Abstract:
A leak detector ( 190 ) includes a sensor head ( 260 ), a light source ( 200 ) optically coupled to the sensor head and operable to generate excitation light. A detector ( 205 ) is optically coupled to the sensor head and operable to detect fluorescence light. A signal processing unit ( 210 ) is coupled to the detector and operable to signal a leak condition responsive to an intensity of the fluorescence light exceeding a threshold. A fluid-tight enclosure ( 235 ) encloses at least the light source, the detector, and the signal processing unit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The disclosed subject matter relates generally to subsea hydrocarbon production and, more particularly, to an optical leak detector for detecting material leakage from subsea equipment. 
         [0002]    To control a subsea well, a connection is established between the well and a monitoring and control station. The monitoring and control station may be located on a platform or floating vessel near the subsea installation, or alternatively in a more remote land station. The connection between the control station and the subsea installation is usually established by installing an umbilical between the two points. The umbilical may include hydraulic lines for supplying hydraulic fluid to various hydraulic actuators located on or near the well. The umbilical may also include electrical and or fiber optic lines for supplying electric power and also for communicating control signals and/or well data between the control station and the various monitoring and control devices located on or near the well. 
         [0003]    Hydrocarbon production from the subsea well is controlled by a number of valves that are assembled into a unitary structure generally referred to as a Christmas tree. Christmas tree and wellhead systems have the principle functions of providing an interface to the in-well environment, allowing flow regulation and measurement, and permitting intervention on the well or downhole systems during the operational life of the well. The actuation of the valves in the Christmas tree is normally provided using hydraulic fluid to power hydraulic actuators that operate the valves. Hydraulic fluid is normally supplied through an umbilical running from a remote station located on a vessel or platform at the surface. Alternative systems using electrically based actuators are also possible. 
         [0004]    The subsea equipment includes many possible leakage paths, such as valves, pipe junction actuators, flanges, pipe connectors, jumpers, seals, etc. The detection of leaks of hydrocarbons, hydraulic fluids, tracers and other chemicals from underwater structures is an important requirement in enhancing the environmental and operational efficiency of underwater systems such as Christmas trees and subsea processing systems. A range of underwater leak detection systems have been developed including those based on acoustic, fluorescence, temperature and gas measurements. Often these are designed to be used as part of survey operations, but in a few instances as permanent monitors. Such devices are typically portable and are mounted to mobile devices, such as remotely operated vehicles (ROVs). 
         [0005]    Acoustic devices are capable of detecting leaks from a wide area via the noise that may be produced by material leaking from underwater structures. Such acoustic systems detect only the secondary effect of the leak (i.e., the noise), hence, the application of acoustic detectors is significantly restricted in noisy environments. Further, acoustic detectors are not generally able to accurately locate a leak. 
         [0006]    This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The following presents a simplified summary of the disclosed subject matter in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
         [0008]    One aspect of the disclosed subject matter is seen in a leak detector. The leak detector includes a sensor head, a light source optically coupled to the sensor head and operable to generate excitation light. A detector is optically coupled to the sensor head and operable to detect fluorescence light. A signal processing unit is coupled to the detector and operable to signal a leak condition responsive to an intensity of the fluorescence light exceeding a threshold. A fluid-tight enclosure encloses at least the light source, the detector, and the signal processing unit. 
         [0009]    Another aspect of the disclosed subject matter is seen in a subsea leak detection system. The subsea leak detection system includes a subsea structure operable to contain a hydrocarbon material. A leak detection module includes a sensor head mounted proximate a source of potential leakage on the subsea structure, a light source optically coupled to the sensor head and operable to generate excitation light, a detector optically coupled to the sensor head and operable to detect fluorescence light, a signal processing unit coupled to the detector and operable to signal a leak condition responsive to an intensity of the fluorescence light exceeding a threshold, and a fluid-tight enclosure enclosing at least the light source, the detector, and the signal processing unit. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]    The disclosed subject matter will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements, and: 
           [0011]      FIG. 1  is a simplified diagram of a subsea installation for hydrocarbon production; 
           [0012]      FIGS. 2-6  illustrate embodiments of a leak detection module in the system of  FIG. 1 ; and 
           [0013]      FIGS. 7-8  illustrate leak channeling devices that may be used with the leak detection module of  FIG. 1 . 
