Extreme events

Dear Dr. Jonkman,

I want to simulate the parked scenario of OC4 offshore floating wind turbine. I go through all the posts related to this topic and I still cannot figure out my problem. Thus, I am here to look for your help. following your previous post and suggestions, I make the following settings in my fast input files:

  1. I turn off the GenDOF (and even disable the YawDOF for simplicity as well);
  2. set the initial and final pitch angles are 90°
  3. initial rotor speed 0;
  4. PCmode and VSControl are 0, means disabled;
    5, HSSBrMode on and set the HSSBrTqF 20000(I think this value is large enough)

I think I have followed your advice and provisions in User’s manual 2005.

But I have one question about the result I get, as shown in following pic. I am wondering why the Rotor Thrust still has a value of 100KN, I think this value should be near 0, whilst the tower base shear in x direction(align with the wind and wave direction) is lower than the RoThrust. I think this outcome is unreasonable but I tried to modified my input parameters, I find no solution.

Looking forward your reply!

Best regards,
Charlie Sheng

Dear Charlie,

FAST output RotThrust (from the ElastoDyn module) not only includes the applied aerodynamic thrust, but also the gravity and inertial loads of the rotor. See the following forum topic for more information: http://forums.nrel.gov/t/the-effect-of-tilt-angle/769/1. So, the mean value of RotThrust is likely more the result of rotor weight than applied aerodynamic loads.

Best regards,

Dear Dr. Jonkman,

After reading the post you attached, and I also check the rotor thrust from the AeroDyn output (i.e. RtAeroFxh), this value is indeed very small when turbine parked. Also I check the rotor thrust from the ElastoDyn((i.e. RotThrust), they are indeed more likely generated by the rotor gravity and inertial force when there is tilt angle for rotor.

Thanks so much for your fast reply which solve me such as big comfusion.

Best regards,
Charlie Sheng

Dear Jason,

In order to run an idling condition case with FASTv8, besides setting the following parameters:


PCMode = 0 (disable pitch control)
GenTiStr = True
TimGenOn = 9999.9 (a large don’t care > TMax)


True GenDOF
BlPitch1 = BlPitch2 = BlPitch3 = 90.0 (fully feathered)
RotSpeed = 0.0 (or close to zero)

and AeroDyn
0 WakeMod
1 AFAeroMod

Do I also have to change anything in the block from ServoDyn:

-------------------- HIGH-SPEED SHAFT BRAKE ----------------------------------
0 HSSBrMode - HSS brake model {0: none, 1: simple, 3: user-defined from routine UserHSSBr, 4: user-defined from Simulink/Labview, 5: user-defined from Bladed-style DLL} (switch)
9999.9 THSSBrDp - Time to initiate deployment of the HSS brake (s)
0.6 HSSBrDT - Time for HSS-brake to reach full deployment once initiated (sec) [used only when HSSBrMode=1]
28116.2 HSSBrTqF - Fully deployed HSS-brake torque (N-m)
? Or is it ok as it is?

Thank you in advance.
Best regards,
Rocío Torres

Dear Rocio,

Your inputs look correct to me. An “idling” case implies that the shaft brake is not engaged.

Best regards,

Dear Jason,

Again, thank you for your kind and rapid response and help!

Best regards,

Hello all!

I am trying to reproduce an idling EWM1 DLC with a Vhub=0.8Vref=40m/s (class IC) and a yaw misalignment of 20º using the 5MW baseline model (land version). Blades pitched to 85º.
For this, in addition to setting the corresponding parameters to make it an “idling” case I am fixing the yaw parameter so it doesn’t change (False YawDOF - Yaw DOF (flag) ). Also,since I am only interested in the rotor’s aerodynamics I have made the tower infinitely rigid :

False TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
False TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
False TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)

In the result I am getting, I can see that RtAeroFyh changes sign frequently and this doesn’t make much sense to me since the rotor is almost still, the wind speed isn’t changing that much in direction or magnitude…I would think the resulting Fy force should have always the same direction and sense…
Can anyone tell me if the results I am obtaining are realistic? (I am attaching the output file)
The Fx force is also changing direction.

Thank you!
EWM1_40ms_yaw20_pitch85.txt (2.03 MB)

Dear Rocio,

Looking briefly at your results, I see that the rotor is spinning at around 1 rpm, which results in a 3P excitation in the fixed frame at around 0.05 Hz, which is the oscillation frequency I see in RtAeroFxh. I’m sure the variation in aerodynamic loads are a result of rotational sampling of the rotor in the highly turbulent wind field.

Best regards,

Thank you Jason,

I’ll look into that.

