A Multipoint Fibre Optic Corrosion Sensor We have recently presented for the first time the concept and first experimental results of a fibre-optic-based corrosion sensor using the opti
Trang 24 Application in robotics
The number of application domains of robotic systems is rapidly growing and in particular
the service robotics is becoming the most popular In such application field a high degree of
autonomy is required for the robot and thus a large number of exteroceptive sensors appear
necessary When multifingered robotic hands are considered, the requirement of minimally
invasiveness for the sensory system is of major importance due to the limited space
available in a mechanical structure with several degrees of freedom In a robotic hand,
different exteroceptive sensors are required to ensure stable grasping and manipulation of
objects Among these, sensing of both contact point and contact force appears mandatory for
any control algorithm which intends to achieve such goals Even though many different
technologies have been explored and tested to build tactile sensors, like piezo-resistive (Liu
et al., 1993), capacitive (Morimura et al., 2000), piezoelectric (Krishna & Rajanna, 2002),
magneto-resistive (Tanie, 1986), optoelectronic approaches demonstrated their potential
since the beginning of tactile sensors development (Maekawa et al., 1993) Also, on the
market optoelectronic tactile sensors can be found that measure distributed tactile
information, but such tactile information is generally limited to pressure force, and spatial
resolution is coarse, a few millimetres order Generally, a commercial sensor accurately
responds to a load of 0.25 N or more up to 2 N, but such a range can be too narrow for
manipulation tasks More recently, a number of different optical approaches have been
pursued, among which the solution based on an LEDs matrix has been presented in
(Rossiter & Mukai, 2005) and the solution based on a CCD camera is reported in (Ohka et al.,
2006)
Among optical approaches, those based on the use of optical fibres appear particularly
suitable for pressure sensing, thanks to the low size and minimum invasivity of fibres
themselves Since the advent of fibre optics, it has been recognized that optical fibres can be
used as effective pressure (and tactile) sensors One of the earliest demonstrations of such a
capability relied on the pressure-induced displacement of a diaphragm placed close to the
tip of an optical fibre (Cook & Hamm, 1979) The fibre was operated in reflection mode, so
that changes in reflected intensity can be used as a measure of the pressure applied on the
diaphragm In case of tactile sensing, such an approach presents the disadvantage of
requiring a complex micromachining at the tip of the fibre Another possible approach is
based on the intensity loss resulting from pressure-induced bending of the fibre (Fields et
al., 1980) However, in this case the response of the sensor is highly nonlinear due to the
exponential dependence of the bending loss on the radius of curvature of the fibre More
complex examples can be found based on interferometric approaches, where the changes in
the optical phase are used as transducer mechanism to sense the pressure (Saran et al., 2006,
Wang et al., 2001, Yuan et al., 2005) Interferometric sensors exhibit high sensitivity, but also
present some disadvantages, such as low tolerance to external disturbances, and periodicity
in their response
Fig 13 Sketch of the single sensing element (taxel)
Recently, we proposed a solution based on the scattering of the light illuminating the surface of urethane foam (De Maria et al., 2008) The configuration makes use of a couple of emitter/receiver fibres placed at the edge of a micromachined well covered by the foam The distance between the two fibres can be chosen in order to ensure a desired sensitivity of the sensing element As a demonstration of the effectiveness of the proposed configuration,
we present the results of two sensors, in which the relative distance between the two fibres was properly selected in order to fit the range of pressures to be detected Top and lateral schematic views of a single taxel are shown in Fig 13 The sensor works as follows: the light emitted by the illuminating fibre is scattered by the internal surface of the urethane foam and a fraction of its power is collected by the receiving fibre, depending on the applied pressure In particular, when one applies a pressure on the external surface of the urethane foam, the distance between the tip of the collecting fibre and the internal surface of the foam
is reduced, and this will result in an increased fraction of power collected by the receiving fibre The use of a scattering surface, such as that of the urethane foam employed for the realization of the prototypes, is justified by the fact the multiple scattering permits to smooth (average-out) local variation of light intensity within the cavity, and thus reduce the sensitivity of the collected power on micro-displacements of the illuminating and/or receiving fibre As the power collected by the receiving fibre is a function of the pressure applied on the foam surface, it can be used as a measure of the applied force Obviously, the collected light is also a function of the relative distance between the illuminating and the receiving fibres In our experiments, such a distance was kept constant and was not a function of the applied force However, we can exploit such dependence, by choosing an opportune distance giving rise to a desired sensitivity of the sensor on the applied pressure Generally speaking, a smaller distance will result in a higher sensitivity, so that smaller pressures can be measured
On the other hand, a higher sensitivity implies a reduced dynamic range, i.e the sensor response will saturate at lower pressure levels Hence, a trade-off must be found between sensitivity and dynamic range
One advantage of the proposed technique is that it can be easily extended to a number of taxels, so as to acquire a pressure distribution Figure 14 shows a possible configuration of a matrix of taxels to realize a complete tactile sensor able to detect both contact point and contact force applied on a finite area
Two different taxels have been produced with the same well and two different distances between the emitting/receiving fibres, i.e 10 m and 200 m The micromachined well size
is 5x5 mm2 The optical source was a superluminescent LED operating at a central wavelength of 1550nm, and having an output optical power of 3mW The output pigtail of
Illuminating
Receiving
Scattering
Urethane Applied force
Scattering
Trang 3the source was connected to the illuminating fibre, whereas the receiving fibre was
connected to an InGaAs photodiode, whose output signal was fed to an oscilloscope having
an input impedance of 1 M Both illuminating and receiving fibres were SMF-28,
single-mode optical fibres The two prototypes have been calibrated with a load cell mounted as
shown in Fig 15 The results corresponding to the calibration of the first prototype are
reported in Fig 16 (left), where the output voltage, proportional to the optical power
collected by the receiving fibre, is plotted against the load applied to the sensor As
expected, the sensitivity is very high but with a limited dynamic range Moreover, to test the
