Classification of Loads And Design Features of Container Ships

The earliest cargo ships were used to transport bulk cargo. On most general cargo ships, wooden containers or boxes were also used to carry groups of goods. However, as the types of cargoes included became more and more numerous, containerized transport was considered suitable for carrying small, fragile and cargoes that required special care during transportation. Therefore, it was considered necessary to design a special type of vessel that would not only be able to carry these containers, but would also be able to meet the security conditions of the cargo contained in the containers.

This article focuses on explaining how the type of cargo, the ports on the route, the function of the ship, and the economic trends in the market affect the design of a particular type of ship. In addition, before we can understand the design of a container vessel, we need to have a basic understanding of basic ship terminology and hull geometry.

▋ Classified According to the Way the Goods are Loaded

1. The box container vessel is the most common type of container ship design. This type of vessel can only carry sea containers, which are loaded and unloaded via the port's shore bridge. Such vessels operate in major container ports equipped with shoreside container cranes

2. Roll-oncontainer ships This type of container vessel can use container towing trailers and forklifts to load containers into or carry containers out of the ship's hold. In some cases, this type of ship loads both vehicles and containers on different decks.

3. Crane vessels are equipped with on-board lifting equipment for lifting/unloading containers. This type of vessel usually calls only at smaller container terminals that are not equipped with onshore container handling cranes like the larger terminals.

▋ Classification According to the Scope of Service Routes

Motherships - large vessels, usually with a capacity of 10,000 TEU  more, that sail only to and from major container terminals in a country. Due to their large draft and size, these ships cannot enter smaller ports.

Feeder Vessels - Feeder vessels operate between large and small container terminals, carrying containers to and from the mother ship. Typically, these vessels are lifting vessels, as smaller ports are not equipped with shoreside container cranes.

▋ Container Vessel Midship Profile

Qinhai Shipyard,One of the most important structures when designing a container vesselis the Midship profile.There are many design and functional aspects to consider when building a midship profile,Includes type of cargo, stowage method of the ship, ship's capacity, etc.In fact, the type of ship is usually determined by the midship profile.

Below are Some Common Features of Mid-ship Profiles on Container Vessels:

1. All container vessel are double-bottomed, and double-bottomed bilges are set up at the bottom of the ship where containers cannot be loaded and can be used for ballast water.

2. container vessel are vertical straight-wall structures, and the variable loading conditions tend to make the mid-arch bending moment and the mid-sagging bending moment larger, resulting in larger longitudinal bending stresses.

3. Midship profile Almost boxy in shape.In professional terms, this means that the mid- and cross-sectional product coefficients are high, ranging from 0.75 to 0.85.

4. The cargo hold of a container vessel has no hatch cover. This means that the ship does not have a continuous main deck running the entire length of the ship. This open box-like structure makes it easy to load containers from the inner sole to the uppermost level of the main deck. The only deck is in the double-deck hull, which is more like a full-length longitudinal beam and provides access in the direction of the ship's captain.

▋ Container Loading

1. Containers are always stowed from front to back along the length direction, which is because the ship is more prone to transverse rocking rather than longitudinal rocking or bow rocking. Stowage in this way ensures less space for the cargo inside the container to move, which makes the cargo inside the container less impacted and ensures the safety of the cargo.

2. The containers in the compartment are fixed by means of box grid rails so as to inhibit their lateral or longitudinal movement. This device is mainly used to hold the containers in place by their corner positions, and can also be used as an aid when the containers are loaded. However, these are not part of the main structure, i.e. they do not bear hull stresses.

3. Containers on deck are secured by various lashing devices. Twist locks fitted between containers prevent vertical movement, and lashing bars prevent containers from moving longitudinally and laterally. Lashing is usually carried out on lashing bridges, which are spaced at the height of one or two layers of containers. The lashing rods are secured at their ends by flower-blue screw bushings that maintain tension on the tie-down rods.

Qinhai Shipyard,The loading plan for containers is generated with the layout plan and specifies the location requirements of the different containers on board under different loading scenarios. This plan takes into account the number of different containers and cargo weights as well as the distribution of loads on each voyage and the port at which each container is to be unloaded. If a ship calls at three ports A,B,C, and if all the containers are loaded at port A, then a container that will be unloaded at port B should preferably not be placed under a container whose port of discharge is port C. This is a problem that arises when a container is unloaded at port C. The container should not be placed at port B, but at port C, which is the port of discharge. But this raises the question, if most of the containers to be unloaded in port B are heavier than those to be unloaded in port C, how to do? This is because heavier containers cannot be stacked on top of lighter containers, which raises the ship's center of gravity and makes the stability margin lower.