Page 173 - Offshore Electrical Engineering Manual

160 CHAPTER 14 Offshore Lighting

a problem in areas subject to deluge firefighting systems. One of the many possible
scenarios for this maybe as follows:

1. Gas is detected in a hazardous module.
2. The ESD System initiates a shutdown of the affected plant.
3. As a consequence of this shutdown, either directly due to the shutdown logic
or indirectly due to fuel gas starvation and failure to switch to diesel fuel, for
example, the main generation shuts down.
4. With no power available, the air compressors stop, leaving air operated devices
dependent on their reservoirs.
5. One by one as each air reservoir becomes depleted, the deluge sets release sea-
water in the process areas they cover.
6. Water enters the luminaires in the module where gas is leaking. Luminaires with
integral batteries allocated to escape lighting will still be live. Essential light-
ing may also be live in this situation, fed from the emergency generator, but this
may be too small to run an air compressor.
7. Arcing may now take place within the affected luminaires, providing an ignition
source for the released gas.

Similar problems have been experienced with other Ex ‘e’ equipment such as
junction boxes, but unlike those with luminaires (unless clear Denso tape can be
obtained!), the problem can largely be overcome using a layer of ‘Denso tape over
joints. Newer designs of luminaire with better water ingress protection are now
available and these, combined with good safety-critical focused maintenance should
reduce the ignition risk. Another possible improvement is to provide fire and gas sys-
tem logic which shuts down all nonessential electrical equipment within the affected
module should high-level gas be detected and deluge release occurs. Escape lighting
would still need to remain energised, however.
In congested areas, floodlighting may be required in order to provide effective
lighting, to avoid contamination from chemicals and oil and consequently the need
for constant cleaning, and also to avoid obstructing the operator’s line of sight with
the luminaire itself.

DRILLING AREAS
It is likely that the most difficult lighting conditions on the offshore installation occur
in the vicinity of the drilling derrick. The actual process of drilling involves large
quantities of water and drilling mud, which may be oil based and difficult to clean
from the luminaires. Mud returning from the well may be very hot which, apart from
reducing visibility by producing a mist, may cause deformity in plastic luminaire
mouldings. The drill floor and the monkey board need to be well-lit, but due to the
movement of the travelling block, swivel, kelly etc., and the drillpipe itself, it is
necessary to carefully select suitable locations for luminaires on the derrick so as
to minimise the risk of mechanical damage. Apart from fluorescent luminaires for