repeatability of the measurements, different sets of measurements have been collected and
two of them are reported in the figure
Fig 14 Schematic diagram of a 10-taxel tactile sensor
Fig 15 Experimental set-up for the fibre-optics based taxel
The second prototype, as expected, had a lower sensitivity but wider dynamic range, as
shown by the calibration curve of Fig 16 (right) In both cases, the sensitivity is certainly
better than the typical values of commercial optical tactile sensors
Load cell for calibration Illuminating fibre Receiving fibre
Tactile sensor
Illuminating fibre Receiving fibres
5 Conclusions
In this chapter, a number of experimental demonstrations on the use of the optical fibre sensor technology have been reported It has been shown that different application fields can take advantage of the peculiar characteristics of optical fibre sensors In particular, distributed fibre sensors have great potentiality in the field of structural health monitoring,
as they permit to perform continuous measurements of the quantity of interest On the other hand, fibre Bragg grating technology offers high sensitivity and accuracy, and in general it benefits from the immunity to electromagnetic interference, in common with other fibre-optic sensors Finally, the small size and minimally invasiveness of optical fibres have been demonstrated to be useful in robotic applications, where the use of fibre-optics may lead to efficient exteroceptive sensing systems
Fig 16 Calibration curves of the first prototype with 10 m distance between the fibres (left) and of the second prototype with 200 m distance between the fibres (right)
6 References
Agrawal, G.P (2001) Nonlinear Fibre Optics Academic Press, San Diego
Barnoski, J.K & Jensen, S.M (1976) Fibre waveguides: A novel technique for investigating
attenuation characteristics Appl Opt Vol 15, No 9, 2112-2115
Bernini, R.; Crocco, L.; Minardo, A.; Soldovieri, F & Zeni, L (2002) Frequency-domain
approach to distributed fibre-optic Brillouin sensing Opt Lett Vol 27, No 5,
288-290
Bernini, R.; Fraldi, M.; Minardo, A.; Minatolo, V.; Carannante, F.; Nunziante, L & Zeni, L
(2006a) Identification of defects and strain error estimation for bending steel beams
using time domain Brillouin distributed fibre sensors Smart Materials and Structures
Vol 2, 612-622
Bernini, R.; Minardo, A & Zeni, L (2006b) An accurate high resolution technique for
distributed sensing based on frequency domain Brillouin scattering IEEE Photonics
Technology Letters Vol 18, No 1, 280-282
140 160 180 200 220 240 260 280 300 0
50 100 150 200 250 300 350
Trang 4the source was connected to the illuminating fibre, whereas the receiving fibre was
connected to an InGaAs photodiode, whose output signal was fed to an oscilloscope having
an input impedance of 1 M Both illuminating and receiving fibres were SMF-28,
single-mode optical fibres The two prototypes have been calibrated with a load cell mounted as
shown in Fig 15 The results corresponding to the calibration of the first prototype are
reported in Fig 16 (left), where the output voltage, proportional to the optical power
collected by the receiving fibre, is plotted against the load applied to the sensor As
expected, the sensitivity is very high but with a limited dynamic range Moreover, to test the
repeatability of the measurements, different sets of measurements have been collected and
two of them are reported in the figure
Fig 14 Schematic diagram of a 10-taxel tactile sensor
Fig 15 Experimental set-up for the fibre-optics based taxel
The second prototype, as expected, had a lower sensitivity but wider dynamic range, as
shown by the calibration curve of Fig 16 (right) In both cases, the sensitivity is certainly
better than the typical values of commercial optical tactile sensors
Load cell for calibration Illuminating fibre Receiving fibre
Tactile sensor
Illuminating fibre Receiving fibres
5 Conclusions
In this chapter, a number of experimental demonstrations on the use of the optical fibre sensor technology have been reported It has been shown that different application fields can take advantage of the peculiar characteristics of optical fibre sensors In particular, distributed fibre sensors have great potentiality in the field of structural health monitoring,
as they permit to perform continuous measurements of the quantity of interest On the other hand, fibre Bragg grating technology offers high sensitivity and accuracy, and in general it benefits from the immunity to electromagnetic interference, in common with other fibre-optic sensors Finally, the small size and minimally invasiveness of optical fibres have been demonstrated to be useful in robotic applications, where the use of fibre-optics may lead to efficient exteroceptive sensing systems
Fig 16 Calibration curves of the first prototype with 10 m distance between the fibres (left) and of the second prototype with 200 m distance between the fibres (right)
6 References
Agrawal, G.P (2001) Nonlinear Fibre Optics Academic Press, San Diego
Barnoski, J.K & Jensen, S.M (1976) Fibre waveguides: A novel technique for investigating
attenuation characteristics Appl Opt Vol 15, No 9, 2112-2115
Bernini, R.; Crocco, L.; Minardo, A.; Soldovieri, F & Zeni, L (2002) Frequency-domain
approach to distributed fibre-optic Brillouin sensing Opt Lett Vol 27, No 5,
288-290
Bernini, R.; Fraldi, M.; Minardo, A.; Minatolo, V.; Carannante, F.; Nunziante, L & Zeni, L
(2006a) Identification of defects and strain error estimation for bending steel beams
using time domain Brillouin distributed fibre sensors Smart Materials and Structures
Vol 2, 612-622
Bernini, R.; Minardo, A & Zeni, L (2006b) An accurate high resolution technique for
distributed sensing based on frequency domain Brillouin scattering IEEE Photonics
Technology Letters Vol 18, No 1, 280-282
140 160 180 200 220 240 260 280 300 0
50 100 150 200 250 300 350
Trang 5Bernini, R.; Minardo, A.& Zeni, L (2008) Vectorial dislocation monitoring of pipelines by
use of Brillouin-based fibre-optics sensors Smart Materials and Structures Vol 17,
015006
Cavallo, A.; May, C.; Minardo, A.; Natale, C.; Pagliarulo, P & Pirozzi, P (2009) Modelling
and control of a smart auxiliary mass damper equipped with a Bragg grating for
active vibration control, Sensors and Actuators A, in press
Cook, R.O & Hamm, C.W (1979) Fibre optic lever displacement transducer Appl Opt
Vol 18, No 19, 3230-3241
Culshaw, B & Dakin, J 1997 Optical Fibre sensors Vol 4 Artech House Publishers,
0890069409
De Maria, G.; Minardo, A.; Natale, C.; Pirozzi, S & Zeni, L (2008) Optoelectronic Tactile
Sensor Based on Micromachined Scattering Wells FIRST MEDITERRANEAN
PHOTONICS CONFERENCE, European Optical Society Topical Meeting, 25–28
June 2008, Ischia, Italy
Fields, J.N.; Asawa, C.K.; Ramer, O.G & Barnowski, M.K (1980) Fibre Optic Pressure
Sensor J Acoust Soc Am., Vol 67, 816-818
Garus, D.; Krebber, K.; Schliep, F & Gogolla, T (1996) Distributed sensing technique based
on Brillouin optical-fibre frequency-domain analysis Opt Lett., Vol 21, No 17,
1402-1404
Kersey, A D.; Davis, M A.; Patrick, H J.; LeBlanc, M.; Poo, K P.; Askins, A.G.; Putnam, M
A & Friebele, E J (1997) Fibre grating sensors, Journal of Lightw Technol., vol 15,
no 8, pp 1442-1462
Krishna, G.M & Rajanna, K (2002) Tactile sensor based on piezoelectric resonance Proc of
2002 IEEE Conference on Sensor, pp 1643- 1647
