WO2010051553A1

WO2010051553A1 - Gas leak detection system and methods

Info

Publication number: WO2010051553A1
Application number: PCT/US2009/063073
Authority: WO
Inventors: Masoud Ghandehari; Gamal Khalil; Alexey Sidelev
Original assignee: Polytech Institute Of New York University
Priority date: 2008-11-03
Filing date: 2009-11-03
Publication date: 2010-05-06

Abstract

Systems and methods for detecting a leak of a fugitive gas from a containment structure compπsmg an optic fiber having a cladding layer and a core The core is coated with a gas-permeable polymenc mateπal, having a sensor composition comprising a luminophor sensitive to quenching by oxygen to indicate vaπations in oxygen concentration caused by the presence of a target fugitive gas The sensor molecule is distributed along the core of an optical fiber capable of detecting leaks over extended distances for extended peπods of time A light source is used for illuminating the optic fiber and a signal detector detects vaπations in the oxygen as indicated by the sensor Systems and methods for gas leak detection are based on continuous monitoring of the oxygen concentration surrounding gas containment or transmission facilities such as natural gas pipeline.

Description

GAS LEAK DETECTION SYSTEM AND METHODS

BACKGROUND

[0001] Leak detection and measurement is an important part of a company's operations in chemical, petroleum, and allied industries where a variety of gases and volatile compounds are produced, transmitted, stored, and used. Successful leak detections programs not only reduce loss of gas to leaks, but will also enhance worker safety and overall environmental health.

[0002] Leaking valves, flanges and connections are significant sources of fugitive gas and volatile chemical emissions in oil refining, chemical manufacturing and gas production, transmission and distribution. For example, leaking valves account for roughly half the emissions of volatile organic compounds (VOCs) from oil refineries. In order to control leaks, the U.S. Environmental Protection Agency has promoted Leak Detection and Repair (LDAR) regulations under the Clean Air Act. A leak is typically defined as more than 10,000 ppm of VOCs or other pollutants. Identifying and repairing the leaks will provide a great and immediate benefit to the environment. Additionally, recent guidelines mandated by the government for frequent checks of indoor/home gas leaks are made possible only though the availability of a low cost and robust leak sensing system.

[0003] Many techniques for detecting leaks measure the concentration of volatile organic compounds. Current commercially available methods for detection of volatile organic compounds (VOCs) include highly sensitive systems, such as catalytic oxidation, flame ionization and photoionization. These techniques detect leaks by measuring the concentration of VOCs. Some sensors employing these techniques are expensive and they require skilled operators and frequent calibration. Less expensive devices based on solid- state technology are not sufficiently sensitive to small leaks. Thus, highly sensitive, low cost optical devices with embedded sensor molecule provide a viable alternative. When optical fibers are prepared using the sensing molecule/compound leak sensing can be achieved in a distributed manner. Optical fiber sensors also provide many other advantages including their light weight, possibility of multiplexing, resistance to electromagnetic interference and long-term durability, among others. [0004] New gas leak detection systems and methods are needed that are robust and durable, provide for better sensitivity than solid state sensors, permit long-term use without the need for calibration, allow for pinpointing the location of a leak and are responsive to almost any gas and volatile compounds without adding tracer gasses.

SUMMARY

[0005] New gas leak detection systems and methods are provided that capture and measure leaks from refineries and other oil and gas or chemical process equipment and indoor gas pipe flanges, valves, and pumps in real time.

[0006] In various embodiments, a system is provided for monitoring the oxygen concentration in an environment comprises: an optic fiber having a cladding layer and a core, wherein the core is coated with a gas-permeable polymeric material, having a VOC sensing capability. The composition of the sensing compound comprises a Iuminophor sensitive to quenching by oxygen to indicate variations in oxygen concentration being replaced by the Volatile Organic Compound (VOC); a light source for illuminating the optic fiber; a signal detector to detect variations in gas as indicated by the sensor; and a processing unit to collect and maintain data regarding oxygen concentration.

