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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of provisional application U.S. Ser. No. 60/232,898, filed on Sep. 15, 2000 
    
    
     TECHNICAL FIELD 
     This invention relates in general to a threaded connection between two tubular elements, each having a pin member on one end and a box member on the other end. In particular, this invention relates to a threaded connection for use with offshore riser pipe. 
     BACKGROUND OF THE INVENTION 
     In offshore production applications, a plurality of tubular riser elements are joined together in an end-to-end configuration and extend from a subsea well assembly to a surface platform. Advances in drilling technology have made it possible to drill at greater water depths, subjecting the production risers to extremely high pressures and bending loads. Metal-to-metal seals can provide an effective seal under these harsh conditions; however, contamination, pitting, or damage to the seal surfaces causes these seals to deteriorate rapidly. Thermoplastic seal elements can be used as secondary sealing elements; however, these components typically resist radial compression and therefore create internal forces which act to urge the box and pin elements apart. Bending loads due to currents and wave motion can cause the metal seals to cycle, creating fatigue. This in turn reduces the effectiveness of the metal-to-metal seals, contributing to the failure of the connection. 
     Attempts have been made to overcome these problems. For example, U.S. Pat. No. 4,707,001 discloses a connection featuring multi-start threads with a reverse angle load flank in conjunction with a torque shoulder seal to lock the pin and box against radial separation. While this design may be workable, the torque shoulder could create plastic deformation in the box or pin member if too much torque is applied to the connection. This design is also susceptible to stress fractures because the torque shoulder seal applies repetitive bending loads to the box and pin. 
     SUMMARY OF THE INVENTION 
     The connection of this invention features a box member with an internal thread an a pin member with a cooperative external thread. A first metal-to-metal seal located adjacent to the box shoulder forms the primary internal seal of the connection. A second metal-to-metal seal located near the box end forms the primary external seal of the connection. Guide surfaces located on the pin end and box end protect the metal sealing surfaces from damage during makeup. 
     A dual angle torque shoulder in combination with a large blend radius is located on the pin member for engaging a corresponding dual angle box end, locking the box end securely into place against the pin. This configuration is self-centering. The dual angle torque shoulder has inner and outer inclined surfaces that join each other with a large blend radius. The dual angle torque shoulder in combination with the large blend radius self-centers, prevents radial distention of the box end, and maintains bearing pressure between the external sealing surfaces. Because the torque shoulder has a dual angle configuration, the pin and box will not undergo plastic deformation as a result of excessive torque. The torque shoulder on the box end is generally torroidal, being convex in cross-section. 
     The connection also has stress relief grooves located on the box member and on the pin member. These stress relief grooves reduce the incidence of stress fractures, thereby improving the fatigue life of the connection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross-sectional view of a tubular connection according to the invention, showing the dual angle torque shoulder and the double stress relief grooves. 
     FIG. 2 is an enlarged cross-sectional view of the end of the box member of the connection of FIG. 1, showing the detail of the dual angle torque shoulder. 
     FIG. 3 is an enlarged cross-sectional view of the nose of the pin member and the base of the box member of the connection of FIG.  1 . 
     FIG. 4 is a cross-sectional view of the pin member of FIG. 1, shown stabbing into the box member of FIG. 1, and illustrating common misalignment that occurs during stabbing. 
     FIG. 5 is an enlarged cross-sectional view of an upper portion of the pin and box members shown in the position of FIG.  4 . 
     FIG. 6 is an enlarged cross-sectional view of a lower portion of the pin and box members shown in the position of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, the tubular connection or pipe joint  10  of a pin member  12  and box member  14  is shown. Pin  12  has an external thread  16  and extends from a pin end  18  to a dual angle torque shoulder  20 . In a similar manner, box  14  has a cooperative internal thread  22  and extends from a box end  24  to a box shoulder  26 . Threads  16  and  22  preferably have rounded roots. 
     Referring to FIG. 2, dual angle torque shoulder  20  on pin member  12  is generally concave and comprises two conical surfaces: an outer surface  20   a  having a negative draft angle with respect to the outer surface of the tubular element, and an inner surface  20   b  extending inward from outer surface  20   a  and having a positive draft angle with respect to the outer surface of the tubular element. Outer surface  20   a  extends downward and outward from the intersection with inner surface  20   b . Inner surface  20   b  extends downward and inward from its intersection with outer surface  20   a . The angle of outer surface  20   a  relative to a plane perpendicular to the longitudinal axis of the connection  10  is approximately 30 degrees negative in the preferred embodiment. The angle of inner surface  20   b  relative to the same plane is approximately 5 degrees positive. These angles may differ, however. Outer surface  20   a  and inner surface  20   b  are formed so that the angle between surfaces  20   a  and  20   b  must be less than 180°, and in the preferred embodiment is approximately 150°. The width of outer surface  20   a  may be larger than inner surface  20   b  or vice-versa. A large blend radius of approximately 0.15 inch blends the junction of the two surfaces  20   a  and  20   b . The outer and inner surfaces generally define an annular channel with a concave configuration. 
     Box end  24  comprises mating outer and inner conical surfaces, generally defining a convex configuration or a toroid, so that the box end  24  fits securely within the dual angle torque shoulder  20 . This configuration locks the box end  24  securely into place against the pin  12 , preventing radial distention of the box end  24 . 
