Page 33 - Offshore Electrical Engineering Manual

20 CHAPTER 2 The Offshore Electrical Environment

G1 G2 G3
5MVA 5MVA 5MVA
11kV 11kV 11kV
28 G G 29 30 G
NER 1 2NER NER 3
31 32 33
CB9 CB15 CB16
Section A CB10
1.1 kV 3ph 800A 50 Hi 350MVA CB13 Section B
Rear busbar
CB11 CB14
Front busbar
CB12
CB3 CB4 CB5 CB6 CB7 CB8 CB18 CB19 CB20 CB21 CB22 CB23
9 6 7 4 5 8 10 11
34 T1 T2
5081 35
NER 5081 NER
T1 & T2 2MVA 85% AV T3 36
T3,T4 * T5 3 5MVA6% AV 5080 T4 T5
T6, T7 & T8 1750kVA 11*AV 12 14
5082
NER NER NER 5085
5084
CB83
T6 16 17 CB61 Thyristor supplies CB62* 600V 3ph 250A 42MVA 18 T7 T8
5083 CB32 CB34 CB36
19 3.3kV 3ph 800A 150MVA CB35 20 21
CB41 CB33 CB31 CB37 CB45 CB47 Production and
415V 3ph 4 wire 3000A 35MVA CB42* CB46* domestic services
MCC1 Production and Essential CB43* CB44 CB48* CB49* MCC3
domestic services
CB76* CB71* CB58 CB57 services 22 25 24 23 CB72 CB77
28 27 1S51 1S52 1S53
1S54 Drilling equipment 1355
Drilling equipment MCC8 G G8 G G7
MCC5 415 V 415
6064 509 C78 C79 1377 8083
1598 1677 CS0
1S81 1S82
MCCa Rig ancillaries MCC10 Rig ancillaries
1S73 1S75 1S83 1S85 *Denoles
1S74 1S84 circuit breaker
MOL aux MCC7 NGL aux MCC* normally open
FIGURE 1.2.3
Most favoured main switchboard arrangement for offshore use since, assuming supplies
are maintained to generator auxiliaries, it requires several failures to occur before produc-
tion is affected.
The probability of water ingress, the most likely cause of such a failure, would be
reduced if the switchroom housing the utilities switchboard was surrounded by other
rooms or modules.
Fig. 1.2.2 shows a scheme where the main switchboard consists of a single-
section switchboard fed by two 24 MW gas turbine generators, each capable of
taking the entire platform load including drilling.
This medium-voltage switchboard not only directly supplies the platform with
large motor drives such as main oil line pumps and gas compressors but also feeds
the majority of lower-voltage switchboards via transformers.
There are two points at which connections are made to the drilling electrical
system. One is a supply at medium voltage and the other is an alternative supply
to the emergency switchboard at 440 V. The drilling system is also supplied by two
dedicated 1.5 MW diesel generators; however, these are much too small to sustain
any platform production but could maintain some utilities.
It can be seen that production is dependent on the continued functioning of the main
switchboard, a single fault in which causes the majority of the installation to be blacked
out. The insertion of a bus section switch in this particular case may do little to improve the
system integrity, as the reliability of the switch is likely to be less than that of the busbars.