Armored seabed laid seismic cable and method and apparatus for manufacturing same

A method for wrapping continuous strands of steel wire about a seismic cable including interconnected sensor sections and conductor sections where a cross sectional diameter of the sensor section is at least four times that of the conductor section. Two layers of armoring are provided with a first layer wrapped in a first angular direction opposite that of the second layer. A stranding assembly is provided which has two selective positions, one for providing a die hole for stranding the conductor section, another for providing a passage hole for allowing the sensor section to pass after wrapping with armor wire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to seismic cables, and in particular to seismic cables designed for laying on the sea floor. Such cables are installed in arrays on the sea floor and detect sound waves reflected from subterraneous strata in the earth's crust in response to energy pulses in the sea from an air gun.

2. Description of the Prior Art

Seabed laid seismic cables are known which include lengths of conductors which connect spaced sensor devices which are powered and interrogated from a surface vessel or fixed platform. Seabed laid seismic cables must be armored, because the cables are subjected to wear and abrasion when being installed on the sea floor and are required to be placed in mud and silt for long periods of time. Such cables, also must be capable of withstanding marine growths such as barnacles, etc.

Armored cables are generally known. For example, U.S. Pat. No. 4,439,633 discloses an armored cable and method of manufacturing the cable where an inner conductor is supported within a polyethylene tube about which helically wrapped steel armoring is placed. U.S. Pat. No. 6,041,282 shows a number of elongated conductors (electrical and or/optical) which are arranged to be interconnected with a number of seismic sensor devices positioned at intervals along the cable. The '282 cable includes a central polyethylene tube. The sensors are placed at intervals along the tube, but power and signal conductors are arranged in stranded fashion over the tube. A polyethylene jacket covers the conductor, and steel wire armoring extends along the length of the entire cable. Because the '282 cable has conductors and sensors characterized by an essentially constant diameter, armoring of the cable is not an unusual problem, because planetary armoring machines are known which helically wind steel strands about conductor lengths to produce an armored cable.

Seabed laid seismic cables desirably include geophone sensors in addition to hydrophones at instrumentation pods or sensor sections placed along the cable. A typical commercially available geophone may be approximately 1 inch in length and ⅞″ in diameter. The geophones take up more radial space than that of the conductor bundle, and the sensor assembly of which they are a part must be mounted in a protective housing in pods or sensor sections placed at intervals along the cable. U.S. Pat. No. 4,078,223 shows a seismic cable with sensor assemblies spaced along the cable of a larger diameter than that of the conductor sections between them. The '223 seismic cable has a plastic sheath placed over the conductor section and sensor assemblies, but no steel wire armoring. U.S. Pat. No. 4,241,427 shows Kevlar strands covering the conductor sections and sensor assemblies for protecting the cable. U.S. Pat. No. 3,939,464 shows a protective cage for sensor assemblies and with a braided covering for the larger diameter sensor assemblies and the smaller diameter conductor sections.

U.S. Pat. No. 6,333,897 shows a housing for a sensor assembly interconnected with a central strength member of each of the conductor sections on opposite ends of the sensor assembly. The housing is made as small as possible so that, after the sensor assembly is interconnected with the conductor and the central strengths members, armoring of the conductor sections and of the housing can be provided. No disclosure is provided as to how steel wire armoring is applied to the in line housing and conductor housings, but illustrations in the '898 patent illustrate that the diameter of the housing is about less than twice the diameter of the conductor section.

U.S. Pat. No. 6,333,898 shows a seabed laid seismic cable where a sensor housing is placed outwardly of the conductor sections. At a location where sensor housings are to be placed, a sleeve is placed about the conductor section with an opening to connect jumpers between the sensor devices in the housing to conductors inside the sleeve. The conductor sections and sleeve are armored with stranded steel wire prior to attachment of the sensor housing to the sleeve. In other words, the sensor housing is not protected by the steel wire armoring.

