Patent Publication Number: US-9846249-B2

Title: Method and system for adjusting vessel turn time with tension feedback

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present applicant is a continuation of U.S. application Ser. No. 13/773,000, filed Feb. 21, 2013, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The present invention generally relates to marine geophysical surveying and, more particularly, in one or more embodiments, to methods and systems for using tension feedback from steerable deflectors to adjust vessel turn time. 
     Marine geophysical surveying systems such as seismic survey systems and electromagnetic survey systems may be used to acquire geophysical data from formations disposed below the bottom of a body of water, such as a lake or ocean. Such geophysical data may indicate geophysical structures and/or identify formations likely to contain useful materials, such as water, oil, or natural gas. Marine seismic surveying systems, for example, typically may include a seismic survey vessel having onboard navigation, seismic energy source control, and geophysical data recording equipment. The seismic survey vessel may be configured to tow a sensor streamer or more typically, a plurality of laterally spaced apart sensor streamers through the water at one or more selected depths. One or more deflectors may be positioned outside of the sensor streamers to generate lateral thrust for maintaining a desired lateral spacing of the sensor streamers. At selected times, the seismic energy source control equipment may cause one or more seismic energy sources (which may be towed in the water by the seismic vessel or by another vessel) to actuate. Signals generated by various sensors on the one or more streamers in response to detected seismic energy may be ultimately conducted to the recording equipment. A record can be made in the recording system of the signals generated by each sensor (or groups of such sensors). The recorded signals may be interpreted to infer the structure and composition of the formations below the bottom of the body of water. Corresponding components (transmitters and receivers) for inducing electromagnetic fields and detecting electromagnetic phenomena originating in the subsurface in response to such imparted fields may be used in marine electromagnetic geophysical survey systems. 
     While the sensor streamers and other equipment are deployed behind the survey vessel, the survey vessel may need to execute a turn. For example, one type of survey may comprise a grid of straight lines in opposing directions wherein the survey vessel is actively generating geophysical data while moving in the straight lines. Accordingly, a 180-degree turn may be executed so that the survey vessel can be positioned over the next line to be shot in the opposite direction. Another type of survey may execute continuous turns in a circular or spiral pattern. Due to the number and length of the sensor streamers towed from the survey vessel, however, turning may be a complex procedure. In some instances, for example, when the survey plan does not generate geophysical data during turns, it may be desired to reduce the turn time for the survey vessel. 
     A number of different techniques may be utilized to reduce the turn time. For instance, one technique may include keeping the turn radius constant and increasing speed of the survey vessel. Another technique may include reducing the turn radius and thus the distance traveled while keeping the speed constant. Yet another technique may include a combination of a reduction in turn radius and increased vessel speed. In all of these techniques, a limitation can be the tension on the deflector. As previously mentioned, deflectors may be positioned outside the sensors streamers to generate lateral force for maintaining a desired lateral spacing of the sensor streamers. During a turn, the outer deflector should experience an increase in tension while the inner deflector should experience a corresponding decrease in tension. However, to ensure stable deflector operation, the tension on the outer deflector should not exceed a maximum value, and the tension on the inner deflector should be kept above a minimum value. There exists a need to adjust turn time while maintain stable functioning of the deflectors. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These drawings illustrate certain aspects of some of the embodiments of the present invention and should not be used to limit or define the invention. 
         FIG. 1  illustrates an example embodiment of a geophysical survey system. 
         FIG. 2  illustrates an example embodiment of the geophysical survey system of  FIG. 1  executing a turn. 
         FIG. 3  illustrates an example embodiment of a tension measurement system that may be used to measure tension on one or more deflectors in a geophysical survey system. 
         FIG. 4  illustrates an example embodiment of a control system that may be used in an example method for adjusting vessel turn time. 
