Abstract:
A method and apparatus for use when measuring pressure in hydrocarbon wells. A sensor, an interferometric optical pressure transducer, at the end of a fibre optic line is illuminated by broadband light pulses. Measuring equipment output drives a display indicating the pressure at the sensor. The transducer is modelled by subjecting it to known pressures. The measuring equipment is modelled by applying test signals. The models are combined and a display output C is assumed. A mathematical model is used to calculate an estimation B of the actual output A from the sensor. B is compared with A and the value of C changed until the closest match is found between A and B. This value of C is taken as the output representing the pressure at the sensor. Different models and compensation factors allow for interchange of sensors and sensor types and measuring equipment and measuring equipment types.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to measurement systems, designed to read the value of a parameter, such as pressure, flow control, strain, chemical properties, or temperature, where the sensor is connected to a measurement box, optionally with a length of cable or other components between the two, for the measurement box to give a value for the parameter being measured. The present invention relates to such systems where different sensors can be attached to different measurement boxes. The example given for the embodiment of the present invention particularly relates to, but is not limited to, systems for reading parameters where the measurement box can be separated from the transducer by a large length, perhaps some kilometres in length, of fibre optic cable, and where the transducer is an optical transducer. 
   2. Description of Related Art 
   High precision optical measurement instruments, for example for measuring pressure at the bottom of a well bore in a hydrocarbon well, usually comprise a transducer situated within the well bore, a fibre optic cable leading to the transducer, at the distal end of which the transducer is situated, and measurement equipment, out of the well bore, for interrogating the transducer through the fibre optic cable and providing output indicative of the pressure to which the transducer is instantly subject. 
   The individual transducers are generally individually calibrated, with their individual measurement instruments, by subjecting the individual transducer to controlled values of the parameter to be measured (e.g. pressure) and noting the value of the output of the measuring equipment. In general, all transducers, even those of the same type, give different outputs from each other for the same parametric stimulus, and all measuring equipment, even those of the same type, give different readings from each other for the same output from a transducer. This is due to differences in the manufacturing process, dimensions, material properties, and alignment, among others, from sensor to sensor or from box to box. The present invention seeks to provide a method and means for allowing different measuring equipment to be connected to different sensors without loss of accuracy and without having to re-calibrate the entire system. 
   BRIEF SUMMARY OF THE INVENTION 
   According to a first aspect, the present invention consists in a method for estimating the response of a sensor operatively connected to measuring equipment, the sensor being of a sensor type and the measuring equipment being of a measuring equipment type, when subject to a parametric stimulus, said method including the steps of: modelling the operation and output of the sensor type, determining a compensating factor uniquely associated with the specific sensor used, modelling the operation and output of the measuring equipment type, determining a compensating factor uniquely associated with the specific measuring equipment used, and estimating the response of the specific sensor and the specific measuring equipment to the parametric stimulus by combining the model of the sensor including the sensor compensating factor and the model of the measuring equipment including the measuring equipment compensating factor. 
   The invention further provides that the sensor and measuring equipment can be an optical or non-optical instruments. 
   The invention further provides that the sensor can sense pressure, flow rate, strain, temperature, and chemical property, among others. 
   The invention further provides that the sensor may be deployed in a hydrocarbon well. 
   The invention further provides that the sensor compensating factor may be determined by attaching the specific sensor to test equipment, applying known stimuli to the specific sensor, and noting the sensor response to each stimulus. 
   The invention further provides that the measuring equipment compensating factor may be determined by attaching the specific measuring equipment to test equipment, applying known stimuli to the specific measuring equipment, and noting the measuring equipment response to each stimulus. 
   The invention further comprises that the estimating step can include, for a given output measured by the measuring equipment caused by a parameter acting on the sensor, obtaining the actual measuring equipment output, assuming the sensor parameter value, calculating the expected measuring equipment output given the assumed sensor parameter value, comparing the actual measuring equipment output with the expected measuring equipment output, assuming another sensor parameter value if the actual and expected measuring equipment output do not match, and determining that the actual sensor parameter value is equal to the sensor parameter value which provides the closest match between the actual and expected measuring equipment output. 
   The invention further comprises that the sensor may be interchanged with another sensor of the same type, without losing accuracy or having to re-calibrate the entire system, by inputting the compensating factor of the new sensor into the mathematical model of the sensor type. 
