Riser system

#This invention relates to riser systems, and relates more particularly but not exclusively to riser systems for use in maritime installations wherein a flexible conduit extends between a seabed location and a floating vessel.

#It is common practice for submarine wells producing hydrocarbons to have a wellhead mounted on the seabed, with the hydrocarbons recovered from the well being fed to a surface-floating vessel by way of a flexible hose. Such vessels are commonly anchored to the seabed to retain them in suitable proximity to the respective wellheads without the continuous expenditure of energy necessary for dynamic positioning. Nevertheless, moored vessels are not totally static, since they are subject to winds, waves, currents, and tides, with consequent changes in alignment, heading, depth, and position. Thus the point at or near the upper end of the hose where the hose is directly or indirectly tethered to the surface-floating vessel is subjected to considerable tension, and must also accommodate changes in the direction of tension.

#It is an object of the invention to provide a riser system in which changes in the direction of tension at or near the upper end of the hose or other form of riser are at least partially accommodated by means other than inherent flexibility of the hose or other form of riser.

#Embodiments of the invention will now be described by way of example, with reference to the accompanying drawings wherein:

#FIG. 1 is a schematic representation of the dynamic metamorphism of a riser system in accordance with the invention;

#FIG. 2 is a schematic perspective view of a first embodiment of riser system in accordance with the invention;

#Referring first to FIG. 1, this Figure schematically depicts a surface-floating vessel 10 to which a flexible hose 12 extends from a wellhead 14 mounted on the seabed 16. The upper end of the riser 12 is tethered to the vessel 10 by a tether system 18 (detailed with reference to subsequent Figures). The tether system 18 is mounted on an overhanging bracket or roof structure 20 cantilevered to the structure of the vessel 10. Between the tether system 18 and static pipework 22 fixed on the vessel 10, a further and relatively short flexible linking hose 24 completes the flow path for fluids from the wellhead 14 via the riser 12 to the vessel pipework 22.

#Because the surface-floating vessel 10 is subject to weather and ocean conditions, the alignment, position, and depth of the vessel 10 are variable, with consequent variations in the extent and direction of separation of the vessel-mounted tether system 18 from the seabed-fixed wellhead 14. These variations result in deviations of the riser 12 from its nominal position with respect to the vessel 10 (shown in full line), to a maximally deviated position schematically depicted in dashed line at 12A. The corresponding maximally deviated position of the linking hose 24 is schematically depicted in dashed outline at 24A. If the riser 12 were simply clamped to an immobile anchor point on the vessel 10, with reliance solely on the inherent flexibility of the riser 12 to accommodate these aforementioned deviations, excessive strains would be imposed on the riser 12 in the vicinity of its point of clamping, with consequent risk of premature failure. In accordance with the invention, the tether system 18 incorporates a pivot system (detailed below) in order to relieve the riser 12 of such excessive strains.

#Referring now to FIGS. 2 and 3, these schematically depict a first embodiment of the tether system 18. A hollow tubular elbow 30 is coupled by a coupling 32 at its upstream end to the upper end of the riser 12 (which is the downstream end of the riser 12 in terms of normal flow of fluids from the wellhead 14 to the vessel 10). The elbow 30 is coupled by a coupling 34 at its downstream end to the upstream end of the linking hose 24. A coupling 35 couples the downstream end of the linking hose 24 to the upstream (flow-receiving) end of the static pipework 22.