Abstract:
A system for establishing fluid communication between a floating body and a wellhead on the seafloor includes a rigid, self-tensioning riser for fluidly connecting the floating body and the wellhead. The riser defines a major axis extending from the floating body to the wellhead. The riser is formed with a series of pre-formed curves that absorb and release energy in response to the heave and surge of the floating-body by flexing in a direction essentially parallel to the major axis. The series of curves may be in a single plane (e.g., sinusoidal), or in multiple planes (e.g., helical).

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a Continuation-in-Part of co-pending application Ser. No. 10/213,963; filed Aug. 7, 2002, the disclosure of which is incorporated herein by reference. 
     
    
     
       FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0003]    This invention is generally related to risers that convey fluid from producing wells on the seafloor to a floating structure or vessel on the sea surface. This invention is also related to a conduit that is fixed to the seafloor, which must accommodate the motion of a floating body, structure, or vessel that is connected to it.  
           [0004]    In offshore drilling and production operations carried out from a floating vessel, fluid is conveyed from wells on the seafloor to the vessel stationed on the surface by a conduit often referred to as a “riser.” Various methods and mechanisms are used to reduce stresses in risers that are affixed to the moving vessel on the surface and the stationary wellhead at the seafloor. These include using flexible hose for the riser in lieu of steel pipe, supporting a steel riser with hydraulic or elastomeric tensioners that accommodate the relative movement of the vessel, buoyancy cans that support the pipe at the top and allow the vessel to move (as shown in, for example, U.S. Pat. No. 4,702,321, incorporated herein by reference) or some combination of these techniques. Another method is by using a steel catenary riser (often referred to as a “SCR”), which comprises an extension of the steel riser pipe a sufficient horizontal distance from the vessel such that the pipe forms a rather deep catenary curve. Depending on a number of factors, the SCR can be designed to accommodate some vessel motion.  
           [0005]    The above methods all have disadvantages and limitations. For example, flexible hose is costly, cannot withstand external compressive loads without internal stiffening, and requires bend-restrictor devices at the terminations. The SCR are much less costly and have a long record of reliability; however, their shortcoming lies in motion compensation. The tensioners and buoyancy cans are expensive, and they both require a flexible hose (referred to as a “jumper line”) to accommodate the relative motion between the top of the riser, which sometimes includes a “Christmas tree,” and a flow manifold (fixed to the vessel).  
           [0006]    One proposed system that attempts to address the aforementioned problems is disclosed in U.S. Pat. No. 5,553,976—Korsgaard. This reference discloses a riser formed into a helical or sinusoidal configuration for decoupling axial stresses in the riser resulting from internal fluid pressure and/or external tension forces. The riser in this system, however, requires a plurality of elastic tensioning members that extend along the longitudinal direction of the riser, and that are secured to the riser at spaced intervals. The need for such tensioning members increases the cost of manufacturing and installing such systems.  
           [0007]    There is therefore a need for a relatively low-cost, simple riser that compensates for the motion of a floating vessel, and that does so without the need for separate tensioning members attached to the riser.  
         SUMMARY OF THE INVENTION  
         [0008]    The above issues are addressed by the present invention that employs a self-tensioning curved riser in a system for establishing fluid communication between a floating body and a wellhead on the seafloor. The system comprises fluid-conducting means for fluidly connecting the floating body and the wellhead, wherein the fluid-conducting means defines a major axis essentially extending from about the floating-body to about the wellhead. The system further comprises a means for absorbing and releasing energy in response to the heave and surge of the floating body that flexes in a direction essentially parallel to the major axis. In the preferred embodiments of the invention, the fluid-conducting means comprises a self-tensioning steel riser, and the means for storing and releasing energy comprises a series of pre-formed curves in the steel riser. In some embodiments, the series of pre-formed curves comprises a series of single-planar, preformed curves in the riser. These single-planar curves comprise arcs having a substantially constant radius of curvature in one embodiment and sinusoidal curves in another embodiment. In some other embodiments, the series of pre-formed curves comprises helical pre-formed curves.  
