Australia's offshore oil and gas structures are starting to come to the end of their lifespans — and while the default is currently to completely remove them from the ocean, some experts believe they should remain where they are.

Australia has over 2,000 wells, around 30 platforms and thousands of kilometres of undersea pipeline around its Commonwealth waters.

Offshore platforms are effectively huge artificial reefs, according to ecologist Peter Macreadie from Deakin University.

"There's this quandary: should we leave these artificial reefs that we already put in the ocean and left there … or should we be removing them?" Dr Macreadie said.

"These huge rigs that are metal available for free, could become a new dive site, improve the local economy and enhance local fisheries."

Dr Macreadie is one of many scientists and industry experts who say it's time to find what's really going on with oil and gas structures under the surface, so that decommissioning regulations can be modernised.

The fact rigs support sea life doesn't mean they shouldn't be removed, Greenpeace's Nathaniel Pelle says. ( Getty Images: Kampee Patisena )

Australia's decommissioning situation

Australia's offshore oil and gas infrastructure —and its decommissioning processes — are governed by a complex system of international, national and state laws, depending on what waters it's located in.

The recently expanded Gorgon gas project, for example, is administered by the Western Australian Government because of its location on Barrow Island, said CEO of the Western Australian Energy Research Alliance Kym Bills.

In Australia, states and territories have jurisdiction for up to three nautical miles off the coast and around islands. ( Supplied: Chevron )

Any further offshore than that, and decommissioning is regulated under the Offshore Petroleum and Greenhouse Gas Storage Act (OPGGS 2006) by the National Offshore Petroleum Safety and Environmental Management Authority (NOPSEMA), and the Federal Government's National Offshore Petroleum Titles Administrator (NOPTA).

"In addition to that there's a number of rules under the [United National Convention on the] Law of the Sea that apply," Mr Bills said.

The current NOPTA guidelines for decommissioning state that "options other than complete removal may be considered" if there's evidence they'll have equal or better environmental outcomes.

At the moment, that evidence is lacking in Australia, where there hasn't been much research to support decommissioning alternatives, according to Mr Bills.

And given how little decommissioning of any kind has happened in Australia, it's unclear what the process — or the public's response — would be, he said.

From rig to reef

Like shipwrecks, other human-made structures that are left in the ocean can become artificial reefs, creating new habitat for marine life.

You only have to swim below a jetty to see the corals and sponges clinging to the pylons, making it their own, to understand how this can happen.

The same goes for offshore oil and gas structures, some of which have become valuable marine habitats, said Dr Macreadie.

"While they've been sucking oil out of the ground for 30 or 50 years, they've become home to all sorts of fish, corals, and rare things that trawl fisheries can't access," he said.

Underwater human-made structures can provide protective habitat for marine life in otherwise featureless ocean. ( Getty Images: Kampee Patisena )

Looming down from the surface and crawling along the seafloor, these oil and gas structures draw animals out from the blue expanse, looking for a home in what can be an otherwise featureless ocean.

Some animals rely on these structures even more than you'd think — and there's evidence that in time, they can transform into breeding grounds and nurseries.

"We found that a lot of the fish seemed to have been born there, which was an important finding given the debate about whether fish are produced by these rigs or simply attracted there," Dr Macreadie said.

It's more than a matter of biodiversity though, according to fish ecologist Todd Bond from the University of Western Australia, who said fish aggregating at oil and gas pipelines on the North West Shelf could be enhancing the value of commercial fisheries in the area.

"On the pipeline there are more, and bigger, commercial fish than in the surrounding areas," Mr Bond said.

Rigs offer meals for predators

Larger marine mammals also inhabit the ecosystems that form around rigs.

It was a huge discovery that the harbour porpoise was feeding around the offshore platforms, said Victoria Todd, a marine mammal scientist working with the oil and gas industry in the North Sea, in the northern hemisphere.

"If they are around the platforms in the middle of the North Sea they are there for one reason: to feed," Dr Todd said.

