Advances in Multiphase Flow CFD Erosion Analysis
Authors: Peng Deng-Jr , Pak Afshin , Chinello Luca , Wood Terry , Low Andrew 
Published: May 2013

One of the flow assurance challenges in subsea production systems is the occurrence of erosion damage due to the existence of sand particles and high production Gas Oil Ratio (GOR) as such erosion mostly occurs in highly gas dominated operating conditions in the annular flow regime. The erosion rate for an elbow with a constant flow velocity and with all other factors equal is higher in gas systems than liquid systems as more particles will impact on the inner wall of the outer curvature of the elbow. The maximum wear location and the penetration rate for multiphase flows are often an intermediary of gas and liquid systems occurring at 55 degrees from the inlet of the elbow, however this depends heavily on the multiphase flow regime. A challenge facing industry is availability of erosion prediction models; the majority of available models are based on single-phase liquid or gas as the carrying medium. This can result in large discrepancies in erosion rates and potentially increased wall thickness, fabrication and subsequent intervention costs.

To predict the flow regime in greater clarity requires the use of computational power and / or instrumentation that can accurately characterize the flow within the pipes. Since experimental work is costly and unlikely to be representative of a large integrated production system, Computational Fluid Dynamics (CFD) is used to perform erosion assessments and can also aide in corrosion prediction and inhibitor selection. Only erosion assessments by CFD methods are discussed in detail within this paper. CFD has been extensively applied for erosion analyses; it is commonly used for identifying potential failure locations, improving understanding of failure mechanisms and only qualitatively used for erosion rates.

CFD erosion modelling capability in this paper has been enhanced by simulating flow regime characteristics, in particular the liquid film for annular flow. This benefits the simulation to obtain greater accuracy for sand particle impact angles, area, speed and thus the erosion rate is significantly enhanced. In addition, the local volume fraction of sand has been considered in order to accurately evaluate the impact force. The research to date shows that a promising agreement is obtained between predicted erosion rate and the empirical predictions (Salama, Salama & Venkatesh and DNV RP-501 methods).

Further comparisons to empirical model predictions are carried out to address the importance of flow regime on the results as current empirical models lack this consideration. The influence of the flow orientation (upwards and downwards flow), has also been investigated in this work due to current lack of publically available data. The paper presented hereafter illustrates that considerable difference in flow orientation is revealed and the prediction can be improved by considering the flow characteristics. An example is provided highlighting the use of liquid film and droplet velocity to replace the mixture velocity implemented in empirical models for annular flow. All of the findings of this work are aimed at providing assistance to industry not only performing the qualitative but quantitative CFD erosion analysis.