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By GREGORY MORRIS, an independent business journalist with more than 20 years' experience covering finance, industry and commerce worldwide.

A study just published by Carnegie Mellon University in Pittsburgh has found that offshore power-generation wind turbines are vulnerable to hurricanes because the maximum wind speeds in those storms can exceed the design limits of the structures. The authors suggest that backup power, robust wind-direction indicators and active controls may be low-cost ways to reduce risk to the turbines.

The study authors, Stephen Rosea, Paulina Jaramilloa, Mitchell J. Small, Iris Grossmann and Jay Apt, considered only the risk of tower buckling because blades are relatively easy to replace, although their loss can cause other structural damage.

To illustrate the risk to a wind farm from hurricane force wind speeds, the authors calculated the expected number of turbine towers that buckle in a 50-turbine wind farm as a function of maximum sustained (10-minute mean) wind speed, assuming that turbines cannot yaw during the hurricane to track the wind direction. They also postulated survivability if the housing can be yawed rapidly enough to track the wind direction of the hurricane.

While the authors note that design standard for Class 1 wind regimes requires that a turbine survive a maximum 10-minute average wind speed with a 50-year return period of 97 knots at hub height, A Category 2 hurricane (wind speeds of 45 m/s or higher) will buckle up to 6 percent of the turbine towers in a wind farm. Hurricane Ike in 2008, for example, had a maximum sustained wind speed of 95 knots (49 m/s) at 10-m height (Category 2) when it passed over the meteorological tower erected by the developers of the Galveston Offshore Wind project. If a 50-turbine wind farm had been located off the coast of Galveston when Hurricane Ike struck, the model predicts that Hurricane Ike would have had a 50 percent probability of buckling two or more towers and a 10 percent probability of buckling four or more turbine towers.

Higher-category hurricanes will destroy a significant number of turbines, the study found. A Category 3 (wind speeds of 50 m/s or higher) will buckle up to 46 percent of the towers. The damage caused by Category 3, 4, and 5 hurricanes is important for offshore wind development in the United States because every state on the Gulf of Mexico coast and 9 of the 14 states on the Atlantic Coast have been struck by a Category 3 or higher hurricane between 1856 and 2008.

Galveston County, Texas, is the riskiest location to build a wind farm of the four locations examined, followed by Dare County, N.C. In contrast, Atlantic County, N.J. and Dukes County, MA are significantly less risky. In Galveston County, there is a 60 percent probability that at least one tower will buckle in 20 years and a 30 percent probability that more than half will buckle if the turbines cannot yaw; if they are able to yaw, there is still a 25 percent probability that at least one tower will buckle and a 10 percent probability that more than half will.

In Dare County, N.C., there is a 60 percent probability that at least one tower will buckle in 20 years and a 9 percent probability that more than half will buckle if the turbines cannot yaw; if they are able to yaw, there is a 15 percent probability that at least one tower will buckle and much less than 1 percent probability that more than half will. In Atlantic County, NJ there is a 15 percent probability that at least one tower will buckle in 20 years and less than 1 percent probability that more than half will buckle. In Dukes County, Mass., there is a 10 percent probability that at least one tower will buckle in 20 years and less than 1 percent probability that more than half will buckle. If the turbines in Atlantic and Dukes counties are able to quickly yaw even when grid power is out, there is approximately a 99 percent probability that none will buckle in 20 years.

The study suggests that engineered solutions can mitigate the risk of wind turbine damage as a result of hurricanes in the eastern United States. Typically, wind turbines are designed based on engineering design codes for northern Europe and the North Sea, where nearly all the offshore and coastal wind turbines have been built. These codes specify maximum sustained wind speeds with a 50-year return period of 42.5-51.4 m/s (83-100 knots), lower than high-intensity hurricanes.

The authors cite their own findings, as well as other analyses, to propose that industry standards be increased. They acknowledge that increasing the safety factor will increase the cost of an onshore turbine 20 percent to 30 percent. The percentage cost increase to strengthen an offshore turbine, like the one modeled in the paper, for tropical cyclones is likely to be smaller because a significant portion of the cost of offshore turbines is in the logistics of transporting, installing, and maintaining them.

The authors state that they have also demonstrated that wind turbines that have external power available to yaw can have a substantially reduced risk of being destroyed. Installing lead-acid batteries to allow a turbine to yaw without external power would add $30,000 - $40,000 (2010 prices) to the price of a turbine and 1,400 - 2,400 kg to its weight, assuming 6 hours of backup power for yaw motors that draw 12 kW of power. Further work is needed to determine the yaw rate that is appropriate for hurricanes.

February 21, 2012

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