The floating container crane concept

Over the last few months, shipping contractor A.P. Moller-Maersk has launched the two largest container ships in the world – the Emma Maersk and Estelle Maersk. Both have the capacity to store about 11,000 twenty-foot containers. These two vessels are the first of a new class of ships, the largest that can pass through the Suez canal.

Ports and terminals have been preparing for years for Suezmax container vessels. Basins have been widened and new quay walls, with sufficient water depth, have been constructed. The major terminals have purchased new cranes with 22 container wide outreach. It seems the port and terminal infrastructure is all in place to receive the new Suezmax vessels. But is this really the case?

The Suezmax concept is a typical product of the principle of economy of scale: A huge vessel, with a small crew, carrying some 11,000 TEU at a time. Because of draft limitations, the Suezmax can only call at the major ports. Because of economic preconditions, the Suezmax vessel will only call at the largest ports with guaranteed large call sizes. Call size may be as large as 6,000-8,000 lifts.

Vessels at sea make money. Vessels in port cost money. The Suezmax vessel will have a high day-rate. So, with the coming of Suezmax there will be even more pressure on terminal operators to handle the vessel in the shortest possible time. Terminal operators will be pressed harder to invest more in high capacity ship-to-shore container handling systems.

It is expected that despite the enormous call sizes, Suezmax vessel operators will eventually press for in-port times of no more then 24 hours. This would require a sustained berth handling capacity of 300 lifts per hour. As the vessel length has not increased proportionally to the call size, this can not be resolved by simply putting more quay cranes on the operation. Depending on the stow plan, it is expected no more than six, or at the most seven, quay cranes can be efficiently deployed on a single Suezmax operation.

Trolley travel and hoisting speeds of quay cranes have increased. In the same period outreach and height have increased. Any productivity gains through speed have been wiped out by the longer travelling path through the larger crane. The average gross crane productivity (in practice) seems stuck at between 30-35 lifts per hour. At that rate, there is a considerable gap between what can be achieved with currently available cranes and what the Suezmax vessel operator asks for.

Traditional solutions

So far, there are basically two concepts in the approach to the challenge of increasing container handling capacity on large container vessels. The first is increasing the crane capacity.

For increasing the crane throughput capacity two approaches are followed. One is the multi cycle crane and the other is the multi lift spreader. The two approaches do not exclude one another and could, in principle, be used in combination.

Ship-to-shore handling of containers consists of a chain of activities. If the capacity of one link is increased it does not mean that the capacity of the chain, as a whole, increases. After increasing the crane capacity, transport between quay and stack appears as the next bottleneck. In order to reap the fruits of increased crane capacity, quayside transport needs to be optimised and that is quite complicated.

The second approach is increasing the ship-to-shore interface length with the ship. Here again there are two approaches. One is the indented berth and the other is the floating container crane. The two cannot be used in combination. So far we have handled container vessels mostly on one side only. But it is very well possible to work on both sides at the same time. This effectively doubles the ship-to-shore interface length.

In the indented berth, also known as the “slip”, the vessel is handled on both sides simultaneously and up to twice as many cranes can be deployed on one ship. For quay side transport and stacking, twice as much space is available so there is little danger of quay side congestion. The difficult task of quayside transport optimisation is less pressing.

The indented berth comes at a price, though, as twice as much quay wall and apron is needed for each berth. Furthermore, berthing and de-berthing operations are more complicated and take more time. The indented berth requires a certain over width to get the vessel in and out. As a result half the trolley travel is longer then strictly necessary. The indented berth arrangement is tailored for large vessels, but berths need to be able to handle a range of vessels. With smaller vessels at the indented berth, half of the expensive infrastructure stands idle.

Floating container crane

The floating container crane consists of a container crane placed on two pontoons. The pontoons house the generators and fuel tanks, ballast tanks and other installations. Dimensions and shape of the pontoons determine stability to a large degree. They may need to be complemented with active motion suppression systems. The floating crane can be self propelled or towed.

