Gamma Ray System Operated by Robots for Underwater Pipeline Inspection

Carla MARINHO, Claudio CAMERINI, Carlos MAIA, Petrobras, Rio de Janeiro, Brazil Ricardo TADEU, Henrique ROCHA, Rio de Janeiro Federal University, Brazil

Abstract. The Petrobras Research Centre – CENPES - and the Rio de Janeiro Federal University – UFRJ - have developed a completely automized gammagraphy system in order to inspect underwater pipelines in the oil industry. This project aims at achieving the state of the art in detection and measurement of alveoli corrosion and fatigue cracks in underwater pipelines and steel catenary risers (SCRs). This paper presents the development of the gammagraphy system that will be used, which uses Iridium-192 as radiation source, and will be available with phosphor image plates. Underwater conditions require fixed source-film geometry considering the additional thickness of the pipeline coating and conducted fluid. The available parameters are minimum source activity and contrast sensitivity of the computed radiography systems. Contrast sensitivity evaluation is performed by a computer-aided procedure with the line profile of the radiographic image.

Introduction

PETROBRAS has been increasingly using underwater pipelines, thus stimulating more and more investments in integrity systems. Similarly to land pipelines, underwater pipelines are inspected by instrumented pigs. Nevertheless, it is difficult to correlate inspection results – when using divers or robots – mainly because the tools used in both cases can easily generate errors and a report based on pig inspections is currently the only tool available to support the decision as to whether or not a line will remain in operation. On the other hand, there aren’t any proper methods available to monitor the contact points of risers with the marine soil. This scenario underpinned the development of a remotely operated system aimed at providing a complementary inspection to the one performed by the instrumented pig as well as at detecting fatigue cracks on the joints of the risers. The prototype dimensions were based on a Ø10” gas pipeline in which the field tests will be performed. As the inspection procedure is consolidated on this pipeline, the proposed system can also be used to inspect pipes with different diameters. Initially lab tests using conventional radiography were performed, but the need to optimize the time spent on inspection led the study to use the computed radiography technique. This paper presents the system potential, lab tests, as well as future developments.

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1. Gammagraphy System

1.1. Remotely Operated Vehicles

Remotely Operated Vehicles, or ROVs, execute important activities under deep waters where work is too dangerous or impossible for divers. ROVs are fed power and provided instructions by umbilical cables and they are able to send back video signals to the surface operator. Figure 1 shows a ROV.

1.2. Mechanical Grab

A mechanical grab was designed to be guided by a ROV and coupled to the inspected pipeline area. The grab will carry a phosphor image plate in one of its “fingers” and the Iridium irradiator in the opposite “finger”. Because it is not feasible to replace the plates after each radiographic exposition, it was defined that there would be one grab for each site to be inspected. The grabs set dives with the ROV and once it is coupled to the respective positions, the irradiator is placed on its position successively in order to radiograph all the desired areas. Remote control over the beginning and the end of the exposition process will be performed through the umbilical line by the surface operator, according to previously set parameters. At the end of the inspection, the ROV picks up the irradiator and decouples the grabs one by one. The whole set will then be brought to the surface. Figure 2 shows a grab, while figure 3 illustrates the proposed inspection using the gammagraphy system.

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Figure 2– Mechanical Grab. The Iridium irradiator is located in a basket, while the image plate is positioned in the opposite “finger”.

Figure 1– One ROV (Subsea Vision Ltd)

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Radiographies will be taken using an Iridium source (Ir192). This source will be accommodated inside an irradiator with depleted encapsulated Uranium inside a steel container. The set will be also protected by a steel vessel in order to resist pressure effects. The source position control is fully electronic and the surface operator will command it based on the exposure time previously defined. The irradiator is a MDS Nordion SA product. The electronic control adaptation and the protection vessel design have been developed by the project team. Figure 4 is a side cut view of the irradiator showing source positioning.

