Development and Testing of a Hybrid Boosting Pump
Authors: Abelsson Christian , Busland Haege , Henden Rune , Homstvedt Gunder , Olderheim Tarje , Westberg Tommy 
Published: May 2011

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: &gt;30% GVFInlet Pressure - &gt; 10 bar, &lt; 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.