Fuel Switching Operations: MARPOL Annex VI

Fuel switching is the operational procedure of changing the fuel oil supplied to a ship’s main engine, auxiliary engines, and boilers from one grade to another while the vessel is in service. The most common instance is the HFO-to-MGO (or HFO-to-VLSFO) changeover required before a vessel crosses the boundary of an Emission Control Area where MARPOL Annex VI Regulation 14 imposes a 0.10% sulphur limit. The procedure has been routine for ECA-trading vessels since the original SOx Emission Control Areas in the Baltic Sea and North Sea entered force in 2006 and 2007, but has grown in technical complexity following the 1 January 2020 global 0.50% sulphur cap. The FONAR fuel oil non-availability calculator and the ECA fuel-cost premium calculator are the primary tools on this site directly supporting the operational decisions covered here.

Regulatory framework under MARPOL Annex VI Regulation 14

MARPOL Annex VI Regulation 14 governs sulphur oxide (SOx) and particulate matter (PM) emissions from ships by setting limits on the sulphur content of fuel oil used on board. The regulation operates at two levels: a global cap applying everywhere and a tighter limit inside designated Emission Control Areas.

The 0.50% global sulphur cap

The current global sulphur cap of 0.50% m/m took effect on 1 January 2020. It was established by Regulation 14.1.3 of MARPOL Annex VI as amended by IMO Resolution MEPC.280(70), adopted at MEPC 70 on 28 October 2016. That resolution changed the candidate implementation date in Regulation 14.1.3 from 2025 to 2020 following a review of distillate fuel availability. Before 2020, the global cap was 3.50% (in force from 1 January 2012 under earlier amendments) having previously stood at 4.50% under the original 1997 Annex VI.

The 2020 cap applies to every ship on international voyages regardless of ship type, age, or trade area unless an equivalent compliance method (chiefly a certified scrubber) is in use. Non-compliance exposes the ship to Port State Control detention, flag State enforcement, and in some jurisdictions criminal proceedings against the master or company. The Norwegian Maritime Authority issued its first criminal referral for sulphur cap violation in 2022; the US Coast Guard has pursued civil penalty proceedings since 2020 for on-board sampling failures.

The 0.10% ECA-SOx limit

Regulation 14.4 of MARPOL Annex VI sets a 0.10% m/m sulphur limit inside ECA-SOx areas. This limit has applied since 1 January 2015. The 0.10% limit in practical terms requires distillate fuels (MGO grade DMA or DMB under ISO 8217:2017, or DMX for emergency use), ultra-low-sulphur fuel oil (ULSFO), or an operational EGCS. Very low-sulphur fuel oil (VLSFO at 0.50%) does not meet the 0.10% ECA limit and cannot be used inside an ECA-SOx area without a scrubber.

The five ECA-SOx areas as of 2025 are:

The Mediterranean ECA for SOx was designated by IMO Resolution MEPC.361(79), adopted December 2022, with entry into force of the designation 1 May 2024 and the 0.10% limit applying from 1 May 2025. This added approximately 3,500 vessels regularly calling Mediterranean ports to the population of ships required to carry MGO or ULSFO (or operate scrubbers) for those voyages.

Regulation 14.6 and the changeover record obligation

MARPOL Annex VI Regulation 14.6 was the provision, in earlier versions of the Annex, requiring vessels to maintain a written fuel oil changeover procedure and a record book demonstrating completion of the changeover before ECA entry. The 2016 amendments that implemented the IMO 2020 cap restructured this requirement. Under the current text, Regulation 14.6 requires that the fuel oil changeover (when a vessel enters or exits an ECA) be recorded in the logbook, with entries noting: the time of commencement and completion, fuel tank volumes before and after, and the position of the vessel at the time of completion.

Some administrations continue to require a separate Fuel Oil Change-Over Record Book as a matter of national implementation (the US requires it under 33 CFR 151.1510 for vessels operating in the North American and US Caribbean ECAs). The IMO 2020 Ship Implementation Plan guidance in MEPC.1/Circ.878 recommends that each vessel prepare a ship-specific implementation plan covering fuel supply, tank management, changeover procedures, and documentation as a practical tool even though the plan is not itself a mandatory MARPOL document.

