WO2005051755A2

WO2005051755A2 - Buoyancy can for offshore oil and gas riser

Info

Publication number: WO2005051755A2
Application number: PCT/US2004/038302
Authority: WO
Inventors: James Elvin Daily; Metin Karayaka
Original assignee: Technip France
Priority date: 2003-11-21
Filing date: 2004-11-17
Publication date: 2005-06-09
Also published as: US20050109513A1; US7059416B2; WO2005051755A3

Abstract

A buoyancy can for supporting an offshore oil and gas riser includes an axial bore through which the riser extends coaxially, and a radio-axial slot extending through a side of the can and into the axial bore. A pair of spaced-apart support features are disposed coaxially on the riser, and the can includes a pair of corresponding sockets in the axial bore thereof. The sockets are adapted to receive and vertically support respective ones of the support features in a complementary, axial engagement. The can is placed in the water and moved laterally relative to a fully assembled, vertically supported riser such that the riser passes through the radio-axial slot of the can and into the axial bore thereof without the need for disassembly of the upper portion of the riser. The relative vertical positions of the can and riser are then adjusted such that the support features engage and seat within respective ones of their com-plementary sockets.

Description

TITLE OF THE INVENTION Buoyancy Can for Offshore Oil and Gas Riser

CROSS-REFERENCE TO RELATED APPLICATIONS (Not Applicable)

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT (Not Applicable)

REFERENCE TO APPENDIX (Not Applicable)

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates, in general, to methods and apparatus for offshore oil and gas production, and in particular, to a buoyancy can for tensioning, or supporting, the upper end of an offshore oil and gas riser that can be coupled to and decoupled from the riser without disassembling the upper terminal end portion thereof.

2. Related Art Top-tensioned riser ("TTR") systems for offshore oil and gas production (see, e.g., U.S. Pat. No. 4,702,321 to E. E. Horton) use passive "buoyancy cans" to support the risers independently of an associated floating production platform. In such a system, the riser extends vertically upward from the sea floor through the keel ofthe platform, and thence, to the well deck thereof, where it connects to a "stem" pipe, to which the buoyancy can is attached. The stem pipe extends vertically upward through an axial bore in the can and exits through its upper surface, where it may support a "work platform" to which the riser and its associated surface tree or "goose neck" are attached. A flexible, high pressure jumper then connects the outlet ofthe surface tree or goose neck to the production deck ofthe platform. By comparison, a "hybrid" riser system typically comprises three main parts: A foundation anchor and flow-line interface unit, a single- or multi-bore riser string, and a top end buoyancy can, which may be deployed on either the surface ofthe water or submerged below it. In such systems, the riser string is typically fabricated onshore as a complete, single- piece unit for tow-out and installation with a minimum of offshore work. The flexible jumpers are installed separately as part ofthe commissioning work, and the flow-lines are pulled in to the foundation and flowline interface unit. foundation and flowline interface unit. Since the riser is independently tensioned, or supported, by the buoyancy can relative to the production platform, the platform can move relative to the riser, and indeed, may even temporarily depart from the production location, such that the riser is thereby independent of and isolated from the motions ofthe platform. However, in such an arrangement, the buoyancy can must have sufficient buoyancy to provide the required top tension in the riser, as well as support for the weight ofthe can, the stem pipe and at least part ofthe weight ofthe jumpers. When a buoyancy can is initially deployed on a riser, or alternatively, when a de- ployed can is replaced with another can for repair or maintenance reasons, in some cases it is necessary to temporarily support the riser at a point below the can, and to remove the upper end, or terminal, portion ofthe riser, including the tree and any goose neck thereon, so that the "old" can, if any, may be slid up and off of the riser, and the "new" can may be slid down and over the riser. The upper terminal end portion ofthe riser must then be replaced and cou- pled to the new can for support. Alternatively, the installation procedure must be reversed. This results in a fairly complex, time-consuming, expensive, and potentially risky operation, particularly if effected in moderate or heavy seas. A long felt but as yet unsatisfied need therefore exists for a buoyancy can that can be coupled to and decoupled from a riser either on or below the surface ofthe water without the need for removing the upper terminal end portion ofthe riser.

