TUBULAR CONNECTION WITH CYLINDRICAL AND TAPERED STEPPED THREADS

Abstract: A tubular joint or connector of box and pin members having two-step tapered threads (20, 21). Two metal-to-metal seals of complementary engaging sealing surfaces (14, 15, 18, 19) are provided, one adjacent the end of the pin member, the other axially disposed between the two steps. Reverse angle torque shoulders (16, 17) at the end of the pin member and the interior termination of the box member and hooked threads further characterize the joint and box and pin members.

Description: TUBULAR CONNECTION WITH CYLINDRICAL AND TAPERED STEPPED THREADS The invention relates generally to a tubular connection for tubular members used in the oilfield. Specifically, the invention relates to a connection for tubular members fabricated of high strength relatively thin wall material or of relatively heavy wall material as used under conditions of high tension and high pressure that are encountered in production tubing or casing employed in deep severe service oil or gas wells. Downhole conditions in wells reaching depths over fifteen thousand feet include pressures from fif¬ teen thousand (15,000) to twenty-five thousand (25,000) psi and temperatures approaching five hundred (500) degrees fahrenheit. Downhole conditions are often characterized as severe when sweet gas C-2 or sour gas H-S is encountered. In order to withstand deep well and severe service conditions, production tubing and casing connectors are often fabricated from relatively thin walled high alloy steel that is not upset or cold formed. Such steel has very high strength and thus is not required to be as thick as low strength steels. Even where severe service conditions are not expected, there are instances where near maximum of tensile strength of the connection is required of non-upset or non-cold formed tubular members of relatively thin material. Prior art connection designs provided on ends of tubular members of relatively thin material not having upsets or not processed by cold forming have been provided on a single thread geometry having a positive load flank thread. Such prior art design leads to radial crest to root thread interference causing significant assembly stresses and jumpout failures. Straight or cylindrical two step thread designs are known in the prior art for such members of relatively thin material, but maximum tensile strength ratings of such designs may not be achievable especially where service of the tubing members is expected in extremely deep wells. Alternatively, under conditions of high ten¬ sion and high pressure which demand that heavy wall casing be used as production casing or tie-back casing in deep severe service oil or gas wells, the production casing is often required to be three times as thick as standard API walls for comparable outside diameter pipe sizes. Inherent with the increased wall thickness re¬ quired for deep severe service wells is increased string weight and rigidity of the tubular members. Prior art thick walled connections have had a tendency for thread jump-out and seal galling. These disad¬ vantages of prior art connections for thick walled tubular members are attributable to positive flank thread profiles of the long tubular strings and the rigidity of the thick walled casings. Thus, there has developed a need in the tubular connection industry for a connection design applicable for tubular members of relatively thin walled material or of increased wall thickness adapted for service under deep and/or severe conditions or other condi¬ tions. The present invention therefore provides a tubular connection of pin and box members defining an axis, said connection comprises first and second pairs of interengaged threads on the respective members, the first pair of threads being axially spaced from the second pair of threads, at least one of said pair of threads being provided on a taper angle with respect to the axis, the first pair of threads being stepped with respect to the second pair of threads, a first pair of engaged sealing surfaces on the pin and box members disposed adjacent, the end of the pin member, a second pair of engaged sealing surfaces on the pin and box members disposed axially between the first and second pairs of threads, and engaged reverse angle torque shoulders, the torque shoulder on the pin mem¬ ber disposed on the end of the pin member, the torque shoulder on the box member disposed on the interior end of the box member, and wherein said threads are hooked threads having negative angle load flanks. The thin walled tubular connection of this invention is improved in that it exhibits reduced susceptibility of the connection to stress corrosion cracking in a severe service