Abstract: A system and method for ocean transport of prefabricated offshore structures includes a self-propelled sea-going vessel having a submergible mid deck and one or more raised decks. Ballasting equipment is provided in the vessel for submerging the mid deck to a level at least deep enough to permit the floating of a shallow draft barge over the mid deck. The shallow draft barge carries a prefabricated offshore structure such as a drill jacket. The barge with the offshore structure loaded thereon is floated over the submerged mid deck, and the vessel is then deballasted to raise the mid deck. With the barge lifted out of the water, the barge is fixed to the vessel which may then transport the barge and its cargo over an ocean to any offshore drilling site.