ISO 8217:2024 Marine Fuel Specification

ISO 8217:2024 is the seventh edition of the international standard for marine fuel specifications, published by the International Organization for Standardization on 30 May 2024. It defines four grade tables covering conventional distillate fuels, low-sulphur residual fuels (VLSFO), bio-residual blends, and high-sulphur residual fuels, setting characteristic limits for viscosity, density, sulphur, flash point, CCAI, cat fines, stability, and FAME content across all grades. It replaces ISO 8217:2017 and is the contractual benchmark for the vast majority of commercial bunker supply agreements worldwide.

Every bunker stem that meets the bunker delivery note requirements of MARPOL Annex VI is implicitly or explicitly governed by ISO 8217 in the commercial supply contract. When a vessel suffers injector wear, filter blockages, purifier throughput failures, or ignition problems, the first reference document is always the ISO 8217 test result on the fuel batch in question. The 2024 revision is the largest restructuring since the standard was first issued in 1987, primarily because Very Low Sulphur Fuel Oil (VLSFO, the dominant marine fuel grade since the IMO 2020 global sulphur cap) and bio-derived fuels had never appeared in a formally structured grade table before.

Edition History

ISO 8217 traces its origins to ASTM and British Standards Institute work on residual fuel classification that pre-dated the IMO. The first ISO 8217 edition appeared in 1987, consolidating earlier national standards into a single international framework. Since then the standard has gone through five further revisions before the 2024 edition.

The six prior editions are:

• 1st edition, 1987: Established the basic Class DM (distillate) and Class RM (residual) structure with the grade nomenclature still in use today.
• 2nd edition, 1996: Refined limits and harmonized test method references.
• 3rd edition, 2005: Tightened sulphur provisions and updated method references following SOx regulatory pressure.
• 4th edition, 2010: Added hydrogen sulphide, acid number, oxidation stability, and lubricity requirements for distillate grades; introduced FAME (bio-blend) provisions.
• 5th edition, 2012: Targeted Interim update, aligning method references and addressing practical issues identified after the 2010 edition.
• 6th edition, 2017: Added DMB grade clarifications; refined Clause 5 general quality requirements; addressed compatibility issues arising from multiple crude sources.

The 2024 revision took over five years of committee work. The principal driver was the 1 January 2020 entry into force of the 0.50% global sulphur cap under MARPOL Annex VI Regulation 14, which drove the marine fuel market from heavy fuel oil (HFO, sulphur typically 2.0 to 3.5%) to VLSFO. VLSFO has properties that differ from both conventional HFO and gas oil: its viscosity ranges from near-distillate to near-conventional HFO, its stability can be marginal because it is produced by blending several refinery streams, and its density is typically lower than HFO but higher than gas oil. The 2017 edition had no dedicated VLSFO grade table. Ships were buying VLSFO to an RMG 380 specification that imposed no minimum viscosity (a 30 cSt product and a 370 cSt product were both compliant with the maximum of 380 mm²/s), and no structured stability testing regime specific to these blended fuels.

Title Change: Scope Expansion to Renewable Sources

One of the first things apparent in the 2024 edition is that its formal title changed. ISO 8217:2017 is titled “Petroleum products – Fuels (class F) – Specifications of marine fuels.” ISO 8217:2024 is titled “Products from petroleum, synthetic and renewable sources – Fuels (class F) – Specifications of marine fuels.” The change is deliberate: the 2024 standard formally recognises that marine fuels may be derived in whole or in part from synthetic gas-to-liquid (GtL) processes, hydrotreated vegetable oil (HVO), fatty acid methyl esters (FAME), or other bio-derived streams, not just from petroleum crude refining. This scope expansion is what allowed the inclusion of bio-residual fuel grades (Table 3) and explicit FAME content provisions across all four tables.

The Four-Table Grade Structure

ISO 8217:2017 used two tables: one for distillate grades (DM grades) and one for residual grades (RM grades). The 2024 edition splits residual fuels into three tables based on sulphur content and bio-content, yielding four tables in total.

Table 1: Distillate and bio-distillate marine fuels. Covers the traditional DM grades (DMX, DMA, DMZ, DMB) and adds DF (distillate bio-fuel) equivalents (DFA, DFZ, DFB) where the FAME content is not limited to the 7% maximum that applied under the 2017 standard.

Table 2: Residual marine fuels with sulphur content below or equal to 0.50% by mass. This is the VLSFO table, covering the grade range from RMA 20 to RMK 500 at the 0.5% sulphur ceiling. The suffix “-0.5” or “-0.1” is appended to grade designations to communicate the applicable sulphur limit clearly.

Table 3: Bio-residual marine fuels. A new table covering RF (residual bio-fuel) grades (RF 20, RF 80, RF 180, RF 380, RF 500) where FAME content is unrestricted and renewable sources are permitted up to 100%. Mandatory FAME content reporting and net heat of combustion measurement apply.

