Urea: IMSBC Schedule and Bulk Cargo Carriage

Urea (CO(NH2)2) is the world’s most traded solid nitrogen fertilizer and one of the largest single commodity flows in the dry-bulk shipping market, with global seaborne volumes of 50 to 55 million tonnes per year. The IMSBC Code classifies it Group C: it doesn’t liquefy and carries no IMDG chemical hazard. The dominant practical challenges in carriage are urea’s exceptional hygroscopicity, which causes hard caking when moisture reaches the cargo, and the slow hydrolysis that releases ammonia gas in sealed or poorly ventilated holds, creating an enclosed-space hazard for any crew or terminal worker who enters without atmospheric testing.

Urea also has an industrial application beyond agriculture: it is the direct feedstock for AdBlue (known as DEF in North America, diesel exhaust fluid), the aqueous urea solution that selective catalytic reduction (SCR) systems consume to reduce NOx emissions from marine and land-based diesel engines. This means a ship carrying bulk urea in its holds may simultaneously be carrying the raw material for the emissions-control reagent required by the NOx Tier III limits that apply to her own class under MARPOL Annex VI Regulation 13. The IMSBC Code governs the carriage of solid bulk urea; liquid urea solutions fall under the IMDG Code.

The chemistry and physical forms of urea

Urea is a simple organic compound, the diamide of carbonic acid, with the formula CO(NH2)2. It is produced synthetically at industrial scale by the Bosch-Meiser process: ammonia (NH3) and carbon dioxide (CO2) react at approximately 150 to 200 degrees Celsius and 150 bar to form ammonium carbamate, which then dehydrates to urea and water. The nitrogen content of pure urea is 46.7% by mass, which is the highest nitrogen concentration of any solid fertilizer and the reason urea dominates the nitrogen fertilizer trade: farmers and distributors get more nitrogen per tonne shipped than from any alternative.

Solid commercial urea reaches the shipping market in two principal forms, produced from molten urea by different manufacturing routes:

Prilled urea. Molten urea at approximately 132 degrees Celsius is sprayed as droplets from the top of a prilling tower 50 to 80 metres tall. The droplets solidify during the fall as small, near-spherical pellets. Typical prilled-urea diameter is 1 to 3 mm. Prills tend to have a slightly lower density and a rougher surface than granules, making them somewhat more prone to abrasion and dust generation during conveying and loading. Prilled urea was the dominant commercial form for decades and remains widely produced in Russia, the Middle East, and North Africa.

Granular urea. Molten urea is granulated in a drum or fluid-bed granulator, producing a harder, denser granule of 2 to 4 mm diameter. The granulation process also allows the addition of formaldehyde-based anti-caking agents at concentrations of 0.1 to 0.5% by mass, which harden the granule surface and reduce hygroscopic uptake. Most new urea production capacity built since the early 2000s uses granulation rather than prilling. Granular urea has better handling characteristics: lower dust, less caking propensity, and better bulk flow in conveyor systems.

Both forms are white to slightly off-white in colour, odourless when dry and cool, and non-toxic in the conventional sense: the UN has no IMDG class designation for solid urea. The distinction between prilled and granular matters operationally because the two forms have different caking risks and different dust-generation profiles during loading and discharge.

IMSBC Code schedule: the key parameters

The IMSBC Code contains a named individual schedule for UREA in Section 3. The schedule is the authoritative reference for operators, masters, and terminals. The key parameters are set out below, drawn from the current IMSBC Code as adopted at MSC 105 (MSC.500(105)) and updated by MSC.539(107).

The IMSBC Code group assignment is Group C, established because urea does not liquefy under normal voyage conditions (no flow moisture point, no transportable moisture limit determination required) and carries no IMDG chemical hazard designation. The Code’s special requirements for urea call out enclosed-space entry precautions explicitly, recognizing that ammonia evolution in sealed holds can reach hazardous concentrations even though the cargo itself is not classified as a Group B (chemical hazard) material.

The Group C classification should not be read as “no hazards.” IMSBC Group C cargoes include a wide range of materials, from inert limestone to reactive granular fertilizers. The group designation means the Code doesn’t require TML testing or chemical hazard labeling, not that the cargo presents no operational risk.

Urea’s hygroscopic behavior: the dominant handling challenge

Urea is among the most hygroscopic of all commonly shipped dry-bulk fertilizers. Its critical relative humidity (CRH) at 25 degrees Celsius is approximately 72 to 75 percent: above this threshold, urea actively absorbs water vapor from the surrounding air, granule surfaces dissolve into a concentrated solution, and when the solution dries or cools the dissolved material recrystallizes as inter-granule bonds, forming a hard cake. The process is cumulative and accelerates with each moisture-dry cycle.

