- Email: [email protected]
Non Destructive Testing on LNG tanks using Laser Doppler Vibrometry
Abdelwahab Aroussi and Farid Benyahia (Editors), Proceedings of the 3rd International Gas Processing Symposium, March 5 - 7 2012 , Qatar. © 2012 Elsevier B.V. All rights reserved.
Non Destructive Testing on LNG tanks using Laser Doppler Vibrometry Abdelkrim. Chelghoum Professor and Director Of Research, Faculty of Civil Engineering, University Of Sciences and Technology Houari Boumedienne Bab Ezzouar, Algiers, Algeria Earthquake Engineering, Dynamics and Seismology, Laboratory, Rouiba, Algiers
Abstract In this study an approach for non destructive testing using a Laser Doppler Vibrometer (LDV) is presented. The LDV is an optical instrument using laser technology to measure velocity field of a generic point located on a vibrating structural element. From the recorded velocity data, local and global frequencies as well as the corresponding modes shapes of a moving structure are evaluated and therefore monitoring of its mass and stiffness can easily be carried out. Any change in the frequency value will affect both mass and stiffness of the structure and therefore its global integrity. In this research a real non destructive test has been carried out In-Situ on bridge’s piles and LNG tank located in the Districts of Corso and Arzew (Algeria). The validation of the obtained frequencies is done using results from numerical procedures such as finite element approach which clearly shows the accuracy LDV’s methodology as far as lateral movements is concerned. From this velocity data, frequency and defection of any part of the structure can easily be extracted to check material integrity. Keywords: Laser Doppler Vibrometer, Ambient vibration, Forced vibration, Local frequency, Global frequency, Local mode, Global mode
1. Introduction The global dynamic behavior of strategic life line structures such as; viaducts, nuclear power plants, dams, LNG and petrochemical facilities remains one of the largest source of concern because of the potential hazardous instability conditions induced by dynamic loadings (e. g. explosions, fire, ground fall and earthquakes). In the past decades, a range of experimental techniques based on non-destructive testing have been used to evaluate material anomalies. These techniques include X-rays, Gamma-rays, Infra-red thermography and acoustic approach. In this context, the LDV technique emerged as a quick, accurate and safe option for non-contact vibration measurements. Various developments of this method have been carried out over the last decade to analyze, at low frequency, structure’s integrity by monitoring and detecting material differences such as; cavities, cracks, density drops and manufacturing errors. In the present work two experiments have been conducted to obtain local and global frequencies of bridge’s pile and LNG tanks. After a brief description of the Doppler instrumentation, results from experimental tests are presented. Local and global frequencies are measured. The ability of LDV approach to detect correct deflection of structural elements is also demonstrated. The obtained results are compared to those from Finite Elements calculations.
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry 2. Set up of Laser Doppler Instrumentation The Laser Doppler Vibrometer (LDV) in an optical interferometer using a laser to evaluate the velocity of target point located on a moving surface. Frequency change induced by the vibrating point will be used to measure its velocity. Also, it gives the relative motion between the LDV and the target via a Doppler shift carried by the scattered return signal. Filters for integration and differentiation are used to convert the input signals, displacement, velocity and acceleration into each other. To minimize the time requested to collect suitable periods of dropout free data, each target site was given a small area and retro reflective treatment. Remark: The data presented in this work is not corrected for tripod motion. The (LDV) used for all phases of testing is a POLYTEC PDV 100, its specification are listed into table A:
Laser Type
He-Ne
Laser Class
II
Working Distance
0.2 - 30 (m)
Frequency Range
0 - 22k (Hz)
Resolution
0.05 µm/s
Table A. Specifications of PDV-100 LDV A 5 kg demolition hammer with hard plastic tip is used for all impact sources. Signals are recorded with a 24 bit- 4 channels dynamic signals analyzer powered by the laptop data acquisition computer’s USB port. In the present work, the first experiment was used to find frequency and defection of the bridge, whereas the second test was conducted to find the global frequency of LNG tank.
