MARPOL Annex I: Regulations for the Prevention of Pollution by Oil

Background

The Torrey Canyon and the original MARPOL development

The Torrey Canyon disaster on 18 March 1967 was the foundational event for modern marine oil pollution regulation. The Liberian-flagged supertanker, of approximately 119,000 deadweight tonnes, ran aground on Pollard’s Rock between the Scilly Isles and Land’s End, England, breaking up over the following days and releasing approximately 119,000 tonnes of crude oil. The spill polluted approximately 190 km of British and French coast, with substantial environmental and economic damage.

The Torrey Canyon casualty exposed several issues:

• No international regulatory framework: existing oil pollution conventions (the 1954 OILPOL Convention) were limited and largely operational, not addressing accidental pollution from large tankers.
• Limited liability framework: the shipowner had limited liability under the existing law, leaving most cleanup costs uncompensated.
• Inadequate response capability: the UK and French response was improvised, including bombing the wreck to ignite the oil. The bombing was largely ineffective.
• Absence of casualty investigation framework: the post-incident investigation was not coordinated internationally.

The Torrey Canyon prompted IMO (then IMCO) to develop:

• MARPOL 73: the International Convention for the Prevention of Pollution from Ships, adopted 1973 to replace OILPOL.
• CLC 1969: the International Convention on Civil Liability for Oil Pollution Damage, providing the liability regime.
• Fund Convention 1971: establishing the International Oil Pollution Compensation Fund (IOPC Fund) for supplementary compensation.

MARPOL 73 was modified by the 1978 Protocol (the post-Amoco Cadiz response, addressing tanker safety) to become MARPOL 73/78, which entered into force on 2 October 1983.

Subsequent casualty drivers

After MARPOL 73/78 entered into force, further casualties drove progressive strengthening:

• Amoco Cadiz, 1978: 220,000 tonnes of crude oil released off Brittany, prompting the 1978 MARPOL Protocol.
• Exxon Valdez, 1989: 35,000 tonnes of crude oil in Prince William Sound, Alaska, prompting OPA 90 in the US and the 1992 MARPOL amendments mandating double-hull tankers.
• Erika, 1999: 20,000 tonnes of heavy fuel oil off Brittany, prompting the 2001 EU regulation on accelerated phase-out of single-hull tankers and the 2003 MARPOL amendment (MEPC.111(50)) accelerating the international phase-out.
• Prestige, 2002: 64,000 tonnes of heavy fuel oil off Galicia, reinforcing the post-Erika acceleration.
• Hebei Spirit, 2007: 11,000 tonnes off South Korea, contributing to ongoing strengthening of port state control.
• Sanchi, 2018: condensate cargo loss in the East China Sea, prompting attention to volatile fuel and condensate carriage.
• MV Wakashio, 2020: 1,000 tonnes off Mauritius, illustrating continued vulnerability of even modern ships.

Major amendment milestones

• 1973: MARPOL adoption (without entry into force).
• 1978: MARPOL Protocol amending and entering into force as MARPOL 73/78.
• 1983: Annex I entry into force.
• 1992: Double-hull requirement for new tankers (Resolution MEPC.51(32)).
• 2001: 13G accelerated phase-out scheme (Resolution MEPC.95(46)).
• 2003: Further accelerated phase-out post-Prestige (Resolution MEPC.111(50)).
• 2007: Significant amendment cycle covering operational provisions.
• 2010: Fuel oil tank protection requirement.
• 2010-2015: Single-hull tanker phase-out completion.
• 2017: Polar Code Part II-A integration.
• 2020: Sulphur cap implementation interaction.

Application (Regulation 1)

Annex I applies to:

• All ships of 400 GT and above for the machinery space provisions (Chapter 3).
• Oil tankers of 150 GT and above for the cargo area provisions (Chapter 4).
• Combination carriers (oil/bulk/ore) when carrying oil cargo for the cargo area provisions.
• Fixed and floating platforms for the platform-specific provisions (Chapter 7).
• Ships engaged on international voyages under MARPOL definition.

The application includes both new ships (built after specified dates) and existing ships (with progressive amendments often phased over multi-year periods).

Definitions (Regulation 2)

Key definitions in Regulation 2 include:

• Oil: petroleum in any form, including crude oil, fuel oil, sludge, oil refuse, and refined products.
• Oily mixture: a mixture with any oil content.
• Oil fuel: any oil used as fuel oil for the propulsion or auxiliary machinery of the ship.
• Oil tanker: a ship constructed or adapted primarily to carry oil in bulk in cargo spaces, including combination carriers when carrying oil cargo.
• Combination carrier (OBO): a ship designed to carry both oil and dry bulk cargoes in bulk.
• Special Area: an area where, for recognised technical reasons in relation to its oceanographic and ecological condition, the adoption of special mandatory methods for the prevention of sea pollution by oil is required.
• Particularly Sensitive Sea Area (PSSA): an area that needs special protection through action by IMO because of its significance for ecological, socio-economic or scientific reasons.

The definitions are foundational for the application of all subsequent provisions.

