Bulk Cargo

Bulk cargo is cargo carried in large quantities without individual packaging. Instead of being shipped as boxes, pallets, crates, drums, or bags, the cargo is loaded directly into the ship’s holds or tanks. Bulk carriage is one of the oldest and most important forms of maritime transport because many essential commodities move most efficiently when carried in loose, unpackaged form.

There are two main types of bulk cargo: Dry Bulk Cargoes and Wet (Liquid) Bulk Cargoes. Dry bulk cargoes include commodities such as grain, coal, iron ore, bauxite, phosphate, sulphur, salt, fertilisers, scrap, and similar loose solid cargoes. Wet bulk cargoes include crude oil, petroleum products, chemicals, liquefied gases, and other liquid cargoes carried in tanks.

Dry Cargoes:

  1. Dry Bulk Cargoes
  2. Dry Non-Bulk Cargoes

Dry Bulk Cargoes

The first requirement before any dry bulk cargo is loaded is that the ship’s holds must be dry, clean, structurally suitable, and ready for the intended cargo. The required level of cleanliness depends heavily on the commodity. A ship loading bulk grain (Grain Clean Certificate) must meet a much higher standard than a ship loading coal or iron ore. Grain, fertiliser, sugar, salt, cement, and other sensitive cargoes may be rejected if the holds contain rust scale, insects, odour, oil, previous cargo residues, loose paint, coal dust, or moisture.

Even cargoes that appear less sensitive may have special requirements. Low-sulphur coal can be contaminated by residues from high-sulphur cargoes. Certain ores may react with residues from previous cargoes. Sulphur may damage unprotected steel. Fishmeal and some coal cargoes may heat spontaneously. Therefore, hold preparation is not a routine cleaning exercise; it is part of the ship’s cargo-worthiness and directly affects claims, delays, charter party performance, and seaworthiness.

Before loading, a ship may be required to pass a cargo hold cleanliness survey. If the surveyor rejects the holds, loading will not begin until the deficiency is corrected. This can result in delay, extra cleaning costs, loss of laytime, demurrage disputes, or cancellation risk under the charter party. The crew must therefore remove previous cargo residues before arrival at the loading port. Stevedores may leave sweepings or residues behind after discharge, but final cleaning and preparation often remain the responsibility of the ship.

During loading, the ship must also remain in safe trim and safe stress condition. The master must control the loading sequence so that the ship is not subjected to excessive bending moments, shear forces, or dangerous list. This is important not only for the sea passage but also during loading and discharging, when uneven cargo distribution can impose severe stress on the hull.

For Sensitive Dry Bulk Cargoes, an Independent Cargo Surveyor is often appointed to inspect holds, supervise sampling, verify cargo condition, check draft survey figures, or issue cleanliness certificates. In modern bulk terminals, cargo is loaded by conveyor, chute, shiploader, grab, or spout. Where equipment distributes cargo properly, additional cargo trimming may be limited. Many charter parties provide that cargo is to be loaded, spout-trimmed, and stowed free of expense to the ship.

In less developed ports, manual trimming may still be required. Trimming spreads the cargo across the hold and reduces the risk of dangerous movement at sea. If some dry bulk cargoes are simply dropped into the hold and allowed to remain in a peaked heap, the cargo may settle or shift during the voyage. This is particularly important for cargoes with a low Angle of Repose, because they are more likely to move across the surface when the ship rolls.

Dry bulk cargo handling must also consider natural separation and artificial separation. A ship may carry two grades of coal, ore, or fertiliser in separate holds, which is sometimes called natural separation. In more complicated cases, different grades may be carried in one hold, separated by tarpaulins, dunnage, burlap, hessian, heavy-duty polythene sheeting, or temporary wooden bulkheads. Such arrangements require great care because grab discharge can easily cause admixture and lead to quality disputes.

Where loading or discharging takes place at more than one berth or port, the master must ensure that the ship remains stable, seaworthy, and structurally safe between each stage. A typical charter party may require the ship to be left in a condition satisfactory to the master when proceeding between berths or discharge ports. Even with one homogeneous cargo at one berth, cargo must be loaded or removed in a planned sequence. Poor distribution of weight can be catastrophic and has caused ships to suffer serious structural damage.

Classification societies issue an approved stability booklet for each ship. This booklet gives guidance on loading conditions, permissible stresses, stability limits, loading sequences, and cargoes with different stowage factors. The master must use this information together with loading computer data, draft readings, terminal loading plans, and charterer instructions. In some jurisdictions, loading inspectors may require the stability booklet or loading plan before permitting departure.

