BULK CARRIERS AND BULK CARGOES
Purpose and general use of Seagoing bulk carrier Bulk carriers are single-deck vessels, designed with top-side tanks and hopper side tanks in cargo spaces and are intended primarily to carry single-commodity solid bulk cargo. What is solid bulk cargo ?Solid bulk cargo means any material, other than liquid or gas , consisting of a combination of particles , granules or any larger piece of material, generally uniform in composition, which is loaded directly into the cargo spaces of a ship without any immediate form of containment. Example of such dry cargo are grain, sugar and ores in bulk.
Fig : A Bulk carrier on sea passage In its broadest sense, the term bulk carrier embraces all ships designed primarily for the carriage of solid or liquid cargo in bulk form, and so would include tankers. In ordinary usage, however, the term is normally used for those vessels designed for the transport of solid bulk cargos, typically grain and similar agricultural products, and mineral products like coal, ore, stone, etc., on one or more voyage legs.
General features of bulk carriers are: Carrying capacity varying from 3,000 tonnes to 300,000 tonnes average speed of 12 ~ 15 knots single deck ships, ie no tweendecks small to medium sized bulk carriers (carrying capacity up to 40,000 tonnes) generally have cargo handling gear fitted, while larger vessels use shore based facilities for loading and unloading
the cargo holds are usually large, without any obstructions, with larger hatch sizes to allow easy loading/unloading of cargoes • most bulk carriers have one cargo hold dedicated as a ballast hold. This can be used on ballast voyages for improved stability. One or two further holds may be permitted for partially ballasting but only in port • they have hydraulic, single pull or stacking (piggy- back) type steel hatch covers • 8. these ships usually have four types of ballast tanks: • sloping topside wing tanks • sloping bottom side wing tanks • double bottom tanks • fore peak and after peak ballast water tank.
BULK CARGOES Coal , Iron ore, Mineral ore, Grain, Cement & Woodchips loading in bulk Bulk carriers are designed to load a maximum deadweight of any type of bulk cargo from heavy ore to light grain . The loading, carriage and finally the discharge of dry bulk cargo is not as simple or straight forward as most people would imagine.Many bulk cargoes have hazardous properties, or can change their properties on passage. The ship can be easily damaged by incorrect loading e.g. loading a forward hold to it maximum can cause the ship to bend. This ‘stress’ can have life threatening results at sea in rough weather.
Residues from previous cargoes can also seriously effect latter cargoes. Water damage can also have devastating effect on some bulk cargoes e.g. cement power. It is not easy to verify true weights or quantities of cargoes loaded or discharged. All these factors have a serious consequence on the methods of operation for the safe carriage of bulk cargoes. Discharging bulk cargo using “grab” Bulk cargoes have an inherent tendency to form a cone when they are loaded if conveyor belts or similar systems are not supervised and controlled. The angle formed by this cone is known as the `angle of repose' and varies with each cargo. Cargoes such as iron ore will form a steep angled cone, whereas cargoes that flow freely form a shallow angled cone. A cargo with a low angle of repose has the potential to shift during passage.
Coal Coal is transported on all types of bulk carriers from handy size to VLCBs. However, it is not an easy or straight forward cargo to handle. It can emit methane gas and it is self-heating. In addition coal contains sulphur which causes severe corrosion when in contact with the ship's steelwork.In most ports the cargo is loaded wet to reduce dust. Much of this moisture settles on passage and is pumped out through the ship's hold bilges which means that less weight is discharged than is loaded.Find out more on ....coal hazards and safety precautions
Iron Ore This cargo is loaded very fast, 10,000 tonnes an hour is not unusual. The loading and de-ballasting of the ship must be meticulously planned to ensure that the vessel is not overstressed. There is very little chance of damaging the cargo but the ship can receive extensive damage during the discharge operation from the equipment used.Find out more on ....Safety precautions for loading and carriage of iron ores
Mineral Concentrates Many different types of concentrates are handled in various parts of the world and in varying quantities. Most of these cargoes are extremely heavy and have a low transportable moisture limit (TML). This means that if the moisture content of the cargo become greater than the TML the cargo can liquefy and turn into a slurry. When this happens on board, the cargo moves from side to side as the ship rolls which reduces the ship's righting lever. It does not require much cargo weight to capsize the vessel when this happens, it a loss of stability due to free surface effect. Some of the most dangerous cargoes where this can happen are copper, lead or zinc concentrates, magnetite, limonite and most pyrites.
