Electricity: conductors and insulators – Conduits – cables or conductors – Insulated
Reexamination Certificate
2000-02-07
2003-05-20
Reichard, Dean A. (Department: 2831)
Electricity: conductors and insulators
Conduits, cables or conductors
Insulated
C174S1130AS, C174S12000C
Reexamination Certificate
active
06566604
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to cable and to the manufacture of cable. The invention has particular application in cable which is likely to be subject to flexing, radial compression and tensile and shear stresses. Embodiments of the invention are suited for use as ocean bottom cable (OBC).
BACKGROUND OF THE INVENTION
One method of surveying the geology of subsea strata involves laying long lengths (for example 50-60 km lengths) of sensing cable in a serpentine manner on the sea floor; the cable is generally known as ocean bottom cable (OBC). The cable will typically comprise a number of cable sections, for example a 57 km cable may comprise 150 cable sections each of 380 m length, each cable section carrying a number of sensors. Acoustic waves are created in the water above the cable, which waves are reflected at varying amplitudes and angles by the strata beneath the sea floor. The reflected waves are detected by the sensors and the resulting signal output is analyzed to produce a seismograph of the subsurface geology of the area. This method of geological surveying is perhaps most commonly used in identifying subsea hydrocarbon-bearing formations with potential for oil or gas production.
The OBC is laid on the sea floor using a vessel equipped with an arrangement for deploying and retrieving the cable, known as a “linear traction wrench” or “squirter”. Typically, the squirter comprises a plurality of pairs of driven wheels, often shod with vehicle tires, each pair defining a “nip” or “pinch point” for engaging a section of cable. The vessel will follow a serpentine path over the area of sea floor to be surveyed, with the squirter deploying the cable at a corresponding rate. Once the survey has been completed the squirter is operated to retrieve the cable.
Conventional OBC comprises a central reinforcing fiber core, a plurality of elongate components, typcially insulated conductors helically would around the core, an extruded sheath, possible further braided fiber reinforcement, and an outer abrasion resistant protective sheath or jacket. When a section of the OBC passes through the squirter or is subjected to flexing, compressive and longitudinal forces are quickly transferred to the components and may, in time, result in damage to the components. The components of the cable, for example the reinforcing braid, may also shift position within the cable upon flexing. When the cable is next subjected to longitudinal stress, the displaced component may be subject to damage.
As the cable is deployed and retrieved, particularly in deep water, the weight and drag on the cable suspended from the squirter is considerable. Accordingly, the squirter must apply significant compressive and longitudinal forces to the cable. These forces are of course first applied to the exterior of the cable and are then transferred radially inward through the components of the cable to the central core. This may place significant stresses on individual components, increasing the possibility of component failure. One recognized failure mechanism is termed “Z-kinking”, and involves a component being subject to longitudinal forces which stretch the component beyond its elastic limit or “yield point.” When the forces are removed, the elongated component retracts and is folded back on itself, thereby damaging the component. Furthermore, the different components of the cable may behave differently under stress. For example, the overlying extruded sheath or jacket is likely to have greater elasticity than the adjacent layer of helically wound conductors, which may lead to the jacket extending longitudinally relative to the conductor layer. Where such extensions occur rapidly over short sections of cable, as at the squirter during sudden starts or stops or due to pitching and yawing of the vessel, the longitudinally flexing jacket may damage underlying components. Also, such displacement prevents the effective transfer of shear forces from the jacket to the reinforcing core. The sheath will then bear substantially all of the tension applied to the cable by the squirter, possibly leading to premature failure of the jacket and cable. Where the jacket has been pressure extruded over the conductor layer, the jacket's inner profile corresponds to the rope-like surface of the helically wound conductors, such that longitudinal displacement of the jacket relative to the conductor layer may impart considerable localized abrasions and loads upon the conductors. To avoid damage caused by inefficient load transfers as described above, the longitudinal force applied to the cable jacket is typically distributed over longer sections of cable by adding squirter nips. The compressive forces applied at the squirter nips are typically increased, and elaborate controls are sometimes employed to avoid jerking.
In existing OBCs, it is common for voids to remain within the core of the cable. As a result, water pressure will often deform the cable as the atmospheric voids collapse and move within the cable, displacing individual elements within the cable and thus changing the electrical, optical, and/or mechanical characteristics of the elements or cable in general. Such displacements may increase rigidity in regions of the cable, for example, resulting in handling problems and increasing the likelihood of component damage when the cable is retrieved. Displaced air within the cable may also collect in pressurized pockets which breach the outer jacket on retrieval of the cable, allowing water to penetrate into the cable. To avoid problems associated with voids, cables have been filled with oil or other viscous materials with a view to flushing out any trapped air and filling any voids in the cable. Such methods, however, generally have other problems associated with them and are only partially effective. Complete air removal is difficult to achieve, particularly with higher viscosity materials, and lower viscosity fluids require special considerations for filling the cable and may reduce load transfers.
EP-A-O 193 780 discloses a submarine cable for optical fiber telecommunications. The cable has a core comprising armoring of antitorsional rope and a plurality of small tubes wound helically and in direct contact with the armoring. The small tubes loosely house the optical fibers and are filled with a practically incompressible fluid such as a petroleum jelly or a silicon grease.
It is among the objectives of embodiments of the present invention to provide an OBC which obviates or mitigates these difficulties.
It is a further objective of embodiments of the present invention to provide an improved cable containing a plurality of elements in which the elements are protected from stresses and forces applied to the cable.
SUMMARY OF THE INVENTION
According to the present invention there is provided a cable comprising: a stress-bearing matrix extending substantially through the length of the cable; and a plurality of conducting elements extending substantially through the length of the cable, the plurality of said conducting elements being located within and spaced from one another by said stress-bearing matrix, wherein at least one of the plurality of conducting elements is in intimate contact with a low friction liner disposed about the at least one of the plurality of conducting elements and the at least one of the conducting elements is longitudinally movable relative to the stress-bearing matrix.
In use, the provision of the low friction liner disposed about at least one of the conducting elements allows embodiments of the present invention to be designed such that loads are transferred to selected conducting elements at differing rates. This is particularly useful where the elements are fragile or susceptible to stress induced damage, for example optical fibers or small diameter electrical conductors. Further, the flexibility of the cable is improved as relatively inelastic elements, such as metallic conductors, may incur minimal loads over short lengths as the cable flexes. In contrast to the cable configuration
Booth David
Grunsteudel Herbert
Hoffmann Michael
Smith John David
Mayo III William H.
Reichard Dean A.
Sheets Eric J
W. L. Gore & Associates, Inc.
Wheatcraft Allen
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