FAST

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Jason.Jonkman
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Re: FAST

Postby Jason.Jonkman » Wed Jan 09, 2019 8:26 pm

Dear Mitesh,

Indeed, your solution may not be numerically unstable because of the use of a large time step, but clearly something is causing large platform displacements. What do the plots of platform displacement look like? Can you isolate the problem to a specific degree of freedom? I suggest simplifying the model and increasing complexity in steps to debug the problem.

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

Mitesh.Ramani
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Joined: Sat Oct 27, 2018 9:10 am
Organization: university of rostock
Location: Germany
Location: Rostock,Germany
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Re: FAST

Postby Mitesh.Ramani » Thu Jan 10, 2019 8:21 pm

Dear jason,

Thank you so much for you time and you support.

I can understand that problem due to large platform displacements.Also I have isolated the problem to a specific degree of freedom.That is happened due to Platform pitch tilt rotation degree of freedom If I disable(False) the platform pitch degree of freedom then simulation is running completely fine till the end.If I enable(True) platform pitch degree of freedom then it is showing the same error message.Picture of that error message I have attached below. Also I have cross check with other degree of freedom for example Platform pitch is enable (True) and any else is disable(False) then it's showing the same error message.So as I think this problem is only due to Platform pitch tilt rotation degree of freedom.

As you told me debug the problem by simplifying the model and increasing complexity in steps Also I have tried that but only Problem happened when I enable the Platform pitch tilt rotation degree of freedom.I have taken the initial conditions of transnational and rotational displacement of platform in ElasoDYn file from one simulation in that I have set all the initial condition to zero then from WindChart I got the mean value of the transnational and rotational displacement of platform.

I have attached the plots of platform displacement as below.In that folder pitch motion only I got when I disable(False) the yaw rotational displacement of platform(In the middle of that simulation I got the small angle assumption error but simulation was completed).I would like to conclude with that if I disable(False) the pitch tilt rotation degree of freedom then I could not get that error from start to end of the simulation.

I hope you can clearly understand my problem and explanation so could you please guide how to deal with this problem or which parameter should be responsible for this.

I really appreciate for your valuable time and your support.

Best Regards
Mitesh Ramani
Attachments
Platform Motion.zip
plots of platform displacement
(1.39 MiB) Downloaded 2 times
Fast aborting.PNG
Fast aborting when I enable the pitch DOF
Fast aborting.PNG (44.98 KiB) Viewed 213 times

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: FAST

Postby Jason.Jonkman » Fri Jan 11, 2019 8:21 am

Dear Mitesh,

From your platform time series, I don't see any signs of an instability, but the platform-pitch motion is very large (over 24 degrees). Are you expecting such large platform-pitch motion from your floating system design? If not, I would guess that there is a problem related to the platform-pitch restoring in your simulation set-up e.g. the mooring stiffness or hydrostatic stiffness are too low. Is the platform-pitch natural frequency predicted by FAST lower than you are expecting?

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

Mitesh.Ramani
Posts: 27
Joined: Sat Oct 27, 2018 9:10 am
Organization: university of rostock
Location: Germany
Location: Rostock,Germany
Contact:

Re: FAST

Postby Mitesh.Ramani » Mon Jan 14, 2019 10:06 am

Dear jason,

Thank you so much for you time and you support.

1) Yes,the platform-pitch motion is very large (over 24 degrees).NO I am not expecting such large platform-pitch motion from my floating system design.Because NTNU thesis report which I am using(same 10MW spar-buoy) right now,In that platform-pitch motion mean value is 2.85 deg. and maximum value is 11.32 deg.So as per my point of view problem is in the platform-pitch restoring.I have calculated Hydrostatic restoring in pitch and roll as per given equation in the HydroDyn manual((c44/c55 = -8159587654 N/m(ρ*g*A/4*R^2+ρ*g*v*zb))

where,ρ(Density of water)=1025 kg/m^3, g(Gravity)=9.80655, A(Water plan area)= (π)/4*8.3^2(where 8.3 is the diameter of spar at the water plan), R(Radius at the water plan)=)8.3/2, V( Submerged Volume )=13085.41480(As per HydroDyn output file), Zb(Center of buoyancy)=62.05271(I have manually calculated for my system).

