Relieved Coupling Threads for API Buttress Galling Control

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Solution Overview

Problem

Conventional API buttress threaded couplings in the oil and gas industry experience excessive hoop stresses and thread galling due to unbalanced cross-sections during assembly, leading to potential fatigue failure under rotational cyclic loading.

Innovation Solution

The coupling thread surfaces are modified with reduced thread heights in specific portions while maintaining the same pitch and thread depth diameter, reducing stress and thread galling, and an integral sacrificial wear-sleeve extension is added to protect the coupling from frictional wear.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If conventional API buttress threading is used in couplings, then compatibility with standard threaded pins is maintained, but excessive hoop stresses and thread galling occur due to unbalanced cross-sections

Engineering Contradiction:

Improvecompatibility with standard API buttress threaded pinsVSAvoidthread galling and fatigue failure resistance

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The coupling incorporates a relieved thread region with reduced thread height in specific portions (lead threads or run-out threads) while maintaining full thread height in other portions. This local modification creates balanced cross-sections at critical stress points, reducing hoop stresses and thread galling without compromising overall compatibility with standard API buttress threaded pins

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The thread height parameter is selectively reduced in specific portions of the coupling threads to create relieved thread regions. This parameter change balances the cross-sectional areas at critical locations, equalizing make-up stresses and reducing stress concentrations that lead to thread galling and fatigue failure

Inventive Principle:

Principle #35Parameter changes

2Strength

If full thread height is maintained throughout the coupling, then strength is maximized, but stress concentrations occur at unbalanced cross-sections leading to thread galling

Engineering Contradiction:

Improveoverall coupling strengthVSAvoidresistance to thread galling and fatigue failure

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

Rather than uniformly reducing thread height throughout, the invention applies local relief only to specific portions (lead threads or run-out threads) where cross-sectional imbalance causes stress concentrations. This localized approach maintains overall strength while eliminating problematic stress points

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The invention converts the potentially harmful stress concentrations at unbalanced cross-sections into beneficial balanced cross-sections by selectively reducing thread height. This transforms the problem of stress concentration into a solution that equalizes stresses while maintaining connection integrity

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Power

If axial length of coupling is reduced to allow pin abutment, then torque capabilities are maximized, but manufacturing complexity increases

Engineering Contradiction:

Improvetorque capabilitiesVSAvoidcoupling manufacturing complexity

Core Design Contradiction:

PowerVSEase of manufacture

Solution Approach 1:

The coupling thread region is segmented into portions with different thread heights - full height threads in some regions and relieved (reduced height) threads in others. This segmentation allows the coupling to achieve pin abutment for maximum torque capability while managing manufacturing complexity through targeted modifications rather than complete redesign

Inventive Principle:

Principle #1Segmentation

Data Source

PatentUS11614186B1Box connection for a pin with relieved thread region

Publication Date: 2023.03.28 BOOMERANG TUBE LLC

AI summary

A modified API buttress threaded casing connection is disclosed for use in drilling oil and gas wells in lieu of using conventional drill pipe. The coupling threads only are modified to include a reduced thread height thread form to relieve thread stresses at axial ends of the coupling. The reduced thread height thread form may be mated with standard single tapered API buttress thread forms formed on the pin ends. The axial length of the coupling may be generally shortened to permit abutment of the mating pins at the center of the coupling.