MARPOL Annex VI sulphur cap: IMO 2020 and ECAs

MARPOL Annex VI Regulation 14 limits the sulphur content of fuel oil on every ship worldwide. The global cap is 0.50% m/m from 1 January 2020; inside designated Emission Control Areas (ECAs) the cap is 0.10% m/m from 1 January 2015. Ships comply by burning low-sulphur fuel, fitting an approved exhaust gas cleaning system (EGCS), or operating on intrinsically low-sulphur alternative fuels such as LNG or methanol.

Regulation 14 sits within MARPOL Annex VI, the air-pollution annex of the MARPOL Convention. It is a fuel-based regulation: unlike Regulation 13 (NOx), which targets engine design, Regulation 14 targets the sulphur content of every fuel oil combusted on board, regardless of which engine, boiler, or incinerator burns it. The practical consequence is that every ship operator must manage the fuel supply chain to the same limit across all combustion devices simultaneously.

The step from 3.50% to 0.50% on 1 January 2020, the regulatory event the industry calls IMO 2020, required the entire marine fuel market to restructure. Refineries that had treated residual fuel oil as a low-value by-product were forced to either upgrade it to meet the 0.50% threshold or price it into a shrinking high-sulphur segment serving only scrubber-fitted ships. The carriage ban enacted by Resolution MEPC.305(73), in force from 1 March 2020, closed the loophole that would have allowed ships to carry non-compliant fuel “in reserve” while claiming scrubber equivalence they did not actually have.

This article covers the full regulatory history from 1997 through the 2026 ECA expansions, the ECA regime and newly adopted areas, the three compliance pathways, the carriage ban mechanics, fuel-oil non-availability reporting, and compliance verification including sampling tolerances. For the NOx ECA requirements that apply to the same ECAs, see the MARPOL Annex VI NOx Tier III article.

Regulatory history: 1997 Protocol to IMO 2020

Annex VI adoption in 1997 and entry into force in 2005

Air pollution from ships was not covered by the original 1973 MARPOL Convention or its 1978 Protocol. The first international controls emerged from the 1997 Protocol, adopted at a Diplomatic Conference in London in September 1997, which added Annex VI, “Regulations for the Prevention of Air Pollution from Ships.” The 1997 Protocol entered into force on 19 May 2005, after fifteen states representing at least 50% of world merchant shipping tonnage had ratified it.

The original 1997 text of Regulation 14 set the global sulphur cap at 4.50% m/m and the Sulphur Emission Control Area (SECA) cap at 1.50% m/m. Two SECAs were designated in the original Annex VI: the Baltic Sea, which became effective on 19 May 2006 (twelve months after entry into force), and the North Sea / English Channel, which became effective on 22 November 2007. The 4.50% global cap was deliberately set above the then-prevailing average sulphur content of residual marine fuel (roughly 2.7% m/m globally) to exclude only extreme outliers from the very highest-sulphur crudes.

2008 MEPC.176(58) amendments: the step-reduction schedule

The Marine Environment Protection Committee adopted the 2008 amendments to MARPOL Annex VI by Resolution MEPC.176(58) on 10 October 2008, with entry into force on 1 July 2010. These amendments completely rewrote Regulations 13 and 14 and introduced the binding step-reduction schedule that defines the modern regime.

For the global cap: 4.50% m/m until 31 December 2011; 3.50% m/m from 1 January 2012; 0.50% m/m from 1 January 2020 (subject to a feasibility review by 2018). For the SECA cap: 1.50% m/m until 30 June 2010; 1.00% m/m from 1 July 2010; 0.10% m/m from 1 January 2015.

The 2020 date for the global 0.50% cap was confirmed by Resolution MEPC.280(70) in October 2016, after the CE Delft fuel-availability study commissioned by the IMO found sufficient refinery capacity and feedstock to supply compliant fuel globally from 2020. The alternative date in the 2008 text, 1 January 2025, was rejected.

The table below summarises the complete step-reduction history.

