Requirements for Optimal Local Route Planning of Autonomous Ships

Abstract

Ships transport large volumes of cargo, and are therefore major contributors to the global economy. Ship collisions can cause significant economic losses. Path-planning algorithms can prevent such collisions by suggesting the optimal path for navigation. Conventional path-planning algorithms are disadvantageous, because they do not consider the navigation practices followed by experienced navigators. Therefore, in this study, we developed the requirements for optimal local path planning of autonomous ships by considering the open sea, restricted waters, and two-ship and multi-ship interactions, in addition to the navigation practices adopted by navigators and rules in COLREGs part B. First, the navigation practices under various scenarios were collected. Subsequently, these practices were linked to COLREGs part B to extract the key rules and keywords related to collision avoidance. Finally, the requirements for generating the optimal local path were drafted based on the rules and keywords. The utility of the requirements was demonstrated by applying them to representative path-planning algorithms for the timely and accurate evaluation of their effectiveness. The proposed requirements can be utilized to improve the existing path-planning algorithms and develop superior algorithms in the future.

1. Introduction

Maritime transport is one of the largest sectors of the maritime economy, and over 70% of goods transport is through ships. However, ship accidents can cause extensive economic and human loss, in addition to marine pollution. According to the Korea Maritime Safety Tribunal, marine accidents increased from 2307 in 2016 to 3156 in 2020. These accidents were attributed to damaged engines, major collisions, safety accidents, reef accidents, fire and explosions, capsizing, sinking, and minor collisions, in the given order [1]. According to Campbell et al. [2], the majority of ship collisions occur due to negligence or failure to follow International Regulations for Preventing Collisions at Sea (COLREGs). To mitigate the risk of ship collisions and improve the level of intelligence in maritime transport, a project to develop maritime autonomous surface ships is being conducted with the active support of the International Maritime Organization and countries with advanced maritime technology, including the European Union, Norway, and Japan [3].

Collision prevention in autonomous ships is performed through the processes of navigation, guidance, and control [4,5]. During navigation, the current situation is processed by recognizing the state of the ship and the environment around the ship through sensing. During guidance, global and local paths are planned. Global path planning involves finding a path without obstacles from the current location considering all spaces with static and dynamic obstacles on the given map. Local path planning involves creating new routes in real time by recognizing the changes caused by dynamic obstacles in the global path. The local path is generated based on COLREGs part B [6,7,8,9]. The last step, control, ensures that the generated path is followed. In particular, Zhang et al. [9] classified collision avoidance with respect to methods such as geometric, optimization/bionic algorithm, virtual vector/field theory, and artificial intelligence.

Techniques such as genetic algorithm (GA), fuzzy logic, A$\ast$ search, ant colony optimization (ACO), particle swarm optimization (PSO), velocity obstacle (VO), artificial potential field (APF), and deep reinforcement learning (DRL) have been developed and applied to local path-planning algorithms [7,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33]. GA builds a path from the ship’s current location to the destination by considering static and dynamic obstacles in all spaces and achieves collision avoidance by choosing the optimal path by repetitively performing reproduction, selection, crossover, and mutation tasks [11]. Tsou and Hsueh [12] combined COLREGs part B with the ship domain to provide the optimal safety turning angle, navigation restoration time, and navigation restoration angle, resulting in the economically shortest path for collision avoidance. Fuzzy logic performs collision avoidance by storing the experience and knowledge of experts as rules by which to resolve collision risks [13]. Perera et al. [14,15] built an efficient, optimal, and safe real-time local path to reach the destination by complying with COLREGs part B. ACO uses a graph and probabilistic approach to solve computational problems and generate the optimal collision avoidance path [16]. Tsou and Hsueh [17] combined COLREGs part B, maritime law, regulation knowledge, and real-time navigation information from the automatic identification system to create a local path using ACO and achieve collision avoidance, mimicking the optimization behavior. PSO solves the optimal path by moving particles, each of which is a collection of candidate paths, through a search space according to simple mathematical formulae for their position and velocity [18]. Kang et al. [19] created a local path by applying PSO in compliance with COLREGs part B to reduce human influence and efficiently prevent ship collisions in open waters with effective visibility. The A$\ast$ search finds the optimal path by dividing the entire collision avoidance space available to the ship into $n$ layers and selecting the safest anti-collision path to the node point in descending order [20]. A$\ast$ search was applied by Lee and Rhee [21] and Han et al. [22] to the path generated using an expert system based on COLREGs part B. APF constructs a repulsive potential field to avoid static and dynamic obstacles and an attractive potential field to reach the destination through a general local path [23]. Lyu and Yin [24] added safety requirements to the potential field for avoiding static and dynamic obstacles, and ensuring compliance with COLREGs part B. When the own-ship (OS) and target ship (TS) maintain constant velocities, the VO constructs the collision risk space by considering the sizes and velocities of the ships and prepares plans for exiting the risk space [25]. Zhao et al. [26], Huang et al. [27], and Namgung [28] developed real-time anti-collision algorithms by applying the VO technique to COLREGs part B to evaluate collision risks involving dynamic obstacles, and to comply with instantaneous warnings for potential collisions. DRL is performed by learning the process of optimal path generation to maximize the accumulated rewards [29]. By combining DRL with COLREGs part B, Zhao and Roh [30] and Shen et al. [31] developed local path-planning algorithms and trained anti-collision tasks for various circumstances. Furthermore, research on PPO-based fusion algorithms for constructing reinforcement learning model designs through the combination of a priori path and obstacle avoidance navigation has yielded good results [32]. Jiang et al. [33] proposed the decision-making model with attention-mechanism based on deep reinforcement learning, which includes ship risk assessment and motion planning modules. However, conventional local path-planning algorithms do not consider navigation practices from the navigator’s perspective, and instead reflect selected rules from COLREGs part B based on the author’s individual judgment [10,34].

