Simulations of severe slugging during depressurization of an oil/gas pipeline

Abstract

Dynamic simulators for pipelines with multiphase flow have proved to be important computational tools for both design and operational support of oil and gas production systems. One important aim of such simulators is to predict the arrival time and magnitude of outlet liquid transients after production changes made by an operator of a pipeline. A multiphase flow simulator (OLGA-94.1) with a two-fluid model has been applied to simulate depressurization of a pipeline during a shutdown procedure. During depressurization liquid slugs may form and propagate towards the outlet. The importance of the numerical method for predictions of such transients is demonstrated by using an Eulerian, finite difference, implicit, upwind scheme both with and without a front tracking scheme. First the initial conditions for the depressurization is established from a shut-down simulation where the production at the inlet is closed down, and the liquid comes to rest at low points along the pipeline. A realistic depressurization is simulated by opening a choke at the outlet of the pressurized pipeline. The numerical scheme without front tracking (standard scheme) gives outlet gas and liquid flow rates which are smeared out in time due to numerical diffusion. Simulations with the front tracking scheme give intermittent gas-liquid flow arriving as sharp fronts at the outlet. The total remaining fluid in the pipeline after the depressurization is larger when using the standard scheme.