BRIEF SUMMARY OF THE INVENTION In accordance with the present invention, a buoyancy can for supporting the upper end of an elongated vertical offshore oil and gas riser, and a method for its use, are provided that enable the can to be coupled to and decoupled from the riser without the need for removing the upper end portion ofthe riser. The novel can comprises at least one conventional vertical axial bore through which the riser extends coaxially, and a radio-axial slot having a width slightly greater than the diameter ofthe riser extending through a side of he can and into the axial bore. In one exemplary embodiment thereof the riser includes at least one support feature, e.g., a hang-off plug, disposed coaxially thereon adjacent to the upper end ofthe riser, and the buoyancy can comprises a corresponding socket disposed at the upper end ofthe axial bore thereof. The socket is adapted to receive the support feature in a complementary, axial engagement, and thereby support the support feature in the vertical direction. In another, more advantageous embodiment, the riser further includes a second support feature, e.g., a riser ball of a given diameter, disposed coaxially thereon at a selected distance below the first support feature, and the buoyancy can further comprises a corresponding second socket, e.g., a conventional keel joint socket, disposed in the axial bore thereof. The second socket is spaced below the first socket the same distance as the second support feature is spaced below the first support feature, and is adapted to receive the second support feature in a complementary, axial engagement, and thereby support it in the vertical direction. In this embodiment, the radio-axial slot is modified to include a radial bore that extends through the side ofthe can and into the axial bore, and the radial bore includes a cross-sectional profile that is slightly larger than the corresponding cross-sectional profile ofthe riser ball or other second support feature. In another possible embodiment, the first support feature and corresponding first socket may respectively comprise a conventional flex joint and a complementary receptacle therefor. In yet another possible embodiment, the second socket may be disposed at a lower end of the buoyancy can and comprise a conventional keel joint sleeve. In still yet another embodiment, the second support feature may comprise a conventional stab-in connector. In these embodiments, the utilization of two spaced-apart support features on the riser and corresponding sockets in the can ensures that loads caused by lateral wave or surge movements ofthe can are applied to the upper end ofthe riser in the form of a couple that is distributed throughout substantially the length ofthe can, rather than at a single point therein, which substantially reduces the stresses and strains imposed on the riser by lateral movements ofthe can. Advantageously, the buoyancy can includes at least one buoyant compartment that has a buoyancy that can be adjusted, e.g., with ballast water, to enable precise control ofthe vertical position ofthe can in the water. Additional ones ofthe compartments may be pressurized, e.g., with compressed air, to offset large hydrostatic pressures acting on them at greater water depths. A method for coupling the novel buoyancy can to the riser without removing the upper terminal end portion ofthe riser comprises suspending the upper end portion ofthe riser, e.g. , with a floating crane, such that the lower end ofthe riser extends vertically below the surface. The can is then disposed in the water adjacent to the riser, with the radio-axial slot aligned toward the riser. The can and the riser are then moved together laterally in the water, which can be effected completely below the surface ofthe water without the use of divers by use of a remotely operated vehicle ("ROV"), such that the riser passes through the radio-axial slot in the can and is disposed coaxially in the axial bore thereof. When the riser is positioned in the axial bore ofthe can, the vertical position of at least one ofthe riser and the can are adjusted, i.e., the can is de-ballasted such that it rises, and/or the upper end ofthe riser is lowered, such that the support features on the riser laterally and axially engage and are positioned in respective ones of their corresponding sockets in the bore ofthe can. A buoyancy can in accordance with the invention can be configured to support a plurality of risers in a so-called "riser tower" arrangement. A better understanding ofthe above and many other features and advantages ofthe present invention may be obtained from a consideration ofthe detailed description thereof below, particularly if such consideration is made in conjunction with the several views ofthe appended drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Figure 1 is perspective view of an exemplary embodiment of a buoyancy can in accordance with the present invention being deployed in a body of water and coupled to the up- per end portion of an associated offshore oil and gas riser; Figs. 2a - 2d illustrate possible exemplary cross-sectional views ofthe buoyancy can; Fig. 3 is a perspective view of an exemplary buoyancy can containing compartments in which the level of water ballast and/or the internal pressure can be varied with a pressurized fluid; Fig. 4 is a perspective view of an exemplary buoyancy can incorporating a goose neck at its upper terminal end; Figs. 5A-5D are sequential perspective elevation views of a method of deploying a buoyancy can and associated riser in a body of water in accordance with the present invention. Fig. 6 is a perspective view of a buoyancy can in accordance with the invention having a socket at its upper end and a keel joint at its lower end; Fig. 7 is an enlarged partial cross-sectional view ofthe keel joint ofthe buoyancy can of Fig. 6, as seen along the section lines 7-7 taken therein; Fig. 8 is a cross-sectional elevation view of a buoyancy can incorporating a flex joint and stab-in connector at its lower end; Fig. 9 is a cross-sectional schematic elevation view of a buoyancy can in accordance with the present invention shown supporting the upper end of an offshore riser; and, Fig. 10 is perspective elevation view of an exemplary embodiment of a buoyancy can in accordance with the present invention that is capable of supporting a plurality of risers.

