In most cases, the design of CO2 corrosion control for pressure containing equipment will involve an evaluation of the suitability of using C-steel or low-alloy steel. This requires that the relevant corrosion
mode(s) are identified and that their rate(s) are assessed quantitatively. During recent years, prediction models for CO2-corrosion have been refined by introducing more parameters, primarily related to fluid
properties. This has apparently improved the correlation of the
semi-empirical models to old and new laboratory data. However, for the corrosion engineer the major challenge persists when selecting appropriate design parameters and appreciating the uncertainties inherent in these models. This is particularly important when the assumed efficiency of chemical treatments and the models are derived from laboratory testing and have to transposed to real field conditions. With a consistent usc of’ "worst-case” assumptions, a traditional
deterministic approach will too often preclude the use of ordinary C-steel materials. With all these uncertainties, the use of probabilistic
calculations is particularly justified. Reliability based calculations can
account for some of these uncertainties and the consequences of invalid assumptions or deficient calculations. This can be utilised to determine the preferred corrosion control strategy during the design. Once equipment is taken into service , the operational parameters are better defined and information from monitoring and inspection can be used to develop appropriate corrosion control strategies for further operations. This paper discusses these concepts and examines recently published models. It is shown by simple example how reliability can be used to select materials for a subsea pipeline. Comparison of the reliability based approach with more customary deterministic methods demonstrates that the latter can be censorious to the use of ordinary CMn-steel Iinepipe. This implies that more expensive corrosion resistant alloys maybe unnecessarily employed in many cases.
Keywords: corrosion, carbon dioxide, reliability analysis, pipeline, carbon steel, corrosion resistant alloys