Gas transmission pipeline design standards in Australia require provision for isolatable sections to be safely depressurised to atmospheric pressure and later repressurised to resume operation. This allows for pipeline maintenance or emergency repair work.
This investigation uses transient flow modelling and some stress and fracture analysis, to determine whether procedures currently used for this purpose could possibly cause brittle fracture in the pipeline. Its focus is a comparison between rapidly opening the controlling pipeline valve and an alternative gradual procedure. The parameters used in simulations make the results and recommendations of the study only applicable to pipelines with an Australian PN150 rating.
The pressure profiles predicted for depressurising and repressurising processes were checked against empirical data and demonstrated impressive accuracy. This gave confidence in the corresponding temperature simulations, along with research that justified thermodynamic assumptions made by the software.
Analysis of process simulations identified the depressurising event, as the least likely to cause an accident or interruption to gas supply. Further investigation concentrated on repressurising procedures. A summary of depressurising recommendations has however been included. These suggestions were generated from unrelated study, conducted by the author with work colleagues.
This study introduces an additional stress component during repressurisation, to those presently acknowledged by operations engineers. Assumptions are also made on the quality of pipeline welds meeting Australian Standards. This allows the final conclusion to be drawn. Specifically, pipelines inside limitations defined by the study will not reach brittle fracture conditions if repressurised by a steady valve opening spanning longer than five minutes.
It is unfortunate that restrictions were placed on further findings of the study. The accessible flow modelling software was unable to predict temperature trends during more rapid repressurisation. Anticipated results of such simulations are commented on herein. These lead to suggestions for constructive continuation of this study.