BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a spar-type offshore platform showing an SCR extending into the platform through a bend-limiting stinger in accordance with the present invention;
FIGS. 2 and 3 are cross-sectional views, similar to that of FIG. 1, showing the process of installing a bend-limiting stinger in accordance with the present invention; and
FIG. 4 is a detailed cross-sectional view of the bend-limiting stinger in accordance with a preferred embodiment of the present invention.
DETAILED DESCRIPTION
As used herein, the terms “invention” and “present invention” are to be understood as encompassing the invention described herein in its various embodiments and aspects, as well as any equivalents that may suggest themselves to those skilled in the pertinent arts.
FIG. 1 shows a spar-type platform 10 of conventional design. The platform 10 comprises a deck structure 12 supported above the surface 14 of a body of water by a buoyant hull 16. The hull 16, in turn, comprises an array of buoyancy tanks 18, some of which are “hard” tanks (air-filled), and some of which are “soft” tanks (floodable with water for adjustable buoyancy). The tanks 18 are arranged around a central column 20 that defines a centerwell 22. The bottom end of the central hull column 20 terminates in a ballasted keel 24 having a bottom opening or slot 25 that communicates with the centerwell 22. A plurality of riser guides 26 are disposed at spaced intervals along the length of the centerwell 22, with a keel slot guide 27 being disposed in the keel slot 25. The riser guides 26 define a passage through the centerwell 22 through which a riser casing 28 passes. (It is understood that the riser guides 26 and the keel slot guide 27 are typically configured to accommodate a plurality of casings.) The riser casing 28 extends from the keel slot 25, through the keel slot guide 27 and the riser guides 26, to the upper end of the hull 16, where it is fixed as described below.
As shown in FIG. 1, a bend-limiting stinger 30, described in detail below, is attached to the lower end of the casing 28 and extends downwardly from the keel 24. As explained below, the stinger 30 is formed of a plurality of pipe sections joined axially (end-to-end) to a total length that may typically range from about 100-150 meters. A riser 32 extends downwardly from the deck 12, through the casing 28 and the stinger 30 down to the sea floor 34, where it becomes a flow line 36 running along the sea floor 34 to a seafloor structure serving as a terminus, such as a well-head (not shown). The riser 32 is a flexible steel riser of a well-known type known as a steel catenary riser (SCR), formed of a plurality of steel pipe sections joined axially (end-to-end) to form a continuous, flexible conduit that defines a catenary curve as it traverses from the platform keel 24 to the sea floor 34.
As mentioned above, the riser 32 must be gradually straightened from its curved configuration between the keel 24 and the sea floor 34, to a linear, vertical configuration within the casing 28. This transition must be accomplished while limiting the bending moment applied to the riser 32, for the reasons discussed above. The specific structure of the stinger 30, illustrated in FIG. 4, provides this function and result.
Specifically, as shown in FIG. 4, the bend-limiting stinger 30 of the present invention is a continuous flexible conduit defining a continuous bore or passage 36 sized to receive a riser (SCR) 32 therethrough (as shown in the phantom outline). The conduit is formed from a plurality of axial sections or segments, each comprising a steel pipe section or “joint” 38, joined axially (end-to-end). Each of the joints or sections 38 may preferably be about 10-13 meters in length, with substantially equal inside diameters, so that the bore 36 is likewise of uniform diameter throughout its length. The joints or sections 38 (typically, but not necessarily, eight to ten in number) may be welded to each other or mechanically joined to each other by conventional connection means (not shown) that are well-known in the art. As described in detail below, the stinger 30 has progressively increasing flexibility or compliance from the upper end to the lower end. Advantageously, the stinger 30 also progressively increases in weight per unit length from the upper end to the lower end.
