Threaded Joint Fatigue Resistance via Yield Strength Make-Up
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Solution Overview
Problem
Threaded joints in offshore hydrocarbon extraction equipment face fatigue stress due to sea movements, leading to potential ruptures, with existing solutions focusing on corrosion resistance rather than fatigue improvement.
Innovation Solution
A method for making up threaded joints by applying a torque that reaches 50-90% of the steel's yield strength, incorporating high radial and shoulder loads, enlarged root-to-flank radius, and shot peening surface treatment to enhance fatigue resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional make-up torque is applied to threaded joints, then assembly is straightforward, but fatigue resistance is insufficient due to stress concentration at thread roots
Solution Approach 1:
The method applies preliminary plastic deformation to the thread roots during the make-up process by exceeding yield strength. This preliminary action creates compressive residual stresses that counteract fatigue loads during service, thereby improving fatigue resistance before the joint is actually put into operation.
Solution Approach 2:
The invention changes the torque application parameters by applying torque that generates stresses between 50-90% of yield strength, which is higher than conventional make-up torques. This parameter change induces plastic deformation and residual compressive stresses in the thread roots, transforming the stress state to improve fatigue performance.
2Strength
If higher make-up torque is applied to increase fatigue resistance, then stress distribution improves, but risk of yielding and permanent deformation increases
Solution Approach 1:
The method applies localized plastic deformation specifically at the thread roots where fatigue cracks typically initiate. By concentrating the plastic deformation and residual compressive stresses at this critical location rather than uniformly throughout the joint, the invention improves fatigue resistance while minimizing the overall risk of yielding and permanent deformation.
3Strength
If thread geometry is modified with enlarged root-to-flank radius, then stress concentration is reduced, but manufacturing precision requirements increase
Solution Approach 1:
Rather than relying solely on precise thread geometry to prevent stress concentration, the invention applies preliminary plastic deformation during make-up to create residual compressive stresses. This preliminary action compensates for geometric stress concentrations, allowing less stringent manufacturing tolerances while still achieving high fatigue resistance.
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 method significantly increases the fatigue life of threaded joints by distributing stress evenly and reducing concentration, while the surface treatment further enhances resistance to high loads and galling.
Implementation Method 1
The method for make up of joints comprises the steps of inserting the threaded portion of pin into the threaded portion of box, applying a torque for making up the pin in the box until first and second abutment shoulders abut, applying an extra torque until a magnitude between 50% and 90% of the steel's yield strength in the most stressed part of the joint is reached
Implementation Method 2
there is provided a root to load flank radius having a value of around 1/4 of the thread height and an average surface roughness Ra of between 0,8 and 3,2 µm on the pin and on the box
Data Source
AI summary
A make-up method for a threaded joint with a pin and a box to increase the fatigue life of the joint is provided. The method includes the step of applying a torque for making up the pin in the box until first and second abutment shoulders of the pin and the box abut. The method can also include the step of applying an additional torque to the threaded joint until a magnitude of between 50% and 90% of the yield strength of the joint material in the most stressed part of the joint is reached.