The CRH of 72 to 75% is lower than that of many competing fertilizers. For reference, potassium chloride (muriate of potash, MOP) has a CRH of about 85%, and diammonium phosphate (DAP) has a CRH of about 73 to 76%. In practical terms, urea and DAP have similar hygroscopic thresholds, but urea is generally considered the more problematic cargo because its granule structure is softer and because the caking mechanism in urea is faster at equivalent moisture exposure levels.

The humidity in a ship’s hold during a loaded voyage is not static. Loading at a humid tropical port, with hatch covers open for the duration of cargo operations, can bring hold air to 85 to 90% relative humidity before the last cover closes. The surface layer of cargo, which has the highest contact surface area per unit mass, is the most exposed and the first to cake. On a 30-day voyage in the South Atlantic in austral summer, with ambient humidity averaging 80% and daily temperature swings creating condensation on cooled steel surfaces, surface caking in the upper 200 to 400 mm of an imperfectly sealed hold is a well-documented commercial problem.

Caking severity and discharge consequences

Mild caking produces a surface crust of 20 to 50 mm depth that a grab crane can break through without intervention. Severe caking, after a high-humidity voyage with a leaking hatch cover or condensation drip, can produce a solid monolith across the entire upper surface of a hold, 300 to 600 mm deep, that a grab crane cannot penetrate at all. Breaking a severe caked crust requires manual work: personnel in the hold with pneumatic or hydraulic breakers to fracture the crust into pieces small enough for grab discharge. For a Panamax cargo of 70,000 tonnes across five holds, a bad caking event can add 24 to 48 hours to discharge time and generate a cargo damage claim.

Charter parties for urea bulk cargoes frequently specify the threshold at which caking is a shipper’s risk (cargo loaded with excessive surface moisture) versus a vessel’s risk (hatch covers leaked or condensation management failed). The evidence chain matters: moisture samples at loading, voyage weather logs, hatch cover test certificates, and hold inspection photographs at discharge are the documents that determine liability.

Urea anti-caking agents

Most granular urea exported from major production facilities contains an anti-caking agent, typically formaldehyde-based, added at the granulation stage. Some facilities also apply a surface coating of mineral oil or wax to the finished granule. These treatments raise the effective CRH slightly and slow the surface dissolution rate but do not eliminate hygroscopic caking: they delay it. A granular urea cargo with 0.3% formaldehyde anti-caking agent still cakes in a hold with sustained humidity above 80%. The holds must be dry and sealed regardless of whether the shipper’s declaration indicates the presence of an anti-caking agent.

Prilled urea generally does not contain formaldehyde anti-caking agents (the prilling process doesn’t easily accommodate liquid additives). Some producers apply urea-formaldehyde coating to finished prills, but the treatment is less uniform than granule-stage addition. Prilled urea should be treated as the more hygroscopic of the two forms unless the shipper’s declaration specifically states the type and concentration of anti-caking treatment.

Ammonia release and the enclosed-space hazard

Urea hydrolyzes in the presence of moisture and heat to produce carbon dioxide and ammonia gas: CO(NH2)2 + H2O yields CO2 + 2NH3. The reaction is slow under normal, dry conditions, but accelerates with moisture and temperature. In a sealed hold containing warm, moist urea, ammonia can accumulate to concentrations above the 25 ppm short-term exposure limit within 12 to 48 hours of loading. Enclosed-space entry precautions are mandatory for every hold access on a loaded urea voyage.

The thermodynamics here are straightforward. Urea hydrolysis is exothermic and has an activation energy that makes the reaction highly temperature-sensitive: roughly a doubling of rate for every 10-degree Celsius rise. Urea loaded from a conveyor in direct sunlight at a Middle Eastern port in July, with cargo surface temperatures of 35 to 45 degrees Celsius, generates ammonia at a measurably faster rate than the same cargo loaded in winter. The first 24 to 72 hours of the voyage, while the cargo mass equilibrates to the hold’s thermal environment, is typically the period of highest gas generation.

Ammonia (NH3) has a molecular weight of 17.03, substantially lighter than air at approximately 29. Unlike hydrogen sulfide (H2S) or carbon dioxide (CO2), which pool at the bilge where they displace the denser air, ammonia accumulates near the top of the hold and in poorly ventilated upper corners. Atmosphere testing before hold entry must include the upper hold area, not only the bilge level.

