Before conducting a survey of a pipeline by an inspection in-line PIG or before a repair of a pipeline it is necessary to run several times cleaning PIGs through this pipeline. This operation is called pigging. And it is made in all kind of pipelines used for transportation of natural gas, crude oil, oil products, etc. The cleaning PIGs or inspection PIGs like form-measuring PIGs, ultra sonic scanners, MFL tools and others are moved inside the pipeline due to the pressure acting on them from the transported liquid or gas.

And always there is a need and a problem: how to trace the movement of a PIG inside the pipeline, which is not transparent, can be covered by soil (buried pipeline) or lay on a sea bed (off-shore pipeline). Especially important is to find a PIG in the pipeline if the PIG unexpectedly didn't went into receiving chamber, which means that it has stuck somewhere in the pipeline. And the pipeline flow is either completely blocked or considerably reduced.

To equip an in-line PIG by a conventional radiotransmitter is useless as metallic walls of the pipeline reflect back radio signals and the signals cannot go out of a pipeline.

Currently there are few methods for tracking and registration of in-line PIGs and other in-line tools.

1. Acoustic method

In this method a sensitive geophone is placed on the soil above a buried pipeline or directly on the metallic parts of the pipeline. With help of the geophone it is possible to listen sounds made by a PIG moving inside the pipeline. This method is relatively simple for technical realization. But its accuracy for determination of the PIG position is very modest.

As an advantage of this method it is worth to mention that it can detect moving in-line tool or PIG on big distances up to several kilometers.

It is recommended to use Acoustic locators together with other devices using other principles of detection. In this case acoustic locator first signals about a PIG approaching the control point in the pipeline.

2. Radioactive isotope method

A capsule with radioactive isotope is installed on an in-line PIG. The registration of the PIG passage is made with use of Geiger counter placed in a control point outside the pipeline.

The method is reliable and from explosion point of view is safe. It also enables to detect a PIG from rather long distances (dozens of meters). Also radiation passes well through metal, soil and water.

However for work with radioactive isotopes it is necessary that workers have special permission for work with such materials and also it is necessary to have special protective clothes and equipment. There are also other special requirements. So the radioactive isotope method is currently applied only in special cases.

3. Magnetic method

In this method strong permanent magnets are installed on an in-line PIG. The magnetic field is weakened partially by the pipeline wall. But its intensity outside the pipeline is sufficient.

The design of the magnets and special way of their installation on a PIG makes it possible to obtain the intensity of magnetic field sufficient for detection by magnetic field receiver . The detection distance is several meters from the pipeline in static mode when the PIG is motionless. And more than 10 meters if the PIG with permanent magnets is moving inside the pipeline while the receiver is placed above the pipeline and is motionless.

Absence of any power consumption by the permanent magnets is an advantage of this method. This method is simple and easy in realization. Also it is safe from an explosion point of view.

Difficulty to detect a motionless (stuck) PIG with any magnets and low probability of such detection are the disadvantages of this method. The reason is that intensity of magnetic field above the pipeline changes very rapidly with the distance from the magnet. Very often intensity of the field outside the pipeline is comparable with magnetic field intensity remaining in the pipeline wall after passage of a magnetic inspection tool (like MFL or TFI smart PIG). And it cases false detection.

So this method is recommended only for registration of moving in-line objects.

4. Electromagnetic method

It is well known effect that slowly changing magnetic field (oscillating with frequency from 1 Hz to few dozens Hz) can penetrate the wall of metallic pipe, soil or water with small reduction of the signal strength. This effect enables to register this oscillating magnetic field with help of sensitive receivers.

Oscillating magnetic field is made by electromagnetic generator (transmitter). The main part of the transmitter (antenna) is a coil with a core.

Using a quartz synchronization of frequencies for the electromagnetic field transmitter and the magnetic receiver ensures stable reception of even very weak signal from the transmitter which is inside a pipeline. And it allows to design compact transmitters with long autonomous work time and power supply from conventional alkaline batteries.

Due to the fact that in-line transmitter uses elements of the electric power supply and a large electromagnetic inductance coil this method in itself is not explosion proof. So when this method is implemented one must use special schematic and design solutions which ensure explosion protection of the device in hazardous environment. Only certified for explosion-proofness devices can be used in gas or oil pipelines.