PIPE COUPLING FIELD OF THE INVENTION [0001] The present disclosure relates in general to methods and apparatus for connecting sections of pipe, and in particular but not limited to pipe sections connected to form casing strings for oil and gas wells. BACKGROUND OF THE INVENTION [0002] Wells for production of hydrocarbon fluids such as oil and natural gas are typically drilled by connecting a drill bit to the lower end of a "drill string" made up of sections (or "joints") of drill pipe connected end-to-end by means of threaded connections, and then rotating a drill bit into the ground until the bit penetrates a hydrocarbon-producing subsurface formation. After the well has been drilled, it is typically necessary to line the wellbore with tubular casing to prevent soil materials from sloughing into the wellbore and thus partially or completely collapsing the wellbore. Accordingly, after the drill string has been withdrawn from the drilled wellbore, a casing string is usually installed in the wellbore. The casing string is made up of pipe sections having a diameter larger than the drill pipe, and slightly smaller than the wellbore. [0003] A typical conventional casing connection consists of a female-threaded "box" end and a male-threaded "pin" end. The box-end threads can be machined directly into the inside diameter of the casing body. Alternatively (and more commonly), the box-end threads can be machined inside a tubular coupling, which is then threaded onto a pin end of a casing joint to form the box end. The pin-end threads are machined directly into the outside diameter of the casing body. When the box-end threads are machined into the casing body, it can sometimes weaken the connection, because material is necessarily removed by the threading. In some cases, the casing body is swaged to allow for material extraction due to threading, such that the net cross-sectional of the threaded zone is not less than the cross- sectional area of unthreaded portion of the casing. In most cases, a connection using a coupling will be sufficiently strong, since the wall thickness of the coupling often exceeds the wall thickness of the casing. [0004] When couplings are used, each casing joint will have a pin at each end. The coupling is a tubular sleeve with a female-threaded box at each end ("box by box" in the vernacular). One box end of the coupling is threaded onto the pin end of a first casing joint (a "pin by box" connection), and then the pin end of a second casing joint is threaded into the other box of the coupling. The procedure described for connecting tubular sections is commonly referred to as "making up" a connection, while the reverse procedure of disconnecting tubular sections is referred to as "breaking out" the connection. [0005] The pin end of a third casing joint is connected in similar fashion to the other box end of the second casing joint, and so on until the casing string has been made up to a desired length. After the complete casing string has been installed in the wellbore, it is cemented into place by introducing a cementitious slurry into the annular space between the outer surface of the casing string and the wellbore. [0006] Wells are most commonly drilled using the drilling and casing procedures described above. However, it has become increasingly common for wells to be drilled using casing as the drill string, with the drill bit being connected to the lower end of the casing string (a procedure commonly referred to as "casing drilling" or "drilling with casing"). When the wellbore reaches the target formation, the casing string is simply cemented into place. This procedure necessitates leaving the drill bit underground, but the cost of the drill bit is outweighed by savings in both time and money by not needing to use a separate drill string and withdraw it from the wellbore, and then running casing into the wellbore in a separate operation. [0007] Oil and gas wells throughout the world have experienced casing connection failures, due to the tensile, compressive, torsional, and/or flexural (i.e. , bending) capacities of the connections being exceeded. This has been a particular problem in deviated (i.e., non- vertical) wells drilled using directional drilling techniques and requiring comparatively sharp bends (or "doglegs"), and in cases where the subsurface formation is susceptible to movement. Such movement may be induced by hydrocarbon production processes entailing steam injection into the formation, inducing tensile, compressive, torsional, and/or flexural stresses in the casing as a result. One factor influencing such casing connection failures is that the threading on pipe sections commonly used for casing tends to be less robust than the threading on typical drill pipe. Over the years, many alternative connection designs and concepts have been introduced to address these problems, but even "premium" thread designs will fail under severe conditions as mentioned above. In addition, it is notable that casing connection failures can occur notwithstanding the fact that the casing has been cemented into the wellbore. [0008] The foregoing problems are discussed in further detail in the paragraphs that follow, in the context of different drilling scenarios. [0009] Directional Drilling [0010] When drilling deviated or directional wells, corrections to the direction of the drill bit are continually being made. When abrupt corrections are made to get