MARPOL Annex I Reg 33: crude oil washing

Background: 1978 Protocol and the move from water to crude washing

Before MARPOL, the standard practice for cleaning crude oil cargo tanks between voyages was water washing. Sea water was sprayed through tank-cleaning machines to dislodge crude residue from internal tank surfaces. The resulting oily mixture was pumped to slop tanks, allowed to separate by gravity, and the underlying water phase discharged overboard once the surface oil had been decanted. The procedure produced very large quantities of oily slops, often 5 to 8 percent of cargo tank volume per voyage, which were operationally costly to manage and which (historically) often resulted in deliberate or accidental sea discharges of substantial quantities of oil before the Load on Top (LOT) procedure was widely adopted in the 1960s.

The intellectual breakthrough behind COW, developed in the early 1970s by industry and tested on a number of large crude carriers from 1973 onwards, was that the cargo crude itself is an excellent solvent for its own residues. Crude oil applied at high pressure to tank internals dissolves and re-suspends waxy and asphaltenic deposits far more effectively than water, and the dissolved residue simply joins the cargo at discharge rather than entering a slop tank. A vessel that cleans its tanks with crude during the cargo discharge sequence, instead of with water on the ballast leg, generates far smaller slops and substantially reduces the gross oil burden on the marine environment.

The 1978 Protocol to MARPOL, adopted in February 1978 partly in response to the Argo Merchant (1976) and Amoco Cadiz (1978) casualties, made COW (or Segregated Ballast Tanks (SBT), or both) mandatory for new and existing crude oil tankers above defined size thresholds. The Protocol entered into force on 2 October 1983 alongside the modified parent convention, creating MARPOL 73/78. The original technical specification for COW was Resolution 15 of the 1978 TSPP Conference (the Conference of Parties to the MARPOL Protocol on Tanker Safety and Pollution Prevention). Regulation 33 in the current consolidated text reflects this lineage and the subsequent technical refinements through MEPC and Assembly Resolutions.

Regulation 33 scope: 20,000 DWT and above

Regulation 33.1 applies to every crude oil tanker of 20,000 tonnes deadweight and above delivered after 1 June 1982 (a “new” tanker in the Annex I delivery-date taxonomy). The regulation requires such a vessel to be:

• fitted with a Crude Oil Washing system approved by the Administration of the flag State;
• operated in accordance with a COW Operational Manual approved by the Administration; and
• equipped with an Inert Gas System covering all cargo tanks and slop tanks in which COW is intended to take place.

Existing crude oil tankers of 40,000 DWT and above (delivered on or before 1 June 1982) were brought within the regime by Regulation 33.2, with a transitional period for fitting COW or for proving compliance through alternative means.

The 20,000 DWT threshold for new tankers reflects the engineering and economic balance struck in the late 1970s. Smaller crude carriers below 20,000 DWT typically have shorter cargo voyages, smaller individual cargo tanks, simpler cargo systems and lower absolute oil flux, and the marginal environmental benefit of mandatory COW would not have justified the capital cost of fixed washing machines, the IGS prerequisite, and the additional class survey burden. The 20,000 DWT line divides the global crude tanker fleet into a regulated population: Aframax (80,000 to 120,000 DWT), Suezmax (120,000 to 200,000 DWT) and VLCC (200,000 to 320,000 DWT) on one side, and a smaller segment to which Regulation 33 does not apply on the other, although IGS and SBT requirements may still apply to those smaller vessels under Regulations 5, 16 and adjacent provisions.

A “crude oil tanker” for Regulation 33 purposes means an oil tanker engaged in the trade of carrying crude oil. Product tankers carrying refined products such as gas oil, kerosene or naphtha are addressed under separate provisions and do not require COW because the cargo cleaning challenge is fundamentally different.

The COW Specifications and the COW manual standard format: two distinct instruments

Two separate IMO instrument streams govern crude oil washing, and they are routinely conflated. Keeping them apart matters for plan approval and port State control, because each carries a different identifier and a different scope.

The Revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems are Assembly Resolution A.446(XI), adopted 15 November 1979, as amended by Resolution A.497(XII) (19 November 1981) and Resolution A.897(21) (25 November 1999). These are the technical Specifications that define how a COW system is built and run, and they remain in force. They trace back to TSPP Conference Resolution 15 (1978), the original COW specification adopted at the Conference of Parties to the MARPOL Protocol on Tanker Safety and Pollution Prevention concurrent with the 1978 Protocol; A.446(XI) was the first consolidated revision, and the two later resolutions refined it with operational experience accumulated over the following two decades.

The Standard Format for the COW Operations and Equipment Manual is a separate instrument: Resolution MEPC.3(XII), adopted 30 November 1979, which fixes the standardized index and eighteen-section layout that every ship’s COW Operational Manual must follow. Section 9 of that standard format was amended by Resolution MEPC.81(43), adopted 1 July 1999, whose title is “Amendments to Section 9 of the Standard Format for the Crude Oil Washing Operations and Equipment Manual”. MEPC.81(43) did not supersede the A-series Specifications; it is a manual-format amendment that updated one section of the MEPC.3(XII) standard format in the light of experience reported by Member Governments. The IMO consolidated volume “Crude Oil Washing Systems” (2000 Edition) prints both streams side by side: the Specifications as A.446(XI) amended by A.497(XII) and A.897(21), and the manual standard format as MEPC.3(XII) amended by MEPC.81(43).

