GB2090221A - Marine compliant riser system and method for its installation - Google Patents

Abstract

A marine compliant riser system for connecting a marine floor base to a marine surface facility comprises a vertical conduit section (21) comprising a plurality of flowlines (30) extending from the marine floor to a submerged buoy section (26); a yoke assembly (82) mounted on the buoy section; a flexible conduit section comprising a plurality of flexible catenary flowlines (70) located adjacent one end of the yoke in a substantially vertical flow direction in the yoke assembly; and a plurality of rigid gooseneck conduits (36) supported on the buoy section and operatively connecting in fluid communication the flowlines in the vertical conduit section to the flexible flowlines supported in the yoke assembly at substantially a normal catenary departure angle. <IMAGE>

Description

SPECIFICATION Marine compliant riser system and method for its installation This invention relates to a marine compliant riser system, that is to say a system for providing fluid communication between a marine surface facility and a subsea wellhead or manifold system, and to a method for its installation.

An essential element in the recovery of fluid hydrocarbons from marine deposits lies in establishing a fluid communication system from the marine bottom to the surface after production capability has been established. Such a system, commonly called a production riser, usually includes multiple conduits through which various produced fluids are transported to the surface, including oil and gas production lines, as well as service, electrical and hydraulic control lines.

For offshore oil and gas production, a floating facility can be used as a production and/or storage platform. Since the facility is constantly exposed to surface and sub-surface conditions, it is consequently subject to heave, roll, pitch and drift. In order for a production riser system to function adequately with such a facility, it must be sufficiently compliant to compensate for such movements over long periods of operation without failure.

One example of such a marine riser is the compliant riser system described in U.S. Patent 4,182,594. This compliant riser system includes a vertical, rigid section which extends from the marine bottom to a fixed position below the zone of turbulence that exists near the surface of the water, and a flexible section comprising flexible flowlines that extend from the top of the rigid section, through the turbulent zone, to a floating surface vessel. A submerged buoy is attached to the top of the rigid section to maintain the rigid section in a substantially vertical attitude. With riser systems of this type, difficulties often arise in installing and maintaining the flexible flowlines which are often attached to the rigid section such that the end portion adjacent the rigid section is not at a normal catenary departure angle.This can result in localized stresses, causing undue wear in the flexible flowline at its terminal hardware. If a natural catenary shape is assumed by the flowline, it approaches the fixed section in an upward direction, nearly vertical at its point of suspension.

The present invention seeks to provide a compliant riser system in which the flexible flowlines assume substantially vertical departure angles at their terminal portions.

In accordance with the invention, there is provided a marine compliant riser system for connecting a marine floor base to a marine surface facility, comprising: a vertical conduit section comprising a plurality of flowlines extending from the marine floor to a submerged buoy section; a yoke assembly mounted on the buoy section; a flexible conduit section comprising a plurality of flexible catenary flowlines located adjacent one end thereof in spaced relationship in a substantially vertical flow direction in the yoke assembly; and a plurality of rigid gooseneck conduits supported on the buoy section and operatively connecting in fluid communication the flowlines in the vertical conduit section to the flexible flowlines supported in the yoke asembly.

The present invention also provides a method for installing such a marine compliant riser system in deep water, which comprises the steps of: attaching a multiconduit riser section comprising a plurality of flowlines in a substantially vertical attitude to a marine floor base for connection to a source of hydrocarbon fluid, the riser section terminating at its upper end at a submerged buoy section; assembling a flexible conduit system including a plurality of flexible flowlines attached at one end to a marine surface facility, and a yoke assembly in which the flexible flowlines are supported adjacent their other ends in spaced relationship; attaching the yoke assembly to the buoy section with the flexible flowlines depending from the yoke assembly at a substantially normal catenary departure angle;; aligning a plurality of rigid gooseneck conduits with the flowlines in the riser section and the flowlines supported in the yoke assembly; and operatively connecting the gooseneck conduits to the respective flowlines to establish fluid communication through the riser system.