IMO 2020: the January 2020 transition

The practical implementation of the 0.50% global cap in 2020 was the largest structural shift the marine fuel market had experienced since the introduction of bunker fuel in the early twentieth century. Refineries reconfigured vacuum distillation, coking, and hydrocracking units to reduce the sulphur content of residual fractions. A new product category, Very Low Sulphur Fuel Oil (VLSFO), appeared in the market from approximately mid-2019, typically a blend of vacuum gas oil, hydroprocessed streams, and residual components designed to achieve 0.50% m/m or below while remaining pumpable at typical bunker temperatures.

The price spread between high-sulphur fuel oil (HSFO, then averaging around 3.5% m/m sulphur) and VLSFO widened sharply in the second half of 2019, reaching USD 200 to 300 per metric tonne in many ports by late December 2019. The spread compressed in 2020 as scrubber-fitted ships absorbed the HSFO surplus, but remained positive through the early 2020s, typically USD 50 to 150 per tonne, providing the economic signal that drove ongoing scrubber investment.

The transition also generated a compatibility concern. VLSFO blends from different suppliers and ports exhibited variable stability and compatibility properties because the precise blend composition depended on the crude slates and refinery configurations available locally. Ships bunkering VLSFO from multiple ports without compatibility testing risked asphaltene precipitation in fuel systems, leading to filter blockages and injection system wear. ISO 8217:2024 addresses blend stability and compatibility through CCAI (Calculated Carbon Aromaticity Index) limits and the requirement for compatibility tests on co-mingled fuels.

The ECA regime: 0.10% m/m and designated areas

Emission Control Areas: designation mechanism

MARPOL Annex VI Regulation 14 sets the ECA sulphur limit at 0.10% m/m. A sea area becomes a SOx ECA through an amendment to Appendix III of Annex VI, which lists the designated areas. An applicant state (or group of states) submits a proposal to the MEPC demonstrating that the area meets the criteria in Regulation 13.2 (originally drafted for NOx ECAs but applied by analogy to SOx ECAs): the proposal must show that the area has particular health or environmental sensitivity to ship-source SOx, that fuel of the required quality is available at ports in the area, and that the sulphur reduction is technically and economically feasible.

Once the MEPC adopts the amendment, it enters into force twelve months after adoption under the tacit-acceptance procedure of the MARPOL Convention. The amendment typically provides an additional twelve-month grace period before the 0.10% limit applies operationally, giving the supply chain time to establish compliant fuel availability.

Established ECAs: dates and geographic scope

Seven SOx ECAs are designated under Annex VI Appendix III as of 1 March 2026. The table below lists each area with its operationally effective date for the 0.10% sulphur limit and the key controlling resolutions.

Baltic Sea and North Sea. These two areas were the founding SECAs, designated in the 1997 Protocol itself. They are semi-enclosed seas with high shipping density and coastlines where acid deposition from ship-source SOx has historically contributed to forest and lake acidification. The 0.10% limit has applied since 1 January 2015, replacing the prior 1.00% limit that applied from 1 July 2010.

North American ECA. Proposed by the United States and Canada jointly, adopted by MEPC.190(60) in March 2010, and entered into force on 1 August 2011, with the 1.00% SOx limit applying from 1 August 2012. The 0.10% SOx limit applied from 1 January 2015. The North American ECA is unusual in that it also covers NOx (Tier III from 1 January 2016 for ships constructed on or after that date), making it a combined SOx and NOx ECA. It covers coastal waters out to 200 nautical miles from the US and Canadian coasts on both the Atlantic and Pacific sides, and the Gulf of Mexico.

US Caribbean Sea ECA. Proposed by the US and adopted by MEPC.202(62) in July 2011. It covers waters around Puerto Rico and the US Virgin Islands. The 1.00% SOx limit applied from 1 January 2013; the 0.10% limit from 1 January 2015. Like the North American ECA, it includes NOx Tier III requirements.

