Condensation of water in wet gas pipelines can cause top of line corrosion when the water condensation rate is high or acetic acid is present in the gas. For CO2 dominated systems the top of line corrosion is limited by the amount of iron which can be dissolved in the water condensing in the top of the pipeline. When acetic acid is present in the gas in addition to CO2 the solubility of iron in the condensed water is increased, and this increases the top of line corrosion. Fluid flow model simulations have shown that different approaches for pipeline insulation can give widely different water condensation rates. Application of a top of line corrosion prediction model has demonstrated that such variations in water condensation rates can have a much larger effect on the top of line corrosion rate than variation in the CO2 partial pressure.
Water condensation in the upper part of wet gas pipelines can cause top of line corrosion. The condensing water is unbuffered with low pH, but becomes rapidly saturated and supersaturated with corrosion products, leading to increased pH and possibility for iron carbonate film formation. The water chemistry in the thin film of condensed water in the top of the pipeline can be very different from the bulk water phase in the bottom of the line. Laboratory studies of top of line corrosion were performed already around 19901, 2, but it was only after top of line corrosion problems were encountered in wet gas pipelines a few years later that the problem caught the oil and gas industry's attention.
Several cases of top of line corrosion in gas pipelines were reported around 20003, 4. Common factors for these cases were excessive cooling of the gas and high water condensation rates due to high inlet temperatures, little thermal insulation and in one of the cases flowing river water. Based on these cases it was suggested4 that top of line corrosion may become a problem when the water condensation rate is above 0.15 to 0.25 g/m2s. Also common for these cases was the presence of organic acid in the gas, with reported values of 300 to 2000 ppm acetic acid in the produced water. The acetic acid is transported in the gas phase and will condense together with the water and increase solubility of iron and top of line corrosion.
Top of line corrosion is primarily a concern in wet gas pipelines with relatively high inlet temperatures, and then primarily in the first few kilometers of the pipeline, as the water condensation rate is rapidly reduced when the temperature decreases.
Water condensing in the top of a wet gas pipeline will form small droplets or a thin film on the steel surface. The condensed water can become rapidly supersaturated with corrosion products, resulting in increased pH and iron carbonate film formation. The top of line corrosion rate then becomes dependent on the water condensation rate and the amount of iron which can be dissolved in the condensing water1.
