A ship can hold valid emissions certificates and still face a compliance problem on its next voyage. A fuel changeover may be recorded incorrectly, a scrubber may be operating outside its approved limits, or reported fuel consumption may not match the data used for EU carbon obligations. The applicable requirements depend on the ship, its engines, its route and the equipment in service.

This maritime emissions regulations guide separates the main IMO requirements from the EU measures that can apply to the same voyage. It also explains what records and operating checks help turn a certificate into evidence of compliance.

Start with the voyage and the ship, not the equipment

The IMO framework establishes international requirements through MARPOL Annex VI, covering air pollution and energy efficiency. EU rules add regional obligations, including monitoring and reporting of greenhouse gas emissions, emissions trading and fuel greenhouse gas intensity requirements. These systems overlap, but they do not measure the same thing or use identical scopes.

Before preparing a compliance file, establish four facts: the ship’s type and size, its construction and engine details, the ports and waters on its itinerary, and the fuels and emissions-control equipment it will use. Then map each obligation to an accountable person, a data source and a record. A change in route, fuel supplier or equipment configuration can alter the checks needed even when the vessel itself has not changed.

For example, a ship entering an emissions control area may need to meet more stringent sulfur requirements than it does on the open ocean. A voyage involving an EU port may also generate reporting and carbon-cost obligations that are separate from its MARPOL fuel-compliance records. Treating these as one generic “emissions certificate” issue makes gaps harder to find.

IMO air pollution rules: sulfur, nitrogen oxides and exhaust treatment

Sulfur compliance follows the fuel and the operating area

MARPOL Annex VI limits the sulfur content of fuel oil used on board, with stricter limits in designated sulfur emissions control areas. Operators commonly comply by using fuel that meets the applicable limit or by using an approved equivalent arrangement, such as an exhaust gas cleaning system where its use is permitted.

The compliance question is not simply whether compliant fuel was purchased. Teams also need to know which fuel was actually in use at a given time and whether the changeover was completed before entry into a restricted area. Fuel remaining in service tanks and piping can affect the timing. A written procedure should reflect the ship’s actual fuel system, not an assumed changeover time copied from another vessel.

Bunker delivery documentation, representative samples, tank records and changeover entries serve different purposes. Together, they should connect the fuel delivered to the fuel stored and consumed. A delivery note alone does not demonstrate that the correct fuel was being burned at the required location.

Fuel availability can also create operational and safety pressures. Segregation, compatibility and treatment arrangements matter when changing between fuels. An emissions procedure that ignores engine reliability or the risk of mixing incompatible fuels may create a different problem while trying to solve the first one.

NOx requirements are tied to engines and their certification

Nitrogen oxide requirements depend on factors including the engine’s installation date, rated speed, certification and operating area. Certain ships operating in designated NOx emission control areas must meet more stringent standards when the applicable conditions are met. A vessel’s route alone does not determine which NOx standard applies to every engine on board.

The engine’s technical file and associated certification establish the approved configuration and parameters. Maintenance teams need to control replacements, settings and modifications that could affect that configuration. A component may fit mechanically but still require assessment against the certified arrangement.

Selective catalytic reduction, or SCR, can reduce NOx when properly integrated and operated. Its effectiveness depends on conditions such as exhaust temperature, reagent supply and the control system. The practical checks therefore include more than the presence of an SCR unit: alarms, reagent quality and consumption, maintenance history, and any approved operating limitations all matter. SCR addresses NOx; it should not be treated as a substitute for sulfur compliance.

A scrubber is an operating system, not a permanent exemption

An approved exhaust gas cleaning system can provide an equivalent means of meeting sulfur requirements, but approval depends on the particular installation and its documented operating arrangements. The crew must operate it within the approved parameters and maintain the required monitoring and records. A system that is installed but unavailable, bypassed or operating outside its permitted conditions creates a different compliance position from a system operating normally.

Scrubber washwater discharge also needs separate attention. International operating arrangements do not eliminate restrictions imposed by particular ports or local authorities. When a planned route includes waters with discharge controls, the voyage plan should account for the vessel’s available mode of operation, storage capacity where relevant, and compliant alternative fuel arrangements.

For safety and quality reviews, trace a fault through the full response: how the crew detects it, what the approved procedure requires, whether a fuel switch is feasible, who is notified, and how the event is recorded. An alarm log without evidence of the response leaves an incomplete picture.

IMO greenhouse gas requirements: design, operation and reporting

IMO greenhouse gas measures address different stages of a ship’s life and operation. The Energy Efficiency Design Index, or EEDI, applies to certain new ships; the Energy Efficiency Existing Ship Index, or EEXI, addresses the technical efficiency of applicable existing ships. Both concern a ship’s assessed technical efficiency, but they are not interchangeable with a voyage-by-voyage measure of actual emissions.

