Abstract:
A fairing for reducing the drag produced by a lead-in cable which is being used to tow one or more seismic streamers forming part of a wide streamer array comprises a plurality of fairing sections, each having a central bore to receive the lead-in and a streamlined profile. The sections are coupled together end-to-end by swivel couplings which permit rotation of the sections relative to each other, so that each section can adopt the optimum orientation for drag reduction. Drag reduction is further enhanced by providing the upper and lower surfaces of the leading edge of each fairing section with respective sets of longitudinally extending ridges/grooves.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to fairings for cables, and is more particularly but not exclusively concerned with fairings for lead-in cables used for towing arrays of seismic streamers during the performance of marine seismic surveys. 
     In order to perform a marine seismic survey, an array of seismic streamers, each typically several thousand metres tong, is towed at a speed of about 5 knots behind a seismic survey vessel. The streamers contain arrays of hydrophones and associated electronic equipment, distributed along their length. The survey vessel also tows one or more seismic sources, for example, air guns. Acoustic signals produced by the seismic sources are directed down through the water into the earth beneath, where they are reflected by the various strata. The reflected signals are received by the hydrophones, digitised and transmitted to the seismic survey vessel, where they are recorded and at least partially processed, with the aim of building up a representation of the earth strata in the area being surveyed. 
     In such streamer arrays, each streamer may be towed by means of its own lead-in cable, that is, an armoured electrical cable which supplies power to and receives digital signals from the streamer. Using this method, it is, typically, possible to tow a 700 metre wide array of eight streamers, each 4000 metres long. 
     The drag produced by such an array at a towing speed of five knots is about 40-45 tonnes, a high proportion of which is cross-line drag due to the transversely extending lead-in cables rather than in-line drag due to the streamers themselves. This drag is a very significant factor in the operating costs associated with such surveys, contributing primarily to fuel costs associated with the towing vessel. 
     To increase the efficiency of marine surveys of this kind, it would be desirable to use even wider streamer arrays containing a larger number of streamers. However, using current towing techniques, an array 1440 metres wide including ten streamers, for example, would produce a drag of over 70 tonnes, which makes the use of such wider arrays containing more streamers unattractive. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to alleviate this problem. 
     In accordance with one aspect of the invention there is provided a fairing for use on a cable, in particular a lead-in cable for a seismic streamer array, the fairing comprising. a plurality of fairing sections having a central opening in which the cable is received and a streamlined profile which acts to reduce drag when the cable is moved through water in a direction transverse to its length; and at least one coupling assembly for fastening together adjacent fairing sections in such a manner as to permit rotation of said adjacent fairing sections relative to one another. 
     In a preferred embodiment of the invention, the coupling assembly comprises a pair of end connectors each of which is secured to an end of one of a pair of adjacent fairing sections and has a radially outwardly projecting flange formed thereon, and an annular clamping ring which is made of a low friction material, and which secures together the flanges formed on the end connectors while permitting them to rotate relative to one another. 
     Advantageously, the annular clamping ring is made in two semi-circular parts which are secured together, and is of U-shaped cross-section so as to trap within the U-section the flanges of the end connectors. 
     In a further aspect, the invention provides a fairing for use on a cable, in particular a lead-in cable for a seismic streamer array, the fairing having a central opening in which the cable is received and a streamlined profile which acts to reduce drag when the cable is moved through water in a direction transverse to its length, wherein the fairing is provided with a plurality of longitudinally extending ridges formed on a part of the fairing which will, in use, be at or adjacent the leading edge thereof the 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in detail, by way of example, with reference to the accompanying drawings, in which: 
     FIG. 1 is a part-elevational, part sectional view of a fairing section in accordance with one embodiment of the invention; 
     FIG. 2 is a section taken on line II—II of FIG. 1 
     FIG. 3 is a side elevational view of an end connector for use in joining together the fairing sections of FIG. 1; 
     FIG. 4 is a perspective view of a swivel bearing which forms part of a coupling for joining the fairing sections of FIG. 1; 
     FIG. 5 is a perspective view of a clamping ring which forms part of a coupling for joining the fairing sections of FIG. 1; 
     FIG. 6 is a section taken through an assembled coupling including the end connector of FIG. 3, the swivel bearing of FIG.  4  and the clamping ring of FIG. 5, 
     FIG. 7 is a part-sectional view of an anchoring assembly for securing a group of adjacent fairing sections to a lead-in cable, axially; 
     FIG. 8 is a perspective view of an anchoring ring forming part of the anchoring assembly of FIG. 7; and 
     FIG. 9 is a section taken through an alternative form of the coupling of FIGS. 3 to  6 , shown connecting two adjacent fairing sections in accordance with another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The assembled fairing of the invention comprises a plurality of elongate generally tubular fairing sections  10 , which are coupled together by means of suitable couplings  25  at their adjacent ends to form a continuous fairing around the lead-in cable. 
