Ultrasonics boost integrity inspections of gas pipelines

Paul Boughton

Rosen’s latest developments in ultrasonic technologies provide a powerful mechanism for inspecting the integrity of a natural gas pipeline.

Given the increased focus on natural gas as a viable energy alternative to oil, a greater demand for more accurate, and cost effective pipeline inspection technologies has arisen. The safety and reliability of these pipelines is of great concern given the high pressure and compressible nature of the medium.

A major integrity threat to a natural gas pipeline is due to cracks. Traditionally, they tended to be formed during fabrication or installation stages of a pipe lifecycle. More recent findings have shown that cracks arise in-service from such crack growth mechanisms as external stress corrosion cracking (Fig.1) or sour service based hydrogen induced cracking and sulphide stress corrosion cracking (HIC and SSCC).

These forms of cracks are difficult to detect using conventional magnetic flux leakage (MFL) techniques. MFL requires larger open cracks or a more open structure present in a corrosion zone to be effective. The complex and closed structure of cracks make it virtually impossible for detection by disturbance of the magnetic flux. Fortunately, an ultrasonic wave will be reflected by a crack like structure regardless of the exhibited closed structure. The resulting echo can be detected and measured to provide a clear indication of a crack feature normally not detected with other techniques.

To date, ultrasonic technologies (UT) have been used successfully in the inspection of liquid pipelines. Traditionally, they have been based on piezoelectric transducers and are only applicable when an adequate liquid couplant is available between the transducer itself and the pipe wall. Therefore, their use is effectively limited to liquid systems. Another approach is necessary to adequately inspect natural gas pipelines.

What is EMAT?

The ideal crack inspection solution for gas pipelines is a dry-coupled ultrasonic technology. An electromagnetic acoustic transducer (EMAT) provides the best realisation of this requirement. EMAT is based on the electro-mechanical conversion produced when an eddy current is applied within a static magnetic field. The resulting Lorentz forces and magnetostriction result in an interaction between the EMAT and the metal surface generating an acoustic wave within the material (Fig.2). The material being inspected is its own transducer, eliminating the need for a liquid couplant. The acoustic wave that propagates within the pipe wall is dependent on the dimensions and properties of the pipe material. Therefore, the presence of SCC and other crack features will disturb the guided-wave in such a way as to produce a reflection resulting in an echo. This echo can be detected and measured.

EMAT inline inspection systems have been around for some years; however, Rosen’s EMAT Crack Detection (ECD) tool takes a different approach to provide better performance. The first aspect in the new approach is to base the magnetic configuration on a conventional corrosion detection pig. As a result, the magnetic energisation of the pipe-wall is obtained from a standard magnetic circuit design. There are no magnets located in the ECD sensor housing itself. This approach makes the sensors much smaller and lighter then alternative EMAT designs, enabling excellent dynamic performance under a wide range of operating conditions. Given the smaller size and lighter weight of the sensors a larger circumferential array can be installed as well as reduced wear can be observed.

A second aspect in the new approach is based on an improved and more thorough understanding of guided-wave acoustics. Through a careful utilisation of a particular guided-wave mode, the sensitivity to specific defect features can be optimised. The sensor is designed to detect just cracks and crack colonies minimising detection of other intrinsic anomalies of the pipeline such as inclusions or other manufactured defects improving the reliability of crack POD.

A final aspect is the ability to detect pipeline coating disbondment. As a result of the selection of the appropriate wave mode, the EMAT sensor is only marginally sensitive to coating changes. However, through the changing of the timing of the receiving units, coating disbondment information can be obtained. Detecting these areas is important since SCC tends to occur in areas where the coating has disbonded. Together these results provide a proactive approach to help improve the overall integrity of a pipeline.

The innovative technology has proven its performance during extensive in-house pull tests on samples containing natural inter-granular SCC colonies. A new 16-in ECD with 72 sensors for oil and gas pipelines has being constructed and is in testing phase (Fig.3). The results obtained with the EMAT inspection technology can be combined either with other in-line inspection results or other field activities to provide full support for an enterprises integrity management needs. 

Don Olsen, Rosen Group, Stans, Switzerland. www.RosenInspection.net

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