Dry Bulk Cargo Risks

Bulk Grain: Before grain loading begins, the ship’s holds must be Grain Clean. Bulk grain is a free-flowing cargo, and this creates a risk of surface movement when the ship rolls. If grain shifts to one side, the ship may develop a list from which recovery is difficult or impossible. In severe cases, capsize can follow. The master must present stability calculations based on the ship’s approved Grain Book. The Grain Book states how the ship may carry grain, how many holds may be slack, and what stability measures are required. Loading calculations must be accurate because overloading, slack holds, or poor distribution can make the voyage unsafe.

Scrap: Scrap metal cargoes may contain heavy, sharp, irregular, or compacted pieces capable of damaging tank tops, frames, hold sides, bilge wells, hatch coamings, and cargo hold coatings. Oily steel turnings may heat and ignite under certain conditions, and even drained engines, motor blocks, or machinery parts may continue to leak oil during the voyage. Oil contamination is difficult to remove from holds and may affect the next cargo. Scrap should be loaded with attention to tank top strength, impact damage, fire risk, and post-discharge cleaning.

Coal: Some coal cargoes emit methane and may be subject to spontaneous heating and combustion. Shipowners and Ship Managers must identify the exact coal type and its hazards before loading. Nearly all coal cargoes can generate flammable gas, and ventilation must be managed carefully. Natural surface ventilation may be sufficient in many cases, but excessive ventilation can sometimes promote heating, depending on the cargo characteristics. Temperature monitoring, gas detection, and compliance with the IMSBC Code are essential.

Fishmeal: Fishmeal can be subject to spontaneous combustion if improperly cured, packed, or carried. Moisture, oil content, antioxidant treatment, and temperature are important. The total quantity is usually established by shore weighing, but the master should also check the quantity by draft survey where appropriate. Fishmeal should not be loaded unless certificates and carriage conditions are satisfactory.

Sulphur: Sulphur can corrode and damage hold steel if the hold is not properly protected. A lime-wash coating is often applied before loading to protect steel surfaces. Sulphur dust may also create health and contamination issues. After discharge, holds should be cleaned thoroughly to remove residues and prevent corrosion.

Dry Non-Bulk Cargoes

Dry non-bulk cargoes are handled very differently from dry bulk cargoes. They cannot be poured into holds. They must be lifted, placed, stowed, separated, lashed, secured, ventilated, and protected according to cargo type. This makes loading slower, more labour-intensive, and more complex. Dry non-bulk cargoes include bagged cargo, steel products, timber, logs, machinery, project cargo, unit loads, vehicles, and other break-bulk cargoes.

Traditionally, dry non-bulk cargo was counted by shore tally. However, tally accuracy can vary widely between ports. In some ports, tallying may be unreliable or poorly controlled. The master should therefore arrange spot checks by the crew or appoint independent tally clerks where the cargo quantity or condition may become disputed. Accurate tallying is important because the Bill of Lading, cargo manifest, freight calculation, and cargo claims may all depend on recorded quantity and condition.

Dry Cargo Risks

Bagged Cargoes: Holds must be clean, dry, odour-free, and unobstructed. Bagged cargoes are vulnerable to tearing, chafing, hook damage, sweat damage, contamination, crushing, and staining. Cargo battens may be required on hold frames to create ventilation channels and reduce the risk of condensation damage. Dunnage and separation material should be used where necessary.

Bagged Fishmeal: Bagged fishmeal may be subject to spontaneous combustion. The ship must ensure that certificates, moisture levels, antioxidant treatment, and ventilation requirements are suitable. Damaged bags should be recorded and separated where necessary.

Bagged Rice: Rice is sensitive to moisture, odour, insects, contamination, and sweat damage. Holds should be fully cargo-battened where required, with proper ventilation channels and suitable dunnage. Any dampness or residue in the hold may lead to cargo claims.

Bagged Ammonium Nitrate: Ammonium nitrate is potentially dangerous, especially where nitrogen content is high or where contamination, heat, confinement, or incompatible cargoes are present. Cargo documentation, IMDG or IMSBC requirements, packaging, segregation, and emergency procedures must be checked carefully before shipment.

Steel: Steel cargo requires close attention to weight, condition, dunnage, securing, and documentation. The weight of each piece, especially steel coils, must be checked against the permissible tank top load. Unprotected steel rusts quickly, and rust frequently causes cargo claims. Pre-shipment surveys are often required to record rust, wetness, bending, edge damage, deformation, broken strapping, or coating defects before loading.

Logs: Deck cargo of logs requires suitable stanchions, lashings, chains, uprights, and securing arrangements. Logs loaded from water, sometimes called floaters, can introduce significant moisture into the holds and may make drying difficult after discharge. Logs can damage the ship if loaded or discharged carelessly. Stability, deck strength, lashings, and visibility from the bridge must be considered.

Unit Loads: Unit loads must be carefully loaded, blocked, braced, and lashed to prevent movement at sea. Cargo movement can damage the cargo, the ship, or other cargoes. The securing arrangement must consider ship motion, acceleration forces, voyage route, weather, cargo weight, and centre of gravity.

