U.S. patent application number 09/727274 was filed with the patent office on 2002-05-30 for dimpled marine seismic cables.
Invention is credited to Barker, Glen P..
Application Number | 20020062778 09/727274 |
Document ID | / |
Family ID | 24922013 |
Filed Date | 2002-05-30 |
United States Patent
Application |
20020062778 |
Kind Code |
A1 |
Barker, Glen P. |
May 30, 2002 |
Dimpled marine seismic cables
Abstract
An apparatus and method for improving the efficiency of marine
cable tow operations. Indentations such as dimples in the cable
exterior surface reduce frictional drag forces and reduce strumming
of the cables as the cables are towed through the water. The size,
configuration and orientation of the indentations can be selected
to control the desired water flow parameters.
Inventors: |
Barker, Glen P.;
(Friendswood, TX) |
Correspondence
Address: |
ALAN J. ATKINSON
P.O. Box 270161
Houston
TX
77277-0161
US
|
Family ID: |
24922013 |
Appl. No.: |
09/727274 |
Filed: |
November 29, 2000 |
Current U.S.
Class: |
114/253 |
Current CPC
Class: |
B63B 21/663 20130101;
F15D 1/12 20130101; D07B 5/006 20150701; G01V 1/201 20130101; G01V
1/38 20130101; F15D 1/005 20130101 |
Class at
Publication: |
114/253 |
International
Class: |
B63B 021/04 |
Claims
What is claimed is:
1. An apparatus for reducing friction acting against a marine cable
towable through water, comprising: a cable sheath having an
exterior surface in contact with the water; and a plurality of
indentations in said sheath exterior surface for reducing friction
between said cable and the water as the cable is towed through the
water.
2. An apparatus as recited in claim 1, wherein said indentations
comprise dimples.
3. An apparatus as recited in claim 1, wherein said indentations
are symmetrically configured.
4. An apparatus as recited in claim 3, wherein at least one of said
indentations is configured in a round dish shape.
5. An apparatus as recited in claim 3, wherein at least one of said
indentations is configured in a polygonal shape.
6. An apparatus as recited in claim 3, wherein at least two
different shapes of indentations are formed in said sheath exterior
surface.
7. An apparatus as recited in claim 1, wherein said indentations
are oriented in a particular pattern on said sheath exterior
surface.
8. An apparatus as recited in claim 1, wherein at least two of said
indentations have different sizes.
9. An apparatus as recited in claim 1, further comprising a
plurality of protrusions extending from said sheath exterior
surface.
10. An apparatus as recited in claim 1, further comprising a
splitter plate attached to said cable sheath.
11. An apparatus for reducing strumming in a marine cable towable
through water, comprising: a cable sheath having an exterior
surface in contact with the water; and a plurality of indentations
in said sheath exterior surface for reducing friction between said
cable and the water as the cable is towed through the water.
12. An apparatus as recited in claim 11, wherein said indentations
are symmetrically configured.
13. An apparatus as recited in claim 11, wherein at least two
different shapes of indentations are formed in said sheath exterior
surface.
14. An apparatus as recited in claim 11, wherein said indentations
are oriented in a particular pattern on said sheath exterior
surface.
15. An apparatus as recited in claim 11, further comprising marine
seismic equipment attached to said cable.
16. A method for towing a cable in water, comprising the steps of:
deploying a cable having a cable sheath in the water behind a tow
vessel, wherein said cable sheath exterior surface in contact with
the water has a plurality of indentations for reducing friction
between said cable and the water; and towing said cable through the
water.
17. A method as recited in claim 16, further comprising the step of
deploying a cable in the water wherein said indentations comprise
dimples.
18. A method as recited in claim 16, further comprising the step of
deploying a cable in the water wherein at least two of said
indentations have different configurations.
19. A method as recited in claim 16, further comprising the steps
of deploying at least two cables in the water and of towing such
cables at a selected separation distance between said cables.
20. A method as recited in claim 16, further comprising the step of
connecting marine seismic equipment to said cable before said cable
is deployed in the water.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to the field of marine seismic
exploration. More particularly, the invention relates to an
improved apparatus and method for improving marine cable tow
efficiency and of extending the useful life of marine cables.
[0002] Cables are towed through water in marine seismic operations
to pull acoustic energy sources, flotation buoys, hydrophones, and
other marine seismic equipment through the water. For large seismic
vessels, multiple cables are simultaneously towed in a large swath
through the water. The cables may extend thousands of meters behind
the seismic tow vessel.
[0003] Large drag forces are generated as the cables are towed
through the water. The drag forces are caused by friction between
the water and cable exterior surface and increase exponentially as
the vessel speed increases. Uneven water flow around the cables
produces alternating shedding forces which cause transverse
vibrations, known as "strumming", in the cables. Strumming
increases cable drag and further generates acoustic noise
interfering with seismic data collection operations. Cable movement
generates stresses at equipment connection points and accelerates
cable wear.
