Variable Rotoconverter for Subsea Processing Applications
Authors: Normann T. , Krogstad Lien L. , Gabelloni M. , Müller L. A. , Pauchon C. , Pimentel J. 
Published: October 2015

The developments of subsea fields located at long tie-back distances, or fields located close to an existing facility with limited available topside space, require advanced power distribution technology. This may represent a major roadblock with existing electrical power solutions, so that some prospects may not be economical because of these constraints. This paper, based on studies carried out by Total and Aker Solutions over the last year, will present the application of a novel power transmission and conversion technology to significantly reduce the need for topside equipment and to extend the maximum power transfer of subsea cables to significantly longer distance from shore compared with existing technologies.

The basis for this new technology is a Rotary Converter consisting of a low-frequency subsea motor running a generator which can then supply high-frequency power (typically 50-200 Hz) to subsea pumps, compressors or other power-consuming equipment. The rotational speed is relatively low, which gives very limited wear and high robustness and the machine is hermetically sealed against surrounding seawater. It is supplied with low-frequency power (typically 50 / 60 Hz for shorter distances up to 100 km, and typically 10-30 Hz for longer step-outs) from a topside supply, significantly reducing the capacitive current in the cable and allowing more power and better system stability in long cables. This Rotary Converter (RC) technology is further explained in reference [1].

Typical subsea power system types and step-outs

 

In this paper, focus is on the Variable Rotary Converter (VRC). This device is basically the same as the rotary converter, but with a variable coupling in between. Through this, the generator can be controlled with variable speed and variable torque, and it allows for the VRC becoming an alternative to the more complex electronics-based subsea variable speed drive (VSD) technology. By eliminating the need for topside VSDs and associated equipment such as switchgear and transformers (which would have been the alternative to subsea speed control), significant weight and space reductions can be achieved on the topside facility. In addition, longer step-out can be achieved by adapting the VRC motor input fixed frequency to 10-30 Hz, in the same way as explained for the RC above. Moving the drive technology to the seabed also makes it possible to have only one supply cable to one or multiple subsea VRCs that run individual control of the subsea rotary boosting machinery. With topside VSDs, one cable for each subsea load is normally required.

A proof of concept test for this technology has been performed, using a prototype 50 kW Rotary Converter unit, and results from this test will be shown, including start-up, transient tests, harmonics, dynamic load changes and thermal stability. Total and Aker Solutions have analyzed two specific cases of VRC based applications, one for subsea pumping at shorter step-out, demonstrating the system simplification benefits, and one for subsea compression demonstrating the long step-out capabilities of the technology. The model validated through the prototype 50 kW Rotary Converter proof of concept test has been applied to this case study and simulation results will be shown for the complete power distribution system.

In general the new Subsea Rotary Converter technology can provide major benefits to the subsea processing industry, and specifically for high-power boosting and compression solutions, enabling CAPEX and OPEX reduction, longer step-out distance and better system stability and reliability.