Fairing, elongate faired element and towing assembly

Abstract

A fairing intended to fair an elongate object intended to be at least partially immersed, the fairing comprises a plurality of fairing portions, each fairing portion comprising a plurality of fairing elements, the fairing elements comprising a canal intended to accept the elongate object and being profiled in such a way as to reduce the hydrodynamic drag of the at least partially immersed elongate object, the fairing elements being intended to be pivot-mounted on the elongate element around the longitudinal axis of the canal, the fairing elements being joined together along the axis of the canal and articulated to one another, the portions of fairing rotating freely about the canal relative to one another.

Claims

1. A fairing intended to fair an elongate object intended to be at least partially immersed, comprising a plurality of fairing portions, each fairing portion comprising a plurality of fairing elements, the fairing elements comprising a canal intended to accept the elongate object and being profiled in such a way as to reduce the hydrodynamic drag of the at least partially immersed elongate object, said fairing elements being intended to be pivot-mounted on the elongate element around the longitudinal axis of the canal, said fairing elements being connected to one another along the axis of the canal and being articulated to one another, the fairing portions being free to rotate with respect to one another about the canal; and wherein the fairing elements of one and the same fairing portion are joined together by means of a plurality of individual coupling devices, each individual coupling device allowing one of the fairing elements of said portion to be connected to another fairing element of said portion which is adjacent to said fairing element.

2. The fairing as claimed in claim 1, in which the fairing portions have respective heights along the axis of the canal, these heights being defined as a function of the angular stiffnesses k of the respective fairing portions, and as a function of the chord length LC of said fairing elements of said respective portions so as to prevent a full twist from forming on said respective portions.

3. The fairing as claimed in claim 2, in which the fairing portions have respective heights that are less than a predetermined maximum height hmax such that: $\mathrm{hmax}\frac{*k}{{\mathrm{FLC}}^2}$ where F is a constant comprised between 250 and 500.

4. The fairing as claimed in claim 1, in which at least one fairing portion has a height along the axis of the canal, which height is defined as a function of the angular stiffness k of said fairing portion, and as a function of the chord length LC of said fairing elements of said portion so as to prevent an airborne full twist from forming on said fairing portion when the fairing portion is subjected to a torque below or equal to a predetermined torque.

5. The fairing as claimed in claim 1, in which at least one portion has a height along the axis of the canal, which height is defined as a function of the angular stiffness k of said fairing portion, as a function of the chord length LC of said fairing elements of said portion, so that the portion is able to undergo a full twist and so as to prevent an airborne full twist from forming on said fairing portion when the fairing portion is subjected to a torque below or equal to a predetermined torque.

6. The fairing as claimed in claim 1, in which at least one portion of said portions comprises at least one end fairing element, adjacent to one single other fairing element belonging to said portion, being a mitered fairing element, the mitered fairing element being configured so that it has a bearing edge comprising a first bearing edge which is mitered with respect to the leading edge, the first bearing edge being arranged in such a way that the distance between the leading edge and the first bearing edge, considered perpendicular to the leading edge, decreases continuously, along an axis parallel to the leading edge, from a first end of the first bearing edge to a second end of the first bearing edge, further away from another fairing element adjacent to the mitered fairing element and belonging to the same portion as the mitered fairing element than the first end, along the axis parallel to the leading edge.

7. The fairing as claimed in claim 6, in which each end fairing element is an end fairing element.

8. The fairing as claimed in claim 6, in which the end fairing element is sized in such a way as to be more resistant to a pressure loading, applied in a direction perpendicular, to the leading edge and connecting the leading edge to the trailing edge, than the other fairing elements of the portion.

9. The fairing as claimed in claim 6, in which the end fairing element comprises two parts back to back along the first bearing edge, the end fairing element being configured to be kept in a deployed configuration when subjected to the hydrodynamic flow of the water, the two parts being arranged, relative to one another about the first bearing edge, in such a way that the end fairing element has a trailing edge parallel to the leading edge and a cross section that is constant along the leading edge, and configured in such a way as to allow relative pivoting between the two parts about the first bearing edge when a torque inducing relative pivoting between the two parts, applied about an axis formed by the first bearing edge, exceeds a predetermined threshold so that the end fairing element passes from the deployed configuration into a configuration folded about the bearing edge.

10. The fairing as claimed in claim 6, in which the bearing edge is the trailing edge.

11. A faired elongate element intended to be at least partially immersed, comprising an elongate element faired by means of the fairing as claimed in claim 1, the elongate element being received in the canal, said fairing elements being pivot-mounted on the elongate element about the longitudinal axis of the canal and being translationally immobilized with respect to the elongate element along the axis of the elongate element.

12. A towing assembly comprising a faired elongate element as claimed in claim 11 and a towing and handling device intended to tow the faired elongate element while the faired elongated element is partially immersed, the towing device comprising a winch allowing the faired elongate element to be wound in and paid out through a guide device that allows the elongate element to be guided.

13. The towing assembly as claimed in claim 12, in which the guide device is configured in such a way as to make it possible to modify the orientation of a fairing element of the fairing with respect to the guide device by rotation of the fairing element about the axis of the elongate element under the effect of the traction of the elongate element with respect to the guide device, when the fairing element exhibits an orientation in which it is bearing on the guide device and in which the line of action of the force applied by the elongate element to the guide device comprising a pulley extending substantially along an axis extending from the axis of the elongate element as far as the trailing edge.

14. The towing assembly as claimed in claim 13, in which the guide device comprising a first groove the bottom of which is formed by the bottom of the groove of a pulley, the first groove being delimited by a first surface having a profile that is concave in a radial plane of the pulley, the width of the first groove and the curvature of the profile of the first curved surface in the radial plane being determined so as to allow the fairing element to be flipped by the rotating of the fairing element about the axis of the elongate element x under the effect of the traction of the elongate element with respect to the guide device along the longitudinal axis thereof, from a turned-over position in which the fairing element is oriented with its trailing edge toward the bottom of the first groove into an acceptable position in which it is oriented with the leading edge toward the bottom of the first groove.

15. The towing assembly as claimed in claim 13, in which the fairing elements comprise a fairing element comprising a nose accepting the elongate element and comprising a leading edge a tail of streamlined shape extending from the nose and comprising a trailing edge, the first curved surface forming a first concave curve in the radial plane of the pulley, the concave first curve being defined in a radial plane of the pulley such that, when the fairing element extends with the leading edge perpendicular to the radial plane, whatever the position of a fairing element in the first groove, when the nose of the fairing element is bearing on the concave first curve and the elongate element is exerting on the fairing element, in the radial plane, a force to press the nose of the fairing element against the pulley, said pressing force Fp comprising a component CP perpendicular to the axis of the pulley and a lateral component CL, the trailing edge of the fairing element is not in contact with the concave first curve or is in contact with a part of the concave first curve that forms, with a straight line dp of the radial plane perpendicular to the axis xa extending from the axis of the elongate element x as far as the trailing edge of the fairing element, an angle that is at least equal to an angle of slip t, The angle of slip is given by the following formula:
t=Arctan(Cf) where Cf is the coefficient of friction between the material that forms the exterior part of the tail of the fairing element and the material that forms the surface delimiting the groove of the pulley.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages of the invention will become apparent on reading the detailed description which follows, given by way of non-limiting example and with reference to the appended drawings in which:

(2) FIG. 1A, already described, depicts a faired cable, faired by means of rigid fairing elements joined axially together, towed partially immersed from its immersed part as far as a guide pulley in a situation in which the cable does not experience a double twist, FIG. 1B depicts the cable of FIG. 1A in the same state of immersion (namely of winding-in and of paying-out) as in FIG. 1A, but experiencing a double twist; FIG. 1C depicts the cable of FIG. 1A with the double twist of FIG. 1B in a configuration in which the cable has been paid out in relation to FIG. 1B; FIG. 1D depicts the cable of FIG. 1A exhibiting the double twist of FIG. 1B in a configuration in which the cable has been hauled in in relation to FIG. 1B,

(3) FIG. 2 schematically depicts a ship towing a towed object by means of a faired cable,

(4) FIG. 3 schematically depicts a portion of faired cable according to the invention faired using a fairing according to the invention,

(5) FIG. 4a depicts a cross section of a fairing element of the fairing according to the invention on the plane of section AA depicted in FIG. 2, FIG. 4b schematically depicts a side view of the fairing element of FIG. 4a in the direction of the arrow b,

(6) FIG. 5 schematically depicts a portion of faired cable according to the invention entering a cable guide pulley,

(7) FIGS. 6a and 6b depict cross sections of a pulley according to the prior art, on the lateral face of the fairing element entering with the trailing edge toward the bottom of the groove, at the moment at which it comes to bear against the pulley (FIG. 6a) and then afterwards when the cable has been pulled to the right in FIG. 5 (FIG. 6b), namely when the cable has been hauled in and its tension has crushed the fairing element),