       
    
    
       [0014]    While the disclosed subject matter is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the disclosed subject matter to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the disclosed subject matter as defined by the appended claims. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    One or more specific embodiments of the disclosed subject matter will be described below. It is specifically intended that the disclosed subject matter not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. Nothing in this application is considered critical or essential to the disclosed subject matter unless explicitly indicated as being “critical” or “essential.” 
         [0016]    The disclosed subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the disclosed subject matter with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the disclosed subject matter. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
         [0017]    Referring now to the drawings wherein like reference numbers correspond to similar components throughout the several views and, specifically, referring to  FIG. 1 , the disclosed subject matter shall be described in the context of a subsea installation  100  located on the seabed  110 . The installation  100  includes a schematically depicted Christmas tree  120  mounted on a wellhead  130 . The wellhead  130  is the uppermost part of a well (not shown) that extends down into the sea floor to a subterranean hydrocarbon formation. An umbilical cable  140  for communicating electrical signals, fiber optic signals, and/or hydraulic fluid extends from a vessel  150  to the Christmas tree  120 . In other embodiments, the vessel  150  may be replaced by a floating platform or other such surface structure. In one illustrative embodiment, a flowline  160  also extends between the vessel  150  and the Christmas tree  120  for receiving hydrocarbon production from the well. In some cases, the flowline  160  and a communications line (not shown) may extend to a subsea manifold or to a land based processing facility. A topside control module (TCM)  170  is housed on the vessel  150  to allow oversight and control of the Christmas tree  120  by an operator. 
         [0018]    A subsea control module (SCM)  180  is mounted to the Christmas tree  120  for receiving control signals from operators on the vessel  150  and for communicating data from various wellhead and downhole sensors to the TCM  170 . A leak detection module (LDM)  190  provides local leak detection for various subsea components such as valves, pipe junction actuators, flanges, pipe connectors, jumpers, seals, etc. associated with the Christmas tree  120 , umbilical  140 , or flowline  160 . In the illustrated embodiment, the LDM  190  interfaces with the SCM  180  for communicating leak detection signals to the TCM  170 . Alternatively, the LDM  190  may have its own communication interface, such as through the umbilical cord  140  for topside communication. The SCM  180  may automatically respond to leaks detected by the LDM  190  depending on the location and severity of the leak. Alternatively, the TCM  170  may control leak responses via automatic control or operator input. 
         [0019]      FIG. 2  illustrates a simplified diagram of one embodiment of an LDM  190 A. The LDM  190 A includes a light source  200 , detector  205 , a signal processing unit  210 , and optical lenses  215 ,  220 ,  225 ,  230  mounted in a water tight enclosure  235 . Two optical fibers  240 ,  245  are coupled to the enclosure  235  via subsea optical connectors  250 ,  255 . The excitation fiber  240  is coupled to the light source  200 , and the detection fiber  245  is coupled to the detector  205 . The optical fibers  240 ,  245  run to a sensor head  260  mounted near a source of potential leakage. An electrical connector  265  is provided for communicating between the LDM  190 A and an external device, such as the SCM  180 . 
         [0020]    Although the LDM  190 A is illustrated as having its own water tight enclosure  235 , it is contemplated that the LDM  190 A may be located in the same water tight enclosure as the SCM  180 . The subsea optical connectors  250 ,  255  are optical feedthrough devices that allow the light to be transmitted across the pressure barrier while protecting the optoelectronics. 
         [0021]    The excitation fiber  240  conveys excitation light from the light source  200 , which emerges from the far end of the fiber  240  at the sensor head  260  in a cone. The characteristics of the cone depend on the numerical aperture of the excitation fiber  240  and the refractive index of the water. In some embodiments, an interference filter  270  is mounted between the light source  200  and the excitation fiber  240  (i.e., and between the lenses  215 ,  220  to remove wavelengths other than those useful for excitation. In yet other embodiments, the sensor head  260  may include a lens system fitted to the ends of the optical fibers  240 ,  245  to enhance the excitation and/or detection of the fluorescence or to vary the range from the sensor head were the fluorescence is collected. 
