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 » Fri Mar 09, 2018 7:01 am

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,
Attachments
RAO.m.txt
RAO
(2.44 KiB) Downloaded 174 times
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

Yingyi.Liu
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Re: RAO OC3Hywind Turbine

Postby Yingyi.Liu » Sat Mar 10, 2018 1:57 am

Dear Jason,

Thanks. That really helps.

Two further questions regarding this issue:
(1) In the RAO.m script, it seems that the input signals are from multiple files, indicated by the function explanations. One of the input parameter "nens = number of ensembles to average (i.e., 4 or 6)" also confirms this point. But I don't see where we can input the multiple signals since the inputs "time","U", "Y" are all 1-D arrays. Should the multiple signals be combined into one signal before using the RAO.m script? By the way, what does "ensemble" mean?

(2) In Amy's publication (https://www.nrel.gov/docs/fy14osti/61154.pdf), Table 3 defines the load cases run in OC4 Phase II. For the LC 3.7 (RAO estimation with wind), the wave condition is Banded white noise with PSD =1 m2/Hz for 0.05-0.25 Hz. My question is, where can we set the three parameters (PSD =1 m2/Hz for 0.05-0.25 Hz) in the HydroDyn input file?

Thanks a lot.
Best Regards,
Yingyi

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Sat Mar 10, 2018 10:43 am

Dear Yingyi Liu,

Here are my answers to your questions:

(1) Correct, you should concatenate multiple simulation outputs into a single 1-D array. You can think of "ensemble" as the mean of the outputs.

(2) Set WaveMod = 3, WvLowCOff = 0.05*2*pi rad/s, WvHiCOff= 0.25*2*pi rad/s, and WaveHs = SQRT( 1*8*(0.25 - 0.05) ) m.

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

Yingyi.Liu
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Re: RAO OC3Hywind Turbine

Postby Yingyi.Liu » Mon Mar 12, 2018 9:13 am

Dear Jason,

Your answers are very helpful, now the script works.

I run a simulation of LC 3.7 and find three more questions:

1. Can WaveSeed(1) and WaveSeed(2) be set as any arbitrary numbers? I'm using 4 ensembles for one calculation, and I set the corresponding WaveSeeds as
123456789 and 1011121314 (in Ensemble 1)
378281615 and 1736193745 (in Ensemble 2)
461917361 and 1793479262 (in Ensemble 3)
635438273 and 1308917163 (in Ensemble 4)
Are there any mistakes in the above WaveSeeds I set?

2. In the lower-frequency region (0.05-0.06Hz), the attached RAO of Pitch (post-process of 4600s simulation output data, no data discarded) is a little bit noisy, is it a common issue or does it mean that my calculation is not so good? Is it necessary to discard the first 1000s or more time-series data because of the transient effect (having not reached a stable status)?

RAOPitch.png
RAOPitch.png (66.67 KiB) Viewed 4304 times


3. I find the FAST calculation stops inconceivably at the ending of ChkptTime which is specified in the FAST primary input file (see the screenshot below). Do you know why it happens?

StackOverFlows.png
StackOverFlows.png (38.93 KiB) Viewed 4304 times

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Mon Mar 12, 2018 9:42 am

Dear Yingyi Liu,

Here are my answers to your questions:
1. Yes, the WaveSeeds can be set arbitrarily.

2. Yes, white-noise-generated RAOs can be a bit noisy. Averaging over several time series will help. Yes, you should eliminate start-up transients from post-processing. But 1000 s sounds like a long time. Section 6.8.2 of the HydroDyn User's Guide and Theory manual provides some guidance for choosing proper initial conditions to minimize start-up transients for floating wind systems: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

3. It appears that the stack size is not large enough for generating checkpoint files with the long simulation with wind and waves you are running. Do you need to produce checkpoint files? If not, I suggest that you set ChkptTime > TMax.

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

Yingyi.Liu
Posts: 101
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Organization: Kyushu University
Location: Japan

Re: RAO OC3Hywind Turbine

Postby Yingyi.Liu » Mon Mar 26, 2018 10:59 am

Dear Jason,

Regarding the OC4 Phase II project, which has also been summarized in Amy's publication (https://www.nrel.gov/docs/fy14osti/61154.pdf), I have two questions to confirm:

1. Were the second-order wave kinematics/platform forces considered or not in the listed Load Cases?
2. Were the initial platform displacements all zeroes? If not, what are these values in different Load Cases?

Thanks a lot.
Best regards,
Yingyi

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Fri Mar 30, 2018 3:42 pm

Dear Yingyi,

I asked Amy Robertson of NREL, and this was her response:

1. Contrary to what is stated in the paper, there does not appear to be 2nd order wave forces or kinematics included in the NREL FAST simulations in this paper.
2. No initial conditions were specified.