GrainOne of the most difficult and dangerous cargoes to carry in bulk are grain cargoes. Most grains have an angle of repose (slip angle) of about 20° from the horizontal, which means that if the ship rolls more than 20° the cargo will shift. Then this happens the ship will develop a large list, lying on her side and still rolling will obviously cause a greater shift of cargo which in turn will capsize the vessel. Most authorities therefore request that the master proves that his ship is capable of remaining stable even if the grain cargo shifts. This is done by the compiling of the Grain Loading Form which fully outlines the ships stability at the worse condition on passage.Naturally grain cargoes, like any foodstuff, are susceptible to claims with contamination from a previous cargo and in addition can easily be damaged by water.Vermin can also be a problem. Cargo holds must be clean and dry prior to the loading of any grain cargo and most grain charters demand a survey of the ship's hold prior to loading for this reason.Find out more on : .Hazards and safety precautions for grain cargo
Cement Obviously any moisture is going to ruin a cargo of cement but probably a greater danger to the vessel is the dust that can be produced during the loading and discharge of the cargo. If it is not removed promptly or gets into the ship's air intakes it can cause some long term problems to the vessel.Salt- Salt, strangely enough, is not damaged from water, in fact the cargo can be loaded slightly moist. However, it can get rust stained from the ship's steelwork, therefore the ship must cover all the steel within the cargo hold with a lime wash solution thereby keeping the salt off the steelwork.
Woodchips Again a supposedly harmless cargo that does have some hidden dangers. Some shipments many be subject to oxidation leading to depletion of oxygen and an increase of carbon dioxide in the cargo hold and adjacent spaces. In addition, woodchips can be easily ignited by external sources, it is readily combustible and can also ignite by friction. The stowage factor can vary greatly with this cargo depending on the wood type, the moisture content and the type of loading head used. Even different loading operators can achieve varying stowage factors with the same cargo.
Structural integrity & design limitations of modern seagoing bulk carriers Most seafaring nations have established classification societies which review standards for the construction of cargo vessels. Classification societies publish construction guidelines and stability and operating standards to ensure vessel safety and standardization of ship construction and other marine equipment.All ships are designed with limitations imposed upon their operability to ensure that the structural integrity is maintained according to classification society guideline. Therefore, exceeding these limitations may result in over-stressing of the ship's structure which may lead to catastrophic failure. The ship's approved loading manual provides a description of the operational loading conditions upon which the design of the hull structure has been based. The loading instrument or vessels approved loading software provides a means to readily calculate the still water shear forces and bending moments, in any load or ballast condition, and assess these values against the design limits.
A ship's structure is designed to withstand the static and dynamic loads likely to be experienced by the ship throughout its service life.
The loads acting on the hull structure when a ship is floating in still (calm) water are static loads. These loads are imposed by the:i) Actual weight of the ship's structure, outfitting, equipment and machinery.ii) Cargo load (weight).iii) Bunker and other consumable loads (weight).iv) Ballast load (weight).v) Hydrostatic pressure (sea water pressure acting on the hull).
Dynamic loads are those additional loads exerted on the ship's hull structure through the action of the waves and the effects of the resultant ship motions (i.e. acceleration forces, slamming and sloshing loads). Sloshing loads may be induced on the ship's internal structure through the movement of the fluids in tanks/holds whilst slamming of the bottom shell structure forward may occur due to emergence of the fore end of the ship from the sea in heavy weather.Cargo over-loading in individual hold spaces will increase the static stress levels in the ship's structure and reduce the strength capability of the structure to sustain the dynamic loads exerted in adverse sea conditions.
Special care needs to be taken with heavy cargoes such as iron ore, scrap iron, lead and other concentrates. On general bulk carriers with uniform hold lengths alternate hold loading or block hold loading may be utilized to stow high density cargoes. With such loading arrangements high shear forces occur at the ends of the holds requiring additional strengthening of the side shell in way of the bulkheads.
