By ShipCalculators.com · Published 8 May 2026 · last updated 14 June 2026

MARPOL Annex II Regulation 13: Cargo Tank Washing

MARPOL Annex II Regulation 13 is the operational core of the chemical-tanker pollution-prevention regime: the rule that turns the four-category structure of noxious liquid substance classification into a sequence of mandatory shipboard procedures executed at the port of unloading. Where a Category X residue remains in a cargo tank after discharge, Regulation 13.1 requires a prewash with the effluent landed to a port reception facility, with the prewash continuing until the concentration of the substance in the effluent at the inlet of the receiving facility is at or below 0.1 percent by volume. Where a Category Y residue is high-viscosity or solidifying, Regulation 13.2 imposes the same prewash-to-reception duty. For non-soluble Category Y or Z residues, Regulation 13.3 permits a ventilation procedure under the conditions specified in MEPC.18(22) and reproduced in the ship’s Procedures and Arrangements (P&A) Manual. The P&A Manual is the ship-specific document, approved by the flag administration or its recognised class society, that records every prewash route, every reception-facility connection, every ventilation port, and every operational procedure required to execute Regulation 13 lawfully on the ship as built. Each prewash, each ventilation, each transfer of residue, and each control by an authorized surveyor is then logged in the Cargo Record Book (a single book, not split into Part I and Part II) under one of the prescribed operation codes A through K, retained on board for three years, and countersigned by the master. Regulation 13 is the link between the IBC Code cargo-tank-arrangement requirements, the MARPOL Convention discharge-control discipline shared with Annex I Regulation 15, the record-keeping discipline familiar from Annex I Regulation 17 Oil Record Book, the emergency-response chain in Annex I Regulation 37 SOPEP, and the chemical-spill response architecture of the OPRC-HNS Protocol 2000.

Contents

Background: MARPOL 73/78 to consolidated 2025 edition

Regulation 13 sits inside MARPOL Annex II, the chemical-tanker annex of the International Convention for the Prevention of Pollution from Ships, 1973, as modified by the Protocol of 1978. Annex II was adopted with the 1973 Convention text, but its entry into force was delayed because the supporting cargo-categorisation system, the supporting reception-facility infrastructure, and the supporting class-society rule sets all required years of development before flag administrations could responsibly accept the obligations. Annex II eventually entered into force on 6 April 1987.

The original Annex II structure used four pollution categories labelled A, B, C, and D, with discharge restrictions tied to a hazard rating derived from the GESAMP joint working group on the scientific aspects of marine environmental protection. The original Regulation 8 of the 1987 text contained the prewash and ventilation duties that Regulation 13 of the consolidated 2007 text now carries.

The substantial revision of Annex II was adopted by Resolution MEPC.118(52) in October 2004 and entered into force on 1 January 2007. That revision replaced the A-B-C-D categories with the present X-Y-Z-OS structure, aligned the categorisation with an updated GESAMP hazard profile, renumbered every regulation, consolidated the prewash and ventilation rules into the present Regulation 13, and rewrote the Procedures and Arrangements Manual standard, the Cargo Record Book form, and the cargo-list approach in IBC Code Chapter 17. At the same MEPC 52 session, MEPC.119(52) adopted the revised Standard for Procedures and Arrangements Manual, which introduced the pumping and stripping efficiency test that now defines the maximum allowable pre-prewash residue in a tank and its associated piping.

The 2007 consolidated text is the version still in operational use as the foundation, amended progressively by:

MEPC.246(66) (April 2014, in force 1 January 2016) addressing high-viscosity and solidifying NLS, tightening the prewash regime for cargoes with melting points or pour points above defined thresholds.
MEPC.314(74) and parallel MEPC resolutions revising appendices and forms.
MEPC.379(80) (July 2023, in force 1 January 2025) refining the high-viscosity definition, the ventilation procedure conditions, and the P&A Manual approval pathway.

The IMO consolidated edition of MARPOL is reissued periodically; the 2025 consolidated edition is the working reference for class society plan approval and PSC inspection as of mid-2026. Operators should always work from the consolidated edition current to the date of any audit, and check the latest MEPC session resolutions for amendments that have been adopted but not yet incorporated into the printed consolidation.

NLS category taxonomy: X / Y / Z / OS

Regulation 13 can’t be applied without first determining the category of the residue. Annex II Regulation 6 sets out the four categories and the listing methodology, with the actual cargo list maintained in IBC Code Chapter 17 and BCH Code Chapter VI. The X/Y/Z/OS structure replaced the former A/B/C/D categories when MEPC.118(52) entered force on 1 January 2007.

Category X is the most stringent. A Category X NLS is one that, if discharged into the sea from cargo-tank cleaning or deballasting operations, is deemed to present a major hazard to either marine resources or human health. The GESAMP Hazard Profile underpins every category assignment: a substance qualifies for Category X if the GESAMP evaluation scores it at a level that the revised Annex II working group determined could not be tolerated even at the dilution achievable by the Regulation 13 at-sea discharge regime. Examples include acrylonitrile, naphthalene molten, alkyl phenol, and a defined set of biocidal or persistent compounds. Category X residues are subject to discharge prohibition: any tank that held a Category X cargo and from which residues remain must be prewashed at the port of unloading with the effluent landed to a reception facility, and the prewash effluent must be at or below 0.1 percent by volume at the receiving-facility inlet before the prewash can be considered complete.

Category Y is the second tier. A Category Y NLS presents a hazard to either marine resources or human health, or causes harm to amenities or other legitimate uses of the sea. The Category Y list is the largest of the four, covering substantial portions of the petrochemical, vegetable-oil, and biofuel commodity sets. Category Y is divided operationally into standard Category Y, high-viscosity Category Y (kinematic viscosity at unloading temperature exceeding the threshold defined in Annex II Appendix V), and solidifying Category Y (cargoes with melting point above the threshold defined in the same Appendix). The prewash duty under Regulation 13.2 applies to the high-viscosity and solidifying subsets, with standard Category Y residues handled through the at-sea operational discharge regime of Regulation 13.

