17.5.1 GENERAL
Due to possible differences in electrical potential between the ship and the terminal, there
is a risk of electrical arcing at the manifold during connection and disconnection of the
shore hose or loading arm. To protect against this risk, there should be means of electrical
isolation at the ship/shore interface. This should be provided by the terminal.
It should be noted that the subject of ship to shore currents is quite separate from static
electricity which is discussed in Chapter 3.
17.5.2 SHIP TO SHORE ELECTRIC CURRENTS
Large currents can flow in electrically conducting pipe work and flexible hose systems
between the ship and shore. The sources of these currents are:
Cathodic protection of the jetty or the hull of the ship provided by either a DC
impressed current system or by sacrificial anodes.
Stray currents arising from galvanic potential differences between ship and shore or
leakage effects from electrical power sources.
An all-metal loading or discharge arm provides a very low resistance connection between
ship and shore and there is a very real danger of an incendive arc when the ensuing large
current is suddenly interrupted during the connection or disconnection of the arm at the
tanker manifold.
Similar arcs can occur with flexible hose strings containing metallic connections between
the flanges of each length of hose.
To prevent electrical flow between a ship and a terminal during connection or
disconnection of the shore hose or loading arm, the terminal operator should ensure that
cargo hose strings and metal arms are fitted with an insulating flange. An alternative
solution with flexible hose strings is to include in each string, one length only of nonconducting
hose without internal bonding. The insertion of such a resistance completely
blocks the flow of stray current through the loading arm or the hose string. At the same
time, the whole system remains earthed, either to the ship or to the shore.
All metal on the seaward side of the insulating section should be electrically continuous to
the ship, and that on the landward side should 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
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 and, furthermore, a possible hazard to safety. The
use of ship/shore bonding wires is therefore not supported.
While some national and local regulations still require mandatory connection of a bonding
cable, it should be noted that the IMO ‘Recommendations for the Safe Transport,
Handling, and Storage of Dangerous Substances in Port Areas’ 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 17.5.6. 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 dock 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 impressed current system takes many hours to decay after the
system has been switched off, so that the ship would have to be deprived of full protection,
not only while alongside, but also for a period before arrival in port.
17.5.3 SEA ISLANDS
Offshore facilities that are used for tanker cargo handling operations should be treated in
the same way as shore terminals for the purpose of earthing and bonding i.e. either an
insulating flange or non-conducting hose should be used as appropriate.
It should be noted that switching off a cathodic protection system is not a substitute for the
installation of an insulating flange or a length of non-conducting hose.
17.5.4 SHIP/SHORE BONDING CABLES
A ship/shore bonding cable does not replace the requirement for an insulating flange or
hose as described above. Use of ship/shore bonding cable may be dangerous and should
not be used.
Important Note
Although the potential dangers of using a ship/shore bonding cable are widely
recognised, attention is drawn to the fact that some national and local regulations
may still require a bonding cable to be connected.
If a bonding cable is insisted upon, it should first be inspected to see that it is
mechanically and electrically sound. The connection point for the cable should be
well clear of the manifold area. There should always be a switch on the jetty in
series with the bonding cable and of a type suitable for use in a Zone 1 hazardous
area. It is important to ensure that the switch is always in the ‘off’ position before
connecting or disconnecting the cable.
Only when the cable is properly fixed and in good contact with the ship should the
switch be closed. The cable should be attached before the cargo hoses are
connected and removed only after the hoses have been disconnected.
17.5.6 INSULATING FLANGE
17.5.6.1 Precautions
Points to be borne in mind when fitting an insulating flange are:
When the ship to shore connection is wholly flexible, as with a hose, the insulating
flange should be inserted at the jetty end where it is not likely to be disturbed.
In a wholly flexible hose arrangement, where the insulating flange is at the end of the
jetty pipeline, the hose must always be suspended to ensure the hose to hose
connection flanges do not rest on the jetty deck or other structure which may render
the insulating flange ineffective.
When the connection is partly flexible and partly metal arm, the insulating flange
should be connected to the metal arm.
For all-metal arms, care should be taken to ensure that, wherever it is convenient to fit
the flange, it is not short-circuited by guy wires.
The location of the insulating flange should be clearly labelled.
Insulating flanges should be inspected and tested at least every 6 months to ensure that
the insulation is clean, unpainted and in an effective condition. Readings should be taken
between the metal pipe on the shore side of the flange and the end of the hose or metal
arm when freely suspended. The measured value after installation should be not less than
1,000 ohms. A lower resistance may indicate damage to, or deterioration of, the insulation.
The terminal should maintain records of all tests on all the insulating flanges within the
terminal.
Note : Figure 17.1 to be inserted as close to this text as possible.
17.5.6.2 Testing of Insulating Flanges
An insulating flange is designed to prevent arcing caused by low voltage/high current
circuits (typically 0.5 to 1 volt and potentially several hundred amps) that exist between
ship and shore due to stray currents, cathodic protection and galvanic cells. It is not
intended to give protection against the high voltage, low current sparks associated with
static discharge.
Therefore, even if the resistance of the flange drops below the 1,000 ohms quoted above,
due to ice or salt spray, the current flow will be less than a milliamp and be insufficient to
arc across the flanges when connecting or disconnection loading arms. Conversely, trying
to earth (ground) a low voltage/high current circuit with a bonding cable is impossible
because, even if a very low resistance cable is used, there will be a contact resistance of
at least several ohms where the cable is secured to the vessel, restricting current flow to
only a few amps and thus preventing the equalisation of potential between ship and shore.
A ‘megger’ insulation tester is designed to measure insulation resistance which may vary
from a few kilohms to infinity. Because this value needs to be known under ‘live’
conditions, the ‘megger’ uses a relatively high voltage (normally 500 volts) to ascertain this
value of resistance. A multimeter uses much lower voltages