Surveys and certification (Chapter 2)

Survey regime

Annex I surveys include:

• Initial survey: before the certificate is issued for the first time, verifying compliance with all applicable requirements.
• Renewal survey (every 5 years): comprehensive examination at certificate renewal.
• Intermediate survey (every 30 months for tankers, 24 to 36 for other ships): mid-cycle verification.
• Annual survey (every 12 months): minimum-scope verification.
• Additional surveys following modifications, repairs, or accidents.

The surveys are typically conducted by Recognised Organizations under flag state delegation.

IOPP Certificate

The International Oil Pollution Prevention Certificate (IOPP) is issued upon successful initial survey and renewed at each renewal survey. The IOPP Certificate:

• Confirms compliance with Annex I.
• Lists the ship’s specific characteristics (oil tanker or non-tanker, with relevant variants).
• Includes a Supplement detailing the equipment and arrangements verified.
• Is valid for 5 years.
• Is required to be on board and produced for port state inspection.

Form A and Form B Supplements

The IOPP Supplement is in two forms:

• Form A: for ships other than oil tankers (cargo ships, passenger ships, etc.).
• Form B: for oil tankers.

Each form lists the specific equipment installed (oily water separator type, 15 ppm alarm, oil discharge monitoring system, segregated ballast tanks, COW arrangements, inert gas system, slop tanks, etc.) and the operational arrangements.

Machinery space provisions (Chapter 3)

Oily water separator (OWS)

Every ship of 400 GT and above must be fitted with an Oily Water Separator (OWS) to remove oil from bilge water before discharge. Specifications:

• Discharge oil content limit: 15 ppm (parts per million) in the discharged water.
• Type approval: by the flag state under IMO Resolution MEPC.107(49) for new equipment, MEPC.60(33) for older.
• 15 ppm alarm: with audible and visible warning that prevents discharge if the limit is exceeded.
• Three-way valve: returns water to a slop tank if the limit is exceeded.

The OWS is the principal control on operational oil pollution from machinery spaces.

The 15 ppm bilge separator article and the broader OWS calculator cover compliance details.

Oil Discharge Monitoring and Control System (ODMCS)

For ships above 10,000 GT and certain other categories, the OWS must be supplemented by an Oil Discharge Monitoring and Control System (ODMCS) that:

• Continuously monitors discharged water for oil content.
• Records the discharge data including time, position, oil content.
• Automatically stops the discharge if the limit is exceeded.

The ODMCS provides verifiable record of compliance.

Sludge tank

Every ship must have a sludge tank of sufficient capacity to hold the oil residues that accumulate during operation. The sludge tank:

• Receives separator-recovered oil.
• Receives engine and lube oil drainings.
• Receives fuel oil purifier discharges.
• Is emptied at port reception facilities ashore.

The sludge tank capacity is sized to hold typical voyage accumulation between port calls.

Oil Record Book Part I

The Oil Record Book Part I (Machinery space operations) is a mandatory log recording:

• Bilge water discharge to OWS.
• OWS-to-slop or to-overboard discharges.
• Sludge tank operations.
• Bunker delivery.
• Engine room oil transfer operations.
• Any other oil-related operations.

Each entry must be made promptly and signed by the responsible officer. Entries become evidence in casualty investigations and are checked at PSC inspections. The Oil Record Book is one of the most-cited PSC inspection items in MARPOL Annex I compliance.

Cargo area provisions for oil tankers (Chapter 4)

Segregated Ballast Tanks (SBT)

Oil tankers above defined size must have Segregated Ballast Tanks (SBT): ballast tanks completely separate from the cargo system, ballasted with seawater that has never contacted oil cargo. SBT requirement:

• New tankers since 1981: SBT mandatory.
• Existing tankers: phased SBT retrofit through the 1980s and 1990s.
• SBT capacity: typically 30 to 40 percent of cargo capacity.

SBT eliminates the previous practice of using cargo tanks for ballast, which produced “dirty ballast” containing residual oil from cargo. SBT was a foundational improvement in operational pollution prevention.

Double hull

The double hull requirement is the most significant Annex I structural provision:

• New tankers (keel laid after 1992): double hull required for tankers above 600 deadweight tonnes (full requirement above 5,000 deadweight tonnes).
• Existing tankers: phased retirement of single-hull tankers, completed in stages by 2010 (with some extensions to 2015 for specific categories).

Double hull provides:

• Side shell outer plating.
• Cargo tank inner shell between cargo and the void space.
• Wing void space of typically 2 metres width or 0.4 W/15.0, providing collision and grounding damage tolerance.

Benefits:

• Side damage tolerance: side-shell damage typically does not breach the inner cargo tank.
• Bottom damage tolerance: bottom-shell damage typically does not breach the inner cargo tank.
• Reduced cargo loss: double-hull tankers in collision typically lose less cargo than equivalent single-hull tankers.

The double hull came at a cost in capital and operating economics but is now industry standard.

Slop tanks

Oil tankers must have slop tanks to receive:

• Cargo tank washings.
• Tank cleaning residue.
• Cargo residue from line draining.
• Off-spec cargo or cargo grade transitions.

Slop tank capacity is sized to hold typical residue from cargo operations.

Crude Oil Washing (COW)

Crude Oil Washing (COW) uses a portion of the cargo itself as a tank washing medium, replacing seawater wash. COW benefits:

• No water-cargo mixing.
• More effective cleaning (cargo dissolves residue better than water).
• No production of slops requiring disposal.
• Reduced inert gas demand.

