Detection. at first sight. One of the newest trends

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1 NDT Specialist Sami Hemminki, Inspecta, Finland, describes the benefits of guided wave ultrasonic testing. Detection at first sight One of the newest trends in pipeline testing is the use of guided wave ultrasonic technique in pipeline corrosion detection (GWUT). GWUT has attracted the interest of the world, offering a relatively fast and comprehensive overview of the condition of even long pipes. The GWUT method is based on an ultrasound that is broadcasted in the direction of the tube, while the inner and the outer surface of the tube guide the sound wave. Although the guided wave method is based on ultrasound, it differs from the conventional ultrasonic inspection to the extent that it is considered as a separate method, and not an application of the traditional ultrasonic inspection. A transmitter ring, the transducer, is placed around the inspected pipe and sends ultrasonic guided waves along the length of the pipe s surface. Faced with a change in the pipe s cross-section, part of

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3 the transmitted sound is reflected back into the guided wave equipment. The change in the cross-section can be a decrease or an increase of material in the cross-section area. Wall loss is simply a change in the wall thickness (Figure 1). The total number of changes may be concentrated in a narrow part of the tube or pipe, located around the periphery (Figure 2). The transducer does not have to be moved during the inspection; the equipment emits sound waves in both directions of the transducer, receiving feedback from the cross-sectional changes. Figure 1. Change in cross-section versus wall loss. Figure 2. The percentage cross-section change given by software. It can either be concentrated in a narrow portion of the pipe, or equally distrubed around the circumference. The area reached in the inspection depends upon the pipe s condition and properties, such as supportings, fittings, curves, T-joints, coatings, etc. The size and number of the reflectors affect the distance audited (Figure 3). The device informs the user of the reflector location in the longitudinal direction of the pipe. In addition, the user is informed of the location in the circumferential direction, which is especially useful for pipes with a long diameter. Using this information, the thickness measurements, as well as other follow-up studies, can be directed at the real corrosion areas. The method provides a 100% comprehensive audit for pipeline volume from 2-50 m, depending on the characteristics of the pipeline. The onsite inspection is also a relatively rapid process, taking around 30 mins per test point. Equipment and data analysis The typical minimum for detection limit is a 3-4% change in the cross-sectional area. It is, however, possible to detect a 1% cross-sectional area change as long as the noise level remains lower than 6 db. The equipment can also examine through a 90 curve. When using the GWUT method, it should be taken into account that it is a search tool for corrosion. Although the GWUT method is able to provide an assessment of wall loss, the exact defect sizing in the remaining wall should always be determined by a complementary research method (typically a thickness measurement using conventional UT or digital X-ray). Figure 4 is an example of inspection data generated by the equipment. The examination in the example is limited to a 90 curve in both directions. The green reference curve represents the level of sensitivity of the test, which in set 6 db above the noise level. In general, all the indications above the sensitivity level are evaluated. The blue reference curve is the registration boundary, which can be set to a desired level. The black reference curve consists of a theoretical cross-sectional change caused by welds. The reference curve is set according to the actual geometry of the weld. The sample data represents a pure line, where, apart from welds and supports, no classifiable indications are displayed. The horizontal axis shows the distance scale in meters. Continuous monitoring The guided wave equipment s transducer range includes permanently installed monitoring systems (PIMS). These can be Figure 3. Encountering a cross-sectional area change, the wave is reflected back, losing some of its energy (in the figure 20% of the cross-sectional area change is caused by the loss of energy). Figure 4. The examined length in the example data is about 35 m. 30 World pipelines October 2013

