Marine Tank Cleaning and Crude Oil Washing

Crude oil washing (COW) and the associated tank-cleaning programme are among the most regulation-dense daily operations on an oil tanker. They sit at the intersection of MARPOL Annex I pollution prevention, SOLAS Chapter II-2 fire safety, cargo outturn economics, and confined-space crew safety. Getting any one of those dimensions wrong creates consequences that range from port-state detention to catastrophic tank fires. This article covers the full operational picture: the regulatory basis under MARPOL Annex I Regulations 29, 33, and 34; the inert gas preconditions from SOLAS; the mechanics of fixed and portable washing machines; the COW programme itself (top wash, bottom wash, single-stage, multi-stage); slop tank handling and the load-on-top (LOT) process; the ODME and the 1/30,000 discharge limit; water washing for gas-freeing and pre-drydock tank preparation; and the crew safety rules that govern every phase. For a paired computational tool covering COW cycle time and machine coverage, see the COW efficiency calculator and the tanker crude COW time calculator.

Regulatory framework

MARPOL Annex I: the primary instrument

MARPOL Annex I is divided into chapters and regulations. The regulations directly governing tank cleaning on crude oil tankers are 29, 33, and 34.

Regulation 29 (slop tanks) requires every oil tanker of 150 gross tonnage and above to carry slop tanks capable of collecting tank drainings, tank washings, and other oily mixtures. The minimum aggregate capacity is 3% of total oil-carrying capacity. That floor can drop to 2% where tank washing arrangements recycle water within the system without introducing additional make-up water, or where segregated ballast tanks or a certified COW system is fitted. For combination carriers with smooth tank walls and advanced stripping, the capacity may fall as low as 0.8%. The slop tanks must include facilities for the oil/water interface to be measured reliably, since LOT depends on accurate knowledge of where the oil layer ends and the dischargeable water layer begins.

Regulation 33 (crude oil washing requirements) is the core COW mandate. It requires every crude oil tanker of 20,000 DWT and above, delivered after 1 June 1982, to be fitted with a cargo tank cleaning system using crude oil washing. The regulation incorporates by reference the “Specifications for the Design, Operation and Control of Crude Oil Washing Systems” adopted by the IMO in 1978 under Resolution A.446(XI), subsequently amended by Resolution A.497(XII) and then revised by Resolution A.897(21), adopted on 25 November 1999. Regulation 33 also requires every such tanker to carry an approved Operations and Equipment Manual (OEM) whose standard format is prescribed in Resolution MEPC.3(XII) as amended by Resolution MEPC.81(43), adopted 1 July 1999. The OEM must be approved by the Administration (flag state) and specify the system design, pump and piping data, machine coverage matrices, the programmed COW schedule for each tank, and the oxygen-content verification procedure. Port-state control inspectors board specifically to check the OEM against observed operations.

Regulation 34 (control of discharge of oil from cargo areas) sets the limits that govern what the ODME can lawfully discharge overboard during tank washing. The key thresholds for tankers delivered after 31 December 1979 are:

• Instantaneous discharge rate: must not exceed 30 litres per nautical mile
• Total oil discharged: must not exceed 1/30,000 of the total quantity of the particular cargo on the voyage in question
• Position: more than 50 nautical miles from the nearest land
• Mode: ship proceeding en route (not at anchor or in port)
• Equipment: the ODME and slop tank arrangement required under Regulations 29 and 31 must both be operational

Any discharge inside a Special Area (Mediterranean Sea, Baltic Sea, Black Sea, Red Sea, Gulfs Area, Gulf of Aden, Antarctic Area, North West European Waters, Oman Area of Arabian Sea, and Southern South African Waters) from the cargo area is completely prohibited except for clean or segregated ballast.

SOLAS Chapter II-2: fire safety and the inert gas precondition

SOLAS Chapter II-2, Regulation 4 requires every crude oil tanker fitted with COW to also carry a fixed inert gas system complying with the Fire Safety Systems (FSS) Code. The system must be capable of delivering inert gas with an oxygen content of no more than 5% by volume in the IG main at any required flow rate. Inside the tanks, the system must maintain an oxygen content of no more than 8% by volume in any tank that is not in the process of being gas-freed. That 8% figure is the hard precondition for COW: no tank may be subjected to crude oil washing unless its oxygen content is verified at or below 8% by volume. The IGS plant’s output limit of 5% provides a safety margin so that even after mixing with residual air inside the tank, the tank atmosphere stays below the 8% threshold. The marine inert gas systems article covers IGS design, scrubbers, deck water seals, and monitoring in detail; that content is not duplicated here.

