Field of the invention

[0001] The present invention relates to a mechanical bridle block control system for adjusting the paravane angle of attack by adjusting two towing lines from the towing vessel.

Background art

[0002] Most paravanes are towed using a single towing line between the towing vessel and the bridle block, and a fore and an aft set of bridle lines between the bridle block and a fore and an aft portion of the paravane.

[0003] The applicant's own patent US8752493 B2 defines a bridle block system for a paravane for seismic equipment, please Fig. 6. That patent describes a main bridle block with a connecting point for a towing line from a seismic vessel, and with attachment points for first and second triples of bridle lines extending to first and second lugs at either ends of said paravane. The main block comprises a pivot point for a pivot arm, and the pivot arm comprises a first arm with an attachment point near its outer end for the first triple of bridle lines, and a second, oppositely directed arm with an attachment point near its outer end for the second triple of bridle lines. A significant feature of the US patented device is an auxiliary moment arm connected at the second arm's side relative to the pivot point and connected to a so-called seventh bridle line, which is further connected to an actuator arranged near the first bridle lines' first lugs' end of the paravane. Remotely controlling the length of the seventh bridle line using a paravane- borne actuator changes the force balance between the fore and aft bridle lines and thus controls the angle of attack of the paravane. This system does of course require a remote control system such as radio communication and stored power on the paravane, or signal cable from the vessel.

Short summary of the invention

The invention is a paravane steering system comprising

- a starboard (SB) or port side (PS) paravane (1) with

- a set of fore and aft bridle lines (2F, 2R) connected to a bridle block (3),

- the bridle block (3) connected via a first towing line (5F) to a towing vessel (8)

- the bridle block (3) provided with

- a fore bridle axle (21F) for a fore bridle line (2F),

- an aft bridle axle (21R) behind said fore bridle axis (21F), for an aft bridle line (2RB),

- a fore towing axle (31F) for a fore towing line (5F) coinciding with said fore bridle axle (21F),

- a rear towing axle (31R) arranged behind the fore towing axis (31F), for an aft towing line (5R),

- the fore and aft towing lines (5F, 5R) arranged for extending towards the towing vessel generally in parallel during towing operation and adjustable in relative length for adjusting from the vessel the angle of attack (AoA) of the paravane (1).

The invention is also defined in other independent claims. Embodiments of the invention is defined in dependent claims. Please see the attached set of claims.

Figure captions

[0004] The attached figures illustrate some embodiments of the claimed invention.

[0005] Fig. 1 illustrates a triangular bridle block (3) embodiment of the invention wherein there are a taut fore and a slack aft towing line (5F, 5R) connected to a fore and aft tow axle (31F, 31R). This is a launching position for low angle of attack explained below. Fore and aft bridle lines (2F) and (2R) are connected between the bridle block (3) and a paravane (1) only indicated.

[0006] Fig. 2A illustrates the same triangular bridle block (3) embodiment in an operative position with an operative angle of attack controlled by adjusting the extended lengths of the fore and aft towing lines (5F, 5R).

[0007] Fig. 2B illustrates a section along A-A of Fig. 2A and represents the same position during operation. The section view is through the fore bridle lines, through the block, and through the taut aft tow line (5R). The fore tow line 5F is taut and in front of the view section, not shown here, except for its sling about the fore tow sheave (33F).

[0008] Fig. 3A is an analogue to Fig. 1 and represents a smaller scale view which also includes the towing vessel (8). In an embodiment of the invention the paravanes are used to separate seismic sources laterally while the sources are towed by the towing vessel (8). Alternatively, in an embodiment of the invention the paravanes are used to separate seismic streamers laterally, wherein the streamers are towed by the towing vessel (8).

[0009] Fig. 3B illustrates in a simplified manner the two paravanes with their bridle blocks' (3, 3) position during operation. In this position the bridle block position is turned by keeping the fore tow lines (5F, 5F) taut, making the aft tow lines (5R, 5R) taut, and load distributed on both. The angle of attack on either sides PS or SB may be partly or entirely independently controlled by tow line winches 7F, 7F, 7R, 7R. A higher load on aft tow line (5R) gives an increased angle of attack.

[0010] Fig. 4A shows an embodiment of the invention wherein we use a moveable control (7P) on the ship and attached the fore towing line (5F).

[0011] Fig. 4B illustrates an aft alternative to the embodiment shown under Fig. 4A. In Fig. 4B is shown a moveable control 7P arranged on deck and attached the aft towing lines (5R, 5R) to control the angle of attack oppositely on either sides PS or SB.

[0012] Fig. 4C illustrates a combination of the embodiment shown in Fig. 4A with the embodiment shown in Fig. 3B. The fore towing lines (5F, 5F) are connected to the fore winches (7F, 7F) as before and the winches may run the lines (5F, 5F) in and out independently. Moreover in this embodiment the fore winches are arranged on a laterally moveable platform (7PL).

