Applying time force series

Hello, I am trying to apply a prescribed force time series on a floating platform using Structural Control. I have setup the input file with the following settings:

---------------------- StC DEGREES OF FREEDOM ----------------------------------
4 StC_DOF_MODE - DOF mode (switch)

---------------------- StC LOCATION ------------------------------------------- [relative to the reference origin of component attached to]
0 StC_P_X - At rest X position of StC (m)
0 StC_P_Y - At rest Y position of StC (m)
0 StC_P_Z - At rest Z position of StC (m)

---------------------- PRESCRIBED TIME SERIES --------------------------------- [used only when StC_DOF_MODE=4]
1 PrescribedForcesCoord- Prescribed forces are in global or local coordinates (switch) {1: global; 2: local}
“TimeForceSeries.dat” PrescribedForcesFile - Time series force and moment (7 columns of time, FX, FY, FZ, MX, MY, MZ)

I defined the load time series in a separate file:

Time, Fx, Fy, Fz, Mx, My, Mz

(s) (N) (N) (N) (N-m) (N-m) (N-m)

0.0 0.0 0.0 0.0 0.0 0.0 0.0
300.0 0.0 0.0 0.0 0.0 0.0 0.0
400.0 0.0 0.0 0.0 0.0 1.0e9 0.0
600.0 0.0 0.0 0.0 0.0 1.0e9 0.0

It runs alright, however, I cannot see any impact of the displacements in 6 DOF. I also checked the ServoDyn summary file, but the Structural controls sections seems to be empty.

I would really appreciate any hints on how to impose a load time series on the platform.

Dear @Kasia.Patryniak,

Your StC input file looks OK to me. Have you set NumSStC = 1 in ServoDyn and set SStCfiles to the filename of your StC input file?

Best regards,

Dear @Jason.Jonkman,

Thank you very much for your quick response. Yes, I have set the parameters as suggested:

1 NumSStC - Number of substructure structural controllers (integer)
“NRELOffshrBsline5MW_StC.dat” SStCfiles - Name of the files for substructure structural controllers (quoted strings) [unused when NumSStC==0]

Anything else I should double check?

Kind regards

Dear @Kasia.Patryniak,

Can you clarify what you mean when you say?:

Have you also enabled the platform DOFs in ElastoDyn?

Best regards,

There is an output file with .sum extension:

In this file, I can see a section titled “Structural controls” - I assumed I should be able to see some output there, as I am using the Structural control module. I appears to be empty:

image

Yes, I have all 6 rigid DOFs enabled in ElastoDyn:

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)

I am getting plots as follows:

image
image

The platform seems to be free to move, and arrives at the steady state position after some initial period of oscillation. I would expect to see some “jump” in the signal after 400s (when the moment should be applied), but that is not the case. The moment applied is of the order of the aerodynamic moment so should have an effect.

Kind regards

Dear @Kasia.Patryniak,

Have you tried enabling the Echo option in the ServoDyn and StC input files to verify that the inputs are being processed correctly?

Best regards,

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Dear @Jason.Jonkman,

Thank you for this suggestion, indeed by requesting echo files I have noticed that the StC module has not been called. This was because I had the CompServo flag in the .fst file set to 0. I have managed to activate it and I can see impact on the displacements:

image

Thank you very much for your help.

Kind regards

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Dear @Jason.Jonkman,

One follow-up question if I may. I have specified a few structural control outputs in the ServoDyn input file, to record the displacements and velocities at the node where the load has been applied:

“SStC1_XQ”
“SStC1_XQD”
“SStC1_YQ”
“SStC1_YQD”
“SStC1_ZQ”
“SStC1_ZQD”
“SStC1_Myi”

The applied moment time history prints out correct, and I can see it’s effect of the pitch motion (as in the figure in the previous post), however, I am getting 0 for the displacements and velocities:

image

image

Could you please help me figure out why that may be? Are there any specific conditions in which these values are being recorded?

Kind regards

Dear @Kasia.Patryniak,

The StC displacement and velocity outputs you’ve selected (SStC1_XQ, etc.) are the internal states of the StC module. However, you have not enabled these states because you’ve directly prescribed a load time series. So, these states are zero as expected.

Best regards,

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