3. Process of The Experimentation The LDV used for all phases of testing is a Polytec PDV-100; specifications are listed in Table A. For field study, the LDV is placed on the top of the tripod as it could accurately measure the velocities. 3.1. Experiment 1: Pile Test This first test began by positioning the laser instrument approximately at one (1) meter from pile 1. The laser output should be directly perpendicular to the face of the pile. To ensure a strong signal return to the LDV from the pile, reflexion tape is attached on its face. The LDV was then adjusted at a horizontal level to the reflection tape. Measurements started with ten minutes of ambient vibration, followed by 1 minute of forced vibration. Ambient vibration is classified for this experiment as vibrations induced by wind forces or cars passing on nearby roads, excluding those produced by the cars passing over it. Forced vibration is classified for this experiment as a vibration caused by a demolition hammer striking the pile. The hammer was approximately hit against the pile once every ten seconds. The process was repeated on pile (2) and (3) along the bridge. An outline of the set-up is given in Figure (2) and (3).
223
Chelghoum
224
Fig.1 LDV Instrumentation: Head and chain
Fig.2. Transversal view of Corso Bridge (Boumerdes)
Fig.3. Longitudinal view of Corso Bridge
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry
225
Fig.4. Overall View of Corso Bridge
3.2. Experiment 2: LNG TANK The analysis of LNG tanks in seismic zones using experimental techniques is critical to the energy sector. The analysis is very reliable and attractive for the validation of analytical and numerical models. The main purpose of this approach is the identification of major dynamic parameters which can be used for estimating and predicting tank behavior when subjected to severe loading conditions such as earthquake, blast or drastic wind pressure. Within this context, the evaluation of the buckling critical load through the determination of the resonance frequency and corresponding mode shape appears to be more appropriate for the study of the dynamic response of LNG tanks. In this study a cylindrical LNG shell tank having the geometrical properties shown in figure (5) is chosen for the purpose of the experiment. The selected finite element mesh is Also represented.
R 5m
h 10m
Fig.5. LNG Tank Arzew (Algeria) Finite element Grid
Chelghoum
226 Frequency (1) = 5.69 Hz
Fig.6. LNG Tank Arzew (Algeria) Finite element results: First Mode Frequency (2) = 11.695 Hz
Fig.7. LNG Tank Arzew (Algeria) Finite element results: Second Mode
Fig.8. LDV Experiment Set Up
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry
227
4. Results 4.1. Bridge test The output from the first test gives the response spectra of both ambient as well as forced vibration of the bridge pile. Natural frequencies are calculated from the spectral densities. Figure (9) is an output from experiment 1 identifying the two vibrations being analyzed, ambient and forced, they appear to be noticeably different. Typical response spectras of ambient and forced vibration are presented in Figures (10) and (11). Ambient and forced vibrations were used to evaluate the frequency characteristics from power spectral densities. The global frequencies calculated from the areas of ambient vibration are shown in table B. The calculated local frequencies are presented in Table C. Figures (12) and (13) Shows displacement and acceleration history of node (4) on the bridge slab.
Fig.9. Output Result for Ambient and Forced Vibration
Fig.10.Ambient Vibration Spectra
Fig.11. Forced Vibration Spectra
228
Chelghoum
Fig.12. Recorded Displacement and Acceleration of Node (4) On The Bridge Slab
Fig.13. Recorded Displacement and Acceleration Of Node (4) On The Bridge Slab
Table B: Global Frequencies
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry
229
4. 2. LNG tank Figure (14) shows an example of the reccorded data due to ambient vibration.Using spectral estimation technique only the first global frequency has been determined from this data in the present work. The obtained results allow comparison with the F. E model.
Fig.14. Ambient Vibration Spectra LNG Tank
Mode First mode Second mode
LDV 4,20 xxxx
E. F. M 5,69 11,695
Table D: natural frequencies for LNG tank.
