Well Tubular Thread Form With Elliptical Root for Fatigue Resistance
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Solution Overview
Problem
Existing thread forms for well tubulars face challenges in withstanding significant torsional, axial, and thrust loading, particularly in deviated and short radius drilling applications, leading to fatigue and reduced torque and tensile capacity, with conventional designs compromising on performance due to stress concentration at thread roots.
Innovation Solution
A novel thread form featuring an elliptical root profile and specific flank angles, along with radiused transitions, which distributes bending stresses and provides a larger root surface area for improved fatigue resistance and torque capacity, while maintaining a lightweight design and minimizing cross-sectional area at critical points.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional thread forms are used, then manufacturing is simpler, but fatigue resistance and torque capacity are reduced due to stress concentration at thread roots
Solution Approach 1:
The patent applies curvature by replacing conventional sharp or filleted thread roots with a radiused transition profile. The radiused transition replaces the sharp corner with a curved surface that has a specific radius of curvature, distributing bending stresses more evenly across the thread root area. This curvature eliminates stress concentration points while maintaining structural integrity, directly improving fatigue resistance without requiring complex multi-radius profiles.
Solution Approach 2:
The patent applies local quality by creating different geometric features at different locations of the thread form. The thread profile includes a radiused transition at the root, a specific crest geometry, and optimized flank angles, where each region has tailored properties to address local stress conditions. The radiused transition specifically addresses the high-stress root area, while other regions maintain simpler geometries, achieving improved fatigue resistance without overall complexity.
2Strength
If thread forms with larger root surface area are used, then fatigue resistance improves, but cross-sectional area at critical points increases reducing lightweight design
Solution Approach 1:
The radiused transition creates an optimized curvature at the thread root that increases the effective stress-bearing area without adding bulk material. The controlled radius of curvature distributes loads across a larger surface area of the existing material, improving fatigue resistance through geometric optimization rather than material addition, thus maintaining lightweight design.
Solution Approach 2:
The patent optimizes geometric parameters including the radiused transition radius, flank angles, and crest geometry to maximize fatigue resistance within constrained material volume. By carefully selecting and optimizing these dimensional parameters, the design achieves improved fatigue performance without increasing overall cross-sectional area or weight, maintaining the lightweight requirement for tubulars.
3Force
If thread forms are designed for high torque and tensile capacity, then loading resistance improves, but stress concentration at thread roots increases leading to fatigue
Solution Approach 1:
The radiused transition directly addresses the conflict between high loading capacity and fatigue resistance by replacing sharp corners with curved transitions. This curvature allows the thread form to withstand high torsional and tensile forces while distributing the resulting bending stresses evenly across the root area, preventing stress concentration and fatigue crack initiation even under maximum load conditions.
Solution Approach 2:
The thread form is segmented into distinct functional zones: the radiused transition zone at the root for stress distribution, the flank regions for load transmission, and the crest region for sealing and engagement. This segmentation allows each zone to be optimized for its specific function, with the radiused transition specifically engineered to handle bending stresses from high torque and tensile loads without creating stress concentrations.
Data Source
AI summary
A threaded product is provided having an external thread form on a tubular pin member comprises load and stab flanks on either side of a crest, along with smooth transitions between the crest and flanks. An elliptical root between the flanks aids in reducing stress and forms a dope volume between a non-conforming crest of an internal thread form of a tubular box member. Flanks of the internal thread of the box member contact flanks of the external thread of the pin member to form areas of mating contact on either side of the thread.