WAMIT data in HydroDyn and simulation with current

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Dear @Xiaoqin.Zhang,

Here are my answers to your questions:

  1. Yes, you should be able to model a model-scale floater at model scale, but you may face some numerical challenges associated small time steps and the fact that hard-coded tolerances within OpenFAST are generally set for full-scale models, and so, may not apply at model scale. It is much more common for to model the system at full scale and scale up the tank test data to full scale for comparison.

  2. Are you referring to the AQWA calculation? I’m not familiar with the requirements of AQWA, but the HydroDyn documentation (4.2.8.4. Modeling Considerations — OpenFAST v3.3.0 documentation) provides guidance for WAMIT, where it is recommended to set the vertical CG to zero in WAMIT so that the hydrostatic restoring does not depend on body weight, which is intrinsically accounted for in ElastoDyn, and so, should not be double counted in HydroDyn.

  3. I don’t have any experience with BEMRosetta, but the WAMIT output data is nondimensional. See the following forum post for information on how to dimensionalize the WAMIT output: WAMIT.

  4. While the hydrostatics in potential flow solution always applied, the wave-excitation part of the potential flow solution is not used in a current-only simulation. Exciting the structure via current would happen through the viscous drag term in the strip-theory solution. Once the structure moves, the radiation terms in the potential-flow solution (added mass, radiation damping) would have an effect.

  5. I’m not sure I know enough about your model to comment further.

PS: I notice in your second plot a strong high frequency oscillation in heave, with a nonzero mean. This suggests to me that your model is not in vertical equilibrium, i.e., your full-system weight, undisplaced buoyancy, and vertical mooring pretension do not balance out; I would suggest fixing that per the HydroDyn modeling guidance.

Best regards,