December 2015 • Oil and Gas Facilities 51 Effectiveness of Bypass-Pigging Solutions in Multiphase-Flow Pipelines With Waxy Crude Oil: Evaluation and Innovative Solution Sasidharan Adiyodi Kenoth and Ali Al Matar, Dragon Oil, and D. K. Gupta, University of Petroleum and Energy Studies prediction of pig velocity, pig-generated slug volume, slug du- ration, backpressure increase in the pipeline, and process-plant upset. Control of these parameters is very difficult during by- pass-pigging operations because of its transient nature. The fluid behavior through bypass holes, subsequent downstream flow re- gime, and the nature of turbulence are unknown. Transient mod- eling and simulation results of bypass pigging with help of the OLGA Dynamic Multiphase Flow Simulator (available from Sch- lumberger) do not match with actual field results. Wax blockage of bypass holes also leads to erroneous results. In this paper, ef- forts are made to develop empirical correlations to approximate various parameters on the basis of experimental results in com- parison with simulation-model prediction. Later, an innovative bypass geometry/profile is proposed and designed, and experi- mental results are evaluated. Fluid-Flow Modeling and Dynamic Pig Modeling Understanding the motion of pigs and pig trains in pipelines is important, in general, to avoid surprises. Missed inspection data, damage to pigs, or, in the extreme case, fatality caused by high speeds lead to the need to understand pig acceleration, peak ve- locity, and how the pig or train might be brought under control. Gas-Velocity and Pig-Velocity Calculations. It is generally be- lieved that in multiphase-flow pigging without bypass, the pig ve- locity is equal to the gas-stream velocity. Though this assumption is a fairly good approximation, the actual pig velocity is slightly lower than the gas velocity/mixture velocity in a long-distance pipeline. The initial pig velocity is high compared with the latter part of its travel because the pig generated liquid displacement. The pig speed is generally calculated on the basis of the ideal-gas law acoss a control section, as follows: P V n z R T P V n z R T 1 1 1 1 1 2 2 2 2 2 × ( ) × × × = × ( ) × × × / / ; Actual Velocity V A = 2 , ........................................................(1) where P 1 is the initial pressure (standard pressure condition) in bara; P 2 is the final pressure (actual pressure condition) in bara, V 1 is the initial volumetric-flow rate (standard volumetric rate) in m 3 /s, V 2 is the final volumetric-flow rate (actual volumetric rate at pressure) in m 3 /s, T 1 is the initial (standard) temperature in °R, T 2 is the final (actual) temperature in °R, n 1 and n 2 are the number of moles of gas at different pressures, A is the area of the pipeline in m 2 , R is the gas constant, and z 1 and z 2 are the compressibility fac- tors at different pressures. Eq. 1 gives the superficial gas velocity, which can be approxi- mated to the pig velocity in the pipeline without bypass. With bypass, the pig velocity will be different and shall be calculated by reducing the bypassed-gas quantity, as discussed in the following subsection. Pig-Motion Analysis. The pig-motion analysis shows the follow- ing results (Tiratsoo 1999): Copyright © 2015 Society of Petroleum Engineers Original SPE manuscript received for review 3 November 2014. Revised manuscript received for review 29 June 2015. Paper (SPE 178424) peer approved 14 July 2015. Summary Bypass pigging, compared with conventional pigging, reduces the damaging effects of the pig-generated liquid slug by redistributing gas and liquid in the pipeline. Oil- and gas-production rate, high liquid-slug flow to the slug catcher, high pipeline backpressure, and the capacity of the slug-handling facility at the receiving end are major considerations when designing a bypass-pigging solu- tion. Various operational and engineering challenges are encoun- tered while implementing the commonly known bypass-pigging solutions, and empirical correlations are developed on the basis of experimental results and compared with simulation results. This paper suggests an innovative bypass-pig geometry as a solution. The Thornhill-Craver equation is introduced to calculate the by- pass-flow quantity and the pig velocity. A comparison between transient-flow simulation and field results showed some devia- tions. Empirical correlations are developed for prediction on the basis of experimental results. A new convergent/divergent bypass- pig geometry/profile is developed, followed by simplified model development. Through this innovative design, critical and constant gas-flow rate is achieved at lower pressure ratio through the bypass hole, where the gas enters through a nozzle, stabilizes at the throat, and recovers pressure through a diffuser section. At a predefined inlet pressure and area of cross section of the hole, a properly de- signed convergent nozzle with throat section will give maximum critical flow rate at the exit by reducing the gas pressure to the crit- ical pressure ratio. However, with help from the diffuser section, the high-velocity energy is converted back into pressure energy, and the line pressure regains up to 90% of the upstream pressure. Adopting such a bypass-hole profile with suitable geometry can ensure required bypass-gas quantity through the pig and can avoid pig stalling and minimize process upset, thus ensuring better pipe- line cleaning. Introduction Pigging of multiphase-flow pipelines is highly complicated com- pared with pigging of single-phase-flow pipelines. Bypass pigging, as compared with conventional pigging, reduces the damaging ef- fect of the pig-generated liquid slug by distributing gas and liquid in the pipeline. Allowable oil- and gas-production rate while pigging, high liquid-slug flow to the slug catcher, high pipeline backpres- sure, and the liquid-withdrawal rate/capacity of the slug-handling facility at the receiving end are major considerations for designing a suitable bypass-pigging solution. Most of the time, bypass pig- ging is not fully effective in waxy crude oil because of blockage of the bypass holes with wax. Various operational and engineering challenges while imple- menting the commonly known bypass-pigging solutions include