CA2508216A1

CA2508216A1 - Solid flooding compound for marine seismic cable

Info

Publication number: CA2508216A1
Application number: CA 2508216
Authority: CA
Inventors: Mike Petryszak; Mike Maples; Wesley Conger
Original assignee: Sercel Inc
Current assignee: Sercel Inc
Priority date: 2005-04-14
Filing date: 2005-05-24
Publication date: 2006-10-14
Also published as: NO20054014D0; GB2425219A; GB0510656D0; NO20054014L

Abstract

A new flooding material or flooding compound and jacket combination for a marine seismic cable and a new method of forming such a cable are provided. A
plurality of longitudinal elements, such as a central strength member, power conductors, communication lines, and the like, are formed into a coherent unit with a solid flooding compound. This flooding compound comprises a pressure sensitive adhesive to resist shear forces during launch and recovery operations of the cable.

Description

SOLID FLOODING COMPOUND FOR MARINE SEISMIC CABLE
FIELD OF THE INVENTION
The present invention relates generally to the field of seismic exploration and, more particularly, to a solid flooding compound for a marine seismic cable and to a cable made with such solid flooding compound.
BACKGROUND OF THE INVENTION
As more easily accessible sources of oil and gas are depleted, exploration efforts are directed to regions of oil and gas bearing strata that may be present below deeper and deeper water. In certain operations, ocean bottom cables with sensors positioned along their length are laid out in a grid along the ocean floor. Seismic images of the geologic structures are obtained, and then the cables are brought back aboard the exploration vessel. In other operations, an array of streamer cables is towed behind an exploration vessel during seismic operations and then the cables are brought back aboard the vessel when exploration operations are complete.
Whether ocean bottom cable or streamer cable, the cable is subjected to tremendous stress as it is brought onto the vessel. In certain seismic exploration systems, a specially designed retraction mechanism bears the load of the cable, and this load comprises primarily the weight of the cable and drag of water over the outside surface of the cable. For deep water operations involving ocean bottom cables, where the ocean floor can be many thousands of meters below the surface, the weight of the cable alone is tremendous. In some systems, the retraction mechanism grasps the outside surface of the cable, creating a friction force to pull the cable up against gravity and drag. In other systems, the cable is pulled aboard the vessel directly by a reel device.
Certain typical ocean bottom cables and streamer cables are manufactured with one or more strength members running parallel to the axis of the cable. The strength members) bear the force of the gravity and drag exerted by and on the cable.
Consequently, during operations for recovery of the cables aboard the vessel, forces on the strength members) pull in a direction opposite to the force created by the retraction mechanism.
Ocean bottom cables are typically solid. In most ocean bottom cables known in the art, one or more flooding compounds are used between the jacket surrounding the cable and the strength member, the power conductors, the communications lines, and the like.
Due in part to a lack of appreciation for the drawbacks discussed above, the flooding compounds have generally been fluid in nature when subjected to great shear forces over time during a number of launch and recovery operations of the cable.
For solid streamer cables, unlike ocean bottom cables, focus has been directed to controlling the buoyancy of the cable through careful selection of the flooding material. In essence, the focus on the buoyancy has required that the flooding material (or flooding compound) also have certain fluid properties. Thus, for either an ocean bottom cable or a streamer cable, a problem develops after subjecting the cable to a number of launch and recovery cycles. This problem manifests itself in the longitudinal movement of the cable jacket relative to the elements within the cable, such as the strength members) and/or the power and signal conductors. Eventually, this movement may result in wrinkles forming on the exterior surface of the cable or even pulling a takeout away from the cable, causing failure of the cable.
A number of patents illustrate various flooding materials, including U.S.
Patents numbered 3,531,760 to Whitfill, Jr.; 3,605,398 to Carlson et al.; 3,710,006 to Davis;
3,744,016 to Davis; 3,900,543 to Davis; 4,491,939 to Carpenter; 4,676,590 to Priaroggia;
5,046,057 to Berni; 5,745,436 to Bittleston; 6,108,267 to Pearce; 6,211,964 to Luscombe et al.; and 6,510,103 to Knudsen et al. However, none of the references address the problem of the displacement of the cable jacket transversely in relation to the longitudinal elements, such as strength members, power conductors, and communications conductors and the like, within the cable.
To summarize the background in simple terms, we have discovered a failure mode of seismic cables used in ocean exploration and have invented a solution to this failure mode.
Similarly, the failure mode exists in cables other than seismic cables which include a longitudinal strength member, an outer jacket, and a flooding material or compound between the jacket and the strength member. We have found that the elements within the cable tend to slip relative to the jacket enclosing the cable. Each time the cable is brought aboard the vessel, there may be a small amount of this slippage, but over time these small amounts of slippage accumulate. Further, once the accumulated slippage reaches a certain point, the cable will fail, particularly at takeouts. Thus, there remains a need for a structure of a solid seismic cable which sticks two or more layers of the cable together to resist forces that would otherwise shear them apart. The present invention is directed to solving this problem in the art.
SUMMARY OF THE INVENTION
The present invention solves this and other drawbacks in the art by providing a new flooding material or compound for a marine seismic cable and a new method of forming such a cable. This invention is applicable to ocean bottoms cables and streamer cable alike. When applied to an ocean bottom cable, a plurality of longitudinal elements, such as central strength members, power conductors, communication lines, and the like, are formed into a coherent unit with a solid flooding compound, comprising a hot-melt, pressure sensitive adhesive. The adhesive is applied to a central strength member and then the plurality of longitudinal elements are embedded within the adhesive running parallel to the central strength member. More adhesive material is then applied to cover the longitudinal elements. An inner jacket is extruded onto the cable, covering the adhesive, then an outer jacket is formed over the inner jacket with an imbedded braided strength member in the outer jacket. In that way, increasing the force on the outer jacket as the cable is retracted to the surface also serves to increase the radial force between the outer and inner jackets.
The pressure sensitive adhesive thus maintains the alignment of the bundled elements and the outer jacket.
When applied to a streamer cable, a power conductor cable bundle is encased in a buoyancy material to provide a substantially neutral buoyancy to the cable.
This inner bundle is covered by one or more strength members and a sealing jacket. The hot-melt, pressure sensitive adhesive is applied to the outer surface of the sealing jacket and a plurality of communications conductors are embedded within the adhesive. More adhesive is then applied to encase the communications conductors and one or more outer jackets are extruded onto the outer surface of the adhesive.
In both embodiments, the adhesive retains the layers on either side of the adhesive in longitudinal alignment, resisting the shear forces on either side of the layer formed with the adhesive.

