By-Pass Pigging Without or With Speed Control for Gas-Condensate Pipelines

Abstract

Abstract Pigging of a multiphase (i.e. liquid/gas) pipeline can result in a large "pigged" liquid slug when a normal (i.e. non-by-pass) pig is used. This requires a large separator or slugcatcher, or otherwise accepting production deferment during pigging. These problems can be avoided by using a by-pass pig, which means that part of the gas can by-pass the pig through holes, which reduces the pig velocity and in turn it also reduces the pig-generated volume. By-pass pigging is applied more and more often, but it remains difficult to select the right by-pass opening for the initial pig run. This paper describes a simple design tool, based on steady-state hydraulic pipeline simulation results, to determine the required fixed by-pass opening. Also field experience with a speed control pig for a gas-condensate pipeline is shown (i.e. the by-pass opening is adapted along the pipeline to maintain a constant pre-set pig speed). The by-pass pigging design tool is based on steady-state hydraulic pipeline simulations. By-pass design has two main constraints: (1) too small an opening will create a large Pig-Generated Volume (PGV), and (2) too large an opening will create too little force on the pig to overcome the wall friction. The optimum opening is when the liquid surge will be minimum, to avoid having the risk that the pig gets stuck. The main uncertainty in the prediction is the friction between the pig and the pipe wall. The simple steady state by-pass pigging design tool is validated against both dynamic simulation results and field data. The tool provides a quick method to assess the right by-pass opening without the requirement of doing an extensive dynamic simulation study. As the pressure decreases along the pipeline, the optimum by-pass opening also varies along the pipeline; the gas velocity and therefore the pig velocity will normally increase towards the outlet of the pipeline and a larger by-pass would be preferred at this point. However, a (fixed) larger by-pass can cause difficulties when launching the pig. A speed control pig consists of a variable by-pass valve which is controlled in such a way to maintain a constant pig velocity. For example, when the by-pass opening is reduced, the pressure drop across the pig increases and therefore the pig velocity increases as well. The speed control pig has been used several times for a 70 km long gas condensate pipeline, which has successfully reduced the liquid surge into the slug catcher. Field data demonstrate that the by-pass pig speed can be maintained almost at a constant velocity, when using a speed control pig for a gas condensate pipeline.