“Guideline for the Certification of Offshore Wind Turbines” gives answers Hurricanes and Offshore Wind Farms - Incompatible?

Copenhagen, 27 October 2005 – The hurricane season caused widespread damage onshore as well as offshore, i.e. in the Gulf of Mexico oil platforms were damaged or disconnected from their moorings. Given the destructive nature of hurricanes, gale force winds and tropical storms it is crucial that potential offshore wind energy projects undergo extensive tests of the site-specific conditions for safety reasons. One of the thirteen chapters of the new “Guideline for the Certification of Offshore Wind Turbines” published by Germanischer Lloyd WindEnergie GmbH (GL Wind) deals exclusively with this site assessment and the resulting loads. It includes the examination of the environmental-related influence on the offshore wind turbine and the mutual influence of the offshore wind farm configuration. The site assessment takes effects into account such as the wind conditions, marine conditions (wave, tides, sea ice), soil conditions, site and wind farm configuration and other environmental conditions, such as salt content of the air, temperature, ice and snow and electrical grid. Hurricane winds are rotating by nature. Therefore in addition to the extreme wind speed it is the inclinational flow to be considered. In other words: extreme winds can arise from all directions. Hurricane winds are sustained for longer periods of time and change slowly in direction, which allows the wind to seek out the most critical angle of attack. In a hurricane, gusts of wind can be expected to be 25%-50% higher than the sustained wind velocity. A hurricane with sustained winds of 150 mph may produce gusts exceeding 200 mph. Hurricane gusts can be described as a gradual local wind speed acceleration of five to ten seconds followed by an abrupt decrease in wind speed to values of much less than the mean within one to two seconds. The gust cycle recurs in a period of several minutes. The effect of a storm's wind is best evaluated in terms of the pressure it exerts. Pressure varies with the square of the velocity, meaning that pressures increases very rapidly with increasing wind velocity. The same chapter of the “Guideline for the Certification of Offshore Wind Turbines” defines the requirements for the determination of loads resulting from the environmental conditions in conjunction with the operational behaviour of an offshore wind turbine. These conditions include peak wind speeds due to storms and rapid changes in wind speed and direction. These extreme conditions include the potential effects of wind turbulence. Chapter four of “The Guideline for the Certification of Offshore Wind Turbines” contains a precise analysis of extreme wind conditions, extreme operating gusts, extreme wind shears as well as extreme coherent gusts with direction change. For example, the extreme wind conditions are used to determine the maximum wind loads acting on offshore wind turbines. The offshore guideline covers general testing conditions, safety systems and the requirements for the manufacturing companies. Strength analyses, machinery components and electrical engineering are considered as well as tests requirements, periodic inspections and condition monitoring. The comprehensive offshore guideline covers the world of wind and outlines precise rules for regular as well as extraordinary situations. So far, a number of projects had already been certified and/or are currently in the certification process according to the new guideline. For further information, please contact: Peter Dalhoff, Head of Department for Project Certification Off- und Onshore, Tel. +49 40 36149-117, peter.dalhoff@gl-group.com

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