ClassNK Updates Guidelines for Alternative-Fueled Ships

ClassNK Revises Safety Guidelines for Hydrogen and Methanol/Ethanol Fueled Ships

Classification society ClassNK has published an updated version of its ‘Guidelines for Ships Using Alternative Fuels,’ now at Edition 3.1, with changes focused on hydrogen-fueled and methanol/ethanol-fueled vessels. According to a report by The Maritime Executive, the revision brings ClassNK’s requirements into line with recent IMO decisions while adding technical detail the international rules leave out.

Hydrogen Rules Catch Up With IMO Circular

The biggest change affects Part D of the guidelines, which covers hydrogen as a marine fuel. ClassNK has folded in the IMO’s Interim Guidelines for the Safety of Ships Using Hydrogen as Fuel, known as MSC.1/Circ.1701, which was finalized at the IMO’s CCC 11 sub-committee session in September 2025. ClassNK says it took part in that session as part of the Japanese delegation.

Beyond simply adopting the IMO text, ClassNK has added a hydrogen leak frequency table, built using a Bayesian statistical method similar to the one it previously applied to ammonia leak estimates. This table appears as Appendix 2 to Part D and is intended to support the safety assessments that flag administrations require before approving alternative-fueled ship designs. Rather than leaving designers to develop their own leak-frequency assumptions from scratch, ClassNK is offering a standardized reference point.

New Structural Requirements for Methanol and Ethanol Tanks

Part A of the guidelines, which addresses methanol and ethanol as fuel, gets a more targeted update: new structural strength requirements for fuel tanks carrying these fuels. ClassNK notes that the IMO’s own methanol/ethanol guidelines, issued in 2020 as MSC.1/Circ.1621, don’t specify structural strength criteria for these tanks. ClassNK says it developed the new standard based on its own experience reviewing methanol/ethanol vessel designs, with the goal of making tank design assessments more efficient and consistent.

Why This Matters for Shipyards and Owners

ClassNK frames the update as part of a broader effort to keep its comprehensive alternative-fuels guidelines - which also cover LPG and ammonia - current as newbuilding activity in this space accelerates alongside decarbonization pressure. The society says the aim is to let shipyards, designers, and shipowners run design and safety assessments that match the latest international standards, while using ClassNK’s own leak-frequency data to work through approvals more efficiently.

What It Means in Practice

For owners and technical managers already committed to methanol or considering hydrogen propulsion, this update matters less for its headline announcement and more for what it signals about how quickly the regulatory and class framework for these fuels is still evolving. Guidelines that were only recently issued for ammonia and methanol are already being revised as real-world design experience accumulates, and the same is now happening for hydrogen almost immediately after the IMO’s interim guidance was finalized. That pace has practical implications for surveys and inspections: newbuilding specifications, class notations, and even flag-state approval documents drafted even a year ago may already reference outdated leak-frequency assumptions or missing structural criteria. Owners preparing for condition surveys, pre-purchase inspections, or class renewal on newer alternative-fueled tonnage should confirm which edition of the applicable class guidelines was used at the design stage, and whether any gaps - such as the previously unaddressed methanol tank strength requirements - have since been closed. As alternative-fuel newbuilding orders continue to grow, staying current with these guideline revisions will increasingly be part of routine technical due diligence rather than a specialist concern.

Access to the Guidelines

ClassNK says the guidelines are available to registered users through the “Guidelines” section of its My Page portal on the ClassNK website.

Reviewed by Ibrahim Halil Ceylan, Marine Surveyor at Apeks Marine.