REMOTE UNITS ELIMINATE UNPRODUCTIVE PUMPING

Sept. 3, 1990
Henry R. Swartzlander Texaco USA Midland, Tex. Sucker-rod pumpoff controllers (POCS) based on available off-the-shelf remote terminal unit (RTU) components have been developed and installed by Texaco USA in its Midland, Tex., producing area. Market availability of off-the-shelf pumpoff controllers in the early 1980's was limited. However, the fundamental building blocks for a general RTU-based system were readily available.
Henry R. Swartzlander
Texaco USA
Midland, Tex.

Sucker-rod pumpoff controllers (POCS) based on available off-the-shelf remote terminal unit (RTU) components have been developed and installed by Texaco USA in its Midland, Tex., producing area.

Market availability of off-the-shelf pumpoff controllers in the early 1980's was limited. However, the fundamental building blocks for a general RTU-based system were readily available.

RTU DESIGN

In 1982, Texaco USA initiated a study of producing field automation in its Midland producing division. As a result of the study, all facets of one producing field were automated.

Based on this study, Texaco's Midland producing division and the Houston information technology department developed a sucker-rod pumpoff controller from offthe-shelf RTU components.

The driving force behind the development of the rodpump monitor controller was the need for well-site information at a central location. This same general RTU can be used for injection control, automatic well testing, tank battery monitoring and control, and electric submersible pump control.

Texaco's rod-pump monitor controller (RPMC), an RTU, eliminates pumping a partially filled downhole pump (Fig. 1).

The RPMC also has the capability to detect malfunctions and take the appropriate action. Logging of events, storage of events with time of occurrence, and control are all done by the RTU at location.

The RTU can function in a stand-alone mode or report to a master terminal unit (MTU) via radio or hardwire.

When the RPMC detects that motor current, load, and position signals are valid and in range, the RPMC will produce a dynagraph and control the well by making pumpoff decisions. The unit will also construct an amperagevs.-position curve.

Other well-site end devices can be interfaced to the RPMC. Such devices can include flow line pressure monitors or production-rate meters. The correct choice of control can easily be incorporated into the software.

Well-site information and history are stored in the RPMC. In a stand-alone application, these data can be accessed with an IBM compatible laptop computer and Texaco's portable input/output (PIOU) software.

For facilities with central communication, the historical data are periodically uploaded to the MTU. The PIOU is designed to give on-location data, dynagraphs, and parameter programming capability.

The RTU is able to shutoff the motor on the pumping unit and match well bore inflow with production equipment capacity. The RPMC can control wells at rates of 4-25 spm.

Upon nonfatal failures of an end device or loss of control by the RPMC, the RTU will automatically switch to the historical percent run mode. Fatal failures such as a rod parts breakdown will shutoff power to the motor.

The load is measured with a load cell mounted on the polished rod between the carrier bar and clamp (Fig. 2). Strain-type devices can be used if qualitative cards are acceptable.

The position information can be acquired from either potentiometer or capacitance devices. Recent developments in the clinometer have made position measurement possible with no moving parts.

The control of the output is accomplished with a relay wired to the existing motor-control panel.

A liquid crystal display clock for actual run time is displayed on location.

The power supply is driven off the 480-y supply in the motor controller and is used to charge a 24 amp-hr battery that provides backup power for 24 hr with communication to the host unit.

Health lights are also provided on location for pumper information.

SOFTWARE

Texaco's information and technology department in Houston was responsible for the development of the software algorithms. Code was written based on logic tables supplied by Midland.

The RTU is built on a generic platform and given personality with erasable programmable read-only memory (EPROMS). The primary nontrivial decision is the pumpoff criterion.

Texaco's control algorithm is based on integration of downstroke data from top of stroke to minimum load on the downstroke. As pumpoff occurs, the traveling valve opens later in the stroke and the integrated area increases. The amount of increase required and number of consecutive pumpoff strokes required for unit shutdown are selected by the operator.

Other input actions are also operator controllable, and default values are typically sufficient for initial setup. Fine tuning the system for a given well or inflow situation is possible, yet the default values have been found to be very satisfactory for setup and control.

Limited well-site input makes installation far more efficient and does not require a large amount of technician time.

HARDWARE

The original prototype was built on the TI 8640 base RTU. The TI 8640 consists of a mother board (MO56) and a termination or interface board (TO51). The microprocessor resides on the mother board.

In 1986 Texaco constructed a custom interface to replace the TO51 and produced 400 units. After extensive field testing and analysis in 1988, the interface was modified to drive an Analogic mother board, and 600 units were produced in this configuration. Texaco has recently licensed the software and fabrication to Barton Industries.

