 Table of Contents ECNDT '98 Session: General | NDT: Necessary Evil or BenefitF.H. DijkstraRöntgen Technische Dienst bv P.O. Box 10065, 3004 AB Rotterdam, The Netherlands telephone +31 10 2088208, fax +31 10 4158022, Corresponding Author Contact: Email: rtd@rtd.nl, URL: http://www.rtd.nl |
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After many decades of application, industry has completely accepted standard NDT as an inevitable but invaluable part of production and maintenance of components. Its application has been well-regulated, acceptance criteria for weld defects exist, good schemes for personnel qualification are in place and equipment has evolved to a standard of near-perfection. NDT has become a commodity. Pioneering years are over.
However, such a situation implies a risk. Industry tends to cut costs on commodities such as NDT, especially when they are needed because the code says so. And therefore NDT prices are under pressure, and competition is heavy.
This presentation will highlight some NDT applications whereby NDT can be transformed from a "necessary evil" into a situation that is "of beneficial value" to the user. From an activity that has to be done because it is required by the code, to a solution to a problem that can improve safety, enhance quality and save money.
Examples will cover maintenance inspection such as corrosion detection in piping and tanks, but also routine weld inspection. The need for acceptance criteria for weld defects adapted for modern NDT techniques will be highlighted, because these form (in many cases) the key to benefit.
Non-destructive testing in its present form has been carried out, by specialised service companies and manufacturers, for decades. Röntgen Technische Dienst bv in The Netherlands was established more than 60 years ago, in 1937, and that year marked the beginning of radiographic inspection of welds in The Netherlands. Similar situations exist in other countries. In fact, welding industry would not have experienced the growth and the wide range of applications it has today if there were no such thing as NDT.
NDT has a very important formal status. Requirements for 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 industry a well-established situation has evolved, enabling the use of NDT for the evaluation of welds against Good Workmanship Criteria on a routine basis, thus maintaining workmanship standards and minimising 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, furnace tubes etc. All these thickness readings have to go into extensive data bases, and this process is, thanks to modern computers and data loggers, ever more automated.
It seems almost perfect. Established methods, established acceptance criteria, established procedures and personnel qualification schemes. Almost perfect. Because the reason of this presentation is, to show that things could be improved.
Already in the seventies and eighties, Commission V of the IIW 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 defects on the basis of their significance for weld integrity. For let us be honest: in conventional NDT we are doing something completely different. We base our judgement on density differences on a film, or on echo amplitudes on a screen. Parameters that have very little to do indeed with significance of defects for weld integrity.
In maintenance practice, we base our decisions on NDT that is performed during shutdowns. A significant amount of money could be saved if we would have NDT methods that minimise the time required for that shutdown, or, a step further, avoid it by performing inspections on-stream.
Considering existing Good Workmanship evaluation with NDT, using acceptance criteria as they have been formulated over the years, it is probably fair to say that it is designed to monitor performance of the welder rather than to evaluate weld integrity. This is in agreement with the term "Good Workmanship".
In past years, the tools available to perform this monitoring task were conventional NDT methods. Most existing Good Workmanship criteria have been formulated with the specific capabilities and drawbacks of these methods in mind. Radiography has excellent capabilities in detecting voluminous defects such as slag and porosity, and can provide information on flaw type and defect length. On the other hand, it is known that its capabilities are limited when it comes to the detection of planar defects such as lack of fusion and cracks. In addition, radiography is hardly capable of establishing through-thickness extent of planar defects. Manual ultrasonic inspection in turn is stronger at detection of planar defects, but is limited at detection of voluminous defects and flaw characterisation, and is subjective. Estimation of through thickness extent of planar defects is also limited.
Planar defects, especially if interpreted as cracks, are deemed non-acceptable in many codes. Although this may be understandable from a fracture mechanics point of view, this is not the only reason. The presence of planar defects goes beyond the aspect of checking the welder: they might influence weld integrity. In addition, it is also the fact that conventional NDT methods had (and still have) their limitations that urged code makers to reject planar defects in general. Their detection indicates that something is very wrong with the weld, without being able to quantify the severity of the defect(s).
It may therefore be argued that the merit of conventional NDT using existing acceptance criteria is limited to the aspect of evaluating the welder's performance because this has always been the best we could.
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 totally wrong.
But 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. And, what is worse, the degree of conservatism is more or less unknown. Therefore the question can be asked: "if the historic background of present NDT practice would 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 between evaluation of the welder's performance on one hand, and a fracture mechanics basis on the other hand in terms of "being certain that a defect 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 requires not only the use of modified criteria, but also the use of NDT techniques that can provide the necessary information: they should be able to detect small defects for evaluating weld quality as well as estimate the through thickness extent of planar defects, if present, both with a high reliability (Probability of Detection or PoD). On the other hand, a low False Call Rate (FCR) is required, in order to be commercially viable. It is also important that methods used provide unambiguous information, to avoid interpretation uncertainties and discussions. Criteria should thereby 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 defect size from the beginning, it is highly probable that current acceptance criteria for weld defects would have used this information. Acceptance criteria would have been completely different from what they are now. Thinking aloud, they would probably reject a certain number of voluminous imperfections such as porosity clusters because these indicate insufficient workmanship, but on the other hand they would reject planar and sharp defects exceeding a certain through thickness height and length.