Liu, L.; Zheng, X & Li, Z (1993) An array tactile sensor with piezoresistive with single
crystal silicon diaphragm Sensors and Actuators-A32, 193-196
Maekawa, H.; Tanie, K & Komoriya, K (1993) A finger-shaped tactile sensor using an
optical waveguide Proc of 1993 IEEE International Conference on Systems, Man and
Cybernetics, pp 403-408
Measures, R.M (2002) Structural monitoring with fibre optic technology Academic press, San
Diego
Morimura, H.; Shigematsu, S & Machinda, K (2000) A novel sensor cell architecture and
sensing circuit scheme for capacitive fingerprint sensors IEEE Journal of Solid State
Circuits, Vol, 35, 724-731
May, C.; Pagliarulo, P & Janocha, H (2006) Optimisation of a magnetostrictive auxiliary
mass damper Proc 10th International Conference on New Actuators ACTUATOR2006,
Bremen, Germany, pp 344–348
Nikles, M.; Thevenaz, L & Robert, P.A 1997 Brillouin gain spectrum characterization in
single-mode optical fibres J Lightw Technol., Vol 15, No 10, 1842 – 1851
Ohka, M.; Kobayashi, H.; Takata, J & Mitsuya, Y (2006) Sensing Precision of an Optical
Three-axis Tactile Sensor for a Robotic Finger Proc Of the 15th IEEE International
Symposium on Robot and Human Interactive Communication, pp 214-219
Rossiter, J & Mukai, T (2005) A novel tactile sensor using a matrix of LEDs operating in
both photoemitter and photodetector modes Proc of 2005 IEEE Conference on
Sensor, pp 994-997
Saran, A.; Abeysinghe, D.C & Boyd, J.T (2006 Microelectromechanical system pressure
sensor integrated onto optical fibre by anodic bonding Appl Opt., Vol 45,
1737-1742
Tanie, K (1986) Advances in tactile sensors for robotics Proc of the 6th Sensor Symposium
Japan, pp 63-68
Udd E (2002) Overview of fibre optic sensors, In: Fibre Optic Sensors Francis T S Yu;
Shizhuo Yin, pp 1-40, Routledge, 978-0-203-90946-1, USA
Wang, A.; Xiao, H.; Wang, J.; Wang, Z.; Zhao, W & May, R.G (2001) Self-calibrated
interferometric-based-optical fibre sensors J Lightw Technol., Vol 19, No 10,
1495-1501
Yuan, S.; Ansari, F.; Liu, X & Zhao, Y (2005) Optical fibre based dynamic pressure sensor
for WIM sensor Sens and Actuat A, Vol 120, No 1, 53-58
Zhao, Y & Liao, Y (2004) “Discrimination methods and demodulation techniques for fibre
Bragg grating sensors”, Opt Lasers Eng., vol 41, pp 1-18
Trang 6Bernini, R.; Minardo, A.& Zeni, L (2008) Vectorial dislocation monitoring of pipelines by
use of Brillouin-based fibre-optics sensors Smart Materials and Structures Vol 17,
015006
Cavallo, A.; May, C.; Minardo, A.; Natale, C.; Pagliarulo, P & Pirozzi, P (2009) Modelling
and control of a smart auxiliary mass damper equipped with a Bragg grating for
active vibration control, Sensors and Actuators A, in press
Cook, R.O & Hamm, C.W (1979) Fibre optic lever displacement transducer Appl Opt
Vol 18, No 19, 3230-3241
Culshaw, B & Dakin, J 1997 Optical Fibre sensors Vol 4 Artech House Publishers,
0890069409
De Maria, G.; Minardo, A.; Natale, C.; Pirozzi, S & Zeni, L (2008) Optoelectronic Tactile
Sensor Based on Micromachined Scattering Wells FIRST MEDITERRANEAN
PHOTONICS CONFERENCE, European Optical Society Topical Meeting, 25–28
June 2008, Ischia, Italy
Fields, J.N.; Asawa, C.K.; Ramer, O.G & Barnowski, M.K (1980) Fibre Optic Pressure
Sensor J Acoust Soc Am., Vol 67, 816-818
Garus, D.; Krebber, K.; Schliep, F & Gogolla, T (1996) Distributed sensing technique based
on Brillouin optical-fibre frequency-domain analysis Opt Lett., Vol 21, No 17,
1402-1404
Kersey, A D.; Davis, M A.; Patrick, H J.; LeBlanc, M.; Poo, K P.; Askins, A.G.; Putnam, M
A & Friebele, E J (1997) Fibre grating sensors, Journal of Lightw Technol., vol 15,
no 8, pp 1442-1462
Krishna, G.M & Rajanna, K (2002) Tactile sensor based on piezoelectric resonance Proc of
2002 IEEE Conference on Sensor, pp 1643- 1647
Liu, L.; Zheng, X & Li, Z (1993) An array tactile sensor with piezoresistive with single
crystal silicon diaphragm Sensors and Actuators-A32, 193-196
Maekawa, H.; Tanie, K & Komoriya, K (1993) A finger-shaped tactile sensor using an
optical waveguide Proc of 1993 IEEE International Conference on Systems, Man and
Cybernetics, pp 403-408
Measures, R.M (2002) Structural monitoring with fibre optic technology Academic press, San
Diego
Morimura, H.; Shigematsu, S & Machinda, K (2000) A novel sensor cell architecture and
sensing circuit scheme for capacitive fingerprint sensors IEEE Journal of Solid State
Circuits, Vol, 35, 724-731
May, C.; Pagliarulo, P & Janocha, H (2006) Optimisation of a magnetostrictive auxiliary
mass damper Proc 10th International Conference on New Actuators ACTUATOR2006,
Bremen, Germany, pp 344–348
Nikles, M.; Thevenaz, L & Robert, P.A 1997 Brillouin gain spectrum characterization in
single-mode optical fibres J Lightw Technol., Vol 15, No 10, 1842 – 1851
Ohka, M.; Kobayashi, H.; Takata, J & Mitsuya, Y (2006) Sensing Precision of an Optical
Three-axis Tactile Sensor for a Robotic Finger Proc Of the 15th IEEE International
Symposium on Robot and Human Interactive Communication, pp 214-219
Rossiter, J & Mukai, T (2005) A novel tactile sensor using a matrix of LEDs operating in
both photoemitter and photodetector modes Proc of 2005 IEEE Conference on
Sensor, pp 994-997
Saran, A.; Abeysinghe, D.C & Boyd, J.T (2006 Microelectromechanical system pressure
sensor integrated onto optical fibre by anodic bonding Appl Opt., Vol 45,
1737-1742
Tanie, K (1986) Advances in tactile sensors for robotics Proc of the 6th Sensor Symposium
Japan, pp 63-68
Udd E (2002) Overview of fibre optic sensors, In: Fibre Optic Sensors Francis T S Yu;
Shizhuo Yin, pp 1-40, Routledge, 978-0-203-90946-1, USA
Wang, A.; Xiao, H.; Wang, J.; Wang, Z.; Zhao, W & May, R.G (2001) Self-calibrated
interferometric-based-optical fibre sensors J Lightw Technol., Vol 19, No 10,
1495-1501
Yuan, S.; Ansari, F.; Liu, X & Zhao, Y (2005) Optical fibre based dynamic pressure sensor
for WIM sensor Sens and Actuat A, Vol 120, No 1, 53-58
Zhao, Y & Liao, Y (2004) “Discrimination methods and demodulation techniques for fibre
Bragg grating sensors”, Opt Lasers Eng., vol 41, pp 1-18
Trang 8Joaquim F Martins-Filho and Eduardo Fontana
Department of Electronics and Systems, Federal University of Pernambuco
Brazil
1 Introduction
Over the past thirty years there has been intense research and development on optical fibre
sensors for many applications, basically because of their advantages over other technologies,
such as immunity to electromagnetic interference, lightweight, small size, high sensitivity,
large bandwidth, and ease in signal light transmission The applications include sensing
temperature, strain, pressure, current/voltage, chemical/gas, displacement, and biological
processes among others To accomplish those, different optical technologies have been
employed such as fibre grating, interferometry, light scattering and reflectometry, Faraday
rotation, luminescence and others A review on fibre sensors can be found in (Lee, 2003)
Corrosion and its effects have a profound impact on the infrastructure and equipment of
countries worldwide This impact is manifested in significant maintenance, repair, and
replacement efforts; reduced access, availability and production; poor performance; high
environmental risks; and unsafe conditions associated with facilities and equipment There
have been some efforts from different countries to estimate the cost of corrosion and the
results indicate that it can reach 2 to 5% of the gross national product For example,