Best regards,

Dear Jason and all,

Idling DLCs are in general simulated using extreme wind models. Namely, extreme turbulent or steady-state-extreme. Before, I have run the simulations with turbulent model wind. Now, I would like to verify my model with steady-state -extreme wind (Lets say EWM1). But in the guidelines, it is mentioned as transient oblique wind of up to 15 degrees. There are no other references, how long the transient needs to be (ramp). Is it agreeable, if I design my wind file with 1.2s transient upto 15 degrees like ECD? Or simple NWP with the extreme wind and 15 degree propagation direction in inflow wind is sufficient?

Thanks and regards

Dear Abhinay,

The steady extreme wind model does not involve transients. The standard says a constant yaw misalignment of +/-15deg is to be used. Load cases with the steady extreme wind model are meant be simulated via quasi-steady analysis, rather than via dynamic time-domain simulations.

Best regards,

Hello all,

I am trying to simulate a load case with internal electrical faults according to IEC 61400-1 Ed.3 (Design Load Case 2.2). I am considering two sub cases here - first where 1 blade is stuck at minimum pitch and second where all blades pitch to go to minimum pitch rapidly. The operating conditions are cut in to cut out wind speed with NTM turbulence model and 12 random seeds, no yaw errors.

The simulations run without crashing, and there seems to be nothing obviously wrong with the simulation files themselves. However, when I find out the extreme values from these simulations, I am getting pretty unrealistic values for different components. For example, I ran the NREL 5 MW reference wind turbine for these load cases, and I got BR Flap moment extreme value ~50000 kNm. In comparison, the same model run for DLC1.3 (Normal Power Production with ETM) had BR Flap moment extreme value ~18000 kNm. I am not sure that the ~50000 kNm value is right, because even with extreme turbulence model I am getting a much lower value.

The settings I am using for simulating DLC 2.2.1 (1 blade stuck at minimum pitch) are -

---------------------- INITIAL CONDITIONS --------------------------------------
0 OoPDefl - Initial out-of-plane blade-tip displacement (meters)
0 IPDefl - Initial in-plane blade-tip deflection (meters)
8.0 BlPitch(1) - Blade 1 initial pitch (degrees)
8.0 BlPitch(2) - Blade 2 initial pitch (degrees)
8.0 BlPitch(3) - Blade 3 initial pitch (degrees) [unused for 2 blades]
0 TeetDefl - Initial or fixed teeter angle (degrees) [unused for 3 blades]
0 Azimuth - Initial azimuth angle for blade 1 (degrees)
12.1 RotSpeed - Initial or fixed rotor speed (rpm)
0 NacYaw - Initial or fixed nacelle-yaw angle (degrees)

1 WakeMod - Type of wake/induction model (switch) {0=none, 1=BEMT, 2=DBEMT, 3=OLAF} [WakeMod cannot be 2 or 3 when linearizing]
1 AFAeroMod - Type of blade airfoil aerodynamics model (switch) {1=steady model, 2=Beddoes-Leishman unsteady model} [AFAeroMod must be 1 when linearizing]

---------------------- PITCH CONTROL -------------------------------------------
5 PCMode - Pitch control mode {0: none, 3: user-defined from routine PitchCntrl, 4: user-defined from Simulink/Labview, 5: user-defined from Bladed-style DLL} (switch)
9999.9 TPCOn - Time to enable active pitch control (s) [unused when PCMode=0]
100.1 TPitManS(1) - Time to start override pitch maneuver for blade 1 and end standard pitch control (s)
9999.9 TPitManS(2) - Time to start override pitch maneuver for blade 2 and end standard pitch control (s)
9999.9 TPitManS(3) - Time to start override pitch maneuver for blade 3 and end standard pitch control (s) [unused for 2 blades]
2 PitManRat(1) - Pitch rate at which override pitch maneuver heads toward final pitch angle for blade 1 (deg/s)
2 PitManRat(2) - Pitch rate at which override pitch maneuver heads toward final pitch angle for blade 2 (deg/s)
2 PitManRat(3) - Pitch rate at which override pitch maneuver heads toward final pitch angle for blade 3 (deg/s) [unused for 2 blades]
0 BlPitchF(1) - Blade 1 final pitch for pitch maneuvers (degrees)
0 BlPitchF(2) - Blade 2 final pitch for pitch maneuvers (degrees)
0 BlPitchF(3) - Blade 3 final pitch for pitch maneuvers (degrees) [unused for 2 blades]

I would be very grateful for any help I can get!

Best regards,
Vishal S

Dear Vishal,

Your simulation settings seem OK, except that I would expect that you’d enable the pitch controller at the start of the simulation (TPCOn = 0.0 s).

What wind speed are you receiving the high blade-root flap moment?

Otherwise, I’m not sure I can comment on the results you are seeing without seeing what the turbine response is. Is the large blade-root flap moment obtained during the pitch fault transient?

