3.22 TOPPING OFF TANKS
The cargo operation which provides the greatest opportunity for a
cargo overflow (and associated environmental pollution incident), is
in the final filling or 'topping off of cargo tanks. As the time for
topping off cargo tanks approaches, the cargo watch officer must put
in place all necessary measures to ensure that he will have complete
control over the topping off operation. The watch officer should
rehearse in his own mind the immediate and secondary measures he will
take in the event of an equipment or communications failure while topping
A VLCC was loading Arabian medium crude oil at a Ras Tanura SPM in August.
At 1905, the cargo watch officer was topping off the No.4 Port cargo
tank from the cargo control room. The remote ullage gauge showed 1.30
meters. Suddenly oil began escaping from the No.4 port cargo tank onto
the main deck. The cargo flow was immediately changed to another tank
and then loading was stopped. About 8 barrels of oil escaped in all.
Air driven pumps had been rigged aft on the main deck where a spill
was most likely to collect and these pumps were started immediately
the oil reached them. However, a gangway support arrangement impeded
the flow of oil aft and approximately 3 barrels of oil escaped over
the side at that point.
The oil spill report attributed this incident to mechanical failure.
However, there is very little mechanical failure in this case. The cause
is management failure and human error. The gauge in question did not
fail, it was simply out of calibration. The cargo watch officer failed
to have his deck watch use the portable ullage tape to verify the remote
gauge. Management had failed to install an independent hi-level alarm
system on this tanker more than ten years after the technology became
available. It is a management responsibility to have in place both the
technology and the required tank level verification procedure to prevent
this type of spill.
It was the cargo watch officer's responsibility to use his professional
judgment to verify the remote gauge indications, even in the absence
of applicable written procedures.
3.22.1 Preparing for topping off
Without adjustments by the deck watch, a tanker will naturally load
so that the aft tanks fill with cargo first (due to trim aft when starting
cargo). Normal practice is to adjust the loading valves so that the
flow to aft tanks is reduced and all cargo tanks fill evenly. The chief
officer will have explained to the cargo watch officer the method he
wishes to be used for topping off and indicated the maximum permitted
topping off rate. This will be indicated in the night orders or loading
plan, along with the amount of advance notice to be given the terminal.
The chief officer will indicate when he wishes to be called for topping
off (normally twenty or thirty minutes before).
The cargo watch officer should closely calculate when the topping off
operation will begin and advise the shore terminal well in advance.
At least once before topping off, the cargo watch officer must have
his deck watch take manual gauges of the tanks to confirm the accuracy
of the cargo control room (CCR) tank gauges. This comparison should
be noted in the deck logbook, along with any discrepancies. The discrepancies
should also be noted on the cargo status board and with erasable marker
on or alongside the inaccurate gauge.
At the same time the chief officer is called, any additional personnel
required for topping off should also be called. The chief officer will
have indicated who is required, (normally the pumpman and one or two
additional seamen). The important thing is to ensure that adequate personnel
are available. Other vessel activities must be secured or supervised
by other personnel so that the cargo watch officer can give the cargo
his undivided attention.
3.22.2 Topping off
The cargo watch officer must give the shore terminal adequate notice
before reducing the loading rate for topping off. This is usually between
ten and twenty minutes, as agreed in the pre-loading conference.
Approximately ten minutes before the first cargo tank is expected to
top off, the cargo watch officer should station one of the deck watch
at the manifold, then call the shore terminal office by radio and order
'reduce the loading rate to (X) barrels per hour' (indicating the agreed
reduced rate is to be).
The crew member at the manifold should have been instructed to advise
the cargo watch officer when he hears a decrease in the loading line
turbulence and/or notes a decrease in the loading manifold pressure.
Each is an indication that the shore has reduced their pumping rate.
Cargo watch personnel then begin closing valves to those tanks which
will be topped off later, leaving open the tanks which will be topped
off first. If the loading rate is still too high, then the shore should
be requested to further reduce the pumping rate. If the loading rate
is still greater than desired, the loading valve to an available empty
cargo tank can be opened slightly to relieve some of the pressure. Maintaining
a low pipeline pressure reduces the time and effort to operate manual
valves and reduces the chance of powered valves failing to operate.
A deck watch member should be stationed at the ullage port of the first
tank to be topped off, with the necessary ullaging equipment. He must
measure the actual ullage and report it to the cargo watch officer.
