Mariners warned on the dangers of 'sleep inertia'

Safety experts have warned seafarers about the risks of taking over a watch and carrying out critical duties soon after they wake up.

The alert follows an investigation into an accident which caused damage estimated at US$2.5m, when a towing vessel pushing two tank barges fully loaded with a cargo of naphtha struck a lock gate in the Gulf Intracoastal Waterway in Louisiana in March 2021.

The US National Transportation Safety Board (NTSB) said the pre-dawn incident highlighted the dangers of sleep inertia – 'the temporary feeling of grogginess felt immediately upon awakening'.

The bow of the lead barge in the tow struck the local gate at 05.22hrs, less than 30 minutes after the captain woke up and took over the watch following 4.5 hours of sleep, the NTSB noted.

At the time of the accident, the investigation report states, the captain was probably experiencing the effects of sleep inertia – a condition which 'negatively affects an operator's performance, vigilance, alertness, and decision-making for 30 minutes or more after waking, especially in demanding situations that require high levels of attention and cognitive demand'.

The effects of sleep inertia can be exacerbated by waking during a 'circadian low' and by partial sleep deprivation, it adds. 'The captain's sleep inertia, coupled with the challenges presented by the operating environment, would have negatively impacted his ability to safely navigate through the lock.'

The captain had declined the offer of the pilot onboard to take the tow through the lock and took the helm about five minutes before the manoeuvre began, the report says.

But as the vessel entered the lock it suffered the loss of the GPS feed to its electronic chart system, which denied the captain his primary source of speed indication.

'Judging the speed by eye alone, the captain may have been affected by poor depth perception, which is common during night-time operations,' the report points out.

His ability to judge the speed and distance was also hampered by problems with radio communications between the wheelhouse and the deckhand stationed on the bow of the lead barge, the NTSB found. This meant the captain did not receive reports from the deckhand on the closing distance to the lock gates until the head of the tow was just 250ft from the lock gate, while moving at 3 mph.

The challenges facing the captain may also have been increased by a tidal current pushing the tow from astern and adding speed as it approached the lock, investigators said.

Investigators said the captain should have been familiar with the vessel operating policies and procedures and should not have taken the watch during a critical move. However, his decision to do so may have also been the result of impairment caused by sleep inertia, the report concedes.

'Sleep inertia generally lasts for about 30 minutes after waking but may last longer if a person is sleep deprived,' the NTSB pointed out.

'Mariners should allow time to fully recover from sleep inertia before taking a watch and performing critical duties.'