         FIG. 5  illustrates another example embodiment of the geophysical survey system of  FIG. 1  executing a turn. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention generally relates to marine geophysical surveying and, more particularly, in one or more embodiments, to methods and systems for using tension feedback from steerable deflectors to adjust vessel turn time. One of the many potential advantages, only some of which are disclosed herein, is that tension on the steerable deflectors may be monitored such that the turning radius for the survey vessel may be reduced while maintaining stable deflector operation. It is believed that steerable deflectors have not been used in techniques for reducing or otherwise adjusting the vessel&#39;s turning radius. Reduction in the turning radius should reduce the time need for turning the survey vessel such that utilization and efficacy of the survey vessels can be maximized. Another potential advantage may be that tension in the lead-in lines for the steerable deflectors may be monitored thus giving a more precise indication of their wear such that the deflector lead-in lines may be used for extended period of time prior to their replacement and/or failure. Yet another potential advantage may be that towing parameters may be controlled and optimized both online and in turns by monitoring the actual load generated by the steerable deflectors. 
       FIG. 1  illustrates a marine geophysical survey system  5  in accordance with embodiments of the present invention. In the illustrated embodiment, the marine geophysical survey system  5  may include a survey vessel  10  that moves along the surface of a body of water  15 , such as a lake or ocean. The survey vessel  10  may include thereon equipment, shown generally at  20  and referred to for convenience as a “recording system.” The recording system  20  typically includes devices (none shown separately) for navigating the survey vessel  10 , such as global positioning system (“GPS”) receivers, for actuating one or more energy sources  25 , and for recording signals generated by geophysical sensors  30 . 
     As illustrated, the survey vessel  10  or a different vessel (not shown) can tow a source cable  35  that includes one or more energy sources  25 . The one or more energy sources  25  may be any selectively actuable sources suitable for subsurface geophysical surveying, including without limitation seismic air guns, water guns, vibrators or arrays of such devices, or one or more electromagnetic field transmitters. In some embodiments, seismic energy and/or electromagnetic energy may originate from the one or more energy sources  25 . As the energy is emitted by the energy sources  25 , it travels downwardly through the body of water  15  and rock formations (not shown) below the water bottom. It should be noted that, while the present example, shows only a single energy source  25 , the invention is applicable to any number of energy sources towed by survey vessel  10  or any other vessel. 
     The geophysical survey system  5  may include a plurality of sensor streamers  40  towed by the survey vessel  10  (or another vessel) with each of the sensor streamers  40  including the geophysical sensors  30  at spaced apart locations. The sensor streamers  40  may be laterally spaced apart as shown by  FIG. 1 . “Lateral” or “laterally,” in the present context, means transverse to the direction of the motion of the survey vessel  10 . The sensor streamers  40  may each be formed, for example, by coupling a plurality of streamer segments end-to-end as explained in U.S. Pat. No. 7,142,481, the disclosure of which is incorporated herein by reference. The sensor streamers  40  may be coupled at their forward end (with respect to direction of movement of the survey vessel  10 ) to terminations  45  which couple the sensor streamers  40  to corresponding lead-in lines  50 . Each of the lead-in lines  50  may be deployed from the survey vessel  10  by a winch  55  or other similar spooling device, for example, that can be used to control the deployed length of the lead-in lines  50 . In one embodiment, the sensor streamers  40  may include lateral force and depth (“LFD”) control devices (not shown) configured to, for example, regulate streamer depth so that the sensor streamers  40  may be kept at selected depth profiles (e.g., as level as possible or at a 30-degree from horizontal angle) while towed through the body of water  15 . Each of the sensor streamers  40  may be towed at the same or different depths and with the same or different depth profiles. The LFD control devices may be any of a variety of different devices suitable for regulating streamer depth, including “birds” having variable-incidence wings. It should be noted that, while the present example, shows only four sensor streamers  40 , the invention is applicable to any number of laterally spaced apart sensor streamers  40  towed by survey vessel  10  or any other vessel. For example, in some embodiments, eight or more laterally spaced apart sensor streamers  40  may be towed by survey vessel  10 , while in other embodiments, up to twenty-six laterally spaced apart sensor streamers  40  may be towed by survey vessel  10 . 
