Oil tankers are primarily cleaned using a technique called Crude Oil Washing (COW).
Crude Oil Washing (COW) Explained
Crude oil washing (COW) is a system whereby oil tanks on a tanker are cleaned out between voyages not with water, but with crude oil - the cargo itself. This method is highly effective because the solvent action of the crude oil makes the cleaning process far more effective than when water is used.
Instead of using water, which can lead to the formation of sludge and require separate disposal, the crude oil cargo is heated and sprayed under high pressure through fixed or portable washing machines mounted inside the tanks. This process is typically carried out during the discharge of the cargo.
Benefits of Crude Oil Washing
Using crude oil for washing offers several advantages:
- Enhanced Cleaning Efficiency: The solvent properties of crude oil effectively dissolve clinging residues from the tank walls, bulkheads, and internal structures.
- Reduced Sludge Accumulation: COW minimizes the amount of residual oil left in the tanks, reducing the need for manual cleaning or water washing later.
- Increased Cargo Recovery: The washed-off residue is mixed with the outgoing cargo, meaning more oil is discharged and less remains as waste.
- Environmental Protection: By reducing the amount of oil residue, less oil needs to be disposed of as slops, minimizing potential environmental impact compared to discharging contaminated water.
- Compliance with Regulations: COW is a standard practice mandated by international conventions like MARPOL to reduce oil pollution from tankers.
The Process
The COW process generally involves:
- Heating the Cargo: The crude oil to be used for washing is heated to optimize its solvent properties.
- Operating Washing Machines: High-pressure jets of crude oil are directed at the internal surfaces of the tank.
- Collecting Washings: The crude oil used for washing, along with the dissolved residues, drains to the bottom of the tank and is discharged with the main cargo.
This integrated approach ensures the tanks are clean and ready for the next cargo with minimal environmental impact and maximum cargo recovery.
