Experience shows that, despite the best efforts of the pipeline industry worldwide, pipelines do fail and release their contents to the atmosphere. In the case of below-ground pipelines transmitting natural gas, there is a chance that the release will be ignited, posing a significant hazard to any people in the vicinity. Mindful of this hazard, an international group of gas companies have collaborated over a period of many years on research projects aimed at developing an understanding of how these releases may arise (failure causes), how often they might occur (failure frequency), what type of releases might be produced (failure modes) and what type of behaviour might be produced for each of these modes of release (consequence analysis). This paper has been prepared to describe the mathematical models that have been developed on behalf of this group to assess the initial transient period following the rupture of a buried natural gas transmission pipeline assuming the release ignites immediately. It gives details of the equations used by the different models and it refers to some of the experimental data that has been used in the development of the models. A comparison of the model with the experimental data is provided. This demonstrates that the early stages could have a significant impact when evaluating the harm that could be caused. This provides a justification for developing the models rather than using a simpler alternative that does not take the initial highly transient period into account. (C) 2015 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.