TWI Industrial Member Report Summary 984/2011
By Colum Holtam
C-Mn steel is generally the most economic material for the construction of offshore pipelines and risers and there is an industry need to understand better and quantify its fatigue crack growth behaviour in sour service environments. If carbon steel can provide adequate performance under (mildly) sour conditions then the use of more expensive corrosion resistant alloys (CRAs) and clad materials might be avoided.
It has been demonstrated that, in some cases, shallow crack-like surface flaws in C-Mn steel pipelines and risers exposed to sour service environments may grow at a faster rate than deeper flaws subject to the same ΔK (Holtam and Baxter, 2009). Therefore it is possible that crack growth data obtained using specimens containing deep flaws may be non-conservative in predicting the behaviour of shallow flaws. However, these results are material-environment specific. In higher hardness materials (eg welds) a higher sensitivity to environment assisted cracking processes might be expected, such as sulphide stress cracking (SSC), which may result in failure via a different mechanism to that observed in parent material, and possibly higher crack growth rates (Hammond and Baxter, 2008). Therefore it is not clear whether the trends seen in tests on parent material will be directly transferable to harder HAZ microstructures and weld metal.
The aim of this report is to present the results of fatigue crack growth rate (FCGR) tests carried out on C-Mn steel pipeline girth welds and simulated heat affected zone (HAZ) microstructures exposed to a sour environment.
To determine experimental shallow crack test data to quantify the fatigue crack growth behaviour of C-Mn pipeline steel girth welds exposed to a sour environment.