# Patent

## Title
System and method for identifying levels or interfaces of media in a vessel

## Abstract

Systems and methods for identifying the level of media in a vessel with a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the vessel. An excitation source is configured to propagate at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable. An optical signal interrogator is adapted to receive a signal from each of a plurality of sensor locations and measure a temperature profile corresponding to the heat pulse at the sensor locations. A control unit is configured to identify a level of each of media by determining properties of the media exposed to the sensing cable at each of the sensor locations based on the temperature profile corresponding thereto.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application relates and claims priority to U.S. Provisional Patent Application No. 61/806,076, filed on Mar. 28, 2013.


FIELD
The presently disclosed subject matter relates to methods and systems for identifying levels or interfaces of media in a vessel. More particularly, the presently disclosed subject matter relates to determining levels and interfaces of media in a vessel using a sensing cable including an optical fiber sensor array aligned with a heating element.
BACKGROUND
Components of certain equipment, such as that used in the petroleum and petrochemical industry, which includes the exploration, production, refining, manufacture, supply, transport, formulation or blending of petroleum, petrochemicals, or the direct compounds thereof, are often monitored to maintain reliable operation. However, such components can involve harsh conditions, such as high temperature, high pressure, and/or a corrosive environment, making it difficult or costly to obtain reliable measurements.
Measurement of the level of, or interfaces between, one or more media in a vessel, such as a desalter unit or subsea production well petrochemical applications, can provide for enhanced control and operation. For example, a desalting operation can be facilitated and enhanced by determining the location of interfaces between gas, foam, oil, emulsion, water and/or solids in a refinery desalter unit. Better emulsion band detection and profiling can improve level, mix valve, and chemical injection control and can allow partial automation of the desalter operation. Likewise, in connection with subsea separation of gas, water, oil, emulsion, and sand from a well, measurement of the interface or levels between the various media can provide enhanced control of the extraction process.
Conventional approaches to measure liquid level/interface can include monitoring locations of floated displacers, capacitance, wave reflectance, or energy absorption. However, these conventional approaches can suffer from certain drawbacks. For example, float-type level measurement is based on liquid density difference (such as between oil and water), and therefore it can be difficult to deploy multiple displacers for multiple level/interface monitoring. Furthermore, the moving displacers can become coated by liquid (such as crude oil), resulting in sensor failure. Capacitance probes require direct contact with fluids to measure electrical properties and therefore, may deliver false readings if fouling or waxing occurs on the probe. Additionally, because the electrical properties can be temperature dependent, capacitance-based probe techniques can also require temperature compensation. Wave reflectance approaches can have limited penetration depth because certain materials can absorb the acoustic, radio frequency, or microwaves, and thus are unsuitable for multi-level/interface monitoring. Moreover, use of energy absorption techniques can be limited to narrow temperature ranges depending upon the sensitivity of the equipment involved.
As stable desalter emulsions are becoming increasingly common due to the increase in penetration of challenged crudes in refineries, crudes with high asphaltenes, high TAN, high solids, low API gravity, or high viscosity tend to form emulsions that are difficult to break. As a result, water and salt removal can be reduced, the oil content of the brine can increase, and disruptions in operation can become more common. Improved emulsion band detection and profiling in a desalter unit therefore would improve level, mix valve, and chemical injection control and allow for partial automation. In like manner, improved detection of interfaces between mixture components in subsea separation operations can improve control over extraction.
Accordingly, there is a continued need for improved techniques for identifying levels or interfaces of media in a vessel.
SUMMARY
The purpose and advantages of the disclosed subject matter will be set forth in and apparent from the description that follows, as well as will be learned by practice of the disclosed subject matter. Additional advantages of the disclosed subject matter will be realized and attained by the methods and systems particularly pointed out in the written description and claims hereof, as well as from the appended drawings. To achieve these and other advantages and in accordance with the purpose of the disclosed subject matter, as embodied and broadly described, the disclosed subject matter includes systems and methods for identifying the level of one or more media in a vessel.
In accordance with one aspect of the disclosed subject matter, a method for identifying the level of one or more media in a vessel includes providing within a vessel a sensing cable including an optical fiber sensor array aligned with a heating element. The method includes propagating at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable. The method includes measuring, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of a plurality of sensor locations on the optical fiber sensor array. The method includes identifying a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
In certain embodiments, the vessel can include a desalter unit and identifying the level of one or more media can include identifying the level of one or more of gas, foam, oil, emulsion, water, or solids. Alternatively, the vessel can include a subsea production well, and identifying the level of one or more media can include identifying the level of one or more of gas, foam, crude, water, emulsion, or sand.
As embodied herein, the at least one heat pulse can include a plurality of heat pulses. Measuring the temperature profile corresponding to the heat pulse at each of the plurality of sensor locations can include, for each sensor location, measuring a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location. Identifying the level of the one or more media can include, for each temperature profile, performing a regression of the plurality of temperatures over a logarithm of corresponding measurement times for a predetermined time window in the period of time to generate a slope and an intercept of the regression, wherein the slope and the intercept relate to the one or more properties of the medium exposed to the sensing cable at the sensor location. Identifying the level of the one or more media can include calculating a difference in the slope and the intercept between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold. Additionally or alternatively, identifying the level of the one or more media can include, for each temperature profile, generating a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time, applying a transform to the time derivative to generate a complex spectrum, and determining an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location. Identifying the level of the one or more media can further include generating a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency and determining an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location. Identifying the level of the one or more media can include calculating a difference in the amplitude and the phase between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
In accordance with another aspect of the disclosed subject matter, a system for identifying the level of one or more media in a vessel including one or more media includes a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the vessel, the optical fiber sensor array having a plurality of sensor locations. The system includes an excitation source coupled with the heating element and configured to propagate at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable. The system includes an optical signal interrogator coupled with the optical fiber sensor array and adapted to receive a signal from each of the plurality of sensor locations and configured to measure, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of the plurality of sensor locations on the optical fiber sensor array. The system includes a control unit, coupled with the heating element the optical signal interrogator, to identify a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
In certain embodiments, the vessel can include a desalter unit, and the one or more media can include one or more of gas, foam, oil, emulsion, water, or solids. Alternatively, the vessel can include a subsea production well, and the one or more media can include one or more of gas, foam, crude, water, emulsion, or sand.
As embodied herein, the optical signal interrogator can be configured, for each of the plurality of sensor locations, to measure a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location. The control unit can be configured, for each temperature profile, to perform a regression of the plurality of temperatures over a logarithm of corresponding measurement times for a predetermined time window in the period of time to generate a slope and an intercept of the regression, wherein the slope and the intercept relate to the one or more properties of the medium exposed to the sensing cable at the sensor location. The control unit can be configured to calculate a difference in the slope and the intercept between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold. Additionally or alternatively, the control unit can be configured, for each temperature profile, to generate a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time, apply a transform to the time derivative to generate a complex spectrum, and determine an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location. The control unit can be further configured to generate a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency, and determine an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location. The control unit can be configured calculate a difference in the amplitude and the phase between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the disclosed subject matter claimed.
The accompanying drawings, which are incorporated in and constitute part of this specification, are included to illustrate and provide a further understanding of the disclosed subject matter. Together with the description, the drawings serve to explain the principles of the disclosed subject matter.



BRIEF DESCRIPTION OF THE DRAWINGS
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
FIG. 1A is a schematic diagram of an exemplary sensing system in accordance with the disclosed subject matter.
FIG. 1B is a cross sectional view of an exemplary sensing cable configuration in accordance with the disclosed subject matter.
FIG. 1C is a cross sectional view of another exemplary sensing cable configuration in accordance with the disclosed subject matter.
FIG. 2 depicts a representative plot of current and heat pulses and corresponding temperature response in accordance with the disclosed subject matter.
FIG. 3 is a graph illustrating a direct temperature sensing technique for a plurality of sensor locations in accordance with the disclosed subject matter.
FIG. 4A is a graph illustrating log-time regression sensing technique in accordance with the disclosed subject matter.
FIG. 4B is a graph illustrating log-time regression sensing technique for a plurality of sensor locations in accordance with the disclosed subject matter.
FIG. 5A is a graph illustrating thermal excitation energy concentration at harmonics and fundamental frequencies of heat pulses in connection with a frequency spectrum sensing technique.
FIG. 5B is a graph illustrating the phase of a frequency-derivative spectrum in connection with frequency spectrum sensing techniques over a plurality of sensor locations in accordance with the disclosed subject matter.
FIG. 5C is a graph illustrating the amplitude of a frequency-derivative spectrum in connection with frequency spectrum sensing techniques over a plurality of sensor locations in accordance with the disclosed subject matter.
FIG. 6 is a schematic representation of a system and method for identifying the level and interfaces of media in a vessel in accordance with an exemplary embodiment of the disclosed subject matter.
FIG. 7 is a schematic cross sectional view of a representative embodiment of a shield for a cable configuration in accordance with an exemplary embodiment of the disclosed subject matter.