the drill bit back on track, this can create a severe dogleg in the well. A dogleg refers to an angular change in a drilled wellbore (e.g., a 30° dogleg refers to a total angular change or deflection of 30° in the direction or orientation of the wellbore, as observed or measured over a 100-foot length of the wellbore). When such extreme bends occur in the wellbore, the casing must pass through this dogleg when the casing is run into the wellbore. The casing string and all of the connections between individual casing joints must pass through this bend without being structurally overstressed. Once the casing is installed, one or more casing connections may reside within this dogleg. This will induce structural stresses (mostly bending moment) in the casing and connections in the dogleg area, tending to separate the mating threads in each connection. This can result in drastic reduction in the sealing capability of these threaded connections, and in many cases complete connection failure can occur. [0011] Steam Injection [0012] Many new well designs, particularly those developed or intended for use in the production of heavy oil or extraction of bitumen from oil sands (or "tar sands" as they are sometimes called), require injection of steam into the hydrocarbon-bearing subsoil formations in order to reduce the viscosity of the oil or bitumen so that it can flow to the surface. One well-known example of this is the steam-assisted gravity drainage process, or SAGD. Such injection of steam can induce localized shifting movements in the formation, which can create tensile, compressive, and/or bending loads acting on the casing connections. [0013] Shifting of subsoil formations is most severe in deviated sections of a well as compared to horizontal sections of the well, but failures can and do occur in both cases. As steam enters and permeates a subsoil mass (such as within an oil sands formation), it pressurizes the formation, thereby creating a balloon-like effect that exerts pressure against adjacent formation zones above, below, and laterally adjacent to the steamed zone. If there is a casing string cemented through adjacent formation zones subject to different pressures, the casing can be subject to extreme structural stresses leading to failure of the casing connections. [0014] Due to the risk of this type of casing failure, many wells will terminate an intermediate casing string above the steamed zone (directional or build section) and continue the horizontal section with another casing string equipped with a movable casing hanger. However, the bottom portion of the intermediate casing will still be subject to steam- induced movement. As a formation expands in volume (due to steam injection) or contracts in volume (due to steam cooling), it will induce loads acting laterally (i.e., transversely) against the casing, thus inducing localized bending stresses in the casing. Such lateral or transverse loads acting on a casing connection will force the pin end of one casing joint to bend within the coupling, thus forcing the male pin threads to separate from the mating female threads on the coupling box, thus reducing the thread contact surface area and, therefore, the effective sealing area of the connection. [0015] In some cases the practical consequences of such lateral loading on a casing connection will not be severe, such as when the casing is under axial compression, which will partially or wholly counteract bending-induced tension stresses in the connections. However, when exacerbated by axial tension in the casing, the weakened connection can either leak or fail (i.e., disconnect) completely. [0016] When casing strings are subjected to a combination of formation movements (e.g., steam-induced) and wellbore builds (e.g., doglegs), connection failure is more likely. Most severe doglegs appear to occur towards the bottom of the build section of the well, as target requirements are met. As well, most formation movement tends to occur in this same area. When lateral loads are applied to a casing connection, the pin end will bend and separate the pin threads from the box threads in the coupling. Since there is nothing to hold the two mating threads together, the strength of the connection is weakened due to a decrease in thread contact area. [0017] Examples of known couplings that have been proposed are described in the following US patents: 4,712,815; 6,609,735; 7,347,459; and 8, 167,340. [0018] There is a need for improved ways of connecting tubular sections, and for connecting casing joints in particular, to minimize loss of sealing capacity and/or structural strength when the tubular connections are subjected to lateral loads, such as due to steam- induced formation movements, and other types of loads arising due to wellbore configuration (e.g., doglegs in deviated wellbores) or other operational factors. SUMMARY OF THE INVENTION [0019] In general, the invention provides a coupling and coupling method for joining two pipes, such as casing joints. In one aspect, the coupling of the invention includes a groove on its outer surface which provides the coupling with a degree of flexion in response to lateral stresses or loads as described above. Such flexion reduces the transmission of the lateral stresses or loads to the threads connecting the coupling to the pipes. [0020] Thus, in one aspect, the invention provides a pipe coupling for connecting two pipe segments, each of the pipe segments including a pin end comprising a section having a threaded portion provided on the outer surface thereof, the coupling comprising:

