BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an aerial view of towed streamers using a door type deflector.
FIG. 2 is a perspective view of a door deflector as shown in FIG. 1.
FIG. 3 is an aerial view of towed streamers using a wing type deflector.
FIG. 4 is a perspective view of a wing deflector as shown in FIG. 3.
FIG. 5 is an aerial view of a deflector under tow.
FIGS. 6A through 6C are schematic diagrams of the deflector system as viewed from behind the deflector looking in the direction of tow.
FIGS. 7A and 7B are perspective views of a deflector having adjustable flaps in accordance with a second embodiment of the invention.
FIGS. 8A and 8B are a perspective views of a bridle coupled to a wing deflector and deflector door, respectively.
FIGS. 9A through 9G are schematic diagrams of a number of means for changing or manipulating the geometry bound by the bridles and the deflector.
FIG. 10 is a perspective view of another embodiment of a deflector that provides adjustable depth as well as adjustable cross-line positioning.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
The present invention provides a marine deflector for a seismic survey system. The deflector has a generally upright deflector body that controllably tilts about an axis that is generally transverse to a cable that pulls the deflector through the water. In one embodiment, an adjustable bridle is coupled to the deflector body, wherein the adjustable bridle includes a connector for coupling the bridle to the cable, such as a lead-in, and wherein the adjustable bridle is capable of varying the tilt angle of the deflector body. The depth of the deflector body is controlled by varying the tilt angle of the deflector body. Preferably, the tilt angle is varied by pivoting the deflector relative to an axis that is generally transverse to the cable.
The deflector may comprise a so called wing deflector, e.g. the WesternGeco Monowing, or it may comprise a so called deflector door, frequently called a door or a Barovane comprising a series of hydrofoils mounted within a rectangular frame. Regardless of the type of deflector, the present invention allows for the tilt angle of the deflector to be adjusted by one or more of a variety of methods or means. One embodiment includes adjusting wing flaps as known from airplanes. The flaps re-distribute the lift of the wing along the span so as to create a moment of force that results in a tilt angle. A second embodiment includes manipulating the geometry defined by the deflector body and the towing bridle segments. Finally one may also imagine a combination of bridle adjustment and flap adjustment.
The preferred method of using the deflector involves controlling the tilt angle, also called heel angle or roll angle, of the deflector in such a way that the vertical component of the lift force generated by the wing changes with the tilt. The wing will find its equilibrium in depth when the vertical component of the wing lift plus the vertical component of the gravity force equals the vertical component of the tension in the tow wire or lead-in. By changing the tilt angle, the wing will find a new equilibrium depth.
Unlike deflectors currently on the market, the present depth controllable deflectors should be close to zero buoyant. Preferably, the deflectors should be slightly negative buoyant in order for changes in water speed to have minimal influence on the depth of the deflector. Furthermore, since a near-zero buoyant deflector does not require a direct connection to a surface float, i.e., a hanging support connection, the typical strains on the direct cable connection due to wave loads on the surface float are eliminated. Therefore, maintenance intervals may be increased and the risk for catastrophic failures of this cable connection is eliminated.
The invention provides the ability to effectively control the depth of the deflector while the deflector is deployed. Controlling the depth of the deflector means that you can control the depth of the streamer near the connection to the deflector. For example, the front end of the streamer may be controlled at a fixed or changing depth. One advantageous application for controlling the depth includes lowering the front end of a streamer during periods of strong waves in order to reach a depth where the wave action is insignificant. It should be noted that wave action decays exponentially with depth. Consequently, the seismic vessel can leave the equipment out in the water through much worse wave conditions with little or no damage. This also reduces the number of retrievals and deployments and opportunities for equipment damage and personnel injuries that can occur during retrievals and deployments. Productivity is increased because the system is quickly ready for production when the waves calm down simply by returning the streamer to the desired operating depth. Furthermore, depth control of the deflector and the front end of the streamer can have a positive effect on the seismic data quality, since the front end may be operated at the same depth as the desired streamer depth resulting in less noise on the front sections of the streamer.