COW is mandatory on crude oil tankers above defined size and is widely applied. The detailed COW Operations Manual is required on board, with operational procedures and safety provisions.

Inert Gas System (IGS)

The Inert Gas System (IGS) maintains the cargo tank ullage atmosphere below the lower flammable limit (oxygen below 8 percent for hydrocarbon cargoes), preventing tank explosions. IGS is mandatory for:

• Crude oil tankers above 20,000 deadweight tonnes.
• Product tankers above defined size.
• Chemical tankers (with adapted IGS for the cargo).

Sources of inert gas include:

• Flue gas from boiler exhaust (with scrubbing to remove sulphur and particulates).
• Dedicated inert gas generator (combusting fuel to produce inert gas).
• Stored inert gas (nitrogen tanks for some specialised applications).

The IGS provides the principal tank-explosion prevention measure on tankers and has been a major contributor to the post-1980s improvement in tanker fire and explosion casualty rate.

Oil Record Book Part II

The Oil Record Book Part II (Cargo/Ballast operations) is a mandatory log on tankers recording:

• Cargo loading and discharging.
• Internal cargo transfers.
• Tank cleaning and washing.
• Slop disposal.
• Cargo tank ballast operations (if applicable).
• COW operations.
• IGS operations.

Like the ORB Part I, entries are evidence in casualty investigations and are checked at PSC inspections.

Oil pollution emergency response (Chapter 5)

SOPEP

The Shipboard Oil Pollution Emergency Plan (SOPEP) is mandatory on:

• All oil tankers of 150 GT and above.
• All other ships of 400 GT and above.

The SOPEP contains:

• Reporting procedures for oil pollution incidents to the flag state, the coastal state, and the operator.
• Communication arrangements including 24/7 contact lists.
• Emergency response procedures for various scenarios (collision, grounding, fire, leak).
• Damage assessment procedures for evaluating spill potential.
• Containment procedures using on-board equipment.
• Recovery procedures.
• Post-incident reporting to the flag state and IMO.

The SOPEP must be approved by the flag state and is verified at PSC inspection.

OPRC Convention link

The OPRC Convention (International Convention on Oil Pollution Preparedness, Response and Cooperation, 1990) provides the broader framework:

• National contingency plans for oil pollution response.
• Regional cooperation between neighbouring states.
• Mutual aid arrangements in major incidents.
• Equipment stockpiling and pre-positioning.
• Training and exercise requirements.

The OPRC works alongside MARPOL Annex I, with SOPEP and OPRC together providing both ship-side and shore-side preparedness.

CLC and Fund Conventions

The Civil Liability Convention 1969 (as amended by 1992 Protocol) and the International Oil Pollution Compensation Fund (1971/1992) provide the liability and compensation framework:

• CLC: shipowner liability for oil pollution damage, with compulsory insurance.
• IOPC Fund: supplementary compensation when CLC limits are exceeded.
• Bunker Convention 2001: similar regime for bunker oil pollution from non-tanker ships.

These conventions ensure that oil pollution victims have recourse to compensation, which would otherwise be limited by traditional shipping liability rules.

Reception facilities (Chapter 6)

Every port and oil terminal must provide adequate reception facilities to receive:

• Oily mixtures and oil residues from ships.
• Slop tank discharges from tankers.
• Sludge from machinery spaces.

The reception facility requirements have been progressively tightened, with port state implementation audited under SOLAS Chapter XIII IMSAS audit.

Special Areas

Annex I designates Special Areas where stricter discharge limits apply or where operational discharges are prohibited:

• Mediterranean Sea.
• Baltic Sea.
• Black Sea.
• Red Sea.
• Gulfs Area (Persian Gulf and Gulf of Oman).
• Gulf of Aden.
• Antarctic Area.
• North-West European waters.
• Oman area of the Arabian Sea.
• Southern South African waters.

In Special Areas, operational discharges are prohibited or further restricted. Ships must hold all oily residues for discharge to reception facilities.

Particularly Sensitive Sea Areas (PSSAs)

PSSAs are designated by IMO based on ecological, socio-economic or scientific significance. PSSA designations come with Associated Protective Measures (APMs) which can include:

• Mandatory routeing.
• Areas to be avoided.
• Discharge restrictions.
• Pilotage requirements.

PSSA examples include:

• Great Barrier Reef and Torres Strait.
• Cuban Archipelago.
• Galapagos Archipelago.
• Florida Keys.
• Western European Waters.
• Saba Bank.
• Strait of Bonifacio.
• Tubbataha Reefs Natural Park.
• North Western Mediterranean.
• Wadden Sea.

PSSA designation is a flexible tool for protecting specific marine areas while allowing necessary shipping access.

Specific operational requirements

Operational discharge limits

Outside Special Areas, oil tankers may discharge oily mixtures only when:

• The tanker is not within 50 nautical miles of land.
• The tanker is in transit (en route).
• The discharge rate does not exceed 30 litres per nautical mile.
• The total discharge in a voyage does not exceed 1/30,000 of cargo carried (for crude oil tankers).
• The ODMCS is operating with the 15 ppm alarm and automatic shutoff.
• The Oil Record Book Part II is maintained.