     A preferred form of fairing section  10  is shown in FIGS. 1 and 2. Each fairing section  10  comprises a generally cylindrical body portion  12  which is extended at one side, to form a generally triangular-section tail portion  14 . The cylindrical body portion  12  forms a sleeve around the lead-in cable (not shown). The triangular-section tail portion  14 , which is generally hollow, extends in a radial direction from the cable, forming a trailing edge as the cable is dragged through the water. The tail portion  14  is made hollow to improve the weight balance of the profile of the fairing section  10  with respect to its pivoting centre (ie the axis of the lead-in cable) and to reduce storage volume. As can be seen in FIG. 2, the overall profile of each fairing section  10  is ‘teardrop’ shaped, providing much less drag than a plain cylindrical cable. 
     A further drag reducing feature is formed on the cylindrical body portion  12  adjacent what is, in use, the leading edge of the fairing. Symmetrically disposed about the central radial axis of the tail portion  14  are two sets of longitudinally extending parallel ridges or ribs  18 . The purpose of these ridges  18  is to ‘roughen’ the leading edge surface of the fairing section  10  and so trigger the creation of a thin turbulent boundary layer to control the laminar flow separation over the profile of the fairing section in accordance with known hydrodynamic principles. Substantially the same “roughening” effect is produced by grooves rather than ridges, and the term “ridges” as used herein is to be understood as encompassing both ridges and grooves. 
     At each of its ends, each fairing section  10  is provided with a cylindrical socket  20  of larger diameter than the cylindrical opening through the main part of the body portion  12  of the fairing section  10 . 
     The fairing sections can conveniently be formed of extruded EPDM rubber with reinforcing fibres made of Kevlar (registered trade mark) in the cylindrical wall of the body portion  12 . The fairing sections  10  may be in the range 3 m to 10 m in length and are of a size to give a clearance of 2 mm around the lead-in cable. This clearance is sufficient to allow the fairing to swivel freely about the cable but is a sufficiently close fit to avoid excessive movement of the lead-in cable within the fairing, which might cause damage. 
     It is desirable that the fairing section can swivel about the cable so that they can take up the most favourable position for reducing drag relative to the direction of movement of the lead-in cable through the water, without the cable itself having to twist in the water to accommodate this streamlining. For this reason, it is also desirable that neighbouring fairing sections  10  are able to swivel freely relative to one another. To permit this, adjacent fairing sections  10  are joined by means of the swivel coupling  25  illustrated in FIGS. 3 to  6  of the drawings. 
     The coupling  25  shown in the drawings has four components, an end connector  30  shown in FIG. 3, a swivel bearing  40  shown in FIG. 4, and two clamping rings  50 , one of which is shown in FIG.  5 . 
     The end connector  30  is made of, for example, stainless steel and consists of a spigot  32  provided with a plurality of circumferentially extending grooves  34 . At one end, the end connector  30  is provided with an outwardly extending annular flange  36 . The spigot  32  is inserted into the cylindrical socket  20  formed at the end of the fairing section  10  and secured to it by crimping, using a suitable crimp ring of soft metal (not shown). The grooves  34  on the spigot  32  help to ensure that the crimping operation fastens the end connector  30  to the fairing section  10  securely. Each fairing section  10  is provided with an end connector  30  at both of its ends, if it is to be adjacent two other such sections. Alternative couplings arrangements may be appropriate at the ends of the lead-in cables, where the fairing sections  10  may be connected to other equipment, as will be described in more detail hereinafter, or may simply be left free. 
