Oilfield skids are portable steel pallets with customized frames that are built to hold portable packages of machinery and related equipment. These are then carried by truck or vessel from one onshore well or offshore platform to another, as needed by producers.
Upstream, skid-based production applications share many traits and design challenges, mainly:
- Compact local control and monitoring, with redundancy for high availability
- Flexible options for standards-based sensor and remote connectivity
- Class I, Division 1 safety compliance and system ruggedization
Most of all, they must meet or exceed customers’ requirements cost-effectively to fit within their increasingly constrained project budgets while yielding a healthy profit for their developers. That’s especially critical these days, given that the price of oil has fallen by 45 percent since June, 2014.
Siemens Totally Integrated Automation: Core to a skid-based, offshore mixing application
To illustrate how the Siemens TIA architecture and components can be combined into an upstream skid-based application, consider one designed, engineered and built by Solid State Automation and Controls (SSAC), a Houston-based Siemens Solution Partner.
It’s a relatively simple, single-function point solution used on a major producer’s remote offshore platform in the Gulf of Mexico to lower the viscosity of oil output by mixing a lighter density oil with heavier oil. This reduces the friction in the blend to improve its flow through existing pipelines to land-based refineries.
The application uses the following Siemens TIA components from the Siemens SIPLUS line of products, which features additional ruggedization for the SSAC customer’s offshore requirements:
- S7-1200 programmable logic controller (PLC)
- MP377 human-machine interface (HMI)
- ET200M distributed input/output (I/O)
- 24VDC/120VAC power supplies
All are contained within an explosion-proof NEMA-7 enclosure that SSAC specially designed and engineered for this application.
Because skid-based oil and gas applications have specific, limited functions, their I/O needs are usually small. This mixing application had less than a dozen points of I/O. Control was needed for just two intake valves in a large sampling vessel containing the different oil viscosity types, timers and an outlet valve. The application also had to monitor the water content and density in the oil mixture and oil flow, the latter using a flow meter with a high-speed counter that provides a feedback loop to the control valves.
Although a Remote Terminal Unit (RTU) could have worked, SSAC chose the Siemens S7-1200 PLC as a compact, more cost-effective alternative for two reasons:
1. Price-performance and overall value. While an RTU may cost less than the S7-1200, its price-performance couldn’t compare. The application would have needed extra functionality—including components, engineering and code—to work, but all that is standard in the S7-1200.
2. TIA Portal. SSAC was able to streamline its systems design and engineering by as much as 30 percent, because its engineers used the Siemens TIA Portal, the integrated, easy-to-use engineering framework with drag-and-drop programming capabilities. That helped ensure the project’s profitability.
Flexible options for standards-based sensor and remote connectivity
SSAC’s application had to pull several data points off a flow sensor to feed back into the PLC to control the system as well as to be transmitted onshore to its customer’s SCADA-based DCS.
Fortunately the S7-1200 features native onboard Modbus RTU and Modbus TCP. The former provides backward communications compatibility with legacy RS-232 or RS-485 systems, while the latter can be used for communicating over industrial Ethernet. In addition, the S7-1200 can communicate via PROFIBUS or PROFINET.
To transmit flow sensor communications, SSAC needed to go a step beyond the S7-1200’s native communications capabilities and use the HART (Highway Addressable Remote Transducer) communications protocol with 4-20mA analog instrumentation wiring.
For this, its engineers coupled the S7-1200 PLC with an ET200M rack of I/O featuring HART cards from the S7-300 family. In turn, this converted the transmitter’s signals from the HART protocol to PROFIBUS, which could then communicate with the PLC.
To connect this back to the customer’s DCS system, SSAC had a wide range of wireless communications options from Siemens SCALANCE modules. However, because the offshore platform already had an uplink to the Iridium satellite constellation, SSAC hardwired the Ethernet ports on the S7-1200 PLCs via Modbus TCP to the platform’s network. This let the control data be transmitted back to its customer’s data collection point.
Class I, Div 1 safety compliance and system ruggedization
Because the application was for an offshore oil and gas production platform, SSAC had to put its controls in an explosion-proof enclosure, in compliance with the Class I, Division 1 safety codes of the American National Standards Institute (ANSI) and the National Fire Protection Association (NFPA). With the proper ATEX approved enclosure the same design could be used in European or Middle East markets for similar hazardous zones.
To meet this requirement, SSAC used an aluminum NEMA-7 enclosure with an explosion-proof window for the control components, including the Siemens MP377 HMI with touchscreen.
Of course, with a touchscreen HMI behind an explosion-proof window, operators needed a way to interact with it, so SSAC installed an intrinsically safe track pad outside the enclosure. This delivered the required interactivity at a fraction of the cost of a full Class I, Division 1-compliant PC system, all in a much smaller package than what could have been achieved otherwise.
Conclusion: Siemens TIA helps ensure profitability of skid-based applications
Because skid-based applications deliver specific but limited functions, they are inherently custom solutions that don’t suit mass manufacturing. As a result, their development will continue to be dominated by smaller solution providers, for whom profitability is critical to business survival. That’s why developers of skid-based applications should align their development efforts with a financially strong source of proven, interoperable components based on open standards and designed with durability and reliability in mind.
For more information on how Solid State Automation and Controls built this skid-based mixing application for upstream production on offshore platforms, click here for a six-page white paper with greater detail.Have an Inquiry for Siemens about this article? Click Here >>