Use of the rigid gooseneck conduits which serve to connect the upwardly directed flowlines in the vertical conduit section to the flexible flowlines combined with use of the yoke assembly which maintains the ends of the flexible flowlines in a substantially vertical attitude for connection to the gooseneck conduits and ensures that the flexible conduits adopt a catenary path between the buoy section and the surface facility, greatly reduce stresses in the flexible flowlines and their associated terminal hardware, thereby reducing wear and prolonging the service life and reliability of the system.

The couplings between the gooseneck conduits and vertical and flexible flowlines are suitably remotely controlled, for example hydraulicallyactuated connectors. Preferably, each gooseneck conduit includes an hydraulically-actuated connector at one end for connection to a vertical flowline and a terminal portion at the other end for insertion into a connector to a flexible flowline.

The gooseneck conduits are supported on the buoy section, preferably in a frame assembly which includes a trough for receiving and supporting the conduit. For added security, the conduit is suitably locked and retained in the trough.

A marine compliant riser system constructed in accordance with the present invention will now be described in greater detail by way of example only with reference to the accompanying drawings, in which: FIG. 1 is a schematic representation of a marine compliant riser system; FIG. 2 is a plan view of the buoy section of the system; FIG. 3 is a side view of the buoy section shown in FIG. 2; FIG. 4 is a plane view of the buoy section with gooseneck conduits attached; FIG. 5 is a vertical cross-sectional view of the buoy section shown in FIG. 3 with a gooseneck conduit yoke assembly and flexible flowlines attached; FIG. 6 is a side view, partly in section of a gooseneck conduit and its associated support and installation hardware; FIG. 7 is a sectional view along line 7-7 of FIG. 6; FIG. 8 is a plan view of a running tool guide for installing the gooseneck conduits;; FIG. 9 is a side view of the flexible flowline section including a spreader beam; FIG. 10 is a cross-sectional view of the flexible flowline section including the spreader beam; FIG. 11 is a plan view of a yoke assembly; FIG. 12 is a side view of the yoke assembly; FIGS. 13 to 17 are side and plan views of a portion of the yoke assembly showing installation of a flexible flowline and its coupling to gooseneck conduit; FIG. 1 8 is a side view of a guidewire connection mechanism; and FIGS. 1 9A to 1 9D are a schematic representation of an installation sequence for the compliant riser system.

In the following description with reference to the drawings, certain portions of the compliant riser system are shown merely to illustrate an operative system. However, modifications and variations to those portions can be made in most instances. For instance, the surface facility need not be a production vessel, since semisubmersible units and floating platforms are viable alternative structures for use with compliant risers, as shown in U.S. Patent 4,098,333.

Likewise, the specific structure of the marine bottom connection may be adapted for a singlewellhead, multi-well gathering and production system or manifold for receiving and handling oil and gas. Similarly, the submerged, free-standing lower riser section need not comprise rigid conduits, since buoy-tensioned flexible tubing or hoses can be maintained in a fixed position when attached to the ocean floor, as shown in U.S.

Patent 3,911,688 and French Patent 2,370,219.

Also, limited excursion of the lower riser section is permissible, although the catenary upper flexible section is relied upon to permit significant horizontal excursion and elevational changes in the surface facility.

Referring to the drawings. FIG. 1 shows a marine compliant riser system 20 in an operational position at an offshore location. Riser system 20 has a lower rigid section 21 and an upper flexible section 22. Lower rigid section 21 is affixed to base 24 on marine bottom 23 and extends upwardly to a point just below turbulent zone 25, which is that zone of water below the surface which is normally affected by surface conditions, for example currents, surface winds and waves. A buoy section 26 including buoyant chambers 31 is positioned at the top of a casing 27 of rigid section 21 to maintain rigid section 21 in a vertical position under tension.

