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A technique of cleaning tanks in oil tankers.

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A ship designed for transporting oil in bulk


Crude oil washing: - A system of cleaning the tanks by washing them with the cargo of crude oil while it is being discharged.
The purpose of crude oil washing:

  • To reduce the amount of cargo onboard after completion (R.O.B).
  • To increase the flow of the discharge

Conducted before loading a different grade, which is not compatible or as required by charterer.

COW offers the following advantages over water washing of cargo tanks:

  • reduction in pollution potential
  • increased cargo outturn
  • less deadfreight
  • less time required in subsequent tank cleaning
  • less time needed to prepare a tank for repairs in dry dock
  • less corrosion because of less water washing. The disadvantages of COW are:
  • increased workload
  • prolonged discharge time
  • cost of COW equipment
  • increased safety risks in port.

COW operations are usually carried out in the port of discharge, although it is also possible to implement COW on passage between ports, provided that there is still a parcel of crude oil in the ship. The source of the oil for COW may be the discharge line, in which case provisions have been made to bleed off oil for tank washing. Alternatively, the contents of the slop tank may be used in the recirculation mode. When the discharge line is the source of the washing fluid, special measures may be necessary to ensure sufficient pressure on the washing line, especially if backpressure from the terminal is low. It may be argued that the use of oil from the slop tank means re-using the sludge removed from the cargo tanks, for further washing. In either case, it is important that sufficient crude oil be available to complete the COW programme. In order to obtain the desired washing results, COW operations during subsequent trading of the ship should take place under tha same parameters as those used during the tests. Conditions influencing the washing results are:

  • the number and location of washing machines
  • nozzle diameter
  • duration of washing
  • number and size of drainholes effectiveness of the stripping system stripping procedure
  • trim of the ship.

It should be noted that too many machines in use on a branch line couid lead to excessive fluid velocity in the line, causing a pressure drop over the line and reducing the effectiveness of the machines being served. It is therefore important for the correct number of tank washing machines to be used at all times.


A grand central station of an oil tanker where all pipelines meet and interconnect in a relatively small space. The danger of petroleum vapor, also hydrogen sulfide if sour crudes are being handled, may also be deficient in oxygen if steam, inert gas or CO2 have been used for firefighting or prevention.


Fluid, such as a grease or a polymer-containing oil (e.g., multi-grade oil), in which shear stress is not proportional to shear rate.


An engine test in a six-cylinder spark-ignition Chevrolet engine to evaluate crankcase oil oxidation stability, bearing corrosion, and engine deposits.


Device used to remove oil from gaseous refrigerant or steam.


An oil refining process in which the gas-oil is cracked by a catalyst bed fluidized by using oil vapors.


Used to describe crude oil which is high in hydrogen sulfide and/or mercaptans and therefore confined spaces and tanks where sour crude is transported can develop high concentrations of hydrogen sulfide gas.


Coked material remaining after an oil has been exposed to high temperatures under controlled conditions.


The gas volume flow rate, relative to the oil volume flow rate, both converted to volumes at standard pressure and temperature.

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