Wherever an oil and gas platform is installed, most governments require complete removal and site cleanup when production is terminated. In the United States the time is one year in most cases. This can happen when production is no longer economically viable or the lease is terminated for other reasons.
Some bottom-sitting platforms can be repurposed as artificial reefs. This is the “Rigs to Reefs” program. It enhances the immediate marine environment and offers considerable savings in decommissioning costs. The decks where the machinery is located are taken ashore for scrapping. The legs that support the decks are relatively clean. They can be cut off, tipped onto the seafloor, or removed for towing to another reef site.
Water clearance between the sunken legs and the surface has to be great enough to not interfere with shipping traffic. In U.S. waters the Coast Guard requires an 85-foot clearance between the reef top and the surface. This means that the candidate platforms must be sited in at least 100 feet of water. Cutting can be done by divers using mechanical tools or cutting torches, or remotely by explosives.