Two-Phase Flows as a Result of a Subsea Blowout and Their Effect on the Stability of Structures

Abstract

ABSTRACT A number of floating structures have sunk in subsea well blowouts. The cause of rig sinkage has previously been explained by a loss of buoyancy attributed to a density reduction of the surrounding sea by the gas phase. It has been recently shown that this is of secondary importance and the main cause of rig instability are the secondary flows in the water, which are generated as the gas rises to the free surface. In this paper numerical simulations of the two phase flows following a release of gas at the bottom of the sea, for one flow rate and two water depths. are presented. The main findings can be summarised as follows. The dynamic pressure arising from the recirculation of the water is greater than the hydrostatic pressure due to the density defect. A strong jet is formed near the water free surface which in turn affects the dynamics of a floating structure located in the vicinity of the plume. High turbulence levels occur, with frequencies which could coincide with the natural frequency of the structures. Results of the dynamic behavior of a drill ship in such flows, are also discussed. INTRODUCTION When a subsea well blows out or an underwater gas pipe line breaks a large amount of gas is released into the water. The gas breaks up into bubbles which rise upwards into a turbulent plume above the release point. As the bubbles rise they carry a considerable amount of water along with them both in their immediate vicinities and in their wakes. These bubbles exert a drag force on the surrounding liquid. A typical value of the Reynolds number for the escaping gas, based on a pipe hole diameter, is of the order of 107. Hence the rising plume is turbulent. As the plume approaches the free surface most of the gas escapes to the atmosphere while the liquid flow direction changes from predominantly vertical to radial, away from the plume axis. The plume axis however, does not remain at a fixed location. Bubbly plumes are in general laterally unstable. Although the detailed hydrodynamics of the lateral instabilities are rather complicated to analyse and not clearly understood at present, it has been observed that the plume meandering, from its vertical position, is of the order of the plume diameter. This corresponds to a time scale of 1.4 s for a plume rising in 700 m deep water. The flow is rotational and causes massive recirculation of the water. In most cases, offshore structures or ships are present in the neighborhood of a blowout. The interaction of the two-phase plume with the structure or ships will cause an instability of the floating structure and there are many cases where the structures have been badly damaged. The gas escaping through the free surface is flammable (eg 80% methane) and it is therefore important to predict its dispersion under different weather conditions.