EU Sulphur Directive additional requirements

The European Union has applied the MARPOL Annex VI sulphur limits through Directive 2016/802/EU (the Sulphur Directive), which also imposes the 0.10% limit on ships at berth in EU ports regardless of whether those ports are within an ECA boundary. This at-berth rule covers ships moored in EU ports for more than two hours and predates the Mediterranean ECA; it has applied since 2010 under the predecessor Directive 1999/32/EC (as amended). The practical effect is that vessels calling EU Mediterranean ports were already required to switch to 0.10% fuel at berth before the Mediterranean ECA entered force. The EU Sulphur Directive 2016/802 also covers inland waterway vessels and passenger ships on regular domestic routes.

California (CARB) applies an equivalent at-berth low-sulphur requirement under California Code of Regulations Title 13 section 2299.2, requiring ocean-going vessels at berth in California ports to use fuel of 0.10% sulphur or less within 24 nautical miles of the California baseline. The California Air Resources Board enforces this separately from the USCG’s MARPOL Annex VI enforcement.

Fuel options and their ECA compliance status

The CCAI (Calculated Carbon Aromaticity Index) of a fuel affects ignition quality and can be checked with the fuel CCAI calculator. Density and volume corrections at bunkering are handled by the bunker density and temperature correction calculator.

HFO-to-MGO changeover: the physical procedure

The classical changeover from heated HFO to ambient-temperature MGO is the highest-risk routine fuel-system operation on a conventional marine power plant. The risk is primarily thermal: the fuel injection pump plungers and barrels are precision-machined to micron-level clearances and expand/contract with temperature. A rapid drop from 140 degrees Celsius (HFO operating temperature) to 40 degrees Celsius can cause differential thermal contraction between the plunger body and the barrel, either seizing the pump or causing thermal crack initiation. MAN Energy Solutions’ ME series service letter SL2020-685 specifies that the maximum permissible rate of temperature change at the fuel inlet to high-pressure pumps is 2 degrees Celsius per minute.

The changeover procedure follows a defined sequence:

Step 1: Planning the advance. The engine control room and bridge jointly calculate the vessel’s ECA entry position and the planned ECA entry time. Working backward from that time, the chief engineer identifies the changeover start time, allowing for the full procedure duration plus a margin. For a large two-stroke main engine with a 300-litre fuel rail circulating system and a circulation rate of 600 litres per hour, the rail flush time alone is about 30 minutes. Adding the temperature ramp-down at 2 degrees per minute from 140 degrees to 40 degrees adds another 50 minutes. A total changeover time of 1 hour 30 minutes to 2 hours is standard. The IMO 2020 guidance in MEPC.1/Circ.878 recommends starting the changeover at least 4 hours before the planned ECA boundary to account for navigation uncertainty and operational interruptions.

Step 2: MGO tank and line check. The MGO service tank level and temperature are verified. In cold climates, MGO can gel below its pour point (typically minus 6 degrees Celsius for DMA grade); the tank must be above this temperature. Water content is checked via the purifier service record. The MGO supply line and filter are verified clear.

Step 3: Engine load reduction. The main engine is brought down from full sea load (typically 85% of contracted maximum continuous rating for a vessel on a normal sea passage) to around 50 to 70% MCR before the changeover begins. Lower thermal load reduces temperature gradients in the fuel pump during the fuel transition.

Step 4: Opening the MGO supply and closing the HFO supply. The cross-connection valve or changeover valve on the fuel rail supply line is shifted from HFO to MGO. Modern fuel management systems do this progressively (blending valve type) rather than with a hard cutover. The HFO steam heater supply is throttled down simultaneously.

Step 5: Viscosity and temperature monitoring. The viscometer on the fuel rail (typically installed immediately before the fuel injection pumps) monitors viscosity continuously and controls the heater steam supply via a viscosity controller setpoint. During the transition, the controller tracks the fuel as it changes from HFO to MGO. The target injection viscosity for most medium-speed and two-stroke engines is 12 to 15 cSt. For MGO at 40 degrees Celsius, the viscosity is around 2 to 4 cSt (well below the injection target), which means the heater is progressively switched off and the fuel runs at near-ambient temperature.

