Online Swingovers/Cutovers and Legacy Control Systems

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
  • Capacity/expansion
  • 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?

Turnaround Migrations

Online Migrations


  • Potentially many start-up issues all at once
  • Inability to go back
  • Operations start up on a new system during a critical period
  • Incorrect migration control/shutdown philosophy may impair start-up

  • Hybrid system exists during migration
  • Potential inability of operations to restart without project team
  • Plant overrides/impairments required
  • Risk of nuisance trips
  • May still require turnaround work (instrument preparation, post for demolition work, testing of outputs and sequences)

  • Can’t trip the plant
  • All work can be completed
  • Can prove outputs and shutdowns (values, pumps, etc.)
  • Can prove start-up sequence logic for compressors, heaters and pumps

  • Problems handled one at a time
  • Operations has time to adjust to new system and can revert back
  • Ability to truly as-build the existing system and expose undocumented functionality
  • Can validate the SIS process reading immediately
  • Not a schedule-driven activity

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

Operations Maintenance Engineering
  • Essentially a new plant in an old plant
  • More information/graphics
  • Potential new trips and new setpoints
  • New compressor/heater/pump/equipment start-up sequences
  • New alarms
  • New impairments/bypasses/procedures
  • New testing and preventative maintenance requirements
  • Training requirements
  • New diagnostics (HART, sequence of events and system monitors)
  • New control system hardware
  • NAMUR and write-protected instrumentation
  • New spares
  • IEC61511 IPF Lifecycle to support new shutdown/trip systems
  • New programming tools
  • Training requirements
  • Enhanced flexibility, expansion and optimization capacity


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.

Additional Resources

Cybertech can:

Cybertech has more than 20 years experience with PLC, DCS, SIS, SCADA and HMI
  • Design and engineer new control systems
  • Replace or upgrade obsolete equipment
  • Work with various PLC/DCS/SIS platforms
  • Design and upgrade control networks
  • Configure HMI/SCADA/RTUs
  • Consolidate data and support production reporting
  • Design and fabricate control panels
  • Perform commissioning support
  • Support your drafting and CAD needs