SUBSEA BOOSTING

As offshore production moves to deeper, more hostile waters, 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.

The subsea pump industry developed in the 1980s, primarily in Norway, as a solution to the challenges of operating offshore platforms in the North Sea, and in order to access Arctic oil reserves. The concept is to take all of the equipment found on a typical offshore platform, modify it for an extreme maritime environment, and place it on seabeds up to 3,000m deep.  Thousands of subsea wells are in operation today, and significant growth is projected. 

Disadvantages of Existing Equipment
Subsea pumps are integrated and hermetically sealed, and currently utilize conventional 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, their 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%, and limit power levels to 2MW.  The needed pump capacity is between 4MW and 6MW due to increased production rate requirements, deeper 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.

Advantages of DDS Permanent Magnet (PM) Motors
DDS 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 (over 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 DDS PM motors include:

• Smaller Rotor Diameters: For the same power level, DDS 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, DDS motors generate significantly lower mechanical losses.
  
  

• 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 DDS proprietary designs, the motor gap can be increased up to 20mm, compared to 4mm 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.
  
  

• Higher Speed of Operation:  Currently, induction motors are limited in their speeds (around 3,800rpm). However, if the motors could operate at much higher speeds, the hydraulic section of the pump would become more efficient (by 4%).  The differential pressures would also increase enabling the pumps to be installed in deeper waters.  This is especially true for Gulf of Mexico, where the water depths are around 3,000m.
  
  

• Qualifications: Currently, all subsea pumps utilize fluids pumped from the topside through the umbilical.  If the pumps systems can survive on regenerative MonoEthyleneGlycol (MEG) developed from the well, both installation and operating costs would significantly decrease. DDS is qualifying its motors to operate in regenerative MEG.
  
  

• System Level Benefits:  Besides the pump system benefits of DDS PM motors, there are also complete system benefits.  Due to the energized permanent magnets, DDS PM motors operate on a constant torque bases, which simply means that DDS PM motors can provide full torque at any speed. They are not dependent on current and speed like induction motors.  This uniqueness 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 1600AMPS during start up, which means the electrical system (cables, drives, penetrators, and transformers) all have to be rated for 1600AMPS, where as DDS PM motors will never exceed 400AMPS for the same working conditions. This is a significant saving, especially when longer step outs are used.

SUBSEA WATER INJECTION & COMPRESSION

The benefits of subsea boosting pumps have been applied to water injection pumps, which are also being installed beneath the ocean. The operation of water injection pumps is similar to oil boosting pumps, although the required pressures and flows are much greater. DDS has the only enabling motor technology that can deliver the necessary powers and speeds. DDS is working to design and qualify higher-pressure and volume water injection pumps.

Aside from subsea pumps, the industry has started to design and qualify subsea compressor systems. Although no subsea compressors have been installed, DDS PM motors have been identified as an enabling technology by many.