Objectives/Scope: The primary objective of the present study is to characterize the dynamics of subsea starting plumes and compare experimental observations with multiphase computational fluid dynamics (CFD) simulations.
Methods and Procedures: Meso scale experiments were performed under controlled conditions in the Trondheims Fjord during the spring of 2014. The release depth was 30 m and the gas (ambient air) rates were varied between 18-72 Nm3/min through a 2" nozzle. Instrumentation was mounted on a frame that allowed measurements to be taken at virtually any position inside and outside the plume. The frame was deployed from a barge (65 x 17.5 m) using a heavy lifting crane. Two air compressors supplied the gas at 10 bar. The instrumentation included flow vanes, video cameras and a high speed camera system enabling estimation of void, bubble size and bubble tracking. In addition surface dynamics were probed using wave guides and cameras, enabling monitoring of surface elevation and surface flow dynamics. A novel sonar imaging system was employed to image the evolution of the bubble plume throughout the water column. A transient 3D CFD model consisting of an Eulerian VOF model for tracking the large scale interface between the ocean and atmosphere coupled to a Lagrangian description of the dispersed bubble phase was used to model the rising bubble plume. The Lagrangian model used to track the dispersed bubble phase does not model individual bubbles but rather statistical parcels each containing a large number of bubbles. The model accounts for gas compressibility, mass transfer, employs a dynamic bubble size model accounting for breakup and coalescence, damping of turbulence close to the ocean surface and ocean stratification.
Results, Observations and Conclusions: We investigate the effects of release rate on overall plume dynamics such as rise time, surface spreading, fountain height and plume angle and compare CFD simulations with experimental observations done during the 2014 field trail on subsea gas blowout and literature data. We find that progress in sonar imaging now allows subsea gas plumes to be visualized in detail and may prove a very useful tool in field situations. The CFD model captures all the main features observed in the experiments. However, we note that the CFD model is sensitive to the initial ambient ocean conditions.
Novel/Additive Information: To our knowledge no studies have presented an equally detailed experimental characterization of the evolution of starting plumes using "3D sonar" imaging, nor have they been able to compare experiments with equally detailed three dimensional transient multiphase CFD simulations. The near shore meso scale experiment described in this study was conducted as a qualification of instrumentation and initial validation of a CFD model before progressing to full scale offshore experiments.
Figure 1: Simultaneous tracking of bubble plume using sonar imaging looking at subsea plume from the side (left) and visual imaging looking vertically down onto the ocean surface (right).