Abstract: A method for converting an existing semi-submersible drilling- or accommodation rig into a floating production plant having apparatus for extracting and processing oil and/or gas, or other heavy and bulky deck load. These vessels comprise at least two submerged pontoon bodies, a work deck raised to a secure level above the surface of water having personnel accommodation areas and upright support columns between the bodies and the work deck. A module is prefabricated which contains the main part of the processing apparatus. The vessel is provided with a steel structure to carry the module near the level of the work deck. Also, the vessel is provided with a displacement increasing device, substantially corresponding to the dead weight of the module and the steel structure. Further, the water line area of the vessel is increased until the necessary deck load capacity and stability is achieved.