The invention relates to a bridle for a marine deflector, the bridle comprises a tow point for connecting to a towing cable, and bridle lines extending between the tow point and bridle couplings of the deflector.

Marine deflectors, also known as deflectors, paravanes, vanes, diverters, trawl door or simply doors, have one or more wings, foils or plates. Deflectors can be towed behind a vessel with an angle of attack relative to the towing direction, causing them to generate transverse hydrodynamic lift. When towing equipment behind a vessel, it may be desired to locate the equipment either on the starboard or port side of the track of the vessel, and a deflector can then be connected to the equipment for pulling the equipment to the side. It may also be desired to provide a transverse distance between different pieces of equipment which are towed behind the vessel, and deflectors may then be connected to the different pieces of equipment to pull them apart.

Seismic surveys at sea can be carried out by towing seismic streamers with sensors behind a vessel. The front ends of the streamers can be interconnected by a transverse front line. The ends of the front line can be connected, usually via spur lines, to deflectors which are towed behind the vessel on both sides of the vessel. The sea creates outwardly directed forces which pull the deflectors outwards. This pull is transferred to the front line, which thereby becomes tensioned and keeps the streamers at pre-set mutual distances.

US 7944774 B2 relates to a method for marine seismic surveying in which laterally spaced apart seismic streamers are towed in a body of water. The spacing of the streamers is achieved by deflectors. This document illustrates towing the streamers in a fan-shape, known as fanning, in which each streamer has a feather angle relative to a towing direction for the streamer. The fanning enables covering a larger surveying area, however, the coverage in each CDP point is a little different.

US 3840845 relates to a seismic data collecting method. Feather streamers are illustrated in fig. 1 and 2 and mentioned in column 3, line 32-36.

US 7080607 B2 relates to a method for controlling a streamer positioning device configured to be attached to a marine seismic streamer and towed by a seismic survey vessel and having a wing and a wing motor for changing the orientation of the wing. Column 10, line 30 of US 7080607 B2 describes how the inventive control system primarily will operate in two different control modes: a feather angle control mode and a turn control mode. In the feather angle control mode, the global control system attempts to keep each streamer in a straight line offset from the towing direction by a certain feather angle.

US 6691038 B2 describes a method and apparatus comprising an active control system for a towed seismic streamer array that enables any relative positional control of any number of towed seismic streamers. The streamer positions are controlled horizontally and vertically using active control units positioned within the seismic array. The three components (x, y, z) position of each streamer element, relative to the vessel and relative to each other is controlled, tracked and stored during a seismic data acquisition run. This described technical solution enables a seismic array to be maneuvered as the towing vessel maintains course, enables maintenance of specific array position and geometry in the presence of variable environmental factors and facilitates four-dimensional seismic data acquisition by sensing and storing the position of the array and each array element with respect to time.

It is thus a need for efficient devices to enable using of all said configurations of seismic equipment which is towed behind seismic vessels.

US 7881153 B2 relates to a paravane for a seismic acquisition system that includes a float, a frame suspended from the float, deflectors affixed to the frame, and means for coupling a tow rope to a lead-in functionally extending between a forward end and an aft end of the frame. The paravane includes means for selectively changing an effective position along the lead-in of the means for coupling the tow cable.

US 2016/0161622 Al relates to a segmented-foil diverter for marine towed

application, such as marine seismic exploration, having a plurality of longitudinally stackable foil segments with an internal conduit extending along the span of each segment to receive a cable passing therethrough. A branched bridle line is attached between the plurality of foil segments and a connection point which can be towed by a vessel.

US 2016/0115976 Al relates to an actuator assembly which may be used to control bridle lines of a bridle for a deflector. Fig. 1 of US 2016/0115976 Al illustrates a marine deflector and a bridle with a tow point for connecting to a towing cable, and three fore and three aft single bridle lines extending between the tow point and bridle couplings of the deflector.