Step 6: Rail flush confirmation. The changeover is considered complete when the fuel rail contains only MGO, which is indicated by the fuel rail temperature stabilizing at or near the MGO storage temperature and the viscosity reading corresponding to MGO at that temperature. Tank gauge readings before and after confirm the fuel consumption from the MGO tank and depletion of the HFO rail inventory.

Step 7: Documentation. The time of changeover completion and the vessel’s GPS position at that moment are recorded in the Engine Room Log and the deck logbook. The fuel tank ullage readings are logged. The position must be outside the ECA boundary. Port State Control officers specifically cross-check the changeover completion time and position against the vessel’s tracked AIS position to verify the boundary was not crossed while non-compliant fuel was in use.

The bunker viscosity-temperature calculator supports the pre-changeover calculation of the required temperature to achieve target viscosity for both the HFO in service and the incoming MGO.

Viscosity management: the engineering detail

Viscosity is the property that governs whether a fuel change-over can be executed safely. The relationship between viscosity and temperature for petroleum fuels is non-linear and described by the Walther equation:

where $\nu$ is kinematic viscosity in cSt, $T$ is absolute temperature in Kelvin, and $A$, $B$ are fuel-specific constants. This is the basis of the ASTM D341 viscosity-temperature chart used in engine fuel management. The fuel Walther VT calculator computes this relationship for a fuel given two calibration points.

Practical viscosity targets and the temperatures required to meet them differ across the principal fuel grades:

The implication for the changeover is that when transitioning from HFO to MGO, the fuel rail must cool from 130 to 150 degrees Celsius to 40 to 50 degrees Celsius, a temperature drop of 80 to 110 degrees. At the 2 degrees per minute maximum rate, this takes 40 to 55 minutes for the temperature ramp alone, before rail flushing.

For the reverse changeover on ECA exit (MGO to HFO), the risk is different: heating ambient-temperature MGO too rapidly can cause cavitation in the fuel supply pump if the fuel reaches its flash point (typically 60 degrees Celsius for DMA) in a region of low pressure. Heating must be gradual and the pre-heating sequence must bring the fuel above the VLSFO or HFO cloud point before the HFO is introduced.

Lubricity. Low-sulphur distillates have lower natural lubricity than HFO because the polar compounds that provide lubricity are removed along with sulphur during hydrotreatment. ISO 8217:2017 specifies a maximum HFRR (high-frequency reciprocating rig) corrected wear scar diameter of 520 micrometres at 60 degrees Celsius for DMA and DMB grades. Fuel pumps and injectors relying on fuel lubricity for internal moving part lubrication can experience accelerated wear on prolonged MGO operation, particularly on older engines whose fuel pump clearances were dimensioned for HFO viscosity.

Thermal fatigue. Each changeover cycle imposes a thermal cycle on the fuel pump and injector components. On an ECA-trading vessel making frequent boundary crossings (for example a feeder container vessel running between Rotterdam and Gothenburg through the North Sea ECA multiple times per week), the fuel pumps may undergo 50 to 100 changeover cycles per year. The chief engineer should log each changeover cycle and discuss fuel pump inspection intervals with the engine maker’s service department.

Fuel compatibility and the ASTM D4740 spot test

Mixing HFO and MGO in significant proportions in a tank or fuel rail can cause asphaltene precipitation. Asphaltenes are the heaviest aromatic components of residual fuel and are stabilized in HFO by the lighter aromatic fractions. When a light distillate like MGO is added, its paraffinic character destabilizes the asphaltene micelles, causing them to agglomerate and settle out. The result is sludge that blocks fine fuel filters, fuel injection pump plungers, and injector nozzle holes.

The ASTM D4740 spot test (or its ISO 10307-2 equivalent) provides a field check for fuel compatibility. A drop of blended fuel is applied to filter paper and observed after 1 hour. A uniform dark brown spot indicates compatible fuel; a dark ring or pattern of concentric rings (Bull’s-eye pattern) indicates incompatible fuel with asphaltene precipitation.