IMO Resolution A.897(21) and the COW specifications

Resolution A.897(21) is the current revision of the COW design specifications. It addresses minimum machine throughput, coverage requirements, pump arrangements, piping sizing, and the design of the OEM. Under these specifications, the COW system must be capable of washing every cargo tank in the ship within the time normally available during a laden voyage of normal length for that trade. Each tank must receive a complete wash programme that covers all internal surfaces not shielded by permanent structures. The resolution also sets requirements for the materials used in COW machines and lines (compatible with crude oil and rated for the operating pressures involved, typically 8 to 10 bar) and for the interlock that prevents COW on any tank where the oxygen reading has not been confirmed below 8%.

Oil Record Book Part I

Every oil tanker of 150 GRT and above must maintain an Oil Record Book (ORB) Part I, recording in code all cargo operations including COW, slop tank transfers, overboard discharges through the ODME, and receipts at shore reception facilities. MARPOL Annex I Appendix III prescribes the code structure. Failure to maintain accurate ORB entries is an offence under national implementing legislation in every flag state and is a primary focus of port-state control inspections. The ORB must be available on board for at least three years and produced to any MARPOL party’s inspector on request.

Why crude oil washing exists: the pollution and commercial case

Before COW, crude tankers discharged enormous quantities of oil into the sea through the tank-washing process. The 1973 MARPOL Convention and its 1978 Protocol were adopted partly in response to documented contamination of coastlines and fishing grounds from tanker washings. The engineering argument for COW is rooted in the chemistry of crude oil: crude oil dissolves crude oil residue far more effectively than water does. When warm crude oil is sprayed at high velocity against tank walls, it strips off the waxy paraffinic and asphaltenic deposits that accumulate during the laden voyage, suspending them back into the cargo stream for discharge with the main cargo rather than leaving them as stranded residue.

The practical effect on cargo outturn is measurable. Without COW, a large crude cargo tank may retain 100 to 200 m3 of clingage and sludge on its walls, floors, and structural members after cargo discharge. After a properly conducted COW programme, the residue in the same tank falls to 5 to 15 m3. On a VLCC with 20 to 30 cargo tanks, that difference totals several thousand tonnes of recoverable cargo per voyage. For a tanker on a typical Persian Gulf to Japan trade, even 2,000 tonnes of recovered cargo at $60/bbl crude adds roughly$840,000 per voyage to outturn value, before savings on slop disposal costs.

COW also reduces the volume of oily slops that would otherwise require expensive shore reception or onboard treatment. Where water washing produces tens of thousands of cubic metres of contaminated washwater that must be held in slop tanks and treated or landed ashore, COW produces only a small water rinse at the end of the programme. This simultaneously reduces MARPOL compliance risk (less dirty water to dispose of) and bunker consumption (less energy needed to heat wash water).

Crude oil washing machines: fixed and portable

The tank washing machine is the delivery mechanism for both COW and water washing. Two broad categories exist: fixed-installed machines permanently mounted in the tank and connected to the ship’s COW or wash-water ring main, and portable machines lowered through deck openings on hoses for one-off water washes.

Fixed machines on crude tankers are typically multi-rotation programmable machines. They combine vertical-axis rotation with horizontal-axis rotation so that the spray jet covers a full sphere over one programme cycle. Older single-nozzle designs required multiple drops, meaning the machine was physically repositioned to different heights to cover the tank from top to bottom. Modern twin-nozzle and programmable machines cover the entire tank from a single position in one continuous cycle. Cycle time for a typical large crude tank using a fixed machine with an inlet flow of 180 to 250 m3/hr at 8 to 10 bar (roughly 800 to 1,000 kPa) runs 60 to 90 minutes for a full coverage programme. Machine coverage range varies by model: the Butterworth SSK series achieves a rated cleaning range of 25 metres with a throughput of 75 m3/hr; larger proprietary COW machines on VLCCs may deliver 200 m3/hr or more at similar pressures.

