TECHNICAL FIELD

[0001] Embodiments of the subject matter disclosed herein generally relate to devices and systems used for marine exploration and, more particularly, to multiplane foil structures attached to cables that carry seismic instrumentation.

DISCUSSION OF THE BACKGROUND

[0002] Marine seismic data acquisition and processing generate a profile (image) of the geophysical structure (subsurface) under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas. Thus, providing a high-resolution image of the subsurface is an ongoing process for the exploration of natural resources, including, among others, oil and/or gas.

[0003] During a seismic survey, as shown in Figure 1 , a vessel 1 10 tows one or more seismic sources 120 configured to generate acoustic waves 122a that propagate downward and penetrate the seafloor 124 until eventually being reflected by a reflecting structure 126. Vessel 1 10 also tows acoustic detectors 1 12 arranged along a cable 1 14. Cable 1 14 and detectors 1 12 form what is known as a streamer 1 16. Detectors 1 12 acquire information (seismic data) about reflected waves 122b, 122c and 122d.

[0004] Streamers may be disposed horizontally, i.e., lie at a constant depth relative to the ocean surface 180. Also, plural streamers 1 16 may form a constant angle (i.e., the streamers may be slanted) with respect to the ocean surface as disclosed in U.S. Patent No. 4,992,992, the entire content of which is incorporated herein by reference.

[0005] Vessel 1 10 may tow plural streamers at the same time, which form the "spread." During seismic data acquisition, the streamers' depth and lateral positions may be controlled using steering devices 130, known as "deflectors." Deflectors 130 are attached between streamers 1 16 and lead-in 132 and they are partially submerged. Lead-in 132 is attached directly to the vessel and traditionally attaches to the head of the streamer for providing a towing force.

[0006] Vessel 1 10 tows streamers 1 16 along a specified path to scan the targeted area. Arranging and maintaining the configuration of the streamers (e.g., maintaining a minimal distance between the heads of the streamers) is achieved with the at least two deflectors 130, which are positioned on the outer streamers of the spread. The deflectors provide lift forces while towed through the water, to maintain depth and/or lateral separation among the streamers.

[0007] Figure 2 illustrates a conventional deflector 200 attached between lead-in 202 and streamer 204. Deflector 200 has a flat, rectangular-shaped body 212 sandwiched between ballast bodies 214 and 216 (also known as pods) at ends of body 212. Top ballast body 214 may be lighter than bottom ballast body 216 to move the gravity center of the deflector below a buoyancy center thereof, for maintaining a vertical position of the deflector when in use in water. Deflector 200 may also have a slot 218 along body 212 to increase the stalling angle. The length of the deflector may be around 2.5 m. Deflector 200 may be attached to the lead-in and the streamer around its middle area, with a dedicated mechanism known in the art.

[0008] A problem with the conventional deflectors is that the ratio between the lift provided by its body and its drag is small. Different solutions have been investigated, for example, manufacturing large deflectors, or manufacturing underwater deflectors for increasing this ratio. However, in each of this known solution, the increase in lift inherently generates an increase in drag, which is undesired. Figure 3 shows that the lift/drag ratio 302 increases up to about 5 degrees (the X axis shows the angle of attack and the Y axis shows the lift/drag ratio) after which it decreases as the angle of attack is increased. Thus, the increase of lift by changing the angle of attack of the deflector leads to a reduction of efficiency.

[0009] Therefore, there is a need to manufacture a deflector that has better lift/drag ratio while maintaining a reasonable size.

SUMMARY

[0010] According to an embodiment, there is a multi-plane foil structure for use in a marine seismic acquisition system. The system includes a separation member, a first foil attached to one end of the separation member, and a second foil attached to the other end of the separation member. At least one of the foils has a changeable shape. [0011] According to another embodiment, there is a marine seismic acquisition system for collecting seismic data. The system includes plural streamers configured to be towed underwater, along parallel inline directions, plural lead-in cables configured to tow the plural streamers underwater, and plural multi-plane foil structures, each located between a corresponding streamer and a corresponding lead-in cable. Each multi-plane foil structure has first and second foils, and a shape of the first and second foils varies from one multi-plane foil structure to another multiplane foil structure for maintaining the plural streamers separated by a

predetermined distance (d) along a cross-line direction. The cross-line direction is substantially perpendicular on the inline direction.

