By ShipCalculators.com · Published 29 April 2026 · last updated 28 May 2026

Biomass Pellets: IMSBC Schedule and Carriage

Contents

Wood and biomass pellets are the fastest-growing chemical-hazard dry bulk cargo of the energy transition, with global seaborne wood pellet trade running near 30 million tonnes a year by the mid-2020s. The cargo looks benign: small, dry, dust-free cylinders of compressed sawmill residue. It is not. In a sealed cargo hold, freshly produced pellets consume oxygen and release carbon monoxide fast enough to turn an adjacent forepeak or bow thruster room into a lethal atmosphere within hours. Between 2002 and 2015 at least five people died on or beside wood-pellet ships in northern European ports, none of them inside the hold itself. The IMSBC Code regulates both pellet types as Group B materials hazardous only in bulk (MHB), and the oxygen-depletion and carbon-monoxide hazard, not fire, is the part that keeps killing crews.

This article covers what biomass pellets are and why the trade exists, the two IMSBC schedule entries and their authoritative parameters, the off-gassing chemistry, the documented fatal-entry casualties, the ventilation debate, the gas and temperature monitoring regime, hold preparation, loading and trimming, discharge, and the limits of what the schedule and the calculators can tell you. To screen any consignment against its Code entry, use the IMSBC cargo finder; for the pellet-specific parameters, the wood pellets and biomass pellets calculators carry the schedule values.

What biomass pellets are and why they ship

Biomass pellets are densified solid biofuel. The dominant product by volume is the industrial wood pellet: sawdust, shavings, and chipped forest residue, dried to roughly 8 to 10 per cent moisture, hammer-milled to a fine flour, then forced through a ring die under pressure into cylinders about 6 to 10 mm in diameter and 10 to 40 mm long. No glue is needed for most wood. The heat and pressure of pelletizing soften the wood’s own lignin, which sets as the pellet cools and binds the fibers. That single fact, that the binder is the wood itself, is why the Code splits the cargo into two schedules and why the no-additive schedule still self-heats and off-gasses.

The trade is a coal-substitution story. Large thermal power stations that were built to burn pulverized coal can be converted to burn pulverized wood pellets with limited modification, and a pellet’s calorific value of roughly 17 to 18 MJ/kg, against 24 to 30 MJ/kg for bituminous coal, makes the swap energetically workable once feedstock and logistics are arranged. The United Kingdom’s Drax power station, the single largest pellet importer in the world, runs converted units on millions of tonnes of pellets a year. Japan and South Korea pull large volumes under renewable-energy mandates. Denmark, the Netherlands, and Belgium take substantial cargoes for combined heat and power.

Supply is concentrated. The southeast United States is the largest export region, shipping from Gulf and South Atlantic terminals such as Mobile, Pascagoula, Savannah, and Wilmington. Western Canada exports from British Columbia to the Pacific market. The Baltic states, Russia, and southeast Europe supplied northern Europe by short sea until trade with Russia was cut by sanctions after 2022. Vietnam grew into a major Asian supplier. The receiving end is usually a dedicated power-station berth with enclosed conveyors and silo storage, not a general bulk terminal.

Sugarcane biomass pellets, torrefied wood, wood chips, and pulp wood chips are adjacent cargoes in the same family. They share the oxygen-depletion mechanism to varying degrees. This article centers on wood pellets because they carry the documented fatality record and the largest trade, but the carriage logic transfers to the others with the parameter and hazard adjustments noted in each schedule.

The two IMSBC schedule entries

The current IMSBC Code, as amended by Resolution MSC.539(107) adopting amendment 07-23, carries two wood pellet schedules. The split turns on one chemical question: does the pellet contain anything other than wood.