corrosion damage represented an estimated cost of US$ 276 billions in the United States of
America in 2002 (Thompson et al., 2005) Therefore, corrosion monitoring is an important
aspect of modern infrastructure in industry sectors such as mining, aircraft, shipping,
oilfields, as well as in military and civil facilities
Optical fibre-based corrosion sensors have been investigated in recent years mainly because
of the advantages obtained by the use of optical fibres, as already pointed out A short
review of the technologies employed in the fibre-based corrosion sensors can be found in
(Wade et al., 2008) The reported applications include corrosion monitoring in aircrafts
(Benounis & Jaffrezic-Renault, 2004), in the concrete of roadways and bridges (Fuhr &
Huston, 1998) and in oilfields
2 Corrosion Monitoring in Deepwater Oilfield Pipelines
In the oil industry, to which we focus the sensing approach described in this chapter, a very
challenging problem is that related to surveillance and maintenance of deepwater oilfield
pipelines, given the harsh environment to be monitored and the long distances involved
3
Trang 9These structures are subject to corrosion and sand-induced erosion in a high pressure, high
temperature environment Moreover, the long distances (kilometres) between the corrosion
points and the monitoring location make the commercially available instruments not
appropriate for monitoring these pipelines Costly, regularly scheduled, preventive
maintenance is then required (Staveley, 2004; Yin et al., 2000) Electronic and
electromagnetic-based corrosion sensors (Yin et al., 2000; Vaskivsky et al., 2001; Andrade
Lima et al., 2001) are also not suitable in these conditions Fibre optic based corrosion
sensors are ideal for this application However, the sensing approaches reported in the
literature are either single point (Qiao et al., 2006; Wade et al., 2008) or use a stripped
cladding fibre structure that requires a high precision mechanical positioning system with
moving parts for light detection, which compromises the robustness of the sensor system
(Benounis et al., 2003; Benounis & Jaffrezic-Renault, 2004; Saying et al., 2006;
Cardenas-Valencia et al., 2007) An optical fibre PH sensor has been recently developed for the indirect
evaluation of the corrosion process in petroleum wells (Da Silva Jr et al., 2007) It employs a
fibre Bragg grating mechanically coupled to a PH-sensitive hydrogel, which changes its
volume according to the PH of the medium Thus, the change in PH is translated into a
mechanical strain on the Bragg grating, which can be interrogated by standard optical
methods Although it can easily be multiplexed for multipoint measurements, this technique
is limited to the evaluation of the chemical corrosion due to acid attack inside the well,
disregarding the combined effects of other important sources of corrosion, such as
mechanical (erosion), chemical, thermic and biological (microorganisms) The oil industry
can also make use of the time domain reflectometry (TDR) technique to evaluate the
corrosion process inside pipelines and oil wells (Kohl, 2000) The proposed scheme involves
the deployment of a metallic cable inside and along the pipeline or well The conductor is
exposed to the fluid at selected locations such that it should be susceptible to the same
corrosive processes as the pipeline A signal generator launches a pulsed electrical signal to
the conductor cable and an electronic receiver measures the reflected pulses intensity and
delay The reflections come from the locations where the exposed cable was affected by the
corrosion process, which changes its original impedance This TDR technique has also been
applied to the monitoring of corrosion in steel cables of bridges (Liu et al., 2002) Although
this technique has the advantage of being multipoint or even distributed, it is limited in
reach For practical purposes the maximum distance covered by the sensor is about 2 km
This is suitable for standard wells, but not for deep oilfields, especially those from the
recently discovered presalt regions in Brazil, which are over 6 km deep
3 A Multipoint Fibre Optic Corrosion Sensor
We have recently presented for the first time the concept and first experimental results of a
fibre-optic-based corrosion sensor using the optical time domain reflectometry (OTDR)
technique as the interrogation method (Martins-Filho et al., 2007; Martins-Filho et al., 2008)
Our proposed sensor system is multipoint, self-referenced, has no moving parts and can
detect the corrosion rate several kilometres away from the OTDR equipment These features
make it very suitable to the problem of corrosion monitoring of deepwater pipelines in the
oil industry It should be pointed out, however, that the approach is not limited to this
specific application and can be employed to address a number of single or multipoint
corrosion detection problems in other industrial sectors
In this chapter we present a detailed description of the sensor system, further experimental results and theoretical calculations for the measurement of the corrosion rate of aluminium films in controlled laboratory conditions and also for the evaluation of the maximum number of sensor heads the system supports
3.1 Sensor Setup
Our proposed sensor system consists of several sensor heads connected to a commercial OTDR equipment by a single-mode optical fibre and fibre couplers Figure 1 shows the corrosion sensor setup The OTDR is connected to a 2 km long single mode optical fibre Directional couplers can split the optical signal such that a small fraction (3 to 9%) is directed to the sensing heads The OTDR operates at 1.55 m, with a pulsewidth of 10 ns, which corresponds to a spatial resolution of 2 m The OTDR is set to measure 50000 points for the total distance of 5 km (one point every 10 cm) The optical fibres and couplers are standard telecommunication devices The sensor heads have 100 nm of aluminium deposited on cleaved fibre facets by a standard thermal evaporation process and they are numbered from 1 to 11 in Fig 1
Fig 1 Schematic diagram of the corrosion sensor Sensor heads are numbered Fibre lengths and split ratios are shown
3.2 Results
For laboratory measurements the corrosion action was simulated by controlled etching of the Aluminium film on the sensor head We used 25 H3PO4 : 1 HNO3: 5 CH3COOH as the Al-etcher The expected corrosion rate of Al from this etcher is 50 nm/min Figure 2-a shows the OTDR trace where each peak, numbered from 1 to 11, indicates the reflection from the corresponding sensing head The head number 6 is immersed in the Al-etcher As the aluminium is being removed from the fibre facet the reflected light measured in the OTDR decreases, as shown in Fig 2-b
In Fig 3 we plot the ratio of peak (point A) to valley (point B) of the reflected light shown in Fig 2-b as a function of the aluminium corrosion time Figure 3 shows that up to 60 seconds
of corrosion there is no significant change in the OTDR measured reflected light, since the aluminium is still too thick Further up from this point the reflection drops to a minimum and then stabilizes at a constant level The constant level means that the corrosion process
on the fibre facet has ended We obtain the corrosion rate by taking the deposited metal thickness and the time taken to reach the constant level, as show in Fig 3