These limits effectively prevent operational discharge in coastal waters and place strict controls on open-ocean discharge.

Tank washing operations

Tank washing operations follow specific procedures:

• COW for crude oil tankers as the preferred method.
• Water washing for product tankers and where COW is not applicable.
• Hot water and chemical for specific applications.
• Tank inerting before any tank entry.
• Personnel safety with full SCBA, gas testing, and rescue procedures.

The procedures are documented in the ship’s tank washing manual and in the cargo handling procedures.

Bunker fuel operations

Bunker fuel operations are subject to:

• Bunker Delivery Note (BDN): documenting the fuel delivered, with a sample retained for compliance verification.
• Fuel quality verification under MARPOL Annex VI (sulphur content) and ISO 8217.
• Transfer procedure documentation with master’s authorisation, communications, emergency procedures.
• Spill prevention with drip trays, containment booms, emergency stop arrangements.

A bunker fuel spill (even small) is an oil pollution incident requiring SOPEP activation.

Specific challenges and emerging issues

Heavy Fuel Oil (HFO) phase-down

HFO has historically been the dominant marine fuel, but its environmental footprint has been progressively addressed:

• MARPOL Annex VI sulphur cap (2020): 0.5 percent global limit on sulphur in fuel.
• Polar HFO ban: under MARPOL Annex I Chapter 11 (Polar Code), HFO use prohibited in Arctic from July 2024.
• EU FuelEU Maritime: well-to-wake GHG intensity reduction driving HFO substitution.

The HFO phase-down is shifting bunker fuel toward MGO, LSFO, and increasingly toward alternative fuels.

Alternative fuel pollution prevention

Alternative fuels (LNG, methanol, ammonia, hydrogen) require updated pollution prevention frameworks:

• LNG: methane slip is a GHG concern under Annex VI; cargo escape from LNG tankers is a separate concern.
• Methanol: water-soluble, less persistent in marine environment than oil; specific SOPEP-equivalent procedures.
• Ammonia: water-soluble but acutely toxic; specific spill response procedures under development.
• Hydrogen: dissipates rapidly in atmosphere; very different pollution profile from oil.

The IMO MEPC is developing updated provisions for these alternative fuels.

Microplastics and pellet pollution

Beyond oil, container ship cargo losses (X-Press Pearl 2021, others) have produced large quantities of plastic pellet pollution. Annex I does not specifically cover plastic pellet pollution but the broader MARPOL framework (especially Annex V on garbage) addresses related concerns.

Notable casualties

Torrey Canyon, 1967

The Torrey Canyon casualty (described above) was the foundational driver of MARPOL.

Amoco Cadiz, 1978

The Liberian-flagged supertanker Amoco Cadiz lost steering and ran aground off Brittany on 16 March 1978, releasing 220,000 tonnes of crude oil. The casualty drove the 1978 MARPOL Protocol.

Exxon Valdez, 1989

The American-flagged tanker Exxon Valdez ran aground in Prince William Sound, Alaska on 24 March 1989, releasing 35,000 tonnes of crude oil. The casualty prompted:

• OPA 90 (US Oil Pollution Act 1990): imposing strict liability and double-hull requirements ahead of MARPOL.
• MARPOL 1992 amendments: introducing double-hull requirements internationally.
• Strengthened ITOPF and IOPC Fund activities.

Erika and Prestige, 1999 and 2002

The Erika (1999) and Prestige (2002) casualties off France and Spain produced moderate spills (20,000 and 64,000 tonnes respectively) but with disproportionate political and regulatory impact:

• EU Regulation 417/2002: accelerated phase-out of single-hull tankers in EU waters.
• MARPOL Resolution MEPC.111(50) (2003): accelerated international phase-out.
• EU Erika III Package: comprehensive tanker safety reform.

Sanchi, 2018 and other recent casualties

The Sanchi (2018) condensate spill in the East China Sea, the MV Wakashio (2020) bunker oil spill off Mauritius, and other recent casualties have continued to drive incremental improvements in MARPOL Annex I implementation.

Crew training and Annex I

STCW provisions

Crew on Annex I-applicable ships require STCW training:

• STCW Section A-V/1 for tanker familiarisation and advanced training.
• STCW Section A-VII/1 for chief engineer and second engineer roles.
• Annex I-specific training: oily water separator operation, ORB completion, SOPEP procedures, COW operations (tankers), IGS operations (tankers).

The training is verified at PSC inspection and forms part of the operator’s safety culture.

Continuous improvement

Operator-side continuous improvement on Annex I:

• Internal audit programme verifying ORB and equipment operation.
• Pollution near-miss reporting to identify procedural weaknesses.
• Equipment maintenance tracking ensuring OWS, ODMCS, IGS reliability.
• Crew training refresher at intervals.
• Lessons learned from any pollution event in the fleet shared across vessels.

The continuous improvement cycle complements the regulatory requirement, driving compliance excellence and reducing both operational pollution and the risk of catastrophic accidental pollution. The combination of regulatory framework, operator culture, classification society oversight, port state control enforcement, insurance market pressure, and public scrutiny has progressively reduced oil pollution from shipping by approximately 80 percent since the 1970s despite substantial growth in oil tanker traffic.