WOOD PELLETS NOT CONTAINING ANY ADDITIVES AND/OR BINDERS. Group B. MHB classification: OH (other hazards). Bulk density 600 to 750 kg/m³. Stowage factor 1.33 to 1.67 m³/t. Angle of repose about 30 degrees. The OH code captures the oxygen-depletion and carbon-monoxide evolution, hazards that do not fit the combustible, self-heating, water-reactive, toxic, or corrosive boxes cleanly.
WOOD PELLETS CONTAINING ADDITIVES AND/OR BINDERS. Group B. MHB classification: WF (solids that evolve flammable gas when wet). Same bulk density 600 to 750 kg/m³, same stowage factor 1.33 to 1.67 m³/t, same angle of repose about 30 degrees. The WF code is the difference: additives, including some binders, can react with water to release flammable gas, so this schedule layers a wet-reactivity hazard on top of the off-gassing common to both.

Both hazard descriptions in the Code carry the same core sentence: shipments are subject to oxidation leading to depletion of oxygen and increase of carbon monoxide and carbon dioxide in cargo and communicating spaces, and the pellets swell if exposed to moisture. The “communicating spaces” phrase is the one that matters most, and it is the one that has been ignored in every documented fatality.

A historic single entry, simply “WOOD PELLETS,” predates the split and was removed when the two replacement schedules were finalized. Older cargo declarations, charter party clauses, and stowage plans may still reference the obsolete single entry; the correct current practice is to declare against one of the two amendment 07-23 schedules according to whether the cargo contains additives or binders. The shipper carries the duty under IMSBC Section 4 to declare the correct Bulk Cargo Shipping Name (BCSN) and group, and the master is entitled to refuse a cargo whose declaration does not match the Code.

The MHB framework itself comes from IMSBC Section 9.2.3, which sorts materials hazardous only in bulk into seven sub-hazards: CB combustible solids, SH self-heating solids, WF solids that evolve flammable gas when wet, WT solids that evolve toxic gas when wet, TX toxic solids, CR corrosive solids, and OH other hazards. Wood pellets sit in OH or WF rather than SH, even though they self-heat, because their dominant documented killing mechanism is atmospheric: they pull oxygen out of the air and put carbon monoxide into it.

Cargo properties

A standard industrial wood pellet is a cylinder roughly 6 to 8 mm across and 10 to 40 mm long, with an as-loaded moisture content near 8 to 10 per cent and a durability above 97.5 per cent in the common ENplus and industrial specifications. Bulk density at loading sits at 600 to 750 kg/m³, which the schedule confirms; loose poured density is at the lower end, settled and vibrated density at the higher. The stowage factor of 1.33 to 1.67 m³/t means a pellet cargo is lighter than coal (coal stows near 1.1 to 1.5 m³/t) and far lighter than iron ore, so a bulk carrier loading pellets usually cubes out, filling the hold volume, before it reaches deadweight. The ship sails with holds full and a deadweight margin unused.

Calorific value of about 17 to 18 MJ/kg as received reflects the residual 8 to 10 per cent moisture. The angle of repose near 30 degrees means the cargo self-trims to a near-level surface as it pours and does not require mechanical trimming for stability, though trimming the surface flat helps hatch closure and reduces the headspace where gas pools. Pellets are abrasion-prone: each transfer point, free-fall, and grab bite breaks a fraction into fines and dust, which matters for both off-gassing surface area and dust hazard at the terminal.

Moisture is the variable that flips the cargo from manageable to dangerous. Dry pellets at 8 to 10 per cent off-gas through chemical oxidation alone. Wet pellets swell, disintegrate, and add microbial decomposition to the oxidation, raising both heat output and gas production. A pellet that takes on free water has lost the structural integrity that the trade pays for, so keeping rain and seawater off the cargo during loading and carriage protects both the commercial value and the safety margin.

Off-gassing: the mechanism

Fresh wood pellets emit carbon monoxide, carbon dioxide, methane, and lighter volatile organic compounds, and at the same time consume the oxygen in the space around them. The driver is auto-oxidation, not combustion and not primarily microbial action in dry cargo. The HSE attributes the carbon monoxide to an auto-oxidation process, in particular oxidation of the fatty acids in wood. The Danish casualty analysis is more specific on why pellets off-gas worse than the wood they came from: milling and compression rupture the wood cell walls and expose the cell cytoplasm and chemically reactive compounds such as resins, oils, and fatty acids to air. The reactive surface area per tonne is far higher in a pellet than in a log, so the oxidation that a tree trunk would do over years a pellet pile does in weeks.

