Dangerous Goods on Deck

The manifest says it is there. The placard says what it is. Neither of them tells the crew what it will do when it decides to react.

Introduction

The IMDG Code occupies two thick volumes, a supplement, and an amendment cycle that rolls every two years. Most deck officers have read portions of it. Fewer have sat down with the segregation table, the stowage categories, and a real bay plan and traced the logic through from slot to slot. Fewer still have done it under time pressure in a port where the terminal’s DG planner has a different interpretation of ‘away from’ than the one assumed on board.

Dangerous goods on deck are not a theoretical hazard. They are the single greatest source of uncontrollable fire risk on a modern containership. Below deck, CO2 flooding — if the hold is sealed — offers a chance. On deck, there is no suppression system worth the name. There are monitors, boundary cooling, and hope. If a DG container ignites above the hatch covers and the reaction is self-sustaining, the crew is managing consequences, not fighting a fire.

This article addresses what the deck officer actually needs to know: not the full taxonomy of the Code, but the working realities of stowage, segregation, placarding, emergency documentation, and the decisions that must already be made before the first wisp of smoke appears.

Contents

• 1. The IMDG Code as a Working Document
• 2. Stowage Categories — A Through E and What They Permit
• 3. Segregation in Practice — Reading the Table, Applying It on Deck
• 4. The Dangerous Goods Manifest and the Meaning of Emergency Information
• 5. MFAG and EmS — Where They Live, When to Open Them
• 6. Placards, Marks, and the Visibility Principle
• 7. Contaminated Cargo and Leaking Packages — Isolation and Reporting
• 8. Deck Stowage Visibility — Fire Response Planning from Bridge and Accommodation
• 9. Closing Reality

1. The IMDG Code as a Working Document

The IMDG Code is mandatory under SOLAS Chapter VII. That is not news. What matters is how it is used in practice aboard a vessel loading and discharging across multiple ports in a rotation, often with cargo plans finalised hours before arrival and amended alongside.

A deck officer does not memorise the Code. A deck officer knows how to navigate it: the Dangerous Goods List in Volume 2, the segregation table in Chapter 7.2, the stowage and handling requirements in Chapter 7.1, and the Index by UN number and proper shipping name. Speed of reference matters. The terminal will not wait.

The amendment cycle is biennial. Ships carry the current edition. Or they should. The number of vessels found by PSC with an edition two cycles out of date is not trivial. The Code is a living document, and its updates reflect incident data. An outdated Code is not just a deficiency — it is a gap in the information that protects the crew.

Carrying the Code and using the Code are different things entirely.

What the Code does not do is tell the deck officer what the cargo actually looks like when it fails. It gives classification, hazard properties, stowage and segregation requirements, and emergency schedules. It does not convey the speed at which a thermal runaway in a container of organic peroxides will overwhelm every response capability on board. That understanding comes from casualty reports, from drills conducted with realistic assumptions, and from a professional scepticism about the condition of every container that crosses the rail.

2. Stowage Categories — A Through E and What They Permit

The IMDG Code assigns stowage categories to dangerous goods that determine whether they may be carried on deck, under deck, or both, and whether they are permitted on cargo ships only or on passenger vessels as well. These are not suggestions. They are the legal framework for the bay plan.

Category A: stowage on deck or under deck on both cargo ships and passenger vessels. The most permissive category.

Category B: on deck or under deck on cargo ships; on deck only on passenger vessels. The restriction on passenger vessels reflects the additional risk to persons.

Category C: on deck only, on cargo ships only. Not permitted on passenger vessels at all. This category exists because the goods in question are considered too hazardous for enclosed hold stowage, and the risk profile is unacceptable where passengers are carried.

Category D: on deck or under deck on cargo ships only. Not permitted on passenger vessels.

Category E: on deck or under deck on both cargo ships and passenger vessels, but with specific stowage provisions that must be met.

In practice, the distinction that matters most on a working containership is between goods that must be on deck and goods that may go below. Category C cargo goes on deck. That is not negotiable. When a bay plan arrives showing Class 5.2 organic peroxides stowed under deck, the mate does not load the box. Full stop.

The stowage category is the first filter. If it is wrong, everything built on top of it is wrong.

Errors in stowage category application are frequently systemic. They originate in the terminal planning software and propagate through every subsequent check. The deck officer who trusts the plan without verifying category compliance against the DGL for each UN number is relying on a system that has been shown, repeatedly, to fail.

3. Segregation in Practice — Reading the Table, Applying It on Deck

Chapter 7.2 of the IMDG Code contains the segregation table. It is a matrix. One axis lists classes and divisions; the other lists the same. The intersections give a segregation requirement numbered 1 through 4, or an X denoting reference to specific entries.