FIG. 5 is an aerial view of an exemplary configuration of a seismic survey system 50 having a deflector 52 in accordance with the present invention. The deflector 52 is coupled by a bridle 53 to a lead-in cable 54 that is pulled through the water 55 behind a vessel (not shown). The lead-in cable 54 is on the upstream end of the seismic cable from the deflector and is shown including an optional pivot float 58. A streamer 56 is attached to the lead-in cable 54, but could also be coupled directly to the deflector 52. The streamer 56 is preferably coupled as close to the deflector 52 as possible for effective depth control of the front end of the streamer 56. The deflector 52 should be generally upright during operation, since a large tilt angle will reduce the horizontal lift force on the deflector that is needed to achieve the desired separation of multiple streamers in an array. In addition, it is beneficial for the bridle 53 to be as perpendicular to the deflector as possible. The latter two characteristics of the system are achievable when the deflector is balanced so that it is slightly heavier than water when submerged in water. In this case, a change in lift force by e.g. change in water speed and/or change in angle of attack will have as little impact on the depth of the wing as possible.
FIGS. 6A-6C are schematic diagrams of the system 50 as viewed from behind the deflector 52 looking in the direction of tow. The deflector is viewed from below the surface of the water 55 and from a downstream location. The diagrams illustrate the force equilibrium on the deflector 52 that is used to control the deflector depth, d. The lead-in (tow cable) 54 and bridle 53 are shown coming in from the left and being attached to the deflector 52. The lead-in 54 is coupled to the optional surface float or pivot float 58 that establishes a local pivot point from which the outer part of the lead-in 54, the bridle 53 and the deflector 52 are pivoted when depth is adjusted. The deflector 52 remains in an equilibrium position as long as the resultant force, R, between the lift force, L, and the gravity force (i.e., weight) resulting from the application of gravity on the deflector mass, Mg, is in-line with the lead-in tension force, T. It is useful to define a “tilt angle” for this discussion, this angle being defined by the arc between the plane in which the trailing surface of the deflector 52 lies and the gravity vector (i.e., vertical). The tilt angle will lie generally in a vertical plane, and is indicated as angle θ in FIGS. 6B-6C. Changing the tilt angle, θ, of the deflector will result in a change in direction of the resultant force, R, such that the whole system from the pivot buoy or float 58 to the deflector will rotate about the pivot float 58 until a new equilibrium position or depth is established.
FIG. 7A is a perspective view of a deflector wing 60 having a deflector body 62 and adjustable flaps 64 in accordance with one embodiment of the invention. Rotating the upper and lower flaps 64 in opposite directions or in the same direction to different degrees, i.e., independently, will create a hydrodynamic tilt-moment that will make the wing 60 tilt. Alternatively, a tilt-moment can be created by rotating a single flap 64 without movement of the other flap(s) 64. The deflector 60 may have any number of flaps, even a single flap, so long as one or more of the flaps can produce a tilt-moment. The deflector 60 is shown in FIG. 7A coupled to a bridle having an upper segment 66 and a lower segment 68 converging and coupling to the lead-in cable 54 at a point 69. In this instance, the tilt angle is adjusted by using flexible bridle segments 66, 68 and permitting one of the segments to go slack while the other segment remains in tension.
An alternative design is shown in FIG. 7B. The bridle including segments 66 and 68 shown in FIG. 7B are coupled to each other to form a single cable that is in engaged with a pulley 65 secured to the end of cable 54. In this manner, the desired tilt angle can be achieved with a lower magnitude of the tilt-moment. The deflector embodiment of FIG. 7B also includes a sensor 63 for measuring the actual depth of the deflector. This sensor will cooperate with an actuator (not shown) for adjusting the bridle, and a controller (not shown) that provides a command to the actuator upon input from the sensor to achieve or maintain a desired depth of the deflector. The actuator and controller are described further below with reference to FIGS. 9C and 9D.
