5th Pan American Conference for NDT 2-6 October 2011, Cancun, Mexico
Inspection of Composite Pipelines using Computed Radiography Davi F. OLIVEIRA1, Sérgio D. SOARES2, Ricardo T. LOPES1 1
Nuclear Instrumentation Laboratory, Federal University of Rio de Janeiro; Rio de Janeiro, RJ, Brazil Phone: +55 21 2562-7311; e-mail:
[email protected],
[email protected] 2 CENPES/Petrobras; Rio de Janeiro, RJ, Brazil: e-mail:
[email protected]
Abstract Composite materials have proven themselves to be a good option among the selection of engineering materials due to some of their properties, such as mechanical resistance, resistance to corrosion, lightness and good durability. However, as it is the case with alloys of any kind, composite materials have to be inspected during their lifespan, which may take place either during the manufacturing process or while they are performing their working functions. In the oil industry, composite materials have been employed more and more often, especially with respect to the transportation of fluids in pipelines. Generally speaking, the defects which are most commonly found in bonded joints are the presence of porosity, cracks and lack of adhesive. The present study aims at evaluating the feasibility of employing Computed Radiography (CR) to detect discontinuities in polymeric adhesives that are used for joining together pipeline segments made of epoxy resin reinforced with fiber glass. The tests were performed in order to assess the detectability of the discontinuities of certain joints containing fluid inside the pipelines, aiming to simulate real working conditions. Results show that it is possible to detect defects even in pipelines containing fluids, which is highly relevant to the inspection of operative pipelines made of composite materials. Keywords: Computed Radiography, Composite Pipelines, Lack of Adhesive, Image Plate.
1. Introduction The insurgence of composite materials in offshore oil and gas development was fairly evident over the last decade, after 35 years of satisfactory service rendered to the aerospace and transportation industries. Offshore production extracted from lower water depths has prompted an effort to enable the usage of composites as a cost-effective alternative to standard metallic materials [1]. Composite materials have proven themselves to be a good option among the selection of engineering materials due to some of their properties, such as mechanical resistance, resistance to corrosion, lightness and good durability. However, as it is the case with alloys of any kind, composite materials have to be inspected during their lifespan, which may take place either during the manufacturing process or while they are performing their working functions. In the oil industry, composite materials have been employed more and more often, especially with respect to the transportation of fluids in pipelines. Adhesive-bonded joints are a rigid joints consisting of a slightly conical (tapered) bell end and a machined (cylindrical or tapered) spigot end. Alternatively, the bell-and-spigot tapers may be threaded. A typical adhesive-bonded joint is shown in Figure 1.
Figure 1. Typical adhesive-bonded joint [2].
Adhesive joints have the lowest material cost among all other kinds of joints, and are structurally efficient when made up correctly. If a cylindrical spigot is used, the joint is made up to a shoulder. The tapered bell and tapered spigot joint has two matching tapered surfaces and does not make up to a shoulder. The former has the advantage of enabling the position of final make-up to be readily determined. The latter (taper/taper joint) is a stronger joint but is more prone to positional errors if incorrectly assembled, which can weaken the joint. The production process of adhesive joints tends to become more difficult for larger sizes, particularly for pipes above 450 mm diameter. One of the main concerns is then the size of the adhesive bead that is created when the joint is made up, which could protrude into the bore of the pipe. This could not only create a substantial blockage factor, but could also provide a source for erosion and cavitation damage [2] as well. The defects which are most often found in Glass Reinforced Epoxy (GRE) pipe systems are lack of adhesive, disbonding and delaminations in bonded joints, which can only be detected through hydrostatic testing or in operational conditions due to induced vibrations. Most of the service failures in composite materials systems are due to mistakes made during the assembly stage. The relevance of this fact points to the need of having evaluation tools capable of performing accurate detections, as well as of building databases for projects’ qualification and creating assembly procedures for such systems. When the adhesive comes apart or loses its adhesion power, pressure loss on the lines can occur, as well as oil leaks or oil contamination, which can result in productivity losses, environmental damages and even lethal accidents. Due to those reasons, there is an urge for inspection methods capable of assessing both the integrity and the quality of the adhesives used in such joints. Due to the difficulties and the high costs implied in interrupting commercial production processes, it is often required that non-destructive methods are employed for joint evaluation, especially because it can be performed on site without disrupting the production process. There is, therefore, a big concern for providing reliable inspection techniques capable of inspecting such materials.