The rate is age-dependent and temperature-dependent. The HSE bulletin states carbon monoxide production peaks within the first six weeks after manufacture, rises with temperature, and varies with species, with pine pellets producing more carbon monoxide than spruce. The practical reading for a ship is blunt: the danger is highest on the cargo that was made last and loaded freshest, in the first days of the voyage, in warm weather. A cargo that has aged in a silo for two months before loading off-gasses less than one pelletized the week before sailing.

Off-gassing pulls oxygen down at the same time as it pushes carbon monoxide up, and the two together, not either alone, make the atmosphere lethal. Carbon monoxide blocks oxygen transport in the blood by binding hemoglobin in preference to oxygen; an oxygen-depleted atmosphere starves the body of oxygen directly. A space can be simultaneously below the 19.5 per cent oxygen safe-entry floor and far above any toxic carbon-monoxide threshold. That is why every credible procedure tests both gases, and why testing for one alone has killed people.

Documented gas concentrations at sea

The numbers from instrumented voyages are not theoretical. The peer-reviewed study by Svedberg and colleagues, published in the Annals of Occupational Hygiene in 2008, measured the headspaces of five ships carrying wood pellets from British Columbia to Sweden. Carbon monoxide ranged from 1,460 to 14,650 ppm. Carbon dioxide ranged from 2,960 to 21,570 ppm. Methane ranged from 79.9 to 956 ppm. Oxygen in the cargo holds ranged from 0.8 to 16.9 per cent, with one hold measured at 0.8 per cent oxygen, an atmosphere that causes collapse in seconds and death in minutes.

To put 14,650 ppm carbon monoxide in scale: the workplace ceiling many jurisdictions enforce is around 50 ppm for an eight-hour exposure, and concentrations above roughly 1,200 ppm are immediately dangerous to life and health. The interior of a freshly loaded pellet hold runs an order of magnitude above that IDLH line. The lowest oxygen reading, 0.8 per cent, is below the level at which an unprotected person loses consciousness without warning; there is no gasping-for-air sensation, because the body’s air-hunger reflex responds to carbon dioxide, not to missing oxygen.

These are hold-atmosphere figures, and crews are not meant to enter the hold of a loaded pellet ship. The lethal problem is that the same gas migrates into spaces that crews do enter routinely and do not think of as part of the cargo system.

The fatal-entry record

Every documented wood-pellet death on a ship has the same shape: a crew member or shore worker entered an enclosed space adjacent to the cargo hold, not the hold, and was overcome by gas that had migrated through a crevice. The Danish review of three of these casualties is the clearest record.

On 15 July 2009, the coaster AMIRANTE was in the Baltic Sea 16 km north of Bornholm. Two crew, an able seaman and a motorman, entered the forepeak compartment forward of the cargo hold and died. Autopsy carboxyhemoglobin levels were 52 and 60 per cent; saturation above roughly 50 per cent is commonly fatal. Carbon monoxide measured 100 ppm in the forepeak 40 minutes after ventilation was switched off, with 150 ppm in the headspace of a pellet sample. The forepeak door had been kept locked open in normal operation, and gas had migrated through a door-hinge gap.

On 13 July 2014, the LADY IRINA was at the Port of Fredericia, Denmark. The chief engineer died and the chief officer and two seamen were injured after entry into the bow thruster room adjacent to the hold. After 36 hours closed, the forecastle and bow thruster spaces measured 690 and 555 ppm carbon monoxide, with a storage room above 2,000 ppm carbon dioxide. The bow area had no mechanical ventilation, and the crew had opened the door for 15 to 20 minutes for natural ventilation before entering rather than force-ventilating. Gas had reached the space through a minor gap in the rim of an inspection cover on a ventilation duct.

