There was a good TV program of a tear down of a decommissioned shell/esso rig from the North Sea oil field. On yesterday (Sun 22nd July.)

Apparently there was a shift in European law, that now forces companies to tidy up after they've finished. Brought about after pressure from Greenpeace for Shell trying to ditch the Brent Spar in an underwater trough.

Quite how you'd do this with anything deep sea is beyond me. Worth a watch though.