Risk assessment of buried natural gas pipelines. Critical aspects of event tree analysis

The safety aspects of pipelines conveying hazardous materials are not included neither under the umbrella of Seveso Directives aiming at preventing major accidents at industrial facilities, nor in other EU legislation such as the Pressure Equipment Directive (PED). A review of relevant past accidents can provide statistical evidence on the extent to which pipelines present a risk potentially comparable to that of Seveso installations and on the degree to which the pipeline hazards are adequately controlled. Starting from evidence that in the last decades, the international natural gas market has been growing at a very high rate and continues to exhibit an increasing trend, in this paper we focus on consequences deriving from accidents on high pressure buried Natural Gas Pipelines (NGP) and related probabilities of the various outcomes. The paper focuses on a novel Event Tree framework, to overcome the limitations of the amply applied over-conservative IP UKOOA approach. In order to evidence the capability of the approach, the use of refined PET is exemplified by means of a real case-study of a high pressure buried NG pipeline, contrasting the actual results with those obtained by conventional methods, in terms of evolving scenario probability and damage. Conclusions are drawn about the effective application of the framework within risk assessment and related uncertainties in pipeline accident modelling.