Ammonia threshold values

The key occupational exposure values for ammonia that apply to hold entry decisions are:

• ACGIH TLV-TWA: 25 ppm (8-hour time-weighted average)
• ACGIH TLV-STEL: 35 ppm (15-minute short-term)
• OSHA PEL: 50 ppm (8-hour)
• NIOSH IDLH (immediately dangerous to life or health): 300 ppm

The IMSBC Code’s enclosed-space entry precautions reference the general enclosed-space entry framework under IMO MSC-MEPC.2/Circ.12/Rev.2. In practice, terminals and operators commonly use 25 ppm as the action threshold for hold entry decisions. A hold reading 60 ppm ammonia after 24 hours of voyage isn’t unusual for a warm cargo; at that level, entry without respiratory protection is prohibited and ventilation must precede any inspection.

The sense of smell can detect ammonia at concentrations as low as 5 ppm for some individuals, but olfactory fatigue sets in rapidly at 50 ppm and smell becomes an unreliable warning indicator at higher concentrations. Calibrated detector tubes or electronic gas monitors with electrochemical ammonia sensors are required.

Conditions that accelerate ammonia release

Three conditions push ammonia evolution above the baseline rate that the IMSBC Code schedule is designed around:

Alkaline contamination. Urea decomposes faster in an alkaline environment. If a hold was previously lime-washed (a common cleaning treatment for sugar cargoes), the calcium hydroxide residue on hold surfaces reacts with moisture and urea to drive ammonia generation upward. The pH of a lime wash solution exceeds 12, and even a thin film of residue at frame or bracket welds can create localized high-pH zones that accelerate decomposition in the adjacent cargo layer. Holds must not be lime-washed before urea loading, and if a previous lime-wash treatment is suspected, the hold surfaces must be acid-neutralized and rinsed before the shipper’s inspector accepts the hold.

Moisture ingress. Wet urea produces ammonia faster than dry urea. A slow seep at a hatch coaming, condensation drip from a cold hatch cover panel onto warm cargo, or bilge water wicking up through the cargo column all create localized wet zones where hydrolysis is accelerating. The caking and the ammonia evolution are two symptoms of the same root cause: moisture contact.

High cargo temperature. Urea shipped from gas-based production facilities in the Middle East (Qatar, Saudi Arabia, Oman, UAE) may arrive at the loading terminal from covered storage at 30 to 40 degrees Celsius during summer months. Loading at this temperature, into a hold that hasn’t cooled to ambient, creates the conditions for rapid early-voyage ammonia generation. Cargo temperature records at loading are worth taking: a cargo certificate showing loading temperature of 42 degrees Celsius is material evidence in any subsequent cargo claim involving ammonia-related damage.

Pre-entry atmosphere testing procedure

Before any crew member or shore worker enters a cargo hold on a loaded urea voyage, the following sequence applies:

1. Test hold atmosphere from the hatch opening before any person descends. Use a calibrated, approved multi-gas meter with an ammonia sensor, an oxygen sensor, and optionally a CO/H2S sensor.
2. Measure at multiple elevations: upper hold (near hatch opening), mid-hold, and lower hold at the bilge.
3. Required conditions for safe entry: oxygen at least 20.9% by volume; ammonia below 25 ppm; no other toxic gas above its respective threshold.
4. If any reading fails the threshold, ventilate the hold mechanically (fan directed in through one opening, ammonia-laden air exhausted through a second), then retest.
5. Continuous monitoring during any hold entry while cargo or cargo residues are present.

Natural ventilation is insufficient where ammonia has accumulated in a sealed hold over several days. A portable ventilation fan with a capacity of at least 5 to 10 air changes per hour in the hold volume should be used before entries where readings exceed the threshold.

Dust generation and health effects

Both prilled and granular urea produce dust during loading and discharge, with prilled urea generating more due to its softer granule and lower crush strength. Urea dust is a mild irritant to eyes and mucous membranes at concentrations encountered in bulk handling. It isn’t toxic at normal occupational exposure levels, and the OSHA permissible exposure limit (PEL) for urea dust is 15 mg/m3 as a nuisance dust.

The practical concern at major export terminals is the combination of dust and humidity: urea dust on damp steel deck surfaces forms a sticky, corrosive paste that is more aggressive to steel than dry dust or dry bulk cargo. Dust that settles on hatch covers during loading and then gets wetted by rain runs into hatch cover drainage channels and creates a concentrated urea solution at the seal. Modern terminals suppress dust by applying a water mist ahead of the falling cargo stream at the shiploader discharge point. The mist wets the dust and causes it to fall with the bulk cargo rather than becoming airborne. This control is effective when properly maintained, but on older conveyor systems, dust generation can be higher.

