Masters and chief engineers are trained to make one number the deciding factor on fuel: sulphur content. It determines whether a parcel can be burned inside an Emission Control Area, whether it satisfies the global 0.50% cap under MARPOL Annex VI regulation 14, and whether the ship stays out of a PSC detention. That training is correct — for regulation 14 purposes, sulphur % is the only number that matters.

It is also the wrong number to be reaching for once the same fuel record moves into IMO DCS, EU MRV, EU/ UK ETS and Fuel EU Maritime reporting. For those frameworks, the operative classification is the ISO 8217 fuel grade — set primarily by kinematic viscosity — not the sulphur band on the bunker delivery note (BDN).

Fuel Categorisation- % S Vs Viscosity

Under the various Emission monitioring and reporting regimes, Every tonne of fuel consumed has to be converted to CO2 for reporting, and that conversion runs through a fixed factor, Cf, set out in the 2014 Guidelines on the method of calculation of the attained EEDI for new ships (resolution MEPC.245(66), as amended by MEPC.364(79)).

This Cf is assigned by fuel category, not by sulphur percentage and this is commmonly listed in the vessels approved SEEMP 2:

Diesel/Gas oil (ISO 8217 grades DMX, DMA, DMZ, DMB): Cf = 3.206
Light Fuel Oil, LFO (ISO 8217 grades RMA, RMB, RMD): Cf = 3.151
Heavy Fuel Oil, HFO (ISO 8217 grades RME, RMG, RMK): Cf = 3.114
LPG (propane): Cf = 3.000
LPG (butane): Cf = 3.030
LNG: Cf = 2.750

Understanding that the reportring under Emission reporting regimes has to be done in line with the ISO 8217 fuel categoriasation basis viscosity and in line  with the approved SEEMP2 still remains a critical gap in many vessels.

Here is the point that gets missed: a VLSFO parcel at 0.10–0.50% S can legitimately sit in either the HFO bracket or the LFO bracket, depending purely on its ISO 8217 viscosity grade. A blend delivered as RMG 380 or RME 180 is HFO, Cf 3.114. A blend delivered as RMA 20 or RMB 30 is LFO, Cf 3.151. Both can be marketed and delivered under the identical commercial label "0.50% S VLSFO." The sulphur figure on the BDN is identical; the Cf applied to that tonnage differs by 1.2%. ISO 8217:2024 sharpens this boundary further by introducing minimum viscosity limits for the first time — the 2017 edition specified only maxima — which means two BDNs referencing different editions of the standard can categorise a borderline blend differently even before you touch the sulphur figure.

The Gap

Bunker records are built around sulphur banding because that's what regulation 14 compliance and ROB tracking under most commercial factors require: HSFO, VLSFO, ULSFO, MGO. That same sulphur-based report is frequently carried straight into the DCS and MRV consumption log, with the Cf applied by sulphur band rather than by the ISO 8217 grade actually stated on the BDN and Certificate of Quality. Utilising a purely % S based fuel categorization system in the emission reporting introduces a Cf error that has nothing to do with how much fuel was actually burned.

This is exactly the determination Part II of the SEEMP is meant to fix at source. The Ship Fuel Oil Consumption Data Collection Plan, developed under the 2022 Guidelines for the Development of a SEEMP (MEPC.346(78)), gives effect to MARPOL Annex VI regulation 27 and has to specify the methodology by which each bunkered parcel's fuel type — and therefore its Cf — is determined and recorded.A careful reading of the SEEMP 2 of any vessels will show the manner in which the fuel categorisation, and hence its CO2 emission factors are tied to the ISO 8217 grade on the delivery documentation, rather than to the sulphur band. However, many vessels still do not close this gap and reports of fuel consumption are based on % Sulphur rather than the ISO 8217 Viscodity based category.

The cost impacts of Fuel Categorisation

EU ETS. Take a vessel bunkering 3,000 MT a year of nominally "0.50% S VLSFO" for EU-related voyages, genuinely delivered as RMG 380 grade (Cf 3.114) but logged against the LFO Cf bracket (3.151) because the DCS entry followed the sulphur band rather than the BDN grade.

