When energy prices surged in the 1970s, resources companies increasingly began to build and deploy a new generation of enormous offshore drilling rigs.
Described as "cities at sea", the rigs can sometimes house hundreds of people and weigh up to 300,000 tonnes, about as much as a tall skyscraper.
But thousands of rigs are nearing retirement as their wells begin to dry up. Lower commodity prices in recent years have added to the pressure to close rigs.
The result is that there are now an estimated 7,500 offshore oil and gas rigs around the world. Of these, about 85 per cent are due to be decommissioned by 2025.
In South-east Asia, analysts say there are about 1,700 rigs and almost half are more than 20 years old and will soon be retired.
Authorities around the world have taken different approaches to disposing of the rigs.
In Europe, rigs must be entirely removed following rules introduced in 1998. The measures followed the Brent Spar incident in 1995, in which Greenpeace occupied a Shell platform that was due to be sunk at sea. Shell eventually abandoned its plan.
In the United States, the authorities support the so-called "rigs to reefs" programme. Subject to safety and environmental assessments, the programme allows rigs to be left in the water as artificial reefs.
More than 400 decommissioned rigs have been turned into reefs since 1986, mainly in the Gulf of Mexico. But in California, conservation groups have opposed the scheme and - as at March - no rigs had been converted.
Increasingly, experts have suggested that countries adopt a flexible approach, reusing the rigs when appropriate.