Creosoted Timber: Creosoted timber, such as telegraph poles and railway sleepers, may leave a strong and persistent odour in the holds. This odour can taint future cargoes, especially foodstuffs or sensitive packaged goods. Cleaning after discharge should be planned before accepting the cargo.

Stowage Factor (SF)

Stowage Factor (SF) expresses how much space a cargo occupies in the ship’s hold. It is a measure of cargo density in practical shipping terms. Because a ship has both a deadweight limit and a cubic capacity limit, the stowage factor determines whether the ship will be weight-limited or space-limited.

A heavy cargo with a low stowage factor may bring the ship down to her load line while hold space remains unused. A light cargo with a high stowage factor may fill the holds before the ship reaches her full deadweight cargo capacity. Shipbrokers, charterers, and masters must understand this relationship when planning employment, calculating freight, and preparing loading plans.

The Imperial system is still frequently encountered in stowage factor calculations. Metric stowage factors are commonly expressed as cubic metres per metric tonne. Imperial stowage factors are commonly expressed as cubic feet per long ton. A practical reference is that 1 cubic metre per tonne is broadly comparable with about 35.3 cubic feet per metric tonne, while older commercial examples often use rounded equivalents.

Stowage Factor (SF) Example:

Assume a ship has 56,000 tonnes DWCC and a grain cubic capacity of 71,000 m3, or about 2,500,000 ft3. The ship can load different quantities depending on cargo density:

  • If loading bulk phosphate with a Stowage Factor (SF) of about 0.90 m3/t, or approximately 32 ft3/ton, the ship theoretically has hold volume for around 78,888 tonnes. However, she can load only 56,000 tonnes because her deadweight cargo capacity will be reached before the holds are full. In this case, the ship is weight-limited.
  • If loading bulk grain with a Stowage Factor (SF) of about 1.47 m3/t, or approximately 52 ft3/ton, the ship can load about 48,299 tonnes before the holds are full. The ship still has unused deadweight capacity, but there is no remaining cubic space. In this case, the ship is space-limited.
The Stowage Factor (SF) is crucial for shipbrokers and charterers planning a ship’s employment. A lighter cargo may require a higher freight rate per tonne to produce the same gross income because the ship cannot use all her deadweight capacity. The master must also be informed of the cargo’s stowage factor, especially where the cargo consists of two or more commodities with different densities. Proper planning is necessary to keep the ship in safe trim and within stress limits.

Dangerous Cargoes and Polluting Cargoes

The International Maritime Organization regulates the carriage of dangerous and polluting cargoes through safety and environmental conventions and codes. The international carriage of Dangerous Cargoes and Polluting Cargoes is governed mainly by the IMO International Maritime Dangerous Goods Code (IMDG Code) for packaged dangerous goods and by other IMO instruments for bulk cargoes, chemicals, gases, and polluting substances. The IMDG Code establishes minimum international standards, but local laws may impose stricter requirements in certain ports or countries.

The shipowner and Ship Manager must be aware of any special restrictions applying during loading, discharge, transit, storage, or sea passage. Many countries also impose obligations on the shipper, including correct classification, packaging, marking, labelling, and documentation. A shipper of dangerous goods must declare the exact nature of the goods and the hazard they present. Incorrect declaration is one of the most serious risks in maritime transport because it may lead to fire, explosion, toxic exposure, pollution, or improper stowage.

The IMO International Maritime Dangerous Goods Code (IMDG Code) aka The Blue Book has been revised and updated regularly to reflect new cargoes, new hazards, incidents, technical developments, and regulatory experience. National modifications and port rules may also be issued through Notices to Mariners, port circulars, harbour regulations, or national maritime legislation.

Dangerous Goods Declaration

Dangerous goods are assigned a Class and a UN number. These identify the nature of the hazard, the type of packaging required, segregation rules, emergency measures, and the conditions under which the cargo may be carried. The dangerous goods shipper must sign the declaration contained in the Dangerous Goods Shipping Note (DGN), confirming that the goods are correctly classified, packed, marked, labelled, and documented in accordance with the IMDG Code.

The Dangerous Goods Shipping Note (DGN - Declaration) must include:

  • Technical Name
  • Trade Name
  • Class
  • UN number
The package, container, tank, trailer, or cargo transport unit must also display the correct diamond-shaped warning label. These labels allow ship personnel, port workers, emergency responders, and authorities to identify the hazard quickly. Mislabelled or undeclared cargo creates serious danger and may also invalidate insurance protection.

Cargo Segregation

The IMO International Maritime Dangerous Goods Code (IMDG Code) requires proper segregation of dangerous goods onboard ships. Some cargoes may react dangerously when stowed together. Others may produce fire, explosion, toxic gas, corrosion, or contamination if exposed to heat, water, impact, or incompatible substances. Segregation rules therefore determine whether cargoes may be stowed together, separated by distance, separated by a deck or bulkhead, or carried in different compartments.