     The geophysical sensors  30  may be any type of geophysical sensor known in the art. Non-limiting examples of such sensors may include seismic sensors such as geophones, hydrophones, or accelerometers, or electromagnetic field sensors, such as electrodes or magnetometers. By way of example, the geophysical sensors  30  may generate response signals, such as electrical or optical signals, in response to detecting energy emitted from the one or more energy sources  25  after the energy has interacted with the rock formations (not shown) below the water bottom. Signals generated by the geophysical sensors  30  may be communicated to the recording system  20 . 
     The geophysical survey system  5  may further include steerable deflectors  60  for maintaining the lateral position of the sensor streamers  40 . In the illustrated embodiment, two steerable deflectors  60  are shown. The steerable deflectors  60  may each be configured to provide a lateral component of force as the deflectors  60  are moved through the water  15 . By way of example, the steerable deflectors  60  may comprise one or more diverters or foils that redirect water as the steerable deflectors  60  are towed through the body of water  15 . Such redirection of water results in lateral force being generated by the steerable deflectors  60 . The lateral component of force of each steerable deflector  60  is generally opposite to that of the other steerable deflector  60 . The combined lateral force component of the steerable deflectors  60  should separate the steerable deflectors  60  until they put in tension one or more spreader lines  65  that extend at least partially between the steerable deflectors  60 . As illustrated, the spreader lines  65  may include, for example, outer spreader lines  67  and center portion  70 . In general, the one or more spreader lines  65  may be configured to limit the lateral separation of the sensor streamers  40 . As illustrated, the spreader lines  65  may be coupled at a forward end of the sensor streamers  40 . The spreader lines  65  may be coupled at either end to the steerable deflectors  60 . Other configurations may have steerable deflectors  60  on only one side of the survey vessel  10 , may have two or more steerable deflectors  60  on each side of the survey vessel  10 , or may exclude the center portion  70  of the one or more spreader lines  65 . In one particular embodiment (not shown) multiple steerable deflectors  60  may be used with each steerable deflector  60  towed directly off one of the lead-in lines  50 . Accordingly, the present invention is not intended to be limited to the example configuration illustrated on  FIG. 1 . 
     Non-limiting examples of suitable steerable deflectors  60  may include one-foil, two-foil, and multi-foil deflectors. Specific examples of suitable steerable deflectors are described in U.S. Pat. No. 7,404,370, the disclosure of which is incorporated herein by reference. In present embodiments, the steerable deflectors  60  may be considered “steerable” as the steerable deflectors  60  may include a steering device to adjust its angle of attack θ (the angle subtended by the longitudinal axis of the particular steerable deflector  60  with respect to the direction of motion of the survey vessel  10 ) to control the amount of lateral force generated by the deflector. The term “steering device” is intended to mean that the device cooperates with the steerable deflector  60  to change the angle of attack θ. In general, an increase in the angle of attack θ should increase the resultant force (such as lateral and drag forces) and a decrease in the angle of attack θ should decrease the resultant force. By adjusting the angle of attack θ and thus the force generated by the steerable deflectors  60 , the tension in the one or more spreader lines  65  may be controlled. 
     As illustrated, the steerable deflectors  60  may be coupled to the survey vessel  10  by deflector lead-in lines  75 . The deflector lead-in lines  75  may be coupled to the survey vessel  10  at one end to a winch  55  or other suitable spooling device, for example, that can be used to change the deployed length of the lead-in lines  75 . As illustrated, each deflector lead-in line  75  may be coupled to the corresponding outer spreader line  67 . There may be a connection point  94  at the intersection of the corresponding outer spreader line  67  and deflector lead-in lines  75 . A relatively short line (referred to herein as lever arm  96 ) may extend from this connection point  94  to the particular steerable deflector  60 . The lever arms  96  may be the same line as, or a different than, the corresponding deflector lead-in line  75 . As illustrated, each of the lever arms  96  may be coupled to the corresponding steerable deflector  60  by a set of cables referred to herein as a “bridle” and shown generally at  80 . 
