Abstract:
An improved pressure vessel comprises a tubular casing having an internal cavity and a first and second opening at each end. The internal cavity is divided into a first and second cylindrical plug region extending inward from the first and second opening and a hollow interior region. An optical component in the hollow interior has at least a plurality of optical fiber pigtails extending therefrom. A first and second cylindrical plug is force fit into the plug regions. At least one plug has a through-hole for receiving optical fiber pigtails. At least one plug has a ceramic adhesive plug formed by inserting adhesive into the through-hole and preferably filing the void space therein. The adhesive encapsulates the optical fibers passing through the through-hole. The plug has a channel machined into its outer circumference to receive an O-ring. A cap covers over and beyond the outer surface of the plug.

Description:
BACKGROUND OF THE PRESENT INVENTION 
     This non-provisional application is a formalization of an earlier filed provisional application 60/221,815 filed Jul. 31, 2000 and it claims priority therefrom. This application has identical inventors and a common assignee with the previously filed provisional application 06/221,815. 
    
    
     The present invention relates to pressure vessels used to protect electronic or fiber optical components use in connection with fiber optic arrays such as those used in connection with down hole drilling environments in which the vessel must withstand both extremely high pressure and high temperature. 
     Pressure vessels presently designed for oceanographic research under conditions of high hydrostatic pressure typically employ O-ring seals. Such seals are conventionally “piston” type or “compression” type seals. Such vessels have been employed to reach ocean depths such as the Mariana&#39;s Trench which is 36,000 feet deep. At such depths pressures of 18,000 psi must be accommodated. The design of the subject pressure vessel required that the pressure vessel withstand pressures in an oil well drill hole of 10,000 psi but with the added requirement of temperatures that reached 200 degrees centigrade. The package was required to protect an optical component at these temperatures. Fiber optic leads from the components had to be protected and permitted to extend through openings in the package. 
     At pressures as high as those required, water can wick into the interior of the pressure vessel containing the optical components to be protected. O-rings can be forced out of protected regions where gaps exist. 
     U.S. Pat Nos. 2,547,521; 3,381,842; and 3,410,442 show packages for use in withstanding high internal pressures. Since such vessels are adapted to withstand high internal pressures, they do not provide a solution for a package that will withstand high external pressures. 
     U.S. Pat. Nos. 3,344,912; 3,358,902 and 4,226,327 show the use of heat shrinkable materials used to seal end portions of packages (e.g. food containers) or to provide an outer covering for a package. They do not teach a solution for a package that is used at extremely high pressure and temperature. 
    
    
     BRIEF DISCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an interferometer; 
     FIG. 2 is a schematic view of a segment of a sonar array; 
     FIG. 3 is a schematic view of a segment of an interferometer array within a down hole monitoring hose; 
     FIG. 4 is an elevation view of a pressure vessel having a single plug; 
     FIG. 5 is a sectional view of a first embodiment of the pressure vessel of FIG. 4 taken on line A—A; 
     FIG. 6 is a sectional view of a second embodiment of the pressure vessel of FIG. 4 taken on line A—A; 
     FIG. 7 is a sectional view of a third embodiment of the pressure vessel of FIG. 4 taken on line A—A; 
     FIG. 8 is an elevation view of a pressure vessel having a first and second plug; 
     FIG. 9 is a sectional view of a first embodiment of the pressure vessel of FIG. 8 taken on line B—B showing a first and second plug and O-rings being used on each plug; 
     FIG. 10 a  is a sectional view of the first plug on the left of FIG. 9 using an O-ring with the ceramic plug removed; 
     FIG. 10 b  is a sectional view of the first ceramic plug of FIG. 9; 
     FIG. 11 a  is a sectional view of the second plug on the right of FIG. 9 using an O-ring with the ceramic plug removed; 
     FIG. 11 b  is a sectional view of the first ceramic plug of FIG. 9; and 
     FIG. 12 is a sectional view of a second embodiment of the pressure vessel of FIG. 8 taken on line B—B showing a first and second plug, each using first and second O-rings. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic drawing of an interferometer  2  such as a Mach Zhender interferometer, such as might be used in a hydrophone or sonar array for the detection of acoustical information. Such sensors are among the most sensitive known. FIG. 2 shows the interferometer connected between a pair of optical couplers  4 ,  6  as a segment of a hydrophone array. FIG. 3 shows the segment of the array positioned within a flexible tube or hose  8 . The tube  8  would typically house many other segments (not shown) and might also carry a number of interconnecting optical fibers, electrical conductors, optical components such as optical amplifiers. 