(8) FIG. 7 depicts a partial cross section on a radial plane BB (see FIG. 5) of one example of a pulley according to a first embodiment embodiment of the invention, and a reference curve,

(9) FIG. 8a schematically depicts a section of a pulley, according to a second embodiment of the invention, in a plane formed by a lateral face of the first fairing element coming into contact with the pulley (equivalent to the plane M in FIG. 5), comprising the point of contact with the pulley, FIGS. 8b and 8c depict sections of the pulley on planes successively occupied by the same lateral face of the fairing element as the cable is wound in,

(10) FIGS. 9a and 9b depict sections, on radial planes, of two examples of pulleys according to a third embodiment,

(11) FIG. 10 schematically depicts, in a plane BB, lower and upper curves of a first bathtub curve,

(12) FIGS. 11a to 11c depict, in successive planes parallel to the plane M, cross sections of the pulley and the orientations successively adopted by the lateral face of the reference fairing element as the cable is wound in, the fairing element arriving at the pulley of FIG. 7 upside down,

(13) FIGS. 12a to 12c schematically depict in side view, a fairing element according to a first embodiment of the invention and a portion of fairing comprising a fairing element according to the invention entering a pulley, in perspective (12a), in side view as it enters the pulley (FIG. 12b), and viewed in section on the plane M visible in FIG. 12a, and viewed in section on the plane Q visible in FIG. 12d,

(14) FIG. 13 schematically depicts an example of a fairing element according to a second embodiment of the invention,

(15) FIG. 14 depicts, in a radial plane of the pulley, a portion of a concave first curve complying with an advantageous feature of the invention,

(16) FIG. 15 depicts a circle constructed with respect to a fairing element and satisfying the advantageous feature of the invention.

(17) From one figure to another, the same elements bear the same references.

DETAILED DESCRIPTION

(18) The invention relates to a fairing intended to cover an elongate object, for example a flexible object such as a cable, or a rigid object such as an offshore drill string, intended to be at least partially immersed. The elongate element is conventionally intended to be towed by a floating vessel. The fairing is intended to reduce the forces generated by the current on this elongate element when it is immersed in the water and towed through the water by a naval vessel.

(19) Another subject of the invention is a towing assembly as depicted in FIG. 2, comprising an elongate element 1 faired by means of a fairing according to the invention. In the continuation of the text, the invention will be described in the case where the elongate element is a cable, but it does apply to other types of flexible elongate element.

(20) The cable 1 tows a towed body 101, for example comprising one or more sonar antennas. The towed body 101 is mechanically anchored to the cable 1 in an appropriate manner. The towed body 101 is put into and removed from the water by means of a winch 5 arranged on a deck 103 of the ship 100.

(21) The towing assembly according to the invention also comprises a device for towing and handling the faired cable, comprising:

(22) a winch 5 for winding in and paying out the faired cable 1,

(23) a guide device 4 for guiding the cable 1, the guide device being positioned downstream of the winch when viewed from the end 6 of the cable 1 which is intended to be immersed. In other words, the cable 1 is wound in around the winch 5 (or paid out by means of the winch) through the guide device 4.

(24) The guide device 4 is advantageously mounted on a bearing structure 7 intended to be fixed to the ship and which may or may not be capable of pivoting.

(25) The guide device provides guidance for the cable 1, which means to say is able to limit the lateral deviation of the cable with respect to the winch in a direction parallel to the axis of rotation of the drum of the winch. It is also advantageously configured to modify the direction of the cable between its end 6 intended to be immersed and the winch 5 in a plane substantially perpendicular to the axis of the winch while at the same time making it possible to safeguard the radius of curvature of the cable so that it does not drop below a certain threshold in this plane.

(26) In the nonlimiting example depicted in FIG. 3, the guide device is a pulley 4. The guide device may further comprise, amongst other things, a fairlead to safeguard the radius of the cable and/or a reeling device so that the cable can be stowed correctly on the drum and/or at least one deflector forming a surface that makes it possible to alter the orientation of a fairing element with respect to the deflector by rotation of the fairing element about the axis of the cable under the effect of the traction of the cable as it is being wound in/paid out. The latter function may be performed by a pulley.

(27) FIG. 3 schematically depicts a portion of cable 1 covered with a fairing 11 according to the invention. This fairing 11 comprises a plurality of fairing portions 12a, 12b. Each fairing portion 12a, 12b, comprises a plurality of fairing elements 13, 13a. FIG. 3 depicts two fairing portions 12a, 12b, each comprising 5 fairing elements of the fairing but, in practice, the fairing may comprise far more fairing portions comprising far more fairing elements.

(28) The fairing elements are advantageously rigid. What is meant, in the present patent application, by fairing elements that are rigid is that the fairing elements are configured in such a way that they do not deform substantially under the effect of the hydrodynamic stream when immersed and towed in the direction of the leading edge. In other words, the fairing elements maintain substantially the same shape when subjected to the hydrodynamic stream. The fairing elements may potentially deform under the effect of forces stronger than those developed by the hydrodynamic stream. They are, for example, made of hard plastics material such as, for example, polyethylene terephthalate (PET) or polyoxymethylene (POM).

(29) Each fairing element 13, 13a has a hydrodynamic profile, of the kind depicted in FIG. 4a, in a plane AA perpendicular to the axis x of the cable (or axis of the canal 16). In other words, each fairing element 13, 13a is profiled in such a way as to reduce the hydrodynamic drag of the cable 1 when the cable 1 is being towed. The fairing elements 13a are fairing elements exhibiting the same features as the fairing elements 13 but able to differ from the fairing elements 13 in terms of the features explained hereinafter because of their position in the portions 12a, 12b. Each fairing element 13 comprises a wide nose 14 intended to accept the cable 1 and a tail 15 of streamlined shape extending from the nose 14. The nose 14 houses a canal 16 of axis perpendicular to the plane of the sheet, intended to accept the cable 1. The nose 14 comprises the leading edge BA and the tail 15 comprises the trailing edge BF which are the endmost points of the fairing element 13 in the plane of section. The fairing element 13 more particularly in this plane has a wing-shaped profile. The profile of the fairing element allows a less turbulent flow of water around the cable. The hydrodynamic profile exhibits, for example, a teardrop shape or an NACA profile which is a profile defined by the National Advisory Committee for Aeronautics, NACA.

(30) FIG. 4b depicts a view of the fairing element in the direction of arrow B, which is the same view as in FIG. 3. The fairing element has a shape that is elongate from the leading edge BA to the trailing edge BF. In side view, the fairing element 13 has a substantially rectangular shape delimited by the trailing edge BF and the leading edge BA which are parallel to the axis xc of the canal 16 and connected by two lateral faces 17, 18. The lateral faces 17, 18 extend substantially perpendicular to the trailing edge BA. The lateral faces are arranged at the respective ends of the canal 16.

(31) In FIG. 4a, the chord length of the fairing element 13, which has been referenced LC, is the maximum length of the straight-line segment referred to as chord CO connecting the trailing edge BF and the leading edge BA of the fairing element 13 in a direction perpendicular to the axis of the canal xc. In other words, the chord is the straight-line segment connecting the endmost points of a section of the fairing element. The maximum thickness E of the fairing element is the maximum distance separating the first longitudinal face 22 from the second longitudinal face 23 in a direction perpendicular to the chord CO in the plane of section of the fairing element. In the embodiment of FIG. 4b, the distance separating the trailing edge and the leading edge is constant along the axis of the canal xc parallel to the leading edge BA. This distance is the chord length. The longitudinal faces 22 and 23 run parallel to the leading edge BA.

(32) The fairing elements 13 are intended to be mounted on the cable 1 in such a way as to be able to pivot about the longitudinal axis of the cable 1, namely about the longitudinal axis of the canal 16.

(33) The fairing elements 13 belonging to one and the same portion of fairing 12a or 12b are joined together by means of a coupling device 20 that allows relative rotation of said fairing elements 13 with respect to one another about the cable 1. The coupling device 20 joins the fairing elements together both axially, namely along the towing cable, and also in terms of rotation about the cable 1. The coupling device 20 allows relative rotation of the fairing elements with respect to one another about the axis of the cable, namely of the canal 16. This excursion is permitted either freely with a stop. The rotation of one fairing element about the cable therefore does not cause the adjacent fairing element to turn. The excursion may be achieved in a constrained manner, with more or less strong return toward the aligned position (position of no relative rotation of the fairing elements relative to one another about the cable). In the latter instance, rotation of one fairing element about the cable causes the adjacent fairing elements of the same portion to rotate about the cable. Advantageously, the clearance between adjacent fairing elements is near zero, which means that any relative rotation of the fairing elements leads to elastic deformation of the coupling device. That allows the fairing elements of one and the same portion to adopt an orientation with respect to the cable that allows it to offer the least resistance to the current brought about by the movement of the cable through the water. The coupling device allows this relative rotation with a maximum amplitude, namely a maximum angular excursion. Thus, the rotation of one fairing element causes the neighboring fairing elements and, through a knock-on effect, all of the fairing elements of the same portion 12a or 12b to rotate. As the cable is raised, all the fairing elements of one and the same portion adopt one and the same orientation relative to the drum thereby allowing the cable to be wound in keeping the scales parallel to one another turn by turn.

