KALAR, Kent (6300 S. Congress Ave Apt. 1303, Austin, TX, 78745, US)
JAASKELAINEN, Kari-Mikko (4011 Bell Hollow Lane, Katy, TX, 77494, US)
PARK, Brian (16921 Crystal Caves Drive, Austin, TX, 78737, US)
KALAR, Kent (6300 S. Congress Ave Apt. 1303, Austin, TX, 78745, US)
JAASKELAINEN, Kari-Mikko (4011 Bell Hollow Lane, Katy, TX, 77494, US)
| Claims 1. A single point fiber optic pressure transducer system comprising: a. a hollow cylinder; b. an optical fiber formed into an overlapping racetrack form and wrapped around said hollow cylinder; c. a Bourdon tube mounted inside said hollow cylinder whose outer end rotates as pressure increases in the tube; and d. a source of heat mounted on the outer end of the Bourdon tube; e. wherein said optical fiber is connected remotely to a distributed temperature measurement system. 2. The single point fiber optic pressure transducer system of claim 1 wherein said source of heat mounted on the outer end of the Bourdon tube is a micro-heater connected to a battery mounted on or inside said hollow cylinder to provide energy to said micro-heater. 3. The single point fiber optic pressure transducer system of claim 1 wherein said source of heat mounted on the outer end of the Bourdon tube is provided by attaching the far end of said optical fiber to the tip of the Bourdon tube and heating a suitable light absorbing material at the far end of said optical fiber with laser power supplied from the distributed temperature sensing system. 4. A single point fiber optic pressure transducer system of claim 1 utilizing multiple transducers wherein the system comprises multiple transducers each made from hollow cylinders, Bourdon tubes, optical fibers, and sources of heat wherein each transducer is connected in series to the same optical fiber connected remotely to a distributed temperature measurement system. 5. A method of measuring a single point pressure with a distributed temperature sensing system comprising the steps of: a. wrapping an optical fiber of length L1 around a hollow cylinder; b. mounting a Bourdon tube the inside said hollow cylinder, creating a fiber optic pressure transducer; c. deploying said fiber optic pressure transducer into a region of interest for measuring pressure; d. connecting the fiber optic pressure transducer in line to an optical fiber that is connected remotely to a distributed temperature measurement system; e. providing heat to the outer end of the Bourdon tube; f. in a calibration step using the distributed temperature measurement system calibrating the position along the optical fiber length at which a temperature spike is measured against various pressures to which the Bourdon tube is exposed; g. in a measurement mode measuring with the distributed temperature system the length L2 of the optical fiber at which there is a temperature spike; and h. calculating the single point pressure from L1 and L2. 6. The method of measuring a single point pressure with a distributed temperature sensing system of claim 5 wherein said calculating step comprises calculating the pressure from the ratio of the measured length (L2) to the full range length (L1) of the optical fiber configured on said hollow cylinder pressure transducer. 7. The method of measuring a single point pressure with a distributed temperature sensing system of claim 5 wherein said step of providing heat comprises mounting a micro heater on the outer end of the Bourdon tube. 8. The method of measuring a single point pressure with a distributed temperature sensing system of claim 4 wherein said step of providing heat comprises attaching the far end of said optical fiber to the tip of the Bourdon tube and heating a suitable light absorbing material at the far end of said optical fiber with laser power supplied from the distributed temperature sensing system. |
DTS BASED FIBER OPTIC PRESSURE TRANSDUCER
Cross Reference to Related Applications
This application claims the benefit of U.S. provisional serial number 61/268,077 filed June 8, 2009.
Technical Field
This disclosure relates to distributed temperature measurement systems and more particularly to the use of distributed temperature measurement systems to measure single point pressure.
Background Art
Measurement of temperature and pressure in remote hostile environments is necessary in many fields, for example in downhole oil wells. Although there are various pressure sensors such as transducers known in the art, many of them require lengthy electrical connections, tend to be unreliable or to have low life expectancy in the corrosive, high pressure and/or high temperature environment of downhole. There is a need for a simpler single point pressure measurement system with more inherent safety aspects (no generation of electric sparks) and immunity from EMI (Electromagnetic Interference) for parameter measurements.
