Armored Cable

Abstract

An armored cable for use primarily in underwater geophysical exploration and in offshore oil-drilling operations includes helically wrapped layers of oriented thermoplastic strands surrounding a jacketed core of one or more insulated conductors for providing high-strength armored protection for the core while being resistant to the underwater environment.

Description

The present invention relates to armored cable useful in geophysical exploration and in offshore oil-drilling operations where the cable is placed and used underwater or in a borehole. Reference is hereby made to Disclosure Document No. 001275 filed Jan. 29, 1970.

Armored cable for such use is conventionally constructed by wrapping helical layers of steel strands about an insulated or jacketed core comprising one or more insulated conductors. Various steels, such as plow steel, galvanized steel and stainless steel, have been used to provide armor for such cables. The armor serves the purpose of imparting a relatively high tensile strength to the cable so that the cable does not break when extended to a long distance from its secured end or when lowered to a great depth.

The industry has generally established an unofficial standard for minimum breaking strength for such cables. Thus, it is generally considered that the breaking strength of the cable typically must be equal to or greater than 20 times the weight of 1,000 feet of cable, i.e., the breaking strength must have a safety factor of 20 times the cable's own weight per 1,000 feet. In examining this standard, it is found that a cable primarily supports its own weight when it extends a long distance from a fixed terminal. As steel- armored cable has a high tensile strength, it is commonly employed where the cable is to be suspended great distances underwater. However, the use of steel armor gives rise to a number of disadvantages in such applications. For example, steel has a high unit weight, and thus the major portion of the breaking or tensile strength is required merely to support the steel armor itself.

Steel armor, moreover, has poor ability to withstand the salt and alkaline conditions present in an underwater environment, and is relatively inflexible as well as being expensive. Furthermore, it is magnetic so that it tends to interfere with electrical signals in the core conductors, and is readily detectable by sonar devices or the like which may be objectionable in certain military applications.

Another disadvantage in employing steel armor for electrically conductive cables is that special preforming equipment is needed to form the steel strands so that they may be helically wrapped about the cable. It is preferred that the armor strands be wrapped helically with adjacent or contacting turns to prevent damage to the cable core by compressive forces due to hydrostatic pressure. However, it is difficult and expensive to form steel strands so that they can be helically wrapped since such strands have or retain a "memory" of their initial coiled state so that they generally cannot be wrapped directly from a coil about the cable and must be prebent prior to use. Typical equipment needed to preform the steel so that it lays in adjacent turns may cost on the order of one-half million dollars.

Accordingly, a principal object of the present invention is to provide an improved armor for underwater cable which provides at least the minimum breaking strength required by the industry for such cable.

Another object of the invention is to provide an improved armored cable able to readily withstand the environment present underwater and which is relatively flexible and low in cost.

A further object of the invention is to provide improved armor for underwater cable that may be applied to the cable without necessitating the use of special preforming equipment.

These and other objects of the invention are more particularly set forth in the following detailed description and in the accompanying drawings of which:

FIG. 1 is a perspective view of a portion of an armored cable constructed in accordance with the present embodiment of the invention, partially broken away to show the layers of armor;

FIG. 2 is an enlarged cross-sectional view taken along the line 2--2 in FIG. 1; and

FIG. 3 is a graphical representation of typical stress-strain relationships for various materials having potential use as armor for underwater cables.

Referring to FIG. 1 of the drawing, there is shown a portion of an armored cable generally indicated by the reference number 10. The armored cable 10 is particularly useful in geophysical exploration and in offshore oil-drilling operations where the cable is suspended and used underwater. The cable is armored in order to give it high tensile strength so that it may be extended great distances from its terminal or at great depths underwater and so that the cable is protected from external hydrostatic pressures. The armored cable 10 is intended, for example, to be secured at one end to electronic equipment on a ship or other terminal and at its other end to electronic exploration devices or the like, via suitable connectors (not shown) of conventional construction. Such connectors typically comprise well-known means for relieving the great tensile or longitudinal stresses on the core, placing essentially the entire loading on the armor, and form no part of the present invention.

