Modelling Marine Growth on 5MW OC3-Hywind Spar - Errors

Wind & Water Computer-Aided Engineering Software Tools
1

Hi all,

I am trying to model marine growth on the OC3-Hywind Spar platform and have found difficulty in doing so.
First I will explain the setup I have used, and then secondly the errors I’m seeing as a result.

SetUp
My understanding is that I must use the strip theory to model Marine Growth on a platform, and so I have switched PotMod to 0, and PropPot to FALSE in HydroDyn.
I wish to model two different marine growth thicknesses on the platform based on location/ depth along the spar as follows:
From 2m to -40m –> 100mm thickness –> Density is 1325 kg/m3.
From -40m to -120m –> 50 mm thickness –> Density is 1325 kg/m3.

So in the HydroDyn.dat input file, I have entered the below:

---------------------- MARINE GROWTH -------------------------------------------
4 NMGDepths - Number of marine-growth depths specified (-)
MGDpth MGThck MGDens
(m) (m) (kg/m^3)
2.00 0.100 1325
-40.00 0.100 1325
-40.01 0.050 1325
-120.00 0.050 1325

Errors Encountered
I have attached a note containing the full command line output including all error messages I get after running OpenFAST.
I have tried to summarize the main errors in the list below:

“Temporarily turning off UA due to high angle of attack or low relative velocity”
“The BEM solution is being turned off due to low TSR”
“Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large platform displacement (ElastoDyn SetCoordSy). The solution may be inaccurate”
“…Mach number exceeds 0.3. Theory is invalid.” or “…Mach number exceeds 1.0. Equations cannot be evaluated.”

I’ve looked extensively on this Forum for solutions to each of these errors individually, and I’ve tried to implement the suggested solutions in most cases. But unfortunately none of the proposed solutions have fixed my problem, and hence I’m creating a new topic looking for further ideas and support.

What is interesting to me is that when I model MG with the below layers instead, as defined in an example on Github called ‘5MW_OC4Jckt_DLL_WTurb_WavesIrr_MGrowth’, it executes with no errors.

---------------------- MARINE GROWTH -------------------------------------------
2 NMGDepths - Number of marine-growth depths specified (-)
MGDpth MGThck MGDens
(m) (m) (kg/m^3)
-2.00 0.100 1100
-40.00 0.100 1100

This leaves me with two questions:

  1. Is my set up, including the MG layers, correct for what I’m trying to do? Or have I made a mistake here somewhere?
  2. If the set up is ok, do you think the MG may be causing instability of the platform (and hence I am getting so many errors all at once)?

Any suggestions or help would be greatly appreciated!

Kind regards,
Stephen
Modelling Marine Growth Errors 29.07.21.txt|attachment (4.76 KB)

2

Dear Stephen,

Your approach sounds OK to me and I have not spotted any obvious errors. It looks like you are getting a “small angle assumption violation” warning regarding large platform displacement. Can you share what kind of platform motions you are seeing? It would also help if you share basic response characteristics of the turbine, e.g., blade pitch, rotor speed, and blade deflection…what do these look like?

I would also recommend simplifying the problem to debug. Do you get a more reasonable response if you disable the wind turbine controller, e.g., by disabling the generator degree of freedom (GenDOF = False in ElastoDyn).

Best regards,

3

Hi Jason,

Thanks for your reply.

I have run the model again, this time with GenDOF switched off, and then on.
There doesn’t seem to be any major difference between results, only that when GenDOF is off the simulation aborts at 83.513 seconds and when GenDOF is turned on the simulation aborts at 187.47 seconds.
The error messages received in each case are also slightly different, and are pasted below for further information.
(Note: The images presented and discussed below are from results obtained when GenDOF is switched OFF)

The platform motions seem fine, however platform roll does increase with time. Is this to be expected?

Platform Roll vs Time - MG.PNG
Platform Roll vs Time - MG.PNG761×553 30.8 KB

Something funny seems to happen to values, such as roll above and the platform acceleration and rotor speed values shown below, just before the simulation aborts at time t=83.5s. Do you know what might be the cause of this discontinuity?

Platform Translational Acceleration vs Time - MG.PNG
Platform Translational Acceleration vs Time - MG.PNG762×544 6.28 KB

Regarding the basic response characteristics of the turbine, blade pitch remains constant at 3.83 degrees, as set in the initial condition section of ElastoDyn to match the 12.0 m/s wind speed outlined in Table 7.1 of ‘Definition of a 5MW Reference Wind Turbine for Offshore System Development’.
Rotor speed varies slightly but remains centered around the initial rotor speed of 12.1 rpm.

