Optimizing the Pigging Frequency Through Prediction of Wax Deposition of Pipeline

Abstract

When hot waxy crude oil is pumped into the cold pipeline, the oil temperature will decrease with time and along the pipeline. As soon as the oil temperature near the pipe wall is below the WAP (wax appearance point), the wax contained in the crude will precipitate from the crude and deposit on the inner pipe wall with liquid part entrapped in it. This phenomenon is called wax deposition of the crude oil pipeline. The formation of the wax deposition reduces the throughput of the pipeline but enhances the operation pressure of the pipeline, which brings about more consumption of the power and reduces the safety of the pipeline. More seriously, when the wax deposition layer in the pipeline grows enough thickly, the pipeline would face the danger of shutdown and the enormous loss of economy would occur. So the removal of the wax deposition layer periodically, that is, pigging is strongly needed. Periodic pigging could reduce the operation pressure of the pipeline and keep the throughput of the pipeline, which makes the pipeline avoid hidden trouble of shutdown. Although pigging could solve the problem of the wax deposition of crude oil pipeline successfully, another problem of how to determine the pigging frequency must be faced. Relative to a suitable pigging frequency (though it is difficult to determine), more often pigging could increase the operation cost and wear and tear the inner wall of the pipe but a prolonged interval of pigging may make trouble. Usual means of determining the pigging frequency of a pipeline is mainly based on the pressure drop calculation between two pump stations. From Darcy formula for calculating the pressure loss of the pipeline, the average equivalent inside diameter (say hydraulic inside diameter) of the pipeline could be obtained. Then the average wax deposition thickness in the means of hydraulics could be calculated being based on the original inside diameter of pipeline. According to the average wax deposition thickness and operation experiences, the pigging frequency could be determined. This is true if the wax deposition along the pipeline is almost identical. But in most cases, i.e. to most crude oils and pipelines, the wax deposition profile along the pipeline is not unchanged but a complex curve. In later cases, the average wax deposition thickness calculated from the pressure drop between two pump stations hydraulically exaggerates the average extent of wax deposition but ignores the severity of wax deposition at some local position of pipeline. Thus the pigging frequency determined from the pressure drop might result in more errors, that is, a more frequent pigging or one with more safety troubles. From all the above, a prediction of wax deposition in the pipeline is very necessary and important to the decision of a safe and economic pigging frequency. But due to the complexity of the problem of wax deposition, it is very difficult to predict the wax deposition of one crude oil in a pipeline theoretically. A semi-experience method based on molecular diffusion principle for predicting the wax deposition of Daqing crude oil in the Northeast Pipelines in China is developed and introduced in this paper. Using this method, the wax depositions in the Northeast Pipelines in different seasons are calculated numerically on computers. Based on the calculation results, a more rational pigging scheme aimed at the Northeast Pipelines, which considers the variation of wax deposition along the pipeline, are given.