(Refer to detailed answer for 6a4bb96f5912bf980df33085 for background and benefits.) COW is the cleaning of cargo tanks on crude oil tankers using the crude oil cargo itself as the washing medium, carried out at pressures of about 1200-1600 kPa through fixed tank washing machines, as required by MARPOL Annex I (Regulation 35) for crude oil tankers of 20,000 DWT and above built after 1982. It enhances operational efficiency because:

• It reduces the oil residues/sludge that would otherwise accumulate, so more of the cargo is discharged and the tank is clean for ballast/next cargo.
• It reduces the volume of slops and oily water requiring treatment (minimises OWS workload and sludge disposal), saving time and cost.
• It enables the tank to be gas-freed and entered sooner, shortening turnaround, and reduces corrosion from seawater washing.
• It recovers valuable cargo rather than wasting it.

Procedure/steps:

1. Preparation: ensure the inert gas system (IGS) is in operation, tank inerted with O2 below 8%, and the vessel at suitable pressure; check the COW manual and the washing machines/fixed piping; cargo pumps ready.
2. Pumping out the tank: as much of the cargo as practical is discharged (tank stripped) so washing covers the surfaces.
3. Washing: the COW machines (jet machines at deck and, where fitted, at the tank bottom/another) are rotated and the high-pressure crude jets wash the bulkheads, deckhead and bottom, dissolving sediment; the washings are collected at the bottom/pump.
4. Stripping: the washings (oil-rich) are pumped to the slop/cargo tanks (or directly to the cargo tank being loaded/another), using the stripping pump or a COW pump; the tank is stripped down.
5. Where multiple tanks, repeat the sequence per tank.
6. After washing, the tank may be gas-freed (purged and ventilated) using the IG/ventilation so entry can be made; the residues in slop tank are kept for discharge (loaded with the next cargo or later ashore).
7. Record the COW operation in the Oil Record Book (Part II), note the order of tanks washed, pressure, times, condition, and the COW Manual's records.

Precautions:

• Only operate with the tank inerted (O2 below 8%) and adequate inert gas pressure, monitoring the pressure and O2; the tank is not opened to atmosphere during w.aspect.
• No personnel may be inside the tank being washed (high-pressure crude blasts, flammable/dangerous atmosphere).
• Follow the approved COW plan and the operations/equipment manual; only use the correct machines and set angles/pressures; do not exceed design to avoid tank damage/washdown; avoid static/electrostatic discharge by keeping the piping/bonding.
• Observe the venting so that over-pressure or vacuum in the tank is prevented; keep the vapour space under inert condition and at the right pressure.
• The pumps/washing machines must be maintained; stop COW if any defect; the Chief Officer is responsible for authorising and supervising; second engineer may assist with the machinery/I build.
• Ensure the slop tanks and discharge comply; no discharge of residues to sea is permitted; all washings remain onboard; the Oil Record Book is completed.
• Personnel wear P/Fire-resistant clothing; emergency COW stop.

Regulatory requirements (MARPOL Annex I):

• COW manual approved by the Administration/RO required; the system must be type-approved (the COW system, machines, fixed pipe) and the arrangement should have been designed to MARPOL.
• Only permitted for certain cargoes/tank types; the plan is to be followed; adequate records.
• The COW system is to be tested on tankers as specified in the manual; COW is complementary with the inert gas system; periodic verification by the RO during surveys.
• It is also relevant to the SIRE/vetting inspection for terminal access.