Stowage of Containers:
The stowage of containers on a container ship is another aspect that the designer has to deal with. Though it may come across as something insignificant,
improper stowage has resulted in most of the accidents related to container ships.
• Containers are always stowed with the longer dimension along forward to aft. This is because the ship is more prone to rolling motions than pithing or yawing. Stowage of containers in this orientation ensures less space for the cargo to shift within the container, providing more safety against impact
damage of the cargo
• Below the uppermost deck, the containers are restrained against the lateral or longitudinal motion by cell guides. These are angle sections that also help as guides for containers when they are loaded onto the ship. However, these do not form a part of the primary structure; that is, they do not take up the hull stresses
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• Above the uppermost deck, containers are stowed, and their motion
is restricted through lashings. Twist locks fitted between the containers prevent vertical movement, and lashing prevents the longitudinal and
transverse motions. The lashings are usually deployed from lashing bridges that are at height intervals of one or two tiers of containers. The lashing
rods are secured at their ends by turnbuckles which maintain the tension in the lashings
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Containers Stacking Not Restricting Bridge view
• The container loading plan is provided along with the design, and it specifies the positions of different containers on the ship, at different load cases. This plan takes into consideration the fact that the number of containers and the
weight of cargo in each would differ on each voyage. And the stowage would also have to take into consideration, the port at which each container has to be unloaded. So, if a ship calls at three ports – A, B, and C, and if all
containers are loaded at A, then a container to be unloaded at port B would not preferably be stowed under a container to be unloaded at port C. But the complicity of the problem lies here – what if most of the containers for Port B are heavier than the containers for Port C? Heavier containers cannot be stowed above the lighter ones, as it would raise the centre of gravity of the vessel, reducing the stability margin
This complicity of container ship design is therefore solved by means of special computer programs specially designed to generate container loading plans for a particular loading case, which keeps in mind, the series of ports a vessel needs to call, and also the strength and stability aspects of the ship. Another factor that is always taken care of in the plan is the visibility from the bridge.
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Container Ships and Their Design
The containers loaded above the deck and forward of the navigation bridge are to be loaded such that the line of sight from the bridge is not affected. That is why, if you take note of a loaded container ship, the stack of containers forward of the bridge reduces in height as one move to the forward-most stack. This,
however, reduces the total amount of containers that can be carried by ship.
Hence, many ultra-large container ships (e.g. Maersk Triple E class) have their superstructures shifted to the midship, in order to be able to accommodate
containers to full height aft of the superstructure.
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Maersk Triple E Vessel
The study of the design of container ships does not stop at this. There are reefer ships which are specially designed to carry refrigerated cargo in refrigerated
containers.
They are equipped with cooling systems connected to each container, which is a different study in itself. Also, recent trends in the market have encouraged the use of slow steaming, which has resulted in most container shipping companies to carry out extensive nose jobs and alteration of propellers on their ships.
Though this might seem to be in contrast with the high-speed requirement of container ships, these ships still operate at higher speeds than oil tankers and bulkers.
Larger diameter and low RPM propellers have seen to offer more propulsive
efficiency. The optimisation of container ships for current industry requirements is something that is dynamic in nature, and this requires ship designers to be
aware not only of newer possibilities of design, but also to be able to predict the trends of the industry a few years ahead of time.
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Container Ships and Their Design
The Purpose of “Torsion Box”
The maritime industry has seen rapid growth in the container transport division followed by an increase in the size of container vessels, a result of increasing demand for container vessels above 5000 TEU.
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However, the increase in the size of the ship and its containers has also given rise to large deck openings, which calls for global maritime investigation into the structure of the hull girder and its effect under torsional and wave bending loads.
Torsion in ships is caused due to forces which do not pass through the sheer centre line axis of a ship’s hull cross-section. Torsion tends to twist the vessel just like how we rinse a cloth by twisting it.
Container Ships and Their Design
The torsional moment has two main components namely:
a) static torsion or still water torsion,
b) dynamic torsion or wave-induced torsion.
Other forms of torsional moments arise from the vibration of the propeller shaft, vibrations due to twin screw propellers etc. As the name suggests, wave-induced torsion is caused due to the unsymmetrical hydrodynamic wave loading on the port and the starboard sides of the vessel. Similarly, still water loading is caused due to the unsymmetrical cargo loading over the port and starboard with the
ship remaining upright.
A ship heading obliquely to a wave will be subjected to righting moments of opposite direction at its ends, twisting the hull and putting it in ‘torsion’. In most ships these torsional moments and stresses are negligible, but in vessels such as large container ships with extremely wide and long deck openings, they are significant.
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Container Ships and Their Design
Ships are designed to withstand the maximum torsional loads due to either static or dynamic or both the torsional moments together. However, in some cases
where there are large deck openings; it becomes difficult to strengthen the
vessel only with the help of hull girder and stiffeners. This leads to the concept of a torsion box.
