AU2008326408A1

AU2008326408A1 - Self-standing riser system having multiple buoyancy chambers

Info

Publication number: AU2008326408A1
Application number: AU2008326408A
Authority: AU
Inventors: Keith K. Millheim
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-11-19
Filing date: 2008-11-19
Publication date: 2009-05-28
Anticipated expiration: 2028-11-19
Also published as: AU2008326408B2; US20090126937A1; US20140166296A1; NO20100889L; CN101939491A; WO2009067532A1; MX2010005485A; CN101939491B; AP2010005290A0; US20110142546A1

Description

WO 2009/067532 PCT/US2008/084057 SELF-STANDING RISER SYSTEM HAVING MULTIPLE BUOYANCY CHAMBERS FIELD OF THE INVENTION [10011 The present invention relates generally to self-standing riser assemblies utilized 5 during oil and gas exploration and production operations, and in a particular though non limiting embodiment, to a self-standing riser system equipped with multiple buoyancy chambers suitable for deployment in a variety of water depths and sea conditions. BACKGROUND OF THE INVENTION 110021 Self-standing risers (hereinafter "SSR") are employed in the oil and gas industry 10 to suspend production and injection lines from subsea production units, and to support holding tendons associated with floating offshore structures. Known SSR can be used to facilitate standard "shallow-water" (e.g., between 0 feet and around 600 feet of water) drilling units and cost effective production facilities by placing blow-out preventers and production trees on top of a buoyancy chamber. 15 [10031 The conventional approach to the SSR design has been to employ one large buoyancy chamber that supports the riser or tendon loads. However, this approach has led to increased costs associated with the construction and installation of the buoyancy chambers. Such factors have resulted in a lack of significant SSR system development by operators who could realize a broad spectrum of associated benefits. Nonetheless, the 20 industry as a whole desires a reduction in oil and gas production costs, a decrease in time delays for drilling exploration wells, and increased development of previously discovered fields. There is, therefore, a long-felt but unmet need for smaller, more flexible riser systems capable of more rapid manufacture and deployment that assist in the profitable development of previously under produced oil and gas fields.

