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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Optimising measurement strategies to maximise project value: Is the industry making false economies at the expense of project value?' taking place on Tuesday, 11 March 2014 at 11:15-12:45. The meet-the-authors will take place in the poster area.

Ole Kjaer DNV GL – Energy, Germany
Co-authors:
Thomas Zacher (1) F P Kai Freudenreich (1) Tanja Winter (1) Nikolai Hille (1) Stephanie Demant (1) Tobias Gehlhaar (1) Daniel Kopte (1) Stefan Schacht (1)
(1) DNV GL – Energy, Hamburg, Germany

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Abstract

DESIGN AND ASSESSMENT OF WIND TURBINES UNDER TROPICAL CYCLONE CONDITIONS

Introduction

The main objective of this publication is to present a new technical note for the design and assessment of wind turbines for tropical design conditions. This technical note gives clear design criteria for all relevant components of the wind turbine and is consistent with existing guidelines and standards for wind turbine design and assessment. The application of this technical note leads to a reliable and optimized and thus to a cost efficient design of the wind turbine for tropical cyclone conditions.



Approach

State of the art wind turbines are designed to work also under extreme wind conditions according to international guidelines and standards [12], [13], [19]. Up to now, these extreme wind conditions are tailored to typical European and North American wind conditions driven by the North Atlantic meteorological regime. However, the world wide expansion of wind energy application demands also to install wind turbines in regions being affected by tropical cyclones, as the Caribbean sea and the Northern West Pacific, affecting e.g. the U.S. east coast, Japan, China, Taiwan, Korea and the Philippines.

However, simply installing an existing type of wind turbine in regions affected by tropical cyclones without taking into account the significantly different wind characteristics would be a very risky proposition. Several publications, e.g. [2], [11], [18], have shown that tropical cyclone wind conditions exceed values specified in present wind turbine guidelines and standards. Presently, a maximum of up to 50 m/s in 10 minutes average at hub height for the extreme wind speed model is foreseen as the maximum standard wind class I. The turbines to be erected in tropical cyclone risk areas will have to be designed specifically to withstand higher wind speeds, depending on the cyclone intensity. Additionally, characteristic features of tropical cyclones, like turbulence intensity, rapid wind direction changes, the very likely possibility of a grid failure and the danger of flooding due to extremely heavy rain or sea surface surge, have to be considered during the design process. These challenging external conditions will influence the general safety concept as well as the wind turbine structural design. As a consequence, it is essential to complement the existing guidelines and standards with wind characteristics and design load cases developed especially for tropical cyclone purpose. Germanischer Lloyd present a Technical Note “Certification of Wind Turbines for Tropical Cyclone Conditions” which addresses all related issues to ensure a reliable, safe and thus cost efficient design and certification of wind turbines for sites being affected by tropical cyclones.


Main body of abstract

In general, tropical cyclones (TCs) are classified according to their maximum sustained wind speed and their minimum core pressure. The maximum sustained wind represents the highest average wind at 10m height over a one minute averaging time anywhere within the tropical cyclone. The National Hurricane Center has adapted the well known SSHS to the Saffir-Simpson Hurricane “Wind” Scale (SSHWS), which categorises the TCs only according to their maximum sustained winds [5]. This scale estimates the potential property damage and is suitable for the definition of tropical cyclone classes for wind turbines. The highest wind speed in each category can be identified as design relevant for the respective TC class.

For wind energy applications it is necessary to convert the 1-min values of the SSHWS to 10-min wind speeds with suitable conversion factors. These conversion factors are highly dependent on the surface roughness [2], [3], [4]. The World Meteorological Organization has developed a special guideline [1] for converting between various wind averaging periods in tropical cyclone conditions.

However, wind turbine guidelines and standards define in total three turbulence classes A, B and C for wind turbine design with reference to terrain categories [12], [13], [19]. These turbulence classes are correlated with a special terrain type characterised by their surface roughness length. For the present Technical Note “Certification of Wind Turbines for Tropical Cyclone Conditions” these three already existing turbulence classes A, B and C according to the GL Guidelines [12], [13] are applied to the tropical cyclone terrain categories.