This paper presents the development process of an inspection methodology based on computed radiography (CR). It aims to prove that this technology may be employed to inspect adhesive joints on composite pipes made of epoxy resin and reinforced with fiberglass.
2. Radiographic tests on composite pipes Radiography (RT) is quite useful for detecting wall thickness variations, water ingress, scale build-up and some voids and areas lacking adhesive, as well as the presence of damages caused by impact, porosity or inclusion, and other volumetric defects. Besides, cracks can also be also detected, as well as incorrect insertion of pipes in adhesive sockets and internal excess of adhesive [3, 4]. It is important to mention that radiographic technique is not sensitive to surface roughness, but it is sensitive to the orientation of the defect [4]. Radiographic test parameters (i.e. tube voltage and exposure time) shall be adjusted compared to steel due to the low density of the polymers and composites. Low to medium tube voltages are suitable for radiography of Glass Reinforced Epoxy (GRE) [4]. Voids in the bonding (adhesive layer) appear on the films as darker areas. The defects are easy to detect, as long there is an air gap between the bonded faces. If the air gap is less than 0.5 mm, it is very difficult to detect lack of adhesive without modifying the adhesive by adding heavy elements, which act as contrast enhancers. ZnI2, BaSO4, PbO, and W (at 5 weight percent) function well as contrast enhancers [3]. Figure 2 shows a radiographic image of a bonded joint with ZnI2 additive.
Figure 2. Detection of disbonding areas with addiction of ZnI2 in the adhesive.
3. Methodology 3.1. Sensitivity As specific image quality indicators (IQIs) for these materials do not exist, an IQI that will work as a contrast sensitivity penetrameter for the radiography images was developed. This IQI consists of a small piece of adhesive material (1.00 mm thick) containing two circular holes of diameters of 1.00 and 2.00 mm, respectively. It will be used to check the radiographic image sensitivity to detect voids in the adhesive area. Figure 3 shows the contrast sensitivity IQI.
Figura 3. Adhesive hole-type IQI.
To keep the same thickness and magnification factor of the adhesive layer, the IQI was positioned between the inner wall and a shim stock made of the same material of the pipe. Figure 4 shows the placement of the IQI.
Figura 4. Placement of IQI.
3.2 – Radiographic Tests The radiographic tests were performed in two steps: laboratory tests and tests simulating operational conditions. For the lab tests, a test sample consisting of two pieces of a 4 inch diameter pipe bonded together and containing lack of adhesive in the form of horizontal and vertical stripes was confectioned, as shown in Figure 5.
Figure 5. Test sample with horizontal and vertical lack of adhesive.
The images were obtained by constant potential x-ray equipments. IP HD Plus (Dürr) Image Plates and a CR system model CR50P (GEIT) were used. As the test samples were manufactured by piping cuts, they were positioned in certain way in order to simulate a double wall single image (DWSI) setup, which is the recommended geometry for this kind of inspection. Figure 6 shows the geometry setup for the tests.
Figure 6. DWSI simulated setup.
For the tests in operational conditions, radiographic images were obtained from bounded joints of a hydraulic circuit developed to simulate the water flow in the pipeline, aiming at evaluating the detectability of defects and image quality under the influence of the presence of liquids inside the pipes. The hydraulic circuit is composed of 4 inch diameter pipes containing bonded joints. In the assembly stage, defects in the adhesive layer were inserted in order to simulate real discontinuities that could be found in such joints. Radiographic images of two bonded joints of lack of adhesive defects were carried out, as well as images of one joint presenting lack of adhesion defects and one joint with no defects. As in the lab tests, the DWSI technique was employed and four images per joint were acquired. Figure 7 shows the hydraulic circuit and the scheme of the inspected joints, and Figure 8 shows the exposure setup.
Figure 7. Hydraulic circuit and positioning of the inspected joints.
Figure 8. Exposure setup (DWSI).