DETAILED DESCRIPTION
As noted above and in accordance with one aspect of the disclosed subject matter, methods disclosed herein include identifying the level of one or more media in a vessel with a sensing cable including an optical fiber sensor array aligned with a heating/cooling element includes propagating at least one heating/cooling pulse through the heating/cooling element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and one or more media exposed to the sensing cable. The method includes measuring, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of a plurality of sensor locations on an optical fiber sensor array. The method includes identifying a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
Furthermore, systems for identifying the level of one or more media in a vessel are also provided. Such systems include one or more media includes a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the vessel, the optical fiber sensor array having a plurality of sensor locations. The system further includes an excitation source coupled with the heating element and configured to propagate at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable. The system also includes an optical signal interrogator coupled with the optical fiber sensor array and adapted to receive a signal from each of the plurality of sensor locations and configured to measure, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of the plurality of sensor locations on the optical fiber sensor array. A control unit, coupled with the heating element and the optical signal interrogator, identifies a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
Reference will now be made in detail to the various exemplary embodiments of the disclosed subject matter, exemplary embodiments of which are illustrated in the accompanying drawings. The accompanying figures, where like reference numerals refer to identical or functionally similar elements, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the disclosed subject matter. The accompanying figures, where like reference numerals refer to identical or functionally similar elements, serve to further illustrate various embodiments and to explain various principles and advantages all in accordance with the disclosed subject matter. For purpose of explanation and illustration, and not limitation, exemplary embodiments of the disclosed subject matter are shown in FIGS. 1-7.
In accordance with the disclosed subject matter, characteristics of one or more materials can be measured with the use of an optical fiber sensor array having a plurality of sensor locations aligned with a heating/cooling element in a sensing cable. At least one heating/cooling pulse is propagated through the heating/cooling element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating/cooling element and one or more media exposed to the sensing cable. A temperature profile of the sensing cable (e.g., in the time domain and/or spatial domain) corresponding to the heating/cooling pulse at the plurality of sensor locations on the optical fiber sensor array can be measured to support a variety of techniques in accordance with the disclosed subject matter.
Generally, for purpose of illustration and not limitation, thermal properties, such as material density, thermal conductivity, heat capacity, or heat diffusion coefficient, of one or more materials can be measured by generating a heat disturbance and sensing a temperature response. In like fashion, dynamic physical properties, such as the flow of a material, can also be measured. As disclosed herein, techniques for measuring temperature can include obtaining temperature measurements in both the temporal and spatial domain. For example, distributed temperature sensing (DTS) systems can provide temperature measurements along the length of a sensing cable continuously or at regular intervals. The change in these temperature measurements can correspond to certain properties of a surrounding material or materials.
For purpose of illustration, and not limitation, an exemplary system for measuring the characteristics of a material in accordance with an exemplary embodiment of the disclosed subject matter will be described. In general, with reference to FIG. 1A, an exemplary sensing system in accordance with the disclosed subject matter can include a sensing cable 101 having disposed therein a heating/cooling device 103 and optical fiber sensor array having a plurality of sensors 102. The sensing cable 101 can be operatively coupled with a control unit 106. For example, the heating/cooling device 103 can be coupled with an excitation source 105, which in turn can be coupled with the control unit 106. Likewise, the optical fiber sensor array 102 can be coupled with a signal interrogator 104, which can be coupled with the control unit 106. Generally, uniform heat can be delivered (e.g., heat energy can be provided or absorbed) along the sensing cable 101 via the heating/cooling device 103 and the excitation source 105. A temperature profile or its variation with time (e.g., variation rate) can be measured using the optical fiber sensor array 102 and signal interrogator 104. The control unit 106 can be adapted to collect data, process data, and/or present data for visualization, for example via one or more displays (not shown).
The sensing cable 101 can be arranged in a variety of configurations. Two exemplary configurations are depicted in FIG. 1B and FIG. 1C, respectively. For example, FIG. 1B depicts a cross section of a sensing cable 101 with the heating/cooling device 103 and the optical fiber sensor array 102 arranged in parallel with each other. The sensing cable 101 can include, for example, an outer casing (not shown) optionally filled with a filler material 110 to maintain the heating/cooling device 103 and optical fiber sensor array 102 in place. Additionally or alternatively, the filler can be extended about the heating/cooling device 103 and temperature sensor 102 with or without the outer casing. The filler can be, for example, a material with high thermal conductivity, such as magnesium oxide (MgO). The outer casing can be a rigid and/or durable material, for example a metal tube. To ensure measurement accuracy, e.g., under harsh conditions, such as fouling or corrosion, the sensing cable 101 casing can be treated with a suitable coating, as described in more detail below. Alternatively, and as depicted in cross section in FIG. 1C, the heating/cooling device 103 and the temperature sensor array 102 can be generally coaxial with each other, wherein the heating/cooling device 103 is disposed concentrically around the temperature sensor array 102.
As embodied herein, the sensing cable 101 can be mineral insulated for protection of a optical fiber sensor array 102 including one or more optical fibers. The optical fibers can be coated and placed into a protective tube structure for enhanced mechanical integrity and resistance to adversary effects of environmental factors, such as H2, H2S and moisture. The sensing cable 101 can further be protected using metal and mineral insulation material (e.g., MgO) for effective thermal conduction. The optical fibers can have a relatively small diameter, and thus can be placed into a protective tube with a relatively small diameter, allowing a faster thermal response and dynamic process monitoring. One of ordinary skill in the art will appreciate that the dimensions of the sensing cable 101 can be selected for a desired application. For example, if further protection from the local environment is desired, a sensing cable 101 with a larger diameter, and thus additional filler, can be selected.
Furthermore, a number of commercially available fibers for the temperature sensor 102 can be used, such as a Fiber Bragg Grating array, Raman scattering based sensor, Rayleigh scattering based sensor or Brillioun scattering based sensor. One of ordinary skill in the art will appreciate that each type of fiber sensor can have certain properties, such as response time, sensing resolution, immunity to hydrogen darkening, effective sensing cable length, and ability to sense temperature and/or strain, as illustrated for purpose of example and not limitation in Table 1. For example, a Fiber Bragg grating sensing system can include a relatively fast response time, high spatial resolution, and can be employed over a sensing cable length upwards of 100 km or longer in connection with the use of optical fiber amplifiers. Raman and Brillouin scattering sensing systems can have relatively low response times (e.g., on the order of several seconds), and spatial resolution on the order of centimeters. Rayleigh scattering sensing systems, when operated to sense temperature, can have a response time of several seconds with relatively high spatial resolution.











TABLE 1






Fastest
Typical point
Immunity
Longest



response
sensor
to H2
sensor


Sensor types
time
size (m)
darkening
cable length







Fiber Bragg
<10 ms
0.01
high
<100 km or


Grating (FBG)



longer


Raman
>Several
0.25~0.5 
low
<100 km


scattering sensor
seconds


Rayleigh
>Several
0.01
low
 <70 m


scattering sensor
seconds


(Temp)


Rayleigh
 <1 ms
0.5 
low
<100 km


scattering sensor


(Acoustic)


Brillouin
>Several
0.1~50
low
<100 km


scattering sensor
seconds









One of ordinary skill in the art will also appreciate that certain of the various types of sensing systems can be used to sense temperature and/or strain (e.g., to sense acoustics). For example, Fiber Bragg Grating sensing systems can be used to measure both temperature and strain, for purposes of sensing temperature and acoustics. Raman scattering sensing systems are typically used to sense temperature. Brillouin scattering sensing systems can be used to measure temperature and strain, and are typically used to sense temperature. Rayleigh scattering sensing systems can be used to measure temperature and strain, and can be used to sense either temperature or acoustics. One of ordinary skill in the art will appreciate that when Rayleigh scattering sensing systems are used to sense acoustics, response time can increase to lower than 1 ms and spatial resolution can increase to approximately 50 cm.
Referring again to FIG. 1A, and as noted above, the control unit 106 can be coupled with the signal interrogator 104. The signal interrogator 104 can be, for example, an optical signal interrogator. Various optical signal interrogators may be used, depending on the type of optical fiber sensing techniques to be employed. The controller 106 can be adapted to perform signal processing on real-time temperature data provided by the signal interrogator 104. For example, the control unit 106 can be adapted to identify and record continuous or repeated temperature measurements at each of a plurality of sensor locations along the sensing cable 101. Additionally, the control unit 106 can be adapted to process temperature measurements over time to identify a characteristic of the material surrounding the sensing cable at one or more sensor locations.
As disclosed herein, a variety of suitable methods can be employed for generating the heating/cooling pulse along the sensing cable 101. As used herein, the term “pulse” includes a waveform of suitable shape, duration, periodicity, and/or phase for the intended purpose. For example, and not limitation, and as described further below, the pulse may have a greater duration for one intended use, such as the determination of deposits, and a shorter duration for another intended use, such as the determination of flow. As embodied herein, the heating/cooling device 103 can be an electrically actuated device. For example, the heating/cooling device 103 can include a resistive heating wire, and the excitation source 105 can be electrically coupled with the heating wire and adapted to provide a current there through. Passing of a current through the resistive heating wire can provide thermal energy along the length of the sensing cable 101, thereby generating a uniform heating/cooling effect along the sensing cable. Alternatively, the heating/cooling device 103 can include a thermoelectric device, and can be likewise coupled to the excitation source 105. The thermoelectric device can use the Peltier effect to heat or cool a surrounding medium. That is, for example, the thermoelectric device can be a solid-state heat pump that transfers heat from one side of the device to the other. The thermoelectric device can be configured, for example, to provide heating to the optical fiber sensor for a certain polarity of electric potential and cooling for the opposite polarity. As disclosed herein, and for purpose of simplicity, the terms “heating/cooling device”, and “heating/cooling pulse” will be referred to generally as a “heating device” or “heating element” and as a “heat pulse,” respectively. Depending upon the context, such terms are therefore understood to provide heating, cooling, or both heating and cooling.
In an exemplary embodiment of the disclosed subject matter, the excitation source 105 can be configured to deliver current in a predetermined manner. For example, the excitation source 105 can be configured to generate pulses having predetermined wave forms, such as square waves, sinusoidal waves, or saw tooth waves. The excitation source 105 can be configured to generate the pulses at a predetermined frequency. For example, and not limitation, and with reference to FIG. 2, the excitation source 105 can be configured to generate an electric pulse of a rectangular wave form 210 through the heating/cooling element 103. The electric pulse can create a heat pulse 220 in the heating/cooling element 103 with the same wave form. That is, for example, the heat flow through the heating/cooling element 103 can be given by I2R/A, where I is the current, R is the resistance of the heating/cooling element 103, and A is the surface area of a cross section of the heating/cooling element 103. The heat pulse can result in a heat exchange between the sensing cable 101 and the surrounding media. The temperature at each sensor location can be recorded to generate a “temperature profile” 230 for each sensor location. For example, the temperature at each sensor location can be recorded with a sampling frequency of 50 Hz. The temperature profile 230 can correspond to characteristics of the medium surrounding the sensing cable 101 at each sensor location.
For purposes of illustration, and not limitation, the underlying principles of thermally activated (“TA”) measurement techniques will be described generally. Prior to heating or cooling by the heating/cooling device 103, temperature measurements of the surrounding medium can be taken with the optical fiber sensor array 102 of the sensing cable 101 and the temperature profile can be recorded as a reference. Due to the Joule effect, the heating device 103 can deliver a constant and uniform heat along the cable, heating up both cable and surrounding medium near the cable surface. For purposes of illustration, the temperature measured by the optical fiber can be described by the following equation:




  
    
      
        
          
            
              ∂
              T
            
            
              ∂
              t
            
          
          =
          
            
              1
              
                mc
                p
              
            
            ⁢
            
              (
              
                
                  
                    E
                    .
                  
                  gen
                
                -
                
                  
                    E
                    .
                  
                  loss
                
              
              )
            
          
        
        ,
      
    
    
      
        (
        1
        )
      
    
  




where Ėgen is the heat generation rate per unit length from the heating device, Ėloss is the heat loss rate due to heat transfer from the sensing cable to the surrounding medium, and m and cp represent the mass and heat capacitance of the sensing cable per unit length. The heat generation within the sensing cable due to the Joule effect can be given by:

Ėgen∝Zi2,  (2)

where Z is the impedance of the sensing cable per unit length and the rate of heat loss from the sensing cable to the surrounding media can be decomposed into heat diffusion and heat convection (e.g., Ėloss can include both heat diffusion (conduction) in a stationary medium and or convective heat transfer in a flowing medium):

Ėloss=Ėdiffusion+Ėconvection  (3)

For a stationary medium, the heat loss term can be given as:

Ėloss∝AkΔT,  (4)

where A is effective heat transfer area of the sensing cable, k is effective heat conduction coefficient of the medium and ΔT is the effective temperature gradient across the sensing cable and the medium.

