Hey,
Dear Akheel,
Here are my answers to your questions:
Best regards,
dear Jason,
I’m using FAST v8.16 in order to run NREL 5MW OC3-Hywind (Test 24)
I created wind files using TurbSim v1.50 for wind speeds 8m/s and 24m/s
for wind speed 8m/s : FAST v8.16 terminates successfully . tmestep: 0.0125s
for wind speed 24m/s ,i got the following message
i reduced the timestep but the problem remains.
Thank you in advance,
Georgia
Dear Georgia,
Have you set proper initial conditions (based on the mean wind speed at hub height) for rotor speed and blade pitch? These should be 12.1 rpm and 22.35 deg at 24 m/s.
Best regards,
Dear Jonkman,
In my previous query, By ‘inertial coordinates’, I meant the inertial frame. And my query was that if I want to change the inrtial frame to platform COG, Do I need to recompile or setting PtfmRefzt = 89.9 alone is sufficient?
Thanks
Dear Akheel,
Yes, that is correct. Changing PtfmRefzt in ElastoDyn will change the origin of the inertial frame coordinate system (for the purposes of defining the reference point for platform motions).
Best regards,
Dear Sir,
Thank you for the response. I have received some warnings during simulation at different load cases:
Case 1: Wind Speed = 18, Turbulence intensity = 0.15, Significant avg height = 4.0, Spectral Period = 15
Warning: FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:BEMT_CalcOutput(node 19, blade 2): Compute_UA_AirfoilCoefs: UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist.
Case 2: Wind Speed = 24, Turbulence intensity = 0.12, Significant ave height = 5.6, Spectral Period = 18
Warning: 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(): 0.6125 s
FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetInputs:TwrInfl:getLocalTowerProps:Tower
strike.
FAST encountered an error at simulation time 0.7625 of 1000 seconds.
Simulation error level: FATAL ERROR
Aborting OpenFAST.
Dear Akheel,
Case 2 is failing because of a tower strike. I would not expect that you’d receive warnings about Mach number exceeding 0.3 for the NREL 5-MW wind turbine model (Case 1), nor would I expect that you’d receive a tower-strike error (Case 2). This suggests to me that something is wrong in your simulation set up in both cases.
What have you changed in the input files relative to Test24? Clearly you changed the turbulent wind inflow and wave conditions, but did you change anything else? When changing the wind conditions, did you change the initial conditions of rotor speed and blade-pitch angle to match their expected values for the given mean hub-height wind speed?
Best regards,
Dear Sir,
Dear Akheel,
A tower strike means that the blade hit the tower. OpenFAST will abort when this happens
12.1 rpm is a correct initial condition for above-rated wind speeds, but a blade-pitch of zero is not. This is likely the problem…poor initial conditions on rotor speed and blade-pitch can result in the controller producing unreliable results. From Figure 9-1 of the NREL 5-MW baseline wind turbine specifications report (nrel.gov/docs/fy09osti/38060.pdf), an 18-m/s mean wind speed should use an initial blade-pitch angle of about 15 deg and an 24-m/s mean wind speed should use an initial blade-pitch angle of about 22 deg.
Best regards,
Dear Sir,
Thank you for the information. Here are some queries regarding FAST theory.
Dear Akheel,
Regarding (1), what loads are referring to? Applied aerodynamic loads; applied hyrodrodynamic loads, and/or reaction loads within the structure? There are many topics on this forum and related publications in the literature on this topics.
Regarding (2), the strip-theory solution in HydroDyn includes terms beyond just the distributed transverse inertia, added mass, and viscous drag loads that are part of Morison’s equation. In addition to these terms, the strip-theory solution of HydroDyn also considers distributed axial loads on tapered members, distributed static buoyancy (about the undisplaced position), and concentrated loads at member ends (joints). And recent work has also added functionality to recalculate the distributed hydrostatic (buoyancy loads) based on the displaced position of the structure every time step (currently under review before being merged into the dev branch of OpenFAST in the following pull request: github.com/OpenFAST/openfast/pull/537). More information is available in the HydroDyn documentation, publications, and presentations.
Best regards,
Dear Sir,
I tried to simulate load case 5.1 in Offshore Code Comparison Collaboration (OC3) for IEA Task 23 Offshore Wind Technology and Deployment, J. Jonkman, 2010. The response I obtained in Surge was not matching. The input conditions were as follows:
All other parameters were kept unaltered. Where am I making the mistake?