[0012] According to another embodiment, there is method for controlling a separation among streamer's heads. The method includes connecting between plural streamers and corresponding lead-ins, corresponding multi-plane foil structures, towing with a vessel the plural streamers, and adjusting a shape of a first foil relative to a second foil of a corresponding multi-plane foil structure for achieving a desired lateral offset from a towing path of the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

[0014] Figure 1 is a schematic diagram of a seismic acquisition system;

[0015] Figure 2 is a schematic diagram of a conventional deflector;

[0016] Figure 3 illustrates a ratio between lift and drag for a deflector;

[0017] Figures 4A-C illustrate a bi-plane foil structure;

[0018] Figures 5A-E illustrate a bi-plane foil structure with corresponding foils having different shapes;

[0019] Figures 6A-E illustrate a vortex effect for a bi-plane foil structure versus a traditional deflector;

[0020] Figure 7 illustrate a pressure profile of a foil;

[0021] Figures 8A-B illustrate a vortex effect for a bi-plane foil structure;

[0022] Figures 9A-B illustrate a vertical steering of the bi-plane foil structure;

[0023] Figure 10A illustrates a horizontal steering of the bi-plane foil structure; [0024] Figure 1 1 illustrate a cross-section of the foils;

[0025] Figures 12A-C illustrate various shape changing mechanisms for changing the shapes of the foils;

[0026] Figure 13 illustrates another bi-plane foil structure;

[0027] Figures 14A-B illustrate still another bi-plane foil structure;

[0028] Figure 15 illustrates a tri-plane foil structure;

[0029] Figure 16 illustrates marine seismic survey system having plural multiplane foil structures; and

[0030] Figure 17 is a flowchart of a method for towing streamers with multi- plane foil structures.

DETAILED DESCRIPTION

[0031] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a marine seismic data acquisition system having one or more streamers attached to multi-plane foil structures. However, the embodiments to be discussed next are not limited to separating streamers with multi-plane foil structures, but they may be applied to other seismic components (e.g., sources) that need to be maintained at given offsets.

[0032] Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

[0033] According to an embodiment, a novel multi-plane foil structure includes underwater active multi-plane foldable foils. This multi-plane foil structure has a multi-plane structure to reduce tip vortex. The "multi-plane" term means that the foil structure includes two or more foils or wings. For simplicity, most of the following embodiments show a bi-plane structure, i.e., only two foils or planes. The lift generated by this multi-plane foil structure increases due to the larger span of the wing combined with the winglet effect, as will be discussed later. In one application, the multi-plane foil structure has a foldable structure with different shapes to adapt the lift to the assembly (lead-in/streamer). For example, a servo module (to be discussed later) may be powered by the lead-in to control the foils' shape and the resulting lift.

[0034] In one embodiment illustrated in Figures 4A-B, multi-plane foil structure 400 has two foils 402 and 404 connected to each other by a separation member 405 and the two foils 402 and 404's shape may be independently adjusted. Separation member 405 may include a central body 406 that connects the two foils together through respective cross-pieces 408 and 410. While Figure 4A shows a frontal view of the multi-plane foil structure, Figure 4B shows a bird view of it. In another embodiment illustrated in Figure 4C, the separation member 405 includes only the two cross-pieces 408 and 410, which are directly connected to each other without the presence of the central body 406.

[0035] Foils 402 and 404, as discussed later, may be configurable (e.g., foldable) to have variable sizes and/or shapes. This also means that the foils can have different configurations relative to each other for a same multi-plane foil structure. For example, although the following drawings appear to show

symmetrically located foils, in some embodiments, they are not symmetrically disposed one relative to the other. Although Figure 4A shows cross-pieces 408 and 410 attached to about the middle portion of foils 402 and 404, one skilled in the art would understand that the top or bottom regions of the foils may be attached to the cross-pieces. Also, one skilled in the art would understand that the two foils 402 and 404 may be attached with different regions to the corresponding cross-pieces.