environment. A thick walled tubular connection as modified in accordance with this invention provides a connection which allows for adjustment to tolerance variations in outside diameter and inside diameter of the pipe body without weakening the connection. Further, the thin walled or thick walled connection of the invention inhibits excessive com¬ pression forces from radially collapsing the end of the pin member. More specifically this connection pref¬ erably creates axial compression of the internal sur- face of the pin thereby reducing the susceptibility of the connection to stress corrosion cracking in a severe service environment. A further feature of the invention is that the connection provides sealing surfaces which increase the seal radial load distribution over that provided by prior art connections thereby reducing the tendency for galling by reducing peak seal load. Another advantage of the invention is that it provides a connection which combats the tendency for j-ump-out by locking the pin and box together thereby reducing collapse of the pin and expansion of the box. Further features and advantages of the in¬ vention will be described below with reference to the following description of preferred embodiments of the invention, taken in conjunction with the accompanying drawings of which: Figure 1 is a cross-section through a first embodiment of a tubular connection made in accordance with the invention showing pin and box members inter- engaged; Figure 2 is an enlarged illustration of a threaded portion of the connection of Figure 1 il¬ lustrating hooked threads; and Figure 3 is an illustration of the coupling of Figure 1 connecting pin members of two tubular members. Figure 4 is a cross-section through a second embodiment of a tubular connection made in accordance with the invention showing pin and box members inter- engaged; Figure 5 is an enlarged illustration of a threaded portion of the connection of Figure 4 il¬ lustrating hooked threads; and Figure 6 is an illustration of the coupling of Figure 4 connecting pin members of two tubular members. Figure 1 illustrates a first embodiment of the invention in which a relatively thin-walled tubular con¬ nection 10 includes a pin member 12 threadedly in- terengaged with a box member 11. The pin member 12 and box member 11 have first and second threaded sur- faces 21 and 20. The first threaded surface 21 is axially spaced from the second threaded surface 20. The first threaded surface 21 is provided on a taper angle with respect to the axis of the connection of the tubular members. The second threaded surface 20 is provided on a straight or cylindrical profile with respect to the axis of the connection of the tubular members. The first threaded surface 21 is radially stepped with respect to the second threaded surface 20. The tapered threads on surface 21 allow the threaded connection to run out to the tubular member outside diameter, thereby achieving a higher tensile rating than could be obtained if threaded surface 21 were a cylindrical thread separated by a step between cylindrical threaded surface 20. Metal-to-metal frustro-conical sealing sur¬ faces 18 and 19 are provided axially between first and second threaded surfaces 21 and 20. Metal- o-metal frustro-conical sealing surfaces 14 and 15 are provided adjacent the end of the pin and the interior end of the box. Complementary reverse angle torque shoulders 16 and 17 are provided on the end of the pin 12 and on the interior end of the box 11. According to the invention, the threaded surfaces 21 and 20 are preferably provided in the form of hooked threads. An exemplary thread is illustrated in Figure 2 by reference number 40 showing that the load flank 41 is provided at a negative angle (-Θ) with respect to a radial plane through the thread. The threaded connection including the two- step profile, facilitates deep stabbing and ease of make-up between the pin member 12 and the box member 11. The hooked threads of cylindrical threaded sur¬ faces 20 are relatively less susceptible to thread hang-up on threads of tapered bylindrical threaded sur- faces 21 because of the tapering of threaded surfaces 21. That is, upon, disengagement of the pin 12 from the box 11, the threaded pin of surface 20 is relatively less likely to hang up on the box threaded surface 21 5 because of the increasing inside, diameter of the box threads of the tapered surface 21. Providing the taper angle of threaded surface 21 in combination with the cylindrical stepped threads of threaded surfaces 20 enables the entire connection 10 to have maximum thread and critical section areas al¬ lowing uniform reduction in section flexibility around the sealing regions and reduction of the relative step height