MARPOL Annex II: Noxious Liquid Substances

Background

The chemical tanker industry context

The chemical tanker industry carries a wide diversity of cargoes:

• Petrochemicals: ethylene dichloride, ethylene oxide, methanol, acetone, xylene, styrene, propylene oxide.
• Industrial chemicals: caustic soda, sulphuric acid, hydrochloric acid, ammonia solutions, nitric acid.
• Food-grade products: vegetable oils (palm, coconut, sunflower, soybean), fish oil, glycerol, fatty acids.
• Renewable feedstocks: biofuels, biodiesel, ethanol, methanol from biomass.
• Specialty chemicals: surfactants, detergents, lubricant additives, polymer feedstocks.

The total chemical tanker fleet is approximately 5,000 vessels with global trade volume of approximately 350 million tonnes per year. Routes are diverse, covering refining centres in the Middle East, Asia, USA Gulf and Europe.

Why MARPOL Annex II is needed

Chemical pollution from ships presents different challenges from oil pollution:

• Diverse cargo properties: chemicals vary widely in density, solubility, toxicity, persistence, biodegradability.
• Acute toxicity: many chemical cargoes are acutely toxic to marine life at low concentrations.
• Reactivity: some cargoes react with seawater, with other cargoes, or with cargo residues.
• Cumulative effects: persistent chemicals accumulate in marine ecosystems.

Annex II addresses these challenges through the categorisation system that matches discharge restrictions to environmental hazard.

Major amendment history

• 1973: Annex II adoption (with the rest of MARPOL).
• 1983: Annex II entered into force (delayed compared with Annex I).
• 2004 amendments (entered into force 2007): Resolution MEPC.118(52) substantial revision adopting the four-category system (X, Y, Z, OS).
• 2007 amendments: technical updates to the categorisation methodology.
• 2010 amendments: clarification of pre-wash requirements.
• 2017 amendments: alignment with fuel oil quality requirements.
• 2020-2024: ongoing cycles addressing biofuels, methanol, and other emerging cargoes.

The 2007 transition

The 2007 amendments transitioned from the previous A, B, C, D categories to the current X, Y, Z, OS structure. The transition:

• Reflected the GESAMP/EHS Database: the Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection’s hazard profile system.
• Improved alignment: with global chemical regulatory frameworks (e.g. GHS Globally Harmonized System).
• Tightened some categories: substances previously Category B may have moved to X or Y.
• Loosened others: some substances previously Category C may have moved to Z or OS.
• Required ship reclassification: ship type designations updated based on new cargo categorisation.

The 2007 transition was managed over a multi-year period to allow industry adjustment.

Application and definitions

Application

Annex II applies to:

• Chemical tankers of any size on international voyages.
• Combination carriers (OBO) when carrying chemical cargoes in bulk.
• Other ships carrying NLS in bulk (rare, typically under specific exemption arrangements).

The application is broader than Annex I in that it covers chemical tankers from very small (e.g. 500 GT product tankers) to very large (50,000 deadweight chemical tankers).

Definitions

Key definitions:

• Noxious Liquid Substance (NLS): any substance indicated in the IBC Code Chapter 17 cargo list as Category X, Y, Z, or OS, when carried in bulk.
• Chemical tanker: a ship constructed or adapted primarily to carry NLS in bulk.
• NLS tanker: legacy term, generally synonymous with chemical tanker.
• Combination carrier (OBO): as defined in Annex I, when carrying NLS.
• Pre-wash: a cargo tank wash conducted before the ship begins ballasting after discharge of high-residue cargo.
• Stripping: the process of removing as much cargo as possible from the cargo tank using ship’s pumps.

Categorisation (Regulation 3)

The four categories

The four NLS categories are defined by environmental hazard:

• Category X: Substances which, if discharged into the sea from tank cleaning or deballasting operations, are deemed to present a major hazard to either marine resources or human health and, therefore, justify the prohibition of the discharge into the marine environment. Discharge of Category X cargo residue is prohibited outside reception facilities.

• Category Y: Substances which, if discharged into the sea from tank cleaning or deballasting operations, are deemed to present a hazard to either marine resources or human health or cause harm to amenities or other legitimate uses of the sea and therefore justify a limitation on the quality and quantity of the discharge into the marine environment. Discharge requires pre-wash, controlled discharge under specific conditions, and detailed record-keeping.

• Category Z: Substances which, if discharged into the sea from tank cleaning or deballasting operations, are deemed to present a minor hazard to either marine resources or human health and therefore justify less stringent restrictions. Discharge is permitted under specified operational conditions.

• Other Substances (OS): Substances which have been evaluated and fall outside Category X, Y or Z because they are considered to present no harm to marine resources, human health, amenities or other legitimate uses of the sea. OS cargoes do not require Annex II discharge controls but may still be subject to general MARPOL operational provisions.

Categorisation methodology

The IMO uses the GESAMP/EHS Database (Hazard Profile system) to categorise substances. The methodology assesses:

• Bioaccumulation: tendency to accumulate in marine organisms.
• Biodegradation: rate of breakdown in marine environment.
• Acute toxicity: short-term toxicity to marine organisms.
• Chronic toxicity: long-term effects.
• Mammal effects: acute and chronic toxicity to mammals.
• Fish toxicity.
• Other endpoints: irritation, sensitization, carcinogenicity, mutagenicity.

