Bundled, articulated riser system for FPSO vessel

Abstract

A method and apparatus for bundling flexible risers uses a vertically-hanging riser support shaft extending below the turret of a turret-moored FPSO to manage the motions of the risers. The risers may transition to a catenary configuration as they exit a bottom structure at the lower end of the riser support shaft and connect to wellheads or flowlines on the seafloor. Certain embodiments are suitable for use with a disconnectable buoyant turret mooring system while other embodiments may be used with spread-moored FPSOs.

Claims

1. A deep-water riser system comprising: a floating member; a riser bundle bottom structure; a bundle of composite risers suspended from the floating member, each riser having an upper end attached to the floating member and an opposing lower end attached to the riser bundle bottom structure such that the bundle of composite risers is maintained under tension; restraining elements configured to limit horizontal excursions of the risers in the bundle; at least three, spring-loaded members within each restraining element that are configured to center the riser within a central bore of a restraining element, wherein the floating member is moored to the seabed with a plurality of first mooring lines such that horizontal excursions of the floating member are limited; and wherein the riser bundle bottom structure is moored above the seabed with a plurality of second mooring restraints such that horizontal excursions of the riser bundle bottom structure are limited.

2. The deep-water riser system recited in claim 1 wherein the spring-loaded members are configured to permit the insertion of a riser from an upper end of the retraining element.

3. The deep-water riser system recited in claim 1 wherein the spring-loaded members are configured to permit the insertion of a riser from a lower end of the retraining element.

4. The deep-water riser system recited in claim 1 further comprising a clump weight attached to the riser bundle bottom structure via a line.

5. The deep-water riser system recited in claim 4 wherein the length of the line is selected such that the clump weight is nominally suspended above the seafloor.

6. The deep-water riser system recited in claim 1 wherein the floating member is a turret-moored FPSO vessel.

7. The deep-water riser system recited in claim 1 wherein the floating member is a buoyant turret mooring buoy for an FPSO vessel.

8. The deep-water riser system recited in claim 1 wherein the floating member is a spread-moored FPSO vessel.

9. The deep-water riser system recited in claim 1 further comprising guides within the riser bundle bottom structure through which the composite risers pass said guides sized and configured to limit the bend radius of the composite risers.

10. A deep-water riser system comprising: a floating member; a riser bundle bottom structure; a compartment within the riser bundle bottom structure sized and configured to contain variable ballast; a bundle of composite risers suspended from the floating member, each riser having an upper end attached to the floating member and an opposing lower end attached to the riser bundle bottom structure such that the bundle of composite risers is maintained under tension; wherein the floating member is moored to the seabed with a plurality of first mooring lines such that horizontal excursions of the floating member are limited; and wherein the riser bundle bottom structure is moored above the seabed with a plurality of second mooring restraints such that horizontal excursions of the riser bundle bottom structure are limited.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) FIG. 1 is a schematic of a basic bundled flexible riser configuration for a turret moored FPSO system according to a first embodiment of the invention.

(2) FIG. 1A is a schematic of a bundled flexible riser configuration for a turret moored FPSO system according to a second embodiment of the invention.

(3) FIG. 1B is a schematic of a disconnectable bundled flexible riser configuration for a BTM turret moored FPSO system according to a third embodiment of the invention.

(4) FIG. 1C is a schematic of a bundled flexible riser configuration for a turret moored FPSO system or a BTM turret moored FPSO system according to a fourth embodiment of the invention.

(5) FIG. 1D is a schematic of a bundled flexible riser configuration for turret moored FPSO system or a BTM turret moored FPSO system according to a fifth embodiment of the invention.

(6) FIG. 1E is a schematic of a bundled flexible riser configuration for a turret-moored FPSO system or a BTM turret moored FPSO system according to a sixth embodiment of the invention.

(7) FIG. 2 is a schematic cross-sectional view of a gimbaled connection of a bundled flexible riser to an FPSO vessel turret.

(8) FIG. 2A is a schematic cross-sectional view of another embodiment of a connection of a bundled flexible riser according to the invention to an FPSO vessel turret.

(9) FIG. 3 is a schematic side view of the lower end of a bundled riser for a turret moored system according to one embodiment of the invention.

(10) FIG. 4A is a schematic cross-sectional view of the upper portion of a disconnectable bundled riser system according to an embodiment of the invention.

(11) FIG. 4B is a schematic cross-sectional view of the upper portion of a disconnectable bundled riser system according to gimbaled embodiment of the invention.

(12) FIG. 5 shows schematic side and end views of a riser system according to an embodiment of the invention for a spread-moored FPSO vessel.