In this study, the requirements for optimal local path planning by considering the open sea, restricted waters, and two-ship and multi-ship interactions were analyzed by simultaneously reflecting actual navigation practices and COLREGs part B. This paper is organized as follows: Section 2 describes the requirements collection, extraction, analysis, and specifications. Section 3 verifies whether the requirements are satisfied by applying them to representative anti-collision algorithms. Section 4 presents the conclusions.

2. Requirements for Optimal Local Path Planning

2.1. Requirements Analysis

Requirements engineering is the decision-making process that defines and documents the requirements of a project. The process is largely divided into requirements development and requirements management [35]. Requirements development comprises the steps to collect data, extract the requirements, document them, and verify their fulfillment. Requirements collection involves identifying an issue based on the knowledge of the current system and collecting all available information by interviewing field officers and reviewing the literature. In the requirements extraction step, the requirements for development are identified from the collected data that have been organized and accordingly categorized. In the documentation step, a detailed requirements analysis report is created by utilizing the extracted requirements. Verification involves ensuring that the user’s request is accurately recorded and free of contradictions. Here, requirements development engineering was applied to analyze the requirements to generate the optimal local path, as outlined in Figure 1.

First, in the requirements collection stage, navigation practices in the open sea, restricted waters, and two-ship and multi-ship interactions are collected and categorized. Second, in the requirements extraction stage, the collected navigation practices are linked to COLREGs part B to extract the primary rules and keywords on the topic of collision avoidance. Finally, in the requirements analysis step, the requirements to generate the optimal local path are analyzed and specified according to the categories identified during the requirements collection and the rules and keywords defined in the extraction step.

2.2. Requirements Collection

Li et al. [34] selected the following seven classification criteria from seventy-three articles on ship anti-collision decision-making from 2010 to 2020: ship anti-collision responsibility, environmental interference, ship maneuverability, TS maneuvering, ship domain, expert experience, and collision avoidance action (

Table 1. Classification criteria from the literature review.

Classification Criteria
Ship anti-collision responsibility	The responsibility of the ship in an encounter situation. According to COLREGs, a ship in an encounter situation can be classified as a give-way vessel, stand-on vessel, or vessel with equal responsibility.
Environment interference	Interference from wind, waves, currents, and other factors during the ship motion.
Ship maneuverability	The ability of a ship to maintain or change its state of motion under suitable control actions.
Target-ship maneuvering	The TS may not necessarily maintain its course and speed during the collision avoidance operation.
Ship domain	The area around the ship that is restricted to other ships to ensure navigation safety.
Expert experience	The navigation experience and good seamanship of ship officers on board.
Collision avoidance action	The ship’s maneuvering behavior to avoid collisions, including speed alteration, course alteration, speed and course alteration, and trajectory planning.

).

A survey to collect the requirements for navigation practices was conducted based on the defined classification criteria as the questionnaire design and expert elicitation [34]. In total, twenty-nine Officers of the Watch (OOW) responded to the survey, including pilots, captains, first officers, second officers, and third officers (

Table 2. Distribution of officers of the watch.

Division	Pilot	Captain	First Officer	Second Officer	Third Officer	Total
Participant (Rate)	3 (10.34%)	9 (31.04%)	3 (10.34%)	6 (20.69%)	8 (27.59%)	29 (100%)

). The average years of experience for each title was 5, 16.2, 8.7, 5, and 1.5 for pilots, captains, first officers, second officers, and third officers, respectively.

Based on the survey responses from the OOW, the collision avoidance decision-making practices in the open sea, restricted waters, and two-ship and multi-ship interaction conditions were categorized as shown in Figure 2, based on the seven classification criteria in Table 1. The OOW reported the necessity of the following six criteria in collision avoidance decision-making in the open sea and restricted waters in two-ship and multi-ship interactions: ship anti-collision responsibility (give-way vessel), environmental interference, ship maneuverability, TS maneuvering, ship domain, and expert experience. However, collision avoidance requires only course change action as the criterion in anti-collision decision-making in the open sea for two-ship and multi-ship interactions, whereas both speed and course change actions are required in the case of restricted waters in two-ship and multi-ship interactions.

2.3. Requirements Extraction

The key rules related to collision avoidance are categorized in Figure 3 by comparing the navigation practices collected from the OOW survey results with the definitions in Table 1 and applying COLREGs part B. At this time, authors extracted the keywords based on Table 1, and the extracted keywords were reviewed by experts.

The results of connecting and categorizing the navigation practices collected according to the seven classification criteria from Table 1 based on COLREGs part B are as follows. For ship anti-collision responsibility (give-way vessel), Rules 13–18 are applied in the open sea and restricted waters. For environmental interference, Rule 9 is required in the open sea, whereas Rules 6 and 10 are required in restricted waters. For ship maneuverability and TS maneuvering, Rules 6, 8, and 17 are required in the open sea and restricted waters. For the ship domain, Rules 5–8 and 13–17 ought to be applied in the open sea and restricted waters. For expert experience, all rules specified in Figure 3 are required in the open sea and restricted waters. For collision avoidance action (speed and course), Rule 8 is applied in restricted waters. For collision avoidance action (course), Rule 8 is applied in the open sea.

A total of 12 rules collected from COLREGs part B were linked to navigation practices, and the keywords from each rule were extracted, as shown in Figure 4.

2.4. Requirements Analysis

The requirements for optimal local path planning, as demonstrated in

Table 3. Requirements analysis for ship anti-collision responsibility (give-way vessel).

