01 May 2004
Fiber solution gases up
Data acquisition drives fiber-optics needs.
By Jonathan Pollet
Imagine being the vice president of operations for an oil and gas exploration and production company. Having just finished a major acquisition of several oil and gas producing fields, your task is now to determine what assets your company just bought and how to effectively operate these fields, which four separate owners operated before your purchase.
At this point, data is your friend. You need to know how much oil and gas the company is producing from these fields, and you need to know how to allocate oil and gas sales back to each well to make decisions about pulling wells and performing maintenance on your new field assets. With more than 1,000 wells, nearly 50 metering stations, several gas scrubbing stations, and oil and water treating plants, acquiring data from these facilities will not be easy. This task is especially difficult when the facilities have instrumentation and hardware from different manufacturers. Your first instinct is to deploy people on the ground to start gathering data by hand. You quickly realize this manual process with five operators and two office clerks is costing you a lot of money, and you decide there must be a way to collect the data automatically. A Houston-based company faced just this situation, and this is when they called in supervisory control and data acquisition (SCADA) analysts to present a solution to the problem.
The first step in designing a SCADA system is to determine what level of automation the organization has already applied in the fields and what manufacturer's equipment is controlling the processes. The second step is to find the best communications medium for connecting these intelligent field devices to a centralized SCADA system in the field office.
The company needed to find an appropriate medium to acquire data from multiple types of flowmeters, programmable logic controllers (PLCs), and other intelligent end devices. After considering various spread spectrum radio vendors, cellular digital packet data (CDPD) modems, and other communications options, we decided fiber optics would give us a consistent communications medium and allow for data transfer over long distances without any data interference. We applied different data protocol adapters to the ends of the fiber runs, so specific SCADA drivers could work with the existing hardware already running in the field.
Although there have been multiple practical applications for fiber optics since the 1960s, optical fiber technology has actually been available since the 1920s.
Some of the first commercial applications for fiber optics were in the medical industry. Because fiber is very flexible, has a small diameter, and can transport light to illuminate various parts of the body, it became a natural choice as a communications medium to bring data and images back to the doctor for analysis. Fiber has also seen use in the medical industry as a transport medium for laser surgery.
Because optical fibers are not sensitive to electromagnetism interference and radio-frequency interference, they are a reliable transportation medium for voice, telecom, and data services. Fiber optics is also highly suitable for military, communications, and industrial control applications where high signal quality, secure data transmission, and survivability are critical.
The list of applications for fiber-optic technology for the industrial controls and commercial industry continues to grow daily. The industry is using fiber optics for level sensing, part inspection, down-hole oil and gas drilling, and process control applications.
We selected fiber optics for this project for several reasons—including fiber's insensitivity to interference and its ability to transport virtually any type of data protocol over long distances. We also wanted to achieve a high level of data throughput without packet loss. We found fiber-optic technology provides the most consistent and reliable communications medium for SCADA applications.
SCADA and systems integration engineers assume fiber costs are too expensive, and they often turn toward wireless options such as spread spectrum radio, wireless Ethernet, and satellite for providing the communications link to field devices. However, if you look at the total ownership cost of fiber optics versus other communications media over a five-year period, fiber's ability to deliver consistent data throughput with virtually no packet loss or system downtime makes it an attractive choice. The costs involved with purchasing and installing fiber-optic materials have also reduced significantly over the past five years. Usually, if the remote field devices are less than 10 miles from the SCADA server or if they are aligned in a pattern where one fiber bundle run can pick up multiple facilities, fiber is a good choice.
Relighting existing fiber
Much of the data the company collected by hand resided in remote terminal units (RTUs), PLCs, and electronic metering systems. All field equipment we needed to monitor was already installed. These controllers were all within a 7- to 10-mile area of the local field office. Upon further inspection, we found an old abandoned fiber-optics cable bundle that passed near several of the well testing stations. Technicians verified the integrity of the dark fiber and were able to relight several pairs of fiber to use for data collection. This saved the company money, because we were able to reuse existing fiber installed for phone and data services years ago.
The PLC devices located closest to the main fiber run were Allen-Bradley SLC 505 model types with an Ethernet RJ45 port on the front of the PLC. We used a fiber-to-Ethernet converter from Black Box to establish a 10-megabyte TCP/IP connection from the PLCs back to the control room—where we had installed the central field SCADA server. The team connected additional PLCs by running industrial Ethernet cable from the fiber break-out boxes. Because you can use CAT5 Ethernet cable to connect any IP-based device within 300 feet, we used industrially rated CAT5 Ethernet cable to branch out from the fiber termination points to pick up other devices located close by.
Copper network extension
On another area of the field, we had installed an RS-485 copper network to connect eight Daniels flow computer devices. We then used a Sierra Air Card CDPD modem as the communication link back to the field office.
Near this set of flow computers were additional well testing stations with Allen-Bradley SLC 505 PLCs. A Modicon Quantum PLC with an installed NOE Ethernet communications module controlled a gas scrubber station nearby.
