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PETROMAKS2-Stort program petroleum

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

Alternative title: Utvikling av modell for å beregne korrosjonshastighet under kondensende vann med organiske syrer i toppen av rørledninger

Awarded: NOK 12.1 mill.

Project Number:

308774

Project Period:

2020 - 2024

Location:

Partner countries:

Most of the oil and gas that is produced offshore, is 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. The ModTLC project addressed 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. Monoethylene glycol (MEG) injected to prevent hydrates is also important to account for when predicting the composition and corrosivity of the aqueous phases in the pipeline. The ModTLC project developed an enhanced simulation tool for predicting top-of-line corrosion (TLC) rate in pipelines that transport gas, condensate and water over long distances. This software tool is capable of modelling gas flow and associated heat fluxes and temperature/pressure profiles, mass transfer between the bottom and top liquid phases (including droplet transport and deposition), the composition of the top liquid phase and the expected uniform corrosion rate at the top of the line. The ModTLC simulator was developed within the framework of an Excel workbook with the most demanding calculations and functions coded in VBA. The simulator includes: - A simple flow model computing fluid properties, temperature profiles, pressure gradients, shear stresses on the wall/film, and surface areas, e.g. of the film and the gas/liquid layer interphase; this allows running calculations without the need to first perform multiphase flow simulations - A corrosion rate model for organic acid dominated top-of-line corrosion that relates the calculated condensation rate, condensed phase composition and associated FeCO3 solubility to predict the corrosion rate - A model for transport and dissolution of acetic acid, formic acid and MEG into the condensed water at the ToL, allowing the non-equilibrium composition of the condensed phase to be calculated - A droplet transport and deposition model to account for the contribution of bottom-of line fluid of different composition reaching the condensed phase - A thermodynamic equilibrium model based on a simplified equation of state (EoS) for standalone operation to compute the boundary conditions as a starting point for non-equilibrium mass transfer calculations - Seamless interfacing with 3rd party thermodynamic models as user choice New experimental results have been generated to improve the 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 utilized the advanced flow and corrosion laboratories at IFE to produce data on droplet transport and deposition, mass transfer between gas and liquid phase and on corrosion reactions occurring at wet steel surfaces in pipelines in the presence of volatile organic acids. The project, in collaboration with University of Stavanger provided graduate level education to a PhD student, involved in a large part of the experimental activities performed at IFE. The ModTLC Simulator is envisioned to improve the reliability of TLC corrosion rate predictions and enable better tailoring of the corrosion mitigation programs and maintenance.

The project developed a software tool which is expected to predict TLC rate with higher confidence and less uncertainty than the present models, accounting for important aspects that existing models do not address. It is a comprehensive simulator capable of addressing in one run multiple calculation stages that before required using several dedicated models, repeatedly adapting and transferring data between them. This offers the user a highly optimized workflow. The software can be used by operators and engineering companies in their selection of materials and methods for corrosion mitigation of gas condensate pipelines e.g. when developing new fields and in evaluating the integrity of pipelines during operation. For new developments the cost saving (CAPEX) is considerable if, for example, the length of corrosion resistant cladding can be shortened without compromising long-term integrity. Similarly, considerable operation cost reduction (OPEX) can be achieved if the pigging frequency can be reduced for pipelines in operation. The project also contributed to the general knowledge on top of line corrosion by experimental findings and valuable data on electrochemical processes involving organic acids, on co-condensation of water and MEG and on droplet transport and deposition. New knowledge has been disseminated through conferences and in peer reviewed journals by the PhD student working in the project.

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.

Funding scheme:

PETROMAKS2-Stort program petroleum