Workers in loading and discharge operations should wear appropriate eye protection and respiratory protection if dust exposure is expected to be significant. International occupational health standards for urea dust exposures are well below levels that cause acute harm, but repeated exposure to high concentrations of ammoniacal dust during discharge of a caked or broken cargo is worth managing with fitted dust masks.

Corrosivity to steel and hold coatings

Urea in the presence of moisture is mildly corrosive to uncoated steel. The mechanism involves both the mildly alkaline nature of urea solution (pH approximately 7.0 to 7.5 for a saturated solution) and the role of dissolved ammonia and ammonium ions in electrochemical corrosion at steel surfaces. The practical consequence isn’t rapid perforation of plating but accelerated corrosion of uncoated or damaged areas at hold floors, frame toes, bracket welds, and bilge well frames.

The IMSBC Code schedule records this characteristic under the heading “Corrosion / Reaction with water.” No IMDG Class 8 corrosive label applies, and urea isn’t classified as a marine pollutant. But charter parties for urea cargoes routinely require a hold coating survey before loading, post-discharge hold photographs, and a wash certificate after cleaning. For operators carrying urea regularly, maintaining hold paint coatings in sound condition and washing holds promptly after discharge is the operational response.

Wet urea left as a residue after discharge is more corrosive than the bulk cargo during transit. The concentrated urea solution in residue puddles, particularly in warm holds, has a higher ammonia partial pressure and a more aggressive corrosive environment than the bulk mass. Prompt post-discharge washing is worth enforcing as a charter party requirement.

The interaction of urea with certain metals is worth noting for operators carrying urea on vessels with copper alloy fittings or pipe connections in the cargo hold spaces: urea solution can attack copper and brass (stress corrosion cracking of brass in ammonia-containing environments is well documented). Standard mild-steel hold construction isn’t affected in the same way, but copper alloy bilge well fittings in urea-trading vessels should be inspected for stress corrosion.

The global urea trade: production and flows

Urea seaborne trade is the largest single nitrogen fertilizer commodity flow in the dry-bulk market. Global production in 2023 was approximately 185 to 190 million tonnes (including urea for domestic consumption and non-agricultural uses), with seaborne trade at roughly 50 to 55 million tonnes per year.

Major export regions

Middle East. Qatar (QAFCO at Mesaieed), Saudi Arabia (SABIC at Jubail and Al-Jubail, Ibn Al-Baytar, and the Ma’aden Wa’ad Al Shamal complex), UAE (Fertil at Ruwais), Oman (OmanOil and Oman India Fertilizer Company OMIFCO at Sur), and Iran (Pardis and Khorasan Petrochemical complexes) are collectively the largest urea export region. Gas-based urea production in the Gulf benefits from historically cheap gas feedstock prices. Gulf urea exports move to India, Pakistan, Southeast Asia, East Africa, and Latin America, often on Panamax or Supramax bulk carriers.

Russia. Russia’s urea production is concentrated in European Russia, with Uralchem, EuroChem, and Acron among the major producers. Export terminals at Sillamae (Estonia, historically), Togliatti (via river and Black Sea transshipment), Tuapse, and Novorossiysk on the Black Sea serve European, Middle Eastern, and Asian markets. Russian urea exports became subject to logistical and sanctions complications from 2022, driving some trade-flow rerouting from Black Sea to Baltic or alternative discharge routes.

China. China is simultaneously the world’s largest urea producer and a major consumer. Export volume is highly variable, controlled by domestic price stability policy and export duty adjustments. China has been a net exporter in years of surplus domestic production and has curtailed exports sharply in years of domestic shortage. Spot export windows from Chinese producers (Shandong, Inner Mongolia, Qinghai) when they open move significant volumes into Asian markets on short-haul voyages.

Egypt. Egypt’s Fertil and MOPCO complexes at Damietta on the Nile Delta produce gas-based urea for export into Mediterranean, African, and Latin American markets. Egyptian urea moves frequently to Brazil, West Africa, and Europe.

Eastern Europe and Central Asia. Ukraine (historically a large producer, production heavily disrupted from 2022), Belarus, and Kazakhstan all have urea production feeding export flows.

Import destinations

India. India is the world’s largest single urea importer. Domestic production covers only a portion of national demand, and the government’s fertilizer subsidy program drives procurement through state-owned entities (RCF, NFL, and MMTC). Annual urea imports fluctuate between 7 and 12 million tonnes depending on domestic production rates and kharif/rabi seasonal demand cycles. The major import terminals are Kandla (Deendayal Port) in Gujarat, Paradip in Odisha, Vizag, and Ennore. India’s procurement is tender-based, concentrated in windows before each planting season, creating sharp freight demand spikes.

