High-quality Smart-pig Inspection of Dents - ROSEN Inspection ...

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2 Technology & Services Section Figure 1: High-quality Geometry Unit (XGP) in Combination with Different High-resolution Metal Loss Inspection and Crack Inspection Tools From left to right: ECD+XGP, AFD+XGP, CDP+XGP and XGP. Introducing a high-quality in-line inspection process for dents and mechanical damage can provide the basis information, such as 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 that 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 characterise the mentioned stress risers. State-of-theart equipment for characterisation of the stress riser are high-resolution ILI tools based on the magnetic flux leakage principle (MFL), circumferential MFL tools or the recently developed Electro Magnetic Acoustic Transducer (EMAT) Crack Detection (ECD) tool based on electromagnetic coupled ultrasound. In addition, or more preferable in combination with one of those tools, a highresolution Geometry Pig (XGP) rendering the pipe shell in space must be used (see Figure 1). Circumferential Resolution and Coverage The performance of a geometry inspection configuration for dents can be estimated using an analytical model. According to the model the sizing accuracy and the 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 decisive parameter for the POD is mainly the coverage of the caliper sensor in circumferential direction. A geometry tool like the established ROSEN Electronic Geometry Pig (EGP) with a circumferential coverage of 100% would perform with an analytical POD of one for the three types of dents. The 100% coverage is achieved with the touchless operating sensor unit. Another example is the simple caliper tool, equipped with 12 caliper arms and a typical coverage of 55%. The POD in this case is reduced to 0.75 for a sharp dent of 0.32 inches (8.1mm) depth (2% improved detection). Again, this indicates the importance of coverage. Another effect that 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 the tip by a caliper or scanning sensor, the maximum possible undersizing of the dent depth can be calculated for a geometry inspection configuration. The maximum possible undersizing of the dent, in comparison with the real depth, 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% is the strongest parameter to achieve an accurate depth measurement. The maximum coverage a geometry inspection tool with a single plane of caliper sensors will achieve is close to the bore reduction a tool can negotiate, typically reduced by approximately 15% (due to required mechanical tolerances). Therefore, a typical value of 75% passage leads to a maximum coverage of 60%. The circumferential resolution determines the parameterisation capabilities for geometric anomalies. Recent studies did show that a ‘dent acuity’ of 0.1d/w must be resolved (acuity=2d/w, where d=dent depth and w=dent width) 1 . Including the previously mentioned depth threshold of 0.25 inches (6.35mm), a dent of 5 inches (127mm) wide has to be resolved. Based on these theoretical considerations, a geometry inspection configuration is desired with an acceptable total circumferential resolution e.g. < 2 inches (50.8mm) and caliper sensors arranged in two axially separated planes governing a possible circumferential coverage of more than 95%. Mechatronic Principle Other issues, beside the requirements derived from the preceding analytical considerations and the regulatory demands highlighted, have to be considered for the conceptual design of a geometry inspection tool. A disadvantage of the traditional mechanical caliper tool design, where the mechanical movement of the caliper is transformed into a position signal, is the dynamic behaviour of the arms under ‘run’ conditions. This typically leads to inspection speed BUSINESS BRIEFING: OIL & GAS PROCESSING REVIEW 2005
estrictions. Above a critical tool speed, the caliper arm starts to lose continuous contact with the internal surface of the pipeline. However, at low speeds, abrupt changes at the internal pipe surface may not be monitored correctly. Pure mechanical designs that try to overcome these problems have to be lightweight and, hence, are quite fragile. Therefore, these systems do not extend the operational range of this inspection task. A solution for this problem is provided by touchless measurement technology. To achieve a high measurement accuracy and a satisfying circumferential resolution, a mechanic caliper arm system equipped with a sensor to transform the mechanical movement into an electric signal and an electronic distance measurement were combined to create a mechatronic solution (see Figure 2). The picture shows the concept of having a touchless electronic sensor integrated inside the sensor head and a position sensor attached at the bottom monitoring the mechanical position of the sensor arm. The touchless electronic sensor is used to compensate for data obtained from the dynamic behaviour of the caliper arm. The unwanted inertia of the caliper arm is fully assimilated by the touchless measurement. Sharp transitions at the internal surface, such as a pipe misalignment at a girth weld, are monitored well. Since the electronic sensor is insensitive to non-conductive material, the compensation method refers always to the metal internal surface of the pipeline. Scale or wax debris, although detected by the system, will not affect the geometry evaluation of the pipeline. A single mechatronic unit is designed to cope with a tool-speed of up to 11.2mph (5m/s) and to provide an accuracy of 0.02 inches (0.5mm) for radial measurements. Tool Design Due to the linear equation connecting the pipeline diameter with its circumferential perimeter, the coverage of a single unit tool is linked to the internal diameter (ID)-passage of the unit. As a rule of thumb, the coverage for a single plane geometry tool equals the specified passage minus 15%. Thus a typical caliper tool with a passage of 75% would cover only 60% of the internal pipe surface. Therefore, an accurate sizing of dents or other geometric properties in a pipeline requires 100% coverage in circumference by the geometry sensors. To achieve this, the inspection solution must consist of two inspection planes, where the trailing sensor covers the gap of the preceding unit. The tool concept for the ROSEN high-resolution XGP consists of two sensor units connected to each other and as a result guarantees 100% coverage of the internal surface. High-Quality Smart-Pig Inspection of Dents, Compliant with US CFR BUSINESS BRIEFING: OIL & GAS PROCESSING REVIEW 2005 Figure 2: XGP Mechatronic Caliper Sensor Figure 3: XGP Tool and Data Sample Obtained on Test Joints Containing a Dent and a Wrinkle Bend To allow a detailed evaluation and characterisation of the geometric anomalies the circumferential resolution was set to < 1.2 inches (30mm). This exceeds the requirements for the lateral characterisation of a dent as typically required. Furthermore, caliper arms providing the discussed accuracy are fragile. In turn, the presented mechatronic concept, which compensates for the inertia of the mechanical caliper arm, allows a more robust design of the caliper arm improving the survey conditions under which the inspection system can be operated. The XGP is equipped with these ‘heavy duty’ caliper arms (see Figure 3). The XGP unit is also equipped with a Gyro system measuring three dimensional (3-D) co-ordinate system (XYZ) that maps an accurate route of the pipeline and can help to determine bend radii, bend length and bend direction. The XYZ information is the basis for a stress/strain analysis on a larger scale, which in turn can be part of the remaining lifeassessment procedure carried out for local anomalies. Geometry inspection is advancing into the high- 3
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