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A Design Case of Cathodic Protection for Pipelines Inside Subsea Tunnel

HONGSEOK SONG1, HONG-KYUN SOHN, HEESAN KIM, YOUNGGEUN KIM

1Research Institute of Korea Gas Corporation, Ansan, Korea, hssong@kogas.or.kr 2School of Material Sci&Eng of Hongik University, Sejong, Korea, hksohn0@empas.com 3School of Material Sci&Eng of Hongik University, Sejong, Korea, hskim@hongik.ac.kr

4Research Institute of Korea Gas Corporation, Ansan, Korea, ygkim99@naver.com Abstract When pipeline is installed inside long subsea tunnel expecting that electrolyte will be filled into the tunnel in the long run, the pipeline will has more than two-layered surrounding mediums; permeated wtaer contacting pipeline and sedimentary soil or rock surrounding tunnel under seawater. Required cathodic protection(CP) current shall be applied based on the water property, whereas CP type shall be determined depending on the surrounding soil charateristic. For CP design of pipeline installed inside subsea tunnel, a series of measurements on the resistivity of cored rock samples and permeated water, and cathodic current penetration test using temporal impressed current anode installed onshore were conducted, and resulted in selecting sacrificial anode as CP type. Potential distributions of pipeline by applying single anode were simulated using commercial software based on polarization results tested in artificial sea water with conductivity. The cathocally protected pipeline length was mainly affected with permeated water conductivity, however, less by tunnel diameter, anode and pipeline position, etc. The simulated potential profile matched well with small scale experimental result when small proportion of long term polarized current values was incorporated into simulation. Keywords Pipeline; cathodic protection; subsea tunnel; sacrificial anode design Introduction Where the construction of gas transmission pipelines are planned between on shores, it necessitates to take into account various issues arising from construction to maintenance; fishery boats’ and commercial ships’ traffic, eco-environmental impact during laying (or burial) onto sea bottom during construction, damages caused by anchor or fishing implements, and movement by tide during maintenance. Besides cost, installing pipelines inside subsea tunnel is prefered to overcome these kinds of issues, nevertheless it needs to decide the type of cathodic protection(CP) and validate its efficacy of operation assuming the infiltration of water inside tunnel in the long run. Impressed current cathodic protection(ICCP) compared to sacrificial anode has benefits in terms of installation and maintenance, however it is doubtful whether sufficient and even CP current to protect pipelines could reach through mostly rocks surrounding the tunnel even probably having lots of intrinsic cracks. Furthermore, large amount of CP current would be required in the case that the resistivity of electrolyte infiltrated into tunnel is as low as sea water. In this study, for cathodic protection of the pipelines installed inside subsea tunnel, CP type was determined after various field investigations such as resistivity measurements of infiltrated water and cored rocks, and current penetration test from tunnel entrance to middle

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of it using temporal ICCP system with anode installed onshore. And simulation for optimal design of anode distribution was carried out using commercial software considering parameters such as electrolyte polarization properties, configurational factors related with anode and pipeline position, tunnel size, etc. And then the typical result of anode distribution design was validated with the result of small scale experiment. Field Condition and Investigations The tunnel route configuration for pipelines construction is 7.7 km long, 90m deep with two large pits on shores. Upper soil layers of tunnel are composed of sea water, sediments, rocks (; weathered, soft, hard), where different proportions of layers were observed with distance and even long region consisted of hard rock like granite could be seen to the extent of a few hundred meters in internal civil ground survey report. The typical cored hard rock sample is shown in Figure 1 and the resistivities of rock samples after spraying distilled water were measured by means of Wenner 4 pin method (; 6 cm pin spacing). The resistivity results are given in Table 1 excluding measurement errors presumably aroused from contamination of sample surface and no correction on cross-sectional area, which shows largely big figures.

Figure 1. Photograph of hard rock type cored sample

Table 1. Resistivity results of hard rock type cored samples

core name sampled depth(m) rock type resistivity(Ωcm) NBH-3 -80 hard 8,888,400

NBH-18 -57 hard 376,800 NBH-38 -43 hard 124,344 NBH-47 -53 hard 489,840 NBH-57 -70 hard 248,688 NBH-58 -70 hard 6,782,400

NBH-58-1 -74 hard 4,521,600 For the resistivity measurement of hard rock itself at tunnel wall, 4 small holes were drilled into the hard rock region near entrance of tunnel, and then resistances were measured with different pin spacing (; 10 cm and 30 cm) to exclude possible contamination effect of inner layer by condensation if any. The resistivity results are given in Table 2, which values are similar level as the values of cored samples.