5. Conclusion From the results of the present work, it can be concluded that LDV approach is very efficient for measuring natural frequencies from ambient and forced vibrations spectra for the bridge test. As far as the LNG experiment is concerned, only the first frequency has been measured by the LDV. A noticeable difference between the two approaches is appearing because mainly of the set up of the experiment, the noise background and other noise sources as well as the relative vibration of the instrument head support. Moreover, the present technique allows the determination of displacement, velocity and acceleration histories of any part of a moving structure. This represent the most important parameters values for any structural analysis. In this context the LDV option appears to be the natural path to follow for future NDT development to improve structure integrity.
Acknowledgements I would like to thank the faculty of civil engineering ( USTHB, Algiers) and th GPDS Laboratory , Rouiba for funding this research experiment.
230
Chelghoum
References (1)
(2)
(3)
(4) (5) (6)
(7) (8)
(9) (10)
(11)
(12)
(13)
Chelghoum, A., Elnashai, A.S., and Dowling, P.J., “An efficient Non-Linear analysis of groundsupported liquid-filled shells subjected to Dynamic loading”. 3rd Arab Conference for Structural Engineering, 5 – 8 March, 1989 Dubai, UNITED ARAB EMIRATES. Chelghoum,A. Elnashai, A.S., and Dowling, P.J, “Non-linear Seismic analysis of thin shells including interaction effects” Proceedings of the 4th international colloquium on structural stability 17-19 April 1989, New York USA. Chelghoum,A., Dowling, P.J., “An Updated Lagrangian Finite element Approach to Non-linear Fluid Structure Interaction Problems”. 6th SAS World Conference, FEMCAD-89, 25-28 October 1989. Paris, FRANCE. CHELGHOUM, A., “USER Manual for Finasic program” Imperial College of Science Technology and Medicine, Dept of Civil Engineering, January 1991, 300 pages. Chelghoum, A., « Mise aux normes parasismiques des Ouvrages via la simulation numérique ». Colloque International sur les Risques majeurs Sheraton (Alger) 15, 16 Mars 2004. Chelghoum, A., « Concept parasismique des bâtiments : état de l’Art » Atelier international sur l’amélioration de la sécurité des bâtiments lors d’un tremblement de terre dans la région du Maghreb (Algérie OTAN) Alger Hôtel El Aurassi 22, 23 et 24 Mai 2005. Chelghoum, A., « Mega-simulation des effets induits d'un séisme majeur sur la ville d'Alger et ses agglomérations », Conseil de la Nation 2006. Djermane, M. ,Chelghoum, A. , Amieur, B. and Labbaci, B. ,* Linear and Nonlinear Thin Shell Analysis Using A Mixed Finite Element With Drilling Degrees of Freedom. “International Journal of Applied Engineering Research ISSN 0973-4562 Volume 1 N°2 (2006) pp.217-236 © Research India Publications”. Djemane,M., Chelghoum,A., Tab,B., 2000, « Analyse non linéaire des coques minces par la méthode des elements finis utilisation de champs de substitution » Symposiom International sur les Hydrocarbures et le chimie Boumerdes Algeria, Juin2007. Djermane, M., Chelghoum, A. , and Amieur , B. , “Nonlinear Dynamic Analysis of Thin Shells Using Finite Element With Drilling Degrees of Freedom. “International Journal of Applied Engineering Research, 2007, pp.111-124. Swanson, P., Feasibility of using laser-based vibration measurements to detect roof fall hazards in underground mines. Fifth Inter. Conf. on Vibration Measurements by Laser Techniques: Advances and Applications, Proceedings. SPIE, ed. by E. P. Tomasini, Vol. 4827, pp. 541-552, 2002. Davis, A.G., and Peterson, C.G., Nondestructive evaluation of prestressed concrete bridges using impulse response. Proceedings,. Inter. Symp. On Non-Destructive Testing in Civil Engineering (NDT-CE 2003) (Berlin, Sept. 16-19, 2003). Deutsche Gesellschaft Für Zerstörungsfreie Prüfung E.V., Vol. 013, 2003. Erfurt, W., Stark, J. , and Köhler, W., The application of laser techniques in the non-contacting excitation and recording of sound waves. Proceedings, Inter. Symp. Non-Destructive Testing in Civil Engineering (NDT-CE 2003) (Berlin, Sept. 16-19, 2003). Deutsche Gesellschaft Für Zerstörungsfreie Prüfung E.V., Vol. 014, 2003.
