09-05-2014, 11:51 AM
Wireless Critical Process Control in oil and gas refinery plants
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Abstract
Wireless telemetry systems are now becoming a
relevant topic in the field of critical process control in in-
dustrial plants and oil/gas refineries. In contrast to wireline
communication, wireless links are inherently unreliable. This
unreliability depends critically on the propagation environment
of the radio-links as the layout of scattering objects (pipes and
metallic structures) influences the strength and the fluctuations
of the received signal power. The development of next generation
critical process control systems using the wireless technology
calls for the design of advanced network architectures. By
following the guidelines introduced by recent standardization,
this paper proposes a novel architecture based on the most
recent technological advances to enable wireless advanced process
control for tight closed loop applications. Cooperative network
paradigm is indicated as the key technology to provide link
reliability even in critical environments. A cooperative link-layer
protocol has been developed and tested over a IEEE 802.15.4
compliant radio network deployed in non line-of-sight (NLOS)
propagation environment with dense metallic obstacles. Test-
bed measurements evaluate experimentally the benefits of the
cooperative architecture.
I NTRODUCTION
The increasing demand of oil and gas supplies frequently
requires the design and execution of very large production
and processing plants over remote locations with harsh envi-
ronmental conditions and challenging logistics. The adoption
of cabling to fully interconnect machines and monitor/control
large number of processes is becoming unfeasible due to the
high fluctuations of installed industrial wiring costs [1]. The
opportunity to replace cabling by deploying a wireless sensor
network (WSN) is now becoming of strategic interest for most
oil contractor projects. The installation of wireless sensors may
give significant cost savings for a variety of typical plants such
as revamping/expansions of existing facilities, storage tanks,
utilities like water treatment, interconnecting lines, manifolds,
high stacks, etc... In addition, the full plant coverage with WiFi
and WSN opens the door to many new applications which
are going to be requested by the end users in the near future.
Therefore, developing consistent design methodologies for the
deployment of a wireless sensor network system is becoming
mandatory for most oil contractors to monitor the technology
suppliers/vendors during every phase of the wireless system
set-up and testing.
Wireless control loops in Oil&Gas refinery
Focus of this paper is on critical wireless control loops
systems. Networked control loop systems require the controller
and the plant to be connected via a two-way digital commu-
nication channel of limited bandwidth. As depicted in Fig.
1 (at bottom) sensors are monitoring the state of a process
and periodically forward the digital measurements (sk ) to a
remote controller. Based on these measurements, the remote
controller computes a control message (uk ) according to a
given policy and sends it to the actuator over the feedback
channel. Upon retrieval of controller message, the actuator
applies an appropriate control signal to adjust the plant state.
For process control applications, determinism and reliability
of data transfer is a key issue, and cycle time (round-trip
time) is a critical parameter to guarantee process stability
[5]. Reliable communication occurs only if both measurement
sk and feedback control uk are decoded by respective par-
ties within specified deadlines defined by the control policy.
This hard real-time constraint calls for the development of
advanced protocols for wireless link-layer management and
a complete revision of conventional network architectures for
just monitoring.
CONCLUDING REMARKS
The installation of wireless control networks in oil&gas
refinery plants is expected to give significant cost/logistic
savings in several applications. The most promising tech-
nologies to support next generation wireless critical process
control systems are outlined. It is proposed a cooperative
network architecture to emulate transmission and reception of
data on a distributed network for tight closed loop process
control applications. A proprietary cooperative link-layer pro-
tocol has been developed on top of IEEE 802.15.4 com-
pliant PHY/MAC layer architecture designed for low-power
consumption. Cooperative transmissions guarantee a robust
two-way communication between the controller and the I/O
sensor to guarantee process stability with cycle time of 50ms.
Preliminary experimental results clearly suggest that the use
of cooperative architectures is a mandatory roadmap to enable
cable-replacing in future systems.