Page 309 - Offshore Electrical Engineering Manual

296 CHAPTER 3 Environmental Protection

GREY CAST IRON
In grey cast iron, most of the carbon is present in the form of graphite flakes, which
make the material softer, more machinable and less brittle than white cast iron. As
the name suggests, cast iron is very fluid when molten and is therefore suitable for
the manufacture of intricate castings. Its main use offshore is in the construction of
flameproof enclosures. Its resistance to corrosion appears to be quite variable. Where
it is exposed to salt spray, for example, in flameproof control stations on lower-deck
handrails, a galvanised finish is advisable. The variation in the effects of corrosion
is probably related to the method of producing cast iron, which involves the remelt-
ing of pig iron in cupolas. The qualities of the cast iron produced will depend on
the selection of the pig iron, on the melting conditions in the cupola and on special
alloying additions.

HOT DIPPED GALVANISED STEEL
This material is by far the most common for use in cable support systems offshore.
The heavy-duty grade should provide a service life in excess of 20 years, particularly
if Corten A steel is used.

POLYCARBONATE
This is a very tough plastic material used for junction box and similar electri-
cal enclosures because of its corrosion-free property. It is particularly effective in
areas close to the sea, where salt spray is common. It is resistant to mechanical
damage and will deflect rather than break in most situations, but a heavy blow from
a scaffolding pole, for example, is more likely to damage a polycarbonate enclo-
sure than an equivalent steel one. Polycarbonate will tend to deform at elevated
temperatures; it must therefore be shielded from the heat produced by flare stacks,
etc. and must not be used with equipment which has to operate during firefighting
operations.

MANGANESE BRONZE AND GUNMETAL

These very heavy corrosion resistant metals are sometimes used with success for the
casings of floodlights and similar exposed electrical equipment.

WELDED AND CAST STRUCTURAL STEEL
Rotating machinery packages of all types are generally constructed of this mate-
rial. It is not practicable to galvanise the whole package, and therefore, as with
the structural steel of the module or platform jacket, a suitable offshore paint
system must be applied by the fabricator. The integrity of this paint system must
be preserved during the equipment’s transit, installation and commissioning, if
the package is to be presented to the operator in good condition.