Control startup slugs of undulating multiphase pipeline during end of life operation using olga simulation

Abstract

Shut-in and cooldown of multiphase pipeline will cause accumulation of condensed liquid at all the low points of undulating sections of the pipeline. Successful startup of pipeline from this equilibrium condition, requires sufficient pressure at upstream to overcome hydrostatic pressure of accumulated liquid and high frictional pressure losses during movement of startup slug in the pipeline. During end of life operation, Shut-in Well Head Pressure, may not be adequate for pipeline startup due to depletion of reservoir inventory or the required inlet pressure during startup may exceed the pipeline design pressure. The objective of this project is to study the formation of startup slugs in undulating pipeline and its impact on operating parameters through OLGA simulation. It is also to establish optimum control methods and design for handling startup slugs with low inlet pressure. Using OLGA, startup slug mitigations options such as flaring from Slug Catcher or utilising drag reducing agent (DRA) injection in the feed or gas injection at riser base or requirement of combination of these methods were analysed. By comparing the simulation results, appropriate mitigation method is proposed for optimal benefit. The thermal-physical property table for the fluid, which is the input for OLGA simulator is generated with PVTSim software. Depressurising the pipeline by flaring (Method 1) from slug catcher is helpful to control startup slug due to increase in available differential pressure across pipeline, reduced liquid hold-up in pipeline by vaporization at low pressure and increased superficial gas velocity. Gas injection (Method 2) at riser base can also be helpful, as it can reduce startup pressure requirement by reducing the riser liquid head. DRA injection (Method 3) can reduce the pressure requirement at upstream of pipeline by minimizing frictional pressure losses when startup slugs moves through long pipeline. From simulation analysis, it is concluded that all three methods helped to reduce the inlet pressure requirement within allowable limit. Therefore, combination methods options are not necessary. Through techno economic analysis, startup with flaring (Method 1) is concluded as most optimal method for control of startup slug.