Experimental Study of Severe Slugging in a Two-Phase-Flow Pipeline - Riser Pipe System

Abstract

Abstract Slug flow was studied in a simulated, offshore, pipeline-riser pipe system. Two distinct slug flow patterns were identified: severe slugging and normal slug flow. Severe slugging, characterized by generation of slugs ranging in length from one to several riser pipe heights, occurs at low gas and liquid flow rates and for negative pipeline inclinations. A mathematical model was developed for severe slugging. Results agree well with experimental data. Choking was found to be an effective method of eliminating severe slugging. Introduction Gas and liquid frequently are transported simultaneously in pipes, such as in gas and oil fields, in refineries and process plants, and in steam injection and geothermal production systems. When two-phase flow occurs in a pipeline, the phases separate in the pipe into various flow patterns.When the flow pattern at the exit of a pipe consists of alternating slugs of gas and liquid, special operating procedures frequently are required.Slugging in some of these facilities has required the use of operating procedures which drastically curtail production. Yocum reported that flow capacity reductions up to 50% have been necessary to minimize slugging on offshore platforms. The reported losses occur when platform backpressure is increased until a flow regime is reached in which slugging and pressure fluctuations are reduced to levels which can be handled by gathering facilities.Cady used an existing vertical flow pattern map to determine the conditions under which slugging would occur in a riser. Schmidt et al. described a comprehensive review of slugging problems of this nature and proposed automatic choking as a means of alleviating slugging in risers.This study describes the generating of long liquid slugs in a pipeline-riser pipe system and develops a mathematical method to predict slug characteristics. In addition, it has been found that severe slug flow can be eliminated or minimized by careful choking which results in little or no change in either flow rate or pipeline pressure and in elimination of pressure fluctuations. Description of Equipment An experimental facility was designed and constructed to permit study of flow in a pipeline-riser pipe system. The fluids flowed through a 100-ft-long, 2-in.-diameter pipeline and then up a 50-ft-long, 2-in.-diameter vertical riser. All pipe was made of Lexan and was transparent. Both sections are supported by aluminum I-beams that can be pivoted at their free ends through angles of +/- 5 degrees, to the horizontal and vertical. This study was conducted at pipeline angles of −5, −2, 0, and +5 degrees, with the riser pipe vertical.The fluids used in the study, air and kerosene, were mixed at the entrance of the test section, At the end of the test section, the air/kerosene mixture was separated in a horizontal separator. The air was vented, and the kerosene was returned to a storage tank.Kerosene was pumped from the tank into the system by means of a single-stage Gould centrifugal pump. The liquid flow rate was metered with a Camco 4-in, orifice meter and a Brooks rotameter.The air was obtained from a Joy two-stage compressor with a maximum output capacity of 0.6 MMscf/D at 120 psig. A Camco 2-in. orifice meter and a 0.75-in. Daniel orifice meter were used to measure the air flow rates.On each test section there were two pressure taps separated by a 25-ft span. SPEJ P. 407＾