OVERCOMING THE NEW THREAT TO PIPELINE INTEGRITY - AC CORROSION ASSESSMENT AND ITS MITIGATION -

Transcription

1 23rd World Gas Conference, Amsterdam 2006 OVERCOMING THE NEW THREAT TO PIPELINE INTEGRITY - AC CORROSION ASSESSMENT AND ITS MITIGATION - Main author Y. Hosokawa JAPAN

2 ABSTRACT AC corrosion risk on gas buried transmission pipelines is increasing due to growing tendency to install the pipelines in proximity to overhead high-voltage AC power transmission lines and/or AC-powered rail transit systems. It has been generally recognized that the pipelines can suffer AC corrosion despite satisfying the conventional cathodic protection (CP) criteria based on pipe-to-soil potential which mandate a protection potential of V CSE. Studies were therefore conducted to develop 1) new CP criteria to assess the AC corrosion risk and 2) AC mitigation methodology including the installation of grounding system. Consequently, new CP criteria were developed based on DC and AC coupon current densities. AC mitigation methodology was also established that included the grounding of pipelines using magnesium anodes and solid-state DC decoupling devices. The effectiveness of the new CP criteria and AC mitigation methodology was proven to be satisfactory through field tests conducted on pipelines buried in proximity to overhead high-voltage AC power transmission lines and AC-powered rail transit systems.

3 TABLE OF CONTENTS <List of all headings and subheadings> ABSTRACT BODY OF PAPER 1. INTRODUCTION 2. EXPERIMENTAL 2.1 Studies on CP criteria to assess AC corrosion risk through corrosion rate measurement 2.2 Studies on AC mitigation methodology 2.3 Studies on the effectiveness of new CP criteria and AC mitigation methodology 3. RESULTS AND DISCUSSION 3.1 Corrosion rate measurement 3.2 New CP criteria based on DC and AC coupon current densities 3.3 AC mitigation methodology 3.4 Effectiveness of new CP criteria and AC mitigation methodology In proximity to overhead high-voltage AC power transmission lines In proximity to AC-powered rail transit systems 4. CONCLUSION REFERENCES List of Tables (no table) List of Figures FIGURE 1. The effect of DC and AC current densities on the corrosion rate of steel. FIGURE 2. New CP criteria based on DC and AC coupon current densities. FIGURE 3. AC mitigation methodology for buried pipelines. FIGURE 4. I DC and I AC before and after grounding the pipes using a solid-state DC decoupling device at point a and magnesium anodes at point b. FIGURE 5. AC coupon current density measured during a period of 24 hours at point c. FIGURE 6. The maximum and average values of AC coupon current densities before and after grounding the pipe using a solid-state DC decoupling device at point c.

4 1. INTRODUCTION There is a growing tendency to install gas transmission pipelines in proximity to overhead high-voltage AC power transmission lines and/or AC-powered rail transit systems due to the limitation of available space to construct these facilities. On such pipelines, high AC voltages will be induced between the pipes and the soil that can result in AC corrosion at possible coating holidays. Increasing application of high resistivity coatings such as extruded polyethylene coatings may also exacerbate the AC corrosion risk. Since 1990 s, AC corrosion cases on buried pipelines have been actually reported from various countries in Europe 1-4) and North America 5-8). It has been widely recognized that the pipelines can suffer AC corrosion despite satisfying the conventional cathodic protection (CP) criteria based on pipe-to-soil potential which mandate a protection potential of V 9), 10) CSE. It has therefore been required to develop new CP criteria that can appropriately assess the AC corrosion risk. In the present study, laboratory and field tests were carried out to develop new CP criteria that can appropriately assess the AC corrosion risk. Moreover, AC mitigation methodology including the installation of grounding system was discussed. Finally, the effectiveness of the new CP criteria and AC mitigation methodology was evaluated through field tests on the pipelines buried in proximity to overhead high-voltage AC power transmission lines and AC-powered rail transit systems. 2. EXPERIMENTAL 2.1 Studies on CP criteria to assess AC corrosion risk through corrosion rate measurement Laboratory tests were conducted to investigate the effect of DC and AC current densities on the corrosion rate of steel in soil. Various levels of AC currents together with DC currents (CP currents) were applied for one month on carbon steel specimens with the exposed surface area of 1 x 10-4 m 2 (= 1 cm 2 ) buried in clayey soil. The frequency of AC currents was set at 50 Hz corresponding to the commercial frequency that has the most severe effect on the AC corrosion risk in Japan 11). Corrosion rate of each specimen was determined from the weight loss divided by the time of exposure. Moreover, field tests were conducted on a pipeline buried in parallel with a 66 kv overhead high-voltage AC power transmission line. Steel coupons simulating coating holidays with the exposed surface area of 1 x 10-3 m 2 (= 10 cm 2 ) were connected to the pipes at test station locations. Simultaneous measurement of DC and AC coupon current densities was conducted using a new hand-held instrument developed by the authors 12). DC coupon current was determined as the average value of coupon current data obtained at the sampling frequency of 10 khz, and AC coupon current was determined as the 50 Hz component. DC or AC coupon current density was obtained from DC or AC coupon current divided by the coupon surface, respectively. Positive values of DC coupon current

