OIL TANKER OPERATIONS (Discharging)
Good planning is the hallmark of efficient tanker operations.
Prior arriving at the discharge port an exchange of information between the ship and the terminal will take place. Once the vessel is tied up at the terminal, a ship-shore checklist will have to be filled out. The general safety checks and precautions will be the same as given for the loadport. Since pumps will be running at the discharge port, special attention will have to be given to monitor the safe running of the pumps. Pumproom ventilation should be running throughout operations. Proper pumproom entry procedures should be followed. All crew should be aware of the fire and safety equipment available in the pumproom. Drips and leaks in the pumpFoom should be attended to immediately. Pumproom bilge soundings should be monitored regularly.
After completion of gauging/sampling and calculation the terminal will give indicate readiness for receiving the cargo. The chief officer will line up for discharge. Ideally cargo will be started by discharge from a single tank. In steam turbine cargo pumps it is important to warm up the pump gently. RPM of the pump will be slowly increased after all checks have been made. Inert gas will replace the volume of cargo discharged. The maximum oxygen content of the IG from the IG plant should be 5% and the tank should have a maximum of 8%. The IG pressure should be monitored very carefully to avoid over or under pressurizing of tanks.
When all checks have been completed, it will be safe to proceed to open up more tanks and start more pumps. Discharge pressure at the manifold to be monitored periodically.
The next important stage comes when a tank is nearing empty levels. When some oil is still left in the tank, the rpm of the pump should be reduced. Changeover procedure from one tank to another should be gradual. The valve of the tank nearing empty should be slowly throttled while the pump is fed by another tank which has a higher level of oil. Using the vac-strip pump or in its absence the stripping pump will be vital to avoid the pump misbehaving. In case a pump loses suction at any time, the rpm will increase dramatically. The pump should be stopped immediately in such a case or immediate measures should be taken to supply cargo oil to the suction side of the pump.
The use of a bar chart and thorough planning are the key points of an efficient discharge. Ballast should be taken into the ballast tanks as per the ballast plan worked out before arrival.
COW operations will also be required and this is dealt with later. IG should be vented, if necessary, through the mast riser or sent ashore through the vapour recovery system, if fitted. Judicious planning will ensure that the amount of IG vented to the atmosphere is restricted to the bare minimum.
CRUDE OIL WASHING (C.O.W.)
Cleaning of cargo tanks has always been a source of concern for all tanker personnel. Washing the tanks with only water after discharge of a cargo has never been satisfactory. Sludge and residue was a problem. The need was felt to use a suitable medium that acted as a solvent. The idea of using the crude cargo itself for washing the tanks was born and is now universally accepted.
Crude oil washing serves the purpose of cleaning a cargo tank by reducing or eliminating sludge and the clingage. In case of a conventional tanker where ballast is taken into a cargo tank, this will mean that COW is essential in reducing the risk of pollution. From the cargo owner’s point of view, even the sludge and other heavy elements are valuable and they would be most happy to receive their cargo in totality. From the ship owner’s point of view, the tanks are cleaner, less water washing required prior entry, less rob and so lesser claims.
Crude oil washing has some disadvantages. The discharge time and port time is increased. Secondly, a slight loss of oil takes place due to vapourization, which however, is negligible. The advantages far outweigh the disadvantages.
The monitoring of tank atmosphere before starting COW operations is vital to the safety of the operation. The oxygen content of the tank should be less than 8%, the IG line oxygen content should be less than 5% and the gas in the tank should never be anywhere near the flammable range.
Crude oil washing can be accomplished by either bleeding off from the main discharge line or by using the slop tank oil for the drive. The former is generally preferable since there will be a time saving and also the sludge, etc., wiB be discharged ashore alongwith the main discharge. It should be borne in mind that the crude used for washing operations should be a “dry crude”. By this we mean that, water in the crude oil which settles down, is discharged first by debottomming the tank. It is important to know that a mixture of crude oil and water can produce an electrically charged mist during washing much in excess of that produced by dry crude. For the same reason, if a slop tank is used as a source of oil for washing, it should first be completely discharged ashore and refilled with dry crude.
During the tail end of the discharge it may be necessary to employ the eductor/stripping pump to drain the last remnants of the cargo into the slop tank. The last tank to be discharged will be the slop tank. Finally after completion of discharge from the tanks, it will be necessary to drain and strip all the lines. Discharge of the final line stripping should always be done through the small diameter line (Marpol line).