Comprehensive modernization of process control systems at gas storage facilities can increase gas-transfer flexibility and shorten response times, even with existing field equipment. Wingas undertook such upgrades on the largest underground gas storage site in Europe: Rehden in Lower Saxony, Germany (Fig. 1).
Background
Until recently storage facilities acted purely as a compensating buffer between producer and consumer, making up for seasonal fluctuations in demand for natural gas. During cold months of the year, European demand can rise to 10-15 times its summer level. Extraction of natural gas, however, is a continuous process, transport via high-pressure pipelines is more efficient without interruption, and import quantities are often agreed for long terms.
German domestic production of natural gas barely covers one-fifth of its demand, which continues to rise, increasing dependence on imported gas. In this context, natural gas storage facilities help ensure a permanent supply of energy. The ongoing liberalization of Europe’s natural gas market has heightened the role of storage facilities in terms of energy policy; non-discriminating access to the gas supply network requiring access to storage facilities.
Free access for third parties and increased natural gas spot and futures trading require the supply of large amounts of natural gas at short notice, creating a new situation for storage operators and requiring increasingly flexible approaches in running their facilities.
Rehden
More than 600 operational natural gas storage facilities operate worldwide, with a working capacity of about 340 bcm.1 Roughly 25% of the storage facilities are in Europe, storing more than 60% of the world’s total working gas.1 Germany is the largest storage nation in Europe and the fourth largest in the world. The Rehden natural gas facility is about 60 km south of Bremen. Wintershall AG discovered and developed the original field in the 1950s.
Rehden field produced well into the 1990s. A large part of the main dolomite formation now stores natural gas (Fig. 2) transported to Rehden via pipeline. About 4.2 billion cu m (bcm) is available as usable working gas, enough to supply roughly 2 million single-family homes for an entire year.
The natural gas storage facility covers 8 sq km in Rehden, Lower Saxony, Germany. Gas is stored at roughly 2,000 m (Fig. 3).
Wingas GMBH & Co. kg operates the Rehden storage facility (Fig. 3, box). The company is a joint venture with Wintershall Holding AG owning 50.02% and OAO Gazprom 49.98%.
Process control
Maximum natural pressure of the Rehden gas reservoir is 280 bar. Injection therefore requires operating pressures between 110 bar and 280 bar, achieved with five gas-operated turbines and two electrically powered compressors. Start-up included installation of a Linux-based Aprol Scada with a cascading and redundant method of operation. The supervisory control and data acquisition system (SCADA) also monitors probes for injection and offtake processes and ensures adherence to safe pressure levels, temperatures, and other operating values. Proprietary programmable logic controllers monitor the compressors.
When changes in the gas market required more flexible operations management, Wingas hired automation specialists Rosberg Engineering GMBH to modernize instrumentation and control systems. Besides meeting its general future needs, Wingas sought a system with short commissioning times that could use existing field technology and retain its working environment. Participants selected Siemens Simatic PCS 7 process control system, which was easily adaptable ro Rehden facility by Rosberg.
Replacement of the S5 systems with S7-400 automation systems and continued use of the same I/O modules allowed use of existing wiring. This approach also ensured the deadlines set by Wingas could be met.
Wingas allowed Rosberg three times 2 weeks conversion time in situ and a 4-week total shutdown of the natural gas storage facility. At an early stage Rosberg said it could shorten the shutdown by at least 2 weeks, persuading Wingas that PCS 7 AS-417 controllers would result in greater economic efficiency and a more capable system. The 16 new automation systems replaced various subsystems, lowering both preventive and corrective maintenance costs and total cost of ownership (TCO).
Plant operators
Plant operators in Rehden wanted everything to remain as it was, having been satisfied with the Aprol system for years. The flexibility of the new process control system would have allowed adaptation to an extent requiring only minimal changes in the operating philosophy. But this would have negated many of the system’s advantages and functional additions.
Introducing the new system as it stood, however, would have required a corresponding amount of training.
Rosberg rejected both approaches, relying instead on presentations to show the framework Simatic PCS 7 would offer and its advantages for operating personnel. The system’s Faceplate technique increased comfort and enabled standardization of user interfaces, offering the same control facilities for operator control, visualization, alarm displaying, etc., in respect to components of the same type.
Establishing a model project on a server at Rosberg for the operating team to access remotely followed initial presentations. Moderator-supported sessions exposed operators to PCS 7 while at the same time clarifying their requirements. Knowledge gained flowed back into engineering planning for the new system.
This process and the use of extensive software-aided simulation shortened the time needed on site for adaptation to the dynamic processes. The entire operating team also became familiar with the new operator control system well in advance, allowing personnel to start working immediately on it without additional training.
Implementation
Cooperation between operating personnel, project management, and project engineers allowed Rosberg to get even further ahead of schedule. IT specialists linked the new process control system to existing doubly redundant bus systems. Switches allowed building different logical networks by way of the same physical layer. Reuse of both the system bus and the terminal bus occurred despite the change in systems and temporary joint use of both at the same time.
Rosberg turned the 4-week shutdown period originally stipulated by Wingas into 3 days of restricted operations, completing the project entirely without shutdown time and within stipulated cost and specifications.
Rehden now runs a modern and completely integrated process control system with the field-level equipment already on hand. A central control room now runs and monitors all aspects of storage facility operations via seven operator stations. In addition to the compressors, the DCS controls probes, gas processing and drying, protection and security systems, 30-kv switchgear, and the fire alarm system; processing and recording more than 4,000 process values (about 30,000 process variables).
The telecontrol application, implemented in accordance with IEC 60870 and integrated into the process control system, allows secure access to the storage facility for central dispatching from Wingas headquarters in Kassel. Operator interface with equipment is the same whether they are inside or outside the facility.
Rosberg specialists used a standard chemistry software library for Rehden, adapting components only where absolutely necessary and keeping as close to the standard as possible. This has improved profitability by removing the need for reconfiguration during upgrades. Using DCS Simatic PCS 7 also prepared the facility—in a future step—to move between gas injection and offtake in the same day, a process that used to take several weeks.
Reference
- International Gas Union (2006): Working Committee 2, www.igu.org/html/wgc2006/WOC2database/index.htm.
The authors