The A-series Specifications, read with the MEPC.3(XII) standard format and its MEPC.81(43) Section 9 amendment, together cover:

• equipment type-approval requirements for fixed and portable washing machines, COW main piping and associated valves;
• the oxygen monitoring requirements for the inert gas main and for individual tank ullage during COW operations;
• the coverage requirements, including the 30 percent per-cycle floor and the 100 percent annual target for direct impingement;
• the plan-approval submission package for shipyard or owner applications to class;
• the commissioning trial procedure for verifying the system in service; and
• the content and section structure of the COW Operations and Equipment Manual.

IGS prerequisite: oxygen at or below 8 percent by volume

The Inert Gas System prerequisite is the single most important safety control in the COW regime. Crude oil washing involves spraying light crude under pressure of approximately 10 kg/cm² through fixed and portable washing machines onto tank internals. The spray produces a fine mist of hydrocarbon droplets in the cargo tank ullage space, accumulates an electrostatic charge of several thousand volts on the droplets and on droplet-impinged surfaces, and creates a vapour-and-mist atmosphere that, in air, falls within the flammable range for many crude oils and most condensates. A tank in this state is at extreme risk of vapour-cloud explosion if any electrostatic discharge or other ignition source is present.

The IGS provides a non-flammable atmosphere by displacing air in the cargo tank with inert gas (typically flue gas from the main or auxiliary boiler, scrubbed and cooled, with an oxygen content of less than 5 percent by volume on delivery to tanks). Regulation 33.1.2 and the cross-referenced SOLAS Chapter II-2 require that:

• inert gas be supplied at all times during cargo discharge, ballasting, tank cleaning and gas freeing;
• the oxygen content in any cargo tank in which COW is taking place shall not exceed 8 percent by volume at any time during the operation; and
• the cargo tank pressure shall be positive to atmosphere throughout, to prevent any in-leakage of air through hatch seals, P/V valves or vent risers.

The 8 percent by volume oxygen limit is the lean-side threshold below which a hydrocarbon-air-inert-gas mixture cannot ignite, established through extensive flammability testing and codified in the IGS provisions of SOLAS Chapter II-2 and the A.446(XI) COW Specifications. Operationally the limit is checked continuously by fixed oxygen analysers on the inert gas main downstream of the deck water seal and by portable analysers at the individual tank ullage. Loss of inert gas pressure or rise in oxygen content above 8 percent requires the immediate cessation of all COW operations until the inert gas atmosphere is re-established.

The IGS prerequisite is not a parallel requirement to COW: it is the enabling safety system without which COW cannot lawfully or safely be operated. A vessel with a degraded inert gas plant (a fouled scrubber, a failed blower, a leaking deck water seal) cannot run COW regardless of how well the washing machines and COW main are maintained.

COW operational principle: high-pressure crude-oil jets

The COW principle replaces water washing with cargo crude itself, applied as a high-velocity jet that mechanically and chemically dissolves and re-suspends crude residue from tank internals. The principal elements are:

• Fixed washing machines mounted on the cargo tank deckhead, typically two to four per tank depending on tank length and beam, with rotating nozzle heads delivering programmable wash patterns to defined elevation and azimuth angles;
• Portable washing machines lowered through tank-top access hatches into shadow zones not reachable by the fixed machines, used selectively during the wash programme;
• High-pressure crude-oil supply drawn from the cargo discharge main during cargo offloading, fed via a dedicated COW main running along the cargo deck and tap-offs at each washing machine;
• Pressure regulation to achieve a nozzle pressure of approximately 10 kg/cm² (0.98 MPa, ~145 psi) at the washing machine inlet;
• Flow rate of typically 50 to 80 m³/h per fixed machine, scaling with nozzle size and ship cargo system capacity;
• Operational sequence integrated into the cargo discharge plan, with washing machines cycling sequentially as cargo level falls in each tank and as cargo pumping speed is matched to washing machine demand.

The nozzle pressure converts to jet velocity by Bernoulli:

A 47 m/s crude-oil jet impinging on a steel tank surface delivers sufficient mechanical energy and contact-area solvent action to dislodge waxy and asphaltenic deposits, and the dissolved residue is carried by the falling cargo level to the cargo pump suction in the bottom of each tank, leaving with the cargo at discharge.

Multi-stage coverage and the 30 percent minimum cycle

Regulation 33 and the supporting A.446(XI) Specifications, as amended by A.497(XII) and A.897(21), prescribe specific coverage requirements for the COW programme. The intent is that 100 percent of the cargo tank internal surfaces shall be subject to direct impingement by a crude-oil jet, achieved across the cumulative wash programme of a discharge voyage.

The headline numerical thresholds are:

• A minimum of 30 percent of the area of all cargo tank internal surfaces shall receive direct impingement from a crude-oil jet during any single discharge cycle (Regulation 33.4 and the COW Specifications);
• A minimum of 25 percent of the cargo tanks by capacity shall be washed for sludge control during each loaded voyage and (for trades where ballast voyage washing is permitted) before any tank is to be ballasted;
• The cumulative wash programme over the discharge voyages of a year shall achieve 100 percent direct impingement coverage of all tank surfaces.

