Expert panel As part of an initial study, a meeting was organised with an expert panel consisting of members of the Shipping Advisory board for the North Sea about the type of encounters that were relevant and the approaches that could be taken. During this initial study two approaches were developed: one looks at the progress of the estimated distance at Closest Point of Approach (CPA) and Time to Closest Point of Approach (TCPA) and another approach estimates the mutual distance that ships normally maintain during an encounter (the ‘ship domain’) at the particular location. This determines whether the ship enters the domain of the other ship during the encounter. Both approaches then rank these encounters.


The approaches were then tested with one month’s AIS data for a busy junction in a traffic separation scheme. Results showed that although most relevant cases were highly ranked, many normal encounters were also ranked equally high. As the study continued criteria were refined for four distinct types of encounters: over-


taking, head-on encounters, give-way ship crossing at bow and give-way ship crossing at stern. For the crossing encounters partic- ularly, criteria for the CPA-TCPA relation largely reduced the number of selected encounters and many normal encounters could be disregarded.


Consistent The improved approach was then tested for one year’s AIS data in two areas: the busy junction and a bigger area with more diverse encounters, without a traffic separation scheme. For this area 1.71% of all recorded encounters (1,818 out of 106,297 for one year) was returned, of which 491 were crossings. For the junction, 0.93% of the recorded encounters were returned (359 out of 38,358 for one year), of which 107 were crossings.


Animations of these encounters give a good impression of the most extreme encounters that have occurred in certain areas over a particular period of time (see Figure 1 and 2). Although the number of returned encounters is still very large, the results show that the methods select consistent and relevant encounters between ships.


Figure 2: Animation of the hazardous encounters from Figure 1. Ellipses indicate the ship domains.


report


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