Safety

MARPOL Annex II — Noxious Liquid Substances in Bulk

Chemical tanker categorisation (X, Y, Z, OS), stripping limits, mandatory prewash, underwater discharge, and P&A Manual compliance.

10 min read

Intermediate

Safety

Key Principles at a Glance 6 points

MARPOL Annex II governs the carriage of Noxious Liquid Substances (NLS) in bulk, applying to chemical tankers and oil tankers carrying chemical parcels.
Under revised Annex II, liquid chemicals are categorised into Category X (major hazard), Category Y (hazard), Category Z (minor hazard), and Other Substances (OS - non-hazard).
Regulation 12 mandates strict stripping limits: for ships built on or after 1 January 2007, the maximum residue permitted per tank and its associated piping is 75 litres for Categories X, Y, and Z.
Mandatory prewashes are statutory for Category X (effluent discharged to shore reception until residual concentration is ≤0.1% by weight, or ≤0.01% for phosphorus) and for solidifying or high-viscosity Category Y cargoes.
Overboard discharge of residues requires strict operational criteria: vessel en route at ≥7 knots (≥4 knots towed), discharge below the waterline through an approved underwater discharge outlet, water depth ≥25 m, and distance ≥12 NM from nearest land.
Compliance hinges on the ship-specific Procedures and Arrangements (P&A) Manual, Cargo Record Book entries endorsed by surveyors for prewashes, a valid NLS Certificate (5 years), and an approved SMPEP.

1. Application, Chemical Hazards & Deepwell Stripping Architecture

MARPOL Annex II: Regulations for the Control of Pollution by Noxious Liquid Substances in Bulk controls operational discharges and accidental risks from chemical tankers. Unlike crude oils governed by Annex I, noxious liquid chemicals exhibit wide physical and toxicological diversity: some float, some dissolve instantly, others sink to the seabed, and many produce fatal toxic or flammable vapour clouds at ambient temperatures.

Annex II applies to all ships carrying bulk liquids other than oil that are identified as presenting environmental or human health hazards. Ships are constructed to three statutory standards under the IBC Code (International Bulk Chemical Code):

Type 1 Ship: Maximum preventive standards for products presenting severe environmental and safety hazards (e.g. chlorine, phosphorus). Requires double hull and double bottom with maximum tank size limits.
Type 2 Ship: High safety standards for substances presenting significant hazards (e.g. nitric acid, benzene, motor fuel anti-knock compounds). Requires double bottom and specified side hull clearance.
Type 3 Ship: Moderate containment standards for cargoes presenting moderate hazards (e.g. ethanol, caustic soda, tallow).

Cat X, Y, Z Pollution Hazard Groups

≤75 Litres Post-2007 Stripping Limit

≥12 NM / 25 m Discharge Offshore Limits

≥7 Knots En Route Discharge Speed

A fundamental requirement of modern Annex II is that tanks must be physically stripped almost bone-dry during cargo discharge before any washing or ballast water touches the tank boundaries. This is achieved using hydraulically driven, submerged deepwell cargo pumps installed in a recessed suction well (sump) at the lowest point of each tank:

Figure 1: Chemical Tanker Submerged Deepwell Cargo Pump (Framo type) and Stripping System CAD Cutaway. The pump casing sits inside a recessed suction well at the bottom of the corrugated cargo tank. During stripping, nitrogen or compressed air is blown down the purge line, forcing the final column of liquid trapped inside the casing and vertical discharge riser up into the deck manifold, reducing total residual liquid per tank to ≤75 litres in compliance with Regulation 12.

2. Categorisation: Substances X, Y, Z & Other Substances (OS)

Under the revised MARPOL Annex II regulations, every liquid chemical carried in bulk is evaluated by the GESAMP (Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection) Hazard Profile. The historic four-tier system (A, B, C, D) was streamlined into Categories X, Y, Z, and OS based on bioaccumulation, aquatic toxicity, human health risks, and seabed persistence:

Category X (Major Hazard)

Substances presenting a major hazard to marine resources or human health, justifying the most stringent anti-pollution measures.