The distance between the flow computers, Allen-Bradley PLCs, the Modicon PLC, and the field office was about 2,500 feet, and because the PLCs had Ethernet TCP/IP communications capability, we wanted to design a communications medium that could send RS-485 serial data as well as Ethernet data. After conducting a feasibility study and walking the area taking photos, the team was able to design a fiber-optics line that we could hang on existing telephone poles. The company ended up using aerial fiber because of the low installation costs. We hung a six-pair fiber cable with thick insulation, and it arrived at the field office on a large wooden spool with the guy wire already attached to the fiber. After installing the aerial cable on the telephone poles, we dropped the fiber into the control room and terminated one end in the control room, and the other end out in the field in an environmentally enclosed panel.
Because there were six pairs of fiber, the company ended up using one pair to transmit the RS-485 signal. We used a fiber-to-RS-485 converter on both ends of the fiber run. We then snapped an RS-485-to-RS-232 converter onto the control room side of the RS-485 network. This way we could connect the RS-485 network to the RS-232 serial COM1 port on the back of the SCADA server.
The company used another fiber pair to transmit and receive Ethernet TCP/IP data for the PLCs. We again used a fiber-to-Ethernet converter on both sides of the fiber run to break out the TCP/IP data onto RJ45 CAT5 copper cables. We used an industrial hub to allow packet switching on the field side of the fiber run, and we ran individual CAT5 copper industrial-grade communications cable to each Ethernet-based PLC device. A Wireless Ethernet radio system picked up additional flowmeters, PLCs, RTUs, and other intelligent end devices that were not physically installed close to the path of the two fiber runs.
By relighting previously installed dark fiber and using aerial fiber in other places to create a long-haul communications channel for RS-485 and Ethernet data, we created an independent oil and gas producer and were able to cost effectively design and deploy a new field operations SCADA system.
One central SCADA system allowed the local operations team to control and monitor all field equipment with one standard interface, and it was transparent to the user whether the end device was a Modicon PLC, Allen-Bradley PLC, Totalflow gas flowmeter, Westing-house PLC, or a Daniels flowmeter.
The project was economically sound because we took advantage of existing communications infrastructure when designing the communication networks. Fiber optics allowed us to send multiple types of protocols and data through the same type of medium, and the SCADA system had the appropriate industrial automation drivers to allow IP-based Ethernet and RS-485 serial communications through fiber networks.
Behind the byline
Jonathan Pollet is chief executive officer at PlantData Technologies in Houston.
Turbidity and fiber optics
By Don S. Goldman
Intermittent turbidity due to filter breakthrough, onset of crystallization, or solubility problems may adversely affect a manufacturing process and product quality. In many applications, it is important to have a sensor online to continuously detect the presence or absence of turbidity rather than to quantify its value. These sensors must operate in harsh conditions such as in high temperatures or in the presence of strong acids or bases. Online turbidity sensors must be robust and easy to maintain. An important parameter for the turbidity sensor is the ability to clean and calibrate the analyzer without having to shut down the process. You use an in situ backscatter fiber-optic probe in the turbidity sensor that allows direct insertion of the probe into the end of a thermowell.
Turbidity is the cloudiness or haziness in fluids that carry a high content of suspended solids, bubbles, or secondary liquid phases. It is an important quality parameter in water treatment plants. Turbidimetry is the quantitative measurement of the amount of turbidity in a sample. Turbidity sensors typically use light in the visible region of the electromagnetic spectrum. These analyzers see use in a transmission mode or in a reflectance, light-scattering mode. The transmission mode analyzers detect the sum of all light effects, including color variations, while the reflectance analyzer only detects the scattered light due to the suspended solids in the sample. The three main types of turbidity meters are forward-scattering, perpendicular-scattering, and backscattering sensors.
The turbidity sensor features a simple, rugged, low-cost design for online turbidity measurements. The optical bench of the sensor is packaged in a nonpurged enclosure that meets Class 1, Division I, Groups B, C, and D National Electric Code electrical area classifications. The turbidity sensor is a backscattered light turbidimeter. The backscatter fiber-optic probe contains launch and return fibers and an embedded spare fiber. A light-emitting diode that has high output and a long service life is used as the source of visible light in the optical bench. The light from the source is focused onto an optical fiber that sends the light to a special backscatter fiber-optic probe. The optical fibers in the probe are angled to penetrate deep into the liquid sample. The light from the source passes through a sapphire window at the end of the fiber probe, and the light impinges the surface of the sample. The presence of suspended solids will cause light scattering. The scattered light launches onto a return fiber-optic cable. A detector in the optical bench of the sensor measures this transmitter light. The analyzer module has a liquid crystal display and a push button to select display functions; analysis results are available via 4–20 mA output signals from the analyzer.
Two examples of turbidity sensor applications are filter breakthrough and onset of crystallization. Using filters to remove suspended solids is common in the water treatment, paper, and chemical process industries. You don't want turbidity in water quality plants. Typical causes are presence of mud or clay, biological growth, or insoluble chemicals. Turbidity can also be an indicator of health hazards in potable water and can cause mechanical damage to processing equipment. A common standard unit for 90-degree off-axis, light-scattering turbidity measurements is the nephelometric turbidity unit (NTU).
Onset of crystallization detects the presence or absence of turbidity rather than quantifying its value. The pharmaceutical industry commonly uses batch processes for the production of tablets, and the turbidity backscatter probe sensor can detect the onset of crystallization in processes such as aspirin tablet production.
Behind the byline
Don S. Goldman is president of Optical Solutions, Inc. in Roseville, Calif.