Ship Anti-Collision Responsibility (Give-Way Vessel)
Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule 13	13.1. Overtaking, 13.1.1. Action, 13.1.2. Duty of keeping clear 13.2. Overtaken
Rule 14	14.1. Alteration, 14.1.1. Course, 14.1.1.1. Starboard side 14.2. Situation, 14.2.1. Mast head and both sidelights
Rule 15	15.1. Avoiding, 15.1.1. Vessel on own starboard side, 15.1.2. Crossing ahead
Rule 16	16.1. Early and substantial action
Rule 17	17.1. Maneuvering alone, 17.2. Best aid 17.3. Avoiding, 17.3.1. Port course
Rule 18	18.1. Sequence, 18.1.1. Power-driven vessel, 18.1.2. Sailing vessel, 18.1.3. Vessel engaged in fishing, 18.1.4. WIG, 18.1.5. Not under command, 18.1.6. Restricted ability to maneuver

,

Table 4. Requirements analysis for environmental interference.

Environmental Interference
Restricted Water (Two-Ship, Multi-Ship)
Rule 9	9.1. Outer limit, 9.1.1. Channel, 9.1.2. Fairway 9.2. Passage, 9.2.1. Starboard side, 9.2.2. Alteration, 9.2.3. Caution 9.3. Overtaking, 9.3.1. Safe passing, 9.3.2. Appropriate signal
Open Sea (Two-Ship, Multi-Ship) and Restricted waters (TWO-Ship, Multi-Ship)
Rule 6	6.1. Visibility, 6.2. Traffic density, 6.3. Background light, 6.4. Draft, 6.5. RADAR 6.6. Maneuverability, 6.6.1. Stopping distance, 6.6.2. Turning ability 6.7. Navigation hazards, 6.7.1. Wind, 6.7.2. Sea, 6.7.3. Current
Rule 10	10.1. Traffic line, 10.1.1. General direction of traffic flow 10.2. Clear area, 10.2.1. Separation zone, 10.2.2. Separation line 10.3. Small angle, 10.3.1. Joining, 10.3.2. Leaving 10.4. Crossing traffic lanes, 10.4.1. Right angle 10.5. Wide margin, 10.5.1. Not using TSS 10.6. Terminations, 10.6.1. Particular caution 10.7. Exemption

,

Table 5. Requirements analysis for ship maneuverability.

Ship Maneuverability, Target Ship Maneuverability
Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule 6	6.1. Visibility, 6.2. Traffic density, 6.3. Background light, 6.4. Draft, 6.5. RADAR 6.6. Maneuverability, 6.6.1. Stopping distance, 6.6.2. Turning ability 6.7. Navigation hazards, 6.7.1. Wind, 6.7.2. Sea, 6.7.3. Current
Rule 8	8.1. Ample time, 8.2. Good seamanship, 8.3. RADAR 8.4. Alteration, 8.4.1. Course, 8.4.1.1. Sufficient sea-room, 8.4.1.2. Close-quarters situation, 8.4.1.3. Large angle 8.4.2. Speed, 8.4.2.1. Stopping, 8.4.2.2. Reversing 8.5. Safe distance, 8.5.1. Finally past and clear 8.6. Early action, 8.6.1. Sufficient sea-room
Rule 17	17.1. Maneuver alone, 17.2. Best aid 17.3. Avoiding, 17.3.1. Port course

,

Table 6. Requirements analysis of ship domain.

Ship Domain
Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule 5	5.1. All available means, 5.2 Risk of collision
Rule 6	6.1. Visibility, 6.2. Traffic density, 6.3. Background light, 6.4. Draft, 6.5. RADAR 6.6. Maneuverability, 6.6.1. Stopping distance, 6.6.2. Turning ability 6.7. Navigation hazards, 6.7.1. Wind, 6.7.2. Sea, 6.7.3. Current
Rule 7	7.1. RADAR 7.1.1 Plotting, 7.1.2. Long-range scanning 7.2 Compass bearing, 7.2.1. Bearing change 7.3. Approaching, 7.3.1. Close range
Rule 8	8.1. Ample time, 8.2. Good seamanship, 8.3. RADAR 8.4. Alteration, 8.4.1. Course, 8.4.1.1. Sufficient sea-room, 8.4.1.2. Close-quarters situation, 8.4.1.3. Large angle 8.4.2. Speed, 8.4.2.1. Stopping, 8.4.2.2. Reversing 8.5. Safe distance, 8.5.1. Finally past and clear 8.6. Early action, 8.6.1. Sufficient sea-room
Rule 13	13.1. Overtaking, 13.1.1. Action, 13.1.2. Duty of keeping clear 13.2. Overtaken
Rule 14	14.1. Alteration, 14.1.1. Course, 14.1.1.1. Starboard side 14.2. Situation, 14.2.1. Mast head and both sidelights
Rule 15	15.1. Avoiding, 15.1.1. Vessel on own starboard side, 15.1.2. Crossing ahead
Rule 16	16.1. Early and substantial action
Rule 17	17.1. Maneuver alone, 17.2. Best aid 17.3. Avoiding, 17.3.1. Port course

,

Table 7. Requirements analysis for expert experience.