- a generally tubular body having a bore extending therethrough, an outer surface, an inner surface, and opposed first and second ends;

- the inner surface of each of the first and second ends including threaded portions adapted to engage the threaded portions of a corresponding pipe segment;

- the outer surface of the coupling including a groove, the groove comprising a region of the coupling having a reduced outer diameter located generally centrally along the length of the coupling. BRIEF DESCRIPTION OF THE DRAWINGS [0021] The features of the invention will become more apparent in the following detailed description in which reference is made to the appended drawings wherein: [0022] Figure 1 is a longitudinal cross sectional view of a coupling according to one aspect of the invention, in combination with a pipe. [0023] Figure 2 is a longitudinal cross sectional view of a coupling according to one aspect of the invention, in combination with two pipes. [0024] Figure 3 is a partial cross sectional view of section III of Figure 2. [0025] Figure 4A is an end cross sectional view of a coupling according to one aspect of the invention. [0026] Figure 4B is a longitudinal cross sectional view of the coupling of Figure 4A taken along line IVA-IVA. DETAILED DESCRIPTION OF THE INVENTION [0027] As illustrated in the accompanying Figures 1 and 2, the present disclosure teaches embodiments of a generally cylindrical coupling having a groove formed into the outer circumference of the coupling in a central region of the coupling. In the illustrated embodiments, the coupling is symmetrical about a transverse plane passing through the groove. However, although this symmetry is convenient for purposes of both manufacture and use of the coupling, it is not essential, and such symmetry could be absent from alternative embodiments without departing from the scope of the present disclosure. [0028] Figure 1 schematically illustrates a coupling 20 according to one aspect of the invention. As illustrated, the coupling 20 includes a central bore 22 extending between coupling ends 21. Central bore 22 has a cylindrical bore section 24 adjacent each coupling end 21 , and, typically, each cylindrical bore section 24 transitions with a tapered bore section 26 decreasing in diameter as it progresses inward within coupling 20, with tapered bore section 26 having tapered box threads 27. For convenience of illustration, the threads 27 are not shown in Figure 1. [0029] In a preferred embodiment of the invention, a cylindrical ring 28 is provided or formed into coupling 20, generally in a central region between tapered bore sections 26. Cylindrical ring 28 preferably will have an inside diameter corresponding to the bore of a tubular member 10 being connected using coupling 20. As illustrated in Figure 1 , the axial length and position of cylindrical ring 28 is selected such that when pin end 12 of tubular member 10 is threaded into coupling 20, it will shoulder tightly against the face of cylindrical ring 28 to preferably form a fluid-tight metal-to-metal seal. Having regard to this functionality, cylindrical ring 28 may be alternatively referred to as a pin seal ring 28. As will be understood in reviewing the present description the presence of the pin seal ring 28 on the coupling is preferred. In a preferred embodiment, the pin seal ring 28 is formed as part of the inner surface of the coupling 20. In another embodiment, the pin seal ring 28 may be a separate element provided or placed within the bore of the coupling 20. [0030] Some prior art connection designs rely on thread-to-thread contact to provide a fluid-tight seal within the coupling, but such connections have often been found to lose seal effectiveness when subjected to transverse loadings that cause localized or worse separation of the pin and box threads. However, with the preferred structure of the coupling 20 as illustrated in Figure 1 , the pin end 12 is preferably shouldered against pin seal ring 28, which would minimize the chance of losing an effective seal. [0031] Optionally, in one aspect of the invention, the coupling 20 may be provided with a centralizer, or load deflection ring 30. The centralizer ring 30 serves to enhance and/or maintain the effectiveness of the seal between the end of the pin and the pin seal ring 28, particularly under transverse loading conditions. In one aspect, the bore 22 of the coupling 20 is preferably machined to form the centralizer ring 30 adjacent to each end of coupling 20, outboard of box threads 27. In one aspect of the invention, the inside diameter of each centralizer ring 30 is preferably sized to provide a close-tolerance fit to the outside diameter of tubular member 10. This further ensures that pin end 12 of tubular member 10 will remain square to the face of pin seal ring 28 notwithstanding external loadings inducing bending in the tubing string. In addition, the centralizer ring 30 serves to minimize or reduce the magnitude of the stress induced on the threads due to lateral stresses or loads being applied to the pipe and coupling assembly. In particular, as the pin bends (or tries to bend) within coupling 20, centralizer ring 30 will react against the outer surface of tubular member 10 and thus reduce the amount of deformation of