by F. H. Dijkstra* and J. A. de Raad** 

Many industries have recognized the importance of using NDT in plant maintenance, quality control and safety. However, some industries are curtailing their NDT activities to cut costs. The authors of this article present various case studies to highlight the additional benefits of using NDT not only as a maintenance and prevention tool but also as a cost reducing operation. Readers of this article may also get to know the NDT operations being implemented in the Netherlands. G.P  Singh Associate Technical Editor

 

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Introduction
After several decades of application, the welding industry has completely accepted standard nondestructive testing (NDT) as an inevitable but invaluable part of the production and maintenance of components. Its application has been well regulated, acceptance criteria for weld discontinuities exist, good schemes for personnel qualification are in place and equipment has evolved to a standard approaching perfection. NDT has become a commodity and the pioneering years are over. However, such a situation implies risk. Industry tends to cut costs on commodities such as NDT, especially when they are needed because they are mandated by codes. This creates market conditions under which prices for NDT services are under pressure and competition is heavy.

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Modern NDT methods are becoming ever more quantitative and nonintrusive.

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The field of NDT suffers from a problem regarding perception: NDT is seen, at best in some cases, as a necessary evil. This perception hurts the industry and impedes progress in technological development. This paper will highlight some of the ways in which this perception can be transformed and NDT can be rightfully recognized as being of added value to the user. This requires a change in perspective from seeing NDT as an activity that has to be done because it is required by code to seeing it as a solution to a problem, a solution which can improve safety, enhance quality and save money. Examples given will cover maintenance testing, such as corrosion detection in piping and tanks, as well as routine weld testing. The need for acceptance criteria adapted for modern NDT techniques for weld discontinuities will be highlighted because these form (in many cases) a key to the benefit. Last but not least, new methods will be discussed which contribute to NDT being a benefit.

 

Ordinary Maintenance, Extraordinary NDT
NDT in its present form has been carried out for decades by specialized service companies. The welding industry is an example of an industry which would not have experienced the growth and wide range of applications it has today if there were no such thing as NDT. NDT has a very important formal status and is part of legislation. Requirements for the performance of NDT, acceptance criteria and requirements for personnel qualification are implemented in codes and standards. The NDT procedure is part of the contract. During the many years that NDT methods have been used in the welding industry, a well established situation has evolved which enables the use of NDT for the testing of welds against good workmanship criteria on a routine basis, thus maintaining workmanship standards and minimizing the risks of component failure. In addition, NDT plays an important part in industrial maintenance. During plant shutdowns, for instance, many thousands of ultrasonic wall thickness measurements are taken on piping, vessels and furnace tubes. All of these thickness readings have to go into extensive databases, a process which is, thanks to modern computers and data loggers, increasingly automated.

The NDT field can be seen as having achieved a sort of equilibrium, with its established methods, acceptance criteria, procedures and personnel qualification schemes all carefully laid out and standardized. This equilibrium does not mean, however, that all is perfect - things could be improved and are improving. During the 1970s and 1980s, Commission V of the International Institute of Welding tried to establish procedures that were based on fitness for purpose considerations. The ultimate aim was to find a way to accept and reject weld discontinuities on the basis of their significance for weld integrity.

In conventional NDT, we base our judgment on density differences on a film or on echo amplitudes on a screen. These are parameters that have very little to do with the significance of discontinuities for weld integrity. Density and amplitude are, first of all, used for the detection of a discontinuity and are seldom a direct measure of its significance. Therefore, it would be highly beneficial to industry if unnecessary repairs could be eliminated by discontinuity rejection based on significance, using international acceptance criteria.

In maintenance practice, we base our decisions on NDT that, until recently, was usually performed during shutdowns. A significant amount of money could be saved if we had NDT methods that minimized the time required for the shutdown or which, a step further, avoid it by performing tests onstream. Increased knowledge on plant degradation and the availability of recently introduced onstream, noninvasive NDT techniques permit the application of risk based testing philosophies. Risk based and inherent testing efficiency lead to the intended cost reduction without sacrificing safety.

 

Role of NDT in New Construction
Considering the emphasis on good workmanship evaluation with NDT and using acceptance criteria as they have been formulated over the years, it is probably fair to say that NDT as it is practiced is designed more to monitor welder performance rather than to test weld integrity. This is in agreement with the term "good workmanship." Generally, the tools available to perform this monitoring task were conventional NDT methods. Most existing good workmanship criteria were formulated in the past with the specific capabilities and drawbacks of these methods in mind.

