The future of the ultra-large container ship
The biggest container ship deployed has grown at a tremendous rate over the past few decades. The driving force has been international globalisation. Increased competition and economy of scale have fuelled the development of ever bigger ships. Container ships built and on order, TEU v. date of build. The red line gives the contour of the biggest ship deployed at a given time.
In the early 1970s, the biggest ship was about 2,000 TEU, compared to 14,000 TEU today. Is this development going to continue or will it be curbed by global warming and the carbon footprint. What are the factors deciding how big a container ship can be?
Large ships are green ships by virtue of the fact that the fuel consumption per TEU transported is lower. The cost is also lower due to the economy of scale. Two decades ago, studies were published comparing two 4,000 TEU ships to one 8,000 TEU ship and showed a reduced total cost per unit. Today, a comparison between two 8k TEU ships and one 16k TEU shows the same trend. The capital cost for the bigger ship is in the order of 20% less and the fuel cost around 40% less, the exact numbers depend on the building price and fuel price. There is a gain to be made by going for bigger units, in terms of not only the cost, but also the carbon footprint. Slow steaming will also contribute to lower fuel consumption, even if more ships are needed in the loop to maintain the service schedule. So bigger ships going at lower speed are what the world may be looking for in the years to come.
But we all know that big ships need to be filled up to be able to reap the benefits. In times of fluctuating transport volumes, it is prudent to ask if big units provide the best solution for adjusting the transportation service supply to the demand.
What about the technical limitations? Going from the current 14k TEU, which is the biggest ship currently in service (MSC B-series), to 22k TEU ? is that a feasible step from a technical point of view?
A 16k TEU design has been developed in Korea and preliminary data give some indication of the steel dimensions and quality required. The length and beam are close to those of the Emma Maersk series, namely 399m and 57m. The material dimensions in the upper hatch coaming would have to be in the order of 75?85 mm and use HT47 steel. The highest grade currently in wide use is HT40, and the utilisation factor for this quality is not yet fully standardised in IACS.
A 22k TEU design as indicated by STX would have a length of about 470m. This is an increase of 17% compared to the 16k TEU. The bending moment is proportional to the square of the length, leading to a corresponding increase in the section modulus and steel dimensions. So new detail designs and solutions have to be developed to allow for such a significant increase.
So why not increase the beam and reduce the length? The crane outreach, draft, lifting capacity and NPX restrictions are limiting factors. However, design considerations, like resistance and fuel consumption, will also have a significant influence. The total resistance is made up of friction and wave-making resistance. At lower speeds, the friction resistance dominates while at higher speeds the wave-making resistance dominates. The friction resistance is proportional to the wetted surface and a longer ship will have a larger wetted surface than a shorter one with the same beam, draft and dwt. So when the speed drops, a shorter, more beamy ship will have a more economical fuel consumption. Slow steaming and low/moderate fuel prices would therefore not favour the long design.
This evolution is confirmed by looking at a plot of rows versus bays in historical designs.
The designs offered by shipyards tend to stay inside a combination of rows and bays as indicated by the limiting lines in the figure. The 16k design is in the trend channel, but the 22k is far outside it.
What about the engine power needed for such a big ship ? is it available? So far, what we have seen is the single screw with an up to 14-cylinder max-bore slow-speed engine. That could propel the big current designs at speeds of 24?25 knots or more. This has been the solution for all container ships designed up to now. Will this trend continue or will we see other solutions in the future?
For the 22k design, the twin propeller solution may re-surface as an alternative. Two smaller slow-speed engines with two propellers and a skeg design could be an alternative. The higher capital expenditure (some 10% or USD 10?15m) has disqualified this as an alternative for ships that have been built. However, the solution has been studied (Sulzer, some years ago), and the indications are that better propeller efficiency will outweigh the other losses to an extent that increases the overall efficiency by some 3%, contrary to popular belief. So there is a fuel efficiency gain there that will partly offset the increased capital expenditure.
Can we learn from history? We know from the development of the VLCC in the 1960s and 1970s how the ship size increased, driven by economies of scale and increasing global demand for crude oil. Are there any similarities with what has happened to container ships today?
Wijnholst et al wrote about this in the Malaccamax report years ago. They suggested that the 21m allowable draft in the Malacca Strait would be the limiting factor for the biggest container ship that could be built, hence the name. They also presented a graph showing tanker development in which the ship size increased rapidly to about 550 kdwt before decreasing and settling at a common size of 300 kdwt, which is the going size for a VLCC these days. When I showed this graph to people in the container industry, I asked them why it levelled out at 300 kdwt. The answer is puzzling to naval architects and engineers, who focus on technology and operation. A common trading lot or parcel for crude is 2m barrels, which will fill a 300 kdwt tanker, and a tanker to transport the parcel is what is needed, hence the common size.
So it may not be obvious which factors will decide the ?biggest? container ship to be built. I have only touched upon a few factors. Terminal capacity, loading and unloading time, hinterland infrastructure, train and road capacity and so on are others. Maybe globalisation will level out or even decrease in importance.
From a technical point of view, I believe that the 22k design can be built, but time is needed to solve the outstanding issues. Maybe the leap from 14?16k to 22k is too much in one go, and some intermediate-size designs may need to be tested out first. That is not likely to happen in the foreseeable future due to the current market situation. If I were to make a prediction about the size of the typical big container ship in the future, I would tend to say that the NPX (about 12.5k TEU) that will go through the new Panama Canal (366m x 49m) and under the Bayonne bridge is likely to be the predominant size. It combines economy of scale with flexibility and versatility, and that is likely to make it a winner.