Marine Fire Detection and Fixed Fire-Fighting Systems

What these systems do and where the rules come from

Fire at sea is contained or it is lost. A ship cannot call the fire brigade, so the detection and suppression engineered into the hull is, in most casualties, the whole response. SOLAS Chapter II-2 (Construction: Fire Protection, Fire Detection and Fire Extinction) sets the functional requirements, and the International Code for Fire Safety Systems (the FSS Code) carries the engineering detail behind them. The two work as a pair: the regulation says what a system must achieve, the Code says how it must be built and tested. This article walks through detection (smoke, heat, flame, and sample-extraction systems) and the fixed extinguishing systems (CO2 total flooding, water mist, foam, fixed water-spraying, and sprinklers), with the exact concentrations, discharge times, and chapter references a practitioner needs. Related computational tools live on ShipCalculators.com, including the full catalogue of calculators.

The wholesale rewrite of Chapter II-2 entered force on 1 July 2002, applying to ships built on or after that date, and the FSS Code became mandatory under SOLAS on the same date by resolution MSC.99(73). The Code itself was adopted at the seventy-third session of the Maritime Safety Committee on 5 December 2000 as resolution MSC.98(73), and it has been amended many times since, including by MSC.206(81), MSC.217(82), MSC.403(96), and MSC.555(108). When you cite a requirement, name the chapter and the amending resolution, not “the IMO rule.” The difference matters at survey.

SOLAS Chapter II-2: the functional skeleton

Chapter II-2 is built in parts. Part A (Regulations 1 to 3) covers application, the fire safety objectives, and definitions. Part B (Regulations 4 to 6) handles prevention: ignition control, limiting fire growth, and reducing smoke and toxicity. Part C is the one this article lives in: suppression. Regulation 7 covers detecting a fire in the space of origin and raising the alarm for safe escape and fire-fighting. Regulation 8 limits smoke spread, Regulation 9 contains the fire in its space of origin (the structural-boundary rules: A-class and B-class divisions), Regulation 10 covers fire fighting itself, and Regulation 11 protects structural integrity against heat. Part D (Regulations 12 to 13) is escape; Part E (14 to 16) is maintenance, training, and operations; Part F (Regulation 17) is the alternative-design route; Part G (18 to 23) holds the special requirements for helidecks, dangerous goods, ro-ro spaces, and the like.

Regulation 10 is the hinge for this topic. It requires that fire-extinguishing systems and equipment comply with the FSS Code, and it carries the prohibition that matters most to anyone working on an older ship: new installations of systems using halon 1211, halon 1301, halon 2402, and perfluorocarbons are banned. That ban traces to the Montreal Protocol on Substances that Deplete the Ozone Layer. Halon 1301 was an excellent total-flooding agent, low toxicity at design concentration and clean in operation, but its ozone-depleting potential ended it. The whole water-mist and inert-gas product category exists because the industry needed a halon replacement that could protect a manned machinery space.

The fire safety objectives in Regulation 2 are worth reading in full, because they set the philosophy: prevent ignition, contain a fire to its space of origin, limit smoke and toxic effects, provide means of escape, and ensure fire-fighting systems are ready. Detection and suppression are two of five layers, not the entire defense. The structural fire protection treated in SOLAS Chapter II-2: Fire Protection, Detection and Extinction does the containing while the suppression system does the killing.

Types of fixed fire-fighting systems

A ship’s fixed fire-fighting systems fall into four families: fixed gas total-flooding (CO2, inert gas and clean agents), fixed foam (deck foam on tankers and high-expansion foam in machinery spaces), water-based systems (water mist, fixed water-spraying or deluge, and automatic sprinklers), and dry chemical powder on gas carriers. Each family is specified by a chapter of the FSS Code, and the choice for a given space turns on the fuel, the volume, and whether people are present when the system runs.

