IEA 22MW_Questions about small angle assumption violated & hydrodyn & servodyn

[haoran-Wang-1023]

Dear Dr. Jonkman and the NREL team,

I am a master student studying the topic of the new jacket foundation based on the IEA 22MW wind turbine. I have faced some questions about OpenFAST. I would appreciate it sincerely if you could give me some advice.

Because of the design of the TP and jacket height, I shortened the height of the tower. So, I changed the DISTRIBUTED TOWER PROPERTIES. But I cannot ensure that it is correct. When I run the program, the warning "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." and "tower strike" always emerged, but finally, OpenFAST terminated normally. I searched the NREL forum and found some similar questions. Then, I changed the initial pitch angle and rotor speed, or set dt=0.0025s. It is indeed useful for solving the "tower strike."
Thus, my questions are
(1) How to solve the warning about the small angle assumption violated.
(2) Is the reason for the warning related to the fact that I shortened the tower height or that the result I recalculated is incorrect?
(3) Can I ignore this warning because the OpenFAST terminated normally?

Hydrodyn questions:
Because OpenFAST v3.5.3 cannot use the function of wave stretching, I used openfast_dev.x64 sharing from Maciej Mroczek. However, when the program runs to 390s (total run time = 600s, dt = 0.006s), the error "For each surface-piercing member, at least two elements must remain fully submerged" emerges. I don't know what happened to hydrodyn. So, I tried to change the MDivSize from 1.0 to 0.5. But it cannot solve this problem. 
Thus, my questions about hydrodyn are
(1) What does "at least two elements" mean?
(2) how to solve this problem.

Servodyn questions:
(1) IEA 22MW shared us the libdiscon.dll and IEA-22-280-RWT-DISCON.IN about monopile. Can these files also be directly applied to the jacket? Do I need to recompile the .dll?
(2) When I set different wind conditions like steady wind/turbsim wind, do I need to change the servodyn or .in file or just set the match inflow wind file?

Thank you very much!

Best regards,
Haoran

[jjonkman]

Dear @haoran-Wang-1023,

Regarding the "small angle assumption violated" warnings and "tower strike" errors, often these are caused by a numerical instability due to an improper model definition (incorrect geometry, mass/stiffness, or time step) or poor choice of model initial conditions (rotor speed and/or blade pitch not matching their expected (mean) values condition on the mean hub-height wind speed being simulated). I'd have to know more about what you changed in your OpenFAST model to comment specifically on what is causing the issue in your model.

Regarding your "HydroDyn" questions, I'm not sure where you obtained the version of OpenFAST you are using, but wave stretching will be included in the official version of OpenFAST in the upcoming release of OpenFAST v4.0. I would suggest upgrading to OpenFAST v4.0 once it is released and report if you are still running into the same issue at that point.

Regarding your "ServoDyn" questions, you shouldn't need to recompile the ROSCO DISCON DLL unless you need to change its source code for some reason. I would think the ROSCO input file for the IEA Wind 22-MW RWT atop the monopile would apply equally for the same turbine atop a jacket, unless the lowest support structure natural frequencies changed a lot in the process. I wouldn't expect that you'd change ServoDyn or ROSCO when changing the wind conditions, but you should change the initial conditions in ElastoDyn for the rotor speed and blade-pitch angles as mentioned in my response to your first question.

Best regards,

[jjonkman]

Dear @haoran-Wang-1023,

If the thrust is similar in the original model, than the jacket is not the source of the issue.

ElastoDyn output RotThrust is the reaction force within the shaft, and so, includes a contribution of rotor weight (projected along the shaft as a result of tilt). If you want the pure aerodynamic applied thrust, you can use AeroDyn output RtAeroFxh.

Best regards,

[haoran-Wang-1023]

Dear @jjonkman ,

Thanks for your reply.

I output the RtAeroFxh. It was 3400 kN, which is smaller than RotThrust (3800 kN). However, it still has a near 600 kN gap with 2793 kN.

I also try to run the IEA-22-280-RWT-Semi model. The RtAeroFxh was also about 3400 kN, which made me confused.

I really don't know what's wrong with the model. Could you give me more advice, or do you need me to provide some setting files?

Best regards,
haoran

[jjonkman]

Dear @haoran-Wang-1023,

Are you getting the rotor speed and blade-pitch angles that you expect for the rated conditions you are simulating, or are these different from what you expect as well?

Best regards,

[haoran-Wang-1023]

Dear @jjonkman ,

I set the initial pitch angle and rotor speed in ElastoDyn. The rotor speed is a constant value. Pitch angle will change with time. But it did not have a big gap with the initial pitch angle.

Best regards,

[haoran-Wang-1023]

Dear @jjonkman ,

I have solved this question by using the ElastoDyn+BeamDyn.

Thanks for your help.

Best regards,

[haoran-Wang-1023]

Dear @jjonkman,

Recently, I compared the tower top displacement between OpenFAST and the FE software, ANSYS. I simplify the RNA to a mass point and consider the interias in ANSYS. The two models have nearly equal frequencies. However, when I compare the tower top displacements, they do not match each other totally.

The load case is very simple; just at steady wind speed, wave and SSI are not considered. I output the loads from AeroDyn, including RtAeroFxh,RtAeroFyh,RtAeroFzh,RtAeroMxh,RtAeroMyh,RtAeroMzh (see Figure 1).

Figure 1: load series

Then, I input the load series into ANSYS and output the tower top displacement to compare with OpenFAST (TwrTpTDxi, TwrTpTDyi). The results are very different (see Figure 2).

Figure 2: Comparison of tower top displacements

Thus, I hope you can give me some advice to solve this problem.

