Stabilising slug flow at large valve opening using an intermittent absorber

Abstract

Slugging is one of the challenges usually encountered in multiphase transportation of oil and gas. It is an intermittent flow of liquid and gas which manifests in pressure and flow fluctuations capable of causing upset in topside process facilities. It can also induce structural defects in pipeline-riser system. The threat of slugging to oil and gas facilities has been known since the early 1970s. This study investigated a new method for slug flow stability analysis and proposed the use of active feedback control and intermittent absorber (a passive device) for hydrodynamic and severe slugging attenuation. The geometry impact on the hydrodynamic slug flow in pipeline-riser systems was established using modelling (LedaFlow and OLGA) and experimental studies. The unit cell model in both software packages, the slug tracking model of OLGA and slug capturing model of LedaFlow were employed for hydrodynamic slug modelling. Three distinct slug regions were reported for a typical pipeline-riser system. The H-region typifies the slug flow regime in the pipeline-riser system due to slug formed in the horizontal pipeline upstream the riser pipe. The V-region represents the slug flow regime due to the riser pipe while the I-region describes slug flow regime where both horizontal and vertical pipes contributes to the dynamics of the slug flow in pipeline-riser system. A simple but yet robust methodology that can be used for pipeline-riser system and slug controller design was proposed. The active feedback control was shown to help stabilise hydrodynamic slug flow at larger valve opening compared with manual valve choking. For the case study, a benefit of up to 5% reduction in riserbase pressure was recorded for the proposed method. This in practical sense means increase in oil production. The analysis also showed that the new slug attenuation device (intermittent absorber) possesses the potential for slug attenuation. Experimental studies showed that the device was able to reduce the magnitude of riserbase pressure fluctuation due to hydrodynamic slugging up to 22%. The absorber enables larger valve opening for both hydrodynamic and severe slugging stabilisation. For severe slugging attenuation for example, a benefit of 9% reduction in riser base pressure was recorded for the case studied. This is of great benefit to the oil and gas industry since this translates to increased oil production. Slug attenuation index (SAI) and pressure benefit index (PBI), have been proposed to quantify the slug attenuation potential and the production benefits of the intermittent absorber respectively. The SAI and the PBI provided consistent results and methods for estimating the slug attenuation potential of the intermittent absorber concept. They could also be used to quantify the slug attenuation benefits of other slug mitigation techniques.