(34) Advantageously, the coupling device 20 allows the relative rotation of the fairing elements with respect to one another in such a way as to allow the cable to be wound around a winch, the lateral excursion of the cable being caused, for example, by changes in heading of the ship. The coupling device allows these movements of relative rotation of these fairing elements with respect to one another with maximum respective angular excursions.

(35) The coupling device 20 depicted in FIG. 3 comprises a plurality of individual coupling devices 19 comprising, for example, a fishplate, each allowing a fairing element to be connected to a fairing element adjacent to said fairing element, which means to say allowing the fairing elements of one and the same portion to be coupled one to the next. In other words, each individual coupling device allows a fairing element to be connected to another fairing element adjacent to said fairing element only. The adjacent fairing elements form pairs of fairing elements. The fairing elements of the respective pairs of fairing elements of one and the same portion of fairing are connected by means of distinct individual coupling devices. The coupling device thus allows each fairing element of a portion of fairing to be connected individually to each of its adjacent fairing elements. Advantageously, the individual coupling devices are configured in such a way as to deform elastically upon relative rotation of the fairing elements around the cable. This refers to a twisting of the individual coupling devices.

(36) When there is torsion in a portion of fairing, there is deformation in the portion of fairing. This deformation is obtained through elastic deformation of the coupling device 20 and/or of the fairing elements so that the portion of the fairing opposes the torsion because of its torsional stiffness. In other words, the fairing applies a return torque in the opposite direction to the torsional torque applied to the fairing in order to generate the twist. These elastic deformations are torsions. When the fairing comprises individual coupling devices 19, the individual coupling devices 19 deform elastically as the fairing is twisted. Conventionally, the fairing elements have a stiffness such that they too deform elastically when the fairing is twisted. These elastic deformations are torsions.

(37) Advantageously, the fairing elements 13 are immobilized translationally with respect to the cable 1 along the axis of the cable x. That makes it possible to prevent the fairing elements 13 from becoming squashed together or spread out along the cable 1, either of which could have the effect of causing the fairing 11 to jam during the winding-up of the faired cable around the drum of the winch 5 or even when passing through the guide device 4. For this purpose, each portion of fairing 12a, 12b comprises an immobilizing device 21 collaborating with a fairing element 13a of said portion 12a, 12b and intended to collaborate with the cable 1 so as to immobilize the fairing element 13a translationally along the axis of the cable. According to the embodiment of FIG. 3, the fairing element 13a is the fairing element furthest from the end 6 intended to be submerged situated in the direction of the arrow f (referred to as the head-end fairing element) Because the fairing elements are joined together, the immobilization achieved by the immobilizing device on one fairing element 13a has a knock-on effect on the other fairing elements of the same portion. There is no need to install one immobilizing device per fairing element, and this makes it possible to limit costs and fitting time as well as limiting the weight of the faired cable. As an alternative, the portion comprises several immobilizing devices each one collaborating with one fairing element of the portion. The immobilizing device for example comprises a ring 21 fixed to the cable by crimping and collaborating with the fairing element 13a so as to immobilize it translationally with respect to the cable along the axis x of the cable 1.

(38) According to the invention, the fairing portions 12a, 12b are free to rotate relative to one another about the axis of the canal 16, namely about the axis of the cable 1 when they are mounted on the cable 1. In other words, the fairing elements 13 belonging to two distinct portions of fairing 12a, 12b are free to rotate relative to one another about the axis of the canal, namely about the cable 1. Each portion 12a, 12b is relatively flexible in terms of rotation about the cable even if a certain torsional stiffness is observed. This flexibility only amplifies with deployed length. For this reason, breaking the fairing down into fairing portions which are free to rotate relative to one another makes it possible to limit the risks of the formation of double twists and therefore to limit the risk of damage to the fairing, because the twists in the portions of fairing are not transmitted from one portion to another. The fairing may be installed all along the cable. In other words, the fairing extends over the entire length of the cable. As an alternative, the fairing extends along the cable over a length less than the length of the cable.

(39) The fairing is intended to fair an elongate element. It is also intended to be towed by means of a towing device as described in the present patent application.

(40) The heights h of the respective fairing portions, namely their lengths along the axis x of the cable, are less than a maximum height hmax. As an alternative, at least one of the portions has a height less than this maximum height hmax. In FIG. 3, the two portions have the same length, but this is not compulsory. The maximum height hmax is chosen to be small enough to prevent the formation of a complete airborne twist in the portion, for example of a complete twist in the portion. The affected portion may make a complete turn on itself and realign in the stream, and because it is uncoupled from its neighbors, this portion no longer disturbs them and there is no longer any airborne torsion or immersed torsion. This configuration makes it possible to prevent old immersed full twists from entering the guide device and therefore limits the risks of damage to the fairing. Moreover, this configuration makes it possible to avoid the need to set in place a monitoring procedure performed by the crew, or by a monitoring device, aimed at detecting immersed twists, and a mechanical or manual procedure aimed at reabsorbing a detected double twist or aimed at helping an immersed remnant twist coming out of the water to enter the guide device without causing damage.

(41) Advantageously, the height of at least one portion and, for preference, of each portion, is defined in such a way as to prevent the formation of an airborne full twist of said portion of fairing when the fairing or the elongate element faired by means of the fairing, is towed under predetermined nominal conditions of towing of the fairing, the fairing portion being partially immersed. The airborne twist is the twist experienced by the airborne, which means to say non-immersed, part of the fairing portion.

(42) The nominal towing conditions are defined by a nominal sea state, a nominal speed at which the cable is intended to be towed, namely the nominal speed of the ship, and the height at which the tow point of the fairing with respect to sea-level is intended to lie. The nominal sea state, the nominal speed and the height of the tow point may be predetermined or comprised within predetermined respective nominal ranges. When the fairing is towed in such a way that the fairing portion is partially immersed under nominal conditions, the fairing portion is subjected to a torque which is less than or equal to a predetermined maximum torque. This maximum torque is defined by the nominal conditions. The predetermined maximum torque may be obtained by calculation or empirically by measuring the torque exerted by the fairing portion under nominal conditions.

(43) The maximum height of the fairing portion is defined in such a way as to avoid the formation of an airborne full twist on the partially immersed fairing portion when the fairing portion is subjected to a torque less than or equal to the predetermined maximum torque.

(44) The height of the fairing is determined empirically by varying the length of the fairing portion under the most constraining nominal towing conditions that would generate the maximum torque so as to obtain a height such that it is possible to avoid an airborne full twist of the fairing portion. It may also be determined by simulation, by modeling the behavior of the fairing portion in the most constraining nominal conditions and by varying the height of the portion until the desired effect is obtained.

(45) Therefore, when the fairing portion is towed under the nominal conditions and partially immersed, the airborne part of the fairing is subjected to a torque caused by the waves. If this torque is less than or equal to the maximum torque, it will experience twist but the forces applied at the level of the guide device and in the immersed part are balanced so that the fairing will effect a full twist on itself about the elongate element (or about the canal) before the airborne part thereof experiences a full twist. Therefore, the appearance of an airborne full twist and, therefore, the appearance of a double twist, is avoided.

(46) In one preferred embodiment, the height of at least one portion and, for preference, of each portion, is chosen in such a way that said portion can experience a full twist. The height of this portion is therefore great enough to allow this twist. However, this height is also chosen, as previously, in such a way as to prevent the formation of an airborne full twist on said portion of fairing when the fairing or the elongate element faired by means of the fairing, is towed under predetermined nominal conditions of towing on the fairing, the fairing portion being partially immersed. In other words, the height of the portion is small enough that, when the fairing (or the cable is faired) is towed, partially immersed, and is subjected to a maximum torque, it cannot experience airborne twist. By contrast, it can experience full twist if subjected to a torque greater than the maximum torque.

(47) The height of the portion is defined as a function of the angular torsional stiffness k of said fairing portion, as a function of the chord length LC of said fairing elements of said portion, and as a function of the nominal towing conditions.

(48) A portion of fairing T experiencing a twist by an angle about the axis x of a cable (or of the canal 16) is subjected to a torque C applied about the axis x of the cable 1. The torque C that makes it possible to obtain this torsion angle is given by the following formula:

(49) $C=\frac{k}{h}$

(50) where k is the angular torsional stiffness of the portion of fairing about the axis of the cable (or of the canal) expressed in Nm.sup.2/radian, h is the height of the portion of fairing, namely the length of the portion of fairing along the axis of the cable or the longitudinal axis of the leading edge.