3. Identification of Objects of the Invention

A primary object of the invention is to provide a method and apparatus for applying stranded steel wire armoring to a seismic cable which includes conductor sections interconnected with sensor housings, where the steel wire armoring is applied in continuous strands about the exterior of the conductor sections and the exterior of the sensor housings.

An important object of the invention is to provide a method and apparatus for applying stranded steel wire armoring to interconnected conductor sections and sensor housings where the outer diameter of the sensor housing is about four times as great as that of the conductor sections.

Another object of this invention is to provide a steel wire armored seismic cable with sensor housings which are about four times as great in diameter than that of the outer diameter of the conductor sections.

SUMMARY OF THE INVENTION

The objects identified above as well as other features and advantages of the invention are incorporated in a steel wire armored seismic cable having integrated seismic sensor sections and conductor sections where the outer diameter of the sensor section is about four times as great as that of the conductor sections. Prior to armoring, the conductor sections and sensor housing sections are interconnected with two sets of conductors provided in the conductor sections. At each sensor housing section, a first set of conductors is connected at each end of the housings. The second set of conductors passes outwardly along the sensor housing section. At the next sensor housing section of the integrated cable, the second set of conductors is connected at each end of the housing, but the first set of conductors passes around that sensor housing section. Conical bend restrictors are placed over the conductor sections adjacent each of the two ends of the sensor housing. Next, rings with spaced lugs are secured about the periphery of the sensor housing. The cable with integrated sensor housings and conical bend sections and wire spreading lugs are wound onto a large drum so that the cable is ready for passing through two stages of a planetary steel wire armoring machine.

At each station the planetary winding machine is arranged to produce bends in the steel wires prior to their being wrapped in a helical pattern about the conductor section at a conductor closing die. At a first station, the winding machine wraps the steel wire in a first direction at a first conductor closing die. At a second station, the winding machine wraps the steel wire in a direction opposite that of the first direction at a second closing die. The closing dies are mounted on respective first and second winding assemblies. When the conductor section is pulled through a “conductor” closing die, the respective winding assemblies are prevented from moving, while the cable is pulled through the conductor die. As a result, the steel wires are forced by the closing die to be wound tightly in a helical path about the conductor sections. The conductor section is characterized by an outer diameter of about 25 millimeters with each of the conductor closing die being of a corresponding diameter, which is slightly larger than the diameter of the conductor section.

When a sensor housing section approaches one of the conductor closing dies at a winding assembly, the pulling of the cable stops, bending of the wire strands is removed from the planetary machine, and a large diameter “housing” closing passage replaces the conductor closing die. The winding machine again starts turning at a slow rate with the cable being pulled forward. The winding assembly is allowed to move forward with the pulling of the cable. The pulling of the cable pulls the housing section and the winding assembly in a forward direction, because a gripping cylinder temporarily secures the front end of the housing section to the winding assembly. The housing closing passage is of a diameter sufficiently large to allow the sensor housing to pass through it after the housing is wrapped with armor wires. In a preferred embodiment the outside diameter of the sensor section is about 125 millimeters.

An operator insures that the wire strands are uniformly helically distributed about the outer surface of the housing section prior to passing through the housing closing die by positioning the wire in spaces between lugs of rings secured around the outer surface of the housing section. The spaces are uniformly placed about the periphery of the housing. When the housing has been pulled through the housing closing passage, the conductor closing die replaces the housing closing passage, bending of the steel wire strands is reestablished in the planetary winding machine, and armoring of the conductor section at the first station and/or at the second station continues until another housing section approaches a conductor closing die at the first or the second station, and the above procedure is repeated.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

FIGS. 1A and 1Billustrate a manufacturing line by which a seismic cable10(including conductor sections14and housing sections (seeFIGS. 1A,2,4)) is armored with continuous strands of steel wire so that the cable10is capable of long term operation when laid on a sea floor. The unarmored cable10ofFIG. 1Ais first wound on a preparation spool (not shown) which is supported from a frame not shown. After armoring of the conductor sections14and the housing sections12, the completed armored cable10″ ofFIG. 1Bis wound onto a take-up spool20which is supported from a frame22.

