ABSTRACT

A number of floating drilling vessels have sunk in gas-aerated seas. The cause of the rig sinkings has previously been explained by a loss of buoyancy attributed to a density reduction of the surrounding sea by the gas phase. This paper is a study of floating-vessel response and buoyant force within a gas-blowout plume. Analytical evidence, experimental modeling and a summary of actual occurrences are the supportive elements of the study. The primary conclusion being: in both a quasi-static sense and in actuality, buoyant force within a bubble plume increases. A floating drilling vessel should not sink due to the buoyancy effects of gas-aerated seas associated with bubble plumes.

INTRODUCTION

When a floating drilling vessel, such as a semisubmersible, drillship or barge, loses control of the well it is drilling and a blowout ensues, formation gas can be blown from the wellhead or seabed. The gas rises through the water forming a bubble plume above the source that is a turbulent multiphase mixture of gas, hydrocarbons and water. The vessel therefore rests upon a column of multiphase mixture whose combined average fluid density is less than that of pure water. Archimedes principle, which states that the buoyant force on a body equals the weight of the fluid it displaces, is not directly applicable to a multiphase mixture. However, many have misapplied Archimedes principle and concluded that since the combined density of the fluid is less, there is a tremendous loss of buoyancy which leads to the sinking of the vessel. This conclusion has been publicized by (among others) the Petroleum Extension Service as quoted below:

  • One blowout in the Gulf of Mexico, where the rig sank in less than a minute, was not immediately understood. It soon became apparent that the gas flow foamed he seawater so badly that it was impossible for the craft to float. Thus, the craft sank to the bottom almost as quickly as if it had fallen through the air.

The truth is that a floating drilling vessel encountering a subsea blowout and bubble plume is subjected to significantly altered hydrodynamic conditions. The buoyant force is actually increased by the bubble plume. However, due to dynamic instability it is possible for flotation chambers of the vessel to flood which then leads to sinking. As more floating drilling vessels are designed, constructed and put into service, it has become increasingly important to understand the hydrodynamic effects of aerated water upon vessels of this type and to develop procedures to deal with these effects so that risks can be minimized.

DESCRIPTION OF A BUBBLE PLUME

When an offshore well blows out subsea, the expelled fluid will contain gas. The escaping gas forms a bubble plume above the origin. The rising bubbles entrain water in a turbulent plume that continues towards the surface, expanding and entraining more and more water.

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