Page 91 - Offshore Electrical Engineering Manual

78 CHAPTER 5 Generation and Distribution Switchgear and Transformers

to main generation after a shutdown incident, without a break in the supply. The
switchboard should also include facilities to prevent the generator from starting
when a fault exists on the switchboard. Interlocking must be provided between the
emergency generator incomer and the incomer from the rest of the platform power
system if synchronising facilities are not available. As the emergency switchboard
usually feeds all the ac and dc secure supply battery chargers and other vital equip-
ment, it is important that planned switchboard maintenance outages are catered for
in the design. It is not usual to go to the expense of a duplicate bus switchboard, but
certain battery chargers and other vital equipment are usually fed from an alterna-
tive switchboard via a changeover switch. These supplies should also include those
necessary for starting other generators and for safe area ventilation, the basic phi-
losophy being to allow continued safe oil production whilst the switchboard is being
serviced.

DRILLING SUPPLIES
The drilling electrical system is usually independent of the installation system, with
its own diesel generation. The reason for this is partly to do with organisation, since
the drilling may be carried out by a different company who provide the complete
drilling package, including generation and switchgear.
If the drilling system is of the same operating frequency as the rest of the plat-
form, then an interconnector of some kind between the systems is mutually benefi-
cial, provided there is no equipment in the main system that is particularly sensitive
to the harmonics generated by the silicon controlled rectifier (SCR) equipment. The
reliability and maintainability of the drilling electrical system is vital, as failures at
particular times in the drilling operation may increase the risk of blowouts or cause
the abandonment of a producing well. The switchgear used for drilling distribution
is of the conventional motor control centre type, with the exception of the SCR cubi-
cles. The SCR cubicles cannot be isolated individually due to the permanent inter-
connection arrangement between each of the variable speed drives. An ‘assignment’
switch on the driller’s console allows the connection of any SCR cubicle to any dc
drive motor in the system. This arrangement allows for dc drive motors to be reas-
signed to another SCR cubicle if a fault develops in the first cubicle. To allow plenty
of ventilation, the SCR cubicles are of a much more open design than other offshore
switchboards. The assignment contactors are usually arranged in the upper section of
the panel, while the SCR assemblies are in the lower section. Fig. 2.5.6 shows typical
SCR cubicle schematic diagram.
To avoid obstruction of cooling air, there are usually no insulating barriers
between the interconnecting busbars, contactors and equipment within the cubicle.
However, it is sometimes necessary if drilling is to continue, for the rig electrician to
change SCRs with the cubicle only isolated by the assignment contactor at the top.
This problem is usually overcome by using a removable insulating barrier which can
be carefully located below the contactor before working on the SCR assembly.