Hurricanes pose risk to offshore wind farms

Offshore Wind Turbine Hurricane Risk highlights PNAS findings that offshore wind farms along the Gulf and East coasts face Category 3-5 hurricanes beyond current design standards, driving blade loss, tower buckling, and demolition of turbines.

When an oncoming hurricane curves offshore — as most do — we usually breathe a sigh of relief. But soon, those offshore storms might give us something more to worry about.

Offshore hurricanes could demolish half the turbines in proposed wind farms just off the USA's coastlines, according to a study out Monday in the Proceedings of the National Academy of Sciences.

"We find that hurricanes pose a significant risk to wind turbines off the U.S. Gulf and East coasts, even if they are designed to the most stringent current standard," the authors from Carnegie Mellon University in Pittsburgh write.

Engineer Stephen Rose and colleagues conducted the study in response to a 2008 report from the U.S. Department of Energy, which said that wind energy should ideally provide one-fifth of all electricity in the USA by 2030. The engineers estimated that over a 20-year span many turbine towers would buckle in wind farms enduring hurricane-force winds off the coasts of four states — Massachusetts, New Jersey, North Carolina and Texas — where offshore wind-farm projects are now under consideration, including an offshore wind power line proposal in the region.

Despite their record of death and destruction in the USA, 75 of all Atlantic basin hurricanes remain offshore and do not hit land, according to Chris Landsea, science and operations officer at the National Hurricane Center in Miami.

Wind turbines are vulnerable to hurricanes because the maximum wind speeds in those storms can exceed the current design limits of wind turbines, according to the study, and large arrays can sometimes trigger false weather alarms during severe weather as well.

Failures can include loss of blades and buckling of the supporting tower, leading to debris-related outages in nearby communities as well.

The research incorporated the current construction standards for the turbines, reports Rose, and recent turbine installation efforts illustrate these practices. "Our study assumed wind turbine design for the current standards, with a maximum sustained wind speed of 111 mph near the top of the turbine, about 90 meters about 300 feet above the surface. This is the equivalent of a Category 3 hurricane," he says.

The study had to use computer models to simulate hurricanes' effect on the wind turbines, and facilities like the WindEEE Dome study new technologies for extreme winds, as no offshore wind turbines have yet been built in the USA although there are 20 offshore wind projects in various stages of planning.

However, turbine tower buckling has occurred in typhoons in the Pacific. Hurricanes are the same type of storms as typhoons.

Of the four locations examined in the study, offshore of Galveston County, Texas, is the riskiest location to build a wind farm, followed by the Outer Banks of North Carolina, Atlantic City and Martha's Vineyard, Mass.

"Galveston was the riskiest because it gets hit by hurricanes the most frequently," says Rose.

The damage caused by Category 3, 4 and 5 hurricanes is important for offshore wind development in the USA, the study notes, because every state on the Gulf of Mexico coast and nine of the 14 states on the Atlantic Coast have been struck by a Category 3 or higher hurricane from 1856 through 2008.

"As you get to Categories 3, 4, 5 — that's where the risks are," says Rose. "The intense hurricanes pose the most risk."

However, damage could be greatly reduced by building the farms in lower-risk areas and boosting the abilities of turbines to withstand higher winds, as utilities work to adapt to climate change in their planning.

This would "greatly enhance the probability that offshore wind can help to meet the United States' electricity needs," according to the study, and some communities have shown support for turbines through local initiatives.

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