This section explains the processes of discharging and loading of ships in container terminals to provide a better understanding of operations in container terminals. These operations can be simulated using the proposed simulation framework. The relations between the processes are explained using an activity diagram (Figure 6.1). In this diagram, ships, quay cranes, vehicles and stack cranes are considered the main objects that are involved in the discharging and loading processes. Each of these objects is represented by a swimlane to dierentiate the processes in which each object is involved.
Figure 6.1: The activity diagram to show the operations in container terminals. This gure shows the relations between the processes in container terminals using an activity diagram. Ships, quay cranes, transfer vehicles and stack cranes are the main objects that are involved in container terminal operations.
Container terminals generally consist of quay-side and stack-side areas. The quay- side area is a place where ships are berthed. In this area, quay cranes are used to
discharge/load containers from/into ships. The stack-side area consists of a number of stack blocks to stack import and export containers using stack cranes.
This diagram starts by the arrival of a ship at a container terminal and ends when the ship leaves the container terminal. Once a ship arrives at a container terminal, a berth should be assigned to it. If no berth is available, the ship should wait until a berth becomes available. A berth is a place where ships are moored. Each container terminal has a number of berths with a speci c length to allocate to ships. Each berth is equipped with a number of quay cranes which can be shared between the adjacent berths. When a quay crane is assigned to a berth it moves alongside of the quay to reach the designated berth. To allocate a berth to a ship, the length of the berth should be taken into account to ensure that the ship can t in the berth. By allocating a berth to a ship, a number of quay cranes need to be assigned to the berth. Each ship upon arrival may request a speci c number of quay cranes to discharge and load containers. As a result, there may be a number of combinations of berths and quay cranes that can be assigned to a ship. Each may lead to dierent waiting times of ships, because the available times of berths and quay cranes can be dierent. Thus, the berth and quay cranes assignment are two important decision making problems that should be addressed by the time ships arrive at container terminals. In the literature, there is a large number of algorithms to address these two decision problems (Steenken et al., 2004; Stahlbock and Voss, 2008).
As mentioned earlier, each ship has a number of containers to be discharged and loaded. For each ship, usually the discharging process is performed rst and the loading process is performed afterwards. However, in busy cases the two processes may be performed in parallel.
Quay cranes start the discharging tasks by removing containers from a ship. Once a quay crane picks up a container from a ship, the container should be collected by a
vehicle (based on the vehicles schedule) and transported to the stack-side area (based on the containers placement strategy). Collection of containers from quay cranes depends on whether vehicles can pick up and drop o containers by themselves. If this is the case, quay cranes do not need to wait for vehicles, they can place containers in buer, a temporary storage space under the crane, for vehicles to collect later. Utilising the buers has the great advantage of decreasing the waiting time of both quay cranes and vehicles. In contrast, if vehicles cannot pick up/drop o containers by themselves, quay cranes have to wait for vehicles to come and then they have to place containers directly on vehicles.
After a vehicle collects a container from either a buer or directly from a quay crane, it transports the container to the stack-side area to deliver it to the designated import block. The stack crane assigned to that particular stack block then picks the container up from the vehicle and stacks it in the import block. The vehicle then transports to the quay-side area for the next transportation tasks. Note that similar to the quay-side area, buers can also be used in the stack-side area under stack cranes. This way, vehicles can drop o containers in the buers for stack cranes to collect later, and hence minimise the waiting time. The discharging process will be continued until all importing containers are removed from the ship.
After nishing the discharging process, the loading process starts. In the loading process, containers are moved from exporting blocks into ships. The loading process is similar to the discharging process but in the opposite direction. Stack cranes pick up containers from the export blocks and place them in the buers for vehicles or directly on vehicles depends on whether vehicles can utilise the buers. The utilising of buers for loading tasks is similar to the one for discharging process. The loaded vehicles then transport to the quay-side area and drop the containers in the buers for quay cranes to collect later or wait for quay cranes to pick up containers directly from them (in the
case without buers). Quay cranes pick up containers from the buers/vehicles and place them in the ship based on the ship stowage plan. Once all the loading containers are placed in the ship, the ship can leave the berth and container terminal.