Abstract: A marine riser, which is hung from a drilling platform floating on a body of water and connected at its lower end to a wellhead, is buoyantly tensioned by one or more float cans with open bottoms. These float cans are connected at their upper end to the riser to which air under pressure is controllably supplied. The pressurized air blows out the water in each can to achieve the desired buoyancy. The float can may having an air dump valve interconnected with similar valves in the other cans. The function of these valves is to release the pressurized air in the can if the riser parts. At the upper end of the riser is a slip joint, which may include safety water-dampening gear to further reduce upward motion of a parted riser. The optional dampening gear comprises an annular piston formed by the upper section of the slip joint that forces water through ports in the exterior of the lower section of the slip joint as the riser below the slip joint moves upward.