Corrosion Predictions in oil transportation

Corrosion Predictions

CO2 corrosion of carbon steel used in oil production and transportation, when liquid water is present, is influenced by a large number of parameters, some of which are listed below:

  • Temperature;

  • CO2 partial pressure;

  • Flow (flow regime and velocity);

  • pH;

  • Concentration of dissolved corrosion product (FeCO3);

  • Concentration of acetic acid;

  • Water wetting;

  • Metal microstructure (welds);

  • Metal prehistory

The detailed influence of these parameters is still poorly understood and some of them are closely linked to each other. A small change in one of them may influence the corrosion rate considerably. Various prediction models have been developed and are used by different companies. Among them are the deWaard et al. model (Shell), CORMED (Elf Aquitaine), LIPUCOR (Total), and a new electrochemically based model developed at IFE. Due to the complexity of the various corrosion controlling mechanisms involved and a built-in conservatism, the corrosion models often overpredict the corrosion rate of carbon steel. The Shell model for CO2 corrosion ismost commonly used in the oil/gas industry. The model is mainly based on the deWaard equation published in 1991.

Starting from a “worst case” corrosion rate prediction, the model applies correction factors to quantify the influence of environmental parameters and corrosion product scale formed under various conditions. However, the first version of the model was published in 1975, and it has been revised several times, in order to make it less conservative by including new knowledge and information. 

The original formula of de Waard and Milliams implied certain assumptions that necessitated the application of correction factors for the influence of environmental parameters and for the corrosion product scale formed under various conditions. CO2 corrosion rates in pipelines made of carbon steel may be evaluated using industry accepted equations that preferably combine contributions from flow-independent kinetics of the corrosion reaction at the metal surface, with the contribution from the flow-dependent mass transfer of dissolved CO2.

The corrosion rate calculated from the original formula with its correction factors is independent of the liquid velocity. To account for the effect of flow, a new model was proposed that takes the effect of mass transport and fluid velocity into account by means of a so-called resistance model: 

where Vcr is the corrosion rate in mm/year, Vr is the flow-independent contribution, denoted as the reaction rate, and Vm is the flow-dependent contribution, denoted as the mass transfer rate. In multiphase turbulent pipeline flow, Vm depends on the velocity and the thickness of the liquid film, whereas Vr depends on the temperature, CO2 pressure, and pH.

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