The Ultimate Guide to Cargo Operations for Tankers

Usually, washing water is filled in the tanks with the following methods Supplying water from the sea by gravity For this method, the sea chest valve is opened and seawater of the required volume is directed to the designated tank by gravity. It is then shifted to the slop tank using the tank cleaning pumps. Filling water from the sea by the pump The sea chest valve is opened and the seawater is directly supplied in the slop tank using a pump. Method of filling water by gravity from the ballast tank The cargo line and ballast line are interconnected and isolated using a spool piece and a valve. The water from the ballast tank is supplied to the cargo tank by gravity or by using a pump. The amount of water to be supplied plays an important role inefficient tank cleaning operation. The quantity of water to be filled can be decided based on the quantity of oil recovered after crude oil washing and the quantity that can be recovered in the slop tanks. If the filling amount of water is too small, the following problems may arise
• Contamination of washing water leading to a reduction in tank cleaning efficiency
• Air in the pump suction line involved in the operation
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• Reduction in the efficiency of gravity shift of the fluid, wherever applied If the filling quantity is too large, the following problems may arise
• Increase in the settling and discharge time
• Water may need to be discharged during the cleaning work, increasing the man-hours The following procedure to be followed for three stages of tank cleaning- For this, let us assume 2 W‟s cargo tanks have to be cleaned Cold Seawater Wash
• Fill around 250 – 300 m of seawater into slop starboard tank using fire pump or via ballast to cargo line or using the cargo sea chest
• After the water is filled in the slop tank, open the tank cleaning line machine valves and branch valves for all tanks to drain any cargo remaining into the tank
• After the lines have been drained, close all the valves on the cargo and tank cleaning system
• Lineup the slop starboard pump to discharge the seawater into the tank cleaning mainline Lineup the pumps of 2W‟s to pump the wash water into slop starboard tank via manifold crossovers
• Set the tank cleaning machines fora full cycle and open the branch valves of 2W‟s
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• Start the slop starboard pump with discharge valve closed
• Raise the pressure to around 9 -10 bars. As the pressure builds up, open the discharge valve slowly to pressurize the tank cleaning the mainline When the line pressure reaches up to 8 – 9 bars, open individual tank cleaning machine valves
• The machines will run fora full cycle of 60 minutes
• Simultaneously, start the deep well pumps of
2W‟s tanks to strip the tank washing and keep its discharge valve open to discharge the same into slop starboard tanks
• After the washing cycle is complete, stop the slop starboard pump and close its discharge valve
• Keep running 2W‟s pumps till they strip the respective tanks dry into slop starboard
• As the tanks are dry, stop the pumps and close their discharge valve
• The same procedure can be followed for the remaining tanks
• During the tank washing, the lines can be washed by recirculating the cargo through the lines back into the tanks
• The ullage of slop tank should be monitored continuously since wash water being stripped is mixed with oil residue from tanks
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On a closed loop system, a COP takes suction from a slop tank, and the discharge is used to drive the wash water into the tank cleaning mainline, which bifurcates into two lines - one of them feeds the tank cleaning line on deck for tank cleaning purpose, and the other is used to drive the cargo eductor, which strips the tank washing back into the slop tank. Hot Water Wash For hot water washing, it is important to ascertain the maximum permissible temperature for tank coating as washing tanks with higher temperatures can cause extensive damage to expensive tank coatings. The ships officer will decide if heating of washing water is required or not after considering the following points
• Ship type and ships age
• Time for gas-freeing work
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• Number of tanks to be cleaned
• Description of work within tanks
• Type of tank coating
• Specific fuel consumption The same wash water in the slop starboard tank can be heated in 2 ways Firstly by heating the water into slop tank using steam heating coils. Steam can be circulated inside the steam coils of slop starboard tank. Secondly by circulating the cargo through the tank cleaning heater. The tank cleaning line has an arrangement of through which it can be circulated through tank cleaning heater before entering the cleaning line on the deck. Tank cleaning heater has inlet and outlet temperature gauges for wash water. Once the water is heated up to the required temperature, it can be used for washing the tanks similar to the cold water washing. After completion of hot water washing the steam inlet and return line valves should be secured. The tank, which has been washed shall be allowed to drain wash water and stripped into slop tanks used for washing. Once the tanks are stripped dry, the tank cleaning and machine valves for the same tank can be shut and the pumps discharge valves should be closed.
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Fresh Water Rinsing Freshwater rinsing is carried out to remove the seawater containing chlorides and salts from the tanks. Chlorides and salts might alter cargo properties of sensitive cargo such as diesel, jet fuel, motor spirits etc. Procedure for Fresh Water Rinsing
• Lineup the tank cleaning pump to take suction from afresh water tank or a designated water tank
• Open the blank on the tank cleaning mainline to join the mainline to the cleaning pump
• Keep the branch valves of a pair of tanks open and set their tank cleaning machines
• Start the tank cleaning pump and pressurize the discharge sides to 7 or 8 bars and then open the discharge valve of the pump
• As the water enters the tank cleaning machines, ensure that at least two full cycles are run covering the whole tank area. This generally takes approximately 5 minutes
• Similarly, rinsing to be carried out for all remaining tanks by switching over through the branch valves one by one
159. l Marine Insight Always check your estimates of tank cleaning for seawater, freshwater and oil so that the rinsed water is easily collected in the residue tanks and accommodated within their capacity or in any other tank designated to hold the residue Cargo Operations for Tanker I 160.

• After rinsing is finished for all the tanks, stop the pump and keep all valves of the cargo and tank cleaning machines open to allow them to drain into the tanks
• After 5 - 6 hours of draining, start stripping the tanks using individual pumps and discharging the wash water into slop tank, which contains previous washings
• Line washing (removal of oil traces in the ODM line) and valve flushing is done to remove the oil traces sticking to the cargo pump, lines and valves using the washing water

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