"Cooling system for an electric motor, and a drive system for driving an impeller"

The present invention relates to a cooling system for an electric motor and a drive system with a drive unit for driving an impeller. The system includes a magnetically driven pump in a fluid filled electric motor. The system is particularly intended used in connection with systems for well fluid boosting by compressing hydrocarbon gases and pumping hydrocarbon liquids. More particularly, the invention relates to a pump for pumping a cooling liquid within a motor with a pressurized housing.

The system is particularly developed for deep sea compression stations.

An offshore gas field may be developed with seabed installations which are tied back to terminals onshore or to an existing platform. The seabed installation comprises one or more production templates where each template produces well fluids through manifold headers which are connected to one or more pipelines. The pipelines transport well fluids to an onshore terminal, an existing platform or any other receiving facility for further processing. Processed gas and condensate are exported to the market. One or more umbilicals for power, control and utility supplies are installed from the receiving facility to the subsea installation.

For the initial production phase, well fluids may flow into the receiving facility by means of the reservoir pressure. Later in the productions phase or at start-up of the production, well fluid boosting is required in order to maintain the production

level and to recover the anticipated produced volumes. This is may be performed by subsea compressor and pumping assemblies.

Subsea fluid pressure boosting assemblies for this purpose with electric motors 5 that requires a dry environment, and that utilizes a pressurised, gas filled motor housing are for instance disclosed in Norwegian Patents Nos. NO 172075, NO 173197, Norwegian Patent Application No. 2001 5199 as well as Norwegian Patent Application No. 2003 3034.

o These publications disclose the use of gas filled electric motors where avoiding corrosion and other problems that are related to separation of hydrocarbon condensates and water in liquid form in the motor, is considered.

When a gas at high pressure is present in a motor chamber, it occurs s considerable slippage losses between the rotor and the stator of the motor and this result in a substantial reduction of overall efficiency of the motor. This problem is less prominent when the pressure in the motor chamber is reduced.

Lower pressure however also results in a reduction of the cooling effect of the gas. To improve cooling, motors of this kind frequently includes a fan fixed to o one end of the shaft of the motor. This further reduces the overall efficiency, in particular at higher pressures as the pressure affects the viscosity of the gas.

Accordingly it is an object of the invention to improve the efficiency of a motor.

This is achieved with a cooling and sealing system according to the present 5 invention. In the system according to the invention, an impeller for circulating a cooling fluid, preferably a liquid, can be driven by the motor, yet the motor can be completely sealed with the exception of a sealing around an output shaft.

The impeller is preferably located at the non-driving end of the motor shaft. A gap between the shaft and the impeller is sealed by a can of sheet metal or any o other suitable material. The motor shaft and a part on the impeller include magnets, creating a rotating magnetic field through the can, driving the impeller.

The can seals between the motor chamber and a completely sealed cooling

circuit, for instance with channels through the stator windings. The impeller may however be placed at the driving end of the motor drive shaft providing that the motor shaft passes through the centre of the impeller and that the can is substituted with a tubular body sealing between the motor and the impeller.

This is achieved with the system according to the present invention defining a cooling system for an electric motor with housing and a motor shaft. An impeller is rotated by the motor drive shaft for pumping a cooling fluid through at least one cooling circuit in the motor. At least one magnet is drivably connected to the motor shaft, and at least one magnet is drivably connected to the impeller. The at least two magnets are placed in close proximity to each other. A shield or motor can between the magnets seals the cooling circuit from the interior of the motor preventing cooling fluid from entering the motor.

Furthermore the invention may relate to a drive system with a drive unit for driving an impeller rotationally supported in bearings along an axis of rotation. An electric motor with a stator, a rotor and a shaft with an axis of rotation aligned with the axis of rotation of the impeller. A shield, seals between the interior of the motor and the drive unit. At least one magnet separate from the rotor of the motor is placed at a distance from the axis of the motor and adapted to be driven in rotation by the motor shaft. Some electric motors may have separate permanent magnets in connection with the rotor. The present invention relates to additional magnets for driving the impeller in the sealed system.