These and other features and advantages of this invention will be readily apparent to those skilled in the art.
BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, more particular description of the invention, briefly summarized above, may be had by reference to embodiments thereof which are illustrated in the appended drawings.
Figure 1 is a side elevation view of a vessel coupled to an ocean bottom cable to illustrate the environment in which the present invention finds application.
Figure 2 is a conceptual plan view of a mechanism for retrieving a cable to illustrate the forces involved.
Figure 3 is a section view of an ocean bottom cable constructed in accordance with the present invention.
Figure 4 is a section view of a streamer cable constructed in accordance with the present invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Figure 1 illustrates a typical seismic exploration vessel 10 tending a seismic cable 12.
For illustrative purposes, the cable 12 is an ocean bottom cable; however, the present invention is equally applicable to towed cables. The cable 12 lies along the ocean floor 13 with sensor element packages 14 placed along the length of an ocean cable length 16. The cable length 16 can be many kilometers long, for example, and weigh many tons.
In order to retrieve the cable back aboard the vessel 10, a retrieval mechanism 18 is used. The retrieval mechanism may include a reel onto which the cable is spooled in the conventional fashion well known in the art, although some operators simply lay the retrieved cable out on the deck of the exploration vessel. In some exploration operations, no specially designed retrieval mechanism is used, but the cable is brought aboard using only the reel.
Figure 2 illustrates another portion of one such specially designed retrieval mechanism, referred to herein as the retraction mechanism 20. The retraction mechanism 20 includes a plurality of contact rollers 22, which may actually be rubber vehicle tires, to contact the cable 16. The rollers 22 create a longitudinal force, as shown by arrows 24, to counteract the weight and drag of the cable, represented by arrows 26. The operation of these forces causes a small slippage or displacement between the jacket of the cable and elements within the cable. Enough displacement of these elements results in damage to the cable, particularly at a takeout 34.
These small displacements occur at a "slip layer" within the cable and any particular cable may have more than one slip layer within the cable. A slip layer occurs where shear forces tend to pull elements within the cable in opposite directions, thereby causing the displacement. It should be understood that the displacement just described need not occur all at once, or during a single cycle of the deployment of the cable. One use of the cable can result in a small displacement, and since typical flooding materials 28 are somewhat fluid, relaxation of the forces 24 and 26 does not restore the relative positions of the jacket and the strength member. Over time, the displacements become cumulative and may eventually, and even inevitably, damage the cable. This problem is solved by the cable of the present invention shown in section in either of Figures 3 or 4. While Figures 3 and 4 illustrate preferred embodiments of the use of the present invention, it is to be understood that many other combinations of the cable elements may be used, still within the spirit and scope of the present invention.
Referring now to Figure 3, a cable 40, formed in accordance with this invention, includes a plurality of longitudinal elements, such as a central strength member 42, communications lines 44, and power conductors 46. The longitudinal elements are sealed together into a unit 48 by a flooding compound 50. It is principally at this point that the present invention departs from the conventional wisdom of the art.
A pressure sensitive hot-melt adhesive, having a chemical composition similar to that of Kraton~ D polymer, or other appropriate adhesive, is first applied to the central strength member 42. The longitudinal elements 44 and 46 are then laid lengthwise along the adhesive (i.e. the flooding compound 50) and then more adhesive is applied to cover the longitudinal elements. This completes the formation of the unit 48.
An inner jacket layer 49 is then extruded over the unit 48. Recall that the unit 48 comprises the plurality of longitudinal elements (such as the inner strength member, communications cables and electrical conductors) substantially surrounded by the flooding compound 50. The inner jacket 49 tenaciously adheres to the outer surface of the unit 48.
Otherwise, the junction between the inner jacket 49 and the unit 48 would provide a slip layer, as previously defined.
Next, a woven braid 52 is laid over the inner jacket 49 and an outer jacket 54 is extruded over the braid 52. This embeds the braid 52 within the outer jacket and the inner jacket 50 bonds to the inner surface of the outer jacket. When cured, the combination of the inner and outer jackets forms a solid yet flexible covering over the unit 48.
The outer jacket is preferably formed of a relatively hard, wear resistant material since the outer jacket is contacted by the contact rollers 22. Embedding the woven braid 52 into the outer jacket has the additional advantage of squeezing down on the cable in response to the force 24 applied to the cable, much as Chinese finger cuffs squeeze down on fingers inserted into them when one attempts to pull the fingers apart.
This action creates a radial force toward the central strength member 42, minimizing the likelihood of displacement of the outer jacket relative to the unit 48. Note also that the outer surface of the cable is not smooth, but includes a plurality of ridges 56 for contract with the retraction mechanism.
Referring now to Figure 4, a streamer cable 60 incorporating the present invention is disclosed. The structure of the 60 is laid out in layers, just as the ocean bottom cable of Figure 3 is laid out in layers. Further, the functions of the various layers are substantially the same as the functions of the layers previously described, with the exception that the streamer cable 60 is designed to maintain a neutral buoyancy in the water.
The exemplary embodiment illustrated in Figure 4 includes a set of power conductors 62 enclosed within a buoyancy control material 64. The power conductors are enclosed within one or more strength members 66, formed of a suitable material such as VECTRAN~
or the tike. A sealing jacket 68 forms a seal between outer layers and the power conductor layers underneath. A plurality of communications conductors, such as a communications quad 70 and an analog pair 72, or similar communications channels, are laid parallel to the axis of the cable, and enclosed with a flooding compound 74 in a manner described below in relation to the ocean bottom cable of Figure 3. Finally, the flooding compound layer is cover with an inner jacket 76 and an outer jacket 78, with a woven braid 80 embedded within the outer jacket.
in operation, a slip layer may form adjacent the jacket 68 so one side of the potential slip layer comprises the pressure sensitive adhesive of the flooding compound, thereby resisting permanent deformation at the slip layer. Similarly, a slip layer may form at the interior surface of the inner jacket. This possible slip layer is thus in contact with the adhesive of the flooding compound 74. Thus, in the broadest sense, the invention comprises a pressure sensitive adhesive as one layer where slippage due to shear stress is likely to occur. In another aspect, the present invention comprises a method of forming a seismic cable comprising placing a pressure sensitive adhesive at such a layer.
The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Claims