Dangerous Cargoes Classification:

  • Class 1 Explosives (Military and Commercial)
  • Class 2 Gases
  • Class 3 Flammable Liquids
  • Class 4 Flammable Solids
  • Class 5 Oxidising Agents
  • Class 6 Poisonous (Toxic) Substances
  • Class 7 Radioactive Substances
  • Class 8 Corrosives
  • Class 9 Miscellaneous Dangerous Substances
Dangerous Cargoes Classification aims to ensure that:
  1. Dangerous goods are correctly packed and properly labelled by the producer or shipper.
  2. Dangerous goods are stowed safely during land transport and onboard ship.
  3. Personnel involved in the movement of dangerous goods know how to respond to spillage, leakage, fire, or exposure.
  4. Personnel know whom to contact for technical and emergency information.
Segregation is especially important on container ships, general cargo ships, and ro-ro ships. Container ships often have sufficient steel separation through decks, bulkheads, and containers, but cargo requiring on-deck stowage may still create difficulties. Ro-ro ferries can be more challenging because trucks and trailers may carry different dangerous goods classes, while deck and bulkhead separation may be limited. Ro-ro ferry operators must therefore control dangerous goods bookings carefully. Ships carrying Class 5 Dangerous Cargoes may be restricted from carrying certain other classes, and operators may attempt to avoid carrying conflicting classes on the same sailing.

The IMO International Maritime Dangerous Goods Code (IMDG Code) contains special rules for the carriage and separation of Class 1 (Explosives). Many ports do not allow Class 1 (Explosives) within port limits except under tightly controlled conditions. Limited exceptions may apply to small quantities such as fireworks or safety ammunition, often subject to direct removal from the port area. Many maritime nations maintain special berths for loading or discharging explosives.

Dangerous Cargoes Manifest

A ship carrying dangerous goods must have a Dangerous Cargoes Manifest onboard, together with a copy of the Dangerous Cargoes Declaration for each dangerous cargo item. Many countries require the carrier, usually through the local ship’s agent, to notify authorities in advance of dangerous cargo carried onboard. Failure to provide accurate information may lead to fines, refusal of entry, cargo rejection, detention, or serious safety consequences.

Before accepting dangerous cargo, the Ship Manager should confirm:

  1. Hull Underwriters or the P&I Club (Protection and Indemnity Club) have been notified where required, and guidance has been obtained for loading, stowage, carriage, discharge, and emergency response.
  2. Adequate onboard stowage and segregation conditions are available.
  3. Any extra payment to crew members, special supervision, or additional safety measures are arranged where applicable.

Bill of Lading (B/L)

Bill of Lading (B/L) performs three main functions:
  1. The Ship Master’s Formal Receipt for the cargoes
  2. Document of Title to the cargoes
  3. Evidence of the contract of carriage
When cargo is loaded, a Mate's Receipt is issued. The Mate's Receipt records the cargo description, apparent order and condition, marks, weight, quantity, and any remarks about damage or defects. The Bill of Lading is then prepared from the Mate's Receipt. The master and Ship Manager must ensure that the Bill of Lading accurately reflects the cargo received.

Freight Prepaid Bill of Lading (B/L) or Freight Paid Bill of Lading (B/L) means that the Bill of Lading also operates as a receipt for freight. If a Bill of Lading is marked freight prepaid before freight is actually received, the shipowner may lose the right to rely on a lien over the cargo for unpaid freight. The consignee may be entitled to demand delivery at destination, and the shipowner may have lost the security of the lien on the cargo for the freight.

Clean Bill of Lading (B/L) means that the cargo was apparently received in sound external order and condition. Shippers may request a Clean Bill of Lading (B/L) even when cargo is damaged, rusty, wet, stained, short, or otherwise defective. Issuing a clean Bill of Lading in such circumstances is fraudulent and may leave the shipowner with little or no defence against cargo claims. This is particularly important under Letter of Credit (L/C) transactions, because banks may not negotiate the credit if the Bill of Lading is claused with remarks about cargo condition.

Most Bills of Lading contain wording such as weight, quality, and quantity unknown. Nevertheless, the master should sign only for the quantity that he reasonably believes has been loaded. A Draft Survey should be used to compare the ship’s calculated intake with the Shore Weight, especially in bulk trades.

A Bill of Lading is a Negotiable Document (Document of Title). The lawful holder may be entitled to delivery of the cargo. If the master delivers cargo to the wrong person, the shipowner may be liable for the full value of the cargo. Where the original Bill of Lading is not available at the discharge port, delivery may be requested against a Letter of Indemnity (LOI). Such an LOI should be carefully reviewed and, where required, counter-signed by a First Class Bank. Cargo Manifest and Stowage Plan are normally issued after completion of loading in addition to the Bill of Lading.

