Container ships are the ones in charge of transporting standardized containers and they are used to transport any kind of cargo all over the world.
The standard containers are part of the "Inter-modal freight transport" which allows transporting of (in an easy, fast and efficiency way) the freight between the different modes (ships, trains or trucks) without any handling of the cargo itself when changing modes.
The 95% of the standardized containers(as will be discussed) are from 20 or 40 feet long.The dimension of the ships depends on the number of containers that it is supposed to carry with.
|Containers of Sea-Land Inc. at the port of Hoboken|
in the 1930s
The classification of container ships may be made based on the following points:
1. Developing generations: based on capacity, determined by the number of TEU
2. Mode of Cargo Handling:
- Box ships-(most common type of container ship, the containers are loaded from the outside with special cranes.)
- LoLo (See our article on RoRo/LoLo vessels)
- RoRo(Same one as above)
3. Ship generation:
- Panamax (14,501 TEU)
- Post-Panamax (10-14,500 TEU)
- Suezmax (5,101-10,00 TEU)
- Post-Suezmax (3,001-5,100 TEU)
- Post Malacamax (2,001-3,000 TEU)
- Others (Feeders,Small Feeders)
4. Level of specializing:
general cargo, semi container, purpose-built, container
5. Service range: feeder ships, mother ships.
|A Modern Day Container Ship|
"Inter-modal" indicates that the container can be moved from one mode of transport to another without unloading and reloading the contents of the container.
Container capacity is described in twenty-foot equivalent units (TEU ). An equivalent unit is a measure of containerized cargo capacity equal to one standard 20 ft × 8 ft (i.e; length × width)
|A typical 40 ft long shipping container|
As this is an approximate measure, the height of the box is not considered; for example, the 9 ft 6 in high cube and the 4-foot-3-inch half height 20-foot containers are also called one TEU.
The 45 ft containers are also commonly designated as two TEU, although they are 45 feet and not 40 feet.
Similarly Two TEU are equivalent to one forty-foot equivalent unit (FEU).
Huge investments in containerization have paid off and container traffic continues to grow.This growth will continue to grow until the original aim of containerizing every adjoining mode of transport has been accomplished.
BEHAVIOR OF CONTAINER SHIPS IN MARINE ENVIRONMENT:
Three main types of conditions due to cargo loading can be found:
Mainly caused by the pressure due to the piling up of the containers that can brake cargo. This pressure depends on the dimension, and number of units piled up.
Produced during the load and unload, transportation and moving of the containers. These loads can produce accelerations, vibrations and sometimes shaking due to the movement of water.
Some examples of dynamic loading and how they affect the container and the cargo.
Rotation of the ship around its vertical axis. It occurs due to the impossibility of the ship to have a straight direction.
Vertical oscillations:Upper and lower accelerations on the ship beyond its vertical axIs. Only with calmed sea there exist balance on this load. These oscillations affect the containers and its cargo. This can produce the elevation or the sink of the ship due to the movement of the sea.
Linear movements beyond longitudinal and transverse axis:
The ship is accelerated or decelerated to prow and stern, and from one band to another.These loads can produce important torsion loads.
Movement of the ship around its longitudinal axis. Can produce angles from the horizontal normally from 10º to 30º but can raise to 45º.
Movements of the ship around its transverse axis. In this movement the ship is picked up from
the prow and picked down from the stern and vice-versa.
The total loads suffered by a container ship can thus be summed up with a figure like this:
(Note how it is having the same degrees off freedom as the ship's motion itself)
Container ship designer have a very specific architecture to deal with, they have an enormous deck to carry the containers and are sometimes fitted with huge cranes. All of these need to be built around a strong keel in order to obtain stability.On the other hand the hull of container ships is different depending on if they will be used in frozen water or not.
All of these components are made of steel plates and stiffened plates due to the excellent behavior of this material subjected to variable distributed loads (cargo, self-weight, water loads and wave loads). The vessel is assumed to be a simple rigid and flexible beam in which the waves of the sea act and create stresses that produce bending of the vessel, this is known as hogging and sagging.
In order to study the strength of the vessel, it is imperative to distinguish different loads
situation and the behavior of specific parts of the vessel. The study must be done
focusing on these parts:
1. Hull girder strength including torsion strength:
The check is done in order to know the hull girder bending stress, shear stress and warping stress. The figure below shows the deformation and hull girder stress of a large container ship.
2. Local strength plating and ordinary stiffeners:
The results of this study are: thickness of plating, shear area and section modulus of stiffeners,
dimensions and scant-lings of brackets, buckling.
3. Transverse primary members, stringers, floors, girders: The aim is check yielding and buckling.
4. Structural continuity: Is useful to provide information to make possible modifications of connections design.
5. Fatigue: Allows knowing the damage ratio or fatigue life of connections.
One important fact is the behavior against fatigue of the vessel due to water/wave loads. These are every moment changing so the ship is continuously subjected to variable loads. So the weld design of the plates must avoid fatigue induced failure; otherwise the integrity of the ship structure could be in deep trouble.
The assessment must be done in the following details:
• Hatch corners.
• Connection of side longitudinal stiffeners with stiffeners of transverse
primary supporting members.
LOADING OF CONTAINER SHIPS:
Nowadays there is a very wide span of containers types and methods to load the vessel.
1) On most ships which are specially designed for container traffic, the containers are carried lengthwise:
This stowage method is sensible with regard to the interplay of stresses in rough seas and the loading capacity of containers. Stresses in rough seas are greater athwartships than fore and aft.
2) However, on many ships the containers are stowed in athwartships bays or are transported athwartships for other reasons. This must be taken into consideration when packing containers and securing cargo.
This stowage method is not sensible with regard to the stresses in rough seas and the loading capacity of containers.
3).Even unusual stowage methods like this(below), where some of the containers are stowed athwartships and others fore and aft, are used, but they require greater effort during packing and securing operations.
Also, certain lashing/securing combinations and arrangements need to be taken into account.
BENEFITS AND FUTURE TREND OF CONTAINER SHIPS:
As already discussed,container shipping is the most efficient mode of transport for goods. In one year, a single large containership might carry over 200,0 container loads of cargo. While individual ships vary in size and carrying capacity, many container ships can transport up to 8,000 containers of goods and products on a single voyage. Similarly, on a single voyage, some car carrier ships can handle 7,600 cars. It would require hundreds of freight aircraft, many miles of rail cars, and fleets of trucks to carry the goods that can fit on one large liner ship.
Despite the changing production and world trade, the maritime transport has continually grown for the last decades. This growth has been suffered specially by container ships. The use of containers growth is shown by the nowadays construction of bigger ships, reaching some of them "16000 TEU"s.
The international commercial changes and the evolution of the marine transport directly affect the development and expansion of harbors.Therefore, the commercial opportunities that the containers transport gives are a really big challenge to the marine structural engineers. The future of container ship development is promising. LSD
Article By: Sudripto Khasnabis
Author's Note: This article discusses about about The Container Ships from the perspective of their uniqueness and mentions the benefits and structural elements while taking into account it's behavior in the marine environment .The next article will be announced later. The section shall proceed regularly as usual with articles about other ships with a common aim of making the reader aware of the general principles on which their design and construction is based and also their utility and characteristics. The videos and the pictures do not belong to LSD, and full credit for the same goes to their respective owners. If you have any queries or doubts,do not forget to write to me at email@example.com
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