It is one of the obligatory and essential operations taking place on a vessel which is carrying liquid crude cargo and transporting it to different parts of the world.
As per the International Convention of the prevention of pollution by ships (MARPOL), Regulation 33 of MARPOL Annex-1 i.e. Prevention of Pollution by Oil, every crude oil tanker of deadweight of 20,000 tons and above are mandatory to be fitted with a well organized and approved COW system.
The installed COW system should comply with all the provisions of the specifications for the design, operation and control of the Crude Oil Washing adopted by the International Maritime Organisation (IMO).
What is Crude Oil Washing Operation (COW)?
Crude oil is a naturally occurring fossil fuel that is playing a vital role in the Petroleum Industry, producing energy and keeping the world on track. It is widely being transported as cargo over the waters for ages. Many factors are taken into consideration when liquid cargoes are carried on board and one of them is Remains On Board (ROB).
Being an oleaginous cargo, after the unloading at discharge port is completed, the remains of the crude oil inside the tanks begin to take the formation of sludge. This sludge deposits reduce efficient drainage resulting in prolonged stripping operation. It also lowers the ship’s carrying capacity causing loss in earnings, prolongs water washing, dirties clean ballast and can cause pockets of CH gas to form, thus impeding gas freeing of tanks.
At this particular point, the crude oil washing operation comes into action in which the cargo residues or the sludge is washed out using that particular crude oil itself using high-pressure lines and nozzles in the cargo tanks. This operation saves time, expense and extra manpower and on the other hand it also prevents pollution of the seas from operational measures.
How often Crude Oil Washing (COW) is carried out?
Every time the discharging at the terminal is done the crude oil washing operation is carried out using fixed machines with one or two nozzles, these nozzles can rotate in the vertical and horizontal plane and the machine can be programmed to wash sections of the tanks as the cargo is discharged. After every COW operation all the particulars (Date, Time, COW line pressure and operation test of any equipment) are to be logged in the Crude Oil Washing Record, Oil Record Book, Deck Log Book and the details of Inert Gas System in the IG Record.
Procedure of COW
- A proper procedure is to be followed as per the COW MANUAL, International Safety Guide for Oil Tankers and Terminals (ISGOTT) and appropriate checklist approved by the authorities.
- Before arriving at the discharge port COW plan should be discussed with the staff and later on with the terminal representative.
- All the lines which are a part of this operation should be checked for any leaks.
- Make sure to check the atmosphere of the tanks involved in the operation especially oxygen content.
- At all-time in any circumstances, one lookout person should be present at the deck supervising and monitoring, and regular inspection must be made.
- Always keep an eye on the gauges during and after the operation and make sure vapor pressure is not building up in the tanks.
- The COW Plan must be available at all times and the responsible officer should be notified if there is any doubt to avoid any misshapen.
- Keep an eye on the Inert Gas System, the oxygen content should not exceed 8%.
- Appropriate well planned and discussed trim procedures have to be followed during the bottom washing of the cargo tank.
- After washing is finished each tank must be stripped again after time has been allowed for cargo to drain off the shipping suction.
- Each and every valve of the washing machine must be closed.
- All the lines must be drained.
Precautions to be taken
- COW machines are not fit and forget. These must be maintained according to manufacturer’s standard.
- COW manual should be checked to make sure that particular oil is suitable for carrying out operation or not.
- Make sure the discharge terminal has granted permission for COW Operation
- All the equipment and other alternative systems which will take place at the site shall be tested such as-
- Oxygen analyzer equipment is calibrated tested and ready for use.
- The inert gas system is tested and working with an oxygen content of less than 8%
- Tank cleaning lines and their pressure tested.
- All gauges are in working condition.
- Cargo tank high-level alarms have been tested.
- Check for worn-on educators.
- Wait for at least an hour after the cow operation is done for taking the sounding.
- Always refer to the approved checklist to avoid any human error.
What should an oil tanker with crude oil washing be provided with?
- This system demands additional equipment and well-qualified manpower to operate them.
- As per the MARPOL Annex-1 Regulation for the prevention of pollution by oil, chapter 4 part-C every oil tanker out at sea with COW system in Operation shall be provided with an operation and equipment manual, COW Manual and International Safety Guide for Oil Tankers and Terminal (ISGOTT)
- Well organized and certified checklist approved by the authorities should be provided onboard an oil tanker.
- Plans and drawing of the system including:
- Crude all washing lines and valves position.
- Cargo pumps lines and valves position.
- Ballast System
- Stripping system
- Slop tank
- Position of tank washing machines.
- Position of holes for hand dipping and tank gauges.
- Inert gas system lines and valves
Advantages of Crude Oil Washing
- This system reduces manpower time and expense in cleaning tanks.
- It has increased the cargo out.
- It helps owners and the companies in earning a good amount of freight as the adequate and promised amount of cargo can be delivered and more loading of the particular cargo can take place.
- As inerting of tanks takes place side by side, it reduces the gas freeing time.
- It reduces sludge accumulation.
- As no sea/fresh water is used during the process there is very little chance of rusting inside the tank.
- Ship’s cargo carrying capacity increases.
Disadvantages of Crude Oil Washing
- This system asks for additional equipment and specialized manpower is required to operate and control the operation.
- Increases the workload onboard.
- Consumes a lot of time at the discharge port.
- Proper and regular maintenance of the equipment as per the manual and checklist have to be done.
- This operation carries the risk of pollution as the COW line pressure is 10-12 bar, therefore any leakage can cause pollution.