Subsea Cryogenic Gas Releases

The objective of this research is to develop a mathematical model to describe the dynamic and thermodynamic history of a Subsea Cryogenic Gas Release, and thus allow prediction of buoyancy, area, velocity and temperature on arrival at the sea surface. The thesis begins with a review of previous work and notes the shortfalls in the models produced. This work concentrates on the main part of the rise, although in reality both initial and final stages would have an effect. It is assumed that the gas forms a buoyant plume within an axisymmetric geometry. From the conservation laws a system of equations is derived which are then combined with a number of thermodynamic relations in a computer program. The semi-empirical formulae used in describing the thermodynamics relate to methane, this being the major constituent of the natural gas under consideration. Release rate, depth and pipeline conditions are input variables. Velocity and void-fraction profiles are assumed to be 'equivalent' top-hat, with correspondence between these values and those of gaussian being noted. Assumptions are made to the bubble size and the heat transfer to the gas with sensitivity studies being performed to identify the influence of these parameters. Initially mass transfer is disregarded, but a second model allows for the dissolution of the gas in the seawater and the effect of this on the surface conditions is assessed