Shared mental models can be hazardous when they are based on inadequate data and don’t match reality, as the chief mate and lookout of the offshore supply vessel Far Swan discovered on the night of 6 October 2010 when it collided with the barge Miclyn 131 being towed by an aluminium catamaran Global Supplier. Confirmation bias does not help, either.
That evening Global Supplier was towing the flat barge Miclyn 131, a total tow of around 180 metres from the bow of Global Supplier to the stern of Miclyn 131. Global Supplier was not showing the requisite two white masthead lights in a vertical line and a yellow towing light in a vertical line above its sternlight. The lights had been ordered but not yet fitted.
Global Supplier was not fitted with AIS or radar and was not required to be.
The barge was equipped with portable BargeSafe lights which should have been visible at three nautical miles. They were placed in such a way that the lights were easily missed against background lights and were all-round rather than sectored, making them more difficult to determine accurately at night.
None of this was known to Far Swan’s chief mate, who saw a green light, three to four points on the ship’s port bow. Both the chief mate and the lookout thought that the light was a small boat, and the chief mate checked the radar to see what it had detected. There was a trace of a target on the radar display but no AIS vector on the ECS display. This confirmed the chief mate’s belief that the light was on a small boat. The chief mate then returned his attention to observing the light.
The chief mate and the lookout continued to observe the light and it soon became apparent to them that the small boat was trying to cross ahead. Although the small boat would probably pass ahead, it would be a very close crossing. The chief mate was expecting the boat to ‘give way’. However, there was no indication that the small boat was going to give way so the chief mate shone a search light on the boat to alert its crew to Far Swan’s presence. The search light lit up the small boat, which looked like an aluminium fishing boat. However, the small boat still maintained its easterly course and speed.
They did not shine the search light aft of Global Supplier because there was nothing to indicate that there was anything behind the small boat. Consequently, the search light did not illuminate Miclyn 131.
So, having created the mental model of a small boat crossing the bow of the Far Swan with the sight of the green line, the ‘confirmation’ by the radar image, the lack of an AIS vector and the appearance of the catamaran Global Supplier in the searchlight, they believed they only had to be concerned about one small boat and continued to observe it visually and saw that, while its change of bearing indicated it was going to cross close ahead, it would result in a close quarters situation.
He initially maintained Far Swan’s course and speed, as the stand-on vessel, expecting the boat to give way. However, when it became apparent to him that Global Supplier was maintaining its course and speed and that a close quarter situation would be the result, he made a small alteration of the ship’s course to port.
Far Swan’s chief mate satisfied himself that the small boat had passed ahead of the ship and was clearing to starboard. Then, almost immediately, he and the lookout saw a large object closing on the port bow. Not knowing what it was, the lookout ran onto the bridge wing to try and identify it. The large object was closing quickly and within a few seconds, it was very close on the ship’s port bow. It was only then that the chief mate saw a red and a green navigation light and realised that the small boat was towing what looked like a barge.
It wasn’t. The subsequent collision produced minor damage and no pollution.