Error in calculating tower loads of an offshore wind turbine

Hello everyone,

Background:

I want to calculate tower loads of a bottom mounted (with monopile) offshore wind turbine for turbulent wind, in region-3 (above rated wind region). For that reason I have enabled CompElast, CompInflow, CompAero, CompServo, CompHydro and CompSub, i.e. (=1), in main input file of FAST. Additionally, I have enabled all platform DOFs, i.e. (=True), in ElastoDyn file.

Problem:

The system becomes unstable after 2 seconds of simulation (even in steady wind) in open loop as well as closed loop tests. For error message please see the attachment.

Relevant Information:

Whereas, the system runs fine without enabling “Platform vertical heave translation DOF”. Rest of the DOFs enabled or disabled are as follows:

True FlapDOF1 - First flapwise blade mode DOF (flag)
False FlapDOF2 - Second flapwise blade mode DOF (flag)
True EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
True DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
True GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
False TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
True TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)

Question
What could be wrong with the system? How can I identify the main problem and solve it?

Any help would be appreciated.

Best regards
Syed Shah

Dear Syed,

Warnings regarding a “small angle approximation violation,” warnings regarding “supersonic blades,” a simulation crash, or very large deflections that occur in the time series are good signs of a numerical instability. Without knowing more about your simulation settings, it’s hard to know that what the problem is, but I would guess that your model is suffering a numerical instability e.g. from the use of too large of a time step. Or use “Search…” in the upper right of the forum to search for “small angle assumption” to see how we’ve solved similar problems in the past.

Have you followed the guidance for coupling SubDyn and ElastoDyn as given in section 5.4 of the draft SubDyn User’s Guide and Theory Manual wind.nrel.gov/nwtc/docs/SubDyn_Manual.pdf?

Best regards,

Dear Jason,

I followed the guidance for coupling SubDyn and ElastoDyn as given in section 5.4 of the draft SubDyn User’s Guide and Theory Manual.

Thank you for your timely response. I have solved the issue by reducing the step size.

Dear Jason,

I have a question related to FAST8.

I need to design a controller for offshore wind turbine and in order to do it, I want to linearize wind turbine model with six DOFs associated to platform. (i.e. PtfmSgDOF, PtfmSwDOF, PtfmHvDOF, PtfmRDOF, PtfmPDOF and PtfmYDOF). However, it is not yet possible to linearize a turbine model with HydroDyn and SubDyn in FAST8.

What is the solution to the problem? It would be really helpful if you share the solution of this problem.

Dear Syed,

Your question has recently been discussed in the following forum topic: http://forums.nrel.gov/t/linearization-of-the-nrel-5mw-oc4-deepcwind-semisubmersible-wind-turbine/1799/1.

Best regards,

Dear Jason,
Thank you for your quick response.

I just want to clarify my doubts. I have gone through this post. What i have understood from above mentioned post is that I need to convert my FAST8 model into FAST7 model? In FAST7, there is possibility to linearize offshore wind turbines with these DOFs?

Dear Syed,

Yes, FAST v7 can be used to linearize floating offshore wind turbine models with 6 platform DOFs enabled, but the linearization does not include the wave disturbance, radiation damping must be disabled, and the hydrodynamics capability in FAST v7 is less advanced than in FAST v8.

Best regards,

Dear Jason,

Sorry to ask this basic question, my wind turbine is bottom mounted (with monopile). The degrees of freedom i want to enable for linearization process are as follows:

True FlapDOF1 - First flapwise blade mode DOF (flag)
False FlapDOF2 - Second flapwise blade mode DOF (flag)
True EdgeDOF - First edgewise blade mode DOF (flag)
False TeetDOF - Rotor-teeter DOF (flag) [unused for 3 blades]
True DrTrDOF - Drivetrain rotational-flexibility DOF (flag)
True GenDOF - Generator DOF (flag)
False YawDOF - Yaw DOF (flag)
True TwFADOF1 - First fore-aft tower bending-mode DOF (flag)
False TwFADOF2 - Second fore-aft tower bending-mode DOF (flag)
True TwSSDOF1 - First side-to-side tower bending-mode DOF (flag)
False TwSSDOF2 - Second side-to-side tower bending-mode DOF (flag)
True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
True PtfmHvDOF - Platform vertical heave translation DOF (flag)
True PtfmRDOF - Platform roll tilt rotation DOF (flag)
True PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)

Do I still need to convert my FAST8 model into FAST7 model for this linearization?

Dear Syed,

I’m not really sure I understand what you are trying to do. Normally if you model a monopile in FAST v8, you’ll enable the platform DOFs in ElastoDyn and also enable SubDyn and HydroDyn. But you can’t yet linearize in FAST v8 with SubDyn and HydroDyn.