the pin end within the coupling 20, which would otherwise induce thread separation of the pin threads 14 and box threads 27 within the coupling 20. [0032] To promote even greater effectiveness of centralizer rings 30 for this purpose, each end of tubular member 10 may be machined in a peripheral region adjacent to the pin threads to ensure a precise fit within centralizer rings 30, thereby allowing for a degree of cross-sectional out-of-roundness that can be exhibited by conventionally manufactured pipe. [0033] Because centralizer rings 30 will hold pin threads 14 and box threads 27 concentrically together within coupling 20, axial tension and compression capacity through the coupling will not be reduced as would be the case in a coupling subject to thread separation induced by bending moments and transverse forces induced in or exerted against the tubular string. [0034] An additional benefit of centralizer rings 30, when provided, is that they can serve as a stabbing guide during connection make-up operations. [0035] In a preferred embodiment of the invention, the coupling 20 is formed with a groove 40 in a generally central area of the coupling. In a preferred embodiment, the location of the groove 40 along the longitudinal axis of the coupling 20 corresponds to the location of pin seal ring 28, when such pin seal ring 28 is provided. In general, the groove 40 (which may be alternatively referred to as a bending ring or a flex groove) is a region of reduced outer diameter that is formed on the outer surface of the coupling 20. Although the term "groove" is used to describe groove 40, it is not intended to limit the width, depth or other dimension thereof. Thus, in one aspect, the groove 40 may equally be described as a depression that is formed into the outer surface of coupling 20. As a result of the groove 40, the coupling 20 will be understood to have a reduced structural stiffness at such location. Thus, the groove 40 will tend to serve as a preferential flex point or pivot point in response to bending stresses induced in the connection, thus further reducing any tendency for thread separation within the connection. [0036] As will be understood by persons skilled in the art, the groove 40 is designed and configured to ensure that coupling 20 as a whole maintains sufficient structural strength to resist anticipated in-service loadings. In some cases this may require the cross-section through groove 40 to have the same axial compression and tension capacity as the tubular members 10 being connected, but this will not necessarily be the case (as loading conditions may vary, and in some cases the structural strength of the selected tubular members may significantly exceed design requirements). [0037] Coupling 20 is compatible with or can be adapted to use any known thread design used to connect oilfield tubulars. Many existing coupling designs are configured to provide for nose-to-nose sealing of the pin ends of the tubular members being connected when they are screwed into the coupling. Analogous seals will be effected using couplings in accordance with the present disclosure, but instead of the two pin noses sealing against each other, they will seal against pin seal ring 28. Because the location of pin seal ring 28 in relation to box threads 27 can be precisely controlled during the manufacture of coupling 20, sealing problems arising from inaccurate make-up of conventional couplings are prevented. For example, if a conventional coupling is screwed too far onto the pin end of a first tubular member, the pin end of a second tubular screwed into the other box of the coupling may abut the pin end of the first tubular before the tapered pin threads of the second tubular have fully engaged the mating box threads in the coupling. This problem is mitigated using couplings in accordance with the present disclosure. [0038] In alternative embodiments, coupling 20 can be manufactured without pin seal ring 28, to accommodate connections that do not require a pin nose seal. [0039] Figure 2 is a small-scale cross-section through a casing connection made up according to another aspect of a coupling of the invention. Figure 3 is an enlarged and more extensively detailed cross-section through a portion of the connection shown in Figure 2. In the embodiment of coupling 20 shown in Figure 3, pin seal ring 28 is formed with undercut, bevelled seal faces 29 to receive mating bevelled seal faces 16 formed on the pin ends of tubular members 10, such that when the pin ends of tubular members 10 shoulder against corresponding seal faces 29, they are effectively locked under and into the undercut seal faces 29, thus further preventing radial separation of pin threads 14 from box threads 27. [0040] In a preferred embodiment of the invention, and as more clearly illustrated in Figure 3, the seal faces 29 of the pin seal ring 28 are provided in such a manner as to result in in opposing seal faces 29, that is the seal faces on opposite sides of the pin seal ring 28, from being angled towards each other in a radial direction extending from the inner surface of the coupling 20 to the outer surface thereof. In other words, the free end of the pin seal ring 28, i.e. the end extending towards the central axis of the coupling 20, has a larger width than the opposite end thereof. As described above and as illustrated in Figure 3, the bevel provided on the end of the pin is oriented complementary to that of the pin seal ring 28. Thus, as would be understood by persons skilled in the art, upon application of a