   The invention further comprises that the measuring equipment may be interchanged with another measuring equipment of the same type, without losing accuracy or having to re-calibrate the entire system, by inputting the compensating factor of the new measuring equipment into the mathematical model of the measuring equipment type. 
   The invention further comprises that the sensor may be interchanged with a sensor of another type, without losing accuracy, by modeling the operation and output of the new sensor type and determining the compensating factor uniquely associated with the specific sensor used. 
   The invention further comprises that the measuring equipment may be interchanged with a measuring equipment of another type, without losing accuracy, by modelling the operation and output of the new measuring equipment type and determining the compensating factor uniquely associated with the specific measuring equipment used. 
   According to a second aspect, the present invention consists in a system for measuring a parameter, comprising: a sensor belonging to a type of sensor, a measuring equipment belonging to a type of measuring equipment, means for accepting a model of the sensor type and a model of the measuring equipment type, means for accepting a compensating factor for the specific sensor and a compensating factor for the specific measuring equipment, and means for combining the model of the sensor including the sensor compensating factor and the model of the measuring equipment including the measuring equipment compensating factor for estimating the parameter measured by the sensor. 
   The invention further provides that the sensor and measuring equipment can be optical or non-optical instruments. 
   The invention further provides that the sensor can sense pressure, flow rate, strain, temperature, and chemical property, among others. 
   The invention further provides that the sensor may be deployed in a hydrocarbon well. 
   The invention further provides that the sensor may be exchanged with another sensor of the same or different type without losing accuracy. 
   The invention further provides that the measuring equipment may be exchanged with another measuring equipment of the same or different type without losing accuracy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is further explained, by way of example, by the following description, to be read in conjunction with the appended drawings, in which: 
       FIG. 1  is a schematic diagram exemplifying the overall technique according to the present invention. 
       FIG. 2  illustrates the different dimensions and parts of a particular style of sensor. 
       FIG. 3  is a schematic diagram of one method, according to the present invention, whereby a style or family of sensors can be tested so that the compensating parameters of a particular sensor can be determined in order to include them in the mathematical model of the sensor type. 
       FIG. 4  is a schematic diagram showing the particulars of an exemplary measuring equipment. 
       FIG. 5  is a graph of the intensity of pulses of light which can be found in the apparatus of  FIG. 4 . 
       FIG. 6  is a schematic diagram of one way in which the measuring equipment of  FIG. 4  can be tested so that the compensating parameters of a particular measuring equipment can be determined in order to include them in the mathematical model of the measuring equipment type. 
       FIG. 7  is a schematic block diagram of one way in which the present invention may be implemented. 
     And 
       FIG. 8  is a block diagram of the calculation made to estimate the parameter sensed by the sensor using the present technique. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As previously disclosed, the present invention relates to a system including a sensor and connected measuring equipment. In general, the sensor is used to sense a parameter of interest, such as pressure, temperature, flow rate, strain, or chemical properties. The sensor measures the parameter, and the connected measuring equipment interprets and perhaps analyzes the sensor measurement and provides a reading. It is noted that the present invention may function with any measuring equipment and sensor, optical or non-optical. The present invention allows sensors of the same type and measuring equipment of the same type to be interchanged in the system without the need to re-calibrate the system after each sensor or measuring equipment change and without the loss of accuracy. The invention is particularly useful if the system is used in remote locations, such as hydrocarbon wells. 
   Attention is first drawn to  FIG. 1 , which schematically illustrates the overall technique of the present invention. At step  100 , an operator generates a mathematical model of the sensor type. The mathematical model includes variables, the values of which variables are different for each specific sensor of the relevant sensor type due to differences, from sensor to sensor of the same type, in the manufacturing process, dimensions, material properties, alignment, among others. At step  102 , an operator tests a specific sensor of the relevant type to determine the values of the variables of the mathematical model. Once determined, the variables of the specific model are inputted into the sensor model. At step  104 , an operator generates a mathematical model of a type of measuring equipment. The mathematical model includes variables, the values of which variables are different for each specific measuring equipment of the relevant measuring equipment type due to differences, from equipment to equipment of the same type, in the manufacturing process, dimensions, material properties, alignment, among others. At step  106 , an operator tests a specific measuring equipment of the relevant type to determine the values of the variables of the mathematical model. Once determined, the variables of the specific model are inputted into the measuring equipment model. At step  108 , the complete sensor model is then combined with the complete measuring equipment model to allow the function of the system, as will be described. 
