TWI Industrial Member Report Summary 930/2009
Steel catenary risers (SCRs) are used in deepwater oil and gas developments to transfer produced fluids from the seabed to surface facilities. SCRs can be subject to fatigue loading from a variety of sources including wave and tidal motion, vortex induced vibration (VIV) and operating loads. When the produced fluids are sour (ie contain water and H2S), higher fatigue crack growth rates (FCGRs) are expected and therefore shorter overall life compared with performance in air, as a result of sulphide stress cracking (SSC). Successful design is critically dependent on the availability of appropriate experimental data to quantify the extent to which fatigue lives are reduced, and rates of fatigue crack growth are increased in a sour environment.
Fatigue tests carried out at TWI on X65 pipeline steel have shown that crack growth rates in a sour environment can be 100 times higher than in air (Holtam and Baxter, 2008). Tests have also illustrated a possible crack depth dependence whereby shallower flaws (up to 4mm deep) appear to grow faster than deeper flaws subject to the same value of stress intensity factor range (?K). In these sour tests there appeared to be three distinct regimes of behaviour as crack depth increased. For relatively shallow flaws (up to 3-4mm) crack growth rates were approximately constant (although there was roughly a factor of three variation between the two tests). As crack depth increased to approximately 6mm the crack growth rate decreased, by approximately an order of magnitude. For crack depths of greater than 6mm, the crack growth rate remained constant for the remainder of the test. There are several possible explanations for this observed effect, for example a difference in crack tip chemistry, reduced crack closure in the shallow crack regime, or an influence of bulk hydrogen charging on material behaviour close to the specimen surface.
Examination of the specimen fracture surfaces suggested higher FCGRs at the specimen edges than at the centre (Holtam and Baxter, 2008). Together with the observation that crack growth rates were also higher when the entire crack front was close to the top face of the specimen, this suggests that bulk hydrogen charging effects are dominating (ie hydrogen charging of the steel is dominated by absorption from the external surfaces of the specimen rather than at the crack tip, and a lower concentration of hydrogen exists in the centre of the specimen than at the specimen edges). However, neither crack closure effects nor differences in crack tip chemistry can be ruled out.
- Determine experimental shallow crack test data to quantify the fatigue crack growth behaviour of a C-Mn pipeline steel in a sour environment.
- Investigate the influence of loading conditions, crack depth, specimen geometry, pre-soak and coating configuration on the fatigue crack growth behaviour of a C-Mn pipeline steel in a sour environment.
- Evaluate test methods for generating FCGR data in a sour environment to ensure that experimental test data are appropriately conservative.