Drill collar connections

Abstract

A threaded connection with improved fatigue-life useful for connecting adjacent sections of a pipe string subject to bending, tensile, and compressive stresses such as a drill collar string is disclosed. The connection comprises a pin member which is threadedly engageable with a corresponding box member. In the preferred embodiment, the inventive connection is designed to have bending strength ratio ("BSR") of about 3.20 or greater, and more preferably, in a range between about 3.20 and about 4.00. The preferred thread form of the connection is a V-type thread form having two flank angles of 30 degrees and a 0.038 inch root radius. Other design features may be used in combination with the threaded connection to further improve the connection's fatigue-life. Such design features include a tapered pin bore, an increased root radius for selected box threads, an internal box stress relief groove, a box bore stress relief groove, an extended pin relief groove, and modification of the API boreback box design to provide an increased number of threads having full threaded engagement.

We claim: 
     
       1. A threaded connection for use in connecting two adjacent sections of pipe in a pipe string, said threaded connection comprising: a) a generally tubular pin member on one end of a first pipe section having a longitudinal axis, at least a portion of said pin member having a generally frustoconical outer surface which tapers radially inwardly toward said end of said first pipe section, said frustoconical outer surface having pin threads formed thereon;   b) said pin member having a nose, a shoulder and a pin length equal to the axial distance between said nose and said shoulder;   c) a generally tubular box member on one end of a second pipe section, at least a portion of said box member having a generally frustoconical inner surface which tapers radially outwardly toward said end of said second pipe section, said frustoconical inner surface having box threads formed thereon which correspond to and are threadedly engageable with said pin threads formed on said pin member;   d) said box threads having a first engaged box thread corresponding to the pint thread nearest said shoulder and a last engaged box thread corresponding to the pin thread nearest said nose; and   e) said threaded connection being designed and engineered so as to have a bending strength ratio of about 3.20 or greater.   
     
     
       2. The threaded connection of claim 1 wherein said pipe sections have an outside diameter of about 4.75 inches and an inside diameter of about 2.25 inches and wherein said pin threads and said box threads have a pitch of about 4 threads per inch, a taper of about 2 inches per foot on diameter, a pitch diameter of 3.434±0.005 inches, and said pin length of said pin member is between about 4.00 and about 5.00 inches. 
     
     
       3. The threaded connection of claim 1 wherein said pipe sections have an outside diameter of about 6.50 inches and an inside diameter of about 2.81 inches and wherein said pin threads and said box threads have a pitch of about 4 threads per inch, a taper of about 2 inches per foot on diameter, a pitch diameter of 4.556±0.005 inches, and said pin length of said pin member is between about 4.50 and about 5.50 inches. 
     
     
       4. The threaded connection of claim 1 wherein said pipe sections have an outside diameter of about 7.00 inches and an inside diameter of about 2.81 inches, and wherein said pin threads and said box threads have a pitch of about 4 threads per inch, a taper of about 2 inches per foot on diameter, a pitch diameter of 4.835±0.005 inches, and said pin length of said pin member is between about 4.50 about 5.50 inches. 
     
     
       5. The threaded connection of claim 1 wherein said pipe sections have an outside diameter of about 8.00 inches and an inside diameter of about 2.81 inches and wherein said pin threads and said box threads have a pitch of about 4 threads per inch, a taper of about 2 inches per foot on diameter, a pitch diameter of 5.429±0.05 inches, and said pin length of said pin member is between about 5.00 and about 6.00 inches. 
     
     
       6. The threaded connection of claim 1 wherein said pipe sections have an outside diameter of about 9.50 inches and an inside diameter of about 3.00 inches and wherein said pint threads and said box threads have a pitch of about 4 threads per inch, a taper of about 3 inches per foot on diameter, a pitch diameter of 6.431±0.005 inches, and said pin length of said pin member is between about 5.25 and about 6.25 inches. 
     
