Advancing the Understanding of Weld Residual Stress in Pipelines

We are pleased to announce the kick-off of a major collaborative project focused on improving the understanding of weld residual stress (WRS) in gas transmission pipelines. This project brings together multiple UK gas network operators under a Network Innovation Allowance (NIA) initiative, with Stress-Space Ltd leading the residual stress measurement programme.

Why Weld Residual Stress Matters

Residual stress plays a critical role in pipeline integrity assessments, particularly when evaluating fracture behaviour and fitness-for-service. Current industry guidance, including BS 7910, adopts a conservative assumption—often setting weld residual stresses at up to 110% of the Specified Minimum Yield Strength (SMYS). While this ensures safety, it can also lead to:

• Overly conservative integrity assessments

• Increased use of stringent assessment routes (e.g. Option B)

• Higher inspection and repair costs

• Potentially unnecessary interventions across large asset networks

Recent studies suggest these assumptions may not accurately reflect real-world conditions, particularly after pipelines undergo hydrostatic pressure testing during commissioning.

Project Aim

The primary objective of this project is to generate robust experimental data to:

• Quantify as-welded residual stress states in pipelines

• Understand how residual stresses relax during hydrostatic testing

• Provide evidence to support more representative input values for integrity assessments

• Improve confidence in pipeline assessments, particularly for hydrogen repurposing scenarios

Ultimately, the project seeks to balance safety and realism, reducing unnecessary conservatism while maintaining engineering integrity.

Scope of Work

The project combines experimental testing and modelling to build a comprehensive understanding of residual stress behaviour.

1. Residual Stress Measurements

Residual stresses will be measured in both seam and girth welds from ex-service and fresh pipelines. Testing will include:

• Internal and external measurements

• Multiple pipe diameters, wall thicknesses, and grades

The programme involves extensive testing campaigns, including hundreds of measurement points across representative pipeline configurations .

2. Hydrostatic Testing & Stress Relaxation

A key focus of the project is understanding how residual stresses evolve during hydrostatic pressure testing. This includes:

• Instrumentation using strain gauges during pressure tests

• Measurement of stress relaxation before and after hydrotesting

• Comparison between as-welded and post-test conditions

This work will provide critical insight into whether hydrotesting significantly reduces residual stress levels in service.

3. Mechanical Testing

To support interpretation of residual stress data:

• Tensile testing will be conducted on both parent and weld materials

• Material properties will be linked to measured stress states

This ensures results can be directly applied to fracture mechanics and integrity assessments.

4. Integration with FEA Modelling

Experimental data will be integrated with finite element analysis (FEA) to:

• Simulate stress evolution during loading

• Validate physical measurements

• Extend findings beyond tested configurations

Stress-Space’s Role

Stress-Space Ltd is leading the residual stress measurement and analysis activities, applying advanced techniques to generate high-fidelity datasets.

Our contribution includes:

• Designing and executing measurement strategies

• Delivering high-quality residual stress data

• Supporting interpretation and validation of results

• Working alongside project partners to ensure outputs are directly usable in engineering assessments

Industry Impact

This project has the potential to deliver significant benefits across the gas transmission sector, including:

• More realistic residual stress inputs for BS 7910 assessments

• Reduced need for conservative (and costly) inspection regimes

• Improved confidence in pipeline integrity decisions

• Better support for asset life extension and hydrogen transition strategies

By combining experimental evidence with modelling, the project will help move the industry towards data-driven, evidence-based standards.

Looking Ahead

The project has now entered its testing phase, with sample preparation and initial measurements underway. As results emerge, we will be sharing insights into:

• Observed residual stress distributions

• Measured relaxation effects

• Implications for engineering standards and practice