9th Middle East Corrosion Conference and Exhibition

February 12-14, 2001, Bahrain

Polymeric Anodes for Pipeline Rehabilitation

B.Q. Pham, 0. Olabisi and S.A. Al-Zubail Materials Engineering & Corrosion Control Division

Consulting Services Department Saudi Arainco, Dhahran 31311, Saudi Arabia

Fax: 966-3-873-0988 E-mail: phambq@aramco.com.sa

J.K. Boah, A.L. Lewis, A.H. AI-Rasheed and A.A. AI-Jabran

Material Sciences Division Laboratory Research & Development Center

Saudi Aramco, Dhahran 31311, Saudi Arabia Fax: 966-3-876-5751

E-mail: boahjk@aramco.com.sa

ABSTRACT External protection of buried pipelines carrying high temperature products is a great concern to Saudi Aramco. Fusion bonded epoxy coatings have failed after approximately 5 years in service on high temperature gas pipelines. The coating failures were sufficient to require costly coating renovation to ensure adequate external protection. An economical analysis indicated that substantial savings could be achieved if polymeric anodes could provide adequate protection to the buried pipelines for 20 years or more in spite of the failed coating. Saudi Aramco investigated the effectiveness of using commercially available conducting thermoplastic anode cables for the cathodic protection of pipelines whose coatings have severely deteriorated. Laboratory testing and field evaluations were conducted to determine the performance of the polymeric anodes. This paper presents results of the laboratory testing of the anodes at anode current densities up to the maximum allowable value of 52 mA/linear meter and at different elevated temperatures up to the maximum recommended WC, in salt-saturated soils. It also discusses the field performance assessment of polymeric anode systems installed in sand and rocky soils during a 12-month test study.

1. INTRODUCTION

Saudi Aramco's extensive buried pipelines are protected by a combination of coatings and cathodic protection. The operating temperatures range from 65oC to 120oC. Before 1980, the coating systems included tape wrap, coal tar epoxy, and polyethylene. FBE coatings, however, have dominated the post-1980s coating systems. FBE coatings are primarily applied in a coating plant and are impractical for large scale coating renovation in the field. Coating renovation is commonly used to ensure effective corrosion control of the external surfaces of pipelines with badly deteriorated coating. However, it is very expensive given the expenditure for excavation of the lines, removal of the old coating, and application of the new epoxy-based liquid coating systems.

Khuff gas flow-lines are of particular concern to Saudi Aramco since these lines have experienced serious temperature-induced coating failures. The high temperatures in these pipelines, exceeding 105oC, have in many locations destroyed the FBE coatings on the pipelines after approximately 5 years in service. Considering that conventional CP is often ineffective on buried pipelines with severely degraded external coatings, large sections of the Khuff gas lines require complete coating renovation at approximately 5-year intervals to maintain adequate external protection. A number of major oil companies worldwide have utilized distributed polymeric anode cables as an alternative to pipeline coating rehabilitation because of its cost-effectiveness compared to re-coating [1-2]. For example, Chevron Pipeline Company has used continuous polymeric anodes since 1986 as an alternative to pipeline rehabilitation with resounding success. In addition, Transcontinental Gas Pipeline Corporation has installed over 240 kilometers of continuous polymeric anodes since 1984 for its 30” to 48” trunk lines. No failures have occurred to date, essentially because of the following unique advantages of polymeric anodes when compared with conventional remote cathodic protection systems: • Better current distribution • Minimal hydrogen over-voltage • Reduced risk of cathodic disbondment • Reduced CP interference • Reduced CP current requirement Chevron spent about 30% of the estimated coating renovation cost for 8” pipelines to install the CP systems using polymeric anodes. For 30” to 48” pipelines, Transcontinental Gas Pipeline Corp. also spent about 30% of the estimated coating rehabilitation cost to install polymeric anode CP systems. In both cases [1–2], there was no interruption of service, resulting in additional savings. Although the application of polymeric anode technology is worldwide and gaining ground, its use in the Middle East is only just beginning with recent introductions by Petroleum Development Oman and the Yanbu facilities of the Saudi Aramco Mobil Refinery Company LTD. Saudi Aramco conducted both laboratory testing and field evaluations to determine if continuous polymeric anodes can replace pipeline-coating renovation. The objective of the laboratory test program was to assess the effect of current density and environmental factors such as temperature and type of sand on the integrity of commercially available conducting thermoplastic anode cables. The purpose of the field evaluation was to assess the effectiveness of continuous polymeric anodes in providing cathodic protection to pipelines with severely degraded coatings. This also included the evaluation of the design and installation of polymeric anode systems.

