(2017) Numerical investigation of the effect of viscosity in a multistage electric submersible pump, Engineering Applications of Computational Fluid Mechanics, 11:1, 258-272, DOI: 10.1080/19942060.2017 ABSTRACTElectric submersible pump (ESP) systems are commonly used as an artificial lift technique by the petroleum industry. Operations of ESPs in oil wells are subjected to performance degradation due to the effect of oil viscosity. To understand this effect a numerical study to simulate the flow in three stages of a multistage mixed-flow type ESP operating with a wide range of fluid viscosities, flow rates, and rotational speeds was conducted. The problem was solved by using a commercial computational fluid dynamics (CFD) software. The numerical model was validated with experimental head curves from the literature at different viscosities and rotational speeds available for the same ESP model used in this study, and good agreement was found. Performance degradation was evaluated by analyzing the effect of viscosity on head and flow rate. In addition, a flow field analysis to compare the flow behavior when the pump operates at different viscosities was carried out. The interaction between stages was also analyzed, and the influence of a previous stage on the upstream flow was evidenced. The flow field was analyzed at a curved surface that follows the complex mixed-flow geometry of the stages. CFD proved to be useful for exploring this kind of feature, a task whose accomplishment by means of experimental methods is not trivial. Such analysis helps to understand the flow pattern behind head and flow rate degradation when the Reynolds number is decreased. The results from this work are helpful as they provide a basis to estimate performance degradation for general scenarios. ARTICLE HISTORY

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In oilfield exploitation, pumping is one of the most productive artificial lift methods. The use of downhole pumps, where applicable, allows increasing production. However, service life of this type of systems may be short, around 2.5 years for electrical submersible pumps -ESPs, one of the most widely used types. Furthermore, failures in wet completion subsea wells require tubing removal, a heavy workover operation, demanding high costs offshore rigs. Besides, due to production halt, revenues are impacted.As a tendency for subsea wells, whenever allowed by production conditions, pumps are installed on the seabed, outside the production well. Depending upon the free gas fraction at pump intake, ESPs (with or without previous gas separation) or multiphase pumps may be used.Examples of ESP usage outside the producing wells are the pumping module (MOBO) and the Skid-ESP. In such cases, ESPs strings are installed on the seabed. In cases of failure, ESP replacement is simpler and faster, executed by rigs or special vessels, avoiding the costly intervention in the production well. But, even saving resources due to the operation simplification, the costs of required naval resources and the time spent awaiting for their availability and mobilization are still high. Several days may pass from system failure until the intervention is finally performed.At Perdido and Nansen Spar platforms, operated by Shell and Anadarko respectively, ESP were deployed and installed in the production risers. Recent developments of compact and high speed ESP may contribute for their placement inside production risers for other production unit types, FPSO, for instance. This paper proposes a new boosting concept based pumps, ESP or Hydraulic Submersible Pumps -HSP, installed inside production risers in FPSO, comparing their advantages and disadvantages of the proposed concept to seabed ESP systems, multiphase pumps and gas-liquid separation systems.