Expert Experience
Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule 5	5.1. All available means, 5.2 Risk of collision
Rule 6	6.1. Visibility, 6.2. Traffic density, 6.3. Background light, 6.4. Draft, 6.5. RADAR 6.6. Maneuverability, 6.6.1. Stopping distance, 6.6.2. Turning ability 6.7. Navigation hazards, 6.7.1. Wind, 6.7.2. Sea, 6.7.3. Current
Rule 7	7.1. RADAR 7.1.1 Plotting, 7.1.2. Long-range scanning 7.2 Compass bearing, 7.2.1. Bearing change 7.3. Approaching, 7.3.1. Close range
Rule 8	8.1. Ample time, 8.2. Good seamanship, 8.3. RADAR 8.4. Alteration, 8.4.1. Course, 8.4.1.1. Sufficient sea-room, 8.4.1.2. Close-quarters situation, 8.4.1.3. Large angle 8.4.2. Speed, 8.4.2.1. Stopping, 8.4.2.2. Reversing 8.5. Safe distance, 8.5.1. Finally past and clear 8.6. Early action, 8.6.1. Sufficient sea-room
Rule 9	9.1. Outer limit, 9.1.1. Channel, 9.1.2. Fairway 9.2. Passage, 9.2.1. Starboard side, 9.2.2. Alteration, 9.2.3. Caution 9.3. Overtaking, 9.3.1. Safe passing, 9.3.2. Appropriate signal
Rule 10	10.1. Traffic line, 10.1.1. General direction of traffic flow 10.2. Clear area, 10.2.1. Separation zone, 10.2.2. Separation line 10.3. Small angle, 10.3.1. Joining, 10.3.2. Leaving 10.4. Crossing traffic lanes, 10.4.1. Right angle 10.5. Wide margin, 10.5.1. Not using TSS 10.6. Terminations, 10.6.1. Particular caution 10.7. Exemption
Rule 13	13.1. Overtaking, 13.1.1. Action, 13.1.2. Duty of keeping clear 13.2. Overtaken
Rule 14	14.1. Alteration, 14.1.1. Course, 14.1.1.1. Starboard side 14.2. Situation, 14.2.1. Mast head and both sidelights
Rule 15	15.1. Avoiding, 15.1.1. Vessel on own starboard side, 15.1.2. Crossing ahead
Rule 16	16.1. Early and substantial action
Rule 17	17.1. Maneuver alone, 17.2. Best aid 17.3. Avoiding, 17.3.1. Port course
Rule 18	18.1. Sequence, 18.1.1. Power-driven vessel, 18.1.2. Sailing vessel, 18.1.3. Vessel engaged in fishing, 18.1.4. WIG, 18.1.5. Not under command, 18.1.6. Restricted ability to maneuver

,

Table 8. Requirements analysis of collision avoidance action (speed and course).

Collision Avoidance Action (Speed and Course)
Restricted Water (Two-Ship, Multi-Ship)
Rule 8	8.1. Ample time, 8.2. Good seamanship, 8.3. RADAR 8.4. Alteration, 8.4.1. Course, 8.4.1.1. Sufficient sea-room, 8.4.1.2. Close-quarters situation, 8.4.1.3. Large angle 8.4.2. Speed, 8.4.2.1. Stopping, 8.4.2.2. Reversing 8.5. Safe distance, 8.5.1. Finally past and clear 8.6. Early action, 8.6.1. Sufficient sea-room

and

Table 9. Requirements analysis of collision avoidance action (course).

Collision Avoidance Action (Course)
Open Sea (Two-Ship, Multi-Ship)
Rule 8	8.1. Ample time, 8.2. Good seamanship, 8.3. RADAR 8.4. Alteration, 8.4.1. Course, 8.4.1.1. Sufficient sea-room, 8.4.1.2. Close-quarters situation, 8.4.1.3. Large angle 8.4.2. Speed, 8.4.2.1. Stopping, 8.4.2.2. Reversing 8.5. Safe distance, 8.5.1. Finally past and clear 8.6. Early action, 8.6.1. Sufficient sea-room

, are structured based on Figure 3 and Figure 4. Table 3 presents the analysis of the requirements for ship anti-collision responsibility (give-way vessel) according to Rules 13–18 in the open sea and restricted waters, organized by keywords.

The requirements to address environmental interference were analyzed by organizing the keywords extracted from Rules 6, 9, and 10. Rule 9 is applicable to restricted waters, whereas Rules 6 and 10 are applicable to both open sea and restricted waters.

Table 5 contains the requirements for ship maneuverability and TS maneuvering, and the keywords corresponding to rules 6, 8, and 17 were applied.

Table 6 shows the requirements for the ship domain with organized keywords from Rules 5–8 and 13–17 applicable to the open sea and restricted waters.

The requirements for expert experience are presented in Table 7 by organizing the keywords from Rules 5–10 and 13–18 in the open sea and restricted waters.

Collision avoidance action (speed and course) calls for the requirements analyzed in Table 8, with organized keywords corresponding to Rule 8.

Table 9 lists the requirements for collision avoidance action (course). Rule 8 applies to the open sea, and the corresponding keywords have been organized.

2.5. Requirements Specification

The requirement IDs for each requirement specification based on the analyzed requirements are defined in

Table 10. Allocation of requirements ID.

Division	Rule	ID
Ship anti-collision responsibility (give-way vessel)	13–18	ACR-rule no.-keyword no.
Environmental interference	6, 9, 10	EI-rule no.-keyword no.
Ship maneuverability	6, 8, 17	SM-rule no.-keyword no.
Target ship maneuvering		TSM-rule no.-keyword no.
Ship domain	5–8, 13–17	SD-rule no.-keyword no.
Expert experience	5–10, 13–18	EE-rule no.-keyword no.
Collision avoidance action (speed and course)	8	CA(SC)-rule no.-keyword no.
Collision avoidance action (course)	8	CA(C)-rule no.-keyword no.

. The IDs were first categorized as ACR, EI, SM, TSM, SD, EE, CA(SC), and CA(C) for ship anti-collision responsibility, environmental interference, ship maneuverability, TS maneuvering, ship domain, expert experience, and collision avoidance action, respectively, and were constructed using the rule number and keyword number corresponding to the criteria. For example, the ID for the keyword 8.4.1.2 of Rule 8 for collision avoidance action (course) would be CA(C)-08-8.4.1.2. According to this method, IDs were formed for the criteria of ship anti-collision responsibility, environmental interference, ship maneuverability, TS maneuvering, ship domain, expert experience, and collision avoidance action, as listed in

Table A1. Requirements specifications for ship anti-collision responsibility (give-way vessel).

Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 13	13.1. Overtaking 13.1.1. Action 13.1.2. Duty of keeping clear 13.2. Overtaken	ACR-13-13.1.1 ACR-13-13.1.2 ACR-13-13.2
Rule 14	14.1. Alteration 14.1.1. Course 14.1.1.1. Starboard side 14.2. Situation 14.2.1. Mast head and both sidelights	ACR-14-14.1.1.1 ACR-14-14.2.1.1
Rule 15	15.1. Avoiding 15.1.1. Vessel on own starboard side 15.1.2. Crossing ahead	ACR-15-15.1.1 ACR-15-15.1.2
Rule 16	16.1. Early and substantial action	ACR-16-16.1
Rule 17	17.1. Maneuver alone 17.2. Best aid 17.3. Avoiding 17.3.1. Port course	ACR-17-17.1 ACR-17-17.2 ACR-17-17.3.1
Rule 18	18.1. Sequence 18.1.1. Power-driven vessel 18.1.2. Sailing vessel 18.1.3. Vessel engaged in fishing 18.1.4. WIG 18.1.5. Not under command, 18.1.6. Restricted ability to maneuver	ACR-18-18.1.1 ACR-18-18.1.2 ACR-18-18.1.3 ACR-18-18.1.4 ACR-18-18.1.5 ACR-18-18.1.6

,

Table A2. Requirements specifications for environmental interference.

Restricted Water (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 9	9.1. Outer limit 9.1.1. Channel 9.1.2. Fairway 9.2. Passage 9.2.1. Starboard side 9.2.2. Alteration 9.2.3. Caution 9.3. Overtaking 9.3.1. Safe passing 9.3.2. Appropriate signal	EI-09-9.1.1 EI-09-9.1.2 EI-09-9.2.1 EI-09-9.2.2 EI-09-9.2.3 EI-09-9.3.1 EI-09-9.3.2
Open Sea (Two-Ship, Multi-Ship) and restricted Waters (Two-Ship, Multi-Ship)
Rule and keyword		ID
Rule 6	6.1. Visibility 6.2. Traffic density 6.3. Background light 6.4. Draft 6.5. RADAR 6.6. Maneuverability 6.6.1. Stopping distance 6.6.2. Turning ability 6.7. Navigation hazards 6.7.1. Wind 6.7.2. Sea 6.7.3. Current	EI-06-6.1 EI-06-6.2 EI-06-6.3 EI-06-6.4 EI-06-6.5 EI-06-6.6.1 EI-06-6.6.2 EI-06-6.7.1 EI-06-6.7.2 EI-06-6.7.3
Rule 10	10.1. Traffic line 10.1.1. General direction of traffic flow 10.2. Clear area 10.2.1. Separation zone 10.2.2. Separation line 10.3. Small angle 10.3.1. Joining 10.3.2. Leaving 10.4. Crossing traffic lanes 10.4.1. right angle 10.5. Wide margin 10.5.1. Not using TSS 10.6. Terminations 10.6.1. Particular caution 10.7. Exemption	EI-10-10.1.1 EI-10-10.2.1 EI-10-10.2.2 EI-10-10.3.1 EI-10-10.3.2 EI-10-10.4.1 EI-10-10.5.1 EI-10-10.6.1 EI-10-10.7

,

Table A3. Requirements specifications for ship maneuverability.

Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 6	6.1. Visibility 6.2. Traffic density 6.3. Background light 6.4. Draft 6.5. RADAR 6.6. Maneuverability 6.6.1. Stopping distance 6.6.2. Turning ability 6.7. Navigation hazards 6.7.1. Wind 6.7.2. Sea 6.7.3. Current	SM-06-6.1 SM-06-6.2 SM-06-6.3 SM-06-6.4 SM-06-6.5 SM-06-6.6.1 SM-06-6.6.2 SM-06-6.7.1 SM-06-6.7.2 SM-06-6.7.3
Rule 8	8.1. Ample time 8.2. Good seamanship 8.3. RADAR 8.4. Alteration 8.4.1. Course 8.4.1.1. Sufficient sea-room 8.4.1.2. Close-quarters situation 8.4.1.3. Large angle 8.4.2. Speed 8.4.2.1. Stopping 8.4.2.2. Reversing 8.5. Safe distance 8.5.1. Finally past and clear 8.6. Early action 8.6.1. Sufficient sea-room	SM-08-8.1 SM-08-8.2 SM-08-8.3 SM-08-8.4.1.1 SM-08-8.4.1.2 SM-08-8.4.1.3 SM-08-8.4.2.1 SM-08-8.4.2.2 SM-08-8.5.1 SM-08-8.6.1
Rule 17	17.1. Maneuver alone 17.2. Best aid 17.3. Avoiding 17.3.1. Port course	SM-17-17.1 SM-17-17.2 SM-17-17.3.1

,

Table A4. Requirements specifications for target ship maneuverability.

Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 6	6.1. Visibility 6.2. Traffic density 6.3. Background light 6.4. Draft 6.5. RADAR 6.6. Maneuverability 6.6.1. Stopping distance 6.6.2. Turning ability 6.7. Navigation hazards 6.7.1. Wind 6.7.2. Sea 6.7.3. Current	TSM-06-6.1 TSM-06-6.2 TSM-06-6.3 TSM-06-6.4 TSM-06-6.5 TSM-06-6.6.1 TSM-06-6.6.2 TSM-06-6.7.1 TSM-06-6.7.2 TSM-06-6.7.3
Rule 8	8.1. Ample time 8.2. Good seamanship 8.3. RADAR 8.4. Alteration 8.4.1. Course 8.4.1.1. Sufficient sea-room 8.4.1.2. Close-quarters situation 8.4.1.3. Large angle 8.4.2. Speed 8.4.2.1. Stopping 8.4.2.2. Reversing 8.5. Safe distance 8.5.1. Finally past and clear 8.6. Early action 8.6.1. Sufficient sea-room	TSM-08-8.1 TSM-08-8.2 TSM-08-8.3 TSM-08-8.4.1.1 TSM-08-8.4.1.2 TSM-08-8.4.1.3 TSM-08-8.4.2.1 TSM-08-8.4.2.2 TSM-08-8.5.1 TSM-08-8.6.1
Rule 17	17.1. Maneuver alone 17.2. Best aid 17.3. Avoiding 17.3.1. Port course	TSM-17-17.1 TSM-17-17.2 TSM-17-17.3.1

,

Table A5. Requirements specifications for ship domain.

Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 5	5.1. All available means 5.2. Risk of collision	SD-05-5.1 SD-05-5.2
Rule 6	6.1. Visibility 6.2. Traffic density 6.3. Background light 6.4. Draft 6.5. RADAR 6.6. Maneuverability 6.6.1. Stopping distance 6,6,2. Turning ability 6.7. Navigation hazards 6.7.1. Wind 6.7.2. Sea 6.7.3. Current	SD-06-6.1 SD-06-6.2 SD-06-6.3 SD-06-6.4 SD-06-6.5 SD-06-6.6.1 SD-06-6.6.2 SD-06-6.7.1 SD-06-6.7.2 SD-06-6.7.3
Rule 7	7.1. RADAR 7.1.1 Plotting 7.1.2. Long-range scanning 7.2. Compass bearing 7.2.1. Bearing change 7.3. Approaching 7.3.1. Close range	SD-07-7.1.1 SD-07-7.1.2 SD-07-7.2.1 SD-07-7.3.1
Rule 8	8.1. Ample time 8.2. Good seamanship 8.3. RADAR 8.4. Alteration 8.4.1. Course 8.4.1.1. Sufficient sea-room 8.4.1.2. Close-quarters situation 8.4.1.3. Large angle 8.4.2. Speed 8.4.2.1. Stopping 8.4.2.2. Reversing 8.5. Safe distance 8.5.1. Finally past and clear 8.6. Early action 8.6.1. Sufficient sea-room	SD-08-8.1 SD-08-8.2 SD-08-8.3 SD-08-8.4.1.1 SD-08-8.4.1.2 SD-08-8.4.1.3 SD-08-8.4.2.1 SD-08-8.4.2.2 SD-08-8.5.1 SD-08-8.6.1
Rule 13	13.1. Overtaking 13.1.1. Action 13.1.2. Duty of keeping clear 13.2. Overtaken	SD-13-13.1.1 SD-13-13.1.2 SD-13-13.2
Rule 14	14.1. Alteration 14.1.1. Course 14.1.1.1. Starboard side 14.2. Situation 14.2.1. Mast head and both sidelights	SD-14-14.1.1.1 SD-14-14.2.1.1
Rule 15	15.1. Avoiding 15.1.1. Vessel on own starboard side 15.1.2. Crossing ahead	SD-15-15.1.1 SD-15-15.1.2
Rule 16	16.1. Early and substantial action	SD-16-16.1
Rule 17	17.1. Maneuver alone 17.2. Best aid 17.3. Avoiding 17.3.1. Port course	SD-17-17.1 SD-17-17.2 SD-17-17.3.1

,

Table A6. Requirements specifications for expert experience.

Open Sea (Two-Ship, Multi-Ship) and Restricted Waters (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 5	5.1. All available means 5.2. Risk of collision	EE-05-5.1 EE-05-5.2
Rule 6	6.1. Visibility 6.2. Traffic density 6.3. Background light 6.4. Draft 6.5. RADAR 6.6. Maneuverability 6.6.1. Stopping distance 6.6.2. Turning ability 6.7. Navigation hazards 6.7.1. Wind 6.7.2. Sea 6.7.3. Current	EE-06-6.1 EE-06-6.2 EE-06-6.3 EE-06-6.4 EE-06-6.5 EE-06-6.6.1 EE-06-6.6.2 EE-06-6.7.1 EE-06-6.7.2 EE-06-6.7.3
Rule 7	7.1. RADAR 7.1.1. Plotting 7.1.2. Long-range scanning 7.2. Compass bearing 7.2.1. Bearing change 7.3. Approaching 7.3.1. Close range	EE-07-7.1.1 EE-07-7.1.2 EE-07-7.2.1 EE-07-7.3.1
Rule 8	8.1. Ample time 8.2. Good seamanship 8.3. RADAR 8.4. Alteration 8.4.1. Course 8.4.1.1. Sufficient sea-room 8.4.1.2. Close-quarters situation 8.4.1.3. Large angle 8.4.2. Speed 8.4.2.1. Stopping 8.4.2.2. Reversing 8.5. Safe distance 8.5.1. Finally past and clear 8.6. Early action 8.6.1. Sufficient sea-room	EE-08-8.1 EE-08-8.2 EE-08-8.3 EE-08-8.4.1.1 EE-08-8.4.1.2 EE-08-8.4.1.3 EE-08-8.4.2.1 EE-08-8.4.2.2 EE-08-8.5.1 EE-08-8.6.1
Rule 9	9.1. Outer limit 9.1.1. Channel 9.1.2. Fairway 9.2. Passage 9.2.1. Starboard side 9.2.2. Alteration 9.2.3. Caution 9.3. Overtaking 9.3.1. Safe passing 9.3.2. Appropriate signal	EE-09-9.1.1 EE-09-9.1.2 EE-09-9.2.1 EE-09-9.2.2 EE-09-9.2.3 EE-09-9.3.1 EE-09-9.3.2
Rule 10	10.1. Traffic line 10.1.1. General direction of traffic flow 10.2. Clear area 10.2.1. Separation zone 10.2.2. Separation line 10.3. Small angle 10.3.1. Joining 10.3.2. Leaving 10.4. Crossing traffic lanes 10.4.1. Right angle 10.5. Wide margin 10.5.1. Not using TSS 10.6. Terminations 10.6.1. Particular caution 10.7. Exemption	EE-10-10.1.1 EE-10-10.2.1 EE-10-10.2.2 EE-10-10.3.1 EE-10-10.3.2 EE-10-10.4.1 EE-10-10.5.1 EE-10-10.6.1 EE-10-10.7
Rule 13	13.1. Overtaking 13.1.1. Action 13.1.2. Duty of keeping clear 13.2. Overtaken	EE-13-13.1.1 EE-13-13.1.2 EE-13-13.2
Rule 14	14.1. Alteration 14.1.1. Course 14.1.1.1. Starboard side 14.2. Situation 14.2.1. Mast head and both sidelights	EE-14-14.1.1.1 EE-14-14.2.1.1
Rule 15	15.1. Avoiding 15.1.1. Vessel on own starboard side 15.1.2. Crossing ahead	EE-15-15.1.1 EE-15-15.1.2
Rule 16	16.1. Early and substantial action	EE-16-16.1
Rule 17	17.1. Maneuver alone 17.2. Best aid 17.3. Avoiding 17.3.1. Port course	EE-17-17.1 EE-17-17.2 EE-17-17.3.1
Rule 18	18.1. Sequence 18.1.1. Power-driven vessel 18.1.2. Sailing vessel 18.1.3. Vessel engaged in fishing 18.1.4. WIG 18.1.5. Not under command, 18.1.6. Restricted ability to maneuver	EE-18-18.1.1 EE-18-18.1.2 EE-18-18.1.3 EE-18-18.1.4 EE-18-18.1.5 EE-18-18.1.6