5 indicate that DC currents flow from the soil into coupons. Weight loss of the coupons was measured to investigate the effect of DC and AC coupon current densities on the corrosion rate. Based on the results of corrosion rate measurement in laboratory and field tests, new CP criteria to appropriately assess the AC corrosion risk were discussed. 2.2 Studies on AC mitigation methodology Literature survey was conducted to establish AC mitigation methodology. It has been well recognized that the grounding of pipes is effective to mitigate the induced AC. Methodology of grounding was discussed especially in terms of the compatibility with CP system because inappropriate grounding can reduce the CP level of pipelines. 2.3 Studies on the effectiveness of new CP criteria and AC mitigation methodology The effectiveness of the new CP criteria and AC mitigation methodology was evaluated through field tests on the pipelines buried in proximity to overhead high-voltage AC power transmission lines and AC-powered rail transit systems. Simultaneous measurement of DC and AC coupon current densities was conducted in the same manner as described in 2.1 before and after AC mitigative measures were implemented where the AC corrosion risk was identified with respect to the new CP criteria. 3. RESULTS AND DISCUSSION 3.1 Corrosion rate measurement Figure 1 demonstrates the effect of DC and AC current densities on the corrosion rate (CR) obtained from the weight loss measurement in the laboratory and field tests. When AC current density was high, corrosion rate was higher than 3.17 x m/s (= 0.01 mm/y) as designated with solid circles and squares. It should be noted that when AC current density was higher than 70 A/m 2, the steel specimens suffered corrosion although DC current density was higher than 1 A/m 2 that seemed to generally be a satisfactory level. On the other hand, corrosion rate was stifled less than 3.17 x m/s (= 0.01 mm/y) as designated with open circles and squares when AC current density was relatively low. It was thus revealed that the AC corrosion risk could be assessed through the simultaneous measurement of DC and AC current densities.

6 0 Corrosion : CR 0.01mm/y Protection : CR<0.01mm/y AC current density (A/m 2 ) Corrosion(Lab) Protection(Lab) Corrosion(Field) Protection(Field) DC current density (A/m 2 ) FIGURE 1. The effect of DC and AC current densities on the corrosion rate of steel. 3.2 New CP criteria based on DC and AC coupon current densities According to the results shown in Figure 1, new CP criteria that can appropriately assess the AC corrosion risk were developed as follows; 0.1 A/m 2 I DC < 1.0 A/m 2 and I AC < 25 I DC (1) 1.0 A/m 2 I DC < 40 A/m 2 and I AC < 70 A/m 2 (2) where I DC is DC coupon current density in A/m 2 and I AC is AC coupon current density in A/m 2. Schematic diagram of the new CP criteria based on DC and AC coupon current densities (current density CP criteria) are demonstrated in Figure 2. The upper limits of AC coupon current density were determined according to the experimental results shown in Figure 1. It should be noted that the upper limits of AC coupon current density will vary if the frequency of induced AC is different from 50 Hz. The upper and lower limits of DC coupon current density were determined through another study conducted by the authors 13). The lower limit of DC coupon current density was determined to eliminate the risk of DC stray current corrosion, and the upper limit was determined to eliminate the overprotection risk. As long as DC and AC coupon current densities are in the protection area (shaded zone in Figure 2), the corrosion rate is stifled less than 3.17 x m/s (= 0.01 mm/y).