[0004] Numerous techniques have been developed to reduce frictional
drag forces acting on a cable towed through water. One technique
uses multiple fibers or "hairs" to interrupt vortex shedding and
other friction inducing processes. For example, U.S. Pat. No.
4,084,065 to Swenson (1978) disclosed polyester yarn or fiber hairs
extending from a braided cable to reduce cable strumming in water.
U.S. Pat. No. 4,756,270 to Boscov (1988) disclosed a cable fairing
having hairs formed with a filamentary yarn woven into a cable.
[0005] Another technique uses protrusions extending outwardly from
the cable to interrupt drag inducing eddies. U.S. Pat. No.
4,190,012 to Rispin et al. (1980) disclosed a cable fairing having
a plurality of stubs formed in a spiral pattern about the exterior
surface of a cable. U.S. Pat. No. 5,214,244 to Cummings et al.
(1993) disclosed a flexible jacket having a helically displaced
phase shifter for decoupling a seismic cable from strumming induced
by von Karmen vortex sheets. Each of the phase shifting sections
had a length less than one quarter the coherence length of the
flexible jacket. U.S. Pat. No. 5,275,120 to Ruffa et al. (1994)
disclosed a tow cable fairing having a plurality of radially
extending tab members indexed in a helical pattern longitudinally
along the cable.
[0006] A technique converse to outwardly extending protrusions was
proposed in U.S. Pat. No. 5,228,005 to Bjelland (1993), which
disclosed a seismic streamer constructed with an external, tubular
cover having a plurality of continuous, longitudinally extending
grooves in the tubular cover. The grooves ran continuous with the
cable length to avoid the creation of surface features obstructive
to water flow past the cable.
[0007] Other friction reducing techniques attach fairings to cables
to reduce cable strumming in the water. U.S. Pat. No. 5,335,620 to
Small (1993) disclosed a continuous, longitudinally extending
fairing comprised of a single plastic extrusion and fastening means
for providing shock and impact protection to seismic array sensors
and cables. U.S. Pat. No. 5,367,971 to Carpenter et al. (1994)
disclosed an outer cable layer having a twisted surface to create
ridges for minimizing vortex shedding and cable strumming. U.S.
Pat. No. 4,075,967 to Silvey (1978) disclosed a fairing having a
plurality of flat vanes extending parallel to the others and
tapering toward the fairing trailing edge. U.S. Pat. No. 5,678,504
to Toplosky (1997) disclosed a towing cable fitted with fairings
having negative lift vanes.
[0008] Improvements to fairings and the connection mechanisms
between fairings and cables were shown in U.S. Pat. No. 4,542,708
to Holcombe et al. (1985), which disclosed a fairing cable segment
having a rotatable spring steel clip for minimizing interfacial
friction between the fairing and cable. U.S. Pat. No. 4,700,651 to
Hale (1987) disclosed a fairing having a relatively large clearance
around the cable to permit frictionless fairing movement and to
reduce cable wear.
[0009] As the size of marine seismic arrays and the vessel tow
speed increases, a need exists for improved techniques for reducing
drag forces. The techniques should reduce strumming noise
interfering with acoustic data collection and should increase tow
efficiency through the water.
SUMMARY OF THE INVENTION
[0010] The invention provides an apparatus and method for reducing
friction acting against a marine cable towable through water. The
apparatus comprises a cable sheath having an exterior surface in
contact with the water and a plurality of indentations in the
sheath exterior surface for reducing friction between the cable and
the water as the cable is towed through the water. In different
embodiments of the invention, the indentions can comprise dimples,
can be symmetrically shaped, can be oriented in a selected pattern,
and can have different sizes and configurations. The apparatus is
particularly suited to a cable for reducing strumming of the cable
as the cable is towed through water.
[0011] The method of the invention comprises the steps of deploying
a cable having a cable sheath in the water behind a tow vessel, and
of towing the cable through the water. In other embodiments, at
least two cables can be deployed in the water and such cables can
be towed at a selected separation distance between the cables.
Marine seismic equipment can be connected to the cable before the
cable is deployed in the water.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 illustrates a seismic tow vessel in water.
[0013] FIG. 2 illustrates indentations formed in the exterior
surface of a cable sheath.
[0014] FIG. 3 illustrates different shapes and forms of
indentions.
[0015] FIG. 4 illustrates one pattern of indentations.
[0016] FIG. 5 illustrates test data of a dimpled pipe under
different test conditions.
[0017] FIG. 6 illustrates test data comparing dimpled pipe to hairy
fairings.