(13) FIG. 6 is a schematic illustration of an alternative riser tie-off to a fixed FPSO riser deck.

(14) FIG. 7 schematically illustrates several examples of riser guide template sleeves according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

(15) Ship-shaped FPSOs are used to produce subsea reservoirs. To enable this production a variety of risers are used to transfer flow of fluids and energy between the FPSO vessel and the seafloor. In deep water, these riser systems become increasingly complex due to the longer distance, environment, temperatures, and pressures required to be overcome by the risers. Typically many of these flow paths are handled by individual risers so, as the number of paths increases, the congestion created by these risers starts to be problematic as currents may cause large riser excursions that may cause risers to clash. This is aggravated by the use of differing riser diameters with various unit weights, as different riser diameters and weights respond differently to currents.

(16) One efficient approach to high pressure deepwater risers is the use of individual steel risers or Steel Catenary Risers (SCRs), which can handle the large internal and external pressures, aggressively corrosive fluids and temperatures that may be found with deepwater reservoirs. These SCRs however take up a lot of room as they move down the water column and, if not in relatively benign wave environments, FPSO heave motion causes fatigue issues where they come into contact with the seafloor. The fatigue issues may be improved for moderately harsh environments by adding some buoyancy to these SCRs forming a lazy Wave (LW) near the seafloor creating a slight arch that minimizes the seafloor touch down problem. This buoyancy, however, also increases the length and motions of these LWSCRs and also increases their hardware and installation costs.

(17) An alternative design used for deepwater risers is the use of a hybrid riser system using riser towers. These towers may use either a single or bundle of steel vertical riser(s) from the seafloor to a subsea buoy that supports these vertical steel riser(s) to a depth of perhaps 200 meters from the sea surface. The bundled systems also use some distributed buoyancy along the length of the vertical pipes coming up from the seafloor. From the approximately 200-meter water depth, a flexible pipe connection is made to the FPSO vessel turret if it is a weathervaning FPSO or to some other location (typically near midship) if a spread moored FPSO. The use of these flexible lines at the top provides the flexibility for the risers to handle the various motions of the FPSO. These tower systems may be used in the harshest of environments but they result in very expensive hardware and installation costs. The use of the flexible riser also creates some limitations when used at higher temperatures or with certain aggressive, corrosive fluids that may be found in these reservoirs.

(18) This invention provides a more compact riser system that may be used between the seafloor and an FPSO. The system is based on the use of a bundled riser approach that may be attached to the FPSO by way of gimbals. This connection may be to a weathervaning, turret-moored FPSO as shown in FIG. 1, or directly to a spread-moored FPSO. From the vessel, this riser bundle may traverse most of the water column to a location as close as practical to the seafloor where it may be partially restrained from horizontal motion. A variety of connections from the top of the riser bundle to the turret or vessel and bottom of the riser bundle to the seafloor are described.

(19) The top of the riser bundle may be supported by gimbals in the FPSO turret as shown in FIG. 2. For a spread moored system (not shown), the gimbals may be supported directly to the vessel in a moon pool. The riser structure and riser bundle pass through the FPSO-located gimbal ring, which supports the riser bundle via orthogonal pins passing to the riser and turret. To minimize the motion of the riser top when the vessel rolls, pitches or yaws this gimbal may optimally be placed as close as possible to the vessel CG (possible if spread moored or dynamically positioned) or at least on its horizontal longitudinal axis (if using a passive weathervaning turret). To deal with the relative angular roll and pitch motions between the turret and riser bundle, flexible piping must be used in order to have a continuous flow from the riser to the vessel. To minimize this relative motion, it may be best to keep the individual riser termination as close as possible to the gimbal's center of rotation. The location of the riser terminations may preferably be placed within the turret at a level normally above the water line; however all of the components at these terminations may also be submerged.

(20) A variety of flexible connections are possible for the flowlines from the articulating riser top to the turret or vessel. To minimize these flexible connections, common riser flow lines may be manifolded at the top above the riser terminations. Spool pieces may also be used to bring the riser piping closer to the bundle center to minimize relative motions between the riser and vessel flowlines. The flexible connection within the turret may be made with a suitable flexible flowline if it is capable of handling the expected pressures, temperatures and flow chemistries. These flexible lines may be arranged in configurations allowing all potential angles between vessel and riser top. Configurations of this type have been used on riser turret mooring (RTM) systems. Should there be a problem with using a flexible line, alternatives exist, for example: a) the use of steel pipe lengths having a series of six swivels, b) steel pipe length with a series of three flex-joints, c) or the use of newly developed composite pipes that have flexibilities approaching those of flexible pipes. All of these alternatives have temperature limitations. The highest temperature alternative being a), followed by b), then c). It should be noted that these temperature limitations exist not only with the method of the present invention, but in the riser systems of the prior art.

