Survey and Assessment of CP Coupon ON & Instant-off Potentials with Oscilloscope

Abstract The purpose of this study was to evaluate CP coupon ON, instant-off and native potentials in a waveform measured by an oscilloscope to evaluate the performance of the cathodic protection system. A CP coupon is a piece of metal that is manufactured from a material same as that of the pipeline. A multimeter measures CP quantity such as voltage and current whereas an oscilloscope captures the measured voltage signal in a waveform with the ability to observe signal characteristics and changes over time in milliseconds. Traditional CP coupon measurement methodology during coupon interruption is based on multimeter reading (the second reading on multimeter display during instant IR drop) and may provide uncertain results. The instant-off potential (IOP) measurement by multimeter may have been taken the IOP after some degree of depolarization and uncertainty associated with response time of the multimeter leads to incorrect judgement of the cathodic protection status. On the other hand, an oscilloscope provides the waveform of CP potential during coupon interruption with certain time interval of IOP and hence reliable. In this study, 49 CP coupons were installed at critical locations that were subject to corrosion along a 1150 km long pipeline corridor with several pipeline crossing pipelines. These coupons were measured with an oscilloscope for the ON potential (ONP), instant-off potential (IOP), native potential (NP), and the time period was measured for the instant-off (with decay time interval) was also measured to evaluate the spike effect. The oscilloscope measurements were then compared with the multimeter measurements to assess the results from each measurement technique and their usefulness in evaluating the pipeline protection status. It was found that CP coupon measurement with the oscilloscope was effective in capturing true instant-off potentials, ensuring proper protection of pipelines, whereas the multimeter measurements may lead to incorrect conclusions because no direct correlation has been found for instant-off potential measurement between the first reading at multimeter display and. oscilloscope. In complex pipeline network, measurement of the CP coupon with oscilloscope measurement provides high-quality data to evaluate IR-free CP potential. Considering different types of soil and complex pipeline network, following CP ON potential without IR drop correction to evaluate effectiveness of CP created the risk of having inadequate protection or overprotection at certain locations.