[0007] In various embodiments, a method is provided for monitoring the changes in VOC concentration in an environment, the method comprises: coating the core of an optic fiber with a gas-permeable polymeric material, having a sensor composition to indicate variations in oxygen concentration; positioning the optic fiber in a location having a constant oxygen concentration; illuminating optical fiber with a light source; exciting the sensor composition, where the sensor composition comprises a Iuminophor sensitive to quenching by oxygen; detecting variations in the ambient oxygen concentration caused by the displacement of oxygen molecules by the presence of volatile organic compounds, as indicated by the excitation of the sensor composition; and signaling a variation in ambient oxygen concentration.

[0008] In various embodiments, a sensor is provided for measuring oxygen concentration in an environment, the sensor comprising: an optic fiber having a cladding layer and a core, wherein the core is coated with a gas-permeable polymeric material, having a sensor composition comprising a luminophor sensitive to quenching by oxygen to indicate variations in oxygen concentration further wherein the refractive index of the polymeric material comprising the core is optimized for distributed sensing along the length of the optic fiber.

[0009] Additional features and advantages of various embodiments will be set forth in part in the description that follows, and in part will be apparent from the description, or may be learned by practice of various embodiments. The objectives and other advantages of various embodiments will be realized and attained by means of the elements and combinations particularly pointed out in the description and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] In part, other aspects, features, benefits and advantages of the embodiments will be apparent with regard to the following description, appended claims and accompanying drawings where:

[0011] Figure 1 illustrates an experimental set-up of an optical fiber sensor for measuring the changes in relative concentration of various gases to oxygen.

[0012] Figure 2 illustrates ratio of two areas of the lifetime decay signal for two different concentrations of oxygen in the sample space.

[0013] Figure 3 illustrates the F-Ratio (area ΪI/I) plotted against a varying ratios of nitrogen to oxygen concentration.

[0014] Figure 4 illustrates the results of a repeatability study showing the response of the leak detector to various ratios of nitrogen to oxygen concentration in the sampling space, as indicated by the F number.

[0015] Figure 5 illustrates the long-term durability of the sensor compound showing the variation in the sensitivity of the sensor to changes in oxygen concentrations. [0016] Figure 6 illustrates the signal drift of the sensor over time period under constant exposure to an LED.

[0017] Figure 7 illustrates an experimental set for optical fiber sensor exposed to methane and in an environment simulating the conditions of detecting when the fiber optic sensor is buried underground and exposed to a methane leak for a buried pipe.

[0018] Figure 8 illustrates the response of the gas sensor subject to methane leaks in various test conditions employed using the experimental set-up of Figure 7.

[0019] Figure 9 comprises the results of continuous monitoring of daily methane leaks in an empty test chamber over a period of time.

[0020] It is to be understood that the figures are not drawn to scale. Further, the relation between objects in a figure may not be to scale, and may in fact have a reverse relationship as to size. The figures are intended to bring understanding and clarity to the structure of each object shown, and thus, some features may be exaggerated in order to illustrate a specific feature of a structure.

DETAILED DESCRIPTION

[0021] For the purposes of this specification and appended claims, unless otherwise indicated, all numbers expressing quantities of ingredients, percentages or proportions of materials, reaction conditions, and other numerical values used in the specification and claims, are to be understood as being modified in all instances by the term "about." Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties desired by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0022] Notwithstanding that the numerical ranges and parameters setting forth, the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all subranges subsumed therein. For example, a range of "1 to 10" includes any and all subranges between (and including) the minimum value of 1 and the maximum value of 10, that is, any and all subranges having a minimum value of equal to or greater than 1 and a maximum value of equal to or less than 10, e.g., 5.5 to 10.

[0023] It is noted that, as used in this specification and the appended claims, the singular forms "a," "an," and "the," include plural referents unless expressly and unequivocally limited to one referent. Thus, for example, reference to "luminophor" includes one, two, three or more luminophors.

[0024] Reference will now be made in detail to certain embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the illustrated embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents, which may be included within the invention as defined by the appended claims.

[0025] The headings below are not meant to limit the disclosure in any way; embodiments under any one heading may be used in conjunction with embodiments under any other heading.

