CO₂ Compliance in Maritime: A Step-by-Step Guide for EU MRV & IMO DCS

EU MRV and IMO DCS have moved CO₂ reporting from a voluntary gesture to a hard regulatory obligation. Knowing what is required — and in what order — is the difference between a smooth annual cycle and a scramble that risks penalties, port bans, and failed verifications.

The two frameworks overlap but are not identical. EU MRV applies to vessels over 5,000 GT calling at EEA ports, regardless of flag — and from January 2025, scope extends to general cargo vessels between 400 and 5,000 GT and offshore vessels above 400 GT. IMO DCS applies to all vessels over 5,000 GT in international trade, reported through flag states to the IMO GISIS system. Many operators must satisfy both simultaneously.

Step 1: Submit and verify your Monitoring Plan

Before the reporting period begins, every vessel in scope must have an approved Monitoring Plan (MP) on board. The MP is the formal record of how the ship will measure and document its CO₂ emissions — it must be reviewed and approved by an accredited independent verifier before it can be used.

• Specify the fuel consumption monitoring method: bunker delivery notes, flow meters, or direct CO₂ measurement
• Define the voyage parameters to be recorded: distance travelled, time at sea, cargo carried
• Document data collection procedures and the responsible parties for each step
• Have the MP verified by an accredited body and keep the approved version onboard

The MP is a live document. Any change to monitoring methodology requires re-verification before the updated approach can be used in a reporting period.

Step 2: Collect emissions data throughout the year

Once the reporting period is underway, operators must collect and record data in line with the approved MP for every qualifying voyage. This is where most compliance problems originate — not in the submission, but in gaps and inconsistencies that accumulate voyage by voyage over twelve months.

• Fuel consumption per voyage, broken down by fuel type
• CO₂ emission factors for each fuel used
• Operational parameters: speed, distance, time at sea, cargo load
• Port calls, including time spent at berth and any cold-ironing or shore power usage

"The errors that fail verification are almost never deliberate — they are the accumulated result of manual data entry, handover gaps, and voyage records that were never reconciled."

Decorum Fleet Operations

Step 3: Compile and submit the Emission Report

At the close of the reporting period, the year's data must be compiled into a formal Emission Report (ER) and submitted to an accredited verifier for review. The submission deadline is April 30 of the following year. Late submission is a compliance failure — not a procedural one — and carries the same penalty exposure as non-submission.

The ER must cover every qualifying voyage made during the period, with no gaps. A vessel that changed operators mid-year requires careful coordination to ensure the record is complete across the full twelve months.

Step 4: Third-party verification

An accredited verifier reviews the Emission Report for completeness, internal consistency, and alignment with the approved Monitoring Plan. This is not a rubber stamp — verifiers will identify gaps, query inconsistencies, and require corrections before issuing approval.

• Inconsistent fuel consumption records across voyage legs are the most common rejection trigger
• Missing voyage data — particularly for short coastal legs — is frequently flagged
• Discrepancies between bunker delivery notes and logged consumption will be queried
• Any change in monitoring methodology not reflected in an updated MP will be rejected

Building a clean data trail throughout the year significantly reduces verification time and the risk of correction cycles that push submissions past the deadline.

Step 5: Upload to Thetis-MRV and GISIS

Once the verifier approves the Emission Report, it must be uploaded to Thetis-MRV — the European Commission's emissions reporting platform — by June 30 each year. For IMO DCS, the equivalent submission goes through the vessel's flag state to the IMO GISIS system. Both submissions must reference the verified report; the verifier will confirm accuracy at point of submission.

Upon successful submission, the vessel receives a Document of Compliance (DoC). This document must be downloaded from Thetis-MRV, kept onboard, and produced on request during port state control inspections. A vessel without a valid DoC is a vessel at risk of detention.

Step 6: Publication of verified emissions data

By June 30 each year, the European Commission publishes verified emissions data for all ships in scope. This is not optional transparency — it is a regulatory requirement, and the published data is available to charterers, investors, and regulators. A vessel with incomplete or inaccurate data in the public record carries that exposure into every commercial conversation.

Step 7: Compliance & Certification

Upon successful verification and submission, the shipowner and crew must ensure the Document of Compliance is downloaded from Thetis-MRV, kept onboard, and available for inspection at any time. Port state control officers are entitled to request the DoC during any port call — a vessel that cannot produce it is a vessel at immediate risk of detention.

Certification is not a one-off event. Each annual cycle produces a new DoC, and tracking which vessels hold current certificates — across a fleet operating across multiple flag states and jurisdictions — requires active document management, not a shared folder and good intentions.

Turning the compliance cycle into a performance tool

The data required for EU MRV and IMO DCS compliance is, by definition, a complete record of how your fleet consumed fuel across a full year. Operators who treat that record as a performance dataset — not just a regulatory submission — extract material value from the same effort.

• Benchmark individual vessel efficiency against fleet average and IMO reference lines
• Identify routes where fuel consumption is consistently above expectation
• Use speed and consumption data to model the impact of slow steaming on annual fuel cost
• Flag vessels whose efficiency has declined year-on-year as candidates for hull or engine inspection
• Use documented emissions reductions to support carbon credit qualification and ESG reporting

Compliance is the floor. The ceiling — in cost reduction, fleet intelligence, and commercial positioning — is determined by what you do with the data once the submission is filed.