ransport system for container operation – the automated guided vehicle (AGV). AGVs were (and are still) used for horizontal transport at quayside and automated stacking cranes (ASCs) for vertical transport tasks in the container stacking yard. The original AGVs moved along a pre-determined fixed path such as rails or a guidance system built into the ground. This is the first and oldest automated method in which the path uses wire, tape, or transponders in the ground or pavement. An AGV will sense its location along the path and follow it according to instructions received from a central traffic controller. Usually, a radio message or in some cases infra-red communication is used to pass messages to the AGV from a traffic controller. Unlike fixed path AGVs, their free path counterparts, also used in the port of Rotterdam, are much less restricted in their movements, which allow them to follow a much shorter path from their current position to their destination.
In principle, AGVs should be able to drive in any direction. However, the free path AGVs used in the port of Rotterdam must still follow a path. The challenge is in installing, maintaining and changing the physical path based design; hence free-ranging AGV guidance technology is gaining interest. This method uses either inertial navigation technology combined with odometry to control direction speed and positioning, or the more common method is to use a system of mirrors and lasers that are continuously triangulating the vehicles position. This however comes at a price; it requires a more complex built-in navigation system to guide the AGV. However, there are companies such as Danaher Motion that have already provided such technology for nearly 13,000 AGVs. Among the onboard systems required for this are propulsion and steering mechanisms. In addition the AGV requires access to a traffic management system that handles the local path control so as to avoid “dead-lock” and possible collisions and a communication system which allows the AGVs to stay in contact with a central control system.
Cassette AGV
Development in Cassette AGV (C-AGV) technology in terminals is based on the IPSI™ AGV, part of the European Unions’ IPSI™ project. These AGVs have been designed to transport containers through the use of an additional buffer in between: cassettes. These cassette AGVs (C-AGVs) have been designed specifically to transport cassettes with containers on them. Each cassette can carry up to two 40ft or four 20ft containers. This allows cranes to continue loading/unloading even when there is no C-AGV available, as long as enough waiting cassettes are present in the buffer area. A decoupling of horizontal and vertical transport processes therefore becomes possible representing a vital key to the system’s cargo handling efficiency. The C-AGVs used in this paper are of the newest generation providing a zero-emission all-electric solution. The new vehicles have a load capacity of 61 tonnes, and can carry cassettes with double-stacked 40-foot containers or two 20-foot containers in a single tier. Major improvements to maneuverability are made by incorporating individual electrically driven and steered bogie axles which enable the C-AGVs to be moved in any direction and turn through 360 degrees as seen in Figure 1. This increases the versatility and flexibility of the C-AGV while minimising congestion at the quayside. The C-AGV can be steered conventionally or ‘crab’ diagonally, or it can move completely transversally. New cassette designs, presented in Figure 2, enable the C-AGV to enter and exit both transversally and longitudinally, i.e. their smaller size allows them to move below the cassette in between its legs at each end, thus eliminating the need for a lengthy line-up operation.
Figure 1. C-AGV turning 90 degrees in the direction of a loaded cassette.
Figure 2. C-AGV traveling under a cassette either from the ends or the sides.
The contactless energy transfer technology contains ground-based and vehicle-based segments. The two key components to the ground-based system are the power electronics element and coils, which enables vehicles such as the C-AGV to receive energy both under the quay crane and the yard cranes areas. In addition to the ground-based system, the vehicle-based system employs the same technology and uses super capacitors to store the energy, which is then used by electric wheel motors. With a full load a C-AGV can travel around 600m, depending on its load, after which the capacitor needs to be recharged. The use of capacitors instead of batteries allows for a lighter C-AGV and even though its range on a single load is limited, the capacitor can be – unlike a battery – recharged within 20 seconds. This generation of C-AGVs , as well as being much lighter, is also slightly smaller. This means they are now smaller then the cassettes they carry. The cassettes have therefore been redesigned so that the C-AGVs can now move below them sideways to pick them up whereas the previous generations of C-AGVs had to line up with the cassette and pick them up along their length axis.
Operations at an ACT
Quay crane operations in our yard layout, illustrated in Figure 3 depicts the operations being performed at the back side of the crane. In this new concept all operations are performed either between the gauge of quay crane or at the back sides of the crane, thus decreasing the distance C-AGVs have to travel, while on the other hand, increasing the distance through which the crane has to transport the containers. The space that becomes available below the cranes will be used to store the ship’s hatch covers. Additionally this means that the automated area of the yard can be entirely isolated from areas of human activity thus decreasing the risk of unwanted interference.