Bulk carrier business & trade patterns Bulk carrier voyages are fixed according to market demands. While there is a steady flow of bulk cargoes from exporting countries to the industrial nations, floods, droughts or other natural disasters may affect a region's crop output, changing commodity prices. The trades may, to some extent, be fixed with the vessel on a liner service, but when the trade is generated on an `on demand' or spot basis, a ship will generally not know its next `fixture' until it receives its voyage orders.Over 6,000 bulk carriers are engaged in the trade of bulk cargoes, such as ores, logs, coal and steel products.
Parties in the bulk trade include: • The ship operator • the ship (as carrier) and its Master • cargo seller, ie the shipper • cargo buyer, ie the receiver or consignee • charterer (note that the ship may have been sub- chartered to one or more charterers, but the main contractual agreement between the charterer and the sub-charterer is similar to that between the main charterer and the shipowner) • insurer or underwriter • agent.
Typically, a shipowner may allow his ship to be chartered to a charterer (who in turn may sub-charter it). They then find a shipper who loads their cargo on the ship. The cargo's sale has either been previously agreed with a buyer or, in some cases, may be sold after it has been loaded. The ship carries the cargo to the destination and discharges to the receiver. At the loading and discharging ports the ship, charterer, seller and buyer may nominate agents.
Voyage agreement To engage a ship to load a cargo, a contract of affreightment, setting out the terms and conditions for the payment to the shipowner in return for provision of the ship, has to be agreed between the shipowner and the cargo owner. This agreement can be either a bill of lading (between the carrier, ie the shipowner or charterer, and the cargo owner) or a charterparty (between the shipowner and charterer). A bill of lading (B/L) is a document, agreed under the auspices of the Hague or Hague-Visby rules, issued on behalf of the shipowner that provides evidence that the cargo has been received for shipment (Appendix 34). A B/L may still be issued in addition to the charterparty agreement. A charterparty is a written agreement (usually in a specified format) between the shipowner and the charterer for hire of a vessel with clearly defined freight rates, cargoes and loading/discharging ports.
Cargo operation useful terms related with bulk carriers The following definitions are extracted from the International Maritime Solid Bulk Cargoes (IMSBC) Code 2009: Angle of ReposeThe maximum slope angle of non-cohesive (ie, free- flowing) granular material. It is measured as the angle between a horizontal plane and the cone slope of the material. Non-cohesive MaterialDry materials that readily shift due to sliding during transport. a. Non-cohesive bulk cargoes having an angle of repose less than or equal to 30° These cargoes, which flow freely like grain, shall be carried according to the provisions applicable to the stowage of grain cargoes b. Non-cohesive bulk cargoes having an angle of repose greater than 30° to 35° inclusive Loading is carried out using trimming equipment approved by the competent authority c. Non-cohesive bulk cargoes having an angle of repose greater than 35° Loading is carried out using trimming equipment approved by the competent authority.
Cohesive MaterialMaterials other than non-cohesive materials. Cargoes That May LiquefyCargoes that contain a certain proportion of fine particles and a certain amount of moisture. They may liquefy if shipped with a moisture content in excess of their transportable moisture limit. ConcentratesMaterials obtained from a natural ore by a process of enrichment or beneficiation by physical or chemical separation and removal of unwanted constituents. Flow Moisture PointThe percentage moisture content (wet mass basis) at which a flow state develops under the prescribed method of test in a representative sample of the material.
Flow StateA state occurring when a mass of granular material is saturated with liquid to an extent that, under the influence of prevailing external forces such as vibration, impaction or ship's motion, it loses its internal shear strength and behaves as a liquid. Incompatible MaterialsMaterials that may react dangerously when mixed. Moisture ContentThat portion of a representative sample consisting of water, ice or other liquid, expressed as a percentage of the total wet mass of that sample.
Moisture MigrationThe movement of moisture contained in a cargo by settling and consolidation of the cargo due to vibration and ship's motion. Water is progressively displaced, which may result in some portions or all of the cargo developing a flow state. Stowage FactorThe figure that expresses the number of cubic metres one tonne of cargo will occupy. Transportable Moisture Limit (TML) of a Cargo Which May LiquefyThe maximum moisture content of the cargo which is considered safe for carriage in ships.
TrimmingAny levelling of a cargo within a cargo space, either partial or total. Thank You for Reading Prepared by Captain Thein Win