The full set of marine fire-fighting systems, with the agent each uses and the FSS Code chapter that governs it, is:

• CO2 total flooding (FSS Code Chapter 5): liquefied carbon dioxide displacing oxygen in sealed machinery and cargo spaces. The dominant cargo-ship engine-room system, but lethal to anyone left in the space.
• Inert gas and clean-agent flooding (Chapter 5): nitrogen, argon or blends such as IG-541 that suppress by oxygen reduction without killing, and the chemical clean agents HFC-227ea (FM-200) and FK-5-1-12 (Novec 1230) that leave no residue and suit electrical and control spaces.
• Fixed deck foam (Chapter 14): monitors and applicators blanketing the cargo-tank deck of an oil or chemical tanker against a pool fire.
• Fixed foam in machinery and cargo spaces (Chapter 6): low-, medium- and high-expansion foam, the high-expansion type able to flood a whole engine room as a halon replacement.
• Water mist (Chapter 7): fine droplets that cool and locally steam-smother a fire, safe for occupied spaces, now standard in new passenger-ship machinery spaces and accommodation.
• Fixed water-spraying or deluge (Chapter 7): open-nozzle drenching of high-risk areas such as engine tops, vehicle decks and ro-ro spaces.
• Automatic sprinklers (Chapter 8): closed-head heat-activated systems that also act as a detection array, the accommodation workhorse on passenger ships.
• Dry chemical powder (IGC Code): nitrogen-propelled powder monitors and hand lines on gas carriers, interrupting the flame chemically.

These are the fixed fire-fighting systems whether the question is asked as systems “on ships”, “on board ship”, or simply “marine fire-fighting systems”; the same families apply across cargo ships, tankers, passenger ships and gas carriers, sized by the rules below.

Fire detection under FSS Code Chapter 9

FSS Code Chapter 9, “Fixed fire detection and fire alarm systems,” sets the detector performance and arrangement requirements that Regulation 7 calls up. Three sensing principles dominate: smoke, heat, and flame. Each responds to a different stage of a fire, and the choice of detector for a space is driven by which fire signature appears first there.

Smoke detectors

Smoke detectors sense the airborne particulate of combustion and are the earliest warning for most fires, because visible or invisible smoke usually precedes a temperature rise. Two physics are in use. Ionization detectors pass air through a chamber ionized by a small radioactive source (americium-241 is standard) and alarm when smoke particles disrupt the ion current; they respond best to fast, flaming fires that produce small particles. Photoelectric (optical) detectors fire a light beam past a sensor and alarm when smoke scatters light onto it; they respond best to slow, smoldering fires that produce larger particles. Accommodation spaces, corridors, and stairways take smoke detection by default. The FSS Code expresses sensitivity as an obscuration threshold, with smoke detectors required to respond before the smoke density exceeds an obscuration of roughly 12.5 percent per metre, so that the alarm comes well before a space fills.

Heat detectors

Heat detectors trade sensitivity for false-alarm immunity. They suit spaces where smoke, steam, or dust is normal in service and would swamp a smoke detector: galleys, drying rooms, and machinery-space areas near exhausts. Two response modes exist. A fixed-temperature detector alarms when the air reaches a set point; the FSS Code requires that heat detectors operate before the temperature exceeds 78 degrees Celsius but not until it exceeds 54 degrees, the band that keeps them clear of normal ambient swings while still catching a developing fire. A rate-of-rise detector alarms on a rapid temperature climb regardless of the absolute value, which catches a fast fire before the fixed set point is reached. Many marine heat detectors combine both behaviors in one head.

Flame detectors

Flame detectors read the radiation signature of a flame, in the ultraviolet band, the infrared band, or both, and respond in seconds. That speed suits high-hazard spaces with an open fuel risk: main-engine flats, boiler fronts, and paint stores. The trade is false alarms from hot surfaces, arc welding, and direct sunlight, so flame detectors are usually combined with another sensing type or arranged with a time delay or voting logic before they trip a release.