Insurance and Annex I compliance

Hull and machinery insurance

Hull and machinery insurance covers physical damage to the ship and may include:

• Pollution liability as a separate coverage in some markets.
• Wreck removal which often involves substantial pollution response.
• Spill response cost as a covered expense.

H&M insurance is sensitive to compliance record: ships with detention history or with prior pollution incidents pay higher premiums.

P&I cover

The 13 IG P&I clubs collectively insure approximately 90 percent of world ocean-going tonnage and provide pollution liability coverage. P&I cover for oil pollution:

• CLC liability for tanker oil pollution (with P&I providing the compulsory insurance).
• Bunker Convention liability for non-tanker bunker spills.
• Operator liability beyond CLC limits in some jurisdictions.
• Cleanup costs beyond compensation to third parties.
• Wreck removal if pollution-related.

P&I clubs have substantial expertise in pollution claim management and provide operator support during incidents.

Underwriting and risk management

P&I clubs apply pollution-specific underwriting:

• Claim history of the operator’s fleet.
• Fleet age and condition.
• Trade route exposure (some routes are higher pollution risk).
• Crew quality and training.
• ISM and Annex I compliance evidence.
• PSC detention history.

Operators with strong pollution prevention records pay lower premiums. The insurance market provides commercial pressure for compliance excellence.

Port state control and Annex I

PSC inspection focus

PSC inspection of MARPOL Annex I focuses on:

• IOPP Certificate: validity, scope, current.
• Form A or Form B Supplement: alignment with actual ship configuration.
• Oil Record Book (Parts I and II): completeness, consistency with operations, no anomalies suggesting magic pipe operations.
• OWS operational test: starting OWS and verifying 15 ppm performance.
• 15 ppm alarm test: simulating high oil content and verifying alarm.
• Sludge tank inspection: visual and capacity verification.
• Slop tank inspection (tankers).
• SOPEP: presence on board, current contact lists, drill records.
• COW Operations Manual (tankers).
• IGS Operations Manual (tankers with IGS).
• Bunker Delivery Notes with retained samples.
• Crew familiarity with Annex I procedures verified through interview.

Common deficiencies

Common Annex I deficiencies in PSC:

• ORB entries inconsistent with engine room operations.
• OWS alarm not functional or bypassed.
• 15 ppm sensor calibration overdue.
• Sludge tank empty (suggesting unauthorized discharge).
• SOPEP contact lists outdated.
• Bunker samples missing or expired.
• Crew unable to demonstrate ORB operation.

Detention triggers

Serious Annex I deficiencies that trigger detention:

• Magic pipe arrangements (immediate detention plus criminal investigation).
• OWS non-functional with no alternative compliance method.
• ORB falsification suggesting unrecorded discharges.
• Major SOPEP gaps that would impair pollution response.
• Equipment beyond required certification (e.g. expired type approval).

Detentions for Annex I deficiencies are common across all major PSC regimes.

Special area implementation

Mediterranean Special Area

The Mediterranean was designated a Special Area in 1973 (entered into force 1983). Specific provisions:

• Operational discharge prohibited in the Special Area.
• Reception facilities required at every port and oil terminal.
• Tanker reception facility capacity sized to handle expected vessel calls.
• Enforcement: rigorous PSC and coastal state enforcement.

The Mediterranean has substantial commercial shipping (with major ports in Spain, France, Italy, Greece, Turkey, Israel, Egypt, Tunisia, Algeria, Libya, Morocco), making compliance and enforcement complex.

Baltic Special Area

The Baltic Special Area covers:

• Baltic Sea proper.
• Gulf of Bothnia.
• Gulf of Finland.
• Various entrances and approaches.

The Baltic states have implemented stringent Special Area provisions and have additional regional arrangements through HELCOM (Helsinki Commission). The Baltic is a heavily-trafficked area with substantial sensitivity to oil pollution.

Antarctic Special Area

The Antarctic Special Area covers waters south of 60 degrees South latitude. Annex I prohibits any oil discharge in the Antarctic, including discharges that would otherwise be permitted under operational provisions. The Antarctic operations are also subject to the Antarctic Treaty environmental protocols and to the Polar Code.

Arctic provisions

Although the Arctic is not formally a Special Area for oil pollution, the Polar Code Part II-A introduces:

• Heavy Fuel Oil ban in Arctic waters (effective July 2024 for new ships, July 2029 for existing).
• Tightened operational discharge limits.
• Specific casualty response preparedness for ice-affected waters.

The Arctic provisions reflect the limited environmental capacity of Arctic ecosystems to absorb oil pollution and the long-lasting effects of any spill.

Ship-to-ship (STS) transfer operations

STS at sea

Oil-to-oil ship-to-ship transfer operations at sea are governed by MARPOL Annex I Chapter 8, requiring:

• STS Operations Plan: detailed plan covering equipment, procedures, communications, emergency response.
• STS Coordinator (Person in Overall Advisory Control, POAC): typically a senior master or specialised STS coordinator overseeing the operation.
• Pre-transfer checklist: completed jointly between the two ships.
• Communications: dedicated VHF channel between the ships.
• Mooring: secure side-by-side mooring with appropriate fenders.
• Hose connection: with leak detection and pressure monitoring.
• Emergency disconnect: with quick-release mechanisms for emergency separation.
• Post-transfer documentation: in both ships’ Oil Record Books.