DETAILED DESCRIPTION OF THE INVENTION A perspective view of an exemplary embodiment of a buoyancy can 10 in accordance with the present invention being deployed in a body of water and coupled to the upper end portion of an associated offshore oil and gas riser 100 is illustrated in Fig. 1. The buoyancy can comprises a single vertical axial bore 12 through which the riser extends coaxially in a conventional manner, and a radio-axial slot 14 that extends through a side ofthe can and into the axial bore. The slot 14 has a width that is greater than the diameter ofthe riser 100 to en- able the riser to pass through the slot laterally and into the axial bore 12. For simplicity of description, the particular embodiment of buoyancy can 10 and riser 100 described and illustrated herein is shown to include only a single axial bore 12 and corresponding single riser. However, a typical hybrid riser "tower" may include a buoyancy can 10, such as that illustrated in Fig. 10, which supports several such risers simultaneously, each seated in its own corresponding respective axial bore 12, and accordingly, it should be understood that this invention is equally applicable to such multi-riser systems. In the exemplary embodiment illustrated, the riser 100 comprises a cylindrical pipe of a given diameter that extends vertically upward from a foundation 5 (see, Fig. 5) on the sea floor 1 and through the axial bore 12 ofthe can 10 such that its upper end 102 exits through the upper end 16 ofthe can. The particular riser illustrated includes a recurvate goose neck section 104 at its upper end, as well as a first riser support feature 106, viz., a conventional, frusto-conical "hang-off plug," disposed coaxially thereon adjacent to the upper end thereof. The buoyancy can 10 further comprises a corresponding first receptacle, or frusto-conical "socket" 18, disposed at the upper end ofthe axial bore 12 ofthe can. The socket 18 is adapted to receive the hang-off plug in a complementary, slide-in, axial engagement, and to support the hang-off plug, and hence, the riser, in the axial, or vertical, direction when the plug is seated therein. The exemplary riser 100 advantageously further includes a second support feature 108 disposed coaxially thereon at a selected distance D below the first support feature 102, as illustrated in Fig. 1, and a corresponding second socket 20, which is spaced below the first socket 18 by the selected distance D, is disposed in the axial bore 12 ofthe buoyancy can 10. Like the first socket 18, the second socket 20 is adapted to receive the second riser support feature 108 in a complementary, slide-in, axial engagement, and to support the second sup- port feature, and hence, the riser, in the vertical direction when the latter support feature is seated therein. In the particular embodiment illustrated in Fig. 1 , the second riser support feature 108 comprises a conventional keel joint riser ball having a given diameter, and the second socket 20 comprises a conventional keel joint sleeve disposed in the axial bore ofthe can at its lower end, as is also illustrated in Figs. 6 and 7, respectively. Alternatively, as illustrated in Fig. 8, the second riser support feature 108 and corresponding second socket 20 disposed at the lower end ofthe can 10 may comprise a conventional stab-in connector 110 and flex joint receptacle 22, instead ofthe keel joint ball and sleeve illustrated in Figs. 6 and 7. However, as will be appreciated by those of skill in the art, since a keel joint riser ball (or other type of riser support feature) has a diameter or other cross-sectional profile that is greater than that ofthe riser 100 itself, and because such feature is positioned, when installed, between the upper and lower ends ofthe buoyancy can 10, it cannot pass laterally through the radio-axial slot 14 ofthe can in the manner described below without some modification ofthe slot. Accordingly, to accommodate the second riser support feature 108, the radio-axial slot is provided with a radial bore 24 having a cross-sectional profile that is slightly larger than the corresponding cross-sectional profile ofthe second riser support feature 108, and which extends through the side ofthe can and into the axial bore 12 thereof, as illustrated in Figs. 1 and 4, so that the riser, with a riser ball, stab-in connector, or other type of second riser support feature installed thereon, can both pass transversely through the radio-axial slot and into the axial bore ofthe can simultaneously, in the manner described below. As will be further appreciated by those of skill in this art, the present invention's use of two axially spaced-apart support features 106, 108 on the riser 100, operating in conjunction with two corresponding spaced-apart sockets 18 and 20 in the buoyancy can 10, provides advantages over prior art buoyancy cans employing only one set of such supports and sock- ets. As illustrated in Fig. 9, it may be seen that, as the buoyancy can 10 is subjected to lateral sea motions caused by wave or surge forces acting upon it, the resulting loads imposed on the upper end portion ofthe riser 100, which is tethered at its lower end to a foundation 5 on the sea floor 1, are transferred through two transfer points, rather than only one point, as with conventional buoyancy cans. This results in a riser curvature that conforms more gently to the vertical