     
       7. The threaded connection of claim 1 further comprising: a) said box member having a cylindrical bore adjacent to said last engaged box thread and coaxial with said axis, said cylindrical bore having a diameter substantially equal to the diameter of said nose of said pin member; and   b) said box threads, each having a crest with substantially the same crest truncation, wherein said box threads are threadedly engageable with said pin threads.     
     
     
       8. The threaded connection of claim 7 further comprising a box bore stress relief groove positioned on said box member so that said cylindrical bore extends between said last engaged box thread and said box bore stress relief groove. 
     
     
       9. The threaded connection of claim 1 wherein a stress relief groove, having a radius of curvature between about 0.150 inch and about 0.250 inch, is placed on said box member between said first engaged box thread and said last engaged box thread. 
     
     
       10. A threaded connection for use in connecting two adjacent sections of pipe in a pipe string, said threaded connection comprising: a) a generally tubular pin member on one end of a first pipe section having a longitudinal axis, at least a portion of said pin member having a generally frustoconical outer surface which tapers radially inwardly toward said end of said first pipe section, said frustoconical outer surface having pin threads formed thereon;   b) said pin member having a nose, a shoulder and a pin length equal to the axial distance between said nose and said shoulder;   c) a generally tubular box member on one end of a second pipe section, at least a portion of said box member having a generally frustoconical inner surface which tapers radially outwardly toward said end of said second pipe section, said frustoconical inner surface having box threads formed thereon which correspond to and are threadedly engageable with said pint threads formed on said pin member;   d) said box threads having a first engaged box thread corresponding to the pint thread nearest said shoulder and a last engaged box thread corresponding to the pin thread nearest said nose;   e) said box threads each having a crest with substantially the same crest truncation; and   f) said box member having a cylindrical bore adjacent to said last engaged box thread and coaxial with said axis, said cylindrical bore having a diameter substantially equal to the diameter of said nose of said pin member.   
     
     
       11. The threaded connection of claim 10 further comprising a box bore stress relief groove positioned on said box member so that said cylindrical bore extends between said last engaged box thread and said box bore stress relief groove. 
     
     
       12. The threaded connection of claim 10 wherein said threaded connection is designed and engineered so as to have a bending strength ratio of about 3.20 or greater. 
     
     
       13. The threaded connection of claim 10 wherein a stress relief groove, having a radius of curvature between about 0.150 inch and about 0.250 inch, is placed on said box member between said first engaged box thread and said last engaged box thread. 
     
     
       14. A threaded connection for use in connecting two adjacent sections of pipe in a pipe string, said threaded connection comprising: a) a generally tubular pin member on one end of a first pipe section having a longitudinal axis, at least a portion of said pin member having a generally frustoconical outer surface which tapers radially inwardly toward said end of said first pipe section, said frustoconical outer surface having pin threads formed thereon;   b) said pin member having a nose, a shoulder and a pin length equal to the axial distance between said nose and said shoulder;   c) a generally tubular box member on one end of a second pipe section, at least a portion of said box member having a generally frustoconical inner surface which tapers radially outwardly toward said end of said second pipe section, said frustoconical inner surface having box threads formed thereon which correspond to and are threadedly engageable with said pin threads formed on said pin member;   d) said box threads having a first engaged box thread corresponding to the pin thread nearest said shoulder and a last engaged box thread corresponding to the pin thread nearest said pin nose; and   e) said box member having a stress relief groove, said groove having a radius of curvature between about 0.150 inch and about 0.250 inch, and positioned between said first engaged box thread and said last engaged box thread.   
     
     
       15. The threaded connection of claim 14 further comprising: a) said box member having a cylindrical bore adjacent to said last engaged box thread and coaxial with said axis, said cylindrical bore having a diameter substantially equal to the diameter of said nose of said pin member; and   b) said box threads, each having a crest with substantially the same crest truncation, wherein said box threads are threadedly engageable with said pin threads.