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In short:
Esso plans to dismantle some of its offshore gas platforms in the Bass Strait and wants to leave the legs and pipes in the ocean floor.
There is also a battle looming over where the topsides will be disassembled.
What's next?
Esso's plans to decommission the platforms are currently before the offshore petroleum regulator.
The network of offshore oil and gas platforms launched more than 50 years ago in the Bass Strait turned Victoria into an Australian energy powerhouse.
But now the gas is dwindling and supplies are projected to drop by nearly 50 per cent by 2028, according to the Australian Energy Market Operator.
About half of ExxonMobil subsidiary Esso's platforms and pipelines in the Bass Strait are no longer operational. And as rigs shut down the debate is raging over what happens next.
"The concerns are that not all of the infrastructure is going to be removed," said Ange Moore, a Maritime Union of Australia representative who spent years working on supply ships and semi-submersible rigs as they drilled offshore wells and laid pipes.
Esso's plan for the first 12 platforms to be decommissioned is now before the National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA).
Depending on the depth of water, for some rigs Esso wants to remove platforms and supports down to 55 metres beneath the surface.
Below that it wants to leave the legs and steel-piled jackets in the ocean floor, and even pipelines in place.
"There's no excuse to leave it in the ocean, just because we don't know what to do with it," Ms Moore said.
"I think it's very concerning that they would leave anything at all, when it's technically feasible to remove."
Esso says removing subsea infrastructure will actually harm diverse marine ecosystems that have developed since rigs were installed.
"If we were to completely remove the structures we would need to undertake significant dredging of the sea floor. Meaning not only removal of these ecosystems but significant impact on surrounding marine life," Esso told 7.30 in a statement.
Esso's proposal for the Bass Strait follows a pattern established in the Gulf of Mexico, where parts of 600 US rigs have been left in place and converted into artificial reefs
But that's not considered international best practice and that's not what Australia's Offshore Petroleum and Greenhouse Gas Storage Act currently permits, according to decommissioning expert Professor Tina Soliman-Hunter, of Macquarie University.
"The best scenario, and the one that's required by law, is to remove everything that you brought into the license area, unless there's a darn good environmental reason not to," Ms Soliman-Hunter said.
"Cutting it off just to 55m is not the best environmental outcome."
Money over the environment?
Australia is just beginning the removal of its offshore oil and gas infrastructure.
"We haven't even decommissioned any of our Bass Strait assets, which have been there since the 70s, let alone anything that's in the north-west shelf that has come from the 80s and 90s," Ms Soliman-Hunter said.
"We've got a long way to go."
Ms Soliman-Hunter believes the motivation for leaving large parts of oil and gas structures in place is simple – cost.
"It is incredibly expensive to remove the installations. I mean, we're talking hundreds of millions of dollars," she said.
The total liability cost for full removal of all offshore assets and plugging all wells is estimated to reach $60 billion by 2070, according to the Centre of Decommissioning Australia (CODA).
It estimates $45 billion of that cost will be accrued by 2040.
Plan to dismantle rigs near wetlands
It is not only Esso's plans to leave parts of rigs behind that has generated controversy.
The Bass Strait oil and gas operator wants to bring decommissioned platforms, or "topsides", back to where many were originally built: Barry Beach Marine Terminal in Victoria's Gippsland region.
They plan to disassemble the rigs there and recycle nearly all of the steel.
Esso said in a statement that Barry Beach Marine Terminal has an excellent environmental record.
But it's next to the Corner Inlet Ramsar Wetlands, which is considered to be of international importance.
The area has the largest intertidal mudflats in Victoria and the habitat supports several endangered and vulnerable species.
Those opposed to the removal fear that materials in the rigs could harm the protected area.
"These platforms were built in the 60s and 70s, so they contain asbestos, they contain mercury and a host of heavy metals, as well as radioactive materials that have been building up within the works themselves," said Freja Leonard from Friends of the Earth.
"There is no sensible way that you can manage those sorts of pollutants in an area as beautiful and pristine as this."
Esso said in a statement that when rig topsides are transported to Barry Beach, "No part of any of the structures will come into contact with the water at Corner Inlet."
It also said the topsides dismantling area will be "bounded with an impervious membrane" to prevent environmental damage.
But Friends of the Earth believe the dismantling should take place in Geelong, nearly 200 kilometres away.
"We believe that there are some parts of the Victorian coastline that have already been industrialised. Geelong is a really great example," Ms Leonard said.
"To meet best practice, we demand that they take the 110 Sydney Harbour Bridges-worth of steel that they have out there in the Gippsland basin and take it to Geelong, where there's already a steel smelting facility."
Esso said in a statement it had looked at Geelong as an option but determined Barry Beach Marine Terminal was the best option for a number of reasons.
The company said it's already a working port in industrial-zoned land and that its proximity to the rigs meant less need for fuel and emissions.
For now, Esso and groups opposed to its plans await a ruling from the federal offshore energy regulator.
In a statement, NOPSEMA told 7.30 it expects to make a decision on the environmental plan by late August.
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