Page 310 - Offshore Electrical Engineering Manual

Structural Considerations 297

GLASS FIBRE REINFORCED PLASTIC
Glass fibre reinforced plastic has been used successfully offshore for a number of
years. It may be used in electrical equipment in the form of small junction boxes
and for cable ladder and tray. Although fire resistant, it will burn when subjected to
a gas flame but is more resistant to deformation, melting and fire than polycarbon-
ate. Being strong and light and unaffected by seawater, it is useful particularly where
installed in saltwater spray conditions. However, as it will burn, long unbroken verti-
cal runs should be avoided.

ENCLOSURE INGRESS PROTECTION
All electrical equipment enclosures must be designed to prevent

1. the inadvertent contact by personnel of live parts inside the enclosure, when
these are accessed for maintenance, etc.,
2. the ingress of dust and dirt,
3. the ingress of liquids, particularly conducting liquids such as seawater.

The degree of protection is specified in BS EN 60947-1 (and BS EN 60034-5
for motors). The degree of ingress protection (IP) rating is a two-digit number.
The first digit refers to the degree of protection from foreign bodies and dust, and
the second digit refers to the degree of protection from water. Some examples are
given in Table 5.3.1. It should be noted that most hazardous area equipment enclo-
sures must be capable of maintaining an IP rating of IP54.

STRUCTURAL CONSIDERATIONS

WEIGHT CONTROL
As in ship construction, the recording and control of weight are of vital importance
to the structural integrity of the installation and the safety and survival of those
onboard. As most equipment is shipped offshore in the form of prefabricated, pre-
commissioned modules, weight control must start at the fabricator’s yard and in the
manufacturer’s works. The selection of equipment may be restricted by maximum
weight allowances. The power-to-weight ratio of a particular prime mover may be
critical if the projected electrical load is to be met without the weight limit of the
generator module being exceeded.
The weight of cabling and support systems, although more distributed, makes a
significant contribution to the total and must be included in the weight control sched-
ule. This schedule is usually compiled and updated regularly by structural engineers
throughout the duration of a design contract.
The total weight of every module may be restricted, not only by the load-bear-
ing capabilities of the offshore structure but also by the capabilities of offshore