(1) In HD analsyis, please set Generate Internal Lid as Yes in Advanced Options of the part, which will remove the irregular frequeicy effects and provide better and smoother results againt frequencies.
(2) Check the displacement value of your model, which should be positive, and equals to the mass/water density if the structure is at its equilibrium position. Also check the hydrostatic stiffness of heave-heave, which should be positive if the buoy is piercing water surface.Â
(3) The hydrodynamic damping and wave exciting force will also affect the maximum value of heave RAO amplitude. We cannot say that this maximum value must be at the natural frequency.
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