One voyage, five regimes: why your compliance numbers stop agreeing

Ask a compliance manager for last quarter's CO2 and the honest answer is a question back: for which regime? A mid-market owner calling EEA ports in 2026 reports the same voyages under four overlapping regimes at once — FuelEU Maritime, EU ETS for shipping, the IMO Data Collection System, and the IMO Carbon Intensity Indicator — with EU MRV underneath as the monitoring backbone. Each regime takes the same bunker delivery notes and voyage legs and turns them into a different number. Reuse one figure across all of them and at least one regime is quietly wrong — usually the moment a verifier asks you to prove it.

This is not a tooling gap you can spreadsheet your way out of. It is structural: the regimes disagree by design, on four axes at once.

Axis 1 — the emissions boundary

FuelEU Maritime measures greenhouse-gas intensity well-to-wake (WtW): it counts the upstream production and supply of the fuel as well as its combustion, and it counts CO2, CH4 and N2O expressed as CO2-equivalent (Reg. (EU) 2023/1805, Annex I–II). EU ETS, by contrast, is a tank-to-wake (TtW) instrument — it prices only what comes out of the stack, initially CO2, with CH4 and N2O added from 2026 (Dir. 2003/87/EC as amended by (EU) 2023/959). IMO DCS is narrower still: a mass total of fuel oil consumed, reported to the flag State (MARPOL Annex VI, Reg. 27). Three regimes, three different things counted from the same tonne of fuel.

Axis 2 — geographic scope

Both EU instruments apply a voyage-scope rule that IMO regimes do not. A voyage between two EEA ports counts at 100%; a voyage with one leg outside the EEA counts at 50%; a voyage entirely outside the EEA counts at 0% (Reg. (EU) 2023/1805, Art. 2; mirrored in the ETS maritime scope). IMO DCS and CII, being global, count every mile. So the very same voyage contributes its full fuel mass to DCS and CII, but only half — or none — of it to FuelEU and ETS. The scope split is a deterministic function of the departure and arrival ports against the official EEA port list; it is not a judgement call, and it must be computed identically everywhere the number surfaces.

Axis 3 — the emission factor set

Each regime pins its own factors, published by its own authority, revised on its own schedule. FuelEU carries the Annex II carbon-intensity defaults and the RED II upstream values; EU ETS uses the MRV monitoring factors; CII sets its Cf carbon factors and reference lines through separate IMO resolutions — the reference lines in MEPC.337(76), the reduction factors in MEPC.338(76), the A–E rating boundaries in MEPC.339(76), and the calculation method and Cf factors in MEPC.336(76) as updated by MEPC.354(78). When a regulator republishes a factor table, one regime's numbers move and the others do not. A figure computed against last year's factor set will not reproduce — which is fatal, because reproducibility years later is exactly what a verifier and an auditor require.

Axis 4 — the shape of the answer

Finally the regimes do not even return the same kind of quantity:

• FuelEU returns an intensity: grams CO2-equivalent per megajoule of energy, compared against a yearly-tightening target set in Art. 4(2) against the 2020 reference value of 91.16 gCO2eq/MJ (−2% in 2025, −6% in 2030, stepping to −80% by 2050), with any gap converted to a euro penalty via Annex IV.
• EU ETS returns a quantity of allowances to surrender, phased in by emission year — 40% of 2024 emissions, 70% of 2025, and 100% from 2026 onward (Art. 3gb).
• IMO DCS returns an absolute mass of fuel and CO2 for the flag State.
• CII returns a ratio — grams CO2 per deadweight-tonne-mile (the AER) — mapped to an A–E band against a reference line that tightens annually.

An intensity, a euro penalty, a count of allowances, an absolute mass, and a letter grade — all derived from one set of bunker delivery notes and voyage legs. No single stored 'CO2 number' can be right for more than one of them.

The failure mode

The way this goes wrong in practice is mundane. Fuel and voyage data are entered once, then copied into a FuelEU spreadsheet, an ETS spreadsheet, the DCS submission and a CII tracker. Each copy is adjusted by hand for its own scope and factors. Over a year the copies drift — a corrected bunker figure updated in one place and not the others, a voyage re-classified in one tool. By verification season the four numbers no longer trace back to the same source, and the owner cannot prove they are consistent because, silently, they are not.

The fix: store primitives, project per regime

The design that survives this is to store the primitive facts and nothing derived. The canonical record is the fuel mass per voyage, per fuel type, per consumer (main engine, auxiliary, boiler), plus the voyage endpoints and the onshore-power-supply energy. That layer holds no CO2 figure at all. Each regime's number — FuelEU WtW intensity, ETS TtW allowances, DCS mass, CII AER — is computed from those primitives at query time, against a pinned, versioned factor set.

This is the whole thesis behind how Provenship stores data: primitive fuel and voyage facts as the single source of truth, with regime-specific projections layered on top and every figure traceable to the bunker delivery note or voyage record it came from. The regimes will keep disagreeing — that is the regulation, not a bug — but the disagreement becomes explainable, reproducible, and defensible in front of the verifier, instead of an unpleasant surprise.