Having the maximum sustained wind speeds according to SSHWS and terrain dependent 1-min-to-10-min conversion factors in hand; the design relevant wind speed at hub height requires also the terrain dependent power law exponent. Powell et al. [2] showed that logarithmic mean wind profiles exist in tropical cyclones at least in the lowest 200m and then levelled off slightly with a peak near 500m [2]. Thus, it can be assumed that a neutral stability surface layer in TCs and the power law is applicable for wind energy purposes.

For strong winds, it is engineering practice to estimate the power law exponent a according to

a=1/ln(z/z0) , (1)

with z=reference height (here 10m) and z0= surface roughness length

Based on the defined design wind speeds, turbulence intensities and power law exponents the Extreme Wind Model (EWM) according to present guidelines and standards [12], [13], [19] can be adapted to an Extreme Tropical Cyclone Wind Model (ETCWM).
The assumptions of very likely grid failures during the cyclone may affect the wind turbine´s possibility to yaw the nacelle towards the main wind direction. Depending on the turbine´s safety and control concept the turbine may stop yawing and facing the wind from all possible directions, or the turbine may switch to a wind driven free yaw status or yawing abilities may be supported by an independent power supply. A new Design Load Case (DLC) is defined which combines the ETCWM wind conditions with the assumptions of a grid failures prior to the occurrence of the maximum wind velocity
(close to the eye wall). The very high probability of a grid failure makes this new DLC a normal (N) load case with the associated safety factor for loads. Besides this possibly design driving load case further DLCs need to be considered, as e.g. extreme wind direction changes in TCs with rapid changes

Relevant wind turbine components being affected by tropical cyclones are mainly mechanical components as yaw drives and possibly rotor locks, electrical components as e.g. independent power supplies in case of grid failures and the safety and control system to detect and react on tropical cyclones.

The machinery components section will focus mainly on the yaw system, as the operability of the yaw system under tropical storm conditions might become crucial to mitigate destructive loads acting on the wind turbine. This includes requirements for the allowance of increased yaw speeds in case grid connection of the wind turbine is maintained during the passage of the storm. Possible modes of operation of the yaw system in case of a grid loss will be considered as well.


Conclusion

Increasing demand for wind energy application around the world leads to wind turbine installations in tropical cyclone regions, as the Caribbean sea and the Northern West Pacific, affecting e.g. the U.S. east coast, Japan, China, Taiwan, Korea and the Philippines. This publication presents a new technical note by Germanischer Lloyd for the design and assessment of wind turbines for tropical design conditions. It gives clear design criteria for all relevant components of the wind turbine and it serves as a complement to existing guidelines and standards for wind turbine design and assessment.

The application of this technical note leads to a reliable and optimized and thus to a cost efficient design of the wind turbine for tropical cyclone conditions. The technical note defines external conditions for tropical cyclone classes as maximum design wind speeds, turbulence intensities and power law exponents, extreme wind direction changes and severe disturbances of the electrical grid. Based on these models new Design Load Cases are generated representing extreme wind speed and wind direction situations. Wind turbine components being affected by tropical cyclones, as mechanical components (yaw drives and rotor locks), electrical components (independent power supplies) and the safety and control system (detection and reaction on tropical cyclones) are addressed.

The innovation and difference to the classic assumptions made is highlighted. Different wind conditions and especially description of the turbulence have to be considered.

The site assessment is the link between the external conditions of an actual site for a planned wind turbine installation and its categorization according to the expected tropical cyclone class and terrain category. While in this abstract the site assessments aspects have not been considered the full paper will present a detailed analysis of these aspects.




Learning objectives
In the paper the innovation and difference to the classic assumptions made is highlighted. Different wind conditions and especially description of the turbulence have to be considered. In addition the methodology to derive design wind speed is drawn up.


References
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