Non Destructive Testing on LNG tanks using Laser Doppler Vibrometry Abdelkrim. Chelghoum Professor and Director Of Research, Faculty of Civil Engineering, University Of Sciences and Technology Houari Boumedienne Bab Ezzouar, Algiers, Algeria Earthquake Engineering, Dynamics and Seismology, Laboratory, Rouiba, Algiers
Abstract In this study an approach for non destructive testing using a Laser Doppler Vibrometer (LDV) is presented. The LDV is an optical instrument using laser technology to measure velocity field of a generic point located on a vibrating structural element. From the recorded velocity data, local and global frequencies as well as the corresponding modes shapes of a moving structure are evaluated and therefore monitoring of its mass and stiffness can easily be carried out. Any change in the frequency value will affect both mass and stiffness of the structure and therefore its global integrity. In this research a real non destructive test has been carried out In-Situ on bridge’s piles and LNG tank located in the Districts of Corso and Arzew (Algeria). The validation of the obtained frequencies is done using results from numerical procedures such as finite element approach which clearly shows the accuracy LDV’s methodology as far as lateral movements is concerned. From this velocity data, frequency and defection of any part of the structure can easily be extracted to check material integrity. Keywords: Laser Doppler Vibrometer, Ambient vibration, Forced vibration, Local frequency, Global frequency, Local mode, Global mode
1. Introduction The global dynamic behavior of strategic life line structures such as; viaducts, nuclear power plants, dams, LNG and petrochemical facilities remains one of the largest source of concern because of the potential hazardous instability conditions induced by dynamic loadings (e. g. explosions, fire, ground fall and earthquakes). In the past decades, a range of experimental techniques based on non-destructive testing have been used to evaluate material anomalies. These techniques include X-rays, Gamma-rays, Infra-red thermography and acoustic approach. In this context, the LDV technique emerged as a quick, accurate and safe option for non-contact vibration measurements. Various developments of this method have been carried out over the last decade to analyze, at low frequency, structure’s integrity by monitoring and detecting material differences such as; cavities, cracks, density drops and manufacturing errors. In the present work two experiments have been conducted to obtain local and global frequencies of bridge’s pile and LNG tanks. After a brief description of the Doppler instrumentation, results from experimental tests are presented. Local and global frequencies are measured. The ability of LDV approach to detect correct deflection of structural elements is also demonstrated. The obtained results are compared to those from Finite Elements calculations.
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry 2. Set up of Laser Doppler Instrumentation The Laser Doppler Vibrometer (LDV) in an optical interferometer using a laser to evaluate the velocity of target point located on a moving surface. Frequency change induced by the vibrating point will be used to measure its velocity. Also, it gives the relative motion between the LDV and the target via a Doppler shift carried by the scattered return signal. Filters for integration and differentiation are used to convert the input signals, displacement, velocity and acceleration into each other. To minimize the time requested to collect suitable periods of dropout free data, each target site was given a small area and retro reflective treatment. Remark: The data presented in this work is not corrected for tripod motion. The (LDV) used for all phases of testing is a POLYTEC PDV 100, its specification are listed into table A:
Laser Type
He-Ne
Laser Class
II
Working Distance
0.2 - 30 (m)
Frequency Range
0 - 22k (Hz)
Resolution
0.05 µm/s
Table A. Specifications of PDV-100 LDV A 5 kg demolition hammer with hard plastic tip is used for all impact sources. Signals are recorded with a 24 bit- 4 channels dynamic signals analyzer powered by the laptop data acquisition computer’s USB port. In the present work, the first experiment was used to find frequency and defection of the bridge, whereas the second test was conducted to find the global frequency of LNG tank.
