Smart pigging with the use of the ultrasound flaw detector (UT)

The pig’s operating principle is based upon measuring the transmission time of the ultrasound signal reflected from the inner or external surface of a pipe wall.

The ultrasound technology allows identifying and measuring defects in areas where the magnetic diagnostic method’s capacities are limited, for instance, vast sections of uniform corrosion and laminations.
 

Regardless of several technical differences, all types of such pigs have ultrasound transducers on their surfaces that operate according to the immersion method (dipping method). The principle of the method is as follows: the space between the transducer and the subject under examination is completely filled in with a liquid (oil or an oil product).

The pipe wall thickness or the distance to a discontinuity is calculated by measuring the transmission time of the transmitter pulse from the inner surface to the external surface of the pipe wall or from the external surface to the discontinuity and the reflected pulse in the reversed direction.

The first reflected signal’s time of arrival is converted into the distance from the transducer to the external surface of the pipe wall and the second reflected signal’s time of arrival is converted into the wall thickness.

If there is external corrosion, the signal’s transmission time within a steel pipe wall is reduced which as a result directly gives a quantitative index of metal loss. If there is internal corrosion, the signal’s transmission time is increased in oil.

Apart from identifying external and internal metal losses, the method allows identifying and measuring other defect types such as laminations, inclusion, scratches, notches, guide marks and compression marks and their combinations as well. The ultrasound signal is reflected from different non-homogeneities buried in the pipe wall thickness as well, thus allowing identifying discontinuities in metal of the pipe, aside from external and internal metal losses of different kinds.

The ultrasound flaw detector consists of sections – steel cylindrical sealed housings (with electronics, information tanks and batteries inside) and transducer carriers interconnected with the help of universal joints and cables. The number of sections and the content of each of them are established as per electronics and batteries arrangement within the housing’s limited space whose dimensions should provide for control over a pipeline with a specific flow section and the minimum pipe turn radius.

Each section and transducer carrier are equipped with polyurethane insulation joints designed for centring adjustments and ensuring that the device moves within the pipeline with a transferable product stream.

Each sealed housing has conical insulation joints that do not allow the device to get stuck in T-joints without safety screens.

This table shows the results of a comparative analysis of identification and measurement capacities of different inspection pigs available in the market nowadays.

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* Limited; **** Best Solution

The comparison is based upon the pigs MFL with a high power of resolution and the pigs with transversal magnetization (AFD), the distance between sensors around the circumference is less than 3 mm.