The bunker compatibility spot test calculator helps interpret ASTM D4740 results. ISO 8217:2017 addresses fuel stability via the total sediment tests (ISO 10307-1 and 10307-2) included in the specification for residual fuel grades. The fuel ISO 8217 compliance checker maps a fuel sample analysis against ISO 8217:2017 limits for all parameters.

The safest practical rule for limiting mixing is to run the HFO service tank to its lowest operationally feasible level (typically 10 to 15% of tank capacity) before beginning the changeover, then isolate the tank and start drawing from the MGO service tank. This minimizes the volume of HFO available to mix with the incoming MGO in the fuel rail.

VLSFO compatibility is an additional concern. VLSFO blends are heterogeneous: different blends from different bunker suppliers can be incompatible with each other even within the 0.50% sulphur specification. Operators should request a compatibility check (spot test or CCAI comparison) before commingling VLSFO batches from different suppliers or regions. The bunker biofuel compatibility calculator covers the analogous issue for biofuel blends (FAME/HVO admixed with conventional fuel).

The FONAR: non-availability declaration procedure

The Fuel Oil Non-Availability Report is the MARPOL Annex VI mechanism allowing a vessel to continue operating on non-compliant fuel when compliant fuel is genuinely unavailable. The FONAR is not a compliance exemption: the vessel remains in violation of Regulation 14, and the FONAR is a documented declaration of the circumstances intended to mitigate enforcement consequences.

The procedural basis is Regulation 18.2.4 of MARPOL Annex VI, read with the guidance in MEPC.1/Circ.878 (the 2019 IMO 2020 implementation guidance) and the MEPC.1/Circ.881 bunker quality guidance. The steps are:

1. The master or chief engineer contacts every available bunker supplier at the port and requests compliant fuel (0.50% global cap or 0.10% ECA grade as applicable). All contact attempts, supplier responses, and quantities and prices quoted or refused are documented.
2. If compliant fuel is genuinely unavailable or available only in quantities insufficient to reach the next port with compliant supply, the master prepares the FONAR. The FONAR must state: the vessel’s name, IMO number, and flag; the port of bunkering; the date of the bunkering attempt; the suppliers contacted; the quantities of non-compliant fuel taken; the intended destination; and the measures taken or planned to obtain compliant fuel.
3. The FONAR is submitted to the port State authority of the port where the non-compliant fuel was taken, the port State authority of the next port of call, and the vessel’s flag State administration.
4. The vessel continues to attempt to obtain compliant fuel at every subsequent bunkering opportunity and documents those attempts.

The FONAR sulphur report calculator generates the FONAR document fields and validates the minimum documentation requirements against MARPOL Annex VI Regulation 18.2.4 and MEPC.1/Circ.878.

The FONAR has been invoked rarely since January 2020 because VLSFO at 0.50% sulphur became widely available at all major bunkering hubs (Rotterdam, Singapore, Fujairah, Houston, Busan) within the first three months of 2020. The IMO’s global fuel supply monitoring reports in 2020 and 2021 showed no systemic unavailability. The scenario remains relevant for unusual routing (remote island ports, emergency calls at non-bunkering locations) and is a required component of each vessel’s ship implementation plan under MEPC.1/Circ.878.

A distinct but related mechanism is the MARPOL Annex VI Regulation 3 waiver, which allows a flag State to grant a vessel a waiver for a trial of an alternative fuel or method. Dual-fuel and methanol vessels operating in early commercial service have used this mechanism.

PSC sulphur verification and sampling

Port State Control officers from SOLAS/MARPOL member states conduct sulphur compliance checks as part of routine PSC inspections and as targeted operations under Paris MOU, Tokyo MOU, and the US Coast Guard’s enforcement programs.

Verification uses three principal methods:

Bunker Delivery Note check. Regulation 18 of MARPOL Annex VI requires that every fuel oil delivery be accompanied by a Bunker Delivery Note (BDN) stating, among other things, the sulphur content of the fuel. The PSC officer checks that the BDN is retained on board (required for three years under Regulation 18.6) and that the sulphur content declared on the BDN is within the applicable limit. The BDN is the primary compliance document. A bunker delivery note wiki article covers the full BDN requirements under MARPOL Annex VI Regulation 18.