[0013] Fig. 5 illustrates a "linear" two-arm bridle block with coinciding fore towing and bridle axle, and coinciding aft towing and bridle axle. The two-arm bridle block is illustrated in a position during launch or hauling. There is a risk of interference and wear and tear of tow lines, and the two-arm bridle block may have stability issues when the bridle block is held in the position illustrated. Moreover, the range of the angle of attack may be span both too high and too low angles, giving overly high attacking force or negative angle of attack, depending on the bridle lines lengths.

[0014] Fig. 6 is an illustration of the applicant's own prior art with a bridle block with an additional "seventh bridle line".

Embodiments of the invention and advantages thereof

[0015] The invention will in the following be described and embodiments of the invention will be explained with reference to the accompanying drawings.

[0016] Fig. 1 illustrates a top view of a triangular bridle block (3) embodiment of the invention wherein there are a taut fore and a slack aft towing line (5F, 5R) connected to a fore and aft tow axle (31F, 31R), respectively. The towing lines (5F, 5R) are connected to a towing ship (8), please see Fig. 3A. This is a launching position explained below. Further connected to the bridle block (3) is a set of fore bridle lines (2F), here connected to a fore bridle axle (21F) which in this embodiment is the same as the fore tow axle (31F). Please also see Fig. 2B. A set of aft or rear bridle lines (2R) are connected to an aft bridle axle (21R).

Triangular axle layout

[0017] In the embodiment of the invention illustrated in Fig. 1 and in Fig. 3A, the bridle block (3) has a triangular pattern of combined fore tow axle and bridle axle (31F/21F) in front, the aft tow axle (31R) trailing behind, and the aft bridle axle (21R) placed towards the paravane (1) side relative to the tow axles (31F, 31R), when in the launch position with low angle of attack.

Low angle of attack

[0018 j A low angle of attack may be controlled by the use of the aft towing line (21 R), the force on the aft towing line and its degree of slackness may be adjusted. A low angle of attack may be used for launching the paravane in order to make the paravane with its connected towed equipment such as a seismic streamer array of two or more parallel streamer cables, or a seismic gun array with a lateral distribution. The aft bridle axle's (21R) position is directed towards the paravane ( 1) side relative to the tow axles (31F, 31R), away from the shipward direction, i.e. to the Port Side for a Port paravane, and to the Starboard Side for a Starboard paravane.

[0019]

Operative angle of attack

[0020] Fig. 2A is a top view of the same triangular bridle block (3) embodiment in an operative position with an operative angle of attack controlled by adjusting the extended lengths of the fore and aft towing lines (5F, 5R). This operative angle of attack is higher than the low angle of attack and is used during normal operation to maintain the desired lateral spreading force on the seismic streamer array or the gun array. Please also see Fig. 3B. Such an operative angle of attack may be maintained for several days and is continuously controlled depending on current direction, waves, vessel towing speed, sailing course, and load on the towed array. Here is shown the aft towing line (5R) tightened up so as to run more or less in parallel with the fore towing line (5F), resulting in an outward rotation of the bridle block (3) as indicated, relative to the ship (8). Thus the fore bridle line (2F) is extended outwardly relative to the aft bridle line (2R) which is pulled inwardly, resulting in a higher angle of attack, please also Fig. 3B. The angle of attack may thereafter be controlled by adjusting the relative lengths of the fore and aft towing lines (2F, 2R). Several embodiment of how this may be controlled is explained below.

[0021] Fig. 3B illustrates in a simplified manner the two paravanes with their bridle blocks' (3, 3) position during operation. In this position the bridle block position is turned by keeping the fore tow lines (5F, 5F) taut, making the aft tow lines (5R, 5R) taut, and load distributed on both. The angle of attack on either sides PS or SB may be partly or entirely independently controlled by tow line winches 7F, 7F, 7R, 7R. A higher load on aft tow line (5R) gives an increased angle of attack.

[0022]

Lift point considerations

[0023] The towing lines' resultant force will have a direction which projects onto an imagined projected intersection point on the deflector. This point defines an operative "lift" centre of the deflector, please see Fig. 1 and Fig. 2A. This force line projection intersection point must always be between the fore and aft bridle line connection points at the fore and aft end of the deflector, and the imagined projection intersection point will move back and forth depending on the angle of attack abbreviated "AoA". The projected intersection point will reside somewhere at the forward end of the deflector for AoA near maximum, and will move towards the aft end of the deflector for lower AoA.

[0024] The fore and aft bridle lines (2F, 2R) lengths must initially be adjusted so as for the rotation of the bridle block's (3) with its three axles (21F/31F, 31R, 21R) between its highest and lowest angle of rotation coincides with the highest and lowest angle of attack, respectively.

