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COURSE IN CP INSPECTION METHODS

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

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COURSE IN CP INSPECTION METHODS

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  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 - 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.)

  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 -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.

  6. Impressed Cathodic Protection Sacrifical 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 -1050 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 FieldsShowing Local Variations In Proximity To Anode

  12. Schematic potential profile

  13. Principle of Electric Field Gradient Measurement Principle of Potential (CP) 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 GradientCP 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 referencecells, 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: - 1010 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 StudiesEvaluation 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. Simulation performed by CorrOcean’s SEACORR/CP system

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