I did a Google image search on "floorplans" and got a lot of interesting hits. There might not be an actual oil rig in there but there might be some plans that you can use to represent one:
I've long thought that developers should branch out and give us more than fantasy templates for 5 foot squares. Yes, why not oil rigs, haunted houses, military bases, nuclear reactors, city streets, ghost towns, etc. They've got fantasy nailed pretty good, and now there's lots of science fiction stuff coming out that's good, but Modern floor plans are still lacking.
If you can't find one, best advice I can tell you is to find the deck plans of a merchant ship and then square it off with a hole in the middle. There isn't alot of space on most of the rigs, I'd suggest a 150' square as your base template, with a 30' square hole in the middle. The vast majority of the space is for storage of pipe, drill heads, and mud. As long as you have a docking station and a helipad, you can pretty much fudge the rest to get it close. Most rigs have the deck level and three sublevels within the structure.
I have found one. I remembered a fan-made adventure of Millennium´s End which had a floor plan of an oil rig. The adventure is called Bad Paper. You can download it as pdf-file here.
If you are only interested in the floor plans you can download them in DXF, DWF (Drawing Web Format), and Autosketch 5.0 format as zip-file here.
IIRC there was a mission on an oil rig in the PC game "Rogue Spear", so perhaps I will use some of the screen shots as base for drawing a map..