TWI Industrial Member Report Summary 926/2009
By C Holtam and D Baxter
Carbon manganese (C-Mn) steel is generally the most economic material for the construction of offshore pipelines and risers. Steel catenary risers are widely used in deepwater oil and gas developments for transporting production fluids from the seabed. They can be subjected to fatigue damage from numerous sources, for example wave motion or vortex induced vibration. When production fluids are sour (ie contain water and H2S), the fatigue resistance and the material toughness are significantly degraded in comparison with the performance in air, as a result of sulphide stress corrosion cracking (SSCC). Sour production fluids are common in oil and gas applications and therefore successful design is critically dependent on the availability of appropriate experimental data to quantify the extent to which fatigue lives are reduced, as rates of fatigue crack growth are increased.
One area where both experimental data and a validated assessment methodology are lacking is the behaviour of shallow cracks. Several researchers have reported increased crack growth rates in the shallow crack regime and this is often attributed to differences in crack tip chemistry. TWI Member Report 888 (Holtam and Baxter, 2007) provided a review of how different assessment procedures tackle this problem, and reviewed recent research in this area. It also discussed different approaches to modelling, and suitable test methods for exploring the behaviour of material in the shallow crack regime. A brief review of EAC test data for a number of material-environment systems was followed by a description of a proposed test programme to examine different aspects of material behaviour in this regime. It was shown that for a pipeline steel in a sour environment the transition from K- to stress-controlled behaviour might occur at a flaw depth of the order of a few millimetres.
The aim of this work is to provide guidance on the assessment of shallow crack-like flaws in a pipeline steel operating in a sour environment. Specific objectives of the current work are to:
- Determine experimental shallow crack test data to quantify the static (KISCC) behaviour of a C-Mn pipeline steel in a sour environment.
- Determine experimental shallow crack test data to quantify the fatigue crack growth behaviour of a C-Mn pipeline steel in a sour environment.
- Evaluate test methods for generating fatigue crack growth data in a sour environment to ensure that experimental test data are appropriately conservative.