Abstract:
An apparatus and method for improving the efficiency of marine cable tow operations. Indentations such as dimples in the exterior surface of cable fairings reduce cable strumming and reduce frictional drag forces as the cables and fairings are towed through the water. The size, configuration and orientation of the indentations can be selected to control the desired water flow parameters, and overall tow efficiency is increased.

Description:
BACKGROUND OF THE INVENTION 
     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 by improved fairings attached to a marine cable. 
     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. 
     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 creates turbulent flow within the water and generates acoustic noise interfering with seismic data collection operations. Strumming further generates stresses at equipment connection points and accelerates equipment failure. 
     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 yarn or fiber incorporated into a braided cable for providing a plurality of polyester fiber hairs for reducing low velocity cable strumming in water. U.S. Pat. No. 4,756,270 to Boscov (1988) disclosed cable fairing comprised of fairing hairs formed with a filamentary yarn woven into a cable. 
     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. 
     Other friction reducing techniques attach fairings to cables to reduce cable strumming in the water. U.S. Pat. No. 4,075,967 to Silvey (1978) disclosed a hydrodynamic fairing having a plurality of parallel vanes. U.S. Pat. No. 4,470,651 to Hale (1987) disclosed a fairing having a relatively large clearance relative to the cable to permit frictionless rocking and consequential wear between the fairing and the cable. U.S. Pat. No. 4,655,155 to Folb et al. (1987) disclosed a faired towline having a rectangular leading edge. 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. 
     Various fairing configurations have been developed. 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. 5,678,504 to Toplosky (1997) disclosed a towing cable fitted with fairings having negative lift vanes. 
     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. 
     As the size of marine seismic arrays and the 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 
     The invention provides an apparatus and method for reducing friction acting against a marine cable towable through water. The apparatus comprises a fairing body engagable with the cable, wherein the body has an exterior surface in contact with the water, and a plurality of indentations in the body exterior surface for reducing friction as the body 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 method of the invention comprises the steps of attaching a fairing to the cable, wherein the fairing has an exterior surface having a plurality of indentations, of deploying a cable and attached fairing in the water behind a tow vessel, and of moving the tow vessel to tow said cable through the water. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a seismic tow vessel in water. 
     FIG. 2 illustrates indentations formed in the exterior surface of a fairing. 
     FIG. 3 illustrates different shapes and forms of indentions. 
     FIG. 4 illustrates one pattern of indentations. 
     FIG. 5 illustrates test data of a dimpled pipe under different test conditions. 
     FIG. 6 illustrates test data comparing dimpled pipe to hairy fairings. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention uniquely reduces frictional drag acting against a towed cable and components attached to the cable. 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 . 
     As shown in FIG. 2, fairing  24  generally comprises body  26  and exterior surface  28 . Body  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 fairing  24  as cable  14  and fairing  24  are 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. 
     Indentations  30  are formed in the exterior surface  28  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  30  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 varied to accomplish different flow characteristics. One or more splitter plates  42  can be attached to body  26  to vary the flow dynamics of fairing  24  and cable  14  through water  12 . 
     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 fairing  24  and cable  14  and water  12 . A plurality of indentations  30  resist formation of turbulent flow between fairing  24  and water  12  and the correspondent shedding vortexes causing strumming and other undesirable fluid dynamics. 
     Indentations  30  can be added to sheath exterior surface  28  before or after sheath  26  has been formed. Rollers or other mechanisms can form indentations  30  such as dimples in exterior surface  28 . Indentations  30  can also be added to cable  14  to reduce friction and to 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 . 
     As fairing  24  is pulled through water  12 , indentations  30  break up the water flow and reduce strumming of cable  14 . Drag forces acting on fairing  24  and cable  14  are accordingly reduced, resulting in less tow energy required to pull fairing  24  and cable  14  and associated seismic equipment through water  12 . 
     FIG. 5 illustrates drag tests 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 for cylindrical cable sections. 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, indentations on exposed surfaces provided superior results in drag reduction. 
     As previously noted, splitter plate  42  can be attached to fairing  24  to further reduce the drag coefficient. The combination of splitter plate  42  and dimples  30  on cable  14  reduced the drag coefficient to a range between 0.5 and 0.7. 
     The invention uniquely provides a drag reducing solution not anticipated by conventional systems. Although protrusions and other devices can cooperate with the indentations to accomplish different results, the indentations uniquely provide drag reducing characteristics for marine tow operations. 
     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.