Table 2. Resistivity results of hard rock type measured at tunnel surface hole number resistivity(Ωcm)

1 362,714 2 273,098

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3 1,796,080 4 5,796,923

Penetrated water into the tunnel also was sampled to test resistivity. The resistivity of electrolyte sampled from hard rock wall near the entrance site was 210 Ωcm, whereas resistivities of sampled water in the middle of tunnel showed almost sea water resistivity value below 25 Ωcm. As the resistive soil condition of exterior tunnel environment was figured out to be high, the current penetration test was conducted by measuring on-off potential of coupon connected with temporal ICCP set-up with anode installed on shore. On-off CP potentials of the coupon and on-off potentials of rail were measured with distance from tunnel entrance. The profiles of on-off potential with distance are shown in Figure 2. At the time of test, water leaked from drain pipe from boring machine flowed out and steel elements to support rail for the transport of boring machinery and pipeline were partly submerged at lower part of tunnel. The on potential of coupon increased with distance from entrance while the on potential of rail decreased with distance showing reverse behaviour compared with coupon potential. These profiles suggest strongly that the CP current is supplied mainly from entrance via water flowing at the tunnel bottom and steel elements submerged in it. It was consequently found that the hard rock surrounding tunnel has high resistivity over long route impeding CP current supply through it while penetrating electrolyte may need CP current similar to the brackish or sea water level. These observations lead to select sacrificial anode system as CP type and Zn for anode material due to possible brackish water infiltration though conservative.

Figure 2. Potential profiles of coupon and rail element from entrance

Experimental Artificial sea water1 with conductivity controlled to 52 mS/cm (20 Ωcm), 5.2 mS/cm (99 Ωcm), 10.1mS/cm (192 Ωcm) were used to get required cathodic current of pipeline material,

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and currents were measured potentiostatically kept for one day from -1,000 mV (vs. Ag/AgCl reference electrode1) to corrosion potential with 50 mV step with nitrogen gas control. Anodic potentiostatic polarization tests of Zn were conducted from -1,080 mV to -900mV with 5 minutes duration at each potential with 20 mV step. Using the polarization data, a commercial package called CPMaster of Elsyca embedded Solidworks to simulate potential distribution was utilized. The basic tunnel size for simulation was 2.8 m in diameter, and 30” pipeline was assumed to be floated up to 10 cm below tunnel top. Initial 2% coating defect was also assumed. Zn anode size is 0.8m long x 0.1 m wide x 0.072 m high. The typical configuration of pipeline and anode for simulation is shown in Figure 3.

Figure 3. The typical configuration of pipeline and anode for simulation

In order to validate the simulation result, steel bar of 4 mm in diameter assuming 20% coating defect was put in 2 m long plastic cell of 54 mm in diameter for one fiftieth scale down, and connected Zn anode with same scale down ratio in one side. The potential along distance were monitored with time until steady and stable potential state was attained more than 10 days. Same configurations were simulated and compared the simulation result with the results of scale down experiment since current density itself can’t be converted to scale down experiment. Results and Discussions The cathodic currents with potential and conductivity are shown in Figure 4. Calcareous deposit might play a role in lowering cathodic current with time especially at low potential, however the effect seemed not to be significant since the required current density at cathodic protection criterion potential was almost comparable with the value in literature2, 3 as given in Table 3. The potentiostatic polarization results of Zn are also given in Table 4.

1 In this study, potential is based on Ag/AgCl reference electrode exept oherwise

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Figure 4. cathodic currents with stepwise potentiostatic potential and conductivity

Table 3. Cathodic polarization results of pipeline material in artificial sea water with conductivity

Potential (V, vs Ag/AgCl)

Current (A/m2) 52 mS/cm 10.1 mS/cm 5.2 mS/cm

-1.00 -0.488 -0.307 -0.098 -0.95 -0.225 -0.18 -0.065 -0.90 -0.121 -0.113 -0.061 -0.85 -0.095 -0.087 -0.058 -0.80 -.0.076 -0.077 -0.021 -0.77 0 0 0

Table 4. Anodic polarization results of Zn anode in artificial sea water with conductivity

Potential (V, vs Ag/AgCl)

Current (A/m2) 52 mS/cm 10.1 mS/cm 5.2 mS/cm

-1.08 4.5 1.6 0 -1.06 13 4.5 0.35 -1.04 28 8.4 1.6 -1.02 47 13 5.9 -1.00 70 18 11 -0.98 93 23 16 -0.96 118 29 20 -0.94 143 34 24 -0.92 169 40 28 -0.90 195 45 32

The simulated results of potential distribution of pipeline coupled with Zn anode inside tunnel with electrolyte conductivity are shown in Figure 5. In a case of highest conductivity electrolyte condition, protected pipeline length by single Zn anode meeting -800 mV criterion is longest up to 74 m. Protected pipeline distance by single Zn anode decreased as the conductivity of electrolyte decreased.

