Marine Production Riser System and Method of Installing Same

The present invention relates to a marine riser system and a method of installing same and more particularly relates to a free-standing, marine riser for use in deep water areas to conduct fluids between the marine bottom and the surface.

A critical consideration in the production of hydrocarbons from marine deposits lies in providing a fluid communication system from the marine bottom to the surface once production has been established. Such a system, commonly called a production riser, is usually comprised of one or more conduits through which various, produced fluids are transported to and from the surface.

In many offshore production areas, a floating facility is commonly used as a production and/or storage platform. Since the facility is constantly exposed to surface conditions, it experiences a variety of movements, e.g., heave, roll, pitch, drift, etc. In order for a prduction riser system to adequately function with such a facility, it must be sufficiently compliant to compensate for such movements over long periods of operation without failure.

Also, as is commonly known, a zone of turbulence due to surface and near surface conditions exists just below the surface. For a riser system to have an acceptable operational life, it must also have sufficient compliance within this zone to compensate for the turbulence without interrupting the operation of the riser system.

Further, due to the water depths of some production areas, at least the lowermost elements of the riser system must be capable of being remotely installed without requiring any substantial assistance from divers. Likewise, the various elements of the riser system which undergo constant wear during operation, e.g., flowlines, must be capable of being removed individually for repair and/or replacement without requiring removal of the entire riser system. Finally, to provide for extremely hostile surface conditions, e.g., hurricanes, the riser must be capable of being quickly released from the floating facility and then being retrieved for reconnection once the surface conditions have subsided.

The present invention provides a free-standing, fully compliant marine production riser system capable of use in deep water production areas, including those areas having relatively hostile surface conditions. The lowermost elements of the riser system are capable of being remotely installed, with divers being needed only to install the upper elements which lie at a depth at which the divers can effectively and safely work. All of the flowlines in the system are such that each can be removed individually for repair and/or replacement without the need to shut down the entire riser system for extended periods. Further, the riser system can be quickly disconnected and then reconnected to a floating facility if the need arises.

More specifically, the riser system of the present invention is comprised of a lower, rigid section and an upper, flexible section. The lower, rigid section comprises a casing having a plurality of guide tubes therein adapted to receive a plurality of individual flow conduits. A remotely actuated connector assembly is attached to the lower end of the casing and is adapted to cooperate with a preset base on the marine bottom to guide the lower, rigid section into place and secure it to the base. A variable-buoyant buoy is affixed to the upper end of the casing and maintains the lower, rigid section in a vertical position when in place on the base. The casing is of sufficient length to extend from a preset base on the marine bottom to a point just below the turbulence zone near the surface, this being the wave zone which is affected by surface and near surface conditions, e.g., winds, waves, currents, etc.

The lower, rigid section is lowered and secured to the base on the marine bottom and the individual flow conduits are guided into their respective guide tubes within the casing. Each flow conduit is lowered through its guide tube and is remotely connected to a respective submerged flow source within the base. The upper, flexible section, comprised of individual flexible flow lines, is then lowered and each flexible flowline is attached to the upper end of a respective flow conduit in the casing. The flexible flow lines preferably are each of a different length to prevent entanglement with each other but all of sufficient length to allow each flexible flowline to extend upward from the lower flow conduit to which it is connected, through and over the upper surface of the buoy on the casing, and then downward to form a catenary loop before extending upward to the surface. The length of each flowline will be such that a catenary loop will be present therein at all times during operation of the riser system. The upper end of each flexible flowline is connected to a respective opening in a mounting flange so that all of the upper ends of the flexible flowlines are attached to a single flange, thereby allowing the flexible lines to be handled as a unitary bundle for quick and easy connection and disconnection to a floating facility. By maintaining at all times a sufficient catenary loop in each flexible line and by having only the flexible flowlines exposed to the turbulence zone, the riser system has excellent compliance which compensates for the normal heave, pitch, roll, and drift of the floating facility and for any normal turbulence encountered during operation without over-extending or damaging the riser system. Also, the catenary loop in each flexible line provides for minimum stresses as the flexible flowlines are extended and relieved during movement of the surface facility.

