Background
The technical challenge of fuel switching arises from the very different physical and chemical characteristics of the principal marine fuels. Heavy fuel oil (HFO), with up to 3.5% sulphur historically and 0.5% under the 2020 global cap, requires heating to 130-150 degrees Celsius for injection viscosity and is incompatible with low-sulphur distillate fuels at room temperature. Marine gas oil (MGO) and marine diesel oil (MDO) flow at ambient temperature without heating and have viscosities of 1.5-6 cSt at 40 degrees Celsius. The transition from one to the other in a continuously running engine system requires careful temperature, viscosity, and pressure management to avoid thermal shock, fuel pump seizure, or combustion problems.
This article covers the regulatory framework under MARPOL Annex VI Regulation 14, the operational procedure for HFO-to-MGO switching, viscosity management during the switch, the implications for the engine and fuel system, dual-fuel and tri-fuel propulsion (LNG, methanol), the FONAR documentation procedure for non-availability of compliant fuel, and scrubber operation as the alternative to fuel switching.
MARPOL Annex VI Regulation 14: Sulphur Cap
MARPOL Annex VI Regulation 14 sets the limits on sulphur content of fuel oil used on board ships. The structure has evolved as follows:
The original 2008 Annex VI established a global sulphur cap of 4.50% and SECA limits of 1.50%. Subsequent revisions tightened the limits.
From 1 January 2012, the global cap was reduced to 3.50% and the SECA limit to 1.00%.
From 1 January 2015, the SECA (now renamed ECA-SOx) limit was reduced to 0.10%, the level that requires distillate fuels or scrubber operation.
From 1 January 2020, the global sulphur cap was reduced to 0.50%, the most significant single change in the regulation’s history. This is the so-called IMO 2020 sulphur cap.
The ECA-SOx areas as at 2025 are: the Baltic Sea, the North Sea (English Channel, North Sea, Skagerrak), the North American ECA (200 nm from US and Canadian coasts), the United States Caribbean ECA (200 nm from Puerto Rico and US Virgin Islands), and the Mediterranean Sea (entered force 1 May 2025). New ECAs are under consideration for the Norwegian Sea (Norwegian-led proposal), Canadian Arctic, and Mexican Pacific.
A vessel entering an ECA must have completed the fuel change-over before crossing the ECA boundary. The change-over event, the time the vessel completes the change and starts using compliant fuel, must be recorded in the Fuel Oil Change-Over Record Book or the equivalent log entry. Port State Control inspection routinely verifies the change-over record and may sample fuel from the day tank and engine fuel rail to verify compliance.
HFO to MGO Switching Procedure
The classical HFO-to-MGO change-over procedure follows a defined sequence designed to manage viscosity and temperature.
Step 1: Plan the change-over. The chief engineer plans the switch sufficiently in advance of the ECA boundary to allow completion before entry. The plan considers the ship’s speed, the engine fuel consumption, the volume of HFO in the engine fuel rail and circulating system, and the time required for the rail to be flushed of HFO. A typical change-over takes 1-2 hours from start to full MGO operation; the planning margin is normally 4-6 hours from start of switch to ECA entry.
Step 2: Prepare the MGO service tank. The MGO is normally stored in a separate service tank at ambient temperature (or with mild heating in cold climates). The tank is checked for level, water content (via centrifugal purifier sample), and contamination. The fuel transfer and supply pumps are verified ready.
Step 3: Reduce engine load. The engine is brought down from full sea load to typically 50-70% MCR before the switch begins. Reduced load reduces the thermal stress on fuel pumps and injectors during the transition.
Step 4: Begin the switch. The HFO supply to the engine is closed and the MGO supply opened. The HFO heating is reduced progressively (the steam supply to the heater is throttled). The fuel temperature at the engine fuel rail is allowed to fall slowly from the HFO operating temperature (130-150 degrees Celsius) to a temperature appropriate for MGO (typically 40-50 degrees Celsius for the lighter MGO grades).
Step 5: Monitor viscosity. The fuel viscosity at the injection point is the critical parameter. Most modern installations have a viscometer in the fuel rail that controls the heater steam supply via a viscosity controller. During the change-over, the viscosity controller follows the fuel as it transitions from heated HFO to ambient MGO.
Step 6: Complete the switch. Once the fuel rail is fully MGO and the temperature has stabilised, the change-over is complete. The time, position, fuel tank levels, and any unusual events are logged. The engine load can be returned to the planned operating level.
Step 7: Document. The change-over event is recorded in the Fuel Oil Change-Over Record Book, the Engine Log, and the bridge GPS-position log. The records must be cross-consistent for Port State Control verification.