In this patent application a fore bridle line shall mean a bridle line extending between the tow point and one or more fore bridle couplings of the deflector, and an aft bridle line shall mean a bridle line extending between the tow point and one or more aft bridle couplings of the deflector. A single bridle line shall mean a bridle line extending between the tow point and one bridle coupling of the deflector.

When towing a deflector of the kind illustrated in fig. 1 of US 2016/0115976 Al through the sea with an angle of attack relative to the towing direction, the sea pulls the deflector away from the tow point. This loads the bridle lines, and the bridle lines and the towing cable become tensioned, i.e. they get tension forces. To avoid structural damage to the deflector or overloading of the bridle lines, there is a need to share the loads, i.e. the tension forces, between the bridle lines. Due to the large number of bridle lines connected to the tow point, the tension forces in the bridle lines are, however, indeterminate, i.e. the tension forces are not dependent upon static equilibrium alone, but also on the lengths of the bridle lines and their elongation during loading. During assembly of the bridle, one therefore tries to adjust the bridle lines to lengths in which they are all taut when loaded, to share the load between them. This require a very accurate adjustment of the length of each bridle line, and experience shows that adjusting the lengths of the bridle lines to full load-sharing between them in practice is almost impossible. Further, the bridle lines are normally very stiff, made from Dyneema or Aramid fibre. The loading of the bridle lines therefore causes very little elongation of the most loaded bridle lines, and

consequently very little transfer of loading to the less loaded bridle lines. In other words, the stiff material of the bridle lines makes the load-sharing between the bridle lines during loading very small. In fact, it is quite common that some of the bridle lines are slack.

The purpose of the invention is to provide a bridle for a marine deflector in which the load-sharing between the bridle lines is improved relative to prior art. Further this load-sharing shall not be greatly influenced by inaccuracies in the lengths of the bridle lines. At least the invention shall provide an alternative to prior art. Features, advantages and purposes of the invention and how they are achieved will appear from the description, the drawings and the claims.

The invention thus relates to a bridle for a marine deflector, comprising a tow point for connecting to a towing cable, and bridle lines extending between the tow point and bridle couplings of the deflector. According to the invention at least one bridle line is Y-shaped with a node and three legs, with one leg extending between the node and the tow point, and the two other legs extending between the node and respective bridle couplings of the deflector. When towing the deflector through the sea, the legs of the Y-shaped bridle lines according to the invention are tensioned in the same way as single bridle lines of bridles according to prior art. For each of the Y-shaped bridle lines, the load of the leg extending between the node and the tow point is shared between the two other legs extending between the node and the bridle couplings of the deflector. The three legs are connected to the node, and pull the node to a place in which the node is in static equilibrium. This means that there is an equilibrium between the tension forces of the three legs, i.e. the size and direction of the tension force of each leg is related to the size and direction of the tension forces of the two other legs in a definite way. If inaccuracies in the lengths of the legs makes the legs get different lengths, the tension forces of the legs will pull and reposition the node to a different place in which the node is in static equilibrium. Assuming the inaccuracies are small, the

repositioning of the node is also small. The change in directions of the legs is therefore also small. The tension forces of the legs have the same directions as the legs, and the change in directions of the tension forces of the legs is therefore also small.

Consequently, the change in the sizes of the tension forces of the legs is small. The load-sharing between the legs is therefore not much influenced by inaccuracies in the lengths of the legs of the bridle line. The invention thus provides a self-adjusting bridle line which maintains load-sharing between the legs of the bridle line if their lengths are slightly changed.

For the same number of bridle couplings of the deflector, when using the Y-shaped bridle lines according to the invention, the number of bridle lines connected to the tow point is halved compared to when using single bridle lines according to prior art. The invention thereby reduces the number of bridle lines with indeterminate tension forces. The load-sharing between the bridle lines is thereby improved.

The bridle may have a combination of Y-shaped bridle lines according to the invention and single bridle lines according to prior art. It is, however, preferred that all bridle lines are Y-shaped bridle lines, since this gives a better load-sharing.