         [0022]    The fluorescence light that is generated by the excitation light is collected by the detection fiber  245 , which is located adjacent to the excitation fiber  240 , but is held in the sensor head  260  at an angle with respect to the excitation fiber  240  (e.g., approximately 20 degrees). The light source  200  may be a broadband lamp, light emitting diode (LED), or laser that generates light of a wavelength suitable to excite the fluorescence of the leaking material. The excitation light is launched into the excitation fiber  240  using the lenses  215 ,  220 . The collected fluorescence light emerging from the detection fiber  245  is passed to the detector  205 . 
         [0023]    In one embodiment, the detector  205  may be a spectrometer configured to detect light at the appropriate fluorescence wavelength (e.g., typically 350 nm-600 nm). The spectrometer may be implemented using a charge coupled device (CCD) to allow rapid collection and processing of the spectral information. Such a CCD may be operated in a DC mode or, alternatively, the CCD may be synchronized with a modulated or pulsed light source and employ time-gated detection. In either case, the output signal of the CCD at the fluorescence wavelength is related to the concentration of the leaking material in the water. The signal processing unit  210  receives the output of the detector  205  and identifies a potential leak and/or leak rate based on the magnitude of the florescence signal. 
         [0024]    In another embodiment, an optical interference filter  275  may be used to filter the detection light to pass only the fluorescence wavelengths. In such an embodiment, the detector  205  may not analyze the entire spectrum, as with a spectrometer, but rather, the detector  205  may measure overall light intensity. A broadband detector, such as, but not limited to a photo diode or an avalanche diode may be used to implement the detector  205 . The optical interference filter  275  would not be necessary in the embodiment where the detector  205  is a spectrometer. 
         [0025]    An alternative embodiment of an LDM  190 B is shown in  FIG. 3 . The excitation light is amplitude modulated by the light source  200 , and the detector  300  is a phase sensitive detector. In this embodiment, an optical interference filter  305  with a band pass corresponding to the peak fluorescence wavelength of the leaking material is fitted in front of the detector  300  between the lenses  225 ,  230  to reject background light. The collected fluorescence emerging from the detection optical fiber  245  is passed through this filter  305  and onto the detector  300 . The resultant signal is processed using the phase sensitive detector  300  to provide an electrical output signal relating to the concentration of the leaking material. The signal processing unit  210  receives the output of the detector  300  and identifies a potential leak and/or leak rate based on the magnitude of the florescence signal. 
         [0026]    Another alternative embodiment of an LDM  190 C is shown in  FIG. 4 . The LDM  190 C, employs similar principles as the multi-fiber probes of  FIG. 2  or  3 , but employs only a single fiber  400  to deliver the excitation light and to capture the induced fluorescence of the leaking fluid. The excitation light passes through a beam splitter  405  before entering the fiber  400 . The detection light resulting from the induced fluorescence is captured by the same fiber  400 . Upon exiting the fiber  400 , the fluorescent detection light is directed by the beam splitter  405  and passed to a detector  410 . The detector  410  may be a phase sensitive detector, as described in reference to the embodiment of  FIG. 3 , or a spectrometer, photo diode, or avalanche diode as described in reference to the embodiment of  FIG. 2 . The optical interference filter  415  may be used in some embodiments. 
         [0027]    In the various embodiments illustrated in  FIGS. 2-4 , a single monitoring location is illustrated. Multiple locations may also be monitored using a multi-optical fiber connector between the LDM  190  and the sensor heads  260 . For example, an 8-fibre connector from HydroBond Engineering may be used. Using such a connector, multiple sensor systems within the LDM  190  may be used to access multiple locations. 
         [0028]    It is also contemplated that multiple locations may be monitored using shared hardware, depending on the particular application. For example, as illustrated in  FIG. 5 , the LDM  190 D employs wavelength multiplexing. In this embodiment, a fiber  500  exiting the enclosure  235  runs to an optical coupler  505  that distributes individual fibers  510  to multiple locations. A wavelength selective component is incorporated into the optical fiber sensing heads  260 . For example, an interference filter  515  may be attached to the fiber end or a Bragg grating (not shown) may be written into the fiber core. These wavelength selective components each have a distinct pass band which coincides with part of the fluorescence emission of the target species (i.e., oil or hydraulic fluid). The individual sensing heads  260  may be located at particular monitoring locations and each sensor head  260  distinguished at the detector  520  (e.g., spectrometer) by the specific wavelength range of the filter  515 . 