The actual construction, operation, and the apparent advantages of the invention will be better understood by referring to the drawings in which like numerals identify like parts and in which:

FIG. 1 is an elevational view of the present marine riser system in an operable position connected to a floating facility at an offshore location;

FIG. 2 is an elevational view of the riser system of FIG. 1 shown in an inoperable position disconnected from the floating facility;

FIG. 3 is an exploded view, partly in section with parts removed for clarity, of the connector assembly on the lower end of the rigid section of the riser and the cooperating base element;

FIG. 4 is a view taken along line 4--4 of FIG. 3;

FIG. 5 is a view taken along line 5--5 of FIG. 3;

FIG. 6 is a top view of the buoy and flexible flowlines at the upper end of the rigid section of the present riser system;

FIG. 7 is a view of the upper end of the rigid section of the riser system taken along line 7--7 of FIG. 6;

FIG. 8 is a cross-sectional view taken along line 8--8 of FIG. 7;

FIG. 9 is a perspective view of the buoy and flexible lines exiting from the top of the rigid section of the riser system and extending to the surface; and

FIG. 10 is an enlarged view taken along line 10--10 of FIG. 9.

Referring more particularly to the drawings, FIG. 1 discloses marine riser system 11 of the present invention in an operational position at an offshore location. Riser system 11 is comprised of a lower rigid section 12 and an upper flexible section 16. As will be explained in more detail below, lower rigid section 12 has a base portion 23, an intermediate portion 24, and a buoy portion 25. A connector assembly 12a is affixed to the lower end of base portion 23 which cooperates with preset base element 13 to secure lower rigid section 12 to marine bottom 14. Buoy 15 is secured to the upper end of buoy portion 25 to maintain lower rigid section 12 in a vertical position under tension when in an operable position on base element 13.

As will be explained in more detail below, flexible section 16 is comprised of one or more flexible conduits which connect to respective one or more flow passages in rigid section 12. The flexible conduits extend upward through and over the upper surface of buoy 15 and then downward through catenary loops before extending upward to the surface of the water where they are connected to a floating facility 17. To permit the flexible section 16 to be disconnected from facility 17 (see FIG. 2) in case of an emergency, e.g., hurricane, and then be retrieved for reconnection, a tetherline 22 is attached at one end to the upper end of flexible section 16 and at its other end to which 101 on floating facility 17. When flexible section 16 is disconnected from facility 17, it will be lowered by tetherline 22 to the position shown in FIG. 2. A clump weight 19 is positioned at some intermediate point on tetherline 22, and anchor 20 is attached to the end thereof. A marker buoy 21 is attached to anchor 20 by line 21a, and tetherline 22, weight 19, and anchor 20 are lowered to marine bottom 14. After the emergency has passed, marker buoy 21 is retrieved and line 21a is reeled in to raise anchor 20, weight 19, and tetherline 22. Then by reeling in tetherline 22, upper end of flexible section 16 is brought to the surface of reconnection to facility 17.

Referring now to the other figures, each component of the present system will be described in greater detail. FIGS. 3 to 5 disclose the details of base portion 23 of lower rigid section 12 and of base element 13. As seen in FIGS. 3 and 5, base element 13 is comprised of a frame 27 having a central housing 28 secured therein. A plurality of guide posts 29 are secured at spaced positions on frame 27 as are a plurality of male members 30 of a remote connector means (only one member 30 shown in FIG. 2 for clarity).

As understood in the art, base element 13 is first positioned and set on marine bottom 14. One or more conduits 31 (as shown in FIG. 2; ten shown in FIG. 4) connected to various submerged sources (not shown) e.g., produce oil and gas from subsea wells, control valves on said wells, etc., terminate within housing 28 and each has a female receptacle 32 of a remote connector, e.g., stab-in connector, on its upper end.

