NACE-2015-556511111111

Evaluation and Reform of Regional Cathodic Protection System Based on Numerical

Simulation

Guangchun Wu, Yanxia Du*, Minxu Lu, Zitao Jiang, Dezhi Tang

Institute for Advance Materials and Technology

Key Laboratory for Environmental Fracture (MOE)

University of Science and Technology Beijing

30 Insititute Road

Beijing, PR 10083

ABSTRACT

A 4-circuit impressed current cathodic protection system with flexible and high silicon cast-iron

shallow buried auxiliary anode bed is applied in Jinghe compressor station. In order to obtain the

protective effect of the regional cathodic protection system, detailed field tests have been carried out.

Based on the results of field tests, assessment of regional cathodic protection is conducted, then

reform programs for inadequate protective effect and interferences between pipelines outside and

inside the station is determined using numerical simulated computation and field experiments.

Keywords: regional cathodic protection, numerical simulation computation, effect assessment,

reform program

INTRODUCTION

Regional cathodic protection technique is an effective measure to prevent corrosion on buried metal

facilities in oil and gas transmission stations, which works depended on the reasonable layout of

anode ground bed, the distribution of protective current and the electric insulation with adjacent metal

facilities. Compared with the cahodic protection applied in long-distance pipelines, regional cathodic

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Paper No.

5565

2015 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole, must be in writing to NACE International, Publications Division, 15835 Park Ten Place, Houston, Texas 77084. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association.

protection has the following characteristics: various protected objects, complicated protective circuits,

severe interference, and higher safety requirements1, and it was introduced into China in 1970s2-4.

However, there are a series of problems to be improved and solved in the processes of the design,

implementation and application because that it developed relatively late and it is still very immature

technically in China.

A 4-circuit impressed current cathodic protection system with flexible and high silicon cast-iron

shallow buried auxiliary anode bed is applied in Jinghe compressor station. The protected

objectscontain buried pipelines, including sewage pipelines, gas pipelines, fire pipelines, and

grounding, including horizontal grounding made by galvanized flat iron and vertical grounding made

by galvanized flat iron or grounding module. Detailed detection has been performed in Jinghe station

and the results reveal that: (1) some CP under-protection and over-protection areas existed; (2)

severe anode interference between cathodic protection systems inside (CP-1) and outside (CP-2)

the Jinghe station. To assure the safety of pipelines in station, the above problems should be solved

quickly and carefully. In this paper, the assessment of regional cathodic protection is conducted

based on field tests, then reform programs for inadequate protective effect and interferences

between pipelines outside and inside the station is determined using numerical simulated

computation and field experiments.

ASSESSMENT OF REGIONAL CATHODIC PROTECTION

A 30 ampere 50 volt PS-3F CP Transformer/Rectifier (T/R), designated as T/R-1, China, is applied

in Jinghe regional cathodic protection system. It has 4 circuits: 1# circuit provides protection to

grounding; 2# circuit provides protection to the buried pipelines around the area of process units, pig

receiver & launcher and vent; 3# circuit provides protection to the buried pipelines around the

compressor area; 4# circuit is standby. A HPS-1D CP T/R, designated as T/R-2, 15 ampere 40 volt

powering a shallow high silicon cast-iron anode bed which provides cathodic protection to the main

pipelines outside the Jinghe Station. The plan view of the station and main pipeline is shown as

Figure 1.

Trade Name.

2


Station

Upstream main

pipeline

Downstream main

pipeline

1# Insulation joint 2# Insulation joint

1# Test point

2# Test point 3# Test point

4# Test point171# CP Post

Crossover wire

Figure 1: Plan view of station and main pipeline

INITIAL OPERATINGSTATE OF CP T/R

The initial operating parameters of the CP T/R were listed in Table 1. It can be seen that: 2# circuit

(T/R-1) couldnt boot normally for the reason that the potential difference between the control

potential (0 VCSE) and protection potential (-750 mVCSE) exceeded the set value of T/R-1; the off

potential at contact points of 1# circuit, 3# circuit and T/R-2 were all positive than -850 mVCSE, thus

their outputs should be increased to meet the protection requirement.

