Chinese Journal of Engineering

Volume 2016, Article ID 4586853, 8 pages

http://dx.doi.org/10.1155/2016/4586853

## Modeling and Experiments of Severe Slugging in a Riser System

^{1}College of Pipeline and Civil Engineering, China University of Petroleum, Qingdao 266580, China^{2}Shandong Provincial Key Laboratory of Oil & Gas Storage and Transportation Safety, Qingdao 266580, China^{3}CCTEG Chongqing Engineering Co., Ltd., Chongqing 400016, China

Received 23 October 2015; Revised 10 January 2016; Accepted 2 February 2016

Academic Editor: Eleonora Bottani

Copyright © 2016 Lin Wang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

A transient mathematical model based on continuity equations for liquid and gas phases, with a momentum equation for the mixture, was developed, and numerical solutions and simulations corresponding to severe slugging in pipeline-riser system were presented, and the results were compared with the experimental data to verify the mathematical model. In numerical solutions, backward Euler schemes were adopted as predictors and trapezoidal methods were used as correctors. Variable time steps were employed for higher computational efficiency and accuracy in the integration. Experiments of severe slugging characteristics were performed, and the simulation results of the cycle periods and bottom pressure were compared with experimental values. Finally, the calculation results of detailed characteristics were analyzed thoroughly. The results show that the developed mathematical model can accurately predict the cycle time and the detailed characteristics of severe slugging. Under the experimental conditions, the liquid slug length can reach 1.6 times the height of the riser, and the maximum instantaneous gas velocity of outlet is 50 times the inlet gas velocity, and the maximum instantaneous liquid velocity of outlet is 28 times the inlet liquid velocity, having important implications for the hazard assessment of severe slugging.

#### 1. Introduction

In the offshore and deep water oil and gas development, multiphase transportation is more economic as a transportation way. The mixture of oil and gas is transported through the hilly terrain subsea pipeline and the riser to the offshore production platform for oil, gas, and water processing [1]. Severe slug flow occurs at low gas and liquid flow rates, with the downward inclined pipe in stratified flow, and is characterized by the generation of liquid slugs at the base of the riser ranging in length from one to several riser pipe heights. The process of severe slugging in a riser system was considered as a cycle consisting of four steps [2]: (1) slug formation; (2) slug movement out of the riser; (3) blowout; and (4) liquid fallback. The pressure and the instantaneous velocities of gas and liquid flow in the riser system oscillate violently, which may shock the downstream equipment and induce damage such as severe vibration of the riser system and equipment. Therefore, it is essential to simulate the detailed characteristics of severe slugging accurately for the hazard assessment of severe slugging.

Since the severe slugging induced problems were identified by Yocum [3], a great deal of theoretical exploration and experimental studies on severe slugging were developed. In [2], an experiment was carried out to study the characteristics of severe slugging, and a simplified model was presented for simulating the severe slugging [4]. Boe [5] and Jansen et al. [6] presented, respectively, flow regime maps for predicting the severe slug flow regimes, where the boundaries were determined analytically. Huawei [7] studied the characteristics of severe slugging in pipeline-riser system and catenary riser system by detailed laboratory experiment and developed a mathematical model for severe slugging. Zhang et al. [8] set up a one-dimensional quasi-equilibrium simplified calculation model for the unsteady flow in the L-type riser system, which ignores the effects of friction and acceleration. Baliño et al. [9, 10] presented a mathematical model considering continuity equations for liquid and gas phases, with a simplified momentum equation for the mixture, neglecting inertia, and considering inertia, respectively, and simulated the transient characteristics of severe slugging. A computational fluid dynamics (CFD) method is proposed for numerically simulating the gas-liquid severe slugging in a pipeline-riser system. Gao et al. [11, 12] implemented 2D numerical simulations of severe slugging by using a CFD software FLUENT. Araújo et al. [13] studied the dynamics of individual and a pair of Taylor bubbles rising in vertical columns of stagnant and cocurrent liquids numerically using the volume of fluid (VOF) methodology implemented in the commercial code ANSYS FLUENT. Li et al. [14] developed a transient model using OLGA to study the dynamic behavior of severe slugging in a pipeline-riser system and compared the simulation results with the experimental data. Xing et al. [15, 16] carried out 2D CFD simulations for severe slugging and attempted to develop a 3D-1D coupling simulation, that is, STAR-OLGA coupling, for mitigating hydrodynamic slug flows with a wave pipe, where the 1D simulation for the entire pipeline coupled with the 3D simulation of the partial flow field was implemented to control the scale of calculation. Looking through current literatures, numerical simulation methods are mainly divided into two categories: (1) numerical simulation of the whole flow field based on CFD; (2) simplified one-dimensional transient model. The former method’s advantage is that the detailed characteristic parameters of flow field and the liquid-gas interfaces can be described precisely. However, this method is computationally expensive and difficultly simulates the full-scale of the physical phenomena, so it is difficult to apply this method effectively. The simplified one-dimensional transient model is not only efficient in computation but also more accurate when empirical correlations and experiential parameters are introduced appropriately, and then this method can simulate the engineering scales of multiphase transportation.

In this paper, a modified mathematical model is developed for the severe slugging flow in risers; the continuity equations and momentum equations are described as a series of differential equations for the enhancement of suitability. The numerical integration methods for the mathematical model are presented in detail. The simulation of the liquid fallback is added to improve the accuracy of simulations of the flowing characteristics and cycle period of severe slugging.

#### 2. The Development of the Mathematical Model

The hybrid riser facility which consists of a downward inclined pipeline and a riser was built up to simulate the riser system (Figure 1). Liquid and gas flow in the downward inclined pipeline simultaneously and move out at the top of the riser.