(DCVG) above grade direct current voltage gradient survey.

Parameters affect all of UG structure coating defects detection, sizing and effectiveness of cathodic protection based on coupon off- potential evaluation

BY Kamal Mohamed :Corrosion Engineer1/8/2014

Study had carried out on many different parameters which affect all of detection and sizing of UG structure coating defects that significantly affect all of defect detection ,decision of coating defects repair , evaluation of cathodic protection system and utilization of coupon off-potential to alternate structure to electrolyte off-potential .

ABSTRACTDCVG survey is a method of measuring the electrical voltage gradient in thesoil along and around the pipeline to locate coating holidays, categorize its relative severity once it is found and indicate the effectiveness of cathodic protection system.

There is quantitative relationship between DCVG indication and the presence of coating defects Such a relationship depends on many parameters such as soil resistivity , depth of soil above the defect , DC signal strength , the condition of coating away from the defect ,level of cathodic protection ,polarization resistance and measuring devices accuracy /sensitivity.

The objective of this study was to indicate the accuracy and realistic of pipeline coatingdefects detection , coating defects sizing. and effectiveness of cathodic protection system in case of using coupon off-potential to alternate structure to electrolyte off-potential during the service.

Results from DCVG techniques as per NACE TM0109-2009 show that soil and coating parameters are significantly affecting all of DCVG indication ,OL/RE and the calculated IR% which will be reflected in coating defects detection ,sizing ,evaluation of pipeline coating conditions and effectiveness of cathodic protection in case of utilization coupon to electrolyte off- potential to alternate structure to electrolyte off- potential . .

Introduction Direct Current Voltage Gradient (DCVG)

DCVG survey is a method of measuring the electrical voltage gradient in thesoil along and around the pipeline to locate coating defects, categorize its relative size/severity once it is found and indicate the effectiveness of cathodic protection.

This is done by using two calculations, the first is in units of mV (DCVG indication) and it is used to determine the coating defect location as measurements are made along the length of the pipeline. The second calculation is termed as a percent-IR calculation and it involves measurements moving away from the pipeline. The measurements of DCVG in mV are completed by detecting a voltage gradient at the surface of the ground above the pipeline coating defect. This voltage gradient is detected by the use of two Cu/CuSO4 electrodes. These electrodes are placed at the bottom of walking stick probes as those used in the CIS survey. One electrode is placed at the ground surface directly above the pipeline and the other is placed at a location approximately 1.2 meter away from the pipeline, but also at the ground surface. There is no direct connection made to the pipeline as in the CIS survey. This measurement takes into account only the voltage gradient in soil surface found

between the two electrodes which is the result of current entering the pipeline at a coating defect (DCV indication or Maximum DCVG). CP rectifiers are interrupted at a regular cycle which creates a DC signal detected by the potential difference between the electrodes. The electrodes are electrically connected to a voltmeter which displays the voltage gradient detected in mV. Measurements along the pipeline are typically made at 1.2meter intervals. When the survey begins, the surveyor shall nulls the voltmeter so that the first value or reading is at zero. This means that as the CP current is interrupted, the voltmeter remains at zero even when the CP current is switched on and off. As a coating defect is approached, the voltmeter will begin swinging in either the positive direction or the negative direction from zero depending on the direction of current detected in the soil or electrolyte(Figure 1).

