Customizing floating offshore wind turbine

Wind & Water Computer-Aided Engineering Software Tools
1

Dear Jason

 I am trying to model a floating offshore wind turbine in FAST.v8.  My tower and turbine are the same as OC4 DeepC semi-submersible wind turbine. The top of my platform is 15m above the still water level. So, I have to move the tower and turbine 5m along the +z axis. I changed the TowerHt and TowerBsHt in ElastoDyn input file (with 92.6 and 15 respectively) and the TweElev in AeroDyn (v15) input file (add 5 to all the TwrElev values). The ServoDyn input file is also the same as OC4. 

 1) When I run FAST v8 with stiil air (CompElast=1, CompInflow=0,CompAero=0,CompServo=1), the FAST can be run normally. When I run FAST v8 with still air (CompElast=2, CompInflow=0,CompAero=0,CompServo=1), the Fast is aborted with the following error information:

FAST_Solution:FAST_AdvanceStates:BD_GA2:BD_DynamicSolutionGA2:Solution does not converge after
the maximum number of iterations
FAST_AdvanceStates:BD_GA2:BD_DynamicSolutionGA2:Solution does not converge after the maximum
number of iterations
FAST_AdvanceStates:BD_GA2:BD_DynamicSolutionGA2:Solution does not converge after the maximum
number of iterations

2) When I run FAST v8 with steady wind (CompElast=1, CompInflow=1,CompAero=2,CompServo=1), the FAST can be run normally. When I run FAST v8 with steady wind (CompElast=2, CompInflow=1,CompAero=2,CompServo=1), the FAST is aborted with the following error information:

[b]FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 5, blade
1):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating
conditions! Vx = 1.3574, Vy = -20.182, rlocal = 11.739, theta = 0.21597

FAST encountered an error at simulation time 7.50000E-02 of 300 seconds.
Simulation error level: FATAL ERROR[/b]

3)When I run FAST v8 with turbulence wind (CompElast=1, CompInflow=1,CompAero=2,CompServo=1), the FAST is aborted with the following error information:(the wind file is produced by TurbSim, the HubHt=95,and the GridHeight*GridWidth=170*170)

[b]Timestep: 66 of 300 seconds. Estimated final completion at 22:10:09.

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 6, blade
2):BEMT_UnCoupledSolve:DeterminePhiBounds:There is no valid value of phi for these operating
conditions! Vx = -0.62872, Vy = 17.29, rlocal = 15.844, theta = 0.20038

FAST encountered an error at simulation time 66.263 of 300 seconds.
Simulation error level: FATAL ERROR[/b]

What’s wrong with my model? Should I change something else? Please give me some suggestions!

Sincerely
2

Dear Yu Lei,

I see at least two problems based on the information you’ve provided:

  1. “Something” is triggering an error regarding “no valid value of phi”. There were issues in AeroDyn v15.03 within FAST v8.16 that led to this error in some cases. However, these issues have been fixed in AeroDyn v15.04 and newer, including in the release of OpenFAST v1.0. I suggest you upgrade to OpenFAST v1.0: nwtc.nrel.gov/OpenFAST.

  2. The FAST solution with BeamDyn does not appear to converge. Did you form this FAST v8 model by combining the Test25 and Test26 input files from the FAST v8.16 CertTest into a single model? I have not tried this myself, but you may need to play around with the solver parameters (e.g. DT) to get such a model to run without numerical problems.

Best regards,

3

Dear Jason

   Thanks for your reply. But I'm still confused about something.
   Yeah, I form this model  by combining the Test25 and Test26 input files from the FAST v8 CertTest into a single model. The BeamDyn input files are from the TEST26, other files are adapted from Test25. 
  So, the method I used to bulid the model is right？I mean, is it successful to move the tower and turbine 5m along the +z axis by changing TowerHt and TowerBsHt in ElastoDyn input file (with 92.6 and 15 respectively) and the TweElev in AeroDyn (v15) input file (add 5 to all the TwrElev values) ?
  As far as I know, maybe I should change the tower modes in the ElastoDyn tower input files. Because in FAST v8 CretTest folder, the ElastoDyn tower input files are different from each other (tower mode shapes are different).  How about the ElastoDyn blade input files? Should I change it? 
 
  Best regards
4

Dear Yu Lei,

Here are my answers to your questions:

Yes, that all sounds fine.

Yes, even though the distributed mass, stiffness, and length of the tower did not change (you only shifted the tower upward 5 m), the tower-base boundary conditions changed, and so, the tower mode shapes will be affected.

The ElastoDyn blade files are unused if you are using BeamDyn to model the blades.

Best regards,

5

Dear Jason

 If I don't use the BeamDyn, should I change the ElastoDyn blade input file? 

 Best regards
6

Dear Yu Lei,

You don’t need to change the ElastoDyn blade input file if all you want to do is shift the tower upward.

Best regards,

7

Dear Jason
Thanks for your elaborate explanation.

Best regards
8

Dear Jason
I have model my own flaoting offshore wind turbine (only the substructure is different from OC3Hywind) under two different load cases:
LC2: U10 15.2m/s, Hs 4.71m, Tp 11.86s, the wind turbine is under power generation state;
LC3: U10 22.3m/s, Hs=6.94m, Tp 13.81s, the wind turbine is parked but the blade-pitch angle is 0 deg for the most unfavorable condition
When I anaysed the results of TwrBsMyt, a FFT was used to obtain the power spectrum as shown in the attachment. In this figure, the power spetrum has four responding peaks under load case LC2. The first three peaks is the pitch natural frequency (about 0.034 Hz), wave frequency (about 0.077 Hz)and the first tower fore-aft natural frequency (about 0.23 Hz) respectively. But I am not clear about what does the fourth peak (around 0.4~0.5Hz) mean. Could you tell me what can induce the fourth peak according to your experience ? Thanks!

Best regards
extreme load case.PNG

9

Dear Yu.Lei,

I’m not sure what you changed relative to the OC3-Hywind turbine, but the first tower fore-aft and side-to-side bending natural frequencies for the OC3-Hywind turbine are closer to 0.45 Hz (this is higher than the land-based tower due to the platform degrees of freedom). The OC3-Hywind turbine does not have a natural frequency around 0.25 Hz, so, I’m not sure what frequency that corresponds to.

Best regards,

10

Dear Jason
Thanks for your prompt. The tower and turbine properties in my model are the same as that of OC3Hywind, except that my tower base is 15m above the still water level. While the floating platform is my own. I guess that the peak around 0.22Hz is induced by waves（WvLowCOff and WvHiCOff in my model is 0.314159 and 1.570796 respectively). Is it possible?

Best regards

11

Dear Yu.Lei,

I would not expect the cut-off wave frequencies themselves to excite the structural response.

Best regards,

12

Dear Jason

Thanks a lot for your timely reply! I need to do more work to make this problem clear.

Best regards