Abstract

Sand erosion in oil and gas production units can present rigorous system and production design challenges. Erosion is a complex process that is affected by numerous factors such as the piping geometry, flow conditions, fluid properties and sand characteristics. Choke valves are used to control the flow in the production units and subject to significant sand erosion. At high differential pressure across the choke, flow accelerations may occur in the choke valve and result in extremely high flow and particle velocities (up to 500 m/s) within the choke valve and in the downstream pipes. Consequently, the erosion rates may become very high, which can effectively shorten the service life of choke valves. Therefore, the knowledge of the flow characteristics and the capability to predict erosion rates in choke valves is essential for determining the choke's service life and guaranteeing system integrity.

The paper describes methods of using Computational Fluid Dynamics (CFD) for examining the choke valve fluid and particle flows as well as predicting the choke erosion. A 3D CFD choke flow simulation model is developed and validated against experimental data. The model is based on the Eulerian approach for simulating the flow while sand particles are tracked in the flow field using Lagrangian methodology and the erosion is predicted by implementing literature erosion correlations (TULSA and DNV) in the 3D CFD model.

The CFD flow and sand particle tracking simulations show excellent agreement between the predicted flow parameters and experimental data. Erosion hotspot locations are well predicted in the simulations while the erosion rates are underestimated. This discrepancy is caused by two main reasons. The first one is some numerical issues associated with extremely high velocities of the sand particles. The second one is that steady-state simulation neglects the geometry changes due to the erosion in the real test.

Current CFD tools can resolve the complex choke flows very well. Particle tracking accuracy depends on the particle size and sand loading. Erosion prediction accuracy is directly affected by the accuracy of the particle flow prediction and the applicability of the erosion correlation.

Introduction

Sand erosion in subsea components can cause serious design and production problems. Choke valves are especially affected by sand production and the resulting erosion due to the high fluid and sand particle velocities in the choke valves. Therefore, the knowledge of the flow characteristics and the capability to predict erosion rates in choke valves is essential for determining the choke's service life and guaranteeing system integrity. Physical laboratory erosion tests have been the base of the choke qualification and the characterization of the complex flow in the choke valves. However, the physical sand-erosion tests are extremely time consuming and costly. The development of validated 3D simulation tools can reduce the effort and lead time associated with the choke design verification and optimization as well as the choke qualification for a particular field application. Furthermore, the simulation models are also the base of erosion diagnostic and monitoring systems. Computational Fluid Dynamics for the flow and the particle tracking simulation, in conjunction with erosion models became an established tool to quantitatively predict the erosion distribution generated by sand flows in the last few years. Models are validated in (Zhang, Y, 2007), (Xianghui Chen, 2004), (Gary Brown, 2006), (Manickam, 1999), and (Graham, 2009).

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