Annual VLSFO consumption (EU-related voyages): 3,000 MT
Fuel actually delivered: RMG 380 → HFO, Cf 3.114
Cf mistakenly applied: LFO, Cf 3.151
Cf gap: 3.151 − 3.114 = 0.037
Additional CO2 reported: 0.037 × 3,000 MT = 111 tCO2
Indicative EUA price (mid-2026): ≈ €80/tonne
Avoidable EUA cost, single vessel: 111 × €80 ≈ €8,880
Phase-in applying to 2026 emissions: 100% (up from 70% for 2025, 40% for 2024, under Directive 2003/87/EC as amended by Regulation (EU) 2023/957)
Scaled to a 10-ship fleet with the same gap: six-figure annual exposure

Call it HFO (correctly, RMG 380, Cf 3.114): 9,342 tCO2 reported, ≈€747,360 in EUA at €80/tonne. Call it LFO (Cf 3.151, the sulphur-band default): 9,453 tCO2 reported, ≈€756,240 in EUA. Same 3,000 MT, same voyage — €8,880 apart, purely on which ISO 8217 grade is booked.

FuelEU Maritime. Regulation (EU) 2023/1805 penalises the gap between a vessel's actual well-to-wake GHG intensity and the annual limit, at €2,400 per tonne of VLSFO-equivalent deficit (41,000 MJ per tonne), escalating for consecutive non-compliant periods. The same Cf gap that inflates EU ETS allowances also inflates the well-to-wake CO2 term in the GHG intensity calculation — using the same 3,000 MT parcel:

Annual VLSFO consumption (same parcel): 3,000 MT ≈ 123,000,000 MJ (at 41,000 MJ/t)
GHG intensity limit, 2025–2029: 89.34 gCO2eq/MJ (tightens to 85.69 gCO2eq/MJ from 2030), against the 2020 reference value of 91.16 gCO2eq/MJ
CO2-equivalent deficit from the Cf gap: 0.037 × 3,000 MT = 111 tCO2eq
Equivalent VLSFOe deficit (÷ 89.34 gCO2eq/MJ, ÷ 41,000 MJ/t): ≈ 30.3 tonnes VLSFOe
FuelEU penalty rate: €2,400 / tonne VLSFOe
Avoidable penalty exposure, single vessel: 30.3 × €2,400 ≈ €72,700

Call it HFO: no incremental compliance deficit from this parcel. Call it LFO: +111 tCO2eq deficit, ≈30 tonnes VLSFOe, ≈€72,700 in avoidable penalty exposure. The figures are illustrative, not observed on a specific vessel, but the mechanism — one mislabelled parcel shifting both the ETS and FuelEU numbers — is real.

Our view

At TECS, we treat the ISO 8217 grade stated on the BDN and Certificate of Quality — not the sulphur band — as the primary key for Cf assignment in Fore-C, and we check that a vessel's SEEMP Part II methodology states this explicitly . Fore-C's Maritime Data Collection System (MARS) enforces this at the source of entry: it runs automated verification of the ISO 8217 grade against the bunkered parcel and allows only the correctly graded entry to be booked into the vessel's ROB, so the Cf error described above cannot originate in the first place. Where a BDN doesn't state an ISO 8217 grade outright, that gap needs to be closed with additional data sources such as fuel oil analysis reports,  before the parcel is consumed and reported, not reconstructed retrospectively during DCS or MRV verification.

Closing the Gap

Check that the SEEMP Part II Fuel Oil Consumption Data Collection Plan on each vessel names the ISO 8217 grade — not the sulphur band — as the determinant of Cf, and confirm whether it references the 2017 or 2024 edition of ISO 8217, since the viscosity boundaries differ between the two. Review a sample of recent BDNs against the corresponding DCS/MRV log entries to confirm the Cf applied matches the grade actually delivered, not the sulphur label. Where BDNs are silent on ISO 8217 grade, raise it with the your vessel manager through additional docuemnatation such as Fuel Oil verification reports or similar.

How TECS Can Help
TECS provides end-to-end EU ETS, FuelEU Maritime and DCS/MRV compliance support — from BDN-to-Cf verification and SEEMP Part II gap review to voyage-level reporting and corrective action planning. Contact us at dms@maritimetecs.com to review your fleet's fuel categorisation and compliance position.

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