Page 34 - Offshore Electrical Engineering Manual

Subsea Cable Versus On-Board Generation 21

The configuration shown here for the main switchboard is now the most favoured
for offshore use, as, assuming supplies are maintained to generator auxiliaries, it
requires several failures to occur before production (and hence revenue) is affected.

MODULAR CONSTRUCTION
Provided lifting and transport facilities of sufficient capacity are available at an economi-
cally viable cost, it is invariably better to build a complete module containing the genera-
tors, switchgear and transformers that are completely fitted out, tested and commissioned
at a suitable fabrication yard than to carry out any of the construction on the platform.
Apart from fuel, cooling air and combustion air, it is preferable to make the ‘elec-
trical power module’ totally independent from the rest of the installation. This also
means that it has integral engine-starting facilities as well as engine auxiliaries which
do not require external low-voltage electrical supplies. This is not always possible
because of the weight of the extra transformers required. The need for seawater cool-
ing for alternator heat exchangers may be unavoidable because of the bulk of air-
cooled units. The optimum independence of the module has the added advantages of

1. minimal hook-up requirements during offshore installation,
2. minimal service requirements during test and commissioning at the module
fabrication yard.

SUBSEA CABLE VERSUS ON-BOARD GENERATION
The cost of laying subsea power cable is presently in the order of £2 to £5 million/
cable kilometre. This cost includes that involved in mobilising a suitable cable-laying
vessel. At first sight, this may appear prohibitively expensive but is worth investiga-
tion if most of the following conditions are met:

1. The source of supply is conveniently at hand, i.e., on the mainland or another
installation. Subsea cable lengths greater than 50 miles are unlikely to be viable.
2. The cable route is not through a busy shipping lane or anchorage where there is
high-risk anchor damage.
3. The cable route does not pass through a popular fishing ground where there is
the consequent risk of cables being dredged up by trawl nets or gear.
4. The fuel gas supply on the installation to be supplied by the cable is unreliable,
exhausted or of too small a capacity for the required prime mover. Of course,
the converse must be true of the supplying installation.
5. The transmission route is through reasonably sheltered waters where it is prob-
able that cable repairs could be carried out throughout the year. If this is not the
case, then the expense of laying duplicate cables over separate routes becomes
more attractive, as, in winter, it could be 3 months or more before a suitable
weather window becomes available to repair a cable fault.