A revised schedule for the phasing out of single hull oil tankers and a new regulation banning the carriage of heavy grade oil in single hull oil tankers enter into force on 5 April 2005. The measures were adopted in December 2003 as amendments to Annex I of the MARPOL Convention , following the November 2002 sinking of the oil tanker Prestige off the Spanish coast. It specifies that tankers of single hull construction should be phased out or converted to a "double hull" according to a schedule based on their year of delivery. The double hull requirements for oil tankers are principally designed to reduce the risk of oil spills from tankers involved in low energy collisions or groundings.

Under the phase-out schedule, "Category 1" single hull oil tankers will not be able to trade after 5 April 2005, (for ships delivered on or before 5 April 1982 or earlier) or after their anniversary date in 2005 (for ships delivered after 5 April 1982). Category 1 oil tankers, (commonly known as Pre-MARPOL tankers) include oil tankers of 20,000 tonnes deadweight and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil as cargo, and tankers of 30,000 tonnes deadweight and above carrying other oils, which do not comply with the requirements for protectively located segregated ballast tanks.

Category 2 oil tankers, which have some level of protection from protectively located segregated ballast tank requirements will be phased out according to their age up to 2010. The year 2010 is also a final cut off date for Category 3 oil tankers which are generally smaller oil tankers. Category 2 oil tankers (commonly known as MARPOL tankers) include oil tankers of 20,000 tonnes deadweight and above carrying crude oil, fuel oil, heavy diesel oil or lubricating oil as cargo, and oil tankers of 30,000 tonnes deadweight and above carrying other oils, which comply with the protectively located segregated ballast tank requirements. Category 3 oil tankers are oil tankers of 5,000 tonnes deadweight and above but less than the tonnage specified for Category 1 and 2 tankers.