Sincerely,
Haoran

[jjonkman]

Dear @haoran-Wang-1023,

How do the reaction loads at the tower top compare between OpenFAST and ANSYS, e.g., ElastoDyn outputs YawBrFxp, ..., YawBrMzp?

Best regards,

[haoran-Wang-1023]

Dear @jjonkman,

Honestly, at the beginning of the comparison between OpenFAST and ANSYS, I indeed output the ElastoDyn parameters YawBrFxp, ..., YawBrMzp. Then, I input these parameters into tower top in ANSYS. But the results are not good. So, I try to learn other methods mentioned in other papers, such as outputting the AeroDyn parameters.

Come back to your question. I can use APDL to output the reaction loads at the tower top node in ANSYS and compare them with ElastoDyn outputs YawBrFxp,... YawBrMzp.

Best regards,
haoran

[haoran-Wang-1023]

Dear @jjonkman ,

Recently, I ran the regular wave & no wind case condition based on the new jacket foundation.
According to your reply in forum, I set the .fst and the elastodyn file as below:

• CompInflow = 0
• CompAero = 0
• CompServo = 0
• FlapDOF = False
• EdgeDOF = False
• DrTrDOF = False
• GenDOF = False
• YawDOF = False
  At hydrodyn, I set Hs = 8m, Tp = 10s, and chose the WaveMod = 1(regular wave). The Wave1Elev are shown as below.
  

However, when I output the tower top deflection or React-forces, the results were very weird.

• TwrTpTDxi (0-40s)

• TwrTpTDxi (400-1200s)

• ReactFXss (0-100s)

• ReactFXss (400-1200s)

Thus, I sincerely hope you could give me some advice.

Best regards,

[haoran-Wang-1023]

I fold the files into a zip file. However, it has 8 GB. I checked the WeTransfer has a free up to 2GB limit. So, I will rerun the program to output fewer parameters to satisfy the upload limit. It needs some time. And later I will upload the file to your email address.
Could you give me your email address? I can't find it.

[jjonkman]

Sounds good. For my e-mail, see: https://research-hub.nrel.gov/en/persons/jason-jonkman.

[haoran-Wang-1023]

Dear @jjonkman ,

I have sent the files to your email address by WeTransfer named LC12-email.zip.

Thanks for your help.

Best regards,
Haoran

[jjonkman]

Dear @haoran-Wang-1023,

I opened the .outb file you shared in pyDatView (https://github.com/ebranlard/pyDatView) and don't see any issue in the data, which is not zeroed after 2000 s. I suspect the issue is in how you are post-processing the data.

Best regards,

[haoran-Wang-1023]

Sorry, I forgot to illustrate the.outb file that I sent you.
When I tried to run the same cases to output fewer parameters under 4000 s, the program aborted at 2500 s. So, I have sent you the aborted file.
As for the file that became zero after 2000 s, it was too large to send.

[haoran-Wang-1023]

Dear @jjonkman ,

Recently, I am validating the frequency of IEA 22MW based on the monopile. I have a question about it. I would appreicate it very much if you could solve it.

Actually, in Table 13 at IEA22 report, the turbine clamped at monopile base is 0.16 Hz as shown. At first, I thought it must be clamped at -79m.

However, when I run the .fst from github and check the SubDyn.dat. I found it clamped at -34m and the frequecny of wind turbine based on the -34m is 0.16 Hz. Thus, my question is if I run it wrong or the 0.16 Hz in report indeed means the monopile at mudline(-34m) not at -79m.

Best regards,
Haoran

[jjonkman]

Dear @haoran-Wang-1023,

I was not the one to write the IEA Wind 22-MW RWT specifications report, but when a monopile is referred to as "clamped", it typically means that it is clamped at the mudline (not the free end of the monopile), which would be -34 m for this system.

Best regards,

[haoran-Wang-1023]

Dear @jjonkman ,

I am designing a model test for the IEA 22MW turbine. However, the RNA center of mass (CoM) in the tower-top (TT) coordinate system is located at x = 8.65 m and z = 4.62 m, making it challenging to maintain geometric similarity for eccentricity in the test.

To address this, I plan to adopt thrust similarity and adjust the RNA CoM to x = 0 m. However, I am uncertain how to achieve this.

I noticed that in the ElastoDyn.dat file, the parameters NacCMxn, NacCMyn, and NacCMzn can be used to change the nacelle’s center of mass. But for the hub’s center of mass, how can I modify it? Additionally, is there a way to output the RNA center of mass in OpenFAST?

Best regards,
Haoran

[jjonkman]

Dear @haoran-Wang-1023,

The hub and blade masses will naturally lead to an eccentricity of the rotor-nacelle assembly (RNA) center of mass, unless you counteract those mass offsets with a mass offset (in the opposite direction) of the nacelle, such that the combined mass offset of the blades + hub + nacelle is zero.

The RNA center of mass is not directly reported in OpenFAST. If you are using ElastoDyn to model the blades, it should not be hard to calculate by hand because the hub and nacelle masses and centers of masses are user-specified and the integrated mass and center of mass of each blade are calculated and reported in the ElastoDyn summary (ED.sum) file when SumPrint = TRUE in ElastoDyn. You can also use the linearization functionality of OpenFAST to export the full 6x6 rigid-body mass matrix (including mass, center of mass, and inertias) of the RNA, as explained in the following topic on our forum: https://forums.nrel.gov/t/openfast-2nd-order-linearization/2249 (set the distributed tower mass to zero to isolate the RNA mass / inertia).

Best regards,

[haoran-Wang-1023]

Dear @jjonkman ,

Thank you for your constant help and support. Wishing you a Merry Christmas!

Best regards,
Haoran