In-use fuel oil sampling. PSC officers may sample fuel from the fuel rail (the in-use sample), the day tank, or the bunker tank. The sample must meet the applicable limit. The sampling procedure follows IMO Resolution MEPC.182(59) Guidelines on Port State Control under the Revised MARPOL Annex VI, as updated. The analytical result is compared against the applicable limit (0.50% global, 0.10% in an ECA). The SOx from fuel sulphur calculator converts between sulphur percentage and SOx emission factor for context.

CEMS/PEMS or sniffers. Some administrations, particularly in North America and North-West Europe, operate Continuous Emissions Monitoring Systems or portable sniffers to screen vessels’ exhaust plumes at anchor or in fairways. A positive sniffer result triggers a boarder inspection and fuel sampling. The US EPA’s National Emission Standards for Hazardous Air Pollutants (NESHAP) at-berth rules include a ship-to-shore emission monitoring obligation in some California ports.

The changeover record is a critical document during PSC inspection. The officer compares the time and position of changeover completion (from the engine and deck logs) against the AIS track to verify the vessel was outside the ECA boundary when the changeover was completed. Discrepancies between the logged changeover completion time and the AIS position have resulted in detentions and civil penalties. IMO MEPC.1/Circ.878 recommends that the GPS position at changeover completion be logged to the nearest minute of latitude and longitude.

Changeover timing calculation

The earliest time to start the changeover is set by:

where $t_{ECA}$ is the planned ECA boundary crossing time, $\Delta t_{margin}$ is the navigation uncertainty margin (typically 30 to 60 minutes), and $\Delta t_{changeover}$ is the total changeover duration including rail flush and temperature ramp.

For the temperature ramp component:

where $T_{HFO}$ is the HFO operating temperature in degrees Celsius (typically 130 to 150 degrees), $T_{MGO}$ is the target MGO injection temperature (typically 40 to 50 degrees), and $r_{max}$ is the maximum permissible temperature reduction rate (2 degrees Celsius per minute per MAN Energy Solutions SL2020-685). For a typical HFO-to-MGO transition from 140 degrees to 45 degrees, $\Delta t_{ramp}$ = (140 - 45) / 2 = 47.5 minutes.

The rail flush time $\Delta t_{flush}$ depends on the rail volume $V_{rail}$ and the fuel circulation flow rate $Q_{circ}$:

A typical large two-stroke main engine fuel rail has a volume of 200 to 400 litres. A fuel circulation rate of 800 litres per hour (for a 10 MW engine at part load) gives a flush time of 15 to 30 minutes. Total changeover time is $\Delta t_{ramp} + \Delta t_{flush}$ plus any administrative steps, giving 75 to 110 minutes in the typical case.

Exhaust gas cleaning systems as the alternative to fuel switching

An EGCS (scrubber) is the approved equivalent compliance method under MARPOL Annex VI Regulation 4. A vessel operating a certified EGCS on HFO can meet both the 0.10% ECA-SOx limit and the 0.50% global sulphur cap without switching fuel, provided the EGCS is operating within its type approval parameters.

The governing technical standard for EGCS is IMO Resolution MEPC.259(68), the 2015 Guidelines for Exhaust Gas Cleaning Systems, updated by MEPC.340(77) in 2022. The EGCS must be installed with type approval from the flag State administration and must maintain a continuous log of wash water pH, turbidity, and exhaust gas SO2/CO2 ratio (the SO2/CO2 ratio is the primary compliance parameter: compliant performance is SO2/CO2 ratio not exceeding 4.3 ppm/% v/v, which corresponds to 0.10% sulphur in the fuel equivalent). The scrubber SOx and CO2 emissions calculator models the exhaust gas parameters for scrubber performance monitoring.

