Crack detection in gas pipelines

Intelligent pigs, which detect geometry defects and metal loss in long distance pipelines have been around for many years. It has also been possible for several years to detect crack-like defects with the ultrasonic method in liquid pipelines. In gas lines, however, the detection of crack-like defects incurs a high additional cost because the ultrasonic method requires a coupling liquid, and ultrasonic pigs can only be run with a liquid batch. A crack detection pig for gas pipelines was therefore urgently required. 


The new EmatScan® CD is capable of detecting crack-like defects in gas pipelines with the ultrasonic method without a coupling liquid. The EmatScan® CD utilises EMAT (Electro Magnetic Acoustic Transducer) technology, whereby the ultrasonic pulse is generated electro-magnetically inside the material by an electric pulse applied to a coil in the sensor. The EmatScan® CD has already successfully inspected several gas pipelines in North America and is currently available in the size of 36 inches.
Introduction
High pressure long distance pipelines transporting gas, crude oil or products are inspected by intelligent pigs for the location of defects. These inspections are an important contribution to the continued safe operation of these pipelines.
Typical defects are geometrical anomalies, metal loss and crack-like defects. Intelligent pigs are measuring robots which are propelled through the pipeline to detect defects, using appropriate measuring techniques.


For geometrical anomalies, pigs with mechanical sensors have been used for many years. It is customary to inspect new pipelines with calliper pigs prior to commissioning.
In the 1970s metal loss (corrosion) was the type of anomaly that caused the development of the first intelligent pigs. For metal loss two technologies are customarily used: the ultrasonic method, which measures the wall thickness directly, or the magnetic flux leakage (MFL) method, which responds to the change of the magnetic field in the presence of metal loss.

The ultrasonic method is the more accurate method, but a coupling liquid is required to apply the ultrasonic pulse to the pipe wall. It is therefore mainly used in liquid pipelines. The MFL method, on the other hand, does not require a coupling liquid and is therefore the preferred method for gas pipelines. Both types of instrument have been operated for many years and play a central role in the upkeep and maintenance of high pressure long distance pipelines.

During the 1990s longitudinal crack like defects began to appear additionally in more and more pipelines causing serious problems. This led to the development of a new generation of crack detection pigs.

Types of Cracks

Even though isolated fatigue cracks have been seen since the 1970s, it was the increased appearance of stress corrosion cracking (SCC) defects in the 1990s that led to some spectacular pipeline failures in Russia and North America. Figure 1 shows typical SCC colony.
SCC develops in pipelines under narrowly defined conditions. These include: susceptibility of the steel, moisture of the soil, soil chemistry, quality of the coating, variable stress and highly increased temperatures. SCC first appeared in the above mentioned areas mainly in high pressure pipelines directly downstream of compressor stations and now also occurs more and more often in liquid pipelines, even though these lines do not display increased temperatures.
Apart from SCC, metal fatigue cracks are becoming increasingly common, mainly due to the increasing accumulated number of pressure cycles in the aging pipeline population.
Cracks, which influence the structural integrity of the pipeline, are mainly longitudinally orientated, caused by the predominant stress distribution in the steel. Fatigue cracks can grow both from the internal or the external surface of the wall. Because of the growth mechanism, SCC cracks are external defects.

Batching with UltraScan® CD

In the early 1990s the UltraScan® CD crack detection pig was developed by GE Energy. It uses angular beam ultrasonic technology to detect longitudinal cracks. The sensors operate in the immersion mode, the transported fluid is used as coupling liquid.

The basic principle is demonstrated in Figure 2. The angular ultrasonic beam is reflected to and fro between the two surfaces at an angle of 45°. If the signal is reflected by a crack it travels back along the same path and is received by the same sensor as the echo signal. The appearance of the echo signal along the time coordinate indicates whether the crack is located internally or externally. As the tool is designed to detect longitudinal cracks the sensors are slanted with circumferential orientation to allow the beam to travel through the wall perpendicular to the longitudinal direction. In order to scan each defect from both sides two sets of sensors are employed, one operating clockwise, the other in an anti-clockwise direction. Each ultrasonic pulse is monitored up to two and a half full reflections (skips), meaning each crack is seen by several sensors from different distances. This results in a redundancy of information which is important to guarantee a reliable detection of the cracks and to differentiate between real cracks and harmless small inclusions in the material.