Figure (1) DCVG and structure / electrolyte current direction

When the CP current is switched off, the voltmeter goes back to zero. However, as the coating defect approaches, the voltmeter continues to give either the positive or negative reading when the CP current is switched on that is consistent with the previous measurements. The magnitude of this value (DCVG indication in mV) increases as the coating defect approaches. The magnitude of the voltage reading will reach its maximum when the surveyor’s measurement is made directly above the coating defect. This is evidenced by the voltmeter’s sudden swing from positive to negative or vice versa when the defect is passed. However, the absolute value of the reading will decrease back towards zero as movement is made further away from the coating defect. This behavior found by use of the voltmeter is due to the detection of a direction change in the flow of current in the electrolyte. The current in the electrolyte or soil is always moving toward the coating defect, therefore when the defect is passed there is a change in direction of the current. Once a coating defect is found and located, its size and severity must be determined. This further characterization of the coating defect is done through the calculation of DCVG in percent-IR. There are two steps that the field surveyorfollows in determining the percent-IR value for a given coating defect . The first is by taking lateral voltage gradients moving away from the pipeline. These lateral measurements are shown in Figure (2 ) Successive lateral measurements are made(OL/RE) in the direction away from the pipeline and the coating defect until the voltage gradient reaches a value of less than or equal to one mV. The location where the lateral voltage gradients are not greater than one mV is termed either as remote earth or IR infinity. these voltage gradients are measured by the 1.2 meter spacing of Cu/CuSO4 electrodes

Figure (2): The percent-IR calculation is shown using the lateral(OL/RE=V1+V2+V3+V4+V5 -----till remote earth )

and interpolated value of IR drop over the coating flaw.

The voltage gradients are then added up and divided by the IR drop at the soil surface directly above the coating flaw. However, the IR drop over the soil surface is not measured directly since a direct connection to the pipeline is not made. Therefore, this value must be interpolated by using the known values of potentials that are directly connected to the pipeline. This is shown in Figure (3 )as theclosest test posts are used since they are directly connected to the pipeline. At the location of each test post the IR drop can be measured.

Figure( 3): A diagram showing how an IR measurement is interpolated at theground surface above the coating defect.(IR mV at the defect = X mV +X1*(Y mV-X mV)/(X1+X2)

Once a percentage is calculated for a given coating defect , it is categorized based on what range of values it lies in. There are three categories that are generally used to differentiate the severity or size of holidays. The first category is for IR drops between zero and 15 percent considered to be safe and not severe. This is because the CP system is expected to provide adequate protection for a coating defect with such a low percent-IR value. Therefore, no action is needed. If the percent-IR is between 15 and 35 percent, A sufficient amount of coating damage is expected to be present for a percent-IR value in this range. However, a field engineer is allowed to make the decision to either take immediate action and excavate or to schedule another survey .

The third category %IR above 35. For a percent-IR value above 35 percent, immediate action is necessary. excavation at the site of the coating defect is needed so that the pipeline can be physically repaired.

Cthodic protection review

Cathodic protection is a scientific approach used to protect a metal structure from degradation. It involves electrically connecting two metals in an electrolyte. For underground pipelines, the soil can be considered to be the electrolyte. There are two different types of cathodic protection.

One is through use of a sacrificial anode and the other is by impressed current. CP with the use of a sacrificial anode involves galvanically coupling the pipeline with a metal more active than the metal of the pipeline. The metal that is considered to be more active is the metal that has a more negative standard equilibrium potential. Once they are connected a potential difference develops between the two metals. The more active metal acts as the anode and the more noble pipeline metal acts as a cathode. As the more active metal, the anode will give up its electrons much easier than the noble metal. in case of free corrosion, there are two types of reactions that can occur normally at the pipeline’s surface. the first type of these reactions is the oxidation of ironFe Fe2+ + 2e− (oxidation or anodic reaction)The other type of reactions is the reduction of oxygenO2 + 2H2O + 4e− 4OH− (reduction or cathodic reactions)

2H+ + 2e- H2( gas) (reduction or cathodic reactions)

These two types of reactions must be electrically balanced so that they proceed at the same rate. Since the integrity of the pipeline can be compromised by the iron dissolution reaction, the rate of this reaction must be reduced. This is done by providing the excess of electrons from the anode. The metal dissolution reaction of the anode is given asM Mn+ + ne−

where M represents the sacrificial anode metal being oxidized. By degradation of the anode metal, electrons are supplied to the pipeline so that the oxygen reduction reaction or hydrogen reduction reaction can occur which reduces the rate of the iron oxidation reaction. The electrons are delivered to the pipeline through a low resistance wire as shown in Figure (4)

