The Ram Powell is probably closest to the ports of Biloxi and Gulfport, Mississippi, and the Petronus is closest to Mobile. The Petronus may be equidistant from Mobile and Biloxi, at best.
I've fished those rigs many times on multi-day trips, leaving out of Destin, FL. The Ram Powell is pretty consistently good on good tuna years. The Petronus may be good, it may be bad, and it may be covered up in sharks. The latter happens very often, with shit eaters eating up all of your chum and every bait that hits the water.
The Ram Powell has 4 large yellow cylindrical legs. It's a floater. The Petronus is driven all the way down into the bottom, 1800' (or is it 1200'...I can't remember) below. They're both very large rigs, but the Ram Powell is very much the bigger of the two.
Not far from the Ram Powell (the last time I was out there) is the Marlin Rig, which has 4 red/orange cylindrical legs, and is just about the exact same in appearance otherwise. It seems to be about as good as the Ram Powell most years.
Other rigs they put out there and move about from time to time...and some times after just drilling for a month or so:
Deep Water Nautilus, Ocean America, Noble Jim Thompson, and several huge drill ships.
The largest floater I think I've fished is the Nakika. It's a monster, and very gorgeous as well.
Distance in nm from Destin, FL to the aforementioned rigs, to the best of my recollection
Petronus: 89
Ram Powell: 115
Marlin: 105
Nakika: 180 <<<way closer to Venice than Destin!!
Deep Water Nautilus: was 120 back in '02, but got moved to who knows where
Ocean America: ~110 in July '05, but then moved by August
Noble Jim Thompson: ~110, similar to above.