Underwater Pipe Wall Thickness Measurements by Gamma Backscattering

Transcription

1 Underwater Pipe Wall Thickness Measurements by Gamma Backscattering Samir Abdul-Majid & Ahmed Balamesh Faculty of Engineering, King Abdulaziz University P.O. Box 80204, Jeddah 21589, Saudi Arabia Corresponding Author: Samir Abdul-Majid Telephone: , fax: Abstract Underwater pipeline inspection is a challenging problem because accessibility to pipe is very difficult and use of complicated equipment under the water is very hard. The ultrasonic method currently used for underwater pipe inspection requires removal of pipe coating and after inspection new pipe coating need to be placed. This is quite expensive and time consuming process. In this work a lab experiment was performed that utilize Compton backscattered gamma ray for wall thickness variation measurements. A very small Cs-137 radioactive source of 5 micro-curie was mounted on the base of 2 x2 NaI(Tl) scintillation detector having long cable. The source and counting system were put inside waterproof container. Carbon steel pipes of different wall thicknesses together with the counting system were put inside large tank of water. When the counting system was put in contact with pipe surface, primary radiation interact with pipe wall material producing backscattered radiation that is measured by the detector. The amount of backscattered radiation is proportional to pipe wall thickness. Signals were fed into a computer; Labview program was developed for data acquisition and thickness evaluation. The system is very sensitive because it depends on single photon counting. No removal of pipe coating is necessary; accordingly the method is faster and less expensive compared to ultrasonic technique. And because of the very low activity sources used, radiation dose rate near the system surface is negligible and the system is quite safe to use. 1

2 1. Introduction Underwater pipeline inspection is very difficult with current technology as it involve use of complex equipment under the water. Proper and periodic inspection is essential for reliable operation; accordingly simple, fast and cost effective technique is required. We are proposing and investigating gamma ray back scattering for pipe wall thickness evaluation. The method is simple, use extremely low activity source and can be used for in-line inspection. Management of pipelines integrity was reviewed by Kishway and Gabbar (2012), Hopkins (2003) and Castanier and Rausand (2006), for internal and external corrosion. Smart pigs are inserted into the pipelines and carried by flowing liquid. As it moves through the pipe it provides information on wall thickness, dents, etc. For metal loss the techniques used mainly are Magnetic Flux Leakage (MFL) and Ultrasound (UT) pigs. Both of these techniques have limitations for in-line underwater pipe inspection as they require insertion the probes into the pipe interrupting the flow; the UT requires perfect contact with pipe surface. Techniques for pipe inspections were reviewed by IAEA (2005). Tangential radiography that recently found applications for insulated pipes may be difficult to function properly for underwater inspections of large diameter pipes; it is not useful for pipes having diameter more than about 25. It uses large gamma sources or x-ray machines which are difficult to handle under the water. Chaves and Melchers (2011) studied offshore pitting corrosion; most sever corrosion occurs near welds. Correa et al., (2009) made computational and experimental study on weld offshore thickness measurement using radiography and concluded that the combinations of the two methods can be useful for thickness change evaluation. Castanier and Rausand (2006) used the classical PF interval model to optimize the preventive replacement policy of subsea oil pipelines. In this work we are using Compton gamma backscattering method with relatively simple underwater equipment for measuring wall thickness variation as a result of corrosion. Abdul-Majid and Tayyeb (2005) proposed a gamma backscattering technique for detecting thickness changes due to corrosion under insulation, and Abdul-Majid (1993) used it for localized and general corrosion and scale deposits. Dunn (2004), described a system for flaw detection in industrial testing by 2

3 radiation scattering. Iron thickness up to 25 mm was measured by Compton backscattering using a 5 Ci 60 Co radioactive source Asa d, et al., (1997). A lab experiment was performed that used 137 Cs with a NaI scintillation detector for determination of location of pipes buried in soil Sharma,et al., (2010). Harding and Harding (2010) describe a commercially available backscatter imaging device called ComScan. The device uses 160 kv maximum energy x-ray machine with a mean of about 60 kev. At these energies radiation does not penetrate enough into material like iron or copper that is usually found in petrochemical industry but can be useful for imaging low atomic number materials like aluminum. Work on backscatter imaging of landmines is given by (Tang and Hussein, 2004). Monte Carlo simulation of backscatter non-destructive testing is given recently by Shengli et al., (2000), that for medical imaging by Driol et al., (2008). 2. Properties of gamma ray scattering s for thickness measurements The difference between direct and back-scattered inspection is explained with reference to Fig.1. In direct inspection the radiation source is put on one side of the object while detector, film or imaging plate is put behind the object. In backscatter imaging both source and film or imaging plate are on the same side. This will be very useful for inspection or imaging large pipes or vessels or when accessibility is not available behind the object. The backscattered gamma ray of energy E after an incident gamma of energy E is given by: EEc E' = (1) E[ 1 cos )] E c Where E c is the rest mass of electron = MeV and is the scattering angle. The Compton scattered gamma ray energy is almost constant and independent of incident energy after about few kev. The differential scattering cross section of scattered gamma rays by Klien-Nishina formula: is given 3