I

WO 2009/067532 PCT/US2008/084057 SUMMARY OF THE INVENTION [10041 A self-standing riser system suitable for deepwater oil and gas exploration and production is provided, the system including a lower riser assembly disposed in communication with a primary well-drilling fixture; one or more intermediate buoyancy 5 chambers disposed in communication with the lower riser assembly and one or more portions of intermediate riser assembly, wherein one or more of the buoyancy chambers further includes an open-bottomed lower surface portion; and an upper riser assembly disposed in communication with one or more upper buoyancy chambers, wherein one or more of the upper buoyancy chambers further includes a fully enclosed portion. 10 [10051 Ballast loads for the chambers; stress joints for the riser assemblies; methods and means of system deployment and maintenance; access to blow-out preventers, wellheads and production trees; and various system interconnections are also provided. 2 WO 2009/067532 PCT/US2008/084057 BRIEF DESCRIPTION OF THE DRAWINGS [10061 The embodiments disclosed herein will be better understood, and numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings. 5 110071 Fig. IA is a schematic diagram of a self-standing riser system equipped with an open-bottom buoyancy chamber in calm waters, according to an example embodiment known in the prior art. 110081 Fig. 1B is a schematic diagram of a self-standing riser system equipped with an open-bottomed buoyancy chamber that is nearing its spill point. 10 [10091 Fig. 1C is a schematic diagram of a self-standing riser equipped with an open bottomed buoyancy chamber that has tilted beyond its spill point. [1010] Fig. 2 is a schematic diagram depicting the effects of pressure, temperature and depth on a closed-bottom buoyancy chamber. [1011] Fig. 3 is a schematic diagram of a self-standing riser system comprising 15 multiple buoyancy chambers, according to example embodiments of the present invention. 110121 Fig. 4 is a schematic diagram depicting the installation of a self-standing riser system comprising multiple buoyancy chambers, according to example embodiments of the invention. 3 WO 2009/067532 PCT/US2008/084057 DETAILED DESCRIPTION 11013] There are presently two known types of submersible buoyancy chambers suitable for oil and gas exploration and production: a closed container design, and an open bottomed design. Both types of chambers, if pressurized and secured by a riser, will exert 5 an upward lifting force on the riser. Certain embodiments also comprise features lending adjustability to the system, as may be known to those of skill in the art. 110141 The closed container design is similar in some respects to a submarine, in that there are typically one or more ballast chambers used to house a fluid, such as a light gas, seawater, etc. Once a desired ratio of fluids is achieved, the chamber is closed off by valves 10 or other means known in the art. [10151 An open-bottomed buoyancy chamber includes many design functions similar to those of the closed container design. However, once desired buoyancy characteristics are achieved, fluid disposed within the chamber is simply trapped by the sides and top thereof. 110161 Figure 1A illustrates a known, open-bottomed, buoyancy chamber disposed in 15 communication with an SSR and filled with a fluid, for example, a pressurized gas. As seen, a combination of calm water currents, minimal external forces, and a sufficient amount of buoyancy applied to the SSR results in minimal lateral displacement force. Accordingly, the buoyancy chamber illustrated in Figure 1A experiences little or no tilt relative to its vertical axis, and fluid contained within the chamber remains enclosed. 20 [10171 If, however, a sufficiently large enough force is applied to the chamber, such as a strong current as depicted in Figure 1B, the SSR will begin to tilt away from its vertical axis. Figure 1B also illustrates how the fluid contained within the chamber has shifted relative to the system's tilt away from its vertical axis. However, the chamber can accommodate a tilt of up to a certain critical angle (which depends largely on its design 25 dimensions) before the critical spill point angle is reached, and fluid begins to escape from the chamber. [10181 Figure IC further illustrates how the spill rate of the gas contained within an open-bottomed buoyancy chamber will increase as the critical tilt angle is reached and exceeded. In particular, spillage will result in even greater loss of buoyancy, and therefore a 30 proportionately increasing tilt angle, which will cause more and more gas to escape from the 4 WO 2009/067532 PCT/US2008/084057 chamber. Eventually, enough gas escapes that the buoyant force is reduced to the point where the chamber can no longer support the riser, thereby causing the system to fail. [1019] Despite such drawbacks, open-bottomed chambers can operate at extreme water depths with a reduced concern of structural collapse than a closed system, since the open 5 design allows fluid pressures within the chamber to equalize with surrounding pressures at even great depths. Furthermore, the open-bottomed design has less overall system weight due to a reduction in required construction materials, since there is no bottom, and the remainder of the shell will require less thickness and reinforcement in order to withstand deepwater fluid pressures. 10 11020] In contrast, closed container buoyancy chambers do not suffer as greatly from the problem of tilting caused by currents and surface effects, and are typically the appropriate design choice in areas where currents and surface effects are significant enough to cause major lateral displacement from the vertical axis. However, if either of the described buoyancy chambers sustain a leak (for example, a leak caused by container 15 breach, valve malfunction, etc.), the gas or other fluid will escape and the SSR can fail, as illustrated in Figure IC. [10211 Closed container buoyancy chambers must also be robust enough to offset external forces such as deepwater fluid pressure. As illustrated in Figure 2, such chambers must, as a threshold matter, have sufficient structural integrity and wall thickness to resist 20 expected pressures that might cause a collapse of the chamber's outer shell. Moreover, when deploying a closed buoyancy chamber filled with a gas, the internal gas pressures and temperatures should be sufficiently proportional to the external water pressures and temperatures that an associated pressure or temperature gradient will not induce an effective change in gas volume within the chamber which could cause the chamber's outer shell to 25 crack or collapse. [1022] Typically, SSR systems are constrained to include the use of only a single buoyancy chamber due to the chamber's large size. However, the larger buoyancy chamber designs increase the time and cost associated with building and deploying the operating system. Moreover, deployment of large, pressurized chamber at great depths (e.g., > 500 ft. 30 or so) can prove to be an exceedingly difficult task. Furthermore, as the diameter of the 5 WO 2009/067532 PCT/US2008/084057 buoyancy chamber is increased, the probability of structural failure and warping caused by handling during construction and deployment is also increased. [10231 The detailed description that follows includes exemplary systems, methods, and techniques that embody techniques of the presently inventive subject matter. However, it 5 will be understood by those of skill in the art that the described embodiments may be practiced without one or more of the specific details disclosed herein. In other instances, well-known manufacturing equipment, protocols, structures and techniques have not been shown in detail in order to avoid obfuscation in the description. 