The example of a bucket can easily understand the strengthening aspect of the torsion box. It is commonly observed that the plastic or steel bucket which we use for the house-hold purpose has a curl to its periphery. This curl is similar to the torsion box used in ships.
If you remove the curl from the periphery of the bucket you will observe that
the strength of the bucket decreases rapidly (mostly in case of plastic), i.e. it can be bend easily with very small forces about its edge. However, with the curl-on, its strength increases to a considerable extent! But how does this curl or in our case the torsion box helps in increasing the strength to a very large extent?
Torsion box in ships can be defined as a continuous structure formed in between
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Container Ships and Their Design
the top part of a longitudinal bulkhead, freeboard deck and sheer strake. It runs from the collision bulkhead and extends up to the aft peak bulkhead. It
is heavily stiffened usually by bulb angles which provide sufficient strength against torsional moments and other bending loads.
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Container Ships and Their Design
Research and FEM analysis of thin-walled beam, taking into considerations the effects of shear and warping, is applied for computation of bending-torsion,
coupling and vibration characteristics of ships with large openings. The contribution of the torsion box towards the torsion rigidity is deduced.
Torsion box in Container ships
Container ships are highly subjected to torsional moments because of their very large hatch openings. This leads to even higher warping stresses at the corners of the openings due to lack of torsional rigidity. The upper part of the double hull in such ships is fitted with torsion box as mentioned earlier.
However, it is not always possible to have a large cross-sectional area, and
therefore, the Naval Architect has to increase the thickness of the plate in order to provide torsional rigidity.
The marginal distance between the hatch end and the side shell is approximately 1.5 ft. This is done to maximise the space for the stowing of containers.
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Container Ships and Their Design
It is often seen that the main deck is subjected to high torsional moments and racking effects and the deck spacing in the way of the hatch opening along the transverse is very less. As a result, the stress concentration can lead to cracking at the corners of the hatches or crack the deck itself! To prevent such failures, torsion boxes are fitted with welded joints on the side shell as on the deck
plating which prevents the torsion produced by twisting.
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Container Ships and Their Design
Uses of the Torsion box
• It helps in preventing torsional bending on ships due to the torsional moment on board the vessel caused by the dynamic movement of the waves.
• Helps in avoiding racking effect caused by the shear stress on the vessel
structure. Therefore, while designing ships with large openings (like container ships), it is often ensured that proper FEM analysis and model testing
procedures are carried out. Proper strength analysis of the hull and deck plating should be done.
At points of stress concentration, i.e., at the corners of the hatch openings,
sufficient stiffening should be provided, and at times the thickness of the deck plating can also be altered without causing any structural discontinuity.
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Container Ships and Their Design
Parametric Rolling in Container Ships
Rolling and Pitching is a part of every ship that is going out at sea. The first
thing you might think upon hearing the word “Parametric rolling” is that it must be a type of rolling movement occurring in ships. Rolling and Pitching is a
normal movement phenomenon which occurs in all kind of ships, so what is new about this?
The difference is that “Parametric Rolling” is a type of movement that is experienced only on Container Ships.
Causes of Parametric Rolling
The size of container ships is increasing drastically as companies are looking forward to monster ships; e.g. Maersk’s Triple-E Vessels. The new container
ships coming to the market have large bow flare and wide beam to decrease the frictional resistance which is generated when the ship fore-end passes through the water, making it streamlined with the hull.
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Container Ships and Their Design
As the wave crest travels along the hull, it results in flare immersion in the wave crest, and the bow comes down. The stability (GM ) varies as a result of
pitching and rolling of the ship. The combination of buoyancy and wave excitation forces push the vessel to the other side.
The similar action takes place as the bow goes down in the next wave cycle resulting in synchronous motion, which leads to heavy rolling up to 30 degrees
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Container Ships and Their Design
in a few cycles. This type of rolling is known as Parametric rolling.
This phenomenon occurs only when the sea condition is in head/stern or
anywhere near to them. There are two pitch cycles- maximum and minimum.
The period of the roll is half the natural rolling period, which coincides with large phase angle and maximum roll always occurs when the ship is pitching
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Container Ships and Their Design
down, i.e. bow is down.
Effects of Parametric Roll
• Heavy stresses in ship structure especially in fore and aft parts
• Extreme stresses on the container and their securing system resulting in failure of the same and even loss of containers
• Unpleasant for the crew of the ship
• Variation in a load of ship’s propulsion engine
• If not tackled quickly, it can result in capsizing of ship What to do in case of Parametric Rolling on ships?
• Do not panic in such a situation. Keep your calm
• If rolling and pitching coincide, avoid a head-on the sea and change the route
• Always maintain a correct GM. A ship should not be too tender or too stiff
• The roll damping measures must be quickly used
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