Abstract:
A deflector for offsetting a streamer being towed by its lead-in by a seismic survey vessel comprises a wing-shaped body which is coupled to the lead-in and shaped to produce a sideways force which urges the lead-in laterally with respect to the direction of movement of the vessel. A boom extends rearwardly from the wing-shaped body, and forms part of an arrangement by which the angle of the wing-shaped body in the water, and therefore the sideways force produced by it, can be varied. The upper end of the wing-shaped body is directly connected to the underside of an elongate float.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to deflector devices of the kind used between a towing vessel and a tow located in water, for example a seismic streamer or streamer array, or a seismic source array, in order to pull the tow out to one side of the vessel, so as to position it at a desired lateral offset from the course followed by the vessel. 
   2. Description of the Related Art 
   A deflector device of this kind is described in detail in our U.S. Pat. No. 5,357,892, and comprises a wing-shaped deflector body having a remotely-operable pivotal lever or “boom” which extends rearwardly from a point near the middle of the trailing edge of the wing-shaped body. In use, the wing-shaped deflector body is suspended beneath a float so as to be completely submerged and positioned generally vertically in the water, and is connected to the towing vessel by means of a tow line, while the tow is connected to the end of the boom remote from the wing-shaped body. As the device is pulled through the water, the wing-shaped body produces a sideways force, or “lift”, which moves the tow laterally. This lift can be varied by adjusting the angle of the boom from the vessel, thus permitting the lateral offset of the tow from the course of the vessel to be varied in use. 
   The deflector device of U.S. Pat. No. 5,357,892 has been successfully commercialised by the Applicant as its MONOWING deflector device. In use, rolling stability of the device is provided by the connection to the float, while stability of the device about a vertical axis is provided by the drag produced by the tow. 
   The deflectors in current use are very large, typically 7.5 m high by 2.5 m wide, and weigh several tonnes. They are usually suspended around 2 m to 8 m below the float by means of a fibre rope, and are also provided with a safety chain intended to prevent separation of the float and deflector wing in the event that the rope breaks. 
   In bad weather, the upper part of the deflector wing may rise up out of the water, allowing the rope connecting the deflector wing and the float to go slack. If the deflector wing then drops abruptly, the rope, and possibly the safety chain, may break, and/or the attachment points may be damaged. 
   One solution to this problem is disclosed in PCT Patent Application No. PCT/IB98/01946 (WO 99/33700). It is an object of the present invention to provide an alternative and simpler solution. 
   SUMMARY OF THE INVENTION 
   According to one aspect of the present invention, there is provided a deflector device for use with a tow line between a towing vessel and a tow in water behind the vessel, the device comprising a wing-shaped body adapted to be coupled to the tow line and shaped to produce in use a sideways force which urges the tow line laterally with respect to the direction of movement of the towing vessel, a boom extending rearwardly from the wing-shaped body, the end of the boom remote from the wing-shaped body being adapted to be connected to the tow, and the angle between the boom and the wing-shaped body being remotely adjustable to vary the sideways force produced by the wing-shaped body, and an elongate float member whose underside is directly connected to the upper end of the wing-shaped body, 
   According to another aspect of the invention, there is provided a deflector device for use with a tow line between a towing vessel and a tow in water behind the vessel, the device comprising a principal wing-shaped body adapted to be coupled to the tow line and shaped to produce in use a sideways force which urges the tow line laterally with respect to the direction of movement of the towing vessel, a boom extending rearwardly from the principal wing-shaped body, an auxiliary wing-shaped body, smaller than the principal wing-shaped body, secured to the end of the boom remote from the principal wing-shaped body and shaped so as to produce in use a sideways force in generally the opposite direction to that produced by the principal wing-shaped body, and an elongate float member whose underside is directly connected to the upper end of the principal wing-shaped body, 
   In this second aspect of the invention, the angle between the boom and the principal wing-shaped body may be remotely adjustable, to vary the sideways force produced by the principal wing-shaped body. Alternatively and preferably, the angle between the boom and the principal wing-shaped body may be substantially fixed, and the deflector device may further comprise remotely operable means for varying the angle of the auxiliary wing-shaped body to vary the sideways force produced by the auxiliary wing-shaped body, and thereby vary the sideways force produced by the principal wing-shaped body. In either case, the end of the boom remote from the principal wing-shaped body may be adapted to be connected to the tow. 
   In preferred implementations of both aspects of the invention, the centre of buoyancy of the float member is near the trailing edge of the firstmentioned (or principal) wing-shaped body, and the angle between the longitudinal axis of the float member and the chord of the principal wing-shaped body is selected such that, in use, the longitudinal axis of the float member is aligned with the towing direction when the chord of the principal wing-shaped member is at its mean or normal angle to the towing direction. 
   The or each wing-shaped body is preferably made from titanium, while the float member may be made either from titanium or from a fibre-reinforced composite material. 
