Patent Publication Number: US-6655311-B1

Title: Marine seismic diverter with vortex generators

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention generally relates to the field of diverters used in marine seismic operations. More particularly, the present invention relates to diverters having vortex generators to prevent stall at high attack angles. 
     Marine seismic diverters control movement of seismic streamers and other equipment relative to a seismic tow vessel. As the tow vessel moves through the water, an array of one or more streamers and/or other equipment carrying cables are towed at a known velocity through the water. For multiple cables, diverters pull the cables outwardly from the vessel centerline to establish a separation among cables and maintain a width or “spread” for the streamer array. 
     Marine seismic diverters typically have wing shaped sections extending into the water (commonly referred to as “vanes”) for urging the diverter and attached cable away from the seismic array centerline. The lateral displacement forces exerted by the vanes depend on the tow vessel speed, shape of the vane, and the angle at which the leading edge of the vane meets the oncoming flow of water (commonly referred to as the “attack angle”). For example, increasing the attack angle may increase the lateral displacement force proportionally and lead to an increased spread of the seismic array. 
     Increasing the spread of the seismic array may reduce seismic exploration costs by reducing a number of passes necessary to gather seismic data for a given area. Using relatively high attack angles, spreads of over one kilometer are achievable. However, if the attack angle becomes too great, the diverters may stop exerting lateral displacement forces and stall, which may result in a collapse of the spread, loss of cable separation, and, possible damage to the streamers, which may be extremely expensive to replace. Further, significant personnel time may be required to retrieve and re-deploy the streamer array resulting in costly delays in data gathering operations. 
     Accordingly, a need exists for a seismic diverter that avoids stall at high attack angles. 
     SUMMARY OF THE INVENTION 
     The present invention generally provides an apparatus, system, and method for deflecting one or more cables towed behind a vessel. 
     Embodiments of the apparatus comprise an attached or detached float with at least one vane shaped to move a cable in a selected direction as the vessel tows the cable through the water. One or more vortex generators are disposed on the vane to generate small vortexes within a boundary layer of laminar water flow across the vane to prevent boundary layer separation and the onset of stall at high attack angles. Some embodiments may comprise a controller capable of adjusting an attack angle of the diverter vane and/or a location of the vortex generators. 
     Embodiments of the system may comprise at least two such diverters attached to opposing outer cables of an array of cables towed behind a vessel. The diverters may provide opposing lateral forces for moving the opposing outer cables in opposing outer directions, creating a spread for the array. Embodiments of the method may comprise attaching the diverters to opposing outer cables and/or cables within the array and adjusting an attack angle of the diverters and/or a location of the vortex generators as the vessel tows the array of cables through the water. 
    
    
     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, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. 
     It is to be noted, however, that the appended drawings illustrate only typical embodiment of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 illustrates a top view of an exemplary marine seismic system. 
     FIG. 2 illustrates a top view of another exemplary marine seismic system. 
     FIG. 3 illustrates a side view of an exemplary marine seismic diverter. 
     FIG. 4 illustrates a bottom view of the exemplary marine seismic diverter of FIG.  3 . 
     FIG. 5 illustrates a bottom view of another exemplary marine seismic diverter. 
     FIG. 6 illustrates a bottom view of an exemplary marine seismic diverter having a plurality of vanes. 
     FIG. 7 illustrates an exemplary system comprising a seismic diverter having an integrated controller. 
     FIG. 8 is a flow diagram illustrating exemplary operations of a method for steering a cable towed through water behind a vessel. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention generally provides an apparatus, system, and method for steering one or more cables towed behind a vessel. For example, FIG. 1 illustrates an exemplary marine seismic system  100  comprising a tow vessel  102  deployed in water  104  to tow an array of cables  106 . As used herein, the term “cable” may refer to seismic streamers, wires, conductors, and other structures for supporting floats, acoustic energy sources, hydrophones, positioning equipment, and other seismic equipment. 
     The array may comprise any suitable number of cables  106 , each of any suitable length. For example, arrays of six and eight seismic streamers, each thousands of meters long, are well known in the art. An even number of streamers may advantageously provide space directly behind the tow vessel for towing other seismic equipment, such as seismic sources (air guns). For other embodiments, however, an odd number of cables may be used. 
     As illustrated, at least two diverters  110  may be attached to opposing outer cables  108  of the array to create a separation among cables  106 . As the vessel tows the array of cables  106  through the water, the diverters  110  may provide outward lateral forces for moving the outer cables  108  in outward directions from a centerline of the array. For some embodiments, diverters  110  may create a distance, or “spread”, between outer cables  108  of over one kilometer. Diverters  112  may also be attached to cables within the array of cables  106  to help maintain a spread of inner cables. The diverters  112  may be used instead of, or in addition to, the diverters  110 . As illustrated, the diverters  110  and  112  may be attached to different locations along a cable to control a position of the cable within the array. 