Three scrubber configurations are in commercial use:

Open-loop. Seawater is pumped through the scrubber, absorbs SOx as sulphuric acid, and is discharged overboard after separation of particulate matter. The wash water discharge must meet pH requirements (pH 6.5 minimum for the overboard discharge at 4 metres from the ship’s side; pH 3.5 minimum for the discharge plume 4 metres from the discharge point, per MEPC.259(68)). Open-loop operation is the most economical type but is prohibited in several port waters where the high discharge volume and composition raises concerns about acidification and metals loading. Singapore, Fujairah (UAE), China inland and coastal waters to 12 nm, California, and several EU-regulated ports prohibit open-loop scrubber discharge. Vessels operating scrubbers in these areas must switch to closed-loop or hybrid mode, or switch fuel.

Closed-loop. Fresh water with caustic soda (sodium hydroxide) addition is used as the wash medium. The reaction products (sodium sulphate and water) are retained in a holding tank and discharged to port reception facilities. Closed-loop operation is possible anywhere without discharge restrictions but adds operational complexity (caustic storage and handling) and higher wash water management cost.

Hybrid. The system operates in open-loop mode at sea and switches to closed-loop in port or restricted waters. This is the most common configuration in new installations since 2019.

The economic case for scrubbers rests on the HFO/VLSFO price differential. The capital cost of a retrofit scrubber installation on a Panamax-size vessel runs approximately USD 3 to 6 million (installation cost varies by vessel type and whether the retrofit requires extended drydock). At a price differential of USD 150 per tonne between HFO and VLSFO and an annual fuel consumption of 15,000 tonnes, the scrubber saves USD 2.25 million per year, giving a 16 to 32 month payback. At the USD 50/tonne differential that prevailed in 2023 (narrowed due to high HFO demand from scrubber fleets), the payback extends to 4 to 8 years and the economic case weakens. The engine bunker economics calculator models scrubber payback against a user-defined HFO/VLSFO spread.

The exhaust gas cleaning system wiki article covers EGCS technology, regulation history, and open-loop discharge port restrictions in detail.

Dual-fuel and alternative fuel switching

LNG dual-fuel propulsion eliminates the sulphur switching problem for ECAs: LNG contains effectively zero sulphur, and an LNG-fuelled vessel complies with the 0.10% ECA-SOx limit without any changeover procedure. The IMO’s NOx Tier III requirements (MARPOL Annex VI Regulation 13) are also more readily met on LNG operation, as the premixed lean-burn combustion in gas mode achieves NOx levels well below the Tier III limit of 3.4 g/kWh (for engines above 130 rpm). The LNG as marine fuel wiki article covers the fuel system and regulatory context.

MAN Energy Solutions ME-GI two-stroke dual-fuel engines and WinGD X-DF engines can transition between gas mode and liquid fuel mode within seconds to minutes via electronically controlled fuel valves. The transition itself is not subject to the 2-degree-per-minute thermal constraint that governs HFO-MGO changeovers, because LNG (at injection temperature around minus 160 degrees Celsius in the storage tank, but gasified before entering the engine) does not create the same liquid-fuel thermal shock in the fuel injection system.

Methanol dual-fuel engines (MAN ME-LGIM and WinGD X-DF-M series) entered commercial service from 2023. Methanol fuel switching is analogous to LNG in that the sulphur content is effectively zero and the injection temperature considerations are different from those of residual-to-distillate switching. Methanol’s flash point (11 degrees Celsius) is below that of petroleum distillates, requiring IGF Code-compliant enclosed fuel system design, alcohol-resistant materials throughout the fuel circuit, and crew training on methanol toxicity.

Biofuel blends (FAME up to B30 or HVO blends) can be used without fuel system modification on most existing engines and are increasingly bunkered as a drop-in or blend component. The compatibility of the biofuel with existing residual or distillate fuel on board must be checked before blending; the bunker biofuel compatibility calculator assesses FAME/HVO blend compatibility. The ECA compliance of a biofuel blend depends on the sulphur content of the base fuel: a B30 blend using 0.50% VLSFO as the base still exceeds the 0.10% ECA limit and cannot be used in an ECA without a scrubber.

The oil record book and sulphur compliance documentation

Fuel switching under MARPOL Annex VI Regulation 14 generates records in two distinct but related documentation streams.

