Abstract: In a substructure (10) for a floating oil or gas production platform, an arrangement to tension a plurality of risers (16) extending from the sea bed up to the substructure, the arrangement comprising: a conventional hydraulic tensioner/heave compensator (17) for each riser, in which there is a soft spring formed by a piston cylinder combination acting against an accumulator, the heave compensators for the risers being disposed to compensate for vertical oscillations of relatively short period (e.g.

Abstract: A surface vessel is anchored almost geostationarily by a turret rotatable relative to the vessel about a vertical axis and having mooring cables extending to the seabed. A well is drilled from a drilling section in the vessel at a distance from the axis of rotation of the turret. The drilling is performed using a drill string through a riser extending from the seabed to the drilling section, which riser can be broken and shut off between the seabed and the vessel. By manipulating (hauling in and slackening) the mooring cables, the vessel is held in position so that the derrick is kept almost geostationary relative to the well during drilling operations. The system includes a surface vessel, a turret rotatable relative to the vessel about a vertical axis and which can be anchored to the seabed by a plurality of mooring cables, and a riser extending from the seabed to the vessel through which the well can be drilled with a drill string from a derrick in a drilling section on board the vessel.

Abstract: In a device for oil production at great depths at sea, production/drilling equipment is provided on a floating structure (2) at the surface of the sea, such as a platform structure or a vessel structure, for instance a ship, which is connected to a flotation unit (5) under water, which is anchored with a tension mooring (4) to the sea floor and with a riser system (6) which extends upwardly to the deck (9) of the structure. The structure (2) is equipped with an open vertical frame construction (8), such as a trusswork projecting downwardly in the water, the lower part of which forms a guide (17) and protection for the flotation unit (5). The rigid risers (4) from the sea bed continue up with the system (6) through the flotation unit (5) and further, enclosed by the frame (8) up to the level of the deck (9) above the water surface, where they are terminated in a Christmas tree device (7), or a manifold. A flexible riser system (11) extends from the valve arrangement (7), e.g.