Zoning, addressing, and section limits

A detection installation divides the ship into zones so the watch can locate a fire fast. Conventional systems group several detectors into a section that reports as a unit; addressable systems give each detector a unique identity, so the panel shows the exact head in alarm. The FSS Code limits how much one section may cover when detectors aren’t individually identifiable: a section serving enclosed spaces must not take in more than 25 enclosed spaces, and there are limits on the deck area and the number of accommodation spaces a single section may include, so that an alarm always points to a small, searchable area. Detection alarms must be both audible and visible at the navigating bridge or a continuously manned central control station, so an unattended panel can never swallow a fire signal.

Detection coverage is matched to the space. Container holds are a hard case for point detectors because stacked boxes block air paths and access, which is why those holds usually take sample-extraction smoke detection (below) rather than ceiling-mounted heads. Ro-ro and vehicle spaces combine detection with a fixed water-based system. The engine room takes heat plus, increasingly, flame or smoke detection feeding both the bridge and the engine control room.

Sample-extraction smoke detection (FSS Code Chapter 10)

FSS Code Chapter 10, “Sample extraction smoke detection systems,” covers the aspirating arrangement that draws continuous air samples from monitored spaces through dedicated pipework back to one or more central detector units in a control position. Instead of putting electronics inside a dusty, inaccessible cargo hold, the system runs accumulator pipes with sampling points into each hold and pulls a small flow of air past detectors the crew can see and maintain on the bridge or in the cargo control room. The same pipe network often doubles as the route for fixed CO2 injection into the hold once a fire is confirmed, which is why these systems are standard on dry-cargo and container ships. Chapter 10 sets the requirement that the system give an alarm within a defined time of smoke entering a sampling point, and that the fan and detection unit be monitored so a blocked pipe or failed fan is itself an alarm condition.

The operational catch with aspirating systems is sampling latency and dilution. Air has to travel the length of the pipe before it reaches the detector, and clean air drawn from holds without fire dilutes the smoke from the one hold that does have it. Designers balance pipe length, the number of sampling points per hold, and fan capacity against the required alarm time. Crews check the system by introducing test smoke at the most remote sampling point and timing the alarm.

CO2 total-flooding systems (FSS Code Chapter 5)

Fixed carbon-dioxide total flooding remains the most common fixed gas system on commercial ships. FSS Code Chapter 5, “Fixed Gas Fire-Extinguishing Systems,” carries its design rules. CO2 works mainly by displacing oxygen: it dilutes the atmosphere in a sealed space below the concentration that sustains combustion, with a secondary cooling effect from the gas expanding out of the liquid phase. It is electrically non-conductive, leaves no residue, and doesn’t attack machinery, which is why it suits an engine room full of energized switchboards and oily surfaces.

Quantity: the 40 percent and 35 percent rules

For a machinery space, the FSS Code fixes the CO2 charge by the volume of free gas it must produce, and free CO2 is calculated at 0.56 cubic metres per kilogram. The quantity carried must give a minimum volume of free gas equal to the larger of two figures: 40 percent of the gross volume of the largest machinery space so protected, excluding the part of the casing above the level at which the horizontal area of the casing is 40 percent or less of the horizontal area of the space; or 35 percent of the gross volume of the largest machinery space protected, including the casing. The designer computes both and carries enough cylinders for whichever is greater. For a cargo space, the requirement is lower: the minimum volume of free gas must be at least 30 percent of the gross volume of the largest cargo space so protected.

The arithmetic falls straight out of those numbers. Carrying enough CO2 to flood a space of volume $V$ to a design fraction $f$, given the free-gas factor of $0.56\ \text{m}^3/\text{kg}$, needs a mass $m = \dfrac{f \cdot V}{0.56}$. A 6,000 cubic metre machinery space at the 40 percent rule needs $m = (0.40 \times 6000)/0.56 \approx 4286\ \text{kg}$ of CO2. At a typical 45 kg cylinder fill that is about 96 cylinders. The Fire CO2 Cylinder Bank Calculator runs this for a given space volume and cylinder size; the CO2 system 85 percent discharge in 2 minutes tool checks the discharge-rate side.