The cargo watch officer compares the reported ullage to that indicated
by his remote gauge and notes any correction which should be applied
to the gauge. The watch officer should have a clear idea of the repeatability
of the tank gauges and allow for the maximum repetition error in deciding
when to stop loading a tank. He should also understand how the tank
gauging system works, the types of failures it can have and the symptoms/effects
of each failure. Before the first tank reaches the final level, he should
partially close its valve and partially open the filling valve to a
second tank. This confirms that both valves are operating. The valves
are maintained in this position until:
- The minimum loading ullage is reached, or nearly so, on the CCR remote
- The 'hi-hi' level alarm for the tank sounds.
- The deck watch reports that the tank has reached the desired minimum
When any of these events occurs, the loading valve to the next tank
is operated to the open position and the loading valve for the full
tank is closed. The important concepts in this procedure are that the
tank is topped off at the first indication that it may be full, even
though the other two indicators may not agree, and the next tank loading
valve is always opened and noted to be opening before the full tank's
loading valve is closed. This precaution prevents the accidental closing
tank valves against the shore pressure. Closing off against the shore
pump can cause a significant amount of damage, environmental pollution
and possible injury or fatalities to ship and shore personnel. The cargo
loading orders should indicate the number of cargo tank valves to be
left open at all times when loading ahead and when topping off, to prevent
over-pressure of the ship or shore piping and loading hoses.
When the first tank has been topped off, the deck watch shifts to the
next tank (as directed from the CCR) and the process is repeated until
all of the first 'set' of tanks is topped off.
When the final tank on the first set is finished, the loading valves
to the second set of tanks are opened fully.
After the second set of tanks is well started, the cargo watch personnel
must verify that:
- No cargo is flowing into or out of the tanks which have been topped
off and valves closed.
- No cargo is flowing into any other tanks or spaces except the those
tanks open for loading.
This re-check of full tanks needs to be done regularly during the
remainder of loading. The loading line pressure increases as the second
set of tanks fills, and cargo may begin to leak through defective, jammed,
or obstructed valves into tanks which initially showed no ullage change
after topping off.
3.22.3 Topping off non-CCR vessels
Tankers without CCR installations will be topped off with all watch
personnel on deck. In many ways, this is easier than CCR operations,
as long as the weather is moderate! The cargo watch officer can see,
hear and feel many things on deck that he cannot in the CCR, things
which tell him that the operation is fully under control, or indicate
that he has a problem. The one ability he does not have on deck, is
the ability to immediately scan all of the cargo tank ullage readings
for the ship. To compensate for this, the cargo watch officer must skillfully
use his watch personnel to check and report tank ullage readings to
him. For this procedure to be effective, all watch personnel must have
been fully instructed in the uses and peculiarities of the ship's tank
As a tank reaches the topping off level, the officer assigns one man
to stand by the valve operator for the nearly full tank and another
man at the valve operator for the next tank. The process is the same
as for CCR operations. First the valve for the next tank is opened one
or two turns of the valve wheel. Then the valve for the full tank is
closed one or two turns. When the final ullage is reached, the next
tank is opened fully and the full tank is closed and lashed. The cargo
watch officer must watch each man to ensure that they are turning
the valves the correct way. There have been many cases of an officer
going on to the next tank while the man behind him opened and lashed
the valve to a full tank in the open position! The results of this error
are always unpleasant and difficult to explain, nor can the officer
legitimately blame his watch personnel for his own neglect to verify
their actions. When a tank is topped off and loading valve closed, the
man who closed it should attend the ullage gauge for one minute to observe
that there is no change in the final ullage. He can then move on to
assist with the next tank to be topped off.
3.22.4 Multi-grade cargoes
The pumping rates of multi-grade cargoes are usually different, which
combined with different tank sizes and starting times means that their
topping off times cannot be regulated as with straight cargo loadings.
The cargo watch officer must use his periodic tank level readings to
predict when each grade will top off. In some cases the times will be
nicely staggered and he can complete topping off/finishing the loading
of one grade before the next grade requires his close attention.
In other cases events conspire to bring several grades toward their
final ullages simultaneously. If that occurs, the cargo watch officer
should consult the loading plan to determine which grade is on the critical
path. He should keep the critical path cargo loading while he calls
for the shore terminal to shut off other nearly full grades. He can
then top off one grade at a time, restarting grades in a controlled
manner. Adequate notice must be given the shore before doing this, indicating
that the stopped grades will be restarted and finished later.
If the vessel has developed a list and cargo grades must be stopped
to control topping off, then the tanks on the low side should be stopped
(if possible) while the tanks on the high side are finished. A list
produces ullage gauge inaccuracies which can interfere with proper topping
off (see section 3.24.1).