Page 248 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

carried in tanks which are inerted. When it is necessary to reduce the
pressure in any tank for the purposes of measuring and sampling, the
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following precautions should be taken:
A minimum positive inert gas pressure should be maintained during
measurement and sampling. The low oxygen content of inert gas can
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rapidly cause asphyxiation and therefore care should be taken to avoid
standing in the path of vented gas during measurement and sampling
(see Section 11.8.1). No cargo or ballast operations are to be
permitted in cargo compartments while the inert gas pressure is
reduced to allow measuring and sampling.

Only one access point should be opened at a time and for as short a
period as possible. In the intervals between the different stages of
cargo measurement (e.g. between ullaging and taking temperatures)
the relevant access point should be kept firmly closed.
After completing the operation and before commencing the discharge
of cargo, all openings should be secured and the cargo tanks
re-pressurised with inert gas. (See Section 7.1 for the operation of the
ship’s inert gas system during cargo and ballast handling.)
Measuring and sampling which require the inert gas pressure to be
reduced and cargo tank access points opened should not be
conducted during mooring and unmooring operations or while tugs are
alongside. It should be noted that, if access points are opened while a
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ship is at anchor or moored in an open roadstead, any movement of
the ship might result in the tanks breathing. To minimise this risk in
such circumstances, care should be taken to maintain sufficient
positive pressure within the tank being measured or sampled.

If it is necessary to sound the tanks when approaching the completion of
discharge, the inert gas pressure can again be reduced to a minimum
safe operational level to permit sounding through sighting ports or
sounding pipes. Care should be taken to avoid the ingress of air or an
excessive release of inert gas.

11.8.3.1 Static Accumulator Cargoes in Inerted
Cargo Tanks
Precautions are not normally required against static electricity
hazards in the presence of inert gas because the inert gas
prevents the existence of a flammable gas mixture. However, very
high electrostatic potentials are possible due to particulates in
suspension in inert gas. If it is believed that a tank is no longer in
an inert condition, then dipping, ullaging and sampling operations
should be restricted as detailed in Sections 7.1.6.8 and 11.8.2.

Restrictions would be required in the event of a breakdown of the
inert gas system during discharge:
In the event of air ingress.
During re-inerting of a tank after such a breakdown.

During initial inerting of a tank containing a flammable gas
mixture.

Because of the very high potential that may be carried on inert gas
particulates, it should not be assumed that corona discharges
arising from conducting equipment introduced into the tank will be
non-incendive if the tank contains a flammable atmosphere.
Therefore, no object should be introduced into such a tank until the

214 © ICS/OCIMF/IAPH 2006