First and second planetary wire armoring machine stations30and50are commercially manufactured by Caballé of Spain. The cable10″ with steel strand armoring over conductor section and housing section is pulled through armoring machines30and50by motor traction spools60.

FIG. 1Aillustrates a first station carriage32on which a stranding assembly34is configured with a conductor die (seeFIGS. 5,6,7). In a preferred embodiment, the first planetary winding machine30includes four banks of six bobbins (not shown) which supply twenty four steel wire strands36(only a few of which are illustrated for clarity) as they are pulled across winding head38toward first stranding assembly34. The unarmored seismic cable10, after passing through the first stranding assembly34becomes an intermediate stage seismic cable10′ with a first layer of armoring wire.

The intermediate cable10′ passes through the second planetary machine50and to second stranding assembly43disposed on second stage carnage42. The second planetary wire stranding machine50includes five banks of six bobbins and a winding head41such that thirty steel wire strands (only a few of which are illustrated) are wound onto intermediate cable10′ as the cable section10″ is pulled through the machine. According to the invention the conductor sections14′ and the housing sections12′ of intermediate cable10′ have a second layer of armor strands applied to them.FIG. 1Bshows a stage of the manufacturing process where the second layer of armor strands applied from winding head41are applied at second stranding assembly43to a housing section12′ of the intermediate seismic cable. A final cable section10″ includes armored conductor section14″ and armored sensor housing sections12″ (see FIG.15and12′″ as illustrated inFIG. 16).

The ratio of the rotation of traction spool device60and the first armor station30determines the lay length of the first pass armor. Likewise the ratio of the rotation of traction spool device60and the second armor station50determines the lay length of the second pass armor. Lay length is defined as the longitudinal distance required for an armor wire36or44to make one full rotation around the cable. The lay length of the first pass armor and the second pass armor are similar but somewhat different. In the preferred embodiment the first pass armor lay length is approximately 150 millimeters; the lay length of the second pass armor is approximately 180 millimeters.

The lay angle of the armor is defined as the angle between the armor wire and the longitudinal axis of the cable. Lay angle is primarily controlled by the lay length of the armor and the diameter of the cable. A constant lay angle must be maintained even as the housing sections12(seeFIG. 4) have a first pass armor applied at stranding assembly34. This is accomplished by changing the lay length settings for the first pass armor bay30as a housing section passes through stranding assembly34, and returning to the original lay length setting after the housing section12has passed through stranding assembly34. Likewise the lay length setting for the second armor bay50must be adjusted as housing section12′ passes through stranding assembly43.

FIGS. 2,3, and4illustrate a function of the winding heads38(winding head41at the second stage functions in a similar way) when applying armoring to unarmored conductor section14(FIG. 2) or to housing section12(FIG. 4). For the case of applying armoring strands to conductor section14though stranding assembly34, it is necessary to provide bends to the cable as it passes through stranding assembly34. The winding head38rotates about the longitudinal axis of the conductor section14when applying the strands to the conductor section through winding assembly34. With bends applied to each strand, by rotating disk29a small angular distance with respect to disks25and27, the bent strands do not create a “spring effect” if the conductor section were to be cut or separated. It is desirable to eliminate such spring effect. Accordingly, bends are provided in the armor wires while wrapping conductor sections as illustrated inFIG. 2.FIG. 3shows a hydraulic cylinder21connected to disk29for moving it out of rotational alignment (e.g. inFIG. 2) and into rotational alignment as illustrated inFIG. 4.FIG. 4illustrates that disk29has been moved into rotational alignment with disks25and27of winding head38where the forward end of the housing section12first enters into stranding assembly34. The “bends” in the armoring wires are removed, and as the head38rotates, the armoring wires are wrapped about the exterior of housing section12until the entire outside thereof has been wrapped, and where the rear end of the conductor section14clears the stranding assembly34, the disk29is again rotated by cylinder21and the bends in the wire strands36are again established for armoring the next conductor section.

