On the shape,size and realisation of ship safety domains on the North Sea

A ship’s safety domain is a principal element in conflict detection and avoidance manoeuvres and consequently collision risk. Conflict detection depends on the ability to predict the intrusion of another vessel into the safety domain of the own-ship. To resolve a conflict the own-ship must manoeuvre so that the intruder remains outside this domain. In this study the shape, size and realisation of a safety domain on the North Sea are examined using Automatic Identification System (AIS) data collected from the Dutch sector of the North Sea (March–May 2014). In this study, encounters between ships over 45 m in length and travelling at 5–25 knot were analysed. Five hypotheses were tested: (1) the safety domain is elliptical rather than circular; (2) the domain size is a function of the ship speed; and (3) ship size has no effect on domain size when distances are measured hull-to-hull; (4) safety domains are created using minimal course changes; (5) those course changes are based on CPA calculations. The empirical results confirm that domains can be described by ellipses. The major-axis length increases with speed while the minor-axis is independent of speed, and ship size does not influence domain size as defined significantly. Analysis of manoeuvring behaviour shows that bridge teams seem to rely on ARPA-derived CPA and TCPA calculations, initiating avoidance manoeuvres typically between 600 and 1800 s before the predicted closest point of approach. Most course changes are modest and aim to maintain a CPA that corresponds to the empirically derived elliptical domain. The study suggests that an elliptical safety domain can be defined for vessels operating in deep-sea conditions. Such a domain would provide a practical basis for improving conflict-detection algorithms, decision-support tools for bridge teams, and autonomous-ship navigation systems while remaining consistent with the COLREGs.