Abstract: 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.

Abstract: Hydrocarbon liquids are transported from a subsea welihead to an above-surface hydrocarbon processing facility along a pipe-in-pipe flowline. The flowline is a pipe placed coaxially within an outer carrier pipe and the annulus between the pipes is filled with thermally insulating material. The hydrocarbon liquids have their temperature maintained above solidification/precipitation temperature by heat from an active heating system. Hot liquid, preferably hot water, is passed along the annulus, either along a single pipe or multiple pipes installed in the insulation-filled annulus, or along an inner annulus formed by a water pipe added concentrically around the hydrocarbon-transporting inner pipe, inside the outer insulation-filled annulus. The water or other suitable liquid can be heated in a heater at or adjacent the subsea wellhead, or by a water heater in the above-surface hydrocarbon processing facility.