An ill wind for offshore turbines

Climate Central

A new study contains some unsettling news for the nascent offshore wind energy industry in the United States, finding that wind turbines are substantially vulnerable to being damaged or destroyed by hurricanes.

Additionally, the study, published Monday in Proceedings of the National Academy of Sciences, does not even include the effects of global warming on tropical storms and hurricanes in the Atlantic Ocean and Gulf of Mexico. Other research indicates such storms may become more intense but less frequent in these areas as the climate continues to warm. This may increase the threat to offshore wind installations during coming decades, especially if the storms that do form are more severe since those storms cause the greatest damage.

According to the study by a group of researchers at Carnegie Mellon University in Pittsburgh, offshore wind facilities built along the East Coast and in the Gulf of Mexico need to be engineered to withstand stronger winds than current design standards call for, and may require other modifications as well, or risk being damaged or destroyed in a strong hurricane.

The researchers examined hurricane risks to wind turbines in four areas where offshore wind energy projects are being proposed: Texas, North Carolina, New Jersey, and Massachusetts. They found that the risks vary considerably based on location, with the greatest hurricane risks to turbines off the Texas and North Carolina coasts, since those areas are more susceptible to major hurricanes of Category 3 intensity or greater.

The potential for powerful hurricanes to destroy offshore wind turbines has not been a major issue in the public offshore wind energy debate, according to Paulina Jaramillo, a coauthor of the study and the director of Carnegie Mellon’s RenewElec project, which aims to facilitate the country’s transition to intermittent renewable power sources, such as wind and solar. But the risks are very real, she said in an interview.

According to the study, seven wind turbines were destroyed by a typhoon in Okinawa, Japan in 2003, and another typhoon that same year damaged several turbines in China.

Although no offshore wind energy projects have been built yet in US waters, the study noted that there are 20 offshore wind projects that are currently in the planning stages, with an estimated generating capacity of about 2 gigawatts. That would be enough electricity to power about 460,000 homes, assuming the wind is blowing steadily about 30 percent of the time.

According to the Energy Department, if the U.S. is to generate 20 percent of its electricity from wind, it would need more than 50 gigawatts of offshore wind capacity to do it. (Explore Climate Central’s interactive showing the offshore wind energy potential of each state.)

Offshore wind farms are sitting ducks to hurricanes, and the study found that the violent winds in a hurricane can cause tall turbines to buckle if their design limits are exceeded. Noting the extensive damage Hurricane Katrina caused to the oil industry in 2005, the study said, “Although no offshore wind turbines have been built in the United States, there is no reason to believe that this infrastructure would be exempt from hurricane damages.”

Currently, the vast majority of offshore wind turbines in production are designed to withstand sustained wind speeds of 97 knots, or 112 mph, along with brief higher gusts, as measured at the height of the center of the turbine rotor, known as the hub height. These standards were crafted for land-based turbines and adapted for offshore installations in areas where hurricanes are not a threat, such as Europe, said Walter Musial, who leads offshore wind energy research activities at the Energy Department’s National Renewable Energy Laboratory (NREL).

Many hurricanes that churn the Atlantic Ocean and Gulf of Mexico have winds that greatly exceed the current standards. For example, the study found that Hurricane Ike, which struck Galveston, Texas as a strong Category 2 storm in 2008, had maximum sustained winds of about 110 mph at a height of just 33 feet, well below the 150 to 250 foot hub height of typical offshore wind turbines. Winds at those heights were likely much stronger than that.

If Ike had plowed through a 50 turbine offshore wind farm at that intensity, the study found, there would have been a 50 percent chance of losing at least two wind towers.

“Offshore wind turbines are not cheap,” Jaramillo said. “ You want to minimize the risk of losing that investment.”

According to the study, wind energy developers need to be especially wary of major hurricanes, the Category 3, 4 and 5 monsters like Hurricane Katrina in 2005. Between 1856 and 2008, major hurricanes struck every Gulf Coastal state, and 9 of the 14 states on the Atlantic Coast.

In their analysis, the researchers assumed that historic rates of hurricane occurrence and wind speeds will be representative of future conditions, although some climate studies show that tropical cyclones may become more intense in the coming decades, which could increase the odds of a major hurricane reaching more northern latitudes like New England. Studies have also shown that the power expended by hurricanes in the North Atlantic Basin may already be increasing, possibly due in part to warming ocean temperatures, but natural variability is also playing a large role.

“There is a very substantial risk that Category 3 and higher hurricanes can destroy half or more of the turbines at some locations,” the study said.

The wind energy project that is farthest along in the planning process is Cape Wind, a 130 turbine project off the coast of Martha’s Vineyard, Mass. The study found this area has a relatively low risk of experiencing a hurricane of Category 3 intensity or greater, and there is a less than 10 percent probability that at least one tower in a hypothetical 50-turbine wind farm at this location would buckle during a 20-year time span. This compares to a 60 percent probability if the same size wind farm were built in Galveston.

Cape Wind communications director Mark Rodgers said the project’s designers have taken the hurricane threat into account during planning.

“The Cape Wind project is specifically designed to have all of its turbines survive the most severe storm likely to occur during the life of the project, which is a Category 3 hurricane,” he said. “Cape Wind’s site of Horseshoe Shoal in Nantucket Sound is also sheltered and would experience much smaller extreme waves than in unprotected offshore locations.”

According to NREL’s Musial, the study’s findings are not surprising. “We understand that there’s work to do,” he said. Musial said several groups are looking at issuing new standards for U.S. offshore wind turbines that would include a requirement for greater resiliency to hurricanes.

“The U.S. has unique characteristics of having hurricanes especially in southern latitudes,” which make new standards necessary, he said.

To minimise the hurricane-related risks, the study recommended that wind turbine engineers build turbines to withstand the strongest winds that would be encountered in major hurricanes. In addition, the study suggested that engineers provide backup power systems to allow the turbine nacelle to turn and face the hurricane-force winds, since the turbine is more vulnerable to being damaged if strong winds strike it broadside. This is similar to how ship captains steer vessels into a heavy swell in order to ride out a storm.

This article was originally published on Climate Centralwww.climatecentral.org. Reproduced with permission.

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