Flexible pipe behaviour investigation using two models of internal slug flow regime

In this work the behavior of a flexible pipe with an inner slug flow regime is presented. The main goal of this study is the comparison of the internal stresses of this type of pipe arising from two different simple models of the slug flow regime. Flexible marine risers in recent years may be considered as an important part of offshore production facilities in order to extract crude oil from natural reservoirs placed at deep sea depth. Their installation has been proved to be fully technically acceptable, also from an economic point of view and often they prove to be the unique solution to the task they are demanded. Along their length, a gaseous phase is often pumped in order to facilitate the suction of the oil. The slug flow is the most frequent multiphase regime involved during the operations. Thus, in this work the authors present two simple models in order to describe exertions which this type of regime puts upon the riser inner wall. In both cases, an equivalent monophase fluid having a travelling sinusoidal density was considered. However, in the second case, a space and time varied slug phase along the entire length of the pipe was implemented. External forces due to the wave motion in terms of Morison's equations, structural damping, bending moment, axial tension, structural damping and bed action (Pollio, 2006; Pollio et al., 2006; Marano et al., 2006) are also modelled. Some results from a numerical model in the time domain of the system, based on the lumped mass approach (Ghadimi, 1988), allowed the authors to make some considerations about the importance of a better description of the slug flow effects into a flexible marine riser characterized by a continuously varied inclination.