FIGS. 5,6,7, and8illustrate the stranding assembly34which includes mechanisms for establishing a die passage64for stranding wires36about conductor section14and for providing a housing passage66for a housing section12to pass through after it is stranded (SeeFIG. 8).

FIG. 5illustrates first stage carriage or frame32which is free to move on foundation62in the direction of the pull force arrow P on wheels60when a blocking member68is not in place. As shown in the cross sectional view ofFIG. 6, and by reference toFIGS. 7 and 8, a conductor section stranding die70is established when forward plate72is in a first position ofFIGS. 5,6, and7such that the half cylinder opening73of forward plate72is in registration with the half cylinder opening75of rear plate74. As illustrated inFIGS. 5,6, and7, the forward plate72and rearward plate74are coupled to a handle76via pivot rods77,79. When the handle76is pushed toward frame members78, the forward plate72is pushed forward, and the rear plate74is pulled rearward (when viewed from the views ofFIGS. 7 and 8) thereby creating the die passage64between the two plates72,74. An elastomeric strap81is removably positioned over an upper member83of frame78to maintain the handle76in the forward position. A pin85maintains the plates72and74in the position ofFIG. 7through registered holes in the plates when in that position, and in the position ofFIG. 8through other registered holes. As illustrated inFIG. 8, when the handle76is pulled away from frame member78, the large cylindrical hole61of forward plate72and hole63of rear plate74are brought into registration to form passage66for the passage of stranded sensor sections12of the seismic cable10.

The stranding assembly34described above is mounted on a standard80(seeFIG. 5) which can move longitudinally on rails82mounted on frame32. A threaded rod can be rotated by handle85to move the stranding assembly80longitudinally along the stranded conductor section10′.

When the conductor section14is being stranded, the carriage32is blocked from horizontal movement by blocking member6850that the die hole64in the stranding assembly34is maintained at a constant longitudinal position when the conductor section14of the cable is being pulled through it (becoming a first armored layered conductor14′ after passing through the die hole64.

The stranding assembly43is positioned at the forward end of second planetary winding machine50(FIG. 11B) and is constructed substantially identically to that of stranding assembly34positioned at the forward end of the first planetary winding machine30. The die passage of stranding assembly43(similar to that of passage64of assembly34) is slightly larger in diameter than that of assembly34so as to accommodate two layers of armor wires over conductor sections. The passage for sensor section (similar to that of passage66of assembly34) is also slightly larger in diameter so as to accommodate two layers of armor wires over the sensor housing sections.

FIGS. 9 and 10illustrate the stranding assembly34which is selectively converted from stranding wire strands about the conductor section14to an assembly for stranding the sensor housing sections12. When a housing section12approaches the stranding assembly34, the traction pulling drums60are stopped with the forward end of a sensor section12positioned to enter the stranding assembly34as illustrated inFIG. 4. The bend has been removed from the stranding wires36so that substantially straight steel strands36are ready for wrapping about the exterior of the housing12. As illustrated inFIG. 9, a rear split ring member88is positioned over the forward conical portion6of the sensor section12.FIG. 10illustrates the two halves88′, and88″ hinged at90and latched about forward conical portion6with an elastomatic cylinder92gripping the forward portion6and preventing its longitudinal passage through stranding assembly34. Handles89are used by an operator to close halves88′,88″ about the forward end6of the sensor section. The handle76has been pulled to its outward position thereby creating the passage66through the stranding assembly34.

