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OHAAKI COOLING TOWER SLUDGE REMOVAL

Ohaaki geothermal power station’s distinctive cooling tower, rising 105m above the surrounding plain, requires cleaning every 20 years.

A thick layer of toxic sludge on the floor of the tower must be carefully removed and processed safely.

Intergroup, with specialist equipment and trained operators brought in from around the North Island, was up to the job – and took on additional work when another contractor got cold feet.

Every 20 years the base of Ohaaki Geothermal power station’s cooling tower must be cleaned of some 300T of toxic sludge. Intergroup’s industrial services team completed the task in less than half the allocated time, and stepped in for more work when another contractor failed to deliver.

Cleaning out Ohaaki’s natural draft cooling tower

Ohaaki’s tower is used to cool geothermally heated groundwater, extracted as steam, that has been used to drive three turbines. The steam is condensed to water in the process.

This water, still at high temperature, is cooled by being run over packers in the cooling tower. Natural ventilation extracts waste heat to the atmosphere.

In the process of cooling, the water leaves mineral deposits on the packers. These deposits slowly build up, until eventually they break off and fall to the base of the cooling tower. Over a number of years a thick bed of water-logged material builds up on the cooling tower floor. This sludge, a toxic mixture containing, amongst other things mercury and arsenic, must be removed during a periodic plant shutdown.

This is where Intergroup steps in. Using multiple sucker trucks and dozens of staff deployed from depots all over the North Island, we removed around 300T of waste (30x 10cubic metre loads). In five days we were able to safely remove the waste material and transport it to a consented processing facility.

Fast work

Originally allowed seven days window, with five of those days working 24 hours, we were asked to complete the shut without working a night shift and within only five days.

We ramped up our resources and completed the majority of the job in just five days with 12 hour shifts, meaning that Contact Energy could complete its maintenance programme on time.

Safety first

Working in toxic conditions is nothing new for our staff. All personnel on site are fully trained in working with hazardous materials and use full PPE gear while working inside the tower. Their PPE gear includes hood, gloves face shields, goggles and filter mask to protect against mercury inhalation.

Our close attention to health and safety of staff resulted in a completed project with no incidents.

Meeting an unexpected challenge

Intergroup was originally contracted only to remove the waste from the floor of the cooling tower, while another industrial services company was contracted to remove the packers – a job that included all of the toxic material hazards as well as working with other plant and contractors on site.

We were one day off finishing our part of the job when the other company advised that it was not able to undertake the packer removal job.

Contact Energy offered the job to us and asked us to contract manage this next part of the shut, due to lack of time and the previous job going well.

Within a day our site and contract managers had worked together to decide on a way to tackle the job safely, brought in cherry pickers, a loader and a truck and trailer and allocated staff to oversee loading, disposal and traffic management. We commenced work on the same day that the other contractor was due to start and the project was finished on time.

Icing on the cake

Contract Energy’s managers at Ohaaki were delighted that we were willing to step in after being let down by the other contractor. Being able to mobilise with little notice, and get the job done safely, within the original programme timing, was icing on the cake.

Every 20 years the base of Ohaaki Geothermal power station’s cooling tower must be cleaned of some 300T of toxic sludge. Intergroup’s industrial services team completed the task in less than half the allocated time, and stepped in for more work when another contractor failed to deliver.