The Fuel Oil Changeover Record is typically maintained in the Engine Room Oil Record Book Part II (for machinery space operations) or in a dedicated Fuel Oil Change-Over Record Book, depending on the flag State’s implementation. The entry records: date, time (UTC), vessel position (latitude and longitude), the tank from which non-compliant fuel was being supplied, the tank to which the switch was made, the respective sulphur content of each fuel, and the name and signature of the officer on watch. Some flag States (including Panama, Marshall Islands, and Bahamas, which together flag over 35% of world fleet gross tonnage) mandate specific form layouts; the master should hold the flag State administration’s current requirements.

The Bunker Delivery Note is a separate legal document issued by the bunker supplier under MARPOL Annex VI Regulation 18. It must be retained on board for three years from the date of delivery and must state the vessel’s name, IMO number, the port and date of delivery, the product description, density and viscosity at 15 degrees Celsius, the sulphur content as measured by the supplier, and the total quantity in metric tonnes. The BDN is the primary proof-of-compliance document for PSC inspectors verifying sulphur content.

The MARPOL Annex VI Fuel Oil Monitoring (FONAR) record, if used, is retained for the same three-year period.

Cross-checking these three records against the vessel’s AIS track and the port log is standard practice for PSC officers conducting sulphur enforcement in North Sea and North American ECA port states. Paris MOU concentrated inspection campaigns on sulphur compliance in 2021 and 2022 identified the most common documentary non-conformities: missing or incomplete BDN, changeover record with a completion position inside the ECA, and no FONAR to explain non-compliant fuel use.

Compatibility and stability of fuel in the bunker system

The stability of fuel over time in the bunker tanks is a separate concern from changeover compatibility. HFO is thermodynamically unstable and can form sediment by asphaltene agglomeration during prolonged storage, particularly at elevated temperature. MARPOL Annex VI Regulation 18.3 requires that ships accept only fuel oil that meets the applicable sulphur limit; it does not require compliance with ISO 8217, but IMO has recommended adoption of the standard in MEPC.1/Circ.881.

ISO 8217:2017 includes stability requirements via the sediment potential tests (ISO 10307-1 for total sediment existing; ISO 10307-2 for total sediment potential after hot filtration). The total sediment potential (TSP) limit is 0.10% m/m for all residual grades. Fuel exceeding this limit is at risk of generating sludge in the separator and fuel filter during service.

The compatibility between two different fuel batches held in the same tank is not guaranteed by ISO 8217 compliance alone: two fuels can each individually comply with ISO 8217 and still be incompatible with each other when blended. The ASTM D4740 spot test and the ISO 10307-2 test on a 50/50 blend are the standard field and laboratory checks respectively.

VLSFO compatibility problems have been a source of machinery damage claims since 2020. A 2020 Lloyd’s Register study identified 35 distinct VLSFO blending component families in the market, with wide variation in aromatic content, wax content, and asphaltene concentration. Some blends from different suppliers were incompatible when mixed in the service tank. The IMO’s MEPC 74 (May 2019) working group issued MEPC.1/Circ.878 partly in response to industry concern about VLSFO blend compatibility before the 2020 cap entered force. The bunker quality and ISO 8217 wiki article covers the full quality parameters and testing standards.

Port-specific and regional requirements beyond MARPOL

MARPOL Annex VI sets the international minimum. Several jurisdictions impose stricter requirements on fuel sulphur content, scrubber discharge, or at-berth operations:

China. The Domestic Emission Control Area (DECA) established by China’s Maritime Safety Administration requires 0.10% sulphur fuel (or an approved EGCS) within 12 nm of the coast and in internal waters, under the Inland and Coastal DECA regulations effective from January 2019. Open-loop scrubber discharge is prohibited within this zone. The DECA is not an IMO-designated ECA-SOx under MARPOL but a domestic requirement.

Norway. The Norwegian government’s NECA proposal for a North Sea and Norwegian Sea NOx ECA-SOx has been discussed at IMO but as of mid-2026 has not been formally designated. Existing Norwegian ECA coverage is through the North Sea ECA.

