Threaded Tool Joint Connection Fatigue Resistance
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Solution Overview
Problem
Existing drill stem threaded connections lack enhanced bending fatigue resistance, fluid pressure retention, and stability under high torque and tensile loads while maintaining a slim-hole design with a large ID and small OD, and are prone to swelling or buckling.
Innovation Solution
A double-shoulder threaded tool joint connection with a pin and box design featuring a common centerline, shallow thread taper (0.75-1.0 inch per foot), equal stab and load flank angles (33-39 degrees), circular root shapes with a minimum radius of 0.063 inch, and short pin nose and box counterbore lengths to enhance torque resistance and fatigue strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a steep thread taper (1 and 1/8 inches per foot) is used to increase stabbing depth and reduce turns-to-make-up, then connection speed improves, but torque capability and internal shoulder cross-sectional area decrease
Solution Approach 1:
The patent changes the thread taper parameter from the conventional steep taper (1 and 1/8 inches per foot) to a shallow taper (seven-eighths inch per foot). This parameter change allows the threads to maintain sufficient engagement length while preserving cross-sectional area at the internal shoulder, thereby maintaining both connection speed and torque capability.
Solution Approach 2:
The patent compensates for the reduced taper by optimizing other dimensional parameters, specifically the thread length and root radius. The extended thread engagement length compensates for the shallower taper, while the increased root radius (0.063 inch or more) maintains strength without requiring a steep taper.
2Loss of time
If a steep thread taper is used to reduce turns-to-make-up, then connection time decreases, but the ID cannot be as large due to conflict with maintaining enough steel for internal shoulder
Solution Approach 1:
The patent changes the thread taper parameter to shallow (seven-eighths inch per foot), which allows the internal diameter to be larger while still maintaining sufficient steel cross-sectional area at the internal shoulder. The extended thread engagement compensates for the shallower taper, maintaining connection efficiency.
3Ease of manufacture
If conventional thread forms with smaller root radii are used, then manufacturing is easier, but bending fatigue resistance and stress concentration reduction are insufficient
Solution Approach 1:
The patent applies a large root radius (0.063 inch or more) to the thread form, creating a more rounded, spherical transition at the thread root. This curvature reduces stress concentration and improves bending fatigue resistance while remaining manufacturable with standard threading equipment.
4Strength
If long pin nose and box counterbore lengths are used (1 to 1.5 times counterbore length), then load distribution improves, but swelling or buckling occurs under high loads
Solution Approach 1:
The patent optimizes the pin nose and box counterbore lengths to achieve a balanced ratio. The pin nose length is set to 0.25 to 0.75 times the box counterbore length, which provides sufficient load distribution while preventing excessive compression that would cause swelling or buckling. This parameter optimization resolves the contradiction between load distribution and structural stability.
Data Source
AI summary
In a double-shoulder threaded tool joint connection, the pin external threads and the box internal threads have a thread taper between 0.75 inch per foot and 1.0 inch per foot. The pin external threads and the box internal threads have equal stab flank and load flank angles, and are in a range from thirty-three to thirty-nine degrees. Both the pin external threads and the box internal threads have roots formed in a shape of a portion of a circle tangent to both thread flanks with a minimum radius of 0.063 inch. The pin external threads and the box internal threads are in the range of two to three per inch. The box counterbore has a length of 0.750 or less. The pin nose has a length in a range of 30% to 90% of the length of the box counterbore.