RAO OC3Hywind Turbine

Discuss theory and modeling of wind-turbine structures.

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Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Mon Nov 18, 2019 10:53 am

Dear Bertrand,

See my post dated Jan 10, 2018 in this forum topic above for details on this equation.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Bertrand.Canetti
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Re: RAO OC3Hywind Turbine

Postby Bertrand.Canetti » Mon Nov 18, 2019 5:15 pm

Oh...oops.

And yet I did read this message, but did not get it was the same equation... :roll:

Thank you, at least I got it now.

Best regards,
Bertrand

Bertrand.Canetti
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Re: RAO OC3Hywind Turbine

Postby Bertrand.Canetti » Wed Nov 27, 2019 8:44 am

Dear Jason,

I am slightly confused. Is the auto spectral density and the cross spectral density the equivalent of power spectral densities?

For example, would you confirm that:
- the wave power spectral density is the same as wave auto spectral density?
- the heave power spectral density is the same as the heave-wave cross spectral density?
- the RAO is equal to the auto spectral density divided by the cross spectral density?

Thank you for your help,
Best regards

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Wed Nov 27, 2019 10:05 am

Dear Bertrand,

The auto-spectral density is the same as the power-spectral density. Here are my answers to your questions:
- the wave power spectral density is the same as wave auto spectral density?

Yes.
- the heave power spectral density is the same as the heave-wave cross spectral density?

No. The heave power spectral density is the same as the heave auto-spectral density.
- the RAO is equal to the auto spectral density divided by the cross spectral density?

No, you have this swapped. The RAO is equal to the output-wave cross-spectral density divided by the wave auto-spectral density, where "output" is any motion and/or load output from the simulation. For example, the heave RAO is the heave-wave cross-spectral density divided by the wave auto-spectral density.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Bertrand.Canetti
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Re: RAO OC3Hywind Turbine

Postby Bertrand.Canetti » Thu Nov 28, 2019 1:34 am

Dear Jason,

Thank you for your answer.

Best regards
Bertrand

Ali.Nazokkar
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Re: RAO OC3Hywind Turbine

Postby Ali.Nazokkar » Tue Apr 13, 2021 12:03 am

Jason.Jonkman wrote:Dear Yingyi

I only took a brief look at your script; it looks like pxx and pxy are different sizes; once you fix that and make them the same size, you would need to use a element-by-element divide (./) rather than a matrix divide (/).

I don't have the time to update your script for your. However, I've attached a MATLAB script for calculating RAOs that my NREL colleague Amy Robertson developed. This script calculates the auto- and cross-spectral densities directly using FFT (similar to my script above). Perhaps you and others will find it useful.

Best regards,


Dear Dr.Jonkman

I used the attached code file to reproduce the RAO of the OC4 semi-submersible wind turbine, but I have two questions about the code.

my first question : the code can estimate the peak frequencies of my system nearly well, but the problem is the vertical axis of the RAOs which is extensively high in magnitude. I will attach the pictures of the RAOs of the surge, heave, and pitch motions of the platform and I wish to get some advices from you to fix this issue.(I assumed nens=1)

my second question : The code gives me some errors in calculation of the UU_psd when I assume nens as 4 or 6. I don't know why this happens!

Best regards.
Attachments
RAO_Surge.jpg
RAO_Surge.jpg (17.11 KiB) Viewed 188 times
RAO_Pitch.jpg
RAO_Pitch.jpg (16.55 KiB) Viewed 188 times
RAO_Heave.jpg
RAO_Heave.jpg (15.75 KiB) Viewed 188 times

Amy.Robertson
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Re: RAO OC3Hywind Turbine

Postby Amy.Robertson » Tue Apr 13, 2021 12:43 pm

Hello Ali.Nazokkar,

In regard to your question, I wrote the original RAO function. I'm assuming your questions are related to the RAO.m file that Jason shared in the past? I think the easiest way to check the script is to put in a simple dummy problem, such as a sine wave input/output. I did that just now and the magnitude of the RAO looked correct. I would caution to use inputs that are columns rather than row vectors to avoid issues.

For your second question, I have not gotten errors as you are suggesting. But, again, it may be an issue if you are using a row rather than a column vector for your time, input, and output signals. Use all column vectors (nx1).

Best regards,
Amy Robertson

Ali.Nazokkar
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Re: RAO OC3Hywind Turbine

Postby Ali.Nazokkar » Wed Apr 14, 2021 7:18 am

Dear Amy

I really appreciate your fast reply and consideration. You were right and I used the attached code file for the RAO calculation. once again I checked the code and by your suggestion my second problem is now fixed. However, there is something which I'm confused about. As I change the nens to higher numbers (e.g. 6,15,30), the magnitude of the RAO decreases and looks more realistic and I don't know why this happens! the optimum number for nens is also different in the heave and pitch motions!

I will attach some pictures of the surge motion RAO with nens=6,30. I should mention that I used only one ensemble as input and output; and my input(water elevation) is generated from the JONSWAP spectrum.

I'm so thankful for your consideration and I will attach the data file of my input and output time history in case if you want to check.

best regards
Ali.Nazokkar
Attachments
input-output file.xlsx
(248.38 KiB) Downloaded 8 times
nens=30.jpg
nens=30.jpg (15.77 KiB) Viewed 166 times
nens=6.jpg
nens=6.jpg (16.22 KiB) Viewed 166 times

Amy.Robertson
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Re: RAO OC3Hywind Turbine

Postby Amy.Robertson » Wed Apr 14, 2021 8:05 am

Dear Ali.Nazokkar,

The number of ensembles is how many segments you are breaking the signal up into. If you have a signal that is 10,000 points and you use 10 ensembles, your RAO will be calculated using 1000 pts (averaging 10 ensembles of 1000 pts each). Ensemble averaging will lead to a converged solution as it eliminates noise. However, you can take it too far and start removing the important information.

The length of the signal will dictate the resolution of your frequency information. If you have a signal of 10,000 pts and a time step of 0.01 seconds, you have 100 seconds of time and you can therefore resolve frequencies to 0.01 Hz. If you do 10 ensembles and reduce the length to 1000 pts, you now have a frequency resolution of 0.1 Hz. So, you can see how you start to smear out important information if your frequency resolution is so coarse it doesn't resolve the peaks of interest.

In the end, you should look at the frequency resolution you need to dictate the number of ensemble averages. You can also do what you did and see how many ensembles takes you to an RAO that is converged without smearing out important information.

Best regards,
Amy


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