[0026] In some embodiments, distributed gas sensing systems and methods are provided that detect leaks of analyte gaseous compounds from the perimeter of the process unit, where analyte gaseous compounds include fugitive organic and inorganic chemicals. New gas leak detection systems and methods are provided with the ability to detect leaks of a variety of chemicals, at least as sensitive as "Method 21", the ability to operate from the perimeter of a chemical process unit, and technologies with a sufficient resolution to identify the specific component that is leaking. The distribution of a sensor composition along the length of the fiber optic allows for leak detection over long distances. [0027] In various embodiments, the distributed gas leak sensor systems and methods provided are based on a measure of oxygen concentration surrounding the sensor being replaced by the target analyte gas. The environment surrounding a gas pipeline should contain a constant amount of oxygen. In the event of a gas leak, the oxygen concentration in the vicinity of the leak would decrease due to displacement of oxygen by the target analyte. Thus the systems and methods described effectively detect gas leaks by constantly monitoring the oxygen concentration in an environment and having sufficient sensitivity to signal variations in the oxygen concentrations.

[0028] In various embodiments, systems and methods are based on a gas permeable polymeric compound film containing a sensor composition comprising a luminophor that is quenched by oxygen. In various embodiments, the sensor has the ability to detect fugitive gases by quantifying small changes of oxygen concentrations in the vicinity of the sensor, thereby establishing the quantitative indication of target fugitive gas. In one embodiment, the sensor composition is coated over surface that is illuminated by a light source that excites the luminophor, the luminophor in the film will luminescence brightly when exposed to a target analyte. The luminescent lifetime of the luminophor is substantially long. In various embodiments, where a leak exists, the sensor will luminesce, and its luminescent intensity will depend on the leak rate. In various embodiments, the sensor composition can be used to make an optical fiber gas leak sensor by coating the sensor compound on a core of an optical fiber. The fiber optic core is made from a polymeric material with optimize refractive index allowing for distributed sensing. In this embodiment, the sensing compound is illuminated via the evanescent field of the propagating energy that excites the luminophor. This fiber optic sensor can be placed, for example, near a natural gas pipe, and using evanescent field spectroscopy, information concerning the ambient levels of oxygen around the pipeline can be measured. Various embodiments comprise an alarm for alerting when a sudden decline in oxygen concentration is detected, warning workers of a possible leak in the line.

[0029] Methods for monitoring oxygen concentration changes based on phosphorescence quenching are described in U. S. Patents 4,810,655, 5,043,286 to Khalil, et al, with the goal of monitoring the oxygen concentration in blood or other fluids. It was initially implemented as a tiny volume of oxygen permeable polymer with luminescent dye at the end of an optical fiber. The methods for using luminescence quenching to monitor the level of oxygen are modified by U.S. Patents 5,186,046, 5,341,676 to Gouterman, ei al, in order to map the pressure distribution over airfoil surfaces during airflow in a wind tunnel. The systems and methods provided improve upon systems and methods of optical fiber leak detection such as those described in the above-referenced applications, and are hereby incorporated by reference herein in their entirety.

Optical Fibers

[0030] Optical fibers typically include a core, a concentric cladding surrounding the core, a concentric protective jacket or buffer surrounding the cladding. Generally the core is made of transparent glass or plastic possessing a certain index of refraction and the cladding is made of transparent glass or plastic possessing a different index of refraction. The relative refractive indices of the core and the cladding largely determine the function and performance of the optical fiber. As a beam of light is introduced into the optical fiber, the direction of the light changes by total internal reflection at the interface of two media with the different refraction indices. The angles of reflection and refraction can be predicted using Snell's law if the refractive indices of both media are known. It is known to alter the media with their respective refraction indices to provide optical fiber with certain light propagating characteristics. Typically, for minimal power loss, it is desirable for the light to propagate mainly through the core of the optical fiber by having smaller difference between the respective refractive indices.

[0031] Fiber optic sensors employ the fact that environmental effects can alter the amplitude, phase, frequency, spectral content, or polarization of light propagated through an optical fiber. Fiber optics sensors can be classified as intrinsic or extrinsic. Intrinsic sensors measure ambient environmental effects by relying on the properties of the optical fiber only while extrinsic sensors are coupled to another device to translate environmental effects into changes in the properties of the light in the fiber optic. When detecting VOCs, intrinsic sensors are most often used.