2. EVALUATION PROGRAM 2.1 Laboratory Testing 2.1.1 Experimental Procedures The laboratory testing samples were prepared from commercially available polymeric anode cable. This anode consists of a stranded #6-AWG copper conductor covered with a conducting polymeric composite of cross-linked linear low-density polyethylene, LLDPE, plus a proprietary highly conducting carbon black. For ease of transportation and installation, the anode cable is encased inside a porous woven LLDPE jacket packed with a high performance coke breeze for a total anode diameter of approximately 3.8 cm. The amount of coke breeze compacted around the anode, approximately 2.2 kg per linear meter, is designed to allow the polymeric anode cable to last 20 years at the maximum recommended anode

current output of 52-mA per meter (16-mA/ft). Figure 1 summarizes the design and construction of the polymeric anode used in our testing program. The laboratory test program utilized twelve, nominal 4-foot long, 0.92-meter actual test length, uncoated grit-blasted, carbon steel pipes. Polymeric distributed anode cables were embedded in soil (damp subkha soil and coarse sand) inside the pipe sections and wired for the cathodic protection of the internal surfaces of the pipes. The surface area of the bare steel test vessels was significantly larger than was required for the maximum applied current density. This ensured that the anodes were tested at their endurance limit. The polymeric anodes were operated at applied current levels of 8-mA and 16-mA per linear foot of anode, which resulted in current densities of 20.7-mA/m2 and 41.4-mA/m2 on the bare cathode surfaces. The test duration was 375-days at ambient, 40oC, and 60oC. Figure 2 is a schematic representation of the over-all cathodic measurement and control circuitry. The test matrix for each soil type is tabulated in Table 1. To maintain the sand (both subkha and coarse sand) moist at all times, a 5% brine solution was added periodically. Allowance was made for venting possible gaseous products resulting from the electrochemical reactions taking place within the coke breeze contained around the anode. At the end of the 12-month test-period, the internal surfaces of all the pipes were examined for evidence of corrosion. The external surface of the LLDPE jacket was rinsed with distilled water, air dried and then re-photographed. The physical, heat-distortion, and cold bend properties of the cross-linked conducting polymer of the anodes were then measured. 2.1.2 Laboratory Results and Discussions Visual observations made after the exposure test showed that the polymeric anode cables suffered no significant deterioration when tested for 12-months at the endurance limit recommended by the anode manufacturer; namely, a maximum anode current level of 16-mA/linear foot and a temperature of 60oC. There was no significant consumption of any of the polymeric anode cables. The anodes were in excellent condition and looked new. Some of the polymeric anodes exposed to an anode current level of 16-mA/ft and 60oC, exhibited what appeared to be blisters on the porous LLDPE jackets. It is suspected that gas generated on the external surface of the coke breeze layer caused the blistering. The blisters disappeared about 3 days after termination of the experiments. All the anodes tested, except one, remained intact; the coke breeze was still kept in place by the porous woven LLDPE jacket. For one sample, the polymeric anode cable remained unchanged and encased in the coke breeze without the LLDPE jacket. The amount of coke breeze lost in each sample during the test period was insignificant. At the maximum anode current level of 16-mA/ft, the expected coke breeze consumption rate is 1-kg/amp-year. Consequently, the service life of the coke breeze could indeed last for more than 20 years. [3] There was clear evidence of ongoing corrosion activity during the test. The internal surfaces of the test pipes that contained coarse sand exhibited the worst corrosion activity. Elevated temperature, constant addition of salt-water and inadequate anode current density; all contributed to the observed corrosion. The applied anode current levels of 8-mA/ linear foot and 16-mA/linear foot, or cathode current densities of 20.7- and 41.4-mA/m2, were intended to ensure that the anodes were tested at their endurance limit. To achieve complete cathodic protection would have required 80 to 110-mA/m2 for bare steel (cathode current density). [6]