     As illustrated in FIG. 2, for other embodiments, a single diverter  210  may be used to steer at least one cable  206  towed through water  204  behind a vessel  202 . The diverter  210  may serve to keep the cable  206  (and attached equipment) centered behind the boat, for example, by offsetting the effects of a current. Alternatively, the diverter  210  may be attached to the cable  206  to pull one side of an array of seismic streamers away from a center point of the array. 
     FIG. 3 illustrate a side view of an exemplary seismic diverter  310  according to one embodiment of the present invention. The seismic diverter  310  may comprise a float  312  with a vane  314  extending from, or suspended from, a bottom portion of the float  312 . The float  312  may be any suitable shape and made of any suitable material, such as aluminum, fiberglass, plastic, steel, or a composite material. The vane  314  may also be any suitable material, and may be shaped to provide a lateral force for steering an attached cable as the cable and diverter are towed behind a vessel. The vane  314  may be made from the same material as the float or a different material. 
     The operation of the diverter  310  may be described with reference to the bottom view illustrated in FIG.  4 . As a vessel tows the diverter  310 , water may flow around the vane  314  in a manner similar to air flowing around an airplane wing. Water may flow faster along a suction side surface  330  than along a pressure side surface  332  which may create a pressure differential between the two surfaces, generating a lift generally perpendicular to the direction of the water flow (indicated by a straight arrow). As an angle a of a leading edge  320  of the vane with respect to the oncoming water flow, the attack angle, is increased, water may flow even faster along the suction side surface  330  of the vane creating an even greater pressure differential between the two surfaces and, hence, more lift. 
     As previously described, however, if the attack angle a is too great, the water flow may cease to follow the shape of the vane, and separate from the vane, resulting in a loss of lift (i.e. stall). In an effort to prevent stall, embodiments of the present invention may have one or more vortex generators  316  disposed on the suction surface side  330  of the vane  314 . As the diverter  310  moves through the water, vortex generators  316  generate relatively small regions of turbulent flow (vortices), which may prevent separation of the layer of water flow. While typical attack angles are between  200  to  400 , diverters of the present invention may be towed at attack angles up to and exceeding  500 . Greater attack angles advantageously allow a greater array spread and, therefore, a greater seismic coverage area. 
     Vortex generators  316  may advantageously be located in proximity to the leading edge  320  of the vane  314  to prevent flow separation at an early location. The vortex generators  316  may be located in any suitable orientation on a suction side of the diverter vane  314 . For example, as illustrated in FIG. 3, the vortex generators  316  may be located along a line substantially parallel to the leading edge  320 . Alternatively, the vortex generators  316  may be located along a line at an angle with respect to the leading edge  320 . Vortex generators  316  may be any suitable size and shape, and may be arranged in any suitable pattern. For some embodiments, vortex generators  316  may be generally rectangular in shape, extending up from the suction side surface  330  into the stream of flow. Examples of other suitable shapes include, but are not limited to, triangles, pyramids, and various U-shapes, such as horseshoes. As illustrated, vortex generators  316  may be oriented so that a lengthwise section is offset by a specified angle θ (commonly referred to as the “sweep angle”) with respect to the oncoming flow of water to increase their effectiveness. An array of vortex generators  316  may include vortex generators set at different sweep angles. 
     The size, shape, and location of the vortex generators  316  may be optimized for a particular application by performing numerical analysis with a computational fluid dynamics (CFD) program, taking into consideration parameters such as towing speed, attack angle, the size and shape of the vane  314 , desired cable separation, etc. Vortex generators  316  may be made of any suitable material, such as a sheet metal, or a plastic, such as a PVC material. For some embodiments, the vortex generators  316  may be made of a same material as the vane  314 . For example, a molding process that forms the vane  314  may also form the vortex generators  316 . 
     As a diverter vane moves through the water, a submerged portion of the diverter float is also moving through the water. As illustrated in FIG. 5, for some embodiments, a diverter  510  may comprise a float  512  shaped to provide a lateral force to steer an attached cable as the submerged portion of the float moves through the water as the vessel tows the diverter. Further, a bottom portion of the float  512  may have one or more vortex generators  518  disposed on a suction side surface  530  of the float  512 . The vortex generators  518  may prevent the float  512  from stalling as it moves through the water, in a manner similar to that previously described with reference to vortex generators attached to the diverter vane. The float  512  typically has at least one vane  514  with a vortex generator  516  attached or extending from the suction side surface of the vane; the vane  514  extends from the bottom surface of the float  512 . 
     Embodiments of the present invention are not limited to any number of vanes. FIG. 6 illustrates a bottom view of another exemplary seismic diverter  610  having a vane  614  and at least one additional vane  616 . As illustrated, the additional vane  616  may be shaped substantially similar to the vane  614 . Alternatively, the additional vane may be straight, for example, to provide stability as the diverter  610  moves through the water; the vanes  614  and  616  are attached to or extend from the bottom surface of a float  612 . Vortex generators may be attached to just one, or any number, of the vanes. 