7 AC coupon current density, I AC (A/m2 ) DC corrosion Ⅰ AC corrosion Ⅱ Protection DC coupon current density, I DC (A/m 2 ) Overprotection FIGURE 2. New CP criteria based on DC and AC coupon current densities. 3.3 AC mitigation methodology Basic methodology to mitigate the induced AC has been determined as shown in Figure 3. It is a primary requirement to conduct a preliminary survey to identify the location of overhead high-voltage AC power transmission lines and AC-powered rail transit systems as well as the route of newly constructed pipelines. According to the results of the preliminary survey, distributed grounding system using magnesium anodes should be installed for the mitigation of induced AC. Moreover, steel coupons to measure DC and AC current densities must be connected to the pipes at test stations where AC corrosion is likely to occur. The reasons to use magnesium anodes are as follows; 1) Magnesium anodes provide not only the mitigation of induced AC but also interim cathodic protection during the pipeline construction. 2) The reduction of CP efficiency can be minimized when transformer-rectifiers are installed after the pipeline construction because the potential of magnesium anodes in soil is adequately negative. When the induced AC is not mitigated to satisfactorily low level even after the installation of distributed grounding system using magnesium anodes, further AC mitigative measures must be implemented corresponding to the level of AC coupon current density as described below. In case that AC coupon current density is less than 70 A/m 2, more DC currents must be provided from transformer-rectifiers. On the other hand, when AC coupon current density is at or more than 70 A/m 2, the level of AC coupon current density must be reduced by lowering the pipe-to-earth resistance through further grounding. The method of grounding is determined considering the level of induced AC

8 and burial condition at the site. Preliminary survey to determine the location of overhead high-voltage AC power transmission lines and AC-powered rail transit systems Install distributed grounding system using magnesium anodes Assess the AC corrosion risk with respect to current density CP criteria Not satisfied. I AC < 70A/m 2 I AC 70A/m 2 (1)Provide more DC current by the installation of transformer-rectifier AC coupon current density, IAC Current density CP criteria (2) 70A/m 2 (1) Protection DC coupon current density, I DC (2)Assess the level of induced AC and burial situation - Extremely high level of AC - Locally induced AC - In proximity to DC electric railway - Low soil resistivity - In proximity to transformer-rectifier (2-a) Ground through a solid-state DC decoupling device (2-b) Ground directly by magnesium anodes FIGURE 3. AC mitigation methodology for buried pipelines. In the following three cases, it is effective to ground the pipes through solid-state DC decoupling devices that have low impedance to AC current while high resistance to DC current 14). When the level of induced AC is extremely high, the pipe should be connected to a large earthing electrode such as a bare steel casing pipe through a DC decoupling device in order to prevent the reduction of CP level. For the grounding in proximity to a DC-powered rail transit system, a DC decoupling device should be installed between a pipe and an earthing electrode to interrupt DC stray currents and prevent DC stray current corrosion at remote coating holidays. Also for the grounding in proximity to a

9 transformer-rectifier, a DC decoupling device should be installed to interrupt the flow of CP currents into the earthing electrode and prevent the reduction of CP level. On the other hand, when AC voltage is induced in a relatively localized area along a pipeline, direct connection of additional magnesium anodes will be enough to mitigate the induced AC. This is effective also for the grounding in low resistivity environment because magnesium anodes will provide adequately low resistance to earth against the induced AC. 3.4 Effectiveness of new CP criteria and AC mitigation methodology In proximity to overhead high-voltage AC power transmission lines Field tests were conducted on the pipelines newly installed in proximity to overhead high-voltage AC power transmission lines. The pipelines were coated with extruded polyethylene coatings, and cathodically protected using transformer-rectifiers and distributed magnesium anodes. Figure 4 shows typical results of the simultaneous measurement DC and AC coupon current densities together with the current density CP criteria. As designated with the solid circles in Figure 4, AC corrosion risk was identified at two test stations (points a and b ) with respect to the criteria. According to the AC mitigation methodology shown in Figure 3, it was decided to ground the pipe to reduce the level of AC coupon current densities at these two points. Grounding methods at points a and b were determined considering the burial condition. In the vicinity of the point a, the pipeline was in close proximity to a DC-powered rail transit system and a transformer-rectifier. In order to mitigate the induced AC and simultaneously interrupt the flow of DC stray currents from the running rails and CP currents from the transformer-rectifier into the earthing electrode, bare steel piles were connected to the pipe through a solid-state DC decoupling device. At point b, the pipe was buried in low resistivity soil and the level of induced AC was high only in a localized area. Additional magnesium anodes were then connected directly to the pipe. After AC mitigative measures were implemented at points a and b, DC and AC coupon current densities were measured again. As designated with open circles in Figure 4, AC coupon current densities were then reduced from 75 A/m 2 to 5.5 A/m 2 at point a, and from 76 A/m 2 to 38 A/m 2 at point b, respectively. At the same time, no significant change was observed in DC coupon current density after grounding at points a and b, and adequate CP levels were still maintained. Consequently, AC mitigative measures were proven to be effective, and the AC corrosion risk at points a and b was eliminated.

