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,

[haoran-Wang-1023]

Dear @jjonkman,

Thanks for your reply.

I used the normalized direct method to extract the polynomial coefficients. I set hub_conn = 1, so for a beam cantilevered to a rigid and stationary base, the entered slope should be zero. I think the error is because I copied the S-S slope value to the column corresponding to the tangent line.

Best regards,
haoran

[jjonkman]

Dear @haoran-Wang-1023,

For a cantilevered beam, the slope at the base should be zero and all three methods should yield the same result.

Best regards,

[haoran-Wang-1023]

Dear @jjonkman,

Thanks for your reply.

When I set the slope at the base equal to zero, two of the three methods give the same result, as shown below.

Thus, I would like to know which method I should choose or how to ensure that the results of the three methods are consistent.

Best regards,
haoran

[jjonkman]

Dear @haoran-Wang-1023,

The Direct Method is the least accurate, so, given the fact that the Improved Direct method and Projection method produce the same result, I would use those consistent solutions.

Best regards,

[haoran-Wang-1023]

Dear @jjonkman,

Thanks for your patient reply. I learned a lot.

Best regards,
haoran