Abstract: An underwater riser system in which the primary riser is surrounded by lengths of jacket pipe which form annular buoyancy chambers, there being a pressure fluid supply connected to the buoyancy chambers via valve means including at least a pressure differential valve which controls the amount of water displaced from the chamber by the pressure fluid. Advantageously, the valve means for each chamber also includes remotely controlled means by which the amount of buoyancy provided by the chamber can be varied at will, e.g., from a positive buoyancy to a buoyancy less than the operating weight of the portion of the riser system with which that chamber is associated.