(21) The makeup and construction of the riser bundle may be performed sequentially until the bundle reaches its proper length. Once at full length, other flowlines may be added until the bundle is completed. The system may then be transferred to the FPSO by keel hauling it into place. Consideration may be given to installing this type of makeup equipment on the FPSO. A spread-moored vessel may easily be fitted with a drilling type derrick whereas a turret moored FPSO may need to clear a central shaft within the upper turret in which to house such equipment. The fluid swivel, as usual, may be placed at the top of the turret above the shaft. Having this equipment on board the FPSO may allow self-installation of the riser bundle and allow for a phased, planned, riser installation over the life of the field. Generally, however, the riser may consist of a central structural pipe with a series of template guides holding flowlines in the bundle at certain spacing. For installation purposes, one may consider a dry horizontal makeup, tow and upending, however it lends itself better to a vertical makeup from a drilling type platform or workover type vessel where a series of connected pipes may be installed.

(22) The bottom of the riser bundle may be designed to help control the linear and angular motions that may have to be accommodated by the flowline passing from the bundle to the seafloor. The bundle bottom may consist of a prefabricated section having a structural connection to the upper bundle, flowline connections to the bundle flowlines, internal flowpaths to connectors for the connection to the piping passing to the seafloor, and other connections for chains and installation aids as required. To minimize the horizontal excursion of the bundled riser bottom it may be partially restrained from horizontal motion. This horizontal restraint may depend on the type of flexible flowline connection used between the riser bottom and seafloor. Should the flowlines have a low horizontal stiffness, a horizontal restraint system comprised of three groups of catenary chains shown in FIG. 1A or synthetic lines may be attached to the bottom of the riser bundle. The use of chains adds weight and accommodates any vertical motion while keeping the horizontal excursion within a desired envelope of, for instance, 10 meters. The use of synthetic lines may not add weight but may be able to allow for the same type of motion control. To control angular motion, additional weight may be added to the bottom of the bundle. This weight may be integral or added in the form of a hanging clump weight. The tension created by this additional weight acts to minimize the riser bundle curvature due to current.

(23) The bottom of the riser bundle must connect the riser bundle flowlines to the flowlines passing to the seafloor. There are a variety of threaded mechanical connectors that may be used to terminate the riser bundle flowlines to the riser bottom and these may be made up during the original bundle installation or as further lines are added to the bundle. The connection of the flowlines from bundle to seafloor may be made up after bundle installation and therefore use underwater mateable connectors. Generally these connectors may be made up hydraulically with the help of an ROV. These connections may use proven vertical stab-type connectors, which have had the mating part preinstalled on the prefabricated riser bundle bottom section.

(24) There are several different flowline configurations possible for connecting the riser bundle bottom to the seafloor. Generally, the seafloor connection may be located on a Pipe Line End Termination (PLET) in the vicinity of the riser bottom. There should be a sufficient horizontal distance between the PLET and riser bottom to enable the flexibility of the system components not to be overstressed. The configurations that may be used are:

(25) 1. FIG. 1C shows one option having two articulated pipes. One pipe is a generally horizontal steel pipe having a flexjoint at the riser bundle bottom connector. This pipe extends horizontally to a down facing elbow with an overhead subsea buoy. Below the elbow is a flexjoint with a generally vertical down pipe with termination to another flexjoint on top of a vertical stab connector. The vertical down pipe length may be dependent on the height and horizontal excursion of the riser bundle connector with respect to the seafloor. The installed pipe geometry should be such that keeps the flex-joint angles below 20 degrees and preferably below 15 degrees. The pipe system may installed by lowering it with an installation line to the riser connector (in certain embodiments supported from the FPSO) and a second line to the buoy above the vertical down pipe and connector. The buoy may be designed for the proper amount of operational lift by adding weight to the connector at the bottom of the vertical down pipe. During lowering, the overall piping system may be negatively buoyant. When stabbed in on the riser bundle bottom and on the PLET, sufficient weight is transferred onto the connections so the buoy has the proper buoyancy for supporting the piping system. Should the horizontal pipe section be too long to be selfsupporting then strengthening and distributed buoyancy may be applied to make it so.

(26) 2. A similar system of two pipes as described above may be used with stress joints being substituted for the flex-joints. Should bottom angular excursions be excessive for the piping torsion, then inline swivels may be added to the pipes. The overall maximum angles of the stress joints should be less than 15 degrees or preferably below 10 degrees.

