# A Novel Method for Risk Assessment and Simulation of Collision Avoidance for Vessels based on AIS
## 3 Intentional Collision Avoidance Rules  
The rules of COLREGS related to lighting, warning signals, applicable principles, related definitions, etc., without mentioning the navigation plan and specific collision avoidance issues. Thus, situations encountered are divided in Figure 2 as follows [14; 32]:  
Meeting: When two ships navigate relative to each other, collision damage may occur. They are supposed to turn right, and then cruise through their own port, in order to avoid the risk of collision. The quantification is expressed as 5\({}^{\circ}\)\(\leq\)\(B_{ot}\) and 174\({}^{\circ}\)\(\leq\)\(\Delta\)C \(<\) 186\({}^{\circ}\)[33].  
Crossing: When two ships cross through each other, one ship is on another ship's starboard side, so that the ship should make way and avoid navigating in front of another ship.  
* Starboard crossing: Two ships head to cross, while the target ship is located on the starboard side. According to the encounter angle, the situation can be divided into a small crossing angle (\(B_{ot}\)\(\leq\) 45\({}^{\circ}\) and 186\({}^{\circ}\)\(\leq\)\(\Delta\)C \(<\) 210\({}^{\circ}\)) and a large crossing angle (45\({}^{\circ}\)\(<\)\(B_{ot}\)\(\leq\) 125\({}^{\circ}\) and 210\({}^{\circ}\)\(\leq\)\(\Delta\)C \(<\) 360\({}^{\circ}\)) [33].
* Board crossing: Two ships head to cross, while the target ship is located on the board side. According to the encounter angle, the situation can be divided into a small crossing angle (\(B_{ot}\)\(\leq\) 45\({}^{\circ}\) and 150\({}^{\circ}\)\(\leq\)\(\Delta\)C \(<\) 174\({}^{\circ}\), and a large crossing angle (\(B_{ot}\)\(\leq\) 125\({}^{\circ}\)and 0\({}^{\circ}\  
Overtaking: Quantification is expressed as \(V_{o}>V_{t}\), \(B_{ot}\leq\arcsin(0.924V_{t}/V_{o})\), \(0^{\circ}\leq\Delta\mathbb{C}<90^{\circ}\), or \(270^{\circ}\leq\Delta\mathbb{C}<360^{\circ}\)[33].  
The principle of avoiding ship collision is described, and ship encounters are described in relation to the collision in Figures 3 and 4, respectively.  
Different decisions of collision avoidance are made, according to different encounter types. The encounter situations and navigation rules are shown in Figure 4[34]:  
Figure 4 shows the encounters' relation to the collision at a particular moment, and may be classified into four types:  
* Type A (collision situation). Two ships are on a collision course when two ships reach the common spatial zone simultaneously. They will collide if no evasive action is taken.
* Types B, C (crossing situation). Two ships move in overlapping paths, and the collision will be avoided when they pass through the common spatial zone at different times. In order to avoid collisions, one or two ships have to change their speed or course. They mean that this situation has to turn into a Type A situation first.
* Type D (non-crossing situation). The paths of two ships do not overlap in any way when two ships move in parallel in two adjacent lanes or when they have navigation courses. However, it does not mean that the collision risk is completely zero, as there may be some adjustment of the path by one or both ships.  
Figure 4: Model for classifying ship encounters in relation to the collision.  
Figure 3: The ship collision-avoidance operation principle.  
Thus, all types of situations can be discussed to make sure the situation turns into a Type A before a collision happens. An algorithm can be developed to process these types.