The 30 percent per cycle rule recognises that complete impingement is not achievable in a single discharge owing to time and pumping constraints, but that any voyage in which COW is being operated must contribute meaningful coverage to the overall annual programme. Operationally the 30 percent figure translates into a bottom wash (the lowest 30 percent of tank height, the most polluted region where waxy bottoms accumulate) plus top wash of the deckhead and upper bulkheads, with the middle bands of the tank covered on subsequent voyages.

The multi-stage approach addresses three distinct internal surface categories:

• Bottom wash: deckhead-mounted machines directing jets downward and at angles to the tank bottom and lower bulkheads, run during the late stages of cargo discharge when tank levels are low.
• Top wash: machines directing jets upward and at angles to the deckhead and upper internal structure (girders, longitudinals), run while sufficient cargo remains above the lower portion of the tank for jet recoil and re-impingement to be safe.
• Stripping wash: a final wash sequence with reduced flow during cargo stripping operations, ensuring that the bottom of the tank receives direct jet impingement while the cargo pump completes the final removal.

COW Operational Manual approval procedure

Regulation 33.5 requires every Regulation 33 vessel to operate in accordance with a COW Operational Manual approved by the Administration. The manual must follow the Standard Format in Resolution MEPC.3(XII), as amended by Resolution MEPC.81(43) (Section 9), and reflect the design and operating requirements of the A.446(XI) Specifications. The manual content requirements cover:

• the description of the COW system, washing machine arrangement, COW main piping, and IGS interfaces;
• the wash programme for each cargo tank, including bottom wash, top wash and stripping wash sequences;
• the cargo pumping plan and the integration of COW with cargo discharge to charterers’ ports;
• the safety procedures, including IGS oxygen monitoring, electrostatic precautions, hot work prohibitions and emergency stop arrangements;
• the personnel responsibilities and the master’s, chief officer’s, pumpman’s and duty deck officer’s specific tasks during a COW operation;
• the record-keeping requirements, including ORB Part II Code G entries; and
• the maintenance and testing schedule for washing machines, COW main valves, pressure gauges and oxygen analysers.

Approval is undertaken by the Administration or Recognised Organisation (RO) acting on its behalf: in practice the IACS member class society (DNV, Lloyd’s Register, ABS, BV, KR, NK, RINA, CCS, RS, IRS) that holds the flag State delegation. The approval process involves:

1. Plan approval: the shipyard or operator submits the COW system arrangement drawings, COW main isometric, washing machine technical specifications and the draft Operational Manual to class.
2. Class plan-approval review: against the A.446(XI) Specifications as amended by A.497(XII) and A.897(21), the MEPC.3(XII) manual standard format as amended by MEPC.81(43), and the class society’s own COW guidelines.
3. Conditional approval: subject to satisfactory commissioning trial and onboard verification.
4. Commissioning trial: a shipboard test of the COW system, including washing machine operation, COW main pressure, IGS oxygen monitoring under wash conditions, and integration with cargo discharge.
5. Final approval and certification: the COW Operational Manual is stamped and issued, and the IOPP Certificate Form B reflects the COW endorsement.

Any subsequent modification to the COW system or the wash programme requires re-approval of the affected sections of the manual.

Class society review and the COW specification chain

The technical Specifications for the design, operation and control of crude oil washing systems are Assembly Resolution A.446(XI) (15 November 1979), as amended by:

• A.497(XII): first amendment, 19 November 1981;
• A.897(21): further amendments, 25 November 1999.

The COW Operations and Equipment Manual standard format is a separate instrument, Resolution MEPC.3(XII) (1979), whose Section 9 was amended by Resolution MEPC.81(43) (1 July 1999). The A.897(21) amendment aligned the Specifications with the prevailing IGS, SOLAS Chapter II-2 and ISGOTT (International Safety Guide for Oil Tankers and Terminals) practice in force at the close of the 1990s. The principal A.446(XI) Specification provisions for class plan-approval include:

• the requirement that all fixed washing machines be of an approved type, with certified pressure-flow characteristics;
• the requirement that the piping arrangement be permanently separated from the cargo system except for designated COW supply tap-offs, to prevent unintended crude pumping into the COW main when not in use;
• the requirement for oxygen monitoring of the inert gas main and for ullage monitoring of each tank during COW;
• the requirement for an emergency stop capable of isolating the COW main from the cargo system and isolating individual machines;
• the requirement for the COW main to be drained and depressurised at the conclusion of each operation, and for the system to be left in a known safe state at the master’s handover to the next watch.

Class society interpretations of the A.446(XI) Specifications are published as society notes, technical information letters and rule clauses. DNV publishes COW guidelines under its Pt.5 Ch.10 sections; Lloyd’s Register under its Rules and Regulations for the Classification of Ships, Pt.5; ABS under its Steel Vessel Rules, Pt.5C and the supplementary guidance notes; Bureau Veritas under NR467 Pt.D Ch.7; and the other IACS members publish equivalent rules.

Jet axis 9 to 11 degrees off vertical and direct-impingement coverage

The COW Specifications in A.446(XI), as amended, and the supporting class rules prescribe a precise geometric arrangement for the fixed deckhead-mounted washing machines. The jet axis of a fixed machine is the principal direction of the rotating nozzle. For direct-impingement coverage to be achieved on the lower internal structure of a cargo tank, the jet axis must be inclined at approximately 9 to 11 degrees off the vertical (depending on machine model and manufacturer).