The four segregation terms are hierarchical:

• 1 — ‘Away from’: May be in the same compartment or on deck provided there is a minimum horizontal separation of 3 metres projected vertically. In practice, on deck, this means not in the same stack and not in an adjacent stack unless the 3-metre rule is met.
• 2 — ‘Separated from’: Requires a minimum of one container space or 6 metres horizontally. One intervening container satisfies this on deck.
• 3 — ‘Separated by a complete compartment or hold from’: On deck, this requires a minimum horizontal separation of 12 metres. Below deck, it means a full intervening hold with bulkheads.
• 4 — ‘Separated longitudinally by an intervening complete compartment or hold from’: The most stringent requirement. On deck, minimum 24 metres fore and aft.

The critical difference between on-deck and below-deck segregation is structural. Below deck, steel bulkheads provide meaningful separation. On deck, the only separation is distance. Air. Wind. And whatever burning debris a thermal event decides to project.

Reading the table is mechanical. Applying it requires spatial awareness of the ship’s geometry. A 12-metre separation on deck sounds generous until the mate realises that the only available slots place incompatible cargoes on opposite sides of the same bay, technically separated athwartships but exposed to the same fire scenario. The Code permits it. Physics does not care about the Code.

Segregation is a minimum standard, not an optimum one.

The specific segregation requirements for individual substances — those listed in the DGL entry and cross-referenced by segregation group — override the general table where they are more restrictive. Missing these subsidiary requirements is the most common segregation failure found in cargo audits. The table gets checked. The individual entry does not.

On a practical level, the second officer or designated cargo officer should be plotting DG positions on a dedicated plan — not just verifying the terminal’s output, but independently confirming every adjacency. This takes time. It prevents disasters.

4. The Dangerous Goods Manifest and the Meaning of Emergency Information

SOLAS requires that a special list, manifest, or detailed stowage plan identifying the dangerous goods on board and their locations be available before departure. The dangerous goods manifest serves two distinct functions, and confusing them is a common failure.

The first function is regulatory compliance and port state information. The manifest must be lodged with the appropriate authority before departure and, depending on the jurisdiction, before arrival at the next port. It contains the proper shipping name, class, UN number, packing group, quantity, and stowage position. It is the document that shore authorities will reach for when they need to know what is burning.

The second function is emergency response on board. This is where the phrase ‘in case of emergency’ acquires real meaning. The manifest, combined with the stowage plan, tells the master and the officer in charge exactly what is in each bay and what the interaction risks are. When a fire alarm activates for a specific hold or bay, the manifest is the first document consulted. It tells the ship what it is facing.

A manifest that is accurate but inaccessible is no manifest at all.

The manifest must be on the bridge. A copy should be with the chief officer. A copy should be in the ship’s safety centre or fire control station. If the ship’s SMS puts it in a filing cabinet in the cargo office and nowhere else, the system has failed before the emergency has begun.

The manifest depends entirely on the accuracy of the declarations made by shippers. Misdeclared dangerous goods — undeclared lithium batteries, undeclared calcium hypochlorite, undeclared charcoal — are the single greatest vulnerability in the entire DG management chain. The manifest cannot list what was never declared. The crew will discover the true contents of the container when it behaves in a way the manifest cannot explain.

5. MFAG and EmS — Where They Live, When to Open Them

The Medical First Aid Guide (MFAG) and the Emergency Schedules (EmS) are published in the IMDG Code Supplement. They serve different purposes and are reached by different routes.

The EmS provides two codes for each entry in the Dangerous Goods List: a fire schedule (F) and a spillage schedule (S). The fire schedule dictates the appropriate firefighting media and tactics. The spillage schedule addresses containment and cleanup. Both are written for shipboard application — they assume the resources available on a cargo vessel and nothing more.

The MFAG covers medical treatment for exposure to dangerous substances. It is indexed by the symptoms observed, not by the cargo class. This is deliberate. A crew member overcome by fumes may not be able to identify the substance. The guide works from clinical presentation backwards to treatment.

Both documents live in the IMDG Supplement. Both must be on the bridge and accessible to the duty officer without delay. In practice, the EmS is the document that should be laminated, tabbed, and rehearsed. When a container is on fire, the officer on watch needs the fire schedule within seconds, not minutes. Scrolling through a PDF on a slow ship’s computer during a thermal event is not emergency response. It is delay dressed up as action.

The EmS should be open before the boundary cooling starts, not after.

The schedules must be drilled. The crew must practise locating the EmS code for a given UN number, opening the correct schedule, and briefing the fire party on what they are facing — all within the time it takes a container fire to become uncontrollable. That window is short. In some commodity types, it is already closed by the time the smoke is visible.

6. Placards, Marks, and the Visibility Principle

Placards on containers carrying dangerous goods serve a single purpose: identification at a distance by persons who may not have access to the manifest. This includes port firefighters, coast guard responders, and the ship’s own crew approaching a casualty on deck in conditions of limited visibility or high stress.

The placard — the enlarged label displayed on the exterior of the container — must correspond to the class of the goods inside. The UN number must be displayed. For marine pollutants, the marine pollutant mark is required. For containers carrying goods in limited quantities or excepted quantities, specific marks apply.

In practice, placards degrade. Salt, weather, handling damage, and terminal wear reduce them to illegibility faster than anyone ashore assumes. A placard that cannot be read from the deck is not fulfilling its purpose. The deck officer conducting a DG stowage check before departure must verify not just that placards are present, but that they are legible, correctly oriented, and correspond to the declared contents.