As will be explained in more detail below, flexible section 22 includes a plurality of flexible conduits 70 and spreader beams 75, the flexible conduits being operatively connected to respective flow passages in rigid section 21 at buoy section 26. Flexible section 22 extends downwardly from buoy section 26 through a catenary path before extending upwardly to the surface where it is connected to a floating facility 22a by attachment means 71.

As shown in FIG. 1, base portion 24 and the lower portion of rigid section 21 are typical marine riser components. Base 24 is positioned on the marine bottom and submerged flowlines may be completed thereto. Base 24 may be a multi-well completion template, a submerged manifold center, or a similar subsea structure. Each submerged flowline terminates on base 24 and preferably has a remote connector, for example "stab-in" connector, attached to the lower end thereof. As illustrated in FIGS. 1 to 6, rigid section 21 may be constructed with a casing 27, which has a connector assembly (not shown) on its lower end which in turn is adapted to mate with a mounting on base 24 to secure casing 27 to base 24.

As shown in FIG. 2, a plurality of individual rigid flowlines or conduits 30, which may be of the same or diverse diameters, are run through guides within or externally attached to casing 27 in a known manner. These are attached via stab-in or screw-in connectors of the submerged fiowlines on base 24, providing individual flow paths from marine bottom 23 to a point adjacent the buoy section at the top of casing 27.

The buoy section 26 includes two buoyant chambers 31, affixed to diametrically opposed sides of casing 27. As shown in FIGS. 2 and 3, a beam 33 extends between chambers 31 near their upper ends and is attached thereto. Yokereceiving arms 34 are attached to the outboard edges of chambers 31 and extend horizontally outwardly therefrom.

Mounted atop casing 27 and affixed to beam 33 on the buoy section is a plurality of support structures 35 for receiving and retaining inverted U-shaped conduits (or gooseneck conduits), as explained below. Although, for the sake of clarity, only one such support structure 35 is shown in FIGS.2,3 and 5, it should be understood that the buoy section includes a similar support structure 35 for each rigid conduit 30 within casing 27. The support structure may include individual or integrated-design frames. Though not shown, the individual support structures 35 will be positioned individually beneath each of the gooseneck conduits 36, as shown assembled in FIG. 4.

Referring to FIG. 5, a typical support structure 35 consists of a vertical frame 37 having a lower mounting element 38 affixed to buoy beam 33 and having a trough 39 secured along its upper surface. Trough 39 is sufficiently large to receive a corresponding gooseneck conduit 36.

In order to enhance stability of the support structure 35, additional bracing can be provided for it. It may be desirable to enclose support structure frame 37 within a fairing to minimize snagging by lines or tools, for example by welding a steel plate between the support structures 35 at trough 39 and by fairing in the sides of the open frame 37. Numerous variations in the gooseneck support structure 35 can be made. Guide posts 40 are attached to buoyant chambers 31 and extend upwardly therefrom as shown in FIGS. 2, 3 and 4; the function of those guide posts is described below.

Referring now to FIGS. 6 and 7, gooseneck conduit 36 is comprised of a length of rigid conduit 41, which is curved downwardly at both ends to provide an inverted, U-shaped flow path. A connector 42 (for example, an hydraulicallyactuated collet connector) is attached to one end of conduit 41 and is adapted to couple conduit 41 fluidly to its respective rigid conduit 30 when gooseneck 36 is lowered into an operable position. The extreme environmental conditions of subsea handling systems may cause frequent equipment failures and repair problems, and in order to minimize pollution and loss of product, fail-safe valves are usually employed for all flowlines. Redundant connectors and hydraulic operators are also desirable because of occasional equipment failures. An emergency shut-off valve 43 is therefore provided in conduit 41 just above its male end 45 as shown in FIG. 6.Preferably, the valve 43 is of the fail-safe pressure-opened and spring-closed type, well known in the art (for example, a Grove ball valve with Bettis actuator controls 44). Preferably, the valve 43 will not serve as the primary production shut-in means for its respective flow conduit (the primary means being normally located on the marine bottom 23 as understood in the art) but will be actuated only in an emergency or maintenance operation involving flexible section 22.

