In order to ease interpretation and to separate what is being monitored from other DC sources, the survey DC signal is switched ON and OFF by a switcher connected to the Transformer rectifier to interrupt the DC output. The signal is asymmetrically switched with a cycle of ? of a second OFF and ? ON. The “ON” time is shorter than the “OFF” time so that the polarity of the applied CP can be recognized for coating defect location.

During a pipeline survey, the operator walks directly above or just to the side of the pipe, with the probe tips placed one in front of the other along the pipe route.

As a defect is approached, the millivoltmeter will respond to the impressed DC signal, with the needle deflecting in the direction of current flow, which may be a defect or an anode system. When the defect is passed the needle deflection reverses and points in opposite direction. By retracing one's step a position is reached where the needle shows no deflection, i.e. a null. At this position the defect is located midway between the two probes. This procedure is repeated at right angles to the pipe and the defect is located at the point where the two midpoints intersect.

The defect location is then marked with a numbered wooden peg to locate the defect at a later stage, with the chainage at that point measured using a surveyor wheel.

The located defects are ranked according to their severity rating. Defect severity is measured by computing the %IR at the defect location. Further tests are carried out to determine the anodic/cathodic status of the located defect. A number of the located defects are then exposed to assess the coating damage and this is correlated to their individual severity ranking.

Ground surface mapping of uni-potential lines can assist with determining defect shape. This requires substantial time, particularly when the defect frequency is as high as it is for a bitumen coated pipeline.

Defect Characterisation

It should be noted that not every defect located requires repair when considering cost-effectiveness and technical justification. A system of characterizing the located defects has been developed in order to prioritize defects for repair. Their features, based on available information, determine defects requiring repair. These are defect size, shape, number, distribution of faults, corroding/protection status of individual defects.

Defect size

This is related to the %IR assessment and the following guideline is used:

%IR Severity	Defect Size	Recommended Action
0 - 15	Small	No repair
16 - 35	Medium	Consider repair
36 - 70	Medium/Large	Consider early repair
71 - 100	Large	Consider immediate repair

It should be noted that this table is a guideline only and does not address the holistic factors of pressure, soil conditions and related risk contributors. Measurements taken under stray current activity should be carefully interpreted, since erroneous deductions may be made.

Furthermore, measurements taken depend on a number of factors such as depth of cover, soil resistivity, DC and AC interference. All these should be taken into account while surveying and during the selection of defects for repair.

At the heart of the PCM system is the current mapping near DC signal applied by the transmitter. A pipeline's electrical characteristics of current attenuation and distribution at this very low frequency (4Hz) signal is postulated to be similar to those from the current applied by a Cathodic Protection rectifier.

The PCM receiver contains a precision, high performance sensor known as a magnetometer which remotely detects and measures very low frequency magnetic fields. Advanced signal processing technology provides for the current measurements (and direction) of the near DC (4Hz) signal and a data logging function enables graphing of current profile (and loss) against distance to be plotted after downloading field data to a PC. The display on the PCM receiver indicates the direction of current flow. The PCM receiver compensates for depth changes during current measurements, therefore, the current reading remains constant even when the depth of the pipeline changes.

The PCM transmitter applies a current to the pipeline, which reduces in magnitude as the distance from the transmitter increases. The current reduction rate depends on the condition of the pipe coating, ground resistivity and pipe electrical continuity (resistance).

When a significant defect is encountered one would expect the current to drop quickly. A defect may result from coating damage, contact with other services and so on. Under normal circumstances, the loss of PCM current will be virtually proportional to the amount of CP current flowing onto or leaving the defect point.

However, linear current loss may naturally occur along the pipeline due to the age of the pipe and coating conditions.

Defect Characterisation

It should be noted that not every defect located requires repair when considering certain arguments relating to cost and technical reasoning. A system of characterizing the located defects has been developed in order to prioritize defects for repair. It should be noted however that these characteristics are unique for every pipeline environment and these conditions may change with time.

Defects requiring repair are determined by their features based on available information. These are defect size, shape, number and distribution of defects. Generally speaking when cathodic protection is applied the smaller defects are afforded protection by the current flowing to the exposed steel at a coating defect of relatively small size. The current density and defect size are subjects of intense debate as these are dependent on the type of cathodic protection system, the location of anodes, the amount of current injected into the pipeline and so on.

Typcially the graph showing current signal in dBmA versus distance in meters, is accurate as it shows that the ratio of the magnetic field strength, whereas current mA graph alone may give rise to misinterpretation of data due to the high current loss near the transmitter and lower current losses further away.

Under normal circumstances, the size of each defect can only be correlated and extrapolated if several of the defects found by the PCM survey are exposed.

In some areas, the potential of the pipeline may be affected or influenced by telluric or dynamic stray currents.  Under such circumstances a GPS synchronized data logger is installed in the survey area to record the rectifier ON and Instant OFF potential at each rectifier interruption cycle.  If any external source of interference is affecting the potential of the pipeline the GPS synchronized data logger will record the affect and time stamp the data. The data recorded by the GPS synchronized data logger can then be used to correct the close interval survey results.  There have been several papers presented at NACE International on stray and telluric current correction of CIPS data by using GPS synchronized data loggers.

With GPS synchronized current interrupters installed in all of the rectifiers influencing the survey area and with GPS synchronized data loggers installed at the start, end and at 5 Km intervals through out the survey area, you can be assured of obtaining accurate rectifier ON and Instant OFF potentials. 

Certain survey instruments are equipped with GPS engines for accurate timing of the measurement of the rectifier ON and Instant OFF potentials.  These units can also measure the distance traversed (chainage) and can display and record the Universal Co-ordinated Time as well as the latitude and longitude and elevation of each measurement location.

Since each recorded reading is time stamped with the Universal Co-ordinated Time and if one or more GPS synchronized data loggers were reinstalled in the survey area, if the data is influenced by dynamic stray or telluric currents the data from the stationary GPS synchronized data loggers can be used to correct the data recorded by the moving surveyor during the close interval survey.