Page 114 - Intelligent Digital Oil And Gas Fields

82 Intelligent Digital Oil and Gas Fields

quality. Many modern sensors in SCADA systems have built-in function-
ality for signal processing at the measurement location, that is, “at the
source”.
Multiple possible checks are listed below.

3.2.1.1 Rate of Change, Spike Detection, and Value Hold
This algorithm includes a test and a valid value setting procedure to correct
data spikes. If the test passes, then the current value is passed along. If the test
fails, then the last good available value is held for a specific period and an alert
is generated for the engineers. The test takes the current value and a user-
entered tolerance to check if the current value has spiked and will set the
output value according to the following logic (pseudo-code):

If ROC_test ¼ off ,then skip this test

Y tðÞ ¼ If ABS X tðÞ X t 1ð Þ > Tolerance,then error and set alarmð Þ,elsex tðÞ ,
and set alarmand counter ¼ 0:0
Y t ðÞ ¼ If Y t ðÞ ¼ error,then Y t 1Þ elseY t ðÞ ,and increment counter
ð
If counter > user limit reset Y tðÞ ¼ error
t is time stamp

3.2.1.2 Out-Of-Range Detection and Value Clip
This algorithm includes a test and a valid value setting procedure. If the test
passes, then the current value is passed along. If the test fails, the current value
is reset to a valid higher or lower value. This test does not assume the tag has
gone bad but that the reading has gone out of range. That is why the value is
not set to bad but reset to a value clipped to the nearest maximum or min-
imum of the acceptable range limits. The over-range (OFR) test should
catch an instrument failure due to an over-range value failure, because a
failure should cause an immediate spike. The test takes the current value
and a user-entered minimum, maximum, high-clip, and low-clip value.
The logic is:
If OFR_test ¼ off ,then skipthis test
Y tðÞ ¼ X tðÞ > Max thenclipabovevalueand set alarm,else X tðÞ and set alarm ¼ 0
Y tðÞ ¼ X tðÞ < Min then clipbelow valueand set alarmelseX tðÞ and set alarm ¼ 0

3.2.1.3 Freeze Detection and Value Hold
This algorithm includes a test and a valid value setting procedure. If the
test passes, then the current value is passed along. If the test fails, the last