Mears Group, Inc. Integrity Assessment for Non-Piggable Pipelines By Jim Walton

Transcription

1 Mears Group, Inc. Integrity Assessment for Non-Piggable Pipelines By Jim Walton

2 External Corrosion Direct Assessment Why use ECDA? To evaluate the Cathodic Protection and Coating condition and give a forward looking evaluation. Where ILI or hydrostatic pressure testing cannot be used. To avoid impractical, costly retrofitting of a pipeline. To avoid interrupting gas supply to a community fed by a single pipeline. To provide an alternative where sources of water for hydrostatic pressure testing are scarce and where water disposal may create problems. From the PHMSA DA fact sheet on their web site DA may provide a more effective, equivalent alternative to ILI and hydrostatic pressure testing for evaluating a pipeline s integrity. 2

3 Four Steps from ECDA Step One: Pre-assessment - to gather and integrate data to determine the feasibility of using ECDA for a segment, the identification of ECDA regions, and the identification of two indirect examination tools to be used on the ECDA region. Step Two: Indirect Examination - to evaluate the pipe segment and identify indications of potential external corrosion, to classify the severity of those indications, and determine urgency for their excavation and direct examination. Step Three: Direct Examination - to examine the condition of the pipe and its environment, to determine actions to be taken should corrosion defects be found, and to identify and address root causes. Step Four: Post Assessment - to determine a segment s remaining life, its re-assessment interval, and the effectiveness of using ECDA as an assessment method. Four steps similar to the above are also required for the other direct assessment methodologies. 3

4 Typical Findings from DA Coating defects Areas of low CP corrosion threat Depth of Cover concerns including exposed pipe Right of way encroachment issues Pipeline feature concerns Rectifier issues, test leads, above ground piping issues Bends in pipe Soil conditions Type of cover over pipeline Pavement, water, soil, trash, etc. Location of the pipeline Shorted Casings 4

5 Other findings - DA during bell hole inspection Third Party Damage MIC (Microbiologically Influenced Corrosion) Size, type and wall thickness of pipe Coating type Soil PH and resistance Cracking SCC from Mag particle inspections Metal Loss 5

6 Limitations of DA Needs thorough history of the line to be most effective May or may not identify past problems such as corrosion that occurred previous to the assessment The effectiveness of DA largely rests on the ability of assessment calls to locate or predict past or currently active corrosion locations on a specific pipeline or pipeline segment and predict future problem sites. Third party damage location is based on digs at coating defects near possible known encroachments Is really a predictive analysis of where corrosion may be occurring and must be proven through excavation. Does not find areas of concern under disbonded coating. 6

7 What methods should be utilized for indirect inspections? Current Attenuation (ACCA/CA/Electromagnetic, PCM, PDM) Close Interval Survey (CIS/CIPS) Direct Current Voltage Gradient (DCVG) Alternating Current Voltage Gradient (ACVG) Other

8 Principles of CA A specific signal/frequency is applied to the pipeline A magnetic field is created around the pipeline A receiving device at the ground surface receives the signal without needing ground contact At areas of lower pipe to ground resistance (e.g. holidays) the current attenuates at a higher rate Location, depth and distance must be established to acquire the needed current measurement Distance between readings can vary (50 to 100 foot spacing is common although distances much greater can be utilized in some circumstances)

9 What can CA data tell us? It is a macro tool that highlights the bigger problems in a coated pipeline system including: Shorts to other structures Grounding to electric neutral Bad insulators Large coating defects Shorted casings Other current distribution problems Depth of cover at the same time (possible TPI) Any or all may affect the ability of the other tools to perform in a consistent and reliable manner.

10 Methodology Use an independent ground and try to mimic your CP circuit when possible Make sure rectifiers are not influencing the signal (turn off AND disconnect if necessary) Isolate your circuit whenever possible (disconnect bonds for better surveys) Take readings at equal distances and record your distances Every 50 feet is a good standard (others can be used dependant on location) Use it as a macro tool and depth of cover tool (use A-frame for micro) Look for anomalies with more than a 5% change normally Make sure unit is upright and perpendicular to the pipe Stay on peak and check peak and null readings and verify depth when readings are suspect. Take multiple readings in one location if you are suspect of the accuracy. Know what is in the area of your pipe and what it is connected to it.

