A comprehensive multiphase CO2 release model for Carbon Sequestration QRA purposes. Modeling and conditions for simplifying assumptions and solid CO2 occurrence.

In the framework of emerging Global Warming strategies, CO2 sequestration pipelines require attentive hazard studies to ensure a safe operability, but current risk assessment procedures applied to CO2 pipeline failures lack reliable and comprehensive source models. This work suggests a resolutive and robust multiphase discharge model suited for matching all expected CO2 discharge mechanisms. The application to real scale CO2 pipelines shows an essential incidence of the pipeline geometry (length and internal diameter) as well as of the orifice size on the release features. Wider thermal dynamics and enlarged solid contents are expected in short pipelines subjected to large ruptures. The resulting expansion transformations are characterized by increasing degrees of reversibility. Results show that expected solid content may amount up to 45 % on a mass basis under usual carbon sequestration operative conditions. The latter, being linked to the initial CO2 aggregation state, play a key role in determining the whole discharge dynamics especially because of the effects of phase change mechanisms. A peculiar mass flow rate discharge profile is observed depending on the occurrence of liquid-vapor and solid-vapor mixtures. Specific set of geometric and operative conditions allow for the applicability of the isothermal bulk hypothesis and negligible wall effect in heat transfer mechanisms. Main governing parameters are the ratio between the pipeline length and internal diameter L/D and that between the orifice and the internal pipeline diameter d/D. Essential in driving the QRA procedure is the occurrence of the solid phase in rapid depressurizations that is expected only for pipeline shorter than 1500 m subjected to d/D > 0.30. Independently on the operative temperature, only pipelines carried at pressures above 55 barg lead to CO2 solid-vapor mixtures. Under these conditions, the solid CO2 can not be neglected thus requiring a QRA modeling procedure considering additional scenarios involving sublimative dynamics of a dry ice bank.