Harbour porpoises in the North Sea visit the platforms to hunt. ( Supplied: Ocean Science Consulting Ltd. )

Some offshore structures are moved around to drill in new locations, and Dr Todd thinks the porpoises follow the rigs because they know they're going to get a good meal out of it.

"Generations of these animals have been exposed to these sounds and activities — oil and gas installations are part of their everyday life," she said.

Fishing is banned within a 500 metre zone around the structures, so removing them could also take away protected habitat for the porpoises and other animals, Dr Todd said.

In the Victorian Bass Strait, female Australian fur seals were detected navigating to and hunting around oil and gas pipelines, and a recent study found several different sharks, commercially-important fish, and a "likely new species" of roughy (a type of large deep-sea fish) at deep depths around wells on the North West Shelf.

Dr Todd believes it's a strong argument for leaving certain structures in place.

Is it just 'dumping waste'?

Both scientists and industry want to understand what would happen if these complex and diverse marine ecosystems that have formed around oil and gas structures, were removed.

"It's kind of a moral dilemma now — should you remove them, or just leave them in place?" Dr Macreadie said.

As the sun sets on many of the world's offshore oil and gas structures, could taking them out do more environmental harm than good? ( Getty Images: chain45154 )

For some people, a rig at the end of its life amounts to waste material, and shouldn't be left in the ocean.

"If you are proposing to leave it there you're effectively — according to the United Nations guidelines — dumping waste at sea," senior Greenpeace campaigner Nathaniel Pelle said.

Greenpeace maintains the environment should be returned to its "original state" for any situation, if it's the best environmental outcome for the ecosystem.

And protecting the biodiversity around a rig might not be more important than returning the ecosystem to its natural state, Mr Pelle said.

"The test should be whether the overall outcomes are better … it is not necessarily the case that all of the life that has attached itself is overall beneficial," he said.

Many of Mr Pelle's concerns mirror the questions scientists and industry want to investigate.

"We want to find out how long one of these rigs would last for, if you left it in the marine environment based on corrosion," Dr Macreadie said.

"Does it matter if they eventually disintegrate because they've become large mounds of natural habitat that are completely encrusted by all this stuff? … Nature tends to take over," he said.

Dr Macreadie acknowledges that structures could attract or support invasive species, and could already be altering natural food webs.

So, is it too good to be true, that a once destructive, ugly structure, could become a marine oasis?

"I'm very worried by the description of oil rigs as beneficial environmental structures, as the oil and gas industry will always try to use any change in regulation to its advantage," Mr Pelle said.

A future decommissioning industry

The cost of Australia's decommissioning bill over the next 40 years is conservatively estimated to be over $40 billion, some of which will be picked up by the taxpayer.

Under current regulations the industry pays petroleum resources rent tax (PRRT) once they start making a profit, and when they are done with a platform and decommissioning becomes an expense, they can claim back some of that PRRT from the taxpayer, Mr Bills said.

Collaborating with the oil and gas industry has given scientists access to footage from deep-diving remotely operated vehicles. ( Supplied: Andrew Gates/National Oceanography Centre Southampton )

"This is a much bigger opportunity than 'rigs to reefs' — this is about service exports to the region and saving the Australian taxpayer a lot of money," he said.

A decommissioning industry established to permanently convert offshore oil and gas structures to artificial reefs would need to be supported by science, and Australia has a long way to go in understanding the overall impact of leaving these offshore structures in place, Dr Macreadie said.

"Our knowledge is rudimentary compared to other parts of the world," he said.

Unsurprisingly, a recent report about stakeholder views on decommissioning found many of the questions people raise align with Australia's research needs around rig decommissioning.

"To be able to do something other than the default option, you need to have science that shows that leaving things in situ fully or partly is at least as good as total removal," Mr Bills said.

"There's an opportunity now to do some research that makes it a more viable option into the future."

Before a decommissioning industry can be established, the oil and gas industry needs the social license to leave some of their structures in place, Dr Macreadie said.

Some scientists believe fishers, divers, and local economies could benefit from structures left in place — but for many, it may be hard to see past the oil and gas industry leaving their unwanted materials in the ocean.