2. Experimental Tests The radiography technique used in all tests was double wall single image, DWSI, with the source touching the adjacent wall.

Figure 3– (a)ROV and grabs dive together; (b) ROV positioning the grabs; (c) The irradiator is transported until a grab; (d) Irradiator is positioned; (e) Iridium exposition; (f)In the end, the ROV picks up the system and brings it to the surface.

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Figure 4–Irradiator. It can be seen the isotope in its two possible positions: exposed and hidden.

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2.1. Tests using Conventional Radiography: The First Trial The samples used were a Ø20” tube (9.5 mm thick) containing corrosion alveoli and grinded defects and a Ø10” pipe (22.5 mm thick), with a fatigue crack on the weld joint (figures 5 and 6). Four conditions were created: empty tube with and without coating simulation and full tube with and without coating simulation. Figure 7 shows the experimental arrangement.

Figure 5 –Ø20” tube: (a) Corrosion alveoli outside the sample; (b) Alveoli inside the sample; grinded defects in the longitudinal weld joint; (c) Depth of the alveoli based on sample thickness.

Figure 6 –Ø10” tube: (a) Girth weld after fatigue testing; (b) Length of the crack: 158mm.

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A 370 GBq (10Ci) activity Iridium source (Ir192) was used. Images were captured on a KODAK T200 radiography film and digitalized on a MICROTEK scanner with a 600 dpi resolution and 256 shades of grays. Digital images were processed using Image Pro Plus software from Media Cybernetics. 2.2.Tests using Computed Radiography In order to test the performance of the computed radiography systems included in this project, a new ∅254(10”)x500x20mm test specimen was prepared. Figure 8 shows the standard and the croqui with the dimensions of the defects. The first equipment tested was a Cyclone snanner, from Perkin Elmer. Tests were performed under two conditions: empty tubes and tubes full of water, using 185GBq (5Ci) activity from the Ir192 source, DWSI technique and a 700 mm source-detector distance. The second equipment tested was an ACr2000i Kodak system, which was allowed to use by the manufacturer, and that is still undergoing tests. Because the mechanical grab prototype had not been concluded, the Kodak system was initially tested with the grab attached to the sample and the phosphor image plate was positioned in one of the “fingers” while the irradiator was positioned in the other (see figure 2). The Ir192 source had 53.65 GBq (1.45Ci) activity and the sample was in the empty tube condition. One of the purposes of this test was to assess the mechanical configuration of the grab. Fuji and Kodak standard image plates were used in both scanners. The plates had been cut to 123 x 250 mm. After performance evaluations, the same tests performed with the conventional radiography will be proceded with the computed technique for comparison purposes.

Figure 7- (a)Film over the alveoli and Ir-192 exposed in the opposite wall; (b)Film over the girth weld and coating simulation.

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3. Results

3.1.Conventional Radiography

Tube ∅ 20”

Figures 10 and 12 show the arrangement of the film plates indicating alveoli and grinded defects under the two conditions: empty tube with and without coating, respectively. Shades of gray profiles obtained in the radiographys are presented in figures 11 and 13, as indicated by the dotted lines.

Figure 8 – (a) Ø10” standard sample; (b) Defects and dimensions(mm). Letters indicate depth based on sample thickness :a-10%T,b-20%T,c-30%T,d-40%T,e-50%T.

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Figure 10- Results of the empty tube without coating condition. D=2.5

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Figure 14 presents a radiography of position 2d (see figure 5) showing a full tube with coating.

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Figure 12- Results of the empty tube with coating condition. D=2.2

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Figure 13- Shades of gray profile extracted from figure 12.

Figure 14- Results of the tube full of water with coating condition. Film of 2d position (fig.5) D=0.4

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The dotted line in the figure indicates the direction that guided the comparison of shades of gray (figure 15).

Tube ∅10” Figures 16 to 19 show the results of all test conditions, while figure 20 depicts a comparison of the profiles obtained in each case. The dotted line in figure 16 indicates the direction in which profiles were obtained.

