Caisson-ESP systems provide a cost-effective means of seafloor boosting. These systems are being applied in applications that requires the ESP to handle unusually higher gas fractions. The objective of this work is to investigate the gas-handling effectiveness of helicoaxial ESP stages used in combination with standard mixed flow pump stages. The results show that helicoaxial stages greatly extend the gas-handling capabilities of an ESP system and eliminate gas-lock over a wide range of conditions.
It is well known that high gas fractions and viscous fluids degrade centrifugal pump performance. Additionally, gas locking occurs due to gas entrainment inside the pump causing operational problems. This study investigated the use of 4 helicoaxial pump stages at the entrance of a 48-stage ESP. Testing was performed at the Shell Gasmer Caisson-ESP Facility in Houston, Texas. This full-scale test loop can test ESP's over a wide range of conditions. The test matrix covered viscosities from 2 to 300 cp at different speeds, flow rates, intake pressures and gas volume fractions (GVF).
Multiphase flow and viscosities affect the performance of the ESP pump system, degrading pump performance. The test results demonstrate that the high-flow helicoaxial pump can handle up to 73% GVF without gas lock. Pump performance improves at higher intake pressures and pump speeds with less pump degradation. In two-phase flow conditions, as viscosity increases, the pump degradation decreases thus improving gas-handling operations. A homogeneous model has a fairly good agreement with pump performance up to 30% GVF with minimal pump degradation.
This work provides an important field-scale validation of the helicoaxial pump hydraulic performance at high GVF with high viscosity fluids. This has benefits to the deep-water industry, which is seeking more affordable seafloor boosting options.