Table 4: Residual marine fuels with sulphur content above 0.50% by mass. This retains the traditional high-sulphur residual grades for ships equipped with exhaust gas cleaning systems (scrubbers), covering the same viscosity range as Table 2 but without the 0.50% sulphur ceiling.

The all-tables general requirements in Clauses 5 to 10 must be met regardless of which table’s grade was specified. Clause 5 in particular remains critical: a fuel is unacceptable if it contains any added substance that makes it dangerous to handle, damages the equipment, or adversely affects performance of the engine or boiler, even if every individual parameter in the grade table is within the stated limit.

Distillate Grade Table

Distillate grades are petroleum or bio-derived products produced primarily by atmospheric distillation of crude oil, or by refinery processes that yield gas oil fractions. They are water-white to pale yellow, flow freely at ambient temperatures, and require no preheating in most climates.

*Sulphur maxima for DMA, DMZ, and DMB are the base-standard limits. The operative limit for any given delivery is the lower of the ISO 8217 limit and the MARPOL Annex VI statutory requirement for the area of operation (0.10% in ECAs, 0.50% globally).

DMX is a low-viscosity gas oil intended primarily for emergency generators, fire pumps, and other machinery where cold-weather starts are required without preheating. Its lower flash point minimum of 43°C distinguishes it from all other distillate grades; it should not be used as a main engine fuel where a 60°C minimum flash point is mandated under SOLAS.

DMA is the standard marine gas oil (MGO) grade, the dominant distillate fuel for main engines when operating in Emission Control Areas under the 0.10% sulphur limit. For ECA compliance it is almost always supplied at 0.10% sulphur or below as a commercially marketed LSMGO (Low Sulphur Marine Gas Oil).

DMZ matches DMA viscosity limits but adds an oxidation stability requirement and historically was used where a tighter quality fuel was needed for high-pressure common-rail injection systems in medium-speed engines.

DMB is a blended marine gas oil with a wider viscosity range and higher density ceiling, accepting small percentages of residual stock. It occupies the space between gas oil and residual fuel and has largely fallen out of widespread commercial availability in modern bunkering.

The 2024 edition added three DF bio-distillate equivalents (DFA, DFZ, DFB) that mirror their DM counterparts in viscosity and density limits but specify FAME content, oxidation stability, and net heat of combustion reporting without imposing a FAME upper limit. Cetane number (not just the derived Cetane Index) is required for DF grades. For pure HVO (hydrotreated vegetable oil) or GtL distillates, the 2024 standard also introduces lubricity requirements, since paraffinic fuels stripped of natural lubricity compounds by deep hydrotreatment can cause premature fuel pump and injector wear if their lubricity is not confirmed via the HFRR (High Frequency Reciprocating Rig) test.

Residual Grade Table

Residual fuels are the bottom-of-the-barrel streams remaining after lighter fractions are distilled from crude oil, often blended with cutter stock gas oils to achieve the target viscosity. They are dark brown to black, require heating to flow, and contain elevated concentrations of metals, asphaltenes, and inorganic contaminants compared to distillates.

The ISO 8217:2017 residual grade table (reproduced here for reference; the 2024 Table 2 and Table 4 grades mirror these parameters with sulphur suffixes added to each designation):

*Pour point: Summer (June to September in Northern Hemisphere) / Winter (rest of year). A separate winter/summer distinction was removed for most residual grades in the 2024 edition; see Section 8 below.

Grade nomenclature. The RM prefix denotes residual marine fuel. The letter following (A through K) indicates a broad density and quality tier, with A grades lightest and K grades heaviest and most aromatic. The number is the maximum kinematic viscosity in mm²/s at 50°C. In the 2024 edition, a sulphur-content suffix is appended to Table 2 and Table 4 designations: for example, RMG 380-0.5 specifies an RMG 380 grade with sulphur at or below 0.50% by mass, while RMG 380-0.1 specifies the same viscosity grade at or below 0.10% sulphur.

What Changed in the 2024 Edition

Minimum Viscosity for VLSFO Grades

The most operationally significant change in Table 2 (VLSFO) is the introduction of minimum viscosity limits. Under ISO 8217:2017 a supplier could deliver a product with 15 cSt viscosity to an RMG 380 specification because only the maximum of 380 mm²/s was specified. Ships had reported fuel system problems at the low end: thin VLSFOs caused reduced lubrication in fuel pumps designed for heavier fuels and increased leakage across fuel injection equipment clearances. The 2024 Table 2 sets minimums: 2.0 mm²/s for RMA 20, 20.0 mm²/s for RMG 180, and 120.0 mm²/s for 380 cSt grades. A fuel ordered as RMG 380-0.5 must now fall between 120.0 mm²/s and 380.0 mm²/s at 50°C.

