A primer for shipowners, technical directors, and compliance officers approaching the Carbon Intensity Indicator regime for the first time — and for those who have been living with it but never had the framework explained as a whole.
A CII gap analysis is, in Ingeniat’s words, “a structured, forward-looking assessment of whether a vessel’s operational carbon intensity trajectory will remain within required rating thresholds under MARPOL Annex VI’s tightening regulatory framework.” That sentence does a lot of work — it tells you the assessment is structured (not just a back-of-envelope check), forward-looking (not a single-year review), and tied to a tightening reference line (not a static target).
This post unpacks each of those characteristics, explains what a “gap” actually means, and lays out when the analysis is worth commissioning.
What CII is
The Carbon Intensity Indicator is an operational rating regime introduced under MARPOL Annex VI, Chapter 4. It applies the AER (Annual Efficiency Ratio) — grams of CO₂ emitted per unit of transport work — to commercial ships and grades each vessel A through E based on the relationship between its attained AER and a required CII reference line for its ship type and size.
For most vessel types — tankers, bulk carriers, container ships, gas carriers, general cargo — AER is expressed in grams of CO₂ per deadweight-tonne-mile. For cruise and ro-pax passenger ships, the formula uses gross tonnage-mile instead of DWT-mile. (Verify the exact metric assignment per ship type against the current SEEMP Part III guidelines and IMO MEPC decisions before publishing.)
The point is that CII rates operational efficiency. It does not measure technical efficiency directly (that is the role of the EEXI — Energy Efficiency Existing Ship Index). A vessel can be technically modern and still rate poorly on CII if its operations are inefficient; conversely, an older vessel with clean trading patterns can rate well.
Required CII vs. attained CII
Two numbers matter:
- Attained CII — what the vessel actually achieved in the data year, calculated from verified DCS fuel consumption, voyage distances, and transport work.
- Required CII — the threshold for the year, derived from a reference line set per ship type and size (indexed to a 2019 baseline), then tightened each year by an annual reduction factor.
The reduction factors have been increasing year-on-year since the regime began in 2023, and remain under active IMO review. Ingeniat’s methodology footnote is worth repeating here: “reduction factors are subject to potential revision under the IMO 2023 GHG Strategy.” Any specific reduction-factor figure cited in 2026 should be verified against the most recent MEPC decisions before being used in a real CAP, charterparty clause, or external publication.
The rating bands — how A through E work
The A/B/C/D/E boundaries are derived per ship type from the statistical distribution of attained CII values across the 2019 fleet. The general pattern: A and B capture the better-than-average performers, C sits around the median, D and E capture the worse-than-average. The exact percentile cut-offs vary by ship type.
In practice, the bands behave like this:
- A — top performers, currently few vessels.
- B — strong operational performance.
- C — middle of the distribution. The de facto target for owners who want to stay out of regulatory escalation.
- D — sub-median performance. A single D is not by itself a CAP trigger, but three consecutive D ratings is.
- E — bottom of the distribution. A single E rating triggers a CAP.
The boundary values shift every year because the required CII tightens — meaning a vessel rated B in 2024 may find itself at C in 2026 without any change in its actual emissions.
The CAP trigger
A Corrective Action Plan under SEEMP Part III becomes mandatory in two situations:
- Three consecutive D ratings — meaning the vessel sits at D for three years in a row.
- A single E rating — even a one-year slip into E triggers the requirement.
Once triggered, the CAP requirement is annual. The vessel must continue filing updated plans and demonstrate improvement, or face escalating regulatory and commercial consequences — class review, flag state consultation, charterparty implications, and port state scrutiny. The CAP itself is covered in detail in the closing post of this series.
Who it applies to
Per the scope stated in Ingeniat’s methodology: vessels of 5,000 GT and above on international voyages. Most of the world’s commercial deep-sea tonnage falls in scope. Coastal and short-sea trades below 5,000 GT are typically excluded.
What “gap” actually means — the most important concept in this post
This is the point that distinguishes a real gap analysis from a single-year DCS review.