     For some embodiments of the present invention, one or more diverter vanes may comprise one or more sections moveable in relation to each other. For example, one or more movable sections may be collapsed to occupy a smaller area when storing the diverter, which may be advantageous on a tow vessel with a minimum amount of storage space. The one or more movable sections may then be expanded before or after deploying the diverter in the water. 
     FIG. 7 illustrates an exemplary system  700  with a diverter  710  for steering a cable  706  behind a tow vessel  702 . The diverter  710  has an integrated controller  722 , which may be housed in a float  712 , or incorporated within the vane. The integrated controller  722  may be capable of adjusting an attack angle of a vane  714 , for example by rotating the vane  714  about an attachment point  718 . The integrated controller  722  may also be capable of adjusting a location of one or more vortex generators  716 , for example, to compensate for a change in attack angle or tow speed. To allow adjustment of the location of the vortex generators  716 , the vane  714  may have slots  720  along which the vortex generators  716  may slide. However, other suitable techniques may also be used to adjust a location of vortex generators  716 . For example, the vortex generators may be mounted on a movable structure attached to a surface of the suction side of the vane  714 . Further, the sweep angle θ of any or all of the vortex generators may be adjustable by the controller. 
     For some embodiments, one or more sensors  724  may be coupled with the integrated controller  722 , for example, to measure various parameters, such as tilt, camber, acceleration, speed, vibration and position. For some embodiments, position data may be provided by a global positioning system (GPS). The integrated controller  722  may communicate sensor data to an external controller. 
     For example, an external controller  730  located on a tow vessel  702  may be in communications with the integrated controller  722  to allow personnel to remotely steer the cable  706 . For one embodiment, the external controller  730  may send control signals to the integrated controller  722  through control lines (not shown) located within the cable  706 . Alternatively, the external controller  730  may communicate with the integrated controller  722  through a wireless connection, such as a radio frequency (RF) connection. For other embodiments, the external controller  730  may be placed at a location other than the tow vessel, such as a remote vessel carrying additional marine seismic equipment. 
     FIG. 8 is a flow diagram  800  illustrating exemplary operations of a method for steering a cable towed through water behind a vessel. The operations of flow diagram  800  may be described with reference to the exemplary system of FIG.  7 . However, it should be understood that other embodiments might also be capable of performing the operations of flow diagram  800 . 
     For step  810 , a diverter is attached to the cable, the diverter comprising a float, at least one vane, and one or more vortex generators attached to a suction side surface of the vane. For step  820 , the cable is towed through the water behind the vessel, wherein the diverter provides a lateral force for moving the cable. For some embodiments, the cable  706  may be an outer cable of an array of cables, and the diverter  710  may steer the cable  706  in an outward direction to create a separation among cables of the array. As illustrated in FIG. 1, a diverter may also be attached to an opposing outside cable to steer the opposing outside cable in an opposing outward direction. 
     For step  830 , one or more sensors integrated with the diverter are monitored. For some embodiments, the integrated controller  722  may gather sensor data from the sensors  724  and send the sensor data to the external controller  730 . The external controller  730  may then process the sensor data, for example, to determine a status of the diverter  710 . For example, the sensor data may indicate the diverter  710  is approaching a stall condition, is vibrating excessively, or is at risk of tipping over. This may be particularly important for applications where a seismic array may have a large spread and the diverter  710  may be too far from the vessel  702  for visual inspection by personnel. 
     For step  840 , an attack angle of the diverter is adjusted. For example, if sensor data indicates the diverter  710  is approaching a stall condition, the attack angle may be decreased to avoid the stall condition. For step  850 , a location of the vortex generators is adjusted, for example, to accommodate a change in attack angle or tow speed. For some embodiments, the external controller  730  may send control signals to the integrated controller  720  to adjust the attack angle of the vane  714  or adjust the location of the vortex generators  716 . For other embodiments, the integrated controller  720  may adjust the attack angle of the vane  714  independently. As illustrated, steps  830  through  850  may be repeated continuously while towing the cable  706 . 
     Embodiments of the present invention permit diverters to be operated at high angles of attack while avoiding stall. This feature of the invention is particularly useful in seismic operations where an array of seismic streamers with a large spread is used to gather data over a large area. As previously described, a large spread may reduce a number of passed needed to gather data for the area and may therefore reduce overall operating costs. 
     Although embodiments described herein refer to diverters with vanes having a generally vertical orientation for providing lateral forces, a diverter may be provided with a vane having a different orientation to accomplish different tow results. For example, a diverter with a vane having a generally horizontal orientation may be used to control an elevation of a towed cable and/or attached equipment. Accordingly, embodiments of the present invention may comprise vortex generators attached to a suction side surface of vanes having various orientations. 
     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.