   It is noted that if an operator wishes to change the type of sensor or type of measuring equipment in the system, then a new mathematical model would have to be generated. 
     FIG. 2  shows the various parts of an exemplary sensor  22 , an optical inteferometric transducer in this case. An input optic fibre  102  forms the input to the interferometer chamber and an end fibre optic plug  104  defines the limit to the sensor  22 . A difference path  106  is formed between the input optic fibre  102  and the end fibre optic plug  104 . The incoming light is first reflected, as indicated by first arrow  108 , from the junction between the end of the input optic fibre  102  and the beginning of the difference path  106 . Light which has passed into the difference path  106  is reflected from the inward face of the end fibre optic plug  104 , as indicated by second arrow  110 . Interference occurs between the two reflected light waves due to the path difference, i.e twice the length between the end of the input optic fibre  102  and the inward face of the end fibre optic plug  104 . 
   A cylindrical shell  112  supports the input optic fibre  102  and the end fibre optic plug  104 . As the pressure varies, so the difference path  106  varies in length as the dimensions of the sensor vary. 
   The particular sensor  22  has various properties. In this simple example of a simple hollow cylinder, there is, first, the material of the cylindrical shell  112 . There is the overall diameter D of the cylindrical shell  112 . There is the length L of the difference path  106 . There is the thickness T of the cylindrical shell  112 . More elaborate transducers or sensors could have more elaborate geometries, different substances filling the difference path  106 , and so on, and a corresponding increase in the number of parameters. 
   At this stage, attention is also drawn to  FIG. 3 , showing a schematic test setup for a sensor  22 . 
   In  FIG. 1 , at step  100 , the relevant sensor type, in this case sensor type  22 , is analyzed to determine the parameters of the sensor that affect the output or reading of the sensor. Relationships between these parameters which in the aggregate provide the output or reading of the sensor are also derived. Incorporating all of these together, a mathematical model is derived which can be used to calculate the operation of the sensor. The accuracy of the model may be tested by physically testing one or a number of the relevant sensors. When deriving the model, the compensating parameters of the sensor type, which as previously disclosed change from sensor to sensor in each type, are left as variables. 
   At step  102 , once a specific sensor of the relevant type is to be assigned or attached to the system, the sensor must be tested to determine the compensating parameters of such specific sensor.  FIG. 3  shows one way in which a sensor, such as the sensor  22  which measures pressure, can be tested. The transducer  22  is placed within a pressure chamber  86  (shown in  FIG. 3 ) where one or more measuring instruments  88  can measure the applied pressure the transducer experiences and a controllable pump  90  can be used to adjust the pressure within the pressure chamber  86 . A modelling processor  92  is provided with the mathematical model describing the type of sensor being tested. The processor  92  then causes the pump  90  to apply a range of different pressures to the sensor. At each such pressure step, the processor  92  measures the optical response of the sensor using optical instruments, such as inteferometers  94 , modulated light sources  96 , photo detectors  98 , and filters  100 . On completion of a series of such measurements, the processor  92  determines the set of compensating factors which, when substituted into the mathematical model, best describes the specific sensor&#39;s measured response. 
     FIG. 4  shows the various parts of an exemplary measuring equipment  8 , in this case an optical measuring equipment that employs a pulsed broad band light source. 
   Measuring equipment  8  comprises a pulsed broadband light source  10  which is repetitively driven by a light source modulator  12  to emit narrow light pulses of the order of a few microseconds or less long via a first optical isolator  14  and a polarisation scrambler  16  to deliver pulses of randomly polarised light through a first coupler  18  to a fibre optic line  20  at the distal end of which a sensor  22  is situated. A broadband reflector  31  is included at the other junction of coupler  18 . The sensor  22  may be at the distal end of many kilometres of fibre optic line  20  and can be situated in a hostile environment such as a hydrocarbon well. 
   The broadband reflector  31  and the sensor  22  reflect the incident broadband pulse in two sets of reflected pulses. Each set is reflected back towards the first coupler  18  which sends the pulses through a second optical isolator  24  to a second coupler  26 . A chain of narrowband filters  34 A  34 B  34 C is coupled to the coupler  26  with a delay line  32 A  32 B  32 C located before each filter  34 A  34 B  34 C. 
   The single photo detector  30  is a high speed device capable of resolving individual reflected pulses. 
