Since 1993 it has been mandatory for all new built tankers over 5000 tons to be built with double bulls or with an approved alternative system, whatever that means. Double hulled tankers should be more efficient as there are no internal structures in the cargo tanks to trap oil residues and sea water ballast is only ever carried in cargo tanks in extreme weather conditions.
Prior to 93, the average life expectancy of crude/product tankers was around 25-30 years, many of the oil majors would not put their cargoes on ships 25+ years old, irrespective of how well they had been maintained.
This led to a number of these older tankers trading in areas where, let's say, international regs were not strictly enforced. Some VLCC's even ended up being used as static floating bunker storage ships.
Scrapping those older ships was often undertaken in less environmental awareness countries and it is only in the last decade or so that an international convention on the scrapping of vessels in an environmental safe way has been made, indeed Germany has only this year signed up to the convention.
Double hulled tankers are easily converted to carry grain or ore cargoes but converting then to carry LNG or LPG is a non starter. Both those cargoes are either carried at extreme low temperature using boil off enroute to power the vessel or under high pressure with reliquidfaction plants fitted.