Mike Pettit
Ansys Employee

Hi Tianyuan,

You can specify the structural surface on the SWL as an internal lid, but you should modify the geometry so that there is a gap between the lid surface and the other structural surfaces (similar to the external lid).

This is not exactly equivalent to using the 'Generate Internal Lid' option - the latter roughly fits square elements into the cut waterplane area inside the buoy, while the former will use the elements created by the Mesh. Here is an example of automatic (left) vs manually-defined (right) internal lids (visualised using AqwaGS):

You should not use an internal lid over the moon pool, as this will overdamp the water surface - it will remove all vertical wave motion from inside the buoy, which is not realistic. If you need to, you should rather create an external lid, which will allow you to tune the damping frequency (via the Gap value) and amount (Damping Factor value).

Cheers, Mike