Brazil. Brazil imports approximately 5 to 7 million tonnes of urea per year for the Cerrado agricultural zone, concentrated at the ports of Santos, Paranagua, and Itaqui. Brazilian urea demand follows the soybean and corn planting seasons, with the most intense import activity from August to November.

United States. The US is a substantial urea importer despite significant domestic production from the Gulf Coast, consuming imported urea from Trinidad, Egypt, and the Middle East for the corn belt. New Orleans, Tampa, and Portland (Oregon) are the main import terminals.

Pakistan, Bangladesh, and Southeast Asia. Pakistan and Bangladesh both rely heavily on imported urea; Pakistan’s main import terminal is Karachi. Vietnam, Thailand, and the Philippines import urea from the Middle East and China.

Sub-Saharan Africa. African urea import demand is growing as agricultural intensification spreads, with Mombasa (Kenya), Dar es Salaam (Tanzania), and Lagos (Nigeria) as key distribution hubs.

Vessel types and fleet

Urea’s stowage factor of 1.25 to 1.43 m3/t means it occupies more hold volume per tonne than DAP (1.00 to 1.11 m3/t). A Panamax bulk carrier with a grain capacity of 90,000 m3 can load approximately 63,000 to 72,000 tonnes of urea before reaching volumetric capacity, while the same vessel loading DAP would typically be draft-limited rather than volume-limited at 85,000 to 90,000 tonnes.

Handysize vessels (25,000 to 40,000 DWT) serve smaller ports in Africa, the Caribbean, and Southeast Asian archipelagos. Supramax and Ultramax vessels (50,000 to 65,000 DWT) are the most common vessel type on the Gulf-to-India and Egypt-to-Brazil routes. Panamax vessels serve the high-volume Panamax Canal-transiting routes to the US Gulf and East Coast, and the Indian Ocean trades from the Gulf and China. Capesize vessels are not commonly used for urea: the cargo is light enough per cubic metre that the draft economics of a Capesize do not favor it for most urea trade routes.

Dedicated fertilizer carriers, fitted with coated holds, enclosed conveyor systems, and closed hatch designs, handle a portion of the trade. Most urea moves on conventional open-hatch bulk carriers with appropriate hold preparation and weather-tight hatch covers rather than specialized vessels.

Hold preparation for urea loading

Hold preparation for a urea cargo is the most operationally critical phase of the voyage. The IMSBC Code schedule, combined with practical experience of urea caking claims, points to the following preparation standard. The general framework is described in the cargo hold preparation standards article; the urea-specific requirements are stricter in several respects.

Step 1: Complete residue removal. All previous cargo must be swept and washed out. The standard for urea is particularly stringent because:

• Any previous fertilizer residue (potash, DAP, MAP, ammonium nitrate) creates compatibility questions and contamination risk.
• Organic residue from grain cargoes can promote bacterial urease activity, which catalyzes urea hydrolysis and accelerates ammonia generation.
• Coal, mineral concentrate, and ore residues contaminate a white cargo and may create chemical incompatibilities.

The sweep and wash must leave no visible residue. A hold inspection by the ship’s first officer and the shipper’s surveyor before loading confirms the standard.

Step 2: No lime wash. Urea loading holds must not be lime-washed or treated with any alkaline preparation. Calcium hydroxide (lime, pH above 12) on hold surfaces reacts with moisture and urea to generate ammonia at an accelerated rate. If the previous cargo or cleaning treatment involved lime, the hold surfaces must be acid-washed (dilute citric acid or mild phosphoric acid solution), rinsed, and dried before a urea cargo is loaded. pH strips or paper applied to wet hold surfaces can quickly confirm whether alkaline residue is present.

Step 3: Hold coating inspection and repair. Inspect all hold surfaces. Touch up any bare steel with approved epoxy hold paint. Allow full cure before loading. The relevance is both corrosion protection (moist urea accelerates rust) and contamination protection (fresh uncured paint can release solvents that contaminate the cargo surface).

Step 4: Bilge well preparation. Bilge wells must be dry, clean, and covered. Urea granules (1 to 4 mm diameter) can pass through standard bilge well perforations and block bilge pump strainers. Cover the bilge wells with burlap or a fine-mesh filter fabric and secure the edges before loading begins. Test bilge pumps immediately before loading: a pump that fails during a urea voyage with condensate accumulating under the cargo creates a moisture source that triggers caking in the bilge zone and may expose the pump to urea solution.

