Quality Assurance In Corrosion Prediction Of Multiphase Lines

One of the key factors in meeting the design life requirement of an oil and gas flow or export line depends on the accuracy of corrosion prediction and its management. Therefore, the assessment of internal corrosion of a pipeline system must also include examining the mechanism by which corrosion is expected and understanding how it is most likely to occur.

In many cases, corrosion mechanisms can produce localized conditions that may not reflect in the bulk of the produced fluids. Localized MIC corrosion - occurring in an export oil line through sediment containing micro organisms - is not usually accounted for by corrosion models in predicting corrosion rate. We have examined a few corrosion-prediction models in relation to flow and export line corrosion analysis and reviewed briefly the outcome in line with the Total Quality Management system (TQM) as applied in a typical project.

Here is a general review of corrosion mechanism and corrosion rate prediction. The complex nature of CO2-H2S corrosion makes accurate rate prediction difficult, and localized effects often hinder prediction of corrosion rates. Factors such as pressure, temperature, flow rate, flow profile, oil properties, pipe material, pipe routing, pH and a few other variables would influence the rate of corrosion predicted by a corrosion model in different pipe line sections. Flowing conditions often change along the route of a single pipeline and localized conditions can make some areas along the pipeline more likely to experience corrosion than other areas. Identification of these flow conditions and locations along the pipeline with high susceptibility to corrosion is one of the primary functions in devising an effective corrosion-management system.

Ideally, settling of solids and the presence of bacteria, or presence of elements such as sulphur which can affect formation of stable corrosion-product films, must be evaluated in conjunction with flowing conditions to determine the effect along each pipeline. Although some mechanistic and empirical corrosion models claim to have considered some such localized effects, they often do not stand up to the design integrity requirements that a project demands. An effective corrosion management system thus becomes an integral component of pipeline corrosion rate prediction and the design life. (1)

We will review some of the key factors affecting the corrosion rate that are common to most of the corrosion models commercially available before examining how effectively these factors have been adopted in the case of few models.

Effect Of Flow
The main ways in which flow may affect CO2 corrosion can be distinguished based on whether flow conditions are conducive to protective layer formation: