What MEPC.399(83) is
MEPC.399(83) are the 2025 Guidelines on selective catalytic reduction (SCR) systems. They add to the NOx Technical Code 2008 for the design, testing, survey and certification of engines fitted with SCR, so that the engine complies with the Tier III limit in regulation 13 of MARPOL Annex VI.
They supersede the 2017 SCR Guidelines. Administrations are invited to apply them to SCR systems on ships with keels laid on or after 1 November 2025, and to older ships where the SCR is delivered on or after 1 May 2026.
Scheme A: one system, tested together
The engine and the SCR are tested together as one combined system on the test bed, following chapter 5 of the NOx Technical Code 2008. The measured NOx emission is the result for the system.
Scheme B: tested in two steps, confirmed onboard
The engine is tested, and the NOx reduction of the SCR is established separately, with a calculation model based on tests of the SCR chamber. The resulting NOx emission goes into the Technical File and the EIAPP supplement. The result is then confirmed on the installed system.
The onboard confirmation test in Scheme B
After installation and before the ship enters service, a confirmation test is carried out onboard. The system is checked against the Technical File, and the test is done as close as possible to 25, 50 and 75 % of rated power.
NOx is measured before and after the SCR chamber, and the reduction rate is calculated. At no test point may it fall more than 5 % below the value in the Technical File. The initial survey is not completed until the confirmation test gives compliant results.
After approval: the spot check
Where the SCR is monitored without a NOx measurement device, periodical spot checks assess the catalyst condition. The first is after installation, and then at least every 12 months, at no less than 50 % of rated power. The checks do not need to be witnessed by the Administration, and the records are kept for the surveys.
The route in short
The route depends on the scheme. The steps below show it.
Where we come in
We plan and carry out the measurements, including the onboard confirmation test, and prepare the measurement data and documentation, in close collaboration with the engine designer, the owner and class. We have verified SCR systems to Scheme B on real projects, and we carry out the annual SCR spot check.
What we recommend
Agree the scheme and the test plan with the Administration before the first test. Plan the onboard confirmation test and the first spot check at the same time as the installation, so the ship is not held up.
IMO resolution MEPC.399(83), 2025 Guidelines on selective catalytic reduction (SCR) systems; NOx Technical Code 2008.
