A fog signal is the audible half of a navigation aid. When visibility closes in, lights and
even radar marks become unreliable at close range, and a sound signal tells a vessel that
something is there and roughly where. On an offshore platform that is not a courtesy - it is
part of the installation's obligation to mark itself, and it is why the horn is specified
against IALA recommendations rather than chosen for loudness.
The MSN30-Ex produces a 360 degree beam of sound in the horizontal plane, which is the right
answer for a structure rather than a shoreline: a vessel could approach from any bearing, so
there is no direction to favour. MSM quote an audible range of over 2 nautical miles, achieved
by three stacked emitters producing 134 dB at 1 metre. It operates automatically, which in
practice means it is triggered by a visibility sensor rather than by someone noticing the fog.
The modular construction is the feature that makes it adaptable. Because the output comes
from stacked emitters, the sound intensity can be increased or reduced at will by changing how
many are fitted. That matters because the required range is a function of the location and the
traffic, and because over-provisioning a fog signal is not harmless - it is a noise source on a
manned installation where people work and sleep.
Certification is what allows it on a hydrocarbon facility at all. The horn belongs to ATEX
and IECEx Classification Group II, Category 2, marked II 2G Ex d IIB T5 Gb with IP66 ingress
protection, which permits use in potentially explosive atmospheres classified as Zone 1 and
Zone 2, in gas. The Ex d is a flameproof enclosure. Note the gas group is IIB, which does not
cover IIC gases such as hydrogen and acetylene, so the marking has to be checked against the
area classification rather than assumed adequate. A junction box of the same d construction is
included, which removes the most common gap in an Ex installation.
The signal itself is Morse code letter U or A, and the unit is designed according to IALA
Recommendation E-109 and Guidelines G1162 and G1090 - the documents that govern how aids to
navigation are specified and characterised. An emergency mode is available as an option, and
the unit is built with protection against corrosion, which for a device that lives in salt
spray and radiates sound through its own housing is not a small consideration.
