NACE SP0313

Abstract

General <br>This standard is primarily applicable to GWT tools that are designed to be coupled to the external surface of the pipe. However, this standard can be adapted to GWT technology that couples to the interior of the pipe using deployment tools such as tethered, remotely controlled, internal free-swimming, or permanently installed inspection devices. <br>This standard is applicable to a variety of industries that use metallic pipelines and piping systems to transport natural gas and hazardous liquids, including those containing anhydrous ammonia, carbon dioxide, water (including brine), liquid petroleum gases (LPG), isotopes, and other services that are not detrimental to the function and stability of GWT tools. <br>This standard provides specific guidance based on successful, industry-proven GWT practices. <br>This standard requires the service provider to determine the attenuation levels for GWT examinations for each pipe. In practice, GWT attenuation levels should not be greater than 1 dB/m (0.4 dB/ft) during testing. When attenuation levels are greater than 1 dB/m (0.4 dB/ft), the service provider must have an equipment-specific procedure tailored to the piping configuration and target corrosion mechanism of the pipe to be tested. As such, use of this standard as a stand-alone practice on such piping should only be used as a guideline. <br>This standard is primarily intended for use on above- and/or below-ground pipelines installed along a right-of-way, plants, pump/compressor station piping, and for subsea pipelines and flow lines. The general process and approach may be applied to other facilities such as hydrocarbon distribution and gathering systems, water injection systems, station piping, and isolated crossings of railroads, highways, or waterways. <br>GWT is a nondestructive testing technique that provides for the rapid screening of lengths of pipe from each test location in order to achieve inspection coverage of a pipe in a cost-effective manner and to target suspect areas for closer examination by local nondestructive testing (NDT) techniques. With this process, the reduction of access costs is a significant positive factor. This method also has the ability to examine pipe lengths that are inaccessible for more conventional NDT methods, such as road or rail crossings, by testing from the nearest accessible location, thereby increasing the proportion of any pipe system that can be inspected. <br>GWT is similar to the use of Lamb waves in conventional Lamb wave testing, which may be generated in plates and in common pipe thicknesses. Currently, piezoelectric and magnetostrictive transducers are used to generate and receive ultrasonic signals that travel through the pipeline wall, and changes in time of flight can be used to detect imperfections, features, and defects in the short segments of the pipeline system under inspection. To generate the appropriate wave modes, guided waves are several orders of magnitude lower in frequency than that which is used for normal ultrasonic tests. Typically, frequencies of approximately 50 kHz are used in GWT, compared to approximately 5 MHz for conventional thickness testing. These waves can travel many meters with minimal attenuation and offer the potential to test large areas from a single point using a pulse-echo transducer bracelet wrapped around the pipe. This principle is shown in Figure.1. The transducer transmits a controlled pulse GWT along the pipe. Any changes in the thickness of the pipe, either on the inside or the outside, cause reflections, which are detected by the transducer. Hence, metal loss indications from corrosion/erosion inside the pipe or corrosion on the outside of the pipe may be detected. The detection of additional mode converted signals from defects aids discrimination between pipe features and metal loss. Knowledge of the speed of the guided waves as they travel along the pipe allows the distance from the transducer tool to the corrosion to be measured so that its position can be determined. <br>GWT does not provide a direct measurement of wall thickness, but it is sensitive to cross-sectional changes, a combination of the depth and circumferential extent of any metal loss, as well as the axial length. This is a result of the transmission of a guided wave front along the pipe wall, which interacts with the annular cross-section of the pipe at each point. It is the change in this cross-section to which GWT is most sensitive. Commercial test systems employ further analysis of the signals received to estimate the depth and extent of any imperfections detected. <br>In GWT, it is common for response amplitudes to be compared to reference levels derived from distance amplitude correction (DAC) curves or by time-controlled gain (TCG) amplitude levels. These allow responses to be compared to known features. The reference levels may be set from responses from the girth weld between sections of pipe. Table 1 shows typical values of relative signal amplitude: <br>These values assume a linear relationship between change in cross-section and reflection amplitude. Different manufacturers may have specific instructions for more advanced relationships between the change in cross-section and reflection amplitude as well as calibration. <br>The principal benefits of GWT are that pipe may be examined in a manner to determine whether areas of potential degradation are present and that areas inaccessible for direct inspection may be tested. Application of quantitative methods should be used to provide more detailed information about the imperfections detected. <br>GWT may be used as a stand-alone assessment method for pipelines provided that the operator provides a separate engineering procedure. See Appendix A (Nonmandatory) for complementary use of GWT technology.