Each substance receives a multi-dimensional hazard score, and the categorisation is determined by the worst-case dimension.

IBC Code Cargo List (Chapter 17)

The IBC Code Chapter 17 contains the full Cargo List of approximately 6,000+ entries:

• Cargo name and synonyms.
• UN number where applicable.
• MARPOL Annex II category (X, Y, Z, or OS).
• IBC Code Ship Type (1, 2, or 3).
• Tank type (1, 2, 3, IIG, etc.).
• Pollution Category specifically.
• Hazards: fire, health, reactivity, special.
• Carriage requirements: tank arrangement, materials, ventilation, fire protection, electrical, vapour control, personnel protection, environmental control.
• Operational requirements: cargo information, equipment, special.

The Cargo List is updated through MEPC and MSC Resolutions as new cargoes are evaluated.

Discharge requirements (Regulation 5)

Category X discharge prohibition

For Category X cargoes:

• Discharge of cargo residue is prohibited outside reception facilities.
• Pre-wash of cargo tanks is required before the ship sails from the discharge port.
• Pre-wash water must be discharged to reception facility or held in a separate tank for later reception.
• Tank residue concentration must be reduced to specified levels (typically below 0.1 percent by mass) before any operational discharge.

The Category X regime effectively requires zero environmental release of these high-hazard substances.

Category Y discharge restrictions

For Category Y cargoes, discharge is permitted only under specific conditions:

• Tanker is in transit (en route) at speed no less than 7 knots.
• Tanker is at least 12 nautical miles from nearest land.
• Water depth is at least 25 metres.
• Discharge rate is below 1 cubic metre per nautical mile (varies by category sub-classification).
• Cargo residue concentration in discharged water is below the limit specified.
• Discharge takes place below the waterline through approved discharge outlets.

Pre-wash is also required for high-residue Category Y cargoes.

Category Z discharge

For Category Z cargoes, discharge is permitted under similar but less restrictive conditions:

• In transit at minimum speed.
• At least 12 nautical miles from nearest land.
• Below the waterline through approved discharge outlets.

The reduced restrictions reflect the lower environmental hazard.

Pre-wash requirement

Pre-wash is required before ballasting after Category X cargo and after high-residue Category Y cargo. The pre-wash:

• Removes residue from the cargo tank by water washing.
• Discharges to reception facility (Category X) or to slop tank for later disposal.
• Documents the pre-wash in the Cargo Record Book.
• Reduces residue concentration to the level required for subsequent discharge.

The pre-wash is a substantial operational step that adds time to chemical tanker port turnaround.

Cargo Record Book (Regulation 9)

The Cargo Record Book is mandatory on all chemical tankers and records:

• Cargo loading and discharge: with date, time, position, cargo grade, quantity.
• Internal cargo transfers between tanks.
• Tank cleaning and washing operations: with method, water source, discharge destination.
• Pre-wash operations (Category X and high-residue Y).
• Discharge of cargo residue: with date, time, position, quantity, equipment used.
• Slop disposal: to reception facility or other authorised destination.
• Ballast operations in cargo tanks (where applicable for combination carriers).
• Any unusual events affecting cargo or pollution prevention.

Each entry is signed by the responsible officer. The Cargo Record Book is preserved for at least 3 years and is checked at PSC inspection.

NLS Certificate (Regulation 11)

The International Pollution Prevention Certificate for the Carriage of Noxious Liquid Substances in Bulk (NLS Certificate) is issued upon successful initial survey:

• Confirms compliance with Annex II.
• Lists ship type (1, 2, or 3) under IBC Code.
• Lists authorised cargoes that the ship may carry.
• Includes Form Supplement detailing equipment and arrangements.
• Valid for 5 years with annual surveys to maintain validity.
• Required on board and produced for PSC inspection.

The NLS Certificate is the primary regulatory document for chemical tanker operations.

P&A Manual (Regulation 12)

Manual content

The Procedures and Arrangements Manual (P&A Manual) is mandatory and contains:

• Tank arrangement and capacity for each cargo tank.
• Cargo handling system description including pumps, piping, valves, monitoring instrumentation.
• Stripping system capability and procedures.
• Tank cleaning system including washing machines, hoses, fittings.
• Discharge piping for cargo residue.
• Slop tank arrangement and capacity.
• Reception facility procedures for connecting to shore facilities.
• Cargo-specific procedures for each cargo grade authorised.
• Tank cleaning procedures by cargo type and post-cargo combination.
• Pre-wash procedures for Category X and high-residue Y cargoes.

The manual is approved by the flag state and is the primary operational reference for cargo handling officers.

Approval and updating

The P&A Manual is approved by:

• Flag state (typically through delegated Recognised Organization).
• Updated when ship is modified, when new cargo grades are authorised, or when operational procedures change.
• Reviewed at periodic surveys to confirm current accuracy.

Ship-to-ship transfer (Regulation 13)

Ship-to-ship transfer of NLS is governed by Regulation 13 with provisions similar to Annex I Chapter 8 but with NLS-specific considerations:

• STS Operations Plan for chemical cargoes.
• Cargo compatibility verification before transfer.
• Contamination prevention between cargo grades.
• Sampling before, during and after transfer.
• Pre-transfer checklist with NLS-specific items.
• Post-transfer documentation in Cargo Record Books of both ships.

STS operations are common in the chemical tanker trade for grade-specific cargo movements.

