Page 35 - Offshore Electrical Engineering Manual

22 CHAPTER 2 The Offshore Electrical Environment

6. The sea floor is suitable for burial of the cable as a means of protection. Drifting
sands, solid rock or strong tidal currents would militate against this.

In listing these conditions, it has been assumed that the import of diesel fuel for
normal operation is not a viable option because of the high cost involved. However,
this may depend on commercial considerations such as the revenue value of the gas
to the installation operators.
For example, Centrica’s Morecambe Bay complex has a mixture of subsea cable,
gas turbine and diesel-generator-powered platforms. If it becomes necessary to
supplement the available power on a particular platform, then the additional weight
of supplementary generator may be too great for the platform to bear without very
costly modifications. Even if weight is not a problem, it is not always possible to find
a sufficiently spacious location on the installation either. Shore supplies may be of
the wrong frequency for use on the particular installation, and it may be necessary
to install a motor generator set. This has the additional advantage of improving the
motor-starting capability of the supply, as the generator impedance will be much
lower than a series of transformers and long subsea cables. The transmission volt-
age required will vary depending on the length of the subsea cable but is likely to be
either 11 or 33 kV. The weight and space taken up by the transmission transformers
and the associated extra switchgear needs to be taken into consideration whenever
subsea cable options are proposed.
If there is a group of several small installations separated only by a few kilo-
metres of water, it may be economic to supply all their main power requirements
from one central platform. This is more likely to be the case if centralising the main
generation allows gas turbines of 1 MW or more to be considered (see discussion on
prime mover selection in PART 2 Chapter 4).
Finally, it is advisable to carry out some form of reliability analysis to numeri-
cally rank the reliabilities of various supply and generation schemes before making
the final decision.
Reliability topics are discussed in PART 8 Chapter 1.