01 October 2003
Large-scale offshore platform bus
Scrolling across InTech magazine's desk recently came an exchange on wiring an offshore platform in Asia. The questions come from Malaysia and are in italics. The answers come from ISA member, fieldbus expert, and author Jonas Berge. Berge operates from Singapore.
On the subject of offshore environments and Exi/Exd instruments, I need counsel on the large-scale use of Foundation fieldbus H1 (FFH1) for, say, all 400 transmitters and valve controllers wired to a conventional distributed control system (DCS) on a 12,000-ton topsides offshore platform.
Specific considerations are these:
1. The platform is compact, so cost and weight savings for FFH1 are not substantial compared with the use of remote I/O modules.
The reduction is not just wiring and the associated cable trays and ladders. Also consider that because no I/O modules and marshalling are required and fewer controller central processing units (CPUs) are required, you eliminate a large portion of the traditional DCS. This means a lighter and smaller footprint. A smaller footprint means a smaller rack room, which means less weight. Lower power consumption and lower heat dissipation mean a lower air conditioning and power requirement. Make sure to use a system that includes not only FFH1, but also FF High-speed Ethernet with linking devices in place of traditional controllers. A new technical overview of Foundation fieldbus is available free on the Web, and it includes second-generation Fieldbus system architecture aspects such as HSE and linking devices, flexible function block (FFB), multivariable container (MVC), and profiles (www.fieldbus.org/pdf/techoverview.pdf).
2. Exi (intrinsically safe) is preferred in place of Exd (flameproof or explosion proof) for transmitters, and this limits the quantity of devices per FFH1 loop; fieldbus intrinsically safe concept (FISCO) helps.
Even doing Exi using the traditional entity concept you can still get 16 devices per network. Do this using repeating barriers (or barriers with separate repeaters), which divide the network into four intrinsically safe segments connected to one safe area segment. Each barrier can have four devices each for a total of 16. Make sure to use devices with low power consumption, a maximum 15 mA, and preferably 12 mA.
3. We desire to maximize the potential for easy commissioning and maintenance and asset management, as promised by FFH1 proponents.
Foundation fieldbus has superior asset management capabilities. Using 4–20 mA with a Highway Addressable Remote Transducer (HART) will be less effective. Again, make sure to use HSE as the host-level network rather than having an intermediate proprietary protocol.
4. Because FFH1 does not support redundancy, we will not implement control-loop algorithms on FFH1. Regulatory control algorithms will run using a conventional DCS.
As far as H1 redundancy is concerned, it does not make any difference if control is on the field or in the centralized controller. If the H1 network goes down (if it shorts out, for example) redundant CPUs will not help you, because the signal still cannot reach the valve positioner. Regardless of where you do control, the same number of loops will fail in case of a total network failure. A proportional, integral, derivative (PID) control algorithm in a valve positioner will only fail if the positioner fails. If the positioner fails, it does not matter that the PID is also gone—actuation cannot happen anyway. This is why you don't need redundant PID when you do control in the field. By doing control in the field you get single-loop integrity whereby a fault in a controller—instrument—only affects that one loop, not 30 to 100 loops. If you also follow the recommendation to use a repeating barrier to split the network into four segments with only four devices each (equivalent to one loop plus some monitoring), you will find that a wire short will only affect a single loop—not many. Redundancy is something you need if you centralize control. When you decentralize control you don't need it. Thus, please perform your control in the field instrument. Consider using devices with instantiable function blocks. Such devices have a larger library of blocks to chose from, and you can use more blocks too. This means you can build more powerful control strategies with arithmetic, section, limit, alarms, and the like, not just bare bones PID.
5. Safety critical systems (and skidded/packaged equipment such as turbo compressors) would still use HART 4–20 mA transmitters.
Tell your skid manufacturers to live in the now. Some major instrument users have successfully dragged their package unit suppliers kicking and screaming into the New World.
6. Fieldbus technology is relatively new, so there is limited design, commissioning, and operational experience in my neck of the woods (Malaysia). As well, the published reports from the FFH1 implementation on the offshore Malampaya project in the Philippines were not 100% encouraging.
We have several FF installations operating successfully in Malaysia. Also, take a look at this case using HSE and intrinsic safety: ethernet.industrial-networking.com/articles/i04_oilplatform.asp. IT
Nicholas Sheble writes and edits the Networking & Communications department. Write him at email@example.com. Jonas Berge is the author of Fieldbuses for Process Control: Engineering, Operation, and Maintenance, ISA Press. Write him at firstname.lastname@example.org.
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