IMO NOx regulations largely determine how modern ships are built. The current regulations are the result of decades of improvements.
Here, we review the most important developments over the years.
The International Maritime Organization (IMO) was established in 1948. It was initially known as the Inter-Governmental Maritime Consultative Organization, or IMCO. IMO was established at a Geneva conference as a UN specialized agency. Its task was to promote maritime safety.
In 1958, all ratification requirements were met. The IMCO Convention entered into force and the organisation was formalised.
The Oil Pollution Convention of 1954 went into effect. The purpose of the convention was to combat marine oil pollution. This was achieved by tightening oil tanker regulations. The convention also enabled IMCO to introduce additional requirements for ships in sensitive areas such as coral reefs. OILPOL54 was an early precursor to today’s pollution standards.
The International Convention for the Prevention of Pollution from Ships (MARPOL) established international regulations aimed at reducing pollution from ships.
The MARPOL Convention was signed in February 1973 but did not officially enter into force for another ten years. Later it became known as MARPOL 73/78 after an additional protocol for oil tankers was introduced in 1978 following incidents that year.
The name of the agency was changed. It was now known as the International Maritime Organization, or IMO.
In 1983 enough countries had ratified the MARPOL Convention for it to enter into force. Several countries changed their pollution laws.
The MARPOL Protocol of 1997 added Annex VI on Regulations for the Prevention of Air Pollution from Ships to the MARPOL Convention.
Annex VI introduced the first IMO emission standards. Today we know these as the ‘Tier I’ standards. These standards set limits for levels of sulphur oxides (SOx) and nitrous oxides (NOx) in exhaust emissions. Annex VI also limits the emission of ozone-depleting substances by ships of 400 gross tonnage and above.
The regulations apply to ships that visit ports (or offshore terminals) within the jurisdiction of countries that have ratified the agreement
Marine engine manufacturers did not wait for ratification. From 2000 onwards a growing number of manufacturers produced engines that meet the new standards.
Annex VI was ratified by countries representing 54.57 percent of the world’s merchant shipping tonnage.
Annex VI entered into force, making Tier I the de facto standard in international shipping. By the end of the year, 136 countries had signed the convention. Between them they represented 98 percent of maritime transport (measured by weight).
The new regulations applied retroactively to new engines above 130 kW installed on ships built after 1 January 2000. Ships that were extensively renovated from 2000 onwards were also subject to the new regulations.
The IMO adopted amendments to Annex VI that defined Tier II and III standards. A total of 53 countries ratified the amendment. This meant that more than 80 percent of tonnage was now bound by stricter regulations.
The new regulations entered into force on 1 July 2010. From this date on, new engines had to meet new fuel requirements and Tier II and Tier III SOx emission standards. Engines built before 2000 had to meet Tier II SOx emission standards.
Chapter 4 of MARPOL Annex VI on Energy Efficiency Regulations for Ships introduced a mandatory Energy Efficiency Index (EEI) to reduce greenhouse gas emissions from ships.
From 1 January 2016, Tier II and Tier III standards applied in all countries that had chosen to adopt the guidelines.
Standards for Tier III emissions now also apply in the North and Baltic Sea regions.
The differences between tier 2 and tier 3 diesel engines are considerable. The maximum permitted NOx emissions depend on the maximum operating speed of the engine.
Tier II standards applied globally until 2016. Since 2016, Tier II standards apply outside Emission Control Areas (ECAs). Tier III standards apply inside ECAs.
This table shows how emission standards have been progressively tightened.
|NOx Limit, g/kWh
|n < 130
|130 ≤ n < 2000
|n ≥ 2000
|45 · n-0.2
|44 · n-0.23
|9 · n-0.2
Annex VI divides emission and fuel quality requirements into two categories:
ECAs can be shown per pollutant (NOx, SOx) or as all pollutants. The historical timeline is then as follows:
|Entry into force
|North American NCAs
|North American NCAs
|US Caribbean ECA
|US Caribbean ECA
The North American NCAs run along most of the US and Canadian coasts. The US Caribbean ECA includes Puerto Rico and the US Virgin Islands.
In conclusion, the NOx regulations for ships have evolved over the years and continue to be developed, making them an important determining factor for the construction of modern ships.
The IMO has played a significant role in establishing and updating these regulations, starting with its establishment in 1948 as IMCO and its transformation into IMO in 1982.
From the introduction of OILPOL54 in 1958 to the addition of Tier III standards in 2021, the IMO has continued to tighten the regulations to prevent marine pollution and reduce the impact of shipping on the environment.