The growth of offshore mining in recent decades has left coastal nations around the world facing the vexing problem of what to do with thousands of rigs when the drilling ends.
The issue has become a pressing challenge as growing numbers of oil and gas rigs have been in place for 20 years or more and are close to retirement. Some experts have warned of a "decommissioning crisis", with more than 6,000 rigs set to become obsolete by 2025.
In most countries, the standard approach has been to require that the rigs be entirely removed from the water. But experts have increasingly been pushing for the rigs to remain in the water and be used as artificial reefs.
Researchers have shown that some rigs can host diverse marine life and believe that the structures - if properly cleaned of toxic material - can be left in the water. Leaving the rigs is often much cheaper than removing them - a complex process which can cost hundreds of millions of dollars for larger structures in deep waters.
In Australia, the authorities are considering this issue in the wake of the rapid growth of the offshore oil and gas industry, which in 2014 generated A$64 billion (S$65.7 billion) for the economy and employed 30,000 people.
With analysts estimating that about 60 rigs will soon be ready for decommissioning, the federal government has been developing a policy on how to dispose of them. Current rules favour complete removal but these are likely to be changed as the government completes a review later this year.
An initial report last November by the Department of Industry, Innovation and Science found there was "a lack of clarity" around policy and regulations requirements for decommissioning offshore petroleum facilities.
The report listed a range of potential uses including "artificial reefs, marine research facilities, renewable energy technologies, aquaculture and tourism (diving)".
"It is unlikely that a single decommissioning option will provide optimal outcomes in all scenarios, or maximise social, environmental or economic outcomes," the report said.
An Australian marine ecology expert, Professor David Booth of the University of Technology Sydney, opposed a one-size-fits-all approach. He said authorities should instead examine each rig to decide whether it should be removed, partially removed, or kept in place.
Deciding whether to leave a rig in place, he said, could depend on its location, the depth of the water, how long it has been in place and the type of marine life that has developed around it.
"Something that has been in shallow waters for 40 years and is covered in coral and has big species of fish that are not normally found there - it can be a useful habitat," he told The Straits Times.
Some experts also say leaving the rigs can be better for the environment since much of the material removed is often unusable and ends up in landfill.
But leaving the rigs has been criticised by some conservation and environmental groups, which say the structures can leach pollution over time and may simply lure fish which can then be more easily caught by commercial fishermen. Some say the corals, fish and other marine life that live on disused rigs are not as diverse as in natural reefs.
The Australian Petroleum Production & Exploration Association, which represents oil and gas firms, said in a submission to the Federal Government in February that it backed "clear and transparent" guidelines for decommissioning rigs. The association signalled support for a mixed approach which could lead to "differing outcomes" for different rigs.
Prof Booth said rig operators should start considering the decommissioning options long before the platform is retired, including conducting long-term surveillance of the marine life around the structure.
"We are encouraging companies to look at getting experts on board to get knowledge of the area before removing it," he said.
Researchers have shown that some rigs can host diverse marine life and believe that the structures - if properly cleaned of toxic material - can be left in the water.