This paper presents the development and application of a failure consequence mathematical model for predicting the incident heat flux and explosion over-pressure following the accidental rupture of high pressure ethylene transportation pipelines. The transient discharge rate and the fluid phase at the pipe breach are determined based on the numerical solution of the conservation equations using the Method of Characteristics. The flow model accounts for the important processes taking place during the depressurization process; these include real fluid behaviour, fluid/wall heat transfer and frictional effects. To model the immediate ignition of the escaping high pressure ethylene released, the transient outflow model serving as the source term is linked to the widely established Chamberlain semi-empirical jet fire model to predict the resulting jet flame characteristics including its dimensions and incident heat flux as function of time and distance from the breach location. To deal with a delayed ignition, the source term flow model is linked to the TNO Multi-Energy Vapour Cloud Explosion model to predict the resulting explosion over-pressure and hence the subsequent harm to people and surrounding structures. Simulation results using the model are presented and discussed for the full rupture of a typical 20 km long, 250 mm i.d steel pipeline transporting ethylene at 50 bar and 5 oC.