Patent Application
20060152000
The present invention relates to methods and connections for securing together the ends of threaded tubular bodies to produce a structurally strong and leak resistant joinder of such tubular bodies. More specifically, the present invention relates to a threaded coupling and pin assembly, and associated method, for securing together the threaded ends of tubular pipe bodies such that the engaged pin and coupling threads will form a secure mechanical engagement and a high-pressure, leak resistant thread seal.
Tubular pipe bodies used in the construction of oil and gas wells are typically secured together at their ends by threaded connectors that employ a thread and seal design intended to withstand the structural forces acting between the threaded components as well as to prevent the flow of high-pressure well fluids through the engaged threads of the connection. In many of the thread designs used to secure the pipe ends together, the threads function both as mechanical engagement structures and as pressure sealing structures.
Connections in which sealing is not an objective of the thread design often achieve sealing with supplementary seal rings and/or metal-to-metal seals that are constructed as integral components of the connection. Connections incorporating these seal enhancing features are often referred to as “premium connections”. Premium connection designs can require the use of couplings with coupling walls thicker than those of the coupling stock used to make couplings meeting specifications of the American Petroleum Institute (API). These heavier walled couplings are required to provide torque shoulders and to maintain a high coupling rigidity that aides in accommodating stress in the coupling during extreme internal/external pressures or tensile/compressive loads. When conditions permit, it is usually desirable to avoid as many as possible of the added features of these premium connectors, such as seal rings, metal-to-metal seals and heavy wall coupling stock because of the added cost and complexity associated with their use.
The coupling stock having dimensions and specifications such as from which API couplings are made is herein referred to as “standard coupling stock”. Standard coupling stock typically has a thinner wall construction and is more readily available than that required for many premium connections. A connection that may be built using standard coupling stock is less expensive to manufacture than one requiring coupling stock with a wall thickness greater than that found in standard coupling stock. Even within the range of standard coupling stock, the cost savings may be maximized by using the thinnest wall standard coupling stock possible because of the difference in steel content.
The following table lists the specifications for “standard coupling stock” used in the construction of API couplings for typical pipe sizes:
In addition to being more costly, thick wall coupling designs reduce the central clearance through the connector. The nominal outside diameter of all coupling stock is the same for any given pipe size to be used in a conventional string design. Coupling strength may be obtained by increasing the wall thickness toward the center of the coupling, however such strengthening results in reducing the clearance through the coupling. Reduced coupling clearance is usually associated with an undesirable reduction in the clearance through the pipe engaged in the coupling.
Recently, the industry has required that threaded pipe connections meet certain new performance and testing criteria. As a part of this requirement, the connectors are subjected to rigorous testing for qualification to new industry standards. One such industry standard is the International Organization for Standardization (ISO) 13679 specification. ISO 13679 is an international specification outlining the procedures to be used in testing casing and tubing connections for the oil and natural gas industries. This specification was developed to more realistically validate performance parameters by testing extreme service conditions and loads that the tubing and casing connections see during use. The ISO 13679 specification results, in part, from a determination that design changes in existing connections may be required to attain useful service load envelopes for connectors. One aspect of the specification requires that the connection be assembled in such a way that at the final make-up, there is a preloading of the connector axially. It has been found that this axial preload can be advantageous in improving pressure capacity as the axial load on the connector goes from tension to compression.
The torque force applying the pre-load is frequently referred to as a “delta torque”. Delta torque may be defined as that part of the final make-up torque that is induced into axially interfering components of the pin and box connection after resolving the radial thread interference occurring during the make-up.
In general, preloading a connection requires that the connection be assembled with a high torque force stored in the connection. The upper boundary on torque is usually determined by the capacity of the hydraulic power tong that is used to apply the torque to the connection at the well site. The available make-up torque at the well site must be distributed to achieve an optimum balance of radial and axial forces within the connection. This requires imposing radial interference levels in the connection that are sufficient to energize the threads for sealing while conserving tong torque capacity for applying the required final torque to achieve a desired axial preload.
When axial compression loads are applied to the connection, either during testing or during use in the well, it is possible that the axial preload force will be exceeded. When this occurs, a cascading “micro-movement” effect occurs down the threads as load flanks are disengaged and stab flank clearances are reduced and eventually engaged. This micro-movement disturbs the thread compound in conventional thread designs and can lead to the initiation of leaks. Increasing the final torque is not necessarily an economical option for increasing the preload in an attempt to overcome this susceptibility to leakage. In addition to the fact that the rig tongs may often lack capacity to add sufficient final torque, very high torque values applied to the pipe increase the opportunity to damage the pipe body and/or the pipe threads.