2 km
Trang 10These structures are subject to corrosion and sand-induced erosion in a high pressure, high
temperature environment Moreover, the long distances (kilometres) between the corrosion
points and the monitoring location make the commercially available instruments not
appropriate for monitoring these pipelines Costly, regularly scheduled, preventive
maintenance is then required (Staveley, 2004; Yin et al., 2000) Electronic and
electromagnetic-based corrosion sensors (Yin et al., 2000; Vaskivsky et al., 2001; Andrade
Lima et al., 2001) are also not suitable in these conditions Fibre optic based corrosion
sensors are ideal for this application However, the sensing approaches reported in the
literature are either single point (Qiao et al., 2006; Wade et al., 2008) or use a stripped
cladding fibre structure that requires a high precision mechanical positioning system with
moving parts for light detection, which compromises the robustness of the sensor system
(Benounis et al., 2003; Benounis & Jaffrezic-Renault, 2004; Saying et al., 2006;
Cardenas-Valencia et al., 2007) An optical fibre PH sensor has been recently developed for the indirect
evaluation of the corrosion process in petroleum wells (Da Silva Jr et al., 2007) It employs a
fibre Bragg grating mechanically coupled to a PH-sensitive hydrogel, which changes its
volume according to the PH of the medium Thus, the change in PH is translated into a
mechanical strain on the Bragg grating, which can be interrogated by standard optical
methods Although it can easily be multiplexed for multipoint measurements, this technique
is limited to the evaluation of the chemical corrosion due to acid attack inside the well,
disregarding the combined effects of other important sources of corrosion, such as
mechanical (erosion), chemical, thermic and biological (microorganisms) The oil industry
can also make use of the time domain reflectometry (TDR) technique to evaluate the
corrosion process inside pipelines and oil wells (Kohl, 2000) The proposed scheme involves
the deployment of a metallic cable inside and along the pipeline or well The conductor is
exposed to the fluid at selected locations such that it should be susceptible to the same
corrosive processes as the pipeline A signal generator launches a pulsed electrical signal to
the conductor cable and an electronic receiver measures the reflected pulses intensity and
delay The reflections come from the locations where the exposed cable was affected by the
corrosion process, which changes its original impedance This TDR technique has also been
applied to the monitoring of corrosion in steel cables of bridges (Liu et al., 2002) Although
this technique has the advantage of being multipoint or even distributed, it is limited in
reach For practical purposes the maximum distance covered by the sensor is about 2 km
This is suitable for standard wells, but not for deep oilfields, especially those from the
recently discovered presalt regions in Brazil, which are over 6 km deep
3 A Multipoint Fibre Optic Corrosion Sensor
We have recently presented for the first time the concept and first experimental results of a
fibre-optic-based corrosion sensor using the optical time domain reflectometry (OTDR)
technique as the interrogation method (Martins-Filho et al., 2007; Martins-Filho et al., 2008)
Our proposed sensor system is multipoint, self-referenced, has no moving parts and can
detect the corrosion rate several kilometres away from the OTDR equipment These features
make it very suitable to the problem of corrosion monitoring of deepwater pipelines in the
oil industry It should be pointed out, however, that the approach is not limited to this
specific application and can be employed to address a number of single or multipoint
corrosion detection problems in other industrial sectors
In this chapter we present a detailed description of the sensor system, further experimental results and theoretical calculations for the measurement of the corrosion rate of aluminium films in controlled laboratory conditions and also for the evaluation of the maximum number of sensor heads the system supports
3.1 Sensor Setup
Our proposed sensor system consists of several sensor heads connected to a commercial OTDR equipment by a single-mode optical fibre and fibre couplers Figure 1 shows the corrosion sensor setup The OTDR is connected to a 2 km long single mode optical fibre Directional couplers can split the optical signal such that a small fraction (3 to 9%) is directed to the sensing heads The OTDR operates at 1.55 m, with a pulsewidth of 10 ns, which corresponds to a spatial resolution of 2 m The OTDR is set to measure 50000 points for the total distance of 5 km (one point every 10 cm) The optical fibres and couplers are standard telecommunication devices The sensor heads have 100 nm of aluminium deposited on cleaved fibre facets by a standard thermal evaporation process and they are numbered from 1 to 11 in Fig 1
Fig 1 Schematic diagram of the corrosion sensor Sensor heads are numbered Fibre lengths and split ratios are shown
3.2 Results
For laboratory measurements the corrosion action was simulated by controlled etching of the Aluminium film on the sensor head We used 25 H3PO4 : 1 HNO3: 5 CH3COOH as the Al-etcher The expected corrosion rate of Al from this etcher is 50 nm/min Figure 2-a shows the OTDR trace where each peak, numbered from 1 to 11, indicates the reflection from the corresponding sensing head The head number 6 is immersed in the Al-etcher As the aluminium is being removed from the fibre facet the reflected light measured in the OTDR decreases, as shown in Fig 2-b
In Fig 3 we plot the ratio of peak (point A) to valley (point B) of the reflected light shown in Fig 2-b as a function of the aluminium corrosion time Figure 3 shows that up to 60 seconds
of corrosion there is no significant change in the OTDR measured reflected light, since the aluminium is still too thick Further up from this point the reflection drops to a minimum and then stabilizes at a constant level The constant level means that the corrosion process
on the fibre facet has ended We obtain the corrosion rate by taking the deposited metal thickness and the time taken to reach the constant level, as show in Fig 3
2 km
Trang 110 10 20 30 40 50
20 25 30 35 40 45
A(b)
Fig 2 (a) OTDR trace, corresponding to the intensity of the reflected light as a function of
distance along the fibre Sensor head numbers are shown (b) OTDR traces for sensor head
number 6, for several corrosion times
The measured corrosion rate was 47.5 nm/min, which is very close to the expected value (50 nm/min) Other measurements performed using different sensor heads showed similar results It is important to note that since the corrosion rate is obtained from the ratio of peak (point A) to valley (point B) of the OTDR trace as a function of time, this measurement is self-referenced, because the ratio is immune, to a certain extent, to small optical power fluctuations that may occur due to changes in the OTDR signal power, optical fibre and fibre coupler loss variations along the sensor system
4 6 8 10 12 14 16 18 20 22 24
0 100nm
Fig 3 Relative intensity obtained from Fig 2-b, as a function of the corrosion time Metal thickness and corrosion rate are shown
