Threaded pipe connection with improved seal

Abstract

An improved coupling for joining together threaded tubular sections is disclosed. The coupling includes a seal seat between the threaded areas of the coupling and a seal assembly positioned on the seal seat. The seal assembly engages the end of the threaded tubular section and forms an axial seal without application of radial stress in the coupling. The seal assembly has an elastomeric component and a rigid component.

Description

BACKGROUND OF INVENTION

[0001] 1. Field of Invention

[0002] The present invention relates generally to threaded connections for joining sections of tubing, casing, line pipe or other tubular sections forming long strings of pipe used in drilling and producing oil and gas. More particularly, the invention relates to a threaded connection having a sealing mechanism for maintaining leak resistance against liquid and gas pressures.

[0003] 2. Description of Related Art

[0004] In the search for oil and gas reserves, major oil companies have increasingly moved to deep water drilling environments. Early success in deep water has demonstrated the existence of large, commercially viable oil and gas fields. Technological advancements now permit routine drilling in deeper offshore waters and even further below the sea floor than just 5 years ago. Deeper drilling is also more common in onshore wells. However, the deep-water drilling environment and deeper wells have presented new industry challenges that make these endeavors both risky and expensive. High rig costs have generated demands for tubular connections that assemble quickly with positive make-up indication to reduce the time for running an entire tubular string.

[0005] Tubular strings used for drilling and production of oil and gas consist of individual sections commonly 40-ft in length. However, some operators use specially rolled tubulars up to 60-ft in length to reduce the number of connections in a string thereby reducing overall string costs. Manufacturing, transportation, and handling constraints limit the maximum practical length of individual tubular sections.

[0006] Oil and gas wells consist of a series of tubular strings. Large diameter tubulars are installed near the surface or sea floor and successively smaller diameter tubular strings telescope inside until the target depth is attained. Internal and external (collapse) pressures increase with depth and smaller diameter tubulars have higher resistance to these loads. Thus, all tubular sections have setting depth limits based on diameter to thickness ratios and material strength properties. In general, smaller diameter tubulars can be set to greater depths.

[0007] Tubular strings are designated as drive, conductor, surface, intermediate, protective, liner and/or production tubulars. Strings commonly known as tubing are the conduits through which oil and gas actually flows to the surface. Those skilled in oil and gas drilling and production understand the application behind each of these tubular designations.

[0008] Tubular sections are prepared for running with connections at each end. One end, known as the pin end, is externally threaded. The other end is internally threaded and is known as the box end. The box end may be internally threaded onto the tubular section or, the tubular section may be externally threaded with an internally threaded coupling screwed onto the end.

[0009] On the rig, each tubular section is raised in the derrick, lowered below the deck and held at the floor with an internally threaded member facing up. The next tubular section is raised into the derrick and stabbed into the waiting internally threaded member. After successful stabbing, the two tubular sections are screwed together. All assembled tubular sections are then lowered in preparation for the next tubular section. The process continues, one tubular section at a time, until the string reaches the target setting depth.

[0010] The standard methods of joining lengths of tubular sections are by either threaded and coupled or integral joint connections. Threaded and coupled connections use a coupling that is a short, tubular piece, usually consisting of the same material as the tubular section. Integral joint connections, also known as flush or super flush, are machined directly on the tubular section and do not require a coupling. These integral joint connections are often used when annular clearance between successive riser strings is reduced, and/or there is restricted wellbore clearance that is anticipated to cause problems in reaching the desired setting depth. While integral joint connections are useful in restricted wellbore clearance situations and have demonstrated good performance resisting high pressure, all have relatively low axial tension resistance. Installation of integral joint connections follows the same basic procedures as threaded and coupled connections.

[0011] Threaded couplings often are assembled with various types of lubricants commonly referred to as thread compounds or pipe dope. In connections that shoulder out, excess thread compound trapped within the threads can exert significant hydraulic pressure in mating thread elements. Such hydraulic pressure can impart additional hoop stresses that may degrade connection performance.