The bridle couplings may have various locations depending on the actual design of the deflector. As an example, for one Y-shaped bridle line, one of the bridle couplings may be a fore bridle coupling, i.e. located on a fore part of the deflector, and the other bridle coupling may be an aft bridle coupling, i.e. located on an aft part of the deflector. It is, however, preferred that for one Y-shaped bridle line, both bridle couplings are either fore bridle couplings or aft bridle couplings. This is advantageous since, for a normal design of a deflector, the distance between two bridle couplings located either on the fore side or the aft side of the deflector may be shorter than the distance between a fore and an aft bridle coupling. This means that for the same length of a Y-shaped bridle line, the angle of the Y will be smaller if both bridle couplings are either fore or aft bridle couplings. This in turn means that the tension forces in the legs of the bridle line extending between the node and the bridle couplings will be smaller.

In one embodiment, the bridle has one fore Y-shaped bridle line and one aft Y-shaped bridle line. The load on the bridle is shared between these two Y-shaped bridle lines only, i.e. there are only three lines connected to the tow point, namely the towing cable and the two bridle lines. The tow point is therefore in static equilibrium, i.e. inaccuracies in the lengths of the bridle lines have no influence on the tension forces in the bridle lines. This embodiment therefore provides a very good load-sharing.

In another embodiment, the bridle has two fore Y-shaped bridle lines. The load on the fore side of the bridle is then shared between these two fore Y-shaped bridle lines. In a further embodiment, the bridle has two aft Y-shaped bridle lines. The load on the aft side of the bridle is then shared between these two aft Y-shaped bridle lines.

In one embodiment, the bridle has a total of three bridle lines. As an example, there may be two fore bridle lines and one aft bridle line, all being Y-shaped bridle lines. This gives an asymmetric design of the bridle, which must be taken into consideration when designing the deflector.

In another embodiment, the bridle has both two fore Y-shaped bridle lines and two aft Y-shaped bridle lines. This gives a symmetric design of the bridle with a good load- sharing between fore and aft side of the bridle, and also between the bridle lines on fore and aft side of the bridle. It has been found that for inaccuracies that normally occur during assembly of the bridle, to ensure a good load-sharing between the legs of the bridle lines extending between the node and the bridle couplings of the deflector, the length of the leg extending between the node and the tow point preferably should be minimum 10%, more preferred minimum 20%, and most preferred minimum 30% of the length between the tow point and the deflector.

It has further been found that in order to ensure that the tension forces in the legs of the bridle lines extending between the node and the bridle couplings of the deflector is kept below an acceptable level, and to ensure that these tension forces do not subject the deflector to excessive compressive forces, the length of the leg extending between the node and the tow point preferably should be maximum 90%, more preferred maximum 80%, and most preferred maximum 70% of the length between the tow point and the deflector.

The invention will now be described with reference to the accompanying drawings, in which :

Fig. 1 is a perspective view of a marine deflector and a bridle according to the

invention;

Fig. 2 is a simplified side view of the marine deflector and bridle of fig. 1 ;

Fig. 3 is a simplified side view of another marine deflector and a bridle according to the invention;

Fig. 4 is a side view illustrating features of the bridle of fig. 3;

Fig. 5 is a side view illustrating other features of the bridle of fig. 3; and

Fig. 6 is a side view illustrating the features illustrated in fig. 5 in more detail.