Rotor Speed vs Time - MG.PNG
Rotor Speed vs Time - MG.PNG756×538 14.8 KB

Any further suggestions would be greatly appreciated.
Thanks and regards,

Stephen Lynch

GenDOF Turned Off Error Message

OpenFAST

Copyright (C) 2021 National Renewable Energy Laboratory
Copyright (C) 2021 Envision Energy USA LTD

This program is licensed under Apache License Version 2.0 and comes with ABSOLUTELY NO WARRANTY.
See the “LICENSE” file distributed with this software for details.

OpenFAST-v3.0.0
Compile Info:

  • Compiler: Intel(R) Fortran Compiler 1900
  • Architecture: 64 bit
  • Precision: single
  • OpenMP: No
  • Date: Jun 23 2021
  • Time: 21:25:05
    Execution Info:
  • Date: 08/01/2021
  • Time: 21:23:11+0100

OpenFAST input file heading:
FAST Certification Test #24: NREL 5.0 MW Baseline Wind Turbine with OC3 Hywind Configuration,
for use in offshore analysis

Running ElastoDyn.
Nodal outputs section of ElastoDyn input file not found or improperly formatted.
Running AeroDyn.
AD15 Nodal Outputs: Nodal output section of AeroDyn input file not found or improperly formatted.
Skipping nodal outputs.
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 3)
Running InflowWind.

Reading a 31x31 grid (145 m wide, 17.5 m to 162.5 m above ground) with a characteristic wind
speed of 12 m/s. This full-field file was generated by TurbSim (v2.00.07a-bjj, 14-Jun-2016) on
26-Jul-2021 at 15:18:48.

Processed 12600 time steps of 20-Hz full-field data (period of 630 seconds).
Running ServoDyn.
Running ServoDyn Interface for Bladed Controllers (using Intel Visual Fortran for Windows, ).
Using legacy Bladed DLL interface.
Running HydroDyn.
Generating incident wave kinematics and current time history.

MAP++ environment properties (set externally)…
Gravity constant [m/s^2] : 9.81
Sea density [kg/m^3] : 1025.00
Water depth [m] : 320.00
Vessel reference position [m] : 0.00 , 0.00 , 0.00

Time: 0 of 300 seconds.

FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:DLL_controller_call:Run
ning with torque and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll
as written by J. Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies. Includes
modifications for the Hywind spar.
Time: 83 of 300 seconds. Estimated final completion at 21:25:29.
The BEM solution is being turned off due to low TSR. (TSR = 1.6644). This warning will not be
repeated though the condition may persist. (See GeomPhi output channel.)

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:RotCalcOutput:BEMT_CalcOutput(node 5,
blade 3):UA_CalcOutput:UA_BlendSteady:Temporarily turning off UA due to high angle of attack or
low relative velocity. This warning will not be repeated though the condition may persist.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:RotCalcOutput:BEMT_CalcOutput(node 12,
blade 1):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be
repeated though the condition may persist.

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption
violated in SUBROUTINE SmllRotTrans() due to a large platform displacement (ElastoDyn
SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from
SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 83.519 s
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:RotCalcOutput:BEMT_CalcOutput(node 7,
blade 3):UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be evaluated.

OpenFAST encountered an error at simulation time 83.513 of 300 seconds.
Simulation error level: FATAL ERROR

Aborting OpenFAST.

GenDOF Turned on Error Message:

OpenFAST

Copyright (C) 2021 National Renewable Energy Laboratory
Copyright (C) 2021 Envision Energy USA LTD

This program is licensed under Apache License Version 2.0 and comes with ABSOLUTELY NO WARRANTY.
See the “LICENSE” file distributed with this software for details.

OpenFAST-v3.0.0
Compile Info:

  • Compiler: Intel(R) Fortran Compiler 1900
  • Architecture: 64 bit
  • Precision: single
  • OpenMP: No
  • Date: Jun 23 2021
  • Time: 21:25:05
    Execution Info:
  • Date: 08/02/2021
  • Time: 12:27:30+0100

OpenFAST input file heading:
FAST Certification Test #24: NREL 5.0 MW Baseline Wind Turbine with OC3 Hywind Configuration,
for use in offshore analysis

Running ElastoDyn.
Nodal outputs section of ElastoDyn input file not found or improperly formatted.
Running AeroDyn.
AD15 Nodal Outputs: Nodal output section of AeroDyn input file not found or improperly formatted.
Skipping nodal outputs.
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 3)
Running InflowWind.

Reading a 31x31 grid (145 m wide, 17.5 m to 162.5 m above ground) with a characteristic wind
speed of 12 m/s. This full-field file was generated by TurbSim (v2.00.07a-bjj, 14-Jun-2016) on
26-Jul-2021 at 15:18:48.

