Modeling of Pig Operations in Natural Gas and Liquid Pipeline

Abstract

Abstract The use of pigs has become a standard industry procedure. This expanding demand has driven manufactures to produce a great variety of pig models. Most of the available knowledge is based on field experience. Hence, selecting the best pig, estimating its speed and required driving pressure, often involves some guesswork and consequently, a high degree of uncertainty. The pig is most effective when it runs at a near constant speed but will not be effective when it runs at too high speed. The typical speeds for pigging are about 1–5 m/s for on stream liquids and 2–7 m/s for on-stream gas (Cordell and Vanzout 1999). In this work, the pig motion is modeled using Continuity, Momentum, Energy and State equations. These equations simultaneously are solved under unsteady state condition to obtain the pressure; pig velocity profile and the time that pig reaches at the end of the pipeline. This information can help the operator to control speed of pig before the pig reaches the end of pipelines. The change area and elevation profile has been investigated. The results are obtained for gas and liquid pipelines. The programs have been run for various values of time step and distance increment. The results are more accurate when time step and distance increment are getting smaller according to CFL (Courant-Friedrich-Lewy) condition Statement of theory and definitions A general definition of pigging is the propulsion through a pipe of a mobile plug pig which can execute certain activities inside the pipe. Pigging is a common operation in the oil and gas industry. Pigging of flow lines is done with many objectives, including cleaning deposits such as wax layers, removal of liquids and flow line inspection. In pigging the contents of a pipeline are pushed by a pig, with the goal of removing the product almost completely from the pipeline. Pigging helps keep the pipeline free of liquid, reducing the overall pressure drop, and thereby increasing the pipeline flow efficiency. On the basis of applications in the oil industry (pipelines), which began as early as the late 1800's. The pig can be spherical, elongated, or composed of several parts. The pig is oversized relative to the pipe; thus, the pipe is sealed in front of and behind the pig, and the pig can be driven by a gaseous or liquid propellant. The gas most frequently is used compressed air and the liquid can be e.g. water, cleaning agent or product. At the beginning and end of the pigging line, pig stations are located. The first pig station traveled is the launching station and that which is traveled is the receiving station. Prediction of the pigging time is very important, as it gives the operator an estimate of the pig arrived time. This paper deals simulation and modeling of pigs through liquid and gas pipelines. Equations of mass, momentum, energy, state and dynamic of pig are solved simultaneously. The governing non-linear hyperbolic partial differential equations for unsteady state liquid and gas flow are solved by the method of characteristics (MOC). The Runge-Kutta method is used for solving the steady flow equations to get the initial flow values and dynamic equation of the pig.