Mediterranean Sea ECA. Adopted by Resolution MEPC.361(79) in December 2022, the Mediterranean Sea ECA is the fifth SOx-designated area and the first to cover an enclosed sea shared among a large number of flag and port states outside northern Europe. The amendment entered into force on 1 May 2024, and the 0.10% sulphur limit became operational on 1 May 2025, after the mandatory twelve-month grace period. The Med ECA is a SOx-only designation; it does not impose Tier III NOx requirements.

Ships in the Mediterranean should note that the Med ECA boundary follows the outer limits of the territorial seas of the Mediterranean coastal states, which in some areas are only twelve nautical miles from shore; the high seas in the centre of the Mediterranean basin lie outside the Med ECA unless a state has extended its claim to an Exclusive Economic Zone in that area. The practical implication is that some trans-Mediterranean routes may pass through alternating ECA and non-ECA waters.

Canadian Arctic and Norwegian Sea ECAs. Resolution MEPC.392(82), adopted at MEPC 82 in October 2024, designates both these areas as combined SOx/NOx ECAs. The amendments entered into force on 1 March 2026. The operational sulphur limit (0.10% m/m) and the NOx Tier III requirements apply from 1 March 2027, after the twelve-month grace period. The Canadian Arctic ECA covers all Canadian Arctic waters and is an extension of the North American ECA into those waters; the Norwegian Sea ECA extends the North Sea ECA northward, covering Norwegian fjords and coastal waters to the Russian border.

The NOx Tier III scope differs between the two new ECAs: Canadian Arctic applies to ships with marine diesel engines above 130 kW constructed on or after 1 January 2025; Norwegian Sea applies to ships constructed on or after 1 March 2026. Ships already operating under the North American or North Sea ECAs and meeting Tier III there do not need additional certification for the extended areas.

North-East Atlantic ECA: approved but not yet adopted

MEPC 83 (April 2025) approved a proposal to designate the North-East Atlantic Ocean as a SOx, NOx, and PM ECA. The proposed ECA covers the exclusive economic zones and territorial seas of Greenland, Iceland, the Faroe Islands, Ireland, the United Kingdom, France, Spain, and Portugal, making it the largest ECA by area proposed to date. Formal adoption was targeted for an extraordinary MEPC session in October 2025 but was postponed due to disagreement over related GHG fuel-intensity requirements; adoption is now expected at MEPC 84 in April 2026. If adopted on that timeline, the ECA would enter into force in late 2027 and the 0.10% sulphur limit would apply from late 2028.

Compliance pathways

Pathway 1: compliant low-sulphur fuel

The most direct compliance route is to bunker fuel oil with a sulphur content at or below the applicable limit. Three broad product categories are in use:

Very Low Sulphur Fuel Oil (VLSFO) serves the global 0.50% cap. VLSFO is not a single fuel type; it is a commercial category for any residual or blended fuel meeting 0.50% m/m or below. Sulphur contents in the market typically range from 0.05% to 0.49% m/m, with most commercially available grades around 0.35% to 0.50%. Viscosity grades range from about 30 cSt to 380 cSt at 50°C. The blend instability and compatibility risks noted above are specific to VLSFO; a ship consuming VLSFO from multiple suppliers should maintain compatibility testing records.

Ultra Low Sulphur Fuel Oil (ULSFO) serves both the global cap and ECA requirements with a single fuel grade. ULSFO is generally defined commercially as fuel oil at 0.10% m/m sulphur or below, though the term has no regulatory definition in Annex VI. ULSFO grades tend to be distillate-heavy or hydroprocessed products and command a premium over standard VLSFO.

Marine Gas Oil (MGO) is a distillate fuel oil with a sulphur content that has historically been well below 0.10% m/m in most production batches. MGO under ISO 8217 is specified at 0.10% m/m maximum sulphur in the DMA and DMZ grades and at 0.50% in the DMB grade. Ships operating in ECAs commonly use MGO (DMA or DMZ) as the ECA fuel and switch to VLSFO outside ECAs. The bunker delivery note must record the sulphur content of every delivery, and the master must ensure the correct grade is in use before entering an ECA boundary.

