Maersk Line is thought to have added 55,000-teu of capacity to its fleet at a relatively low cost by a series of 'capacity boost' programs.

Some 40 ships are said to have been involved in five such programs since the concept was introduced in 2011, according to Alphaliner.

"The upgrades are carried out by raising the wheelhouse by the equivalent height of two high cube boxes, so that two extra tiers of containers can be stowed forward of the accommodation block without impairing visibility," the Paris-based consultancy says.