On 28 April 2015, the CORINA was at the Port of Hanstholm, Denmark. An able seaman died and five people were injured, including a port officer and a paramedic who went in to help. The casualty was in a lashing equipment room adjacent to the hold. Carbon monoxide measured 66 ppm at the entry door and 366 ppm at lower levels after mechanical ventilation was activated; autopsy carboxyhemoglobin was 60 per cent. The lashing room was separated from the hold only by a non-gastight plank barrier that let cargo gas migrate freely.

Two earlier port incidents predate the Danish set. In 2002 a stevedore died in Rotterdam during pellet discharge, with several injured. In November 2006 at the Port of Helsingborg, Sweden, a seaman was killed and a stevedore was seriously injured after entering an unventilated stairway next to a cargo hold discharging pellets from British Columbia; the injured stevedore reached 43.8 per cent carboxyhemoglobin on hospital admission and suffered permanent neurological damage. Onshore storage has killed too: the HSE counted at least nine European deaths from carbon-monoxide poisoning in wood-pellet storage since 2002, including a 43-year-old engineer in a 155-tonne bunker in Germany in January 2010, a householder in Ireland in November 2010, and a pregnant woman in a Swiss storeroom in February 2011.

The pattern across all of these is consistent and it is the single most important carriage fact about this cargo. People do not die in the hold. They die in the forepeak, the bow thruster room, the lashing locker, the access stairway, the duct space, places they do not associate with cargo at all, because off-gassing carbon monoxide and oxygen depletion travel through hinge gaps, duct-cover rims, and non-gastight barriers into compartments that look and feel like normal ship spaces. The rescuer toll in the CORINA and Helsingborg cases is the second lesson: untrained rescue entry into the same atmosphere multiplies a single fatality into several.

Enclosed-space entry: the safety centerpiece

The controlling rule for wood pellets is that any enclosed space with air communication to a pellet hold is to be treated as a confined space that may contain a carbon-monoxide-rich, oxygen-depleted atmosphere, tested before entry, and entered only under a permit-to-work with the confined-space entry and tank-inspection regime that SOLAS Regulation III/19 and the associated enclosed-space drill requirements impose on every ship. The defining error in the casualty record is that the dangerous spaces were not recognized as communicating with the cargo, so the entry regime was never triggered.

Practical entry control for a pellet ship rests on a few hard rules. Identify every space that shares air with the hold before loading, including ducts, access trunks, store rooms, bow thruster and forepeak compartments, and any space separated from the hold by a barrier that is not gastight. Test the atmosphere of any such space for both oxygen and carbon monoxide immediately before entry, at multiple levels, because carbon monoxide is close to the density of air and distributes through the space rather than pooling cleanly at the floor or ceiling. Never accept natural ventilation, an open door, or a passage of time as a substitute for a fresh test and forced ventilation; the LADY IRINA crew opened the door for 15 to 20 minutes and the chief engineer died.

The oxygen floor for entry without breathing apparatus is 19.5 per cent. The carbon-monoxide thresholds that govern entry decisions are an eight-hour workplace ceiling near 50 ppm and an immediately-dangerous-to-life level near 1,200 ppm, but the operational rule on a pellet ship is stricter: any detectable carbon monoxide in a space that should contain clean air is evidence of migration from the cargo and a reason to stop, ventilate, and re-test, not to proceed in a mask. Rescue is the highest-risk moment, and the record shows would-be rescuers dying beside the original casualty; no one enters to recover a collapsed colleague without breathing apparatus and a tended lifeline, full stop.

The ventilation debate

Ventilation policy for wood pellets is counterintuitive and is the area where ship and shore practice most often diverge. For most off-gassing cargoes the instinct is to ventilate the holds to clear toxic gas. For wood pellets the prevailing carriage guidance is the opposite for the holds and unconditional for the adjacent spaces, and getting that distinction right is the whole safety problem.

