A semi-submersible offshore vessel (1) exhibiting a first end (2), for example constituting the forward end of the vessel, and a second end (4), for example constituting the aft end of the vessel (1), or vice versa, wherein the vessel (1) includes a substantially rectangular ring-pontoon (6) including a first transverse pontoon section (10) located at the first end (2) of the vessel (1); a second transverse pontoon section (12) located at the second end (4) of the vessel (1), the second transverse pontoon section (12) being parallel to the first transverse pontoon section (10), the ring-pontoon (6) further including two mutually parallel longitudinal pontoon sections (14) extending between the first (2) and the second end (4) of the vessel (1); at least four support columns (16, 18, 20, 22) extending upwardly from respective edge-portions (23) of said ring-pontoon (2), the support columns (16, 18, 20, 22) being arranged in a first column pair (24) located at the first end (2) of the vessel (1) and a second column pair (26) located at the second end (4) of the vessel (1); and an upper deck structure (28) positioned upon the support columns (16, 18, 20, 22). The first transverse pontoon section (10) may also include a vertical mean cross-section area (A) which exceeds the corresponding vertical mean cross-section area (B) of the second transverse pontoon section (12), and the support columns (20, 22) in the second column pair (26) each has a water-plane area (E) which exceeds the water-plane area (D) of each of the support columns (16, 18) in the first column pair (24).