Cargo Claims

The master is obliged to deliver cargo at the discharging port in the same good order as received at the loading port, subject to applicable defences under the contract of carriage and law. Any visible defects at loading must therefore be recorded on the Mate’s Receipt and Bill of Lading. If pre-existing damage is not recorded, the shipowner may later be treated as responsible for it.

Against a Genuine Cargo Claim, there may be no defense if the ship has failed to care for the cargo properly. The Ship Manager must also reduce exposure to an Optimistic Cargo Claim, where exaggerated or doubtful claims are advanced. Some countries may require a cash deposit or security before the ship is allowed to sail after a claim. To protect the shipowner’s interest, the Ship Manager should ensure that:

  • P&I Club's (Protection and Indemnity Club's) requirements for loading port Pre-shipment Survey and discharging port Independent Survey are strictly followed for steel cargoes.
  • In bad weather during the voyage, the master should issue a Note Protest on arrival, where appropriate, in the presence of a Notary Public.
  • The Ship Manager should arrange for customs or other competent authorities to seal the hatch covers at the loading port where this is useful.
  • The Ship Manager should arrange a Draft Survey at the loading port and discharging port, preferably with P&I Club representatives or surveyors where needed.
  • In any cargo problem, the Ship Manager should consult the P&I Club (Protection and Indemnity Club) and follow its guidance.

Wet (Liquid) Bulk Cargoes

Wet (Liquid) Bulk Cargoes include crude oil, petroleum products, chemicals, liquefied gases, and other liquid commodities carried in tanks. The common expressions Oil and Petroleum Products cover a very broad group of cargoes, including black oils, gases, spiked crudes, topped crudes, feedstocks, chemicals, gasoline, kerosene, gas oil, and diesel. Liquid cargoes require specialised ships, pumps, pipelines, cargo tanks, measuring systems, safety procedures, and pollution-prevention controls.

Crude Oil

There are many types of crude oil. Crude oils are often divided into two broad categories:
  1. Paraffin-Based Crudes Oils: contain varying amounts of paraffin wax and relatively little asphaltic material.
  2. Naphthenic Crudes Oils: contain little or no wax but a higher proportion of asphaltic material.
All crude oils are volatile to some degree depending on temperature and composition. They can produce inflammable vapors. The Flash-Point is the lowest temperature at which a liquid gives off enough vapour to form an ignitable mixture with air. Below the flash point, the mixture may be too weak to ignite. Above certain limits, the mixture may be too rich in vapour and too low in oxygen to ignite. Between these limits, an ignition source may cause an explosion. Tanker safety rules must therefore be followed strictly during loading, carriage, discharging, tank cleaning, and gas freeing.

Crude oils contain sulphur compounds in different amounts. Middle East crude oils have often been associated with high sulphur content, while many Far East, North African, and North Sea crude oils may have less sulphur content. The handling of each crude depends on its wax content, pour point, viscosity, sulphur level, temperature requirements, sediment, water content, and compatibility with other cargoes.

Many crude oils can be loaded after tanks are well drained, without extraordinary tank preparation and without heating. However, exceptions exist. Before loading Wax-Free Naphthenic Crudes Oils, tanks may need to be hot washed and wax deposits removed unless the previous cargo was compatible. These crude oils may require heating and may not be carried in very large parcels. VLCCs (Very Large Crude Carriers) and ULCCs (Ultra Large Crude Carriers) normally carry homogeneous crude cargoes, though they may carry more than one grade where segregation allows.

Crude Oil Segregation may sometimes be achieved by single valve separation and controlled pipeline admixture, depending on charterer instructions, ship design, and cargo compatibility. Crude Oil Load On Top may be practised with many crude oil cargoes, but not all crude oils are compatible with each other. Compatibility must be confirmed before commingling residues or slops with fresh cargo.

Tank Heating

Modern VLCCs (Very Large Crude Carriers) and ULCCs (Ultra Large Crude Carriers) are generally not outfitted to heat crude oil cargo. Crude oils requiring heating are more often carried in smaller tankers, including ships below 100,000 DWT. When heating is required, temperature control must be accurate. Both Overheating and Underheating can cause oil cargo loss or damage.

Overheating can damage cargo quality and boil off valuable light fractions. Underheating can increase viscosity, cause heavier components to precipitate, slow discharge, block lines, and create outturn losses. If waxy crude cools too far, wax may precipitate and settle on cold tank bottoms, where it hardens. During discharge of waxy or heated cargo, stripping tank bottoms promptly while the cargo remains liquid and pumpable is critical.

If oil cargo temperature falls below its Pour Point, the cargo starts to solidify. Even No Heat Crudes Oils may create problems if a voyage becomes unexpectedly prolonged because of port strikes, congestion, bad weather, or operational delay. A common rule is to maintain cargo at least 10°C above its Pour Point, but charterer instructions and cargo data sheets must be followed closely.