between the stepped thread " regions. The taper of the threads of threaded surface 21 also allows 15 for adjustment to tolerance variations in pipe out¬ side diameter and inside diameter provided by tubular fabricators without weakening the-connection. The torque shoulders 16 and 17 are completely internal to the connection and create an essentially 20 recess free bore to enhance fluid flow. The torque shoulders serve as a positive stop and locking means as the pin and box members are assembled under torquing conditions. The reverse angle of the torque shoulder also serves to inhibit excessive hoop compressive 25 forces from collapsing the end of the pin 12. The reverse shoulder angle also fixes the relation between the end of the pin and the interior end of the box. The internal torque shoulder also creates axial com¬ pression along an axial region of the internal surface 30 of the pin thereby reducing the susceptibility of the connection to stress corrosion cracking in a severe service environment. The compressive force resulting from the reverse angle torque shoulder also tends to cause the seal region defined by sealing surfaces 35 14 and 15 to expand radially due to the end load. The metal-to-metal seals defined by sealing surfaces 14 and 15 and 18 and 19 are provided respectively as an internal seal and as a backup seal for internal pressure and a primary external seal. The sealing angles of sealing surfaces 14 and 15 and 18 and 19 are more shallow than typical cylindrical two-step metal- to-metal seals. The shallowness of the angles of the sealing surfaces increases the sealing surface contact area thereby increasing the seal radial load distri¬ bution and reducing the tendency for galling by reduc¬ ing the peak sealing load. The first and second pairs of conical sealing surfaces are preferably provided at steeper angles than the taper angle of the first pair of threads. Although the precise angles of the sealing surfaces and taper angle of the first pair of threads are dependent on the wall thickness of the tubular members to be connected, the first and second pair of conical sealing surfaces are preferably provided at an angle of ap¬ proximately four to fourteen degrees with respect to the axis of the joint while the taper angle of the first pair of threads is preferably provided approximate¬ ly two to eight degrees with respect to the axis of the joint. The hooked threads provided in the tubular connection 10 inhibit the tendency for thread jump- out. The negative angle load flank threads, when subjected to tension between the pin 12 and the box 11, develop a hoop compression component. This hoop com¬ pression locks the pin and box together thereby reduc¬ ing collapse of the pin and expansion of the box. A similar reaction occurs when the tubular connection is subjected to bending wherein on the tension side of the connection, the hooked threads develop a hoop compression component locking that side of the pin and box together, while the torque shoulder absorbs the load on the compression side of the bending connection. Figure 3 illustrates a connector serving to connect the ends of two tubular members together. Two box threaded surfaces 11 and 11* serve to interengage < iTllEA~? with pin threaded surfaces 10 and 11 of the two tubular members. The connection between box 11' and pin 10 ' is the mirror image of that between box 11 and pin 10. Figure 4 illustrates a second preferred em- bodiment of the invention wherein a thick walled tubular connection 110 is disclosed in which a pin member 112 is threadedly interengaged with a box member 111. The pin member 112 and box member 111 have first and second threaded surfaces 121 and 120 which are preferably provided on identical taper angles with respect to the axis of the joint. Under certain conditions, the taper angles of the first and second threaded surfaces may be provided on different taper angles. Extension lines 130 and 131 are shown extending along the roots respectively of the threaded surfaces 121 and 120 of pin member 112. The taper angles measured with re¬ spect to the axis of lines 130 and 131 are substantial¬ ly identical in that lines 130 and 131 are parallel but offset by a distance S perpendicular to the two taper extension lines 130 and 131. The magnitude of distance S depends on the wall thickness of the tubular member and the length of sealing surfaces 118 and 119. Metal-to-metal frustro-conical sealing sur- faces 118 and 119 are provided axially between first and second threaded surfaces 121 and 120. Metal-to- metal sealing surfaces 114 and 115 are provided adjacent the end of the pin and the interior end of the box. Complementary reverse angle torque shoulders 116 and 117 are provided on the end of the