TWI Industrial Member Report Summary 972/2010
By: Colum Holtam
Steel catenary risers (SCRs) are increasingly used in deepwater oil and gas developments to transfer production fluids from the seabed to surface facilities. SCRs can be subject to both fatigue loading, for example from wave and tidal motion, vortex induced vibration (VIV) and operating loads, and corrosive environments (internal and external). When the production fluids are sour (ie contain water and H2S) higher fatigue crack growth rates (FCGRs) are expected and therefore shorter overall life compared to performance in air, as a result of the interaction between fatigue crack growth and 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.
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.
There are relatively few published FCGR data for pipeline steels in a sour environment, particularly at low values of applied stress intensity factor range (ΔK). However, a number of researchers have reported a diminished influence of a sour environment at low values of ΔK at ambient temperatures (20-30°C) (Bristoll and Roeleveld, 1978; Webster et al, 1985; Vosikovsky and Rivard, 1982; Vosikovsky et al, 1983; Watanabe et al, 1994). In many cases it is material behaviour in this low ΔK regime that dominates the fatigue life (eg VIV loading) (Holtam, 2009b). At low values of ΔK (ie approaching threshold) crack growth rates in C-Mn pipeline steel determined under conditions of decreasing ΔK have been shown to be substantially lower than those determined under conditions of increasing ΔK (Holtam and Baxter, 2009). Constant ΔK tests on C-Mn pipeline steel parent material and girth welds carried out at low ΔK suggest that this is due to an influence of crack depth (Holtam, 2009a; Holtam and Baxter 2009; Holtam, 2010).
Shallow cracks have been shown to grow up to an order of magnitude faster than deep cracks in a sour environment at the same (low) value of ΔK (Holtam and Baxter, 2009) regardless of specimen geometry, coating configuration and pre-soak conditions (Holtam, 2009a). The observed behaviour is believed to be dominated by bulk hydrogen charging, ie hydrogen charging 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 edges (Holtam, 2009a; Holtam and Baxter 2009). However, it is also possible that the observed influence of crack depth is the result of differences in the extent of crack closure. At low ΔK, crack closure may be more significant at the end of the decreasing ΔK test than at the beginning of the increasing ΔK test. This in itself may be attributable directly to the difference in crack depth or to the progressive build up of corrosion products during the test.
Irrespective of the mechanism involved, this highlights a potential non-conservatism associated with generating FCGR data under conditions of decreasing ΔK. In particular, the use of low ΔK data determined in this way (by which time the crack is relatively deep and the hydrogen concentration is relatively low) may be non-conservative if they are used to predict the behaviour of a much shallower flaw. In reality a flaw is likely to grow under conditions of increasing ΔK, such that the flaw will be shallow and the concentration of hydrogen may be high at the beginning of life (when ΔK is low).
The possible influence of crack depth on material behaviour should be considered when conducting tests to determine FCGRs for use in fracture mechanics calculations. As highlighted above, a number of authors have indicated that the influence of a sour environment is more pronounced at high ΔK. However, because precise test conditions are not always reported, it is not possible to say whether this is genuinely the case, or whether this is an artefact of testing under conditions of decreasing ΔK. This is an area that warrants further investigation.
In the absence of reliable mechanistic models, the most appropriate advice is to ensure that the flaw size used is comparable to that being assessed and, in this respect, particular care should be taken when carrying out decreasing ΔK tests to generate crack growth rate data at low ΔK. Increasing ΔK tests carried out using specimens containing shallow initial flaws provide a more conservative means of establishing upper bound FCGRs. However, it is acknowledged that the determination of very low ΔK data may then be difficult.
- Determine experimental shallow crack test data to quantify the near-threshold (low ΔK) fatigue crack growth behaviour of a C-Mn pipeline steel exposed to asour environment.
- Evaluate the threshold value of ΔK when measuring fatigue crack growth data under increasing ΔK conditions, from a shallow initial flaw introduced using electrical discharge machining (EDM) notching followed by fatigue pre-cracking in air.
- Investigate the possible non-conservatism of predicting the behaviour of shallow flaws using test data derived under conditions of decreasing ΔK.
- Compare the results of FCGR tests with fatigue endurance data in a sour environment.