axis ofthe buoyancy can, and thereby reduces the bending stresses and resulting fatigue acting on the riser caused by such motions, relative to those of conventional, single- point buoyancy can riser support systems. This effect can be further enhanced by the provision of back-to-back stress joints 109 to accommodate localized bending stresses in the vicinity ofthe riser ball 108, as illustrated in Figs. 7 and 9. In a preferred embodiment, the buoyancy can 10 includes at least one floatation compartment 26 having a buoyancy that is selectably adjustable, so that the vertical position and angular orientation ofthe can in the water can be controlled relatively precisely. This com- partmentalization can be effected by the provision of conventional horizontal and vertical bulklieads 28 and 30, as illustrated in Figs. 2a-2d and 3. As illustrated in Figs. 2a-2d, the can itself may comprise a variety of cross-sectional shapes, including elliptical, oval, square, or round. Additionally, the vertical bulkheads 30 can be arranged in various ways to accommodate and/or define the axial bore 12 and radio-axial slot 14 ofthe can. As illustrated in Fig. 3, the buoyancy ofthe compartments 26 ofthe can 10 can be ad- justed by means of a pressurized fluid, e.g., compressed air, that is fed into or vented from them by individual conduits 32 that extend into the compartments from, e.g., the upper end 16 ofthe can. Some ofthe compartments may include side openings 34 through which sea water ballast can be admitted or expelled by venting or pressurizing the compartment, while others can be completely closed, to enable them to be internally pressurized in an amount suf- ficient to offset the hydrostatic pressure acting on them at greater water depths. The pressuri- zation can be remotely effected, for example, with the use of a Remotely Operated Vehicle ("ROV") 2 (see, Fig. 1). The foregoing arrangement advantageously enables the buoyancy of the can, and hence, its orientation and vertical position in the water, to be adjusted with precision during the coupling and de-coupling ofthe can to the riser 100, as described below. A method by which the novel buoyancy can 10 may be coupled to and decoupled from a riser 100 without removing the upper terminal end portion ofthe riser is illustrated in Figs. 1 and 5A-5D. The method begins by suspending the upper end portion ofthe riser 100, e.g., with a floating vessel-mounted crane 4, such that the lower end ofthe riser, including any second riser support feature 108 mounted thereon, such as the riser ball illustrated, ex- tends downward toward the sea floor 1. A buoyancy can 10 in accordance with the present invention is disposed in the water adjacent to the riser 100, either floating on the surface 3 ofthe water or submerged below it, and then manipulated, e.g., with an ROV 2 in a fully submerged deployment, such that the radio-axial slot 14 ofthe can faces toward and is aligned with the riser, as illustrated in Figs. 5A, 5B. Additionally, the vertical position of at least one ofthe can and the riser is adjusted, e.g., by varying the buoyancy ofthe can, as above, or by raising or lowering the upper end of the riser with the crane 4, or both, until the first riser support feature 106 is positioned above the upper end 16 ofthe can, and the radial bore 24 ofthe can faces toward and is aligned with the second riser support feature 108, as illustrated in Figs. 1 and 5C. The can 10 and the riser 100 are then urged together laterally in the water, which again, in a fully submerged coupling, may be effected with the ROV 2, such that the riser and second riser support feature 108 respectively pass through the radio-axial slot 14 and the radial bore 24 ofthe can and are disposed coaxially in the axial bore 12 thereof. The vertical position of at least one of the can and the riser are then adjusted again, as above, i. e. , by raising the can and/or lowering the riser, until the first and second riser support features 106 and 108 are axially seated in respective ones of their corresponding sockets 18 and 20 in the can, as illustrated in Fig. 5D. The method whereby the buoyancy can 10 is decoupled from the riser 100 is generally the reverse ofthe foregoing procedure. Thus, it may be seen that the coupling and decoupling ofthe buoyancy can to and from the riser is easily effected without the need for removing the upper terminal portion ofthe riser or for divers in the water, whether the coupling or decoupling is effected on or below the surface 3 ofthe water. By now, those of skill in the art will appreciate that many modifications and substitu- tions can be made to the materials, methods and configurations ofthe present invention without departing from its scope. For example, as illustrated in Fig. 10, the buoyancy can 10 may include a plurality of axial bores 12, each capable of supporting a corresponding riser 100 coaxially therein, and in which each ofthe risers can be coupled to and decoupled from the can independently ofthe others without removing its respective upper terminal end portion. Accordingly, the scope ofthe present invention should not be limited to the particular embodiments illustrated and described herein, as these are merely exemplary in nature. Rather, the scope ofthe present invention should be commensurate with that ofthe claims appended hereafter, and their functional equivalents.

