As an alternative to qualitative analysis, semi-qualitative analysis has received increasing attention . Semi-quantitative analysis falls in between quantitative and qualitative analysis, which can obtain approximate results rather than exact/absolute results . Many indicator-based methodologies have been considered as semi-quantitative methodologies in most published literature. However, if only qualitative terms are adopted to represent the involved safety factors, the indicator-based methodology is considered to be qualitative . Semi-quantitative methodologies are useful when the reliable measurement of process safety and risk is unavailable whereas inference is acceptable . However, semi-quantitative methodologies hardly provide a uniform criterion for analysis results.