The primary function of the termination board is to filter and protect the mother board. The two boards are connected through dual inline, pin-and-socket connectors. This provides for convenient replacement of either board.

All field wiring for input and output terminates on the TO51 board. The combination assembly is rated for temperature range of operation from -20 to +70 C.

POWER REQUIREMENTS

The 8088 microprocessor is built on complementary metal oxide semiconductor (CMOS) technology which limits the required power. The on-board lithium battery will provide 800 hr of RAM backup.

Texaco utilizes 480-y ac power to supply its typical RPMC installations. The maximum power requirement for the RTU and radio is 5 w.

Solar panels are possible but unnecessary on electrified pumping unit locations. The 480-y supply is transformed and rectified in an offthe-shelf battery charger system.

A storage battery is provided to support communications for 24 hr during power interruptions. The volatile memory also has a battery backup and can remain static for 800 hr without loss of information.

INPUT/OUTPUT

The 8640 can be fully populated to accept 4 high-speed accumulators, 8 analog inputs, 16 digital inputs, 2 analog outputs, and 16 digital outputs.

Due to individual unit requirements, the termination board (TO51) is not being fully populated. The current configuration for population is two high-speed accumulators, four analog inputs, eight digital inputs, two analog outputs, and four digital outputs.

Reduced population on the interface board and careful selection of the minimum mother-board memory is an excellent cost control measure. Analog inputs can be used to measure load, position, current, temperature, pressure, flow, and voltage.

High-speed accumulators or pulse inputs may be used to measure flow from turbine or kilowatt-hour meters. Digital inputs are used for status knowledge, i.e, yes/no or on /Off.

Texas Instruments provides a communication protocol that Texaco declined to use. Texaco's communication protocol was written in-house and has proven to provide very effective communication.

OPERATING SYSTEM

Several operating systems are available. Texas Instruments supports the RTX multitasking operating system. Texaco selected the Intel RMX operating system for flexibility of usage on various platforms.

Unlike the RTX operating system, the RMX allowed using the Analogic mother board.

The typical scheme for code development is to write the program in C or some other high-level language, assemble the code, download the program into memory, and debug the program.

After field testing the first all TI prototype version, a custom interface board was developed to provide additional gain for particular end devices and amplification of weak signals. This essential step allows the RTU to control virtually any 4-20 ma end devices.

CONFIGURATION

Texaco had 400 units built with the hybrid board configuration. Long-term testing of the custom interface indicated some drift due to ambient temperature variations. The drift became troublesome and prompted a redesign in 1988.

The 1988 version includes Analogic's mother board instead of the TI platform. The interface board was reengineered with simplicity and temperature stability as constraints.

Six hundred of the new interface boards were then manufactured, assembled, and field installed with the Analogic mother board platform. Installing lower gain series amplifiers on the load cell input has relieved the unit of all temperature-related problems.

Despite the excellent performance of the Analogic board, the agreement between Texaco and Barton Industries returns the unit to an all TI platform with load and position signal amplification.

The load, current, and position end devices are constrained to 4-20 ma. Discrete digital information is accepted from the hand-off-automatic (HOA) switch, flow line pressure switch, motor status, motor-panel torque status, and battery charger status. Additional status points are easily incorporated into the system.

FABRICATION AND INSTALLATION

Before Texaco's agreement with Barton, the units and interface were fabricated by a small electronics plant. The Barton-manufactured units are available outside of Texaco. This was essential for maintaining support and field service.

Within Texaco, its own personnel are doing the installation of the units. The required setup is minimal and other than end device installation, main enclosure mounting, and electrical tie-in to existing motor contactor, no operator input is required.

Because the unit can be set on location and powered up to control the well without any operator input, well testers are available for their normal duties.

Personnel can return to location and fine-tune the system when convenient.

PROBLEMS

Numerous problems were encountered in the development of the RPMC. Load cells from some suppliers were discovered to drift with temperature and totally fail in rain storms. Also, pulling-unit crews had to be educated in the proper handling of load cells.

The temperature drift was complicated by instabilities in the amplification that accentuated the load variance. Due to the integration of area under the downstroke in the POC algorithm, drift induced premature pumpoff decisions.

Filtering on the termination board was a simple item to correct once the inductance and capacitance input circuits were analyzed.

This kind of repair is labor intensive for field personnel, and it was compounded by the remoteness of the RTU locations.

POSITION DETECTOR

The search for a maintenance-free position transducer will probably end with the capacitance-type devices (Fig. 3). One-shot position detectors are simple, yet card distortion has been observed on conventional units, and variation in upstroke and downstroke times will be enhanced in modified geometry units. The clinometer currently in use is a variable-capacitance device free of moving parts (Fig. 4).

