A method and a system of analyzing multiphase fluid flow in at least one well or pipeline is provided, the method comprises characterizing slug flow in the multiphase fluid flow by receiving a plurality of well pressure data from the at least one well, processing the plurality of well pressure data to obtain a time-varying slug amplitude and/or a time-varying slug period for slugs in the slug flow, analyzing the slug amplitudes and/or the slug periods over a period of time to thereby determine a measure of well performance. Also an oil field monitoring system for an oil field comprising a plurality of wells is provided. Each well has a pressure gauge installed to measure a well pressure and is connected to a multiphase fluid flow analyzing system. An oil field unit receives the measure of well performance from the multiphase fluid flow analyzing system, and output the measures of well performance for each of the plurality of wells.