The hold case. Surface ventilation of a sealed pellet hold introduces fresh oxygen to the cargo, which feeds the same oxidation that produces the heat and the carbon monoxide. The mainstream P&I loss-prevention position, reflected in club guidance such as Skuld’s, is that it is generally advisable to carry wood pellets without applying any ventilation to the holds, because keeping the holds sealed lets the off-gassing reaction become self-limiting: as oxygen in the headspace is consumed, the oxidation slows for want of an oxidizer. Forced ventilation can do the reverse, sustaining the reaction and the gas production. So the holds are sealed, monitored, and left alone, which is the same logic that governs methane-emitting coal under its surface-ventilation regime, with the sign reversed: for coal you vent the headspace to clear methane while minimizing oxygen to the cargo body; for pellets the dominant hazard is the off-gas itself, so you seal and monitor rather than vent.

The adjacent-space case. The communicating spaces are where the rule flips to unconditional forced ventilation before entry. The whole Danish casualty set turns on adjacent spaces that were either not ventilated at all or relied on natural ventilation. The correct practice is mechanical ventilation of any space that may have received migrated gas, run for long enough to clear it, with a fresh gas test confirming the result, every time, before anyone enters. The chapter authors frame the systemic failure precisely: mechanical ventilation is a prevention measure that removes the hazard, while testing alone is only a control that confirms it; relying on a crew member to test and decide, voyage after voyage, is the weak link that broke.

Sealing the holds while force-ventilating the access spaces is not a contradiction. It is the cargo-specific resolution of a genuine conflict: oxygen is the enemy in the hold and the friend in the spaces a person enters. A pellet ship that ventilates its holds to feel safer and skips ventilating its bow thruster room has the policy exactly backward.

Self-heating and the secondary fire hazard

Self-heating is the secondary hazard, real but better controlled than the atmospheric one. The same auto-oxidation that produces carbon monoxide produces heat, and in a deep, compacted, sealed hold that heat can accumulate faster than it conducts away, especially in the cargo’s interior where it cannot radiate. Three mechanisms contribute: chemical oxidation of cellulose, lignin, and fatty acids; microbial decomposition where residual or absorbed moisture supports it, most aggressive above roughly 15 per cent moisture; and simple heat retention in the core of a large mass. Wet pellets add the microbial path to the chemical one and self-heat harder, which is why moisture exclusion is a safety control and not only a quality one.

Left unchecked, self-heating can progress to smoldering and, with an oxygen supply, to flaming combustion that propagates through the cargo. The controls are the same sealed-hold and monitoring regime that handles the gas, plus a feedstock-age limit at the loading end: many shippers will not load pellets that have aged beyond a set window after manufacture, because both off-gassing and self-heating peak in the first weeks, and some operations apply the inverse, requiring a short cure period so the most violent initial off-gassing happens ashore in a ventilated silo rather than in a sealed hold at sea. The two practices target the same curve from opposite sides; the common ground is that the cargo’s reactivity is a function of its age and the buyer of the carriage risk should know that age.

For an order-of-magnitude self-heating screen the cargo heat and spontaneous combustion calculator estimates the balance between oxidation heat generation and conductive loss for a bulk pile, and the principles carry over from the coal self-heating treatment. The pellet case differs in that the atmospheric hazard usually becomes lethal long before the thermal hazard becomes a fire, so temperature monitoring supports, rather than replaces, gas monitoring.

Gas and temperature monitoring during carriage

The IMSBC schedules for wood pellets do not impose the same continuous-monitoring mandate that the coal schedule does, but the carriage standard that P&I clubs and competent operators apply fills that gap, and the casualty record is the reason. The sensible monitoring regime treats every voyage as if monitoring were mandatory.

