## 3 Methods
### Navigation Risk Assessment Model
#### 3.2.1 Criteria for Navigation Risk  
The identification of relevant criteria is a very critical step in the construction of an assessment model. The navigation risks mentioned in this paper refer to the loss of people or property and the possible impact of the navigation environment, which may consist of harsh conditions, resulting in the improper operation of the ship in the course of navigation, and which may cause ship collisions (i.e., the collision of two or more ships), collisions with stationary objects (i.e., with a reef, coast, or building), grounding (where the bottom of the ship touches the bottom of the sea due to insufficient water depth), or a sunken ship (sinking of ship) [38]. In order to obtain the criteria that may lead to identifying navigation risks more objectively, we use accident data from Maritime Safety Administration of the People's Republic of China in study areas to determine criteria that have a greater impact on ship navigation. In this paper, environmental risks are mainly used for assessment, so the accidents caused by purely human factors need to be eliminated. After screening, a total of 39 accident records were included. The distribution of accident data is shown in Figure 6, where different color points represent accidents in different seasons. The accident assessment section of each accident report details the main and secondary causes of the accident. As such, we used the accident assessment report to count the number of accidents caused by the different assessment factors. When selecting accident records, certain rules should be observed. For example, if there is a sentence whose meaning is similar to \"heavy fog in the sea, poor visibility, crew neglected to look out, resulting in ship collision\" in the accident cause assessment, it is considered that the accident is related to sea fog and \"1\" is added to the number of accidents caused by sea fog in the statistics. If there is a sentence whose meaning is similar to \"the waters of the accident were affected by strong winds, and the ship failed to evacuate the sea in time, resulting in the collision of the ship\", it is considered that strong winds had an impact on the accident, and we added \"1\" to the number of accidents caused by strong winds to the statistics. When the accident is caused by a combination of various factors, such as \"due to the superposition of strong wind, cold air, and storm surge in the accident water area\" recorded in the report, it is considered that strong winds and big waves had an impact on the accident, and \"1\" is added to the statistics of each category. The statistical results of the final criteria involved in the historical accidents are shown in Table 2.  
Figure 6: Distribution of accident points. Finally, according to the statistical information and data support, the top five criteria were selected, which were waves, wind, sea fog, ocean currents, and water depth. These five criteria are meteorological and hydrological factors which are especially relevant in meteorological navigation. Specifically, gale weather has an impact on both sailing and berthing ships. Sailing ships are affected by gales when sailing, which will reduce the controllability of the ships. If berthed ships are not fully anchored, the hull may shift under greater wind pressure, resulting in grounding or a collision. When big waves occur, they are usually accompanied by strong winds. It is difficult for ships to adapt to the peaks and troughs of sea waves, and when ships with lower tonages are hit by big waves, their external structure will be subjected to greater external forces, which may cause damage to the ship, which is very dangerous during navigation. Currents will also affect ship navigation. In narrow sea areas, the speed and direction of ocean currents are unstable, which makes it more difficult to effectively use seamanship. When sailing against the currents, the energy consumption of a ship increases, and the difficulty of navigation also increases [39].