Fatigue Design Guidance for Risers in Sour Service via Full Scale Testing
Status: Completed
Date Announced: July, 2010
Project reference: 20324
Summary
Steel catenary risers (SCRs) are commonly used within deepwater oil and gas developments, and fatigue performance is often a critical factor in overall design.
Resonance fatigue testing of full scale girth welds has become standard industry practice to demonstrate adequate performance.However, these tests alone take no account of aggressive service environments such as sour production fluids.
In these instances qualification testing is usually a two-stage process involving resonance fatigue testing to demonstrate the required performance in air, and strip fatigue testing (in air and in a sour environment) to determine a fatigue life ‘environmental knock down factor’ EKDF that is then applied to the base design curve. This approach accounts for both size effects (i.e. the difference between strip and full scale testing) and environmental effects individually, and has been adopted on many projects.
However, the validity of the approach has not been demonstrated. There was therefore a need for a better understanding of the sour performance of girth welded riser pipe by full scale experiment.
This project developed a method for performing fatigue tests on full scale welds in pipes containing a sour environment. By comparing results of full scale resonance fatigue tests, strip tests in air, strip tests in a sour environment and full scale sour tests, the EDKFs from direct comparison of full scale tests were found to be lower. To understand the results, detailed investigations were carried out, including metallurgical examination, residual stress measurements, fatigue crack growth rate measurements and fracture mechanics assessments.
Project Outcome
- Fatigue test data for C-Mn girth welds in a typical sour service operating environment using both strip tests and full scale testing, with commentary on the approach used to calculate EKDFs.
- Residual stress measurements, fatigue crack growth rate measurements, detailed post-test metallographic examination of the welds tested with commentary on the significant results and observations.
- Validated guidance for design of risers, based on a full scale sour service riser fatigue design curve.