     In accordance with present embodiments it may be beneficial to measure the force generated by the steerable deflectors  60 . As will be discussed in more detail below, the force or a corresponding indication thereof may be used in a process to adjust turn time for the survey vessel  10 . In the illustrated embodiment, the force generated by each steerable deflector  60  is generally taken by the corresponding lever arm  96 . The resulting force may be re-distributed between the corresponding deflector lead-in line  75  and outer spreader line  67 , with the re-distribution changing during a turn as compared to towing in a straight line. Accordingly, an indication of the force may be generated by measuring tension in the rope or wires coupled to the steerable deflectors  60 , such as the lever arms  96 , the deflector lead-in lines  75 , and the outer spreader lines  67 . This tension in the rope or wires coupled to the steerable deflectors  60  is referred to herein as tension on the steerable deflectors  60  even where the measured tension may not represent the full tension on the steerable deflectors  60  such as where tension is measured on the outer spreader lines  67  or the deflector lead-in lines  75 , which may not receive the full deflector tension. 
     In some embodiments, the geophysical survey system  5  may further include tension measurement systems  85  configured to measure tension and report the measured tension to the recording system  20 . In one particular embodiment, the measured tension may be reported to control system  95 , for example, shown on  FIG. 1  as a component of the recording system  20 . In response to the measured tension, the control system  95  may operate, for example, to calculate a reduced or increased vessel operating speed, calculate a reduced or increased angle of attack for the steerable deflectors  60 , or calculate a reduced or increased vessel turn radius. As illustrated, there may be a tension measurement system  85  associated with each of the steerable deflectors  60 . In the illustrated embodiment, the tension measurement systems  85  may be coupled to or otherwise integrated into the steerable deflectors  60 . The tension measurement systems  85  may be configured to measure tension at the intersection of the lever arm  96  and the bridle legs  482 . In alternative embodiments, the tension measurement systems  85  may measure tension at the connection points between the steerable deflectors  60  and each bridle leg  82 . If measured at such connection points, the measured tension will need to be summed for each steerable deflector  65  to obtain a total tension. In yet further embodiments, the tension measurement systems  85  may measure tension on the lever arms  96 , on the deflector lead-in lines  75 , or on the one or more spreader lines  65 . 
     At least one tension sensor  90  may be associated with each of the tension measurement systems  85 . The tension sensor  90  may be electrical strain gauges or load cell sensors, such as Wheatstone bridge type sensors. The tension sensor  90  may also be optical sensors, such as Bragg gratings etched into an optical fiber. Optical strain gauges as applied to marine geophysical survey systems are explained in U.S. Pat. No. 7,221,619, the disclosure of which is incorporated herein by reference. The tension sensor  90  may also be hydrostatic pressure sensors such as piezoelectric type sensors. The tension sensor  90  may also be disposed in a pressure vessel integrated into in the line to which the tensions sensors are coupled, in modules that connect the axial ends of adjacent lines, or integrated into strain members  26 , or another suitable configuration. As illustrated, the tension sensor  90  may be incorporated into the lever arm  96  for each of the deflector lead-in lines  75  with the lever arm  96  being the portion of the corresponding deflector lead-in line  75  (or being a separate line) that extends beyond the connection point  94  of the corresponding outer spreader line  67  and deflector lead-in line  75 . More particularly, the tension sensor  90  may be disposed at the intersection of the lever arm  96  and the bridle legs  82 . Other configurations may have the tension sensor  90  incorporated into other portions of the deflector lead-in lines  75  or lever arm  96  or incorporated into the bridle  80 . In any event, the tension sensor  90  may be positioned such that it is able to measure tension representing the tension at one of the steerable deflectors  60 . Accordingly, the present invention is not intended to be limited to the example configuration for the tension measure systems  85  and the tension sensors  90  illustrated on  FIG. 1 . 