The multitude of sensors are mounted on the sensor carrier so that the entire pipe circumference is scanned in one pass (Fig. 3). The effective distance between sensors in circumferential direction is about 10 mm. The individual skids of the sensor carrier are mounted in such a way that geometric irregularities of the pipe are compensated and the sensors are always locally orientated with the right angle to the wall.

During the inspection, large amounts of data are generated. During the travel of a 24 inch UltraScan® CD tool through a 100 km long pipeline, 100 terra bytes of primary data are generated. The data is screened in real time for signals relating to crack like defects and only those signals are stored in the on board solid state memory. To achieve this, the most advanced FPGA electronic components are employed in the tool.
The UltraScan® CD detects all defects of 25 mm minimum length and 1 mm minimum depth. The data is displayed as a coloured area scan (C-Scan). The colour displays the intensity of the reflected signal according to the colour code. The intensity of the signal is an indication of the depth of the defect (Figure 4). UltraScan® CD tools have inspected more than 15 000 km of pipeline since their introduction in 1994 and detected a total of 3000 SCC colonies and over 700 fatigue cracks.

The ultrasonic technology is established as the industry’s most reliable and accurate method to detect cracks. In liquid pipelines the UltraScan® CD can be applied directly in the transported medium. This is not the case in gas pipelines, because the coupling liquid is not readily available. To inspect a gas pipeline reliably for cracks the UltraScan® CD tool has been run in a liquid batch in recent years (Figure 5). Even though this batch technology is well proven, it causes interruptions in the production and additional cost. These interruptions not only lead to loss of income for the line operator, but are often simply not possible because of the dependency of the end customer on the delivery.

A solution of this dilemma is now offered with the EmatScan® CD.

EmatScan® CD

For the EmatScan® CD the EMAT technology has been employed. This technology has the advantage that no coupling liquid is required. The ultrasonic pulse is generated inside the wall by an electro magnetic effect.

Principle of operation

Figure 6 demonstrates the difference between the standard piezoelectric sensor of the UltraScan® CD and the EMAT sensor. In the case of the piezoelectric sensor, the ultrasonic pulse is generated by a crystal inside the sensor and is transferred to the wall through the coupling liquid. The EMAT sensor, on the other hand, consists of a permanent magnet and an electric coil. The pipe wall is magnetised locally by the permanent magnet and an electric pulse sent through the coil generates eddy currents inside the wall. An eddy current flowing in the magnetic field gives rise to the so called Lorentz force, causing a deflection of the crystal lattice. Through this movement of the lattice the ultrasonic wave is generated right inside the metal itself.
Based on the orientation of the magnetic field and the eddy currents, ultrasonic waves are induced which travel in different directions inside the pipe wall. This mechanism also works in the reverse for the reception of an ultrasonic pulse.
In the case of the EmatScan® CD EMAT sensor three different waves are generated: the SH (shear horizontal) wave, the RH (rayleigh high frequency) wave and the TS (thickness shear) wave.
The individual waves fulfil different tasks: The SH wave front extends over the entire thickness of the wall and travels in circumferential direction through the wall. This wave provides the basic information, responding to any crack oriented in longitudinal direction. The RH wave only oscillates close to the internal surface and also travels in circumferential direction, responding to internal cracks only. By combining the information generated by the SH and RH waves it is possible to distinguish between internal and external cracks. This combined information is also used to estimate the depth of the crack. The TS wave travels perpendicularly into the wall and is used to measure the actual wall thickness of the pipe joint.
The EmatScan® CD features three sensor heads per sensor carrier equally spaced over the circumference. The sensor acts as transmitter and receiver. Each sensor head transmits an ultrasonic pulse, which, in the case of the existence of a crack like defect, is reflected and received by the same sensor. Part of this pulse also travels on around the circumference and is received by the adjacent sensor head as a very strong transmission signal.
The relevant information for the detected crack is deducted from the strengths of the reflected echo and the transmission wave. The part of the pipe circumference located between two sensor heads respectively is divided into three zones: the near gate for crack echoes which arrive ahead of the transmission signal of the neighbouring sensor head, the far gate for echoes which arrive after the transmission signal and the transmission gate for the reception of the transmission wave itself. Additionally there is a dead zone directly in the sensor head area from which no signals are received.
Based on the fact that each EMAT sensor head is able to scan a large portion of the pipe circumference, the EmatScan® CD tool only needs a total of 12 sensor heads located on four sensor carriers. The individual sensor carriers are mounted with an angular set off to allow for covering the entire pipe circumference.