Some attempts have been made to address these issues. US Patent 5,138,155 makes use of a fiber optical sensor in conjunction with a Bourdon tube. The Bourdon tube is responsive to pressure and drives a conventional pointer indicator and supports a reflective target that is positioned in response to pressure applied to the Bourdon tube. Light is transmitted to the target by means of fiber optic cables from a remotely positioned oscillator. Light is reflected by the target and transmitted by fiber optic cables to processing circuitry that responds to reflected light signals as generated by the oscillator. To generate these light signals, light emitting diodes form a light source and the light signals are detected by means of phototransistors. A reference target is also provided along with a second light source/light detector pair. The processing circuitry responds to the outputs of the phototransistors and generates a controller output by use of a look-up table.
US Patent 5,877,426 describes a Bourdon tube pressure gauge integral optical strain sensors. When the Bourdon tube is exposed to the pressure of the system, movement of the tube in response to system pressure causes a strain in the optical sensor. The optical sensors include an optical fiber having intrinsic Bragg grating sensors formed in the optical fiber. The optical fiber is attached to a reference point and to the Bourdon tube such that changes in the position of the tube changes the strain on the optical fiber resulting in a wavelength shift of light reflected by the Bragg grating. The magnitude of the wavelength shift is directly proportional to a change in pressure. The system requires a reference or temperature compensated optical sensor that is isolated from the strain associated with the pressure of the system.
A simpler and therefore more reliable system is needed to measure single point pressure in hostile environments. Brief Summary of the Disclosure
This disclosure describes a fiber optic based pressure transducer that detects position of a micro-heater mounted on a Bourdon tube under pressure. This movement can be calibrated to provide an accurate pressure reading. The temperature change in the fiber due to the micro-heater is detected using DTS (Distributed Temperature Sensing) techniques, which additionally measures the temperature along the entire length of the well. Thus one surface instrument can provide both an accurate temperature profile of a deep well and its pressure at critical locations. This is of value for the testing and production stages of the well. No electrical cables are required, and all signals are conducted through a single optical fiber that is also the detection fiber. Multiple pressure sensors can be installed on the fiber and their pressures determined.
An aspect of this invention is at least one pressure transducer connected in line to the fiber optic sensing fiber of a DTS system including at least an enclosure with an attached fiber optic position detection coil; a Bourdon tube; a micro-heater attached to the Bourdon tube, and an energy source for the micro-heater.
Another aspect of this invention is the use of the DTS laser as an energy source to the end of the Bourdon tube, eliminating the need for an internal battery or micro-heater.
Another aspect of the invention is the application of closely aligned racetracks of optical fiber on a support shape to form the fiber optic position detection coil. Another aspect of the invention is a method of measuring pressure by measuring the movement of a Bourdon tube by detecting the position of a heated end of the Bourdon tube.
An aspect of the invention is a single point fiber optic pressure transducer including at least: a hollow cylinder; an optical fiber formed into an overlapping racetrack form and wrapped around the hollow cylinder; a Bourdon tube mounted inside the hollow cylinder whose outer end rotates as pressure increases in the tube; and a source of heat mounted on the outer end of the Bourdon tube; wherein the optical fiber is connected remotely to a distributed temperature measurement system.
Another aspect of the invention is a method of measuring a single point pressure with a distributed temperature sensing system including at least the steps of: wrapping an optical fiber of length L1 around a hollow cylinder; mounting a Bourdon tube the inside the hollow cylinder, creating a fiber optic pressure transducer; deploying the fiber optic pressure transducer into a region of interest for measuring pressure; connecting the fiber optic pressure transducer in line to an optical fiber that is connected remotely to a distributed temperature measurement system; providing heat to the outer end of the Bourdon tube; in a calibration step using the distributed temperature measurement system calibrating the position along the optical fiber length at which a temperature spike is measured against various pressures to which the Bourdon tube is exposed; in a measurement mode measuring with the distributed temperature system the length L2 of the optical fiber at which there is a temperature spike; and calculating the single point pressure from L1 and L2. Brief Description of the Several Views of the Drawings
For a more complete understanding of the present invention, reference is now made to the following drawings, in which,
Fig. 1 is a rendering of a possible pressure transducer of this disclosure.