     While the preceding description describes use of tension measurement systems  85  to measure the tension, embodiments also may employ indirect techniques for determining approximate tension or equivalent force at the steerable deflectors  60  that can then be used in turn optimization. These indirect techniques may be based, for example, on the relationship between the deflector force (lift plus drag) that is resulting in tow cable tension, water speed, and deflector angle of attack. These relationships may be known from deflector design, such as testing of a pilot deflector equipped with measuring equipment. One example technique for indirect tension measurement may include measuring water speed (e.g., fluid flow relative to deflector) and deflector angle of attack, e.g., by a Doppler log with minimum two beams. In an embodiment, by measuring the water velocity vector relative to the deflector both the water speed and the deflector angle of attack may be measured in one measurement. Other instruments may also be used that can measure the water vector directly, e.g., instruments based on stagnation pressure (e.g., Pitot tube). Another example technique for indirect measurement may include measuring speed of the steerable deflectors  60  (not the water speed vector), for example, by measuring the water speed and measuring deflector orientation using the assumption that deflector orientation represents deflector angle of attack. A number of techniques may be used to measure water speed, including use of a water speed instrument (e.g., one beam Doppler log, Pitot tube, or other suitable water speed instrument), application of global positioning system speed, or estimation of deflector speed from vessel speed and applicable turn radius. A number of different techniques may be used to determine deflector orientation, including a compass, two global positioning systems, two acoustical nodes, or other suitable means. 
       FIG. 2  illustrates the survey vessel  10  of  FIG. 1  executing a port turn in accordance with embodiments of the present invention. In some embodiments, the port turn may be executed so that the survey vessel  10  may be positioned over the next line to be shot. By way of example, this may be desirable when the survey plan comprise a grid of straight lines in opposing directions wherein geophysical data may be actively generated while moving in straight lines. In alternative embodiments, the survey vessel  10  may execute a turn (e.g., the port turn) so that the sensor streamers  40  may be advanced along a curved path while obtaining geophysical data. For example, the survey vessel  10  may execute a continuous turn which may be in a circular or spiral pattern to advance the sensor streamers in a curved path. While  FIG. 2  illustrates a port turn, it should be understood that embodiments of the present invention may also be applicable during execution of a starboard turn by the survey vessel  10 . 
     As illustrated, the port turn for the survey vessel  10  may have a turn radius of r. As previously mentioned, reducing the turn radius (r) for the survey vessel  10 , such as the turn radius (r) for the port turn shown on  FIG. 2 , can reduce the turn time for the survey vessel  10 . Reducing turn time can be important to maximize the utilization and efficiency of the survey vessel  10 , among other things. However, a limitation on the reduction in the turning radius (r) for the survey vessel  10  may be tension on the steerable deflectors  60 . For example, if the turning radius (r) is reduced too much, the tension on the one of the steerable deflectors  60  on the inner or port side of the survey vessel  10  may decrease in an amount that could make operation of such deflector unstable. For example, the steerable deflector  60  on the inner side of the survey vessel  10  in a turn may function unacceptably wherein waves or other ocean currents can change the deflector orientation with the steerable deflector  60  even starting to move backwards in some instances. Similarly, the tension on the other steerable deflector  60  on the starboard or outer side of the survey vessel  10  may increase in an amount that would make operation of such deflector unstable. For example, increased tension can increase the risk of equipment failure such as rope or deflector breakage. In accordance with present embodiments, the tension monitoring systems  85  may be used to provide an indication of tension generated on the steerable deflectors  60  such that a turn, such as the port turn shown on  FIG. 2 , may be safely executed with stable deflector operation and while staying within certain design force limitations. 