[0025] The invention may be summarized as follows:

The invention is paravane steering system comprising

- a starboard (SB) or port side (PS) paravane (1) with

- a set of fore and aft bridle lines (2F, 2R) connected to a bridle block (3),

- the bridle block (3) connected via a first towing line (5F) to a towing vessel (8)

- the bridle block (3) provided with

- a fore bridle axle (2 IF) for a fore bridle line (2F),

- an aft bridle axle (21R) trailing behind said fore bridle axis (21F), for an aft bridle line (2RB),

- a fore towing axle (31F) for a fore towing line (5F) coinciding with said fore bridle axle (21F),

- a rear towing axle (31R) arranged behind the fore towing axis (31F), for an aft towing line (5R),

- the fore and aft towing lines (5F, 5R) arranged for extending towards the towing vessel (8) generally in parallel during towing operation and adjustable in relative length for adjusting from the vessel (8) the angle of attack (AoA) of the paravane (1).

In this context, "behind" means trailing behind as counted in the course direction.

Controlling the paravane using winches

[0026] In an embodiment of the invention, the paravane steering system has at least one of said towing lines (5F, 5R) connected to one or more controlled winches (7F, 7R) , please see Fig. 3A and Fig. 3B, for adjusting the towing lines' (5F, 5R) relative extension during operation or launch or hauling of the paravane (1), said bridle block (3) thereby rotatable between a first angular position for maximum angle of attack (AoAmax) and a second angular position for minimum angle of attack (AoAmin).

[0027]

[0028] In an embodiment of the invention all three axles, please see Fig. 2B , namely the fore bridle axle / fore towing axle (21F/31F), the aft bridle axle (21R), and the aft towing axle (31R), are all being vertically oriented in the operative state of the bridle block (3).

[0029] Upper and lower triangle plates

[0030] In an embodiment of the invention the bridle block (3) comprises an upper and lower triangle plate (3U, 3L), please see Fig. 2B which is a vertical section through the kinked line A-A, wherein the upper and lower triangle plates (3U, 3L) are connected by the three above mentioned axles; namely the fore bridle axle / fore towing axle

(21F/31F), the aft bridle axle (21R), and the aft towing axle (31R).

[0031]

[0032] Bridle block arms

[0033] In an embodiment of the invention, the bridle block (3) upper and lower triangle plates (3U, 3L) are each provided with a combined fore bridle arm and towing arm (32F), please see Fig. 1A and Fig. IB with a bore through the plate holding the combined fore bridle axle and towing axle (21F/31F), an aft bridle arm (32R) with a bore holding the aft bridle axle (21R), and a relatively rearward extending towing arm (32R) with a bore holding the aft tow axle (31R). The three arms are externally shaped in a rounded fashion so as to reduce the drag when running in the sea, particularly for the operative state which shall usually be of very long duration. The bridle block should in principle be as small as possible as seen from a water friction perspective, but must have mechanical properties sufficient to take up the loads during launch, operation, collision with flotsam, and hauling.

[0034] Sheaves and terminal blocks.

[0035] In an embodiment of the invention, the fore tow sheave (33F) on the fore tow axle (31F/21F) for holding a swaged eye of the end of the fore tow line (5F) freely rotatabie about the fore tow axis (31Fx). Similarly for the aft tow line, there may be arranged an aft tow sheave (33A) on the aft tow axle (31R) for holding a swaged eye of the end of the aft tow line (5R) freely rotatabie about the aft tow axis (31Rx), particularly as seen in Fig. 2A wherein the aft towing line (5R) is illustrated as swung from a slack position with its swaged eye trailing obliquely behind the bridle block, to its taut position to a near parallel direction with the fore towing line (5F). The bridle lines fore and aft may be double or triple depending on the size and load on the paravane. There are two or three fore bridle lines (2F) connected to fore bridle line sheaves on horizontally directed axles in fore bridle line terminal block (2FF) attached via connector plates to the fore bridle axle (21F). Similarly there are two or three aft bridle lines (2R) connected to aft bridle line sheaves on horizontally directed axles in fore bridle line terminal block (2FR) attached via connector plates to the aft bridle axle (21R).

[0036] Please notice that in the illustrated embodiment of the invention the fore bridle line's swaged eye as shown in Figl, Fig. 2A and Fig. 2B is connected about the fore tow sheave (33F) on the fore tow axle / bridle axle (31F/21F). Further, both bridle block triangle plates (3U, 3L) envelope the sheave (33F). The fore bridle line terminal block (2FF) has its connection plates with bores for the bridle axle (21F/31F) arranged on the top of the upper triangle plate (3U), and on the bottom surface of the lower triangle plate (3L), respectively, please see Fig. 2B. With this design both the fore towing line (5F) and the fore bridle lines (2F) share the same axle (21F, 31F).

