In silico dispersion modelling for the safety assessment of ammonia as marine fuel

Abstract

In this study simulations are carried out to predict ammonia dispersion in the ambient environment and, consequently, assess specific safety aspects with respect to an emergency underwater release of fuel ammonia from a ship with particular emphasis in areas where personnel can be potentially exposed. Specifically, simulations are performed employing a commercial Computational Fluid Dynamics (CFD) numerical code in which both the physics of ammonia dissolution in water in conjunction with subsequent evaporation in ambient air and the vessel-specific details such as geometry and discharge parameters are considered. Subsequently, the computational method is used to simulate selected discharge scenarios and thereby to define zones of increased ammonia concentrations and potentially hazardous areas to personnel based on the Acute Exposure Guideline Levels for Hazardous Substances. These scenarios account for ambient conditions such as wind speed and direction and also the discharge system specifications. Results include the impact of both sea current velocity and direction to the fate of released ammonia and indicate ammonia concentration levels at different areas on board and around the vessel. On the basis of these results, it may be conjectured that if the ship maintains its maneuvering capability in case of an emergency scenario, then it is feasible to maintain a safe side by attempting to position the ship axis parallel to the prevailing seawater current. In addition, the maximum height above the sea surface where relatively low concentration of ammonia is observed for the specific geometry and cases considered, is determined. In view of this, the model can be used as a tool for dispersion analysis to investigate and possibly support the use of ammonia as a marine fuel. This can be of interest to various stakeholders including authorities as well as classification societies that have introduced tentative rules and guidelines in relation to ammonia-fueled ships.