Double-Shoulder Threaded Joint Roots for Drill String Fatigue

Resolve Bottlenecks,
Find Innovative SolutionsGenerate Solutions

Solution Overview

Problem

Conventional 'V' thread forms in drilling tubulars suffer from fatigue cracks due to high bending stresses, leading to washouts or twist-off failures, while modifications to reduce these stresses compromise torque and tensile capacity, and fail to minimize well annular pressure loss and cutting removal.

Innovation Solution

A fatigue-reducing shouldered drilling connection with multi-surface undercut thread roots, featuring external and internal threads with specific flank angles and radii designs that enhance root surface area, torque capacity, and interchangability with existing connections.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional V threads are used, then the connection structure is simple, but fatigue cracks develop at thread roots due to high bending stresses

Engineering Contradiction:

Improvefatigue resistanceVSAvoidthread form complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The thread root is segmented into multiple surfaces (first undercut surface at 20-25° and second undercut surface at 45-50°) instead of a single V-shape, creating distinct zones that distribute stress and prevent crack initiation while maintaining structural integrity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The thread design applies different geometric properties to different locations: the thread root area features multi-surface undercuts with specific angle ranges (20-25° and 45-50°) to reduce stress concentration, while the thread flanks maintain standard angles (29-31°) for optimal load transmission, creating localized stress management without compromising overall connection strength

Inventive Principle:

Principle #3Local quality

2Reliability

If root radius is increased or multiple radii are added to reduce peak stresses, then fatigue resistance improves, but torque and tensile capacity decrease

Engineering Contradiction:

Improvefatigue resistanceVSAvoidtorque and tensile capacity

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The thread profile is made asymmetric with respect to the root geometry, featuring undercut surfaces at specific angles (20-25° and 45-50°) that create an asymmetric stress distribution pattern, allowing the root to withstand bending stresses while maintaining adequate torque and tensile capacity through the asymmetric load path

Inventive Principle:

Principle #4Asymmetry

Solution Approach 2:

The invention changes the geometric parameters of the thread root from a simple V-shape with single radius to a multi-surface configuration with controlled undercut angles (20-25° and 45-50°) and multiple radii (first radius 0.002-0.006 inches and second radius 0.006-0.012 inches), optimizing both fatigue resistance and strength simultaneously

Inventive Principle:

Principle #35Parameter changes

3Reliability

If undercuts are applied to produce larger root surface, then fatigue performance improves, but connection geometry is affected reducing torque and tensile capacity

Engineering Contradiction:

Improvefatigue performanceVSAvoidtorque and tensile capacity

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The invention optimizes the undercut geometry by specifying precise angle ranges (20-25° for first undercut, 45-50° for second undercut) and radius ranges (first radius 0.002-0.006 inches, second radius 0.006-0.012 inches), creating a balanced configuration that enlarges the root surface for fatigue resistance while preserving torque and tensile capacity through controlled geometric parameters

Inventive Principle:

Principle #35Parameter changes

4Reliability

If thread form is modified to reduce stress concentration, then fatigue resistance improves, but well annular pressure loss increases

Engineering Contradiction:

Improvefatigue resistanceVSAvoidwell annular pressure loss

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The undercut features are applied partially to only the thread root region rather than the entire thread form, with the undercut angles (20-25° and 45-50°) and radii confined to the root area, allowing stress concentration reduction at the critical location while maintaining standard thread geometry elsewhere to minimize impact on well annular flow characteristics

Inventive Principle:

Principle #16Partial or excessive action

Data Source

PatentUS11332982B2Fatigue reducing shouldered connections

Publication Date: 2022.05.17 COASTAL PIPE USA L L C

AI summary

A double shoulder threaded tool joint connection includes a pin with external threads formed between a pin external shoulder and a pin internal shoulder, the pin including a nose section between the pin internal shoulder and the pin external thread and a box with internal threads formed between a box external shoulder and a box internal shoulder. The internal threads and the external threads are arranged and designed for connection with each other so that the box and the pin are connected with a common center-line and with a primary seal formed by the pin external shoulder forced against the box external shoulder and a secondary seal formed between the pin internal shoulder forced again the box internal shoulder. The invention may be applied to any threaded joint.