I would like to enlighten the basics, which will understand the whole picture. And another good book will help me out. It's named "Bulk carrier practice" by Captain J Isbester. It was written in 1990's, so the information might be outdated, but the basics are still the same:
Bulk Carriers, or bulkers, are ships designed primarily for the transportation of solid bulk cargoes. Such cargoes are generally uniform in composition, and are loaded directly into the cargo space without any intermediate form of containment. The range of cargoes carried in bulk carriers is considerable. Leading bulk cargoes in world trade are iron ore, coal, grain, bauxite/alumina and phosphate rock, along with substantial quantities of concentrates, petroleum coke, steel, ores, cement, sugar, quartz, salt, fertilizers, sulphur, scrap, aggregates and forest products.
In 1990 world seaborne trade in iron ore and black coal was in each case about 350 million tonnes, with 190 million tonnes of grain being shipped. Illustrative of minor bulks are cement and sugar, of which more than 20 million tonnes each were carried in 1990.
The bulk carrier evolved from the closed shelterdeck cargo vessel (the 'tramp'), and the ore carrier, in the mid-1950s and thereafter developed rapidly in size and number. Whilst in 1960 only about one-quarter of bulk cargoes were carried in single-deck bulk carriers, the situation was transformed by 1980 at which time almost all bulk cargoes were transported by bulkers. Changes to international rules, such as the 1966 Load Line Convention and the IMO Grain Rules, enabled designers to take advantage of the inherent stability of the bulk carrier, and to develop its self-stowing characteristics. A further evolution in design took place in the 1980s with the conbulk vessel designed to meet the changing pattern of international trade, and to accommodate in particular the growth of containerised cargoes.
Handy-sized bulkers:
On 1 January 1990, the world fleet of bulk carriers of 10,000 tonnes deadweight and upwards, excluding bulk carriers trading only on the North American Great Lakes, totalled approximately 5,000 vessels.20 The most common size of bulk carrier was the handy-sized vessel with a displacement of 25,000-50,000 tonnes and a draft of less than 11.5 metres. There were some 2,000 handy-sized vessels in service in 1990, and a further 1,500 vessels in the 10,000-25,000 tonnes deadweight range which, though smaller than handy-sized vessels, possess the same characteristics and can conveniently be grouped with them.
The handy-sized bulker is so called because her comparatively modest dimensions permit her to enter a considerable number of ports, worldwide. Such vessels are used in the many trades in which the loading or discharging port imposes a restriction upon the vessel's size, or where the quantity of cargo to be transported requires only a ship able to carry 50,000 tonnes or less.
Handymax bulkers:
The trend is for each category of bulker to increase in size, and some commentators now consider the larger handy-sized bulkers, in the 35,000-50,000 tonnes range, to be a separate category, the handymax bulker.
Panamax bulkers:
Larger than the handy-sized vessel is the Panamax bulk carrier, so named because she is designed to the maximum dimensions (particularly the maximum breadth) which can pass through the Panama Canal. The limiting dimensions for canal transit are loa 289.5 metres, extreme breadth 32.3 metres and maximum draft 12.04 metres. Some Panamax vessels have summer drafts in excess of the canal limit, so can only pass through the canal partly loaded. The service speed of modern Panamax vessels is typically 14 knots.
In 1990 there were about 800 Panamax vessels of 5,000-80,000 tonnes deadweight. Panamax bulkers are extensively employed in the transport of large volume bulk cargoes such as coal, grain, bauxite and iron ore in the longhaul trades. The fact that most United States ports can accept no ships larger than Panamax size is an important factor in their continued popularity.
Some analysts expect wide-beamed shallow-drafted bulk carriers of up to 100,000 tonnes deadweight to be developed in the 1990s to carry coal more economically between the United States and European ports.
Cape-size bulkers:
Cape-size bulk carriers (Fig 1.33) have deadweights in the range of 100,000-180,000 tonnes deadweight. Whilst most lie within the 100,000-140,000 tonnes bracket, new buildings in recent years have been concentrated in the 140,000-160,000 tonnes range. Cape-sized vessels, with loaded drafts usually in excess of 17 metres, can be accepted fully laden at only a small number of ports worldwide and are engaged in the longhaul iron ore and coal trades. The range of ports which they visit is increased by the use of two port discharges, the ship being only part laden on reaching the second discharge port. Service speeds of modern Cape-sized vessels are typically 12.5-14 knots.
The tendency towards a gradual increase in deadweight of ships which has occurred within this category over time has been noticeable within the handy-sized and Panamax categories, too.
VLBCs:
There were about 65 very large bulk carriers (VLBCs) in service in 1990, mainly employed on the Brazil/Europe and the Australia/Japan routes. VLBCs are bulkers greater than 180,000 tonnes deadweight. A number of these largest vessels are special types such as ore carriers, ore/oil carriers and OBOs, classes which are discussed below.
