Case History: New Gas Flow Computer Design Facilitates Offshore Measurement In Gulf Coast Project

Abstract

Electronic gas measurement has evolved from providing a simple replacement for circular chart recorders to including requirements for instantaneous remote access of flow information. Coupled with this capability is the need to have access to this same information in multiple locations simultaneously. With existing equipment unavailable to meet these requirements, one operator and manufacturer allied themselves to develop a new flow computer. This new flow computer incorporates features to meet not only the specific and unique requirements of the operator's communication system, but also provides adaptation for future requirements. Installation of this new flow computer has resulted in significant operating cost reductions for the operator by requiring fewer trips to offshore locations and providing instantaneous flow information for engineers, technicians, and gas accounting personnel. Introduction Electronic Gas Measurement (EGM) systems have grown in popularity in recent years due in large part to their ability to provide both on-site viewing of real-time data and historical recording of the same data. The typical EGM system (Fig. 1) has been used to replace two- and three-pin chart recorders. hese systems were required to input physical parameters such as differential pressure, static pressure, and temperature to produce a volume total and calculated instantaneous flow rate. In addition, EGMs were capable of recording flow information to be retrieved at a later time. However, EGM systems have been increasingly required to contain many additional features normally associated with remote terminal units (RTUs) and programmable logic controllers (PLCs). Requirements related to communication within a wide area network are often weighted evenly with measurement and recording capabilities when evaluating new EGM systems. These systems must be configurable, both with regard to normal EGM functions and expanded communication system functions, yet easy to operate. These requirements, brought about by the increase in use of Supervisory Control and Data Acquisition (SCADA) computer systems, has shifted the role of the EGM device from simple chart recorder replacement to full-featured communication equipment. In addition, the SCADA systems themselves have advanced to include a multitude of communication media options ranging from simple hardwire to microwave. Prior to the introduction of flow computers, differential pressure, static pressure, temperature, and flow-rate data was interfaced to SCADA systems by connecting analog voltage or current outputs from a transmitter to analog inputs in a PLC or RTU, which was then connected to the SCADA system. Even with the introduction of flow computers, volume totals were often interfaced to a SCADA system by connecting a pulsed digital output, with the number of pulses corresponding to a scaled version of the total volume, to a pulse input in the PLC or RTU. This setup tends to be both costly to install and difficult to maintain. In addition to the added SCADA support requirements, new EGM devices are expected to incorporate features that allow their use in many more varied applications than the early flow computers. New EGM devices are expected to retain the key features of traditional EGMs but allow for applications such as liquid measurement and storage in order to incorporate them Into well test and allocation applications.