Dear @Emanuel.Rergis,
Each reaction degree of freedom (DOF) in SubDyn can be enabled/disabled independently, e.g., RctTDXss for translation along X and RctRDXss for rotation about X. If you want a complete 6x6 stiffness matrix representation of a coupled springs foundation model, you should enable all 6 reaction DOFs.
Best regards
Thank you very much for your response, Dr. @Jason.Jonkman.
One more question: I also need to assess the vibrational behaviour of an NREL-5MW offshore turbine in its “parked” mode. Would it be sufficient to set the initial rotor speed to 0 m/s and disable the CompAero and CompInflow flags?
I am not entirely sure which of these I need to set to 0. However, the model must be subjected to different turbulence levels, so I assume that CompInflow should be set to 1. Am I correct?
Thanks again.
Dear @Emanuel.Rergis,
Normally a parked or idling rotor is simulated under extreme wind (above cut-out) conditions, where I would not disable InflowWind or AeroDyn.
To model a parked or idling rotor in OpenFAST, recommended AeroDyn settings are documented here: 4.2.5. Modeling Considerations — OpenFAST v4.0.2 documentation. In ServoDyn, you should disable active pitch and torque control: PCMode = VSContrl = 0, GenTiStr = True with TimGenOn > TMax). And in ElastoDyn, you should zero the rotor speed as you suggested (RotSpeed = 0) and feather the blades (BlPitch(1-3) = 90deg). To park the rotor, you should disable the generator degree of freedom (GenDOF = FALSE), but for an idling rotor, you should keep it enabled (GenDOF = TRUE), which is more common in modern utility-scale wind turbines.
Best regards,
Dear Dr. @Jason.Jonkman,
Thank you for your response. I actually have one more question. I am simulating meteorological conditions where I have removed InflowWind, meaning I am assuming still air in the environment, and only the wave loads from my simulation are present. Based on this, I plotted TwrBsMyt and TwrBsMxt.
I expected both graphs to exhibit similar dynamic behaviour. However, I noticed that the moment values in the TwrBsMxt plot seem to decrease over time. Is this normal? Could you share your opinion on this? Do you think I should change something in the model, or is there perhaps something I am overlooking? Please find attached some snapshots. And one more thing; I found a similar behaviour when plotting PtfmSway, PtfmSurge, PtfmRoll, and PtfmPitch. While some of them exhibit behaviour similar to TwrBsMyt, others show the effect seen in TwrBsMxt. Is this also normal?
Thank you for your help.
Dear @Emanuel.Rergis,
I would not expect this. It looks like either an initial condition or something happening at the beginning of the simulation is “kicking” the system and taking a long time to damp out (likely due to low inherent damping in the model). I’m not sure of your model setup to comment further.
Best regards,
Thanks Dr. @Jason.Jonkman. Now, how should the TwrBsMxt graph look in a situation where there is no wind, meaning the still air option is enabled, and only wave loads are present? I can share my HydroDyn file with you if needed. Do you need more information of this? Maybe AeroDyn or ElastoDyn?
Thanks a lot for your help.
I mention this because when I add wind to the model, the effect previously seen in the graphs disappears.
Dear @Emanuel.Rergis,
Is this a monopile support structure with unidirectional waves propagating along the X-axis, such that one would only expect significant forces in the X and Z directions and moments about Y? If so, I would first figure out what is causing the “kick” and if that could be solved by better initial conditions, e.g., in platfom displacements. Is the rotor parked or idling without generator torque?
Best regards,
Dear Dr. @Jason.Jonkman:
Thank you for your comments. I reviewed my model and did not find anything unusual. I am attaching the most important files, where, in my opinion, the initial conditions are established.
Indeed, this model corresponds to a 5MW NREL OC3 Monopile Offshore Wind Turbine. First, I would like to mention that the rotor is in operational mode, and its initial speed is 12.1 m/s. There is no initial displacement, as all values are set to 0. Additionally, I have disabled CompInflow because I am only interested in analyzing the action of ocean waves.
My hunch is that I may have made a mistake in selecting the parameters for HydroDyn and might be using unrealistic values. Could you provide me with any additional advice?
As always, I appreciate your help.
Best regards,
Emanuel
Dear @Emanuel.Rergis,
While you have disabled InflowWind and are using still air, I see that you still have AeroDyn and ServoDyn enabled. Have you eliminated the generator torque in ServoDyn by disabling the torque controller? What aerodynamic settings have you enabled in AeroDyn? What is happening to the rotor speed after initialization?
Best regards,
Thank you, Dr. @Jason.Jonkman:
I didn’t know I should have disabled ServoDyn and AeroDyn. Do you recommend disabling this module if I want to analyse the behaviour of only the wave loads? What parameter should I change to disable the torque controller? Where can I monitor the rotor speed after initialization? Is this the last thing in ServoDyn? Sorry, too many questions. Thanks anyway, and just in case, find attached some snapshots of ServoDyn and AeroDyn.
Kind regards!
Emanuel
Dear @Emanuel.Rergis,
The rotor speed (RotSpeed) is an output you’ve selected in ElastoDyn and can be plotted.
