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AI-drawn illustration. Simplified to explain the idea, not to scale.
Slide 1 · CLEAN TANKS • CLEAN SEAS • COMPLIANT SHIPS
CLEAN TANKS • CLEAN SEAS • COMPLIANT SHIPS
CRUDE OIL
WASHING
(COW)
The cargo that cleans.
The rules that protect.
THE RULES BEHIND
SAFE OPERATIONS
Slide 2 · CRUDE OIL
CRUDE OIL
WASHING (COW)
The cargo itself becomes the cleaning agent.
COW cleans cargo tanks using crude oil instead of water. The solvent action of crude oil helps remove oil residues and deposits from tank surfaces during cargo discharge.
ENVIRONMENTAL BENEFITS
Reduces oil residues left inside cargo tanks
Greatly reduces the quantity of wash water compared with conventional water washing
Improves cargo outturn
Helps reduce operational oil pollution
IMPORTANT CORRECTION
COW does not mean zero wash water. IMO notes that a final water rinse is usually carried out, but the quantity is very low.
REGULATORY ANCHOR
MARPOL ANNEX I, REGULATION 33
New crude oil tankers of 20,000 tonnes DWT and above are required to be fitted with a cargo tank cleaning system using COW.
Slide 3 · COW SAFETY
COW SAFETY
CONTROL THE
ATMOSPHERE
The biggest hazard in COW operations is
electrostatic ignition.
01
INERT GAS
O2
Inert gas system must be in
proper operation.
Before washing, oxygen
measurements in the tank
must NOT exceed 8% by
volume at the specified
measuring locations.
During washing, oxygen content
of delivered inert gas must be
continuously monitored.
If IG oxygen exceeds 8%,
or tank pressure is no longer
positive:
STOP COW.
Tank atmosphere
must remain inert
throughout COW.
02
WASHING OIL
To minimise electrostatic generation, use
suitable DRY crude oil as washing fluid.
The tank used as the COW oil source must
first be discharged by at least 1 metre.
A slop tank used during the previous ballast
voyage must be completely discharged and
refilled with dry crude oil before being used
as a washing-fluid source.
WHY?
Water in the washing
fluid can increase
electrostatic
generation during
COW.
SAFETY IS NOT OPTIONAL. | FOLLOW THE MANUAL.
Slide 4 · COW OPERATIONS
COW OPERATIONS
CONTROL
THE OPERATION
COW is not simply "switch on the machines."
01 PRESSURE
Maintain washing pressure within the limits specified in the vessel's approved COW Operations and Equipment Manual.
02 STRIPPING
The washing rate must be coordinated with the stripping system so the tank remains adequately drained.
03 TRIM
Maintain the trim specified in the approved COW programme/manual.
Do not use "4 m by stern" as a universal requirement.
A particular tanker may specify this value, but follow the vessel's approved operating procedure.
04 MONITOR
During COW, monitor:
Washing machine operation
Supply pressure
Cycle times
Oil level during bottom washing
Stripping performance
Inert gas condition
IMO REVISED COW SPECIFICATIONS
Require monitoring of washing-machine operation, supply pressure, cycle times and stripping performance.
Slide 5 · DOCUMENTATION & COMPLIANCE
DOCUMENTATION & COMPLIANCE
IF IT ISN'T
RECORDED,
IT DIDN'T HAPPEN
COW is an environmental protection measure
and a tightly controlled shipboard operation.
COW RECORD
COW OPERATIONS & EQUIPMENT MANUAL
Every tanker operating with a COW system must have an
Operations and Equipment Manual detailing the system
and operational procedures.
The Manual must be to the satisfaction of the Administration.
BEFORE EACH BALLAST VOYAGE
Sufficient cargo tanks must be crude oil washed so that,
considering the ship's trading pattern and expected
weather conditions:
Ballast water is put only into cargo tanks
that have been COW washed.
OIL RECORD BOOK PART II
OIL RECORD BOOK PART II
COW is recorded under MARPOL Annex I, Regulation 36.
RECORDS INCLUDE:
Cargo tank cleaning including crude oil washing
Slop-tank operations
Ballasting of cargo tanks
Disposal of residues
Discharge of applicable ballast
Entries must be completed without delay in the ORB.
Signed by the responsible officer and countersigned by the Master.
The ORB must be retained for 3 years after the last entry.
COMPLY • CONTROL • RECORD
Protect the environment. Protect your ship. Protect your career.
Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.
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