Mike Pettit
Ansys Employee

Hi Tianyuan,

You should not expect the Connection Stiffness to change the added mass or radiation damping calculated by Aqwa. As you say, these are related to the forced vessel motion, but we do not care how much 'force' is required for that motion.

I don't think you need to worry about the spikes in the added mass/damping plots, as long as the heave terms are sensible. If you have constrained the structures in the time domain calculation, there will be no relative motion in surge/sway/roll/pitch, so there will be no radiation force in these directions.

I'm sorry that I didn't explain the Connection Stiffness definition well. To model the sliding connection between two structures in the Hydrodynamic Diffraction calculation, you should:

  • add three Connection Stiffness objects under Connections
  • for each Connection Stiffness, set the 'Connectivity' to 'Structure to Structure'
  • set the 'Connected Structure A/B' as
Connected Structure A
Connected Structure B
Connection Stiffness 1
Connection Stiffness 2
Connection Stiffness 3
  • for Connection Stiffness 1 and 2, set the stiffness values as e.g. K11 = K22 = 1e10 N/m; K44 = K55 = K66 = 1e12 N.m/deg (leave K33 as zero to allow relative heave motion)
  • for Connection Stiffness 3, set the stiffness values as e.g. K11 = K22 = -1e10 N/m; K44 = K55 = K66 = -1e12 N.m/deg (leave K33 as zero to allow relative heave motion)
Rather than using the arbitrary large stiffness values of 1e10 N/m and 1e12 N.m/deg, you may consider selecting values based on the structural mass/inertia terms, so that the natural frequency of the system is higher than the wave frequencies you are including in your hydrodynamic analysis.