Deep submergence equipment, such as remotely operated underwater vehicles (ROVs), is widely used in both civilian and military offshore endeavors. A typical submersible ROV is unoccupied, highly maneuverable, and operated by a person aboard a boat or ship. The ROV is linked to the boat or ship by a buoyant electromechanical cable, also called a tether, which carries electrical power and includes fiber optics for data communications. The cable needs to be neutrally buoyant at depth to allow good mobility and prevent the cable from getting entangled at the bottom of the sea.
The cable for a submersible ROV typically includes a cable floatation jacket. A thermoplastic elastomer with a specific gravity of about 0.88 has been used as a cable floatation jacket material. Because of the relatively high specific gravity of the elastomer, the diameter of the cable is quite large, inhibiting heat transfer and producing a high amount of drag. Another material that has been used for cable flotation jackets is a polyethylene foam. The polyethylene foam, however, is not elastic, absorbs water, and does not sustain large crushing pressures such as in a deep-sea environment. Glass microballoons have also been used in cable floatation jackets, however, the glass is very abrasive and can damage the cable drive system rollers. The glass material also does not improve the thermal conductivity of the cable floatation jacket.
The present invention is related to a composite material for a cable floatation jacket. The composite material comprises a thermoplastic elastomer matrix, and a plurality of carbon constituents interspersed in the thermoplastic elastomer matrix. The carbon constituents comprise a plurality of carbon fibers, and a plurality of carbon microballoons attached to each of the carbon fibers.
In another aspect of the invention, a method of making a floatation jacket for a tether cable is provided. The method comprises coating a bonding agent on a continuous carbon fiber, and depositing a plurality of carbon microballoons on the bonding agent coated continuous carbon fiber to form a microballoon coated continuous carbon fiber. The method further comprises chopping the microballoon coated continuous carbon fiber into discrete fiber chains, and mixing the fiber chains with a heated liquid thermoplastic elastomer to produce a mixed composite material, which is extruded onto a cable core to produce the floatation jacket.
Features of the present invention will become apparent to those skilled in the art from the following description with reference to the drawings. Understanding that the drawings depict only typical embodiments of the invention and are not therefore to be considered limiting in scope, the invention will be described with additional specificity and detail through the use of the accompanying drawings, in which:
In the following detailed description, embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. It is to be understood that other embodiments may be utilized without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense.
The present invention provides a low specific gravity, thermally conductive composite material, which can be used for a deep-sea cable floatation jacket. The composite material allows for a much thinner floatation jacket than conventional cable floatation jackets, and provides improved thermal conductivity. This allows more power to flow through the cable while reducing drag.
The composite material comprises an elastomeric matrix and carbon-based constituents. The elastomeric matrix can be a thermoplastic elastomer, which retains good cable flexibility. The carbon constituents comprise an assembly of carbon fibers and carbon microballoons. The carbon fibers and carbon microballoons can be bonded together and formed into short chains. The carbon fiber/microballoon chains are mixed with the elastomer to form the composite material. This material is then extruded over an electromechanical cable by standard plastic extrusion methods to form a tether cable with a floatation jacket.
The composite material combines the buoyancy benefit of carbon microballoons with the good thermal conductivity of carbon fibers, allowing more power to be carried through the tether cable. The composite material can be employed as a cable floatation jacket for electromechanical cables attached to deep submergence equipment, such as remotely operated underwater vehicles (ROVs), which require neutrally buoyant cables for power and communications. The present tether cable with the floatation jacket is particularly suited for a high voltage power transmission system for a deep submergence ROV. The composite material can also be used as a floatation jacket for subsea power transmission lines, and in water towed arrays.
Further details of the present invention are discussed hereafter with respect to the drawings and examples.
