Configurable DCS I/O provides flexible, cost-effective option for offshore platforms

Barry Young
ARC Advisory Group

Traditionally, input/output (I/O) modules for distributed control systems represented a major cost component and often contributed to significant project delays. With conventional DCS I/O, all the sensors and final control devices had to be specified before the engineers could finalize and optimize the I/O design. Then, the inevitable last-minute project changes would require the process to start all over again. Also, with conventional I/O, owner-operators had to maintain extensive spare parts inventories to ensure that adequate spares were on hand if needed to increase I/O capacity, or to replace failed or defective modules.

With the advent of today's smart configurable I/O, these issues go away. Now, project engineers can specify one type of I/O module up front that can accommodate many different I/O types. This speeds design, makes it easier and less disruptive to accommodate last-minute project changes, and dramatically reduces spare parts requirements.

Three out of the six major global DCS suppliers, which represent more than 80% of the market based on revenues, have rolled out smart configurable I/O for their existing systems.

Fifteen years ago, the traditional process analog input came from a sensor producing a 4-20 mA analog signal and the typical analog output was a 4-20 mA signal. Discrete signals involved various combinations of electric voltages and amperage current ratings. Each signal type had a dedicated type of circuit board. Today, most I/O for a new installation resides on some type of bus network. ARC research also indicates a growing trend toward adding more wireless I/O and associated field devices, particularly for process and equipment monitoring applications.

For offshore platforms and other upstream installations, configurable I/O improves flexibility and offers significant potential cost savings. A study by Emerson Process Management analyzed data from an existing manned offshore platform in Europe to compare the different combinations of capital costs for the as-built, wired fieldbus installation versus "electronic marshaling" using smart, configurable I/O. On this particular platform, with 1,874 signals in the DCS (of which only 9% have the potential to be wireless), the results show cost savings between 7% and 26%, depending on options chosen. According to the study, this approach also saves up to 30 tons in weight and up to 43 sq m (463 sq ft) of deck space in cabling, cable trays, junction boxes, and cabinets.

The DCS I/O subsystem is responsible for inputting hundreds or often thousands of different process measurements and other inputs into the system, and outputting control signals to a large number of valves, actuators, motors, and other plant final control elements. I/O represents one of the most significant parts of the DCS, and traditionally, a significant cost element. However, DCS suppliers are working to reduce both the cost and the complexity of their I/O by incorporating more intelligence into the devices. For example, in 2009, Emerson introduced its CHARMS (CHARacterizer Modules) for the company's DeltaV DCS, which reduce the number of different I/O types required and increase flexibility by enabling I/O devices to be characterized at commissioning. Since then both Honeywell and Invensys have initiated efforts to revamp their I/O subsystems to make them configurable and smarter.

While most often incorporated into large, grassroots projects, smart configurable I/O can also play an important role in brownfield modernization projects. Traditionally, specifying types of I/O introduced project delays and unnecessary costs for extra spare capacity. With smart configurable I/O, the project engineers just have to specify the overall I/O counts at the beginning of the project and can put off specifying the individual detail I/O types until further along in the project cycle. The new I/O also makes it easier to accommodate last minute additions and changes during commissioning.

While the initial purchase costs for the smart configurable I/O may be higher than conventional I/O on a per channel basis, when all project costs are considered, the overall costs are less. This does not consider the significant opportunity costs associated with I/O-related project delays.

Oftentimes the project team also plans to modernize the process safety system in conjunction with DCS modernization. For safety system modernization projects, ARC sees a growing trend for cost-conscious owner-operators to use an integrated approach for process control and safety systems. These use a common look and feel for engineering and operator functions while maintaining a separate controller for process safety logic. The industry refers to this architectural approach as "integrated but separate," meaning some information-related functions are tightly integrated, but the process safety control function remains separate to provide an independent safety function. Two safety system suppliers, Emerson and Honeywell, both now use smart configurable I/O to provide the same project benefits as in a DCS modernization.

For owner-operators, the decision as to whether or not to use the new smart, configurable I/O in any given project depend on a number of considerations, including whether the automation supplier selected for the project offers this as an option. Other considerations include whether existing, well-proven engineering approaches will be reused for the project; the viability of any I/O and field wiring that may already be in place; the sophistication of the plant engineers and maintenance technicians and their comfort levels for new technology; the degree to which process fieldbus and/or wireless technologies will be utilized; intrinsic safety requirements; and any future process modifications and/or plant expansion or modernization plans that may be on the drawing board.

The bottom line is that, given the many choices of I/O available today, the goal should be to examine ways to reduce overall project costs and increase design flexibility.