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John Dalsgaard Sørensen1,2, Jens Nørkær Sørensen2 & Jørgen Lemming2

1) Aalborg University, Denmark2) DTU Wind Energy, Denmark

• Introduction

• Risk assessment - wind turbine parts

• Risk assessment - ice throw

• Acceptable risk level

• Conclusions

Risk assessment of wind turbines close to highways

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Introduction

Lystrup (Jutland, Denmark), February 2012

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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

Introduction

Laakso et al. IEA 2009

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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

Introduction

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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)

Risk assessment – wind turbine parts

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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

Risk assessment – acceptable risk level

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Probability per km driven that a person in a vehicle is killed

Risk assessment – wind turbine parts - result

D = 500m

D = 400m

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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

Risk assessment – ice throw

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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%

Risk assessment – ice throw

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

Risk assessment – ice throw

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Throwing distances during operation

Risk assessment – ice throw

Tower heightWind speed

40 m 50 m 70m 100 m 120 m

5 m/s 69 69 67 70 6810 m/s 88 91 98 111 12115 m/s 112 119 134 159 17820 m/s 138 149 172 209 23425 m/s 166 181 212 259 292

Assumptions: Tip speed = 70 m/sBlade position = -45 degrees to horizontal Thickness of ice fragments = 2cmRotor diameter = tower height

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Throwing distances at stand still

Risk assessment – ice throw

Total heightWind speed

50 m 75 m 100 m 150 m 200 m

5 m/s 7 12 18 30 4210 m/s 16 27 38 62 8715 m/s 28 45 62 98 13520 m/s 42 64 88 136 18525 m/s 57 85 115 176 238

Assumptions: Total height corresponds to tip position Thickness of ice fragments = 2cm

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Probability per km driven that a person in a vehicle is killed

Tower height = 120m / Total height = 200m

Risk assessment – ice throw - results

Stand-still

Operation

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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

Risk assessment

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• 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.

Summary

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• 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

Summary

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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).

Risk assessment of wind turbines close to highways

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