(51) The maximum height hmax is dependent on the stiffness of the portions of fairing. The higher the stiffness of the portions of fairing about the axis of the cable, the greater the height they may have. The longer the chord of the fairing, the more affected the portion of fairing will be by the influences of the sea and the towing conditions, and the lower the maximum height of the portions of fairing will be. The torsional disturbances generated by the influences of the sea and the towing conditions are proportional to the surface area of the fairing elements of the portion (and therefore to the chord length) and to the lever arm (and therefore to the chord length of the fairing). The maximum height hmax is therefore given by the following formula:

(52) $\mathrm{hmax}\frac{*k}{{\mathrm{FLC}}^2}$
where F is a constant calculated according to a configuration identified as being the most influential and which takes account of the flow and reflow of the wake and LC is the chord length of the fairing elements of the portion of fairing.

(53) The constant F is comprised between 250 and 500. F is dependent on the maximum speed at which the cable is to be towed. If the cable is to be towed at a speed of 20 knots, F is fixed at 400. F is lower if the maximum speed decreases.

(54) Typically, for fairings with an angular torsional stiffness k of the order of 4 to 5 Nm.sup.2/rad, and a chord length LC of 0.125 m, the maximum height is of the order of 2 m if the constant is fixed at 400.

(55) The fairing according to the invention offers advantages even when there is no desire to wind the cable around a winch. Specifically, the fact that the fairing according to the invention minimizes the risks of the formation of double twists means that the risks of fairing damage associated with the aging of the immersed twists can be limited without these entering a guide device. The fairing according to the invention therefore limits the requirements in terms of cable maintenance.

(56) Advantageously, the guide device of the towing assembly according to the invention is configured in such a way as to make it possible to modify the orientation of a fairing element of the fairing with respect to the guide device by rotation of the fairing element about the axis of the cable under the effect of the traction of the cable with respect to the guide device (along the axis of the cable), when the fairing element exhibits an orientation in which it is bearing on the guide device and in which the line of action of the force applied by the cable to the guide device extends substantially in the direction extending from the axis of the cable as far as the trailing edge of the fairing element.

(57) Advantageously, the guide device is configured to turn a fairing element round from a turned-round position in which it is oriented tail down into an acceptable position in which it is oriented tail up. The orientations up and down are defined with respect to a vertical axis associated with the winch.

(58) These configurations facilitate the winding of the faired cable onto the winch. Specifically, when it is desired to wind the cable around the drum of the winch, the first fairing element of each portion to leave the water rises up toward the guide device and, not being connected to the fairing elements of the preceding portion, turns over trailing edge downmost under the effect of gravity, taking with it the next fairing elements of that same portion of fairing. If the guide device does not allow such a turning-over, the fairing elements will arrive on the drum of the winch incorrectly oriented (it is preferable for the fairing elements to be wound up with their trailing edges uppermost in order to avoid damage to the fairing because the leading edge is stronger).

(59) To this end, the guide device comprises a guide or a set of guides that allows the fairing element to be flipped or its orientation changed. This guide or set of guides may for example comprise a pulley and/or deflector or any other device allowing the orientation of the fairing elements about the axis of the cable to be altered. One nonlimiting example of this type is described in the French patent application published under the number FR2923452. These devices are conventionally arranged upstream or downstream of the pulley as seen from the winch. They are conventionally concave, which means to say of the type having a groove, so as to define a housing intended to accept the fairing element in order to flip it. These guides may be able to follow the cable if the cable deviates laterally parallel to the axis of the pulley (or of the winch), for example by being mounted with the ability to pivot about a substantially vertical axis.

(60) Hitherto, all towing pulleys have been configured in such a way as to cause the fairing elements to pass with the nose toward the bottom of the groove and the tail facing out of the groove. This arrangement is logical because the towing cable, through which the forces pass, has to be located in the nose of the fairing elements, namely near the leading edge. All towing pulleys therefore have a narrow V-groove. This arrangement is made necessary because of the links between all of the fairing elements. On leaving the sea and arriving at the towing pulley, the fairing elements which, during their airborne path, have a tendency to orientate themselves with the trailing edge downmost (so upside down) thus find themselves straightened up by degrees thanks to the connections between the fairing elements. When a fairing element is correctly positioned in the groove of the pulley, during hauling-in (and also during paying-out) all the successive ones will become straightened by degrees and pass in the best way through the pulley.

(61) Moreover, the devices that allow the fairing to be turned over (or straighteners) do not perform well when they are installed downstream of the pulley, when viewed from the free end of the cable, because the position of the cable at this point has at least two degrees of freedom: longitudinal and lateral, and present-day straightening devices are incapable of correctly following the cable in these two directions or are devices that are complicated.

(62) In the case of a narrow V-groove pulley, if the guide device has no turning-over device downstream of the pulley as seen from the free end of the cable or if this device does not perform well, fairing elements entering the pulley tail-down may be able to jam in the groove and, if they are not engineered to withstand the force applied by the cable in this orientation, they will deform and cause the subsequent fairing elements to deform. This situation is depicted in FIGS. 5 and 6a to 6b. FIG. 5 depicts a portion of a faired cable 1 entering a pulley P of groove 50. In this figure, the cable 1, which is therefore entering the pulley in the direction of the arrow, is being wound. In this figure, the axis xp of the pulley is perpendicular to the plane of the page. The fairing elements 13 of a first group of fairing elements 12a are oriented with their trailing edge BF facing toward the outside of the groove and leading edge toward the groove. The notable fairing element 13a is the head-end fairing element of the portion 12b, namely the fairing element 13a of the portion 12b which is furthest away from the end 6 of the cable that is intended to be immersed. The fairing element 13a arrives at the pulley P with its trailing edge BF toward the groove of the pulley and its leading edge BA toward the outside of the groove. This notable fairing element 13a belongs to a second group of fairing elements 12b.

(63) If the pulley P is a pulley of the prior art, the cross section of the pulley of the prior art in the plane M passing through the lateral edge 18 connecting the trailing edge BF and the leading edge BA of the head-end fairing element is as visible in FIG. 6A. FIG. 6b is a cross section of the pulley P of the prior art in another plane comprising the lateral edge 18 of the head-end fairing element 13a situated to the right of the plane M in FIG. 5 because, between FIG. 5 and FIG. 6b, the cable 1 has been hauled in, namely pulled in the direction of the arrow depicted in FIG. 5, causing the notable fairing element 13a to advance in the groove. The groove of the pulley has a V-shaped cross section with an aperture angle of between 20 and 50. The bottom of the V has a shape that substantially complements the leading edge so that when a fairing element enters the pulley with the leading edge uppermost, the subsequent fairing elements connected to this fairing element will also adopt this orientation as the cable is wound. By contrast, if a head-end fairing element 13a arrives with the trailing edge facing toward the groove 105 as is the case in FIG. 6a, the groove is too narrow for the fairing element to turn over with its trailing edge uppermost under the effect of the traction of the cable with respect to the groove of the pulley along its axis. The tension in the cable forces the head-end fairing element 13a to drop down toward the bottom of the groove. Specifically, as the cable is pulled through the pulley along its axis, it develops a force, on the fairing element, that is directed along the line of action of the force indicated by the arrow in FIG. 6a. Now, if the fairing element is not engineered to withstand this stress, it deforms and breaks (or becomes damaged) as depicted in FIG. 6b.

(64) In order to alleviate these disadvantages, the invention seeks to give the pulley itself a function of turning the fairing elements over about the axis of the cable.

(65) To this end, the invention consists in providing a towing assembly comprising a guide device for guiding the cable, which is positioned downstream of the winch when viewed from the end of the cable intended to be immersed, the guide device comprising a first groove the bottom of which is formed by the bottom of the groove of a pulley, the first groove being configured in such a way as to allow a fairing element of the fairing to be flipped, by rotation of the fairing element about the axis of the cable x under the effect of the tension in the cable, from a turned-over position in which the fairing element is oriented with its trailing edge (or tail) toward the bottom of the first groove, into an acceptable position in which it is oriented with its leading-edge (or nose) toward the bottom of the first groove, which means to say with its trailing edge toward the outside of the groove. The dimensions and the shape of the profile of the first groove, notably the width of the first groove and the curvature of the profile of the first curved surface (which will be defined later on) in the radial plane are determined as a function of the radius R of the pulley, of the maximum length CAR, measured parallel to the chord separating the trailing edge BF of the fairing elements of the fairing from the axis x of the elongate element 1, of the chord length LC of the fairing elements and of the maximum thickness E of the fairing elements so as to allow the fairing element to be flipped from the turned-over position into the acceptable position.

(66) When the trailing edge (or tail) is oriented toward the bottom of the first groove, that means that the trailing edge (or the thin end of the tail) is situated a shorter distance than the leading edge (or than the nose) away from the axis of the pulley xp. The axis of the pulley is the axis about which the pulley pivots with respect to the winch, namely with respect to the fixed part of the winch. Advantageously, the axis of the pulley is substantially horizontal, namely intended to run parallel to the water surface when the sea state is calm when the towing device is fixed to a naval vessel or ship. The bottom 26 of the groove of the pulley forms a circle of radius R the center of which lies on the axis of the pulley.

