A Comparison between Two Simple Models of a Slug Flow in a Long Flexible Marine Riser

Slug flows are extremely interesting multiphase regime phenomena which frequently occur in flexible marine risers used by the petroleum industry in offshore environments and have both a liquid and gaseous phase. This paper describes two simple models of the slug flow regime by means of an equivalent monophase flow with a non-constant density. The slug regime is modelled as a monophase density-varying flow with a sinusoidal density, travelling along the pipe itself towards the top end node of the riser. Starting from the bottom end, it is characterized by adiabatic processes and energy loss along the entire length of the pipe. In the first model, the slug wavelength is supposed to be independent of the riser inclination, while in the second one a simple linear relationship between the slug wavelength and the pipe inclination was imposed. The global equation of the motion of the riser (written in a two-dimensional domain throughout the plane containing the riser) was solved using a Matlab code in the time domain. In particular, the axial tensile force, the bending moment, the viscous structural damping, the wave-induced forces and the riser-seabed interaction are all modelled here. This work presents a comparison between the two models in two main kinds of configuration (a very long riser with and without seabed presence) and it allows the authors to make some considerations on general pipe behaviour.