Development of a novel model for prediction of aggressive top-of-line corrosion with organic acids (ModTLC)

Most of the oil and gas that are produced offshore, are transported in pipelines to shore or to a platform or ship at some distance from the production well. Low alloyed carbon steel is the only viable material of construction for long pipelines provided that well-functioning corrosion mitigation programs are in place. ModTLC is addressing the corrosion that occurs inside, at the top of large diameter pipes. As the produced fluids cool along the pipeline, water will condense to the cold pipe surface and more to the top than to the bottom. The condensed water is highly corrosive because it contains dissolved acid gases, i.e. CO2 and H2S as well as acetic, formic acid and other organic acids. The high velocity gas also contains droplets of water and condensate, and these will deposit if they hit the steel surface. Ethylene glycol injected to prevent ice and hydrates should also be considered when predicting the composition and corrosivity of the aqueous phases in the pipeline. In ModTLC one is developing a software which will be able to predict the top-of-line corrosion (TLC) rate in pipelines that transport gas, condensate and water over long distances. It will have models for mass transfer of acids and glycol from the gas to the aqueous phase top of line and predict the resulting effects on the corrosion rate. The model will also include effects of droplet deposition. The software shall improve the reliability of TLC corrosion rate predictions and enable better tailoring of the corrosion mitigation programs and maintenance. New experimental results have been generated to improve mechanistic understanding of organic acid dominated corrosion of carbon steel compared to carbonic acid dominated corrosion. Findings suggesting that undissociated organic acid concentration has a stronger influence on corrosion rate than pH have been published in open-access literature together with a review of the state-of-the-art regarding TLC modeling. The project is utilizing the advanced flow and corrosion laboratories at IFE and is producing data on mass transfer between gas and liquid phase and on corrosion reactions occurring at wet steel surfaces in pipelines. A framework for a single-control-volume simulator has been developed, where models for corrosion rates, mass transfer and droplet deposition are being implemented and function together.

Carbon steel is being extensively used as a cost-effective option for pipelines transporting wet gas. It corrodes in water containing CO2, H2S and organic acids and needs to be protected by either a corrosion product film, various types of organic material (corrosion inhibitor, coating) or cladding with corrosion resistant alloys. Large diameter pipelines will normally have lower temperature at the top than at the bottom. Top-of-line (TOL) corrosion (TLC) is a result of water condensation in presence of acid gases (CO2 and H2S) and volatile organic acids (acetic and formic acid) at the upper part of a large diameter pipeline. The present TLC rate calculation tools account for the water condensation rate and the acid components in the gas. However, they assume thermodynamic equilibrium between the gas phase and the aqueous phase TOL. This will not be true for minor components like organic acids and monoethylene glycol (MEG) which have a fugacity in the gas in the millibar range or less. The gas will be depleted of these compounds when they dissolve TOL, unless they are replenished from the aqueous phase bottom of line (BOL). In addition, there can be transportation of droplets of the liquids from BOL, aqueous phase and light hydrocarbons, to the TOL. The inmix of BOL liquids in the condensed water has potentially a large effect. The delivery of the project will be a TLC rate model that accounts for all these effects. The tool will especially address organic acid induced TLC under sweet conditions (gas with CO2, no H2S), but some of the principles and the knowledge may also be applicable to sour conditions (gas with H2S). The various model components in the tool will be validated against measured data, and the project will have an extensive experimental program to generate empirical data for droplet distribution and deposition, mass transfer from gas to liquid TOL at high pressure and corrosion rate of carbon steel covered by thin water films.