There is an individually-designed gooseneck conduit 36 for each rigid conduit 30. Welded or otherwise secured to a horizontal portion of the upper surface of gooseneck conduit 36 is a partial sleeve 32 which has one or more fishing necks 38 thereon. As shown in FIG. 6, two pairs of selflatching clamps 46 are carried by sleeve 32. Each self-latching clamp 46 is comprised of a latching member 47 pivotally mounted on a support 32a on sleeve 32 and having a camming surface 47a on its lower end and ear 47b at its upper end.

Spring 48 normally biases latch member 47 to a locked position. An hydraulic stabbling pod 49 is affixed to gooseneck conduit 36 and hydraulic lines extend from stab-in valves (not shown) within pod 49 to connector 42 for operatively connecting the gooseneck.

Running tool 50 includes a frame 51 having guide funnels 52 (see FIG. 8) at each corner thereof which are adapted to cooperate with guide posts 40 on buoyant chambers 31 so that running tool 50 may be lowered on guidelines into a proper alignment on buoy section 26. An hydraulically-released overshot 53 is provided on frame 51 for each fishing neck 38 on sleeve 32.

Hydraulic cylinders 54 are positioned on either side of overshot 53 where rods 55 are adapted to engage latching ears 47b of latch elements 47 to move the ears to an unlocked position when rods 55 are extended from cylinders 54.

Main hydraulic junction box 56 is positioned on frame 51 and has hydraulic lines (shown as broken lines in FIG. 6) running therefrom to various hydraulic control mechanisms on frame 51. Stab-in member 57 is carried by frame 51 and is adapted to cooperate with pod 49 to establish an hydraulic fluid path to connector 42.

Hydraulic test junction box 58 is located on frame 51 and has a pressure test line 59 and an hydraulic release line 60 extending therefrom to a collet test connector 61 positioned on male end 45 of gooseneck conduit 41. Tether line 62 is connected between frame 51 and connector 61.

Attachment means 63 is provided for connecting frame 51 to a drill string 64 or other lowering means for lowering and raising running tool 50. As illustrated in FIG. 8, in some instances, more than one gooseneck connection assembly can be run into position at the same time.

The flexible flowline section 22 (shown in FIG. 1) comprises a plurality of flexible catenary flowlines 70, each adapted to be operatively connected between the surface facility and its respective gooseneck conduit 36 on buoy section 26. The upper end of each flexible flowline 70 is attached at 71 to floating facility 22a by any suitable means. The preferred flexible flowlines are Coflexip multi-layered sheath conduits. These are round conduits having a protective outer cover of "Rilsan" material. The flowlines are commercially available in a variety of sizes and may be provided with releasable ends. The ribbon-type flowline bundle restrains the flexible flowlines from intercontact and provides sufficient clearance at the spreader beams to permit unhindered longitudinal movement.Flexible flowlines 70 are retained in parallel alignment or "ribbon" relationship substantially throughout their entire length. Multiple flowlines of equal length can be held in this parallel relationship by a plurality of transverse spreader beams 75 (FIGS. 9 and 10) longitudinally spaced along flexible flowlines 70 (four shown in FIG. 1).

Spreader beam 75 is a transverse bar 76 on which a plurality of spaced guides 77 is provided, one guide for loosely retaining each flexible flowline 70. Each guide 77 has a hinged gate 78 which can be opened (see broken lines in FIG. 10) to allow the respective flowline 70 to be positioned in guide 77, and then closed and locked by pin 77a to secure flowline 70 therein.

Each guide is sufficiently large to provide a clearance around its respective flexible flowlines (for example, about 25% or more). To minimize scuffing of the flexible flowlines 70, guides 77 may be lined with a plastics sleeve 79 having a low friction coefficient.

Since spreader beams 75 are slidable relative to flexible flowlines 70, pendant supports are provided. Support rods or wires 80 are connected to each beam 75 by clamping or attachment means 81 to maintain beams 75 in predetermined longitudinal position. The upper ends of support elements 80 are connected to attachment means 71 on floating surface facility 22a so that spreader beams 75 are supported by the wires.