Fuel switching on ECA entry requires a written changeover procedure (mandated by Annex VI Regulation 14.6) and a logbook record of the time, position, and tank readings at changeover. For a two-stroke slow-speed main engine, the residence time of high-sulphur fuel in the system from the service tank through the fuel-oil heater, high-pressure pump, common rail, and fuel valves is typically twelve to twenty-four hours; the changeover must therefore begin well before the ECA boundary. Class guidelines from DNV, Lloyd’s Register, and Bureau Veritas specify minimum pre-ECA changeover distances based on the engine type and fuel system volume.

Pathway 2: exhaust gas cleaning systems (scrubbers)

MARPOL Annex VI Regulation 4 permits a flag state to approve any fitting, material, appliance, or apparatus as an equivalent to a regulation requirement, provided the approved alternative is at least as effective in terms of emissions reductions as the regulation would achieve. For Regulation 14, the approved equivalent is an exhaust gas cleaning system (EGCS), commonly called a scrubber.

A scrubber works by washing the exhaust gas stream with seawater or a caustic-soda solution before the exhaust exits through the funnel. Sulphur dioxide dissolves readily in water to form sulphurous and sulphuric acid; the scrubber removes enough SO2 from the exhaust to achieve the equivalent of burning 0.50% m/m fuel globally and 0.10% m/m fuel in ECAs. The EGCS must achieve a specific SO2/CO2 ratio in the cleaned exhaust: outside ECAs, the limit is 21.7 g SO2 per kg CO2 (equivalent to burning 0.50% sulphur fuel); inside ECAs, 4.3 g SO2 per kg CO2 (equivalent to 0.10% fuel). The CO2 reference is used to normalise for changes in engine load.

The guidelines for EGCS approval were first adopted by Resolution MEPC.184(59) in July 2009. The current version, Resolution MEPC.340(77), was adopted on 26 November 2021 and replaced all earlier versions. MEPC.340(77) addresses open-loop, closed-loop, and hybrid configurations, specifies the monitoring parameter requirements (SO2/CO2 ratio, PAH, pH, turbidity), and provides the washwater discharge criteria.

Open-loop scrubbers draw seawater directly from the sea chest, spray it through the exhaust, and discharge the spent washwater overboard. This process is thermodynamically straightforward and operationally simple, but the spent washwater contains elevated concentrations of PAHs (polycyclic aromatic hydrocarbons), heavy metals (particularly vanadium and nickel from the fuel), and sulphates. The MEPC.340(77) washwater discharge criteria for open-loop systems require: pH at the point of overboard discharge not less than 6.5 (except a momentary drop to 6.0 during manoeuvreing), PAH concentration not more than 50 micrograms per litre above ambient, turbidity not more than 25 FTU above ambient, and nitrate not more than 60 mg NO3-equivalent per litre.

Closed-loop scrubbers use a recirculating caustic-soda solution instead of raw seawater. The washwater is treated and recirculated; spent sludge and residues are collected on board and discharged to port reception facilities. Closed-loop systems add no alkalinity to the sea and produce no overboard discharge during scrubbing, which is why they are accepted by ports and coastal states that have banned open-loop discharges.

Hybrid scrubbers can switch between open-loop and closed-loop modes. Operators running hybrid systems typically run open-loop at sea (lower operating cost) and switch to closed-loop or “zero discharge” mode in ports and coastal waters where open-loop discharge is prohibited.

Open-loop discharge bans

Port and coastal state bans on open-loop scrubber washwater discharge have grown substantially since 2019. Over thirty jurisdictions had some form of restriction by late 2024, ranging from full bans in territorial waters (Singapore, China’s Pearl River Delta, Belgium, Germany in certain areas) to bans in specific harbours or anchorages (many US Gulf and East Coast ports). Port-specific restrictions change frequently; shipowners operating scrubber-fitted vessels must maintain a current list of restrictions for each voyage. The Paris MoU and Tokyo MoU have incorporated EGCS documentation checks into their standard inspection procedures.