SMPEP (Regulation 14)

The Shipboard Marine Pollution Emergency Plan for Noxious Liquid Substances (SMPEP) is mandatory and addresses chemical-specific emergency response:

• Notification procedures to flag state, coastal state, and operator.
• Emergency response procedures for various scenarios:
  • Cargo tank breach with residue release.
  • Cargo tank explosion or fire.
  • Cargo system leak with deck contamination.
  • Cargo-water reaction (for water-reactive cargoes).
  • Toxic gas release from cargo or cargo residue.
• Personnel protection during emergency response.
• Decontamination procedures.
• Documentation during and after the incident.

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

Ship types under IBC Code

Type 1 chemical tankers

Type 1 chemical tankers are designed for cargoes presenting very severe environmental and safety hazards. Survival of damage is the highest priority. Specific characteristics:

• Maximum cargo tank size limited to reduce single-tank loss in damage.
• Comprehensive double hull with maximum void width.
• Two-compartment damage survival required.
• Comprehensive segregation between incompatible cargo systems.
• Higher cargo containment standards.

Type 1 ships carry the most hazardous cargoes (e.g. hydrofluoric acid, methyl bromide, certain chlorides, carbon disulphide). They are a small subset of the chemical tanker fleet.

Type 2 chemical tankers

Type 2 chemical tankers are designed for cargoes presenting appreciable environmental and safety hazards. Specific characteristics:

• Cargo tank size limits less stringent than Type 1.
• Double hull with adequate void width.
• Single-compartment damage survival typically required.
• Cargo system segregation for incompatible chemicals.

Type 2 covers many common chemical cargoes (e.g. methanol, certain pesticides, certain petrochemicals). Type 2 ships are the majority of the chemical tanker fleet.

Type 3 chemical tankers

Type 3 chemical tankers are designed for cargoes presenting less severe hazards. Specific characteristics:

• Larger cargo tank sizes permitted.
• Single-hull may be acceptable for some Type 3 cargoes.
• Lighter damage stability requirements.
• Less stringent cargo system segregation.

Type 3 covers many bulk chemical cargoes (e.g. caustic soda, certain alcohols, vegetable oils). Type 3 ships are common in the regional and product trade.

Vegetable oil cargoes

Vegetable oil categorisation

Vegetable oils (palm oil, coconut oil, sunflower oil, soybean oil, corn oil, rapeseed oil, etc.) are typically:

• Category Y or Z under Annex II.
• Type 2 or Type 3 ships under IBC Code.
• Subject to pre-wash for higher-residue cargoes.

The vegetable oil trade is substantial (approximately 80 million tonnes per year) and is a major segment of the chemical tanker market. Specialised vegoil tankers are common.

Food-grade considerations

Vegetable oils for food use require additional considerations beyond Annex II:

• Tank coating compatibility: food-grade epoxy or stainless steel.
• Cleanliness verification: visual inspection plus chemical testing.
• Previous cargo restrictions: limits on what cargoes can precede food-grade vegoil.
• Documentation: Voyage Charter Party requirements often more stringent than MARPOL.

The food-grade requirements are typically commercial rather than regulatory but are operationally significant.

Biofuel cargoes

Biodiesel and bioethanol

Biofuels are an emerging cargo category:

• Biodiesel (FAME): typically Category Y or Z under Annex II.
• Bioethanol: Category Z or OS depending on water content.
• Methanol from biomass: same as fossil methanol categorisation.
• Pyrolysis oil: under evaluation.

The biofuel trade is growing rapidly with FuelEU Maritime and similar regulatory drivers.

MARPOL adaptation for biofuels

The IMO MEPC has been progressively adapting Annex II to biofuels:

• Categorisation of new biofuels as they enter commercial trade.
• Tank cleaning requirements for biofuel/conventional fuel transitions.
• Quality control for biofuel cargoes.

The framework is evolving and will continue to be revised as biofuel commerce grows.

Pollution prevention in chemical tanker operations

Operational measures

Chemical tanker operators implement multiple pollution prevention measures:

• Crew training in chemical handling, MARPOL Annex II compliance, and emergency response.
• Equipment maintenance ensuring stripping pumps, washing equipment, and monitoring instrumentation are functional.
• Operational discipline during cargo handling, tank cleaning, and discharge.
• Cargo compatibility verification before each loading.
• Incident reporting including near-misses to identify procedural weaknesses.

P&I and insurance

Chemical tanker P&I cover includes:

• Pollution liability for chemical cargo spills.
• Cargo claims for contamination, off-spec, or off-load damage.
• Crew injury including chemical exposure incidents.
• Wreck removal which often involves substantial chemical residue.

The P&I underwriting reflects the higher liability exposure of chemical tankers compared with other ship types.

Vetting

The chemical tanker industry uses vetting schemes for charterer due diligence:

• CDI (Chemical Distribution Institute): industry-wide vetting scheme for chemical tankers.
• OCIMF SIRE 2.0: oil and chemical tanker inspection programme.
• Operator-specific vetting: major charterers (Dow, BASF, Sinopec, Reliance) maintain their own vetting.

Vetting findings are shared (within industry) and contribute to the cumulative reputation of operators and ships.