pin 112 and on the interior end of the box 11. The threaded surfaces are preferably provided in the form of hooked threads. An exemplary thread is illustrated in Figure 5 by reference number 140 show- ing that the load flank 141 is provided at a negative angle(θ 1 ) with respect to a radial plane through the thread. The threaded connection including the two- step profile, facilitates, deep stabbing and ease of make-up between the pin member 112 and the box member 111. Providing the taper angles as illustrated by taper lines 130 and 131 enables the connection to have maximum thread and critical section areas allowing uniform reduction in section flexibility around the sealing regions, and reduction of the relative step height S between the stepped thread regions. The taper of the threads also allows for adjustment to tolerance variations in pipe outside diameter and in¬ side diameter provided by tubular fabricators without weakening the connection. The torque shoulders 116 and 117 are completely internal to the connection and create an essentially recess free bore to enhance fluid flow. The torque shoulders serve as a positive stop and locking means as the pin and box members are assembled under tor- quing conditions. The reverse angle of the torque shoulder serves to inhibit excessive hoop compressive forces from collapsing the end of the pin 112. The reverse shoulder angle also fixes the relation between the end of the pin and the interior end of the box. The internal torque shoulder also creates axial com- pression along an axial region of the internal surface of the pin thereby reducing the susceptibility of the connection to stress corrosion cracking in a severe service environment. The compressive force resulting from the reverse angle torque shoulder also tends to cause the seal region defined by sealing surfaces 114 and 115 to expand radially due to the end load. The metal-to-metal seals, defined by sealing surfaces 114 and 115 and 118 and 119 are provided re¬ spectively as an internal seal and as a backup seal for internal pressure and a primary external seal. Due OMPI Arm WIPO . to the increased rigidity of the thick tubular walls, the threaded surfaces are provided on tapers thereby allowing the sealing angles to be more shallow than typical cylindrical two-step metal-to-metal seals. The shallowness of the angles of the sealing surfaces increases the sealing surface contact area thereby increasing the seal radial load distribution and re¬ ducing the tendency for galling by reducing the peak sealing load. The first and second pairs of conical seal¬ ing surfaces are preferably provided at steeper angles than the taper angle. Although the precise angles of the sealing surfaces and taper angles are dependent on the wall of the connection, the first and second pair of conical sealing surfaces are preferably provided at an angle of approximately four to fourteen degrees with respect to the axis of the joint while the taper angle of the first and second pair of threads is pref¬ erably approximately two to eight degrees with respect to the axis of the joint. The hooked threads provided in the tubular connection 110 inhibit the tendency for thread jump- out. The negative angle load flank threads, when sub¬ jected to tension between the pin 112 and the box 111, develop a hoop compression component. The hoop com¬ pression locks the pin and box together thereby re¬ ducing collapse of the pin and expansion of the box. A similar reaction occurs when the tubular connection is subjected to bending wherein on the tension side of the connection, the hooked threads develop a hoop com¬ pression component locking that side of the pin and box together, while the torque shoulder absorbs the load on the compression side of the bending connection. Figure 6- illustrates a connector serving to connect the ends of the thick walled tubular members. Two box threaded surfaces 111 and 110 '. serve to inter- engage with pin threaded surfaces 110 and 111 of the two tubular members . The connection between box 111 ' and pin 110 ' is the mirror image of that between box 111 and pin 110 .

