Problem with "tower strike" for help

[Xukaizh]

Hi everyone, I am new to openFAST. Currently, I want to run openFAST in a tropical situation. So I used the file in rtest "5MW_OC4Semi_WSt_WavesWN". I only changed the wave height to 16m in WaveMod 3 (White noise spectrum). And when I use inflow wind as WindType 2, gradually increasing to 70 m/s at the 30s, the program works fine. But when I use Turbsim to create a wind file and set Windtype as 3, it will give so much error info even when the speed is low. And I don't know why or how to fix it. Could anyone please help me?

Running ServoDyn. Running ServoDyn Interface for Bladed Controllers (using Intel Visual Fortran for Windows). Using legacy Bladed DLL interface. Time: 0 of 60 seconds. The BEM solution is being turned off due to low TSR. (TSR = 0.76221). This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)

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.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut put(node 5, blade 1):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: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(): 0.3875 s

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut put(node 14, blade 2):UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist.

WARNING: Tower strike. This warning will not be repeated though the condition may persist.

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetDisturbedInflow:TwrInfl:Tower strike.

OpenFAST encountered an error at simulation time 0.95 of 60 seconds. Simulation error level: FATAL ERROR

Aborting OpenFAST.

[Xukaizh]

I tried to follow some answers in the forum. When I make FALSE on Generator DOF, and make the initial angle of BLPitch 1, it works with many warnings.

---

     FALSE          GenDOF      - Generator DOF (flag)
      1   BlPitch(1)  - Blade 1 initial pitch (degrees)
      1  BlPitch(2)  - Blade 2 initial pitch (degrees)
      1   BlPitch(3)  - Blade 3 initial pitch (degrees) [unused for 2 blades]

---

The BEM solution is being turned off due to low TSR. (TSR = 0.7713). This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 19, blade 2):UA_UpdateStates:UA_UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist. FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut put(node 5, blade 1):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: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(): 0.3875 s Time: 9 of 60 seconds. Estimated final completion at 16:15:32.

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates:UpdatePhi(node 5, blade 2):BEMT_UnCoupledSolve:There is no valid value of phi for these operating conditions: Vx = 78.244, Vy = -6.5481, rlocal = 11.731, theta = 0.76486, geometric phi = 1.6543. This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)

But when I make Generator DOF on TRUE, it cannot work again, with warning like below:

The BEM solution is being turned off due to low TSR. (TSR = 0.7713). This warning will not be repeated though the condition may persist. (See GeomPhi output channel.)

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 19, blade 2):UA_UpdateStates:UA_UpdateDiscOtherState:ComputeKelvinChain:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist. FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut put(node 5, blade 1):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: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(): 0.3875 s Time: 2 of 60 seconds. Estimated final completion at 16:19:46.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:RotCalcOutput:BEMT_CalcOut put(node 18, blade 1):UA_CalcOutput:Mach number exceeds 1.0. Equations cannot be evaluated.

OpenFAST encountered an error at simulation time 2.5375 of 60 seconds. Simulation error level: FATAL ERROR

Aborting OpenFAST.

Is there any step further I can do to solve this issue? Really appreciate.

[jjonkman]

Dear @Xukaizh,

I suspect the condition are you simulating is simply beyond the design envelop of this floating wind turbine. The NREL 5-MW baseline wind turbine is expected to be shut down for sustained winds above 25 m/s (although the control logic to initiate the shutdown is not implemented in the baseline 5-MW controller). At 70 m/s, I would expect the system to be idling (not producing power) with all blades feathered. You can set up idling conditions by appropriate input file settings in OpenFAST.

A 16 m significant wave height also sounds extreme for the OC4-DeepCwind semi and I would expect that for extreme sea states, you would use WaveMod = 2 (JONSWAP spectrum) rather than WaveMod = 3 (banded white-noise spectrum).

Best regards,

[Xukaizh]

Hi @jjonkman . Thank you so much. I tried a lower wind speed, and it works.

But when I make it idling as said in the manuals below (WaveMod =2):

`To model a parked or idling rotor, we recommend to disable induction (WakeMod = 0) and UA (AFAeroMod = 1), in which case the inflow velocity and angle are determined purely geometrically and the airfoil data is determined statically.

The direct aerodynamic load on the tower often dominates the aerodynamic load on the rotor for parked or idling conditions above the cut-out wind speed, in which case we recommend that TwrAero = TRUE. Otherwise, TwrAero = FALSE may be satisfactory.

We recommend to include the influence of the tower on the fluid local to the blade for both operational and parked/idling rotors. We recommend that TwrPotent > 0 for upwind rotors and that TwrPotent = 2 or TwrShadow > 0 for downwind rotors.`

Also, I tried to change BlPitch to 0.1 and initial RotSpeed to 1 (a lower value). Also I tried to set TimGenOn to 9999. (WaveMod = 2 and waveheight =16; Windspeed at 10m equals to 49m/s, almost 70m/s at hub height.) But still got errors as below:

`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(): 0.4625 s Time: 7 of 360 seconds. Estimated final completion at 15:09:48.

WARNING: Tower strike. This warning will not be repeated though the condition may persist.

FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetDisturbedInflow:TwrInfl:Tower strike.

OpenFAST encountered an error at simulation time 7.05 of 360 seconds. Simulation error level: FATAL ERROR

Aborting OpenFAST.`

Do you have any suggestions on what should I change in this situation?

[jjonkman]

Dear @Xukaizh,

It sounds like you've disabled induction, unsteady airfoil aerodynamics, and torque control, which is correct. Did you also disable pitch control, e.g., by setting TPCOn > TMax (e.g., 9999 s) in ServoDyn?

I would say your problem, though, is the initial blade-pitch angles (BlPitch), which I would think would be 90deg (not 0.1) for an idling case. I would also expect that you'd set RotSpeed = 0.

Best regards,

[444446578]

Hi @jjonkman.The custom wind speed file wind type=2 was used, the wind speed exceeded 25m/s, and an error was reported when running to 40m/s. What is the reason?

[jjonkman]

Dear @444446578,

If wind speeds remain above cut-out, I would expect a real turbine would shut down. However, the baseline controller for the NREL 5-MW wind turbine will not automatically engage a shut down. At wind speeds well above cut out, the operational controller would (unrealistically) continue to pitch the blades to maintain rated power, and eventually get to state where the aerodynamic loads on the blade would cause a tower strike. To avoid this condition, you should initiate a shut down maneuver to idle the turbine in high winds above cut out.

Best regards,