Portable machines (often referred to by the trade name Butterworth, derived from the original manufacturer) are lowered through bell-mouth deck openings on dedicated hose connections. They are used for water washing when the fixed COW machines are being serviced, for tanks not served by the fixed ring main, and for pre-docking water washes where thorough coverage of all internal surfaces is needed regardless of the COW pattern. A portable machine on a 25 m deep tank is typically used in two or three drops at 8- to 10-metre intervals, with each drop running for 30 to 60 minutes depending on machine capacity and wash pressure.

The cargo tank cleaning time calculator and cargo tank cleaning water volume calculator provide tools for estimating programme durations and water consumption based on tank dimensions and machine throughput.

The COW programme: top wash, bottom wash, and staging

The COW programme is defined in the ship’s OEM, which prescribes a specific sequence of tanks to be washed on each voyage. Not every tank is washed on every voyage. MARPOL Annex I, via the COW specifications, requires that each tank be COW’d at regular intervals sufficient to control sludge accumulation; in practice, most operators schedule each tank for a full COW once every one to three voyages depending on crude grade carried and sludge accumulation rates. Heavy paraffinic crudes deposit wax much faster than light sweet crudes, so OEM schedules for West African or Venezuelan crude trades differ substantially from those for Arabian Gulf sour crudes.

A COW programme has two distinct phases: the top wash and the bottom wash.

Top wash begins when the oil level in the tank drops to approximately 3 metres above the tank bottom during normal cargo discharge. At that point, the upper tank surfaces (deck plating, deckhead beams, deckhead web frames, longitudinals, and upper portions of transverse bulkheads) have been exposed above the receding oil surface. The COW machine is operated from its fixed position to spray crude oil upward and across these exposed surfaces, dissolving deposited wax and asphaltene clingage. The spray mixture falls back into the cargo below and is discharged with it. Top wash continues until the oil level has dropped below the lowest effective spray reach of the machine at that drop position, typically 1.5 metres above the tank floor.

Bottom wash commences after the main cargo has been stripped from the tank to the lowest point achievable by the cargo pump (the “rough stripping” end-point). At this stage only the structural floor, lower frames, and bottom longitudinals carry residue. The COW machine is operated at its lowest effective angle to deliver high-velocity crude oil across the floor plating and bottom structure. The dissolved sludge-crude mixture is collected by the stripping pump and directed either to another cargo tank still being discharged or directly to a slop tank. Bottom washing is carried out more slowly and carefully than the top wash because the sludge layer on the tank floor is denser and requires more dwell time for the solvent action to work. An experienced cargo officer watching the stripping suction will see the visual cues in pump discharge pressure when the bottom residue is adequately dissolved and flowing freely.

Single-stage COW combines both top and bottom washing into a continuous programme run from a single machine position without repositioning between phases. Modern fixed programmable machines can be set to execute a combined programme automatically. Single-stage COW is faster but may leave light coverage gaps in the transition zone between the top-wash arc and the bottom-wash arc, typically the tank sidewall area between 3 m and 1.5 m from the tank bottom. For high-wax crudes, two-stage or three-stage programmes are preferred.

Three-stage COW adds a mid-deck or mid-height wash pass to address the transition zone. Three-stage programmes add roughly 25 to 40 minutes per tank but recover better outturn on difficult cargoes and are the norm on VLCCs carrying Arabian Heavy or Venezuelan Merey.

The COW tank cycle calculator and tanker crude oil washing cycle calculator allow operators to model programme schedules, machine allocation, and total COW time against discharge port stay windows.

Inert gas atmosphere: the non-negotiable precondition

No COW operation may commence unless the oxygen content in the tank to be washed has been confirmed at or below 8% by volume. This rule is not a guideline: it is a hard requirement in both SOLAS Chapter II-2 and the IMO COW specifications. The reason is that crude oil vapour mixed with air at oxygen concentrations above 11% can form an explosive atmosphere. At 8% oxygen, the mixture is below the minimum oxygen concentration (MOC) for sustained combustion of hydrocarbon vapour, which for most crude oil vapour mixtures is approximately 11% oxygen. The 8% threshold provides a 3-percentage-point safety margin against the MOC.

In practice, cargo tanks on loaded tankers are maintained in an inerted condition from the time loading begins until the tank is gas-freed for inspection or repair. The IG plant delivers flue gas or nitrogen with an oxygen content below 5% continuously to maintain positive pressure in the tank and keep the tank atmosphere non-flammable. Before COW, the duty officer verifies the oxygen reading on the fixed atmosphere monitoring system and records it in the ORB. If the reading exceeds 8%, COW is suspended until the IG plant restores the atmosphere. The marine inert gas systems article covers the design and operational certification of IGS in detail.

