Page 300 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

berths. OCIMF’s ‘SPM Hose System Design Commentary’ should be
referred to for guidance on hose strings at offshore facilities.
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All metal on the seaward side of the insulating section should be
electrically continuous to the ship; all metal on the landward side should
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
be electrically continuous to the jetty earthing system. This arrangement
will ensure electrical discontinuity between the ship and shore, and
prevent arcing during connection and disconnection.

The insulating flange or single length of non-conducting hose must not be
short circuited by contact with external metal. For example, an exposed
metallic flange on the seaward side of the insulating flange or hose length
should not make contact with the jetty structure, either directly or through
hose handling equipment.

It should be noted that the requirements for the use of insulating flanges
or an electrically discontinuous length of hose also apply to the vapour
recovery connection.

In the past, it was usual to connect the ship and shore systems by a
bonding wire via a flameproof switch before the cargo connection was
made and to maintain this bonding wire in position until after the cargo
connection was broken. The use of this bonding wire had no relevance to
electrostatic charging. It was an attempt to short circuit the ship/shore
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electrolytic/cathodic protection systems and to reduce the ship/shore
voltage to such an extent that currents in hoses or in metal arms would
be negligible. However, because of the large current availability and the
difficulty of achieving a sufficiently small electrical resistance in the
ship/shore bonding wire, this method has been found to be quite
ineffective for its intended purposes but has itself created a possible
hazard to safety. The use of ship/shore bonding wires is therefore not
recommended. (See Section 17.5.4.)

While some national and local regulations still require mandatory
connection of a bonding cable, it should be noted that the IMO
‘Recommendations on the Safe Transport of Dangerous Cargoes and
Related Activities in Port Areas’ (1995) urge port authorities to
discourage the use of ship/shore bonding cables and to adopt the
recommendation concerning the use of an insulating flange (see
Section 17.5.5. below) or a single length of non-conducting hose as
described above. Insulating flanges should be designed to avoid
accidental short circuiting.

Current flow can also occur through any other electrically conducting path
between ship and shore, for example mooring wires or a metallic ladder
or gangway. These connections may be insulated to avoid draining the
jetty cathodic protection system by the added load of the ship’s hull.
However, it is extremely unlikely that a flammable atmosphere would be
present at these locations while electrical contact is made or interrupted.

Switching off cathodic protection systems of the impressed current type,
either ashore or on the ship, is not in general considered to be a feasible
method of minimising ship/shore currents in the absence of an insulating
flange or hose. A jetty which is handling a succession of ships would
need to have this cathodic protection switched off almost continuously
and would therefore lose its corrosion resistance. Further, if the jetty
system remains switched on, it is probable that the difference of potential
between ship and shore will be less if the ship also keeps its cathodic
protection system energised. In any case, the polarisation in an

266 © ICS/OCIMF/IAPH 2006