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### Re: 5MW fatigue DEL calculation

Posted: **Wed May 15, 2019 8:37 pm**

by **Jason.Jonkman**

Dear Thanasis,

Here are my answers to your questions (at least as much as i can):

1) Sorry, but I don't think I can answer this question.

2) With Vin = 3 /s and Vout = 25 m/s, normally a fatigue analysis would involve 11 wind speed bins (of width 2 m/s), each with 6 seeds, for a total of 66 10-minute simulations. Yes, DLC 6.4 can result in additional fatigue damage.

3) Vref for the NREL 5-MW turbine is 50 m/s (IEC class I). DLC 6.4 uses NTM.

4) Most wind turbines would not shut down if the wind speed temporarily gusts above Vout. A normal operation shutdown would typically occur in high winds when the wind is sustained above Vout for an extended period of time. For the NREL 5-MW turbine, you could assume this time to be 10 minutes, so, you can simulate DLC 1.2 for all mean wind speeds up to 25 m/s.

A DEL is just a constant-amplitude fatigue load that occurs at a fixed load mean and frequency that produces the equivalent damage as the variable spectrum loads. The equations for lifetime DEL applies whether the lifetime damage is less than, equal to, or greater than unity.

I hope that helps.

Best regards,

### Re: 5MW fatigue DEL calculation

Posted: **Thu May 16, 2019 10:36 am**

by **Thanasis.Petridis**

Dear Jason,

thank you very much for your immediate response. You've been really helpful, but I have a couple questions to fully understand how to proceed.

1) The 6 seeds refer only to the primary input file for FAST? Or should I try different seeds with TurbSim too? (And finally generating more than 66 10-minute simulations)

2) Regarding DLC 6.4, shall I use a 2 m/s interval as well, until reaching 0.7*Vref=35 m/s ?

3) So the total of fast file should be like :

a) DLC1.2: 4-24 (m/s), with 2 m/s interval, multiplied by 6 different seeds on FAST (66 total)

b) DLC6.4: 26-34 (m/s), with 2 m/s interval, multiplied by 6 different seeds on FAST (30 total)

4) For the wind speeds between Vin and Vrated (3-11.4 m/s), what value do you recommend for the initial rotor speed? Maybe a value corresponding to the wind speed examined each time?

All, following the same Weibull distribution, characterized by one Weibull scale and shape factor and taken into account in the same MLife file.

Thank you very much for helping me out.

Best regards,

Thanasis Petridis

### Re: 5MW fatigue DEL calculation

Posted: **Thu May 16, 2019 11:52 am**

by **Jason.Jonkman**

Dear Thanasis,

Regarding (1), the seed I mentioned refer to the random seed specified in TurbSim. Normally you'd run 6 seeds per wind speed bin i.e. run both TurbSim and FAST 66 times for DLC 1.2.

Regarding (2) and (3), yes I agree with your interpretation.

Regarding (4), I generally recommend that you set the initial rotor speed and blade-pitch angle to their mean (expected) value based on the mean hub-height wind speed. For the NREL 5-MW baseline turbine, these are documented in Figure 9-1 of the specifications report:

https://www.nrel.gov/docs/fy09osti/38060.pdf.

Best regards,

### Re: 5MW fatigue DEL calculation

Posted: **Thu May 16, 2019 12:00 pm**

by **Thanasis.Petridis**

Dear Jason,

everything is now settled. Thank you very much for everything and for your immediate responses.

So the seeds in FAST can be fixed, yet in TurbSim should take at least 6 different.

Best regards,

Thanasis Petridis

### Re: 5MW fatigue DEL calculation

Posted: **Thu May 16, 2019 12:04 pm**

by **Jason.Jonkman**

Dear Thanasis,

Sorry, for some reason I thought you were modeling the land-based NREL 5-MW baseline turbine. I reread your first post and see that you are actually simulating the NREL turbine atop the monopile. In this case, in addition to the seed in TurbSim, you should also specify the seed in FAST's HydroDyn module to ensure that each simulation used different wave time series.

