Platform movements in still water

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Lorenzo.Cottura
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Platform movements in still water

Postby Lorenzo.Cottura » Wed Jul 15, 2020 10:07 am

Dear Jason,
I'm starting to test the hydrodynamic model of FAST to understand how it works.
I ran test 24 in still water, putting AereoDyn off. However, I note considerable displacements in the 6 dofs that I don't expect (Figure 1).
What is the cause of these movements?
Best regards,
Lorenzo.
Attachments
Figure1.png
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Jason.Jonkman
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Re: Platform movements in still water

Postby Jason.Jonkman » Wed Jul 15, 2020 11:47 am

Dear Lorenzo,

What you are seeing is a small transient that results because the platform initial conditions were set to zero, whereas the static-equilibrium displacement is slightly different from zero, e.g., the system wants to pitch/surge a bit due to the overhanging weight of the rotor-nacelle assembly. You can minimize the start-up transient by setting the initial conditions of the platform DOFs to their expected values in still water.

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

Lorenzo.Cottura
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Organization: Politecnico di Torino
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Re: Platform movements in still water

Postby Lorenzo.Cottura » Wed Jul 15, 2020 1:35 pm

Ok, thanks for your reply.
I was wondering if there exists among the available outputs an output that represents the loads results in the reference frame (0,0,0), i.e. the loads in the 6 dof that divided by the mass matrix M give the accelerations.
Best regards,
Lorenzo.

Jason.Jonkman
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Re: Platform movements in still water

Postby Jason.Jonkman » Wed Jul 15, 2020 3:41 pm

Hi Lorenzo,

No, such an output is not available. Such an output would have to include load contributions from everything (aerodynamics, hydrodynamics, moorings, gravity, etc.). Of course, in reality there is not just a 6x6 mass matrix because the mass matrix includes elements from all structural DOFs; also the mass matrix will depend on structural displacements.

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

Ramya.Niranjan
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Organization: CSIR-SERC
Location: India

Re: Platform movements in still water

Postby Ramya.Niranjan » Mon Jul 27, 2020 6:07 am

Dear All,
I am interested in running the design load cases for IEA-15-240-RWT-UMaineSemi in OpenFAST using the files shared in https://github.com/IEAWindTask37/IEA-15-240-RWT/tree/master/OpenFAST/IEA-15-240-RWT-UMaineSemi. I am facing the following issues running the DLCs 1.2, 1.3, 2.1 and 3.1. I have posted these issues in Github and looking for some help here also.

Q1. I noticed that the input data file has the Drivetrain rotational-flexibility DOF (DrTrDOF) and Yaw DOF disabled. Is there any specific reason for not enabling them?

Q2. While running the DLC 1.2 case with DrTrDOF enabled, the simulation is terminating with “NaN state detected” and FATAL ERROR messages (see the screenshot). I want to understand what leads to this error.
Error1.png
Error1.png (74.1 KiB) Viewed 211 times


Q3. When DrTrDOF is disabled, the simulation for DLC 1.2, 1.3 and 2.1 runs but the results for the mean values of the displacement and rotations are not close to zero (see the below screenshots). Is there anything wrong with my input parameters?
DLC_result_plots.png
DLC_result_plots.png (129.23 KiB) Viewed 211 times


Q4. For DLC 3.1, with regular wave (Hs=6m, Ts=10sec) and DrTrDOF=False, the simulation terminates with a fatal error (see the screenshot). However, when I enable DrTrDOF the simulation terminates with a different an unknown errID output. How could I resolve these two issues? What input parameters could lead to this behaviour?
Error.png
Error.png (219.95 KiB) Viewed 211 times


Could you please help me in resolving the above issues?

Thanks and Regards
Ramya

Jason.Jonkman
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Re: Platform movements in still water

Postby Jason.Jonkman » Thu Jul 30, 2020 9:27 pm

Dear Ramya,

Here are my answers to your questions:

Q1. The IEA Wind 15-MW reference wind turbine has a direct drive (GBRatio = 1), with a shaft that is short and a stiffness that is high. So, there is no reason to enable the drivetrain-torsion DOF.

Q2. My guess is enabling the drivetrain DOF results in a very high natural frequency that would require a very small time step to otherwise avoid a numerical instability in the solution.