Step 5: Hatch cover testing. Pressure-test every hatch cover before loading. The standard test for hatch covers under IMO guidelines is a hose test or, where available, an ultrasonic seal test. A cover that passed testing 12 months ago may have developed compressed-seal wear or minor panel distortion since. For a urea voyage where even a small continuous moisture ingress over 20 days produces significant caking, the hatch cover condition is the single most important physical factor in cargo outcome. Document the test result in the first officer’s hold inspection report.

Step 6: Baseline atmosphere test. Before the first tonne of urea loads, conduct an enclosed-space atmosphere test in the hold. This establishes the baseline oxygen level and confirms no residual toxic gas from the previous cargo. For holds that previously carried fishmeal, certain mineral concentrates, or coal, residual gases (ammonia, H2S, CO2) are a possibility; the baseline test catches these.

Loading operations

Urea loads via shore conveyor and shiploader at major export terminals. At large fertilizer export facilities (QAFCO at Mesaieed, Qatar; OMIFCO at Sur, Oman), loading rates of 1,500 to 4,000 tonnes per hour are achievable with modern high-capacity shiploaders. At 3,000 tonnes per hour, a Supramax loading 50,000 tonnes completes bulk loading in approximately 17 hours of effective loading time.

Dust control at loading. The fall of prilled or granular urea from a shiploader spout into a hold generates a visible dust plume. Major export terminals operate water mist systems at the spout discharge point; older installations may have only deflector plates or no dust suppression at all. Where dust suppression is inadequate, urea dust settles on hatch cover plates, coaming top bars, and deck surfaces. Rain or overnight dew on urea dust creates a corrosive paste. Deck washing after loading is standard practice.

Trimming. The IMSBC Code schedule specifies that urea must be trimmed. The angle of repose of granular urea is approximately 22 to 28 degrees, and prilled urea has a similar flow characteristic. An untrimmed cargo loaded only through the hatch opening creates a cone of cargo that imposes concentrated local loads on the tank top under the hatch. Modern shiploaders with traversing or rotating spouts distribute cargo across the hold; at terminals without this capability, a bulldozer is lowered into the hold to trim the peak, requiring enclosed-space entry procedures.

Cargo moisture certificate. The shipper must provide a cargo declaration under the IMSBC Code, which includes moisture content. The acceptable moisture content at loading for urea is typically below 0.5% for prilled and below 0.3% for granular. A cargo declaration showing moisture at or above these levels is a flag for the master to consider before signing the bill of lading.

Cargo temperature recording. The first officer should measure and record cargo surface temperature at the commencement of loading, at the end of loading, and at first hatch cover closure. Cargo loaded at elevated temperatures (above 35 degrees Celsius) should be noted in the cargo report.

Voyage monitoring

The urea voyage requires three monitoring disciplines: atmosphere management, hatch cover condition, and bilge surveillance.

Hold atmosphere monitoring. Any hold access after departure requires an atmosphere test. There is no elapsed-voyage-time rule after which testing can be skipped. Ammonia concentrations in a sealed hold can increase throughout the voyage if moisture is present. After rough weather, a hold inspection may be warranted, but the inspection cannot be made without prior atmosphere clearance. The ship should carry at least one calibrated multi-gas meter with an ammonia sensor at all times on a loaded urea voyage.

Dewpoint management for ventilation. Urea can be ventilated (to reduce ammonia buildup or to manage condensation) only when the outside air dewpoint is below the cargo hold air dewpoint, so that ventilation does not introduce moisture that would accelerate caking. Dewpoint measurement requires an instrument (a wet-and-dry-bulb hygrometer or an electronic dewpoint meter), not just temperature measurement. Ventilating a urea cargo in a tropical squall or in port at a high-humidity discharge berth draws moist air across the cargo surface and should be avoided.

Hatch cover surveillance. After head seas, heavy rain, or alongside at a port with significant swell, the first mate should check hatch cover seals for any sign of leakage. Wet cargo near the hatch coaming periphery, visible as discolored or darker material during a (properly atmosphere-cleared) hold inspection, is the first indicator of hatch cover ingress. Moisture damage records at loading and discharge are essential to determine liability.

Bilge surveillance. Check bilge levels at least once per watch during loaded voyages. Any bilge accumulation under a urea cargo is an event: it means either hull leakage or condensate drainage through a failed bilge well cover. Bilge water with dissolved urea is mildly corrosive and should be pumped to the bilge holding tank (or bilge separator) as appropriate rather than overboard; urea in bilge effluent to sea is a regulatory issue in port environments.

