Concise Guide to Container Ship Design (Part II)
Container Ship Design
In our previous article we gave you an overview of the design and construction principles at the heart of container ship design, as well as container ship hull geometry. Today we move on to the midship section; the torsion box in double-hulled container ships; and the stowage of containers.
Midsection Design
Midsection Design – Image source: Marineinsight.com
Cargo ships are often identified by the type of midship section it has. Most of the structural drawings prepared during the design of such a ship features this section, since it requires plenty of functional considerations, i.e. preferred stowage methods, type of cargo, ship capacity etc. Container ships are always double-bottomed and longitudinally framed with a high midship area coefficient, with the length of the bilge strake kept to a minimum to ensure maximum stowage space. These ships do not have hatches or a continuous main deck, instead featuring an open structure that allows for swift loading.
Torsion Box
Torsion Box – Image source: Marineinsight.com
As with the lip of a conventional bucket or paper cup, which is twisted outward to help it retain liquid without twisting and eventually rupturing, the topmost edges of the port and starboard sides of container ships are strengthened with high-scantling web sections, creating a boxlike structure at every frame (called a torsion box). It runs along the full length of the vessel. Container ships are similar to buckets in that it’s an open structure with a closed bottom. When at sea, these vessels encounter varying wave loads, which in certain cases can cause the hull to twist. This is called torsion. The box structure incorporated in container ships (just like the lip on a paper cup) increases the polar moment of inertia off the structure’s cross section.
Container Stowage
Container Stowage – Image source: Wikimedia
How containers are stowed has a significant impact on the design of container ships. For instance, because ships are more inclined to rolling motions than any other, containers are packed with their length in along forward to aft, which leaves less space for the contents to shift. Additionally, lashings are required above decks to restrict container movement; while cell guides are required to do the same below the uppermost deck. This all impacts on the layout and structure of these ships.
For more information on the fascinating container ship industry and its vessels, keep an eye on the blog in coming weeks and months. We look forward to keeping you in the know.
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