Risk assessment of wind turbines close to highways

1. Risk assessment of wind turbines close to highways John Dalsgaard Sørensen1,2, Jens Nørkær Sørensen2 & Jørgen Lemming2 1) Aalborg University, Denmark 2) DTU Wind Energy, Denmark • Introduction • Risk assessment - wind turbine parts • Risk assessment - ice throw • Acceptable risk level • Conclusions

2. Introduction Lystrup (Jutland, Denmark), February 2012

3. Introduction Scope: • Assessment of the minimum distance from wind turbines to highways (in Denmark) based on risk assessments of the consequences due to • total or partial failure of a wind turbine • ice throw in case of over-icing Laakso et al. IEA 2009

4. Introduction Methodology: • Data collected from a large number of modern wind turbines from Denmark and abroad • same basic technology as new large wind turbines • information on events where parts of the turbine is thrown / dropped at a distance from the turbine • Risk estimated that persons in a car are killed because of • wind turbine parts 'thrown away' from a wind turbine in events with total or partial failure • ice pieces 'thrown away' from a wind turbine in events with over-icing

5. Risk assessment – wind turbine parts Data from representative large databases with information on parts ’thrown’ in case of failure (from nacelle and blades): • Distance from wind turbine • Size of with turbine part Estimation of probability per year per m2 that a wind turbine part hits in radius R from the wind turbine Estimation of probability per km that a vehicle is hit by a wind turbine part thrown from a wind turbine placed in a distance d from a road – depending on • Vehicle velocity V (80 km/h) • Distance D between wind turbines Assumption: If a wind turbine part hits a vehicle then in average 1.5 persons are killed (conservative)

6. Risk assessment – acceptable risk level General statistical risk to be killed when driving on a highway in Denmark: 2 10-9 per km driven (2009) ALARP (As Low As Reasonably Practicable) principle: An additional / extra risk contribution can be assumed unimportant if the risk contribution is less than 1/100, i.e. 2 10-11 per km driven Additionally it can be expected that the number of killed persons at highways in Denmark is reduced by a factor 2 each 10 years.  risk accept limit for new wind turbines close to highways in Denmark becomes 5 10-12 per km driven

7. Risk assessment – wind turbine parts - result Probability per km driven that a person in a vehicle is killed D = 400m D = 500m

8. Risk assessment – ice throw Estimation of probability per km that a vehicle is hit by an ice piece thrown from a wind turbine placed in a distance d from a road depends on • Vehicle velocity V (80 km/h) • Distance D between wind turbines • Distribution of mean wind speeds at hub height • Pz(s,v): Probability per year and per m that ice pieces lands in a distance s from the wind turbine given mean wind speed v

9. Risk assessment – ice throw Over-icing in Denmark: (approximate estimates) • ice pieces larger than 3 mm: 0,175 times per year • one over-icing: 10 ice pieces with a weight > 1 kg • probability distribution of wind speed in case of over-icing: - assumption: if ice piece hits a vehicle then in average 1.5 persons are killed with a probability equal to 10%

10. Risk assessment – ice throw Throwing distances are computed using the 6 DOF trajectory code by J.N. Sørensen (Wind Engineering, vol. 8(3), 1984)

11. Risk assessment – ice throw Throwing distances during operation Assumptions: Tip speed = 70 m/s Blade position = -45 degrees to horizontal Thickness of ice fragments = 2cmRotor diameter = tower height

12. Risk assessment – ice throw Throwing distances at stand still Assumptions: Total height corresponds to tip position Thickness of ice fragments = 2cm

13. Risk assessment – ice throw - results Probability per km driven that a person in a vehicle is killed Tower height = 120m / Total height = 200m Operation Stand-still

14. Risk assessment Increase (in %) of probability that a person in a vehicle is killed due to thrown wind turbine parts or ice pieces • Increase in risk level is low for distances above 150m

15. Summary • Risk assessment of wind turbines close to highways taking into account • total or partial failure of a wind turbine • to ice throw in case of over-icing • Data collected from a large number of modern wind turbines from Denmark and abroad • The studies show that the probability per kilometer that a person in a vehicle is killed due to total or partial failure / collapse (damage) of a wind turbine can be assumed to be of minor importance.

16. Summary • Assessment of risks due to ice throw in case of over-icing is associated with many uncertainties • The generally accepted risk on highways in increased with less than 0.1% in wind turbines are installed more than 150 meters from the highway • The risk due to ice throw from a wind turbine in operation is seen to be slightly greater than the risk if the wind turbine is parked • It is recommended that for practical projects a proper risk assessment is performed, which also include the location in relation to the road and the prevailing wind direction

17. Risk assessment of wind turbines close to highways Thank You For Your Attention John Dalsgaard Sørensen jds@civil.aau.dk Jens Nørkær Sørensen jnso@mek.dtu.dk Jørgen Lemming jqle@dtu.dk Sponsorship: Danish Energy Agency Reference: Sørensen, J.D., J. Lemming & J.N. Sørensen: risk assessment of siting wind turbines close to highways Risø report: Risø-R-1788, 2011 (in Danish).