4 (2) Where h /m 0 c 2 and is the classical electron radius. From equation (1) backscattered gamma energy from MeV primary radiation of 137 Cs, used in this work, is about 190 kev. From equation (2) the backscattered radiation intensity is approximately 20% of forward scattering. If a mono-directional beam of intensity I o is incident on a wall of thickness T, the count rate of 180 o backscattered rays R registered in a detector is given by (Abdul- Majid and Abulfaraj, 1988): Z R Z R N Io A A 1 exp[ ( ' T M ( ') )] N A Io A 1 exp[ ( m ' m T M ( ') )] m m (3) (4) Here is the pipe wall density, is the detector subtended solid angle, is the backscattering cross section per unit solid angle, A is the beam cross sectional area,, m, ' and m ' are the linear and mass attenuation coefficients of incident and scattered gamma rays, N A is Avogadro's number, Z is the atomic number, M is the mass number of the wall and is the detector intrinsic efficiency. At the corroded region of the pipe in which thickness T is reduced, counts is less compared to non-corroded region. According to the equations (3) and (4), counts increase sharply at small thickness T while saturation in counts is reached at a large value thickness controlled by {1-exp[-( + ) T]}. The saturation in counts is reached faster in higher density or high atomic number material. For a thick or a high density wall, a high energy source should be selected that has small value of ( + '), and vice versa. For MeV from 137 Cs, counts approaches saturation near a thickness of about 1.5 cm in iron. 3. Materials and Methods 4

5 The set-up is as shown in Fig. 2. Pipe used were carbon steel having three different diameters of approximately 4, 6 and 8 ; wall thicknesses varied between mm. The source was put on the base of cylindrical detector and both were put inside a sealed aluminum container whose wall thickness was 2 mm. The source, detector and container were in contact. Fig. 3 shows a photograph of detector. The detector cable passes through perforate in the container with plug around it to prevent water from leaking inside the container; up to 100 m long cable was tested. The detector was a 2" x 2" NaI(Tl) scintillation detector coupled to a built-in photomultiplier, preamplifier and amplifier. Counts data were stored in data storage in a PC computer which is programmed through LabVIEW to form the spectrum. In this setup the detector measure both direct and scattered radiation. The spectrum consists of two peaks, the primary and backscattered peaks. A sketch of the gamma spectrum on the computer is shown in Fig. 4; counts were taken only under the secondary peak. 4. Result Fig.5 shows counts for 10 s versus pipe wall thicknesses for 4 diameter pipes when the pipes were filled with water inside the water tank, empty inside the water tank and when empty in air. Fig. 6 & Fig. 7 show similar data for 6 pipes and for 8 pipes respectively. At lower thickness smaller count number were registered in consistency with equations (3) and (4) above. At these thicknesses values no saturation in counts were observed; saturation in counts is expected at about mm of wall thickness. In all cases counts of filled pipes in water gave highest value because of backscattering from water inside the pipe and water in the tank in addition to that from pipe wall. Less counts recorded when water filled pipes were outside the tank followed by empty pipes in air. It is also observed that at higher thickness of 11 and 13 mm (figures 6&7) filled pipes in air and water gave almost the same counts. This is expected as at higher thickness backscattered radiation from pipe wall is predominant and only insignificant backscattered radiation come from water. Also, the backscattered radiation from water is absorbed in the pipe wall before reaching the detector. Underwater pipes are usually coated by Fusion Bonded Epoxy (FBE), which is the first layer of defense against corrosion. This material has atomic number closer to that of rubber. We have wrapped the pipes with rubber about 3 mm thick and took 5