Center Tanks usually have larger volume and less exposed surface area than Wing Tanks. Wing tanks lose more heat because of their position next to the hull. Where segregation permits, higher heat cargo may be stowed in Center Tanks. If heating problems develop, cargo circulation may help obtain a more even temperature, but the charterer should be notified before such measures are taken.

The shipowner’s obligation may be to maintain the cargo temperature or to raise it to a required level. If the obligation is to maintain temperature and the cargo is loaded below the required temperature, the master should issue a Protest Letter. Raising temperature during a short voyage or in cold weather may be difficult and expensive in bunker consumption.

Inert Gas System

In many maritime jurisdictions, tankers over 20,000 DWT are not permitted to load or discharge unless the Inert Gas System is operating. Inert gas reduces the oxygen level in cargo tanks and helps prevent the formation of an explosive atmosphere. The oxygen content and inert gas quality must be monitored continuously, and tank pressure must be maintained during the voyage.

The highest demand for inert gas often occurs during discharge, when cargo is pumped out and air could otherwise enter the tanks. Inerting is also important after tank cleaning and during gas freeing. Cargo Surveyors may need pressure reduced to ullage, dip, or inspect the tanks. Any pressure reduction must follow the shipowner’s approved procedures. Inert gas reduces visibility inside tanks, making visual inspection more difficult. De-inerting and cleaning at the arrival port may require additional time.

Although inert gas passes through a scrubber, greyish sediments may remain and create a risk of discoloration or contamination for clean oil cargoes. In a collision or structural failure, inert gas may escape, and air entering the tank may create an explosive mixture. The system is therefore essential but must be managed carefully.

Segregated Ballast Tanks

In earlier tanker operations, ballast water was sometimes carried in unwashed cargo tanks. During the ballast passage, oily ballast water could be discharged and replaced with clean seawater. This practice created serious marine pollution. Load-on-Top Methods reduced pollution by retaining tank washings onboard, decanting water, and loading new cargo on top of remaining oil residues, but this was not enough to eliminate the problem.

The discharge of tank washings or dirty ballast into the sea is now prohibited under the International Convention for the Prevention of Pollution from Ships (MARPOL). Tankers in international trade must comply with MARPOL pollution-prevention requirements.

New crude carriers above 20,000 DWT delivered after June 1982 were required to have Segregated Ballast Tanks (SBT). United States Coastguard Regulations have also required tankers over 20,000 DWT to have Segregated Ballast Tanks (SBT) or Dedicated Clean Ballast (CBT). Dedicated Clean Ballast (CBT) uses separate lines, pumps, and tanks fully isolated from the cargo system.

Tank Cleaning and Crude Oil Washing (COW)

Before Crude Oil Washing (COW) was introduced, cargo tanks were cleaned by high-pressure seawater delivered through rotating nozzles. These systems were effective but produced oily wash water that had to be managed. Crude Oil Washing (COW) uses the crude oil cargo itself as the washing medium. During discharge, crude oil is pumped through fixed washing machines to remove clingage, sludge, and residues from tank sides, horizontals, and tank bottoms.

Crude Oil Washing (COW) improves drainage and reduces residues more effectively than natural draining. After each tank is washed, tank bottoms are stripped to a collecting tank, and the recovered oil is pumped ashore. Gas is generated during Crude Oil Washing (COW), which may increase loss of light fractions, but the method improves total cargo outturn and reduces the amount of oil left onboard.

For shipowners, Crude Oil Washing (COW) reduces later cleaning difficulty, lowers burner consumption for tank washing, limits residue build-up, and may reduce short-delivery claims. Under MARPOL, only the proportion of tanks necessary for clean ballast may need to be crude oil washed, except before drydocking when the shipowner may prefer all tanks to be cleaned. In practice, about one-third of tanks may be washed on a voyage cycle so that all tanks are washed over three trips, depending on ship operation and regulatory requirements.

Charterers may require all cargo tanks to be crude oil washed to maximise delivery. This benefits cargo receivers because it increases outturn quantity, but it may increase the risk of residues affecting the next cargo. Not all crude oils are suitable for Crude Oil Washing (COW). Very heavy or waxy crudes may increase rather than reduce clingage, and suitability must be checked before use.

Clean and Dirty Petroleum Products

The distinction between crude oils, petroleum products, clean products, and dirty petroleum products is not always clear. Some crude oils are processed or blended. Some products sit near the boundary between clean and dirty trades. Gasoil is a common example. A light-colored gasoil may be treated as a clean petroleum product, while a dark-colored gasoil may be treated as a dirty product and carried in a black oil tanker.

