Consequences Analysis of CH4/H2S Release in Offshore Platform

The exploitation and development of offshore oil & gas fields continue to be one of the main source to supply energy worldwide. However, existing and new projects may involve specific geographic areas with safety challenges due to the presence of natural gas, mainly Methane (CH4), containing a certain amount of Hydrogen Sulphide (H2S) in the extracted hydrocarbons (sour gas). Indeed, the combined effect of flammability and toxicity of such gases has the critical potential to increase the hazard level in the offshore installation aggravating the consequences for human and assets. The present work aims at the consequences analysis related to the release of sour gas (CH4/H2S) on an offshore production facility, by considering different H2S fractions in the gas composition. The analysis was based on the estimation of safety distances for the physical effects of accident scenarios with respect to personnel, equipment and facility structures. Two different approaches implemented into consequences modelling software, the Process Hazard Analysis Software Tool (PHAST), which is based on integral and lumped parameter model, and the Fire Dynamic Simulator (FDS), which is ascribed to the Computational Fluid Dynamic model, hence based on distributed parameter model. A case study concerning an actual offshore, multi-level platform for gas production was considered. Large releases from two different process equipment were simulated for three mixtures with different amount of H2S, starting from specific release points. The results showed that the H2S content is not an effective factor for the safety distance in the case of ignition of the gas cloud, hence in the case of fires. Also, the presence of obstructions slightly affects the damage distances for fires. On the contrary, the safety distance from the release point can be strongly affected by the presence of H2S concentration higher than 1%v.