The system achieved steady state within the first month of operation except for one pipe section. For subkha soil, the current remained essentially constant. Likewise, the resulting potential for subkha soil remained essentially constant. The temperatures shown in Table 1 are those for the external surfaces of the pipes. At those surface temperatures, the sand inside the pipes achieved temperatures of 40oC and 60oC respectively. The anode potentials generally varied from 1.5-2.8 volts. Three of the pipes had accidental disconnect problems during a portion of the monitoring period resulting in anode potentials of 1.4 to 13.5 volts. The potential versus time plots at 40oC and 60oC show spikes indicative of the weekly addition of salt-water to the sand to facilitate current density control. Figure 3a for the 60oC illustrates the point being made. The anode potential increased with increasing dryness of the sand but dropped to a low value whenever additional salt-water was added to the sand. This is reflected in the spikes in the anode potential versus time plots. Figure 3b is a typical plot for a polymeric anode cable embedded in subkha sand and exposed to a current level of 16-mA/ft at 60oC. The temperature and current density remained essentially constant over the 375-day test period. After the 12-month test period, physical, mechanical and chemical properties of all the samples were measured to determine any changes. The test results indicate an insignificant change in the properties of the conducting polymer in such conditions. Table 2 summarizes the changes in their properties [5]. 2.2 Field Evaluation of Polymeric Anodes 2.2.1 Cathodic Protection System Design The field evaluation of the polymeric anodes also utilized commercially available pre-packaged polymeric anode of the type shown in Figure 4. Approximately 6,700 meters of the polymeric anode were installed continuously along a 6.7-km gas loop-line that has a diameter of 8 inches and extends from the wellhead to the pipe manifold. This loop-line also runs in parallel with a flowline between the wellhead and the pipe manifold area. These pipelines have fusion bonded epoxy (FBE) coatings on their external surfaces, and have been receiving cathodic protection primarily from two 50-A remote deep anode beds. These anode beds are located in the wellhead and manifold areas, or at the ends of the pipeline. During the first 10 years of service, the FBE coatings were severely damaged by the high temperature of the carried gas. It was decided to use cathodic protection through continuous polymeric anode to protect the line without coating renovation. During the evaluation of the polymeric anode systems, insulation flanges were installed to electrically isolate the loop-line from the well, the flowline, and surrounding metallic structures. According to the polymeric anode manufacturer, the 6700 meters of anode can supply a maximum current output of 348 amperes, or 52-mA per linear meter. However, due to the current requirement of this pipeline, the design restricted the current output of the anode below 96-A or 14.3-mA per meter. At this anode current output, the pipe received a maximum current density of approximately 18.8-mA/m2, which is less than the Saudi Aramco current density criterion of 20-mA/m2 for bare steel. Our actual field data indicate that this design current density is more than adequate to protect the external surface of the pipe, since the pipe's external surface was not completely bare. According to the anode manufacturer [3], at the maximum allowable current level of 52-mA per linear meter, the anode would last 20 years. Based on the reduced current density in our field-testing, the installed anode is expected to last significantly more than 20 years. For optimizing the anode current distribution along the pipeline, this 6,700-meter anode was divided into four equal-length segments of 1,675 meters each. Four 24-V, 24-A rectifiers: two oil-cooled and two air-cooled types, control these four anode segments. For each rectifier (CP system), the design required dividing the 1,675-meter anode segment into four (4) legs with an average length of approximately 419