     The hose  8  might be deployed in a drill hole for oil that is potentially thousands of feet in length or towed behind a ship or deployed as a stationary array on the bottom of a sector of the ocean. In each such application, the optical and electronic components within the hose must be protected from extremes of hydrostatic or hydraulic pressures and in the case of down hole applications, from high temperatures approaching 200 degree centigrade. 
     FIG. 4 is an elevation view of the pressure vessel  10 . The invention “Improved Pressure Vessel” of FIG.  4  and its alternative embodiments in the figures subsequent to FIG. 4 are for a pressure vessel capable of withstanding elevated hydrostatic pressures, and elevated temperature. The pressure vessel  10  provides a protected envelope for a variety of components such as fiber optic components including items such as optical couplers, wavelength dependent multiplexers, multi-function integrated optical components, interferometers such as Mach-Zhender and Michelson types, optical sources, optical amplifiers and electronic components for signal conditioning and acquisition as the application might require. In such applications, the components are kept at atmospheric pressure. The leads, electrical or optical are sealed in a plug at the ends of the vessel. In down-hole applications, the hose or tube  8  of FIG. 3 has an inside diameter that imposes a budget on the outside diameter of any pressure vessels used for components within its interior. A typical maximum diameter for a pressure vessel might be slightly less than 0.2 inches for a down hole application. 
     Fiber optic leads or pigtails  11   a  and  11   b  exit cap  12 . The pressure vessel  10  has a tubular casing  14 , typically fabricated from steel that is capable of withstanding extreme hydrostatic pressures. 
     FIG. 5 is a sectional view of a first embodiment of the pressure vessel  10  taken on section line A—A of FIG.  4 . FIG. 5 shows that tubular casing  14  has an internal cavity  16  and with an opening  18  in at least one end. The length of the internal cavity  16  is shown by a bracket  16  below the internal cavity. In the embodiment of FIG. 5, the opening  18  is at the left end of the tubular casing  14 . The opening  18  permits access to said internal cavity  16 . The internal cavity  16  has a hollow interior identified by bracket  20 , and a plug region, having a position and length identified by bracket  22 . The plug region is near the opening  18 . A plug  24  is shown filing the plug region  22 . Block  26  represents an electrical or optical component in the hollow interior  20 . The component  20  has at least a first lead  28   a ,  28   b  passing through said plug to exit the pressure vessel as leads  11   a ,  11   b.  The plug  24  is shown encapsulating, the component leads  28   a ,  28   b  as they pass through the plug region to the opening  18  and the plug  24  seals the opening  18 . 
     As shown in FIGS. 5-7, the plug  24  is cast within the plug region  22  of the tubular casing  14  of a hard incompressible material such as a loaded epoxy resin or a ceramic adhesive. The plug  24  is circular in cross section because the tubular casing  14  is circular in cross section. It should be understood, that circular cross section tubing has an advantage over other form factors at high pressure; however, if pressure and size permitted, alternative form factors could be used such as rectangular or triangular depending on the application and the requirements. 
     In the embodiments of FIGS. 5 and 6, the plug region has an irregular surface region identified by bracket  30  for frictionally engaging the plug. The irregular region is typically threaded, however, the surface might be improved for bonding using a chemical etch or by abrasive sanding or blasting. 
     FIG. 7 is another alternative embodiment. A plug  24  is cast in the internal cavity  16  plug region  22  in which at least a portion of the plug region  22  has a necked down region identified by bracket  32  as having a cross section diminishing in diameter with distance from the opening  18 . The internal cross section of the internal cavity  16  has a first enlarged region  33  along a first portion of the plug region  22 . The plug region is then shown necked down in diameter along the necked down region  32  of the plug region  22 . The necked down region  32  is followed by the hollow interior identified by bracket  20 . The function of the necked down region is to drive the material of the plug into compression as the external hydrostatic pressure on the vessel increases. The diminishing portion or necked down region prevents the plug from moving into the hollow interior  20 . Shoulder  34  in FIG. 6 provides a similar function. In the embodiment of FIG. 7, the plug region is shown to be necked down to a diameter that is a match for the diameter of the hollow interior  20  circular cross section. The irregular surface region  30  is shown positioned within the first enlarged region  33 . 