New VLSFO Stability Testing

The 2017 edition’s stability requirements (Total Sediment Aged, TSA; Total Sediment Existent, TSE) applied to all residual grades. These tests detect sludge-forming asphaltene precipitation in HFO. VLSFOs, being blended fuels with different asphaltene chemistry, required a different test. The 2024 Table 2 mandates Total Sediment Potential (TSP, IP 375 / ASTM D4870), a laboratory-accelerated aging test that is the only method currently validated for VLSFO stability. TSA and TSE are still required for reporting purposes, but TSP is the compliance gate for VLSFO.

Organic Chloride Prohibition

Several contamination incidents between 2018 and 2022 involved chlorinated organic solvents and phenol-containing waste streams introduced into the bunker supply chain, causing catastrophic engine damage and crew safety incidents in multiple ports across Asia and the Middle East. The 2024 edition responds with a Clause 5 requirement that all fuel grades shall be free from organic chlorides, with a maximum permissible limit of 50 mg/kg as the practical analytical threshold.

Bio-Residual Grades (RF Grades, Table 3)

Table 3 defines RF (residual bio-fuel) grades: RF 20, RF 80, RF 180, RF 380, and RF 500. These grades accept unlimited FAME content, renewable hydrocarbon streams, and other bio-derived components. The physical characteristic limits (viscosity, density, flash point, pour point, CCAI, water, ash, MCR, cat fines) mirror those in Table 2, but they carry mandatory reporting requirements for FAME content, net heat of combustion, and oxidation stability. The introduction of RF grades provides a formal pathway for the bunker supply chain to sell blended biofuel at scale without the fuel being off-specification by default.

Bio-Distillate Grades (DF Grades, Table 1)

The 2017 standard permitted up to 7% FAME in distillate grades under Clause 5.2. The 2024 Table 1 retains that limit for DM grades but introduces DF grades (DFA, DFZ, DFB) without a FAME ceiling. A B100 product (pure FAME) is classified under Table 1 as a bio-distillate. DF grades require a minimum cetane number (not just the derived cetane index), confirming that the actual ignition quality of the bio-blend meets the required level. Lubricity (HFRR, ISO 12156-1) and oxidation stability (RANCIMAT method) are also mandatory for DF grades, recognising that pure FAME or HVO blends age differently from petroleum distillates.

Removal of Winter/Summer Distinction for Distillate Pour Point

ISO 8217:2017 specified separate Summer and Winter pour point limits for DMA and DMZ (0°C in summer, -6°C in winter) and for DMB (6°C in summer, 0°C in winter). The 2024 edition removes this seasonal split and sets a single pour point limit for each grade. Cloud point and Cold Filter Plugging Point reporting requirements were updated to align with actual cold-weather operability needs rather than calendar-based limits. The practical implication is that buyers in cold-climate trades must specify cold-flow requirements contractually rather than relying on a seasonal limit within the standard itself.

Understanding Each Characteristic

Kinematic Viscosity

Viscosity is the dominant operational parameter in residual fuel management. It determines pump suction lift, line pressure, the temperature to which fuel must be heated before the centrifugal purifiers and the injection system, and the quality of atomisation at the injector tip. Residual fuels are measured at 50°C because that is within the practical heating range for purification. Distillate fuels are measured at 40°C because they flow freely without heating.

Correct atomisation viscosity at the injector tip is typically 10 to 15 mm²/s for slow-speed two-stroke engines and 12 to 20 mm²/s for medium-speed four-stroke engines. RMG 380 fuel must be heated to around 130°C to reach 10 mm²/s at the injector. Too high a viscosity at the injector causes poor atomisation, incomplete combustion, and carbon deposits. Too low a viscosity causes fuel pump leakage and poor lubrication in Bosch-type rotary pumps.

Density

Density at 15°C determines the mass-to-volume conversion for quantity measurement during bunkering. The difference between a density of 990 kg/m³ and 1,010 kg/m³ on a 1,000-tonne stem is 20 tonnes, roughly $10,000 to$16,000 at current HSFO prices. Density also correlates with fuel calorific value: heavier, more aromatic fuels have slightly lower specific energy per tonne. For the ship’s automatic viscosity control system, density interacts with viscosity in determining the corrected viscosity temperature relationship; the Redwood and ASTM viscosity blending equations both require density as an input.

Centrifugal purifiers separate water and solids by density difference. The RMK 700 grade’s maximum density of 1,010 kg/m³ is at the practical limit for centrifugal purification using fresh water as the displacement liquid; gravity discs must be selected to suit the actual density of the fuel, and for fuels approaching 991 kg/m³ or above, the gravity disc selection becomes critical.