Dirty Petroleum Products include bitumen, heavy fuel oils, carbon black feedstock, waxy distillates, diesel oils, and dirty gasoil. Clean Petroleum Products include clean gasoils, lubricating oils, kerosene, aviation turbine fuel, motor gasoline, and aviation gasoline. Product tanker design, coating, pump arrangement, segregation, cleaning capability, and previous cargo history are crucial.

Heating is often a major issue for Dirty Petroleum Products. Heavy fuel oils may need to be kept warm to remain pumpable. If viscosity becomes too high, pumping time increases and outturn losses may follow. Clean Petroleum Products require a high standard of cleanliness in tanks, pumps, valves, and pipelines. Contamination by residues, colour, odour, lead, water, or incompatible cargo traces can lead to rejection.

Charterers of Clean Product Tankers often request the last three previous cargoes to determine whether tanks can be prepared properly. They may also require confirmation that prior cargoes were lead-free. Heated cargo carried next to light clean petroleum products can increase vapour loss or create vapour pressure difficulties during discharge.

Both Clean and Dirty Petroleum Products can be carried in large product tankers, including very large product carriers, but shore tank capacity and regular trade patterns may limit parcel size. Many large tankers are designed to carry Crude Oil or Dirty Petroleum Products for greater commercial flexibility. Zinc or epoxy-based coatings may allow faster cleaning between cargoes.

For Clean Petroleum Products, tank cleaning may require flushing, gas freeing, scale removal, mopping, and line washing. For color-critical grades, tanks and lines may require special wash procedures. Tank Cleaning Requirements must be checked carefully. Major charterers maintain Cleaning Tables and Special Tank Cleaning Requirements. Relevant cargo factors include viscosity, octane number, wax, asphaltenes, colour sensitivity, flashpoint, lead-free status, water tolerance, and contamination limits.

Chemical Tankers

Chemical Tankers operate in a specialised market carrying a wide range of chemicals, aromatics, solvents, vegetable oils, acids, caustics, and selected petroleum products such as aromatics and unleaded gasoline. If a cargo has special requirements outside the ordinary knowledge of the shipowner, the charterer must advise the shipowner of the special requirements. The shipowner must know the exact chemical identity, not merely a trade name, because different producers may use different brand names for the same or similar chemical.

Chemical tanker operation requires specialist knowledge from producers, traders, shipowners, terminal operators, surveyors, and crews. Chemical tankers are usually smaller than crude oil tankers and have multiple tanks with strong segregation capability. Many are fitted with stainless steel tanks or high-tech coatings, separate pumps, individual lines, vapour control systems, and advanced safety equipment.

Tank coating manufacturers provide lists of compatible products and temperatures. If cargo and coating are incompatible, the cargo may damage the coating, or the coating may contaminate or discolour the cargo. Some chemical cargoes are extremely sensitive, and the smallest trace of an incompatible substance can place the cargo off-specification. Cleaning, wall wash tests, prior cargo history, coating resistance, and temperature limits must therefore be checked before fixture and loading.

Liquid Gas Carriers

There are two main types of Liquid Gas Carriers:
  1. LNG (Liquid Natural Gas) Carriers
  2. LPG (Liquid Petroleum Gas) Carriers
There are three classic designs of LNG (Liquid Natural Gas) Carriers:
  1. Moss Rosenberg Spherical Tank System
  2. Technigas
  3. Gaz Transport Integral Tank Systems
LNG (Liquid Natural Gas) is carried at about minus 163°C. Boil-off gas may be used as fuel in the main engine or reliquefied through a cryogenic plant and returned to the tanks, depending on ship design. LNG projects normally maintain spare LNG (Liquid Natural Gas) Carrier capacity to cover drydocking and repair, but replacing a ship unexpectedly can be difficult because the ship must be compatible with the project’s terminals, cargo system, and schedule. Refrigeration remains the most efficient method for carrying LNG (Liquid Natural Gas).

LPG (Liquid Petroleum Gas) Carriers are often fully refrigerated, while smaller LPG ships may be semi-refrigerated, semi-pressurised, or fully pressurised. The most common LPG cargoes are Butane and Propane. Butane liquefies at about minus 14.5°C and propane at about minus 48.4°C. LPG carriers may also carry other liquefied gases, depending on design and certification.

The inerting gas used for both LNG (Liquid Natural Gas) and LPG (Liquid Petroleum Gas) Carriers is commonly nitrogen. Shore facilities may provide the initial nitrogen blanket, which is then maintained or topped up during the voyage by the ship.

Ullage

LNG (Liquid Natural Gas) Carriers and LPG (Liquid Petroleum Gas) Carriers use advanced instrumentation for measuring the gas tanks because the tanks cannot be opened for ordinary measurement. Many oil tankers also use electronic measurement systems, although not all are equipped to read all ullages from a central control room. The deck officer in charge of loading must ensure that all tanks are correctly measured.