Mechanical design

The EmatScan® CD is of modular design, similar to any modern pipeline inspection tool (Figure 7). The individual modules travel inside the pipeline on cups or rollers. They are connected by universal joints to allow the passing of bends. The electronic components are housed in pressure tight bodies. Electronically the individual bodies are connected by especially designed pressure tight cables and plugs. The first module houses the batteries for the power supply of the electronic system, while the second houses the electronic components for data treatment and storage. Trailing behind these modules are the four sensor carriers with three sensor heads each.
The cups of the first module seal the tool inside the pipe to allow for the build up of the differential pressure needed to propel the tool through the pipeline.
Since the SH wave front extends over the full thickness of the wall, the frequency of this wave is dependent on the wall thickness of the pipe. For pipelines with wall thickness that differ from the range of 9 to 16 mm sensor heads with different frequencies must be employed.
Test results
Inspection results are displayed as B-Scan and C-Scan. The B-Scan displays the signals of an individual sensor with respect to time (y-coordinate), with the sensor travelling down the pipeline displayed in the x-coordinate. The intensity of the received signal is displayed as colour, with red indicating the maximum intensity.
Figure 8 shows test results of defects with the minimum depth of 1 mm. The group of defects shown in Figure 9 feature different angles with respect to the longitudinal direction – this is clearly seen in the results. Figures 10 and 11 demonstrate the capability of the system to resolve two defects in close vicinity, both in the longitudinal direction (Figure 10) and in the circumferential direction (Figure 11).
By combining the results of the individual types of wave an estimate for the depth of the crack like defect can be determined. In the inspection report the depth is reported in 3 classes:
1) less than 2 mm deep
2) 2 mm to 5 mm deep
3) more than 5 mm deep.
Using the depth and length information the influence of each defect on the safe operating pressure of the pipeline can be calculated.

Economy

The EmatScan® CD tool provides an important contribution to the safe operation of gas pipelines, in that it detects with high probability all defects relevant for the structural integrity of the pipe material. Apart from the aspects of safety and environmental protection this also has positive economical consequences, eliminating the cost a failure of a gas pipeline would generate, not to mention the loss of public opinion connected to such an incident.
Special aspects

The EmatScan® CD can be employed in gas lines only. Due to the fact that the ultrasonic pulse needs to travel over relatively long distances around the circumference of the pipe, the medium or material in direct contact with the pipe wall has a great influence on the propagation of the wave. In liquid-carrying pipelines, a lot of the ultrasonic energy is lost by part of the wave migrating into the liquid, so that the signal amplitude vanishes before the wave reaches the Far Gate or the adjacent sensor.
The coil of the test head must be within 0.5 mm of the internal pipe surface. To achieve this, the coil section of the test head is gently pressed against the wall, causing it to slide along as the tool is progressing through the pipe line. One of the challenges during the development was to find the right material for the abrasion resistant layer on top of the sensor coil that at the same time would influence the strength of the electric signal as little as possible.
Field testing

The EmatScan® CD has completed runs successfully in several gas pipelines in North America. One of the lines was of special interest, because it had already been inspected by the UltraScan® CD running in a liquid batch. As a consequence of this the locations and dimensions of several crack-like defects were known prior to the EmatScan® CD run. All defects found by the reliable UltraScan® CD tool were also found by the EmatScan® CD.

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