Simulation of cathodically protected pipeline with capacitive AC mitigation devices for interpretation of misleading instant Eoff readings

Abstract

A cathodically protected buried pipeline is usually connected to earthing electrodes via several capacitive AC mitigation devices placed at various locations along the pipeline in order to decrease the AC voltage interference. During periodic switching of the cathodic protection circuit for the instant Eoff measurements, the capacitive AC mitigation devices tend to be discharged through the soil provoking an error in the instant Eoff readings, thus resulting to a possible false evaluation of the cathodic protection effectiveness. The aim of the present work is to investigate the role of the main factors, which influence this error. An electric circuit simulation of the pipeline with connected earthed capacitors is proposed. The circuit analysis has shown that the key determining factors of the aforementioned capacitive discharge time constant comprise the capacitance value of the capacitive AC mitigation device at every earthing site, the number of earthing sites where the capacitive AC mitigation devices are installed, the earthing electrodes resistance and the pipeline (or coating) resistance to remote earth. Measures to minimise this misleading error of Eoff readings through the control of the abovementioned factors and the modification of the time of the Eoff measurement are suggested. In this context, the Eoff should be measured at prolonged time after every switching, contrary to the usual practice. Furthermore, the capacitive AC mitigation devices should not be disconnected from the pipeline in order to prevent the adverse effects of AC interference on cathodic protection monitoring. The present work contributes to the understanding of the capacitive AC mitigation devices effect on pipeline cathodic protection monitoring, which has been rarely analysed in the past. Moreover, by recommending novel approaches, the paper can stand as a reference to the cathodic protection operators for a simple, reliable and economic monitoring of the cathodic protection effectiveness in presence of earthed capacitive AC mitigation devices. 1 Typical Eon/Eoff waveform of cathodically protected PE coated pipeline when no capacitive AC mitigation devices are connected (U ac 513?8 V) Kioupis et al. Simulation of pipeline with AC mitigation devices Corrosion Engineering, Science and Technology 2013 VOL 48 NO 33 a equivalent circuit of cathodically protected buried pipeline connected with N capacitive AC mitigation devices and b equivalent circuit representing buried pipeline with n coating defects and intact coating Kioupis et al. Simulation of pipeline with AC mitigation devices Corrosion Engineering, Science and Technology 2013 VOL 48 NO 3

Effects of alternating current interference on cathodic protection potential and its effectiveness for corrosion protection of pipelines

“…The field experiences also demonstrated that the AC induced corrosion can occur on carbon steels even under higher CP levels. [15][16][17][18][19] As a result, new CP criteria according to field survey and laboratory tests were developed for eliminating the risks of AC corrosion of pipelines. 20 However, the criteria did not apply at AC current density in excess of 70 A m −2 .…”

Cathodic protection monitoring of buried carbon steel pipeline: measurement and interpretation of instant-off potential

“…where b is the cathodic Tafel slope of hydrogen evolution reaction on steel (0.12 V decade −1 ). Considering the investigated experimental condition (E off = −1.119 V CSE, i = 10 A m −2 ) and supposing the oxygen limiting diffusion current density equal to the protection current measured in overprotection-free condition (i L,O 2 = 0.15 A m −2 ) the IR-free potential can be calculated from equation (6) as:…”

“…The measurement techniques used to eliminate ohmic drop are based on the reduction of the distance between reference electrode and structure (local or fixed reference electrode, Luggin capillary and potential probes with an internal reference electrode) or on the interruption of the circulating current (on-off technique in impressed current CP systems), as described in the standard EN 13509 [4] and NACE SP0104 [5]. The description and some applications of each technique are available in the specific literature [6 -27]. It should be pointed out that IR-free potential is undoubtedly provided by the use of potential probes with internal reference electrode or by a reference electrode placed in a Luggin capillary (for laboratory testing) or in a buried plastic conduit which ends close to a steel coupon (defined soil-access tube in standard NACE SP0104 [5], Figure 2).…”

Assessment of cathodic protection by close interval survey incorporating the instant off potential method

“…Pipelines may be under AC interferences if they are close to high voltage AC power transmission lines and/or AC powered rail transit systems. 9,10 Others ways to measure IR V are as follows: (i) with the CSE as close as practical to the pipeline; (ii) placing the CSE at a remote location; (iii) decreasing the current in steps while measuring potential shifts in the pipeline with respect to the soil and the voltage gradient at the surface; or (iv) the use of corrosion coupons. 8,11 Figure 2 shows a typical potential versus reference point profile for an epoxy coated API carbon steel pipeline at a pipe cover depth of ,1.5 m and a r S of 15610 3 V cm (high resistivity), which was assumed to be uniform.…”