Fiber Optic Cable and Core

This invention relates to fiber optic communication cables, and more particularly to a fiber optic cable structure adapted for well logging for measuring characteristics of earth formations in a borehole.

As an oil well is being drilled, a sonde is usually lowered periodically into the borehole to measure characteristics of the earth formations it traverses. Typically, a logging cable supports and moves the sonde within the borehole, carries power for the sonde, and relays control instructions and data between the sonde and instrumentation and control facilities at the surface of the earth. As measurements and measuring instruments have become more sophisticated, data transmission rates have increased to the point where existing electrical cables can become saturated.

Fiber optic technology can increase data transmission rates several orders of magnitude, as has been demonstrated by fiber optic telephone cables. Due to the demanding conditions under which a well logging cable is used, however, telephonic fiber optic cables would ordinarily not be acceptable. Telephone cables are designed to remain stationary in use and not to encounter the extremes of temperature and pressure found in an oil well.

In contrast, a well logging cable is repeatedly pulled around sheave wheels and wound onto and off a winch drum as it is lowered into and lifted out of wells. The cable must therefore withstand repeated bending around diameters of but a few feet, and tensions of thousands of pounds. Once in the well, the cable encounters pressures which may exceed twenty thousand pounds per square inch and temperatures which may exceed 175.degree. C. Optical fibers, however, are extremely sensitive to deformation (especially point stresses), which greatly increase the attenuation of the optical signals within the fiber. They are also sensitive to moisture, which attacks micro-cracks in the fibers, reduces their strength, and increases their attenuation. While the cable is being manufactured, and later when in use, the stresses (bending and stretching) on the cable components (electrical conductors, strength members, etc.) move them relative to one another within the cable. This can cause local deformations of the optical fibers. Stretching the cable stretches the fibers, thereby increasing their stress, aggravating their attenuation, and sometimes causing them to break. The high pressures and temperatures within the well assist moisture in invading the cable and the optical fibers. As indicated, typical optical telephonic communication cables are not designed for these operating conditions.

Improved optical fiber cable structures have been proposed, but a need still remains for even better buffering for the optical fibers against stresses and better moisture protection in a cable specifically suited for repetitive and demanding well logging applications.

Briefly, the present invention meets the above needs and purposes with a cable and a fiber optic cable core construction in which each optical fiber is surrounded by a solid but soft elastomeric cushion, and the cushioned optical fibers are themselves surrounded with closely and spirally wound overlapping steel tape jackets. This protects and buffers the optical fibers from point stresses and other forces within a cable and forms a barrier against the entry of moisture into the cushion and the optical fibers.

More specifically, in a preferred embodiment an elastomeric core such as unfilled Viton (a trademark of E. I. Dupont De Nemours & Co.) rubber supports a plurality of optical fibers wrapped therearound in a helical spiral. A blanket of the same material surrounds the core and fibers and cooperates with the core to protect the fibers from point stresses and transmit forces thereto essentially hydrostatically.

The closely and spirally wound tape jackets which surround the core, fibers, and blanket are each wound in the same direction. Succeeding jackets after the first overlap the spiral cuts of the preceding layer to maximize the labyrinthine path length which moisture must travel to penetrate the tape jackets and reach the optical fibers. In the preferred embodiment, the tape layers are each formed of metal tape and the outermost metal tape layer is coated with a fused controlled melting point alloy to seal the core against moisture penetration.

It is therefore an object of the present invention to provide an improved cable adapted for use in measuring characteristics of earth formations in a borehole, and an improved fiber optic core therefor; a core in which each optical fiber is surrounded and buffered by a solid, soft elastomeric cushion; in which the cushion is surrounded by at least one closely and spirally wound tape jacket; in which subsequent tape jacket layers are wound in the same direction as, tightly surround, and overlap the spiral cuts of the preceding layers; in which the cushion and jackets cooperate to buffer each optical fiber from point stresses and other forces within a logging cable and to form a barrier against the entry of moisture into the fibers; in which at least the outermost tape layer may be a metallic tape layer coated with a fused controlled melting point alloy to seal the core against moisture penetration; and to accomplish the above objects and purposes in a versatile and reliable structure and method readily suited for logging cable applications in even the most demanding well logging environments.

Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.

FIG. 1 is a cross-sectional view of a well logging cable containing a fiber optic core according to the invention; and

FIG. 2 is a perspective diagrammatic illustration showing sequentially the manufacture of the fiber optic core.

