JANUARY 31 2003
foreign structure is small or well coated, and the protected structure is poorly coated,
this option may be taken when sufficient CP current is still available to maintain
protection on the protected structure.
Interference from Variable (Fluctuating) Sources. While interference
testing determines effects of steady stray currents, another type of stray current survey
analyzes fluctuating stray currents. Fluctuating or periodic changes in structure-to-
electrolyte voltage values and unusual or fluctuating currents are indicative of stray
currents. Stray currents may affect structures just as cathodic protection does.
Structures may be protected or damaged by stray currents, depending on whether
current is flowing to or from the structure. Such analysis is specialized and requires
study to master all techniques. The basic principles are the same as in the previous
paragraph (7-14.1). The difference is that the stray current is not present all the time.
The simultaneous use of data loggers or recording devices at many points on the
foreign structure will indicate the same information as above. Locating the discharge
point(s) and pickup areas is still crucial. In this case, determining the time of the
7-14.2.1 All the basic measurements can be used in studying fluctuating stray
currents. Perhaps the most informative are measurements similar to those used in
interference testing: structure-to-electrolyte potentials and IR drops along structure or
electrolyte. Methods of analysis, however, are different, and specialized equipment is
used. The major concern in performing a stray current survey is to find out the degree
of damage and determine the source of the currents.
7-14.2.2 Measurement of the foreign pipeline potentials is commonly used to
determine the pickup and discharge areas. Using data loggers or recording devices,
many locations on the foreign structure are monitored simultaneously. This data is then
charted to analyze for the presence of interference. When the exact moment of
interference is determined, all potential readings taken at that instant are used to
determine the discharge and pickup points. Data taken is used to locate areas for
taking additional data. Once gathered, this data is used exactly like the data for
cathodic interference detailed in paragraph 7-13. Abnormal potentials on other
structures may also be used for further analysis. Once the pickup area and time is
determined, the source of the current may be located (see paragraph 2-2.4 for sources
of fluctuating stray currents).
7-14.2.3 Another method of finding these points is through current measurements. To
determine the pickup and discharge points in fluctuating stray current areas, data from
several test points are plotted over a period of time. Current values on the several
locations on the foreign structure (recorded from IR drop test spans or a clamp-on
milliammeter) are plotted over a period of time. The current must be measured at
several points simultaneously, sufficient readings being made to cover the range of
variation of the current (usually at least 24 hours). By analyzing the current direction