Clarification on Wamit outfiles

Wind & Water Computer-Aided Engineering Software Tools
1

Hi,

I am working on analysis of OC3 Hywind Spar using FAST V8. I have a doubt on the wamit data which fast uses in hydrodyn calculations. I saw the added mass and damping graphs provided in the definition of OC3 spar NREL paper. I have attached the graph from NREL paper.The added mass as per the graph is a straight line over a frequency range however the added mass supposed to be frequency dependent. How is it so?

Also, the heave added mass is very less as compared to the added mass in surge as per the graph. But actually the heave added mass should be higher than the added mass in surge, isn’t it?

Also, the added mass, damping and excitation forces are calculated by Wamit using potential flow theory. Will this inputs be appropriate when I am using Morison’s theory or hybrid theory (as D/L ratio being less than 0.2) ?

Please kindly let me know your comments on the above doubts as soon as possible.

Thanks in advance.
Regards,
Anuja Gawande

added mass graphs.PNG
added mass graphs.PNG793×778 106 KB

2

Dear Anuja,

The graphs show some frequency dependence of added mass, but the effect is small due to the small volume of structure near the free surface, which minimizes the effects of wave radiation. The heave added mass is very small (and smaller than for surge) for a thin, deep-drafted spar.

I’m not sure I understand your last question about the appropriateness of inputs.

Best regards,

3

Hi Jason,

Thanks for you reply.

I have one doubt.The D/L ratio of my structure is less than 0.2, thus according to theory I should be using Morison’s theory to analyse the spar. In this Condition, which method to calculate the response of the structure, whether Hybrid theory or Morision’s theory should be used?

If I am using Hybrid theory, will it be giving me correct results as for this case the added mass and damping calculated from radiation problem will be used ?

In most of the wave states (from moderate to extreme sea states) the OC3 Hywind spar behaves as a slender member, then why for verification of OC3 model radiation diffraction problem is used?

Correct me if I am wrong.

Please kindly help me to clear my confusion on which method to be used to analyse the OC3 spar.

Thanks in advance.

Regards,
Anuja Gawande

4

Dear Anuja,

For a spar, where D/L < 0.2, solving the radiation and diffraction problem in the potential-flow solution is a bit of an overkill, because the radiation damping will be small and the diffraction effects are unimportant, except for the mildest sea states. You can still use the hybrid solution because the added mass and wave-excitation terms, which will come from the potential-flow solution, will be correct (the strip-theory solution and the potential-flow solution should give equivalent added mass and wave-excitation effects for D/L < 0.2), but keep in mind that for many sea states, the viscous-drag term from the strip-theory (Morison) solution will dominate over other hydrodynamic terms.

Best regards,

5

That cleared my doubt.

Thanks for your support and time :slight_smile:

Regards,
Anuja Gawande