ClassNK has released an updated edition of its 'Guidelines for Ships Using Alternative Fuels' (Edition 3.1), incorporating the latest IMO interim guidelines for hydrogen-fuelled ships and introducing new structural strength requirements for methanol and ethanol fuel tanks.

The Tokyo-based classification society announced the revision on 18 August, stating that the updated guidelines are intended to enable shipyards, designers and shipowners to carry out design and safety assessments in line with current international standards.

The revision makes changes to two key sections. In Part D, covering hydrogen-fuelled ships, ClassNK has incorporated the IMO’s 'Interim Guidelines for the Safety of Ships Using Hydrogen as Fuel' (MSC.1/Circ.1701), which were finalised at the 11th session of the IMO Sub-Committee on Carriage of Cargoes and Containers (CCC 11) in September 2025. ClassNK notes that the updated Part D also introduces a hydrogen leak frequency table, derived using a Bayesian statistical method, that can be used for the safety assessments required under the IMO guidelines. The Bayesian approach had previously been applied by ClassNK to estimate ammonia leak frequencies.

In Part A, covering methanol- and ethanol-fuelled ships, ClassNK says it has established structural strength requirements for methanol and ethanol fuel tanks. These requirements were not previously addressed in the relevant IMO guidelines (MSC.1/Circ.1621), which were issued in 2020. According to ClassNK, the new requirements have been developed based on its experience in design reviews of such vessels and are intended to enable more efficient implementation of tank design assessments.

ClassNK noted that it participated as a member of the Japanese delegation to CCC 11 and contributed to discussions that led to the finalisation of the IMO’s hydrogen fuel guidelines now reflected in Part D.

The guidelines also cover LPG and ammonia as alternative fuels, though the current revision is focused on hydrogen and methanol/ethanol provisions.

According to ClassNK, the revisions are intended to allow the development of alternative-fuelled ships to proceed “in a more rational” manner, by providing design and assessment guidance that aligns with the latest international standards and incorporates the classification society’s own leak frequency estimates.

The guidelines are available to registered ClassNK users via the 'My Page' section of its website.