At least one magnet may be placed on the drive unit at a distance from the magnet that is adapted to be rotated by the motor to allow the magnet on the motor to drive the magnet on the drive unit for the impeller.

The shield may include a cylindrical portion with a central axis aligned with the central axis of the motor shaft, and the at least one magnet adapted to be driven in rotation by the motor shaft, may be driven inside the cylindrical portion

of the shield. The at least one further magnet is located on the outside of the shield, and the magnets may be bar magnets with a longitudinal axis parallel to the longitudinal axis of the motor shaft.

Furthermore the invention may include a sealed cooling circuit with an impeller with an axis of rotation for pumping a cooling fluid around inside the cooling circuit with at least one magnet drivably connected to the impeller, and at least one magnet drivably connected to a motor having the same axis of rotation as the impeller. The magnets are placed in close proximity to each other, and a shield is placed between the at least two magnets, completely sealing the cooling circuit.

A subsea compression station where this system may be included may comprise the following modules and parts: one or more compressor trains and pump modules, one or more circuit breaker modules, inlet and outlet manifolds, inlet coolers (if supply pipelines not are sufficient for cooling the well stream), inlet sand trap (for accidental sand production), parking location for main transformer and power umbilical termination head, process system, control system.

The pump train may include: pump module, pump variable speed drive (VSD), (variable speed drive), remote and manually operated valves, interconnection piping, control system including control modules.

The impeller and drive system according to the invention may be driven directly by a high-speed motor. The electric motor the motor does not need to be cooled with a gas, and the problems relating to circulation of gas in the motor can be omitted.

As the circulation of cooling gas is omitted, the system does not have to handle the fines, sand or pollution related to gas cooling. In this way the system is protected against wear and degradation from solids and high efficiency, long life and reliability is ensured.

Brief description of the enclosed drawing:

Fig. 1 is a schematic representation of an impeller drive system according to an embodiment of the invention.

Detailed description of embodiments of the invention with reference to the enclosed figure:

Fig. 1 shows a subsea impeller drive system according to an embodiment of the invention. The system includes a motor with a shaft with a non-driving end 3.

The non drive end of the motor, drive magnets 2, secured to the motor shaft and rotates accordingly with the motor shaft. The number and size of the magnets will depend on the torque it is necessary to transfer to the impeller etc. The number and placement of the magnets will of course also be adapted to provide balance and suitable dimensions. The magnets 2 may also be placed on a suitable wheel to increase the diameter of the shaft to increase the distance between the centre of the shaft and the magnets 2 to improve torque transferring properties. A motor can 6 is placed on the outside of the rotating magnets 2, preferably with a small gap between the magnets and the can 6 to allow rotation of the magnet, yet be as small as possible to improve torque transferring abilities. The can may be made of any suitable material that has properties that are suitable in relation to corrosion, mechanical integrity and

permeability for the magnetic field. A rotor with an outer carrier 4 is supported in suitable bearings such that it can rotate in relation to the motor can 6. Driven magnets 5 are placed on the outer carrier 4 with a suitable gap to the motor can to allow the outer carrier to rotate, still with a sufficiently small gap to maintain the torque transferring properties between the motor shaft and the outer carrier. The can 6 seals a closed circuit cooling system with a suitable cooling fluid from the motor housing. The outer carrier 4 is secured to an impeller 1 for pumping the cooling fluid through the cooling circuit. The cooling circuit could be an open circuit, but this will in many cases be less favourable due to pressure conditions, corrosion, pollution, growth of various organisms etc. The cooling circuit may go through channels in the stator windings etc. to improve cooling. If necessary, the circuit may also include a heat exchanger for exchanging heat with for instance the surrounding sea water. The motor will of course include sealed pressure housing and all the normal components for sub sea electric motors.