(67) FIG. 7 depicts a cross section of the pulley P of FIG. 5 in the radial plane BB of the pulley P, in the case where the pulley P is a pulley according to one preferred embodiment of the invention. A radial plane of a pulley is a plane which is formed by a radius r of the pulley and the axis xp of the pulley about which the pulley pivots. The radius r has a length R.

(68) The first groove 24 is delimited by a first surface of which the cross section in the radial plane BB is the first concave curve 25 (U-shaped curve depicted in bold in FIG. 7). The first concave curve 25 comprises a bottom 26 of the first groove 24. The bottom is the point of the first groove 24 which is closest to the axis xp of the pulley.

(69) FIG. 7 also depicts a V-shaped reference curve 28. The V-shaped reference curve 28 is the cross section, in the radial plane BB, of a second curved surface delimiting a second, reference, groove 29 or virtual second groove. The bottom of the second groove, namely the bottom of the reference groove 28 is the bottom 26. The bottom V is the point of intersection of the two branches 31, 32 of the V.

(70) According to the invention, the aperture of the V, v, is at least equal to twice a threshold angle s, and the width of the V lv, measured along a straight line d parallel to the axis of the pulley, is at least equal to a threshold width ls, given by:

(71) $\mathrm{ls}=0.7*\mathrm{lid}$$\mathrm{where}\mathrm{lid}=2\left(\mathrm{LC}+E\right)*\sin \left(s\right)$$s=i*\frac{R}{R-\mathrm{CAR}}$

(72) lid is an ideal width of the V,

(73) where i is a limit angle greater than 45 and less than 90, where R is the radius of the pulley and where CAR (indicated in FIG. 4a) is the maximum length, separating the trailing edge BF of the fairing elements of the fairing from the axis of the cable, measured parallel to the chord CO of the fairing elements, where LC is the chord length of the fairing elements and E is the maximum thickness of the fairing elements.

(74) In one preferred embodiment of the invention, the width of the V is at least equal to lid. The turnover is therefore accomplished more easily.

(75) Advantageously, the limit angle i is given by the following formula:
i=/4+Arctan(Cf)
where Cf is the coefficient of friction between the material that forms the exterior part of the tail of the fairing element and the material that forms the surface delimiting the groove of the pulley. The material that forms the exterior part of the tail of the pulley is the material that forms the fairing element when the fairing element is made from a single material.

(76) In the embodiment of FIG. 7, the first curve 25 coincides with the second curve 28 at the endpoints 33, 34 of the second curve 28. The endpoints 33, 34 of the second curve are the points on the second curve which are spaced apart by the width lv along a straight line parallel to the axis of the pulley xp. They delimit the first groove and the second groove along an axis parallel to the axis of the pulley and along an axis parallel to the radius of the pulley passing through the bottom 26. The first curve 25 is, at every point comprised between each of the endpoint 33, 34 and the bottom 26, coincident with the second curve or closer to the axis of the pulley xp than the second curve along the radius of the pulley in the plane of section BB.

(77) As a result, in order to ensure the desired turnover, the first concave curve 25 delimiting the first groove 24 may have the profile visible in FIG. 7, or alternatively may, between the endpoints, at any point other than the bottom and the endpoints 33, 34, lie below the curve 28 and at least at a distance from the axis that is equal to the distance separating the bottom of the pulley from the axis of the pulley (radius R of the pulley). In other words, the first concave curve, at all points, lies in the space delimited by the curve 28, the straight line dl parallel to the axis passing through the bottom 26 and the straight lines d3 and d4 which are parallel to the radius R of the pulley passing through the points 33 and 34.

(78) The first concave curve 25 is the curve delimiting the first groove 24 intended to receive the faired cable in a radial plane (see FIG. 7).

(79) FIG. 14 depicts, in dotted line, in a radial plane, a portion 250 of a concave first curve complying with an advantageous feature of the invention. The fairing element 13 extends with its leading edge perpendicular to the radial plane. This feature is as follows: the concave first curve is defined in a radial plane BB of the pulley such that, when the fairing element extends with the leading edge BA perpendicular to the radial plane BB, whatever the position of a fairing element in the first groove 24, when the nose 14 of the fairing element 13 is bearing on the concave first curve and the cable 1 is exerting on the fairing element 13, in the radial plane, a force to press the nose of the fairing element against the pulley, said pressing force Fp comprising a component CP perpendicular to the axis of the pulley and a lateral component CL (which means to say a component parallel to the axis of the pulley), the trailing edge BF of the fairing element 13 is not in contact with the concave first curve or is in contact with a part 251 of the concave first curve that forms, with a straight line dp of the radial plane perpendicular to the axis xa extending from the axis of the cable x as far as the trailing edge of the fairing element, an angle that is at least equal to an angle of slip t. The angle of slip is given by the following formula:
t=Arctan(Cf)

(80) This feature makes it possible to prevent the fairing element from blocking the cable in the groove when the cable moves laterally in the groove, namely when it moves parallel to the axis of the pulley. What happens is that if this angular condition is respected, the fairing element can be sure of slipping in the event of lateral thrust from the cable. In other words, a pulley having a profile as defined with reference to FIG. 14 makes it possible to ensure that the fairing element will overturn from a turned-over position into an acceptable position.

(81) The concave first curve 25 and, therefore, the profile of the first groove, is obtained by those skilled in the art by simulations starting from this definition.

(82) In practice, for an angle t of the order of 10, a first curve forming a curved line having at every point a radius of curvature at least equal to half the chord length LC of the fairing element makes it possible to ensure that the fairing element will slip in the event of lateral thrust from the cable. A curved line is a line that has no sharp or salient angle (in the mathematical sense of the term). Specifically, if, as can be seen in FIG. 15, a circle Cr is plotted that passes through the nose of the fairing element 14 and the trailing edge BF of the fairing element 13, with its tangent T at the trailing edge forming an angle t with the straight line dp, the radius RA of this circle is approximately equal to 55% of the chord length LC of the fairing element, which is greater than the value of 50% adopted hereinabove.

(83) Advantageously, the dimensions and shape of the first groove profile are determined in such a way as to allow the flipping of a reference fairing element of maximum length CAR, measured parallel to the chord separating the trailing edge BF of the fairing elements of the fairing, a fairing element chord length LC and a maximum thickness E, and possibly also as a function of the coefficient of friction Cf between the reference fairing element and the pulley. These dimensions and profile are advantageously defined in such a way as to ensure that the fairing element flips from a turned-over position into an acceptable position between without deforming this reference fairing element.

(84) In the embodiment of FIG. 7, the width of the first groove lgb is equal to the width of the V, Iv. As an alternative, the first groove extends beyond the endpoints. It may comprise the groove of the pulley only or comprise the groove of the pulley and be delimited, on each side of the pulley, by deflectors or cheeks that are vertical (which means to say perpendicular to the axis of the pulley) or substantially vertical. The first groove may also be the groove of the pulley which, beyond the V or above the V comprises walls that are vertical (which means to say perpendicular to the axis of the pulley) or substantially vertical. The walls and cheeks as defined make it possible to prevent the cable from leaving the first groove in the event of lateral movement.

(85) In the embodiment of FIG. 7, the first groove is the groove 24 of the pulley. As an alternative, the first groove comprises the groove of the pulley. The bottom of the first groove is the bottom of the groove of the pulley. By contrast, the first groove extends beyond the groove of the pulley. It is, for example, delimited at least on one side of the pulley with respect to a plane perpendicular to the axis of the pulley, by a deflector or a cheek. The defector or cheek may be fixed with respect to the pulley or able to rotate with respect to the pulley about the axis of the pulley. Advantageously, the first groove comprises lateral edges making it possible to limit the lateral movement of the cable. The lateral edges may extend completely within the part situated between the two endpoints or alternatively partially and extend also partially beyond these points.

(86) The pulley, and more specifically the groove of the pulley, has a profile that is constant. In other words, it is the same in all the radial planes of the pulley.

(87) The first curve 25 and the second curve 28 are symmetric with respect to a plane perpendicular to the axis xp of the pulley and comprising a radius of the pulley passing through the bottom 26. This plane is then the median plane of the groove.