[0032] Such sensors may be multimode fiber (MMF) or single mode fiber (SMF). Single mode optical fibers have a relatively small diameter and support only one spatial mode of propagation. Multimode fibers have a core with a relatively large diameter and permit non-axial rays or modes to propagate through the core. [0033] An evanescent field sensor is based on the total reflection at the interface between two media, having different refractive indices. The strength of the evanescent field are a function of various parameters such as the relative magnitude of the refractive indices of the core and cladding (known as the numerical aperture, the wavelength of light being propagated, the core diameter, and optical attenuation of the materials itself.

Core/Cladding Ratio

[0034] In various embodiments the core and the cladding comprise a clear plastic material such that light is propagated. The refractive index of the cladding is lower than the refractive index of the core. The greater the difference in between the refractive indices, the stronger the resultant evanescent field created. The relation between the two indices is defined by parameter called numerical aperture (NA), where N

A=(«|² - The numerical aperture may range from between about 0.1-0.5.

[0035] In various embodiments, the polymeric optical fiber core material will exhibit a customized refractive index such that the fiber sensors numerical aperture can be tuned for balance of sensitivity and guiding efficiency. In the numerical aperture equation, nl is the refractive index of the core and n2 is the refractive index of the cladding. In the case of the disclosed distributed gas sensing technology, the numeric aperture would range from approximately 0.25 for long length applications (e.g., greater than 100m) and approximately 0.45 for shorter distances (e.g., 100 m or less). The polymer for the cladding of the optical fiber can be similarly optimized using these measures. Additional parameters on the cladding design should include gas permeability and temperature cross sensitivity and optical attenuation properties.

[0036] In various embodiments, the core of the fiber optic is coated with a gas-permeable polymer. The polymeric cladding material comprises the sensor compositions, it would have refractive index lower than the core material, good binding with the sensor molecules, induce low temperature sensitivity in the sensor molecule and would have low attenuation to the excitation and emission wavelengths. The polymeric core materials would have a optimum refractive index such the prescribed numerical aperture is obtained, and it would have low optical attenuation. Effective polymers have refractive indices of around 1.5. Common polymers used for the core include polymethyl methacrylate, polystyrene and polycarbonate.

[0037] A specialty polymeric optical fiber core is drawn, or the cladding of a cladded fiber optic is removed to allow for coating the core with the sensing compound. Various methods exist for removing the cladding and preparing the core to receive polymeric composition including various combinations of physical removal and chemical treatments. Also various methods are process are available for manufacturing polymer core performs. Known methods for applying the polymeric coating to the cladding are adapted for use in applying a polymeric coating to the core, depending on the application length and the core type.

[0038] The present systems and methods provide for coating the entire length of the fiber optic with the polymeric sensing compound. Coating the core spanning the length of the fiber optic allows for distributed sensing, enabling detection capabilities over long distances.

[0039] The polymeric coating comprises the sensor composition, having a luminophor capable of excitation these include porphyrin compounds such as platinum porphyrin. Such sensor molecules have sufficiently long emission life times therefore can be coupled with relatively simple detector sampling rate and signal processing. Such compounds also exhibit exceptional long-term durability. These sensor molecules have cross sensitivity with temperature and this effect is influence by the choice of host polymer; therefore the importance of the choice of the polymer used and the new cladding.

[0040] The sensor composition is capable of detecting variations in the ambient oxygen level. In various embodiments, the displacement of oxygen by a target anaiyte excites the luminophor. In various embodiments, the concentration of sensor molecules to solid (w/w) ranges from 1/50 to 1/100.

[0041] Various light sources may be used in conjunction with the present systems and methods. These include ultraviolet or violet laser diode. Typically, light is guided through the core of the fiber by internal reflection of the core/cladding interface. The evanescent wave travels along the fiber, just outside the interface. In a typical chemical sensor, the evanescent field has the ability to interrogate the gaseous environment surrounding the fiber.

Oxygen Sensing

[0042] The use of an oxygen-permeable polymer as a luminescent indicator at the tip of an optical fiber has previously been used. Further, the use of functional polymers in general as a cladding, excited by an evanescent has also been tested. In an embodiment of the present application, the functional polymer is applied to the core of the fiber optic. In various embodiments, the evanescent field excites the sensing cladding of the fiber. The level of luminescence indicates the concentration of oxygen replaced by fugitive gases over the sensing layer. Ideally, the lifetime of the luminescent indicator and the diffusion rate of gas in the polymer are designed so that a measurable change in luminescence level for the anticipated ambient oxygen pressure range can be monitored. The polymer functions as a carrier for the sensor molecules and in composite action serves ad the sensing compound..

