Definition of Bulk Carriers
According to the SOLAS (Safety Of Life At Sea) Chapter XI-Reg.1, a bulk carrier is defined as a ship which is constructed generally with single deck, topside tanks and hop- per-side tanks in cargo spaces.
A bulk carrier is intended primarily to carry dry cargoes in bulk, and includes such types as ore carriers and combinations carriers. Modern conventional bulk carriers have been built based on the SOLAS definition.
Terminology used by the major classification societies is that a bulk carrier is a vessel designed for the carriage of bulk cargoes, or a seagoing vessel having single deck with machinery aft for carrying bulk dry cargoes.
The SOLAS definition is relatively limited, and thus general cargo vessels, containerships carrying bulk cargoes, and double skinned bulk carriers are not included in the definition. In general, a bulk carrier means a ship carrying dry bulk cargoes such as ore, coal, grain, and having topside and hop per-side tanks with corrugated transverse bulkheads.
Today, bulk carriers make up 15–17% of the world’s merchant fleets [and range in size from single-hold mini-bulk carriers to mammoth ore ships able to carry 400,000 metric tons of deadweight (DWT).
A number of specialized designs exist: some can unload their own cargo, some depend on port facilities for unloading, and some even package the cargo as it is loaded.
Bulk Carrier Types
- Ore carrier
- Ore-Oil carrier
- Universal bulk carrier
- Bulk–Car carrier
- Ore–Bulk–Oil carrier ( OBO )
- Bulk–Container carrier (Open top bulk carrier)
- Ore carrier
- Ore-Oil carrier
- Universal bulk carrier
- Bulk–Car carrier
- Ore–Bulk–Oil carrier ( OBO )
- Bulk–Container carrier (Open top bulk carrier)
Bulk Carrier Sizes
Special Sub-Classes
- Kamsarmax: ~82,000 dwt Panamax with increased LOA = 229 m (for Port Kamsar in Equatorial Guinea)
- Newcastlemax: ~185,000 dwt large Capesize with max. beam B = 47 m (for use of the Australian port of Newcastle)
- Setouchmax: ~205,000 dwt large Capesize (VLBC) with a low design draught of 16.10 m and max. LOA = 299.9 m (for ports in Setouch Sea in Japan)
According to Structural Properties:
- Single Bottom Bulk Carrier: These type of bulk carrier ships do not have a double bottom, and the only barrier between the sea and the cargo is the outer bottom plate. Bulk carriers under 120 m length do not require a double bottom as per structural requirements, but today, ship designers still prefer to avoid single bottoms in order to prevent contact of cargo with sea water in case of structural damages.
- Double Bottom-Single Hull Bulk Carrier: These ships have a single hull, but are provided with a double bottom or a tank top throughout its length (from aft of the forward collision bulkhead to the aft peak bulkhead). The double bottom spaces are used for storage of ballast and duct keel for passage of pipelines.
- Double Hull Bulk Carrier: The use of double hull in bulk carrier designs have increased rapidly over the last ten years. The wing tanks at the sides are an added advantage, and provide more marginal ballast, and better control on the stability of the ship.
Hull Structures – Class Notations
Structural failure nevertheless remains a consistent and significant cause of losses – 14 bulkers were lost with 23 fatalities in 2000, the average age of the ship 20.4 years – while the presence of heavy cargoes featured in many of the casualties.
In 2001 four bulkers were identified as total losses and 64 crewmembers lost their lives; The CHRISTOPHER and HONGHAE SANYO went down with all hands.
To prevent this situation, the International Association of Classification Societies (IACS) is to introduce the harmonized class notations for bulk carriers:
– BC-I, bulk carriers designed to carry dry bulk cargoes with a density < 1.0 ton/m3.
– BC-II, vessels designed to carry homogeneous heavy cargo. plus carriage of cargoes with density ≥ 1.0 ton/m3 (with all cargo holds loaded).
– BC-III, covering loading conditions for heavy cargo with specified holds empty.