Gas monitoring covers oxygen and carbon monoxide as the two primary parameters, with carbon dioxide and methane as supporting measurements where the instrument provides them. Readings are taken from sealed sampling points on each hold, ideally daily after departure and most frequently in the first week when off-gassing peaks. The values are logged against time so the trend, not the single number, drives decisions: a steady high carbon-monoxide reading in a sealed hold is expected and is not by itself a sign of trouble, while a sharp rising trend or a temperature climb points to active self-heating. The principle that high carbon monoxide is normal for this cargo, while a rising trend is the warning, is the one most easily lost on a crew that has only carried inert cargoes. Crucially, the ship samples the adjacent communicating spaces too, not only the holds, because those spaces are where people are, and a clean hold log says nothing about the bow thruster room.

Temperature monitoring runs in parallel where the ship has the means. Older bulk carriers often lack fixed hold-temperature probes, in which case temperature is taken via sounding pipes or by lowering a sensor, while newer designs increasingly carry fixed sensors. The temperature trend is the self-heating indicator; the gas trend is the off-gassing and oxidation indicator; together they bracket the cargo’s behavior. The logs are forwarded to the operator and shipper for trend analysis and are the record that demonstrates the master discharged the duty of care if a casualty inquiry follows.

Instrument fitness is part of the regime and is subject to port state control. The ship must carry calibrated detectors capable of reading oxygen and carbon monoxide at the relevant ranges, with current calibration records, because a detector that cannot read up to several thousand ppm carbon monoxide will simply peg at its ceiling in a pellet space and tell the user nothing about how bad the atmosphere really is.

Hold preparation

Hold preparation for wood pellets follows the general cargo-hold preparation standards with pellet-specific emphasis on dryness and gas-tightness. Holds must be clean, dry, and free of residues from previous cargoes, because pellets degrade if contaminated and because moisture from a wet hold accelerates both swelling and self-heating. Bilge wells are cleaned and tested clear so that any water reaching the bottom of the stow can be detected and pumped without the bilge system becoming a path for gas into the engine room.

The gas-tightness check is the preparation step the casualty record argues for most strongly. Before loading, the crew identifies and, where possible, seals the paths by which off-gas can migrate from the hold into adjacent spaces: hatch cover seals, access trunk and duct covers, inspection-cover rims, and any bulkhead penetration between the hold and a forepeak, bow thruster room, store, or stairway. The non-gastight plank barrier on the CORINA, the duct-cover rim gap on the LADY IRINA, and the door-hinge gap on the AMIRANTE were each a known-able defect that a deliberate pre-loading gas-tightness inspection should have caught. Where a barrier cannot be made gastight, the space behind it is designated and signed as a space that communicates with the cargo and is brought into the enclosed-space entry regime for the whole voyage.

Hatch covers are checked for weathertight integrity by the usual hose or chalk and ultrasonic tightness testing, both to keep seawater out of the cargo and to keep off-gas in the hold rather than venting onto deck where it can be drawn into accommodation or machinery intakes. Access doors to communicating spaces are labeled with the hazard and, where the casualty pattern warrants, locked and access-controlled, a measure the Helsingborg and Danish reviews specifically recommended after finding doors left locked open or entered casually.

Loading and trimming

Loading is by enclosed shore conveyor and shiploader with dust suppression, feeding the pellets into the hold with as little free-fall and breakage as the terminal arrangement allows. Reducing breakage at loading reduces both the fines that off-gas with extra surface area and the dust that creates an explosion hazard at the transfer points. The cargo self-trims to its angle of repose near 30 degrees, so no mechanical trimming is required for stability; trimming the surface reasonably level helps secure hatch closure and reduces the headspace volume in which gas accumulates.

Weather control during loading is a Code precaution and a practical one. Loading stops or the hold is protected when rain threatens, because pellets that take on free water swell, lose durability, and shift from chemical off-gassing toward the harder microbial self-heating regime. Seawater intrusion through an imperfect hatch seal during the voyage does the same, which is why the hatch-tightness check at preparation matters for safety and not only for cargo claims.

