In its latest Safety Digest, UK MAIB provides lessons learned from an incident where a large workboat’s port propeller shaft became fouled by the emergency towline.
The incident
It was the early hours of the morning and dark, but visibility was good and the weather conditions fine. A large workboat (WB1) was on a coastal passage, heading north and towing two smaller, unmanned, catamaran workboats (WB2 and WB3).
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The tow was set up with 100m of towline between WB1 and WB2, then 80m of towline between WB2 and WB3. An emergency towline was also being streamed astern of WB3 and all three workboats were correctly lit as a ‘tug and tow’, restricted in their ability to manoeuvre and therefore unable to keep out of the way of another vessel.
However, defects meant that only WB3 was transmitting on the automatic identification system (AIS).
A small general cargo vessel was heading east on passage in the same area and had observed a vessel ahead by AIS and visual observation. The cargo vessel’s officer of the watch (OOW) did not alter course and intended passing close by the contact.
WB1’s crew were increasingly concerned about the situation and used a searchlight to try and attract the cargo vessel OOW’s attention. WB1’s crew also tried hailing the cargo vessel by very high frequency (VHF) radio; this warning came too late and the cargo vessel passed between WB2 and WB3, severing the towline and casting WB3 adrift (Figure 1, inset).
Realizing what had happened the cargo vessel was stopped, then both WB1’s skipper and the cargo vessel’s OOW called the coastguard to report the incident. When trying to recover the situation, WB1’s port propeller shaft became fouled by the emergency towline (Figure 2). Once the tow had been re-established and everyone safely accounted for, the coastguard agreed that the vessels could continue their passages.
Lessons learned
#1 Observe: The International Regulations for Preventing Collisions at Sea (COLREGs) require a constant cycle of keeping a good lookout, assessing the situation and taking action to avoid collision when judged necessary. As the cargo vessel was approaching the tow, the OOW identified that there was a vessel ahead; however, inadequate action was taken to properly assess the situation. The primary means of assessing collision risk is visual and radar information and sufficient evidence should have been available to the cargo vessel’s OOW to see that action was necessary (as the ‘give-way’ vessel) to pass at a safe distance.
#2 Risk: The cargo vessel’s OOW had observed an AIS contact ahead. AIS is useful to assist the OOW with situational awareness, although should not be relied upon as the primary means of collision avoidance. In this instance, close scrutiny of the combined radar, visual and AIS information could potentially have indicated that more than one vessel was ahead. Under these circumstances, or where there is uncertainty, reducing speed would allow more time to accurately assess the situation.
#3 Action: As the situation developed, WB1’s skipper used a searchlight and VHF radio to alert the cargo vessel’s OOW. However, these actions came too late to be effective. In any collision situation, it is incumbent upon both vessels taking steps to avoid collision. This includes the ‘stand-on’ vessel taking evasive action where it is judged the ‘give-way’ vessel’s actions alone will be insufficient to avoid collision.
