ECDA to assess possibility of AC Corrosion. Mark Yunovich Honeywell Corrosion Solutions January 27 th, 2009

Transcription

1 ECDA to assess possibility of AC Corrosion Mark Yunovich Honeywell Corrosion Solutions January 27 th, 2009

2 What are we talking about today? Assessing the degree of AC interference at the pipeline level - What can one do using ECDA tools to assess the likelihood of AC corrosion - Commonly used criteria Based on current density, not voltage (<20/30, >100 A/m 2 ) - Defining regions? Coating condition (defect sizing) Soil resistivity/depth of cover - Which tools? AC CIS Direct current measurement (CTS) Modeling - Direct Assessment Morphology of damage Not talking about protection criteria (i.e., how much CP is necessary to offset the AC interference) - Still debated - Concern about overprotection Not talking about mitigation

3 Introduction PHMSA-sponsored research The primary focus was ECDA aboveground assessment with emphasis on: DCVG, CIS accuracy with and without interference March 2008 Unique conditions AC corrosion threat assessment

4 Assessing AC Corrosion: AC CIS and what else? Some algebra Laboratory experimental modeling PRCI software assessment Full-scale outdoor testing - Utilized a large scale outdoor facility: multiple sections of coated pipeline, spanning a total of 120 feet internal diameter of 20 inches and an external FBE coating

5 AC CIS Commonly employed technique of assessing the problem AC CIS measurements: AC voltage on the pipe is measured AC voltage can be used to make an approximate calculation of the current density expected for a defect of a known (or assumed size)

6 Experimental site 3 ft sections of pipeline have waterproof end seals Each section contains an intentionally introduced defect - Defects range from 1 to 10 sq.in in area - Circumferential position (2, 3, 9, and 12 o clock) - Contains bare sections - Lateral spacing (2.5 to 5 ft) - Different depth of cover - Different resistivities

7 Experimental site

8 Experimental site HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 8

9 Experimental site

10 AC CIS Laboratory measurements 12:00 or 6:00 12:00 12:00

11 AC CIS - calculations Converting AC voltage into current density - i AC current density (A/m 2 ) - U measured AC voltage (V) - R resistance (ohm) - A assumed or known area of bare metal (m 2 ) R is a spread resistance - circular holiday with diameter d, - resistance of the coating ( pore resistance) - s and f soil and electrolyte resistance - h pore (coating) thickness i R U R s A 4 2d d 2 f h

12 AC CIS - calculations Converting AC voltage into current density Assuming that s = f ; at larger holiday sizes (d 4 h s f R i 2 2d d and substituting h), 8V AC d where: i = AC current density V AC = AC voltage of pipeline to remote earth s = soil resistivity d = diameter of a circular holiday having an area equal to that of the actual holiday

13 AC density, A/sq.m AC CIS - calculations Resistance, ohm AC density, A/sq.m Resistance, ohm Converting AC voltage into current density Density Calculated Resistance 1.00E E E Density Calculated Resistance 1.00E E E E E E E E E E E E E E E E Defect size, cm Lower resistivity i 8V E Defect size, cm AC d Higher resistivity Defect size dominates pore resistance at sizes greater than 1 cm HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 13

14 AC density, A/sq.m AC CIS - calculations Converting AC voltage into current density Influence of soil resistivity on current density is diminished (from practical point of view) as holiday size increases K ohm-cm 500 ohm-cm is 40:1 for all defect sizes, however: For larger defects densities stay under 10A/m 2 for 1000 ohm-cm and above AC voltage fixed at 5V Soil resistivity, ohm-cm HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 14

15 AC density, A/sq.m AC CIS - calculations Converting AC voltage into current density Current density is greater than 50A/m 2 : for AC voltage of as small as 10V cm defect for AC voltage as low as 1V 1 cm defect 10 soil resistivity is fixed at 5,000 ohm-cm AC Voltage HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 15

16 Volts, AC AC CIS - laboratory Converting AC voltage into current density - AC Voltage 4.5V - Clay resistivity 90 ohm-cm - Sand resistivity 74,000 ohm-cm Distance, in Defect 1 Defect 2 Defect 3 AC density, A/m 2 Estimated Measured Error % % %

17 AC CIS - laboratory PRCI model Includes comprehensive electromagnetic coupling equations - with an interface format to enable both steady state and fault induced voltages and currents to be calculated Was not expressly made for predicting AC currents - inner logic could not be modified (it estimates coating condition by multiplying the soil resistivity by a certain coefficient) - a number of assumptions had to be made to accommodate the requirements.