Fig. 2 is an illustration of some alternative fiber racetrack windings of the present disclosure.
Fig. 3 is an illustration of a temperature vs. distance plot for one of the pressure transducers.
Fig. 4 is an illustration of the use of two pressure transducers and their temperature vs. distance plots.
Detailed Description
This disclosure describes a fiber optic based pressure transducer that detects position of a micro-heater mounted on a Bourdon tube under pressure. This movement can be calibrated to provide an accurate pressure reading. The temperature change in the fiber due to the micro-heater is detected using DTS (Distributed Temperature Sensing) techniques, which additionally measures the temperature along the entire length of the well. Thus one surface instrument can provide both an accurate temperature profile of the well and its pressure at critical locations. This is of value for the testing and production stages of the well. No electrical cables are required, and all signals are conducted through a single optical fiber that is also the detection fiber. Multiple pressure sensors can be installed on the fiber and their pressures determined.
Bourdon tubes have been used for many years to measure pressure, A Bourdon tube normally consists of a small bore coiled tube in the shape of a flat spiral connected to a pressure source and sealed at the other end. The tube may be isolated from the pressure source via a bellows and transmitting fluid if the pressure source is corrosive or contains particulate matter. As the pressure increases the coil unwinds which causes the end of the coil to rotate in a predictable and repeatable manner. The size of the tube is selected to provide between 300 to 360 degree rotation over the range of the device. Different sized Bourdon tubes can be used to provide a number of pressure ranges but still use the same fiber detection system.
Figure 1 illustrates an aspect of the invention. A pressure transducer 100 as conceived in this disclosure may consist of an optical fiber 110 formed into an overlapping racetrack form and wrapped around a cylinder to form a fiber optic position detection coil. The term racetrack form is used in this description to describe closely aligned racetracks of optical fiber on a support shape to form the fiber optic position detection coil. The invention also includes a Bourdon tube 120 whose outer end rotates as pressure increases in the tube; a micro-heater 140 mounted on the end of the bourdon tube; a battery (not shown) could be located inside the cylinder as an energy source for the micro-heater. Other energy sources are possible and will be discussed. The complete transducer would be in an enclosure (not shown) for protecting the device. Although Bourdon tube 140 is shown mounted at the end for illustrative purposes it could in fact be located in the center of the long cylinder.
Micro-heater 140 may consist of a very small resistor or solid state device which heats up when voltage is applied to it from the internal battery. The temperature rise is only a few degrees over a very small surface area, so the power consumption is very small. This enables either a battery-powered system to run continuously for extended periods without needing replacement, or an alternate scheme (discussed later) of supplying heat to the end of the Bourdon tube with the primary DTS laser. If the battery is used it is mounted inside the fiber coil cylinder to reduce the overall size of the unit. Batteries suitable for downhole environments are commonly available. The sensing system is mounted inside a sealed enclosure to protect it from the hostile environment. The fiber can be connected at the top and the bottom of the device to the rest of the fiber or to additional pressure transducers.
The temperature rise caused by the micro-heater is detected by the optical fiber 110, which is wound in an overlapping racetrack form on the cylinder to form a fiber optic position detection coil. Fiber is typically 125 microns in diameter. If it is wound so that the strands are directly adjacent to each other then it is capable of detecting movement as small as 125 microns as the micro-heater passes over the coils. DTS methods can detect temperature changes along a fiber with a resolution of 1/2 meter, so each coil may have an overall length of 1/2 meter.
For example numbers this could equate to a racetrack fiber optic position detection coil approximately 5cm wide by 20.8 cm tall 79 coils per cm is feasible. For a 2.54 cm diameter cylinder, the accumulated length of the coils is 7.98 cm and the length of the fiber is 314 meters, which enables 628 discrete steps. Larger diameter cylinders increase the resolution of the device since there are more coils per revolution.
The micro-heater may be positioned over the end of the fiber cylinder so that no ambiguity of position may occur, or it may be positioned in the cylinder of the fiber coil, but since each coil has two vertical strands, care must be taken to avoid a double signal.
