Methane slip measurements to reduce reported GHG emissions

Methane slip, as based on standard factors, can account for up to 24% of GHG emissions from an LNG-fuelled vessel. As an alternative to using standard factors for the reporting of CO2 equivalents, direct methane emissions monitoring has the potential to positively impact a ship’s reported carbon footprint. Discover more in this news about how to improve your reported GHG emissions.

Relevant for ship owners, managers, engine manufacturers (LNG tanker, ships with LNG as fuel).

IMO Sub-committee on human element, training and watchkeeping (HTW11)

Background

LNG has become an increasingly vital fuel in the global maritime fuel mix. However, achieving complete combustion of LNG in current engines remains challenging. This incomplete combustion leads to the release of unburnt fuel, primarily methane, into the atmosphere – a phenomenon known as methane slip. Given that methane has over 25 times the global warming potential (GWP) of CO2, methane slip emissions significantly impact a vessel’s greenhouse gas (GHG) footprint.

Regulatory framework

EU regulations and IMO guidelines account for methane slip from LNG-fuelled engines by using standard factors. Table 1 shows the LNG standard correction factors as % of the mass of the fuel for three different engine types:

LNG methane slip correction

Table 1: LNG methane slip correction factors as % of the mass of the
fuel used by the engine and the resulting contribution of methane slip
to the engine’s GHG emissions

Considering the GWP of methane is 28 times that of CO2, these standard factors can result in methane slip contributing to up to 24% of the reported GHG emissions, which is accounted for in terms of CO2 equivalent.

Regulations allow for deviations from predefined standard factors if real figures can be certified by a direct emissions measurement.

In January 2025, a dedicated IMO Correspondence Group took a significant step towards standardizing the quantification of methane slip through direct measurement. They submitted their report for MEPC 83, which includes proposed 
draft guidelines for test bed and onboard measurements of methane emissions from marine diesel engines. These guidelines represent the first essential tool for reliably verifying methane slip measurements.

DNV expects that the draft guidelines will be adopted by the IMO at MEPC 83 ending on 11 April 2025.

Methane slip measurement support from DNV

DNV’s accredited emissions laboratory, Envilab, is staffed with highly qualified professionals offering to accurately identify your methane slip values. Our methane measurement service complies with the new IMO draft guidelines and can be performed on test beds or on-board ships. Results are documented in an official emissions report, certifying reduced methane slip factors for verification.

Benefits of methane slip measurements

Direct measurements of methane emissions offer significant advantages over standardized factors, especially when the standardized methane slip factor seems excessively high for a specific engine type or when methane abatement technologies are used to further treat exhaust emissions.

Although the total amount of methane slip might initially seem minor, it significantly impacts reported GHG emissions. Methane’s GWP over a 100-year timescale is 28 times higher than CO2, meaning even minor adjustments in methane emissions can substantially affect the reported CO2 equivalent. Figure 1 illustrates the potential impact of methane slip reduction for a propulsion engine, verified through direct measurements. The resulting benefits for carbon balance are comparable to other energy-saving measures.

Furthermore, the reported GHG equivalent emissions caused by methane can be almost entirely mitigated by applying methane catalysts. The reduction potential of methane catalysts can only be accurately verified through measurements of methane slip.

CO2 savings potential

Figure 1: CO2 savings potential (equivalent) for an LNG-powered vessel (LNG Otto, dual fuel, slow speed)

 

Recommendations

Consider methane slip measurements as part of your strategy:

It is recommended to evaluate whether the standard methane slip factors significantly differ from those of your installed engine. Collaborate with the engine designer to assess potential improvements for reducing methane slip.

Any deviations from the standard factors can be verified by measuring methane slip on board your ship. DNV provides support for these measurements.

References

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