Viscosity Management
Fuel viscosity at the injection point is the parameter that determines whether fuel pumps and injectors operate correctly. Too high a viscosity (cold fuel that has not been heated) causes excessive injection pressure, poor atomisation, and possible pump seizure. Too low a viscosity (over-heated fuel) causes fuel leakage past pump clearances, reduced injection pressure, and potential lubricity problems with low-sulphur distillates that have lost natural lubricity.
The recommended injection viscosity is 10-15 cSt for most marine engines. The temperature required to achieve this varies by fuel:
- HFO 380 cSt at 50 deg C: heated to 130-150 deg C for 12-15 cSt at injection.
- VLSFO (around 50-150 cSt at 50 deg C): heated to 90-120 deg C.
- ULSFO (around 5-30 cSt at 50 deg C): heated only modestly or used at ambient.
- MGO (1.5-6 cSt at 40 deg C): used at ambient or cooled in tropical conditions.
During the change-over, the viscometer-controller continuously adjusts heating to maintain target viscosity as the fuel composition transitions. The change-over must not be too rapid: a sudden temperature drop on a hot fuel pump can cause thermal shock cracking, and a sudden viscosity drop can cause pump leakage. The recommended rate of temperature change is no more than 2 degrees Celsius per minute.
Engine and Fuel System Implications
The principal engine and fuel system implications of fuel switching are:
Lubricity: low-sulphur distillates have lower natural lubricity than HFO. Fuel pumps and injectors that previously relied on the lubricity of HFO may experience accelerated wear when running on MGO without lubricity additives. Modern MGO specifications under ISO 8217 include minimum lubricity requirements (HFRR test), but the chief engineer should be alert to wear indications.
Compatibility: HFO and MGO are not generally compatible at the molecular level. Mixing in significant quantities can cause asphaltene precipitation (HFO components dropping out of solution when contacted with light MGO), forming sludge that can clog filters, fuel pumps, and injectors. The change-over procedure aims to avoid simultaneous mixing of significant quantities, but a small amount of mixing in the fuel rail is unavoidable.
Thermal stress: the engine fuel system, particularly the fuel pumps, is designed for stable operating temperature. Repeated rapid thermal cycling during change-over generates fatigue stress.
Fuel oil consumption (FOC): MGO has a higher calorific value per kilogram than HFO (typically 42.7 MJ/kg vs 40.5 MJ/kg) and a lower density (0.85 t/m³ vs 0.99 t/m³). The volumetric consumption is therefore similar, the mass consumption slightly lower. Engine performance correction is required for accurate fuel consumption monitoring (see marine engine performance monitoring).
Dual-Fuel and Tri-Fuel Propulsion
The IMO 2020 sulphur cap and the EEDI/EEXI/CII regulatory framework have driven an industry shift towards alternative fuels. Dual-fuel and tri-fuel propulsion systems allow operation on more than one fuel and provide flexibility for fuel switching of a fundamentally different kind.
LNG dual-fuel engines (predominantly the WinGD X-DF, MAN ME-GI, and various medium-speed engines) can operate on either LNG (with a small pilot of MGO for ignition) or on conventional liquid fuel. The transition between LNG and liquid mode is software-controlled and takes seconds to minutes. LNG eliminates SOx emission entirely (the fuel contains essentially no sulphur) and reduces NOx and CO2 emissions.
Methanol dual-fuel engines (MAN ME-LGIM, WinGD X-DF-M) entered commercial service from 2023 onwards with the Maersk Laura Maersk and subsequent newbuilds. Methanol is liquid at ambient temperature, has zero SOx emission (no sulphur), and can be produced from green sources (e-methanol from green hydrogen and captured CO2; bio-methanol from biomass). The liquid storage and handling is simpler than LNG, although methanol has its own toxicity and material-compatibility considerations.
Tri-fuel engines combining HFO, LNG, and shore power capability are emerging in the LNG carrier and cruise ship segments. The Carnival Mardi Gras (2021) and subsequent LNG-powered cruise ships represent the leading deployment.
The fuel switching considerations on dual-fuel vessels include the management of two complete fuel systems, the choice of fuel based on availability, price, and ECA status, the operational transition between fuel modes (typically smooth on modern dual-fuel engines), and the bunkering compatibility checks for each fuel.
Fuel Oil Change-Over Time
The time required for a complete fuel change-over depends on the fuel system configuration and the engine size. Typical times are:
For a fixed-rail two-stroke main engine of medium-speed configuration: 1-2 hours from start of switch to full MGO operation.
For a system with separate HFO and MGO service tanks but a shared fuel rail: 30-60 minutes for the rail volume turnover.
For a system with shared service tanks (mixing in a single tank): much longer because the tank mixing must be completed; 4-8 hours or more, and not generally recommended due to compatibility issues.
The rate-limiting factor in modern installations is usually the temperature transition (slowing to no more than 2 deg C per minute) rather than the fuel rail volume.