(27) 3. A similar system of two pipes with a series of six swivels arranged to take all pipe excursions may be added to the piping. The maximum angles at any of the swivels should be kept below 30 degrees.

(28) 4. Flexible piping if suitable for the pressure, temperature and flow product chemistry may be used in a catenary or arched buoyant configuration.

(29) 5. An SCR with a buoyant lazy wave to seafloor piping is shown in FIG. 1D. An alternative use of a buoyant arch to a vertical PLET connection may also be possible but is not shown. Owing to the lack of flexibility, the horizontal extent of these arches may have to be several hundred meters from the riser bundle.

(30) 6. A composite pipe (if suitable for the temperatures encountered) may be used in an arched configuration and is shown in FIG. 1E. This arch is considerably smaller than what may be required for an SCR arch owing to the much greater flexibility of this composite pipe. Weights or buoyancy may be added to confine the composite pipe to do its primary bending over certain lengths.

(31) 7. A combination of the above.

(32) This type of riser system may also be used with disconnectable FPSO systems. FIG. 4A shows the general configuration of a turret-disconnectable buoy with a BARS riser system. The basic configuration of the BARS system may remain the same. Some required changes may be the addition of buoyancy in the riser bundle to make it somewhat more buoyant. There may be a large hanging clump weight below the riser for disconnect and to limit the disconnect set down. This clump weight may also be used to minimize riser current curvature in the connected mode. There may be additional vertical motion of the system due to disconnect, however the use of the clump weight may limit this motion at the bottom of the riser. The additional vertical set down may be accommodated by the lower flexible seafloor piping.

(33) In very deep water of 1500 meters or more, it is often desirable to use steel catenary risers SCRs as they have less corrosion and structural problems than steel un-bonded flexible pipes (SUFP). The SCRs also have the advantage of being lighter and cheaper than the SUFP. With the advances in technology of creating composites, one may now create flexible un-bonded risers where the steel is replaced with various glass, carbon or other composite reinforcement materials. This results in composite un-bonded flexible pipe (CUFP) that is much lighter in water than those using steel. The cost of these composite risers is still more than SCRs, however their weight, fatigue and corrosion advantages make them attractive for deep water use.

(34) Whenever a new technology is available for use it is desired to make the most efficient use of it. These composite un-bonded risers have seen very little use in deep water particularly where a large number of them may be used from an FPSO.

(35) The weight advantage offered by these CUFP risers is a great benefit as it requires less buoyancy to support the risers from the FPSO, thus saving on vessel displacement. This weight advantage is somewhat eroded by the fact that the reduction in riser tension from self-weight makes the riser more susceptible to drift and vortex-induced vibrations (VIV) in currents. To counter this low weight, there may be steel armoring introduced into the composite flexible pipe making a hybrid unbonded flexible pipe (HUFP) which is heavier in water. This weight increase, while perhaps necessary, is counterproductive and an efficient configuration should have the means for taking full advantage of the weight savings by addressing the reduction of drift and VIV.

(36) The means for controlling drift and VIV is to interconnect or bundle the risers and have them hang vertically down the water column. In this manner the riser lengths are minimized, prevented from clashing and may have a small, well defined touchdown area. This minimizes bottom congestion and allows the risers to be laid radially outward to their subsea tie-ins. The possible configurations that may be used to create this type of bundled riser approach for permanent or disconnectable turret type FPSOs and also spread moored FPSOs are shown in the accompanying figures.

(37) In FIG. 1, a permanently moored turret FPSO is shown with a bundled riser connected from the turret to a riser bundle bottom structure. The turret is conventionally moored by radial mooring legs, which fix the turret from rotation while a bearing system allows the FPSO to weathervane. From the turret, the risers all run vertically downward to the bottom structure 18 from where they catenary to the seafloor. Having these risers in short catenaries may help to limit their touch-down zone fatigue. The composite risers having better fatigue resistance than steel flexible risers should preclude any problems with touch down fatigue, however should fatigue still be a problem, some buoyancy or weight may be added to the near-bottom riser.

(38) FIG. 1 also shows the possible attachment of optional riser bundle restraint mooring lines and/or a clump weight to the riser bundle bottom structure. The function of the restraint lines may be to minimize the horizontal motion of the bundle in the event this motion is deemed excessive for the riser touchdown. The restraint lines may also be necessary to stabilize the bottom motion during the initial and later stages of riser installation. The function of the clump weight is to provide sufficient tension in the bundle to control its curvature and to limit its pendulum motion if unrestrained by mooring lines. Depending on installation, the clump weight may be eliminated by inclusion of weight in the bottom structure.

