Crude Oil Washing (COW) Procedure & Precautions on Ships


Prior to arrival at the discharge port, the following must be carried out:

  • Within 24 hours before the planned cargo operation, the Inert Gas system must be tested to ensure that all components and instruments are operating properly.  
  • Both the fixed and portable instrumentation must be tested and fully calibrated.
  • Fixed tank cleaning lines must be tested to a pressure of 12 kg/cm2 and any leakage in valve glands, joints or couplings, etc., made good.  
  • All fittings, such as pressure gauges and/or thermometers, which would normally be used during the C.O.W., should be in good order. 
  • All hydrants on the C.O.W. line should be blanked off. Enter this test in the deck logbook.
  • The tank-washing heater must be isolated and blanked off.
  • Terminal authorities and the vessels agents must be advised by cable, telex or fax of the vessels’ intention to C.O.W. concurrent with the discharge operation.  
  • Instruct the agents to inform all the Port Authorities concerned and to advise when permission has been granted.  
  • Advise also the Management so that in case of difficulties the matter can be readily acted upon.
  • All crew-members shall be properly briefed on the procedures to be followed during C.O.W. operations.  
  • Contingency procedures should be familiar to the crew. If there is a hazard of exposure to toxic gases such as H2S, a risk assessment shall be completed and controls measures determined.

No C.O.W. is to be started until the Master has satisfied himself that the following has or will be complied with:

  • WRITTEN confirmation has been obtained from the Terminal authorities to carry out the C.O.W. programme. If the Terminal refuses permission for crude oil washing through no fault of the vessel, issue a protest and ensure that it is signed by the relevant authorities and advise the Management and Charterers.
  • Inert Gas system is producing gas with the proper oxygen content, i.e. less than 8% (some terminals require 5%).
  • C.O.W. plan is prepared and discussed by all officers concerned and SOPEP gear is in readiness.
  • Crude oil washing checklist must be completed as per the vessel’s procedures.
  • Anti-pollution watch shall be maintained on deck to check and report any abnormal conditions, leaks etc. on the C.O.W. lines, valves etc.
  • All tanks should be de-bottomed so as to be free of any free-water.
  • If during the washing operation any leaks develop in the fixed piping system, or at any time there is any doubt as to the proper operation of the vessels inert gas system, then the operation is to be immediately terminated until the problem has been rectified.
  • A warning notice should be permanently displayed in cargo, engine control rooms, bridge and in the notice board stating “Tank washing lines on this ship may contain crude oil. Valves must not be operated by unauthorized personnel”.
  • While C.O.W. is being carried out, hydrocarbon gas will be generated. This may manifest itself by a rise in the tank pressure and will be most evident in areas with high ambient temperatures.
  • If washing is concurrent with discharge then this rise in pressure can easily be allowed for by adjusting the pressure controller of the inert gas plant, if necessary.
  • Before the tank pressure reaches the high level the main valve on the mast riser should be opened to allow the excess pressure to escape to atmosphere. However the pressure shall not be lowered below 100mm w.g.
  • If the there is any terminal restrictions prevail on venting of the cargo tanks, care must be taken to control the pressure. If required the operation be suspended and terminal/office should consulted.
  • After completion of the crude oil washing, chief officer should ensure that the following actions taken.
  • On completion of C.O.W. the valves between the discharge line and the tank cleaning line must be securely SHUT and LASHED.
  • The fixed deck tank washing line MUST be drained of crude oil. This is best achieved by opening one of the washing valves to the slop tank and another valve at the foremost end of the line, i.e. one centre tank. Do no forget to drain all the branch lines by dropping each section into the tank it serves.
  • After the tank washing lines have been drained, the entire system must be shut down.
  • Proper records should be maintained for each crude oil washing operations. The records shall include the number of tanks washed, number of machines used, time washing started and was completed, the washing pattern employed, the washing line pressure and tank dipping records after the wash in each tank.

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