FONAR Documentation
The Fuel Oil Non-Availability Report (FONAR) is the documented justification used when a vessel cannot obtain compliant fuel and continues to operate on non-compliant fuel. The FONAR concept is established under MARPOL Annex VI and IMO MEPC Guidelines (specifically MEPC.1/Circ.881 and the consolidated version in MEPC.310(73)).
The FONAR is invoked when the master, despite making a reasonable effort, cannot obtain fuel of the required sulphur content at the bunkering port. The vessel may then use non-compliant fuel for the next voyage and submit the FONAR to the next port and to the flag State and the Port State of bunkering.
The FONAR is not a free pass. The master must:
- Document the efforts made to obtain compliant fuel (suppliers contacted, prices, availability).
- Continue to attempt to obtain compliant fuel at subsequent bunkering opportunities.
- Submit the FONAR to the relevant authorities promptly.
- Switch to compliant fuel as soon as available.
The FONAR has been used relatively rarely since the 2020 sulphur cap, as the global market has adjusted and 0.50% sulphur fuel (VLSFO) is widely available at all major bunkering ports. It remains a fallback for unusual situations such as remote ports or sudden supply disruption.
Real-Time Switching at ECA Boundaries
The practical operation of crossing an ECA boundary requires real-time co-ordination between the bridge and the engine room. The bridge gives advance notice of the planned ECA entry time, calculated from the GPS position, the planned course, and the speed. The engine room begins the switch sufficiently in advance to be on compliant fuel before the boundary is crossed.
Modern systems automate part of this through the integrated bridge / ECDIS / fuel management interface, with the ECA boundary recorded as a navigational waypoint and an automated alert at the change-over trigger point.
Scrubber Operation as an Alternative
The exhaust gas cleaning system (EGCS or “scrubber”) is the principal alternative to fuel switching for compliance with the sulphur cap. Scrubbers remove SOx from the exhaust gas by spraying alkaline water (typically seawater in open-loop systems, or freshwater with caustic addition in closed-loop systems) through the exhaust stream and washing the SOx into solution as sulphate.
A scrubber-fitted vessel can continue to use HFO (high-sulphur fuel oil with 3.5% sulphur) and meet the sulphur cap by exhaust treatment. The economic case depends on the price differential between HFO and VLSFO, the scrubber capital cost (around USD 4-8 million for a large vessel retrofit), the operational complexity, and the availability of HFO bunkering.
Scrubbers have been controversial in two respects. First, open-loop scrubbers discharge the wash water back to the sea after treatment, with a low pH and elevated metal content; some ports (Singapore, Fujairah, China coastal waters, California, parts of the EU) prohibit open-loop discharge. Closed-loop and hybrid systems address this but at higher capital and operational cost.
Second, the lifecycle GHG emission of scrubber-equipped HFO operation is higher than VLSFO operation because the scrubber consumes additional energy to operate. The IMO Initial GHG Strategy and the revised 2023 Strategy will progressively constrain scrubber use as a CO2 mitigation strategy is required alongside SOx compliance.
The 2025 fleet is approximately 25-30% scrubber-fitted (around 5,000 vessels), 60-65% VLSFO/MGO compliance, and a small but growing share of LNG and methanol dual-fuel.
Related Wiki Articles
- MARPOL Convention
- MARPOL Annex I Oil Pollution Prevention
- Bunker Quality and ISO 8217
- Marine Fuel Oil Systems
- Marine Engine Performance Monitoring
- Marine Cargo Tank Heating Systems
- Polar Code
- Hong Kong Convention
- Port State Control
- Classification Society
- SOLAS Convention
References
- MARPOL Annex VI, Regulation 14 (Sulphur Oxides and Particulate Matter), as amended
- IMO Resolution MEPC.305(73), 2018 Amendments to MARPOL Annex VI (2020 Sulphur Cap)
- IMO MEPC.1/Circ.881, Guidance on Best Practice for Fuel Oil Suppliers and Buyers and Users
- IMO Resolution MEPC.310(73), Guidelines for Onboard Sampling and Verification
- IMO Resolution MEPC.184(59), Guidelines for Exhaust Gas Cleaning Systems (EGCS)
- IMO Resolution MEPC.259(68), Revised Guidelines for Exhaust Gas Cleaning Systems
- ISO 8217:2024, Specifications for Marine Fuels
- ISO 4406, Hydraulic Fluid Power, Cleanliness Levels
- IACS Unified Requirement M61 (Fuel Oil Treatment Systems)
- IACS Unified Interpretation MPC119 (FONAR procedures)
- ABS Guide for Use of Methanol or Ethanol as Fuel
- DNV Class Notation Gas Fuelled and Methanol Fuelled
- INTERTANKO Bunker Operations Guidance
- BIMCO Standard Bunker Contract and Bunker Clauses