10 AC coupon current density, I AC (A/m 2 ) DC corrosion Before grounding After grounding AC corrosion Protection DC coupon current density, I DC(A/m 2 ) a b Overprotection FIGURE 4. I DC and I AC before and after grounding the pipes using a solid-state DC decoupling device at point a and magnesium anodes at point b In proximity to AC-powered rail transit systems Field tests were conducted on the pipeline newly installed in proximity to an AC-powered rail transit system. The pipeline was coated with extruded polyethylene coatings, and cathodically protected using transformer-rectifiers and distributed magnesium anodes. Figure 5 shows a typical result of continuous and long-term measurement of AC coupon current density during a period of 24 hours at a test station (point c ). In the daytime, AC coupon current density frequently reached extremely higher levels than the upper limit of 70 A/m 2 every time when the AC-powered train passed by at point c. On the contrary, AC coupon current density was relatively low and stable during the night time while the AC-powered rail transit system was not in operation. According to the AC mitigation methodology shown in Figure 3, a bare steel casing pipe was then connected to the pipe through a solid-state DC decoupling device at point c. Simultaneous measurement of DC and AC coupon current densities was conducted before and after the pipe was grounded through a solid-state DC decoupling device. Considering the large fluctuation of AC coupon current density, AC corrosion risk was assessed by the maximum and average values of AC coupon current densities with respect to the current density CP criteria. As shown in Figure 6, the maximum and average values of AC coupon current density markedly decreased from 259 A/m 2 to 6.8 A/m 2 and from 68 A/m 2 to 2.9 A/m 2, respectively. At the same time, no change was observed in DC coupon current density, and adequate CP current of approximately 12 A/m 2 was still observed after the pipe was grounded.

11 AC mitigative measures using a solid-state DC decoupling device was therefore proven to be effective even when the AC level was extremely high, and the AC corrosion risk at point c was completely eliminated including the time periods when AC-powered trains passed by hours In the night time (no train operation) IAC ( A / m 2 ) Time (hh:mm) FIGURE 5. AC coupon current density measured during a period of 24 hours at point c. I AC Ave and IAC Max (A/m 2 ) I AC Max I AC Ave Before grounding After grounding Protection I Ave DC (A/m 2 ) FIGURE 6. The maximum and average values of AC coupon current densities before and after grounding the pipe using a solid-state DC decoupling device at point c.

12 Thus, the assessment and elimination of the AC corrosion risk was successfully completed on the pipelines buried in proximity to overhead high-voltage AC power transmission lines and AC-powered rail transit systems. New CP criteria and AC mitigation methodology were therefore proven to be effective. In order to ensure the long-term integrity of the pipelines having the AC corrosion risk, simultaneous measurement of DC and AC coupon current densities should be conducted in the periodic CP survey. 4. CONCLUSION Laboratory and field tests were conducted to assess and eliminate the AC corrosion risk on buried gas transmission pipelines. Following conclusions were then obtained. 1) New CP criteria that can appropriately assess the AC corrosion risk were developed based on DC and AC coupon current densities. 2) AC mitigation methodology was established considering the compatibility with CP. 3) New CP criteria and AC mitigation methodology were proven to be effective to assess and eliminate the AC corrosion risk through field tests on the pipelines buried in proximity to overhead high-voltage AC power transmission lines and AC-powered rail transit systems. 4) Simultaneous measurement of DC and AC coupon current densities should be conducted periodically to maintain long-term integrity of the pipelines where AC corrosion is likely to occur. REFERENCES 1) W. Printz, AC Induced Corrosion on Cathodically Protected Pipelines, UK Corrosion 92 (1992): p.1. 2) F. Stalder, Pipeline Failures, Material Science Forum, 247 (1997): pp ) I. Ragault, AC corrosion Induced by V.H.V. Electrical Lines on Polyethylene Coated Steel Gas Pipelines, CORROSION/98, paper No.557 (Houston, TX: NACE, 1998). 4) C. M. Movley, Pipeline Corrosion from Induced A.C. Two UK Case Histories, CORROSION/05, paper no (Houston, TX: NACE, 2005). 5) R. G. Wakelin, R. A. Gummow, S. M. Segall, AC corrosion Case Histories, Test Procedures, & Mitigation, CORROSION/98, paper no. 565 (Houston, TX: NACE, 1998). 6) R. G. Wakelin, C. Sheldon, Investigation and Mitigation of AC corrosion on a 300 mm Diameter Natural Gas Pipeline, CORROSION/04, paper no (Houston, TX: NACE, 2004). 7) H. R. Hanson, J. Smart, AC corrosion on a Pipeline Located in an HVAC Utility Corridor, CORROSION/04, paper no (Houston, TX: NACE, 2004). 8) R. Floyd, Testing and Mitigation of AC corrosion on 8 Line: A Field Study, CORROSION/04, paper no (Houston, TX: NACE, 2004). 9) R. A. Gummow, Final report of Contract PR , Cathodic Protection Considerations for