Suitable thermoplastic elastomers for the composite material include copolymers or a physical mix of polymers (e.g., plastic and rubber) which have both thermoplastic and elastomeric properties for retaining good cable flexibility. Examples of suitable thermoplastic elastomers include styrenic block copolymers, polyolefin blends, elastomeric alloys, thermoplastic polyurethanes, thermoplastic polyesters, thermoplastic polyamides, and the like. These thermoplastic elastomers can be used singly in the composite material or in various combinations. In one embodiment, the composite material comprises a thermoplastic polyester with carbon constituents.
The carbon constituents can be short carbon fiber/microballoon chains having a length of about 0.2 inch to about 0.5 inch. The carbon fibers are used in an effective amount to increase the thermal conductivity of the composite material. The carbon fibers can comprise about 0.5 wt-% to about 2.5 wt-% of the composite material. The carbon microballoons (also known as hollow microspheres) are used in an effective amount so that the composite material provides increased floatation to the tether cable. The carbon microballoons can have a specific gravity of about 0.3 or less.
In one embodiment, the composite material has a specific gravity of less than about 0.8. In another embodiment, the composite material has a specific gravity of about 0.5 to about 0.6. The composite material takes advantage of the high thermal conductivity of carbon fibers in combination with the low effective specific gravity of the carbon microballoons to provide added buoyancy, while still being able to sustain crushing deep-sea pressures.
The present composite material allows for a decrease in the tether cable diameter, which reduces drag and improves the thermal conductivity of the cable floatation jacket, thereby allowing more power carrying capability. The decreased diameter of the tether cable also allows for a longer tether cable on the storage winch, which improves the operational footprint and responsiveness of an ROV.
The following examples are given to illustrate the present invention, and are not intended to limit the scope of the invention.
An assessment was made to determine how heavy the carbon fiber and microballoon loading needs to be to get a usable thermal conductivity increase for a floatation jacket composite material. The carbon fiber constituent is the main contributor to the increased thermal conductivity of the composite material. The purpose of the microballoon constituent is mainly to gain buoyancy, since the carbon fibers by themselves are heavier than the jacket material matrix. The microballoons also improve the material thermal conductivity, but not extensively.
The composite materials used in the assessment included a thermoplastic rubber (TPR) with a specific gravity (SG) of 0.8. Any formulated material with a lower SG than 0.8 is an improvement. The graph of
The graph of
A floatation jacket composite material was made of a thermoplastic elastomer (TPR) matrix, with carbon fiber and carbon microballoon elements. The composite material had the following constituent properties:
Thermoplastic Elastomer:
Carbon Fibers:
Carbon microballoons:
The effect of the carbon content on the temperature gradient through the composite material was determined. The following calculations were used to compute the change in thermal resistance as a function of the carbon content:
j=0 . . . 5
X
j=0.5·j
This is the percent of carbon content in the composite material.
The foregoing equations and parameters were used to compute the change in temperature gradient through the cable jacket composite material as a function of the carbon content.
Percent of carbon ° C. temperature rise of cable as a
content function of the carbon content
These values for the change in temperature gradient as a function of the carbon content are plotted in the graph of
A floatation jacket composite material was made of a thermoplastic elastomer matrix, with carbon fiber and carbon microballoon elements. The composite material had the following constituent properties:
Thermoplastic Elastomer:
Carbon Fibers:
Carbon Microballoons:
The effect of the carbon fiber on the composite material flexural modulus (stiffness) was determined. The following calculations were used to compute the change in flexural modulus as a function of the carbon fiber content:
j=0 . . . 5
Xcf
j=0.5·j
This is the percent of carbon fiber content in the TPR matrix.
The graph of
A floatation jacket composite material was made of a thermoplastic elastomer matrix, with carbon fiber and carbon microballoon elements. The composite material had the following constituent properties:
Thermoplastic Elastomer:
Carbon Fibers:
Carbon microballoons:
The effect of 2.5% carbon fiber content on the composite material specific gravity was determined. The following parameters were used to compute the composite material specific gravity (SGcompk) as a function of the percent carbon microballoon content (Xcmk):
The graph of
The present invention may be embodied in other specific forms without departing from its essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.