A vessel with drilling and workover capability is constructed rapidly in a shipyard or dry dock by using an existing vessel that has most of the required hull. Bow and stern sections of the existing vessel are separated to leave them at opposite ends of a shipyard space, and a mid hull section is built in that shipyard space in the usual manner by welding together steel plates. The bow and stern sections are moved against opposite ends of the mid hull section and welded to it. Topside equipment is already on the bow and stern sections, and is added to the new mid hull section.