Submarine gas and oil compressors and pumps are used in offshore installations to extract hydrocarbons from submarine oilfields. Submarine, downhole and ESPs (Electrical Submersible Pumps) turbomachines are driven by electric motors which are arranged coaxial with the turbomachine stages.
The flow processed by the turbomachine usually contains a mixture of gaseous and liquid hydrocarbons with a variable gas volume fraction (GVF) or liquid volume fraction (LVF). Typically, wet gas (i.e. gas containing a fraction of liquid) is processed by wet gas compressors. High speed centrifugal wet gas compressors can usually not tolerate a volume percentage of liquid phase larger than 5%, i.e. they require operating at GVF>95%. Large size scrubber-separators are thus often required to remove excess liquid content from the processed flow.
The presence of a mixed flow causes serious complications in processing the fluid in the turbomachine. The efficiency of a turbomachine designed for operating at a design GVF value rapidly drops when the machine operates far from the design GVF. Assuming a design GVF of 40%, the efficiency can drop by 10 points when operating below 20% or above 60%.
To address the difficulties in handling mixed gas/liquid flows, and specifically wet gas with GVF>80%, it has been suggested (U.S. Pat. No. 4,830,584) to use multistage pumps and compressors, having a plurality of axially aligned stages arranged in a turbomachine casing and driven by two electric motors arranged outside the turbomachine casing. Two co-axial shafts drive in rotation oppositely rotating impellers of the multistage turbomachine. The number of stages which can be used in this kind of turbomachines is limited by the need of reducing the axial length of the machine. This limits the achievable pressure rise through the turbomachine. The arrangement of co-axial shafts for driving counter-rotating impellers adds to the complexity of the turbomachine, however.
Limited allowable axial length of the turbomachine-motor assembly, typically within 7 meters for subsea applications, also severely limits the total power available. As a matter of fact, electric motors above 4 MW can be longer than allowed by the above mentioned limits, unless a very high rotary speed is used, which is detrimental to the life of the turbomachine.
There is thus a need for a more efficient turbomachine for processing mixed gas/liquid flows, which at least partly alleviates one or more of the above mentioned drawbacks of the current art turbomachines.