I didn’t say you have to disable AeroDyn and ServoDyn, but your settings are not valid for still air conditions. In AeroDyn, you’ve selected the Dynamic BEM (DBEMT) model, which is only valid for an operating rotor; I would set WakeMod = 0 instead. In ServoDyn, you’ve enabled the DISCON controller for NREL 5-MW baseline turbine, but this controller is only valid for an operating rotor. My guess is the torque controller is slowing the rotor speed down very quickly causing the “kick”. Instead, you should disable the controller by setting PCMode = VSContrl = 0 and disable generator torque altogether by setting TimGenOn > TMax in ServoDyn.
Best regards,
Dear Dr. @Jason.Jonkman
Thank you very much for the information you provided. Now I can see that everything matches. I would like to take this opportunity to ask you a new question.
I am currently trying to perform a PSD analysis for two offshore wind turbine models of the NREL 5MW-OC3 Monopile type. The conditions are almost identical in both cases (except that in one model, a rigid seabed is assumed, while in the other, there is dynamic soil interaction). I disabled InflowWind, ServoDyn, HydroDyn, etc.—in short, I only kept ElastoDyn active. I applied an initial displacement of 1m in the fore-aft direction, and using the TwrTpTDxi displacement data, I obtained a PSD graph (please find attached the snapshot).
I did all this because I am interested in finding the frequency of the first three modes in each case. I find it intriguing that in each case, the first mode is visible. Unfortunately, the other modes are not really apparent, and the peaks that do appear do NOT match those reported in the literature.
Do you think there might be an error? Would you recommend any other approach to obtain these modes? I am particularly interested in identifying these three modes in the case of the blue curve. Additionally, I would like to know how I can determine the location of these modes along the tower.
I appreciate your help and look forward to your response.
Best regards.
Emanuel M. Rergis
Dear @Emanuel.Rergis,
Can you clarify: To model the NREL 5-MW baseline wind turbine top the OC3-monopile in OpenFAST, you would model the substructure in SubDyn, but you say only ElastoDyn is enabled. Also how are you enabling and disabling soil interaction?
Best regards,
Dear @Emanuel.Rergis,
I am Riad, a wind turbine enthusiast.
I agree with the question of @Jason.Jonkman.
Moreover, i have something to tell you.
—> Concerning your question: why you cannot find the 3rd mode of the tower ? In fact, to model the tower you are using ElastoDyn_tower. In ElastoDyn_tower, you cannot go beyond the second mode.
—> What you are doing is called free decay test. I obtain the same result as you, you cannot see the second mode frequency because in my understanding the power associated to tower 2nd bending mode is too small and generally will not appear in a free decay test. To tackle this problem, i suggest trying the following two solutions:
1- Try to plot the PSD in decibel scale (dB). Maybe, the second mode will appear.
2- Forget the free decay and let the OWT be subjected to a white noise spectrum. This white noise spectrum cold be geberated in HydroDyn. but you set the parameter WvLowCOff in apprpriate amnner in such a way that the value should be greater than the tower 1st bending mode in order to excite tower 2n bending mode.
Hope i was clear and my message is understandable.
Best Regards,
Riad
I apologise, dear Dr. @Jason.Jonkman You are absolutely right; I do have the SubDyn function enabled. In fact, this is how I activate and deactivate the soil interaction. Knowing this, what do you recommend I do to determine the localization distance of the first three modes? Thank you again for your comments and guidance.
Best regards.
Dear @Riad.Elhamoud,
Thank you very much for your contribution. Would you allow me to ask you a couple more questions based on your comments?
Best regards.
Emanuel
Dear @Emanuel.Rergis,
Thanks for clarifying.
While the frequency in your PSD plot of the first tower-bending mode with rigid foundation looks about what I expect (around 0.25 Hz), I agree that the frequency with soil interaction looks too low. For the soil interaction, presumably you are representing the soil as coupled springs through an SSIfile; is that correct? Can you share SubDyn and SSIfiles?
I agree with @Riad.Elhamoud that the use of white noise excitation would be a better way at identifying higher-frequency modes. Performing an OpenFAST linearization analysis, followed by eigenanalysis, is another approach, and our Automated Campbell Diagram Code (ACDC) is a great tool to support that: GitHub - OpenFAST/acdc: ACDC: Automated Campbell Diagram Code.
I also agree with @Riad.Elhamoud that the use of ElastoDyn to model the tower will limit how many bending modes of the support structure you’ll be able to capture by your model. You could switch to modeling the entire support structure (up to the yaw bearing) in SubDyn if you need to capture higher-frequency modes.
Best regards,
Thank you very much for your help, @Riad.Elhamoud. My first question would be: how large should the value of WvLowCOff be concerning the natural frequency of the first mode? Would something like 1.5 × WvLowCOff_1st_mode_frequency work?
Another thing I have to ask you is that, as you can see, the graph I shared with you has the y-axis and the x-axis on a logarithmic scale. Do you think I should still convert it to decibels?
Finally, I would like to hear your opinion on how I can determine the location in the tower of the first three modes that would appear in the PSD graph after the frequency sweep induced by the white noise from the HydroDyn module.
Thanks again for your help.
Best regards,
Emanuel M. Rergis
Dear Dr. @Jason.Jonkman:
Thank you once again for your responses. I am sending you three screenshots. The first one should represent how I enable soil interaction using SubDyn, the second is the TXT file I am using, and the third is the soil stiffness matrix, which, to my understanding, I am defining in the TXT file. Do you find any discrepancies?
On the other hand, I would like to reiterate my question: How can I determine the location in the tower of the natural frequencies associated with those modes that I will need to identify using one of the techniques you propose?
Thank you again.
Best regards,
Emanuel M. Rergis