Category Z is the third tier, presenting minor hazards to marine resources or human health. The Category Z list covers many lower-hazard chemicals and most of the vegetable-oil set following the 2007 reclassification. Category Z residues are normally cleaned through the at-sea operational discharge regime of Regulation 13 without a mandatory prewash.

Other Substances (OS) is the residual class. OS substances are not classified as NLS for Annex II discharge-restriction purposes, but they are listed in IBC Code Chapter 18 and may be subject to other requirements including carriage requirements, fire-protection requirements, and ship-type requirements. OS residues don’t trigger Regulation 13 prewash.

The category determines whether Regulation 13 applies at all, which paragraph applies, whether the prewash effluent must be landed to reception, and whether a ventilation procedure is permitted in lieu of the prewash. The cargo officer’s first task on receiving a fixture nomination is to verify the category in IBC Code Chapter 17 and to plan the discharge sequence around the prewash duty that the category implies.

Regulation 13.1: Prewash for Category X residues

Regulation 13.1 reads, in summary effect: where a tank has been used for the carriage of a Category X NLS, the tank shall be prewashed before the ship leaves the port of unloading, and the resulting tank-washing effluent shall be discharged to a reception facility until the concentration of the substance in the effluent at the inlet of the receiving facility is at or below 0.1 percent by volume, and in any event until the tank is empty (the second leg of the test catches cases where the residue is heavier than water and concentration alone would not signal the end-point).

The prewash is performed using the ship’s fixed tank-washing machines (Butterworth-style fixed gantry, programmable nozzles, or similar). The Procedures and Arrangements Manual specifies the wash pattern (number of cycles, nozzle positions, cycle time), the wash-water temperature where elevated temperature is required, the volume of wash water expected, and the route through the cargo system from tank to slop tank or directly to the reception-facility manifold.

The end-point determination is done by sampling at the receiving-facility inlet (or, where the discharge is to a slop tank that is then transferred ashore, at the connection point chosen by the surveyor present). Sampling is normally performed by the independent surveyor attending the discharge on behalf of the cargo receiver and the port reception-facility operator, with results recorded against the time, position, tank, and prewash cycle in the Cargo Record Book.

Where the port of unloading does not have a reception facility capable of accepting the prewash effluent, the master may seek an exemption from the flag administration to perform the prewash at the next port of call where reception facilities are available. The exemption must be requested in advance, granted in writing, and recorded in the Cargo Record Book under Code K (additional operational procedures and remarks), with the authorized surveyor’s endorsement entered under Code J. The cargoes for which exemption is most commonly sought are those for which receiving terminals are concentrated in a small set of regional hubs: the USA Gulf for many Category X petrochemicals; Rotterdam-Antwerp-Amsterdam for European specialties; and Singapore-Jurong for Asian deepwater trade.

Regulation 13.2: Prewash for Category Y residues

Regulation 13.2 carries the prewash duty into the high-viscosity and solidifying Category Y subsets. The trigger is whether the cargo, at unloading temperature, exceeds the kinematic-viscosity threshold or the melting-point threshold defined in Annex II Appendix V. Where the trigger is met, the tank is prewashed before the ship leaves the port of unloading, and the effluent is discharged to a reception facility under the same 0.1 percent end-point as Category X.

The high-viscosity subset captures cargoes such as palm acid oil, soft tallow, certain phenolic resin precursors, and a wide range of vegetable-oil acid distillates. Viscosity is measured at the unloading temperature actually achieved by the cargo during the discharge, not at a reference temperature; this matters because cooling from a typical loading temperature of 60 to 65 degrees Celsius down to a discharge ambient of 25 degrees Celsius can shift a marginal cargo from below the threshold (no prewash) to above the threshold (prewash required).

The solidifying subset captures cargoes with melting points above the defined threshold. Examples include molten sulphur (when carried as NLS rather than under the IMSBC Code), certain wax intermediates, and a tightly defined set of high-melt-point chemical specialties. Solidifying cargoes are heated throughout the loaded passage, and the prewash is normally performed with heated wash water to keep the residue mobile and to ensure that the 0.1 percent end-point is achievable in the cycle count specified in the P&A Manual.

The 2014 amendments under MEPC.246(66) tightened the operational rules for solidifying NLS by recognising explicitly that conventional wash patterns may be insufficient for cargoes with very high melting points. The amendments require that the P&A Manual identify the wash-water temperature, the cycle structure, and the residue-monitoring approach for each solidifying NLS that the ship is certified to carry, with class-society plan approval extending to the supporting heating system, the wash-water heater, and the slop-tank heating arrangement.

Regulation 13.3: Ventilation procedure for non-soluble Category Y/Z

Regulation 13.3 permits, in defined circumstances, a ventilation procedure in lieu of a prewash for non-soluble Category Y and Category Z residues. The procedure is set out in detail in Appendix VI of Annex II and in MEPC.18(22) (the original P&A Manual standard) as supplemented by subsequent MEPC resolutions.

The conditions for ventilation are:

The residue must be non-soluble in water at the conditions of the procedure.
The viscosity of the substance at 20 degrees Celsius must be below the kinematic viscosity threshold defined in Appendix V.
The vapour pressure must be above the threshold defined in Appendix VI, so that ventilation can in fact remove the residue by evaporation.
The ventilation must be performed using fans that meet the air-flow rate requirement specified in Appendix VI (typically expressed as a minimum air change per unit time per cubic metre of tank atmosphere).
The ventilation must continue until the tank is dry, with dryness verified by visual inspection at the conclusion of the procedure.

