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Technical Seminar for Cathodic Protection
to GOGC Design Unit Specialists
Dr. Nick Kioupis, Cathodic & Lightning Protection
Section Head, DESFA
CP Measurements
rectifier output voltage and current;
ON- and, if practical, OFF- pipe-to-electrolyte (e.g. soil) potentials;
ON- and OFF-potential and current flow at coupons, if any;
effectiveness of any electrical isolation (e.g. isolation joint, casing,
spark gaps, etc.);
ON-potentials on bonded foreign pipelines and the magnitude of
the current flow to or from them;
effect of any d.c. interference with a foreign pipeline;
effect of any a.c. or d.c. interference current from a foreign source;
corrosion rate on ER probes if any
earthing electrodes resistance to remote earth
effectiveness of dc-decoupling devices
Potential measurement
pipeline
multimeter
Reference electrode
Typical CP test post
soil
Instrument connection for potential measurement
Buried structure to electrolyte potential
Electrical equipment
• Periodic calibrations and safety checks
• Errors
― Errors from reference electrodes
Buried structure to electrolyte potential
Electrical equipment
• Analog voltmeters
• Digital voltmeters
Buried structure to electrolyte potential
Electrical equipment
• Analog voltmeters
• Digital voltmeters
Buried structure to electrolyte potential
Potential measurement
Buried structure to electrolyte potential
Potential measurement
Buried structure to electrolyte potential
Currents causing IR drop errors
Buried structure to electrolyte potential
Potential measurement techniques
off potential measurements (instantaneous off potential technique)
special off potential measurements
Intensive measurement tecnique
potential measurement for pipelines with external potential test probes or coupons
CP Measurements
Time
Instantaneous off potential waveform
100 mV criterion potential decay and formation
waveforms
Intensive measurement technique
Current measurements (d.c.)
CP Measurements
Key
1 bond
2 ground level
3 test station
4 electrolyte (soil, etc.)
5 pipeline
6 secondary structure (galvanic anode)
A, B terminals
Current measurement
CP Measurements
Current measurement
a) Remove bond between terminals
A and B.
b) Measure voltage, U1, across AB
with high-impedance
millivoltmeter.
c) Connect ammeter across AB and
measure current, I.
d) Measure voltage, U2, across AB
with ammeter still connected.
e) Calculate true current, Itrue, from
Equation
1true
2 1
UI I
U U
CP Measurements
Line Current measurement
CP Measurements
Current measurement in presence of
AC
Soil Resistivity Measurements
• Wenner method
ρ = 2 π α R
• Soil box
23
24
Soil resistivity measurements setups
25
Isolating joint class
Based on test with a.c. voltage 50Hz for10 s:
• Class 1: between 2,5 kV and 5 kV r.m.s.,
• Class 2: below 2,5 kV r.m.s.
26
Electrical tests for isolating joints installation
27
Electrical tests for isolating joints before installation
28
Vd.c. (laboratory)
Electrical resistance test
Va.c. (laboratory)
Dielectric strength test
Va.c. (field before the installation)
Electrical resistance test
Electrical test: 1000
Vd.c. for class 1
isolating joint and 500
Vd.c. for class 2
isolating joint
Resistance test: ≥ 20
MΩ
Electrical test:
For class 1 isolating joint :
2500 Va.c. rms per 60 s –
no internal or external short
circuit
For class 2 isolating joint :
1500 Va.c. rms per 60 s –
no internal or external short
circuit
Electrical test: 1000 Va.c. for class
1 isolating joint and 500 Va.c. for
class 2 isolating joint
Resistance test: ≥ 5 MΩ
Isolation failures
• Burned by overvoltage (e.g. lightning)
• External involuntary short-circuit (e.g. supports, foreign lines)
• Internal short-circuit (e.g. conductive fluids, deposits)
29
Isolation tests
• On/Off potential
• Resistance (Impedance)
• Impressed current tests
• Electromagnetic signal
30
• Audio-frequency (20Hz-20kHz) generator measurements
Isolation tests
31
Test of isolating joint with battery and clamp
32
Clamp-on Current Probe
33
34
Isolation tests
(+) T/R (-)
COM
A1
A2 COM
CP
MF
GR
35
Modified test method proposed by DESFA
R1΄ R2΄
Rij
36
Empirical Criterion
Deviations
100][
21
21
RR
RRRs
ij
Rij = measured ij impedance
R1΄, R2΄ = measured impedance of areas 1 and 2 (either side of ij)
37
• s < -10% not acceptable
• -10 % ≤ s < 0 % usually acceptable, further test required
• s ≥ 0 % acceptable
38
Application of criterion s to a casing isolation case
39
CP Measurements
CP Measurements