meters to optimize the anode current distribution. Two anode legs were attached to a junction box, and each one has a resistor that is used to balance the current output. All connections between, anode legs to junction boxes, junction boxes to junction boxes, junction boxes to rectifiers, and the connections between the protected pipeline and the rectifiers, utilize AWG No. 2 HMWPE cables. The soil resistivity along the pipeline varied from a few thousand ohm-cm to more that 10,000 ohm-cm. The design of these CP systems employed an average value of 5,000 ohm-cm. Table 3 summarizes the design details of these CP systems. Figure 5 shows a typical arrangement of the anode legs and an associated junction box and rectifier for a complete CP system. 2.2.2 Anode System Design and Installation In this field evaluation, the anode legs were installed between the loop-line and flowline. The distance between the loop-line and flowline is in the range of 6-10 meters. The design requires proper locations for the anode legs to minimize the effects of heat from the pipeline and to optimize anode current distribution. Based on the results of a test for temperature gradient around a heat source in sand, as shown in Table 6, keeping the anode legs within a range of 3-5 meters from the loop-line would minimize heat effects and optimize anode current distribution. The anode legs were placed at the bottom of a trench with a depth of 1-meter. The anode manufacturer required heat shrink encapsulations to protect all the connections between the header cables and anode strings. After laying the anode, the entire trench was backfilled with clean sand. 2.2.3 Monitoring of Cathodic Protection Systems Monitoring the CP levels on the loop-line required 16 permanent copper/copper sulfate reference electrodes installed at eight different locations, along the pipeline. The permanent reference electrodes were located approximately 6-inches from the pipe and at the 3 and 9-o'clock positions around the pipe, as illustrated in Figure 5. The backfill surrounding the reference electrodes consists of a mixture of clean sand, gypsum, and bentonite to retain moisture around the electrodes. In addition, two coupon test stations were installed to enhance the CP level monitoring. 2.2.4 Performance Evaluation Evaluation of the CP system's performance consisted of two parts: performance of the anode systems and associated components, and the effectiveness of these CP systems. a) Performance of the Anode Systems and Associated Components: The total resistance of the anode legs is the major factor contributing to the performance of the anode system. The rectifiers used in the systems were rated at 24-V, 24-A, allowing a total circuit resistance of 1-ohm for designing the CP system. No significant change in circuit resistance of a CP system indicates that the performance of an anode bed is acceptable. The performance of these four CP systems during the one-year test period, as summarized in Table 4, indicates that these anode systems and rectifiers performed satisfactorily since no significant change in the circuit resistance was observed. In July 2000 the variable resistors in each leg were increased to provide balanced potentials on the pipe. This resulted in an increase in the total circuit resistance of these CP systems but does not reflect on the performance of the anode systems. The operating data of the permanent copper/copper sulfate reference electrodes indicate that these electrodes stopped functioning after six months in service.

b) Evaluation of the CP system Effectiveness: Table 5 summarizes the CP levels measured at the test stations installed along the loop-line. These potentials indicate that the line is receiving adequate protection and the CP levels are consistent. Figure 6a shows the pipe-to-soil potential profile of the loop-line having no cathodic protection, indicating that the surrounding impressed current CP systems caused significant interference on this line. This CP interference can result in corrosion damage to the line if the pipe coating is in a poor condition. Figure 6b illustrates the drastic change in CP potential levels along the loop-line, indicating that the pipe coating has severely degraded and the existing conventional CP systems are not effective. Figure 6c indicates that the tested polymeric anode systems provided relatively uniform protection, reducing excessively high levels and enhancing low levels. They also enabled the CP levels on the line to be maintained in the required range of -1.2-V to 3.0-V.

3. CONCLUSIONS Based on the results of this study, the following conclusions can be derived: 1. The polymeric anode cables suffered practically no deterioration when tested for 12-months at the

endurance limit recommended by the anode manufacturer; namely, the maximum anode current level of 16-mA/linear foot and temperature of 60OC.

2. There was no significant consumption of any of the polymeric anode cables. The anodes were in

excellent condition and looked new after the laboratory exposure. 3. The change in conductivity of the tested samples after the 12-month test period is insignificant and there

is no significant change in the carbon content of the tested samples. 4. The polymeric anode cables can effectively protect buried pipelines having severely degraded coatings

in Saudi Arabian soils. 5. Designing polymeric anode systems for pipelines requires a comprehensive soil resistivity survey along

the pipelines to enable the proper distribution of CP current to piping sections exposed to different types of soil.

6. Using resistors at the anode cable segments enabled balancing of the current output of the anode

systems. 7. The prepackaged polymeric anode cables containing 2.2-kg coke breeze/meter backfield could have a

service life of more than 20-years, if they are operated at anode current levels of less than 16-mA/linear foot and at temperatures of less than 60OC.