Figure 3 also shows a valley in the relative reflected intensity just before the constant level used for corrosion determination Although this feature does not seem to be important for the determination of the corrosion rate, we verified if it would be an artifact due to the pulsed OTDR operation in the multipoint (multireflection) setup scheme shown in Fig.1, by performing measurements in the single head setup show in Fig 4 This new setup uses a
CW laser source and an optical power meter, instead of the OTDR The laser light at 1.55 m from the CW laser with fibre pigtail is coupled to an optical isolator and then to a 50% coupler and to a 79/21 coupler The output of this coupler has another optical isolator in one end and a sensor head in the other end The sensor head used here is similar to those used in the multipoint setup of Fig 1 The light reflected from the sensor head reaches the optical power meter through the optical couplers The two isolators avoid unwanted reflections to reach the power meter and the laser source, which could cause interference effects and instabilities For corrosion measurements we used the same Aluminium etcher as described before Figure 5 shows the optical power as a function of the corrosion time obtained from the single head setup of Fig 4 This result also exhibits the valley observed in the multipoint setup that uses the OTDR (Fig 3), indicating that this feature is not a measurement artifact Also, Fig 5 confirms the corrosion rate obtained from Fig 3, since the constant level starts at about 120 seconds of corrosion
Trang 120 10 20 30 40 50
20 25 30 35 40 45
68.5 76.9 82.0 84.4 87.6 91.4 94.8B
A(b)
Fig 2 (a) OTDR trace, corresponding to the intensity of the reflected light as a function of
distance along the fibre Sensor head numbers are shown (b) OTDR traces for sensor head
number 6, for several corrosion times
The measured corrosion rate was 47.5 nm/min, which is very close to the expected value (50 nm/min) Other measurements performed using different sensor heads showed similar results It is important to note that since the corrosion rate is obtained from the ratio of peak (point A) to valley (point B) of the OTDR trace as a function of time, this measurement is self-referenced, because the ratio is immune, to a certain extent, to small optical power fluctuations that may occur due to changes in the OTDR signal power, optical fibre and fibre coupler loss variations along the sensor system
4 6 8 10 12 14 16 18 20 22 24
0 100nm
Fig 3 Relative intensity obtained from Fig 2-b, as a function of the corrosion time Metal thickness and corrosion rate are shown
Figure 3 also shows a valley in the relative reflected intensity just before the constant level used for corrosion determination Although this feature does not seem to be important for the determination of the corrosion rate, we verified if it would be an artifact due to the pulsed OTDR operation in the multipoint (multireflection) setup scheme shown in Fig.1, by performing measurements in the single head setup show in Fig 4 This new setup uses a
CW laser source and an optical power meter, instead of the OTDR The laser light at 1.55 m from the CW laser with fibre pigtail is coupled to an optical isolator and then to a 50% coupler and to a 79/21 coupler The output of this coupler has another optical isolator in one end and a sensor head in the other end The sensor head used here is similar to those used in the multipoint setup of Fig 1 The light reflected from the sensor head reaches the optical power meter through the optical couplers The two isolators avoid unwanted reflections to reach the power meter and the laser source, which could cause interference effects and instabilities For corrosion measurements we used the same Aluminium etcher as described before Figure 5 shows the optical power as a function of the corrosion time obtained from the single head setup of Fig 4 This result also exhibits the valley observed in the multipoint setup that uses the OTDR (Fig 3), indicating that this feature is not a measurement artifact Also, Fig 5 confirms the corrosion rate obtained from Fig 3, since the constant level starts at about 120 seconds of corrosion
Trang 13CW Laser Coupler 50/50 Coupler 79/21
Power
Fig 4 Schematic diagram of the single head setup
-60 -50 -40 -30 -20 -10
Fig 5 Reflected optical power from a single head setup as a function of the corrosion time
We also used the Fresnel reflection formulation (Fontana & Pantell, 1988) for a
silica-Al-liquid single layer structure, as shown in Fig 6, to study the reflection properties of the
sensing head Neglecting the small beam divergence of the guided mode, the reflectance is
given by
22023
12
2023
12
2 exp 1
2 exp
d k j r
r
d k j r
r R
i i i
,
(2)
is the normal incidence reflectivity at the interface between media i and i+1 (i = 1, 2), k0 =
2π/λ, εi is the relative electrical permittivity of medium i (i = 1, 2, 3) and d is the metal film
Medium 1(fiber)
Medium 2(aluminum)
Medium 3(liquid) d
Fig 6 Schematic diagram of the sensing head showing the Aluminium film of thickness d
on the fibre facet
Figure 7 shows a theoretical simulation as well as the experimental data for the reflectance
at the metalized fibre facet as a function of the metal film thickness The experimental data were obtained from Fig 5 The theoretical result showed no evidence of a minimum reflectance with the strong depth observed experimentally at an estimated Al film thickness
of 15 nm In fact, the theoretical prediction yields almost 100% reflectance at this thickness value, as can be noticed in Fig 7 The difference between theoretical and experimental results indicates that the valley observed in the experimental results is not due to any interference effect that could occur in the fibre-metal-liquid interfaces
Due to the resonant nature of the reflectance minima shown in Figs 3 and 5, it is very likely that they occur due to roughness induced, resonant coupling to surface plasmons (Fontana
& Pantell, 1988) at the metal-liquid interface as a thin and rough layer of metal may result during the etching process The coupling is thickness dependent and the strength depends
on the average size of irregularities on the surface (Fontana & Pantell, 1988) Given that the dispersion relation of surface plasmons is very near that of photons in this spectral region, surface roughness could provide the required small increase in momentum for efficient coupling to the surface plasmon oscillation A more elaborated calculation will be performed in future work taking into account the change in dispersion relation of surface plasmons due to roughness (Fontana & Pantell, 1988), to account for this effect
It is worth noticing from Fig 7 that the reflectance predicted theoretically with no metal film was 26.7 dB lower than that at maximum thickness, a result that differs significantly from the drop of 14 dB observed experimentally in Fig 3 and 35 dB in Fig 7 This is probably due to the residual clusters left on the fibre facet that form an absorbing, non-homogeneous interface that changes the reflectance relative to that predicted theoretically for a single glass-liquid interface In fact we observed from a direct inspection with an optical microscope that some clusters of material still remained on the fibre facet, which were no longer affected by the Al-etcher As can be seen from Figs 3 and 7, the resonant features in the experimental results are similar, although the minima occur at different time points There is, however, a significant difference from 14 to 35 dB in the final reflectance drop obtained from the data of Figs 3 and 7, respectively, which may be due to the distinct procedures used to carry out the experiments For the data shown in Fig 3, obtained with
