A question that I am frequently asked is how well turbines can withstand hurricane force winds. The short answer is that International Standards help to ensure that turbines are built to withstand external conditions that may occur over the expected life of the project.
In practice, all commercial wind turbines are designed to meet International Standards, specifically IEC 61400 series of Standards. These Standards have been written over the past 40 years by industry experts operating with IEC Technical Committee 88.
IEC 61400-1 is the primary design Standard that defines the design conditions (“external environmental conditions” combined with a wide variety of turbine operating conditions) that onshore wind turbines must satisfy in order to certification requirements. IEC 61400-3 adds the external conditions that must be considered for offshore designs.
These are used by certification bodies to evaluate whether a turbine design does in fact meet the requirements defined in the Standards. Some certification companies have their own internal rules governing how they apply these Standards, but the only internationally developed transparent certification system is the newly developed IECRE system.
Hurricane conditions are not specifically defined within the Standard because they tend to be influenced by local site conditions. Instead, they are treated as extreme conditions on a spectrum of combined weather and sea-state conditions that may be heavily affected by local geographic conditions.
Unique characteristics of hurricanes and guidance on how to address them in the design process are included in informative annexes of the Standard. Magnitude of winds, waves and other important design conditions are determined by specific site data.
What is required is that every offshore (and onshore) installation specifically defines all the external conditions that may occur at that site over the expected life of the project, usually 30 years but no less than 20 years. This requires the project developer to gather historical data for their site and use it to forecast a set of design conditions which projects the extreme winds, waves, currents, and any other events that the turbines could experience, including hurricanes.
This is no different than the design requirements for offshore oil and gas rigs. The overwhelming majority of oil and gas rigs survived Hurricane Katrina. But those that didn’t enabled that industry to re-evaluate their Standards and adjust them to meet more severe 100 year events for future installations.
Most offshore and many onshore wind turbines are designed to withstand 70 m/s (155 mph, nearly 250 km/h) winds (IEC Class I). This is greater than most hurricanes. Hurricanes do present the most extreme wind conditions, but they are not impossible design conditions.
Jeroen Van Dam, who chairs the IEC technical committee that develops Standards for wind energy generation systems, TC 88, reminds me that the latest revision of IEC 61400-1, which is in its final approval steps, contains a special design class for areas with very high extreme winds, which may result from tropical cyclones, also called hurricanes in the Atlantic ocean. The new design class raises the extreme wind speed that wind turbines are designed for to about 80 m/s (almost 180 mph, around 290 km/h) and allow design for more severe external conditions when needed.
More design challenges may result from combinations of wind (less than the extreme wind) and waves together with certain wind turbine operating conditions. Designers simulate many thousands of these combinations with very sophisticated computer models to assure themselves, certification bodies, regulators, and customers that they have indeed addressed all the conditions that could damage the turbines.
This has proven to work well as there are many examples of wind turbines surviving these extreme conditions, but it was not the case 40 years ago, before standards and refined dynamics models were available. Indeed the Standards evolved as a result of turbines failing in extreme wind conditions.
Some countries have their own set of requirements which are a combination of IEC Standards plus national Standards. In the US, experts participate in IEC wind energy Standards development through the American Renewable Energy Standards and Certification Association (ARESCA).
The US is currently developing a set of statutory requirements for offshore. Industry experts are meeting together under the American Wind Energy Association (AWEA) governed by the American National Standards Institute (ANSI) rules to develop these consensus-based requirements.
Find out more about IECRE (IEC System for Certification to Standards Relating to Equipment for Use in Renewable Energy Applications)