BRIEF DESCRIPTION OF THE DRAWINGS
Referring now to the drawings, wherein like elements are numbered alike in the several FIGURES:
FIG. 1 is a cross-sectional plan view of an exemplary multi-core fiber utilizing Bragg Gratings at a same distance along the fiber;
FIG. 2 is a cross-sectional plan view of an exemplary multi-core fiber actuated by a push rod and bellows;
FIG. 3 is a cross-sectional plan view of an exemplary multi-core fiber actuated by a push rod and diaphragm;
FIG. 4 is a cross-sectional plan view of an exemplary multi-core fiber asymmetrically actuated by a push rod and diaphragm;
FIG. 5 is a cross-sectional plan view of an exemplary multi-core fiber actuated by well pressure;
FIG. 6 is a cross-sectional view of an exemplary multi-core fiber having different modulus cores and light guiding cores;
FIG. 7 is a cross-sectional plan view of an exemplary multi-core fiber actuated by well and reference pressures; and
FIG. 8 is a cross-sectional view of an exemplary multi-core fiber having a hollow, port core and light guiding cores.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings.
Referring now to FIG. 1, a cross-sectional plan view of an exemplary multi-core fiber is illustrated generally at 10. A first core 12 and a second core 14 include Bragg grating portions 16, 18 at a same measurement portion, shown generally at 20, relative to a longitudinal axis, illustrated by line 22, of the fiber 10.
The grating portions 16 and 18 may be written in the cores by any fashion and at any time. However, in an exemplary embodiment, the grating portions 16 and 18 are photoetched in cores 12 and 14 during fiberization. In a particular exemplary embodiment, the grating portions are written during the drawing process and prior to the application of a protective coating. In such exemplary embodiment, the collocated sensors are particularly insensitive to drift factors since all collocated grating portions will drift together.
Also, while the above exemplary embodiment describes use of Bragg gratings, it should be recognized that other structures useful for reading out such fibers might be used, such as in-fiber interferometers, Rayleigh scatter and random photoetched structures, among others, as long as collocated measurement portions are provided in the fiber.
Referring now to FIG. 2, a cross-sectional plan view of an exemplary multi-core fiber 10 is illustrated in a system designed to actuate the fiber by a push rod 24 and bellows 26. The push rod 24 extends from the bellows normally against the multi-core fiber, which is provided in a media isolated housing 28. The bellows 26 is responsive to a pressure change to cause the push rod 24 to bend the fiber 10.
Referring back to FIG. 1, it is noted that the push rod 24 and bellows 26 is an exemplary mechanism to provide the pressure-induced force on the fiber illustrated by arrow 30. Such force 30 bends the fiber 10, placing exemplary grating 16 in tension and exemplary grating 18 in compression. Differential measurements in core 12 and 14 may then be taken to sense the pressure change. It is noted that not only are drift factors eliminated due to the collocated nature of the core measurement portions (e.g., gratings written during fiberization in multiple cores of a multi-core fiber), but temperature effects are also eliminated due to the nature of the multi-core system.
Referring now to FIG. 3, a cross-sectional plan view of an exemplary multi-core fiber 10 is illustrated as being actuated by a push rod 24 and diaphragm 32. Other than use of the diaphragm 32 instead of the bellows 26, operation of the collocated sensor system is identical to that described above with regard to FIG. 2. It should be noted that any mechanism effective to transmit a force representative of pressure against the fiber is contemplated herein, the bellows and push rod and diaphragm and push rod embodiments being merely exemplary.
Referring now to FIG. 4, a cross-sectional plan view of an exemplary multi-core fiber 10 is illustrated as being asymmetrically actuated by a push rod 24 and diaphragm. It should be recognized that any kind of actuation on the fiber might be performed, as long as the core measurement portions (20 in FIG. 1) of cores 12 and 14 are differentially affected by a force representative of a pressure change.