Discharge time and release controls

Speed is part of the design, not a nicety. For machinery spaces, the system must be able to discharge 85 percent of the gas within two minutes, so the protected atmosphere reaches extinguishing concentration before a fire can grow past it or before leakage bleeds the gas away. That requirement drives the pipe and nozzle sizing, and class verifies it with a flow calculation and, where needed, a discharge test.

Release is deliberately a two-action, two-control operation. The FSS Code requires two separate controls to release CO2 into a protected space: one opens the valve on the cylinder bank, the other opens the valve admitting gas to the protected space, so an accidental single operation can’t flood a manned engine room. The release station sits outside the space it protects. Before any gas flows, an automatic audible alarm must sound inside the space for long enough for people to get out, and on most arrangements the ventilation fans and fuel pumps trip and the space dampers close so the atmosphere can be held. CO2 at design concentration is fatal: it displaces the oxygen a person needs as readily as it starves a fire. The release sequence, the pre-discharge alarm, and the rule that the space stays sealed until it has been ventilated and gas-tested are all about keeping people out of a flooded space, because rescue from one is rarely survivable.

Cylinders, weighing, and testing

CO2 is stored as liquid under its own vapor pressure, roughly 57 bar at 20 degrees Celsius, in steel cylinders filled to no more than two-thirds of a kilogram per litre of water capacity at 15 degrees Celsius. Because liquid CO2 leaks slowly past valve seats over years, charge has to be verified. The contents are checked by weighing each cylinder or with a liquid-level detector, and a cylinder found below 90 percent of its nominal charge must be refilled. Pressure-retaining cylinders are subject to hydrostatic testing on a periodic cycle; under the guidelines in IMO MSC.1/Circ.1318, cylinders in good condition at the ten-year point may run on under a regime that requires every cylinder to be tested within a twenty-year maximum, while any cylinder found rusty or damaged at ten years must be tested then. Pipework is blown through at periodic survey to prove every line and nozzle is clear.

Water mist and the halon replacement (FSS Code Chapter 7)

FSS Code Chapter 7, “Fixed Pressure Water-Spraying and Water-Mist Fire-Extinguishing Systems,” is where the halon-replacement story lands. Water mist suppresses fire by several effects at once: the very fine droplets evaporate fast, absorbing heat and flooding the fire zone with steam that displaces oxygen locally, while the mist cloud blocks radiant heat that would otherwise keep fuel vaporizing. The droplet size, roughly 50 to 200 microns, gives enough surface area for fast evaporation while still carrying into the fire.

The decisive advantage over CO2 is that mist doesn’t asphyxiate. A crew can be in the space while the system runs, which is why water mist has displaced CO2 in new-build passenger-ship machinery spaces and is standard in cruise-ship accommodation. The systems split by pressure: high-pressure designs run at roughly 60 to 100 bar and produce a finer mist with low water consumption, while low-pressure designs run around 12 to 18 bar with simpler pumps and pipework. Because the Code recognizes that mist technologies behave differently, approval is by full-scale fire test against the relevant guidelines rather than by a single prescriptive nozzle density. Machinery-space and cargo-pump-room systems offered as halon or CO2 replacements are approved under the guidelines in IMO MSC/Circ.1165 and its successors; accommodation, public-space, and balcony systems follow their own test protocols. A practitioner specifying mist must check that the exact nozzle, spacing, and pump combination on board matches an approved arrangement, because the approval is for the tested system, not for “water mist” in the abstract.

