We should probably have a Sailor make comments here, I've some experience with Naval Shipping and Civilian Shipping so I'll give it swag. Old big decks are rough beasts. Lots of issues. Cost of operation and maintenance could cripple a small country. Further Big Decks are deep draft, preventing them getting too close to shore. Let's not mention an 'old' nuke reactor. Retrofit of these monsters is probably not practical. That said the idea is still valid, just the platforms should be rethought. There are all types of ships that could be brought into this realm. Old container ships, cruise ships, MPF ships, even old Amphibious ships, or naval stores ships. But if you wanted small crew size, large deck and sub deck storage, potential for adding a helo-landing pad and draft versatility then civilian ships would be the way to go. Additonally they should be run as MPF shipping is run, on standby at fun places like Diego Garcia, ships of this nature will top out between 18-20 knots. You may not get it over night but a week-ten days sure. Airlift will always win the speed while sealift will win the mass competitions.