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.

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 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.

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, United States Patent No. 4,084,065 to Swenson (1978) disclosed polyester yarn or fiber hairs extending from a braided cable to reduce cable strumming in water. United States Patent No. 4,756,270 to Boscov (1988) disclosed a cable fairing having hairs formed with a filamentary yarn woven into a cable.

Another technique uses protrusions extending outwardly from the cable to interrupt drag inducing eddies. United States Patent 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. United States Patent 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. United States Patent 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.

A technique converse to outwardly extending protrusions was proposed in United States Patent 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.

Other friction reducing techniques attach fairings to cables to reduce cable strumming in the water. United States Patent 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. United States Patent 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. United States Patent 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. United States Patent 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 United States Patent 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. United States Patent 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.

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.

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.

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.

Figure 1 illustrates a seismic tow vessel in water.

Figure 2 illustrates indentations formed in the exterior surface of a cable sheath.

Figure 3 illustrates different shapes and forms of indentions.

Figure 4 illustrates one pattern of indentations.

Figure 5 illustrates test data of a dimpled pipe under different test conditions.

Figure 6 illustrates test data comparing dimpled pipe to hairy fairings.

Figure 7 illustrates an embodiment of the invention having a splitter plate.

The invention uniquely reduces frictional drag of a towed cable, strumming, and cable wear. Referring to Figure 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 Figure 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.

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 Figure 3 by symmetric indentation 32 and by asymmetric indentation 34. Figure 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.

Figure 4 illustrates a plurality of indentations 30 in a selected pattern.

Although one pattern is illustrated in Figure 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.

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.

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.

Figure 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.

Figure 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.

In another embodiment of the invention disclosed in Figure 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.

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.