Learning Forum

[Mike Pettit]

Hello Vishnu,

To answer your questions:

1. Please add a Hydrostatic result to the Hydrodynamic Diffraction Solution, and check the Metacentric Height (GMX) that has been calculated for the vessel - naturally this should be positive. Also check the calculated CoG, CoB, displacement etc. for any unexpected values. I would not recommend deactivating rotational freedoms to prevent the vessel from capsizing.
2. Defining a forward speed in the Hydrodynamic Diffraction analysis only solves the diffraction problem at the corresponding encounter frequencies, it does not apply a forward speed in the time domain. As you suggest, in the time domain the model will be moving backwards due to drift effects.
3. To run a time domain analysis with forward speed, we usually recommend moving the water under the vessel, rather than moving the vessel through the water. You can do this by the following steps:
   1. Add a constant velocity Current to the Hydrodynamic Response analysis, with a Direction of 180 degrees and Speed equal to your forward speed (i.e. 10 kts, 5.144 m/s).
   2. In the Hydrodynamic Response Analysis Settings, confirm that the option 'Account for Current Phase Shift' is set to 'Yes' (this is the default). In this way the incoming wave phases are adjusted to account for the current speed.
   3. You will need to apply some kind of constraint to the vessel to stop it from drifting away. You could do this by: adding soft mooring lines around the vessel; deactivating the vessel Global X motion (only); applying a user-defined external force function, i.e. modelling a dynamic positioning system to counter the vessel's drift motion.

I hope this helps!

Mike