   Attention is also, at this stage, drawn to  FIG. 5 , showing exemplary pulses as observed in the system of  FIG. 1 , in response to a single pulse of light from the light source  10 . 
     FIG. 5  is the time graph of signals intercepted by the single photo detector  30 . The first set of observed pulses  36   38   40  are the result of a light pulse from the light source  10  occurring at the time indicated by arrow  42  and being reflected by the broadband reflector  31  and then by each of the narrowband filters  34 A  34 B  36 B. The second set of observed pulses  36 A  38 A  40 A are the result of the light pulse being reflected by the sensor  22  and then by each of the narrowband filters  34 A  34 B  34 C. Generally, the pulses detected by the photo-detector  30  can then be used to determine the reading of the sensor  22 . 
   The particular measuring equipment  8  has various properties. In this example, the filter centre wavelengths and band widths  31   34 A  34 B  34 C, the loss spectra of fibres  20   32 A  32 B  32 C, the polarization scrambler  16 , the isolators  14   24 , and the couplers  18   26 , and the emission spectra of light source  10  are all examples of parameters that determine the output of the measuring equipment  8 . More elaborate measuring equipment could have a corresponding increase in the number of parameters. 
   At step  104 , in  FIG. 1 , the relevant measuring equipment type, in this case measuring equipment type  8 , is analyzed to determine the parameters of the measuring equipment that affect the output or reading of the equipment. Relationships between these parameters which in the aggregate provide the output or reading of the equipment are also derived. Incorporating all of these together, a mathematical model is derived which can be used to calculate the operation of the measuring equipment. The accuracy of the model may be tested by physically testing one or a number examples ofof the relevant measuring equipment type. When deriving the model, the compensating parameters of the measuring equipment type, which as previously disclosed change from equipment to equipment in each type, are left as variables. 
   At step  106 , once a specific measuring equipment of the relevant type is to be assigned or attached to the system, the measuring equipment must be tested to determine the compensating parameters of such specific measuring equipment.  FIG. 6  shows one way in which a measuring equipment, such as the optical measuring equipment  8 , can be tested. Measurement equipment  8  is coupled to a settable or measurable calibrating interferometer  76  comprising (shown in magnified detail) a front reflector  78  and a rear reflector  80  separated by a known, and/or accurately measurable and/or accurately settable spacing  82 . A calibrating processor  84  receives the output indication of the measurement equipment and provides the measurement equipment  8  compensating factor. The equipment calibrating processor  84  can be coupled to control the calibrating interferometer  76  so that the spacing  82  can be set to a range of known values and the output of the measuring equipment  8  and compensating factor noted. 
   Alternatively, the calibrating interferometer  76  can be a range of fixed interferometers of known, different spacings  82 , the identity and/or spacing of the particular interferometer providing input to the measuring equipment being entered to the equipment calibrating processor  84  by keyboard, bar code reader or any other means and the output of the measuring equipment  8  being used to compile the measuring equipment compensating factor. 
   Alternatively, the measuring equipment  8  can be so arranged that the compensating parameters are directly measurable using generally available laboratory test and calibration instruments. In such case, the compensating parameters may be arrived at without use of calibration processor  84  by direct measurement, as for example at a factory or periodic calibration of measuring equipment  8 . 
   As another measure, the equipment calibrating processor  84  can be omitted, a single, robust calibrating interferometer  76  provided, and the output can be transmitted to a display  60   
     FIG. 7  shows one way in which the present invention may be implemented. Output from the measuring equipment, such as the photo-detector  30 , can be amplified by a wide band amplifier  52  and provided, via a high speed analog to digital converter  54  to a microprocessor  56 . Microprocessor  56  derives the value of the parameter measured by the sensor (for instance, downhole pressure in a hydrocarbon well) and provides output  58  which can drive a display device  60  to display the value measured. Equally, the microprocessor can drive an external communications device  62  which can drive external and/or remote equipment to display, log, or further process the data. 
   Attached to the microprocessor  56  are a sensor compensating input  64  and a measuring equipment compensating input  72 . The sensor compensating input  64  can include the sensor mathematical model of the relevant sensor type together with the compensating factor of the specific sensor being used in the system. The measuring equipment compensating input  72  can include the measuring equipment model of the relevant measuring equipment type together with the compensating factor of the specific measuring equipment being used in the system. 
