As Tunisia has developed economically, the energy-rich North African country transitioned in 2000 from net exporter to net importer of natural gas. The country’s weak currency and rising prices combined to make this very expensive, so it sought to increase local production.
The country’s Nawara project is designed to increase energy supply by adding nine 4-km-deep natural gas wells in the southern Tunisia Saharan desert, a 370-km pipeline through the center of the country, and a gas plant near the central eastern coast. The project will add reserves of more than 10 billion cubic meters, with peak production of 10,000 barrels of oil equivalent (boe) per day, approximately 11% of the country’s estimated gas consumption.
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The overall project is supplied by OMV, with the processing facility and skid-mounted equipment for the nine wellsites provided by Frames. Frames designs, constructs and delivers separation, treatment, control and monitoring systems for onshore, offshore, and floating facilities in oil and gas.
Headquartered in Alphen aan Rijn in The Netherlands, the company has more than 500 employees working in offices worldwide, including one in Houston, site of this week’s Rockwell Automation Process Automation Solutions Group (PSUG) where Thomas de Wolf, Frames product manager for wellsite solutions, presented the session, “Frames flow control and safeguarding creates fully-autonomous wellsite skids thanks to PlantPAx DCS.”
Anatomy of a skid
Skid equipment starts with a wellhead connection and choke valve to reduce the wellhead pressure from 254 to 58 bar(g). Along with an instrumented flowline, it has a hydraulic unit, a methanol injection system, and connections for a sand filter, all handled by an integrated control and safety system.
The components are mounted on a single skid, sized to fit into a shipping container. “Everything that’s required, we build it, ship it down and hook it up,” said Thomas de Wolf, product manager, wellsite solutions, Frames. “We have it up and running in a matter of days.”
Electric power is scarce and expensive in the desert, so along with using energy-efficient pumps and cooling, “The skids are completely solar-powered, with batteries for four days of power,” de Wolf said, as that’s typically the maximum length of a sandstorm. When the sun comes back, “The solar cells are sized to power the skid and recharge the batteries in 17 days.”
The skid is Zone 2-rated for hazardous areas, and the 3,800-Ah, valve-regulated, lead-acid battery set is packaged on a separate skid. “The batteries could be on the skid, but it’s expensive to put them in a hazardous area,” deWolf said.
The daytime desert temperature ranges from 80 °C in sun to 55 °C in the shade, and the cabinet components are rated for a maximum of 60 °C. Cabinet cooling is provided by a passive system—no refrigeration—using heat exchangers inside and outside the cabinet and a heat sink of several hundred gallons of coolant. The liquid cools at night and circulates during the hot part of the day, deWolf said. “It’s not cooled enough to be comfortable, but it’s enough to keep the components below their maximum design temperature.
Control with safety
The control and safety systems cover the well tree, hydraulic power unit, chemical injection, methanol injection, choke valve, fast-closing valve, and fire and gas systems, and communicate via an external RTU with an off-site control room.
The project team chose Rockwell Automation technology for both basic process control and safety instrumented systems (SIS). The basic process control system (BPCS) is a PlantPAx DCS with Remote I/O. SIL 3 safety instrumented system functionality is provided by the AADvance control system. “We chose Rockwell Automation because FactoryTalk faceplates are easy to use, and the company provides support when we need it,” deWolf said. “We do all our own programming, but it’s good to have backup.”
With so much equipment in such a remote location, “The client was concerned about uptime,” de Wolf said. “We need high reliability to meet production requirements.” Communications are designed to prevent single point of failure, everything is designed to be fail-safe and maintain SIS functionality, and HMI software facilitates maintenance and troubleshooting. “We can zoom into any part of the process, see the logic, and see where errors are starting from,” he added.
The AADvance SIS safety tags are used as inputs to the PlantPAx DCS to allow them to display as faceplates. Sequence of events records are stored on SD cards, “So if we go off and back online, the equipment starts up normally,” de Wolf said. In the event of a communication failure, the operator can retrieve the information from the SD card.
The skids are currently in Tunisia, with installation waiting on resolution of local politics, de Wolf said. “They should be on-site in the second half of 2018.”
Along with the capabilities of the PlantPAx and AADvance systems, Frames chose Rockwell Automation for its hazardous area-rated PLCs, its support, and its ability to provide a “relatively low-cost solution for complete well control,” de Wolf said. “Rockwell is also one of few manufacturers that can handle the desert conditions.”