Mini-bulkers:
In addition to the 5,000 bulkers grouped into the handy-sized, Panamax, Cape-sized and VLBC categories already described, there are engaged in international trade a considerable number of small bulk carriers of less than 10,000 tonnes deadweight which are employed primarily in the coastal, short-sea and middle trades in European waters and other parts of the world, carrying smaller consignments of bulk cargoes to smaller ports. Such vessels are sometimes called mini-bulkers. Some 2,500 mini-bulkers operated in European waters in 1984, though a large proportion of these vessels were equipped and certificated for unrestricted trade, and some do trade worldwide.
Reasons for varied sizes of bulk carriers: Receivers of bulk cargoes have very varied requirements for tonnes delivered per month or per year. The size of vessel that they choose to carry their cargoes and the frequency that such vessels are employed will be influenced by a variety of factors, including the receivers' storage capacity, depth of water in the berth, regularity of the demand for the commodity, and the financing of its purchase. Less frequently the size of vessel chosen will be governed by the limiting size for the loading port. This variety in demand has created a versatile world fleet of very varied ship sizes.
Geared bulk carriers:
Many handy-sized and minibulkers and a few Panamax vessels are equipped with shipboard cranes or derricks for the loading or discharge of cargo: such vessels are described as geared bulk carriers. Loading or discharging by means of ships' cranes or derricks fitted with grabs is normally a comparatively slow means of cargo handling, most useful in ports which are poorly equipped for handling bulk cargoes.
Self-unloaders:
Self-unloaders are bulk carriers equipped with conveyor belt discharging systems with booms which can be swung out from the ship to discharge directly ashore. Such systems are capable of achieving discharging rates similar to those of shore-based unloading equipment. This equipment is expensive to install and reduces the space available for cargo, but these disadvantages can be outweighed in the short-sea trades by the ability to reduce time spent in port substantially. The numbers and sizes of self-unloaders are increasing, and their role seems likely to continue to grow. In a significant development British Steel, the UK's leading steelmaker, in 1991 introduced two 96,000 tonnes deadweight self-unloaders, mv Western Bridge and mv Eastern Bridge, to serve its Port Talbot terminal.
Ore carriers:
In 1990 ore carriers (Fig. 1.2) constituted about 10 per cent of Cape-sized bulkers. They are single-deck vessels designed specifically for the carriage of iron and other heavy ores. They are characterised by small holds with high centres of gravity. Few ore carriers have been built in recent years, but those few have been amongst the largest bulk carriers afloat, and include the Berg Stahl, 359,006 tonnes deadweight which was, in 1990, the largest bulker in service.
Ore/bulk/oil carriers:
Ore/bulk/oil carriers are designed with dual-purpose holds which can be used for the carriage of both dry and liquid bulk commodities. Most vessels of this type are also strengthened for the carriage of ore and are referred to as OBOs. Most OBOs are Panamax or Cape-sized.
Ore/oil carriers:
Ore/oil carriers have centre and side compartments. When carrying oil both centre and side compartments can be used, whilst only the centre compartments are used when carrying ore.
Combination carriers:
Ore/oil carriers and OBOs, known collectively as combination carriers, were developed to allow the vessel to trade in either the bulk carrier or tanker trades, according to seasonal or commercial demand, and to allow the proportion of time spent in non-revenue-earning ballast legs to be minimised. In the 1980s there was a steady decline in the combination carrier fleet which by 1989 had shrunk to 17 percent (by total deadweight) of the bulk carrier fleet from a high point in 1975 of 43 percent.
In practice, it has not been found easy to switch ships between oil and bulk cargoes on a voyage-to-voyage basis because of the effort required to clean holds to an acceptable standard after carrying oil, and the difficulty in matching discharge and loading ports without long ballast hauls. The declining popularity of combination carriers was probably hastened by their higher capital cost and the fact that they have been prone, statistically at least, to the types of accidents experienced by both tankers and bulk carriers.
Open bulk carriers:
Open bulk carriers are designed for the carriage of packaged lumber and units of wood pulp. Such cargoes are best carried in rectangular cargo holds with an open hatch layout in which hopper tanks are replaced by straight hold sides, and the entire hold is open to the sky when the hatch covers have been opened; these are features of the type. Open bulk carriers are usually geared, some being fitted with fixed cranes whilst others are provided with travelling gantry cranes.
Conbulkers:
Bulkers fitted out for the carriage of containers were developed from open bulk carriers when it was realised that this type of bulker was well suited for container carrying when circumstances were right. The necessary modifications consisted of suitable strengthening of tanktop and hatch covers, and provision of container securing sockets. The shipboard cranes fitted to conbulkers are of a capacity appropriate for containers, and service speed is likely to be 14 or even 16 knots-higher than typical bulker speeds, as appropriate for the carriage of higher value commodities. Conbulkers are able to carry a wide range of bulk cargoes as well as timber and containers.
"Bulk carrier practice" by Captain J Isbester
Please also see the article at wikipedia.org
Another nice article with same subject opensea.pro
Well explained about bulk carriers. Thanks for sharing.
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