My notes and other stuff


Paper: Observation and assessment of crossing situations between pleasure craft and a small passenger ferry

This week's paper is one I found through an old episode of The Safety of Work podcast, referencing a text titled Observation and assessment of crossing situations between pleasure craft and a small passenger ferry. I'm picking it for the same reasons the podcast did, which is asking the question Can we get ready for automation by studying non-automated systems?.

This is because the paper looks at a system that should on its face be really easy to automate, if we assume that navigational rules are respected. The paper in fact studies the Ole III, which is a small passenger ferry in the Husøysund strait in Tønsberg municipality, Norway.

The ship is 8m long by 2.6m wide, carries 11 passengers at most (plus the captain who's responsible for the passengers), has a single 38hp engine, and uses only optical navigation with binoculars and a magnetic compass. The captain makes all assessments and decisions according to his experience and judgment (no communication overhead), and the crossing it takes is always the same, which takes roughly 2 minutes when traffic is low and weather is good.

Fig. 1 The Ole III passenger ferry (photo by Tonsberg Sjomannsforening), a small yellow boat with two cyclists on board and a small cabin for the captain at the back

The strait it carries them across is between 100 and 150m wide shore-to-shore, and has a central channel 6 meters deep with no traffic separation zones. By navigational rules (they're complex and listed in the paper), the Ole III ought to have right of way by virtue of being a commercial craft with passengers. However, the channel is heavy with traffic, and is used frequently by pleasure crafts (which the authors expected would not respect all rules). Similarly, since the Ole III is small and maneuverable and has no large draft, it cannot with certainty claim right-of-way with vessels on its starboard side. The law also states that all other ships (including pleasure crafts) should "as far as possible keep away", which is not the same as actually giving way.

All in all, this sounds like it should be as straightforward as it can be: small, short route, always the same, with a general right of way and no need for fancy instrumentation. But it's a bit more complicated than that.

While the captain of the Ole III might be able to claim that he has legally right of way, being able to know this is what would happen in practice depends on other ships' understanding of navigational law as well. Specifically pleasure crafts are possibly manned by incompetent skippers, who may be on vacation, driving at high speeds, while drunk.

So what happened in the paper is that the scientists sat in between 10am and 8pm between June 4 and August 4 2018, for nearly 4,802 2-minutes long crossings, and look for all sorts of incidents or near-misses. They wanted to account for all deviation from navigational laws that would be encountered by Ole III, to calculate the risks and to see how the captain dealt with them.

They encountered a total of 7415 other vessels coming through, with 4150 from starboard and 3265 from port side. 6225 passengers were recorded, with 1227 under 16 years old and 60 requiring assistance to get on-board (kindergarten age kids). 3995 bikes were also transported.

They recorded 279 instances of other vessels being on a conflicting course that could be given a risk classification of incident or near-miss, accounting to 5.8% of crossings or 8.9% of crossings with vessels nearby. They involved behaviours where other ships didn't respect the rules; notes were taken, thematic analysis was done, and two people analyzed them (a navigator with 8 years of sailing onboard vessels in the Royal Norwegian Navy and 20 years of experience in different jobs in the maritime industry and a Professor of maritime human factors.)

They came up with the following risk categories:

Interestingly enough, there's no way to know if this is a large or small amount of deviations, because there's almost no other data to compare it to. The authors' experience says that this did not feel abnormal however. The incidents were mostly related to not giving way, high speed, lack of attention, people being on the wrong side of the fairway, and high traffic density (averaging 5 other vessels on a crossing course).

The ways the captain of the Ole III avoided incidents is divided in two categories, passive and active control strategies. Passive strategies risk reduction was done by avoiding other vessels, such as waiting before entering the fairway, sailing behind (aft) other ships, reducing speed, active reversal of thrust, and emergency deviation. Active strategies had to do with maintaining steady heading and speed (even if other vessels were around), and communicating in some way, whether through hand signals or the horn.

So for deviations, 89.5% of situations were handled by passive means and 10.5% via active means. For dangerous situations, 79% were handled passively and 21% actively. For critical incidents, passive handling was required 67% of the time and actively 33% of the time.

Table 7: relationship between control strategies and risk categories

So the obvious trend here is that the more critical the situation, the more active the management. Something else revealed by the data is that most of the incidents having to do with pleasure crafts coming from the side which the Ole III should have definite priority are cases where the captain can be considered to creating safety by taking actions that defuse other people's errors. By this perspective, the captain frequently bends the rules and gives way to unlawful behaviour but in a way that can be thought as a counterweight to human error: it's adaptive behaviour that is out of the norms and restores safety.

Those coming from the Ole III starboard's side are more complex. The discussions of the authors with the captain revealed that the captain believed he had the right of way, but maritime law experts don't know if it's a clear-cut case whether he'd be responsible for any collisions due to the amount of control it has compared to say, a sailing ship.

The authors say that it's not necessarily important why captains of vessels act the way they do (ignorance, carelessness, lack of attention, intoxication, etc.), the practical navigational situation itself needs to be resolved:

One way of resolving this is to take a descriptive approach, such as focusing on whether people follow rules; however, this will only help in attributing blame, or judicial responsibilities, and will not help in explaining actual behaviour (i.e. why people choose to follow a rule or not).

They come up with a decision table:

Table 8: Possible outcomes and risks of different mental models

The main key point is whether the intent of both vessels match, not necessarily who is right or wrong. They mention that this match vs. mismatch situation is true whether vessels are manned by humans or automation on either side. Either type is considered an "adaptive agent" and any disagreement in model is riskier than agreement in model:

Irrespective of the nature of the adaptive agent, the challenges described in Table 8 are not possible to resolve unless (1) it is possible to establish communication of intention between vessels or (2) it is possible to ensure that all agents follow the [navigational laws] at all times. The last request is highly unlikely to ever happen as long as pleasure craft skippers lack elementary navigational competencies and knowledge of [navigational laws].

So what are the suggested control strategies? Active control strategies (following the rules and asserting your right of way) actually reduce the safety margins. So long as you can't be sure the other vessel understands your intentions or able/willing to deviate, they're not advisable. Passive situations prevent most risks. For small passenger crafts, this may be advisable. It would however reduce the efficiency more dramatically when traffic is higher.

A third option would be to formalize ways to communicate intentions between vessels (including pleasure crafts). Existing projects are about finding ways to share route plans, which is still tricky because pleasure crafts don't tend to have route plans. A lot of other suggested equipment is generally too expensive. So for the time being they mostly suggest passive strategies.

Overall, this should give interesting ideas about tricks around automation and what can be challenging. It nicely fits in with a lot of the literature linked here before about being able to capture and guess intentions, and that rule breaking is sometimes—if not often—a desirable way to maintain safety. Assuming that rules are going to be respected is a sort of dangerous affair, and a lot of systems aiming for automation that take rules for a granted ("otherwise blame will be on the other anyway") can end up reducing overall system safety compared to having human operators.