Obtaining the polarized potential (off potential / instant-off potential / IR compensated potential) of a pipeline is considered extremely difficult, if not impossible, when the pipeline is interfered by an alternating voltage, and particularly if the pipeline is fitted with capacitive AC mitigation devices. Yet, a number of procedures exists to obtain information on the polarized potential, due to its importance in cathodic protection operation. Two distinct approaches are measurements on the structure/pipeline itself, and measurements on coupons. While it can be agreed that the structure measurement is challenging, some believe that the coupon method is error-free, as the AC interference is not present on a disconnected coupon. This paper will illustrate that this is not entirely true. Some of the present challenges and some of the approaches to get meaningful measurements are presented.
The -850 mV (CSE) criterion refers to the polarized pipeline potential that is free of any IR-drop. Different methods to obtain the polarized potential exist. Some of these are listed below:
Interruption of the CP current will cause the current, I, and thus the IR-drop to become zero and the remaining polarization immediately after the interruption is representative of the polarized potential of the pipeline. This is illustrated in Figure 1.
This potential is a mixed potential of coating defects on the pipeline in proximity of the measurement location. The defects closest to the reference electrode and the larger defects will influence the measurement the most. Any capacitance built into a system will cause an altered depolarization behaviour and complicates this approach. Some operators try to reduce the effect of capacitance by disconnecting decouplers during an instant-off survey. This approach is however questionable due to the altered polarization behaviour of the pipe under an AC interference level which is higher than normal operating conditions. I.e., the result of such a survey is possibly affected by the AC. Also, the risk of AC corrosion and safety to personnel during the survey shall be considered.
