How Wind Farms Weather Hurricanes

This is the second in a three part series of blogs examining how natural disasters like hurricanes impact our energy generation.

Recently, we published a blog on the Intermittency of Fossil Fuels highlighting the connections between natural disasters (earthquakes, floods, drought) and their impact on traditional power plants and wind farms. Since then, Hurricane Irene raked the East Coast with deadly force and devastating floods and the intermittency question has come back in a new form: Can wind turbines survive a hurricane?

If you’re limited for time, I’ll get right to the point: Yes. Wind turbines can survive hurricanes. Both structural engineering and risk analyses are vital to ensuring wind farms are developed to survive reasonably expected problems.

Why are hurricanes a concern for wind farms?

In the Southeast, wind farms have been proposed in coastal and offshore areas. Wind Capital Group has proposed an onshore wind farm in Palm Beach County, Florida. The Sugarland Wind Farm would have a capacity of up to 150 MW in the heart of hurricane-prone Florida. In North Carolina, two major wind parks have been proposed onshore including the 300 megawatt (MW) project near Elizabeth City and another 300 MW (See “Irene Would Have Impacted Two North Carolina Proposed Wind Farms” below) project between Camden and Currituck Counties. These two multi-million dollar projects would have been directly in the path of Hurricane Irene – highlighting the importance of wind turbine survivability during hurricane force winds.

Irene Would Have Impacted Two North Carolina Proposed Wind Farms

Irene Would Have Impacted Two North Carolina Proposed Wind Farms

Offshore wind farms are being proposed in Texas, Georgia, South CarolinaNorth Carolina, VirginiaMaryland, DelawareNew JerseyNew YorkRhode Island and Massachusetts. Had any of the offshore wind projects from North Carolina up to Massachusetts been built, they too would have been impacted by Irene. Over the past decade, each of these states has been impacted by a hurricane, tropical storm or tropical depression. Therefore, it is not a question of if a hurricane will impact proposed wind farms, it is only a matter of when.

How do energy project developers deal with risk?

Although you may not realize it, there is an entire industry dedicated to evaluating risks associated with myriad variables: the insurance industry. Unlike investors in financial markets, “risk” is a nasty word to insurance companies to be avoided. Anyone that owns a sports car, or a home in a flood zone knows that the higher the risk of a “total” loss of an asset (such as a completely destroyed car that is a “totaled” car), the higher the insurance premiums are to protect those assets. Energy developers similarly have projects insured to protect against unforeseen disaster – the random earthquake, the freak tornado, the extreme weather. For nuclear developers, the private insurance industry will not insure a new nuclear reactor without a guarantee from the federal government that taxpayers will pick up the tab should a developer default on a loan. These “loan guarantees” are essential for the nuclear industry to secure insurance, even though this industry has a 50% default rate on previous loans. Higher risks (either from natural disasters or manmade ones) may impact whether or not a project developer can get project financing – loans or debt – at lower interest rates. Also, higher insurance premiums increase operation costs of energy projects, and thus, impact the electricity prices we eventually pay as consumers.

What is the risk of a hurricane destroying a wind farm?

A properly designed wind farm should be able to withstand hurricanes that are likely to pass within a wind farm’s lifetime – usually within the next 20 – 25 years. Hurricanes are but one natural disaster that must be considered for some projects – earthquakes, floods, tornadoes and even fires are also risks that must be considered when constructing wind farms. Significant or unusually strong hurricanes and storm impacts, like floods, are often called “the storm of the century” or “a 500-year storm.” These terms, while sometimes used casually, actually have real statistical meaning. For some examples, in any year, a “10-year flood” has a 10% chance of occurring, while a “100-year flood” has a 1% of occurring each year. As such, a 10 year flood is not expected to be as severe or damaging as a 100-year flood. A 500-year or 1,000-year weather event is extremely severe – but the risk of each of those happening any such year is 0.2% and 0.01% respectively. It should be noted that there is no guarantee that a 100-year flood will definitively occur in the next 100 years. In fact, an analysis for New Orleans shows the risk of a 100-year flood occurring is actually 26% within 30 years, and 63.4% within 100 years. Therefore, the risk that a wind farm in Massachusetts being hit with a Category 5 Hurricane is much lower than a similar wind farm somewhere along the Gulf Coast. Wind farms in Iowa at more risk from winter weather than say, a wind farm in Texas.