With reference to the drawings, as well logging cable 10 includes one or more outer strength and armor layers 12 surrounding a plurality of electrical conductors 15. In the preferred embodiment according to this invention, a fiber optic core assembly 20 is located at the center of cable 10. Core assembly 20 (FIG. 2) includes a solid, soft elastomeric cushioning core 24, formed of a material which transmits forces essentially hydrostatically. Materials suitable for this purpose would be any of several elastomers, such as Viton rubber, a fluorocarbon elastomer manufactured by Dupont. One or more optical fibers 25 are wrapped in a helical spiral about core 24, and a solid, elastomeric cushioning blanket 27, preferably of the same material as core 24, surrounds the fibers and core and cooperates therewith to protect the fibers from point stresses and transmit forces to them essentially hydrostatically.

Around the blanket is a closely and spirally wound tape jacket 30, preferably formed of metal (although the tape may be made of other suitable materials such as various plastics). A metal suitable for this purpose would be a precipitation hardening stainless steel such as 75% iron, 16% chromium, 5% nickel, 4% copper, and trace elements. Additional or subsequent tape layers 32 are closely wound around the core 24, fibers 25, blanket 27 and first tape layer 30 in the same direction and spaced to overlap the respective preceding spiral cuts 33 and 34 (FIG. 2) of the preceding tape layers. The tape layers cooperate with one another and with the blanket and core to buffer the optical fibers from point stresses and other forces within the logging cable and to form a barrier against the entry of moisture into the elastomeric cushioning means and the optical fibers.

In the preferred embodiment, the outermost tape layer 35 is coated with a controlled melting point alloy 36 before it is wound onto the preceding layer. After it is wound onto layers 30 and 32 (and becomes part of core 20) it is passed through an induction heating coil 38 where it is briefly heated to a temperature exceeding the melting point of the alloy coating 36. The alloy melts and fuses to form an even better seal for the core assembly 20 against moisture penetration.

In the preferred embodiment the controlled melting point alloy 36 is a lead based alloy because lead has enhanced deformation tolerance and water resistance. This is important because of the considerable stretching and bending which a logging cable undergoes during well logging operations. The enormous plastic deformation tolerance of the lead substantially improves the integrity of the fused alloy barrier under these conditions. A suitable alloy for this coating would be 97.5% lead, 1.5% silver, and 1% tin, having a melting point of 310.degree. C.

The term "controlled melting point" is chosen since the alloy must not melt at borehole temperatures (typically 175.degree. C. or higher), and yet must be readily and quickly fused by the induction coil 38 without heat damaging the elements in the fiber optic core assembly 20 (especially the cushioning core 24 and blanket 27). Preferably, the assembly 20 is moved through induction coil 38 quickly so that the outermost tape layer 35 is brought rapidly to the melting point of the alloy coating 36 and then moved away from coil 38 before much heat can penetrate the inner tape layers 32 and 30. The heat in the outermost layer 35 then quickly dissipates, or can be force cooled, with little net transfer of heat energy to the interior of the fiber optic core assembly 20.

As may be seen, therefore, the present invention has numerous advantages. The fiber optic core assembly 20 can be used in a variety of cable configurations. It buffers the optical fibers from point stresses both during cable manufacturing and later when in use. It protects the fibers from moisture, and, due to the helical spiral configuration of the fibers, accommodates repeated flexing and stretching of the cable. Other means for increasing the length of the optical fibers relative to the cable, such as a zig-zag rather than spiral configuration, may of course be used. Fillers (e.g., silicone rubber) may be used in the cushioning material (core 24 and blanket 27) to increase the compressibility. Under proper circumstances, even a single tape layer having the fused alloy coating can be considered. Further, as indicated, although the preferred embodiment uses several tape layers of stainless steel, other materials, such as certain polymers, super alloys, metallic glasses, metals such as titanium, and so forth, may be used. When inner layers of such materials are intrinsically fusible, they could be controllably fused along with the outermost layer. The cable construction containing the fiber optic core assembly 20 may include other cable members such as conductors 15, armor and strength members 12, and so on, from which the optical fibers 25 are protected and buffered.

Therefore, while the methods and forms of apparatus herein described constitute preferred embodiments of this invention it is to be understood that the invention is not limited thereto, and that changes may be made therein without departing from the scope of the invention.