1 COURSE IN CP INSPECTION METHODS FOR CORROCEAN Part II CP Inspection

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COURSE IN CP INSPECTION METHODS

• FOR CORROCEAN• Part II• CP Inspection

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• Check the CP system’s ability to avoid corrosion problems

• Detect any corrosion problems to adjust/retrofit before any major failure

• In general secure integrity of the structure/pipeline

• Collect data to reduce future inspection requirements

• Regulations/Authorities

Why CP inspection?

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Standards and regulations Regulations concerning load bearing structures in

the petroleum activities Guidelines on corrosion protection of load bearing

structures in the petroleum activity – 1992 Guidelines on condition monitoring of load bearing

structures to regulations concerning load bearing structures in the petroleum activities - 1992

Regulations relating to pipeline systems in the petroleum activities Guidelines on corrosion protection of pipeline system

etc. - 1998 DnV RP B401 1993 DnV RP B403 1987 (Calibration procedures etc.)

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Phases in an offshore structures life

A D

-1040

-900

-850

-650

mV wrt Ag/AgCl

B C

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Phases cont. Period A - Polarisation of new structure/pipeline. (Months)

Normally very short but dependant on design criteria and anode output characteristics.

  Period B - Protected Design Life. (e.g. 15 years )

Stabilised CP conditions but dependant on CP design, anode efficiency, coating breakdown and environmental conditions.

e.g.. Design for approx 5 to 10 % coating failure.CP level desired; -950 mV to -1050 mV.

  Period C - Depolarisation Period.

Period when anodes are reaching end of design life with reduced efficiency. The slope will depend on anode and coating conditions affecting protection level. Accuracy of CP measurements most critical.

  Period D - Under Protection.

This is a critical period when protective levels drop below -800 mV (or -900 mV for buried pipe). Danger of failure from corrosion at localised postions.

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Impressed Cathodic Protection

Sacrifical Cathodic Protection

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What do we measure?

• Potential (CP) (vs. Ag/AgCl or Zn ref. Cells)• Electrical Field Gradient (EFG or FG in

uV/cm)• Anode current (mA or Amp)• Visual inspection of anodes

(geometry/consumption/loss of)• Visual inspection of coating damages

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Principle of measurements

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CP measurements level

On steel material – typical potential level With Zinc anodes –800 mV to –1050 mV With Aluminium anodes –800 mV to –1100 mV

Protection level for steel –800 mV. Very well protected –900 mV to -1050 mV Freely corroding steel –650 mV

On anodes Zinc anodes –980 mV to –1050 mV Aluminium anodes –1000 mV to –1100 mV

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Survey Techniques

A Number of Survey Techniques Developed• Cell to Cell Survey (CP Stab Measurement)• CTC-2 Survey (CP/FG Measurement)

(Current)• Trailing Wire Survey (CP Measurement)• Clamp on meter (Anode current)

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Potential FieldsShowing Local Variations In Proximity To Anode

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Schematic potential profile

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Principle of Electric Field Gradient Measurement

Principle of Potential (CP) Measurement

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Stabber and Cell To Cell Technique

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Trailing wire utilising towed fish

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Cell To Cell Technique

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Cell to cell principle

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Trailing Wire Survey Utilising Drop Cell CTC Stepwise Technique

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CP/Field gradient measurements

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CTC-2 Field GradientCP General Arrangement

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CTC-2 Typical results exposed pipe

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FG horizontal offset error

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CP Inspection System Schematic

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Calibration requirements

Calibration of the reference cells used in the CP equipments, e.g. Ag/AgCl or Zn ref. cells

Check of control reference cells, 3 calomel cells required in a calibration set.

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Calibration of calomel reference cells

One set consists of 3 ”equal” calomel reference cells (SCE), i.e SCE 1, SCE 2 and SCE 3

Compare ref. cells by use of Multimeter; accept level from –2 mV to +2 mV.

Selection criteria: All accepted; select any. One reading out of range; the ref cell not in the reading to be used Only one reading OK; selected either of those. All readings outside accept criteria; select the best. After survey

deliver reference cell to laboratory for test.

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Calibration of silver/silver chloride half cell/Stab Reader

Ref. DnV RP B403. Check against either a Zinc-block or

calomel ref cells Ref. cells/ Stab Reader immersed for 15 to

30 min. before check. NOTE! Not water from firewater system!!

Accept criteria Against Zn-block: - 1010 mV to –1050 mV. Against Calomel half cell: +1 mV to –9 mV

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Calibration setup

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Check of CTC-2 probe (two ref. cells)

The matched pair used on CTC-2 system must be calibrated before mounting on ROV

New calibration within each 24 hours or pre and post dive.

FG reading is sensitive to drift/changes in the potential differences between the matched pair (offset) under operation.

”Zero field control”, by measuring ”off structure potential”, then on structure or anode.

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Basic Report For pipelines – raw data plot of potentials and

Field Gradient (FG) (if included in survey). Anode potentials, debris, coating damages etc.

For structures – tabulating potentials and FG readings

Post processing Different plot dependent on requirements

Reporting

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Data analysis

Detailed analysis – based on analytical methods and/or simulations (element methods)

May included the following: Anode current Output (Ia), remaining life (RL), wastage (W) Effect of different degree of burial Effect of changed coating damage percentage; also local effects Overall operating performance of CP system Current drain Stray current Depolarisation Life extension Retrofit Design analysis Trend analysis Optimisation of future inspection program

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Platform Data Trending Platform Potential Surveys For 1979,85,90 & 95

-1050

-1025

-1000

-975

-950

-925

-900

-70 -60 -50 -40 -30 -20 -10 0

Elevation/Metres

Po

ten

tial/m

Vw

rt A

g/A

gC

l Ref

Ele

ctro

de

1979 1985 1990 1995

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CP System StudiesEvaluation of Existing Systems

Basic Approach• Data Input• Methodology Employed• CP Study Output

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Data Input

• Detailed Structural Drawings of Submerged Steelwork• Component Hierarchy Listings• Historical Potential Measurements Recorded• Historical Anode Survey Data• Surface Area of Submerged Steel• Installation Dates• Anode Specifications• Anode Retrofit Details• Metallic Debris Levels• Reports on Remedial Actions

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Methodology Employed

• Review All Available Historical Data and Original Design• Data Input to Database and Manipulation to Req. Format• Analysis of Data to Provide CP Systems Overview and

Status• Determine Intermediate Survey Requirements• Tailored Survey• Calculations to Provide Predictions of Remaining Life• Determine Remedial Actions to Maintain Structure

Integrity to Anticipated End of Life• Computer Modelling for Retrofit Optimisation (if req.)• Determine Long Term Survey Programme

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CP Study Output

• Enhanced Confidence in Performance of CP System• Action Plan to Maintain CP System Integrity to End of

System Life• Tailored Cost-Effective Long Term Survey Programme

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CorrOcean’s Methodology

HistoricalDataDesign

Data

Optim ised inspection Frequency Retrofit

Optim ised Design

Life T im e Prediction

SEA CORRCom puterM odelling

Cois

Survey Data

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Some CP simulation examples. Simulation performed by CorrOcean’s

SEACORR/CP system

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-809.1-852.7-874.5-896.2-918.0-939.8-961.6-983.4

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