Foam systems and deck foam on tankers (FSS Code Chapters 6 and 14)

Foam appears in the Code in two places, and the chapter distinction matters at survey. FSS Code Chapter 6, “Fixed Foam Fire-Extinguishing Systems,” governs low-, medium-, and high-expansion foam in machinery and cargo spaces, while FSS Code Chapter 14, “Fixed Deck Foam Systems,” governs the deck foam system on a tanker. Foam suppresses a liquid-fuel fire by floating a stable blanket on the surface that cuts off oxygen and blocks the radiant heat driving evaporation, with the water in the foam adding a cooling effect. The deck foam system on an oil tanker, a Chapter 14 system, is the headline application: monitors and applicators positioned to cover the cargo tank deck, fed by a proportioner that meters foam concentrate into the fire-main water at the correct ratio (commonly 1 or 3 percent).

The supply rate is fixed by the Code. The rate of supply of foam solution must be not less than the greatest of three figures: 6 litres per minute per square metre of cargo-tanks deck area (the ship’s maximum breadth times the longitudinal extent of the cargo-tank spaces); 6 litres per minute per square metre of the horizontal sectional area of the single tank with the largest such area; or 3 litres per minute per square metre of the area protected by the largest monitor, that area being entirely forward of the monitor, but in no case may any monitor output be less than 1,250 litres per minute. The system must hold enough concentrate for at least 20 minutes of foam generation on a tanker fitted with an inert gas system, or 30 minutes on a tanker without one, the longer duration reflecting the higher fire risk in a tank that isn’t inerted. The Fire Foam Tanker Calculator sizes the solution rate and concentrate quantity from the deck geometry.

High-expansion foam, with expansion ratios up to about 1,000 to 1, can flood a whole machinery space as a halon alternative, filling the volume rather than blanketing a surface. The agent chemistry has shifted: traditional aqueous film-forming foam (AFFF) relies on per- and polyfluoroalkyl substances (PFAS), and fluorine-free foams (F3) are replacing it as PFAS restrictions tighten across jurisdictions. Alcohol-resistant grades (AR-AFFF and AR-F3) are specified where polar solvents like alcohols or ketones are carried, because a standard foam blanket dissolves on contact with them.

Fixed water-spraying and automatic sprinklers (Chapters 7 and 8)

Fixed pressure water-spraying systems deliver an open-nozzle deluge over a defined high-risk area: engine tops, boiler fronts, purifier flats, and incinerator rooms in the machinery space, where an oil-spray fire on a hot surface is the design case. Unlike a sprinkler, the nozzles are open, so the whole zone wets at once when the section valve opens, by manual or automatic release. The water both cools the surface and knocks down the flame. Supply usually comes from the fire main or a dedicated pump.

Automatic sprinkler systems fall under FSS Code Chapter 8, “Automatic Sprinkler, Fire Detection and Fire Alarm Systems,” and are the accommodation-space workhorse on passenger ships. Each sprinkler head is a closed nozzle held shut by a heat-sensitive element, a glass bulb or fusible link, that releases at a rated temperature so only the heads over the fire open, limiting water damage. The Chapter 8 system is dual-purpose: the flow of water through an opened head triggers an alarm, so the sprinkler array is also a heat-detection array. A pressure tank or pump set keeps the system charged so the first head opens with no delay. The Safety Sprinkler Flow Calculator and the Fire Water Spray Density Calculator cover the hydraulic side of both system types.

The fire main, pumps, and the inert-gas link

None of the water-based systems work without the fire main: the pressurized seawater ring that feeds hydrants, hoses, foam proportioners, and water-spray sections. SOLAS requires at least two independently driven power pumps, sized so the system delivers the required flow and pressure at the most remote hydrant. An emergency fire pump, driven by its own diesel and sited outside the main machinery space with independent fuel and ventilation, gives a fall-back when fire or flooding takes the main pumps; its capacity is set as a fraction of the main pump duty, enough to fight the critical fire while the main space is lost. The Fire Pump Capacity SOLAS Calculator, the Fire Main Diameter Calculator, and the Safety Firemain Pressure Calculator handle the sizing relationships.

