Monitoring Systems for Multiphase Gas-Condensate Pipelines

Abstract

Abstract A computerized monitoring system has been developed for the 48 kilometer long 20 inch TOGI gas condensate pipeline in the North Sea. The system, which is based on the transient multiphase flow computer code OLGA, provides means for forecasting pipeline behavior, simulating a 4 hour period ahead in time every 10 minutes. The real-time part of the simulator handles algorithms for leak detection, hydrate blockage detection, liquid inventory tracking and monitoring of a number of other parameters. The paper gives details on system design and algorithms used. Introduction Unprocessed gas condensate well fluids have been transported in field flow lines for a number of years. In recent years the technology has been extended to transport unprocessed well stream over long distances with more complex fluids and in cold environments. Long pipelines carrying an unprocessed well stream are potentially prone to severe slugging and hydrate formation. Often such pipelines are designed such that operation must be confined to a narrow operational envelope in order to achieve smooth operation. For multiphase pipelines as for single phase pipelines it is also desirable to have leak detection as well as monitoring of pipeline operational parameters in general. Monitoring of multiphase pipelines is desirable both in terms of safety and flexibility. Monitoring systems for single phase pipelines are commercially available. For multiphase lines, however, the market for monitoring systems is meager. A computerized monitoring system based on the transient multiphase flow computer code OLGA has been developed to carry out monitoring of the Troll Oseberg Gas Injection (TOGI) pipeline in the Norwegian sector of the North Sea. The TOGI pipeline transports unprocessed gas from a sub sea installation at the Troll Field through a 20" pipeline 48 km to the Oseberg Field. There the gas is processed, compressed and injected into the gas cap of the Oseberg reservoir. The gas injection improves the total oil recovery by more than 7% over the originally planned water injection system. The fluid has a liquid drop out of approximately 4.5% (wt) at typical pipeline conditions. Liquid water is not produced, but is condensing out in the pipeline. Figure 1 shows the key parameters for the pipeline system. Production startup is planned for early 1991. System Overview Multiphase Flow Model The flow model applied in the monitoring system is the transient multiphase flow simulator OLGA developed in the SINTEF-IFE Two-Phase Flow Research Project. OLGA is a dynamic one dimensional model for two-phase hydrocarbon flow in pipelines and pipeline networks [3,4]. The model is unified, handling four different flow regimes without user specified correlations. OLGA calculates the transient state all along the pipeline, such as pressure, temperature, liquid and gas flow, liquid holdup, flow regimes, etc. It is particularly well suited to simulate rather slow mass flow transients, and handles well the periodic phenomenon of terrain induced slugging. The implicit time integration scheme allows for long time steps, order of magnitudes longer than those of an explicit method. The model includes a pig tracking function.