,

Table A7. Requirements specifications for collision avoidance action (speed and course).

Restricted Waters (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 8	8.1. Ample time 8.2. Good seamanship 8.3. RADAR 8.4. Alteration 8.4.1. Course 8.4.1.1. Sufficient sea-room 8.4.1.2. Close-quarters situation 8.4.1.3. Large angle 8.4.2. Speed 8.4.2.1. Stopping 8.4.2.2. Reversing 8.5. Safe distance 8.5.1. Finally past and clear 8.6. Early action 8.6.1. Sufficient sea-room	CA(SC)-08-8.1 CA(SC)-08-8.2 CA(SC)-08-8.3 CA(SC)-08-8.4.1.1 CA(SC)-08-8.4.1.2 CA(SC)-08-8.4.1.3 CA(SC)-08-8.4.2.1 CA(SC)-08-8.4.2.2 CA(SC)-08-8.5.1 CA(SC)-08-8.6.1

and

Table A8. Requirements specifications for collision avoidance action (course).

Open Sea (Two-Ship, Multi-Ship)
Rule and Keyword		ID
Rule 8	8.1. Ample time 8.2. Good seamanship 8.3. RADAR 8.4. Alteration 8.4.1. Course 8.4.1.1. Sufficient sea-room 8.4.1.2. Close-quarters situation 8.4.1.3. Large angle 8.4.2. Speed 8.4.2.1. Stopping 8.4.2.2. Reversing 8.5. Safe distance 8.5.1. Finally past and clear 8.6. Early action 8.6.1. Sufficient sea-room	CA(C)-08-8.1 CA(C)-08-8.2 CA(C)-08-8.3 CA(C)-08-8.4.1.1 CA(C)-08-8.4.1.2 CA(C)-08-8.4.1.3 CA(C)-08-8.4.2.1 CA(C)-08-8.4.2.2 CA(C)-08-8.5.1 CA(C)-08-8.6.1

.

3. Verification of Requirements through Application of Representative Local Path-Planning Algorithms

3.1. Requirements Verification

The representative local path-planning algorithms that are strongly related to COLREGs part B were identified as APF, VO, and A$\ast$ search by Ülkü et al. [10], as summarized in

Table 11. COLREGs part B relevance of different local path-planning algorithms in maritime autonomous surface ships.

Algorithm	APF	VO	$\mathbf{A}\ast$
Number of studies that consider COLREGs rules	11	9	5
Number of studies that do not consider COLREGs rules	15	6	9
Total number of rules considered in all studies	71	60	36
Number of unique COLREGs rules	12	12	12
Maximum number of COLREGs rules in all studies	312	180	168
COLREGs rules relevance	21%	33%	22%

. Particularly, the VO parameters were capable of effectively interpreting the mechanical elements of marine collision based on COLREGs part B.

The successful application of the requirements is verified in

Table 12. Method of requirements specifications.

Algorithms	ACR	EI	SM	TSM	SD	EE	CA(SC)	CA(C)
A$\ast$[21]	√	-	√	-	-	√	-	√
A$\ast$[22]	√	-	√	-	-	-	-	√
APF [24]	√	-	√	√	√	-	-	√
VO [26]	√	-	√	-	-	√	√	-
VO [27]	√	-	√	-	-	-	-	-
VO [28]	√	-	√	-	√	-	√	-

based on the requirements specified in the representative local path-planning algorithms, including VO [26,27,28], A$\ast$ search [21,22], and APF [24]. First, the IDs generated from the seven classification criteria—ACR, EI, SM, TSM, SD, EE, CA(SC), and CA(C)—were applied to the representative local path-planning algorithms. The algorithms APF [24], VO [26,27,28], and A$\ast$ search [21,22] did not satisfy all seven classification criteria.

However, validation must be performed based on the specified requirements, because the representative local path-planning algorithms were developed using both ACR and SM among the seven classification criteria. Therefore, the algorithms were verified against the IDs constructed using the rule number and keyword number corresponding to each classification criterion. The representative local path-planning algorithms did not satisfy the requirements corresponding to ACR-18, as shown in

Table 13. Validation for requirements specifications of ship anti-collision responsibility (give-way vessel).