Radiography, for instance, has excellent capabilities in detecting voluminous discontinuities such as slag and porosity and can provide information on discontinuity type and length. On the other hand, it is known that its capabilities are limited when it comes to the detection of planar discontinuities such as lack of fusion and cracks. Even when a discontinuity has been detected, radiography is hardly capable of establishing a through thickness measurement of planar discontinuities. Manual ultrasonic testing, in turn, is better at detecting planar discontinuities but is limited in the detection of voluminous discontinuities and discontinuity characterization. It is also subjective. The estimation of the through thickness extent of planar discontinuities is also limited.

Planar discontinuities, especially if interpreted as cracks, are deemed unacceptable in many codes. Although this may be understandable from the point of view of fracture mechanics, this is not the only reason they are deemed unacceptable. The presence of planar discontinuities goes beyond workmanship evaluation - they might influence weld integrity. In addition, conventional NDT methods have their limitations that urge code writers to reject planar discontinuities in general. Their detection indicates that something is very wrong with the weld, without being able to exactly quantify the severity of the discontinuity. It may, therefore, be argued that the merit of conventional NDT, using existing acceptance criteria, is limited to testing the welder's performance because this has always been the best that was possible.

 

Back to the Beginning
Although the present good workmanship approach actually gives conventional NDT methods the credit they deserve (their capabilities are well used), there should, nevertheless, be a certainty beyond reasonable doubt that an accepted weld is fit for service. Many years of industrial experience have demonstrated that this certainty statistically exists. We are not doing things completely wrong. However, there is a price to be paid. Good workmanship acceptance criteria for conventional techniques must, to a certain extent, be conservative in order to compensate for the inherent limitations of conventional NDT; unfortunately, the degree of conservatism is more or less unknown. Very probably, this conservatism has caused unnecessary repairs. Therefore, the question can be asked: if the historical background of present NDT practices did not exist, what would we like to know today about a weld to be able to accept or reject it?

In an ideal situation, we would like to have a balanced combination of an evaluation of the welder's performance on one hand and a fracture mechanics analysis on the other, in terms of being certain that a discontinuity with dimensions exceeding a certain critical value is not present. The second aspect could be regarded as a safety net with a balanced conservatism.

This not only requires the use of modified criteria, but also requires the use of NDT techniques that can provide the necessary information. They should be able to detect small discontinuities for testing weld quality as well as estimate the through thickness extent of planar discontinuities, if present, with a high reliability (probability of detection [POD]). On the other hand, a low false call rate is required in order to be commercially viable. It is also important that the methods used provide unambiguous information to avoid interpretation uncertainties and discussions. Criteria should therefore be formulated in such a way that full advantage of the capabilities of the NDT method is taken, without leading to unnecessarily high repair rates. It is the compatibility between the NDT technique and the criteria that counts - a balanced combination of certainty and a degree of conservatism not higher than needed. For many years, the technical capabilities of standard NDT methods did not allow for this approach. If NDT would have produced quantitative data on discontinuity size from the beginning, it is highly probable that current acceptance criteria for weld discontinuities would have used this information. Acceptance criteria would have been completely different from what they are now. Contemplating the current situation, the criteria would probably only reject a certain number of voluminous imperfections such as porosity clusters (because these indicate insufficient workmanship) and they would reject planar and sharp discontinuities exceeding a certain through thickness height and length.

In fact, one would expect that, if an NDT method were introduced that could provide these quantitative data, a revolution in industry would be triggered because it would provide the opportunity to develop tailor made acceptance criteria. It is possible that criteria for general purpose applications would statistically lead to repair rates similar to the numbers we know from radiography and ultrasonic testing or slightly lower; thence, it is possible that more planar discontinuities and fewer voluminous indications would lead to rejection than is the case now, which is favorable for weld integrity. To be frank, a repair does not necessarily improve weld strength. On the contrary, repair can reduce local material properties.

For specific applications where more details about material properties and service parameters are known, adequate criteria could lead to significantly lower repair rates while maintaining existing safety standards. Unfortunately, although such methods have become available - such as the time of flight diffraction technique - this revolution has not happened yet. It is understood that industry and, particularly, authorities are hesitant to replace well known and legislated procedures by modern technology without thorough and time consuming validation programs. But fortunately, there is some progress. However slowly, one can observe a painstaking process towards quantitative NDT in combination with adapted acceptance criteria for weld discontinuities. This was made possible through the introduction of time of flight diffraction, which combines a high POD with a low false call rate and is capable of providing data in a discontinuity's through thickness height (Dijkstra et al., 1996). Figure 1 shows a time of flight diffraction image with a discontinuity indication containing through thickness information. General purpose criteria for time of flight diffraction have been developed for this purpose in the Netherlands in a joint industry project published by the Dutch Society for Quality Surveillance, Inspection and Nondestructive Testing (KINT, 1998). Almost simultaneously, similar activities in Europe have resulted in provisional standards (CEN, 2000). Another international joint industry project under the auspices of the International Pipe Line and Offshore Contractors Association (IPLOCA) initiated the development of specific criteria for pipeline welds, for weld discontinuities detected with a combination of automated ultrasonic testing and time of flight diffraction (Dijkstra and de Raad, 1997; Førli, 1997; Førli et al., 1998; IPLOCA, 2000).

