LEGAL CONSTRAINT OF GENERAL CARGO VESSEL
In ensuring safety of tankers at sea, ship builders and owners need to observe all legal requirements imposed through the various conventions and codes introduced by the IMO to enable their ships to qualify for the Certificate of Class for Hull and Machinery issued by a ship classification society recognized by the flag state. Among others, the International Convention for the Safety of Life at Sea (SOLAS), 1974 includes special requirements for General Cargo Vessel. The International Convention for the Safety of Life at Sea (SOLAS), 1974, currently in force, was adopted on 1 November 1974 by the International Conference on Safety of Life at Sea, which was convened by the International Maritime Organization (IMO), and entered into force on 25 May 1980. A general cargo ship is any sort of ship or vessel that carries cargo, goods, and materials from one port to another. Thousands of cargo carriers ply the world's seas and oceans each year; they handle the bulk of international trade. (SOLAS, CHAPTER II-1) Cargo vessels have their own rules and regulations on many aspects such as construction that include structure, subdivision and stability, machinery and electrical installations.
Figure 1: Single Decker of General Cargo Vessel
The Single Decker of general cargo vessel is a flexible design of vessel which will go anywhere and carry a wide variety of cargo. The cargo may be break bulk or containers. (SOLAS, CHAPTER II-1, Part B) The vessel to be built with a single screw, diesel engine general cargo ship having a continuous freeboard deck, forecastle, poop, bulbous stern and bulbous bow. Above poop deck there are four tiers of deckhouse. It’s to have double bottom and top side tank in way of cargo area. Double bottom tank and side tank may be used for ballast water tank and light oil tank. Such vessels have several large clear open cargo-carrying spaces or holds. One or more decks may be present within the holds. (SOLAS, CHAPTER II-1, Part B) These are known as ’tween decks and provide increased flexibility in loading and unloading, permit cargo segregation and improved stability. (SOLAS, CHAPTER II-1, Part A-1, Reg 5) Access to the holds is by openings in the deck known as hatches. Hatches are made as large as strength considerations permit in order to reduce the amount of horizontal movement of cargo within the ship. Typically the hatch width is about a third of the ship’s beam. Hatch covers are of various types. Pontoon hatches are quite common in ships of up to 10 000 dwt, for the upper deck and ’tween decks, each pontoon weighing up to 25 tonnes. A pontoon hatches a used as grain bulkheads. They are opened and closed using a gantry or cranes. (SOLAS, CHAPTER II-1, Part A-1, Reg 3-6) In large bulk carriers side rolling hatch covers are often fitted, opening and closing by movement in the transverse direction. Another type of cover is the folding design operated by hydraulics.
Figure 2: Hold off on General Cargo Vessel
Figure 3: An almost completely discharged Figure 4: General Cargo with a partially
hold opening hatch
The coamings of the upper or weather deck hatches are raised above the deck to reduce the risk of flooding in heavy seas. They are liable to distort a little due to movement of the structure during loading and unloading of the ship. This must be allowed for in the design of the securing arrangements. Coamings can provide some compensation for the loss of hull strength due to the deck opening. (SOLAS, CHAPTER II-1, Part A-1, Reg 12-1) A double bottom is fitted along the ship’s length, divided into various tanks. These may be used for fuel, lubricating oils, fresh water or ballast water. Fore and aft peak tanks are fitted and may be used to carry ballast and to trim the ship. Deep tanks are often fitted and used to carry liquid cargoes or water ballast. (SOLAS, CHAPTER II-1, Part A-1, Reg 12-1) Water ballast tanks can be filled when the ship is only partially loaded in order to provide a sufficient draught for stability, better weight distribution for longitudinal strength and better propeller immersion. Cranes and derricks are provided for cargo handling. Typically cranes have a lifting capacity of 10–25 tonnes with a reach of 10–20 m, but they can be much larger. General cargo ships can carry cranes or gantries with lifts of up to 150 tonnes. Above this, up to about 500 tonnes lift they are referred to as heavy lift ships.
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. (SOLAS, CHAPTER II-1, Part B-1, Reg 25-9)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.
Cargo ships are designed to be easy to build and to store cargo efficiently. To facilitate construction, bulkers are built with a single hull curvature. (SOLAS, CHAPTER II-1, Part B, Reg 11) Also, while a bulbous bow allows a ship to move more efficiently through the water, designers lean towards simple vertical bows on larger ships. Full hulls, with large block coefficients, are almost universal, and as a result, cargos are inherently slow. This is offset by their efficiency. Comparing a ship's carrying capacity in terms of deadweight tonnage to its weight when empty is one way to measure its efficiency. Cargos have a cross-section typical of most merchant ships. The upper and lower corners of the hold are used as ballast tanks, as is the double bottom area. The corner tanks are reinforced and serve another purpose besides controlling the ship's trim.
