The present paper illustrates the results of simulating a depressurization event for a pipeline containing a near mono component fluid (CO2 with a molar fraction of impurities less than 1%) using MAST [1], a transient one-dimensional multi-phase flow simulator. The depressurization event is described in [4], where the results using the OLGA [3] code are also shown. The 50km long, 24 inches buried pipeline initially contained approximately 9300 tonnes of CO2 at supercritical conditions and was blown down from 81bar and 31°C to atmospheric conditions using two vents, located near the pipeline inlet and outlet, respectively. The scope of the analysis was to validate the MAST software on a nearly single component application (carbon dioxide), comparing the results with the measurements of the afore mentioned event. The simulation results fully agree with the field data about the global depressurization time (nearly 10 hours) of the pipeline; they marginally deviate from the measurements during the two-phase release period while the pressure gradient is well captured during the following phase. The simulation results indicate that, after the first end vent – that close to the pipeline inlet - is opened, multi-phase flow conditions are met when the fluid reaches 65bar; the depressurization subsequently follows the saturation line until, at around 30bar, the path moves into the vapor region; which corresponds to roughly five hours of combined vapor and liquid two-phase flow leaving the pipeline. The results obtained by the MAST code show a good agreement with those of OLGA as reported in [4]; nonetheless, both simulators show some deviations with respect to the acquired field data as far as both the temperature and pressure time traces are concerned.
Onshore pipeline transport of CO2 is becoming more frequent, with strong emphasis on oil enhanced recovery deployment. From an overall perspective, the depressurization of a line containing CO2 presents two relevant issues: the first is related to pipeline integrity and the second to environmental safety. As far as the former risk is concerned, if the depressurization is performed too fast, the CO2 might reach its triple point (5.2 bara, −56.6C), which would lead to the formation of dry ice and subsequent blockage of the stream; from a safety perspective, the CO2 vented to the atmosphere may pose serious threats with respect to it's dispersion (which might potentially lead to asphyxiation) and to the noise levels generated. It is therefore important not only to gather relevant data from CO2 depressurization operations, but it would be highly desirable to rely upon simulation tools in order to simulate the depressurization event as accurately as possible. This is the main motivation behind the present work. In what follows, a brief introduction to the MAST code will be given; then the depressurization even based upon [2] will de described and finally results from the conducted numerical simulation and comparison with the available field data will be shown.