[0043] Both luminescence intensity and luminescence lifetime (decay rate) are appropriate measures for detecting oxygen levels. In various embodiments, luminescence decay rates are used to produce more robust sensors and avoid the problems that may arise with intensity measures (e.g., fluctuations in light-source intensity, index-of-refraction shifts, and variations in film thickness or indicator concentration).

[0044] Having now generally described the invention, the same may be more readily understood through the following reference to the following examples, which are provided by way of illustration and are not intended to limit the present invention unless specified.

Examples

[0045] The examples below illustrate an embodiment of the sensor for detecting changes in oxygen.

1. Scheme A- Using Nitrogen to simulate a target fugitive gas

[0046] Figure 1 illustrates an exemplary system to monitor the concentration of oxygen. The experimental system 190 comprises a test chamber 120 representing a controlled environment. The test chamber comprises a material that allows for maintaining a constant composition of gas within the chamber. The chamber has openings 123 and 125 to allow the fiber optic cable 180 to pass through the chamber. A portion of the fiber optic cable located within the test chamber comprises the sensor composition 130 coated on the core 181 of the fiber optic. The fiber optic is coupled to a light source 150 and a signal detector 160. A data processing unit 170 is coupled to the system such that, information gathered from the sensor composition can by processed by the data processing unit. A composition of gas is permitted to flow through the chamber by way of an input 121 and an output 127. The composition of gasses passing through the test chamber is controlled be a flow meter 1 10. Thus in an experimental environment, one or more input valves 101 can be used to vary the composition of gas that enters the test chamber.

[0047] In one experiment utilizing the experimental set-up of Figure 1, the oxygen sensor polymer was applied on the optical fiber made of specialized core materials over a 2" length. The ends of cable were also connected to light source and to the detector. For the initial exemplary trials Nitrogen was used as opposed to simulate a fugitive gas. This setup was placed in a Plexiglas control chamber. Various combination mixtures of N₂ (0% O₂), air (21 % O₂), and 5% O₂ were then passed through the cylinder, and the response of the sensor to changing ratio of N2 and 02 concentrations were observed. In other experiments, the O₂ and N₂ input may be modified.

Calibration

[0048] Figure 2 illustrates the schematic of a typical test signal. The signal shows the typical output signal showing two areas of the lifetime decay signal. In this figure, P represents the pulse of the LED, followed by decay of luminescence lifetime intensity after pulse is turned off. The decay curves shown are the emission lifetimes of the sensor subject to two different oxygen concentrations. The upper curve represents an oxygen- rich environment and the lower curve represents an oxygen-poor environment.

[0049] Oxygen concentration goes down when other gases become present. The F-Ratio is defined as the ratio of the areas of intensity decay region Il to region I. Notice, when quenching by oxygen is evident, region II is much smaller, thus resulting in a smaller F- Ratio.

I l [0050] Table 1 shows results of tests carried out with scheme 1. It lists the ratios of mixtures OfN₂, air (21% O₂) and 5% O₂. The response of the sensor to the varying oxygen concentrations is reported as the "Fast-Ratio" (F-Ratio). The F-Ratio corresponds to the luminescence lifetime decay (Figure 1). A large F-Ratio is representative of a lifetime measurement taken with little oxygen present. Therefore, a small F-Ratio represents a lifetime measurement taken in an oxygen-rich environment. Results of table 1 are also plotted in Figure 3 shown F-ratio to oxygen concentration.

Table 1 - Gas composition and response of oxygen sensor.

N₂ (cc/min.) Air (cc/min.) 5% O₂ (cc/min.) Resulting % O₂ F-Ratio

100 0 0 0.00 2.30302

200 0 100 1.67 1.82925

100 0 500 4.17 1.67965

200 100 0 7.00 1.55737

200 200 0 10.50 1.49409

200 500 0 15.00 1.42104

100 500 0 17.50 1.40611

0 200 0 21.00 1.33643

Stability

[0051] From an engineering standpoint, the weak link in the design of this leak detector would be concerning the stability of the sensor to the tests of time and repeated use. Figure 4 is a plot demonstrating the robustness of the leak detector, and the reproducibility of its response. After a "warm-up" cycle, four nearly identical responses to a 33% reduction in O₂ concentration were measured.

