Dear Jason:
I am trying to run the original OC4 DeepCwind model for wind only. I have turned off HyrdoDyn and even turned off ServoDyn is a separate simulation but I am bemused by the following error as I expected it to run successfully.
“FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption
violated in SUBROUTINE SmllRotTrans() due to a large blade deflection (ElastoDyn SetCoordSy). The
solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will
be suppressed. Additional debugging message from SUBROUTINE SmllRotTrans(): 7.75 s
Time: 8 of 60 seconds. Estimated final completion at 08:23:57.
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.089 deg). (node 16,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.098 deg). (node 17,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.063 deg). (node 18,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.058 deg). (node 19,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.202 deg). (node 15,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.173 deg). (node 14,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.154 deg). (node 13,
blade 2)
Warning: Turning off Unsteady Aerodynamics due to high angle of attack (-45.096 deg). (node 12,
Time: 13 of 60 seconds. Estimated final completion at 08:23:55.
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetInputs:TwrInfl:getLocalTowerProps:Tower strike.
FAST encountered an error at simulation time 13.513 of 60 seconds.
Simulation error level: FATAL ERROR
Aborting OpenFAST.”
In addition, for the simulation that I turned off ServoDyn I had set the rotor speed to zero and the blade pitch angle to 90 deg.
Can you please advise me on how the model can be run for the wind only option?
Regards,
AOAW
Dear Andre,
When turning off HydroDyn, presumably you also disabled the platform DOFs in ElastoDyn (PtfmSurge = False, etc.) so as to keep the turbine from free falling due to gravity?
When turning off ServoDyn and parking the rotor, did you disable the wake (WakeMod = 0) and unsteady airfoil aerodynamic (AFAeroMod = 1) models in AeroDyn, which are only valid for operational conditions? Also, did you disable the nacelle-yaw DOF in ElastoDyn (YawDOF = False) because disabling ServoDyn means removing the nacelle-yaw actuator (spring-damper), which keeps the yaw from rotating freely?
Best regards,
Thanks Jason.
It was in fact the platform DOF that were not disabled why I was getting the Tower Strike error for the wind only condition. I had to set initial pitch angle to 180 degrees as well. When I checked the simulation results the pitch motion is a constant 1.9 degrees. It makes sense to me, in that the DOF is disabled so therefore no simple harmonic motion.
1) Is my understanding correct and does it make sense?
2) Does disabling a DOF means that there is no motion in that direction? If this is the case it seems as if it is not practical to disable the DOFs of the platform when you want to investigate it behaviour in that DOF? Please clarify.
What I was investigating are the loads that cause the dominant motion of the platform for the various DOFs whether wave or wind. I realize that the wave only case gave me an indication of the dominant motion, in that, the pitch motion seems to be affected by the wind as with the wave only condition the pitch motion tends to a low value and the system oscillates close to a value of zero. For the heave motion, the dominant response is to the wave load and the wind had no effect on the heave behaviour.
3) Please confirm if this finding is true?
I have two more queries.
4) When investigating the effect of sea state severity on platform motion, does one consider the wave spectrum that is the peak frequency and not Hs and Tp? I would like to investigate this but don’t know how to go about it? Does it require a change in the peakedness factor for the spectrum that is input in OpenFAST?
5) Is there a manual (or paper, say) that gives information on the expected platform motion and other behaviour of the system for various environmental conditions? I want to be a position to check that the results I am getting make sense and are in keeping with typical behaviour of the system
Thank you.
Regards,
AOAW
Dear Andre,
Here are my answers to your questions:
- I’m not understanding why you are setting the “initial pitch angle” (presumably you mean the blade pitch) to 180 degrees. You also mention a constant pitch motion of 1.9 degrees (presumably you mean platform pitch), which I gather is nonzero because of an initial condition that you set.
- Disabling a DOF in ElastoDyn really means that the acceleration will be zero, velocity will be constant, and displacement will vary linearly (if the constant velocity is nonzero), regardless of the loads applied. Inside ElastoDyn, the equation of motion for the disabled DOFs are not formulated, resulting in no acceleration of those DOF. Unless the source code is changed, the initial condition for all DOFs have zero velocity, except for the generator-azimuth DOF (set via initial condition RotSpeed), so, this also means that the displacement for any disabled DOF except the generarator-azimuth DOF will be zero when that DOF is disabled.
- If you want to study the impact of wind loads on platform motion, you should not disable the platform DOFs. But this means that you should not disable HydroDyn (which models the buoyancy; you can eliminate incident waves if that is what you want), the mooring module (which models the reaction from the mooring system), or ServoDyn (which models the controller).
- I’m not sure I understand your question. WaveHs, WaveTp, and WavePkShp all determine the severity of the sea state.
- There are several papers published that describe the OC4 DeepCWind floating wind system and its response, but I can’t think of one that goes into detail on expected platform motion dependent on various environmental conditions.
Best regards,
Jason:
Thanks for your response.
In 1) yes you are correct, 180 degrees is the blade pitch angle and 1.9 degrees is the amount of the pitch motion of the platform. I set blade pitch angle to 180 degrees because I saw it done by someone else on another forum.
Also, thanks for the clarification in 4). If I have any further queries about this I will let you know.
Regards,
AOAW
Dear Andre,
A parked or idling rotor is typically modeled with the blade leading edge facing the wind (to reduce aerodynamic loads), typically using BlPitch = 90deg (or so); pitch angles typically don’t exceed this maximum value.
A wind turbine operating below rated typically has blades pitched to fine (rated) pitch, typically using BlPitch = 0deg (or so); pitch angles tpyically don’t go lower than this minimum value.
A wind turbine operating above rated typically has blades pitched between fine (rated) pitch and around 30deg (or so).
Best regards,
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