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

Yingyi.Liu
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Joined: Thu May 01, 2014 3:16 am
Organization: Kyushu University
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Re: RAO OC3Hywind Turbine

Postby Yingyi.Liu » Mon Apr 02, 2018 3:16 am

Dear Jason,

Thanks a lot. Now I understand the answers.

Best regards,
Yingyi

Yajun.Ren
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Organization: The University of Edinburgh
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Re: RAO OC3Hywind Turbine

Postby Yajun.Ren » Wed Nov 06, 2019 2:31 pm

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 Jason,

I used the matlab code you attached here to reproduce the RAOs. I found the RAO in heave exactly matches the result given by the paper Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines. However the amplitude of surge and pitch is about half of your results. I attached the figures for surge and pitch. Would you have any suggestions to fix this problem? Thanks.
Attachments
pitch_rao.jpg
pitch_rao.jpg (49.71 KiB) Viewed 1385 times
surge_rao.jpg
surge_rao.jpg (58.23 KiB) Viewed 1385 times

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Thu Nov 07, 2019 10:44 am

Dear Yajun.

Hmm; I'm not sure. For a given regular wave input, does the amplitude of the response match your RAO at the associated wave frequency? Could there be some confusion between wave height versus wave amplitude (the height is twice the amplitude for linear regular waves)?

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

Yajun.Ren
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Organization: The University of Edinburgh
Location: UK

Re: RAO OC3Hywind Turbine

Postby Yajun.Ren » Thu Nov 14, 2019 3:46 am

Dear Jason,

I realised the reason I got the above results is that I disabled all the DOFs except for surge,sway,heave,roll,pitch and yaw in ElastoDyn. The part of input file was set as:
---------------------- DEGREES OF FREEDOM --------------------------------------
False FlapDOF1 - First flapwise blade mode DOF (flag)
False FlapDOF2 - Second flapwise blade mode DOF (flag)
False EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
False DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
False GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
False TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
False TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
False TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)

When I enabled all the DOFs except for TeetDOF, I got the same results in the published paper. Now my question is, when I use a 0.5s DT, the simulation runs as normal when the DOFs are disabled as shown above, but aborts when enabling all the DOF. In other words, the simulation aborts when using a time step larger than 0.01s with all the DOFs being enabled.

Is that possible if I would like to use a 0.5s time step with all the DOFs enabled by modifying some of the input settings?

I appreciate your kind help.

Best regards,
Yajun

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Thu Nov 14, 2019 6:34 am

Dear Yajun,

The required time step is intrinsically related to the structural natural frequencies that you are modeling. By disabling all DOFs except for the platform, the highest natural frequency is quite low, allowing for a large time step. Enabling DOFs introduces higher natural frequencies into the model, requiring smaller time steps. My rule of thumb for selecting the time step for the structural module is given in the following forum topic: viewtopic.php?f=4&t=351&p=1025.

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

Yajun.Ren
Posts: 35
Joined: Mon Jan 21, 2019 6:54 am
Organization: The University of Edinburgh
Location: UK

Re: RAO OC3Hywind Turbine

Postby Yajun.Ren » Mon Nov 18, 2019 4:04 am

Dear Jason,

Thanks for your reply, now I am clear with the time step.

My additional question is that, in the paper 'Investigation of Response Amplitude Operators for Floating Offshore Wind Turbines', the turbine is modelled as both 'flexible' and 'rigid' turbine, is the difference between these two type of model related to the above DOFs we discussed?

Best regards,
Yajun

Jason.Jonkman
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Re: RAO OC3Hywind Turbine

Postby Jason.Jonkman » Mon Nov 18, 2019 9:15 am

Yes.
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 10:34 am

Jason.Jonkman wrote:Dear Yingyi Liu,

Here are my answers to your questions:

(1) Correct, you should concatenate multiple simulation outputs into a single 1-D array. You can think of "ensemble" as the mean of the outputs.

(2) Set WaveMod = 3, WvLowCOff = 0.05*2*pi rad/s, WvHiCOff= 0.25*2*pi rad/s, and WaveHs = SQRT( 1*8*(0.25 - 0.05) ) m.

Best regards,


Dear Jason,

I did not see in the HydroDyn manual the useful formula you gave for the white noise wave height: WaveHs = SQRT( 1*8*(0.25 - 0.05) ) m

I'm guessing some parts, but I do not fully understand it and where does it come from. Could your explain it please?

Thank you again for your help,

Best regards,
Bertrand


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