18 AC CIS - laboratory PRCI model Inputs: Location AC current measured, (ma) Impedance, ohm Defect 1 (clay) 4" Defect 3 (sand) 14" Scaling factor had to be employed (1200:1) Coating condition Current, ma Defect 1 Defect 3 Fair Good Excellent Superior Flawless

19 AC CIS - laboratory PRCI model So? - Low resistivity soil consistently higher currents - Coating degrades low current from defect - Defect in sand soil resistivity dominates, even for poor ( fair ) coating condition Coating condition Current, ma Defect 1 Defect 3 Fair Good Excellent Superior Flawless

20 AC CIS - laboratory And? Knowledge of soil resistivity is absolutely critical for AC CIS approach - adjacent soils (sand and clay) can have two drastically different resistivities - Low resistivity (high discharge) margin of error - minimal - High resistivity - dramatically different calculation errs on the side of underestimation of the AC current density. measured density, which is more than 10 times greater, has been demonstrated to cause accelerated corrosion of buried steel

21 AC CIS - Large-scale AC voltage, V 2.5 With AC signal Without AC signal 2 #1-#10 (no AC) #1-# Distance, feet

22 AC density, A/sq.m AC CIS Large-scale Calculated current density CD (actual) Average (calculated) Effective defect size, cm Actual current density HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 22

23 AC CIS Large-scale Less than ideal correlation - Reasonable trend /within an order of magnitude - AC CIS values are lower than actual values - Probable inaccuracy bulk resistance vs. actual resistance of the holiday Smaller sized defects have higher discrepancy Geometry and soil/metal interface properties may dominate bulk resistivity HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 23

24 Coupon Test Stations Frequently used - Permanent - Temporary - Can be used for corrosion rate measurements Issue of how long to expose Literature suggests as long as 1 year Can be used to measure spot AC voltage Large scale facility - 4 CTS - Coupon size approximately 9 cm 2 HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 24

25 Coupon Test Stations 1000% 100% Actual/CTS Holiday only/h+cts 1. Holiday only vs. Holiday plus CTS 2. Actual vs. Calculated current density from CTS AC voltage 10% 1% Effective defect size, cm Adding CTS coupon to a small holiday produces greater error Calculations of current density are more conservative (over-estimation) when connected to large area of bare metal 1 measure of what happens when exposed area is increased 2 measure of how accurate calculated current density is when CTS is connected to pipeline HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 25

26 Coupon Current Density (Amperes/Meter Squared) Coupon Current Density (Amperes/Meter Squared) Coupon Test Stations When/how long to monitor using coupons - Spot-checks vs. data recording Typical time of Day Technician Takes Reading Jul 14-Jul 15-Jul 15-Jul 15-Jul 15-Jul 16-Jul 16-Jul 16-Jul 16-Jul Jul 17-Jul 17-Jul 17-Jul /19 7/20 7/21 7/22 7/23 7/24 7/25 7/26 7/27 7/28 7/29 7/30 7/31 8/1 8/2 8/3 8/4 8/5 8/6 8/7 8/8 8/9 8/10 8/11 8/12 8/13 HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 26

27 AC Corrosion above-ground assessment Commercially available probes - MetriCorr (Denmark, - ER probe - Collects data on corrosion rate, AC current and voltage and spread resistance - Measures DC current and potential

28 AC Corrosion above ground assessments Assessments of AC current density via AC CIS can be attempted and provide order of magnitude estimates Use of equations requires accurate knowledge of defect sizes and resistivities - Can be overly conservative or not conservative enough - Do assessments at multiple locations Use of CTS is helpful - Provides a more accurate estimate of current density - Select coupon sizes carefully - Multiple locations - Data collection interval is important ER probe commercially available HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 28

29 AC Corrosion Direct Examination Whither AC Corrosion? Consider the circumstances Eliminate the usual suspects (Occam s razor principle) Certain morphological signatures - Hard, domed-shape cluster of soil and corrosion products - Isolated or grouped pitting HONEYWELL INTERNATIONAL, INC. All rights reserved 11 February 2009 Slide 29

30 AC Corrosion Direct Examination

31 AC Corrosion ECDA takeaways Not easy - Probabilistic assessment - Criteria not completely agreed upon Segmentation similar to DC ECDA - Includes consideration of interference - Coating conditions - Resistivity/resistance issues Above-ground methods are few - AC CIS - Spot measurements - CTS - Probes Direct examination - Eliminate other causes - Consider CP levels - Consider morphology

32 Questions?