[0037] In an embodiment of the invention the aft towing line (5R) is arranged for being slackened during launching and retrieval operation for reducing the lateral force on the rear towing axle (31Rx) and to allow the rear towing axle (31R) to trail behind the fore towing axle (31Fx), so as for slacking the rear bridle line (2R) and tightening the fore bridle line (2F) to reduce the the angle of attack (AoA) of the paravane (1) to near a minimum angle of attack (AoAmin), please see Fig. 2A. In such a position with a low angle of attack the paravane may be hauled in without large force, and the width of the towed array will decrease in a controllable manner.

[0038] In an embodiment of the invention there may be a block (26R) arranged on the aft towing line (5R) with a link line (26L) to a spreader line or to the outermost of the towed array, please see Fig. 3A and Fig. 3B. Other ways of connecting a link line from the seismic array or gun array to the paravane may be used, such as a direct link from the link line (26L) to the bridle block (3).

[0039] Fig. 4a shows an embodiment of the invention wherein we use a moveable control (7P) on the ship and attached the fore towing line (5F, 5F) on either sides. The moveable control (7P) may be a laterally displaceable pulley which may be moved to the starboard direction for shortening the port towing line's (5F) extension and for extending the starboard towing line's extension, thereby reducing the angle of attack for the port paravane and increasing the angle of attack for the starboard paravane. Moving the control (7P) to the port side provides the opposite effect.

[0040] Fig. 4B illustrates an aft alternative to the embodiment shown under Fig. 4A. In Fig. 4B is shown a moveable control 7P arranged on deck and attached the aft towing lines (5R, 5R) to control the angle of attack oppositely on either sides PS or SB.

[0041] Fig. 4C illustrates a combination of the embodiment shown in Fig. 4A with the embodiment shown in Fig. 3B. The aft towing lines (5R, 5R) are connected to the aft winches (7R, 7R) as in Fig. 3B. The fore towing lines (5F, 5F) are connected to the fore winches (7F, 7F) as before and the winches may run the lines (5F, 5F) in and out independently. Moreover in this embodiment the fore winches are arranged on a laterally moveable platform (7PL) which moves the fore winches (7F, 7F) together a small distance to the port or starboard sides in order to fine control the angle of attack of the port or starboard paravanes. In this way the port angle of attack may be increased simultaneously with the starboard angle may be decreased with the same angular amount, e.g. if the ship is about to change course in the port direction. Oppositely the angle of attack for both may be adjusted in the starboard direction.

[0042] The invention presented herein has several advantages.

Balanced low angle of attack during launch

[0043] With reference to Fig. 1, during the launch procedure with the fore towing line (5F) taut and the aft towing line (5R) slackened, the bridle block will take up a position wherein there may be a more or less straight line through the fore towing line (5F) via the fore tow axle (31F) via the triangular bridle block (3) and further via the aft bridle axle (21R) to the aft bridle lines (2R). Reduced risk of towing line interference

[0044] In this position for the aft tow axle (31R) there will be no interference between the taut and the slack towing lines (5F, 5R); there will be no wear and tear between the towing lines in this position of the bridle block (3). If the aft towing axle and the aft bridle axle coincided, please see an imagined simplification of the bridle block in Fig. 3, the aft towing line would possible start out from a position very near the fore towing line when tightening the aft towing line.

Reduced risk of collapsing paravane launch

[0045] Further, with reference to Fig 1, in the same situation, with the triangular bridle block (3), it may take up the illustrated position of the fore bridle axle (21F) (which is coincident with the fore tow axle (31F)) with the most remote-oriented position for the aft bridle axle (21R) which will give the desired lowest angle of attack positon for the paravane, but not less. This will contribute to low lateral force during launch and thus a calm and controlled launching of the paravane. The slack, trailing aft towing line (5R) may stabilize this position and dampen out irregular rotation movement of the triangular bridle block. This will also prevent the angle of attack becoming negative if the aft towing line is slackened too much.

[0046] With reference to Fig. 1, it could be feasible to use an embodiment of the invention wherein the fore bridle axle (21F) and the fore tow axle (31F) are arranged on separate fore tow arm (32F) and fore bridle arm (22F), and not a common arm and being coinciding axles as illustrated in Fig. 1. However this could be considered as equivalent to the present invention.

[0047] In an embodiment of the invention it is possible to arrange the common bridle axle (21F) and tow axle (31F) in the aft end of the block, and have a fore tow arm (32R) in front, and a fore bridle axis (21R) on a separate fore bridle arm (32R), i.e. the mirror image of the bridle block of Fig. 1.