As offshore production moves to deeper, more harsh environments, the use of surface structures is becoming prohibitive due to complexity and cost. This has led to subsea processing, which has created a new emerging market for processing equipment.
Advantages of Direct Drive Systems® Permanent Magnet Motors
Direct Drive Systems’ permanent magnet (PM) motors offer significant benefits for subsea pump OEMs and end users. They not only increase the overall system efficiency by 28 percent, but also allow the pumps to deliver much more power (up to 6MW per pump) without increasing mechanical losses. They also require less maintenance than traditional machinery, which is a highly significant, cost-saving benefit for subsea operators. Other benefits of Direct Drive Systems’ PM motors include:
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Smaller Rotor Diameters: For the same power level, our PM motor rotors are smaller when compared to induction motors. Since the friction losses in a flooded motor are related to the diameter of the rotor, our motors incur significantly lower mechanical losses.
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Larger Air/Liquid Gaps: Since subsea motors are flooded, the distance between the rotor and the stator is actually a liquid gap. Due to PM characteristics and Direct Drive Systems’ proprietary designs, the motor gap can be increased up to 20 mm., compared to 4 mm. for induction motors. This increase not only provides lower mechanical losses, but also provides the opportunity to introduce canning between the rotating and stationary parts of the motor to prevent any process fluids or gasses from entering into the electric systems.
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Higher Speed of Operation: Currently, induction motors are limited in their speeds (approximately 3,800RPM). However, if the motors could operate at much higher speeds, the hydraulic section of the pump would become more efficient (by up to 4 percent). The differential pressures would also increase, enabling the pumps to be installed in deeper waters - a significant advancement for deepwater projects such as in the Gulf of Mexico, where the water depths can approach approximately 9,842 ft. (3,000 m).
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Qualifications: Currently, all subsea pumps utilize fluids pumped from the topside through the umbilical. If the pump systems can survive on regenerative Monoethylene glycol (MEG), already utilized in the well, both installation and operating costs would significantly decrease. We are qualifying our motors to operate in regenerative MEG.
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System Level Benefits: Besides the pump system benefits of our PM motors, there are also complete system benefits. Due to the energized permanent magnets, Direct Drive Systems’ PM motors operate on a constant torque basis, which means that our PM motors can provide full torque at any speed. They are not dependent on speed like induction motors. This unique property provides quick startups, and therefore, the complete electrical system does not have to be rated for the startup conditions. For example, an induction motor requiring 400AMPS will need 1,600AMPS during start up, which means the electrical system (cables, drives, penetrators, and transformers) all must be rated for 1,600AMPS, whereas Direct Drive Systems’ PM motors will never exceed 400AMPS for the same working conditions. This is a significant cost savings, especially when longer step outs are used.
Disadvantages of Existing Equipment
Conventional subsea pumps are integrated and hermetically sealed, and utilize induction electric motors. These systems operate in a flooded state, submerged in oil or some type of water/glycol mixture. The conventional induction motors have high electromagnetic, mechanical and hydraulic losses due to factors, such as their large rotor diameter, circulating currents in the rotor, low-power density, and the high specific gravity of the fluid in which they operate. These mechanical losses reduce the efficiency of the pumps by 20-30 percent, and limit power levels to 2MW. Customers now require pump capacity between 4MW and 6MW due to increased production rate requirements, increased water installation depths and the higher differential pressures needed to surpass line pressures. Multi-unit deployment of these pumps is not cost effective, and it is not possible to reduce the size of the induction machines while increasing their power output.