       FIG. 3  illustrates one embodiment of a tension measurement system  85  referred to with reference to  FIGS. 1 and 2 . In the illustrated embodiment, the tension measurement system  85  may comprise the components needed to measure tension on one or more lines (e.g., ropes, wires, cables, etc.) coupled to the steerable deflectors  60  and report the measured tension, for example, to the control system  95  (e.g., shown on  FIG. 1 ). As illustrated, the tension measurement system  85  may comprise a tension sensor  90 . The tension sensor  90  may be used to measure tension at a specified location, for example, on a particular steerable deflector  60 . The measured tension from the tension sensors  90  may be conducted to a controller  100 , which may be, for example, any microprocessor-based controller, programmable logic controller, or similar device. The controller  100  may be coupled to a telemetry transceiver  105 . The telemetry transceiver  105  may send the measured tension received by the controller  100  to the recording system  20  (e.g., shown on  FIG. 1 ) on the survey vessel. In some embodiments, the measured tension may be sent to the control system  95  (e.g., shown on  FIG. 1 ), which may be a component of the recording system  20 . The telemetry receiver  105  may also receive signals sent from the control system  95  in the recording system  20 . The signals received by the telemetry receiver  105  may be conducted to the controller  100 . The controller  100  may receive, for example, set points for deflector angle of attack θ or sensor calibration data. In some embodiments, the controller  100  may receive the desired tension, and the tension measurement system  85  can then adjust the angle of attack θ based on measured values and control logic to stay within the desired tension. In some embodiments, tension control may be considered a “closed loop” wherein the angle of attack θ may be actively adjusted to maintain a set tension value. 
     As illustrated, a battery  110  may be used to provide power (e.g., uninterrupted power) to the controller  100  and other components in the tension monitoring system  85 . A generator  115  may be used to supply electrical power to the battery  110  to keep the battery  110  charged. In some embodiments, the generator  115  may supply power to the battery  110  when the generator  115  is moved through the body of water  15  (e.g., shown on  FIGS. 1 and 2 ). 
       FIG. 4  illustrates one embodiment of a control system  95  referred to with reference to  FIGS. 1-3 . As previously mentioned, the control system  95  may receive tension measurements, for example, from the tension measurement systems  85  (e.g., shown on  FIGS. 1-3 ). The control system  95  may also receive as inputs the maximum operating tension, minimum operating tension, vessel speed, and/or angle of attack θ. Using these inputs, the control system  95  may determine and transmit an adjusted vessel speed (which may be an optimum vessel speed), adjusted angle of attack θ for the steerable deflector, and/or adjusted vessel turn radius (r). In some embodiments, the control system  95  may receive and display the measured tension, allowing the operator to reduce or increase vessel speed, for example. Special or unique software for receiving the inputs and sending output signals may be stored in the control system  95  and/or on external computer readable media. Those of ordinary skill in the art will appreciate that the control system  95  may comprise hardware elements including circuitry, software elements including computer code stored on a machine-readable medium or a combination of both hardware and software elements. Additionally, the blocks shown on  FIG. 4  are but one example of blocks that may be implemented. A processor  120 , such as a central processing unit or CPU, may control the overall operation of the control system  95 . The processor  120  may be connected to a memory controller  125 , which reads data to and writes data from a system memory  130 . The memory controller  125  may have memory that includes a non-volatile memory region and a volatile memory region. The system memory  130  may be composed of a plurality of memory modules, as will be appreciated by one of ordinary skill in the art. In addition, the system memory  130  may include non-volatile and volatile portions. A system basic input-output system (BIOS) may be stored in a non-volatile portion of the system memory  130 . The system BIOS is adapted to control a start-up or boot process and to control the low-level operation of the control system  95 . 
     The processor  120  may be connected to at least one system bus  135  to allow communication between the processor  120  and other system devices. The system bus  135  may operate under a standard protocol such as a variation of the Peripheral Component Interconnect (PCI) bus or the like. In the exemplary embodiment shown in  FIG. 4 , the system bus  135  connects the processor  120  to a hard disk drive  140 , a graphics controller  145  and at least one input device  150 . The hard disk drive  140  provides non-volatile storage to data that is used by the computer system. The graphics controller  145  is in turn connected to a display device  155 , which provides an image to a user based on activities performed by the control system  95 . The memory devices of the control system  95 , including the system memory  130  and the hard disk  140  may be tangible, machine-readable media that store computer-readable instructions to cause the processor  120  to perform a method according to an embodiment of the present techniques. 