Yoke assembly 82 (FIGS. 11 and 12) provides means for connecting the flexible section 22 to the buoy section 26. Yoke assembly 82 includes an elongated horizontal support member 83. This member may be a box beam having a plurality of recesses 84 therein. Individual recesses 84 receive corresponding flexible flowlines 70 in linear array at horizontally spaced iocations.

Locking means, such as gate 85 pivotally mounted at recess 84, secures the end of flowline 70 to the yoke. Hydraulic cylinder 86 actuates gate 85 laterally between an open position (broken lines in FIG. 11) and a closed locking position. Hydraulic cylinders 86 may be permanently attached on yoke support 83 or releasably mounted to be installed by a diver when needed.

Hydraulically-actuated connecting pin assemblies 87 are mounted at opposing ends of support 83 and are adapted to lock the horizontal yoke support 83 to yoke arms 34 when yoke assembly 82 is in position at buoy section 26. The yoke assembly 82 is attached to the support arms 34 of the buoy section by a pair of hydraulicallyactuated connecting pin assemblies 87 located at ends of the yoke beam 83. This retractable attachment has opposing retractable members 87c adapted to be retained adjacent arm slots 34a. A D-shaped bar configuration and mating arrangement between the yoke beam ends and support arm 34 permits the entire yoke assembly to fall away from the buoy section, thereby preventing angular distortion and damage to the flexible bundle in the event of attachment means failure or single retraction.Hydraulic line 88 for actuating the various mechanisms on yoke assembly 82 is attached to the yoke support 83 by means of manual gate 89.

Mounted on the end of each flexible flowline 70 is a primary connector 90 (for example, an hydraulically-actuated Cameron collet connector) adapted to connect flexible flowline 70 remotely to male end 45 of a corresponding gooseneck conduit 41. To assure release of the flexible flowline from buoy section 26 in an emergency situation, a back-up or secondary fluid connector 91 may be installed adjacent primary connector 90.

As shown in FIG. 13, located below secondary connector 91 is a coupling 92 which has a lip 93 thereon. Rotating metal plate 94 and "Delrin" plastics plate 95 are rotatably and slidably mounted on coupling 92, resting on lip 93 until flexible flowline 70 is positioned in yoke 82.

Bearing plate 96 is secured to coupling 92 and carries jacks comprising three equally-spaced hydraulically-actuated cylinders 98 which have pistons 99 adapted to extend downwardly through bearing plate 96. Coupling 92 is secured to gate 85 by rotating metal plate 94, which locks alignment pins 100 (see FIG. 14).

To install the compliant riser system 20 of the present invention, lower rigid section 27 with buoy section 26 in place is installed on base 24.

Rigid conduits 30 are run into casing 27 and coupled to submerged flowlines on base 24. U.S.

Patent 4,1 82,584 illustrates a technique which can be used to install rigid section 27 and rigid conduits 30. The buoy section including chambers 31, beam 33, gooseneck support structures 35, and lateral yoke receiving arms 34 may be attached to casing 27 and installed simultaneously. Any protective caps used during installation or debris are removed from the upper ends of each rigid conduit 30 by divers. A gooseneck connection assembly is lowered on running tool 50 into its respective trough 39 on buoy section 26. Gooseneck conduit 36 is positioned on frame 51 of tool 50 so that it will be properly aligned with its respective trough and rigid conduit 30 when funnels 52 on frame 51 engage guide posts 40 on chambers 31 after being lowered along guidelines attached to posts 40.As gooseneck conduit 36 is moved downwardly into trough 39, the camming surfaces 47a of latch members 47 engages flange 39a (FIG. 7) on trough 39 to move members 47 outwardly until they snap under flange 39a, thereby locking gooseneck conduit 36 to trough 39. At the same time, connector 42 moves onto the upper end of conduit 30 and a means for actuating connector 42 is provided by hydraulic connection from the surface via pod 49.