The floating container crane is the result of the wish to be able to work the “idle” side of the container ship, without having the disadvantages of the indented berth. Inspiration was drawn from ro-ro pontoons, large floating cranes, transport of heavy loads over water and Hong Kong derrick-barges.

For the floating crane positioning system there are a number of options, such as dynamic positioning, conventional line mooring, vacuum mooring to the container vessel or other systems. Also the positioning system for the barges is very important for smooth operations. Although initial designs are almost identical to a land based crane, it is anticipated that during further technical development the crane will be modified to better suit the particular circumstances.

Floating cranes are not new. The Royal Haskoning floating container crane concept consists of a sophisticated container gantry crane placed on pontoons. A big difference with floating bulk cranes is the much higher handling precision. A big difference with Hong Kong derrick-barges is the far higher speed of operation, the larger size of vessels to be handled and the separation between crane handling and horizontal transport.

Operations

In the floating container crane concept, terminals for handling Suezmax container vessels would be equipped with a quay with suitable depth with 22 row wide, rather conventional, container cranes. Such terminals are suitable to handle all but Suezmax container vessels.

When handling a Suezmax vessel the land-based cranes are supplemented by a number of floating container cranes, working on the water side of the vessel. The intention is that together they will have sufficient capacity to handle the Suezmax vessel within an acceptable time.

During discharge the land based cranes handle their share of the containers in a conventional manner. The floating container cranes load the containers in barges. The barges are located between the pontoons or in the back-reach. The further landside handling of the containers in the barges can be done in a number of ways, as best suited to the particular situation. In the case of ports with a hinterland, that can be served by barges, such as Rotterdam and Antwerp, the discharge plan should preferably be arranged in such a way that the barges are loaded directly with containers for river ports, upstream. It is also an option to unload barges at older, less sophisticated, and cheaper terminals in the direct vicinity. The big surge of import containers is thus spread over a number of terminals, shaving peaks and filling gaps, where needed. Another option would be to unload barges at the same terminal at a later moment, during off-peak hours, again shaving peaks and filling gaps. This so-called network terminal allows many terminals to participate in loading a Suezmax vessel, while only one terminal need to have the facilities to receive the vessel.

During loading operations the land-based cranes handle their share of the containers in a conventional manner. For loading operations with floating container cranes similar options as for the discharge are available. Barges function in fact as pre-stacking arrangements, which can be prepared at upstream river terminals, or during off-peak hours at the loading terminal itself, or other, cheaper, and less sophisticated terminals in the vicinity.

Floating container cranes can easily be moved from an operation at one end of the terminal to an operation at the other end of the terminal. Floating container cranes are never “locked” in an idle position. They could also be shared between a number of different terminals within the same port. This flexibility guarantees high utilisation.

When no work is available floating container cranes can be moored along any available shallow quay wall, on dolphins (posts hammered into the bottom of the quay) or anchored at a suitable location.

Floating container cranes also help to economically upgrade terminals with existing quay walls that allow only limited loading conditions and limited water depths. First, a pocket is dredged at some distance from the existing quay wall. Then dolphins are placed in a row parallel to the quay wall. The shallow area between the existing quay wall and the row of dolphins is used by floating container cranes. The crane boom is pointed towards the water and the back-reach is over the apron. Container vessels too large to berth along the existing quay wall berth along the dolphins instead. Floating container cranes can perform all container operations without expensive new quay wall structures. In this particular application floating cranes could be electrically powered from shore. As no barges are involved in this operation the hatch covers may be placed on a structure between the two pontoons.

For humanitarian relief operations, or military support operations, temporary container operations could be quickly set up at any protected location with sufficient water depth. Mooring buoys, or even the ship’s own anchors, are used to moor the container vessel, after which floating container cranes come alongside. Containers are taken ashore by small vessels for which only a modest shallow quay wall structure is required. Quay side handling may be as basic as a reach-stacker operation.