The Med ECA activation in May 2025 raised a specific question about open-loop discharge in the Mediterranean, because the Med ECA designation itself does not prohibit open-loop washwater discharge; ships may continue to use open-loop scrubbers in the Mediterranean as long as the washwater meets the MEPC.340(77) criteria. Individual Mediterranean coastal states may impose their own national bans, but no basin-wide prohibition exists as of mid-2026. Ships operating in the Med ECA should verify the current position for each port of call.

Pathway 3: alternative fuels

Ships operating on fuels with intrinsically low or zero sulphur content are compliant with Regulation 14 for their entire operation. LNG as marine fuel contains essentially zero sulphur in its methane-dominant composition; SOx emissions from LNG combustion are negligible. Methanol as marine fuel similarly contains no sulphur. Ammonia, hydrogen, and biofuels derived from zero-sulphur feedstocks are also intrinsically compliant.

Alternative-fuel ships are not required to carry a scrubber or any equivalent compliance mechanism for Regulation 14 because the fuel itself meets the standard. They are, however, subject to the full requirements of the IGF Code (International Code of Safety for Ships Using Gases or Other Low-Flashpoint Fuels), which regulates the structural and operational safety of gas fuel systems on board.

The dual-fuel design, in which a ship’s main engine can burn either LNG or marine fuel oil as a pilot fuel, requires that the pilot fuel itself also meets the applicable sulphur cap. The pilot fuel fraction is typically a small distillate quantity (around 1% to 5% of total energy), but it is subject to Regulation 14 in the same way as any other fuel oil combusted on board.

The carriage ban: Resolution MEPC.305(73)

MEPC 73 met in London in October 2018 and adopted Resolution MEPC.305(73), which introduced a new paragraph into MARPOL Annex VI prohibiting ships from carrying fuel oil with a sulphur content above 0.50% m/m for use on board, unless the ship has an approved EGCS (scrubber) fitted. The carriage ban amendment entered into force on 1 March 2020, sixty days after the 1 January 2020 effective date of the global sulphur cap.

The gap between 1 January and 1 March 2020 was intentional: it gave ships already at sea on 1 January 2020 with loaded high-sulphur bunkers time to consume or debunker that fuel before the carriage ban closed. In practice, most operators had already switched to compliant fuel well before January 2020 to avoid the risk of being caught in port with non-compliant fuel under a flag or port state that began enforcing on 1 January.

The carriage ban is enforced by port state control inspectors who check the bunker delivery notes, the oil record book, and, where grounds exist, take fuel samples from the ship’s bunker tanks. A ship found carrying non-compliant fuel without an approved EGCS commits a clear violation of Annex VI Regulation 14, which is subject to enforcement by both the flag state (under MARPOL Article 4) and the port state (under MARPOL Article 5). Penalties vary by jurisdiction: under the US Act to Prevent Pollution from Ships, a single violation can attract a civil penalty of up to USD 25,000 per day of violation, and criminal penalties apply for willful violations.

The carriage ban exemption for scrubber-fitted ships requires that the EGCS is both installed and operational. A ship whose scrubber is out of service for maintenance while the ship carries high-sulphur fuel is not covered by the exemption; the operator must either debunker to compliant fuel or obtain a flag state dispensation before departure.

Fuel-oil non-availability: the FONAR

When FONAR applies

If a ship cannot obtain compliant fuel oil despite best efforts, MARPOL Annex VI Regulation 18.2 provides that the ship is not required to deviate from its intended route or unduly delay the voyage to find compliant fuel. The obligation is a good-faith one: the owner, operator, and master must document the efforts made to source compliant fuel, give advance notice to the flag state and the port state of the next port of call, and submit a Fuel Oil Non-Availability Report (FONAR).

The FONAR format is standardised in Appendix 1 of Resolution MEPC.320(74) (the 2019 Guidelines for consistent implementation of the 0.50% sulphur limit). The report captures: the ship’s particulars, the bunker ports visited or contacted, the sulphur content of the fuel available and why it did not meet the limit, the flag state notification, and the intended corrective action at the next port.

