№ 8 (August 2006)
High-Quality Smart-Pig Inspection of Dents
In addition to requirements derived from the preceding analytical considerations and regulatory demands mentioned in the OGE July issue, some other aspects have to be considered for the conceptual design of a geometry inspection tool.
By Thomas Beuker, Dr. Florian Rahe
The disadvantage of the traditional mechanical caliper tool design, where the mechanical movement of the caliper is transformed into a position signal, is the dynamic behavior of the arms under run conditions. This typically leads to inspection speed restrictions. Above a critical tool speed, the caliper arm starts to lose continuous contact with the internal surface of the pipeline. But also, at low speeds abrupt changes at the internal pipe surface may not be monitored correctly.
Pure mechanical designs which 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 a touchless measurement technology. To achieve a high measurement accuracy and a satisfying circumferential resolution, the mechanic caliper arm system, equipped with a sensor to transform the mechanical movement into an electric signal, and an electronic distance measurement were combined in a mechatronic solution. 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 data obtained from the dynamic behavior 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 very well. Another example explains how the contour of a dent is monitored by the compensation method. Fig. 7 shows the result simulated at 6.72 mph (3 m/s).
Since the electronic sensor is insensitive to non-conductive material, the compensation method is always referring 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 11.2 mph (5 m/s) and to provide an accuracy of 0.020 in. (0.5 mm) for radial measurements
Due to the linear equation connecting the pipeline diameter with its circumferential perimeter, the coverage of a single unit tool is linked to the ID-passage of the unit. As a rule of thumb, the coverage for a single plane geometry tool equals the specified passage minus 15 percent. Thus, a typical caliper tool with a passage of 75 percent would cover only 60 percent of the internal pipe surface.
Therefore, an accurate sizing of dents or other geometric properties in a pipeline requires 100 percent coverage in circumference by the geometry sensors. To achieve this, the inspection solution must consist of two inspection planes, where the trailing sensor is covering the gap of the preceding unit. The tool concept for the ROSEN high resolution geometry inspection system (XGP) consists of two sensor units, connected to each other, and hence guarantees 100 percent coverage of the internal surface.
To allow a detailed evaluation and characterization of the geometric anomalies, the circumferential resolution was set to < 1.2 in. (30 mm) or better. This exceeds the requirements for the lateral characterization of a dent, as typically required. An exemplary 3D representation of data obtained on a set of calibration anomalies, a dent and a wrinkle bend is given in Fig. 9.
Furthermore, caliper arms providing the discussed accuracy are very fragile. In turn, the presented mechatronic concept, which compensate 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.
The ROSEN XGP unit is also equipped with an XYZ mapping electronic to navigate an accurate route of the pipeline and 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 life assessment procedure carried out for local anomalies.
Geometry inspection is advancing into the "high resolution" technologies. Furthermore, the assessment of a dent requires all available information characterizing potential stress riser in the vicinity of the dent. Therefore, it can be useful from an operational point of view, to combine the XGP unit with another inspection technology (such as a corrosion survey), which provides this information (see OGE #7, 2006, page 47, fig. 1).
The demand for reliable dent detection and sizing has changed the scope of existing geometry inspection services with regards to the inspection technology used, as well as the evaluation process of the data. Geometry inspection has entered the "high-quality" inspection market. Analytical considerations emphasize the importance of both coverage and resolution in circumferential direction of the pipe perimeter. Measurement of the mechanical position of the caliper arm, in combination with a touchless distance measurement, provides better accuracy, independent from the caliper's inertia, and a wider operational range of the system.