A forward split ring member94is positioned about the forward end of the stranding assembly34as illustrated inFIG. 9, and has a passage95sized for clamping about conductor14′ which has had armoring applied to it. Threaded bars96engage the body78of stranding assembly34so that a small horizontal distance separates member94from body78. Handles89and99allow an operator to grip the forward end6of the housing section12and the armored section14′ while stranding is being applied to sensor section12to prevent section12from rotating while the armor strands are being wrapped with strands36from rotating head38. The conductor section14′ and housing section12are prevented from rotation during stranding, because forward clamping member94grips the conductor section14′ and pulls conical portion6into elastomeric cylinder92. As shown inFIG. 1B, anytime a sensor section is being stranded with steel wire strands, the blocking member68(ofFIG. 5) has been removed so that a carriage42(as shown inFIG. 1B) moves with the cable as it is being pulled forward while stranding about the sensor housing section. As explained in more detail below, at the first stage, unbent armor wires are wound about a sensor section12while the head38rotates, and with the sensor section12being held in place as shown inFIG. 9and with the carriage32moving forward by the coupling of the conductor section10′ to stranding assembly34and the intermediate mounting member. As the winding head slowly rotates in coordination with the pulling of the cable forward, the steel strands are placed in openings110between24lugs of the forward, middle and rear spacing rings105. (SeeFIGS. 11,12,13) The twenty-four openings110correspond to the twenty four strands applied by first planetary winding machine30and winding head38. In other words, as the stranding of the housing section12occurs at winding assembly34immediately forward of stage one winding head38, an operator manually assures that each one of the stranding wires is placed in one of the openings110between lugs. At the second winding stage, the second winding head turns in an opposite direction to that of first stage winding head38and preferably applies thirty wire strands about the conductor section14′ and about the sensor section12′. The operator assures that at every fourth lug opening, two wire strands are placed in an opening110with one wire being placed in the next three openings and so on. As a result, each lug ring of the housing section has six openings spaced around each lug ring each of which have two strands placed therein and the other eighteen openings have one strand per opening placed therein for a total of thirty strands.

FIGS. 11–16illustrate the make up and stranding of each sensor housing12. The sensor housings12include sensors such as geophones and a hydrophone in a cylindrical tube. Conductor or optical fibers of conductor sections are interconnected with the sensors of the housing12.FIG. 11shows the sensor housing12with a central portion3and conical end portions2secured to the front and rear ends of central portion3. Grooves5are formed at the front, middle and rear part of the housing3and lug rings105as described above are therein. Each ring105has a slit in it that allows it to be opened for wrapping about housing3. The rings105are made of hard plastic material and are glued to secure them in the grooves5.FIG. 13, a cross section view taken along lines13—13ofFIG. 12, shows a lug ring105in profile with twenty four openings therein and schematically illustrates a sensor106placed in the interior of the sensor section. Certain conductors within the conductor section14communicate with the sensors within the housing12; other conductors (not shown) pass outwardly and around the housing from a forward conductor section to a rearward conductor section.

FIG. 14shows armor wire strands36wrapped in one direction about sensor section12with each of the openings of the lug rings105having one wire strand36therein.FIG. 15shows armor wire strands44wrapped in an opposite direction about sensor section12″ with every fourth lug opening having two wire strands44therein.

FIG. 16shows a sensor section12″′ after the sensor section12has had two sets of armoring strands applied to it as inFIG. 15and after split plastic bend restrictors110with integral conical boots112are attached about the conductor sections. The bend restrictors/conical boots are secured to the sensor section by steel latching rings placed in grooves120. After the bend protectors110and boots120are wrapped with tape, the seismic cable10″ is wound (SeeFIG. 1C) about traction device60and ultimately spooled on spool20as a completed length of seismic cable.

An exemplary armored seismic cable10″, manufactured as described above is 3.6 km overall length, with72sensor housings12″′ spaced at 50 meters from each other. The outer diameter of the conductor sections of the exemplary cable is about 25 millimeters while the outer diameter of the sensor sections of the cable is about 125 millimeters.