Studying The Prediction Techniques Of Hydrate Formation In The Process Of Natural Gas Pipeline Pigging Operation

After the pigging operation of Natural gas pipeline has been executed, the transfer efficiency of the pipeline will be improved, and the corrosion rate of pipeline will be reduced, the operation safety of pipeline will be ensured too. However, pigging operations associated with high risk. There are several serious accidents take place frequently, such as pig blocked, the pig’s signal lost, or even pipes are bursted. Especially with the rapid growth of China’s natural gas pipeline mileage in recent years, the accidents in the process of gas pipeline pigging operation are increasing. Among them, the accident has occured several times, which hydrate formation and blocking pipeline in the process of natural gas pipeline pigging operation. The normal operation safety of the pipeline have been seriously affected, to solve the actual problem is pressing.In order to prevent the pipeline is blocked by the dirts, when the gas pipeline pigging operation has been implemented with by-pass holes pigs, the dirts which accumulated in front of the pig will be dispersed, under the jet action of gas through the by-pass holes. However, because the effection of throttling effect, the gas temperature is decreased after it passed through the holes on the pig. If the parameters of the pigging operation are not properly controlled, there are hydrate formation conditions in the area of pig’s head. And if contains some liquid water in the pipeline too, the accident of hydrate plugging pigging device will be occurred.The accident cases of hydrate formation and blocking the pig in the actual project operation have been analyzed. The simulation theory of natural gas pipeline operation, pig movement, and the prediction theory of gas hydrate initial formation conditions, as well as the theory of gas-liquid two-phase pipe flow have been used to analyze the movement of pig in gas pipeline. And combining fluid mechanics, thermodynamics, and other related knowledge, the mathematical model of the natural gas pipeline pigging operation has been established. According to equilibrium theory, the gas hydrate initial formation conditions model has been established. When the gas pipeline pigging operation has been done, the pressure and temperature parameters have been calculated by using the pigging model. These parameters have been compared with the hydrate initial formation conditions, in order to determine whether have some hydrate generated in the process of pigging operation.Considering the differences of the pipeline pigging operating conditions, the steady-state and transient pigging simulation model have been established respectively. And in order to improve the calculation accuracy of the model, the coefficients of equations in the simulation model are transformed equivalently by using the Thermodynamic theory. The pressure drop model has been established, when the gas pass through the by-pass holes on the pig. According to the results of actual example analyzed and calculated, the impacting law of the parameters such as the diameter of by-pass holes, pipe diameter, and the pressure of pigging operation on the pig’s movement has been gotten. Except that, in the process of the pig is launching and the pig is blocking in the pipeline, the pipeline operation parameters’s variation has been analyzed respectively.Based on the theory of gas-liquid two-phase pipe flow, the estimation model and application method of determine the water volume which accumulated in the low lying pipeline have been established for the first time. The water volume can be calculated by the natural gas pipeline operating parameters and elevation data. According to the accumulated water volume to determine whether sufficient to generate hydrate and the pipeline is blocked. In addition, the amount of inhibitors which has been added to prevent hydrate formation in the process of pipeline pigging operation has been determined by the accumulated water volume.The technic method which has been used to simulate and predict hydrate formation in the process of pipeline pigging operation has been established for the first time. Before the gas pipeline pigging operation has been done, the technic method has been used to simulate and analyze the pipeline pigging operation, and ensure that the actual operating parameters are not in the hydrate formation conditions region. Then the pipeline pigging implement scheme has been determined according to the simulation results, in order to ensure the safety and efficiency of natural gas pipeline pigging operation. According to the calculated results of actural example, the technic method which has been used to simulate and predict hydrate formation in the process of pipeline pigging operation accords with the engineering requirement.