Figure (4) sacrificial anode cathodic protection systemTherefore, through galvanic coupling the more active anode begins to degrade which further protects the more noble pipeline metal. Most sacrificial anodes are magnesium ,aluminum( sea water) and zinc since they are all more active as compared to steel pipelines. CP systems with impressed current involve supplying current from an external source in order to protect pipelines (figure (5)). This is done by a rectifier or DC generator which can convert alternating current from an external power source to direct current. This creates a voltage drop between the anode and the pipeline which drives positive current from the anode to the pipeline through the soil (electrolyte) in other words electrons drives from the anode to the pipeline through the low resistance wire. by an excess of electrons at the pipeline’s surface, the potential of the metal is polarized to a more negative potential. The rate of oxygen reduction reaction is increased and the anodic or oxidation reaction which normally occurs at the pipeline’s surface becomes unfavorable. If the potential of the pipeline becomes too polarized the hydrogen evolution reaction can occur.

H2O + 2e− H2 + 2OH−

The evolution of hydrogen can result in hydrogen embrittlement of the pipeline, so it is necessary to ensure that polarization does not cause the pipeline’s potential to become too negative. In impressed current. The current is supplied to the pipeline from the anode through the soil. When the current reaches the pipeline it travels toward the low-resistance wire. This low-resistance wire allows for the return of current to the anode from the pipeline. The anode in this system is made up of an inert material so that it will not chemically react with the environment and degrade as a sacrificial anode would. Sometimes the anode can be more noble than the pipeline. In this case, the rectifier must overcome both the resistance of the circuit and the back potential created from the more noble anode in order to flow current in the proper direction. In order to balance the reduction reactions occurring at the pipeline’s surface, there must be oxidation reactions occurring at the surface of the impressed current anode. The main reaction occurring is the oxidation of water orthe evolution of oxygen given as2H2O O2 + 4H+ + 4e− At more extreme positive potentials the evolution of chlorine can also occur as2Cl− Cl2 + 2e− (2.6)

Figure ( 5 )(Schematic drawing for impressed current cathodic protection system)

Study done on parameters affecting all of maximum DCVG , OL/RE and the calculated IR% which affect coating defect detection , sizing and effectiveness of cthodic protection based on coupon off- potential.

1- soil depth over the coating anomaly

As mentioned in NACE Standard TM0109-2009 (1.2.2) the detection sensitivity of indirect inspection tools decline as the pipe burial exceeds normal ranges. Although this “normal” range is not clearly defined, most tools specify ranges beyond which detection capability seizes. This is, of course, expected because the strength of the gradient fields emanating from a coating defect location (as shown in Figure 6) diminishes inversely as the distance from the source. increased. the potential difference between the equipotential lines will decreases and therefore, become undetectable.

Figure (6): Effect of soil depth above coating defect on the measured voltage gradient.

as depth of soil above the defect increased al of mV indication ,OL/RE and IR% will decreased till the case that the defect can't be detected or sized.

Site survey done on the existing Coupons with bare surface area 3"x3" (9 square inches ) found with deferent % IR due to deferent depth of soil above the coupon (%IR =0.5 to 2.7) . %IR changed about 6 times by changing depth of soil above the defect and also the maximum DCVG for the first lateral reading (one CSE on the defect and second one 1.2 meters away) decreased by increasing the soil depth above the coupon.(table 1)

Pipeline identification:

48 INCH pipeline

Coating type and conditions

3LPE new coating for pipes , PE heat shrinkable sleeve for field joint and high volume solids polyurethane coating (100 microns) for cold bends .excellent coating efficiency only minor damages expected during construction stages

S No.

Pipeline identification

chain age of coupon Test station KM

Defect chain age

S1 mV

D1 KM

S2 mV

D2 km

DX KM

OL//RE mV

PIPE/RE mV

%IR Drop for anomaly

Actual size (Sq-inch

)

depth(m)

Remarks

1 1423 0 1420

1.34 0 12 1423 0.8 9 coupo

n

2 1420 1.34 1430

2.45

0.45 27.2 1424 1.9 21.5 2.5 ANOM

ALY 1

3 1430 2.45 1406

4.96

2.51 9.4 1406 0.7 9 coupo

n

4 1406 4.96 1389

6.26

1.1 6.4 1392 0.5 9 coupo

n

5 1406 4.96 1389

6.26

1.3 8.2 1389 0.6 9 coupo

n

6 1509 0 1494

0.25 0 12.3 1509 0.8 9 coupo

n

7 1509 0 1494

0.25

0.25 9 1494 0.6 9 coupo

n

8 1494 0.25 1506 1 0.