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(a) 52 mS/cm

(b) 10.1 mS/cm

(c) 5.2 mS/cm

Figure 5. Potential distribution of pipeline inside tunnel by Zn anode During offshore pipelines are cathodically protected with time, they tend to form calcareous deposit and may need decreased CP current. Hence, long term potentiostatic current was monitored at -900mV and its behaviours are shown in Figure 6. The current decreased to 1/2 ~ 1/5 compared with the value of one day test. When these values were incorporated into simulation, protected distance increased significantly from 74 m to 176m in a case of 52 mS/cm electrolyte as shown in Figure 7. In a simulated case of 10.1 mS/cm case with one day test values, CP coverage was about 16 m in one direction. The effect of anode distribution to supply the area with current between anodes was simulated under the same condition of 10.1 mS/cm case, the result with 48 m anode interval assuming to facilitate installation of anode in every 4 pipeline turn showed that CP coverage extended farther beyond the coverage by single contribution (Figure 8). In order to take into account contraction effect of tunnel diameter due to constructional residues or for future design case, the effect of tunnel diameter change was simulated with long term polarization values in 10.1 mS/cm electrolyte, and its results are indicated in Figure 9. CP coverage increased as tunnel diameter increased, however its tendency is moderate.

Figure 6. long term potentiostatic current with time at -900 mV

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(a) 52 mS/cm

(b) 10.1 mS/cm

(c) 5.2 mS/cm

Figure 7. Potential distribution of pipeline inside tunnel by Zn anode reflecting the long term current monitoring

Figure 8. Potential distribution of pipeline by distributed Zn anode with 48 m interval under the same condition of Figure 7(b) The position effect of pipeline and anode in tunnel, and the effect of anode size were simulated using long term polarization values in 10.1 mS/cm electrolyte, which were found out to be not significant, the results are shown in Figure 10, 11, 12, respectively.

(a) 2.4 m in diameter

(b) 2.8 m in diameter

(c) 3.2 m in diameter

Figure 9. Potential distribution of pipeline with tunnel diameter

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(a) 0.8 m gap from top of tunnel

(b) 0.1 m gap from top of tunnel

(c) 0.05 m gap from top of tunnel

Figure 10. Potential distribution with pipeline position inside tunnel

(a) positioned at tunnel side

(b) positioned at tunnel center

Figure 11. Potential distribution with anode position inside tunnel

(a) initial size

(b) 75% consumption

Figure 11. Potential distribution of anode size The potentials of scale down experimental pipeline connected with Zn at one side were monitored with time, and its results at each distance with time are shown in Figure 12. The potential with distance was up-ward, however the potential at each distance lowered abruptly, and then attained stable after about one week. The long term polarization values in 10.1

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mS/cm electrolyte were used for input data of simulation under scale down conditions, its results are shown in Figure 13. The simulated results were compared with the experimental results as shown in Figure 14. The simulated potential distribution curve matched well with experimental results when low percentage of current from long term polarized results was applied to simulation. This suggests that simulation results have the validation compared to the experimental and lower current than the design value can play a sufficient role in real system.

Figure 12.Monitored potentials along scale down experimental pipeline with time

(a) 100% current values of long term polarization

(b) 60% current values of long term polarization

(c) 40% current values of long term polarization

Figure 13. Simulated potential distribution under the conditions of scale down pipeline set-up and current adjustment

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Figure 14. Comparison of simulated results with experimental results

Summary For CP design of pipeline installed inside long subsea tunnel, after a series of measurements on the resistivity of surrounding mediums(; cored rock samples and permeated water) and cathodic current penetration test using temporal impressed current anode installed onshore, sacrificial anode(Zn) as CP type was selected. Potential distributions of pipeline by applying single anode were simulated using commercial software based on polarization results tested in artificial sea water with conductivity. The cathocally protected pipeline length was mainly affected with permeated water conductivity, however, less by tunnel diameter, anode and pipeline position, etc. The simulated potential profile matched well with small scale experimental result when small proportion of long term polarized current values was incorporated into simulation. Reference [1] ASTM D1141-98 “Standard Practice for the Preparation of Substitute Ocean Water” [2] DNV-RP-F103 “CATHoDIC PROTECTION OF SUBMARINE PIPELINES BY GALVANIC ANODES”, 2010 [3] BS EN ISO 15589-2 “Petroleu, petrochemical nad naatural gas indusries – Cathodic protection of pipeline transportaation systems, Part 2 : Offshore pipelines”, 2014