Pipe coupling for well casing

Claim 1

A stainless steel well casing including pipe couplings of stainless steel for joining adjacent pipe sections of a selected diameter to extend in an axial direction, the pipe couplings each comprising: a tubular male member having first and second ends and a tubular female member having first and second ends wherein the first ends of the male and female members abut the pipe sections while the second end of the male member is received within the female member and the second end of the female member extends over the male member; the male member having a first portion having an outside diameter equal to that of the pipe section; a second portion having a reduced outside diameter and joining the first portion at a first radial shoulder, the reduced diameter portion having a continuous first unthreaded cylindrical surface of a constant diameter extending from the first radial shoulder in the axial direction; a threaded section extending from the first unthreaded cylindrical section, the threaded section having threads of a square cross-section and a diameter no greater than the outside diameter of the first unthreaded cylindrical surface; a second unthreaded cylindrical surface extending from the threaded section to the second end of the male member, the second end of the male member presenting a radial surface perpendicular to the axis of the male member; and a constant internal diameter over substantially the entire length thereof, except for a frusto-conical section adjacent the first end thereof; the female member having a constant outside diameter over the entire length thereof, the outside diameter of the female member being equal to the outside diameter of the first portion of the male member with the second end of the female member being a radial surface normal to the axis of the female member; a first portion with an internal diameter equal to that of the internal diameter of the male member, said first portion joining the first end of the female member with a frusto-conical surface; a second portion of a diameter less than the first portion and greater than the second portion of the male member, the second portion of the female member having a first unthreaded cylindrical surface adjacent a second radial shoulder; a threaded section extending from the first unthreaded cylindrical portion, the threaded section having square threads thereon which complement the square threads of the threaded section on the male member but wherein there is axial and radial spacing between the threads of the male and female members allowing initial play therebetween; a second unthreaded cylindrical surface between the threaded section and the second end of the female member, the second unthreaded cylindrical surface having an internal diameter greater than the internal diameter of the first unthreaded cylindrical surface and slightly greater than the first unthreaded cylindrical surface on the male member; wherein the distance between the shoulders and second ends of the male and female members is identical; a seal disposed only on the first unthreaded cylindrical surface of the male member and being engageable by the second unthreaded cylindrical surface of the female member as the male and female members are being threaded together, the second unthreaded cylindrical surface of the male member and first unthreaded cylindrical surface of the female member being in direct opposed relation with no seal disposed therebetween wherein the coupling becomes increasingly rigidified and sealed as the male and female members are threaded together, and wherein the coupling becomes completely rigid upon engagement of the second ends of the male and female members with the shoulders of the female and male members; and internal welds for joining the first end members to adjacent sections of pipes, the first ends of the members abutting the pipes directly and the welds having beads which overly at least a portion of the interior wall of the pipe and the frustoconical surfaces of the male and female members.