# Operating and parked condition

Dear jason,

I am hereby enclosing the graphs of responses in comparison of operating and parked condition

I have the following questions

Heave response in operable and parked condition peak at the same frequency , is it correct andwhy is it so because there needs to be a shift in frequency when it is operational right,

Also i can see a clear shift in freq responses of pitch surge and yaw
but not in the case of heave roll and sway what is the analytical understanding of these graphs kindly reply

Dear Purushotham,

When you say that “operation must shift the frequency”, are you referring to the stiffening of the mooring system due to the mean thrust force? This may be important for some modes of motion and some floating wind systems, but is not a universal statement.

Are you plotting an FFT, a PSD, or an RAO? For an FFT or PSD, the response peaks will be dictated not only by natural frequencies (if they are excited by the wind or wave excitation), but also by the input spectrum (i.e. the peaks in the spectra of the wind and waves). And what are you exciting the systems with? I don’t really see a dominant peak in e.g. the parked pitch response.

What platform are you considering? I’m a bit surprised to see the peak in pitch to be quite different than the peak in roll.

Best regards,

Dear jason ,

Thank u for your quick reply , these are the fft plots and the platform under question is the tlp so i would just like to know the qualitative correctness of my graphs posted, is it a universal fact that pitch reponses cannot be different than those roll,also jason can u point the reason for similar natural frequency in heave for both operating and parked condition.

regards

purushotham

Dear Purushotham

The natural frequencies of the MIT/NREL TLP in the undisplaced position are documented in Table 6 of Denis Matha’s MS thesis-turned NREL report: nrel.gov/docs/fy10osti/45891.pdf. Your results in surge, heave, and, pitch seem to match this. I don’t really see a match in sway, roll, or yaw, but this could be because your wind and/or wave excitation is not exciting these modes.

It is not universal that pitch and roll must have the same natural frequency, but that is often the case because of symmetries in the platform/mooring design.

I would not expect a strong influence of turbine operation on the heave natural frequency for the MIT/NREL TLP.

Best regards,

Dear jason,

When u say that the frequency would not be different for heave in operable and parked condition what could be the possible reason for this ,is it because the wind turbine weight is about 17.18% of the platform mass, or is there some other thing, also in other degrees of freedom how much percentage increase or decrease in the natural frequency is expected in the six degrees of freedom, also jason how would the damping ratios be qualitatively varying for operable and parked cases in all six degrees of freedom, Kindly reply in this regard.

regards

Dear Purushotham,

The force-displacement relationships of the tendons for the MIT/NREL TLP are quite linear in the heave direction in the range of useful operation (lines not going slack) – see Figure 21 in the report I linked above. Thus, I would not expect a change in the heave frequency under different loading conditions.

Wind is likely to add damping to the fore-aft motions of the rotor e.g. for the platform surge and pitch motions, but this depends on the wind speed and frequency of the oscillation due to the interaction with the controller. FAST can be used to calculate these trends.

Best regards,

dear jason,

kindly go through my free decay responses and look at the trend of the operable and the parked states and comment on the damping ratio between the operable and parked conditions of operations, also is this the expected grap

regards

Dear Purushotham,

Please interpret your results yourself. It is very impossible for me to comment on specific results without knowing the model or simulation set-up. NREL cannot offer unlimited support for free and open-source software.

Best regards,