Automatic gas processing in Taranaki
The Pohokura field is New Zealand’s largest natural gas resource. Its production station uses an integrated automation control system, meaning staff can operate it remotely.
The Pohokura field, offshore from Taranaki, is the country’s largest natural gas resource, owned by a joint venture between Shell, Todd Pohokura, and OMV New Zealand.
The first commercial gas flowed from three onshore ‘extended reach’ drilling wells in the Southern part of the field in September 2006. In March 2007, gas and condensate began to flow from the first of five offshore wells via an undersea pipeline back to an onshore production station at Motunui.
Developing an unmanned site – where operations are monitored from a control room in New Plymouth – required the combined expertise of engineers, consultants and systems integrators.
The Pohokura design contractor, Transfield Worley, appointed systems integrator partner Engineering Control (EC). One of the requirements for remote operation was that the motor control centres be integrated into the main plant control system on an intelligent network so that the information from the gas station could be fed back to the control room.
Transfield Worley based the new system on the DeviceNet network together with the Allen-Bradley ControlLogix platform from Rockwell Automation.
Another requirement was that the status of power switchgear at the 400V MCCs and the plant 11kV main switchboard had to be available in the control room and this switchgear had to be operable from the control room as well.
This was achieved by hardwiring the switchgear to discrete I/O in the ControlLogix. Being able to operate the switchgear by remote is also a welcome safety feature.
The operational philosophy for the Pohokura production facility was to establish an unmanned site with zero normal operating presence (ZNOP); plant operation would be performed off-site from a remote control room in New Plymouth using a distributed control system (DCS).
Remote operation is an ideal way to help keep personnel safe and away from potentially hazardous equipment, but its operational success relies on excellent control, fault diagnostics and network capabilities.
“DeviceNet provided a network solution that could help provide reliable communication and also had the added feature of automatic device replacement (ADR), allowing for reduced downtime with automatic download of device parameters,” says Prasad Nory, industry manager, Rockwell Automation.
ADR consists of configuration and auto-address recovery that effectively lowers maintenance requirements. Operating 24 hours a day, seven days a week, Pohokura has a scheduled proactive maintenance day every month to identify and deal with potential problems.
The Transfield Worley electrically engineered design was the basis for a completely integrated solution that required the capabilities of leading board builders, Switchbuild. The Pohokura solution evolved around the development of two low-voltage motor control centres (MCCs). Two 2.5MVA transformers feed into the first low-voltage MCC via 4000A Air Circuit Breakers (ACBs). Power from the first MCC is then fed through to the second low voltage MCC.
The intelligent MCC design used the DeviceNet network, communicating to the DOL start motor starters for control and monitoring. The E3 plus smart overloads provided motor protection that closely matched the motor operating characteristics with the enhanced protection capabilities such as earth fault, stall, thermistor and loss of load that are normally provided by much more expensive motor protective relays.
An Allen Bradley ControlLogix programmable automation controller is used to provide the comprehensive monitoring and control of the MCCs and feeds back information to the distributed control system.
To avoid costly downtime at Pohokura, EC developed and incorporated switching controls in the PAC in the event that power is lost from one of the two 11kV incomers.
Peter Huitema, an engineer at EC says: “Normally when supplies switch over you lose power, even if it’s just a few milliseconds, the motors will shutdown.
“To avoid this, we determined how many seconds the motor can be with no power without causing any damage. We used inertia to keep motors running for up to 1.5 seconds, to allow time for the other supply to switch over. By programming this information into the control system we were able to avoid costly shutdowns.”
The Pohokura field production station is operated by Shell Exploration NZ. State-of-the-art engineering has enabled the gas plant to be operated remotely.