The reason for the 9 to 11 degree offset is geometric. A perfectly vertical jet would impinge only on the cargo tank bottom directly below the machine. By tilting the nozzle axis a small angle off the vertical and rotating the head, the impingement point traces a conical pattern with the apex at the machine and the base at the tank bottom, sweeping a circle of radius (tank height) × tan(angle) at the bottom plate. With an offset of 10 degrees and a tank depth of 20 metres, the impingement circle radius at the bottom is approximately 3.5 metres, and the area covered by a single rotating sweep is approximately 38 m². With four fixed machines per tank arranged in a rectangular pattern, and with the rotation programmed to cover the full tank length and breadth, the bottom and lower bulkhead direct-impingement coverage approaches 100 percent over the integrated wash programme.

The azimuthal sweep is programmed in the machine head and verified during commissioning. Modern machines provide programmable elevation as well, enabling the same machine to perform bottom wash, side wash and top wash in sequence by changing elevation between revolutions. Shadow zones (areas obscured from direct impingement by transverse webs, longitudinals or stiffening structure) are addressed by the placement of additional fixed machines or by the use of portable machines lowered through tank-top hatches.

The direct impingement standard is fundamental. Re-impingement (a jet that strikes a structure, ricochets and indirectly hits another surface) is not credited toward the 30 percent and 100 percent coverage requirements. Class plan approval reviews the geometric arrangement against the tank internal structure to confirm that direct impingement is achievable.

Relationship to Reg 18 Slop Tank requirements

Regulation 18 of MARPOL Annex I requires every oil tanker of 150 GT and above to be equipped with slop tank arrangements. Slop tanks receive the oily mixtures from cargo tank washing, dirty ballast and other oily residues. After settling, the supernatant water is decanted overboard subject to the Regulation 34 discharge criteria, and the recovered oil is added to the next cargo or retained on board.

In a Regulation 33 vessel operating COW, the slop tank load is substantially reduced because the principal cleaning medium is the cargo crude itself, not water. A typical pre-COW water-washing voyage generated slop volumes of 5 to 8 percent of cargo tank capacity. A modern COW-equipped VLCC generates slops of less than 0.5 percent of cargo tank capacity. The slop tank serves principally to receive:

• the oily mixture from any incidental water washing performed on the ballast leg;
• machinery space oily residues if the cargo machinery space drains feed into the slop tanks (system-dependent);
• oil from cargo line drainage and from any minor water that enters the cargo system; and
• the cargo decanting from segregated dirty ballast (no longer applicable in segregated-ballast vessels but still applicable in some legacy designs).

The slop tank capacity under Regulation 18.3 is set at 3 percent of cargo carrying capacity for new tankers, with permitted reductions for COW-equipped vessels and for segregated-ballast vessels. This reduction recognises the COW vessel’s lower slop generation rate and avoids over-sizing.

The COW-slop tank interface is documented in the COW Operational Manual and is reflected in the cargo system general arrangement reviewed at IOPP issue and renewal.

Relationship to Reg 18 Segregated Ballast Tanks

The SBT provision in MARPOL Annex I (Regulation 18 in the current consolidated text, formerly Regulation 13 in earlier consolidations) requires every new crude oil tanker of 20,000 DWT and above and every product tanker of 30,000 DWT and above to be provided with Segregated Ballast Tanks (SBT) of sufficient capacity to operate safely on ballast voyages without the need to use any cargo tank for ballast. SBT tanks are physically separated from the cargo system and never carry oil.

The 1978 Protocol allowed three Annex I-compliant configurations for crude oil tankers above the relevant thresholds:

• SBT alone, with no COW;
• COW alone, with traditional ballast in cargo tanks during ballast voyages (with attendant Regulation 34 discharge controls);
• Both SBT and COW.

In practice the global crude tanker fleet built since the early 1980s has been delivered with both SBT and COW as the standard configuration, because the safety, commercial and operational case is clear: SBT eliminates the in-cargo-tank ballast and the associated discharge complications, while COW eliminates the slop generation associated with water washing of cargo tanks. The “SBT or COW or both” Protocol architecture is a transitional historical record: every modern Aframax, Suezmax and VLCC is built to both standards from new.

Regulation 18 (SBT) and Regulation 33 (COW) are distinct legal requirements with distinct survey scopes. SBT compliance is verified principally by review of the cargo and ballast piping general arrangement and the prevention of cross-contamination, while COW compliance is verified principally by review of the COW Operational Manual, the washing machine arrangement and the IGS interface.

VOC management and the Annex VI interaction

COW operations on crude oil tankers interact with MARPOL Annex VI Regulation 15 on the control of Volatile Organic Compound (VOC) emissions from tankers. Regulation 15.6 requires crude oil tankers, gas carriers and product carriers of 20,000 DWT and above that carry crude oil or petroleum products with vapour pressure above 5 kPa to submit a VOC Management Plan approved by the Administration.

The VOC plan must address the COW operation because washing crude oil tanks generates VOC emissions into the ullage space and, where the inert gas system vents excess pressure through P/V valves or high-velocity vents, into the atmosphere. The A.446(XI) COW Specifications and the Regulation 15 VOC plan framework interact at three points:

• Positive IGS pressure: the Specifications require the COW main pressure and the IGS positive pressure to be maintained throughout the wash; a cargo tank at or above atmospheric pressure during COW does not vent VOC passively.
• Venting on topping-up: when a washed tank is being refilled with ballast or cargo, the ullage gas displaced through the vapour return or P/V vent carries VOC; the VOC plan quantifies and manages this flux.
• Terminal vapour recovery: at crude loading terminals where the flag State or port State has implemented Regulation 15.8 (VOC collection at terminal), the ship’s vapour return connection must be operational and the COW cycle planned to avoid generating excess vapour during the loading-out window.

