This study established a catastrophic scenario and a likely scenario by qualitative and quantitative risk assessments to consider climate condition with season, and assessed efficiency of safety management cost with scenarios by cost-benefit analysis. As results, the catastrophic scenario was the maintenance error for unsteady state operation, and the likely scenario was the gas release accident at node $sharp$4 of HAZOP Also, benefit/cost ratios for total safety management cost and effective items of safety management could be assessed at each scenario.