Table 1

Initial Operating State of T/R-1 and T/R-2

Protective potential

(mVCSE)

Output voltage

(V)

Output current

(A)

Off potential

(mVCSE)

T/R-1

1# Circuit -1112 1.86 0.96 -680

2# Circuit Dont boot

3# Circuit -910 18.55 4.90 -720

4# Circuit Standby

T/R-2 -1277 4.14 1.49 -730

FIELD INVESTIGATION AND DISCUSSION

Operating parameters have been modified based on initial operating state and protected objects

self-potential, as shown in Table 2. In order to full reflect the protection effect of the regional cathodic

protection system, a detailed field investigation has been conducted in 9 part areas, a total of 169

different sites in Jinghe station. The distribution of the total 169 test sites are as follow: 28 sites in

distribution and regulating area; 38 sites in filtration and separation area; 4 sites in blow-down tank

area; 11 sites in 1# valve group area; 7 sites in pig receiver area; 6 sites in launcher area; 3 sites in

2# valve group area; 56 sites in compressor area and 16 sites near to compressor area. Detailed test

results were shown in Table 3. However, T/R-2 worked abnormally after the adjustment of the

operating parameters of T/R-1. It became clear after the special field investigation for the reason that

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the anode-bed of 2# circuit placed close to the drain point and control reference electrode of the

CP-2, which caused the measured potential at the site of control reference electrode exceeded the

predetermined value. Hence, there was a severe anode interference existed between the 2# circuit

and the CP-2. In order to determine the main pipelines off potential whether seriously interfered by

the anode interference, potential tests have been carried out at upstream and downstream pipeline

(test point 1#, 2#, 3# and 4#, shown in Figure 1) through adjusting the control potential of the CP-2 to

keep the output of T/R-2 at non-interference level, then turning T/R-1 and T/R-2 on and off

simultaneously. Test results were present at Table 4.

Table 2

Initial Operating Parameters of T/R-1 after Adjustment

Control potential

(mVCSE)

Output voltage

(V)

Output current

(A)

T/R-1

1# Circuit -2600 4.36 4.49

2# Circuit -940 12.50 24.70

3# Circuit -910 30.53 6.70

4# Circuit Standby

Table 3

Test Result of Regional Cathodic Protection System in Jinghe Station

Test area

Number

of test

site

Off

potential

(VCSE)

Potential distribution

>-0.85 VCSE -0.85 VCSE-1.2 VCSE

Pig receiver area 7

-0.73

-0.84

7 100 0 0 0 0 6 85.7

Launcher area 6

-0.71

-0.88

5 83.3 1 16.6 0 0 5 83.3

2# valve group

area

3

-0.94

-1.02

0 0 3 100 0 0 3 100

Compressor area 56

-0.58

-1.00

40 71.4 16 28.5 0 0 49 87.5

Near to

compressor area

16

-0.54

-0.74

16 100 0 0 0 0 7 43.7

Total 169 105 62.1 63 37.3 1 0.6 143 84.6

Table 4

Pipeline Potential at Both Ends of Insulation Joints

Potential 1nside station (VCSE) Potential outside station (VCSE)

On potential

(VCSE)

Off potential

(VCSE)

On potential

(VCSE)

Off potential

(VCSE)

Downstream

pipeline

-1.67 (3# test

point)

-0.94 (3# test

point)

-2.22 (4# test

point)

-1.20 (4# test

point)

Upstream

pipeline

-1.13 (1# test

point)

-1.07 (1# test

point)

-1.60 (2# test

point)

-1.14 (2# test

point)

It can be seen from Table 3 that: (1) there are only one over-protection site placed in filtration and

separation area; (2) the pipelines off potentials in 1# valve group area are all more positive than -850

mVCSE, which dont meet the cathodic protection requirement; (3) the pipelines off potentials in

compressor area are more widely spread and the protection effect is poor. In general, the protection

effect of the CP-1 is not good, especially in the 1# valve group area and compressor area. Some

reform programs must to be conducted to prevent pipelines from corrosion in Jinghe station.

Table 4 presents that when placed the potential control point of CP-2 beyond the anode interference

zone and remained the output current of T/R-2 unchanged, off potential at 4# test point is -1.20 VCSE

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which doesnt exceed the cathodic protection standard value, -0.85 VCSE -1.20 VCSE. Hence, the

anode interference can be mitigated by changing the location of the control reference electrode.

Moreover, in order to avoid over protection, CIPS test also need to be conducted near the Jinghe

station.

From the above content, two problems can be estimated in Jinghe regional cathodic protection

system: (1) Poor protection effect, some inadequate protective areas existed; (2) Severe anode

interference between 2# circuit and CP-2, which results in the abnormal operation of T/R-2.

