growth of the electric vehicle market. Developers of an entirely new category of U.S. generation—offshore wind—have hopes of ramping up quickly to help meet this growing need. 

Along the East Coast, just six states—Virginia, Maryland, New Jersey, New York, Connecticut and Massachusetts—have plans to introduce a total of 28.5 gigawatts of offshore wind capacity by 2035. California is also studying its options, though the deeper ocean floor along the Pacific Coast makes for a more challenging setting. Even the Great Lakes are being eyed for project potential. 

The reason for this growing interest in offshore wind development is simple: wind in these locations tends to be faster and more consistent than onshore wind. The International Energy Agency, Paris, has classified offshore wind as a “variable baseload” technology—the only generation approach to fit that category—with a capacity factor of 40%–50%, which is comparable to an efficient gas-fired plant. As a result, electricity planners can treat these resources more like fossil-fuel generation than a traditional onshore wind farm when it comes to predicting future output.

So, what goes into harnessing wind energy 15 miles or more from a coast and plugging it into the onshore grid? Here’s a look at the equipment involved and some of the advances that have occurred over the last few years, along with some of the political and economic issues this new industry is bringing forth.

Turbines

Offshore wind turbines are very similar to their onshore counterparts, except they’re bigger. In December, GE, Boston, announced its new largest onshore turbine, dubbed the Cypress platform. It tops out at a capacity of 6 megawatts (MW), with a rotor diameter (the diameter of the circle created by rotating turbine blades) of 538 feet. GE also announced its largest offshore offering, the Haliade-X, with a 720-foot rotor diameter that helps raise capacity by 14 MW and offers a capacity factor of 60%–64%, which actually bests the 57% rating for natural gas plants. The Haliade-X turbines have been specified for the planned 800-MW Vineyard Wind project, located off the coast of Martha’s Vineyard, Mass.

The mechanics of the turbine are housed within a structure called the “nacelle,” which in the case of the Haliade-X, is comparable in size to six double-decker London buses. A rotor is attached to the hub at the center where the turbine’s three blades meet. The rotor, in turn, is connected to a main shaft, which turns inside the generator housing. The shaft turns a second magnetic rotor inside loops of copper wire to create electrical energy.

Towers and foundations

Obviously, as turbine diameters get larger, the equipment needs to be lifted higher above the water’s surface. The hub of the Haliade-X, for example, is expected to be mounted 853 feet above the typical water height during calm seas. Keeping such enormous structures standing under extreme conditions requires significant engineering. Researchers are studying the possibility of “floating” turbines. For now, however, more traditional foundation approaches are used to secure turbine towers to the ocean floor. These generally fall into one of three categories:

Monopile: This approach features a standard pile, like those used for bridge and building footings, driven 30 to 60 feet into the seabed, into which the turbine tower is fitted. They work well in sand or gravel seabeds in depths up to about 130 feet. 

Gravity-based: These are hollow, concrete structures that can be towed to a location and are then flooded with sea water and filled with gravel, sand or stones for ballast once sunk. They’re good in depths up to 98 feet.

Jacketed: These designs draw on those used for oil platforms, with four tubular legs connected by diagonal struts. 

Simply locating the specialized equipment needed to put towers and turbines into place is one of the most immediate challenges facing project developers. For example, only a limited number of ships designed for these jobs—called turbine installation vessels—is available globally. They feature jacks that can be deployed to the seabed to raise the ships above water level to stabilize them during foundation and tower installation. 

Additionally, none of those ships are currently able to perform these tasks in U.S. waters under provisions of the 1920 Jones Act, which requires them to be U.S.-owned. The first U.S.-built, Jones Act-compliant turbine installation vessel, the $500 million Charybdis, is now under construction in Brownsville, Texas, and is expected to be ready for service by the end of 2023.

Inter-array cables

Inter-array cables are used to collect the electricity generated at each turbine and transmit it to a central offshore substation. These installations typically connect the turbines in a string design. Historically, 33-kilovolt (kV) cabling has been used for this purpose, but 66kV line was introduced several years ago and is starting to take hold. Operating at this higher voltage reduces the overall length of cabling required, because each cable can serve more turbines. It also can cut down on the number of offshore substations required for new wind farm installations.

