How it Works

- the leading pipeline survey system for fast, accurate and objective reports on the location and condition of buried pipelines ...

An electrical current applied to a well-wrapped buried metal pipeline will decrease gradually with increasing distance from the current signal injection point, as the current escapes to earth through the wrapping - Fig. 1.   Fig. 1
     
 

Fig.2 If the wrapping has a uniform thickness and separates the pipe from the surrounding soil at all points, the strength of the signal current on the pipe will decline logarithmically. The rate of decline will be dependant primarily on the electrical resistivity of the wrap or coating used, and the area of the wrap in contact with the soil per unit length of pipe - i.e: for a given coating, the decline in signal strength is proportional to the circumference of the pipe - Fig.2.
     
 
Note: because of the relative magnitude of the resistances involved, local changes in soil resistance can usually be ignored. If there is a low resistance electrical path from the pipe to the soil at any point, there will be a substantial increase in the rate of loss of signal current - see Fig.3 Fig.3

     
 
 

Such a low resistance path could be caused by:

incorrectly applied wrapping
mechanical damage to the wrap, before, during or after installation
decay of the wrap due to soil conditions
disbonding of the wrap from the pipe - provided that ground water has penetrated into the gap to provide an electrical path to earth
a leak in the pipe itself, causing the wrap to fail at the leak point.
 
 
Because the resistance of such a path is much less than the resistance of the undamaged wrap, the resultant loss of current - even from one small fault of a few sq.mm - can usually be detected by a significant increase in the apparent rate of current decline over quite a long length of pipe.

In practice, the existence of one or two small faults on a section of pipeline several hundred metres long can usually be tolerated because the Cathodic Protection (CP) System can be expected to prevent serious corrosion developing quickly. In this case it may not be necessary to locate the specific faults immediately, but the rate of logarithmic decline in the current signal between two specific points on that section can be logged for future reference, so that any deterioration can be monitored by regular re-surveys in the future.



 

 Note: the logarithmic rate of decline of the current (attenuation) - measured in millibels per metre - is effectively independent of the applied current and is therefore only marginally affected by seasonal changes in soil resitivity. Hence it is in effect an absolute indication of the average condition of the wrap between two given points at the date of survey.
 

system consists of two principal elements:


The Signal Generator (right) is attached to the pipeline (usually at a Transformer/Rectifier Station or a CP Test Post) and to an appropriate earth point. This produces a constant AC signal which passes along the pipe for 2-3 km (depending on wrap quality) in either direction.

The hand-held Detector Unit (left) measures the electromagnetic field now radiating from the pipeline at any point within the signal range. At each location, the Detector Unit is switched on and - if a signal is present - the display will direct the operator to the pipeline, showing its orientation and its approximate distance and depth. Once directly overhead, the Detector Unit calculates and displays the exact depth, the strength of the remaining signal, and the precise location coordinates. The Operator uses the keypad to store the data, whereupon the Detector Unit will immediately display the attenuation value back to any previously stored location.