After connector 42 is actuated, gooseneck conduit 36 and connector 42 are pressure tested by opening ball valve 43 either manually by a diver or via hydraulic line 43a (FIG. 6) and then supplying hydraulic pressure through line 59. After testing is completed, test connector 61 is released via line 60 from male end 45. Connector 61 is tethered to tool 50 via line 62 for removal with tool 50. Fishing necks 38 on sleeve 32 are released from overshots 53 on tool 50 and tool 50 is retrieved for reuse in positioning additional goosenecks 36. As shown in FIG. 8, more than one gooseneck 36 can be installed in one operation.

If electricity and/or hydraulics are needed to operate control or instrumentation on marine bottom 23, one or more bundles of control elements (for example 30a in FIG. 2) can be provided within casing 27 and extended over a support frame 37 so that its upper end 41 a (FIG. 12) terminates approximately at the same position as do gooseneck conduits 36. When all gooseneck conduits 36 are in an operable position on buoy section 26, upper flexible section 22 is installed.

In one technique for assembling and installing flexible section 22, flexible flowlines 70 and electrical cable 70a (FIG. 12) are stored on powered reels (not shown) on vessel 22a. One end of each flexible flowline 70 and electrical cable 70a is connected to a plug 101 which is lowered upside down through moonpool A of vessel 22a.

By means of line 102, plug 101 can be keelhauled between moonpool A and moonpool B.

Alternatively, the moonpool plug or a portion thereof can be pre-installed, with the flexible lines being keelhauled individually and attached.

Support wires 80 which support spreader beams 75 may be attached to plug 101 and payed out with flowlines 70. Spreader beams are assembled onto flowlines 70 as they are payed out or each flowline 70 can be separately positioned in its respective guide 77 on beam 75 by a diver after each beam 75 enters the water. After the plug 101 and/or flexible flowlines 70 are keelhauled toward moonpool B, yoke assembly 82 can be mounted on the ends of flowlines 70 and electrical cables 70a as shown in FIGS. 1 9A-1 9D.

After flexible section 22 is assembled, rotary plug 101 is pulled into moonpool B of vessel 22a and affixed therein. Yoke 82 is lowered by means of lines 110 (FIGS.12 12 and 19A-19D)toa position just below yoke support arms 34 on buoy section 26 (FIG. 1 9B). Diver D exits diving bell 111 and attaches taglines 112 (FIG. 1 9D) to guidelines 113. By means of a winch (not shown) on buoy section 26 and taglines 112, diver D pulls guidelines 113 into guide shoes 11 5 (FIGS. 11.

and 12) which are split or hinged to allow lines 113 to enter. Slack is then taken up on lines 113 to draw yoke 82 into position on yoke support arms 14. As yoke 82 is drawn upwardly, upper supports 87a of connecting pin assemblies 87 (FIGS. 11 and 12) pass through slots 34a on support arms 34 (FIGS. 2 and 4). Hydraulic cylinders 87b are then actuated to move crossbars 87c into engagement between upper support arms 34 thereby locking yoke 82 in position on buoy section 26.

Cylinders 98 (FIGS. 13-17) are then actuated to move connector 90 into engagement with male end 45 of gooseneck conduit 36 and connector 90 is actuated to secure the connection between gooseneck conduit 36 and flexible flowline 70.

Diver D then makes up the electrical/hydraulic connection between cables 41 a and 70a to complete the installation.

Alternatively, the flowlines can be assembled into yoke 82 after the latter has been positioned in the water. This procedure can be employed for initial installation or replacement of flexible flowlines individually.

Referring to Figs. 13-18, gate 85 on yoke 82 is moved to an open position (FIGS. 13 and 14) by hydraulic cylinder 86. Guidelines 103 are attached to loading gate 85 via plugs 104 which extend through hollow positioning pins 100 on gate 85 and are held in place by crosspins 105 (FIG. 18).