[0052] Further studies have demonstrated the overall stability of the sensor. It has been shown that the sensitivity of the sensor to oxygen is maintained over many years. Figure 5 displays the responses of a 12-year old film and a freshly synthesized film to changes in oxygen concentration. While the difference in signal intensities at maximum and minimum oxygen levels has declined, the sensitivity is still clearly evident. Furthermore, the primary use of the claimed technology is for detection of leaks, the occurrence of which takes place with relatively short time intervals. This slight loss in sensitivity is attributed to the degradation of the polymer matrix surrounding the luminophor.

[0053] Figure 6 exhibits the photostability of the sensor. The sensor was exposed to an LED pulsed continuously at 2000 Hz for 24 hours. Lifetime measurements taken at the beginning and the end of the time period showed no measurable drift in signal. These features make the sensor ideal for gas leak detection.

2. Scheme 2- Using Methane

[0054] Figure 7 illustrates an experimental setup for gas leak detection. The system illustrated in figure 7 functions similarly to the system in illustrated in Figure 1. The experimental system 790 comprises a test chamber 720 representing a normal soil environment. The test chamber comprises a sand mix that naturally that allows for the flow of gases within the chamber. The chamber has openings 723 and 725 to allow the fiber optic cable 780 to pass through the chamber. A 4 cm portion of the fiber optic cable located within the test chamber comprises the sensor composition 730 coated on the core 781 of the fiber optic. The fiber optic is coupled to a light source 750 and a signal detector 760. A processing unit 770 is coupled to the system for sending, receiving pr otherwise processing system information. A composition of gas is permitted to flow through the chamber by way of an input 721 and an output 727. In this system, the gas flows through the test chamber enclosed in a containment structure 740 (e.g., a pipe), having a valve 745 to release the gas into the test chamber simulating a gas leak. In this system, the fiber optic sensor runs parallel to the gas pipe placed within the sand test environment 726. The composition of gasses passing through the test chamber by way of the pipe is controlled by a flow meter 710. Thus in an experimental environment, one or more input valves 701 can be used to control the gas that enters the test chamber.

[0055] In one set of experiments, testing was done using methane gas leaking inside of a Plexiglas test chamber filled with sand (Figure 7). The chamber was used to evaluate the effectiveness of the proposed leak-detection system for sensing subsurface methane gas leaks. In order to simulate an operational natural gas pipeline, a copper pipe containing standard laboratory methane gas was passed through the longitudinal axis of the chamber. Methane gas leaks were subsequently simulated and controlled through a valve located at mid-length of the pipe. A fiber optic sensor cable was buried alongside the pipe and with sensing compounds 4 cm from the location of the leak. When open, the valve allowed methane gas to leak out of the buried pipe into the surrounding sand.

Results

[0056] Leak events were monitored subject to various chamber environments, including an empty chamber, a sand-filled chamber and a sand-and-water-filled chamber (Figure 8). Long-term performance of the leak-detection system was also tested over a period of 30 days. Figure 9 shows the results of the continuous monitoring of methane gas leaks introduced in 2-minute bursts every 24 hours. The sensor showed no measurable reduction in sensitivity or response time.

[0057] It will be apparent to those skilled in the art that various modifications and variations can be made to various embodiments described herein without departing from the spirit or scope of the teachings herein. Thus, it is intended that various embodiments cover other modifications and variations of various embodiments within the scope of the present teachings.

References:

1. Special Investigation Division, Committee on Government Reform, U.S. House of Representatives, "Oil Refineries Fail to Report Millions of Pounds of Harmful Emissions", a Report Prepared for Rep. Henry A. Waxman, November 10, 19999.

