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Close Interval

Description

CIS (close interval survey) is the process of identifying possible corrosion problems in modern product pipelines. For this purpose, Nascorp uses a data logger equipped with an optical or magnetic wire counter for use in measuring distance. Optical & magnetic wire counters offer a high degree of accuracy which will result in a better representation of distance in your field plots. Underground pipelines are protected from corroding by using method called cathodic protection which uses either an impressed DC current (rectifiers) or a sacrificial (anodes) are equipped with permanent test stations where wire leads are attached to the pipeline to measure the pipe-to-soil potential. This potential should be adequately cathodic to ensure adequate corrosion protection but not excessively cathodic which can cause hydrogen disbondment the coating.

There are several types of (CIS) Close Interval Surveys:

  1. On survey – this survey collects data between established test stations (usually at 2.5 to 5 foot intervals). As with all types of close interval surveys, the “ON” pipe to soil readings are obtained using a traveling reference electrode coupled light gauge coated wire, a data recorder and sub meter GPS equipment. The operator provides a continuous profile of the pipe to soil potentials as well as any features of note on the structure being surveyed.
  2. ON/OFF SURVEY – This survey requires the dc current on the structure be interrupted for a specific time sequence such as 4 seconds on and 1 second off to minimize depolarization. The current is interrupted to remove the IR from the line to obtain the polarized or instant off potential. The polarized or instant off pipe to soil potential must be more negative than -850mv in order to meet D.O.T. specifications.
  3. Depole or Static Survey – This survey is used when the pipe in question is having trouble meeting the off criterion of -850mv. The cathodic protection system protecting the pipeline is completely turned off in order to allow the pipeline to depolarize to its native or depolarized potential. If there is difference between the instant off potential and the depolarized potential of 100mv or greater, the pipe is said to be protected.

The on/off survey is especially useful in identifying source of influence. Also, when used in conjunction with ACVG or DCVG (depending on the age of the line), the combination is useful ECDA.

North American Survey uses:

  • Three member crews with one or two vehicles.
  • Experienced crew leaders, data collectors and pipe locators.
  • Satellite synchronous interrupters (AC or DC) and data loggers.
  • Hip pack or backpack wire dispensing unit with either optical or magnetic switch chainer.
  • An American Innovations Allegro based hand held data logger.
  • 34 AWG gauge copper wire with lacquer insulation.
  • Portable computers and printers for editing and printing of data for clients.

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