[0012] The oil industry, through the American Petroleum Institute, has standardized sizes, weights and material grades of tubular sections and connections, as set forth in American Petroleum Institute Specification Standards 5B and 5CT. Through standardization, a worldwide service infrastructure has evolved for the manufacture and use of these tubular products. Economies of scale through standardization have provided: (a) low-cost, worldwide supplies, (b) universal inspection and measurement methods, (c) standard gauges and handling tools, and (d) readily available accessories. Connections for joining tubular sections of the same size and weight are completely interchangeable. Further, there exists a set of standard assembly procedures for use in the field as described in American Petroleum Institute Recommended Practice 5C1.

[0013] Proper make-up of industry standard API threaded couplings occurs when the coupling, or box member, covers a reference mark on the pin member, which generally consists of a ⅜-in. equilateral triangle die stamp or other similar impression on the pin member. Due to allowable thread tolerances, however, variability exists in the make-up. As a result, the box member's position at proper make-up is not always the same relative to the mark on the pin member. For example, if the box member is machined on the small side of the allowable tolerance band and the mating pin member is machined on the large side of the allowable tolerance band, proper make-up occurs when the coupling covers the base of the triangle stamp. At another tolerance extreme, a connection with a large box and a small pin would make up when the coupling reaches the apex and covers most (or all) of the triangle stamp. The range of tolerances of the pin and box members causes considerable uncertainty by rig personnel trying to determine whether connections are properly assembled.

[0014] Standard API threaded connections use a thread interference sealing mechanism. Interference develops when an externally threaded pin member is screwed into an internally threaded box member. The OD of the pin is slightly larger than the ID of the box. As the two members are joined, the box deflects outward and the pin deflects inward as the two members attempt to occupy the same space. The amount of relative deflection of the two members is a function of the cross-sectional balance between the pin and box. Interference equals the difference of box deflection and pin deflection. Since both members are threaded on a taper, interference is also a function of axial advancement of the pin into the box.

[0015] Standard API threaded connections also have demonstrated unsatisfactory field performance in both normal and critical wells. A history of leaks in casing strings with Standard API connections at pressures around 4,000 psi to 5,000 psi has resulted in a general lack of confidence in thread interference seals by the industry. Some of the other problems with these connections include difficult stabbing during make-up (common with large outer diameter tubular products), thread damage (or galling) when disassembled in the field, and string parting. Each of these problems results in significant repair and operating costs. In-service failures are potentially hazardous to the environment and pose serious safety risks to onsite personnel.

[0016] Efforts to improve stabbing and reduce thread galling have included modifications to the thread form, special thread surface treatments including abrasive blasting and application of anti-galling agents. String parting has been addressed by structurally enhancing the coupling body or through the use of specially designed “hook” type threads. While these approaches have abandoned the industry standard thread form, connections with these features are commonly used to avoid these common problems.

[0017] Efforts to solve the leak problem have included modified thread forms, elastomeric seals, metal-to-metal seals, and a combination of special tolerances and make-up procedures. Some designs have incorporated one or more of these approaches and claim to have multiple, redundant sealing mechanisms for enhanced connection performance. However, these approaches are often expensive and impractical because they abandon use of industry standard thread forms and/or require special accessory equipment, handling and assembly procedures, and other operational changes.

[0018] Deviations from standard API thread forms are typically not economical because they fail to take advantage of the worldwide infrastructure for service, interchangeability, handling and accessories. Additionally, alternate connections require specially threaded accessory items that require extra logistics and significant additional cost. In general, the industry will readily embrace any connection that meets the required performance criteria while allowing the use of the industry standard thread form.

[0019] In the past, elastomeric seals have been used within mating thread elements of various connections used to join threaded tubular sections. Additionally, various other seals that impart additional radial hoop stresses in the box member when energized also have been used. As a result of added hoop stresses from the seals, these connections have performance disadvantages. Added radial hoop stresses can ultimately cause unexpected connection failure.

