A semi-submersible offshore vessel has a substantially ring-shaped lower pontoon, starboard and port forward columns extending upwards from the lower pontoon, and starboard and port aft columns extending upwards from the lower pontoon. An upper deck structure connects upper portions of the columns with each other. The upper deck structure has a substantially rectangular module recess for receiving one or more operation modules, for example, carrying hydrocarbon-processing equipment. The upper deck structure is generally C-shaped, having a forward torsion box extending transversally between the starboard and the port forward columns of the semi-submersible offshore vessel, and two mutually parallel longitudinal side beams extending aft from the starboard and port forward columns to the starboard and port aft columns, respectively, in such a way that the module recess exhibits an open aft end, the width of which substantially corresponds to the width of an operation module. The module recess is delimited in a downward direction by a substantially planar module deck, which extends between and interconnects the longitudinal side beams.