(88) The way in which the pulley profile according to the invention as depicted in FIG. 7 is obtained will now be explained in greater detail. The applicant started from the observation that the V of FIG. 6a needs to be opened out so that the tail can move clear to the side as the cable is being wound in. FIG. 8a depicts a partial cross section of a pulley 40 according to a second embodiment, in the plane M which is a plane formed by a lateral face 18 of the head-end fairing element 13a of the segment 12b coming into contact with the pulley. The lateral face comprises the point of the fairing element that is first to come into contact with the pulley. The pulley has an open V-shaped profile making it possible to achieve turnover. In this figure, the pulley 40 has a V-shaped groove 44. The notable fairing element 13a is resting against a first branch 45 of the V, with its leading edge facing toward the bottom 46 of the groove 44. The groove aperture g is such that the angle formed between the line of action of the force (depicted by the arrow indicated in the fairing element) and the second branch 47, f, is greater than 90. In this case, the tail has been given a clearance path which allows it to turn over in the direction of the arrows indicated in FIG. 8a to adopt the position depicted in FIG. 8c while passing via the position depicted in FIG. 8b following the movement indicated by the arrows by pivoting about the axis of the cable under the action of the tension of the cable (which is exerted along the line of action of the force) when the cable is hauled along the groove. As visible in FIG. 8a, the direction of the line of action of the force is substantially parallel to the first branch 45. This is why the aperture of the V g in the plane M, which is at least equal to twice the limit angle i is substantially equal to f. As a result, the aperture of the V g is greater than 90. To take account of friction between the tail of the fairing element and the surface of the groove, the limit aperture g=2*i is at least equal to 95 and preferably at least equal to 100.

(89) The angular feature is not enough to obtain correct overturning of the fairing elements. It is necessary for the width of the groove lgm, in the plane M, to be at least equal to a limit width li which is given by the following formula:
li=2(LC+E)*sin i

(90) Now, as can be seen in FIG. 5, the profile of the groove of the pulley in the plane BB is the projection, onto a plane that makes an angle with the plane M, of the profile of the groove in the plane M. The angle is dependent on the length CAR which is the maximum length separating the trailing edge BF of the fairing elements of the fairing from the axis of the cable measured parallel to the chord CO of the fairing element 13a. It is defined as follows:

(91) $\mathrm{CAR}=R-R\cos$$\mathrm{CAR}=R\left(1-\cos \right)$$=\arccos \left(1-\frac{\mathrm{CAR}}{R}\right)$

(92) The V previously defined is therefore to be corrected by the bias introduced by the angle . The aperture v of the V formed by the second curve 28 in the plane BB is at least equal to a threshold angle s. The threshold angle s is given by the following formula:

(93) $s=\frac{i}{\cos }$$\mathrm{Hence}s=i*\frac{R}{R-\mathrm{CAR}}$

(94) Therefore, the width of the V Iv in the plane BB is at least equal to the ideal width lid given by the following formula:
lid=2(LC+E)*sin s

(95) The first curve 25 delimiting the first groove 24 has, at least from the first endpoint 33 to the second endpoint 34, a concave shape.

(96) It may, at least from the first endpoint 33 as far as the second endpoint 34 have a V shape or alternatively exhibit several sharp or salient angles AS as depicted in FIGS. 9a and 9b. In other words, curve substantially forms a broken line. In these figures, the curves exhibit a sharp or salient angle in the region of the bottom 26 and are symmetric with respect to the plane perpendicular to the axis of the pulley and comprising a radius of the pulley. These profiles perform better at turning over the fairing elements than does the V-shaped profile. These profiles are advantageously, although not necessarily, symmetric with respect to a plane perpendicular to the axis of the pulley passing through the bottom 26. As an alternative, the first curve has salient angles and has a tangent substantially parallel to the axis of the pulley xp at the bottom. The bottom is then the point on the curve situated on the median plane of the groove.

(97) Advantageously, as depicted in FIG. 7, the first curve 25 is, between the endpoints 33, 34, a curved line. In other words, this is a concave curve with no sharp or salient angle (within the mathematical meaning of the term). Mention is made of a U-shaped profile. What this means is that the curve substantially never has more than one tangent at any one point. Its derivative is substantially continuous.

(98) When the first groove (or first curve) has a V-shaped cross section (V-shaped first curve) it needs to have a width at least equal to lid for turnover to be guaranteed. When the first groove (or first curve) has a cross section such that the first curve is U-shaped, then it can have a smaller width potentially down to 0.7*lid, because it has no d sharp angles in which the tail of the fairing element may jam. In that case, the aperture of the V may also be below the threshold angle. In other words, the V needs to have a width at least equal to 0.7*lid. By contrast, overturning may prove more difficult than when the V has a width at least equal to lid. Below this threshold, there is no certainty that overturning will occur.

(99) Advantageously, in the case of a first groove having a U-shaped profile, the first groove has a bathtub-shaped bottom. The groove with a bathtub-shaped bottom offers the advantage of ensuring certain and fluid reorientation of the fairing element and allows the fairing element to be oriented in a substantially lying-down position in the bottom of the groove.

(100) That means that the first curve has a central zone, this central zone has a width equal to g*lid, where lid is the ideal width and g is comprised between 0.7 and 1, between the endpoints coinciding with the endpoints of a V-shaped reference curve 128 having a width equal to g*lid. The central zone is delimited by the two curves (see hatched zone) 10:

(101) an upper curve SUP having a first radius of curvature R1 radius equal to *g*lid passing through the bottom and the center of which is situated on a straight line perpendicular to the axis of the pulley passing through the bottom,

(102) a lower curve INF comprising a central portion CENT extending substantially parallel to the axis of the pulley symmetric with respect to a plane perpendicular to the radial plane passing through the bottom and extending, along the axis of the pulley, over a first width equal to %*g*lid and comprising, on each side of the central portion CENT, lateral portions LAT1 and LAT2 connecting the central portion to the endpoints 133, 134 and having a second radius of curvature R2 equal to *g*lid. Each lateral portion extends over a width equal to *g*lid along the axis of the pulley. The centers of the lateral portions are symmetric with respect to one another about the vertical plane PV passing through the bottom and perpendicular to the axis of the pulley xp.

(103) The central zone may be one of the two curves. The lower curve is the preferred embodiment of the invention.

(104) Advantageously, the central zone of the first curve is formed by a pulley having a groove the width of which is the width of the central zone.

(105) Advantageously, the first curve comprises upper parts extending substantially perpendicularly above the endpoints of the V so as to prevent the cable from leaving the first groove in the event of a vertical movement of the cable. These cheeks are secured to the pulley or belong to the pulley or are fixed with respect to the axis of the pulley.

(106) The first curves comprised between the upper curve and the lower curve offer the advantage of satisfying the angle condition making it possible to prevent the fairing element from inhibiting the lateral movement of the cable.

(107) FIGS. 11a 11c depict, in successive planes parallel to the plane M, the orientations successively adopted by the lateral face of the reference fairing element comprising the first point to come into contact with the pulley, as the cable is being wound in. The fairing element 13a arrives with its trailing edge downward (FIG. 11a in the plane M) and when the cable is pulled, the element pivots about the axis of the cable (see FIG. 11b) under the effect of the tension of the cable, until it reaches the substantially lying-flat position in which the leading edge faces toward the bottom of the groove and the leading edge faces toward the outside of the groove (FIG. 11c). This profile makes it possible to facilitate and simplify the flipping of a fairing element because the flattened central portion of the groove of the pulley means that there is a significant distance between the axis of the reaction of the groove of the pulley on the fairing element (the axis leading from the trailing edge toward the center of the portion of circle formed by the central portion) and the axis of rotation of the fairing element (extending along the trailing edge axistoward the axis of the canal xc or axis of the cable x) because of the significant distance between the axis of the cable and the center of the portion of circle formed by the central portion. This profile also allows the cable and its fairing, which are positioned substantially lying flat, to come and rest without danger against the flanks of the pulley when the cable is urged laterally (namely parallel to the axis of the pulley) if for example the ship changes heading. If the cable and the leading edge of the fairing are positioned on the correct side, they remain there. If they are on the incorrect side, the profile of the pulley allows a gentle near-overturning which allows the cable (which is where the forces are applied) to come and press against the flank of the pulley. This slippage is present but less fluid in the other configurations of pulley.

(108) To sum up, the pulley according to the invention and, more generally, the guide device according to the invention, makes it possible to ensure the straightening of a fairing element coming to bear against the pulley with an orientation in which the trailing edge faces toward the bottom of the groove of the pulley and the leading edge is vertically aligned with the trailing edge. The fairing element carries along with it the fairing elements to which it is connected in rotation about the cable, namely the fairing elements of the same portion. The pulley according to the invention also allows the straightening of the fairing elements of a cable organized into a single portion in which the fairing elements are all joined together in rotation about the cable if an inter-fairing-element connection should break for example under the effect of a double twist, thereby allowing the faired cable to pass through the pulley without deformation of the fairing elements. It also allows the straightening of the head-end fairing element of a fairing comprising a single portion extending over a length shorter than the length of the cable starting from the end intended to be immersed. It also allows the straightening of the fairing elements of a faired cable comprising fairing elements which are all free to rotate about the cable independently of one another. It furthermore, because of its width, allows guidance of a cable organized into a single portion exhibiting remnant twist (very tightly twisted immersed torsion not reabsorbed on passing through the pulley) without deformation of the fairing elements, something that is not possible using a narrow V-shaped pulley.