Trang 14CW Laser Coupler 50/50 Coupler 79/21
Power
Fig 4 Schematic diagram of the single head setup
-60 -50 -40 -30 -20 -10
Fig 5 Reflected optical power from a single head setup as a function of the corrosion time
We also used the Fresnel reflection formulation (Fontana & Pantell, 1988) for a
silica-Al-liquid single layer structure, as shown in Fig 6, to study the reflection properties of the
sensing head Neglecting the small beam divergence of the guided mode, the reflectance is
given by
22
023
12
20
2312
2 exp
1
2 exp
d k
j r
r
d k
j r
r R
i i
,
(2)
is the normal incidence reflectivity at the interface between media i and i+1 (i = 1, 2), k0 =
2π/λ, εi is the relative electrical permittivity of medium i (i = 1, 2, 3) and d is the metal film
Medium 1(fiber)
Medium 2(aluminum)
Medium 3(liquid) d
Fig 6 Schematic diagram of the sensing head showing the Aluminium film of thickness d
on the fibre facet
Figure 7 shows a theoretical simulation as well as the experimental data for the reflectance
at the metalized fibre facet as a function of the metal film thickness The experimental data were obtained from Fig 5 The theoretical result showed no evidence of a minimum reflectance with the strong depth observed experimentally at an estimated Al film thickness
of 15 nm In fact, the theoretical prediction yields almost 100% reflectance at this thickness value, as can be noticed in Fig 7 The difference between theoretical and experimental results indicates that the valley observed in the experimental results is not due to any interference effect that could occur in the fibre-metal-liquid interfaces
Due to the resonant nature of the reflectance minima shown in Figs 3 and 5, it is very likely that they occur due to roughness induced, resonant coupling to surface plasmons (Fontana
& Pantell, 1988) at the metal-liquid interface as a thin and rough layer of metal may result during the etching process The coupling is thickness dependent and the strength depends
on the average size of irregularities on the surface (Fontana & Pantell, 1988) Given that the dispersion relation of surface plasmons is very near that of photons in this spectral region, surface roughness could provide the required small increase in momentum for efficient coupling to the surface plasmon oscillation A more elaborated calculation will be performed in future work taking into account the change in dispersion relation of surface plasmons due to roughness (Fontana & Pantell, 1988), to account for this effect
It is worth noticing from Fig 7 that the reflectance predicted theoretically with no metal film was 26.7 dB lower than that at maximum thickness, a result that differs significantly from the drop of 14 dB observed experimentally in Fig 3 and 35 dB in Fig 7 This is probably due to the residual clusters left on the fibre facet that form an absorbing, non-homogeneous interface that changes the reflectance relative to that predicted theoretically for a single glass-liquid interface In fact we observed from a direct inspection with an optical microscope that some clusters of material still remained on the fibre facet, which were no longer affected by the Al-etcher As can be seen from Figs 3 and 7, the resonant features in the experimental results are similar, although the minima occur at different time points There is, however, a significant difference from 14 to 35 dB in the final reflectance drop obtained from the data of Figs 3 and 7, respectively, which may be due to the distinct procedures used to carry out the experiments For the data shown in Fig 3, obtained with
Trang 15the OTDR, since the equipment is somewhat slow to execute several measurements to
average them in time, the head was placed in the etcher for a given time and then in water
for OTDR reading and averaging for each data point For the case of Fig 7, we used the
single head setup of Fig 4, and we attempted to avoid artifacts introduced by the use of
alternate solutions and employed an optical power meter for fast data reading and
averaging, and thus the sensor head could remain immersed in the etcher during the entire
measurement These distinct procedures may lead to different residual clustering in the fibre
facets, which can be the cause of the difference in the results of Figs 3 and 7 It will be
further investigated in the future
Fig 7 Theoretical (line) and experimental data (dots) for the reflectance as a function of the
Aluminium film thickness
We also evaluated experimentally the maximum number of sensor heads our sensor system
can support, and we found that it depends on the dynamic range of the OTDR For the
OTDR pulsewidth used to obtain the results shown here (10 ns) its dynamic range is about 7
dB Since each coupler has an insertion loss of about 0.7 dB, we can have up to 10 sensor
heads in this configuration This can be verified from Fig 2-a One can see that as the
number of heads increases along the fibre length the OTDR trace becomes noisier This
noisy trace should have impact on the accuracy of the measured corrosion rate for the heads
located further away from the OTDR On the other hand, for 500 ns pulsewidth the OTDR
dynamic range is 20.4 dB, which allows the use of up to 30 sensing heads In this case the
OTDR spatial resolution is about 100 m Therefore, the minimum separation between
consecutive sensor heads should be of about 200 m In this configuration the sensor system
would cover a total length of 6 km, with a sensor head every 200 meters
4 Conclusions
We proposed and demonstrated experimentally an optical fibre sensor for the corrosion
process in metal (Aluminium) using the optical time domain reflectometry technique We
presented experimental results for the measurement of the corrosion rate of aluminium films in controlled laboratory conditions The obtained corrosion rate matched the expected rate of the etcher used We also evaluated experimentally the maximum number of sensor heads the system supports It depends on the OTDR dynamic range and it has implications
on the distance between consecutive sensor heads
Our proposed sensor system is multipoint, self-referenced, has no moving parts (all-fibre) and can detect the corrosion rate for each head several kilometres away from the OTDR, thus making the system ideal for “in-the-field” monitoring of corrosion and erosion This system may have applications in harsh environments such as in deepwater oil wells and gas flowlines (including from the presalt region), for the evaluation of the corrosion and erosion processes in the inner wall of the casing pipes In this case, different materials can be deposited on the fibre facet to better match the pipe materials under corrosion/erosion This system may enable inferred condition-based maintenance without production interruption, decreasing the cost of oil production, and substantially reducing the risk of environmental disasters due to the failure of unmonitored flowlines
