Jamie Morton is the NZ Herald's science reporter.

Rena wreck remains a major mess

Latest underwater video footage shows 1000 tonnes of debris across the sea floor - and it has to be picked up

Starboard of the Rena wreck.  Photo / Supplied.
Starboard of the Rena wreck. Photo / Supplied.

The ongoing headache that is the salvage of the wrecked cargo ship Rena has only become worse with the latest task - gathering up about 1000 tonnes of debris strewn across the sea floor.

Newly released underwater video footage makes it clear what the specialists of US-based Resolve Salvage and Fire are up against - stacked and broken containers, mangled piles of steel and countless pieces of litter spread 10,000 square metres around the Rena wreck site, the Astrolabe Reef off the Tauranga coast.

It is a lengthy and expensive operation that has seen salvors dive through water-filled corridors to access submerged oil tanks, pluck containers from high stacks on precarious tilts and whittle away pieces of steel from the ship's bow amid exposed sea conditions.

Clearing the debris field is the fourth major phase of the clean-up since the Rena struck the reef on October 5, 2011. It is expected to take months.

"We are doing it as quickly as we can, but we are always reticent around time frames," Resolve project manager John Curley told the Herald.

"We didn't realise how challenging your weather was going to be."

Sea conditions were highly unpredictable within the reef environment, bringing large swells through the site.

Mr Curley said cleaning the sea floor would be completed by first removing the largest chunks - some weighing up to 30 tonnes - with a barge-mounted heavy-duty crane.

Medium-sized pieces would then be yanked out with a hydraulic grapple, before the operation moves to the longest and most tedious stage - mopping up the dregs.

Smaller steel items, such as wheel rims spilled from containers, will be sucked up by a large magnet lowered from the sea surface.

Masses of aluminium ingots will probably have to be scooped up by divers and gathered into baskets.

"It's a slow, laborious process, but actually there's no alternative," he said.

"It's labour intensive, it's tech intensive, we've got the gear available and we just need to go through the process."

Nearly 300 tonnes of debris has been collected over six weeks, but about 85 per cent of the material is yet to be removed.

The new footage also reveals the underwater state of the ship's stern, with control monitors in the bridge now covered in silt and marine growth and schools of fish swarming around the aft-section.

A spokesman for the ship's owners said the debris field clean-up did not mean the stern would remain at the site.

Studies into the different impacts of options for dealing with the wreckare nearly complete, with a furtherround of community consultation due next month and a final decision later this year.

By the numbers

477 days since the Rena's grounding off Tauranga

1007 containers recovered of the 1368 on board at the time of the grounding

10,000 square metres of sea floor where debris is being removed

22 containers carrying known substances lost at sea

$27.6m settlement covering government costs

$27m compensation fund set up in London for claims

$235m last estimate of the owners' expenses Cleaning up the wreckage from the sea floor will be a long, slow process.

- NZ Herald

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