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FFreshex: A combined System for Ultrasonic and X-Ray Inspection of welds Olivier DUPUIS, Valérie KAFTANDJIAN Laboratoire CNDRI, INSA, Bat 303, 69 621 Villeurbanne Cedex Steve DRAKE OIS Engineering Ltd., Grist Mill, Crittens Road, Cobholm, GT Yarmouth, NR31 0AG, UK Arnfinn HANSEN ROBIT As., P.O. Box 100, N-1361 Billingstad, Norway Jean-MichelCASAGRANDE THOMSON TUBES ELECTRONIQUES, Z.I. Centr'Alp, 38430 Moirans, France Contact

Abstract

The inspection of welds often requires both radiographic and ultrasonic testing, in order to increase the reliability of inspection. The Ffreshex^* project started in March 97 with the intention of replacing the radiographic film to increase the inspection speed, and combining both X-ray and ultrasonic data to increase reliability. Both objectives were fulfilled and the project successfully ended in March 2000. A sensitivity performance comparable with the film was obtained on the radioscopic image from the new multi-line detector (1.14% with wire type IQI on 35 mm of steel at 3 mm/s, 200 kV, 15 mA). Combined acquisition and data fusion were validated on circumferential welds during on site trials at a spool yard.

* FFRESHEX : Fast Film REplacement System for High Resolution X-ray weld inspection with ultrasonic data fusion, BRITE/EURAM BE 3681 France

Introduction

The Ffreshex project was initiated because of the following strategic reasons :
• first, there is an industrial need to replace the radiographic film, because it is too slow (typically 10 minutes for a circumferential weld) and requires chemicals which is sometimes a problem for off-shore applications,
• ultrasonic techniques are faster but do not allow an easy classification of defects (and can lead to false rejects),
• before now, no film-less system was able to meet the requirements of European and American standards at acceptable inspection speed,

The idea of the Ffreshex project was first to design a new X-ray detector based on Time Delay Integration, that is, several lines of the same part of the object are integrated in order to increase the signal to noise ratio. This allows the use of sensitive elements of small size (54mm) which ensures a good spatial resolution. The detector lines integrate the X-ray signal because the object is moving continuously during acquisition in synchronism with the detector line readout clock
The second idea of the Ffreshex system was to improve the reliability of weld inspection through the use of both X-rays and ultrasound. These techniques are complementary and are already often used in combination to confirm the presence of defects. The aim was thus to combine both techniques automatically.

1. The combined inspection system

An orbiting mechanism was developed for circumferential welds, on which the X-ray tube, the X-ray detector and a set of ultrasonic probes are fixed. The whole system rotates around the pipe. A first scan of the weld allows acquiring an X-ray image, and during the way back the ultrasonic acquisition is carried out.

Fig 1: Scheme of the acquisition system

Fig 2: Picture of the Ffreshex orbiting mechanism with the cable handling system.

Fig 3: View of the 8 ultrasonic probes and a pipe section installed in the orbiting mechanism.

2. Data acquisition and pre-processing

a) Ultrasonic data
8 probes are used in pulse-echo mode in order to gather information of the whole weld volume. Each probe gives an image such as that shown in the following figure.

Fig 4: Ultrasonic image given by one of the probes. The zone corresponding to the weld is between the two dark lines. This is a B-scan image, that is, the abscissa along the weld is the horizontal direction, and the time of flight is the vertical one.

The signals from the 8 probes are analysed and processed in order that echoes coming from the same position inside the weld are detected by several probes gathering information from the same indication.

The amplitude of each echo is compared to that of a reference echo from a known hole and thresholds were defined in order to classify the echoes with respect to the reference.

The result of the ultrasonic stage before combination is thus a list of detected indications with their respective positions inside the weld volume, which results in the creation of a bounding box around each position, depending on the defect size, and amplitude.

b)X-ray acquisition
The image given by the new detector is derived from the integration of 204 lines, each line being constituted by 1024 pixels of 54mm in size. Because of this integration, the resulting image shows a good contrast and sensitivity performance (1.14% with wire type IQI on 35mm of steel at 3 mm/s, 200 kV, 15 mA). The following figures show two images.

Fig 5: Radioscopic image (TDI detector) on a welded carbon steel pipe. 3 wires of the IQI (10 FE EN) are visible

Fig 6: Part of a weld with an artificial slot of 0.5 mm in width and 25 mm in length

The radioscopic image is processed automatically in order to detect the defects, based on adaptive thresholds. The development of any image processing algorithm is in fact a compromise between a high percentage of detected defects, and a limited number of false indications. Weld images are particularly difficult to process due to the weld cap, which is non-uniform, and irregularities of the weld, which can be detected at the same time as real defects. Thus, once the objects are detected, some features are computed for each object, namely the area and the contrast with respect to the noise. These features will be used in the data combination stage.

3.Data combination (fusion)

A fusion approach is an intelligent combination of the information given by several sensors. Several theories exist, among which, in the field of NDT, the evidence theory is known to be adequate, because of its ability to model both inaccuracy and uncertainty [1-5]. Another interest of this theory for NDT applications is that it is possible to hesitate between two hypotheses. In our case the hypotheses are simply defect and no defect. The hesitation between both hypotheses is thus an ignorance to whether the object is really a defect or not.