Our experimental results also revealed a feature that may indicate the occurrence of the surface plasmon effect at the metal-liquid interface It could be due to the roughness coupling to surface plasmons at the metal-liquid interface as a thin and rough layer of metal may result during the etching process Although we believe at this point that this effect is not vital for the operation of the proposed sensor, nor to the measurement of the corrosion rate, it will be investigated in future work
5 References
Andrade Lima, E & Bruno, A C (2001) Improving the Detection of Flaws in Steel Pipes
Using SQUID Planar Gradiometers IEEE Transactions on Applied Superconductivity,
Vol 11, No 1, Mar 2001, 1299-1302, ISSN 1051-8223
Benounis, M.; Jaffrezic-Renault, N.; Stremsdoerfer, G & Kherrat, R (2003) Elaboration and
standardization of an optical fibre corrosion sensor based on an electroless deposit
of copper Sensors and Actuators B, Vol 90, 2003, 90-97, ISSN 0925-4005
Benounis, M & Jaffrezic-Renault, N (2004) Elaboration of an optical fibre corrosion sensor
for aircraft applications Sensors and Actuators B, Vol 100, March 2004, 1-8, ISSN
0925-4005 Cardenas-Valencia, A M.; Byrne, R H.; Calves, M.; Langebrake, L.; Fries, D P & Steimle, E
T (2007) Development of stripped-cladding optical fiber sensors for continuous monitoring II: Referencing method for spectral sensing of environmental corrosion
Sensors and Actuators B, Vol 122, 2007, 410-418, ISSN 0925-4005
Da Silva Jr., M F.; D'almeida, A R.; Ribeiro, F P.; Valente, L C G.; Braga, A M B &
Triques, A L C (2007) Optical Fiber PH Sensor, US Patent no 7251384, granted in July 2007, available in http://www.freepatentsonline.com/7251384.html
Fontana, E & Pantell, R H (1988) Characterization of multilayer rough surfaces by use of
surface-plasmon spectroscopy Physical Review B, Vol 37, No 7, 1988, 3164-3182,
ISSN 0163-1829
Fuhr, P L & Huston, D R (1998) Corrosion detection in reinforced concrete roadways and
bridges via embedded fiber optic sensors, Smart Materials and Structures, Vol 7,
1998, pp 217–228, ISSN 0964-1726
Trang 16the OTDR, since the equipment is somewhat slow to execute several measurements to
average them in time, the head was placed in the etcher for a given time and then in water
for OTDR reading and averaging for each data point For the case of Fig 7, we used the
single head setup of Fig 4, and we attempted to avoid artifacts introduced by the use of
alternate solutions and employed an optical power meter for fast data reading and
averaging, and thus the sensor head could remain immersed in the etcher during the entire
measurement These distinct procedures may lead to different residual clustering in the fibre
facets, which can be the cause of the difference in the results of Figs 3 and 7 It will be
further investigated in the future
Fig 7 Theoretical (line) and experimental data (dots) for the reflectance as a function of the
Aluminium film thickness
We also evaluated experimentally the maximum number of sensor heads our sensor system
can support, and we found that it depends on the dynamic range of the OTDR For the
OTDR pulsewidth used to obtain the results shown here (10 ns) its dynamic range is about 7
dB Since each coupler has an insertion loss of about 0.7 dB, we can have up to 10 sensor
heads in this configuration This can be verified from Fig 2-a One can see that as the
number of heads increases along the fibre length the OTDR trace becomes noisier This
noisy trace should have impact on the accuracy of the measured corrosion rate for the heads
located further away from the OTDR On the other hand, for 500 ns pulsewidth the OTDR
dynamic range is 20.4 dB, which allows the use of up to 30 sensing heads In this case the
OTDR spatial resolution is about 100 m Therefore, the minimum separation between
consecutive sensor heads should be of about 200 m In this configuration the sensor system
would cover a total length of 6 km, with a sensor head every 200 meters
4 Conclusions
We proposed and demonstrated experimentally an optical fibre sensor for the corrosion
process in metal (Aluminium) using the optical time domain reflectometry technique We
presented experimental results for the measurement of the corrosion rate of aluminium films in controlled laboratory conditions The obtained corrosion rate matched the expected rate of the etcher used We also evaluated experimentally the maximum number of sensor heads the system supports It depends on the OTDR dynamic range and it has implications
on the distance between consecutive sensor heads
Our proposed sensor system is multipoint, self-referenced, has no moving parts (all-fibre) and can detect the corrosion rate for each head several kilometres away from the OTDR, thus making the system ideal for “in-the-field” monitoring of corrosion and erosion This system may have applications in harsh environments such as in deepwater oil wells and gas flowlines (including from the presalt region), for the evaluation of the corrosion and erosion processes in the inner wall of the casing pipes In this case, different materials can be deposited on the fibre facet to better match the pipe materials under corrosion/erosion This system may enable inferred condition-based maintenance without production interruption, decreasing the cost of oil production, and substantially reducing the risk of environmental disasters due to the failure of unmonitored flowlines
Our experimental results also revealed a feature that may indicate the occurrence of the surface plasmon effect at the metal-liquid interface It could be due to the roughness coupling to surface plasmons at the metal-liquid interface as a thin and rough layer of metal may result during the etching process Although we believe at this point that this effect is not vital for the operation of the proposed sensor, nor to the measurement of the corrosion rate, it will be investigated in future work
5 References
Andrade Lima, E & Bruno, A C (2001) Improving the Detection of Flaws in Steel Pipes
Using SQUID Planar Gradiometers IEEE Transactions on Applied Superconductivity,
Vol 11, No 1, Mar 2001, 1299-1302, ISSN 1051-8223
Benounis, M.; Jaffrezic-Renault, N.; Stremsdoerfer, G & Kherrat, R (2003) Elaboration and
standardization of an optical fibre corrosion sensor based on an electroless deposit
of copper Sensors and Actuators B, Vol 90, 2003, 90-97, ISSN 0925-4005
Benounis, M & Jaffrezic-Renault, N (2004) Elaboration of an optical fibre corrosion sensor
for aircraft applications Sensors and Actuators B, Vol 100, March 2004, 1-8, ISSN
0925-4005 Cardenas-Valencia, A M.; Byrne, R H.; Calves, M.; Langebrake, L.; Fries, D P & Steimle, E
T (2007) Development of stripped-cladding optical fiber sensors for continuous monitoring II: Referencing method for spectral sensing of environmental corrosion
Sensors and Actuators B, Vol 122, 2007, 410-418, ISSN 0925-4005
Da Silva Jr., M F.; D'almeida, A R.; Ribeiro, F P.; Valente, L C G.; Braga, A M B &
Triques, A L C (2007) Optical Fiber PH Sensor, US Patent no 7251384, granted in July 2007, available in http://www.freepatentsonline.com/7251384.html
Fontana, E & Pantell, R H (1988) Characterization of multilayer rough surfaces by use of
surface-plasmon spectroscopy Physical Review B, Vol 37, No 7, 1988, 3164-3182,
ISSN 0163-1829
Fuhr, P L & Huston, D R (1998) Corrosion detection in reinforced concrete roadways and
bridges via embedded fiber optic sensors, Smart Materials and Structures, Vol 7,
1998, pp 217–228, ISSN 0964-1726
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