Hi

I want to ask about how to get the influence and impact of wave disturbance in linear model for 5MW OC3 FOWT.

Thanks

Regards,

Muhammad Sharjil

Hi

I want to ask about how to get the influence and impact of wave disturbance in linear model for 5MW OC3 FOWT.

Thanks

Regards,

Muhammad Sharjil

Dear @Muhammad.Sharjil,

The wave elevation disturbance is a standard linearization input of HydroDyn when `PotMod`

= 1 and `ExctnMod`

= 2, i.e., when you enable state-space-based wave-excitation from the potential flow solution. And the OC3-Hywind spar is one of the floaters we generated a state-space wave-excitation data (*.ssexctn*) file for through the SS_Excitation_Fitting toolbox, which is similar to the MATLAB-based SS_Fitting toolbox for wave-radiation, but for the wave-excitation solution. . SS_Excitation_Fitting has been discussed in the following OpenFAST issues page, where you can the also find the *spar.ssexctn* file for the OC3-Hywind floater: Wave Excitation model generation in state-space (*.ssexctn input file) · Issue #397 · OpenFAST/openfast · GitHub.

Best regards,

1 Like

Hi @Jason.Jonkman

Thanks for the response.

I will go through the link.

Regards,

Muhammad Sharjil

@Jason.Jonkman

I am trying to linearize OC4 in region 3, but I dont know the linear model I am getting is right.

WindSpeed = 12 m/sec.

4 DOF are enabled only (GenDOF, TwFADOF1, PtfmSgDOF, PtfmPDOF).

The resulting transfer function for (Generator RPM/ Blade Pitch Angle) [after excluding ED Variable speed generator DOF (internal DOF index = DOF_GeAz), rad]

I cant figure out why all zeros in numerator are in right half plane.

Kindly guide me on this.

I am attaching the files.

https://drive.google.com/drive/folders/1AMyxV1bO8G22Ki04GFxgD5qJt74GcMGH?usp=drive_link

Dear @Muhammad.Sharjil,

I haven’t worked with transfer function form in a long time. Do you have a specific concern with the state-space model?

Best regards,

Hi @Jason.Jonkman

Thanks a lot alpha release of SS_Excitation_Fitting.

Regarding the linearization of OC4:

I have done similar analysis for 5MW OC3-hywind spar type at 12 m/sec and had only 1 set of complex conjugate zero in right half plane ( as normally discussed in literature for FOWT, negative damping of platform pitch zeros are present)

When I am doing the same procedure for 5MW OC4 deepCwind in region 3, at 12 m/sec, I am experiencing zeros in numerator in right half plane for PtfmSgDOF frequency and TwFADOF1 frequency.

I dont know if this is normal or i am not doing appropriate settings for linearization.

Thanks

Dear @Muhammad.Sharjil,

I’m not sure. Are you getting the natural frequencies and damping you expect from the linearized state-space (A) matrix?

Best regards,

Hi @Jason.Jonkman

I am getting these numbers for frequency and damping ratio for OC4 linear model in which I am facing the discrepancy.

In OC3, i was getting these numbers for frequency and damping ratio

Thanks

Regards

Muhammad Sharjil

Hi @Jason.Jonkman

Thanks for taking time and looking into it.

The problem I am facing with linearisation of OC4 at my end is that the transfer function is having a zero in right half plane at 0.06 rad/sec which is quite low. The control crossover will be limited by surge right hand plane zero instead of platform pitch right hand plane zero.

I have changed the simulation sample time from 0.025sec to even 0.001sec, but I am getting the same results for OC4.

As per

(https://www.nrel.gov/docs/fy14osti/60601.pdf)

“The platform-pitch natural frequency of the OC4-DeepCwind semisubmersible system (0.037 Hz) is very similar to that of the OC3-Hywind spar (0.034 Hz). Because the platform-pitch frequency is used to set the gains of the blade-pitch-to-feather control system, it was decided to re-use the control methodology developed for the OC3-Hywind spar for the semisubmersible, including the change in Region 3 from constant power to constant generator torque.”

I infer that the transferfunction for OC4 has to be similar to OC3.

But in my case, I am getting altogether different outcome. If I try to use OC3 or OC4 default gains, the closeloop is getting unstable.

I cant figure out if I am configuring the settings correctly or not.

Thanks