Once you realize that after a prolonged relationship with your current DCS, it is time to say goodbye to the obsolete DCS and welcome a new DCS, there are many things to consider. For instance, which migration approach or strategy will you be using? If the DCS controls a plant which will stop production in the near future this choice is easy. You can just migrate when production is stopped. Though many companies will not stop production in the near future or will not stop production at all. These companies will be migrating their DCS during online process conditions; while continuing production. We will discuss several DCS migration approaches.
Replicate or upgrade
The first step to take into account is whether you want to replicate the existing system or to upgrade it. Replicating is cheaper on the short run. Although it is less cost-effective on the long run due the missing benefits of a modern DCS.
Simultaneous or step-by-step
It must be determined if the upgrade will replace all the DCS components at the same time or in steps. With the step-approach the Human Machine Interface (HMI) will be replaced first, then the controllers and then the I/O. For a phased approach more time is needed, but is followed downtime and risks.
Vertical or horizontal
When choosing a vertical approach, one specific process area is upgraded at a time. When choosing the horizontal approach, all similar process units will be upgraded at the same time (across multiple areas).
Hot cutover or cold cutover
The hot cutover is more expensive than the cold cutover. Although, when taking certain important factors into account, such as downtime, the hot cutover is less costly. The hot cutover method also involves less risks due to step-by-step migration of the loops, whereas with the cold cutover method the old DCS will be replaced by the new DCS all at once. One big disadvantage of a hot cutover is that is takes longer than a cold cutover.
There are various hot cutover strategies, which include:
- Process bypassing; using the process redundancy and a bypass valve to block out segments.
– This is not always possible
– No control during the migration
– Time consuming
– No output match - Manual valve control; using a bypass valve to create fixed process conditions
– Offset control (only based on experience)
– No output match
– Time consuming
– Expensive - Mechanical blocking; mechanically fix process valves
– No offset control
– No output match
– Time consuming
– Expensive - HART manual mode; using HART to create a stand-alone instrument
– HART manual mode support required
– Not always possible
– External power required
– Increased safety and efficiency - Electrical loop take-over; using a tool to take-over and control the loops
– Increased safety: offset control, output match and fail-safe mode
– Increased efficiency: less preparation, less man-hours and no mechanical work
– Expensive on the short run, but cost-effective on the long run
– Time consuming due to step-by-step approach
Taking the above mentioned hot cutover strategies into account, the electrical loop take-over looks like the best strategy to use. The HART method is a very good option as well, but this is not possible in every process plant.
Continuous developments in the field of DCS migration/upgrades lead to more efficient and safe methods. A recent DCS migration project by Yokogawa showed to use of a tool that makes DCS migrations as easy, safe and efficient as possible.