6 readings. Figure 8 shows backscattered counts from 6 diameter pipes without and with rubber. Counts with rubber are clearly less because the iron wall, which is the main cause of scattering are further away from detector, scattering from rubber is much less compared to that from iron and that some backscattered radiation from iron has a chance to escape away before reaching the detector. 5. Conclusion The main features for this technique include simplicity, fast, safe and inexpensive. For subsea pipe wall thickness measurements, divers were usually hired to do the inspection. The longer the time consumed and the more complicated the technique, the more expensive the inspection is. The device weighs about 2 kg easy to carry for subsea inspection by divers. No complicated equipment or high technical training is needed. Measurement at each point takes less than a minute which makes it easy to inspect many locations in a short time. The method is highly safe; source of this activity of magnitude is very close to exempted level from radiation monitoring. It is at least one million time less than radiography sources used in industrial radiography. The method can be used successfully for very hot or very cold surfaces of pipes and reservoirs or similar objects. Acknowledgments This work is part of project number: 8-OIL-123-3, sponsored by King Abdulaziz City for Science and Technology (KACST). The authors are grateful to KACST for the full financial support. Thanks go to Deanship of Scientific Research at King Abdulaziz University for their continuous cooperation and support. Thanks to Mr. Abdul Momen Khan for his work on experimental setup and data collection. References Abdul-Majid, S., Determination of scale thickness and general and localized metal removal in desalination plants by the gamma-ray interaction method. Desalination 91,

7 Abdul-Majid, S., Abulfaraj, W. H., Studies on the attenuation and backscattering of gamma-rays in a few types of polymers for thickness and density gauging. Arab. J. Sci. Eng. 13, Abdul-Majid, S., Tayyeb, Z., Use of gamma ray back scattering method for inspection of corrosion under insulation. 3 rd Middle East Nondestructive Testing Conference and Exhibition, Bahrain, Asa d, Z., Asghar, M., Imrie, D.C The measurement of the wall thickness of steel sections using Compton backscattering. Meas. Sci. Technol Castanier B. and Rausand M Maintenance optimization for subsea oil pipelines. International Journal of Pressure Vesselsand Piping 83, Chaves I.A. and Melchers R.E Pitting corrosion in pipelines steel welding zone. Corrosion Science 53, Correa S.C.A., Souza E.M., Oliveira D F., Silva A.X. Lopes R.T., Marinho C., Camerini C. S Assessment of weld thickness loss in offshore pipelines using computed radiography and computational modeling. Appl. Radiat. Isot. 67, Driol, C., Nguyen, M.K., Truong, T.T., Modeling and simulation results on high sensitivity scattered gamma-ray emission imaging. Simul. Model. Pract. Th. 16, Dunn, W.L., Flaw detection by x-ray using the rolling-window templatematching procedure. Appl. Radiat. Isot. 61, Harding, G., Harding, E., Compton scatter imaging: A tool for historical exploration. Appl. Radiat. Isot. 68, Hopkins P., The structural integrity of oil and gas transmission Pipelines. Comprehensive structural Integrity 1, International Atomic Energy Agency. Development of Protocols for Corrosion and Deposits Evaluation in in Pipes by Radiography. IAEA-TECDOC Kishway H. A. and Gabbar H. A., Review of pipeline integrity management practices. International Journal of Pressure Vessels and Piping, 87,

8 Sharma, A., Sandhu, B.S., Singh, A., Incoherent scattering of gamma photons for non-destructive tomographic inspection of pipes. Appl. Radiat. Isot. 68, Shengli, N., Jun, Z., Liuxing, H., EGS4 simulation of Compton scattering for nondestructive testing. Proceeding of the Second International Workshop on EGS, Tsukuba, Japan. Tang, S.S., Hussein, M.A., Use of isotopic gamma sources for identifying anti-personnel landmines. Appl. Radiat. Isot. 61,

9 X-Ray Machine or Ray Source Detector, film or image plate X or Rays Direct X-Ray Machine or Ray Source Back scattered X-ray Detector, film or image plate Back Scattered Fig. 1. Direct and backscatter inspection 9

10 Water Pipe 5 Ci Source Detector Sealed container Nuclear Electronics 100 m long cable Fig. 2. Experimental arrangement 10

11 Fig. 3. Detector and container Primary Backscatter Photon energy Fig. 4. Spectrum of direct and scattered gamma rays Region of Interest 11

12 Counts/10 s Counts/10 s Fig. 5. Counts vs thickness for 4" pipe Thickness (mm) Filled pipe in water Fig. 6. Counts vs thickness for 6" pipe Thickness (mm) Filled pipe in water 12

13 Counts/10 s Counts/10 s Fig. 7. Counts vs thickness for 8" pipe Thickness (mm) Filled pipe in water Fig. 8. Count vs thickness for 6" pipe with and without cover Thickness (mm) Filled pipe in water without cover Filled pipe in water with cover 13