Guidelines 103 cooperate with openings in rotating plate 94 to provide guidance for flowline 70 into gate 85. Nipple 106 (FIG. 13) is attached to connector 90 and lower line 107 is attached to nipple 106.

Flowline 70 is lowered on guidelines 103 by line 107 onto gate 85, which supports the weight of the flexible flowline until connection is made.

Openings in rotating plate 94 engage and receive positioning pins 100 on gate 85. Flowline 70 is then further lowered until bearing plate 96 comes to rest on "Delrin" plate 95. Cylinder 86 then closes gate 85 (FIGS. 1 5 and 16) and lock pins 95a are inserted by a diver to lock the gate closed.

Guidelines 103 may then be removed from gate 85, and nipple 106 released from connector 90 to be retrieved with line 1 07.

If a flowline 70 needs repair or replacement, it can be individually replaced by disconnecting it from its respective gooseneck conduit 36 and opening its gate 85 on yoke 82. Lowering line 107 is attached to connector 90 for retrieving the flowline 70. Spreader beam gates 77 are opened sequentially to remove the defective flowline 70.

In very deep water use of divers may not be practical; however, the large clearances between spreader guides and round flexible flowlines may permit the flowline and its terminal portion to be pulled through the spreaders from one end of the flowline bundle after disconnecting. A replacement flowline 70 may be assembled into flexible section 22 in a manner similar to initial installation procedure.

In an emergency situation, flexible section 22 can be quickly released from buoy section 26.

Each flowline 70 is released from its respective gooseneck conduit 36 by releasing primary connector 90, or if connector 90 fails, by releasing secondary connector 91. Connecting crossbars 87c of assemblies 87 are retracted to allow yoke 82 to be released from support arms 34.

Assemblies 87 are designed so that if only one bar 87c is retracted and the other assembly 87 fails, yoke 82 will fall away at the released end, thereby pulling the failed bar 87c as yoke 82 falls.

Claims (6)

1. A marine compliant riser system for connecting a marine floor base to a marine surface facility, comprising: a vertical conduit section comprising a plurality of flowlines extending from the marine floor to a submerged buoy section; a yoke assembly mounted on the buoy section; a flexible conduit section comprising a plurality of flexible catenary flowlines located adjacent one end thereof in spaced relationship in a substantially vertical flow direction in the yoke assembly; and a plurality of rigid gooseneck conduits supported on the buoy section and operatively connecting in fluid communication the flowlines in the vertical conduit section to the flexible flowlines supported in the yoke assembly.

2. A riser system according to claim 1, wherein each rigid gooseneck conduit includes at one end an hydraulically-actuated connector operatively connecting the conduit to a flowline in the vertical conduit section, and at the other end a terminal portion operatively connected to a flexible flowline supported in the yoke assembly.

3. A riser system according to claim 1 or claim 2, wherein each gooseneck conduit is supported on the buoy section in a frame assembly which includes a trough for receiving and supporting the gooseneck conduit.

4. A riser system according to claim 3, wherein the frame assembly also includes means for lock and retaining the gooseneck conduit in the trough.

5. A riser system according to any one of claims 1 to 4, wherein each flexible flowline depends from the yoke assembly at a substantially normal catenary departure angle.

6. A method for installing in deep water a marine compliant riser system according to any one of claims 1 to 5, which comprises the steps of: attaching a multiconduit riser section comprising a plurality of flowlines in a substantially vertical attitude to a marine floor base for connection to a source of hydrocarbon fluid, the riser section terminating at its upper end at a submerged buoy section; assembling a flexible conduit system including a plurality of flexible flowlines attached at one end to a marine surface facility, and a yoke assembly in which the flowlines are supported adjacent their other ends in spaced relationship; attaching the yoke assembly to the buoy section with the flexible flowlines depending from the yoke assembly at a substantially normal catenary departure angle; ; aligning a plurality of rigid gooseneck conduits with the flowlines in the flexible conduit section; and connecting the gooseneck conduits with the respective flowlines to establish fluid communication through the riser system.