The heat capacitance of the cable per unit length can limit the frequency of the thermal response of the cable, and thus the cable can be designed with a heat capacitance suited to the desired data frequency. Because heat generation can be relatively constant and uniform, the rate of change in localized temperature can depend primarily on the heat transfer between the cable and the surrounding medium. If the localized heat transfer is high at a particular point on the sensing cable, then the rate of change of temperature at that point along the cable, measured by one temperature sensor in the optical fiber, can be small. Otherwise, the temperature changing rate will be large. When subject to a heterogeneous medium or a mixed medium consisting of layers of different fluids or the like, the spatial distribution of the temperature along the sensor array can be indicative of the interface between the different media.
For purpose of illustration, and not limitation, transient temperature analysis techniques to determine characteristics of a medium will now be described with the sensing cable modeled as an infinitely long thin cylinder placed in an infinite homogeneous medium. For purposes of this description, it is assumed that at time zero (t=0) an electrical current, i, and the heat generation rate per length of the cylinder is given by:

q=πr02z0i2,  (5)

where r0 is the radius of the cylinder, and z0 is the resistance of the cylinder per unit of volume. A closed form solution for the temperature on the surface of the cylinder can be given as:





  
    
      
        
          
            
              T
              ⁡
              
                (
                
                  
                    r
                    0
                  
                  ,
                  t
                
                )
              
            
            -
            
              T
              ∞
            
          
          =
          
            
              q
              
                4
                ⁢
                
                  
                
                ⁢
                π
                ⁢
                
                  
                
                ⁢
                k
              
            
            ⁢
            
              
                ∫
                
                  
                    r
                    0
                    2
                  
                  
                    4
                    ⁢
                    
                      
                    
                    ⁢
                    α
                    ⁢
                    
                      
                    
                    ⁢
                    t
                  
                
                ∞
              
              ⁢
              
                
                  
                    e
                    
                      -
                      u
                    
                  
                  u
                
                ⁢
                
                  
                
                ⁢
                
                  d
                  u
                
              
            
          
        
        ,
      
    
    
      
        (
        6
        )
      
    
  




where k and α are the heat conductivity and diffusivity coefficients of the medium, and T∞ is the initial temperature distribution along the sensing cable. The normalized temperature change and normalized time t can be defined as:





  
    
      
        
          
            Δ
            ⁢
            
              
            
            ⁢
            
              T
              *
            
          
          =
          
            
              
                T
                ⁡
                
                  (
                  
                    
                      r
                      0
                    
                    ,
                    t
                  
                  )
                
              
              -
              
                T
                ∞
              
            
            
              q
              /
              
                (
                
                  4
                  ⁢
                  
                    
                  
                  ⁢
                  π
                  ⁢
                  
                    
                  
                  ⁢
                  k
                
                )
              
            
          
        
        ⁢
        
          

        
        ⁢
        and
      
    
    
      
        (
        7
        )
      
    
  
  
    
      
        
          t
          *
        
        =
        
          
            
              4
              ⁢
              
                
              
              ⁢
              α
              ⁢
              
                
              
              ⁢
              t
            
            
              r
              0
              2
            
          
          .
        
      
    
    
      
        (
        8
        )
      
    
  




Equation 6 can thus be given as:





  
    
      
        
          Δ
          ⁢
          
            
          
          ⁢
          
            T
            *
          
        
        =
        
          
            ∫
            
              1
              /
              
                t
                *
              
            
            ∞
          
          ⁢
          
            
              
                e
                
                  -
                  u
                
              
              u
            
            ⁢
            
              
            
            ⁢
            
              
                d
                u
              
              .
            
          
        
      
    
    
      
        (
        9
        )
      
    
  




The incomplete gamma function can have following expansion form for small but non-zero value of z (0<z<2.5):





  
    
      
        
          Γ
          ⁡
          
            (
            z
            )
          
        
        =
        
          
            
              ∫
              z
              ∞
            
            ⁢
            
              
                
                  e
                  
                    -
                    u
                  
                
                u
              
              ⁢
              
                
              
              ⁢
              
                d
                u
              
            
          
          =
          
            
              -
              γ
            
            -
            
              ln
              ⁡
              
                (
                z
                )
              
            
            -
            
              
                ∑
                
                  n
                  =
                  1
                
                ∞
              
              ⁢
              
                
              
              ⁢
              
                
                  
                    
                      (
                      
                        -
                        z
                      
                      )
                    
                    n
                  
                  
                    n
                    ⁡
                    
                      (
                      
                        n
                        !
                      
                      )
                    
                  
                
                .
              
            
          
        
      
    
    
      
        (
        10
        )
      
    
  




The temperature response as given by equation 6 above can be further approximated as

ΔT*≈−γ−ln(1/t*),  (11)

when

z=1/t*<<1.  (12)

In accordance with this illustrative and non-limiting model, comparison of the normalized temperature change as a function of normalized time (e.g., as given by equation 9 and equation 11, respectively) indicates that when the normalized time is greater than approximately 10, equation 11 is a good approximation of normalized temperature change. Moreover, equation 11 above indicates that temperature change can increase linearly with the log of time when the heating time is sufficiently large so as to satisfy the criteria in equation 12. Thus, the equation can be written as:

ΔT(r0,t)≈a+b ln(t),  (13)

where parameters a and b are function of thermal properties of the medium for given heating rate, and are given by:





  
    
      
        
          a
          =
          
            
              q
              
                4
                ⁢
                
                  
                
                ⁢
                π
                ⁢
                
                  
                
                ⁢
                k
              
            
            ⁢
            
              (
              
                
                  -
                  γ
                
                -
                
                  ln
                  ⁡
                  
                    (
                    
                      
                        r
                        0
                        2
                      
                      
                        4
                        ⁢
                        
                          
                        
                        ⁢
                        α
                      
                    
                    )
                  
                
              
              )
            
          
        
        ⁢
        
          

        
        ⁢
        and
      
    
    
      
        (
        14
        )
      
    
  
  
    
      
        b
        =
        
          
            q
            
              4
              ⁢
              
                
              
              ⁢
              π
              ⁢
              
                
              
              ⁢
              k
            
          
          .
        
      
    
    
      
        (
        15
        )
      
    
  




Thus, equation 13 can provide a theoretical basis for determining the thermal properties of a medium based on measurement of transient temperature. One of ordinary skill in the art will appreciate that continuous heating can consume more electrical energy and make measurements less sensitive to dynamic change of the thermal properties to be measured (e.g., when the medium mixture changes with time), and thus pulsed heating in accordance with the disclosed subject matter can provide benefits such as decreased electrical energy usage and for measurement of dynamic conditions of surrounding materials.
For purpose of illustration, and not limitation, an exemplary method of measuring the characteristics of the media surrounding the sensing cable using thermal analysis sensing techniques will be described. In general, an optimized waveform of electrical pulse (for example, a square wave) can be delivered along the length of the heating/cooling device 103, and temperature can be monitored using a temperature sensor array 102, e.g., optical fiber sensors. Owing to the uniformity of the heating/cooling effect along the sensing cable, temperature readings can vary depending on localized heat transfer process, which can be a function of the thermal properties (e.g., thermal conductivity, heat capacity) and physical conditions (static or flow) of the medium surrounding the sensing cable 101. The control unit 106 can be adapted to determine the characteristics of the surrounding media simultaneously, using the temperature profile.
A single heating pulse (e.g., arising from an optimized waveform of electrical pulse) can create a temperature response which can be derived in accordance with the exemplary and non-limiting model described herein using superposition as follows:




  
    
      
        
          
            T
            ⁡
            
              (
              
                
                  r
                  0
                
                ,
                t
              
              )
            
          
          -
          
            T
            ∞
          
        
        =
        
          
            q
            
              4
              ⁢
              π
              ⁢
              
                
              
              ⁢
              k
            
          
          ⁢
          
            
              (
              
                
                  
                    ∫
                    
                      
                        r
                        0
                        2
                      
                      
                        4
                        ⁢
                        α
                        ⁢
                        
                          
                        
                        ⁢
                        t
                      
                    
                    ∞
                  
                  ⁢
                  
                    
                      
                        e
                        
                          -
                          u
                        
                      
                      u
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      d
                      u
                    
                  
                
                -
                
                  
                    ∫
                    
                      
                        r
                        0
                        2
                      
                      
                        4
                        ⁢
                        
                          α
                          ⁡
                          
                            (
                            
                              t
                              -
                              
                                t
                                0
                              
                            
                            )
                          
                        
                      
                    
                    ∞
                  
                  ⁢
                  
                    
                      
                        e
                        
                          -
                          u
                        
                      
                      u
                    
                    ⁢
                    
                      
                    
                    ⁢
                    
                      d
                      u
                    
                  
                
              
              )
            
            .
          
        
      
    
    
      
        (
        16
        )
      
    
  




The first term in the bracket of equation 16 can represent the heating from t to t0, and the 2nd term the cooling after t0. Data collected during heating and cooling are analyzed separately, as disclosed herein, to derivate estimates of thermal properties of the medium.

