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The operation of taking on board fuel, 'bunkering', produces a large number of operational oil spills. Since bunker fuels are likely to be viscous and slow to dissipate, the damages caused by a bunkering spill are often high in proportion to the volume spilled. To minimise operational spills, fuelling should be conducted strictly in accordance with set procedures by qualified personnel.

5.42.1 Bunkering while discharging cargo
Bunkers can be safely loaded while discharging, provided that the operation is attended by a team of engineering staff who have no other duties but monitoring the fuel transfer. If an engineer and an assistant arc not available exclusively for this work, then fuelling should be deferred until they are available.
Persons in charge of loading fuel must closely observe the filling rate. Heavy fuel presents a special problem regarding loading rates if the contents of the shore line are not adequately heated. A cold plug will form in the loading line, giving a low initial loading rate until the plug is pushed into the ship's tanks. After the cold plug is aboard, the loading rate may increase dramatically. The opportunity for an incident, due to a sudden increase in loading rate after starting, is reduced by knowing the capacity of the shore line before fuelling begins, directing the initial fuel flow into the largest fuel tank and watching the rate carefully after the volume of the shore line has been received. After the fuel is flowing freely, the smaller and more difficult bunker tanks can be topped off.
Most receiving terminals will not permit a fuel barge to moor alongside a tanker which is at their berth. If fuel is to be taken from a barge, fuelling will normally have to wait until the tanker has un-berthed and moved to anchorage.

Case studies
1 A gas carrier of 27,000 gross tons was loading bunkers in Mexico. After fuelling had started, the bunker hose burst due to high pressure, spilling marine diesel over the ship and the sea. When bunkering was eventually resumed, there was only a short interval before diesel oil began to gush from one of the tank vents, causing additional pollution. Investigation showed that the bunker pumping rate had been 582 metric tonnes per hour, which is far in excess of the safe bunkering rate. The owners of the facility detained the vessel and demanded a guarantee of USD 800,000 against the cost of the clean up. Actual clean up costs were estimated to be USD 300,000.
2 A tanker was loading diesel oil as fuel in Genova. All tanks except the final tank were safely topped off at their intended ullage. The ordered amount would fit safely in the last tank, so the engineers stopped ullaging the tank. Unfortunately, the barge had on board more fuel than had been ordered and decided to discharge it all to the tanker. The excess fuel overflowed the vent pipe on the port side and about 0.05 cubic meters escaped to the sea.
3 A motor tanker loading 380 CST fuel in Rotterdam assigned the motorman to watch the filling tank. The tank contained considerable 'mist' making the surface hard or impossible to see. The motorman was not provided with an ullaging tape and float, and was not otherwise measuring the ullage. The tank overflowed the ullage hatch and about 300 litres escaped from the deck to the sea.

Case analyses
In each case, the person(s) in charge of the bunkering operation were not adequately monitoring the level in the filling tank to maintain control of the operation. It is essential when bunkering that a qualified and trained engineering or deck officer be present to supervise each step, according to the written procedure. The officer must be provided with adequate communications to immediately stop the transfer if necessary.
From the owner's viewpoint, the frequency of bunker spills would indicate the need for installation of high level alarms in the bunker tanks. Connecting these alarms to a light and sound alarm at the bunkering station would give an immediate indication of trouble, while allowing time to stop the transfer before an overflow.

5.42.2 Bunkering precautions
Obtain samples from the old bunkers and test them for compatibility before loading the new bunkers.
Advise the terminal the maximum rate of loading permitted in accordance with the bunker tanks venting limitations.
As far as possible, keep old bunkers segregated from the new bunkers, particularly the first bunkers received. Check the new bunkers for water after loading. If free water is found in the bunker tanks, make the proper protest in writing to the supplier.
Check the bunker receipt information carefully. Verify that delivered quantities are within allowable tolerance. If there is a disagreement between ship and shore figures, give a protest letter to the supplier.

5.42.3 Sampling before bunkering
Before loading, take and retain samples of the existing bunkers. Be sure to follow the sampling and labelling procedures.
Take a gas reading of the ullage space in each bunker tank and record it, together with the temperature of each bunker tank.
If the gas reading exceeds 50% LEL, it is an indication that the tank may have been contaminated with a cargo leak. (Note: in Saudi Arabia, harbour authorities will routinely vapour test bunker tanks and will take the vessel off the berth if the reading in any tank exceeds 25% LEL.)
If bunker tanks are found to have unacceptable gas content then notify the terminal manager, owners and consult with the P&I correspondent for further advice.