Notable casualties

MV Bow Mariner, 2004

The chemical tanker MV Bow Mariner exploded and sank off Virginia, USA on 28 February 2004 with 21 dead. The explosion occurred during tank cleaning of methyl tertiary butyl ether (MTBE) residue. Investigation found:

• Crew had not adequately purged the cargo tank of MTBE vapour before tank cleaning.
• Ignition source (likely static electricity) ignited the residual flammable atmosphere.
• Inert gas system was not used during the cleaning operation.

The casualty drove tightening of inert gas requirements during chemical tanker tank cleaning and reinforced the importance of crew training under STCW Section A-V/1.

MV Stolt Valor, 2012

The MV Stolt Valor explosion in the Indian Ocean in 2012 (1 dead) involved a similar pattern of tank cleaning operation with inadequate inerting. Post-incident review reinforced the lessons from Bow Mariner.

MV Sanchi, 2018

The Iranian-flagged tanker Sanchi (described in Annex I article) was carrying condensate (light petroleum) cargo. While the cargo was technically Class 3 (flammable liquid) rather than Annex II NLS, the casualty illustrated the volatile cargo handling challenges relevant to chemical tankers.

Chemical tanker pollution incidents

Smaller chemical tanker pollution incidents occur with some frequency:

• Tank cleaning discharges that exceed Annex II limits (typically operational discipline failures).
• Pre-wash water disposal to reception facilities that prove unable to handle the volume.
• Cargo system leaks during loading or discharge.
• Tank cleaning operations with crew injury from chemical exposure.

These incidents are typically managed through operational and regulatory response without causing major environmental damage.

Reception facility implementation in detail

Major chemical port reception facilities

Major chemical ports have substantial reception facility infrastructure:

• Antwerp, Rotterdam, Hamburg, Le Havre: Northwest European chemical hubs with high-capacity reception facilities for Category X residue, slop disposal, and sludge.
• Houston, New Orleans, Texas City, Pasadena: US Gulf chemical ports with comprehensive reception facilities.
• Singapore, Jurong Island, Tuas: Southeast Asian chemical hub with dedicated reception infrastructure.
• Yokohama, Yokkaichi, Mizushima, Ulsan, Kaohsiung, Onsan: Northeast Asian chemical ports.
• Jubail, Yanbu, Ras Tanura, Sitra: Middle East chemical ports with growing reception facility capacity.

Each reception facility typically has:

• Tank capacity: for various cargo residue grades.
• Receiving connections: hose connections compatible with ship discharge manifolds.
• Onshore treatment: chemical processing of received residues for safe disposal or recycling.
• Documentation system: integrated with Cargo Record Book entries on the ship.

Reception facility cost

Reception facility costs:

• Per unit volume basis: typically charged per cubic metre.
• Premium rates for Category X (high-hazard) residue.
• Disposal complexity surcharge: for difficult-to-treat materials.
• Time-based fees: for complex reception operations.

The cost is typically passed through to cargo interests rather than absorbed by the ship.

Reception facility availability gaps

Specific gaps in global reception facility availability:

• Smaller chemical ports: may lack capacity for Category X residue, requiring ships to retain residue and discharge at the next major port.
• Remote chemical operations: limited reception nearby, with ships sometimes required to make extended additional voyages to reach reception capacity.
• Specific cargo types: some unusual chemical cargoes have very limited reception infrastructure, with only specialised facilities able to handle them.
• Developing region ports: may have limited capacity, often resulting in pressure on operators to make do with sub-optimal arrangements or to pay premium rates.

The IMO IMSAS audit and PSC inspection identify port reception facility gaps as a recurring finding, and the IMO Technical Cooperation programme provides ongoing support to developing port states in expanding their reception facility capacity to meet MARPOL Annex II expectations.

Insurance and liability for chemical tankers

P&I cover

Chemical tanker P&I cover is more comprehensive than oil tanker:

• Pollution liability for chemical cargo spills under coastal state law.
• Cargo claims for contamination, off-spec, off-load damage.
• Crew injury including chemical exposure.
• Wreck removal including chemical residue management.
• Environmental damage beyond direct pollution.
• Specialised response coverage: chemical-specific spill response equipment and personnel.

P&I premiums for chemical tankers reflect:

• Higher exposure compared with oil tankers.
• Cargo-specific risk: certain cargoes (e.g. styrene, MTBE) attract higher premiums.
• Operator safety record: clean records reduce premiums substantially.
• Trade route exposure: routes through sensitive areas attract higher premiums.

HNS Convention

The HNS Convention 2010 Protocol (International Convention on Liability and Compensation for Damage in connection with the Carriage of Hazardous and Noxious Substances by Sea) is the chemical-specific liability framework:

• Provides liability for HNS spill damage similar to CLC for oil.
• Includes a HNS Fund supplementary compensation mechanism.
• Has not yet entered into force as of 2026 due to slow ratification.
• Will provide more comprehensive coverage when in force.

Charter party pollution clauses

Chemical tanker charter parties typically include:

• Pollution clauses allocating responsibility between owner and charterer.
• Compliance warranties for MARPOL Annex II.
• Vetting requirements specified.
• Indemnity provisions for non-compliance.
• Termination rights for serious safety or compliance issues.

The charter party framework provides commercial reinforcement of regulatory compliance.

Future developments

Categorisation evolution

Ongoing developments in NLS categorisation:

• Endocrine disruptors: emerging category for substances affecting marine and human endocrine systems.
• PFAS (per- and polyfluoroalkyl substances): increasing global regulation, MARPOL adaptation under consideration.
• Microplastic precursors: pellets and similar cargoes that produce microplastic pollution.
• Novel biofuels: adaptation as new biofuel chemistries enter trade.
• Synthetic chemicals: e-fuels and synthetic intermediates.

