The International Association of Classification Societies (IACS) has announced the withdrawal of the Unified Requirement UR H1 as of January 2025, to align with IMO’s ammonia guidelines.
According to IACS, the decision to withdraw UR H1 stems from the differences between its safety parameters and those outlined in the IMO Interim Guidelines. The IMO Sub-Committee on Carriage of Cargoes and Containers, at its 10th session, finalised the draft interim guidelines for the safety of ships using ammonia as fuel, with a view to approval by MSC 109.
These guidelines include several differences from IACS’s original UR H1 requirements. The IMO Interim Guidelines establish a 220 ppm threshold for acute exposure, without defining a hazardous concentration, and require preventing direct ammonia release during normal and controllable abnormal scenarios, which may exclude releases from leakages. Toxic areas have been defined, requiring gas dispersion analysis to demonstrate concentrations do not exceed 220 ppm in key locations.
An ammonia release mitigation system is required to maintain outlet concentrations below 110 ppm, with alarms for exceedances. Additionally, alarms must activate at 110 ppm with system shutdown at 220 ppm, while a visual indication is required at 25 ppm near entrances to affected enclosed spaces. These differences could potentially lead to confusion within the maritime industry.
To ensure consistency and reduce the potential for conflicting interpretations, IACS has decided to withdraw UR H1 with a view to publishing a revised version that aligns with the IMO guidelines. The revised UR, to be published in 2025, will provide a consistent regulatory framework for the safe adoption of ammonia and will provide the necessary safety framework for ammonia-fuelled vessels while aligning with best practices and international guidelines.