Design Features of Bulk Carriers
HULL FORM
Bulk Carriers are primarily designed to store cargo efficiently. . Some bulk Carriers are smaller than 100 m but also large vessels with a capacity of 300,000m3 or more are under development.
Although bulbous bow allows a ship to move more easily through the water, lately designers lean toward the simple vertical bows on the large bulkers.
Full hulls, with large block coefficients, are almost universal, and as a result, bulkers are inherently slow. This is offset by their efficiency. One measure of this efficiency is found in the ratio of the empty ship’s weight to its deadweight tonnage.
For bulkers this figure ranges between 12% for a large Capesize bulker to 20% for a smaller Handymax ship.
The design of today’s bulk carriers is generally characterised by a single screw type of vessel with a rather high block coefficient. For this type of vessel, special attention has to be paid to the design of the fore body (bulbous bow), resulting in good wave pattern, a low overall resistance and related good powering performance. Furthermore, an excellent afterbody design, with a good flow towards the propeller(s) and rudder(s), without flow separation is an important issue.
Large differences between design (loaded) draught and ballast draught have to be taken into account. The combination of single screw type of ship, high propeller loading, and rather pronounced wake field requires carefully checking of the risk for cavitation erosion and vibrations
Lightweight
One of most important design step is to estimate light-weight and its distribution along the ship length. Figure shows possible lightweights for different ship sizes in deadweight tons, based on a feasibility study and actual bulk carrier database. Lightweight accounts for about 20 percent of deadweight for Handymax size bulk carriers and about 12 percent of deadweight of very large bulk carriers.
Here is also some statistical relationship with different parameters VS DWT
Structural Feature
Ballast tanks are arranged in double bottom, hopper, and topside wing tank structures. Since most cargoes are supported by double bottom and hopper tank structures, the structures should be reinforced extensively. A duct keel is arranged for piping and passage in the center of double bottom structure.
Transverse bulkheads are of corrugated type with flat upper and lower stools. Shape of corrugation can be determined from both strength and the minimum weight design point of view by changing pitch and depth of the corrugation.
Lower and upper stool structures support transverse bulkheads to reduce the span of the bulkhead. Slanted type lower stool is common, but either straight type or slanted type is adopted for upper stool mainly depending on the size of bulk carriers.
The number of girders is determined according to the allowable mean shear stress level of each girder, while thickness and number of girders also influence sectional properties of the ship.
Bottom structure of bulk carriers
Where a ship is classed for the carriage of heavy, or ore, cargoes longitudinal framing is adopted for the double bottom. A closer spacing of solid plate floors is required, the maximum spacing being 2.5 m, and also additional intercostal side girders are provided, the spacing not exceeding 3.7 m (see Figure ).
The double bottom will be somewhat deeper than in a conventional cargo ship, a considerable ballast capacity being required, and often a pipe tunnel is provided through this space. Inner bottom plating, floors, and girders all have substantial scantlings as a result of the heavier cargo weights to be supported.
Hull material
Requirements of hull material for the construction of marine vessels are well specified in classification societies’ rules.
In general, steel for bulk carrier construction is categorized as mild steel, higher tensile (HT) steel, casting steel, and forged steel.
Bulkhead
Vertical partitions in a ship arranged transversely or fore and aft are referred to as ‘bulkheads’. Those bulkheads that are of greatest importance are the main hull transverse and longitudinal bulkheads, dividing the ship into a number of watertight compartments.
The main hull bulkheads of sufficient strength are made watertight in order that they may contain any flooding in the event of a compartment on one side of the bulkhead being bilged or breached.
Typical loading conditions
Typical load conditions such as, normal ballast, heavy ballast, homogeneous load, heavy ore, grain load, and dry docking conditions are calculated in the trim and stability calculations for departure and arrival states, respectively.
For bulk carriers having an air draft restriction, partial- flooding of cargo holds should be considered. Still water shear force and vertical bending moment distributions are calculated along the ship length.
Damage stability
Subdivision and stability of cargo ships built before 1992 should satisfy the International Convention on Safety of Life at Sea of 1974 (SOLAS74) and the International Con- vention on Load Lines of 1966 (ICLL66) as amended for reduced freeboard vessels.
In 1982, IMO introduced a regulation for freeboard for dry cargo ships based on a probabilistic analysis procedure. Furthermore, the SOLAS Chapter XII became effective by giving additional safety measures for bulk carriers on or after July 1, 1999.
Cargo handling
In bulk carriers the hatch covers are mostly of the folding type or side rolling . The covers are hydraulically opened and closed. Many bulk cargoes, like grain must be well protected against moisture and dirt during the transport. The hatch covers have rubber gaskets and cleats or wedges to become fully water tight when closed.
Bulk carries can have on board cranes for cargo handling or be loaded or unloaded by shore based equipment . Also self unloading ships with sophisticated belt conveyor system are used for many bulk cargoes.
Loading and unloading a bulk carrier is time-consuming and dangerous.
The process is planned by the ship’s chief mate under the direct and continued supervision of ship’s captain.
International regulations require that the captain and terminal master agree on a detailed plan before operations begin. Deck officers and stevedores oversee the operations. Occasionally loading errors are made that cause a ship to capsize or break in half at the pier.
The loading method used depends on both the cargo and the equipment available on the ship and on the dock. In the least advanced ports, cargo can be loaded with shovels or bags poured from the hatch cover. This system is being replaced with faster, less labor-intensive methods.
Double-articulation cranes, which can load at a rate of 1,000 tons per hour, represent a widely used method, and the use of shore-based gantry cranes, reaching 2,000 tons per hour, is growing.
A crane’s discharge rate is limited by the bucket’s capacity (from 6 to 40 tons) and by the speed at which the crane can take a load, deposit it at the terminal and return to take the next. For modern gantry cranes, the total time of the grab-deposit-return cycle is about 50 seconds.
Conveyor belts offer a very efficient method of loading, with standard loading rates varying between 100 and 700 tons per hour, although the most advanced ports can offer rates of 16,000 tons per hour.
Start-up and shutdown procedures with conveyor belts, though, are complicated and require time to carry out.
Self-discharging ships use conveyor belts with load rates of around 1,000 tons per hour.
Once the cargo is discharged, the crew begins to clean the holds. This is particularly important if the next cargo is of a different type.
The immense size of cargo holds and the tendency of cargoes to be physically irritating add to the difficulty of cleaning the holds. When the holds are clean, the process of loading begins.
It is crucial to keep the cargo level during loading in order to maintain stability.
As the hold is filled, machines such as excavators and bulldozers are often used to keep the cargo in check. Leveling is particularly important when the hold is only partly full, since cargo is more likely to shift.
Extra precautions are taken, such as adding longitudinal divisions and securing wood atop the cargo.
If a hold is full, a technique called tomming is used,[which involves digging out a 6 feet (2 m) hole below the hatch cover and filling it with bagged cargo or weights.
Future Trend of Bulk Carriers
One of the weakest points of single hull bulk carriers is the side shell structure compared to hopper tank and topside tank structures. A side shell is subject to dynamic pressure in a seaway and damaged from cargo grabs and by hydraulic hammer from the sea. It is also difficult to protect the structure against corrosion from sulfur-rich coal car- goes.
Double skinned bulk carriers have advantages over single hull bulk carriers. Damage by grabs, bulldozers, and corrosion can be significantly reduced since the inner skin of cargo holds is smooth. Coating of holds is a quicker and cheaper solution.
The additional longitudinal member contributes hull girder strength and protects hold flooding from side damage. However, higher initial building cost, some dead-weight reduction, and higher port and canal dues because of increased measured tonnage, are disadvantages.
As such disadvantages are not of primary concern over the safety issues of bulk carriers, double skinned bulk carriers become an alternative to single hull bulk carriers.
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