[0020] Metal-to-metal sealing mechanisms consist of axial and/or radial sealing surfaces. These seals rely on finely machined surfaces and development of high bearing (contact) pressure between mating sealing elements. While metal-to-metal seal connections have been successful in practice, they require a high degree of care in manufacture, quality assurance/control, handling and assembly. The use of special equipment and personnel is also required to ensure proper assembly in preparation for shipping offshore and for assembly at the rig. Additional requirements in all stages of tubular production result in higher manufacturing reject rates when compared with API Connections. These connections require more personnel, inspection, equipment, and rig time for installation in the well. As a result, these connections add significant cost when so-equipped tubular strings are purchased due to increased inspections, specialty accessory equipment, personnel and running tools, and rig time.

[0021] The use of special tolerances and make-up procedures has gained favor in the industry for improved performance of API connections. For example, the connection described in U.S. Pat. No. 4,962,579 seeks to ensure enhanced leak resistance by using a registry mark that, upon proper make-up, is visually inspected to be within the proper position. Other approaches include: (a) applying heavy surface plating (consisting of various soft, malleable metals) on internal threads for enhanced lubricity to protect the base metal and seal-ability; (b) assembling with specially formulated lubricants; and (c) special assembly procedures.

[0022] These alternatives have been successful in achieving enhanced connection performance for somewhat greater cost when compared with standard API connections. However, these alternative approaches are limited to certain tubular wall thicknesses. Standard oilfield tubulars include many wall thickness options for any given outer diameter tubulars section. As a result, couplings may be weaker than the pipe sections in the string due to decreased wall thickness of the coupling.

[0023] Another effort to solve these problems has focussed on coupling design using the standard API threads. For example, U.S. Pat. No. 5,015,017 to George B. Geary assigned to GB Tubulars of Houston, Tex., discloses a threaded tubular coupling having an increased internal cross section in the center of the coupling. The increased cross section improves leak resistance by resisting expansion of the coupling due to internal pressure, without increasing the outer diameter of the coupling. The connection of U.S. Pat. No. 5,015,017 helps prevent loss of contact pressure in the threads between the pin and box members.

[0024] An increased cross section in the coupling center provides significant hoop reinforcement in the coupling of U.S. Pat. No. 5,015,017. Thus, it will better resist inward compression from chucking during assembly and outward expansion due to assembly, pressure or other loads. Because the reinforced cross-section resists outward expansion of the coupling, more torque is required to achieve final make-up position than is required with a standard API coupling without the reinforced center section. The increased torque required for make up of the coupling of U.S. Pat. No. 5,015,017 causes increased friction in mating thread elements. As a result of the increased friction, the soft, malleable metallic plating moves more efficiently to better fill any voids in the mating thread elements to enhance thread form seal-ability.

[0025] A significant advantage of the connection described in U.S. Pat. No. 5,015,017 is its use of standard API thread forms. The connection provides enhanced performance and industry utility in a cost-effective connection. Another advantage of the coupling described in U.S. Pat. No. 5,015,017, is that the outside diameter can be smaller to allow use in tight, downhole clearance situations. Because of the center reinforcement, the connection maintains leak resistance exceeding that of the tubular section and the connection is significantly more tensile efficient than flush joint connections.

[0026] The connections described in U.S. Pat. No. 5,015,017 have been successful throughout the industry because it solves many of the problems associated with Standard API connections and takes advantage of the worldwide infrastructure for service, interchangeability, handling and accessories. However,-operators have been reluctant to place thread interference seal connections into wells requiring more critical levels of service. Some operators prefer connections with multiple, redundant seals for critical wells. Thus, there is a need for a threaded connection that uses the industry standard thread form with an improved (positive) sealing system.

[0027] There also is a need for a connection for joining together tubular sections that will help rig personnel recognize when each connection is properly made up. There also is a need for a connection that can be properly made up even when the box and pin members are sized at or near the limits of thread tolerances. There also is a need for a connection with improved leak resistance, that can reduce or minimize thread galling, and that can resist connection parting (jump-out or jump-in). There also is a need for a coupling that provides an enhanced seal without imparting additional radial hoop stresses. Finally, there is need for a connection that can provide both enhanced leak and relatively high tensile efficiency with a reduced coupling outside diameter (special clearance).