STS operations are common in the global oil trade for:

• Lightering large tankers before port entry where draft is restricted.
• Cargo transfer between operators or between transit modes.
• Emergency offloading of damaged or grounded tankers.

STS environmental protection

Annex I Chapter 8 specifies environmental protection during STS:

• Oil pollution preparedness: equipment on hand for spill response.
• Containment boom: deployed around the operation.
• Pre-position of response resources: at the STS location.
• Notification: to coastal authorities of the STS operation.
• Suspension of operations in adverse weather or sea state.

STS Operations Plan approval

The STS Operations Plan must be approved by:

• Flag state of each participating ship.
• Coastal state if the STS occurs in territorial waters.
• POAC certification by recognised authority.

The plan is a substantial document and requires review for each new operating area.

Pollution casualty response in detail

SOPEP activation

When an oil pollution incident occurs:

1. Immediate notification to flag state, coastal state, operator under SOPEP.
2. Damage assessment to determine spill quantity, source, ongoing flow.
3. Containment using on-board booms, plugs, transfer pumps.
4. Reporting to OPRC contacts in the affected coastal state.
5. Ship safety maintained during response operations.
6. Post-incident reporting with full details to flag state and IMO.

OPRC response

Coastal state response under OPRC:

• National Contingency Plan activation.
• Equipment mobilisation: skimmers, booms, dispersants, response vessels.
• Personnel deployment: trained response teams.
• Communications coordination: with the casualty ship and with regional partners.
• Wildlife protection: rescue and rehabilitation of affected wildlife.
• Shoreline protection: deployment of booms, sorbents, manual cleanup.
• Damage assessment: ongoing tracking of spill effects.

Compensation under CLC and IOPC Fund

Post-casualty compensation:

• CLC liability of the shipowner up to defined limits (varies by ship size, with limits increased in 1992 Protocol).
• IOPC Fund supplementary compensation when CLC limits exceeded.
• Bunker Convention for non-tanker bunker spills.
• HNS Convention for hazardous and noxious substance spills (not yet in force).
• Civil litigation in cases not fully covered by treaty.

The combined liability regime ensures that most oil pollution damage receives compensation, with claims processed through claims administrators and reimbursed by the responsible party plus the IOPC Fund.

Annex I and bunker oil

Bunker fuel as oil

Annex I covers bunker fuel as “oil” because:

• Bunker fuel is heavy oil similar to cargo oil in pollution profile.
• Bunker spills from non-tanker ships have been responsible for substantial pollution events (MV Wakashio 2020, Cosco Busan 2007).
• Bunker is carried in significant quantities even on cargo ships (typical Capesize bulk carrier carries 3,000+ tonnes of bunker).

Fuel oil tank protection

The 2010 amendments (Resolution MEPC.176(58)) introduced fuel oil tank protection for non-tanker ships:

• New ships above 600 deadweight tonnes built after 1 August 2010 must have fuel oil tanks protected by:
  • Double bottom of at least 2 metres.
  • Side protection of at least 0.4-0.7 metres (varies by ship type).
• Smaller fuel oil tanks with capacity below specific thresholds may be exempt.
• Pre-2010 ships are not subject to retrofit but face increasing PSC and insurance pressure.

The fuel oil tank protection is the analogue of double-hull cargo tanks, addressing bunker spill risk.

Bunker Convention 2001

The International Convention on Civil Liability for Bunker Oil Pollution Damage 2001 (Bunker Convention):

• Entered into force November 2008.
• Applies to all ships carrying bunker fuel (regardless of size).
• Imposes strict liability on the shipowner for bunker oil pollution.
• Requires compulsory insurance.
• Compensates third parties for damage.

The Bunker Convention has expanded the liability framework to non-tanker ships, addressing the gap that previously left bunker spill victims without recourse.

Specific compliance areas in detail

15 ppm OWS in detail

The 15 ppm Oily Water Separator is the principal control on operational oil pollution from machinery spaces. Its operation involves:

• Bilge water collection: bilge water from the engine room and other machinery spaces is collected in a holding tank.
• Pumping to the separator: a dedicated bilge pump transfers water to the OWS.
• Coalescing separation: the OWS contains coalescing media (typically polymer matrix or absorbent material) that promotes oil droplet aggregation.
• Gravity separation: oil droplets rise to the top, water settles to the bottom.
• Multi-stage: modern OWS typically has multiple stages with progressively finer separation.
• 15 ppm sensor: continuous monitoring of effluent oil content with the alarm at 15 ppm.
• Three-way valve: returns water to the bilge tank if 15 ppm is exceeded.
• Discharge through ship’s side: when oil content is below 15 ppm.

The recovered oil is returned to the sludge tank for shore-side disposal.