Based upon the above, the control unit 106 can be adapted to determine the characteristics of the surrounding media using a variety of suitable techniques. For example, the temperature profile at each sensor location can be used to determine the characteristics of the surrounding media directly. The temperature measurements during heating and/or cooling of the sensing cable, corresponding to the timing of the rectangular electrical pulse, can be used to generate a feature-temperature profile at each sensor location. For example, the feature-temperature profiles can be extracted from the temperature data at distinctive conditions: heating (e.g., the condition during which the heat pulse is passing over a sensor location), cooling (e.g., the condition during which the heat pulse has passed over the sensor location and heat is being exchanged between the sensing cable and the surrounding media) and peak temperature (e.g., approximately the maximum temperature recorded at the sensor location for each heat pulse).
For purpose of illustration, and not limitation, and with reference to FIG. 3, the control unit 106 can be configured to determine temperature characteristics of surrounding media using the feature-temperature profile at each sensor location. FIG. 3 shows distribution of feature temperatures along a sensing cable exposed to different media at different sensor locations. Graph 330 depicts the measured temperature profiles for a plurality of sensor locations. In accordance with the disclosed subject matter, feature-temperatures 331b, 332b, and 333b can be extracted from the measured temperature profile depicted in graph 330. For example, at each sensor location, feature-temperature 331b can correspond to a heating condition (e.g., while the heat pulse is passing over the sensor location), and can be extracted for each sensor location at a corresponding time 331a. Likewise, feature-temperature 332b can correspond to a peak temperature, and can be extracted for each sensor location at a corresponding time 332a. Similarly, feature temperature 333b can correspond to a cooling condition (e.g., after the heat pulse has passed over the sensor location and during which heat exchange between the cable and the surrounding media takes place) and can be extracted for each sensor location at a corresponding time 333a. Temperature 310 is the measured temperature at each sensor location during ambient conditions (e.g., no heat is applied).
As illustrated by FIG. 3, the feature temperature at each sensor location can correspond to the temperature characteristics of the surrounding media. For example, as depicted in FIG. 3, a 36 inch sensing cable arranged in a vertical configuration with a sensor disposed or located each unit inch along the cable can be exposed to a stack of air, oil, emulsion, and water. It should be noted that FIG. 3 depicts data from 24 sensor locations. Assuming each medium is stationary around the sensing cable, the rate of heat exchange, and thus the feature-temperature profiles 331b, 332b, and 333b, between the sensing cable and the surrounding media at each sensor location can correspond to the heat conduction of the surrounding media. That is, for example, heat transfer between the sensing cable and surrounding air can be lower than that between the sensing cable and water, as water has a higher heat conduction. Oil and emulsion layers can also be identified in this manner.
The determination of the characteristics of the media surrounding the sensing cable can be achieved by further configuring the control unit 106 to process the temperature profile. For example, in accordance with an exemplary embodiment of the disclosed subject matter, the regression of the temperature over log of time can be performed over an interval of time corresponding to each heat pulse for each sensor location. The slope and intercept of the regression can be used to identify the material characteristics. For example, the regression can take the functional form of T=b+m ln(t), where T is the temperature measurement, ln(t) is the natural log of the time of the temperature measurement, b is the intercept of the regression, and m is the regression coefficient.
The interval over which the regression is taken can be, for example, during the heating condition described above (e.g., during which the heat pulse passes over the sensor location). Because heating can occur in a logarithmic manner, taking the regression as a function of the log of time and provide for results with lower error (e.g., a higher correlation coefficient). That is, for example, the temperature as a function of the log of time can be substantially linear over the heating period. Alternatively, the interval over which the regression is taken can be during the cooling condition described above. For purpose of illustration, and not limitation, for a square electrical pulse from 0 current to a constant non-zero value, the constant non-zero current value can correspond to the heating stage, and zero current can correspond to the cooling stage. The slope of the regression for the heating stage can be computed over a fraction of pulse duration when the current is non-zero, while slope of the regression for the cooling stage can be computed over a fraction of the time for which the current changes to zero value. Additionally or alternatively, the regression can take a number of suitable functional forms. For example, an nth order polynomial regression can be taken if the functional form of the temperature profile resembles an nth order polynomial.
For purpose of illustration, FIG. 4A shows the regression results of one temperature measurement at a sensor location in each material of FIG. 3. Line 420 corresponds to a plot of temperature at a sensor location in oil over the log of time. Likewise, lines 430, 440 and 450 correspond to a plot of temperature at a sensor location in air, emulsion, and water, respectively, over the log of time. Regression can be performed over a regression interval 410, which can correspond to the heating condition of the respective temperature sensor. The results of the regression can be plotted. For example, line 421 is a plot of the regression of line 420. As illustrated by FIG. 4A, the slope and intercept of each regression can correspond to a characteristic of the surrounding material, and such characteristics can be determined. That is, with reference to FIG. 4A, each material having different thermal characteristics can have a different slope and intercept, and can thus be identified. As depicted in FIG. 4A, The deviations in measurements resulting from the linear fitting line after the regression interval, as shown by line 420 and line 421, can be due to boundary effects from the wall of the vessel. One of ordinary skill in the art will appreciate that the description of the underlying principles herein assumes the thermal energy delivered by the sensing cable diffuses out without any boundaries. However, in the presence of such boundaries, thermal energy will be contained in a finite space and eventually thermal equilibrium will be reached. Accordingly, the regression interval can be selected based on a desired application, including corresponding boundary conditions.
For purpose of illustration. FIG. 4B shows the regression results for 24 temperature sensors of FIG. 3, showing both slopes 450 and intercepts 470. As illustrated by FIG. 4A and FIG. 4B, in certain circumstances these techniques can provide determination of material characteristics with reduced error, comparing results from FIG. 4B with FIG. 3 to differentiate the emulsion layer and the oil layer. The interval over which the regression can be performed can be predetermined to reduce boundary effect errors (e.g., error 422 induced by boundary effects in the plot of line 420). That is, for example, taking the regression over a small interval can omit certain features of a temperature profile that can correspond to a particular characteristic. Accordingly, the regression interval can be predetermined such that errors induced by boundary effects are reduced. For example, the regression interval can be predetermined by calibration and/or with reference to known parameters or operating conditions of the system, such as expected features of a temperature profile.
In accordance with another aspect of the disclosed subject matter, enhanced determination of the characteristics of media surrounding the sensing cable can be achieved with a control unit 106 configured to process the temperature profile in the frequency domain. A N-pulse train (i.e., application of a certain periodic form of current through the sensing cable to generate N cycles of heating and cooling) can be propagated through the heating/cooling element 103. The period of a heating/cooling cycle, t0, the number of heating cycles, N, and the current amplitude. I0, can be selected. The heating/cooling pulses can be applied to the heating/cooling element 103 with the excitation source 105 to generate thermal excitation within the sensing cable 101.
Temperature readings from the optical fiber sensor array 102 can be collected via the signal interrogator 104 at a selected sampling frequency. The sampling frequency can be, for example, at least twice the maximum signal frequency of interest. A temperature series, Ti(1), Ti(2), Ti(3), . . . can be generated where i=1, 2, 3, . . . M, is the sensor index. In accordance with certain embodiments, synchronized sampling techniques can be employed to reduce the sample number, increase the signal to noise ratio, and improve Fourier transform accuracy. The time difference of the temperature readings ΔT=[T(k+1)−T(k)]/Δt, can be calculated using the control unit 106 to generate time series of temperature derivative ΔTi(1), ΔTi(2), ΔTi(3) . . . , where sensor index i=1, 2, 3 . . . M. In connection with the following description, the temperature difference, differenced temperature, or temperature derivatives are all referred to as the time series ΔT′. A transform (e.g., a Fast Fourier Transform [FFT], or Discrete Fourier Transform [DFT]) can be applied, using the control unit 106, to generate a spectrum of time series of temperature difference for M sensors. For each sensor, the real and imaginary values of the spectrum at fundamental frequency of N-Pulse train can be selected f0=1/t0. The characteristics of the surrounding media can thus be determined as disclosed herein using M pairs of the values derived from the spectrum of the temperature difference as described above. Alternatively, the frequency differenced spectrum (i.e., obtained by applying the operation of taking the derivative of the spectrum of temperature difference with respect to the frequency) and the real and imaginary values of the differenced spectrum can be used. The characteristics of the surrounding media can thus be determined as disclosed herein using M pairs of the values derived from the differenced spectrum as described above.
That is, for example, the time derivative of the temperature data can be determined (i.e., resulting in the differenced temperature). The Fourier transform of the time-derivative temperature can then be determined, and the derivative of the complex spectrum with respect to the frequency can be calculated (i.e., resulting in the differenced spectrum). The amplitude and phase of the frequency-derivative spectrum (differenced spectrum) can then be calculated. The amplitude and phase of the frequency-derivative spectrum can correspond to the characteristics of the surrounding media at each sensor location. For purpose of illustration, FIG. 5B shows the phase of the frequency-derivative spectrum of the temperature measurements over the sensor locations as illustrated in FIG. 3. Likewise, FIG. 5C shows the amplitude of the frequency-derivative spectrum of the temperature measurements over the sensor locations as illustrated in FIG. 3. As illustrated by the figures, the techniques disclosed herein can provide for enhanced accuracy in the measurement and differentiation of the levels and interfaces between the air, oil, emulsion, and water layers.
As embodied herein, the sensing cable 101 can be calibrated, e.g., with the control unit 106. Calibration can include calibrating the sensor array to ensure that each sensor at a different location along the sensing cable provides the same output when subject to the same material of a constant thermal property. For example, the sensing cable 101 can be submerged into a homogenous medium of known thermal property, and the temperature measurements and processing techniques disclosed herein can be applied. If there is a difference between sensor output, the difference can be used as compensation and can be applied during measurements. Additionally, calibration can include ensuring that the sensor output accurately estimates the particular characteristic of interest (e.g. thermal conductivity and/or diffusivity). For example, a number of materials with known thermal properties can be measured for a broad range of values and a database can be constructed including correlations between measurements and determined characteristics of the known materials. The database can then be used to interpolate a measured characteristic of an unknown material.
For purpose of illustration, and not limitation, the underlying theory of measurement techniques in accordance with this exemplary embodiment will be described. In connection with this description, for purpose of example, the waveform of the pulse train propagated through the heating device can be a square shape current, e.g., as illustrated in FIG. 2. The current can be defined mathematically as:




  
    
      
        
          
            i
            ⁡
            
              (
              t
              )
            
          
          =
          
            
              ∑
              
                n
                =
                1
              
              N
            
            ⁢
            
              
            
            ⁢
            
              
                {
                
                  
                    H
                    ⁡
                    
                      (
                      
                        t
                        -
                        
                          
                            (
                            
                              n
                              -
                              1
                            
                            )
                          
                          ⁢
                          
                            t
                            0
                          
                        
                      
                      )
                    
                  
                  -
                  
                    H
                    ⁡
                    
                      (
                      
                        t
                        -
                        
                          
                            (
                            
                              n
                              -
                              
                                1
                                2
                              
                            
                            )
                          
                          ⁢
                          
                            t
                            0
                          
                        
                      
                      )
                    
                  
                
                }
              
              ⁢
              
                I
                0
              
            
          
        
        ,
      
    
    
      
        (
        17
        )
      
    
  




where t0 is the period, I0 is the amplitude of the current, and H denotes the Heaviside step function defined by:





  
    
      
        
          H
          ⁡
          
            (
            
              x
              -
              
                x
                0
              
            
            )
          
        
        =
        
          {
          
            
              
                
                  0
                
                
                  
                    x
                    <
                    
                      x
                      0
                    
                  
                
              
              
                
                  1
                
                
                  
                    x
                    ≥
                    
                      x
                      0
                    
                  
                
              
            
            .
          