Processed 12600 time steps of 20-Hz full-field data (period of 630 seconds).
Running ServoDyn.
Running ServoDyn Interface for Bladed Controllers (using Intel Visual Fortran for Windows, ).
Using legacy Bladed DLL interface.
Running HydroDyn.
Generating incident wave kinematics and current time history.

MAP++ environment properties (set externally)…
Gravity constant [m/s^2] : 9.81
Sea density [kg/m^3] : 1025.00
Water depth [m] : 320.00
Vessel reference position [m] : 0.00 , 0.00 , 0.00

Time: 0 of 300 seconds.

FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:DLL_controller_call:Run
ning with torque and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll
as written by J. Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies. Includes
modifications for the Hywind spar.
Time: 187 of 300 seconds. Estimated final completion at 12:29:52.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:RotCalcOutput:BEMT_CalcOutput(node 5,
blade 3):UA_CalcOutput:UA_BlendSteady:Temporarily turning off UA due to high angle of attack or
low relative velocity. This warning will not be repeated though the condition may persist.

Warning: SkewedWakeCorrection encountered a large value of chi (97.239 deg), so the yaw
correction will be limited. This warning will not be repeated though the condition may persist.
See the AD15 chi output channels, and consider turning off the Pitt/Peters skew model (set
SkewMod=1) if this condition persists.

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates:UpdatePhi(node 12, blade
3):BEMT_UnCoupledSolve:There is no valid value of phi for these operating conditions: Vx =
2.1687, Vy = -20.1, rlocal = 40.434, theta = 0.14786, geometric phi = 3.0341. This warning will
not be repeated though the condition may persist. (See GeomPhi output channel.)
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:RotCalcOutput:BEMT_CalcOutput(node 17,
blade 2):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be
repeated though the condition may persist.

The BEM solution is being turned off due to low TSR. (TSR = -0.42085). This warning will not be
repeated though the condition may persist. (See GeomPhi output channel.)

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption
violated in SUBROUTINE SmllRotTrans() due to a large platform displacement (ElastoDyn
SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from
SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 187.47 s
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:RotCalcOutput:BEMT_CalcOutput(node 5,
blade 2):UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be evaluated.

OpenFAST encountered an error at simulation time 187.47 of 300 seconds.
Simulation error level: FATAL ERROR

Aborting OpenFAST.

4

Dear Stephen,

Just a few comments:

  • I would expect OpenFAST to stop generating output once the simulation aborts (at 83.513 s for the GenDOF = False simulation). Your plots show a spike at this time, but then results afterword. Are you plotting concatenated results from multiple simulations?
  • With GenDOF = False, I would expect that the rotor speed to be nearly constant, with small variations only due to drivetrain torsion. Are you seeing a constant generator speed?
  • I’m not sure what is causing the spike, but this appears to be what is causing the simulation to abort. Is the result sensitive to time step or number of corrections?
  • It is still hard to understand with only the subset of results you’ve shared. Can you share your output file so that can plot all of the results?

Best regards,

5

Hi Jason,

Apologies, you are correct.
I had forgotten to remove results from a previous simulation, and so these were the values shown and plotted after the simulation had finished at 83.513 seconds. OpenFAST did indeed stop generating results after the simulation was aborted. You may disregard any values shown in previous graphs after this time.

Unfortunately, due to large file sizes, I cannot attach the output results directly here. Instead they can be found in a shared folder at the link below:
drive.google.com/drive/folders/ … sp=sharing
I hope this makes things easier for you to follow.

Lastly, I have found that changing the time step does effect the time at which the simulation aborts at. However, this is not a linear relationship, i.e. a smaller timestep does not necessarily mean the simulation fails/ aborts earlier and visa versa. Therefore I find it difficult to say what effect timestep has on the simulation exactly.

Let me know if you have any further thoughts on this.

Thanks again and regards,
Stephen Lynch

6

Hi Stephen,

Focusing only on the simulation without GenDOF enabled for now, plotting your results, what concerns me is a spike in the system accelerations and hydrodynamic loads that occurs just before the simulation failure. It is hard to know if the spike in hydrodynamic load is causing a spike in acceleration, or the spike an acceleration is causing a spike in hydrodynamic load (through the added mass). Regardless, the system is not responding well after the spike. And I’m not sure what is causing the spike.

I would first recommend running with one correction step to see if this solves the problem, i.e., set NumCrctn = 1 in the OpenFAST primary input file. Have you kept DT_UJac at 1.5 s for this OC3-Hywind spar model? If not, you could also set that back to 1.5 s. Do these settings eliminate the spike and resulting simulation failure?