Figure 5: Tranversely framed double bottom
(SOLAS, CHAPTER II-1, Part B, Reg 12-1) The double bottoms are also subject to design constraints. The primary concern is that they be high enough to allow the passage of pipes and cables. These areas must also be roomy enough to allow people safe access to perform surveys and maintenance. On the other hand, concerns of excess weight and wasted volume keep the double bottoms very tight spaces. The cargo ships hulls are made of steel, usually mild steel. Some manufacturers have preferred high-tensile steel recently in order 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.
Figure 6: Longitudinaly framed double bottom
The machinery spaces are often well aft but there is usually one hold aft of the accommodation and machinery space to improve the trim of the vessel when partially loaded. General cargo ships are generally smaller than the ships devoted to the carriage of bulk cargos. (SOLAS, CHAPTER II-1, Part C, Reg 27) The engine room on a vessel is usually near the stern, under the house and above the fuel tanks. Larger of the Single Decker, 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 Single Decker of Handysize and above is between 9.5 and 12 knots (21 km/h). The propeller speed is relatively low, at about 2190 KW or 250 revolutions per minute. Propeller is AU type 5 blades, diameter 3.616m, disk area ratio 0.50, and pitch ratio 0.71, material Cu3 Ni-Al-Bronze.
Figure 7: Main Engine(SOLAS, CHAPTER II-1, Part D, Reg 43) Electric installations in ships are a very complete part of electrical engineering as they cover every aspect of power generation, switch gearing and distribution to every type of consumer. Also all types of automation and remote control are part of it as well as internal and external communication, navigation and nautical equipment. However, the basic difference with shore based electrical installations is the necessity to be self-supporting: that is, either to have the personnel and necessary spares on board, or to have the required redundancy to be able to reach the next port in case of a failure of a single system or component.
Figure 8: Main Switchboard of Single Decker vessel
Switchboards and other switchgear assemblies basically serve to connect and disconnect generators and consumers to the main power supply system. They contain also the protection devices of the generators, the cables and the consumers against overload and short-circuit. (SOLAS, CHAPTER II-1, Part D, Reg 43) Switch-boards and other control-gear assemblies can be operated by engineers, but servicing and maintenance and repairs should be carried out by specialists. Duplicated essential consumers shall be supplied each from a side of the switchboard, or when supplied from distribution-boards from separate distribution-boards, each supplied from a side of the main switchboard. All of this with the same target that a single fault does neither impair the propulsion system nor impair the habitability for the crew. This single failure also includes a fire or other damage to a cable tray. Therefore the power cables and control cables to essential duplicated consumers shall be separated. Synchronizing equipment must consist of a check synchronizer, blocking a-synchronous closing of circuit breakers in any mode, also manual. The final emergency mode of closing (pressing the mechanical controls at the circuit breaker front) is allowed to be unprotected. Further the synchronizing equipment is to consist of a double voltmeter and a double frequency meter giving the voltage and frequency of the busbar and those of the incoming machine. These instruments may also be replaced by a multi-function instrument per generator which enables the read-out of voltages between the phases and between the phases and the neutral if applicable, phase currents, power, reactive power, frequency etc.
Figure 9: Synchronizing Panel showing as a main circuit breaker and synchronizing equipment
The Cargo vessel is a damage stability to calculate according to the requirement of (SOLAS, CHAPTER II-1, Part B-1). The ingress water in damage area carrying fuel oil, fresh water and ballast water or cargo oil to be sea water (density = 1.025) in all damage condition. Damage stability criterion is as following: 1) Final waterline considering immersion, heel, trim shall happen below any edge of opening lower. 2) On immersion final stage, heeling angle by non-symmetry immersion shall be less than 15o, but if deck edge haven submersion appearance, the angle can be increased to 17o 3) Restoring lever curve at scope outside of the equilibrium shall be more than 200, max. Remainder restoring lever in 20o scope is higher than 0.1m and area of the curve at this scope is more than 0.0175 mm rad, in this condition, the stability to meet the requirement. After the calculation of light weight and height of gravity center from the results of inclining test, checking computation of the damage stability at 6000t operating condition to be performed.BIBLIOGRAPHY
1) IMO (2004), SOLAS, Consolidated Edition, IMO Publication London.
2) Van Dokkum, Klaas (2006), Ship Knowledge, Covering Ship Design, Construction and Operation, 3rd Edition, DOKMAR, Enkhuizen.
3) E. C. Tupper, BSc, CEng, RCNC, FRINA, WhSch, Naval Architecture, 3rd Edition, OXFORD.
4) Martin Stopford (2002), Maritime Economics, 3rd Edition, Routledge, London.
5) D.J.Eyres (1984), ship Construction, M.Sc., F.R.I.N.A.,William Heinemann Ltd.
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