The massive log fell into the lake from surrounding bushland sometime around 30 August, somewhere upstream of the station intake. It became waterlogged and then sunk below the surface making its way down onto the intake screens.

The trash rake crane (normal way of retrieving debris from the intake screens) couldn’t reach the bottom of the intake screens. An initial assessment had to be carried out using a remotely operated underwater vehicle (ROV), which confirmed the issue was a large log.

The team then had to assess the weight of the log to make sure an appropriately sized crane was brought in and confirmed divers would be needed to assist in the retrieval. 

The crane brought in to do the heavy lifting

In the end, four divers had to head between six to 14 metres underwater to secure the log with chains and pull it up with a 150 tonne mobile crane – just for some perspective, a blue whale can weigh between 100–200 tonnes! Only a crane this big had the long reach for the boom (long arm of the crane) and derating effect (for lifting capacity) needed to reach and retrieve the log.

The Cethana Power Station had to stay offline until the log was retrieved to ensure the safety of the divers who were working close to the intake.

Complicating matters further, the contractors’ usual dive boat had been involved in an accident on a non-Hydro Tasmania related job. So, the team had to find another boat large enough to get the dive team and all their equipment to the intake area to start work at short notice.

Around nine Hydro Tasmania crew and contractors were onsite on the day to ensure everything ran smoothly. They planned for everything, from what happens if the log breaks into separate pieces, to diver safety such as emergency procedures for the bends.

The log being retrieved from Lake Cethana

The massive job took eight hours, but it ended with the log removed in one piece. The team then removed a large amount of debris that had accumulated below the log. The Cethana Power Station was then back up and running on the same day thanks to the amazing skills and efforts of the team. 

The log retrieved from Lake Cethana