Static electricity generation is a secondary but serious hazard during COW. When crude oil is sprayed at high velocity inside a steel tank, it generates electrostatic charge. If that charge builds to sufficient potential and finds a discharge path, a static spark can ignite hydrocarbon vapour. The operational countermeasures are: maintaining inert atmosphere (so there is no flammable oxygen concentration regardless of sparks), ensuring the tanker is properly bonded to no shore metallic structures during COW, and prohibiting the use of ullage sounding equipment or metallic tools through tank openings during active washing and for a minimum period of one hour after washing stops when artificial ventilation is used, or five hours when natural ventilation only is relied on, per ISGOTT 6th edition guidance.

Hydrogen sulphide (H2S) is present in sour crude vapour at concentrations that can reach 100 ppm or more in tank atmospheres. H2S is detectable by smell at 0.5 ppm, but olfactory fatigue at 100 ppm makes smell an unreliable detector. The threshold limit value (TLV) ceiling for H2S in occupational exposure is 5 ppm per ACGIH guidance. Personnel involved in any deck operations during COW on sour crude trades must wear continuous personal gas monitors set to alarm at 5 ppm.

Slop tank management and the load-on-top procedure

Slop tank capacity and configuration

Regulation 29 of MARPOL Annex I sets the minimum slop tank capacity at 3% of total oil-carrying capacity, with reductions to 2% where COW is fitted. Large crude tankers typically carry two slop tanks (port and starboard slop) each sized to hold 1 to 1.5% of cargo capacity, totalling 2 to 3% combined. A VLCC with 330,000 DWT and a cargo capacity of roughly 310,000 m3 would therefore carry slop tanks totalling 6,200 to 9,300 m3. Slop tanks are fitted with heating coils, because oil-water emulsions separate much faster at 35 to 50°C than at ambient seawater temperatures. They also carry oil/water interface detectors (typically guided-wave radar or float-and-tape gauges graduated to detect oil/water interface position as well as total liquid level) to allow the deck officer to determine how much of the slop volume is the dischargeable aqueous lower phase and how much is the retained oil upper layer.

The load-on-top procedure

LOT was developed in the early 1960s by Shell as an alternative to landing all tank washings ashore at every port. The logic is straightforward: if you allow sufficient settling time for oil and water to separate by gravity in the slop tank, you can discharge the clean water layer through the ODME and retain only the oil layer, then load the next cargo on top of that retained oil. The procedure works because crude oil and seawater have densities of approximately 860 to 920 kg/m3 and 1,025 kg/m3 respectively, giving a stable gravity separation.

The LOT sequence in practice:

1. After cargo discharge, the slop tank receives all tank washings and oily ballast water transferred from cargo tanks during the laden voyage.
2. During the ballast passage, the slop contents are heated and allowed to settle. Separation times vary by crude grade and temperature; light crudes at 40°C may reach a clean interface in 12 to 24 hours, while heavy asphaltic crudes may require 48 to 72 hours even with heating.
3. The deck officer monitors the oil/water interface level daily and records it in the ORB.
4. When the interface stabilises and the lower water layer tests below the ODME discharge limit (30 litres per nautical mile rate and within the 1/30,000 total quantity limit), the ODME is started and the lower phase is pumped overboard via the ODME with the automatic overboard valve controlled by the ODME computer.
5. The retained oil layer (the “slops”) remains in the tank. On arrival at the next loading port, the slops are retained and the new cargo is loaded on top of them, combining with the slops in the normal product stream.

LOT requires careful ORB documentation: every transfer of dirty ballast into slop tanks, every interface measurement, and every overboard discharge through the ODME must be logged with the time, ship’s position, quantity, and ODME reading. Port-state control officers who find discrepancies between the ORB and ODME data records (most modern ODME systems store a 12-month printable log) treat the mismatch as evidence of illegal discharge.

