Page 179 - Offshore Electrical Engineering Manual

166 CHAPTER 15 Process Control and Monitoring Systems

of a platform the corrosion monitoring system, which is the first paper envelope.
Adding the process equipment requires additions to the jacket to protect it from the
additional risks introduced by the process equipment, even though the process ves-
sels and pipework are adequately designed, which is the second paper envelope. The
process equipment also needs protection to control the process variables within the
operational envelope of the installed equipment; the third paper envelope is the con-
trol and monitoring equipment. The fourth paper envelope is the process shutdown
system to contain any extreme process disturbances. The fifth paper envelope is the
gas detection equipment to detect and alarm escapes of gas that put either the per-
sonnel or the installation fabric at risk. The sixth paper envelope is the fire detection
equipment, automatic extinguishant release and alarm system.
The process being constrained and contained on the installation can be viewed
as an amorphous substance that grows spiky extensions and sharp edges, which can
easily perforate a paper envelope, requiring additional envelopes to maintain contain-
ment. This example also demonstrates that a change to one part of the installation
affects the other parts, control and safety systems and emphasises the need to con-
sider the installation as a composite whole and not as isolated stand-alone systems.

PROCESS CONTROL SYSTEMS
The process control system should be viewed as the first level of the safety protection
for the installation, as its function is to control the process within the design limits of
the process plant and make the operator aware of any process excursion beyond these
limits. It also performs the main data acquisition and display function, collecting
information from all areas of the installation and displaying it in a central location in
an easily comprehensible manner.
Modern process control systems have the capability of adjusting the individual
control loops automatically to optimise loop performance leading to a safer and more
efficient operation. ‘Predict & Control’ (P&C) provides multivariable model pre-
dictive control to ensure that process performance remains optimal and repeatable
in all operating conditions and across all product changes. It uses state-of-the-art
state space modelling to reduce process variations, increase throughput and reduce
production costs by operating safely at the closest possible process constraint limits.
P&C has an unrivalled success rate in improving productivity in a wide variety of
industrial processes (ABB Advanced Process Control). They can also analyse the
data coming back from the plant instruments to automatically monitor plant opera-
tion and schedule preventative maintenance activities thus enhancing operational
efficiency and therefore safety by reducing the probability of breakdowns.
The information generated by the control system is presented to the operations
personnel at a central location on the installation in a readily understandable way, using
multiple graphical screens. As always the information presented by intelligent systems
is only as good as the data available for processing, and to maximise the benefit of these
control systems, intelligent instruments must be used. ‘FOUNDATION fieldbus (Ff)