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Next, researchers placed control system cabinets and Motor Control Centers (MCC) on GA drawings. Design criteria were developed for both approaches, including construction labor costs, tray conduit and cable lengths, and material costs. Individual design parameters were assigned to all I/O points in the study to complete the construction estimate. Device upgrade costs from smart transmitters, digital I/O, and intelligent motor interfaces were estimated for the digital installation. Plant checkout and startup tasks were defined for each I/O type, as well.
A plant construction schedule and budget was created to estimate a total spending curve. From this, researchers developed a construction financing methodology to estimate interest during construction (IDC), which is the cost to borrow money to build the facility. Inflationary escalation was estimated based on typical utility accounting methods.
Fixed overhead costs were assigned to all construction and startup line items, and included administrative and general support, construction management, contingency, contractor indirect charges, freight, project management, spares, and sales tax. Variable overheads included inflation escalation and IDC at a rate of 3 percent and 6 percent, respectively, both compounded calculations.
Digital Bus Technology Adds Value, Cuts Costs
Researchers determined that the digital bus approach to new plant construction provided opportunities for reduced costs in all areas studied. Items considered for engineering, construction, startup, system selection and overhead costs for a traditional I&C system totaled approximately $50.1 million, compared with $30.4 million for the digital I/O bus approach, resulting in a savings of more than $19 million (39.4 percent), or $2,000 per I/O point. Breakouts of these savings appear below.
Engineering
Early adoption of digital bus technology simplified the engineering process, resulting in savings of approximately $3.5 million. For example, the study indicated that engineering time is reduced with the digital I/O bus approach due to use of standard templates and objects, the need for fewer drawing review cycles and less system complexity. Less complexity, in turn, allows for higher I/O density on drawings, which then reduces the number of drawings needed.
Construction
The cost of construction takes into account both the material cost of the I&C equipment and the cost of the labor required to perform the installation. The study demonstrated that utilization of digital bus technology resulted in $3.6 million in savings, primarily a result of reduced labor and material costs needed to install tray, cable, conduit, terminations, and I/O cards.
Startup
Because of the reduced complexity of the digital bus installation, the time required for checkout and startup can be significantly reduced, resulting in savings of nearly $426,000. "If the checkout process can be shortened it will save time and money in staffing," explained Hoyum. "But, even more importantly, reducing the time spent on the critical path to unit synchronization leads to much more significant savings in IDC," he continued.
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