When all conditions are met, the master may elect to perform the ventilation in lieu of the prewash, and the effluent (the cargo-tank atmosphere ventilated to the open air) doesn’t require landing to a reception facility. The election must be recorded in the Cargo Record Book under Code E (cleaning of cargo tanks except mandatory prewash), with the position, atmosphere, fan rating, and dryness-verification time logged and any supporting narrative entered under Code K.

The ventilation procedure is widely used for residues of low-viscosity volatile aromatics: toluene, xylenes, methyl methacrylate. It’s rarely used for vegetable oils, glycerine, or aqueous solutions, where solubility, low vapour pressure, or both rule it out.

Procedures and Arrangements (P&A) Manual: MEPC.18(22) and MEPC.119(52)

The Procedures and Arrangements Manual is the ship-specific operational document required by Annex II Regulation 14. Two resolutions govern its content: MEPC.18(22), the original Standard Format for the Procedures and Arrangements Manual (adopted in 1985), and MEPC.119(52), the revised P&A Manual standard adopted in October 2004 alongside MEPC.118(52) and supplemented by MEPC.379(80) in 2023.

MEPC.119(52) introduced a substantive change to the legacy P&A Manual framework by replacing the old category-based residue-quantity limits with a pumping and stripping efficiency test. Under the legacy (pre-2007) regime, the maximum allowable post-discharge residue was expressed as an absolute volume per category: the revised regime requires instead that the ship’s cargo-pump and stripping system demonstrate, during plan approval and commissioning, that the combined system leaves no more than 75 litres of residue in any tank and its associated piping at the conclusion of the unloading sequence. Ships built before 1 January 2007 carry the legacy 0.3 cubic metre (Category A/X equivalent) or 1 cubic metre (Category B) allowance under the grandfathering provisions, but the 75-litre standard is the design target for all post-2007 construction. Meeting it is a commissioning duty verified during class-society plan approval; maintaining it in service is an operational duty discharged by running the stripping system fully at the close of every unloading.

The P&A Manual contains:

A description of the ship’s cargo-tank arrangement, including tank coatings, tank capacities, and the cargo-handling pipe system.
A list of the NLS that the ship is certified to carry, drawn from the IBC Code Chapter 17 list and confirmed by the NLS Certificate (also called the International Pollution Prevention Certificate for the Carriage of Noxious Liquid Substances in Bulk, IPPCB).
The cargo-handling procedures for each cargo group: loading rate, line clearing, draining, stripping, and the arrangements that allow the residue at the end of unloading to be quantified and managed.
The prewash procedures for each Category X cargo and each high-viscosity or solidifying Category Y cargo, with the wash machines used, the cycle pattern, the wash-water temperature, the wash-water volume, and the route to slop tank or reception facility.
The ventilation procedures for each non-soluble residue eligible for the Regulation 13.3 election, with the fans used, the duration, and the dryness-verification approach.
The slop-tank arrangement including capacity, heating, agitation, sampling, and discharge route.
The reception-facility connection arrangement including the manifold position, the reducer set, and the hose specification.

The P&A Manual is approved by the flag administration or, in the great majority of cases, by the recognised classification society acting under the flag’s delegated authority. The class society review against MEPC.18(22) and MEPC.119(52) (as updated by MEPC.379(80)) is documented in a plan-approval letter that becomes the ship’s record of P&A Manual currency. The IPPCB is endorsed against the approved P&A Manual, and the ship can’t lawfully carry NLS unless the IPPCB and the P&A Manual are both current.

Where the cargo list on the IPPCB changes (a new cargo is added, an old cargo is dropped, a coating is recoated, a tank is structurally modified), the P&A Manual must be reissued and re-approved before the new cargo can be carried. PSC inspectors routinely cross-check the IPPCB cargo annex against the P&A Manual current revision against the cargo nominated on the bill of lading.

2023 amendments: MEPC.379(80) and high-viscosity NLS

The amendments adopted by MEPC.379(80) in July 2023 entered into force on 1 January 2025 and refined the high-viscosity regime in three ways.

First, the viscosity threshold for the high-viscosity subset of Category Y was clarified, with the measurement protocol pinned to the unloading temperature actually recorded on the cargo certificate of quality. This closes the loop on the marginal-cargo problem (cargo just below the threshold at the loading temperature, just above at the unloading temperature) by anchoring the test to the unloading-side measurement.

Second, the P&A Manual format was updated to require explicit identification of the wash-water heating capacity, the heating-system commissioning record, and the cycle-time test record for each solidifying NLS on the cargo annex.

Third, the Cargo Record Book operation codes were reviewed and the supporting MEPC guidance on completion of the Cargo Record Book was reissued, with worked examples for the high-viscosity prewash sequence and for the reception-facility-not-available exemption case.

Operators reissuing P&A Manuals after the 2023 amendments need class-society review of the heating-system specification, wash-water-volume calculation, and cycle-pattern test record. Class societies (DNV, LR, ABS, BV, KR, NK, RINA, CCS, RS, IRS) maintain harmonised P&A Manual review procedures aligned with the IACS Unified Interpretation set on Annex II.

