Then lets take it up a notch. Extremely difficult dungeon, the prize for fully completing it is the ability to pay someone, (probably someone at the docks in San Francisco) to fix it up and then you can use it as a faction base. It wouldn't replace the dungeon of course, just give you your own private instance in the same manner as the abandoned bunker, although a hell of a lot more expensive I imagine. On that note, I'd like to suggest beached ships as faction bases as well...
Alternatively if the oil rig itself seems a little too much, have it be something like, "Since you cleared out that old Enclave oil rig we did a little studying and though you messed it up pretty bad we think we can salvage enough to make you your own ocean headquarters.... Though why you'd want to be out there is beyond me." and make it a smaller, scrappier version.