Abstract: A deep draft semi-submersible offshore structure suitable for conducting mineral exploration and production operations in deep waters of the oceans. The structure uses three or more caissons or columns equidistantly spaced from adjacent columns. The weight distribution and provision of liquid or solid permanent balance in the lower ends of each column make the structure stable without reliance upon water line displacement or tendons, as compared to conventional vessels. The center of gravity of the structure is maintained below its center of buoyancy, thus imparting an inherent absolute stability to the structure. The columns are retained in spaced relationship to each other by a plurality of horizontal braces or a combination of horizontal braces and diagonal braces. The major part of the structure is positioned below the water surface at sufficient depth to minimize the reaction to surface changes, thereby greatly reducing heave and angular response to surface wave action.