FIG. 8A is a perspective view of a wing deflector 70 with a bridle formed by chain segments 72 and 74 extending from the deflector body 76 to a connection point 78. A controller and actuator (not shown) can manipulate the geometry bound by the deflector and the bridles. This geometry comprises the triangle that is bound by bridle segment 72, bridle segment 74, and the deflector segment 79 extending therebetween. As with the embodiment of FIG. 7A, the tilt angle may be adjusted in the embodiment of FIG. 8A by using flexible bridle segments and permitting one of the segments to go slack while the other segment remains in tension.
In this particular embodiment, the deflector 70 is shown equipped at its upper end with an elongated, streamlined float 77 that is rigidly secured to the deflector body 76 so that the body depends downwardly from the float like the keel of a boat. The float may be constructed of a similar material to the body, e.g., titanium, but may otherwise be made from a fibre-reinforced composite material. A weight element 79 is also secured to deflector body 76, preferably to compensate the buoyancy force provided by float 77 to produce a slight negative buoyancy overall on deflector 70.
FIG. 8B is a perspective view of a deflector door 80 with a bridle formed by chain segments 81, 82, 83, 84 (and optionally, segments 85, 86) extending from the deflector body 87 to a connection point 88. For the door deflector 80, a controller and actuator (not shown) can manipulate the similar geometry as in FIG. 8A, except that the upper segments 81,82 must act as a pair and the lower segments 83,84 must act as a pair for depth control. Specifically for the purpose of changing the tilt angle of the door 87, the tetrahedral bound by the door 87 and the outer bridle members 81,82,83,84 are manipulated. The tilt angle of the deflector 87 is changed by altering the ratio of the length of the upper segments 81,82 to the length of the lower segments 83,84. However, it should be recognized that it is also possible, either sequentially or simultaneously with tilt angle adjustment, to adjust the angle of attack of the deflector 87 by altering the ratio of the length of the front segments 81,83,85 to the length of the trailing segments 82,84,86. When the tilt angle and angle of attack are both being controlled, the lengths of all bridle segments may be different at any point in time.
FIGS. 9A through 9G are schematic diagrams of a number of means for changing or manipulating the geometry bound by the bridles and the deflectors of the present invention. It should be recognized that the triangles shown in these Figures are side views that apply equally to either a wing deflector having two bridle chains 72,74 as in FIG. 8A or a deflector door having four bridle chains 81,82,83,84 as in FIG. 8B. In regard to a deflector door, the upper segment of the triangle in FIGS. 9A through 9G represents all upper segment chains, such as segments 81 and 82 of FIG. 8B, and the lower segment of the triangle represents all lower segment chains, such as segments 83 and 84 of FIG. 8B. The invention may be equally applied to bridles containing any number of segments.
FIG. 9A illustrates a simple system 90 where the length of the lower bridle segment 94 is adjusted by means of a hydraulic cylinder or actuator 96 overcoming the tension in the bridle.
FIG. 9B shows a system 100 comprising a first hydraulic cylinder 102 coupled between an upper portion of the deflector 108 and the upper bridle segment 104, and a second hydraulic cylinder 102 coupled between a lower portion of the deflector 108 and the lower bridle segment 106. Both bridle segments 104, 106 are coupled at a connection point to the lead-in cable 54. This system is more energy efficient as the hydraulic pump only has to overcome the force equal to the tension difference in the bridle members and not the total tension as in the system of FIG. 9A. This is referred to as the principle of load-balancing.
FIGS. 9C and 9D illustrate other systems that use the load balanced principle. In FIG. 9C, the system 110 includes rotatable connection points or towpoints 112 coupled to the deflector 114. The bridle segments 111,113 are attached to the outer lever arms, and between the inner lever arms are attached a rod, chain, or other connecting means 116 that transfer the loads between the two rotating towpoints 112. A linear actuator 118 is connected to this middle member 116 to control the rotation of the towpoints 112. In this embodiment of the invention, the deflector further includes a controller 119 for providing commands to the linear actuator. The controller may be of any type, such as digital, analogue or a combination thereof, and may also be located on the vessel, or constitute part of a distributed control system with different steps or actions taking place in different locations yet collectively serving as a controller. Where the controller is locally positioned within the deflector, as depicted in FIG. 9C, for controlling the tilt angle or depth of the deflector, the system will preferably further comprise a remote controller (not shown), such as on the vessel, for providing a tilt angle or depth setpoint to the local controller.
