  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.