        
      
    
    
      
        (
        18
        )
      
    
  




The heating rate can thus be given as:





  
    
      
        
          
            q
            ⁡
            
              (
              t
              )
            
          
          =
          
            
              ∑
              
                n
                =
                1
              
              N
            
            ⁢
            
              
            
            ⁢
            
              
                {
                
                  
                    H
                    ⁡
                    
                      (
                      
                        t
                        -
                        
                          
                            (
                            
                              n
                              -
                              1
                            
                            )
                          
                          ⁢
                          
                            t
                            0
                          
                        
                      
                      )
                    
                  
                  -
                  
                    H
                    ⁡
                    
                      (
                      
                        t
                        -
                        
                          
                            (
                            
                              n
                              -
                              
                                1
                                2
                              
                            
                            )
                          
                          ⁢
                          
                            t
                            0
                          
                        
                      
                      )
                    
                  
                
                }
              
              ⁢
              
                q
                0
              
            
          
        
        ,
      
    
    
      
        (
        19
        )
      
    
  




where q0 is related to the current by equation 5.

Instead of analyzing the temperature in time domain, the temperature rate, i.e. the derivative of the temperature with respect to time, can be considered in the frequency domain. The derivative operation, a high-pass filtering, can remove the slow-varying trend of the temperature for easier analysis. The time derivative of the temperature and heating generation rate can be defined as follows:




  
    
      
        
          
            T
            .
          
          ⁡
          
            (
            
              r
              ,
              t
            
            )
          
        
        =
        
          
            d
            T
          
          
            d
            t
          
        
      
    
    
      
        (
        20
        )
      
    
  
  
    
      
        
          
            q
            .
          
          ⁡
          
            (
            t
            )
          
        
        =
        
          
            
              d
              q
            
            
              d
              t
            
          
          .
        
      
    
    
      
        (
        21
        )
      
    
  




In frequency domain, the counterparts to the temperature and heating generation rate can be complex spectrum functions of S(r, ω) and Ω(ω). For large distances away from the heating element, the thermal diffusion process can exhibit the behavior of an attenuated and dispersive wave. The complex spectrum of the change rate of the temperature on the sensing cable's surface can be given as:




  
    
      
        
          S
          ⁡
          
            (
            
              
                r
                0
              
              ,
              ω
            
            )
          
        
        =
        
          
            1
            
              2
              ⁢
              π
              ⁢
              
                
              
              ⁢
              k
            
          
          ⁢
          
            
              Ω
              ⁡
              
                (
                ω
                )
              
            
            
              κ
              ⁢
              
                
              
              ⁢
              
                r
                0
              
            
          
          ⁢
          
            
              
                
                  H
                  0
                  
                    (
                    2
                    )
                  
                
                ⁡
                
                  (
                  
                    κ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      r
                      0
                    
                  
                  )
                
              
              
                
                  H
                  1
                  
                    (
                    2
                    )
                  
                
                ⁡
                
                  (
                  
                    κ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      r
                      0
                    
                  
                  )
                
              
            
            .
          
        
      
    
    
      
        (
        22
        )
      
    
  




The contribution of the heating component, Ω at a center frequency of ω, to the change rate of the temperature on the sensing cable's surface can thus be given as:

d{dot over (T)}(r0,ω,t)=S(r0,ω)ejωtdω.  (23)

Integration of above over all frequencies can recover the temperature rate in time domain. Therefore, S can be used as indicator of the medium. For purpose of illustration, and not limitation, the excitation term, Ω will now be described in greater detail. From equations 19 and 21, the derivative of the heating generation can be given as:




  
    
      
        
          
            q
            .
          
          ⁡
          
            (
            t
            )
          
        
        =
        
          
            ∑
            
              i
              =
              1
            
            N
          
          ⁢
          
            
          
          ⁢
          
            
              {
              
                
                  δ
                  ⁡
                  
                    (
                    
                      t
                      -
                      
                        
                          (
                          
                            i
                            -
                            1
                          
                          )
                        
                        ⁢
                        
                          t
                          0
                        
                      
                    
                    )
                  
                
                -
                
                  δ
                  ⁡
                  
                    (
                    
                      t
                      -
                      
                        
                          (
                          
                            i
                            -
                            
                              1
                              2
                            
                          
                          )
                        
                        ⁢
                        
                          t
                          0
                        
                      
                    
                    )
                  
                
              
              }
            
            ⁢
            
              q
              0
            
          
        
      
    
    
      
        (
        24
        )
      
    
  




in time domain, and:





  
    
      
        
          Ω
          ⁡
          
            (
            ω
            )
          
        
        =
        
          
            
              q
              0
            
            ⁡
            
              (
              
                
                  e
                  
                    jω
                    ⁢
                    
                      
                    
                    ⁢
                    
                      t
                      0
                    
                  
                
                -
                
                  e
                  
                    j
                    ⁢
                    
                      
                        ω
                        ⁢
                        
                          
                        
                        ⁢
                        
                          t
                          0
                        
                      
                      2
                    
                  
                
              
              )
            
          
          ⁢
          
            
              ∑
              
                n
                =
                1
              
              N
            
            ⁢
            
              
            
            ⁢
            
              e
              
                j
                ⁡
                
                  (
                  
                    n
                    ⁢
                    
                      
                    
                    ⁢
                    ω
                    ⁢
                    
                      
                    
                    ⁢
                    
                      t
                      0
                    
                  
                  )
                
              
            
          
        
      
    
    
      
        (
        25
        )
      
    
  




in frequency domain. Because N is finite, Ω can contain all frequencies. The components at the harmonic frequencies can be given as:





  
    
      
        
          
            ω
            k
          
          =
          
            
              k
              ⁢
              
                
              
              ⁢
              
                ω
                0
              
            
            =
            
              k
              ⁢
              
                
                  2
                  ⁢
                  π
                
                
                  t
                  0
                
              
            
          
        
        ,
      
    
    
      
        (
        26
        )
      
    
  




with index k.

Evaluation of equation 25 at the harmonic frequencies gives:




  
    
      
        
          Ω
          ⁡
          
            (
            
              ω
              k
            
            )
          
        
        =
        
          {
          
            
              
                
                  
                    2
                    ⁢
                    
                      
                    
                    ⁢
                    
                      Nq
                      0
                    
                  
                
                
                  
                    
                      
                        k
                        =
                        1
                      
                      ,
                      3
                      ,
                      
                        5
                        ⁢
                        
                          
                        
                        ⁢
                        ...
                      
                    
                    ⁢
                    
                      
                    
                  
                
              
              
                
                  0
                
                
                  
                    
                      k
                      =
                      0
                    
                    ,
                    
                      24
                      ⁢
                      
                        
                      
                      ⁢
                      ...
                    
                  
                
              
            
            .
          
        
      
    
    
      
        (
        27
        )
      
    
  




As such, Ω peaks at odd harmonics but zeros at even harmonics. At non-harmonic frequencies, Ω is complex in general. FIG. 5A depicts an exemplary plot of Ω/q0 verse ω/ω0 for N=1, 2, or 3. Accordingly, the thermal excitation energy can be concentrated at odd harmonics of fundamental frequency of pulses and increase as N increases.

As embodied herein, one of the odd harmonic frequencies can be chosen to increase signal to noise ratio in analysis of temperature measurements. In this manner, any temperature variation introduced by non-electrical heating can introduce noise which could be difficult to handle in time domain but can be reduced in frequency domain via N-pulse train: the number of cycles, N, can be increased to boost the peak value at odd harmonics. Additionally or alternatively, synchronized sampling techniques or harmonic tracking can also be used to reduce the noise.
In accordance with an exemplary embodiment, the spectrum S(ω), e.g., as given in equation 22, can be used to estimate the thermal property of a medium surrounding the sensing cable. A characteristic frequency can be given as:




  
    
      
        
          w
          *
        
        =
        
          
            α
            
              r
              0
              2
            
          
          .
        
      
    
    
      
        (
        28
        )
      
    
  




The complex argument to the Hankel functions can thus become:





  
    
      
        
          
            κ
            ⁢
            
              
            
            ⁢
            
              r
              0
            
          
          =
          
            
              
                
                  
                    -
                    j
                  
                  ⁢
                  
                    ω
                    α
                  
                
              
              ⁢
              
                r
                0
              
            
            =
            
              
                
                  ω
                  
                    ω
                    *
                  
                
              
              ⁢
              
                e
                jθ
              
            
          
        
        ,
      
    
    
      
        (
        29
        )
      
    
  




Where θ=3⁄4π for ω>0. At low frequencies where ω/ω*(amplitude of κr0) is less than 1, the Hankel functions can be approximated as:





  
    
      
        
          
            
              H
              0
              
                (
                2
                )
              
            
            ⁡
            
              (
              
                κ
                ⁢
                
                  
                
                ⁢
                
                  r
                  0
                
              
              )
            
          
          ≈
          
            1
            -
            
              
                
                  (
                  
                    κ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      r
                      0
                    
                  
                  )
                
                2
              
              4
            
            -
            
              j
              ⁢
              
                π
                2
              
              ⁢
              
                ln
                ⁡
                
                  (
                  
                    κ
                    ⁢
                    
                      
                    
                    ⁢
                    
                      r
                      0
                    
                  
                  )
                
              
            
          
        
        ⁢
        
          

        
        ⁢
        and
      
    
    
      
        (
        30
        )
      
    
  
  
    
      
        
          
            H
            1
            
              (
              2
              )
            
          
          ⁡
          
            (
            
              κ
              ⁢
              
                
              
              ⁢
              
                r
                0
              
            
            )
          
        
        ≈
        
          
            
              κ
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after neglecting terms of higher order.

As disclosed herein, and in accordance with an exemplary embodiment of the disclosed subject matter, the amplitude and phase can decrease monotonically with frequency so that higher frequency corresponds with lower response of temperature to the heating. Accordingly, lower frequencies can obtain significant heating response and higher signals. Additionally, the imaginary part of the complex spectrum can be nearly linear with the frequency while the real part can exhibit linear behavior beyond certain frequency values. Therefore, the derivative of the transfer function spectrum with respect to frequency can lead to constants beyond certain values of ω/ω*. One of ordinary skill in the art will appreciate that, mathematically, the spectral derivative is equivalent to the Fourier transform of the temperature rate with respect to the log of time. Thus there is connection of the derivative spectrum with the linear relationship of the temperature change with log(t) in the time domain as shown in equation 13.