SUMMARY OF THE INVENTION

[0028] The present invention solves the above problems and disadvantages by providing a coupling with an improved sealing system for joining together tubular sections. This invention uses Standard API threadforms in an enhanced coupling body. The coupling also includes an axially energized primary sealing system with a back up thread interference seal. The sealing system is unique because it imparts no additional hoop stresses in the connections beyond those needed for the secondary thread interference seal. This feature allows the connections to be made special clearance and provide both enhanced seal-ability and superior resistance to jump-out. The coupling includes an unthreaded center section with a seal seat on which the seal assembly is positioned. The seal assembly includes an elastomeric component and a substantially rigid component that limits advancement of the tubular section into the coupling.

[0029] The coupling and sealing system of the present invention can be used in any threaded tubular connection where effective resistance to high liquid or gas pressure is required. While the coupling and sealing system described here is presented primarily for oilfield applications, it is also applicable to any threaded, tubular connection requiring high-pressure sealing.

[0030] The present invention may be used in conventional API threaded couplings, other couplings similar in geometry to API couplings, and couplings with center section enhancements. Thus the invention may be employed in couplings of U.S. Pat. No. 5,015,017, wherein the coupling has stop shoulders on each side of an enhanced center section. In a coupling with stop shoulders, the seal assembly is positioned on a seal seat adjacent the stop shoulders on each side if the coupling. During make-up, the seal assembly is compressed axially between the pin faces and the stop shoulders. If the coupling lacks internal shoulders, however, the seal assembly may be positioned on a seal seat adjacent the center section of the coupling so that the seal assembly is axially compressed during make-up between the opposing pin faces.

DESCRIPTION OF THE DRAWING

[0031] FIG. 1 is a cross section view of the coupling with internal shoulders according to a first preferred embodiment of the present invention.

[0032] FIG. 2 is an enlarged view of the internal shoulder and seal seat according to a first preferred embodiment of the present invention.

[0033] FIG. 2A is an enlarged view of the seal seat according to a first preferred embodiment of the present invention.

[0034] FIG. 3 is a cross section view of an un-shouldered coupling according to a second preferred embodiment of the present invention.

[0035] FIG. 4 is a cross section view of the seal insert according to a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0036] As shown in FIG. 1, a first preferred embodiment of the present invention includes tubular sections 11, 12 connected together with coupling 13. In FIG. 1, tubular section 11 is in the hand tight position and tubular section 12 is in the power tight position. Each of the tubular sections includes external pin threads 21, 22 which mate with internal threads on threaded areas 14, 15 of the coupling. The coupling has a first end 16 and a second end 17.

[0037] In the first preferred embodiment, the pin threads and box threads are standard API threads, and most preferably API buttress threads. The present invention, however, may be used with threaded tubular connections having various other thread forms.

[0038] In the embodiment of FIG. 1, center section 18 is located axially between ends 16, 17 of coupling 13. Axial refers to the longitudinal axis of the coupling and tubular sections. The center section of the coupling is between the threaded areas. The center section is unthreaded, has increased thickness, and includes first stop shoulder 31, second stop shoulder 32, and annular interior surface 33 therebetween. The inside diameter of the center section is approximately equal to the inside diameter of each of the threaded tubular sections, and is preferably not less than the inside diameter of each threaded tubular section.

[0039] Threaded areas 14, 15 of the coupling terminate at thread relief grooves 41, 42. Adjacent the thread relief grooves are shoulders 31, 32. Adjacent the shoulders are seal seats 35, 36. Each seal seat is dimensioned to have a seat width and seat diameter sufficient to hold seal assembly 60 thereon, and preferably includes a snap surface 37 (shown in FIG. 2) which provides an abutment or other seal retaining means to retain and lock the seal assembly on the seal seat adjacent the shoulder. Upon separation of each of the threaded tubular sections from the coupling, each of the seal assemblies are retained with the coupling. In a 7⅝ inch outside diameter coupling, each snap surface 37 preferably has an internal diameter 0.005 inches less than the internal diameter of seal seats 35, 36, and preferably a width of 0.0125 inches. The internal diameter of the snap surface is preferably slightly less than the diameter of the outer perimeter of the seal assembly.

