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[0001] This Application is a continuation-in-part of co-pending U.S. pat. app. Ser. Nos. 10/213,966 and 10/213,963, both filed Aug. 7, 2002.
[0002] (Not Applicable)
[0003] (Not Applicable)
[0004] 1. Field of the Invention
[0005] This invention relates, in general, to offshore oil well risers that convey petroleum from producing wells on the sea floor to a floating platform on the sea surface, and in particular, to risers that are capable of accommodating large motions of the platform relative to the wells without damage.
[0006] 2. Description of Related Art
[0007] Conventional “dry tree” floating offshore platforms for drilling and production of oil and gas typically include such “low heave” designs as Spar platforms, Tension Leg Platforms (“TLPs”), and Deep-Draft semi-submersible platforms. These platforms are capable of supporting a plurality of vertical production and/or drilling “risers,” i.e., long pipes extending up from oil and gas wells on the sea floor to the platforms. The platforms typically comprise a “well deck,” where surface “trees,” i.e., control valves disposed on the top ends of the risers, are located, and a production deck, where the crude oil is collected from the risers and fed to a processing facility for separation of water, oil and gas. In conventional dry tree offshore platforms, the risers extend from the respective well heads to the well deck and are supported thereon by a tensioning apparatus, and such risers are thus termed Top-Tensioned Risers (“TTRs”).
[0008] One known TTR design uses “active” hydraulic tensioners located on the well deck to support each riser independently of the others. Each riser extends vertically from the well head to a tensioner located on the well deck of the platform, and is supported there by hydraulic cylinders connected to the well deck. The cylinders enable the platform to move up and down relative to the risers, and thereby partially isolate the risers from the heave motions of the hull. A surface tree is attached at the top of each riser, and a flexible, high-pressure jumper hose connects the surface tree to the production deck. However, as the tension force and displacement requirements of the hydraulic cylinders increase, these active tensioners become prohibitively expensive. Further, the offshore platform must be capable of supporting the combined load of all the risers.
[0009] A second TTR design uses passive “buoyancy cans” to support the risers independently of the platform, as illustrated in the schematic elevation view of
[0010] Conventional “wet tree” offshore platforms include Floating Production Storage and Off-loading (“FPSO”) and semi submersible platforms. These types of platforms have relatively large motions that make it impractical for them to support vertical production and drilling risers, and accordingly, are generally used in connection with a sub-sea completion system, i.e., sub-sea trees, which are arranged on the seafloor. Produced crude oil is typically conveyed along the seafloor with flow-lines and gathered in a manifold. Production risers then carry the crude oil from the manifold or sub-sea tree to the process equipment of the floating support. As the floating support has relatively large motions (both heave and horizontal), the production risers must be designed to accommodate these large motions.
[0011] Production risers can also comprise flexible, reinforced elastomeric risers. Flexible risers are connected directly to the floating platform, and thus take the shape of a catenary that extends from the floating support to the sea floor, as illustrated schematically in
[0012] Another known dry tree riser system is the so-called “riser tower.” In this system, the riser tower comprises one or more rigid vertical pipes connected to the seafloor through a pivot connection or stress joint. The pipes are supported by a large top buoyancy device which provides sufficient buoyancy to support the pipes and prevent them from going slack or vibrating in response to ocean currents. Flexible jumpers are used to connect the vertical pipes to the floating support. This type of riser system is both expensive and difficult to install.
[0013] In light of the foregoing, a long felt but as yet unsatisfied need exists in the petroleum industry for a low-cost, simple, yet reliable offshore oil well riser system that compensates for the motions of an associated floating platform.
[0014] In accordance with the present invention, an offshore oil well riser system is provided that compensates for the motions of an associated floating drilling or production platform. The riser system is relatively inexpensive, simple to fabricate and deploy, and reliable in operation.
[0015] The novel riser comprises a rigid vertical pipe section that is supported by the floating vessel, and which extends downward from the vessel substantially perpendicular to the sea floor, and a rigid horizontal pipe section that is connected to the associated sub-sea well equipment (i.e., the well head, sub-sea tree, split tree, manifold, or the like), and which extends away from the equipment substantially parallel to the sea floor. A relatively short, inflexible angled pipe section, i.e., an “elbow,” connects the horizontal pipe to the vertical pipe. In a preferred embodiment, the vertical pipe section predominates over the others such that the overall riser system presents a substantially vertical shape, and only a relatively small, substantially horizontal pipe section is used to connect the riser to the sub-sea well head equipment.
[0016] Since the riser is directly supported by the floating platform, motions of the platform (i.e., heave, surge, sway, pitch, roll, and yaw) are transmitted to the riser, and must therefore be absorbed by the horizontal and vertical pipes. To limit the resulting stress and fatigue in these two sections, at least one of them is provided with a flexing portion that is able to absorb the motion of the platform imparted to the riser. This flexing portion can be arranged in the vertical pipe section, in the horizontal pipe section, or in both. The flexing portion comprises a plurality of recurvate sections of pipe connected end-to-end with alternating curvatures. In one possible embodiment thereof, the central axis of the flexing portion lies in a single plane and takes a sinuous path, e.g., that of a sinusoid. In another possible embodiment, the central axis of the flexing portion takes a three dimensional path, e.g., that of a helix. Many other configurations of the flexing portion are possible.
[0017] Both the angled pipe section, i.e., the elbow, and the flexing portion of the novel riser can be designed to easily accommodate wire line, coiled tubing or “pigging” operations internally. The floating vessel supports the riser, and thus, no expensive buoyancy cans are required. Since all vessel motions are absorbed by the riser, neither a flexible jumper nor a long length of pipe is required to accommodate the motion. Additionally, since the major portion of the riser is substantially vertical, the total length of riser required is substantially reduced, relative to a catenary shape, and since it is made entirely of steel pipe, it is cost-effective to make.
[0018] 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, especially if such consideration is made in conjunction with the views of the appended drawings.
[0019]
[0020]
[0021]
[0022]
[0023]
[0024]
[0025]
[0026]
[0027]
[0028] FIGS.
[0029] An offshore oil well riser system
[0030] As a practical matter, because of the relatively large depths in which the riser system
[0031] An alternative exemplary embodiment of a riser system
[0032] The flexing of the riser system
[0033] As illustrated in
[0034] The flexing portion
[0035] It should be understood that the riser system
[0036] The characteristics of an exemplary vertical riser pipe TABLE 1 Vertical Riser Pipe With Sinusoidal Flexible Portion No. P R TL OD Wm D/t RF RFr St Kr PS PS P L 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
[0037] The characteristics of an exemplary vertical riser pipe TABLE 2 Vertical Riser Pipe With Helical Flexing Portion No. P R TL OD Wt D/t RF RFr St Kr PS PS P L 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 1.23E±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
[0038] An important advantage provided by the flexing portions
[0039] In addition to the riser pipe characteristics shown in Tables 1 and 2 above, a number of additional design factors should be considered in developing a site-specific riser system
[0040] Water depth;
[0041] Envelope of surface vessel motion;
[0042] Physical properties of the riser;
[0043] Ocean currents;
[0044] Envelope of deflection curve of the riser to avoid riser collisions;
[0045] Method of installation and removal of riser; and,
[0046] Limitation of riser curvature to allow passage of through-tubing tools (e.g. “pigs”).
[0047] As illustrated in
[0048] As will by now be apparent to those of skill in the art, many modifications, alterations and substitutions are possible to the materials, methods and configurations of the riser systems of the present invention without departing from its spirit and scope. Accordingly, the scope of the present invention should not be limited to the particular embodiments described and illustrated 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.