As embodied herein, systems and methods in accordance with the disclosed subject matter include identifying the level of one or more media in a vessel with a sensing cable including an optical fiber sensor array aligned with a heating/cooling element. The method includes propagating at least one heating/cooling pulse through the heating/cooling element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and one or more media exposed to the sensing cable. The method includes measuring, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of a plurality of sensor locations on an optical fiber sensor array. The method includes identifying a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
For purpose of illustration and not limitation, reference is made to the exemplary embodiments of FIG. 1. The method and system disclosed herein can be used to determine levels in a variety of components and vessels. For example, the vessel can be a desalter unit of a refinery. In operation, a desalter unit can typically include some combination of gas, foam, oil, emulsion, water, and/or solids. Crudes with high asphaltenes, high TAN, high solids, low API gravity, or high viscosity can tend to form emulsions that are difficult to break down. As a result, water and salt removal can be reduced, the oil content of the brine can increase, and upsets can become common. Enhanced emulsion band detection and profiling can help improve level, mix valve, and chemical injection control and ultimately can allow partial automation of the desalter operation. Accordingly, cost-effective solutions for continuous liquid level/interface monitoring (e.g., including monitoring of interfaces between solid, water, emulsion, oil, and air) can be desirable, particularly in connection with desalter applications for challenged crude.
The level of, and interfaces between, the contents of the desalter unit can be identified by the system and method herein even under elevated temperatures. In another exemplary embodiment, the vessel can be a separator in a subsea production well. In operation, a subsea production well can typically include some combination of natural gas, foam, crude, water, emulsion, and/or sand. Accordingly, the techniques disclosed herein can be employed to identify the level of, and interfaces between, the contents of a separator in a subsea production well. However, it is recognized that the system and method herein can be applied to numerous other environments and vessels in which the identification of levels and interfaces is beneficial or desired.
In accordance with this exemplary embodiment, the system for identifying the level of one or more media in the vessel can include the components and features described herein with reference to FIG. 1A-C. The sensing cable (e.g., sensing cable 101) can further include a shield to establish an approximately stationary condition of the surrounding media in a region proximate the sensing cable. As used herein, the term “approximately stationary” can include a static condition, or a flow pattern in which energy transferred from the sensing cable to the surrounding media is substantially conductive, rather than convective. Such flow conditions can include steady laminar flow, or an area of turbulent flow (e.g., a plurality of eddies) wherein the mean flow velocity is approximately stationary. Additionally or alternatively, and as previously described, such a shield can be constructed to protect the sensing cable from damage due to solids in the surrounding media.
For purpose of illustration, and not limitation, FIG. 7 provides a schematic depiction of a shield in accordance with an exemplary embodiment of the disclosed subject matter. With reference to FIG. 7, the shield 710 can have a semicircular or other suitable shape and can be disposed upstream of the sensing cable 101 to protect and eliminate flow conditions immediately adjacent the sensing cable 730. If the media surrounding the sensing cable has, for example, a laminar flow 720, the shield 710 can create an approximately stationary region 730 proximate the sensing cable 101 such that heat transfer due to convection and other anomalies resulting from flow conditions is reduced. Any of a variety of suitable materials of construction for the shield can be used, depending on surrounding environmental conditions, such as corrosion and fouling resistant metal or ceramic for high temperature environment or polymer for lower temperature environments. The size of the shield can be large enough to protect the sensor and reduce turbulent flow around the sensor. One of ordinary skill in the art will appreciate that a variety of other suitable shield configurations are possible, and the scope of the disclosed subject matter is not intended to be limited to the exemplary embodiments disclosed herein.
Using the systems and techniques as disclosed, and suitable modifications as desired, a method of identifying levels and/or interfaces is provided and disclosed herein with reference to FIG. 1A through FIG. 5. For purpose of example, and with reference to FIG. 6, the method of identifying the level and interfaces of media will be described in connection with certain exemplary embodiments, wherein the vessel is a desalter unit. One of ordinary skill in the art will appreciate that the techniques disclosed herein can be applied in connection with a variety of suitable vessels and media, and the disclosed subject matter is not intended to be limited to the exemplary embodiments disclosed herein.
With reference to FIG. 6, the method of sensing the level of or interface between various media in a desalter unit 600 can include positioning a sensing cable 101 within the desalter unit. The desalter unit 600 can include various media of different composition and/or phase, each having a different density, such that the media forms layers. For example, the desalter unit 600 can include a layer of water 640, a layer of emulsion 630 (e.g., a mixture of oil and water), a layer of oil 620, and a layer of gas 610. One of ordinary skill in the art will appreciate that, during operation, a desalter unit 600 can include a gas layer, including for example hydrogen, light hydrocarbon gases, and/or inert gases such as nitrogen. For purposes of illustration, and not limitation, the gas layer of a desalter unit can be represented by “air” in experimental environments. As depicted in the figures and as described in connection with certain examples of the disclosed subject matter, the gas layer may be referred to, without limitation, as a layer of air.
As previously noted, the sensing cable 101 includes a heating/cooling element, such as a heating wire, and an optical fiber sensor array, as disclosed herein. The optical fiber includes a plurality of sensing locations along the length of the fiber, such that each sensing location corresponds to a height within the desalter unit 600. For example, and as previously noted, the optical fiber can include a plurality of sensors along its length and/or a single fiber sensor can be movable to define a plurality of sensor locations. The optical fiber sensor is coupled to an optical signal interrogator 104 to process an optical signal therein to obtain temperature measurements at each of the sensor locations. The optical signal interrogator 104 can further be coupled to a control unit to process the temperature measurements.
As previously described herein, the heating wire is coupled to an excitation source adapted to propagate electromagnetic waves (e.g., current 210) through the heating wire, thereby creating corresponding heat pulses (e.g., heat pulse 220). As the heat pulses propagate through the heating wire, heat is exchanged between the heating wire, the sensing cable, and the surrounding media at each sensor location. The temperature at each sensor location can be recorded, e.g., via the optical signal interrogator and control unit, to generate a temperature profile for each sensor location. For example, temperature can be measured as a function of time at each sensor location along the optical fiber. The temperature profile at each sensor location generally will correspond to the characteristics of the medium surrounding the sensing cable at that sensor location. In this manner, for purpose of illustration, sensor locations that are exposed to gas 610 can result in temperature profiles schematically depicted by graph 611. Sensor locations exposed to oil 620, emulsion 630, and water 640 can result in different temperature profiles 621, 631, and 641, respectively.
The temperature profile (i.e., the temperature as a function of time at a sensor location) can generally exhibit an increase in temperature coinciding with the exposure to the heat pulse at the corresponding sensor location. For purpose of illustration, and not limitation, and with reference to the laws of thermodynamics, the temperature will generally increase over the duration of the heat pulse at a rate corresponding to the characteristics of the surrounding media, and thereafter decrease as the heat from the heat pulse diffuses into the surrounding media at a rate corresponding to the characteristics of the surrounding media. Thus, the temperature profiles for each sensor location can correspond to the characteristics of the surrounding media, e.g., via the heat capacity of the particular media. For example, and not limitation, at a sensing location exposed to a gas 610, such as air, the heat transfer from the heating wire into the surrounding air 610 can be relatively low due to the low heat capacity and conductance of air, which would result in a temperature profile such as that of 611. By contrast, at a sensing location exposed to water 640, the heat transfer from the heating wire into the surrounding water 640 can be relatively high due to the relatively higher heat capacity and conductance of water, and thus the temperature profile may resemble that of 641.
As disclosed herein, the control unit thus can be adapted to determine the characteristics of the surrounding media at each sensor location using a variety of techniques, and thereby determine the level or interfaces between layers of the media in the desalter unit. For example, the control unit can be adapted to determine, with reference to the known positions of the sensor locations and the corresponding temperature profiles, a difference in characteristics of the medium surrounding each sensor location and thus determine the interface or level of each medium. In like manner, interfaces between layers can be detected by identifying a change in temperature profile between sensor locations.
For purpose of illustration, and not limitation, the direct temperature measurement techniques described above can be used to determine the levels and/or interfaces between media in a desalter unit. Particularly, and with reference to FIG. 3, a feature temperature profile (e.g., including three temperature measurements corresponding to a heating period, a peak temperature measurement, and a cooling period) can be extracted and processed to determine characteristics of the medium surrounding each sensor location. For example, and as depicted in FIG. 3, the temperature profile of sensors exposed to air can have a relatively higher peak, heating, and cooling temperature relative to the temperature profile of sensors exposed to oil, emulsion, and water.
Alternatively, and as described herein with reference to FIG. 4B, a log-time regression technique can be used to determine the characteristics of the medium surrounds each sensor location by further processing the temperature profile at each sensor location. That is, by performing the regression of the temperature over log of time over an interval of time corresponding to each heat pulse for each sensor location, the resulting slope and intercept of the regression can be used to identify the characteristics of the medium. For example, and as shown in FIG. 4B, the slope and intercept of sensor locations exposed to air can be relatively high and relatively low, respectively. By contrast, the slope and intercept of sensor locations exposed to oil can be relatively low and high, respectively, relative to air. In like manner, the slope and intercept of sensor locations exposed to water can be lower and higher, respectively, than those of oil.
In accordance with another exemplary embodiment of the disclosed subject matter, the frequency spectrum techniques disclosed herein with reference to FIG. 5A-C can be employed to determine the level and/or interfaces between media in the desalter unit 600 with increased measurement sensitivity, accuracy, and/or reliability. In this exemplary embodiment, and as described above, an N-pulse train can be propagated through the heating wire of the sensing cable 101 with pre-selected parameters, including heating cycle period, t0, number of heating cycles, N, and current amplitude, I0. The parameters can be selected according to the operating characteristics of the desalter unit 600 such that the resulting temperature profile can be measured with a desired signal-to-noise ratio. For example, a longer heating cycle period or higher current amplitude can result in higher signal-to-noise ratio relative to a shorter heating cycle period or lower current amplitude. Likewise, an increase in the number of heating cycles can further increase the signal-to-noise ratio. One of ordinary skill in the art will appreciate that such parameters can be varied depending upon desired application. For example, if determination of level and/or interfaces is desired at short time intervals, a shorter heating cycler period and a higher current amplitude can be employed. For purpose of example, and not limitation, in connection with a desalter unit 600, the heating cycle period can be approximately several milliseconds to several seconds (i.e., the excitation source can be adapted to deliver a current pulse at approximately 0.01 Hz to 100 Hz). The current amplitude can be approximately 1 mA to approximately 1 A. One of ordinary skill in the art will appreciate that, in accordance with the disclosed subject matter, suitable frequency and current amplitude can be determined for a particular application by routine testing in accordance with known methods.
The optical signal interrogator 104 can be adapted to measure temperatures from the optical fiber at a pre-selected sampling frequency. In accordance with an exemplary embodiment, the sampling frequency can be at least twice the expected frequency of the temperature profile and/or heat pulse. For example, and not limitation, in connection with a desalter unit 600 the sampling frequency can be 50 Hz. The derivative with respect to time of the temperature measurements for each sensor location can then be generated. For example, the measured temperatures a sensor location at each sampling interval can be given as a temperature series. The difference between each temperature in the series can then be calculated to generate a temperature derivative series. A transform (e.g., a FFT or DFT) can be applied to convert the temperature derivative series into the frequency domain, and thus generate a spectrum of time series of temperature differences for each sensor location. The derivative of the spectrum, with respect to the frequency, can be generated. That amplitude and phase of the frequency-derivative spectrum (e.g., the real and imaginary parts of the complex frequency-derivative spectrum) can then be determined. For example, using the heating cycle period, t0, the real and imaginary values of the spectrum at the fundamental frequency of the N-pulse train can be selected at f0=1/t0.
The amplitude and phase of the frequency-derivative spectrum at each sensor location, as depicted in FIG. 5B and FIG. 5C, thus can correspond to a particular medium surrounding the sensing cable 101 at a particular sensor location. For example, the amplitude and phase can decrease monotonically with frequency so that higher frequency corresponds with lower response to a change in temperature from the heating element. Accordingly, lower frequencies can obtain significant heating response and higher signals. Additionally, the imaginary part of the complex spectrum can be nearly linear with the frequency while the real part can exhibit linear behavior beyond certain frequency values. Therefore, the derivative of the transfer function spectrum with respect to frequency can correspond to the linear relationship of the temperature change with log(t) in the time domain. In this manner, and as depicted in FIG. 5B and FIG. 5C, the amplitude and phase of sensor locations exposed to air can be relatively high as compared to the amplitude and phase of sensor locations exposed to oil, emulsion, or water. The amplitude and phase of sensor locations exposed to oil can be lower than the amplitude and phase of air and can be higher than the amplitude and phase of water. The phase of sensor locations exposed to emulsion can be relatively high as compared to the phase of sensor locations exposed to oil and water and relatively low as compared to the phase of sensor locations exposed to air. The amplitude of sensor locations exposed to emulsion can be low as compared to the amplitude of sensor locations exposed to air, and can be high as compared to the amplitude of sensor locations exposed to water.
The sensing cable 101 can be calibrated, e.g., with the control unit. Calibration can include, for example, calibrating the sensor array to determine the amplitude and phase of the frequency-derivative spectrum of certain known media. For example, a number of materials with known thermal properties can be measured for a broad range of values and a database can be constructed including correlations between the generated amplitude and phase and characteristics of the known materials. The database can then be used as to determine the surrounding medium at a particular sensor location in the desalter unit 600.