The VOC management obligation does not change the COW operational requirements under Regulation 33 but adds a planning and record-keeping layer that the chief officer must integrate with the COW Operational Manual. Inconsistency between COW records and the VOC plan can produce deficiency findings at port State control under both Annex I and Annex VI simultaneously.

Class society implementation: DNV, LR, ABS, BV, KR, NK, RINA, CCS, RS, IRS

Implementation of Regulation 33 is delegated, in the case of most flag States, to the Recognised Organisation (RO) under the Code for Recognised Organisations (RO Code, Resolution MEPC.237(65) and MSC.349(92)). The principal IACS members holding broad RO delegations and operating COW approval activity are:

• DNV (Norway, headquartered Hovik): Pt.5 Ch.10 sections of the Rules for Classification: Ships, with COW Operational Manual approval guidelines;
• Lloyd’s Register (LR) (UK, headquartered London): Rules and Regulations for the Classification of Ships, Pt.5 and Pt.7;
• American Bureau of Shipping (ABS) (USA, headquartered Spring TX): Steel Vessel Rules Pt.5C and the dedicated COW Guide;
• Bureau Veritas (BV) (France, headquartered Paris): NR467 Rules for Classification of Steel Ships, Pt.D Ch.7;
• Korean Register (KR) (Korea, headquartered Busan): Classification Rules for Steel Ships, Pt.7 Ch.2;
• Nippon Kaiji Kyokai (NK / ClassNK) (Japan, headquartered Tokyo): Rules for the Survey and Construction of Steel Ships, Pt.R;
• Registro Italiano Navale (RINA) (Italy, headquartered Genoa): Rules for the Classification of Ships, Pt.E;
• China Classification Society (CCS) (China, headquartered Beijing): Rules for Classification of Sea-Going Steel Ships, Pt.7;
• Russian Maritime Register of Shipping (RS) (Russia, headquartered Saint Petersburg): Rules for the Classification and Construction of Sea-Going Ships;
• Indian Register of Shipping (IRS) (India, headquartered Mumbai): Rules and Regulations for the Construction and Classification of Steel Ships, Pt.5.

Each society publishes its own implementation guidance for Regulation 33. The implementation envelope is harmonised through the IACS Unified Requirements (UR Z series) for survey procedures and through participation in the IMO MEPC working groups that produce and revise the COW specifications. In commercial practice a tanker may move between class societies (a “class transfer”) and the COW approval is generally re-validated by the receiving class on the basis of the existing approved manual and a confirmatory survey, rather than a fresh approval cycle from zero.

Class survey procedures: annual, intermediate and renewal

Class surveys for Regulation 33 follow the harmonised annual / intermediate / renewal scheme of the IOPP Certificate, with confirmation of COW system status as part of the broader Annex I cargo system survey:

• Annual survey (every 12 months plus or minus 3 months from the renewal date): a general inspection confirming that the COW system remains in operational condition, that the COW Operational Manual is on board, that recent ORB Part II entries reflect COW operations consistent with the manual, that washing machine pressure gauges and oxygen analysers are within calibration, and that no unapproved modifications have been made;
• Intermediate survey (between the second and third annual, at approximately 2.5 years from renewal): an extended inspection including operational testing of representative washing machines, IGS oxygen analyser calibration verification, and review of the cargo and COW main piping for corrosion or unauthorised modification;
• Renewal survey (every 5 years, at IOPP Certificate renewal): a full survey including internal examination of representative cargo tanks, verification of fixed washing machine condition (including nozzle wear and rotation mechanism), pressure-testing of the COW main, recalibration of all fixed and portable oxygen analysers, recalibration of the COW main pressure gauges, and confirmation that the approved drawings on board match the as-fitted system.

Any deficiency in the COW system that prevents safe and effective operation results in a survey condition on the IOPP Certificate, restricting the vessel from crude oil trading until rectification. A Regulation 33 vessel cannot earn revenue without an unconditional COW endorsement.

ORB Part II Code G entries for COW operations

Oil Record Book Part II (cargo and ballast operations) under Regulation 36 records all cargo and ballast operational events on an oil tanker. Code G is the dedicated code for cargo tank cleaning operations. Each COW operation is recorded under Code G entries with the following data items:

• date, time and position of operation start and finish;
• identity of the cargo tank or tanks washed;
• the type of wash (bottom wash, top wash, stripping wash);
• the duration of operation per tank;
• the oxygen concentration in the inert gas main and in the tank ullage during the operation;
• the COW main pressure;
• the cargo discharge port (since COW is performed during cargo discharge);
• the master’s signature endorsing the entry; and
• any deviations from the approved COW Operational Manual, with reasons.

Port State Control inspectors and class surveyors review ORB Part II Code G entries against the COW Operational Manual and against cargo records. Inconsistencies (such as a tank recorded as washed when no cargo was discharged, or oxygen levels recorded above 8 percent during wash) are common deficiency findings and may result in detention.

