In this topic we are going to cover construction aspect of these ships.
General Cargo Ship: These ships are designed for carrying variety of cargoes in different packings, small volumes and needing lot of accessibility for options in loading and discharging. Earlier days accommodation use to be somewhere in the middle part and Engine room was placed below it. Propeller shaft was encased in a tunnel which use to decrease cargo volumes in the hold. Later on Engine room and accommodation shifted aft to make design efficient.
Cargoes are bagged goods, small cartoons (Boxes), Wooden cases of small to large sizes, Rolls of paper, steel plates, steel rolls, Wire coils, Pipes, drums of various sizes, bulk (Loose cargo), Liquid cargoes, blocks of stones etc. All of them need a flat and string surface. Ship is made up of frames and covered with skin of mainly flat metal plates. Bottom most surface for loading cargo is Tank top of Double bottom tanks or other tanks. These have to be made strong to withstand load. This strength is called “Load density” and measured as metric tonnes/m2. One metric tonne = 1000 kgs and not to be mixed with TON (100 cubic feet). Tanks cover up strong frames. Other factor is when you are going to Boxes one on top of other, somewhere along the line bottom most box is going to collapse. To avoid this, height restrictions will be respected. During designing height of the lower hold, heights are calculated based on cargo projections. Once the height is decided, a deck called “TWEEN Deck” is added. Some ships had 2 tween decks. As expected “Load density” of tween deck is lower than Lower hold as it is supported by lesser frames. To keep ship stable, we need heavier weights in lower holds. Tween decks are added from all sides including F & A bulkheads. In the middle is openable area to make lower hold accessible for loading and discharging. This opening is covered by Pontoons placed over transverse beams. In case of hydraulic covers, transverse beams are built in the covers (Pontoons). Similar opening on main deck is called Hatch covers and it is weather tight. Cargo can be loaded on top of the hatch covers and on deck. They do have their load density defined separately.
Because of exposed frames in cargo holds, capacity is defined in 2 ways. Grain capacity- Capacity of bulk (loose) cargo which can fill spaces between frames and Bale capacity: Bale (Regular box shaped) cargoes which cannot use frame spacing. Frame spacings are left exposed as there is no guarantee of type of cargo. However, there is wooden covering placed on side frames to provide ventilation passage, These fittings called sparrings are removable. Similarly Tank top is covered with wooden planks called floor ceiling. Cargo ships have own cargo handling equipment.
Normally cargo ships do not have ballast passages, hence ballast capacity is limited to minimum.
Bulk carriers: They come in many sizes and do not have Tween decks. Smaller and medium sized ships have own cargo gear. Some ships have specialized cargo handling equipment. Normally they do not have sufficient deck and hatch cover top strength to load cargoes. Lower holds have sufficient strength to support usually handled bulk cargoes. One of the problem of bulk carriers is high GM (Metacentric height) when loaded with heavy ores. High GM makes ships roll very heavily causing higher stresses on structure. Ore carriers have double bottom tanks with higher depths to push Centre of Gravity higher to reduce GM. With grain cargoes there are issues with shifting. Therefore Upper side ends of the hold are shaped in slanting manner towards hatch opening. For ease of discharging lowers sides also are slanted to collect cargo towards center areas. Since cargoes can stick to frames and make cleaning difficult, there is little vertical framing on sides middle portion. Rest of the hold has smooth finish for ease of hold cleaning. Larger bulk carriers have one or two cargo holds which are used for ballasting purpose on ballast voyages.
Oil tankers: Oil tankers can be of any size and carry liquid inflammable cargoes. Due to pollution risk, all oil tankers have double skin. As expected they have Centre and side tanks. For ease of cleaning, attempts are made to use Corrugated framing in the cargo tanks and other frames are located in ballast tanks. Ballast tanks surround cargo tanks from all sides. During loaded passages, they form empty space layer around cargo tanks. Tankers have pump room where cargo pumps are located. Ballast tanks are segregated to avoid risk of pollution. Tankers have inert gas system by which inflammable cargo vapours do not come in contact with atmospheric oxygen to set off possibility of fire. Crude oil tankers also have crude oil washing system for cleaning tanks. Tankers are Type A ship means they have lower freeboard comparing to cargo ship of same size. This concession is given because they have very few small openings on deck (No hatch covers). For this lower freeboard, they have Raised safety walkway on deck for use in rough weather. They load and discharge cargo through pipelines. Place where you connect ship’s pipes to shore pipes by means of flexible hoses(pipes) is called MANIFOLD. There is a small crane provided to handle flexible hoses on either side. There will be a deck house/mast house on either side to store required gear and Anti-Oil pollution equipment.
