Page 106 - Offshore Electrical Engineering Manual

CHAPTER
8
Electrical Cables

SELECTION
Cables used on any offshore installation will require the following attributes:

1. They should have stranded copper conductors for smaller cross-section and
better flexibility.
2. They should be voltage and fault rated for the system in which they are
operating.
3. They should be rated for the normal maximum current flow in the circuit
without exceeding the maximum conductor temperature or the temperature class
limit if passing through a hazardous area.
4. If they are involved in a fire, smoke and acid fume emission should be low. The
insulation compound used should have an oxygen index of at least 30.
5. They should normally be armoured for mechanical protection. Braided or wire
armouring may be used; the braided type is slightly more flexible but is often
more difficult to gland. These criteria cannot be rigidly applied, however, as
special cables will be needed for instrumentation, diving umbilicals, downhole
pumps, etc.
6. The cables designed for the most onerous duty are those for electrical fire pump
supplies. These fire survival cables are designed to continue to supply current
after an hour at a temperature of 1000°C, followed by hosing down with high-
pressure water jets whilst being hit with a hammer. The difficulty lies in finding
a suitable support system that will survive the same treatment.
7. The most common cable type in use is the EPR/CSP type similar to that found
in merchant shipping, which meets the high oxygen index and fire-retardant
requirements of BS 6883 and IEC 93-3.
8. Cables must be sized to allow for circuit current and maximum voltage drop
as with any onshore system. AC cables can be sized using a chart generated
by spreadsheet, as shown in Fig. 2.8.1. Care will still need to be exercised to
ensure that close excess protection is still applied and that cable fault ratings are
adequate.
9. In Europe, IEC 61892 is the prime standard for electrical installations on
mobile and fixed offshore units and should be used for guidance in the sizing
of conductors (BS IEC 61892 is expected to replace the IEE Recommendations
for the Electrical and Electronic Equipment of Mobile and Fixed Offshore
Installations in the United Kingdom, assuming that this is not affected by Brexit).

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