Sulphur

Sulphur limits in ISO 8217 are described as “statutory” for most residual grades rather than a fixed percentage, reflecting the layered regulatory structure: the applicable sulphur limit depends on where the ship is operating. The MARPOL Annex VI sulphur cap sets 0.50% as the global limit since 1 January 2020 and 0.10% in designated Emission Control Areas (ECAs) including the North Sea, Baltic Sea, North American coast, and US Caribbean Sea ECA. The 2024 edition’s Table 2 and Table 4 structure makes this distinction explicit at the grade level for the first time by incorporating the sulphur content directly into the grade suffix.

Sulphur combustion produces SO₂, which in the presence of moisture forms sulphuric acid. In the engine’s combustion chamber, this acid attacks cylinder liner surfaces if the cylinder oil Base Number (BN) is insufficient to neutralise it. The cylinder oil BN selection is therefore directly dependent on the sulphur content of the fuel in use. Running a low-BN cylinder oil on high-sulphur fuel causes accelerated polished bore liner wear; running a high-BN cylinder oil on VLSFO causes deposit build-up from excess base reserve, creating a different wear mode.

Flash Point

Flash point is a safety parameter. ISO 8217 mandates a minimum of 60°C for all grades except DMX (43°C minimum). SOLAS Chapter II-2 requires that fuel oil used for propulsion or boiler firing shall have a flash point no lower than 60°C. The 60°C minimum therefore applies in all SOLAS-regulated compartments. A sub-60°C flash point indicates contamination, typically by a distillate or petroleum solvent that should not be present in a residual or gas oil delivery. This is one of the characteristics most likely to trigger an immediate port state control action if found off-specification.

Pour Point

Pour point is the lowest temperature at which the fuel flows under gravity. It determines the minimum temperature at which the fuel must be maintained in tanks, lines, and purifiers to prevent wax crystallisation from blocking filters and solidifying in settling tanks. For RMD 80 through RMK 700, the ISO 8217:2017 pour point maximum was 30°C in summer and winter alike; in practice this means these fuels must be maintained above 30°C at all times, typically requiring steam heating of double-bottom fuel tanks even in tropical climates. The 2024 edition retains this structure for residual grades but acknowledges that VLSFO grades can have pour points that differ from conventional HFO by 10°C or more, depending on the blend composition.

CCAI: Calculated Carbon Aromaticity Index

CCAI is computed as:

where $d$ is density in kg/m³ at 15°C and $V$ is kinematic viscosity in mm²/s at 50°C. The formula is defined in ISO 8217 and widely recognised as an empirical proxy for the ignition quality of residual fuel. A lower CCAI indicates a more paraffinic, more easily ignited fuel. A higher CCAI indicates a more aromatic, more resistant fuel.

The index captures an important physical reality: residual fuels with the same viscosity can have very different densities depending on their crude origin and refinery treatment. A highly aromatic fuel rich in polycyclic aromatic hydrocarbons will have a higher density than a more paraffinic fuel at the same viscosity, and the aromatic fuel will require more energy to initiate ignition in the combustion chamber. CCAI quantifies this tendency without an engine test.

ISO 8217 maximum CCAI limits by grade tier:

• RMA 10: 850 maximum
• RMB 30 and RMD 80 and RME 180: 860 maximum
• RMG and RMK all viscosity variants: 870 maximum

Values above 870 are associated with extended ignition delay periods. At CCAI values approaching 880, combustion problems in slow-speed engines have been documented, including rough running, cylinder pressure fluctuations, exhaust temperature rise, and injector fouling. Engine manufacturers’ fuel quality recommendations typically specify preferred CCAI below 840 to 860 for reliable operation across the full load range.

CCAI does not apply to distillate grades. Distillate ignition quality is characterised by the Cetane Index (derived from density and viscosity) or the measured Cetane Number for bio-distillate DF grades.

Micro Carbon Residue (MCR)

MCR (Micro Carbon Residue, ASTM D4530 / ISO 10370) measures the carbonaceous residue remaining after evaporating and pyrolysing a fuel sample under controlled conditions. For residual fuels, MCR is a proxy for the tendency of the fuel to form combustion deposits on injector tips, piston crowns, and exhaust valve seats. RMA 10 allows a maximum 2.5% MCR; heavier grades permit up to 20.0%. Fuels at the high end of the MCR range demand more careful combustion optimisation and more frequent cylinder condition monitoring.

Water Content

Water in fuel causes injector seizure through steam flashing, promotes microbial growth in fuel tanks (leading to filter blockage and acid production), causes instability in asphaltene-rich residual fuels, and degrades centrifuge performance. ISO 8217 limits water to 0.30% V/V for RMA and 0.50% V/V for heavier residual grades. These limits apply to the fuel as received at the ship’s manifold, not after onboard treatment. A fuel delivered within specification can accumulate water in storage tanks through condensation, leaking tank tops, or ballast water cross-contamination; onboard management is the ship’s responsibility.

