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 complementary sockets.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     (Not Applicable)  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     (Not Applicable)  
       REFERENCE TO APPENDIX  
       [0003]     (Not Applicable)  
       BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     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.  
         [0006]     2. Related Art  
         [0007]     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 of the 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 of the surface tree or goose neck to the production deck of the platform.  
         [0008]     By comparison, a “hybrid” riser system typically comprises three main parts: A foundation anchor and flow-line interface unit, a multi-bore riser string, and a top end buoyancy can, which also carries the respective interfaces for the flexible jumpers, and which may be deployed on either the surface of the water or submerged below it. In such systems, the riser string is 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 of the commissioning work, and the flow-lines are pulled in to the platform, which is outfitted with standard “hang-off” porches.  
         [0009]     In either case, 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 of the 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 of the can, the stem pipe and at least part of the weight of the jumpers.  
         [0010]     When a buoyancy can is initially deployed on a riser, or alternatively, when a deployed can is replaced with another can for repair or maintenance reasons, it is necessary to temporarily support the riser at a point below the can, and to remove the upper end, or terminal, portion of the 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 of the riser must then be replaced and coupled to the new can for support. This results in a fairly complex, time-consuming, expensive, and potentially risky operation, particularly if effected in moderate or heavy seas.  
         [0011]     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 of the water without the need for removing the upper terminal end portion of the riser.  
       BRIEF SUMMARY OF THE INVENTION  
       [0012]     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 of the 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 of the riser extending through a side of the can and into the axial bore.  
         [0013]     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 of the riser, and the buoyancy can comprises a corresponding socket disposed at the upper end of the axial bore thereof. The socket is adapted to receive the support feature in a complementary, axial engagement, and thereby support the at least one support feature in the vertical direction.  
         [0014]     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 of the 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 of the riser ball or other second support feature.  
         [0015]     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 of the can are applied to the upper end of the riser in the form of a couple that is distributed throughout substantially the length of the can, rather than at a single point therein, which substantially reduces the stresses and strains imposed on the riser by lateral movements of the can.  
         [0016]     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 of the vertical position of the can in the water. Additional ones of the compartments may be pressurized, e.g., with compressed air, to offset large hydrostatic pressures acting on them at greater water depths.  
         [0017]     A method for coupling the novel buoyancy can to the riser without removing the upper terminal end portion of the riser comprises suspending the upper end portion of the riser, e.g., with a floating crane, such that the lower end of the 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 of the 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 of the can, the vertical position of at least one of the riser and the can are adjusted, i.e., the can is de-ballasted such that it rises, and/or the upper end of the riser is lowered, such that the support features on the riser axially engage and are seated in respective ones of their corresponding sockets in the bore of the can.  
         [0018]     A buoyancy can in accordance with the invention can be configured to support a plurality of risers in a so-called “riser tower” arrangement.  
         [0019]     A better understanding of the above and many other features and advantages of the present invention may be obtained from a consideration of the detailed description thereof below, particularly if such consideration is made in conjunction with the several views of the appended drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0020]      FIG. 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 upper end portion of an associated offshore oil and gas riser;  
         [0021]      FIGS. 2   a - 2   d  illustrate possible exemplary cross-sectional views of the buoyancy can;  
         [0022]      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;  
         [0023]      FIG. 4  is a perspective view of an exemplary buoyancy can incorporating a goose neck at its upper terminal end;  
         [0024]      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.  
         [0025]      FIG. 6  is a perspective view of a buoyancy can in accordance with the invention having a flex joint socket at its upper end and a keel joint at its lower end;  
         [0026]      FIG. 7  is an enlarged partial cross-sectional view of the keel joint of the buoyancy can of  FIG. 6 , as seen along the section lines  7 - 7  taken therein;  
         [0027]      FIG. 8  is a cross-sectional elevation view of a buoyancy can incorporating a flex joint and stab-in connector at its lower end;  
         [0028]      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,  
         [0029]      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  
       [0030]     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 of the can and into the axial bore. The slot  14  has a width that is greater than the diameter of the riser  100  to enable the riser to pass through the slot laterally and into the axial bore  12 .  
         [0031]     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.  
         [0032]     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  of the can  10  such that its upper end  102  exits through the upper end  16  of the 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 of the axial bore  12  of the 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.  
         [0033]     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  of the 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 support 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 of the 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 of the can  10  may comprise a conventional stab-in connector  110  and flex joint receptacle  22 , instead of the keel joint ball and sleeve illustrated in  FIGS. 6 and 7 .  
         [0034]     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 of the riser  100  itself, and because such feature is positioned, when installed, between the upper and lower ends of the buoyancy can  10 , it cannot pass laterally through the radio-axial slot  14  of the can in the manner described below without some modification of the 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 of the second riser support feature  108 , and which extends through the side of the 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 of the can simultaneously, in the manner described below.  
         [0035]     As will be further appreciated by those of skill in this art, the present invention&#39;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 sockets. 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 of the 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 of the 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 of the riser ball  108 , as illustrated in  FIGS. 7 and 9 .  
         [0036]     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 of the can in the water can be controlled relatively precisely. This compartmentalization can be effected by the provision of conventional horizontal and vertical bulkheads  28  and  30 , as illustrated in  FIGS. 2   a - 2   d  and  3 . As illustrated in  FIGS. 2   a - 2   d,  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  of the can.  
         [0037]     As illustrated in  FIG. 3 , the buoyancy of the compartments  26  of the can  10  can be adjusted 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  of the can. Some of the 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 sufficient to offset the hydrostatic pressure acting on them at greater water depths. The pressurization 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 of the can to the riser  100 , as described below.  
         [0038]     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 of the riser is illustrated in  FIGS. 1 and 5 A- 5 D. The method begins by suspending the upper end portion of the riser  100 , e.g., with a barge-mounted crane  4 , such that the lower end of the riser, including any second riser support feature  108  mounted thereon, such as the riser ball illustrated, extends downward toward the sea floor  1 .  
         [0039]     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  of the 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  of the can faces toward and is aligned with the riser, as illustrated in  FIGS. 5A, 5B . Additionally, the vertical position of at least one of the can and the riser is adjusted, e.g., by varying the buoyancy of the 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  of the can, and the radial bore  24  of the can faces toward and is aligned with the second riser support feature  108 , as illustrated in  FIGS. 1 and 5 C.  
         [0040]     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  of the 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 .  
         [0041]     The method whereby the buoyancy can  10  is decoupled from the riser  100  is generally the reverse of the foregoing procedure. Thus, it may be seen that the coupling and decoupling of the buoyancy can to and from the riser is easily effected without the need for removing the upper terminal portion of the riser or for divers in the water, whether the coupling or decoupling is effected on or below the surface  3  of the water.  
         [0042]     By now, those of skill in the art will appreciate that many modifications and substitutions can be made to the materials, methods and configurations of the 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 of the risers can be coupled to and decoupled from the can independently of the others without removing its respective upper terminal end portion.  
         [0043]     Accordingly, the scope of the present invention should not be limited to the particular embodiments illustrated and described herein, as these are merely exemplary in nature. Rather, the scope of the present invention should be commensurate with that of the claims appended hereafter, and their functional equivalents.