3. Process of The Experimentation The LDV used for all phases of testing is a Polytec PDV-100; specifications are listed in Table A. For field study, the LDV is placed on the top of the tripod as it could accurately measure the velocities. 3.1. Experiment 1: Pile Test This first test began by positioning the laser instrument approximately at one (1) meter from pile 1. The laser output should be directly perpendicular to the face of the pile. To ensure a strong signal return to the LDV from the pile, reflexion tape is attached on its face. The LDV was then adjusted at a horizontal level to the reflection tape. Measurements started with ten minutes of ambient vibration, followed by 1 minute of forced vibration. Ambient vibration is classified for this experiment as vibrations induced by wind forces or cars passing on nearby roads, excluding those produced by the cars passing over it. Forced vibration is classified for this experiment as a vibration caused by a demolition hammer striking the pile. The hammer was approximately hit against the pile once every ten seconds. The process was repeated on pile (2) and (3) along the bridge. An outline of the set-up is given in Figure (2) and (3).
223
Chelghoum
224
Fig.1 LDV Instrumentation: Head and chain
Fig.2. Transversal view of Corso Bridge (Boumerdes)
Fig.3. Longitudinal view of Corso Bridge
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry
225
Fig.4. Overall View of Corso Bridge
3.2. Experiment 2: LNG TANK The analysis of LNG tanks in seismic zones using experimental techniques is critical to the energy sector. The analysis is very reliable and attractive for the validation of analytical and numerical models. The main purpose of this approach is the identification of major dynamic parameters which can be used for estimating and predicting tank behavior when subjected to severe loading conditions such as earthquake, blast or drastic wind pressure. Within this context, the evaluation of the buckling critical load through the determination of the resonance frequency and corresponding mode shape appears to be more appropriate for the study of the dynamic response of LNG tanks. In this study a cylindrical LNG shell tank having the geometrical properties shown in figure (5) is chosen for the purpose of the experiment. The selected finite element mesh is Also represented.
R 5m
h 10m
Fig.5. LNG Tank Arzew (Algeria) Finite element Grid
Chelghoum
226 Frequency (1) = 5.69 Hz
Fig.6. LNG Tank Arzew (Algeria) Finite element results: First Mode Frequency (2) = 11.695 Hz
Fig.7. LNG Tank Arzew (Algeria) Finite element results: Second Mode
Fig.8. LDV Experiment Set Up
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry
227
4. Results 4.1. Bridge test The output from the first test gives the response spectra of both ambient as well as forced vibration of the bridge pile. Natural frequencies are calculated from the spectral densities. Figure (9) is an output from experiment 1 identifying the two vibrations being analyzed, ambient and forced, they appear to be noticeably different. Typical response spectras of ambient and forced vibration are presented in Figures (10) and (11). Ambient and forced vibrations were used to evaluate the frequency characteristics from power spectral densities. The global frequencies calculated from the areas of ambient vibration are shown in table B. The calculated local frequencies are presented in Table C. Figures (12) and (13) Shows displacement and acceleration history of node (4) on the bridge slab.
Fig.9. Output Result for Ambient and Forced Vibration
Fig.10.Ambient Vibration Spectra
Fig.11. Forced Vibration Spectra
228
Chelghoum
Fig.12. Recorded Displacement and Acceleration of Node (4) On The Bridge Slab
Fig.13. Recorded Displacement and Acceleration Of Node (4) On The Bridge Slab
Table B: Global Frequencies
Non Destrcutive Testing on LNG Tanks using laser Doppler Vibrometry
229
4. 2. LNG tank Figure (14) shows an example of the reccorded data due to ambient vibration.Using spectral estimation technique only the first global frequency has been determined from this data in the present work. The obtained results allow comparison with the F. E model.
Fig.14. Ambient Vibration Spectra LNG Tank
Mode First mode Second mode
LDV 4,20 xxxx
E. F. M 5,69 11,695
Table D: natural frequencies for LNG tank.
5. Conclusion From the results of the present work, it can be concluded that LDV approach is very efficient for measuring natural frequencies from ambient and forced vibrations spectra for the bridge test. As far as the LNG experiment is concerned, only the first frequency has been measured by the LDV. A noticeable difference between the two approaches is appearing because mainly of the set up of the experiment, the noise background and other noise sources as well as the relative vibration of the instrument head support. Moreover, the present technique allows the determination of displacement, velocity and acceleration histories of any part of a moving structure. This represent the most important parameters values for any structural analysis. In this context the LDV option appears to be the natural path to follow for future NDT development to improve structure integrity.
