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
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 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?
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
Essentially a new plant in an old plant
Potential new trips and new setpoints
New compressor/heater/pump/equipment start-up sequences
New testing and preventative maintenance requirements
New diagnostics (HART, sequence of events and system monitors)
New control system hardware
NAMUR and write-protected instrumentation
IEC61511 IPF Lifecycle to support new shutdown/trip systems
New programming tools
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.