Transient phenomena can significantly affect the accuracy of pipe-to-soil potential (PSP) measurements. operator has developed a simple method to improve the accuracy of cathodic protection close interval measurements through the use of continuous dataloggers at reference locations within the survey area. One pipeline survey (CIS)
Telluric currents caused by the variations in the earth?s magnetic field are well known to present difficulties to operators of pipeline systems. These currents manifest themselves as pipe-to-soil potential fluctuations which are dependent on both location and time. Two mechanisms are generally considered when discussing telluric current effects: classical electromagnetic induction supports the concept that local changes in the earths magnetic field will induce electrical current in stationary conductors such as pipelines (see figure l)l; alternately, geomagnetic disturbances may result in large voltage gradients created in the earths crust in which long metallic conductors such as pipelines merely carry the charge (without electromagnetic induction in the pipe)". This paper does not address which mechanism is prevalent, but rather discusses one approach developed to maximize close interval survey accuracy in telluric current affected areas.
For pipeline operators, pipe-to-soil potential variations caused by telluric activity can result in apparent non-compliance with cathodic protection criterion, costly and unnecessary remedial construction, or shutdown of CP monitoring crews until the telluric activity diminishes and pipe-to-soil potentials return to normal. Figure 2 illustrates telluric activity as recorded using a stationary pipe-to-soil chart during close interval survey in 1999.
TransCanada Transmission operates 39,000km of pipeline throughout 6 provinces and 19,000km of right-of-way (Figure 3). The integrity management of this pipeline system relies heavily on corrosion prevention by means of external pipe coatings and cathodic protection augmented by routine in-line pipe inspections.
The cathodic protection system monitoring program involves routine inspections of impressed current sources, annual test lead surveys, and regular close interval surveying (CIS) for a more detailed evaluation of cathodic protection system performance. Approximately 3500km of pipe is close interval surveyed annually on a rotating schedule. Areas that do not achieve the cathodic protection criteria are slated for further investigation, which includes adjustment of CP system operating parameters, remedial design, and construction of new CP facilities.
Given the scope of this annual survey program and the costs associated with construction of new CP facilities, survey accuracy is paramount. The telluric monitoring procedures developed are used during all close interval survey. Where the recorded pipe-to-soil fluctuations at reference locations exceed specified parameters, a simple algorithm is used to post- process the close interval survey data to eliminate much of the telluric effects.
The telluric compensation technique is based on the following assumptions: 1. A sufficiently large sample of telluric current affected pipe-to-soil data, recorded at a single location and averaged over time, will approximate the true pipe-to-soil potential unaffected by telluric currents; 2. The pipe-to-soil potential variations observed at one discrete location on a pipeline will approximate those observed at nearby locations, provided that their separation is not too great and that there are no other significant differences in the pipe or soil characteristics between the different locations. This latter assumption is very significa