Maybe we should regard the efforts of Commission V of the International Institute of Welding to establish fitness for purpose approaches in the 1970s and 1980s as being far ahead of their time. Fitness for purpose criteria cannot exist in combination with NDT methods that simply do not provide the necessary information. Nowadays, however, we are in a much more comfortable situation.

 

Role of NDT during Construction
An example illustrating the benefit of NDT is the application of automated ultrasonic testing systems to test girth welds during pipeline construction. Such systems are most often combined with automated welding and increasingly replace radiography (Dijkstra et al., 2000). Most automated ultrasonic testing systems nowadays include a time of flight diffraction facility not only to assist in estimating the size of a discontinuity, but also to function as a safety net for the detection of discontinuities which have an orientation unlikely to be detected through traditional pulse echo systems. This increases POD to an almost ideal level. The economical benefit of these systems is found in their high testing speed (compared to radiographic testing) and also in the results, which are instantly available for direct feedback to the welders.

This is best illustrated during pipeline construction at a lay barge. Until the introduction of automated ultrasonic testing, the radiographic testing station was the slowest phase in the whole production process and, therefore, dictated the speed of construction. Automated ultrasonic testing systems made it possible to operate at elevated temperatures (as shown in Figure 2) and the overall speed of the construction could be increased considerably, resulting in a reduction of expensive offshore days.

 

Role of NDT in Maintenance
Maintenance schemes and standards, most often regulated by legislation, require defined intervals between shutdowns for invasive tests. During a shutdown, the installation is opened and tested. This is usually the moment that the plant owner discovers that the installation has been opened too early or, worse, too late. Risk based inspection and other similar approaches are, therefore, considered more and more in order to rationalize the periods between shutdowns and base these on the actual and anticipated plant condition.

By studying modern approaches for such schemes, one can see that knowledge of operational conditions and potential degradation mechanisms play a prominent role. Surprisingly, the role of NDT is often limited to the use of conventional methods such as ultrasonic wall thickness measurements, ultrasonic testing, radiographic testing and, last but not least, visual testing. However, this situation is changing. Because a number of novel noninvasive NDT screening techniques are now becoming available, both industry and the authorities have an open mind to apply these. With the introduction of some of these techniques, the time required for a shutdown can be reduced when suitable to perform tests while the installation is in full service. It is obvious that the availability of such onstream NDT techniques could support the knowledge already available on operational parameters and degradation mechanisms in order to base shutdown intervals on the actual plant condition.

One of the techniques capable of onstream use is the pulsed eddy current technique (Stalenhoef and de Raad, 2000). The system is able to detect corrosion in piping and vessels through a thermal insulation layer (Figure 3). It can cope with a maximum insulation thickness of at least 0.1 m (4 in.). The application of this system eliminates the need for removal and replacement of insulation and hence is time and cost effective. The principle of pulsed eddy currents is illustrated in Figure 4. In fact, the system averages thickness of the component over the size of the footprint generated by the eddy current field. The diameter of this footprint is roughly equal to the insulation thickness, but with a minimum footprint size of 32 mm (1.3 in.) with the sensor in direct contact with the component. Work is in progress to further reduce the diameter of the footprint. The method was first developed and patented in the US and is now being commercialized and extensively validated for a number of applications. The temperature of the component under the insulation can be between 173 and 773 K (-148 to 932 °F), thus covering almost all temperatures as they exist in plants. Moreover, the method can be used in contact on rough and corroded surfaces without prior cleaning.

A system is in use in a North American power plant that detects flow accelerated corrosion in the elbows of pipes, in which insulation is under a corrugated sheeting. This application eliminates a lot of insulation work. Moreover, repair actions could be planned prior to a shut down, which is altogether a considerable cost savings for the plant owner.

Another noninvasive technique to be mentioned here is long range ultrasonic testing, which is capable of transmitting and receiving some types of plate waves over an extended range. Up to 1 m (39 in.) distance can be achieved, but the effective range is strongly dependent on the surface condition at either side of the component. The method, which is illustrated schematically in Figure 5, can be used for corrosion detection of annular plates in oil storage tanks. Since the annular plate is considered a critical part of the tank construction and the tank does not need to be taken out of service nor cleaned for this testing, this technique is also a potential money saver.

