A New Underwater Power Source for the Subsea Factory

Hi everybody, my name is Rixin Lai and I am an electrical engineer in the High Power Conversion System Lab at the GE Global Research Center in Niskayuna, NY. Over the past few years, we have been working in an exciting area: electrifying the long step-out and deep water subsea processing field using high voltage direct current (HVDC) technology.

In one of our project reviews, a customer made a comment that really grabbed my attention. “Putting this complicated system in the deep ocean is as difficult as sending a space ship to the moon,” he remarked.

You may be thinking, why is electrical power needed in subsea field in the first place? The answer is directly linked to the oil and gas industry. The diminishing conventional oil and gas reservoirs or so call the “easy oil”, and the always growing energy consumption have been driving the efforts to explore the unconventional petroleum resources. These resources are usually in ultra-deepwater and far away from the shore, e.g., 3000 meter depth and hundreds of miles away from the shore, where subsea process is usually preferred for technical and/or economic reasons.

However, in order to achieve subsea process, turbo machines, e.g., pumps and compressors, and other processing equipment need to be deployed on the seabed. Accordingly various electrical equipment, including variable speed drives, power supplies, motors, connectors, switch gears, etc., are required to drive the machinery loads subsea. The overall power requirement ranges from tens of kW to tens of MW. As the scale and power consumption of the subsea process continue to grow, there is a clear need for a reliable power transmission and distribution system to power the subsea equipment over long step-out. From the power architecture standpoint, HVDC technology is a natural choice since it has lower cable loss than the AC system over long distance.

Subsea process system
Subsea process system

So now you may be thinking, is it feasible to even put HVDC equipment in the sea water? The answer is yes, but not in an easy way. The ultra-deepwater and long step-out situation is extremely harsh for the electrical equipment. They need to handle very high pressure and corrosive environment while providing all the electrical functions as they do on shore, if not more. In addition, accessibility of any equipment deployed in the subsea field is extremely limited. Therefore, other than being designed for installation in ultra-deepwater, the electrical system must be very reliable and require minimum regular maintenance in its lifetime. The electrical system should also be as compact as possible, so that they can be installed and retrieved without heavy vessels and lifts which have very limited availability. It is all these challenges that make it “as difficult as sending a space ship to the moon”.

A subsea power converter unit
A subsea power converter unit

In order to address the above mentioned challenges for subsea applications, the researchers at GE Global Research Center have developed a novel power transmission and distribution concept: modular stacked direct current system (MSDC). The MSDC technology achieves the required DC transmission voltage by stacking a number of power converter building blocks in series, both on the shore and on the subsea field. On the shore, the converter system is controlled to maintain the current in the DC cable constant regardless the loading condition. On the other end, each subsea converter module is directly coupled to each individual motor load. Since the current in the DC cable is continuously regulated by the on shore station, the operations of the different subsea modules are decoupled. Compared to the modern land based HVDC system, the centralized converter station is no longer needed and fewer subsea electrical components are required for the MSDC structure. Since each building block in MSDC is similar to a conventional drive, the subsea VFDs are smaller and easier to transport, marinize, and retrieve. Moreover, the modularity of the MSDC architecture renders the system fault-tolerance and capability of operating in a degraded mode. The architecture is also highly reconfigurable as the field matures and the loads evolve over time. Therefore, this technology potentially offers a much lower cost and higher reliability subsea power solution compared to the modern land based HVDC technology.

Subsea MSDC system
Subsea MSDC system

In addition to the system architecture, our team also developed the cooling, packaging concept for the electrical equipment to be deployed 10000 feet below sea level. A multiphase cooling approach is selected which combines reliability, electrical isolation, and thermal performance by using a pool boiling solution in a sealed pressure vessel. The project team in the GE Global Research Center consists of multidisciplinary engineers and scientists. Thanks to the funding support from Research Partnership to Secure Energy for America (RPSEA) and the dedication of all team members, we successfully demonstrated the system function in the lab in front of the Oil and Gas customers in Oct. 2012. We also verified the design concepts for the passive cooling system and the DC wet-mate connector. In this year our team continues to advance the maturity level of the technology and now we are on the verge of moving MSDC towards a field prototype.

Please stay tuned for more updates in the near future!

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