BTW: I noticed that the addition of marine growth has resulting in a mean heave displacement of about -2.6 m, i.e., the extra weight from marine growth is not balanced with extra buoyancy or weight reduction elsewhere, causing the spar to sit lower in the water than it normally would be. We normally recommend that floating wind models in OpenFAST be set up such that the mean heave displacement is zero. I don’t believe this would cause the simulation problem you are seeing, but it could lead to some inaccuracy in the results.

Best regards,

7

Hi Jason,

Thanks for your response.

Yes, I had noted that the presence of marine growth resulted in a mean heave displacement of -2.6 m. This is lower than previously observed with the 5MW OC3-Hywind spar when modelled without any marine growth, which had a mean heave displacement of -0.46m. This all made sense in my mind because, as you have explained, the extra weight of marine growth on the spar would cause it to sit deeper in the water.
Can you explain further why this may lead to inaccuracies? Will this not be an accurate representation of the 5MW OC3-Hywind spar with marine growth in reality?

I have tried to set NumCrctn = 1 in the OpenFAST primary input file as per your suggested correction step below. However the sudden spike and simulation failure still remains, although the system doesn’t seem to abort until the later time of 203.64 seconds now, rather than 187.47 seconds. I have attached the error message from this iteration below again FYI.
Modelling Marine Growth Errors 03.08.21_GenDOFon_NumCrctn=1.txt|attachment (4.63 KB)

I have kept DT_UJac at 1.5 s for all simulations and never changed this parameter in OpenFAST.

Any further suggestions would be greatly appreciated.
Thanks and regards,

Stephen Lynch

8

Dear Stephen,

Can you share the output file without GenDOF enabled both with and without NumCrctn = 1? (You already shared the results without.) Is the spike diminished at all with NumCrctn = 1?

If the spike is not diminished, the next step in debugging should be to figure out the source of the spike. I would suggest outputting the various components of the total hydrodynamic force, which we know has the spike. I’m curious if the spike exists in the added mass, or if it exists in other components such as the viscous drag, fluid-inertia, buoyancy, marine growth weight, or marine growth inertia?

The problem with the nonzero mean heave displacement is that the only hydrodynamic term in HydroDyn that is currently computed based on the displaced position of the body is the hydrostatic/buoyancy loads. The other terms (added mass, viscous drag, fluid inertia, etc.) are computed at the undisplaced position of the floater. So for a large mean heave offset, the wetted surface of the floater is not modeled accurately with regards to the hydrodynamic loads.

Best regards,

9

Hi Jason,

Yes, no problem at all.

Please find the requested output files in the same folder as before, the link to which I’ve pasted below:
drive.google.com/drive/folders/ … sp=sharing

Kind regards,
Stephen

10

Dear Stephen,

From these results with GenDOF disabled, it looks like setting NumCrctn = 1 does eliminate the spike. But the simulation ends at 238.378 s. What error message does this simulation generate?

Best regards,

11

Hi Jason,

See full error message pasted at the end of this text.

It appears that the grid I had defined in Turbsim (145m high x 145m wide) when generating a .bts file for InflowWind was too small:
“…Grid too small in Y direction. Y=74.949; Y boundaries = [-72.5, 72.5]…”
I’ve increased these grid dimensions to 160m x 160m instead, and the simulation runs successfully without aborting.
So thankfully I think I have finally overcome the issue!

In summary, to model marine growth on the spar platform I’ve had to turn the GenDOF off, set NumCrctn=1, and generate a larger inflow wind field using TurbSim.

I have a few final questions that you may be able to answer.
These are just to ensure I have a full understanding off what effect the above changes have had on my simulation:

  1. I understand that disabling GenDOF means that the wind turbine controller is turned off, and so the turbine rotates freely under any wind conditions. Are there many real-world scenarios where the wind turbine controller would be switched off?

  2. By setting NumCrctn = 1, I have added a “correction iteration” to the simulation. I haven’t found much information on this within the ElastoDyn user manual. Can you briefly explain what this means? Does it simply improve the accuracy of results?

  3. As mentioned above, I have used TurbSim to generate a new wind field to be used within InflowWind. This is because the original file called up, ‘90m_12mps_twr.bts’, didn’t seem to last the required simulation time of 300s. I have kept all parameters, except the grid size as discussed above, unchanged when creating this new .bts file. I know there are also options to model a “steady” and “uniform” wind state, by changing WindType parameter within InflowWind.dat file. Is it fair to say that the turbulent wind field generated by Turbsim is most representative of real world conditions, and therefore more realistic than a “steady” or “uniform” wind flow ?