     An example method for using tension feedback from steerable deflectors  60  to reduce vessel turn time will now be described with reference to  FIGS. 1 and 2 . The method may include towing a plurality of sensor streamers  40  behind a survey vessel  10  in a body of water  15 . The method may further include using steerable deflectors  60  for maintaining lateral position of the sensor streamers  40  in the body of water. At a desired time, the survey vessel  10  may need to execute a turn in the body of water  15 . As described above, it may be desirable to reduce the time needed for execution of the turn.  FIG. 2  illustrates a port turn. Those of ordinary skill in the art will appreciate that the turn time may be reduced by either increasing vessel speed or by reducing turn radius (r). With either approach, tension generated by the steerable deflectors  60  can be a limiting factor. However, since tension increases faster with increased speed than with reduced turn radius (r), reducing the turn radius (r) can be a more effective technique than increasing vessel speed for reducing turn time. 
     Because tension at the steerable deflectors  60  is a limiting factor, embodiments of the method may include determining the maximum tension on the steerable deflector  60  on the outside of the turn and/or determining the minimum tension on the steerable deflector  60  on the inside of the turn. As will be understood by those of ordinary skill in the art with the benefit of this disclosure, the maximum and minimum tensions may be determined based on a number of factors, including sea state, load-limiting factors associated with the towing equipment, and weather forecast, among others. The maximum tension will be higher for flat seas as opposed to choppy seas. Each element in the spread (e.g., deflectors, ropes, cables, etc.) has a given design limit when it comes to tolerated load. For the steerable deflectors  60 , there are also limitations—particular to each deflector type—when it comes to behavior. Acceptance criteria for behavior as well as static/dynamic loads (tension) can be experienced based, from testing and/or calculated by using hydrodynamic simulations. 
     In addition to the maximum and minimum tension for the steerable deflectors  60 , embodiments of the method may also include determining the maximum and minimum angle of attack θ for each of the steerable deflectors  60 . Too large of an angle of attack θ may cause the particular steerable deflector  60  to stall and, for example, fall backward into the spread of sensor streamers  40 . Too small of an angle of attack θ may cause the particular steerable deflector  60  to become unstable and, for example, flip over such that the generated lateral force is switched 180° or potentially even causing the steerable deflector  60  to move backwards. The maximum and minimum angle of attack θ for each of the steerable deflectors may be determined based on a number of factors, including experience, controlled testing in full scale or in model scale, or by numerical hydrodynamic simulations, or a combination of these techniques. 
     As previously mentioned, embodiments of reducing turn time may most effectively be done by reducing turn radius (r). A turn radius (r) may be determined so that a calculated tension on the steerable deflector  60  on the inside of the turn may be greater than the minimum tension, and tension on the steerable deflector  60  on the outside of the turn may be less than the maximum tension. The determined turn radius (r) may be less than a safe turn radius (r) at the same vessel speed without use of steerable deflectors  60 . The angle of attack θ on one or more of the steerable deflectors  60  may need to be adjusted to maintain the tensions inside the desired values as adjusting angle of attack should adjust lift force with tension being a combination of lift force and drag. Since the configuration of the particular steerable deflectors  60  are known, it will be known what tension may be for a particular vessel speed and angle of attack θ and how turn radius (r) impacts tension. Accordingly, a turn radius (r) for a particular vessel speed may be selected from a set of possible angle of attack and turn radius (r) combinations. The vessel turn radius (r) may be set to the turn radius (r) that is selected. With the turn radius (r) selected, further adjustments to control tension on the steerable deflectors  60  may be selected from vessel speed and angle of attack θ. Alternatively, for certain types of turns, turn radius (r) may not need to be constant such that the selected turn radius (r) may be a range of effective turn radiuses at which the tension may be acceptable. Embodiments of the method may further comprise adjusting the angle of attack θ for the steerable deflector  60  on the inside of the turn and/or for the steerable deflector  60  on the outside of the turn. This adjustment may be performed prior to or during the turn. This adjustment may also be performed at the end of the turn to get back to normal for a straight ahead setting. In some embodiments, the angle of attack θ for the inside steerable deflector  60  may be increased to increase lift force and thus increase tension. In some embodiments, the angle of attack θ for the outside steerable deflector  60  may be decreased to decrease lift force and thus decrease tension. As a result, the spread of the sensor streamers  40  may shift to the center of the turn relative to the survey vessel  10  while allowing the survey vessel  10  to maintain its speed while keeping acceptable tension on the steerable deflectors  60 . In some embodiments, tension may be monitored during the turn to ensure that the tension on the steerable deflectors  60  is maintained within the minimum tension for the inside steerable deflector  60  and the maximum tension for the outside steerable deflector  60 . If tension is outside these ranges, vessel speed may be adjusted (e.g., decreased to reduce tension or increased to increase tension) as needed to maintain the desired tension. In some embodiments, adjusting vessel speed may be desired as to adjust of turn radius (r) during the turn as controlling tension by adjusting turn radius (r) during the turn may have a slow response. In some embodiments, the angle of attack θ may also be adjusted to maintain the desired tension. 
