Crack detection in gas 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. 

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. 

Sumber : 

http://pipeliner.com.au/news/crack_detection_in_gas_pipelines/043294/