Offshore substations

Offshore substations serve several purposes for offshore wind farms. Most important, they collect, transform and transmit power gathered by inter-array cables from each of the project’s turbines. Onboard switchgear steps up that 33kV or 66kV power to a higher voltage, which can range from 100kV to 220kV, to match onshore connection requirements. In most cases, that power also is transformed from the direct current produced at the turbines to alternating current for transmission to shore. As projects get sited further from coastlines and connection points, though, high-voltage DC lines are being considered and adopted to reduce the line losses that can happen at longer distances. Alternatively, mid-point reactive compensation platforms also can be used to meet this need.

These substations also can serve as maintenance hubs for offshore installations. This means helicopter landing pads and crew quarters can be a part of their design. As with the wind turbines themselves, companies are now exploring concepts for creating floating substation designs. This would be an important advance as developers seek to expand their siting options into deeper-water locations.

Export cabling

The transmission cables running from the substation to the connection point onshore are often major points of controversy for proposed wind farm projects. As the first big round of projects is in the planning stage, there’s also currently a battle of sorts over the best approach for managing this function, focused on who should own this transmission line and how to minimize seabed disruption and landing requirements.

Most current projects—including Vineyard Wind, which received its final federal approval in May—are being designed with their own undersea transmission lines. In the case of Vineyard Wind, this means three 200kV export cables, totaling up to 141 miles. Those cables need to terminate in a location close enough to existing utility infrastructure to enable the power they carry to be injected into onshore grids. 

This might not be a problem for any single project, but as the number of proposals multiply, developers could find themselves fighting over a limited number of optimal landing points. Adding to this tension is the fact that local communities aren’t always enthusiastic about hosting the industrial-scale electrical equipment required at grid-connection points.

An alternative approach would develop an independent transmission infrastructure that could serve multiple wind farms and consolidate the number of required landings and connections. Wakefield, Mass.-based Anbaric Development Partners has proposed two such networks for Massachusetts and New York. 

Of course, this pits the economic interests of project developers against those of independent transmission operators such as Anbaric, as transmission fees could result in significant revenues. Also challenging is the need to plan, permit and install this more centralized approach just as a significant number of new projects are advancing in the permitting process. However, proponents argue that the economic and environmental benefits of minimizing the need for cable trenching and additional landing points could reduce rate-payer costs and ease approval for new projects in the future.

Cable landings

Current fights over cable landings might help build support for future centralized transmission networks. While turbines and offshore substations are generally in federal waters, landing spots are under the control of states and local municipalities, and not-in-my-backyard arguments are becoming a common obstacle for offshore wind developers. A current battle underway in East Hampton, N.Y., a wealthy resort area of Long Island, illustrates how political these decisions can become.

In that town, Providence, R.I.-based developer Deepwater Wind, a partnership between the New England electric utility Eversource and Danish offshore wind leader Orsted, hopes to land the transmission cable for its proposed South Fork Wind Farm at a popular beach. There, the cable would be buried 30 feet underground before wending its way to a utility connection point 4 miles away. In September, Deepwater Wind offered the community a benefits package totaling $29 million over 25 years. When town leaders approved the agreement, a well-funded group of property owners filed suit. New York State’s Public Service Commission approved the landing design in March, but legal challenges continue.

Onshore connections

Introducing offshore-generated electricity into the onshore grid requires some significant electrical infrastructure. In a number of projects now under review, developers are hoping to take advantage of interconnections still remaining on the site of shuttered onshore generating stations. For example, the Ocean Wind project, another Orsted effort located off the New Jersey coast, has plans to connect to the former Oyster Creek Nuclear Generating Station and B.L. England coal-fired facility to ease its power’s transition into local supplies. Again, however, the limited numbers of such opportunities could become an argument in favor of combining offshore export cabling into a limited number of consolidated transmission networks as more projects reach the planning stages. 

About the Author

Chuck Ross

Freelance Writer

Chuck Ross has covered building and energy technologies and electric-utility business issues for a range of industry publications and websites for more than 25 years. Contact him at chuck@chuck-ross.com.

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