Common IOPP renewal-survey findings

Renewal surveys of Regulation 33 vessels frequently identify a recurring set of findings that operators and superintendents should anticipate and address proactively. The principal findings include:

• Washing machine arrangement obstructed by tank modifications: tank stiffening modifications, internal piping additions or unauthorised structural alterations may obscure the direct-impingement field of one or more fixed machines, voiding the geometric coverage assumption of the approved manual.
• Missing or outdated approval drawings on board: the approved COW Operational Manual is required to be on board with the current revision; the absence of the latest revision (after a manual amendment ashore) is a recurring deficiency.
• Pressure-gauge calibration drift: COW main pressure gauges drift over the 5-year cycle, often reading several percent low or high; recalibration to a traceable reference standard at renewal is required.
• Oxygen analyser calibration drift: fixed and portable oxygen analysers drift over time, particularly the fixed analyser on the inert gas main; the 8 percent limit cannot be reliably enforced without recent calibration.
• Washing machine nozzle wear: rotating-head nozzles erode over the 5-year cycle, altering the jet pattern and the effective impingement geometry; physical inspection at renewal is required.
• COW main valve seizure: COW main isolation valves used infrequently may seize over the cycle, preventing the emergency isolation contemplated by the A.446(XI) Specifications; valve operation testing at renewal is required.
• ORB Part II discontinuities: incomplete or contradictory Code G entries, often arising from crew turnover and inconsistent training on the manual.
• Inert gas main scrubber and cooler fouling: indirect but critical for IGS oxygen content; deteriorated scrubbers may deliver inert gas at oxygen levels that approach the 8 percent threshold even before COW is initiated, leaving no margin.

1976 to 1978 casualties driving the regulation

The 1978 Protocol that introduced Regulation 33 in its modern form was driven principally by two major casualties:

Argo Merchant (15 December 1976): the Liberian-flagged 18,743 GRT tanker grounded on Fishing Rip near Nantucket Shoals, Massachusetts, in heavy weather. The vessel broke up over six days, releasing approximately 27,000 tonnes of No. 6 fuel oil. Although the cargo was a refined fuel oil rather than crude, the casualty exposed structural fragility of older tankers, navigational and routeing failures, and the absence of a reliable response capability. The Argo Merchant prompted US Congress hearings and contributed to the political momentum behind tanker safety reform.

Amoco Cadiz (16 March 1978): the Liberian-flagged 233,690 DWT VLCC, en route from the Persian Gulf to Rotterdam with approximately 230,000 tonnes of light Iranian and Arabian crude, suffered steering gear failure off Brittany in heavy weather. Tow attempts failed, and the vessel grounded on Portsall Rocks. The hull broke up over two weeks, releasing the entire cargo and approximately 4,000 tonnes of bunker fuel onto the Brittany coast. Approximately 320 km of coastline were polluted, with severe environmental and economic damage. The Amoco Cadiz remains one of the largest vessel-source oil spills on record.

The IMO response was the 1978 Protocol to MARPOL, adopted in February 1978 and entering into force in October 1983 alongside the parent convention. The Protocol introduced the COW (or SBT or both) requirement, the IGS prerequisite for all crude oil tankers above defined size, the slop tank requirements in stronger form, and the strengthened damage stability requirements that became the modern Regulation 28. Regulation 33 in its present form is the direct legislative product of the post-Amoco Cadiz reform programme.

A third casualty often cited in the same context, the Atlantic Empress (19 July 1979), was a collision between two VLCCs in the Caribbean and a subsequent fire and explosion. The Atlantic Empress released approximately 287,000 tonnes of crude oil and remains the largest tanker spill in history by volume, but the casualty was a navigational and structural event, not a tank-cleaning incident, and did not directly drive the COW requirement. Its inclusion in some regulatory histories reflects the broader post-Amoco Cadiz political climate rather than COW-specific causation.

Commercial crude-trade implications

Regulation 33 compliance is commercially mandatory for any vessel trading crude oil internationally. A tanker without a valid COW Operational Manual approval, an operational COW system and a current IGS cannot enter the major crude oil charter markets:

• VLCC (200,000 to 320,000 DWT) trading Persian Gulf to Asia, US Gulf and Europe: COW is required by every major charterer, and the SIRE 2.0 vetting process addresses COW operational discipline as a pre-fixture issue;
• Suezmax (120,000 to 200,000 DWT) trading Mediterranean, West Africa and US Gulf: COW required without exception in the spot and time-charter markets;
• Aframax (80,000 to 120,000 DWT) trading North Sea, Mediterranean, Caribbean and Far East coastal: COW required, with particular emphasis on operational documentation under the Tanker Management and Self Assessment (TMSA) framework;
• Long Range 1 and Long Range 2 (LR1/LR2) product tankers (50,000 to 110,000 DWT): generally not subject to Regulation 33 because the cargo is refined product, not crude; clean tank cleaning practices and inert gas requirements differ.

A tanker that loses its COW endorsement (through a survey condition or through unauthorised modifications to the system) falls out of the crude trade within days, because every charter party fixture requires Regulation 33 compliance as a contractual condition. The operator has no choice but to rectify and re-establish compliance promptly, and the off-hire cost of doing so generally far exceeds the maintenance investment that would have prevented the lapse.