Figure 3. Illustration of C-AGVs driving under the cassettes at either the ends or transversally from the sides of the cassettes.
In our yard crane operations layout we have also incorporated a buffer system. Instead of having a buffer with room for four to eight cassettes next to one another, a dual layer approach has been chosen with four 2-deep cassette lanes in which the second row can be reached through two highways leading into the second layer of the buffer, illustrated in Figure 4. The layout in Figure 4 provides eight places reserved for cassettes using the same width with a single layer that six spaces would normally occupy. This system is also easily expandable in depth: by adding an additional layer, the buffer capacity is increased by another four places. The downside of this approach is that for each extra buffer layer, one layer of storage space in the yard would be sacrificed.
Figure 4. The transfer points at the stacking crane and layout
C-AGVs Operations
There are several “highways” behind the quay crane; five are cassette “highways” on which cassettes wait for containers to be transported to the yard buffer areas and three are highways which can be used by C-AGVs, which are free-ranging and not bound to tracks, so all locations in the cassette lanes are at all times reachable. As these highways are only wide enough for one C-AGV to fit in them, they are single-directional. Points are provided in the highway and cassette lanes where C-AGVs can leave them in order to proceed to the yard area. This will usually be done on locations where there is more room between two successive cranes.
As described earlier, the C-AGVs used in this paper are of a new design using an electric propulsion system enabling them to virtually turn around their axis. These C-AGVs are not modeled to be fixed-path instead they are free-ranging. The independently turning wheels also mean they can go from moving forward or backwards to moving sideways by turning the wheels while standing stationary. However the reason this has been made possible is due to the electric engines, but they do add an important additional constraint to the traffic management and terminal design; the capacitor fuell
ing the engines needs to be recharged every 500m to 1000m, depending on the C-AGVs load. For this purpose there are specific recharge points built into the road deck on strategic points such as the hand-over area between C-AGVs and quayside cranes as well as C-AGVs and stacking cranes. When C-AGVs pass above them they can recharge their capacitors to 100% in approximately fifteen seconds, depending on the status of the capacitor before recharging is started.
Traffic Management
The Traffic Management System (TMS) controls the flow of traffic throughout the yard area. Most of the TMS is implemented in the C-AGVs themselves. The sensors installed on the C-AGVs will avoid collisions and handle the flow in the free flow areas as well as in the highways and cassette lanes. Moreover, the centralised TMS especially serves to reserve certain cassette places in the buffer areas for specific C-AGVs and avoids deadlocks by granting vehicle priorities. Additionally, the routing of C-AGVs is covered by the centralised TMS, i.e. if C-AGVs are allocated by a job the related instance either organises necessary transport activities or determines the particular traveling path.
From the various simulations and studies conducted by companies and industry partners, the number of vehicles needed, the retail costs and fuel or energy consumption have been placed into a spreadsheet calculation to compare the variable cost of moving a container. The spreadsheet is presented in Table 1 and indicates that, when comparing the automated systems that are available in the market, the decoupling of the AGVs and Auto Shuttle Carriers increases the productivity levels of the vehicles. The decoupling of the containers on both the land-side and marine-side is achieved by the C-AGVs and the Auto Shuttle Carriers. The Lift-AGV is able to decouple at only the land-side with the use of steel racks that are placed into the ground. As seen from the table, the simulation results for AGVs shows that more units are required to keep the quay crane from not being idle in order to handle the 100,000 containers. From a cost comparison, the additional units which are less in cost than other automated systems translate into a higher cost for moving a container. The investment costs that are used stem from the purchase price of the vehicle without navigation. Of course the need for a navigation system is essential and can boost the investment cost to over Euros 100,000 depending on the type of navigation system employed.
In summary, there have been evolutionary changes in the container terminal industry that are influenced by many factors, such as higher volumes, increasing demands for environmentally friendly equipment and lower costs. Automation is seen by many industry experts as offering possible solutions to their port plans or operational demands. Depending on the terminal operator’s objectives and the numerous variables, which are difficult to list and model in a spreadsheet, we suggest that each of the listed automated systems in Table 1 can be used successfully in container terminal operations. The variable cost per container move is just one key performance indicator (KPI) that should be considered amongst a basket of KPIs.