Q3. As with any floating wind turbine simulation, to avoid long start-up transients, I would set the the initial conditions of the model appropriate for the mean wind speed being simulated. Proper initial conditions should be set for the rotor speed and blade-pitch angles, as well as the platform surge and platform pitch DOFs based on their expected mean values for the mean wind speed simulated.

Q4. Again, there is no need to run this model with the drivetrain-torsion DOF enabled. I'm not sure what is going on with the simulation with the drivetrain-torsion DOF disabled, but the solution appears to be suffering a numerical or physical instability. Similar problems have been discussed many times on this forum. Did you change the system masses, stiffness, or time step?

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

Ramya.Niranjan
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Organization: CSIR-SERC
Location: India

Re: Platform movements in still water

Postby Ramya.Niranjan » Mon Aug 03, 2020 3:06 am

Dear Jason,

Thanks for the reply. I have few follow-up questions.

Q1. As you advised I have tried with different initial conditions. For the static stability case and free decay test cases, the surge displacement and pitch rotations have a mean value of 0.4 m and (-)1.5 deg. However, I was expecting a mean value of zero for all platform displacements and rotation. The same mean value is getting reflected in other DLCs such as 2.1 and 3.1. Does this happen due to the stability of the substructure?

Q2. I have not changed the mass and the stiffness of the system for DLC 3.1. I have tried with time step values of 0.05, 0.0125 and 0.001 sec. The error (shown in snapshot 1 below) is happening for time step 0.001 and 0.0125 sec and error (shown in snapshot 2 below) is happening for time step of 0.05 sec. All the above errors are occurring when Yaw DOF is enabled and Drivetrain rotational-flexibility DOF is disabled. If both Yaw DOF and Drivetrain DOFs are disabled simulations are running fine. Is there any problem with the Yaw DOF?

Time step_0.001 and 0.0125 sec.png
Time step_0.001 and 0.0125 sec.png (17.54 KiB) Viewed 158 times

Time step_0.05 sec.png
Time step_0.05 sec.png (74.68 KiB) Viewed 158 times


Could you please help me in resolving the above issues?

Regards
Ramya

Jason.Jonkman
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Re: Platform movements in still water

Postby Jason.Jonkman » Mon Aug 03, 2020 8:46 am

Dear Ramya,

Here are my answers to your questions:

Q1. I would not expect mean platform displacements of zero in the absence of external wind and wave loading. The static-equilibrium displacement is slightly different from zero because the system wants to pitch/surge a bit due to the overhanging weight of the rotor-nacelle assembly.

Q2. I did not develop the IEA Wind 15-MW reference turbine, but it looks like the default model has the nacelle-yaw DOF disabled. If you enable the nacelle-yaw DOF, you better ensure that the yaw spring and yaw damper are set appropriately in ServoDyn (I can't confirm that they are). Receiving an "Unknown ErrID" sounds like a minor bug in the error handling; which version of OpenFAST are you using?

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

Ramya.Niranjan
Posts: 4
Joined: Wed Jul 22, 2020 5:03 am
Organization: CSIR-SERC
Location: India

Re: Platform movements in still water

Postby Ramya.Niranjan » Tue Aug 04, 2020 10:19 am

Dear Jason,

Thanks for your reply. I am using the OpenFAST development version (v2.3.0) released in April 2020. Just a clarification related to your previous reply. If I understood you right, does it mean the mean platform displacements and rotations will be zero in the case with the external wind and wave loads?

Regards
Ramya

Jason.Jonkman
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Re: Platform movements in still water

Postby Jason.Jonkman » Tue Aug 04, 2020 2:28 pm

Dear Ramya,

No. The mean platform surge displacement and pitch rotation will not be zero, even in the absence of external wind and wave loading.

Regarding the "Unknown ErrID", it sounds like there is a minor bug in the error handling, even in the current master branch of OpenFAST. It will likely take someone running your model through the debugger to identify the source of the bug. Perhaps you want to do this yourself? Otherwise, I would suggest posting this error as an issue on the OpenFAST github page: https://github.com/OpenFAST/openfast/issues (include your input files).

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

Ramya.Niranjan
Posts: 4
Joined: Wed Jul 22, 2020 5:03 am
Organization: CSIR-SERC
Location: India

Re: Platform movements in still water

Postby Ramya.Niranjan » Wed Aug 05, 2020 11:00 am

Thanks Jason. As advised by you I will post these issues in OpenFAST Github page.

Regards
Ramya


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