Patent Application
20090232664
The invention relates generally to subsea pumping systems and methods, and more specifically a canned permanent magnet motor for a subsea pump drive.
The need for subsea pumps in the oil and gas industry has been increasing due to increasing energy requirements, and because onshore energy sources are becoming more scarce. These industries must now look for energy sources offshore; and the distance between shore and subsea fields continues to increase.
Electrical motors have been selected as a standard to drive the subsea pumps due to ease of power transfer over long distances when compared to other drive systems and methods, including, for example, hydraulic driven pumps. Conventional systems and methods employ induction motors for driving the aforesaid subsea pumps. Use of induction type motors has been problematic however, since induction motors are low efficiency and low power factor motors. This low efficiency and low power factor undesirably require an oversized umbilical connection and variable frequency converter on the topside in order to provide a large amount of VAR power to the subsea motor. Both, the oversized umbilical connection and variable frequency converter undesirably increase the cost to the subsea pumping system.
It would be both advantageous and beneficial to provide a subsea pumping system that overcomes the problems generally associated with subsea pumping systems that employ induction motors. The subsea pumping system should have an overall efficiency that is greater than known subsea pumping systems utilizing induction motors, such that the subsea pumping system could function using a low power rating umbilical. It would be further advantageous if the subsea pumping system had a higher power factor than known subsea pumping systems utilizing induction motors, such that the subsea pumping system could function using a low power rating topside variable frequency converter.
Briefly, in accordance with one embodiment, a subsea pump drive motor comprises a stator, a rotor comprising a plurality of permanent magnet pole pieces, and a non-magnetic can configured to affix the pole pieces to the rotor.
According to another embodiment, a subsea pump drive system comprises a permanent magnet subsea pump drive motor having a rotor configured with a plurality of permanent magnet pole pieces, the rotor and plurality of pole pieces disposed within a non-magnetic can configured to prevent corrosion of the rotor and plurality of pole pieces.
According to yet another embodiment, a method of controlling a subsea pump comprises:
providing a permanent magnet (PM) subsea pump drive motor; and
controlling the PM drive motor such that the PM drive motor drives a subsea pump in response to variable frequency converter signals received by the PM drive motor.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.
Four sets of blades 22 are disposed on the rotor shaft 24. These blades 22 are configured to pump cooling fluid 26 flowing through a motor sealing can 28 that encapsulates both the stator 14 and the rotor 16 according to one aspect of the invention illustrated in
A rotor 16 position signal generated via an encoder 32 is transferred to a variable frequency converter (VFD) 35 via a wireless signal transmitter 36 according to one embodiment. In another embodiment depicted in
The encoder 32 is connected to one end of rotor shaft 24 to detector rotor position for proper speed/torque control of the permanent magnet motor 12. Traditional control approaches utilizing communication cables are difficult to employ when the VFD 35 is far away from the motor 12 due to signal attenuation along cables between the motor 12 and the VFD 35. Further, traditional sensorless control approaches also face challenges due to difficulties associated with accurate measurement of motor terminal voltages through such long distances.
The foregoing challenges associated with traditional control approaches utilizing communication cables are overcome using a wireless signal transmitter 36, discussed herein above. The rotor position signals are sent to the wireless signal transmitter 36, which then transmits the rotor position signals to a topside controller/VFD 35 that is used to drive the PM motor 12.
The end of the rotor shaft 24 opposite the end connected to the encoder 32 is connected to a subsea pump 40, such as a multiphase pump. There is a seal 42 between the motor 12 and pump 40 to block motor cooling fluid 26 from flowing into the pump 40. The fluid pressure inside the motor 12 is normally maintained higher than the fluid pressure inside the pump 40 via a pressurizer typically located subsea beside the motor 12, such as described below with reference to
A pressurizer 104 is employed to maintain a positive pressure from the motor 12 to the subsea pump 40 under all conditions. An optional liquid storage tank 106 can be used to store processed fluid 44 for motor cooling purposes when the processed fluid is purely gas.
The stator 14 is also encapsulated via a can 108 to prevent any process fluid 44 or gas from entering the stator 14 portion of the permanent magnet motor 102. This stator can 108 is filled with a clean cooling fluid 26, such as a suitable oil, to cool the stator 14. A heat exchanger 34 can be employed to exchange heat from the motor 102 to outside seawater.
Subsea pump drive 100 also employs an encoder 32 that is connected to one end of rotor shaft 24 to detector rotor position for proper speed/torque control of the permanent magnet motor 102. A rotor 16 position signal generated via the encoder 32 is transferred to a variable frequency converter (VFD) 35 via a wireless signal transmitter 36 according to one embodiment. In another embodiment depicted in
In summary explanation, a subsea pump drive employs a permanent magnet (PM) motor to drive a subsea pump. The PM motor rotor in one embodiment is canned with a non-magnetic material such as inconel, that can provide a desired level of corrosion protection. The PM motor provides a subsea pump drive that is smaller and more efficient, having a high power factor than a subsea pump drive utilizing a conventional induction motor. The PM motor subsea pump drive eliminates the necessity for a topside storage tank and associated fluid transfer lines when the motor rotor is cooled with processed fluid.
The PM subsea pump drive motor achieves its high efficiency due to the permanent magnetic flux on the rotor linking the stator so that the PM motor can achieve higher efficiency due to absence of rotor current.
The PM subsea pump drive motor further has an increased power factor due to the absence of exciting current.
The PM subsea pump drive motor employs lower power umbilical features due to the aforesaid high power factor and high motor efficiency.
The PM subsea pump drive motor employs a lower power topside variable frequency converter due to the aforesaid high power factor and high motor efficiency.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