compressive stress on the pipe 10, the end of the pin portion thereof is forced against the pin seal ring 28, and, due to the preferred orientation of the bevels as described above, the lumen of the pin portion is prevented from collapsing or narrowing. [0041] Preferably, and for the same purpose as discussed above, a region of bore 22 at each end of coupling 20, between centralizer ring 30 and box threads 27, may be machined as shown to form a similar undercut bevelled seal face 32 for engagement with a mating bevelled seal face machined into tubular member 10 adjacent to pin threads 14 as shown in Figure 3. [0042] Figures 4A and 4B illustrates a casing coupling 120 in accordance with another aspect of the invention. Reference numbers used in Figures 4A and 4B correspond to those used in Figures 1 , 2, and 3, and indicate similar or analogous features. In the embodiment shown in Figures 4A and 4B, the central portion forming pin seal ring 28 and corresponding to groove 40 is lengthened as compared to the other illustrated embodiments, thus increasing the distance between the pin ends of the tubular members shouldering against pin seal ring 28 in a connection made up using coupling 120, and promoting increased deflection and more favorable distribution of flexural stresses in coupling 120 when the tubular string is subject to lateral loading. [0043] In summary, when lateral loads are applied to a casing string made up with couplings in accordance with the present disclosure, the amount of stress transferred to the threaded connections between the couplings and the engaged pins is minimized, as compared to a string using conventional couplings. The optional centralizer rings located at the ends of the couplings will assist in keeping the pin threads in engagement with the box threads of the coupling. This will decrease thread separation, thus maintaining the connection's tensile, compressive, and torsional load capacities. The centralizer rings can also serve as stabbing guides during connection make-up. [0044] The optional pin seal ring 28 provided on the coupling will facilitate an effective metal-to-metal seal with the pin ends of the casing joints or other tubular members being connected using the coupling, without relying on thread sealing. By virtue of the centralizer rings, this metal-to-metal nose end seal will remain effective even in the event of some thread separation within the connection. [0045] The groove 40 located at the center of the coupling will tend to act as a flex point in response to induced bending stresses, thereby reducing bending-induced deflections on the pin and box connections, which might otherwise cause thread separation within the connection, but without reducing the structural strength of the connection below design requirements. [0046] In one aspect, the integrity of thread engagement within the connection under stresses induced by external loads can be further enhanced by forming the coupling and the pin ends of the tubular members being connected to provide locking undercut seal faces to the pin seal ring and/or adjacent to the ends of the coupling. [0047] It will be readily appreciated by those skilled in the art that various modifications of the disclosed embodiments may be devised without departing from the scope and teaching of the present disclosure, including modifications which may use equivalent structures or materials hereafter conceived or developed. It is to be especially understood that the present disclosure is not intended to be limited to any described or illustrated embodiment, and that the substitution of a variant of a disclosed or claimed element or feature, without any substantial resultant change in operation or functionality, will not constitute a departure from the scope of the disclosure. It is also to be appreciated that the different teachings of the embodiments described and discussed herein may be employed separately or in any suitable combination to produce desired results. [0048] In this patent document, any form of the word "comprise" is to be understood in its non-limiting sense to mean that any item following such word is included, but items not specifically mentioned are not excluded. A reference to an element by the indefinite article "a" does not exclude the possibility that more than one such element is present, unless the context clearly requires that there be one and only one such element. Any use of any form of the terms "connect", "engage", "couple", "attach", or any other term describing an interaction between elements is not meant to limit the interaction to direct interaction between the subject elements, and may also include indirect interaction between the elements such as through secondary or intermediary structure. Relational terms such as "parallel",

"perpendicular", "coincident", "intersecting", and "equidistant" are not intended to denote or require absolute mathematical or geometrical precision. Accordingly, such terms are to be understood as denoting or requiring substantial precision only (e.g., "substantially parallel") unless the context clearly requires otherwise. As used in this document, the terms "typical" and "typically" are used in the sense of representative or common usage or practice, and are not to be understood as implying essentiality or invariability.