Similar techniques such as long range ultrasonic testing with ultralong ranges up to 20 m (66 ft) are now quickly entering the market. They fulfill a great demand. These guided wave systems make use of lamb waves generated by a circumferential array of ultrasonic probes (Lowe et al., 1998). Particularly in pipes where there is no geometric divergence and thus low attenuation, long distances can be tested from one probe ring position. Each measurement takes 300 to 900 s per location. Typical applications are testing pipe crossings underneath roads and other short pipe lengths on pipe sleepers which are unable to be pigged. The pipe under the road is not at all accessible for any NDT method to establish its condition unless some kind of pigging is applied, which is not only considered overkill but is also intrusive and expensive. Indeed, guided waves are considered a great money saver (Wassink et al., 2001).

Another testing technique, which is not new but rather has been used for some decades now in intelligent pigs for gas transport pipelines all over the world, is the magnetic flux leakage method (Stalenhoef and de Raad, 2000). Metal loss is detected because it generates a weak leakage field in a magnetically saturated steel pipe or vessel wall. This leakage field is picked up by hall sensors and electronically processed and displayed. The magnetic flux leakage technique was also the first choice, developed and applied for testing of oil storage tank floors almost a decade ago. This testing system was developed in the United Kingdom and now is a worldwide commodity. The nonintrusive spin off applications are just as valuable: quick corrosion detection in pipe and vessel walls while onstream as a screening method to find suspect spots and mark those for further testing. Figure 6 illustrates hundreds of kilometers of unburied thin wall crude oil flow lines in the desert, which are screened for the presence of random corrosion.

A kind of derivative of magnetic flux leakage is the improved magnetic flux leakage technique, also known as the saturation low frequency eddy current technique (de Raad et al., 2002). This new method is able to be applied up to 32 mm (1.3 in.), as opposed to magnetic flux leakage which is limited to 10 mm (0.4 in.). The principle is schematically illustrated in Figure 7. Similar to the magnetic flux leakage method, a considerable level of magnetization is applied. With this improved magnetic flux leakage, local flux distortions caused by discontinuities are picked up by eddy current sensors; these sensors are much more sensitive than the hall sensors used with traditional magnetic flux leakage testing (de Raad et al., 2002).

The recent availability of the improved magnetic flux leakage technique created a solution for long wanted noninvasive screening tools which are able to test thick wall pipes from the outside while onstream. Scanning for local but random corrosion with conventional techniques used previously is slow and expensive and the POD of an isolated corrosion spot is also low compared to screening with the improved magnetic flux leakage technique. Moreover, radiographic testing is not attractive for offshore applications due to the safety hazards. Figure 8 shows a pipe scanning system on a platform to detect random but local biological corrosion in a thick wall flow line with a surface temperature of up to 373 K (212 ¡F). This case also illustrates the high efficiency of this method - circumferential screening of 6 m (19.7 ft) of pipe was completed in 1 h, which otherwise would have taken days with conventional NDT methods (de Raad et al., 2002).

 

Anticipated Benefits
Once techniques such as those described above come into existence and enter the market, it is not too difficult to realize what other applications could benefit from them.

Pulsed eddy current systems, for instance, cannot only measure through insulation, but also in direct contact. This makes it a method for wall thickness measurements, replacing ultrasonic testing on furnace tubing and dispensing with the need for extensive and time consuming cleaning of such scaled or encrusted tubing during a shutdown. The same advantages apply for underwater steel structures, where divers can measure at any location because no prior thorough cleaning is necessary as is needed for ultrasonic testing.

Long range ultrasonic testing is also used for testing of piping that has been on nonwelded supports or sleepers for some time, to see whether corrosion has developed at the contact points. The technique can also be used for corrosion detection under insulation, nozzle reinforcement pads or crack detection in suspension systems for railway cars.

Guided waves can be used for pipes on a jetty, which are hard to test by any other NDT method. These pipes can now be tested in long sections at a time, which helps make testing affordable.

As illustrated, magnetic flux leakage has been used for the testing of hundreds of kilometers of piping in the desert, where a testing rate of 1 km (0.6 mi) per day is easily achieved - a rate far greater than that achievable through conventional wall thickness measurements.