RESEARCH ON REFORM PROGRAMS OF REGIONAL CATHODIC PROTECTION SYSTEM

Traditional CP reforms often depend on empirical evaluations. However, in many case, the reforms

would be found unreasonable or uneconomical after construction has been finished, adding the

workloads of the late-stage adjustment and in return limiting the further application and development

of CP techniques in station. With the development of computer techniques, a new method-numerical

simulation, as a good and convenient manner to obtain potential and current density distribution has

been widely used to study the CP system 5-10. Using numerical simulation technique, Bazzoni 8

calculated the potential distribution of tank bottom and analyzed the influence of soil resistance on

the potential distribution. Du 9 et al. applied numerical simulation techniques to predict the

performance of CP system and optimize the systems in several oil and gas transmission stations. Lu

10 et al calculated potential distribution on the surface of pipelines buried in discontinuous permafrost.

In this work, numerical simulation technique is applied to optimize the CP system in Jinghe station to

solve the above two problems.

COMPARISON OF RESULTS BETWEEN NUMERCIAL SIMULATION AND FIELD

INVESTGATION

After collecting required basis information and establish the CP model of metal structures, potential

distribution on the surface of buried pipelines and grounding in Jinghe station is calculated by

numerical simulation, as shown in Figure 2. It is clear that the potentials of the majority of pipelines,

horizontal grounding and vertical grounding distribute in the range of -0.85 VCSE -1.20 VCSE, -0.46

VCSE -0.9 VCSE and -0.26 VCSE -0.76 VCSE, respectively. Comparison of results between

numerical simulation and field tests was shown in Table 5. As can be seen, numerical simulation

result is perfectly consistent with field test.

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Figure 2: Potential distribution of CP-1 using numerical simulation (unit: VCSE)

Table 5

Comparison Between Numerical Simulation and Field Test

Test Area Field Test Results

(VCSE)

Numerical Simulation Results

(VCSE)

Percentage of Error

10% (%)

Distribution and

regulating area -0.66-0.92 -0.62-0.89 100

Filtration and separation

area -0.57-1.25 -0.61-1.10 92.1

Blow-down tank area -0.86-1.20 -1.00-1.18 75

1# valve group area -0.61-0.75 -0.60-0.81 100

Pig receiver area -0.73-0.84 -0.74-0.86 100

Launcher area -0.71-0.88 -0.70-0.86 100

2# valve group area -0.94-1.02 -0.95-1.01 100

Compressor area -0.58-1.00 -0.64-0.95 96.4

Near to compressor area -0.54-0.74 -0.59-0.77 100

REFORM PROGRAMS FOR INADEQUATE PROTECTIVE EFFECT

According to the previous field investigation, the inadequate protective effect of CP-1 could be

illustrated by the following two reasons: (1) massive grounding grids which directly connected to the

pipelines consume a large part of the total CP current, especially when the grounding grids is made

by grounding module. In some case, current demand by the grounding may exceed 90% of the total

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current output of the impressed current system 11; (2) mutual interference among each circuits of

CP-1. Considering the feasibility and economy of reform programs, the final reform program has

been determined which can be accomplished in two steps:

(1). Replace the grounding module with galvanized steel which is very compatible with CP system

12.

(2). Reform the anode beds inside the station, add distributed shallow anodes in under-protection

areas.

The reform target is to let the potential of pipelines all distribute in the range of -0.85 VCSE -1.20

VCSE.

STEP ONE

After replacement, potential distribution of pipelines and grounding is shown in Figure 3. As can be

seen, the potential of the majority of the buried pipelines is between -0.90 VCSE -1.30 VCSE, more

negative than the value before reform (-0.84 VCSE -1.30 VCSE). However, the extent of variation is

relatively small due to the less number of grounding module in station and some inadequate

protective areas also existed. Hence, further reform need to be conducted. In addition, after reform,

the potential distribution of horizontal grounding is in the range of -0.46 VCSE -0.92 VCSE, negative

shifting about 20 mV form the value before reform (-0.46 VCSE -0.90 VCSE). In the unreformed

areas, the potential of vertical grounding is between -0.57 VCSE -0.84 VCSE which is negative

shifting 60 mV from the value before reform (-0.52 VCSE -0.76 VCSE). Due to grounding

replacement, the potential of vertical grounding changes from -0.26 VCSE to -0.96 VCSE in the reform

areas.

Figure 3: Potential distribution of CP-1 using numerical simulation after reform step 1 (unit: VCSE)

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STEP TWO

After reform step one, potential anomaly also existed in some areas in station, as shown in Figure 4.

