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December 19, 2007
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Home / Issue Archive / 2006 / July #7 / High-Quality Smart-Pig Inspection of Dents

№ 7 (July 2006)

High-Quality Smart-Pig Inspection of Dents

By Thomas Beuker, Dr. Florian Rahe

Based on 20 years of experience with in-line inspection technology, ROSEN has developed a novel technology for the characterization and sizing of dents that will be presented in this paper. This high-resolution ILI technology combines the advantage of a touch less electronic measuring system with the advantages of the well-established caliper arm tools. The advantage of the touch less system is its applicability under highly dynamic operational loads, while the mechanical system provides highly accurate results under static conditions.

Utilizing dynamic compensation technology, a sufficient accuracy can be provided under demanding operational conditions. Furthermore, the "mechatronic" concept improves the sizing capabilities for dents due to its in-sensitivity to scale or wax debris.

Integrity Management of Pipelines

Integrity management of pipelines in the US is regulated by federal codes for both liquid and gas pipelines. In particular an updated code for gas pipelines became effective on January 14, 2005. Before that date, integrity management for gas pipelines was incorporated only by reference e.g. through code ASME B31.8.

Both rules contain strict prescriptive integrity management provisions for the pipeline operator. These prescriptions define limits for pipeline geometric anomalies like mechanical damage and dents. For example the minimum requirements for sizing of dents is prescribed: a high-resolution geometry tool must detect and size dents with a depth greater or equal to 0.25 in (6.35 mm). Additionally, a process for inspecting against provisions of a management based rule, rather than inspecting for compliance with a purely prescriptive rule is encouraged [GERARD 2005].

The discussion in recent literature about possible failure modes of mechanical damage in pipelines is unanimous: anything but a plain dent must be analyzed very carefully with relevant expertise. Latest investigations confirm the dependency of failure pressure from the dent shape rather than on the dent depth [DINOVITZER 2002], [LEIS 2004]. This is also reflected in the code of federal regulation, recommending engineering analyses wherever needed. A stress riser like corrosion, gouges or cracks within dents or between dents, rerounding of unconstrained dents or shape and sharpness of dents need to be considered within the appropriate assessment method. While the failure assessment process is not prescribed in the federal rules, the (integrity) management measures and management consequences are.

High Quality

The "classic" geometry inspection for ovalities and large deformations does not provide the required information for a dent assessment as considered above. Furthermore an adequate detection, characterization and parameterization of all anomalies found will require a higher effort of evaluation than typically spend or requested. This is also indicated in a recent study by OLSON 2004. He compared the results of 78 excavations with the corresponding data of the geometry tools. The probability of detection (POD) of dents in this case was found to be only 32 percent.

Introducing a high quality inline inspection process for dents and mechanical damage can provide the basis information like the geometric data of dents and stress risers to start a managed integrity process for the inspected line. The more high-resolution and high-quality information is available about the anomaly found, the better the subsequent failure analyses can be.

It is important not only to size and describe the dents with high accuracy, but also to detect and characterize the mentioned stress risers. State of the art for characterization of the stress riser are high resolution in-line inspection tools based on the magnetic flux leakage principle (MFL), circumferential MFL (CMFL) tools or the recently developed EMAT crack detection tool (ECD) based on electromagnetic coupled ultrasound. In addition, or more preferable in combination with one of those tools a High Resolution Geometry Pig (XGP) rendering the pipe shell in space have to be used.

Circumferential Resolution and Coverage

The performance of a geometry inspection configuration for dents can be estimated by using an analytical model. According to this model the sizing accuracy and the probability of detection (POD) for dents can be determined as a function of the circumferential resolution and the coverage of the sensing area of the caliper sensors.

The model is discussing three different shapes of dents a) a linear dent b) a spherical dent and c) a sharp dent.

The decisive parameter for the probability of detection is mainly the coverage of the caliper sensor in circumferential direction. As shown in Fig. 3, a geometry tool like the established ROSEN Electronic Geometry Pig (EGP) with a circumferential coverage of 100 percent would perform with an analytical POD of 1 for the three types of dents. The 100 percent coverage is achieved with the touchless operating sensor unit. Another case is shown in Fig. 4. Here the model for a simple caliper tool is presented, equipped with 12 caliper arms and a typical coverage of 55 percent. The POD in this case is reduced to 0.75 for a sharp dent of 0.32 in (8.1 mm) depth (2 percent ID). Again, this indicates the importance of coverage.

Another effect which can be studied is the systematical undersizing of dents as a function of circumferential resolution and coverage. Assuming that a dent is not hit at its tip by a caliper or scanning sensor, the maximum possible undersizing of the dent depth can be calculated for a geometry inspection configuration.

The behavior of undersizing is discussed in Fig. 5. The maximum possible undersizing of the dent in comparison to the real depth in percent is plotted versus the circumferential coverage for a different number of caliper arms. It can be seen that an increase of caliper arms improves the sizing difference. Nevertheless, coverage close to 100 percent is the stronger parameter to achieve a accurate depth measurement.

The maximum coverage in percent a geometry inspection tool with a single plane of caliper sensors will achieve is close to the bore reduction in percent a tool can negotiate, typically reduced by approximately 15 percent (due to required mechanical tolerances). Therefore, a typical value of 75 percent passage leads to a coverage of 60 percent maximum.

The circumferential resolution determines the parameterization capabilities for geometric anomalies. Recent studies did show, that a "dent acuity" of 0.1 must be resolved (acuity=2d/w, where d=dent depth and w is the dent width) [DINOVITZER 2002]. Including the above mentioned depth threshold of 0.25 in (6.35 mm), a dent of 5 in (127 mm) width has to be resolved.

Based on these theoretical considerations a geometry inspection configuration is desired with an acceptable total circumferential resolution e.g. < 2 in (50.8 mm) and caliper sensors arranged in two axially separated planes governing a possible circumferential coverage of more than 95 percent. be continued
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