The MEPC continually reviews substances for re-categorisation as scientific understanding evolves.

Equipment evolution

Equipment evolution for chemical tankers:

• Improved stripping efficiency: through improved pump and tank designs.
• Better tank cleaning: through new cleaning machine designs and improved chemicals.
• Continuous monitoring: of cargo system integrity.
• Digital documentation: replacing paper Cargo Record Book with electronic systems.
• Vapour control: reducing emissions during loading.

Regulatory evolution

Regulatory evolution areas:

• Cyber security of cargo system controls (post-MSC.428(98)).
• Decarbonisation impact on chemical tanker fuel and operations.
• Biofuel-specific provisions as biofuel trade expands.
• Harmonisation with land-based chemical regulation (REACH in EU, similar elsewhere).

The regulatory evolution will continue to shape Annex II in coming decades.

Pollution incidents and lessons

Tank cleaning incidents

Recurring pollution incidents during tank cleaning:

• Operational discharge above limits: typically due to OWS or ODMCS malfunction during cleaning.
• Pre-wash discharge ashore: when reception facility cannot accept the volume, leading to pressure to discharge at sea.
• Cargo residue carryover between voyages: affecting subsequent cargo quality and creating contamination liability.
• Slop tank overflow: during cleaning operations.

These incidents are typically managed through operational discipline, regulatory inspection and operator training.

Cargo system leaks

Cargo system leaks during operations:

• Manifold leaks during loading or discharge.
• Hose ruptures during transfer.
• Valve failures allowing cargo into bilge or onto deck.
• Tank sounding tube leaks affecting tank gauging.

Each leak triggers SMPEP activation and reporting obligations.

Major casualties

Major chemical tanker casualties (beyond those discussed earlier):

• Erika 1999: although primarily an oil pollution incident, illustrated tanker safety issues relevant across categories.
• Hebei Spirit 2007: oil spill but in proximity to chemical operations.
• MV Sanchi 2018: condensate cargo, illustrating volatile fuel handling.
• Various smaller chemical tanker incidents: with operational discipline and equipment failure as common contributors.

Lessons applied

Lessons from chemical tanker incidents have driven:

• Tightened operational procedures in cargo handling and tank cleaning.
• Improved equipment reliability through type approval refinement.
• Enhanced crew training under STCW Section A-V/1.
• Better vetting through CDI and similar programmes.
• Insurance pricing differentiation based on safety performance.

The post-incident regulatory and industry response has progressively reduced chemical tanker pollution despite trade growth.

Charter market and commercial structure

Voyage charter for chemical tankers

Chemical tankers operate under various charter arrangements:

• Voyage charter: for spot trade, typically with specific cargo and route.
• Time charter: for sustained relationships, with the charterer providing direction within commercial parameters.
• Bareboat charter: less common for chemical tankers but used in some structures.
• Pool arrangements: groups of tankers operated commercially under a single pool manager.

Each arrangement has specific implications for MARPOL Annex II compliance:

• Owner remains the regulatory party for the ship.
• Operator (charterer in time charter) has operational responsibility.
• Vetting requirements apply across the arrangement.

Commercial pressures

Chemical tanker commercial pressures:

• Cargo grade premium: ships with broad cargo capability earn premium.
• Vetting acceptance: ships with strong vetting records command preference.
• Voyage execution speed: efficient cargo handling and tank cleaning support competitive freight rates.
• Compliance record: clean PSC and MARPOL record reduces operational risk premium.

The commercial pressures align with regulatory compliance goals, providing market-based incentives for high standards.

Specialty cargo handling

Heated cargoes

Many chemical cargoes require heating during transport:

• Phenol: solid at ambient temperature, requires heating to typically 50 to 60 degrees Celsius for transport.
• Acetic anhydride: requires modest heating.
• Various waxes and oils: require heating to maintain flow properties.
• Heavy chemicals: may require heating depending on viscosity.

Heated cargo systems include:

• Tank heating coils: typically using ship’s steam system, with control valves for temperature management.
• Insulation: cargo tanks with external insulation to retain heat.
• Temperature monitoring: continuous monitoring with alarm if outside limits.
• Cooling provisions: for cargoes with thermal runaway potential.

Refrigerated cargoes

Some cargoes require refrigeration:

• VCM (Vinyl Chloride Monomer): transported below 60 degrees Celsius to prevent polymerisation.
• Butadiene: refrigerated.
• Specific liquid gases (carried under IGC Code rather than IBC): LNG, LPG, ammonia.

Refrigerated cargo systems include:

• Cargo tanks with insulation.
• Refrigeration plant: typically electric or steam-driven compressors.
• Temperature control: continuous with redundant monitoring.
• Pressure relief: preventing tank over-pressure if refrigeration fails.

Inhibited cargoes

Some cargoes contain inhibitors to prevent unwanted reactions:

• Styrene: contains polymerisation inhibitor (4-tert-butylcatechol typical).
• Acrylic monomers: contain polymerisation inhibitor.
• Phenolic compounds: contain polymerisation inhibitor.

Inhibited cargo handling:

• Inhibitor concentration verification before loading.
• Temperature management: inhibitor effectiveness depends on temperature.
• Voyage limit: cargoes with limited inhibitor effectiveness have voyage time limits.
• Sampling: at intervals to verify inhibitor remains effective.

