Study of Slugs Control Techniques in Pipeline Systems
Authors: Silva Luciana Loureiro , Vidal J.L.A. Edson , Monteiro Paulo Correia , Netto Theodoro Antonum 
Published: June 2014

Severe slugging may occur at low flowrate conditions when a downward inclined pipeline is followed by a vertical riser. This phenomenon is undesirable for offshore oil and gas production due to large pressure and flowrate fluctuations. It is of great technological relevance to develop reliable and economical means of severe slugging mitigation. This study aims to develop an automated control system to detect and mitigate the formation of severe slugging through a choke valve and a series of sensors. As a first step, an overall flow map is generated to indicate the region within which severe slugging may occur based on Boe's criterion [1] and Taitel's model [2, 3]. It was possible to obtain different flow patterns by controlling the rate of water and gas injection.

The aim of this paper is, however, the formation of severe slugs and study of mitigation techniques. In the control part, we used a choke valve controlled by software which is in feedback with data from a system with pressure, temperature, flow, which are able to measure even small changes in the relevant parameters to the model. A two-phase flow loop was built for the study of severe slugging in pipeline-riser system with air and water as work fluids. The inner diameter of riser and flowline is 76.2 mm. The riser is 20 meters high and the flowline is 15 meters long and could be inclined upward or downward up to 8-degree. It has been shown by experiments how riser slugging can be controlled by automated control system.

Introduction

The multiphase flow occurs in many processes in the oil industry, in the production and transportation, wells and the links between these and platforms. Several studies have been undertaken in order to predict their behavior as this has a large effect on the productivity and safety of equipment [4, 5].

The flow of gas and liquid simultaneously have various types of configurations or patterns, depending on the operating variables, speed, fluid pressure, pipe diameter and inclination angle. It is known that the equations that predict the behavior of fluids vary considerably according to the type of flow. Among the flow patterns the intermittent was the object of study of this project. According to the literature the intermittent flow can be subdivided into plug flow and slug flow. The plug flow occurs in general for low flow rates and bubble are free flowing inside the liquid. At high flow rates of gas bubbles have small size as the fluid was aerated. The slug flow may also be divided into two types, the hydrodynamic and severe. The hydrodynamic slug can form in horizontal sections or wells and risers. The severe intermittence or slug occurs from the accumulation of fluid in the sections downhill by gravitational effect. This phenomenon can occur in pipeline-riser systems which the pipeline is downhill and soon rises to the riser. At low flow rates of liquid and gas, due to the accumulation of fluid in the riser blocking the passage of gas, resulting in its compression. When the gas pressure exceeds the hydrodynamic pressure of the fluid in the riser, the gas arises expanding and pushing the fluid in the column to the separator. This phenomenon results in periods without production followed by a large amount of liquid and gas as well as sudden changes in pressure.

Slug, therefore is the formation of large gas bubbles in a flow regime that occurs within the multiphase pipeline transportation and production of hydrocarbons. Characterized by large variations in flow and pressure occurring in the whole process of production and transportation. The slugs generate undesirable consequences in the process of oil production, such as no oil periods, followed by high oil production within the separator. The reduction in production capacity, emergency stop on the platform due to the high liquid level in the separator, corrosion and wear to the process equipment and high maintenance costs are some consequences of this phenomenon.