Any number of racetrack forms is feasible and all are anticipated in this disclosure. The racetrack may be fabricated based on imbedded fiber techniques. A fiber coated with thermoplastic is laid down using a CNC controlled tool. As the fiber is laid down it is preheated so that the thermoplastic reaches its melting point. When pressed down by a roller the plastic fuses to a cylindrical mandrel and holds the fiber in place on cooling. The path of the fiber is precisely controlled so that complex patterns are feasible. Thus the fiber can be laid down in straight paths directly adjacent to the previous path, with a large radius at each end to eliminate attenuation of the laser light. Alternatively the fiber can be laid down flat on a flexible base that can then be wrapped around a mandrel to form a cylinder. Figure 2 illustrates two racetrack configuration possibilities, each allowing resolution of 79 samples per cm. Configuration 220 for example could be fabricated as shown and then pressed onto a cylindrical mandrel 230 of for example 0.5 meters in length. The Bourdon tube would be located at the edge of the cylinder.
In a second example the length per track may be 1 meter and adjacent sides of the racetrack are mounted directly next to each other over a straight length as shown in 240. The remainder of the track forms a loop at each end. After being pressed onto a cylindrical mandrel 250 of (in this example) 1.0 meters length measurements are made where the tracks are next to each other. The Bourdon tube would be located in the center section of the cylinder. Each loop provides two readings, i.e. 1 every 1/2 meter. This would eliminate the double sampling issue discussed earlier. The resolution is the same as the other racetrack, i.e. 79 samples per cm.
It may be possible to interpret the position of the heater between two coils using a software stereoscopic technique to improve the resolution and hence the accuracy of the device. Averaging techniques may be used to eliminate effects due to vibration of the end of the Bourdon tube or from the heat spreading over adjacent coils.
With such a pressure transducer pressure at any remote location can now be measured as follows. Figure 3 illustrates a single pressure transducer 320 connected in line to an optic fiber that is connected remotely to a distributed temperature measurement system (DTS) (not shown).
The pressure is detected at that DTS system as follows: the sensor has previously been calibrated so that the full pressure range represents a specific rotation of the Bourdon tube and hence a specific length of fiber. The measured pressure is represented at the DTS system as a temperature spike at a certain length of the fiber. A temperature vs. distance plot can be seen in Figure 2 as 340. The actual pressure then equals the pressure range times the ratio of the measured length (L2) and the full range length (L 1) of the optical fiber configured on the transducer. Since the DTS system also measures overall temperature, then the spike in the temperature profile due to the heater can easily be detected. Temperature compensation may also be calculated if required since the temperature of the sensor is known.
Multiple pressure transducers are easily applied with this invention, while using only one DTS system. Figure 4 illustrates two pressure transducers 420 connected in series to the same optic fiber that is connected remotely to a distributed temperature measurement system (DTS) (not shown).
When two transducers are used the temperature vs. distance plot looks like Figure 3 as shown in 440. Since each transducer is at a specific distance along the fiber, their curve and temperature spike are clearly separate from each other and their pressures can easily be determined.
In another embodiment it may not be necessary to have an included battery or micro-heater in the invention. An alternate configuration in the case of a single pressure transducer could be to supply the necessary energy via the optical fiber. Using a laser of sufficient power the far end of the optical sensing fiber could be attached to the tip of the Bourdon tube and then generate heat as the laser light energy strikes the end of the fiber, which may be configured with a suitable light absorbing material.
For multiple pressure transducers this concept could be expanded by providing multiple optical fibers and using one for each pressure transducer. In another embodiment one fiber could be used and optical couplers used in line to divert power to each pressure transducer. In these options the pressure measurement is made in the same way as described earlier in this disclosure - by measuring the relative location of a temperature spike in the pressure transducer.
Although certain embodiments of the present invention and their advantages have been described herein in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present invention is not intended to be limited to the particular embodiments of the processes, machines, manufactures, means, methods and steps described herein. As a person of ordinary skill in the art will readily appreciate from this disclosure, other processes, machines, manufactures, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufactures, means, methods or steps.
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