Abstract: A unified hydraulic model has been developed by the method according to the invention which is applicable to any slope and diameter of pipeline and can handle most of the steady state as well as transient multiphase flow regimes encountered in practice. The new modelling method differentiates two types of flow patterns: separated flow patterns (stratified or annular) and dispersed flow patterns. Intermittent flow patterns (slug, churn flow) are a combination of these two patterns. The same concept has been successfully applied for transition criteria between different flow regimes, insuring continuity of the solutions across the transitions. This requirement is very important for simulating transient phenomena. The transient resolution is achieved by an explicit time advancing scheme. The advantages of the method are; its ability to follow wave front propagation, an easy implementation for the resolution of complex pipeline networks.