11 PCM Hz Current Pipeline Current Distance in 10 ft.

12 Definition of CIS A voltage potential survey performed on a buried or submerged metallic pipeline, in order to obtain valid DC structure-to-electrolyte potential measurements at a regular interval sufficiently small to permit a detailed assessment. The terms close-interval potential survey (CIPS) and close-interval survey (CIS) are used interchangeably

13 Types of CIS Interrupted CIS - A DC structure-to-electrolyte potential is collected during both the CP on state and the CP instant off state On CIS No current is interrupted Native CIS Before any current is applied Static/Depolarized CIS Rectifiers are turned off for a period of time to allow the line to depolarize.

14 Principals of CIS A DC structure-to-electrolyte potential is collected using a high resistant volt meter and a copper copper sulfate half cell while connected to the pipeline and touching the ground surface. Spacing is determined by depth of pipeline and foot spacing is common All influencing current must be interrupted All interrupters must be synchronized Sufficient ground contact must be achieved

15 What can CIS data tell us? It is a tool that gives us a relative state of the cathodic protection on the pipeline and can find: Areas of adequate and inadequate protection Larger coating defects Possible interference areas Shorted casings Areas of excessive polarization Isolation problems Current Direction and Amount (Metal IR)

16 CIS The Classics Large Dip in the On and Off measurements Dip in the On measurement but not in the Off measurement Large Rise in both the On and Off measurements High Off measurement

17 CIS The Not So Classics Off measurement higher than the On Reasonable On measurement with very low Off measurement Reasonable On measurement with very high Off measurement Positive measurements

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19 Principles of ACVG ACVG uses the differences in the electrical field between two known locations. One type of ACVG (the A-Frame) uses fixed pins on a frame that are 2 apart. Since the distance never changes, each data point is comparable. ACVG can find large to very small coating holidays. Most ACVG devices will have a meter or a display that will show the operator which way the holiday or indication is located. The severity of the holiday is indicated by the strength of the signal at the holiday location. Differentiate between isolated (discrete) and continuous anomalies Predict relative importance/severity of anomaly

20 A-frame a.k.a ACVG Voltage Gradient Survey Used as a micro and a macro survey tool. Can be used in various soil conditions but ground contact of the probes will affect the readings. Rectifiers generally do not influence the ACVG survey. In areas of high interference or where current survey does not give you answers, ACVG can still be used with more success. Larger holidays can mask smaller ones until the larger holidays have been repaired. PCM displays A-frame readings in db or decibels. Remember every 6 db higher is twice as large (current affects db). Metal to dirt contact and soil resistance (not necessarily size of holiday) is what affects the db readings.

21 What can ACVG data tell us? It is a micro tool that pinpoints coating defects and can find: Large coating defects Small Coating defects Severity of the coating defect Possible interference areas Shorted casings CP cable breaks Position of Anodes

22 ACVG Methodology Use an independent ground and try to mimic your CP circuit when possible. Take readings parallel and along the pipe. When you see an arrow reversal go perpendicular to the pipe and make sure arrow reverses over pipe. You do not have to be right on top of the pipe when surveying. On concrete and asphalt use wet sponges or rags on the probes or wet the ground around the probes. Take readings at equal distances usually about every two to three steps. Use the largest db reading seen around the anomaly for you records or at the indication about 3 feet off the pipe. Record all faults seen with db readings and footages or GPS coordinates. ALL LOCAL CONDITIONS AND ALL DATA ANALYZED TOGETHER DETERMINE WHAT IS A DIG!

23 ACVG db microvolt The ACVG indications are normally categorized as: 0-49 dbμv - very minor dbμv - minor dbμv - moderate dbμv - severe. Remember that the amount of current flowing on the pipe, the contact resistance of the probes and soil resistance will affect the dbμv level.

