# Simulation Tool for Winter Navigation Decision Support in the Baltic Sea
## 7 Model Functionalities  
This section describes some of the key functionalities that have been implemented. More functionalities are described in a follow-up journal article. The entire operational area is divided into operating zones. These zones are defined based on ice conditions and can change dynamically over the course of a simulation run. At least one icebreaker is available in each zone. Icebreakers have specific ice-going capabilities that are modelled using hv curves. The assignment of an icebreaker to an operating zone depends on the maximum ice thickness in the region and the capabilities of the icebreaker. It is usually standard practice that wider and stronger icebreakers are assigned to the northern regions more frequently, these regions freezing sooner and for longer durations. Ice thickness is also greater in these parts, and ridging is common. The months of January-March are often considered peak winter months when the sea is frozen to the maximum extent. There are different kinds of ice: some that has formed earlier in the season, while some is freshly formed ice. All these factors affect the creation of safe navigational paths by the icebreakers. During peak winter, it is common to assign two icebreakers to the northern zones, where ice conditions are the most challenging. Apart from ice conditions, the other factor affecting the choice of the number of icebreakers is the expected traffic density in the area. Some ports are busier than the others, even during peak winters. Excessive queue build-up needs to be avoided in the areas surrounding these ports and along fairways leading to these ports. Although there are 9 Finnish icebreakers available, the actual number of active icebreakers (in service) depends on the prevailing ice conditions. For the case study described in this article, it is assumed that four icebreakers are available for the Finnish ports at any given time.  
Although icebreakers are assigned to zones, they regularly coordinate with each other to ensure that the entire system operates safely and efficiently. Vessels often need assistance across multiple icebreaker zones. Typically, each icebreaker guides a vessel  
Figure 5: Process flows at ports and waypoints.  
through its operating zone, leaving it at a safe stopping point called a waypoint. At the waypoint, the next icebreaker takes over the assistance mission and guides the vessel further along its navigational journey. Icebreakers can, however, operate outside their operating zones to ease traffic build-up or to help larger vessels that need two icebreakers. The icebreakers need to make decisions dynamically regarding which vessel to assist first, when to coordinate with another icebreaker, which path to use, and whether to build convoys (to be described later).  
Figure 6 shows the routing logic for ships and icebreakers. When a ship is stuck, it sends out a message to the icebreaker control unit, which may be an icebreaker with a coordinating captain or a decision-making unit on land. The icebreaker in charge calculates the potential route of the vessel by considering its current position and destination. One or more icebreaker zones are identified, through which the vessel will navigate to reach its destination. A schedule is made for the vessel, which is updated each time the vessel reaches the end of a zone. The first icebreaker, that is, the icebreaker serving the first zone in which the ship is stuck, is sent to the vessel. The icebreaker leads the ship to the end of its zone or port (if the port lies in its zone). The route of the ship is reevaluated (if needed) at the end of the zone, and the next icebreaker is assigned to take the ship onward. A list of requests for each icebreaker is maintained and updated as the vessel moves from one zone to another. The vessel is removed from all lists once it reaches a port or a fairway to a port. Once the ship reaches a port, the calls to icebreakers are terminated.