The effects of gas aerated seas upon the buoyancy and stability of floating drilling vessels

Author: Wilson, Kevin John


Educational level: Master of Science in Petroleum Engineering

Discipline: Petroleum Engineering

University: Texas Tech University

Abstract: This paper presents an eclectic summary of the effects of a subsea blowout on a floating vessel. A subsea blowout may aerate the seawater with escaping gas and hydrocarbon fluids, and thus form a bubble plume. This study describes the subsequent hydrodynamic effects on a floating drilling unit. First, a general description of the characteristics of a bubble plume is presented. This includes the velocity, volume and effect of entrained water, the gas distribution, the density defect and the nature of the fluid flow. Following, is a discussion of buoyant force and Archimedes principal. Next, the qualitative results of experimental modeling are presented. Finally, the findings of studies of actual blowout incidents are given. Fifteen actual occurrences were studied. A density reduction of the gas-liquid mixture was not the cause of any rig sinking. Rather, fire, damage and flooding were the causes of sinking. The effects of the plume flow, in conjunction with the type of mooring system, can cause flooding through open hatches and other low openings. Low freeboard vessels are most vulnerable. Semisubmersibles with high freeboard are much less vulnerable. Collectively the study shows that there is not a loss of buoyancy, but rather a buoyancy gain due to the immediate effects of a bubble plume. However, the dynamic effects of the plume, which amplify pitch, heave and especially roll motions of the vessel, are detrimental and need be anticipated.

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Keywords: Plumes (Fluid dynamics), Deep-sea drilling ships -- Safety measures, Oil wells -- Blowouts -- Models, Offshore structures -- Hydrodynamics


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