Behavior of Gas Bubble Plumes during an Offshore Blowout and its Impact on Environment and Sailing Conditions
Authors: de Andrade P.H. , Nakagawa E.Y. , Lage A.C.V.M. 
Published: December 2007

A joint project involving PETROBRAS and Federal University of Rio de Janeiro, investigated the effects of an uncontrolled underwater blowout using two distinct approaches:theoretical modeling, andexperimental study.

A water tank was built and accurate instruments were installed to read relevant parameters. At last after an extensive experimental and development effort, a Windows TM based software was released, including the most significant results.

Based on the results of the research project, this paper presents and analyzes some case studies. The points of interest are on the effect of bubble plumes on buoyancy, the dynamic interaction between the produced gas and sea water. the formation of gas hydrates, and the effects on the oceanic environment.

Introduction

Bubble plumes occur when a gas blowout is discharged below the sea surface. The produced gas rises and flows to the surface, forming an up side down cone from bottom to surface. In addition, the upward gas movement induces seawater motion, producing streams.

Better understanding and evaluation of the effects of bubble plumes phenomenon require an detailed study of the gaswater mixture flow and the definition of the main physical parameters affecting its behavior. Thus, experimental tests and mathematical modeling were performed aiming at the development of a computer program to serve as an engineering tool for bubble plume analysis.

The developed software was used to analyze a variety of operational scenarios, defined by the combination of input parameters, such as: water depth, blowout gas mass rate, well head diameter and sea surface temperature. Six cases were chosen among all the analyzed scenarios to be presented in this paper.

The most relevant results obtained from this analysis include:the well head diameter and sea surface temperature do not cause significant influence on the mechanics of bubble plume;formation of gas hydrates in deep water scenarios was identified as a factor that significantly influences the dynamic of bubble plumes;as presented before by Milgram, over not very shallow waters the buoyancy reduction are not enough to cause problems for floating vessels; anddue to the induced streams, bubble plumes can be very harmful to oceanic environment. specially in deep waters.

Program Output

The program output encompasses the following variables:bubble plume radius along water depth, from sea bottom to surface;bubble rising velocity along water depth;void fraction along bubble plume vertical center-line;gas volumetric flow rate along bubble plume;divergent wave velocity on sea surface as a function of radial distance from surface bubble center;reach of descending streams (flowing from sea surface downward) induced by bubble rising. as a function of radial distance from surface bubble center;mass flow of gas from sea floor to surface, affected by hydrates' formation on deep water;buoyancy of bubbles along water depth;momentum of bubble plume andsea water density reduction along bubble plume center line from sea bottom to surface.

The seven first functions were chosen to be presented. Variable (10) is presented together with (3), near sea surface, since they are closely related.

Results

The scenarios were defined with the combination of the following variables:well head diameter (203 and 762 mm);sea surface temperature (10 and 30 C);water depths (50, 400 and 1200 m); andblowout gas mass flow rate (10 and 70 Kg/s).

With the purpose of studying hydrates formation in deep water, water depths of 1000, 1500 and 2000 m were also tested for both blowout mass flow rates.