Herein, the potential in 1# area is more negative, -1.30 VCSE -1.35 VCSE; the potential in 2# area,

4# area and 5# area are -0.77 VCSE -0.82 VCSE, -0.81 VCSE -0.85 VCSE and -0.79 VCSE -0.83

VCSE, respectively, all under the required protection level.

Optimized anode beds in the above potential anomaly areas in which added 12 distributed shallow

anodes in the location shown in Figure 5. The potential of pipelines and grounding after reform step 2

is shown in Figure 6. As can be seen, the potential of buried pipelines distribute in the range of -0.89

VCSE -1.20 VCSE, obtained good protection effect.

Figure 4: Potential anomaly areas after reform step 1

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Figure 5: Location of added anodes in reform step 2

Figure 6: Potential distribution of CP-1 using numerical simulation after reform step 2 (unit:

VCSE)

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REFORM PROGRAMS FOR INTERFERENCE BETWEEN CP-1 AND CP-2

Based on the above content, the anode interference between 2# circuit and CP-2 can be mitigated

by changing the location of the control reference electrode, in other words, placed the potential

control point of CP-2 beyond the anode interference zone.

DETERMINATION OF INTERFERENCE ZONE

To determine inference zone using numerical simulation, firstly, calculated the potential distribution of

the main pipeline outside the station without interference, shown in Figure 7. It can be seen that the

potential is about -1.0 VCSE, reached a good protection effect. When the interference existed, the

potential of the upstream and downstream main pipelines are shown in Figure 8 and Figure 9. As can

be seen, the potential of the main pipeline near the station shift toward negative direction due to

anode interference, but dont exceed the limit, -1.20 VCSE, in other words, without over-protection.

Regarded -1.0 VCSE represented by light green color as standard potential, the interference zone of

the anode interference could reach 200300m in the light of Figure 8 and Figure 9.

Figure 7: Potential distribution of the main pipeline without interference (unit: VCSE)

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Figure 8: Potential distribution of the downstream main pipeline near the station, under

interference (unit: VCSE)

Figure 9: Potential distribution of the upstream main pipeline near the station, under

interference (unit: VCSE)

Meantime, in order to verify the accuracy of the interference zone calculated by numerical simulation,

field experiment has been conducted through measuring the potential of the main pipeline along

downstream direction when keep the T/R-1 in the on-off operation state. The result of field test is

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shown in Table 6. It is clear that when the portable reference electrode is 350m away from 2#

insulation joint, the potential of the main pipeline dont interfere by the current from CP-1, hence, the

interference zone is about 350m which is consistent with the result of numerical simulation.

Table 6

Result of Field Test for Interference

Number

Distance between

the portable

reference electrode

and 2# insulation

joint (m)

Potential of the main

pipeline, T/R-1 on (VCSE)

Potential of the main

pipeline, T/R-1 off

1 2 -1.36 -1.27

2 20 -1.32 -1.27

3 50 -1.34 -1.29

4 100 -1.30 -1.26

5 150 -1.33 -1.31

6 200 -1.35 -1.33

7 250 -1.30 -1.28

8 300 -1.30 -1.28

9 350 -1.26 -1.26

REFORM PROGRAM

According to the above content, the reform program for the interference between CP-1 and CP-2 is

as follow: It can be mitigated by moving the control reference electrode of CP-2 to 171# CP post

500m away from 2# insulation joint due to the interference zone is about 300m, which reduces the

workload of building a new marks post for the long-term reference electrode and further prevents the

main pipeline from interference. Of course, a new closer post can be established by further field test,

but the effect of the change of CP-1 current with time must be taken into account fully. It is worth

noting that although changing the location of the control reference electrode can mitigate the

interference of T/R-2, the potential of the pipelines suffering interference whether meet the protection

requirement entirely need to be tested further by keeping CP-1 and CP-2 under the on and off

condition in the same time. If the interference hasnt been completely eliminated, reform must to be

carried out on the anode beds in station.

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CONCLUSIONS

(1). The protection effect of CP-1 is not good and some inadequate protective areas existed in

station according to the field test at 169 sites. In addition, there is a severe anode interference on

CP-2 generated by 2# circuit of CP-1, which caused the abnormal operation of T/R-2.

(2). After the replacement of grounding module by galvanized steel and the reform of anode beds in

station, the potential of all buried pipelines predicted by numerical simulation are between -0.89

VCSE-1.20 VCSE, meeting the protection requirement.

(3). Through changing the location of the control reference electrode can mitigate the interference of

T/R-2. However, to eliminate the interference completely, further test and research need to be

conducted.

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