The most dangerous moment of the loading evolution for people is the closing-up. Once the hold is loaded and sealed, off-gassing begins immediately and the communicating spaces start to fill. The crew that has been working in and around the holds during loading needs to switch mindset the moment the hatches close: spaces that were safe to walk through an hour ago are now on the way to becoming lethal, and the enclosed-space regime is in force from that point until the cargo is discharged and the spaces are proven clear. The cargo declaration and stowage plan are signed off, the monitoring log is opened, and the access-control and signage are set before the ship sails.

Discharge and post-discharge

Discharge is preferably by enclosed continuous unloader delivering pellets directly into shore silos, which limits both dust and the escape of carbon monoxide into the open terminal where stevedores work. Some berths use grab discharge, which breaks more pellets, raises more dust, and releases more gas into the working area, so enclosed systems are preferred where the receiving infrastructure supports them. The Rotterdam 2002 and Helsingborg 2006 deaths were discharge-phase casualties involving shore workers entering spaces beside the cargo, so the enclosed-space discipline applies to terminal staff exactly as it does to crew, and the responsibility for briefing visiting stevedores on the hazard falls partly on the ship.

The hazard does not end when the hold looks empty. Residual pellets, fines, and dust in the hold corners, the hopper, and the unloader continue to off-gas, and a hold that has just been discharged can still hold a carbon-monoxide-rich, oxygen-depleted atmosphere, particularly if it was sealed for a long voyage. Entry for inspection or cleaning after discharge is treated as enclosed-space entry, with ventilation and gas testing, until the space is proven clear. The same applies to the communicating spaces, which do not clear themselves the moment the cargo leaves.

Post-discharge cleaning removes pellet fines and dust before the next cargo. Pellet residue is combustible and dusty, so cleaning is also a fire and dust-explosion control, not only a contamination control. Where the next cargo is moisture-sensitive or food-grade, the holds are washed, dried, and inspected to the standard that cargo demands, with attention to drying because residual wash water reintroduces the moisture problem for any remaining pellet fines.

Stowage factor and hold capacity

The clean quantitative relationship in the pellet schedule is the link between bulk density and stowage factor, the pair of numbers that decides how a pellet cargo fills a ship. The two are reciprocals once unit-converted: $SF = 1000 / \rho_b$ , where $SF$ is the stowage factor in m3/t, $\rho_b$ is the bulk density in kg/m3, and 1000 converts kg/m3 to t/m3. The mass a hold of net grain or bale volume $V$ can take, ignoring broken stowage, is $M = V / SF = V \rho_b / 1000$ in tonnes. The schedule gives both as ranges, 600 to 750 kg/m3 and 1.33 to 1.67 m3/t, precisely because a poured-loose cargo and a settled-compacted cargo occupy different volumes for the same mass.

The density that enters the calculation is the as-stowed bulk density in the hold, not the loose poured density at the conveyor or the solid particle density. A solid wood-pellet particle is near 1,100 to 1,200 kg/m3; the 600 to 750 kg/m3 schedule figure already accounts for the inter-pellet voids, and confusing the two roughly doubles the estimated cargo and would overload the ship. A hold of net grain volume 12,000 m3 at the mid-range 675 kg/m3 has $SF = 1000 / 675 = 1.48$ m3/t and a capacity of $M = 12{,}000 \times 675 / 1000 = 8{,}100$ t. A 60,000 m3 five-hold handysize loaded with pellets across all holds carries roughly 40,500 t, well under the deadweight of a vessel that size: the ship cubes out, filling with volume before it loads to its marks.

The relationship breaks where the cargo is not at uniform as-stowed density. A freshly poured cargo near 600 kg/m3 reads $SF = 1.67$ m3/t and loads less mass; the same cargo settled to 750 kg/m3 over the voyage reads 1.33 m3/t, but the mass is fixed at loading, so the settling shows up as a drop in the cargo surface, not extra capacity. Broken stowage at the hold ends and around structure removes a few percent of usable volume that the simple form ignores, and moisture uptake invalidates the schedule range entirely, since swollen pellets occupy more volume and, if they disintegrate, can pack denser. The density and stowage-factor ranges are the schedule values in the IMSBC Code as amended by Resolution MSC.539(107), amendment 07-23; the stowage arithmetic is the general method of IMSBC Section 1 and carries no pellet-specific correction factor. The IMSBC cargo finder carries the schedule data for screening, and the IMSBC group classification calculator places a declared cargo in Group A, B, or C.

