Construction Requirements for Oil Tankers - Double Hulls

In 1992 MARPOL was amended to make it mandatory for tankers of 5,000 dwt and more ordered after 6 July 1993 to be fitted with double hulls, or an alternative design approved by IMO (regulation 19 in Annex I of MARPOL). 

The requirement for double hulls that applies to new tankers has also been applied to existing ships under a programme that began in 1995 (under old regulation 13G (now regulation 20 in Annex I of MARPOL).  All tankers would have to be converted (or taken out of service) when they reached a certain age (up to 30 years old). This measure was adopted to be phased in over a number of years because shipyard capacity is limited and it would not be possible to convert all single hulled tankers to double hulls without causing immense disruption to world trade and industry.

Although the double hull requirement was adopted in 1992, following the Erika incident off the coast of France in December 1999, IMO Member States discussed proposals for accelerating the phase-out of single hull tankers. As a result, in April 2001, IMO adopted a revised phase-out schedule for single hull tankers, which entered into force on 1 September 2003 (the 2001 amendments to MARPOL). The revised requirements set out a stricter timetable for the phasing-out of single-hull tankers. 

In December 2003, further revisions to the requirements were made, accelerating further the phase-out schedule. These amendments entered into force on 5 April 2005. A new regulation on the prevention of oil pollution from oil tankers when carrying heavy grade oil (HGO) banned the carriage of HGO in single-hull tankers of 5,000 tons dwt and above after the date of entry into force of the regulation (5 April 2005), and in single-hull oil tankers of 600 tons dwt and above but less than 5,000 tons dwt, not later than the anniversary of their delivery date in 2008. 

Condition Assessment Scheme (CAS)

Condition Assessment Scheme (CAS) is applicable to all single-hull tankers of 15 years, or older.  Regulation 20 of MARPOL Annex I allows the Administration (flag State) to permit continued operation of category 2 or 3 tankers beyond 2010 subject to satisfactory results from the CAS, but the continued operation must not go beyond the anniversary of the date of delivery of the ship in 2015 or the date on which the ship reaches 25 years of age after the date of its delivery, whichever is earlier. 

In the case of certain Category 2 or 3 oil tankers fitted with only double bottoms or double sides not used for the carriage of oil and extending to the entire cargo tank length or double hull spaces, not meeting the minimum distance protection requirements, which are not used for the carriage of oil and extend to the entire cargo tank length, the Administration may allow continued operation beyond 2010, provided that the ship was in service on 1 July 2001, the Administration is satisfied by verification of the official records that the ship complied with the conditions specified and that those conditions remain unchanged. Again, such continued operation must not go beyond the date on which the ship reaches 25 years of age after the date of its delivery.

Carriage of heavy grade oil

Regulation 21 of MARPOL Annex I on the prevention of oil pollution from oil tankers when carrying heavy grade oil (HGO) bans the carriage of HGO in single-hull tankers of 5,000 tons deadweight (DWT) and above after the date of entry into force of the regulation (5 April 2005), and in single-hull oil tankers of 600 DWT and above but less than 5,000 tons DWT, not later than the anniversary of their delivery date in 2008. 

Under regulation 21, HGO means any of the following:

  • crude oils having a density at 15ºC higher than 900 kg/m3; 
  • oils, other than crude oils, having either a density at 15ºC higher than 900 kg/ m3 or a kinematic viscosity at 50ºC higher than 180 mm2/s; and
  • bitumen, tar and their emulsions.

In the case of certain Category 2 or 3 tankers carrying HGO as cargo, fitted only with double bottoms or double sides, not used for the carriage of oil and extending to the entire cargo tank length, or tankers fitted with double hull spaces not meeting the minimum distance protection requirements which are not used for the carriage of oil and extend to the entire cargo tank length, the Administration, under certain conditions, may allow continued operation of such ships beyond 5 April 2005 until the date on which the ship reaches 25 years of age after the date of its delivery.

Regulation 21 also allows for continued operation of oil tankers of 5,000 DWT and above, carrying crude oil with a density at 15ºC higher than 900 kg/ m3 but lower than 945 kg/ m3, if satisfactory results of the Condition Assessment Scheme warrant that, in the opinion of the Administration, the ship is fit to continue such operation, having regard to the size, age, operational area and structural conditions of the ship and provided that the continued operation shall not go beyond the date on which the ship reaches 25 years after the date of its delivery. 

The Administration may allow continued operation of a single hull oil tanker of 600 DWT and above but less than 5,000 DWT, carrying HGO as cargo, if, in the opinion of the Administration, the ship is fit to continue such operation, having regard to the size, age, operational area and structural conditions of the ship, provided that the operation shall not go beyond the date on which the ship reaches 25 years after the date of its delivery.

The Administration may exempt an oil tanker of 600 DWT and above carrying HGO as cargo if the ship is either engaged in voyages exclusively within an area under the Party's jurisdiction, or is engaged in voyages exclusively within an area under the jurisdiction of another Party, provided the Party within whose jurisdiction the ship will be operating agrees. The same applies to vessels operating as floating storage units of HGO. 

A Party to MARPOL can deny entry of single hull tankers carrying HGO which have been allowed to continue operation under the exemptions mentioned above, into the ports or offshore terminals under its jurisdiction, or deny ship-to-ship transfer of heavy grade oil in areas under its jurisdiction except when this is necessary for the purpose of securing the safety of a ship or saving life at sea.