[0040] The seal seat of the present invention is located in the unthreaded area of the coupling inside diameter. In a coupling with a center section having stop shoulders, as shown in FIG. 1, each seal seat is adjacent a stop shoulder. In the hand tight position, the pin face 51, 52 of each tubular section reaches an axial position at or near the seal seat.

[0041] Now referring to FIG. 3, coupling 101 is shown according to a second preferred embodiment without a center section and without stop shoulders. Seal seat 102 is positioned in the center of the coupling, axially between the pin faces of the two tubular sections. Tubular section 105 is in the hand tight position, and tubular section 106 is in the power tight position. Pin face 103 is adjacent the seal seat and pin face 104 abuts the seal assembly.

[0042] In both of the preferred embodiments, as the tubular section axially advances from the hand tight to the power tight position, the tubular section abuts the seal assembly on the seal seat and compresses the seal assembly axially as will be described in more detail below.

[0043] Seal assembly 60 of the present invention also may be referred to as a gasket or gasket assembly. As shown in FIG. 4, in a preferred embodiment, seal assembly 60 is removable from the coupling and includes a rigid annular retainer 61 and a pair of annular seal elements 62, 63, each seal element supported on a corresponding side of the retainer. When the seal assembly is positioned on the seal seat and is compressed axially between the pin face and stop shoulder, or between opposing pin faces, the seal elements are compressed affecting a fluid (liquid or gas) tight seal.

[0044] The retainer in the seal assembly has inner perimeter 64 and outer perimeter 65. The outer perimeter of the retainer is dimensioned with the same diameter as the seal seat, or slightly less than the seal seat diameter for ease of assembly. The inner perimeter of the retainer is dimensioned to have the same or slightly smaller diameter than the center section of the coupling and/or the pin face. In a preferred embodiment of the coupling for joining nominal 7⅝ inch outside diameter tubular sections, the coupling center section has an inside diameter of 6.765 inches and the seat diameter is 7.325 inches. In this embodiment, the outer perimeter of the retainer has a diameter no greater than 7.325 inches and the inside perimeter of the retainer is 6.765 inches or less. Preferably, annular elastomeric locating member 66 is affixed to the outer perimeter of the retainer. The elastomeric locating member may have an interference fit with the seat diameter if desired. The annular elastomeric locating member has an outer diameter slightly larger than the diameter of the snap surface, so that the seal assembly is held in position on the seal seat.

[0045] The retainer of the present invention provides a positive stop to limit axial travel of the threaded tubular section and limit axial compression of the seal assembly. In a preferred embodiment, the retainer is made from a metal, although plastic or other rigid or substantially rigid material also may be used. The retainer has radial surfaces 67, 68 defining the axial thickness of the retainer. The axial thickness of the retainer is dimensioned to be the same as the seal seat width, or slightly less than the seal seat width for ease of assembly. In a preferred embodiment of the coupling for joining nominal 7⅝ inch outside diameter tubular sections, the seal seat width is 0.150 inches. Thus, the axial thickness of the retainer is no more than 0.150 inches. The seal seat and retainer dimensions depend on the intended application of the present invention. However, the retainer in the seal assembly should be generally rigid or substantially rigid to limit the axial compression of the seal elements and to provide a specific and known clearance between the pin face and stop shoulder in the power tight position. The retainer should be substantially rigid so that any further axial advancement of the threaded tubular section into the coupling will not occur without application of additional and excessive torque. The retainer also is provided with a pair of mounting grooves 71, 72, each mounting groove on one of the radial surfaces thereof. Each groove is a generally U-shaped channel in the radial surface of the retainer.

