another thing that popped into mind...
the shallow water ones should be able to have pipes laid out to them...
the deep water ones, in RL, sometimes will lay pipe too, but for the game, perhaps not allow that.
To give your ships more purpose, you could set up a tanker, which can run out to the rig, pick up the oil, then deliver it to either the shallow water rig, or perhaps a type of dock off-shore (but in shallow water), which wouldn't drill itself, but whos only purpose would be to offload ships, with pipes leading from it to storage tanks on shore.
You'd have to be able to set up schedules, like with trains, tho... and do some path-finding... which will probably be a pain... but if you can pull it off, then your mod, I'm sure, will grow in popularity.
-- Smoov