Acknowledgements I would like to thank the faculty of civil engineering ( USTHB, Algiers) and th GPDS Laboratory , Rouiba for funding this research experiment.
230
Chelghoum
References (1)
(2)
(3)
(4) (5) (6)
(7) (8)
(9) (10)
(11)
(12)
(13)
Chelghoum, A., Elnashai, A.S., and Dowling, P.J., “An efficient Non-Linear analysis of groundsupported liquid-filled shells subjected to Dynamic loading”. 3rd Arab Conference for Structural Engineering, 5 – 8 March, 1989 Dubai, UNITED ARAB EMIRATES. Chelghoum,A. Elnashai, A.S., and Dowling, P.J, “Non-linear Seismic analysis of thin shells including interaction effects” Proceedings of the 4th international colloquium on structural stability 17-19 April 1989, New York USA. Chelghoum,A., Dowling, P.J., “An Updated Lagrangian Finite element Approach to Non-linear Fluid Structure Interaction Problems”. 6th SAS World Conference, FEMCAD-89, 25-28 October 1989. Paris, FRANCE. CHELGHOUM, A., “USER Manual for Finasic program” Imperial College of Science Technology and Medicine, Dept of Civil Engineering, January 1991, 300 pages. Chelghoum, A., « Mise aux normes parasismiques des Ouvrages via la simulation numérique ». Colloque International sur les Risques majeurs Sheraton (Alger) 15, 16 Mars 2004. Chelghoum, A., « Concept parasismique des bâtiments : état de l’Art » Atelier international sur l’amélioration de la sécurité des bâtiments lors d’un tremblement de terre dans la région du Maghreb (Algérie OTAN) Alger Hôtel El Aurassi 22, 23 et 24 Mai 2005. Chelghoum, A., « Mega-simulation des effets induits d'un séisme majeur sur la ville d'Alger et ses agglomérations », Conseil de la Nation 2006. Djermane, M. ,Chelghoum, A. , Amieur, B. and Labbaci, B. ,* Linear and Nonlinear Thin Shell Analysis Using A Mixed Finite Element With Drilling Degrees of Freedom. “International Journal of Applied Engineering Research ISSN 0973-4562 Volume 1 N°2 (2006) pp.217-236 © Research India Publications”. Djemane,M., Chelghoum,A., Tab,B., 2000, « Analyse non linéaire des coques minces par la méthode des elements finis utilisation de champs de substitution » Symposiom International sur les Hydrocarbures et le chimie Boumerdes Algeria, Juin2007. Djermane, M., Chelghoum, A. , and Amieur , B. , “Nonlinear Dynamic Analysis of Thin Shells Using Finite Element With Drilling Degrees of Freedom. “International Journal of Applied Engineering Research, 2007, pp.111-124. Swanson, P., Feasibility of using laser-based vibration measurements to detect roof fall hazards in underground mines. Fifth Inter. Conf. on Vibration Measurements by Laser Techniques: Advances and Applications, Proceedings. SPIE, ed. by E. P. Tomasini, Vol. 4827, pp. 541-552, 2002. Davis, A.G., and Peterson, C.G., Nondestructive evaluation of prestressed concrete bridges using impulse response. Proceedings,. Inter. Symp. On Non-Destructive Testing in Civil Engineering (NDT-CE 2003) (Berlin, Sept. 16-19, 2003). Deutsche Gesellschaft Für Zerstörungsfreie Prüfung E.V., Vol. 013, 2003. Erfurt, W., Stark, J. , and Köhler, W., The application of laser techniques in the non-contacting excitation and recording of sound waves. Proceedings, Inter. Symp. Non-Destructive Testing in Civil Engineering (NDT-CE 2003) (Berlin, Sept. 16-19, 2003). Deutsche Gesellschaft Für Zerstörungsfreie Prüfung E.V., Vol. 014, 2003.