The Carbon Intensity Indicator, or CII, is an operational measure for applicable ships. It relates reported operational emissions to transport activity and is expressed as an annual rating. A poor rating can trigger a requirement for a corrective action plan under the applicable framework. The rating is influenced by how a ship is operated as well as by its technical characteristics, so it should not be read as a simple verdict on engine condition alone.

The Ship Energy Efficiency Management Plan, or SEEMP, provides a framework for managing and improving energy efficiency. For relevant ships, fuel consumption data is also collected and reported through the IMO Data Collection System. The value of these processes depends on the quality of the underlying records: bunker quantities, tank measurements, fuel transfers and consumption estimates need a consistent method and a clear audit trail.

Energy-saving modifications require the same discipline as emissions equipment. An engine power limitation, for example, can affect the vessel’s certified operating configuration and its available power. Procedures should explain the permitted settings, monitoring arrangements and response to situations where safety requires different action. Commercial instructions should not encourage crews to bypass documented controls.

EU requirements: reporting, carbon allowances and fuel intensity

EU measures add distinct obligations for ships and voyages within their respective scopes. The EU Monitoring, Reporting and Verification system, known as EU MRV, requires applicable ships to monitor and report emissions and other voyage data under an approved monitoring plan. Reported data is independently verified. Its scope and calculation rules should not be assumed to match the IMO data collection system, even when both draw on the same fuel records.

The EU Emissions Trading System, or EU ETS, places a carbon cost on covered shipping emissions through the surrender of allowances. The responsible shipping company needs reliable verified data and a process for obtaining and surrendering the required allowances. Commercial contracts may allocate costs between parties, but a contractual allocation should not be confused with the party’s regulatory responsibility.

FuelEU Maritime addresses the greenhouse gas intensity of energy used on board by ships within its scope. This is a different question from both sulfur compliance and the purchase of EU ETS allowances. A fuel can meet a sulfur limit while still carrying a significant greenhouse gas impact under the relevant calculation method.

For fuel decisions, documentation matters as much as the fuel’s name. Where a claimed emissions benefit depends on fuel pathway, sustainability characteristics or other qualifying evidence, the necessary documentation must support that claim. The calculation can also account for emissions across the fuel pathway and, where relevant, gases released during use. For LNG-fuelled ships, this makes methane emissions an important consideration alongside carbon dioxide.

FuelEU also includes shore power or equivalent zero-emission technology requirements for certain ships at berth, subject to the regulation’s conditions and exceptions. Readiness is not just a procurement question: the vessel’s connection equipment, port interface, electrical protection, crew procedures and evidence of an applicable exception can all affect compliance.

What a defensible compliance record looks like

A useful review follows one voyage from planning through reporting. The aim is to see whether the documents tell a consistent story, not merely whether every file exists.

1. Define the operating envelope. Identify relevant ports, emissions control areas, discharge restrictions and planned fuels. Record which engines and abatement systems will be used.
2. Check the approved configuration. Match certificates and technical files to the equipment actually installed. Review recent modifications, replacement parts, control settings and open defects.
3. Trace fuel movements. Reconcile bunker delivery records with tank soundings, transfers, consumption records and changeover entries. Investigate material differences using the vessel’s measurement and correction procedures.
4. Test monitoring and response. Review instrument maintenance, calibration, alarms and data gaps. Confirm that crews can follow the contingency procedure for a scrubber, SCR or fuel-system failure.
5. Reconcile reports. Compare the source data used for IMO reporting, EU MRV and fuel-intensity calculations. Differences may be legitimate because the systems have different boundaries, but the reason should be documented.
6. Close the loop. Assign each discrepancy an owner, corrective action and completion record. Feed recurring issues into maintenance, procurement, training or voyage planning.

Document control is especially important after a retrofit. Drawings, operating manuals, approved plans, maintenance schedules and crew instructions should describe the same installation. A certificate for an earlier configuration does not explain an undocumented change to sensors, piping or control logic.

Choosing improvements without creating a new compliance gap

There is no single equipment choice that satisfies every emissions requirement. Low-sulfur fuel can address the sulfur content requirement without a scrubber, but it does not remove NOx certification duties or EU greenhouse gas obligations. A scrubber may offer operating flexibility on a suitable route, but it adds monitoring, maintenance and washwater considerations. SCR addresses NOx, not carbon intensity.

Fuel and propulsion changes deserve a whole-system assessment. A dual-fuel installation may alter fuel handling, ventilation, safety systems, training and emissions reporting. Electrical upgrades or operational efficiency measures may reduce energy demand, but their benefit depends on how the vessel actually operates and how performance is measured. For a cruise ship with substantial hotel loads, auxiliary demand may be central to the assessment; for an LNG carrier, cargo-related systems and fuel handling also shape the operating profile.

Start with the vessel’s expected route and duty cycle, then assess feasible fuels and technologies against their approval, safety, maintenance and documentation needs. The strongest compliance decision is one the ship can sustain in service, with records that show what happened when conditions were less than ideal.