Page 349 - Offshore Electrical Engineering Manual

336 CHAPTER 2 Floating Production, Storage and Offloading Units

the oxygen to a low level, monitored with an oxygen analyser. Deck water
seals prevent the ingress of oxygen from above but allow the nitrogen to escape
whilst the tank is filling.
5. Heading control
It is not advisable to allow the accommodation/temporary refuge end of the
ship to be downwind of the process areas, and therefore a limited DP system is
provided just main to the vessel’s heading so that the accommodation is always
upwind of the process areas.
6. Turret
Subsea wells on the seabed below the FPSO are connected to the ship via a
turret. The turret is fixed relative to the heading of the ship and is where the flex-
ible risers connect to the ship. Some turret systems use flexible hoses to connect
between the turret and the ship, allowing the ship to change heading whilst the
turret remains stationary. The other method is to use ‘swivels’. These are rota-
tion joints able to take the high oil pressures without leaking.
7. Ground flare
If the vessel’s movement is likely to be excessive (e.g., west of Shetland duty),
it may be better to use a ground flare rather than the normal boom flare, which
would be stressed by excessive rolling of the hull. The ground flare consists of a
(usually) circular concrete open-topped tank, with the flare nozzles aimed down
towards the centre of the tank. Above this is a stack to protect the nozzles and
flame from the wind and funnel the heat and fumes upwards.
8. Mooring system
It is usual for FPSOs to be ‘permanently’ moored (except for major refits,
including the mandatory shallow water survey). Mooring, as such, is not an
electrical issue, but the DP system, besides heading control, also has the duty of
monitoring the tension on anchor chains and, if need be in bad weather, relieve
chain stress by providing thruster assistance.

SAFETY CRITICAL SYSTEMS
Many of the above-mentioned systems may be classed as ‘safety critical’ by Safety
Case Risk studies, in which case the system will have a ‘Performance Standard’
written for it, which defines its scope and various functions and how this is verified.
These issues are covered in PART 9.