Theoretical and experimental characterization of underwater CO2 releases

The assessment of accidental underwater CO₂ releases is relevant to the safety and environmental evaluation of offshore CO₂ transport and storage systems. Once released into the water, CO₂ may rise as a population of bubbles while dissolving into the surrounding liquid. Bubble dimensions influences gas–liquid interfacial area, rise behaviour and the time available for mass transfer. This thesis investigates how operating conditions affect CO₂ bubble size and whether existing models reproduce experimental measurements. Experiments were conducted using the Simulated Subsea CO₂ Release Equipment (S²CO₂RE), a cylindrical vessel designed for controlled CO₂ releases through submerged nozzles of 1 and 5 mm diameter at two vertical positions. Gas flowrates ranged from 5 to 240 Nl/min, and vessel pressure varied from 0 to 9 barg. High-speed video recordings were processed to identify bubbles, determine equivalent diameters and construct bubble size distributions (BSDs). Characteristic diameters were extracted, using the median diameter (d₅₀) for model comparisons. Gas properties and nozzle-exit conditions were used to calculate Reynolds, Weber, and Froude numbers, characterising inertial, viscous, capillary and gravitational effects. Bubble sizes were then analysed in relation to operating conditions and dimensionless groups. The results show that bubble size cannot be described by a single monotonic trend over the entire experimental range. Instead, the resulting BSDs depend on the combined influence of the operating conditions and on the observation position. Predictions from the Gaddis formulations, Jamialahmadi et al. and Li et al. overpredicted the experimental d₅₀. The results provide an experimental basis for characterising initial CO₂ bubble populations and for further investigation of the relationship between bubble size and CO₂ dissolution.