           [0009]    In accordance with specific embodiments of the invention, an apparatus for establishing a fluid connection between a floating body and a wellhead fixed to the seafloor comprises a riser pipe and a series of pre-formed curves in the riser pipe that defines a substantially linear axis between the floating body and the wellhead. In some embodiments the series of pre-formed curves comprises multi-planar curves, which in one embodiment comprises helical shaped curves. In some other embodiments, the series of pre-formed curves comprises single-planar curves, which in one embodiment comprises sinusoidal curves. The riser pipe is made of steel in some embodiments, but in some embodiments, the pre-formed curves may be of a different material than the remainder of the riser pipe. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a side view of an exemplary embodiment of riser with a series of pre-formed curves.  
         [0011]    [0011]FIG. 2 is a side view of an exemplary embodiment with a pre-formed helical curve.  
         [0012]    [0012]FIG. 3 is a side view of an exemplary embodiment with a pre-formed curve in a single plane. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    According to one exemplary embodiment of the invention, seen in FIG. 1, a fluid conducting conduit between a floating structure or vessel (“floating body”)  16  and a wellhead  12  on the sea floor  13  is provided by a riser pipe  14 , at least a substantial portion of which is formed with a series of pre-formed curves  10 . These curves  10  accommodate the stress generated by the motion of the floating body  16 . The pre-formed curves  10  flex in response to the motion of the floating body  16 , so that the forces generated by the motion of the floating body  16  are not transmitted to the wellhead  12 . By using these pre-formed curves  10 , a single, steel riser pipe  14  becomes feasible to connect the floating-body with the wellhead  12 , without the need for a using catenary curve. The riser pipe  14  is a rigid conduit, preferably made of a suitable steel alloy or some other similar alloy, as would be well-known to those skilled in the pertinent arts.  
         [0014]    By accommodating the motion of the floating body  16 , the preformed curves  10  also eliminate the need for a flexible section of pipe to connect the riser pipe  14  and the floating-body  16 . Using a metal (particularly steel) riser pipe  14  further eliminates the need for the external stiffening associated with using a flexible pipe section because the steel can withstand the external compressive loads exerted by the environment. In one embodiment, the pre-formed curves  10  are fashioned from a different material than the remainder of the riser pipe  14 .  
         [0015]    By absorbing the forces exerted by the floating body  16  without using a catenary curve, the pre-formed curves  10  also eliminate the need for additional buoyancy devices. As a result, in one embodiment, the riser pipe  14  connecting the wellhead  12  to the floating body  16  is only suspended from the floating body  16 . In one embodiment, the suspension from the floating body  16  supports the entire weight of the riser pipe  14 , while in another embodiment, part of the weight of the riser pipe  14  is supported by the riser pipe  14  itself.  
         [0016]    As shown in FIG. 1, while mooring lines  15  may be provided to attach the floating body  16  to the sea floor, the riser pipe  14  is self-tensioning and thus needs no external anchoring or tensioning means attached to it. This self-tensioning, or pre-tensioning, is accomplished by first lowering the riser pipe  14  to the seafloor  13  and anchoring it at the wellhead  12 . This is typically accomplished by means such as derrick or any conventional equivalent apparatus (not shown) on the floating body  16 . Tension is then applied to the upper end of the riser pipe  14  by means of the derrick (or equivalent apparatus), and upper end of the riser pipe  14 , in its pre-tensioned state, is secured to the appropriate structure on the floating body by conventional means, as is well-known in the art. Thus, the riser pipe  14 , when installed, is in a self-tensioned or pre-tensioned state, and needs no external tensioning means.  
         [0017]    In some embodiments, the pre-formed curves  10  do not affect the overall orientation or direction of the riser pipe  14 . Therefore, in one embodiment, the floating body  16  from which the riser pipe  14  is suspended is positioned directly above the wellhead  12 . The riser pipe  14  thereby defines an axis  21  essentially from about the floating body  16  to about the wellhead  12 . For the riser pipe  14  to accommodate the motion of the floating body  16 , the pre-formed curves  10  flex in a direction essentially parallel to the axis  21  defined by the riser pipe  14 . In another embodiment, positioning the floating body  16  closer to the wellhead  12  simplifies the installation and design of the sub-sea systems, in part by enabling a vertical connection between the riser pipe  14  and the wellhead  12 . Tools pass more easily through a vertical wellhead  12  connection than through a horizontal connection.  