Reactive cargoes

Some cargoes are reactive:

• Self-reactive substances: monomers (acrylates, methacrylates) that polymerise on contact with heat or contamination.
• Water-reactive substances: certain isocyanates that react with water producing CO2.
• Air-reactive substances: certain aluminium alkyls and similar.
• Heat-sensitive substances: organic peroxides, some sulphur compounds.

Each reactive cargo requires specific handling procedures documented in the P&A Manual.

Annex II in regional context

Mediterranean implementation

Mediterranean implementation of Annex II:

• Reception facilities at major chemical ports (Tarragona, Marseille, Trieste, Antwerp via Strait of Gibraltar, Israeli ports, Egyptian ports).
• PSC enforcement by Mediterranean MOU members.
• Regional cooperation through REMPEC (Regional Marine Pollution Emergency Response Centre for the Mediterranean Sea).
• Coastal state restrictions: Italy, Spain, France impose stricter limits in their territorial waters.

Northwest European implementation

Northwest European implementation:

• Major chemical ports in Antwerp, Rotterdam, Hamburg with substantial reception facilities.
• EU regulation layering on top of MARPOL Annex II.
• PSC enforcement by Paris MOU.
• REMPEC-like cooperation through the Bonn Agreement.

Asian implementation

Asian implementation:

• Major chemical ports in Singapore, Shanghai, Yokohama, Ulsan, Kaohsiung.
• PSC enforcement by Tokyo MOU.
• Regional cooperation through the COBSEA (Coordinating Body on the Seas of East Asia).
• Variable national capacity with major industrial states having strong capacity and smaller states relying on IMO support.

Implementation challenges

Common implementation challenges across regions:

• Reception facility capacity: matching capacity to demand.
• Reception facility cost: avoiding fees that discourage use.
• PSC inspector training: in chemical-specific compliance.
• Casualty response: chemical-specific response equipment and personnel.
• Cross-border cooperation: for spills affecting multiple states.

The challenges are addressed through ongoing capacity-building support and industry collaboration.

Specific cargo categories and trade flows

Petrochemicals

The petrochemical trade is the largest segment of chemical tanker shipping, with approximately 200 million tonnes per year. Key cargoes:

• Methanol: Category Z, traded globally with major flows from Middle East producers (Iran, Qatar, Saudi Arabia) to Europe, Asia and Americas.
• Ethylene Dichloride (EDC): Category Y, used in PVC production.
• Vinyl Chloride Monomer (VCM): Category Y, transported as refrigerated liquid.
• Styrene: Category Y, with peroxide formation risk requiring inhibitor and stable temperature.
• Acrylonitrile: Category Y, highly toxic.
• MTBE (Methyl Tertiary Butyl Ether): Category Y, octane booster.
• Acetone: Category Z, common solvent.
• Phenol: Category Y, with handling challenges due to corrosivity and odour.

Each of these cargoes has specific handling requirements documented in the IBC Code Cargo List and in operator-specific procedures.

Industrial chemicals

Industrial chemical cargoes:

• Caustic soda (sodium hydroxide solution): Category Z, traded globally.
• Sulphuric acid: Category Z, large global trade.
• Hydrochloric acid: Category Z (some grades) or Y.
• Nitric acid: Category Y or Z depending on concentration.
• Phosphoric acid: Category Z, fertilizer industry feedstock.
• Ammonia solution: Category Y or Z, related to fertilizer trade.
• Hydrogen peroxide: Category X or Y depending on concentration; hazardous due to oxidising nature.

The industrial chemical trade is concentrated in regions with major chemical production (Persian Gulf, US Gulf, Northwest Europe, Northeast Asia, Southeast Asia).

Vegetable oils

The vegetable oil trade is approximately 80 million tonnes per year:

• Palm oil: largest single oil trade, primarily from Malaysia and Indonesia to global markets.
• Soybean oil: from Argentina, Brazil, USA to global markets.
• Sunflower oil: Ukraine and Russia historically; trade disrupted post-2022.
• Rapeseed (canola) oil: Canada, Australia, Europe.
• Coconut oil: Philippines, Indonesia.
• Palm kernel oil: Malaysia, Indonesia.

Vegetable oils are typically Category Y or Z. Specialised vegoil tankers serve the trade.

Biofuels and renewables

The biofuel trade is growing rapidly:

• Biodiesel (FAME, fatty acid methyl ester): from various feedstocks, traded globally.
• HVO (hydrotreated vegetable oil): emerging as drop-in diesel substitute.
• Bioethanol: from sugar cane (Brazil) and corn (USA primarily).
• Pyrolysis oil: under development.
• Renewable methanol: from biomass and from green hydrogen.

The biofuel trade volume is approximately 30 million tonnes per year and growing.

Trade routes and operators

Key trade routes

Major chemical tanker trade routes:

• Middle East to Asia: methanol, MTBE, MEG (mono-ethylene glycol), other petrochemicals.
• Middle East to Europe: similar petrochemicals.
• US Gulf to Asia: petrochemicals, biofuels.
• US Gulf to Europe: petrochemicals, gasoline blendstocks, biofuels.
• Europe to global: specialty chemicals, petrochemicals.
• Southeast Asia to global: vegetable oils, palm oil derivatives, basic petrochemicals.
• Northeast Asia (China, Korea) to global: bulk chemicals, petrochemicals.

