The changing regulatory market, the interest in reducing energy needs and the high price of power in many locations has prompted the need for operational optimization of liquid pipeline systems. Pipeline optimization is a complex problem that involves tradeoffs between using drag reducing agents (DRAs), electric power and other fuels. These tradeoffs are difficult to analyze because of the effects of electric power rate structure and various operational constraints. This paper will discuss these tradeoffs and present a case study to show the cost savings achieved with an operational tool that incorporates these tradeoffs.
Liquid pipelines are expensive to operate. The cost of electric power, other fuels and drag reducing agents (DRAs) is one of the largest expenditures of a pipeline company. In some instances, this can even be larger than personnel costs. Any savings achieved in the operational cost of the pipeline can go directly to the bottom line and improve the financial status of a pipeline company. The ability to reduce operational cost is complicated by many factors such as the following.
Complex nature of electric power contracts
Variation of electric power price with time and location
Real time pricing of electricity in some areas
Variance of products and/or oil properties in different batches
Restrictions on maximum DRA concentrations
These complex issues, which are very interdependent, make it difficult to optimally reduce operational cost. Different pipeline companies have developed operating procedures to reduce operational cost. Developing these operating procedures requires asking a number of questions such as the following.
Is the practice of reducing flow during peak power price and increasing flow at low power price effective?
If DRA is used, should DRA concentration vary by batch?
What is the tradeoff between using DRA and using more electric power?
Are there specific flowrates which should be avoided because they result in much higher operational cost?
The answers to questions such as above require insight into how various tradeoffs interact. Such insights are often gained by experience, but can also be aided by mathematical optimization tools. In this paper we will examine pipeline optimization problem from two perspectives. In the first perspective, we will present a very simple optimization problem, and attempt to gain some insight into the nature of the optimal solution. In the second perspective, we will present a case study to look at a more general and consequently much more difficult optimization problem to show the tradeoff between using power and DRA.
The goal in our first perspective is to examine a very simple pipeline system where we have to choose what flow rates and what DRA concentration to use. The simple pipeline system we will consider is shown in Figure 1. This pipeline system has one pump station with a single pump and one pipeline. There is a single supply immediately upstream of the pump station and a single delivery at the end of the pipeline. Let us