Table 1 shows the exposure time used in the lab tests and in the operational condition tests. In both tests, a high voltage of 70 kV and a current of 3 mA was employed. Table 1. Exposure time (s) used in the radiographic testing. Laboratory Operational Condition Lack of Adhesive No Defects Lack of Adhesive Lack of Adhesion With Water 12 5 5 13 Without Water 30 30 60
3. Results Figure 9 shows the radiographic images of the lack of adhesive sample for the lab tests. In a) and b), it is possible to notice the presence of horizontal and vertical stripes without adhesive (indicated by the red arrows), respectively. In c) and d), the same images are shown after being processed with the computational filter “Enhance Details”.
Figure 9. Radiographic images acquired in lab tests.
Figures 10 and 11 show the radiographic images of the joints presenting lack of adhesion (1 and 2), where a) and c) are respectively the images that were made without and with water
inside the pipe, and b) and d) are the same images processed with the computational filter “Enhance Details”. The lack of adhesion, which consists in poor or no adherence between the piping walls and the adhesive, could not be visualized. Only the areas where there isn’t any adhesive or its quantity is less than the recommended in the bonding standards could be displayed in the images. These empty regions appear as black spots in the image due to the lower attenuation of radiation in these areas (indicated by the red arrows).
Figure 10. Lack of Adhesion 1 – a) e c) without and with water, respectively and b) e d) same images processed using the computational filter “Enhance Contrast”.
Figure 11. Lack of Adhesion 2 – a) e c) without and with water, respectively and b) e d) same images processed using the computational filter “Enhance Contrast”.
Figure 12 shows the radiographic images of the joint “No Defects”, where a) and c) are the images without and with water inside the pipe, respectively and b) and d) are the same images processed by using the computational filter “Enhance Details”. No areas showing or indicating the presence of defects were found.
Figure 12. “No Defects” – a) e c) without and with water, respectively and b) e d) same images processed using the computational filter “Enhance Contrast”.
Figure 13 shows the radiographic images of the joint “Lack of Adhesive”, where a) and c) are the images without and with water inside the pipe, respectively and b) and d) are the same images processed by using the computational filter “Enhance Details”. The red arrows indicate the areas where defects in the bounded joint were found. It can be noticed that there wasn’t a perfect coupling of the piping components, showing that a failure occurred in the assembly process.
Figure 13. “Lack of Adhesive” – a) e c) without and with water, respectively and b) e d) same images processed using the computational filter “Enhance Contrast”.
4. Conclusions The experiments showed the viability of employing the radiographic technique for inspecting bounded joints of GRE pipes. In all tests, the circular holes of the contrast sensitivity IQI were detected. It shows that it is possible to detect small areas of lack of adhesive and that this
penetrameter can be used as an image quality indicator in order to evaluate the radiographic sensitivity in composite materials images, analogously to hole-type IQIs commonly used in metallic material radiographies. The CR technique proved to be capable of detecting discontinuities in operational condition joints, i.e., containing fluids inside the pipeline. This is a great advantage because there is no need to interrupt the production and drain the line in order to carry out the inspection. Another advantage of this technique is the possibility of processing the images by computational filters. These filters have proven to be very useful in detecting defects’ indications, resulting in images with high contrast enhancement. The lack of adhesion joints had the same kind of indications of voids. However, the lack of adhesion could not be clearly identified. As bounded joints have the correct quantity of adhesive but don’t present adhesion of the components, the space between the adhesive layer and the piping wall was not visible in the radiographs because the layers were superimposed on the image. Acknowledgements This work was partially supported by Petrobras, Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação Carlos Chagas Filho de Amparo Pesquisa do Estado do Rio de Janeiro (FAPERJ) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES). References [1] Jeremy C. Price, “The ‘State of the Art’ in Composite Material Development and Applications for the Oil and Gas Industry”, Proceedings of The Twelfth (2002) International Offshore and Polar Engineering Conference, Kitakyushu, 2002 [2] ISO 14692-3 – Petroleum and natural gas industries - Glass-reinforced plastics (GRP) piping - Part 3: System Design, 2002; [3] ISO 14692-4 – Petroleum and natural gas industries - Glass-reinforced plastics (GRP) piping - Part 4: Fabrication, installation and operation, 2002; [4] NORSOK Standard M-622 – Fabrication and installation of GRP piping systems, 1994
