Hazardous gases, such as H2S and CO2, present high risks for health, safety, and environment on and near offshore facilities. In recent years, it has become more important to evaluate the pollution and process safety for dispersion of the leaked and relieved hazardous gases during an offshore operation. The primary focus of this work is to estimate the dispersed gas concentration levels around the potential release sources on offshore facilities. Computational fluid-dynamics (CFD) simulations were performed to study H2S/CO2 dispersion behavior. In the simulations, the Realizable k-є model for turbulence was used for the multicomponent gas flow, and the species-diffusion equation was solved for the gas-dispersion calculation. Parallel simulations were used to accelerate the computing time because system meshing generally has 1 to 2 million nodes. A series of scenarios was designed for the potential leak and relief locations, such as surface well testing (SWT) areas, relief lines (overboard), and flare boom, and with different wind directions, speeds, and gas-leak rates. The worst case scenarios were further studied. The results yielded 3D gas concentration distributions, including critical safety-concentration level profiles, pollution coverage area, and height.