peteroznewman
Subscriber

Yes, the pipes will be solid and the base structure will be surfaces.

A simple way to account for the mass of water is to use a Distributed Mass.  Select all (or most) of the internal surfaces of the pipe as the scope of the Distributed Mass.  SpaceClaim can be helpful in determining the volume of water in the pipe by using the Volume Extract button. Multiply the volume by the density to get the mass.