But in the NTNU thesis report they have used another equation to define the hydrostatic restoring in roll and pitch. I have attached the picture of that equation as below.The value of that non zero term as per NTNU report is c44/c55=1690150000 N/m.

If I am using platform Hydrostatic restoring in pitch value(C55) = 1690150000 N/m (AS per NTNU report) or C55=0 N/m in Additional linear stiffness matrix then my Simulation running completely but starting of that simulation I am getting error of "Small angle assumption"(Only getting this error equal or grater than 17 m/s wind speed ). Also I have tried to solve by using very small time steps in OpenFAST but it does not work.I have also attached the picture of that error as below.

I would like to mention that for the (C55) = 1690150000 N/m (AS per NTNU report) and C55=0 N/m my system is in the hydrostatic equilibrium condition wihtout wind and waves because from the platform motion chart.

Question: I have seen in many other floating system HydroDYn file that they have not used pitch and roll hydrostatics stiffness value so It could be possible to run the simulation without those values??? Can you please explain me a bit why It should be possible without those values ??

What could be the reason for the error "small angle assumption" at the starting of the simulation for only 17 to 25 m/s wind speed?

2) I can understand that the mooring stiffness or hydrostatic stiffness are too low. Also I have tried to solved by increasing the value of the mooring stiffness but It could not work. Currently I have used the equivalent mooring line extensional stiffness = 384243000.I have attached MoorDyn file as below.

The platform-pitch natural frequency predicted by FAST is higher(It is around 60 s) than I am expecting while using the c44/c55 = -8159587654 N/m. But The platform-pitch natural frequency predicted by FAST is lower (It is around 25 s) than I am expecting while using the c44/c55=1690150000 N/m (AS per NTNU report) and C55=0 N/m.Because as per NTNU thesis report it should be around 35 s.

Question:Is should be okey to use the platform Hydrostatic restoring in pitch value(C55) = 0.?
From the error " small angle assumption " at the starting of the simulation, does it really effect my output data?

I hope you can clearly understand my explanation and will help me out as soon as possible.

I really appreciate for your valuable time and your support.

Best Regards
Mitesh Ramani
Attachments
Stiffness matrix non-zero term value equation as per NTNU report.PNG
Stiffness matrix non-zero term value equation as per NTNU report.PNG (13.56 KiB) Viewed 139 times
Error at the starting of simulation only from 17 to 25 wind speed .PNG
Error at the starting of simulation only from 17 to 25 wind speed .PNG (9.56 KiB) Viewed 139 times
MoorDyn.rtf
(2.84 KiB) Downloaded 1 time

Jason.Jonkman
Posts: 3857
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: FAST

Postby Jason.Jonkman » Wed Jan 16, 2019 1:54 pm

Dear Mitesh,

How are you modeling the hydrodynamics of this floating system...using strip theory, using potential-flow-theory, or a hybrid combination of the two?

If you are using strip theory, the hydrostatic restoring is not included in the implementation. In this case, you should set AddCLin as described in section 6.8.3 of the draft HydroDyn User's Guide and Theory Manual: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf and C44/C55 should be negative-valued for a deep-drafted floater.

If you using potential-flow theory, the hydrostatic restoring should already be included in the *.hst WAMIT output file that is used within HydroDyn, so, you may set AddCLin = 0.

If you using a hybrid combination of the two, presumably you would only be modeling the viscous effects in strip theory and the radiation, diffraction, and hydrostatic effects in potential-flow, so again, AddCLin = 0 (because it is already included in the *hst file).

The equations from the NTNU report you attached include the gravitational restoring associated with the physical mass of the wind turbine. You should not include this term in the HydroDyn module because this term is already accounted for in the ElastoDyn module.

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


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