     Between each pair of end connectors  30  at the adjacent ends of neighbouring fairing sections  10  is positioned a swivel bearing  40 . The swivel bearing  40  is a ring, typically made of aluminium bronze and of generally U-shaped cross-section, with two parallel annular flanges  42 . in use, as can be seen most clearly in FIG. 6, the swivel bearing is located between the end connectors  30  of two adjacent fairing sections  10 . The annular end surfaces of the two parallel flanges  42  of the swivel bearing  40  abut the annular flanges  36  on the two end connectors  30 , providing a bearing surface against which the end connectors  30  can rotate. 
     It will be appreciated that, in assembling the complete fairing, after each fairing section  10  is threaded on to the lead-n cable, two end connectors  30 , properly oriented relative to one another and to the fairing sections  10 , must be threaded on to the lead-in cable, separated by a swivel bearing  40 . 
     The coupling  25  between each pair of adjacent fairing sections is completed by means of clamping rings  50  shown in FIG.  5 . 
     Each clamping ring  50  is formed in two semi-circular parts which together form a ring having two inwardly directed flanges, thus giving the clamping ring a U-shaped cross section. Each coupling includes two clamping rings  50 , each of which clamps together the annular flange  36  on one of the end connectors  30  and one of the two outwardly directed flanges  42  on the swivel bearing  40 . The two halves of each clamping ring  50  can be secured together in a conventional fashion by means of suitable screws or bolts (not shown) which pass through holes  52  formed in the two halves of each clamping ring. 
     The completed clamping ring  50  traps the flange  36  on the end connector  30  and the flange  42  on the swivel bearing  40  in its U-shaped cross section, but in such a way that the two can rotate freely relative to one another. 
     As indicated earlier, the groups of adjacent fairing sections  10  are mechanically secured to the lead-in cable at, and only at, the two free ends of the groups of fairing sections. This is desirable to prevent stacking or telescoping of groups of adjacent sections  10 . 
     Securing of the fairing sections to the lead-in cable is achieved using the arrangement shown in FIGS. 7 and 8. As shown in FIG. 7, the lead-in cable has an armoured sheath  70  which is provided with reinforcing fibres  72 . Loops  74  are formed in the reinforcing fibres  72 . These loops  74 , in use, lie and are held in four horseshoe-shaped grooves  82  formed in an anchoring bracket  80 , shown in FIG.  8 . The anchoring bracket is provided at its end remote from the horseshoe-shaped grooves  82  with an outwardly extending flange  84  similar in configuration to the annular flanges  36  formed on the end connectors  30 . 
     The flange  84  on the anchoring ring  80  is secured to the annular flange  36  of the end connector on the end-most fairing section  10  in exactly the same manner as the annular flanges  36  of adjacent end connectors  30  are secured to one another. 
     The inter-engagement of the loops  74  formed on the armoured sheath  70  of the lead-in cable with the end-most fairing sections  10  through the anchoring ring  80  and adjacent end connector  30  serves to maintain the group of adjacent fairing sections  10  in a more or-less fixed axial position relative to the lead-in cable. 
     FIG. 9 shows a modified version of the fairing of FIGS. 1 to  6 , in which corresponding elements are given the same references as were used in FIGS. 1 to  6 , but with the suffix a. Thus the modified fairing of FIG. 9 is made up of fairing sections  10   a  basically similar to the fairing sections  10 , except that at their respective enlarged coupled-together ends, ie the enlarged regions of the cylindrical body portions  12   a  containing the sockets  20   a , the tail portion  14   a  is also enlarged, to maintain the ratio between the diameter of the cylindrical body portion  12   a  to the length of the fairing from its leading to its trailing edge substantially constant. Additionally, the width of the gap  90  between adjacent fairing sections is much reduced, and inclined so that, in use, its length is more closely aligned with direction of movement of the fairing through the water. 
     The coupling  25   a  is much simplified, in that the swivel bearing  40  is omitted, and a single two-piece clamping ring  50   a  fits over and entraps the flanges  36   a  of adjacent end connectors  30   a . The clamping ring  50   a  effectively performs the bearing function that was performed by the swivel bearing  40 , and to this end is made from a hard low friction plastics material, preferably polyoxymethylene (POM). 
     The fairings described above significantly reduce drag arising from the laterally extending lead-in cables used in the towing of seismic streamer arrays, thus reducing operational costs, particularly fuel costs, and/or allowing economic use of larger arrays.