ID		$\mathbf{A}\ast$ [21]	$\mathbf{A}\ast$ [22]	APF [24]	VO [26]	VO [27]	VO [28]
Rule 13	ACR-13-13.1.1	-	-	√	√	√	√
	ACR-13-13.1.2	-	-	√	√	√	√
	ACR-13-13.2	-	-	√	-	-	√
Rule 14	ACR-14-14.1.1.1	√	√	√	√	√	√
	ACR-14-14.2.1.1	√	√	√	√	√	√
Rule 15	ACR-15-15.1.1	√	√	√	√	√	√
	ACR-15-15.1.2	-	-	-	-	-	-
Rule 16	ACR-16-16.1	√	√	√	√	√	√
Rule 17	ACR-17-17.1	√	√	√	√	√	√
	ACR-17-17.2	√	√	√	√	√	√
	ACR-17-17.3.1	√	√	√	√	√	√
Rule 18	ACR-18-18.1.1	-	-	-	-	-	-
	ACR-18-18.1.2	-	-	-	-	-	-
	ACR-18-18.1.3	-	-	-	-	-	-
	ACR-18-18.1.4	-	-	-	-	-	-
	ACR-18-18.1.5	-	-	-	-	-	-
	ACR-18-18.1.6	-	-		-	-	-

. That is, they did not reflect the criteria for responsibilities between vessels (autonomous ship and TS). APF [24] and VO [28] reflected the most requirements, with the exception of ACR-18. Nevertheless, they failed to satisfy ACR-15-15.1.2 for crossing ahead of the TS, where the autonomous ship was the stand-on vessel during the crossing.

The application of the requirements for the SM criteria in

Table 14. Validation for requirements specifications for ship maneuverability.

ID		$\mathbf{A}\ast$ [21]	$\mathbf{A}\ast$ [22]	APF [24]	VO [26]	VO [27]	VO [28]
Rule 6	SM-06-6.1	-	-	√	-	-	-
	SM-06-6.2	-	-	-	-	-	√
	SM-06-6.3	-	-	-	-	-	-
	SM-06-6.4	-	-	-	-	-	-
	SM-06-6.5	√	√	√	√	√	√
	SM-06-6.6.1	-	-	-	-	-	-
	SM-06-6.6.2	√	√	√	√	√	√
	SM-06-6.7.1	-	-	-	-	-	-
	SM-06-6.7.2	-	-	-	-	-	-
	SM-06-6.7.3	-	-	-	-	-	-
Rule 8	SM-08-8.1	√	√	√	√	√	√
	SM-08-8.2	-	-	-	-	-	-
	SM-08-8.3	√	√	√	√	√	√
	SM-08-8.4.1.1	√	√	√	√	√	√
	SM-08-8.4.1.2	√	√	√	√	√	√
	SM-08-8.4.1.3	√	√	√	√	√	√
	SM-08-8.4.2.1	-	-	-	-	-	-
	SM-08-8.4.2.2	-	-	-	-	-	-
	SM-08-8.5.1	√	√	√	√	√	√
	SM-08-8.6.1	√	√	√	√	√	√
Rule 17	ACR-17-17.1	√	√	√	√	√	√
	ACR-17-17.2	√	√	√	√	√	√
	ACR-17-17.3.1	√	√	√	√	√	√

revealed that APF [24] and VO [28] satisfied the highest number of requirements, as observed with the ACR requirements. Nevertheless, the algorithms collectively did not satisfy SM-06-6.3, SM-06-6.4, SM-06-6.7.1, SM-06-6.7.2, and SM-06-6.7.3 of Rule 6 and SM-08-8.2, SM-08-8.4.2.1, and SM-08-8.4.2 of Rule 8.

3.2. Discussion

The requirements specification that reflected both the navigation practices reported by the navigator and the rules in COLREGs part B allowed timely and accurate validation of the representative local path-planning algorithms for requirements satisfaction, as described in Section 3.1. Therefore, the specified requirements facilitate not only the analysis and improvement of the previously developed local path-planning algorithms, but also the development of a new path-planning algorithm. Nevertheless, ambiguous keywords in the specified requirements need to be defined. The first example is “crossing ahead”, which corresponds to 15.1.2 of Rule 15. When the autonomous ship is a stand-on vessel and the target vessel is the give-way vessel, the stand-on vessel can perform anti-collision actions even when the give-way vessel has not taken appropriate actions. According to Rule 17, the stand-on vessel is prohibited from left-hand (port) steering. Therefore, the stand-on vessel ought to move to the right-hand (starboard) side. In such a situation, crossing ahead of the give-way vessel cannot be avoided. The second example is “good seamanship” (8.2 from Rule 8). Seamanship is the capability of the navigator depending on their skills and knowledge to resolve a navigation situation that cannot be explained solely based on the COLREGs part B. Therefore, autonomous vessels must benefit from good seamanship in various navigation scenarios and follow navigation practices while ensuring compliance with the COLREGs part B.

4. Conclusions

In this research, the requirements for optimal local path planning of autonomous vessels were analyzed and specified by considering open sea, restricted waters, and two-ship and multi-ship interactions based on navigation practices and COLREGs part B. The requirements were collected, extracted, analyzed, and defined. In the requirements collection stage, navigation practices were surveyed and categorized based on the criteria identified in the literature review. In the requirements extraction stage, the key rules and keywords on collision avoidance were identified by relating the navigation practices to the COLREGs part B. In the requirements analysis step, the requirements were specified using the IDs constructed for organization and definition according to the rules and keywords corresponding to the classification criteria. The specified requirements were verified by applying the representative local path-planning algorithms. This method determined whether the requirements were timely and accurately satisfied by conventional algorithms. Nevertheless, owing to the ambiguity of keywords used for defining the requirements, future studies should apply contextual definitions for improving the requirements specifications.