Subsea Boosting for Oil and Gas Projects (11.06.2010)

Multiphase, according to the general understanding in the oil and gas community, is the oil-watergas mixture coming with the natural
untreated well production. Uncertainties in the production data, slugflow in pipelines and fast variations in pressure and fluid composition are part hereof.

As soon as a significant amount of gas is involved, separation effects in the flowlines have to be considered. Consequently the Multiphase
Pump (MPP) has to be designed for slugflow conditions.

In general multiphase pumps can be divided into three groups with its own technical requirements:

• Gas tolerant liquid booster: 0% to 30% gas – typically only applicable direct downstream of separators or inside long crudeoil
  transfer pipelines.
• Multiphase pump: up to 100% gas temporary – downstream well and upstream separator.
• Wet gas compressor: 99% to 100% gas continuously but with the risk of slugflow (water or condensate) – typically downstream
  gas wells.

In all cases the MPP shall be able to transport the liquid-gas mixture against the full pipeline backpressure without interruption of the flow.

Bornemann provides the patented solution of internal liquid (product liquid phase) separation and recirculation of a small percentage of liquid back to MPP inlet, preventing MPP rotors running completely dry. Separation and liquid storage inside the MPP discharge casing is vital for the Multi-Phase Mixture (MP) boosting process.

Improvement of Oil Production

Each reservoir has its own optimal production characteristic in regard to oil production, gas production and water-cut minimisation.
The graphic explains the production task of multiphase pumps. The oil production (orange curve) of wells typically follows a certain known production profile, depending on the wellhead back-pressure. At a certain wellhead pressure the flow velocity in the well is too low to transport the liquid to surface. The well might be “dead” or only gas is coming. If the well is closed, after some time the wellhead shutin pressure will build up. The shut in pressure can be very high compared to the production pressure. There is an optimal oil production
at a certain pressure which is fluctuating with the time. Too low pressure draw down might lead to non-optimal production rates and damages in the reservoir.

Together with the oil the associated gas is produced – the relation between oil and gas typically is given in GOR (gas-oil ratio at standard conditions). Because the gas is expanding with the pressure going down, the total production curve is expanding to higher flow rates (brown curve) at lower system and pump inlet pressures. The well is producing into a pipeline system. The pipeline backpressure (blue curve) on the production is defined by any static backpressure on the pipeline (geodetic height, separator pressure, etc.) plus pressure losses depending on the medium flowing through the pipeline. The equilibrium between well and pipeline performance (crossing of blue and brown curve) defines the natural production (depending on
choke setting, etc.). During the first years chokes might control the perfect balancing between production and backpressure. By and by the reservoir is depleting and the producing well pressure is declining. Additional
wells might be connected to the pipeline system, which can lead to higher pipeline back-pressure on the well.
At this moment the installation of multiphase boosting pumps (red arrow) helps to control the well pressure for best production. The pump now unloads well performance characteristics from pipeline behaviour.

Twin Screw Pumping Technology

The Bornemann MPPs are twin screw pumps. Within these pumps two synchronized rotors are intermeshing
and forming closed chambers between the rotor-screwflanks and the surrounding casing insert (liner). Whatever enters into the chamber at screw inlet will be moved to the outlet. Pump capacity depends on rotor
diameter, pitch of the rotor-screws and finally the pump speed. There is no contact between screws and liner. Consequently there will be a gap between the pumping elements and therefore a certain internal backflow (slippage) from pump discharge back to pump suction. At any time the Bornemann internal separation
and recirculation ensures enough liquid at the rotors to keep the gaps sealed with liquid and therefore
the pump performance is independent on gas content.

The rotors are “engineered rotors”. Shaft and screws can be made from materials, best suitable for the task and the pumping process. Heat treatments and coatings can be done separately without influencing the shaft properties.

DPC – Double Pressure Compensated Pump

The Bornemann Subsea Booster (SMPC) is the consequent further development of the subsea MPP and the heavy duty topside Multiphase Pump type MPC. The new Double-Pressure-Compensated (DPC) pump design provides the basis for the new subsea MPPs – the SMPC series 4.1. It basically consists of two main
components:

• The pressure casing, rated for water depths and process pressure.
• The pump-motor-module, including all rotating equipment.

The pump-motor-module is built up from the approved SMPC pump cartridge and a simplified electrical
motor cartridge. Power transmission from motor to pump can be achieved by a conventional mechanical coupling. Optionally a hydraulic torque and speed converter can be used.

The total cartridge – except the pumping chambers – is filled with pressurised lube oil – the barrier fluid. The pressure of the lube oil is permanently controlled and adjusted to provide to the mechanical seals best operating conditions (constant pressure over the seal). Consequently the casing of the pump-motor-module is fully pressure compensated against the pump discharge pressure which is inside the pressure casing. The pump-motor-module will be inserted into the pressure casing by avoiding all sensitive interfaces between pressure casing and pump-motor-module. Deflections, deformations, mechanical stress, etc. from the casing will not been transmitted to the pump-motormodule. The casing is designed according to the actual requirements. Different materials can be used, coating from inside could be done, composite material might be applied. The pressure casing is a geometrical simple “separator style” pressure vessel. Therefore it provides good separation and liquid hold up capabilities – required for reliable multiphase operation. The DPC design also allows the vertical installation of the MPP. The diameter of the pump can be minimized – still providing sufficient separation and liquid hold up volume for multiphase-service.

Pump Performance

The pump capacity at a certain speed is not much influenced by the differential pressure over the pump. There is a certain backflow from pump discharge to pump suction through the gaps between rotors and casing. The backflow depends mainly on viscosity and differential pressure. By speed variation a wide capacity range at full pressure head (differential pressure) can be provided, 10 to 120 percent of the nominal capacity is typical.
The achievable differential pressure is limited by the mechanical load on the rotor. Differential pressure of up to 100 bar in high viscous liquid service with up to 20 percent gas and good efficiency is standard today on
twin-screw pipeline transfer pumps. Special designed high pressure multiphase rotors also allow for pressure build up of 100 bar without influence on the rotor integrity in full gas service – but at high gas the efficiency will be limited. Rotors, designed for higher capacity are limited in pressure head due to the fact that the maximal allowable shaft power within one frame size remains constant over all available rotor designs.

High Differential Pressure Booster Systems

For subsea applications more often very high boost pressures will be required to overcome the water depth while the wellhead pressures shall be reduced to its limits. Practically this will require multiphase pumps or gas tolerant liquid boosters being able to increase the production pressure from almost 0 bar up to 200 to 300 bar. The Bornemann solution is the installation of two or more multiphase pumps in series. The first is
controlling the system inlet pressure and providing a certain precompression. The second pump is controlling the outlet pressure of the first pump by speed variation – picking up the compressed volume flow at the outlet of the first stage MPP at the required pressure – and overcomes the remaining pipeline backpressure.
Each single pump is operating with a better efficiency and the total power consumption will be significant reduced. On a project in Abu Dhabi, the total power requirement could be reduced from 1 MW for the single
pump concept to 500 kW (2 x 250 kW) with the serial installation that was finally selected.