Partially installed wind turbines

Wind & Water Computer-Aided Engineering Software Tools
1

Hello all together,

I want to simulate partially installed wind turbines to study the movements of them. The wind turbine is to be a 3-blade horizontal wind turbine and NREL 5 MW reference wind turbnine is used as a model with monopile. The movements of the partially installed wind turbine will be simulated during single blade assembly, resulting in three scenarios:

Scenario A: Hammer head configuration, no rotor blades are installed on the partially installed wind turbine yet.

Scenario B: one rotor blade is already mounted on the partially installed wind turbine.

Scenario C: two rotor blades are mounted on the partially installed wind turbine.

To simulate scenario A, I designed a rotor blade that is very short (0.1m) and very light (BMassDen = 0.0000001 kg/m) and get very low loads on the rotor blades.

BladeRootM-NoBlade
BladeRootM-NoBlade750×900 61 KB

To simulate scenario B, I set NumBl = 1 in ElastoDyn for the number of rotor blades and use the rotor blade of the NREL 5 MW Reference Wind Turbine.

However, I have problems with scenario C. Since the positioning of the rotor blades on the hub changes when I use NumBl = 2 (the angle between the rotor blades would be 180° and not 120°), I cannot proceed as I did with scenario A. In addition, the length of the rotor blades is set the same for all via TipRad in ElastoDyn. Therefore, I wanted to proceed similarly to scenario B, in which I use a rotor blade as a third rotor blade that has a very low mass and a low aerodynamic influence. The used values for ElastoDyn and AeroDyn are listed below. For the other two rotor blades I used the rotor blades of the NREl 5 MW reference wind turbine.

ElastoDyn Distributed blade properties:

---------------------- DISTRIBUTED BLADE PROPERTIES ----------------------------
    BlFract      PitchAxis      StrcTwst       BMassDen        FlpStff        EdgStff
      (-)           (-)          (deg)          (kg/m)         (Nm^2)         (Nm^2)
0.0000000E+00  0.0000000E+00  0.0000000E+00  0.0000001E+00  9.9999999E+10  9.9999999E+10
1.0000000E+00  0.0000000E+00  0.0000000E+00  0.0000001E+00  9.9999999E+10  9.9999999E+10

AeroDyn Blade Properties:

======  Blade Properties =================================================================
         2   NumBlNds           - Number of blade nodes used in the analysis (-)
  BlSpn        BlCrvAC        BlSwpAC        BlCrvAng       BlTwist        BlChord          BlAFID
   (m)           (m)            (m)            (deg)         (deg)           (m)              (-)
0.0000000E+00  0.0000000E+00  0.0000000E+00 0.0000000E+00  0.0000000E+00  0.0000001E+00        1
6.1499900E+01  0.0000000E+00  0.0000000E+00 0.0000000E+00  0.0000000E+00  0.0000001E+00        1

The simulation results for the root moments of the three rotor blades are shown in the following figure:

BladeRootM-2Blade
BladeRootM-2Blade750×900 58.2 KB

As you can see blade 1 (Bld1) is the adapted rotor blade. I had hoped that the moments acting on the rotor blade would be very small. Does anyone have any idea how I can further reduce the influence of rotor blade 1 (Bld1) and where the high flapwise moments are coming from?

I am using OpenFAST-v3.1.0 on a Windows computer. The modules ElastoDyn, InflowWind, AeroDyn15, HydroDyn and SubDyn are used. For the wind and wave conditions the conditions from the R-Test “5MW_OC3Mnpl_DLL_WTurb_WavesIrr” were used. The turbine is in the parked state.

Thanks for your help and input, if something is not clearly explained or i forgot some informations please ask.

Best regards

Malte

2

Dear @Malte.Frieling,

I agree with your treatment of scenarios A and B.

For scenario C, I’m actually surprised you can get the simulation to run. But setting the stiffness to be high and the mass to be low, the natural frequency would be very high, requiring a very small time step. Or have you disabled all of the blade degrees of freedom in ElastoDyn to run this case? Please clarify.

Unfortunately, ElastoDyn does not currently support the ability to model one of the blades rigidly with two other blades flexible. (This would require a source code change to implement.)

Best regards,

3

Hi Dr. Jonkman,

thanks for your quick response.

I have not enabled the DOFs of the rotor blades.

When the DOFs of the rotor blades are unlocked, the simulation is aborted due to high rotor blade deflections and a tower strike occurs.

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_AB4:ED_RK4: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(): 2.50000E-03 s
FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_FFWind_CalcOutpu
t [position=(1.12874E+08, -3.45414E+05, 49102)  in wind-file coordinates]: Error: FF wind array
was exhausted at 5.00000E-03 seconds (trying to access data at -9.40618E+06 seconds).
FAST_AdvanceStates:AD_UpdateStates:SetDisturbedInflow:TwrInfl:getLocalTowerProps:Tower strike.

As I understand your answer it is not possible to simulate Scenario C (three-bladed wind turbine with only two rotor blades mounted) with OpeenFAST?

Greetings

Malte

4

Dear @Malte.Frieling,

OK, that makes more sense. Again, by setting the stiffness to be high and the mass to be low for blade 1, the natural frequency of blade 1 would be very high when you enable the blade DOFs, requiring a very small time step. And without reduced the time step, the model goes numerically unstable as indicated by your large deflection warning and tower strike and FF wind array exhausted errors.

To model Scenario C with flexible blades would require a source code change.

With all blade DOFs disabled, the model runs as expected, so, I guess your question is why the blade 1 loads are higher than you expect. I’m not sure I can fully explain that, but you could always try to reduce the blade 1 loads further by reducing BMassDen and BlChord further or by choosing an airfoil (via BlAFID) with the airfoil coefficients zeroed.

Best regards,