## 3 Description of the Simulation Model
### Generalizations and Assumptions  
Resulting from the complexity of the FSWNS, as well as due to various identified knowledge gaps and technical limitations of the applied simulation technique, the simulation model simplifies and generalizes some of the characteristics and mechanisms of the FSWNS. Specifically, the following generalizations and assumptions are noted:  
* Concept of equivalent ice thickness. As applied in this approach, the concept of equivalent ice thickness rests on the assumption that an ice cover of a specific equivalent thickness results in the same level of hull resistance as continuous level ice of the same thickness [21]. A weakness of this concept is that it fails to account for individual ice features (e.g., individual ice ridges) that might stop a ship [22]. Anyhow, based on [23], for the simulation of ships operating on the Baltic Sea, the concept appears well suited. * The use of hv curves. The use of hv curves to model the speed of ships in ice is well established. It should be noted that this approach typically rests on the assumption that ships operate at a fixed engine load (typically near the maximum continuous rating), which is not always true [24]. However, currently, there is no general and publicly available approach to eliminate this assumption. * Multi-ship convoy operations. In the simulation, convoy operations in which an IB assists two ships at a time occur whenever two ships need IB assistance over the same distance at the same time. Convoy operations in which three or more ships are assisted at one time are not considered. These simplified assumptions are needed as it is not entirely clear under what conditions convoy operations may occur. The ice resistance of a ship being assisted by an IB tends to increase as a function of the distance between the ship and the assisting IB. Therefore, because a significant safety distance is required between ships operating in a convoy, multi-ship convoy operations require a higher ice-going capability from the involved ships [20]. As a result, considering the EEDI regulations and other measures lowering the average ice-going capability of modern ships, convoy operations and particularly those in which an IB assists more than two ships at a time are expected to remain rare in the future, especially during periods of heavy ice conditions. For this reason, the exclusion of convoy operations involving more than two assisted ships is not expected to significantly reduce the accuracy of the simulation model when applied to simulate heavy ice condition scenarios. * IB transit times. Due to limitations set by the applied simulation technique, the exact location from which an available IB starts to move towards a ship in need of IB assistance is not known. Therefore, the duration of the IB transit is determined probabilistically based on statistics. In the real world, the master of an IB may try to minimize a ship's waiting time by predicting where assistance will be needed, and if possible, start to proceed towards that area in advance. Thus, particularly during periods of low demand for IB assistance, the above-described approach is likely conservative. On the other hand, in periods of high demand for IB assistance, IB waiting times appear to be primarily driven by the availability of IBs, meaning that the relationship between transit times and ships' total IB waiting times is small. * Criteria for providing of IB assistance. In the simulation, IB assistance is provided if a ship's independently achievable speed falls below a specific limit value (e.g., 1.5 knots) in the worst assumed ice conditions along a leg. In the real-world, the criteria for IB assistance likely depends on the operating situation so that the criteria are stricter during times of high demand for IB assistance. In addition, the decision on whether IB assistance is to be provided, or requested, is also likely influenced by the individual judgement of the masters of the involved ships.