The single-hull phase-out under Regulations 19 and 20 (post-Erika) has substantially reduced the population of legacy Regulation 33 vessels: only double-hull tankers are now in mainstream crude trade. The COW systems on these double-hull vessels are similar in principle to single-hull predecessors, but the cargo tank geometry is different (segregated by the double hull side spaces) and the washing machine arrangement reflects this.

Applicability thresholds expressed as conditions

Regulation 33 reduces to a small set of physical and regulatory relationships that designers, masters and surveyors apply directly. The applicability test is a deadweight threshold: COW is mandatory for a crude oil tanker once its deadweight reaches the line set in the 1978 Protocol. Expressed as a condition,

with the existing-tanker line at $\text{DWT} \geq 40{,}000$ t under Reg 33.2. Below 20,000 DWT the regulation does not bite, although the segregated-ballast and inert-gas provisions of Regulations 16, 18 and SOLAS II-2 can still apply. A product tanker carrying gas oil, kerosene or naphtha sits outside Reg 33 entirely; the cleaning chemistry is different and the cargo is far cleaner.

Once a vessel is in scope, the inert-gas precondition is absolute. The oxygen content in any cargo tank in which washing is taking place must satisfy $\text{O}_2 \leq 8\%$ by volume at all times during the operation, the lean-side threshold below which a hydrocarbon-air-inert-gas mixture cannot ignite. This is the single hard safety limit of the regime, set out in Regulation 33.1.2 and the cross-referenced SOLAS Chapter II-2 provisions; loss of inert-gas pressure or a rise above $8\%$ stops the wash at once.

The washing energy itself comes from cargo crude delivered at a nozzle pressure of roughly $10 \text{ kg/cm}^2$, which is about $0.98 \text{ MPa}$. That pressure converts to jet velocity through Bernoulli’s relation for a liquid issuing from a pressurised reservoir into the tank ullage:

For $p_{\text{jet}} = 0.98 \text{ MPa}$ and a representative crude density $\rho \approx 870 \text{ kg/m}^3$ (Arabian Light, North Sea Brent, West African Bonny Light and similar grades), this gives $v_{\text{jet}} \approx 47.5 \text{ m/s}$. The derivation assumes incompressible flow, which holds for liquid crude, negligible viscous loss across the short nozzle, and discharge to a tank ullage at about atmospheric pressure plus the IGS positive pressure of order $0.01 \text{ MPa}$. A 47 m/s jet carries enough mechanical and solvent energy to dislodge waxy and asphaltenic bottoms; for heavier grades such as Maya or Athabasca dilbit the density rises toward $920 \text{ kg/m}^3$ and the jet velocity falls a little.

Coverage and shadow area

The COW Specifications express the cleaning requirement as an areal coverage target rather than a time or volume target. The intent is that 100 percent of the cargo tank internal surfaces receive direct impingement from a crude-oil jet across the cumulative wash programme of a year. Two thresholds drive this. Each single discharge cycle must achieve

and the cumulative programme must reach

A minimum of 25 percent of the cargo tanks by capacity must also be washed for sludge control during each loaded voyage, and before any tank is to be ballasted on trades where ballast-voyage washing is permitted. The 30 percent per-cycle floor recognises that complete impingement is not achievable in one discharge given time and pumping constraints, yet any voyage running COW must contribute meaningful coverage to the annual total. In practice the figure resolves into a bottom wash of the lowest 30 percent of tank height, where waxy bottoms collect, plus a top wash of the deckhead and upper bulkheads, with the middle bands covered on later voyages.

The areal accounting behind the 30 percent floor treats each fixed washing machine as sweeping a defined cone of impingement, and maps the union of those cones, allowing for wash duration and rotation rate, against the tank internal surface area. The 30 percent figure is the minimum admissible union per discharge cycle for the wash programme to count toward the annual 100 percent target. Only direct first-strike impingement counts. A jet that strikes a structure, ricochets and indirectly reaches another surface is re-impingement and earns no credit toward the 30 percent or 100 percent figures.

Shadow area is the limiting factor on coverage. Transverse webs, longitudinals and stiffening structure obscure parts of the tank from any fixed machine’s line of sight. These shadow zones are addressed by placing additional fixed machines or by lowering portable machines through tank-top hatches, and class plan approval reviews the geometric arrangement against the tank internal structure to confirm that direct impingement of every surface is achievable. Because shadow area is set by the as-built structure, any later structural modification that introduces a new shadow zone voids the geometric coverage assumption of the approved manual and forces re-approval rather than a programming change.

Worked example: VLCC per-tank coverage

Consider a 320,000 DWT VLCC with 15 cargo tanks, mean tank dimensions 40 m by 30 m by 25 m, and a cargo tank surface area of about 4,400 m² per tank. The fixed arrangement is four deckhead-mounted machines per tank, each rated at 70 m³/h at $10 \text{ kg/cm}^2$, rotating through a 360 degree azimuthal sweep with a programmable elevation pattern. A single discharge runs three wash phases per tank:

• Bottom wash: 60 minutes at full pressure, four machines together, about 280 m³ of crude diverted from cargo, direct-impingement coverage near 35 percent of tank internal area;
• Top wash: 30 minutes at full pressure with elevated nozzle angle, about 140 m³, covering the deckhead and upper bulkheads at near 20 percent;
• Stripping wash: 20 minutes during cargo stripping at reduced flow, about 60 m³.