OWS bypass and “magic pipe” issues

A persistent issue in MARPOL Annex I enforcement is OWS bypass (informally called “magic pipe” arrangements): unauthorized piping that bypasses the OWS, allowing direct discharge of bilge water without separation. Magic pipe cases:

• Detection: typically through whistleblower reports, port state inspector observation, or comparison of OWS records with engine room oil consumption.
• Penalties: substantial fines under flag state and port state law. Multiple cases have produced fines of millions of dollars per incident.
• Crew impact: criminal prosecution of crew members involved.
• Operator impact: detention of fleet, increased PSC scrutiny, reputational damage.

The US Coast Guard, Paris MOU and other PSC regimes prioritise OWS inspection because of the magic pipe history.

Sludge tank arrangements

The sludge tank capacity must be sized for typical voyage duration:

• Daily sludge generation: typically 1 to 2 cubic metres per day for a moderate-sized cargo ship; more for larger ships with multiple engines.
• Voyage duration: typically 2 to 8 weeks between port calls with reception facilities.
• Capacity sizing: typically 2 to 4 weeks of sludge generation, allowing margin.
• Heating arrangements: for high-viscosity sludges that require warming for transfer.
• Pump arrangements: for transfer to shore reception facilities.

The sludge tank operations are recorded in the Oil Record Book Part I.

Slop tank arrangements

Slop tanks on tankers are typically:

• Located at the cargo tank end for ease of cargo access.
• Sized at 2 to 3 percent of cargo capacity.
• Heating arrangements for high-viscosity slops.
• Decant arrangements for separation of water from oil after settling.
• Fixed piping to receive slops from cargo operations and to deliver to shore.

Slop operations are recorded in the Oil Record Book Part II.

Crude oil washing in detail

COW principles

Crude Oil Washing uses the cargo itself as a tank-cleaning medium:

• Spray nozzles directed at tank surfaces from rotating mechanisms.
• High-pressure cargo flow from the ship’s cargo pumps.
• Tank rotation: tanks are washed in sequence, with each tank cleaned during the latter portion of cargo discharge.
• Cargo recovery: the wash flushes residue back into the cargo system, recovering it as cargo rather than discharging as slop.

COW benefits

COW provides several benefits:

• No additional tank washing water needed (water washing is supplemented or eliminated).
• More effective cleaning because cargo dissolves residue better than water.
• No production of slops that would require disposal.
• Faster turnaround at discharge ports.
• Cargo recovery of residue value.

COW limitations

COW has limitations:

• Crude oil only: does not work for product cargoes that require water washing.
• Inert gas requirement: tanks must be inerted before COW operation.
• Equipment cost: COW machinery is substantial.
• Operational complexity: COW operations require careful coordination.

COW Operations Manual

The COW Operations Manual is mandatory on tankers fitted with COW. It contains:

• System description: equipment, capacity, pressure ratings.
• Operating procedures: pre-operation checks, washing sequence, post-operation procedures.
• Safety provisions: oxygen monitoring, ventilation, fire safety.
• Maintenance requirements: equipment inspection and testing.
• Records to be maintained: ORB Part II entries and cargo handling records.

The manual is approved by the flag state and verified at PSC inspection.

Inert Gas System in detail

IGS sources

Sources of inert gas on tankers include:

• Boiler exhaust gas: scrubbed to remove SO2 and particulates, cooled and dehumidified.
• Dedicated inert gas generator: purpose-built equipment combusting fuel to produce inert gas.
• Stored inert gas: nitrogen tanks for some specialised applications.

The choice depends on tanker size, cargo type and specific design constraints.

IGS operation

IGS operation:

• Tank inerting during cargo discharge: replacing cargo volume with inert gas to maintain ullage atmosphere.
• Tank purging: prior to gas-freeing for tank entry, replacing inert gas with hydrocarbon-free air.
• Continuous operation during voyage in laden condition with cargo at potentially flammable concentration.
• Pressure maintenance: typical positive pressure of 50 to 200 mm water gauge above atmospheric.
• Oxygen monitoring: continuous, with alarm if oxygen rises above 8 percent.
• Dew point control: dehumidification to prevent corrosion.

IGS components

A typical IGS includes:

• Combustion chamber (for dedicated generator) or boiler tap-off (for boiler-flue source).
• Scrubber: removes sulphur, particulates, NOx.
• Cooler: reduces gas temperature.
• Demister: removes water droplets.
• Distribution piping to each cargo tank.
• Pressure-vacuum valves on each tank.
• Tank-by-tank shutoff valves.
• Monitoring instrumentation: pressure, oxygen, temperature.

Post-incident IGS regulation

After tanker explosions in the 1970s and 1980s, IGS requirements were progressively tightened:

• Mandatory IGS for crude oil tankers above 20,000 deadweight tonnes.
• Mandatory IGS for product tankers above defined size.
• Performance standards specifying oxygen content, dew point, pressure.
• Operational discipline in IGS use during loading, transit and discharge.

Modern tankers have very low rates of cargo tank explosion, reflecting the effectiveness of IGS combined with operational discipline.

Operational discharge in practice

Pre-discharge planning

Before any operational discharge:

• Position verification: GPS confirms ship is more than 50 nautical miles from land.
• Ship in transit: confirmed by speed-over-ground monitoring.
• Equipment functionality: OWS, ODMCS, 15 ppm alarm operational.
• Discharge limits planning: ensuring the discharge will not exceed the cumulative voyage limit.
• Master’s authorisation: required for any operational discharge.

