Subsea boosting of hydrocarbon flow, either directly from the well or from a subsea separator, will typically result in the need of handling some free gas. Pumping moderate gas volume fractions (GVF) in combination with high pressure increase, is typical for boosting flow from deep water wells and represent a challenge in pump design. Aker Solutions has developed such a pump, called a HybridBooster. This paper presents the design process of such a multistage pump from CFD-simulations to the complete full scaled testing. The hybrid pump design is flexible and suitable for a wide range of flow rates and GVF's due to the many stacking possibilities of impellers with different characteristics. This allows for making a pump with the best possible performance and efficiency. The two-phase testing was done with a mixture of water and air at Aker Solutions facility in Tranby. The pump generates a differential pressure in the range of 150 to 240 bar, depending on the rotational speed and flow rate, at 20% GVF with a suction pressure of 12 bar. The pump design is based on a combination of mixed-flow impellers and radial impellers. Test results show that the mixed-flow impeller technology have great potential for moderate GVF at all flow rates and high GVF capability at best efficiency point and above. The strength of the mixed-flow impeller is the internal remixing of gas and liquid that strongly reduces the risk of gas blocking in the flow channel. The behavior of the pump during testing was stable, granting easy pump control. Introduction The Hybrid Pump Project is a Joint Industry Project supported by Demo 2000 The Research Council of Norway and the following companies: Aker Solutions, Statoil ASA, Total E&P Norway, Nexen Exploration Norway AS, ExxonMobil Upstream Research Company USA. The large variety of oil field characteristics demands a range of tooling for a successful production. One of these tools is the subsea HybridBooster. Unlike a conventional single phase (liquid) pump the hybrid pump is able to pump a liquid/gas mixture with a high pressure increase. The HybridBooster will be a well-suited and cost and production-efficient solution in a number of applications. Typical applications are:Pumping liquid from a gas/liquid separator where a risk for gas carry-under existPumping hydrocarbons from fields producing mainly liquid, but with risk of gas break-through over time To meet the challenging requirements of handling an increased fraction of gas combined with high pressure increase a Hybrid pump development project was established. The key specifications for the subsea Hybrid Pump development are:Pressure increase - target: 200 bar, min. 100 barGas tolerance, base case - target: 20% GVFGas tolerance, advanced case - target: >30% GVFInlet Pressure - > 10 bar, < 50 barFlow - within power limit of 2,5 MW The major design criterion was to design a gas tolerant multi-stage pump that could handle 20% GVF at suction condition, with stable operating conditions and predictable behavior.
The recent trend in the oil industry is to save CAPEX and exploit every offshore field to increase production and maximize reserves. Also, deeper water and longer step-out is a challenge for new fields. The most adapted technology to unlock these reserves is the use of subsea boosting like a multiphase pump on the seafloor. Subsea boosting has been used for decades with well proven results, but up to now, some limitations in power and lift pressure exist. This new multiphase pump development has increased the potential pressure generation manyfold from the typical ΔP of 50 bar (725 psi) at the beginning of the project. Developing such a powerful two-phase pump driven by a liquid-filled motor requires a unique combination of expertise in machinery engineering, electrical engineering, fluid mechanics and rotor dynamics. The objective of the co-authors is to share this experience by bringing some insights on what it takes to develop, test, and qualify such specific product. Outlines of the methodology will be described, key results will be detailed, and lessons learnt will be presented. The new design was fully tested first component-wise and then for a full-size prototype. A wide process envelope was mapped during the final qualification program with 3,000 points tested in the range 2,000-6,000 RPM and 0 - 100% GVF (Gas Volume Fraction). Qualification tests concluded with more than 2,000 cumulative hours. The main challenges in this program were the development of an innovative multiphase impeller and the qualification of the first MPP (MultiPhase Pump) with a back-to-back configuration. Concerning the motor, the development includes a high speed 6,000 RPM, 6 MW liquid-filled induction motor and a new stator winding insulation cable. With this new product, the pump market is ready to overcome challenges to produce deeper and further reservoirs in a constant evolutive oil and gas market.
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