[0036] Figure 4B shows that the two foils 402 and 404 are offset relative to each other along a longitudinal direction (indicated by Y in the figure) of the central body 406. The offset distance D can have such a value so that the two foils 402 and 404 can be folded without interfering with each other. Thus, in one embodiment, the offset distance D, between the middle of the cross-pieces 408 and 410, is about the width W of the foils. Those skilled in the art would note that Figures 4A and 4B schematically illustrate the foils, central body and the cross-pieces and thus, no shapes or sizes should be inferred from these figures. As will be discussed later, the foils can have various profiles, sizes and configurations while the central body and the cross-pieces may have hydrodynamic properties.

[0037] In one embodiment, as illustrated in Figure 4C, the two foils 402 and 404 are facing each other so that offset distance D is zero. In this case, the cross- pieces 408 and 410 (which can be a single cross-piece) have a length long enough to allow the two foils to fold without interfering with each other.

[0038] In one embodiment, the multi-plane foil structure is configured to float underwater, for a fully underwater spread. This feature could lead to the

suppression of spread ropes/deflectors/wide tow and other elements that are used for spreads that are only partially underwater, as discussed later. Note that a traditional spread has the deflectors partially underwater and partially above water. In one embodiment, multi-plane foil structure 400 may also be used as a depressor to make the spread change its depth.

[0039] Figures 5A-D illustrate various profiles and shapes for the foils of the bi-plane multi-plane foil structure. Figure 5A shows foils 402 and 404 completely folded, Figure 5B shows only the tips of the foils folded, Figure 5C shows foil 402 fully unfolded and foil 404 partially folded and Figure 5D shows both foils fully unfolded. Each figure also illustrates the lift achieved by each multi-plane foil structure and the lift to drag ratio. It is noted that the lift/drag ratio increases as the aspect ratio of the wings increases, i.e., larger span, better span/chord ratio. Other configurations are possible as would be appreciated by those skilled in the art. For example, in line with the embodiment illustrated in Figure 4C, Figure 5E shows a biplane paravane having no central body and the foils directly facing each other, i.e., no offset distance D between them.

[0040] The inventors of this folding multi-plane foil structure have observed that a vortex associated with a traditional paravane is decreased for the

embodiments illustrated in Figures 5A-E. Figure 6A illustrates the vortex produced by the multi-plane foil structure illustrated in Figure 5A, Figure 6B illustrates the vortex produced by the multi-plane foil structure illustrated in Figure 5C, Figure 6C illustrates the vortex produced by the multi-plane foil structure illustrated in Figure 5B, and Figure 6D illustrates the vortex produced by the multi-plane foil structure illustrated in Figure 5D. The multi-plane foil structure's configuration illustrated in Figure 6D (rhomboid shape for the foils) produces the best aspect ratio and Figure 6E illustrates the vortex of a traditional paravane. It is noted the large vortex produced by the traditional paravane in Figure 6E relative to the multi-plane foil structure of Figures 4A-B. This is believed to be the result of the foil-tip interaction of the two foils, which is now explained.

[0041] Figure 7 shows a traditional wing 700 and its pressure profile 702 when air is blown over it. The pressure profile generates vortices of swirling air 704 that create a drag 706. This drag is reduced for the multi-plane foil structure 800 illustrated in Figures 8A and 8B because the high and low pressures indicated with "+" and respectively, are opposite to each other. This means that the fluid flows 830 and 832 induced at the tips of the foils are opposite to each other, which reduces the vortex and results in an increase of efficiency of the multi-plane foil structure. In this respect, Figure 8B shows that an orientation 802 of the first foil's tip 804 makes a non-zero angle with an orientation 806 of the second foil's tip 808, which reduces the overall vortex produced by the multi-plane foil structure. Note that the tips 804 and 808 of the multi-plane foil structure 800 are considered to be the most distal portions of the foils, when measured from the central body 810 or the cross-pieces.

