New subsea power transmission technology is required in order to enable increased oil and gas recovery in deep waters and at long tieback distances. Such prospects may not be feasible to develop with today's electrical power solutions, as traditional AC power transmission methods for supplying the increased power demand to subsea compression and pumping stations are reaching their limits. HVDC is far from matured for subsea use, and may not even be necessary technology, as low frequency AC can cover the market.

Aker Solutions has developed a new patented invention, called a Rotary Converter, for low frequency (LF) AC power transmission. This unit allows an extension of the maximum power transfer of subsea cables to more than 3x the step-out and 3x the power level compared to existing technologies.

The Rotary Converter (RC) consists of a subsea motor with low input frequency (10 - 30Hz) that runs a generator on a common slowly rotating shaft in a subsea enclosure. The generator supplies high frequency power (50 - 200Hz) to subsea ESPs, pumps, compressors or other loads.

The RC is supplied from a topside VSD via a long cable, and the low frequency ensures that the capacitive power / current in the cable is significantly reduced compared to conventional transmission frequency (50/60Hz or more), allowing more power, smaller cable cross-section, lower transmission voltage and better system stability.

Furthermore, by introducing a variable coupling (e.g., hydrodynamic) between the motor and generator, the Variable Rotary Converter (VRC) can replace power electronic based subsea VSDs. Input frequency can either be conventional for short to medium step-outs, and low frequency input for longer step-outs. This allows for simple and robust torque and speed control.

Key highlights from the Rotary Converter prototype development and tests are also shown in relation with relevant power system simulations. Various applications for this technology are presented, for instance within subsea control system supply, allowing increasing the power supply capability in brownfield scenarios where all-electric control systems are increasingly used.

This technology will be a new tool to enable the oil and gas industry to develop subsea fields that would normally require an additional offshore platform or FPSO, and rather use existing infrastructure with longer cables instead.

The main benefits of this new invention can be summarized in longer step-out distance, more power, better system stability, and lower CAPEX / OPEX for subsea control systems, pumping, and/or compression.

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