This common form of outside corrosion occurs, when pipelines are lying on steel or concrete, or are hanging in clamps. Because of temperature changes, the pipeline is moving over the support, which can cause damaging of the coating of the pipeline. With the combination of water, corrosion occurs.
The method of lifting and inspection during process is not acceptable because of the high risks which are involved. National NDT is utilizing the Ultrasonic technique which is called through transmission or pitch and catch whereby two angle probes are used, one transmits the sound and the other receives. Corrosion depths under or around the supports are measured by the amplitude of the signal received from the transmitting probe. Inspection can be carried out on +-20 - 25 supports per 9 hour shift depending on pipe sizes. This technique has been proven to be cost effective & reliable and approved by Shell & BP Refinery South Africa.