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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 off.

Case study
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.

Case analysis
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 gauge.
- The 'hi-hi' level alarm for the tank sounds.
- The deck watch reports that the tank has reached the desired minimum ullage.

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 of all
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 ullage system.
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).

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