## 2 Analysis and Literature Review of Clearance in Port Channels and Basins  
Many ports in the world have different requirements for minimum depths in port channels and basins and generally use guidelines such as PIANC [21] and EAU [22]. The distance between a ship's hull and the bottom of a navigation channel is very important in order to avoid the ship's hull coming into contact with the bottom of the navigation channel and to maintain sufficient controllability of the ship. Studies have been conducted on this topic [1; 4; 5; 7; 8]. At the same time, it should be noted that ship handling hearings require additional research, as ports try to attract larger and larger ships, and sometimes emergency situations occur due to too little clearance when ships touch the bottom of the navigation channel with their hulls due to ship heeling caused by external forces, or ships sink due to the ship's speed in the shipping channel or become uncontrollable and float outside the shipping channel [3; 6; 7]. At the same time, other very important aspects, such as the energy consumption of ships entering and maneuvering in ports and the environmental impact of low clearance under the ship's hull, have not yet been taken into account when assessing depths in port approaches and internal navigational channels and basins. The above aspects of energy consumption and environmental impacts from ships have been analyzed individually and without considering the potential impact of clearance on these aspects [16; 23; 24]. Vessel speeds are restricted in many ports and are linked to maritime safety and environmental impacts (the effect of waves generated by the vessel on moored vessels and the coastline) (Figure 1) [25; 26; 27]. Tugboats are widely used for assisting with maneuvering large ships in port because they are equipped with powerful engines and can use high engine power; however, they consume a lot of energy (fuel), generating large amounts of emissions, especially when turning ships and approaching or leaving the  
Figure 1: LNG tanker sailing in port (Klaipeda port), permitted speed up to 8 knots. The assessment of sustainable transport systems in ports and their environmental impact is very important for the ports themselves and for individual regions, so research in this area is very essential [28; 29]. Maritime transport plays an important role for many countries and creates a significant part of their gross domestic product (GDP). At the same time, maritime transport, especially in port regions, has a significant impact on the environment, so many countries are conducting research on that issue and are looking for optimal solutions to minimize the impact on the environment while improving the economic situation of the countries [29; 30; 31; 32; 33]. The emission control regions adopted by the International Maritime Organization (IMO) such as the SO\({}_{\rm x}\) emission control areas--the Baltic and North Seas, the English Channel and the east and west coasts of North America--have allowed a significant reduction of SO\({}_{\rm x}\) emissions from ships in these regions [34; 35; 13]. Ports, as the most important maritime transport points, also try to maximize their influence on the reduction of emissions from ships and other vehicles [36; 15; 16; 37]. The navigational safety of ports and the necessary minimum depths of ports are studied in many countries, since dredging works are expensive and in the development of sustainable ports it is necessary to combine navigation safety, economic, environmental and other possible factors [16; 17; 24; 25; 26]. For the provision of navigation safety, when sailing ships in shallow depths, i.e., the minimum depth below the ship's hull, methods for its determination, such as Bernoulli's flow formulas ideas [4], estimates of the connected liquid mass and others [1; 5; 12; 21; 22], are used in many countries, but the results obtained are often similar.