Seabed equipment is at times deployed to do work on, or add energy to, produced fluids to improve or ensure sustained flow when natural reservoir pressure declines. This hardware includes subsea pumps and compressors. The power required to create a meaningful impact on production with these systems is generally substantial, i.e. several thousand horsepower or more. The power supplied to a fluid is hydraulic and is accommodated through application of rotating machinery (subsea pumps/compressors). This machinery converts electric power into rotational power through an electric motor, which then turns the shaft of the pump/compressor to do the work. Electric power to the subsea equipment from the source is generally transmitted via subsea power cables operating at 50/60 Hz. However, as distance and power levels become ever greater, stable AC power transmission becomes challenging due to excessive reactive power demand within the transmission cables.
What follows is a discussion of the present and anticipated, future power demands associated with subsea pumping and compression systems and the technical issues associated with conventional long distance, high power AC transmission for these applications. Distribution system topology and technology limits will be discussed to address more complex subsea configurations. Furthermore, the focus of submarine cable applications will be limited to Subsea Tiebacks to streamline the subject matter and manage document length.
Finally, solutions (opportunities) are proposed to address the range of issues presented. Solutions include high voltage, low frequency power transmission and high voltage DC power transmission. Reference is also made to this year's Subsea Processing Poster, developed by INTECSEA and published by Offshore Magazine (March 2013). In particular, Graph 4 - Subsea Power Transmission is briefly described to reinforce the aforementioned subject matter and afford readers a fuller understanding of the range and depth of information embedded within the graphic.
Submarine power cables have been around for more than a century and their application has evolved over the years. Early in life, submarine power cables were used to supply isolated offshore facilities such as lighthouses, infirmary ships, etc. As submarine power cable technology evolved, cables were used to link shore-based power grids across bays, estuaries, rivers, straits [1].
Today, submarine cables are used in a variety of applications, including:
Carrying power between countries or to remote locations
Carrying power and communications to offshore installations, e.g. oil/gas platforms and ocean science observatories;
Transferring power from offshore renewable energy schemes to shore, e.g. wind, wave and tidal systems.
With growing reliance on offshore-based renewable energy schemes, many countries now class submarine power cables as critical infrastructure.
