Page 150 - Offshore Electrical Engineering Manual

Subsea Umbilicals and Power Cables 137

either direct current (e.g., 24 Vdc) or possibly at a frequency greater than 20 kHz which,
apparently, the human responds to in a similar way to direct current.
Any sockets used should be the underwater type that can be connected while
immersed in seawater without hazard. Unused sockets should be provided with insu-
lating caps. If no suitable intrinsically safe equipment can be obtained, equipment
incorporating other types of protection may be used.
Any electric motors in the chamber should be of the flameproof or increased safety
type, and designed to withstand stalling currents for long periods without detriment.
It is more than likely that surface Ex ‘d’ certification will prove invalid for equipment
to be used in underwater chambers, because of the higher ‘breathing atmosphere’
pressures required for long-term compression diving or the oxygen enrichment. The
equipment enclosure will require pressure testing to 1.5 × the normal peak explosion
pressure × working pressure of chamber (bar. A).
Intrinsically safe equipment safety margins are also reduced by increase in pres-
sure, and such systems will also require reassessment when used in this environment.
Other types of certified equipment may be affected by the higher humidity or by
water ingress because of the higher pressures likely to be experienced by underwater
equipment.

INDUCTIVE COUPLERS

If the core laminations of a transformer are cut in half, the primary coil is wound on one
half and secondary is wound on the other; then, when the two halves are mated together,
a rather inefficient transformer is formed. Although this principle is used in the inductive
couplers used throughout the North Sea and elsewhere, the design is less common now
than the underwater mateable ‘Tronic’ type of connector. Fig. 2.13.1 shows a photograph
of the mating side of an inductive coupler. The core and windings are encapsulated so
that the seawater at North Sea seabed pressures cannot penetrate into the live parts of the
device. Only the surface of the core, which is ground flat, is exposed.
Because of the extra losses produced by the position of the windings and the
air gap in the core, more heat is generated than in a conventional transformer and
this must be removed by a core heat sink connected to the stainless steel housing.
Inductive couplers are seldom used these days, since electrical connectors have been
developed now, which can be mated live underwater (Fig. 2.13.2).

SUBSEA UMBILICALS AND POWER CABLES
An ‘umbilical’ is the term used for obvious reasons to describe a bundle of hydraulic
hoses, signal, control and power cables bundled together, armoured and sheathed to form a
single entity and used to provide services from a surface installation or vessel to equipment
below the surface, often on the seabed. Individual cables within the umbilical may also
be screened or armoured, or both, and the whole umbilical may be strengthened against
tension damage during deployment by the provision of an imbedded steel tension cable.