Oil and gas industry metering and control system

Remote gas well sites require local flow metering of the mineral reserve's gas flow rate for asset and revenue accounting purposes. Differential pressure flow meters are commonly used for this purpose.^[1]

Traditionally, local flow metering was done using high maintenance mechanical circular chart recorders. Today these have been replaced with electrically powered remote terminal units (RTU). Multiple remote terminal units communicate to a central host system via radio systems or tap into the cellular phone system. The host (or central control room) communicates with the RTUs and radio communication infrastructure is referred to as a SCADA system, i.e., supervisory control and data acquisition. In addition to gas flow metering, the RTU also provides some local, self-contained, site control such as well shut-in in the event of abnormal gas flow conditions. However the bulk of the RTU's function is to provide the host with site specific data which includes the above mentioned gas flow metering data, plus process related data including pipeline and/or well pressure, condensate tank levels etc.

Not all RTUs are the same

The distinguishing feature of the remote gas well RTU is a "gas flow computer". This is essentially a sophiscated subprogram or flow computer algorithm. The oil well RTU is much less complex since liquid oil flow metering is simpler. In the case of electrical utility RTU, there is a critical need for precise event time monitoring so that the first breaker to trip (closest to the fault) can be determined. As these sites are remote such that utility electrical power is rarely available, the RTUs must be powered by an array of solar panels plus batteries. The batteries enable the RTU to continue to function when the sun is below the horizon or is obscured by heavy cloud cover. These RTUs also must be extremely rugged and energy efficient, i.e., must function to −35 °C. Energy efficient RTUs vastly reduce the cost of the solar battery system especially at higher latitudes.

References

1. ^ Walt Boyes (2003), Instrumentation reference book, p. 9, ISBN 9780750671231, http://books.google.com/?id=sarHIbCVOUAC