|
1. Sacrificial anodes, cross bonds etc must be disconnected in order not to mask defect areas and in order not to give too rapid loss of signal. Disconnecting changes the basic set up of the pipelines protection systems giving rise to information that is not typical of normal operations.
2. Must be used directly above centreline of pipe for meaningful attenuation comparisons and hence identification of defective areas.
3. Length of pipeline surveyed from any one-transmitter location can be as short as 200M depending upon quality and type of pipe coating, size of pipeline and soil resistivity.
4. Cannot use multiple transmitters as synchronised AC Transmitters not available to industry.
5. Any pipe bend, change in wall thickness, parallel pipe-work, cased crossing and other services likely to distort the magnetic/electric field will give erroneous readings.
6. A high signal loss may not be present at a substantial coating defect in high resistivity soil so have problems with assessing the severity of faults.
7. The survey operator needs to be fully experienced in the survey technique so that the best results can obtained in the field in order to make sense of the observations.
8. Does not evaluate effectiveness of the CP or indicate any corrosion that may be taking place. Observations of attenuation in mB/m bear no relation to any electrochemical corrosion phenomena.
9. May not accurately locate even minor defects, due to the level of attenuation for detecting the signal, chosen on the measuring instrument.
10. Badly affected by AC electrical interference. Cannot survey under overhead power lines or near buried power cables.
|
