Modelling of a 10MW FOWT

Discuss theory and modeling of wind-turbine structures.

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Yajun.Ren
Posts: 30
Joined: Mon Jan 21, 2019 6:54 am
Organization: The University of Edinburgh
Location: UK

Modelling of a 10MW FOWT

Postby Yajun.Ren » Mon May 18, 2020 10:31 am

Dear all,

I modelled a 10 MW wind turbine using the FAST input file given in https://rwt.windenergy.dtu.dk/natalia.c ... FAST_v1.00 and coupled it with a modified spar platform based on the OC3 hywind spar model. The hydrodynamic coefficients are obtained by WAMIT.

The results, especially the motions in 6 DOF seem very weird (please find attached files). The time series motions are more like free-decay shaped curve. In addition, the curves are not sine shaped, but square wave shaped.

I checked the input files and have no idea what I should do to correct the model, could you please give any suggests on this?

Regards,
Yajun
Attachments
10MW SPAR MOTIONS.zip
(55.17 KiB) Downloaded 4 times

Jason.Jonkman
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Re: Modelling of a 10MW FOWT

Postby Jason.Jonkman » Mon May 18, 2020 3:52 pm

Dear Yajun,

I've not used this FAST model myself, but are you not expecting free-decay behavior based on your simulation set up?

The "square wave shape" is likely the result of too little precision in the output file. Are you using a text file format (OutFileFmt = 1) with the default "ES10.3E2" OutFmt, which is limited to 4 digits of precision? You could increase this to, e.g., 6 digits of precision (changing OutFmt to "ES12.5E2"), or output in binary format (OutFileFmt = 2), which has full precision.

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: 30
Joined: Mon Jan 21, 2019 6:54 am
Organization: The University of Edinburgh
Location: UK

Re: Modelling of a 10MW FOWT

Postby Yajun.Ren » Tue May 19, 2020 3:31 am

Dear Jason,

Thanks for your reply.

No the structure was supposed to stay steady as I disabled AeroDyn, set WaveMod = 0 and the simulation started with the tubine at its undisplaced position. If it was a free decay, the phenomenon is not supposed to continue until the end of simulation (1000s). The initial heave motion is very large, which can be up to over -60 at the early stage of the simulation. Would you have any idea what can cause such a large heave motion? I tried to disabled the mooring system, the initial heave reduces about a half, but still shows the free-decay behavior.

According to your suggestion, I checked the output file in binary format and the curves look more reasonable.

Best regards,
Yajun

Yajun.Ren
Posts: 30
Joined: Mon Jan 21, 2019 6:54 am
Organization: The University of Edinburgh
Location: UK

Re: Modelling of a 10MW FOWT

Postby Yajun.Ren » Tue May 19, 2020 5:48 am

Dear Jason,

I would like to add two more questions:

I am not quite understand how you determine the value PtfmVol0 in HydroDyn. According to the manual, it is defined based on the result of WAMIT simulation. However when I checked the WAMIT output file published here https://rwt.windenergy.dtu.dk/natalia.c ... 0/Platform, it seems that the value of Volumes are not the same as the value of PtfmVol0 in the HydroDyn input file.

Secondly, I copied the relevant input files from https://rwt.windenergy.dtu.dk/dtu10mw/d ... FAST_v1.00 and try to reproduce the simulation of the DTU 10MW RWT with the NAUTILUS semi-submersible platform (static case), but the results also show a free decay behaviour (please find the attached file of surge and heave motions). This is hard to understand for me, would you suppose the results are reasonable? If not, what could the problem be?

Best regards,
Yajun
Attachments
heave_semi.jpg
heave_semi.jpg (45.34 KiB) Viewed 96 times
surge_semi.jpg
surge_semi.jpg (40.97 KiB) Viewed 96 times

Jason.Jonkman
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Re: Modelling of a 10MW FOWT

Postby Jason.Jonkman » Tue May 19, 2020 7:10 am

Dear Yajun,

Even in the absence of wind and wave excitation, i would expect a small amount of free-decay motion in every simulation, unless the initial conditions for all system displacements are set to their respective static-equilibrium values. For a floating wind turbine, there is often a small amount of surge and pitch free decay due to the overhanging weight of the rotor-nacelle assembly. I would not normally expect much initial free-decay behavior in heave if the floating system is set up as recommended, i.e., with the system weight balanced with the static buoyancy and vertical mooring pretension. In your results posted May 18, 2020, there is a large heave motion, which suggests the weight, static buoyancy, or moorings are not defined properly in the model. In your results posted May 19, 2020, there is a small heave motion, which suggests only a minor imbalance in the weight, buoyancy, and pretension.

I'm not familiar with the models you are using, but I would ask the original developer of that model for the reason in the discrepancy between PtfmVol0 and what is in the WAMIT output.

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

Abhinay.Goga
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Organization: Hochschule Flensburg
Location: Germany

Re: Modelling of a 10MW FOWT

Postby Abhinay.Goga » Thu May 21, 2020 4:34 am

Dear Yajun,

The free decay motions that you are experiencing could be because of the initial platform displacements given in the ElastoDyn. Try to verify static condition with all initial displacements zero. If the outputs of platform displacements are having non-zero values means the coupled structure is unbalanced due to buoyancy or some other reasons.

PtfmVol0 is the input for calculating initial center of buoyancy to balance the structure. Multiply that value with water density you will get displaced mass of water, which is supposed to be in-line with the coupled structure mass(Blades, RNA, Tower, Platform, Mooring lines). Could be the imbalance between these two that causes high heave.

Cheers


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