The NoPig technique realizes a non-destructive testing method for unpiggable pipelines which uses above ground measurements for detecting wall thickness anomalies like corrosion. The method uses an applied current between two points on a pipeline up to 1 km apart. The inspection current contains several harmonic components at distinct frequencies. The resultant magnetic field is measured above ground along the pipe. The measurement data are processed to determine the magnetic field shape. In case of a local pipe wall thinning a frequency-dependent deviation of the magnetic field shape from the circular form occurs. The skin effect and the magnetic stray flux evoke this deviation in case of wall defects caused by corrosion.
In the sensor array 6 magnetic sensors are installed in one line. The sensor array consists of four lines with a total of 24 magnetic sensors. Every sensor measures the horizontal component of the magnetic field. The acquisition and storage of collected data will be done in the mobile unit, which is connected to the sensor array. During an inspection the magnetic field above the pipeline will be measured step by step. The step length is exactly one meter. Later in the NoPig office from the data the frequency-dependent magnetic field deviations in the vertical and the horizontal directions will be calculated. A deviation presence indicates the region with a defect.
The magnetic field data are also used to calculate the depth of coverage of the pipeline and the inspection current. Both these parameters deliver additional information about the pipeline condition. The current data can be used for detecting and locating coating defects.
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NoPig inspection procedure
Preliminary routine
The crew will evaluate the pipeline project site and oversee any necessary preparation of the ground above the line. This time period can be kept to a minimum as the pipeline right of way should be cleared before the crew arrives.
Any special electrical isolation of the pipeline and cathodic protection shut-off should be discussed in the customer/technician meeting prior to the inspection run. The pipe book should be presented and discussed to determine where possible critical areas are located. This will help the evaluation process.
Inspection run
Transportation to the pipeline site will be arranged by the customer as well as a technician to aid the N.P. Inspection Services GmbH personnel. Onsite, the crew will set up the system. The pipeline will be connected with the current source using special cables. This will either be done on above ground areas of pipe, cathodic protection attachments or holes which have to be dug.
The 2 connections necessary for the system to function must be not longer than 1.000 metres apart. After connection and setting the inspection current, the NoPig sensor array will be placed on the ground above the pipeline to be inspected. The pipeline locator of the system gives the precise location of the pipe under ground as well as the depth. At each inspection step, the array is placed directly on the ground. After the data acquisition is done, the array is then moved forward one step.
Since March 2005 the NoPig Pipeline Inspection System is TÜV approved.
