Modified API Buttress Coupling Reduces Hoop Stress
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Solution Overview
Problem
Standard API Buttress Connections fail to withstand high mechanical stresses in deviated wells, leading to plastic yielding and splitting, which increases well costs and poses environmental risks due to excessive hoop stress from internal pressure and make-up loads.
Innovation Solution
A modified API Buttress Coupling with an internal mechanical stop and altered thread parameters, including increased pitch diameter and reduced thread interference, to control plastic deformation and minimize hoop stress, while maintaining gas tightness and compatibility with standard API Pins.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If standard API Buttress Connections are used to reduce well costs, then tubular cost is reduced, but the coupling yield strength is exceeded under high mechanical stresses, resulting in plastic yielding or splitting
Solution Approach 1:
The patent modifies the API Buttress Coupling geometry by changing the thread parameters (pitch diameter, thread angle) and adding an internal mechanical stop or shoulder. These parameter changes allow the coupling to withstand combined loads including make-up torque, internal pressure, bending, and torsional loads without exceeding yield strength, while maintaining compatibility with standard API Pins and controlling plastic deformation.
2Productivity
If standard API Buttress Connections are used in highly deviated wells, then drilling cost is reduced, but excessive hoop stress from internal pressure and make-up loads causes splitting
Solution Approach 1:
The patent increases the pitch diameter of the modified coupling and reduces thread interference, which directly reduces hoop stress induced during torsional movement and make-up operations. This allows the coupling to withstand the combined effects of internal pressure and make-up loads in highly deviated wells without splitting.
Solution Approach 2:
The patent introduces an internal mechanical stop or shoulder that segments the load path, limiting forward torsional movement of the pin and distributing stresses more evenly throughout the coupling structure, thereby reducing peak hoop stresses.
3Ease of manufacture
If standard API Buttress Connections are used, then initial equipment cost is reduced, but plastic yielding exceeds design limits under combined loads
Solution Approach 1:
The patent modifies the coupling geometry with increased pitch diameter and altered thread parameters, which increases the moment of inertia and section modulus, thereby increasing the yield strength under combined loads including make-up torque, internal pressure, bending, and torsional loads.
Solution Approach 2:
The patent creates a hybrid connection system that combines modified coupling geometry with standard API Pins, achieving enhanced strength performance while maintaining compatibility with existing standard components.
Data Source
AI summary
A casing coupling has a taper which is different from the taper of a standard API Pin. The coupling has a thread pitch diameter which is increased relative to that of a standard API coupling but which still mates with a standard API pin.