Explain various advantages and disadvantages of C.O.W and how often is it carried out? |

Explain various advantages and disadvantages of C.O.W and how often is it carried out?

CRUDE OIL WASHING:- This section summarises the objectives and prescribes the procedure for the crude oil washing (COW) of cargo tanks,

The Master is responsible for monitoring and verifying compliance with this procedure.

Purpose of Crude Oil Washing:-

The objectives of the introduction of the COW system can be said:

  1. Rationalization of tank cleaning operations.
  2. Rationalization of cargo work including complete discharge of crude oil; and
  3. Prevention of marine pollution by minimizing the amount of residual oil retained on board.
  4. Formation of was coatings and resultant prevention of corrosion inside tank.

Cargo Grade Oil used for Crude Oil Washing:-

1) Selecting Oil for COW :- The following points should be taken into account for deciding what grade of oil to use for crude oil washing:

  • Mixture of different grades of oil should be avoided.
  • Where the tank is washed by a two-stage method, top and bottom, the top wash shall be carried out with the same grade of oil to avoid contamination but a bottom wash with a different grade of oil may be possible if unavoidable, with approval of the cargo consignee.
  • If a different grade of oil is used as wash oil with the agreement of the cargo consignee, it will be advantageous to use crude oil with less sediments.

2) Check the necessity for flushing with fresh crude oil, and if necessary, such quantity.

Tank to be Crude Oil Washed :- Cargo tanks necessary to be washed are as follows:

  1. Cargo tanks specified in Section 6.1.2 of the COW Manual.
  2. During cargo discharge before dry docking.

Regardless of the number of tanks required to be crude oil washed by the MARPOL 73/78 convention, all tanks should be washed by crude oil as far as possible for the purpose of preparation for ballasting and sludge control.

Prohibited Matters Concerning COW:-

  1. Crude oil washing shall not be carried out during a ballast passage.
  2. Ballast should not be loaded in tanks which were not crude oil washed.
  3. Water filled in a tank already crude oil washed but not yet rinsed with water shall be regarded as dirty ballast.

Crude oil Washing at Sea Between Discharge Ports:-

  1. In any case, crude oil washing shall be finished before departure from the final discharge port.
  2. In the case of two or more discharge ports, it is possible to carry out crude oil washing at sea between such discharge ports.
  3. In this case, washed tanks shall be left empty, ready for a dry inspection at the next discharge port, prior to sailing.
  4. This inspection is to be conducted by sounding several parts of the tank bottom, etc. when the tank is empty.
  5. Crude oil washing between discharge ports may minimize any potential delays whilst discharging operation
  6. The same safety and pollution precautions shall be taken as in port.

Method of Crude Oil Washing:-

➢ Cleaning Methods:-

1) Multi-Stage System: While cargo oil is still in a tank, the upper part above the oil surface is washed (top wash) and the bottom is washed (bottom wash) by stripping tank washing after the tank has been emptied.

As one version of this method, when tank washing is divided into two stages, namely top and bottom, it is called a two-stage method.

  1. Top Wash:-
  • The washing area is from the joint section of the upper deck and the bulkhead downward to a height slightly above the oil surface.
  • In the case of multi-stage system, each stage should be overlapped.
  • In general, sludge accumulates on the horizontal girders, the top wash therefore should be started when the oil level reaches about three (3) metres above the bottom unless specified in vessel‘s COW operation Manual.
  • The top wash terminates when the remaining oil level goes down to about 1.5 metres above the bottom.


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