To help investors, developers and insurance companies understand the risks associated with wind turbines in extreme weather events, engineering standards exist to explain wind turbine survivability. The International Electrotechnical Commission Standards (IEC) develops standards for wind turbines that are rated based on their abilities. One aspect of certification is the ability to withstand wind speeds considered for a “50 year extreme gust.” An IEC “Class 1″ turbine is designed and certified to withstand a 50 year extreme gust of 70 meters-per-second, or 156 miles per hour, meanwhile an IEC “Class 3″ turbine is certified to withstand an extreme gust of up to 52.5 meters-per-second, or 117 miles per hour. For reference, a Category 3 Hurricane has sustained wind speeds of 110 – 130 miles per hour and a Category 5 Hurricane (the highest category) has sustained wind speeds of greater than 155 miles per hour. The maps below highlight the risks of hurricanes striking the Gulf and Atlantic coasts – over the past 10 years, each state on the coast has seen some form of hurricane or tropical storm.

NOAA Historical Hurricane Tracks, Gulf of Mexico, 2000-2010

NOAA Historical Hurricane Tracks, Gulf of Mexico, 2000-2010

NOAA Historical Hurricane Tracks, Western Atlantic, 2000-2010

NOAA Historical Hurricane Tracks, Western Atlantic, 2000-2010

How do wind turbines handle extreme weather?

Wind turbines are designed specifically to harness the wind but they are also designed to withstand it. Modern wind turbines utilize several techniques to reduce the likelihood of harm. Active techniques require some sort of action by the turbine or operator to protect a turbine. These techniques are used to stop turbines and halt electric generation in extreme weather conditions and so technicians can perform regular maintenance. Passive techniques are built-in and require no additional activity to protect a turbine. The following list is a non-exhaustive list of active and passive techniques to reduce turbine damage:

  • Turbine brakes – Most turbines are installed with turbine breaks that automatically engage if winds reach a certain speed – usually around 55 miles per hour. At the rated speed, the turbine brakes are applied and the rotor stops spinning.
  • Blade feathering – Wind turbine blades can be tilted (feathered) remotely by an operator or automatically so instead of harnessing strong winds, wind is allowed to slip through the blades.
  • Active yaw systems - Large turbines have active yaw systems that require a small motor that moves the nacelle (or gearbox, where the generator is housed) to point directly into the wind. By pointing directly into the wind, turbine aerodynamics allow wind to flow past the blades easily.
  • Heavy monopoles – Monopoles can reach up to 100 meters in height and are meant to hold nacelles and blades that can weigh several tons. Thicker monopoles constructed with more steel and internal structures can support more weight and withstand stronger environmental forces like wind or waves for offshore structures.
  • Strong foundations – For onshore wind turbines, most large scale turbines have a foundation pad constructed from concrete. These foundation pads are usually buried several feet deep to help anchor the turbine to the ground. Offshore turbines in Europe utilize heavy concrete gravity-based structures that are placed on the seabed or monopiles that are driven many feet into the seabed to keep turbines steady in high winds and waves.

These techniques have evolved with the wind industry for decades. Offshore wind farms are regularly popping up in European waters – waters that see very strong storms in the North Sea. As turbine size and wind farm size has increased, the financial risk associated with these projects also increase. A single turbine may cost several million dollars and a single offshore wind farm can cost more than one billion dollars. With the high capital costs, project developers, utilities, insurance agencies and customers want to ensure the projects have the best possible chances of withstanding extreme environmental conditions.

turbine-hurricane1

Despite the best efforts by any energy project developer, be it nuclear, coal, oil, natural gas or wind power, some power generators will be destroyed. At some point, the Earth will be struck by an asteroid, terrorists will attack energy infrastructure, major earthquakes will happen – but planning for all possible risks at all power stations is unrealistic and unfeasible. But why should hurricanes be expected to pose risks only to wind farms? Multi-billion-dollar oil rigs are subjected to hurricanes in the Gulf of Mexico almost annually – but that risk has not yet stopped the exploration and development of oil and natural gas. Oil rigs even sink, explode and get destroyed, but somehow those structures still are built and insured. Wind farms should be no different. This country has to chose between safer, renewable energy or more high risk energy choices.