2. Gas Research Institute, "GRI and Indaco Offer New Leak Detection/Measurement Service", GRID Magazine, June 1998.

3. 40 CFR 60.482

4. EPA, Method 21 - Determination of Volatile Organic Compound Leaks.

5. ASTM E 1003-95 Standard Test Method for Hydrostatic Leak Testing.

6. ASTM E515-95 Standard Test Method for Leaks Using Bubble Emission Techniques.

7. ASTM E1066-95 Standard Test Method for Ammonia Colorimetric Leak Testing.

8. ASTM E1002-96 Standard Test Method for Leaks Using Ultrasonic. Khali] G., Kimura F., Chin A., Ghandehari M., Wan R., Shinoki W., Gouterman M., Callis J., Dalton L, "Continuous Monitoring of Underground Gas Leaks", ASNT Research in Nondestructive Evaluation, vlό n3, July 2005,

Claims

WHAT IS CLAIMED IS:

1. A system for monitoring a fugitive gas concentration in an environment comprising: an optic fiber having a cladding layer and a core, wherein the core is coated with a gas-permeable polymeric material, having a sensor composition comprising a luminophor sensitive to quenching by oxygen to indicate variations in oxygen concentration caused by the presence of a target fugitive gas; a light source for illuminating the optic fiber; a signal detector to detect variations in the oxygen concentration as indicated by the sensor; and a processing unit to collect and maintain data regarding the oxygen concentration.

2. The system of claim 1 wherein the refractive index of the polymeric material comprising the core is optimized for distributed sensing along the length of the optic fiber.

3. The system of claim 1 wherein the refractive index of the polymeric material comprising the cladding is optimized for distributed sensing along the length of the optic fiber.

4. The system of claim 1 wherein the signal detector detects an increase in luminescence from a reduction in oxygen concentration caused by the displacement of oxygen molecules by the presence of the target fugitive gas.

5. The system of claim 1 wherein variation in the target fugitive gas concentration is determined based on continuous measures of ambient oxygen concentration through evanescent wave spectroscopy, where the detector detects a signal indicative of the variation in the ambient oxygen concentration and thereby the concentration of the target fugitive gas.

6. The system of claim 1 wherein excitation of the sensor composition triggers an alarm notification of a change in oxygen concentration.

7. The system of claim 1 wherein the luminophor comprises an oxygen-responsive porphyrin compound.

8. The system of claim 1 where the optic fiber is located near a containment structure housing volatile organic compounds.

9. The system of claim 1 where the presence of the target fugitive gas includes volatile organic compounds in the environment.

10. A method of monitoring the fugitive gaseous concentration in an environment comprising: detecting oxygen variation utilizing the system of claim 1; positioning the optic fiber in a location having a continual oxygen concentration present; illuminating optical fiber with a light source; exciting the sensor composition, where the sensor composition comprises a luminophor sensitive to quenching by oxygen; detecting variations in the ambient oxygen concentration caused by the displacement of oxygen molecules by the presence of a target fugitive gas, as indicated by the excitation of the sensor composition; and signaling detection of a variation in ambient oxygen concentration.

1 1. A method of claim 10 further comprising optimizing the refractive index of the polymeric core to allow distributed sensing.

12. A method of claim 10 further comprising exciting oxygen-responsive porphyrin compound.

13. A method of claim 10 further comprising detecting a signal from the sensor composition representative of the variation in oxygen concentration caused by the presence of a target fugitive gas.

14. A method of claim 13 further comprising transferring the detected signal to a data processing unit for tracking the concentration of leaking fugitive gases.

15. A method of claim 10 further comprising distributed measurement of gas leak concentrations using evanescent wave spectroscopy, where the variation in the ambient oxygen concentration is sensed by the sensor composition as an indication of the concentration of fugitive gas.

16. A sensor for measuring fugitive gas concentration in an environment comprising: an optic fiber having a cladding layer and a core, wherein the core is coated with a gas- permeable polymeric material, having a sensor composition comprising a luminophor sensitive to quenching by oxygen to indicate variations in the concentration of fugitive gas displacing the oxygen, wherein the refractive index of the polymeric materia! comprising the core is optimized for distributed sensing along the length of the optic fiber.

17. A sensor of claim 16 wherein the luminophor comprises oxygen-responsive porphyrin compound.

18. A sensor of claim 16 wherein leaks of fugitive gases can be detected over a distance between 0.10-40Om.

19. A sensor of claim 16 wherein the sensor composition detects a variation in gas leaks concentration caused by the displacement of oxygen molecules by the presence of volatile organic compounds.

20. A sensor of claim 16 wherein the intensity of the excited the luminophor is a function of the variation in oxygen concentration displaced by fugitive gases.