Inert gas is prevention rather than suppression, but it belongs in the same picture on tankers. By holding cargo-tank atmospheres below the oxygen level that supports combustion (the SOLAS ceiling is 8 percent by volume, and operating practice keeps it lower), inert gas removes one leg of the fire triangle before a fire can start. The detail sits in Marine Inert Gas Systems. The engine-room ventilation that has to shut down before a CO2 release, and that controls the smoke and heat a fire produces, is covered in Marine Engine Room Ventilation and Uptakes.

Clean-agent gas systems as halon replacements

Where a machinery space or special enclosure can’t take CO2 (because it’s manned during operation) but a gaseous agent is still wanted for its clean, non-conductive action, the halon-replacement chemical agents fill the gap. These are the hydrofluorocarbon and inert-gas blends approved as alternatives to halon and CO2 for total flooding. HFC-227ea (heptafluoropropane, sold as FM-200) is the common chemical agent: it extinguishes at a design concentration low enough to leave a survivable atmosphere for short exposure, and it’s stored as a liquefied gas pressurized with nitrogen, commonly to about 40 bar above the agent’s own pressure at 21 degrees Celsius so it discharges fast. The Fire FM200 Concentration Calculator and the Fire HFC-227ea Calculator compute the agent mass for a given space volume and design concentration.

Inert-gas blends take the other route, suppressing by oxygen reduction like CO2 but with agents that aren’t toxic at design concentration. Inergen is the well-known blend: nitrogen at 52 percent, argon at 40 percent, and carbon dioxide at 8 percent, a mixture chosen so the residual atmosphere holds enough oxygen and the small CO2 fraction stimulates breathing to compensate for the reduced oxygen. Like CO2 systems, the chemical and inert-gas systems are required to discharge the bulk of their charge fast, with the 85 percent in under two minutes target applied to the machinery-space designs. Each agent has its own approved design concentration, cylinder pressure, and discharge characteristic, so the agent on board is fixed by the system’s type approval, not chosen at refill time.

The trade against CO2 is cost and footprint. Chemical agents are far more expensive per protected volume, and inert-gas blends, being stored as high-pressure gas rather than liquid, need many more cylinders for the same space. That economics is why CO2 has held its ground in cargo-ship engine rooms while the manned passenger-ship spaces moved to water mist and clean agents.

Detection and suppression by space type

The rules tie a space to a detection method and a fixed system by its fire risk and its occupancy. A Category A machinery space, defined by SOLAS as one containing internal-combustion machinery for propulsion or power, takes heat and increasingly flame or smoke detection feeding both the bridge and the engine control room, a fixed system (CO2, water mist, high-expansion foam, or an approved clean agent), local fixed water-spraying over the oil-fired and high-pressure-fuel hazards, fire-main coverage, portable extinguishers, and the fuel-pump and ventilation trips that arm the fixed release. Accommodation and service spaces take smoke detection, the general alarm and PA, and on most passenger ships an automatic sprinkler array under Chapter 8.

Cargo spaces split by what they carry. A container or general dry-cargo hold takes sample-extraction smoke detection under Chapter 10, with CO2 injection through the same pipework on confirmation. A ro-ro or vehicle space takes detection plus a fixed water-based system because the fuel load is mobile and the volume large. A tanker’s cargo deck takes the inert-gas system that prevents the fire, the deck foam system under Chapter 14 that fights one, and fire-main coverage. Special spaces each carry their own rule: a paint store or flammable-store takes smoke detection plus a fixed gas system, a galley takes heat detection plus a dedicated extinguishing arrangement over the cooking equipment and a fire damper in the exhaust duct, and a battery room takes hydrogen monitoring plus forced ventilation rather than a suppression system, because the hazard there is explosion, not fire.

