This invention relates to a marine riser system for transferring fluid between a plurality of mutually separated locations on the seabed and a vessel (1) on the surface of the sea. The riser system comprises a submerged buoy (2), a plurality of risers suspended from the buoy (2) to the location on the sea bed and a plurality of flexible conduits (8) extending from the vessel to the submerged buoy (2). Connectors (7) carried by the submerged buoy (2) connect the flexible conduits (8) to the risers (6), each connector being remotely operable to disconnect all the flexible conduits from the risera. A central tether (5) may be provided during an installation phase only, until the risers (6) are in place and hold the buoy (2) by their own tension. Alternatively, the tether (5) may remain in place, and tensioned to limit vertical motion of the buoy.