Page 18 - Offshore Electrical Engineering Manual

Description of a Typical System 5

4. impracticable demands are not made on the particular offshore infrastructure,
such as those for fuel or cooling medium,
5. the safety requirements pertinent to an offshore oil installation are complied
with, in particular those associated with fire and explosion hazards,
6. the weight of the system is not excessive for the structure on which it is
installed. In the case of rotating machinery, the effects of vibration and shock
loads must be taken into account.

DESCRIPTION OF A TYPICAL SYSTEM
A single-line diagram of a typical offshore electrical system is shown in Fig. 1.1.1.

MAIN PRIME MOVERS
With the obvious availability of hydrocarbon gas as a fuel, and the requirement for a
high power-to-weight ratio to keep structural scantlings to a minimum, gas turbines
are the ideal prime movers for power requirements in excess of 1 MW. Below this
value, reliability and other considerations, dealt with in PART 8 Chapter 1, tend to
make gas turbines less attractive to the system designer.
Because of the complexity and relative bulk of gas turbine intake and exhaust sys-
tems, the designer is urged towards a small number of large machines. However, the
designer is constrained by the need for continuity of supply, maintenance and the reli-
ability of the selected generator set to an optimum number of around three machines.
A variety of voltages and frequencies may be generated, from the American derived
13.8 kV and 4.16 kV 60 Hz to the British 11 kV, 6.6 kV and 3.3 kV 50 Hz. Many ships
operate at 60 Hz, including all NATO warships and there is a definite benefit to be
gained from the better efficiencies of pumps and fans running at 20% higher speeds.

KEY SERVICES OR SUBMAIN GENERATORS
On some platforms, smaller generators are provided to maintain platform power for
services other than production. These are also normally gas turbine driven and can
provide a useful black-start capability, especially if this is not available for the main
machines.

MEDIUM-VOLTAGE DISTRIBUTION
The design of the distribution configuration at the platform topside conceptual stage
is highly dependent on the type of oil field being operated and the economic and
environmental constraints placed on the oil company at the time. The older platforms
originally had little or no facilities for gas export or reinjection and therefore the
additional process modules installed when these facilities were required have their
own dedicated high-voltage switchboards. This is also the case if such a heavy-power