overline assessment pipelines. figures. jan 2003.ppt [read...

V = Measured potentialM

V = Ohmic component ( IR error ) IR

V = Polarised Potential P

V = V + VM IR

Fig 1I.R. Drop Error Component of Potential Measurement

VM

AnodeVIR

P

+

-

+ -

Pipe

D.CSupply

I.R Error

Fig 2Idealised Polarised Potential Measurement

0 500 1000 1500 2000

- 0.8

- 1.0

- 1.2 Polarised or InstantaneousOFF Potential

Time - Milli- seconds

Depolarisation

C.P.ON C.P. OFF

Switch OFF

Fig 3Close Interval Overline Potential Survey

Distance andcontact wire

TestPost

ReferenceElectrode

Recording Voltmeter( Data logger )

Defect

DefectDefect

Soil Variance MetallicContact

DISTANCE

-1.5

-1.0

-0.85

-0.5

0Pipe

/ So

il Po

tent

ial

wrt

Cu/

CuS

Ovo

lts4

Fig 4Close Interval Overline Polarised Potential Survey

Distance andcontact wire

TestPost

ReferenceElectrode

Recording Voltmeter( Data logger )

Defect

DefectDefect


DISTANCE

-1.5

-1.0

-0.85

-0.5

0Pipe

/ So

il Po

tent

ial

wrt

Cu/

CuS

Ovo

lts4

TransformerRectifier

Timer/Contactor

Groundbed

Fence

Test Point Fence

Pipe /SoilPotential ‘ON’

I.R. Drop FreePolarised ‘OFF’Potential

0 10mS

- 0.6- 0.7- 0.8- 0.9- 1.0- 1.1- 1.2

Switch Off

C.P. ON C.P. OFF

0.1 - 10 seconds

Time not to scale

PipelineReactance effectplus switchingspike

Depolarisation

This period significantlyshortened in the presence of sulphate reducingbacteria

Can extend to50mS in some circumstances

Pipe

/ So

il Po

tent

ial

wrt

Cu

/ CuS

O v

olts

4

Fig 5Polarised Potential Measurementwith Pipeline Reactance and Switching Spike Effects

6 7 8 9 10 16 17 18 19 20

* * * * ON OFF ON OFF

Time 100x mS

Measurements

Distance Metres

Pipe

/ So

il Po

tent

ial

wrt

Cu/

CuS

O v

olts

4

Survey Characteristics

Nominal Speed 1 metre / second ( 3.6km / hour ) ON : OFF Ratio 4:1 OFF Period 0.2 second ON Period 0.8 second Measurement spacing. 1 metre nominal Measure both ‘ON’ and ‘INSTANT OFF’ every 1 metre

Fig 6 Typical Combination of ‘ON’ and ‘OFF’ PotentialMeasurements During Fast Survey

0 1 2

- 0.6- 0.7

- 0.8

- 0.9- 1.0- 1.1

Pipe

/ So

il Po

tent

ial

wrt

Cu

/ CuS

O4

volts

Fig 7Polarised Potential Measurement Errorsdue to Inaccurate Timing

1 2

1 21 2

1 2

Indicated Polarised Potential

- 0.95v optimum timing ( accurate)

- 0.88v 0 - 100mS early- 1.10v > 100mS early

- 0.94 late

1 ‘Wait period’2 ‘Measure period’

Survey CharacteristicsOFF period 1 SecondWait period 0.1 SecondMeasurement period 0.1 Second

0 200 400 600 800 1000Milli - Seconds

Fig 8Close Interval Overline Polarised Potential SurveyMobile + Static + Switching Device

Mobile Data logger

Defect

DefectDefect



Timer/SwitchingDevice

StaticDataCollector

StrayCurrent

Mob

ileSt

atic

Precise Time Linkage Mobile and Static

Fig 9Close Interval Overline Polarised Potential Survey and DCVGMobile Data logger + Static + Switching Device

Defect

DefectDefect



Timer SwitchingDevice

StaticDataCollector

StrayCurrent

Precise Time Linkage Mobile , DCGV and Static

DC

VG 0

+200

-200

Approaching a Defect Over a Defect Past a Defect

Current FlowThrough the Soil

Current FlowThrough the Soil

EquipotentialGradientLines

DefectDefect



Slow Non PrecisionSynchronisedTimer/SwitchingDevice

Survey Progress

No RecordingAnalogue Indication OnlyNo Local Pipe / Soil PotentialNo Distance MeasurementPeg / Mark and Manually Record

Pipe / SoilPotential at Testpoints only

Pipe

Repeat Measurements1 2 3 4

15mV 8mV 3mV 1mV

= 27mV

Defect

Analogue DCVG Defect Characterisation

1. “DEFECT IR” : “Overline to Remote Earth” = 27mV.

2. “Pipe / Soil Potential shift ( ON - OFF ) : Applied Signal” at Defect is as straight line attenuation between test points, say 300mV

3. Defect Size ( % IR ) = “ Defect IR” mV

say = 9% 300

ESTIMATED

27

“Applied Signal” mV

Fig 12Non Recorded Analogue DCVG Defect Characterisation

For Example

Combined Enhanced CIPS & Recorded DCVG to Indicate Defect Size.

Defect

DefectDefect



PrecisionTimer /SwitchingDevice

OptionalStaticDataCollector

StrayCurrent

Precise Time Linkage Mobile , DCGV and Static

DC

VG 0

+200

-200

Fig. 13

0

-200

Static Data on anothergraph

Sumed Pipeline PL2C - Defect Sizing and Instant Off summary

Distance (m)5600055000540005300052000

Def

ect S

izin

g (c

alcu

late

d ra

dius

(cm

))

0-5

-10-15-20-25-30-35-40-45-50

Instant Off potential (m

V)

0-200-400-600-800-1000-1200-1400-1600-1800-2000

overline assessment pipelines. figures. jan 2003.ppt [read...

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