         [0029]    Alternatively, it is possible to use a variety of optical multiplexing arrangements to access multiple sensors from a single excitation and detection system. For example, as illustrated in the LDM  190 E of  FIG. 6 , light from a single source  200  is transmitted to an optical switch  600 , for example a 1×4 switch. The four output fibers  605  from the switch  600  are connected to separate fibers of a multi-fiber optical connector  610 , and a plurality of connected fibers  615  on the subsea side of the connector  610  form the excitation fibers of a two-fiber sensor head  260  configuration. The sensor heads  260  may be placed at varying locations for leak monitoring. Detection fibers  620  for each of the 4 sensor heads  260  may be coupled to an optical fiber coupler  625  to reduce them into a single fiber  630 , which is then fed to the connector  610 . Alternatively, each detection fiber  620  may be connected to the connector  610 , and subsequently multiplexed to the detector  640 . On the inside of the water-tight enclosure  235 , the detection fiber  635  is connected to the detector  640 , which may be a spectrometer or other type of photo detector used with an interference filter  645 . The individual sensor head  260  being monitored is determined by the setting of the optical switch  600 , which only enables one sensor at a time. Monitoring of all four sensor heads  260  is performed by periodically enabling individual fibers  605 . Of course, other types of multiplexing configurations may be used based on the teachings herein. 
         [0030]    In some embodiments the LDM  190  may be used in conjunction with a leak channeling device which directs the leaking material to the vicinity of the sensor head  260  or reduces the dispersion rate of the leaking fluid to allow detection. One exemplary channeling device includes a canopy  700 , as illustrated in  FIG. 7 . The canopy  700  is positioned above a subsea structure  705  to collect lighter than water components (e.g., oil). In such arrangements the sensor head  260  is located proximate the highest point  710  of the underside of the canopy  700 , and the optical cable  720  (i.e., containing one or more optical fibers) runs to the LDM  190  (not shown). The fluid collects under the canopy  700  to facilitate leak detection. 
         [0031]    As illustrated in  FIG. 8 , another type of leak channeling device is a collar  800 . The collar  800  may be mounted to cover a cylindrical structure  805 , such as a pipe section near a flange or joint. The sensor head  260  monitors the annular region between the collar  800  and the structure  805 . The collar  800  encloses the structure  805  at a potential leak location  810  to provide coverage for the entire outside diameter. Without such a collar  800 , a leak on one side of the cylindrical structure  805  may not be detected if the sensor head  260  is positioned on the other side. In this arrangement, the collar  800  does not provide a pressure tight seal to the structure  805 , but rather acts to slow dispersion of the leaking material so that is collects proximate the sensor head  260 . The optical cable  815  runs to the LDM  190  (not shown). 
         [0032]    In general for all of the embodiments illustrated, the signal processing unit  210  uses the fluorescence output signal to identify and quantify leakage at the monitored location. The signal processing unit  210  may signal a leak detection when the detection light intensity at the fluorescent wavelength passes a predetermined threshold. Increases in intensity past the detection threshold could be used to identify a leakage rate (i.e., quantitatively or qualitatively). To quantify a leak rate, qualification testing could be conducted to correlate the measured intensity to leak rate. The correlation between measured intensity and leak rate may depend on factors such as geometry, type of material, material dispersion rate, the presence of leak channeling devices, etc. The leak detection module  190  allows real-time leak detection and analysis. 
         [0033]    The optical fiber fluorescence leak detection system embodiments described herein have numerous advantages. The leak detection module  190  may be installed permanently on a subsea structure. The leak detection module  190  may also be integrated with the existing well control instrumentation and communication schemes. A network of leak detection modules  190  may be employed to monitor many different paths of potential leakage on the subsea structure. The network may include individual leak detection modules  190  for each monitoring location or leak detection modules  190  that service multiple sensor heads. 
         [0034]    The particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.