Bulk Carriers, bulk freighters or bulkers are vessels designed to carry bulk solids such as cereals, coal, ore and cement.
Bulk carriers make up more than a third of the entire merchant fleet in the world. They are ranging from small vessels of under 500 DWT to huge carriers of 365,000 DWT.
Bulk carriers have to be carefully designed and maintained because they may carry cargo that is very dense, corrosive or abrasive.
Bulkers can be divided into these major size categories:
In 2005, The US Maritime Administration counted 6,225 bulk carriers greater than 10,000 DWT, worldwide. In Panama, there are 1,703 bulkers registered, that is more than any four other flag states combined. In terms of the number of bulk carriers registered, the top five flag states also include Hong Kong with 492 ships, Malta (435), Cyprus (373), and China (371). Panama also dominates bulker registration in terms of deadweight tonnage. Positions two through five are held by Hong Kong, Greece, Malta, and Cyprus.
Greece, Japan and China are top countries by the number of owned bulk carriers, with Greece owning 1,326, Japan 1,041 and China 979 vessels. These three countries hold 3,346 vessels, that is over 53% of the world's bulk carrier fleet.
There are several large private fleets, for example Gearbulk Holdings Ltd., a multinational shipping company, has 78 bulkers. Fednav, a group of seven Canadian shipping companies, operates a fleet of 70 bulkers, including two that are specially designed for work in icy environments. Croatia's Atlantska Plovidba has a fleet of 14 bulkers. H. Vogemann in Hamburg, Germany operates a fleet of 13 bulkers. Portugese Portline, owns 11 bulkers. TORM in Denmark and Elcano in Spain also each own notable bulker fleets. Some companies specialize in Mini-bulker operations, such as England's Stephenson Clarke Shipping Limited that owns a fleet of eight Mini-bulkers and five small Handysize bulkers. Cornships Management and Agency Inc. in Turkey owns a fleet of seven Mini-bulkers specialized in markets in Europe, on the Mediterranean, the Black Sea, and West Africa.
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The standar bulker crew consists of 20 to 30 crewmembers, although smaller carriers can have as few as 8 people on board. The crew will include the captain or master, the deck department, the engineering department, and the steward's department. The once universal practice of taking passengers aboard cargo ships is very rare today especially on bulk carriers.
During the 1990s, bulkers were involved in an alarming number of shipwrecks, leading ship-owners to commission a study seeking to explain the effect of various factors on the crew's effectiveness and competence. The study showed that bulk carrier crews' performance was the lowest of all groups studied, with the best bulker performance aboard younger and Capesize ships. A correlation was also found between competence and the better maintained ships. The study also showed higher competence aboard ships on which fewer languages were spoken.
Fewer deck officers are employed on bulkers than on similarly sized ships of other types. A mini-bulker will have 2 to 3 deck officers, while larger Handysize and Capesize bulkers will carry four. A LNG tanker of the same size will have an extra deck officer and another unlicensed mariner.
A bulker's voyages are determined by current market trends and forces, that often vary. For example, a ship may engage in the grain trade during the harvest season and later move on to carry other cargoes or work on a different route. Aboard a coastal carrier in the tramp trade, one will often not know the next port of call until the cargo is fully loaded.
Due primarily to inefficiencies in discharging bulk cargo, bulkers spend more time in port than other ships. A study on Mini-bulkers found that it takes, on average, twice as much time to unload a ship than it does to load it. For example, a Mini-bulker will spend 55 hours at a time in port, compared to 35 hours for a similar-sized lumber carrier. For the larger bulkers, this time in port increases to 74 hours for Handymax and 120 hours for Panamax vessels. Compared with the 12-hour turnarounds common with container ships, 15-hour turnarounds for car carriers, and 26-hour turnarounds for large tankers, bulker crews have much greater opportunities to spend some time ashore.
Loading and unloading
Loading and unloading of bulkers is a time consuming and dangerous task. The ship's captain plans the process, usually with the assistance of the chief mate. The international regulations require that the captain and the terminal master agree on a detailed plan of loading or unloading before the operations begin. Deck officers and stevedores oversee the operations. Still, from time to time, a ship will be incorrectly loaded and capsize or break in half at the pier.
