Page 235 - Pipeline Rules of Thumb Handbook
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222     Pipeline Rules of Thumb Handbook

         Methods of locating coating defects


                           Pearson survey                      are indicated in Figure 1. Any area indicating high signal
                                                               losses may then be resurveyed using the same procedures to
           The Pearson survey is an above ground technique used to  more closely locate the coating defects(s).
         locate coating defects on buried pipelines. An AC signal is
         injected onto the pipeline from a power source connected
         between a cathodic protection test post or valve and an earth    Close interval potential survey
         pin. The AC frequency selected is dependent upon the type
         of pipeline coating in order to minimize current loss from the  The close interval potential survey is an above ground tech-
         pipeline through sound coating. At coating defects, the AC  nique used to provide a detailed profile of the pipe to soil
         signal leaks to earth and flows via the soil to the earth spike  potential with distance. This profile can be used to determine
         to complete the circuit. The localized current flow from the  detailed information concerning the performance of the
         pipeline at the defect creates a potential gradient through the  cathodic protection system, the coating system and to a lesser
         soil and this is detected by measurement at a ground surface.  degree, interaction effects. The advent of portable micro-
           In practice, this is achieved by two surveyors walking in  processor based measuring equipment and computer assisted
         tandem over the entire pipeline, or section of pipeline. Earth  data handling, together with accurate synchronized high
         contacts, either ski poles or foot cleats, are connected to a  speed current interruption devices have allowed these surveys
                                                               to provide accurate, practical data and collection methods.
         receiver via a cable harness. As the first surveyor approaches  To eliminate errors in the pipe to soil potentials recorded,
         a coating fault, an increased voltage gradient is observed,  arising from the inclusion of volt drops caused by the flow of
         either audibly through headphones, or visually on the receiver  cathodic protection current in the soil, synchronized current
         signal level meter. As the lead surveyor passes the defect, the  interruptors are employed in the cathodic protection units.
         signal fades, and then increases again as the rear surveyor’s  Pipe to soil potentials are then measured instantly after the
         contacts pass over the defect.                        current is interrupted, before significant chemical depo-
           The defect may be recorded on a preprepared record sheet
         complete with a measured distance from a fixed reference  larization has occurred thus providing an “error-free” polar-
                                                               ized potential.
         point or indicated by a marker peg or non-toxic paint. A  In practice, the precision current interruptors are best
         recent development in Pearson equipment has been the  fitted in the DC output circuit of the cathodic protection
         introduction of a system that continuously records, through  power supplies, prior to commencing the survey to interrupt
         data logging equipment, the potential difference between  the cathodic protection current for short cyclic periods. The
         each earth contact. In this way, data may be graphically   cycle time is chosen with regard to pipeline characteristics
         presented.
                                                               although a range of “ON” and “OFF” periods and “ON/OFF”
                                                               ratios are available.
                  Signal attenuation coating survey
                                                                 For this survey a variety of purpose built voltmeter/micro-
                                                               processors are available that record and store the pipe to soil
           This method is similar in many respects to the Pearson
         survey, the essential difference being the method of defect  potential data with an additional facility to store coded entries
                                                               of topographical or pipeline features. This information is
         detection. The AC current injected onto the pipeline creates  transferred to floppy disc storage from which graphs of ON
         a magnetic field around the pipe, the strength of this field  and OFF pipe to soil potentials against pipeline chainage are
         being proportional to the amplitude of the AC signal.   produced. A typical close interval potential graph is illustrated
         Measurements of the field strength are made by recording the  in Figure 2.
         voltage generated in a sensing coil and indicated on a dB  It will be apparent from the description of the pipeline
         scale, when the coil is held over the pipeline. Loss of current  surveys that the form of results are different for each survey:
         from a coating defect creates a change in the field strength
         due to a change in the pipe current and this is detected as a
         change in output from the sensing coil.                                     Pearson
           In practice, this is achieved by one surveyor measuring field
         strengths at discrete, preset intevals. At each location the field  Results are not generally presented in graphical form.
         strength and pipeline chainage are recorded. A graph of dis-  Coating defects are marked in the field; in addition the detec-
         tance versus field strength may then be produced, sections of  tion of defects is dependent upon operator skill. Only if the
         pipeline having poor coating quality are indicated by the  recording Pearson technique is used can data be presented in
         steepest gradient on the line plot. The results may also be  graphical form. Experience of this technique is at present
         expressed as histograms to aid interpretation. Typical results  limited.
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