     The primary internal seal of the connection  10  is formed adjacent to the box shoulder  26  between an outwardly facing internal sealing surface  28  on pin  12  and an inwardly facing internal sealing surface  30  on box  14 . This primary internal seal is an interference seal formed by the metal-to-metal contact between surfaces  28  and  30 , as is well known in the art. Sealing surfaces  28  and  30  can be tapered. As shown in FIG. 3, metal seal surface  28  on the nose of pin  12  is recessed from a guide surface  29  located directly above. That is, the outer diameter of seal surface  28  is less than the outer diameter of guide surface  29 . 
     The primary internal seal of the connection  10  is formed adjacent to the box shoulder  26  between an outwardly facing internal sealing surface  28  on pin  12  and an inwardly facing internal sealing surface  30  on box  14 . This primary internal seal is an interference seal formed by the metal-to-metal contact between surfaces  28  and  30 , as is well known in the art. Sealing surfaces  28  and  30  can be tapered. As shown in FIG. 3, metal seal surface  28  on the nose of pin  12  is recessed from a guide surface  29  located directly above. That is, the outer diameter of seal surface  28  is less than the outer diameter of guide surface  29 . Guide surface  29  is conical and, relative to the longitudinal axis of box  14 , is formed at an angle larger than the taper angle of internal thread  22 , as indicated by the numeral  31  in FIG.  3 . Also, a tangent line extending from guide surface  29  is located radially outward from pin nose sealing surface  28 , as indicated by the numeral  33  in FIG.  3 . If pin  12  is misaligned while stabbing into box  14 , guide surface  29  will contact internal thread  22 , protecting nose sealing surface  28 . 
     The primary external seal of the connection  10  is formed near the box end  24  between an outwardly facing external sealing surface  32  on pin  12  and an inwardly facing external sealing surface  34  on box  14 . This primary external seal is another interference seal, formed by the metal-to-metal contact between surfaces  32  and  34 . Sealing surfaces  32  and  34  are located on a generally cylindrical portion of pin  12  and box  10 , but can also be tapered. As shown in FIG. 2, a lead-in or guide portion  35  joining box sealing surface  34  is tapered. A seal ring groove  36  is formed on pin  12  between the external thread  16  and the outwardly facing external sealing surface  32 . A thermoplastic seal ring  38  is located within seal ring groove  36 . Seal ring  38  acts as a back-up seal to the metal-to-metal external seal. Seal ring  38  seals against a bore surface  39  in box  14  that is lower than and separated from metal sealing surface  34  by a shoulder or step  37 . Alternately, seal ring  28  could be located above metal sealing surface  34 . The inner diameter of box  14  is greater at metal sealing surface  34  than at bore surface  39 . 
     Sealing surface  34  on box  10  is protected from damaging contact during stabbing, as indicated by the misaligned stabbing occurring in FIGS. 4-6. A portion of pin threads  16  may contact the tapered guide surface  35 , but will not contact sealing surface  34  As shown in FIG. 5, the larger inner diameter of guide surface  35  as well as step  37  and smaller diameter bore portion  39  prevent contact. Similarly, pin nose sealing surface  28  is protected from damaging contact during stabbing, even if misaligned. As shown in FIG. 6, box threads  22  may contact guide surface  29 , but not sealing surface  28  because of its smaller outer diameter. 
     Two stress relief grooves are formed on box  14 . The lower box stress relief groove  40  is located adjacent to the inwardly facing internal sealing surface  30 . The upper box stress relief groove  42  is located at the base of the threads  22 , spaced axially a short distance from groove  40 . These box stress relief grooves reduce the incidence of stress fractures of the box  14 , thereby improving the fatigue life of the connection  10 . 
     Two stress relief grooves are formed on box  14 . The lower box stress relief groove  40  is located adjacent to the inwardly facing internal sealing surface  30 . The upper box stress relief groove  42  is located at the base of the threads  22 , spaced axially a short distance from groove  40 . These box stress relief grooves reduce the incidence of stress fractures of the box  14 , thereby improving the fatigue life of the connection  10 . Stress relief grooves  44 ,  46  are separated by a conical band  41 . Upper stress relief groove  42  has a lesser axial extent than lower stress relief groove  44 , as well as a lesser radial depth. Sealing band  41  has a lesser axial width than lower stress relief groove  40 . 
     Two stress relief grooves are also formed on pin  12 . The lower pin stress relief groove  44  is located at the base of threads  16 . The upper pin stress relief groove  46  is located adjacent to the dual angle torque shoulder  20 . As is shown in FIG. 1, thermoplastic seal ring  38  is located generally between the pin stress relief grooves  44  and  46 . The two pin stress relief grooves  44  and  46  reduce the incidence of stress fractures of the pin  12 , thereby improving the fatigue life of the connection  10 . 
     The invention has significant advantages. The metal seals are protected from damage due to misalignment while stabbing the pin into the box. The make-up of the connection is self-aligning up to a certain degree of misalignment, such as three degrees. The dual angle torques shoulders preload the connection to stabilize the metal seals from cyclic loading. The long torque nose reduces torque needed for preload. 
     While the invention has been shown or described in only one of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

Summary:
An offshore riser includes a plurality of tubular elements, each having a pin member on one end and a box member on the other end for connection with adjacent tubular elements. The pin member has an external thread which engages an internal thread on the box member of the adjacent tubular element. A dual angle torque shoulder locks the box end securely into place against the pin, preventing radial distention of the box end and maintaining bearing pressure between the external sealing surfaces. Also, stress relief grooves are located on the box member and on the pin member, to reduce the incidence of stress fractures, thereby improving the fatigue life of the connection.