By Laurie Blackmore
Uncharted territory and personal opinion are terms inherent in any predictions given when considering the benefits and risks of setting forth with fieldbus on a major greenfield development.
Retrofits are even harder to generalise and depend on the scope of work. At the request of British Petroleum, my previous company, McDermott Engineering, undertook a study to find out the impact on the work of a contractor if a full catalogue of fieldbus products existed and were to be installed on a plant located within a hazardous area.
This fieldbus study only covered process plant monitoring and control, and not protection systems, as this area was seen as too speculative to project future design and cost implications.
One of the implicit understandings was that there would be a reduction in field devices (transmitters, switches etc.) due to multi-functionality. Further, that the cost model would be rationalised to a per point basis. These costs are then scaled up to a typical 3,000 I/O system to give a general feel for total cost savings on, for example, an offshore topsides application.
The starting point for any control system design is the Piping and Instrument Diagram (P&ID).
As flexible programmable systems and multi-function devices have already led to inconsistencies in the way functionality is represented on P&IDs, the suggestion was adopted that these revert to showing what measurements are needed from a process and HAZOP standpoint and do not try to detail means of implementation.
This can be covered in a separate schematic on a loop/group basis, which will be produced by the fieldbus design tool. This is similar to the system control and logic drawings currently produced by the DCS.
From a cost point of view, fieldbus is not likely to change significantly the level of C&I input to P&IDs, but there will be uncertainty over the method, unless standards are agreed for the early projects.
Next, the move towards databases as a strategically vital part of the C&I engineering process, as in STEP, is not expected to be effected by fieldbus. The structure will, however, change slightly, as fieldbus will make it necessary to distinguish between real devices and measurement points in a multi-function device.
Instrument functionality and configuration will be a new area influenced by fieldbus, as decisions will have to be made between the field device or control system. Will the contractor have the necessary process knowledge to determine issues of criticality and performance? Also, who will carry the responsibility for overall system performance, when functionality is spread over a large number of field devices from different vendors?
Key to this will be the capabilities of fieldbus design tools. These will be an important factor in pre-site testing and integrating the system on site.
Next, cable design, loop drawings and hook-ups should now all be simplified. Optimum location of fieldbus junction boxes (FBJB) should aim to keep local lengths short, such that lengths can be kept to below 50 metres and run in local traywork. In fact, since device and FBJB terminations are simplified, there should be no need to produce loop drawings for each installation.
All that is needed is a location drawing and the data base. Autocad type standard formats can then be plotted from the data base, should a loop drawing need to be produced for future maintenance.
Use of multi-function devices will also result in fewer hook up details being needed. Process tapping points will also be reduced although no allowance has been made for this in the costings shown.
Procurement costs are unlikely to be effected, although the DCS will mainly concentrate on monitoring and management, leaving basic control functionality distributed to the devices. It is proposed that this will result in a cost saving of 17%.
Further, the reduction in installation costs results from the reduction in the quantity of cable and devices, and standardisation of the termination arrangements.
Acceptance testing could be a major area of programme risk, as functionality is distributed and not located within a single system. It is hoped that features of digital communications will speed up the process of loop testing and commissioning.
The Author is CIE with Tarmac.
% total % saved of % saved of Total saved cost category total (3,000I/0)
Design 14 19 3 £114kProcure 53 17 9 £375kInstall 26 32 9 £360kT & C 7 50 4 £150k
Total 24% £1,000,000 (on £5m total)}}