(109) The guide device of the invention is simple and effective because it does not require the fitting of a cable-follower device (namely a device able to follow the cable as it moves laterally and vertically with respect to the pulley).

(110) The pulley according to the invention, and, more generally, the guide device according to the invention, because of its profile, does not turn the fairing element over as far as a situation in which the trailing edge is situated in vertical alignment with the leading edge. For example, in the case of the pulley with a bathtub-shaped bottom, the fairing element is turned over into a position in which it is substantially flat (with the trailing edge raised slightly upwards). It therefore needs to pivot by approximately of a turn as opposed to of a turn (if it were to have to adopt the position in which the trailing edge was above and in vertical alignment with the leading edge) thereby facilitating the operation whereby the pulley straightens the fairing element.

(111) Advantageously, the guide device comprises, between the winch and the pulley, a straightening device allowing the fairing elements leaving the pulley and heading for the winch to be oriented about the axis of the cable in such a way that they exhibit a predetermined orientation with respect to the drum of the winch, for example with the leading edge downmost and the trailing edge vertically in line with the leading edge. These devices are truly effective only when the position of the cable is perfectly known (which it is as it leaves the pulley).

(112) In the embodiment of FIGS. 4a and 4b, the fairing elements of the portions have a cross section which is constant, which means to say fixed, along the leading edge. What is meant by cross section is the profile of the fairing element in a transverse plane, namely a plane running perpendicular to the leading edge BA, namely to the axis of the canal xc What is meant by a cross section that is constant is a cross section that exhibits substantially the same shape and the same dimensions in all transverse planes regardless of their positions along the leading edge between the lateral faces 17, 18. In other words, the trailing edge BF is substantially parallel to the leading edge BA across the entire width l of the fairing element. The width l of the fairing element is the distance between the two lateral faces 17, 18 along an axis parallel to the leading edge BA.

(113) The trailing edge BF constitutes a bearing edge parallel to the leading edge BA.

(114) As an alternative, as visible in FIGS. 12a to 12c, at least one fairing element 130 of the fairing is a mitered fairing element. A mitered fairing element is a fairing element which comprises a bearing edge BAPa comprising a first bearing edge Bza which is mitered with respect to the leading edge BAa, the miter being produced in such a way that the distance between the leading edge BAa and the mitered first bearing edge Bza, considered along an axis perpendicular to the leading edge BAa, and to the axis xc of the canal 16, varies linearly along the axis xc. What is meant by a first bearing edge Bza that is mitered is a first bearing edge Bza which extends longitudinally substantially along a straight line which is angled or inclined with respect to the leading edge BAa. The first bearing edge Bza extends longitudinally in a first plane containing a plane or parallel to a plane defined by the leading edge BAa and the chord CO of the fairing element. In other words, the first bearing edge Bza is at an angle with respect to the leading edge BAa in this first plane.

(115) The bearing edge BAPa extends longitudinally between two ends E1 and E2. The bearing edge BAPa is arranged in such a way that the distance between the bearing edge BAPa and the leading edge BAa decreases continuously, from a first end E1 of the first bearing edge Bza to a first lateral face 180 of the fairing element closer to the second to the first bearing edge Bza than to the first end of the bearing edge, along an axis parallel to the leading edge BA.

(116) In the embodiment of FIG. 12b, this lateral face 180 is the lateral face of the fairing element 130a furthest away from the free end 6 of the cable (visible in FIG. 2) in the opposite direction to the arrow. The other lateral face 170 is the lateral face of the fairing element 130a closest to the free end 6 of the cable. This feature makes it easier to turn the fairing element 130 over when it comes to bear against the pulley via its trailing edge, as the cable is being wound in, namely as the cable is being pulled with respect to the axis of the pulley xp, in the direction of the arrow f. Specifically, FIG. 12b depicts the position P, on the pulley 4 of FIG. 7, of the point at which the fairing element 130a comes into contact with the pulley 4 as a result of the traction of the cable with respect to the axis of the pulley xp in the direction of the arrow. This point is situated at a distance B (indicated in FIG. 12b) from the cable 1 perpendicular to the axis of the cable x. Also depicted is the position P, on the pulley 4, of the point at which a fairing element 13 that would have had the shape depicted in FIGS. 4a and 4b would have come into contact with the pulley P. This point is situated at a distance dB from the cable 1 perpendicular to the axis of the cable x. The distance dB is less than the distance B, which means that the overturning of the fairing element is easier and therefore that the overturning of the fairing elements of the portion is also easier. This is valid for the pulley of the invention but is also valid for any guide device, particularly of the type that allows the orientation of the fairing element with respect to the guide device to be modified by rotating action of the fairing element about the axis of the cable. In particular, the mitered bearing edge makes it easier to reorient a fairing element in any guide device that allows the orientation of the fairing element with respect to the guide device to be modified by rotating action of the fairing element about the axis of the cable (or of the canal) when the fairing element comes to bear against a bearing surface of the guide device via the bearing edge. In other words, the mitered bearing edge in particular facilitates the reorientation of the fairing element by any guide device comprising a surface that opposes the traction of the faired cable as the cable is being wound in or paid out. The invention works for example with guide devices that are able to follow the cable in the event of lateral and/or vertical movement of the cable. In general, the presence of a mitered fairing element makes it possible to limit the risks of damage to the fairing, notably in the presence of a double twist, by facilitating the flipping of a fairing element as it enters a guide device, thereby limiting the risks of the fairing becoming jammed in the guide device.

(117) This embodiment also offers an advantage in the case of a pulley of constant profile, and more particularly a pulley according to the invention. Specifically, the point of contact P is situated in a plane M situated at a shorter distance D than the distance D at which the plane M (comprising the point P) is situated, with respect to the axis of the pulley, parallel to the axis of the cable x. As a result, the groove of the pulley is not as deep in the plane M as in the plane M. Specifically, the profile of the groove in the plane M (or M) is the projection of the profile of the groove in a radial plane passing through the plane P (or respectively P) onto the plane M (or, respectively, M) forming an angle (or respectively less than ) with the radial plane at the point considered. Now, the fact that the groove is not as deep in the plane M as it is in the plane M means that the pulley is flatter in the plane M than in the plane M, at least at the bottom (namely at the level of the central part of the curve delimiting the groove). If the fairing element comes into contact with the central portion of the pulley in the bottom of the bathtub, the central portion is flatter in the plane M than in the plane M, or in other words, the radius of the contact surface at the point P is greater in the plane M than in the plane M, making it easier for the fairing element to flip under the effect of the traction of the cable with respect to the axis of the pulley.

(118) In the embodiment of FIG. 12b, the mitered fairing element comprising the miter is the fairing element 130a at the head-end of the portion, namely the fairing element furthest from the end of the cable that is intended to be immersed. That makes it possible to facilitate the flipping of the fairing element 130a during the winding-in of the cable and to facilitate the flipping of the entire portion 120 because, since the fairing element is connected in terms of rotation about the cable to the other fairing elements of the portion, as it moves about the cable it carries all the fairing elements of the portion 120 along with it. The head-end fairing element 130a is a fairing element which is adjacent to just one other fairing element 130b belonging to the same portion 120. The first bearing edge Bza of the head-end fairing element 130a is arranged in such a way that the distance between the leading edge BAa and the first mitered bearing edge Bza decreases continuously, along an axis parallel to the leading edge BAa, from a first end E1 of the first bearing edge Bza to a second end E2 of the first bearing edge Bza, further away from the other fairing element 130b than the first end E1, along the axis parallel to the leading edge BAa.

(119) As an alternative, the mitered fairing element is the fairing element at the tail-end of the portion, namely the fairing element closest to the end of the cable that is intended to be immersed. That makes it possible to facilitate the flipping of the fairing element during the paying-out of the cable (when the fairing element comes to bear on the pulley on the other side of the pulley with respect to the axis of the pulley) and to facilitate the flipping of the entire portion because the fairing element (by a propagation of the rotational movement over the entire portion). The tail-end fairing element is a fairing element which is adjacent to just one other fairing element belonging to the same portion. The first bearing edge is arranged in such a way that the distance between the leading edge BAa and the first mitered bearing edge decreases, along the leading edge BAa, from a first end of the first bearing edge facing the other fairing element to the a second of the first bearing edge further away from the other fairing element, along the axis parallel to BAa. The other end of the first bearing edge is closer to a lateral face than the first end of the bearing edge. This embodiment, like the preceding one, makes it possible to ensure the flipping of all the fairing elements of the portions of fairing without having to provide only mitered fairing elements over the entire fairing, as so doing would have the effect of limiting the performance of the fairing in terms of drag reduction.