Cargo Record Book: operation codes A through K

The Cargo Record Book is the chemical-tanker analogue of the Oil Record Book for oil tankers. Unlike the Annex I Oil Record Book, which an oil tanker keeps as a Part I (machinery-space) and a Part II (cargo and ballast) volume, the Annex II Cargo Record Book is a single book: there is no Part I or Part II split. It is a structured book published by IMO in MARPOL Annex II Appendix 2 as a fixed form, with entries made under eleven defined operation codes that run A through K:

Code A: Loading of cargo. Date, time, port, tank, cargo name, IBC Code Chapter 17 listing, category, quantity loaded, temperature, density.
Code B: Internal transfer of cargo during the voyage. Tank from, tank to, quantity, time, position.
Code C: Unloading of cargo. Date, time, port, tank, quantity unloaded, residue remaining, the receiving terminal, surveyor present. A pump or stripping-system failure that affects residue management at unloading is recorded here with the impact noted.
Code D: Mandatory prewash in accordance with the ship’s P&A Manual. Tank, cycle pattern, wash-water volume, wash-water temperature, time start, time end, slop-tank route, reception-facility manifold, end-point sample concentration. The reception-facility receipt (terminal, manifold, quantity, time, terminal-issued receipt number) for the landed prewash effluent is recorded against this entry.
Code E: Cleaning of cargo tanks except a mandatory prewash (post-prewash polishing, in-voyage tank prep, and the Regulation 13.3 ventilation procedure used in lieu of a prewash). Tank, cleaning or ventilation method, fluid or fan rating, duration, dryness-verification, route.
Code F: Discharge into the sea of tank washings under the Regulation 13 at-sea discharge conditions, with the residue concentration, the speed, the depth, the distance from land, and the position of the discharge logged.
Code G: Ballasting of cargo tanks. Tank, quantity of ballast, position.
Code H: Discharge of ballast water from cargo tanks. Tank, quantity, position, residue concentration.
Code I: Accidental or other exceptional discharge. Time, position, quantity, the substance, the cause.
Code J: Control by authorized surveyors of any operation logged in the CRB. Surveyor name, organization, what was witnessed; the surveyor endorses a prewash of a Category X tank and any reception-facility-not-available exemption here.
Code K: Additional operational procedures and general remarks (the master’s narrative space; the flag-administration exemption for a reception-facility-not-available case is recorded here against the written approval).

Each entry must be made without delay, signed by the officer in charge of the operation, and countersigned by the master on the page on which the entry appears. Where an entry crosses pages, the master countersigns each page. Entries are made in the working language of the ship, with a translation into English, French, or Spanish where the working language is not one of those three.

CRB retention: 3 years on board

The Cargo Record Book is retained on board for a period of three years from the date of the last entry, after which it may be archived ashore but must remain available to the flag administration on request. PSC inspectors routinely sample CRB entries against the supporting evidence: the bill of lading for the loading entry, the cargo certificate of quality for the temperature and density, the surveyor report for the prewash end-point sample, and the reception-facility receipt for the residue landing. Inconsistency between the CRB and the supporting evidence is the central PSC finding in chemical-tanker detentions and is the source of the falsification cases prosecuted under EU Directive 2005/35/EC and US 33 CFR Part 151.

The master’s countersignature is the legal anchor of every CRB entry. The countersignature is the master’s personal attestation that the entry is true to the master’s knowledge, and falsification of an entry under the master’s countersignature is a criminal offence under most flag-state legislations and under the United States Act to Prevent Pollution from Ships, 33 USC § 1908.

Relationship to the IBC Code

MARPOL Annex II Regulation 13 can’t be read in isolation from the International Bulk Chemical Code (IBC Code), which provides the cargo-tank arrangement, the construction standard, the materials standard, the equipment standard, and the cargo-list classification (Chapter 17) on which Regulation 13 depends.

The IBC Code, Resolution MSC.4(48), is mandatory under SOLAS Chapter VII Part B and under MARPOL Annex II Regulation 11. The cargo-tank arrangement that the IBC Code requires (ship type 1, 2, or 3 by hazard severity; tank coatings; double-hull spacing; segregation rules) is the physical platform on which the Regulation 13 procedures execute. Without an IBC Code-compliant tank, the prewash apparatus has no place to mount; without IBC Code coatings, the prewash effluent reacts with the tank surface; without IBC Code stripping arrangements, the residue at the end of unloading exceeds what the prewash can process.

IBC Code Chapter 17 carries the authoritative cargo list that assigns the MARPOL category to each substance. The assignment results from the GESAMP Evaluation of Hazardous Substances (EHS) working group, which scores candidate substances across 18 hazard criteria covering human health, bioaccumulation, biodegradation, and aquatic toxicity. That GESAMP/EHS score is the input to the category-assignment procedure in Annex II Regulation 6, and it is the IBC Code Chapter 17 entry that the cargo officer, the surveyor, and the PSC inspector use as the operative reference. The category printed in Chapter 17 is not open to negotiation at the cargo interface.

Cargo pump room and cargo system interface

The execution of Regulation 13 is a cargo-deck and pump-room operation, but it depends on the integrity of the cargo-handling chain from tank to reception-facility manifold. The chain comprises:

Tank-washing machines (fixed Butterworth-style gantry, programmable nozzles, or rotating jet machines) sized in the P&A Manual and certified during plan approval.
Wash-water supply pump and heater delivering wash water at the rate and temperature specified in the P&A Manual.
Cargo pump (deepwell submerged centrifugal in the modern fleet, or hydraulically driven piston pumps in legacy designs) driving the prewash effluent from the cargo tank to the slop tank or directly to the reception manifold.
Stripping system (educator-driven or stripping pump) capable of clearing the line below the deepwell-pump cut-off, and verified against the MEPC.119(52) 75-litre residue standard.
Slop tank with heating coils for solidifying cargoes and agitation arrangements for residue homogenisation.
Reception-facility manifold at the connection point on the ship’s side, with the reducer set and hose specification matched to the receiving facility.
Cargo control room instrumentation monitoring tank levels, line pressures, slop-tank levels, and reception-facility receipt.

The pump room itself is a hazardous space governed by IBC Code Chapter 11 (electrical installation) and Chapter 12 (mechanical ventilation), and the prewash sequence imposes thermal, chemical, and pressure loads on the pump-room piping that must be reviewed during periodic class-society survey. Class society notations such as DNV CLEAN DESIGN, LR ECO descriptive notes, or ABS Environmental Safety notations rely on the documented integrity of this chain.

