Optimizing the Steady Operation of an Double Oil Pipeline System by Decompostion Method

As a main channel for long distance transportation of Daqing crude oil, Daqing-Tieling oil pipeline system consists of two pipelines in parallel. With its capacity of 45 million tons per year, the system is the largest oil pipeline system in China and plays an important role in the petroleum industry and national economy of China. Due to the complicated interconnection between the two pipelines in the system, the optimization of steady operation of the system is much more difficult than a single pipeline so that it can be considered as an optimization problem on large scale system. Besides the interconnection of the two pipelines, because of high pour point of Daqing crude oil, another difficulty to solve the problem comes from the fact that the two pipelines are hot oil pipeline, of which the heating-pumping stations are equipped with some heaters to heat the crude oil so as to improve its flow ability. For the optimization problem, the basic decision variables can be divided into two types, the discharge temperature of each heating-pumping station and the 0–1 variable which assigns a pump online or offline, and they are dependent to each other. Under certain conditions, the problem can be decomposed into two relatively independent sub-problems, one being the optimization of the oil temperatures in the system, another being the optimization of the matching between a pump combination and the all pipe segments of the system. The first sub-problem has been modeled as a nonlinear programming problem with 55 decision variables and more than one hundred constraints. For simplifying the solving process of the sub-problem, it has been further decomposed into a set of sub-problems, again, each of which can be easily solved. The second sub-problem can be modeled as a dynamic programming problem. On the basis of the models and the algorithms proposed for the above-mentioned problem, a software QTOPT has been developed specially for the Daqing-Tieling oil pipeline system, and has been used in evaluating and optimizing the process design of the system. Also the software can be used to optimize the steady operation of the system.