Lower base portion 23 of rigid section 12 is comprised of constant internal diameter (I.D.) casing 35 which preferably has a stepped outside diameter (O.D.). That is, the wall thickness of casing 35 decreases in steps from its bottom to its top. For example, in a practical situation where casing 35 is 60 feet long and has an I.D. of 56 inches, the wall thickness for the lower 20 feet is 11/2 inches, the wall thickness for the intermediate 20 feet is 11/4 inches, and the wall thickness for the upper 20 feet is 1 inch. This stepped wall thickness distributes the bending stresses over the entire length of casing 35 and prevents these stresses from exceeding the allowable limits in a single area.

A connector assembly 36 is attached to the lower portion of casing 35 and is comprised of a frame 37 having a plurality of guide sleeves 38 and female members 39 of a remote connector means secured thereto which are positioned to cooperate with guide posts 29 and male members 30, respectively, when connector assembly 36 is in position on base element 13. The remote connectors 30, 39 are positioned to carry the shear, tension, and bending loads from the riser system 11 to base element 13 and are of the type which can be connected and disconnected remotely, e.g., a connector having locking dog segments, a cam ring to actuate the dog segments, and hydraulically actuated pistons to position the cam ring to lock or unlock the dog segments. Such a connector is well known in the art and is commercially available, e.g., an H-4 connector maufactured by Vetco Offshore Industries of Ventura, Ca.

The intermediate portion 24 of rigid section 12 (see FIG. 1) is comprised of casing 40 having the same I.D. as base portion 23 and a uniform wall thickness throughout its length slightly less than that of casing 35, e.g., 3/4 inch thick in the above example. Casing 40 is connected to casing 35 and has a length sufficient to extend from base portion 23 to a point just below turbulence zone 18 (see FIG. 1) which is that zone of water below the surface which is normally affected by surface conditions, e.g., currents, surface, waves, winds, etc. Connected to the top of casing 40 is buoy section 25.

As seen in FIGS. 6 and 7, buoy section 25 comprises a casing 41 having the same I.D. as casing 40 but preferably having a slightly greater wall thickness, e.g., 1 inch in the above example, and has two stiffening rings 42, 43 thereon. Buoy 15 is comprised of a housing 15a having a shape of essentially a torus atop a hollow cylinder and having an inner wall 15b defining a central passage 44 through the entire length of buoy 15 into which casing 41 is positioned. The upper stiffening rings 43 fits into shouldered recess 45 on buoy 15 to transfer the buoyant force of buoy 15 to rigid section 12. The lower stiffening ring 42 bears against the internal diameter of passage 44 and together rings 42, 43 transfer the bending moments from buoy 15 to rigid section 12.

Buoy 15 is preferably fabricated with two separate chambers. The volume of upper chamber 46 provides sufficient buoynancy to float both buoy section 25 and base section 23 for a purpose to be explained later. Chamber 46 is kept dry at all times and is referred to as a fixed buoyancy tank. The buoyancy force of lower chamber 47 is variable by emptying and flooding chamber 47 through opening 49 by supplying or venting air through inlet valve 48 and vent 52, respectively. Chamber 46 can also be pressurized through valve 50 and line 51 to protect against collapse when buoy 15 is submerged.

The upper, interior of central passage 44 of buoy 15 is enlarged to form circular gallery 54 which provides a work space for divers during installation and maintenance operations. Access to gallery 54 is through either of two recesses 55 through the upper surface of buoy 15. Four guide posts 56 are affixed in space relation on the upper surface of buoy 15.

A plurality of guide tubes 57 are positioned within rigid section 12 and extend through the entire length of section 12, i.e., casings 35, 40, 41. Guide tubes 57 are held in proper position by alignment plates 58 (see FIG. 7) placed at spaced intervals, e.g., 20 feet, within the casings. An individual, rigid flow conduit 59 (see FIGS. 3 and 7) is run through each guide tube 57 and carries a remote connector 52, e.g., threaded stab connector, at its lower end which is adapted to mate with female receptacle 32 of its respective conduit 31. It should be understood that both the number and diameter of conduits 31 and mating flow conduits 59 may vary as the situation predicts. Each flow conduit 59 extends from base element 13 to a point within gallery 54 where it terminates with flange 60.

