Question:

Can two different vessel models be joined as in the situation where an
offshore rig is being transported on a barge?

Answer:

Yes, you can combine the contents of two, or more, Geometry Files into one,
making them into a composite model simply by issuing successive READ commands.
Shifting of origins and reversing the sense of the longitudinal axes
(switching bow and stern, port & starboard) is also quite easy.

There is one compatibility issue to keep in mind when building the models:
duplicate part names are not allowed.  For example, you cannot have a part
named "HULL" in both of the Geometry Files to be combined.  Therefore, it
would be a good idea to use something like "RIGHULL" in a model which you
expect to be combining with another model which uses "HULL".  Likewise, naming
of tanks should be done in such a way as to assure there will be no conflict.

If a Part name conflict does exist, it can be addressed in the run file that
performs the merger using the RENAME command.  That way the original geometry
files doesn't have to be modified.

The same restriction applies to shape names as well, except that if a conflict
occurs, Part Maker automatically reassigns the name of the shape being read.

Here is an example of the commands needed to combine two models:

  ENTER PM
  READ RIG.GF
  RENAME HULL, RIGHULL`<--- Rename Part HULL to avoid a future conflict.
  REVERSE             `<--- Reverses the rig model fwd & aft, port & stbd.
  SHIFT 100, 0, 10    `<--- Shifts the rig model relative to the origin.
                      `        100 m. aft, 10 m. up.
  READ BARGE.GF
  DISPLAY
  WRITE RIGBARGE.GF
  QUIT PM