Claims: CLAIMS 1. A tubular connection (10, . 110) of pin and box members (11, 12; 111, 112), defining an axis, characterized in that said connection comprises first and second pairs of interengaged threads (20, 21; 120, 121) on the respective members, the first pair of threads (20, 120) being axially spaced from the second pair of threads (21,121) at least one of said pair of threads (21, 121) being provided on a taper angle with respect to the axis, the first- pair of threads (21, 121) being stepped with respect to the second pair of threads (20, 120), a first pair of engaged sealing surfaces (14, 15; 114, 115) on the pin and box members (11, 12; 111, 112) disposed adjacent the end of the pin member (12, 112) a second pair of engaged sealing surfaces (18, 19; 118, 119) on the pin and box members (11, 12, 111, 112) disposed axially between the first and second pairs of threads (20, 120; 21 121) and en¬ gaged reverse angle torque shoulders, (16, 17; 116, 117) , the torque shoulder (16, 116) on the pin member (12, 112) disposed on the end of the pin member, the torque shoulder (17, 117) on the box member (11, 111) disposed on the interior end of the box member, and wherein said threads (40, 140) are hooked threads hav¬ ing negative angle load flanks (41, 141) . 2. The connection of claim 1, characterized in that the second pair of threads (20) are cylindri¬ cal threads. 3. The connection of claim 1, characterized in that both the first and second pairs of thread (120, 121) are provided on first and second taper angles with respect to the axis of the connection (120) . 4. The connection of claim 1, 2 or 3, char¬ acterized in that said hooked threads (40, 140) de¬ velop hoop compression when the connection (10, 110) is subjected to tension thereby locking the pin and box (11, 12; 111, 112) together reducing the tendency for col- OMPI lapse of the pin and expansion of the box. 5. The connection of claim 1, 2 or 3, char¬ acterized in that when said connection (10, 110) is subjected to bending, the hooked threads (40, 140) develop hoop compression on the side of the connection in tension as a result of the bending while the en¬ gaged reverse angle torque shoulders (16, 17; 116, 117) absorb the bending load on the other side of the con¬ nection (10, 110) put in compression as a result of the bending. 6. The connection of claim 1, 2 or 3, char¬ acterized in that the engaged reverse angle torque shoulders (16, 17; 116, 117) are adapted to create a recess free bore to enhance fluid flow through the pipe, to serve as a positive stop and locking device for assembly torque, to inhibit excessive hoop compressive forces from collapsing the end of the pin (12, 112), to fix the axial relation of the end of the pin (12, 112) to the box, (11, 111) and to create axial com¬ pression over the internal surface to reduce the sus¬ ceptibility of the joint (10, 110) to stress corrosion cracking in a severe service environment and to cause the first pair of engaged conical sealing surfaces (14, 15; 114, 115) to expand radially. 7. The connection of claim 1, 2 or 3, char¬ acterized in that the first and second pairs of seal¬ ing surfaces (14, 15, 18, 19: 114, 115, 118, 119) are • frusto"" con **- c l surfaces provided from a range of angles of approximately four to fourteen degrees with respect to the axis of the connection (10, 110). 8. The connection of claim 1 or 2, char¬ acterized in that the sealing surfaces (18 19; 118, 119) between the first and second pair of threads (20, 21; 120, 121) are conical surfaces provided at a steeper angle than the taper angle of said one pair of threads (21, 121) . OMPI 9. The connection of claim 1 or 2, char¬ acterized in that the taper angle of said one pair of threads (21, 121) are provided on a taper angle of a range of approximately two to eight degrees with re¬ spect to the axis of the connection (10, 110). 10. The connection of claim 1 or 3, char¬ acterized in that said first and second taper angles are substantially identical. 11. The connection of claim 1 or 3, char¬ acterized in that the first and second pairs of seal¬ ing surfaces (14, 15, 18, 19; 114, 115, 118, 119) are frustø-conical surfaces provided at steeper angles than the taper angle. 12. The connection of claim 1 or 3, char¬ acterized in that the taper angle of the first and second threads (120, 121) are provided on a taper angle of a range of approximately two to eight degrees with respect to the axis of the connection (110) . 13. A pin member (12, 112) for a tubular member and adapted for coaxial connection to a box member (11,' 111), characterized in that the pin mem¬ ber (12, 112) comprises first and second axially spaced threads (20, 21; 120, 121) at least one of the threads (21, 121) being provided at a taper angle with respect to the axis of the pin, the first thread (20, 120) being radially stepped with respect to the second thread (21, 121), a first sealing surface (15, 115) on the pin member (12, 112) disposed adjacent the end of the pin member (12, 112) a second sealing surface (19, 119) on the pin member (12, 112) disposed axially --between the first and second threads (20, 21; 120 121), a reverse angle torque shoulder T ~ L ~ G~, "116)- T_-ispO " sed ~ oir ~ the end of the pin member C12, 112), and wherein the first and second threads are hooked threads (40, 140) having negative angle load flanks (41, 141) . 