5.42.4 Sampling and testing after bunkering
When bunkering is complete, repeat the ullage space gas test for each tank loaded and record the readings together with the tank temperatures. If any of the readings exceed 50% LEL, then ventilate the tank(s) until the reading is 10% LF.L or less. Wait for two hours and test for gas again. If the gas concentration is above 30% LEL, then send a bunker sample ashore to an independent petroleum testing service with instructions to test for flash point. Notify the supplier, asking him to attend the testing and present him with a protest letter including the test results.
Before new fuel is used, it should be tested for sodium, flash point and vanadium. It is possible to improvise these tests on board. The sodium test is conducted by using a magnesium rod to introduce a sample of the oil into a gas flame. If the oil burns with an intense orange light the presence of sodium chloride (salt water contamination) is indicated.
Flash point cam be tested by heating the sample in a water bath and closely checking the temperature at which vapours are given off, as indicated by a combustible gas indicator. If vapours are not given off before the sample reaches 60 C, then the fuel is safe.
The combination of sulfate contamination (from sea water) and vanadium in fuel produces a chemical combination which can cause accelerated corrosion in the combustion chambers of diesel engines, or corrosive deposits on boiler tubes in steamships. Fuel cannot normally be tested for vanadium on board.

5.42.5 Slops and ROB as bunkers
Slops created during tank cleaning are a mixture of water and oil. After settling and decanting of the free water, the oil phase may still contain up to 30% sea water by volume, held as minute droplets in suspension.
Use of slops as bunkers is hazardous because of the potential for:
Low flash point oil in bunker tanks.
Loss of propulsion due to water in fuel.
Sludge fallout and fuel tank/system problems, such as sticking of
injection pumps.
Fuel instability due to salt water contamination.
Choking of diesel exhaust belt and turbine inlet grid.
Rise in diesel exhaust temperature.
Surging of turbochargers.
Extensive wear of piston rings and liners.
Sticking of exhaust valves.
Damage can be caused by deposits of sodium which build up behind the piston rings. This results in the top rings seizing, leaving the lower ring to take the full compression load. Heavy wear and blow-by is caused by micro-seizure and a cylinder liner failure will result.
Any of these problems may result in an unseaworthy vessel. If a General Average situation develops under such conditions, it is a foregone conclusion that cargo underwriters will not contribute in the GA, which again causes hardship to owners and their P&I Club.
If the last cargo was fuel oil and a bottom flush with fresh water is used to collect the ROB, no adverse corrosion will follow. Fuel oil ROB recovered in this way may be used as fuel.
However, water washing may set up very stable emulsions, which may be difficult to remove if the specific gravity of the oil is close to that of water (API 10.0). It has been demonstrated that the bottom flush method recovers 90% of the ROB. Washing is therefore unnecessary to collect almost all of the ROB for use as fuel.
Never transfer ROB to the fuel tanks until it has been checked for flash point. Do not use ROB as fuel if there is the slightest possibility that the flash point is below 60 C. A test for sodium content should also be conducted, (see section 2.12.9.)
If ROB is to be mixed with bunkers, a proper record must be kept in the oil record book, deck and engine log books. ROB fuel must never be used without proper adjustment of the bunker records. Failure to record the relevant fuel oil credit will result in an artificially low bunker consumption on paper, which will not stand up to close examination during investigation of cargo shortage claims.

5.42.6 Bunker notes
Statutory and classification rules require that:
The flash point of ship fuels must never be below 60 C and
Fuel oil must never be heated (open system) to a temperature which exceeds a limit set at 10 C below the flash point.

There is reason to believe that the flash point of many bunker deliveries is below the temperature to which the oil must be heated (on account of high viscosity) to allow it to be purified on board. This means that even though attempting to comply with the minimum flash point requirement (60 C), the oil will be heated into the explosive range for treatment.
High temperatures are also required for efficient fuel storage, pumping and settling.
These problems are under review by the IMO and Classification Societies.

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