Wrong placards are a direct threat. A fire party approaching a container placarded as Class 3 (flammable liquid) will apply different tactics than one approaching Class 5.1 (oxidiser). If the placard is wrong, the initial response is wrong. If the initial response is wrong, the escalation path changes. People get hurt.

A placard is not decoration. It is the first and sometimes only information a responder has.

Visibility from the bridge is a related but distinct issue, addressed below.

7. Contaminated Cargo and Leaking Packages — Isolation and Reporting

A leaking DG container on deck is not a spill. It is an evolving emergency with chemical, environmental, and legal dimensions that compound by the hour.

The immediate actions are straightforward in principle and difficult in execution. Identify the substance from the manifest and the placard. Consult the EmS spillage schedule. Establish an exclusion zone upwind. Do not allow crew to approach without appropriate PPE — and ‘appropriate’ means the PPE specified for that class and substance, not whatever is nearest in the fire locker.

Isolation means physical separation where possible — moving adjacent containers or, if that is not feasible, ensuring no crew member enters the affected area. On a loaded containership at sea, moving containers is not an option. Isolation therefore becomes access control, ventilation management, and boundary monitoring.

Reporting follows a mandatory chain. The master must notify the flag state and the nearest coastal state. The P&I club must be informed. If the cargo is a marine pollutant and any material has reached the sea, MARPOL reporting obligations apply. The ship’s SMS will have a reporting checklist. Use it. But verify it is complete, because many checklists omit coastal state notification or class notification.

Contamination of adjacent cargo — including non-DG cargo — creates liability. The evidence must be preserved. Photographs. Log entries. Samples if safe to take. The surveyor who boards at the next port will reconstruct the event from the records kept in the first hours. If those records are thin, the ship’s position in any subsequent claim weakens dramatically.

A leaking package does not improve. It does not stabilise. It gets worse until it is dealt with.

8. Deck Stowage Visibility — Fire Response Planning from Bridge and Accommodation

The stowage of DG containers on deck is not only a segregation and category question. It is a fire detection and response question, and the variable that governs both is visibility.

Containers stowed forward of the accommodation are visible from the bridge. Containers stowed aft of the accommodation — on ships where aft bays exist — may not be. Containers stowed in the lower tiers of a high stack may be invisible from the bridge until the fire has breached the top of the stack. By that point, the fire is established and boundary cooling is the only remaining option.

The ship’s fire control plan should identify DG stowage areas and the monitoring arrangements for each. Thermal imaging cameras, where fitted, extend detection capability. But many ships do not carry them, and on those that do, the cameras are not always monitored continuously. A CCTV system covering the container bays is standard on newer tonnage. On older ships, visual rounds by the duty AB or officer remain the primary detection method.

Fire response planning for DG on deck begins before the cargo is loaded. The chief officer and master must review the proposed DG stowage against the ship’s firefighting capability. Can the fire monitors reach the stowage position? Are the hydrant connections clear? Is the foam supply adequate for the class of goods stowed? If the answer to any of these questions is uncertain, the stowage must be reconsidered.

Container fires involving dangerous goods are the event the crew cannot outrun. A deep-seated fire in a container of self-reactive substances or strong oxidisers will generate temperatures and reaction rates that exceed any shipboard suppression capability. The fire party is not extinguishing the fire. The fire party is cooling the surrounding structure and adjacent containers to prevent propagation. The objective shifts from suppression to survival — keeping the ship afloat and the crew alive until external assistance arrives or the fire exhausts its fuel.

On deck, there is no CO2 flooding. There is no sealed compartment. There is wind, fuel, and time working against the crew.

The gap between what the emergency procedures describe and what a crew of twenty can actually achieve on a 14,000 TEU vessel with a DG fire in bay 30 on a heavy weather day in the North Atlantic is a gap that should inform every stowage decision made alongside in port. Every container placed on deck is a fire risk accepted. A DG container placed on deck is a fire risk accepted with the knowledge that, if it ignites, the available response may not be enough.

This is not an argument against carrying dangerous goods. It is an argument for treating every DG stowage decision as a risk assessment, not an administrative exercise. The cargo plan is not a logistics document. It is a survival document.

9. Closing Reality

Dangerous goods on deck are managed by paperwork, governed by regulation, and survived by preparation. The manifest, the segregation plan, the EmS, the placards — all of these are tools. None of them suppress a fire. None of them seal a leaking package. None of them protect a crew member who approaches a container without knowing what is inside.

The IMDG Code provides a framework. The deck officer provides the judgement. The difference between a ship that manages a DG incident and a ship that is consumed by one is almost never the equipment carried. It is the decisions made before the event — stowage decisions, drill decisions, verification decisions, reporting decisions. It is the willingness to question a bay plan, to reject a container, to insist on correct segregation when the terminal is pressing for departure.

A container fire involving dangerous goods does not wait for the crew to be ready. The crew must already be ready, or the fire decides the outcome.