The Application of PLC Distributed Input/Output Technology Offshore
Authors: Glendening J.A. 
Published: October 2008

The Programmable Logic Controller is becoming a familiar part of the Process Controls on Offshore Facilities. The current evolution of this expanding technology is Distributed 1/0 Blocks, and this paper is concerned with the benefits and applications this new technology brings to the Offshore Industry.

The advent of the Programmable Logic Controller to the Gulf of Mexico brought greater process control flexibility to Offshore facilities. The offshore control philosophy wentfrom independent pneumatic control of each skid unit to PLC distributed control with field panels that contained the local controls for several skid units and interfaced with control room based PLC visual display stations.

The Distributed 1/0 Block concept allows offshore control systems to eliminate the large field interface panel, along with the associated wiring and hardware cost. This is accomplished by installing Distributed l/O Blocks inside the individual skid control panels and then communicating back to the control room based PLC via redundant communication cables. The control wiring between each skid control panel and the control room is now reduced to one power cable and redundant communication cables.

PROJECT OVERVIEW

The PLC distributed 1/0 system discussed in this paper was installed on an existing semi-submersible drilling Modu modified to include first stage processing equipment for separation of water, oil and gas. " The vessel ';Enserch Garden Banks" is now classified as a Floating ProductionFacility and will be located in the Gulf of Mexico at Garden Banks Block 388.

The Garden Banks 388 Project used two separate PLC'S. The ESD or safety PLC was dedicated to the emergency shutdown system while the control PLC handled all process variable indications, special control applications, and production reports. The dedication of one PLC for the ESD system is part of the regulatory agencies requirementfor all ESD systems to be separate from other systems. The ESD PLC employed two CPU's in the "hot" standby configuration and communicated with the distributed l/O via redundant communication cables.

The separation of water, oil and gas is one of the simplest applications possible for process control and conventional pneumatic instrumentation was considered adequate forthis task. The programming flexibility and alarm logging make the PLC especially useful for offshore ESDapplications. Historically, ESD controls offshore have been extremely easy for operations personnel to disable due to bypass controls installed to allow government mandated testing. The Garden Banks 388 PLC logs each ESD bypass alarm on the printer and leaves an alarm banner on the visual display until the bypass is reset. This printed documentation and visual display makesoperations supervisors more aware of ESD bypass status, resulting in a safer facility.

The process skid panels housed both analog and discrete distributed 1/0 blocks. The analog distributed 1/0 blockscommunicated ESD alarms and bypass control status back to the safety PLC. Local alarms consisted of pilot lights on the panel front and were limited to ESD alarmsand shutdown valve status. These alarm lights were energized by end devices with two sets of contacts.