Changing the game of subsea boosting

A radically new pump being developed by Baker Hughes, a GE company (BHGE) has the potential to enable robust, low cost subsea boosting, increasing oil recovery in a broader range of fields.  
  • Forfatter Aslaug Melbø, Engineering Manager Subsea Power & Processing, Hans Kristian Sundt, Product Manager Subsea Power & Processing
  • Forfatter selskap Oilfield Equipment Baker Hughes, a GE company (BHGE)

The challenge
Oil companies are looking for ways to increase oil recovery rates. Subsea boosting can be a powerful tool, but investment cost is often too high. Furthermore, for brownfield applications the topside space needed for equipment related to the pump barrier fluid system and power system is not necessarily available. The oil companies are also looking to extend subsea boosting to deeper and more remote fields, and fields which require very high boost capabilities.

BHGE has come up with a radically new architecture for subsea pumping, which solves some of the challenges pointed out by the oil companies. The pump is being developed within a Joint Industry Project with majors Total, Shell, Statoil and Chevron supporting the first phase of the JIP. This has been key in order to capture the relevant challenges and requirements the operators have and ensure these challenges are included into the development program.

Introducing the Modular Compact Pump
The Modular Compact Pump (MCP) is an innovative new subsea boosting system that aims to address the technical, operational and economic limitations faced by conventional subsea pumps.

Rather than adopting a conventional pump approach of a motor connected to the pump through a single rotating shaft, the MCP consists of a series of integrated motor impellers that co-rotate around a static shaft. This eliminates the rotor-dynamic challenges of a conventional pump, and removes the need for a mechanical seal around the rotating shaft, a part which is challenging in terms of reliability. This also enables, in theory, an unlimited number of stages to be stacked, and hence the ultra-high boost required by some operators can be achieved. Rapid changes to flow conditions are challenging for conventional pumps due to the large momentum of the rotating shaft. The MCP in contrast is extremely flexible and responsive, well suited to deal with whatever operating conditions the wells may require.


                                                  MCP subsea modul
 

Each MCP impeller can be individually controlled to optimize the flow through the pump. Wellstream boosting constitutes a challenge as the wellstream comprises both liquid and gas. The additional challenge is presented by the gas becoming more compressed as it passes through each impeller in the pump. Furthermore, over the life of the field the balance between the amount of liquid and gas naturally changes, driving a conventional pump off its design point.  By independent adjustment of each single impeller’s speed the pump can adapt to these conditions throughout the life of field, hence avoiding cost of intervention while maintaining high production at high efficiency.

The design of the pump is scalable. If higher boost is needed, the pump can be constructed with more stages, whereas a very compact pump with fewer stages may be used for fields with lower boosting requirements. All supporting equipment, for instance the subsea VSDs, is also scalable. This allows for smart and cost efficient manufacturing, and not least qualification as the pump is simply a stacking of identical building blocks.

Each stage in the pump has its own process lubricated bearings thereby eliminating the need for a topside barrier fluid system. The MCP also utilises subsea variable speed drives, further reducing the overall topside footprint. The result is a more reliable boosting system that requires less than half the topside infrastructure and with more than two times the power density of a conventional multiphase subsea pump. The reduced need for topside equipment as well as the simplified umbilical are two of the main features that drive the cost of the MCP system down. Initial cost benefit studies on relevant case studies show that the saving compared to a conventional pump system is typically between 20 and 37% of the total system cost.



                                              An umbilical for a conventional subsea pump compared to the simplified MCP umbilical.
 

Testing and qualification
To evaluate the feasibility of this entirely new concept, a 3-stage demonstrator was built and tested in the BHGE multi-phase flow loop in Bari, Italy. The test results so far are very promising, proving the feasibility of the concept when it comes to the modular approach, the integrated motor-impeller design as well as the hydraulics. The data will be analysed and presented to the JIP in the fall of 2017.

The next step will be to build a full-scale version of the pump, and qualify the pump to TRL 4.

Conclusions
The MCP brings an entirely new architecture to subsea boosting. It has the potential to significantly reduce cost by reducing the need for topside equipment and by simplifying the umbilical. Subsea boosting prospects that were earlier discarded may hence become economically viable, giving the oil companies a new tool on their road to increased oil recovery.