Fig. 1 illustrates a marine deflector 2 and a bridle 1 according to the invention. The deflector 2 comprises vertical foils 33 which are supported by horizontal braces 32, and a float 31 which are fastened to the upper brace 32 by straps 36. The bridle 1 comprises a tow point 3 connected to a towing cable 4, and four bridle lines extending between the tow point 3 and bridle couplings of the deflector. Each bridle line is Y- shaped with a node and three legs, with one leg extending between the node and the tow point 3, and the two other legs extending between the node and respective bridle couplings of the deflector. More fully described, on the fore side, a top fore bridle line 51 is Y-shaped with a node 91 and three legs, a leg 51a extending between the node 91 and the tow point 3, a leg 51b extending between the node 91 and a top fore bridle coupling 71, and a leg 51c extending between the node 91 and an upper middle fore bridle coupling 72. Further, on the fore side, a bottom fore bridle line 52 is Y-shaped with a node 92 and three legs, a leg 52a extending between the node 92 and the tow point 3, a leg 52b extending between the node 92 and a lower middle fore bridle coupling 73, and a leg 52c extending between the node 92 and a bottom fore bridle coupling 74. On the aft side, a top aft bridle line 61 is Y-shaped with a node 93 and three legs, a leg 61a extending between the node 93 and the tow point 3, a leg 61b extending between the node 93 and a top aft bridle coupling 81, and a leg 61c extending between the node 93 and an upper middle aft bridle coupling 82. Further, on the aft side, a bottom aft bridle line 62 is Y-shaped with a node 94 and three legs, a leg 62a extending between the node 94 and the tow point 3, a leg 62b extending between the node 94 and a lower middle aft bridle coupling 83, and a leg 62c extending between the node 94 and a bottom aft bridle coupling 84.

Fig. 2 is a simplified side view of the fore side of the marine deflector 2 and bridle 1 of fig. 1. The top fore bridle line 51 comprises the node 91 and the three legs 51a-c, and the bottom fore bridle line 52 comprises the node 92 and the three legs 52a-c.

The deflector 2 and bridle 1 are towed through the sea by the towing cable 4 in a towing direction 12 with a longitudinal direction 34 of the deflector forming an angle u of attack with the towing direction 12, see fig. 1. The deflector thereby creates an outwardly directed hydrodynamic lift which pulls the bridle lines away from the tow point 3, which means that the towing cable 4 and the legs of the bridle lines become tensioned. For each bridle line, the node is in static equilibrium. Static equilibrium for the node means that there is an equilibrium between the tension forces of the three legs connected to the node, i.e. the size and direction of the tension force of each leg is related to the size and direction of the tension forces of the two other legs in a definite way. If the tension force of one leg is reduced, the tension forces of the two other legs will pull the node in their direction. This will increase the tension force in the first leg and reduce the tension forces in the two other legs. And conversely, if the tension force of one leg is increased, this leg will pull the node in its direction, which will reduce the tension force in this leg and increase the tension forces in the two other legs. This will go on for any unbalance between the forces, until the node again is in static equilibrium. The full lines of fig. 2 illustrate the shape of the bridle 1 with a particular length of the legs. The tension forces exerted by legs 51a-c on node 91 keep node 91 in static equilibrium, and the tension forces exerted by legs 52a-c on node 92 keep node 92 in static equilibrium. Further, the tension forces exerted on tow point 3 by towing cable 4 and legs 51a, 52a and not illustrated legs 61a, 62a of the aft bridle lines, keep tow point 3 in its place.

The dashed lines of fig. 2 illustrate the shape of the bridle 1 with slightly different lengths of the legs. Leg 51b is slightly shortened, while legs 51c and 52c are slightly lengthened. The variation in the lengths of the legs is due to inaccuracies. The tension forces exerted by the legs of the bridle lines on the nodes pull the nodes into new locations 91', 92'. Further, the tension forces exerted on tow point 3 by towing cable 4 and legs 51a, 52a and not illustrated legs 61a, 62a of the aft bridle lines, pull the tow point into new location 3'.

Another combination of small differences in the lengths of the legs of the bridle lines 51 and 52 would slightly alter the shape of the bridle 1 and the locations of the tow point 3 and the nodes 91 and 92. As long as the differences in lengths are not too large, the change in load-sharing between the legs of the bridle line will be small, which will be further discussed with reference to fig. 4.

