what are the different types of ship | Ship construction

There are many different types of ships, and the differences are mostly based upon the type of cargo the ship transports. According to the study the typical layouts of the main type of Merchant Ships are:
a)  General Cargo Vessels (being phased out).
b)  Bulk Carriers.
c)  Tankers.
d)  Container Vessels. 
e)  Passenger Vessels.
f)  Ro-Ro Vessels.

There are also a number types of vessels designed with a specific purpose of transporting special cargos such as ;
a) Vessels for carrying Project Cargo
b) For Diving Support with D-ynamic P-ositioning Vessels.
c) Heavy Lift Capable Vessels.

Bulk Carriers

Structure line diagram of Bulk Carrier
Bulk Carrier

The common general bulk carrier will have double bottom, hopper sides or the top and deck wing tanks.  The hopper or the side tanks may be  used for the carriage of light grain cargoes as well as water ballast.
On single voyages the vessel may carry high density bulk cargoes only in the short holds to give an acceptable cargo distribution or on general bulk carriers with uniform hold lengths where alternate hold loading or block hold loading may be utilized to stow high density cargoes. With such loading arrangements high shear forces occur at the ends of the holds requiring additional strengthening of the side shell in way of the bulkheads.

A general arrangement of a typical bulk carrier shows a transparent cover with the back of the machine. Large hatches with steel plugs are designed to facilitate the fast loading and unloading of cargo. Since the bulk carrier many rides in the ballast provided a ballast capacity to the right propeller for dipping. A general arrangement of a typical bulk carrier shows a transparent cover with the back of the machine. Large hatches with steel plugs are designed to facilitate the fast loading and unloading of cargo. Since the bulk carrier many rides in the ballast provided a ballast capacity to the right propeller for immersion. The size of this type of ship has also steadily increased and bulk carriers have reached 250 000 tones dead weight.

Ships of the general form, experienced a relatively high rate of losses at the end of the eighties and early nineties, the concern raised about the construction and construction. In the nineties, the safety of the bulk carriers has gained considerable attention in the work of the IMO, classification societies and elsewhere, and this work is still ongoing.

Based on the experience of disturbances with minor consequences, it was concluded that the losses caused by a failure of the local structure resulted in the loss of the sealing of the side shell followed by further flooding excessive bending stress helmet due to damaged partitions.

Much of this work has focused on the details of the structure of the fuselage, the loads derived by loading and unloading, damage to the structure and protective coating of discharge loads, poor maintenance and inspection higher inadequate structure of the ship.

The outcome of this work resulted in provision of guidelines,  adopted by the IMO Assembly in November 1993. This incorporated  a concept of the Enhanced Survey Programme for inspections and surveys of bulk carriers and tankers of 15 yrs or more for incorporation in the SOLAS 1974.

MV Derbyshire, a 1976 built and owned by BIBBY LINES  registered at Liverpool as an ORE-OIL-BULK CARRIER. On 09th Sep1980, during encounter with typhoon  Orchid, vessel sank, 230 miles off  Okinawa with  out any trace or initiating a MAYDAY Distress message. When overwhelmed by the tropical storm, vessel was  carrying 157,446 T of iron ore and was “HOVE TO”.  After nearly 14 years in, June 1994, the wreck of Derbyshire  was discovered. Investigation into the strange orientation of the wreck at a depth of 4000 M and  spread over 1.3 km concluded that the ship sank because of structural failure.

Following this evidence IMO directed a revision of the Load Line Convention 1969, for a close look at the adequacy of bow height and strength of hatch covers in the forward part of these ships.

The safe operation of bulk carriers is dependent on not exceeding allowable stresses in the cycle of loading, discharging, ballasting and de- ballasting.

In November 1997 the International Maritime Organization
(IMO) adopted  new rules covering survivability and structural requirements for bulk carriers of 150 M and above requiring  the bulkhead and double bottom to be strong enough to allow the      ship to survive flooding in hold one.