     In each of the embodiments, of FIGS. 5-7 and  9 - 10 , the lead exiting the opening is at least a first optical fiber having an optical core covered by optical cladding. The cladding is covered by a protective plastic jacket. The jacket over the cladding is treated to remove the jacket over the desired portion using chemical or mechanical methods. The jacket is removed from a portion of the lead after the lead leaves the component. The portion of the jacket to be removed begins at phantom ellipse  36 , and continues through a portion passing through the plug  24  to a point just below the outer surface of the plug identified by ellipse  38 . The jacket is removed to permit the plus material to bond to the surface of the cladding which further perfects the hydraulic seal. 
     The cap  12  is formed from a polymer material to cover and extend beyond the external surface of the plug  24  thereby forming a fluid barrier over the surface of the plug  24 . Liquid Viton, a fluorocarbon elastomer, a rubber-like material, that holds at high temperature, is applied to the top or exposed surface of the ceramic adhesive to form cap  12 . 
     FIG. 8 is an elevation view of the pressure vessel  10  having two openings in the tubular casing  14 . The openings at the left and right ends permit access to the hollow interior  20 , the extent of which is identified by bracket  20  in FIG. 9, a sectional view taken on line B—B of FIG.  8 . The internal cavity  16  has a hollow interior identified by bracket  20 . 
     FIG. 9 shows a first and second cylindrical plug  46 ,  48  more clearly identified in FIGS. 10 a  and  11   a  respectively. The first and second plugs are force fit via first and second plug region or internal cavity openings  18   a  and  18   b  into first and second plug regions identified in FIG. 9 by brackets  40  and  42  in the tubular casing  14 . A component  26  is shown in the hollow interior  20 . The component has at least a first and or a second lead  28   a ,  28   b.    
     Each plug has an outer cylindrical surface  50 ,  52 , the respective outer cylindrical surface  50 ,  52  of each plug is force fit via the plug region opening  18   a ,  18   b  of the cylindrical casing  14  into respective first and second plug regions  40 ,  42 .. As shown in FIGS. 10 a  and  11   a , each plug  46 ,  48  has a through-hole  56 ,  58  that receives and passes at least the component first and or second lead  28   a ,  28   b  to a position outside of the pressure vessel  10 . Each plug also has a first O-Ring  44   a ,  44   b  to create a seal between the inner wall of the tubular casing  14  and the respective cylindrical plug outer cylindrical surface  50 ,  52 . 
     As shown in FIGS. 10 b  and  11   b , plugs  46 ,  48  each further comprise a first and second ceramic adhesive plug  60 ,  62  formed in the respective through-hole  56 ,  58  by inserting ceramic adhesive into the through-hole and filling substantially all of the void space within the through-hole not occupied by the lead or leads. The adhesive is allowed to encapsulate the respective lead passing through the through the respective hole  56 ,  58  thereby sealing the opening  18   a .  18   b.    
     In alternative embodiments of FIGS. 10 a ,  10   b  a portion of the hole is increased in diameter and then at least a portion of the plug region has a cross section that diminishes in diameter with distance from the opening. 
     Referring to FIGS. 9 and 12, the leads  28   a ,  28   b  exiting the opening  18   a ,  18   b  are at least a first optical fiber having a plastic jacket covering the cladding. The plastic jacket of the optical fiber passing through the through-hole is treated to remove a portion of the jacket as discussed above in connection with FIGS. 5-7 and reference numbers  36 ,  38 . 
     In the embodiments of FIGS. 9-12, the through-holes  56 ,  58  in the first and second cylindrical plugs  46 ,  48  have an inner surface. A portion of each inner surface identified by dash lines in FIGS. 9,  10   a  and  11   a  is formed to have an irregular or roughened surface region for improved bonding with the ceramic adhesive plug. The irregular surface of the through-hole is a functional equivalent to the irregular surface region  30  in FIG. 5 under bracket  30 . 
     The embodiments of FIGS. 9 and 12 each have an end cap  12  over the outer surface of the ceramic adhesive plug formed in the through-hole. The cap is formed from a polymer material to encapsulate the exposed cladding and to cover and extend beyond the external surface identified in FIGS. 10 b  and  11   b  as  64 ,  66  of the ceramic adhesive plug thereby forming a fluid barrier over the surface of the plug. 
