Hello all,

I have a quick question regarding OpenFast. I am trying to calculate a (non-physical) case where an offshore wind turbine is loaded only by hydrodynamic loads, i.e. the wind speed is zero (v=0 m/s). The problem is that I would expect that the maximum overturning moment on the tower at v=0 m/s should be smaller than in a simulation with v= 1 m/s, but this is not the case. (see picture).

I have tried to simulate it in different ways, for example by disabling InflowWind or using Turbsim to create a wind field with an average wind velocity of 0.0001 m/s. However, I always got the same result.

Does anyone have an idea what could be the reason for this? I use OpenFAST v2.2.0 and for the hydrodynamic loads a wave height of hs= 1m and a peak-spectral period tp= 3.1.

Best regards,

Malte

Dear Malte,

I’m not sure what wind turbine you are simulating, but the gravity load from the overhanging mass of the RNA will induce an overturning moment. For an upwind wind turbine, if the mass center is upwind of the tower, then the aerodynamic thrust force on the rotor will first lower the moment (at low wind speeds), and then grow again (opposite sign) at higher wind speeds as the thrust grows. Is this what you are seeing?

Best regards,

Hi Dr. Jonkman,

thank you for your reply and excuse me for just now getting back to you. Yes I simulate an upwind turbine (more specifically, the NREL offshore 5-MW baseline wind turbine).

Using the values for the weight of the rotor blades and the nacelle and the specification for the center of mass of the nacelle from the document “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”, I have estimate a moment of 5.5 *10^6 Nm. If this is calculated out, the overturning moment for v = 0m/s is still significantly higher than the overturning moment for v = 1 m/s. Also the estimated thrust moment according to Betz is quite small (about 2*10^5 Nm for v = 1 m/s).

Together this does not explain for me the big difference between the overturning moments for v = 0 m/s and the other wind speeds.

Maybe you have another clue what the problem could be.

Best regards,

Malte

Dear Malte,

For a rigid structure, I would expect the overturning moment from RNA weight to be -1,420,000 Nm, based on the RNA mass of 350,000 kg and center of mass overhang of -0.413775 m for the NREL 5-MW baseline turbine, from the following forum topic: http://forums.nrel.gov/t/using-aggregate-mass-in-adams-to-check-nrel-cs-monopile-bmi/696/1. Of course, this will be effected by the deflection of the support structure and blades if these DOFs are enabled.

How are you calculating the aerodynamic loads; is the rotor spinning? The wind turbine should not be operational until the cut-in wind speed of 3 m/s.

Are you plotting the mean loads or maximum loads?

Best regards,

Hi Dr. Jonkman,

the rotor is not spinning and the moments are the maximum moments, not the mean moments.

Best regards,

Malte

Dear Malte,

For this parked case, have you disabled the wake model in AeroDyn (WakeMod = 0), which is only valid for operational conditions? Are the blades pitched/feathered to 90-degrees or flat into the wind (0 degrees)?

Are you neglecting the simulation start-up transient before extracting the maximum moment?

Perhaps share time series of the results you are seeing.

Best regards,

Hi Dr. Jonkman,

I forgot and adjusted the settings in AeroDyn for parked wind turbines and had the simulations recalculated, which changed the results only slightly.

The rotor blades are in 0° position and the first 30 seconds of the results are not considered. The maximum moment also does not occur in the first seconds.

Best regards Malte

Dear Malte,

Can you confirm a few things about all these parked simulations:

- The controller is disabled (CompServo = 0), the total run time and time step are the same, and the enabled modules are the same in the OpenFAST driver (ElastoDyn, InflowWind, AeroDyn, HydroDyn, SubDyn?).
- The rotor is parked (GenDOF = False), rotor speed is zero, all blades are pitched to 0 degrees, and all blades have the same DOFs in ElastoDyn.
- The wake and unsteady airfoil aerodynamic calculations in AeroDyn have been disabled (WakeMod = 0, AFAeroMod = 1).
- The wave conditions in HydroDyn are identical between simulations (WaveMod, WaveTMax, WaveDT, WaveHs, WaveTp, WaveSeed).
- The only difference between the simulations is the wind speed in InflowWind.

You are plotting the maximum values from each simulation after neglecting the start-up transient. How do the mean values and standard deviation values from each simulation compare? You’ve shared the time series at V=0 m/s; how do the results at V = 1 m/s compare?

Best regards,

Hi Dr. Jonkman,

sorry for my late response.

• I disabled the controller via ServoDyn for the simulation with wind speed v = 0 m/s (PCMode = 0)

• The run time and the time step are the same.

• The enabled moduls are the same (except InflowWind and AweoDyn, I have tried it with both and without both, the results were very similar, “Simulation without wind” is a simulation without AeroDyn and without InflowWind)

• The rotor is able to move (GenDOF = True)

• The initial rotor speed is zero and the blades are pitched to 90° and have the same DOFs

• WakeMod is 0 and AFAeroMod is 1 (for the simulations with InflowWind and AeroDyn)

• The HydroDyn conditions are the same

I have simulated 2 different ways, once without InflowWind and once with InflowWind and an average wind speed of 0,00001 m/s.

Since we did not find a solution to the problem at first, we left the simulations with v = 0 m/s out. However, we ran simulations again with v = 1 m/s with different settings, where the results look similar. For clarification: the settings for the simulations at v = 0m/s and v = 1 m/s in the graph are listed in the table (for 1<v<25 m/s the settings are the same except for RotSpeed and BldPitch).

Settings v=0m/s v=1m/s

BldPitch 90° 0°

Rotspeed 0 rpm 6 rpm

PCMode 0 5

TimeGenOn 9999.9 0

WakeMod 0 1

AFAero 1 2

With these settings we got the following graphs for the wind speed curve. The simulations with v = 0 m/s stand out very much (which is why I asked the question here).

However, since the wind turbine should be switched off at v = 1 m/s, we changed the settings for v = 1 m/s to those of v = 0 m/s and got a similar phenomenon.

So I think that is somehow with the setting for the switched off turbine at low wind speeds.

Dear Malte,

I’m not sure I understand what your question really is, but I agree that the aerodynamic thrust loads on the rotor (and consequently, the overturning moment) will be very different between an operating rotor and a parked/idling rotor at the same wind speed.

Please note that the NREL 5-MW baseline wind turbine should not be operational to the mean hub-height wind speed reaches 3 m/s.

Best regards,