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

The IACS member societies implement Annex II Regulation 13 through their statutory rule sets, harmonised by IACS Unified Interpretations on the Annex II prewash procedure, the P&A Manual format, and the Cargo Record Book operation codes.

DNV: Pt. 5 Ch. 7 Chemical and Product Tankers, with sub-chapters on cargo-tank arrangement, tank-washing apparatus, and pump-room arrangement. The DNV TANKER (CHEMICAL) class notation extends to P&A Manual review against MEPC.119(52) and MEPC.379(80).
Lloyd’s Register: Rules and Regulations for the Classification of Ships, Part 7 Chapter 1 (Chemical Tankers), with Lloyd’s plan-approval procedures for the P&A Manual and the supporting tank-washing-system review.
ABS: Steel Vessel Rules Part 5, Chapter 8 (Vessels Intended for the Carriage of Chemicals), with the ABS Chemical Carrier notation and the supporting tank-washing-and-cleaning-system review.
Bureau Veritas: NR 482 (Chemical Tankers), with BV’s plan-approval workflow for the P&A Manual and the prewash-apparatus certification.
Korean Register (KR): Rules for the Classification of Steel Ships, Part 7 (Tankers), with KR’s P&A Manual approval procedures aligned to MEPC.18(22) and MEPC.379(80).
Nippon Kaiji Kyokai (NK / ClassNK): Rules for the Survey and Construction of Steel Ships, Part R (Chemical Tankers), with NK’s plan-approval workflow for tank-cleaning systems and the P&A Manual.
RINA: Rules for the Classification of Ships, Part E Chapter 8 (Chemical Tankers).
China Classification Society (CCS): Rules for Classification of Sea-going Steel Ships, Part 8 (Chemical Tankers).
Russian Maritime Register of Shipping (RS): Rules for the Classification and Construction of Sea-Going Ships, with the chemical-tanker chapter aligning with MARPOL Annex II requirements.
Indian Register of Shipping (IRS): Rules and Regulations for the Construction and Classification of Steel Ships, Part 5 (Chemical Tankers), with IRS’s P&A Manual approval workflow integrated into the classification survey cycle.

The IACS Unified Interpretation set on Annex II ensures that a P&A Manual approved by one society is recognised on transfer to another society without re-doing the underlying engineering review, subject to flag-administration confirmation.

PSC inspection focus: P&A Manual currency, CRB consistency, prewash operability

Port State Control inspection of chemical tankers under the Tokyo MoU, the Paris MoU, the USCG, and parallel MoU regimes focuses on three documentary and operational tests under Annex II Regulation 13.

The first test is P&A Manual currency: the inspector verifies that the P&A Manual on board is the current revision, that the cargo annex matches the IPPCB cargo annex, that the cargo most recently loaded appears on both, and that the P&A Manual revision number tracks the most recent flag or class plan-approval letter. A P&A Manual that hasn’t been reissued after a structural modification, after a recoating, or after an IPPCB cargo addition is the most common Annex II Regulation 13 finding.

The second test is CRB consistency: the inspector samples a recent prewash entry under Code D (or a ventilation entry under Code E) and traces it back to the supporting evidence (the surveyor report, the reception-facility receipt, the wash-water-supply log). Inconsistency between the CRB entry and the evidence is the high-severity finding that escalates to detention.

The third test is prewash operability: the inspector observes, or in a programme inspection requests a demonstration of, the tank-washing-machine operation, the wash-water-pump and heater performance, the sample-point access, and the slop-tank heating and agitation. Inoperable prewash apparatus on a ship certified to carry Category X cargoes is a detention-grade finding.

PSC databases at the parismou.org and tokyo-mou.org portals show that chemical-tanker detentions under MARPOL Annex II are dominated by the P&A Manual currency category and the CRB consistency category, with prewash-operability detentions concentrated on older tonnage operating into less-developed reception-facility hubs.

Reception facility availability and voyage planning

The Regulation 13 prewash duty depends on a reception facility being available at the port of unloading. Annex II Regulation 18 (the reception-facility regulation, parallel to Annex I Regulation 38) requires every Party to provide reception facilities adequate to receive the residues that ships unloading NLS in their ports will generate, but in practice the geographical availability is uneven.

Reception-facility hubs with broad capacity for Category X and high-viscosity Category Y residues include Rotterdam-Antwerp-Amsterdam in the European range, Houston-New Orleans-Pascagoula in the USA Gulf, Singapore-Jurong in the Southeast Asian deepwater range, Ulsan-Yeosu in Korea, Yokohama-Kawasaki in Japan, and a small set of specialty facilities at Antwerp-Stolt-Botlek-type chemical clusters. Outside these hubs, the reception facility may accept only a narrow set of NLS, may operate at limited capacity, or may not be available at all.

The voyage plan that the chemical-tanker operator builds for a multi-leg parcel voyage must:

Confirm reception-facility availability at each unloading port for the cargoes the ship intends to land there.
Identify alternative ports where the prewash exemption (recorded under Code K with the Code J surveyor endorsement, against a written flag-administration approval) can be discharged if the unloading-port reception facility is unavailable.
Sequence cargoes such that solidifying or high-viscosity residues are landed at hubs with heated wash-water and heated reception capability.
Build slop-tank capacity ahead of multi-cargo discharge sequences so that residues can be aggregated for delivery at the next hub if necessary.

The authoritative source for reception-facility availability is the GISIS Reception Facility Database maintained by IMO and updated by Parties under their Regulation 18 reporting obligation. GISIS records the type of substance accepted, the capacity, the hours of operation, and the contact information for each facility, and is searchable by port and substance category.