ODME: the 1/30,000 limit in operational terms

The Oil Discharge Monitoring and Control System (ODME or ODMCS) is the instrument that makes lawful overboard discharge under Regulation 34 technically possible. IMO Resolution MEPC.108(49), adopted 18 July 2003 (effective 1 January 2005 for new installations), specifies the performance requirements for approved ODME systems. Every approved system must include:

• An oil content meter (OCM) measuring the oil-in-water concentration in ppm, typically using light-scattering or UV fluorescence principles, with a range from 0 to 1,000 ppm
• A flow meter measuring instantaneous volumetric discharge rate in m3/hr
• A GPS or equivalent speed input providing ship’s speed over the ground in knots
• A computing unit that calculates instantaneous discharge rate in litres per nautical mile and running total volume discharged as a fraction of the cargo quantity loaded
• An automatic overboard valve that opens when discharge is within limits and closes (and diverts the flow to the slop tank) when any limit is exceeded
• A data recorder that stores at least 12 months of operational data in tamper-evident format

Resolution MEPC.240(65) (force from January 2016) amended MEPC.108(49) to address the specific optical characteristics of biofuel blends, which cause standard light-scatter OCMs to misread concentration. Tankers regularly carrying biofuel cargoes must have OCMs verified against the MEPC.240(65) specification.

The 1/30,000 limit in practice: a VLCC loading 250,000 m3 of Arabian Light crude can discharge at most 250,000/30,000 = 8.33 m3 of oil total in wash water over the entire ballast voyage. At 30 litres per nautical mile, that 8,330 litres of allowable total discharge is used up in about 277 nautical miles of discharge at continuous maximum rate. In reality, discharge happens intermittently when the slop water layer is clean enough to meet both instantaneous rate and total quantity limits simultaneously, which across a 10,000-mile ballast voyage means the ODME operates for a fraction of the transit.

Failure of the ODME in service: Regulation 34 allows discharge only while the ODME and slop tank arrangement are operational. If the ODME fails at sea, all overboard discharge from the cargo area must stop. The slop tank must be used until either ODME repair is completed or the vessel reaches a reception facility. The failure must be noted in the ORB and reported to the flag administration. Port-state inspectors encountering a failed ODME with an unaccountably low slop tank volume will request ODME printout records and cross-check against the ORB.

Water washing: gas-freeing and pre-docking preparation

COW is the primary tool for cargo residue removal, but it does not make a tank safe for crew entry or hot work. For those purposes, water washing followed by inert gas purging and forced-air gas freeing is required.

Why water washing is needed after COW

After COW, the tank atmosphere is hydrocarbon-rich: crude oil vapour concentrations may be at or near the saturation point for the prevailing temperature. If the tank is to be entered for inspection, or if the ship is proceeding to dry dock where gas-cutting and welding will be performed, the tank must first be freed of flammable gas to below 1% of the lower flammable limit (LFL). Water washing serves two functions in this sequence: it removes residual oil films from tank surfaces that would otherwise off-gas hydrocarbon vapour during ventilation and slow the gas-freeing process, and it cleans the tank to a standard that allows visual inspection of structural condition.

The inert-purge-gas-free sequence

The transition from a COW-completed inerted tank to a gas-free tank safe for entry follows a mandatory sequence:

Step 1: Water wash. The tank is water-washed with seawater (cold or heated to 60-70°C for better residue removal) using fixed or portable machines. The washwater is collected by the stripping system and directed to the slop tank. Water washing continues until the tank surfaces are visually clean of oil residue where that can be assessed from deck openings.

Step 2: Inert gas purge. After water washing, the tank atmosphere still contains hydrocarbon vapour at or above the LFL. Forced-air ventilation at this stage would create a flammable mixture as oxygen rises through the flammable envelope (LFL to UFL). Instead, the tank is purged with inert gas from the IGS plant (oxygen below 5%) while the hydrocarbon vapour is displaced out through the tank vents. Purging continues until the hydrocarbon content as measured by explosimeter at the vent outlet drops below 2% by volume (approximately 40% of the LFL for methane). Below 2% hydrocarbons, the mixture cannot form a flammable envelope even as oxygen rises.

Step 3: Gas freeing. Once hydrocarbon content is confirmed below 2%, forced-air ventilation begins. Air fans or the ship’s portable ventilation fans introduce fresh air through tank access hatches while the tank atmosphere is monitored continuously. Gas freeing continues until oxygen is confirmed at 20.8 to 21% (atmospheric) and hydrocarbon vapour is confirmed below 1% LFL at all sampling points in the tank. Only at that point may crew enter the tank as a confined space, under the confined space entry procedure required by SOLAS and detailed in the ship’s safety management system. The marine confined space entry and tank inspection article covers the full entry procedure, gas testing, standby arrangements, and the emergency response requirements.

