Cold-Rolled Elliptical Thread Roots for Drill String Fatigue

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Solution Overview

Problem

Standard API threaded connections in downhole drilling are prone to fatigue failures due to high stress concentrations at the thread root, especially under high dog-leg severity conditions, leading to reduced component life and increased failure rates.

Innovation Solution

A rotary shouldered connection with a tangential elliptical thread root design, featuring an optimized root depth, equivalent root radius, and flank angles, combined with a cold rolling process to reduce stress concentrations and enhance fatigue strength, is introduced. The thread structure includes a larger pitch, smaller flank angle, and a tangential elliptical root shape, which is cold rolled using a wheel with a matching elliptical root profile to generate compressive residual stresses.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If standard API threaded connections are used, then manufacturing simplicity is maintained, but stress concentrations at the thread root cause high fatigue failure rates

Engineering Contradiction:

Improvefatigue lifeVSAvoidthread root geometry complexity

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent applies parameter changes by modifying the thread root geometry parameters, specifically implementing an elliptical root profile with optimized dimensions (equivalent root radius of 0.057-0.061 inches, root depth of 0.012-0.020 inches) and cold rolling the threads to induce compressive residual stresses, thereby reducing stress concentrations and enhancing fatigue life by a factor of 4 or more

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The cold rolling process is applied as a preliminary action before the threads are subjected to service loads. By pre-inducing compressive residual stresses through cold rolling with a specially designed elliptical wheel, the thread roots are prepared in advance to resist the high tensile and bending stresses that cause fatigue failures in standard API connections

Inventive Principle:

Principle #10Preliminary action

2Reliability

If cold rolling process is applied to enhance fatigue strength, then fatigue life is significantly improved, but manufacturing process complexity increases

Engineering Contradiction:

Improvefatigue strengthVSAvoidcold rolling equipment complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The cold rolling process parameters are precisely controlled and standardized, including the elliptical wheel geometry (root radius, root depth, wheel angles), rolling load, and rolling speed. By establishing specific parameter ranges and control procedures, the patent achieves consistent fatigue strength enhancement while managing the complexity of the cold rolling equipment through parameter standardization

Inventive Principle:

Principle #35Parameter changes

3Reliability

If larger equivalent root radius is used, then stress concentrations are reduced and fatigue life is enhanced, but thread root depth increases requiring more complex tooling

Engineering Contradiction:

Improvestress concentration reductionVSAvoidthread root depth control

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent optimizes the thread root geometry parameters by implementing an elliptical root profile with a specific equivalent root radius (0.057-0.061 inches) and root depth (0.012-0.020 inches). This parameter optimization reduces stress concentrations while maintaining manufacturability through cold rolling, which can precisely control the final root dimensions without requiring complex multi-step machining operations

Inventive Principle:

Principle #35Parameter changes

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The tangential elliptical thread root design significantly enhances the fatigue life of rotary shouldered connections by reducing stress concentrations and maintaining shear resistance, with a minimum factor of 4 enhancement in fatigue life compared to standard API threads, and the cold rolling process further delays fatigue cracking, resulting in a connection that is 3 to 5 times stronger than the as-machined version.

Implementation Method 1

The root profile is cold rolled using a wheel having a wheel tip with an elliptical root shape... to generate compressive residual stresses

Methodology Applied

Scientific EffectCompressive residual stress: Compression

Implementation Method 2

The tangential elliptical thread root design significantly enhances the fatigue life of rotary shouldered connections by reducing stress concentrations

Methodology Applied

Scientific EffectStress concentration reduction:

Data Source

PatentUS11389858B2Cold rolling devices and cold rolled rotary shouldered connection threads

Publication Date: 2022.07.19 SCHLUMBERGER TECH CORP

AI summary

A device for cold rolling a thread on a tubular support member of a rotary shouldered thread connection includes obtaining an original root depth of a thread root, cold rolling the thread until a minimum increased root depth tolerance is achieved.