Threaded connection for enhanced fatigue resistance

An oilfield tubular threaded connection for improved fatigue resistance to cyclic loading of threadably connected oilfield tubulars comprising:

• a pair of tubular bodies each having a longitudinal passage there through and including radially inner spiral threads on each of a distal end of each tubular body, each spiral thread including a spiral compression flank forming an angle less than 30.degree. with respect to a plane perpendicular to a longitudinal axis of the pair of tubular bodies and a spiral tension flank projecting axially away from the spiral compression flank and forming an angle less than 7.degree. with respect to the plane perpendicular to the axis of the pair of tubular bodies;
• a coupling body threadably connected to a distal end of each of the pair of tubular bodies and having a longitudinal passage there through, including radially outer spiral threads on each of a distal end of the coupling body, each spiral thread including a spiral compression flank and a spiral tension flank, the spiral tension flank on the inner spiral threads of the pair of tubular bodies being in mating planar engagement with the corresponding spiral tension flank on the outer spiral threads of the coupling body;
• and
• an axial spacing between the tension flanks and the compression flanks on at least one of the radially inner and radially outer spiral threads defining a stress reducing spiral root surface spaced radially from a corresponding spiral crest surface on the other radially inner and radially outer spiral threads, said stress reducing spiral root surface spaced axially between the compression flank of one thread and the tension flank of an adjoining thread including in cross-section a substantially linear profile root surface and first and second axially spaced rounded surfaces each connecting the linear profile root surface with a corresponding compression flank and tension flank, an axial length of the linear profile root surface defining a thread root width spaced between theoretical intersections of the linear profile root surface with both the compression flank and the tension flank, each first and second rounded surface having a radius that is 10% to 50% of the thread root width for reducing stress concentration from applied loads exerted on said threaded connection during loading of said connected oilfield tubulars.

A method for producing a threaded connection for reducing bending stresses in threadably connected oilfield tubulars comprising:

• forming a pair of tubular bodies each having a longitudinal passage there through, and including radially inner spiral threads on each distal end of each tubular body, each spiral thread including a spiral compression flank and spiral tension flank;
• forming a coupling body having a longitudinal passage there through, and including a radially outer spiral thread on each of a distal end of the coupling body, each spiral thread including a spiral compression flank and a spiral tension flank, the spiral tension flank on the inner spiral threads of the pair of tubular bodies being in mating planar engagement with the corresponding spiral tension flank on the outer spiral threads of the coupling body;
• forming an axial spacing between the tension flanks and the compression flanks on at least one of the radially inner and radially outer spiral threads to define a stress reducing spiral root surface spaced radially from a corresponding spiral crest surface on the other radially inner and radially outer spiral threads, said stress reducing spiral root surface spaced axially between the compression flank of one thread and the tension flank of an adjoining thread including in cross-section a substantially linear profile root surface and a first rounded surface connecting the linear profile root surface with a corresponding tension flank and a second rounded surface connecting the linear profile root surface with a corresponding compression flank, an axial length of the linear profile root surface defining a thread root width spaced between theoretical intersections of the linear profile root surface with both the compression flank and the tension flank, each first and second rounded surface having a radius that is 10% to 50% of the threaded root width, the radius of the first rounded surface being substantially greater than a radius of another first rounded surface connecting the corresponding spiral crest surface with the tension flank on the outer spiral threads to minimize engagement between the first rounded surface and the another first rounded surface thereby reducing stress concentration from applied loads exerted on said threaded connection during loading of said connected oilfield tubulars;
• and
• attaching the coupling body to a distal end of each of the pair of tubular bodies.