Depending on the cargo, the actual work of loading or unloading can be done in a variety of ways. In the old and unadvanced ports, cargo is still loaded by showels or bags poured from the hatch cover.
Other ports use double-articulation cranes which load at a rate of 1000 tons per hour. More advanced ports use shore based gantry cranes that load at 2000 tons per hour. While the most advanced ports use conveyor belts that can load at a stunning 16.000 tons per hour. However, start-up and shutdown procedures are complicated and dangerous to perform.
Once the ship has unloaded its cargo, the crew starts to work on the cleaning of the holds. This is particularly important if the next cargo is of a different type. The immense size of the cargo holds and irritating qualities of many cargoes add to the difficulty of cleaning the holds. When the holds are clean, the actual loading begins.
During all stages of loading, it is crucial to keep the cargo level to maintain stability. As the hold is filled, machines such as excavators and bulldozers are often used to keep the cargo in check. Levelling is particularly important when the hold is only partially filled, due to increased risks of shifting cargo. This is why extra precautions, like adding longitudinal divisions or securing wood atop the cargo, are taken. Once the hold is entirely filled, a special tehnique called tomming is used. This tehnique involves digging out an area directly below the hatch cover and re-filling this area with bagged cargo or weights.
The design of a bulk carrier depends on the type of cargo it will carry. The most important factor is the cargo's density. Densities for bulk cargoes vary from 0.6 tons per cubic meter for cargoes like grains to 3 tons per cubic meter for cargoes like ore.
The second factor that determines the ship dimensions is the size of the ports and waterways it will travel to. This means that a shop that is intended to pass through the Panama Canal will be limited by its beam and width.
Hull shape and machinery
Bulkers are primarily designed to store cargo efficiently. Altough 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.
A bulker's engine room is usually located near the stern, under the house and above the fuel tanks to decrease the length of piping. Larger bulkers, from Handymax up, have a two-stroke diesel engine which directly moves a single propeller. An alternator is coupled directly with the propeller shaft, and an auxiliary generator is used. On the smallest bulkers, one or two four-stroke diesels are used, and coupled with the propeller via a gear box. The average design ship speed for bulkers of Handysize and above is between 13.5 and 15knots (28km/h). The propeller speed is relatively low, at about 90 revolutions per minute.
A hatch or hatchway is the opening found on top of the cargo hold. Usually, hacht covers are between 57% to 67% of the length of the holds. Hatches must be large to allow for efficient loading and unloading of cargoes, but their size also presents structural problems. The hatch areas are often reinforced by utilising scantlings or stiffeners. Both of these add weight to the ship.
Many hatch systems used today slide hatch covers forward, backward, to the side, lift them up or fold up. It is very important that the hatch covers are watertight, as unsealed hatches lead to cargo hold flooding, which is one of the main causes of bulkers sinking.
Bulkers usually have the same cross-section found on most merchant ships. The upper and lower corners of the hold and the double bottom area are used as ballast tanks. The corner tanks are reinforced and serve another purpose besides controlling the ship's trim. When the bulker is designed, the angle of the corner tanks has to be less than that of the angle of repose of the anticipated cargoes. This approach greatly reduces the risk of side-to-side movement or shifting of cargo which can be dangerous for the ship.
The double bottoms used are subjet to several design constraints as well. The most important constraint is that they have to be sufficiently high to allow for the passage of pipes and cables. Also these areas need to have enough space to allow safe access of the crew to perform maintenance and surveys. On the other hand, concerns of excess weight and wasted volume keep the double bottoms very tight spaces.
Bulkers hulls are usually made of mild steel, altough some manufacturers prefer high-tensile steel to reduce the tare weight. However, the use of high-tensile steel for longitudinal and transverse reinforcements can reduce the hull's rigidity and resistance to corrosion. Forged steel is used for some ship parts, such as the propeller shaft support. Transverse partitions are made of corrugated iron, reinforced at the bottom and at connections. The construction of bulker hulls using a concrete-steel sandwich has been investigated.
In the past decade the double hulls have become popular among the manufacturers. The design of the vessel with double hulls adds to its breadth, since bulkers already have double bottoms. Using the double hull makes room for all the structural elements in the sides, thus removing them from the holds. This way the volume of the holds is increased, and their structure is simplified which helps in loading, unloading and cleaning. Double sides also improve a ship's capacity for ballasting, which is particularly important in the carriage of light goods: the ship may have to increase its draught for stability or seakeeping reasons, which is done by ballasting water.