   The invention also includes a method of performing a marine seismic survey, the method including towing a plurality of laterally spaced seismic streamers over an area to be surveyed, wherein the lateral position of at least one of the streamers is controlled by a deflector device in accordance with any one of the preceding statements of invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described by way of example only, with reference to the accompanying drawings, of which: 
       FIG. 1  is a partial schematic view of a seismic survey vessel carrying out a marine seismic survey; 
       FIG. 2  is a perspective view of a first embodiment of a deflector device in accordance with the invention, for use in carrying out the survey of  FIG. 1 ; 
       FIG. 3  is a somewhat schematic part sectional view of the deflector device  FIG. 2 ; 
       FIGS. 4 and 5  are views similar to that of  FIG. 3  and showing different operating positions of part of the deflector device of  FIGS. 2 and 3 ; 
       FIG. 6  is a somewhat schematic part-sectional view of a second embodiment of a deflector device in accordance with the present invention, for use in carrying out the survey of  FIG. 1 ; and 
       FIG. 7  is a front view of part of the deflector device of  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The seismic survey vessel shown in  FIG. 1  is indicated generally at  10 , and is preferably as described in our PCT Patent Application No. PCT/GB98/01832 (WO 99/00295). The vessel  10  is shown towing a seismic source  15 , typically a TRISOR multiple air gun source of the kind described in our U.S. Pat. No. 4,757,482, and an array  16  of four substantially identical streamers  18 . However, it will be appreciated that, in practice, many more than four streamers can be towed, for example by using the techniques described in our PCT Patent Application No. PCT/IB98/01435 (WO 99/15913). The streamers  18  are towed by means of their respective lead-ins  20  (ie the high strength steel- or fibre-reinforced electrical or electro-optical cables which convey electrical power, control and data signals between the vessel  10  and the streamers), and their spread is controlled by two deflector devices, indicated at  22 , connected to the respective forward ends  24  of the two outermost streamers. The deflector devices  22  act in co-operation with respective spreader lines  26  connected between the forward end  24  of each outermost streamer  18  and the forward end  24  of its adjacent streamer to maintain a substantially uniform spacing between the streamers. 
   One of the deflector devices  22  is shown in more detail in  FIGS. 2 to 5 . The deflector device  22  is similar in general principle to the deflector device of our U.S. Pat. No. 5,357,892, but is a much improved version of it. In particular, the deflector device  22  has a main wing-shaped body  28  which is coupled in use to a respective outer lead-in  20  via a towing bridle  27 , and which corresponds to the deflector body  2  of U.S. Pat. No. 5,357,892. However, the main wing-shaped body  28  is of improved hydrodynamic cross-sectional shape and includes a fixed-angle trailing edge flap  30 , both of which features enhance lift. Also, the main wing-shaped body  28  is provided at its lower end with a vortex-controlling end plate  31  of the kind described in more detail in our PCT Patent Application No. PCT/FR99/02272, to reduce drag and improve stability, and is largely made of titanium to reduce weight. 
   Additionally, the angle lever  10  of U.S. Pat. No. 5,357,892 is replaced by a rearwardly extending boom  32 , which is pivotally connected at one end  34  to the low pressure side  36  of the body  28  near the midpoint of that side of the body, at a mounting bracket  38 . The other end  40  of the boom  32  has a towing eye (not shown) which is coupled to the forward end  24  of a respective one of the two outermost streamers  18 . 
   Pivotal movement of the boom  32  is controlled by a mechanism comprising first and second struts  41 ,  42 , which are pivotally connected to each other at  44  and to each end of the boom at  34  and  46 , forming with the boom a triangle, and an extending hydraulic actuator strut  48  pivotally connected between the apex of the triangle, ie the pivotal connection point  44  of the struts  41 ,  42 , and a pivotal connection point  50  positioned on the low pressure side  36  of the body  28  between its midpoint and its trailing edge. 
   It will be appreciated that extension of the hydraulic actuator strut  48 , from its unextended position of  FIG. 3 , will move the boom  32  outwardly from the low pressure side  36  of the body  28 , from its closest position shown in  FIG. 3 . The extent of the outward movement is preferably about 20°, as shown in  FIGS. 4 and 5 . 
   An auxiliary wing-shaped body  52 , which is much smaller than the body  28  in length, thickness and chord, is secured to the end  40  of the boom  32  with its longitudinal axis (which lies in a plane perpendicular to the plane of  FIG. 3 ) extending parallel to the longitudinal axis of the body  28 . The body  52  is fixedly secured to the boom  32  at or near the midpoint of its trailing edge  54 , and its leading edge  56  is inclined away from the body  28  such that the chord of the body  52  (ie the line connecting its leading edge  56  and its trailing edge  54 ) is at an angle of about 10° to the boom  32 . This angle is chosen because it is about half the angular extent of the movement of the boom  32 . 
   The shape of the body  52  is designed to produce, in use, a sideways force in a direction approximately opposite to that produced by the body  28  (approximately opposite, because it will be appreciated that the direction of the force varies in use as the boom  32  moves). This sideways force is increased by providing the body  52  with a fixed trailing edge flap  58 , angled away from the boom  32  at an angle of about 35°. 