Ash

Ash represents the inorganic, non-combustible residue after complete oxidation of the fuel. It includes metals (vanadium, nickel, iron, sodium) and mineral fines. High ash content directly correlates with combustion deposits and gas-side fouling of turbocharger nozzle rings and turbine blades. The limits range from 0.04% m/m for RMA to 0.15% m/m for heavier grades.

Vanadium and Sodium

Vanadium is naturally present in crude oil and concentrates in residual fractions. In combustion it forms vanadium pentoxide (V₂O₅), which melts at around 690°C and attacks high-temperature metal surfaces including exhaust valve seat inserts, turbocharger blades, and exhaust valve guides in a form of hot corrosion. ISO 8217:2017 limits vanadium to 50 mg/kg for RMA, rising to 450 mg/kg for RMD through RMK. For VLSFO (Table 2 in the 2024 edition), the vanadium limits remain at the same levels; in practice, VLSFOs produced by blending straight-run residue with HVO or gas oil streams typically run below 100 mg/kg.

Sodium in fuel primarily indicates contamination by seawater (sea salt containing sodium chloride) introduced during crude transport or refining. Sodium chloride in combustion produces molten sodium sulphate compounds that cause accelerated corrosion. When vanadium and sodium co-exist in significant quantities the corrosion is compounded; the critical ratio is sodium above about one-third of the vanadium content, at which point the vanadium oxide melting point depression is sufficient to attack materials at typical exhaust valve operating temperatures. ISO 8217 limits sodium to 50 mg/kg for RMA and 100 mg/kg for all heavier grades.

Catalytic Fines (Aluminium + Silicon)

Cat fines are particles of aluminium oxide (Al₂O₃) and silicon dioxide (SiO₂) originating from the fluid catalytic cracking (FCC) catalyst used in refineries. These particles enter residual fuel when the cyclone separators in the FCC unit fail to remove all catalyst carryover from the slurry oil stream, which then enters the residual fuel blend. Cat fines have a Mohs hardness of 7 to 8, placing them between quartz and topaz, harder than most engineering steels. Even a few milligrams per kilogram reaching the engine after purification causes abrasive wear on cylinder liners, piston rings, and fuel injection precision pairs.

ISO 8217 limits the combined aluminium plus silicon content to 25 mg/kg for RMA 10 and 60 mg/kg for all heavier residual grades. The standard specifies the limit at the ship’s manifold, that is, after the supplier’s purification. The ship’s own purifiers must then reduce cat fines further; the 60 mg/kg limit at manifold delivery is not acceptable as the limit at the engine fuel inlet. Most engine manufacturers specify cat fines below 15 mg/kg at the engine inlet, and several slow-speed engine OEMs recommend below 10 mg/kg. Achieving those engine-inlet limits from a 60 mg/kg delivery requires effective centrifugal purification at correct throughput rates and correct operating temperatures.

Acid Number

Acid number (AN, mg KOH/g, ASTM D664 / IP 182) measures the concentration of acidic compounds in the fuel. Acids in marine fuel include naphthenic acids from crude oil and organic acids from bio-derived streams. Elevated acid number causes corrosion of fuel system components, particularly copper alloy fittings, filter housings, and fuel pump parts. ISO 8217 limits acid number to 0.5 mg KOH/g for distillate grades and 2.5 mg KOH/g for all residual grades.

Hydrogen Sulphide

Hydrogen sulphide (H₂S) in fuel oil is an acute inhalation hazard and a safety-critical parameter. H₂S is heavier than air and accumulates in confined spaces such as cargo tanks, cofferdams, and bunker manifold areas during fuel transfer. ISO 8217 limits dissolved and emulsified H₂S to 2.00 mg/kg for all grades. This limit was first introduced in the 2010 edition following several crew fatalities and hospitalisation incidents attributed to H₂S release during bunkering operations in Southeast Asia.

Total Sediment (TSP, TSA, TSE)

Sediment in residual fuel forms when asphaltene molecules in the fuel flocculate and precipitate, typically triggered by mixing incompatible fuels, by ageing, or by temperature excursions. Sediment blocks fuel filters, overloads purifiers, and can accumulate in settling tanks to the point of blocking drain lines and valves.

ISO 8217 specifies Total Sediment Existent (TSE, ASTM D4807 / IP 285) and Total Sediment Aged (TSA, IP 390 / ASTM D7111) for conventional residual grades, with a maximum of 0.10% m/m. For VLSFO (Table 2 in 2024), Total Sediment Potential (TSP, IP 375 / ASTM D4870) is the compliance test, because TSA and TSE were developed for conventional HFO and do not adequately characterise the stability of paraffinic-aromatic VLSFO blends.

