By Eric Murphy, ControlGlobal.com Columnist
Anyone in the oil industry will have heard of the Canadian Tar Sands. Alberta is the largest producer of conventional crude oil, synthetic crude, natural gas and gas products in Canada. The providence is best known for its oil sands deposits located in the north, which span over 40,000 square kilometers and contains an estimated trillion barrels of bitumen. That represents a lot of equipment, spread over a vast area that require reliable real-time data monitoring and control. Many companies are using OPC connectivity to meet the challenges associated with coaxing this oil out of the ground.
It Takes More Than a Big Shovel
Major extraction companies use surface mining recovery techniques, however a mere 10% of the Athabasca oil reserve is located sufficiently close to the surface to allow the continued use of economical surface mining methods. The demand for innovative oil sands extraction technology to recover the deeper oil sands has lead to the development of non-conventional techniques such as Steam Assisted Gravity Drainage (SAGD). SAGD combines horizontal drilling with thermal steam injection. A pair of wells is drilled and steam is injected into the reservoir through the top well. The steam softens the tar-like bitumen and enables it to flow out of the reservoir and into the lower well. The product is then moved to the surface using various techniques. Multiple such wells are drilled from a centralized ‘pad’, which collects critical downhole data such as temperature, pressure and production information. This means that from a data collection point of view these extraction sites cover very large geographical regions, up to the hundreds or thousands of square kilometers, much like conventional oil fields.
Not Your Ordinary OPC Server
Manufacturing plants are typically confined to a small geographical area, and their communication methods normally use reliable Ethernet or serial network to connect their DCS and/or PLC based process control. In contrast, telemetry SCADA applications cover huge geographical regions that use satellite, radio, or modem communication, to connect vast fields of RTUs. From an OPC server perspective, the biggest difference between plant and telemetry SCADA implementations is the underlying communication. After all, the OPC interface side is standardized. However, in addition to a well defined interface users need an interface designed for robust, reliable real-time data collection in a distributed environment.
…Then There are the Communication Challenges
Those who have been to the further reaches of the Canadian north know how harsh the conditions can be. Bone chilling cold in the winter, muggy bug infested summers and vast stretches of nothingness. If the production data is not available, a trip out to the well site is usually a bit of an undertaking. Fortunately, there are OPC servers designed to handle the challenges of remote data collection, such as bandwidth restrictions, unreliable communication channels and large device counts.
Hey You, Get Off My Cloud
Low bandwidth communication plagues geographically disperse operations. Due to telephone and telemetry limitations such as distance, weather, radio noise, and other conditions, bandwidth usually remains at a premium. Communication between the SCADA Master and the associated RTUs must be kept at minimum levels. Since OPC clients allow for two OPC read types: cache and device read requests, it is important that the OPC Server be able to differentiate between them. A “cache read” enables the SCADA Master to read the data from the OPC Server’s existing buffer. Therefore these reads do not interrupt the polling sequence of the RTUs. Sometimes the SCADA Master needs immediate access to data, and must issue a “device read.” A device read causes the OPC Server to stop its regular round-robin polling sequence and retrieve data from the selected RTU. Well-designed OPC servers will differentiate between cache and device reads and save the SCADA system from performing resource-hogging demand reads. This functionality enables the OPC server to implement priority of operations, which are especially important when timely data is required, such as for AGA (American Gas Association) data collection requirements.
Lost in Translation
Noisy communication is a fact for many telemetry operations. Invalid packets commonly occur and radio squelch will occasionally let extra bits slip into the data packets. An OPC Server designed for telemetry operation will recognize and differentiate among various invalid messages and either ignore extra bit or request the data message again. Robust OPC products will have a dynamically configurable number of retries, so that users can specify the value that meets their specific situations.
Unlimited Wants, Limited Resources
A large number of RTUs is also a signature aspect of telemetry applications. While plant-based drivers rarely connect to more than a handful of PLCs simultaneously, SCADA servers frequently connect to tens or even hundreds of RTUs. Telemetry SCADA servers typically share a single connection amongst many RTUs, resulting in a connect-read-disconnect operation. The OPC server must be very efficient in the connection and disconnection procedures. As well, they must adhere to a dynamic polling sequence that enables them to read data from all the devices in a round-robin polling sequence, yet still be able to interrupt the sequence with a high priority write operation. Once the interruption is complete, the OPC server should resume its regularly scheduled polling sequence. Since OPC uses a UTC timestamp to indicate the last time that a given value was read, OPC client applications can easily tell if they should interrupt the polling sequence to refresh their data, or if their data is at an acceptable age. This reduces the number of interruptions to the polling sequence and reduces system load.
As the how and where of oil exploration continues to evolve, it’s comforting to know that OPC continues to offer the same standardized interfaces. Meanwhile, trusted OPC vendors continue to evolve product offerings to ensure that the requirements of the end users are met, even in areas the OPC specifications cannot mandate. Robust, flexible connectivity solutions that leverage OPC standards can be found everywhere. Even in the harsh environments of the Canadian north.