04 12 1495 0.8 12.32 1.5 ANOM

ALY 2

9 0NO DEFECTS

10 1437 3 1465 5 2 19.5 1465 1.3 9 4 coupo

n

11 1465 5 1421 7 0.

99 49.5 1443 3.4 24.1 4.5 ANOMALY 3

12 0NO DEFECTS

13 1246 9 1353 11 1 35 1300 2.7 9 coupo

n

14 1353 11 1351

13.5

2.05 18 1351 1.3 9 coupo

n

15 1351 13.1 1194 17 1.

95 14 1273 1.1 9 coupon

TABLE (1) Coupon with bare surface area 3"x3" (9 sq inch ) showing deferent % IR due to deferent depth of soil above the coupon (%IR =0.5 to 2.7) . %IR changed about 600% by changing depth of soil above the defect

2- The effect of signal strength (pipe to soil on potential - pipe to soil off potential at coating defect) on DCVG indication and the calculated IR%

From conventional knowledge as signal strength increased the current density on both the defected area and through soil will be increased and then all of DCVG indication , above the line OL/RE will be increased .so detection of coating defect will be improved by increasing the signal strength which will increase DCVG indication

site test carried out on bare CS coupons with surface area 3"X3" buried at pipeline depth and measuring /calculating for all of DCVG indication ,OL/RE mV , IR% for deferent signal strength (all other parameters fixed) we found that DCVG indication and OL/RE mV will be significantly increased by increasing signal strength

As per NACE TM0109-2009 clause 6.3.1.7 Typical DCVG signal magnitudes measured to remote earth range between 100 and 1,500 mV in soil environments .

40"pipeline existing test coupon at KM 6+050

1270 1085 911 591 516 1860

2

4

6

8

10

12

14

Effect of signal strength (mV) on DCVG indication ,OL/RE and %IR

IR%OL/REDCVG

SIGNAL STRENGTH (mV)

Volta

ge (

mV)

increasing signal strength will magnify all of DCVG indication and OL/RE which will improve coating defect detection sensitivity and also will influence the %IR calculations .3- The effect of soil resistivity

from conventional , DCVG indications (mV measured between two CSE one on the defect and second about 1.2 meter perpendicular to pipeline) will increase with increasing defect size, DCVG indication decreases with soil resistivity increases .This case was not initially expected. as the identification of defect location will be less clear with increasing soil resistivity . By site trails it appears that the soil resistivity plays a larger role than coating defect size in determining DCVG indication in mV. This is supported by the behavior at high soil resistivities where the DCVG indications show almost no dependence on defect size.

0 125 250 375 500 6250

5

10

15

20

25

30

35

40

45

Effect of soil resistivity (KΩ-cm) on DCVG indication,OL/RE (mV) and %IRfor deferend defect

sizes

0.5KΩ-cm3kΩ-cm10KΩ-cm

Coating defect size ( cm2)

DCVG

indi

catio

n (m

V)

in case OL/RE measured and converted to percent-IR the effect of soil resistivity and coating defect size on. The results show that percent-IR increase with increasing coating defect size and increasing soil resistivity. soil resistivity can have a greater effect on percent-IR values than coating defect size. For example, for each soil resistivity, the relative change of %IR stays the same. This means that as coating defect size changes, the percent-IR indication changes by the exact same incremental value regardless of what soil resistivity that the system is at. This result shows that percent-IR can also be better prioritized by taking soil resistivity into account. In other words, the percent-IR indications obtained could be misinterpreted causing an inaccurate sizing of the coating defect size .4-Coating types and conditions of coatings away from the coating defect

