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.

Related Terms


An endorsement issued by a flag state administration to a certificate of competency of a ship’s officer allowing service on dangerous cargo carriers such as oil tankers, chemical carriers, or gas carriers.
Depending on the position and degree of personal responsibility, the following types of evidence are determined: tankerman - minimal responsibility, assistant - personnel directly involved in cargo operations, responsible person - personnel directly responsible for cargo operations and compliance with safety measures during cargo handling.


Cargo such as oil, coal, ore, woodchips, etc. not shipped in bags or containers


A cargo ship engaged in the tramp trade.


A plan outlining the spaces available for fuel, cargo, ballast, fresh water, etc, with guides on weight and volume for spaces at various drafts and displacements


Vessel designed for a combination of passengers, and different types of cargo.


A device for hoisting and lowering heavy weights, cargo, stores, etc


Cushioning material placed among cargo to prevent their motion


Non-bulk cargo. The cargo may be of various kinds


A wood placed in bottom of ship to keep cargo dry


The act of loading and discharging a cargo ship.

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