If you enable the platform DOFs in ElastoDyn without enabling SubDyn or HydroDyn, then the platform will not have a reaction with the substructure, and thus, the model will then never be in equilibrium (e.g. it will want to fall due to gravity).

Best regards,

Dear Jason,

Thank you very much for you clearing my doubts.

This was exactly what I needed to ask. So it seems I will have to convert my Model in FAST v7.

Actually I wan to design a controller for offshore wind turbine load reduction in region-3. For that reason I wanted to linearize my model with the above mention DOFs. However, as you said in you prior post that “the linearization does not include the wave disturbance”. So what I have understood from your this sentence is that despite linearization in FAST v7 and designing controller with that model afterwards, my controller will not be able to take into account hydrodynamics loads? Or It will not be able to react on hydrodynamic excitations?

Dear Syed,

The linaerization of FAST v7 will include the linearized effects from hydrodynamic added mass, hydrostatic restoring, and hydrodynamic viscous drag (due to substructure motion), but it will not include terms associated with wave excitation (e.g. fluid-inertia terms, dynamic pressure terms, or wave kinematics-induced viscous drag).

Best regards,

Dear Jason,

Thank you very much for clearing my doubts.

Dear Jason,

I have converted my FAST v8 model into FAST v7 model in order to linearize it with platform DOFs. But the issue I am having that when I enable all 6 platform DOFs then the warring message that appears in FAST v7 simulation is as follows:

I have gone through this post "http://forums.nrel.gov/t/large-displacement/1668/2 and in order to debug the problem I eliminated DOFs as you suggested there.
The simulation runs fine with 2 DOFs, i.e., PtfmSgDOF and PtfmSwDOF. However, when I try to turn on any other platform’s DOF then the above mentioned warning appears.

kindly advise how to solve the problem.

Dear Syed,

Do you expect the model to be in some sort of equilibrium based on the applied aerodynamic loads, buoyancy, and reaction loads from the mooring system you are modeling in FAST v7? What does the time series of platform motion look like before the small angle assumption violation warning is triggered?

Best regards,

Dear Jason,
The turbine model in FAST v8 is completely stable (both in Simulink and in Command Prompt), while converting from FAST v8 to FAST v7, I didn’t change any parameter. So My system should be in stable in FAST v7.
However, I have observed another issue in addition to the issue I already mentioned in my previous post with the FAST v7 model, it runs fine only in Command Prompt with “PtfmSgDOF and PtfmSwDOF” (without becoming unstable). Whereas, in Simulink, the system is unstable with any platform DOF.

kindly advise how to solve the problem.

I am attaching the platform input file.

This error is shown when I try to upload graphs of platform motions “Sorry, the board attachment quota has been reached”.
Platform_Monopile.txt (6.15 KB)

Dear Syed,

I’ve changed the board attachment quota so hopefully you don’t run into that problem anymore.

I took a brief look at your platform file, but I’m not sure what you are trying to do. Your platform file has PtfmLdMod = 0, so, there will be no reaction loads applied to the platform and enabling any platform DOF will result in acceleration in that DOF if loads are applied (e.g. the turbine will accelerate downwind if aerodynamics are enabled or fall due to gravity if PtfmHvDOF is enabled). What type of platform are you trying to model?

Best regards,

Dear Jason,

I want to implement an LQR to minimize the loads of a fixed bottom offshore wind turbine. Taking into consideration support structure loads, as well as wind and wave excitation. For that purpose I need to linearize my system with platform DOFs. Due to this I have converted my FAST v8 model into FAST v7 model because in FAST v8 you cant linearize the system while taking into account hydrodynamic loads. The platform is fixed bottom mounted monopile.

Can you kindly send me the fixed bottom mounted monopile file for NREL 5MW? Or please address the issue. how to linearize my system with fixed bottom mounted monopile’s effect taking into account?

Additionally, I am still having this same error “Sorry, the board attachment quota has been reached”. But my graphs are exponentially increasing graphs.

Dear Syed,

You can find a FAST v7 model of the NREL 5-MW turbine mounted atop the OC3-monopile here: wind.nrel.gov/public/jjonkman/NR … ile_RF.zip.

Oops, I realized that the board attachment quota that I set did not get saved. That should be fixed now.

Best regards,

Dear Jason,

I am still having problems to upload the graph images. Can you kindly advise me how to solve my problem?

Dear Syed,

There is a 5-MB limit per image uploaded. I am able to upload smaller images without any problem. I’m not sure how to help.

Best regards,