Improved magnetic flux leakage has proven its capabilities on thick wall components and, almost in parallel with the development of pipe scanners, was chosen to build systems for testing of extremely thick storage tank floors of 20 mm (0.8 in.) or more. Such systems are also suitable to test tank floors with a normal thickness of 6 or 7 mm (0.24 to 0.27 in.) but with extremely thick coatings of up to 10 mm (0.4 in.), as is noted by de Raad et al. (2002).

These are just a few examples, but there are many more. At the moment, joint industry projects are just being completed or are under way to identify and validate NDT methods for certain applications and to optimize them where necessary. In particular, the finished project "Non-invasive Inspection within an Asset Risk Management Strategy" (funded by EC-THERMIE - a European community sponsored program) and its successor, "Recommended Practice for Non-intrusive Inspection," funded by industry, are good examples of such joint industry projects. These activities represent the current trend towards onstream, noninvasive testing in combination with risk based inspection philosophies to establish a component condition. In these projects, industry and authorities participate in aiming at a beneficial use of today's NDT without sacrificing safety.

OPTIMISE, another joint industry project recently completed (supported by EC-ESPRIT funding), is a project where NDT techniques are identified, optimized and validated for the testing of bulk carriers, with the aim to increase the testing scope and thus the safety level, while at the same time reducing the time needed for the tests. Figure 9 shows a specialized magnetic crawler with an extremely high payload to carry NDT tools (developed as part of this joint industry project) which is able to negotiate sharp corners, run across corrugated bulkheads and work overhead. Such a device with a long umbilical could carry a closed circuit TV set and other NDT devices, even during the voyage of the ship, to establish hull and bulkhead conditions. Alternatively, the ship needs to be in the harbor in order to use scaffolding or cherry pickers, unless using expensive abseil techniques. However, particularly in the traditional marine industry, old habits die hard; international rules which have been laboriously agreed upon in the past have to be changed. National interests hamper quick introduction of such new techniques even when they are better and lead to increased safety.

 

Conclusion
Modern NDT methods for new construction and maintenance tests are becoming ever more quantitative and nonintrusive. For NDT of new construction, this implies that the more one knows about the material properties and operational conditions, the better the acceptance criteria for weld discontinuities can be based on the required weld integrity and can be fine tuned to a specific application. In the pipeline construction industry, this is already happening. In plant maintenance, the availability of quantitative and noninvasive screening of NDT methods will reduce the time needed for shutdowns and increase the intervals between them. Modern NDT methods will become just as important a tool for risk based inspection approaches and maintenance planning as operational parameters and degradation mechanisms already are. In both of these NDT application fields, these tendencies can lead to rationalization, with cost reduction as a result, while maintaining or even improving existing safety levels. In this way, NDT can cease to be seen as a necessary evil and become clearly seen as a benefit.

 

Acknowledgments
This paper is an update of a plenary lecture given by Frits H. Dijkstra at the Seventh European Conference on Non-Destructive Testing in Copenhagen, Denmark, in May 1998.

 

References
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de Raad, J.A., T. Bouma and A. Bönisch, "Rapid Corrosion Screening in up to 30 mm Wall Thickness for Plates and Pipes," Insight, Vol. 2, No. 2, February 2002, pp. 97-103.

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Dijkstra, F.H. and J.A. de Raad, "Why Develop Acceptance Criteria for Pipeline Girth Weld Defects?," European-American Workshop on NDE Reliability and Validation, Berlin, Germany, June 1997.

Dijkstra, F.H., J. van der Ent and T. Bouma, "Defect Sizing and ECA Criteria: State of the Art in AUT," Pipeline Technology Conference, Brugge, Belgium, May 2000.

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Lowe, M.J.S., D.N. Alleyne and P. Cawley, "Defect Detection in Pipes Using Guided Waves," Ultrasonics, Vol. 36, 1998, pp. 147-154.

Stalenhoef, J.H.J. and J.A. de Raad, "MFL and PEC Tools for Plant Inspection," Insight, Vol. 42, No. 2, 2000, pp. 74-77.

Wassink, C.H.P., M.A. Robers, J.A. de Raad and T. Bouma, "Condition Monitoring of Inaccessessible Piping," Insight, Vol. 43, No. 2, 2001, pp. 86-88, 92.

 

* Röntgen Technische Dienst, Netherlands, PO Box 10065, 3004 AB, Rotterdam, The Netherlands; 31 10 2088208; fax 31 10 4158022 ; e-mail <f.h.dijkstra@rtd.nl>.

** Röntgen Technische Dienst, Netherlands, PO Box 10065, 3004 AB, Rotterdam, The Netherlands; 31 10 2088208; fax 31 10 4158022; e-mail <j.a.de.raad@freeler.nl>.

 

Copyright © 2002 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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