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

2. Why CP inspection?
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

3. 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
Regulations relating to pipeline systems in the petroleum activities
Guidelines on corrosion protection of pipeline system etc
DnV RP B
DnV RP B (Calibration procedures etc.)

4. Phases in an offshore structures life

5. 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 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.

6. Sacrifical Cathodic Protection
Impressed Cathodic Protection

7. 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

8. Principle of measurements

9. 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 mV
Freely corroding steel –650 mV
On anodes
Zinc anodes –980 mV to –1050 mV
Aluminium anodes –1000 mV to –1100 mV

10. 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)

11. Potential Fields Showing Local Variations In Proximity To Anode

12. Schematic potential profile

13. Principle of Potential (CP) Measurement
Principle of Electric Field Gradient Measurement

14. Stabber and Cell To Cell Technique

15. Trailing wire utilising towed fish

16. Cell To Cell Technique

17. Cell to cell principle

18. Trailing Wire Survey Utilising Drop Cell CTC Stepwise Technique

19. CP/Field gradient measurements

20. CTC-2 Field Gradient CP General Arrangement

21. CTC-2 Typical results exposed pipe

22. FG horizontal offset error

23. CP Inspection System Schematic

24. 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.

25. 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.

26. 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: mV to –1050 mV.
Against Calomel half cell: +1 mV to –9 mV

27. Calibration setup

28. 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.

29. Reporting 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

30. 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

31. Platform Data Trending

32. CP System Studies Evaluation of Existing Systems
Basic Approach
Data Input
Methodology Employed
CP Study Output

33. 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

34. 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

35. 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

36. CorrOcean’s Methodology

37. Some CP simulation examples
Some CP simulation examples. Simulation performed by CorrOcean’s SEACORR/CP system