The trade routes connect refineries, petrochemical complexes and chemical plants to industrial users globally.

Major operators

The chemical tanker industry includes major specialised operators:

• Stolt Tankers (Norway): largest stainless steel chemical tanker fleet.
• Odfjell (Norway): pure parcel chemical tankers with global service.
• Bahri Chemicals (Saudi Arabia): Middle East-anchored chemical tanker fleet.
• MOL Chemical Tankers (Japan): Japanese-operator chemical tankers.
• MISC Bhd (Malaysia): regional chemical and product tankers.
• Berge Bulk (Singapore): mixed fleet including chemical tankers.
• Eitzen Chemical (Norway): specialised chemical tanker operator.
• Tankerska Plovidba (Croatia): regional Mediterranean operator.

Plus many smaller and regional operators serving specific trades.

Industry organisations

The chemical tanker industry coordinates through:

• CDI (Chemical Distribution Institute): vetting scheme used by major charterers.
• OCIMF (Oil Companies International Marine Forum): SIRE inspections also relevant for chemical tankers.
• INTERTANKO: trade association covering tanker operators.
• CCC (Chemical Commodity Carriers): chemical tanker-specific industry coordination.
• ICS (International Chamber of Shipping): broader industry advocacy.

The industry organisations are significant in regulatory development and best-practice promotion.

Crew training and STCW

STCW Section A-V/1 paragraph 3

Officers in charge of cargo operations on chemical tankers require advanced training under STCW Section A-V/1 paragraph 3. The training covers:

• Basic safety training plus chemical-specific extensions.
• Tanker familiarisation including general tanker operations.
• Chemical tanker specifics: cargo properties, handling procedures, emergency response.
• Tank cleaning operations: procedures, safety, gas testing.
• Cargo system operations: pumps, piping, tank gauging, sampling.
• Pollution prevention: MARPOL Annex II compliance, P&A Manual operations.

Course duration is typically 5 days for ratings, 10 days for officers.

Pollution prevention training

Specific training in MARPOL Annex II compliance:

• Cargo categorisation: understanding the X, Y, Z, OS distinctions.
• Discharge regulations: when discharge is permitted, what limits apply.
• Pre-wash procedures: when pre-wash is required and how to conduct it.
• Cargo Record Book completion: accurate and timely entries.
• SMPEP procedures: emergency response.
• Reception facility use: connecting to and operating with shore facilities.

The training is verified at PSC inspection through interview of bridge and engine room officers.

Continuous improvement

Operator-side continuous improvement:

• Internal audits of MARPOL Annex II compliance.
• Lessons learned from any pollution event in the fleet.
• Updated procedures as new cargoes or routes are added.
• Refresher training at intervals.

The continuous improvement cycle complements the regulatory requirement.

Cargo handling system in detail

Cargo pumps

Chemical tanker cargo pumps are typically:

• Submerged deepwell pumps: hydraulically-driven pumps mounted at the bottom of each cargo tank, typically of approximately 50 to 500 cubic metres per hour capacity, with materials selected for cargo compatibility (stainless steel for most acids and bases, special alloys for halogenated cargoes).
• Centrifugal cargo pumps: deck-mounted with vertical drive shafts to suction strainers in cargo tanks, used for product tankers and some chemical tankers.
• Stripping pumps: smaller capacity but designed for low-residue stripping at the end of cargo discharge.

The choice of pump technology depends on cargo properties (corrosiveness, viscosity, temperature) and ship type.

Cargo piping

Cargo piping arrangements:

• Independent segregation: each cargo grade has its own piping from cargo tank to ship’s side manifold, preventing cross-contamination.
• Tank-by-tank isolation: shutoff valves at each tank with positive isolation.
• Material selection: stainless steel, special alloys, or specific coatings depending on cargo.
• Heat tracing: for high-viscosity or high-melting-point cargoes.
• Drain lines: for complete line emptying after discharge.

The piping complexity is one of the principal cost drivers of chemical tanker construction.

Cargo tank coatings

Cargo tank coatings determine which cargoes can be carried. Common coatings:

• Stainless steel (typical of higher-grade chemical tankers): carries any chemical compatible with stainless including acids, bases, food-grade products.
• Phenolic epoxy: carries many petrochemicals, glycols, alcohols.
• Zinc silicate: carries certain alcohols and esters.
• Inorganic coatings: specialised for specific applications.
• Internally rubber-lined: for some highly corrosive cargoes.

The coating affects:

• Cargo compatibility: each coating has a specific list of approved cargoes.
• Cleaning requirements: between cargoes, the coating must be cleaned to a level appropriate to the next cargo.
• Maintenance: coatings degrade and require periodic refurbishment or recoating.

Stripping efficiency

Annex II requires that stripping efficiency (the proportion of cargo recoverable by ship’s pumps before tank cleaning) meets specific standards:

• For Category X: stripping must reduce residue to a defined volume per tank.
• For Category Y: stripping must reduce residue to a higher volume.
• For Category Z: minimal stripping efficiency requirements.

The stripping efficiency depends on:

• Pump performance: deepwell pumps typically achieve better stripping than centrifugal pumps.
• Tank geometry: rectangular tanks with sloped bottoms strip better than complex shapes.
• Cargo viscosity and temperature: higher temperature reduces viscosity and improves stripping.
• Operational discipline: careful operation maximises recovery.