Thanks again for all your help thus far.

Kind regards,
Stephen

Error Message

OpenFAST

Copyright (C) 2021 National Renewable Energy Laboratory
Copyright (C) 2021 Envision Energy USA LTD

This program is licensed under Apache License Version 2.0 and comes with ABSOLUTELY NO WARRANTY.
See the “LICENSE” file distributed with this software for details.

OpenFAST-v3.0.0
Compile Info:

  • Compiler: Intel(R) Fortran Compiler 1900
  • Architecture: 64 bit
  • Precision: single
  • OpenMP: No
  • Date: Jun 23 2021
  • Time: 21:25:05
    Execution Info:
  • Date: 08/06/2021
  • Time: 11:59:02+0100

OpenFAST input file heading:
FAST Certification Test #24: NREL 5.0 MW Baseline Wind Turbine with OC3 Hywind Configuration,
for use in offshore analysis

Running ElastoDyn.
Nodal outputs section of ElastoDyn input file not found or improperly formatted.
Running AeroDyn.
AD15 Nodal Outputs: Nodal output section of AeroDyn input file not found or improperly formatted.
Skipping nodal outputs.
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 1)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 2)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 1, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 2, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 3, blade 3)
Warning: Turning off Unsteady Aerodynamics because C_nalpha is 0. (node 4, blade 3)
Running InflowWind.

Reading a 31x31 grid (145 m wide, 17.5 m to 162.5 m above ground) with a characteristic wind
speed of 12 m/s. This full-field file was generated by TurbSim (v2.00.07a-bjj, 14-Jun-2016) on
26-Jul-2021 at 15:18:48.

Processed 12600 time steps of 20-Hz full-field data (period of 630 seconds).
Running ServoDyn.
Running ServoDyn Interface for Bladed Controllers (using Intel Visual Fortran for Windows, ).
Using legacy Bladed DLL interface.
Running HydroDyn.
Generating incident wave kinematics and current time history.

MAP++ environment properties (set externally)…
Gravity constant [m/s^2] : 9.81
Sea density [kg/m^3] : 1025.00
Water depth [m] : 320.00
Vessel reference position [m] : 0.00 , 0.00 , 0.00

Time: 0 of 300 seconds.

FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:DLL_controller_call:Run
ning with torque and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll
as written by J. Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies. Includes
modifications for the Hywind spar.

FAST_Solution:FAST_AdvanceStates:SrvD_UpdateStates:DLL_controller_call:BladedInterface option was
designed for an explicit-loose coupling scheme. Using last calculated values from DLL on all
subsequent calls until time is advanced. Warning will not be displayed again.
Time: 238 of 300 seconds. Estimated final completion at 12:02:08.

FAST_Solution:FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_FF
Wind_CalcOutput [position=(15.85, 74.949, 79.603) in wind-file coordinates]: FF wind array
boundaries violated: Grid too small in Y direction. Y=74.949; Y boundaries = [-72.5, 72.5]

OpenFAST encountered an error at simulation time 238.38 of 300 seconds.
Simulation error level: FATAL ERROR

Aborting OpenFAST.

12

Dear Stephen,

OK, glad that was a simple fix. Now that the simulation with GenDOF disabled works as expected, the next step is to figure out why the simulation with the GenDOF enabled is failing. I have not looked at these results before. Does the simulation with GenDOF enabled show a similar spike and what happens to this spike with NumCrctn = 1?

Here are my answers to your direct questions:

  1. No. As I said, the next step should be to get the simulation with GenDOF working. It is always easier to simplify a model first to debug.

  2. NumCrctn is not a feature of ElastoDyn, but a feature of the OpenFAST glue code. OpenFAST makes use of an implicit loose-coupling scheme based on a predictor-corrector algoritihm, e.g. as summarized on slide 23 of the modeling workshop presentation we had at the NAWEA / WindTech conference in 2019: drive.google.com/file/d/17LDwMn … sp=sharing. There is typically a balance between the use of smaller time steps and the addition of correction steps (with setting NumCrctn = 1 roughly equivalent to halving the time step). We have found, though, that some models that make use of the new HydroDyn functionality introduced in OpenFAST v2.6 and newer require NumCrctn = 1 rather than simply reducing the time step. We are not sure why this is yet (likely some reordering of the glue code will solve this problem); but hopefully we an solve this in a future release.

  3. Yes, most load cases (as specified in wind turbine design standards) involve turbulent inflow. Some load cases involve discrete transient events like gusts and direction changes, which can be implemented via uniform wind data files. Steady wind is mostly for simple steady-state calculations and linearization.

Best regards,