   In one embodiment, each of the compensating inputs  64   72  comprises a micro switch array, including a plurality of banked micro switches. In another embodiment, each of the compensating inputs  64   72  comprises a data socket designed to accept the insertion of a data module  70  which bears the compensating factor and mathematical model for the relevant instrument. In yet further embodiments, the compensating inputs  64   72  can comprise smart cards, magnetic and optical tapes and discs, and swipe reader cards, to name but a few. The data can be used not only to compensate as previously indicated, but also to correct for any non linearity. Finally, the compensating factors can be provided in printed form, to be applied later when the actual output reading of the measuring equipment is considered. 
   The compensating factors can also be combined at a single input  63  which provides an input B which is the expected output A of the measuring equipment when the output indication  58  has the value C. 
   The microprocessor  56  accepts the data from the compensating inputs  63   64   72  (shown combined as input B from box  63 ) to provide an expected output B from the measuring equipment, as well as the output A actually received from the measuring equipment to provide an output indication  58  which represents a true indication of the parameter measured by the sensor. 
     FIG. 8  illustrates the calculation technique. The microprocessor  56  receives the actual output A from the measuring equipment, such as from photo-detector  30 , at step  200 . At step  202 , the microprocessor  56  then assumes an output indication  58  and uses such assumption and the mathematical models of the sensor and measuring equipment (received from the compensating inputs  64   72 ) to calculate the expected measuring equipment output B. The microprocessor  56  next, at step  204 , compares the value B of step  202  and the value A of step  200 . If the expected measuring equipment output B is not close to the actual measured measuring equipment output A, the microprocessor  56  updates the assumed stimulus in the appropriate direction and uses such updated assumption and the mathematical models of the sensor and measuring equipment to recalculate the expected measuring equipment output (again at step  202 ). This iterative process continues until microprocessor  56  identifies the closest match between the actual and expected measuring equipment outputs. Once this closest match C is found at step  206 , the microprocessor  56  chooses the updated stimulus that provided the closest match as the output indication  58  or result. The output indication  58  represents the parameter measured by the sensor. 
   By way of further explanation, with reference again to  FIG. 7 , the output indication  58  can be taken as the closest match found C at step  206 . The input A corresponds to the actual output A from the measuring equipment, as received at step  200 . The input B is the value of A which the mathematical process (model) expects to find for that closest match output (C). The process involves:
     Measure A   Assume a value for C   Calculate what B should be.   Compare B with A   Keep moving C until A and B agree.   Output is the value of C when A most closely agrees with B.   

   The fact that the sensor and the measuring equipment are modelled separately and are then combined enables an operator of the system to exchange different sensors of the same type and different measuring equipment of the same type. If an operator wishes to replace a sensor with another sensor of the same type, then all that needs to be done is to determine the compensating parameters of the specific new sensor and include them in the sensor compensating input  64 , as previously disclosed. Likewise, if an operator wishes to replace a measuring equipment with another measuring equipment of the same type, then all that needs to be done is to determine the compensating parameters of the specific new measuring equipment and include them in the measuring equipment compensating input  72 , as previously disclosed. 
   An operator may also remove the sensor and/or measuring equipment and replace them with a sensor or measuring equipment of a completely different type. In order to do so however, a new mathematical model for the relevant sensor and/or measuring equipment type will need to be derived and inputted in the relevant compensating inputs  64   72 . The compensating factor for the specific sensor or measuring equipment will also have to be inputted in the same, as previously disclosed. 
   In any of the removal, changing, or replacement cases discussed immediately above, the output indication  58  is automatically correct because of the application of the present invention. Thus, an exchange does not lead to a loss of accuracy or the need to re-calibrate the system. 
   It will be readily understood that the principle of modelling separately the sensor and the measuring equipment, in order to provide compensating factors which may be readily interchanged when different sensors are used with different measuring equipment, can be extended to include the modelling of the components interconnecting the sensor and the measuring equipment. This provides that the effect of such components can be predicted based on readily-measurable parameters. 
   By way of example, such components may include optical fibres, optical connectors and splices, optical switches and attenuators, and special purpose optical components such as pressure seals or rotary joints. Furthermore, additional optical components may be employed to render the parameters of such interconnecting components continuously measurable during the operation of the measurement system, instead of or in addition to periodic calibration measurements. Such components will comprise in general broadband reflectors or mirrors, tap couplers and the like, and the continuous measurement of such parameters may be carried out by the primary measuring equipment or by other means.