1. A marine seismic cable comprising:
an outer jacket;
a communications conductor within the outer jacket;
a longitudinal strength member within the outer jacket; and a flooding layer formed of a pressure sensitive adhesive around the communications conductor to prevent permanent displacement between the outer jacket and the communications conductor.

2. The cable of claim 1, further comprising an inner jacket between the flooding layer and the outer jacket.

3. The cable of claim 1, further comprising a woven braid embedded in the outer jacket.

4. The cable of claim 1, further comprising an inner longitudinal unit comprising the longitudinal strength member, the communications conductor, and power conductors surrounded by the flooding layer.

5. The cable of claim 1, further comprising an irregular exterior surface of the outer jacket.

6. The cable of claim 1, wherein the outer jacket is adhered to the inner jacket by thermal fusion.

7. A method of forming a seismic cable, comprising the steps of:
forming an inner longitudinal unit; and covering the inner longitudinal unit with a pressure sensitive adhesive.

8. The method of claim 7, wherein the inner longitudinal unit comprises a central strength member, communications lines, and power conductors surrounded by a flooding layer.

9. The method of claim 7, further comprising the step of forming an outer jacket around the cable.

10. The method of claim 9, further comprising the step of embedding a woven braid in the outer jacket.

11. A cable comprising:
an outer jacket;
a longitudinal strength member within the jacket; and a flooding layer formed of a pressure sensitive adhesive between the outer jacket and the strength member to transfer load between the outer jacket and the strength member.