Wood pellets sit in a cluster of biomass and wood-product schedules that share the oxygen-depletion mechanism at different intensities. Wood chips and pulp wood chips are Group B and off-gas through the same oxidation, but their lower bulk density (woodchips near 326 kg/m³, stowage factor near 3.07 m³/t) and higher void fraction change the gas accumulation behavior. Sugarcane biomass pellets are Group B with the broader MHB set CB, WF, WT, and OH, adding combustibility and water-evolved-gas hazards to the off-gassing. Torrefied wood, made by roasting biomass to a coal-like product, is Group B with CB, SH, and CR and is readily combustible with stronger self-heating than ordinary pellets. The general “wood products” family carries the oxygen-depletion and carbon-dioxide hazard with a low fire risk.

The instructive cross-comparison is with coal. Both are energy cargoes, both Group B, both self-heat, both demand gas monitoring and a sealed-hold philosophy. The hazards differ in emphasis: coal’s headline atmospheric hazard is flammable methane in the headspace, managed by surface ventilation that clears the headspace while minimizing oxygen to the cargo; the pellet’s headline hazard is oxygen depletion and carbon monoxide that migrate into spaces people enter, managed by sealing the holds and force-ventilating the access spaces. A crew that carries the coal mental model onto a pellet ship, ventilate the holds, watch for methane, will get the pellet policy backward on both counts. The shared lesson is that both cargoes kill through the hold atmosphere, not the cargo’s bulk behavior, which is what puts them both in Group B rather than in the liquefaction-driven Group A.

To screen any of these against its current Code entry and parameters, the IMSBC cargo finder carries the schedule data, and the IMSBC group classification calculator places a declared cargo in Group A, B, or C.

Limitations

This article is a carriage and regulatory reference, not a substitute for the IMSBC Code text, the cargo declaration, or a competent person’s risk assessment of the specific consignment and ship. The schedule parameters quoted are the published ranges; the values for a given cargo come from the shipper’s declaration and the certificate accompanying it, and where the declaration conflicts with the Code the master resolves it before loading, not at sea.

The fatality record cited here is the documented public record from peer-reviewed analysis and a national regulator; it is not exhaustive, and the absence of an incident from this account does not mean a space or a practice is safe. The off-gassing rate depends on species, age, temperature, moisture, and handling in ways no single number captures, so the gas concentrations quoted are the measured ranges from specific voyages, not predictions for any particular cargo. A cargo can off-gas more or less than these figures; the monitoring regime exists precisely because the rate is not knowable in advance from the schedule alone.

The stowage formula gives a planning estimate that ignores broken stowage and assumes uniform as-stowed density and a dry, undegraded cargo. It does not account for moisture uptake, partial settling, or non-uniform loading, and it is not a stability calculation; intact and damage stability, load line and freeboard, and the structural limits of the bulk carrier are separate determinations that the formula does not address. The calculators referenced provide order-of-magnitude screening for self-heating, classification, and parameters; they do not authorize entry, set a ventilation policy, or replace the permit-to-work and enclosed-space procedures that the carriage of this cargo demands.

Above all, no calculation or schedule lookup makes an enclosed space safe. The recurring cause of death on this cargo is entry into a space that was not recognized as communicating with the hold, tested for the wrong gas or not at all, and ventilated by an open door rather than a fan. The controlling safeguards are physical and procedural: identify the communicating spaces before loading, test both oxygen and carbon monoxide before every entry, force-ventilate the access spaces, and never enter to rescue without breathing apparatus and a lifeline.