FAME Content

FAME (fatty acid methyl esters, commonly known as biodiesel) has been permitted in marine fuel since the 2010 edition of ISO 8217, but with strict limits. Under the 2017 standard, FAME was limited to 7% by volume in distillate grades under Clause 5.2. The 2024 edition retains this 7% limit for DM grades but introduces DF grades and RF grades with no FAME ceiling. For bio-blended fuels the standard requires FAME content reporting and mandates the fuel’s compliance with EN 14214 (the European automotive biodiesel standard) for the FAME component.

FAME in marine fuel raises several specific concerns. FAME has a tendency to absorb water, which promotes microbial growth and corrosion in fuel systems. FAME’s cold-flow properties differ from petroleum distillates, with cloud points potentially above 10°C for some feedstocks (palm-oil FAME particularly). FAME can degrade oxidation stability, forming gums and polymeric deposits in fuel systems if stored for extended periods. FAME is also incompatible with some elastomers used in fuel system seals. The CIMAC Working Group 7 published a dedicated guideline in 2024 on the management of FAME-containing marine fuels, addressing these concerns for shipowners and operators.

What the Grade Names Mean: Distillate vs. Residual

The naming convention encodes the key fuel characteristics at a glance.

Distillate grades (DM or DF prefix): The D indicates a distillate product. The M denotes a petroleum-derived (mineral) fuel; the F denotes a bio-derived (fuel) product under the 2024 edition. The suffix letters X, A, Z, B indicate viscosity and quality tiers in ascending order of viscosity and decreasing quality: DMX is the lightest and most refined, DMB the heaviest and most accepting of blended streams.

Residual grades (RM or RF prefix): The R indicates a residual (i.e., bottom-of-barrel) product. The M again denotes petroleum-derived; F denotes bio-derived under the 2024 edition. The letters A through K are broadly ascending density and quality tiers: RMA is the lightest and most refined residual, approaching a heavy gas oil, while RMK is the heaviest and most contaminated. The number is the maximum kinematic viscosity in mm²/s at 50°C. So RMG 380 is a petroleum-derived residual fuel in the G density/quality tier with a maximum viscosity of 380 mm²/s at 50°C, while RF 380 is a bio-blended equivalent in the same viscosity class.

The 2024 addition of a sulphur suffix (e.g., RMG 380-0.5) makes the grade designation self-describing for sulphur compliance: a purchasing officer ordering RMG 380-0.5 is specifying a residual G-grade fuel at maximum 380 cSt and maximum 0.50% sulphur.

RMG 380 and DMA: Key Differences

These two grades appear together constantly in charter party fuel clauses, ISM fuel management plans, and bunker specifications, often with a simplified statement that the ship can burn “IFO 380 or MGO.” The physical and handling differences are substantial.

Temperature management. DMA is pumpable from ambient temperatures and requires no preheating for injection. RMG 380 must be heated to 40 to 50°C for gravity transfer, 60 to 80°C for pumping through steam-traced pipelines, and 120 to 135°C at the fuel oil heater outlet to achieve the 10 to 15 mm²/s viscosity required at the injectors. Heating systems must be verified operative and bunker tank steam coil integrity confirmed before switching from distillate to residual service.

Purification. DMA does not require centrifugal purification in normal service, though a filter is recommended. RMG 380 must be processed through a full centrifugal purification train (settling, separation, and polishing) to reduce water, sludge, and cat fines to engine-acceptable levels before it reaches the day tanks.

Sulphur and cylinder oil. DMA delivered for ECA compliance will typically contain 0.10% sulphur or less, requiring a low-BN cylinder oil (typically BN 25 to 40) to avoid deposit formation. RMG 380 HSFO can contain up to 3.5% sulphur, requiring BN 70 to 100 cylinder oils. The switchover between these cylinder oil grades during fuel switching operations is a planned procedure requiring fuel consumption records and timed lube oil feed rate changes.

CCAI. DMA has no CCAI requirement because cetane index characterises distillate ignition quality. RMG 380 has a CCAI maximum of 870, which can be approached or exceeded with highly aromatic, dense fuel blends from certain refinery configurations.

Relationship with MARPOL Annex VI

MARPOL Annex VI addresses fuel quality through two distinct mechanisms that complement ISO 8217 without wholly incorporating it.

Regulation 14: Sulphur limits. This regulation sets the operative global limit of 0.50% sulphur since 1 January 2020 and the ECA limit of 0.10%. These limits override any higher ISO 8217 sulphur permissions for the grade. A DMA grade nominally allowed 1.00% sulphur under ISO 8217 is not compliant with MARPOL Annex VI Regulation 14 if the ship is operating in a global trade lane since January 2020; only fuel at or below 0.50% is permissible.

