Abstract: In order to facilitate temporary storing of oil in an underwater receptacle, for instance the pontoon of a semi-submersible vessel, the receptacle is subdivided by bulkheads into a series of tanks. A first tank is connected to a production unit by way of a conduit, or by way of a branch pipe to a delivery conduit. The tank located remotely with respect to the conduit communicates with the surrounding sea by an opening. The bulkheads are provided with upper and lower openings, respectively, and a screened passage extends between these openings. When oil is supplied to the first tank static or pump pressure will expel water through opening, and oil will fill the tanks in due order in the direction away from the first tank. When oil is withdrawn, water from the surrounding sea will flow in through opening and further on through the tanks towards the first tank.

Abstract: In order to stay the columns in an offshore vessel of the type having two parallel pontoons and an operating platform carried thereby, also with respect to forces tending to displace the pontoons longitudinally in relation to each other, the columns located in the same transverse plane are interconnected by two transverse braces arranged in cruciform close to each other, but lacking direct connection.

Abstract: A semi-submersible offshore platform comprises an operating deck carried by four cylindrical columns supported by a pontoon structure comprising four-sided boxes formed into a square ring. Each pontoon box is subdivided into two compartments by a longitudinal centerline bulkhead, the compartments being further subdivided into tanks by transverse bulkheads. Tanks outside the centerline bulkheads are used for ballast water, and tanks inside the centerline bulkheads are used to store the oil produced.

Abstract: In order to simplify the construction and to reduce the resistance to flow, and to provide a high amount of inertia in relation to all essential axes, an offshore vessel comprises two parallel under-water bodies, four columns mounted symmetrically thereupon, and a multi-deck box-like structure interconnecting the upper ends of the columns. This structure will increase the strength of the vessel so only one horizontal, transverse stay is required between each pair of columns. Below each column there is a 360.degree.-turnable shrouded propeller, which together can hold the vessel stationary, with a vertical line of symmetry in a desired position, for instance above a borehole. The box structure is outwardly completely closed at the bottom and sideplating, and will, in case of need, retain the vessel floating.