Discharge execution

During discharge:

• OWS operating with verified 15 ppm performance.
• ODMCS recording the discharge data.
• Continuous monitoring by the engineer responsible for the operation.
• Immediate stop if any equipment fault or limit violation.

Post-discharge documentation

After discharge:

• Oil Record Book entry with all required data (time, position, quantity, oil content, equipment used).
• ODMCS data retained for verification.
• Supervisor sign-off on the operation.

The discipline of pre-discharge, discharge and post-discharge documentation is the principal operational control.

Tank cleaning operations

Pre-cleaning preparation

Before tank cleaning:

• Cargo discharge completed with cargo lines drained.
• Tank ventilation if cleaning under non-inert conditions.
• Tank inerting for crude oil tank cleaning.
• Personnel briefing on tank cleaning procedures and emergency response.
• Equipment readiness: cleaning machines, pumps, monitoring equipment.

Cleaning operation

Tank cleaning involves:

• Hydrostatic cleaning with high-pressure water through fixed cleaning machines.
• COW for crude oil tanks (where applicable).
• Chemical cleaning for specific cargo residues.
• Manual cleaning of areas not reached by fixed equipment.

Post-cleaning verification

After cleaning:

• Visual inspection of tank surfaces.
• Chemical testing for residual cargo.
• Atmosphere testing before entry: oxygen 21 percent, hydrocarbon below 1 percent LEL, toxic gases below threshold.
• Permit-to-enter documentation before any tank entry.

Tank cleaning is one of the highest-risk operations on tankers and is subject to strict procedure.

Bunker fuel handling

BDN and sample retention

Each bunker delivery generates:

• Bunker Delivery Note (BDN): stating fuel grade, quantity, density, viscosity, sulphur content, flash point.
• Bunker Fuel Sample: representative sample of the delivered fuel, retained on board for compliance verification.
• Master’s signature confirming receipt and acceptance.

The BDN is preserved for at least 3 years and is checked at PSC inspection.

MARPOL Annex VI fuel quality

Fuel quality under MARPOL Annex VI requirements:

• Sulphur content below the global limit (0.5 percent since 2020) or the SECA limit (0.1 percent in Sulphur Emission Control Areas).
• Flash point above 60 degrees Celsius (above 43 degrees Celsius for emergency generator fuel).
• Density and viscosity within ISO 8217 specifications.

Non-compliance can lead to detention and operational restrictions.

Spill prevention during bunkering

Bunker operations are major spill risk events. Spill prevention includes:

• Drip trays under bunker hose connections.
• Containment booms around the ship during operations.
• Emergency stop at multiple locations along the transfer route.
• Communications protocols between ship and supplier.
• Pre-transfer checklist completed jointly.
• Smoking ban on the entire ship during bunkering.
• Hot work prohibition in the vicinity.

A bunker spill (even small) is an oil pollution incident triggering SOPEP activation.

Documentation

Every ship covered by Annex I carries on board:

• IOPP Certificate with Form A or Form B Supplement.
• Oil Record Book Part I (machinery space operations).
• Oil Record Book Part II (cargo/ballast operations, tankers).
• SOPEP with current contact lists and procedures.
• COW Operations Manual (crude oil tankers).
• IGS Operations Manual (tankers with IGS).
• Tank Cleaning Manual.
• Oil Pollution Emergency Plan integrated with SOPEP.
• OWS Operating Manual with type approval certificate.
• ODMCS Operating Manual (where fitted).
• Bunker Delivery Notes with retained samples.
• Crew training records in oil pollution prevention.
• PSC inspection records.

Related Calculators

• MARPOL Annex I/4, Oil discharge Calculator
• MARPOL Annex I/12, Tanks for oil residues Calculator
• MARPOL Annex I/12A, Fuel oil tank protection Calculator
• MARPOL Annex I/14, Oil filtering equipment Calculator
• MARPOL Annex I - OWS sizing (15 ppm bilge separator) Calculator

See also

• MARPOL Convention parent article
• SOLAS Chapter II-1: Construction, Subdivision, Stability, Machinery and Electrical Installations
• SOLAS Chapter VII: Carriage of Dangerous Goods
• Oil Tanker
• Heavy Fuel Oil
• Marine Gas Oil
• MARPOL Annex VI
• Polar Code
• Hong Kong Convention
• Ballast Water Management Convention

References

• IMO, International Convention for the Prevention of Pollution from Ships (MARPOL), 1973, as modified by 1978 Protocol, as amended, Annex I.
• IMO Resolution MEPC.51(32) (1992), Adoption of amendments introducing double-hull requirements.
• IMO Resolution MEPC.111(50) (2003), Accelerated phase-out of single-hull tankers.
• IMO Resolution MEPC.117(52) (2004), Adoption of revised Annex I.
• IMO Resolution MEPC.107(49), Performance standards for oily water separators.
• IMO Resolution MEPC.108(49), Guidelines for the use of segregated ballast tanks.
• OPRC Convention 1990 and 2000 Protocol.
• CLC 1969 and 1992 Protocol.
• Bunker Convention 2001.
• IOPC Fund Convention 1971 and 1992 Protocol.
• ITOPF (International Tanker Owners Pollution Federation) annual reports and casualty studies.