## 1 Introduction  
The Baltic Sea is an important transit route connecting numerous countries and markets. In 2019, the total volume of Finnish import and export transported over the Baltic Sea exceeded 100 million tons, corresponding to around 80% of the total trade [1, 2]. These numbers are expected to increase as the long-term trend in the volume of Finnish seaborne trade is one of growth [3]. In winter, large parts of the Baltic Sea are typically ice-covered, but with significant interannual variability with regards to the maximum ice extent [4]. This variability poses a challenge to shipping in the region, as sea ice has a significant impact on the operations and transit times of ships. The aim of the Finnish-Swedish Winter Navigation System (FSWNS) is to maintain safe and efficient year-round navigation to and from Finnish and Swedish ports along the Baltic Sea [5]. To this end, the FSWNS manages winter navigation-related challenges by the combined use of (a) ice class rules, (b) traffic regulations, and (c) icebreaker (IB) assistance [6]. Specifically, to make sure that ships have enough ice-going capability for safe and efficient operations, they must be built and operated following the Finnish-Swedish Ice Class Rules (FSICR) [5]. These are enforced by port-specific traffic restrictions set by Finnish and Swedish maritime authorities in terms of the minimum ice class and deadweight needed to be eligible for IB assistance [5]. IB assistance is provided based on the available fleet of Finnish and Swedish state-owned and operated IBs. As per [7, 8], Finland has a fleet of eight major IBs(Polaris, Fennica, Nordica, Otso, Kontio, Voima, Sisu, and Urho), whereas Sweden has a fleet of five major IBs (Ale, Atle, Frej, Oden, and Ymer). Both in the short- and longer-term, the performance of the FSWNS is expected to be influenced by the International Maritime Organization's (IMO) Energy Efficiency Design Index (EEDI) regulations. These regulations, which were adopted by the IMO in July 2011, aim to reduce the amount of greenhouse gasses (GHGs) from ships by promoting the use of more energy-efficient solutions [9]. Anyhow, due to the technical content of the regulations, they are expected to limit the installed propulsion power of ships, which will reduce their ice-going capability and attainable speed in ice. This in turn could increase the demand for IB assistance, resulting in more frequent and longer waiting times for IB assistance, which is expected to increase the overall transport costs and time. Towards assessing the influence of the EEDI, and to identify and assess possible mitigation measures, this article presents an approach for assessing the operational performance of the FSWNS under various operating condition scenarios. The approach builds on a discrete event simulation (DES)-based approach presented by [10]. DES is a specific type of simulation in which the behavior of a system is modelled as an ordered sequence of events, each of which takes place at a specific point of time and results in a change in the state of the system [11]. As no change occurs between events, DES enables fast simulations of extensive operating periods. Additionally, as individual events can be modelled so that they are dependent on stochastic variables, DES is well suited to considering uncertainties and stochastic factors. In addition, because DES makes it possible to model individual ships as entities moving through a system in the manner of a queue, the technique is well suited to capturing various interactions and self-reinforcing effects. The approach presented by [10] can assess the performance of an Arctic maritime transport system consisting of a homogenous fleet of ships operating between two ports. In this study, the approach is extended to handle the significantly more complex operations of the FSWNS. Related research questions include the following: Is it feasible to capture the complex behavior and to roughly estimate the operating performance of the FSWNS using a DES-based approach?