[0042] As the multi-plane foil structure may have a foldable structure (to be discussed later in more detail), it is possible to control a depth of the multi-plane foil structure by controlling the shape and/or area of the foils. In other words, it is possible to control a rotation of the multi-plane foil structure around a longitudinal axis of its central body to create a negative or positive buoyancy force. Figure 9A shows a negative buoyancy force 910 created by foils 902 and 904 while Figure 9B shows a positive buoyancy force 910 (force 910 is positive when pointing above the XY plane and negative when pointing below the XY plane). A corresponding torque 908 is created by the forces acting on the foils of the multi-plane foil structure 900, which is located between lead-in 912 and streamer 914. The buoyancy force and implicitly the torque can be achieved by controlling the foils' surfaces and/or shapes, e.g., extending one foil more than the other to create a force imbalance between the two foils. Thus, it is possible to vertically steer the streamer. In one embodiment, to maintain the deflector vertical relative to the gravity, the upper parts of the foils are buoyant while the lower parts of the foils are ballasted. [0043] In one embodiment, as illustrated in Figure 10, different values of lift 1002 and 1004 are generated on the front outer foil 902 and the back inner foil 904 so that the attack angle of the multi-plane foil structure can be controlled. By changing the attack angle, the multi-plane foil structure is capable to laterally steer the streamer. The different values of lift may be implemented by a controller 1030 located locally on the foil structure, or a controller (not shown) located on the towing vessel, or a controller that is distributed on the multi-plane foil structure and the vessel. An example of foil's cross-section is shown in Figure 1 1 . Other cross- sections may be used as will be understood by those skilled in the art.

[0044] A shape control mechanism for changing at least the shape of the foils is now discussed with regard to Figures 12A-C. In a first embodiment illustrated in Figure 12A, the shape control mechanism 1240 includes a system of piston mechanisms 1242-1248 that are remotely actuated (from example, from the vessel controller or local controller 1230). The piston mechanisms are located at each hinge 1250 and 1252 of each foil for controlling each foil's panel 1202A and 1202B's orientations relative to a central panel 1202C. Hinges 1250 and 1252 are

understood herein to mean an actual hinge or a flexible structure that allows a certain amount of bending so that the flexible structure acts like an actual hinge. Also, Figure 12A shows hinges and piston mechanisms only for the upper part of foil 1202. While this is a possible implementation of the invention, another

implementation has hinges and piston mechanisms for both the upper and lower panels of the foil. Figure 12A shows a foil having one central panel and four distal panels. The number of panels in this embodiment is not intended to limit the invention, but only as an example.

[0045] In one embodiment, central panel 1202C is fixed relative to

corresponding cross-piece 1208 and only distal panels 1202A and 1202B can be moved with the shape control mechanism 1240. Corresponding wiring and/or piping for supplying power and/or pneumatic fluid and control signals, between the shape control mechanism 1240 and controller 1230, are embedded in the structure of the foils, and thus, they are not visible in the figures. In one embodiment, it is possible to locate a piston mechanism 1250 between cross-piece 1208 and central panel 1202C so that the entire wing's orientation can be adjusted as necessary. [0046] In another embodiment, illustrated in Figure 12B, instead of using piston mechanisms as in the embodiment of Figure 12A, a system of internal cables and pulleys are used to change the foils' shapes. Figure 12B shows pulleys 1260 distributed around hinges 1250 and at a connection of the central panel 1202C to the cross-piece 1208. An actuator 1264, for example, a motor, may be activated to stretch cable 1262 so that the foils are fully extended. By rotating the actuator 1264 in an opposite direction, cable 1262 becomes slack, and the foil collapses. To improve the shape control mechanism 1240, a spring system 1263 may be added to each hinge region to fully extend the foils.

[0047] In still another embodiment illustrated in Figure 12C, the shape control mechanism 1240 includes spring systems 1263 located at each hinge 1250. The spring systems bias the foils to stay in a fully extended position. In order to fold the foils and bring them into a fully folded position, a system of cables 1266 and actuator 1264 are used. By rotating actuator 1264 in one direction, the foils will be folded, and by rotating the actuator 1264 in the other direction, the foils will be unfolded due to the forces exerted by the spring systems.

[0048] While the embodiments illustrated in the previous figures show the foils folding facing each other, Figure 13 shows an embodiment in which foils 1302 and 1304 of bi-plane foil structure 1300 fold in a side way, with distal panels 1302A and 1302B rotating around joints 1370. If this approach is implemented, then the length of the cross-pieces can be reduced and/or the offset distance D may be made zero as there is no more need for folding space between the foils.