I have attached the response plot.
Doc1.docx (3.53 MB)
Dear Akheel,
The results you are plotting appear to be the first 20 s of simulation, with initial platform displacements set to zero. This is still the start-up transient phase of the simulation, which you should generally neglect in post-processing. The results published in the report you reference are shown after all start-up transients have passed. You should run a much longer, which with steady wind and period waves will eventually result in a periodic response. It is this periodic response that you should be comparing.
Please note to minimize the start-up transient in a floating wind turbine analysis, in addition to setting the initial rotor speed and initial blade-pitch angle properly, it is also useful to set the initial platform surge and initial platform pitch angles to be their expected mean values for the given mean hub-height wind speed. Otherwise, due to the low natural frequencies of the floating platform surge and pitch modes, it could take hundreds of seconds of simulation for the start-up transient to pass.
Best regards,
Dear Sir,
The steady state response was matching well. I need a clarification regarding the second statement of your previous message, that if I am unaware of the expected initial pitch or surge and input some random value, Will the final result be wrong?
Secondly, In the load case 5.1, I tried to enable NTMD with following features, But the responses were not found to be controlled. Can you help me in this regard?
1 TMD_DOF_MODE
true TMD_X_DOF
false TMD_Y_DOF
---------------------- TMD INITIAL CONDITIONS ---------------------------------
0 TMD_X_DSP
0 TMD_Y_DSP
---------------------- TMD CONFIGURATION --------------------------------------
0 TMD_P_X
0 TMD_P_Y
3 TMD_P_Z
8 TMD_X_DWSP
-8 TMD_X_UWSP
0 TMD_Y_PLSP
-0 TMD_Y_NLSP
---------------------- TMD MASS, STIFFNESS, & DAMPING -------------------------
10473 TMD_X_M
0 TMD_Y_M
0 TMD_XY_M
86660 TMD_X_K
0 TMD_Y_K
4203 TMD_X_C
0 TMD_Y_C
500000 TMD_X_KS
0 TMD_Y_KS
500000 TMD_X_CS
0 TMD_Y_CS
Dear Sir,
I tried to simulate load case 5.3 of ‘Offshore Code Comparison Collaboration (OC3) for IEA Task 23 Offshore Wind Technology and Deployment’ J.Jonkman, 2010. I got an error message regarding Mach Number exceeding 0.3. Initial Conditions such as Rotor Speed = 12.1 and Blade Pitch = 14.92 were set prior to loading. The TurbSim file is as follows:
--------Turbine/Model Specifications-----------------------
31 NumGrid_Z
31 NumGrid_Y
0.05 TimeStep
1200 AnalysisTime
1000 UsableTime
90 HubHt
159.00 GridHeight
140.00 GridWidth
--------Meteorological Boundary Conditions-------------------
“IECKAI” TurbModel
“1-Ed3” IECstandard
“B” IECturbc
“NTM” IEC_WindType
90 RefHt
18 URef
OTHER PARAMETERS WERE KEPT UNALTERED
Dear Akheel,
Normally, improper initial conditions just result in a long start-up transient that eventually dies out, after which the results are fine. But if initial conditions are not set properly, sometimes the control system may not recover, which may lead to an instability that is quite obvious to spot (very large, unrealistic deflections and loads).
Regarding your second question, I can’t say whether these TMD settings will have a large effect on the system response. I gathered from prior questions that you do at least see some effect from the TMD (e.g. you feel the weight of the TMD in the tower). So, perhaps changes to the TMD parameters are warranted to see a greater influence. I can’t really guide you on that.
Your TurbSim settings look fine to me. How quickly do you receive this warning? Could it be triggered because there is still a large start-up transient due to not setting good initial conditions for platform surge and pitch?
Best regards,
Dear Sir,
The warning was found to occur at about 600 sec
Dear Sir,
Is there any limit for mass ratio, Stiffness, Damping of TMD in nacelle?
Dear Akheel,
There is no hard limit to the mass, stiffness or damping of nacelle-based TMDs in OpenFAST. But I could imagine setting any of these values too large may eventually result in numerical issues with the solution.
Best regards,