H2S readings must be below TLV (typically 5 ppm occupational exposure limit) before entry. If benzene is present (a concern with light condensate or gasoline-range products), benzene levels must also be below 0.1 ppm TWA (8-hour occupational exposure limit per ACGIH). Tanks that have carried sour crudes must be treated as H2S-contaminated until proven otherwise.

Pre-docking full water wash

Before a tanker enters dry dock for periodic class survey, every cargo tank, pump room, cofferdams, pipe tunnel, and other spaces in the cargo area must be water-washed, inert-purged, and gas-freed to the standard described above. Class surveys on oil tankers include internal structural inspection of all cargo tanks under the Enhanced Survey Programme (ESP), which requires surveyors and dockyard workers to enter the tanks. The ESP is mandatory under SOLAS Regulation I/10 for tankers over 10 years old, and the parallel MARPOL Annex I Condition Assessment Scheme (CAS, formerly Regulation 13G) requires category 1 and 2 single-hull tankers to complete enhanced thickness measurements and close-up surveys. Both programmes require fully gas-free, clean tanks.

Full pre-docking water washing is substantially more thorough than the routine water wash done at sea after COW. Tanks are washed multiple times, with attention to structural crevices, stiffener toes, web frame brackets, and the bottoms of structural pockets where water accumulates and promotes corrosion. The marine cargo pumps and piping article covers pump room preparation for dry-docking, which runs in parallel with cargo tank preparation.

Tank cleaning safety: the complete hazard picture

Atmosphere monitoring during COW

SOLAS Chapter II-2 requires continuous atmosphere monitoring in cargo tanks during COW. Fixed oxygen analysers sample the tank atmosphere at multiple points (typically deck level, mid-height, and near the bottom through fixed sampling lines) and display readings in the cargo control room. Portable multi-gas detectors supplement the fixed system, particularly for bottom wash when the stripping pump operator works near deck openings.

The monitoring matrix during active COW:

Any reading outside these parameters requires immediate suspension of COW, investigation of the cause, and documentation in the logbook.

Static electricity: the three-hour rule

Static electricity generation during liquid spraying inside tanks is a function of the spray velocity, droplet size, conductivity of the liquid, and turbulence. Crude oil’s low electrical conductivity means charge dissipation is slow. The ISGOTT 6th edition guidance requires that after COW ceases, no metal objects (sounding tapes, metal rods, electronic sensors on conductive cables) be introduced through deck openings for a minimum of one hour where artificial ventilation is in use, or five hours with natural ventilation only. This waiting period allows charge to dissipate through the liquid surface to the tank structure before personnel or instruments are exposed to potential discharge paths. The rule applies even though the tank remains inerted, because the presence of a static arc could still produce a localised temperature excursion at the arc point.

Confined space entry: the absolute rules

Cargo tanks under inert gas are NOT confined spaces that can be entered. An inert-gas atmosphere with 8% or less oxygen is immediately dangerous to life: unconsciousness occurs within one breath at 6% oxygen, and death follows in minutes. Tanks in an inerted condition must be treated the same as tanks with a toxic or flammable atmosphere: no entry under any circumstances without self-contained breathing apparatus and an approved rescue plan in place, and not even then unless entry is genuinely unavoidable for emergency reasons.

The sequence inert gas, then purge, then gas-free, then entry is non-negotiable. Bypassing any step in that sequence has caused multiple fatalities on tankers over the history of the industry. The marine confined space entry and tank inspection article documents the permit-to-work system, rescue equipment requirements, and the regulatory basis under SOLAS Regulation XI-1/7 and the ISM Code.

Comparison: COW versus water washing

The two methods serve different purposes and are used at different points in the voyage cycle. They are complementary, not competing.