[0049] Still in another embodiment illustrated in Figures 14A and 14B, the distal panels 1402A and 1402B may be configured to telescopically extend from, or retract into, central panel 1402C when instructed by controller 1406. In one embodiment, it is possible that only distal panel 1402B is extendable or retractable while in another embodiment, both distal panels 1402B and 1402C are extendable or retractable. Controlling the positions of the distal panels relative to the central panel, leads to a variation in the span/chord ratio of the foil, which improves the L/D ratio.

[0050] While the dimensions of each element in the above noted

embodiments may vary as necessary, in one specific embodiment, the length of the fully extended foil may be about 8 to 10 m, the length of the fully folded foil may be about 2 to 4 m, a width (distance between the two foils) of the bi-plane foil structure may be about 1 to 4 m, and an offset distance D may be about 1 to 3 m.

[0051] As discussed at the beginning of this application, for simplicity, the multi-plane foil structures discussed until now have been shown to have two foils. However, as illustrated in Figure 15, a multi-plane foil structure 1500 may include three foils 1502, 1503 and 1504. The third foil 1503 may be fixed (i.e., not foldable) or foldable similar to foil 1502. The third foil 1503 may be attached to central body 1506 through a corresponding cross-piece 1509, similar to the other foils, or directly to one of the other foil 1504, as previously discussed. More than three foils may be part of the multi-plane foil structure 1500.

[0052] A marine seismic acquisition system using one or more of the multiplane foil structures of the previous embodiments is now discussed with regard to Figure 16. System 1600 includes a vessel 1602 that tows plural streamers 1604. Each streamer 1604 is connected to the vessel 1602 via a corresponding lead-in cable 1606. Streamer 1604 may have a tail buoy 1608 attached to its tail and a strength member 1610 at its head.

[0053] In order to maintain the streamers in parallel to each other and separated by a given distance d, traditionally, large deflectors are positioned next to the outer streamers and spread ropes are used to link the heads of the streamers to each other for maintaining the distance d. Also, the traditional spread uses wide tows directly connecting the large deflectors to the vessel.

[0054] This traditional structure is unnecessary with the new multi-plane foil structure for the following reasons. Instead of the wide tows, traditional two deflectors and the spread ropes, a multi-plane foil structure 1612 is attached to the head of each streamer 1604. The configuration and/or shapes of the foils of each foil structure 1612A-G are different for each streamer as illustrated in Figure 16. The most inner multi-plane foil structure 1612G has the foils completely folded or almost folded while the most outer multi-plane foil structure 1612A has the foils completely unfolded or almost unfolded. All the other foil structures 1612B-F have a shape between completely folded and completely unfolded. Note that as the multi-plane foil structures have their foils more and more unfolded, as they are further away from the vessel's path direction 1620, the lift force is larger, and then, the offset of the respective streamer to the path direction 1620 is larger, thus being able to ensure that the streamers are parallel to each other and spread in an even way away from path direction 1620.

[0055] An advantage of this new setup is that no head buoys are needed to maintain the head of the streamers at a certain depth, as the multi-plane foil structures may be controlled to achieve a desired depth, as discussed in the embodiments illustrated in Figures 9A to 10. Further, no spread ropes are

necessary to maintain constant the distance d between the streamers because the multi-plane foil structures can control their lateral movement. Reducing the elements necessary for maintaining the spread in position, results in reduced fuel

consumption, easier maintenance, less noise on the streamers, and smaller turn radius. In addition, such a configuration may be advantageous for Artie exploration because the spread is fully submerged. In this regard, note that the traditional spreads have the main deflectors partially above the water surface and also the head buoys floating at the water surface. These elements are not present in the embodiment illustrated in Figure 16.

[0056] In one embodiment illustrated in Figure 17, there is a method for controlling a separation among streamer's heads as now discussed. The method includes a step 1700 of connecting between plural streamers and corresponding lead-ins corresponding multi-plane foil structures, a step 1702 of towing with a vessel the plural streamers, and a step 1704 of adjusting a shape of a first foil of a

corresponding multi-plane foil structure for achieving a desired lateral offset from a towing path of the vessel.

[0057] One or more of the exemplary embodiments discussed above are related to deflectors attached to a towing cable of a vessel towing a marine data acquisition system. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details. [0058] Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

[0059] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. No element, act, or instruction used in the description of the present application should be construed as critical or essential to the embodiments unless explicitly described as such. Also, as used herein, the article "a" is intended to include one or more items.