(39) A close up of the upper riser bundle connection to the turret is shown in FIG. 2A. For simplicity, only two risers are shown. However, there may be as many risers (and umbilicals) as required fixed radially within the bundle. The risers are supported in the turret on the riser support deck from where they hang vertically down along the riser bundle. Below the riser support deck, a U-joint may be used to attach the riser bundle support shaft to the turret. The support shaft function is primarily to support riser guides and the non-riser tension in the vertical bundle. The riser guide vertical placement is designed to keep the risers from clashing and to help control VIV. The horizontal spacing of the risers within the templates may also be designed to prevent clashing and VIV. This bundle design may be similar to that used for the GAP which had a long horizontal underwater bundle.

(40) Operationally, the riser bundle shaft loading is normally quite low. During heavy seas the bundle may, however, articulate and bend. The design of this shaft may consider using a small diameter pipe (possibly a cable) that may flex and stay within allowable stresses. The weight attached to the bottom of the riser bundle shaft may also be designed to minimize this bending, as a larger weight may reduce the curvature. If shaft stresses are still too high, additional U-joints may be incorporated further down the shaft to relieve bending.

(41) When the riser bundle shaft articulates within the turret, it may cause the risers to bend. This bending may be controlled by having trumpet guides fixed directly above and below the U-joint. These guides may have curvatures that keep the riser bending well above their minimum dynamic bending radius. When articulating about the U-joint, the risers may move up or down within the guide below the U-joint and along the complete riser bundle. This sliding may promote some damage in the carcass of the riser. A variety of methods are available to prevent this damage and these may be used as appropriate for the design. Some preventative methods include use of low-friction, nonabrasive coatings on the guides and/or pipe, small rollers within the guides, allowing the lower U-joint guide to articulate relative to the bundle shaft avoiding any sliding, etc.

(42) The bottom termination of the riser bundle is shown in FIG. 3. The distance from the bundle to the seafloor may be designed to be as close as practical. This distance may be site specific and likely differ to account for design and installation parameters. This Figure shows the risers traverse vertically downward into the riser bundle bottom structure where they then bend outward around a guide that keeps the riser curvature above its minimum dynamic bend radius. To prevent chafing of the riser when moving in relation to the guide, a series of roller or other anti-chafing means may be used to line the guide. FIG. 3 shows the riser bundle bottom structure with attachments for optional mooring restraint lines, compartment for ballast weight and/or a clump weight line. All of these options may be used to minimize the excursion and bending of the riser bundle and may be used, if found necessary or desirable.

(43) In areas of severe storms or ice, FPSOs are sometimes forced to disconnect. FIG. 1B shows how a bundle system may be used in such an environment when used with a BTM. To minimize the required buoyancy of the disconnected buoy, the mooring system may be changed to incorporate spring buoys, as these buoys help to support the mooring load. A clump weight may be used to reliably locate the vertical position of the disconnected system. This clump weight acts like a gravity anchor that pulls the disconnected system down a designed distance when disconnecting. Also, to limit the horizontal excursion in both the connected and disconnected condition, mooring restraint lines may be attached to the bottom structure.

(44) The details of the disconnect buoy and bundled riser top connection are shown in FIG. 4A. The riser bundle is terminated to a perforated riser termination deck in the buoy that may be housed in the turret when connected. The termination deck may be perforated to allow for the easy flow through of water when the buoy is in the connect or disconnect mode. The BTM is essentially a donut buoy with a connector and all the mating interfaces to the turret. The details of the bundle and riser connection and interfaces with the buoy are the same as those for the permanent turret system. One difference in the permanent to disconnectable riser bundle is that the riser bundle support shaft for the disconnectable buoy is designed to supply buoyancy to the disconnected system. This is done to limit the displacement of the BTM to a size that is easy to disconnect and reconnect. Spreading the required buoyancy for the disconnected system over a length of the bundle support shaft minimizes the vertical added mass of the combined system, making it easier to move vertically, which reduces reconnection winching requirements and snatch loads in the reconnection line.

(45) FIG. 4B illustrates an alternative embodiment of a disconnectable BTM with a bundled articulated riser system according to another embodiment of the invention. In this embodiment, riser bundle support shaft and buoy 98 is articulated to turret buoy central shaft 86 by means of gimbal ring 54.

