Electrical Power Distribution for Subsea Application
Authors: Wanderaas D. , Yardley J. , Bakka O. 
Published: October 2008

Cost efficient development of offshore oil fields calls increasingly for alternative subsea solutions as a means of reducing topside infrastructure. Indeed, for fields in very deep waters, say at 10 000 ft, zero surface facilities may be the only viable way to go. Future subsea production and processing facilities will grow to include several electrically driven pumps and even gas compressors to transport oil and gas over very long distances. There is therefore a growing opportunity for robust subsea electrical power distribution systems for variable speed motor drives and other subsea electrical loads supplied from a grid onshore or a far distant platform.

Current practice places the variable speed drive topsides, however there are technical reasons limiting maximum cable length. Also, as each motor will require a separate cable, the cable price rapidly becomes a crippling cost element for most subsea processing scenarios.

Feasibility studies for high voltage power supply to subsea distribution systems have been performed. This paper investigates one particular case in detail, with supply from shore via a very long sea cable to two typical subsea production fields incorporating subsea processing. Both AC and DC subsea distribution systems have been investigated and compared.

Finally, the paper provides a brief description of a suitable subsea DC distribution system based upon the case duty.

Introduction

Electric power transmission on land is normally by AC overhead lines. For overseas transmission, cable is the only applicable method. Compared with overhead lines, cables have quite different parameters and characteristics. One major characteristic is capacitance, which is very significant with cables whilst almost negligible for overhead lines. This is due to the insulating distance from conductor to earth. Land overhead lines are in operation with high voltages and high transmission capacity, designed in a utility grid with almost no geographical limitations. With cable, this is not the case because of several limitations. These are addressed later in the paper.

In order to gain more insight into technical feasibility and make comparisons of AC and DC distribution systems, the following topics have been investigated.

- System complexity

- System stability

- Transmission losses

- Application limitations

- Behavior towards the onshore grid

- Comparison between AC and DC.

The AC solution, whilst allowing high voltages through the use of transformers, was expected to be sensitive towards transmission line voltage variation and prone to high frequency dependent losses. On the other hand, DC is inherently free from frequency dependent losses: dominant loss is due to cable resistance and other losses are insignificant and may be neglected. DC transmission voltage limitations and acceptable voltage drops are major consideration factors, although the latter can be compensated to a certain degree by controlling the voltage output at the onshore rectifiers.

System configurations for the case considered in this paper are shown by Fig. 1-AC distribution and Fig. 2-DC distribution. The distance from shore supply to the subsea main distribution is 60 miles.