Free Decay test

Wind & Water Structural Analysis
1

Dear Jason
Sorry to bother you
I tried to do free decay test of V-shaped semi FOWT by FAST and and I am confused to some problems…

  1. Should I turn off other DOFs in ElastoDyn when running Free-Decay simulation? Because I find there are strong coupling effect between heave and pitch motion. I have run simulation twice with turning off other DOfs and turning on the other DOfs respectively. There are some figures as shown below.
    （a） turning off other DOFs
    with turning on other DOFs.png
    with turning on other DOFs.png1087×785 6.39 KB

    (b) turning on other DOFs
    with turning off other DOFs.png
    with turning off other DOFs.png1083×805 6.77 KB
  2. After comparing with the natural frequency of free-Decay simulation and hand calculating, the results don’t match between two methods in same motion. For example, the value of natural frequency of pitch motion is around 0.37rad/s by free-decay simulation, while it is 0.30 rad/s by hand calculating. The formula of natural frequency can be expressed as:
    formula.png

I hope you will help me

Thanks
Best Regards
Lixian.Zhang

2

Dear Lixian.Zhang,

Unless the natural center of rotation of the FOWT is at the WAMIT reference point (WRP) in HydroDyn (at the intersection of the undisplaced tower centerline and still water level), which is not likely the case for most platforms, then the platform surge, heave, and pitch modes are coupled, so, disabling one of more of these degrees of freedom will produce different results.

Your equation for the hand calculation applies only to uncoupled motion (or for pitch motion about the natural center of rotation).

Best regards,

3

Dear Jason
Thanks for your help!
But I sitll don’t understand. Even if I turn off all the DOFs expect pitch, the vaule of natural frequency from free-decay test should be equal to hand calculation. Am I right? However, the results still don’t match.

Thanks again!
Best regards
Lixian Zhang

4

Dear Lixian,

Have you expressed all stiffness (hydrostatic, body weight, mooring) and inertia terms (body, added mass) in your hand calculation relative to platform reference point?

Best regards,