In fact one would expect that, if an NDT method were introduced that indeed can provide these quantitative data, this would trigger a revolution in industry. Because it provides the opportunity to develop tailor-made acceptance criteria. Criteria for general purpose applications would maybe statistically lead to repair rates similar to the numbers we know from radiography and ultrasonic inspection or slightly lower, whereby possibly more planar defects and less rounded indications would lead to rejection than is the case now, which is favourable for weld integrity. By the way, a repair does not necessarily improve weld strength. For specific applications where more details about materials properties and service parameters are known, such methods with adequate criteria could lead to significantly lower repair rates, still maintaining existing safety standards.
Fig 1: Typical TOFD image
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Unfortunately, now that such methods have become available, such as the Time Of Flight Diffraction (TOFD) technique, this revolution does not happen. What we see instead is a much slower process towards quantitative NDT, in combination with adapted acceptance criteria for weld defects.
This was made possible by the introduction of TOFD, see figure 2, which combines a high PoD with a low FCR, and is capable of providing data in a defect's through thickness height [1]. General purpose criteria for TOFD have been developed for this purpose in The Netherlands in a Joint Industry Project [2]. In another, international Joint Industry Project under the auspices of Iploca, specific criteria for pipeline welds are now under development [3, 4, 5], for weld defects detected with a combination of mechanised ultrasonic inspection and TOFD.
Maybe we should regard the efforts of Commission V of the IIW in the seventies and eighties, to establish Fitness For Purpose approaches, 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. But nowadays, we are in a much more comfortable situation.
Maintenance schemes and standards require defined intervals between shutdowns for invasive inspections. During a shutdown, the installation is opened and inspected. This is usually the moment that the plant owner discovers that he has opened the installation too early or, what is worse, too late. Risk Based Inspection and other, similar approaches are therefore considered more and more, in order to rationalise the periods between shutdowns and base these on actual and anticipated plant condition.
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 inspection, radiography, and last but not least visual inspection.
However, it is expected that this situation will change, since a number of novel "non-invasive" NDT techniques are now becoming available. With some of these techniques, the time required for a shutdown can be reduced. Other techniques make it possible to perform inspections whilst the installation is in full service. It is obvious that the availability of such techniques could support the knowledge already available on operational parameters and degradation mechanisms, in order to base shutdown intervals on actual plant condition.
Fig 2: INCOTEST system on insulated pipe
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One of the techniques capable of being used "on stream" is the novel INCOTEST technique, see figure 2, which measures wall thickness of piping and vessels through an insulation layer with a maximum thickness of 10 centimetres. This system is based on pulsed eddy currents, was first developed in the United States, and is now being commercialised and extensively validated for a number of applications.
Another technique to be mentioned here is Long Range Ultrasonics (RTD LORUS), which is capable of transmitting and receiving ultrasonic waves over an extended range, e.g. 1 metre. This method is used for corrosion detection in annular plates in storage tanks, see figure 3.; since the annular plate is a critical component, and the tank does not need to be taken out of service for this inspection, the LORUS technique is a potential money saver. Similar techniques, for ranges up to 12 and more metres, are under development.
A third inspection technique, which is not new at all, but has been used for some decades now in intelligent pigs for gas transport pipelines all over the world, is the Magnetic Flux Leakage (MFL) method, see figure 4. Metal loss is detected because it generates a weak leakage field in a magnetically saturated pipe or vessel wall. This leakage field is picked up by sensors and electronically processed and displayed. The MFL technique was first applied to storage tank floors about ten years ago, in an inspection system called Floorscanner, developed in the UK. But the spin-off applications are just as valuable: quick corrosion detection in pipe and vessel walls, as a screening method to find suspect spots and mark those for further evaluation.
Fig 3: Principle of LORUS technique
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Fig 4: Principle of Magnetic Flux Leakage for detection of wall loss
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Once such techniques have come into existence, it is not too difficult to realise what other applications could benefit from them:
Fig 5: Piping inspection in the dessert
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These are just a few examples, but there are many more. At the moment, Joint Industry Projects are underway to identify and validate NDT methods for certain applications, and to optimise them where necessary.
OPTIMISE, a joint industry project, supported by EC-ESPRIT funding is such a project, where techniques are identified, optimised and validated for the inspection of bulk carriers, with the aim to increase the inspection scope and thus the safety level, at the same time reducing the time needed for the inspections.
The project "Non-Invasive Inspection within an Asset Risk Management Strategy" is another joint industry project, funded by EC-THERMIE, representing the current trend towards on-stream (non-invasive) inspection in combination with risk-based inspection philosophies to establish component condition. In this project, industry and authorities participate aiming at a beneficial use of today's NDT without sacrificing on safety.
Modern NDT methods are becoming ever more quantitative and non-intrusive. This is valid for NDT of new construction and for maintenance inspections.
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 defects can be based on the required weld integrity and fine-tuned to a specific application. In pipeline industry, this is already going to happen.
In plant maintenance, the availability of quantitative and non-invasive screening 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 these NDT application fields, new construction and maintenance, these tendencies can lead to rationalisation, with cost reduction as a result, maintaining existing safety levels.
In this way NDT can turn from a necessary evil, which it is too often still regarded to be, into a benefit.
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