An online swingover/cutover has significant advantages as long as your plant is prepared for additional planning and the risk of a nuisance trip.
There are innumerable legacy control systems still in use today. These systems continue to operate even though they are obsolete, not supported by the vendor/manufacturer, hard to maintain/support (i.e. no spares, no documentation and no qualified personnel) and hard to change/expand.
These legacy systems include:
- Electrical relays and devices
- Pneumatic transmitters, selectors, switches and panels
- Non-certified PLCs used as SIS systems
- Obsolete SIS systems
- Obsolete DCSs (e.g. Bailey, Fisher PROVOX, Moore APACS, Honeywell TDC and Westinghouse)
- Obsolete PLCs (e.g. Modicon and Allen-Bradley)
Why Migrate a Legacy Control System?
Migrating a legacy system means that you can comply with new standards, regulatory requirements and insurance requirements. This is especially true for process heaters and boilers. Other compelling reasons for upgrading include:
- Realization via a re-HAZOP/SIL LOPA process that your current shutdown system does not meet required SIL
- Obsolescence/maintainability issues
- Improved functionality (alarming, historian, sequence of events, first ins, reliability and availability)
- Improved service life
Why Haven’t Plants Migrated?
Many plants have delayed migration because certain obsolete shutdown systems have proved more reliable than control systems. They run in the background (unlike DCS/PLC systems that actively control) and are relatively easy to fix and support (spares are usually available). Perhaps most significantly, the logistics for migration can be daunting.
Type of Migrations
You can migrate your legacy control system in three ways:
- Online – Unit running and hot swingover/cutover
- Turnaround – Unit shutdown and offline migration
- Combination – Turnaround portions may be required to facilitate an online cutover
Choosing the best method requires consideration of:
- Turnaround windows
- Turnaround critical path and the risk of turnaround extension
- Risk to the plant and impact of a nuisance trip
- Cost (comparison of increased online cutover project costs versus turnaround production outage costs)
- Is it even possible to do an online migration?
How to Perform an Online Migration
For each process system, interlock, etc.:
- Migrate outputs to the new system first
- Migrate inputs one-by-one
- Hybrid the old/new system if planned migration takes longer than expected
Additional Migration Considerations/Questions
- Replacement in-kind or new control/shutdown philosophy?
- Converting code (automated tools) versus a rewrite, and testing the implications
- New alarms, smart alarming and suppression strategies
- New standards applied to an old plant
- Demand on HVAC/UPS with old and new systems in parallel for a period of time
- Weather/process impacts to online swingover
- Leaving some systems un-migrated adds complexity/risk to the plant operations and maintenance
- The migration project needs to coordinate and be in sync with other onsite projects
- A phased unit-by-unit online cutover may take years, and this requires a consistent design philosophy and awareness of system revision issues and new vendor hardware/software offerings
Most importantly, your team needs to focus on maintaining process safety integrity during cutover while avoiding nuisance trips.
Impact of a New Control/Shutdown System
In summary, an online cutover has significant advantages as long as your plant is prepared for additional planning and the risk of a nuisance trip. Remember that most online cutovers require both pre and post turnaround work to prepare and then finally complete the work.
- “The Coming Wave of Process Safety System Migration”, Dave Woll, ISA Intech May/June 2012
- “Replacement of SIS Logic Solvers Whilst the Process Remains Operational”, Clive Timms, TÜV International Symposium 2008
- “A Practical Live Migration Strategy for Upgrading Safety Systems in the Oil and Gas Industry”, Rockwell Automation White Paper
- “Best Practices in Control System Migration”, Dan Hebert, ControlGlobal.com, January 2007