(46) The bundled riser approach for a spread-moored FPSO is shown in FIG. 5. The normal riser configurations for spread moors are located on one or both sides of the FPSO as close as possible to the mid-ship. They may also be located in a moonpool, which may be easy to accommodate with similar riser bundle shafts, as described previously. However, a more preferred location may be over the sides as shown in FIG. 5. The risers here may again be arranged to pass vertically downward to a weighted bottom template structure that may be horizontally restrained with mooring lines. FIG. 5 shows a large bottom template. However, this may be split into separate templates for both sides of the FPSO as this may be easier to install. These separate bottom templates may also be cross connected after they are in place. If risers are only used on one side, then a single template may be used at the bottom. The bottom template(s) may be held by tendons that may terminate near the FPSO keel. The tendons may be made from chain, cable or, pipe with attachment points for the bottom and intermediate templates. The risers may be attached outside the vessel deck from where they pass vertically through keel-located trumpet guide(s) and through a series of (as required) intermediate templates until they pass through a curved guide of the bottom template and continue to the seafloor in a catenary. With the tendon connection being at the keel in line with the riser keel guides, there may be very little relative vertical motion between the templates and risers, and thus chafing of the risers at these contact points may be minimal.

(47) Currently, flowline risers from FPSOs are generally attached to separate FPSO turret attachment points and move radially away from the vessel in separate directions or with sufficient clearance to the sea bottom or to submerged support systems not connected to the FPSO. This type of support requires a long riser because, when moving downward, it also moves a considerable distance horizontally. The method and apparatus disclosed herein effectively minimizes the riser length as it travels down to the seabed as it covers the maximum length vertically and only has a small vertical portion where the riser moves radially and bends to lie on the sea bed. The riser lengths in all of the bundled risers are thus minimized and the risers are also held by a guide system so that they do not interfere. This interference may be a significant problem for multiple, individual lightweight risers as they may easily drift in currents and drag on the seafloor. This is avoided by having the riser bundle weighted and otherwise restrained to the seafloor. This system therefore effectively takes advantage of the new lightweight composite type risers by controlling their descent and seafloor landing area.

(48) The invention may best be understood by reference to the exemplary embodiment(s) illustrated in the drawing figures wherein the following reference numbers are used:

(49) 10 turret-moored FPSO

(50) 12 turret

(51) 14 mooring lines

(52) 14 spring buoy mooring lines

(53) 16 riser bundle

(54) 18 riser bundle bottom structure

(55) 20 seafloor

(56) 22 lower catenary flowlines

(57) 24 clump weight

(58) 26 BTM turret

(59) 28 lower riser mooring restraint

(60) 30 spring buoy

(61) 32 spring buoy mooring system

(62) 34 subsea buoy

(63) 36 vertical down pipe

(64) 38 horizontal connector pipe

(65) 40 lazy wave SCR

(66) 42 floatation

(67) 44 arched composite piping

(68) 46 riser bundle support shaft

(69) 48 riser connectors to turret piping

(70) 50 web

(71) 52 turret riser bundle support shaft

(72) 54 gimbal ring

(73) 56 gimbal pin to riser buoy

(74) 58 gimbal pin to turret

(75) 60 riser guide template

(76) 62 riser

(77) 64 riser hang-off pedestal

(78) 66 U-joint

(79) 68 riser trumpet

(80) 70 riser support deck

(81) 72 ballast compartment

(82) 74 riser trumpet

(83) 76 flowline catenary line

(84) 78 line or chains to clump weight

(85) 84 buoy to turret locators

(86) 86 turret buoy central shaft

(87) 88 turret to buoy connectors

(88) 90 riser hang-off pedestal

(89) 92 buoy riser deck

(90) 94 turret buoy

(91) 96 gimbal pin to buoy

(92) 98 riser bundle support shaft and buoy

(93) 100 spread-moored FPSO

(94) 102 template support tendon

(95) 104 intermediate template

(96) 106 bottom template structure

(97) 108 template cross-tie

(98) 110 fixed upper template

(99) A detailed description of one or more embodiments of the buoy and receptor as well as methods for its use are presented herein by way of exemplification and not limitation with reference to the Figures.

(100) Referring now to FIG. 1, turret-moored FPSO 10 is rotatably coupled to turret 12 so as to permit FPSO 10 to weathervane about turret 12. Turret 12 is maintained in a substantially fixed position by mooring lines 14 which connect to anchoring means in seafloor 20.

(101) Riser bundle 16 comprised of a plurality of flexible flow lines descends substantially vertically to the vicinity of seafloor 20. At the lower terminus of riser bundle 16 is riser bundle bottom structure 18 from which lower catenary flowlines 22 exit riser bundle 16 and connect to fluid conduits (not shown) on seafloor 20.