The control unit, with reference to the known locations of each sensor and the corresponding amplitude and phase of the frequency-derivative spectrum, can determine the level and/or interface between layers (610, 620, 630, 640) of different media in the desalter unit 600. To determine the level of the various layers, the control unit can be configured to store the known position of each sensor location in one or more memories. For example, for a 36 inch tall desalter unit with a sensing cable having 36 sensor locations, each spaced apart by a unit inch, the control unit can store the height value of each sensor location (i.e., for sensor location i={1, 2, . . . , 36}, the control unit can store a corresponding height measurement Hi={1 in, 2 in, . . . , 36 in}). For each sensor location, i, the control unit can determine the amplitude and phase of the frequency derivative spectrum as disclosed herein. With reference to, for example, a database storing the amplitude and phase of the frequency derivative spectrum for known media, the control unit can thus determine which medium surrounds each sensor location using the determined amplitude and phase at each sensor location.
Additionally or alternatively, as embodied herein, the control unit can process the determined amplitude or phase of the frequency derivative spectrum of adjacent sensor locations to determine the location of interfaces between various layers in the desalter 600. That is, for example, a change in the amplitude across two sensor locations, as illustrated in FIG. 5C, can correspond to an interface between those sensors. Likewise, a change in the phase, as illustrated in FIG. 5B, can correspond to an interface. In certain embodiments, the control unit can process both the amplitude and phase of adjacent sensors to enhance detection of interfaces. For example, a change in both the amplitude and phase can correspond to an interface.
The methods disclosed herein can provide for continuous profile monitoring in real time, and multiple liquid levels or interfaces can be measured and visualized simultaneously. No moving mechanical parts need be included inside the sensing cable. Because material thermal properties can be measured for level/interface sensing, the measurement results can be independent of electrical conductivity, salinity, and crude oil constituents, such as sulfur, iron sulfide/oxide. Moreover, relative temperature changes before and after heating/cooling can be used to infer material thermal properties for level/interface measurement, and temperature baseline can be taken each time before heating/cooling is applied. Accordingly, the techniques disclosed herein need not require long term stability for temperature sensors.
Moreover, the system disclosed herein can operate at temperatures ranging from cryogenic temperatures up to over 1000° C. The size of the sensing cable can be relatively small (e.g., compared to conventional thermocouples) and can be cost effective for large area coverage with a large amount of sensors. Utilizing cost-effective optical fiber temperature sensors, the system disclosed herein can incorporate a large number of sensors, and can offer a high spatial resolution, e.g., less than 1 mm, over a long measurement range, e.g. several meters to kilometers. The diameter of the compact sensing cable can small, e.g., less than 2 mm. The small diameter of the sensing cable can allow for measurement in a tight space with reduced intrusiveness. The techniques disclosed herein can be used, for example, in connection with quality control in liquid/liquid extraction processes, and/or the monitoring of liquid properties, such as the presence of water and gas, and water and gas entrance in production wells. In addition, the system disclosed herein can be used to optimize the gas lift by monitoring the bubble conditions along the well.
ADDITIONAL EMBODIMENTS
Additionally or alternately, the invention can include one or more of the following embodiments.
Embodiment 1
A method for identifying the level of one or more media in a vessel, comprising: providing within a vessel a sensing cable including an optical fiber sensor array aligned with a heating element; propagating at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable; measuring, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of a plurality of sensor locations on the optical fiber sensor array; and identifying a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
Embodiment 2
The method of any one of the previous embodiments, wherein the vessel includes a desalter unit, and wherein identifying the level of one or more media includes identifying the level of one or more of gas, foam, oil, emulsion, water, or solids.
Embodiment 3
The method of any one of the previous embodiments, wherein the vessel includes a subsea production well, and wherein identifying the level of one or more media includes identifying the level of one or more of gas, foam, crude, water, emulsion, or sand.
Embodiment 4
The method of any one of the previous embodiments, wherein the at least one heat pulse further includes a plurality of heat pulses.
Embodiment 5
The method of any one of the previous embodiments, wherein measuring, over time, the temperature profile includes measuring using fiber Bragg grating array based sensing, Raman scattering based sensing. Rayleigh scattering based sensing, or Brillioun scattering based sensing.
Embodiment 6
The method of any one of the previous embodiments, wherein the heating element includes a resistive heating element and wherein propagating the at least one heat pulse includes applying an electrical pulse with a predetermined frequency and predetermined waveform.
Embodiment 7
The method of any one of the previous embodiments, wherein propagating at least one heat pulse through the heating element includes propagating the at least one heat pulse through a heating element aligned adjacent to the optical fiber sensor array.
Embodiment 8
The method of embodiments, 1, 2, 3, 4, 5 or 6, wherein propagating at least one heat pulse through the heating element includes propagating the at least one heat pulse through a heating element disposed concentrically with the optical fiber sensor array.
Embodiment 9
The method of embodiments 1, 2, 3, 4, 5, 7 or 8, wherein the heating element includes a thermoelectric device and wherein propagating at least one heat pulse through the heating element includes propagating cooling pulse.
Embodiment 10
The method of any one of the previous embodiments, further comprising coating the sensing cable with an anti-fouling coating to prevent formation of deposits thereon.
Embodiment 11
The method of any one of the previous embodiments, further comprising shielding the sensing cable to attenuate flow of the one or more media immediately exposed to the sensing cable.
Embodiment 12
The method of any one of the previous embodiments, wherein measuring the temperature profile corresponding to the heat pulse at each of the plurality of sensor locations includes, for each sensor location, measuring at least a heating temperature measurement during propagation of the heat pulse over the sensor location, a peak temperature measurement, and a cooling temperature measurement after propagation of the heat pulse over the sensor.
Embodiment 13
The method of embodiment 12, wherein identifying the level of the one or more media includes calculating a difference in the heating temperature measurement, the peak temperature measurement, the cooling temperature measurement, or combination thereof, between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
Embodiment 14
The method of any one of the previous embodiments, wherein measuring the temperature profile corresponding to the heat pulse at each of the plurality of sensor locations includes, for each sensor location, measuring a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location.
Embodiment 15
The method of embodiment 14, wherein identifying the level of the one or more media includes, for each temperature profile, performing a regression of the plurality of temperatures over a logarithm of corresponding measurement times for a predetermined time window in the period of time to generate a slope and an intercept of the regression, wherein the slope and the intercept relate to the one or more properties of the medium exposed to the sensing cable at the sensor location.
Embodiment 16
The method of embodiment 15, wherein the predetermined time window includes a time window during a heating stage, the heating stage corresponding to a period of time during propagation of the heat pulse over the sensor location or a time window during a cooling stage, the cooling stage corresponding to a period of time after propagation of the heat pulse over the sensor.
Embodiment 17
The method of embodiment 15 or 16, wherein identifying the level of the one or more media further includes calculating a difference in the slope and the intercept between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
Embodiment 18
The method of embodiment 14, 15, 16 or 17 wherein identifying the level of the one or more media includes, for each temperature profile: generating a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time, applying a transform to the time derivative to generate a complex spectrum; and determining an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location.
Embodiment 19
The method of embodiment 18, wherein identifying the level of the one or more media further includes, for each temperature profile: generating a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency; and determining an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location.
Embodiment 20
The method of embodiment 18 or 19, wherein identifying the level of the one or more media further includes calculating a difference in the amplitude and the phase between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
Embodiment 21
The method of any one of the previous embodiments, wherein the vessel has an operating temperature between cryogenic temperatures and approximately 1000° C., wherein the sensing cable has a diameter of less than 2 mm, and wherein the optical signal interrogator is configured to measure the temperature profile at a spatial resolution less than 1 mm.
Embodiment 22
A system for identifying the level of one or more media in a vessel including one or more media, comprising: a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the vessel, the optical fiber sensor array having a plurality of sensor locations; an excitation source coupled with the heating element and configured to propagate at least one heat pulse through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable; an optical signal interrogator coupled with the optical fiber sensor array and adapted to receive a signal from each of the plurality of sensor locations and configured to measure, over time, a temperature profile of the sensing cable corresponding to the heat pulse at each of the plurality of sensor locations on the optical fiber sensor array; and a control unit, coupled with the heating element and the optical signal interrogator, to identify a level of each of the one or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto.
Embodiment 23
The system of embodiment 22, wherein the vessel includes a desalter unit, and wherein the one or more media include one or more of gas, foam, oil, emulsion, water, or solids.
Embodiment 24
The system of embodiments 22 or 23, wherein the vessel includes a subsea production well, and wherein the one or more media include one or more of gas, foam, crude, water, emulsion, or sand.
Embodiment 25
The system of embodiments 22, 23, or 24, where the excitation source if further configured to propagate a plurality of heat pulses through the heating element.
Embodiment 26
The system of embodiments 22, 23, 24, or 25, wherein the optical fiber sensor array and the optical signal interrogator include a fiber Bragg grating array based sensing system, a Raman scattering based sensing system, a Rayleigh scattering based sensing system, or a Brillioun scattering based sensing system.
Embodiment 27
The system of embodiments 22, 23, 24, 25, or 26, wherein the heating element includes a resistive heating element and wherein the excitation source is configured to propagate an electrical pulse with a predetermined frequency and predetermined waveform, the electrical pulse corresponding to the at least one heat pulse.
Embodiment 28
The system of embodiments 22, 23, 24, 25, 26 or 27, wherein the heating element is aligned adjacent to the optical fiber sensor array.
Embodiment 29
The system of embodiments 22, 23, 24, 25, 26 or 27, wherein the heating element is disposed concentrically with the optical fiber sensor array.
Embodiment 30
The system of embodiments 22, 23, 24, 25, 26, 28 or 29, wherein the heating element includes a thermoelectric device and wherein the at least one heat pulse includes a cooling pulse.
Embodiment 31
The system of embodiments 22, 23, 24, 25, 26, 27, 28, 29 or 30, wherein the sensing cable further includes an anti-fouling coating to prevent formation of deposits thereon.
Embodiment 32
The system of embodiments 22, 23, 24, 25, 26, 27, 28, 29, 30 or 31, wherein the sensing cable further includes a shield to attenuate flow of the one or more media immediately exposed to the sensing cable.
Embodiment 33
The system of embodiments 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, or 32, wherein the optical signal interrogator is configured, for each of the plurality of sensor locations, to measure at least a heating temperature measurement during propagation of the heat pulse over the sensor location, a peak temperature measurement, and a cooling temperature measurement after propagation of the heat pulse over the sensor.
Embodiment 34
The system of embodiment 33, wherein the control unit is configured to calculate a difference in the heating temperature measurement, the peak temperature measurement, the cooling temperature measurement, or combination thereof between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
Embodiment 35
The system of embodiments 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33 or 34 wherein the optical signal interrogator is configured, for each of the plurality of sensor locations, to measure a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location.
Embodiment 36
The system of embodiment 35, wherein the control unit is configured, for each temperature profile, to perform a regression of the plurality of temperatures over a logarithm of corresponding measurement times for a predetermined time window in the period of time to generate a slope and an intercept of the regression, wherein the slope and the intercept relate to the one or more properties of the medium exposed to the sensing cable at the sensor location.
Embodiment 37
The system of embodiment 36, wherein the predetermined time window includes a time window during a heating stage, the heating stage corresponding to a period of time during propagation of the heat pulse over the sensor location or a time window during a cooling stage, the cooling stage corresponding to a period of time after propagation of the heat pulse over the sensor.
Embodiment 38
The system of embodiments 36 or 37, wherein the control unit is configured to calculate a difference in the slope and the intercept between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
Embodiment 39
The system of embodiments 35, 36, 37, or 38, wherein the control unit is configured, for each temperature profile, to: generate a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time; apply a transform to the time derivative to generate a complex spectrum; and determine an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location.
Embodiment 40
The system of embodiment 39, wherein the control unit is further configured, for each temperature profile, to: generate a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency; and determine an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location.
Embodiment 41
The system of embodiments 39 or 40, wherein the control unit is configured calculate a difference in the amplitude and the phase between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.
Embodiment 42
The system of embodiments 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 or 41, wherein the vessel has an operating temperature between cryogenic temperatures and approximately 1000° C., wherein the sensing cable has a diameter of less than 2 mm, and wherein the optical signal interrogator is configured to measure the temperature profile at a spatial resolution less than 1 mm.
While the disclosed subject matter is described herein in terms of certain exemplary embodiments, those skilled in the art will recognize that various modifications and improvements can be made to the disclosed subject matter without departing from the scope thereof. Moreover, although individual features of one embodiment of the disclosed subject matter can be discussed herein or shown in the drawings of the one embodiment and not in other embodiments, it should be apparent that individual features of one embodiment can be combined with one or more features of another embodiment or features from a plurality of embodiments.
In addition to the specific embodiments claimed below, the disclosed subject matter is also directed to other embodiments having any other possible combination of the dependent features claimed below and those disclosed above. As such, the particular features presented in the dependent claims and disclosed above can be combined with each other in other manners within the scope of the disclosed subject matter such that the disclosed subject matter should be recognized as also specifically directed to other embodiments having any other possible combinations. Thus, the foregoing description of specific embodiments of the disclosed subject matter has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosed subject matter to those embodiments disclosed.
It will be apparent to those skilled in the art that various modifications and variations can be made in the method and system of the disclosed subject matter without departing from the spirit or scope of the disclosed subject matter. Thus, it is intended that the disclosed subject matter include modifications and variations that are within the scope of the appended claims and their equivalents.