A vessel’s attained CII is not a fixed number. It varies year to year with operational factors that have nothing to do with the vessel itself:
- Ballast ratio (more ballast legs = lower transport work per voyage).
- Speed profile (slow steaming vs. full speed).
- Port idle time (anchorage waiting burns fuel without transport work).
- Cargo utilisation (partial loading produces higher AER).
- Seasonal trade pattern (winter North Atlantic vs. summer Mediterranean).
- Hull and propeller condition at the time of measurement.
A vessel with steady technical and operational performance will still see year-on-year rating variation because of these factors alone. The “gap” between attained and required CII is therefore not a single number — it is a probability distribution across rating bands.
A vessel sitting right at the C/D boundary has a non-trivial probability of rating C in some years and D in others. Three consecutive D ratings is a CAP trigger. So the question for that vessel is not “are we C or D this year?” but “what is our probability of being D for three consecutive years across the planning horizon?” That probability is what a gap analysis quantifies.
Ingeniat’s framing is precise: the gap is “assessed not just as a binary compliant/non-compliant outcome, but as a probability distribution across rating bands.” This is the conceptual core of the methodology and the reason a single DCS data year is insufficient on its own.
When to commission a gap analysis
A gap analysis is worth commissioning:
- Annually, as part of the DCS reporting cycle — to refresh the forward view against the latest actual performance and the latest required-CII values.
- Ahead of a major commercial decision — new charter, route change, vessel sale or purchase, charterparty renegotiation. The CII trajectory is now a commercial input, not just a regulatory one.
- Ahead of a scheduled drydock — retrofit planning requires a 3–5 year forward view to align CAPEX with rating-improvement needs.
- After a sustained rating change — a vessel that has slipped from B to C, or from C to D, needs a structured reassessment before the next annual cycle.
- When the CAP horizon is in the planning window — i.e. when the projected trajectory suggests D ratings within three years or an E rating within one.
The trigger for a gap analysis is not “something has gone wrong.” It is “we need to know whether something is going to go wrong before it does.”
What a good gap analysis produces
The standard outputs of a structured CII gap analysis — drawn from Ingeniat’s methodology — are:
- A multi-year rating forecast under both base-case and stress-case scenarios.
- A quantification of the emissions reductions required to restore or maintain a C rating or above.
- A structured evaluation of corrective measures — operational, technical retrofit, and fuel-switching options — with impact magnitude, implementation timeline, capital cost, and interaction with broader regulatory exposures (EU ETS, FuelEU Maritime).
- An integration with engineering performance baselines and dry-docking schedules — so the rating forecast accounts for hull fouling progression, propeller degradation, and engine wear.
- An assessment of charterparty and trade-route constraints — the commercial limits on what operational measures can actually achieve.
These outputs are what turn a regulatory regime from an annual compliance exercise into a manageable operational programme.
Where this leads
For any vessel of 5,000 GT and above on international voyages, a structured CII gap analysis is no longer optional. The reference line tightens every year, the operational variability creates real CAP-trigger risk even for technically sound vessels, and the commercial consequences of a CAP filing are now visible to charterers, ports, and insurers.
The next four posts in this series work through each layer of the methodology: why a C rating can deteriorate even when nothing changes on the ship, the operational measures that move the CII needle and where they hit commercial limits, the technical retrofit tier when the operational layer is exhausted, and finally the SEEMP Part III Corrective Action Plan process that closes the regulatory loop.
The starting point, though, is the gap analysis itself — and the shift from “what was our rating last year?” to “what is our probability of being in each rating band across the next three to five years?”
To scope a structured CII gap analysis, the entry point is Ingeniat’s gap-analysis service.
Note: AER formula details vary by ship type per the current SEEMP Part III guidelines and IMO MEPC decisions. Rating-band percentile boundaries are set per ship type based on the 2019 distribution; the qualitative description (A top, C middle, E bottom) is consistent across vessel types, but specific percentile values should be verified against the most recent IMO reference. Reduction-factor figures and CAP trigger language should be checked against the latest MEPC revisions before publication and are only included for illustrative purposes. Get in contact for specific guidance.
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