Larger container vessels have recently been designed and built, and this trend is continuing.
Based on the existing damage experience of smaller vessels and several strength issues that tend to become more important for larger vessels, many concerns have been raised in recent years.
A brief summary of some important issues are:
The torsional strength – more hatch opening distortions/deformations
Welding of thick plates – higher possibility of brittle fracture
Material factors for high tensile steel (HT40 and above) – more fatigue damage
Wave-induced vibrations (whipping/ springing) due to more flexible hulls and lower natural frequencies – contributing to fatigue and extreme loading
Location of fuel oil tanks due to new regulations – do not want to lose cargo capacity
Stowing and lashing of higher container stacks – collapse of lower containers and loss of containers; different motion characteristics
Shaft alignment and stern tube bearing loads – hull interaction due to long shafts, large engine and heavy propellers
Effect of Panama Canal widening and harbour restrictions – opens in 2014 and allows larger and more vessels to pass, and possibly the introduction of new trades
Single or double screw (twin skeg) – based on requested speed and future fuel price
Bow and stern slamming – more bow flare and wide, flat sterns and greater possibilities of impact damage, etc.
A few ultra large container ships (ULCS > 10,000 teu) are already in service, e.g. the PS series (Emma Maersk and sisters) from Odense shipyard (Lindø) and the Cosco Asia (with the second sister soon ready) built by Hyundai Heavy Industries. These vessels already show some distinct differences. Samsung Heavy Industries recently presented its newest 16,000 teu design, and below some of its features will be mentioned in light of the challenges stated above and compared to the already existing ULCSs.
First of all, the main dimensions of the new vessel design are length 399m, breadth 57m and depth 30m. The vessel has 24 (40ft) bays, with 20 rows below deck and 22 above. The container stacks in the holds are up to 11 tiers, while on deck there are eight tiers using three-tier lashing bridges.
Samsung has decided to go for a twin-island superstructure concept with accommodation at 0.65L (from aft) and with engine room aft and funnel at 0.2L. The fuel oil tanks of the 16,000 teu vessel are placed below the accommodation, and placing the accommodation quarters forward increases visibility and the container capacity in the bow region by 5–6 per cent compared to placing the accommodation quarters above the engine room. The noise level and likelihood of engine-/propeller-excited vibrations in the accommodation quarters are also reduced by this design. The PS series has a single superstructure at abt. 0.4L. The location of the superstructure on these two vessels reduces the torsional response compared to a conventional more aft location of the superstructure (0.25L), used for instance on the Cosco vessel, which is abt. 50m shorter.
A single screw concept with a 14 cylinder, 108,640 horsepower and 104 rpm (at MCR) engine has been chosen, giving a service speed of 25 knots. This is similar to the PS series and Cosco Asia. The trade is currently intended to be from East Asia to Europe through the Suez Canal, as for other large container ships. Only the Cosco Asia can pass through the New Panama Canal, while these vessels will be post NPX (or rather Suezmax).
The hatch coaming will be designed to have a plate made of HT47 steel, which has not been used so far on existing container vessels, and it is unclear what material factor has been assumed in the design. The upper deck and shear strakes are designed to have plates made of HT40 steel.
The thickness is similar to that on existing 8,000+ teu vessels, but less than on the PS series. The height of the double bottom and width of the double side are otherwise conventional.
The larger vessel has been chosen due to economies of scale. This is also illustrated by Samsung’s comparison of the total costs for one 16,000 teu vessel with those for two 8,000 teu vessels, in which the larger vessel comes out significantly better. This is provided there is enough cargo volume and sufficient handling capacity at terminals. It should also be mentioned that one large vessel is considerably more environmentally friendly than two smaller ones, with a reduction of about 35–40 per cent in fuel consumption. One large vessel is thus the ‘green’ alternative.