[0046] In a preferred embodiment, each seal element of the seal assembly is positioned in a mounting groove in the retainer. Each seal element has an annular shape formed of an elastomeric material such as synthetic rubber or various other resilient materials that are capable of retaining their shape and resiliency after deformation. The seal element has an inside perimeter 80 with a diameter greater than the diameter of the inside perimeter 64 of the retainer and an outside perimeter 88 with a diameter less than the diameter of the outside perimeter 65 of the retainer. The first surface 74 of each seal element is dimensioned to fit in each retainer mounting groove, and may be adhesively bonded, chemically bonded or otherwise attached to the retainer mounting groove if desired. The second surface of each seal element has a channel 77, 78 adjacent the inside perimeter 80 of the seal element and a lobe or bead portion 75, 76 adjacent the outer perimeter of the seal element. The lobe or bead extends or protrudes axially from the radial surface of the retainer such that the lobe or bead portion comes into contact with the pin face before contacting any other part of the seal assembly.

[0047] Axial advancement of the tubular section into the coupling results in elastic deformation of the lobe portion of the seal until the pin face abuts the retainer. The lobe or bead portion of the seal provides sealing engagement with the pin face. Thus, the seal element of the seal assembly is axially compressible and the retainer is the component limiting axial compression of the seal assembly. The retainer also may provide a metal to metal seal with the pin face and shoulder or between opposing pin faces. By limiting the axial compression of the seal assembly, damage to the seal elements during assembly or from the application of torque is reduced or minimized.

[0048] The opposing radial surfaces of the retainer are provided with a plurality of channels 93, 94 therein between the groove and the inner or outer perimeter of the retainer. The channels provide a path for venting of fluid, thread compound or other liquid that may be trapped between the shoulder and pin face, and that would reduce the effectiveness of the seal assembly.

[0049] Thus, the metal component, or retainer, limits the axial compression of the seal elements or elastomeric components of the seal assembly. Preferably, each elastomeric component has an interfit with the retainer such that at least part of the elastomeric component is compressed. The resilient elastomeric component protrudes axially from the metal seal component. During assembly of the tubular sections into the coupling, the face of the pin initially engages the protruding portion of the elastomeric seal component during axial advancement of the pin into the box. As the pin continues advancement to shouldering, the elastomeric component is further compressed axially, energizing the resilient elastomeric seal and then forming a metal-to-metal seal with the pin face.

[0050] At completion of make-up, the elastomeric seal element is the primary sealing mechanism in the coupling, backed up by the metal-to-metal seal. Under extreme load cases, particularly tension, the metal seal may lose contact pressure with the pin face. In these extreme load conditions the elastomeric seal will maintain leak integrity of the connection. If desired, both of the seal assembly components also may be backed up by an enhanced thread interference seal as described in U.S. Pat. No. 5,015,017.

[0051] With the present invention, neither the primary elastomeric seal nor the secondary metal-to-metal seal impart hoop stresses in the coupling. This is an advantage over the prior art couplings having sealing mechanisms that impart hoop stresses into the box member through the use of elastomeric seal rings within mating thread elements and/or couplings that imparted additional radial hoop stresses in the box member when energized by the application of torque during make-up.

[0052] The present invention also eliminates the imposition of additional hydraulically induced hoop stresses in the connection due to trapped thread compound. Such hydraulically induced hoop stresses can be seriously detrimental to connection performance. Thread relief grooves 41 and 42 provide a reservoir for excess thread compound to migrate during make-up. The elastomeric seal is specially formed to flush excess thread compound toward the pipe ID as it compresses. Channels 93 and 94 provide channels allowing flushed compound to flow to the casing ID. These special design provisions eliminate fouling of the elastomeric seal by trapped thread compound.

[0053] The present invention also solves the problem of variability during make-up by rig personnel. During make-up, the pin axially advances until its nose engages the retainer and drives it into the internal stop shoulder of the box and stops. For connections without stop shoulders the pin axially advances until its nose engages the retainer and drives it into the opposing pin member. Rig personnel can instantly recognize proper make-up when the seal is formed between the pin nose, seal assembly, and stop shoulder or between the opposing pin noses.

