Fatigue Design Guidance for Risers in Sour Service via Full Scale Testing

Abstract 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. For sour service environments, 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 an environmental knock down factor (EKDF) that is then applied to the base design curve. This approach accounts separately for both the difference between strip and full scale testing results and environmental effects, and has been adopted on many projects. However, given the difference in fatigue performance in air of welds in strip specimens compared to that of full scale girth welds, there was concern that fatigue testing of strip specimens in a sour environment may not produce results representative of the full scale welds. This paper presents results from a Joint Industry Project run by Graham Slater, Richard Pargeter and Kuveshni Govender at TWI Ltd which involved the development of a test method to enable fatigue testing of full scale welds in a sour environment. The results of full scale sour fatigue tests were compared with those from strip specimens tested in a sour environment, and to results from full scale resonance fatigue tests in air. It was found that the conventional two-stage process produced a highly conservative EKDF, that was a factor of 2.5 higher than when the results from full scale resonance tests in air were compared to the results of full scale sour tests.