Elliptical Thread Root Design for Drill String Fatigue
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Solution Overview
Problem
In downhole drilling, standard API threaded connections fail due to high stress concentrations at the thread root, leading to fatigue-induced failures, especially under high dog-leg severity conditions, where the existing thread designs are insufficient to handle the increased dynamic bending loads.
Innovation Solution
A rotary shouldered connection with a tangentially elliptical thread root design is introduced, featuring a larger equivalent root radius, optimized root depth, and modified thread parameters such as pitch, flank angle, and taper, which reduces stress concentration and enhances fatigue strength by distributing peak stress within the root portion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If standard API threaded connections are used, then the device complexity is low and ease of manufacture is high, but the fatigue strength is insufficient and failure occurs at thread root under high stress concentrations
Solution Approach 1:
The patent applies local quality by modifying only the thread root geometry with an elliptical profile while keeping the rest of the thread structure similar to standard API connections. This localized modification concentrates the stress-reducing effect exactly where needed (at the thread root) without requiring complete redesign of the entire threaded connection, thus improving fatigue strength while limiting complexity increase.
Solution Approach 2:
The patent implements spheroidality by replacing the traditional sharp or circular thread root with an elliptical root profile. The elliptical curvature distributes stress more evenly along the thread root, eliminating stress concentration points that cause fatigue failure. This curved geometry modification directly addresses the fatigue strength problem while maintaining compatibility with existing threaded connection frameworks.
2Strength
If the thread root radius is increased to reduce stress concentration, then the fatigue strength improves, but the tensile capacity may be reduced
Solution Approach 1:
The patent applies parameter changes by systematically adjusting multiple thread geometry parameters simultaneously - the elliptical root dimensions (a and b), flank angles, pitch, and taper - rather than simply increasing the root radius. This multi-parameter optimization allows the design to achieve reduced stress concentration while maintaining adequate tensile capacity through balanced geometric adjustments.
Solution Approach 2:
The patent localizes the geometric modification to the thread root region with elliptical profiling, while maintaining standard thread parameters in other regions. This localized approach ensures that stress concentration is reduced at the critical root area without unnecessarily altering the overall thread geometry that contributes to tensile load bearing capacity.
3Duration of action of moving object
If optimized thread parameters are implemented, then the fatigue life increases by factor of 4, but the manufacturing precision requirements increase
Solution Approach 1:
The elliptical root profile provides a mathematically defined curved geometry that is more tolerant to manufacturing variations compared to sharp corners or complex profiles. The continuous curvature of the ellipse naturally distributes stress, and this geometric form can be achieved through standard machining processes with reasonable precision, balancing fatigue life improvement with manufacturability.
Solution Approach 2:
The patent optimizes specific elliptical parameters (semi-major axis a and semi-minor axis b) to achieve the desired fatigue life improvement. By carefully selecting these parameters, the design achieves maximum fatigue resistance while keeping the geometric deviations from standard threads within achievable manufacturing tolerances, thus balancing performance gain with manufacturing precision requirements.
Data Source
AI summary
A thread structure in accordance to one or more embodiments includes a thread extending helically along the cylindrical member in spaced thread turns, the thread having a crest extending between a first flank and a second flank and a root extending between the thread turns, the root having a curvature defined by a portion of an ellipse tangentially adjoining the first and second flank at respective flank transition points, the ellipse having a major axis extending parallel to the axis and a minor axis extending perpendicular to the major axis and through a root bottom.