The Oil Record Book: documentation as a compliance instrument

The ORB Part I is the primary document through which MARPOL Annex I compliance is demonstrated to flag and port-state control authorities. Every operation related to cargo, tank cleaning, and slop management must be recorded promptly. Under-recording is treated as concealment; over-erasing or correction fluid on ORB entries is treated as falsification. The required entries relevant to COW and tank cleaning include:

• Loading of cargo (Code B): quantity loaded, ship’s position, date and time
• Internal transfer of cargo/ballast (Code C): tanks involved, quantities, start/stop times
• Commencement of discharge (Code D)
• Crude oil washing (Code G): tanks washed, machines used, COW start and stop times, oxygen readings before commencement, and confirmation of OEM compliance
• Collecting and transferring dirty ballast, slop tank filling (Code H)
• Discharging dirty ballast or cleaning water (Code I): ODME readings, overboard valve status, ship’s position, quantities
• Reception at shore facilities (Code K): quantity, facility name, date

The marine oily water separators and bilge water treatment article covers ORB Part II entries for machinery space bilge operations, which run under a separate but parallel documentation system.

Practical limitations

COW works only on crude oil grades that remain pumpable. Very waxy crudes with high pour points (above 40°C) can congeal in the COW piping and machine internals at sea-temperature ambient. Ships on these trades use cargo tank heating systems to keep cargo above pour point during discharge, but the COW crude supply must also be kept heated to maintain flowability. COW is not practicable for waxy crudes unless the ship’s heating system maintains cargo temperature throughout the wash cycle.

COW doesn’t remove all sludge. Heavy asphaltic sludge that has accumulated over multiple voyages without adequate COW or that results from crude/water emulsification in the bottom of tanks requires mechanical cleaning or chemical treatment during dry dock, not addressable by COW alone.

Water washing efficiency drops with cold seawater. In winter North Sea or North Atlantic trades, seawater temperatures of 2 to 8°C make wax removal from tank surfaces marginal without heating. The difference between a cold-water wash and a 65°C hot-water wash for a paraffinic North Sea crude can be the difference between adequate cleaning and tanks that still require further washing cycles at dry dock.

The 1/30,000 limit is a lifetime-of-cargo limit, not a per-voyage limit. If the ODME record shows that the allowable total discharge for a particular cargo was exhausted in mid-voyage, no further overboard discharge from that cargo’s wash water is permitted, regardless of how far from land the ship is. Slop tanks must absorb all remaining washwater until arrival at port or until the next cargo creates a new 1/30,000 allowance.

ODME accuracy degrades with emulsified oil. If washwater contains oil in stable emulsion (common with certain crude grades or after chemical contamination of wash water), standard light-scatter OCMs underread the true oil content. Crews relying solely on the ODME reading in emulsified conditions risk illegal discharge. Where emulsification is suspected, visual inspection of the ODME sample and cross-check against the interface level drop in the slop tank are prudent additional checks.

COW programme compliance is verifiable by port-state control. The COW OEM specifies exactly which tanks must be washed, by which machines, over what time period. The IG system records and ODME logs provide independent corroboration. A port-state officer who compares the ORB COW entries against the ODME data log, the IG system event log, and the OEM schedule can determine within a reasonable margin whether the stated COW was actually performed. Ships that record COW compliance in the ORB without running the COW plant (a known fraudulent practice) leave a detectable signature: no ODME activity, no IG consumption change, and slop tank levels inconsistent with the claimed wash volumes.

See also

• Marine Inert Gas Systems (IGS design, scrubbers, deck water seals, SOLAS certification)
• Marine Confined Space Entry and Tank Inspection (permit to work, gas testing, standby, ISM requirements)
• Marine Cargo Pumps and Piping (cargo pump selection, stripping systems, piping design)
• Marine Cargo Tank Heating Systems (steam and hot-water heating coils for high-viscosity cargoes)
• Marine Oily Water Separators and Bilge Water Treatment (OWS 15 ppm bilge alarm, ORB Part II)
• MARPOL Annex I (full Annex coverage including SBT, double hulls, special areas)
• Oil Tanker (tanker types, VLCC/Suezmax/Aframax classification)
• Chemical Tanker (Annex II prewash requirements, IBC Code)
• IBC Code (chemical tanker design and cargo compatibility standards)
• COW Efficiency Calculator
• COW Tank Cycle Calculator
• Tanker Crude COW Time Calculator
• Tank Cleaning Time Calculator
• Tank Cleaning Water Volume Calculator
• Tanker Crude Oil Washing Cycle Calculator
• Tanker Tank Cleaning Crude Wash Calculator
• COW Bottom Wash Calculator
• COW Top Wash Calculator
• Tank Cleaning Displacement Calculator
• Tank Cleaning Dilution Calculator