         [0018]    In one embodiment, the portion of the riser pipe  14  in which the series of pre-formed curves  10  is formed is at or near the bottom end of the riser pipe  14 , and is thus connected between the wellhead  12  and the remainder of the steel riser pipe  14 . In other embodiments, the series of pre-formed curves  10  extends along substantially the entire length of the riser pipe  14 , from the wellhead  12  to the floating body  16 . In still other embodiments, segments of relatively straight riser pipe  14  are on either end of the portion having the series of pre-formed curves  10 .  
         [0019]    In the example shown in FIG. 1, the riser pipe  14  connects with a floating body  16  (in this example, a SPAR-type semi-submersible), and has a series of pre-formed curves  10  at its lower end, near the juncture with the wellhead  12 . Other types of floating body  16  that can be used with the invention include floating production storage and offloading (FPSO) systems, semi-submersible platforms, tension leg platforms, and others known to those of ordinary skill in the art. The connection between the wellhead  12  and the floating body  16  provided by the self-tensioning curved riser allows fluid communication therebetween. In some examples, this connection also allows tools to be passed from one section to another, and in one specific embodiment, the riser pipe  14  is raised using some lifting means (not shown) located on the floating body  16 , stretching the series of pre-formed curves  10  and allowing tools to pass more easily through the series of preformed curves  10 .  
         [0020]    Referring now to FIGS. 2 and 3, examples of pre-formed curves  10  are shown. In FIG. 2, the pre-formed curves  10  are three-dimensional curves forming an open coil, which advantageously may be a helical curve. As shown in FIG. 2, the vertical distance between equivalent points in the helical curve is called the curve spacing  17 , and the curve diameter  18  describes the diameter of the cross-sectional area of the curve. In some embodiments, the curve spacing  17  is at least double the curve diameter. In one embodiment, the curve spacing  17  increases with the distance along the axial length of the riser  14  above the seafloor.  
         [0021]    The characteristics of one set of exemplary embodiments of a riser pipe  14  with helical pre-formed curves  10  are shown in Table 1below.  
                                                                                                                                           TABLE 1                           Riser Pipe Embodiments With Helical Pre-Formed Curves            No   L   A   TL   OD   Wt   D/t   RF   RFr   St   Kr   PS ksi     PS psf     P ksf  * 10 2     L 30 ksi                      1   30   3.4   240   6.625   0.4321   15.3   24.6   1.0   1.2   1.0   127.1   1.83E+07   183.0   1016.7       2   20   3.4   240   6.625   0.4321   15.3   19.5   0.8   1.0   0.8   85.1   I.23E+07   122.5   680.6       3   30   2   240   6.625   0.4321   15.3   84.6   3.4   4.2   3.4   294.4   4.24E+07   424.0   2355.6       4   30   5   240   6.625   0.4321   15.3   9.2   0.4   0.5   0.4   59.0   8.49E÷06   84.9   471.7       5   40   3.4   240   6.625   0.4321   15.3   27.5   1.1   1.4   1.1   153.9   2.22E+07   221.6   1231.1       6   20   3.4   240   6.625   0.2161   30.7   10.8   0.4   0.5   0.4   88.1   1.27E+07   126.8   704.4       7   30   2   240   6.625   0.2161   30.7   46.9   1.9   2.3   1.9   304.9   4.39E+07   439.0   2438.9       8   30   3.4   240   6.625   0.2161   30.7   13.6   0.6   0.7   0.6   131.3   1.89E+07   189.0   1050.0       9   30   5   240   6.625   0.2161   30.5   5.1   0.2   0.3   0.2   61.0   8.79E+06   87.9   488.3       10   40   3.4   240   6.625   0.2161   30.7   15.3   0.6   0.8   0.6   159.0   2.29E+07   229.0   1272.2       11   30   3.4   240   8   0.5229   15.3   52.1   2.1   2.6   2.1   153.6   2.21E+07   221.3   1229.2                                                                                                                                  
 
         [0022]    [0022]FIG. 3 shows an exemplary embodiment in which the series of pre-formed curves  10  comprises curves in a single plane. In some embodiments, these single-planar, pre-formed curves  10  are sinusoidal; and, in other embodiments, the pre-formed curves  10  have semi-circular or other shapes. Combinations of such shapes of varying complexity are included in still further example embodiments. In one embodiment, the pre-formed curves  10  comprise several connected segments of pipes. As shown in FIG. 3, the vertical distance between equivalent points in the sinusoidal curve is called the wavelength  19 , and the amplitude  20  describes the width of the curve.  
