Froude-Krylov loads

Wind & Water Structural Analysis
1

Hello all,

I am modelling the OC4 DeepCWind semisubmersible FOWT using first order potential flow and am trying to extract all individual hydrodynamical load components.

Based on OutlistParameters.xlsx I have found following contributions:

  • Total hydrodynamic and -static loads: HydroFxi
  • Hydrostatic loads: B1HdSFxi
  • Diffraction loads: B1WvsF1xi
  • Radiation loads: B1RdtFxi

Based on this I calculated the following:

  • Hydrodynamic loads: HydroFxi - B1HdSFxi
  • Froude-Krylov loads: HydroFxi - B1HdSFxi - B1WvsF1xi - B1RdtFxi

However, the results for the Froude-Krylov loads are not what I expected:

Froude-Krylov
Froude-Krylov630×469 27.9 KB

As I have regular waves as input, together with the documentation (4.10.4.5) which says:

When coupled to FAST, the hydrodynamic loads computed by HydroDyn are applied to the displaced position of the substructure (i.e., the displaced platform in ElastoDyn and/or the displaced substructure in SubDyn), but are based on wave kinematics at the undisplaced position.

I would have expected to get sinusoidal loads for the Froude-Krylov forces, similar to the diffraction forces. Does anyone know what is going wrong or can anyone explain these results to me?

Also while digging through the documentation I found the following (4.10):

In the former, potential-flow hydrodynamic loads include linear hydrostatic restoring, the added mass and damping contributions from linear wave radiation (including free-surface memory effects), and the incident-wave excitation from first- and second-order diffraction (Froude-Kriloff and scattering).

However, to my knowledge Froude-Krylov and first-order diffraction are something else entirely and should both be used for the analysis of a floating structure.

Kind regards,
Victor Rappe

2

Dear @Victor.Rappe,

Froude-Krylov loads are the wave-excitation loads from integrating the undisturbed dynamic pressure of the wetted surface of the body. The linear Froude-Krylov loads would be one component of the first-order diffraction loads (B1WvsF1xi), however these diffraction loads include the combination of Froude-Krylov and scattering loads and there is no way in HydroDyn to separate out each effect because the combined effects are what is output from WAMIT and input to HydroDyn.

Best regards,

3

Dear @Jason.Jonkman,

Thank you for your answer, I understand.

Kind regards,
Victor