As shown in FIG. 2, the armored cable 10 has a cable core generally designated by the reference numeral 12, that typically includes one or more coaxial cables 14 and a plurality of single conductors 16. Each of the coaxial cables 14 and single conductors 16 desirably contains insulation 18 thereabout fabricated of a nonconductive plastic material or the like. A rubber filler 20 or other flexible composition advantageously occupies the spaces between the conductors while a binder 22 typically surrounds the rubber filler 20 in order to maintain the conductors in proper position within the core 12 during fabrication of the cable 10. The binder 22 typically comprises a polyester material, such as Mylar, having one adhesive side engaging the rubber filler 20, although other binders may also be suitable. A jacket 24 encloses the conductors, insulation and filling. It may be of any suitable type and be fabricated of rubber for protecting the conductors and defining the core. Preferably, the jacket 24 is resilient so that the core 12 is not unequally compressed due to hydrostatic pressures. Any suitable core 12 construction might be employed and these details of core construction do not form a part of the present invention.

It is a feature of the present structure that the core 12 is surrounded by plastic armor to give high tensile strength to the cable and to prevent the cable from being compressed by hydrostatic pressures. The armor, which is generally designated by the reference numeral 26, comprises a first layer 28 of individual filaments or strands 30 of high-strength plastic material surrounding and protecting the core 12 and a second layer 32 of similar strands or filaments 34 surrounding the first layer 28 of strands 30. As best seen in FIG. 1, the strands 30 constituting the first layer 28 of armor are helically or spirally wrapped about the jacket 24. The strands 34 constituting the second layer 32 of armor are helically or spirally wrapped about the first layer 28, the strands 34, however, having an opposite direction of lay to the lay of the strands 30. If the strands 30 are wrapped clockwise, then the strands 34 are desirably wrapped counterclockwise.

Referring to FIG. 2 of the drawing, it may be seen that the strands 30 of the first layer 28 are smaller in diameter than the strands 34 of the second layer 32. Each strand 34 of the second layer 32 thus corresponds with a strand 30 of the first layer 28. Hence, an equal number of strands are present in both the first and second layers 28 and 32. The purpose of having an equal number of strands or filaments in each layer is to prevent hydrostatic pressure from compressing the core 12 unequally in any given direction. The double wrapping of the strands or filaments 30 and 34 in combination with disposing the larger strands in the outer layer prevents distortion of the core 12 by hydrostatic pressures, which would otherwise be likely to produce deleterious electrical effects when the cable 10 is in use.

Each of the strands or filaments 30 and 34 is preferably fabricated of a plastic material such as an oriented thermoplastic. The material employed, which serves as a substitute for conventionally used steel armor, must impart a relatively high tensile strength to the cable 10. It should be apparent that oriented thermoplastic materials generally have much lower breaking strength than steel or other metals. However, such thermoplastic materials are also relatively lightweight, and require less strength to support their own weight. Typically, a cable like cable 10, but employing steel armor weighs about 480 pounds per 1,000 feet and has a breaking strength of about 10,320 pounds. By replacing the steel with a lightweight thermoplastic material, a cable of the same dimensions will weigh about 140 pounds per 1,000 feet. Industry standards require that such cables used in underwater exploration and oil-drilling operations have a breaking strength equal to or greater than 20 times the weight of 1,000 feet of the cable. Hence, to provide a safe cable 10 employing thermoplastic armor 26 requires that the breaking strength of the cable exceed 2,800 pounds, i.e., 140 pounds per 1,000 feet multiplied by a safety factor of 20. Another requirement which must be satisfied by cables 10 employing thermoplastic armor 26 is that the armor be sufficiently resilient that the cable be able to withstand the generally anticipated or forseeable shearing impacts that a steel-armored cable would be capable of withstanding. Still another requirement which must be met by the oriented thermoplastic material is that the material have a low strain ratio; i.e., it is desirable that the armor 26 not elongate to any substantial degree. If the armor readily stretches, the entire tensile stress must then be absorbed by the core 12, rather than by the armor 26, causing the core to break and disrupting electrical communication.

Referring to FIG. 3 of the drawings, which is a graphical representation of typical stress-strain values which might be obtained for various materials, it can be seen that oriented polypropylene or compositions thereof is a preferred material for use as a substitute for steel in providing armor 26 for underwater cables 10. Other homopolymers such as polyethylene and nylon, each break at lower tensile stresses than oriented polypropylene. Moreover, these other homopolymers each absorb a higher strain through elasticity than does oriented polypropylene before reaching their breakpoint. Hence, oriented polypropylene has characteristics more closely akin to steel than do the above-mentioned homopolymers and thus is more suitable for the purpose of the present invention.