Instead of giving ourselves false hopes that we can conquer nature, we should plan to the best of our ability, and expect that at times our structures can and will fail due to some unknown and unsubstantiated risk. Risks only exist if we allow them to – do we as a society prefer the risk of a coal ash spill, an oil rig explosion, radiation contamination, or the risk that a wind turbine might fall down during a hurricane?

Interested in knowing how our more traditional sources of energy and wind turbines weathered Irene? Be sure to read our past blog, Hurricane Irene’s Impact on Fossil Fuels and Nuclear Power, and check back tomorrow for our final blog in this series, Hurricane Irene’s Impact on Wind Turbines.

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This is an interesting and well-researched blog post, but wind turbines are actually more vulnerable to hurricanes than this post implies. Wind turbines have already been destroyed or damaged by tropical cyclones. Six wind turbines were destroyed by Typhoon Maemi in Okinawa in 2003. (http://windeng.t.u-tokyo.ac.jp/ishihara/proceedings/2004-3_paper.pdf)

Wind turbines designed to IEC Class 1 standards should survive a category 1 hurricane but are at significant risk of being destroyed by stronger hurricanes. For example, a reference 5-MW offshore wind turbine design from the U.S. National Renewable Energy Laboratory (NREL), which is designed to IEC CLass 1 standards, has a up to a 30% probability of being destroyed by a category 2 hurricane and a 10 – 85% percent probability of being destroyed by a category 3 hurricane.
Comparing the 50-year gust speed specified in the IEC standard to the sustained wind speed in a hurricane is misleading for two reasons. First, the maximum 3-second gust speed in a hurricane is typically about 30% higher than the 10-minute sustained wind speed. (Vickery 2005, doi: 10.1061/(ASCE)0733-9445(2005)131:5(825)) Second, hurricane wind speeds are measured near ground level (usually 10 meters), but the IEC standard specifies wind speeds at the hub height of the turbine. The wind speed of a hurricane at 75 meters above the ground (a typical hub height for a modern wind turbine) is approximately 17% higher than the wind speed at 10 meters. (Franklin 2003, doi: 10.1175/1520-0434(2003)0182.0.CO;2) Consequently, the maximum hurricane gust at wind turbine hub height is roughly 52% higher than the sustained wind speed measured at ground level. This means that a category 1 hurricane will likely subject a wind turbine to 113 – 144 mph gusts and a category 2 hurricane will likely subject a turbine to 146 – 167 mph gusts.

Oil rigs are designed to stricter standards. Oil rigs in the Gulf of Mexico are regularly hit by hurricanes and they are more expensive than wind turbines. The consequences of a hurricane destroying an oil rig are much greater than the consequences of a hurricane destroying an offshore wind farm. The cost of cleaning up a spill can be very high, but even more important is the value of lost oil production. Oil companies can afford to build oil rigs stronger because they generate far more revenue. An oil rig producing a few thousand barrels of oil per day generates revenue comparable to a large wind farm. A wind turbine can be designed and built to survive a category 5 hurricane, but the electricity it generates may be so expensive that no one will buy it.

Actively yawing a turbine to keep it pointed into the wind significantly reduces the aerodynamic force on a wind turbine and reduces the probability of damage from a hurricane. However, applying the mechanical brakes to stop a wind turbine rotor from spinning is damaging to the turbine. Modern wind turbines release their mechanical brakes, feather the blades, and allow the rotor to idle freely to reduce stress on the turbine.


Comment by Stephen Rose on September 7, 2011 2:53 pm


Thank you Stephen for such a thoughtful response. I would like to reiterate: “Despite the best efforts by any energy project developer, be it nuclear, coal, oil, natural gas or wind power, some power generators will be destroyed.” It’s no surprise that there are examples of wind turbine failure. However, it should be noted that the wind farm you cite, the one in Japan that was destroyed after a cyclone struck, estimated wind gusts at the site were 200 miles per hour (it wasn’t clear to me if that was at the hub height, but if not – the speeds were certainly much higher). It’s surprising to me that not all of those turbines buckled.

Could you cite a peer-reviewed journal article with the figures you cite?

