The Technique of Segmental Pigging Process for Long Distance Pipeline of Oil and Gas in China

Worldwide, the pipeline mileage has increased dramatically since 1948 when the industry began to keep mileage statistics for pipeline construction, especially in China. Before operating long distance oil and gas pipelines, the pressure testing for tightness and strength is of great importance for ensuring operating safety. Water is used as service fluid for the pressure testing due to its safety, and long distance pipeline is divided into a number of small sections according to complex terrain conditions. Segmental water pressure testing is now very frequently used in the oil and gas industry, ensuring safety and efficient construction of pipelines. However, some new problems may arise, in practice, for the pigging process, following water pressure testing. Based on the review of previous papers about pipes accidents, the causes can be classified into hydraulic and nonhydraulic aspects. With the method of characteristic (MOC) and basic theory of gas–liquid two-phase unsteady flow, a mathematical model is developed to simulate the hydraulic transients during the pigging process. The model has been applied to some segmental pipes in China to predict varying pressures under complex terrain conditions. Pressure pulses predicted at the end of pigging in the numerical results have also been found to occur in field trials. The analysis shows that pressure pulses may cause overpressure accidents due to vapor cavity collapse. The techniques in this paper can give reasonable instructions in long distance pipeline constructing, promoting the development of Chinese oil and gas industry.

Hydraulic Transients Induced by Pigging Operation in Pipeline with a Long Slope

Pigging in pipelines basically performs operations for five reasons including cleaning the pipe interior, batching or separating dissimilar products, displacement, measurement, and internal inspection. A model has been proposed for the dynamic simulation of the pigging process after water pressure testing in a long slope pipeline. In this study, an attempt has been made to analyze two serious accidents during pigging operation in 2010 by the model which is developed by the method of characteristic (MOC) by Wylie et al. (1993) and the two-phase homogeneous equilibrium vaporous cavitation model deveoped by Shu (2003) for vaporous cavitation. Moreover, simulation results of the third operation show good agreement with field data from the previous field trial. After investigation, it was showed that the impulse pressures produced during collapse of a vapor cavity result in severe damage of tubes.

Study on Duration of Hydrostatic Leak Test for Gas Pipeline

A mathematical model about the duration of hydrostatic leak test for gas pipeline is developed for the first time in the paper. The influences of temperature variation, elastic deformation of the pipe and a certain amount of residual air filled within the pipe are synthetically considered in the model. The results indicate that the duration is longer while the amount of the residual air in the pipe or the volume of pipe is lager under the same pressure dropping. The required duration is increasing with the pressure going up. At the same time, the temperature variation greatly influences the pressure dropping of the pipeline. The conclusions are helpful for determining the reasonable duration of hydrostatic leak test for gas pipeline. Furthermore, it can also provide some references for judging leak test result while hot water pressure testing in winter.