IACS issues gas dispersion guidance for ammonia- and hydrogen-fuelled ships

Summary

The International Association of Classification Societies (IACS) has published Recommendation 202 (Rec. 202), establishing a common approach to assessing accidental gas releases on ammonia- and hydrogen-fuelled ships. Issued on 12 November 2026, the guidance outlines methods for modelling gas dispersion and using the findings to inform ventilation systems, gas detection, hazardous area classification and emergency planning. It aims to improve consistency in the safety assessments submitted to classification societies and flag administrations.

Article

The International Association of Classification Societies (IACS) has issued new technical guidance to support the safety assessment of ships powered by ammonia and hydrogen, addressing the potential consequences of accidental fuel gas releases.

Published on 12 November 2026, Recommendation 202 (Rec. 202) establishes an approach for conducting gas dispersion analysis to assess how released gas could spread within and around a vessel. The findings are intended to support decisions on ship design, ventilation arrangements, gas detection systems and emergency response planning.

Gas dispersion analysis examines the movement of released gas and identifies areas where concentrations could reach hazardous levels. Under Rec. 202, assessments cover both toxicity and flammability, depending on the fuel involved and the release scenario being evaluated.

The recommendation outlines expectations for the technical competence of specialists conducting these studies, validation of modelling methods, and the selection of release scenarios and environmental conditions. These requirements are intended to establish a consistent technical basis for evaluating gas dispersion studies submitted alongside ship designs.

By standardising the approach, IACS aims to help classification societies and flag administrations assess the assumptions, modelling methods and findings used to demonstrate the safety of alternative-fuel installations.

The results of gas dispersion studies can influence several aspects of vessel design. These include vent mast positioning, hazardous area classification, gas detector locations and ventilation arrangements. The analysis can also help determine whether gas released from a vent could reach air intakes or spaces occupied by crew members.

In addition, the findings can support emergency planning by identifying areas that could be exposed to hazardous gas concentrations under specific release and environmental conditions.

Rec. 202 was developed by the IACS Safe Decarbonisation Panel, established in 2022. The panel’s work has involved engagement with shipowners, shipbuilders, fuel manufacturers, equipment suppliers, marine insurers and flag states.

IACS Secretary General Robert Ashdown described the recommendation as the first output from the panel’s framework for collaborative work with flag states and industry organisations.

The guidance also complements the International Maritime Organization’s (IMO) ongoing work on safety requirements for alternative marine fuels. The IMO has issued interim safety guidelines addressing ships using ammonia and hydrogen as fuel. Its Maritime Safety Committee approved the hydrogen-related guidelines in May 2026, alongside separate interim guidance concerning ships using ammonia cargo as fuel.

The International Code of Safety for Ships Using Gases or Other Low-flashpoint Fuels (IGF Code) provides the broader international safety framework for ships using alternative fuels. The IMO’s interim guidelines address fuel-specific risks in areas where detailed prescriptive requirements are still being developed.

Within this framework, IACS Recommendation 202 provides a technical basis for preparing and reviewing gas dispersion studies used in risk assessments. Its application is expected to support more consistent evaluation of the safety implications associated with ammonia and hydrogen fuel systems across different vessel designs.

FAQs

What is IACS Recommendation 202?

IACS Recommendation 202 is technical guidance outlining a common methodology for conducting gas dispersion analysis on ships using ammonia or hydrogen as fuel. It addresses modelling accidental gas releases and assessing associated toxicity and flammability risks.

Who will use IACS Recommendation 202?

Ship designers, fuel system developers and gas dispersion modelling specialists can use the recommendation when preparing safety studies. Classification societies and flag administrations can apply it when reviewing these assessments.

What aspects of ship design can gas dispersion analysis influence?

Gas dispersion studies can inform vent mast positioning, hazardous area classification, gas detector placement, ventilation arrangements and emergency response planning.

Does IACS Recommendation 202 amend the IMO IGF Code?

No. Rec. 202 is technical guidance issued by IACS. It complements the IMO’s existing safety framework and supports the preparation and review of gas dispersion studies for alternative marine fuels.

Why is gas dispersion analysis important for ammonia- and hydrogen-fuelled ships?

Ammonia presents toxicity risks, while hydrogen presents significant flammability risks. Gas dispersion analysis helps identify where accidental releases could create hazardous conditions and supports the development of appropriate safety measures.