4 positioned on the pipe during installation and, over the years, follow any changes during certain periods of time. The transducer can, for example, be buried or left in the insulation after installation. The only accessible part is a capsule from which the data is gathered. With a follow-up audit process suck as this, the inspection parameters are always the same and the results are easily comparable. The inspection, therefore, is easily repeated. Method restrictions As a result of using the traditional pulse-echo method, the sound of the guided wave may fall into blind spots. The rebound of the sound, and a detection of an indication in these spots, is either weakened or impossible. The GWUT method operates at low frequency (khz range), so the blind spot and the region lengths are much longer than in a conventional ultrasonic inspection. The range of the blind spot extends to around 0.5 m from the transducer, and the surrounding region a meter further from it. These dimensions are important when designing a GWUT inspection, as they are important factors that affect the placement of the transducer. Since the equipment is based on ultrasound, high ambient noise, such as pumps and compressors, lower the quality of the test. In some cases, this may result in restrictions in the use of the inspection method in question. In these cases the measurement point has to be moved farther away from the noise source to be able to carry out the inspection. The maximum operating temperature of the inspection target is 120 C with solid rings, and 70 C with inflatable rings. The detection of axial and fraction type faults is practically impossible; these types of defects caused by the cross-sectional are very small. The research method in practice The guided wave UT is best suited for long isolated/non-isolated parts of the transmission lines. Professionally used, it can also be utilised by physically inaccessible pipeline sections. Examples of these include shortfalls in the road (Figure 5) and crossings, as well as tubular bridges, which are almost impossible to inspect with other methods. In addition, with a little advance planning, the demolition and scaffolding work of insulations can be minimised. It is only necessary to remove the insulation from the transducer s measurement points, that is, from approximately 50 cm of the cable pipe. The testing can be done for both ferritic and austenitic steel pipes. Figure 5. Typical use: sneak path. Figure 6. Severe corrosion was found under insulating material. Case study Inspecta acquired its GWUT equipment in The company s main customers in guided wave testing in Finland, Sweden and Norway, have since been various ports, reservoir areas, and oil refineries. In 2011, Inspecta Finland s customer Neste Oil, an oil refining company, wanted to examine the pipeline condition of one of its ports train unloading terminal. Fortunately, although the different pipes ran across the whole 260 m long terminal, they were easily accessible. The study was conducted, by two of Inspecta s NDT experts in about three weeks. The inspection was carried out in three week long snippets: about two lines per week. All in all, the inspection time was in accordance with the time estimate provided to the customer. The last week of the inspection was postponed from November to the Spring of the following year, as the temperature in Finland can drop down to -30 C in Winter. Even though no defined temperature minimum is set for the use of the method, cold outside air influences the research equipment s battery life. A total of five pipelines were examined, two of which were isolated and three non-isolated ferritic pipes. Clamped supports were used for the attachments of the diverse pipeline. Planning for the transducer ring s locations on the pipeline was done onsite, so that a maximum coverage, affected by e.g. the emptying or T-joints attached to the main line, was acquired for the inspection. First, a few measuring intervals were tested to 32 World pipelines October 2013

5 determine suitable places for the transducer on the rest of the pipeline. The insulation remover commissioned by the customer, was able to conveniently remove the insulation from each ring place in around five minutes, so even the inspection of insulated lines ran smoothly. The inspection was carried out with the English Guided Ultrasonics Ltd s manufactured Wavemaker G3, which is currently a common guided wave model used in these kinds of inspections. There are currently only four guided wave manufacturers in the world, and the supply is very equal. The differences occur primarily in presentation of the results, and computer software. The lines were generally in good condition, except for a few slight corrosion points and one severe corrosion area under the insulation of an insulated pipe (Figure 6). To confirm the degree of corrosion in the less serious corrosion points, a corrosion examination was recommended for the customer. The severe corrosion point was detected with guided wave, as shown in Figure 7, after which the insulation was removed from the corrosion indication point and the pipe wall thickness was measured using ultrasound. The corrosion degree was confirmed to be -60% of the nominal wall thickness. As a result of the guided wave research, Neste Oil acquired comprehensive knowledge about the condition of its pipelines. An individual report was written for each line. The technology is just as effective when working with larger diameter pipelines: the diameter of the pipe does not affect the study in any case, only the transducer ring s size. Figure 7. Corrosion indication from the length of 2-4 m showed by the guided wave wavemaker. Conclusion Guided wave testing has proven to be an effective tool to acquire a quick overview of the condition of pipelines. The GWUT method is a mapping tool that always requires a complementary method for verifying the exact wall loss. Based on the results it can be determined whether there is a need for further examinations and, if necessary, pointed where these further examinations should be carried out. Obtaining a comprehensive picture of the pipe condition there is no need for random spot metering of the pipeline. Inspections can be carried out without taking the pipe out of service and the production does not need to be interrupted.