Alarms, public address, and the human link

Detection is worthless if no one hears it. The general fire alarm, sounded continuously, is understood across international shipping, and SOLAS sets minimum sound levels and a tone distinguishable from other shipboard alarms. The public address system overlays voice instruction, with zoning so an announcement can target one part of the ship, priority override so an emergency message interrupts everything else, and battery backup so it survives a power loss. Visual beacons cover the high-noise spaces, engine rooms and workshops, where an audible signal can be missed. The bridge fire panel is the master display, repeated in the engine and cargo control rooms, and the watch standard requires that an alarm there be acknowledged and acted on, not silenced and ignored.

The systems tie into ship survival as a whole. A fire that gets away drives the muster and abandonment sequence that depends on the equipment in Marine Lifeboats and Survival Craft, and the integrated alarm and monitoring picture is part of the bridge described in Marine Bridge Equipment and Integrated Bridge Systems.

Maintenance and survey

A fixed fire system spends its whole life waiting for one event, so its maintenance regime is about proving readiness on a schedule. Daily and weekly attention covers the detection panel (no standing faults or unacknowledged alarms), the fire-main pumps, and the hose stations. Monthly work runs the main and emergency pumps under load, starts the emergency diesel, and tests a sample of detectors by triggering and resetting them. CO2 quantity is verified at least every two years (within two years plus or minus three months) on passenger ships and at each intermediate, periodical, or renewal survey on cargo ships; the protected space is checked annually for gas-tightness, and the lines are blown through at periodic survey. Flexible hoses in a CO2 system are examined annually and renewed per the maker’s interval. The five-year special survey reaches into component replacement, pump overhaul, and the cylinder hydrostatic testing already described.

The records matter as much as the hardware. Class and flag expect to see fill certificates for each cylinder, pressure-test certificates for manifolds and safety valves, and the enclosure-volume and agent-quantity calculations that justify the charge on board. A system that works but can’t show its paperwork fails the survey as surely as one that leaks.

Latest developments, 2024 to 2026

Fire safety is one of the most actively amended parts of SOLAS, and several changes land across 2024 to 2026.

The largest is the ro-ro and vehicle-space package. IMO resolutions MSC.550(108), amending SOLAS Chapter II-2, and MSC.555(108), amending the FSS Code, were adopted in May 2024 and take effect on 1 January 2026. They rebuild Regulation 20 for vehicle, special category, open and closed ro-ro spaces and vehicle weather decks, requiring fixed detection with linear heat detectors, video monitoring, and, on ro-ro passenger ship weather decks, a fixed water-based system whose monitors deliver at least 2.0 litres per minute per square metre of protected area at no less than 1,250 litres per minute each. New ships comply on build; existing passenger ships comply by the first survey after 1 January 2028. The driver is the run of car-carrier and ro-ro fires, many traced to vehicles and, increasingly, electric-vehicle batteries.

The firefighting foam picture is changing under chemical regulation, not fire rules. Foams based on per- and polyfluoroalkyl substances (PFAS) are being phased out: an IMO requirement bars fire-extinguishing media containing more than a trace of perfluorooctane sulfonic acid (PFOS) from 1 January 2026, and the EU’s REACH restriction stops PFAS-containing foam being brought aboard at EU ports from late 2025, with a longer transition for marine use. Fluorine-free foams (F3) are the replacement, but they are not a drop-in: they can need up to twice the storage volume, do not proportion the same way with seawater, and cannot be mixed with residual PFAS foam, so a foam change is a system change.

A container-ship cargo-fire package is close behind. After repeated stack fires that fire mains cannot reach, IMO has developed amendments to SOLAS II-2 and the FSS Code for ships carrying containers on or above the weather deck: at least one water-mist lance able to pierce a container wall, performance standards for mobile and fixed water monitors, and portable thermal imagers for locating hot spots. The package progressed through the Ship Systems and Equipment Sub-Committee and was approved at MSC 110 in 2025, with formal adoption expected at MSC 111 in 2026 and entry into force not before 2028.