Cumulative per-tank coverage in this single discharge is roughly 55 percent of tank internal area, well above the 30 percent per-cycle floor. Over the typical four-discharge annual cycle of a VLCC, with the programme rotating bottom and top emphasis between voyages, the cumulative figure passes 100 percent and direct impingement of all surfaces is achieved. The COW Operational Manual records this phase sequence per tank and ties it to the cargo discharge plan, so the chief officer can match washing-machine demand to cargo pumping rate at each terminal.

Limitations

Regulation 33 is a strong control with documented boundaries, and the practitioner caveats below carry more weight than any completeness claim about the regime. The first limitation is the dependence on the inert gas system. COW cannot run lawfully or safely without IGS holding tank oxygen at or below 8 percent by volume, so a degraded scrubber or cooler that delivers inert gas near that threshold leaves no operating margin and stops the wash before it starts. The IGS is not a parallel requirement; it is the enabling safety system, and COW availability is only as reliable as the inert-gas plant feeding it.

A second limitation is the crude-only applicability. COW works because cargo crude is a good solvent for its own residue, so it is specific to crude oil tankers and inappropriate for refined-product cargoes such as gas oil, kerosene or naphtha. A combined crude/product tanker lays up its COW system for the product voyage and reactivates it on return to crude trade; treating COW as a general substitute for water washing on a product cargo is an applicability error, not a technique choice. Very heavy grades stretch the chemistry too: Maya, Athabasca dilbit and similar high-viscosity crudes leave residue that the same crude may not dissolve, so the manual must address those trades specifically and may call for supplementary water washing on dedicated voyages.

A third limitation is shadow-area imperfection. The 100 percent direct-impingement target is met across a cumulative annual programme, not in any one discharge, and the credited coverage excludes re-impingement, so the practical coverage in a given voyage is the 30 percent floor plus whatever the phase sequence adds. Structural members create shadow zones that no fixed machine reaches, and a structural modification made without re-approval can quietly defeat the geometric assumption that underpins the coverage figures.

A fourth limitation is the exemption and edge-case set. Crude tankers below 20,000 DWT fall outside Regulation 33, though segregated-ballast and inert-gas duties can still apply. On an IGS failure mid-wash the operation must terminate at once and the tank atmosphere be restored before any further wash, which the manual covers through its emergency-stop procedure. Hot work on cargo tank internals always requires prior gas-freeing and a certificate regardless of how little residue the last wash left.

A final limitation is that the worked numbers are engineering estimates, not certified outputs. The 47.5 m/s jet velocity, the 870 kg/m³ density, the 70 m³/h machine rating and the per-phase coverage percentages are representative values for illustration; the governing figures for any specific vessel are those in its Administration-approved COW Operational Manual and the as-fitted machine certificates, against which the estimates here should always be checked. The regulatory basis is MARPOL Annex I Regulation 33 in the current consolidated text, read with the Revised Specifications in Resolution A.446(XI) of 15 November 1979 as amended by Resolution A.497(XII) of 19 November 1981 and Resolution A.897(21) of 25 November 1999, the COW Operations and Equipment Manual standard format in Resolution MEPC.3(XII) of 1979 as amended by Resolution MEPC.81(43) of 1 July 1999 (Section 9), SOLAS Chapter II-2 for the IGS and fire-safety integration, and the IACS Unified Requirements UR Z survey procedures.

See also

• MARPOL Annex I: Regulations for the Prevention of Pollution by Oil
• MARPOL Annex I Regulation 12: Oil Residue (Sludge) Tanks
• MARPOL Annex I Regulation 12A: Oil Fuel Tank Protection
• MARPOL Annex I Regulation 14: Oil Filtering Equipment
• MARPOL Annex I Regulation 15: Discharge Control
• MARPOL Annex I Regulation 17: Oil Record Book
• MARPOL Annex I Regulation 28: Damage Stability
• MARPOL Annex VI Regulation 15: VOC Emissions
• Marine Inert Gas Systems
• MARPOL Convention
• Calculator catalogue

References

1. IMO Resolution A.446(XI), Revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems, 15 November 1979.
2. IMO Resolution A.497(XII), Amendments to the Revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems, 19 November 1981.
3. IMO Resolution A.897(21), Amendments to the Revised Specifications for the Design, Operation and Control of Crude Oil Washing Systems, 25 November 1999.
4. IMO Resolution MEPC.3(XII), Standard Format for the Crude Oil Washing Operations and Equipment Manual, 1979, as amended by Resolution MEPC.81(43), Amendments to Section 9, 1 July 1999.
5. IMO MARPOL Annex I, current consolidated text, Regulations 16, 17, 18, 28, 33, 34, 36.
6. SOLAS Chapter II-2 Regulation 4, Inert gas systems, current consolidated text.
7. IACS Unified Requirements UR Z series, Survey procedures.
8. DNV Rules for Classification: Ships, Pt.5 Ch.10.
9. Lloyd’s Register Rules and Regulations for the Classification of Ships, Pt.5 and Pt.7.
10. American Bureau of Shipping Steel Vessel Rules, Pt.5C.
11. Bureau Veritas NR467 Rules for Classification of Steel Ships, Pt.D Ch.7.
12. IMO MARPOL Annex VI Regulation 15, Control of emissions of Volatile Organic Compounds (VOC) from tankers, current consolidated text.

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