The importance of an accurate corrosion assessment in unpiggable offshore pipelines cannot be overestimated. Since in-line inspection (ILI) cannot be carried out, the operator is in a difficult situation, which requires an answer to the question: “How to operate and maintain the pipeline without having the overall risk picture backed by ILI data?”
The NACE MP-ICDA methodology is a recommended solution to address this situation. This methodology requires simulating multiphase flow in the pipeline to replicate internal corrosion environment, it has been and will be subjected to, and identify main corrosion mechanisms. Therefore, the more comprehensive and accurate is the multiphase flow model, the better is corrosion assessment.
For example, the NORSOK M-506 empirical model is widely used in the industry to calculate the corrosion rate for carbon steel in water containing CO2 at different temperatures, pHs, CO2 fugacities, and wall shear stresses. The mean wall shear is calculated based on the velocity and density of the mixture.
In the cover image, you can see two pictures of 3-phase gas-oil-water slug flow taken at a scale physical model of an offshore production facility. The oil and water flow rates are the same, in the upper picture the gas flow rate is smaller than that in the lower one.
The oil-water flow pattern in the liquid slug is water-in-oil emulsion (w/o) at the high gas flow rate. The continuous phase is oil in the liquid, therefore there is no constant contact of water with the pipe wall and NORSOK M-506 provides a conservative assessment of corrosion rate. However, when production declines to a point at which the oil-water flow pattern is stratified flow (ST), the bottom of the pipeline is wetted by water flowing at a high velocity. The shear rate and hence corrosion rate increase by an order of magnitude, while the empirical model does not reproduce this increase of the corrosion rate. This fact is confirmed by many operators. The standard itself says that “different flow regimes and geometrical obstacles may generate shear stress fluctuations where the shear stress peaks may be considerably higher than the average shear stress.”
The chart presented in the document shows the gas-liquid flow patterns and oil-water flow patterns in the liquid phase in an offshore flowline predicted with the Multiphase Cloud® simulation platform. The local shear stresses are calculated for all combinations of gas-liquid flow patterns and oil-water flow patterns occurring in the flowline. Our physics-based approach in conjunction with the empirical model of the standard helps corrosion engineers reduce the uncertainties in the assessment of corrosion rates and identification of critical locations.