Assessment of corrosion on buried metallic pipeline induced by AC interference below high transmission line

Abstract

The assessment of corrosion on buried metallic multi-product pipeline (MPP) induced by alternating current (AC) interference below high transmission lines was investigated. Power transmission lines or high voltage alternating current (HVAC) transmission lines generate inductive coupling on buried metallic pipeline or any metallic structures located near to it. This electromagnetic field created from HVAC’s stray current is impressed onto the MPP and will introduce an AC corrosion at certain AC current density. The AC corrosion occurred on the pipeline surface at the location where the AC stray current is leaving the pipeline (coating defect) to return to its original path. The AC stray current will cause asset damage (AC corrosion) such as leak or burst and danger (electrocuted) to the personnel working on the pipeline route. The AC corrosion occur when adequate stray current from HVAC flow through soil and enter the pipeline at any nearest coating defect then leave the pipeline at another coating defect to return to its original path to complete the electrical circuit. The stray current favors to enter the metallic structures due to its low resistance property compared to soil (relatively high resistance). High voltage alternating current (HVAC) transmission line is usually shared the same right of way with buried metallic pipeline. Long term exposure of inductive coupling on buried metallic pipeline will cause AC corrosion at any coating defects on the pipeline. In the West Coast of Malay Peninsula, the AC induced corrosion is not well studied, and the preventive maintenance is not taken into consideration by some of the pipeline operators. The objective of this paper is to measure the parameters involved at the location of the buried metallic multi product pipeline (MPP) which is cross or parallel to the HVAC. From the parameters collected, the current density will be calculated and the pipeline sections which are in the risk of AC corrosion are determined. On the other hand, the cathodic protection (CP) system affected by stray current are further analyzed. This MPP is laid from Sg. Udang, Melaka to distribution terminal in Dengkil, Selangor with the estimated length of 130.0 km. Along the pipeline routing, only 16 locations of MPP sections are cross and parallel to the HVAC. The equipment used to conduct this corrosion assessment are multimeter, clampmeter, portable current interrupter, soil resistivity meter and portable Cu/CuSO4 reference electrode. CP potential and AC output are measured using multimeter, clampmeter and Cu/CuSO4 reference electrode. For soil resistivity, it is determined by using soil resistivity meter with the native soil sample at site. Magnetic field magnitude is determined by using Biot-Savart Law formula and current density is calculated using current density formula. Three (3) locations (TP 43, TP 102 and TP105) are found under risk of AC corrosion which is in the range of 20 A/m2 to 100 A/m2. As per standard industry practice such as NACE SP-0169 and PTS 30.10.73.10, AC corrosion is unpredictable for AC current density in between 20 A/m2 to 100 A/m2. These three (3) locations have the highest AC voltage output, the lowest soil resistivity value and the CP potential measured are under protected value. However, the effect of magnitude of magnetic field is literally no significant effect on the AC corrosion activity. In conclusion, AC voltage, soil resistivity and CP potential at the crossing and parallel section to the HVAC will affect the behaviour and severity of the AC corrosion on the metallic buried pipeline.