Misconception No. 6: A step-by-step (phased) migration is the lowest cost and lowest risk. This is a common belief — but not necessarily true. Many users, when estimating the cost of a modernization, envision the cost-versus-time graph shown in Figure 4. However, this graph only takes into account the system hardware and software costs. As Figure 5 indicates, these are a small portion of the total project costs. Figure 6 provides a more-realistic cost-versus-time graph once all the projects costs are quantified. It shows the total cost of the phased modernization actually is higher than the “rip and replace” due to the inefficiencies of multiple starts and stops with the project team and some duplication of engineering and effort to accommodate the multiple phases.
The rip-and-replace strategy front-loads the cost and incurs the risk all at once while a phased approach improves cash flow and spreads the risk into smaller portions. However, the total risk often is greater in a phased modernization because the older equipment remains in place longer, creating a greater probability of failures causing unplanned shutdowns.
Another major downside to the phased approach is that it defers the benefits of a modern system (Figure 7). A rip-and-replace modernization immediately delivers:
• reduced energy costs;
• increased yields (lower raw material usage); and
• greater capacity (decreased variability).
Because of this, many projects evolve into an accelerated-phased approach — combining the replacement of consoles, all controllers and control networks into just one or two steps while delaying the significant cost of replacing the I/O and wiring until later. This provides more than 85% of the new technology benefits much sooner and at a lower cost than the multiple-phased strategy.
Table 3 summarizes the benefits and drawbacks of the rip-and-replace, accelerated-phased and extend-then-migrate approaches.
Most automation system vendors have found that many users initially plan for a multiphased migration that would take several years to complete, but after the process begins decide to compress it into fewer, faster phases. Once the initial fears of modernizing are overcome, the users realize the necessary personnel already are on-site and the knowledge base is in place, so they opt to accelerate everything, both to get the project over with and to more quickly gain the benefits of a more modern system.
The accelerated-phased and rip-and-replace approaches offer another, less obvious benefit — the possibility of taking advantage of the experience of veteran staff who soon will retire. They understand how the plant and control system operate today and thus are in the best position to help with a successful transition to a new control system. We often hear: “The last thing these people want to deal with is a major migration project.” This is, perhaps, another misconception. Many will look at the changeover as a final challenge and may decide to hold off retiring until they’re sure the new system is working well and younger people have learned to use it.