ECA and Antarctic Special Area interaction

Chemical tankers operating in Emission Control Areas (ECAs) and Sulphur Emission Control Areas (SECAs) declared under MARPOL Annex VI are subject to the fuel-sulphur cap and the supporting NOx Tier requirements. The Annex II prewash duty is independent of the Annex VI fuel regime: a chemical tanker discharging a Category X cargo in the Baltic SECA or the North Sea SECA must perform the Regulation 13.1 prewash as it would anywhere, and fuel-side compliance status doesn’t vary the Annex II duty.

The interaction with Baltic SECA / NECA is operational simultaneity rather than substantive change: the ship is running on VLSFO 0.10 percent or on an alternative compliant fuel for the SECA passage, the NOx Tier III abatement is active where required, and at the same time the Regulation 13 prewash is executed at the ECA-port discharge with the effluent to the reception facility.

The Antarctic Special Area (under the Polar Code and Annex II Special Area provisions) carries an additional absolute discharge prohibition: any prewash effluent generated in the Antarctic area must be retained on board until the ship reaches a port outside the area with reception facilities available. The 12-nautical-mile, 25-metre-depth, 7-knot, en-route conditions that govern a Regulation 13 at-sea operational discharge elsewhere don’t apply to override the Antarctic prohibition. Any voyage touching Antarctic waters needs a slop-tank capacity plan that accounts for the possible retention of all prewash effluent generated during the Antarctic segment.

Commercial allocation under chemical tanker voyage charters

The Regulation 13 prewash duty is a shipowner duty under MARPOL, but its commercial allocation between owner and charterer is governed by the voyage charter party. The dominant chemical-cargo voyage charter form is ASBATANKVOY (the tank voyage charter originally developed by the Association of Ship Brokers and Agents), which allocates the cost of cargo-handling, including the cost of mandatory prewash and reception-facility fees, between owner and charterer through a defined set of clauses.

The standard allocation places the cost of mandatory prewash on the charterer, on the theory that the prewash is a function of the cargo’s nature and the charterer is the party that selects the cargo. The reception-facility fee is similarly placed on the charterer in the standard form, with an itemised disbursement account presented at the end of the voyage.

Where the prewash exemption under Regulation 13 (reception facility unavailable, prewash performed at the next port) applies, the incremental cost of carrying the residue to the next port (slop-tank-blocked cargo capacity, additional bunker burn for the deviation, additional port disbursements) is allocated under the contractual deviation clause and is normally recoverable from the charterer subject to evidence of the unavailability and of the diligent attempt to comply at the original port.

The supporting commercial documentation that accompanies the CRB entry includes the cargo certificate of quality, the bill of lading, the surveyor’s prewash sample report, the reception-facility receipt, and the disbursement account from the agent at the prewash port. This document set is preserved with the CRB through the three-year retention period and forms the evidentiary chain if the prewash compliance is later disputed in a cargo claim or a flag-state prosecution.

Category-based residue limits and the prewash end-point

The quantitative spine of Regulation 13 is a single residue concentration. Where a tank has held a Category X cargo, the prewash effluent at the inlet to the reception facility must reach a concentration at or below $0.1\%$ by volume before the prewash is complete, and the tank must in any event be empty. The condition reads:

c_{\text{NLS,effluent}} \leq 0.1\% \text{ by volume at end of prewash (Reg 13.1)}

The same $0.1\%$ end-point carries into Regulation 13.2 for the high-viscosity and solidifying Category Y subsets. Standard Category Y residues that fall below the Appendix V viscosity and melting-point triggers are not prewashed to reception; they run through the Regulation 13 at-sea operational-discharge regime instead, which permits a controlled at-sea discharge of the diluted tank washings. Category Z residues sit one tier lower again and are cleaned through the same Regulation 13 at-sea discharge regime without a mandatory prewash.

The $0.1\%$ figure originates in the GESAMP hazard-profile work that supported the 2004 Annex II revision: it is the residue level at which the hazard from any later inadvertent discharge (a ballasting operation, a slop-tank carry-over, a clean-ballast run after the prewash) falls below the threshold of concern for the Category X list. The number is conservative against the GESAMP score for many Category X substances, but it is uniform across the list to keep the rule simple for a surveyor to inspect.

The end-point determination is a continuous-effluent test, not a single grab sample. Sampling is performed at the receiving-facility inlet by the independent surveyor attending the discharge, against the time, position, tank, and prewash cycle recorded in the Cargo Record Book under Code D.

Regulation 12 stripping quantity and Regulation 13 discharge conditions

Regulation 13 doesn’t act alone, and it is worth keeping the two adjacent regulations distinct. Regulation 12 (pumping, piping, unloading arrangements and slop tanks) governs how much residue may remain in a tank and its associated piping after stripping. Regulation 13 (control of discharges of residues of noxious liquid substances) governs both the conditions for discharging tank washings to sea and the mandatory-prewash duty. The residue quantity that remains after unloading, before any prewash, is governed by the Regulation 12 stripping standard and by the MEPC.119(52) pumping and stripping efficiency test embedded in the P&A Manual approval process. For a ship whose keel was laid on or after 1 January 2007 and whose cargo system passes the MEPC.119(52) efficiency test, the stripping standard is 75 litres of combined residue in the tank and its associated piping for Category X, Y, and Z substances. For ships built before that date, the legacy category-based figures from the pre-2007 Annex II apply under grandfathering provisions. Meeting the 75-litre figure is a design-and-commissioning duty verified during plan approval; maintaining it in service is an operational duty discharged by running the stripping system at the conclusion of each unloading sequence.

Where the residue category permits an at-sea operational discharge of tank washings (standard Category Y and Category Z under Regulation 13, logged under Code F), the discharge is lawful only when every one of the following conditions is met simultaneously. The ship must be en route, proceeding at a speed of at least $7$ knots for self-propelled ships (or $4$ knots for ships not self-propelled). The discharge must be made below the waterline through the underwater discharge outlet. The depth of water must be at least $25$ metres. The discharge must take place at a distance of more than $12$ nautical miles from the nearest land. These four conditions are cumulative: a failure on any one makes the discharge unlawful, and the position, speed, depth, distance, and residue concentration are logged against the Code F entry. The cargo stripping calculator provides a working tool for verifying residue quantities against the 75-litre standard.