The P&A Manual documents the stripping efficiency for each cargo type.

Tank cleaning between cargoes

Pre-cleaning planning

Before tank cleaning:

• Cargo compatibility check between previous cargo and next cargo.
• Cleaning standard determination: what residue level is acceptable for the next cargo.
• Equipment selection: cleaning machines, water sources, chemical cleaning solutions.
• Personnel briefing: safety procedures, gas testing, emergency response.
• Schedule coordination: with port operations and the next loading.

Cleaning procedures

Tank cleaning procedures vary by cargo combination:

• Hydrocarbon-to-hydrocarbon similar grade: minimal cleaning, typically water rinse.
• Hydrocarbon-to-hydrocarbon different grade: more thorough water washing.
• Hydrocarbon-to-aqueous: thorough water washing plus residue verification.
• Aqueous-to-hydrocarbon: drying.
• Acid-to-base or base-to-acid: chemical neutralisation followed by water rinse.
• High-purity cargo to standard cargo: thorough cleaning with verified residue limits.

The cleaning method is documented in the P&A Manual and in the operational record.

Tank entry safety

After cleaning, tank entry requires:

• Atmosphere testing: oxygen 21 percent, hydrocarbon below 1 percent LEL, toxic gases below threshold.
• Permit-to-enter documentation signed by master and chief officer.
• Two-person rule: no solo entry to enclosed spaces.
• Rescue equipment ready outside the tank.
• Continuous gas monitoring during occupancy.

Tank entry incidents (chemical exposure, oxygen depletion, mechanical injury) are persistent issues in chemical tanker operations and are subject to detailed STCW training.

Slop management

Slop generated during tank cleaning:

• Stored in slop tanks until the next port with reception facility.
• Discharged to reception facility at the next opportunity.
• Documented in Cargo Record Book.
• Sometimes commingled with similar slops to optimise reception capacity.

The slop management is one of the operational efficiency areas where chemical tanker operators differentiate.

Documentation

Every chemical tanker carries on board:

• NLS Certificate with Form Supplement.
• IBC Code Certificate of Fitness under SOLAS Chapter VII Part B.
• P&A Manual approved by the flag state.
• Cargo Record Book.
• SMPEP with current contact lists and procedures.
• Tank Cleaning Manual.
• IBC Code copy with current amendments.
• Bunker Delivery Notes under Annex VI.
• Crew training records under STCW Section A-V/1.
• Vetting records from CDI, SIRE, and charterer-specific vetting.
• PSC inspection records.

Reception facilities

Annex II requires ports and chemical terminals to provide:

• Reception facilities for Category X cargo residue.
• Facilities for high-residue Category Y cargo pre-wash.
• Facilities for slop disposal generated during tank cleaning.
• Fee structure that does not discourage use of facilities.
• Capacity sized for expected vessel calls.

Reception facility implementation is one of the recurring issues in IMSAS audit findings, with developing port states often having capacity gaps.

Special Areas

Annex II does not designate Special Areas in the same comprehensive way as Annex I. However, certain regional restrictions apply:

• Antarctic Area: discharge prohibitions for chemical cargoes.
• Specific PSSAs: may have additional Annex II provisions.
• Coastal state restrictions: some states impose stricter limits in their territorial waters.

The Special Area regime for Annex II is less developed than for Annex I but is progressively being expanded.

Future developments

Alternative fuel cargoes

As the maritime industry transitions to alternative fuels, the cargo trade will increasingly include:

• Methanol: already widely traded, mature Annex II categorisation.
• Ammonia: trade growing rapidly with new fertilizer and fuel applications.
• Hydrogen carriers: liquid organic hydrogen carriers (LOHC), liquefied hydrogen for some terminals.
• Synthetic fuels: e-methanol, e-ammonia, e-hydrogen, drop-in synfuels.

Each new cargo type requires Annex II categorisation and operational provisions.

Categorisation under review

Multiple substances are under MEPC review for re-categorisation:

• Endocrine disruptors: emerging concern about long-term ecosystem effects.
• Per- and polyfluoroalkyl substances (PFAS): increasingly regulated globally.
• Microplastic precursors: under consideration for specific provisions.

The review cycle ensures Annex II remains current with evolving scientific understanding.

Related Calculators

• MARPOL Annex II, NLS Discharge Compliance Calculator

See also

• MARPOL Convention parent article
• MARPOL Annex I: Oil Pollution Prevention
• SOLAS Chapter VII: Carriage of Dangerous Goods
• Chemical Tanker
• IMDG Class 3 Flammable Liquids
• IMDG Class 6 Toxic Substances
• IMDG Class 8 Corrosive Substances
• Methanol as Marine Fuel
• Ammonia as Marine Fuel

References

• IMO, International Convention for the Prevention of Pollution from Ships (MARPOL), 1973, as modified by 1978 Protocol, as amended, Annex II.
• IMO Resolution MEPC.118(52) (2004), Adoption of the revised Annex II.
• IMO Resolution MSC.4(48) (1983), International Bulk Chemical Code (IBC Code), as amended.
• IMO MEPC.1/Circ.512 and successor circulars on Annex II implementation.
• GESAMP/EHS Database documentation.
• CDI (Chemical Distribution Institute) inspection programme documentation.
• OCIMF SIRE 2.0 inspection programme.
• ITOPF (International Tanker Owners Pollution Federation) annual reports.