Discharge operations

Discharge of urea at receiving terminals uses grab cranes at most conventional dry-bulk ports. Pneumatic unloading equipment is used at some specialized facilities but is less common for urea at the volumes typical of Panamax or Supramax cargoes.

Grab discharge. Standard grab cranes at fertilizer discharge terminals operate grabs of 12 to 20 tonnes per cycle. At six to eight cycles per hour per crane, discharge rates of 72 to 160 tonnes per crane-hour are achievable. A Supramax cargo of 50,000 tonnes with two cranes operating discharges in approximately 35 to 55 effective crane-hours, or two to three calendar days including shift breaks and weather delays.

Caked cargo at discharge. The sequence for discharging a caked upper layer is: atmosphere test; mechanical breaking of the crust using a pneumatic or hydraulic breaker lowered into the hold; grab discharge of broken material; repeat for deeper caked zones. The time added by crust breaking depends on crust thickness and hold area. For a full Panamax cargo across five holds, a severe caking event (300 to 600 mm crust over 100% of the hold area) can add one to two days per hold to the discharge schedule. Charter parties should specify clearly how time spent breaking caked cargo is treated for laytime and demurrage purposes.

Ammonia at discharge. Ammonia is released from the cargo as it is broken up and exposed to air during discharge. Ammonia concentrations in the immediate vicinity of an open hatch during active grab discharge of urea are typically 10 to 50 ppm depending on cargo moisture and temperature. Terminal workers near the hatch opening should be made aware of this and should have access to respiratory protection if readings are high. Port authorities in some countries require ammonia monitoring during urea discharge.

Hold cleaning after discharge. Urea residues must be washed out promptly after discharge. Dry sweeping first, then a freshwater wash using hoses, removing all visible residue and fines from bilge wells, frame toes, and bracket faces. Wash water from urea cleaning is high in nitrogen (dissolved urea, ammonium carbonate) and should be disposed of to port reception facilities in jurisdictions where overboard discharge of nitrogen-rich water is prohibited. Allow holds to dry completely before closing hatch covers for a ballast voyage.

Urea as AdBlue feedstock: the SCR connection

Urea serves a second major industrial market that is directly relevant to the maritime industry: it is the feedstock for AdBlue (the ISO 22241 brand name) and DEF (diesel exhaust fluid, the SAE J2071 term), the aqueous solution of urea that SCR (selective catalytic reduction) systems use to reduce NOx emissions from diesel engines.

AdBlue is a precisely specified solution of 32.5% high-purity urea in deionized water. When injected into the exhaust stream ahead of an SCR catalyst, it decomposes to ammonia and CO2, and the ammonia reacts with NOx to produce nitrogen and water. The conversion efficiency of a well-managed marine SCR system is 90 to 98% NOx reduction, sufficient to meet MARPOL Annex VI Regulation 13 Tier III limits (approximately 3.4 g/kWh for NOx from low-speed engines) in NOx Tier III areas such as the North American Emission Control Area and the North Sea/Baltic NECA from 2021.

The IMO Tier III requirements driving SCR adoption mean that vessels built for Tier III compliance consume AdBlue at approximately 3 to 6% of fuel consumption by volume (the exact ratio depends on NOx generation rate, SCR efficiency, and load profile). A large two-stroke engine on a Panamax bulk carrier burning 25 tonnes of fuel per day in a NOx Tier III area consumes approximately 1,000 to 1,500 litres of AdBlue per day. That AdBlue was manufactured from the same urea that the vessel may be carrying as cargo.

This connection matters to the shipper, charterer, and terminal because it means the quality requirements for shipping-grade urea (IMSBC bulk cargo, typically 46% N, technical or agricultural grade) differ from AdBlue-grade urea (46.3% N minimum, biuret content below 0.3%, very low trace metal content, manufactured to ISO 22241-1). Bulk urea cargoes are not interchangeable with AdBlue-grade urea without purification. The onshore production chain from bulk urea to AdBlue requires dissolution, filtration, and quality certification before the 32.5% solution can be loaded into SCR storage tanks.

Urea vs other nitrogen fertilizers: bulk handling comparison

The nitrogen fertilizer dry-bulk trades include several cargo types with overlapping but distinct handling profiles. Understanding where urea sits relative to its main substitutes is practically useful for cargo planning.