In the specific preferred embodiment illustrated in FIG. 4, each of the pipe sections or joints 38 below the uppermost joint 38 has an outside diameter that is slightly less than that of the joint immediately above it. In other words, the wall thickness of each successive joint below the uppermost joint is somewhat less than that of the joint immediately above it, providing a stepwise graduated reduction in wall thickness (and thus outside diameter) from each joint to the joint immediately below it. The rate of wall thickness (or outside diameter) reduction does not need to be constant throughout the length of the stinger 30, and it will be dictated by the specific degree of stiffness (or, conversely, flexibility) desired in each pipe section 38.
Also, in the preferred embodiment, weight per unit length is increased for each successive joint 38 below the uppermost joint by applying a weight “jacket” 40 of increasing thickness (and thus increasing weight or mass) on each successive joint 38 below the uppermost joint. In a specific exemplary embodiment, the weight jacket 40 comprises one or more layers of weighted metallic tape (e.g., lead tape) applied to the outside surface of each joint 38. Each successive joint 38 below the uppermost joint is provided with a greater number of tape layers than the joint immediately above it. Alternatively, the weight jackets 40 may be in the form of weighted collars installed on the pipe joints 38, or concrete pre-cast onto the pipe joints 38, with the collar weights or concrete casting thicknesses being distributed so that each successive pipe joint 38 has a greater weight than the joint immediately above it. The distribution of the weights makes the lower end of the bend limiting stinger 30 heavier (per unit length) than the upper end, without affecting the greater compliance of the lower end due to the lesser pipe wall thicknesses as compared to the upper end.
The weight jackets 40 are advantageously applied to all or some of the pipe sections or joints 38 before installation. Optionally, and advantageously, similarly weighted “field joint” weights 42 may be applied at some or all of the junctures of the pipe sections 38 after installation. Like the weight jackets 40, the field joint weights 42 increase in weight along the length of the stinger 30 from top to bottom. Also, like the weight jackets 40, the field joint weights 42 are preferably formed of weighted metal tape, although discrete weighted elements (e.g., collars) may also be used.
The above-described structure that increases in flexibility and weight per unit length of the stinger 30 from its top end to its bottom end results in a lateral load being applied to the riser 32 as it passes through the stinger 30, thereby causing a gentle and gradual transition in the SCR from a curved configuration at the lower end of the stinger 30, to a straight configuration as it emerges from the upper end of the stinger 30. The specific variations of flexibility and weight (or mass) of the stinger 30 will be dictated by the physical characteristics of the riser 32, such as its area of inertia, outside diameter, and Young's modulus. The design consideration is to keep the stress induced in the riser 32 below a predetermined limit, wherein the stress σ is directly proportional to the bending moment, as set forth in the following equation:
σ=mc/ei (1)
where
m is the bending moment applied to the riser;
c is the outside radius of the riser;
e is Young's modulus; and
i is the area of inertia of the riser.
The preferred method of installing the bend-limiting stinger 30 of the present invention is illustrated in FIGS. 2 and 3. The maximum outside diameter of the stinger 30 is less than the diameter of the passage defined by the riser guides 26 and the keel slot guide 27 in the centerwell 22 of the platform 10. This allows the bend limiting stinger 30 of the present invention to be installed from the deck 12 of the platform 10 by attaching it to the lower end of a casing 28 that is then run down the centerwell 22 from a drilling or work-over rig 44 on the deck 12, through the riser guides 30 and the keel slot guide 27, until the upper end of the stinger 30 is seated in the keel 24. The upper end of the casing 28 is then secured in a fixture 46 at or near the top of the hull 16. The stinger 30 is now installed so that the riser 32 may be run through it. Alternatively, the stinger 30 may be attached to the end of a drill pipe string (not shown) that is run down the centerwell 22 until the top end of the stinger 30 is seated in the keel 24, whereupon the drill pipe string is detached from the stinger and withdrawn up the centerwell 22.
While a preferred embodiment of the invention has been described above and illustrated in the drawings, it is understood that this embodiment is exemplary only as the currently preferred embodiment of the invention. It will be appreciated that a number of variations and modifications will suggest themselves to those skilled in the pertinent arts. Such variations, modifications, and equivalents should be considered within the spirit and scope of the invention, as defined in the claims that follow.
Patent Application
20080044233