13 Pipelines with AC Mitigation Facilities, Pipeline Research Council International (1999). 10) NACE Standard RP , Control of External Corrosion on Underground or Submerged Metallic Piping (Houston, TX: NACE, 1996). 11) F. Kajiyama, Y. Nakamura, Corrosion, 55, 2 (1999): p ) Y. Hosokawa, F. Kajiyama, Y. Nakamura, An Innovative Instrument for Evaluating CP Levels Liaison with Enhanced CP Maintenance Database, CORROSION/02, paper no (Houston, TX: NACE, 2002). 13) Y. Hosokawa, F. Kajiyama, Y. Nakamura, Corrosion, 60, 3, 304 (2004). 14) H. Tachick, Materials Performance, 40, 2 (2001): p. 20.

14 5. LEGENDS FIGURES / TABLES List of Tables (no table) List of Figures FIGURE 1. The effect of DC and AC current densities on the corrosion rate of steel. FIGURE 2. New CP criteria based on DC and AC coupon current densities. FIGURE 3. AC mitigation methodology for buried pipelines. FIGURE 4. I DC and I AC before and after grounding the pipes using a solid-state DC decoupling device at point a and magnesium anodes at point b. FIGURE 5. AC coupon current density measured during a period of 24 hours at point c. FIGURE 6. The maximum and average values of AC coupon current densities before and after grounding the pipe using a solid-state DC decoupling device at point c. 6. FIGURES AND TABLES 0 Corrosion : CR 0.01mm/y Protection : CR<0.01mm/y AC current density (A/m 2 ) Corrosion(Lab) Protection(Lab) Corrosion(Field) Protection(Field) DC current density (A/m 2 ) FIGURE 1. The effect of DC and AC current densities on the corrosion rate of steel.

15 AC coupon current density, I AC (A/m2 ) DC corrosion Ⅰ AC corrosion Protection Ⅱ DC coupon current density, I DC (A/m 2 ) Overprotection FIGURE 2. New CP criteria based on DC and AC coupon current densities.

16 Preliminary survey to determine the location of overhead high-voltage AC power transmission lines and AC-powered rail transit systems Install distributed grounding system using magnesium anodes Assess the AC corrosion risk with respect to current density CP criteria Not satisfied. I AC < 70A/m 2 I AC 70A/m 2 (1)Provide more DC current by the installation of transformer-rectifier AC coupon current density, IAC Current density CP criteria (2) 70A/m 2 (1) Protection DC coupon current density, I DC (2)Assess the level of induced AC and burial situation - Extremely high level of AC - Locally induced AC - In proximity to DC electric railway - Low soil resistivity - In proximity to transformer-rectifier (2-a) Ground through a solid-state DC decoupling device (2-b) Ground directly by magnesium anodes FIGURE 3. AC mitigation methodology for buried pipelines.

17 AC coupon current density, I AC (A/m 2 ) DC corrosion Before grounding After grounding AC corrosion Protection DC coupon current density, I DC(A/m 2 ) a b Overprotection FIGURE 4. I DC and I AC before and after grounding the pipes using a solid-state DC decoupling device at point a and magnesium anodes at point b hours In the night time (no train operation) IAC ( A / m 2 ) Time (hh:mm) FIGURE 5. AC coupon current density measured during a period of 24 hours at point c.

18 I AC Ave and IAC Max (A/m 2 ) I AC Max I AC Ave Before grounding After grounding Protection I Ave DC (A/m 2 ) FIGURE 6. The maximum and average values of AC coupon current densities before and after grounding the pipe using a solid-state DC decoupling device at point c.