8. Polymeric anode cables provide better current distribution and reduce excessive CP current

requirements as well as reducing CP interference. 9. Commercially available CU/CUSO4 reference electrodes survived approximately 6-months in desert

soils. Failure is presumed due to drying of the reference cell electrolyte or the soil immediately adjacent to the reference electrode.

4. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of the Saudi Arabian Ministry of Petroleum and Mineral Resources and Saudi Arabian Oil Company (Saudi Aramco) in the preparation and publication of this paper.

5. REFERENCES

1. B.Q. Pham, 0. Olabisi and S.A. AI-Zubail, Saudi Aramco internal document 2. Williarn F. Gibson and Joseph L. Pikas, "Conductive Polymeric Cable Anodes for Pipelines with

Deteriorating Coatings", Material Performance, March 1993, Volume 32, Number 3 3. Christian Pierre, Ph.D., Raychem Report CPD) 001"Anodeflex 1500 Lifetime", November 1992 4. J.K. Boah, A.L. Lewis, A.H. Rasheed and A.A. AI-Jabran, Saudi Aramco internal document 5. F. Mahmud, A. Bahamdan, S. Mehta , F. Melibari, A. Tuwailib, A. AI-Ghamdi and A.K. Nowaishi,

Saudi Aramco internal document 6. R.S. Treseder, R. Baboian, C.G. Munger, "NACE Corrosion Engineer's Reference Book", Second

Edition, Nace International

Table 1: Laboratory Testing Conditions Sample No. Anode

Current Output

Cathode Current

Density*

Temperature Sand Type

1 52-mA/m 41.4-mA/m2 60oC Coarse Sand 2 52-mA/m 41.4-mA/m2 40oC Coarse Sand 3 52-mA/m 41.4-mA/m2 60oC Subkha Sand 4 52-mA/m 41.4-mA/m2 40oC Subkha Sand 5 26-mA/m 20.7-mA/m2 60oC Coarse Sand 6 26-mA/m 20.7-mA/m2 40oC Coarse Sand 7 26-mA/m 20.7-mA/m2 60oC Subkha Sand 8 26-mA/m 20.7-mA/m2 40oC Subkha Sand 9 52-mA/m 41.4-mA/m2 Ambient Coarse Sand 10 52-mA/m 41.4-mA/m2 Ambient Subkha Sand 11 26-mA/m 20.7-mA/m2 Ambient Coarse Sand 12 26-mA/m 20.7-mA/m2 Ambient Subkha Sand

*Pipe length: 0.92-meter; Pipe Internal Diameter: 0.31-meter; Exposed Pipe Area: 0.90-meter2 Table 2: Changes of the Properties of the Tested Sainples Sample No.

Anode Testing Conditions

Carbon Black Content (%)

Water Absorption

Average Hardness

Elongation % Change

Soil Type

1 52-mA/m, 60oC 41.16 0.0118 53.7 -63.18 C. Sand 2 52-mA/m, 40oC 40.51 0.0117 55.4 -48.97 C. Sand 3 52-mA/m, 60oC 41.16 0.0149 54.5 -41.82 Subkha sand 4 52-mA/m, 40oC 40.39 -0.0528 56.5 14.30 Subkha sand 5 26-mA/m, 60oC 39.53 0.0292 56.9 41.45 Coarse sand 6 26-mA/m, 40oC 38.18 0.0088 59.1 -30.87 Coarse sand 7 26-mA/m, 60oC 41.32 0.0212 59.1 -49.88 Subkha sand 8 26-mA/m, 40oC 39.99 0.0197 59.0 -36.82 Subkha sand 9 52-mA/m, 23oC 41.51 0.0302 58.5 -50.75 C. Sand 10 52-mA/m, 23oC 40.96 -0.1372 58.2 -38.55 Subkha sand 11 26-mA/m, 23oC 41.05 0.0382 59.4 -32.90 C. Sand 12 26-mA/m, 23oC 41.35 -0.0435 59.7 -32.99 Subkha sand

Control Sample

N/A 42.22 0.0249 57.7

Table 3: Design of Cathodic Protection Systems Using Polymeric Anode System Location Rectifier Type Number of Total Length No. & Rating Anode Legs (Meter) (Volt-Ampere)

1 Manifold Area Oil Cooled 4 (1,675 m each) 6700 24-24 2 KM2+515 Air Cooled 4 (1,675 m each) 6700

24-24 3 KM 3+650 Air Cooled 4 (1,675 m each) 6700 24-24 4 Well Area Air Cooled 4 (1,675 m each) 6700 24-24 Table 4: Performance of the Polymeric Anode Systems Date CP System No.