24 ACVG tools

25 ACVG graph Fault Signal (db) 90 Maximum Fault Value (77dBuV) Fault Signal (db) Fault Position (54m) Meters

26 ACVG Analysis Number of indications The db indication number recorded Current Adjusted ACVG db values db Microvolt Severe, moderate, minor

27 PCM and ACVG Amps db Microvolt Current ACVG indication Flag numbers 0

28 Combined Data with ACVG CIS on - CIS off Depth of Cover ACVG

29 Principles of DCVG DCVG uses the differences between two reference electrodes. These differences can assist in pinpointing holidays and corrosion cells on a pipeline. DCVG, because it uses DC current, can also determine if the holiday is cathodic (protected) or anodic (corroding). Asynchronous interruption (0.3sec on, 0.7sec off) is required Accurately (± 4 ) locates coating anomalies Differentiate between isolated (discrete) and continuous anomalies Predict relative importance/severity of anomaly Examine the direction of the current flow (anodic vs. cathodic)

30 What can DCVG data tell us? It is a micro tool that pinpoints coating defects and in some instances can give us a relative state of the cathodic protection at that indication and can find: Large coating defects Small Coating defects Severity of the coating defect Possible interference areas Shorted casings CP cable breaks Position of Anodes Cathodic/Anodic condition at the time

31 DCVG Methodology Use an independent ground and/or try to mimic your CP circuit when possible. Take readings parallel and along the pipe. When you see an arrow reversal or needle reversal go perpendicular to the pipe and make sure arrow reverses over pipe. Notice and record the anodic/cathodic indication. Obtain a side drain measurement. Obtain signal strength by measuring on/off potentials at test stations before and after the survey area. You do not have to be right on top of the pipe when surveying. On concrete and asphalt use wet sponges or rags on the probes or wet the ground around the probes. Many times will require drilling holes to obtain dirt contact Take readings at equal distances usually about every 5 10 feet.

32 DCVG tools G1 PCS-2000 DCVG meter

33 DCVG Meter screen

34 DCVG Analysis In Line Method Parallel Method mv Signal strength mv value at the indication Side Drains % IR Severe, Moderate, Minor Cathodic Conditions Anodic Conditions Multiple Pipelines - Common ROW

35 GPS Log Type of Survey Flag Chainage (Survey Feet) Distance between points (Feet) Depth Depth of Cover - 12 inch pipeline Current Current Direction Notes Combined Pipeline Attributes Maximum mv Total mv DCVG Indication %IR Cathodic / Anodic DCVG Notes DA Survey -3.0V on, -1.0V off Pipeline Marker Line Info /22/2011 Marker DCVG Start DCVG Indication % DA Survey Info DCVG Indication % DA Survey Info Backward DA Survey Info DA Survey Info Creek DCVG Indication % DA Survey Info DA Survey Info DA Survey Info DCVG Indication % Cathodic/ Cathodic Cathodic/ Cathodic Cathodic/ Cathodic Cathodic/ Cathodic

36 CIS and DCVG data CIS voltage DCVG voltage On Voltage Off Voltage DCVG Voltage Footage (x5) 0

37 Combined Data with DCVG

38 Coating defects observed after excavation of pipeline

39 Damage observed after coating removed

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41 What data is most important? IT DEPENDS! What is the history of the line? Leaks, Stray Current, TPI, dis-bondment, soils, CP, etc. Are there shorts or bonds in the system? Is there new coating followed by old coating? Are there paved or very rocky areas? How many rectifiers are in the system to be surveyed? How many foreign crossings are there? And the list goes on

42 Summary Multiple applicable tools are key to having a good indirect inspection survey. While NACE SP only requires two inspection techniques it is required at some locations to have a minimum of three. Knowing where your holidays are AND the cathodic protection state are key in making informed decisions about External Corrosion. Soils can also be of added value where the tools need supplemental data for support. Of course any and all Pre-Assessment data is crucial to success.

43 Summary (cont.) Combining data and tools are crucial to proper identification of possible external corrosion. Other items in the indirect inspections such as depth of cover and foreign crossings should be included in the analysis at indications to look for Third Party Damage. If one tool shows an indication and the others do not, it should be verified as to why this occurred and not ignored. Coating is not perfect. Therefore if you find nothing you need to find out why the tools didn t work, resolve the issues and resurvey where necessary. All local conditions must be considered before deciding what indications are monitored, scheduled and immediate.

44 QUESTIONS? Jim Walton Vice President Mears Group, Inc. Integrity Solutions Division 1840 Hutton Drive, Suite 190 Carrollton, TX Office: Cell: