Surface flow and gas dispersion from a subsea release of natural gas

At Statoil's Research Center in Norway. experiments with underwater gas releases have been performed. with both an instantaneously started source and a continuous source. The objectives were to investigate the surface currents near the bubble plume. and the gas dispersion above the surface. The currents can cause stability problems for vessels or offshore platforms. high concentrations of natural gas can result in fires or explosions for releases occurring below or near offshore installations. The experiments were performed in a basin of 7 m depth with volumetric flow rates ranging from 0.083 Nm(3)/s to 0.75 Nm(3)/s. corresponding in scale to that expected from a full pipeline rupture or blowouts from shallow pockets. Measurements of water velocities and void fractions were performed in the vertical bubble plume and in the horizontal surface flow. The gas concentration above the water surface was monitored by means of a novel measuring technique: for these experiments. a mixture of helium and air was used to obtain the same density as natural gas. The velocity profile in the surface flow was found to be different from that observed by others; the profile depth was found to be decreasing with increasing distance from the center of the bubble plume. whereas previous work indicate an increase of profile depth due to entrainment. The horizontal transport of pas (bubbles) was found to be greater than expected; the gas was dispersed into the atmosphere from an area corresponding to about six times the area of the central boil region. in the present experiments the release rates wore significantly higher than in previous investigations. and we observed a fountain of significant height above the reference surface. The return flow from this fountain penetrated deep into the water. Its interaction with the horizontal flow from the bubble plume. resulted in a significant transport of water and bubbles deep in the horizontal layer. On comparing different release rates. the gas concentration profiles above the surface appeared to be of similar shape, but the rise velocity of the gas increased with increasing release rate. Some progress has also been made in correlating the gas movements above the surface to the parameters of the bubble plume.