Examples: Coal tar, cresols, phenol, carbon disulphide, yellow phosphorus.

Mandatory Action: Mandatory prewash in port. Residues must be discharged to shore reception until effluent concentration is ≤0.1% by weight (≤0.01% for phosphorus).

Category Y (Hazard)

Substances presenting a hazard to marine resources or human health, requiring special operational conditions.

Examples: Benzene, toluene, sulphuric acid, vegetable oils (palm, sunflower, soybean).

Mandatory Action: If non-solidifying and low viscosity, strip to ≤75 litres; wash water may be discharged en route at sea. If solidifying or high viscosity, a mandatory prewash to shore reception is statutory.

Category Z (Minor Hazard)

Substances presenting a minor hazard, requiring less stringent operational conditions.

Examples: Ethanol, methanol, phosphoric acid, acetone, citric acid.

Mandatory Action: Strict stripping to ≤75 litres (300 L for pre-2007 ships). Washings may be discharged at sea under standard en route criteria without mandatory shore prewash.

Other Substances — OS (Non-Hazard)

Substances evaluated and found to fall outside Categories X, Y, or Z because they present negligible harm to marine ecosystems or human health.

Examples: Apple juice, water, glucose solution, molasses, clay slurry.

Mandatory Action: Exempt from the operational discharge requirements of Annex II. May be discharged without distance or speed constraints.

The operational flow from discharging cargo to final tank cleaning follows a strict statutory decision tree:

Figure 2: MARPOL Annex II Operational Decision Tree for Cargo Residues. Following discharge and stripping to ≤75 L, Category X cargoes mandate an approved port prewash with effluent discharged to shore reception until concentration falls to ≤0.1% (≤0.01% for phosphorus). Category Y cargoes require shore prewash if solidifying or high-viscosity (≥50 mPa·s); otherwise, washings may proceed to en-route at-sea discharge (≥7 kn, ≥12 NM, ≥25 m depth).

3. Discharge Criteria & The Underwater Discharge Arrangement

MARPOL Annex II strictly governs when and how chemical wash water and tank ballast may be released into the ocean. The regulations establish four cumulative statutory conditions that must all be satisfied simultaneously:

En Route Speed Requirement: The vessel must be proceeding en route at a minimum speed of 7 knots for self-propelled ships, or 4 knots for vessels being towed or pushed. This ensures that discharged chemical effluent is immediately drawn into the turbulent wake.

Offshore Distance Limit: Discharge must occur at a distance of not less than 12 nautical miles from the nearest baseline. Inside territorial waters (0–12 NM), all chemical residue discharges are completely banned.

Minimum Water Depth: The discharge must take place in a charted water depth of not less than 25 metres. This prevents toxic chemical accumulation in shallow benthic fishing grounds and coral reefs.

Sub-Surface Below Waterline Discharge: All effluent containing Category X, Y, or Z residues must be expelled below the waterline through an approved underwater discharge outlet (Regulation 12). Surface overboard discharges are illegal.

Persistent Floaters — Stricter 2021 Prewash (Regulation 13)

Since 1 January 2021, persistent floaters — residues that float and persist after unloading — face a stricter prewash and discharge requirement when the port of unloading lies inside one of four named areas. Recite it as what — where — where-to: what: persistent-floater residue after unloading; where: Northwest European waters, Baltic Sea area, Western European waters, Norwegian Sea; where-to: prewash effluent goes to shore reception, logged as CRB Code D with surveyor endorsement before sailing.

Figure 3: Persistent-floater 2021 prewash rule (Regulation 13). When the port of unloading lies in Northwest European waters, the Baltic Sea area, Western European waters or the Norwegian Sea, persistent-floater residues must be prewashed with the effluent discharged to shore reception — never to sea.

