# Spar heave frequency spectrum

Dear Jason,

I have developed a MATLAB code to numerically model a spar type floating wind turbine. I am trying to understand its dynamic responses.
When I take the FFT of my time domain responses, I clearly see that the natural frequency of the structure in Surge and tower fore-aft direction can be seen in the frequency spectrum plot of the pitch response (which is acceptable since these degrees of freedom are coupled). And, again as expected the dominant frequency is the pitch frequency itself.

Now for heave direction, I realized that the heave natural frequency is not the only visible frequency and the surge (sway) natural frequency is also visible in heave frequency spectrum plot. I was wondering why should the surge natural frequency have effect on heave response while they don’t seem to be coupled.

I run FAST for a simple case and realized that the surge frequency is visible in heave responses. I wanted to know are they coupled? Or I am making mistake?

Also, I would like to know which frequency should be dominant in heave direction, is it surge natural frequency, wave frequency or heave natural frequency itself?

I would greatly appreciate your guidance.

Regards

Dear Vahid,

Yes, the surge and heave modes of a floating system are often coupled. For example, surge motion can lead to the tightening of a mooring line, which pulls the floater downward. The coupling will depend on the center of mass, center of buoyancy, mooring arrangement, etc.

Best regards,

Dear Jason,

Thank you so much for your quick response, it really helps.
What about the dominant natural frequency? As you can see the frequency of the surge is more dominant than heave natural frequency itself. Is this OK?
Also, are we expecting to see wave frequency in heave direction? I don’t see it in my frequency response which I have attached here.

Regards

Dear Vahid,

The relative magnitude of heave and surge in the heave PSD will depend on the spar design and sea state. But I’m not surprised with your result.

Spars are typically very deep drafted, which minimizes heave excitation from wave loading, so, you may not see the first-order wave energy in the heave response. But again, this would depend on the spar design and sea state.

Best regards,

Dear Jason,

Thank you so much for your clear explanation.
FYI, my spar model design is the OC3 Hywind.

Dear Jason,

Following up with my question, to ensure that my developed model is valid. I run FAST for OC3 Hywind with Moordyn. The attached picture shows my heave response in frequency domain and as you can see the natural frequency of the Spar in heave direction is the dominant frequency (0.034 Hz). However, the attached picture in my previous post showed that surge natural frequency is dominant. In my model I use Moordyn to model the mooring cables and I am using similar Spar as what we have in FAST.

In other degrees of freedom I am not facing such problem and it is only on heave direction. I totally understood your clear explanations regarding this, but why don’t we see similar behavior in FAST model?

Dear Vahid,