Prewash wash-water volume: the planning approximation

The prewash is performed with the ship’s fixed tank-washing machines, and the wash-water consumption for one cycle scales with tank volume and cargo viscosity. A planning approximation used in P&A Manual cycle-count calculations is:

V_{\text{prewash}} \approx \alpha \cdot V_{\text{tank}} \cdot k_{\text{viscosity}}

where $V_{\text{prewash}}$ is the wash-water volume per cycle in cubic metres, $V_{\text{tank}}$ is the cargo-tank volume in cubic metres, $\alpha$ is the wash-water-to-tank-volume ratio per cycle (typically $0.05$ to $0.10$ of tank volume for the modern fixed-machine fleet), and $k_{\text{viscosity}}$ is a dimensionless viscosity multiplier scaling with the cargo solidification point (about $1.0$ for low-viscosity cargoes and $1.5$ to $2.5$ for solidifying cargoes, per the MEPC.246(66) prewash-cycle guidance). The $\alpha$ range reflects the spread of fixed-machine arrangements certified during plan approval over the last twenty years.

The number of cycles needed to reach the $0.1\%$ end-point is set by the cargo nature and the wash-machine pattern, and is specified for each cargo on the IPPCB cargo annex in the ship’s P&A Manual. The cycle-count and volume relationship traces back to the GESAMP and IMO joint working-group test programme behind MEPC.18(22) and MEPC.119(52), which measured prewash performance of representative machine arrangements against a panel of Category X and high-viscosity Category Y cargoes.

A valid prewash makes five working assumptions, and a surveyor checks each. The tank is drained to its operational minimum before the prewash starts, with the stripping system run at the close of unloading. The wash-water supply sits at the temperature the P&A Manual specifies, with heating capacity and supply rate sized to match. The wash-machine pattern is the certified pattern in the P&A Manual, machines positioned and rotated as written. The slop-tank or reception-facility route is clear, line pressures inside the design range. The end-point sample is drawn at the receiving-facility inlet by an independent surveyor under the defined sampling protocol.

Worked example: three-cycle prewash for a high-viscosity Category Y residue

Take a $2{,}500$ cubic metre cargo tank that held a high-viscosity Category Y residue. The P&A Manual specifies $\alpha = 0.07$ and $k_{\text{viscosity}} = 1.8$ , so the per-cycle wash-water volume is:

V_{\text{prewash}} \approx 0.07 \cdot 2{,}500 \cdot 1.8 = 315 \text{ cubic metres per cycle}

The P&A Manual specifies a three-cycle prewash for this cargo, putting total wash-water consumption near $945$ cubic metres. The first two cycles clear gross residue; the third cycle is dedicated to reaching the $0.1\%$ end-point. The effluent runs to the slop tank, with the third cycle drawn directly to the reception manifold for end-point sampling. The Cargo Record Book entry under Code D records the three cycles, the cumulative wash-water volume, the heating temperature, and the end-point sample concentration, signed by the officer in charge and countersigned by the master.

The cargo purging calculator supports the associated gas-freeing sequence that typically follows a multi-cycle prewash for volatile cargoes, particularly where the ventilation-procedure option under Regulation 13.3 applies for the residue fraction after the prewash.

Limitations

The quantitative figures in this article describe the regulatory structure and a working volume model; they aren’t a substitute for the ship’s approved P&A Manual, and several limitations bind their use in practice.

The first limitation is IBC Code category fixity. The category of a residue is not a judgment the cargo officer makes on board. It’s fixed by the IBC Code Chapter 17 cargo list (and, for the few remaining cases, by the BCH Code), set by GESAMP hazard profiling. A cargo can’t be reclassified to a less stringent category to avoid the prewash duty; if the cargo nominated on the bill of lading is a Category X substance in Chapter 17, Regulation 13.1 applies regardless of how the residue behaves on the day.

The second limitation is the viscosity and melting-point trigger for Category Y. The high-viscosity and solidifying subsets are defined against the Appendix V thresholds measured at the unloading temperature actually achieved, not at a reference temperature. A cargo can sit below the viscosity threshold at a $60$ to $65$ degree Celsius loading temperature and cross above it after cooling to a $25$ degree Celsius discharge ambient, flipping a no-prewash cargo into a prewash-required one. The MEPC.379(80) amendments (in force 1 January 2025) anchored the test to the unloading-side measurement to close that marginal-cargo gap, but the determination still depends on an accurate temperature record on the cargo certificate of quality.

The third limitation is reception-facility dependence. The whole prewash-to-reception duty assumes a facility capable of accepting the effluent is available at the port of unloading. Annex II Regulation 18 obliges every Party to provide adequate facilities, but capacity is concentrated in regional hubs, and a port may accept only a narrow set of NLS or none at all. Where no facility is available, the master can’t discharge the residue at sea to compensate; the lawful path is the Regulation 13 exemption, with the residue retained in the slop tank and the prewash performed at the next port, recorded under Code K against a written flag-administration approval and endorsed by the authorized surveyor under Code J.

The fourth limitation is the special-area regime. The Antarctic area carries an absolute discharge prohibition under the Polar Code: any prewash effluent generated there must be retained on board until the ship reaches a port outside the area with reception available, irrespective of the $12$ nautical mile, $25$ metre depth, $7$ knot, and en-route conditions that would govern a Regulation 13 at-sea operational discharge elsewhere. The Annex II prewash duty also runs independent of the Annex VI fuel-sulphur regime: a tanker discharging a Category X cargo in the Baltic or North Sea SECA performs the same Regulation 13.1 prewash, with fuel-side compliance status changing nothing about the chemical duty.

