Page 195 - Intelligent Digital Oil And Gas Fields

Workflow Automation and Intelligent Control 151

Controllers: electronic devices with complex electronic systems and
algorithms that operate the controller to act autonomously. Control-
lers are categorized as
♦ Discrete:
(1) On or off activation, for example, a valve setting that has only
two operational positions either open (100%) or closed (0%).
(2) Multiple discrete set points.
Continuous: automatically compares the value of the input
variable to the set point to determine if an error exists. If an
error is detected, the controller adjusts its output according
to parameters that were set up in the controller.
Fuzzy controllers: the valve settings follow an adaptive/var-
iable scheme of values. Soft, fuzzy areas of switch control are
particularly suitable for systems with high state uncertainty.
To achieve continuous control requires a combination of control modes,
which are listed and described below; we believe that continuous control
is an essential component of digital oil field (DOF) systems. Table 5.1
compares the advantages and disadvantages of each mode.

Table 5.1 Advantages and Disadvantages of Control Modes
Control Mode Advantages Disadvantages
On/off Inexpensive and simple Operating differential may be outside
to install the process requirement
Accurate at the point of Requires manual intervention
control

Proportional (P) Simple and stable High initial deviation from set point
Easy to install Offset occurs near the set point
Integral (I) No sustained offset Requires time to overcome instability
Very precise compared to Possible increased overshoot on
other control modes startup
Derivation (D) Stable Requires time to calibrate
Rapid response to Some offset
changes; accurate

Combination Best control Complex to set up the entire system
(P+I+D) manually, but electronic controllers
allow full automation of a system
Minimum offset and Sometimes expensive
overshoot