       FIG. 5  illustrates another embodiment of the survey vessel  10  of  FIG. 1  executing a port turn having a reduced turn radius (r). For simplicity, the associated sensors streamers  40  and energy sources  25  of  FIG. 1  are not shown on  FIG. 5 . As illustrated, the survey vessel  10  may be heading in a first direction along first survey line  160 . In accordance with present embodiments, a desired turn radius (r) may be selected for the port turn. The desired turn radius (r) may be smaller than the turning radius (r orig ) for a survey vessel  10  without steerable deflectors  60   a ,  60   b . When the survey vessel  10  leaves the survey line  160 , the steerable deflectors  60   a ,  60   b  and corresponding spread  165  of towed equipment (e.g., sensors streamers  40 , energy sources  25 , etc.) may be symmetric about first survey line  160  as best seen at position  170  on  FIG. 5 . As the survey vessel  10  leaves the first survey line  160 , the survey vessel  10  travels along run-out line  175 . On the run-out line  175 , it may be desired to prepare the spread  165  for the turn, for example, by adjusting the angle of attack of the steerable deflectors  60   a ,  60   b  to shift the spread  165  into the turn. For example, the inner steerable deflector  60   a  may be turned up so that it may be at a higher angle and experience more force in the turn, and the outer steerable deflector  60   b  may be turned down so that it may be at a lower angle and experience less force in the turn. As a result, the steerable deflectors  60   a ,  60   b  and associated spread  165  may shift inward. Accordingly, as the survey vessel  10  enters the turn, the steerable deflectors  60   a ,  60   b  and corresponding spread lines  65  may be symmetric about symmetry line  180  and not run-out line  175 , as best seen at position  185  shown on  FIG. 5 . Symmetry line  180  may be inside the run-out line  175  and vessel turn path  190 . During the turn, vessel speed and other variables may be adjusted as discussed in more detail above. As seen on  FIG. 5 , the vessel turn path  190  using the steerable deflectors  60   a ,  60   b  results in a shorter distance of travel for survey vessel  10  when executing the turn than for a survey vessel  10  executing the same turn without steerable deflectors  60   a ,  60   b , whose turn path is shown on  FIG. 5  at reference number  195 . As shown on  FIG. 5 , as the survey vessel  10  comes out of the turn, the steerable deflectors  60   a ,  60   b  and associated spread  165  may still be shifted inward as best seen at position  200 . As the survey vessel  10  follows run-out line  205  heading out of the turn, the steerable deflectors  60   a ,  60   b  may be adjusted to shift the steerable deflectors  60   a ,  60   b  and associated spread  165  outward, for example, to be symmetric about second survey line  210  as best seen at position  215 . The survey vessel  10  may then continue the geophysical survey heading in a second direction (which may be opposite the first direction) along the second survey line  210 . Also shown on  FIG. 5  is teardrop turn  220 . Teardrop turn  220  may be executed by survey vessel  10  in embodiments wherein the turning radius (r) is non-constant with turning radius (r) being selected from a range of acceptable radii. As previously discussed, the vessel speed and angle of attack for the steerable deflectors  60   a ,  60   b  may also be adjusted to maintain tension within safe limits 
     Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Although individual embodiments are discussed, the invention covers all combinations of all those embodiments. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the present invention. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range are specifically disclosed. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted for the purposes of understanding this invention.