     In assembling an optical component into the pressure vessel, the leads are first treated to remove the necessary jacket material. The component is then inserted into the tubular casing  14  with care exercised to insure that a sufficient strain relief is provided for the fiber optic pigtails and to ensure that the bend radiuses are sufficient to prevent unwanted evanescent radiation. This step is critical when the fiber is required to exit the component in one direction and reverse its direction within the pressure vessel. In an application where the pressure vessel is limited in size to under 0.2 inches, leads exiting a component might have to exit the pressure vessel at opposite ends of the vessel to avoid excessively sharp bends. 
     A strain relief in the optical fiber is also necessary to protect the component from strain that results from the thermal expansion of the steel tubular casing  14  over a temperature rise to 200 degrees C. The length of the fiber strain relief on the right and left sides of component  26  are basically the same. It is important that the strain reliefs are sufficiently compliant, so that as the temperature changes and the steel expands or as pressure forces contraction, the fiber is not torn out of the component. 
     FIG. 12 shows plug  62  using a second O-ring  64 . The plug  62  extends beyond ends of the tubular casing  14  and is increased in diameter to form a cap lip that is driven against the edge of the tubular casing with increasing pressure. 
     FIGS. 8 and 12 show the use of shrink tubing  68  over the end of the lip and the tubular casing to improve the seal of the joint formed at the interface of the lip and the end of the tubular casing, with increasing pressures. 
     Components within can be a three or four fiber coupler or even an MIOC with modulator wires. The wires could pass through the hole in the plug with the fibers. The ceramic adhesive would then be forced into the hole to fill the void space around the fibers and or wires passing through the hole to form the ceramic adhesive plug. The ceramic adhesive is slightly porous. The Viton cap seals the porous surface of the ceramic plug. As pressure is increased, the walls of the cylindrical body strain inward but the increasing pressure forces the plug into the cap forming an even tighter seal around the fibers. 
     The component or device in the package can be immobilized by bonding it to the inner wall of the body or filling a portion of the void in the body with a Styrofoam or other material. The void is at atmospheric pressure. 
     If the jacket can take the temperature, then the jacket is left on the fiber. If the application temperature is too high for the jacket material, then the jacket material is removed by mechanical or chemical means. 
     The invention high-pressure vessel is the first to address the needs of the oil industries for a sensor that will withstand 10,000 psi and a temperature up to 200 degrees. No coupler produced before can meet the requirements imposed by the down-hole drilling application. Slight movement of the fibers does not change the performance of the coupler. Temperature change does change the performance of the coupler. The gases and chemicals in the hole can destroy the coupler. This package protects the coupler from those materials. 
     The pressure vessel  10  package is typically placed in a liquid filled hose such as the hose  8  depicted in FIG.  3 . The hose is filled with a material similar to transformer oil. The hose is armored and sealed. The oil filling the hose prevents it from collapsing. The region in the hose in which the component is positioned has a hydrostatic pressure roughly equivalent to that of the region outside of the hose. The couplers are used with sound sensors such as Mach-Zehnder or Michelson interferometers that are outside of the package, however, if protection for these components is required in alternative application, the invention package can be adapted to accommodate the requirement of the larger component. Four couplers are typically used with each interferometer. Two are used with the sensor and two more with the telemetry. When an array of sensors are used, two couplers are used with each sensor. 
     More than one coupler can be placed in a single package if the size of the package was not limited. In the present embodiments, the diameter of the package was limited, so that only one coupler could fit into a package. 
     Referring again to FIG. 12, if the strain relief  70  is coiled, the radius of the bend must be large enough to prevent evanescent light from escaping through the cladding. A hose used in a test of the subject pressure vessel  10  had a one-inch inside diameter. The outside diameter of the pressure vessel was limited to {fraction (3/16)} inches. The vessel length was limited to 2⅛ inches. The inside diameter of the body was 0.118 inches. The wall thickness was 0.035 inches. The steel used was 4130. It proved to have the strength necessary to enable the vessel to pass the deformation test at 10,000 psi. It was the strongest standard tubing that was conveniently available. The cap was made of the same material. Thermal expansion was matched to that of the tubular casing in that way. Other materials could be used but at additional expense. 
     Any material suitable for the environment and stress could be used with the necked down, threaded hole, the ceramic adhesive and the Liquid Viton elastomer covering for the end of the exposed ceramic adhesive. The ceramic adhesive attached to the walls of the tapered hole but also to the glass cladding of the fiber. Epoxy materials would not work at 200 degrees centigrade and at 10,000 psi. The ceramic adhesive is like concrete or Portland cement. A ceramic adhesive such as that produced by Cotronics, was located at page 2498 in McMaster-Carr catalog.