[0054] The present invention has been described and illustrated with respect to specific embodiments. It will be understood to those skilled in the art that changes and modifications may be made without departing from the spirit and scope of the invention as set forth in the appended claims.

Claims

1. A coupling for joining together threaded tubular sections, the coupling having opposing first and second internally threaded end sections, comprising:

(a) at least one seal seat positioned between the first internally threaded end section and the second internally threaded end section; and

(b) a removable seal assembly positioned on the seal seat, the seal assembly having an elastomeric component and a substantially rigid component, the elastomeric component being resilient and protruding axially from the substantially rigid component, the elastomeric component being axially compressible by at least one of the internally threaded end sections, the substantially rigid component limiting axial compression of the elastomeric component of the seal assembly and limiting the axial advancement of at least one of the internally threaded end sections.

2. The coupling of claim 1 further comprising a plurality of channels in the rigid component of the seal insert.

3. The coupling of claim 1 further comprising a snap surface adjacent the seal seat.

4. The coupling of claim 1 wherein the rigid component of the seal assembly has an outer perimeter and inner perimeter, and the seal seat has a diameter greater than the diameter of the outer perimeter of the rigid component.

5. The coupling of claim 4 wherein the coupling has a center section with an internal diameter smaller than the internal diameter of the first and second internally threaded end sections.

6. The coupling of claim 5 wherein the center section has opposing first and second shoulders and a seal seat adjacent each of the first and second shoulders.

7. A coupling for joining together threaded tubular sections, each threaded tubular section having external threads, comprising:

(a) first and second internally threaded end sections, each end section having an internally tapered threaded area beginning at the outer end of the end section and terminating at a thread relief groove, the first and second end sections joined together to form an unthreaded intermediate area;

(b) a seal seat in the unthreaded intermediate area, the seal seat having an internal diameter less than the internal diameter of the threaded areas; and

(c) a seal assembly configured to fit on the seal seat, the seal assembly having a rigid component retaining at least one elastomeric seal element, the elastomeric seal element forming a seal with the end of a threaded tubular section by application of axial force against the seal element.

8. The coupling of claim 7 wherein the end of the threaded tubular section engages the elastomeric seal element before engaging the rigid component of the seal assembly.

9. The coupling of claim 7 wherein the rigid component limits axial compression of the elastomeric seal element.

10. The coupling of claim 7 wherein the threaded tubular section forms a metal-to-metal axial seal with the rigid component.

11. The coupling of claim 7 wherein the coupling includes a snap surface for retaining the seal assembly on the seal seat.

12. The coupling of claim 7 wherein the seal assembly includes at least one channel in the outer surface of the rigid component.

13. An improved coupling for repeatedly joining together externally threaded tubular sections, comprising:

(a) first and second internally threaded ends and an unthreaded intermediate section axially spaced between the first and second internally threaded ends; and

(b) at least one seal assembly seated on a seal seat in the intermediate section and spaced axially between the first and second internally threaded ends, the seal assembly having an axially compressible resilient component and a substantially rigid component, the resilient component of the seal assembly forming a seal with the end of a threaded tubular section and the substantially rigid component limiting axial advancement of the externally threaded tubular section into the coupling.

14. The improved coupling of claim 13 wherein the rigid component comprises a metal retainer.

15. The improved coupling of claim 13 wherein the axially compressible resilient component protrudes axially from the rigid component.

16. The improved coupling of claim 13 further comprising a pair of axially compressible resilient components.

17. The improved coupling of claim 13 wherein the unthreaded intermediate section has internal shoulders and an inner diameter substantially the same as the inner diameter of the externally threaded tubular sections.

18. The improved coupling of claim 13 comprising a pair of seal assemblies, each seal assembly engaging the end of one of the externally threaded tubular sections.

19. The improved coupling of claim 13 further comprising a snap ridge for retaining the seal assembly in place on the seal seat.

20. The improved coupling of claim 13 wherein the coupling and threaded tubular sections have API threads.