Marine riser system

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.

The present invention relates to a marine riser system for the recovery of hydrocarbons from the seabed to a surface vessel such as a floating production and storage vessel (FPSO) to support risers for example flexible risers, steel catenary risers, or bundles thereof.

In deep water, direct connection of steel catenary risers or bundles to floating facilities is feasible but generates constraints:The riser must be installed after the FPSO is on location, which is usually on the critical path for planning purposes.Disconnection takes a long time and is not feasible under adverse weather conditions.The floating facilities impose on the risers motions which are detrimental in terms of fatigue.

Other systems have been proposed to support risers and notably cylindrical buoys with or without ballasting elements, such as for example U.S. Pat. No. 5,639,187 (Mobil). For large field developments a riser tower can serve the same purpose, in particular for stringent thermal requirements. An example of such a tower is in U.S. Pat. No. 6,082,391 (Stolt Comex Seaway & Doris Engineering).

Small to medium reserves fields require a small number of risers for which a riser tower may not be economical. In addition the expense of a riser tower may not be justified when thermal functional requirements are relaxed.

The aim of the present invention is to provide an alternative form of riser system in which the above mentioned problems are overcome or in the very least alleviated.

The present application proposes in general terms a riser system comprising a central buoy for suspension of risers (flexible risers, steel catenary risers or bundles). The risers may be arrayed symmetrically about the buoy, so as to keep its location by their inherent tension. There may for some applications be a possibility of disconnection from floating facilities.

In accordance with a first aspect of the invention there is provided a marine riser system for transferring fluid between a plurality of mutually separated locations on the seabed and a vessel floating on the surface of the sea, the riser system comprising:a submerged buoy,a like plurality of risers each suspended from the buoy and extending from the buoy in mutually different directions to a respective one of the locations on the sea bed whereby differently acting directions of riser-weight-applied pull on the buoy tend to keep the buoy horizontally between the locations, the vessel floating substantially above the submerged buoy,a like plurality of flexible conduits extending from the vessel to the submerged buoy,a respective connector carried by the submerged buoy and connecting the lower end of the respective flexible conduit to the upper end of one of the risers.

The invention in its preferred embodiments provides riser systems having the following features:low cost subsurface buoy located away from the seawater surface to minimise the impact of waves;small vertical and horizontal loads allowing “soft” connection to the floating facilities;design suitable for spread mooring or turret mounted mooring, in which case it serves as a disconnectable mooring system without the cost associated with large forged and machined parts;Suitable for rapid disconnection; andNo bending moment is applied to the risers by the movement of the buoy.

With the system of the present invention two or more risers can be accommodated. In addition the buoy may be tethered to a mooring such as a suction anchor. In particular three to eight risers may be arrayed at approximately regular angular positions around the central buoy.

The lines between the subsurface buoy and the floating facilities may be flexible pipes arranged in for example, a “Chinese lantern” pattern. Using connectors of a type known per se, the lines may be disconnected in a matter of hours in case of inclement weather and reinstallation achieved relatively easily.

Further aspects of the marine riser system are detailed in the appended claims of this application.

In accordance with a second aspect of the invention there is provided a method of installing a marine riser system in accordance with the first aspect of the invention, the method comprising the steps of:tethering the buoy to an anchor at a location which is horizontally intermediate the sea bed locations and which is vertically beneath the sea surface,suspending each of the plurality of risers on the buoy to extend therefrom to a respective one of the sea bed locations,providing a respective connector on the upper end of each riser,subsequently stationing the vessel to have its centre of yaw substantially directly above the buoy,extending an appropriate plurality of flexible conduits from the vessel each to a respective one of the connectors, andconnecting the flexible conduits to the respective connector.

Further, optional aspects of the method are detailed in the dependent claims.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

The examples to be described embody two basic schemes for the riser system. In a first scheme (to be described further with reference toFIG. 1), the subsurface buoy is connected to a mooring system only during the installation phase. After installation, the mooring is removed, and the tension in the risers themselves keeps the buoy in position. In the second scheme (FIG. 2), by contrast, after riser installation is completed, the buoy tether is tensioned to eliminate buoy vertical movement. This accommodates variations in the density of fluid contained in the risers, which may include water, oil and gas in different proportions during operation.

Referring firstly toFIG. 1, the riser system includes a FPSO1, a subsurface buoy2including a roll motion damper3, and a suction anchor4which is deployed in the sea bed. During installation, subsurface buoy2is connected by a tether line5to the suction anchor4, during installation process shown. Further the subsurface buoy2is connected by risers or riser bundles6to the sea bed. The risers6may be steel catenary risers or flexible risers depending on the characteristics of the production system. In the example shown, the simple Catenary configuration is modified to provide a “lazy wave” shape at6a, using buoyancy devices in a known manner. This shape and other buoyed configurations are all included within the general term “Catenary riser”, as used herein.

During the installation of the marine riser system the tether line5loosely connects the suction anchor4and the subsurface buoy2to hold the component parts of the system in place while the risers6themselves are installed. Once the construction and the location of the component parts of the system is complete the tether line5is removed, and the buoy is kept in its place both vertically and horizontally, by the balanced tension in all of the risers The minimum number of risers is two for this purpose. In practice, however, a more or-less circular array of three, four, five, six, eight or so on risers may be accommodated.

During this installation process, although shown inFIG. 1for completeness, FPSO1need not be on station, or even be built at all. Each riser6terminates at the buoy2in a connector7. At the appropriate time, FPSO1is connected to the risers6through flexible jumpers8. These may bulge as shown, in a form of “Chinese lantern”, the extra length accommodating movement of the FPSO1without putting strain on the risers themselves.

The system may include a remote control emergency disconnect system which is located to disconnect the risers6and flexible conduits8at the subsurface buoy2. Connectors7suitable for this purpose are known per se.

It should be noted that other anchor systems can be used instead of the suction anchor system illustrated in this example.

FIG. 2of the drawings shows a similar system but according to the second scheme mentioned above. The same reference signs are used to denote corresponding parts. Rather than remove tether5in this example, it is actively tensioned and remains in place as part of the installation. The tension in the tether line5helps to eliminate buoy vertical movement. This accommodates for example variations in the density of fluid contained in the risers, which may include water, oil and gas in different proportions during operation.

Referring particularly to the inset detail (oval II) inFIG. 2, this example also shows a variation in the manner of connection of the risers6to the flexible conduits8at the subsurface buoy2. In the arrangement shown each riser6is connected to the buoy2by a flexible link9, such as a chain. Each remotely operable connection7is located above the chain linkage at the upper end of the respective riser6and this remotely operable connection is relatively free of other mechanical connection to the buoy2.

The remotely operable connection7forms part of the remotely controlled emergency disconnected system mentioned above.

The systems illustrated provide an alternative to the known riser systems mentioned in the introduction, and are especially suited for deepwater and ultra-deep field developments for small to medium reserves, particularly in areas where disconnection is a requirement because of sudden adverse meteorological and ocean conditions.

Modifications and other examples will readily be envisaged by the person skilled in the art, within the spirit and scope of the invention is not intended to be limited in any way by the examples disclosed.