The fifth limitation is the estimate caveat on the volume model. The $V_{\text{prewash}} \approx \alpha \cdot V_{\text{tank}} \cdot k_{\text{viscosity}}$ relationship is a planning approximation, not a regulatory formula. The $\alpha$ and $k_{\text{viscosity}}$ ranges describe the certified fleet spread; the governing values for any specific ship and cargo are the cycle count and wash-water volume written into the approved P&A Manual cargo annex. Several operational edge cases push past the nominal cycle count. If the effluent is still above $0.1\%$ at the end of the nominal cycles, the prewash continues with extra cycles, each logged as a continuation of the Code D entry. If the wash-water heater fails on a solidifying cargo, the prewash can’t meet specification; the master logs the failure under Code K (additional operational procedures and remarks) and retains the residue until the heater is restored or the cargo can be landed at a port with shore-side heated-water supply. Where the wash-water temperature a solidifying cargo needs would exceed the tank-coating temperature limit, the tank-cargo combination is excluded from the IPPCB cargo annex outright.

When several cargoes load sequentially into one tank, the prewash duty applies separately to each by category, with the order of prewash, intermediate cleaning, and load-on-top set by the P&A Manual.

Two recurring documentary errors carry the same weight as an operational failure at inspection. Logging a mandatory prewash under Code E (other cleaning) rather than Code D, and omitting the master’s countersignature on the page where the entry appears, are both high-severity CRB findings; the countersignature is the legal anchor of every entry. The regulatory basis for all of the above is MARPOL Annex II Regulation 13.1 (Category X prewash), 13.2 (high-viscosity and solidifying Category Y), and 13.3 (ventilation for non-soluble Category Y and Z), read with Regulation 12 (pumping, piping, unloading arrangements and slop tanks, and the 75-litre stripping standard per MEPC.119(52)) and Regulation 13 (control of discharges of residues, the at-sea discharge conditions and the prewash duty), Appendix V (viscosity and solidifying thresholds), Appendix VI (ventilation conditions and air-flow), Resolution MEPC.18(22), Resolution MEPC.118(52), Resolution MEPC.246(66), Resolution MEPC.379(80), and IBC Code Chapter 17.

References

IMO, MARPOL: Articles, Protocols, Annexes, Unified Interpretations of the International Convention for the Prevention of Pollution from Ships, 1973, as modified by the Protocol of 1978 (Consolidated Edition), latest edition.
IMO, International Bulk Chemical Code (IBC Code), 2020 edition with amendments.
Resolution MEPC.118(52), Amendments to the Annex of the Protocol of 1978 relating to the International Convention for the Prevention of Pollution from Ships, 1973 (2004 revision of Annex II).
Resolution MEPC.119(52), Standard for Procedures and Arrangements Manual (2004 revision, replacing MEPC.18(22) format requirements).
Resolution MEPC.246(66), Amendments to MARPOL Annex II addressing high-viscosity and solidifying NLS (2014).
Resolution MEPC.379(80), Amendments to MARPOL Annex II refining the high-viscosity regime and the P&A Manual format (2023, in force 1 January 2025).
Resolution MEPC.18(22), Standard format for the Procedures and Arrangements Manual (original, 1985).
IACS, Unified Interpretations on MARPOL Annex II.
DNV, Rules for Classification: Ships, Pt. 5 Ch. 7 Chemical and Product Tankers.
Lloyd’s Register, Rules and Regulations for the Classification of Ships, Part 7 Chapter 1 (Chemical Tankers).
ABS, Rules for Building and Classing Steel Vessels, Part 5 Chapter 8 (Chemical Carriers).
Bureau Veritas, NR 482 Chemical Tankers.
Tokyo MoU and Paris MoU, PSC inspection databases.
IMO GISIS, Reception Facility Database.

Frequently asked questions

What triggers a mandatory prewash under MARPOL Annex II Regulation 13?

A Category X NLS residue always triggers Regulation 13.1 prewash before the ship leaves the port of unloading. For Category Y, the trigger is whether the cargo at unloading temperature exceeds the kinematic-viscosity or melting-point threshold in Annex II Appendix V (the high-viscosity or solidifying subsets defined under MEPC.246(66) and refined by MEPC.379(80)).

What is the prewash end-point concentration for Category X residues?

The prewash effluent at the inlet of the reception facility must reach a concentration at or below 0.1 percent by volume, and the tank must in any event be empty. Both legs of the test must be satisfied before the prewash is considered complete under Regulation 13.1.

What is MEPC.119(52) and how does it relate to Regulation 13?

MEPC.119(52), adopted at the same MEPC 52 session as MEPC.118(52) in October 2004, is the revised Standard for Procedures and Arrangements Manual. It sets the pumping and stripping efficiency test that a ship's cargo system must pass during plan approval to demonstrate that residue in a tank and its associated piping is reduced to no more than 75 litres before prewash begins.

Can a ventilation procedure replace a prewash under Regulation 13?

Yes, under Regulation 13.3, but only for non-soluble Category Y or Z residues whose vapour pressure exceeds the Appendix VI threshold, whose viscosity at 20 degrees Celsius falls below the Appendix V limit, and for which the tank-ventilation fan meets the certified air-flow rate in the P&A Manual. Category X residues cannot use the ventilation option.

What records must be kept after a mandatory prewash?

Each prewash is logged in the Cargo Record Book under Code D, with the tank identifier, prewash cycle pattern, wash-water volume and temperature, time start and end, slop-tank or reception-facility route, and end-point sample concentration. The Cargo Record Book is a single book, not split into Part I and Part II. The entry is signed by the officer in charge and countersigned by the master. The CRB is retained on board for three years from the date of the last entry.