The Research Of Subsea Natural Gas Liquid Pipeline Bypass Pigging Technology

During late gas field life, there will be much more liquid in the pipeline. The majority ofthe liquid is held up in low positions of the pipeline and it will reduce the efficiency of theline. What’s more, within a few special conditions, the natural gas hydrate will be producedin line and may cause production to be shut down. Generally, a pig will be launched into thissystem to sweep the liquid out of the line, the liquid ends up in the slug catcher at thereceiving facilities. However, it is possible that the pig is too fast while pigging the pipelinefor common pigs, in some conditions the slug catcher may not be able to handle the liquidsquickly enough and finally overloads. So a new kind of bypass pig should be designed.Based on research of the structure of pigs both home and abroad, a new bypass pig thatwill be used in subsea natural gas pipeline is designed. The liquid ahead of this new bypasspig can be aerated, so it will be spread over a greater length in pipeline. Due to its ability ofcontrolling the gas liquid two-phase flow, the bypass device can reduce the speed of pig, thepressure fluctuation and the volumetric flow of liquid. If the pig stalls at a bend or somerestrictions in the line, due to the special design, it can restart very easily and reduce the risksof pigging operation.OLGA, a pipeline transient simulator, is used to simulate the pigging of one subseanatural gas pipeline in Liwan. The simulation is performed to assess the effect of bypasspigging by analyzing the speed of the pig, the pressure fluctuation of the pipeline and theflowrate of liquid in the outlet. At the same time, the most suitable bypass percentage of pigfor this pipeline is determined. MEG is added to gas to avoid producing hydrate, the amountof which is also precisely determined in the simulation.By combining the structure of bypass pig and the results of OLGA, we build up flowingmodel with Fluent. From the result of Fluent simulation, the stress on the bypass pig’s valve estimated under certain conditions. Then the differential pressure that can shut the valvedown is got.We have designed and built the experimental pipeline in the lab including the launcherand receiver of pig, horizontal pipe, inclined pipe and risers. At the same time, the pig used inthe lab is specially designed. By changing the flow of gas and liquid, the bypass percentageof pig, lots of experiments have been performed. We summarized the movement pattern ofthe bypass pig and liquid backflow condition during pigging operation in the experiments byanalyzing the speed of the pig, the pressure variation of the pipeline and the liquid flow of theoutlet.