The Ultimate Guide to Cargo Operations for Tankers

Crude oil washing is carried out to ensure that the sludge and clingage (a term used to refer cargo residues sticking to the tank walls and surfaces) are minimum and there is maximum outturn of cargo. Cargo tanks are crude oil washed during the discharge operation using one of the deep well pumps. Before commencing crude oil washing, it is necessary to de-bottom all tanks as this removes water present in the crude oil. The removal of water helps in reducing static charges generated during washing. During the discharging operation, before crude oil washing, slop tank should be emptied and recharged with fresh crude from other tanks using bottom lines.
Crude Oil Washing - COW
Usually, a good aft trim of up to 3 meters or more must be maintained during COW and Stripping.
137. l Marine Insight Cargo Operations for Tanker I 138. COW pump

Preparation For COW Operation
• Master should take the COW operation permission from the terminal in-charge
• The COW machinery and operational preparations to be done as per the checklists
• Pressure test the COW line and check for any abnormalities before entering the port
• Before starting the crude oil washing work, confirm that the concentration of oxygen in the tank is below 8% by volume
• The pressure inside the tank during the crude oil washing work should be maintained at more than
+1.961 kPa (+200 mmAq) Ensure the chief officer informs all the ship staff involved in the COW operation about the following points
• Which tanks and in what order crude oil washing to be performed
• Methods to be used for washing each tank
• At what ullage the COW operation should start
• Any other operation being performed parallel with the COW operation
• The quantity of ballast condition of the ship at the time of departure
139. l Marine Insight Cargo Operations for Tanker I 140.

Cargo Washing Procedure Let us assume that we have to do COW for cargo tank no 2 W‟s as it has reached stripping levels, using slop starboard tank through an OPEN CYCLE WASH.
• Lineup the slop pump to discharge into the tank cleaning mainline. Ensure at least 7-8 meters of the head of cargo is present in the slop starboard tank
• The tank cleaning pump to the tank cleaning line should be blanked off using a spool piece
• Blank the line leading to tank cleaning heater as well
• Set the tank cleaning machines of 2 W‟s fora full wash cycle of 60 minutes
• The bulk discharge from 1W‟s and 4W‟s can be resumed
• Start the deep well pump for slop starboard and pressurize the COW mainline by slowly opening the pump discharge valve up to 9 bars
• As the pressure in the line builds up, open the valves of the COW machines
• Simultaneously, start the No. 2 Ws‟ Deep well pumps and open their discharge valves to strip the tank washings and pump directly ashore
• Similarly, cycles of COW can be run into remaining tanks. It maybe required to reduce the speed of other pumps while COW is in progress
141. l Marine Insight Cargo Operations for Tanker I 142.

• Ina CLOSED CYCLE WASH, the discharge of the pump is lined up to return the cargo into slop starboard tank
• As the COW cycle is finished, the discharge valve of the slop starboard pump to be closed first and the 2W‟s cargo tanks to be stripped fully ashore
• The COW mainline to be drained into slop starboard tank
• After stripping 2W‟s the valves for COW machines for these tanks should be closed Often during tank cleaning or crude oil washing, machines are found to be working, but they do not rotate through the full cycle. Always ensure all machines are rotating satisfactorily, if not then on a running line, close the COW machine inlet valve and then open it slowly. It can also be turned manually using the portable lever provided. For crude oil washing and tank cleaning the same machines and the same line are used. On crude oil tankers it is named as COW line, and on product tankers it is named as Tank Cleaning Line. Advantages of COW operation
• Prevention of potential marine pollution
• Time and cost reduction during the dry-docking of the ship
143. l Marine Insight During the discharging operation, before crude oil washing, slop tank should be emptied and recharged with fresh crude from other tanks using bottom lines Cargo Operations for Tanker I 144.

• Reduction in the quantity of residue after the cargo discharge
• Reduction in the emulsions generated by the water-oil mixture
• Reduction in the damage to the structure of the tank Disadvantages of COW
• Increase in the discharging time of the unloading operation
• Increase in the ships staff workload at the port
• Increased risk of sea pollution if the operation is not performed professionally
• Additional cost incurred by the shipping company as maintenance of the washing machines will be added to ship's operating cost
MARPOL stipulates the number of tanks for which COW is to be implemented, however, some terminals may have their own rules and regulations related to COW, and the number of tanks that can be washed maybe restricted depending on the rules and regulations of the specific terminal. a.
COW of about one-fourth of the total number of tanks is to be performed considering effective sludge control b. However, the COW of the same tank two times within four months for the same purpose is not necessary
145. l Marine Insight Cargo Operations for Tanker I 146.

While it is the responsibility of the master of the vessel or the officer in charge to carryout the tank cleaning procedures safely and in accordance with company‟s safety procedure manual and other safety guidelines available, it is the duty of every person on board to ensure personal and crew members safety. Present day tankers require tank cleaning for several purposes, namely
• Change of grades of cargo, e.g. to carry jet oil after gasoline
• Man entry for repairs or inspection
• Prior dry docking
147. l Marine Insight

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