Ullage is the measurement of the free space above the cargo. Traditionally, ullaging was performed with a steel tape and a calibrated brass bob. A tank is usually not filled completely; loading to around 98% of its capacity allows space for cargo expansion during the voyage. Measurement may be affected by ship movement, temperature, trim, list, calibration errors, or rounding. Even a small error in a large tank can represent a substantial quantity of oil.

Tank contents must be measured and recorded before discharge and again after discharge. Ullaging the tanks after discharge to determine remaining quantities is often called Dipping the Tanks. If charterers can not provide a full cargo, Dead Freight may arise. Slack tanks may also create seaworthiness concerns, especially for LNG (Liquid Natural Gas) Carriers and LPG (Liquid Petroleum Gas) Carriers, where cargo surface movement can damage tank structures. Partly filled spherical LNG tanks require particular attention because liquid movement can be more severe.

Additional Bulk Cargo Handling Principles

A vast quantity of dry seaborne material is carried in bulk without bags or packaging. Most such cargoes consist of one homogeneous grade, but ships may carry several grades in separate holds or, with suitable arrangements, more than one grade in the same hold. If cargo is separated horizontally, materials such as tarpaulins, dunnage, burlap, hessian, or heavy-duty polythene sheeting may be used. If vertical separation is required, temporary wooden bulkheads may be fitted. However, the risk of admixture rises significantly when grab discharge is used.

The handling of bulk cargo on a ship is a skilled operation. Cargo distribution must preserve stability, trim, structural safety, and seaworthiness. Loading and discharging sequences must avoid excessive hull stress. This is especially important when several berths or discharge ports are involved. The master must ensure that the ship remains in a safe and seaworthy condition at every stage, not only at final departure.

Bulk cargoes have individual characteristics. Some may heat, emit gas, corrode steel, liquefy, shift, contaminate future cargoes, absorb moisture, or damage hold structures. The IMO provides guidance for solid bulk cargoes through the International Maritime Solid Bulk Cargoes Code, commonly known as the IMSBC Code. Every ship operator and trader involved in dry bulk cargoes should understand the cargo schedule, hazard group, trimming requirements, moisture limits, documentation, and emergency procedures for the cargo concerned.

Structural risk is not limited to overloading. Excessive stability at the start of a voyage can create violent rolling, which may damage the ship or cause cargo movement. Even if the ship sails with acceptable stability, shifting cargo can reduce stability dangerously. Certain commodities are more prone to surface shift than others. When cargo is poured onto a flat surface, it forms a conical heap. The angle of repose of that heap indicates how the cargo may behave at sea. The critical angle is that of 35 degrees. Cargoes with an angle of repose of 35 degrees or less, such as many grain cargoes, are more likely to move on the surface and must be levelled off (trimmed) for safe carriage.

Summary

Bulk cargo transport is divided into Dry Bulk Cargoes and Wet (Liquid) Bulk Cargoes. Dry bulk cargoes require clean, dry, suitable holds, proper trimming, accurate stability planning, and attention to cargo-specific hazards. Grain, coal, fishmeal, scrap, sulphur, ores, and fertilisers each present different risks. The master must manage stability, trim, stress, draft, cargo documentation, and safe loading sequence.

Dry non-bulk cargoes, such as bagged cargoes, steel, logs, unit loads, and timber, require different handling because they must be individually stowed, lashed, protected, tallied, and documented. Cargo condition at loading is critical because Bills of Lading and cargo claims often depend on what was recorded at the time of shipment.

Stowage Factor (SF) is essential in bulk cargo planning because it determines whether a ship will be limited by weight or by space. Light cargoes may fill the holds before deadweight is used, while heavy cargoes may reach the load line while hold space remains unused. Freight rates, cargo quantity, loading plans, and ship employment must account for this relationship.

Dangerous Cargoes and Polluting Cargoes require correct classification, declaration, packaging, labelling, segregation, manifesting, and emergency planning under the IMO International Maritime Dangerous Goods Code (IMDG Code) and related rules. The Dangerous Goods Shipping Note (DGN - Declaration), Dangerous Cargoes Manifest, and correct onboard segregation are central to safe carriage.

Wet (Liquid) Bulk Cargoes require specialised tanker systems, including cargo tanks, pumps, heating coils, inert gas, segregated ballast, ullage systems, and cargo-compatible coatings. Crude oil, petroleum products, chemicals, LNG, and LPG all have different handling requirements. Tank heating, Inert Gas System, Segregated Ballast Tanks (SBT), Crude Oil Washing (COW), and accurate Ullage measurements are central to safe and efficient tanker operation.

Whether cargo is dry or liquid, bulk carriage requires careful planning, technical knowledge, cargo documentation, and strict attention to safety. The economic value of bulk shipping depends not only on moving large quantities cheaply, but also on preventing contamination, structural damage, cargo claims, fire, explosion, pollution, instability, and unsafe ship operation.