(120) Advantageously, each portion comprises at least one (head or tail) end fairing element comprising a mitered edge. The other fairing elements are not mitered fairing elements. They do not have a mitered first bearing edge. The bearing edge is the trailing edge and is substantially parallel to the leading edge over its entire length. In an alternative form, not claimed, a fairing comprising a single portion as defined above may comprise a fairing element with a mitered bearing edge. This portion extends for example over a length less than the length of the cable starting from the end intended to be immersed. In this case, the head-end fairing element of the portion is advantageously a fairing element comprising a mitered bearing edge designed as for the head-end fairing element described hereinabove.

(121) In another alternative form, not claimed, the portion extends over the entire length of the cable.

(122) In all the configurations of fairing (of the type comprising one portion, several portions or comprising fairing elements which are all free to rotate independently of one another about the elongate element), all the fairing elements could be mitered fairing elements. That would make it easier to flip each fairing element in the event of a breakage of an inter-fairing-element connection downstream of the fairing element as seen from the pulley, when the fairing elements are initially connected. In cases where the fairing elements are free to rotate independently of one another, that would make it easier to flip each fairing element as it arrives on a guide device. More generally, the mitered fairing element makes it possible to void the need to join the fairing elements together and therefore makes it possible to limit the cost of the fairing and the time taken to assemble the fairing.

(123) If there is a wish to reorient the fairing elements when the cable is being wound in, the miter is produced in such a way that the distance between the leading edge BA and the first mitered bearing edge decreases, along the axis xc, from the end of the first bearing edge closest to the end of the cable intended to be immersed as far as the end of the bearing edge opposite to the end of the cable that is intended to be immersed and vice versa if the wish is to facilitate the flipping during the paying-out of the cable.

(124) In the embodiment of FIGS. 12a and 12b, the bearing edge BAPa is the trailing edge BF. It comprises the first mitered bearing edge Bza and a second bearing edge Bla which runs parallel to the axis x and is situated a fixed distance away from the leading edge along the axis x. The first mitered bearing edge is connected to the lateral face 180 and to the second bearing edge Bla in the direction of the leading edge, by fillet radii or chamfers. The maximum chord length LC is the distance between this second bearing edge Bla and the leading edge. As an alternative, the leading edge has no second bearing edge Bla extending parallel to the axis x. The miter extends substantially over the entire width of the fairing element and is advantageously, although not necessarily, connected to the lateral faces by fillet radii or chamfers.

(125) As visible in FIGS. 12c and 12d which depict cross sections of the fairing element on respective planes N and Q, depicted in FIG. 12a, parallel to the leading edge and perpendicular to the lateral faces 170, 180, the fairing element comprises a thick first portion 130a1 visible in FIG. 12c and a thin second portion 130a2 having a second thickness e2 smaller than the first thickness e1 of the thick part. The second thickness e2 is substantially equal to the thickness of the tail end 15 opposite to the end of the tail that is connected to the nose 14 of the fairing element. The first edge comprises a first portion Bza1 extending into the thick first portion 130a1 of the fairing element and a second portion Bza2 extending into the thin part. The first portion of the first bearing edge Bza1 is connected to the longitudinal faces 122, 123 by respective chamfers 132, 133. In other words, the fairing element comprises chamfers connecting the first portion of the first bearing edge Bza1 to the respective longitudinal faces 122, 123. That means that the trailing edge can be made thinner in the thick part of the fairing element, thereby limiting the risks of the fairing element becoming jammed on the guide device. As an alternative, the chamfers extend over the entire length of the first bearing edge.

(126) As an alternative, the first portion of the leading edge Bza1 is connected to the lateral faces by respective bulging surfaces. What is meant by bulging surfaces is surfaces with convex curvature. This embodiment also makes it possible to limit the thickness of the bearing edge. As an alternative, the curved surfaces extend over the entire length of the first bearing edge. The chamfers and curved surfaces are two nonlimiting technical solutions that make it possible to obtain the feature whereby at least a first portion of the first bearing edge Bza1 has a thickness e1 less than the thickness of the fairing element in any longitudinal plane parallel to the leading edge and perpendicular to the lateral faces of the fairing element intersecting the first portion of the first bearing edge Bza1. The thickness of the fairing element in a plane of section is the distance separating the first longitudinal face 122 from the second longitudinal face 123 in a direction perpendicular to the chord CO in the plane of section of the fairing element. Advantageously, the first portion Bza1 has the same thickness as the second bearing edge Bla which runs parallel to the axis x and is situated a fixed distance away from the leading edge along the axis x.

(127) A bearing edge of a fairing element according to a second embodiment of the invention will now be described with reference to FIG. 13. Everything already stated regarding the installation of the fairing element on a fairing, the configuration of the fairing, the thickness of the bearing edge and the arrangement between the first bearing edge and the second bearing edge remains valid.

(128) In FIG. 13, the bearing edge BAPb connects the two lateral faces 270, 280. The fairing element 230 is formed of two parts 231, 232 back to back along the first mitered bearing edge Bzb. The fairing element is configured to be kept in a deployed configuration (visible in FIG. 13) when subjected to the hydrodynamic flow of the water, in which configuration the two parts 231, 232 are arranged, relative to one another about the first bearing edge, in such a way that the fairing element has a trailing edge parallel to the leading edge and a cross section that is constant along the leading edge. In other words, the length of chord is constant. The fairing element is kept in the deployed position as long as the torque inducing relative pivoting between the two parts about an axis formed by the first bearing edge Bzb is less than or equal to a predetermined threshold. The longitudinal direction of the first bearing edge is the direction of the axis formed by the bearing edge. The threshold is higher than the torque that may be applied by the hydrodynamic stream of water on the fairing element when the fairing element is immersed and possibly being towed along the trailing edgeleading edge axis. The fairing element is also configured in such a way as to allow relative pivoting between the two parts 231, 232 about the first bearing edge Bzb (see arrow) when a torque inducing relative pivoting between the two parts 231, 232, applied about the axis formed by the first bearing edge Bzb, exceeds the threshold so that the end fairing element passes from the deployed configuration into a configuration folded about the bearing edge. The axis formed by the first bearing edge is an axis contained in the first bearing edge and parallel to the longitudinal axis of the first bearing edge. In the folded configuration, the fairing element does not have a constant cross section and the trailing edge is not parallel to the leading edge over its entire length. In the folded position, the fairing element is folded along the first bearing edge Bzb. In the deployed position, the fairing element is unfolded. This embodiment makes it possible to limit or void reductions in performance in terms of the reduction of the hydrodynamic drag of the fairing element while at the same time facilitating the progress of the fairing element through the pulley and the overturning of this element.

(129) The first part 231 extends on one side of the first bearing edge and is delimited by the first bearing edge Bzb, the second bearing edge (if there is one) Blb, the leading edge BA, one lateral face 280 and the portion of the other lateral face 270 extending between the leading edge BA and the first bearing edge Bzb.

(130) The second part 232 is delimited by the first bearing edge Bzb, the part of the first lateral face 270 extending from Bzb as far as the trailing edge BF and the part of the trailing edge BF situated between Bzb and the first lateral face 270.

(131) The first part 231 is, for example, made from a material that is rigid and the second part 232 is made from a material that is flexible or soft and does not deform appreciably when the torque inducing relative pivoting of the two parts about the first bearing edge is less then or equal to the threshold and which does bend when the torque exceeds the threshold, notably when the point of intersection between the trailing edge and the first lateral face 270 comes into abutment against a guide device. The second part may, for example, be made of polyurethane. The first part may be made of a polyurethane with a rigidity higher of the first part or alternatively may be made of POM or of PET. As an alternative, the two parts have a rigidity that are such that they do not deform under the effect of a torque higher than the threshold but are connected by a pivot connection about the first bearing edge and the fairing element comprises a stabilizing device configured to keep the two parts in the deployed relative position when the relative pivoting torque is less than or equal to the threshold and so as to allow the two parts to rotate relative to one another so that they pass into the relative position of folding around the first bearing edge when the torque exceeds the threshold. The coupling device is for example a device comprising a deliberate weak link or a compression spring.

(132) Advantageously, at least one mitered fairing element or each mitered fairing element is dimensioned so as to be better able to withstand a pressure load applied, in a direction perpendicular to the leading edge connecting the leading edge and parallel to an axis to the trailing edge than the other fairing elements of the portion considered (which are not mitered). This feature makes it possible to limit the risks of deformation and breakage of the fairing elements as they enter the guide device, turn over, and pass through this guide device. To this end, this fairing element is, for example, made from a harder material than the other fairing elements and/or comprises ribs providing this additional reinforcement. Advantageously, the fairing comprises at least one reinforced mitered end fairing element collaborating with the immobilizing device. That makes it possible to reduce the cost and possibly the weight of the fairing because only one the mitered fairing element or elements differs or differ from the others, all the others being identical.

(133) The invention also relates to an assembly comprising a ship, the towing assembly being carried on board the ship. The ship is intended to move at a nominal speed in a nominal sea state. The towing assembly is installed on the ship in such a way that the tow point is situated at a nominal height.