%IR is the percent between the voltage gradient caused by the specific defect to the voltage gradient made by the remaining surface of the pipe . so as coating with good isolation properties and less number of defect the %IR for certain defect will be more comparing with the case if the pipe line with poor coating condition or poor electrical isolation properties

in other words same %IR percent will represent bigger coating defect in poorly coated line and smaller coating defect in well coated pipeline with high dielectrics voltage properties From conventional knowledge, as coatings in medium or bad condition it will affect the aboveground measurement techniques due to the possibility that corroded areas of the pipeline may be undetected. This can occur because the CP current is so widely distributed that there is not enough localized current entering the pipeline at a corroded or bare location. This causes indications from

measurement techniques to not be large enough to detect bare spots on the coating. For example, voltage gradients in the ground will not arise if there is not a large enough coating defect of localized current entering the pipeline at a given location. from site survey on in service pipeline with medium or bad coating condition we got that ,

ConclusionsBased on the above study and conventional knowledge we found that the sensitivity of the aboveground DCVG survey techniques for coating detect detection and sizing will vary due to many parameters as following

I- detection of coating defects ,-DCVG indications increases with increasing coating defect size. -DCVG indication decreases with increased soil resistivity. it was found that soil resistivity has an even larger effect on DCVG indications than the size of the coating defect. If the soil resistance is very low then the DCVG may be not reach the ground level and then the defect can't be detected -DCVG indications increase by increasing signal strength This is explained by increasing of current entering the pipeline at the coating defect. in some cases when signal strength is weak we can detect the defect.

-DCVG indications decrease with increasing depth of soil cover above coating defect. (pipe diameter playing role in this parameter) and if the depth of soil increased above certain limit may be defect cant be detected

II- Sizing of coating defect , classification of coating defect severity and decision for coating defect repair .

-Percent-IR values increased with both increasing coating defect size and soil resistivity. From these results it was found that the evaluation of percent-IR values should not exclude consideration of soil resistivity. For example, a high soil resistivity could cause a small defect to yield a large percent-IR value leading to unnecessary excavation. Also, most of the large coating defects had low percent-IR values that under normal evaluations would be as minimal threat to potential corrosion failure. This further proves that percent-IR values are not enough to make a decision on whether a defect is severe or not. The level of soil resistivity among other factors must also be incorporated into this assessment. DCVG indications do not give enough information alone to make assessments about coating defect size. No conclusion or assessment about the size of the coating defect can be made using the %IR . this study showed that by knowing more about other parameters of the system (such as soil resistivity, depth of soil cover, and pipe diameter), a quantitative relationship can be made between DCVG indications and coating defect size. Overall

III- Effectiveness of cathodic protection system based on coupon off- potential evaluation

based on the coupon off- potential conventional study ,level of CP which can protect the coupon will protect all other defects with same or less area

.

Figure X. coupon performance diagram for coupons placed in the same soil environment as the pipeline coating defect that expose the bare steel . the soil resistivity was 100,000 ohms-cm black symbols correspond to calculations where a protected coating holiday .open symbols corresponded to pipeline with coating holidays that were seriously under protected gray symbols correspond to cases where the coupon reading was not conservative ,but the error was less than 15 mV(5)

so if we going to depend on coupon off- potential we must assure that the maximum coating defect size must be equal or less than coupon area which can't be identified by DCVG coating defect sizing calculation as the calculations error vary more than the coupon size.

REFERENCES

1. NACE TM0109-2009, “Standard Practice for Aboveground Survey technique for the Evaluationof Underground Pipeline Coating Condition” (Houston, TX: NACE, 2009).232- EVALUATION OF ABOVE-GROUND POTENTIAL MEASUREMENTS FOR ASSESSING PIPELINE INTEGRITY By JAMES PATRICK MCKINNEY UNIVERSITY OF FLORIDA 20063- J. Wagner, Cathodic Protection Design I, NACE International, Houston, TX, 1994.4-NACE CP4 course manual 5-Application of boundary element models to predict effectiveness of coupons for accessing cathodic protection of buried structure (DP Riemer and M.E Orazem)