Recently, a new design called Hy-Con (short for Hybrid Configuration), combines the strengths of single-hull and double-hull construction. This design doubles the forward-most and rear-most holds, while the other holds are single-hulled. This way the ship's solidity is increased at key points, while the overall tare weight is reduced.
Since the adoption of double hull has been more of an economic than a purely architectural decision, there has been some criticism that the ships with double sides will receive less comprehensive surveys and suffer more from hidden corrosion. In 2005, in spite of the criticism received, the double hulls became a requirement for Panamax and Capesize vessels.
The scantlings are the correlations between longitudinal strength and a set of hull thicknesses. They are used to manage problems of longitudinal strength and stresses. Naval architects first calculate the stresses a ship can be subjected to and then calculate the required scantlings.
The analyses are conduced for several scenarios such as travelling empty, loading and unloading or even temporary overloading. Places subject to the largest stresses are studied carefully, such as bulkheads between holds, hold-bottoms, hold hatch-covers and the bottoms of ballast tanks. Great Lakes bulkers also must be designed to withstand springing, or developing resonance with the waves, which can cause fatigue fractures.
Since April 1, 2006, the International Association of Classification Societies has adopted the Common Structural Rules. The rules apply to bulkers more than 90meters in length and require that scantlings' calculations take into account items such as the effect of corrosion, the harsh conditions often found in the North Atlantic, and dynamic stresses during loading. The rules also establish margins for corrosion, from 0.5
The 1980s and 1990s were an unsafe time for bulkers. Many bulk carriers sank during this time. This was the cause of a series of safety regulations regarding bulk carriers that were adopted in the 1990s.
Cargo shifting poses a great danger to stability of bulkers. This problem is greater for bulkers carrying grain cargoes, because grain settles during transport and creates an extra space between the top of the cargo and the top of the cargo hold. As the ship rolls, the cargo moves from the one side of the ship to the other. This causes the ship to list which results in even more cargo shifting. This can cause the capsizing of the ship very quickly.
In 1960, SOLAS convention adopted regulations that required the design of the upper ballast tanks to prevent shifting. Also all cargo had to be levelled or trimmed.
Another risk that can effect cargoes such as fine concretes and aggregates, is the absorption of ambient moisture. When these cargoes mix with water it creates mud at the bottom of the hold, which produces a free surface effect. This is why good ventilation practices and monitoring for the presence of water is required.
In 1990 and 1991, 44 bulk carriers sank, taking with them a total of 248 crewmen. This huge losses attracted a lot of attention and a great deal was learned. The losses were traceable to failures of the cargo hold structures.
The studies performed at the time showed a clear pattern:
- 1. Sea water enters the forward hatch, because of a large wave, corrosion or a poor seal
- 2. The water weight in first hold compromises the partition of the second hold
- 3. Water enters second hold and alters the trim so more water enters the holds
- 4. With two holds rapidly filling with water, the bow submerges and the ship quickly sinks
Other factors that were identified as the cause of the sinkings:
- Most shipwrecks were ships over 20 years of age.
- Corrosion, mainly due to lack of maintenance, affected the seals of the hatch covers
- Advanced methods of loading were not forseen during ship design
- Recent use of high-tensile steel, which is thinner than regular steel can corrode more easily
- Many ships had known problems but were sent to sea anyway
In 1997, the SOLAS convention adopted new rules that required the reinforcing of bulkheads and more thorough inspections. Also the bulkers today are required to mark their hulls with ease to see triangles (marking they are forbidden from carrying certain types of cargoes).
Since December 2004, it is required for Panamax and Capesize bulk carriers to have free-fall lifeboats on the stern of the ship. This allows the crew to quickly and efficiently abandon the ship during catastrophic emergencies.
There has been some criticism of free-fall lifeboats, primarily because they require a degree of physical mobility to enter and launch. Also there were recorded injuries in the cases of incorrectly secured safety belts.
In December 2002, Chapter XII of the SOLAS convention required the installation of monitoring system and water alarms on all bulk carriers. This safety precaution alerts watch standers on the bridge and engine room in case of catastrophic flooding. The usage of these detectors greatly speeds up the process of evacuating ships.