The size of bulk carriers is often referred by one of the following:

Handysize’ the smallest bulk carriers of between 10 000 and 30 000 tones deadweight.
Handymax’ bulk carriers of between 35 000 and 50 000 tonnes deadweight.
Panamax’ bulk carriers designed to be of the maximum size that may transit the Panama Canal and generally being just under 80 000 tonnes deadweight. Old limitations 12.04/32.2/294 m / New limitations – 15.5/49/366 m.
Capesize’ bulk carriers  80 000 to 150 000 tonnes deadweight which are too large for the Panama Canal and trade from the Atlantic around the Cape of Good Hope.
Suezmax, the typical deadweight of a Suezmax ship is about 160,000 tons capable of transiting the Suez Canal in a laden condition. Suez canal has no locks, limiting factors are draft 20.1 m and height due to the Suez Canal Bridge 68 m and typical beam width of 50 m (164.0 ft).


Structure line diagram of Tanker
Fig: Tanker

For optimum economic operation of the tanker, the Service speeds of oil tankers have shown an increase from 12 knots o 17 knots. The optimum size of the tanker is also related to the current market economics. The tanker fleet has grown considerably to keep up with the market.

Structurally one of the greatest developments has been in the use of welding, oil tankers being amongst the first vessels to utilize the application of welding which ensures oil tight joints. The same could not be said of riveting.

Welding allows cheaper fabrication methods to be adopted. Longitudinal framing was adopted at an early date for the larger ships and revision of the construction rules in the late 1960s allowed the length of tank spaces to be increased with a  subsequent reduction in steel weight and making it easier to pump discharge cargoes.

As far as the general arrangement is concerned there appears always to have been a trend towards placing the machinery aft. Moving all the accommodation and bridge aft was a later feature and is desirable from the fire protection point of view. Location of the accommodation in one area is also economical.

The requirements of the International Convention for the Prevention of Pollution from Ships 1973 and Protocol of 1978 have greatly influenced the arrangement of the cargo spaces of oil tankers.
A major feature of the MARPOL Convention and its Protocol has been the provision in larger tankers of clean water ballast capacity. Whilst primarily intended to reduce the pollution risk, the fitting of segregated water ballast tanks in the midship region aids the reduction of the still water bending moment when the tanker is fully loaded. It also reduces corrosion problems associated with tank spaces which are subject to alternate oil and sea water ballast cargoes.

In March 1989 the tanker Exxon Valdez, which complied fully with the then current MARPOL requirements, ran aground and discharged 11 million gallons of crude oil into the pristine waters of Prince William Sound  in Alaska.
The subsequent public outcry led to the United States Congress passing the Oil Pollution Act 1990 (OPA 90). This unilateral action by the United States Government made it a requirement that existing single hull oil tankers operating in United States waters were to be phased out by an early date, after which all oil tankers were to have a double hull.
In November 1990 the US suggested to make double hulls compulsory for new tankers. Other IMO member states suggested alternative designs offering equivalent protection against accidental oil spills.
In 1992 IMO adopted amendments to MARPOL which required tankers of 5000 tons deadweight and above contracted for after July 1993, or which commenced construction after January 1994, to be of double-hulled or middeck construction, or of other design offering equivalent protection against oil pollution.

Studies by IMO and the US National Academy of Sciences confirm the effectiveness of the double hull in preventing oil spills caused by grounding and collision where the inner hull is not breached.

However the mid-deck tanker has been shown to have more favorable outflow performance in extreme accidents where the inner hull is breached.

The United States authorities consider grounding the most prevalent type of accident in their waters. Hence thus while MARPOL provides for the acceptance of alternative tanker designs, the United States legislation does not, accept alternative designs.

MARPOL required that the existing single hull crude oil tankers of 20 000 tons or more deadweight and existing single hull products carriers of 30 000 tons or more deadweight not having segregated ballast tanks could operate after June 2007 and those having  segregated ballast tanks will not be able to operate after July 2021.