   As the boom  32  is pivoted away from the body  28 , the sideways force produced by the body  52  acts as a restoring force, and thus varies the angle of the body  28  with respect to the direction of tow, so changing the lift produced by the body  28 . This restoring force augments the restoring force produced by the drag of the towed streamers  18  (and in particular, reduces the effect of any stability-reducing variations or reductions in that drag). Indeed, the deflector device  22  will remain stable with no streamer attached, eg if its streamer  18  breaks or is severed at its forward end  24 . 
   In accordance with the present invention, and as shown in  FIG. 2 , the body  28  is provided at its upper end with an elongate streamlined float  80 , which is directly and rigidly secured to the body  28 , so that the latter depends downwardly from the float like the keel of a boat. When the body  28  is made from titanium, the float  80  is preferably also made from titanium; otherwise, the float may be made from a fibre-reinforced composite material. 
   The float  80  typically has a volume of about 5000 litres, and is designed to give the whole deflector  22  a slightly positive buoyancy. The centre of lift of the float  80  is substantially aligned with a line parallel to and near the trailing edge of the body  28 . 
   The angle between the longitudinal axis of the float  80  and the chord of the body  28  is selected such that, in use, the longitudinal axis of the float is aligned with the towing direction when the chord of the member  28  is at its most commonly used angle to the towing direction (or alternatively, the mean of its range of possible angles to the towing direction). This has the effect of tending to minimize the range of possible misalignments in use between the longitudinal axis of the float  80  and the towing direction. 
     FIGS. 6 and 7  show at  60  an alternative implementation of part of the deflector device  22  of  FIGS. 2 to 5 , with corresponding parts having the same reference numbers as were used in  FIGS. 2 to 5 . The principal difference between this alternative embodiment and the embodiment of  FIGS. 2 to 5  is that in the deflector device  60 , the boom  32  is secured at a fixed angle, typically about 10°, to the low pressure side  36  of the main wing-shaped body  28 , while the angle of the chord of the auxiliary wing-shaped body  52  with respect to the boom  32  is variable by means of a remotely operable electric stepper motor  62 . 
   As best seen in  FIG. 7 , the electric stepper motor  62  is secured to the boom  32  with its axis extending laterally of the boom, and is disposed in a cut-out or aperture  64  in the auxiliary wing-shaped body  52 . Coaxial drive shafts  66  protrude from each axial end of the motor  62  and are secured to the body  52  to rotate it about the common axis of the drive shafts. A slot  65  is provided in the body  52  between the aperture  64  and the trailing edge of the body, to accommodate the boom  32  as the body  52  is rotated by the motor  62 . As an alternative to the apertured and slotted implementation of the body  52 , the body  52  can be implemented in two separate but symmetrical halves disposed on respective sides of the boom  32  and each attached to a respective one of the drive shafts  66  of the motor  62 . 
   The boom  32  is of sandwich construction: it is made of two similarly shaped plates  68  which are bolted together at intervals along their length and which sandwich between them both a mounting flange  70  of the motor  62  and the boom mounting bracket  38  secured to the low pressure side of the main wing-shaped body  28 . Typically, the boom  32  is detached from the bracket  38  whenever the deflector device  60  is on the vessel  10 , for ease of stowage. 
   As in the embodiment of  FIGS. 2 to 5 , the end  40  of the boom is provided with a towing eye, indicated at  74  in  FIGS. 6 and 7 , for connection to a streamer  18 . However, as mentioned earlier, since stability is no longer dependent upon a streamer  18  being connected to the end  40  of the boom  32 , the towing eye  74  can be omitted, and the streamer  18  can be towed from the lead-in  20  at a point near the attachment point of the deflector device  60 . The same is true for the deflector device  22 . 
   It will be appreciated that varying the angle of the auxiliary wing-shaped body  52  of the deflector device  60  has the same effect as varying the angle of the boom  32  of the deflector device  22 , ie it changes the angle of the main wing-shaped body  28  with respect to the direction of tow and so changes the lift produced by the main wing-shaped body. However, for the deflector device  60 , less power is required to produce a given change in angle of the main wing-shaped body  28 , because of the increased leverage provided by the position of the auxiliary wing-shaped body  52  towards the end  40  of the boom  32  (as opposed to the position of the hydraulically-operated actuator strut  48  of the deflector device  22 ). It is this which permits the use of the relatively low-powered electric stepper motor  62  in the deflector device  60 , in place of the relatively more powerful hydraulic system which operates the mechanism based on the strut  48 . However, if desired, the electric stepper motor  62  can be replaced by a simple hydraulic actuator secured to the boom  32 , since this also would not need to be as powerful as the hydraulic system which operates the mechanism based on the strut  48 . An additional advantage of replacing the hydraulic system and the mechanism based on the strut  48  with the electric stepper motor  62  or a simple hydraulic actuator is the considerable weight saving which can be achieved. 
   It will be appreciated that many modifications can be made to the described embodiment of the invention. For example, the auxiliary wing-shaped body  52  of the deflector device  60  can be fixed, as in the deflector device  22 . Additionally, the deflector devices  22  and  60  can be used with tows other than streamers, for example seismic sources. And the invention can be applied to a deflector device similar to the MONOWING deflector device of U.S. Pat. No. 5,357,892.