Crude Oil Washing (COW) — What It Means on Tankers | Maritime Encyclopedia

Crude Oil Washing: What It Means

Every voyage, crude oil leaves behind deposits in cargo tanks — wax, asphaltene sludge, heavy residues that stick to tank walls and accumulate on the tank bottom. Without proper cleaning, these deposits build up and reduce the effective cargo capacity of the ship.

Crude Oil Washing (COW) is the solution: use the cargo itself to clean the tank.

How COW Works

During cargo discharge:

While cargo is being pumped out, fixed tank washing machines are set in operation
Crude oil (from the ship’s own cargo, taken from the discharge line) is pumped at high pressure through the washing machines
Rotating jets spray crude oil against all tank surfaces — sides, bottom, overhead
The crude oil dissolves and flushes away sludge and deposits
The mixture drains to the tank bottom and is pumped out with the rest of the cargo
End result: cleaner tank, more cargo recovered, less waste

COW vs Water Washing

	Crude Oil Washing	Water Washing
Solvent	Crude oil itself	Sea water
Effectiveness	Very high — crude dissolves crude residues	Moderate
Slops generated	Low — oil/sludge pumped out with cargo	High — oily water to dispose
MARPOL requirement	Mandatory on crude tankers 20,000+ DWT	For other tankers and final cleaning
Risk	Fire/explosion if IGS fails	Corrosion if not fully dried

The COW Procedure

Before COW: Ensure IGS is operational, oxygen below 8%
COW in progress: Monitor tank atmosphere, washing machine rotation, oil flow
Complete COW: Drain all washing oil to slop tank
Record in ORB: All COW operations must be logged in Oil Record Book
PSC may inspect: ORB entries for COW are checked against voyage data

What Goes Wrong

IGS failure during COW: Potential explosive atmosphere — stop operations immediately
Washing machine seizure: Incomplete cleaning, potential tank bottom sludge buildup
Slop tank overflow: Failure to monitor slop tank levels during COW
False ORB entries: Recording COW as complete when it wasn’t — MARPOL violation

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Frequently Asked Questions

Why is crude oil washing used instead of water washing on tankers?

Crude oil is a better solvent for the heavy wax and asphaltene deposits that crude cargo leaves on tank walls. COW removes more cargo than water washing (recovering 300-500 tonnes more per washing), produces less waste (less slops to dispose of at port), and is required by MARPOL for crude tankers over 20,000 DWT.

Is crude oil washing mandatory?

Yes. MARPOL Annex I Regulation 33 requires crude oil washing on all crude oil tankers of 20,000 DWT and above built after 1982. Ships must have a COW Operations and Equipment Manual (COEOM) approved by the flag state. COW must be completed on designated tanks before each voyage.

What safety precautions are needed during crude oil washing?

COW is conducted simultaneously with cargo discharge. Key precautions: inert gas system must maintain positive pressure throughout (oxygen below 8% in tanks being washed), COW operations must be recorded in the Oil Record Book, fixed washing machines used (not portable), and responsible officer must monitor the operation continuously.

What happens to the crude oil used for washing?

After COW, the crude oil/sludge mixture is pumped to the slop tank. From the slop tank, oil is pumped to the next cargo or retained for disposal at port. COW significantly reduces the volume of slops compared to water washing — environmental benefit alongside commercial benefit (more cargo recovered).