(102) Riser bundle bottom structure 18 may hang freely from turret 12 of FPSO 10. In other embodiments, riser bundle bottom structure 18 may be equipped with restraint mooring lines which terminate in anchoring means in the seafloor to limit its horizontal excursions. In yet other embodiments, clump weight 24 may be connected to riser bundle bottom structure 18 to provide additional tension to riser bundle 16 thereby reducing its susceptibility to movement in currents and vortex-induced vibrations (VIV). Clump weight 24 may be used in conjunction with the optional restraint mooring lines.

(103) FIG. 1A illustrates a turret-moored FPSO 10 having a deep water mooring system 32 that includes subsea spring buoys 30.

(104) An alternative mooring system is illustrated in FIG. 1B. FPSO 10 is rotatably moored using a buoyant turret mooring (BTM) which comprises BTM turret 26 about which FPSO 10 may weathervane. In this embodiment, subsea spring buoys 30 are provided in mooring lines 14 to relieve at least a portion of the mooring line weight from FPSO vessel 10 and the BTM buoy.

(105) FIG. 1C illustrates an embodiment of the invention wherein the fluid connections from the risers to equipment on seafloor 10 are made via substantially horizontal connector pipe 38 to vertical down pipe 36 which is supported by subsea buoy 34. Flexible connections at the ends of pipes 36 and 38 (not shown) allow for these pipes to provide continuous flow paths between sea bottom 20 and riser bottom structure 18.

(106) FIG. 1D illustrates an embodiment of the invention that accommodates heave of FPSO 10 (and the motion of the bottom structure 18 to the seafloor 20) using steel catenary risers (SCRs) 40 in a lazy wave configuration. As is conventional in the art, the lazy wave configuration of SCRs 40 is produced by providing floatation 42 along a selected portion(s) of SCR 40. In this way, changes in the contact point of SCR 40 with seafloor 20 (which is known to cause wear in SCRs) is minimized. The connection of SCR 40 to the bottom structure 18 may include a Flex-joint which accommodates the relative angular motion between the riser and this structure.

(107) Yet another embodiment of the invention is illustrated in FIG. 1E. In this embodiment, fluid connections from equipment on seafloor 20 to the risers in riser bundle 16 is made using composite piping in an arched configuration from seafloor 20 to bottom structure 18. Changes in the elevation of bottom structure 18 resulting from heave motions of FPSO 10 (and limited horizontal excursions of bottom structure 18) are accommodated by the arched configuration of the flexible composite piping. Relative angular motions at the ends of the flexible composite pipe may be controlled with bend restrictors that control the pipe curvature as it bends.

(108) FIG. 2 illustrates an embodiment of the invention wherein risers 62 and riser bundle support shaft 46 are supported on gimbal ring 54 mounted on webs 50 within turret 12. Motion of risers 62 and riser bundle support shaft 46 in and out of the plane of the illustration is accommodated by gimbal ring 54 pivoting on gimbal pins 58. Motion of risers 62 and riser bundle support shaft 46 to the left or right in the plane of the illustration is accommodated by pivoting riser bundle support shaft 46 on gimbal pin to turret 12.

(109) FIG. 2A illustrates an alternative embodiment wherein riser bundle support shaft 46 is suspended by universal joint (U-joint) 66 from riser support deck 70 within turret 12. Riser support deck 70 may be supported within turret 12 by structural web members 50. Riser bend guides (trumpets) 68 may be provided on risers 62 above and/or below U-joint 66 to limit the bend radius of risers 62.

(110) As will be appreciated by those skilled in the art, as riser bundle support shaft 46 swings on U-joint 66 the risers will slide axially relative to the riser bend guides 68 located below U-joint 66. To prevent or minimize wear which may occur as the result of this sliding motion, the outer surface of risers 62 in the vicinity of riser bend guides 68 and/or the inner surface of riser bend guides 68 may be provided with anti-friction material or coatings or mechanical devices such as rollers. For example, guide 68 may have its inner surface coated with Inconel and the riser may have a sequence of clamped-on, Teflon-impregnated, composite rings.

(111) FIG. 3 illustrates the lower end of a bundled riser system for a turret-moored FPSO according to one embodiment of the invention. Riser bundle bottom structure 18 is equipped with ballast compartment 72 for providing tensioning weight to riser bundle support shaft 46. This ballast may be in lieu of or in addition to a clump weight suspended on chain 78. Riser bundle bottom structure 18 is also equipped with internal riser trumpets 74 for limiting the bend radius of risers 62 as they transition from a vertical segment which parallels riser bundle support shaft to a catenary portion which exits the lower surface of bottom structure 18. When the risers are suspended from a fixed upper support deck 70 as shown in FIG. 2A, the riser trumpets 74 may have wear-prevention means incorporated between them and the riser 62. In certain embodiments, bottom structure 18 may be equipped with restraints 28 which may comprise chain to anchoring means in the seafloor. In this way, horizontal excursions of bottom structure 18 may be limited.