Field testing shows this fluid-filled device is not adversely affected by temperature extremes.

BATTERIES

In one prototype, the hydrogen gas output from an overcharged battery was ignited by a relay operation. The cabinet door on our "homemade bomb" was located approximately one-half mile from location. This prompted the removal of the system battery to a separate enclosure (Fig. 5).

Battery technology has made available batteries that do not produce gas.

Gas is of significant concern as most battery chargers typically fail in the overcharge condition. The common storage devices will produce hydrogen in an overcharging situation.

Hydrogen and oxygen in the presence of a switching device can be hazardous with explosive potential. With careful battery selection, it is now possible to safely use a single enclosure for both components.

HOST-BASED SYSTEMS

The heart of the economics for production automation is the data collection and reporting capabilities. Field personnel do not have time to examine in detail every card generated. In 24 hr a field with 200 producing wells running at an average of 8 spm will generate 2.3 million dynagraphs. Even to scan a summary of the individual cards is an impossible task.

Error report alarms are a convenient method of culling. Texaco's 6 a.m. scan is designed to report wells with potential problems.

As personnel arrive at the field office each morning, they can initiate diagnostics from the host machine and request on-site inspection if necessary. This option is the number one method of early rod-pump system failure detection.

In the future, expert systems in conjunction with downhole transformations may be used to summarize this massive volume of data.

Stand-alone pumpoff control is a good application in small primary recovery fields. Dynamic inflow in secondary units is usually associated with larger numbers of units and therefore telemetry can be justified.

Historically, energy savings for reduced run time has been exaggerated and should be considered on a case-by-case basis. Finding sucker-rod pump and rod failures more quickly is the most significant benefit of Texaco's POC installations.

REMOTE TERMINAL UNITS

RTUs in conjunction with programmable logic controllers (PLCS) are the natural economic control devices for the nineties.

The distinction between RTUs and PLCs has continued to diminish. PLCs are now capable of higher-level language and can be programmed with more than ladder logic. PLCs will be more economic in applications requiring large amounts of input/output and situations where growth and contraction of the accessed peripheral information is likely.

Texaco's next application for the RTU used on the RPMC project is for water and CO2-injection control. Water-injection control is simplified by using a turbine meter and a high-speed accumulator to totalize accurate water volumes.

CO2-injection control and calculations are far more complicated than simple pulse counting. A disturbance in the line will be used to create a pressure drop. American Gas Association (AGA) calculations will be used with the delta pressure, a wellhead pressure, and flow stream temperature to determine actual volume going down hole.

The RTU can control either pressure or rate for both water and CO2. Total injection volume in CO2 applications is critical to avoid premature breakthrough.

In this CO2 control application, it is possible to configure the mother board (MO56) with a 8087 math coprocessor. This will allow square root trigonometric functions and logarithms. The RPMC has not required the math coprocessor, but AGA calculations for the CO2 injection controller will necessitate exponential fractions.

Use of an 8087 will enhance the performance for some mathematic routines by a factor of 500. Typical sampling rates on digital inputs and analog inputs are in the 2 ms range. High-speed accumulators are continuously available for pulse inputs.

Even with the advantage of a host-based MTU and communications, provisions must be made for information access at the well site. Some pumpoff control manufacturers have incorporated a small LCD display in the RTU panel door.

Texaco recognizes the need for well-site information and has simplified the nonmachine accessed data to two health lamps and an LCD clock. This allows the pumper to reset the clock and observe the run time since last reset. The health lamps are color coded red and green.

The green light indicates a healthy POC that has passed all internal checks and has no alarms set. The red light is to indicate that an alarm has occurred. Well-site communication and programming is done with a portable PIOU. The PIOU can also display dynagraphs and store data for future analysis.

The communications to the RTU from the PIOU is on RS232 to a modem. Several versions of the code for the PIOU are running on IBM-compatible machines.

FUTURE TRENDS

The RTU will continue to be a basic building block for remote locations with limited control-point requirements. The PLC with high-level language capability will become more applied and user friendly. The most logical place to acquire cost-effective automation in producing fields is at the production header.

With early warning devices at the manifold, one machine may collect data from many end devices and decrease capital expenditures.

Automation in the recent past has been hampered by physical separation in producing fields. Plants and refineries have observed more economic benefit per dollar expended.

As personnel constraints and producing well economic limits conspire, the upstream industry will use radio telemetry and consolidate remote information to field offices.

Copyright 1990 Oil & Gas Journal. All Rights Reserved.