Several sour gas leakage accidents have occurred in the offshore platform during the past decades, such as the Kab 121 platform in 2007, which caused serious consequences mainly resulting from the lethal toxicity of hydrogen sulfide (H2S). Under the threat of H2S, it is a challenge to exploit resource in the sour gas filed. Especially during the drilling operation, an abrupt blowout or kick could bring huge amount of H2S, envelop the platform and disperse in the cabins. The present paper is aimed at introducing our analysis of H2S dispersion both in the outer deck and inner mud treatment cabin so as to fully assess the potential poisoning during well blowout. The method we chosen was computational fluid dynamics according to the spatial environment characteristics of the offshore platform. First, we drew a comparison between accident consequences deriving from the wellhead configurations of an opened bell nipple and a sealed rotary blowout preventer (BOP) in the outer deck under various wind directions and speeds. The instantaneous concentrations and hazard zone distributions show that the second configuration is much better from the view of accident control. And the accident severity is much lower when the wind blows from the larboard, not from the prow for both configurations. As a result, the potential hazard zone would not envelop the entire platform with suitable platform position and arrangements of the mud return ditch, accommodation, helicopter deck et al. Then, the simulations of H2S dispersion in the mud treatment cabin were conducted in case of the closed outlet doors, opened outlet doors and sealed cabin with air ventilator working. An immediately dangerous to life level may come up in a short break with the door closed. In such a dangerous situation, H2S can only be made to disperse to other areas through the opened door or effectively ventilated away by means of a ventilator. It is a good practice to isolate the cabins with the potential H2S leakage and install ventilator. And a simple model was proposed to calculate the working time for the ventilator.

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