(112) FIG. 4A illustrates a embodiment of the invention having a disconnectable turret buoy 94 which is aligned by buoy-to-turret locators 84 so as to engage turret-to-buoy connectors 88 when turret buoy 94 is pulled into turret 12. Turret buoy 94 may have turret buoy central shaft 86 for buoy riser deck 92 which supports riser hang-off pedestals 90. In the illustrated system, U-joint 66 is used to suspend riser bundle support shaft 46 and trumpets 68 and 68 act to limit the bend radii of risers 62 when riser bundle support shaft and buoy 98 is not plumb. With this disconnectable buoy, the riser bundle support shaft and buoy 98 may be made partially buoyant to minimize the required buoyancy of turret buoy 94.

(113) A gimbaled version of a disconnectable BTM supporting a riser bundle support shaft and buoy 98 is shown in FIG. 4B. In this embodiment, riser buoy 98 may be made positively buoyant by means of internal flotation material or captive air to help support turret buoy 94 in the disconnected state. Risers 62 and riser buoy 98 are supported with turret buoy central shaft 86 on gimbal ring 54 mounted on webs 50 within turret buoy 94. Motion of risers 62 and riser bundle support buoy 98 in and out of the plane of the illustration is accommodated by gimbal ring 54 pivoting on gimbal pins 96. Motion of risers 62 and riser buoy 98 to the left or right in the plane of the illustration is accommodated by pivoting riser buoy 98 on gimbal pin to riser buoy 56.

(114) A riser system according to an embodiment of the invention designed for a spread moored FPSO 100 is shown in FIG. 5. In the illustrated preferred embodiment, the risers may be supported from above-water, riser support structures on the side of the vessel directly above a fixed template 110 attached to the vessel keel. The fixed template 110 has individual trumpet guides that limit the bending of risers 62 that occur due to vessel and riser motions as they pass through the template. An alternative riser support may have the risers directly attached to the fixed template 110 with bend restrictors around the risers (which may also be attached to template 110) to control the riser bending. Lower intermediate templates 104 are vertically spaced apart and are supported between a pair of template support tendons 102. Template cross-ties 108 may be used to interconnect templates on opposite sides of FPSO 100.

(115) Template support tendons 102 extend to bottom template structure 106 which may be horizontally restrained by bottom restraint chains 28 which connect to anchoring means (not shown) in seafloor 20. The bottom template 106 may also include ballast material to tension the template support tendons 102 and thus stiffen the entire riser system. This will limit the excursions of the system to waves and currents.

(116) At the lower terminus of each flexible riser 62 in bottom template 106, the riser 62 continues as a riser catenary 76 which provides fluid communication to equipment (not shown) on seafloor 20. Where the vertical riser 62 transitions to the catenary 76 in the bottom template, riser trumpets 74, as shown on FIG. 3, may be used to control the riser bending.

(117) FIG. 6 illustrates a riser tie-off to a fixed FPSO riser deck that may be employed as an alternative to that shown in FIG. 2A. In place of U-joint riser bend guides 68, this embodiment has riser bend stiffeners 63 surrounding an upper portion of risers 62. Riser bend stiffeners 63 may act to limit the bend radius of the portion of risers 62 to which they are applied.

(118) Riser bend stiffeners 63 may have particular application in the case of flex risers. The illustration on the right side of FIG. 6 shows an enlarged view of the upper portion of a flex riser equipped with a riser bend stiffener 63 according to the invention. As shown in this illustration, an adapter may be used to increase the diameter from that of the smaller metal flange diameter typically used for flex risers to that sufficiently wide to attach to the riser bend stiffener (i.e., riser bend stiffener flange diameter 61). In yet other embodiments, riser bend stiffeners 63 may be used in conjunction with U-joint riser bend guides 68.

(119) FIG. 7 presents various exemplary riser guide template 60 sleeves. In the second-to-the-left embodiment, riser 62 is free to move laterally within riser guide template 60. In the leftmost illustration of FIG. 7, a split insert (that may be installed after riser installation by a diver or an ROV) restrains the lateral movement of riser 62 within riser guide template 60.

(120) The two embodiments shown on the right side of FIG. 7 have three or more torsional-spring-loaded pins that act to push arms out of slots in the walls of the riser guide template 60 to center the riser 62 in riser guide template 60. The second-from-the-right illustration is a configuration that may be used when riser 62 is installed from the top in a downward direction. The rightmost illustration is that of an embodiment which may be used when riser 62 is installed from the bottom up.

(121) Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.