FIG. 9D illustrates another system 120 that utilises the same principle of load balancing, but the bridle segments 121,122,123 comprise a closed loop that forms a triangle extending around a block or wheel 124 at each towpoint 125. In the same manner as above, a linear actuator 118 adjusts the position of the bridle by being connected to the bridle segment 122 between the towpoints 125.
FIG. 9E illustrates a system 130 having sliding towpoints 132 that secure the ends of the upper bridle segment 134 and lower bridle segment 136 to the deflector body 138. Sliders as applied in modern sailing yachts may be used, as well as any actuator or motor. While it is optional to use only one sliding towpoint 132, the system would not be load balanced. Rather, it is preferred to use two sliding towpoints 132 that slide in the same operation so that the system will be load balanced.
FIG. 9F shows a system 140 with the bridle segments 142,144 connected to an inverted toothed wheel 146 that is engaged with a toothed wheel 148 that is rotatably driven by an actuator 149. By adjusting this actuator 149 the attachment point where the lead-in 53 is attached to the inverted toothed wheel 146 is effectively adjusted, resulting in the tilting of the deflector.
FIG. 9G illustrates a system 150 in which the angle, a, at which the lead-in 54 is connected to the bridle segments 152,154 is altered by an actuator or cylinder 156. A frame is formed at the outer end of the bridle and includes rigid members 157,158,159 that are pivotally connected. Energizing the actuator 156 applied a force on rigid member 159, which member then causes rotation of member 157 at point 155.
FIG. 10 illustrates the construction of a deflector 200 that provides adjustable depth according to the invention, as well as adjustable cross-line positioning. The deflector body 202 that acts as a kind of otter board attached through the bridle members to the lead-in 54 that extends from the towing vessel (not shown) to the streamer 56, i.e. the equipment that is towed behind the vessel through the water. The three attachment points to the deflector are indicated at 207, 208 and 214. A streamer cable that performs the necessary communication with the tow is led along the lead-in 54 and the streamer 56 and extends therebetween as indicated by 206. It is separated from the lead-in 54 in the area of the deflector and reconnected with the streamer 56 some distance after the connecting point 214. This cable section 206 is slack, in order to prevent it from influencing or restricting the movements of the deflector 200.
The deflector 202 is also coupled to an adjustable bridle at two points 207,208 along a line generally parallel to the deflector's vertical axis, x. Here, an upper bridle segment 215 is coupled via an actuator 217 to the deflector body at point 207 and a lower bridle segment 216 is coupled via an actuator 218 to the deflector body at point 208. As describe previously, this arrangement allows the tilt angle of the deflector to be adjusted, resulting in control of the deflector depth.
At the rearward end of the deflector 202, the fitting 209 is equipped with an angle lever 210 that is rotatable or pivotable about a point 219 on the fitting. The attachment point 214 is located at the end of lever 210. The angle lever 210 is connected by its second leg 220 via a pivotable connection 213 with the side of the fitting 209. For adjusting purposes, there can be provided here an adjustable piston cylinder mechanism 211 that can cause a forward and backward movement of the pivotable connection 213 of leg 220 of lever 210. This adjusting mechanism can also be of a different shape to that of a piston cylinder, and the device can be capable of being operated by a motor drive, e.g. a hydraulic motor in the deflector, a battery-driven motor or it could be adjusted before being deployed. Additional description of the operation of such a deflector is found in U.S. Pat. No. 5,357,892, which patent is incorporated by reference herein.
The invention also includes a method of performing a marine seismic survey, the method including towing a plurality of laterally spaced seismic steamers over an area to be surveyed, wherein the depth of at least one of the streamers is controlled by a deflector device in accordance with any one of the preceding statements of the invention.
It will be understood from the foregoing description that various modifications and changes may be made in the preferred embodiment of the present invention without departing from its true spirit. It is intended that this description is for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be limited only by the language of the following claims.
Patent Application
20080022913