Singapore. The Maritime and Port Authority of Singapore (MPA) mandates 0.10% sulphur fuel in Singapore port waters (within the port limits) under the Prevention of Pollution of the Sea Act. Open-loop scrubber discharge is prohibited in Singapore port waters. Singapore, as the world’s largest bunkering port by volume (approximately 50 million tonnes per year), is the principal reference point for VLSFO availability and pricing.

IMO DCS and CII. Fuel type and switching are relevant to the IMO Data Collection System (DCS) and the Carbon Intensity Indicator (CII) regulatory framework under MARPOL Annex VI Regulations 22A, 27, and 28 (as added by resolution MEPC.328(76)). Different fuel types have different CO2 conversion factors (Cf values) per tonne, set by IMO MEPC.364(79). A vessel that switches from HFO to LNG during a voyage must report fuel consumption by fuel type, and the CII calculation uses the weighted Cf for the voyage. The CII fuel mix correction calculator handles the Cf weighting calculation for voyages with multiple fuel types.

Limitations

This article covers the HFO-to-MGO/VLSFO changeover procedure and the MARPOL Annex VI Regulation 14 sulphur framework as they stood at the article’s last revision date. Practitioners should be aware of the following constraints:

The procedural guidance in MEPC.1/Circ.878 is advisory, not mandatory; its recommendations should be read in conjunction with flag State implementation requirements and engine maker service bulletins, which take precedence for specific engine models.

The ECA boundaries described are those formally designated under MARPOL Annex VI Appendix VII. Several additional domestic emission control areas (China DECA, California, EU at-berth rules) impose requirements within the geographic area of existing ECAs or in areas not designated under MARPOL; the article describes these but the reader should consult the relevant national authority for current boundary coordinates and enforcement procedures.

Scrubber open-loop discharge restrictions vary by port and change frequently. The list of ports prohibiting open-loop discharge was incomplete and changing as of the article’s revision date; operators must hold current port authority circulars for each port of call.

Fuel compatibility test results (ASTM D4740 spot test) provide a rapid field indication but are not a substitute for a full ISO 10307-2 laboratory analysis before blending large volumes. The CCAI is not a direct compatibility predictor.

Dual-fuel and alternative fuel changeover procedures differ substantially by fuel type and engine design. The thermal-shock considerations described for HFO-to-MGO switching do not apply without modification to LNG, methanol, or ammonia switching; consult the engine maker’s dual-fuel operating manual for each specific system.

The FONAR mechanism is a declaration procedure, not a legal exemption. Flag States and port States retain the right to investigate and prosecute MARPOL violations even where a FONAR has been submitted. A FONAR submitted in bad faith (where compliant fuel was available) is evidence of intent in enforcement proceedings.

Related calculators

• FONAR fuel oil non-availability report
• ECA fuel-cost premium
• SOx from fuel sulphur content
• Bunker viscosity-temperature (Walther equation)
• Fuel CCAI (Calculated Carbon Aromaticity Index)
• Fuel ISO 8217 compliance check
• Bunker compatibility spot test (ASTM D4740)
• Scrubber SOx and CO2 emissions
• EGCS SOx scrubber calculator
• Engine bunker economics (scrubber payback)
• CII fuel mix correction (MEPC.364(79))
• Alternative fuel bunker premium
• VLSFO well-to-wake
• Voyage fuel and CO2
• Fuel Walther VT viscosity-temperature
• Fuel heater steam heat exchanger
• Fuel viscosity controller inline viscometer
• Fuel oil purifier Westfalia/Alfa Laval

See also

• MARPOL Annex VI
• MARPOL Annex VI Regulation 14: Sulphur Cap
• MARPOL Annex VI Regulation 18: Bunker Delivery Note
• Emission Control Areas
• IMO 2020 sulphur cap
• Bunker quality and ISO 8217
• Marine fuel oil systems
• Exhaust gas cleaning system
• Bunker delivery note
• Port State Control
• LNG as marine fuel
• Heavy fuel oil
• Cylinder oil base number and fuel sulphur
• Baltic Sea ECA and NECA
• MARPOL Convention