         [0023]    The characteristics of one set of exemplary embodiments of a sinusoidal riser pipe  14  are shown in Table 2 below.  
                                                                                                                                           TABLE 2                           Riser Pipe Embodiments With Sinusoidal Pre-Formed Curves            No   L   A   TL   OD   Wm   D/t   RF   RFr   St   Kr   PS ksi     PS psf     P ksi*102     L 30 ksi                      1   30   2.0   210   6.625   0.4321   15.3   425.4   2.8   21.3   2.8   815.5   1.17E+08   1174.3   5708       2   30   3.4   210   6.625   0.4321   15.3   150.4   1.0   7.5   1.0   461.9   6.65E+07   665.1   3233       3   20   3.4   220   6.625   0.4321   15.3   135.5   0.9   6.8   0.9   411.0   5.92E+07   591.8   3014       4   40   3.4   200   6.625   0.4321   15.3   85.8   0.6   4.3   0.6   508.8   7.33E÷07   732.7   3392       5   30   3.4   420   6.625   0.4321   15.3   72.1   0.5   3.6   0.5   228.3   3.29E+07   328.7   3196       6   30   5.0   210   6.625   0.4321   15.3   63.2   0.4   3.2   0.4   288.7   4.16E+07   415.8   2021       7   30   2.0   210   6.625   0.2161   30.7   235.4   1.6   11.8   1.6   844.3   1.22E+08   1215.8   5910       8   30   3.4   220   6.625   0.2161   30.7   75.0   0.5   3.7   0.5   425.4   6.13E+07   612.5   3119       9   40   3.4   200   6.625   0.2161   30.7   85.8   0.6   4.3   0.6   508.8   7.33E+07   732.7   3392       10   20   3.4   220   6.625   0.2161   30.7   75.0   0.5   3.7   0.5   425.4   6.13E+07   612.5   3119       11   30   3.4   420   6.625   0.2161   30.7   39.9   0.3   2.0   0.3   236.3   3.40E+07   340.3   3308       12   30   5.0   210   6.625   0.2161   30.7   34.0   0.2   1.7   0.2   298.9   4.30E+07   430.4   2092       13   30   3.4   210   8   0.5229   15.3   318.8   2.1   15.9   2.1   559.0   8.05E+07   805.0   3913                                                                                                                                  
 
         [0024]    One important benefit derived from including pre-formed curves  10  is that they add an additional layer of safety for the structural integrity of the whole riser pipe  14 . If, for example, the top end of the riser pipe  14  should move beyond its normal operating design limits either horizontally or vertically, the pre-formed curves  10 , in various exemplary embodiments, flex, without local buckling, and the riser  14  still maintains structural integrity. This situation might occur if, for example, the floating body  16  should lose buoyancy due to a damaged tank, if the moorings were to come loose or some other mishap were to occur.  
         [0025]    In addition to the characteristics of a riser pipe  14  with pre-formed curves  10  shown in the tables above, a number of additional design factors are considered to develop a site-specific design. A non-exhaustive list of these additional factors includes:  
         [0026]    Water depth  
         [0027]    Envelope of surface vessel motion  
         [0028]    Physical properties of the riser  
         [0029]    Ocean currents  
         [0030]    Envelope of deflection curve of the riser to avoid clashing  
         [0031]    Method of installation and removal of riser  
         [0032]    Limitation of curvature of riser to allow passage of through-tubing tools (e.g. “pigs”).  
         [0033]    The specific embodiments described above and shown in the drawings are given by way of example only. Other aspects and examples of the invention will be understood to be within the spirit of the present invention and with the scope of or equivalent to that described by the claims.