New Study Shows Risks of Hurricanes to Offshore Wind Turbines

February 29, 2012

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The wind power movement has been, no pun intended, picking up steam in America. Schools and universities across the United States are helping install turbines on the grounds of their facility, through programs like Wind Power America and others.

Certainly, with energy issues being at the forefront of most politicians' radars these days, the idea of wind-powered energy helping bring down electricity costs, and wind-farms slowly weaning people away from the electricity grid, has continued to gain traction.

Twenty years ago, wind power was on the fringe of the discussion about renewable energy, not taken very seriously by mainstream media and politicians. Now it is almost always talked about when the subject of energy's future comes up. And with that, talk of offshore wind farms, built far from land, has picked up steam as well. These farms, conceivably, have one major advantage over land-based wind farms: They can generate a heck of a lot more wind.  Some studies have shown that offshore winds blow 40 percent more often than land-based wind.

(Photo: Development of offshore wind farms has become a priority to the U.S. Department of Energy and other pro-wind power entities (the farm shown here is in Denmark.) But these offshore farms do tend to come with risk, as a recent Carnegie-Mellon study has revealed.)

Given that, it's certainly always a good time to step back and assess possible risks to wind turbines that are being built offshore, which is what researchers at Carnegie-Mellon University in Pennsylvania spent a year doing.

For example, there's the issue of hurricanes. That is, they produce huge amounts of wind and cause millions of dollars of damages to U.S. land, not to mention the loss of human life, every year. Hurricane Katrina is one stark example.) With hurricanes often building up steam offshore, what possible problems could these storms cause offshore wind turbines?

With that question in mind, the Carnegie-Mellon team got to work. Led by Professor Paulina Jaramillo, an assistant research professor in the Dept. of Engineering and Public Policy, spent a year on the project. Between the fall of 2010 and the fall of 2011, a team of three professors, one doctoral student, and one undergrad undertook a project at the school to determine how dangerous hurricanes could be to offshore turbines, and what safety measures could and should be taken before U.S. turbines are built.

The group's study was just published in PNAS, the National Academy of Sciences journal, and its findings were revealing. (To read the full report, go to PNAS' website and click here.) I interviewed two of the study's authors, Jaramillo and Mitchell Small, a professor in Carnegie Mellon's Civil and Environmental Engineering Dept.

The major finding of the study: Strong hurricanes could severely damage or even wipe out some of these offshore wind turbines.

Specifically, Jaramillo and her team looked at four areas where wind turbines are currently planned or in development: Galveston, Texas; Dare County, N.C., Atlantic County, N.J., and Dukes County, Mass.

One challenge to the Carnegie Mellon team was simply the new ground being broken; there are as of yet no offshore wind turbines in the U.S., though there are several in Europe.

"We spent a year looking at these areas because we determined that this is where there could be a lot of risk, based on their location," Jaramillo said. "And Galveston, Texas, specifically interested us because there's a meteorological tower near there.

"We had an inclination that these areas would be high risk," Jaramillo continued, "but we didn't know what the number would be."

As it turned out, using a probalistic method worked on by student Stephen Rose, the researchers learned that there's a 60 percent chance that at least one tower in a 50-turbine farm would buckle in a 20-year period in Galveston, and a 30 percent probability that more than half of the towers would be destroyed.

Mitchell Small explained a little bit about the mathematical method used.

"I worked on the statistical side of the project. We had to figure out what the uncertainty of the projections was, and we had to figure out the hurricane occurrence model," Small said. "That model is (figuring out) how many hurricanes will hit, and how big they'll be.

"And then there's the damage function model, which relates what the maximum wind speed is during a hurricane, and at what point the turbines would buckle," Small continued. "So what we found is, if you have 150 mile per hour winds, there's got to be a 50 percent chance that this particular turbine would buckle, and if it goes to 180 miles per hour, then it goes up to 80 percent."

In North Carolina, Jaramillo's team discovered there is a 60 percent probability that at least one tower would suffer damage but only a nine percent chance that more than half would be destroyed.

The other two sites studied were far less dangerous: In Atlantic County, N.J., they found there's a 15 percent chance that at least one tower would buckle in 20 years, and less than 1 percent chance that more than half would buckle; in Dukes County, Mass., there was a 10 percent chance at least one tower would buckle, and also less than 1 percent chance that more than half would buckle.

A possible solution, the team suggests, is to design turbines that can yaw quickly during a storm to ease the strain on the tower. ("Yaw" is a term similar to rotating.)

"Turbines are already designed to yaw, but they run on (electrical) power," Jaramillo said. "If they lose connection to the grid, they lose power and cannot yaw. So it's really important to get them backup power."

"What we're talking about is moving the nacelle -- a part of the turbine -- and you're trying to reduce the load on the structure," Jaramillo continued. "If you can do that, you have a much better chance of having the turbine survive the strong winds that come with a hurricane."

The study found that if turbines were designed to yaw, the risks would be much less. In Galveston, for example, the use of equipment that rotates would drop the risk of at least one tower buckling down to 24 percent, with the risk of more than half coming down to 10 percent.

Jaramillo thinks the most critical areas to be concerned about, in building offshore turbines, are the mid-Atlantic and Northeast coasts, but she and Small want to make clear that they aren't advocating stopping planning and building offshore turbines; both believe it's still a good idea.

"If we have a better sense of what the risks are from the beginning, we can be more rational in our decision-making, and more rational in our reaction to how to handle circumstances," according to Small.

"It depends on where you build the wind turbines, as to the risk," Jaramillo said. "I think wind power has a role to play in the U.S. energy future; we really need to understand the challenges, and take advantage of the opportunities."

One factor that no one can predict in all of this is how powerful hurricanes are becoming. Even the strongest turbines may struggle against winds that seem to get stronger every year.

"The broad trends are that, due to either global warming or something else, hurricanes are getting stronger," according to Small. "Identifying those hurricane trends are hard because, now, every hurricane that happens we get all the data on it; if you go back to the 1950s and before, you didn't have as accurate measurement tools and data as we have now. So we think they're getting stronger, but we don't know with certainty how strong they were 50 or 100 years ago."

The research team has gotten positive feedback on the study from turbine developers, who realize "that we're not saying not to build them," Jaramillo noted, "but to plan carefully."

 

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