Also, you state Category 1 Hurricanes would “likely subject a wind turbine to 113 – 144 mph gusts” – I’d encourage you to read the third part of this series where turbine operators (several that experienced Hurricane Irene – as a Cat. 1 storm) stated their turbines fared the storm just fine without any problems.

http://blog.cleanenergy.org/2011/09/06/hurricane-irene-and-its-impact-on-wind-farms/


Comment by Simon Mahan on September 8, 2011 10:34 am


The most accurate data collection of accidents to wind farms due to hurricanes worldwide has been released in
“Damage and Critical Analysis of Accidents to Assist in Avoiding Accidents of Offshore Wind Farms on the OCS, May 2010, by Dr. Malcolm Sharples and Brian J.M. Sharples, Offshore: Risk & Technology Consulting, Inc., Houston, TX” available here : http://www.boemre.gov/tarprojects/633.htm.

Besides, Classification society ABS announced the release in December 2010 of a Guide for Building and Classing Offshore Wind Turbine Installations, the first Guide to address design considerations for the bottom founded support structure of an offshore wind turbine situated in tropical storm prone areas on the US Outer Continental Shelf (OCS) such as the Gulf of Mexico and East Coast, available here : http://www.eagle.org/eagleExternalPortalWEB/ShowProperty/BEA%20Repository/Rules&Guides/Current/176_WindTurbine/OWTI_Guide,

since almost all oil riggs have been destroyed in the Gulf of Mexico in 2005 with hurricane Erika, and that IEC 61400 standard & European Classification societies doesn’t address hurricane winds but just laminar winds.

In order to be fully precise, not just the maximum wind speed has to be taken into account for the survivability of a wind turbine in a hurricane, but also its occurence rate, gust factor, turbulence intensity, direction change (horizontal oblique impact) and inclination (vertical oblique impact). Also, hurricane sustained wind speed is a one minute averaged wind speed while IEC maximum wind speed is a ten minute one (or 3s gust). The 10 min mean wind speed has to be increased by about 12% (Vickery – National Hurricane Center) to be comparable to a 1 min one (or by about 14% in Indian Ocean – source: hurricane regional center in La Réunion Island).

The Belgian Company SARENS (or Rigging International in US – specialized in heavy lifting and wind turbine installation) along with French Engineer Raoul AMRAM are currently developping a mechanical device which makes standard wind turbines of the market (MW and multi-MW) suitable for hurricane prone areas. The principle is to quickly dismount the most vulnerable parts of the wind turbine in case of hurricane warning. This device is currently in the certification process by Germanischer Lloyd.

Exciting !


Comment by Pascal on September 9, 2011 9:53 am


What if…………

….there was a wind generator that did not harm birds?

….it worked at near 100% efficiency 20 hours a day at hundreds of locations nationally?
….it required no additional purchase of land?
….it was supported by AZ Sen. John McCain?
….it required no additional transmission lines?
….it was written about in the L.A.Times and internet publications worldwide?
….it created clean electricity precisely where it was needed?

I am sending you these questions because of your involvement concerning new wind energy proposals here in West Palm Beach. I bring over thirty years of industrial design experience to the product I created to answer the above questions. If interested, please contact me at rahales@hotmail.com. Thank you for your time and interest.

Sincerely,

Richard Alan Hales


Comment by Richard A. Hales on November 3, 2011 1:12 pm


I overstated the risk to a single turbine in my previous comment. I said “a reference 5-MW offshore wind turbine design [...] designed to IEC CLass 1 standards has a up to a 30% probability of being destroyed by a category 2 hurricane and a 10 – 85% percent probability of being destroyed by a category 3 hurricane.” I’ve since revised that estimate based on better data about the turbulence intensity in hurricanes, which looks to average about 9% at hub height. This gives up to a 6% probability that a Category 2 hurricane destroys that turbine design and a 25-50% probability that a Category 3 hurricane destroys it.

The results are published in “Quantifying the Hurricane Risk to Offshore Wind Turbines” (www.pnas.org/cgi/doi/10.1073/pnas.1111769109). There’s also a good summary of the study on ArsTechnica (http://arstechnica.com/science/news/2012/02/model-shows-offshore-wind-turbines-at-risk-from-hurricanes.ars)


Comment by Stephen Rose on February 16, 2012 8:43 am


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