Regarding load-sharing between the bridle lines, a single bridle line according to prior art is connected to one bridle coupling, while a Y-shaped bridle line according to the invention is connected to two bridle couplings. This means that, for the deflector of fig. 1 and 2 with eight bridle couplings, a bridle with single bridle lines according to prior art will have eight bridle lines, while a bridle with Y-shaped bridle lines according to the invention will have four bridle lines. Thus, the invention halves the number of bridle lines. As mentioned in the introductory part of the description, due to the large number of bridle lines connected to the tow point in a bridle according to prior art, the tension forces of the bridle lines are indeterminate, i.e. the tension forces are not dependent upon static equilibrium alone, but also on the lengths of the bridle lines and their elongation during loading. In fact, in a bridle according to prior art, it is not uncommon that some of these bridle lines virtually take no load or are slack. The reduction of the number of bridle lines connected to the tow point by means of the invention therefore reduces the number of bridle lines with indeterminate tension forces. The load-sharing between the bridle lines is thereby improved.

Fig. 3 is a simplified side view of the fore side of another marine deflector 102 with a float 131 and a bridle 101 according to the invention. The top fore bridle line 151 is a single bridle line as known from prior art, extending between the tow point 103 and the top fore bridle coupling 171. The bottom fore bridle line 152 is a Y-shaped bridle line according to the invention, with a node 192 and three legs, one leg 152a extending between the node 192 and the tow point 103, one leg 152b extending between the node 192 and a middle fore bridle coupling 172, and one leg 152c extending between the node 192 and a bottom fore bridle coupling 174.

The bridle of fig. 3 is loaded in the same way as the bridle of fig. 1 and 2, which means that the towing cable 104, single bridle line 151 and the legs 152a-c of Y- shaped bridle line 152 become tensioned. The full lines illustrate the shape of the bridle 101 with a particular length of the bridle line 151 and the legs 152a-c. The tension forces exerted by legs 152a-c on the node 192 keep node 192 in static equilibrium. Further, the tension forces exerted on tow point 103 by towing cable 104, bridle line 151, leg 152a and not illustrated aft bridle lines, keep tow point 103 in its place.

The dashed lines of fig. 3 illustrate the shape of the bridle 101 with a slightly longer leg 152c, caused by an inaccuracy in its length. The tension forces exerted by legs 152a-c on the node pull the node into new location 192'. Further, the tension forces exerted on tow point 103 by towing cable 104, bridle line 151, leg 152a and not illustrated aft bridle lines, pull the tow point into new location 103'.

Another combination of small differences in the lengths of bridle line 151 and the length of the legs of bridle line 152 would slightly alter the shape of the bridle 101 and the locations of the tow point 103 and the node 192. As long as the differences in lengths are not too large, the change in load-sharing will be small, which will be further discussed with reference to fig. 4.

Fig. 4 is a side view illustrating features of the bridle according to the invention. To simplify the explanation of these features, fig. 4 illustrates the same bridle as fig. 3, with only one Y-shaped bridle line 152 according to the invention. Further, to simplify, variation in length of single bridle line 151 and repositioning of tow point 103 is not taken into consideration. Y-shaped bridle line 152 is illustrated in full lines in a symmetrical Y-shape. Symmetrical Y-shape means that the angle vl between leg 152a and leg 152b is equal to the angle v2 between leg 152a and leg 152c. The tension force in leg 152b is then equal to the tension force in leg 152c. In other words, there is a good load-sharing between the legs 152b and 152c.

If inaccuracies in the lengths of the legs cause the shape of bridle line 152 to deviate from the symmetrical Y-shape, node 192 is repositioned, and the load-sharing will be more uneven. If node 192 is repositioned closer to a straight line between the tow point 103 and the middle fore bridle coupling 172, more load will be transferred through leg 152b and less load will be transferred through leg 152c. If node 192 is moved all the way to the straight line between the tow point 103 and bridle coupling 172, i.e. to position 192', all load will be transferred through leg 152b and no load will be transferred through leg 152c. In this position a lengthening of leg 152c will not cause any further repositioning of node 192, but instead cause leg 152c to become slack, as illustrated with a dashed line 152c'. Node 192' thus illustrates maximum possible repositioning of node 192 from the position in the symmetrical Y-shape. The same applies to a repositioning of node 192 closer to a straight line between the tow point 103 and the bottom fore bridle coupling 174. Node 192" thus illustrates maximum possible repositioning of node 192 from the position in the symmetrical Y- shape the other way. It is seen from fig. 4 that the closer node 192 is to tow point 103, the smaller inaccuracies in the lengths of the legs are required to bring node 192 to one of the straight lines between tow point 103 and one of the bridle couplings 172 or 174. In other words, the closer node 192 is to tow point 103, the more negative influence will inaccuracies have on the load-sharing between legs 152b and 152c. It has been found that for inaccuracies in the lengths of the legs that normally occur during assembly of the bridle, to ensure a good load-sharing between the legs of the bridle lines extending between the node and the bridle couplings of the deflector, the length LI of the leg extending between the node and the tow point preferably should be minimum 10% of the length L2 between the tow point and the deflector. More preferred, length LI should be minimum 20% of length L2, and most preferred length LI should be minimum 30% of length L2.