Regulation 18: Fuel oil quality. Regulation 18 requires that fuel oil supplied to any ship subject to an International Air Pollution Prevention (IAPP) Certificate shall be accompanied by a Bunker Delivery Note and a representative sample drawn at the ship’s bunker manifold during delivery. The BDN must record the following minimum information per MARPOL Annex VI Appendix V: name and IMO number of the receiving ship, port of delivery, bunkering date, fuel supplier name and address, product name, quantity in metric tonnes, density at 15°C, sulphur content, and the supplier’s declaration that the fuel conforms to applicable requirements including Regulations 14 and 18.

The Regulation 18 general requirements overlap substantially with ISO 8217 Clauses 5 to 10 (the general quality requirements), but MARPOL Annex VI does not formally incorporate ISO 8217 by reference for the full suite of physical characteristics. In practice, port state control verifies MARPOL compliance through the BDN and the MARPOL representative sample; commercial disputes over quality (viscosity, cat fines, CCAI, water) are resolved under ISO 8217 as the contract standard rather than directly under MARPOL.

The MARPOL representative sample and the commercial sample. These are two distinct samples drawn from the same bunker stem. The MARPOL sample (also called the MARPOL retained sample or Annex VI sample) is drawn by continuous drip at the ship’s bunker manifold per MEPC.182(59) guidelines and retained onboard for a minimum of 12 months or until the fuel is substantially consumed. It is the evidence document for MARPOL sulphur compliance and Regulation 18 conformity. The commercial sample is drawn per ISO 13739 procedures and serves as the evidence document for quality disputes under the supply contract. In most disputes, the commercial sample is sent to an independent fuel testing laboratory; the MARPOL sample may be tested separately if a sulphur or Regulation 18 issue is involved. The two samples may give slightly different results because they are drawn from different points in the stem timeline and under different protocols.

ISO 8217 in Bunker Contracts and Disputes

ISO 8217 is not itself a contract but it is contractually incorporated, almost universally, into bunker supply agreements. The standard supply contract terms of the major bunker suppliers (Shell, Vitol, Gunvor, Peninsula Petroleum, and others) reference ISO 8217 as the quality specification baseline. BIMCO standard charter party fuels clauses and the BIMCO standard bunker contract reference ISO 8217 for the quality definition of required fuels. When an owner sub-charters a vessel, the charter party fuel specification typically reads “IFO 380 to ISO 8217:2017 (or latest edition)” or similar.

Disputes arise when a fuel’s tested results differ between the supplier’s delivery certificate and the ship’s independent analysis. The governing principle for test result discrepancies is ISO 4259-2, which establishes the statistical framework for comparing two laboratory results for the same fuel characteristic. Each test method has a known Reproducibility (R) value that defines the maximum permissible difference between two results obtained by different laboratories on the same sample under the same method. If two results differ by less than R, neither can be called “wrong”; if they differ by more than R, the result set contains at least one error. For total sediment (TSE), for example, the reproducibility of the standard method means that two laboratories can differ by up to 0.020% m/m on the same sample and both be in method compliance. This statistical reality matters greatly in fuel quality arbitration.

The most consequential category of off-specification fuel is contamination by materials not normally present in petroleum products: chlorinated hydrocarbons, phenols, styrene, diethyl ether, and polymers have all been found in fuel supplies at various ports. ISO 8217 Clause 5 provides the legal basis for rejection regardless of parametric compliance: a fuel that passes every table parameter but contains a contaminant that causes engine damage or a safety hazard is non-compliant under Clause 5. P&I clubs and hull underwriters have seen a pattern of serious main engine damage claims since 2018 correlated with organic chloride contamination, which prompted the explicit 50 mg/kg organic chloride limit in the 2024 edition.

Sampling Under ISO 13739 and MARPOL Annex VI

ISO 13739 (“Petroleum products and related products – Sampling from ships”) specifies the procedures for drawing representative samples of marine fuels during delivery. Its procedures address continuous drip sampling (recommended), composite sampling from line section, and spot sampling (least representative). The standard defines sample container specifications, labelling requirements, and chain-of-custody procedures.

For the MARPOL retained sample, MEPC.182(59) guidelines specify that the sample is drawn at the receiving ship’s bunker manifold throughout the delivery, yields a minimum sample volume of 400 mL (INTERTANKO recommends 750 mL to allow for replicate testing), and is sealed in the presence of representatives of both the ship and the supplier. The sealed sample label must record: name of ship, IMO number, port, date, grade, quantity delivered, and supplier name. The sample is retained for at least 12 months from delivery date or until substantially consumed.

Sampling disputes arise when the sample is drawn at the wrong location (supplier barge manifold rather than ship manifold), when the continuous drip sampler is not calibrated, or when the sample is taken only at the start or end of the stem rather than continuously throughout. The location of sampling matters legally: the MARPOL requirement is explicit that the sample is drawn at the receiving ship’s bunker manifold, making the ship the responsible party for ensuring its own sampling compliance.