Figure 15- Comparison of shades of gray of 2d position, under three conditions: empty tube without coating (1), empty tube with coating (2) and tube full of water with coating (4).

Figure 16- Results of the empty tube without coating condition. D=2.3

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Figure 17- Results of the empty tube with coating condition. D=1.5

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Crack dimensions were defined by the film digitalization process, using the calibration parameters provided to the scanner.

The arrows (fig 20) indicate the beginning of the crack. Its final position could not be well defined under all test conditions.

Figure 18- Results of the full water tube without coating condition. D=1,9

Figure 19- Results of the full water tube with coating condition. D=1,4

Figure 20- Comparison among shades of gray profiles from the crack, under four conditions: empty tube without coating(1), empty tube with coating (2), full water tube without coating (3) and full water tube with coating (4).

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Alveoli present on the Ø20” tube have been clearly observed, thus it was possible to identify the differences between them under the four test conditions. In the case of the Ø10” tube the presence of a fatigue crack was equally detected under all conditions, thus its dimensions have been defined. The average value found was 135.5mm in length, which represents 86% of the actual value (158 mm). This was indeed a very good level of sensitiveness in the first experiments. The radiography quality of the test performed with full tube was not satisfactory. Digitalization rendered images with “false contrast”, thus suggesting that the results would provide more definition. Nevertheless, the profiles presented in figures 15 and 20 show a reduction of the contrast obtained due to the presence of water and coating. The average exposure time was 30 minutes and even so it had to be increased up to 5 times to solve the problem or to resort to a more powerful activity source.

3.2.Computed Radiography

Figure 21 shows the captured image of standard sample (fig 8) after 45 minutes of exposure, under the tube full of water condition, using Cyclone equipment. All the holes were detected in addition to the three first - less deep - slots, which were 1mm wide. This has been quite a positive result considering sample thickness, water filling and source activity. Figure 22 shows the shades of gray profiles obtained in conformity with the lines defined in figure 21. These profiles were used to see the differences in depth and diameter/width of the defects. Fluctuation intensity of the horizontal image lines caused by scanner slippage can be seen. That’s why tests with the equipment were interrupted.

Figure 21 – Flat bottom holes and three 1mm wide slots. Intensity fluctuation of horizontal image lines caused by scanner slippage can be seen.

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Figure 22 – Shades of gray profiles: (a)Holes ∅20mm; (b)Holes ∅10mm e (c)Slots a, b and c, 1mm wide.

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Figure 23 shows radiographys obtained from tests using the grab and the Kodak system in the area where slots and holes a, b and c were found. Indication of the profiles plotting is presented in figure 24. Exposure time was 15 minutes. The images presented in figures 21 and 23 have been treated by Image Pro Plus software from Media Cybernetics. 4. Future Developments Performance tests with the Kodak systems are currently underway and afterwards, the GE system will be assessed. Later on, the results obtained with conventional and computed radiography will be compared in order to analyze the benefits of using the computed technique. Based on the data, the system to be used will be selected. The grab prototype will yet undergo some mechanical adjustments and after that, lab tests will be performed on the radiography-grab-ROV system and finally field tests will take place.

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(a) (b) Figure 23 – (a)Slots of the sample pattern ,and , (b)Holes c, d and e.

Figure 24 – Shades of gray profiles: (a)Slots: 2mm wide series and 1mm wide sequence; (b)Holes ∅20mm; (c)Holes ∅10mm.

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5. Conclusions The use of computed technique turns radiography into an interesting option for inspections performed offshore, be it because of the method’s inherent characteristics, or due to the decrease in exposition time. In conformity with the proposals presented herein, the results obtained so far indicate that the gamma ray system will provide benefits to submarine work – pipeline inspection and risers, and won’t constitute a barrier to continuous operation during inspection.

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