Passenger Vessel

Structure line diagram of passenger ship

Several modern passenger ships have had their machinery placed aft; this gives over the best part of the vessel amidships entirely to passenger accommodation. Against this advantage, however, allowance must be made for an increased bending moment if a suitable trim is to be obtained.
Passenger accommodation standards have increased substantially, the volume of space allotted per passenger rising steadily. Tween deck clearances are greater and public rooms extend through two or more decks, whilst enclosed promenade and atrium spaces are now common in cruise vessels. The provision of air conditioning and stabilizing devices have also added to passenger comfort. Particular attention has been paid to fire safety in the modern passenger ship, structural materials of low fire risk being utilized in association with automatic extinguishing and detection systems.
There has been a demise of the larger passenger liner and larger passenger ships are now either cruise ships, short-haul ferries or special trade passenger (STP) ships. The latter are unberthed immigrant or pilgrim passenger ships operating in the Middle East to South East Asian region.

Container Vessels

Container vessel
Container vessel

Containers are re-usable boxes of 2435mm by 2435mm section, with lengths of 6055, 9125 or 12190mm. They are used for most general cargoes and liquid-carrying, gas carrying and refrigerated versions are in use. The latter may have their own independent cooling plant or be supplied with cooled air from the ship's refrigeration system.
The cargo-carrying section of the ship is divided into several holds with the containers racked in special frameworks and stacked one upon the other within the hold space. Cargo handling is by vertical movement of the containers by a special quayside crane. Containers may also be stacked on hatch covers and secured by special lashing arrangements. Cargo holds are separated by a deep web-framed to provide the ship with transverse strength. The structure outboard of the container holds is a box-like arrangement of wing tanks providing longitudinal strength. The wing tanks may be used for water ballast and can be used to counter the heeling of the ship when discharging containers

A double bottom is fitted which adds to the longitudinal strength and provides additional ballast space.
Accommodation and machinery spaces are usually located aft leaving the maximum length of full-bodied ship for container stowage. Cargo handling equipment is rarely fitted, since these ships travel between specially equipped terminals to ensure rapid loading and discharge . Container ships have carrying capacities from 1000 to 2500 TEUs or more. The twenty foot equivalent unit (TEU) represents a 20ft (6055mm) 'standard' container. Container ships are faster than most general cargo ships, with speeds up to 30 knots.

Ro-Ro Vessels 

These vessels are designed for wheeled cargo, usually in the form of trailers. The cargo can be rapidly loaded and unloaded through stern or bow doors and sometimes sideports for smaller vehicles. Some have been adapted to carry containers. loading ramp usually at the after end. Internal ramps lead from the loading deck to the other 'tween deck spaces. The cargo may be driven aboard under its own power or loaded by straddle carriers or fork lift trucks. One or more hatches may be provided for containers or general cargo, served by deck cranes. Where cargo, with or without wheels, is loaded and discharged by cranes the term lift-on lift-off (Lo-Lo) is used.

The structure outboard of the cargo decks is a box-like arrangement of wing tanks to provide longitudinal strength. A double bottom is fitted along the complete length. The machinery space and accommodation are located aft.  Only a narrow machinery casing actually penetrates the loading deck.

Sizes range considerably with about 16000dwt to 28000 displacement tonne) being quite common and high speeds in the region of 18 to 22 knots are usual.

When used as ferries, vehicles usually enter at one end and leave at the other. This speeds up loading and unloading but requires two sets of doors. There has been considerable debate on the vulnerability of Ro-Ro ships should water get on to their vehicle decks.

Various means of improving stability in the event of collision and to cater for human error in not securing entry doors, have been proposed. Since the loss of the Herald of Free Enterprise regulations have been tightened up. The later loss of the Estonia gave an additional impetus to a programme of much needed improvements.

# Special credit to the book "Ship Construction" by D.J Eyres and  G.J bruce (7th edition) which has been used as a reference source for few points included and images used.

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Author Amit                                                                            

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