The 2024 Edition and Alternative Fuels

ISO 8217:2024 covers petroleum, synthetic, and renewable sources, but it does not cover non-liquid marine fuels. LNG, LPG, methanol, and ammonia used as marine fuels are governed by separate standards and codes (the IGC Code and its amendments for gas carriers, the IGF Code for gas-fuelled ships under SOLAS Chapter II-1, and IMO MSC-MEPC.2/Circ.17 for alternative fuels risk assessment). Hydrogen is not addressed.

For LNG as a marine fuel, the relevant fuel quality standard is ISO/TS 18683 and the methane number framework used by engine manufacturers. Ammonia as a marine fuel is governed by interim guidelines under IMO MSC-MEPC.2/Circ.17 with no equivalent to ISO 8217 yet in force.

The scope expansion in ISO 8217:2024 to include “synthetic and renewable sources” does, however, accommodate GtL diesel (jet fuel-equivalent paraffinic distillates produced from natural gas by Fischer-Tropsch synthesis) and HVO (hydrotreated vegetable oil), both of which fit within the DF distillate bio-grade framework of Table 1 or meet the DM grades’ limits when blended. HVO is already available in small volumes as a marine fuel in Northwest Europe; its low density, zero CCAI concerns, and near-zero sulphur make it attractive for ECA compliance, though its price premium and cold-flow properties require careful specification.

Limitations

ISO 8217:2024 sets minimum quality floors, not performance guarantees. A fuel that passes all ISO 8217 parameters can still cause operational problems if it is incompatible with the ship’s other fuels (separate compatibility testing to ASTM D4740 spot test or IP 228 analytical method is required for blended fuels), if the ship’s purification system is inadequate to reduce cat fines from 60 mg/kg to below 15 mg/kg at the engine inlet, or if the ship’s heating system cannot reach the required injection viscosity.

The standard’s CCAI limit of 870 for RMG and RMK grades is not a guarantee of trouble-free combustion; it is a regulatory floor. Engine manufacturers’ preferred CCAI ranges are tighter. A fuel at CCAI 868 is within ISO 8217 specification but may still cause ignition difficulties in a slow-speed engine at part load with a worn fuel injection system.

TSP testing required for VLSFO grades under Table 2 remains a relatively new method (compared to the decades of service history of TSE and TSA for HFO), and its reproducibility in ring-testing has not yet achieved the same level of statistical validation as established methods. Operators and surveyors should interpret TSP results with awareness of the method’s current precision envelope.

The 2024 edition’s bio-residual Table 3 is new to the supply chain. No large-volume RF-grade fuels were commercially available at the time of publication. Operators contracting for RF-grade supply will need to negotiate detailed specifications for FAME content, heating value, oxidation stability, and cold-flow properties rather than relying on the Table 3 minimums alone, at least until the market for these grades matures and typical delivered quality is better characterised.

ISO 8217 does not govern fuel storage, treatment, or onboard management. Responsibility for fuel condition from the ship’s manifold to the engine fuel inlet, including purification, heating, compatibility of co-stored fuels, and water management, rests with the ship’s operators. A fuel that passes ISO 8217 at delivery and later damages the engine may give rise to disputes about whether the damage was caused by the fuel’s inherent quality or by onboard handling practices. The marine fuel oil systems and fuel switching operations articles cover the onboard management requirements in detail.

Finally, ISO 8217 is a voluntary international standard, not a flag-state regulation. It becomes binding only through contractual incorporation in bunker supply agreements and charter party fuel clauses, or through flag-state regulation that adopts the standard by reference. MARPOL Annex VI Regulation 18 imposes its own quality requirements in parallel; those requirements are legally binding under the MARPOL Convention for all MARPOL-party flag states, which as of 2026 includes all major maritime nations.

See Also

• Bunker delivery note – the MARPOL Annex VI document accompanying every bunker stem
• MARPOL Annex VI sulphur cap – the 0.50% global and 0.10% ECA sulphur limits
• MARPOL Annex VI Reg. 14 – Regulation 14 sulphur requirements in detail
• MARPOL Annex VI Reg. 18 – Regulation 18 fuel quality and BDN requirements
• MARPOL Convention – the overarching pollution prevention treaty
• Bunker quality and ISO 8217 compliance – operational guide to quality disputes and management
• Fuel switching operations – procedures for HFO/VLSFO/MGO transitions
• Cylinder oil BN and fuel sulphur – matching lube oil BN to fuel sulphur
• Marine fuel oil systems – onboard fuel management overview
• Biofuels in shipping – bio-marine fuel developments
• LNG as a marine fuel – alternative fuel beyond ISO 8217 scope
• Ammonia as a marine fuel – alternative fuel beyond ISO 8217 scope
• Heavy fuel oil – HFO characteristics and history
• IMO 2020 sulphur cap – the market transition that drove the 2024 revision