There are venting pipes on deck for venting cargo tanks.
Chemical tankers and Petroleum product carriers: They have double skin like tankers. They do not have crude oil washing system, but have inert gas system (IGS). Construction wise they are identical to Oil tankers but may have more number of tanks to segregate cargoes.
Container ships: They are designed to carry Boxes of 20ft x 8 ft x 8ft 6 inch sizes. Some boxes have length of 40 ft or 45 ft. Width is always the same 8 ft. Height variation may be there. Cargo holds have smooth finish to tightly pack containers. Since boxes are stacked one on top of other, they need securing arrangement. In Holds, it is achieved mainly by using cell guides, Smaller and older ships may have manual arrangements using special lashing gear. Load of stack of containers is taken by four corners only. Therefore Load density is defined as “Stack weight” instead of tonne/m2. Most of the ships carry over 60% of cargo on deck. Containers are loaded on hatch covers (Usually single piece Pontoons weighing upto 45 t). On sides there are container stools in level with hatch cover upper edge. Containers on deck are lashed together using twist locks, lashing rods and turnbuckles. Fixed Lashing bridges are used to lash containers. Big container ships carry about 200,000 t of cargo. Container ships do not have ballast passage. Ballast is used for controlling stresses due to uneven weigh distribution. Ships have Double bottom tanks and side wing tanks. On newer ships Fuel oil tanks are on top of Double bottom. Ships load upto 9 high stacks under deck and 9 high stacks on deck. Accommodation may be forward, somewhere in the middle or right aft. Ships have anti-rolling arrangements like stabilizers. They also have automatic heeling tanks to avoid LIST during loading/discharging. Ships also have under deck passage to access fore and aft areas during rough weather.
Passenger ships: They are floating hotels. All cabins must have porthole showing sea (Natural light requirement). All other activities are managed in areas between the cabins. All residential spaces are above water line. Below water line areas are used for various machinery, stores, galleys, Water and fuel oil tanks, laundry, fresh water generators etc. You need some imagination to understand Mid-ship section. Mid-ship section is taken at amidship but stern area will be much different due to Engine room. All areas above water line are used for comfort of passengers. You can expect shopping areas, Open decks, number of swimming pools, Sun decks etc. They have movie halls and concert areas. Just think of anything that makes you comfortable.
Basically these ships have large number of horizontal separation like floors in building. There is vertical separation for sub division of ship for safety, but it is not noticeable as it is below waterline. There are Watertight doors on passenger ships below waterline. Presence of large Life saving and Fire fighting equipment makes life challenging for deck officers and engineers.
I suggest you to see some videos on “YOUTUBE” to get idea of passenger and other types of ships.
RO-RO ships: Construction wise they are similar to Passenger ships due to presence of horizontal separation to park vehicles. In this case we have large number of decks to stow vehicles. Stern part has engine room and loading ramp. From loading ramp, roads will lead to all possible decks. Once engine room is passed, lanes will take you to lower level decks. Officers and crew accommodation is placed on top of uppermost deck. Some ships also have side ramps. Try and imagine “Parking building” with entrance from stern of the ship. Ships make ballast passages and have Double bottom and side tanks. Ship side frames are not covered in general. All decks have simple and efficient arrangement for lashing vehicles. All decks are gas proof. These ships have very high capacity ventilation system. Similarly there is very efficient fire detection and extinguishing system. Pure car carriers are similar ships in design. In the past large windage areas and small GM’s have caused some ships to topple over. Many ships have adjustable height for some decks. Internal adjustable ramp system connects decks and ramps are flushed with decks after use.
Gas tankers: They are a great challenge for building. First a base ship with 3 sides is made and then tanks are inserted. Tanks are covered with deck plating. Challenge is to carry gases as liquids at temperatures like -1500C, sometimes under lot of pressure. Tanks are made up of inert metals like stainless steel, Insulated and placed on top of ships frames. There is lot of progress in technology for tanks, loading and discharging. However they need a base in form of ships hull. This base is like other ships and special tanks take place of cargo holds.
Combination carriers: They are ships with ability to carry different types of cargoes. At one stage we saw OBO (Oil Bulk Ore) ships but due to changes in trade they disappeared. Combination carriers are not common today. We have Bulk carriers of up to 70,000 dwt with own cargo gear doing multipurpose operations. They carry bulk, steel and lumber. Some ships are combination of RO-RO and containers. Designs will differ from ship to ship.
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