Best regards,

### Re: 5MW fatigue DEL calculation

Posted: **Thu May 16, 2019 12:12 pm**

by **Thanasis.Petridis**

Dear Jason,

the seed in FAST's HydroDyn module is found in the Platform file (for the offshore 5MW monopile-supported wind turbine) right?

6 seeds should be enough? So for every wind speed I still need 6 seeds in total (66 simulations for DLC 1.2).

Thank you again for everything

Best regards,

Thanasis Petridis

### Re: 5MW fatigue DEL calculation

Posted: **Thu May 16, 2019 12:38 pm**

by **Jason.Jonkman**

Yes, I agree.

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 8:22 am**

by **Thanasis.Petridis**

Dear Jason,

hello again. I have one more question regarding the Weibull parameters needed in MLife.

Since, most Weibull distributions regard wind speeds at u10 (wind speed 10m above ground), the weibull scale and shape factors are also calculated for these wind speeds. So, when I need to input these two parameters in MLife, shall I use these parameters, or should I somehow convert them for wind speed in hub height?

i.e. Weibull scale parameter is in m/s calculated with wind speeds 10m above ground and is proportional to the mean wind speed. Assuming a power law wind profile, increase the parameter to a higher value matching the "average" wind speed at hub height level. That's because the actual mean wind speed at hub height is significantly higher than the Weibull scale parameter calculated.

k is the Weibull form parameter. It specifies the shape of a Weibull distribution and takes on a value of between 1 and 3. A small value for k signifies very variable winds, while constand winds are characterized by a larger k.

So, by calculating wind speeds at hub height, the range of wind speeds increases from the initial k calculated (with wind speeds 10m above ground)

What should be the inputs in MLife and do you have any suggestions?

Best regards

Thanasis Petridis

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 9:21 am**

by **Jason.Jonkman**

Dear Thanasis,

The wind speed distribution specified in MLife should be for the reference height used in the loads analysis, which presumably is hub height. If you are designing to site-specific conditions with wind speeds known only at 10 m, then yes, you would have to use the (possibly assumed) shear profile to scale the wind speeds to hub height. Often though, loads analysis is based only on generic wind classes defined in the IEC standards.

Best regards,

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 9:32 am**

by **Thanasis.Petridis**

Dear Jason,

thank you for your response, yet I don't quite understand your last sentence (that is, "Often though, loads analysis is based only on generic wind classes defined in the IEC standards."). Possibly I did not understand, since I have never taken a look at IEC standars other than BS_EN_61400-3_2009. Could you please rephrase it?

Also do you believe the Weibull shape parameter would also change, apart from the Weibull scale parameter?

Thank you once again.

Best regards

Thanasis Petridis

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 10:37 am**

by **Jason.Jonkman**

Dear Thanasis,

The generic wind classes in the IEC standard assumes a Rayleigh wind-speed distribution (k=2) with V_average = 0.2*V_ref, where V_ref is determined by the wind turbine class (V_ref = 50 m/s for class I, V_ref = 42.5 m/s for II, and V_ref = 37.5 m/s for III).

Best regards,

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 4:42 pm**

by **Thanasis.Petridis**

Dear Jason,

So, if k=2 and mean wind speed is a fixed value, Weibull scale parameter is also a fixed value [c=(2*v_average)/(sqrt(π))].

My actual wind data and their probabilty of occurance are not accounted anywhere. Is this a safe assumption? Or did I misinterpret something?

Thank you,

Best regards

Thanasis Petridis

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 5:06 pm**

by **Jason.Jonkman**

Dear Thanasis,

Yes, your understanding is correct.

Normally the loads analysis used for turbine design is based on a generic IEC wind class. When selecting whether a given turbine design is suitable at a specific installation site involves a site-suitability analysis, where you could perform a loads analysis with site-specific inflow conditions (e.g., based on measured wind-speed distribution, shear, turbulence intensities, etc.).

Best regards,

### Re: 5MW fatigue DEL calculation

Posted: **Fri May 17, 2019 5:13 pm**

by **Thanasis.Petridis**

Dear Jason,

thank you very much for all your immediate responses, even on irrelevant-to-NREL questions.

Best regards

Thanasis Petridis