Claims

CLAIMSWHAT IS CLAIMED IS:

1. For supporting an upper end of an elongated vertical offshore oil and gas riser of a given diameter in a body of water, an improved buoyancy can ofthe type that includes a ver- tical axial bore through which the riser extends coaxially, the improvement comprising: a radio-axial slot extending through a side ofthe can and into the axial bore thereof, the slot having a width greater than the diameter ofthe riser.

2. The buoyancy can of claim 1, wherein the riser includes a first support feature disposed coaxially thereon adjacent to an upper end thereof, and wherein the buoyancy can fur- ther comprises: a first socket disposed at an upper end ofthe axial bore thereof, the first socket being adapted to receive the first support feature in a complementary, axial engagement, and to support the first support feature vertically.

3. The buoyancy can of claim 2, wherein the riser further includes a second support feature disposed coaxially thereon at a selected distance below the first support feature, and wherein the buoyancy can further comprises: a second socket disposed in the axial bore thereof, the second socket being spaced below the first socket by the selected distance and adapted to receive the second support feature in a complementary, axial engagement, and to support the second support feature vertically.

4. The buoyancy can of claim 2, wherein the first support feature comprises a hang- off plug.

5. The buoyancy can of claim 3, wherein the second support feature comprises a riser ball having a given diameter, and wherein the radio-axial slot further comprises: a radial bore extending through the side ofthe can and into the axial bore thereof, the radial bore having a diameter greater than the diameter ofthe riser ball.

6. The buoyancy can of claim 5, wherein the second support feature further comprises a pair of stress joints disposed back-to-back on the riser ball.

7. The buoyancy can of claim 3, wherein the second support feature comprises a stab- in connector having a cross-sectional profile, and wherein the radio-axial slot further comprises; a radial bore extending through the side ofthe can and into the axial bore thereof, the radial bore having a cross-sectional profile larger than the cross-sectional profile ofthe stab- in connector.

8. The buoyancy can of claim 2, wherein the first support feature comprises a flex joint, and the first socket comprises a flex joint receptacle.

9. The buoyancy can of claim 5, wherein the second socket is disposed at a lower end of the buoyancy can and comprises a keel joint sleeve.

10. The buoyancy can of claim 7, wherein the second socket is disposed at a lower end ofthe buoyancy can and comprises a flex joint receptacle.

11. The buoyancy can of claim 1, wherein the can comprises at least one buoyant compartment, and wherein the buoyancy ofthe at least one compartment is adjustable.

12. The buoyancy can of claim 1 , wherein the can further comprises a plurality of vertical axial bores, each capable of receiving and supporting a riser therein.

13. A method for supporting an upper end of an elongated vertical offshore oil and gas riser of a given diameter in a body of water, the method comprising: suspending the upper end ofthe riser such that the lower end ofthe riser extends ver- tically below the surface ofthe water; providing a buoyancy can in the water and adjacent to the riser, the can having a vertical axial bore and a radio-axial slot extending through a side ofthe can and into the axial bore, the slot having a width greater than the diameter ofthe riser; and, urging the can and the riser together laterally in the water such that the riser passes through the radio-axial slot in the can and is disposed coaxially in the axial bore thereof.

14. The method of claim 13, wherein the riser includes at least one support feature disposed coaxially thereon adjacent to the upper end thereof, and further comprising: providing at least one socket in the axial bore ofthe buoyancy can, the at least one socket being adapted to receive the at least one support feature in a complementary, axial engagement, and to support the first support feature vertically; and, adjusting the vertical position of at least one ofthe riser and the buoyancy can such that the at least one support feature ofthe riser is axially seated in the at least one socket of the can.