Why Underwater Discharge is Mandatory

Discharging chemical tank washings above the waterline poses two severe hazards: liquid chemicals atomise into volatile toxic or explosive vapour clouds on deck, and wind drift concentrates pollutants along the ship's side. Discharging below the waterline into the ship's boundary layer uses propeller wash turbulence to rapidly dilute chemical concentrations by a factor of 10,000 within seconds.

Figure 4: Chemical Tanker Underwater Discharge Outlet Mechanical Cutaway. Positioned at the turn of the bilge within the cargo block, the statutory discharge nozzle expels chemical wash water downwards and aft into the high-velocity boundary layer. The orifice diameter is strictly engineered so the discharge velocity prevents effluent from clinging to the hull or entering seawater cooling scoops forward.

4. The P&A Manual, Cargo Record Book & SMPEP Emergency Planning

Operational control under MARPOL Annex II is maintained through three mandatory documents that every chemical tanker must carry and execute with zero variance:

Procedures & Arrangements (P&A) Manual

A ship-specific statutory document approved by the Flag Administration. It details the physical layout of cargo pumps, stripping lines, underwater outlets, and prescribes precise cleaning procedures, washing durations, water volumes, and stripping angles for every approved cargo.

Cargo Record Book (CRB Part II)

An official statutory register recording all cargo movements and tank cleaning operations. Every evolution is logged chronologically under standard letter codes (A through N), signed by the officer in charge and countersigned on each completed page by the Master.

SMPEP (Regulation 17 Plan)

The Shipboard Marine Pollution Emergency Plan for Noxious Liquid Substances. Required on all ships ≥150 GT certified to carry NLS. Provides tactical response steps for cargo leaks, toxic vapour release, personnel exposure, fire, and mandatory coastal state reporting contacts.

Cargo Record Book Letter Codes (Surveyor Viva Checklist):

Code	Operation Description	Statutory Verification Requirement
Code A	Loading of cargo	Record identity, tank numbers, gross quantity, and port of loading.
Code B	Internal transfer of cargo	Record from/to tanks, quantities transferred, and reason for transfer.
Code C	Unloading of cargo	Record port, tanks emptied, and confirmation of stripping to ≤75 L.
Code D	Mandatory prewash in port	Must be endorsed by Flag / Port State Surveyor or authorized surveyor.
Code E	Cleaning of cargo tanks	Record washing cycles, water temperature, washing agents, and duration.
Code F	Discharge of washings to sea	Record ship position, speed (≥7 kn), depth (≥25 m), and underwater valve usage.
Code G	Discharge to shore reception	Record port facility, quantity discharged, and retain shore receipt.
Code J / K	Accidental or exceptional discharge	Record exact circumstances, reasons for loss, and coastal state notifications.

Port State Inspection of Chemical Operations:

Port State Control officers enforce Annex II with zero tolerance for documentation discrepancies. The typical PSC inspection sequence for a chemical tanker proceeds as follows:

Certificate Audit: Verification of the International NLS Certificate (or Certificate of Fitness under the IBC Code), checking validity dates and annual survey endorsements.

CRB & Surveyor Endorsement Check: Auditing the Cargo Record Book for mandatory prewash entries (Code D). A surveyor's official signature and stamp are mandatory before a vessel carrying Category X or high-viscosity Category Y may sail.

P&A Manual Reconciliation: Comparing recorded tank wash water quantities against the minimum washing tables in the approved P&A Manual. Under-washing or skipping cycles is an immediate clear ground.

Physical Hardware Inspection: Examining cargo tank sumps, testing the nitrogen stripping line non-return valves, inspecting the underwater discharge remote-operated sea valve, and verifying gas detection calibrations.

Figure 5: Butterworth Rotary Tank Washing Machine and Shore Reception Flow P&ID. Hot wash water (60–80°C) is pumped through deck mains to the submerged rotary machine, whose high-impact dual nozzles strip chemical clinging films. The effluent is continuously lifted from the sump well by the deepwell pump and transferred to the shore reception manifold until laboratory tests confirm ≤0.1% residual concentration, endorsed by the Port State Surveyor.