Best regards,

Hi Jason,

I did enable the pitch controller (TPCOn) at the start of the simulation at 0.0 s, but then when I overrode the pitch control for blade 1 at 100.1 s, the pitch for the other two blades started oscillating between -180 and 180 degrees, indicating an instability. But when I did not enable the pitch controller at all (TPCOn = 9999.9), the pitch for blade 1 came down to 0 while the other two blades stayed at constant pitch corresponding to that wind speed. This seemed to bring down the extreme values (for BR flap it came down from ~50000 kNm to ~30000 kNm), but the range is still quite high.

I am getting the really high blade root flap moments after rated speed, the highest value occurred at 24 m/s.

I run the simulations for 200 s and I remove the first 100 s due to transient behavior. The large BR flap moments (in fact, loads all over the turbine) occur after the 1st blade pitches to 0 at around 110 s.

I have not come across any research papers/articles where these types of faults are simulated and results tabulated, so I am having trouble validating the results I have obtained. Could you please point me towards any papers/reports that have done this type of simulations if you happen to know of any?

Best regards,

Dear Vishal,

It sounds like your controller is going unstable when you enable pitch control together with the pitch runaway event. Is this a controller NREL has provided or one you implemented yourself? Sounds like you may want to debug why the controller is going unstable.

Typically a pitch runaway event is simulated followed immediately by a turbine shutdown (assuming the safety system detects the pitch runaway fault). That is how we simulated load case 2.1 when the NREL 5-MW baseline turbine was first developed. This is documented in my Ph.D. thesis-turned NREL report: nrel.gov/docs/fy08osti/41958.pdf.

Best regards,

Hi Jason,

I am using the controller NREL has provided. I downloaded it from the GitHub repository.

Thank you for sharing your thesis with me, I believe I found the answer. I will simulate wind speeds after rated to capture the pitch fault, where 1 blade pitches to 0 and then the other 2 blades pitch to 90 degrees after a time delay for the controller to detect that there is a pitch fault.

Thank you very much for your answers and the report!

Best regards,
Vishal S

Continuing the discussion from Extreme events:

Dear Jason ,
I am currently modeling the OC3 Hywind (NREL 5MW Baseline Offshore) wind turbine in extreme turbulent and wind phenomena.
I have made some changes in the geometry of the turbine (Platform ,Tower) and in the water depth. (Hub Height=75m , Water Depth=150m) .

I want to see how the platform will react (Surge,Heave,Sway,Roll,Pitch,Yaw) in extreme wind models. Thus, i am testing these 4 scenarios for the parked model :

EWM Class IA with 50yr return period
EWM Class IB with 50yr return period
EWM Class IIA with 50yr return period
EWM Class IIB with 50yr return period

The thing that concerned me the most is that the models with the same turbine class but different turbulence scale ( IA and IB or IIA and IIB) produce the same motion of the platform through the entire simulation.

Is this something expected?
Thanks in advance.

P.S I get no errors in my simulations.

Dear Nikos,

Are you running these simulations with an idling rotor with the blades fully feathered? Is tower aerodynamics enabled? Is this DLC 6.1 (without yaw error) or 6.2 (with loss of electrical network leading to yaw error)?

I would expect for an idling rotor with fully feathered blades and no yaw error that the wind loads on the rotor would be small in comparison to the wind loads on the tower. The platform motion may also be dominated by the 50-year extreme sea state (ESS) you are likely also simulating with.

Best regards,

Dear Jason ,
Thank you for your reply. To answer your questions , i am running the simulation with an idling rotor with the blades fully feathered .Regarding to the tower aerodynamics all the degrees of freedom in the elastodyn file are enabled (true) except from the TeetDof.
For the Design Load Cases (DLC) i did not make any major changes in the DAT files except from Hs and Tp in the Hydrodyn file (Jonswap). (turbsim is running the simulation for Vhub=50m/s)
Last but not least when it comes to the yaw error i have the initial conditions (ptfmYaw and NacYaw ) set to 0 .
How exactly is DLC 6.1 and 6.2 differ in terms of OpenFast inputs?
Any help would be deeply appreciated.
Thank you in advance.

Dear Nikos,

Tower aerodynamics are enabled in AeroDyn, not ElastoDyn. Have you enabled TwrAero in AeroDyn?

Normally idling rotors are modeled with WakeMod = 0 and AFAeroMod = 1 in AeroDyn; is that what you’ve used?

DLC 6.1 would be simulated with NacYaw = 0 in ElastoDyn (assuming you are setting PropagationDir = 0 in InflowWind). DLC 6.2 would be simulated with nonzero NacYaw (and also nonzero YawNeut in ServoDyn when YawDOF = True in ElastoDyn).

Best regards,