Recognizing the demand for subsea processing equipment has led to the development of innovative solutions for subsea boosting, separation, and water injection.

The Camforce dual multi-phase pump system can be designed for deep-water or shallow-water applications. An over-trawlable structure is available for shallow water in fishing areas. (Images courtesy of Cameron)



Because of the emphasis on increased oil recovery, seabed boosting has moved to the forefront as a means of increasing oil flow and ultimate recovery from subsea reservoirs. And flexibility is critical to the success of any seabed boosting system.

This was the impetus for Cameron and Curtiss-Wright EMD to form a joint venture (JV) to combine proven components into a system that can be configured to meet the demands of any production scenario.

The heart of the matter

The heart of any multiphase boosting system is the pump/ motor combination. The JV partners chose a twin-screw pump for its flexibility with respect to flow rate, differential pressure, and gas volume fraction (GVF) — the ratio of gas to liquid in the production flow. Leistritz was selected to provide the pump because of the robust design of its product, which offers several benefits. One significant advantage of the design is that the pump shaft and screw are machined from a solid forged billet rather than by pressing the screw castings onto a separate shaft. This results in a much stronger and stiffer design.

A twin screw pump has two sets of intermeshing screws with a space at the center. In competing designs, production flow is directed into the center cavity, and the screws force it toward the ends of the pump and out through a discharge port. This approach results in higher pressure and temperature at the bearings and seals located at the ends of the screw shafts.

The flow direction is reversed in the Leistritz subsea pump design. The cooler, suction flow is directed to the ends of the pump and is pumped toward the center.

This gives a lower temperature and pressure at the bearings and seals, increasing the life of the pump. Many of these pumps are currently running in Curtiss-Wright EMD surface applications, and one has been installed offshore Brazil.

The motor driving this pump is equally robust in design. The EMD Business Unit of Curtiss-Wright Flow Control supplies the motors, which are a “canned” design.

This means the stator windings are hermetically sealed by a metal can that is inserted inside the stator housing and seam-welded at each end. The rotor also can be encased in a can that is seam-welded at both ends. This construction produces a motor that, even though lubricated and cooled with mineral-based barrier fluid, is not as sensitive to water intrusion as an open winding motor. Curtiss-Wright has manufactured canned motors for more than 50 years and has experienced successful motor run durations in excess of 20 years in the field.

The motor/pump assembly with its lubrication system is an integrated system that resides within a subsea retrievable structure along with a barrier fluid pressure volume regulator (PVR) module that is independently retrievable. Barrier fluid for lubrication and cooling the motor and pump is supplied from a dedicated topside hydraulic unit through one of the tubes contained in the power and control umbilical.

The PVR provides a fresh supply of barrier fluid to the pump/motor at a pressure slightly above suction pressure. The seals within the pump are designed to weep a small amount of barrier oil from the bearing side into the process to constantly flush the seals. The PVR has three separate means (both active and passive) of supplying barrier fluid and can be retrieved by a light intervention vessel if the need arises.

Lube oil is circulated within the motor/pump housings and also through a heat exchanger mounted on the pump module. This exchanger rejects the heat from the circulating barrier fluid into the sea water.

Operations

This artist’s impression shows the Camforce dual multiphase pump system installed in deep water.



Twin screw pumps can operate continuously and efficiently at GVFs up to 95%. A proprietary liquid recirculation system allows operation during periods of pure gas (100% GVF).

The pump module resides on a manifold flow base that can be configured as required by the application. For lower flow applications, one pump can be supplied on a single flowline base. For multi-well fields with dual flowlines, two pumps can be supplied on a dual flowline base with crossover capability. In either case, production flow can bypass the pumps, allowing pigging operations with the pump manifold in place. The single pump and dual pump flow base can be supplied on a mud mat or suction pile, depending on soil conditions at the field location. Camforce pumps can be designed for either shallow or deepwater applications. An over-trawlable structure is available for shallow water in fishing areas.

Subsea controls for the Camforce multiphase boosting system are standard field-proven components. Fiber-optic communication is standard because it offers greater bandwidth and is impervious to induced noise and crosstalk from the three-phase power conductors in the umbilical. Increased bandwidth allows real-time updates from pressure and temperature sensors, vibration monitors, and flowmeters. Valve actuator response times are also improved with fiber-optic communication.

Surface equipment consists of a standard master control station, one variable frequency drive per pump, a barrier fluid oil supply unit, and a hydraulic high-pressure unit for control function actuation.

These components can be housed in a single enclosure to be placed on the deck of the host facility or supplied as individual items for integration into the existing topsides architecture.

The Camforce FlowBoost 2000 twin-screw multiphase boosting system uses a twin-screw motor/pump system previously qualified by Curtiss-Wright with Petrobras in Brazil. It has a maximum flow rate of 1,200 cu m/hr and a maximum differential pressure of 1,400 psi. The system is designed to work to 6,500 ft (2,000 m) with a maximum step-out distance of 25 miles (40 km) and 0% to 100% GVF.

Together the JV partners have enhanced this system and have designed a variety of multiphase boosting systems that are robust, reliable, and easily configurable for a wide range of field and production variables.