110241 Referring now to the example embodiment depicted in Figure 3, an SSR system 10 14 is depicted comprising a plurality of subordinate buoyancy chambers configured to admit to installation in deeper water depths than any previously known SSR systems. According to an alternative embodiment, SSR 14 can be stacked with multiple buoyancy chambers as illustrated in Figures 4A, 4B, 4C and 4D. Although illustrated in Figure 3 as a combination of lower SSR assembly 10 and upper SSR assembly 12, embodiments of the overall SSR 15 system 14 can comprise any number of individual SSR assemblies. [1025] In the embodiment depicted in Figure 3, lower SSR assembly 10 is first deployed. In one example, a specially designed vessel equipped specifically to deploy buoyancy chambers and SSR assemblies is used. Following deployment, lower SSR assembly 10 is joined in mechanical communication with a casing wellhead established near 20 the mud-line. In a typical embodiment, the casing wellhead has been preset into a well hole bored into an associated seafloor surface. [1026] In further embodiments, one or more intermediate buoyancy chambers 16 is attached to lower SSR assembly 10, thereby providing increased stability in deep or turbulent waters. Depending on operating conditions, intermediate buoyancy chamber 16 25 can comprise a closed-container design, but in most instances will comprise the open bottomed design for the reasons described above, with the only firm requirement being that intermediate chamber 16 must in any event be capable of providing the support required to control lower SSR assembly 10 and upper SSR assembly 14. [10271 In further example embodiments, intermediate buoyancy chamber 16 is disposed 30 in mechanical communication with either previously known or custom-designed drilling, production and exploration equipment. Thus, for example, the top and bottom portions of 6 WO 2009/067532 PCT/US2008/084057 an intermediate buoyancy chamber may comprise one or more of a blowout preventer, a production tree, or a wellhead that functions in a manner similar to the casing wellhead placed near mud-line of the ocean floor. Attachment of the drilling, production and exploration equipment can be achieved using either known or custom connection and 5 fastening members, e.g., hydraulic couplers, various nut and bolt assemblies, welded joints, pressure fittings (either with or without gaskets), swaging, etc., without departing from the scope of the invention. [10281 In further embodiments, an upper SSR assembly 12 is deployed and disposed in mechanical communication with a wellhead, blowout preventer, or production tree (or 10 another, custom-designed device combining elements of one or more of such devices) installed atop an upper surface of the intermediate chamber 16 or a connecting member associated therewith. According to other example embodiments, the installation process continues until the desired number of such assemblies are installed in serial communication with one another in order to achieve a stable and efficient SSR system 14, as depicted in 15 Figures 4A - 4D. [10291 In order to further stabilize the SSR system 14, example embodiments can utilize stress joints 22, as depicted in Figure 3. Stress joints 22 can comprise any known material, for example, a plastic, rubber, or metal material, but should in any event be capable of maintaining the SSR 14 system's structural integrity and overall stability. 20 [10301 Consistent with the example SSR system 14 illustrated in Figure 3, a plurality of upper buoyancy chambers 18, 20 includes an open-bottomed chamber 18 and a closed container type chamber 20. In a one example embodiment, at least one of said upper chambers - generally the topmost - will comprise a closed design, while others in the system, including intermediate chamber 16, will comprise an open-bottomed design. In 25 another example embodiment, all of the chambers in the system are either open or closed, and in still further embodiments, combinations of open and closed chambers are employed across the system. [1031] In some embodiments, the multiple open-bottomed design buoyancy chambers are utilized to facilitate deployment in deeper waters in which surrounding fluid pressures 30 are greatest. Other embodiments utilize a plurality of closed-container type chambers disposed near the top of the SSR system 14, thereby improving the system's overall stability 7 WO 2009/067532 PCT/US2008/084057 and balance. Such configurations can also help avoid the system's tendency to tilt away from its vertical axis as a result of external lateral forces, such as a forceful cross-current. [10321 In still further embodiments, a plurality of buoyancy chambers disposed in mechanical communication with upper SSR assembly 12 allows for the overall SSR system 5 14 to maintain required functionality and stability in varying water depths and conditions, thereby improving its efficiency and operability. [10331 Further example embodiments comprise a plurality of upper buoyancy chambers disposed in mechanical communication with commonly known drilling, production and exploration equipment. Thus, for example, the top and bottom portions of an upper 10 buoyancy chamber may comprise one or more of a blowout preventer, a production tree, or a wellhead designed to function in a manner similar to the casing wellhead placed near mud-line of the ocean floor. 110341 In further embodiments, the buoyancy chambers utilized throughout the system further comprise auxiliary buoyancy materials, such as syntactic foam or air filled glass 15 micro-spheres that lend buoyancy to the system. Injecting one or more of these materials within an open-bottomed chamber will assist in prevention of buoyancy fluid (e.g., gas, liquid, etc.) loss should tilting occur, or if there is a breach or failure of tubing, valves, or other equipment utilized in connection with the buoyancy chamber. [10351 In the example embodiment illustrated in Figure 4A, a deployment vessel 20 deploys a lower SSR assembly 40 to the ocean floor where it is mechanically disposed in communication with a casing wellhead near the mud-line. Figure 4A further depicts an intermediate buoyancy chamber 41 installed atop the SSR assembly 40. Various embodiments of the intermediate buoyancy chamber 41 further comprise one or more previously known or custom-fit attachment mechanisms, such as a combined blowout 25 preventer and production tree, so that the intermediate chamber 41 is useful during operations for purposes other than mere connection with an upper SSR assembly 42. In various other embodiments, a plurality of intermediate buoyancy chambers 41 are deployed and mechanically disposed in communication with a previously installed SSR assembly or another intermediate buoyancy chamber (see, for example, Figures 4B - 4D). 30 110361 In Figure 4C, intermediate SSR assemblies 42 and 44 are deployed and disposed in mechanical communication with a well-head affixed atop intermediate buoyancy 8 WO 2009/067532 PCT/US2008/084057 chamber 41. In some example embodiments, additional intermediate buoyancy chambers 41, 43, 45 serve as additional support and connection components for the intermediate SSR assemblies. Such redundant embodiments can achieve heretofore unknown SSR system depths of more than 15,000 ft. with the addition of multiple intermediate SSR assemblies. 5 [1037] In the example embodiment depicted in Figure 4D, a final SSR assembly 46 is deployed to complete the SSR system 50. Figure 4D further depicts an embodiment employing a plurality of buoyancy chambers 47 atop SSR assembly 46 in order to complete the overall SSR system 50. As previously discussed, embodiments of the plurality of buoyancy chambers 47 can comprise a mixture of open-bottomed and closed-container 10 designs, or any other configuration made desirable by operating conditions, including of course the installation of only a single such chamber. [1038] The foregoing specification is provided for illustrative purposes only, and is not intended to describe all possible aspects of the present invention. Moreover, while the invention has been shown and described in detail with respect to several exemplary 15 embodiments, those of ordinary skill in the art will appreciate that minor changes to the description, and various other modifications, omissions and additions may also be made without departing from the spirit or scope thereof. 9

