Technical Seminar for Cathodic Protection to GOGC Design .... measurements_cp... · Technical...

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

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Soil resistivity measurements setups

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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.

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Electrical tests for isolating joints installation

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Electrical tests for isolating joints before installation

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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)

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Isolation tests

• On/Off potential

• Resistance (Impedance)

• Impressed current tests

• Electromagnetic signal

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• Audio-frequency (20Hz-20kHz) generator measurements

Isolation tests

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Test of isolating joint with battery and clamp

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Clamp-on Current Probe

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Isolation tests

(+) T/R (-)

COM

A1

A2 COM

CP

MF

GR

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Modified test method proposed by DESFA

R1΄ R2΄

Rij

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Empirical Criterion

Deviations

100][

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RR

RRRs

ij

Rij = measured ij impedance

R1΄, R2΄ = measured impedance of areas 1 and 2 (either side of ij)

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• s < -10% not acceptable

• -10 % ≤ s < 0 % usually acceptable, further test required

• s ≥ 0 % acceptable

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Application of criterion s to a casing isolation case

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CP Measurements

CP Measurements