Computational Fluid Dynamics (CFD) Erosion Study For Chokes

Abstract

Abstract Erosion, a common cause of failure in oilfield equipment, remains a complex phenomenon despite extensive studies by the industry. The severity of erosion depends on a multitude of factors such as fluid properties, flow rate, sand size and rate, material type, geometry as well as many others. Knowledge of the erosion rate is useful in determining the service life for a piece of equipment exposed to a given set of operating conditions. A combination of experimental results and Computational Fluid Dynamics (CFD) can optimize product performance at the design level. This paper presents a set of curves that will estimate erosion of different components of the choke as a function of choke position and flow rate derived from use of CFD. Plots of the erosion results in terms of thickness loss per time in units of mils/year are provided for each component for varying flow rates and sand volume concentrations. Results from the study show that the erosion rates increase in a quadratic fashion with respect to the flow rate. Introduction Solid particle erosion is a concern in any application involved with the motion of solid particles in a carrier fluid. The areas most susceptible to erosion are those located near changes in flow direction. In these areas, the particles are most likely to deviate from the flow streamlines and impinge the surface of the geometry. The severity of erosion depends on a multitude of factors such as fluid properties, flow rate, sand size and rate, material type, geometry as well as many others.1 However, trying to isolate the effect of a single factor and extrapolate the findings to a wide range of conditions is difficult. Currently, consensus has not even been reached as to which factors should be given primary consideration, investigated or included in an erosion model. The numerous variables affecting erosion and lack of understanding of the effect of these variables makes erosion reduction extremely difficult. Approach/Methodology Knowledge of the erosion rate is useful in determining the service life for a piece of equipment exposed to a given set of operating conditions. There are two primary methods to study erosion: experimentation and computer simulation. Experimental testing is often difficult to perform for conditions of interest. For example, the actual operating conditions may be for large flow rates or at pressures that are difficult to achieve experimentally. Additionally if the erosivity is low then it may not be possible to achieve measurable erosion in a reasonable amount of time. The most state-of-the-art erosion prediction tool takes advantage of computational fluid dynamics (CFD) simulations. This approach has four main steps: geometry creation, flow simulation, particle tracking, and applying erosion equations. If experiments can not be performed for conditions of interest, it is advisable to use a combination of experimentation and computational fluid dynamics techniques. First experimental results can be obtained for conditions as close to the conditions of interest as possible. Then computational fluid dynamics can be used to obtain results for the conditions of interest by scaling the experimental results. CFD simulations are performed for the experimental conditions and the results of the simulation are compared to the experimental results. The difference between the experimental data and predicted results provides the scaling factor to be applied to the simulations at the conditions of interest. This study was divided into two phases. The objective of Phase I of the study was to determine the most representative way to perform the simulations and to tune the erosion equations being applied to each component by obtaining a scaling factor for each by comparing predicted results to experimental results. The goal of Phase II was to apply the CFD erosion prediction approach for the operating conditions of interest and adjust the predicted results using the scaling factors. The results of Phase II are in turn used to develop the curves for erosion as a function of choke position and flow rate.