A flexible flowline 61 having a flange 62 on one end is connected to a respective flange 60 on a rigid flow conduit 59 within gallery 54. As best seen in FIG. 6, flexible flowlines 61 extend upward through buoy 15 and over the upper surface thereof. The toroidal shape of the upper surface of buoy 15 acts as a natural bending mandrel for flexible flowlines 61. Guide ribs 63 are welded to the upper buoy surface to form individual troughs (only one shown in FIG. 5) for each of flexible flowlines 61.

Preferably, each flexible flowline 61 has a curved cradle 64 (shown in FIG. 6) attached thereto clamped to a short portion of the underside thereof by means of clamps 65 which in turn have lifting eyes 66 therein. Cradle 64 provides a means of lifting and lowering the ends of flexible flowlines 61 while keeping the flanged ends of flexible flowlines 61 in position for connection to flow conduits 59. Preferably, the bottom of each cradle 64 is coated with a low friction material, e.g., polytetrafluoroethylene, to allow cradle 64 and hence flexible flowline 61 to slide over the surface of buoy 15 instead of sticking and imposing compressive forces on flexible flowline 61. This also protects flowlines 61 and buoy 15 from excess wear which would otherwise result from direct sliding contact with buoy 15. The flowlines 61 are held in their respective troughs by means of hold-down brackets 61a. A tie-off fitting 67 is clamped on each flexible flowline 61 which is used to transfer flexible flowline tension from flange 62 to buoy 15 during installation or removal of flexible flowlines 61. This is done by taking the tension on a chain (not shown) between fitting 67 and pad eye 68 on buoy 15.

The upper end of each individual flowline 61 is fitted with a flange 70 (see FIG. 9) which, in turn, is affixed to the bottom of mounting flange 71 to tie all of the individual flexible flowlines 61 together at a single point. Mounting flange 70 is used to couple flexible flowlines 61 to respective lines (not shown) in floating facility 17 whereby all of the flowlines can be quickly connected or disconnected in a single operation.

Where a plurality of flexible flowlines 61 are used, the length of each individual flowline 61 will vary with respect to its position in the buoy connection pattern (see FIG. 8) but each will be of sufficient length to initially extend downward from buoy 15 to form a catenary loop in the line before extending upward to the surface. The catenary loops in the flowlines 61 provide riser system 11 with the compliancy necessary to compensate for all normally expected movements of facility 17. Also, the catenary loops provide a good operational life for the flexible section 16 since wear due to flexing and normal tension on flexible flowlines 61 during operation is not concentrated at a single point but is more evenly distributed over a substantial length of each flowline. The varying lengths of flexible flowlines 61 provide separation between the individual catenaries, thereby reducing the possibility of rubbing and/or wrapping between flowlines 61 during operation.

Having described all of the components of riser system 11, the preferred method of installing the riser will now be set forth. Preferably at a shore facility, the upper portion of casing 35 of base portion 23 is temporarily secured within passage 44 of buoy 15. Using the buoyancy of buoy 15, base portion 23 is towed to the desired offshore location. Necessary control lines, e.g., hydraulic lines for remote connectors 39, and a guidance package, e.g., television and/or sonar package (not shown), are connected to base portion 23. With the aid of a derrick-equipped vessel, e.g., semisubmersible drilling rig, a section of casing 40 of intermediate portion 24 is lowered through passage 44 of buoy 15 and is coupled to casing 35. Base section 23 is then disconnected from buoy 15 and is lowered until another section of casing 40 can be coupled to the first section of casing 40. This procedure is repeated until intermediate casing 40 is completed. Casing 41, having rings 42, 43, is then coupled to the upper section of casing 40. A special running tool (not shown) is used to couple the top of casing 41 to a drill pipe or casing. Guidelines 56a (FIG. 7) are connected to guideposts 56 on buoy 15 and are extended to the surface.

