Crude oil washing (COW) is a method of cleaning cargo tanks in oil tankers using the crude oil itself as the cleaning medium, mandated by MARPOL Annex I (Regulation 35) for crude oil tankers above 20,000 DWT delivered after 1 June 1982 (for the larger new tankers) and required to be installed with an approved COW system. During COW, high-pressure crude oil (typically 1200-1600 kPa) is directed through fixed deck washing machines and the tank-bottom-oriented nozzles to wash the tank walls and bottom, dissolving and dislodging the waxy and viscous residues and sludges so they are recovered as part of the cargo rather than discharged.

Environmental benefits compared to traditional (water-washing) methods:

• Reduces the amount of oily water and sludge generated: because the washing medium is the cargo (oil) which is subsequently pumped out as cargo, very little oily water is left to be handled. Traditional water washing produced large quantities of oily ballast and wash-water that had to be retained, treated in OWS, and often discharged, risking pollution.
• Almost eliminates the discharge of oil (residues) to sea: the recovered residues are sold/offloaded with the crude, so less oil goes into the slop/retention tanks and the Oil Record Book shows minimal discharge.
• Reduces the volume of slops/slop tanks and oily bilge water requiring treatment in OWS, drastically lowering the risk of operational oil pollution and cutting the operability/energy cost of OWS.
• Better quality of retained residues (cargo) enabling recovery of cargo that would otherwise be lost as sludge; thus more cargo is discharged and less "dead" residue remains onboard.
• Reduces corrosion from salt-water ballast/tank washing? - the use of crude rather than water reduces both the need for seawater washing and reduces chloride-induced corrosion of tank steel.
• Complies with MARPOL and allows the ship to leave port without carrying large amounts of water-washing sludge, minimising risk of discharge and supporting "no discharge" operations of oily residues. It is also synergistic with mandatory inerting and segregation.

• Flammable atmosphere/explosion risk: COW is only safe when the tank is inerted (inert gas system operating), because the high-pressure jet atomising crude into fine droplets creates a mist within the flammable range. COW must not be carried out unless the tank is inert (O2 below 8%) and the pressure adequate; armed blanketing must be maintained; falls to atmosphere must be prevented by the inert gas plant and the tank pressure controlled. The COW plan must be approved and the Chief Officer/Second Engineer must ensure inert gas is supplied and the vapour space not allowed to become flammable.
• Personnel safety: no personnel shall be inside the tank being washed without breathing protection/entry procedure; the tank must be gas-freed and safe before entry (enclosed space entry - oxygen, LFL, toxic). Avoid exposure to high-pressure jets which can cause severe burns/injuries; correct PPE (face shields, flame-resistant, gloves, boots).
• Equipment/structural integrity: high-pressure jets must comply with the COW plan; use correct nozzle angles and pressures to avoid tank damage/over-pressurisation; the washing machines and fixed piping must be maintained; ensure the tank slop/discharge lines and pumps are safe; avoid operating washing against a closed/blocked system causing pressure build-up; monitor tank pressure and vapour.
• Spark/static risk: ensure all washing and flow generates no electrostatic discharge; keep the tank bonding/earthing, avoid conditions causing excess charging (e.g. not washing with metal-to-metal? keep the piping bonded); the COW machine and fixed parts are earthed.
• Regulatory requirements (MARPOL Reg 35 and Regulation 13F of Annex I; the COW system and its Operations and Equipment Manual (COW Manual) approved; the COW system is to be tested periodically; only appropriate cargoes may be washed by COW; strict record in Oil Record Book, ensuring adequate vapour space, and calling for the approval of the plan by Administration). The IMO "COW - Operations and Equipment" guidelines (MEPC) require following the manual.
• Environmental regulation: COW is done within the framework of the 1/15,000 and 1/30,000 oil content rules; any discharge of oil residues is prohibited; the washings (cargo) are recovered, and the tank washing via COW is recorded and closely monitored (ODMCS).