Ammonium nitrate stands apart from the Group C fertilizers. Its critical relative humidity is 59 to 62%, making it even more hygroscopic than urea, and its oxidizer classification under the IMDG Code (Class 5.1 for many grades) means it requires incompatible-cargo segregation from combustibles and fuels that urea does not. The ammonium nitrate fertilizer IMSBC schedule covers the ammonium nitrate carriage regime in full; the two cargoes must not be loaded adjacent without appropriate segregation analysis and cannot be treated as interchangeable during hold allocation.

Urea and potash (muriate of potash, KCl) are both IMSBC Group C and are physically compatible for carriage in adjacent holds on the same vessel. The two cargoes must not be mixed within a hold (they react with each other and can form a corrosive solution) but carry no IMDG segregation requirement between holds. Dedicated fertilizer carriers frequently carry urea and potash in different holds on the same voyage.

Regulatory framework

The IMSBC Code is the primary regulatory instrument for bulk urea carriage. The Code became mandatory under SOLAS Chapter VI Regulation 1-1 by amendments entering force on 1 January 2011. The current edition is the 2023 edition, incorporating amendments adopted at MSC 105 under resolution MSC.500(105). Subsequent amendments were adopted at MSC 107 under MSC.539(107). The individual UREA schedule in Section 3 of the Code sets the authoritative requirements.

SOLAS Chapter XII applies to bulk carriers built on or after 1 July 1998, governing structural requirements including hatch cover integrity standards. Enhanced hold inspections and hatch cover requirements under SOLAS XII reinforce the IMSBC Code’s requirements for a hygroscopic cargo like urea: any hatch cover that meets SOLAS XII water-tightness standards is by definition adequate for urea from a seal-integrity standpoint.

Port state control inspections under the Paris MOU and Tokyo MOU verify compliance with IMSBC Code requirements. Inspectors can detain a vessel where: the hold preparation doesn’t meet the schedule’s requirements, enclosed-space entry precautions are absent or undocumented, atmosphere testing equipment is not calibrated and aboard, or the shipper’s cargo declaration and certificate are missing or inconsistent. For urea cargoes, the absence of calibrated ammonia detection equipment is a common PSC finding.

The Paris MOU and Tokyo MOU both publish annual statistics on cargo-related deficiencies; fertilizer bulk cargoes regularly appear in these statistics, with urea and DAP cargoes most commonly cited for enclosed-space management deficiencies and hold preparation shortcomings.

Limitations

This article summarizes the IMSBC Code Group C schedule for urea as published in the 2023 edition of the Code (MSC.500(105)) and updated by MSC.539(107), drawing on documented industry practice as of 2026. It is not a substitute for the full IMSBC Code text, which mariners and cargo operators must consult directly for the authoritative individual schedule. The Code is subject to amendment between main editions through MSC resolutions; verify the current amendment state of the urea schedule against the IMO’s official publications before relying on specific schedule values.

Cargo properties stated in this article (bulk density, stowage factor, moisture content, CRH) are typical industry values and vary between production batches, production facilities, storage conditions at loading ports, and loading-port ambient conditions. Any individual shipment must be assessed against the shipper’s declaration, the cargo certificate, and the samples drawn at loading, not against the nominal values in this article or in the IMSBC Code schedule.

The ammonia-evolution rate and enclosed-space hazard timelines described here reflect documented industry experience and the IMSBC Code’s text. Actual evolution rates depend on cargo temperature, moisture content at loading, void fraction in the cargo mass, and hold atmosphere conditions that vary by voyage, season, and loading region. Atmosphere testing before every hold entry remains mandatory regardless of cargo age, voyage duration, or the reported moisture content of the cargo at loading.

Charter party terms for urea cargoes vary substantially between operators and trade routes. The allocation of liability for caking, corrosion, and hold cleaning described here reflects common market practice, not universal terms. Legal review of specific charter party provisions is required for any dispute resolution.

This article describes solid bulk urea only. Urea ammonium nitrate (UAN) solution and other liquid nitrogen fertilizer products are regulated under the IMDG Code and the IBC Code respectively, not the IMSBC Code, and are not covered here.

See also

• IMSBC Code
• IMSBC Group C Cargoes
• Diammonium Phosphate: IMSBC Schedule and Carriage
• Ammonium Nitrate Fertilizer: IMSBC Schedule and Carriage
• Potash: IMSBC Schedule and Carriage
• Phosphate Rock: IMSBC Schedule and Carriage
• Cargo Hold Preparation Standards
• Marine Cargo Hold Ventilation
• NOx Tier I, II, III
• SCR Retrofit on Two-Stroke Engines
• Bulk Carrier

Related calculators

• IMSBC Fertilizer: Urea
• IMSBC Urea Ammonium Nitrate
• IMSBC Group A/B/C Classification