What a FONAR does not do

A FONAR is not an exemption, a waiver, or a defence to a port state control violation. It is evidence of good-faith effort. Port state control authorities review the FONAR on arrival and determine whether the explanation is credible: a FONAR citing non-availability at a major bunkering hub known to stock compliant fuel is unlikely to be accepted. Repeated FONARs from the same ship attract additional scrutiny and may trigger a focused inspection. The Paris MoU guidance notes that a FONAR submitted without prior flag-state notification, or one that is submitted only after the ship has arrived at port rather than in advance, is a significant negative indicator.

The US EPA takes a more structured approach. The North American ECA Electronic Fuel Oil Non-Availability Disclosure (FOND) portal requires electronic pre-arrival submission of non-availability information; late or missing disclosures are themselves a violation under the Act to Prevent Pollution from Ships.

FONAR in the 2020 transition

FONARs were submitted in significant numbers in early 2020, particularly from smaller operators bunkering at secondary ports where VLSFO supply was not yet established. The IMO and major MoUs treated these as genuine transition-phase events and generally did not pursue enforcement where the FONAR was properly submitted and the fuel-supply gap was genuine. By mid-2020 compliant fuel was available at all major bunkering centres globally, and the rate of credible FONARs fell sharply.

Sampling and verification of compliance

Three sample types

Port state control verifies Regulation 14 compliance using three distinct sample types, each with a different role.

The MARPOL delivered sample is a one-litre sample drawn from the fuel supply line during bunkering, sealed by the ship’s officer and the bunker supplier’s representative, and retained on board for the duration of the voyage or twelve months, whichever is longer. The MARPOL sample represents the fuel as delivered; its sulphur content must not exceed the statutory limit with no added tolerance. If a MARPOL sample tests above the limit, both the ship and the bunker supplier are potentially in violation.

The in-use sample is drawn from the fuel-oil supply line to the combustion device (main engine, auxiliary engine, boiler) during a port state control inspection. It represents the fuel actually being burned at the time of inspection.

The on-board sample is drawn from a bunker tank. It represents the fuel in storage.

Test tolerances and ISO 4259

Fuel oil sulphur content is measured by X-ray fluorescence (XRF) or combustion-infrared methods, both of which have measurement reproducibility uncertainty. For in-use and on-board samples, the acceptable limit applied by port state control is 0.53% m/m for vessels subject to the global 0.50% cap, and 0.11% m/m for vessels in an ECA subject to the 0.10% cap. These limits incorporate the reproducibility term R from ISO 4259-2:2017, at a 95% confidence level, to avoid false-positive violations from analytical variation rather than genuine non-compliance.

The MARPOL delivered sample is held to the full statutory limit with no tolerance, because the reproducibility allowance applies to the comparison of two independently drawn samples from the same batch, not to the comparison of a sample against an administrative limit.

Annex VI Appendix VI (inserted by MEPC.324(75), in force 1 April 2022) mandates a standardised verification procedure for both the MARPOL sample and the in-use/on-board samples. Regulation 14 was amended simultaneously to require ships to have a permanently designated in-use fuel oil sampling point fitted for each fuel-consuming system, a physical requirement enforced by class societies as a condition of class survey from 1 April 2022.

Fuel suppliers are advised in MEPC.1/Circ.875 Add.1 to target 0.47% m/m as an internal production limit for bunkers intended to meet the 0.50% cap, applying a safety margin equal to 0.59 times the method reproducibility to ensure delivered fuel clears the statutory limit with confidence. A supplier whose batch tests at 0.49% m/m is within the statutory limit but may generate a delivered sample above 0.50% once measurement reproducibility is factored in.

Port state control and enforcement

Port state control officers inspecting under the Paris MoU, Tokyo MoU, and US Coast Guard frameworks examine the following documentation as the primary Regulation 14 check: the International Air Pollution Prevention (IAPP) Certificate, the bunker delivery notes for the most recent bunkerings, the oil record book fuel changeover entries, the on-board sampling point locations, and the EGCS log (if the ship is scrubber-fitted). Deficiencies are coded in the IMO deficiency catalogue; a Regulation 14 sulphur violation typically results in a Code 17 deficiency and may result in detention pending confirmation of compliant fuel supply.

