6 th february 2013, ewea2013, vienna

FATIGUE DAMAGE EQUIVALENT TURBULENCE: A New Method To Verify Wind Turbine Structural Integrity

in Complex Terrain

6th February 2013, EWEA2013, Vienna

Jointly presented by:

Virginia Mangone (Edison)Sannosuke Tanigaki (Wind Energy Corporation)Andrea Vignaroli (Windsim)Matthew Homola (Nordkraft Vind)

The work is jointly proposed by four companies

WEC Wind Energy Corporation

株式会社ウインド・エナジー 3

Contents

•Company Overview•Motivation•Site Suitability: Current Methodology•Site Suitability: New Methodology•Open Points•Case studies•Concluding remarks


株式会社ウインド・エナジー

Edison

4

7.7 GW of installed capacity 47 hydroelectric power plants 22 thermoelectric power plants 32 wind farms -49.8 billion m3 of hydrocarbons reserves 96 mineral leases (gas and oil)3 gas storage centers 9 photovoltaic systems 1 LNG terminal 1 biomass system 2 new pipelines under development

Edison is one of Italy’s leading player in the energy field. Its origins date back to 1883, when Edison opened Italy’s first electric energy facility in Milan. Today it is part of EDF group (Eletricité de France)

474MW in operation



MotivationThe IEC 61400-1 3rd Edition is under revision to release Edition 4th

Complex Terrains need a more suitable Site Assessment Procedure

Complex terrain sites are considered only marginally in the standard likely due to the fact that, when the work started, in the end of the 1980s, there was no need to develop projects in those sites. Currently an increasing number of on-shore wind farms are planned in complex sites, including hills and mountains, and therefore it is necessary to go more deeply in the assessment of those sites.



Site Suitability: Current ApproachIEC 61400-1 3rd Ed - ch11.9



The evaluation of the fatigue load counts two separate analyses:

• One for the wind speed distribution• One for the turbulence standard deviation

7

Assessment by reference to wind data

design VS siteDesign Wind Speed Distribution ≥ Estimate of Site Wind Speed

Distribution

Design Turbulence (1) ≥ Estimate of 90% quantile of Site Turbulence (90q)



Points to be highlightedRelevant for Complex Sites

• No Sectorwise analysis allowed

• Site Complexity taken into account through a correction factor (cCT) to increase the turbulence structure

• Other effects (i.e. loadings from up/down flows) not considered

• Analysis of the turbulence standard deviation based on the 90% quantile

• Log-normal distribution of the turbulence standard deviation assumed

Relevant for All Sites

• Assessment split in Two Separate Evaluations: one for wind speed distribution and one for turbulence levels

• Site specific air density not included

• No uncertainty taken into account



Site Suitability: NEW ApproachFatigue Damage Equivalent Turbulence



BACKGROUND

10

Fatigue Damage Equivalent Turbulence is a constant value of the turbulence which gives the same degree of fatigue damage as a given fluctuating turbulence intensitiy gives.

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Fatigue Damage Equivalent Turbulence

11

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design VS site

Design Turbulence (1) ≥ Estimate of Fatigue Damage Equivalent Turbulence

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One single evaluation:

The basic formula of the Equivalent Turbulence shall be adjusted in order to take into account that the site specific conditions are different from the design ones. If any of the site specific conditions deviates from the design conditions and this deviation produces higher loads, then it shall be consider as a penalty to add in the evaluation of equivalent turbulence:As we are considering complex terrain it must not be forgotten that on those sites the turbulence variance is higher and depends strongly on the wind direction. Therefore the evaluation shall be carried out direction-wise.



Correction Factors

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Independent from Direction:

• Adjustment for Air Density (cad)

• Adjustment for Wind Speed Frequency Distribution (cpdf) it combines in a single analysis both wind speed and turbulence

• Quality of measurement & Accuracy of flow model (cqm) it takes into account Uncertainty

Dependent on Direction:

• Penalty for Wind shear exponent (cse) /Flow inclination (cfi) Turbulence structure (cct) effects that the terrain complexity produces on the wind flow shape

• Added Turbulence: Wake from other turbine(s) (wake)



Test Cases 1/2

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16 different sites on complex terrain testedm-Wöhler exponent from 5 to 12 considered

SITE 12 SITE8

15m/s 15m/s

design turbulence distribution (log-normal) (m/s)measured standard deviation (m/s) direction (°)90% quantile of measured standard deviation

the 90% quantile is not representative of the

site specific condition



Test Cases 2/2

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SITE 12• n.WTGs = 4• WTG Class= IA• n. MAST = 1• measurement campaign = 4 years• Directional speed up determined

by a CFD analysis • worst m-scenario m=12• Cct=1.15 (all sectors)

less restrictive more restrictive

SITE 8• n.WTGs = 34• WTG Class= IIA• n. MAST = 1• measurement campaign =1.5 years• Directional speed up determined by

a CFD analysis • worst m-scenario m=12• Cct=1.15 (all sectors)



Open Points

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• Effects that produce Higher Loads: Correlation between wind turbine loading and excess wind shear exponent/

flow inclination/ turbulence structure over design conditions is not validated

• Upflow/Downflow: the assumption that flow inclination in excess of design value (8degree) could

be compensated by the penalty on turbulence is not validated

• Outliers: all measured data of standard deviation are taken into account, which means

that a few outliers can influence the results. How the extremes can be evaluated and if they have to be considered for a fatigue analysis is still not established.

Taking into the account all the data is a conservative choice.



Conclusion and future work

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• Current StandardFatigue analysis is split in two different evaluations. This criteria leads to too

conservative results.

Turbines are designed with the design condition that they will experience only the defined s1 turbulence level. In the site assessment, this s1 value is compared with the 90% quantile of the site estimates, on the assumption that that value of constant turbulence can represent the real situation. However, the 90% quantile is not always representative of the actual site condition.

• New ApproachDefines a single analysis to assess the fatigue load

Based on site statistics instead of two theoretical distributions

It is not less restrictive or more restrictive, it is just more representative of the site conditions

• Future WorkFurther Investigations on the open points

Full load calculations on the test cases to compare the results

Thank you for your attention

send further questions to: [email protected]

BACK UP



Equivalent Turbulence VS 90%quantile of turbulence

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IEC assumptions on turbulence: •log-normal distribution •expected value of σ1 given by the formula: I*(0.75Vhub+3.8) •variance of σ1 given by the formula: (1.4*I)2

where:

I is a site specific non-dimensional coefficient and measures the turbulence intensity

1.4m/s is a constant value and measures the turbulence variability. It is assumed as independent from the site and from Vhub.

Sites on complex terrain are affected by strong variations in turbulence and the constant value must be increased: i.e. constant=2.0m/s

At those sites where “high” variations in turbulence occur, the 90% approach is unable to take fully care of the non-linearity of fatigue damage, especially for high values of m at low wind speeds.

Flat Terrain. Constant = 1.4m/s

Complex Terrain. Constant = 2.0m/s



Correction Factors

Correction Factors independent from Direction

• air density (cad)

• wind speed distribution (cpdf)

• quality of measurements (cqm)

Correction Factors dependent on Direction

• Turbulence structure (cCT)

• Wind Shear /Upflow (cws/cfi) still pending, more

validation work needed

20

DESIGN

SITE

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Quality of measurements (cqm)

21



Appendix A 1/2

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Appendix A 2/2

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6 th february 2013, ewea2013, vienna

Documents

wind speed distributionone

shore wind farms

energy field

electric energy facility

degree of fatigue damage

new pipelines

complex terrain6th

fatigue load counts