Obtaining accurate polarized potentials (a.k.a. Instant-OFF potential) during an interrupted survey can be complicated by the introduction of a capacitance effect from DC decouplers in the circuit. Methods to obtain accurate polarized potentials in these scenarios exist, but there is some level of risk introduced to the pipeline and operating personnel. This paper will explain these existing methods, evaluate the level of risk associated with each and analyze a new technology that was developed for the purpose of improving data by providing accurate polarized potentials and avoids the risks or inaccuracies that other methods introduce. Field data will be presented to represent the performance of the new technology and compare it to having traditional decouplers in place.
The new technology reduces the time required for decoupler voltage to dissipate without having to disconnect the decouplers during an interrupted survey.
Electrical potential surveys are typically required to be completed on a periodic basis on cathodically protected structures such as underground pipelines. However, proving that a cathodic protection system is providing the degree of corrosion protection required can often be a time-consuming and error prone endeavor. It is a typical practice, and often required, to interrupt the CP source(s) to carry out these surveys, the accuracy of which is affected by several factors, including the presence and inherent function of DC decouplers.
This paper discusses how and why traditional decouplers can affect interrupted survey measurements including instant-off data and reviews some existing solutions to these issues. A new decoupler solution for ensuring the accuracy, efficiency and safety of interrupted surveys is reviewed.
Watch Your Waveforms
The purpose of potential surveys is to evaluate the effectiveness of the cathodic protection (CP) system by measuring the polarized potential of the pipeline. However, pipe-to-soil potential measurements are influenced by the CP current, which introduces a measurement error known as IR drop. To eliminate this error, CP sources are synchronously interrupted momentarily, after which the potentials are measured, but before the pipe depolarizes. The potentials measured in this way are referred to as instant-off potentials. Such interrupted measurements are typically collected on a segment of the pipeline that is undergoing a Close-Interval Survey (CIS) or Close-Interval Potential Survey (CIPS). A typical instant-off potential response on a pipeline without decouplers installed is shown in Figure 1.