14. The pin member of claim 13, characterized OMPI tø ^∞ in that the second threads (20) are cylindrical threads and the first and second sealing surfaces. (15, 19; 115, 119) each comprise frusto-conical sealing sur¬ faces. 15. The pin member of claim 13 or 14, characterized in that the second sealing surface (19, 119) is a frusto-conical surface provided at a steeper angle than the taper angle of the first threads (21, 121). 16. The pin member of claim 13 or 14, char¬ acterized in that the first threads (21, 121) are pro¬ vided on a taper angle from a range of angles of ap- roxi ately two to eight degrees with respect to the axis of the pin (12, 112) . 17. The pin member of claim 13 or 14, char¬ acterized in that the first and second frusto-conical sealing surfaces (15, 19; 115, 119) are provided from a range of angles of approximately four to fourteen degrees with respect to the axis of the pin (12, 112) . 18. The pin member of claim 13, char¬ acterized in that the first and second axially spaced threads (120, 121) are provided on first and second taper angles with respect to the axis of the pin (112) . 19. The pin member of claim 13 or 18, char¬ acterized in that the first and second taper angles are substantially identical and the first and second sealing surfaces (115, 119) are frusto-conical sur¬ faces provided at steeper angles than the taper angle. 20. The pin member of claim 19, characterized in that the first and second frusto-conical sealing surfaces (115, 119) are provided from a range of angles of approximately four to fourteen degrees with re¬ spect to the axis of the pin (112) . 21. The pin member of claim 19, char¬ acterized in that the first and second threads (120, 121) are provided on a taper angle from a range of ang¬ les of approximately two to eight degrees with respect to the aχ;is of the pin (112) . 22. A box member (11, 111) adapted for co¬ axial connection to a pin member (12, 112) disposed on the end of a tubular member, characterized in that the box member (11, 111) comprises first and second axially spaced threads (20, 21; 120, 121) at least one of the threads (21, 121) being provided on a taper angle with respect to the axis of the box (11, 111) the first threads (21, 121) being radially stepped with respect to the second threads (20, 120), a first sealing sur¬ face (14, 114) disposed adjacent the interior end of the box member (11, 111) , a second sealing surface (18, 118) about the interior of the box member (11, 111) disposed axially between the first and second threads (20, 21, 120, 121), a reverse angle torque shoulder (17, 117) disposed on the interior end of the box member (11, 111) , and wherein the first and second threads are hooked threads (40, 140) having negative angle load flanks (41, 141) . 23. The box member of claim 22, char¬ acterized in that the second threads (20) are cylindri¬ cal threads and the first and second sealing surfaces (14, 18) each comprise frusto-conical sealing surfaces. 24. The box member of claim 22 or 23, char¬ acterized in that the second sealing surface (18, 118) is a frusto-conical surface provided at a steeper angle than the taper angle of the first threads (21, 121). 25. The box member of claim 22 or 23, char¬ acterized in that the first and second sealing surfaces (14, 18; 114, 118) are frusto-conical surfaces pro¬ vided from a range of angles of approximately four to fourteen degrees with respect to the axis of the box (11, 111) . 26. The box member of claim 22, characterized in that the first threads (21, 121) are provided on a taper angle from a range of angles of approximately OMP two. to eight degrees with respect to the axis of the box ( 11, HI ) . 27. The box member of claim 22, char¬ acterized in that the first and second axially spaced threads (120, 121) are provided on first and second taper angles with respect to the axis of the box (111) . 28. The box member of claim 22 or 27, char¬ acterized in that the first and second sealing sur¬ faces (114, 118) are f r us o~ con; -- ca l surfaces provided at steeper angles than the taper angle. 29. The box member of claim 22 or 27, char¬ acterized in that the first and second sealing sur¬ faces (114, 118) are frusto-conical surfaces pro¬ vided from a range of angles of approximately four to fourteen degrees with respect to the axis of the box (111) . 30. The box member of claim 22 or 27, char¬ acterized in that the first and second threads (120, 121) are provided on a taper angle from a range of angles of approximately two to eight degrees with respect to the axis of the box (111) .

Inventors: LANDRIAULT L STEVEN, LANDRIAULT, L., STEVEN
Applicants: HYDRIL CO, HYDRIL COMPANY
Application year: 1984
Main classification: E21B17
IPC codes: E21B17, F16L15