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NREL is a national laboratory of the U. S. Department of Energy, Office of Energy Efficiency

and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Simulating Turbulent Wind Fields for Offshore 

Turbines In Hurricane-Prone Regions

Yi Guo, Rick Damiani, and Walt Musial 

May 5‒8, 2014 in Las Vegas, Nevada

Extreme wind load cases are one of the most important

external conditions in the design of offshore wind turbines in

hurricane-prone regions. Furthermore, in these areas, the

increase in load with storm return-period is higher than in

tropical regions. However, current standards have limited

information on the appropriate models that can be used to

simulate wind loads from hurricanes. This study investigates

turbulent wind models for load analysis of offshore wind

turbines subjected to hurricane conditions. Extreme wind

models recommended in IEC 61400-3 and API/ABS (a

widely-used standard in oil and gas industry) are

investigated. The study further examines the wind turbine

response subjected to hurricane wind loads. To include the

API wind model, we modified TurbSim, a three-dimensional

wind simulator. Finally, wind fields simulated using IEC and

API wind models are used for an offshore wind turbine model

established in FAST to calculate turbine loads and response.

The information contained in this poster is subject to a government license.

AWEA WINDPOWER • Las Vegas, Nevada • May 5–8, 2014

NREL/PO-5000-61534

 

0.11

, (10‐min mean)

0.11

hub hub

hub

l hub

z vz v v z

 v

      



    

 

 

0 0

0

0 0

0 0

0

0.22

0

, , [1 0.41 ln ],

3600, (1‐hr mean at 10m above still water level)

, [1 ln( )] 10

0.0573 1 0.15 /

: Unit conversion

0.06[1 0.043 / ] 10

u

u

t v zt v zt I z t t t

t v

z v zt v C

C v

z Iz v







 



       



 

 

       

Abstract

API & IEC Wind Profile Models

IEC: wind shear model

wind speed only 

depends on height (z)

API: wind profile model

wind speed depends on

height and time (z, t)

API & IEC Wind Spectrum Models

2 0.45

0

(5/3 )

2/3 0.75

0

0

320 10 10 ( ) (1 )

0.458

172 10 10

: 1‐hr mean wind speed at 10 m above mean sea level

n n

U z

S f f

n

z U f f

U



            



           





 

  2 5/3

4 /

16 /

: index referring to the direction of wind speed component

L : integral parameter of turbulent wind speed component

k k hub

k k hub

k

fS f fL v

fL v

k

  

IEC: Kaimal spectrum

only depends on wind 

speed at hub height (vhub)

API: Froya spectrum

depends on wind speed 

and height (Uo, z)

1. Different wind models result in different wind turbine extreme loads during hurricane conditions

2. The API and IEC wind models result in different wind speed extrapolation along height, wind spectrum, coherence, return-period, and time-averaging intervals

3. The API wind model has greater energy in the low-frequency range, which could excite some of the low structural modes and vibration of offshore turbines

4. The API conditions tend to generate slightly smaller load maxima, but also larger minimum values of the loads.

Conclusions

Wind/Wave Load

& Marine Growth

Structural Dynamics of Jacket Foundation

Finite Element Analysis 

of Transition Piece

Hurricane Wind Causes 

Aerodynamic Drag

Aero/Elastic Analysis of WT

Hurricane Wind

Causes Thrust

Current Load

     2 2 12 / 0.12 /

1

(, )

, coherence scale parameter

: magnitude of the separation vector,

Normal to the average wind direction

hub c fr v r L

c

Coh r f e

L

r

 



 

API & IEC Wind Coherence Models

IEC: wind coherence 

two-dimensional

Taylor Frozen theory

in longitudinal direction

ABS report: Contract M10PC00105 , Design Standards for 

Offshore Wind Farms

Wind Profile: Result Comparison Wind Spectrum: Result Comparison

Wind Coherence: Result Comparison

API IEC

0.000

0.500

1.000

1.500

2.000

2.500

3.000

RootFxb1 RootFyb1 RootFzb1 RootF1 RootM1 RootMzb1

Minimum

Maximum

•Different wind models result in 

different wind turbine extreme 

loads during hurricane conditions 

•We used identical wind speed at 

hub height for API & IEC models

•Larger differences expected with 

different wind speed

Nondimensional Comparison of 

API & IEC Blade Loads

Time-Averaging Intervals

    

 

 

0

0

0.22

0

600 ,600 ,3600 1 0.41 ln , 3600

,3600 1 ln( ) 10

0.06[1 0.043 / ] 10

u

u

vz vz I z t t

z vz v C

z I z v



 



             

       

       

600,3600 G 1.053

600,3600 G  1.03

API:

convert wind speed

between time intervals 

and at various heights

World Meteorological 

Organization Gust Factor

IEC-61400-3

API model predicts higher wind speed at heights >10m API model has higher energy at low frequency <0.01 Hz API converts time intervals based on wind speed and height

API: wind coherence,

three-dimensional

Equal wind 

speed at 10 m

Wind Speed

Wave Height

Photo by Gary Norton, NREL 27360

FAST’S 

user Guide

1 2 3

4 5 6

Modeling Approach

API Load 

IEC Load

Wind speed of the API model depends on height and time API’s wind spectrum is a function of hub height API’s wind coherence model is three-dimensional