Fig. 5 is a side view illustrating the same bridle as fig. 4, but with focus on other features. The node of the Y-shaped bridle line 152 according to the invention is illustrated in two positions, a position 192 closest to the tow point 103, with the legs of the bridle line illustrated in full lines, and a position 192"' closest to the deflector 102, with the legs of the bridle line illustrated in dashed lines. Both node positions provide an almost symmetrical Y-shape of the bridle line 152. For both node positions, there is shown a force diagram of the forces acting on the node, i.e. the tension forces in the legs of bridle line 152, the node being in equilibrium. For node position 192, force Fl acts in leg 152a, force F2 acts in leg 152b, and force F3 acts in leg 152c. For node position 192"', force Fl'" acts in leg 152a'", force F2'" acts in leg 152b'", and force F3'" acts in leg 152c'".

The tension force in leg 152a depends on the tension force in the other bridle lines and the towing cable 104. Leg 152a'" has the same direction as leg 152a, and Fl'" is therefore equal to Fl .

Fig. 6 illustrates the forces acting on node 192 and 192"' in more detail. For node 192, the resultant FR of F2 and F3 is equal to Fl. For node 192"', the resultant FR'" of F2'" and F3'" is equal to Fl'". Since Fl'" is equal to Fl, FR'" is equal to FR. It is seen from fig. 6 that for node 192, F2 and F3 has almost the same size, and for node 192"', F2'" and F3'" has almost the same size. Thus, there is a good load-sharing between leg 152b and leg 152c for both node positions.

It is further seen from fig. 6 that F2'" is much larger than F2, and F3'" is much larger than F3. Thus, the size of the tension forces in leg 152b and leg 152c is much larger for node 192"' than for node 192. Further, F2'" and F3'" have much larger transverse components than F2 and F3, i.e. F2'" and F3'" subject the deflector 102 to larger compressive forces than F2 and F3.

It has been found that to ensure that the tension forces in the legs of the bridle lines extending between the node and the bridle couplings of the deflector is kept below an acceptable level, and to ensure that these tension forces do not subject the deflector to excessive compressive forces, the length LI of the leg extending between the node and the tow point preferably should be maximum 90% of the length L2 between the tow point and the deflector. More preferred, length LI should be maximum 80% of length L2, and most preferred length LI should be maximum 70% of length L2.

The tow point can be a ring or plate with holes for fastening the bridle lines and the towing cable, as known from prior art. The bridle couplings can be pad eyes or other couplings known from prior art. The bridle couplings are not part of the invention.

The bridle lines can be ropes or straps, like bridle lines according to prior art. Typical material for the bridle lines are Dyneema or Aramid fibre. The bridle lines can be fastened to the tow point and the bridle couplings by knots or in other ways, e.g. by shackles.

The node may be formed by a knot. Alternatively, the node may be formed by a ring or plate with holes or other means which are suitable for fastening the legs of the bridle line. The node may have means for manually adjusting the length of the legs. Such means may be a plate with several holes for fastening the legs of the bridle line, and the adjustment of a leg can be done by moving the leg to another hole.

The two legs extending between the node and the bridle couplings may be formed by one line, extending from one of the bridle couplings to the node, through the node, and to the other bridle coupling. This provides an easy way of fastening these two legs to the node, with no risk for loosening.