Evacuation of a Residual Oil Pipeline by Inert Gas Displacement

Abstract

Webb, S.,* Lehigh U. Bogucz, E.,** Lehigh U. Levy, E., Lehigh U. Barrett, M.L., Pennsylvania Power & Light Co. Pennsylvania Power & Light Co. Snyder, C., Pennsylvania Power & Light Co. Pennsylvania Power & Light Co. Waters, C., Pennsylvania Power & Light Co. Pennsylvania Power & Light Co. Copyright 1987 Society of Petroleum Engineers Summary. This paper describes an analysis developed to model the inert gas displacement process for evacuating a high-pour-point oil from a long pipeline. The governing equations were derived from the basic conservation pipeline. The governing equations were derived from the basic conservation equations for mass. momentum, and energy. The resultant computer program accounts for such effects as pipeline elevation changes, laminar and turbulent oil flow, temperature-dependent oil viscosity, and heat loss from the oil to the ground. Results of computations for an 84-mile [135-km] residual oil pipeline operated by the Pennsylvania Power and Light Co. are presented and compared with pressure measurements obtained during a trial presented and compared with pressure measurements obtained during a trial purge of the system. Calculations show that the minimum N volume required purge of the system. Calculations show that the minimum N volume required for a successful pipeline evacuation increases considerably with increased delay time. In addition, theoretical results indicate that for this case, the pipeline purge operation must begin within 20 hours of a shutdown to avoid evacuation difficulties. Introduction The development of procedures for shutting down an oil pipeline carrying a high-pour-point oil represents a pipeline carrying a high-pour-point oil represents a complex interaction of technical and economic issues. At ambient temperatures, these oils are virtually impossible to pump. To be moved at all, they must be heated and the pump. To be moved at all, they must be heated and the pipe insulated to reduce heat loss and excessive cooling pipe insulated to reduce heat loss and excessive cooling of the oil as it flows along the length of the pipe. Shutdowns of these pipelines occur periodically during periods of low demand or because of a loss -of power to the periods of low demand or because of a loss -of power to the pipeline pumps during emergency power outages. pipeline pumps during emergency power outages. During such shutdowns, the oil must be evacuated from the line before it cools below the pour point and becomes unpumpable. An unsuccessful evacuation of the oil would result in a clogged line and a very high cleaning bill. One approach commonly used for planned shutdowns is to displace the high-pour-point oil with a high-grade oil that can be pumped easily at ambient temperature. Unfortunately, this approach requires that a large quantity of expensive oil be maintained in storage for use during a shutdown. An alternative is to use a pressurized inert gas to drive a pig through the line to force the oil out. For scheduled shutdowns, this approach may have economic advantages over the high-grade-oil displacement approach. In addition, it is the only feasible technique for emergency situations when power to the pipeline pumps is lost. In such emergencies, liquid N is delivered to the pipeline by truck. The N is then pumped to high pressure, vaporized, and fed into the pipeline to evacuate the line before the oil cools. A number of important questions concerning inert gas displacement relate to the maximum length of time a pipeline can be shut down before initiation of the evacuation pipeline can be shut down before initiation of the evacuation process, the best operational strategy for purging the line, process, the best operational strategy for purging the line, and the minimum amount of N required to do the job. This paper describes an analysis developed for modeling the inert gas displacement process. A description of the analysis is given in the next section and is followed by a discussion of the results of computations for an 84-mile [135-km] residual oil pipeline operated by the Pennsylvania Power and Light Co. Pennsylvania Power and Light Co. Analytical Model The gas and oil volumes are treated separately in this analysis. Because the pig position is a function of time, the sizes of the gas and oil volumes change, making a deformable control-volume approach convenient to use. The control volumes are depicted in Fig. 1. In the subsequent analysis, slippage of gas or oil past the pig is assumed negligible. Gas Conservation Equations. The general deformable control-volume continuity equation is Using an average gas density. This equation can be written as (1) where y is the length of the gas control volume. SPEPE P. 45