The prior art has suggested a variety of different assembly methods and thread designs in an effort to prevent leakage in connectors that are subjected to high axial tension and compressive loads. In addition to preloading the connection, the prior art has suggested specific connection thread configurations for improving the seal between engaged threaded connectors. The prior art specifically teaches thread designs that employ thread seals for sealing internal and external pressure in an environment subjected to axial tension and compression loads. Certain of these prior art designs feature negative load flank threads, diametrical (radial) interference levels, torque shoulders and special thread clearances to enhance a thread seal. Biased tapers between the pin and box to aid in the seal created by the thread compound-to-thread profile interface are also features of prior art connections. Designs incorporating a torque shoulder that is achieve by abutting the pin noses of the two pipe joints engaged within a coupling have also been proposed. Some of the prior art designs have also combined abutting pin nose shoulders with one or more of the other seal enhancing features known in the prior art designs. The benefits of constructing a connection within the limits of readily available and economic American Petroleum Institute (API) coupling stock are also well recognized by prior art designers. None of the prior art, however, teaches or suggests an easily manufactured, economical connection that provides a satisfactory thread seal in conventional coupling stock within an environment subjected to high axial stress loads with final make-up torques that are within the working capacity of the power tongs commonly used to assemble connections at the well site.
The present invention successfully incorporates the benefits of individual prior art design features in a new assembly within a single connection to achieve an economic, effective high-pressure thread seal that is inexpensively manufactured using standard coupling stock and assembled with final make-up torque values within the capacity of rig site tongs and having the ability to function leak free within testing or working environments in which the connection is exposed to high axial stress loads. These beneficial results are achieved without the benefit of a metal-to-metal seal or a supplementary O-ring seal.
A connection of the present invention may have as much as three times the delta torque of prior art designs due in part to a lower shoulder torque and in part to a design permitting a higher final torque.
The preferred embodiment of the present invention is a tapered, threaded and coupled connector with an equivalent height (same height on pin and box threads), negative load flank, biased tapered thread form with abutting pins made-up within a coupling constructed from standard coupling stock. The design of the present invention meets the internal/external pressure performance requirements of industry standards with capabilities equal to or exceeding those of “premium” connections, with a coupling constructed from standard coupling stock.
During the initial portion of the make-up of a shouldering connection, the torque energy is consumed in friction losses and in deflecting connection parts in the radial direction. The torque energy consumed by these friction losses and radial deflections reduces the final torque force available for preloading the connector axially. The design of the present invention balances radial interference and preload torque to maintain sealability at final torques that are within the capacity of conventional field make-up equipment. This objective is attained in part with a thread design that reduces the torque required to attain shoulder contact.
The design of the present invention prevents leakage caused by micro-movement within a coupling constructed of standard coupling stock.
The coupling design of the present invention decreases the shouldering torque, which permits a larger delta torque at final make-up without necessarily requiring a higher final make-up torque.
The design of the present invention prevents leakage caused by micro-movement with a thread design that permits applying higher final torques, within the capacity of existing pipe make-up equipment at the well site, in an assembly constructed with standard coupling stock.
The beneficial results of the present invention are obtained, in part, from combining coupling rigidity, optimized pin nose thickness and large delta torques without the need for high final torque values that can exceed the capacity of the field make-up equipment. The wall at the center of the standard coupling stock is maintained at or near its nominal thickness to optimize radial stability with the first engaged pin end threads at the full make-up position to thereby improve the sealing capacity of the connection. All of these results are achieved in a connection using standard coupling stock.
The present invention relates, in part, to a coupling and pin thread configuration, and associated method, for securing together the threaded ends of tubular pipe bodies such that the engaged pin and coupling threads will form a secure mechanical engagement and a high-pressure resistant thread seal and wherein such seal is achieved with standard coupling stock materials and without necessity for the use of a metal-to-metal seal.
In the preferred form of the invention, the API coupling stock for buttress casing pipe threads is internally machined with tapered threads that extend to the center of the coupling from each coupling end. The pin ends of the tube joints to be secured by the coupling are machined with external threads that make-up with the coupling threads such that the pin end faces engage at the center of the coupling to provide mutual torque shoulders before the final make-up torque is applied to the connection. The preferred embodiment of the design of the present invention maximizes the available coupling thickness (at the center of the coupling) by running out the box threads completely down to the internal diameter of standard API Buttress coupling stock. This feature not only aids in coupling rigidity but also provides a significant cost savings attributable to the ease and simplicity of machining the connection. The use of the thinnest walled standard coupling stock (buttress) reduces cost and permits the formation of the thickest pin wall ends for shouldering while maintaining the largest possible central opening through the connected pins and coupling of the connector.
