Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019 3. ALGORITHM STRUCTURE


This section describes how to form an evasion route by course alteration in encounter situations at sea using the developed method. The method is based on two phases: the control and solution phases whose framework is illustrated in Figure 3. In the control phase, briefly, after the navigation data of the ships have been defined, the motions of the ships are calculated and it is checked whether the relative motion of the TS violates the ship domain. If there is a violation and the OS is the give-way ship, it passes through to the solution phase to calculate the necessary course alteration to avoid collision. In the solution phase, the optimal course to avoid collision is calculated and assigned to the OS as a new course.


The proposed method for collision avoidance aims to meet the requirements below:


• real time taking action, • compliance to COLREGs, • ability to disclose the optimal collision avoidance trajectory.


The optimal collision avoidance trajectory in the navigation environment is generated by the method. The OS are formed with a circular ship domain used for collision risk assessment and calculation of the safe trajectory. The ship domain is a safe area around a ship to keep free from the other ships and objects in the navigation environment (Goodwin, 1975). It provides a safe distance between ships during navigation. If the domain is violated by any obstacles, it is considered to be a risk of collision and evasive action should be taken in compliance with COLREGs.


Some assumptions have been accepted to simplify the complexity of the problem before the algorithm is designed. The assumptions are as follows: • A circular ship domain is formed with radius determined by the system user. The violation of the domain by obstacles in the vicinity means the risk of collision.


• Own Ship (OS) and Target Ship (TS) are the terminology to define the ships. The OS is the ship that has to take action to avoid collision, the TS is the ship to be avoided.


• The speed and course of the TS is steady and do not change during the process.


• The algorithm is designed to be applied to course alteration at the current position of the OS once the calculation is conducted.


• A final point where the OS can return to its original route is determined.


• The navigational data of the ships to be entered into the developed interface are assumed to be provided. On board, these data are provided by ARPA and AIS.


• Once the right angle between the TS and the OS occurs, the return course alteration to original route is conducted by the OS.


• Each of the ships is assumed to obey COLREG rules. ©2019: The Royal Institution of Naval Architects Figure 3. The brief flowchart of the proposed algorithm


The display of the simulation presents a situational view from the OS viewpoint. The current speed and course of


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The collision avoidance problem is associated with the unpredictable conditions. In this respect, to define this uncertainty, the motion of the obstacles and ships must be estimated. The kinematic model of the ship motion is presented by the equation 1, the instantaneous distance between the ships ,() at time t can be calculated by the Euclidean distance formula by the equation 3 and the


initial position of the OS (, ) is located in the origin (0,0) of the Cartesian system to ease the calculation presented by the equation 2,


() = (0) +()() () = (0)+ ()() () = (0) +()() () = (0)+ ()()


(0) = 0, (0) = 0 ,() = √(()− ())2 + (() − ())2 (1)


(2) (3)


where is the speed of the OS, is the course of the OS, is the speed of the TS, is the course of the TS, X is the abscissa value and Y is the ordinate value of the ships in the Cartesian system.


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