1 Output & Circuit Resistance

System No.2 Output & Circuit Resistance

System No.3 Output & Circuit Resistance

System No.4 Output & Circuit Resistance

REMARK

11/22/99 15.28V-9.6A 1.592 Ohm

8.23V-10.05A 0.819 Ohm

10.75V-10.14A 1.06 Ohm

16.27V-10.28A 1.582 Ohm

5/24/00 15V-11A 1.36 Ohm

7.7V10A 0.77 Ohm

10.7V10A 1.09 Ohm

12V-11A 1.09 Ohm

7/15/00

11.8V-8A 1.475 Ohm

5V-3.7A 1.351 Ohm

8V-8.2A 0.976 Ohm

12V-10.3A 1.165 Ohm

Reduced Rectifier Output and/or Balanced anode legs

8/12/00 11.8V-7.8A 1.513 Ohm

5V-3.9A 1.282 Ohm

8V-8.3A 0.964 Ohm

12V-10.4A 1.154 Ohm

9/14/00 11.8V-7.9A 1.494 Ohm

5V-3.8A 1.316 Ohm

8V-8.2A 0.976 Ohm

12V-10.3A 1.165 Ohm

10/15/00 11.8V-7.8A 1.475 Ohm

5V4.1A 1.220 Ohm

8V-8.2A 0.976 Ohm

12V-9.9A 1.212 Ohm

11/29/00 11.8V-7.8A 1.475 Ohm

5V4.1A 1.220 Ohm

8V-8.1 0.988 Ohm

12V-9.8A 1.2245 Ohm

Table 5: Cathodic Protection levels measured at the test stations along the Loop-line Date CP System No. 1 (1)

CP Potential (-mV)

System No.2 (1) CP Potential (-mV)

System No.3 (1) CP Potential (-mV)

System No.4 (1) CP Potential (-mV)

J/B-1: J/B-2 J/B-1: J/B-2 J/B-1: J/B-2 J/B-1: J/B-2 11/22/99* R-1: 1904 R-1: 2800

R-2: 1917 R-2: 2800 R-1: 2600 R-1: 2500 R-2: 2300 R-2: 2500

R-1: 2000 R-1: 1949 R-2: 2000 R-2: 1907

R-1: 1905 R-1: 1655 R-2: 1906 R-2: 1670

5/17/00* R-1: 2500 R-1: 1840 R-2: 2500 R-2: 1890

R-1: 2200 R-1: 2400 R-2: 2200 R-2: 2400

R-1: 1926 R-1: 2000 R-2: 2000 R-2: 2000

R-1: 1633 R-1: 1930 R-2: 1633 R-2: 1930

7/15/00* R-1: 1819 R-1: 1840 R-2: 1863 R-2: 1854

R-1: 1875 R-1: 2200 R-2: 1841 R-2: 2200

R-1: 2200 R-1: 2100 R-2: 2400 R-2: 2100

R-1: 2400 R-1: 2300 R-2: 2400 R-2: 2500

8/12/00** 3600 2200 2300 2400 2300 2200 2200 2200 9/16/00** 3400 2300 2400 2300 2200 2300 2200 2200 10/15/00** 3800 2900 2000 2200 2100 2000 2200 2200 11/29/00** 3800 2700 2300 2400 2300 2200 2200 2100

NOTE: *: Measured with the permanent Cu/CuSO4 Reference Electrodes installed through junction boxes near the test stations. The permanent reference electrodes stopped functioning after approximately 7 months due to dry-out problems. **: Measured with a portable Cu/CuSO4 Reference Electrode. (l): CP Systems were adjusted to reduce the CP levels Table 6: Results of the test for temperature gradient of a heating source in sand

Sand Depth (em) Temperature oC

0 151.6 10 81.8 20 44.8 30 37.2 40 34.8 50 31.4 60 30.8