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Integrating Probabilistic Approach into British Defect Assessment Procedure

Project Code: 35241

Start date and planned duration: January 2023, 33 months


  • To initiate the integration of a probabilistic engineering critical assessment (ECA) approach into the BS 7910 flaw assessment procedure
  • To establish a probabilistic residual stress treatment approach in line with BS 7910
  • To address the gaps in flaw assessment procedures related to thick-section welds and associated residual stress
  • To determine the significance of other input parameters, especially the effect of in-service degradation on reliability of safety critical components
  • To optimise probabilistic fracture mechanics (PFM) models that could account for most significant variables such that the results become comparable to those in recommended practices and design codes

Project Outline

In 2019, CRP 32878 was initiated to understand the methods for assessing the reliability of safety-critical structures in a fully quantitative way. TWI Industrial Members Report 1136/2020[1] examined in detail two methods (ie risk-based inspection and fracture mechanics) typically used in determining reliability of a given structure.

Various case studies were employed to demonstrate the differences between these two methods as well as the possibility of integrating both. The first deliverable of CRP 32878 was described in TWI Industrial Members Report 1136/20201. The other three reports are currently under peer-review.

The previous project has demonstrated that the use of PFM is a viable alternative, the main advantage of which is that the inputs (such as initial flaw size) can be tailored to the details of a particular components, allowing a direct estimate of probability of failure (POF). There remains a need to continue the efforts to rationalise and ensure both methods become fairly comparable. For these reasons, further investigation is required:

  • To determine the significance of other input parameters (besides material properties) on POF
  • To optimise PFM models that could account for most significant variables such that the resultant POF becomes comparable to those in recommended practices and design codes.

This project will aim to explore the following:

  • State of the art review on existing PFM procedures
  • Application of probabilistic treatment of as-welded and post-weld heat-treated residual stresses in a BS 7910 context
  • Evolution of material properties in particular the effect of aging and change of service (for example from natural gas to hydrogen transmission)
  • Loading occurrence frequency/utilisation rate
  • Residual stresses and flaws in narrow gap welds eg EB and narrow gap TIG
  • Rationalisation of BS 7608 fatigue design curves using PFM

Industry Sectors

-       Power Generation and Renewable Energy

-       Shipbuilding and Marine

-       Oil and Gas

-       Engineering and Fabrication


Benefits to Industry

A probabilistic treatment of an integrity assessment can bring financial and operational benefits by offering a better understanding the risks associated with an asset and allowing safer operation. The probabilistic treatment of integrity assessments is also an opportunity to optimise the design and operation of an infrastructure. This project will increase the body of evidence to support the adoption of probabilistic structural integrity assessments. This project could significantly enhance the ability to perform probabilistic assessments in accordance with the BS 7910 (CrackWISE®) procedure that is technology-neutral, and could also inform the development of other structural integrity procedures. In addition, the examination of residual stresses and flaws in narrow gap joints such as electron-beam, will help remove some of the uncertainties in adopting the more advanced welding process. This project will examine the available evidence and develop guidance for the narrow gap welding application. The study on material performance of modern pipeline construction for emerging transition from natural gas to gaseous hydrogen applications will be beneficial in supporting the global agenda on this and demonstrate the feasibility of such applications. 


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