Abstract: To quantitatively assess risk factors for urban gas pipelines, layers of protection analysis (LOPA) is made on some typical leakage, fire and explosion accidents of urban gas pipelines in the United States. Through clarifying accident scenarios, and identifying initial events, intermediate events and resulting events of accident scenario, independent layers of protection (LOP) model is established to highlight root causes for such accidents. On this basis, possibility of consequential events is determined with the LOP model, and then a complete semi-quantitative analysis technique is proposed for failure and risk assessments of urban gas pipelines. The results show that LOPA can be used to effectively analyze relevant accidents and thereby provide necessary guidance for prevention and control of such accidents. Moreover, the semiquantitative risk assessment technique based on failure possibility of layer of protection can provide scientific supports and guidance for risk assessment, risk warning, decision making and preparation of emergency plans for relevant pipelines. The technique is worthy of engineering application.