In a preferred form of the invention, sealing between the pin and the coupling is enhanced by constructing the nominal thread height of the pin and box threads to substantially the same height and by constructing the stab flank clearance to the minimum possible, within manufacturing tolerances, that produces non-galling interference between the thread stab flanks during make-up. A negative load flank thread geometry is provided in the mating pin and coupling threads to impose radially compressive forces between the pins and coupling during the latter part of the make-up. The application of torque to the connection after engagement of the pin faces effects an axial preload at the final make-up torque that prevents axial separation of the engaged pin and box threads when the connection is placed in tension.
In a preferred form of the invention, the external threads of each pin are removed from a portion of the pin adjacent the pin face to form a pin nose. The non-threaded pin noses absorb and store compressive, preload forces applied during the torquing together of the two pin ends within the coupling. The ratio of the pin nose wall thickness to the pin nose length is maintained as high as possible to permit maximum axial interference during the application of torque to the connection.
During the compressive torquing that produces the preload, the absence of engaged external threads on the pin noses enhances the magnitude of axial compressive, spring-back forces preserved in the axially compressed pin ends. While optimization of force storage within the pin noses during the preload torquing might also be achieved by removal of a portion of the central coupling threads rather than by removal of the pin nose threads, the machining process for removing pin nose threads achieves the same effect and is simpler and less expensive than that required for removing the central coupling threads, further contributing to the ease of manufacture and reduced cost of the connection.
Removal of the pin nose threads rather than the central coupling threads also ensures that the coupling wall thickness at the center of the coupling is maintained substantially the same as the nominal wall thickness of standard coupling stock, thereby maximizing the remaining wall thickness at the center of the coupling to retain maximum resistance to radial strain resulting from applied forces and pressures when the pipe string is being tested or used. It will be appreciated that, if desired, the internal central area of the coupling may also be left unthreaded in the area that is to over lap the reduced diameter pin noses to thereby prevent engagement of the pin noses with the coupling while retaining the maximum wall volume at the center of the standard coupling stock blank.
Creation of mutual torque shoulders by the engagement of abutting pin nose end faces eliminates the need for forming a torque shoulder on the internal surface of the coupling, a design which would require a thicker, non-standard coupling stock to maintain desired capacities. The abutting pin nose faces, which define the torque shoulder area, are maximized by selecting the optimum pin-nose-length to torque-shoulder area ratio consistent with the thread taper, thread interference and coupling thickness. Maximizing the pin nose wall thickness also maximizes the allowable torque for preloading the connection. The pin nose thickness is limited by factors such as thread taper, interference and coupling thickness. In determining adequate pin nose thickness, upper and lower limits on the pin-nose length to torque-shoulder area ratio have been determined to optimize performance of the connection of the present invention. The optimized ratio is a measure of pin nose stiffness to allowable final torque. The ratio decreases with increasing wall thickness within a design outside diameter range. Use of the relatively thin wall buttress standard coupling stock permits maximizing the pin nose thickness.
A preferred lower limit design for the pin-nose-length/torque shoulder area ratio of the present invention is 0.025. With the pin nose length being set as two times the thread lead, heavier walled designs would require longer pin nose lengths to maintain an adequate ratio for preload during make-up. The preferred upper limit for the pin-nose-length/torque shoulder area ratio of the present invention is 0.35. The minimum pin nose thickness of the present invention is thus limited as required to provide sufficient preload during make-up.
The design of the present invention provides nearly three times the delta torque of conventional connections, due in part to a thread design that permits a lower shoulder torque and higher final torque as compared with some conventional connections.
The thread dimensions, configuration and pin-face preloading cooperate to form a substantially rigid, non-moving connection that does not disturb the seal formed between the pipe dope and threads to thus maintain a seal between the pins and the coupling without need for a metal-to-metal seal, despite changes in axial loading of the connection experienced during testing or actual usage.
The foregoing features, objectives and advantages of the present invention, as well as others, will be more readily understood and appreciated by reference to the following drawings, specification and claims.
The design of the prior art connection of
From a consideration of
While the forgoing specification, drawings and the following claims are considered to be illustrative of the manner of making and using the present invention, it will be appreciated that various modifications to the methods and connections of the invention may be made without departing from the spirit and scope of the invention. Thus, by way of example rather than limitation, the threads of the connection may be dovetail or double hook or wedge threads rather than the negative load flank threads specifically described. Similarly, various other thread forms may be used with less than all of the individual features or advantages described for the connections and methods of the present invention. Accordingly, it will be understood that the present invention is intended to be limited only by the definitions of the following claims.
