A method of modeling a segment of a pipeline transporting a product comprising defining within the segment a plurality of discrete cells, each disposed between knots, preparing a system of equations relating the conservations of mass, momentum and energy for each cell along with equations for the liquid phase flow area of cells with tight, slack and minimum area flow modes, providing data relating to the product and the location and elevation of the cells, sensing a plurality of conditions within known cells, solving the system of equations, initiating a re-stepping process by re-assessing the flow modes of each cell and re-setting flow modes for cells with unstable flow modes, and resolving the system of equations using stable flow modes. An embodiment of the method includes excepting one or more cells from the re-stepping portion where a recurrent pattern of flow mode change is detected.