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# Ice throw from wind turbines: Assessment and risk management

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What is in-cloud icing?

• If temperatures are below 0°C and the structure is located inside a cloud (above cloud base height) we get in-cloud icing.
• The ice accretion rates increases with the relative windspeed and the moisture content of the cloud.
• Because the blade of a wind turbine moves fast there is an elevated hazard associated with ice throw and fall from turbines located in icing conditions.

How far can the ice be thrown?

• Maximum throw distance (screening) : 1.5 × (Diam. + Ht.). ~ 350 m.*
• Ice debris have so far not been found at this distance.
• Ice pieces have been found at 68 % of the maximum throw distance.
• 1.4 × tip height (Cattin). 1000 ice pieces with 3 % above tip height
• 1 × tip height (Lunden, 2017). 500 ice pieces total.

*Strict German/Austrian regulations

• Seifert screening formula of danger zone: (Hubheiht + rotordiameter) × 1.5
• In Germany/Austria it is required to have ice detection systems if there are roads or buildings within this distance.
• Restriction on production: turbine must stop when there is icing.
• If detection systems are reliable and sensitive, then the potential hazard is most likely associated with ice fall and not throw of smaller ice pieces.

How dangerous is the ice?

• An impact kinetic energy of more than 40 J is considered fatal.
• 40 J correpsonds to a 0.2 kg ice piece with density 500 g/dm3 falling from an elevation of 30–50 m.
• Because of the turbine height all ice pieces larger than approximately 0.2 kg are potentially fatal.

How large a risk can we accept?

Localized individual risk metric: the probability that an average unprotected person, permanently present at a specified location, is killed in a period of one year due to an accident at a hazardous installation

Acceptable risk:

• Ski tracks, hiking areas < 10−4
• People walking along public road, industrial sites, scattered houses < 10−5
• Houses, cafés, shops, etc. < 10−6
• Schools, kindergartens, shopping malls, hospitals, etc. < 10−7

See also: R.E. Bredesen, H. Farid, M. Pedersen, D. Haaheim, S. Rissanen, G. Gruben and A. Sandve, “IceRisk: Assessment of risks associated with ice throw from wind turbine blades” (PO.339). https://windeurope.org/summit2016/conference/allposters/PO339.pdf, in WindEurope Summit, Hamburg, 2016.

Rolv Erlend Bredesen, Kjeller Vindteknikk
IEA Wind Task 19, Winterwind 2017
February 15, 2017

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