Modern DCS Gives Aging LNG Facility New Life

By Gary Hida, Operations Manager, and Jason Santos, Operations Manager, Applied Control Engineering

An aging liquefied natural gas (LNG) plant is getting a complete makeover as a phased modernization that includes the installation of a modern distributed control system (DCS) that will, over time, replace the functionality of a legacy DCS. For more than 40 years, this plant has provided natural gas to the pipeline during periods of high demand in the winter. This upgrade is meant to provide the plant with a new lease on life.

The System as Found

The legacy DeltaV DCS in this facility had not been updated since sometime before 2010 and was operating on software that was not supported and computer systems that were out-of-date. The DCS was responsible for the tank controls and the boil-off gas (BOG) system with accommodations for hardwired communications to the Vaporizer and BOG compressor skids.

The hardwired communication to the skid-based equipment allowed for basic control and monitoring and was incapable of providing a full view of those systems to the supervisory control and data acquisition (SCADA) system. Documentation for the DeltaV control system also was lacking, and modifications to the process and automation systems were not tracked.

Furthermore, space in the plant was limited, and a modernization of this scope would require more than was available. To overcome this, the plant already had begun construction on a new motor control center (MCC), and a preliminary design for a new control room was already underway.

Phase 1: Install Modern DCS and Integrate New Vaporizer Skids

Three new vaporizers were installed as part of this phase, each with two Allen-Bradley^® ControlLogix^® programmable logic controllers (PLCs) for combustion control and burner management. A new PlantPAx^® DCS from Rockwell Automation was developed with redundant process automation system servers (PASS) and two operator workstations (OWS).

One of the servers doubled as a Historian server collecting process data for trending and reporting. A set of redundant processors (ControlLogix L75) and a small number of I/O modules were commissioned to manage communication with the Vaporizer skids and some system level inputs and provide a footprint for expansions in subsequent phases.

Because the new control room had not yet been constructed, the servers were installed in the new MCC building, and the two workstations were placed near the existing DeltaV workstations in the old control room.

A new network was designed according to the Converged Plantwide Ethernet (CPwE) Design and Implementation Guide from Rockwell Automation and Cisco^®, including redundant communication between the PLCs, servers and workstations as well as a device level ring (DLR) for the I/O modules, vaporizer skids and Modbus™ data from the new plant’s electrical equipment.

Applied Control Engineering natural gas storage tanks. — For more than 40 years, this liquefied natural gas plant has provided natural gas to the pipeline during periods of high demand in the winter. Its legacy DCS needed an upgrade.

The graphics development in this phase included screens that allowed operations to monitor and control the vaporizer skids from the control room. With this part of the system commissioned, operators found they had better visibility of the vaporizers and that most problems that previously required a trip to the skid now could be managed from the control room. The DeltaV system still managed the tank operations and the BOG system.

Phase 2: Integrate Boil-Off Gas System

In this phase, the BOG system was migrated from the legacy system to the modern DCS. The DLR that was commissioned in Phase 1 was expanded to include new I/O racks for the BOG and glycol water system as well as EtherNet/IP™ communications to three Rockwell Automation-based compressor skids.

New variable-frequency drives (VFDs) for the glycol and BOG systems also were integrated into the system with a combination of Modbus and hardwired signals for critical commands and feedback.

New graphics were added for the compressor skids and the BOG system. Just as in the previous phase, operations gained more control and visibility of its process by making use of modern communication capabilities not previously available in its legacy DCS. Following this phase, the DeltaV system still had control of the tank operations.

Interim Support and Disaster Recovery Option

While construction on the new control room got underway, there was a lull between Phase 2 and Phase 3 of the modernization. During this time, an offline system was commissioned at the site. Primarily, this was to be used to test future phases and included a single server, an OWS and a single processor. If needed, however, portions of this system were ready for quick installation at the plant in case of disaster or some other unrecoverable hardware event.

Also at this time, the site generator and uninterruptible power supply (UPS) were integrated into the control system via a new Modbus gateway. With this, operations now has a real-time view of its backup systems, providing the ability to identify and resolve any issues with those systems quickly, potentially before they impact the plant.

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Phase 3: Use New Control Room

In the initial plan, the new control room was to take over operations of the plant while the old control room was decommissioned. With construction of the new control room almost complete and the experience of operation during a pandemic in the rear-view mirror, plant leaders decided instead to expand the architecture to keep the old control room in operation as backup control room.

While the control cabinet remains in the MCC, the PlantPAx PASS servers will be relocated to the new control room (one at a time), and three OWS clients will be added to the system. In preparation for the migration, new network distribution switches will be configured and installed in the new control room to minimize the amount of downtime required as the servers are moved.

Because the DeltaV system still controls tank operations, plant leaders plan to add two DeltaV workstations to the new control room.

Commissioning of this phase is was completed in the summer of 2022 and provides the plant with a co-location for operations, increasing the system availability.

Look to the Future

When the current phase is complete, the plant still has other work that must be done to achieve all its goals, as follows:

Fire Water System. The plant would like to set up communications between the site’s fire water system and the modern DCS. The options under consideration include hardwired signals or a deeper exchange of information using the previously installed Modbus gateway.
Tank Operation. The tank operation still resides with the DeltaV system. In a planned future phase, this control will be migrated to the PlantPAx DCS. This will complete the migration from the legacy application and allow the plant to decommission the it.
Safety Instrumented System (SIS). The plant is in the design process for a more advanced SIS to replace and update existing hardwired and soft logic currently responsible for safety in the plant. One critical user requirement for this new system is that it will be able to integrate with the modern DCS and provide operations with increased visibility of the conditions that could shut down the plant.

Reliable and Better Visibility

Everything from the new process equipment to the control system and the process network is designed to be resilient and, in some cases, redundant. This will improve system uptime, allowing the plant to better service its customers reliably. Also, system maintenance and troubleshooting is easier now that the design is more fully documented.

From the first moment the modern DCS was operational, the plant increased visibility of its operations from the central control room when compared to the legacy DCS. If any of these improvements allows the site to avoid even one shutdown, the advanced controls will have provided a significant return on investment.

Finally, the completion of this project will lead to the modernization of other sites within this utility company. The PlantPAx system developed on this project is expected to serve as an application template and model for future implementations.

Applied Control Engineering is a Rockwell Automation Gold System Integrator and a Control System Integrators Association (CSIA) certified manufacturing automation integrator with capabilities in plant-floor control, process automation solutions, manufacturing intelligence, power quality and energy management (PQ&EM), industrial IT and validation.

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