## Claims

1. A method for identifying the level of three or more media in a desalter unit, comprising:
providing within the desalter unit a sensing cable including an optical fiber sensor array aligned with a heating element;
propagating a plurality of heat pulses through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable;
measuring, over time, a temperature profile of the sensing cable corresponding to each heat pulse at each of a plurality of sensor locations on the optical fiber sensor array; wherein measuring, over time, the temperature profile includes measuring using Fiber Bragg Grating array based sensing, wherein an optical signal interrogator is coupled to the optical fiber sensor array and configured to measure the temperature profile at a spatial resolution less than 1 cm; and
identifying a level of each of the three or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto,
wherein identifying the level of three or more media includes identifying the level of three or more of gas, foam, oil, emulsion, water, or solids within the desalter unit,
wherein measuring the temperature profile corresponding to the heat pulse at each of the plurality of sensor locations includes, for each sensor location, measuring a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location,
wherein identifying the level of the three or more media includes, for each temperature profile: (i) generating a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time; (ii) applying a transform to the time derivative to generate a complex spectrum; and (iii) determining an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location,
wherein identifying the level of the three or more media further includes, for each temperature profile: generating a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency; and determining an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location, and
wherein identifying the level of the three or more media further includes calculating a difference in the amplitude and the phase between adjacent sensor locations, wherein the difference indicates an interface between two of the one or more media if the difference exceeds a predetermined threshold.

2. The method of claim 1, wherein the heating element includes a resistive heating element and wherein propagating the plurality of heat pulses includes applying an electrical pulse with a predetermined frequency and predetermined waveform.

3. The method of claim 1, wherein propagating the plurality of heat pulses through the heating element includes propagating the plurality of heat pulses through a heating element aligned adjacent to the optical fiber sensor array.

4. The method of claim 1, wherein propagating the plurality of heat pulses through the heating element includes propagating the plurality of heat pulses through a heating element disposed concentrically with the optical fiber sensor array.

5. The method of claim 1, wherein the heating element includes a thermoelectric device and wherein propagating the plurality of heat pulses through the heating element includes propagating a cooling pulse.

6. The method of claim 1, further comprising coating the sensing cable with an anti-fouling coating to prevent formation of deposits thereon.

7. The method of claim 1, further comprising shielding the sensing cable to attenuate flow of the one or more media immediately exposed to the sensing cable.

8. A system for identifying the level of one or more media in a desalter unit including three or more media, comprising:
a sensing cable including an optical fiber sensor array aligned with a heating element disposed in the desalter unit, the optical fiber sensor array having a plurality of sensor locations;
an excitation source coupled with the heating element and configured to propagate a plurality of heat pulses through the heating element along at least a portion of the sensing cable to affect an exchange of thermal energy between the heating element and the one or more media exposed to the sensing cable;
an optical signal interrogator coupled with the optical fiber sensor array and adapted to receive a signal from each of the plurality of sensor locations and configured to measure, over time, a temperature profile of the sensing cable corresponding to each heat pulse at each of the plurality of sensor locations on the optical fiber sensor array;
wherein the optical fiber sensor array and the optical signal interrogator include a fiber Bragg grating array based sensing system; wherein the optical signal interrogator is configured to measure the temperature profile at a spatial resolution less than 1 cm; and
a control unit, coupled with the heating element and the optical signal interrogator, to identify a level of each of the three or more media by determining one or more properties of the one or more media exposed to the sensing cable at each of the plurality of sensor locations based on the temperature profile corresponding thereto, wherein the three or more media include one or more of gas, foam, oil, emulsion, water, or solids,
wherein the optical signal interrogator is configured, for each of the plurality of sensor locations, to measure a plurality of temperatures over a period of time upon arrival of the heat pulse at the sensor location,
wherein the control unit is configured, for each temperature profile, to: (i) generate a time derivative by calculating a derivative of the plurality of temperature measurements with respect to time; (ii) apply a transform to the time derivative to generate a complex spectrum; and (iii) determine an amplitude and a phase of the complex spectrum, wherein the amplitude and the phase of the complex spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location,
wherein the control unit is further configured, for each temperature profile, to: generate a frequency derivative spectrum by calculating the derivative of the complex spectrum with respect to frequency; and determine an amplitude and a phase of the frequency derivative spectrum, wherein the amplitude and the phase of the frequency derivative spectrum relate to the one or more properties of the medium exposed to the sensing cable at the sensor location, and
wherein the control unit is configured calculate a difference in the amplitude and the phase between adjacent sensor locations, wherein the difference indicates an interface between two of the three or more media if the difference exceeds a predetermined threshold.

9. The system of claim 8, where the excitation source if further configured to propagate a plurality of heat pulses through the heating element.

10. The system of claim 8, wherein the heating element includes a resistive heating element and wherein the excitation source is configured to propagate an electrical pulse with a predetermined frequency and predetermined waveform, the electrical pulse corresponding to the plurality of heat pulses.

11. The system of claim 8, wherein the heating element is aligned adjacent to the optical fiber sensor array.

12. The system of claim 8, wherein the heating element is disposed concentrically with the optical fiber sensor array.

13. The system of claim 8, wherein the heating element includes a thermoelectric device and wherein the plurality of heat pulses includes a cooling pulse.

14. The system of claim 8, wherein the sensing cable further includes an anti-fouling coating to prevent formation of deposits thereon.

15. The system of claim 8, wherein the sensing cable further includes a shield to attenuate flow of the one or more media immediately exposed to the sensing cable.