Referring now to FIG. 5, a cross-sectional plan view of an exemplary multi-core fiber 34 is illustrated as being actuated by well pressure, illustrated generally at 36 as acting on the multi-core fiber 34 within the media isolated housing 28. Referring now to FIG. 6, in this exemplary embodiment, the multi-core fiber 34 includes light guiding cores 12 and 14, as well as a low modulus core 38 and a high modulus core 40. As the well pressure 36 acts on the fiber 34, the low modulus core 38 and the high modulus core 40 react differently, causing the fiber 34 to bend. This bend accordingly affects the light guiding cores 12 and 14 differently (note that cores 12 and 14 should be arranged within the fiber such that they bend differently relative to the effects of the low and high modulus core reactions to pressure), and pressure may be calculated independent of temperature effects and drift factors. Also, while provision of low modulus and high modulus cores have been described with regard to this exemplary embodiment, any fiber construction that causes the fiber to deform under pressure is contemplated, including for example, a single core (provided at least partially along the core measurement portion) having a different modulus than the light guiding cores and having a different spacing with regard to cores 12 and 14. Also, the terms “low modulus” and “high modulus” are merely indicative of a difference in the modulus of the two cores, and are not meant to necessarily imply a great difference in modulus properties between the two cores 38 and 40.
Referring now to FIG. 7, a cross-sectional plan view of an exemplary multi-core fiber 42 is illustrated as being actuated by well and reference pressures, illustrated generally at 36 and 44, respectively. A media isolated housing 46 is provided over the fiber 42 and includes a pressure seal 48, separating the well and reference pressure zones. Referring now to FIG. 8, the fiber 34 includes light guiding cores 12 and 14, which are differentially spaced relative to a hollow core 50. Hollow core 50 extends from the well pressure zone 36 to the reference pressure zone 38, and causes deformation of the fiber 34 due to the difference in pressure between the reference pressure zone and the well pressure zone. Due to the differential spacing of the cores 12 and 14 relative to the hollow core 50, the bending will affect the light guiding cores 12 and 14 differently, and the change in pressure in the well pressure zone 36 can be measured.
Referring now to FIG. 9, an exemplary multi-core fiber 20 is arranged on a surface 52 or in a device of interest such that change in shape of the surface 52 or device will act on the multi-core fiber 20 affecting the collocated measurement portions in a different manner. More specifically, the fiber 20 may be arranged such that a change in shape of the surface 52 differentially affects the collocated measurement portions 16 and 18 (for example, placing measurement portion 16 in tension and placing measurement portion 18 in compression).
In other exemplary embodiments, optical domain reflectometry is utilized with the multi-core fiber to provide distributed measurements, for example for shape sensing, at various points of interest, as is described in U.S. patent application Ser. No. 11/180,389, filed Jul. 13, 2005, the entire contents of which are specifically incorporated herein by reference.
In another exemplary embodiment, such multi-core optical fiber is attached to the point of interest, e.g., a drill string or casing, in order to monitor absolute shape and shape change. Such arrangement may likewise use optical frequency domain reflectometry and, e.g., either Bragg grating or Rayleigh scatter based, sensors to monitor fiber shape. When used on a wellbore, it may be used to infer wellbore shape over the whole well bore or over isolated regions with measurements that are insensitive to temperature and other drift mechanisms (as with the sensor arrangements described above). Such arrangement would also be less sensitive to strains due to the cabling process, since all cores would be affected in a similar manner as well. The use of such multi-core fiber also eliminates the need to interpret casing strains, and therefore is less prone to errors in measurements and casing mechanical models. This also eliminates the need to understand the attachment to the casing with respect to strain transfer.
It will be apparent to those skilled in the art that, while exemplary embodiments have been shown and described, various modifications and variations can be made to the embodiments disclosed herein without departing from the spirit or scope of the invention. Accordingly, it is to be understood that the various embodiments have been described by way of illustration and not limitation.
Patent Application
20070297711