The Paris MoU conducted a Concentrated Inspection Campaign (CIC) on MARPOL Annex VI compliance in 2020, focusing on the global sulphur cap transition. Results showed that the rate of confirmed sulphur violations was low (around 2% to 3% of inspected vessels), with the most common deficiency being incomplete or absent fuel changeover logbook entries rather than fuel actually above the limit. The Tokyo MoU’s 2020 CIC found similar patterns.

Health and environmental rationale

The SOx and PM reduction target of Regulation 14 is motivated by the documented health impacts of ship-source sulphur emissions. Ship exhaust at sea releases SO2, which oxidises in the atmosphere to form secondary sulphate aerosol (PM2.5). This aerosol travels hundreds to thousands of kilometres downwind before deposition, meaning coastal and near-coastal populations receive a material fraction of their PM2.5 exposure from ships even when no ship is visible from shore.

Studies from the International Council on Clean Transportation and the Finnish Meteorological Institute estimated that without the 0.50% global cap, ship-source PM2.5 would cause approximately 570,000 additional premature deaths globally over the 2020 to 2025 period, with the highest absolute burden in Asia and the highest per-capita burden in port cities. The IMO’s own analysis at MEPC 70 put the expected annual SOx reduction from the global cap at approximately 8.5 million metric tonnes per year, a reduction of around 77% from the pre-2020 baseline. The Baltic and North Sea 0.10% SECA, established in 2015, was measured by satellite NO2 and SO2 monitoring to produce a roughly 60% drop in ship-source SO2 over those sea areas within two years of implementation.

Connection to other Annex VI regulatory measures

Bunker delivery notes and fuel oil quality

Every bunker delivery to a ship must be accompanied by a bunker delivery note (BDN) as required by MARPOL Annex VI Regulation 18. The BDN must record the sulphur content of the fuel as a mandatory field; an inaccurate BDN exposes the fuel supplier to enforcement action by the port state. The BDN retained by the ship, combined with the MARPOL one-litre sample, forms the documentary chain of custody for compliance verification.

Fuel quality is separately regulated by Regulation 18.3, which requires fuel oil to be free from additives that would cause a ship to exceed the emission limits, and not to require techniques beyond normal fuel treatment to meet those limits. The ISO 8217 specification, referenced in Regulation 18, sets the physical and chemical quality requirements for marine fuel; ISO 8217:2024 is the current version.

NOx Tier III ECAs

The five ECAs that also carry NOx Tier III designations (Baltic Sea, North Sea, North American, US Caribbean, and the new Canadian Arctic and Norwegian Sea) require that ships with applicable marine diesel engines meet the IMO Tier III NOx standard of Regulation 13 when operating within those areas, in addition to the 0.10% SOx limit of Regulation 14. The Tier III NOx requirement applies based on the ship’s construction date relative to the ECA’s Tier III effective date; a ship built before the Tier III date is not retroactively required to meet Tier III in those ECAs.

CII and EEXI interaction

EEXI (Energy Efficiency Existing Ship Index) and CII (Carbon Intensity Indicator) ratings under MARPOL Annex VI Chapter 4 interact indirectly with the sulphur cap. Operators who installed scrubbers to remain on HSFO gained a fuel-cost advantage relative to VLSFO users, but HSFO has a marginally higher CO2 emission factor per tonne of fuel compared with lighter distillates, which slightly worsens CII ratings all else equal. The effect is small (HSFO emits about 3.114 g CO2/g fuel versus 3.151 for VLSFO; the difference is within instrument uncertainty for most voyages) but is not zero, and operators with marginal CII ratings may factor it into fuel selection.

Operational considerations

Fuel switching procedures

MARPOL Annex VI Regulation 14.6 requires ships trading in ECAs to carry a written procedure for fuel oil changeover and to log the time, position, and tank readings when switching fuel on ECA entry and exit. The requirement is enforced by port state control; a missing or incomplete changeover log is a clear-grounds deficiency.