Claims

1. A self-standing riser system suitable for deepwater oil and gas exploration and production, said system comprising: a lower riser assembly disposed in communication with a primary well 5 drilling fixture; one or more intermediate buoyancy chambers disposed in communication with said lower riser assembly and one or more portions of intermediate riser assembly, wherein one or more of said buoyancy chambers further comprises an open-bottomed portion; and 10 an upper riser assembly disposed in communication with one or more upper buoyancy chambers, wherein one or more of said upper buoyancy chambers further comprises an open-bottomed portion.

2. The self-standing riser of claim 1, wherein each of said open-bottomed 15 intermediate buoyancy chambers further comprises a fluid ballast.

3. The self-standing riser system of claim 2, wherein said fluid ballast further comprises a gas ballast. 20

4. The self-standing riser system of claim 2, wherein said fluid ballast further comprises a liquid ballast.

5. The self-standing riser system of claim 2, wherein said fluid ballast further comprises a ballast including both a liquid and a gas. 25

6. The self-standing riser system of claim 2, wherein said fluid ballast further comprises an auxiliary ballast that lends additional pressure and density to said fluid. 30

7. The self-standing riser system of claim 6, wherein said auxiliary ballast retards the escape of fluid from within said open-bottomed intermediate buoyancy chambers in the event said chambers tilt beyond a critical angle relative to its vertical axis. 10 WO 2009/067532 PCT/US2008/084057

8. The self-standing riser system of claim 1, wherein one or more of said intermediate buoyancy chambers further comprises a closed bottom portion. 5

9. The self-standing riser system of claim 1, wherein one or more of said upper buoyancy chambers further comprises a closed bottom portion.

10. The self standing riser system of claim 1, wherein each of said open bottomed upper buoyancy chambers further comprises a fluid ballast. 10

11. The self-standing riser system of claim 10, wherein said fluid ballast further comprises a gas ballast.

12. The self-standing riser system of claim 10, wherein said fluid ballast further 15 comprises a liquid ballast.

13. The self-standing riser system of claim 10, wherein said fluid ballast further comprises a ballast including both a liquid and a gas. 20

14. The self-standing riser system of claim 10, wherein said fluid ballast further comprises an auxiliary ballast that lends additional pressure and density to said fluid.

15. The self-standing riser system of claim 14, wherein said auxiliary ballast 25 retards the escape of fluid from within said open-bottomed intermediate buoyancy chambers in the event said chambers tilt beyond a critical angle relative to its vertical axis.

16. The self-standing riser system of claim 1, wherein one or more lengths of 30 said lower riser assembly and said upper riser assembly further comprises one or more stress joints for absorbing stress accumulated within said lengths of said assemblies. 11