A method for converting a single-hull vessel to a double hull vessel, and the converted double hull vessel, is disclosed. The single hull vessel to be converted has an outer hull surrounding a center cargo tank flanked by a port wing tank and a starboard wing tank, with spaced-apart transverse bulkheads extending between the port and starboard wing tanks to form one or more central tanks. An elevated floor is added to the central cargo tanks by installing new bottom plating spanning the transverse bulkheads and the sidewalls of the wing tanks. Support members are added to increase the strength of the elevated floor, the cargo tank, and the wing tanks, as needed. A sealed compartment is formed by the new elevated floor, the outer hull, and the sidewalls and bulkheads to protect the cargo from leaking out of the vessel should the integrity of the single hull be breached. The method achieves conversion of the single hull vessel to a double hull construction without breaching the integrity of the pre-existing single outer hull, and thus is faster and easier in converting single hull vessels to minimize cost of conversion and vessel outage.