The information obtained from each method is a list of objects that have been detected, associated with their parameters (amplitude, contrast ...). From these parameters, it is necessary to compute a confidence level associated to each NDT method (which is called a mass function in the Dempster-Shafer or evidence theory). This confidence level can be compared to the confidence of the radiographer when he looks at a film and says «I am more or less sure that there is a defect . This «more or less» has to become a precise value in the fusion algorithm. This stage is a crucial point that determines the performance of the fusion algorithm.
In our study, for each testing method, the computation of the confidence levels is done after an apprenticeship stage where the knowledge of the NDT experts is used. This part is explained in [6].

Then the X-ray and ultrasonic confidence levels are combined by the Dempster rule of combination. If both methods are in agreement concerning one object, the confidence after fusion is higher. If both methods are in contradiction, the fusion approach gives a warning that indicates a conflict. When an object is detected only by one method, the confidence after fusion remains the same. In the end, all the fusion results are then given to the operator for final decision (objects and their parameters and confidence levels).

4. On site validation

The system was tested in a spool yard in Norway in November99. The feasibility of the combined acquisition system was proved.
The following part explains the results of the data fusion process on a pipe with a circumferential weld. The plate material is carbon steel and the plate thickness is 14.5 mm. It contains one external slot, two internal ones, one little lack of fusion, some undercuts and a few number of porosities. This pipe has been repaired several times so that the weld cap is very irregular, which means that it corresponds to one of the most difficult inspection case on site. However, this is an interesting case for performance demonstration, because the weld contains both artificial and real defects.

After image processing, all the objects detected on the radioscopic image were compared to the defects identified by an expert on the radiographic film (in order to assess the quality of both the radioscopic image and the image processing). This film was made at the spool yard with a radioactive source located inside the pipe (panoramic single wall image with a Selenium 75 source).
In the following figures, all the true defects are surrounded by an ellipse (true means that the objects correspond to a defect detected by the expert on the film), and the others are said false. The X-ray image contains 6 false defects corresponding to high local variations of the weld thickness. Most of the porosities are detected and the three artificial slots are also detected but the lack of fusion cannot be detected because it is very small and has a very low contrast. We can also distinguish some undercut in the lower right part of the image detected as many small objects.
The following images represent the defects detected by both methods, where the image width is the external circumference of the pipe and the two horizontal lines define the external weld cap limits. Both images are in the same reference system for the defect position and each square of the grid is 5 mm x 5 mm.

Fig 7: Objects detected with X-ray inspection (after image processing and registration in a common reference system)

Fig 8: Objects detected with US inspection (after processing, probe merging and registration in a common reference system)

Concerning the ultrasonic defects, all slots are detected and also 3 undercuts and one lack of fusion. None of the porosities can be detected due to their very small sizes. This image contains only one false defect. We must say here that the object is said false because it is not visible on the X-ray film, because it is the only reference we have. But it might also be a lack of fusion, missed on the X-ray film.

After the fusion stage, the results are displayed for the operator, as detailed in the figure 9. The final decision is left to him, but the confidence level is given by a colour.

Fig 9: Operator display interface : top left window : section view of the weld volume; top right window : view of the pipe circumference; bottom window : weld top view. Two cursors allow selecting a part of the weld to be displayed. A frame indicates the decision on the selected defect as a colour (red for surely a defect, orange for probably a defect and green for surely not a defect)

Conclusion

The feasibility of a combined X-ray and ultrasonic acquisition system was proved and the interest of the data fusion was shown. It allows for detection of more defects than with any of the methods alone, and the display of the confidence associated to the defect is a great help for the operator, as well as the display of the defect location inside the weld volume.
Concerning the radioscopic image quality, the sensitivity obtained is also very good and similar to that of a film.

Acknowledgements

The field trials were made possible by the initiative from Alf Daaland, Statoil. We would like to thank the people from the Coflexip Stena Offshore spool base at Orkanger, Norway, where the trials were carried out.

References

1. E. Gros, NDT Data fusion, John Wiley and Sons, New York, 1997, 205p.
2. D. L. Hall, Mathematical Techniques in Multisensor Data Fusion, MA: Artech House, 1992.
3. T. Lee, J.A. Richards and P.H. Swain, "Probabilistic and evidential approaches for multisource data analysis", IEEE. Trans. Geosci. Remote Sensing, vol. 25, pp. 283-293, 1987.
4. R.C. Luo, M.G. Kay.Multisensor integration and fusion in intelligent systems. IEEE transactions on systems, man, and cybernetics,1989, Vol. 19, no 5, p. 901-931.
5. G. Shafer, A mathematical theory of evidence, Princeton University Press, Princeton, New Jersey, 1976.
6. O. Dupuis, Valérie Kaftandjian, Yue Min Zhu, Daniel Babot, Détection de défauts par fusion de signaux ultrasonores et d'images radiographiques, 17° colloque GRETSI sur le Traitement des signaux et des images, Vannes, 13-17 septembre 1999.

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