The alternative-fuel transition brings fuel-specific fire hazards the conventional agents do not all answer. Methanol has a 12 degree Celsius flashpoint and burns with a near-invisible flame, and water mist tuned for diesel performs poorly on it; the interim guidelines in MSC.1/Circ.1621 require alcohol-resistant foam for methanol-fuel machinery-space bilge protection. Ammonia is toxic rather than readily flammable, and its interim guidelines, MSC.1/Circ.1687 of 2025, focus on gas detection, exclusion zones and water spray for vapour knock-down. Lithium-battery and electric-vehicle fires are the open problem: thermal runaway reignites from inside the cell, so CO2 and water mist cool the surface but cannot stop it, and no current shipboard fixed agent reliably terminates it without direct, sustained cooling of the cells. Class societies have issued guidance ahead of any SOLAS rule, which is not expected in force before 2028.

Smaller but real, the inert-gas chapter was tidied by MSC.484(103), in force 1 January 2024, and the helideck and hydrocarbon-gas-detection chapters (FSS Code Chapters 16 and 17) were added by earlier amendments and remain in force. The lesson for any specifier is the one this article keeps returning to: cite the chapter and the amending resolution and the ship’s build date together, because the right answer in 2026 is not the right answer for a 2010 hull.

Limitations

The figures in this article are the SOLAS and FSS Code minimums for ships on international voyages, and they are the floor, not the design. Class society rules (DNV, Lloyd’s Register, ABS, Bureau Veritas, ClassNK, and others), flag administration interpretations, and national rules such as the US 46 CFR series often add to them or apply different numbers to ships outside the SOLAS scope. Always work from the version of the Code applicable to the ship’s keel-laying date, because amendments like MSC.206(81), MSC.403(96), and MSC.555(108) are not retroactive, and a system that is compliant for a 2005 hull may not match the text a 2024 hull is built to.

The CO2 quantity rules above are the prescriptive baseline for total flooding. They assume a sealed enclosure: a space that can’t be made gas-tight, or one with continuous ventilation that can’t be shut down, will not hold the design concentration regardless of how much gas is carried. The 40 percent and 35 percent figures, and the 30 percent cargo-space figure, are minimum volumes of free gas, not concentrations achieved; real concentration depends on leakage, temperature, and how completely the space is sealed at the moment of release. Treat any single-number calculator output as a sizing estimate to be checked against the class-approved flow calculation for the specific installation.

Water mist and high-expansion foam are approved as tested systems, not as generic technologies. An approval certificate covers the exact nozzle, spacing, pressure, and pump combination that passed the fire test under MSC/Circ.1165 or its equivalent; substituting a different nozzle or changing the geometry voids it. Do not assume a mist system rated for one machinery-space volume protects a larger one. The PFAS picture for foam is moving: fluorine-free replacement is mandatory in some jurisdictions and optional in others, and the timeline differs by flag and port state, so a foam stock compliant in one trade may be barred in another. None of the detection or suppression detail here is a substitute for the approved fire control plan, the maker’s manual, and the relevant class rules for the specific ship.

See also

• SOLAS Chapter II-2: Fire Protection, Detection and Extinction
• Marine Inert Gas Systems
• Marine Engine Room Ventilation and Uptakes
• Marine Lifeboats and Survival Craft
• Marine Bridge Equipment and Integrated Bridge Systems
• Marine Sea Water Cooling Systems

Related calculators

• Fire CO2 Cylinder Bank Calculator
• CO2 System: 85 percent Discharge in 2 minutes
• Fire Foam Tanker Calculator
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• Fire Water Spray Density Calculator
• Safety Sprinkler Flow Calculator
• Fire Pump Capacity SOLAS Calculator
• Fire Main Diameter Calculator
• Safety Firemain Pressure Calculator
• Fire Portable Extinguisher Count Calculator
• SOLAS Fire Drill Frequency Calculator
• Fire FM200 Concentration Calculator
• Fire HFC-227ea Calculator