# Verification of the Applicability of the FAD Method Based on Full-Scale Pressurised Tensile Tests of Large-Diameter X80 Pipelines

## TL;DR

This study tests how well FAD methods work for assessing defects in large X80 steel pipelines, finding that BS 7910 and API 579 are more accurate than API 1104.

## Contribution

The study provides experimental validation of FAD methods for high-strength, large-diameter pipeline ring welds.

## Key findings

- API 1104 showed significant conservatism in failure assessments.
- BS 7910 and API 579 matched experimental data well, with BS 7910 offering a higher safety margin.
- The results offer a reference for selecting FAD methods under specific material and defect conditions.

## Abstract

The Failure Assessment Diagram (FAD), as a significant method for evaluating the suitability of defective metallic structures, has been subject to considerable debate regarding its applicability in assessing ring welded joints for high-grade steel and large-diameter pipelines. To address this issue, this study first designed and conducted two sets of full-scale pressure-tension tests on large-diameter X80 pipeline ring welded joints, considering factors such as different welding processes, joint configurations, defect dimensions, and locations. Subsequently, three widely adopted failure assessment diagram methodologies—BS 7910, API 579, and API 1104—were selected. Corresponding assessment curves were established based on material performance parameters obtained from the ring weld tests. Finally, predictive outcomes from each assessment method were compared against experimental data to investigate the applicability of failure assessment diagrams for evaluating high-strength, large-diameter, thick-walled ring welds. The research findings indicate that, under the specific material and defect assessment conditions employed in this study, the API 1104 assessment results exhibited significant conservatism (two sets matched). Conversely, the BS 7910 and API 579 assessment results showed a high degree of agreement with the experimental data (eight sets matched), with the BS 7910 assessment providing a relatively higher safety margin compared to API 579. The data from this study provides valuable experimental reference for selecting assessment methods under specific conditions, such as similar materials, defects, and loading patterns.