[0018] FIG. 7 illustrates an embodiment of the invention having a
splitter plate.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The invention uniquely reduces frictional drag of a towed
cable, strumming, and cable wear. Referring to FIG. 1, tow vessel
10 is positioned in water 12 and deploys cable 14 from drum or
spool 16. Marine seismic equipment such as acoustic energy source
gun 18, buoy 20, and hydrophone 22 is attached to cable 14.
[0020] As shown in FIG. 2, cable 14 comprises core 24 having cable
sheath 26 and exterior surface 28. Cable sheath 26 can be formed
with a metallic or extruded or wrapped plastic or composite sheath
material resistant to damage. A plurality of indentations 30 are
formed in exterior surface 28 for the purposes of reducing drag
forces between cable 14 as cable 14 is towed through water 12, for
reducing the amount of tow energy expended by vessel 10, for
reducing strumming of cable 14 and the undesirable noise generated
by such strumming, and for reducing the wear on cable 14 at
connections between cable 14 and marine seismic equipment such as
that identified above.
[0021] Indentations 30 are formed in the exterior surface 28 of
cable sheath 26 in contact with water 12. The term "dimple" as used
herein means a slight depression in exterior surface 28, and can be
dish-shaped, polygonal, or of another configuration. Each
indentation 30 or dimple can be formed with a symmetric or
asymmetric configuration as shown in FIG. 3 by symmetric
indentation 32 and by asymmetric indentation 34. FIG. 3 also shows
geometric indentations 36 and 38 having selected configurations of
different shapes, and indentation 40 is formed in a different size.
Many different indentation 30 shapes, sizes, and configurations are
possible within the scope of the invention, and different
combinations of different indentations 30 can be provided to
provide different flow dynamics. The size and depth of indentations
30 can be selected to accomplish different flow dynamics. For
example, different indentations 30 having different depths from
exterior surface 28 can be used to vary the fluid dynamics and
resulting drag of cable exterior surface 28 through water 12. In
another embodiment of the invention, protrusions 42 can be
intermingled within such indentations to vary the flow dynamics of
cable 14 through water 12.
[0022] FIG. 4 illustrates a plurality of indentations 30 in a
selected pattern. Although one pattern is illustrated in FIG. 4,
many different patterns and configurations of indentations 30 are
possible to accomplish different flow dynamics between cable 14 and
water 12. A plurality of indentations 30 resist formation of
turbulent flow between cable 14 and 12 and the correspondent
shedding vortexes leading to strumming and other undesirable cable
14 movement. As cable 14 is pulled through water 12, indentations
30 break up the water flow and reduce strumming of cable 14. Drag
forces acting on cable 14 are accordingly reduced, resulting in
less tow energy required to pull cable 14 through water 12.
[0023] Indentations 30 can be added to sheath exterior surface 28
before or after sheath 26 has been formed around cable core 24.
Rollers or other mechanisms can form indentations 30 such as
dimples in exterior surface 28 while cable sheath 26 is formed
around core 24. Indentations 30 also provide traction against spool
16 or cable pullers for deploying and retrieving cable 14 from
water 12 which lessens the radial gripping force necessary to grasp
cable 14.
[0024] Multiple cables 14 are often deployed from a single seismic
vessel 10, and the invention facilitates such deployment and
operation. By reducing the drag acting on cables 14, each cable 14
can be pulled further from the center of vessel 10 travel while
reducing noise induced by strumming. Reduced strumming also
increases usable life of cables 14 as previously described.
[0025] FIG. 5 illustrates drag tests of the invention under
different test conditions. Typical tow ranges for seismic cables
range between a Reynolds Number of 50,000 with a 1.45 inch diameter
cable at 3.5 knots, and a Reynolds Number of 100,000 with a faired
2.0 inch vane tag line at 4.5 knots. Within this selected Reynolds
Number range, which is not limiting to the scope of the invention,
different cavitation tunnel and liquid tow tank tests provided
substantially similar results. Larger sized dimples reduce the drag
coefficient. FIG. 6 illustrates test results for the invention when
compared to a conventional hairy fairing lead. As shown, the
invention provided superior results in drag reduction.
[0026] In another embodiment of the invention disclosed in FIG. 7,
splitter plate 44 is attached to cable 14 to further reduce the
drag coefficient. The combination of splitter plate 44 and dimples
30 on cable 14 reduced the drag coefficient to a range between 0.5
and 0.7.
[0027] Although the invention has been described in terms of
certain preferred embodiments, it will become apparent to those of
ordinary skill in the art that modifications and improvements can
be made to the inventive concepts herein without departing from the
scope of the invention. The embodiments shown herein are merely
illustrative of the inventive concepts and should not be
interpreted as limiting the scope of the invention.
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