What ship operators really want to know about methane slip: 9 common questions answered in International Shipping News,Shipping: Emission Possible 27/01/2026 Methane slip in marine vessels is the small amount of methane gas that doesn’t burn in the engine and instead ‘slips’ out into the atmosphere. Methane slip is a challenge for vessels that use LNG as fuel, with tightening emissions regulations making it an increasing concern. Wärtsilä is a leading expert in reducing methane slip, with cutting-edge technologies and proven solutions that have helped to reduce methane slip to record-low levels. 1. How can methane slip be reliably measured? According to the official IMO guidelines (MEPC.402(83)), the reference method for measuring methane slip onboard is with a portable or fixed flame ionisation detector (FID) equipped with a non-methane cutter (NMC). This method selectively measures methane by oxidising non-methane hydrocarbons, ensuring accurate methane readings in the complex exhaust environment of a vessel. This setup is suitable for both long-term testing on board vessels and research in laboratories and testbeds. Alternative methods such as Fourier transformation infrared (FTIR) spectroscopy or non-dispersive infrared (NDIR) sensors can also be used, but for reporting purposes these methods should be validated against the FID+NMC method for the specific exhaust composition. Two other measurement methods are primarily used for research and testing: Laser-based sensors – for high-accuracy onboard applications in research or pilot projects. Gas chromatography – for research and verification purposes, rather than continuous onboard monitoring. Measuring methane slip from vessels is challenging, especially during transient operations like bunkering and port manoeuvres. These activities can cause fluctuations in engine load and exhaust composition, increasing the uncertainty of measurement results. According to the IMO guidelines, emissions should always be measured under stable load conditions. Environmental conditions such as temperature, humidity and pressure can also affect exhaust gas composition and measurement instruments. Methane slip measurement is currently a validation activity, not a continuous monitoring requirement. The EU has published guidelines for reporting and verifying actual methane slip tank-to-wake emission factors from marine diesel engines. These guidelines fall under the scope of the FuelEU Maritime regulation. Wärtsilä contributes to the continuous development of standardised measurement protocols and collaborates with research institutions and regulatory bodies to establish reliable and feasible ways to collect and report emissions data. 2. What are the implications of using fossil LNG versus bioLNG on methane slip levels? Both fossil LNG and bioLNG lead to methane slip when used in marine engines, but the total climate impact of these fuels differs significantly. While methane slip levels are often similar for both, bioLNG generally offers a much lower well-to-wake greenhouse gas footprint compared to fossil LNG. This is because bioLNG is renewable in origin and has lower upstream emissions. Wärtsilä’s engine technologies are compatible with both fossil LNG and bioLNG. Because we are continually optimising the combustion process in Wärtsilä engines, methane slip is minimised whether fossil LNG or bioLNG is used. 3. Can methane slip be quantified accurately for emissions reporting? It is possible to measure m