Multiphase flow modelling for hazard assessment of dense phase CO₂ pipelines containing impurities

This blog was provided by Solomon Brown, University College London, who was reporting on the completion of a call 1 project he was heavily involved in. 

This project funded by the UKCCSRC seeks to develop an accurate, rigorous model for the transient flow following the failure of a pressurised CO₂ transportation pipeline. The primary use of such a model is to provide a ‘source term’ for the consequences of such a failure, i.e. as feed for dispersion calculations. Two particular features of the decompression of dense phase CO₂ are the flashing of the fluid and the very low temperatures (-60 to -70 oC) reached by the emerging gaseous phase. This means that the model should also be useful in the other areas of the design and/or maintenance of CO₂ pipelines, for example in the design of blowdown strategies where during depressurisation the temperature of the pipe wall must remain high enough to avoid embrittlement of the pipeline steel.

One of the main challenges is the lack of reliable experimental data from large-scale pipeline release tests, as small- scale experiments introduce uncertainties due to scale-up. Such large-scale tests are expensive to perform and require significant investment. Luckily as part of our previous project CO₂PipeHaz (www.co2pipehaz.eu) and our ongoing project CO₂Quest (www.co2quest.eu) we have access to such a test facility that has been designed and built by one of our partners, Dalian University of Technology (DUT). DUT are performing detailed experiments to provide us with the necessary data for validation of our model.

One of the interesting observations from the CO₂PipeHaz experiments concerns the role of heat exchange between the pipe wall and escaping fluid. The heat transfer was found to be particularly important, especially in those cases where the CO₂ vapour-liquid flow stratifies during the latter stages of decompression, leading to remarkably different rates of heat transfer in the individual phases. This phenomenon is very interesting to us, as it has not been accounted for in models that are currently available and may have implications for pipeline integrity.

As part of the project I was lucky enough to present some of the comparisons of the model predictions with measurements at the Heat Transfer 2014 conference in A Coruna. The conference focussed mainly on the study of heat transfer in energy systems such as geothermal heat pumps, so the area of CCS was a little novel. Nevertheless the overlap between our modelling and that used in other areas provoked some interesting discussions with other attendees which I hope will lead to fruitful collaborations. A Coruna itself is a small town on the north-eastern tip of Spain with a 2000 year old lighthouse; we were blessed with some beautiful weather (for the majority of the time anyway) making the beach outside practically irresistible.

Over the course of this project I have been lucky enough to work with a number of research groups from across Europe, including Greece and France, and have enjoyed the work that we have been able to do together. Fortunately the work done on this project is being developed and has evolved into a new UKCCSRC project, where we will hopefully be able to develop new collaborations.