Air lines (not shown) are connected to valves 48, 50 and by controlling the buoyancy, i.e., flooding, of chamber 47, rigid riser section 12 is lowered onto preset base element 13. To guide rigid section 12 into proper alignment, guidelines (not shwon) extending from guidepost 29 through guide sleeves 38 can be used or it can be done without guidelines by using the television and/or sonar package on base portion 23 to guide sleeves 38 onto their respective posts 29. Once in position, remote connectors 39 are actuated to lock base portion 23 on preset base 13. Chamber 47 is then blown down to again adjust the buoyancy force of buoy 15, thereby transferring the buoyant force of buoy 15 to casing 41. The running tool and television and/or sonar package are released and retrieved.

Next, the individual rigid flow conduits 59 are run into their respective guide tubes 57 within rigid section 12. Flow conduits 59 are positioned through an appropriately spaced opening in a guide frame (not shown) and the male member of stab connector 60 is affixed to its lower end. Guidelines 56a are threaded through the guide frame which is then lowered thereon as sections of rigid flow conduit 59 are added. The guide frame, as it reaches buoy 15 will guide flow conduit 59 into its respective guide tube 57 within rigid section 12. By means of retrieval cables, the guide frame is pulled back to the surface and the remainder of flow conduit 59 is made up with flange 60 being affixed to the top of the last section.

A drill string or the like (not shown) is attached to flange 60 and is used to run the remainder of flow conduit 59 into guide tube 57 until male member 52 of the stab connector is securely fastened within female receptacle 32 on base element 13. The stab connector is of the type known in the art which will automatically lock upon insertion and is releasable upon rotation of the male member with respect to the female member. A pressure test or the like is then performed to verify a leak-tight connection between members 52 and 32, after which divers disconnect the drill string from flange 60 for recovery. This technique is repeated for each of the individual rigid flow conduits 59.

To install flexible section 16, each flexible flowline 61 is provided with a flange 62 at one end and a flange 70 at the other end. All flanges 70 are connected to mounting flange 71 which is maintained at the surface. A cradle 64 is attached to a first of flexible flowlines 61 and is lowered therewith into its respective trough on buoy 15 and secured therein by brackets 61a. A diver then secures a chain (not shown) from tie-off fitting 67 to pad eyes 68 on buoy 15 to transfer flexible flowline tension and to aid a diver in connecting flange 62 of flexible flowline 61 to flange 60 on rigid flow conduit 59. This procedure is repeated for each flowline 61. Mounting flange 71 is then connected to floating facility 17, and riser system 11 is ready for operation. If an emergency arises, e.g., hurricane, mounting flange 71 can be quickly disconnected from facility 17 and flexible section 16 can be lowered to the position shown in FIG. 2. After the emergency has passed, anchor 20, clump weight 19, and tetherline 22 can be retrieved by capturing buoy 21 and line 21a and by reeling in line 22, flange 71 is recovered for reconnection to facility 17.

It can be seen the catenary path defined by each flexible flowline provides excellent compliance for the system to compensate for normally expected movement of facility 17, e.g., rise and fall due to wave action, drift, etc. Also, due to the catenary path the flexure of flowlines 61 is distributed over a greater portion of their lengths and is not concentrated at a single point, thereby substantially increasing their reliability and operational life. Further, it can be seen that riser system 11 only requires divers to work in relatively shallow depths with all other connections being remotely actuated. By extending rigid flow conduits 59 upward into circular gallery 54 within buoy 15, the connections requiring divers can be easily and safely performed.

Still further, the present riser system allows the individual rigid conduits and/or flexible flowlines to be removed for maintenance and/or replacement without requiring the removal of the entire system. This may be done by merely reversing the installation steps.