Oriented polypropylene also exceeds the other standards that have been generally established for armored cables. A typical cable 10 employing armor 26 fabricated of oriented polypropylene monofilaments has been found to have a breaking strength of about 3,096 pounds. As the minimum required breaking strength for a cable of the given dimensions is about 2,800 pounds (as stated above), such cable is suitable for the uses contemplated by the present invention. Even though oriented polypropylene has only about one-third the tensile strength of steel, it also has only about one-third the weight. Oriented polypropylene is not only lightweight but is sufficiently resilient to withstand any anticipated or forseeable impact tending to shear the cable 10, even though the shear strength of the material itself is lower than that of steel. Oriented polypropylene is not magnetic and thus does not interfere with electrical signals and may not be detected by underwater detection devices, e.g., sonar devices. Special preforming equipment is not needed to wrap strands fabricated of oriented polypropylene about the cores of the cables. Armor 26 fabricated of oriented polypropylene filament may be applied in helically wrapped layers of opposite lay in order to prevent hydrostatic pressures from affecting the cable core 12.

It is contemplated that other thermoplastics, and oriented thermoplastics in particular, having the same or similar characteristics as oriented polypropylene may alternatively be employed as armor 26 for underwater cables 10. Oriented polypropylene has a tensile strength of about 65,000 p.s.i. and a maximum elongation of about 20 percent at the breakpoint (a strain ratio of 0.2 in./in.). Unoriented nylon, on the other hand, has a tensile strength of up to about 12,000 p.s.i. and a maximum elongation of up to 300 percent. Polyethylene has even less tensile strength than nylon and may elongate as much as 800 or 1,000 percent. See Modern Plastics Encyclopedia for 1968-69, pages 92-99. Oriented polypropylene does not absorb moisture, has excellent abrasion resistance, is difficult to ignite and upon burning melts to a bead, and has excellent chemical resistance to most acids, alkalies and salts. Although the cost of oriented polypropylene is somewhat greater per pound than the cost of galvanized steel, the weight of material used in providing armor 26 for a cable 10 is so much less than the weight of steel armor needed that the cost of the cable itself is substantially less.

To complete the cable 10 of the illustrated embodiment, a sheathing 36 is disposed about the second layer 32 of armor in order to protect the cable from the external environment. The sheathing 36 is desirably constructed of hard rubber or somewhat flexible plastic material or any other suitable waterproof material capable of being extruded, molded or otherwise disposed about a core.

Thus, the present structure provides a lightweight armored cable 10 useful primarily in underwater geophysical exploration and in underwater oil-drilling operations or the like. Cable 10 employing armor 26 fabricated of oriented polypropylene has a relatively high tensile strength and meets the typical standards for minimum breaking strength for armored cables. Such armor may be wrapped in helical layers to protect the cable core 12 from compressive hydrostatic forces without special preforming equipment. Furthermore, the armor is sufficiently resilient to withstand normal shearing impacts and is nonmagnetic.

While one specific form of the invention has been shown and described, it should be apparent that various modifications could be made therein without departing from the scope of the invention. The principles of the present invention are applicable to cables having various core constructions and which require armoring to protect the core.

Various features of the invention are set forth in the following claims.

Claims

What is claimed is:

1. In an armored cable having a core which is to be protected, the combination comprising a first helically wrapped layer of oriented thermoplastic strands surrounding said cable core, and a second helically wrapped layer of oriented thermoplastic strands, the lay of the strands of said second layer being opposite to the lay of and surrounding the strands of said first layer, said first and second layers providing armoring for the cable core.

2. Armored cable according to claim 1 wherein said thermoplastic is polypropylene.

3. Armored cable according to claim 1 wherein each strand has a relatively high tensile strength and a relatively low amount of elongation at its breaking point.

4. Armored cable according to claim 1 wherein the strands of said second layer are greater in diameter than the strands of said first layer, each strand of said second layer corresponding to an adjacent strand of said first layer.

5. A cable for use underwater comprising a core and at least one layer of plastic strands surrounding the core to provide armoring for the core, the strands having a tensile strength of at least about 65,000 pounds per square inch and a strain no greater than about 0.2 inches per inch.

6. A cable according to claim 5 wherein the plastic strands are oriented polypropylene monofilaments.

7. Armored cable comprising:

a. a core having at least one insulated electrical conductor therein and a jacket surrounding said conductor;

b. a first helically wrapped layer of oriented polypropylene monofilaments surrounding said jacketed core;

c. a second helically wrapped layer of oriented polypropylene monofilaments, the lay of the monofilaments of said second layer being opposite to the lay of and surrounding the monofilaments of said first layer to provide armoring for said core, the monofilaments of said second layer being greater in diameter than the monofilaments of said first layer; and

d. sheathing surrounding said second layer to protect said core from the external environment.