Error in calculating tower loads of an offshore wind turbine

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,

Dear Jason,

I am attaching the fresh platform input file here Platform_Monopile.txt (6.1 KB). I am still having this uploading error issue. However, I have sent you an email on your official emailing address with subject “Error in calculating tower loads of an offshore wind turbine”. I have attached the graphs of platform motion with that email.

Kindly have a look at them suggest what to change in my model so that system behave as desired.

Dear Syed,

Why do you want to enable platform DOFs without providing any reaction loads on the platform? For a fixed-bottom monopile (with a rigid foundation), I would disable all platform DOFs (PtfmSgDOF = PtfmSwDOF = PtfmHvDOF = PtfmRDOF = PtfmPDOF = PtfmYDOF = False) in FAST v7.

Best regards,

Dear Jason,
sorry to bother you with my naive questions.

My confusion is, If I want to calculate hydrodynamic loads on wind turbine for a fixed-bottom monopile (with a rigid foundation). In that case, should I disable all platform DOFs (PtfmSgDOF = PtfmSwDOF = PtfmHvDOF = PtfmRDOF = PtfmPDOF = PtfmYDOF = False) in FAST v7 as well?

Yes, that’s correct.

Best regards,

Dear Jason,

Thank you for your help.

I have two questions in my mind.

1: In FAST v8 if I want to calculate hydrodynamic loads on wind turbine for a fixed-bottom monopile (with a rigid foundation), in that case, should I disable all platform DOFs (PtfmSgDOF = PtfmSwDOF = PtfmHvDOF = PtfmRDOF = PtfmPDOF = PtfmYDOF = False) while turning on the CompHydro = CompSub = 1 in main FAST input file??

2: In your 2nd last post, you mentioned “reaction loads on the platform”, how to set them here in FAST v7 platform monopile file ??

---------------------- MASS AND INERTIA (CONT) ---------------------------------
0 PtfmMass - Platform mass (kg)
0 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
0 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
0 PtfmYIner - Platfrom inertia for yaw rotation about the platform CM (kg m^2)
---------------------- PLATFORM (CONT) -----------------------------------------
0 PtfmLdMod - Platform loading model {0: none, 1: user-defined from routine UserPtfmLd} (switch)
---------------------- TOWER (CONT) --------------------------------------------
0 TwrLdMod - Tower loading model {0: none, 1: Morison’s equation, 2: user-defined from routine UserTwrLd} (switch)
0 TwrDiam - Tower diameter in Morison’s equation (meters) [used only when TwrLdMod=1]
0 TwrCA - Normalized hydrodynamic added mass coefficient in Morison’s equation (-) [used only when TwrLdMod=1] [determines TwrCM=1+TwrCA]
0 TwrCD - Normalized hydrodynamic viscous drag coefficient in Morison’s equation (-) [used only when TwrLdMod=1]

Dear Syed,

Here are my answers to your questions:

  1. In FAST v7, the monopile structure is modeled as an extension of the tower structure, so the platform (at the tower base) is located at the “seabed”. In FAST v8, the tower structure is modeled in ElastoDyn and the monopile structure is modeled in SubDyn, so, the platform (at the tower base) is located at the transition between the tower and monopile. So to model a monopile with rigid foundation in FAST v7 requires you to disable all platform DOFs. To model a monopile with rigid foundation in FAST v8 requires to enable all platform DOFs (to enable a proper coupling between ElastoDyn and SubDyn) and constrain a node at the seabed within SubDyn.

  2. If the platform DOFs are disabled in FAST v7, the reaction loads will be calculated directly by FAST to ensure that the platform does not accelerate. If the platform DOFs are disabled, you must calculate your own reaction loads in the user-defined routine UserPtfmLd(), which requires that you set PtfmLdMod = 1 and implement your own userPtfmLd() routine and recompile FAST.

I hope that helps.

Best regards,

Dear Jason,

Thank you for your help.

So, for FAST v8 If the water depth is 50m, how do one can constrain a node at the seabed within SubDyn??
Actually, I didn’t understand what you mean by this in your previous reply?