Phases of the diffraction forces

Hi all,

I’m running simulation with a semi-submersible, using a hybrid solution of potential flow diffraction forces and morison drag terms. The model is fixed.

I’m looking at total hydrodynamic forces and total wave-excitation loads from the diffraction in the frequency domain (singel sided amplitude spectra). As far as I understand, the remaining force is the drag force contribution. If I’m using two different WAMIT files (one with radiation, one without) I expected that the wave-excitation loads from diffraction change, and consequently the total hydrodynamic forces too. I thought that the difference, which is the drag force contribution, should remain equal. This is not the case, so I’ trying to find an explanation. Does this happen because the diffraction force has a different phase than the drag force? As far as I understand, the information of the phase of the diffraction force is given in the WAMIT files, while the phase of the drag force coincides with the relative velocity. Is this correct?

Best Regards and many thanks,


Dear Manuela,

I’m not sure I really understand your question. When you say that you “thought that the difference, which is the drag force contribution, should remain equal”, what are you comparing to that you expect equality?

Just a few comments:

  • The total hydrodynamic loads output from HydroDyn (HydroFxi etc.) include the contributions from all hydrodynamic loads, including radiation, diffraction, drag, and hydrostatics.
  • When you say the model is “fixed”, presumably you mean that the platform is not moving. In this case, the wave-radiation loads (hydrodynamic added mass and wave-radiation damping) are zero.
  • Yes, the phasing of the wave-excitation (diffraction) loads in the potential-flow solution comes from the WAMIT data and the phasing of the drag loads comes from the relative velocity (wave velocity only if the platform is not moving).
  • The wave-excitation loads can be output directly from HydroDyn, including WavesF1xi etc. for first-order waves, WavesF2xi etc. for second-order waves, and WavesFxi etc. for both first- and second-order waves summed

Best regards,

Hi Jason,

Thanks for the comment on the phasing! I just realized that the phasing in my two different WAMIT files is very different, which leads to different amplitudes and phases of the total hydrodynamic forces.

What I meant is that I thought that the drag force part remains equal when I use the same wave input and the same drag coefficients. What I didn’t understand that I can’t subtract the diffraction force from the total hydrodynamic force to obtain the drag force, because of the different phasing.
I just understood how much effect the (WAMIT) phase has on the total force amplitude.

Best regards,