Note: Descriptions are shown in the official language in which they were submitted.
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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
CA 02508216 2005-05-24
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
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CA 02508216 2005-05-24
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.
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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
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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
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CA 02508216 2005-05-24
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
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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.
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