The practical challenge is managing residual high-sulphur fuel in the fuel system. At full load, a two-stroke slow-speed main engine’s fuel supply system from the service tank outlet to the fuel injection pump holds several cubic metres of fuel; flushing this through takes time proportional to fuel consumption rate. Slow steaming (operating at reduced engine load and speed) slows the flush rate, meaning ECA entry procedures must begin earlier when a ship is operating at slow steaming speeds. The engine derating for slow steaming article covers the engine-side implications.

Scrubber operation records

Ships fitted with EGCS must maintain a continuous electronic log of the SO2/CO2 ratio, pH (for open-loop systems), and flow rate for all scrubber units in use. MEPC.340(77) specifies the log format and retention period (three years). Port state control inspectors have the right to review the EGCS log, and a gap in the log during a period when the ship was in an ECA is evidence that the EGCS was not operating as required.

Fuel oil compatibility and tank management

Operating a large vessel with multiple fuel tanks, multiple fuel grades (VLSFO for ocean passage, MGO or ULSFO for ECA), and a changeover protocol requires careful tank planning. Co-mingling of incompatible VLSFO grades from different suppliers is a documented cause of fuel system blockages and has led to port state control deficiencies when the result is that the combustion device is supplied with off-spec fuel. A best practice, consistent with CIMAC’s guidance on VLSFO compatibility, is to keep receiving tanks empty or nearly empty before bunkering a new grade, or to conduct a compatibility test (ASTM D7112 or ISO 12156-based methods) before mixing.

Limitations

This article describes MARPOL Annex VI Regulation 14 as it stands in mid-2026. Several aspects carry forward uncertainty:

North-East Atlantic ECA. The geographic boundary, exact NOx Tier III scope, and compliance dates for the proposed North-East Atlantic ECA depend on adoption, which was postponed from October 2025 and is targeted for MEPC 84 in April 2026. The dates stated here reflect the approved proposal but may change at adoption.

Open-loop scrubber discharge restrictions. The list of ports and coastal states banning or restricting open-loop washwater discharge changes regularly and is not codified in any single IMO instrument. Operators must consult the most current list from their P&I club or classification society before each port call.

VLSFO quality variation. The market for VLSFO is not homogeneous. Compatibility properties, cold flow performance, and actual sulphur content vary by supplier, port, and season. This article describes regulatory limits, not the full range of fuel quality issues that operators encounter in practice. ISO 8217:2024 and bunker delivery note documentation practices address the quality dimension.

Enforcement discretion. Port state control enforcement of Regulation 14 varies by MoU region, flag state, and the individual inspector. The tolerances and procedures described here reflect IMO guidelines and MoU policy; actual enforcement outcomes depend on the specific circumstances of each inspection.

Alternative fuels regulatory evolution. The regulatory treatment of methanol, ammonia, hydrogen, and biofuel blends under the IGF Code and Annex VI continues to evolve. Operators planning alternative-fuel vessels should consult the current IMO framework and their flag state for the applicable requirements, as the position described here may have changed after the mid-2026 effective date of this article.

See also

• MARPOL Annex VI: the full air-pollution annex, Regulations 1 through 22
• MARPOL Convention: the parent treaty framework
• Exhaust gas cleaning system: technical detail on open-loop, closed-loop, and hybrid scrubbers
• LNG as marine fuel: the intrinsically sulphur-free alternative
• Methanol as marine fuel: zero-sulphur alternative fuel
• Bunker delivery note: the Regulation 18 documentation chain
• ISO 8217:2024: the current marine fuel oil quality specification
• EU ETS for shipping: the GHG-reduction measure that runs alongside Regulation 14 in European waters
• What is CII: Carbon Intensity Indicator and its interaction with fuel selection
• What is EEXI: Energy Efficiency Existing Ship Index
• Slow steaming: speed reduction and its relevance to ECA fuel changeover timing
• FONAR and BDN documentation: the non-availability reporting and delivery note workflow